Development of Lightweight, Biodegradable Plastic Foam

Fibres with Poly (Lactic) Acid-Clay Nanocomposites

by

Mo Xu

A thesis submitted in conformity with the requirements for the degree of Master of Applied Science

Department of Mechanical and Industrial Engineering University of Toronto

© Copyright by Mo Xu 2013

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Development of Lightweight, Biodegradable Plastic Foam Fibres with Poly (Lactic) Acid-Clay Nanocomposites

Mo Xu

Master of Applied Science

Department of Mechanical and Industrial Engineering

University of Toronto

2013

ABSTRACT

Polymeric fibres influence our everyday life in numerous aspects; the area of applications ranges

from industrial to everyday commodities, textile and non-textile. As the global demand for the

polymeric fibres increases rapidly, new innovative classes of fibres and the manufacturing

processes are sought after. This thesis develops an approach to produce fine cell structure and

low void fraction foams, which is then used in the manufacturing of lightweight, biodegradable

foam fibres. Poly (lactic) acid-clay nanocomposite have been foamed with nitrogen and drawn to

different melt draw ratio to produce foam fibres. The foam fibres are then characterized for

crystallinity, Young’s modulus and the yield stress. While the drawability of foam has been

demonstrated, the crystallinity as well as the mechanical properties of the foam fibres are not

drastically enhanced by drawing, as would be expected. Further drawing processes of the as-spun

foam fibres are recommended.

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In loving memory of my father

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ACKNOWLEDGMENT

Words cannot begin to describe my gratitude and appreciation towards those who have directly

and indirectly helped me throughout my master’s study. Without their help and support, this

learning experience would not have been possible.

I would like to begin by expressing my gratitude to my supervisor, Professor Chul B. Park, for

his valuable supervision and guidance on my research activities, as well as his tremendous

mentorship and support in my personal life. His encouragement helped me through some of the

most difficult times.

I would also like to thank Professor Hani Naguib and Professor Edmond Young for serving on

my exam committee and providing valuable comments and feedbacks.

Special acknowledgement goes to Jed Randall and NatureWorks LLC for donating the

experimental materials, as well as Bill Huang and Ingenia Polymers Corp. for the technical

support provided.

I have had the privilege to work with many talented colleagues in the Microcellular Plastics

Manufacturing Laboratory. Not only are their wisdom and technical advices extremely helpful to

my research work, their friendship made the hard days much easier to get by. Many many thanks

goes out to Prof. Takashi Kuboki, Dr. Saleh Amani, Dr. Amir Ameli, Dr. Reza Barzagari, Dr.

Yanting Guo, Dr. Peter Jung, Dr. Babu Adhikary Kamal, Dr. Mehdi Keshtkar, Dr. Anson Wong,

Dr. Changwei Zhu, Seong-Soo Bae, Eunse Chang, Raymond Chu, Weidan Ding, Mohammed

Hasan, Davoud Jahani, Kamlesh Katihya, Ryohei Koyama, Esther Lee, Sam Lee, Hasan

Mahmood, Tero Malm, Lun Howe Mark, Nemat Hossieny, Reza Nofar, Ali Rizvi, Mehdi Saniei,

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Vahid Shaayegan, Alireza Tabatabaei, Hui Wang, Sai Wang, Stephan Wijnands, Hongtao Zhang,

Anna Zhao. I would like to extend my sincere gratitude to Kara Kim, Konstantin Kovalski,

Brenda Fung, and Jho Nazal for their assistance with various administrative issues.

Last but not the least; I owe a big thank you to all of my family who has always believed in me:

my father and mother who inspired and encouraged me throughout the years, and my loving

fiancé, Tongtong, for the unconditional love and support. Without their inspiration, this long

journey would not have been possible.

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TABLE OF CONTENT

ABSTRACT .............................................................................................................................................................. II

ACKNOWLEDGMENT ............................................................................................................................................. IV

TABLE OF CONTENT .............................................................................................................................................. VI

LIST OF TABLES ..................................................................................................................................................... IX

LIST OF FIGURES .................................................................................................................................................... IX

CHAPTER 1 INTRODUCTION ............................................................................................................................... 1

1.1 PREAMBLE .............................................................................................................................................................. 1

1.2 OVERVIEW OF PLASTIC FOAMS ................................................................................................................................... 1

1.3 ANTICIPATED CHALLENGES FOR THE MANUFACTURING OF FOAM FIBRES............................................................................. 2

1.4 OBJECTIVE OF THE THESIS .......................................................................................................................................... 3

1.5 ORGANIZATION OF THE THESIS .................................................................................................................................... 4

CHAPTER 2 LITERATURE REVIEW AND THEORETICAL BACKGROUND ................................................................. 6

2.1 INTRODUCTION ........................................................................................................................................................ 6

2.2 MICROCELLULAR FOAM PROCESSING ........................................................................................................................... 6

2.2.1 Overview of Microcellular Foaming Processes ............................................................................................ 7 2.2.1.1 Continuous Foaming Process ................................................................................................................................ 7 2.2.1.2 Batch Foaming Process ......................................................................................................................................... 9

2.2.2 Polymer-Gas Solution Formation .............................................................................................................. 10 2.2.2.1 Blowing Agent ..................................................................................................................................................... 10 2.2.2.2 Solubility ............................................................................................................................................................. 11 2.2.2.3 Diffusivity ............................................................................................................................................................ 14 2.2.2.4 Plasticization Effect of Gas .................................................................................................................................. 15

2.2.3 Cell Nucleation .......................................................................................................................................... 18 2.2.3.1 Classical Bubble Nucleation ................................................................................................................................ 19 2.2.3.2 Pseudo-Classical Bubble Nucleation ................................................................................................................... 20 2.2.3.3 Stress-Induced Nucleation .................................................................................................................................. 20 2.2.3.4 Crystal-Induced Nucleation ................................................................................................................................. 21

2.2.4 Cell Growth ................................................................................................................................................ 21 2.2.4.1 Cell Coalescence, Coarsening, and Collapse ........................................................................................................ 22

2.2.5 Nanoclay as a Nucleating Agent ............................................................................................................... 23 2.2.5.1 Property Enhancement of Clay-Based Nanocomposites ..................................................................................... 23 2.2.5.2 Dispersion of Nanoclay ....................................................................................................................................... 24 2.2.5.3 Effects of Nanoclay in Plastic Foaming ................................................................................................................ 24

2.3 MANUFACTURING OF POLYMERIC FIBRES .................................................................................................................... 26

2.3.1 Fundamentals ............................................................................................................................................ 26 2.3.1.1 Fibre-Forming Polymers ...................................................................................................................................... 26 2.3.1.2 Fibre Properties and Characterization ................................................................................................................ 28

2.3.2 The Melt-Spinning Process ........................................................................................................................ 30 2.3.2.1 The Melt-Spinning Equipment ............................................................................................................................ 31

2.3.3 Drawing and Fibre Structure Formation .................................................................................................... 34

2.3.4 Fluid Flow in the Spinning Process ............................................................................................................. 35

VII

2.3.4.1 Shear Flow ........................................................................................................................................................... 36 2.3.4.2 Elongational Flow ................................................................................................................................................ 38 2.3.4.3 Flow Instability during Spinning .......................................................................................................................... 40

2.4 FOAM FIBRE SPINNING ............................................................................................................................................ 41

2.5 SUMMARY AND RESEARCH DIRECTION ....................................................................................................................... 42

CHAPTER 3 LOW VOID FRACTION HIGH CELL DENSITY POLYPROPYLENE FOAM .............................................. 43

3.1 INTRODUCTION ...................................................................................................................................................... 43

3.2 EXPERIMENTAL MATERIALS ...................................................................................................................................... 45

3.3 MATERIAL CHARACTERIZATION ................................................................................................................................. 46

3.3.1 Measurement of Complex Viscosity .......................................................................................................... 46

3.3.2 Foam Expansion Ratio and Void Fraction .................................................................................................. 47

3.3.3 Foam Cell Density Measurement .............................................................................................................. 47

3.4 EFFECT OF PRESSURE DROP RATE AND BLOWING AGENT CONTENT ON THE FOAMING BEHAVIOUR OF PP-NANOSILICA IN

EXTRUSION ................................................................................................................................................................. 48

3.4.1 Experimental Setup ................................................................................................................................... 49 3.4.1.1 Approach for Studying the Effect of Pressure Drop Rate .................................................................................... 50 3.4.1.2 Approach for studying the Effect of Blowing Agent Content .............................................................................. 51

3.4.2 Experimental Results ................................................................................................................................. 52

3.4.3 Discussions ................................................................................................................................................ 57

3.5 EFFECT OF NANOSILICA ON CELL NUCLEATION AND STABILIZATION DURING PP FOAMING .................................................... 58

3.5.1 Experimental Setup ................................................................................................................................... 60 3.5.1.1 Foaming Visualization Procedure ........................................................................................................................ 61 3.5.1.2 Extrusion Foaming Procedure ............................................................................................................................. 63

3.5.2 Experimental Results ................................................................................................................................. 64 3.5.2.1 Foaming Visualization Results ............................................................................................................................. 65 3.5.2.2 Extrusion Foaming Results .................................................................................................................................. 68

3.5.3 Discussion on the Effect of Nucleating Agent in Cell Nucleation and Stabilization ................................... 70

3.6 CONCLUSION ......................................................................................................................................................... 71

CHAPTER 4 FIBRE SPINNING OF LOW VOID FRACTION POLY (LACTIC) ACID- CLAY NANOCOMPOSITE FOAM... 73

4.1 INTRODUCTION ...................................................................................................................................................... 73

4.2 EXPERIMENTAL ...................................................................................................................................................... 75

4.2.1 Materials ................................................................................................................................................... 75 4.2.1.1 Fibre Grade PLA ................................................................................................................................................... 75 4.2.1.2 Nanoclay ............................................................................................................................................................. 75 4.2.1.3 Preparation of Nanocomposite ........................................................................................................................... 76 4.2.1.4 Blowing Agent ..................................................................................................................................................... 77

4.2.2 Experimental Equipment ........................................................................................................................... 78 4.2.2.1 Foam Fibre Spinning System ............................................................................................................................... 78 4.2.2.2 Spinneret Design ................................................................................................................................................. 79

4.2.3 Experimental Procedure ............................................................................................................................ 80 4.2.3.1 Extrusion Foaming Procedure ............................................................................................................................. 80 4.2.3.2 Foam Fibre Spinning Procedure .......................................................................................................................... 82

4.2.4 Sample Characterization and Analysis ...................................................................................................... 83 4.2.4.1 Complex Viscosity Measurement ........................................................................................................................ 83 4.2.4.2 Expansion Ratio ................................................................................................................................................... 83 4.2.4.3 SEM Imaging and Foam Cell Density Characterization ........................................................................................ 84

VIII

4.2.4.4 Differential Scanning Calorimetry ....................................................................................................................... 85 4.2.4.5 Tensile Testing..................................................................................................................................................... 85

4.3 RESULTS AND DISCUSSION ....................................................................................................................................... 86

4.3.1 PLA-Clay Nanocomposite Foam ................................................................................................................ 86

4.3.2 As-Spun PLA Foam Fibre ............................................................................................................................ 95 4.3.2.1 As-Spun Foam Fibre Morphology ........................................................................................................................ 96 4.3.2.2 Foam Fibre Drawability ..................................................................................................................................... 101 4.3.2.3 Foam Fibre Characterization ............................................................................................................................. 103

4.3.3 Tensile Properties of As-Spun PLA Foam Fibre ........................................................................................ 107 4.3.3.1 Comparison of Tensile Properties between Foamed and Unfoamed Fibres ..................................................... 107 4.3.3.2 Factors Affecting Tensile Properties of the As-Spun Foam Fibres ..................................................................... 110

4.4 CONCLUSION ....................................................................................................................................................... 114

CHAPTER 5 HIGH EXPANSION PLA FOAMING- A POTENTIAL STRATEGY FOR PRODUCING FOAM FIBRES ....... 117

5.1 INTRODUCTION .................................................................................................................................................... 117

5.2 EXPERIMENTAL .................................................................................................................................................... 118

5.2.1 Experimental Materials ........................................................................................................................... 118

5.2.2 Experimental Equipment ......................................................................................................................... 119

5.2.3 Experimental Methodology ..................................................................................................................... 119

5.3 RESULTS AND DISCUSSION ..................................................................................................................................... 120

5.4 CONCLUSION ....................................................................................................................................................... 123

CHAPTER 6 CONCLUSION............................................................................................................................... 125

6.1 SUMMARY .......................................................................................................................................................... 125

6.2 KEY CONTRIBUTIONS............................................................................................................................................. 125

6.2.1 Development of a Strategy to Produce High Cell Density Low Void Fraction Foam ................................ 125

6.2.2 Demonstrated the Feasibility of the Foam Fibre Spinning Process ......................................................... 127

6.3 RECOMMENDED FUTURE WORKS ............................................................................................................................ 127

REFERENCES ....................................................................................................................................................... 129

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LIST OF TABLES TABLE 3-1 – DIE GEOMETRY SELECTION TABLE ................................................................................................................... 51

TABLE 3-2 – EXPERIMENTAL MATRIX FOR THE STUDY ON DP/DT ........................................................................................... 51

TABLE 3-3 – EXPERIMENTAL MATRIX FOR THE STUDY ON N2 CONTENT ................................................................................... 52

TABLE 3-4 — PROCESSING CONDITIONS FOR THE FOAMING VISUALIZATION STUDY ................................................................... 62

TABLE 3-5 – PROCESSING CONDITIONS FOR THE EXTRUSION FOAMING STUDY .......................................................................... 64

TABLE 4-1 – EXPERIMENTAL MATRIX FOR THE EXTRUSION FOAMING STUDY ............................................................................. 82

TABLE 4-2 – THERMAL PROPERTIES OF PLA AND NANOCOMPOSITES...................................................................................... 94

TABLE 4-3 — MELT DRAW RATIO OF FOAM FIBRES ............................................................................................................ 96

TABLE 4-4 – THERMAL PROPERTIES OF PLA FOAM FIBRES .................................................................................................. 105

TABLE 4-5 – MEAN AND STANDARD DEVIATION OF PARAMETERS FOR MODULUS .................................................................... 112

TABLE 4-6 – MEAN AND STANDARD DEVIATION OF PARAMETERS FOR YIELD STRESS ................................................................ 114

TABLE 5-1 – EXPERIMENTAL MATRIX FOR PLA EXTRUSION FOAMING WITH CO2 .................................................................... 120

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LIST OF FIGURES FIGURE 2-1 – SCHEMATIC OF THE SPIN-LINE DURING HIGH SPEED SPINNING ............................................................................ 35

FIGURE 3-1 – SCHEMATIC OF THE TANDEM EXTRUSION FOAMING SYSTEM .............................................................................. 49

FIGURE 3-2 – COMPLEX VISCOSITY MEASUREMENTS OF PP-NANOSILICA COMPOSITES ............................................................... 53

FIGURE 3-3 – SEM MICROGRAPHS OF SAMPLES FOAMED AT DIFFERENT DP/DT ....................................................................... 54

FIGURE 3-4 – SEM MICROGRAPHS OF SAMPLES FOAMED WITH DIFFERENT N2 CONTENT............................................................ 55

FIGURE 3-5 – CELL DENSITY COMPARISON AMONG SAMPLES FOAMED AT DIFFERENT DP/DT ....................................................... 56

FIGURE 3-6 – CELL DENSITY COMPARISON AMONG SAMPLES FOAMED AT DIFFERENT N2 CONTENT ............................................... 56

FIGURE 3-7 – EXPANSION RATIO COMPARISON AMONG SAMPLES FOAMED AT DIFFERENT DP/DT ................................................. 57

FIGURE 3-8 – EXPANSION RATIO COMPARISON AMONG SAMPLES FOAMED AT DIFFERENT N2 CONTENT ......................................... 57

FIGURE 3-9 — SCHEMATIC OF THE FOAMING VISUALIZATION SETUP ...................................................................................... 61

FIGURE 3-10 – SCHEMATIC OF THE EXTRUSION FOAMING SETUP ........................................................................................... 63

FIGURE 3-11 – COMPLEX VISCOSITY GRAPHS OF MATERIALS USED IN THE VISUALIZATION AND EXTRUSION STUDY............................ 65

FIGURE 3-12 – SNAPSHOTS OF IN-SITU FOAMING VIDEOS .................................................................................................... 65

FIGURE 3-13 – THE CELL DENSITY VS. TIME ....................................................................................................................... 66

FIGURE 3-14 – AVERAGE CELL NUCLEATION RATE .............................................................................................................. 67

FIGURE 3-15 – AVERAGE CELL GROWTH RATE ................................................................................................................... 67

FIGURE 3-16 – SEM IMAGES OF EXTRUDED FOAM SAMPLES ................................................................................................ 68

FIGURE 3-17 – CELL DENSITY OF EXTRUDED FOAM SAMPLES ................................................................................................ 69

FIGURE 3-18 – EXPANSION RATIO OF EXTRUDED FOAMS ..................................................................................................... 70

FIGURE 4-1 – THERMOGRAVIMETRIC ANALYSIS ON THE NANOCLAY-PLA MASTERBATCH ............................................................ 77

FIGURE 4-2 – SCHEMATIC OF THE FOAM FIBRE SPINNING SYSTEM .......................................................................................... 78

FIGURE 4-3 – GEOMETRY OF THE SHAPING CHANNEL .......................................................................................................... 80

FIGURE 4-4 – EXTRUSION SYSTEM PROCESSING PRESSURE.................................................................................................... 86

FIGURE 4-5 – COMPLEX VISCOSITY OF PLA AND NANOCOMPOSITES ...................................................................................... 88

FIGURE 4-6 – SEM GRAPHS OF PLA FOAMED WITH 0.5WT% N2 .......................................................................................... 91

FIGURE 4-7 – SEM GRAPHS OF PLA+3NC FOAMED WITH 0.2WT% AND 0.5WT% N2 ............................................................. 92

FIGURE 4-8 – CELL DENSITY AND VOID FRACTION OF PLA AND NANOCOMPOSITE FOAM ............................................................ 93

FIGURE 4-9 – DSC FIRST HEATING CURVE ON UNDRAWN FOAM ............................................................................................ 94

FIGURE 4-10 – CROSS-SECTION SEM IMAGES OF FOAM FIBRE SPUN AT 230°C ....................................................................... 98

FIGURE 4-11 – CROSS-SECTION SEM IMAGES OF FOAM FIBRE SPUN AT 215°C ....................................................................... 99

FIGURE 4-12 – CROSS-SECTION SEM IMAGES OF FOAM FIBRE SPUN AT 200°C ....................................................................... 99

FIGURE 4-13 – MACHINE DIRECTION SEM IMAGES OF FOAM FIBRE SPUN AT 230°C .............................................................. 100

FIGURE 4-14 – FOAM FIBRE DRAWABILITY ...................................................................................................................... 101

FIGURE 4-15 – AVERAGE CELL DIAMETER IN FOAM FIBRES ................................................................................................. 102

FIGURE 4-16 – ESTIMATED CELL DENSITY AND VOID FRACTION OF THE FOAM FIBRES SPUN AT 230°C ......................................... 104

FIGURE 4-17 – FIRST HEATING CURVES OF THE FOAM FIBRES OBTAINED FROM THE DSC .......................................................... 106

FIGURE 4-18 – COMPARISON ON YOUNG’S MODULUS OF THE AS-SPUN FIBRES ...................................................................... 108

FIGURE 4-19 – COMPARISON ON THE YIELD STRESS OF THE AS-SPUN FIBRES .......................................................................... 109

FIGURE 4-20 – YOUNG’S MODULUS VS. DENSITY (A), CRYSTALLINITY (B), CELL DENSITY (C), AVERAGE CELL DIAMETER (D) .............. 111

FIGURE 4-21 – YIELD STRESS VS. DENSITY (A), CRYSTALLINITY (B), CELL DENSITY (C), AVERAGE CELL DIAMETER (D) ........................ 113

FIGURE 5-1 – PROCESSING PRESSURE DURING EXTRUSION FOAMING WITH CO2 ..................................................................... 121

FIGURE 5-2 – SEM IMAGES OF PLA FOAM BLOWN WITH CO2 ........................................................................................... 122

FIGURE 5-3 – VOID FRACTION AND CELL DENSITY OF CO2 BLOWN PLA FOAM ........................................................................ 123

1

Chapter 1 Introduction

1.1 Preamble

Polymeric fibres influence our ways of living constantly in numerous aspects; the area of

applications ranges from everyday commodities to industrial, textile and non-textile. The global

demand for the polymeric fibres are forever on the rise, new innovative classes of fibres and the

manufacturing processes are heavily researched on. Plastic foams have been gaining popularity

in the industry for their superior properties. Foaming has also been looked upon as an innovative

technology that can be applied to the conventional fibre-spinning process. The successful

application of foaming in fibre-spinning will generate tremendous amount of interest in the

research field and the industrial world as the topic contains both scientific and commercial values.

1.2 Overview of Plastic Foams

The cellular structure in plastic foams has originally been inspired by naturally occurring cellular

structures, such as ones found in bones and plants. While conventional plastic foams typically

have cell sizes in the range of 100µm and a cell density of less than 106 cells/cc, microcellular

plastics developed at MIT are defined as foams having cell sizes less than 10µm and cell

densities higher than 109 cells/cc [1]. The improved cell morphology and cell density dictate that

microcellular foams enjoy from an array of improved properties including but not limited to:

impact strength [2, 3], toughness [4], fracture strength [3], fatigue life [5], thermal stability, low

dielectric constant [6], as well as thermal and acoustical insulation [7, 8].

Plastic foams can be classified by the type of morphology they possess: close-cell foams and

open-cell foams. As the names suggest, close-cell foams exhibit morphology with individual

2

cells separated from one another by cell walls. Both low expansion and high expansion foams

can be produced with the close-cell morphology. These foams can typically be utilized in

packaging, insulation and some structural applications. On the other hand, open cell foams are

often observed with cells that are interconnected with pores present on cell walls, much like the

structure observed in a sponge. Open-cell morphology is usually seen on foams with high

expansion ratios. They are mostly used in thermal and acoustic insulation applications. When the

open-cell morphology becomes extremely porous, the skeleton foam structure can be classified

as reticulated, a special case for the open-cell foam. Cell wall structure is largely absent in a

reticulated foam, and reticulated morphology is only seen in high and ultra-high expansion foams.

Due to the unique morphology, reticulated foams are mostly suitable in filtration applications.

The foaming process fundamentally involves the generation of a one-phase polymer-gas solution,

and the process of phase separation between the two. This can be achieved in a number of

processes, such as continuous processes like extrusion foaming (profile filaments, sheets, films,

etc.); semi-continuous processes such as injection foam molding, or in batch processes such as

bead foaming. The incorporation of foaming in fibre-spinning can be considered as a variation of

extrusion foaming, however many special considerations must be taken into account.

1.3 Anticipated Challenges for the Manufacturing of Foam Fibres

The most crucial process in the manufacturing of plastic fibres is drawing. During the drawing

step, fibres experience uniaxial stretching along the spin-line causing chains to align and

enhancement in crystallization for semi-crystalline materials. The degree of drawing being

applied to fibres has tremendous effect on the tensile properties of fibres, consequently deciding

the areas of suitable application. Unfortunately, parameters that enhance fibre drawability often

3

work against foaming. The paradox between foaming and drawing is the ultimate challenge

associated with the production of foam fibres.

In order to ensure drawability of fibres, fibre grade resins normally have weaker melt strength

and lower melt viscosity than other grades. However, in extrusion foaming where significant

shear is applied to the melt, the level of stress experienced by the polymer is low due to the low

viscosity; this effectively reduces the cell nucleating power of the given polymer gas system.

Furthermore, fibre-spinning is typically performed at a high temperature to ensure drawability,

contradicting to processing conditions utilized in the practice of foaming. The elevated

processing temperature decreases the material’s melt strength and deteriorates its cell

stabilization ability; the excessive cell growth could lead to cell coalescence and/or cell

coarsening. As the result, the foam morphology produced is expected to be poor with large cells

and low cell densities. The lack of an effective strategy to produce foam with high cell density

and fine cell structure remains a key roadblock. Moreover, it is intuitive that low void fraction

foams are much more desired for applications with an emphasis on mechanical strength, such as

in foam fibres. However, the cell density and void fraction usually show strong coupling effect, it

is challenging to produce low void fraction foam with very high cell density.

1.4 Objective of the Thesis

The objective of the thesis is to develop a lightweight, biodegradable foam fibre as well as its

manufacturing technology. The objective can be divided and completed in three stages: firstly to

determine the processing parameters and window for producing low void fraction fine cell

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structure foam; secondly to demonstrate the feasibility of drawing foam; and lastly to examine

the tensile performance of the foam fibres.

The successful implementation of foaming in an otherwise conventional fibre-spinning process

poses stiff requirement from the foam morphology for reasons mentioned in the previous section.

As such, a strategy to produce foam with desired morphology needs to be developed. In the

development of high cell density low void fraction foams, a fibre grade polypropylene is used in

a series of extrusion foaming experiments. Nano-scaled cell nucleating agent is introduced to

improve the overall foaming behavior of the polypropylene. The foaming behavior is examined

while parameters such as the pressure drop rate, blowing agent content, nucleating agent content,

and temperature are varied. Through this series of fundamental foaming studies, an optimum set

of parameters is fixed.

The optimized foaming methodology is adopted to poly (lactic) acid-clay nanocomposite on the

modified fibre-spinning system. Foam is subjected to different degrees of drawing to

demonstrate the feasibility. The effects of drawing on the foam fibre properties are also

investigated.

1.5 Organization of the Thesis

Since the concept of foam fibre has not been well established in the literature, Chapter 2 provides

a literature survey on fundamental topics that are closely related to foam fibre-spinning. Topics

discussed include foam processing in general, polymer nanocomposite, and conventional fibre-

spinning.

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Chapter 3 summarizes a series of fundamental foaming studies performed on fibre grade

polypropylene. The goal is to develop an effective strategy to produce foam with very fine

cellular morphology and low void fraction, which can be easily adopted for the fibre-spinning

application. Through the foaming trials, the effect of each processing parameter on the foaming

behavior is established and compared among each other. Parameters investigated include the

pressure drop rate, the nucleating agent content, the blowing agent content, as well as the

foaming temperature. In addition, the role of cell nucleating agent in cell nucleation and

stabilization is elucidated through a set of designed experiments involving a static foaming

visualization system and an extrusion foaming system.

Utilizing the foaming strategy developed in Chapter 3, the optimization of foaming with poly

(lactic) acid and nitrogen is carried out and presented in Chapter 4. Nanoclay is utilized as a

nucleating agent for its role in the promotion of cell nucleation; its role in the enhancement of

PLA crystallization is also investigated. To demonstrate the drawability of foam produced with

PLA-clay nanocomposite, foam samples are subjected to drawing upon exiting the spinneret.

Factors affecting foam fibres’ degree of drawing are examined. Tensile properties of the foam

fibres have been measured and compared to that of unfoamed fibres; parameters affecting tensile

properties of the as-spun fibres have also been discussed.

Chapter 5 investigates the foaming behavior of PLA with CO2 at low temperatures. The goal is

to improve the PLA foaming behavior observed in Chapter 4 by enhancing material melt strength.

It is suggested that the foam produced using this methodology can be subsequently heated up and

drawn to foam fibre. Further drawing study is required to complete the investigation.

Chapter 6 provides an overview of the research activity documented in this thesis. It is concluded

with the highlight of major contributions achieved as well as a list of recommended future works.

6

Chapter 2 Literature Review and Theoretical Background

2.1 Introduction

The concept of foam fibre-spinning is not established in the literature. As a result, there is not an

abundant source for the comprehensive theoretical knowledge. The subject of foaming and fibre-

spinning are studied separately. Potential conflicts between the two processes are interpreted and

addressed. The goal is to pinpoint the missing pieces of information and guide the study of foam

fibre-spinning to completion.

2.2 Microcellular Foam Processing

Processing technology for microcellular plastic foams was first developed at MIT in the 1980’s

to address material cost and performance issues. Microcellular plastic foams are characterized as

having cell densities higher than 109 cells per cubic centimetre; cell size in the range of 0.1 to 10

micrometers. Specific density reduction is typically in the range of 5% to 98% [9]. Its high cell

density and small cell size ensures that material cost can be greatly reduced without sacrificing

mechanical strength of the material. Impurities that are present within the material are

theoretically larger in size than these microvoids, hence mechanical failure would initiate at pre-

existing impurities rather than at microvoid sites.

Due to the microcellular nature of this class of materials, microcellular foams display many

superior properties such as: impact strength [2, 3], toughness [4], fracture strength [3], high

7

fatigue life [5], thermal stability, low dielectric constant [6], as well as thermal and acoustical

insulation [7, 8].

2.2.1 Overview of Microcellular Foaming Processes

Microcellular foaming technology is very versatile, and it can be implemented with many

conventional plastic processing technologies including continuous processes such as extrusion,

fibre spinning, injection moulding and blow moulding; microcellular foaming can also be carried

out in batch processes.

2.2.1.1 Continuous Foaming Process

The continuous extrusion process begins from the melting of polymer pellets as they are fed

from the extruder hopper. Blowing agent (typically CO2/N2) is injected inside the extruder barrel

through a gas injection port by using a positive displacement pump (thereafter referred as the

syringe pump). The syringe pump is capable of measuring the output flow rate very precisely; it

is useful for regulating the flow rate of blowing agent being injected at any given pressure. The

weight percentage of blowing agent being injected can be quickly calculated by measuring of the

foam output rate.

Gas-polymer mixture is pushed along the extruder through the rotating action of the screw. The

resistance experienced by the pellets and the rotating screw generate significant heat to help to

melt the polymer; at the same time, the shear fields produced by the screw motion apply

dispersive mixing to the injected blowing agent and polymer matrix. To increase the efficiency

of mixing, irregular mixing blades as well as static mixers are often utilized to redistribute local

gas concentration and increase the interfacial area between the two phases. Mixing elements in

8

the extrusion system also enhance uniform temperature distribution in the polymer melt. High

temperature and high pressure are maintained to expedite the diffusion process.

Once a single phase solution is obtained, adequate cooling is applied to the solution to obtain

quality foam structure. If the melt temperature is too high, the melt will not have the necessary

melt strength to stabilize and maintain the cellular structure of foam before it is solidified. Since

plastics are generally thermal insulating materials, a uniform cooling of the polymer melt is not

easy to achieve. A heat exchanger is often used as a cooling channel, where polymer melt can be

cooled uniformly effectively. A second extruder is sometimes attached at the outlet of the first

extruder, forming a tandem extrusion system; the second extruder is used exclusively for the

progressive uniform cooling of the polymer melt.

Once the one-phase solution is cooled to the desired temperature, it is pushed through the die

where the depressurization takes place. The rapid drop in pressure induces a high degree of

thermodynamic instability that causes phase separation and cells to nucleate. As the pressure

decreases, the solubility of gas in the polymer matrix decreases. The polymer-gas system seeks

for lower free energy state such that new thermodynamic stability can be established; gas

molecules start to cluster and form cell nuclei. These newly formed cell nuclei provide relatively

small mean distance for the gas molecules to diffuse through, free gas molecules are more prone

to be attracted to the existing nearby nuclei where lowered free energy can be achieved than

forming new nuclei [1]. This accumulation of gas molecules into existing voids marks the initial

stage of cell growth. As cells continue to expand, adequate cooling is required to increase melt

viscosity and melt strength, which helps to stabilize the cell structures and suppress excessive

cell growth [10].

9

The design of such continuous microcellular filament extrusion system is discussed in details by

Park et al [11].

2.2.1.2 Batch Foaming Process

Comparing to the continuous extrusion process, the batch foaming process undergoes a simpler

approach. Solid polymer is first placed in a pressurized chamber where it is submerged with inert

blowing agent for an extensive period of time for saturation. The saturation time depends on the

polymer sample size and diffusivity of the gas-polymer system, which is dependent on the

saturation temperature. Depending on the saturation temperature used, there are mainly two

procedures followed by researchers to carry out batch foaming processes:

A. For polymer samples that are saturated at ambient temperature, the foaming chamber is first

depressurized and then immediately heated up in an oil bath. As the polymer softens with the

increasing temperature, gas solubility decreases in the system and cells start to nucleate. Foam

morphology is heavily influenced by the foaming temperature: material is too stiff at low

temperatures to allow for cell expansion, whereas polymer becomes too soft at high temperatures

and cell coalescence dominates. As a result, the gradient heating utilized in this technique is

prone to produce un-uniform foam morphology from core to skin. Saturation process usually

takes a very long time because of low diffusivity at low temperatures.

B: If sample is saturated at around the material’s melting point, the chamber can be directly

depressurized to initiate cell nucleation and growth. Subsequent cooling enhances melt strength

of material and stabilizes cellular structure of foams produced. High saturation temperature

dictates high diffusivity of gas through polymer matrix, reducing saturation time. In addition,

10

heating can be more uniformly applied to the polymer, so that the foam morphology obtained is

more consistent as well.

2.2.2 Polymer-Gas Solution Formation

The foaming of a plastic material is the process of the material expanding due to phase change of

the blowing agent. The entire process can be very briefly broken down to in three sub-processes:

formation of polymer-gas solution at controlled pressure and temperature; cell nucleation upon

depressurization; and cell growth.

2.2.2.1 Blowing Agent

There are two types of blowing agent used in the application of plastic foaming: chemical

blowing agent (CBA) and physical blowing agent (PBA).

CBAs are designed to decompose at targeted temperature to release gas (typically nitrogen or

carbon dioxide). They are dry-blended with the polymer matrix prior to processing. The

decomposition of the blowing agent is triggered by the high processing temperature, activating

nucleation of cells to form the cellular structure inside plastics. Each CBA has a different

decomposition temperature, and hence they need to be specifically selected based on the

processing temperature of the base polymer. CBAs are easy to use, and they do not require

modification to existing processing equipment/infrastructure. However, foams produced with

chemical blowing agents are usually associated with higher cost than those produced with PBAs.

PBAs, on the other hand, are less costly when properly utilized. They are introduced to the

polymer matrix in either a gaseous or liquid state; the blowing agent is then dissolved into

polymer under high pressure and at high temperature or ambient temperature to form the one-

11

phase polymer-gas solution. Foaming occurs when the one-phase solution is subjected to a rapid

depressurization; where the thermodynamic instability of the system induces supersaturation of

the blowing agent causing the phase to separate.

Traditionally, long-chain PBAs such as cloroflorocarbons (CFCs), n-pentane, and n-butane were

widely used in the foaming industry due to the high solubility into the polymer matrix. The long-

chain molecular structures of these PBAs also cause low diffusivity through polymer, making

them effective blowing agents. However, long-chain PBAs are in the process of being phased out

for different reasons. CFCs along with other substances play a major role in the depletion of the

ozone layer, causing harm to human and the environment [12]. The Montreal Protocol signed in

the 1987 has ordered a scheduled phasing-out process of the CFCs. At the same time, n-pentane

and n-butane are considered hazardous due to their highly flammable nature. As a result, inert

gases such as nitrogen (N2) or carbon dioxide (CO2) have been used as alternative PBAs.

These inert gases are chemically stable and environmentally friendly, but they present new

technological challenges. Both CO2 and N2 have lower solubility into polymer than that of long-

chain PBAs; in addition, they both have high diffusivity which makes it easier for them to

penetrate through the polymer and escape after cellular structure has been formed. A more

elaborated discussion on the properties of these blowing agents is carried out in the section

below.

2.2.2.2 Solubility

In a gas-polymer mixture system, the solubility of gas can be defined as the maximum amount of

gas that the polymer can dissolve at a specific temperature and pressure. Since gases like CO2

and N2 have gained wide acceptance in the foaming industry as blowing agents, there have been

12

many studies reported on the solubility of gases in the polymer matrix. In general, the studies

reported involve the experimental measurement of the total amount of gas dissolved in polymeric

matrix upon saturation at high pressure, this is known as the apparent solubility; a correcting

factor is then applied to the solubility measurement obtained to take account for the volume

change experienced by the polymer samples.

Early work on the solubility of gas in polymer have widely employed the pressure decaying

method developed by Newitt and Weale [13]. When utilizing the pressure decaying method, a

polymer sample is first placed in a sealed pressure chamber where it is to be submerged in

gaseous blowing agent at a specific pressure. The chamber pressure decreases as gas is dissolved

in the polymer during the saturation process. The total amount of gas dissolved in polymer can

therefore be indirectly determined by the difference in the chamber pressure between gas

injection and saturation. The mass of gas before and after sorption can be estimated from the

ideal gas law (shown in Equation 2-1) with the pressure measured at Pi, system volume occupied

by gas, and the gas compressibility factor Zi at the specific temperature and pressure. This

method is widely adopted for its simplicity in operation and apparatus setup. Sato et al.

performed solubility measurements of PBAs such as CO2 and N2 on various commodity

polymers such as polypropylene (PP), high-density polyethylene (HDPE), and polystyrene (PS)

using this method [14, 15].

Equation 2-1

Alternatively, the apparent solubility of gas in polymeric materials has been directly determined

by the gravimetric technique. The weight-gain of the polymer sample after gas sorption is

directly measured with a magnetic suspension balance (MSB) in situ at high temperatures. Park

13

et al. employed the MSB in the solubility measurement of CO2 in PC, linear PP and branch PP

[16, 17]. Sato et al. utilized the MSB to measure the solubility of CO2 in PS, poly (vinyl acetate)

and biodegradable polymers [18, 19]. Both of these apparatus are capable of measuring weight-

gain in situ at high temperatures.

As mentioned previously, polymer samples swell as gas is dissolved. The change in sample

volume during sorption causes a discrepancy between apparent solubility measured with either

the pressure decaying method or gravimetric technique and the true solubility of the polymer.

The pressure decaying method relies on the accurate measurement of volume available for gas

occupancy to calculate the amount of gas present, whereas the buoyancy effect experienced by

the polymer sample changes according to changes of the sample volume. In order to more

accurately reflect the true solubility, the volume swelling in polymer melt during sorption has to

be considered. The volume swelling can be predicated empirically by thermodynamic models,

also known as the Equation of State (EOS), or it can be measured experimentally. Many

thermodynamic models have been developed over the years, although the Simha-Somcynsky (SS)

EOS [20] and Sanchez-Lacombe (SL) EOS [21] remain much more widely accepted than others.

On the other hand, Li developed an experimental apparatus to visually observe the swelling of

polymer under the presence of blowing agent at high temperature and high pressure [22]. Li’s

work enables a more direct approach at measuring the volume swelling of polymer.

With the theoretical prediction or experimental measurement of the volume change in polymer,

the apparent solubility is corrected to better reflect the true solubility of gas in polymers. This

information is of significant value in both the continuous foaming process as well as the batch

process. In the continuous foaming process, the desired amount of blowing agent is injected in

14

the extruder by the syringe pump, which regulates the volumetric flow rate. However to maintain

the one-phase state of the gas-polymer solution, it is essential to maintain a pressure higher than

the solubility pressure throughout the extruder. In the batch foaming process, the blowing agent

content introduced is controlled by the pressure at which saturation takes place; therefore setting

blowing agent content is as simple as reading the solubility data off a chart. The rate at which gas

dissolves in the polymer, on the other hand, depends on the diffusivity. This will be addressed in

the following section.

Two of the most important factors affecting solubility of gas in polymer are saturation pressure

and temperature. All of the reported studies confirm that the higher the saturation pressure, the

more gas the polymer can dissolve, hence the higher the solubility [14-19, 22]. However, how

solubility temperature affect solubility of the gas-polymer system vary between the gases used.

For instance, corrected solubility of CO2 decreases as the melt temperature increases [16, 18, 19],

whereas the solubility of N2 increases as polymer melt temperature increases [15, 22].

It is also important to keep in mind that any secondary substance such as fillers or crystals in a

polymer matrix can cause non-uniform gas concentration. Gas solubility in the secondary phase

could be significantly different than the solubility in the primary phase.

2.2.2.3 Diffusivity

As previously mentioned, the length of saturation time largely depends on the rate at which gas

diffusion takes place. Diffusion time is a function of diffusivity and diffusion distance. The

relationship is shown in Eqation 2-2 [23], where tD is diffusion time; h represents diffusion

distance; and D represents diffusivity.

15

Equation 2-2

The diffusion process can be shortened by increasing diffusivity of gas and polymer. It has been

reported that diffusivity of gas in polymer matrix increases with the increase in temperature, but

it appears to be insensitive to pressure change [15]. Diffusivity can be experimentally measured

in sync with solubility, its measurement is based on the rate gas is dissolved in the polymer.

Diffusivity coefficients can be calculated by taking the slope of the first half of a gas sorption

curve [24]. The same approach is taken by Sato et al. on the measurement of diffusivity of

blowing agent in various thermoplastic polymers [15, 18, 19].

In the case of the continuous extrusion foaming process, the diffusion process can be accelerated

by employing convective diffusion [25]. Through the rotational action of the plasticizing screw,

gas bubbles injected get smeared such that interface area between the two phases increases and

the diffusion distance decreases; the redistribution of gas and polymer unifies concentration of

gas, and it assists in speeding up the diffusion process as well [26].

2.2.2.4 Plasticization Effect of Gas

The plasticization effect refers to when a secondary phase, usually consisted with small

molecules substances, reduces the melt properties of the primary polymer matrix material and

induces higher degree of flexibility to the material over a range of temperatures [27, 28]. As the

environmentally friendly inert gases have gained popularity as physical blowing agents in the

foaming industry, they have naturally become the inevitable plasticizer in polymer melts due to

their small molecule sizes. Their plasticization effects need to be addressed as they can affect

many aspect in the foaming process.

16

Doolittle described the mechanism of the plasticization effect with the free volume theory [29].

The free volume is defined as the difference between the volume observed at absolute zero

temperature and the volume measured at any other given temperatures. At absolute zero

temperature, there is no vibration or oscillation on the molecular level; therefore molecules are

nicely packed together, occupying little space. As the temperature starts to elevate, molecules

start to oscillate and occupy an imaginary free volume around them. The same principle applies

when gas is dissolved in the polymer. As gas molecules diffuse through the polymer matrix,

swelling occurs; the additional free volume created makes changes in the polymer chain

conformation easier, which effectively reduces the stiffness of the material [29]. The

plasticization effect affects foaming processes in a variety of aspects including the change in

glass transition temperature, viscosity, diffusivity, and etc.

Glass transition temperature is the temperature below which materials start to become hard and

brittle. The plasticization effect of gas decreases the glass transition temperature of materials,

and the magnitude of decrease in glass transition temperature is dependent on the polymer matrix

and plasticizer used; in general, the higher the gas content, the lower the glass transition

temperature [29]. In the extrusion foaming process, plasticization effect lowers the processing

temperature, which enhances the melt strength of the material, contributing positively in the

prevention excessive cell growth.

The plasticization effect of gas increases the diffusivity of the polymer matrix. Diffusivity of the

matrix is increased due to the polymer swelling phenomenon and the additional free volume; gas

molecules can jump to large hole voids as long as they are able to overcome attraction force from

neighbouring molecules [29]. In extrusion foaming, Chen et al. [28] reported significant increase

17

in cell growth rate as the result of the increase in diffusivity caused by the plasticization effect;

he proposed a diffusion-induced cell growth mechanism. In a batch foaming process, the

increase in diffusivity shortens the saturation process; however the high diffusivity promotes gas

to quickly diffuse out of the polymer as well as excessive cell growth and cell coarsening, hence

reducing final cell count.

Melt viscosity is one of the most important factors in continuous extrusion foaming. It is also

strongly coupled with the material melt strength. It has been reported that plasticization has a

significant impact on the melt viscosity of the matrix material. In a rheological study of

polystyrene mixed with HCFC142b and HFC134a, melt viscosity is shown to decrease by two

orders of magnitude [30]; in the foaming study of low-density polyethylene, viscosity is reported

to reduce by fifty percent [31]. The decrease in melt viscosity is undesirable as it lowers the

probability for cell nucleation, especially when stress-induced nucleation is dominate in

extrusion foaming processes. In addition, the decrease in melt viscosity is an indication of

decreases in material melt strength; this can cause excessive cell growth, which could lead to cell

coalescence. More on this will be discussed later.

Interfacial tension refers to the surface stress between two phases, polymer melt and gas bubbles

in our case. Lee and Flumerfelt have shown that the interfacial tension for low-density

polyethylene with nitrogen reduced by roughly fifty percent [32]. Interfacial tension plays a

significant role in cell nucleation. The reduction in interfacial tension promotes earlier cell

nucleation.

18

2.2.3 Cell Nucleation

Nucleation can be defined as the formation of a new phase from the bulk phase. One of the most

commonly observed form of nucleation is the formation of gas bubbles in a liquid phase in a

boiling process. In the manufacturing process of microcellular foams, cell nucleation takes place

as gas bubbles form from an initially homogenous polymer-gas solution.

The cell nucleation is in generally initiated as the one-phase polymer-gas solution experiences a

rapid depressurization process which causes the solubility of gas in the polymer matrix to

decrease. The sudden change in solubility causes a supersaturation in the system; gas bubbles are

nucleated as the thermodynamically instable system seeks for a metastable thermodynamic state.

The dynamic nature of cell nucleation makes it a dominate factor affecting many aspects of

foaming including the early cell growth, final cell density, the final cell morphology, and etc. As

a result, much research has been devoted in this subject.

In plastic foaming, cell nucleation can be classified into homogeneous nucleation, heterogeneous

nucleation, and pseudo-classical nucleation. The Classical Nucleation Theory developed by

Gibbs [33] consists theoretical predictions of thermodynamic instability limits for homogeneous

nucleation and heterogeneous nucleation. Gibbs theory suggests that there exists a critical bubble

size corresponding to the thermodynamic instability equilibrium point, where the free energy of

the system is at maximum; this energy state is referred to as the free energy barrier. He suggests

that bubbles larger than the critical radius grows spontaneously, and bubbles smaller than the

critical radius collapse. Pseudo-classical nucleation emerged in the plastic foaming industry as

researchers reported that nucleation of gas bubbles actually occurs earlier than that predicted by

19

the Classical Nucleation Theory [34]. It is claimed that the free energy barrier for nucleation can

be lowered if the nucleation is initiated at a pre-existing microvoid site.

2.2.3.1 Classical Bubble Nucleation

Homogeneous nucleation involves the formation of gas bubbles from a homogeneous liquid

phase with no pre-existing cavities or microvoids. According to the classical nucleation theory,

the critical radius of a sustained bubble and the free energy barrier for homogenous nucleation to

take place can be determined from Equations 2-3 and 2-4 respectively, where Rcr represents

critical radius, Whom represents the free energy barrier for homogenous nucleation, is the

interfacial tension between polymer and gas, Pbub,cr is the critical bubble pressure and Psys is the

system pressure [35].

Equation 2-3

( ) Equation 2-4

During foaming, the depressurization process causes Psys to decrease, effectively increasing the

degree of supersaturation (Pbub,cr-Psys). According to Equations 2-3 and 2-4, the higher the degree

of supersaturation, the lower the critical bubble radius and the free energy barrier. This is the

fundamental reason why pressure drop rate has such significant impact on the foaming behavior.

Furthermore, it has also been demonstrated experimentally that the cell density of foam can have

strong dependency on the gas content [36, 37]. While the high gas concentration increases the

initial degree of supersaturation, it has also been shown to decrease the interfacial tension

between polymer and gas [38], hence decreasing the free energy barrier for nucleation.

20

Heterogeneous nucleation takes place when impurities such as nucleating agent particles are

present in the polymer matrix. It takes place by substituting a higher energy state solid-liquid

interface with a lower energy state solid-gas interface. The energy barrier of a heterogeneous

nucleation is significantly reduced from that of a homogenous nucleation, as would be indicated

from Equation 2-5 [39]. Note that F is simply a geometric factor equating to the volumetric ratio

of a heterogeneously nucleated bubble to that of a complete sphere with the equal radius of

curvature. The F term is always less than unity by definition. The complete expression for F can

be found in the literature [39], and is not presented here.

( ) Equation 2-5

2.2.3.2 Pseudo-Classical Bubble Nucleation

The classical nucleation theory is established under the assumption that there exists no microvoid

in the matrix prior to bubble nucleation. However, this assumption is deemed invalid when

Lubetkin et al. experimentally demonstrated that bubble nucleation takes place sooner than

would be predicted by the classical nucleation theory [34]. The pseudo-classical nucleation

proposes that the polymer matrix cannot be perfectly wetted to the impurity particles or fillers

present [40, 41], and the pre-existing voids can serve as seeds for bubble nucleation, reducing the

free energy barrier.

2.2.3.3 Stress-Induced Nucleation

During the cell nucleation process, stress experienced by the polymer melt can induce nucleation

greatly. Guo et al. investigated the correlation between shear stress and the cell density by

conducting foaming with a slit die [42]. They observed higher cell density along the cell wall

21

region where shear is more dominant and lower cell density in regions where shear is less

dominant. Leung et al. visualized the nucleation process of a polystyrene-talc composite in a

static foaming chamber [43]; they observed cells to nucleate around existing cells in a chain

reaction fashion; they attributed the clustering effect of cell nucleation to the extensional stress

imposed by the expanding bubbles. Wong and Park also demonstrated the role of extensional

stress in the enhancement of cell nucleation [44]; they compared the nucleating ability of talc

particles of different sizes and concluded that the larger sized talc particles induce more stress

variation around themselves, enhancing stress-induced nucleation.

2.2.3.4 Crystal-Induced Nucleation

For the foaming of semi-crystalline materials, crystal-induced nucleation is another mechanism

that needs to be taken into considertion. When the polymer is processed below the melting

temperature, crystallization is prone to take place. The crystallization kinetics can be affected by

many factors such as molecular structure, temperature, shear and extensional stress, isothermal

processing time, and etc. While gas cannot be dissolved in crystalline regions, it creates a higher

degree of supersaturation around the region, enhancing cell nucleation; in addition, crystals can

be used as heterogeneous nucleation sites, much like the cell nucleating agent. Too high of a

crystallinity is not beneficial for foaming as it hinges the cell growth and foam expansion [10].

2.2.4 Cell Growth

Upon the simultaneous nucleation of cells, the pressure difference between the nucleated bubbles

and the system pressure in the surrounding solution drives the early stage of cell growth. At the

same time, the high gas concentration in the one-phase polymer gas solution drives additional

gas molecules to diffuse into the already nucleated cells. As cells continue to grow, the pressure

22

difference eventually expires and the cell growth mechanism becomes diffusion dominant.

Diffusion of gas molecules takes place where there is a strong gas concentration gradient, both

between polymer-gas solution and nearby cells, as well as through the foam sample skin. The

ability to cool the foam and stabilize the cellular structure is vital as it determines the foam

morphology as well as the amount of gas that is being diffused out.

2.2.4.1 Cell Coalescence, Coarsening, and Collapse

The combined effect of excessive cell growth and poor material melt strength leads to the failure

to stabilize the cellular structure of foam; it can take place in the form of cell coalescence, cell

coarsening and cell collapse.

During cell growth, the cell walls separating neighboring cells grow increasingly thin. Cell

coalescence takes place when the thin cell wall collapses as the result of stretching, and

neighboring cells join to form one. Cell coalescence is especially undesirable in close-cell foams.

On the other hand, if the difference in gas concentration causes gas to diffuse from one cell to

another, the cell losing gas would eventually decrease to below the critical radius and collapse

while the other cell grows. This phenomenon is called cell coarsening. In additiona, if the cell

collapse happens to be the result of gas molecules being diffused out of foam, the mechanism is

called cell collapse.

As indicated above, the importance of material melt strength in the stabilization of the cellular

structure cannot be overly emphasized. There have been many attempts in the literature to

improve the foam morphology through enhancing material melt strength. Researchers have

compared melt strength and foaming between materials of different molecular structure. They

observed that while linear materials exhibit poor melt strength, branching can significantly

23

enhance the material’s ability to stabilize cell structures [45-48]. Naguib et al. demonstrated the

temperature dependency of foam expansion [10]. While severe cell coalescence, coarsening

and/or collapse takes place at high foaming temperatures, lowering the temperature can help to

enhance the melt strength which prevents excessive gas loss. Okamoto et al. was the first to

discover that when nanoclay is used as a nucleating agent for the foaming of polypropylene, the

clay platelets align along the cell wall as the result of the biaxial stretching during cell growth

[49, 50]. The alignment of nanoclay enhances the cells to withstand the stretching force. This

work unfolded a new area of applications for these nano-sized particles.

The strategy to reduce cell coalescence, coarsening, and/or collapse is of particular interest for

the fibre spinning application: fibre grade resins tend to have weaker melt strength; significant

shear and stretching is also expected to deteriorate any remaining cellular structure. However,

material modification such as branching is not desired as the drawability of material decreases;

processing temperature cannot be too low for the same reason. The use of nanoclay, however,

presents an interesting opportunity for the enhancement of cell stabilization.

2.2.5 Nanoclay as a Nucleating Agent

2.2.5.1 Property Enhancement of Clay-Based Nanocomposites

Nanoclay is a layered silicate mineral material. Montmorillonite (MMT) specifically is a very

popular type of clay in polymer processing due to its large surface area and high surface

reactivity [51]. Its structure is consisted with an aluminum octahedral sandwiched between two

sheets of silicon tetrahedral. Each sheet has a thickness of approximately 1nm, hence the name

nanoclay; the lateral distance between layers, or so called the galaxy, can be as low as 30nm.

24

When dispersed in the polymer matrix, nanoclay is found to significantly improve the

mechanical modulus, strength and thermal stability properties of nylon-6 [52]. Similar trend has

been observed on the compression modulus, flexural strength and fracture toughness of

thermoplastics by other researchers [53]. Relevant to the foaming application, dispersed clay

particles is claimed to increase the effective path length for molecule diffusion and enhance the

gas/moisture barrier properties [54, 55]. The gas barrier property can be very useful in the foam

fibre application to reduce gas loss, especially when fibres are drawn to a fine dimension.

2.2.5.2 Dispersion of Nanoclay

It is challenging to fully de-laminate the clay platelets due to the small galaxy distance. The most

efficient approach to adopted in the literature is to improve the interaction between clay and

polymer [56]. This is typically done by replacing the inorganic cations with more bulky organic

cations to increase the galaxy distance as well as the chemical compatibility. There is a number

of commercial grade organomodified clays, including the one used in this thesis: Cloisite 30B

from Southern Clay. Unfortunately, there is a downfall in using the organic modifiers in

nanoclay. Pavlacky and Webster studied the plasticization effects of the organic modifiers and

concluded that melt viscosity, molecular weight and glass transition temperature of the base

polyester were all lowered by the modifier in the absence of nanoclay [57].

2.2.5.3 Effects of Nanoclay in Plastic Foaming

When utilized in plastic foaming, nanoclay can improve the foaming behavior of a neat polymer

in terms of cell nucleation and stabilization. Its presence in the polymer matrix can also have

strong effects on the thermal and rheological properties. The presence of nano-clay introduces

additional heterogeneous nucleation sites; pre-existing microvoids at the polymer-particle

25

interface can further decrease the free energy barrier and act as seeds for nucleation. Ray and

Okamoto utilized nanoclay in the foaming of poly (lactic) acid and CO2, the foam morphology

improved drastically as 5% of nanoclay is introduced in the matrix; closed-cell structure with cell

sizes of around 1µm was obtained [58]. Similar results were reported for the foaming of

polypropylene [59, 60].

Well dispersed nanoclay dramatically increases the melt viscosity of polymer matrix due to the

strong polymer-clay interaction. However at high strain rates, clay based nanocomposite

commonly experiences more shear thinning than neat materials. This has been experimentally

observed by researchers [58, 61]. The proposed mechanism is that clay platelets tend to re-orient

themselves under high shear, the alignment of clay in the shear direction causes slippage

between polymer and clay [61]. Other researchers claim that the organic modifiers used in the

clay themselves cause significant plasticization and reduce melt viscosity [57].

For polymers with slow crystallization kinetics, low content of nanoclay can act as a crystal

nucleating agent. Nam et al. showed that the crystallization rate of PLA can increase as much as

50% when nanoclay is present in the matrix [62]. The effect of nanoclay dispersion on

crystallization has been studied by Ray and Okamoto [63]; the case of intercalated nanoclay

showed high final crystallinity as well as nucleation density than that of the exfoliated

morphology. Nofar et al. examined the crystallization kinetics of PLA-clay nanocomposite under

dissolved CO2 using a high pressure differential scanning calorimetry [64]; the onset of crystal

formation is delayed by the nanoclay as PLA chain mobility is reduced; however, the number of

crystal nuclei formed at the end was higher for the case where nanoclay is present. It is believed

26

that the formation of these small crystals not only provide additional heterogeneous nucleation

sites, but also enhances the material melt strength which is beneficial in foaming.

2.3 Manufacturing of Polymeric fibres

Polymeric fibres are used everywhere in our day to day lives, their versatility are heavily backed

by their outstanding properties in terms of durability, comfort, dimensional stability and aesthetic

appeal. All of the above mentioned properties depend on the final structure of fibres; therefore

the properties can be controlled by carefully manipulating the processing conditions in the

manufacturing process.

Polymeric fibres can be produced as continuous filaments, although they can be further chopped

to standard lengths and be used as staple fibres. They can be produced with a variety of cross-

sections such as circular, trilobal, and hollow depending on the intended application.

2.3.1 Fundamentals

2.3.1.1 Fibre-Forming Polymers

Polymeric fibres have high length to cross-section aspect ratio. Molecules that comprise these

fibres are preferably linear, and hence consist of bifunctional repeating units. The inclusion of

trifuncitonal units could potentially cause branching of chains, or even cross-linking if the

trifuncitonal unit concentration is high enough [65].

One key aspect of fibre-forming polymers is the flexibility of each molecule. The flexibility of a

molecule determines the potential energy barrier for a molecule to change its conformation.

Flexibility of these chains is strongly affected by the structure in the chain backbone as well as

the type of bonds present: a rigid aromatic ring would significantly stiffen the rigidity of the

27

chain making it harder to rotate or reform. In addition, any attached side-groups could also lead

to an increase in chain rigidity [65]. Chemistry between molecules can also affect the overall

flexibility of the fibre forming polymers. In between molecules, the nature of attractive forces

greatly affect chain interaction, in particular, the ability to form van der Waals forces, strong

dipoles or groups that can form hydrogen bonds will increase attraction forces present between

chains, improving fibre strength and stiffness [65]. Molecule chain flexibility and intermolecular

forces are two predominant factors affecting melting temperature Tm, and glass transition

temperature Tg. There are many polymers suitable to be manufactured into polymeric fibres,

some of the commercially available fibre grade polymers include PET from the polyester family,

Nylon 6,6 and Nylon 6 from the polyamide family, as well as PP and PE from the polyolefin

family.

Molecular weight can be a good indicator on which grade of polymer is more suitable for the

fibre-spinning application. Too long chains would cause over entanglement between molecules,

making it less desired for crystallization, thus reducing fibre strength; on the other hand,

extrudate would be too weak to be pulled into fibres if chains are too short. Every polymer is

composed of chains of different lengths, molecular weights are usually characterized by two

average molar mass: a weight average molar mass, Mw, and a number average molar mass, Mn.

The dispersity ratio Mw/Mn measures the distribution of molar masses [65]. There are other types

of specification used to categorize these polymers; one of them perhaps used more frequently in

the processing industry is the melt flow index (MFI). The MFI measurement of thermoplastics is

documented in the ASTM D1238 testing standard. It measures the mass of polymer extruded

through a standard capillary under standard load in ten minutes. The load as well as testing

temperature is material specific. For example, Polypropylene is usually measured at 230ºC under

28

a 2.16kg of load [66]. There is no simple conversion between MFI and molar mass, but as a

general rule, the lower the molar mass, the higher the MFI, and vice versa.

One of the most dominating factors that affect fibre properties is the molecular orientation of

fibres. A fibre with high degree of orientation and crystallinity is most sought after as it exhibits

substantially superb properties.

A highly orientated fibre is strengthened structurally as molecule chains align along the fibre axis.

The stress originated from this alignment action sometimes induces higher levels of crystallinity

or changes crystalline form. The increase in chain alignment as well as crystallinity often results

in a structure with much higher strength, mechanical modulus as well as elongation [67].

Molecular orientation of as spun fibres can be improved in the drawing stage of the fibre

manufacturing process for both semi-crystalline and amorphous materials.

2.3.1.2 Fibre Properties and Characterization

Mechanical Properties

Most of the direct mechanical properties of a fibre can be evaluated from a regular tensile stress-

strain curve [65]. Fibre tenacity is a measure of tensile stress applied to the fibre at the breaking

point; tenacity is strongly affected by the strength of bonding between adjacent chains, degree of

orientation as well as crystallinity. Elongation to break, on the other hand, measures the amount

of strain experienced by the fibre before breakage; inverse to tenacity, strong inter-molecular

bonding, high degree of chain alignment and crystallinity would lower the value of elongation to

break. The amount of elastic recovery determines what portion of deformation or strain is

elastically recoverable; fibres that have undergone extensive stretching tend to exhibit less elastic

recovery than those that have not. Stiffness of a fibre is measured by the initial modulus, which

29

is the slope of the stress-strain curve at zero stress; for applications used in ropes, fibres with

high initial modulus are desirable, whereas fabric used for day-to-day activities would require

fibres with lower stiffness. Toughness can also be calculated as total energy absorbed till failure;

it can be a critical measurement for applications such as seatbelts.

Thermal Properties

Whether for commercial or industrial application, thermal properties of polymeric fibres are

usually taken into account as one of the decisive factors. Since polymers are very temperature

sensitive, they either melt or degrade under high temperatures. Since polymeric fibres are mostly

processed in the molten state, it is especially important that material does not decompose when in

melt phase. Thermal transition temperatures (Tg, and Tm) are important aspects of thermal

properties. In addition, thermal conductivity and thermal insulation properties affect the utility in

certain applications significantly, thus are also important parameters. Flame retardancy is usually

a desired property in polymers, especially in fibres. Flammability is usually measured by the

limiting oxygen index (LOI) [65]; the higher the LOI, the more resistant the fibres are to ignition.

Other Properties

Some of the other less obvious but often times equally important properties are briefly mentioned

here. Electrical property in fibres commonly relates to their ability to dissipate static charges;

high moisture regain and low electric conductivity is usually the main cause for electrification.

Fibre optical properties have several layers of implications on their applications. When used in

commodity textile applications, the appearance of the end product strongly depend on the

interaction of fibres with visible lights. To evaluate the overall degree of molecular chain

alignment along the fibre axis, optical birefringence is the most commonly used tool. Optical

30

fibres are widely used for data transmission over long range of distance as it is much cheaper to

produce and utilize than other candidate such as glass fibre. One of the limiting factors for the

environment of plastic fibre usage is the photodegradation phenomenon in polymers; chain

scission as the result of sunlight would sometimes prohibit some outdoor applications. Surface

property of a fibre strongly influences its resistance to abrasion. Processing instability is mostly

blamed for poor surface finish.

2.3.2 The Melt-Spinning Process

Depending on specific application, polymeric fibres can be made in a number of methods: melt-

spinning, solution-spinning (dry and wet), gel-spinning, as well as electro-spinning. When the

material is thermally unstable, it is most practical to use solution-spinning method. As the name

suggests, it employs solvent in the spinning process. As a drawback, additional steps are required

in this method to remove solvent from the fibres through either evaporation by heating (dry-

spinning) or coagulation in a secondary fluid (wet-spinning). Higher production cost is

associated with lower production rate and also the extra steps taken. Fibres can also be

manufactured through gel spinning where the material remains partially liquid (gel) during the

spinning process. Electro-spinning is usually used to produce nano-size fibres. It uses electric

charge to draw fibres out of a liquid form. Since melt-spinning is considered to be the most

economic method for the mass production of fibres and the focus of this report, aspects of this

technique are to be discussed in details in the following section.

Melt-spinning is considered as the simplest method to produce cost effective fibres. During this

process, polymer pellets are conveyed and uniformly melted in an extruder; molten polymer is

then transported into a metering pump under pressure where the polymer flow can be strictly

31

regulated; since impurities in the melt greatly reduce fibre integrity and cause breakage to

happen frequently, the melt is forced through a fine filter pack immediately after the gear pump;

filtered polymer is then pushed through a plate with numerous capillaries, the spinneret, to form

into strands of fibres; the extrudate/fibre is then quenched in a cooling channel in the medium of

air or water; solidified fibres go through a lubrication device so they become less sticky; finally it

is guided through a set of godets to a winding device. Since the fibre properties are strongly

affected by the degree of chain orientation, an extra step of drawing may be required to stretch

the as-spun fibres to significantly enhance their mechanical properties. The required final draw

ratio is mainly dependent on the degree of orientation of the as-spun fibre, hence the final

spinning speed.

2.3.2.1 The Melt-Spinning Equipment

A. The Extruder

An extruder is normally composed of a cylindrical barrel, which is being heated by electric

heaters wrapped around it; and a close-fitted rotating screw. Polymer pellets are melted as a

result of heat conduction from the barrel walls and mechanical shearing action created by the

rotating screw. A single screw extruder can be broken down into four distinctive regions: feed

section, compression and melting where pellets first start to melt, metering zone where polymer

melt start homogenize and pressure starts building up, and the mixing zone before the melt exits

the extruder.

B. The Gear Pump

32

As molten polymer is pushed out of the extruder, the rotating action of the screw causes material

flow to oscillate quite a bit. In order to produce continuous fibre, a positive displacement pump is

utilized after the extruder to precisely control flow rate. The gear pump functions such that teeth

from the counter-rotating gear capture and redirect melt to travel around the gear, when teeth

from the two gears mesh again the polymer melt is pushed out [68]. Flow rate is thus controlled

by the volume of material each gear tooth can seize and the rpm it is operating at.

