development of lightweight, biodegradable plastic foam
TRANSCRIPT
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
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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
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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
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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
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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
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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.
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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
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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
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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.
77
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.
78
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
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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
87
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
101
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
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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.
126
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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