the effect of molecular weight on polypropylene foaming · ii . the effect of molecular weight on...
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THE EFFECT OF MOLECULAR WEIGHT ON POLYPROPYLENE FOAMING
by
Kamleshkumar Majithiya
A thesis submitted in conformity with the requirements for the degree of Master of Applied Science
Graduate Department of Mechanical and Industrial Engineering University of Toronto
© Copyright by Kamleshkumar Majithiya 2012
ii
The Effect of molecular weight on polypropylene foaming
Kamleshkumar Majithiya
Master of Applied Science
Department of Mechanical and Industrial Engineering University of Toronto
2012
Abstract
The effect of molecular weight on polypropylene (PP) extrusion foaming was investigated and
the process to make soft touch, largely expanded, high cell density non-crosslinked PP foam
using environmental friendly CO2 is presented. In previous research, when the cell density was
high, cell opening was dominant and large expansion could not be achieved even in HMS PP
materials. The effects of processing and material parameters on the foam morphologies of PP
materials with three different melt flow rate (MFR) were studied using single-screw tandem
foam extrusion system. By selecting proper material and die, and by tailoring the processing
conditions, large expansion (25 fold) and high cell density (>109 cells/cm3) was successfully
achieved in the high MFR PP without any additives. The mechanism of locally induced
crystallization was found to be significantly affecting the foaming behavior of PP and was
successfully verified using SEM, DSC, HPDSC, shear viscosity and solubility measurements.
iii
To my beloved wife, Dhvani, and son, Kavya, for your endless love, strong support, and inspiring encouragement during the journey of my MASc study. I could not have done it without you. Your love is and will always be in my heart.
iv
Acknowledgments
Throughout my graduate studies at the University of Toronto, there are a multitude of people that
have helped, supported and encouraged me to make my experience enriching. I feel grateful and
indebted to my supervisor Prof. Chul B. Park for his valued guidance and encouragement
throughout my research at the Microcellular Plastics Manufacturing Laboratory. Professor Park
taught me how to approach complex engineering problems effectively by identifying the key
issues and focusing on them, and his industrial collaborations have always impressed me how
useful cellular polymers can be.
I also would like to thank Prof. Hani Naguib and Prof. Atalla for their valuable suggestions and
guidance for the sound absorption study I conducted.
I would also like to thank my thesis committee: Prof. Javad Mostaghimi and Prof. Nasser
Ashgriz, for the willingness to serve on my defense examination. I would like to thank my
colleagues and fellow researchers in MPML for their help and friendship over the past years.
Their advice and support have been invaluable. Much of the work throughout this thesis research
would not have been possible without their contributions. My sincere gratitude goes to Nemat
Hossieny, Reza Nofar, Lun Howe, Mahmood Hassan, Ali Rizvi, Peter Jung, Dr. Riza Barzegari,
Dr. Keshtkar, Raymond Chu, Dr. Kuboki, Dr. Kamal, Dr. Ameli, Prof. Wentao Zhai, Dr. Eung
Kee (Richard) Lee, Dr. Changwei Zhu, Dr. Saleh, Mohamed Hassan, Anson Wong, Hui Wang,
Nan Chen, Weidan Ding, Mo Xu, Alireza Tabatabaei Naeini, Davoud Jahani, Vahid, Hongtao
Zhang
v
I wish to thank Mr. Ryan Mendell and Jeff Sansome from the machine shop at the Department of
Mechanical and Industrial Engineering at the University of Toronto for their help to solve the
problems related to machining and allowing me to use equipment from the machine shop
I would like to express my special thanks to the MIE staff members including Konstantin,
Brenda Fung, Jho Nazal, Donna Liu, Sheila Baker, Oscar del Rio, Joe Baptista and Teresa Lai.
I truly feel obligated to acknowledge Sabic, Braskem for providing me materials for my
experiments. Also, I would like to thank the Consortium for Cellular and Micro-Cellular Plastics
(CCMCP) for their funding and support in this research.
Last but not least, I would like to thank my wife, my parents, brothers, sisters, parents-in-law, for
their constant love and prayers for my graduate studies. Without their support, encouragement
and patience it would not be possible.
vi
Table of Contents
Acknowledgments .......................................................................................................................... iii
Table of Contents ........................................................................................................................... vi
List of Tables ................................................................................................................................. ix
List of Figures ................................................................................................................................. x
NOMENCLATURE .................................................................................................................... xiv
Chapter 1 Introduction ................................................................................................................. 1
1.1 Preamble ............................................................................................................................. 1
1.2 Research Motivation ........................................................................................................... 2
1.3 Objectives of the Thesis ...................................................................................................... 3
1.4 Overview of the Thesis ....................................................................................................... 4
Chapter 2 Literature Review and Theoretical Background ..................................................... 8
2.1 Introduction ......................................................................................................................... 8
2.2 Thermoplastic Microcellular Foaming ............................................................................... 8
2.2.1 Formation of homogeneous solution of Polymer and Gas ...................................... 8
2.2.2 Cell Nucleation ..................................................................................................... 13
2.2.3 Cell Growth and Stabilization ............................................................................... 22
2.3 Blowing Agent .................................................................................................................. 23
2.3.1 Chemical blowing agent (CBA) ............................................................................ 24
2.3.2 Physical blowing agents (PBAs) ........................................................................... 25
2.4 Foaming Processes ............................................................................................................ 26
2.4.1 Batch Process ........................................................................................................ 26
2.4.2 Continuous Process ............................................................................................... 27
2.5 Factors affecting Foam Extrusion ..................................................................................... 28
vii
2.5.1 Crystallization Kinetics ......................................................................................... 28
2.5.2 Filamentary Die Design in Foam Extrusion ......................................................... 32
2.5.3 Governing Mechanism of Volume Expansion ...................................................... 33
2.6 Characterization of the Foam Samples ............................................................................. 36
2.6.1 Foam Density ........................................................................................................ 36
2.6.2 Volume Expansion Ratio ...................................................................................... 36
2.6.3 Cellular Morphology and Cell Size Distribution .................................................. 37
Chapter 3 Largely Expanded high cell density Polypropylene Foaming .............................. 44
3.1 Introduction ....................................................................................................................... 44
3.2 Experimental ..................................................................................................................... 48
3.2.1 Material Selection ................................................................................................. 48
3.2.2 Rheological measurement ..................................................................................... 49
3.2.3 Pressure Drop Rate Measurement ......................................................................... 50
3.2.4 Thermal Analysis .................................................................................................. 51
3.2.5 Experimental Set-up and procedure ...................................................................... 52
3.2.6 Foam Characterization .......................................................................................... 54
3.3 Result and Discussion ....................................................................................................... 55
3.3.1 Effect on solubility ................................................................................................ 55
3.3.2 DSC Results .......................................................................................................... 55
3.3.3 Effect of strain rate on shear viscosity .................................................................. 61
3.3.4 Die Pressure during Extrusion Foaming ............................................................... 61
3.3.5 Expansion Behavior of PP Foam .......................................................................... 62
3.3.6 Cell Density Characterization ............................................................................... 65
3.4 Fabrication of foam and foam sheet using pilot scale extruder ........................................ 68
Chapter 4 Effect of Nano-clay on Polypropylene Foaming .................................................... 98
4.1 Introduction ....................................................................................................................... 98
viii
4.2 Statement of the Project Scope ....................................................................................... 100
4.3 Experimental Procedure .................................................................................................. 100
4.3.1 Extrusion Foaming .............................................................................................. 101
4.4 Result and Discussion ..................................................................................................... 101
4.4.1 Effect of Nanoclay content on cell nucleation .................................................... 101
4.4.2 Effect of blending ............................................................................................... 102
Chapter 5 Acoustic Behavior of Perforated Expanded Polypropylene Foam ..................... 108
5.1 Introduction ..................................................................................................................... 108
5.2 Theoretical Background .................................................................................................. 109
5.3 Experimental Procedure .................................................................................................. 112
5.3.1 Materials and Sample Preparation ...................................................................... 112
5.3.2 Characterization .................................................................................................. 112
5.4 Results and Discussion ................................................................................................... 114
5.4.1 Effect of Perforation on Sound Absorption ........................................................ 114
5.4.2 Effect of sample thickness on Sound Absorption ............................................... 115
5.4.3 Effect of Expansion Ratio on Sound Absorption ................................................ 115
Chapter 6 SUMMARY, CONCLUSION & RECOMMANDATION .................................. 121
6.1 Summary ......................................................................................................................... 121
6.2 Conclusion ...................................................................................................................... 122
6.3 Recommendations ........................................................................................................... 125
References .................................................................................................................................. 127
ix
List of Tables
Table 3.1 Theoretically calculated value of pressure drop (∆P) and pressure drop rate (dp/dt) ... 72
Table 3.2 pressure drop (∆P) and pressure drop rate (dp/dt) for Die# 4 used in large tandem ... 72
Table 3.3 Value of parameter- n from Avrami analysis for isothermal crystallization of PP40 .. 72
Table 3.4 Crystallinity and melting temperature of foamed sample of PP40 with 7% CO2 ......... 73
Table 3.5 Crystalinity and melting temperature of foamed sample of PP40 with 7%, 9% and 11%
CO2 ............................................................................................................................................... 73
Table 3.6 Crystalinity and melting temperature of foamed sample of PP40 with 7% CO2 .......... 73
Table 3.7 Crystalinity and melting temperature of foamed sample of PP40 with 7%, 9% and 11%
CO2 ............................................................................................................................................... 74
Table 3.8 Processing windows for high cell density (<108 cells/cm3, green color is for <109
cells/cm3) and high volume expansion ratio (<25 fold) ................................................................ 74
Table 5.1 Design of Experiments ................................................................................................ 117
Table 5.2. Perforation ratio for various samples ......................................................................... 117
x
List of Figures
Figure 1.1 Open-cell and closed-cell structures .............................................................................. 6
Figure 1.2 Approach for the research ............................................................................................. 7
Figure 2.1 Steps of continuous extrusion foaming process [10] ................................................... 39
Figure 2.2 Solubility of carbon dioxide (CO2) and nitrogen (N2) in PS [, 11] ............................. 39
Figure 2.3 Homogeneous and heterogeneous nucleation in a polymer-gas solution [] ................ 40
Figure 2.4 The free energy, ΔG, vs. radius of bubble, r, associated with the homogenous
nucleation [Courtesy: Prof. Park, Lecture notes of MIE1706 Manufacturing of cellular polymers]
....................................................................................................................................................... 40
Figure 2.5 Comparison of energy required for homogeneous and heterogeneous nucleation [45]
....................................................................................................................................................... 41
Figure 2.6 Heterogeneous Nucleation on (a) smooth planar surface and (b) in a conical cavity []
....................................................................................................................................................... 41
Figure 2.7 Schematic of a laboratory-scale batch foaming system .............................................. 42
Figure 2.8 Schematic of a continuous extrusion foaming system ................................................ 42
Figure 2.9 Governing Mechanism of Volume Expansion Ratio [96] ........................................... 43
Figure 3.1 Parameters affecting extrusion foaming process ......................................................... 75
Figure 3.2 Effect of molecular weight on solubility .................................................................... 75
Figure 3.3 Non isothermal DSC cooling thermographs of different MFR PP .............................. 76
Figure 3.4 Isothermal melt crystallization behavior for PP5 ........................................................ 76
Figure 3.5 DSC result - effect of isothermal behavior of PP40 at atmospheric pressure ............. 77
xi
Figure 3.6 Isothermal melt crystallization behavior for PP80 ...................................................... 77
Figure 3.7 Effect of gas pressure in crystallization of PP40 ......................................................... 78
Figure 3.8 High pressure DSC Results - effect of isothermal behavior of PP40 at atmospheric
pressure ......................................................................................................................................... 78
Figure 3.9 Time dependence of relative crystallinity at different isothermal temperature for PP40
....................................................................................................................................................... 79
Figure 3.10 Avrami double-log plots for PP40 under different isothermal temperatures. ........... 79
Figure 3.11 DSC heating thermographs of foam sample (a) MFI 40 PP with 7% CO2 and (b)
MFI 5 PP – 13% CO2 ................................................................................................................... 80
Figure 3.12 DSC heating thermographs - Effect of gas content (a) PP40 and (b) PP5 ................ 80
Figure 3.13 Complex viscosity of PP40 under SAOS at f10Hz measured at 135°C, 135°C and
140°C ............................................................................................................................................ 81
Figure 3.14 Storage modulus (G’) versus frequency for PP40 at different temperature .............. 81
Figure 3.15 Complex viscosity of PP40 under SAOS measured at 140°C at 10Hz, 30 Hz, and
70HZ ............................................................................................................................................. 82
Figure 3.16 Storage modulus (G’) versus frequency for PP40 at different temperature .............. 82
Figure 3.17 Die Pressure vs Die temperature for 5 to 13% gas content ....................................... 83
Figure 3.18 Die Pressure Vs Die Temperature for three types of PP using Die #3 (L/D~ 34) ..... 83
Figure 3.19 Volume Expansion ratio versus die temperature at different gas content for the
foamed samples made from PP5/PP40/PP80 using die#3 ............................................................ 84
Figure 3.20 Volume Expansion ratio of the foamed samples made from PP5 using die#1 ......... 84
Figure 3.21 Effect of Pressure drop rate and pressure drop on volume expansion ratio .............. 85
xii
Figure 3.22 Effect of Molecular weight on Cell Density of the foamed samples made using 7%
CO2 ................................................................................................................................................ 85
Figure 3.23 Effect of Molecular weight on Cell Density of the foamed samples made using 11%
CO2 ................................................................................................................................................ 86
Figure 3.24 Effect of Molecular weight on Cell Density of the foamed samples made using 9%
CO2 ................................................................................................................................................ 86
Figure 3.25 Cell Density of the foamed samples made from PP5 using die#1 ............................ 87
Figure 3.26 Effect of molecular weight and gas contents on the cell nucleation ......................... 87
Figure 3.27 Effect of Pressure drop rate on cell nucleation using two different dies ................... 88
Figure 3.28 Tandem Extrusion System ......................................................................................... 88
Figure 3.29 Volume Expansion Ratio versus die temperature of PP40 for large tandem (1.5”-
2.5”) and small tandem (0.75”-1.5”) ............................................................................................. 89
Figure 3.30 Cell density versus die temperature for large and small tandem ............................... 89
Figure 3.31 Processing windows for high cell density (<108 cells/cm3) and high volume
expansion ratio (<25 fold) ............................................................................................................. 90
Figure 3.32 SEM images, for 7% and 11% CO2 gas content at 120°C for PP5, PP40 and PP80 . 91
Figure 3.33 SEM images, for 7% and 11% CO2 gas content at 125°C for PP5, PP40 and PP80 . 92
Figure 3.34 SEM images, for 7% and 11% CO2 gas content at 130°C for PP5, PP40 and PP80 . 93
Figure 3.35 SEM images, for 9% CO2 gas content at 120°C, 125°C and 130°C for PP40 and
PP80 .............................................................................................................................................. 94
Figure 3.36 SEM images, for 11% CO2 gas content at 105°C, 110°C and 115°C for PP40 and
PP80 .............................................................................................................................................. 95
xiii
Figure 3.37 SEM images, for 7% CO2 gas content at 115°C, 120°C and 125°C for PP40 made
from large tandem and small tandem ............................................................................................ 96
Figure 3.38 SEM Images of Foam sheet produced from MFR40 ................................................. 97
Figure 4.1 SEM images for various nano-clay content at 7% CO2 content ................................ 104
Figure 4.2 SEM images for various nano-clay content at 11% CO2 content ............................ 105
Figure 4.3 Effect of Nano clay content on the cell density for PP40 +11% CO2 ....................... 106
Figure 4.4 Effect of Nano clay content on the cell density for PP40 +11% CO2 ....................... 106
Figure 4.5 Effect of nanoclay content on the expansion ratio of the foamed sample with 7% CO2
..................................................................................................................................................... 107
Figure 4.6 Effect of nanoclay content on the expansion ratio of the foamed sample with 11% CO2
..................................................................................................................................................... 107
Figure 5.1 Samples with Perforation .......................................................................................... 118
Figure 5.2 Impedance Tube Set-up ............................................................................................. 118
Figure 5.3. Effect of Perforation on sound absorption ................................................................ 119
Figure 5.4. Effect of sample thickness on sound Absorption for samples with hole diameter: 1.2
mm and spacing = 2 mm ............................................................................................................. 119
Figure 5.5. Effect of Expansion ratio on Sound Absorption for the samples with hole diameter
=1.75 mm and spacing= 3 mm, thickness= 10 mm .................................................................... 120
xiv
NOMENCLATURE
PP = Polypropylene
MFR = Melt flow rate
HMS = High Melt Strength
MFI = Melt Flow Index
SEM = Scanning Electron Microscopy
DSC = Dynamic Scanning Calorimetry
HPDSC = High pressure DSC
EPP = Expanded polypropylene
EPS = Expanded polystyrene
BA = Blowing Agent
EOS = Equation of State
PE = Polyethylene
PS = Polystyrene
HDPE = High density polyethylene
xv
MSB = Magnetic suspension balance
PVT = Pressure-Volume- Temperature
SS-EOS = Simha–Somcynsky EOS
SL-EOS = Sanchez–Lacombe EOS
𝑆 = Solubility coefficient or Henry’s law constant (cm3[STP]/g-Pa)
𝐶 = concentration of gas absorbed per unit mass of polymer or solubility of the
gas (cm3 /g)
𝑝 = Saturation pressure of gas in Pa
𝑆0 = Pre-exponential factor or solubility coefficient constant (cm3 [STP]/g-Pa)
∆𝐻𝑠 = Molar heat of sorption (J)
𝑅 = Gas constant in J/K
D = Diffusivity
D0 = Diffusivity Constant in cm2/s
𝐸𝑑 = Activation energy for diffusion in J.
CFC = Chlorofluorocarbon
CNT = Classical nucleation theory
𝛾𝑝𝑏 = Surface tension
xvi
𝐴𝑏 = Surface area
𝑉𝑏 = Bubble volume
𝑓0 = Frequency factor
Co = concentration of gas molecules
PMMA = Poly(methyl methacrylate)
PET = Polyethylene terephthalate
𝛾𝑏𝑝 = Surface Tension
rcr = Critical radius
𝐶1 = concentration of gas molecules
𝑓1 = frequency factor of gas molecules
𝑘 = Boltzman’s constant
𝑇 = Temperature in K
∆𝐺ℎ𝑒𝑡∗ = Gibbs free energy for heterogeneous nucleation
𝑁ℎ𝑜𝑚 = Rate of homogeneous nucleation
PVC = Polyvinyl chloride
CBA = chemical blowing agent
xvii
PBA = physical blowing agent
LDPE = Low density polyethlylene
𝑋𝑤(𝑡) = Absolute , crystallinity at crystallization time t,
𝑋𝑢 = ultimate crystallinity for t = ∞.
𝜌𝑎 = Amorphous region density
𝜌𝑐 = Crystalline density
N = Cell density
𝑡𝑃𝑟𝑒𝑚𝑎𝑡𝑢𝑟𝑒 = Premature cell growth time
𝑀0 = Undissolved gas amount per unit volume
VER = Volume Expansion ratio
HMS = High melt strength
dp/dt = Pressure drop rate
ρf = Density of the foamed sample
𝜂 = viscosity (Pa.s)
�̇� = shear rate (1/s)
M = measure of consistency
xviii
𝑡𝑟𝑒𝑠𝑖𝑑𝑒𝑛𝑐𝑒 = Residence time
Q = Volumetric flow
N = Power law model exponent
𝑤𝑝 = Weight fraction of PP
𝐻𝑚 = Melting enthalpy of the sample
𝐻𝑚0 = theoretical, 100% crystalline polypropylene enthalpy
RCPP = Random copolymer of PP
NC = Nano-clay
1
Chapter 1 Introduction
1
1.1 Preamble
Foams refer to spherical gaseous voids distributed in a dense continuum. They exist naturally in
pumice, tree trunks, marine organisms, sponges, woods and cork. It is also made artificially from
engineering material such as polymers, metals, ceramics or composites. Polymer foams have
significant applications in various industries such as sports and leisure products, military
applications, automotive industries and aerospace industries. Most of us will find polymer foams
in our daily life in one form or another, for example, in furniture, packaging, refrigerator
insulation or common applications. Foam materials have very high strength-to-weight ratio. Due
to this attribute, it can be an alternative to solid parts without compromising the mechanical
properties. It will also benefit the automotive industry by reducing weight and fuel consumption
as solid components are replaced with foam made parts. [1,2,3]
The characteristics of polymeric foams are determined by the following structural parameters:
cell density, expansion ratio, cell size distribution, open-cell content, and cell integrity. Foams
can be classified in different ways for example, by nature as flexible and rigid, by dimension as
sheet and board, by weight as low density and high density, by structure as open cell and close
cell, by cell density as fine foam and microcellular. Conventional foams have a cell size greater
than 300μm and a cell density less than 106 cells/cm3. Fine-celled foams are foams having a cell
size between 10μm and 300μm, and a cell density between 106 cells/cm3 and 109 cells/cm3.
Microcellular foams are foams having a cell size in order of 10μm or less and cell densities of
2
109 to 1015cells/cm3. Small cells provide an improved energy absorption capability. The smaller
the cell size, the higher the insulation properties, partly because of the reduced radiation effects
in the cell during heat transfer. Due to this benefit, microcellular foams have received great
attention from researchers around the world. [1,4,5,6]
The closed-cell foams are composed of cells that are not interconnected with other cells and have
cell walls. On the other hand open-cell foams have interconnectivity between the cells. Open
cells allow fluids such as air, water etc. to flow through them. Figure 1.1 shows the SEM images
of open and closed cell foams. Some foam consists of both open and closed cells. Based on their
application foams with either open or closed cells are selected. Closed-cell foams are mainly
used in construction and automotive industries because of their mechanical properties. On the
other hand, open-cell forms are used for many applications such as filtration, separation
absorption, packaging, cushioning and noise control engineering [3,7]
1.2 Research Motivation
In recent years, automotive industries and their suppliers have been experiencing more restrictive
waste disposal guidelines. Consequently, the plastic foam industries have experienced serious
regulatory, environmental and economic pressure. Microcellular crosslinked polyolefin foams
are extensively used in protective packaging and automotive dunnage applications to protect high
quality (class A) surfaces from impact, vibration, and abrasion during handling, storage and
transportation. But these cross-linked foams are not recyclable. Therefore, there exists an unmet
demand for 100% recyclable non-cross linked foam product with the same physical properties
and performance comparable to crosslinked polyolefin foam. This would enable automotive and
3
their suppliers to meet waste disposal regulations and respond to market driven environmental
trends. [8]
Packaging foams are typically found with the average cell size of 1-2 mm but in the literature it
has been proved that smaller cell sizes improves low-abrasion performance. Therefore, it is
essential to produce fine cell foams which achieve the same performance as crosslinked foams
by continuous extrusion process. But it is a real challenge to make the fine cell foam without
surface defects, such as ripples, corrugation and warping due to the rapid expansion rate of the
extruded foam at the die. [9]
Therefore, there is a high importance for the plastic industry to devise new methods to produce
high cell density and high expanded non-crosslinked foams which are 100% recyclable.
1.3 Objectives of the Thesis
The main objective is to investigate the effect of molecular weight on polypropylene foaming
with a goal of achieving largely expanded (more than 25 fold), high cell density (more than 109
cells/cm3), small cell sizes (10-30µm), and very soft non-crosslinked polypropylene foams.
These parameters should be met using a tandem extrusion system by finding the proper material,
processing window and optimum gas content. First, this objective will be achieved without the
use of any additives. Subsequently, the effect of nano particles on cell density and volume
expansion ratio will also be investigated. This research involves 1) the development of effective
strategies for the production of largely expanded high cell density polypropylene foams 2) the
identification of the mechanism behind it. Polypropylene is a semi crystalline material.
Crystallization behavior of PP is a crucial parameter for foam processing in bubble growth
phenomenon. The hypothesis of this research is that the crystallization may also assist in cell
4
nucleation. The second objective is to verify the result with characterizing the samples in DSC
and high pressure DSC to investigate the non isothermal and isothermal behavior of selected
polypropylene materials.
In continuation of this objective, the effort will be also made to investigate the effect of nanoclay
on PP foaming behavior to improve the cell density and volume expansion further.
The final objective of this work is to examine the feasibility of expanded polypropylene(EPP)
bead foams as a sound absorber.
The overall research approach is shown in Figure 1.2
Note : Melt Flow Rate (MFR) term will be used in the thesis to represent the molecular weight
of the material. MFR gives an idea about the molecular weight and the viscosity of the polymers.
Low MFR (or MFI-Melt Flow Index) value means high viscosity and high molecular weight
while high MFR value means low viscosity and low molecular weight.
1.4 Overview of the Thesis
The following chapters outline the framework of research in this thesis:
Chapter 1 presents a brief introduction to the background of foams and foaming technology,
research motivation, objective and thesis outline
Chapter 2 covers the literature review on thermoplastic foaming process. It includes an in-depth
review of polymer/gas solution formation, cell nucleation, and cell growth and stabilization and
an overview of foam extrusion, characterization of polymer foams.
5
Chapter 3 describes the overall experimental procedures such as thermal analysis, rheology test,
foam characterization and continuous extrusion foaming of random copolymer of PP blown with
CO2. In the result and discussion section, the effect of the molecular weight, processing
conditions, amount of gas contents on expansion ratio and cell density and cell uniformity are
discussed systematically. The foaming results are compared with the DSC, HPDSC and rheology
test data to investigate the foaming mechanisms.
Chapter 4 includes experiment performed to investigate on the effects of nanoclay on cell density
and expansion ratio of extruded polypropylene foam. It also covers the effect of the blending of
high viscosity and low viscosity PP resins with nano particles.
Chapter 5 covers the study on acoustic behavior of perforated closed cell expanded
polypropylene (EPP) bead foam structures to develop new application for EPP as an acoustic
material.
Chapter 6 provided a summary of contributions and concluding remarks for this thesis as well as
recommendations for future work, respectively.
6
a) Open Cell Structure b) Close- Cell Structure
Figure 1.1 Open-cell and closed-cell structures
7
SYSTEM
Small Tandem
Large Tandem
MATERIALS
Different MFR PP
Blowing agent
Nucleating Agent
DIE SELECTION
Diameter
Length
PHYSICAL PROPERTIES
Solubility
Thermal Behavior
Reheology
EXTRUSION FOAMING
Temperature
Pressure
Pressure drop rate
Gas Content
CHARACTERIZATION
Cell Density
Volume Expansion ratio
CELL DESNITY
109 cells/cm3
EXPANSION
>25 fold
If cell density
and expansion
ratio are not as
expected, find
the solution,
change variables
and repeat the
process
Figure 1.2 Approach for the research
8
Chapter 2 Literature Review and Theoretical Background
2
2.1 Introduction
This chapter provides basic information on basic principles of thermoplastic microcellular
foaming fundamentals, manufacturing processes, and methods for characterization the foam.
2.2 Thermoplastic Microcellular Foaming
In general, micro-cellular foam process involves three major steps as shown in Figure 2.1: i)
Formation of homogeneous solution of polymer and gas. (ii) Cell Nucleation (iii) Cell growth.
[10]
2.2.1 Formation of homogeneous solution of Polymer and Gas
The first critical step of micro-cellular foaming is to achieve a uniform solution of polymer and
gas. The quality of the solution formation significantly affects the final cell morphology and
mechanical properties of the foam. The homogeneous polymer-gas solution is governed by the
system pressure and gas diffusion in the polymer matrix. The amount of blowing agent (BA)
injected into polymer should be less than its solubility limit in the polymer before foaming to
ensure complete mixing and dissolving of gas into the polymer. The solubility limit is affected
by the system pressure and temperature. If the amount of blowing agent exceeds its solubility
limit, the un-dissolved blowing agent will form large voids. To avoid large voids in the foam
product, it is essential to find the amount of blowing agent that can be absorbed and dissolved
9
into the polymer (i.e. solubility) into the polymer matrix at different processing temperatures and
pressure. Generally, the system pressure should always be higher than the solubility pressure to
avoid any undissolved gas pockets. Therefore reliable solubility data for various blowing agents
in different polymer matrix are crucial to polymer foaming industries. [11,12]
2.2.1.1 Solubility
The dissolved gas in the polymer melts affects physical properties of a polymer melt such as
swollen volume, isothermal compressibility, thermal expansion coefficients, viscosity and
surface tension. Therefore gas solubility data and the effects of the dissolved gas in the polymer
are very important knowledge for the polymer foaming industries [13,14,15,16]. Since the
1950s, many efforts have been made to investigate the solubility of gases in polymer melts
through a different kind of approaches including the experimental measurements and theoretical
thermodynamic calculations. The volumetric and gravimetric methods have been widely used to
measure the solubility of various blowing agents in polymers. However, most of these methods
depend on swollen volume of the polymer/gas mixtures to determine the solubility, particularly
at a higher temperature and higher pressure. Therefore, the swollen volume due to gas
dissolution into polymer or the density of the polymer/gas solution needs to be determined for
measuring the solubility accurately. These can be found either by estimating using any equation
of state (EOS), or by experimental measurement. [17, 18, 19, 20, 21, 22]
Sato et al. used the pressure decay method to determine the solubility of CO2 and N2 in PP, High
Density Polyethylene (HDPE), and Polystyrene (PS). The pressure decay method involves the
measurement of pressure changes inside a chamber as gas sorption by a polymer specimen takes
place. It was a very popular method due to its simple operation and low equipment cost.
However, it was difficult to use this method for molten polymers as it needs a high resolution
10
pressure sensor that can operate at higher temperatures. In addition, this method needs a large
polymer sample which increases the measurement time. Another method uses an electro balance
to directly measure the mass uptake during the sorption experiments. This method measures
solubility with high sensitivity and in short time but it works only at low temperature. To
overcome this, researchers have designed systems to independently control the temperature of
the chamber and electrobalance. But the main disadvantage of this method is the effect of
convection-induced gas density variation on the measurement accuracy. This problem was
solved by another gravimetric method that utilizes a magnetic suspension balance (MSB)
developed by Kleinrahm and Wanger[26]. In this method, the sample is weighted in the
compartment that is separated from an outer chamber and that can measure the gas solubility and
diffusivity in polymer at elevated temperatures and pressures. This set up is widely used by many
researchers to measure solubility in polymer. [23, 24, 25, 26]
It was later found that swelled polymer-gas mixture has a buoyancy effect during gas dissolution.
As a result, the mass reading of the dissolved gas in the MSB shows the apparent solubility
which is lower than the actual solubility. For the theoretical prediction of the corrected swelling
volume and to compensate for the buoyancy effect, various EOS were used in the absence of
accurate pressure-volume- temperature (PVT) data of the polymer-gas mixtures. [27, 12] The
Simha–Somcynsky (SS) EOS, along with five other theoretical equations-of-state (including the
Sanchez–Lacombe EOS), have been extensively tested for polymers and oligomers by Rodgers
[28, 29, 30]. SS-EOS shows excellent capabilities for describing the polymer melt PVT data
over a wide range of temperatures and pressures. Moreover, the validity of SS-EOS for the
prediction of the swelling by gas dissolution has also been verified [21,22]. Some EOSs assume
molecules to be arranged in some pattern while some recent theories, such as Statistical
Association Fluid Theory(SAFT) explained that molecules move freely in continuous space.
11
More recently visualization systems have been developed to directly measure the PVT behavior
of polymer-gas solution and to determine accurate solubility data. [31, 32, 33].
The solubility limit depends on the processing pressure and temperature and can be
approximated by Henry’s law [11]
𝑆 =𝐶𝑝
(2.1)
Where 𝑆 is the solubility coefficient or Henry’s law constant (cm3[STP]/g-Pa), 𝐶 is the
concentration of gas absorbed per unit mass of polymer or solubility of the gas (cm3 /g) and 𝑝 is
the saturation pressure of gas in Pa. [11]
The coefficient 𝑆 is a function of temperature, it is given by,
𝑆 = 𝑆0𝑒𝑥𝑝 �−∆𝐻𝑠𝑅𝑇 � (2.2)
𝑆0 is the pre-exponential factor or solubility coefficient constant (cm3 [STP]/g-Pa), ∆𝐻𝑠 is the
molar heat of sorption (J), 𝑅 is gas constant in J/K, 𝑇 is the temperature in K. Using Equation
(2.1) and equation (2.2) the solubility of gas in a polymer matrix can be estimated. Figure 2.2
shows the solubility of CO2 in PS decreases with an increase in temperature, whereas for N2 the
behavior is reversed, the solubility increases with temperature. [11]
In extrusion system, based on the polymer flow rate, the gas flow rate can be determined so that
the gas-to-polymer weight ratio may be maintained below the soluble limit. [11]
12
2.2.1.2 Diffusivity
In addition to the system pressure, the diffusion of the BA in a polymer is a key parameter.
Diffusivity influences the time needed to dissolve the BA into the polymer to meet the
processing time requirements in a continuous process such as extrusion foaming.
Diffusivity (D) is a function of temperature and can be given by the following equation (2.3)
𝐷 = 𝐷0𝑒𝑥𝑝 �−𝐸𝑑𝑅𝑇�
(2.3)
where Do is the diffusivity coefficient constant in cm2/s, and 𝐸𝑑 is the activation energy for
diffusion in J. When an excess amount of gas is injected, the undissolved gas will form the gas
pockets in the polymer. So it is essential for the formation of uniform polymer gas mixture that
the amount of gas injected is below the solubility limit and sufficient dissolution time is allowed.
If the required time of gas diffusion in polymer matrix is longer than the melt residence time,
uniform solution will not be possible. The diffusion rate can be increased by processing the
polymer/gas mixture at a higher temperature. However, the diffusion process of gas in the
polymer is still not fast enough. It can be improved by using a screw with mixing and energy
transfer capability (e.g. barrier screw, energy transfer screw, and Barr screw). Static mixers can
also be used to enhance distributive and dispersive mixing of the materials. In addition, second
stage cooling screw also can be used to allow enough time for mixing and to achieve the
uniformity of the temperature. Basically, by incorporating a mixing device, gas dissolution can
be enhanced by redistributing the local concentration of gas, and creating striations which
increases the area of the polymer-gas interface and decreasing diffusion distance. [34].
13
The diffusivity also depends on the type of blowing agent. Generally, higher molecular weight
gases show lower diffusivity than lower molecular weight gases under the same temperature and
pressure condition. Therefore it is easier to control the cell nucleation and growth kinetics when
foaming with chlorofluorocarbon(CFC) blowing agents compared to foaming with CO2 and N2.
In summary, to achieve fine cell structure, a sufficient higher system pressure is needed to
maintain a large amount of gas dissolved in the polymer according to Henry’s law and it is also
important to ensure the amount of gas dissolved in the polymer should be below the solubility
limit at particular processing conditions.
2.2.2 Cell Nucleation
Cell nucleation can be defined as the conversion of small group of gas molecules into
energetically stable groups or pockets. A thermodynamic instability, either a rapid heating or
pressure drop will cause the formation of bubbles within polymer melts. To induce cells, free
energy barrier must be exceeded. When the polymer melt has been saturated with a gas, the
system becomes supersaturated as the solubility limit reduces upon either a pressure drop or
temperature increase. As a result, the polymer-gas solution tends to form tiny bubbles in order to
go towards low-energy stable state. The Classical nucleation theory (CNT) [35, 36] is commonly
adopted to explain the nucleation process. The theory classifies two different types:
homogeneous nucleation and heterogeneous nucleation. Figure 2.3 shows a schematic of these
two types of nucleation. Homogeneous nucleation occurs randomly throughout the bulk of pure
polymer/gas solution when a certain amount of blowing agent dissolved in a polymer matrix to
form a second phase such as gas bubble in a primary phase, solution of polymer and gas. It
requires higher nucleation energy than heterogeneous nucleation. Heterogeneous nucleation is
14
generally initiated at certain preferable sites such as phase boundary, or sites provided by
additive particles.
2.2.2.1 Homogenous Nucleation
Colton and Suh[37] modeled the nucleation behavior in microcellular foaming using the CNT.
According to this theory, the work required to generate a bubble of radius r can be given by
equation (2.4)
𝑊 = 𝛾𝑝𝑏𝐴𝑏 − ∆𝑃𝑉𝑏 (2.4)
where the term, 𝛾𝑝𝑏𝐴𝑏 , is the work required to create a bubble with surface tension, 𝛾𝑝𝑏 , and
surface area, 𝐴𝑏, and the second term, ∆𝑃𝑉𝑏 is the work done by the expansion of gas inside a
bubble of volume 𝑉𝑏. The difference between the two terms is the actual work required to
generate a cell. After the substitution of geometric equations of a sphere for 𝐴𝑏 and 𝑉𝑏, equation
(2.4) becomes as follows:
𝑊 = 4𝜋𝑟2𝛾𝑝𝑏 − 43𝜋𝑟3∆𝑃 (2.5)
Figure 2.4 shows the variation of energy (W) with radius (r). In order for the bubble to grow
spontaneously, the maximum energy barrier must be overcome. If the induced energy in the system is
lower than the maximum energy, the bubble, which is smaller than the critical bubble size, collapses.
The amount of free energy can be calculated by differentiating 𝑊 with respect to r from the previous
equation (2.5). [37, 36]
∆𝐺ℎ𝑜𝑚∗ = 16𝜋𝛾𝑝𝑏3
3∆𝑃2 (2.6)
15
where ∆Ghom∗ Gibb’s free energy of forming a critical nucleus
The homogeneous nucleation rate is approximated by equation (2.7), [36, 37]
𝑁ℎ𝑜𝑚 = 𝑓0𝐶0𝑒𝑥𝑝 �−∆𝐺ℎ𝑜𝑚∗
𝑘𝑇 � (2.7)
where 𝑓0, is a frequency factor for gas molecules joining the bubble nucleus, Co is the
concentration of gas molecules in solution in the polymer and k is the Boltzman constant.
As per the CNT, the greater number of cells can be nucleated as the saturation pressure (ΔP)
increases. This result has been practically verified in batch process [38,39]. The saturation
pressure can be approximated correspond to the gas absorption in polymer as per the Henry’s
law equation (2.1). As the dissolved amount of the gas in the polymer increases, the chances of
more cell nucleation is increased. The CNT gives useful information about the relationship
between pressure drop and cell nucleation but it cannot be useful to predict the effect of pressure
drop rate on the cell nucleation. Cell nucleation is greatly governed by Pressure drop rate so it is
imperative to investigate the effect of pressure drop rate on cell nucleation.
Park et al. [40] analyzed the effect of pressure drop rate in the extrusion foaming process by
using various set of dies with different parameters to generate the wide range of pressure drop
rate and characterized the final foam structure. This analysis shows that a greater number of cells
nucleated as pressure drop rate increases which may be explained by the mechanism of cell
nucleation/growth competition [40]. If the pressure drop rate is high, there will be certain
pressure drop in very short time. During this shorter period of time, less gas diffuses into already
nucleated cells and they do not have a chance to grow further. As less gas is used in cell growth,
more gas is available for promoting more nucleation in the polymer-gas solution. As a result,
16
higher cell density foam structure can be achieved with higher pressure drop rate. Consequently,
die selection is very important to get enough pressure drop rate to achieve microcellular foam.
As per equation (2.7), the nucleation rate increases with temperature increases. However, this
behavior is not completely examined for an extrusion process. Researchers found different
behavior with different material and condition. Ramesh et al. [41] verified this effect with PS-
CO2 system. But Goel and Beckman [42] claimed that nucleation rate decreases with the
increasing of temperature for PMMA- CO2 system. Matuana et al. [43] found that foaming
temperature doesn’t have substantial effect on final cell density in a CO2 system. Baldwin [44]
examined this effect with amorphous and semi crystalline PET and CPET. They found that cell
density increases with the increasing of temperature for amorphous PET and CPET for below
100°C and there was not any effect above 100°C but temperature doesn’t significantly affect the
cell density in the case of semi-crystalline PET and CPET.
2.2.2.2 Heterogeneous Nucleation
Heterogeneous cell nucleation is originated at some preferred sites by mixing additives in
polymer and gas solution which is called a nucleating agent. As shown in Figure 2.5 it is more
likely to be promoted at the boundary of the matrix and additives as the free energy barrier for
nucleation is lower than that in the homogeneous nucleation. The mechanism of heterogeneous
nucleation in the polymer has not been investigated in depth due to its complexity. However it is
evidenced that cell density in the foam structure can be significantly improved by mixing some
fillers or additives. [45]
Chen et al. from Trexel Inc. [46], investigated the mechanism of heterogeneous nucleation with
polymer and additives. The hypothesis of this mechanism is that the trapped gas between the
17
polymer and additives creates cells when the system pressure drops during the foaming process.
According to heterogeneous nucleation theory, certain sites or spots in the polymer-gas solution
which contain undissolved gas may become cells and the allowed size of the spot is larger than
the following critical value, [46]
𝑟𝑐𝑟 = 2𝛾𝑏𝑝∆𝑝
(2.8)
where 𝑟𝑐𝑟 is critical radius, 𝛾𝑏𝑝 is the surface tension, ∆𝑝 is the pressure difference between the
bubble and the melt. Micro-pores on the polymer-additive boundaries can behave as cracks or
defects. During the mixing process, the polymer melt may not able to fill these Micro-pores and
gaps between two interfaces completely due to surface tension. From the above equation, the
surface tension force increases as the radius 𝑟𝑐𝑟 becomes smaller. Therefore gap cannot be filled
regardless the pressure difference between the polymer melt and micro-pores is larger. These
pores induce cells nucleation as the gas accumulates in the micro-pores. It was experimentally
verified that a certain amount of gas accumulates at the polymer-additive boundary and the spots
where gas accumulates induce cells if the size of spots is larger than the critical value. This is
the reason why heterogeneous nucleation needs less gas content to produce fine-celled
morphology compared to homogeneous nucleation. [46]
The rate of heterogeneous nucleation is given by expressions similar to Equation (2.6) and (2.7),
𝑁ℎ𝑒𝑡 = 𝐶1𝑓1exp�−∆𝐺ℎ𝑒𝑡∗
𝑘𝑇� (2.9)
Where 𝐶1is the concentration of gas molecules, 𝑓1is the frequency factor of gas molecules
joining the nucleus, 𝑘 is th Boltzman’s constant and 𝑇 is the temperature in K., ∆𝐺ℎ𝑒𝑡∗ is
18
Gibbs free energy and can be expressed for heterogeneous nucleation, which occurs at smooth
planar surfaces as follows
∆𝐺ℎ𝑒𝑡∗ = 16𝜋𝛾𝑏𝑝3
3∆𝑃2 𝐹(𝜃𝑐) (2.10)
where 𝛾𝑏𝑝3 the surface energy of the polymer-bubble interface, ∆𝑃 is the gas pressure used to
diffuse the gas into the polymer. 𝐹(𝜃𝑐) , which is the reduction of energy due to the inclusion of
additives (nucleants), can be expressed as follows:
𝐹(𝜃𝑐) = �14�
(2 + 𝑐𝑜𝑠𝜃)(1 − 𝑐𝑜𝑠𝜃)2 (2.11)
where 𝜃𝑐 is the contact angle of the polymer-additive gas interface.
The surface geometry of the nucleating sites varies from one site to another. It depends on the
nucleating agents themselves, the presence of unknown additives or impurities and nature of
internal Therefore, instead of assuming that all nucleating sites are either smooth planner
surfaces, the cell nucleation can occur in conical cavities that exhibit geometries consistent with
the image shown in Figure 2.6 where the semi conical angles, β are randomly distributed
between 0 and 90° at different nucleating sites. In this case, F(θc,β) is the reduction of energy,
which can be expressed as follows[47, 48]
𝐹(𝜃𝑐 ,𝛽) =14�2 − 2𝑠𝑖𝑛(𝜃𝑐 − 𝛽) +
𝑐𝑜𝑠𝜃𝑐𝑐𝑜𝑠2(𝜃𝑐 − 𝛽)sin𝛽
� (2.12)
The homogeneous and heterogeneous nucleations are not different from each other. The mixed
model describes the nucleation by equation (2.13),
19
𝑁 = 𝑁ℎ𝑜𝑚′ + 𝑁ℎ𝑒𝑡 (2.13)
where 𝑁ℎ𝑜𝑚′ is the rate of homogeneous nucleation reduced by the rate of heterogeneous
nucleation. Modified homogeneous nucleation rate 𝑁ℎ𝑜𝑚′ can be given by equation (2.14),
𝑁ℎ𝑜𝑚′ = 𝑓0𝐶0′𝑒𝑥𝑝 �−∆𝐺ℎ𝑜𝑚𝑘𝑇 � (2.14)
where 𝐶0′ is the concentration of gas molecules in solution after heterogeneous nucleation has
occurred.
Due to lower free energy barrier for nucleation, the interface between the additive and the
polymer matrix is more preferential as sites for nucleation compared to homogeneous nucleation.
By controlling the amount of additives, desired number of bubbles can be generated. Xu. et. al.
[49] described that cell density of extruded PS foam increases with adding talc as the nucleating
agent. Furthermore, the effects of talc on nucleation were found to be different with various
geometry dies. A relatively significant effect was observed with a lower pressure drop rate die.
Han et al. [50] also observed that cell size significantly reduces and cell density increases with
adding a small amount of intercalated or exfoliated nano-clay. At the same time, it is difficult to
achieve uniform large number of micro size cells with additives due to poor dispersion and
agglomeration of additives [51,52]. Furthermore, if the amount of the nucleating agent exceeds
certain critical value, the cell density will not further sensitive to the amount of nucleating agent.
Lee et. al.[53] investigated the gas absorption behavior in polymers with mineral filler such as
HDPE with/without talc, and PVC with/without CaCO3 to explain heterogeneous nucleation. It
was pointed out that the filler-polymer interface helps to create cells in foaming process. Ramesh
et. al [54] prepared a model for heterogeneous nucleation in the blend of PS and high impact PS
20
(HIPS) based on the presence of micro voids. Leung et. al. [55,56] also used PS-CO2 system and
demonstrated that heterogeneous nucleation could take place at a reasonably high rate
theoretically, which qualitatively agreed with experimental observations. However, in all
previous findings of cell nucleation during polymer foaming the experimental data was not in
good quantitative agreement with theoretical predictions without the use of fitting parameters.
Therefore, the explanation of the real mechanism behind cell nucleation by classical nucleation
theory is still controversial.
In summary, the free energy required for heterogeneous nucleation is generally much lower than
that required for homogeneous nucleation. Therefore, additives such as talc, nano-clay or
nanotubes can be added to decrease the energy required to create bubbles and therefore enhance
cell nucleation. However there are certain criteria to be fulfilled for being an ideal nucleant [57].
Three of the most important criterion are: first, highest nucleation efficiency can only be
achieved when the nucleation on the nucleant surface is energetically favored and is relative to
homogeneous and heterogeneous nucleation; secondly, ideal nucleants have uniform size and
surface properties; thirdly, ideal nucleants are easily dispersible.
Effect of Shear Stress, Extensional Stress/Strain on Cell Nucleation
In addition to the amount of additives, the cell density is also sensitive to shear force. Chen et.al.
[58] found that the effect of the shear stress becomes more critical when the saturation pressure
or amount of the gas in the polymer becomes lower and the driving force for cell nucleation at
that time is insufficient. Particular in continuous system such as extrusion foaming, the shear
force has significant effect on heterogeneous nucleation rate. Lee came up with the lump cavity
nucleation model to explain this effect which showed that the cavities on the rough surfaces of
the very small nucleating particles, which are not completely wetted by the polymer melt, can
21
form potential sites for bubble nucleation. When the gas phase in the cavity grows and matures
as a result of diffusion of the dissolved gas into the cavity or a pressure drop, the applied shear
force enhances the chance of detaching it from the cavity, which is promoting the bubble
nucleation.
Han and Villamizar[59]; Han and Han[36]; Taki et al[60]; Tatibou et al. and Gendron[61] tried
to investigate the bubble nucleation and growth phenomena using in-situ observation of
continuous foaming process in the industrial extrusion system through transparent slit dies. In
this study, they concluded that the combined effect of extensional and shear stresses promoted
cell nucleation.
In a study of of devolatilization of molten polymer by Albalak et al. (1990), it was proposed that
bubble expansion can generate the tensile stress in the polymer melt which decreases in local
pressure. This contributes to an increase of superheat that causes secondary micro-bubbles to
nucleate around the bubble.
Taki et al. [60] and Guo et al[62] developed experimental foaming visualization system to
capture the foaming process in high temperature/pressure chambers after depressurization in situ
under static condition using high speed camera. In these studies they applied minimal stresses
however in industrial foaming processes; plastics experience substantial shear and extensional
stresses which affect the final cell morphology (bubble size, distribution and density).
In a study of devolatilization of molten polymer by Albalak et al. (1990), it was proposed that
bubble expansion can generate the tensile stress in the polymer melt which decreases in local
pressure. This contributes to an increase of superheat that causes secondary micro-bubbles to
nucleate around the bubble. Wang et al. [63] verified that that polymer deformation during
22
bubble growth would induce extensional stress in some regions around the nearby talc particles,
reducing the local system pressure and hence, Rcr for cell nucleation. To account for this
pressure fluctuation, Leung et al. [64] modified the expressions for critical radius and energy
barrier for cell nucleation as follows.
𝑅𝑐𝑟 = 2𝛾𝑏𝑝
𝑃𝑏𝑢𝑏 − �𝑃𝑠𝑦𝑠 + ∆𝑃𝑙𝑜𝑐𝑎𝑙� (2.15)
∆𝐺ℎ𝑜𝑚∗ = 16𝜋𝛾𝑝𝑏3
3 �𝑃𝑏𝑢𝑏 − �𝑃𝑠𝑦𝑠 + ∆𝑃𝑙𝑜𝑐𝑎𝑙��2 (2.16)
∆𝐺ℎ𝑒𝑡∗ = 16𝜋𝛾𝑏𝑝3
�𝑃𝑏𝑢𝑏 − �𝑃𝑠𝑦𝑠 + ∆𝑃𝑙𝑜𝑐𝑎𝑙��2 𝐹(𝜃𝑐 ,𝛽) (2.17)
Wong et al. [65] developed a novel batch foaming visualization system to capture the in-situ
foaming process with the capability to apply extensional stress to plastic specimen. They verified
the system using the samples of PS and PS/talc by varying processing temperatures and strain.
They concluded that extensional stress or strain can be a governing factor in foaming under
certain processing conditions.
2.2.3 Cell Growth and Stabilization
After cells are nucleated, they start to expand due to gas diffusion from the polymer matrix as the
pressure inside the cell is higher than the surrounding pressure. Cells tend to grow so as to
decrease the pressure difference between inside and outside. The cell growth mechanism is
influenced by the viscosity, diffusion coefficient, gas concentration, time allocated for them to
grow, hydrostatic pressure or stress applied to the polymer matrix and number of nucleated cells.
23
The cell growth can be controlled by temperature which will change the diffusivity and melt
viscosity. For example, the diffusivity of the gas decreases and melt viscosity of the solution
increases as temperature decreases which will decrease the cell growth. The precise temperature
control is required to keep the gas in the polymer matrix for achieving desirable cell growth and
high volume expansion in microcellular foam [66, 67, 68]. In microcellular foams, the cell size
is very small, cell density is very high, the thickness of wall between two cells is smaller and the
growth rate is faster than in conventional foams. This may also induce undesirable cell
coalescence [69]. The cell coalescence while cell growth, the initial cell density will be dropped.
The deteriorated cell density will affect the mechanical and thermal properties of the foam. The
contiguous cells will begin to connect with each other as cell grows. These adjacent cells tend to
fuse together because the total free energy is lowered by reducing the surface area of cells via
cell coalescence [7]. The shear field generated during the process also causes the nucleated
bubble to stretch and that will promotes the cell coalescence [70]. The cell coalescence is very
hard to prevent. Baldwin et al. [71] tried to avoid cell coalescence in the die by increasing the
pressure of nucleated polymer solution but they found cell coalescence and deteriorated cell
density in the final foam structure. It is hard to maintain the high back pressure in the larger die
to produce the larger cross section foam. It may not be possible to suppress cell coalescence by
just controlling the pressure in the die. Park et al. [70] proposed a solution for avoiding cell
coalescence by increasing the melt strength of the polymer through temperature control in micro
cellular extrusion processing.
2.3 Blowing Agent
The process of thermoplastic foaming can be described by state change. The raw plastic material
is heated and pressurized, a blowing agent (A substance that produces a cellular structure in a
24
polymer mass is defined as a blowing agent) is added. The foam structure is developed by
lowering the pressure, and finally foam product is generated by cooling the polymer matrix. The
blowing agent plays important role in manufacturing and performance of polymer foam.
Blowing agent is the dominant factor affecting the density of the foam, cellular microstructure
and morphology of the foam, which in turn determine the end-use performance. The choice of
blowing agent and choice of processing conditions are inter-linked i.e. both influence each other.
[1-3,72]
There are two methods to introduce a blowing agent into polymer matrix; 1) chemical reactions
and 2) physical mixing. The former method uses chemical blowing agent (CBA) and the latter
method uses physical blowing agent (PBA). Sometimes both are needed for foam extrusion. [1-
3]
2.3.1 Chemical blowing agent (CBA)
CBAs are mixture of chemicals that release gas like CO2 and/ or N2 upon thermal decomposition
at a specific temperature range. CBA are generally used to make high and medium density foam
plastic and rubber. They are rarely used to make foam with densities below 400 kg/m3 because
they are too expensive. For example, CO2 and N2 released form CBA cost about 10 times more
that used from a cylinder. The quantity of the blowing agent needed for the foam processing is
very low typically around 2 wt%. The chemical reactions can be either endothermic or
exothermic depends on the type of chemicals. Endothermic CBAs absorb the heat energy while
decomposition process and they have wider decomposition temperature range. Sodium
bicarbonates and their altered forms falls into the endothermic-grade CBAs categories. These
CBAs releases mainly carbon dioxide gas and water vapor during thermal reaction that helps to
create the foam structure. [1-3, 72]
25
Whereas in exothermic CBAs release heat during the thermal decomposition which is more
spontaneous and is harder to be terminated once the reactions are initiated. Exothermic CBAs
like Azo compounds such as Azodicarbonamide and their derivatives and 4, 4’-oxybis (benzene
sulfonylhydrazide) are commercially used in foam processing of LDPE and EVA. These
compounds manly release Nitrogen gas upon thermal decomposition. In the selection of a CBA
for a particular foaming process, the decomposition temperature, the decomposition rate, the type
of gas they liberate (CO2 or N2), the gas yield (the amount of gas liberated in cm3 per gram of
CBA), and the pressure generated from these gases are the general characteristics which need to
be considered. [3,73]
2.3.2 Physical blowing agents (PBAs)
Physical blowing agents (PBAs) are substances that are injected into the polymer system in
either a liquid or gas phase such as pentane or isopropyl alcohol, have a low boiling point and
remain in a liquid state in the polymer melt under pressure.[2] PBAs are generally used for
making low-density foam under 0.2 g/cm3. Before 40 years, CFC was mainly used as a physical
blowing agent due to its low thermal conductivity, soluble, volatile and nontoxic nature. But it
easily reacts with ozone and damage the ozone layer that raised the serious issue of global
warming. In the 1987, Montreal Protocol was signed to discontinue the manufacturing of
halogenated hydrocarbons to minimize the ozone layer damage. The alternative of halogenated
hydrocarbons, such as butane and pentane were commonly used in the production of low-density
foams because it has relatively low price and can be injected into the foaming equipment
efficiently. But they are flammable and the use of such blowing agents introduces flammability
hazards on the shipping and handling of the finished foam products. Considering these
26
environmental and safety issues, these PBAs are replaced by inert gases such as carbon dioxide
(CO2) and nitrogen (N2). [1, 3]
The process of polymeric physical foaming is divided into three main steps. In the first step,
PBA dissolves and saturates into the polymer at a high pressure. The phase separation between
the dissolved gas and the polymer matrix will occur by releasing the system pressure or
increasing the system temperature. The new phase formation known as nucleation, can originate
from self structural adjustment. Cells of gaseous phase will start to nucleate within the polymer
matrix at the defects or nucleating agents. Dissolved gas will slowly diffuse into these cells and
expand the cells. In the last step, the cell expansion stops and stabilized the cellular structure.
The physical foaming phenomena can be applied in continuous processes such as extrusion and
injection molding, and batch processes such as compression molding to produce cellular foams
for various applications. [1-3]
2.4 Foaming Processes
Foaming process can be carried out by batch or continuous process
2.4.1 Batch Process
In batch process, shaped thermoplastic parts are impregnated with a blowing agent gas in a
pressurized vessel at elevated temperature and pressure for a predetermined period of time,
typically several hours. As shown in Figure 2.7 [74], if the thermodynamic instability induced by
the releasing pressure sharply, the solubility of the gas will be rapidly reduced. If the part
temperature is increased to above its glass transition temperature, small bubbles of saturated gas
will begin to nucleate and grow, creating the cellular structure of the foams [75, 3].
27
The main drawback of this process is that it takes long time to saturate the polymer with gas due
to low diffusion rate of gas into the polymer. The batch foaming process is also not cost
effective. To overcome this drawback, a cost-effective, continuous extrusion process was
developed to produce microcellular foam based on the same principle of thermodynamic
instability.
2.4.2 Continuous Process
Extrusion foaming and injection mold foaming are the continuous processes. They are cost
effective and have higher productivity than batch foaming process. Figure 2.8 shows a schematic
of an extrusion foaming system. There are some basic sequences for continuous extrusion
foaming with a PBA: (a) uniform formation of a polymer/gas solution, (b) cell nucleation, (c)
cell growth, and (d) timely solidification of the polymer melt. The polymer materials are first
melted in an extruder. Blowing agents will either be injected directly into the polymer melt
(PBA) or pre-compounded into the polymer materials (CBA). In PBA based process no
decomposition temperature limitation exists, unlike CBA based process so process temperature
can be below critical temperature. It is also cheaper and can produce better cell morphology. A
very high pressure in the barrel is generated due to the screw motion of the extruder. Such a high
pressure is essential to keep the saturation of the blowing agents in the polymer melt. The large
number of bubbles in the polymer melt can be nucleated by applying thermodynamic instability
induced by lowering the solubility of the gas in the solution by introducing a sharp pressure drop.
The nucleated cells continue to expand when it exits through die and it stops either when all
dissolved gas escapes from polymer matrix or when the polymer matrix turns into too stiff
material due to cooling that is not allowed for further expansion. There are two critical issues
involved in cell growth: Cell coalescence and cell rupture. Park and Behravesh developed
28
strategies for preventing cell coalescence and gas escape. Cell coalescence can be inhibited by
increasing the melt strength by cooling the polymer/gas solution homogeneously. Gas escape can
be prevented by forming solid skin layer by cooling the surface of the extrudate so that the gas
can be blocked from escaping from the polymer. Polymer melt solidify due to glassification or
crystallization when the melt is extruded out of the die and its temperature decreases. Timely
solidification is important, because if the solidification takes long, the gas loss would be too
much and if the solidification is too rapid, the melt will become too stiff to expand and large
volume expansion ratio will not be achievable. [76, 77, 78].
2.5 Factors affecting Foam Extrusion
2.5.1 Crystallization Kinetics
In a continuous foaming process, polymer transforms from solid to molten state and finally, to
solid state to get the final shape of the foam for the applications. The former is called melting
and the latter solidification or crystallization. In general melting is done before introducing gas,
but foaming and solidification takes place at the same time with different rates. Generally,
solidification is relatively slower process than Foaming, and plays a important role in degree of
expansion and final foam properties. [1]
In semi crystalline polymers, crystallites are dispersed into an amorphous region. The fraction of
the polymer that is fully crystalline is known as the crystallinity. Depending upon the polymer
chain structure, crystals can be formed within a certain time to induce the resistance for bubble
expansion to have a fine cell structure. The competing mechanisms between expansion and
material strength to hold expansion is an interesting kinetic topic for achieving optimal foam
structure. [1]
29
The crystallites are nucleated from the melt at certain range of temperature during nucleation and
afterwards continue to grow during the growth phase to form three-dimensional conglomerations
of crystallites known as spherulites. The spherulite growth rate is quicker than the nucleation rate
as the required free energy for spherulite to grow is lower than that of required for nucleation
rate. The crystallization rate increases under stress because the molecular chains orient and
become more packable due to stress. Nucleation takes place in one of the two ways; Thermal or
instantaneous nucleation which occurs at the beginning of the process when nuclei appear
instantaneously. It is assumed that it depends only on temperature and to be independent of time
and cooling rate. Therefore, the grown crystals will be of approximately equal sizes. The other
way of nucleation is thermal or sporadic nucleation which appears in the liquid phase during the
process. And the activated nuclei appear at a constant rate per unit volume. [79]
Normally, two types of nucleation are found for polymer crystallization: Primary and secondary
nucleation. During the primary nucleation, three dimensional crystal growth occurs rapidly and
spontaneously after potential nucleus reaches a critical size. If nucleation occurs without any
preformed nuclei or any foreign surfaces, primary nucleation is also called homogeneous
nucleation. On other hand, secondary nucleation occurs when the chain segments are added to
the existing crystal surface. The main difference between primary and secondary nucleation is in
Gibbs free energy or energy required for the formation of a critical size nucleus.
Gibbs[80, 81, 82] developed the classical nucleation theory based on the assumption, that energy
variations in the supercooled phase can overcome the nucleation barrier caused by the surface of
the crystal. Based on this assumption, Turnbull and Fisher[83] developed a formula to estimate
the primary nucleation rate as a function of the crystallization temperature, using the Williams-
landel-Ferry(WLF)[84] equation which universally describes the temperature dependence of
30
polymer melt viscosity: Based on the surface or secondary nucleation theory , Lauritzen and
Hoffman derived a linear growth rate equation, which comprises fold surface energy, lateral
surface energy, heat of fusion and lamellar thickness terms into the Gibbs free energy to explain
the linear growth rate of spherulites[85].
Crystallization Regimes
Crystal growth rate and spherulite size depends on the relative rates of nucleation and deposition
of chain segments. Hoffman[86, 87] described crystallization regimes, which explain the relative
nucleation and growth rates. A regime transition occurs when the relationship between growth
rate Ġ, and the surface nucleation rate, i, undergoes a change. In regime I, the highest
temperature regime, crystal growing on an existing crystal face is completed before the next
layer is nucleated as the chain section deposition rate on one surface nucleus is so fast. In other
words, Ġ varies as i. In regime II, the nucleation rate is fast compared to the growth rate, i.e.
�̇� 𝛼 𝑖1/2, as a result the nucleation of new crystal layers takes place before deposition on
existing layer is completed. This will cause the downward break in the growth rate curves as one
passes through the regime I to regime II transition. Finally, in the lowest temperature regime,
regime III, the mean separation of the nuclei approaches the width of the molecular stems and, Ġ
varies as i, such that at the transition of regime II and III, an upward break in the growth rate
occurs. Frank and Tosi, Sanchez and DiMarizo, Sadler and Lauritzen, DiMarzio and Passaglia
also contributed to develop the other kinetic theories of crystallization.
Generally, data of linear growth rate and the primary cell nucleation rate are enough to determine
the overall crystallization rate. Many measurements of crystallization also involve the
macroscopic determination of crystalinity as a function of time. Kolmogoroff [88], Johonson
[89] and Mehl and Evans [90] described the macroscopic development of crystalinity in terms of
31
nucleation and linear crystal growth. However the classical theory of Avarmi[91, 92, 93] for
phase transformation kinetics is the most widely used model for the analysis of isothermal
nucleation and crystallization in the polymer processing.
But Avarmi model has some limitations due to its following simplified assumptions. (i) there is
no volume change during crystallization (ii) The sample is completely transformed (iii) there is
constant linear growth rate, (iv)The nuclei have constant shape during growth and (v)there is no
secondary crystallization occurs.
𝑋𝑤(𝑡) = 𝑋(𝑡)𝑋𝑢
= ∫ �𝑑𝑄𝑑𝑡 � 𝑑𝑡𝑡0
∫ �𝑑𝑄𝑑𝑡 � 𝑑𝑡∞0
(2.18)
Where 𝑋𝑤(𝑡) is the absolute crystalinity at crystallization time t, and 𝑋𝑢 is the ultimate
crystalinity for t =∞. For isothermal crystallization experiments, the heat generated is estimated
while the polymer is in the isothermal condition, therefore, 𝑋𝑤(𝑡) can be physically obtained as
the area under the crystallization peak in a plot of heat flow verses time. As the Avarmi model is
expressed in terms of the volume fraction, it is necessary to transform the weight fraction
measured by DSC into a volume fraction. This can be done using following relation:
𝑋𝑣(𝑡) = 𝜌𝑎𝜌𝑐𝑋𝑤(𝑡) (2.19)
Where 𝜌𝑎 is the amorphous region density, and 𝜌𝑐 is the crystalline density.
The resulting crystallization kinetics can be used as a basis for establishing strategies for the
production of low-density, fine-celled polypropylene foams.
32
2.5.2 Filamentary Die Design in Foam Extrusion
Different Length and diameter of the filamentary die induce different die pressure and pressure
drop rates and that helps to get different foam structure. Xu et al.[49] designed three
interchangeable groups of 9 dies to have either different pressure drop rates while having the
same die pressure and flow rates, or different die pressure while having the same pressure drop
rates and flow rates. They assumed that the polymer/gas solution flow through die can be
described by the “Power law” in the flow through a tube which states that the viscosity of the
polymer-gas matrix is shear rate dependent and the pressure drop over the length of a nozzle for
a non-Newtonian fluid in a fully developed flow can be expressed as[94]
𝑃𝑑𝑖𝑒 = −2𝑚𝐿
𝑅3𝑛+1��3 +
1𝑛�
𝑄𝜋�𝑛
(2.20)
The residence time t of the polymer/gas solution in the nozzle can be given by equation (2.21),
𝑡𝑟𝑒𝑠𝑖𝑑𝑒𝑛𝑐𝑒 = 𝐿𝑉𝑎𝑣𝑔
= 𝐿
𝑄 𝜋𝑅2⁄ = 𝜋𝑅2𝐿𝑄
(2.21)
Therefore, the pressure-drop rate can be estimated as follows by equation (2.19):
𝑑𝑝𝑑𝑡
≈ ∆𝑝∆𝑡
= −2𝑚�3 + 1𝑛�
𝑛
�𝑄𝜋𝑅3�
𝑛+1
(2.22)
Using these equations, they measured the pressure drop rate and find the effect on the cell
density of extruded PS foams. The experiment results discovered that geometry of the die is
important parameter to govern the cell density due to its effect on the pressure drop rate across
the die. The die back pressure which depends on the die length and die diameter, significantly
affect the cell morphology.
33
Patrick et al. [95] also investigated the effect of pressure drop rate on cell nucleation and growth
behaviors of non-crosslinked high melt strength Polypropylene. Both the cell population density
and the volume expansion ratio increased as the die pressure drop rate increased and the effect of
die pressure on nucleation and expansion behavior was negligible as long as the die pressure
remained above the solubility pressure of CO2.
2.5.3 Governing Mechanism of Volume Expansion
The main purpose of foaming is to make low density foams with high expansion to save material
cost. Therefore, it is essential to get the desirable volume expansion ratio. Moreover, the
effective control of the volume expansion is necessary to enhance the efficiency of costly
blowing agents. Continuous attempt has been made to understand the mechanism of volume
expansion in foam extrusion.
Behravesh et al. [77] explained that initial bump at the die exit promotes gas loss during volume
expansion. The thickness and the temperature of the cell walls are crucial in determining the rate
of gas escape as the gas escape from foams takes place through cell-to cell diffusion. The
thickness of cell walls gradually decreases as the cells grow. Because of the cooling through
convection at die orifice and isentropic expansion of gas, the temperature of the cell walls
decreases. When the temperature of the cell wall is high enough, the cells grow very fast and the
thickness of cell wall decreases and gas will escape quickly through the hot thin cell walls.
Therefore die temperature determines the volume expansion of extruded foams.
Naguib et al. [68] described the fundamental volume expansion mechanism by analyzing the
experimental results of extrusion foaming with PP foams blown with n-butane and using CCD
system to visualize the expansion behavior. It was concluded that the volume expansion of
34
extruded foams blown with a physical blowing agent is governed either by the loss of gas
through the foam skin or the crystallization of the polymer. Figure 2.9 shows the schematic of
this fundamental mechanism which is typically “mountain shape” curve of volume expansion
verses die temperature. When the processing temperature is high, the diffusivity of gas will be
high and foam will take long time to solidify. As a result, the gas that has diffused into the
nucleated cells may easily escape from the foam. Moreover, as the cell expansion increases, the
thickness of the cell walls becomes thin and the resulting rate of gas diffusion between cells
increases This gas loss through the cell walls decreases the amount of gas that is available for the
growth of cells and that lowers the expansion. In addition, the cells will not solidify rapidly
sufficiently; they tend to shrink due to loss of gas through the foam skin, resulting overall foam
contraction. This shows when the processing temperature is high, gas loss phenomena is a
dominant factor that constrains the volume expansion. [96]
On the other hand, if the processing temperature is too low and close to crystallization
temperature, the polymer melt will be solidified too quickly during the foam process before foam
is fully expanded. The foam cannot be fully expanded, if the crystallization occurs in the
beginning stage of the foaming i.e. before the dissolved gas fully diffuses out of the polymer
matrix and into the nucleated cells. Therefore, it is essential that crystallization should not occur
before all of the dissolved gas diffuses out into the cells. When the polymer melt exits through
die, the temperature of the melt decreases due to the external cooling outside the die and the
cooling effect resulting from the isentropic expansion of the gases. Hence the time for the
solidifying of the polymer melt depends on the processing temperature at the die. So, in order to
provide enough time for the gas to diffuse into the polymer matrix, the processing temperature
should be enough high. This shows that there is an optimum processing temperature for
achieving maximum expansion as shown in the middle section of figure. If the melt temperature
35
is too high, the maximum volume expansion ratio governed by gas loss and it will increases as
the processing temperature decreases. If the melt temperature is too low, the volume expansion
ratio is governed by the crystallization behavior and it will increase as the temperature increases.
[96]
Xu et al. [49] pointed out that the inevitable and unwanted premature cell growth inside a die has
a significant effect on volume expansion ratio. These premature cells grow at the die exit and
resulted in big size cells. This big size cells causes instantaneous expansion at the die exit due to
the pressure drop. These phenomena enhance the gas loss. The amount of the premature cell
growth is estimated by cell density, premature cell growth time, and premature cell growth rate,
which are directly influenced by the die geometry. When the premature cell growth is too much,
the volume expansion ratio of the extruded form will be significantly dropped. Equation was
given to calculate the amount of premature cell growth Mpremature, in a filamentary die.
𝑀𝑝𝑟𝑒𝑚𝑎𝑡𝑢𝑟𝑒 ≈ 43 𝜋𝑁𝐶𝑠 ∙ (𝐷𝑡𝑃𝑟𝑒𝑚𝑎𝑡𝑢𝑟𝑒)
32 + 𝑀0 (2.23)
Where N = Cell density, 𝐶𝑠 = dissolved gas concentration per unit volume, 𝐷= diffusivity,
𝑡𝑃𝑟𝑒𝑚𝑎𝑡𝑢𝑟𝑒 = premature cell growth time, 𝑀0= undissolved gas amount per unit volume
If P< Psolubility all the injected gas cannot dissolve into the polymer melt and term (𝑀0) in the
equation can be given as follows,
𝑀0 ≥ 𝑃𝑠𝑜𝑙𝑢𝑏𝑖𝑙𝑖𝑡𝑦 − 𝑃𝑃𝑠𝑜𝑙𝑢𝑏𝑖𝑙𝑖𝑡𝑦
𝑥 𝐶𝑖𝑛𝑗𝑒𝑐𝑡𝑒𝑑 (2.24)
where 𝑃𝑠𝑜𝑙𝑢𝑏𝑖𝑙𝑖𝑡𝑦 = solubility pressure,
𝐶𝑖𝑛𝑗𝑒𝑐𝑡𝑒𝑑= amount of injected gas.
36
The value of M0 can be minimized by proper mixing of the polymer gas solution and providing
enough time for the gas dissolving and can be removed if the P is maintained above P
solubility.[97] It is also found that the volume expansion ratio would be dropped if the amount of
premature cell growth exceeds some critical value.
2.6 Characterization of the Foam Samples
Thermoplastic foams are usually specified in terms of foam density, volume expansion ratio, and
the cell morphology. These parameters are dependent on the processing conditions as these
parameters indicate the degree of the cell nucleation and the expansion which have been
controlled during the foam processing.
2.6.1 Foam Density
Foam density is one of the structural parameters that directly represent the density reduction of
the unfoamed material. The foam density (ρf) can be calculated as:
𝜌𝑓 = 𝑀𝑉
(2.25)
where M = the mass of foam sample, g V = the volume of foam sample, cm3
Water submerging and displacement is a usual method for determining the bulk density of solid
and closed-cell foam specimens.
2.6.2 Volume Expansion Ratio
The relative density of the form is defined by the ratio of foamed part density to its un-foamed
material density. The relative density of a foam specimen is often used in the evaluation of
37
foam’s volume expansion and it is the reciprocal of its volume expansion ratio. The volume
expansion ratio (VER) of a foam sample can be calculated as the ratio of the bulk density of pure
material to the bulk density of the foam sample as follows:
𝑉𝐸𝑅(𝜑) = 𝜌𝑝𝑜𝑙𝑦𝑚𝑒𝑟
𝜌𝑓 (2.26)
In addition to using the volume expansion ratio, researchers also use void fraction (Vf) to
describe the amount of void in the foam, and it is defined as:
𝑉𝑓 = 1 − 𝜌𝑓
𝜌𝑝𝑜𝑙𝑦𝑚𝑒𝑟 (2.27)
The foam density, volume expansion ratio and the void fraction are related to each other, and all
are represents the material savings that results from the void volume that replaces the original
material.
2.6.3 Cellular Morphology and Cell Size Distribution
The cell morphology of a foam sample is typically examined with the aid of a scanning electron
microscope (SEM). The cell morphology of foam can be characterized by its cell size, cell
density, and cell size distribution. The cell size of the cells in the foam can be measured from the
SEM micrographs with the aid of image utility software. Cell population density is defined as the
number of cells per cubic centimeter volume relative to the unfoamed polymer. The cell density
of the foam structures can be estimated using the following equation:
𝐶𝑒𝑙𝑙 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 = �𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝐶𝑒𝑙𝑙𝑠
𝑎𝑟𝑒𝑎 �32∙ 𝑉𝐸𝑅 (2.28)
38
where number of cells = total number of cells in the area which can be estimated from the
micrographs taken by a scanning electron microscope with the aid of image utility software.
area = defined area, cm2,
VER = volume expansion ratio.
Cell size distribution of a foam specimen can be either estimated from the SEM micrographs or
measured with the mercury porosimetry or mercury immersion technique. The principle behind
mercury porosimetry is that mercury is a non-reactive, non-wetting liquid for most substances
and hence, sufficient pressure has to be applied to force its penetration into porous structure. [98]
39
Figure 2.1 Steps of continuous extrusion foaming process [10]
Figure 2.2 Solubility of carbon dioxide (CO2) and nitrogen (N2) in PS [99, 11]
40
Figure 2.3 Homogeneous and heterogeneous nucleation in a polymer-gas solution [100]
Figure 2.4 The free energy, ΔG, vs. radius of bubble, r, associated with the homogenous
nucleation [Courtesy: Prof. Park, Lecture notes of MIE1706 Manufacturing of cellular
polymers]
41
Figure 2.5 Comparison of energy required for homogeneous and heterogeneous nucleation [45]
(a) (b)
Figure 2.6 Heterogeneous Nucleation on (a) smooth planar surface and (b) in a conical cavity
[101]
42
Figure 2.7 Schematic of a laboratory-scale batch foaming system
Figure 2.8 Schematic of a continuous extrusion foaming system
43
Figure 2.9 Governing Mechanism of Volume Expansion Ratio [96]
44
Chapter 3 Largely Expanded high cell density Polypropylene Foaming
3
3.1 Introduction
Polyolefin foams are usually available in the sheet form. Sheet form is a versatile material with
excellent properties such as high elasticity, high impact strength, and high compressive strength.
Due to these properties, they have unlimited applications a across a wide range of industries such
as Automotive, Furniture, Electronics, Agriculture, Construction, Musical instruments, frozen
foods. But this polyolefin foams are crosslinked as crosslinking helps to improve foamability and
to achieve good mechanical properties. However, crosslinking limits the recyclability. Due to
environmental pressures, automotive industries and their supplier require 100% recyclable, low-
cost replacement product.
Currently, soft touch, fine cell, non-crosslinked polyethylene foams, made by Dow chemical
company, are available for various applications. [102] But polypropylene material has a number
of advantages over polystyrene and polyethylene such as PP materials have higher rigidity
compared with other polyolefins, they provide better strength than polyethylene and better
impact strength than polystyrene and they offer higher service temperature and good temperature
stability. Because of these excellent features and relative to low material costs, PP foams have
been considered to be one of the most promising candidates among thermoplastic foams for
industrial applications. However, PP foams are difficult to manufacture compared to the other
polyolefin foams due to their weak melt strength and narrow processing window. Due to weak
melt strength the cell wall separating the bubbles may not have enough strength to endure the
45
extensional force of the expanding bubbles. These walls may rupture during the foaming process.
Therefore, PP foamed products usually have a high open-cell content that is undesirable for
many applications. [103,104]
Since last decade, continuous efforts have been made to enhance the melt strength and
foamability of PP. Some of these efforts include the crosslinking and blending modification for
PP resins which significantly improved expansion, cell uniformity and formability. [105, 106,
107]. However extensive chain crosslinking foam is not recyclable. Other efforts have also been
made to improve the melt the melt strength of PP materials with long chain branching. [108,
109]. Some industries have developed long-chain branched PP materials with high melt strength
as a foamable grade. It has been verified that long-chain branching improves a strain hardening
of PP, increases its melt strength and extensibility at the strain rates relevant for foaming. Several
studies have shown that larger volume expansions by retarding cell coalescence, more uniform
cell structure than linear PP, wider processing window can be achieved with the use of HMS PP
in an extrusion foaming process with CO2 and isopentane as blowing agents.[102, 110, 111, 112,
113, 114]
Patrick et al. [95] used HMS Homo PP and got good expansion and fine cell structure with cell
density was 106 to 107 cells/cm3 however foam cell structure has too many pin holes and foam
was also too stiff and rough. They also used HMS branched random copolymer of PP and they
got very soft foams and expansion was better than homo PP but cell structure was similar to
homo PP with high open cell. To replace the crosslinked material, the fine cell structure is not
enough to get better mechanical properties. The first step is to achieve microcellular foam with
high expansion to have better mechanical properties with low density. Branching only is not
46
enough to make the foam with high cell density with high expansion to meet the requirement.
But with the crystallization, it may become possible.
Previously, XIANG XU et al. [49] verified that when PS was foamed in extrusion, the
higher cell density resulted in a larger expansion ratio. The reason was: the high cell density will
localize the gas loss. For PS, the thinner cell wall did not induce cell opening so more gas retain
effectively. So PS, microcellular foams indicated that the expansion ratio was more easily
obtained. But for PP, either microcellular foam or a large expansion ratio could be obtained. It is
observed that when the cell density was high, then cell opening was dominant and a large
expansion ratio was not obtained. In other words, the largely expanded PP foam was obtained
only when the cell density was small enough to have a larger cell wall thickness. But
microcellular and largely expanded foams together never have been achieved from PP because of
the lack of melt strength.
Hypothesis
In EPP batch foaming process, generally it is easy to obtain high closed cell content because in
batch process there is no shear force unlike the case of extrusion foaming. The polymer chain
entanglements which is the primary reason of the high viscosity of the melt, provides high melt
strength to the polymer. Due to its high melt strength the call walls can be bi-axially extended
during foaming without rupturing when cells grow. On other hand in extrusion foaming, the
polymer-gas solution encounters very high shear rates that releases the large number of chain
entanglements and decreases the polymer viscosity causing the melt strength of the solution to
decrease. Consequently, it is more difficult to achieve high closed cell content through extrusion
process. Secondly, only part of the PP crystals melt in the batch foaming process and other
47
remnant crystals act as cross-linking points that can significantly enhance the overall melt
strength of the gas-impregnated mini PP pellets.
Similar principle can be applied to extrusion foaming but the main difference in the extrusion
foaming is that the polymer is completely molten unlike the batch process where only a part of
the crystals melt so it is not possible to achieve same crystallinity in the extrusion process as in
the batch process.
The hypothesis for this study is described as follows. During the extrusion foaming process with
the tandem line extrusion after the formation of the homogeneous solution of linear semi
crystalline polymer and blowing agent, the solution passes through a secondary extruder where it
cools down at desired temperature before it exits through die. The second extruder provides large
residence time for cooling as well as proper mixing. In this non-uniform temperature and time
distribution with cooling and large residence time, polymer starts to form very small crystals at
temperatures above its crystallization temperature. These crystals act as a crosslinking point and
molecules connected with these crystals behave as a big molecule leading to an increase in the
melt strength of the polymer matrix. The increased melt strength helps in getting higher
expansion ratios. Furthermore, crystals provide heterogeneous nucleation sites to promote the
nucleation. This phenomenon has not been proved yet for extrusion process but similar
occurrence have been proved for expanded polypropylene bead foaming process. This study has
been done to verify the crystallization effect.
This chapter includes the method of the production of high expansion, high cell density (more
than 109cells/cm3) foams using the tandem extrusion system. The experimental results are
demonstrated that verify the feasibility of the proposed ideas. The effects of processing
parameters such as the temperature, the material parameters such as molecular weight of the
48
material, content of blowing agent, and the die geometry on the production of low-density, high
cell density PP foams are investigated.
3.2 Experimental
3.2.1 Material Selection
Design of Experiments
Material physical properties such as viscosity, surface tension, Crystallization temperature, CO2
solubility and diffusivity; operating parameters screw RPM, Barrel temperature and pressure, die
temperature, die geometry- diameter and length which governs pressure drop and pressure drop
rate, gas flow rate and pressure, significantly affect the final foam structure. By Proper selection
of these parameters would be enabled to produce the optimum foam structure with high cell
density and large expansion ratio. Figure 3.1 shows the parameters that affect the extrusion
foaming process [115]
Molecular weight plays an important role in foaming. Previous studies have shown that high
molecular weight materials are favorable for foaming as they have higher viscosities which
consequently have higher melt strengths. On the other hand, materials with low viscosity are
easier to crystallize with high gas content. Crystals help to get uniform foam structure and closed
cell foam in batch foaming. In this study, to check the influence of one characteristic property of
polymer, molecular weight, on the volume expansion ratio and cell density, three different types
of random copolymer of PP (RCPP) with Melt Flow Rate 5, 40 and 80g/10min at 230 °C and
2.16 kg are used. (PP5, PP40, PP80 respectively) To investigate the effect of pressure drop, two
different sizes of dies {L/D ~8 (L = 0.413” / ø = 0.051”), L/D ~ 34 (L = 0.719” / ø =
49
0.021”)}.were used. To investigate the effect of the amount of the gas content on foaming, for
first die 5%, 7%, 9%, 11% and 13% CO2 and for second die 7%, 9% and 11% CO2 were used.
The plastic materials used in this study, random copolymer of PP (RA12MN40, MFR - 40 g/10
min, ASTM D1238, 230°C/2.16 kg) and, RC-PP (Agility D5001-80, MFR- 80 g/10min, ASTM
D1238, 230°C/2.16 kg) were supplied by Sabic and Braskem respectively. The other RC-PP was
JP-B with MFR- 5 g/10min; CO2 supplied by BOC Gas (The Linde Group) was utilized as a
blowing agent and was a commercial grade with 99.5% purity.
3.2.2 Rheological measurement
The measurement of melt rheology is very important to know the effect of strain rate on the
shear viscosity. From the available three materials, only one material PP40 was selected to study
the effect of shearing on the complex viscosity using small amplitude oscillatory shearing
(SOAS) experiments. Melt rheology measurements were carried out on ARES parallel plate
rheometry. The samples with 25 mm diameter and 1 mm thickness were prepared using a
compression molding machine. After putting the sample between two discs, the gap between the
discs was adjusted to 1 .05 mm and extra molten sample was trimmed off to make a smooth edge
around the sample. The strain sweep test was performed in the range of 0.1 – 100% to determine
the strain in the linear visco-elastic region at frequency of 1 rad/s. Then, dynamic frequency
tests were performed in the interval of 0.1 to 100 rad/s. The samples were heated at 180 °C and
kept at that temperature for 5 min. without any shearing and then cooled down to three different
temperatures 130°C 135°C and 140°C respectively at which point they underwent shearing. The
strain amplitude was fixed to 1% to obtain reasonable linear signal intensities at low frequencies.
Strain sweeps at a series of fixed frequencies were carried out to determine the limits of linear
50
viscoelasticity for each sample. To know the effect of shearing on the complex viscosity of
PP40, three samples measured at isothermal temperature with three different strain amplitudes,
10Hz, 30 Hz and 70 Hz.
3.2.3 Pressure Drop Rate Measurement
The shear viscosity measurements were carried out for all three materials to get the data of shear
viscosity and shear rate. These data were fit to Power-law model for viscosity.
Power-law Model,
𝜂 = 𝑚�̇�𝑛−1 (3.1)
Where 𝜂 = viscosity (Pa.s), �̇� = shear rate (1/s), m (Pa.sn) is a measure of consistency. The value
of the m will be larger for more viscous melt and it is sensitive to temperature. n indicates the
degree of non-Newtonian behavior. For Newtonian fluids the value of n = 1 and for polymer
which exhibit shear thinning behavior, n is less than 1. On a log-log graph 𝜂 vs �̇� is a straight line
and the slope is equal to (n-1).
Used least sum of squares (smaller value, better fit) method
𝐿𝑆𝑆 = �(𝑥𝑚𝑜𝑑𝑒𝑙 − 𝑥𝑑𝑎𝑡𝑎)2 (3.2)
where x in this case is the corresponding viscosity value for each shear rate. Here, the data fit
between 0.1 to 100 s-1 to power law model for viscosity. The value of parameter ‘m’ and ‘n’
were found to calculate the theoretical pressure drop and pressure drop rate for all three dies for
all three materials using following equations (3.3) to (3.5).
51
𝑃𝑑𝑖𝑒 = −2𝑚𝐿
𝑅3𝑛+1��3 +
1𝑛�
𝑄𝜋�𝑛
(3.3)
𝑡𝑟𝑒𝑠𝑖𝑑𝑒𝑛𝑐𝑒 = 𝐿𝑉𝑎𝑣𝑔
= 𝐿
𝑄 𝜋𝑅2⁄ = 𝜋𝑅2𝐿𝑄
(3.4)
𝑑𝑝𝑑𝑡
≈ ∆𝑝∆𝑡
= −2𝑚�3 + 1𝑛�
𝑛
�𝑄𝜋𝑅3�
𝑛+1
(3.5)
The calculated values of pressure drop and pressure drop rate are given in Table 3.1.
3.2.4 Thermal Analysis
The thermal properties of these three random copolymers were examined using a differential
scanning calorimetry (DSC). DSC measurements were conducted on polymers as well as some
extruded foam samples by using a DSC2000 from TA instrument, New Castle, DE, with a
refrigerated cooling system. About 10-12mg samples from the pure resins pellets were sealed in
Aluminum Tzero hermatic DSC pan and heated from room temperature to 200°C at 10°C/min
and then maintain for 10 minutes to eliminate the previous processing thermal history.
Afterwards samples were cooled down at a cooling rate of 10°C/min under a nitrogen purge and
then second heating was performed at a ramp of 10°C/min. A nitrogen flow was maintained at 50
mL/min during DSC tests. The cooling and second heating cycles were used for recording the
cooling (crystallization) and heating (melting) thermographs. The degree of crystallinity was
calculated from the heating cycle in the DSC thermograph. The percent of crystallinity was
measured as the ratio of the heat of fusion of PP materials (the area of the melting endotherm and
the heat of fusion of 100% crystalline polymers which 207J/g [113].
52
𝑋𝑐(%) = ∆𝐻𝑚∆𝐻𝑚0 𝑤𝑝
𝑋100 (3.6)
where 𝑤𝑝 is the weight fraction of PP, 𝐻𝑚 is the melting enthalpy of the sample determined by
the first heating cycle by DSC and 𝐻𝑚0 is the theoretical, 100% crystalline polypropylene
enthalpy (207.1 J/g) [116]
The isothermal crystallization of the polypropylene samples were also investigated by cooling
the samples from the melt condition (200°C) with cooling rate of 30°C/min to various sets of
isothermal temperatures to explore the isothermal melt crystallization of the samples.
The effect of CO2 pressure on the isothermal and non isothermal crystallization and melting
temperature of the polypropylene material were investigated using a HP-DSC (NETZSCH DSC
204 HP, Germany). The melting point and heat of fusion for Indium (IN) was measured under
ambient and high CO2 pressure to calibrate the calorimeter. PP film samples were heated form
room temperature to 200°C at a rate of 10°C/min under the pressure of 45 bar and maintained for
10 min at this temperature to erase the previous thermal and stress histories and to dissolve the
gas in the polymer. Samples were cooled from 200 °C to room temperature or at set temperature
at a rate of 10°C/min and 30°C/min for non-isothermal experiments and isothermal experiments
respectively. The selected isothermal temperatures were 140°C, 135°C, 130°C, 125°C, and
120°C for the PP40.
3.2.5 Experimental Set-up and procedure
Figure 2.8 shows a diagram of the tandem extrusion system used in this study. It consists of the
following components: a 5 hp extruder driver, one 3/4’’ extruder (Brabender: 05-25-000) with a
53
mixing screw; one 1½’’ extruder with a built-in 15 hp variable speed drive unit (Killion: KN-
150); gas injection port for injecting the blowing agent, Eleven band heaters, four pressure
transducers (Dynisco PT462B-10M-6/18) for detecting the pressure at several locations, and 10
temperature controllers and thermocouples for controlling the temperatures of the extrusion
barrel, adapters, and the die, three filament dies with length/diameter ratio L/D ~8 (L = 0.413” /
ø = 0.051”) , L/D ~ 34 (L = 0.719” / ø = 0.021”). The first two dies were used only with the PP5
as it didn’t work for other two low viscous materials due to low die back pressure. The third die
with L/D ~ 34 was used for all three materials.
First, the PP copolymer pellets were fed into the barrel of first extruder through hopper and
completely melted by the rotation of the screw. The precise amount of the CO2 (i.e. 5 to 13 wt
%) was injected into the extruder barrel by a positive displacement syringe pump. The gas
eventually completely dissolved in the polymer by the shear field generated by the static mixer
connected with the end of the first screw. The single phase polymer/gas solution fed into the
second extruder where it was cooled to desired temperature. The cooled polymer/gas solution
entered into the die and eventually it exited through die and experienced a rapid pressure drop.
This rapid pressure drop induce a sudden decrease in the solubility of CO2 in the polymer and
causes a large number of bubbles to nucleate instantaneously in the polymer/gas solution and
eventually, it solidified and cellular foam structure was created.
The experiments with die#1 with PP5 were performed by setting the fixed RPM of 20 for first
extruder and RPM of 3 for the second extruder. The mass flow rate of the system was kept
constant at around 6 g/min. In case of experiments using die # 3, the speed of the first extruder
and second extruder was maintained at 8 RPM and 5 RPM to achieve a polymer/gas flow rate of
10-12 g/min for PP40 and PP80 polymers. On the other hand for PP5, the polymer/gas mixture
54
flow rate was kept at 4-6g/min and a lower RPM was used; otherwise a very high die back
pressure was generated.
The blowing agent contents was examined at 7, 9 and 11 wt %, and the barrel temperatures were
kept at 160°C, 180°C, and 180°C for the first screw and 180°C for the entire length of the second
screw. The melt temperature in the second extruder and in the die was lowered step by step and
samples were collected at each set temperature after the system reached the equilibrium state.
3.2.6 Foam Characterization
Cell Morphology – Cell Density and Cell Size
The solidified foam samples were collected from each designated temperature and then
characterized using a Scanning Electron Microscope (SEM, Hitachi 510). The samples were
dipped in liquid nitrogen and then cryo-factured to expose the cellular morphology. The
fractured surfaces were then sputter-coated with a thin layer of platinum and then observed using
SEM. Area and the number of cells in the area were calculated using the SEM images using the
image processing software- Image-Pro Plus V.6.0, Media Cybermatics. The number of cells per
unit volume (N0) of the foamed sample is estimated from the equation (2.25).
𝐶𝑒𝑙𝑙 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 = �𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝐶𝑒𝑙𝑙𝑠
𝑎𝑟𝑒𝑎 �32∙ 𝑉𝐸𝑅
Volume expansion ration measuring the foam density using the equation (2.23)
𝑉𝐸𝑅(𝜑) = 𝜌𝑝𝑜𝑙𝑦𝑚𝑒𝑟
𝜌𝑓
55
The densities of the foam sample were measured via the water displacement method in
accordance with ASTM D792.
3.3 Result and Discussion
3.3.1 Effect on solubility
Solubility is a greater driving force to nucleate bubbles at a higher blowing agent concentration.
When the polymer melt had more dissolved blowing agent, a greater thermodynamic instability
can be induced as the polymer exited from the die because of solubility drop. Therefore, the
effect of molecular weight on solubility should be known to know the effect of solubility on the
cell nucleation and volume expansion ratio. Figure 3.2 shows the solubility at 180°C at 1200 PSI
for all three PP5, PP40 and PP80. It can be seen that there was a negligible increase of solubility
with the decrease of the molecular weight. