Recent Advance Reactive Extrusion in Polymer

Review

Recent Advances in Reactive ExtrusionProcessing of BiodegradablePolymer-Based Compositions

Jean-Marie Raquez, Ramani Narayan, Philippe Dubois*

This review reports on recent advances in the design of biodegradable polymers built frompetroleum and renewable resources using reactive extrusion processing. Reactive extrusionrepresents a unique tool to manufacture biodegradable polymers upon different types ofreactive modification in a cost-effective way. Partially based on our ongoing research, ring-opening polymerization of biodegradable polyesters will be approached as well as thechemical modification of biodegradable polymers, particularly natural polymers. The develop-ment of environmentally friendly poly-mer blends as well as (nano)compositesfrom natural polymers, including naturalfibers and nanoclays, through reactiveextrusion, as an efficient way to im-prove the interfacial adhesion betweenthese components, will be also dis-cussed.

Introduction

Reactive extrusion is an attractive route for cost-effective

polymer processing, which enhances the commercial

viability and cost-competitiveness of these materials, in

J.-M. Raquez, P. DuboisCenter of Innovation and Research in Materials & Polymers(CIRMAP), Laboratory of Polymeric and Composite Materials,Materia Nova & University of Mons-Hainaut, Place du Parc 20,B-7000 Mons, BelgiumE-mail: [email protected]. NarayanDepartment of Chemical Engineering & Material Science, 2527Engineering Building, Michigan State University, East Lansing,MI-48824, USAJ.-M. RaquezDepartement Technologie des Polymeres et Composites, Ecoledes Mines de Douai, Rue C. Bourseul 941 B.P. 10838, 59508 DouaiCedex, France

Macromol. Mater. Eng. 2008, 293, 447–470

� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

order to carry out melt-blending, but also various chemical

reactions including polymerization, grafting, branching

and functionalization as well.[1,2] In addition, reactive

extrusion processes involve introducing reactive agents at

optimum points in the reaction sequence, homogenizing

the ingredients, and allowing sufficient time for the

completion of the reactions. In a typical reactive extrusion

process, the reactants are fed into the extruder, usually

through a feed hopper. However, various liquid or gaseous

reactants can be introduced at specific points in the

reaction sequence by using injection along the extruder

barrel. The reactive mixture is conveyed through the

extruder, and the reaction is driven to the desired degree of

completion. At this point, and after removing any volatile

by-products, the molten polymeric product is pumped out

through a die and subsequently quenched, solidified, and

pelletized. Therefore, production and processing can be

integrated in one-stage processing.[3,4]

The recent environmental regulations, societal concerns

and growing environmental understanding throughout

DOI: 10.1002/mame.200700395 447

J.-M. Raquez, R. Narayan, P. Dubois

Jean-Marie Raquez was born in Paris (France) in 1977. He graduated in Chemistry from the University of Mons-Hainaut (Belgium)in 1999. He received his PhD degree in Polymer Chemistry under the supervision of Professor Philippe Dubois (University ofMons-Hainaut) on the topic dealing with the controlled synthesis of a biodegradable poly(ester-alt-ether), poly(1,4-dioxan-2-one), through ring-opening polymerization. After a postdoctoral stay with Professor Ramani Narayan (MichiganState University, USA), he moved back to University of Mons-Hainaut as assistant research. In 2007 he was appointedAssociated Professor at ‘‘Ecole des Mines de Douai’’ in France. His research work focuses on the chemical modification andsynthesis of polymer-based (nano)composites issued from renewable resource.Ramani Narayan is University Distinguished Professor at Michigan State University in the Department of Chemical Engineering& Materials Science. He has 115 refereed publications in leading journals to his credit, 18 patents, edited three books and oneexpert dossier in the area of bio-based polymeric materials. Under his supervision, 20 students have obtained their Master’sdegree, ten students their Ph.D. degrees and six are working towards their Ph.D. He has major research programs with industryand serves as consultant for several companies.He has won several awards: Awarded the ‘‘University Distinguished Professor’’ title in 2007 highest and very selective honorthat can be bestowed on a faculty member by the university. Those selected for the title have been recognized nationally andinternationally for the importance of their teaching, research and public service achievements; Governors (State of Michigan)University Award for commercialization excellence; University Distinguished Faculty Award, 2006; Withrow DistinguishedScholar award, 2005 awarded to one faculty in the MSU College of Engineering based on exemplary research accomplishments,national & international recognition; Fulbright Distinguished Lectureship Chair in Science & Technology Management &Commercialization (University of Lisbon; Portugal); the William N. Findley Award for ‘‘significant contributions to theapplication of new technologies within the scope of ASTM Committee D20 on Plastic; Award of Excellence from ASTMcommittee D 20 on Plastics for exemplary technical contributions, sustained participation, and valued leadership; 2006. TheJames Hammer Memorial Lifetime Achievement Award, 2006 for outstanding leadership, and research accomplishments in thefield of Degradable Polymers from the BioEnvironmental Polymer Society (BEPS). Research and Commercialization Awardsponsored by ICI Americas, Inc. & the National Corn Growers Association.He is on the Board of Directors of ASTM International-a premier international standards setting organization. He chairs theASTM committee on Environmentally Degradable Plastics and Biobased Products (D20.96) and the Plastics Terminologycommittee D20.92. He is also the technical expert for the USA on ISO TC 61 on Plastics – specifically for Terminology, andBiodegradable plastics. Dr. Narayan also chairs the scientific committee of Biodegradable Products Institute (BPI), NorthAmericaa biodegradable and biobased plastics trade industry organization (www.bpiworld.org).He is a successful entrepreneur having been responsible for commercializing several technologies.Philippe Dubois graduated in Chemical Sciences from the Facultes Universitaires Notre-Dame de la Paix (FUNDP, Namur,Belgium) in 1987. He received his PhD degree in Chemical Sciences from University of Liege (ULg) in 1991. In the same year, heworked as a postdoctoral fellow for Dow Chemical (Terneuzen, Holland) and the Laboratory of Macromolecular Chemistry andOrganic Catalysis directed by Prof. Ph. Teyssie at ULg. Then he joined the National Fund for Scientific Research (FNRS) at ULg till1997. In 1994, he worked as visiting scientist at the Chemical Research Engineering Department of the Michigan State University(MSU). In Oct. 1997, he moved to University of Mons-Hainaut (UMH) where he obtained the chair of macromolecular chemistryand created/directed the Laboratory of Polymeric and Composite Materials (now ca. 35 people). He has co-authored over 300publications in international journals, 180 personal communications at conferences and is co-inventor of 40 patents. Heco-edited 6 books. He is full professor at UMH and invited professor at FUNDP, MSU, and Faculte Polytechnique de Mons (FPMs).He is Scientific Director at the Materia Nova Research Center in Mons and Director of the Center of Innovation and Research inMaterials & Polymers CIRMAP (with ca. 80 members). He is currently Past-President of the Belgian Royal Chemical Society (hewas the President in 2006/7).

448

the world have triggered renewed efforts in plastic

industry to develop new products and processes compa-

tible with our environment.[5–10] The design of biodegrad-

able plastics is an appropriately eco-efficiency approach to

enhance the environmental quality for many products.

Biodegradable plastics can be converted into useful and

friendly environmental products to minimize the waste

disposed in landfills. Different markets are found in the

realm of the biodegradable polymers, including packaging

(trash bags, wrappings, loose-fill foam, food containers and

laminated papers), disposable non-woven (engineered

fabrics), hygiene products (diaper back sheets and cotton

swabs), consumer goods (fast-food tableware, containers,



egg cartons, razor handles and toys) and agricultural tools

(mulch films and planters).

Unfortunately, the utilization of biodegradable poly-

mers as bulk commodity materials is still restricted to few

applications because of the strong cost-competition with

cheaper petroleum-based polymers, and their limited

thermo-mechanical properties.[8,11] To permit their com-

mercial scale-up, an appropriate, inexpensive and easy

process to manufacture biodegradable polymers is

highly desirable for their commercial viability and cost-

competitiveness. Besides, while maintaining their overall

biodegradability, melt-blending through a reactive way

these biodegradable polymers with inorganic/organic

DOI: 10.1002/mame.200700395

Recent Advances in Reactive Extrusion Processing . . .

fillers as well as other biodegradable polymeric materials

may represent an additional way of reducing the overall

cost for the resulting materials, but also to modify their

thermo-mechanical properties and (bio)degradation rates

efficiently. Developing such biodegradable polymeric

melt-blends/composites with satisfactory overall thermo-

mechanical behavior however requires the ability to

control interfacial energy, to generate dispersed phases

of limited size and strong interfacial adhesion, and to

improve the stress transfer between the component

phases.[12] This can be effectively completed using proper

interface compatibilization between these different com-

ponents during their reactive processing.[13] Undoubtedly,

reactive extrusion (coined REX) serves on all these issues in

the manufacture of high-performance and inexpensive

biodegradable polymeric materials using a one-stage

continuous reactive processing.

Hence, this review aims at highlighting the recent

developments of novel biodegradable polymeric materials

using REX as an efficient processing technique, partially

based on our ongoing research over the past few years. The

production of biodegradable polymers through REXwill be

described, as well as the reactive modification and melt

blending of biodegradable polymers in the preparation of

useful and environmentally friendly products.

Reactive Extrusion Processing

Figure 1. Schematic representation of a reactive extrusion process including a typical screw forpolymerization of CL under inert atmosphere.

Reactions that previously re-

quired heavy equipments, parti-

cularly with batch operations,

can be completed in a more

efficient continuous way through

REX. Extruders have been used to

resolve heat and mass transfer

problems that arise when dra-

matic viscosity of the reaction

medium (when monomer is con-

verted to polymer) increases

within a magnitude order of 105

in batch polymerization pro-

cesses.[14,15] In a batch reactor,

as the polymerization proceeds,

the viscosity increases and after

a certain point the material

becomes unmanageable in terms

of mixing and heat transfer. In

this respect, REX shows to be a

promising technique for polymer

processing. The ability of these

extruders to create new thin sur-

face layers continuously can

increase the degree of mixing



and minimize temperature gradients within the polymer

being processed. Another factor that can be controlled via

operating conditions and geometrical specifications of

screw extruders is the residence time in the system.

Generally, the residence time is substantially lower as

compared to that required in a batch reactor for the same

reaction; reducing therefore long exposure to high tem-

peratures that can cause polymer degradation. The ability

of an extruder to handle materials of high viscosities

without any solvents results in a dramatic cost reduction

in raw materials and in solvent recovery equipments, and

polymers produced are in a ready-to-use form. Finally, for

polymer modification reactions, reactive extrusion pro-

cesses offer natural means for polymerization, chemical

cross-linking and grafting via the inherent capability of

stage feeding of reactive agents (Figure 1). This way, one

can tailor-make specialty polymers that are uneconomical

to produce in large scale operations.

The design of extruders for REX applications involves

the manipulation and integration of information and

knowledge from several distinct areas. Extruders for

reactive processing must deal with the continuously

changing nature of the reactive melt. Mixing phenomena

are considerably delayed when the reactive mixture is

highly viscous, leading to important gradient in chemical

composition and temperature that affect the product

quality. Mixing is an important factor when using an

extruder as a reactor. Especially longitudinal mixing must

www.mme-journal.de 449


450

be given significant consideration for radical reactions

since back-mixing can influence the residence time

distribution, that is the course of the reaction, and can

affect the molecular weight distribution (MWD) of the

final product.[20]

Although both single and twin-screw extruder config-

urations are used in REX processes, twin-screw ones are

increasingly being favored over the single-screw ones.

Themain reasons for this are the extended control of residence

time distribution and mixing, but also their superior heat

andmass transfer capabilities. The single-screw extruder is

currently utilized for simpler jobs likemelting, plasticizing,

and discharging melt for the production of films, pipes,

profiles, etc. The twin-screw extruders, according to their

specific characteristics, can tackle the more complex tasks

such as homogenizing, dispersing pigments and additives,

alloying, reactive compounding, concentrating, devolati-

lizing, polymerizing, etc. The major difference between

single- and twin-screw extruders is the conveying

mechanism. Although in single-screw machines, it

depends on frictional forces in the solids conveying zone

and viscous forces in the melt-pumping zone, in twin-

screw extruders it is largely dependent on the screw

geometrical configuration, and it is of a positive displace-

ment character. The relativemerits and the performance of

twin-screw extruders have been appraised.

Depending on the direction of rotation of the two

screws, twin-screw extruders can be distinguished in

co-rotating and counter rotating machines. In simple

terms, it can be said that co-rotating screws have a radial

and counter-rotating have an axial shearing and plasticiz-

ing effect. Although each type of twin-screw extruder has a

certain uniqueness regarding ingredients, type of reaction,

and polymer produced, and although no machine design

(counter- or co-rotating) provides the complete solution,

co-rotating intermeshing twin-screw extruders have been

found to be suitable for many continuous REX processes.

Compared with the counter-rotating twin-screw extruders

where additional radial forces are present, the two screws

for co-rotating intermeshing twin-screw extruders are set

side by side with minimum clearance between them. The

crest of one screw completely wipes the flights and the

root of the other one. This self-wiping feature eliminates

dead regions where material can stagnate during proces-

sing. Due to the co-rotating design of the screws, high

speeds, thus strong shearing forces, and high outputs can

be obtained, enabling the material to be transferred from

one screw to the other under a constant mixing. In

addition, the intensive and constant surface renewal

creates favorable degassing conditions.

The modular design and assembly arrangement of the

screw and barrel sections of twin-screw machines, along

with the use of special feeding and venting ports provide

adequate flexibility for specific reactive extrusion tasks.



The sequence of screw elements is of prime importance for

the control of the filling ratio inside the extruder. Three

types of elements are widely used in co-rotating inter-

meshing twin-screw extruders: kneading blocks, mixing

gears, and conveying screw elements. Kneading blocks are

primarily used for the dispersion of partially melted

polymer particles and solid additives or reactive agents

into themelt. Mixing gears are superior to kneading blocks

for the thorough distribution of finely divided particles, for

achieving isothermal conditions at a given location in the

barrel, and for the homogenization of two or more feed

streams. Conveying elements are the ones that move

the material through the extruder. The arrangement of

the screw elements is what determines the residence time

distribution in twin-screw extruders. The maximum

practical residence time capability of a typical co-rotating

intermeshing 45:1 Length/Diameter twin-screw extruder

is about 7min, at low screw speeds and with short-

pitch elements. Designs with even longer residence times

(10–75min) are also available. Recently, micro-

compounding using reactive extrusion technology has

emerged as a efficient way of handling small amount of

materials, leading to the preparation of expensive

materials such as (nano)composites, biopolymers or

pharmaceuticals at limited cost.[21]

According to its unique characteristic features, the

reactive extrusion processing technology has provided

different types of chemical reactions:[14,20–31]

a. F
ree radical, anionic, cationic, condensation, and
coordination polymerizations of monomers or oligo-

mers to high molecular weight polymers.

b. C
ontrolled degradation and cross-linking of polymers
by means of a free radical initiator for preparing a

product with controlled molecular weight distribution.

c. F
unctionalization of commodity polymers for produc-
ing materials to be used in grafting applications.

d. P
olymer modification by grafting of monomers or
mixture of monomers onto the backbone of existing

polymers for improving various properties of the

starting materials. Free radical initiators and ionizing

radiation can be used to initiate the grafting reactions.

e. I
nterchain copolymer formation: Usually, this type of
reaction involves combination of reactive groups from

several polymers to form a graft copolymer.

f. C
oupling reactions that involve reaction of a homo-
polymer with a polyfunctional coupling agent/filler in

the preparation of high-performance products.

Many authors have largely reviewed different synthetic

technology for the preparation of biodegradable polymers,

as well as their types of applications ranging from daily

applications to biomedical ones.[6–8,32] Within the scope of

this review, we will focus on the use of reactive extrusion

DOI: 10.1002/mame.200700395


in the design of biodegradable polymeric materials with

useful properties. First, the reactive extrusion synthesis of

biodegradable polymers, particularly aliphatic polyesters

obtained through ring-opening polymerization (ROP) of

cyclic (di)esters will be discussed. These synthetic biode-

gradable polymers have attracted much attention owing

to their physico-chemical properties that can be readily

tailored for more specific applications. Besides, chemical

modification and reactive melt blending of synthetic bio-

degradable polymers, as well as those derived from renew-

able resources will be discussed, again carried out through

REX for the manufacture of useful and fully biodegradable

products. Finally, a special attention will be paid to biode-

gradable polymeric composites, and particularly the

(nano)composites. The reinforcement of biodegradable

polymers using (nano)fillers, particularly layered silicates,

have recently emerged as high-performance materials

having interesting mechanical and barrier properties

achieved at low filler content (less than 5 wt.-%).[33]

Reactive Extrusion (Co)polymerization ofCyclic (Di)esters

Among the synthetic biodegradable polymers, aliphatic

polyesters such as poly(e-caprolactone) (PCL) and polylac-

tides (PLAs) have drawn a lot of interest from both the

academic and industrial media, whose potential applica-

tions cover such widely different fields as packaging for

industrial products, mulching films in agriculture, bio-

resorbable materials for hard tissue replacement and

controlled drug delivery devices.[6,34] PCL has been

thoroughly investigated because of the possibility of

blending this aliphatic polyester with a number of

miscible commercial polymers such as PVC and bisphenol

A polycarbonate. PCL is a highly hydrophobic, biodegrad-

able and semi-crystalline polyester with melting and glass

transition temperatures of ca. 60 8C and �60 8C, respec-tively.[35–37] The homopolymer of L-lactide (or D-lactide),

poly(L-lactide) (or poly(D-lactide)), is semi-crystalline with

melting and glass transition temperatures of ca. 175 8C and

60 8C, respectively. Poly(L-lactide) (PLA) is biocompatible,

and degrades by hydrolytic scission to lactic acid, which is

a natural intermediate in the carbohydrate metabolism.

High tensile strength and low ultimate elongation make

that poly(L-lactide) is rather used for producing porous

scaffolds and load-bearing applications such as in

orthopedic fixations and sutures.[38,39]

Interestingly enough, poly(1,4-dioxan-2-one) (PPDX)

appears to be an attractive candidate as a biodegrad-

able substitute for commodity polymers. This aliphatic

poly(ester-alt-ether) copolymer offers a good compromise

between its processing temperature and the service



temperature range with a melting and glass transition

temperatures of 110 8C and –10 8C, respectively.[40–44]

Aliphatic polyesters can be produced by two different syn-

thetic pathways: step polycondensation of a,v-hydroxyacids

and ROP of cyclic (di)esters. However, the traditional

polycondensation usually requires high temperatures,

long reaction time and a continuous removal of water

to recover quite lowmolecular weight polymers with poor

mechanical properties finally. In contrast, ROP provides a

direct and easy access to the corresponding highmolecular

weight polyester within a few minutes.

Both aluminum mono- and trialkoxides have shown to

be very effective initiators[45–51] to promote the ROP of

various (di)lactones such as e-caprolactone (CL) with high

selectivity (restricted occurrence of termination and

transfer reactions). This allows the preparation of high

molecular weight polyesters and a huge range of novel

macromolecular architectures both in solution and in bulk

(i.e., in absence of any solvent). Next to the aforementioned

aluminum derivatives, tin (II, IV) alkoxides and carboxy-

lates (the latter coupled with alcohols as co-initiators), and

more particularly tin(II) bis(2-ethylhexanoate)

(Sn(Oct)2)[52] have been widely used. Such commercial

catalysts can be more readily handled (i.e., do not require

high vacuum equipment), and are relatively easy to purify

(at least down to ca. 2 mol% of proton containing

impurities) by distillation for semi-quantitative synthetic

work,[49] Furthermore, Sn(Oct)2 has been approved as a

food additive by FDA. The most advocated mecha-

nism[50,51] involves a direct catalytic action of Sn(Oct)2.

Actually, Sn(Oct)2 has been first proposed to activate the

monomer forming a donor-acceptor complex, which

further participates directly in the propagation step.

Sn(Oct)2 is liberated in every act of propagation. It follows

from this mechanism that Sn(II) atoms are not covalently

bound to the polymer at any stage of polymerization.

Recently, Penczek et al. have proposed another more likely

scheme proceeding via the ‘‘active chain-end’’ mechanism.

This last one involves the in-situ formation of Sn-alkoxide

bonds at the chain-ends, as observed by MALDI-TOF and

fully confirmed by kinetic studies.[49,52] Thus, through a

rapid exchange equilibrium, Sn(Oct)2, and most probably

any other covalent metal carboxylates, are first converted

by reaction with protic compounds (ROH) into tin (or other

metal) alkoxides as active centers for polymerization

(Figure 2). The polymerization involves a ‘‘coordination-

insertion’’ mechanism similarly to the previously dis-

cussed mechanism for covalent metal alkoxides and

dialkylaluminum alkoxides (Figure 3).

It is worth noting that we have recently and succinctly

reported on ROP of cyclic (di)esters, and to some extent, to

the chemical modification of biodegradable aliphatic

polylactones through reactive extrusion processing.[53]

However, the scope of that mini-review was restricted



Figure 2. Proposed activation mechanism for catalyzed ROP ofe-caprolactone promoted by Sn(Oct)2.

452

only to our own expertise, without referring to the most

relevant results obtained elsewhere. By contrast, the

following section will summarize in a more systematic

way the main advances in REX obtained by the different

research groups that are involved in the field.

Reactive Extrusion Ring-Opening (Co)polymerizationof e-Caprolactone

PCL is an aliphatic polyester currently prepared by ROP of

CL catalyzed with stannous octanoate (Sn(Oct)2) in

the presence of heavy alcohol (initiator) such as

1-dodecanol,[54] using a batch process with a maximum

number-average (Mn) of 80 kg �mol�1, andmarketed under

the trade names ‘‘TONE’’ and ‘‘CAPA’’ by Dow Chemicals

and Solvay, respectively. However, these PCL polymers

suffer from poor processing characteristics like low

melt-strength, and for certain applications, inadequate

thermo-mechanical properties like tear strength.

Figure 3. ‘‘Coordination-insertion’’ mechanism of the ROP of CL.



Kim at al. reported bulk ROP of CL carried out both in a

laboratory internal mixer (Brabender Plasticorder) and in a

modular intermeshing co-rotating twin-screw extru-

der.[55–57] Various initiators such as titanium n-butoxide,

aluminum triisopropoxide, and sodium hydride were first

used to polymerize CL in an internal mixer. High

conversion in PCL could be effectively obtained when

aluminum triisopropoxide initiated the polymerization of

CL. This polymerization was then investigated through

reactive extrusion processing for various ratios of

monomer to initiator using aluminum triisopropoxide

(Al(OiPr)3) as initiator as well as under a range of different

processing conditions, including barrel temperature pro-

files, throughput, and screw speed. GPC analyses demon-

strated that high molecular weight PCL, together with

significant quantities of PCL oligomers were produced at

different reaction temperatures. Increasing screw speed

and decreasing the throughput caused a severe reduction

in the PCLmolecular weight once themaximummolecular

weight was obtained after the first mixing segment.

