Properties of Lignin and Poly(hydroxybutyrate) Blends · Properties of Lignin and...

Properties of Lignin and Poly(hydroxybutyrate) Blends

A Thesis by Publication submitted in

Partial Fulfilment of the Requirement for the

Degree of

Doctor of Philosophy

Payam Mousavioun

M.Sc., B.Sc. (Chemical Engineering)

Chemistry Discipline

Faculty of Science and Technology

Queensland University of Technology

Queensland, Australia

March 2011

III

Acknowledgment

It is a pleasure to thank everybody who made this thesis possible including my

supervisory team, my family and Queensland University of Technology (QUT).

First of all, I would like to express my deepest sense of gratitude to my principal

supervisor, Dr William Doherty, who trusted me and quickly discovered my

potential and interest in the research area. I am heartily thankful to Dr Doherty

whose encouragement, supervision, guidance and support from the preliminary

to the concluding level enabled me to develop an understanding of the subject. I

would also like to acknowledge the support of my associate supervisors,

Professor Graeme George and Professor Peter Halley, during my research

programme. Without their brilliant advice and very timely and valid hints

throughout the completion of my PhD programme, this thesis would not have

been possible. It is an honour for me to have worked with such a great and

prestigious supervisory team.

I would also like to convey my thanks to QUT for providing me with such a

pleasant research area and facilities. I gratefully acknowledge QUT and the

Centre for Tropical Crops and Biocommodities (CTCB) for the financial

assistance of this project through the Postgraduate Research Awards (QUTPRA)

Grant. The assistance of QUT Research Portfolio is also highly appreciated.

I am indebted to many of my colleagues and friends at the Australian Institute

for Bioengineering and Nanotechnology (AIBN), Centre High Performance

Polymers (CHPP) in the University of Queensland (UQ) and CTCB for

providing a warm research atmosphere, sharing of knowledge, and

encouragement. I will never forget the pleasant times I had with my friends

during our meetings at CHPP group at UQ and other social events.

I must acknowledge my beloved wife and best friend, Parastoo. Without her

love, encouragement and assistance, I would not have started and finished this

research programme.

Lastly, I offer my regards and blessings to all of those who have supported me in

any respect during the completion of the project.

IV

Table of Contents

Abbreviations and Nomenclature ............................................................. XVII

Abbreviation ............................................................................................ XVII

Nomenclature ..........................................................................................XVIII

Abstract .................................................................................................

.......................................................................................... XIX

Keywords .................................................................................................

....................................................................................... XXIII

Research Contributions ........................................................................... XXIV

List of Publications..................................................................................... XXV

List of Chapters According to Publications and Contributions ............ XXVII

Scholarship and Grants ........................................................................ XXVIII

Statement of Original Authorship ........................................................... XXIX

CHAPTER 1 ............................................................................................... 1

Introduction ............................................................................................... 1

1.1. Description of Research Problem ............................................................. 2

1.2. Theories and Literature Review ............................................................... 5

1.2.1. Miscibility theories ........................................................................... 5

1.2.2. Kinetics of thermal degradation ........................................................ 7

1.2.3. Literature review .............................................................................. 8

1.2.3.1. Lignin ........................................................................................ 9

1.2.3.2. Poly(hydroxybutyrate) ............................................................. 16

1.2.3.3. Lignin blends ........................................................................... 22

1.2.3.4. PHB blends .............................................................................. 27

1.2.3.5 Studies on the biodegradation of PHB blends ............................ 28

1.2.3.6. Lignin/PHB blends................................................................... 30

1.3. Account of Research Progress Linking the Research Papers .................. 33

1.3.2. Addendum: Kinetics of bagasse decomposition, Lignin

applications .......................................................................................... 34

1.4. References ............................................................................................. 39

V

CHAPTER 2 ............................................................................................. 44

Chemical and thermal properties of bagasse soda lignin ............................. 44

2.1. Introduction ........................................................................................... 45

2.2. Materials and Methods .......................................................................... 47

2.2.1. Lignin extraction ............................................................................ 47

2.2.2. Lignin fractionation ........................................................................ 48

2.2.3. Lignin characterisation methods ..................................................... 49

2.2.3.1. Elemental analysis ................................................................... 49

2.2.3.2. Ash analysis ............................................................................. 49

2.2.3.3. Bulk density ............................................................................. 49

2.2.3.4. Sugar analysis .......................................................................... 50

2.2.3.5. Purity analysis.......................................................................... 50

2.2.3.6. Characterisation of functional groups ....................................... 50

2.2.3.7. Molecular weight determination ............................................... 53

2.2.3.8. Thermogravimetric analysis (TGA) .......................................... 53

2.2.3.9. Differential scanning calorimetry (DSC) .................................. 54

2.3. Results .................................................................................................. 54

2.3.1. The fractionation process ................................................................ 54

2.3.2. Elemental analysis results ............................................................... 54

2.3.3. Molecular weight and functional groups ......................................... 55

2.3.4. Sugar analysis results ...................................................................... 57

2.3.5. Bulk density results ........................................................................ 58

2.3.6. TGA results .................................................................................... 58

2.3.7. Glass transition temperature ............................................................ 61

2.4. Discussion ............................................................................................. 62

2.5. References ............................................................................................. 64

CHAPTER 3 ............................................................................................. 66

Thermal stability and miscibility of poly(hydroxybutyrate) and soda lignin

blends .................................................................................. 66

3.1. Introduction ........................................................................................... 67


3.2.1. PHB................................................................................................ 69

VI

3.2.2. Soda lignin extraction ..................................................................... 70

3.2.3. Lignin characterisation method ....................................................... 70

3.2.3.1. Elemental analysis ................................................................... 70

3.2.3.2. Ash analysis ............................................................................. 71

3.2.3.3. Sugar analysis .......................................................................... 71

3.2.3.4. Characterisation of functional groups ....................................... 71

3.2.3.5. Molecular weight determination ............................................... 72

3.2.4. Blend preparation ........................................................................... 72

3.2.5. Characterisation of blend samples ................................................... 72



3.2.5.3. Scanning electron microscopy (SEM) ...................................... 73

3.2.5.4. Fourier transform-Infrared spectroscopy (FT-IR) ..................... 73

3.3. Results and Discussion .......................................................................... 73

3.4. Conclusion ............................................................................................ 82

Acknowledgments ........................................................................................ 83

3.5. References ............................................................................................. 84

CHAPTER 4 ............................................................................................. 87

Thermophysical properties and rheology of PHB/lignin blends .................. 87

4.1. Introduction ........................................................................................... 88


4.2.1. PHB ................................................................................................ 89

4.2.2. Lignin extraction ............................................................................ 90

4.2.3. Lignin characterisation.................................................................... 90

4.2.4. Blend preparation ........................................................................... 90

4.2.5. Characterisation of blend samples ................................................... 91



4.2.5.3. Rheological analysis ................................................................ 92

4.3. Results and Discussion .......................................................................... 92

4.3.1. Degradation of PHB ....................................................................... 92

4.3.2. Degradation of PHB/lignin blends .................................................. 94

4.3.3. Thermal properties of PHB/lignin blends ........................................ 97

VII

4.3.4. Rheological properties of PHB/lignin blends .................................. 99

4.4. Conclusion .......................................................................................... 104

4.5. References ........................................................................................... 105

CHAPTER 5 ........................................................................................... 107

Environmental degradation of soda lignin/ poly(hydroxybutyrate) blends

........................................................................................... 107

5.1. Introduction ......................................................................................... 108

5.2. Materials and Methods ........................................................................ 110

5.2.1. PHB.............................................................................................. 110

5.2.2. Lignin ........................................................................................... 110

5.2.3. Lignin characterisation ................................................................. 111

5.2.4. Blend preparation ......................................................................... 111

5.2.5. Polymer film fabrication ............................................................... 111

5.2.6. In situ biodegradation of polymer films in soil .............................. 112

5.2.7. Thermogravimetric analysis (TGA) .............................................. 112

5.2.8. Differential scanning calorimetry (DSC) ....................................... 113

5.2.9. X-ray photoelectron spectroscopy analysis.................................... 114

5.2.10. Scanning electron microscopy (SEM) ......................................... 114

5.2.11. Fourier transform-infrared spectroscopy (FT-IR) ........................ 115

5.3. Results and Discussion ........................................................................ 115

5.3.1. Gravimetric analysis ..................................................................... 115

5.3.2. Thermogravimetric analysis .......................................................... 117

5.3.3. Differential scanning calorimetry (DSC) ....................................... 119

5.3.4. XPS analysis ................................................................................. 121

5.3.5. FT-IR analysis .............................................................................. 123

5.3.6. Macroscopic and microscopic changes.......................................... 126

5.4. Conclusion .......................................................................................... 127

Acknowledgements .................................................................................... 127

5.5. References ........................................................................................... 128

CHAPTER 6 ........................................................................................... 131

Thermal stability and miscibility of poly(hydroxybutyrate) and methanol-

soluble soda lignin blends ................................................. 131

VIII

6.1. Introduction ......................................................................................... 132

6.2. Experimental ....................................................................................... 133

6.2.1. PHB .............................................................................................. 133

6.2.2. Lignin extraction .......................................................................... 134

6.2.3. PHB/Lignin blends ....................................................................... 135

6.2.4. Characterisation of blends ............................................................. 135

6.2.4.1. Differential scanning calorimetry (DSC) ................................ 135

6.2.4.2. Thermogravimetric analysis ................................................... 136

6.2.4.3. Scanning electron microscopy (SEM) .................................... 136

6.2.4.4. Fourier transform-Infrared spectroscopy (FT-IR) ................... 136

6.3. Results and Discussion ........................................................................ 136

6.4. Conclusion .......................................................................................... 143

6.5. References ........................................................................................... 144

CHAPTER 7 ........................................................................................... 146

Conclusions and Further Research ............................................................. 146

7.1. Conclusions ......................................................................................... 147

7.1.1. Thermal properties and miscibility study....................................... 147

7.1.2. Thermophysical and rheological properties of lignin/PHB blends . 147

7.1.3. Environmental investigation of lignin/PHB blends ........................ 148

7.1.4. Thermal properties of PHB blends with different types of lignin ... 148

7.2. Future Research ................................................................................... 149

7.2.1. Study of molecular structure of PHB during blend processing ....... 149

7.2.2. Modeling the viscoelasticity of lignin/PHB blends ........................ 149

7.2.3. Study of antimicrobial effect of lignin ........................................... 150

7.2.4. Study of mechanical properties of lignin/PHB blends.................... 150

7.3. References ........................................................................................... 151

APPENDIX 1 ........................................................................................... 152

Thermal decomposition of bagasse. Effect of different sugarcane cultivars

........................................................................................... 152

A.1.1. Introduction ..................................................................................... 153

A.1.2. Experimental ................................................................................... 155

A.1.3. Results and Discussion .................................................................... 158

IX

A.1.3.1. Bagasse composition ................................................................. 158

A.1.3.2. Thermogravimetry (TG) and derivative thermogravimetry (DTG)

analyses .................................................................................................. 159

A.1.3.3. Kinetic study ............................................................................. 167

A.1.4. Conclusion ...................................................................................... 169

A.1.5. References ....................................................................................... 171

APPENDIX 2 ........................................................................................... 174

Value-adding to cellulosic ethanol: Lignin polymers .................................. 174

A.2.1. Introduction ..................................................................................... 175

A.2.2. Lignin Structure ............................................................................... 177

A.2.3. Lignin Fractionation Processes ........................................................ 180

A.2.3.1. Sulfite process........................................................................... 180

A.2.3.2. Kraft process ............................................................................. 182

A.2.3.3. Soda process ............................................................................. 183

A.2.3.4. Other fractionation processes .................................................... 183

A.2.4. Physical Properties of Lignin ........................................................... 185

A.2.5. Applications .................................................................................... 188

A.2.5.1. Protein-lignin blends ................................................................. 191

A.2.5.2. Starch-lignin blends .................................................................. 193

A.2.5.3. Polyhydroxyalkanoates ............................................................. 195

A.2.5.4. Polylactides and polyglycolides ................................................ 197

A.2.5.5. Epoxy resin blends .................................................................... 198

A.2.5.6. Phenol-formaldehyde resins ...................................................... 200

A.2.5.7. Lignin-polyolefin blends ........................................................... 203

A.2.5.8. Lignin-vinyl polymer blends ..................................................... 206

A.2.5.9. Lignin-polyester blends ............................................................. 208

A.2.5.10. Lignin-containing polyurethanes and lignin-polyurethane blends

............................................................................................................... 210

A.2.5.11. Rubber-lignin blends ............................................................... 211

A.2.5.12. Lignin-graft-copolymers ......................................................... 212

A.2.6. Conclusions ..................................................................................... 215

A.2.7. References ....................................................................................... 217

X

List of Figures

Figure 1-1 The description of the process for mixing two polymers.......... 5

Figure 1-2 Cell wall organisation of typical wood presented by Smook

(1934) ................................................................................... 10

Figure 1-3 The molecular repeating unit of cellulose ............................. 10

Figure 1-4 Structure of hemicellulose monomeric sugar units ................ 11

Figure 1-5 Structure of the H-type monomer unit of lignin. Labelled are

the α , β and γ positions of the aryl ether bonds ...................... 13

Figure 1-6 The structure of a possible lignin macromolecule (Glasser, et

al., 1999)............................................................................... 14

Figure 1-7 The structure of the C9 monomer units of lignin. ................... 14

Figure 1-8 Flow scheme of (a) life cycle of PHB, and (b) PHB

manufacturing process (Ghaffar, 2002) ................................. 18

Figure 1-9 Monomer units of PHB, PHV and their copolymer PHBV .... 19

Figure 1-10 DSC cooling and heating curves of pure PHB and PHB/lignin

blend samples ....................................................................... 31

Figure 1-11 Spherulitic growth rate at various crystallisation temperatures

for both a pure PHB and a PHB/lignin blend (Weihua, et al.,

2004) .................................................................................... 32

Figure 1-12 Fractionation process of soda lignin. ..................................... 33

Figure 1-13 Haake mini lab twin extruder ................................................ 35

Figure 2-1 NMR spectrum of an acetylated lignin (L2) fraction ............. 52

Figure 2-2 Size exclusion chromatogrms of lignin and its fractions ........ 56

Figure 2-3 TGA/DTG curve of L1 performed under nitrogen atmosphere

............................................................................................. 59


............................................................................................. 60


............................................................................................. 60

XI

Figure 2-6 TGA/DTG curve of the starting soda lignin performed under

nitrogen atmosphere.............................................................. 61

Figure 2-7 DSC curves for lignin and its fractions .................................. 61

Figure 3-1 The integral thermogravimetric curves for PHB, soda lignin

and PHB/soda lignin blends. ................................................. 75

Figure 3-2 Plots of T0 and T50 of PHB/soda lignin blends versus soda

lignin content. ....................................................................... 76

Figure 3-3 DSC curves of PHB/soda lignin. ........................................... 77

Figure 3-4 Tgs of PHB and the blends versus soda lignin content. .......... 78

Figure 3-5 SEM image of PHB/soda lignin containing 10 wt% lignin. ... 79



Figure 3-8 FT-IR spectra of the carbonyl stretching region of PHB and

PHB/soda lignin blends. ........................................................ 81

Figure 3-9 Hydrogen bonding interactions between the reactive functional

groups in soda lignin and the carbonyl groups of PHB. ......... 82

Figure 4-1 Isothermal degradation of PHB ............................................. 94

Figure 4-2 Integral thermogravimetric curves for PHB, lignin and 50 wt%

PHB/lignin............................................................................ 95

Figure 4-3 Threshold degradation temperature of PHB/lignin blends ..... 96

Figure 4-4 Activation energy of thermal degradation of PHB/Lignin

blends ................................................................................... 97

Figure 4-5 DSC thermograms of PHB/lignin blends with (a) 40 wt%

lignin, and (b) 80 wt% lignin ................................................ 98

Figure 4-6 Complex viscosity (η *) versus % strain for PHB/10 wt% lignin

........................................................................................... 100

Figure 4-7 Dynamic storage modulus of PHB and PHB/lignin blends .. 100

Figure 4-8 Dynamic loss modulus of PHB and PHB/lignin blends ....... 101

Figure 4-9 Tan δ of PHB and PHB/lignin blends .................................. 102

XII

Figure 4-10 Complex viscosity (η *) of blends of pure PHB and PHB/lignin

blends ................................................................................. 103

Figure 5-1 Buried mass loss of pure and blended PHB with lignin ....... 116

Figure 5-2 Actual and expected mass ratio of lignin/PHB blends after 4

months soil burial. .............................................................. 116

Figure 5-3 Mass ratio on thermal degradation of PHB, lignin, 30 and 60

wt% lignin blends after 4, 8 and 12 months of burial test .... 117

Figure 5-4 PHB random chain scission at temperatures of 170ºC-200ºC

........................................................................................... 118

Figure 5-5 PHB chain scission at temperature of 200ºC- 300ºC ............ 118

Figure 5-6 Survey XPS of (a) PHB and (b) lignin and multiplex scans of

carbon bonds of (c) PHB and (d) lignin ............................... 122

Figure 5-7 Multiplex carbon scan of 20 wt% lignin films at (a) zero time

and (b) 4 months buried. ..................................................... 123

Figure 5-8 FT-IR spectra of (a) PHB, (b) lignin and (c) 4 months buried,

10 wt% lignin/PHB blend ................................................... 124

Figure 5-9 FT-IR spectra of the carbonyl stretching region of (a) Pure

PHB and (b) 10 wt% lignin/PHB blend ............................... 125

Figure 6-1 TGA/DTG curve of ML performed under nitrogen atmosphere.

........................................................................................... 137

Figure 6-2 TGA/DTG curve of PHB performed under nitrogen

atmosphere. ........................................................................ 137

Figure 6-3 The integral thermogravimetric curves for PHB, ML and ML-

PHB blends. ........................................................................ 138

Figure 6-4 DSC curves of ML/PHB blends (refer to Figure 4-5) .......... 139

Figure 6-5 Tgs of PHB and the blends versus ML content. ................... 140

Figure 6-6 SEM image of ML/PHB blend containing 10 wt% ML. ...... 141



XIII


ML/PHB blends. ................................................................. 142

Figure A.1-1 Thermal decomposition curve of bagasse sample 6987

performed under nitrogen atmosphere ................................. 161



Figure A.1-3 Thermal decomposition curve of washed bagasse sample 7087






Figure A.1-6 Thermal decomposition curve of washed bagasse sample 7170




Figure A.1-8 Friedman’s plot for various α values for sample 6987 ......... 169

Figure A.1-9 A comparison of activation energy as a function of the degree

of conversion (α ) of bagasse samples originating from different

sugarcane cultivars.............................................................. 169

Figure A.2-1 Cellulose strands surrounded by hemicellulose and lignin ... 177

Figure A.2-2 Monolignol monomer species ............................................. 178

Figure A.2-3 Significant lignin linkage structures. ................................... 179

Figure A.2-4 Correlation between the glass transition temperature (Tg) and

the degree of condensation .................................................. 187

Figure A.2-5 Correlation between total aggregate surface area observed per

photo and the solubility parameter of the polymer matrix .... 188

Figure A.2-6 Hydrogen bonding between β-1 stilbene and amylose. ........ 194

Figure A.2-7 Miscibility of lignin/PHB blends based on Tg. .................... 197

XIV

Figure A.2-8 Conversion profiles of lignin-based phenol formaldehyde

resins .................................................................................. 202

Figure A.2-9 Potential sites for hydrogen abstraction for free-radical grafting

from lignin .......................................................................... 213

XV

List of Tables

Table 1-1 Tg values of some different types of lignin (Glasser, et al.,

1999) .................................................................................... 16

Table 2-1 Elemental analysis of lignins (wt%) ...................................... 55

Table 2-2 Lignin fractions formulae...................................................... 55

Table 2-3 Molecular weight averages and function groups .................... 57

Table 2-4 Purity of lignins .................................................................... 58

Table 2-5 Tg of lignin and fractions ...................................................... 62

Table 3-1 Molecular weight of soda lignin and lignin components (wt%)

............................................................................................. 74

Table 4-1 Molecular weight of lignin and lignin components (wt%)

(Mousavioun et al., 2010) ..................................................... 90

Table 4-2 Degradation rate constant of PHB at various temperatures .... 94

Table 4-3 Degradation rate constant of 50 wt% PHB/lignin blends at

various temperatures ............................................................. 96

Table 4-4 Thermal properties of lignin/PHB blends using the starting

lignin material....................................................................... 99

Table 5-1 Molecular weight of lignin and lignin components (wt%)

(Mousavioun et al., 2010) ................................................... 111

Table 5-2 Thermal properties of virgin lignin/PHB blends cast films at

different ratios and biodegraded after 4, 8 and 12 months.... 120

Table 5-3 Macro and Microfilms of PHB and lignin/PHB blends (scale

bar= 15 mm for Macro and 40 µm for Microfilms) ............. 127

Table 6-1 Characterisation of ML ....................................................... 135

Table 6-2 Peak associated with crystalline portion of PHB ................. 142

Table A.1-1 Bagasse sugarcane cultivars and the soil types .................... 156

Table A.1-2 Compositional analysis of pre-dried bagasse samples.......... 158

XVI

Table A.1-3 Energy dispersive spectroscopy results of the elemental

composition of the residues obtained after thermal

decomposition of bagasse ................................................... 165

Table A.1-4 X-ray powder diffraction d-values (Å) for bagasse ash ........ 166

Table A.2-1 Molecular weight and functional groups of lignins .............. 185

Table A.2-2 Tg of different lignin types (Gargulak and Lebo, 2000) ....... 186

Table A.2-3 Application of lignosulfonate products based on their surface-

active properties .................................................................. 188

Table A.2-4 Lignosulfonate products in speciality markets ..................... 189

XVII

Abbreviations and Nomenclature

Abbrev ia t ion AIBN Australian Institute for Bioengineering and Nanotechnology CHPP Centre High Performance Polymers CTCB Centre for Tropical Crops and Biocommodities DMF N,N’ dimethylformamide DSC Differential scanning calorimetry EL Ether-soluble lignin FT-IR Fourier transform-infrared spectroscopy HPLC high performance liquid chromatography L1 diethyl ether fractionated lignin L2 methanol soluble fractionated lignin L3 Residual solvent fractionated lignin Lignin/PHB Blend of lignin and PHB, the same as PHB/lignin ML Methanol-soluble lignin NMR Nuclear magnetic resonance NREL National Renewable Energy Laboratory PE Polyethylene PEO poly(ethylene oxide) PHA Poly(hydroxyalkanoate) PHB Poly(hydroxybutyrate) PHBV poly(hydroxybutyrate-hydroxyvalerate) PHH Poly(hydroxyhexanoate) PHO Poly(hydroxyoctanoate) PHV Poly(hydroxyvalerate) PLA Poly(lactic acid) PP Polypropylene PVA polyvinyl alcohol QUT Queensland University of Technology QUTPRA QUT Postgraduate Research Award RACI The Royal Australian Chemical Institute RL Residual lignin SEM Scanning electron microscopy TGA Thermogravimetric analysis TnBACl tetra-n-butylammonium chloride UQ University of Queensland XPS X-ray photoelectron spectroscopy analysis

XVIII

Nomencla ture

A s�� the pre-exponential factor � - the degree of conversion β

°C min-1 the linear heating rate ∆Hm J g-1 melting enthalpy η * Pa.s complex viscosity E� kJ mol�� apparent activation energy ΔE� kJ mol�� energy of vaporation to a gas at zero pressure G' Pa storage modulus G" Pa loss modulus G�� J Gibbs free energy K�� - Gordon-Taylor equation adjustable parameter K� - Kwei equation adjustable parameter Mn g mol-1 number average molecular weight Mw g mol-1 weight average molecular weight σ - solubility q - Kwei equation adjustable parameter R JK�� mol�� the ideal gas constant S� JK�� entropy T K absolute temperature t min time

Tan δ

- the ratio of energy dissipated to energy stored Tcc °C cold crystal temperature Tg °C glass transition temperature Tm °C melting temperature T�� °C the equilibrium melting point V cm3 volume v! cm3 mol-1 the molar volume w - weight fraction W g weight x - ratio against the % methoxyl (OCH3) content

x%&' - mass ratio of PHB X! - Bulk crystallinity X)! - PHB crystallinity

XIX

Abstract

The Queensland University of Technology (QUT) allows the presentation of a

thesis for the Degree of Doctor of Philosophy in the format of published or

submitted papers, where such papers have been published, accepted or submitted

during the period of candidature. This thesis is composed of Seven

published/submitted papers and one poster presentation, of which five have been

published and the other two are under review. This project is financially

supported by the QUTPRA Grant.

The twenty-first century started with the resurrection of lignocellulosic biomass

as a potential substitute for petrochemicals. Petrochemicals, which enjoyed the

sustainable economic growth during the past century, have begun to reach or

have reached their peak. The world energy situation is complicated by political

uncertainty and by the environmental impact associated with petrochemical

import and usage. In particular, greenhouse gasses and toxic emissions produced

by petrochemicals have been implicated as a significant cause of climate

changes.

Lignocellulosic biomass (e.g. sugarcane biomass and bagasse), which potentially

enjoys a more abundant, widely distributed, and cost-effective resource base, can

play an indispensible role in the paradigm transition from fossil-based to

carbohydrate-based economy.

Poly(3-hydroxybutyrate), PHB has attracted much commercial interest as a

plastic and biodegradable material because some its physical properties are

similar to those of polypropylene (PP), even though the two polymers have quite

different chemical structures. PHB exhibits a high degree of crystallinity, has a

high melting point of approximately 180°C, and most importantly, unlike PP,

PHB is rapidly biodegradable.

Two major factors which currently inhibit the widespread use of PHB are its

high cost and poor mechanical properties. The production costs of PHB are

significantly higher than for plastics produced from petrochemical resources (e.g.

PP costs $US1 kg-1, whereas PHB costs $US8 kg-1), and its stiff and brittle

nature makes processing difficult and impedes its ability to handle high impact.

XX

Lignin, together with cellulose and hemicellulose, are the three main components

of every lignocellulosic biomass. It is a natural polymer occurring in the plant

cell wall. Lignin, after cellulose, is the most abundant polymer in nature. It is

extracted mainly as a by-product in the pulp and paper industry. Although,

traditionally lignin is burnt in industry for energy, it has a lot of value-add

properties. Lignin, which to date has not been exploited, is an amorphous

polymer with hydrophobic behaviour. These make it a good candidate for

blending with PHB and technically, blending can be a viable solution for price

and reduction and enhance production properties. Theoretically, lignin and PHB

affect the physiochemical properties of each other when they become miscible in

a composite. A comprehensive study on structural, thermal, rheological and

environmental properties of lignin/PHB blends together with neat lignin and

PHB is the targeted scope of this thesis. An introduction to this research,

including a description of the research problem, a literature review and an

account of the research progress linking the research papers is presented in

Chapter 1.

In this research, lignin was obtained from bagasse through extraction with

sodium hydroxide. A novel two-step pH precipitation procedure was used to

recover soda lignin with the purity of 96.3 wt% from the black liquor (i.e. the

spent sodium hydroxide solution). The precipitation process is presented in

Chapter 2. A sequential solvent extraction process was used to fractionate the

soda lignin into three fractions. These fractions, together with the soda lignin,

were characterised to determine elemental composition, purity, carbohydrate

content, molecular weight, and functional group content. The thermal properties

of the lignins were also determined. The results are presented and discussed in

Chapter 2. On the basis of the type and quantity of functional groups, attempts

were made to identify potential applications for each of the individual lignins.

As an addendum to the general section on the development of composite

materials of lignin, which includes Chapters 1 and 2, studies on the kinetics of

bagasse thermal degradation are presented in Appendix 1. The work showed that

distinct stages of mass losses depend on residual sucrose. As the development of

value-added products from lignin will improve the economics of cellulosic

XXI

ethanol, a review on lignin applications, which included lignin/PHB composites,

is presented in Appendix 2.

Chapters 3, 4 and 5 are dedicated to investigations of the properties of soda

lignin/PHB composites. Chapter 3 reports on the thermal stability and

miscibility of the blends. Although the addition of soda lignin shifts the onset of

PHB decomposition to lower temperatures, the lignin/PHB blends are thermally

more stable over a wider temperature range. The results from the thermal study

also indicated that blends containing up to 40 wt% soda lignin were miscible.

The Tg data for these blends fitted nicely to the Gordon-Taylor and Kwei

models. Fourier transform infrared spectroscopy (FT-IR) evaluation showed that

the miscibility of the blends was because of specific hydrogen bonding (and

similar interactions) between reactive phenolic hydroxyl groups of lignin and the

carbonyl group of PHB.

The thermophysical and rheological properties of soda lignin/PHB blends are

presented in Chapter 4. In this chapter, the kinetics of thermal degradation of the

blends is studied using thermogravimetric analysis (TGA). This preliminary

investigation is limited to the processing temperature of blend manufacturing.

Of significance in the study, is the drop in the apparent energy of activation, Ea

from 112 kJmol-1 for pure PHB to half that value for blends. This means that the

addition of lignin to PHB reduces the thermal stability of PHB, and that the

comparative reduced weight loss observed in the TGA data is associated with the

slower rate of lignin degradation in the composite. The Tg of PHB, as well as its

melting temperature, melting enthalpy, crystallinity and melting point decrease

with increase in lignin content. Results from the rheological investigation

showed that at low lignin content (≤30 wt%), lignin acts as a plasticiser for PHB,

while at high lignin content it acts as a filler.

Chapter 5 is dedicated to the environmental study of soda lignin/PHB blends.

The biodegradability of lignin/PHB blends is compared to that of PHB using the

standard soil burial test. To obtain acceptable biodegradation data, samples were

buried for 12 months under controlled conditions. Gravimetric analysis, TGA,

optical microscopy, scanning electron microscopy (SEM), differential scanning

calorimetry (DSC), FT-IR, and X-ray photoelectron spectroscopy (XPS) were

used in the study. The results clearly demonstrated that lignin retards the

XXII

biodegradation of PHB, and that the miscible blends were more resistant to

degradation compared to the immiscible blends.

To obtain an understanding between the structure of lignin and the properties of

the blends, a methanol-soluble lignin, which contains 3× less phenolic hydroxyl

group that its parent soda lignin used in preparing blends for the work reported in

Chapters 3 and 4, was blended with PHB and the properties of the blends

investigated. The results are reported in Chapter 6. At up to 40 wt% methanol-

soluble lignin, the experimental data fitted the Gordon-Taylor and Kwei models,

similar to the results obtained soda lignin-based blends. However, the values

obtained for the interactive parameters for the methanol-soluble lignin blends

were slightly lower than the blends obtained with soda lignin indicating weaker

association between methanol-soluble lignin and PHB. FT-IR data confirmed

that hydrogen bonding is the main interactive force between the reactive

functional groups of lignin and the carbonyl group of PHB. In summary, the

structural differences existing between the two lignins did not manifest itself in

the properties of their blends.

XXIII

Keywords

Application Bagasse Biodegradation Biomass Blend Bulk density Burial test Cellulose Characterisation Cold crystallinity Crystallinity Degradation Differencial scanning analysis (DSC) Elemental analysis Environmental Extrusion Fractionation Fourier transform infrared spectroscopy (FT-IR) Glass transition temperature (Tg) Gravimetric analysis Hemicellulose Hydrogen bonding Kinetics Lignin Lignin chemistry Lignocellulose materials Melting point Miscibility Molecular weight Nuclear magnetic resonance (NMR) Poly(hydroxybutyrate) (PHB) Properties Rheological analysis Scanning electron microscopy (SEM) Soda lignin Sugar analysis Sugarcane Sustainability Thermal stability Thermogravimetric analysis (TGA) Viscoelasticity X-ray photoelectron spectroscopy (XPS)

XXIV

Research Contributions

The study on composite materials made from lignin and PHB has added to the

body of knowledge on biodegradable plastics by providing the following

outputs:

Provided information at both the macroscopic and microscopic levels that

was used to explain the properties exhibited by lignin/PHB blends.

Established that the Gordon-Taylor and Kwei models can be used to

predict the Tg of lignin/PHB blends and similar composite materials.

Provided data showing regions of miscibility and immiscibility between

lignin and PHB.

Provided data showing regions where lignin acts a plasticiser for PHB

and regions where it acts as a filler. This has enabled areas for easy

processing of such materials to be identified

Provided information on the environmental performance (i.e.

biodegradability) of lignin/PHB blends.

Published five peer-reviewed articles, with another two articles under

review.

XXV

List of Publications

The following publications have been produced as a result of this thesis.

Peer reviewed journal papers:

1. Payam Mousavioun and William O.S Doherty, “Chemical and thermal

properties of fractionated bagasse soda lignin”, Industrial Crops and

Products, Vol 31, 52-58, 2010. Impact factor, 2.103.

2. Payam Mousavioun, William O.S. Doherty and Graeme A. George,

“Thermal stability and miscibility of poly(hydroxybutyrate) and soda

lignin blends”, Industrial Crops and Products, Vol 32, 656-661, 2010.

Impact factor 2.103.

3. William O.S.Doherty, Payam Mousavioun, Christopher M.Fellows,

“Value-adding to cellulosic ethanol: Lignin polymers”, published in

Industrial Crops and Products, Vol 33, 259-276, 2011. Impact factor

2.103.

4. Vanita R. Maliger, William O. S. Doherty, Ray L. Frost, and Payam

Mousavioun, “Thermal Decomposition of Bagasse: Effect of Different

Sugar Cane Cultivars”, published in Industrial & Engineering Chemistry

Research, Vol 50, 791-798, 2011. Impact factor 1.752.

Peer reviewed journal papers under review:

5. Payam Mousavioun, Peter Halley and William O.S. Doherty,

“Thermophysical properties and rheology of PHB/lignin blends”,

Polymer International, 2011. Impact factor 2.137.

6. Payam Mousavioun, Graeme A. George and William O.S. Doherty,

“Environmental degradation of soda lignin/PHB blends”, Polymer

Degradation and Stability, 2011. Impact factor 2.137.

Published peer reviewed international conference paper:

7. Payam Mousavioun, William O.S. Doherty, Graeme A. George and

Peter Halley, “Thermal stability and miscibility of poly(hydroxybutyrate)

and methanol-soluble soda lignin blends”, presentation in 10th AIChE

XXVI

meeting, Salt Lake City, UT, USA, November 2010. CD Rom. Note that-

publication in AIChE journal is categorised as A class, though the present

work was published as an AIChE proceedings.

Poster presentation:

8. Payam Mousavioun, William O. S. Doherty, Graeme A. George and

Peter Halley “Thermal behaviour of PHB/lignin composites”, Poster

presentation, 11th Pacific Polymer Conference, Cairns, Australia,

December 2009.

X

XV

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XXVIII

Scholarship and Grants

Tuition fee waiver Award from Faculty of Science and Technology,

Queensland University of Technology (QUT), 2008-2011.

QUT Postgraduate Research Award (QUTPRA), 2009-2011.

The Royal Australian Chemical Institute (RACI), grant for attending 11th

Pacific Polymer Conference, Cairns, Australia, 2009.

QUT grant -in-aid for attending 10th AIChE meeting, Salt Lake City, UT,

USA, 2010.

XXIX

Statement of Original Authorship

“The work contained in this thesis has not been previously submitted to meet

requirements for an award at this or any other higher education institution. To

the best of my knowledge and belief, the thesis contains no material previously

published or written by another person except where due reference is made.”

Signature Date

1

CHAPTER 1

Introduction

2

1.1. Descr ip t ion of Research Problem It is now widely accepted that in order to help to reduce global warming it is

necessary to use sustainable environmentally friendly plastics instead of the

traditional petroleum-based ones. Many petroleum-based polymers do not

degrade and are usually decomposed by combustion, thereby adding to the

carbon dioxide levels in the atmosphere. Although there are a few commercially

available biodegradable polymers suitable for commodity applications, their cost

is prohibitive. An example is poly(hydroxybutyrate (PHB). However, PHB has

poor mechanical properties and is difficult to process (Khanna and Srivastava,

2005). The main reasons for its poor properties include (a) low Tg, (b)

undergoes secondary crystallisation which occurs during storage at ambient

temperature, and (c) has a low nucleation density which allows large spherulites,

with cracks and splits, to form. Polymer blending is considered to be one of the

most effective methods for lowering the cost of production of these types of

polymers, and in certain cases improves processing and product quality. The

strategy in this project is to blend lignin, an inexpensive biodegradable

amorphous polymer, with high-value biodegradable aliphatic polyester, PHB,

and to investigate the properties of the blends. The project will, therefore

evaluate the physico-chemical properties of lignins, establish suitable processing

conditions for the preparation of the blends, assess the thermal, mechanical and

rheological properties of the blends, and determine the probable environmental

degradation mechanisms of the blends. The overall benefit from the research is

an improved knowledge on the performance and applicability of lignin-based

composite materials. The research activities have been divided into three main

aims:

• Develop composite materials from lignin by investigating the preparation

and characterisation of soda lignins.

• Prepare, characterise and determine the properties of soda lignin/PHB

blends.

• Prepare, characterise and determine the properties of methanol-soluble

lignin/PHB blends. This is to examine whether the structural differences

3

between soda lignin and methanol-soluble lignin will affect the properties

of the corresponding blends derived from them.

Research Aim #1: Develop composite materials of lignin

Sugar cane fibre, bagasse (a lignocellulosic material), is the fibrous residue from

the sugarcane milling process. The Australian sugar industry harvests around 35

million tonnes of sugarcane a year and this is converted into 5 million tonnes of

sugar, 1 million tonnes of molasses and 10 million tonnes of bagasse. There is

now a focus by the industry to increase the income stream by adding value to the

whole sugarcane biomass, including bagasse. Increasing amounts of surplus

bagasse will therefore become available as the Australian sugar industry

continues to move towards increased energy efficiency. Presently, bagasse is

burned for its fuel value to produce steam and electricity for factory operations.

The cellulose component of bagasse (50% of dry matter) has attracted interest as

a potential source of fuel ethanol. The other component of bagasse is lignin

(20% dry matter), a non-toxic amorphous hydrophobic polymer obtained readily

through extraction methods. Its macromolecular structure and low cost makes

lignin and lignin esters a good candidate for blending with aliphatic polyesters

such as PHB.

Lignin is composed of phenylpropane repeat units and possesses aliphatic and

aromatic hydroxyl groups together with vacant para-sites on the aromatic

monomer unit (section 1.2.3.1). This functionality makes lignin amenable to

chemical reactions. However, for lignin to be used as a feedstock to produce

composite materials of consistent quality, it has to be of high purity, susceptible

to chemical reactions, and of narrow molecular weight distribution. Thus, a

process for lignin isolation and purification from bagasse is a sub-objective in

this project. A number of destructive and non-destructive analytical tools were

used for detailed characterisation of lignin, including its molecular weight and

functionality.