C. The Spinneret Pack

The spinneret pack is consisted of a filtering component and the spinneret itself. A simple

filtration system could be composed of layers of fine mesh metal screens, whereas a more

complicated system can employ filter sand or alumina to help eliminate smaller particles. It

should be noted that filter screens introduce additional shearing on the polymer melt, thus they

affect polymer rheology to some extent. A spinneret is a stainless steel disk between 3-30mm

thick with capillary holes. The pattern of holes can be arranged in different fashions, from

concentric circles, parallel holes, and etc. It is normally for a monofilament spinneret to have

between two to four holes [67].

D. Cooling Channel

As the extrudate exits the spinneret head, it needs to be solidified to a desired temperature before

it can come in contact with the godets or being drawn. In the early days, melt-spun fibres were

cooled through natural convection. The thin boundary layer of air surrounding the fibre caused

poor heat transfer ability. Quenching systems have been developed to increase the efficiency of

cooling. The most commonly accepted methods are cross-flow quench, in-flow quench, and out-

33

flow quench. Cross-flow quench is the most widely used method; ventilation is setup along the

spin-line to blow air across the fibres. Its advantage is that quench air temperature does not

increase along the spin-line; however it can be quite challenging to provide uniform cooling for

multifilament spinning. For spinning of staple fibres, in-flow or out-flow quench can be used to

provide more uniformed cooling: cooling air in an in-flow system is directed from outside the

filaments to blow radially inward by a conical shaped channel, the air then streams downward

with the fibres; ring patterned spinneret is used for the out-flow system so that cooling air can be

injected in the centre of the ring, as the air stream travels downward, it is forced to blow radially

outward by the inverse conical chamber.

Any cooling air flow will introduce unnecessary turbulence to the spin-line; however the

turbulence needs to be minimized so that a uniform filament can be produced. Cooling profile

must be closely controlled as drawability of fibre, crystallinity kinetics, and other important

parameters are depended to it.

E. Godet Rollers

Drawing refers to the stretching of the polymeric fibres to increase the degree of chain alignment

and crystallinity to improve mechanical properties. Drawing can take place in stages depending

on the setup; however, the first stage of drawing always takes place immediately after the

extrudate exits the spinneret through the usage of godet rollers. The first roller takes up the spin-

line and applies the first degree of drawing. The as-spun fibre can undergo additional rollers

where the circumferential speed of each roller is gradually increased. Since the fibre relaxes

when the tensile load is removed, the actual draw ratio would be lower than the machine draw

34

ratio. Furthermore, heating element is usually employed between rollers where drawing takes

place since fibres need to be heated to above its glass transition temperature [69].

Some of the advanced processing techniques could incorporate drawing and spinning in one

process if the spinning speed is fast enough. Traditionally, fibre-spinning is carried out in the

range of 600-1500 m/min. The as-spun fibre would then require further drawing to achieve a

draw ratio between 3 and 4.5 [67]. With the advancement in the spinning technology, highly

oriented yarns (HOY) can be spun at a speed of 4000 to 6000m/min, whereas fully oriented

yarns (FOY) are spun at speeds higher than 6000m/min. These as-spun yarns can be used

directly without any further drawing [70].

2.3.3 Drawing and Fibre Structure Formation

The drawing of fibres as well as the fibre structure formation along the spin-line can be depicted

by Figure 2-1. Diameter of the fibres decreases as the extrudate travels away from the spinneret;

in high speed spinning applications, a special neck region appears immediately after the draw

down region, where the diameter of the extrudate rapidly decreases [71, 72]. X-ray diffraction of

the fibres spun at different speeds reveal that the significant enhancement in crystallinity only

occurs when the neck region is present in the spin-line [72]. This indicates that chain orientation

and significant stress-induced crystallization is only possible when the spinning speed is high

enough to form the neck region. In the case of low speed spinning, there is very limited chain

orientation in the draw down region, let alone stress-induced nucleation. Even in the high speed

spinning case, temperature variation in the cross-section could cause different crystal

morphology [73]. Stress-induced aligned crystals only occur around the skin where the melt is

35

cooled down sooner by ambient air; in the hotter core region, spherulite type crystals are more

dominant.

Figure 2-1 – Schematic of the spin-line during high speed spinning

Figure 2-1 only provides a brief understanding of the fibre structure formation during spinning.

The actual fibre-spinning process also involves parameters such as air drag, air cooling, surface

tension, gravity, crystallization kinetics, viscoelastic behavior of the materials, and much more.

Many researchers utilize sophisticated mathematical models which accounts for the energy

momentum and mass balance to simulate the fibre-spinning process [74-77]. However this is not

the focus of this thesis.

2.3.4 Fluid Flow in the Spinning Process

To best understand how the polymer melt responds to external stress during the fibre-spinning

process, it is essential to examine the flow of polymer fluids. Two aspects need to be taken into

consideration: shear flow in the capillary channel, and the elongational flow along the spin-line

during cooling and stretching.

36

2.3.4.1 Shear Flow

When polymer melt passes through the spinneret head, the flow characteristics are dominated by

the shearing effect. Poiseuille was the first to derive a set of equations describing a laminar fluid

flow through a capillary [67]. According to Equation 2-6, for a given polymer flow rate and

capillary length, shear viscosity is proportional to the pressure exerted on the fluid as well as the

fourth power of the radius of capillary channel. Two key assumptions that made this model

possible were: the liquid is Newtonian; all the energy applied to the fluid was used to overcome

the viscous drag of the fluid [67].

Equation 2-6

Shear Thinning

Newtonian fluids, like water, possess constant viscosity when in a laminar flow, so that the shear

rate is directly proportional to shear stress (constant viscosity). However, majority of polymers

used for fibre-spinning applications are non-Newtonian (i.e., PET, Nylon 6, Nylon 6-6, PP, and

PE) which exhibit pseudo-plastic or shear-thinning when in melt form. The viscosity of the melt

decreases with increasing shear rate. This phenomenon is mainly caused by the entanglement of

long chain molecules in polymeric materials; as the shear rate increases, the loss of existing

entanglement becomes higher than the generation of new ones, hence lowering the frictional

resistance between fluid layers. This decrease in viscosity is seen as an advantage in plastic

processing industry as it reduces power requirement for processing [67]. To more accurately

describe shear viscosity of a shear-thinning fluid, an empirical model known as the power-law

equation, also known as the Ostwald-de Waele equation [78], is presented in Equation 2-7. In

37

this equation, k and n are rheological constants found in the log plot of shear stress and shear rate.

This viscosity is considered the apparent viscosity as it neglects the viscoelastic nature of

polymeric materials.

( ) Equation 2-7

Viscoelastic Fluids

Polymeric fluids are viscoelastic fluids, not ideally viscous. During processing, a fraction of the

energy applied to the polymer cannot be used to overcome the viscous drag, but it is instead

stored as elastic energy due to internal friction between molecules [67]. The elastic energy stored

is subjected to relaxation over time as the energy is dissipated. This viscoelastic behaviour can be

best demonstrated by the die-swell phenomenon in any extrusion process. It can be observed that

any extrudate coming out of a die would expand in volume; this is an indication of elastic energy

being released. As the die length increases while maintaining pressure exerted on the polymer

melt, the expansion volume decreases as the polymer is given more time to relax and the elastic

energy is dissipated.

Bagley came up with a simple approach that would consider the viscoelastic nature of fluids [79].

He assumes that a fraction of the total energy applied in a fluid that undergoes capillary flow is

to be held accountable for the elastic energy stored. It is then equivalent to consider that a

fraction of the pressure exerted is stored internally as the elastic energy. The Bagley model

measures the exerted pressures at different die length and shear rate, the results are mapped on a

pressure vs. die length plot. It is apparent that as die length approaches to zero, there is a residue

pressure for each shear rate, the higher the shear rate, the higher the residue pressure, thus the

38

amount of elastic energy stored is also higher. Further extrapolation of these pressure trend lines

showed that they converged to the same negative intercept, b, the effective capillary length. True

shear stress can be computed using this model as shown in Equation 2-8 [79].

( )

[( ⁄ ) ] Equation 2-8

As already discussed, shear viscosity can be significantly affected by processing parameters such

as shear rate, and pressure. It is also strongly dependent on processing temperature.

The relationship between temperature and polymer viscosity is in large non-linear. This follows

that the viscosity is sensitive to changes in free volume, which can be a result of thermal

expansion. Williams, Landel and Ferry proposed an empirical relationship to describe the

dependence between temperature and viscosity shown in Equation 2-9 [80].

( ⁄ ) ( )

( ) Equation 2-9

It should be noted that shear viscosity of polymeric materials can also be affected by molecular

weight and molecular structure. These topics are outside of the scope of this report.

2.3.4.2 Elongational Flow

Upon examining the velocity profile of a shear flow, it can be assumed that the internal fluid

flow has zero velocity along the capillary wall and maximum velocity in the centre. This

parabolic shaped velocity profile from the shearing action would promote orientation of polymer

chains to align with the flow direction. However the degree of orientation caused by the shearing

in the spinneret channel is usually far from being sufficient in the fibre spinning application. As

pointed out by Ziabichi [69], the residence time of polymer fluid inside the spinneret channels is

39

much shorter than that of the relaxation time, hence there is not enough time for polymer chains

to relax in the orientated mode; another evidence is the die swell phenomenon exhibited by fluid.

Ziabichi also seems to be the first person identifying the unique velocity profile of elongational

flow from shear flow [81].

Trouton was reportedly the first to have studied elongational flow [67]. When he attempted to

measure the shear and elongational viscosity of pitch and waxes, he discovered that elongational

viscosity is approximately the same as three times the shear viscosity in strain rates lower than

1s-1

[82]. Materials possessing this behaviour have thereafter been called Troutonian materials.

To investigate elongational viscosity of polymers, Vinogradov et al. measured both shear and

elongational viscosity of polystyrene (PS) over a wide range of strain rate at 130ºC. They found

that PS exhibited Trouton-like behaviour at low elongational strain rate, but tension-stiffening

was observed at higher strain rate [83]. They claimed that the behaviour is due to the unique

molecular structure of polymer materials: flow appears purely viscous at low extension rate since

the network structure can be retained; as material is subjected to higher extensional rate, the

material starts to exhibit the viscoelastic nature where the elasticity starts accumulating making

the material more resistant to flow, thus the raising viscosity. This tension-stiffening

phenomenon was not observed when the measurements were taken at 150ºC. A similar study was

conducted with low density polyethylene (LDPE) over an even broader range of strain rate at a

temperature of 150ºC. Surprisingly as the extensional rate increased beyond the tension-

stiffening range, the elongational viscosity started declining [84]. This decrease in viscosity

under high extensional strain rate is caused by the destruction of network-like structure and the

40

alignment of polymer molecules. It is a similar mechanism to the necking behaviour of plastics

under tensile stress. This is the region fibre spinning can be performed.

2.3.4.3 Flow Instability during Spinning

Upon the brief discussion on the fibre spinning process, it would seem ideal to maximize

material throughput to increase productivity and profitability; however in the real life scenario,

the maximum fibre production rate is restricted throughout the spinning process by processing

instabilities: extrudate swell, melt fracture, and draw resonance.

Extrudate swell, also known as die swell, has already been discussed above. It is caused by the

elastic nature of polymer melts. The swelling experienced by extrudate disrupts proper material

flow upon exiting the spinneret. The effect of die swell can be reduced in a number of ways:

increasing melt temperature; decreasing shear rates by increasing the diameter of the capillary

die (spinneret) or slowing down material flow rate; alternatively, an increase in residence time

inside the spinneret brings the material closer to relaxation [85]. Capillary residence time can be

increased by slowing down material flow rate or increasing the length of capillary. In general,

extrudate swell can be reduced by the appropriate design of the spinneret geometry or lowering

the production rate of fibres. Furthermore, the tensile force exerted on the extrudate strand during

spinning helps to reduce the effect of extrudate swell as it is pulled away from the spinneret.

Melt fracture refers to the distortion of material melt flow. The severity of melt fracture typically

advances with the increase of shear rate experienced by the flow. The shear rate associated with

the onset of melt fracture is known as the critical shear rate. The origin of surface distortions is

associated with materials elongational and shear properties near the die exit, whereas causes for

the volume distortion is linked to the inadequate geometry design of the die entrance region [86].

41

In fibre spinning in particular, melt fracture can be induced by the temperature gradient along the

capillary of the spinneret due to the cooling channel immediately below it. Critical shear rate can

be reduced further by the variation in temperature profile [85].

Instability often appears in the spin-line in the form of draw resonance. Draw resonance mainly

appears in the form of periodic fluctuation in the fibre diameter. Besides producing inconsistent

fibre diameter, draw resonance is also undesirable because its tendency to induce spin breaks.

Similar to the onset of melt fracture, draw resonance is often associated with a critical draw ratio;

for both Newtonian and non-Newtonian materials. Draw resonance can be caused by internal

changes such as the material’s viscosity, elasticity and density, or external disturbances such as

spin-line velocity changes, cooling air velocity and temperature variations, as well as material

flow rate and take-up speed fluctuations [87].

2.4 Foam Fibre Spinning

To the author’s best knowledge, there is no literature publication on the topic of foam fibre

spinning. The continuous manufacturing of foam fibre has not been previously reported in the

literature. Guo et al. investigated the preparation of a novel cellular fibre through the post-

treatment of as-spun PET fibres [88]. They saturated the as-spun fibres in a nitrogen charged

pressure chamber at room temperature; upon depressurization the chamber was heated for 10

seconds to develop the cellular structure. The highest cell density obtained with these batch

foamed fibres was 106 cells/cc.

Because the cellular fibres were prepared in a batch system, the methodology can never be

adopted to mass production. Its contribution is limited in the study of foam fibre spinning as the

processing conditions are far apart.

42

2.5 Summary and Research Direction

In this chapter, theoretical backgrounds on the microcellular foam processing have been

thoroughly reviewed. Special attention was paid to the cell nucleation and cell growth

mechanisms in foaming. It is believed the material’s poor nucleating ability and insufficient cell

stabilization will be most challenging to overcome in the demonstration of foam fibre spinning.

It is noted that nanoclay can be effectively used as a cell nucleating agent to enhance both cell

nucleation and growth.

Literatures in the conventional fibre spinning have also been examined. Drawing of fibre is

evidently the most crucial step as it dictates the structure formation in fibres. It would seem

apparent that the drawability of foam will also pose significant challenge in the process of foam

fibre spinning.

The literature survey points out that there is no effective strategy available to produce foam with

morphology that is friendly to the fibre spinning application. As a result, the first objective of the

thesis is the development of a foaming strategy to produce foam with fine morphology and low

void fraction. Fibre spinning experiments are then carried out with the foam obtained to study

parameters that influence the drawing behavior of foam. Lastly, properties of foam fibres such as

crystallinity, modulus and yield stress are to be examined; correlation between foam fibre

properties and processing conditions are suggested.

43

Chapter 3 Low Void Fraction High Cell Density Polypropylene Foam

3.1 Introduction

Plastics are used everywhere in our day-to-day lives. In recent decades, the cost of raw plastic

resins has skyrocketed. Foaming technology has been widely adopted to produce lightweight

plastic foam products as an important strategy to reduce manufacturing cost as well as the

consumption of raw plastics. Foams can be classified into high density foam and low density

foam, each with their perspective suitable applications. For applications such as packaging, low

density foams are suitable due to their superior compression, and impact properties; for

applications such as blown film extrusion, injection molding, blow molding and fibre-spinning,

high density foam with low foam expansion and high cell density is desirable since it retains

maximum tensile properties of the foamed plastics.

In the manufacturing of plastic fibres, the drawability is one of the most crucial parameters as it

plays the most significant role on the fibre’s properties. In a typical fibre-spinning process, fibre

undergoes multiple stages of drawing operations to obtain the final draw ratio. During drawing,

the stretching force induces polymer chains to align along the fibre axis; the uniaxial orientation

of closely packed chains can induce crystallization, which strengthens the fibre’s mechanical

properties.

In order to enhance material drawability, fibres are typically produced with polymers of weak

melt strength and low melt viscosity. Low melt strength and elasticity allow polymer to deform

under low tensile stress such that fibres can be drawn more easily. However, these materials

44

typically have low extensional and shear viscosity. Materials with low shear and extensional

viscosity perform poorly in cell nucleation. In extrusion foaming where significant shear is

applied to the melt from the screw action as well as the resistance from the die, the level of shear

stress experienced by the polymer is low due to the low viscosity, this effectively reduces the cell

nucleating power of the given polymer gas system.

Moreover, to ensure the drawability of the polymer, fibre spinning is typically performed at a

temperature higher than material’s melting temperature [89, 90], contradicting to processing

conditions normally preferred and utilized in the foaming industry. The elevated processing

temperature further decreases the material’s melt strength and deteriorates its cell stabilization

power. Once the cells nucleate, the low melt strength of the material leads to excessive cell

growth, cell coalescence and/or cell coarsening. As the result, the foam morphology produced is

poor with large cell sizes, and low cell density. Poor morphology causes fibres to break under

much lower tensile stresses during spinning, thus significantly reduce fibre drawability.

An additional challenge in spinning foam fibre is that void fraction of undrawn foam has to be

maintained low enough such that fibres can be stretched to a fine dimension. However, void

fraction is heavily coupled with cell density. It is difficult to obtain foam with high cell density

and low void fraction.

It is clear from the aforementioned challenges that much work is needed to identify a strategy to

produce foam with desirable morphology for the fibre spinning application before attempting to

spin foam fibre.

45

In an effort to develop a strategy to produce high cell density and low void fraction plastic foam,

a series of fundamental foaming studies have been designed and conducted. The effect of

pressure drop rate, blowing agent content, and nucleating agent on the foaming behaviour of

polymer have been investigated individually. The findings of these studies, which can contribute

significantly to further research in the spinning of foam fibres, are summarized in this chapter.

3.2 Experimental Materials

Polypropylene (PP) is a popular material in the plastic processing industry due to its enhanced

properties such as superior mechanical strength and higher service temperature. It is considered

as a promising candidate to replace polyethylene and polystyrene in many foaming applications.

However, it is a challenging material to foam with due to its poor nucleation ability which is

partially caused by its weak melt strength [48, 91]. The development of fine cell structure low

void fraction foam with this material has the potential to make tremendous impact in numerous

commodity and industrial foaming applications.

A fibre grade PP has been selected as the subject of interest. It is a linear PP manufactured by

Total Petrochemicals USA (PP3762). It is reported to have a Melt Flow Rate of 18 grams/10min

and a melting temperature of 165°C [89]. This material is thereafter referred to as the neat PP.

To improve the cell nucleating behaviour during foaming, nanosilica has been utilized in the

experiments as a nucleating agent. Since PP is a hydrophobic polyolefin, it has weak interfacial

adhesion with additives such as nanosilica. A modified PP is normally used as a compatibilizer

to improve adhesion as well as the dispersion of nanoparticles inside a PP matrix. In all

experiments, the nanocomposite has been obtained by diluting a PP-nanosilica masterbatch to the

46

desired nanosilica concentration. The masterbatch consists of 10wt% nanosilica (Aerosil 200),

15wt% of PP grafted maleic anhydride (Fusabond P613 manufactured by DuPont) as a coupling

agent, as well as 75wt% PP carrier.

It is widely accepted in the literature that nitrogen has superior nucleating power as a blowing

agent. Researchers have been able to obtain high cell density yet relatively low expansion ratio

foam using nitrogen with PP and HDPE [92, 93]. Blowing agent used in this series of studies is

99.998% purity nitrogen supplied by Linde Gas.

3.3 Material Characterization

3.3.1 Measurement of Complex Viscosity

Foaming behavior of any given material and equipment system will depend on a number of

factors including materials properties and processing conditions. Characterization on material

properties such as the complex viscosity will not only help to minimize effort for finding the

appropriate processing conditions for different compounds, but also assist with interpreting

experimental results and gaining a fuller understanding of material behavior.

Complex viscosity data presented in this chapter were all obtained from an ARES oscillatory

rheometer by TA Instruments. The measurements were made with a pair of 25mm diameter

parallel plate with a gap of 1mm (sample thickness) at a temperature of 180°C. A strain sweep

test was first performed to determine the material’s linear viscoelastic region, frequency sweep

tests were then carried out within the material’s linear viscoelastic region (5% strain in the

polypropylene cases at the testing temperature) to measure the complex viscosity; frequency

ranges from 0.1 rad/s to 500 rad/s.

47

3.3.2 Foam Expansion Ratio and Void Fraction

Expansion ratio, φ, is a dimensionless ratio between the density of neat polymer or polymer

composite and the density of foam. Density of each is measured with the water displacement

method outlined in ASTM D792; an electromagnetic balance was utilized.

In low density foam applications, the measurement of expansion ratio is usually converted to

void fraction as it is a more direct representation of material saving. The conversion is shown in

Equation 3-1.

(

) Equation 3-1

3.3.3 Foam Cell Density Measurement

As an important property of most foam samples, the cell density data can be characterized from

the cellular structure of foam on the fractured surface. A Scanning Electron Microscope (JEOL

JMS6060) was used to examine the samples, such that both cell sizes and cell density can be

characterized. Cell density can be calculated with respect to per unit unfoamed polymer volume

(Nunfoam) as per Equation 3-2, where n represents the number of cells within the micrograph; A

represents the area of micrograph in cm2.

(

)

Equation 3-2

48

3.4 Effect of Pressure Drop Rate and Blowing Agent Content on the Foaming

Behaviour of PP-Nanosilica in Extrusion

Difficulties associated with the manufacturing of PP foams due to its weak melt elasticity and

melt strength have already been discussed. The present section focuses on approaches that will

improve the foam behavior of polypropylene.

In a typical extrusion foaming process, physical blowing agent such as carbon dioxide and

nitrogen is dissolved and mixed in the polymer matrix under high pressure and shear. As the one-

phase solution exits the extrusion die, the rapid depressurization induces a thermodynamic

instability which drives the cell nucleation and growth [94]. Since the cell nucleation is a

dynamic process, it is obvious that the faster the depressurization occurs, the higher the cell

nucleation rate is. As a result, the appropriate design of die geometry is essential for producing

high cell density foams.

On the other hand, classical nucleation theory dictates that the higher the concentration of gas,

the lower the energy barrier for cell nucleation to take place [33]. However, in the process of

producing low-expansion foams, the blowing agent content is kept low. This causes significant

reduction in the degree of supersaturation, hence the cell density decreases [95]. Nitrogen is

known to have excellent ability to nucleate small cells; the appropriate amount of nitrogen

therefore would have the potential to produce high cell density, yet low expansion foams.

It is established that high cell density can be achieved through high pressure drop rate and high

blowing agent content, the current study investigates the sensitivity of each of these parameters

to the foaming behavior separately, especially comparing them to that of using nanosilica as a

nucleating agent.

49

3.4.1 Experimental Setup

To compare the effectiveness of increasing pressure drop rate, the blowing agent content, to that

of using nanosilica as a nucleating agent, material compounds used throughout of the study

remained the same. Four material compounds were used in the study, namely Neat, 1wt%

nanosilica, 2wt% nanosilica, and 3wt% nanosilica. PP nanocomposites were obtained through

direct dilution of the masterbatch. Material grades have been covered in Section 3.2. Their

complex viscosity measurements were measured in accordance to Section 3.3.

A small tandem extrusion system was utilized to carry out the foaming study. Please refer to

Figure 3-1 for the schematic. The first extruder is a Brabender 0.75” extruder which was used to

plasticise the polymer matrix as well as the injection and mixing of the blowing agent; the

second extruder (Brabender 1.5”) was mainly used to apply uniform cooling to the one-phase

polymer gas mixture to enhance melt strength. Gas injection was carried out with a high

precision high pressure Teledyne ISCO 260D metering pump.

Figure 3-1 – Schematic of the tandem extrusion foaming system

50

The study was carried out in two stages: the first stage examined the effect of the die geometry

(which determines the pressure drop rate) on the foaming behavior while keeping constant

blowing agent content; the blowing agent content was varied in the second stage of the study.

3.4.1.1 Approach for Studying the Effect of Pressure Drop Rate

Based on Bird’s earlier model [96], Xu et al. [97] derived a set of equations that estimate the

amount of pressure drop and pressure drop rate across an extrusion die based on the die geometry,

the material rheological characteristics as well as the material flow rate during processing.

To de-couple the effect of pressure drop and pressure drop rate, the present study utilized the

equations derived and selected two extrusion dies with similar pressure drop, yet two pressure

drop rates that were one order of magnitude apart. The procedure followed for selecting the dies

geometry was as follows: complex viscosity measurement was first carried out on the neat PP at

180°C to determine its power law constants; pilot experimental trials were then conducted to

estimate the experimental material flow rate; the pressure drops and pressure drop rates were

then estimated. The equations used for the calculations are presented in Equation 3-3 and 3-4,

where m and n are the power-law constants of the material, L and R are the length and radius of

the die respectively, Q is the material flow rate. The die geometries as well as their perspective

pressure drop and pressure drop rate are tabulated in Table 3-1.

(

)

(

) Equation 3-3

(

)

(

) Equation 3-4

51

m n Q

(g/min) L (mm) R (mm) dP

(Mpa) dP/dt

(Gpa/s)

Die 1 (low -dP/dt) 3546 0.4151 15 9.52 0.41 -6.46 -0.391

Die 2 (high -dP/dt) 3546 0.4151 15 2.91 0.23 -7.25 -4.17

Table 3-1 – Die geometry selection table

As mentioned earlier, blowing agent content was kept constant at 0.2wt% in the first stage of the

study. For each of the extrusion dies selected, four material compounds were foamed under

various die temperatures. Foam samples were characterized for expansion ratio and cell density.

The sensitivity of pressure drop rate in the foaming behavior of the compounds was examined.

Experimental matrix of this study is shown in Table 3-2.

Die # Nanosilica wt% Die Temp (celcius)

1 (low -dP/dt)

0 180, 170, 160

1 180, 170, 160

2 180, 170, 160

3 180, 170, 160

2 (high -dP/dt)

0 180, 170, 160

1 180, 170, 160

2 180, 170, 160

3 180, 170, 160

Table 3-2 – Experimental matrix for the study on dP/dt

3.4.1.2 Approach for studying the Effect of Blowing Agent Content

In the second stage, the die with the high dP/dt was used while the physical blowing agent (N2)

content was varied between 0.3wt% and 1wt%. Nanosilica was again used as nucleating agent to

enhance the foaming behavior. A temperature sweep between 185°C and 165°C was performed

for each compound at each blowing agent contents. The experimental matrix is presented in

Table 3-3. Each of the foam samples was characterized for expansion ratio and cell density.

52

Insight on the sensitivity of the blowing agent content to the foaming behavior of PP-

nanocomposites was obtained.

N2 content (wt%) Nanosilica wt% Die Temp (°C)

0.3

1 185, 175, 165

2 185, 175, 165

3 185, 175, 165

1

1 185, 175, 165

2 185, 175, 165

3 185, 175, 165

Table 3-3 – Experimental matrix for the study on N2 content

3.4.2 Experimental Results

The dynamic shear viscosity measurements were carried out on all four compounds. As can be

observed from Figure 3-2, the complex viscosity of the 1wt% NS case was significantly higher

than that of the Neat PP case at low frequency ranges, implicating the much enhanced melt

viscosity at low shear rates due to the presence of nanosilica; however as the frequency

approached 100 rad/s, Newtonian plateau appeared in all four material compounds, therefore

reducing the enhancement of viscosity. It is noteworthy that the conventional fillers also

exhibited the enhancement of viscosity, however only at higher contents. The finding suggests

that there is some degree of interaction between the nanosilica particles [98]. As the

concentration of nanosilica increased to 2wt%, the complex viscosity increased across the

frequency range examined. The increase in the melt viscosity can contribute to enhanced melt

strength, which would act favorably in the foaming of the PP nanocomposites. Furthermore,

when the nanosilica content was increased to 3wt%, the viscosity started to decrease as

compared to the 2wt% case. This phenomenon can be explained by the low viscosity of the

coupling agent utilized in the nanocomposite. Since the amount of the coupling agent in the

53

compound is directly proportional to the amount of nanosilica, at higher nanosilica loading, the

more pronounced is the effect of the coupling agent. It is speculated that any further increase in

the concentration of nanosilica would only further decrease the overall viscosity of the

compound.

Figure 3-2 – Complex viscosity measurements of PP-nanosilica composites

PP was foamed with nanosilica at 0wt% (Neat PP), 1wt%, 2wt%, and 3wt%. Comparison SEM

images were prepared in Figure 3-3 and 3-4, examining the effect of pressure drop rate and

blowing agent content on the foam morphology, respectively.

As can be seen in Figure 3-3, the introduction of nanosilica as a nucleating agent improved the

foaming behavior of PP significantly. Neat PP did not yield consistent foam morphology in

54

either the low dP/dt or the high dP/dt case. Whereas when 1wt% of nanosilica was added,

consistent cellular morphology started to appear; as the nanosilica content was further increased,

the improvement in morphology continued but became less significant. The increase in pressure

drop rate also improved the foaming behavior significantly; it was evident from the smaller cell

sizes and higher cell count produced by the high dP/dt case. The higher pressure drop rate caused

more cells to nucleate, and to smaller sizes. The effect of foaming temperature on the cell

morphology was present, but not very significant in the temperature range examined. Samples

foamed at 180°C showed severe cell coalescence, as temperature was decreased, however,

individual cells were more discretely defined and the overall morphology became more

consistent.

Figure 3-3 – SEM micrographs of samples foamed at different dP/dt

Figure 3-4 shows the morphology change between samples foamed with 0.3wt% and 1wt% N2.

Samples foamed with 0.3wt% displayed really similar morphology and trend as samples foamed

at high dP/dt and 0.2wt% N2. The slight increase in N2 content contributed had a positive effect

55

in foaming. On the other hand, all of the samples foamed with 1wt% N2 yielded much higher cell

count and much smaller cell sizes. The cellular structure obtained was dramatically different

from the low N2 content case; cell sizes appeared to be much more consistent with higher cell

wall to cell wall distance. The dependency of morphology on nanosilica concentration and

foaming temperature disappeared within the range studied.

Figure 3-4 – SEM micrographs of samples foamed with different N2 content

In order to compare the sensitivity of pressure drop rate and the blowing agent content to the

foaming behavior, cell density graphs were plotted in Figures 3-5 and 3-6. From Figure 3-5, cell

density increased by close to two orders of magnitude as a result of 2wt% of nanosilica. In

comparison, each of the compounds experienced about an order of magnitude increase in cell

density when pressure drop rate was increased (by one order of magnitude). It is therefore clear

that the effect of nanosilica was much more pronounced than the effect of pressure drop rate in

the final cell density. Die temperature did not seem to be a major factor in the measurement of

the final cell density.

56

Figure 3-5 – Cell density comparison among samples foamed at different dP/dt

Figure 3-6 shows the comparison of cell density of samples foamed with different N2 content.

The figure suggests that when the nitrogen content was increased from 0.3wt% to 1wt%, there

was a two orders of magnitude increase in cell density while other parameters remained constant.

Cell density of higher than 107 cells/cm

3 were obtained when foaming with 1wt% of nitrogen.

The increase in nanosilica content also increased the cell density; however the increase was less

than one order of magnitude.

Figure 3-6 – Cell density comparison among samples foamed at different N2 content

Expansion ratio measured between 1 and 3.2 for the low pressure drop rate cases and between

1.3 and 2.5 for the high pressure drop rate cases (Figure 3-7). The decrease in overall expansion

57

ratio for the high dP/dt case could be attributed to the higher cell density and smaller cell sizes.

Similar trend was observed in Figure 3-8, where the high blowing agent content samples yielded

lower expansion ratio at 1wt%, 2wt%, and 3wt% nanosilica loading. Again, the high cell density

produced by the higher blowing agent content and the small cell sizes were the main factors

contributing to the low expansion ratio.

Figure 3-7 – Expansion ratio comparison among samples foamed at different dP/dt

Figure 3-8 – Expansion ratio comparison among samples foamed at different N2 content

3.4.3 Discussions

As a comparison, the introduction of nanosilica in PP yielded a two orders of magnitude increase

in the cell density measurement; an increase in the blowing agent (N2) content from 0.3wt% to

58

1wt% also generated a two orders of magnitude increase in the cell density of foam samples; on

the other hand, the pressure drop rate played a less significant factor on the final cell density,

contributing to a one order of magnitude increase.

When the pressure drop rate is increased, it effectively increases the degree of supersaturation

causing more cell nuclei to form in the instant. Final cell density increases due to increased cell

nucleation rate and the dynamic nature of the nucleation process. As the nuclei density increases,

more gas is consumed during nucleation reducing the amount of gas that can be diffused to the

neighboring cell; this mechanism drives the cell sizes to decrease and cell wall to cell wall

distance to increase.

When nanosilica is present, the nano-particles provide higher interfacial area for heterogeneous

nucleation to happen; at the same time, its presence may induce local pressure variation which

increases the degree of local supersaturation, hence reducing the overall free energy barrier for

nucleation [43, 44]. In addition, as verified in the shear viscosity measurements, the addition of

nanosilica at low contents (1wt%, 2wt%, and 3wt%) improves the shear viscosity of the matrix,

enhancing its melt strength.

Last but not the least, the increase in the N2 content decreases the energy barrier for cell

nucleation. This is evident in the improvement of foam morphology, increase in cell density, as

well as the decrease in expansion ratio of foams.

3.5 Effect of Nanosilica on Cell Nucleation and Stabilization during PP Foaming

Nucleating agents (NA) are effective tools to enhance the cell density of low-expansion foams,

as already demonstrated in the previous fundamental studies. Previous researchers investigated

59

the foaming behaviors of various micro-sized and nano-sized NAs. Wong and Park [44] studied

the effect of particle sizes on various micro-sized talc on the cell density of polystyrene foams.

They demonstrated through in-situ visualization that the larger sized talc was more effective at

inducing cell nucleation than small talc particles despite of the lower particle density. They

attributed this result to the higher local pressure fluctuation generated around larger talc particles,

which reduced the free energy barrier and promoted cell nucleation. Other researchers suggested

that nano-scaled nucleating agents, such as nanoclay and nanosilica, could be more effective in

inducing cell nucleation due to the large interfacial area between the nanoparticles and the

polymer melt. For example, Lee et al. [99] investigated the effects of nanosilica on

polypropylene (PP) foaming through in-situ visualization and continuous extrusion foaming.

They showed that nanosilica was effective in generating higher cell density at low level contents.

At higher contents, the cell density decreased. They attributed this result to the poor dispersion of

nanosilica at high content levels. Similar results were obtained by Zhai et al. [100] and Lee et al.

[101] in the extrusion foaming of PP with nanosilica, and batch foaming of low-density

polyethylene with nanoclay, respectively.

These studies demonstrated that NAs are effective in inducing cell nucleation. However, their

role in cell nucleation and stabilization has not been individually and comprehensively studied,

especially for the low-expansion foaming application. Furthermore, it has been hypothesized that

the coupling agent used to improve adhesion between nano-particles and polymer matrix could

contribute greatly in the cell nucleation and growth mechanism, which ultimately determines the

final foam morphology. In this context, this study was designed to examine the role that

nanosilica plays in the nucleation, early growth and stabilization of cells in during the foaming

process of PP. The experiment was conducted in two stages: i) Foaming with a visualization

60

system under static conditions; ii) Extrusion foaming. Using this methodology, the cell

nucleation, early stage cell growth could be elucidated by the static visualization study, whereas

the cell stabilization ability could be studied by examining morphology of extruded foam

samples. The findings from this study would be critical to the fundamental understanding of

effects of the NAs on plastic foaming processes. This understanding will be an important step in

generating an effective strategy to manufacture low-expansion high cell density foam with

polypropylene.

3.5.1 Experimental Setup

All of the material grades and description have been covered in Section 3.2.

Since one of the objectives of the present study is to investigate the effect of coupling agent on

the cell nucleation and growth, three material compositions were prepared: neat PP was used as

received; PP with 2wt% nanosilica was prepared via the dilution of the 10wt% nanosilica PP

masterbatch; as well as a compound with PP and 3wt% coupling agent (the same amount of

coupling agent used in the 2wt% nanosilica PP compound). The three material compositions are

denoted as the Neat PP, PP+Nanosilica, and PP+CA throughout this study, respectively.

When preparing the PP+CA compound, special attention was paid to mimic the processing

conditions of that of the PP+Nanosilica, where the PP was first melt compounded in a twin-

screw extruder with 15wt% coupling agent loading (the equivalent amount used in the

masterbatch), the compound was then diluted with PP to 3wt% of CA.

The shear viscosity characteristics of all three experimental compounds were tested using the

same approach detailed in Section 3.3.

61

3.5.1.1 Foaming Visualization Procedure

The foaming visualization system developed by Wong et al. was used to conduct in-situ foaming

observation of the materials described above. This system essentially features a high pressure

foaming chamber with rapid heating and cooling capacity; a solenoid valve for rapid

depressurization of the foaming chamber; a view cell where the foaming process can be observed;

a light source; a high magnification high speed camera; as well as a frame grabber. The

development of the system and more detailed descriptions can be found in Reference [102]. A

schematic of the system is shown in Figure 3-9.

Figure 3-9 — Schematic of the foaming visualization setup

For each material compound, a plastic film of 400 µm in thickness was first prepared by

compression molding; samples were then punched into circular disks and placed inside of the

high-temperature/high-pressure view-cell for each experimental trial. In order to avoid

heterogeneous nucleation that would take place on the contacting interface of polymer and metal

chamber, each polymer sample was placed on top of a thin PET film of the same shape with a 1

mm diameter hole in the middle, exposing an area of the PP sample to gas; foaming was captured

62

only in the suspended region of the PP polymer. N2 was injected into the view-cell while the

setup was maintained at the predetermined foaming temperature. Saturation of gas was allowed

at the specific saturation pressure and time. At the end the saturation time, the pressure valve was

triggered to the open position releasing the high pressure gas; foaming occurred due to the rapid

depressurization and the foaming process was captured with the high-speed camera. From the

foaming videos recorded by the high-speed camera, cell density and cell size data were

characterized at a selected time interval. The analysis methods and the equations used were

described in details in Reference [103].

In order to study the effect of nanosilica and the CA in an isolated manner, the foaming

temperature, saturation pressure, saturation time and average pressure drop rate were kept

constant. Foaming conditions are summarized in Table 3-4.

Compound

N2 Saturation

Pressure

(MPa)

Saturation

Time

(min)

Foaming

Temperature

(°C)

Pressure

Drop Rate

(MPa/s)

Neat PP 13.79 30 180 -10

PP+CA 13.79 30 180 -10

PP+Nanosilica 13.79 30 180 -10

Table 3-4 — Processing conditions for the foaming visualization study

Saturation pressure was selected to allow the dissolution of 2wt% N2 in PP matrix [104].

Blowing agent content in the static foaming experiment was chosen to be higher than that used in

the extrusion foaming study to compensate for the low pressure drop rate with the static case.

Foaming temperature was chosen to erase the crystals such that their effects on foaming could be

isolated, especially since the study had a focus on foaming at the temperature range allowed by

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fibre spinning. 30 minutes saturation time would allow gas to diffuse through a 400 micron thick

sample.

3.5.1.2 Extrusion Foaming Procedure

The extrusion foaming study was conducted on a lab-scaled 0.75” single screw extruder (Refer

to Figure 3-10 for the schematic of the experimental setup). Gas was injected with a high

precision high pressure Teledyne ISCO 260D metering pump. The gear pump was attached after

the downstream to the extruder to regulate the melt flow. A filamentary die with a diameter of

0.25 mm and a length of 2 mm was selected based on findings in Sections 3.4 and 3.5. The die

geometry was selected to maximize the pressure drop rate while maintaining a high extruder

pressure beyond the solubility pressure of 1wt% N2 (6.8MPa) [104]. The pressure drop rate

generated with this die was estimated to be 5.4 GPa/s using the same approach as demonstrated

in Section 3.4. For all experimental trials, processing temperature profile remained the same

from the extrusion barrel up to and including the mixers, while the gear pump and die

temperatures were varied together to study how foaming behavior is affected by temperature.

Foam samples obtained from the extrusion experiments were characterized in terms of their final

foam morphology, cell density, as well as the expansion ratio. Experimental conditions for the

extrusion foaming study is summarized in Table 3-5.

Figure 3-10 – Schematic of the extrusion foaming setup

64

Compound Sample N2

content

Temperature (°C)

T1 T2 T3 TMixer T Heat

Exchanger

TGear

Pump TDie

Neat PP

1 1wt% 180 190 200 200 200 200 200

2 1wt% 180 190 200 200 200 190 190

3 1wt% 180 190 200 200 200 180 180

4 1wt% 180 190 200 200 200 170 170

PP+CA

5 1wt% 180 190 200 200 200 200 200

6 1wt% 180 190 200 200 200 190 190

7 1wt% 180 190 200 200 200 180 180

8 1wt% 180 190 200 200 200 170 170

PP+Nanos

ilica

9 1wt% 180 190 200 200 200 200 200

10 1wt% 180 190 200 200 200 190 190

11 1wt% 180 190 200 200 200 180 180

12 1wt% 180 190 200 200 200 170 170

Table 3-5 – Processing conditions for the extrusion foaming study

3.5.2 Experimental Results

Figure 3-11 shows the shear viscosity measurements of Neat PP, PP+CA, and PP+Nanosilica

compounds. The viscosity dropped significantly when 3wt% of coupling agent was introduced in

the PP matrix, this phenomenon was expected since it is well-known that the highly functional

modified PP used in the coupling agent had extremely low viscosity; this would have further

negative effect to the already poor melt strength of PP. It is noteworthy that by introducing a low

content of nanosilica at 2wt%, the viscosity of the nanocomposite was enhanced by close to one

order of magnitude.

65

Figure 3-11 – Complex viscosity graphs of materials used in the visualization and extrusion study

3.5.2.1 Foaming Visualization Results

Figure 3-12 – Snapshots of in-situ foaming videos

Snapshots of the in-situ foaming videos of all three materials (Neat PP, PP+CA, and

PP+Nanosilica) are shown in Figure 3-12. It can be observed that the PP+Nanosilica case had the

earliest on-set of cell nucleation, demonstrating nanosilica’s ability to induce cell nucleation. The

corresponding cell density with respect to unfoamed volume (Nunfoam) vs. time data is displayed

in Figure 3-13. By differentiating the cell density data with time, the cell nucleation rate for each

66

case was determined and it was plotted in Figure 3-14. Based on Figures 3-13 and 3-14, it was

observed that nanosilica was very effective as a cell nucleating agent: it has the earliest onset

time of cell nucleation, the highest cell nucleation rate, and the highest cell density among all

three experimental compounds foamed under static conditions. In particular, Nunfoam increased by

approximately two orders of magnitude over the Neat PP case. The extraordinary cell nucleating

power of nanosilica could be due to the high interfacial area between nanosilica particles and the

polymer melt due to the small particle sizes. However, investigation to characterize the

dispersion of nanosilica in PP will be needed in the future to confirm this.

The reason behind the higher cell density observed for the PP+CA case was not clear. It was

suspected that the addition of CA might have reduced the surface tension of the polymer-gas

interface and hence the energy barrier for nucleation. However, this could not be confirmed at

present due to the lack of available surface tension data for this PP and PP+CA materials.

Figure 3-13 – The cell density vs. time

67

Figure 3-14 – Average cell nucleation rate

The competition phenomenon between cell nucleation and growth was not clearly observed in

the visualization foaming study. Although the PP+CA and PP+Nanosilica cases yielded orders of

magnitude higher cell nucleation rate than the Neat PP case, there was no statistically significant

trend in terms of the average cell growth rates among the cases (see Figure 3-15). Although the

nanosilica case yielded lower average cell growth rates, which appeared to indicate a retarding

effect on cell growth, there were significant variations in the measurements recorded.

Figure 3-15 – Average cell growth rate

68

3.5.2.2 Extrusion Foaming Results

SEM images of the foam morphology of the extruded foam samples are shown in Figure 3-16. In

the cases of the neat PP and PP+CA, very similar morphology was observed: low cell density

with large and non-uniform cell sizes. There were significant cell coalescence and/or coarsening

for those samples at all of the processing temperatures investigated. Meanwhile, in the case

PP+Nanosilica, the foam morphology improved significantly with higher cell density, smaller

cell sizes, and more uniform cell structure at all of the temperatures investigated.

Figure 3-16 – SEM images of extruded foam samples

The cell density with respect to unfoamed volume (Nunfoam) data for all materials is shown in

Figure 3-17. Cell densities for the PP+Nanosilica case peaked to close to 107 cells/cm

3, which

were approximately two orders of magnitude higher than the neat PP and PP+CA cases.

69

Figure 3-17 – Cell density of extruded foam samples

There did not seem to be a clear effect of the nanosilica, or CA on the foam expansion (Figure 3-

18), as the foam expansion for all materials largely overlapped across all temperatures

investigated. This could be due to the relative high processing temperature range used, hence

significant amount of gas might have diffused out of the polymer-gas mixture during cooling for

all cases. It is noted that the foam expansion ratio among all extruded foam samples were

between 1.2 and 1.8, which demonstrated that low-expansion foams were produced successfully

for all samples.

70

Figure 3-18 – Expansion ratio of extruded foams

3.5.3 Discussion on the Effect of Nucleating Agent in Cell Nucleation and Stabilization

In the in-situ foaming visualization study, the PP+CA case showed a surprisingly similar cell

density as that of the PP+Nanosilica case under static conditions. However, a similar trend was

not observed in the extrusion foaming experiment. It is believed that it was due to stress-induced

nucleation, which is much more significant in the extrusion case due to the continuous flow of

polymer-gas mixture. As Wong and Park showed [44, 105], the stress-induced cell nucleation

became much more significant in the presence of heterogeneity (i.e., nanosilica). This partially

explains why the cell densities were similar for the PP+CA and PP+nanosilica cases in the static

foaming study, but orders of magnitude different in the extrusion foaming study.

Moreover, the addition of nanosilica to PP increases the viscosity of the PP significantly (i.e., the

zero-shear viscosity increased from around 990 Pa-s for the neat PP to 1700 Pa-s for

PP+nanosilica, as measured with shear rheology). The increase in viscosity may have prevented

excessive cell growth that could have led to cell coalescence and/or coarsening, hence the PP’s

ability to stabilize the cell structures is enhanced. It is believed that the cell stabilization effect of

71

nanosilica is an important cause that leads to the significant increase in cell density for the

PP+nanosilica case when compared to the Neat PP case.

It is also worth noting that the cell density for the PP+CA case was significantly higher than the

neat PP case under static conditions in the visualization study, but their final cell densities in the

extruded samples were very similar to each other. It is believed that the low cell density of the

PP+CA case in foam extrusion is caused by excessive cell coalescence and/or collapse due to the

low viscosity of the material. Consequently, the final cell density was similar to the neat PP case

even though more cells could have been nucleated in the PP+CA case as suggested by the static

visualization study.

3.6 Conclusion

In the manufacturing of synthetic fibres, drawing is one of the most crucial processes as it

determines the tensile properties of the fibres. In the context of developing a strategy to produce

fine cell structure and low void fraction foam while ensuring the drawability of material, a series

of fundamental foaming studies were conducted in this chapter.

The role of the pressure drop rate, the blowing agent concentration, as well as the nucleating

agent content are each examined individually in the foaming trials; their effectiveness at

improving cell density are directly compared on a magnitude basis. The cell density increased by

as much as two orders of magnitude when 3wt% of nanosilica is present as the cell nucleating

agent. It is believed that the presence of nanosilica not only provides additional heterogeneous

nucleation sites for cells to nucleate, but also increases the viscosity of the composite, especially

at low nanosilica contents. The increase in melt viscosity is beneficial in both the cell nucleation

and the cell stabilization. The cell density experienced a further two orders of magnitude increase

72

when the N2 content is increased from 0.3wt% to 1wt%. The increase in N2 content increases the

degree of supersaturation at any moment during the nucleation process causing the nucleation

rate to increase significantly. The pressure drop rate also appears to have a strong effect: the cell

density increased by an order of magnitude when the pressure drop rate was increased by

approximately an order of magnitude. The die temperature did not play a significant role within

the range examined.

In order to further understand the role nanosilica plays on the cell nucleation and stabilization

mechanisms, studies were conducted with a static foaming visualization system and an extrusion

foaming system. The effect of the coupling agent on foaming is also investigated. It is discovered

that both the PP+nanosilica and the PP+CA compounds yielded much higher cell nucleation rate

and final cell density than Neat PP in the static visualization chamber, indicating strong

nucleating ability of the compounds. However, in the extrusion foaming study where shear and

extensional stress are dominant, PP+CA exhibited poor foaming behavior similar to that of the

Neat PP, while the presence of nanosilica drastically enhanced the foaming behavior of PP. It is

believed that stress-induced cell nucleation is much more pronounced in the extrusion system

where heterogeneity (nucleating agent such as nanosilica) is present in the compound;

furthermore, the improvement in the melt viscosity due to the nanosilica helps to enhance the

melt strength of material and reduce the amount cell coalescence and/or collapse.

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Chapter 4 Fibre Spinning of Low Void Fraction Poly (lactic) Acid- Clay

Nanocomposite Foam

4.1 Introduction

The annual demand of synthetic fibres, the majority of which is petroleum based PET, has been

estimated to reach 57 million tonnes by 2015 [106]. With the growing market for synthetic fibres,

there is much need for innovations in both the development of new fibres and the manufacturing

processes.

Due to the many superior properties associated with plastic foams, foaming has been looked

upon as one of the innovative strategies that can be applied to conventional fibre spinning. Upon

the successful application of foaming, the foamed fibres will not only be light weight in nature,

but also extremely economically viable because of the significant savings in material

consumption. Furthermore, the application of foaming could potentially improve the ductility as

well as the impact resistance that neat polymers are typically lacking.

In terms of selection of innovative materials, a new class of biodegradable and compostable

material derived from sustainable natural resources is gaining popularity. Poly (lactic) acid (PLA)

is the most commercialized in its class at the present time. Unlike its petroleum based

counterparts such as PET, the production of PLA does not involve the consumption of fossil fuel

resources; it is derived from annually renewable crops like corn and synthesized through either

condensation of lactic acid or ring-opening of the cyclic lactide dimer. At the same time, because

PLA is biodegradable and decomposable, its disposal makes minimal impact to the landfill and

the planet we live in. PLA was primarily used in the biomedical and clinical industry, however it

74

is quickly spreading into commercial and industrial end-user applications. In the manufacturing

of fibres, in particular, PLA is comparable to other popular materials such as PET and PP in

terms of mechanical properties. PLA fibres has found their paths in applications such as appeal,

blankets, duvets, drapes, wipes, hygiene products, filtration, compostable geotextiles, medical

textile scaffold, and etc. [106]. The potential size of the PLA fibre market poses great interest for

the foam industry.

One of the biggest challenges associated with the foaming of PLA is the poor viscosity of

material. In this context, nanoclay has been successfully used by numerous researchers to

enhance the melt viscosity and strain hardening phenomenon, as well as the foaming behavior.

Ray and Okamoto [58] reported foaming of neat PLA and PLA composite with nanoclay in a

batch system. They reported poor foamability of neat PLA mainly due to its low melt strength

which is required to withstand the stretching force during cell growth. The presence of nanoclay

helped to improve foaming by both acting as a cell nucleating agent, as well as enhancing the

melt viscosity to control cell growth and coalescence mechanism. Di et al. also reported the

increase in cell density and decrease in cell size with the presence of nanoclay [107]. They

attributed the increase in melt viscosity to the good interaction of PLA molecules and nanoclay

during mixing. In addition, nanoclay is capable of improving the gas barrier properties in a

polymer matrix [55].

The present chapter presents the feasibility study of foam fibre spinning with PLA- clay

nanocomposite. Strategies developed in Chapter 3 are utilized on the PLA-nanoclay composite to

produce the desired foam morphology for melt spinning; the foam extrudate is then subjected to

stretching to produce single filament foam fibre. The stretchability of fibres is studied against the

75

changes in fibre properties. Tensile properties of the as-spun foam fibres are compared with that

of unfoamed fibres.

4.2 Experimental

4.2.1 Materials

4.2.1.1 Fibre Grade PLA

A linear fibre grade PLA from NatureWorks LLC (Commercial grade Ingeo Biopolymer 6400D)

was selected for this chapter’s foaming and fibre spinning studies. It is a semi-crystalline

material with 2 wt% of D-content and a melt flow rate of 6 g/10min (tested with 2.16kg at 210°C)

[90]. It has a glass transition temperature of 55°C and a melting temperature of 167°C. This

grade of PLA was selected particularly because of the relatively low MFR among other fibre

grade PLA materials, which implicates higher melt viscosity; also the low D-content suggests the

material can crystallize relatively easily.

4.2.1.2 Nanoclay

Nanoclay Cloisite 30B from Southern Clay Products was selected to be used as a nucleating

agent to promote cell nucleation and stabilization. This clay contains about 30wt% organic

modifier (methyl, tallow, bis-2-hydroxyethyl ammonium) to increase the galaxy distance

between clay layers in order to enhance its affinity to polymers [57]. This type of organoclay has

been reported to have good compatibility to PLA matrices [58, 64, 107-109]. Furthermore, the

presence of nanoclay can improve the gas barrier properties of PLA, which prevents excess

blowing agent escaping the material after cells nucleate [54, 55]. Other favorable properties of

nanoclay include crystal nucleating ability, biodegradability, and flame retardant properties [54,

110].

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4.2.1.3 Preparation of Nanocomposite

To ensure the proper mixing of PLA nanoclay composite, the materials were compounded in an

industrial-scale twin-screw compounder to form a 10 wt% nanoclay masterbatch, which can then

be diluted to the desired nanoclay loading for each experimental compound. Details that pertain

the compounding process are not disclosed here due to the commercial value of the intellectual

property. To avoid any differences in the materials’ processing history, all of the neat PLA were

processed in the same compounding condition. Since PLA is a highly hydrophilic material, it is

necessary to dry the PLA resins to reduce the moisture content to minimize hydrolytic

degradation. The material had been dried in a Conair dehumidifying online drier (Conair W15) at

60°C for 8 hours prior to compounding.

A themogravimatric analysis (TGA) was conducted on the compounded masterbatch to test for

the concentration of nanoclay. The masterbatch pallet was heated from room temperature to

600°C at a rate of 20°C/min under nitrogen atmosphere. As shown in Figure 4-1, only 7 wt% of

materials were remaining at the end of the analysis; this indicated that the compounded

masterbatch contained 7 wt% of nanoclay, the missing 3 wt% could be the organic modifier in

the clay.

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Figure 4-1 – Thermogravimetric analysis on the nanoclay-PLA masterbatch

Neat PLA as well as compounds with 1wt%, 3wt% and 5wt% nanoclay were used in this series

of studies. They were obtained by dry blending the masterbatch with processed PLA before each

experiment. All blends had undergone the same drying procedure as mentioned before. The

compounds are referred to as Neat PLA, PLA+1NC, PLA+3NC, and PLA+5NC respectively in

later sections.

4.2.1.4 Blowing Agent

The blowing agent used in this chapter’s foaming studies was 99.98% purity extra dry grade

nitrogen (N2) supplied by Linde Gas. Solubility data of nitrogen in PLA had been measured in a

MSB apparatus with help of equations-of-state, and was reported in Li’s previous work [111].

The solubility pressure at the processing temperature will be reported alongside with actual

processing pressures in the results and discussion section.

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4.2.2 Experimental Equipment

4.2.2.1 Foam Fibre Spinning System

Figure 4-2 – Schematic of the foam fibre spinning system

Figure 4-2 shows a schematic of the foam fibre spinning system. It was mainly consisted of a

0.75” single screw extruder, a gear pump, as well as godet rollers for fibre stretching purposes.

Gas was injected with a high precision high pressure Teledyne ISCO 260D metering pump. In

order to enhance the generation of a uniform polymer-gas mixture, a six-element static mixer

with a diameter of 6.8 mm (Omega FMX-84441-S) as well as a heat exchanger containing

homogenizing static mixers (Labcore H-04669-12) were attached downstream to the extruder.

The gear pump (Oerlikon Barmag ZP504-0-IZ) was attached after the mixers to regulate the melt

flow before it reached to the spinneret. As the extrudate exit the spinneret, they were drawn by

gravity before reaching down to the first godet. A cooling column was constructed around the

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spinning path so that air cooling could be applied to the spin-line, when desired. The fibres were

further stretched by the high-speed rotational motion of the first godet roller. The stretching

action from the gravity as well as the first godet roller constituted the spin-draw of the fibres.

The spin-draw process provided the fibres a draw ratio of around 10:1. The as-spun fibres can be

further drawn by the higher-speed rotational motion of the second godet roller; however this was

not performed in the present study due to complications which will be explained in the results

section.

4.2.2.2 Spinneret Design

As in any foaming process, the depressurization device needed to be carefully selected to provide

adequate back pressure and pressure drop rate. In the foam fibre spinning system described here,

depressurization takes place inside of the spinneret. The modular spinneret utilized could house

six die inserts simultaneously to produce multifilament fibres; at the end of each die insert, there

was a trilobal shaped channel with minimal resistance. The geometry of the shaping channel are

shown in Figure 4-3, all units are in mm. Die inserts selected have a diameter of 0.25mm and a

length of 2mm; this geometry allowed for high pressure drop rate while maintaining reasonable

back pressure.

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Figure 4-3 – Geometry of the shaping channel

The shear rate experienced by the material while traveling through the die will be significantly

higher than that measured by the oscillatory rheometer, as will be shown in the results section.

Due to the lack of rheology information at the processing conditions, the pressure drop rate

induced by the die was not estimated.

4.2.3 Experimental Procedure

4.2.3.1 Extrusion Foaming Procedure

The strategy developed in chapter 3 was utilized in the extrusion foaming study to produce high

cell density low expansion PLA foam. The die was selected based on the generation of the

highest possible pressure drop rate; the amount of blowing agent injected was close to the

maximum amount soluble at the processing pressure.

Neat PLA and the three PLA-clay nanocomposites described in Section 4.2.1.3 were foamed

with 0.5wt% N2. The temperature profile for the extruder and the mixers was maintained

constant while the spinneret temperature was varied. As discussed previously, fibre spinning is

typically performed at temperatures higher than the material’s melting temperature to ensure that

81

the fibres are easily drawable once they exit the spinneret. As a result, the spinneret temperature

was varied between 230°C and 200°C, slightly lower than the recommended melt spinning

temperature [90]. Since PLA is prone to degradation under high temperature and long processing

time due to the hydrolysis reaction, the residence time of material was maintained at around 15

minutes. The speed control for the extruder motor and the gear pump motor was maintained at 10

RPM and 25 RPM respectively. To adjust the system pressure during foaming, four die inserts

were used in the spinneret; however, samples were only collected from the exit of the fixed insert

to maintain experimental consistency. Samples were collected as soon as they exit the die to

avoid any stretching.

To study the effect of N2 content, PLA+3NC compound was foamed with 0.2wt% N2 under the

same processing conditions. However, preliminary SEM images revealed poor foam morphology

from these samples, therefore a full factorial set of experimental studies were not carried out.

The foaming experimental matrix is summarized in Table 4-1.

At the end of the all foaming experiments, all four material compounds were melt compounded

in the equipment in the absence of blowing agent, such that DSC and shear viscosity

measurements can be carried out.

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N2

content Compound

Temperature (°C)

T1 T2 T3 Tmixer Theat exchanger Tgear pump Tspinneret

0.5wt%

Neat PLA

175 200 200 210 215 230 230

175 200 200 210 215 215 215

175 200 200 210 215 200 200

PLA+1NC

175 200 200 210 215 230 230

175 200 200 210 215 215 215

175 200 200 210 215 200 200

PLA+3NC

175 200 200 210 215 230 230

175 200 200 210 215 215 215

175 200 200 210 215 200 200

PLA+5NC

175 200 200 210 215 230 230

175 200 200 210 215 215 215

175 200 200 210 215 200 200

0.2wt% PLA+3NC

175 200 200 210 215 230 230

175 200 200 210 215 215 215

175 200 200 210 215 200 200

Table 4-1 – Experimental matrix for the extrusion foaming study

4.2.3.2 Foam Fibre Spinning Procedure

As shown in Figure 4-B, the fibre spinning system was equipped with two godet rollers; the first

godet roller completes the spin-draw process and heats up the as-spun fibres, and the fibres can

be subjected to further drawing by differentiating the two roller speeds. However during

experiment trials it was determined that the heating element on the first godet roller did not

provide adequate heating to soften the solidified foam fibre. As a result, no further drawing was

possible on the as-spun foam fibres. The collection process of fibre samples is described below.

For each of the extrusion foaming conditions, the foam filament was drawn to different degrees,

after which the foam fibre samples were collected. The degree of stretching or drawing can be

measured by the melt draw ratio (MDR) which is defined by the ratio between fibre velocity

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exiting the spinneret, denoted as vi, and linear velocity at the first godet, denoted as vf. Exiting

velocity, vi, can be estimated from the volumetric material flow rate, Qfoam, and the die diameter,

d, using Equation 4-1 shown below. Linear velocity, vf, was set directly from the system

controller. The vf was at first set at 200 m/min, and increased by increments of 200 until the spin-

line broke. In addition, foam fibres were also collected at the bottom of the system without any

mechanical stretching; the vf in this case is estimated by measuring the length of fibre collected

in a unit time. The godet roller speed and MDR achieved are tabulated in the results section.

Equation 4-1

4.2.4 Sample Characterization and Analysis

4.2.4.1 Complex Viscosity Measurement

Shear viscosity measurement of the four material compounds was carried out using an ARES

oscillatory rheometer from TA Instruments. The measurements were made with a pair of 25mm

diameter parallel plates with a gap of 1mm (sample thickness). The materials were determined to

be in the viscoelastic linear region at a shear strain of 2%, thus all of the frequency sweep tests

were carried out at 2% strain level from 0.1 rad/s to 500 rad/s. To study the temperature

dependency of each compound’s complex viscosity, all compounds were tested at the processing

temperatures of 230°C, 215°C, and 200°C.

4.2.4.2 Expansion Ratio

Typically, the expansion ratio of foam samples can be calculated by taking the density ratio

between foamed and unfoamed polymer, where the density of each is measured with the water

displacement method outlined in ASTM D792. Void fraction of the foam can be converted from

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the expansion ratio using Equation 3-1. In the cases of PLA foam samples, the water

displacement method was used to measure the expansion ratio of undrawn foam samples.

The same methodology was utilized to determine the void fraction of the as-spun foam fibres.

However it should be noted that PLA foam fibres had much bigger surface area than undrawn

filaments due to the much smaller diameters; as a result there was high probability of tiny air

bubbles to be trapped between fibres and on the uneven fibre surfaces, skewing the void fraction

measurement. Therefore the actual void expansion of the foam fibres could be lower than the

measurements obtained.

4.2.4.3 SEM Imaging and Foam Cell Density Characterization

A Scanning Electron Microscope (JEOL JMS6060) was used to examine the cellular

morphology of both undrawn filaments and drawn foam fibres. Undrawn samples were freeze-

fractured with liquid nitrogen to expose the cross-section. The drawn fibres were fished through

a fine hole in a fixture and fractured to expose the cross-section morphology; the foam fibres

were also cut along the spin-line direction to expose the morphology in the machine direction.

Because the cells were subjected to different degree of drawing, their aspect ratios were

calculated by dividing the average cell length with the average cell diameter. The cell aspect

ratio can be used as an important morphology feature.

Cell density of the undrawn foam samples can be calculated with respect to per unit unfoamed

polymer volume as per Equation 3-2. On the other hand, when calculating the cell density of the

foam fibres, the ellipsoid shape nature of the cells were taken into account by utilizing Equation

4-2 [112], where Nd,ellipsoid represents the cell density with respect to unfoamed polymer, L is the

average length of the ellipsoid cells, and D is the average diameter of the ellipsoid cells.

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( ) Equation 4-2

4.2.4.4 Differential Scanning Calorimetry

To determine the degree of crystallinity in final foamed samples, differential scanning

calorimetry (DSC) was conducted on all foam samples, both undrawn and drawn. Model Q2000

from TA Instruments was used in conducting the tests. Samples were heated in aluminum pans

from 20°C to 230°C at a rate of 10°C/min under nitrogen atmosphere. The degree of crystallinity

was obtained directly from the heating curve by using Equation 4-3, where χ is the degree of

crystallinity, ∆Hm is the melting enthalpy, ∆Hcc is the cold crystallization enthalpy, and 93.6 is

the melting enthalpy of 100% crystalline PLA [113]. The crystallinity measurement can be used

to study the effect of cell nucleating agent as well as the drawing operation on the formation of

crystals in foam products.

Equation 4-3

4.2.4.5 Tensile Testing

Tensile testing had been conducted on the fibre samples collected. Yield stress and Young’s

modulus were measured and compared between fibres spun at different draw ratios, as well as

before and after foaming. All of the tensile tests were performed on an Instron 5848 Microtester

with a 500N load cell. For direct comparison purposes, all fibres were tested at a constant

extension rate of 60% initial length per minute as per ASTM D3822. Since the gauge length was

maintained at 20 mm on the tensile tester, the extension rate was set at 12 mm/min.

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4.3 Results and Discussion

4.3.1 PLA-Clay Nanocomposite Foam

The system pressure during foam processing was recorded and plotted in Figure 4-4. Solubility

of N2 in PLA has been measured at temperatures of 180°C and 200°C by Li [111], extrapolation

of the data suggested that the system pressure remained higher than solubility pressure of 0.5wt%

N2 for all experimental runs. It should also be noted that the extrusion system had a built-in

safety feature that caps system pressure to just below 25MPa; as a direct result, Neat PLA and

PLA+1NC could not be cooled to 200°C as originally planned; the safety feature was triggered

by the heightened pressure.

As a general trend, the processing pressure increased linearly with decreasing temperature. This

revealed the temperature dependency of the materials viscosity. It was interesting to observe that

the PLA+3NC and PLA+5NC cases consistently experienced processing pressures lower than

that of Neat PLA and PLA+1NC at all spinneret temperatures examined. This could be resulted

from the differences in the materials’ rheological behavior, as will be discussed below.

Figure 4-4 – Extrusion system processing pressure

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The complex viscosities of the four material compounds were measured at the three processing

temperatures (shown in Figure 4-5). At 230°C, the complex viscosity increased with nanoclay

loading in the composite, showing a strong dependency between melt viscosity and nanoclay

content. As the testing temperature decreased, the complex viscosities of Neat PLA and

PLA+1NC increased significantly, whereas the viscosities of higher nanoclay content

compounds remained largely unchanged. The diminishing temperature dependency of the

composite viscosity is an indication of the formation of nanoclay networks in high clay content

composites; the effect of the clay-polymer and clay-clay interaction on the complex viscosity is

much more pronounced than the effect of temperature. It is also possible that the reduction in

chain mobility imposed by the presence of nanoclay is equivalent to that of the lowering in

temperature.

The viscosity curves of Neat PLA and PLA+1NC displayed the Newtonian plateau at the low

frequency range up to around 100 rad/s, and shear thinning at the higher frequencies; on the

other hand, high nanoclay content compounds (PLA+3NC and PLA+5NC) exhibited more

significant shear thinning throughout the frequency range examined. It is quite noteworthy that

the PLA+3NC viscosity curve crossed over both of the curves for Neat PLA and PLA+1NC at

frequencies between 1 and 500 rad/s depending on the testing temperature. The curve for

PLA+5NC exhibited very similar trend, however the cross-over did not occur in the range of

frequencies examined.

88

Figure 4-5 – Complex viscosity of PLA and nanocomposites

While the complex viscosity measurement obtained in Figure 4-E is useful in explaining material

behavior at low shear rate, it may not be representative of the material flow inside of the

spinneret due to the difference in shear rate. Using Equations 4-4 and 4-5, the shear rate in the

spinneret can be estimated to be 19392 1/s, where is the material flow rate and r is the die

diameter. According to Cox-Merz rule, the shear viscosity of a material is equal to the complex

viscosity when the shear rate is equal to the oscillatory frequency. The maximum frequency

examined in the rheometer is 500 rad/s or 80 1/s, which is more than two orders of magnitude

lower than what is experienced in the spinneret. As a result, the complex viscosity data measured

by the rheometer could not accurately represent the rheological behavior of the material at the

processing condition.

(

)

( ) ( ) Equation 4-4

(

)

( ) Equation 4-5

89

It is in fact hypothesized that at the shear rate representative of the processing condition, the

shear viscosity of PLA+3NC and PLA+5NC would be significantly lower than that of the Neat

PLA and PLA+1NC, as suggested by the processing pressure graph in Figure 4-4. This behavior

has been repeatedly reported in the literature: Ray and Okamoto [58] reported the shear thinning

phenomenon of PLA nanoclay composite in their dynamic rheology test, where similar cross-

over of viscosity curves for compounds with different clay content was observed. Baldi et al.

measured the shear viscosity of polyamide 6-nanoclay composite in a capillary rheometer [61],

and indicated that the nanocomposites appear to be less viscous than neat polymer under high

shear rates; they attributed this to the slip process between polymer melt and clay particles that

were aligned in the shear direction. Other researchers attributed the decrease in nanocomposite

viscosity to the plasticizing effect of the organic modifier (roughtly 30 wt%) in the Closite 30B

nanoclay [57, 114].

Each of the four compounds was foamed with 0.5wt% N2, the foam morphology of the samples

are shown in Figure 4-6. As mentioned earlier, Neat PLA and PLA+1NC could not be foamed at

200°C since the increasing melt viscosity drove the system pressure beyond the critical safety

pressure which triggered the automatic shut-down. As can be directly observed from the SEM

images, the Neat PLA did not foam at either 230°C or 215°C. Large gas pockets were present

due to the inability of cells to nucleate effectively; as a result, most of the gas molecules were

consumed in the growth of the small number of cells. This is similar to the foaming behavior of

Neat PP shown in Chapter 3. When nanoclay was introduced as a nucleating agent, the

morphology of the sample improved dramatically. At 1wt% loading, nanoclay exhibited

significant influence on the cell nucleation which resulted in a much improved cellular

morphology; cell sizes were larger than desired, however they formed good consistency. It is

90

important to point out that the Neat PLA and PLA+1NC cases exhibited similar shear viscosity,

thus the improvement in morphology shown is mainly rooted in the increase in heterogeneous

nucleation sites. As the nanoclay content was increased further to 3wt% and 5wt%, two

competing factors affected the cell nucleation process: the higher nanoclay content increased

heterogeneous nucleation sites due to its large interfacial area with polymer, meanwhile the high

nanoclay content compounds were expected to have lowered the shear viscosity, which limited

shear induced nucleation in the presence of nanoclay [43]. Furthermore, the introduction of

nanoclay has been reported to enhance the material melt strength and elongational viscosity of

polymer melts [58, 61]; hence improving the cell stabilizing mechanism of the composite during

cell growth [50]. At a clay content of 3wt%, the foam morphology improved from the PLA+1NC

case, as evident in the much higher cell count and much smaller cell sizes. A small fraction of the

cells were irregular in shape and large in size; this was caused by insufficient dissolution of

nitrogen in the matrix, or insufficient melt strength which caused excessive cell coalescence

and/or cell coarsening. As the nanoclay content was increased to 5wt%, the cell morphology

showed larger cells with less consistency. Within the processing temperature window examined,

the temperature did not exhibit significant influence on the cell morphology. It appears that

within the range of nanoclay content studied, best cell morphology was obtained with the

PLA+3NC compound. For this reason, PLA+3NC was foamed with 0.2wt% N2 to investigate the

effect of blowing agent content.

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Figure 4-6 – SEM graphs of PLA foamed with 0.5wt% N2

To study the effect of blowing agent content, the PLA+3NC compound was foamed with 0.2wt%

of N2. As proven in Section 3.4, the low blowing agent content caused lower degree of

supersaturation and increased the energy barrier for cell nucleation. The result is evident in the

comparison SEM images shown in Figure 4-7. Samples foamed at 0.2wt% N2 showed much

lower cell count and worsened cell morphology; large gas pockets were formed due to the

excessive cell growth. It is conclusive from the SEM images that the higher N2 content was

needed to produce the desired foam morphology. Therefore, subsequent experiments were

conducted only with 0.5wt% N2; a full factorial experiment was not carried out.

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Figure 4-7 – SEM graphs of PLA+3NC foamed with 0.2wt% and 0.5wt% N2

Cell density of the PLA foam samples was not significantly dependent on the processing

temperature. As shown in Figure 4-8, the more dominant effect on cell density was the use of

nanoclay as a cell nucleating agent. Foam samples produced with PLA+1NC, PLA+3NC, and

PLA+5NC all had cell densities higher than 106 cells/cm

3, which was an improvement over the

Neat PLA case by close to two orders of magnitude. The highest cell density was consecutively

obtained at the highest processing temperature of 230°C by all of the four compounds. On the

other hand, the void fraction of foam samples was not dominated by the nanoclay content; rather,

it displayed a weak correlation with the processing temperature. Samples obtained at 230°C had

the void fractions between 40-50%, higher than the rest of the compounds.

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Figure 4-8 – Cell density and void fraction of PLA and nanocomposite foam

Figure 4-9 and Table 4-2 summarizes the thermal properties of PLA foam samples obtained from

the DSC. The first thing to notice is that as the nanclay content increased from 0% to 3%, the

glass transition temperature (Tg) increased; further increase in the clay content decreased Tg. The

increase in Tg of nanocomposite was commonly observed and was generally attributed to the

reduced chain mobility imposed by the clay particles; the decrease in Tg at high nanoclay content

was previously observed by Miyagawa et al. [115], they believed that the organic modifier used

in clay acted as a plasticizer and promoted the glass transition process. PLA is well known for

the slow crystallization kinetics. Since the extrusion conditions were far off from the ideal

isothermal condition, the foam samples tested in the DSC all showed relatively low degree of

crystallinity. As the nanoclay content was increased, the crystallinity was observed to increase

slightly. This could be attributed to the crystal nucleating ability of nanoclay where the dispersed

nano-particles reduced the mobility of polymer chains to an extent and enhanced the formation

of crystals as well as the crystallization rate. This observation formed good agreement with what

had been reported in the literature [62, 107, 116]. In addition, the introduction of nanoclay

94

caused earlier onset of cold crystallization (Tcc), as well as the narrowing of the cold

crystallization peak; this had also been observed by other researchers [63, 108]. The shift in the

onset temperature for cold crystallization suggested that the clay particles enhance packing of the

PLA chains making it easier to re-crystallize during heating. Although it has been suggested that

high content of exfoliated nanoclay could cause physical hindrance to the chain mobility and

retard the overall crystallinity of the material [107], the phenomenon was not observed in the

scope of the current study. The decrease in melt temperature (Tm) served as an indication that

smaller or less perfected crystals were formed under the influence of nanoclay.

Figure 4-9 – DSC first heating curve on undrawn foam

Compound Tg (°C) Tcc (°C) Tm (°C) Xc (%)

Neat PLA 55.57 102.85 168.27 6.66

PLA+1NC 57.79 99.37 167.31 8.01

PLA+3NC 57.9 95.98 166.17 10.15

PLA+5NC 55.34 93.73 164.81 11.52

Table 4-2 – Thermal properties of PLA and nanocomposites

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4.3.2 As-Spun PLA Foam Fibre

After collection of undrawn foam samples, foam produced in each experimental condition was

drawn to different melt draw ratio (MDR). As previously mentioned, MDR is defined by the

ratio of the collection speed and the exit speed at the spinneret. An increase in MDR should

theoretically increase the degree of chain alignment in the fibre causing enhanced crystallinity

and mechanical properties. For this reason, the drawability of the foam fibres as well as the

property changes induced by the drawability was the centre of the spotlight in this study. Since

the material flow rate was maintained constant throughout this study, the exit velocity remained

unchanged and was calculated to be 60.6cm/s using Equation 4-3. During the collection of foam

fibres, the speed of the first godet roller was increased at increments until fibre breakage

occurred. MDR for each condition was calculated based on the roller speed, and tabulated in

Table 4-3.

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230°C

Neat PLA Collection (cm/s) 86 333 667 1000

MDR 1.4 5.5 11 16.5

PLA+1NC Collection (cm/s) 104 333 667 1000

MDR 1.7 5.5 11 16.5

PLA+3NC Collection (cm/s) 88 333 667 1000

MDR 1.5 5.5 11 16.5

PLA+5NC Collection (cm/s) 65

MDR 1.1

215°C

Neat PLA Collection (cm/s) 79 333 667 1000

MDR 1.3 5.5 11 16.5

PLA+1NC Collection (cm/s) 82 333 667 1000

MDR 1.4 5.5 11 16.5

PLA+3NC Collection (cm/s) 108 333 667

MDR 1.8 5.5 11

PLA+5NC Collection (cm/s) 65 333

MDR 1.1 5.5

200°C

PLA+3NC Collection (cm/s) 80 333

MDR 1.3 5.5

PLA+5NC Collection (cm/s) 62 333

MDR 1 5.5

Table 4-3 — Melt Draw Ratio of foam fibres

4.3.2.1 As-Spun Foam Fibre Morphology

As-spun foam fibre samples were examined under the SEM for their morphology in the cross-

section and the machine direction.

Figures 4-10, 4-11, and 4-12 show the cross-section morphology for foam fibres spun at 230°C,

215°C, and 200°C, respectively. The diameters of foam fibres were shown to decrease with

increasing draw ratio, regardless of foam morphology, material composition and temperature.

MDR seemed to have little effect on the morphology otherwise. Among the four material

compounds, similar morphology trend was observed for the foam fibres as the ones for undrawn

foams. Neat PLA showed extremely poor morphology with singular voids were scattered across

the cross-section; this type of inconsistent morphology would form stress concentrators and were

97

very undesirable. Cellular morphology started to improve when as little as 1wt% nanoclay was

introduced; however decreasing spinneret temperature caused cellular morphology to worsen for

PLA+1NC fibres. The worsened fibre morphology could be attributed to the increase in melt

strength which limited cell growth. PLA+3NC yielded very reasonable cell morphology at 230°C

with the highest apparent cell count and most consistent cell size and cell geometry;

unfortunately the maximum draw ratio obtained with PLA+3NC fibres declined from 16.5 to 5.5

as the spinning temperature was decreased to 200°C. The drop in spinneret temperature caused

the solidification of fibres to take place sooner, and the brittleness of PLA fibres below the glass

transition temperature caused the fibre breakage. The drawability of the PLA+5NC case was the

most interesting of all. At 230°C, the spin-line of the foam was extremely fluid like, possibly due

to the plasticizing effect of the organic modifier in the high loading of clay; the fluidity of the

spin-line made the collection of foam fibres impossible. As the spinneret temperature was

decreased to 215°C, the spin-line solidified enough to be collected. However, further decrease of

the processing temperature caused the spin-line to become extremely brittle causing fibres to

break when stretching was applied, similar to the PLA+3NC case. Combining all of the factors

mentioned above, reasonable drawing could not be obtained with the PLA+5NC compound. It

should be noted that although nanoclay enhanced the foaming behavior of PLA significantly, the

enhanced melt strength and elongational viscosity significantly reduced the drawability of the

foam fibres. This negative influence of elongational viscosity and melt strength on drawability of

polymeric material had also been confirmed by Laun and Schuch [117].

98

Figure 4-10 – Cross-section SEM images of foam fibre spun at 230°C

99

Figure 4-11 – Cross-section SEM images of foam fibre spun at 215°C

Figure 4-12 – Cross-section SEM images of foam fibre spun at 200°C

100

The foam morphology of the fibres in the machine direction is shown in Figure 4-13. Neat PLA

showed the highest cell lengths regardless of the draw ratio experienced. This might have been

caused by the uneven distribution of large gas pockets within the foam fibres. As nanoclay was

introduced, the morphology in the machine direction improved just like the case for the cross-

section morphology. Cells exhibited shorter lengths than those of the Neat case. It was

interesting to observe that the lengths of the cells generally increased with the melt draw ratio.

As the nanoclay content increased, the cell lengths decreased as a result of higher resistance for

extensional deformation caused by the interaction between clay particles. This also highlighted

the decrease in fibre drawability with increasing nanoclay content.

Figure 4-13 – Machine direction SEM images of foam fibre spun at 230°C

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4.3.2.2 Foam Fibre Drawability

The diameters of the foam fibres are plotted against the MDR for each experimental case. This

allows comparison on the amount of deformation that occurred to each compound during

drawing. As shown in Figure 4-14, the four compounds displayed very comparable deformation

when subject to drawing: all diameters decreased to below 200µm at a draw ratio of 5.5; further

increase in draw ratio did not significantly reduce fibre diameters.

Figure 4-14 – Foam fibre drawability

As the processing temperature was decreased, the increase in material melt strength caused the

fibres to be less drawable showing larger diameter at a draw ratio of around 1.5. In addition,

fibres spun at lower temperatures were much more brittle; they often fractured with minimal

elongation as the samples were collected from the godet roller. This could be due to the fact that

they reached solidification point much sooner than fibres spun at 230°C, limiting the opportunity

for molecules to orientate themselves in the tensile direction before the structure was frozen.

Since all as-spun fibres were required to undergo further drawing to enhance molecular

orientation and improve mechanical properties, drawability of the as-spun fibres is an essential

102

property that cannot be neglected. As an unfortunate consequence, it is determined that the fibres

foamed and spun at 215°C and 200°C were not feasible for further studies since they would not

be able to withstand any further drawing.

Figure 4-15 displays the change in the average cell aspect ratio in the foam fibres spun at 230°C.

Due to the poor foam morphology of Neat PLA fibres, the cell aspect ratio did not show

significant trend. For PLA+1NC, the aspect ratio of cells increased with the melt draw ratio as

the cells were subjected to the same stretching action during the spin-draw process. As the

nanoclay content increased to 3%, the enhanced clay-to-clay interaction caused higher

extensional viscosity for deformation to occur; consequently, the cell aspect ratio of PLA+3NC

foam fibres increased slower than the PLA+1NC case and peaked as the draw ratio was

increased.

Figure 4-15 – Average cell diameter in foam fibres

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4.3.2.3 Foam Fibre Characterization

Void fraction of the foam fibres had been measured with the underwater displacement method

outlined previously. Cell density had been estimated based on the cross-section and machine

direction morphology. As shown in Figure 4-16, both cell density and void fraction were

correlated to the nanoclay content. The higher the nanoclay content, the higher are the cell

density and the void fraction for the foam fibres. This, as suggested previously in Chapter 3, was

the result of the enhanced cell nucleation and stabilization ability of the nanocomposites. On the

other hand, with increasing MDR, void fraction decreased slightly. A possible explanation is that

the fibre diameter decreased with increasing MDR; which reduced the diffusion distance for gas

to escape outside of the foam. The measurement of average cell density for the foam fibres

showed two opposite trend. Neat PLA compound showed increasing cell density as the draw

ratio was increased; the increase in cooling rate at high draw ratios could have resulted the

increase in cell density. The two nanocomposites, PLA+1NC and PLA+3NC, showed decreasing

cell density with increasing draw ratio; this could be caused by the increase in cell lengths at

high draw ratios; according to Equation 4-2, the higher the volume of each cell, the lower the

estimated cell density.

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Figure 4-16 – Estimated cell density and void fraction of the foam fibres spun at 230°C

To examine the effect of MDR on thermal properties, the degree of crystallinity in particular,

each foam fibre sample was weighed and tested in a DSC. Thermal properties of the foam fibres

are summarized in Table 4-4, the first heating curves from the DSC are shown in Figure 4-17.

The degree of drawing, measured by MDR, did not pose significant effect on the glass transition

temperatures of all four compounds. For the Neat PLA and PLA+1NC, both the cold

crystallization temperature and the melting temperature decreased with increasing draw ratio.

The decrease in Tcc indicated a more enhanced crystallization kinetics, which was caused by the

enhancement in chain orientation during the fibre drawing process, with or without low content

of nanoclay. The decrease in Tm was due to the formation of smaller or less perfected crystals as

chains were oriented and packed closer together. On the other hand, the correlation between Tcc,

Tm and the draw ratio was not observed for PLA+3NC and PLA+5NC cases; it had been

demonstrated that the interaction between clay particles at high loadings could act as physical

hindrance for PLA reducing its mobility [107].

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The crystallinity of PLA as-spun fibres have been studied by Fambri et al. [118]. The

crystallinity of the as-spun fibre was reported to increase from 5% to 35% while the collection

speed increases from 1.8 to 10 m/min; however, further increase in the collection speed caused

faster cooling on the spin-line, reducing overall crystallinity of the fibre. In the current study,

however, the collection speeds were much higher (in the range of 200-600 m/min); the cooling

effect would be much more dominant. For the Neat PLA and PLA+1NC cases, total crystallinity

increased slightly with the draw ratio, signaling higher orders of chain orientation caused by

drawing. Nanoclay at high loading reduced the material’s overall crystallinity due to the reasons

mentioned above. Even at 200-600 m/min collection speed, the process is still considered low

speed spinning; post-spinning drawing operation is expected to further improve the overall

crystallinity of the foam fibres.

Compound MDR Tg (°C) Tcc (°C) Tm (°C) Xc (%)

Neat PLA

1.0 55.57 102.85 168.27 6.66

1.4 55.16 101.91 167.83 7.75

5.5 55.37 100.84 167.65 9.34

11.0 55.53 99.93 167.01 10.53

16.5 54.89 99.14 166.65 15.09

PLA+1NC

1.0 57.79 99.37 167.31 8.01

1.7 57.79 99.19 166.79 9.92

5.5 57.17 98.43 166.93 12.60

11.0 57.02 97.57 166.59 14.86

16.5 57.68 97.41 167.11 17.20

PLA+3NC

1.0 57.90 95.98 166.17 10.15

1.5 58.31 96.04 166.74 11.82

5.5 58.41 95.84 166.90 13.77

11.0 57.66 95.77 166.52 14.05

16.5 57.83 95.85 166.25 14.08

PLA+5NC 1.0 55.34 93.73 164.81 11.52

1.1 55.77 93.87 165.41 13.02

Table 4-4 – Thermal properties of PLA foam fibres

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Figure 4-17 – First heating curves of the foam fibres obtained from the DSC

107

4.3.3 Tensile Properties of As-Spun PLA Foam Fibre

The biggest concern associated with the concept of foam fibres with internal voids was the

deterioration of the mechanical properties. If the severe reduction in mechanical properties were

to be caused by foaming fibres, it would largely limit the areas of applications where foam fibres

were suitable for. Although it has been mentioned that the as-spun foam fibres examined in this

chapter require further drawing to enhance its properties, it is still valuable to compare the

mechanical properties of these as-spun foam fibres with those of as-spun unfoamed fibres. For

this reason, both the Young’s modulus and the yield stress of the as-spun fibres were examined.

PLA+5NC fibres were not examined in this section as draw ratio could not be obtained beyond

1.5 even at the temperature of 230°C. Young’s modulus is mainly a material property that is

largely dependent on the structure and composition of a material. Yield stress, on the other hand,

reflects largely the homogeneity and morphology of a material.

4.3.3.1 Comparison of Tensile Properties between Foamed and Unfoamed Fibres

Figure 4-18 shows a direct comparison of Young’s modulus of the as-spun unfoamed (left) and

foam fibres (right). As far as unfoamed fibres were concerned, the content of nanoclay in the

compound seemed to be the dominating factor for the Young’s modulus. Modulus measured for

PLA+3NC nearly doubled the modulus measurement of Neat PLA and PLA+1NC; the strong

interaction between clay particles was believed to have caused the significant increase in

modulus. At only 1wt% nanoclay, the content level might have been too low to affect the

material’s tensile modulus. Within the range of MDR achieved for the as-spun fibres, it was not a

strong factor for the modulus measurement. For the as-spun foam fibres, the Young’s modulus

also displayed positive correlation to the nanoclay content. Fibres foamed with 3wt% nanoclay

experienced a 20% drop in Young’s modulus when compared to the conventional as-spun PLA

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fibres (unfoamed Neat PLA case). Since Young’s modulus is a material property, it should not

be strongly affected by the foam morphology of fibres. However, since the presence of voids

inside the foam fibres reduced effective fibre diameter, the actual stress and modulus

experienced by the material would be much higher than calculated based on apparent fibre

diameters. The difference in the void fractions between compounds could explain the difference

in reduction of the Young’s modulus.

Figure 4-18 – Comparison on Young’s modulus of the as-spun fibres

Similar to the measurement of Young’s modulus, the yield stress did not seem to be significantly

affected by the MDR (Figure 4-19). This is probably due to the fact that the drawing speed was

too low to induce significant chain orientation in the tensile direction; this was especially true for

the nanocomposite cases where the clay particles decreased the mobility of PLA molecules.

Further post-spinning drawing process would be required to enhance the crystallinity and the

mechanical properties. With increasing nanoclay content in the material compounds, the yield

stress increased for the unfoamed fibres, while exhibiting decreasing elongation at break. By

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comparison, the opposite trend was observed for the foam fibres where yield stress showed

decreasing trend with increasing nanoclay content. Void fraction of the foam fibres was likely

the most dominant factor affecting foam fibres’ yield stress. Since the void fraction for

PLA+3NC doubled the void fraction for Neat PLA, it is understandable that the yield stress

decreased accordingly. The cellular structure of the foam fibres introduced more potential sites

for crack initiation.

Figure 4-19 – Comparison on the yield stress of the as-spun fibres

The tensile strength and modulus of PLA fibres have been previously reported to be in the range

of 50 MPa and 3 GPa respectively [119]. This is in good agreement with the tensile results

obtained for the unfoamed fibre in the current study. Fambri et al. demonstrated that the tensile

property of the as-spun fibres strongly depended on the draw ratio of as-spun fibres. In their

study, fibres were collected at a draw rate of between 4 and 25, although at extremely low

colleting speed of 1.8-20 m/min. The yield stress of their as-spun fibres were between 60-120

MPa; when the fibres were further drawn at 160°C, the yield stress further increased to a

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maximum of 290 MPa [118]. Similar results were obtained by Yuan et al. [120], as the PLLA as-

spun fibres were treated with hot drawing, the modulus increased from 1.2-2.4GPa to 3.6-5.4GPa;

the yield stress of the hot drawn fibres were above 300MPa. These results pointed out promising

area of improvement by utilizing post-spinning drawing.

4.3.3.2 Factors Affecting Tensile Properties of the As-Spun Foam Fibres

Mechanical properties of fibres have been mainly associated with the drawing speed in the

literature. However, by introducing foaming to conventional fibre spinning, a number of material

and processing parameters previously unaccounted for are introduced, some of which are directly

affected by the foaming process: fibre density, degree of crystallinity, cell density, and cell sizes.

While these material/morphology parameters were coupled in the foaming process, and could not

be individually studied, they were analyzed in a simple linear regression to indicate possible

correlation. Since the content of nanoclay had shown to have great influence on Young’s

modulus and the yield stress (Figures 4-20 and 4-21), it was also included in the analysis. To

decrease the sensitivity of the line regression to the magnitude difference of each parameter,

values for all parameters (including Young’s Modulus and Yield Stress) were normalized to a

standard score using Fisher-z transformation. As a result, every term in the linear regression

expression is unitless.

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Figure 4-20 – Young’s modulus vs. density (a), crystallinity (b), cell density (c), average cell diameter (d)

As previously mentioned, Young’s modulus is a material property which depends on the

micro/macro molecule structure as well as the composition of the material. The linear regression

was carried out with clay content, density and crystallinity. Density and standard deviation of the

parameters used in obtaining the standard score are listed in Table 4-5. The linear regression of

the selected inputs resulted an R2 value of 76.3%. As shown in Equation 4-5, the clay content

was still the strongest influencer of the young’s modulus; this might be because the content of

nanoclay determined the level of particle interaction among clay layers. Density of the foam

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fibres was the second strongest factor as it determined the amount void inside of material. It

should be noted that although both the clay content and density of fibre were positively

correlated to the value of Young’s modulus, they acted as competing factors. Higher clay content

yielded fibres with lower density. The degree of crystallinity, according to the linear regression,

did not affect fibre’s modulus significantly. The low level of variation in crystallinity among

samples might be the cause of this. Parameters associated with the foam morphology were not

examined in the linear regression as they were not considered to be material property parameters.

Modulus Clay content Density Crystallinity

Mean 1.5977 1.3333 0.9947 13.5030

Standard Deviation 0.2712 1.3229 0.0850 2.3905

Table 4-5 – Mean and standard deviation of parameters for modulus

( ) ( ) ( )

Equation 4-5

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Figure 4-21 – Yield stress vs. density (a), crystallinity (b), cell density (c), average cell diameter (d)

Material’s yield stress depends mainly on the homogeneity of the material. As a result, the

morphology of the foam fibres is expected to have significant effect on the yield stress of the

fibre. Linear regression was performed with clay content, degree of crystallinity, cell density and

density. Density and standard deviation of the parameters used in obtaining the standard score

are listed in Table 4-6. The average cell diameter was left out of the analysis as a key parameter

since its correlation was too weak when compared with other parameters. Clay content was again

the most dominating factor on the fibre’s yield stress due to reasons discussed above. It was

reflected in the linear regression equation, which was obtained with a R2 value of 71.7%. Foam

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characters such as the density and cell density could be both positively correlated to the yield

stress. Crystallinity again showed very weak correlation.

Yield Stress Clay content Density Crystallinity Cell Density

Mean 27.2043 1.3333 0.9947 13.5030 9204030.6776

Standard Deviation 4.9933 1.3229 0.0850 2.3905 7329803.9925

Table 4-6 – Mean and standard deviation of parameters for yield stress

( ) ( ) ( )

( ) Equation 4-6

The linear regression analysis used in this section can be used as a simple tool to look for

possible correlation between material parameters and the mechanical properties for the foam

fibres. However the coefficients obtained for each tested parameter were not intended to be used

quantitatively for comparison purposes.

4.4 Conclusion

Utilizing the strategy developed in the previous chapter, the foaming experiments are conducted

with fibre grade PLA with a target of high cell density and low void fraction. Foaming has been

carried out with nanoclay as the cell nucleating agent and nitrogen as the blowing agent. The

introduction of nanoclay significantly increases the heterogeneous nucleating sites promoting

cell nucleation. However, as the nanoclay content increases, the shear viscosity is expected to be

lowered due to the possible slip action between the clay particles and the polymer molecules; it

could also be attributed to the plasticizing effect from the organic modifier used in the clay. An

equilibrium nanoclay content for the two competing phenomenon is found to be 3%; the

compound yields the best foam morphology and the highest cell density. DSC thermographs also

show increased crystallinity in the foam samples as the nanoclay content is increased. It is

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believed that the presence of nanoclay limits chain mobility during the crystallization process,

leading to the higher crystallinity. This theory is further supported by the decrease in cold

crystallization temperature, which suggests improved packing of PLA chains.

The drawability of the PLA nanocomposite foams has been demonstrated in the spin-draw study.

The melt draw ratio for each sample has been calculated and presented as a measure of the

degree of drawing experienced by the foam. A maximum MDR of 16.5 is achieved with Neat

PLA, PLA+1NC and PLA+3NC at the spinneret temperature of 230°C. The best morphology

among all foam fibres is obtained with the PLA+3NC compound, similar to the undrawn samples.

The diameters of all foam fibres decreases to between 100µm and 200µm at the MDR of 5.5,

however further increase in the MDR shows little impact on the fibre diameter. As the MDR

increases, the cell density decreases for the two nanocomposites as the cells are elongated by the

stretching action. The void fraction is shown to decrease with increasing melt draw ratio; as the

draw ratio increases, fibre diameters decrease, and effectively reduces the diffusion distance for

gas to escape. DSC results show weak, yet positive, correlation between MDR and the total

crystallinity of the foam fibres. This suggests while stress induced crystallization was initiated

during the spin-draw process, the draw speed achieved was not sufficient to induce significant

crystal formation. The lack in crystallization is reflected in the tensile test results as well. Neither

the Young’s modulus nor the yield stress measured from the foam fibres is shown to be

dependent on the draw ratio; although, both are shown to be strongly affected by the nanoclay

content. The presence of nanoclay increases the Young’s modulus as the clay particles form

strong interaction between themselves and with PLA. Yield stress of the foam fibres decreases

with increasing nanoclay content; however it is important to keep in mind that the void fraction

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in foam fibres is strongly correlated to the nanoclay content. The decrease in yield stress is likely

a direct result of the decrease in the density of the foam fibres.

As suggested by both the DSC results and tensile measurements, additional drawing is required

to enhance stress-induced crystallization in the as-spun foam fibres. Further drawing can be

made possible by the thorough heating of the as-spun foam fibres in a heating element. Further

studies are required to confirm the feasibility and effectiveness of this strategy.

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Chapter 5 High Expansion PLA Foaming- A Potential Strategy for

Producing Foam Fibres

5.1 Introduction

It has been demonstrated in Chapter 4 the feasibility of producing as-spun foam fibres on a

modified melt-spinning system directly. A tensile study revealed that the maximum melt draw

ratio achieved in the spin-draw process was not sufficient to enhance the tensile properties of the

fibres; post-spinning drawing procedures were deemed necessary to induce additional chain

orientation. Since PLA is in the glassy state at room temperature, as-spun fibres need to be

heated thoroughly to above its glass transition temperature for the drawing process to take place.

Besides the thorough heating of fibres, the foam morphology might also require improvement to

ensure that the cellular fibres are able to sustain the drawing action. With the two concerns in

mind, an alternative approach is suggested: foam is first obtained with improved cell density and

morphology without any spin-drawing, the foam can then be subsequently heated up to the

drawing temperature and drawn to the desired ratio.

There have been several approaches taken by researchers to enhance the foamability and foam

morphology of PLA. Nanoclay is widely used for PLA foaming as a cell nucleating agent; it was

shown to significantly improve the foaming behavior [107, 116, 121], as well as thermal and

rheological properties [58]. Material modifications such as branching and the usage of chain

extenders had been adopted to effectively increase the melt strength and elasticity of PLA in

order to improve its foaming behavior [58, 122, 123]. However, this approach is not desirable for

the foam fibre spinning application as the PLA needs to remain the linear molecular structure to

ensure the drawability. The melt strength of the PLA can also be enhanced by inducing

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crystallization. Nofar et al. demonstrated that the crystallinity of PLA can be improved under the

presence of dissolved CO2 [124]. They attributed the increase in crystallinity to the formation of

more perfected crystals due to the plasticization effect of gas. Mihai et al. [122] claimed that the

biaxial stretching of material during the cell growth is largely accountable for the increase in

crystallinity. Wang et al. [125] investigated means of controlling the crystallinity of PLA foam

through shear and the presence of supercritical CO2. By inducing crystallization in the linear

PLA matrix, they obtained foam samples with high expansion ratio and a high cell density of 107

cells/cm3.

In this context, this chapter presents the foamability study of the fibre grade PLA using CO2 as a

blowing agent and nanoclay as a nucleating agent. The goal is to produce PLA foam with high

cell density and fine morphology using induced crystallization as means to enhance melt strength

and elasticity. Such foaming strategy can be later adopted for the manufacturing of PLA foam

fibres.

5.2 Experimental

The foaming study presented in this chapter serves as an extended feasibility study for Chapter 4.

As a result, the experimental methodology utilized here was very similar to that of the last

chapter. In addition, experimental trials were only performed with the most promising material

compound as found in Chapter 4; a full factorial experimental matrix was not followed.

5.2.1 Experimental Materials

The material compound used was the PLA+3NC compound as seen from Chapter 4. Material

grades and detailed descriptions of the preparation process were covered in Section 4.2.1. Bone

dry carbon dioxide (CO2) was used as a blowing agent as its solubility is much higher than that

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of nitrogen [111]. High blowing agent concentration should not only improve the cell nucleation,

but also enhance the plasticization effect in the polymer melt enhancing the crystallization of

PLA.

5.2.2 Experimental Equipment

The modified melt spinning equipment as shown in Figure 4-B was again used for the foaming

experiment. Equipment setups as well as the die geometry remained the same; this is for the

purpose of minimizing the processing environment variation as seen in Chapter 4, such that

better comparison on the foaming results could be established. Please refer to Section 4.2.2 for

the list of equipment used in the experimental setup.

5.2.3 Experimental Methodology

The experimental methodology was designed to minimize the number of foaming trials

necessary to complete the feasibility study. PLA+3NC compound was foamed with 5wt%, 8wt%,

and 11wt% CO2 respectively. Since the goal of this study was to stimulate PLA crystallization

under the presence of dissolved CO2, processing temperatures were set lower than previously

used in Chapter 4; the lower temperature also reduced the likelihood of hydrolysis reactions

taking place in the extruder. During the 5wt% CO2 trials, RPM for the extruder and the gear

pump were maintained at 8 and 15 respectively, resulting a residence time of around 15 minutes.

However the lowest spinneret temperature achieved was 170°C due to the high system pressure.

During the 8wt% and 11wt% CO2 trials, RPMs were reduced to 5 and 4 respectively, prolonging

the residence time to around 25 minutes; at the same time, processing temperatures were

gradually decreased in the processing equipment from 200°C to 150°C. The increase in the

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residence time and the cooling temperature profile should assist in the crystallization of PLA

before foaming takes place. The full experimental matrix is shown in Table 5-1.

Compound CO2

content

Residence

Time

(min)

Temperature (°C)

T1 T2 T3 Tmixer Theat

exchanger

Tgear

pump Tspinneret

PLA+3NC

5wt% 15 175 185 190 200 200 175 175

175 185 190 200 200 170 170

8wt% 25

175 185 190 200 200 160 160

175 185 190 200 200 155 155

175 185 190 200 200 150 150

11wt% 25

175 185 190 200 200 160 160

175 185 190 165 160 155 155

175 185 190 165 160 150 150

Table 5-1 – Experimental matrix for PLA extrusion foaming with CO2

5.3 Results and Discussion

The system pressure during each experimental trial was plotted in Figure 5-1. As can be seen, all

trials had been conducted at a pressure nearing system capacity, and much higher than the

solubility pressure for 5wt%, 8wt%, and 11wt% CO2 reported by Li [111]. As the spinneret

temperature was decreased, pressure in the system rose as a result of increasing material melt

viscosity.

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Figure 5-1 – Processing pressure during extrusion foaming with CO2

The morphology of foam obtained is shown in Figure 5-2 below. For the 5% CO2 trials, cellular

morphology was poor for both 175°C and 170°C cases; cell sizes varied between below 10 µm

and 80 µm. For the 8% CO2 cases, the decrease in the spinneret temperature resulted in

significant changes in the foam morphology: cellular morphology appeared much more discrete

for the lower temperature with almost no cell coalescence present; cell count increased while cell

sizes decreased. During trials for the 11% CO2 case, the cooling profile applied in the mixers and

the spinneret further enhanced the melt strength. The foam morphology improved drastically at

the temperatures of 155°C and 150°C; cellular structure was shown to have high cell count and

consistently small cell sizes. Further decrease in the processing temperature could not be carried

out due to the increase in system pressure. However, the morphology obtained from the CO2

blow foams was of much improvement.

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Figure 5-2 – SEM images of PLA foam blown with CO2

The improved morphology observed in Figure 5-2 for the 8% and 11% CO2 cases are also shown

in the void fraction and cell density charts presented in Figure 5-3. Both void fraction and cell

density decreasd as the processing temperature decreased for the 5% CO2 trials. This could have

been caused by the decrease in pressure drop rate at the die insert due to the dropping material

flow rate. However, for the 8% and 11% CO2 cases, the processing temperature seemed to be a

significant factor for the void fraction and cell density. The cell density was close to 109

cells/cm3 at the temperature of 150°C for the 11% CO2 case. The increase in cell density was of

two orders of magnitude when compared to the best case scenario in Figure 4-8 when the foam is

blown with N2.

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Figure 5-3 – Void fraction and cell density of CO2 blown PLA foam

While the foams blown with CO2 have shown to have remarkable improvement in terms of

morphology and cell density over the results obtained in Chapter 4, it is only achievable when

processed at lower temperature. At the same time, the CO2 content used in the foam is

significantly higher than N2, which could induce more prominent adiabatic cooling effect during

cell expansion [126, 127]. The two causes together make the spin-drawing impossible without

additional heating. An efficient heating element would have to be implemented downstream of

the extruder before drawability of the CO2 blown foams can be investigated.

5.4 Conclusion

As the as-spun foam fibres obtained in Chapter 4 required additional heating and drawing

processes, high expansion foaming of PLA has been investigated in this chapter as a possible

alternative approach for producing foam fibres. PLA-nanoclay composite is foamed at low

temperatures using CO2 as the blowing agent. High contents of CO2 (5%, 8% and 11%) are used

in the foaming trials to induce the plasticization and possibly crystallization of PLA. The effect

of processing temperature has shown dominant effect on the cell density and void fraction; both

cell density and void fraction increase as the polymer melt strength is enhanced. As 8% and 11%

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of CO2 is injected, the plasticizing effect of gas allows the processing temperature to be dropped

to 150°C, at which point the best foam morphology is obtained. The cell density of foams

produced with CO2 is two orders of magnitude higher than that produced with N2. The foams

obtained in this chapter can be utilized in future drawing studies, provided that thorough heating

can be applied to the foam. The much improved cell density and morphology should enhance the

final draw ratio of the foam fibres.

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Chapter 6 Conclusion

6.1 Summary

In order to fulfill the rapidly growing demand for synthetic fibres, new innovative classes of

fibres and their manufacturing processes are constantly being researched on. The manufacturing

of foam fibre is a promising concept as foaming improves a number of a material’s property

including toughness, fatigue life, and light weight; most importantly, it reduces material

consumption significantly. Unfortunately, there is little research documented so far in the subject

of foaming targeted for the fibre spinning application. This thesis investigated strategies to

produce low void fraction high cell density foam with low viscosity material, and demonstrated

the feasibility of producing as-spun foam fibres.

6.2 Key Contributions

6.2.1 Development of a Strategy to Produce High Cell Density Low Void Fraction

Foam

1. Using the lab-scale tandem foaming extrusion system, the effects of pressure drop rate,

blowing agent content and cell nucleating agent content on the foaming behavior of fibre grade

polypropylene are investigated and compared. It is shown that high cell density low void fraction

PP foam can be best obtained by using nano-scaled nucleating agent, high blowing agent content,

and an extrusion die with high pressure drop rate. The study not only pinpoints a strategy for

producing low void fraction foam, but also offers quantitative comparison in terms of the effect

that each parameter has on the foaming behaviour. A systematic foaming study like this has not

been previously reported, especially for producing low void fraction foams.

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2. The effect of nano-scaled nucleating agent on the cell nucleation and stabilization mechanisms

is elucidated. The cell nucleation and early growth of PP was captured in a static foaming

visualization system. The compound with PP and nanosilica produced the highest cell nucleation

rate and final cell density. Surprisingly, the compound with PP and coupling agent, though

having much lower complex viscosity, outperformed the neat PP in terms of cell nucleation rate

and cell density in the static visualization system. However, when the same compounds were

foamed in the extrusion system, the strong shear and extensional stresses present destroyed the

cellular morphology for the low viscosity compounds; the presence of nanosilica significantly

improved the foaming behavior. The difference in foaming behaviors observed in the two

systems emphasizes the significance of stress-induced nucleation in extrusion systems where

shear and extensional stresses are dominant. It also elucidates the role that nanosilica plays as a

nucleating agent on the cell nucleation and stabilization mechanisms.

3. Using the modified melt-spinning system, foaming of PLA is carried out with nitrogen as the

blowing agent and nanoclay as the nucleating agent. The presence of nanoclay significantly

improves the foamability of PLA; however 3wt% seems like the optimum clay concentration.

1wt% nanoclay does not seem to provide as much heterogeneous nucleation sites, where 5wt%

nanoclay reduces the melt viscosity too much at high shear conditions. The plasticization of high

clay content compounds is believed to be caused by the organic modifier used in the clay, as well

as the slip process between clay and PLA under high shear. In addition, the nanoclay particles

limit chain mobility of PLA and enhance the crystallization in the foaming process.

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6.2.2 Demonstrated the Feasibility of the Foam Fibre Spinning Process

1. PLA foams have been melt-spun on the modified fibre-spinning system as the foam extrudate

exits the spinneret. It is demonstrated in the study that PLA foam retains drawability at high

temperatures; and the as-spun foam fibres maintain the cellular structure. The drawability

depends strongly on the processing temperature. At a spinneret temperature of 230°C, foam

fibres were drawn to the diameters of below 150µm.

2. Both the cell density and void fraction of the foam fibres decrease slightly with increasing

drawing. DSC results reveal that even the highest draw ratio obtained was not sufficient to

initiate stress-induced crystallization for the PLA as-spun foam fibres. Preliminary tensile testing

suggests weak correlation between the tensile properties and the draw ratio. It has been

concluded that additional drawing steps would be necessary to enhance properties of the foam

fibres.

6.3 Recommended Future Works

1. It has been pointed out in Section 4.3.1 that the rheology measurement obtained with the

oscillatory rheometer is only representative of a material’s behavior at shear rates below 80 1/s,

whereas the estimated shear rate that polymer experiences in fibre spinning is orders of

magnitude higher. In order to better understand how PLA-clay nanocomposites behave in the

processing conditions, rheology tests should be carried out in capillary type of rheometers. The

effect of dissolved gas in the composite mixture should also be taken into account.

2. The spinneret temperature has shown tremendous impact on the drawability of the foam fibres.

If efficient heating can be applied to the spin-line, higher draw ratio would be obtained in the one

step spin-draw. It is recommended to retrofit the cooling column downstream of the spinneret to

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include convective heating elements to direct apply heating to the spin-line during drawing.

However, if the degree of drawing is still deemed insufficient, additional heating devices would

have to be deployed between the first and second godet rollers.

3. Once heating can be directly applied to the spin-line, a full factorial set of experimental trials

is recommended to determine the window of heating profiles required to obtain the highest draw

ratio on the foam fibres.

4. The total degree of crystallinity in the drawn foam fibres need to be assessed using the DSC.

The correlation between total crystallinity and the draw ratio can reveal whether the uniaxial

stress induced enough chain orientation and packing.

5. Tensile properties of the as-spun foam fibres have been obtained and presented in this thesis.

However, with the additional heating and drawing processes, much improved modulus and yield

stress are expected from the foam fibres. Tensile properties of the drawn fibres shall be measured.

129

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