Higher molecular weight PCL produced by increasing

ratios of monomer to initiator resulted in a more severe

reduction of the molecular weight during reactive extru-

sion. This was ascribed to both specific mechanical energy

(i.e. the energy consumed per unit of mass of the material

extruded) and the molecular weight.[57,58] A more detailed

study about rheological developments for reactive proces-

sing of PCL upon both experimental and modeling aspects

has been reported elsewhere,[16–19] and has recently been

reviewed in International Polymer Processing.[60–63]

Recently, we have reported that alumi-

num sec-butoxide (Al(OsecBu)3) was a more

suitable initiator in the reactive extrusion

ROP of CL. Although Al(OiPr)3 has shown to

efficiently initiate the ROP of CL, it requires

being sublimated first, and then dissolved

in an organic solvent like toluene in order

to control the ROP of CL. In contrast,

Al(OsecBu)3 is commercially available as a

pure liquid, and therefore does not require

any previous purification step before use.

In addition, Al(OsecBu)3 has shown to be an

efficient initiator in the bulk ROP of CL

carried out in small reactors.[64,65] Inter-

estingly, when Al(OsecBu)3 was used as

initiator in reactive extrusion ROP of CL, a

well-controlled synthesis of PCL in terms of

molecular weight and polydispersity

(Mw=Mn � 1.7) was successfully obtained

at a temperature ranging from 130 to

180 8C in an intermeshing twin-extruder

with the screw configuration made up

with only conveying elements. The con-

veying elements were selected to avoid as

DOI: 10.1002/mame.200700395


Figure 4. Three-arm star-shaped PCL as prepared by ROP of CL initiated with Al(OsecBu)3in reactive extrusion.

much as possible the undesirable thermal degradation

reaction that occurs when kneading elements are used.

Like Al(OiPr)3, the polymerization of CL promoted by

Al(OsecBu)3 proceeds through the so-called ‘‘coordination-

insertion’’ mechanism, yielding polyester chains end-

capped by a growing aluminum alkoxide bond (see

Figure 3).[64,65] Due to the trifunctionality of Al(OsecBu)3,

three-arm star shaped PCL with Mn of each arm as high as

200 kg �mol�1 could be prepared with monomer conver-

sions larger than 95% and within residence times of less

than five minutes in the extruder (Figure 4). In blown film

applications, the resulting PCL displayed significant better

dart and tear properties than commercially available

linear PCL.

As potential compatibilizing agents in polyamides/PCL

blends, lactam-CL block copolymers were prepared

through reactive extrusion as well.[66,67] Continuous

copolymerization of CL with e-caprolactam (CLa) and

v-lauryl lactam (LLa) were carried out in a modular

intermeshing co-rotating twin-screw extruder. Sodium

hydride (NaH) and N-acetyl caprolactam were employed,

respectively, as co-initiators in the synthesis of lactam-

lactone copolymers. In the presence of N-acetyl caprolac-

tam, it is considered that CLa is unable to act as a faster

activator for polymerization of CL than N-acetyl capro-

lactam as described by Goodman et al.[68] In this respect,

NaH was added as a co-initiator together with N-acetyl

caprolactam for polymerization of CLa before the reaction

of sodiocaprolactam with CL. It has been demonstrated

that the order of addition is of prime importance for the

formation of blocky copolymer. Simultaneous feeding of

bothmonomers with NaH and N-acetyl caprolactam in the

first hopper of the twin-screw extruder produces amixture

of homopolymers. In contrast, both high molecular weight

P(CLa-b-CL) and P(LLa-b-CL) block copolymers have been

successfully achieved by adding the lactam (LLa and CLa)

into the first hopper and the CL sequentially into

the second hopper. The respective block lengths of the

copolymer could be adjusted by controlling the feed rate of

each monomer during reactive extrusion. To modulate the



stiffness of lactam part, lactone-lactam

terpolymers having block and random

caprolactam structure, i.e. P(LLa-b-

CLa-b-CL) and P(LLa/CLa-b-CL) have

also been prepared through the same

procedure.

For seat belt applications, new fibers

derived frompoly(ethylene terephthalate)-

block-PCL block copolymers have been

developed through REX.[69] Such fibers

provide the desired load-limiting perfor-

mance for the design of safety seat belt.

The synthesis of poly(ethylene ter-

ephthalate)-block-PCL block copolymers

was carried out by ROP of CL initiated by hydroxyl-

terminated poly(ethylene terephthalate) (PET) together

with Sn(Oct)2. A block copolymer with minimal transes-

terification reactions could be obtained in reactive

extrusion at 290 8C because of the fast distributive mixing

of CL into the high melt-viscosity PET and the short

reaction time. After fiber spinning, these PET-b-PCL block

copolymers exhibited high degree crystalline orientation.

Reactive Extrusion Ring-Opening (Co)polymerizationof L,L-Lactide

A tremendous demand is raising on the synthesis of PLA as

obtained by ring-opening polymerization of L,L-lactide (LA)

because LA derives from renewable resources, avoiding the

depletion of petrochemical resource, and the emission of

green gas (CO2). The cyclic dimer of lactic acid is recovered

after fermentation of corn or sugar beets followed by a

free-solvent polymerization/depolymerization process

(Figure 5).

Interestingly, a continuous one-stage reactive extru-

sion[70–74] has been reported by some of us on the

manufacture of economically viable PLA via ROP of LA

promoted by Sn(Oct)2. This technique requires that the

bulk polymerization be close to completeness within a

very short time (5–7 min), which is predetermined by the

residence time distribution of the extrusion system, and

that the PLA stability is high enough at the high processing

temperature. Although Sn(Oct)2 can promote quite fast

polymerization of LA, it is well-known to provide adverse

effects on both the molecular weight and properties of PLA

as a result of back-biting and intermolecular transester-

ification reactions, not only during the polymerization of

LA, but also during any further melt-processing.[75,76] In

this respect, an equimolar amount of triphenylphosphine

(P(C6H5)3) has been added to Sn(Oct)2 in order to enhance

the rate of LA polymerization significantly, but also to

suppress (or at least delay) any degradation reactions such

as transesterification reactions.[70,71] This kinetic effect has



Figure 5. PLA production via prepolymer and lactide.

454

been accounted for the coordination of the Lewis base onto

the tin atom, making easier the insertion of the monomer

into the metal alkoxide bond of the initiator/propagation

active species. This tin alkoxide bond is formed in situ by

reaction of alcohol and the tin(II) dicarboxylate, and

proceeds through the aforementioned ‘‘coordination-

insertion’’ mechanism.[50,51] The addition of one equiva-

lent of P(C6H5)3 onto Sn(Oct)2 allows reaching an

acceptable balance between propagation and depolymer-

ization rates, so that the polymerization is fast enough to

be performed through a continuous one-stage process in

an extruder.

Using a closely intermeshing co-rotating twin-screw

extruder (with a suitable processing and screw concept),

the equimolar Sn(Oct)2/P(C6H5)3 complex was used as a

catalyst system in ROP of LA yielding high molecular

weight PLA within a residence time of ca. 7 min at high

monomer conversions (ca. 98%) and at a temperature of

about 180–185 8C. Adding alcohol as (co)initiating system

could easily adjust molecular weights of as-recovered PLA.

In addition, by reducing the amount of the catalytic

complex ([LA]0/[Sn]¼ 5 000), the resulting PLA exhibited

good melt-stability during further melt processing such as

melt-spinning. For more specific applications, the

melt-stability for the resulting PLA[77,78] could be further

enhanced by adding stabilizers (Ultranox 626) during

reactive extrusion, without influencing the course of the

polymerization reaction. Interestingly enough, some of us

demonstrated that reactive extrusion processing could

enhance much more the kinetics of ROP of LA in bulk than

the conventional batch polymerization technology such as

glass reactors. Indeed, although the conversions in LA

reached the same equilibrium values (ca. 98%) whichever

the polymerization system, the time required for reaching



this monomer conversion was

approximately 40min using glass

ampoules as reactor, compared to ca.

7min in the reactive extrusion pro-

cess. At lowmonomer conversion, the

relative enhancement on polymeriza-

tion kinetics can be ascribed to the

molten temperature of reaction me-

dium inside the extruder, being a

key-parameter in reactive proces-

sing.[59] In batch polymerization,

depending on the extent of polymer-

ization, there is a temperature gradi-

ent with a bad thermal transfer for

the reaction medium, leading to

longer reaction times. By contrast,

the heat transfer is better due to high

mixing phenomena in extruders.[60,61]

However, at high monomer conver-

sion, the rate of the reaction gets

limited not only by the molten temperature and the

reactivity of the chemicals, but also by the diffusion of the

monomers (and other low molecular weight compounds)

inside the high viscous melt to find a reactive partner,

when high molecular weight polymer is reached. This

physical movement is limited to the Brownian movement

in the glass ampoule, but is well supported in the

twin-screw extruder by the mixing elements and by the

shearing of the polymer inside the intermeshing zone.

The preparation of block copolymers based on PLA has

also been carried out in a co-rotating twin-screw

extruder.[73,74] Different lengths of v-hydroxylated pre-

polymers such as PCL or poly(ethylene oxide) (PEO) as

macroinitiators enable to prepare a multitude of possible

block polymers with the same processing concept and

equipment. Other authors have attempted to prepare

multi-block copolymers based on PLA through a controlled

number of transesterifications.[79] In the first attempt,

various catalysts (nBu3SnOMe, Sn(Oct)2, Ti(OBu)4, Y(Oct)3,

and para-toluene sulfonic acid) were added to promote

these transesterification reactions of PCL against PLA and

PEO prepolymers. In blends of PLA and PCL (50:50 by

weight), the use of nBu3SnOMe was reported to catalyze

the transesterification reactions between PLA and PCL. If

2wt.-% in nBu3SnOMe was added to the blend, some

transesterification reactions occurred during reactive

extrusion, but substantial PLA degradation took place also

as evidenced by the formation of large amounts of LA

monomer (�12wt.-%). Smaller amounts in nBu3SnOMe

were not effective in promoting transesterification under

these reaction conditions. The use of the other studied

catalysts, i.e. Sn(Oct)2, Ti(OBu)4, Y(Oct)3, and para-toluene

sulfonic acid, revealed that no sufficient transesterification

reactions occurred, and in all cases the PCL was recovered

DOI: 10.1002/mame.200700395


unaffected, while PLA degradation was observed as

indicated by the formation of LA and fast decrease in

molecular weight. In this respect, Stevels et al. have

preferred to effectively prepare multi-blocky PLA-b-

PCL-b-PLA and PLA-b-PEO-b-PLA terpolymers with moder-

ate molecular weights at 180 8C by reactive extrusion ROP

of LA initiated with a,v-hydroxyl PCL and PEO in presence

of Sn(Oct)2. High yield (more than 90wt.-% in LA converted

into polymer) could be achieved within a few minutes.

Reactive Extrusion Ring-Opening (Co)polymerizationof 1,4-Dioxan-2-one

Poly(1,4-dioxan-2-one) (PPDX) obtained by ROP of

1,4-dioxan-2-one (PDX) appears to be another attractive

candidate as a biodegradable substitute for commodity

polymers. This aliphatic poly(ester-alt-ether) copolymer

offers a good compromise between its processing tem-

perature and its service temperature range because of a

melting and glass transition temperatures of 110 8C and

�10 8C, respectively.[40,41] Furthermore, PPDX has proven

to be tougher than polylactides and even HDPE with a

tensile strength close to 50 MPa for an ultimate elongation

ranging from 500% to 600%. The use of PPDX materials as

bulk materials is however restricted as a result of its low

ceiling temperature (265 8C). This favors the unzipping

depolymerization reactions from the hydroxyl end-group

in the molten state (Figure 6).