Research Aim #2: Prepare and characterise soda lignin/PHB blends

The incorporation of an amorphous polymer such as lignin or lignin ester should,

in principle, improve the overall properties of PHB by lowering the melting

point, reducing secondary crystallisation, improving processability and reducing

4

brittleness. To optimise processing conditions for the preparation of lignin/PHB

blends, the thermal properties of lignin, the thermal properties of PHB, and the

kinetics of PHB degradation were investigated. The lignin/PHB blends were

assessed by investigating thermal and miscibility properties, as well as

mechanical and rheological properties. Biodegradation studies of the blends

were based on a standard burial soil test.

Research Aim #3: Prepare and characterise methanol-soluble lignin/PHB

blends

The aim of this phase of the project is to study methanol-soluble lignin/PHB

blends in order to establish whether the differences in lignin structure would

affect the properties of lignin/PHB blends. The experimental protocol used to

prepare methanol-soluble lignin/PHB blends was similar to those of soda lignin-

based PHB blends.

A summary of the research plan is outlined as follows: Phase 1 –

Characterisation of lignins; Phase 2 – Preparation, characterisation and

properties of lignin/PHB blends; Phase 3 – Environmental degradation of soda

lignin/PHB blends; Phase 4 – Preparation, characterisation and properties of

methanol-soluble lignin/PHB blends.

5

1.2. Theor ies and L i terature Review

1.2.1. Miscibility theories

Many researchers have studied polymer blending for the development of new

materials and to tailor properties of the blends by exploiting the physical,

chemical, mechanical and thermal properties of the individual components

(Lipatov and Nesterov, 1997).

There are several theories that have been developed to describe compatability

and miscibility of polymers. One of the most famous is the Flory-Huggins

treatment of polymer/solvent interactions in binary polymer systems (Lipatov

and Nesterov, 1997). This theory devised a general scheme which enables one to

deal with the mixing properties of a pair of polymers. It provides a basic

understanding of the occurrence of different types of phase diagrams

independent of temperature and molecular weight. Figure 1-1 illustrates the

process of mixing two polymers, A and B; where nA and nB are moles of the

polymers A and B, and VA and VB are their respective volumes, with V being the

total volume.

Figure 1-1 The description of the process for mixing two polymers

In order to find out whether true mixing would indeed occur, the change in the

Gibbs free energy has to be considered. This change, called the ‘Gibbs free

energy of mixing’ and denoted by ∆-./0, is given by:

nnnnAAAA

VVVVAAAA

nnnnBBBB

VVVVBBBB

nnnnA A A A , , , , nnnnBBBB

V=VV=VV=VV=VAAAA + V+ V+ V+ VBBBB

GGGGAAAA GGGGBBBB GGGGABABABAB

6

∆-./0 2 -45 6 7-4 3 -58 (1-1)

where -4, -5 and -45 denote the Gibbs free energies of the polymers A and B in

separate states and the mixed state, respectively.

The Flory-Huggins treatment represents ∆-./0 as a sum of two contributions:

∆-./0 = 69∆:; + ∆-<=> (1-2)

where the first component 9∆:; is the product of the temperature (T) and the

translational entropy (∆:;), and the second component is the interactions and

motions of the polymers represented by ∆-<=>.

According to equation (1-2), a decrease in ∆-<=> in association with an increase

in ∆:; will lead to a decrease in ∆-./0, which favours miscibility.

Now ∆:; and ∆-<=> can be represented by:

∆:; 2 ?7@4 A BBCD 3 @5 A B

BED8 (1-3)

∆-<=> 2 ?9F7BCBEBGH

8 (1-4)

where ? is the ideal gas constant and I> is the molar volume of a reference unit

(i.e. solvent) common to both polymers. Principally I> can be chosen arbitrarily,

but usually it is identified as the volume occupied by one of the polymer

components in the polymer solution. The decisive factor that describes the

extent of miscibility is the ‘Flory-Huggins interaction parameter’ χ. χ describes

the thermodynamic ‘quality’ of one component to act as a solvent towards

another. Flory-Huggins interaction parameter χ can be estimated from solubility

parameters using the following equation:

χ 2 7KL�KM8MGHNO (1-5)

where σ� σR are the solubility parameters of polymer 1 and polymer 2.

The ability of a polymer to influence the properties of another depends primarily

on its ability to associate and interact with that polymer. Methods for measuring

the association or compatibility on the nano-level (apart from measuring χ)

include electron microscopic techniques and thermal analysis. The presence of

single glass transition temperature (Tg), and the depression of the equilibrium

7

melting point 9.� , and Tm are useful parameters that can also be used to

demonstrate miscibility. The Tg can be determined using differential scanning

calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA). In this

study, all measurements of Tg have been undertaken using only DSC instrument.

It was found that the speciments were too brittle to effectively use the DMTA for

the measurement of Tg.

1.2.2. Kinetics of thermal degradation

It is of practical significance to understand and predict the thermal

decomposition process of polymer blends, since this knowledge will help to

better design the engineering process and to estimate the influence on blend

properties by thermal events. It is necessary to consider the kinetics of

decomposition over a wide range of decomposition temperatures. This limits the

use of the conventional isothermal approach. The non-isothermal approach has

the advantage that the decomposition process can be examined at elevated

temperatures and over a wide temperature range. At these temperatures the

degradation process may follow different mechanisms and so provide useful

practical information for the design engineer.

Yao et al. (2008) describes various methods that are used to calculate kinetic

parameters for the thermal decomposition of compounds based on weight loss.

These include first-order decomposition kinetics with different reaction schemes

involving single or multiple constant heating rate methods (i.e. non-isothermal).

For this work, the Friedman’s method (1964) has been used since the method

was applied for the thermal degradation of polymers.

The general rate equation for a decomposition or degradation process can be

described as:

S�S; ~ USVSO 2 W798X7�8 (1-6)

where � is the degree of conversion, U the linear heating rate (°C min-1), W798 is the rate constant and X7�8 is the reaction rate model, a function which

depends on the actual reaction mechanism. The rate constant, W798 can be

calculated by assuming that the temperature and the degree of conversion, � are

non-dependent functions.

8

In this work,

�2 YZ�YYZ�Y[

(1-7)

where \� is the initial weight, \ is the weight during experiment, and \] is the

final weight of the investigation determinate from the TG thermograms.

The rate constant W798can be represented by the Arrhenius equation as:

W798 2 ^_7�`abc8 (1-8)

where de is apparent activation energy (Wf ghi��8, ? is the ideal gas constant

(8.314 fj�� ghi��), ^ is the pre-exponential factor (gk@��) and 9 is absolute

temperature (j).

For a dynamic TGA process, introducing U, into (1-9) results

S�SO 2 74l8_

7�`abc8X7�8 (1-9)

Equations (1-8) and (1-9) are the fundamental expressions of analytical methods

to calculate kinetic parameters on the basis of TGA data.

The Friedman method, which is a linear differential method of equation 1-8, is:

mU S�SOn 2 6 oa

NO 3 7^X7�88 (1-10)

Then for a given value of � the plots of i@ S�S; vs

�O directly leads to 6 oa

N from

the slope.

The main advantage of using Friedman’s approach or any other iso-conversion

method is that de can be calculated for the main degradation process without

any knowledge of the form of the kinetic equation.

1.2.3. Literature review

Introduction

Polymer blending, a process which involves the mixing of two or more

components by solvent casting or melting, is a cost effective technique to tailor-

make materials with improved physical, chemical, mechanical and thermal

properties.

9

Nowadays, with the high price of crude oil and the associated negative impact of

synthetic polymers, increasing attention is being paid to lignocellulosic biomass

as a provider of chemicals and polymers. Lignin is a component of biomass and

its properties can be exploited in the manufacture of polymer blends.

PHB which is generally obtained via fermentation (and in recent times in plants),

is biodegradable. It is envisaged that the incorporation of lignin/lignin-

derivatives into PHB will produce useful polymers for a wide range of

applications. The review presented here is on lignin, PHB and their polymer

blends.

1.2.3.1. Lignin

Introduction

Lignocellulosic materials refer to plants that are composed of cellulose,

hemicellulose and lignin. Sugarcane bagasse, which is comprised of

lignocellulosic compounds, is one of the most promising industrial residues

obtained from the sugar industries (Pandey, et al., 2000). The lignin extracted

from this source is used in the present research investigation.

The wall of a typical lignocellulosic cell is composed of several layers (Figure 1-

2), which are formed as new cells and created at the cambium layer. The middle

lamella is composed mainly of lignin, and serves as the glue bonding adjacent

cells together. The wall itself is made up of a primary wall and a three-layered

secondary wall, each of which has distinct alignments of microfibrils.

Microfibrils are rope like bundles of cellulose molecules, interspersed with and

surrounded by hemicellulose molecules and lignin (Smook, 1934).

10

Figure 1-2 Cell wall organisation of typical wood presented by Smook (1934)

Cellulose (Figure 1-3), which is a polysaccharide and is the main building

material of all plant cells including sugarcane, makes up about 50% of the dry

weight of bagasse (Doherty and Halley, 2004). Since bonding between and

within glucose molecules is so strong, cellulose molecules are very strong.

Lateral hydrogen bonding between cellulose chains is also quite strong, causing

them to group together to form strands that, in turn, form the thicker, rope like

structures called microfibrils (Milton, 1995).

Figure 1-3 The molecular repeating unit of cellulose

Hemicellulose, the second chemical component of bagasse, makes up 30% of its

dry weight (Glasser, et al., 1999). Unlike cellulose, which is made only from

11

glucose, hemicellulose consists of glucose and several other water-soluble

sugars, such as xylose and arabinose (Figure 1-4), produced during

photosynthesis. The degree of polymerisation (that is, the number of sugar

molecules connected together) is lower for hemicellulose than for cellulose and

branched chains rather than straight chains are formed. Hemicellulose surrounds

strands of cellulose and helps in the formation of microfibrils (Milton, 1995).

Figure 1-4 Structure of hemicellulose monomeric sugar units (a) xylose and (b) arabinose

Lignin is the second most abundant organic substance on earth after cellulose,

and plays several important roles in nature. The word lignin was introduced by

de Candolle in 1819 and is derived from the Latin word lignum, meaning wood

(Sjöström, 1993). Lignin stiffens the plant stem to withstand the forces of

gravity and wind, and makes the wood resistant to vermin. Although lignin

provides plants with a protective barrier against being attacked by

microorganisms, it also plays another important role, since it is recycled in the

natural ecology. When it degrades, it serves the soil as a complexing agent for

minerals and as a moisture-retention aid. Lignin also plays a role in the water

conducting system of plants by sealing the water conducting system against the

hydraulic pressure drop produced by the transport of water from the soil to the

leaves (Glasser, et al., 1999). Lignin makes up around 20% of the dry weight of

bagasse.

(a)

(b)

12

Extraction methods of lignin

For lignin to be used to make new products, it must be removed from the plant.

In addition to the diversity of repeat units and bonding patterns which

characterise natural lignin, is the chemical alteration introduced by each method

of removing lignin from the plant. The recovery process to extract lignin from

woody plants changes the chemical and functional group composition of lignin

(Lora and Glasser, 2002) and makes this material extremely heterogeneous.

Methods for recovering lignin are:

• Alkali (soda) process,

• Sulfite process,

• Kraft process,

• Ball milling,

• Enzymatic process,

• Acid digestion and

• Organosolv process.

Different types of lignin have been described depending on the means of

isolation. These include soda lignin, kraft lignin, organosolv lignin,

lignosulfonate, hydrolytic lignin and Klasson lignin. Ball milled lignin is the

best lignin sample among the many isolated lignins that can be used to study the

chemical structure and reactivity of native lignin. However, there have been no

quantitative relationships found between the structural changes in lignin and the

degree of milling. In this project, lignin will be extracted from bagasse using the

soda process, as this is the process of choice in the bagasse biorefinery project

undertaken at Queensland University of Technology, Brisbane, Australia for the

production of bioethanol. Soda lignin is easily recovered by lowering the pH,

filtering and drying. The purity of extracted lignin, as shown in Table 2-4, was

96.3 wt%. The lignin obtained is hydrophobic and contains no sulfur. Its

solubility properties are different from conventional lignosulfonates obtained

through sulfite pulping.

13

The majority of delignification lignin processes (apart from ball milling and

enzymatic process) involve either acid or alkali mechanisms. The

phenylpropane C9 units in lignin are joined by ether linkages, which readily

undergo both acid and base-induced hydrolysis under specific conditions. Side-

chains may be cleaved depending on the type of substructures, particularly under

alkaline conditions (Doherty and Halley, 2004). In the acid delignification

process α-aryl ether substructures are the most readily broken, but it is likely that

β-aryl ether bonds are also broken under strongly acidic conditions (Figure 1-5).

During delignification, components with the functionalisation of the carbonium

ion intermediates are reactions with aromatic structures (weak nucleophiles)

which form carbon-carbon inter-unit linkages and result in condensation

products. The frequency of such condensation reactions increases with the

acidity of the pulping liquor, and decreases with the concentration of the anion

(e.g., bisulfate anions) (Doherty and Halley, 2004).

Figure 1-5 Structure of the H-type monomer unit of lignin. Labelled are the α ,

β and γ positions of the aryl ether bonds

Lignin structure

Lignin is a large, cross-linked, macromolecule with molecular masses in excess

of 10,000 g mol-1. The degree of polymerisation of natural lignin is difficult to

measure, since it is fragmented during extraction, and since the molecule consists

of various types of substructures, which appear to be repeated in a haphazard

manner (Figure 1-6).

14

Figure 1-6 The structure of a possible lignin macromolecule (Glasser, et al., 1999)

There are three monolignol monomers, methoxylated to various degrees: p-

coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol (Quideau and Ralph,

1992) (Figure 1-7). These are incorporated into lignin in the form of the

phenylpropanoids p-hydroxyphenyl (H), guaiacyl (G), and syringal (S)

respectively (Boerjan, et al., 2003).

(a) (b) (c)

Figure 1-7 The structure of the C9 monomer units of lignin. (a) p-coumaryl alcohol (4-hydroxyl phenyl, H), (b) coniferyl alcohol (guaiacyl, G), (c) sinapyl alcohol (syringyl, S).

OH

OH

OH

OH

OCH3 OH

OH

OCH3

OCH3

15

The polymerisation of lignin can produce a number of bond structures by the

delocalization of and reaction at the free radical sites. The lignin produced by

plants depends not only on the species of plant, but the part of the plant as well.

Therefore, lignin of varying composition exists within a single plant. This

means that the lignin recovered from a lignocellulosic plant will be a mixture of

structure and repeat unit composition that will vary with the source of the wood.

Each class of plants, grasses, softwoods, and hardwoods produces a lignin rich in

one or two types of C9 monolignol repeat unit (Doherty and Halley, 2004).

Hardwoods have a lignin that consists almost entirely of G and S type

monomers. Softwoods also have both G and S types, however the major

component is the S type (Boerjan, et al., 2003). The G predominates in grasses,

but also contains some H monomer units, which enables them to be more

flexible in making combinations with other groups.

Sugarcane bagasse lignin is a grass lignin and has a higher proportion of H

groups and hence a lower methoxyl content (i.e. more monomer units with

vacant ortho- and para-sites), than softwood and hardwood. Based on these

chemical structures, lignin is soluble in polar solvents and insoluble in

hydrocarbons, and hence forms immiscible multi-component systems with non-

polar compounds such as polyethylene (PE) and PP (Doherty and Halley, 2004).

The structural heterogeneity of lignin has also been studied by various methods

in a number of investigations. In several of those studies, lignin was subjected to

fractionation prior to analysis. Robert et al. (1984) fractionated kraft lignin by

successive acidification of kraft black liquor, while Moerck et al. (1986) used

organic solvent, Vanderlaan and Thring (1998) fractionated Alcell® lignin with

an organic solvent and Wallberg et al. (2003) used ultrafiltration. These

fractionations were analysed for functional groups, elemental composition and

molecular weight. The results of these investigations showed that the

fractionation process separated the lignin into distinct molecular weights and that

there were differences in the carboxylic acids, phenolic hydroxyl and methoxyl

contents. The properties of the materials produced were dependent on these

structural properties.

16

The Tg is influenced by such factors as the free volume between polymer chains;

the existence and abundance of attractive forces between molecules (which

obviously relates to solubility); the freedom of molecular side groups, branches

and segments to rotate around intermonomer bonds; chain stiffness; and chain

length. The Tg values of some different types of lignin are shown in Table 1-1.

Tab le 1 - 1 Tg va lues o f so me d i f fe ren t t ypes o f l i gn in (G la sse r, e t a l . , 1999)

Types of lignin Tg (ºC)

Lignin in Wood

- Hardwood

- Softwood

65-85

90-105

Milled wood lignin

- Softwood

- Hardwood

138-160

110-130

Periodate lignin 193

Kraft lignin 124-174

Organosolv lignin 91-97

Steam explosion lignin 113-139

1.2.3.2. Poly(hydroxybutyrate)

Introduction

PHB is a polyhydroxyalkanoate (PHA), which belongs to the group of

polyesters. It was first isolated and characterised in 1926 by the French

microbiologist Maurice Lemoigne (1926). PHB is produced by micro-organisms

(such as Alcaligenes eutrophus or Bacillus megaterium), apparently in response

to conditions of physiological stress. The polymer is primarily a product of

carbon assimilation (from glucose or starch) and is utilised by micro-organisms

as a form of energy storage molecule to be metabolized when other common

17

energy sources are not available. Three enzymes (and others) are needed for

production of the PHB polymer. These enzymes include the

3-ketothiolase (PHBA), acetoacetyl-CoA reductase (PHBB), and

polyhydroxybutyrate synthase (PHBC) (Sticklen, 2008).

PHB-producing bacteria require substrates such as ethanol, sucrose, or glucose,

which are costly. In bacteria, PHB is produced in a diluted aqueous solution.

Therefore, the recovery of PHB from diluted fermentation systems adds to the

cost of fermentation as a means of producing PHB. Recently, significant

attempts have been undertaken to produce PHB from plants (Sticklen, 2008).

Plants produce carbon sources via photosynthesis in concentrated products.

Therefore, the costs of production of PHB in plants may become lower than the

costs of its production in bacteria (Sticklen, 2008).

Production of PHB

The manufacturing process of PHB begins with sunlight (Figure 1-8). Through

photosynthesis, atmospheric carbon dioxide is converted to carbohydrates in

either sugar beets or sugarcane. These carbohydrates are the raw material for the

manufacture of PHB. PHB can be produced from glucose as a raw material, or

from agricultural wastes, such as molasses or material refined from the

processing of sugar beets and lactose, or from a wide variety of sources e.g.

volatile fatty acid fermentation products. The sugar is broken down during

metabolism into C2 building blocks, which are converted, over several steps, to

C4 monomers. Finally, the PHB is polymerised.

18

Figure 1-8 Flow scheme of (a) life cycle of PHB, and (b) PHB manufacturing process (Ghaffar, 2002)

The poly-3-hydroxybutyrate (PHB) form of poly(hydroxyalkanoate) (PHA) is

the polymer used in this project. PHB is probably the most common type of

PHA, but many isolation of this class are produced by a variety of organisms:

these include poly-4-hydroxybutyrate (P4HB), polyhydroxyvalerate (PHV),

polyhydroxyhexanoate (PHH), polyhydroxyoctanoate (PHO), and their

copolymers (Figure 1-9).

(a)

(b)

Sugar Beet Sugarcane

Pre-fermentation

19

Figure 1-9 Monomer units of PHB, PHV and their copolymer PHBV

In the future, research using genetic technology, among others, may prove

successful in producing a bacteria-based plastic that has more desirable

properties and is cheaper to produce than PHB. Also, PHB production may

become cheaper if researchers can find a way to make bacteria produce larger

amounts of polymer within shorter time spans or from waste materials using

cheaper production methods. If PHB becomes as cheap as plastics produced

from petrochemicals, then it will probably become widely used, since it has the

potential to be employed for packaging products such as bottles, bags, wrapping

film and disposable nappies (Sykes, 2001).

PHB is also being evaluated as a material for tissue engineering scaffolds and for

controlled drug-release carriers, owing to its biodegradability, limited

cytotoxicity, optical activity and isotacticity (Hasirci, 2003).

There are manyisolation processes that can be used to obtainPHB. Two typical

ones are:

• (1) The extraction method. Mechanical loads are used to destroy the cell

walls and then the polymer is dissolved in chloroform or another solvent

20

such as methyl chloride, 1,2-dichloroethane, pyridine or propylene

carbonate. The remains of the cell must then be separated by

centrifugation and filtration of the solvent.

• (2) Enzymatic method. Enzymes at 37°C destroy the cell wall. The

PHB is then isolated using the same method as that described in previous

section.

Physical and chemical properties of PHB

Some of the main PHB properties are listed below:

• Water insoluble and relatively resistant to hydrolytic degradation. This

differentiates PHB from most other currently available biodegradable

plastics, which are either water soluble or moisture sensitive.

• Good oxygen permeability.

• Good UV resistance but poor resistance to acids and bases.

• Soluble in chloroform and other chlorinated hydrocarbons.

• Biocompatible and hence suitable for medical applications.

• Melting point = 175-177ºC, and Tg = 4ºC.

• Tensile strength of 40 MPa, which is close to that of PP.

• Sinks in water (while PP floats), facilitating its anaerobic biodegradation

in sediments.

• Non-toxic.

Chemistry behind the brittleness of PHB

Crystal structure and crystallisation conditions are responsible for some of the

properties of many PHB products. A sound knowledge and understanding of

crystallisation mechanisms is necessary for designing materials with better

mechanical properties. In practice, crystals formed by polymer molecules are

imperfect. The crystallinity of most melt-crystallised polymers lies in the range

of 30% - 70%. The lamellae thickness can be measured by small angle X-ray

diffraction and directly by electron microscopy. PHB is stiff and brittle with its

brittleness dependent on its Tg, degree of crystallinity and on its microstructure.

21

PHB poses a low nucleation density (Mahendrasingam, et al., 1995, Withey and

Hay, 1999) resulting in the formation of large spherulites. Spherulites contain

crazes, and splitting occurs around the centre of these crazes, hence producing a

significant structural weak point (Barham and Keller, 1986). If PHB is annealed

at high temperatures, stress and brittleness also increases. Another factor which

contributes to the brittle nature of PHB is the fact that it undergoes secondary

crystallisation at room temperature. This process involves the conversion of

amorphous to crystalline material over time, and occurs in PHB because its Tg of

approximately 4°C is close to that of ambient temperature.

22

1.2.3.3. Lignin blends

Introduction

Polymer blending, which involves the mixing of two or more polymeric

components, has been shown to provide the ability to control or tailor properties

to specific desired goals. In many instances, polymer blending results in the

formation of high performance composite materials, this being a consequence of

synergistic interactions. However, many polymer combinations are not miscible

and exist in two different phases in the polymer matrix. The separation into

phases in the polymer matrix results in high interfacial tension and poor

polymer-polymer interactions. This results in materials with poor mechanical

properties, due to poor stress transfer between the phases.

Feldman (2002) reviewed lignin and its polyblends. The review includes phenol

formaldehyde resin-lignin adhesives, epoxy-lignin adhesives, other adhesives

and sealants with lignin, polyolefin lignin blends, polyvinyl chloride/lignin

blends and rubber/lignin blends. Some of these blends are presented here

together with PHB blends and lignin/PHB blends. Since lignin possesses

attractive properties it has been considered by many researchers to provide

compatibility for different polymer types.

Study on miscibility of lignin blends

A very interesting study reported by Pouteau et al. (2004) investigated the

compatibility of lignin-polymer blends by image analysis using visible

spectroscopy. The study looked at the development of lignin-based blends

lignin. It investigated semi-polar polymers (e.g. lignins), very polar polymers

(e.g. starch) and apolar polymers (e.g. PP). The morphology of the blends

obtained from semi polar polymers was very sensitive to the variation of the

solubility parameters. Over a low range of polymer solubility parameters, both

heterogeneous and homogeneous systems were obtained. The properties of the

blends were improved by a careful choice of polymer type. Furthermore, it was

also considered possible to take advantage of lignin variability to improve the

compatibility of the blend. Only low molecular weight lignins were compatible

with a polar and very polar matrix.

23

Kadla et al. (2004) and Kubo et al. (2002) studied the intermolecular interactions

between lignin and synthetic polymers. Their investigations revealed the

immiscible nature of lignin in polyvinyl alcohol (PVA) and PP. However, the

study demonstrated the misciblity behaviour in poly(ethylene oxide) (PEO) and

polyethylene terephthalate (PET), in which the Tg showed a negative deviation

from the linear mixing rule which indicated specific intermolecular interactions.

Furthermore, from these studies Fourier transformed infra red (FT-IR) analysis

revealed strong intermolecular hydrogen bonding between lignin and PET.

Further work by Kadla et al. (2003) investigated the miscibility behaviour over

the entire blend ratio of lignin with PEO. The results of the FT-IR analyses

revealed a strong hydrogen bonding between the aromatic hydroxyl proton of

lignin and the ether oxygen in PEO.

Tinnemans et al. (1984) investigated the mechanical properties of water-

swellable lignin blends. They specifically worked on acylated kraft lignins with

maleic anhydride-styrene copolymers. The resulting blends exhibited a good

tensile strength and demonstrated a high strain at break, owing to favourable

miscibility of the components.

Lignin/PE and lignin/PP blends

Previous attempts at blending PE with lignin in concentrations > 20 wt% yielded

blends with relatively poor mechanical properties. A new method, based on

blending PE with ethylene-vinylacetate (EVA) copolymer has been developed by

Pavol et al. (2004). On the basis of their study, Pavol and co-workers (2004)

found that the addition of 10 wt% EVA caused 200 wt% increase in tensile

strength, and a 1300 wt% increase in elongation at break, compared to those of

the corresponding unmodified samples. Moreover, a composite material

prepared containing 33.6 wt% lignin displayed acceptable processing and

mechanical properties, and was used successfully in preparing blown-films.

Alexy et al. (2000) used lignin as a natural filler in a low-density PE and PP at

concentrations up to 30 wt%. Their study described the influence of lignin

blending on processing stability, mechanical properties and light and long-term

heat degradation, for both polymer blend types. They also showed that different

degradation behaviors between PE-lignin and PP-lignin blends existed. It was

24

determined that lignin concentration influenced both tensile strength and melt

flow index.

The influence of lignin on the oxidative stability of PP and recycled PP has been

examined by Gregorova et al. (2005) using DSC under non-isothermal

conditions. The results showed that lignin exerts a stabilising effect in both

virgin and recycled PP. The protection factor increases with lignin content in the

PP matrix. Moreover, for the evaluation of heat resistance, the influence of the

lignin content on Vicat softening temperature (VST) was determined. VST

showed that the presence of lignin improves the heat resistance of PP and

recycled PP plaques.

The orientation and property correlations of biaxially oriented PE blown films

have been studied by Chen et al. (2006). Correlations between orientation in

both the machine and transverse directions were found with dart impact and

Elmendorf tear strength. These correlations were linked to underlying

morphology and micro-deformation mechanisms.

Košíková et al. (1993) investigated sulfur-free lignins as composites of PP films.

The results showed that PP films containing 2 wt% - 10 wt% spruce organosolv

lignin and/or beech wood prehydrolysis lignin, had good compatibility and

sufficient tensile strength. Also, the physicochemical properties of the lignin-

containing films indicated compatibility between lignin and PP, and

demonstrated that the film acted as a good UV absorber.

Methods for preparing PE blends with organosolv lignin and methods of making

them has been patented by Bono et al. (1995). Another earlier patent by Bono et

al. (1994) involved producing degradable plastic films with ethylene copolymers

and lignin. The lignin was incorporated in the form of very fine powder with a

grain diameter of about 1 µm - 5 µm. The films were homogeneous and

possessed a thickness of about 15 µm - 25 µm. Improved degradation was

achieved with photoactive and oxidizing agents.

Košíková et al. (2001) have reported on the ability of lignin-degrading

microorganisms, phanerochaete chrysosporium, to attack PE in lignin/PE blends.

The isolation of the oligomer fraction from biodegraded polymer blends

25

indicated that the biotransformation of lignin during the cultivation process was

accompanied by the degradation of the PE matrix.

Lignin-polyurethane blends

The morphology of lignin-polyurethane blends has been studied by Feldman et

al. (1989). In this study, although SEM revealed an even distribution of lignin

particles in the polyurethane matrix, it clearly showed the different morphologies

of the constituent phases. The results were confirmed by DSC analysis which

showed immiscibility.

Ciobanu et al. (2004) studied a polyurethane elastomer blended with flax soda

lignin to form dimethylformamide-cast films containing between 4.2 wt% and

23.2 wt% of lignin. The spectral, mechanical and thermal properties of this new

type of blend were investigated in an attempt to establish their potential

applications. Based on that investigation, films containing more than 9.3 wt%

lignin were found to be heterogeneous. The thermal degradation range of

polyurethane and the blends were quite similar. However the presence of lignin

accelerated decomposition at lower temperatures. The tensile strength increased

by up to 370 %, toughness up to 470 % and the elongation at break up to 160 %

for the blends compared to the pure polyurethane film.

Lignin-epoxy blends

Feldman et al. (1991a, 1991b) studied a bisphenol A-polyamine hardener-based

epoxy adhesive modified by kraft lignin. They investigated the curing of these

blends with up to 40 wt% kraft lignin. The curing process was performed either

at room temperature or above the Tg of the components. However, the result was

an enhanced degree of bonding between components, and the reason for the

improvement was thought to be an association between lignin and the unreacted

amine groups of the hardener. In another study, Feldman et al. (1988) observed

that epoxy blends with 10 wt% and 20 wt% lignin improved the adhesion tensile

strength of the epoxy polymer system. However, blending with 5 wt% and 20

wt% lignin had little effect on the adhesive shear strength (by tension loading),

or on the weatherability of the epoxy system. However, after a post curing

process (4 h at 75ºC), a significant improvement of the adhesive strength in shear

of the epoxy-lignin blends was detected.

26

Lignin-based carbon fibres

One of the most interesting applications of lignin is to use it to make carbon-

fibres because of its low cost, high volume and ability to produce fibres, through

melt-spinning. Griffith et al. (2003) studied the use of high-lignin content blends

which could be melt-spun to produce small rows of 10 m - 20 m non-sticking,

drawable filaments. The study was successful and commercial carbon fibres can

now be produced with kraft lignin.

Kadla et al. (2002) reported producing a fusible lignin with excellent spinnability

to form a fine filament following thermal pretreatment under vacuum. Blending

kraft lignin with PEO further facilitated fibre spinning, but at PEO levels

>5 wt%, the blends could not be stabilised without the individual fibres fusing

together. The carbon fibres produced had an overall yield of 45 wt%. The

tensile strength and modulus increased with decreasing fibre diameter, and were

comparable to those of the much smaller diameter carbon fibres produced from

phenolated exploded lignins. In view of its mechanical properties, the tensile

strength of 400 MPa - 550 MPa and the elastic modulus of 30 GPa - 60 GPa,

kraft lignin should be further investigated as a precursor for general grade carbon

fibres.

Effect of UV irradiation on the thermal stability of lignin blends

On the basis of the study by Bittencourt et al. (2005), different films containing

two types of extracted lignin (i.e. kraft lignin and the acetone solvent fraction of

kraft lignin) with different proportions of polyvinyl alcohol (PVA) were

prepared via solvent-casting. The films, with concentrations up to 25 wt%

lignin, were irradiated with UV light for different time intervals. The results of

this analysis indicated better thermal stability and miscibility for the films

prepared with lignin extracted with acetone. This shows that the composition of

the functional group of lignin has a strong bearing on its miscibility behaviour.

A thermal and FT-IR study of polyvinylpyrrolidone (PVP) and bagasse lignin

blends has been undertaken by Silva et al. (2005). The bagasse lignin was

extracted with formic acid and the blends were cast with dimethyl sulfoxide and

formic acid. Blends were also irradiated with UV light. The results showed

miscibility in PVP-lignin blends with 5 wt% lignin content cast from dimethyl

27

sulfoxide, and miscibility in blends containing 5 wt% and 10 wt% lignin cast

from formic acid. Irradiation with UV light resulted in improved thermal

stability.

1.2.3.4. PHB blends

Most studies reported in the literature on PHB blends deal with miscibility,

thermal and mechanical properties. Little has been reported on their

processability and on their rheological properties. This project will investigate

the processability of lignin/PHB blends by studying detailed thermal events,

viscoelastic behavior and storage and loss modulus.

Avella et al. (2000) in a comprehensive review summarizes the properties of

blends of PHB and poly(hydroxybutyrate-hydroxyvalerate) (PHBV). The

mechanical, morphological, and miscibility properties of blends with polyesters,

polyethers, polyvinylacrylates and polysaccharides were studied, as well as the

biodegradation of the blends. The results from the study showed that the

microstructure of the blends controlled the mechanical and biodegradation

behavior of the blends.

Antunes and Felisberti (2005) studied blends of PHB and poly(ε-caprolactone)

(PCL), which is a semi-crystalline polymer that is used as a biomaterial. PHB

and PCL were blended by melting mixtures in an internal mixer. The blends

compositions varied from 0 wt% to 30 wt% PCL. DMTA, DSC and SEM were

used to characterise the blends. The blends were found to be immiscible with no

indication of interaction either in the amorphous or crystalline state. The

morphology of the blends revealed PHB as the matrix and PCL as the dispersed

phase.

El-Taweel et al. (2004) studied the stress-strain behaviour of blends of PHB (of

molar mass 30,000 g mol-1), with different miscible amorphous polymers (of

molar mass 600 g mol-1 to 200,000 g mol-1). They found that a high extension

ratio was obtained only if the PHB content was less than 60 wt%.

Liu et al. (2004) studied the crystallisation of poly(vinylidene fluoride) (PVDF)

and PHB blends using DSC. They found that solid PVDF possibly acts

heterogeneously, nucleating and accelerating PHB crystallisation.

28

An investigation of PHB blends containing starch or starch derivatives has been

reported by Innocentini-Mei et al. (2003). Their work showed a significant

decrease of both the Tg and the melting point (Tm) for all formulations. Best

results in terms of modulus and Tg were obtained with grafted starch-urethane

blends.

The melting and crystallisation behaviour and phase morphology of PHB blends

with poly(DL-lactide)-co-poly(ethylene glycol) (PELA) have been studied by

Zhang et al. (1997). Compared to pure PHB, the cold crystallisation peak

temperatures (Tcc) of PHB blends shifted to higher temperatures. The growth of

spherulites of PHB in the blends was affected significantly by a 60 wt% PELA

content. Similar results were also obtained by Deng et al. (1993).

An investigation on the thermal properties of PHB blends with cellulose esters

containing acetate, propionate, or butyrate substituents has been reported by

Scandola et al. (1993). They observed a good PHB miscibility with the cellulose

esters. The morphology of blends of PHB with cellulose acetate butyrate (CAB)

by compression molding followed by different thermal treatments has been

carried by Tomasi et al. (1995) The results also showed good miscibility

between CAB and PHB.

Yoshie et al. (1995) using high-resolution solid-state carbon-13 nuclear magnetic

resorance (13C-NMR) and proton (1H NMR) spectroscopy observed hydrogen-

bonding interaction in the amorphous phases of PHB and PVA blends. The DSC

measurement confirmed the compatibility of the blends and showed that the

blends have lower crystallinity than the individual polymers.

1.2.3.5 Studies on the biodegradation of PHB blends

Biodegradation of polymer blends is determined both by the degradability of

blend components themselves and by the blend composition. Ikejima et al.

(1999) studied the environmental biodegradability and crystallisation behaviour

of blend films of PHB with chitin and chitosan. The crystallisation behaviour

was similar between blends and with PHB alone. However, several of the blends

showed faster biodegradation than either of the polymer components.

Zhao et al. (2005) studied the effect of aging on the fractional crystallisation of

PEO component in the PEO-PHB blends. Their investigation confirmed that

29

nearly all the PEO component that had remained trapped within the interlamellar

regions of PHB affected aging.

Nagahama et al. (2005) wrote a review on manufactured biodegradable plastics

through forming PVA-PHB blends, fibre-polymer composites and aliphatic

polyester blends. Their work showed effective biodegradation in the composites

made with PHB. Teryshnaya and Shibryaeva (2006) also studied oxidative

degradation of biomicrobial PHB-low density PE and ethylene-PP rubber-PHB

blends. Their results showed improved degradation of the olefins components.

Kikkawa et al. (2006) investigated the enzymatic hydrolysis of poly(L-lactide)

and atactic PHB blends showed that either poly(L-lactide) or atactic PHB

domains were attacked depending on the kind of enzyme used. The larger

number of enzyme molecules was found on poly(L-lactide) domains suggesting

a higher affinity of the enzyme for poly(L-lactide).

Gonvaleves et al. (2009) investigated the biodegradation of PHBV, PP and their

blends in soil. They found the PHBV degraded faster than PP, and that in the

blends, PP only showed changes in the amorphous region.

The type of environment in which the biodegradation is performed has a

significant effect on the rate of degradation. El-Hadi et al. (2002) found that for

blends of PHB and nucleating agents (e.g. tributyrin), aerobic biodegradation

was easier in river water and compost, than in the soil. Imam et al. (1998)

reported that in a natural composite environment, the weight loss correlated with

the amount of starch present in the blends. Imam et al. (1998) also found that

there was no significant difference in molecular weight decrease between neat

PHBV compared to PHBV/starch blends. Imam et al. (1995), on the other hand,

reported that in an activated sludge environment, the rate of weight loss were

quite similar with neat PHBV and PHBV/starch blends.

Recently, Woolnough et al. (2010) studied the biodegradation of PHB and some

other “green plastics” in mature soil detecting mass loss, topographical changes

and biofilm attachments and found that PHB itself has a better degradability

among polyhydroxyoctanoate and poly(DL-lactide) and polystyrene and ethyl

cellulose.

30

1.2.3.6. Lignin/PHB blends

Based on the published information available, there are five articles specifically

relating to lignin and PHB blends. (Camargo, et al., 2002, Ghosh, et al., 2000,

Mihaela, et al., 2010, Naegele, et al., 2000, Weihua, et al., 2004).

Ghosh et al. (2000) investigated the thermoplastic blends of several

biodegradable polymers with lignin and lignin esters, based on both solvent

casting and melt processing. The biodegradable polymer they used contained

cellulose acetate butyrate (CAB), a starch-caprolactone copolymer blend and

PHB. In addition to organosolv lignin, they investigated organosolvo lignin

esters of acetate, butyrate, hexanoate and laurate. They detected a high level of

compatibility between blends of lignin acetate, lignin butyrate and CAB. They

observed a significant amount of retarded crystallisation of PHB with the

addition of lignin, which result in lower melting points of the blends. The

addition of lignin also increased the modulus of the blends significantly at room

temperature, probably because it increased the crystallinity of PHB.

Weihua et al. (2004) investigated the effect of lignin fine powder on the

nucleation of PHB by studying the kinetics under both isothermal and

nonisothermal crystallisation processes. The DSC results showed that lignin not

only acted as a nucleating agent and decreased the activation energy of the

crystallisation process, but it also increased the number of the spherulites formed

(Figures 1-10 and 1-11). However the size of spherulites had decreased.

31

Figure 1-10 DSC cooling and heating curves of pure PHB and PHB/lignin blend samples showing the melt nonisothermal and cold crystallisation temperature, Tmc and Tcc: (A) cooling, and (B) heating (Weihua, et al., 2004)

32

Figure 1-11 Spherulitic growth rate at various crystallisation temperatures for both a pure PHB and a PHB/lignin blend (Weihua, et al., 2004)

Understanding the mechanical and rheological properties of polymer blends is

necessary for understanding changes in the viscoelastic response and

processability conditions.

None of the studies reported to date on lignin/PHB blends have examined in

detail the macroscopic and microscopic associations between lignin and PHB

that would help explain observed thermal, rheological and biodegradation

properties of blends.

33

1.3. Account of Research Progress L inking the

Research Papers This project commenced with a comprehensive literature review of lignin

chemistry, properties and applications. It was evident from the review that

limited work has been carried out with lignin/PHB blends and no biodegradation

evaluation studies for these blends. The following sections link the various

research papers covering the research program.

1.3.1. Chemical and thermal properties of soda lignin

The first step in making lignocellulosics (such as bagasse) amenable to

enzymatic hydrolysis for the production of sugars and subsequently ethanol is to

pretreat it either by mechanical or chemical means. Sodium hydroxide is one of

the pretreatment options used to fractionate lignocellulosics. The advantage of

using sodium hydroxide is that the lignin component of lignocellulosics can

readily be recovered. The lignin recovered by this process has high ash content

and hence is of low purity (Lora and Glasser, 2002). A purification step is

necessary if the soda lignin is to be used in chemical reactions, such as in resin

synthesis. In the present study, a two-stage process was developed which

improved the purity of soda lignin derived from bagasse. Soda lignin produced

by this process was characterised by physical, thermal and chemical means. The

soda lignin was fractionated into three parts using two solvents, diethyl ether and

methanol, which have different polarities. Figure 1-12 shows the flow diagram

for the fractionation process.

Figure 1-12 Fractionation process of soda lignin.

34

Based on this process, only a very small portion of the lignin sample, ~8 wt%,

was recovered using diethyl ether (EL). The major proportion, ~ 68 wt%, was

methanol soluble (ML), and the residue (RL) makes up the remaining 24 wt%.

The results clearly demonstrated the heterogeneity of soda lignin. The two-stage

lignin precipitation process, and the chemical and thermal properties of soda

lignin and its fraction was published in Industrial Crops and Products

(Mousavioun and Doherty, 2010) titled: “Chemical and thermal properties of

fractionated bagasse soda lignin”.

This study on lignin chemistry showed that the lignin with the highest proportion

of phenolic hydroxyl functional group (i.e. ether-soluble lignin) has the highest

potential to interact with PHB to form miscible blends. However, the proportion

of this lignin type in soda lignin was too small to merit investigation.

1.3.2. Addendum: Kinetics of bagasse decomposition, Lignin applications

During the course of the study on the thermal properties of lignin and PHB, it

was established that to obtain optimised conditions for production of lignin/PHB

blends with minimum PHB degradation, the kinetics of PHB degradation should

be studied. Both isothermal and non-isothermal conditions were used in the

study. Similar Ea values for the decomposition process were obtained using both

approaches. As the non-isothermal approach is rapid, it was decided, as an add-

on to the project, to investigate the kinetics of the thermal degradation of bagasse

from which lignin originates. The results of this work were published in

Industrial Engineering & Chemistry Research (Maliger et al., 2011). The title of

the article is, “Thermal decomposition of bagasse. Effect of different sugarcane

cultivars”. The article is presented in Appendix 1. The contribution by the

author of this thesis for this piece of work was 30 wt%.

Having reviewed hundreds of articles on lignin chemistry, properties and uses, a

review paper was deemed necessary to illustrate the potential of lignin-based

polymers for improving the economics of producing cellulosic ethanol from

lignocellulosics. This review paper was published in Industrial Crops and

Products (Doherty, et al., 2011), with the title: “Value-adding to cellulosic

ethanol: Lignin polymers”. The paper is in Appendix 2. The author of this

thesis contributed 20 wt% towards writing the paper.

35

1.3.3. Thermal stability and miscibility of PHB and soda lignin blends

Literature review showed that there were five articles that specifically relate to

lignin and PHB blends (Camargo, et al., 2002, Ghosh, et al., 2000, Mihaela, et

al., 2010, Naegele, et al., 2000, Weihua, et al., 2004). None of these studies

examined the association and interactions between the functional groups of

lignin and those of PHB, which may lead to a better understanding of observed

thermal stability and miscibility properties of lignin/PHB blends.

As mentioned in section 1.3.2, there was concern about the thermal stability of

PHB during processing. So in the 2nd phase of the project, isothermal

degradation tests with PHB were performed at temperatures from 165°C to

190°C. Results showed 175°C was the optimum processing temperature that

will result in minimum PHB degradation.

To produce blends, lignin and PHB were dried at 100°C for 12 h and then stored

in desiccators under vacuum prior to use. Lignin/PHB blends with lignin

contents from 10 wt% to 90 wt% were mixed in a Haake mini lab twin screw

extruder (Figure 1-13) using the procedure reported by Ghaffar (2002). To

minimise PHB degradation, the temperature of the extruder was maintained at

175°C for 2 min. The polymer blends were extruded as strands then cooled and

pelletised. The pellets were stored in a desiccator to avoid moisture absorption,

prior to use.

Figure 1-13 Haake mini lab twin extruder

Feeder

Screw

Heater

Controller

36

In this study, the thermal properties and miscibilities of PHB and soda lignin

blends were investigated by TGA, DSC, SEM and FT-IR over the entire range of

composition. The most important outcome of the study was that lignin reduced

the initial temperature of decomposition of PHB, but stabilised PHB over a wider

temperature range at higher temperatures. This may be because the carbohydrate

components in lignin start to decompose at an earlier temperature than PHB. A

single Tg, which depicts miscibility, was obtained for blends containing up to 40

wt% lignin. The Tg results correlated well with the SEM and FT-IR data. The

FT-IR data showed that the miscibility of the blends is probably associated with

specific hydrogen bonding interactions between the reactive functional groups in

lignin and the carbonyl groups of PHB. This result showed the anticipation of

improvement in properties of PHB by lignin (outcome of phase 1 of the project)

was valid. Results of this work were published in a paper in Industrial Crops and

Products (Mousavioun, et al., 2010), titled “Thermal stability and miscibility of

poly(hydroxybutyrate) and soda lignin blends”.

At this stage of the project it was concluded that lignin, to a certain extent,

improves the thermal properties of PHB. However, the key question still

remained of whether lignin could enhance the rheological properties of PHB and

hence its processibility.

1.3.4. Combination of thermal stability and rheological properties

A comprehension of the rheological properties of polymer blends is required to

determine changes in the viscoelastic responses and determine blend

compositions suitable for easy processing. The miscibility between components

is a significant parameter that dictates viscoelastic responses. Thus, the next

phase of the project was to study the effect of lignin loading on the viscosity of

PHB and its thermophysical properties. The results have been submitted to

Polymer International, titled “Thermophysical properties and rheology of

PHB/lignin blends”. The conclusion drawn from this work is that lignin not only

affects the thermal stability of PHB (based on Ea values, confirming the mass

loss data to some degree), it also affects PHB crystallisation. The rheological

study showed that lignin contents of 10 wt% and 30 wt% plasticise PHB,

resulting in blends having lower viscosities than PHB alone. For blends

37

containing 60 wt% and 90 wt% lignin respectively, lignin acts as a filler, and

blends have viscosities higher than PHB.

1.3.5. Environmental degradation of lignin/PHB blends

In this phase of study, for various compositions of lignin/PHB blends, four

samples were prepared and buried in the soil. The standard burial test method

was used (Woolnough, et al., 2010). Samples were removed every 4 months for

analysis during the 12 months of the trials. The samples were analyzed before

and after exposure, using gravimetric analysis, TGA, DSC, optical microscopy,

SEM, X-ray Photoelectron Spectroscopy (XPS) and FT-IR. The gravimetric

analysis results showed that lignin significantly protects PHB against

degradation, while the DSC results showed that hydrogen bonding of lignin with

PHB plays a significant role to protect PHB against degradation. XPS data

revealed an accumulation of biofilms on the surface of buried film samples.

XPS and FT-IR confirmed that PHB is the most susceptible component against

degradation. FT-IR analysis showed that low lignin contents (<30 wt%)

accelerate PHB degradation, while high lignin contents retard the process. The

results have been explained using the miscibility concept. Results of this work

have been submitted to Polymer Degradation and Stability, titled

“Environmental degradation of lignin/PHB blends”.

1.3.6. Methanol-soluble lignin/PHB blends

This phase of the project examined the impact the composition of the functional

groups of lignin influenced the properties of lignin/PHB blends. Soda lignin

contains 3× the proportion of xylan and phenolic hydroxyl group than ML. It

however, has 1.5× less carboxylic acid groups. ML/PHB blends with lignin

contents from 10 wt% to 90 wt% were assessed in a similar fashion as soda

lignin/PHB blends. The result of the study was presented and published as a full

paper in the Proceedings (CD-ROM) at the 10th AIChE Annual meeting, Salt

Lake City, UT, USA. The title of the paper is: “Thermal stability and miscibility

of poly(hydroxybutyrate) and methanol-soluble soda lignin blends”.

38

The T0 values of ML/PHB blends were higher than the T0 values of soda

lignin/PHB blends. This may be because of the proportion of xylan in the

composite. Xylans are known to decompose at lower temperatures than

cellulose and lignin.

Tg results of ML/PHB blends indicated that blends containing up to 40 wt% ML

are miscible with PHB. The similar results were obtained for soda lignin/PHB

blends (section 1.3.3). FT-IR spectra showed that for blends up to 50 wt% ML,

there was a small but definitive shift to lower wavenumbers, indicating hydrogen

bonding interactions. The similarities between the results and those of

lignin/PHB blends indicated that the differences observed in the composition of

lignin functional groups were not significant to influence the glass transition

temperature of lignin/PHB blends derived from the two lignin types.

39

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44

CHAPTER 2

Chemical and thermal properties of

bagasse soda l ignin

Payam Mousavioun and W.O.S. Doherty

Centre for Tropical Crops and Biocommodities, Queensland University of

Technology, GPO Box 2434, Brisbane, Australia

Published in Industrial Crops and Products, Vol 31, Page 52, 2010

45

Abstract- A major challenge of the 21st century will be to generate

transportation fuels using feedstocks such as lignocellulosic waste materials as a

substitute for existing fossil and nuclear fuels. The advantages of

lignocellulosics as a feedstock material are that they are abundant, sustainable

and carbon-neutral. To improve the economics of producing liquid

transportation fuels from lignocellulosic biomass, the development of value-

added products from lignin, a major component of lignocellulosics, is necessary.

Lignins produced from black liquor through the fractionation of sugarcane

bagasse with soda and organic solvents have been characterised by physical,

chemical and thermal means. The soda lignin fractions have different physico-

chemical and thermal properties from one another. Some of these properties

have been compared to bagasse lignin extracted with aqueous ethanol.

2.1. I ntroduct ion In the last century, energy sources have been derived from petroleum (30 %),

natural gas (23 %), coal (22 %), renewable (19 %) and nuclear (6 %) (Song,

2002). In the chemical industry, 4 % of crude oil and 31 % of natural gas are

used in the manufacture of platform chemicals and composite materials. The

state of the oil market ($US40-$US100 per barrel) is unpredictable because of

economic and political pressures, and the ever increasing oil demand from

developing Asian countries will probably maintain the current high price of

crude oil. There is the ongoing debate among geologists as to the time frame

when oil reserves will be depleted. There is also the strong push towards

reduced greenhouse gas (GHG) emission. Fossil fuels used in transportation

contribute over 25 % of GHG. It has been estimated that the utilisation of

plant/crop-based feedstock for the production of chemicals in the European

Union could deliver GHG reductions of over 6 M tonnes per annum in the next

decade. As a consequence of these events, there has been coordinated R&D

strategy across the globe for the utilisation of plant/crop-based products.

The International Energy Agency, Energy Outlook in November 2006 stated,

“Rising food demand, which competes with biofuels for existing arable and

pasture land, will constrain the potential for biofuels production using current

technology”. Such a constraint causes feedstock price increase for both food and

46

fuel. So the challenge we now have is to be able to produce transportation fuels

from non-food sources e.g., bagasse, wheat straw, rice stalk, cotton linters,

agricultural wastes, and forest thinning, at an economically competitive price

without government subsidies. To bring down production cost of biofuels,

developing a market for lignin products which have equivalent properties as the

petroleum-based products is necessary.

In 2006, the world produced 1.4 billion tonnes of sugarcane (FAO, 2009). This

equates to ~400 million tonnes of bagasse. Currently, a vast majority of bagasse

is used to produce low value co-generation of power, manufacture of pulp and

paper products and furfural production. However, with continuing

improvements in energy efficiencies of sugar factories, more and more bagasse

will be available for other applications, such the production of cellulosic ethanol.

The advantage bagasse has over other non-food sources is that it is located

centrally due to existing transportation infrastructure. So, co-location of a

cellulosic ethanol plant to a sugar factory gives the opportunity to share

production systems but also to share processing facilities.

Bagasse, a non-wood, consists mainly of cellulose (50 wt%), hemicellulose (30

wt%) and lignin (20 wt%). Lignin is an amorphous large, cross-linked,

macromolecule with molecular masses in the range 1000 g mol-1 to 20,000 g

mol-1. The degree of polymerisation in nature is difficult to measure, since it is

fragmented during extraction and the molecule consists of various types of

substructures which appear to repeat in a haphazard manner. There are three

monolignol monomers, methoxylated to various degrees: p-coumaryl alcohol,

coniferyl alcohol, and sinapyl. Despite improvements in structure elucidation,

the exact structure of lignin is still unknown. The consensus by a number of

workers (Adler, 1980; Sjostrom, 1981; Chen, 1991; Ede and Kilpelaeinen, 1995;

Karhunren et al., 1995a; Karhunren et al., 1995b) is that the two commonest

linkages are ether and carbon-carbon bonds. The phenylpropane β-aryl ether

linkages constitute the largest proportion of the different linkages connecting the

monomeric units. These linkages need to be broken for effective lignin

fractionation from biomass.

The processes used in the extraction of lignin from woody plants are conducted

under conditions were lignin is progressively broken down to lower molecular

47

weight fragments resulting to changes to its physico-chemical properties. Thus,

apart from the source of the lignin, the solvating ability of the solvent to either

lignin or the cellulose, or both, the properties of the solvent to inhibit to C-C

bond formation, the solution pH and the method of extraction influences the

chemical and functional group composition of lignin. Gosselink et al. (2004)

have reported that lignin composition is different not only among plants but also

different between parts of the same plant. The structural heterogeneity of lignin

has also been studied by various methods in a number of investigations. In

several of these studies the lignin was subjected to fractionation prior to the

analysis (Robert et al., 1984; Moerck et al., 1986; Vanderlann and Thring, 1998;

Wallberg et al., 2003). These fractionations were analysed for functional groups,

elemental composition and molecular weight. The results of these investigations

showed that the fractionation process separated the lignin into distinct molecular

weights and that there were differences in the carboxylic acids, phenolic

hydroxyl and methoxyl contents The properties of the materials produced were

dependent on these structural properties

As one of our research goals is to produce cellulosic ethanol from bagasse via

pretreatment with soda and add value to lignin, we have undertaken a

characterisation exercise on soda lignin and examined its heterogeneity through

sequential extraction in order to target products based on structure-property

relationships. Where possible we have compared the results to that of bagasse

lignin obtained through aqueous ethanol extraction, as the lignin obtained by this

process is generally regarded to be of good quality.

2.2. Mater ia ls and Methods

2.2.1. Lignin extraction

Bagasse was obtained from a Mackay Sugar Mill, Queensland Australia. It was

wet depithed and then air dried. Lignin was extracted from bagasse by the soda

process using a 20 L Parr reactor. In this method, 1 kg of bagasse is reacted with

~10.5 L of 0.7 - 1 M NaOH. Once the reactor reaches the operating temperature

of 170°C, it is maintained for 1.5 h. After cooling, the liquid (black liquor) was

removed from the bottom of the reactor and sieved to remove fibrous material.

48

To the black liquor, dilute sulfuric acid was slowly added with stirring to pH 5.5.

Near pH 5.5 an obvious change in the appearance of the solution occurs from

black to murky brown. This change is due to the initial stages of lignin

precipitation. The mixture was stirred for 10 min - 15 min after which

acidification was continued to pH 3. It was then transferred to a 65°C water bath

and stirred using an overhead stirrer for 30 min – 45 min. The mixture was then

vacuum filtered to recover the lignin. The lignin was repeatedly washed with hot

water until all signs of foaming had subsided. It was then left to air-dry before

being further dried in a vacuum oven at 45°C overnight. This procedure

increases the purity of lignin by reducing the inclusion of ash and carbohydrate

components. It is different from other procedures reported in the literature

because it is based on a two-stage acid precipitation process. The initial

precipitation process at pH 5.5 produces lignin particles of high purity which are

then allowed to grow to larger sizes before proceeding to the second precipitation

stage where the proportion of impurities is highest.

Organosolv lignin (OL) was obtained through precipitation of the black liquor

into a dilute H2SO4 solution. The black liquor was obtained by the

delignification of bagasse at 190 oC with 50 wt% aqueous ethanol solution with a

liquor to solid ratio of 10:1, a reaction time of 90 min, in a 20 L Parr reactor.

Crude lignin was dissolved in 0.1 M NaOH equilibrated and precipitated with

H2SO4 at pH 3. The slurry was filtered hot and the residue was washed with

water until the filtrate became colourless. The lignin was air dried and further

dried at 45°C and 100°C, consecutively.

2.2.2. Lignin fractionation

Soda lignin is a complex and heterogeneous mixture with a rather broad

molecular weight distribution. Sequential fractionation was carried out to

separate the lignin into three fractions of distinct molecular weight/size and

chemical functionality. Ether and methanol are the solvents used in this study as

was used by Thring et al. (1996) to fractionate ALCELL lignin.

To fractionate soda lignin, ~ 100 g and 250 mL diethyl ether are added to a large

Schott bottle (1 L) or beaker (1 L). The container is covered and the contents are

then stirred for 20 min before being left to settle for 10 min. The diethyl ether is

49

then decanted into another container. The remaining solid is then subjected to

the same treatment. This is repeated until the supernatant diethyl ether is a light

yellow colour when decanted. The lignin residue is allowed to dry before this

process is repeated using methanol in place of ether. The diethyl ether fraction

(L1) and methanol fraction (L2) are either recovered using the rotary evaporator

to evaporate the solvent, or acid is used to precipitate the lignin, followed by

filtration to recover the solid lignin. All 3 fractions of lignin (L1, L2 and the

remaining residue, L3) are then dried and weighed. The moisture contents were

between 2.4 wt% and 3.8 wt%.

The OL was not fractionated in this study.

2.2.3. Lignin characterisation methods

2.2.3.1. Elemental analysis

Elemental analysis was performed on the lignin samples using a FLASHEA

1112 Elemental Analyser instrument. In preparing the samples for analysis, first

they were dried at 100°C overnight, to remove any moisture. To measure

carbon, hydrogen and nitrogen contents, 2 mg to 4 mg samples were

encapsulated in a tin container, and for measuring oxygen content 2 mg to 4 mg

samples were encapsulated in a silver container. The analysis results were

obtained via gas chromatography, and compared with those of standard

materials.

2.2.3.2. Ash analysis

Crucibles were pre-dried to constant weight in a muffle furnace at 575°C.

Lignin samples (0.5 g - 2 g) were weighed into the crucibles and heated to 105°C

to remove moisture. The crucibles were then heated at 575°C to constant weight.

The weight of ash remaining was calculated as a percentage of the original dry

weight of sample (Sluiter et al., 2008a). An internal reference bagasse material

in which the ash content is known was used as a standard.

2.2.3.3. Bulk density

The bulk density of the original lignin and L2 were determined using the

standard method (ASTM C29/C29M).

50

2.2.3.4. Sugar analysis

Analytical grade glucose was supplied by B.D.H. D- (+) Xylose and D- (+)

arabinose with purity > 99 wt% were supplied by Sigma. D-Cellobiose with

purity ≥ 99 wt% was supplied by Fluka. Standard stock solutions were prepared

with degassed deionised water. Analytical grade concentrated H2SO4 (98 wt%)

was supplied by Merck.

Aliquots of 3 mL of 72 wt% H2SO4 were added to 0.3 g samples of lignin in

pressure tubes. The tubes were placed in a water bath at 30°C for 1 h and stirred

intermittently to completely wet the lignin sample. The acid was then diluted to

4 wt% through the addition of water and the samples were autoclaved in pressure

tubes at 121°C for 1 h (Sluiter et al., 2008b). The samples were filtered through

porcelain crucibles to remove solids and the liquid fraction was analysed by high

performance liquid chromatography (HPLC) for glucose, xylose and arabinose.

The standard sugar solutions were also subjected to acid hydrolysis prior to

HPLC analysis in order to obtain the standard recoverable sugars.

2.2.3.5. Purity analysis

The purity of the lignin samples was calculated from the sum of ash and sugar

results. Due to the nature of the pulping and precipitation techniques, a

significant amount of ash and sugars may be present if the sample is not

copiously washed with distilled water.

2.2.3.6. Characterisation of functional groups

To predict the properties of lignin and its fractions, different functional group

analyses were performed. The functional groups quantified were the methoxyl

group, carboxylic acid functional group, phenolic hydroxyl group and total

hydroxyl group contents.

2.2.3.6.1. Methoxyl content method

The classical method for methoxyl determination of lignins uses hydroiodic acid

to promote demethylation and gas chromatography to determine the percentage

methoxyl content (Girardin and Metche., 1983). A less tedious method involves

the use of proton nuclear magnetic resonance (1H NMR) (Aberu and Freire,

1995).

51

The 1H NMR spectra of samples of acetylated lignins show that syringyl proton

signals occur between 6.28 ppm and 6.80 ppm, while the guaiacyl proton signals

occur between 6.80 ppm and 8.00 ppm. The theoretical ratios between aromatic

and methoxyl protons of guaiacyl and syringyl are 1.00 ppm and 0.33 ppm

respectively. These ratios can actually be measured from the 1H NMR spectra of

acetylated lignins. i.e. x = H(aromatic)/H(methoxyl). Aberu and Freir (1995)

analysed the 1H NMR spectra data of a whole range of acetylated lignins

(aromatic and methoxyl protons occur between 6.4 ppm to 7.1 ppm and 3.5 ppm

to 4.1 ppm respectively, see Figure 2-1) and plotted the ratio x against the %

methoxyl (OCH3) content obtained by the classical hydroiodic acid method for

the same lignins. They then submitted the data to a statistical linear regression

analysis to obtain equation (2-1):

% OCH3 = 28.28436 – 19.750047x (2-1)

For this study, soda lignin samples (~ 1 g) were added to mixtures of

pyridine/acetic anhydride (2:1) and the solutions were stirred at room

temperature for seven days. At the end of this, for each mixture, the pyridine

was removed by rotary evaporation with the periodic addition of ethanol. The

residue was dissolved in a small volume of chloroform and the solution was

added drop-wise with stirring to 200 mL of diethyl ether. The precipitated

acetate was then filtered off on a glass frit and collected solid was dried in

vacuum at 40 oC. The acetylated lignins were then analysed by 1H NMR to

obtain x from which the % OCH3 was calculated from equation 2-1.

52

Figure 2-1 NMR spectrum of an acetylated lignin (L2) fraction

2.2.3.6.2. Carboxylic acids and phenolic hydroxyl groups method

Carboxyl groups are believed to be present in native lignin, in extremely low

concentrations. However, when native lignin is subjected to chemical or

biological treatments, carboxyl groups are frequently detected in significant

quantities. Therefore, quantitative measurements of carboxyl groups may

provide information regarding the degree to which the lignin has been degraded

or modified as a result of treatment.

A titration method was used in this work (Dence, 1992). The saturated KCl

electrolyte generally used in calomel electrodes was replaced by a 1 M aqueous

solution of tetra-n-butylammonium chloride (TnBACl). The titrant (0.05M

TnBAH) was standardised through titration of benzoic acid in N,N’

dimethylformamide (DMF) to a sharp inflection break.

The lignin sample and p-hydroxybenzoic acid were dissolved in a solution of

distilled water, concentrated HCl and DMF. The solution was then titrated with

0.05 M TnBAH. There were three inflections in the titration curve. These

53

correspond to: excess HCl and strong acids present in the sample, carboxylic

acids, and phenolic hydroxyl groups, respectively.

A blank was also run on a solution of p-hydroxybenzoic acid, distilled water,

HCl and DMF.

2.2.3.6.3. Total hydroxyl groups by acetylation method

The amount of total hydroxyl group in lignin was determined by potentiometry

(Gosselink et al., 2004). The acetylation procedure given by Gosselink and co-

workers (2004) is known to be unreliable because the acetylation of lignin is not

complete. Therefore the procedure was slightly modified and the heating time

extended from 1 h to 24 h. Approximately 0.5 g - 0.8 g of air-dried lignin was

added to 10 mL of an acetic anhydride: pyridine (1:4 v/v) mixture. This was

heated overnight in an oil bath at 90°C. After adding 2 mL water and 5 min

stirring, 50 mL ethanol was added. Subsequently, the acetylated lignin was

potentiometrically titrated with a standardised 0.1 M NaOH in ethanol.

2.2.3.7. Molecular weight determination

As lignin from different crops or treatments can be extremely diverse in

structure, it is necessary to determine these differences through analytical

methods. The molecular weight of a polymer can be a good indication of its

strength as well as other physical properties. Size exclusion chromatography is a

simple technique that can be utilized to determine the molecular weight of lignin.

Lignin samples were prepared in eluent (0.1M NaOH) at 0.2 mg.mL-1 just prior

to analysis and filtered through a 0.45 µm syringe filter before running. Sodium

polystyrene sulfonate standards of molecular weights 4,950 g mol-1, 16,600 g

mol-1, 57,500 g mol-1, 127,000 g mol-1, 505,100 g mol-1 and 1,188,400 g mol-1

used to prepare a standard calibration curve. Lignin weight average molecular

weight (Mw) and number average molecular weight (Mn) were calculated using

the equation obtained from the trend line of the standard curve.

2.2.3.8. Thermogravimetric analysis (TGA)

Approximately 10 mg of sample was weighed into an aluminium pan and placed

in the thermogravimetric analyser (TGA). Heating was at a rate of 10°C min-1

and was performed from room temperature to approximately 800°C. The test

was performed in an atmosphere of nitrogen, which was injected at a flow rate of

54

15 mL min-1. A curve of weight loss against temperature was constructed from

the data obtained by the instrument. A derivative of this curve (dTG) was

produced to indicate the temperatures at which maximum rates of weight loss

occurred.

2.2.3.9. Differential scanning calorimetry (DSC)

Approximately 10 mg - 15 mg of lignin was precisely weighed and then

encapsulated in an aluminium pan. The pan was then placed in a DSC-Q100

instrument and heated from 0°C to 200°C at a heating rate of 10°C min-1 (cycle

1). The test was performed in an atmosphere of nitrogen, which was injected at a

flow rate of 15 mL min-1. Samples were then cooled at a rate of

30°C min-1, to -10°C (cycle 2). Samples were then reheated to 200°C at a rate of

10°C min-1 (cycle 3). The plot obtained from this second heating run shows the

Tg as a step transition.

2.3. Resul ts

2.3.1. The fractionation process

Only a very small portion of the original lignin sample, ~8 wt%, was recovered

using diethyl ether (L1). The major proportion, ~ 68 wt% is methanol soluble

(L2), and the residue makes up the remaining 24 wt% (L3). Similar fractional

yields (within 1 wt%) were obtained in repeat experiments, demonstrating the

reproducibility of the fractionation procedure. The results clearly show that

bagasse soda lignin is heterogeneous. For OL, values of 24 wt% for L1, 50 wt%

for L2 and 23 wt% for L3 were obtained. Thring et al. (1996) using a similar

fractionation procedure obtained values of 27 wt% for L1, 53 wt% for L2 and 18

wt% for L3 for an ALCELL lignin extracted from mixed hardwood. Based

solely on the fractionation data, it appears that the organosolv or ALCELL lignin

is more polydispersed than the soda lignin produced in this project.

2.3.2. Elemental analysis results

The elemental analysis results of the lignins are shown in Table 2-1. The data

show that carbon and hydrogen contents decrease from L1 to L3. The nitrogen

contents of the soda lignin and its fractions are lower than that of OL. The

55

protein contents in the soda lignin are in the range 0.13 wt% to 2.13 wt% if a

conversion factor of 6.26 is used. The protein content is low in the soda lignins

because proteins are soluble in alkali and so will easily be removed during the

lignin recovery process.

Tab le 2 - 1 E lementa l ana lys i s o f l i gn in s (wt %)

Sample N C H O

Starting lignin 0.29 63.25 5.95 26.40

L1 0.02 70.38 7.27 21.52

L2 0.26 63.83 6.00 27.47

L3 0.34 43.85 5.10 31.39

OL 0.50 62.10 6.20 29.00

Atomic ratios are calculated using values in Table 2-1, neglecting the nitrogen

contents, to give the empirical formulae of the different lignins (Table 2-2). In

lignin chemistry the empirical formula of the macromolecule is commonly given

as a hypothetical hydroxyphenyl structural unit (Quideau and Ralph, 1992). This

is known as the C9-formula, with six carbon atoms in the benzene ring plus three

carbon atoms making up the propyl side-chain. The results are shown in Table 2-

2. Worth noting, though not surprising, is that the C9 formula of L2 is similar to

that of OL.

Tab le 2 - 2 L ign in f rac t ion s fo rmu lae

Sample Empirical formula C9 formula

Starting lignin C5.27H5.95O1.65 C9H10.16O2.82

L1 C5.87H7.27O1.35 C9H11.15O2.07

L2 C5.32H6.00O1.72 C9H10.15O2.91

L3 C3.65H5.10O1.96 C9H12.58O4.83

OL C5.18H6.18O1.78 C9H10.73O3.15

2.3.3. Molecular weight and functional groups

The results of Size Exclusion Chromatography of soda lignin and three different

fractions is shown in Figure 2-2. In this figure, the intensity of UV absorbance

56

according to the retention time, for different fractions of lignin compared with

soda lignin, is shown.

Figure 2-2 Size exclusion chromatograms of lignin and its fractions

Table 2-3 shows Mn and Mw results of the soda lignins and OL. The molecular

weight of the soda lignins increases from L1 to L3. For these lignins, their

polydispersity are similar, with values around 1.1. This indicates that each

fraction essentially contains lignin of the same chain length. On the other hand,

Vanderlaan and Thiring (1998) obtained values of 1.5 for ether soluble fraction

and 2.3 for methanol fraction from ALCELL lignin.

As shown in Table 2-3, the methoxyl content of the OL is higher than that of the

soda lignins. This means that for bagasse, lignin is demethoxylated to a greater

extent during the soda extraction process (Thring et al., 1996) compared to the

organosolv process.

The methoxyl group content of the soda lignin fractions increases with molecular

weight. This increase is not related to molecular weight but related to the

insolubility of syringyl dominated lignin macromolecule in the ether and

methanol solvents used in the sequential fractionation process (Thring et al.,

1996). The results are also suggestive that the syringyl units are more difficult to

depolymerise during the delignification process.

57

Tab le 2 - 3 Molecu la r we ig h t ave rages and funct ion g roup s

Sample Mn

(g mol-1)

Mw

(g mol-1)

Mw/Mn Methoxyl*

(wt%)

Phenolic OH*

(wt%)

RCOOH*

(wt%)

Total OH*

(wt%)

Starting

lignin

2160 2410 1.12 10.9 5.1 13.6 14.5

L1 500 560 1.12 3.1 4.3 33.8 27.4

L2 2 380 2 670 1.12 11.7 1.5 21.1 15.3

L3 5 350 5 990 1.12 12.5 3.4 6.2 7.3

OL 2 000 2 300 1.15 15.1

* Error in analysis (% ±5)

The differences between the phenolic hydroxyl and the aliphatic hydroxyl for the

various soda fractions are due to the differences in polarity between ether and

methanol. The hydroxyl and carboxylic acid contents were highest with the

ether-soluble L1. Soda pulping generally increases the carboxylic acid contents

of lignins relative to organosolv pulping (Gosselink et al., 2004).

2.3.4. Sugar analysis results

The sugar and ash contents of lignin and its fractions are shown in Table 2-4.

The purity of the original soda lignin, L1 and L2 are comparable to OL. The low

purity of L3 is related to the high ash, xylan and glucan values. As xylan is

present in all the lignin samples, it confirms its strong association with the

phenolic backbone.

Mass balance calculations indicate that the ash content obtained for L3 cannot be

more than 9 wt%. The procedure used in the determination of the ash content of

lignin is based on heating the acid insoluble residue sample at 575°C to a

constant weight. If there is char formation of the carbohydrate residue during the

heating process, the value of the ash that is recorded will be inflated. Sample L3

has very high carbohydrate content (see Table 2-4) and it is therefore proposed

that there was char formation during heating at 575°C.

Energy dispersive spectroscopy indicated that the main element in the ashed

lignin samples is silicon. This is expected since sugarcane bagasse (from which

58

the lignins were extracted) contains high silica content. Minor amounts of

sodium, iron and potassium were also detected.

Tab le 2 - 4 Pu ri t y o f l i gn in s

Sample Ash

(wt %)

Glucan*

(wt %)

Xylan*

(wt %)

Arabinan*

(wt %)

Purity

(wt %)

Starting lignin 2.0 0.2 1.6 <0.1 96.3

L1 0.2 0.0 0.2 <0.1 99.6

L2 1.0 0.1 0.5 <0.1 98.4

L3 18.5 3.9 10.0 0.37 67.2

OL 0.4 1.4 0.6 <0.1 97.6


2.3.5. Bulk density results

The bulk density for the starting soda lignin was 680 kg m-3 and that for L2 was

640 kg m-3. These values are higher values than typically reported for soda

lignins which are between 450 kg m-3 and 500 kg m-3.

2.3.6. TGA results

The results for the thermal decomposition of soda lignin, soda lignin fractions

(i.e. L1, L2 and L3), and OL are in Figures 2-3 – 2-6. The decomposition

profiles of L2 and the starting lignin material are similar, supporting the solvent

fractionation result in which L2 constitutes the largest proportion of the starting

material. The TGA/dTG curves of all the samples predominantly show a two-

step (neglecting water loss) thermal decomposition process, though L2, L3 and

the starting lignin material in addition, have shoulders at higher temperatures.

The first weight loss starting from <100°C for L1 and at later temperatures for

the other lignins is associated with water loss. The second weight loss (i.e. the

first decomposition stage) with a peak temperature of 300–310°C is usually

assigned to the decomposition of hemicellulose (i.e. xylan) present in a

lignocellulosic matrix (Garcìa-Pèreza et al. 2001). The second stage

59

decomposition (peak temperature > 336– 367°C) is due to cellulose (i.e. glucan)

and lignin decomposition (Garcìa-Pèreza et al. 2001).

The weight loss recorded for L1 between 175°C and 310°C is 24 wt% (see

Figure 2-3). This weight loss cannot be associated with hemicellulose

decomposition because from the sugar analysis results of Table 3, the

hemicellulose (.i.e. xylan) content is 0.2 wt%. It is therefore likely that there is a

compound or compounds present in L1 that decompose in the temperature

regions associated with hemicellulose decomposition. This low molecular

weight compound or compounds may be polyphenolic in nature with a high

carboxylic acid content (see Table 2-4).

The maximum decomposition temperature of L1 is 380°C. For L2, L3 and the

starting lignin material, although 75 wt% of the material decomposes at

temperatures lower than 380°C, 25 wt% decomposes at temperatures > 400°C.

In summary, the TGA results clearly show that L1 is different from the other

lignin fractions. More importantly, the TGA results show that soda lignin is

thermally stable around 175°C, and so in preparing thermoplastics and polymer

blends with soda lignin, the working temperatures should be restricted to this

temperature region.


60



61

Figure 2-6 TGA/DTG curve of the starting soda lignin performed under

nitrogen atmosphere

2.3.7. Glass transition temperature

The DSC results for cycle 3 for the soda lignins, as well as OL, are shown in

Figure 2-7. The results were processed using “Universal 4.2E TA” software.

Table 2-5 shows the Tg of the lignins. It shows that the Tg of the soda lignins

increased with increase in molecular weight, and that OL has the same Tg as L2.

The low Tg obtained with L1 suggests that it is not lignin but a related

polyphenolic macromolecule.

Figure 2-7 DSC curves for lignin and its fractions

62

Tab le 2 - 5 Tg o f l i gn in and f ra c t ion s

Sample Tg (°C)

Starting lignin 130

L1 51

L2 130

L3 154

OL 130

2.4. Discuss ion The results of this work have confirmed the heterogeneity that exists in soda

lignin. The lignin fractions obtained via sequential extraction were different in

carboxylic acid, methoxy, phenol hydroxyl, ash and sugar contents, as well as in

molecular weight. There is some similarity in the molecular weight averages

between the soda fraction obtained with methanol i.e. L2 and the organosolv

lignin, OL.

The high purity of L1 and L2 suggests that they can be used in similar

applications as organosolv lignin.

The highest phenolic hydroxyl group content, L1, has the highest potential to

react with oxyalkylating modification reagents such as ethylene oxide and

propylene oxide. This would improve the compatibility between lignin and

polyolefins and improve the dispersion of lignin in the polyolefin network. Also,

with the lowest methoxy content, L1 is likely posseses vacant sites that are

desirable for lignin functionalisation.

In the work reported by Muller et al. (1984) it was found that kraft lignin-based

phenol formaldehyde (PF) resins have superior properties to steam exploded

lignin-based PF resin. The differences were attributed to differences in the

chemical structure between the two lignin types. Kraft lignin was found to have

a higher phenolic guaiacyl content, lower carbon-carbon bonding between

aromatic rings, higher solubility in sodium hydroxide and higher molecular

weight than steam exploded lignin (Muller et al., 1984). So, to a certain degree,

63

the high phenolic content of the starting soda lignin material, L1 and L2 should

translate into good reactivity with formaldehyde in PF and epoxy resins.

Fraction L3, though high in ash content, will also be suitable for making PF

resins because of its relatively high molecular weight and high sugar content (i.e.

glucan and xylan).

For lignin to be used in free radical polymerisation reactions and for the

syntheses of acrylic resins and paints, its solubility can be improved in

monomers such as styrene and methyl methacryalate by reacting its hydroxyl

groups with acid anhydrides. Also its free radical scavenging ability would be

considerably reduced by the formation of a lignin ester. As fractions L1 and L2

are high in phenolic hydroxyl groups they will be expected to readily form the

desired ester, and because they contain a high percentage of carboxylic acid

functional groups they have a greater probability of increasing the elastic

character acrylic resins/paints through hydrogen-bond structuring.

64

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by 1H NMR. An. Bras. Ci. 67, 379–382. Adler, E., 1980. Lignin chemistry-past, present, and future. Wood Sci. Technol.

14, 241–268. Chen, C.L., 1991. Lignins: occurrence in woody tissues, isolation, reactions, and

structure. Int. Fiber Sci. Technol. Ser. 11, 183–261. Dence, C.W., 1992. Determination of carboxylic groups. In: Lin, S.Y., Dence,

C.W. (Eds.), Methods in Lignin Chemistry. Springer, Berlin, pp. 458–464. Ede, R.M., Kilpelaeinen, I., 1995. Homo- and hetero-nuclear 2D NMR

techniques: unambiguous structural probes for non-cyclic benzyl aryl ethers in soluble ligninsamples. Res. Chem. Intermediates 21 (35), 313–328.

Food and Agriculture Organisation, 2009. Methods in lignin Chemistry. Prod STAT database. http://faostat.fao.org/site/567/DesktopDefault.aspx?PageID=567 #ancor.

Garcìa-Pèreza, M., Chaala, A., Yanga, J., Roy, C., 2001. Co-pyrolysis of sugarcane bagasse with petroleum residue. Part I: thermogravimetric analysis. Fuel 80.

Girardin, M., Metche, M., 1983. Microdosage rapide des groupements alkoxyles par chromatographie en phase gazeuse: application à la lignine. J. Chromatogr. A264, 155–158.

Gosselink, R.J.A., Abächerli, A., Semke, H., Malherbe, R., Käuper, P., Nadif, A., van Dam, J.E.G., 2004. Analytical protocols for characterisation of sulphur-free lignin. Ind. Crops Prod. 19 (3), 271–281.

Karhunren, P., Rummakko, P., Sipila, J., Brunow, G., 1995a. Dibenzodioxocin a novel type of linkage in softwood lignins. Tetrahedron Lett. 36 (1), 169– 170.

Karhunren, P., Rummakko, P., Brunow, G., 1995b. The formation of dibenzodioxocin structures by oxidative coupling. Amodel reaction for lignin biosynthesis. Tetrahedron Lett. 36 (25), 4501–4504.

Moerck, R., Yoshida, H., Kringstand, K.P., Hatakeyama, H., 1986. Fractionation of kraft lignin by successive extraction with organic solvents, 1. Functional groups, 13C-NMR-spectra and molecular weight distributions. Holzforschung 40, 51–60.

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Sluiter, A.H.B., Ruiz, R., Scarlata, C., Sluiter, J., Templeton, D., Crocker, D., 2008b. Determination of Structural Carbohydrates and Lignin in Biomass Laboratory Analytical Procedure (LAP); NREL Report No. TP-510-42618.

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In the case of this chapter: Publication title and date of publication or status: “Thermal stability and miscibility of poly(hydroxybutyrate) and soda lignin blends”, Industrial Crops and Products, Vol 32, 656-661, 2010

Contributor Statement of contribution* Payam

Mousavioun Experimental design, Conducted experiments, Data analysis. Signature

Date William O.S.

Doherty Wrote the manuscript, Data analysis.

Graeme A. George Data analysis.



66

CHAPTER 3

Thermal stabil i ty and miscibil i ty of

poly(hydroxybutyrate) and soda l ignin

blends

Payam Mousaviouna, William O.S. Dohertya and Graeme A. Georgeb

a Sugar Research and Innovation, Centre for Tropical Crops and

Biocommodities, Queensland University of Technology, GPO Box 2434,

Brisbane, Australia. b School of Science and Technology, Queensland University of Technology,

GPO Box 2434, Brisbane, Australia.

Published in Industrial Crops and Products, Vol 32, Page 656, 2010.

67

Abstract- The thermal properties and miscibility of poly(hydroxybutyrate)

(PHB) and soda lignin blends were investigated by thermogravimetry analysis

(TGA), differential scanning calorimetry (DSC), scanning electron microscopy

(SEM) and Fourier transform infrared spectroscopy (FT-IR) over the entire range

of composition. Although the addition of soda lignin shifts the onset of PHB

decomposition to lower temperatures, the PHB/lignin blends are thermally more

stable than PHB over a wider temperature range. The thermal behaviour of these

blends as measured by TGA suggests compatibility for the blends containing up

to 40 wt% soda lignin. These results correlate well with the glass transition

temperature (Tg) data where a single Tg was obtained for these blends. At higher

lignin to PHB ratios, two Tgs depicting immiscibility were obtained. The infrared

data show that the miscibility of the blends containing up to 40 wt% soda lignin

is associated with specific hydrogen bonding interactions between the reactive

functional groups in lignin with the carbonyl groups of PHB.

3.1. I ntroduct ion The negative impact of petrochemical-based platform chemicals and industrial

commodities has led to the use of “green” materials to reduce greenhouse gas

and toxic emissions, reduce energy demand, and reduce the use of non-

renewable resources. As a consequence, there has been a focus on the use of

environmentally friendly natural polymers and biopolymers. These polymers

include cellulose, hemicellulose, lignin, starch, proteins, fats, polynucleotides,

glycolide/lactide-based linear aliphatic polyesters, non-glycolide/lactic linear

aliphatic polyesters and aliphatic and aromatic polycarbonates. Of particular

interest is a class of microbially produced polymers known as

polyhydroxyalkanoates (PHAs). PHAs serve as intracellular carbon and energy

storage materials for the algae and bacteria that produce them (Verhoogt et al.,

1994). Polyhydroxybutyrate (PHB) is a member of this class of polymers. PHB

is insoluble in many solvents and has good barrier properties towards water,

oxygen and carbon dioxide (Ghaffar, 2002). It is readily broken down, with the

aid of enzymes, to water and carbon dioxide. These properties combined with

PHB’s potential for sustainable usage, makes it a potential commodity material

in the packaging industry.

68

The reasons why the potential of PHB has not been fully fulfilled, apart from its

prohibitive cost, are its stiff and brittle nature and its thermal instability during

processing. The crystal structure and crystallisation conditions are responsible

for these thermo-mechanical properties. PHB undergoes secondary nucleation at

ambient temperature because of its low glass transition temperature (Tg) and it

possesses a low nucleation density resulting in the formation of large spherulites

(Barham and Keller, 1986). The spherulites contain crazes, and splitting occur

around the centre of these crazes, hence producing a significant structural weak

point (Mahendrasingam et al., 1995). PHB undergoes thermal degradation and

depolymerisation at temperatures close to its melting point and degradation is

further enhanced by high shear rates during melt processing and extrusion.

As a result of these limitations, research efforts have concentrated on modifying

PHB by (a) changing its bulk properties without changing its physical form, (b)

changing its microstructure, (c) making it more resistant to thermal degradation

(d) changing its chemical properties, (e) modifying its solubility so that less toxic

chemicals are used, (f) improving its processability, and (g) lowering production

costs without sacrificing properties. One approach to improve PHB’s properties

is through blending. Polymer blending is a less expensive way of producing

materials with desired properties. The literature contains several investigations

of blends of PHB and other polymers such as poly(ξ-caprolactone) (Antunes and

Felisberti, 2005), poly(vinylidene fluoride) (Chiu et al., 2001), poly(viny

alcohol), poly(lactic acid), poly(vinyl acetate), poly(vinylphenol), poly(DL-

lactide)-co-poly(ethylene glycol), and cellulose esters. These polymers,

depending on proportion, result in good miscibility with PHB. Many of these

compounds have been found to be miscible or partially miscible on the basis of

specific hydrogen-bonding interactions (Kuo et al., 2002, Kuo and Chang, 2001,

Sixun et al., 2003, Yong et al., 2001, Yoshie et al., 1995, Zheng and Mi, 2003).

The main factors affecting the miscibility of polymers are the chemical nature of

the polymer constituents and their molecular weight. The chemical nature of the

polymers accounts for the existence of strong interactions (i.e., negative

enthalpy) between the polymers. Thus, polymers with interacting functional

groups that result in hydrogen-bond formation and ionic interaction would

69

enhance miscibility between polymer systems (Viswanathan and Dadmun,

2002).

Lignin is an amorphous macromolecule composed of phenylpropane repeat units

and possesses aliphatic and aromatic hydroxyl groups as well as carboxylic acid

groups. These interacting functional groups, as well as its amorphous nature

make lignin a good candidate for blending with aliphatic polyesters, such as

PHB. The amorphous nature of lignin may reduce the formation of large

spherulites, retard crystallisation (Ghosh, 1998) and reduce secondary

nucleation, all of which impact on PHB brittleness. Limited studies have been

carried out on PHB and lignin blends. Ghosh (1998), Ghosh et al. (1999) and

Ghosh et al. (2000) prepared blends (from the melt and solution) of PHB,

polyhydroxybutyrate-hydroxyvalerate (PHBV) and cellulose acetate butyrate

with organosolv lignin and organosolv lignin ester. The organosolv lignin and

its butyrate derivative were found to have a high degree of miscibility with PHB,

and the lignin was found to inhibit and retard PHB crystallisation. Ghosh et al.

(2000b) reported that the Tg of the blends increased from that of pure PHB

towards that of lignin/lignin butyrate further confirming some compatibility

between PHB and lignin. The organosolv lignin used in these studies was

obtained through extraction of hardwood with aqueous ethanol. The source from

which lignin is obtained and the method of extraction has a strong bearing on its

properties (Lora and Glasser, 2002). Thus, in this work the miscibility between

PHB and soda lignin was examined using thermal and spectroscopic methods.

The soda lignin was obtained from sugarcane fibre (i.e. bagasse) with sodium

hydroxide as solvent used in the extraction process.


3.2.1. PHB

Bacterial PHB was obtained from Sigma Aldrich. The weight average molecular

weight, Mw as determined by gel permeation chromatography is 440,000 g mol-1

while the number average molecular weight, Mn is 260,000 g mol-1. The Tg of

the PHB is 4°C and the melting point is 173°C.

70

3.2.2. Soda lignin extraction

Bagasse was obtained from a Mackay Sugar Mill, Queensland, Australia. It was

wet depithed (through a 4.2 mm screen) and then air dried. Soda lignin was

extracted from bagasse by the soda process using a 20 L Parr reactor. In this

method, 1 kg of bagasse is reacted with about 10.5 L of 0.7 M - 1 M NaOH.

Once the reactor reached the operating temperature of 170°C, it was maintained

at that temperature for 1.5 h. After cooling, the liquid (black liquor) was

removed from the bottom of the reactor and sieved to remove fibrous material.

To the black liquor, dilute H2SO4 (0.1 M) was slowly added with stirring to pH

5.5. Near pH 5.5 an obvious change in the appearance of the solution occurs;

from black to murky brown. This change is due to the initial stages of lignin

precipitation. The mixture was stirred for 10 min - 15 min after which

acidification is continued to pH 3. It was then transferred to a 65°C water bath

and stirred using an overhead stirrer for 30 min – 45 min. The mixture was then

vacuum filtered to recover the soda lignin. The soda lignin was repeatedly

washed with hot water until all signs of foaming have subsided. It was then left

to air-dry before being further dried in a vacuum oven at 45°C overnight. This

procedure increases the purity of soda lignin by reducing the inclusion of ash and

carbohydrate components. It is different from other procedures reported in the

literature because it is based on a two-stage acid precipitation process. We posit

that the initial precipitation process at pH 5.5 produces lignin particles of high

purity which were then allowed to grow to larger sizes before proceeding to the

second precipitation stage where the proportion of impurities is highest. The

extraction process and recovery procedure resulted in the recovery of 89 wt% of

the starting lignin content (22.2 wt%) in the bagasse.

3.2.3. Lignin characterisation method

3.2.3.1. Elemental analysis

Elemental analysis was performed on the soda lignin sample using a FLASHEA

1112 Elemental analyser instrument. In preparing the sample for analysis it was

dried at 100°C overnight, to remove any moisture. To measure carbon, hydrogen

and nitrogen contents, 2 mg to 4 mg of sample was encapsulated in a tin

container, and for measuring oxygen content the same quantity of soda lignin

71

was encapsulated in a silver container. The results were obtained gas

chromatography, and compared with those of standard materials.

3.2.3.2. Ash analysis

Crucibles were pre-dried to constant weight in a muffle furnace at 575°C.

Lignin samples (0.5 g - 2 g) were weighed into the crucibles and heated to 105°C

to remove moisture. The crucibles were then heated at 575°C to constant weight.

The weight of ash remaining was calculated as a percentage of the original dry

weight of sample (Sluiter et al., 2008a). An internal reference bagasse material

in which the ash content is known was used as a standard.

3.2.3.3. Sugar analysis

Analytical grade glucose was supplied by B.D.H. D- (+) Xylose and D- (+)

arabinose with purity > 99 wt% were supplied by Sigma. D-Cellobiose with

purity ≥ 99 wt% was supplied by Fluka. Standard stock solutions were prepared

with degassed deionised water. Analytical grade concentrated H2SO4 (98 wt%)

was supplied by Merck.

Aliquots of 3 mL of 72 wt% H2SO4 were added to 0.3 g samples of soda lignin

in pressure tubes. The tubes were placed in a water bath at 30°C for 1 h and

stirred intermittently to completely wet the lignin sample. The acid was then

diluted to 4 wt% through the addition of water and the samples were autoclaved

in pressure tubes at 121°C for 1 h (Sluiter et al., 2008b). The samples were

filtered with porcelain crucibles to remove solids and the liquid fraction was

analysed by high performance liquid chromatography (HPLC) for glucose,

xylose and arabinose. This was performed on a Waters system equipped with a

Waters 590 pump and a Waters 410 RI detector. The HPLC configuration

included a Shodex KS-801 column with guard KS-G. The column was operated

at 85°C. The standard sugar solutions were also subjected to acid hydrolysis

prior to HPLC analysis in order to obtain the standard recoverable sugars.

3.2.3.4. Characterisation of functional groups

Lignin possesses different functional groups. The functional groups quantified

were the methoxyl group (Abreu and Freire, 1995), carboxylic acid (Dence,

1992), phenolic hydroxyl group (Dence, 1992) and total hydroxyl group contents

(Gosselink et al., 2004).

72

3.2.3.5. Molecular weight determination

The soda lignin sample was prepared in 0.1M NaOH eluent at 0.2 mg.mL-1 just

prior to analysis and filtered through a 0.45 µm syringe filter before running.

The analysis was performed on a Waters system equipped with a Waters 2487

UV detector set at 280 nm. The column used was a Shodex Asahipak GS-320

HQ with guard Asahipak GS-2G 7B. Sodium polystyrene sulfonate standards of

molecular weights 4,950 g mol-1, 16,600 g mol-1, 57,500 g mol-1, 127,000 g mol-

1, 505,100 g mol-1 and 1,188,400 g mol-1 used to prepare a standard calibration

curve. Lignin weight average molecular weight (Mw) and number average

molecular weight (Mn) were calculated using the equation obtained from the

trend line of the standard curve.

3.2.4. Blend preparation

Soda lignin and PHB were dried at 100°C and 40°C respectively, for 12 h and

then stored in desiccators under vacuum prior to use. Lignin-PHB blends with

lignin contents from 10 wt% to 90 wt% were mixed in a Haake mini lab twin

screw using the procedure reported by Ghaffar (2002). To minimise PHB

degradation, the temperature of the extruder was maintained at 175°C for 2 min.

The polymer blends were extruded as strands then cooled and pelletised. The

pellets were stored in a desiccator to avoid moisture absorption. Similar

processing conditions were carried out for soda lignin and PHB.

3.2.5. Characterisation of blend samples


The thermal decomposition studies were carried out in a TA Instruments Q500.

Approximately 10 mg of sample was weighed into an aluminium pan and

analysed by thermogravimetric analysis (TGA). Heating was at a rate of 10°C

min-1 and was performed from room temperature to approximately 800°C. The

test was performed in an atmosphere of nitrogen, which was injected at a flow

rate of 15 mL min-1. A curve of weight loss against temperature was constructed

from the data obtained by the instrument. A derivative of this curve (DTG) was


occurred.

73


Approximately 10 mg - 15 mg of sample was precisely weighed and then





30°C min-1, to -10°C (cycle 2). Samples were then reheated to 200°C at a rate of



3.2.5.3. Scanning electron microscopy (SEM)

The morphology of the PHB/soda lignin blends was examined using a scanning

electron microscope, type FEI Quanta 200 Environmental SEM at an

accelerating voltage of 15 kV. For this examination the pellets were

compression moulded between two sheets of Teflon using an established

procedure (Ghosh et al., 1999).

3.2.5.4. Fourier transform-Infrared spectroscopy (FT-IR)

IR spectra were collected using a Nicolet 870 Nexus Fourier transform infrared

(FT-IR) spectrometer equipped with a Smart Endurance single bounce diamond

ATR accessory (Nicolet Instrument Corp., Madison, WI). Spectra were

manipulated and plotted with the use of the GRAMS/32 software package

(Galactic Corp., Salem, NH). The spectrometer incorporated a KBr beam splitter

and a deuterated triglycine sulfate room temperature detector. Spectra were

collected in the spectral range 4000 to 525 cm-1, using 64 scans at 4 cm-1

resolution with a mirror velocity of 0.6329 cm.s-1. The measurement time for

each spectrum was around 60 s.

3.3. Resul ts and D iscuss ion The molecular weight and the percentage composition of the soda lignin used in

this work is given in Table 3-1. The purity (determined from the sum of the ash

and sugars) is comparable to organosolv lignin obtained from a previous study in

our laboratory (Mousavioun and Doherty, 2010). The polydispersity (i.e. ratio of

Mw to Mn) of the soda lignin is 1.1 indicating that the lignin polymer consists of

molecules of similar chain length.

74

Tab le 3 - 1 Molecu la r we ig h t o f soda l i gn in and l ign in co mponent s (wt %)

Ash* Glucan* Xylan* Arabinan* Purity

2.0 0.2 1.6 <0.1 96.1


** Error in analysis (% ±5)

The integral thermogravimetric curves for PHB, soda lignin and PHB/soda lignin

blends are given in Figure 3-1. PHB appears to have two main overall

degradation steps while soda lignin degradation is complex constituting of

several processes (Mousavioun and Doherty, 2010). For PHB/soda lignin

blends, degradation occurs in several more stages (Figure 3-1) suggesting that

blending PHB with soda lignin completely changes the decomposition behaviour

of PHB. As shown in Figure 3-1, the decomposition temperature at which the

material has reached 5 wt% degradation (T0) of PHB decreases with the addition

of soda lignin. For example, T0 for pure PHB is 212°C, whereas it is 162°C for

PHB/lignin blend with a composition of 60/40. Also, the temperature at the

maximum rate of weight loss of PHB decreases with soda lignin addition (Figure

3-1). Whilst these results are suggestive that the addition of soda lignin

promotes PHB degradation, it does not, however, give a quantitative assessment

of the overall thermal stability of the blends. As shown in Figure 3-1,

degradation of pure PHB is almost complete by ~260°C, whereas the weight loss

for the blends at this temperature is less than 60 wt%. The lower weight loss is

an indication that the blends are thermally more stable than PHB over a wider

temperature range.

Mn

(g mol-1)

Mw

(g mol-1)

Methoxy**

Phenolic

OH**

RCOOH** Total

OH**

2160 2410 10.9 5.1 13.6 14.5

75

Figure 3-1 The integral thermogravimetric curves for PHB, soda lignin

and PHB/soda lignin blends. Lizymol and Thomas (1993) used TGA to study the thermal properties of

miscible and immiscible polymer blends. For the miscible poly(vinyl

chloride)/poly(ethylene–co-vinyl acetate) blends, they found that increasing the

proportion of poly(ethylene-co-vinyl acetate) in the blend effectively increased

T0 and the temperature at 50 wt% weight loss (T50) more than in proportion to

the proportion of poly(ethylene-co-vinyl acetate) in the blend. For the

immiscible, poly(ethylene-co-vinyl-acetate) / poly(styrene-co-acrylonitrile)

systems, the T0 values of the blends were lower than the proportion of

poly(ethylene-co-vinyl acetate) in the blend, while T50 values were virtually

unaffected. Based on this approach, plots of T0 and T50 of PHB/soda lignin

blends versus soda lignin content were constructed and the results are presented

in Figure 3-2.

76

Figure 3-2 Plots of T0 and T50 of PHB/soda lignin blends versus soda lignin

content. As the soda lignin content increases T0 decreases, but the decrease does not

correspond to the proportional amount of soda lignin added to the blend.

However, at low lignin contents (up to 20 wt%), T50 values increases more than

in proportion to the proportion of soda lignin added (i.e. well above the tie line)

possibly suggesting some degree of miscibility between soda lignin and PHB at

these compositions. At soda lignin concentrations > 20 wt%, the results may

suggest incompatibility between the components. As the PHB/soda lignin

systems are not similar to those of Lizymol and Thomas (1993), the results

shown in Figure 3-2 may simply be an indication of lignin degradation products

reacting with PHB degradation products to form stable species.

The most accepted parameter to assess polymer miscibility is the Tg. A single Tg

of a blend implies complete miscibility between the polymer pairs in their

amorphous fractions, whose value is an average of the individual component’s

Tg. Two or more Tg’s suggest that the degree of miscibility is restricted. Figure

3-3 shows the DSC curves of the blends where the Tgs were obtained using TA

Universal Analysis 2000 software. The exothermic peak at ~80°C (for 50 wt%

77

and 60 wt% lignin) is associated with cold crystallisation temperature of PHB.

Ghaffar (2002) obtained similar cold crystallisation temperatures for PHB and

polyvinyl acetate blends. Why the peak is prominent in some blends and not in

others is not known and is worth future investigation.

Figure 3-3 DSC curves of PHB/soda lignin. (refer to Figure 4-5) Most miscible polymers display a single Tg whose value is dependent on the

proportion of the individual components (Fox, 1956). Figure 3-4 illustrates the

Tgs of PHB and the blends. A single Tg is obtained up to a soda lignin content of

40 wt%, thereafter there are two Tgs. The error in the analysis is % ±5. The Tgs

results therefore give a further indication of miscibility between PHB and soda

lignin at soda lignin contents up to 40 wt%.

78

Figure 3-4 Tgs of PHB and the blends versus soda lignin content. Figure 3-3 also shows that the Tg of the PHB component of the blends increases

with increase in soda lignin content. Similar results were obtained by Ghosh and

co-workers (Ghosh et al., 1999, Ghosh et al., 2000) for organosolv lignin/PHB

blends, though the values obtained in the present study were slightly higher

(Figure 3-4). This could be related to the method of preparation, differences in

the PHB source, or the lignin type as soda pulping generally increases the

carboxylic acid and hydroxyl contents of lignins relative to organosolv pulping

(Gosselink et al., 2004).

To obtain a better idea of interactions between PHB and soda lignin, we

evaluated the Tg data using the well-known Fox, Gordon-Taylor and Kwei

equations. These are:

Fox equation: �Op7qi_@r8 2 sL

Op,L3 sM

Op,M (3-1)

Gordon-Taylor equation: 9t7qi_@r8 27u19w,13j-9u29w,28

7u13j-9u28 (3-2)

Kwei equation: 9t7qi_@r8 27u19w,13juu29w,28

7u13juu28 3 yu�uR (3-3)

where Tz,� {| R and w� {| R are glass transition temperatures of the pure

components and their corresponding weight fractions, respectively. K��, K� and

q are adjustable parameters. As can be seen in Figure 3-4, the PHB/soda lignin

79

blends up to 40 wt% lignin fit nicely to the Gordon-Taylor with a K�� value of

4.15. It also fitted the Kwei equation with K� value of 0.2 and q having a value

of 22. The positive value of q and a relatively high value of K�� (larger than 1)

indicate that strong interactions (ElMiloudi et al., 2009) exist between OH

groups of lignin and the carbonyl groups of PHB for the blends containing up to

40 wt% lignin.

Figures 3-5, 3-6 and 3-7 illustrate typical SEM images of blends. For blends of

PHB/soda lignin containing 10 wt% and 30 wt% lignin, there was no apparent

phase separation, whereas for the blend with a 50/50 composition in weight,

phase separation is observed. For other compositions higher than 50 wt% soda

lignin, phase separation between the components was detected. Thus, the SEM

data follow similar trends as the data obtained from the Tg of the blends.

Figure 3-5 SEM image of PHB/soda lignin containing 10 wt% lignin.

80

Figure 3-6 SEM image of PHB/soda lignin containing 30 wt% lignin.

Figure 3-7 SEM image of PHB/soda lignin containing 50 wt% lignin. Attempts at understanding the miscibility of the PHB/soda lignin blends have

been made using FT-IR, as have been undertaken by previous workers with

similar polymer systems (Dong and Ozaki, 1997). Infrared spectroscopy

provides information on hydrogen bond formation (and other interactions) and

the strength of H-bonds through wavenumber shift, band intensity and band

width of specific signals. Figure 3-8 shows the FT-IR spectra from 1800 cm-1 to

1620 cm-1 of the carbonyl stretching region of PHB and PHB/soda lignin blends.

The PHB spectrum exhibits two peaks, these are at ~1733 cm-1and ~1722 cm-1,

though the first peak at 1733 cm-1 is more of a shoulder to the main peak at 1722

cm-1. The peak at 1733 cm-1 is associated with the amorphous component of

PHB, the peak at ~1722 cm-1 is associated with the crystalline component of

81

PHB (in its preferred conformation). The band at 1685 cm-1 has been reported to

be a crystalline band, although its origin is not known (Guo et al. 2010).


PHB/soda lignin blends. Figure 3-8 also shows that for blends containing 10, 20, 30, 40 and 50 wt% soda

lignin, there is a small but definite shift (2 cm-1 to 4 cm-1) to a lower

wavenumber for the main PHB peak. The shift to a lower wavenumber is

indicative of hydrogen bonding interactions because the stretching frequencies of

participating groups usually move towards lower wavenumbers (Barsbay and

Güner, 2007). Barsbay and Guner (2007) obtained similar small shifts for blends

of dextran and poly(ethylene glycol) cast in water. The present results therefore

indicate that the reactive functional groups of lignin are probably engaged in

hydrogen bonding interactions (or other associations) with the carbonyl oxygen

in PHB (Figure 3-9).

82

Figure 3-9 Hydrogen bonding interactions between the reactive functional

groups in soda lignin and the carbonyl groups of PHB. It is not known why there were no differences in the wavenumber shifts between

these blends containing different proportions of lignin. Figure 3-8 also shows

that there are no shifts observables for PHB blends containing 60 wt% to 90 wt%

of lignin relative to the PHB band at 1724 cm-1, thereby confirming previous

observations concerning the immiscibility of PHB/lignin blends containing these

compositions. It should however, be noted (as shown in Figure 3-8), that there

are noticeable shifts to lower wavenumbers for all the blends for the PHB band

at1742 cm-1. Since this band is of far less intensity compared to the main band at

1722 cm-1, and consequently is less prominent, it may be concluded that there are

some favourable weak interactions between the amorphous part of PHB and

lignin at all lignin proportions.

3.4. Conclus ion Soda lignin was found to improve the overall thermal stability of PHB, though it

reduced the initial temperature of decomposition of PHB. The TGA, DSC and

SEM of the PHB/soda lignin blends suggest that intermolecular interactions

between PHB and soda lignin were favoured at a soda lignin content of up to 40

wt%. These intermolecular interactions were found to be due to hydrogen

83

bonding formation between the reactive functional groups of lignin and the

carbonyl groups of PHB.

Acknowledgments Many thanks go to Dr Llew Rintoul of the School of Physical and Chemical

Sciences and Dr Lalehvash Moghaddam of Sugar Research and Innovation,

Centre for Tropical Crops and Biocommodities both at Queensland University of

Technology, Brisbane, Australia for their assistance in FT-IR analysis.

84

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87

CHAPTER 4

Thermophysical properties and

rheology of PHB/lignin blends

Payam Mousaviouna, Peter Halleyb and William O.S. Dohertya a Sugar Research and Innovation, Centre for Tropical Crops and


Brisbane, Australia. b Centre High Performance Polymers (CHPP), School of Chemical Engineering

and AIBN, St Lucia, The University of Queensland, QLD 4072, Brisbane,

Australia

Submitted to the Polymer International, 2011












In the case of this chapter: Publication title and date of publication or status: “Value-adding to cellulosic ethanol: Lignin polymers”, published in Industrial Crops and Products, Vol 33, 259-276, 2011.


Mousavioun Collating of literature. Signature

Date William

O.S.Doherty Wrote the manuscript.

Christopher M.Fellows Edited and wrote some sections of the manuscript.



88

Abstract- The thermal and rheological properties of poly(hydroxybutyrate)

(PHB) and lignin blends were investigated by thermogravimetry analysis (TGA),

differential scanning calorimetry and rheology over the entire range of

composition. Although the addition of lignin shifts the onset of PHB

decomposition to lower temperatures, the PHB/lignin blends are thermally more

stable in terms of overall weight loss at temperature than PHB over a wider

temperature range. However, the drop in the apparent energy of activation of

decomposition, E� from 112 kJ mol-1 for pure PHB to half that value with

blends, in fact suggests that lignin reduces the thermal stability of PHB and that

the reduced weight loss observed in the TGA curves is associated with the

slower rate of degradation of lignin. The rheology results show that for 10 wt%

and 30 wt% lignin, lignin behaved like a plasticizer forming a single phase with

PHB and reduced the elasticity and viscosity relative to pure PHB. For further

additions of lignin (60 wt% and 90 wt% lignin), lignin then acts as a second

phase and decreased the ability of the system to dissipate energy and increased

the viscosity of the blends. These results are in good agreement with the glass

transition data, showing that critical changes in phase behaviour are dependent

on material composition.

4.1. I ntroduct ion There are concerted efforts to produce biodegradable polymers as alternatives to

the petroleum-based ones. Polyhydroxybutyrate (PHB) and similar bacterial

polyesters have obtained world-wide interest because of their biodegradability,

sustainability, and their durability and plasticity. PHB has remarkably similar

properties to those of PP, but is expensive, stiff and brittle in nature, and is

thermally unstable during processing. The crystal structure and crystallisation

behaviour are responsible for these thermo-mechanical properties. As a

consequence of these properties, polymer blending has been used to try to

modify PHB and improve its properties and lower production costs. Literature

contains several investigations of blends of PHB and other polymers such as

poly(ξ-caprolactone) (Antunes and Felisberti, 2005), poly(vinylidene fluoride)

(Chiu et al., 2001), poly(viny alcohol) (Yoshie et al., 1995), poly(lactic acid)

89

(Zhang et al., 1997), poly(vinyl acetate) (An et al., 1997), poly(vinylphenol)

(Xing et al., 1997), poly(DL-lactide)-co-poly(ethylene glycol) (Zhang et al.,

1997), and cellulose esters (Pizzoli et al., 1994). Addition of these polymers,

depending on proportion, results in good miscibility with PHB. Mousavioun et

al. (2010) have studied the thermal properties and miscibility of PHB and lignin

blends and found that PHB/lignin blends are thermally more stable than PHB

over a wider temperature range. Infrared data showed that the miscibility of

blends containing up to 40 wt% lignin was associated with specific hydrogen

bonding interactions between the reactive functional groups in lignin with the

carbonyl groups of PHB (Mousavioun et al., 2010).

Understanding the mechanical and rheological properties of polymer blends is

necessary in establishing changes in the viscoelastic response and processability

conditions. The compatibility of a dispersed component (i.e. filler) within the

matrix can produce highly non-Newtonian response, such as in a monodispersed

crosslinked polymeric spheres of polystyrene in polymethyl methacrylate (Sun et

al., 1993). Aranguren et al. (1992) found that for polymethylsiloxanes mixed

with silica particles, the dynamic moduli showed a strong strain dependence,

while the frequency dependence was dramatically affected with the

concentration of the silica particles. A similar study but on polystyrene

composites containing monodisperse crosslinked polystyrene beads conducted

by Gandhi et al. (1990) showed a uniform decrease in rheological properties due

to increasing bead concentration or a reduction in the crosslink density of the

beads.

The aim of this work is to determine the thermophysical properties and rheology

of PHB/lignin blends and to determine the role lignin plays in PHB viscosity.

The properties of the blends were examined by thermogravimetric analysis

(TGA), differential scanning calorimetry (DSC) and rheological analysis.


4.2.1. PHB

Bacterial PHB is obtained from Sigma Aldrich. The weight average molecular


90




Bagasse was obtained from a Mackay Sugar Mill, Queensland, Australia. It was

wet depithed (through a 4.2 mm screen) and then air dried. Lignin was extracted

from bagasse by the soda process using 0.7 M sodium hydroxide solution. The

procedure for lignin extraction and purification has been described elsewhere

(Mousavioun et al., 2010).

4.2.3. Lignin characterisation

Lignin composition was determined by the methods described in the paper by

Mousavioun and co-workers (2010). The results of lignin composition are

presented in Table 4-1.

Tab le 4 - 1 Molecu la r we ig h t o f l i gn in and l ign in co mponent s (wt %) (Mousa v ioun e t a l . , 2010)


2.0 0.2 1.6 <0.1 96.1


** Error in analysis (% ±5)


Lignin and PHB were dried at 100°C and 40°C respectively for 12 h and stored

in desiccators under vacuum prior to use. PHB/lignin blends with lignin contents

from 10 wt% to 90 wt% were mixed in a Haake mini lab twin screw using the

procedure reported by Ghaffar (2002). To minimise PHB degradation, the

Mn

(g mol-1)

Mw

(g mol-1)

Methoxy**

Phenolic

OH**

RCOOH** Total

OH**

2160 2410 10.9 5.1 13.6 14.5

91

temperature of the extruder was maintained at 175°C for 2 min. The polymer

blends were extruded as strands then cooled and pelletised. The pellets were

stored in a desiccator to avoid moisture absorption. Similar processing

conditions were carried out for pure PHB.

4.2.5. Characterisation of blend samples


The thermal decomposition studies were carried out in a TA Instruments Q500.

Approximately 10 mg of the sample was weighed in an aluminium pan and

analysed by TGA in non-isothermal and isothermal modes.

In the non-isothermal mode, heating was at a rate of 10°C min-1 and was

performed from ambient to approximately 800°C. The test was performed in an

atmosphere of nitrogen, which was injected at a flow rate of 15 mL min-1. A

curve of weight loss against temperature was constructed from the data obtained

by the instrument.

The isothermal runs were performed at 170°C, 180°C and 190°C. To reach the

desired temperature, samples were pre-heated from ambient temperature to that

temperature at a rate of 10°C min-1, and then degraded isothermally for at least

50 min. The test was performed in an atmosphere of nitrogen, which was

injected at a flow rate of 15 mL min-1. A curve of weight loss against

temperature was constructed from the data obtained by the instrument. The

conversion rate curve was produced to indicate the mass loss conversion (%)

during this time.


Approximately 10 mg of sample was precisely weighed and then encapsulated in

an aluminium pan. The pan was then placed in a DSC-Q2000 instrument and

heated from 0°C to 180°C at a heating rate of 10°C min-1 (cycle 1). The test was

performed in an atmosphere of nitrogen, which was injected at a flow rate of 15

mL min-1. Samples were then cooled at a rate of 30°C min-1, to -10°C (cycle 2).

Samples were then reheated to 180°C at a rate of 10°C min-1 (cycle 3). The plot

obtained from this second heating run shows the Tg as a step transition. It was

difficult to obtain the Tg using DMTA, because of the brittle nature of the blends,

and so this technique was not used in the research study.

92

In addition to Tg, other thermal parameters of PHB/lignin blends were evaluated.

Melting temperature (Tm), melting enthalpy (∆Hm), bulk crystallinity (Xc), cold

crystal temperature (Tcc) and melting crystallisation point (Tmc) are extracted

from DSC thermographs. The bulk crystallinity of a blend was calculated using

the following equation:

}> 2 ∆~�∆~�a�

}>�~5 (4-1)

where ∆�.e0 and }>�~5 are the melting enthalpy and crystallinity of the pure

PHB used in this study. }> is a ratio of ∆�. of the sample and that of the 100%

crystalline PHB (∆��). ∆�� of PHB is assumed to be 146 J g-1 (Barham et al.,

1984). On this basis, the values of ∆�.e0 and }>�~5 used in this study are 92 J

g-1 and 63 % respectively.

4.2.5.3. Rheological analysis

The time and temperature dependent storage modulus (G′), loss modulus (G″)

and complex viscosity (η*) were determined by a Rheometrics Inc. Advanced

Rheometric Expansion System (ARES) with RSI orchestrator software, and

using parallel plate geometry having a plate diameter of 25 mm and a gap of 0.5-

1.0 mm. Disk-type specimens of pure PHB and its blends with diameter of 25

mm and thickness of 0.5 mm were obtained from compression moulded sheets.

The experiments were performed at 175°C over the frequency range of 1 rad s-1

to 100 rad s-1. To minimise the length of the test period and the probable

degradation of PHB, a multi frequency wave was used.

4.3. Resul ts and D iscuss ion

4.3.1. Degradation of PHB

The general rate equation for a decomposition or degradation process can be

described as:

�� 2 k7T8f7�8 (4-2)

where � is the degree of conversion, k7T8 the rate constant, and f7�8 is the

reaction rate model, a function which depends on the actual reaction mechanism.

In this, work

93

�2 YZ�YYZ�Y[

(4-3)

where W� is the initial weight, W is the weight during the experiment, and W� is

the final weight of the investigation determinate from the TG thermograms.

The rate constant k7T8 can be represented by the Arrhenius equation: as

k7T8 2 Ae7��8 (4-4)

where E� is the apparent activation energy (kJ mol��8, R is the ideal gas constant

(8.314 J. K��.mol��), A is the pre-exponential factor (s��), and T is absolute

temperature (K).

For an isothermal TGA process, combination of equations (4-2), (4-3) and (4-4)

results in

�� 2 Af7�8e7�

��8 (4-5)

Equations are the fundamental expressions of analytical methods to calculate

kinetic parameters on the basis of TGA data.

A linear differential method of equation 4-5 is

m�� n 2 6 �� 3 7Af7�88 (4-6)

Then within the linear part of isothermal degradation curves, the plots of ��

vs. �� directly leads to 6 ��

� from the slope from which E� is calculated.

On the basis of the report from Hablot et al. (2008), the degradation temperature

of PHB is close to its melting point. Therefore a series of isothermal gravimetric

analyses was undertaken, in order to find the safe temperature range for

processing PHB with lignin (see Section 4.2.5.1). Figure 4-1 shows the mass

loss ratios at different temperatures. The rate of mass loss shows a sharp

increase from 180°C. On the basis of these results, a working temperature of

175°C was selected for the preparation of PHB/lignin blends (see Section

4.2.5.1).

The degradation rate constants, k, at various temperatures were calculated from

the slope of the linear portion of the curves of Figure 4-1 and the results are

detailed in Table 4-2. The k values increase with an increase in temperature.

94

Figure 4-1 Isothermal degradation of PHB

Tab le 4 - 2 Deg radat ion ra te constan t o f PHB a t va r iou s temp era tu res

Temperature (ºC) Degradation rate constant, k (s-1)

170 1.6×10-6

180 3.3×10-6

190 6.0×10-6

The study by Aoyagi et al. (2002) obtained a k value of 1.84×10-6 at 170°C from

the slope of the inverse number-average degree of polymerisation against

reaction time. The Ea value obtained from the same study was

111 kJ mol-1, while in the present study a value of 112 kJ mol-1 was obtained. As

similar k and E� values were obtained, it is probable that a completely random

chain scission mechanism occurs with PHB during isothermal degradation

(Aoyagi et al., 2002).

4.3.2. Degradation of PHB/lignin blends

The thermal degradation of PHB/lignin blends were evaluated by TGA. Figure

4-2 shows the intergral thermogravimetric curves of PHB, lignin and a 50 wt%

PHB/lignin blend. The curves for the other blends have not been included for

the sake of clarity and have been presented by Mousavioun et al. (2010). The

results of Figure 4-2 show that the addition of lignin reduces the amount of

0

1

2

0 10 20 30 40 50

Mas

s lo

ss r

atio

(%

)

Time (min)

190°C

180°C

170°C

95

material lost over the wide range of temperatures investigated. The lower weight

loss is an indication that the blends are thermally more stable than pure PHB.

Figure 4-2 Integral thermogravimetric curves for PHB, lignin and 50 wt% PHB/lignin

However, if the decomposition temperature at which the material has reached 5

wt% degradation (T0) is examined, the addition of lignin appears to accelerate

the initial degradation process (Figure 4-3). This is due to the lignin component

because its T0 value is ~120°C. Thus the shift in the T0 of the blends to lower

temperatures, through the addition of lignin, provides the opportunity to process

the blends at lower temperatures, possibly limiting PHB degradation via random

chain scission mechanisms (Avella et al., 2000).

0

10

20

30

40

50

60

70

80

90

100

100 200 300 400 500 600 700 800

Mas

s (%

)

Temperature (°C)

TG

PHB

Lignin

50 wt% PHB/lignin

96

Figure 4-3 Threshold degradation temperature of PHB/lignin blends

The same trials were examined to evaluate the degradation constant of

PHB/lignin blends with various ratios and temperatures. For instance, the

degradation rate constant for 50 wt% PHB/lignin blend is shown in Table 4-3.

Tab le 4 - 3 Deg radat ion ra te constan t o f 5 0 wt % PHB/ l ign in b lend s a t va r i ous temp era tu res

Temperature (ºC) Degradation rate constant, k (s-1)

170 3.3×10-4

180 3.9×10-4

190 6.1×10-4

The E� values (error ±10%) of PHB/lignin blends were calculated using equation

4-6 and the results are presented in Figure 4-4. There appears to be a rough trend

of increasing E� with an increase in lignin content. However, it also appears that

over 10 wt% to 60 wt% lignin the E� barely changed. The drop of E� from 112

kJ mol-1 for pure PHB to up to half that value with lignin, in fact suggests that

lignin reduces the thermal stability of PHB and that the reduced weight loss

100

120

140

160

180

200

220

240

0 10 20 30 40 50 60 70 80 90 100

T0

(°C

)

Lignin (wt%)

97

observed in the TGA curves is associated with the slower rate of lignin

degradation in the blend.

Figure 4-4 Activation energy of thermal degradation of PHB/Lignin blends

4.3.3. Thermal properties of PHB/lignin blends

The glass transition temperature (Tg) of a polymer blend gives an indication of

the miscibility and processibility of the material. A single Tg implies complete

compatibility between the components. If blending results in a material having a

lower Tg than the brittle or stiff component, this then means that the blend has

more flexible chains thus lowering stiffness as well as improving processibility.

On the other hand, if the blend has a higher Tg and a higher melt viscosity, there

would be an improvement in processibility with no change in chain stiffness. In

a situation where there are two Tgs, the degree of miscibility is restricted.

However, this does not mean that there are no interactions between the

components.

Figure 4-5 shows two DSC thermograms of PHB/lignin blends which represent

the heat flow at the second heating cycle for (a) 40 wt% lignin, and (b) 80 wt%

lignin respectively. This figure shows that for the blend containing 40 wt%

lignin, lignin not only is miscible in the PHB matrix, but also is is acting as a

0102030405060708090

10 20 30 40 50 60 70 80 90

Ea

(kJ

mol

-1)

Lignin%

98

plasticiser by enhancing the Tg of PHB. This will result in some improvement in

the processibility of PHB.

Figure 4-5 DSC thermograms of PHB/lignin blends with (a) 40 wt% lignin, and (b) 80 wt% lignin

The summary of thermal parameters of PHB/lignin blends are shown in Table 4-4.

The Tg of PHB, as well as its melting temperature (Tm), ∆Hm, Xc and Tmc,

decreases with an increase in lignin content, irrespective of the lignin loading. In

general, when the lignin content ≤ 40 wt%, it raises the Tmc (i.e. the melt cold

crystallisation temperature) of the PHB. However, when the lignin content is

≥50wt%, it reduces the Tmc of PHB. Weihua et al. (2004) obtained similar

results at lower lignin contents.

99

Tab le 4 - 4 The rma l p rop e rt i es o f l i gn in / PHB b lends us ing the s ta rt in g l ign in mate r ia l

Sample Tg (°C)* Tm(°C) ∆Hm (J g-1) Xc (%)** Tmc(°C)

PHB 3 172 92 63 89

Lignin content

10 wt%

7

174

90

61

132

20 wt% 9 173 85 58 121

40 wt% 15 170 78 53 98

50 wt% 18 (130) 167 34 23 82

60 wt% 17 (148) 165 31 21 86

70 wt% 21 (134) 157 9

80 wt% 26 (125) 152 5

90 wt% 43 (131) 158 3

Lignin (130)

* Tg with parentheses is lignin, and the one without is PHB

** Xc is for the bulk crystallisation as opposed to the crystallinity of PHB itsels. (see Table 5-2)

4.3.4. Rheological properties of PHB/lignin blends

Melt rheology of composite blends is essential to understand the molten

structural property relationship and their processibility. Preliminary strain sweep

experiments were carried out to determine the extent of the linear viscoelastic

domain. As an example, Figure 4-6 shows complex viscosity (η*) vs strain for

PHB/10 wt% lignin. In all cases the results show that all samples were stable

and showed a linear viscoelastic response at strains ≤ 10%.

100

Figure 4-6 Complex viscosity (η*) versus % strain for PHB/10 wt% lignin Storage modulus ( G′) and loss modulus (G″) of PHB and blends are presented in

Figures 4-7 and 4-8 respectively. The results show that G′ and G″ generally

increase with increase in frequency.

Figure 4-7 Dynamic storage modulus of PHB and PHB/lignin blends

1

10

100

1000

1 10 100

η*

(Pa

s)

Strain (%)

1

10

100

1000

10000

100000

1 10 100

G' (

Pa)

Frequency (rads-1)

Pure PHB 10% Lignin 30% Lignin 60% Lignin 90% Lignin

101

Figure 4-8 Dynamic loss modulus of PHB and PHB/lignin blends Initially as lignin is added to pure PHB, the G′ values at all frequencies (except at

100 rad s-1) are reduced, for the 10 wt% and 30 wt%. At higher lignin contents,

i.e. at 60 and 90 wt%, the G′ values are higher than that of pure PHB for the

entire frequency range. Similar results are seen for G″.

Figure 4-7 shows a slight drop in the G′ of pure PHB at high frequencies (e.g. at

100 rad s-1), which is absent in the PHB/lignin blends. This result probably

implies that the addition of lignin improves the mechanical property of PHB.

The ratio of G″ to G′ is termed tanδ and is the ratio of energy dissipated to

energy stored in one cycle of deformation. Tan δ is small for elastic solids and

large for viscous fluids. Tan δ plots at various frequencies for pure PHB and the

blends are shown in Figure 4-9.

1

10

100

1000

10000

100000

1000000

1 10 100

G"

(Pa)

Frequency (rad/s)

Pure PHB 10% Lignin 30% Lignin 60% Lignin 90% Lignin

102

Figure 4-9 Tan δ of PHB and PHB/lignin blends

Figure 4-9 shows that for pure PHB materials the tan δ ranges from 4 at low

frequencies (somewhat elastic) to 2 (reduction in elasticisty and increase in

viscous nature) at higher frequencies, as expected for many polymer melts

(Ferry, 1980). With the addition of 10 wt% lignin the tan δ increases markedly

(from 10 at low frequencies to 30 at higher frequencies) which implies that they

behave more like a viscous fluid (less elastically) than pure PHB. Thus the

addition of 10 wt% lignin (and to a lesser extent 30 wt% lignin) to PHB, appears

to significantly decreases the elasticity of PHB. However, for the blends

containing 60 wt% lignin, the tan delta is reduced again (below that of pure

PHB) indicating an increased elastic response. It is possible the lignin is

interacting with the PHB to form an elastic lignin-PHB network, and reduces the

ability of the system to dissipate energy in a viscous manner.

Figure 4-10 depicts the complex viscosity (η*) of pure PHB and PHB/lignin

blends.

1

10

100

1 10 100

Tan δ

Frequency (rad s-1)

Pure PHB 10% Lignin 30% Lignin

60% Lignin 90% Lignin

103

Figure 4-10 Complex viscosity (η*) of blends of pure PHB and PHB/lignin

blends

The complex viscosity (η*) of pure PHB shows a typical shear thinning profile

with increasing frequency. Upon addition of 10 wt% lignin the complex

viscosity reduces appreciably across the frequency range, showing that lignin is

increasing the ease of processibilty. (This is in-line with the reduction in

elasticity and increase in viscous nature as seen in Figure 4-9). Further addition

of lignin to 30% increases the viscosity, but it is still lower than pure PHB for

most of the frequency range. Further addition of lignin then increases the

viscosity further and it becomes greater than the pure PHB for most of the

frequency range. Summarising the results reflected in Figures 4-7, 4-8, 4-9 and

4-10, it could be postulated that addition of lignin up to 10 wt% increases the

viscous dissipation of the system, reduces the viscosity (improves processibility),

and the blend act like a single phase plasticised sample (where the plasticiser

reduces the viscosity of the system). Upon further addition of lignin over 10wt%

acompeting additional effect occurs, which sees the elasticity of the system

increase and the viscosity increase. Here it can be postulated that the lignin is

interacting with the PHB matrix like a filler or a second phase that interferes

1

10

100

1000

10000

1 10 100

η*

(Pa

s)

Frequency (rad s-1)

Pure PHB 10% Lignin 30% Lignin

60% Lignin 90% Lignin

104

with the relaxation processes of the PHB matrix. These postulations are in very

good agreement with the Tg data (table 4), which shows that critical changes

from single phase to dual phase behaviour occur at concentrations greater than

40wt% lignin.

4.4. Conclus ion Lignin was found to affect the thermal stability and crystallisation of PHB. The

TGA and DSC of the PHB/lignin blends suggest that intermolecular interactions

between PHB and lignin were favoured at a lignin content of up to 40 wt%.

Rheological study shows lignin content of 10 wt% and 30 wt% plasticises PHB

and results in the formation of a single phase, while 60 wt% and 90 wt% lignin

present a two phase PHB/lignin blend systems.

105

4.5. References An, Y., Dong, L., Xing, P., Zhuang, Y., Mo, Z., Feng, Z., 1997. Crystallization

kinetics and morphology of poly(β-hydroxybutyrate) and poly(vinyl acetate) blends. Eur. Polym. J. 33, 1449-1452.


Aoyagi, Y., Yamashita, K., Doi, Y., 2002. Thermal degradation of poly[(R)-3-hydroxybutyrate], poly[ε-caprolactone], and poly[(S)-lactide]. Polym. Degrad. Stab. 76, 53-59.

Aranguren, M.I., Mora, E., DeGroot, J.V., Macosko, C.W., 1992. Effect of reinforcing fillers on the rheology of polymer melts. J. Rheol. 36, 1165.

Avella, M., Rota, G.L., Martuscelli, E., Raimo, M., Sadocco, P., Elegir, G., Riva, R., 2000. Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and wheat straw fibre composites: thermal, mechanical properties and biodegradation behaviour. J. Mater. Sci. 35, 829-836.

Barham, P.J., Keller, A., Otun, E.L., Holmes, P.A., 1984. Crystallization and morphology of a bacterial thermoplastic: poly-3-hydroxybutyrate J. Mater. Sci. - Mater. Med. 19(9), 2781-2794.

Chiu, H.J., Chen, H.L., Lin, J.S., 2001. Crystallization induced microstructure of crystalline/crystalline poly(vinylidenefluoride)/poly(3-hydroxybutyrate) blends probed by small angle X-ray scattering. Polymer 42, 5749-5754.

Ferry, J.D., 1980. Viscoelastic properties of polymers, 3rd ed. John Wiley, New York.

Gandhi, K., Park, M., Sun, L., Zou, D., Li, C.X., Li, Y.D., Aklonis, J.J., Salovey, R., 1990. Model-filled polymers. II. Stability of polystyrene beads in a polystyrene matrix. J. Polym. Sci. Pol. Phys. 28, 2707-2714.


Hablot, E., Bordes, P., Pollet, E., Avérous, L., 2008. Thermal and thermo-mechanical degradation of poly(3-hydroxybutyrate)-based multiphase systems. Polym. Degrad. Stab. 93, 413-421.


Pizzoli, M., Scandola, M., Ceccorulli, G., 1994. Crystallization kinetics and morphology of poly(3-hydroxybutyrate)/cellulose ester blends. Macromolecules 27, 4755-4761.

Sun, L., Aklonis, J.J., Salovey, R., 1993. Model filled polymers. XIV: Effect of modifications of filler composition on rheology. Polym. Eng. Sci. 33, 1308-1319.


Xing, P., Dong, L., An, Y., Feng, Z., Avella, M., Martuscelli, E., 1997. Miscibility and crystallization of poly(β-hydroxybutyrate) and poly(p-vinylphenol) blends. Macromolecules 30, 2726-2733.

106



107

CHAPTER 5

Environmental degradation of soda

l ignin/ poly(hydroxybutyrate) blends

Payam Mousaviouna, Graeme A. Georgeb and William O.S. Dohertya

a Sugar Research and Innovation, Centre for Tropical Crops and Biocommodities, Queensland University of Technology, GPO Box 2434, Brisbane, Australia. b Chemistry Discipline, Faculty of Science and Technology, Queensland University of Technology, GPO Box 2434, Brisbane, Australia. Submitted to the Journal of Polymer Degradation and Stability, 2011












In the case of this chapter: Publication title and date of publication or status: Thermal Decomposition of Bagasse: Effect of Different Sugar Cane Cultivars, published in Industrial & Engineering Chemistry Research, Vol 50, 791-798, 2011


Mousavioun

Data analysis. Signature

Date Vanita R. Maliger

Experimental design and conducted experiments.

William O. S. Doherty Wrote the manuscript,

Data analysis.

Ray L. Frost Edited manuscript.



108

Abstract- Blends of lignin and poly(hydroxybutyrate) (PHB) were obtained

using melt extrusion. Film samples were prepared by compression-moulding and

environmental degradation assessed by a burial test in soil for up to 12 months.

The extent and mechanism of degradation of lignin/PHB blends were

investigated by gravimetric analysis, thermogravimetric analysis (TGA),

differential scanning calorimetry (DSC), X-ray Photoelectron Spectroscopy

(XPS), scanning electron microscopy (SEM) and Fourier transform infrared

spectroscopy (FT-IR) over the entire range of compositions. Based on

gravimetric analysis, PHB films disintegrated and lost 45 wt% of initial mass in

soil within 12 months. In contrast, inhibition of PHB degradation by even low

concentrations of lignin was observed. It also showed lignin slowed down the

rate of degradation of blends. TGA showed the degradation profile of PHB and

miscible ratios of lignin and PHB (in lignin content of less than 40 wt%) do not

change with time. However, due to immiscible ratio profiles, the rate of

degradation is more rapid at longer buried time. According to the DSC results,

lignin increased the crystallinity of PHB in miscible portions and decreased the

crystallinity of PHB in the immiscible region. Also, DSC results exhibited

hydrogen bonding of lignin with PHB plays a significant role to protect PHB

against degradation. XPS data revealed an accumulation of biofilms on surface

of buried film samples. FT-IR displays disintegration in PHB, along with an

increase in lignin intensity in blends. SEM monitors surface roughness of buried

samples.

5.1. I ntroduct ion Producing biodegradable plastics and materials has been suggested in response

to increased awareness of the environmental hazards with disused plastics

(Sticklen, 2008). One such material is poly(hydroxybutyrate) (PHB) which is

totally biodegradable and can be produced by fermentation of sugars and other

chemicals or in plants (Sticklen, 2008). PHB has attracted much commercial

interest as a plastic material because its physical properties are remarkably

similar to those of polypropylene (PP), even though the two polymers have quite

different chemical structures. PHB exhibits a high degree of crystallinity, has a

high melting point of approximately 173°C, and most importantly, unlike PP,

109

PHB is rapidly biodegradable (Grassie, et al., 1984). PHB consumption, at

present, is mostly restricted in medical application because of its high cost

compared to synthetic plastics (Grassie, et al., 1984). Two major factors which

currently inhibit the widespread use of PHB are its high cost and poor

mechanical properties. The production costs of PHB are significantly higher

than for plastics produced from petrochemicals, and its stiff and brittle nature

makes processing difficult and impedes its ability to handle high impact. Several

attempts have been made to improve the physical properties of PHB by blending

it with other biodegradable polymers found to be miscible with PHB, such as

poly(ξ-caprolactone) (Antunes and Felisberti, 2005), poly(vinylidene fluoride)

(Chiu, et al., 2001), poly(vinyl alcohol) (Yoshie, et al., 1995), poly(lactic acid)

(Zhang, et al., 1996), poly(vinyl acetate) (An, et al., 1997), poly(vinyl phenol)

(Xing, et al., 1997), poly(DL-lactide)-co-poly(ethylene glycol) (Zhang, et al.,

1997), and cellulose esters (Pizzoli, et al., 1994).

Biodegradation of polymer blends is determined by both degradability of blend

components themselves and the blend composition. Woolnough et al. (2010)

studied the biodegradation of PHB and some other “green plastics” in mature

soil by detecting mass loss, topographical changes and biofilm attachment and

found that PHB itself has a better degradability than polyhydroxyoctanoate,

poly-DL-lactide and ethyl cellulose. In a composite, the component with the

lower degradation rate, might inhibit the degradation of the composite. Kumagai

and Doi (1992) found the degradation of PHB in the blend is restricted by

Polyvinyl acetate (PVAc). PVAc which is non-degradable remains on the surface

of buried PHB/PVAc blends and protects the blend against biodegradation. Also,

Wu (2006) found the mechanical properties of PHB/wood flours became

significantly worse compared with PHB, due to poor compatibility between

those two.

Understanding the biodegradation properties of blends is essential to establish

the biodegradability of blends and tailor the composition of blends to increase

sustainability. Avella et al. (2000) showed that the reinforcement of PHB with

wheat straw does not affect its biodegradation in long term soil burial tests.

Ikejima et al. (1998) found that the degradation profiles of the PHB/Polyvinyl

alcohol (PVA) blend films depended on their blend composition. The blend films

110

with PHB-rich composition showed a higher degradation rate and higher final

degraded ratio than the pure PHB film. Lim et al. (2005) studied the effect of

acidity of soil on degradation of medium-chain-length polyhydroxyalkanoates

(PLA) and found that acidic soils accelerate the degradation of PLA. Scott

(2002) has referred to the fact that lignocellulose, due to its hydrophobicity and

chemical inertness, does not readily degrade either abiotically or biotically and

when it does biodegrade, the lignin tends to accumulate. Lignin biodegrades

slowly to an extent of 17-53 wt% in every 100 days. So, PHB is expected to be

the main target for biodegradation in this study.

The aim of this work is to determine the environmental degradation properties of

PHB/lignin blends and to determine the role lignin plays in either accelerating or

retarding PHB degradation. The structure and environmental properties of the

blends were examined by gravimetric, thermogravimetric analysis (TGA),

differential scanning calorimetry (DSC), scanning electron microscopy (SEM),

X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared

spectroscopy (FT-IR) over a wide range of compositions.


5.2.1. PHB





5.2.2. Lignin

Lignin was extracted from bagasse obtained from the Mackay Sugar Mill,

Queensland, Australia. It was wet depithed in particles less than 4.2 mm and

washed and then air dried. Lignin was extracted from bagasse by the soda

process using 0.7 M sodium hydroxide solution. The procedure for lignin

extraction and purification has been described elsewhere (Mousavioun, et al.,

2010).

111

5.2.3. Lignin characterisation

Lignin composition was determined by the methods described in the paper by

Mousavioun and co-workers (2010). The results of lignin composition are

presented in Table 5-1.

Tab le 5 - 1 Molecu la r we ig h t o f l i gn in and l ign in co mponent s (wt %) (Mousa v ioun e t a l . , 2010)


2.0 0.2 1.6 <0.1 96.1

Mn (g mol-1)

Mw

(g mol-1) Methoxy** Phenolic

OH** RCOOH** Total OH**

2160 2410 10.9 5.1 13.6 14.5

*Error in analysis (% ±2) ** Error in analysis (% ±5)


Lignin and PHB were dried at 100°C and 40°C respectively for 12 h and stored

in desiccators under vacuum prior to use. PHB/lignin blends with lignin contents

from 10 wt% to 90 wt% were mixed in a Haake mini lab twin screw using the

procedure reported by Ghaffar(2002). To minimise PHB degradation, the

temperature of the extruder was maintained at 175°C for 2 min. The polymer


stored in a desiccator to avoid moisture absorption. Similar processing

conditions were carried out for pure PHB. The previous work on PHB/lignin

blends showed that lignin, up to 30 wt% content, is miscible in a PHB matrix

(Mousavioun, et al., 2010).

5.2.5. Polymer film fabrication

Polymer films were prepared using a hot press instrument under 7.5 bar pressure

and 175°C. To produce film samples with the same thickness a rectangular

mould with the thickness of 100µm was used. Film samples were prepared by

compression moulding between two Teflon sheets. All film samples moulded in

rectangular shape with the dimension of 45mm x 30mm and thickness of 100µm.

112

Films were subsequently removed and further dried under vacuum (48h, 25°C)

before standing for a further 24h (25°C, relative humidity 30%) until their

weights had atmospherically equilibrated (0.18-0.52g). The films were then

aged for three weeks to enable their crystallinity to reach an equilibrium value

and then films were carefully inserted in slide frames.

5.2.6. In situ biodegradation of polymer films in soil

In situ environmental degradation was conducted on site in garden soil (Pinjarra

Hills Field Station, University of Queensland, Australia). The procedure

followed was similar to that reported by Woolnough. (2010) All soil was sieved

to particles of less than 2mm in diameter and mixed completely before burial of

polymer samples. The samples were buried as per ASTM D 5988 in three

1.0x0.7 m2 plots (samples buried for 4, 8 and 12 months individually). The soil

of each plot had a pH of 6.7 as measured according to ASTM D 4972, a

temperature of 12-27°C and water content that varied with rain patterns at ~20%.

Polymer films were buried at least 2 cm apart and 20 cm below the soil surface.

The special arrangement of the burial ensured that the total polymer weight did

not exceed 7.7 wt% of soil (ASTM D 6003). Every 2 months soil from one of

the three plots was removed and the temperature, pH and water content were

measured. The experiment continued for 52 weeks. Soil was removed from the

polymer film by immersing in a solution containing 0.25 wt% sodium

hypochlorate, prior to drying under vacuum (84 hr, 25°C) and then weighed. The

soil removal protocol and preparation of films again followed the method

suggested by Woolnough (2010).

5.2.7. Thermogravimetric analysis (TGA)

The thermal decomposition studies were carried out in a TA Instruments Q500

thermogravimetric analyser. Approximately 10 mg of sample was weighed into

an aluminium pan and analysed by TGA by non-isothermal and isothermal

methods.

In the non-isothermal mode, heating was at a rate of 10°C min-1 and was

performed from ambient to approximately 800°C. The test was performed in an

113

atmosphere of nitrogen, which was injected at a flow rate of 15 mL min-1. A

curve of weight loss against temperature was constructed from the data obtained

by the instrument.

The isothermal runs were performed at 170°C, 180°C and 190°C. To reach the

desired temperature, samples were pre-heated from ambient to the set

temperature at a rate of 10°C min-1, and then degraded isothermally for at least

50 min. The test was performed in an atmosphere of nitrogen, which was

injected at a flow rate of 15mL min-1. A curve of weight loss against

temperature was constructed from the data obtained by the instrument. The

conversion rate curve was produced to indicate the mass loss conversion (wt%)

during the time.

5.2.8. Differential scanning calorimetry (DSC)

Approximately 5 mg of sample was precisely weighed and then sealed in an

aluminium pan. The pan was then placed in a DSC-Q2000 instrument and

heated from 0°C to 175°C at a heating rate of 10°C min-1 (cycle 1). The test was

performed in an atmosphere of nitrogen, which was injected at a flow rate of 15

mL min-1. Samples were then cooled at a rate of 30°C min-1, to -10°C (cycle 2).

Samples were then reheated to 180°C at a rate of 10°C min-1 (cycle 3). The plot

obtained from this second heating run shows the Tg as a step transition.

In addition to Tg, other thermal parameters of PHB/lignin blends were evaluated.

Melting temperature (Tm), melting enthalpy (∆Hm), PHB crystallinity (X)!), and

melt cold crystallinity temperature (Tmc) were extracted from DSC

thermographs. The crystallinity of PHB (not that of the blends) was calculated

using the following equation:

X)! 2 ∆&�∆&��

��

(5-1)

where ∆H�� is melting enthalpy and X!%&' and x%&' are crystallinity and mass

ratio of PHB used in this study. X!%&' is a ratio of ∆Hm of the sample PHB and

that of 100% crystalline PHB (∆H0). ∆H0 of PHB is assumed to be 146 J g-1

(Barham, et al., 1984). On this basis, the values of ∆H�� and X!%&' used in this

study are 92 J g-1 and 63 % respectively.

114

To evaluate the interaction between PHB and lignin, a well-known equation

suggested by Kwei is employed.

Kwei equation: 9t7qi_@r8 27sLOp,L��sMOp,M8

7sL��sM83 yu�uR (5-2)


components and their corresponding weight fractions, respectively. K� and q are

adjustable parameters. A relatively higher q indicates a stronger hydrogen

bonding between those two components (ElMiloudi, et al., 2009).

5.2.9. X-ray photoelectron spectroscopy analysis

XPS data were acquired using a Kratos Axis ULTRA X-ray photoelectron

spectrometer, incorporating a 165 mm hemispherical electron energy analyser.

The incident radiation was monochromatic Al Kα X-rays (1486.6 eV) at 150 W

(15 kV, 10 mA) and at 45 degrees to the sample surface. Photoelectron data

were collected at take off angle of 90°. Survey (wide) scans were taken at an

analyser pass energy of 160 eV and multiplex (narrow) high resolution scans,

which focus on a particular atom, at 20 eV. Survey scans were carried out over

1200 eV - 0 eV BE range with 1.0 eV steps and a dwell time of 100 ms. Narrow

high-resolution scans were run with 0.05 eV steps and 250 ms dwell time. Base

pressure in the analysis chamber was 1.0 x 10-9 torr and during sample analysis

1.0 x 10-8 torr. Atomic concentrations were calculated using the Kratos Vision 2

software and a linear baseline.

5.2.10. Scanning electron microscopy (SEM)

The morphology of the lignin/PHB blends was examined using a scanning

electron microscope, FEI Quanta 200 Environmental SEM, at an accelerating

voltage of 15 kV. For this examination the pellets were compression moulded

between two sheets of Teflon using an established procedure (Ghosh, et al.,

1999). To obtain better images of film topography, micrographs were taken at a

tilt of 35˚.

115

5.2.11. Fourier transform-infrared spectroscopy (FT-IR)

IR spectra were collected using a Nicolet 870 Nexus Fourier transform infrared

(FT-IR) spectrometer equipped with a Smart Endurance single bounce diamond

ATR accessory (Nicolet Instrument Corp., Madison, WI). Spectra were


(Galactic Corp., Salem, NH). Spectra were collected in the spectral range 4000

to 525 cm-1, using 64 scans at 4 cm-1 resolution with a mirror velocity of 0.6329

cm s-1. The measurement time for each spectrum was around 60 s.

5.3. Resul ts and D iscuss ion

5.3.1. Gravimetric analysis

To investigate how the mass of buried samples changed with time, there are two

approaches which are shown below.

Figure 5-1 shows how a wide range of polymer films which were blended from

PHB and 10 wt% to 90 wt% lignin compare with PHB with respect to mass

reduction in incremental periods of 4 months burial during a year.

Figure 5-1 clearly shows a significant difference between the degradation rates

of pure PHB compared to those blends with lignin. The obvious effect of even

small concentrations of lignin to retard degradation of PHB is shown in a

different plot of the gravimetric data in Figure 5-2. Figure 5-2 depicts the trends

of actual mass loss of different lignin/PHB blends, after 4 months, compared

with the expected mass loss of those blends in which PHB is assumed to act as

the only degradable component in blend (ie. a rule of mixtures plot). Figure 5-2

shows that once lignin is added to form a PHB blend the degradation is inhibited.

This could arise either by a biochemical protection effect of lignin against attack

by bio organisms on PHB (Dizhbite et al., 2004) or surface segregation of lignin

from the blend inhibiting biofilm formation and access to the PHB (Cronin,

2008) . The possible surface segregation of lignin has been probed by XPS, as

discussed later.

116

Figure 5-1 Buried mass loss of pure and blended PHB with lignin

(Error in analysis % ±5)

Figure 5-2 Actual and expected mass ratio of lignin/PHB blends after 4

months soil burial. (Error in analysis % ±5)

0.5

0.6

0.7

0.8

0.9

1

0 4 8 12

Mas

s ra

tio

Months

Pure PHB 10% Lignin 40% Lignin60% Lignin 90% Lignin

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50 60 70 80 90

Mas

s ra

tio

Lignin %

Actual mass ratio Expected mass ratio

117

5.3.2. Thermogravimetric analysis

The mass ratio of pure PHB, pure lignin and a blend of PHB with 60 wt% lignin

(after burial) during thermal degradation up to 500°C are given in Figure 5-3.

Figure 5-3 Mass ratio on thermal degradation of PHB, lignin, 30 and 60 wt% lignin blends after 4, 8 and 12 months of burial test

PHB is prone to thermal degradation and decomposes by a three-step

mechanism. Firstly, in the temperature range of 170°C to 200°C, volatile

monomeric, dimeric, trimeric and tetrameric species are formed. This

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

200 250 300 350 400 450 500

Mas

s ra

tio

Temperature (°C)

Pure PHB-4m Pure PHB-8m Pure PHB-12m

30% lignin-4m 30% lignin-8m 30% lignin-12m

60% lignin-4m 60% lignin-8m 60% lignin-12m

lignin

lignin

60% lignin

12m 8m 4m

30% lignin4,8,12m

PHB4,8,12m

118

mechanism of random chain scission (Figure 5-4) has been demonstrated by

mass spectroscopy/FT-IR (Grassie, et al., 1984, Hablot, et al., 2008).

Figure 5-4 PHB random chain scission at temperatures of 170ºC-200ºC

When the temperature is increased from 200ºC to 300ºC, the second step of the

degradation occurs. Within this temperature range, PHB oligomers are broken

down to a free monomeric unit of crotonic acid, pictured in Figure 5-5.

Figure 5-5 PHB chain scission at temperature of 200ºC- 300ºC

Based on the third step of the PHB degradation mechanism, when the

temperature reaches 500ºC, the only species observed are carbon dioxide and

propane. PHB appears to have two main overall degradation steps, while lignin/

PHB degradation occurs in several more stages (Mousavioun et al., 2010) as is

shown in Figure 5-3. Figure 5-3 generally shows that addition of lignin increases

the degradation temperature of PHB. Figure 5-3 shows that for PHB and

miscible blends, the thermal degradation profile was independent of burial time.

However, for the immiscible blends (e.g., 60 wt% lignin), longer burial times

increased the rate of degradation.

119

5.3.3. Differential scanning calorimetry (DSC)

Changes in transition temperatures by DSC analysis, give insight into chemo-

physical and structural changes due to biodegradation of lignin/PHB blends.

Thus, in addition to the use of Tg, other thermal parameters of PHB/lignin blends

like melting temperature (Tm), apparent melting enthalpy (∆Hm) and melt cold

crystallinity temperature (Tmc) of blends with different ratios, , permit us to

obtain a relative crystallinity scale (X!) which is useful to compare the samples.

DSC data of the lignin/PHB blends, compared with pure PHB after burial of 4, 8

and 12 months, are summarised in Table 5-2. This table clearly shows that the

degree of crystallinity of PHB is changed by lignin content. In lower lignin

contents (miscible ratios of lignin/PHB blends) addition of lignin, increases the

crystallinity of PHB. To the contrary, higher lignin contents (immiscible ratios

of lignin/PHB blends) has no affect on crystallinity of PHB (Weihua et al. (2004)

obtained similar result at lower lignin content). Accordingly, the chain scissions

that occur in PHB blocks are probably responsible for the decrease in Tm. The

reason for decrease in ∆Hm of PHB phase may be due to irregularity of the chain

formed, due to branching and cross-linking. In general, when the lignin content

is less than 50 wt%, it enhances the crystallisation of PHB. However, when the

lignin is more than 50 wt%, it retards the crystallisation of PHB.

120

Tab le 5 - 2 The rma l p rop e rt i es o f v i rg in l i gn in / PHB b lends ca st f i lms a t d i f fe ren t ra t i os a nd b iodeg raded

a f te r 4 , 8 and 12 month s

Sample Tg (°C)* Tm(°C) ∆Hm (J g-1) })>7%8 * * Tmc(°C)

PHB 3 172 92 63 89

Virg

in li

gnin

/PH

B b

lend

10/90 7 174 90 68.8 132

20/80 9 173 85 72.5 121

40/60 15 170 78 88 98

50/50 18 (130) 167 34 46 82

60/40 17 (148) 165 31 52.5 86

70/30 21 (134) 157 9 N.A. N.A.

80/20 26 (125) 152 5 N.A. N.A.

90/10 43 (131) 158 3 N.A. N.A.

Lig

nin/

PH

B b

lend

af

ter

4 m

onth

s

10/90 10 169.3 75.7 57.7 115

20/80 12 161 (169) 68.9 48.7 96

30/70 18 164 53.9 52.8 103 40/60 10 163.2 47 53.3 88.5 50/50 N.A. N.A. N.A. N.A. N.A. 60/40 15 (131) 154 19.7 32.5 82 70/30 21 (131) 151 3.8 N.A. N.A. 80/20 24 (131) 151 2.5 N.A. N.A. 90/10 60 (135) N.A. N.A. N.A. N.A.

Lig

nin/

PH

B b

lend

af

ter

8 m

onth

s

10/90 11 168.9 81.7 62.2 120.7 20/80 13 163 53.4 46.2 109 30/70 16 159 (168) 58.7 57.1 104 40/60 16 165 54.3 61.6 91 50/50 N.A. N.A. N.A. N.A. N.A. 60/40 17 (139) 161.6 35.17 60 87 70/30 22 (131) 151 2.87 N.A. N.A. 80/20 25 (135) N.A. N.A. N.A. N.A. 90/10 60 (131) N.A. N.A. N.A. N.A.

Lig

nin/

PH

B b

lend

af

ter

12 m

onth

s

10/90 13 169 84 64.4 119 20/80 14 160 (167) 69 58.7 108 30/70 18 163 (169) 65 63 107 40/60 11 163 46 53 96 50/50 N.A. N.A. N.A. N.A. N.A. 60/40 15 (139) 159 32 55 85 70/30 N.A. N.A. N.A. N.A. N.A. 80/20 25 (146) N.A. N.A. N.A. N.A. 90/10 60 (132) N.A. N.A. N.A. N.A.

* Tg without parentheses is pure PHB and the one with parentheses is lignin

** X’ c is for PHB crystallinity as opposed to the bulk crystallinity of PHB (see Table 4-4)

121

Using the outcomes of Figure 5-1, together with the assumption that almost all of

degradation is due to PHB, the q value which satisfies the evaluated Tgs of 4

months buried blends is equal to 52, where Mousavioun and co-workers (2010)

found that q was equal to 22 for the same blend before the burial test. That

significant increase in q could represents a significant increase in hydrogen

bonding of lignin/PHB blend after 4 months burial. This hypothesis is proposed

that the less strongly bonded PHB is susceptible to degradation and the hydrogen

bonding of lignin with PHB plays a significant role to protect PHB against

degradation.

5.3.4. XPS analysis

Survey analysis of pure PHB and lignin are shown in Figure 5-6. Unexpectedly,

the PHB was found to contain a significant nitrogen signal suggesting

contamination. The nitrogen was found to be present at an atomic concentration

of approximately 3.5 %. The signal observed at a BE of 399.97 eV may indicate

that the nitrogen is present in the form of an amide or a polypeptide fragment of

a protein. As noted in the experimental section, the PHB was of bacterial origin

and the nitrogen contamination is most likely a remnant of the fermentation and

extraction process used to produce the PHB (Hablot, et al., 2008) .

The multiplex carbon 1s scan of pure PHB portrayed in Figure 5-6 (c) is

comparable with the standard PHB which is reported by Beamson and Briggs

(1992) except for the effect of the peptide which results in a broadening of

the C-O band in the spectrum due to an underlying C-N band at 286eV in Figure

6 (c).

122

(a) (b)

(c) (d)

Figure 5-6 Survey XPS of (a) PHB and (b) lignin and multiplex scans of carbon bonds of (c) PHB and (d) lignin

Multiplex XPS scans of carbons for 20 wt% lignin content blends are shown in

Figure 5-7. This figure clearly depicts the differences between zero time and 4

months buried films. There is evidence of surface contamination with the

presence of Si 2p signal. This is indicative of a silica/clay mineral. Total

removal of clay and other surface contamination from buried samples is difficult,

despite the cleaning protocol adopted in these trials. The COO content drops

123

from 12.46 wt% to 3.05 wt% after 4 months burial time. This is suggestive of

PHB degradation. Also, the atomic concentration of oxygen involved in O―C

linkages has reduced from 11 to 6 % further supporting PHB degradation. The

CO content increased from 14.7 to 21 wt%, most likely indicating an increase in

lignin content after 4 months burial time. On the basis of these data, the

hypothesis that lignin in the blend completely covered the surface of the film is

not valid.

As the nitrogen content increased in the sample buried for 4 months (Figures 5-6

and 5-7), it shows biofilm attachement resulting from fermentation.

(a) (b)

Figure 5-7 Multiplex carbon scan of 20 wt% lignin films at (a) zero time and (b) 4 months buried.

5.3.5. FT-IR analysis

Figure 5-8 shows the IR spectrum of PHB, lignin and a 4 months aged 10 wt%

lignin/PHB blend. IR spectra of lignin show a strong hydrogen bonded O―H

stretching absorption around 3400 cm-1 and a prominent C―H stretching

absorption around 2900 cm-1 (Pandey, 1999). PHB spectrum exhibits main two

124

peaks, these are at 1733 cm-1 and 1722 cm-1, though the first peak at 1733 cm-1 is

more of a shoulder to the main peak at 1722 cm-1. The peak at 1733 cm-1 is

associated with the amorphous component of PHB, the peak at ~1722 cm-1 is

associated with the crystalline component of PHB (in its preferred

conformation), and the peak at 1655 cm-1 is associated to ─C═C─ stretching

vibration (Li, et al., 2003). The band at 1685 cm-1 has been reported to be a

crystalline band, although its origin is not known (Guo, et al., 2010). All the

buried blends contain some extra peaks compared with PHB and lignin. These

peaks generally are seen from 3697 cm-1 to 3620 cm-1 which have been reported

as indicative of kaolin (Farmer, 1974) which showed there was some soil

remaining on the films.

Figure 5-8 FT-IR spectra of (a) PHB, (b) lignin and (c) 4 months buried, 10 wt% lignin/PHB blend

125

(a)

(b)

Figure 5-9 FT-IR spectra of the carbonyl stretching region of (a) Pure PHB

and (b) 10 wt% lignin/PHB blend

To examine chemical changes in pure PHB and lignin/PHB blends with different

ratios, the IR spectra of the buried cast films were recorded at various times.

126

Figure 5-9 shows the FT-IR spectra from 1850 cm-1 to 1400 cm-1 of the carbonyl

stretching region of PHB and lignin/PHB blends.

Based on Figure 5-9 for all blend ratios of lignin/PHB, decreases in intensity of

amorphous PHB after 4 and 12 months shows that most of PHB lost was from

the amorphous phase. This result confirms the thermal analysis of buried films

which was explained in section 5.3.3 and XPS results in section 5.3.4.

The band at 1655 cm-1 is associated with ─C═C─ group (Figure 5-9). The

intensity of this peak increased at 4 months and remains at that intensity at 12

months for films containing only PHB. However, the peak intensity increased

with burial time for the blend containing 10 wt% lignin. The increase in the

amount of ─C═C─ group, and hence peak intensity, is associated with PHB

degradation.

Figure 5-9 shows the intensity of the peak at 1685 cm-1 (a crystalline band of

PHB) decreases with burial time. This is an indication of decrystallisation of

PHB with burial time.

5.3.6. Macroscopic and microscopic changes

SEM of the blended films of lignin/PHB films and PHB together with the images

of macro films are shown in Table 5-3. Those micrographs are taken with

magnification of 2000. These images which represent the topography of films

show the roughness of films generally increased during the burial test. Also,

Table 3 shows the roughness of PHB film has increased significantly after 12

months and nearly 30% of its surface degraded completely while, the roughness

of those films which has lignin on it, have not changed. Also, microscopic

images show the films with higher lignin content remained smoother after 12

months burial. So, the lignin plays a significant role to resist against degradation

of films with burial time which confirms the gravimetric and FT-IR results

defined in sections 5.3.1 and 5.3.4.

127

Tab le 5 - 3 Mac ro and M ic ro f i lms o f PHB and l i gn in / PHB b lend s (sca le ba r= 1 5 mm fo r Mac ro and 40

µ m fo r M ic ro f i lms)

Macro film Microfilm Virgin 12 months Virgin 12 months

Pure PHB

30/70 Lignin/PHB

90/10 Lignin/PHB

5.4. Conclus ion PHB is the main component of lignin/PHB blend which is susceptible to

biodegradation. Experimental investigations and analysis of lignin/PHB blends,

clearly depict that the lignin makes a considerable effect on biodegradation

properties of PHB. Lignin inhibited the biodegradation of PHB significantly. In

immiscible ratios of lignin/PHB blends the inhibition on degradation was

observed. This might happen because lignin inhibited PHB random chain

scission rate or it protects PHB against biodegradation attack.

Acknowledgements The authors thank Dr Barry Wood (The University of Queensland) and Dr Llew

Rintoul (Queensland University of Technology) for their cooperation in the XPS

and FT-IR aspects, respectively, of this work.

128

5.5. References An, Y., Dong, L., Xing, P., Zhuang, Y., Mo, Z., Feng, Z., 1997. Crystallization

kinetics and morphology of poly(β-hydroxybutyrate) and poly(vinyl acetate) blends. Eur. Polym. J. 33, 1449-1452.





Cronin, D.J., 2008, Formation of multi-component films using anionic liquid, Honours Chemistry Thesis In, Queensland University of Technology, Brisbane.

Dizhbite, T., Telysheva, G., Jurkjane, V., Viesturs, U., 2004. Characterization of the radical scavenging activity of lignins--natural antioxidants. Bioresource Technol. 95, 309-317.

ElMiloudi, K., Djadoun, S., Sbirrazzuoli, N., Geribaldi, S., 2009. Miscibility and phase behaviour of binary and ternary homoblends of poly(styrene-co-acrylic acid), poly(styrene-co-N,N-dimethylacrylamide) and poly(styrene-co-4-vinylpyridine). Thermochim. Acta 483, 49-54.

Farmer, V.C., 1974. The infrared spectra of minerals, Mineralogical society, London.


Ghosh, I., Jain, R.K., Glasser, W.G., 1999. Multiphase materials with lignin. XV. Blends of cellulose acetate butyrate with lignin esters. J. Appl. Polym. Sci. 74, 448-457.

Grassie, N., Murray, E.J., Holmes, P.A., 1984. The thermal degradation of poly(β-hydroxybutyric acid): Part 2--Changes in molecular weight. Polym. Degrad. Stab. 6, 95-102.

Grassie, N., Murray, E.J., Holmes, P.A., 1984. The thermal degradation of poly(β-hydroxybutyric acid): Part 3--The reaction mechanism. Polym. Degrad. Stab. 6, 127-134.

Guo, L., Sato, H., Hashimoto, T., Ozaki, Y., 2010. FT-IR study on hydrogen-bonding interactions in biodegradable polymer blends of poly(3-hydroxybutyrate) and pol(4-vinylphenol). Macromolecules 43, 3897-3907.


129

Ikejima, T., Cao, A., Yoshie, N., Inoue, Y., 1998. Surface composition and biodegradability of poly(3-hydroxybutyric acid)/poly(vinyl alcohol) blend films. Polym. Degrad. Stab. 62, 463-469.

Kumagai, Y., Doi, Y., 1992. Enzymatic degradation and morphologies of binary blends of microbial poly(3-hydroxy butyrate) with poly(ε-caprolactone), poly(1,4-butylene adipate and poly(vinyl acetate). Polym. Degrad. Stab. 36, 241-248.

Li, S.D., He, J.D., Yu, P.H., Cheung, M.K., 2003. Thermal degradation of poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) as studied by TG, TG–FTIR, and Py–GC/MS. J. Appl. Polym. Sci. 89, 1530-1536.

Lim, S.P., Gan, S.N., Tan, I., 2005. Degradation of medium-chain-length polyhydroxyalkanoates in tropical forest and mangrove soils. Appl. Biochem. Biotech. 126, 23-33.


Pandey, K.K., 1999. A study of chemical structure of soft and hardwood and wood polymers by FTIR spectroscopy. J. Appl. Polym. Sci. 71, 1969–1975.


Scott, G., 2002, Degradable polymers, principles and application, In, Kluwer academic publisher, Dordrecht.




Wu, C.S., 2006. Assessing biodegradability and mechanical, thermal, and morphological properties of an acrylic acid-modified poly(3-hydroxybutyric acid)/wood flours biocomposite. Journal of Applied Polymer Science 102, 3565-3574.




130

Zhang, L., Xiong, C., Deng, X., 1996. Miscibility, crystallization and morphology of poly(β-hydroxybutyrate)/poly(d,l-lactide) blends. Polymer 37, 235-241.

131

CHAPTER 6

Thermal stabil i ty and miscibil i ty of

poly(hydroxybutyrate) and methanol-

soluble soda l ignin blends

Payam Mousaviouna, William O.S. Dohertya, Graeme A. Georgeb and Peter

Halleyc a Sugar Research and Innovation, Centre for Tropical Crops and Biocommodities, Queensland University of Technology, GPO Box 2434, Brisbane, Australia. b School of Science and Technology, Queensland University of Technology, GPO Box 2434, Brisbane, Australia. c Centre High Performance Polymers (CHPP), AIBN, St Lucia, The University of Queensland, QLD 4072, Brisbane, Australia

Published in 10th AIChE meeting, Salt Lake City, UT, USA, November 2010

Research Students Centre, Level 4, 88 Musk Avenue, Kelvin Grove Campus, GPO Box 2434. Brisbane QLD 4001 Ph: +61 7 3138 4475 or 3138 5306 e-mail [email protected]

http://www.rsc.qut.edu.au/studentsstaff/ Correct as at: 7-6-10


Thesis by Published Paper The authors listed below have certified* that: 1. they meet the criteria for authorship in that they have participated in the conception, execution, or

interpretation, of at least that part of the publication in their field of expertise; 2. they take public responsibility for their part of the publication, except for the responsible author

who accepts overall responsibility for the publication; 3. there are no other authors of the publication according to these criteria; 4. potential conflicts of interest have been disclosed to (a) granting bodies, (b) the editor or

publisher of journals or other publications, and (c) the head of the responsible academic unit, and 5. they agree to the use of the publication in the student’s thesis and its publication on the

Australasian Digital Thesis database consistent with any limitations set by publisher requirements.

In the case of this chapter: Publication title and date of publication or status: Environmental degradation of soda lignin/PHB blends”, Polymer Degradation and Stability, 2011.


Mousavioun Wrote the manuscript, Experimental design, Conducted experiments, Data analysis.

Signature

Date

Graeme A. George

Aided experimental design, Data analysis.

William O.S. Doherty

Data analysis.



132

Abstract- Poly(3-hydroxybutyrate), PHB is a biodegradable and biocompatible

polymer generally produced by algae and bacteria. The main disadvantages of

PHB include: (a) prohibitive cost, (b) poor processability, and (c) thermal

instability during processing. Lignin (obtained from sugarcane fibre) has been

blended with PHB to ascertain improvements in PHB properties. The properties

of the blends were investigated by differential scanning calorimetry (DSC),

thermogravimetry analysis (TGA), scanning electron microscopy (SEM) and

Fourier transform infrared spectroscopy (FT-IR) over the entire range of

composition. The addition of methanol-soluble lignin increases the thermal

stability of PHB over a wide temperature range. A single glass transition

temperature (Tg), which depicts miscibility, was obtained for blends containing

up to 40 wt% lignin. At up to 30 wt% lignin, the experimental data fitted the

Gordon-Taylor and Kwei models. The Tg results correlate with the SEM and

FT-IR data. The FT-IR data show that the miscibility of the blends is probably

associated with specific hydrogen bonding interactions between the reactive

functional groups in lignin and the carbonyl groups of PHB.

6.1. I ntroduct ion Polyhydroxyalkanoates (PHAs) are a class of environmentally friendly natural

polymers (Verhoogt et al., 1994). Polyhydroxybutyrate (PHB) is a member of

this class of polymers. PHB is insoluble in many solvents and has good barrier

properties towards water, oxygen and carbon dioxide (Ghaffar, 2002). It is

readily broken down, with the aid of enzymes, to water and carbon dioxide.

These properties, combined with PHB’s potential for sustainable usage, makes it

a potential commodity material in the packaging industry.

The reasons why the potential of PHB has not been fully utilised, apart from its

prohibitive cost, are its stiff and brittle nature and its thermal instability during

processing. The crystal structure and crystallisation conditions are responsible

for these thermo-mechanical properties. PHB undergoes secondary nucleation at

an ambient temperature because of its low glass transition temperature (Tg) and it

possesses a low nucleation density resulting in the formation of large spherulites

(Barham and Keller, 1986). The spherulites contain crazes, and splitting occur

around the centre of these crazes, hence producing a significant structural weak

point (Mahendrasingam et al., 1995). PHB undergoes thermal degradation and

133

depolymerisation at temperatures close to its melting point and degradation is

further enhanced by high shear rates during melt processing and extrusion. One

approach to improve PHB’s properties is through blending (Antunes and

Felisberti, 2005, Chiu et al., 2001). Many polymer components in blends have

been found to be miscible or partially miscible on the basis of specific hydrogen-

bonding interactions due to the presence of functional groups (Kuo et al., 2002,

Kuo and Chang, 2001, Sixun et al., 2003, Yong et al., 2001, Yoshie et al., 1995,

Zheng and Mi, 2003). Lignin is an amorphous macromolecule composed of

phenylpropane repeat units that possesses aliphatic and aromatic hydroxyl

groups as well as carboxylic acid groups. These interacting functional groups, as

well as the amorphous nature of lignin, make it a good candidate for blending

with PHB. Limited studies have been carried out on PHB and lignin blends.

Ghosh et al. (2000) and Ghosh (1998) prepared blends (from the melt and

solution) of PHB, polyhydroxybutyrate-hydroxyvalerate (PHBV), cellulose

acetate butyrate with hardwood organosolv lignin and hardwood organosolv

lignin ester. The organosolv lignin and its butyrate derivative were found to

have a high degree of miscibility with PHB, and lignin was shown to inhibit and

retard PHB crystallisation. The source from which the lignin is obtained and its

method of extraction have a strong bearing on its properties (Lora and Glasser,

2002). Thus, in this work the thermal stability and miscibility between PHB and

methanol-soluble fraction of soda lignin was evaluated using TGA, DSC, SEM

and FT-IR.

6.2. Exper imental

6.2.1. PHB


weight, Mw, as determined by gel permeation chromatography is 440,000 g mol-1

while the number average molecular weight, Mn, is 260,000 g mol-1. The Tg of

the PHB is 4°C.

134


Bagasse was obtained from Mackay Sugar Mill in Queensland, Australia. It was

wet depithed (through a 4.2 mm screen) and then air dried. Lignin was extracted

from bagasse by the soda process using a 18.5 L Parr reactor (Model 4555, Par).

In this method, 1 kg of bagasse is reacted with about 10.5 L of 0.7 M - 1 M

NaOH. Once the reactor reached the operating temperature of 170°C, this

temperature was maintained for 1.5 h. After cooling, the liquid (black liquor)

was removed from the bottom of the reactor and sieved to remove fibrous

material. The procedure used to recover and purify the lignin has been described

elsewhere (Mousavioun and Doherty, 2010).

Lignin is a complex and heterogeneous mixture with a broad molecular weight

distribution. To fractionate the lignin, ~ 100 g and 250 mL diethyl ether (AR

grade, Merck) are added to a 1 L beaker. The beaker is covered and the mixture

is stirred for 20 min before being left to settle for 10 min. The diethyl ether is

then decanted into another container. The remaining solid is then subjected to

the same treatment. This is repeated until the supernatant diethyl ether is a light

yellow colour when decanted. The lignin residue is allowed to dry before this

process is repeated using AR grade methanol (supplied by Merck) in place of

ether. The methanol soluble lignin (ML) was recovered using the rotary

evaporator to evaporate the solvent followed by filtration to recover the solid

lignin. The solid was then dried to a constant weight at 100°C for 2 h. The

amount of lignin recovered by this process was 64 wt% of the original starting

lignin material.

Details of the analytical procedures used for lignin characterisation (i.e., ash

analysis, sugar analysis, functional group analysis, molecular weight

determination) have been described by Mousavioun and Doherty (2010).

Table 6-1 gives the composition of ML. The purity of the lignin (98.4 wt%) is

comparable to organosolv lignin (96 wt%) (Mousavioun and Doherty, 2010).

Xylan is the highest proportion of the sugars associated with lignin. The number

average molecular weight (Mn) of the lignin is 2380, while the weight average

molecular weight (Mw) is 2670. The polydispersity (i.e. ratio of Mw to Mn) of

the lignin is 1.1 indicating that the lignin polymer consists of molecules of

similar chain length.

135

Tab le 6 - 1 Cha ra cte r i sa t ion o f ML

Type of analysis wt%

Ash 1.0

Glucan 0.1

Xylan 0.5

Arabinan <0.1

Methoxy 11.7

Phenolic OH 1.5

Total OH 15.3

RCOOH 21.1

6.2.3. PHB/Lignin blends

ML/PHB blends with lignin contents from 10 wt% to 90 wt% were mixed in a

Haake mini lab twin screw mixer using the procedure reported by Ghaffar

(2002). To minimise PHB degradation, the temperature of the extruder was

maintained at 175°C for 2 min (Mousavioun and Doherty, 2010). The polymer


stored in a desiccator to avoid moisture absorption.

6.2.4. Characterisation of blends


Approximately 10 mg -15 mg of the sample was precisely weighed and then





30°C min-1 to -10°C (cycle 2). Samples were then reheated to 200°C at a rate of



136

6.2.4.2. Thermogravimetric analysis

Approximately 10 mg of sample was weighed into an aluminium pan and

analysed by thermogravimetric analysis (TGA). Heating was at a rate of 10°C

min-1 and was performed from room temperature to approximately 800°C. The

test was performed in an atmosphere of nitrogen, which was injected at a flow

rate of 15 mL min-1. A curve of weight loss against temperature was constructed

from the data obtained by the instrument. A derivative of this curve (DTG) was


occurred.

6.2.4.3. Scanning electron microscopy (SEM)

The morphology of the ML/PHB blends was examined using a scanning electron

microscope, type FEI Quanta 200 Environmental SEM at an accelerating voltage

of 15 kV. For this examination the pellets were compression moulded between

two sheets of Teflon using an established procedure (Ghosh et al., 1999).

6.2.4.4. Fourier transform-Infrared spectroscopy (FT-IR)

Infrared (IR) spectra were collected using a Nicolet 870 Nexus Fourier transform

infrared (FT-IR) spectrometer equipped with a Smart Endurance single bounce

diamond ATR accessory (Nicolet Instrument Corp., Madison, WI). Spectra were


(Galactic Corp., Salem, NH). The spectrometer incorporated a KBr beam splitter

and a deuterated triglycine sulfate room temperature detector. Spectra were

collected in the spectral range 4000 to 525 cm-1, using 64 scans at 4 cm-1

resolution with a mirror velocity of 0.6329 cms-1. The measurement time for

each spectrum was around 60 s.

6.3. Resul ts and D iscuss ion The thermal decomposition curve for ML is shown in Figure 6-1. The first

weight loss occurring at ~175°C is associated with water loss. The second

weight loss (i.e. the shoulder) with a peak maximum at 285°C is mainly

associated with hemicellulose (i.e. xylan) decomposition, while the peak at

335°C is associated with cellulose (i.e. glucan) and lignin decomposition

(Garcìa-Pèrez et al., 2001). At 420°C ~50 wt% of ML has decomposed. What is

worth noting in the result presented in Figure 6-1 is that lignin starts to degrade

at ~184°C and that its degradation is complex constituting of several processes.

137

PHB, on the other hand, appears to have two main overall degradation steps as

shown in Figure 6-2.

Figure 6-1 TGA/DTG curve of ML performed under nitrogen atmosphere.

Figure 6-2 TGA/DTG curve of PHB performed under nitrogen

atmosphere.

Figure 6-3 shows the integral thermogravimetric curves for ML/PHB blends with

those of ML and PHB included for comparison. The degradation of ML/PHB

blends occurs in several more stages than PHB, suggesting blending PHB with

ML has completely changed the decomposition profile of PHB. As shown in

Figure 6-3, inset, the decomposition temperature at which the material has

138

reached 5 wt% degradation, or (T0) of PHB increases up to 50 wt% lignin.

Thereafter, the T0 decreases with lignin addition. Figure 6-3 also shows that the

temperature at the maximum rate of weight loss of PHB decreases with the

addition of lignin. The degradation of pure PHB is almost complete by ~280°C,

whereas the weight loss for the blends is less than 85 wt%. The results from the

TGA study, therefore, show that overall the addition of ML increases the thermal

stability of PHB.

Figure 6-3 The integral thermogravimetric curves for PHB, ML and ML-PHB blends.

The most accepted parameter to assess polymer miscibility is the Tg. A single Tg

of a blend implies complete miscibility between the amorphous fractions of the

polymer components, whose value is an average of the individual Tg of the

polymer components. Two or more Tg’s suggest that the degree of miscibility is

restricted. Most miscible polymers display a single Tg whose value is dependent

on the proportion of the individual components (Fox, 1956). Figure 6-4

illustrates the Tgs of the lignin/PHB blends. A single Tg is obtained up to a ML

content of 40 wt%, thereafter there are two Tgs. The Tg results therefore give an

indication of miscibility between PHB and ML at lignin contents up to 40 wt%.

139

The exothermic peak at ~75°C (for 50 wt% and 70 wt%) is associated with the

cold crystallisation temperature of PHB (Ghaffar, 2002). The absence of this

peak in the other blends is not known and is worth further investigation.

Figure 6-4 DSC curves of ML/PHB blends (refer to Figure 4-5)

Figure 6-4 also shows that the Tg of the PHB component of the blends increases

with increase in ML content. Similar results were obtained by Ghosh and co-

workers (1999, 2000) for organosolv lignin/PHB blends, though the values

obtained in the present study were slightly higher (Figure 6-4). This could be

related to the method of preparation, differences in the PHB source, or the lignin

type as soda pulping generally increases the carboxylic acid and hydroxyl

contents of lignins relative to organosolv pulping (Gosselink et al., 2004).

We evaluated the Tg data using the well-known Fox (1956), Gordon-Taylor

(Schneider, 1988) and Kwei (Lin et al., 1989) equations, to obtain a better idea

of interactions between PHB and ML. These equations are:

Fox equation: �Op7qi_@r8 2 sL

Op,L3 sM

Op,M (6-1)

Gordon-Taylor equation: 9t7qi_@r8 27sLOp,L��csMOp,M8

7sL��csM8 (6-2)

Kwei equation: 9t7qi_@r8 27sLOp,L��sMOp,M8

7sL��sM83 yu�uR (6-3)


components and their corresponding weight fractions, respectively. K��, K� and

q are adjustable parameters.

140

As shown in Figure 6-5, ML/PHB blends up to 30 wt% lignin fit to the Gordon-

Taylor model with a KGT value of 3.34. These blends also fit the Kwei model

with Kw value of 0.1 and q having a value of 15. The positive value of q and a

reasonable KGT value indicate some interactions exist between OH groups of

lignin and the carbonyl groups of PHB for the blends containing up to 30 wt%

lignin (ElMiloudi et al., 2009). It should be pointed out that these interaction

parameters are lower than those obtained for soda lignin in a previous study

(Mousavioun et al., 2010). One point of difference between ML and the soda

liginin of previous study is that the soda lignin contained 4 times the amount of

phenolic OH groups present in ML.

Figure 6-5 Tgs of PHB and the blends versus ML content.

Figures 6-6, 6-8 and 6-8 illustrate typical SEM images of blends. For blends

containing 10 wt% and 30 wt% lignin, there was no apparent phase separation,

whereas for the blends with 50 wt% of lignin and higher, phase separation was

observed. Thus, the SEM data confirm the Tg data.

141

Figure 6-6 SEM image of ML/PHB blend containing 10 wt% ML.



Figure 6-9 shows the FT-IR spectra from 1800 cm-1 to 1620 cm-1 of the carbonyl

stretching region of PHB and ML/PHB blends. PHB spectrum exhibits main two

peaks, these are at 1733 cm-1 and 1722 cm-1, though the first peak at 1733 cm-1 is

more of a shoulder to the main peak at 1722 cm-1. The peak at 1733 cm-1 is

associated with the amorphous component of PHB, the peak at ~1722 cm-1 is

associated with the crystalline component of PHB (in its preferred

142

conformation), and the peak at 1697 cm-1 is associated with hydrogen-bonded

carbonyl (Guo et al., 2010). The band at 1685 cm-1 has been reported to be a

crystalline band, although its origin is not known (Guo et al., 2010).


ML/PHB blends.

Tab le 6 - 2 Pea k asso c ia ted w i th c ry sta l l i ne po r t ion o f PHB

Lignin content (wt%) Wavenumber at Max. transmission (cm-1) 0 1722 10 1718 30 1720 40 1720 50 1720 70 1722 80 1722 90 1722

The shift to a lower wavenumber is indicative of hydrogen bonding interactions

(Barsbay and Güner, 2007). As shown in Figure 6-9 (and Table 6-2), for blends

containing 10 wt%, 30 wt%, 40 wt% and 50 wt% ML, there is a small but

definitive shift (2 cm-1to 4 cm-1) to a lower wavenumber relative to the PHB

peak of 1722 cm-1. This implies that the reactive functional groups of ML are

engaged in hydrogen bonding interactions with the carbonyl oxygen in PHB as

has been reported in a previous study (Mousavioun et al., 2010). This explains

143

the compatibility obtained between PHB and lignin for the blends containing up

to 50 wt% lignin. The reason why there were no differences in some of the

wavenumber shifts between these blends containing different proportions of

lignin is not known.

Figure 6-9 also shows that for the PHB band at 1733 cm-1, there is probably a

slight shift to a lower wavenumber (~ 4 cm-1) for the 10 wt% and 30 wt% blends.

Although this band is of less intensity compared the main band at 1722 cm-1 it

shows some favourable interactions between the amorphous part of PHB and

lignin. The slight shift to a higher wavenumber for the other blends may also be

linked to some sort of association between ML and PHB.

6.4. Conclus ion The addition of lignin to PHB has been found to improve the overall thermal

stability of PHB. For blends containing up to 50 wt% lignin, the addition of

lignin raised the initial decomposition temperature i.e. T0 of PHB by a few

degrees. Glass transition temperature and microscopy studies indicated

miscibility with blends containing 10 wt% - 40 wt%. At up to 30 wt% lignin, the

experimental data fitted the Gordon-Taylor and Kwei models. The

intermolecular interactions between the two polymer components were found to

be due to hydrogen bonding formation between their functional groups.

144

6.5. References Antunes, M.C.M., Felisberti, M.I., 2005. Blends of poly(hydroxybutyrate) and

poly(ε-caprolactone) obtained from melting mixture. Polym. Sci. Technol. 15, 134-138.


Barsbay, M., Güner, A., 2007. Miscibility of dextran and poly(ethylene glycol) in solid state: Effect of the solvent choice. Carbohyd. Polym. 69, 214-223.




Garcìa-Pèrez, M., Chaala, A., Yang, J., Roy, C., 2001. Co-pyrolysis of sugarcane bagasse with petroleum residue. Part I: thermogravimetric analysis. Fuel 80, 1245-1258.


Ghosh, I., 1998, Blends of biodegradable thermoplastics with lignin esters, In, Virginia Polytechnic Institute and State University, VA, pp. 139.



Gosselink, R.J.A., Abächerli, A., Semke, H., Malherbe, R., Käuper, P., Nadif, A., van Dam, J.E.G., 2004. Analytical protocols for characterisation of sulphur-free lignin. Ind. Crops Prod. 19, 271-281.

Guo, L., Sato, H., Hashimoto, T., Ozaki, Y., 2010. FTIR study on hydrogen-bonding interactions in biodegradable polymer blends of poly(3-hydroxybutyrate) and pol(4-vinylphenol). Macromolecules 43, 3897-3907.

Kuo, S.W., Chan, S.C., Chang, F.C., 2002. Miscibility enhancement on the immiscible binary blend of poly(vinyl acetate) and poly(vinyl pyrrolidone) with bisphenol A. Polymer 43, 3653-3660.

Kuo, S.W., Chang, F.C., 2001. Effects of Copolymer Composition and Free Volume Change on the Miscibility of Poly(styrene-co-vinylphenol) with Poly(ε-caprolactone). Macromolecules 34, 7737-7743.

Lin, A.A., Kwei, T.K., Reiser, A., 1989. On the physical meaning of the Kwei equation for the glass transition temperature of polymer blends. Macromolecules 22, 4112-4119.

145

Lora, J.H., Glasser, W.G., 2002. Recent Industrial Applications of Lignin: A Sustainable Alternative to Nonrenewable Materials. J. Polym. Environ. 10, 39-48.




Schneider, H.A., 1988. The Gordon-Taylor equation. Additivity and interaction in compatible polymer blends. Die Makromolekulare Chemie 189, 1941-1955.

Sixun, Z., Qipeng, G., Chi-Ming, C., 2003. Epoxy resin/poly(ɛ-caprolactone) blends cured with 2,2-bis[4-(4-aminophenoxy)phenyl]propane. II. Studies by Fourier transform infrared and carbon-13 cross-polarization/magic-angle spinning nuclear magnetic resonance spectroscopy. J. Polym. Sci., Part B: Polym. Phys. 41, 1099-1111.

Verhoogt, H., Ramsay, B.A., Favis, B.D., 1994. Polymer blends containing poly(3-hydroxyalkanoate)s. Polymer 35, 5155-5169.

Yong, H., Naoki, A., Yoshio, I., 2001. Blend of Poly(ɛ-caprolactone) and 4,4'-Thiodiphenol: Hydrogen Bond Formation and Some Solid Properties. Macromol. Chem. Phys. 202, 1035-1043.


Zheng, S., Mi, Y., 2003. Miscibility and intermolecular specific interactions in blends of poly(hydroxyether of bisphenol A) and poly(4-vinyl pyridine). Polymer 44, 1067-1074.

146

CHAPTER 7

Conclusions and Further Research

147

7.1. Conclus ions

In this thesis, the advantages and disadvantages of blending lignin with PHB

have been studied. The properties of the lignin/PHB blends that were studied

are:

� Thermal properties and miscibility

� Thermophysical and rheological properties

� Environmental degradation

� Thermal properties of PHB blends with different types of lignin

7.1.1. Thermal properties and miscibility study.

In this study, lignin was found to increase the overall thermal stability of the

PHB/lignin blend, although it reduces the initial onset temperature of PHB

degradation. Thermal analyses (TGA and DSC) indicate that it is the

intermolecular interaction between PHB and lignin which causes miscibility

within a range of blends. One of the fundamental outcomes of these

investigations is the evaluation of the range of miscibility of these two polymers

and thermal analyses revealed that 40 wt% lignin is the highest amount of lignin

which gives a miscible blend. There was a significant difference between the

properties of miscible and immiscible blends reflected in the thermal, rheological

and environmental degradation properties. One of the factors which control

miscibility is believed to be hydrogen bonding between carbonyl groups of PHB

and hydroxyl groups of lignin.

7.1.2. Thermophysical and rheological properties of lignin/PHB

blends

The mechanical properties of PHB are affected by the high degree of crystallinity

(62 %) and the Tg of 4°C. PHB has a low concentration of nucleation sites so it

has relatively big crystals which make it brittle and susceptible to secondary

crystallisation. Lignin has been shown to reduce the bulk crystallinity of PHB

up to 65% (see Table 4-4). Another benefit which lignin provides to PHB during

blending is a reduction in the melting temperature and so enables PHB to be

148

processed at a lower temperature. Decreasing the processing temperature not

only reduces the risk of thermal degradation of PHB, but also saves energy.

A low concentration of lignin is also found to plasticise PHB. In the miscible

region, lignin lowers by up to 10 times the lignin/PHB blend melt viscosity

which facilitates the processing of PHB and saves energy.

7.1.3. Environmental investigation of lignin/PHB blends

Investigations of lignin/PHB blends on soil burial for up to 12 months showed

lignin does not improve the biodegradation properties of PHB. Lignin not only,

even in low concentration, inhibits the biodegradation of PHB, it decreases the

rate of degradation as burial time is increased. Surface composition analysis

using XPS show a presence of both PHB and lignin on the surface of film

samples. The analysis also revealed the presence of biofilms on the buried films.

The presence of biofilms is evidence of biodegradation. In a future study it is

worth to investigate the antimicrobial effect of lignin on lignin/PHB blends.

7.1.4. Thermal properties of PHB blends with different types of lignin

According to chemical analyses in this study, soda lignin (SL) has a far higher

concentration of phenolic hydroxyl group with lower content of carboxylic acid

and methoxyl groups compared to ML (see Table 2-3). Also, modelling results

based on the Gordon-Taylor equation showed stronger hydrogen bonding in

SL/PHB blends 7K�� 2 22.08 compared with ML/PHB 7K�� 2 3.348. Therefore, the conclusion is that the phenolic hydroxyl group could make a

stronger contribution in hydrogen bonding. The T0 values of ML/PHB blends

were higher than the T0 values of soda lignin/PHB blends. This may be because

of the proportion of xylan in the composite (see Table 2-4). Xylans are known to

decompose at lower temperatures than cellulose and lignin.

Tg results of ML/PHB blends indicated that blends containing up to 40 wt% ML

are miscible with PHB. The similar results were obtained for soda lignin/PHB

blends (section 1.3.3). FT-IR spectra showed that for blends up to 50 wt% ML,

there was a small but definitive shift to lower wavenumbers, indicating hydrogen

149

bonding interactions. The similarities between the results and those of

lignin/PHB blends indicated that the differences observed in the composition of

lignin functional groups were not significant to influence the glass transition

temperature of lignin/PHB blends derived from the two lignin types.

7.2. Future Research

The opportunities for future related research can be classified in the following

areas:

� Study of molecular structure of PHB during processing with lignin

� Modelling the viscoelasticity of lignin/PHB blends

� Study of antimicrobial effect of lignin

� Study of mechanical properties of lignin/PHB blends

7.2.1. Study of molecular structure of PHB during blend processing

In the processing of lignin/PHB blends, PHB is sensitive to thermal degradation.

The degradation temperature of PHB is close to its melting point. An

investigation for monitoring the molecular structure of lignin/PHB blend while

processing could be linked with the proposed methods to evaluate the kinetics of

the degradation reaction of PHB. The FT-IR monitoring of the extruder chamber

for the lignin/PHB blend could be a good approach to detect changes in

molecular structure of PHB and lignin/PHB blends. A near infrared spectroscopy

(NIRS) method is available (Siesler, et al., 2007) for real-time monitoring of the

processing in the Haake Minilab extruder and this could be adapted for these

studies.

7.2.2. Modeling the viscoelasticity of lignin/PHB blends

Data on the complex viscosity of lignin/PHB blends at different temperatures,

lignin content and frequencies could be used to develop a model that can be

used to provide processing conditions for different blend compositions. For

creating such a model, it is essential to trial a frequency sweep of the complex

150

viscosity of lignin/PHB blends at different temperatures. However, the sensitivity

of PHB to degradation in such close range to its melting point makes this

correlation hard. Using FT-IR detection during the rheological study could be

useful to detect when degradation is occurring.

7.2.3. Study of antimicrobial effect of lignin

An investigation of the radical scavenging activity of lignin could clarify why

lignin protected PHB from biodegradation. Lignin is known to be a natural

antioxidant (Dizhbite, et al., 2004) with a variety of functional groups containing

oxygen (for example hydroxyl and carboxylic acid) which could play a

considerable role in antibacterial and antifungal activity (Nada, et al., 1989).

Based on the outcomes of this thesis, lignin resisted the degradation of PHB even

on the surface of buried films which were exposed to the soil. A more detailed

laboratory-based microbiological study could help to understand this.

7.2.4. Study of mechanical properties of lignin/PHB blends

Utilizing lignin in low concentration leads to miscible blends with PHB. Also,

lignin in higher amounts acts as a filler in lignin/PHB blends and moreover,

lignin affects the bulk crystallinity of lignin/PHB blends both of which alter the

mechanical properties of PHB. Both miscible and immiscible blends of

lignin/PHB need to be investigated in terms of mechanical properties such as

tensile strength and elongation at break.

151

7.3. References

Dizhbite, T., Telysheva, G., Jurkjane, V., Viesturs, U., 2004. Characterization of

the radical scavenging activity of lignins--natural antioxidants. Bioresource Technol. 95, 309-317.

Nada, A.M.A., El-Diwany, A.I., Elshafei, A.M., 1989. Infrared and antimicrobial studies on different lignins. Acta Biotechnologica 9, 295-298.

Siesler, H.W., Ozaki, Y., Kawata, S., Heise, H.M., 2007. Near-Infrared Spectroscopy, Wiley-VCH Verlag GmbH.









In the case of this chapter: Publication title and date of publication or status: Environmental degradation of soda lignin/PHB blends”, Polymer Degradation and Stability, 2011.


Mousavioun Wrote the manuscript, Experimental design, Conducted experiments, Data analysis.

Signature

Date

Graeme A. George

Aided experimental design, Data analysis.

William O.S. Doherty

Data analysis.



152

APPENDIX 1

Thermal decomposition of bagasse.

Effect of different sugarcane cultivars

Vanita, R. Maligera, William O.S. Dohertya, Ray L. Frostb and Payam

Mousaviouna

a Sugar Research and Innovation, Centre for Tropical Crops and


Brisbane, Australia. b Inorganic Materials Research Program, School of Physical & Chemical

Sciences, Queensland University of Technology, GPO Box 2434, Brisbane,

Australia

Published in the Journal of Industrial & Engineering Chemistry Research, Vol

50, Page 791, 2011

halla

Due to copyright restrictions, this article is not available here. Please consult the hardcopy thesis available from QUT Library or view the published version online at: http://dx.doi.org/10.1021/ie101559n









In the case of this chapter: Publication title and date of publication or status: Thermal stability and miscibility of poly(hydroxybutyrate) and methanol-soluble soda lignin blends”, presentation in 10th AIChE meeting, Salt Lake City, UT, USA, November 2010. CD Rom.


Mousavioun Wrote part of the manuscript, Experimental design, Conducted experiments, Data analysis.

Signature

Date William O.S.

Doherty

Wrote the manuscript, Data analysis.

Graeme A. George Data analysis.

Peter Halley Data analysis



174

APPENDIX 2

Value-adding to cellulosic ethanol:

Lignin polymers

William O.S. DohertyA, Payam MousaviounA and Christopher M. FellowsB

A Centre for Tropical Crops and Biocommodities, Queensland University of Technology, GPO Box 2434, Brisbane, QLD 4000, Australia B Chemistry, School of Science and Technology, The University of New England, Armidale, NSW 2351, Australia

Published in Industrial Crops and Products, Vol 32, Page 259, 2011

175

Abstract- Lignocellulosic waste materials are the most promising feedstock for

generation of a renewable, carbon-neutral substitute for existing liquid fuels.

The development of value-added products from lignin will greatly improve the

economics of producing liquid fuels from biomass. This review gives an outline

of lignin chemistry, describes the current processes of lignocellulosic biomass

fractionation and the lignin products obtained through these processes and finally

outlines the current and potential value-added applications of these products, in

particular as components of polymer blends.

A.2.1. I ntroduc t ion Concern about the depletion of fossil fuel resources and climate change

attributed to anthropogenic carbon dioxide emissions is driving a strong global

interest in renewable, carbon-neutral energy sources and chemical feedstocks

derived from plant sources. Commercial products, which are capturing an

increasing share of the liquid fuel market, are esters of long-chain fatty acids

from plant oils (biodiesel) and ethanol from the enzymatic digestion and

fermentation of starch or sucrose. As an example of the use of biomass as a

chemical feedstock, a consortium led by Dupont is working to convert maize

starch to the monomer, 1,3-propandiol, using genetically modified Escherichia

coli (Caimi, 2004, Wehner et al., 2007). This monomer can then be used to

prepare poly(trimethylene terephthalate), a polyester which is traditionally

synthesised by the polycondensation of trimethylene glycol with either

terephthalic acid or dimethyl terephthalate (Kurian, 2005, Kurian and Liang,

2008).

Industrial production of fuels and feedstocks from plant sources has concentrated

on those sources that can be most readily and economically processed, such as

oil palm, sugarcane, and corn. However, these compete for arable land with

crops intended for human or animal consumption, putting upward pressure on

food prices and accelerating environmental degradation. For this reason, current

research efforts have concentrated on lignocellulosic biomass from sources that

do not compete with food crops: e. g., agricultural waste products, such as sugar

cane bagasse, wheat straw, rice stalk, cotton linters, and forest thinnings, or

novel crops that can be grown in environments too marginal for food production,

such as switchgrass and eucalypts (Sierra et al., 2008). In order for biomass to

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be a sustainable source of liquid fuel, technologies are required to enable the

economic production of suitable compounds from these sources, the dry mass of

which consists primarily of a matrix of cellulose, hemicellulose, and lignin

intimately mixed on a microscopic scale.

Current research is focussed on increasing the effectiveness and reducing the

cost of cellulase and xylanase enzymes for cellulose and hemicellulose

saccharification, (Maki et al., 2009, Oehgren et al., 2007, Rattanachomsri et al.,

2009), developing enzymes capable of converting the range of sugars produced

by the digestion of hemicelluloses to ethanol, (Bettiga et al., 2009, Yano et al.,

2009) and improving the pre-treatment process for the fractionation of cellulose,

hemicellulose and lignin from biomass (Fox et al., 1987, Kim, 2009, Moxley,

2008). Whatever the means, for producing ethanol from lignocellulosic biomass,

large volumes of lignin will be produced. Current pilot plants producing ethanol

from lignocellulosic material use the residual lignin for energy generation,

sequester it as ‘biochar’ as a carbon sink, or must dispose of it as waste. The

viability of biofuel production would clearly be greatly enhanced by the

development of markets for lignin-derived products. Any value-added lignin

derived product will improve the economics of biomass conversion, while high-

volume bulk commodity applications will also address the problem of waste

lignin disposal. There are a number of physicochemical factors which suggest a

bright future for lignin-based products: (a) compatibility with a wide range of

industrial chemicals; (b) presence of aromatic rings providing stability, good

mechanical properties, and the possibility of a broad range of chemical

transformations; (c) presence of other reactive functional groups allowing facile

preparation of graft copolymers; (d) good rheological and viscoelastic properties

for a structural material; (e) good film-forming ability; (f) small particle size; and

(g) hydrophilic or hydrophobic character depending on origin, allowing a wide

range of blends to be produced (Mousavioun and Doherty, 2010).

The focus of this review is the preparation of possible value-added polymers

derived from the varieties of lignin likely to be generated in significant amounts

from the production of cellulosic ethanol.

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A.2.2. L ignin Struc ture Lignocellulose materials refer to plants that are composed of cellulose,

hemicellulose and lignin. The cellulose microfibrials (formed by ordered

polymer chains that contain tightly packed, crystalline regions) are embedded

within a matrix of hemicellulose and lignin (Figure 3-1). Covalent bonds

between lignin and the carbohydrates have been suggested to consist of benzyl

esters, benzyl ethers and phenyl glycosides (Smook, 2002).

Figure A.2-1 Cellulose strands surrounded by hemicellulose and lignin (Department of energys genomic, http://genomics.energy.gov, 1986)

Lignin is primarily a structural material to add strength and rigidity to cell walls

and constitutes between 15 wt% and 40 wt% of the dry matter of woody plants.

Lignin is more resistant to most forms of biological attack than cellulose and

other structural polysaccharides, (Akin and Benner, 1988, Baurhoo et al., 2008,

Kirk, 1971) and plants with a higher lignin content have been reported to be

more resistant to direct sunlight and frost (Miidla, 1980). In vitro, lignin and

lignin extracts have been shown to have antimicrobial and antifungal activity,

(Cruz et al., 2001) act as antioxidants, (Krizkova et al., 2000, Pan et al., 2006,

Ugartondo et al., 2008) absorb UV radiation, (Toh et al., 2005, Zschiegner,

1999) and exhibit flame-retardant properties (Reti et al., 2008).

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Lignin is a cross-linked macromolecular material based on a phenylpropanoid

monomer structure (Figure A.2-2). Typical molecular masses of isolated lignin

are in the range 1,000 g mol-1 to 20,000 g mol-1, but the degree of polymerisation

in nature is difficult to measure, since lignin is invariably fragmented during

extraction and consists of several types of substructures which repeat in an

apparently haphazard manner. In this review the term ‘lignin’ will be used both

for the in vivo material and the various fractions isolated from living matter,

which invariably undergo some degree of chemical and physical change.

The monomer structures in lignin consist of the same phenylpropenoid skeleton,

but differ in the degree of oxygen substitution on the phenyl ring. The H-

structure (4-hydroxy phenyl) has a single hydroxy or methoxy group, the G-

structure (guaiacyl) has two such groups, and the S-structure (syringyl) has three

(Figure A.2-2). The polymerisation of the phenylpropanoid monomers is

initiated by oxidases or peroxidases. While the precise mechanism is obscure, it

is postulated that radical-radical combination of free radicals produced by

enzymatic dehydrogenation is the key reaction, either under enzymatic control

(Davin et al., 2008) or in a random ‘combinatorial’ manner (Ralph et al., 2004).

(a) (b) (c)

Figure A.2-2 Monolignol monomer species. (a) p-coumaryl alcohol (4-hydroxyl phenyl, H), (b) coniferyl alcohol (guaiacyl, G), (c) sinapyl alcohol (syringyl, S)

Both carbon-carbon and carbon-oxygen bonds between monomers are found in

lignin (Figure A.2-3). The most common functionality, accounting for about

half the bonds between monomers in lignin from most sources, is a carbon-

oxygen link between a p-hydroxy moiety and the β-end of the propenyl group (β-

OH

OH

OH

OH

OCH3 OH

OH

OCH3

OCH3

179

O-4) (Figure A.2-3a) (Chen, 1991, Ede and Kilpelaeinen, 1995, Kukkola et al.,

2004).

(a) (b) (c)

(d) (e) (f)

Figure A.2-3 Significant lignin linkage structures. (a) β-O-4, (b) αααα-O-4, (c) 5-5, (d) β-β, (e) 5-O-5, (f) β-5

The degree of cross-linking possible in lignin, and hence the rigidity of the

structure, is dependent on the degree of substitution. In softwoods, the G

OCH3

O

O

OCH3

OH OCH3

O

OCH3

O

OH

OCH3

O OO

CH3

O

O

OCH3

O

OCH3

OH

OH

OO

CH3

O OCH3

O

OCH3

O

OCH3

OH

180

structure is dominant, while hardwood lignins normally contain a mixture of S

and G structures with S in the majority, while H structures predominate in

lignins found in grasses (Wang et al., 2009).

Recent interest in lignin has been driven by the fact that it forms a large

proportion of the non-food biomass under consideration for the production of

renewable and carbon-neutral liquid fuels and chemical feedstocks. Separation

of cellulose from lignin is one of many technical hurdles which must be

overcome in order for biofuels to be economically produced from cellulose-

containing waste. While biotechnology allows plants to be modified to have a

larger cellulose: lignin ratio (Hu et al., 1999, Sticklen, 2008) and alter lignin

structure to produce lignins which can be more easily separated, (Lapierre et al.,

1999) these strategies will unavoidably run into limits imposed by plant

physiology and thus a significant volume of waste lignin is unavoidable.

A.2.3. L ignin Fract iona t ion Processes The extraction of lignin from lignocellulosic materials is conducted under

conditions where lignin is progressively broken down to lower molecular weight

fragments, resulting in changes to its physicochemical properties. Thus, apart

from the source of the lignin, the method of extraction will have a significant

influence on the composition and properties of lignin. The majority of lignin

extraction and delignification processes occur by either acid or base-catalysed

mechanisms. The chemistry of bond cleavage in lignin by these mechanisms has

been reviewed by Gratzl and Chen (2000).

A.2.3.1. Sulfite process

At present the main commercial source of lignin is from the pulp and paper

industry. The sulfite process which traditionally used to be the main pulping

technology involves the reaction of a metal sulfite and sulfur dioxide (Smook,

2002). The main reactions that take place during the pulping process are: (a) the

reaction between lignin and free sulfurous acid to form lignosulfonic acid, (b) the

formation of the relatively soluble lignosulfonates with the cations, Mg, Na or

NH4+, and (c) the fragmentation of the lignosulfonates. In addition to

lignosulfonates, degraded carbohydrates are also produced. The pulping

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reactions are usually conducted between 140°C and 160°C and the pH of the acid

sulfite process is between 1.5 and 2.0, while the bisulfite process is between pH

4.0 to 5.0 (Smook, 2002). The chemistry of the sulfide process has been

exhaustively reviewed by Wong (1980) and more recently by Alen (2000).

Several purification steps are required to obtain the lignosulfonate fraction with

high purity, including fermentation to convert the residual sugars to ethanol and

membrane fitration to reduce the metal ion content. The lignosulfonate

biopolymer is typically highly cross-linked, with ~5 wt% sulfur content, and

bears two types of ionising groups; sulfonates (pKa ≤ 2) and phenolic hydroxy

groups (pKa ~ 10). Because of the low pKa for the sulfonate groups,

lignosulfonates are water-soluble under most conditions. The physicochemical

properties of lignosulfonates are affected by the metal cation (Na or Ca) of the

sulfite salt used during the pulping process. Sodium sulfite produces more

extended lignin chains that are more suitable for use as dispersants, while

calcium sulfite produces more compact lignin, presumably due to a bridging

effect of chelating Ca2+. The sulfur content (5 wt%) of sulfite lignins is one of

the major factors restricting its use in speciality applications, and so most of its

lignin is currently used for energy generation.

The sulfite delignification process is an acid catalysed process in which there is

cleavage of the α-ether linkages and β-ether linkages of lignin. The process goes

via the quinone methide intermediate or nucleophilic substitution. Generally, less

side-chain cleavage is seen under acid-catalysed rather than alkali-catalysed

reactions. The complete breakdown of the aryl ether linkages leads to the

formation of a reactive resonance-stabilised benzyl carbocation. Under these

conditions condensation reactions occur. The carbocation may form a C-C bond

with an electron-rich carbon atom in the aromatic ring of a lignin fragment or the

protonation of a benzylic oxygen atom may cause inter- or intramolecular

condensation by a SN2 mechanism. The formation of organic acids such as

acetic acid during the delignification process can encourage the formation of the

benzylic carbocation or lead to protonation of a benzylic oxygen atom,

enhancing the SN2 condensation pathway.

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A.2.3.2. Kraft process

The kraft or sulfate process is now the main traditional method for pulping and

hence produces the largest volume of lignin (Smook, 2002). It uses sodium

hydroxide and sodium sulfide under strong alkaline conditions to cleave the ether

bonds in lignin. The delignification process proceeds in three stages. The first

phase occurs around 150°C and is controlled by diffusion. The second stage

occurs between 150°C -170°C, while the final stage occurs at even higher

temperatures. The bulk of the delignification (90 wt%) occurs during the second

stage. The lignin may be recovered from the alkaline liquid remaining after pulp

extraction, the black liquor, by lowering the pH to between 5 and 7.5 with acid

(usually, sulfuric acid) or carbon dioxide (Koljonen et al., 2004). Recent

developments in improving the yield of the kraft process have been reviewed by

Couchene (1998) and Kordsachia et al. (1999) have compared the suitability of

the kraft process for different substrates with the sulfite process.

The kraft process produces lignin with aliphatic thiol groups called kraft lignin.

Kraft lignin is hydrophobic and therefore needs to be modified to improve

reactivity. The high sulfur content (1 wt% - 2 wt%) of kraft lignin is also a

major reason why its main application has been in energy generation in pulp

mills.

The kraft process goes by alkaline hydrolysis in which the β-1,4 links in

cellulose are cleaved, allowing the lignin component of biomass to be extracted.

However, the lignin itself is also susceptible to attack by alkali and except for the

diaryl ether linkages, ethers in lignin readily undergo base-induced hydrolysis

under relatively mild conditions.

In alkaline hydrolysis α-aryl ether bonds are more easily broken than β-aryl ether

bonds, particularly in situations where the substructures contain a free phenolic

hydroxyl group in the para position (Baucher et al., 2003, Sakakibara et al.,

1966). Simple heating of the biomass in water results in substantial cleavage of

the α-ether bonds either through a quinone methide intermediate or through

nucleophilic substitution by a SN2 mechanism (Chakar and Ragauskas, 2004).

In alkaline media intermolecular condensation reactions can occur with

competition between the added nucleophiles and anionic lignin fragments (e.g.,

phenolate anions and carbanions) (Olm and Tisdat, 1979). The extent of

183

condensation will depend on the types of structures initially formed. If a

structure contains good leaving groups at the β-carbon, neighboring group

participation reactions resulting in the cleavage of β-aryl ether linkages will

predominate over condensation reactions (Chakar and Ragauskas, 2004).

A.2.3.3. Soda process

The soda process (which goes by alkaline hydrolysis) was the first chemical

pulping method and was patented in 1845. Soda process led kraft pulping which

now dominates the chemical pulping industry. The soda process is now

becoming the predominant method for the chemical pulping of non-wood

material such as bagasse, wheat straw, hemp, kenaf and sisal. This is mainly due

to the development of both low cost chemical recovery methods and effective

effluent treatment technology. It may also be due to less stringent environmental

legislation for effluent discharge in some countries. The pulping process involves

heating the biomass in a pressurised reactor to 140°C - 170°C in the presence of

13 wt% - 16 wt% alkali (typically sodium hydroxide).

Lignin recovered through extraction with sodium hydroxide is normally referred

to as ‘soda lignin’. Soda lignin from non-wood sources is typically difficult to

recover by filtration or centrifugation because its high carboxylic acid content,

arising from oxidation of aliphatic hydroxy groups, makes it a relatively good

dispersant. Heating is therefore required to encourage coagulation and ensure

filtrable material can be obtained. Soda non-wood lignin recovery has been

patented by Abaecherli et al.(1998). As soda lignin contains no sulfur and little

hemicellulose or oxidised defect structures, it has good potential for use in high

value product.

A.2.3.4. Other fractionation processes

With the push to produce cellulosic ethanol and bio-diesel, additional sources of

lignin will be available through various pre-treatment technologies. Promising

pre-treatment technologies for lignocellulosic biomass involve a combination of

physical, chemical, biochemical and thermal methods. Physical methods include

steam explosion, pulverising and hydrothermolysis (Mosier et al., 2005). The

principal chemical methods are the use of ammonia expansion, aqueous

184

ammonia, dilute and concentrated acids (e.g., H2SO4, HCl, HNO3, H3PO4, SO2)

and alkali (e.g. NaOH, KOH, Ca(OH)2) and ionic liquids. Significantly, all the

approaches under development for production of biofuels from lignocellulosics

are likely to produce lignin with little or no sulfur, increasing the scope for the

manufacture of value-added products.

Organic solvents (e.g. ethanol, formic acid, acetic acid, methanol) produce a

form of lignin, called organosolv lignin. The benefits of organosolv lignin over

sulfonated and kraft lignins include no sulfur, greater ability to be derivatised,

lower ash content, higher purity (due to lower carbohydrate content), generally

lower molecular weight and more hydrophobic (Lora and Glasser, 2002). This

delignification process is not used widely because the pulp produced is of lower

quality than that of soda or kraft process and there is extensive corrosion of the

plant equipment.

A relatively recent development in biomass fractionation is the application of

ionic liquids (IL) to fractionate lignocellulosic materials. Ionic liquids usually

consist of a large asymmetric organic cation and a small anion and typically have

negligible vapour pressure, very low flammability and a wide liquidus

temperature range. Most work on IL as biomass solvents has used

alkylimidazolium IL for dissolving cellulose. The mechanism of dissolution

involves the coordination of small hydrogen acceptors, such as chloride ions, to

the hydroxy groups of cellulose, breaking the strong intramolecular H-bonding

between the cellulose fibres (Spear et al., 2002, Swatloski et al., 2003). An ionic

liquid mixture containing 1-ethyl-3-methylimidazolium cation and a mixture of

alkylbenzenesulfonates with xylenesulfonate as the main anion has been used to

extract lignin from sugarcane bagasse at atmospheric pressure and elevated

temperatures (170°C - 190°C) (Tan et al., 2009). The addition of small amounts

of sodium xylene sulfonate to the ionic liquid mixture aided the cleavage of ether

groups in lignin. This was attributed to the sodium ions coordinating to the ether

oxygen, thereby increasing the carbonium ion character of the ether carbon

atoms and enhancing their susceptibility to nucleophilic attack by the

arylsulfonate groups of the ionic liquid (Tan et al., 2009). Lignins were

recovered from IL by precipitation, allowing the IL to be recycled. Lignins with

molecular weights around 2220 g mol-1 obtained by this process contained

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between 0.6 wt% and 2 wt% ash and about ~1.5 wt% sulfur. Low levels of

hemicellulose (< 0.1 wt.%) were also detected. Fasching et al.(2007) developed a

new facile method for the isolation of lignin from wood using a mixture of N-

methylimidozole and dimethylsulfoxide. Lignin was isolated by pricipitation

using dioxane/water mixture. Other classes of IL, such as alkylphosphonium IL,

which we are currently investigating, solubilise lignin by similar mechanisms as

those under acidic conditions. These lignins were sulfur-free and were of low

molecular weights.

A.2.4. Phys ica l Proper t ies of L ignin The physicochemical state of lignin dictates how and where it can be utilised in

the manufacture of various products. The source from which lignin is obtained

and the method of extraction has a strong bearing on its properties (Lora and

Glasser, 2002). As a highly cross-linked material with widely varying

functionality, lignin may not readily be characterised to give meaningful

molecular weight data, but other parameters more directly relevant to end-use

properties may be assessed. Despite this, the molecular weight data does provide

some useful guide. Table A.2-1 gives the functional groups and molecular

weight of selected lignins. The reactivities of these lignins will impact on the

attributes of the end products. For example, Muller et al. (1984) found that kraft

lignin-based phenol formaldehyde resins have superior properties to steam

exploded lignin-based phenol formaldehyde resins.

Tab le A.2-1 Molecu la r we ig h t and fun ct iona l g ro ups o f l i gn ins

Lignin type Mn (g mol-1) COOH (%) OH phenolic (%) Methoxy (%)

Soda (bagasse) 2160 13.6 5.1 10.0

Organosolv (bagasse) 2000 7.7 3.4 15.1

Soda (wheat straw) 1700 7.2 2.6 16

Organosolv

(hardwood)

800 3.6 3.7 19

Kraft (softwood) 3000 4.1 2.6 14

186

Another important parameter is the glass transition temperature, Tg, which is an

indirect measure of crystallinity and a degree of cross-linking and directly

indicates the rubbery region of the material (Table A.2-2) (Gargulak and Lebo,

2000).

Tab le A.2-2 Tg o f d i f f e ren t l i gn in t ypes (Ga rgu la k and Lebo, 20 00)

Types of lignin Tg (°C)

Milled wood lignin

-Hardwood

-Softwood

110-130

138-160

Kraft lignin 124-174

Organosolv lignin 91-97

Steam explosion lignin 113-139

Lignin Tg will depend on the amount of water and polysaccharides, as well as

molecular weight and chemical functionalisation, but in general the Tg will be

lower the greater the mobility of the lignin molecules. While Tg generally

increases with increasing molecular weight, the impact of structural variation

based on the degree of polymerisation has only recently been established.

Baumberger and co-workers (2002) showed using a series of transgenic poplars

that the variations in Tg were closely related to the degree of polymerisation of

lignin as determined by thioacidolysis. This is illustrated in Figure A.2-4, where

the Tg increases with the degree of condensation, expressed as the fraction of

phenylpropanoid units involved in C-C linkages.

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Figure A.2-4 Correlation between the glass transition temperature (Tg) and

the degree of condensation (% phenylpropanoid units involved in C-C linkages) of milled wood and enzyme lignins isolated from control and transgenic poplars (Baumberger et al., 2002). Data for control plants are shown as open symbols, and data for transgenic plants derived from those controls are shown as closed symbols. Figure redrawn with permission from Baumberger et al. (2002)

The reactivity and physicochemical properties of lignins are dependent to certain

extent, on their molecular weight distribution. Recently, Baumberger et al.

(2007) developed the use of size-exclusion chromatography to measure the

molecular weight distribution of lignin.

More potential applications of lignin can be realised if the miscibility of lignin

with other polymeric materials can be improved. This may be done through the

chemical modification of lignin with appropriate hydrophobic groups (e.g.

butyrate, hydroxypropyl, ethyl) (Ghosh et al., 2000, Uraki et al., 1997) or

through the formation of lignin copolymers (Wang et al., 1992). Pouteau and his

coworkers (2004) examined the compatibility of lignin-polymer blends by image

analysis. A correlation (Figure A.2-5) between the solubility parameter of kraft

lignin (20.5-22.5 (MPa)1/2) and the solubility parameters of different polymers

was obtained. The data shown does not discriminate between the molecular

170

175

180

185

190

45 50 55 60 65 70 75 80

Tg

(C

)

% units involved in C-C bonds

(°C

)

188

weight of lignin fractions, but only low molecular weight lignins are compatible

with apolar and very polar matrices.

Figure A.2-5 Correlation between total aggregate surface area observed per photo and the solubility parameter of the polymer matrix (Pouteau et al., 2004). Figure redrawn with permission from Pouteau et al. (2004).

A.2.5. App l icat ions There are many commercial applications of low value where lignins

(predominantly lignosulfonates) are used because of their surface-active

properties (Gargulak and Lebo, 2000, Stewart, 2008). Table A.2-3 gives the

variety of these lignosulfonate products.

Tab le A.2-3 App l i ca t i on o f l i gno su l fon ate p rodu c ts based o n the i r su r fa ce-a c t i v e p rope r t ies

Products Reference

Concrete additives (Sestauber et al., 1988, Shperber et al.,

2004)

0

10000

20000

30000

40000

15 17 19 21 23 25 27

Ave

rag

e s

urf

ace

per p

ho

to

Solubility Parameter (MPa)1/2

189

Animal feed pelleting aid (Winowiski and Zajakowski, 1998)

Metallic ore processing (Clough, 1996)

Oil well drilling muds (Detroit and Sanford, 1989, Kelly, 1983)

Dust control (Buchholz and Quinn, 1994, Fiske, 1992)

Phenol-formaldehyde resins (Raskin et al., 2002)

Lignosulfonates are also used to produce a number of value-added products for

specialty markets (Gargulak and Lebo, 2000). Table A.2-4 gives the variety of

these lignosulfonate products.

Tab le A.2-4 L igno su l fon ate p rod uct s in spec ia l i t y ma rket s

Products Reference

Vanillin (Bjorsvik and Minisci, 1999, Gogotov, 2000)

Pesticides (Lebo, 1996)

Dispersant for carbon black (Goncharov et al., 2001)

Dyes and pigments (Hale and Xu, 1997)

Gypsum board (Northey, 2002)

Water treatments (Jones, 2004, Zhuang and Walsh, 2003)

Scale inhibitors (Ouyang et al., 2006)

Industrial cleaners (Jones, 2008)

Emulsifiers (Gundersen et al., 2001, Sjoblom et al., 2000)

Matrix for micronutrient (Docquier et al., 2007, Meier et al., 1993,

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fertilisers Niemi et al., 2005)

Wood preservatives (Dumitrescu et al., 2002, Lin and Bushar,

1991)

Battery expanders (Pavlov et al., 2000)

Specialty chelants (Khabarov et al., 2001)

Bricks, refractories and

ceramics

(Pivinskii et al., 2006)

Retention aids in papermaking (Vaughan et al., 1998)

Blending of two or more polymers provides the ability to tailor material

properties to achieve specific goals with higher value. While a particular

homopolymer will have properties that can be tailored by controlling molecular

weight and the degree of branching and crosslinking, blending of polymers

makes a vastly greater range of potential materials properties available. As well

as making simple additive properties accessible, in many instances polymer

blending results in high-performance composite materials as a result of

synergistic interactions between the components. However, many polymer

combinations are immiscible and so exist in two different phases in the polymer

matrix. This separation into phases can result in poor stress transfer between the

phases, thereby lowering the mechanical properties of the blend to that at least of

one of the individual components. When incorporated in blends with natural and

synthetic polymers, lignin generally increases the modulus and cold

crystallisation temperature but decreases the melt temperature. The addition of

plasticisers to such systems have been found to improve the mechanical

properties by reducing the degree of self-association between lignin molecules,

improving lignin-polymer miscibility (Feldman et al., 2001). Because lignin

possesses easily-functionalisable hydroxyl and carboxylic acid groups, its

compatibility with different polymer types has been extensively examined. The

191

following section presents some examples of lignin blends with natural and

synthetic polymers.

Natural polymers are synthesised by living organisms or by enzymes isolated

from living organisms, through sophisticated biosynthetic pathways requiring

carbon dioxide consumption. These ‘environmentally friendly’ polymers include

cellulose, hemicellulose, lignin, starch, proteins, nucleic acids and linear

aliphatic polyesters. The ability to control the hydrophilicity of lignin means that

it could in principle form composite materials with any of these polymers, while

the physicochemical qualities of lignin means that it can in many cases improve

the tensile strength and bulk modulus of these biopolymers, and protect the

composite against oxidative degradation under UV light or elevated temperature.

Feldman (2002) and more recently Stewart (2008) have reviewed lignin blending

with synthetic polymers. The present review will discuss protein-lignin blends,

starch-lignin blends, epoxy-lignin composites and phenol-formaldehyde resins

where all or part of the phenol is derived from lignin, polyolefin-lignin blends,

lignin blends with vinyl polymers, lignin-polyester blends, lignin as a component

of polyurethanes, synthetic rubber-lignin blends, graft copolymers of lignin and

the prospects of lignin incorporation into further polymer systems. Most of these

copolymers and polymer blends are currently in the research phase with the

intent of commercial applications.

A.2.5.1. Protein-lignin blends

Proteins have long been used for the production of plastics and resins (Huang et

al., 2004, Nagele et al., 2000). The main drawbacks of protein-based materials

are high water absorption and the difficulty of separating the proteins from

naturally occurring colourants without denaturation, however these obstacles are

gradually being overcome (John and Bhattacharya, 1999, Otaigbe and Adams,

1997, Zhong and Sun, 2001). As a crosslinked material with a largely aromatic

structure, lignin has the capacity to increase the tensile strength, Young’s

modulus, thermal stability and elongation at break of protein materials.

The addition of soda lignin to soy protein plastics has been shown to reduce

water absorption, as well as improving the mechanical properties of soy

protein/glycerol blends. Blends containing 50 wt% soda lignin have a tensile

192

strength twice that of unblended soy protein (Huang et al., 2003). Thermoplastic

materials comprising lignin and protein blended with natural rubber, have been

patented. These materials have been shown to have improved impact resistance

compared to lignin-free formulations (Nagele et al., 2000).

Hydrogen-bonding interactions are often insufficient to ensure adequate mixing

of lignin with protein. Huang et al. (2004) blended kraft lignin with soy protein

using methylene diphenyl diisocyanate (MDI) as a compatibiliser. MDI will

form urethane links between hydroxy groups on lignin molecules and in the

protein. Only a slight reduction in water absorption was observed, but the

addition of 2 wt% MDI caused a simultaneous enhancement of modulus,

strength, and elongation at break of the polymer blends, which was attributed to

graft copolymerisation and crosslinking (Huang et al., 2004).

An alternative strategy for enhancing the compatibility of lignin with protein,

rather than adding a compatibiliser, is chemical or enzymatic modifications of

the lignin. Blending soy protein with hydroxypropylated soda lignin resulted in a

200 % increase in the tensile strength of the blended material, (Chen et al., 2006,

Wei et al., 2006) without reducing the elongation at break (Huang et al., 2006).

Wei et al. (2006) suggested that improved mechanical properties of protein

blended with hydroxylpropyl lignin molecules were due to: (a) the formation of

supramolecular domains by hydroxylpropyl lignin, (b) the strong adhesion

between the hydroxylpropyl group and soy protein and, (c) the interpenetration

of the soy protein molecules into the supramolecular hydroxylpropyl domain.

Protein has also been incorporated in more complex composite materials, e.g., an

adhesive composition of low molecular weight polyaminopolyamide-

epichlorohydrin resin and protein has been patented (Spraul et al., 2008).

While most processing of gluten protein increases the degree of cross-linking,

incorporation of kraft lignin in gluten reduced protein/protein interactions,

prevented loss of solubility (Kunanopparat et al., 2009). This has obvious

implications for processibility of gluten-based materials, suggesting kraft lignin

is a promising additive for such materials. It was suggested that kraft lignin had a

radical scavenging activity, reacting with the sulfur-centred radicals responsible

for gluten crosslinking.

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A.2.5.2. Starch-lignin blends

The use of starch-based films for packaging materials has increased recently as

they degrade readily in the environment in comparison to conventional synthetic

materials. However, a significant disadvantage of starch films is that they have

very poor water resistance. Blending with hydrophobic polymers can clearly

improve the water resistance of starch, and lignin has a high compatibility with

starch making it an obvious candidate for blending. Baumberger (2002) has

reviewed studies involving starch-lignin films, giving an overview of methods of

preparation, thermomechanical properties, mechanisms of starch-lignin

interactions and potential target applications of starch-lignin blends.

Lepifre et al. (2004) compared the reactivity of films of three soda lignins (one

derived from sugarcane bagasse and the other two from wheat straw) with starch

on exposure to radiation doses of 200 kGy and 400 kGy, using spectroscopic and

chromatographic techniques. Infrared analysis of the bagasse lignin-starch film,

in contrast to the wheat straw lignin, showed evidence of condensation probably

related to the presence of reactive ferulic acid, and that irradiation improved

compatibility of the two polymers.

Lepifre et al. (2004) found that grafting of starch films with lignin gave

significant improvements in water resistance. The higher water resistance of

lignin/starch blends is attributable to the partial compatibility of lignin with the

amylose component of starch, the presence of hydrophobic lignin at the surface

of the material due to surface activity of phenolic groups, and cross-linking

between the starch-rich phase and the lignin-rich phase (Baumberger et al.,

2000). The work by Baumberger et al. (1998) established that reduced water

content and water solubility starch-kraft lignin blends was due to the amount of

water soluble phenolics present in lignin as these hydrophilic compounds are

likely to interact with the starch matrix, through hydrogen bonding, and lead to

increased bonding to lignin. Figure A.2-6 shows the bonding between β-1

stilbene (a component) found in lignin and the amylose portion of starch. The

increase in elongation at break for the starch-kraft lignin blend compared to

starch was attributed to the increased plasticity of the starch matrix due to the

presence of low molecular phenolics and amphiphilic fatty acids.

194

Figure A.2-6 Hydrogen bonding between β-1 stilbene and amylose. Composite films of lignin, starch, and cellulose have been cast from ionic liquid

at room temperature, with the product showing good mechanical properties,

thermal stability, and resistance to gas permeation (Wu et al., 2009).

Ke et al. (2003) studied the effect of amylose content on the mechanical

properties and moisture uptake of starch films. Three dry corn starches with

different amylose contents: Amioca (0 wt% amylose ); HylonV (50 wt%

amylose) and HylonVII (70 wt% amylose); were blended with poly(lactic acid)

at various starch/poly lactic acid ratios and characterised for morphology,

mechanical properties and water absorption. It was shown that starch with 50

wt% or more amylose content effectively reduced moisture uptake than those

with the higher percentage amounts of amylopectin. The explanation given was

that although amylopectin is more crystalline than amylose, its large branched

molecules contains ~75 wt% amorphous structure which readily absorb water

(Ke et al., 2003). Moreover no significant difference in mechanical properties

was observed among starches with varying amylose content, except that the

blend containing 50 wt% amylose had slightly greater strength.

195

Based on the foregoing, to produce starch-lignin blends with improved properties

research should be directed in the following areas: (a) reducing the

hydrodynamic volume of lignin and increasing its phenolic hydroxyl group

content, (b) attaching hydrophobic groups to both starch and lignin, (c) including

high molecular weight plasticisers such as sorbitol and maltitol (Ghosh et al.,

2000), (d) using starch polymer with a high amylose content and (e) forming

lignin esters prior of blending with starch. The use of plasticisers will minimise

starch degradation and improve processability. The acetylation of the hydroxyl

group in low molecular weight lignin will reduce the amount of hydroxyl groups

available for water molecules to attract to and hence improve the water resistance

of the blends. The attachment of acetyl groups to lignin will reduce hydrogen

bonding increasing the free volume of the amorphous component of starch,

thereby reducing Tg. The miscibility of the starch-lignin blends property

relationships could be studied by Fourier transform infrared (FT-IR)

spectroscopy, Raman spectroscopy and differential scanning calorimetry (DSC)

in order to evaluate molecular interactions between the two components.

A.2.5.3. Polyhydroxyalkanoates

Polyhydroxyalkanoates (PHA) are a group of biodegradable and biocompatible

linear aliphatic polyesters mainly composed of R-(–)-3-hydroxyalkanoate units,

produced as carbon and energy storage materials by a range of algae and

bacteria. PHA have been reported with alkanoates ranging in length from C3 to

C14, but the most common are polyhydroxybutanoate (PHB, C4) and

polyhydroxyvalerate (PHV, C5) and copolymers of C4 and C5 alkanoates

(PHBV) (Reddy et al., 2003).

Unlike most biopolymers, PHA are insoluble in water and have low permeability

towards oxygen, carbon dioxide and water. These barrier properties make PHA

good candidates for the production of packaging products like bottles, bags,

wrapping film and disposable nappies. These applications have not been fully

realised because PHB and PHV are relatively stiff and brittle and are thermally

unstable during processing.

Blending with lignin is one possible strategy for overcoming the mechanical

disadvantages of PHA. Ghosh et al. (2000) investigated the thermoplastic blends

196

of several biodegradable polymers with organosolv lignin and organosolv lignin

ester based on both solvent casting and melt processing. Blends of PHB with

lignin are claimed to have a high degree of recyclability (Yao, 2008). On

addition of up to 20 wt% lignin to PHB, improvements were seen in Tg, melting

point, Young’s modulus, and the degree of crystallinity (Ghosh et al., 2000).

The addition of lignin reduced the crystallinity of PHB more than addition of

lignin butyrate, suggesting greater compatibility of PHB with lignin than lignin

butyrate. Recently, Mousavioun et al. (2010) examined the miscibility between

PHB and bagasse soda lignins (having distinct chemical group functionality)

based on the Tg of their blends. A single Tg implies complete compatibility

between the components, while two or more Tg values suggest that the degree of

miscibility is restricted. Figure A.2-7 indicates that with the lignin content <40

wt% there is compatibility between PHB and lignin. The Tg were higher for the

blends obtained from the lignin fraction containing higher xylan and phenolic

hydroxyl content, but lower for higher methoxyl and carboxylic acid content.

This implies that the association between lignin and PHB is probably related to

the chemical functionality of the lignin polymer as the molecular weights of

these lignin fractions are similar, approximately 2400 g mol-1. In fact it was

shown by FT-IR that the miscibility between PHB and soda lignin was due to

hydrogen bond formation between the carbonyl group of PHB and the phenol

hydroxyl group of lignin (Mousavioun et al., 2010).

Blends of lignin butyrate with the slightly more hydrophobic polymer PHBV

gave a significant reduction in crystallinity compared to blends with PHB.

Meister et al. (1993) have reported that grafting lignin with styrene-acrylonitrile

copolymer improves its compatibility with PHB-PHV.

197

Figure A.2-7 Miscibility of lignin/PHB blends based on Tg. Lignin Tg,▲; PHB Tg,■.

Camargo et al. (2002) investigated the thermal, mechanical and optical properties

of bagasse lignin blended with PHB, as well as the biodegradation of the blends.

A significant increase in Tg was observed and the PHB/lignin blend was readily

degraded by the common fungi species Trametes versicolor. The significant

increase in Tg obviously related to the interactions between the reactive

functional groups of lignin and the carbonyl groups of PHB. Weihua et al. (2004)

investigated the effect of 1 wt% lignin on the nucleation of PHB by studying the

kinetics of both isothermal and nonisothermal PHB crystallisation. DSC showed

that not only did lignin act as a nucleating agent, decreasing the activation

energy of crystallisation, but it reduced the size of the spherulites to give a less

brittle material.

A.2.5.4. Polylactides and polyglycolides

Poly(L-lactic acid) (PLA) is a crystalline biodegradable polymer which like PHA

has poor processing properties because of its high crystallinity. Copolymers of

L-lactic acid and L-glycolic acid are frequently used in biomedical applications

to enable the tailoring of flexibility and degradation rate. Li et al. (2003)

examined the thermal and mechanical properties of PLA/lignin blends, with

198

results indicating a strong intermolecular hydrogen-bonding interaction between

PLA and lignin. The tensile strength and elongation at break decreased with

lignin content, while the Young’s modulus remained almost constant up to a

lignin content of 20 wt%. At a lignin content greater than 20 wt%, thermal

degradation of PLA was enhanced. More recently, ring-opening polymerisation

of cyclic lactides with lignin has been used to create graft-copolymer additives

that can significantly reduce the crystallinity and improve the processing and

end-use performance of PLA (Uyama et al., 2008). Graupner (2008) used lignin

to reinforce PLA/cotton composites and compared the mechanical properties of

the composites with those of PLA/kenaf composites. Addition of lignin

appeared to enhance the adhesion between the cotton fibres and PLA, improving

the tensile strength and Young’s modulus, though the impact resistance

decreased (Graupner, 2008). Lignin has also been added to PLA in order to

reduce its flammability, giving performance competitive with commercial

intumescent formulations (Reti et al., 2008).

From the foregoing, it is evident that not much work has been carried out to

understand the interactions between lignin and these crystalline polymers. The

use of solid state nuclear magnetic resonance (NMR) and relaxation methods

should be included in the analytical tools to study the interactions of lignin and

these interacting polymers in order develop a better understanding of the nature

of the blends for property enhancement.

A.2.5.5. Epoxy resin blends

Substitution of lignin for phenol is a possible route toward the preparation of

inexpensive and renewable epoxy-resin adhesives. In fact, entirely renewable

epoxy-resins have been prepared using lignin and epoxides of plant origin

(Hirose, 2006, Watado et al., 2009). A very wide variety of co-monomers and

curing reagents have been applied to prepare lignin-derived epoxy resins (Ebata,

2004, Hirose et al., 2002).

The effect of lignin blending with epoxy resins is strongly affected by the type of

lignin used (Feldman, 2002). Commercial hardwood lignins have been reported

to improve adhesion to epoxy resins more than Indulin, a softwood lignin, a

result which correlates well with the density of hydrogen-bonding groups in the

199

material. Tomlinite (a commercial soda lignin) lignin (20 wt%) gave the highest

adhesive joint shear strength. In general, differences in performance could be

related to the differences in molecular weight and the type and amount of

functional groups (Feldman, 2002). Investigation of the viscoelastic properties

of cured kraft lignin/epoxy resins has been found to have a very broad Tg, which

suggests they may be suitable for application as adhesives or as damping

materials for noise and vibration (Tomita, 1998). Epoxy-resins derived from

lignin have also been applied in concrete formulations (Cheng et al., 2005) and

in a range of fibreboard and plywood products (Okabe et al., 2006).

Feldman et al. (1991a, 1991b) studied a bisphenol A-polyamine hardener-based

epoxy adhesive incorporating kraft lignin. Blends with up to 40 wt% kraft lignin

were cured at room temperature or above their Tg, demonstrated enhanced

bonding between the components. The improvement was attributed to

association between lignin and the unreacted amine groups of the hardener. In

another study, Feldman and Khoury (1988) observed that epoxy blends with 10

wt% and 20 wt% of lignin improved the adhesion tensile strength of an epoxy

polymer system. While blending with 5 wt% to 20 wt% lignin had little effect

on the initial adhesive shear strength or the weatherability of the epoxy-lignin

blend, after post-curing (4 h at 75 ºC) significant improvement of adhesive

strength under shear was observed.

Modifying the lignin structure by ozonisation was found to have little effect on

the properties of epoxy resins prepared from soda lignin and epoxy compounds

(polyethylene glycol diglycidyl ether and bisphenol A diglycidyl ether) which

could be prepared with acceptable shear strengths in applications as wood

adhesive (Nonaka et al., 1996). Only one Tg was observed in resins of this kind,

suggesting formation of interpenetrating polymer networks (Nonaka et al.,

1997). Epoxy resins prepared from lignin, glycerol, and succinic anhydride

which were cured with dimethylbenzylamine at a range of ratios showed a

constant decomposition temperature, regardless of lignin and glycerol content

(Hirose and Hatakeyama, 2006).

200

A.2.5.6. Phenol-formaldehyde resins

Extensive work has been carried out with a number of different lignin types as

substitutes for phenol in phenol-formaldehyde resins. These have primarily been

considered for use in adhesive applications, though there has been some

application of lignin-containing phenol-formaldehyde resins as foams (Frollini et

al., 2004). The state of work in this field to 2002 was comprehensively reviewed

by Feldman (2002). The properties of wood adhesive products produced with

lignin-based phenol-formaldehyde resins have been found to be comparable with

those of commercial resins up to 35 wt% partial replacement with lignin

(Kulshreshtha and Vasile, 2002). A range of different lignins, including

organosolv lignin, soda lignin, and lignosulfonates, have been used in phenol-

formaldehyde resin preparation, and black liquor has even been applied directly

(Nada et al., 2003, Wang et al., 2006). A number of methods for lignin

derivatisation for forming phenol-formaldehyde resins are described in the

literature. These phenolysis methods are (a) the lignin reacts with phenol and the

lignin-phenol complex is then reacted with formaldehyde, (b) the lignin which

reacts with phenol and formaldehyde, and the pre-polymer is then reacted with

phenol, (c) phenol reacts with formaldehyde and the mixture is then reacted with

lignin, and (d) the lignin reacts with formaldehyde and the hydroxymethylated

lignin is then reacted with phenol. The phenolation process may be acid or base

catalysed. The condensation reaction occurs between the ortho or para position

of phenol and the side chain of the phenylpropane units of lignin in which the α-

position is substituted by hydroxyl, etherified lignin residue or a double bond-

carbon linked lignin residue. Incorporation of lignin into phenol-formaldehyde

resins has been demonstrated to delay the first glass transition and speed up

curing (Khan and Ashraf, 2006, Khan and Ashraf, 2007).

Vazquez et al. (1999) and Cetin and Ozman (2002), have shown phenol-

formaldehyde resins prepared using organosolv lignin and subsequent plywood

board formation produced board knife-test results better than those obtained with

a commercial phenol-formaldehyde resin. In contrast, Gardner and McGinnis

(1988) prepared lignin-based resins with kraft lignin and steam-exploded

hardwood lignin showing lower reactivity and poorer physical properties than

the pure phenol resin. This variation in results is consistent with other reports

201

indicating that the method of extraction and the source from which the lignin is

derived has a strong bearing on the properties of the phenol-formaldehyde resin.

For example, Olivares et al. (1988) reported that different fractions of softwood

lignin separated by ultrafiltration after methylation and demethylation gave

different reactivities toward formaldehyde, leading to different mechanical and

water absorption properties. In another example, Piccolo et al. (1997) showed

that during resol synthesis organosolv bagasse lignin acted as a chain extender.

As a result, molded resins prepared with 40 wt% lignin exhibited modulus

extension at elevated temperatures.

Park et al. (2008) studied the partial substitution of phenol in phenol

formaldehyde resin with high-purity bagasse organosolv lignin. Purification by

extraction with cyclohexane/ethanol removed waxes, lipids, tannins in the lignin

prior to synthesis. The Tg of the resins were between 125 ºC and 150 ºC, and this

transition was clearly evident in the resins when the lignin content was increased

from 10 wt% to 40 wt%. Conversion profiles for lignin/phenol-formaldehyde

resins obtained by differential scanning calorimetry are shown in Figure A.2-8,

demonstrating that partial replacement of phenol with lignin increases the rate of

conversion. The conversion profile is relatively unchanged with the addition of

10 wt% lignin, however, the initial rate of conversion increases markedly upon

increase in lignin concentration to 20 wt% and even further when the

concentration is increased to 30 wt%. Further addition of lignin to 40 wt%

decreases the rate of conversion from the 30 wt% value, but the conversion rate

is still higher than the phenol-formaldehyde resin. In the same study, cardboard

coated with lignin/phenol-formaldehyde showed water resistance properties far

superior to untreated cardboard or cardboard treated with an equivalent phenol-

formaldehyde resin (Park et al., 2008, Pizzi, 2003). Phenol-formaldehyde-type

adhesives prepared from lignosulfonate derived from grasses (bagasse, kaigrass

and wheat straw) have demonstrated acceptable performance qualities at up to 70

wt% lignosulfonate content (Akhtar et al., 2009, Liu et al., 2006). The best

adhesive properties on incorporation into phenol-formaldehyde resins of wheat

straw soda-lignin were found for the lower molecular weight fractions (Liu et al.,

2008).

202

Figure A.2-8 Conversion profiles of lignin-based phenol formaldehyde resins (Park et al. 2008)

Peng and Riedl (1994) have shown that the reactivity of lignosulfonate with

formaldehyde is increased when wheat starch was added as a filler, and less

condensation was apparent. Hydroxylmethylation has been reported to produce

resins with improved properties in comparison to unmodified lignin (Yang and

Liu, 2002). It would be interesting to see if the addition of starch to the

hydroxymethylation procedure of lignin described by Muller and Glasser (1984)

would further enhance the reactivity of the hydroxymethylated lignin and

produce resins of improved quality. In another application of multiple natural

products in a composite material, pulverised lignocellulosic materials such as

sisal fibre have been applied as fillers in lignin-based phenolic resin (Frollini et

al., 2004).

From an environmental point of view, an important advantage of using lignin in

partial replacement of phenol-formaldehyde resin is the decrease in

formaldehyde emission during processing (Kulshreshtha and Vasile, 2002). It is

also possible to avoid this volatile and toxic compound entirely, with good

materials properties having been demonstrated for a resin composed of lignin

and the non-volatile aldehyde glyoxal (Mansouri et al., 2007). The overall

0

0.2

0.4

0.6

0.8

1

90 100 110 120 130 140 150 160

De

gre

e o

f C

ure

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consensus is that lignin-based resins generally have weaker adhesive properties,

and a high degree of variability in adhesion performance (Cyr and Ritchie,

1989). The presence of plasticisers or contaminants, such as very low molecular

weight lignin, is suspected to be largely responsible for the low bond strengths

obtained, (Hiro-kuni and Kenichi, 1989, Lora, 2002) while the variability is

probably due to the sensitive dependence of properties on the source and history

of the lignin, as mentioned above.

A.2.5.7. Lignin-polyolefin blends

The main objectives of incorporating lignin in polyolefins are to act as a

stabiliser against oxidation under UV radiation or at elevated temperatures, or

conversely, to enable the biodegradation of the material. Early investigations of

polymer blending found good compatibility between hydrophobic lignin and

high density polyethylene (HDPE) with little change in properties, but poor

compatibility with low density polyethylene (LDPE) (Deanin et al., 1978).

Some improvements in the tensile modulus of LDPE were found with greater

than 20 wt% lignin incorporation, but tensile strength was poor. The differences

observed between HDPE and LDPE suggest that molecular architecture may

play as large a role as chemical structure in determining the compatibility of

lignin in blends, as the interactions between lignin and the many short branching

chains of LDPE may be entropically unfavourable.

Blends of up to 70 wt% hydrophilic lignin (lignosulfonates) were similar for

both HDPE and LDPE, with increases in Young’s modulus and a decrease in

elongation at break for both classes of blend, with sugar-rich lignosulfonates

giving the largest increase in modulus (Kubat and Stroemvall, 1983). Scanning

electron microscopy of these blends suggested a morphology of thin

HDPE/LDPE fibres in a lignosulfonate matrix.

Straw lignin obtained by steam-explosion has been blended with LDPE, HDPE,

and linear low density polyethylene (LLDPE), giving blends that are stabilised

against UV radiation and can be processed by conventional thermoplastic

methods. While modulus was slightly increased in the blends, tensile strength

and elongation at break were impaired (Pucciariello et al., 2004). Significant

improvements have been observed in the thermal oxidative stability of PE

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blended with lignosulfonate, and incorporation of lignosulfonate also had a

significant impact on the rheology of the polymer melt (Levon et al., 1987).

Ammonium lignosulfonate and epoxy-modified lignosulfonate can act as

nucleating agents in PP processing as well as plasticisers and controllers of melt

flow (Darie et al., 2007). Košíková et al. (1993) investigated sulfur-free lignins

as composites of PP films. PP films containing 2 wt% - 10 wt% of spruce

organosolv lignin and/or beech wood prehydrolysis lignin showed good

compatibility between lignin and PP and sufficient tensile strength. The films

acted as good UV absorbers (Kosikova and Demianova, 1992). The influence of

lignin on the oxidative stability of PP has been examined by Gregorova et al.

(2005) by differential scanning calorimetry under non-isothermal conditions. It

was found that lignin exerts a stabilising effect in both virgin and recycled PP,

with a protection factor increasing with lignin content in the PP matrix, though

the increases with small quantities of lignin were less significant than for PE

(Chodak et al., 1986). Surface modification of lignin/PP blends by treatment

with silicon tetrachloride plasma increased tensile and impact strength by

introducing surface cross-linking (Toriz et al., 2002).

Alexy et al. (2000) used lignin as a filler for both LDPE and PP at concentrations

up to 30 wt%, with only small impacts on tensile strength and melt flow index,

but improvements in processing stability and modulus. Resistance to light and

heat degradation was improved for both PE-lignin and PP-lignin blends. Kraft

lignins acylated with long hydrophobic substituents have been used to

compatibilise fractions of different polyolefins in recycled household waste,

giving good values of tensile strength and elongation at break for blends of

LDPE and PP (Tinnemans and Greidanus, 1984). Compatibility of lignin and

hydroxypropyl lignin with PE is low in comparison with more polar monomers,

making them relatively ineffective in improving bulk modulus (Ciemniecki and

Glasser, 1989, Glasser et al., 1988). The compatibility between lignin and PE/PP

can be improved by grafting ethylene/propylene monomers onto lignin prior to

blending to the polyolefin (Casenave et al., 1996). The Young’s modulus of the

lignin-grafted material prepared by Casenave (1996), Ait-Kadi and Riedl was

similar to that of pure PE at up to 64 wt% lignin content. Chemical modification

of soda lignin with stearoyl chloride has also been effective in increasing its

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compatibility with LDPE, giving significant mixing attested to by improvements

in mechanical properties (Vasile et al., 2006).

Organosolv lignin blends based on the compatilisation of PE with ethylene-vinyl

acetate copolymer (EVA) have been investigated by Alexy et al. (2004). The

addition of 10 wt% EVA gave an approximate 200 wt% increase in tensile

strength and a 1300 wt% increase in elongation at break in comparison to blends

without EVA. A composite material prepared containing 33.6 wt% lignin

displayed acceptable processing and mechanical properties, and was used

successfully in preparing blown films. The compatibility of lignin and EVA was

found to increase with increasing content of vinyl acetate for both soda lignin

and hydroxypropyl lignin (Glasser et al., 1988). Tensile properties were inferior

with less than 10 wt% vinyl acetate and the best tensile properties were obtained

with materials containing between 5 wt% and 20 wt% hydroxypropyl lignin and

greater than 25 wt% vinyl acetate. Ciemniecki and Glasser (1989) also observed

that blends of EVA and hydrodxyropyl lignin showed superior strength

properties as the proportion of the polar vinyl acetate component increased.

In another application of EVA, lignin was added to an EVA/LDPE blend to form

a homogeneous blend exhibiting a single glass transition temperature that could

be used to prepare a foam (Zhou and Luo, 2007). LDPE grafted with maleic

anhydride is another compatabiliser that has been successfully used to mix LDPE

and lignin, with scanning electron microscopy indicating more dispersed lignin

in smaller domains in the presence of maleated LDPE, decreasing the melting

temperature and improving stability to thermal oxidation (Li and Luo, 2005). At

25 wt% loading of lignin and 10 wt% maleated LDPE, blown films could be

prepared with excellent properties.

Processes for preparing degradable plastic blends of ethylene copolymers and

organosolv lignin have been patented by Bono (1994). Lignin was incorporated

in the form of powder having a grain diameter of about 1 µm - 5 µm, and

homogeneous films with a thickness of about 15 µm - 25 µm were obtained

showing improved degradation with photoactive and oxidizing agents. The

ability of the lignin-degrading microorganism Phanerochaete chrysosporium to

degrade lignin-PE blends has been reported by Košíková et al. (2001). The

isolation of oligomer fraction from biodegraded polymer blends indicated that

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the biotransformation of lignin during the cultivation process was accompanied

with degradation of the PE matrix.

A.2.5.8. Lignin-vinyl polymer blends

As with polyolefins, vinyl lignin polymers have attracted interest primarily as a

UV and thermal stabiliser. In general, unmodified lignin has poor compatibility

with non-polar vinyl polymers; while the modulus of these blends is increased,

reductions in tensile strength and elongation at break are obtained. Early work

found good compatibility between hydrophobic lignin and relatively polar

poly(vinyl chloride) (PVC), but poor compatibility with polystyrene (Deanin et

al., 1978). However, the source of the lignin can have a considerable impact on

miscibility, and steam-explosion lignin powder has more recently been

successfully blended with atactic polystyrene into a readily processible material

(Pucciariello et al., 2004). Improved blending of polystyrene and lignin has also

been achieved using a copolymer of styrene and vinyl phenol (Henry and

Dadmun, 2009) or cellulose phthalate (Hechenleitner et al., 1997) as

compatibilising agents. In the latter case, a strong dependence of thermal

stability on the hydrophobicity of the lignin used was observed, with the more

hydrophobic lignin fraction promoting stability and the more hydrophilic fraction

reducing stability.

A significant body of research has been carried out on the blending of lignin and

PVC (Banu et al., 2006, El Raghi et al., 2000, Feldman and Banu, 2003, Mishra

et al., 2007). One rationale for this has been to increase the resistance of PVC-

based floor coverings to attack by fungi that can degrade phthalate-based

plasticisers to generate potentially toxic products (Feldman et al., 2003).

Generally homogeneous PVC/lignin blends can be prepared at low lignin

content, with increased rigidity due to the lignin component improving impact

resistance and scratch hardness while reducing flexibility (Mishra et al., 2007).

Larger quantities of lignin with concomitant changes in properties towards

rigidity can be achieved by using plasticisers that can disrupt intermolecular

hydrogen bonding in lignin, (Feldman and Banu, 2003) and in general lignin

may have either an antiplasticising or plasticising effect depending on its

molecular weight and how it is dispersed through the PVC matrix (Banu et al.,

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2006, Feldman et al., 2003). There are reports that thermal stability of PVC can

be improved by the addition of lignin (Szalay and Johnson, 1969). Conversely, a

negative impact of lignin on the stability of PVC to weathering has been

attributed to degradation of lignin under PVC processing conditions (Feldman

and Banu, 1997b). There is some evidence that softwood lignins, generally

having a higher proportion of crosslinked phenol groups, are more effective in

promoting properties of PVC/lignin blends than hardwood lignins (Feldman and

Banu, 1997a).

Polymer blends of hydroxypropyl lignin with poly(methyl methacrylate)

(PMMA), and poly(vinyl alcohol) (PVA) were investigated by Ciemniecki and

Glasser (1989). In both cases compatibility was high, with the lignin being able

to contribute to modulus, while depending on molecular weight the effect of

lignin incorporation could be either plasticising or antiplasticising. In all cases

two-phase materials were produced, but Tg values nevertheless suggest a high

degree of compatibilisation. Blends prepared using injection moulding showed

generally better properties than blends formed by solution casting from organic

solvent (Ciemniecki and Glasser, 1988).

Li et al. (1997) reported a blend of 85 wt% underivatised kraft lignin and

poly(vinyl acetate), prepared with indene and diethyleneglycol benzoate as

plasticisers, which exhibited promising mechanical properties. The modulus and

tensile strength of these blends was strongly influenced by the degree of

association between the lignin molecules. Lignins dissociated by prolonged

incubation in 0.10 M NaOH gave much poorer mechanical properties in the

blends, while lignins associated by incubation in 0.40 M NaOH with a high ionic

strength gave blends with excellent mechanical properties. This work has

significant implications for the entire field of lignin-polymer blends, implying

that the effect of the blended copolymer on non-covalent interactions between

lignin molecules could play a critical role in the properties of blended materials

(Chen and Sarkanen, 2006).

Blends of hydrophobic lignin with water-swellable alternating copolymers of

maleic anhydride have attracted interest as matrices for delivery of agricultural

actives. Acylated kraft lignin was blended with poly(maleic anhydride-alt-

styrene) by solvent casting to give brittle films which could be swollen to up to

208

50 times their dry weight in water or dilute aqueous ammonia (Tinnemans and

Greidanus, 1984).

There has been interest in blends of lignin with hydrophilic polymers such as

poly(vinyl alcohol) (PVA) and poly(ethylene oxide) for application in carbon

fibre synthesis (Kubo et al., 2005, Kubo and Kadla, 2004, Kubo and Kadla,

2005). While kraft or organosolv lignins can be spun into fibres, they produce

carbon fibres that are brittle and difficult to handle, and morphological properties

can be significantly improved using miscible (e.g., PEO) or immiscible (e.g.,

PVA) blends of hydrophilic polymer with lignin (Kubo et al., 2005).

Incorporation of lignin in PVA greatly reduces the crystallinity of the PVA and

reduces Tg, suggesting there are strong hydrogen-bonding interactions between

lignin and PVA despite the fact the these blends remain two-phase systems

(Kubo and Kadla, 2003).

A ‘polyionic complex’ of lignosulfonic acid and poly(vinyl pyridine) can be cast

into a thin film and shows good adhesive properties (Hasegawa et al., 2008).

As with polyolefin/lignin blends, blends of lignin with vinyl polymers have been

shown to be biodegradable, with poly(methyl methacrylate) (PMMA) and

polystyrene blends with as little as 10 wt% lignin degrading under the action of a

number of ‘white rot’ fungus species (Milstein et al., 1996). Conversely, lignin

blending with poly(vinyl alcohol) had little effect on its rate of bidegradation

(Pseja et al., 2006). Incorporation of small amounts of lignin has also been

demonstrated to accelerate the thermal depolymerisation of polystyrene and

PMMA (Mansour, 1992).

A.2.5.9. Lignin-polyester blends

Blends of lignin have been prepared with a wide variety of synthetic polyesters,

in addition to the poly(hydroxy alkanoates) and polylactides/polyglycolides

already discussed. These include poly(ethylene terephthalate) (PET), (Agafitei

et al., 1999) poly(butylene terephthalate), (Xu et al., 2007) poly(trimethylene

succinate) (Li and Sarkanen, 2005) and poly(ε-caprolactone) (Nitz et al., 2001a).

Blending of poly(butylene adipate) and poly(trimethylene succinate) with

acylated, methylated and ethylated kraft lignin is most effective when the lignin

has a broad molecular weight distribution (Li and Sarkanen, 2005). These

209

polyesters appear to be relatively less effective plasticisers of lignin than PEG.

On the other hand, alkylated lignin in combination with aliphatic polyester

plasticisers has produced materials with tensile properties very similar to

polystyrene (Sarkanen and Li, 1999).

Blends of methylated and ethylate kraft lignins with aliphatic polyesters appear

to be a potentially versatile class of thermoplastics, with homogeneous blends

obtained when the ratio of methylene/carboxylate ester in the polyester is

between 2 and 4 (Li and Sarkanen, 2003). As with kraft lignin/poly(vinyl

acetate) blends, the association of the lignin molecules into supramolecular

structures is postulated to be largely responsible for the properties of these

blends (Li et al., 1997). The amount of alkylated kraft lignin necessary to disrupt

crystalline domains of the polyester is least when the ratio of

methylene/carboxylate ester in the polyester is between 2.5 and 3.0 (Li and

Sarkanen, 2002).

Lignin can form homogeneous blends with poly(butylene terephthalate) (Xu et

al., 2007) and PET (Kadla and Kubo, 2004). Soda lignin decreases the melting

temperature and Tg of PET, improving its processability, and accelerates

crystallisation implying it can play a role in PET nucleation (Chaudhari et al.,

2006). The compatibility of lignin epoxy-modified with epichlorohydrin with

PET and a poly(ethylene terephthalate/isophthalate) copolymer was studied by

Agafitei et al. (1999). Optimum compatibility with significant increases in

surface and bulk electrical resistivity, reductions in crystallisation temperature,

and increases in melting temperature, were achieved using 4 wt% -10 wt% lignin

(Agafitei et al., 1999).

Organosolv lignin alkylated with propyl, butyl, or pentyl groups formed

homogeneous blends with poly(ε-caprolactone) (PCL), with crystallisation

studies suggesting the miscibility improved as the length of the alkyl chain

increased. Good elongation at break values were obtained even in blends with 50

wt% alkylated lignin (Teramoto et al., 2009). Unmodified straw lignin strongly

stabilised PCL against UV radiation and increased the blend modulus, but

decreased the observed elongation at break, and the two components of the blend

were shown to be immiscible by dynamic mechanical analysis (Pucciariello et

al., 2008). Compatibilisation of PCL with lignin and wood flour could be

210

achieved by incorporation of PCL grafted with maleic anhydride, giving a blend

with a five-fold improvement in modulus and 100 wt% improvement in yield

stress (Nitz et al., 2001a). Lignin was found to retard the rate of decomposition

of these biodegradable composites (Nitz et al., 2001a).

Sulfur-free lignin compounded with poly(butylene-co-adipate-co-terephthalate)

gave improved mechanical properties, but marked differences were seen between

lignin obtained by alkaline pulping of fibre plants such as sisal and abaca and

alkaline pulping of wood, with the lignin derived from the fibre plants giving

superior modulus and yield stress (Nitz et al., 2001b). The blends obtained were

heterogeneous, with the disperse lignin phase occurring in larger domains when

wood lignin was used.

A.2.5.10. Lignin-containing polyurethanes and lignin-polyurethane

blends

The incorporation of lignin and lignin derivatives into polyurethanes has been

investigated in order to (a) increase crosslinking of the polyurethane networks,

(b) increase Tg, (c) increase tensile strength, (d) increase curing rates and, (e)

increase thermal stability (Hatakeyama and Hatakeyama, 2005, Saraf and

Glasser, 1984).

Extensive work has been undertaken on the development and characterisation of

polyurethanes from lignin grafted with polycaprolactone, which gave two-phase

systems with properties controlled by the degree of association of the PCL

chains (Hatakeyama et al., 1998, Hatakeyama et al., 2001a, Hatakeyama et al.,

2001b). Lignin extended with polyethylene oxide has also been used as a basis

for producing polyurethanes, notably in interpenetrating network systems with

PMMA (Kelley et al., 1989, Kelley et al., 1990). Liu et al. (2002) used

propylene oxide-modified lignin with ethylene glycol and methylene

diisocyanate to prepare polyurethane resins potentially suitable for use in hard

foams, with lignin content of 30 wt% or less. One intriguing application as a

geoengineering material is in situ polyurethane/inorganic composites generated

by injecting lignin and isocyanates into sand (Hatakeyama et al., 2005).

Polyurethane/lignin blends have also been investigated. The morphology of such

blends has been studied by Feldman and Lacasse (1989) While SEM revealed an

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even distribution of lignin particles in the polyurethane matrix, the different

morphologies of the constituent phases could clearly be observed, with

differential scanning calorimetry (DSC) confirming immiscibility. Polyurethane

lignin blends have also been obtained by treating steam explosion lignin from

straw with a range of isocyanates. The presence of ethylene glycol reduced the

yields, and the best results were obtained using an isocyanate terminated

poly(butylene terephthalate) (Bonini and D'Auria, 2007). Ciobanu et al. (2004)

used a polyurethane elastomer blended with flax soda lignin to form

homogeneous solvent-cast films containing between 4.2 wt% and 23.2 wt%

lignin. While the thermal degradation ranges of unmodified polyurethane and the

blends were similar, the presence of lignin accelerated decomposition at lower

temperatures. The tensile strength increased up to 370 wt%, toughness up to 470

wt% and elongation at break up to 160 wt%, for the blends compared to the

unmodified polyurethane film.

A.2.5.11. Rubber-lignin blends

Lignin has attracted most attention as a filler in natural and synthetic rubbers -

that is, as a component of a multiphase mixture, not in a homogeneous blend. It

has been applied as a filler in butadiene-styrene-butadiene and isoprene-styrene-

butadiene rubbers for shoe soles, (Savel'eva et al., 1983) in styrene-butadiene

elastomer, (Kosikova et al., 2003, Kramarova et al., 2007) and in natural rubber

(Kramarova et al., 2007). Soda lignin and calcium lignosulfonate were compared

as fillers in natural rubber, and though neither had properties entirely comparable

to carbon black, soda lignin had better filler properties than calcium

lignosulfonate and showed potential as a low-cost substitute for carbon black

(Lazic et al., 1986). Low molecular weight lignins have been shown to be more

effective in improving the tensile strength of natural rubber than of styrene-

butadiene rubber, being significantly more effective than starch or protein as a

filler for natural rubber but not for styrene-butadiene rubber (Kramarova et al.,

2007).

Lignin-based phenol-formaldehyde resin has demonstrated good mechanical

properties, oil resistance, and resistance to environmental oxidation when used as

a filler in nitrile rubber (Wang et al., 1992).

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Lignin has also been applied in combination with an oligomeric polyester as a

modifier of isoprene rubber and methylstyrene-butadiene rubber (Savel'eva et al.,

1988). The vulcanisation rate of the rubbers increased and optimum

vulcanisation time decreased, and improvements were obtained in the

mechanical properties suitable for applications as tyre rubber (fatigue strength,

adhesion to reinforcing cord). Improved adhesion to textiles in blends with lignin

has also been observed in blends of lignosulfate with natural rubber (Piaskiewicz

et al., 1998) and styrene-butadiene rubber (Lora et al., 1991). While in these

applications lignin incorporation increases the adhesiveness of the material, a

hydrophobically modified lignin has been applied to pre-vulcanised natural

rubber latex in order to decrease the stickiness of natural rubber latex as a

paperboard coating material (Wang et al., 2008).

A.2.5.12. Lignin-graft-copolymers

Apart from the uses of lignin as a filler in thermoplastics and as a copolymer in

thermosetting polymers, there is the potential for lignin to be used in free-radical

copolymerisation with unsaturated polymers. This potential is limited by the

ability of the phenolic hydroxyl groups in lignin to act as radical scavengers,

initiating the formation of quinonic structures (Barclay et al., 1997, Lu et al.,

1998).

The residual double bonds in lignin are 1,2-disubstituted and hence not reactive

towards free-radical attack, but lignin has a high concentration of benzylic sites

that should be susceptible to hydrogen abstraction and hence afford grafting sites

(Figure A.2-9). The chief limitation on achieving grafted copolymers based on

free-radical monomers and lignin is hence not normally the intrinsic reactivity of

the ligand, but that the high polarity of the hydroxyl groups leads to a molecule

insoluble in non-polar comonomers such as styrene and methyl methacrylate.

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(a)

(b)

Figure A.2-9 Potential sites for hydrogen abstraction for free-radical grafting from lignin ; (a) benzylic hydrogen, (b) allylic hydrogen from double bond from dehydrozylation

Lignin has been shown to retard the polymerisation of styrene and methyl

methacrylate, (Rizk et al., 1984) but good yields of PMMA-grafted lignin have

been prepared, (Meister and Zhao, 1992) and successful grafting using

conventional radical initiation has also been achieved with acrylamide, (Ibrahim

et al., 2006, Meister et al., 1991) vinyl acetate, (Corti et al., 2003) cationic vinyl

monomers, (Meister and Li, 1990) acrylic acid, (Maelkki et al., 2002),

acrylonitrile (Chen et al., 1996) and sodium acrylate (Potapov et al., 1990).

Interest in grafting polyelectrolytes to lignin arises from the possibility of

incorporating the thermal and mechanical resistance of lignin into polyelectrolyte

applications for extreme environments, such as additives for drilling muds

(Ibrahim et al., 2006). Chemical grafting of PMMA or polystyrene to lignin

produces surface-active materials which have possible applications as wood

coatings (Chen et al., 1995, Gardner et al., 1993). Contact angle on wood

surfaces coated with lignin-PMMA graft copolymer, a measure of

OCH3

O

OH

OCH3

OH

H

OCH3

O

OH

OCH3

OH

-H

O

OCH3

OCH3

OH

H

O

OCH3

OCH3

OH-H

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hydrophobicity, increased with lignin content, and copolymers of relatively low

molecular weight gave larger contact angles than copolymers of low molecular

weight (Gardner et al., 1993). Sailaja (2005) has reported that lignin grafted with

PMMA using manganese pyrophosphate initiator gave much improved

mechanical properties in blends at up to 50 wt% with PE, in comparison to

blends of PE with unmodified lignin.

A promising means for producing graft copolymers of lignin and free-radical

monomers appears to be initial derivatisation of lignin with more readily

polymerisable moieties, e.g., with isocyanatomethacrylate to give pendant

methacrylate groups readily polymerisable with methyl methacrylate or styrene,

(Glasser and Wang, 1989) or with chloromethylstyrene and methacryloyl

chloride (Da Cunha et al., 1993). Feldman et al. (1991c) carried out free-radical

grafting of maleic anhydride onto lignin in order to facilitate incorporation of the

modified lignin into a polyurethane. They reported both free-radical grafting to

the lignin backbone and a degree of esterification of the phenol hydroxy groups

on treatment with maleic anhydride and a persulfate radical source.

Grafting of methyl methacrylate to lignin using radiation was first reported by

Koshijima and Muraki (1964). Alkoxylation of the phenol groups improved the

effectiveness of radiation grafting, and radiation-curable coatings have been

produced using acrylic acid and propoxylated lignin (Reich et al., 1996).

Radiation-induced grafting of styrene to lignin was facilitated in the presence of

an organic solvent, with better efficiency as the proportion of methanol in the

reactants was increased (Phillips et al., 1972). Increasing moisture content in

wood was correlated with increasing radiation-induced grafting of PMMA to

lignin, presumably a phenomenon related to monomer diffusion within the

matrix (Sutyagina et al., 1987).

Grafting to lignin has also been accomplished through anionic and cationic chain

polymerisation, and chemical (De Oliveira and Glasser, 1994a) or enzymatic

(Huttermann et al., 2000) grafting of complete polymer chains. Oliveira and

Glasser prepared star-like graft copolymers of lignin and poly(caprolactone)

using anionic polymerisation (De Oliveira and Glasser, 1994b) and

heterogeneous composites of these copolymers with poly(vinyl chloride) (De

Oliveira and Glasser, 1994c). While these lignin-PCL copolymers were brittle

215

and had poor mechanical strength on their own (De Oliveira and Glasser, 1990),

they were found to exhibit good plasticisation properties with PVC. Anionic

polymerisation has been used to graft well-characterised polystyrene chains onto

mesylated lignin, producing copolymers suitable for use as compatibilisers for

blends of kraft lignin and polystyrene (Narayan et al., 1989).

Another route to lignin-PCL graft copolymers is by enzymatic polymerisation of

ε–caprolactone (Enoki and Aida, 2007). A similar chemo-enzymatic

polymerisation pathway has also been reported as a means of grafting acrylamide

(Mai et al., 2000a) and acrylic acid (Mai et al., 2001) onto lignin, in a process

where the role of the laccase enzyme appears to be primarily to catalyse the

production of peroxide-derived radicals (Mai et al., 2002). Although grafting of

acrylic acid to calcium lignosulfonates could be successfully carried out with a

hydroperoxide initiator alone, the process was much more effective when the

initiator was used in combination with laccase (Mai et al., 2000b).

A.2.6. Conc lus ions Lignin is a very abundant naturally occurring polymer with good properties for

the applications, of many materials which can play a role in replacing or partly

replacing petroleum-based components in a broad range of composite materials.

Lignin can be isolated in fractions of varying molecular weight and may readily

be functionalised to play a role in a broad range of composite materials. In

addition, lignin can serve as a feedstock for the production of both liquid fuel

and a broad range of commodity chemicals. The importance of lignin in these

applications is likely to increase, as society becomes less tolerant of product

streams that dispose of lignin by landfill or burning and as the exploitation of

lignocellulosic sources for biofuels increase the amount of lignin generated.

Widespread exploitation of these lignocellulosic sources would also dramatically

change the nature of the lignin isolated: today most lignin is hydrophilic sulfated

material produced as a by-product of the pulp and paper industry, but the

thermal, chemical, and biological methods employed in digesting lignocellulosic

material are all likely to give rise to unfunctionalised lignin. For many

applications, this material will be processed in order to improve its quality and

hence lead to the emergence of a viable lignocellulosic biofuels industry. Lignin

of superior quality will afford a significant opportunity to apply it to a much

216

greater extent in polymer composites, controlled-release formulations, and as a

feedstock for fuels and commodity chemicals. Conversely, the development of

these applications on a commercially viable scale will exert a ‘pull’ effect on

lignocellulosic biofuel development, making the industry economically viable at

an earlier stage of fossil fuel resource depletion. Despite hundreds of years of

experience in the pulping of biomass, technically feasible processes for

separation of biomass into its main components still lie mostly below the

threshold of economic viability. The present treatment strategies, whether

thermal, thermochemical or thermomechanical, still require considerably energy

input. Thus an important future research direction is the cost-effective

fractionation of lignocellulosic biomass. Specifically, the processes involved in

lignin recovery from black liquor (such as acid precipitation and membrane

filtration) need to be improved so that better separation, decreased losses during

washing of the precipitated lignin, and improved purity can be achieved.

Research into the use of flocculants, surfactants and ions for effective lignin

isolation from black liquor produced from various fractionation strategies would

also be worthwhile.

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