Again, like other cyclic esters such as CL, aluminum

alkoxides can efficiently promote ROP of PDX, which offers

the possibility for the synthesis of high molecular weight

PPDX through a fast and continuous process in a twin-

screw extruder.[41] Besides these kinetic considerations,

developing PPDX as biodegradable thermoplastics requires

reducing its thermal degradation essentially due to

unzipping reactions, while preserving as much as possible

the semi-crystalline properties of PPDX (high melting

temperature slightly above 100 8C). Therefore, simulta-

Figure 6. Unzipping depolymerization mechanism of v-hydroxyl PPD



neous bulk (co)polymerization of PDX promoted by

Al(OsecBu)3 (([PDX]0þ [CL]0)/[Al]¼ 2 500) was carried out

with lowmolar fractions in CL ranging from 0 to 16 mol-%

in a co-rotating twin-screw extruder at 130 8C. Indeed,random distribution of a few units of CL all along the PPDX

backbone[43] represents the best way to prevent or at least

limit undesirable unzipping reactions of PPDX chains. As

far as the homopolymerization of PDX is concerned, the

polymerization conversion did not exceed more than 65%,

corresponding to the equilibrium value obtained at 130 8C,in a good agreement with previously published data

performed in bulk and in sealed glass ampoules.[41]

Interestingly, with a limited amount in CL, the PDX

(co)polymerization yield reached the completion, attesting

for the formation of a blocky structure for the resulting

P(PDX-co-CL) copolymers. Both 13C and 1H NMR spectros-

copy indicated that multiple short PPDX homosequences

separated from each other by CL unit(s) composed the

resulting copolyester chains. As shown by TGA and DSC

experiments, this allowed enhancing significantly the

thermal stability of PPDX chains, without preventing the

crystallization of the resulting copolymers with a melting

temperature as high as 95 8C at a CL molar fraction close to

11mol-%. These relevant features open the door to the

manufacture of low cost PPDX-based materials at large

scale using a continuous one-step process.

Reactive Extrusion Modification ofBiodegradable Polymers

Chemical modification is usually employed to enlarge the

potential applications of biodegradable polymers. For

instance, to better suit their properties to specific

applications, chemical modification of starch is often

required because of the dominant hydrophilic character,

and unsatisfactory mechanical properties, particularly in

wet environment of starch. Typical examples of reactive

extrusion modification of biodegradable polymers will be

X chains.

outlined in the following sections.[18]

‘‘Grafting onto’’ Reactions ofVarious Monomersonto FunctionalBiodegradable Polymers

Over the past decades, starch, an

anhydroglucose polymer, has attracted

considerable attention as an interest-

ing structural platform for the

manufacture of sustainable and bio-

degradable plastic packaging due to

its natural abundance and low cost.[7]



456

Starch is a polymer made from anhydroglucose (C6H10O5)

units attached either by a-1,4 linkages or by a-1,6

linkages.[80] The a-1,4 linkages yield a linear homopolymer

called amylose with molecular weights ranging from

100000 to 500 000 g �mol�1, while the a-1,6 linkages are at

the origin of the branching points in the polysaccharide

structure called amylopectine with molecular weights

higher than millions (Figure 7). It is worth noting that the

proportion of both amylose and amylopectine is geneti-

cally established and is relatively constant for each species

of plant.

Due to its high functionality in hydroxyl functions,

various types of monomers have been grafted onto

unmodified starch through reactive extrusion. In paper-

making and textile applications, cationic starches have

been successfully prepared through reactive extrusion

in the presence of 3-chloro-2-hydroxypropyltrimethyl-

ammonium chloride (CHPTMA) as monomer, and of an

alkaline catalyst, yielding a quaternary ammonium cationic

starch ether [starch–O–CH2–CHOH–CH2Nþ(CH3)3].

[81] Using

a Clextral BC 45 twin-screw extruder as a reactor, a

reaction efficiency of up to 82% was obtained in extrusion

processing with wheat starch within only a few minutes.

Carr’s et al. proposed a similar study[82] with high reaction

efficiency of up to 90% or more for the system of

unmodified starch reacting with CHPTMA (molar ratios

monomer/starch of 1:2) using NaOH as catalyst in a ZSK 30

twin-screw extruder. Reaction temperature was kept at

70 8C for all experiments. The combined effect of the high

temperature (90 8C), intensive mixing, high-starch solids

(65%), and appropriate level of catalyst contributed to the

unusually high reaction efficiency values (90%), exceeding

maximum values previously reported using laboratory-

batch reaction procedures.

Carr et al. reported grafting of acrylic acid (AA) and

acrylamide (AC) onto starch free-radically promoted

by aqueous ceric ammonium nitrate (CAN), carried out

using a twin-screw extrusion.[83] Presumably, free-radicals

are formed at carbon atoms 2 or 3 in starch that are able to

initiate the polymerization with acrylic compounds as

Figure 7. Structure of amylose (a) and amylopectine (b).



evidenced by electron spin resonance. In the presence of

35 wt.-% in water, and at 80 8C, levels of add-on (wt.-% of

total synthetic polymer) of the starch grafted with

polyacrylic compounds (St-g-PAC) were of 27–44% for

the extrusion process. Interestingly, the average residence

times in the extruder were about 2–3 min, largely below

two hours, which are currently required in a batch process.

When AA was used as monomer, the saponification of

resulting St-g-PAC led to highly water-absorbent materials

for hygienic and cosmetic applications.

Cellulose, a polysaccharide constituted by anhydroglu-

cose (C6H10O5) units attached via b-1,4 linkages, is themost

abundant biomass on the surface of the earth. Cellulose

can be converted into a wide range of derivatives with

desired properties and applications. The most important

derivative from cellulose is cellulose acetate used in many

common applications including toothbrush handles and

adhesive tape backing. The common synthesis of cellulose

acetate is the processing of high-grade cellulose in the

presence of a mixture of methylene chloride and acetic

anhydride.[84] The resulting cellulose acetate is processed

into strands, sheets and films by addition of liquid

additives using the extrusion technique. Cellulose acetate

corresponds to a thermoplastic with good barrier proper-

ties to grease and oil, and is used for the packaging coating

of food. Until the mid-1990s, plastic-grade cellulose

acetates were believed to be non-biodegradable due to

their high substitution degree (SD).[84–87] Between two and

three hydroxyl groups of the glucose repeat unit are most

often acetylated. However, it has been shown that

cellulose acetates with SD up to 2.5 are biodegradable in

stimulated composting.[87] A decrease in SD from 2.5 to 1.7

results in a large increase in the rate of their biodegrada-

tion. In this respect, thermoplastic cellulose acetates are

regaining interest as potentially biodegradable plastics for

composting of plastic waste without encountering the

water-solubility problems typical for starch-based materi-

als. Low molecular weight plasticizers like phthalates,

glycerol, triacetine, or cyclic lactones are preferred for easy

processability.[88] However, their main disadvantage is

related to plasticizer migration

that can account for loss ofmecha-

nical properties. Novel families of

thermoplastic cellulose acetate

starting from cellulose-2,5-acetate

were produced through reactive

extrusion technology that grafted

cyclic lactones, simultaneously

onto polysaccharide, hydroxy-

functional plasticizer, optionally

also hydroxyfunctional fillers.[89]

Organosolv ligin, cellulose, starch,

and chitin were added to rein-

force the polymer matrix. It was

DOI: 10.1002/mame.200700395


demonstrated that cyclic lactones such as CL, glycolide,

and lactide could be successfully grafted onto the rigid

polysaccharide backbone, and hydroxyfunctional plastici-

zers in a twin-screw extruder system (length/diameter

ratio was of 48) at 190 8C. Low molecular weight

hydroxyfunctional plasticizers such as glycerol were

in situ converted into non-migrating higher molecular

weight oligopolyester plasticizer. The simultaneous graft-

ing of cellulose-2,5-acetate and hydroxyfunctional plasti-

cizer substantially improved compatibility between these

two components. These resulting oligolactone-modified

cellulose-2,5-acetate exhibited high compatibility with

fillers such as organosolv ligin, cellulose, starch, and chitin

when these latter were added in a subsequent down-

stream reactive processing.

Acid/acetyl Derivatization Carried out ontoFunctional Biodegradable Polymers

Acid derivatization of starch is a well-known technique to

obtain lower viscosity products, which are dispersible at

higher solids than one made from the native starch and

one whose dispersions are still able to be pumped and

handled.[90–93] Miladinov et al. reported the reactive

extrusion preparation of starch-fatty acid esters contain-

ing 0.01–0.03 mol levels in co-organic acid anhydrides

(acetic, propionic, heptanoic, and palmitic anhydrides)

with regard to the degree of substitution of starch, in the

presence of sodium hydroxide as catalyst.[94] Some

molecular weight reduction of the amylopectin fraction

could be detected that lowered the specific mechanical

energies for REX, particularlywhen heptanoic and palmitic

anhydrides were used as co-organic acid anhydrides.

Reactive extrusion preparation of starch esters has been

carried out using maleic anhydride (MA) as cyclic dibasic

acid anhydrides, yielding a free carboxylic group. Such a

free carboxylic group has shown to be valuable to promote

acid-catalyzed esterification reactions with biodegradable

Figure 8. Hydrolysis (a) and glucosydation (b: for sake of clarity, only thglycerol is represented) reactions present during the in situ maleatio



poly[(butylene adipate)-co-terephthalate], leading to the

formation of a graft copolymer in subsequent downstream

blending operations.[53,95] In situ reactive modification of

starch by 0–8wt.-% inMAas esterification agent and in the

presence of 20wt.-% glycerol as plasticizer has been carried

out through reactive extrusion at 150 8C. When 2.5 wt.-%

MAwas used, the recovery yield for the resultingmaleated

thermoplastic starch (MTPS), as determined by Soxhlet

extraction, was almost complete. This attests for the

reaction of some glycerol moieties to starch backbone

during the extrusion process. Increased MA content

decreased the recovery yield of resulting MTPS due to its

partial solubilization in the solvent used for Soxhlet

extraction (acetone). Besides the in situ esterification of

TPS, intrinsic viscosity, FT-IR and 2D liquid-phase NMR

spectroscopy measurements proved the occurrence of

some hydrolysis and glucosidation reactions as promoted

by MA moieties grafted onto the starch backbone. Such

reactions reduced the relative molecular weight of MTPS

(Figure 8). Therefore, the resulting MTPS had improved

processability due to its reducedmelt viscosity. Indeed, TPS

has found to display a gel-like viscoelastic behavior. This is

related to the formation of a crystalline elastic network

produced by the complexation of amylose molecules with

lipids/plasticizers, and the physical entanglement of

starch chains caused by its high molecular weight. Such

a physical entanglement is responsible for the incomplete

homogenization in the melt blend of TPS with other

polymers such as PCL, affecting the final properties for the

resulting melt-blends.[96] WAXS diffraction analyses con-

firmed the complete disruption of the granular structure of

native starch in MTPS during the reactive extrusion

processing. Tomasik et al.[90] reported a similar chemical

modification of cornstarch using MA and the like, and by

varying amounts of water (18, 20 and 30%) as plasticizer,

the whole process was carried out by extrusion. Carbonate

buffer, either at pH 8 or pH 9, was added during extrusion.

Extrusion of starch with cyclic anhydrides in alkaline

medium represented a facilemethod for the preparation of

e reaction between starch and the hydroxymethylene function fromn of TPS.



458

anionic thermoplastic starches. However, such modified

hydrophilic thermoplastic contained large amounts in

water, e.g. rendering difficult their subsequent melt

blending with hydrophobic biodegradable polymers.

Soy proteins are the first biopolymers derived from

agriculture, which have been used for the manufacture of

molded materials. Among plant protein sources, soy protein

is of relatively low cost with vast available supplies. Soy

proteins are available in three different forms as soy flour,

soy isolate, and soy protein concentrate. Soy proteins are

complex macromolecules containing 20 amino acids that

supply enough available sites to react with coupling

agents. As biomaterials, soy proteins can be masterfully

converted into soy protein plastics through extrusion with

a plasticizers.[97,98] Common plasticizers used in the

manufacture of soy protein plastics include glycerol,

ethylene glycerol, propylene glycerol, 1,4-butanediol, 1,3-

butanediol, poly(ethylene glycol) sorghum wax, and

sorbitol.[99] Thermoplastic processing of proteins with

plasticizers induces marked changes in the connectivity of

the protein network, which limits the processing window.

Cross-linking of proteins occurs in situ as a temperature-

controlled phenomenon, but high shear conditions can

significantly decrease the activation energy of this

reaction.

Interestingly, Vaz et al. used the high functionality of

soy proteins in the reactive extrusion for the design of

biodegradable soy matrix drug delivery systems.[100,101]

Glyoxal was used as a cross-linker, which has dialdehyde

functionality able to react with the free e-amine groups of

the lysine (hydroxylysine) residues of soy protein

(Figure 9). In situ cross-linking reactions of soy protein

were carried out at ca. 130 8C under rotation speed at 100

RPM, in the presence of glyoxal and of an encapsulated

drug (theophylline) through one-stage reactive extrusion

process at pH 4 and 7. The resulting drug delivery systems

offer several advantages as follows: (i) ease of production,

(ii) suitability for a large variety of polymeric matrices,

(iii) applicability to different types of drugs, and

(iv) biodegradability. The drug release patterns could be

adjusted during processing by cross-linking, changing the

net charge (effect of pH), and by a filler reinforcement such

as hydroxyapatite.

Corn gluten meal chemically modified with citric

derivatives has also been prepared in a continuous reactive

extrusion process.[102] Corn gluten meal is a mixture of

corn starch, fiber and corn protein obtained, as a

by-product, from the wet-milling of corn in the ethanol

Figure 9. Acetyl functionalization of soy proteins from e-amine gro(hydroxylysine) residues.



industry. In this study, citric anhydride was reacted with

this corn product in reactive extrusion to generate value-

added, acid-insoluble reaction products with enhanced

metal-binding properties for the treatment of industrial

wastewaters. Pendant carboxylic groups were so obtained

from citric anhydride that reacted with the different

nucleophilic groups of corn gluten meal from the part of

starch/fibers (hydroxyl functions), and corn protein

(hydroxyl, sulfhydryl, and amino groups). Interestingly,

these derivatives exhibited high metal binding ability for,

at least nine metals (Cd2þ, Co2þ, Cu2þ, Fe2þ, Pb2þ, Mn2þ,

Ni2þ, Agþ, and Zn2þ). Respirometry testing revealed that

biodegradability of corn gluten meal was unaffected after

treatment with citric anhydride.

Free-Radical Grafting of Functional Groups ontoBiodegradable Polymers through Reactive Extrusion

In order to incorporate functional pendant groups all along

the polymer backbone, free-radical grafting of unsaturated

monomers such as maleic anhydride, and acrylic acid and

its derivatives has received much attention in the reactive

extrusion technology.[24–26,30,31] The most preferred unsa-

turatedmonomer is actuallymaleic anhydride (MA) that is

useful for further compatibilization reactions with, e.g.,

hydrophilic fillers like starch granules.[103] The reason

of this interest is that MA does not readily polymerize

under the conditions employed in grafting reactions, and is

therefore grafted at high efficiency without the accom-

panying formation of homopolymer. A number of

methods are available in the literature to produce such

a graft polymer that includes melt-grafting, solid state

grafting, solution grafting, suspension grafting in aqueous

or organic solvents, and redox-induced grafting.[104] The

most widespread method is the melt state process

performed by REX, which can alleviate the difficulties

owing to diffusion-controlled grafting reactions of high

molten viscosities polymeric matrix in bulk.

Tang et al. investigated thoroughly the grafting of MA

onto biodegradable aliphatic/aromatic (co)polyesters

through reactive extrusion using dicumyl peroxide,

benzoyl peroxide, and di-tert-butyl peroxide as free-radical

initiators.[1] The effect of various factors such as free-

radical initiator concentration, MA concentration, and

reaction temperature on the percent grafting of MA onto

the copolyesters was investigated. The structures of the

grafted (co)polyesters were characterized using FT-IR and

ups of the lysine

NMR spectroscopy. It has been demon-

strated that MA can be grafted onto any

copolyesters. The grafting reaction takes

place selectively at aliphatic dicarboxylic

acid units of the copolyesters (Figure 10).

Minimal degradation of the polyester

DOI: 10.1002/mame.200700395


Figure 10. Free-radical grafting of PLA backbone.

chains was also observed from intrinsic measurements.

The desired graft content can be controlled by the

aforementioned factors, but there is, however, an opti-

mum radical concentration, which depends on the [free-

radical initiator]/[MA] ratio to promote grafting efficiency,

beyond which the chain scission and termination reac-

tions become predominant. In PLA, low grafting efficiency

was observed due to its limited availability of free-radical

sites on the polymer backbone for grafting.

Some of us have carried out a more consistent study on

the free-radical grafting reaction of MA onto the PLA

backbone using a co-rotating intermeshing twin-screw

extruder (L/d ratio of 14) at two reaction temperatures (180

and 200 8C).[104] For all experiments, 2 wt.-% MA was used,

while varying the free-radical initiator concentration,

i.e. 2,5-dimethyl-2,5-di-(tert-butylperoxy)hexane (Luperox

L101), between 0 and 5 wt.-% by PLA. Triple-detector size

exclusion chromatography (TriGPC), melt-flow index, and

thermal gravimetric analysis were used as main char-

acterization tools. Whichever the processing temperature,

increasing free-radical initiator content increases the

content of MA moieties grafted onto PLA chains, as

determined by back titration of an excess of morpholine

with HCl. However, the molecular weight for the resulting

MA-g-PLA decreased with further addition of Luperox 101

during the reactive extrusion processing. Such a behavior

is more likely due to a competition between the molecular

weight increase through chain branching, and the

molecular weight decrease by b-chain scission triggered

by grafted MA (Figure 11). In contrast, the simple addition

of Luperox 101 to the extruded PLA allows increasing its

molecular weight to some extent through a free-radical

self-branching reaction. However, highly branched PLA

chains until the formation of microgel could occur at

higher temperature as shown by TriGPC. This was more

Figure 11. Free-radical b-scission of PLA backbone.



likely due to the hydrogen radical

abstraction in a-position of the car-

bonyl groups followed by radical

addition onto the carbon–carbon

double bond of the enolate forms in

equilibrium within the polyester

chains. Additional free-radical chain

scissions might occur and participate

in the chain branching process as

well.

Interestingly, when these low-

maleated PLA (0.7 wt.-% MA) were melt-blended with

granular cornstarch (30 and 40 wt.-%) again by reactive

extrusion, improvement in the interfacial adhesion of

PLA-based composites could be successfully achieved.

These grafted reactive functions can therefore react with

the hydroxyl groups of starch macromolecules to form

covalent bonds, and thus, they provide better control of the

size of phase and strong interfacial adhesion.

Reactive Extrusion Oxidation Reactions ofBiodegradable Polymers

Commercial oxidized starches are batch-prepared at room

temperature conditions and low (3%) concentrations of

oxidant (usually hypochlorite) for adhesive applica-

tions.[105] During product isolation (filtration and aqueous

washing), some of the product is dissolved (due to

molecular breakdown) and gets lost. Hypochlorite has

been the oldest and most frequently used oxidant. Other

oxidants such as permanganate, hydrogen peroxide,

persulfate, periodate and dichromate have also been used.

The different oxidation procedures result in variations in

molecular structure and properties. Wing and Willett[106]

used the reactive extrusion to prepare oxidized starches.

Three types of cornstarches (waxy cornstarch, pearl

cornstarch, and amylomaize) were oxidized by a reactive

extrusion-drum drying process with hydrogen peroxide

and a ferrous-cupric sulfate catalyst. A ZSK 30 corotating

twin-screw extruder, which had a temperature profile of

88/115/88/77/71/65/54/48 8C (feed to die) and a screw

speed of 110 rpm, with a starch feed of 180 g �min�1 (10%

moisture) was used. Thermo-chemical oxidation of starch

by reactive peroxide extrusion-drum drying represents a

rapid, continuous procedure for making water-soluble

products containing high carboxyl

and carbonyl contents. Increasing

the peroxide level increased the

carboxyl and the carbonyl content,

whereas increasing the amylose con-

tent decreased the solubility. The

residual granule structure was still

present in high amylose starch



460

extrudates with or without peroxide. Solution viscosities

indicate a significant molecular degradation by reactive

extrusion.

Reactive Extrusion Melt Blending ofBiodegradable Polymers

Polymer melt-blending is a well-used technique whenever

modification of properties is required, because it uses

conventional technology at low cost.[107] The objective for

preparation of novel melt-blends between two or more

polymers is not to change the properties of the entire

components drastically, but to capitalize on themaximum

possible performance of the blend. Substantial efforts have

been realized in the preparation of biodegradable starch-

basedmelt-blends using the reactive extrusion technology,

due to the abundance, renewability and low cost produc-

tion of starch.[108–122] Interestingly, hydroxyl groups cause

starch to behave as an alcohol during chemical reactions

generally. This property of starch is important when

considering reactive melt blending of starch with syn-

thetic polymers. The presence of such large numbers of

hydroxyl groups affords starch hydrophilic properties, and

therefore adds affinity for moisture and dispersability in

water. However, hydrophilicity is undesirable in many

plastic packaging applications, and hence it is a major

limitation in using starch as a homopolymer. Melt-

blending starchwithmoisture resistant and biodegradable

polymer having goodmechanical properties represents the

best method to prepare useful products, while maintain-

ing the biodegradability of overall products.

Besides, fillers such as talc and kaolin are frequently

incorporated in thermoplastics to reduce the costs of

molded products.[123] These fillers improve the properties

of the polymers such as strength, rigidity, durability, and

hardness. Particularly, worldwide for at least twenty years,

there has been a new and intense desire to tailor the

structure and composition of materials applying sizes

about the nanometer. Nanofillers having at least one of

their dimension in the nanometer scale, exhibit high

specific surface areas able to significantly improve the

properties of polymeric (nano)composites at low content

in contrast to microfillers.[33]

Developing such melt-blends/(nano)composites with

satisfactory overall physico-mechanical behavior however

requires the ability to control interfacial tension, to

generate a dispersed phase of limited size and strong

interfacial adhesion, and to improve the stress transfer

between the component phases.[12] Compatibilization is

therefore called upon. Usually, this is achieved by adding

or creating in situ during the blending process, a third

component, often called an interfacial agent, emulsifier, or

compatibilizer. The latter can be a graft or a block



copolymer. Effective compatibilizers must be located at

the interface between the phase domains of the immis-

cible blend. Most importantly, the degree of compatibiliza-

tion in a particular system depends on the reactivity of the

compatibilizer used. It has been found that a compati-

bilizer is most effective when its sections are of higher

molecular weight than the corresponding blend compo-

nents. Several theories have attempted to explain the role

of compatibilizers, of which two mechanisms are con-

sidered plausible. The first mechanism is thermodynamic

in nature in that the compatibilizer reduces the interfacial

tension between the phases. The second mechanism is

kinetic in nature in that the presence of the compatibilizer

at the interface reduces the agglomeration of domains by

steric stabilization.[1,124] Recently, Macosko et al. have

reported an excellent review about reactions at the polymer-

polymer interface for blend compatibilization.[125] It has

been shown that the major factors influencing the

interfacial reaction, and therefore blend compatibilization

are: inherent reactivity of functional polymers used as

compatibilizer, thermodynamic interactions between dif-

ferent polymeric partners, functional group location along

the compatibilizer chain, and the effect of processing

flows. Concerning the last factor, flow in melt mixers is

well-known to accelerate the coupling rate tremendously,

by changing the concentration profile of functional groups

from compatibilizer at the interface and/or increase the

collision probability. However, in this case, it is often

unclear which of the mechanisms is predominately

involved under simple model flows.

Reactive Extrusion Starch-Based Melt-Blending

Starch is largely used as filler for environmentally friendly

plastics since about two decades. Although starch may be

added as filler, its more interesting and technologically

challenging uses have been in the area of using starch as a

binder, as a thermoplastically processable constituent

within thermoplastic polymer blends, and as a thermo-

plastic material by itself.[126] While native starch does not

typically behave as a thermoplastic material by itself, it is

a thermoplastic in the presence of a plasticizer when

heated and sheared.[127,128] Glycerol and water are the

most widely used plasticizers. The role of plasticizers is to

destructurize by cleaving hydrogen bonds between the

starch macromolecules, and by inducing partial depoly-

merization of starch polymers. It contributes to lower the

melting and the glass transition temperatures below its

decomposition temperature (230 8C).[13,129,130] However,

the physical properties of polyol-plasticized starch tend to

be modified after being stored for a long period because of

its re-crystallization (retrogradation), with migration of

plasticizers. To limit the retrogradation phenomenon,

amide compounds as plasticizers such as urea and

DOI: 10.1002/mame.200700395


formamide were added in the plasticized starch prepara-

tion, but the resulting materials were very rigid and

brittle.[131,132] The starch structure may be modified by,

e.g., acetylation to reduce the hydrophilic character of the

macromolecules. This hydrophobic starch acetate was

shown to be useful in starch-based extruded foams.[133–135]

However, this chemical process results again in inferior

mechanical properties and greater product cost for these

starchy materials.[136]

Therefore, some authors have preferred to melt-blend

TPS with biodegradable hydrophobic polymers such as PCL

and cellulose acetate in order to manufacture environ-

mentally friendly products.[108–119,137,138] As excepted,

melt-blending TPS with these polyesters resulted in a

significant improvement in the properties of plasticized

starch. However, although a ‘‘protective’’ polyester skin

layer is formed at the surface of most blends during

injection molding, the moisture sensitivity of TPS can be

not fully addressed.

Averous et al. prepared different compositions of wheat

thermoplastic starch and PCL through a two-stage extru-

sion in order to determine the thermo-mechanical proper-

ties of resulting melt-blends.[139] A large range of blends

was analyzed with different glycerol (plasticizer):starch

content ratios (0.14:0.54) and various PCL concentrations

(up to 40 wt.-%). Whatever the composition, it resulted a

phase-separation occurring in the melt-blend as observed

by DSC and DTMA. Depending on the content in glycerol,

two distinct behaviors could be obtained. When the starch

matrix has a glassy behavior (low content in glycerol),

extrusion blending with PCL resulted in a decrease of

the material modulus, but the impact resistance was

improved. On the other hand, when the starch has a

rubbery behavior, PCL increased the modulus of the mate-

rials. However, ageing studies showed a structural evolu-

tion for the resulting melt-blends after processing during

several weeks. A significant increase of Young’s modulus

and of the maximum strength was observed for all

melt-blends. This is due to post-crystallization and water

evolution inside melt-blends.

Other efforts have been employed to develop a coating

of TPS with hydrophobic biodegradable polyester through

reactive extrusion. Multilayer coextrusion has beenwidely

used in the past decades to combine the properties of two

or more polymers into one single multi-layered structure.

Martin et al. preparedmultilayer films based on plasticized

wheat starch (PWS) and various biodegradable aliphatic

polyesters through flat film coextrusion.[140] PLA, poly-

esteramide (PEA), PCL, poly(butylene succinate adipate)

(PBSA), and poly(hydroxybutyrate-co-valerate) (PHBV)

were chosen as the outer layers of the stratified ‘‘polyester/

PWS/polyester’’ film structure. Different levels of peel

strength were found, depending on the compatibility of

plasticized starch with the respective polyesters. In



particular, PEA presented the best adhesion to the PWS

layer, probably due to its polar amide groups. PCL and PBSA

showed medium adhesion values, and both PLA and PHBV

were the least compatible polyesters. The same trend in

the magnitude of adhesion strength was observed which-

ever the multilayer techniques. However, some inherent

problems rise due to the multilayer flow conditions

encountered in co-extrusion such as an encapsulation

and interfacial instabilities phenomena because of the

difference of hydrophilic balance between TPS and the

polyesters. For formation of in situ bonding, electron

irradiation carried out TPS layers has also been attempted,

but the results were quite unsatisfactory.

Anhydride functionalization of the biodegradable

hydrophobic polymers represents another method of

compatibilization largely utilized in the reactive extrusion

preparation of starch-based melt-blends with useful

end-properties.[1,90–93,138,141–144] For instance, MA and

dicumyl peroxide (DCP) were used as cross-linking agent

and initiator respectively for blending plasticized starch

with a biodegradable aliphatic/aromatic (co)polyesters,

called EnPol1, using a two-stage reactive extrusion process

(L/D¼ 40:1).[145] The first step was the preparation of

maleated polyester, and the resulting polyester was then

melt-blended with 40 wt.-% in starch plasticized with

glycerin as a plasticizer. It has been demonstrated that at a

peroxide initiator constant varying MA content higher

than 2wt.-%were found to be unsuitable for compounding

with starch as observed on the tensile properties for the

melt-blends. In contrast, improvement in Young’smodulus

and stress at break were achieved in all blends containing

less than 1.5 wt.-% of MA. Melt-blends with 1.0 wt.-% in

MA showed larger improvement in break elongation. This

could be explained by the improvement in interfacial

adhesion between plasticized starch and maleated poly-

esters as shown by SEM analyses.

In melt-blends between TPS and polyesters, some

authors[146] have tried to develop chemically modified

TPS as a valuable coupling agent with polyesters through a

reactive extrusion free-radical grafting process using

ricinoleic oxazoline maleate derivatives (Figure 12). Direct

insertion of the fatty acid oxazoline moiety into the starch

backbone represents a suitable way of hydrophobic

modification for the polysaccharide, but also the bifunc-

tionality of the grafting agent provides the opportunity for

reactive coupling of the modified starch with hydrophobic

polymers such as polycondensates with small amounts of

reactive groups, e.g. free carboxylic acid functions. In a

mini-scale extruder, the grafting reaction was promoted

by bis(a,a-dimethylbenzyl) peroxide as free-radical initia-

tor in high grafting yields (between 1 and 30 wt.-% to

starch) for potato starch plasticized with 20 wt.-% in

glycerin. From torque measurement, the grafting was

completed within 15 min. The presence of oxazoline



Figure 12. Reaction scheme for the radical grafting of starch with ricinoleic oxazoline maleate.

Figure 13. Proposed coupling reaction between oxazoline groups and free acid groups inbiodegradable PBAT.

462

moieties onto starch backbone was proved by 1H NMR and

FT-IR spectroscopy.

Subsequently, coupling reactions were carried out from

these TPS derivatives with poly[(butylene adipadate)-

co-terephthalate] (PBAT), biodegradable a,v-hydroxy-

carboxy-polycondensate, again in a laboratory extruder

(Figure 13). PBAT is a biodegradable aromatic/aliphatic

copolyester obtained by polycondensation from tere-

phthalate acid, adipic acid, and 1,4-butanediol, wherein

themaximum amount of terephthalate acid in copolyester

is close to 40 wt.-%, enhancing its mechanical strength,

while retaining the biodegradability of the resulting

copolyester (Figure 14).

Interestingly, a uniform blend could be obtained with

TPS chemically modified by 10% ricinoleic oxazoline

maleate derivatives as shown by SEM. In contrast, the

polymer obtained from untreated TPS exhibits a well-

separated phase. Interestingly, the tensile properties of

films obtained from the PBAT/TPS grafted with 10%

ricinoleic oxazoline maleate derivatives melt-blends

(50:50) were enhanced. Biodegradation studies did not

show an impairment of the degradation values compared

to the unmodified TPS/PBAT melt-blend.

However, although many have attempted for years to

discover the ‘‘perfect’’ starch/polymer blend that would

Figure 14. Synthesis of PBAT.



yield an environmentally sound

polymer while, at the same time,

fulfilling desired mechanical and

cost criteria, such a combination

has been difficult to achieve. The

reason for this is that the empha-

sis has been on finding the opti-

mal polymer or mixture of poly-

mers and other admixtures in

order to ‘‘optimize’’ the properties of the starch/polymer

blend thereby.

Furthermore, a major issue that most authors have not

addressed yet, is that the morphology, particularly the size

of dispersed domain, and thus the mechanical properties

for the resulting melt-blend of both immiscible partners, is

strongly affected by their difference in melt-viscosity. It

is very critical since starch,[141] and also TPS, exhibits high

melt-viscosity related to its high molecular weight

(ranging from 100 000 to 500 000 g �mol�1 and more than

millions, respectively, for the amylose and amylopectin).

As a consequence, at least 20% plasticizer is required in the

extrusion of TPS-based melt-blends at temperatures of ca.

130 8C.[127]

In contrast to this approach, we have originally prepared

through reactive extrusion a novel in situ chemically

modified TPS, so called maleated TPS (MTPS) with

improved processing and high reactivity in blown film

applications.[142] As aforementioned, MTPS was prepared,

through reactive extrusion processing of starch in the

presence of glycerol as plasticizer, and of MA as

esterification agent. In addition to derivatization of starch

backbone with MA, the MA moieties grafted onto the

starch backbone could promote some hydrolysis and

glucosidation reactions that reduced the relative molecu-

lar weight of MTPS. This reduction

in molecular weight is a major

issue in the melt blending of TPS

with its polymeric partner that

most of the authors have not

addressed. In addition to reduced

melt-viscosity, the interest in

using MTPS is the presence of free

carboxylic acid groups, i.e. MA

moieties grafted onto the starch

backbone, which could promote

DOI: 10.1002/mame.200700395


acid transesterification reactions with polyester chains

such as PBAT, leading to graft copolymers[147–149]

(Figure 15). We selected PBAT as a good candidate for

melt-blending with starch-based products due to its

interesting thermal and mechanical properties.[138]

As far as MTPS/PBAT melt-blends are concerned, effect

of polyester and MA contents was studied on the

physico-chemical parameters for the resulting melt-

blends, again prepared through a downstream extrusion

operation. For high polyester fractions, i.e., 60 and 70wt.-%,

PBAT-g-MTPS graft copolymers were obtained through

transesterification reactions promoted by the MA-derived

acidic moieties grafted onto the starch backbone. This was

determined by selective Soxhlet extraction experiments

and FT-IR spectroscopy analyses. At lower polyester

content, no significant reaction occurred more likely due

to an inversion in phase morphology between PBAT and

MTPS. Interestingly, tensile properties of blown films

derived from the PBAT-g-MTPS graft copolymer containing

70 wt.-% polyester, were much higher than those of the

melt-blend of TPS and PBAT performed in the presence of

MA. This difference in mechanical performances resulted

from the low melt-viscosity of MTPS, yielding a finer

morphology of the dispersed phase in the continuous PBAT

matrix, together with an increased interfacial area for the

grafting reaction. Thiswas attested by ESEM. IncreasedMA

content in the preparation of MTPS did not affect the

tensile values, suggesting that the entanglement of PBAT

and MTPS chains, responsible for these values, did not

change after reactive extrusion melt blending. Moreover,

WAXS diffraction analyses evidenced that the MTPS

crystalline structure was completely disrupted in the

‘‘PBAT-g-MTPS’’ reactive blends suggesting the grafting

Figure 15. Proposed mechanism of transesterification reactions betweby the MA-derived acidic moieties grafted onto the starch backbone



reaction/homogenization of the MTPS in the polyester

continuous phase.

Biodegradable Polymeric (Nano)composites Preparedthrough Reactive Extrusion

By definition, a composite material is formed by the

combination of different phases, which have distinct

structural and chemical compositions, leading to a synergy

of physical, chemical and/or mechanical properties

compared to each component taken separately. As

reinforcing elements, one can distinguish fibers, particles,

clays, minerals, and so on. In addition, the matrix can

include various types of materials: organic, mineral, and

metallic. The classification of composites is based on

either the shape of the filler (fibers or particles) or on the

matrix.

In recent years, the use of natural/bio-fiber reinforced

composites has rapidly expanded due to the availability of

natural/bio-fiber derived from annually renewable

resources, as reinforcing fibers in both thermoplastic

and thermosetting matrix composites as well as for the

positive environmental benefits gained by such materi-

als.[150,151] Most of research is also being conducted on the

potential of natural fibers as reinforcement for polymers,

because natural fibers have low density, acceptable

specific strength properties, easy preparation, and biode-

gradability.

Fully green biocomposites were prepared from PLA and

recycled cellulose fibers (from newsprint) by extrusion

followed by injection molding processing.[17] The physico-

mechanical and morphological properties of the result-

en MTPS and PBAT promoted.

ing composites were investi-

gated by varying the amount

of cellulose fibers. Compared

to the neat resin, the tensile

and flexural moduli of the

composites were significantly

higher because of the high

modulus provided by cellu-

lose. Increase in stiffness of

resulting composites was also

confirmed by DMA analyses.

The authors claimed that PLA/

natural fiber composites have

mechanical properties of suf-

ficient magnitude to compete

with conventional thermo-

plastic composites. However,

a significant decrease in ten-

sile strength at high amount

of fibers could be observed,

because of the lack of interac-



464

tion between cellulose and PLA. Cellulose has a strong

hydrophilic character due to three hydroxyl groups per

monomeric unit in contrast to the rather hydrophobic PLA.

Wood fiber was used as a reinforcing fiber in the reactive

extrusion preparation of corn gluten meal-based compo-

sites.[152] Corn gluten meal mainly contains oil (4%), starch

(20%), zein protein (60–70%), which is an alcohol-soluble

protein extracted from corn that has low water resistance.

Homogeneous dispersion ofwood fiber in corn glutenmeal

could be obtained through reactive extrusion in the

presence of glycerol, ethanol, and water as plasticizers.

The mixture of ethanol/water was an excellent plasticizer

to disrupt the intermolecular interaction of zein, leading to

the improvement of melt-mobility. Corn gluten meal-

based composites were successfully prepared with 10–

50wt.-% wood fiber. It is worth pointing out that the

melt-viscosity of the medium increased with increasing

wood fiber content, and with a decreasing water content,

which led to a decrease of melt-mobility. From the flexural

testing, it has been demonstrated that the flexural

strengths of these biocomposites increased after the

addition of 10–30 wt.-% wood fiber, but decreased by

the addition of 40–50 wt.-% wood fiber. These results are

ascribed to the difference in interfacial tension between

wood fiber and corn gluten meal. Morphological studies

revealed that breaking occurred in the matrix for these

bio-composites at high content of wood fiber (30 wt.-%),

breaking occurred at the interface of the fiber and the

matrix.

Chemically pre-treated natural fibers were also

employed in reactive extrusion in order to enhance the

interfacial adhesion between natural fibers and polymers,

and hence the properties of resulting composites. Indian

grass fiber reinforced soy based biocomposites were

accordingly fabricated using twin-screw extrusion and

injection molding technology.[153] Using extrusion tech-

nology, soy protein can be masterfully converted to soy

protein plastics.[154] However, soy protein plastic products

tend to have lower strength and higher moisture

absorption. Currently, biodegradable polymers in the

melt-blend of soy protein plastic are used to overcome

these drawbacks, including polyester amide and PCL,

whose processing windows match that of soy protein

plastic. Liu et al. incorporated PBAT with the soy protein

polymer to form a soy-based bioplastics.[153] To get higher

strength and modulus materials from soy based bioplas-

tics, raw Indian grass and alkaline-treated Indian grass

were added as natural fibers. Alkaline treatment

removes the fraction of lignin contained in this fiber,

and reduces its size. Tensile and flexural properties as well

as the heat deflection temperature of soy-based bioplastics

were improved, but the impact strength of the biocompo-

sites did not improve after reinforcement with raw Indian

grass fiber. The impact fracture of raw Indian grass fiber



reinforced biocomposites was found to occur on the outer

surface of the fiber, due to intrinsic differences in the

morphological structure between the outer and inner

surfaces of the grass fiber as shown by ESEM. Interestingly,

the alkali solution treated Indian grass fiber significantly

increased the tensile strength and impact strength as well

as the flexural strength due to the improved dispersion of

the fiber in thematrix, and the enhanced aspect ratio of the

fiber.

Other authors have preferred to melt-blend biodegrad-

able polymers and natural fibers by adding maleic

anhydride-modified polymers as a compatibilizer. Nitz

et al. reported the melt-compounding of PCL with wood

flour and lignin in a Werner & Pfeiderer twin-screw

extruder.[155] Wood flour contains about 25 wt.-% lignin,

which together with cellulose forms the structural

component of trees and various plants. As a cheap

phenolic biopolymer, lignin offers attractive potential as

filler and additive, especially with respect to the modifica-

tion of biodegradable polymers. Because of the presence of

phenolic groups in lignin, it is expected that lignin fillers

can influence both oxidative stability and biostability of

such compounds. Reactive extrusion technology was

carried out to prepare several families of thermoplastic

PCL compounds containing wood flour and lignin in the

presence of MA-grafted PCL as a compatibilizer. In the first

step, grafting of MA onto PCL was performed with a

reactive extrusion process in the presence of Luperox 101

as free-radical initiator. Appropriate MA/peroxide ratios

led toMA-grafted PCLwith aMA content close to 1.44wt.-%

as determined by titration. When a low amount of this

MA-grafted PCL (less than 2.5 wt.-% of overall product) was

added, attractive properties could be obtained in these

PCL-based composites reinforced with wood flour and

lignin. For a PCL composites containing 2.5 wt.-% in

MA-g-PCL and 40 wt.-% wood flour, Young’s modulus

increased by 450%, and the tensile strength increased by

115% in comparison with the properties of neat PCL. The

mechanical properties of the wood flour composites were

much better as those of the lignin-based compounds.

More than 70 wt.-% lignin was added without mechanical

properties being impaired, while compositions containing

40 wt.-% lignin showed break elongation exceeding 500%.

According to morphological studies, very effective lignin

dispersion was achieved within the PCL matrix due to

better compatibilization provided by MA-g-PCL. Interest-

ingly, biodegradation studies revealed that the addition of

lignin enhanced the biostability of PCL compounds,

affording to enhance the lifetime of PCL-based compounds

in outdoor applications.

Inorganic fillers have also been utilized in the prepara-

tion of biodegradable polymeric composites. Talc is a

common filler used for the improvement of properties of

the polymers such as strength, rigidity, durability, and

DOI: 10.1002/mame.200700395


hardness. Talc has a plate-like geometry in which an

edge-shared octahedral sheet of Mg(OH)2 is sandwiched

between tetrahedral sheets of silica (SiO2). A bacterial

polyester, i.e. PHBV has been melt-compounded with

different talc weight contents (15–50 wt.-%) through

extrusion combinedwith an injectionmolding process.[156]

PHBV belongs to the family of polyhydroxyalkanoates,

which are biocompatible and biodegradable thermoplas-

tics with potential applications in different fields like

agriculture, marine, and medicine. The synthesis of

polyhydroxyalkanoates occurs normally, when there is

an excess of carbon and energy and limitation of at least

one nutrient (N, P, Mg, Fe, and so on) needed for

microorganism growth. When PHBV was melt-blended

with talc, moderate to significant improvements in the

tensile, flexural, and storage moduli of talc-filled PHBV

were obtained compared to those of neat PHBV. This can be

explained by the poor filler dispersion and filler-matrix

adhesion as revealed by scanning electron microscopy in

the talc-filled PHBV composites.

We reported the preparation of new biodegradable and

high-performance talc/PBAT hybrid materials through

reactive extrusion in blown films applications.[157] In the

first step, the polyester backbone was reactively modified

through free-radical grafting of MA in order to improve

the interfacial adhesion between PBAT and talc. The

resulting MA-g-PBAT was then reactively melt-blended

with talc through esterification reaction of MA moie-

ties grafted onto the polyester chains with the silanol

present at the edge surface of talc.[158,159] Sn(Oct)2 and

4-dimethylaminopyridine (DMAP) were studied as ester-

ification catalysts (Figure 16). The interfacial adhesion

between both partners was substantially enhanced as

evidenced by SEM and selective extraction of the polyester

part. As a result, the biaxial tensile properties measured on

blown films prepared from these compatibilized compo-

sites were considerably improved, as compared to those of

the conventional PBAT-talc melt-blends. Extrapolation to a

one-step reactive extrusion process was successfully

achieved by preparation of in situ chemically modified

PBAT-talc compositions containing up to 60 wt.-% talc.

Interestingly, the highest tensile properties were obtained

Figure 16. Surface grafting mechanism of MA moieties onto the silanolof talc.



by melt-blending 50 wt.-% of native PBAT and 50 wt.-% of

such a chemically modified PBAT/talc hybrid filled with

60wt.-% of talc, therefore used as a masterbatch. Such an

approach allowed reducing the degradation of native

polyester chains through undesirable reactions such as

b-scission and transesterification reactions promoted,

respectively, by the MA free-radical treatment and

esterification catalysts used along the reactive extrusion

process. Finally, X-ray photoelectron spectroscopy mea-

surements carried out on the reactivelymodified PBAT-talc

compositions, e.g. containing 60wt.-% talc, attested for the

formation of covalent ester bonds between the silanol

functions available at the edge surface of talc particles, and

the maleic anhydride moieties grafted onto the polyester

backbones.

Recently, polymer-layered silicate nanocomposites have

emerged as a new class of organic-inorganic materials that

have shown unexpected properties such as large increase

in the thermal stability, mechanical strength, and imper-

meability to gases such as water and oxygen. Such

improvements in their properties achieved at low content

in layered silicate (<5 wt.-%) are relied on the interactions

between the clays and polymers, which can yield inter-

calation and/or exfoliation structures. In the intercalated

‘‘hybrid’’ structure, a monolayer of extended polymer

chains is sandwiched between the silicate sheets, resulting

in a well-ordered multilayer of alternating polymer and

inorganic sheets. In the exfoliated (or delaminated)

nanostructure, the silicate nanolayers are individually

dispersed in the polymer matrix. Exfoliation of the silicate

layers usually provides nanocomposite materials with

the highest improvement in properties aforemen-

tioned.[160–167]

Most of works have focused on the development of

(nano)composites based on aliphatic polyesters. Aliphatic

polyesters are among the promising materials for the

production of high-performance, and environment-

friendly biodegradable plastics. However, gas-barrier

properties, melt-viscosity for further processing, etc. are

not often sufficient for various end-use applications.

Okamoto et al. reported detailed studies about the

structure-property relationship in designing desired prop-

functions at the edge

erties for layered silicate nanocom-

posites PLA.[168–170] These PLA/

layered silicate nanocomposites were

prepared by melt-extrusion from

montmorillonite organicallymodified

with octadecylammonium (MMT-C18),

wherein silicate layers of the clay

were intercalated and randomly dis-

tributed in the matrix. MMT-C18 was

prepared by cation-exchange be-

tween octadecylammonium and the

naturally occurring Cloisite Naþ.



466

Incorporation of very small amounts of oligoPCL as

compatibilizer led to better parallel stacking of the silicate

layers, and also much stronger flocculation due to the

hydroxylated edge-edge interaction of the silicate

layers.[168] The PLA/layered silicate nanocomposites exhib-

ited a remarkable improvement of materials properties in

both solid (loss and storage moduli) and melt-states

(melt-viscosity) compared to the matrix without clay.

Other types of organically modified layered silicates

(synthetic fluorine mica, synthetic fluorine mica modi-

fied with N-alkyl-N,N- [bis(2-hydroxyethyl)-N-methyl-

ammonium, and smectite) have also been developed by

Okamoto et al. in reactive extrusion.[169,170] It has been

demonstrated that intercalated, exfoliated, and a mixture

of both structures were achieved in the PLA/layered

silicate nanocomposites. Interestingly, better dispersion

and thus better barrier properties for PLA/layered silicate

nanocomposites could be achieved when smectite was

used as nanoclays. In addition, all nanocomposites

exhibited a remarkable improvement of various materials

properties with simultaneous improvement in biodegrad-

ability compared to neat PLA. Same efforts were provided

by the authors in the preparation of polybutylene

succinate (PBS)/layered silicate nanocomposites through

an extrusion process.[171–173] PBS is chemically synthesized

by polycondensation of 1,4-butanediol with succinic acid.

In all (nano)composites, intercalated structures were

obtained, with a remarkable improvement in tensile

properties, thermal stability, and biodegradation of

resulting PBS-based (nano)composites.

New (nano)hydrids were developed from PCL through

reactive extrusion. Organically layered double hydroxide

was used as nanoclay.[174] Although a very low content of

filler was added into the PCL matrix, no good dispersion,

particularly exfoliation was obtained. However, some

mechanical and physical properties were improved with

respect to neat PCL.

Nanocomposites were made from natural biodegrad-

able polymers chemically modified or not. For instance, to

limit the retrogradation phenomenon of plasticized starch,

green (nano)composites were successfully prepared

through reactive extrusion from activated montmorillo-

nite and thermoplastic cornstarch.[175] The thermoplastic

cornstarch was plasticized with urea, ethanolamine as

plasticizers (one equivalent with respect to starch), but

also with natural montmorillonite activated by ethanol-

amine. Exfoliated structures could be so obtained as

attested by WAXS analyses. TEM and SEM images showed

that the resulting nanocomposites presented reticulating

fiber structure after rapid cooling in liquid nitrogen.

The mechanical properties of (nano)composites evi-

dently improved such as tensile stress, and Young

modulus, but also their thermal stability and water

resistance.



‘‘Green’’ nanocomposites from cellulose acetate were

also prepared through reactive extrusion. Eco-friendly

triethyl citrate was used as a plasticizer, while Cloisite 30B

as organically modified montmorillonite with a methyl

tallow bis(2-hydroxyethyl) quaternary ammonium and

maleic anhydride grafted cellulose acetate butyrate

(CAB-g-MA) were used as organoclay and as compatibili-

zer, respectively.[176,177] CAB-g-MA was previously pre-

pared through reactive extrusion by grafting MA onto

cellulose acetate butyrate as promoted by Luperox 101. The

objective of adding CAB-g-MA was that the MA moieties

react with the hydroxyl functions from Cloisite 30B.

Nanocomposites containing from 0 to 7.5 wt.-% in Cloisite

30B prepared with 5 wt.-% of CAB-g-MA (MA content was

of 0.86 wt.-%) exhibited the best morphology, i.e. the

complete exfoliation of nanoclay within the matrix, but

also the best mechanical properties in terms of tensile and

flexural properties.

In melt-blends derived from hydrophilic plasticized

starch and hydrophobic biodegradable polyester, layered

silicates were also added to improve the compatibility

between both partners. Ikeo at al.[178] developed nanoclay

reinforced biodegradable plastics of PCL/plasticized starch

blends through reactive extrusion. Starch was first

plasticized with glycerin and water, following by its melt

blending with PCL. Maleic anhydride grafted PCL as

recovered through reactive extrusion was added as a

compatibilizer, enabling to improve the compatibility

between thermoplastic starch and PCL. Significant

improvement could be however obtained when natural

montmorillonite (Cloisite Naþ) was added as nanoclay.

Interestingly, the nanoclay acted positively in these

melt-blends after their electron irradiation as attested

by the increase in their modulus and yield stress by

more than 50 wt.-% without any reduction in their

elongation.

In situ REX processing was performed in the preparation

of plasticized starch/PCL nanocomposites.[179,180] Native

wheat starch (ca. 60 wt.-%), PCL (ca. 40 wt.-%), glycerin as

plasticizer, organo-modifiedmontmorillonite, and Fentons

reagent (H2O2 and Fe2þ from ferrous sulfate) were

extruded in a conical twin-screw micro-extruder at

120 8C, and injection-molded at 150 8C. Native starch

was partially oxidized by the peroxide, enabling ester

groups from PCL to cross-link with carbonyl and/or

carboxyl groups as generated from oxidized starch through

a peroxide-initiated free process. Ferrous ion catalyzes the

decomposition of H2O2 into highly reactive hydroxyl

radical that initiates this free-radical chain reaction

process. Addition of 3wt.-% organo-modified clay (MMT-C18)

in this chemically modified starch/PCL blends increased

elongation almost fourfold over that of unmodified starch/

PCL blends. Better solvent-resistance properties were also

achieved.

DOI: 10.1002/mame.200700395


Biodegradable nanoscale-reinforced starch-based pro-

ducts were also prepared from an in situ chemically

modified thermoplastic starch and PBAT through reactive

extrusion. Four nanoclays were employed in this study:

hydrophilic Cloisite Naþ, organophilic Cloisite 30B, Ben-

tone 111, and Bentone 166.[181,182] First, thermoplastic

starch was in situ chemically modified in the presence of

nanoclay previously swollen in glycerol as plasticizer

together with MA. Melt blending of these nanoscale-

reinforced MTPS with PBAT was carried out in the

subsequent downstream blending operation. It is worth

noting that the swelling treatment was beneficial in order

to get a better dispersion of nanoclays within the

starch-based melt-blends. As shown previously, the MA

moieties thus grafted onto the starch backbone were able

to promote some glucosidation and hydrolysis reactions

during the preparation of MTPS, reducing the molecular

weight of native starch. This allows a better interpenetra-

tion of the resulting starch ester into the nanoclay

galleries, togetherwith a beneficial swelling pre-treatment

of nanoclay in glycerol. Interestingly enough, the resulting

formulations exhibited superior tensile strength and high

elongation at break, particularly with Cloisite 30B in

blown film applications. In this case, the presence within

the clay galleries of a quaternary ammonium ion bearing

two primary hydroxyl groups could form strong hydrogen

bond interactions with the MA-derived acidic moieties

grafted onto the starch backbone in the MTPS and to some

extent, to react with MTPS and PBAT through transester-

ification reactions promoted by MA. Within the

PBAT-g-MTPS graft copolymer-cloisite 30B nanocomposite,

WAXS and TEM analyses attested for the partial exfolia-

tion of some clay platelets. As a result, water vapor and

oxygen barrier properties of nanoscale-reinforced MTPS-g-

PBAT nanocompositions were enhanced as compared to

the precursors.

Abbreviations

AA acrylic acid;

Concluding Remarks and Outlook

Reactive extrusion is a versatile tool for cost-effective

polymer processing, which enhances the commercial

viability and cost-competitiveness of these materials, in

order to carry out melt-blending, and various chemical

reactions including polymerization, grafting, branching

and functionalization as well. For instance, the obvious

advantages of reactive extrusion polymerization process

are as follows:

AC acrylamide;

Al(OiPr)3 aluminum isopropoxide;
– S
Ma

�

olvent free-melt process

Al(OsecBu)3 aluminum sec-butoxide;
– C
CAN ceric ammonium nitrate;

ontinuous processing, starting from monomer, and

resulting in polymer or finished product

CHPTMA 3-chloro-2-hydroxypropyltrimethylammo-
– C
nium chloride;

ontrol over residence time and residence time dis-

tribution

cromol. Mater. Eng. 2008, 293, 447–470

2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

– In
tegration of other extrusion streams along with the
polymerization process

Environmental concerns have triggered many efforts in

the development of environmentally friendly plastics, i.e.

biodegradable polymers tominimize thewaste disposed in

landfills. However, the use of these biodegradable poly-

mers as bulk materials is still restricted by their relatively

high production cost and poor mechanical properties com-

pared with commodity plastics such as polyethylene.

This review has hence outlined the substantial research

and the development that have been undertaken in the

realm of biodegradable polymers using high-performance

continuous reactive extrusion. When combined to both an

adapted chemistry such as the right selection of the

catalytic system, and a fine selection of extrusion para-

meters, reactive extrusion has demonstrated a remarkable

ability in the synthesis of biodegradable polymers through

ring-opening polymerization using effective initiating

systems, chemical modification of biodegradable polymers

and reactive melt-blending of natural polymers with

aliphatic (co)polyesters. Biodegradable polymers-based

(nano)composites have effectively been prepared from

common fillers such as wood fiber, but also using layered

silicates as nanoclays through reactive extrusion. What-

ever reactive melt-blending, biodegradable polymeric

melt-blends/composites with satisfactory overall physico-

mechanical behavior however require the ability to control

interfacial tension, to generate a dispersed phase of limited

size and strong interfacial adhesion, and to improve the

stress transfer between the component phases, i.e. the

reinforcement of their interface through the formation of

strong covalent bonds. This can be effectively completed

using proper interface compatibilization between differ-

ent components, as reactive extrusion is able to provide

during the reactive processing of these biodegradable

polymeric melt-blends/composites. The reactive extrusion

technology serves on the sustainability and future growth

of biodegradable polymers, particularly in the realm of

compatibilizing mechanisms, surface modifications, and

advanced processing techniques, and it is through an

understanding of these points that they are expected to

replace more and more commodity plastics.



CL e-caprolactone;CLa e-caprolactam;

DCP dicumylperoxyde;

DMA dynamic mechanical analyzer;

DMAP 4-dimethylaminopyridine;

DSC differential scanning calorimetry;

DTMA dynamic thermo-mechanical analysis;

ESEM environmental scanning electronic micro-

scopy;

FT-IR Fourier Transform Infrared;

LA L,L-lactide;

LLa v-lauryl lactam;

Luperox 101 2,5-dimethyl-2,5-di-(tert-butylperoxy)hex-

ane;

MA maleic anhydride;

MMT-C18 montmorillonite organicallymodifiedwith

octadecylammonium;

NaH sodium hydride;

NMR nuclear magnetic resonance;

REX reactive extrusion;

ROP ring-opening polymerization;

PBAT poly[(butylene adipate)-co-terephathalate];

PBS poly(butylene succinate);

PBSA poly(butylene succinate adipate);

P(C6H5)3 triphenyl phosphine;

PCL poly(e-caprolactone);PEA polyesteramide;

PEO poly(ethylene oxide);

PHBV poly(hydroxybutyrate-co-valerate);

PLA polylactide;

PDX 1,4-dioxan-2-one;

PPDX poly(1,4-dioxan-2-one);

SD substitution degree;

SEM scanning electronic microscopy;

Sn(Oct)2 tin (II) bis(2-ethylhexanoate);

TriGPC triple-detection gel-permeation chromato-

graphy;

WAXS wide angle X-ray scattering.

468

Acknowledgements: This research was partly funded by CornProducts International. The authors are very grateful to ‘‘RegionWallonne’’ and the European Community (FEDER, FSE) for generalsupport in the frame of ‘‘Objectif 1-Hainaut: Materia Nova’’. Thiswork was partly supported by the Belgian Federal GovernmentOffice of Science Policy (SSTC-PAI 6/27).

Received: December 6, 2007; Revised: January 30, 2008; Accepted:January 30, 2008; DOI: 10.1002/mame.200700395

Keywords: biodegradable polymer; chemical modification; melt-blending; nanocomposites; reactive extrusion; ring-openingpolymerization



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DOI: 10.1002/mame.200700395

Recent Advance Reactive Extrusion in Polymer

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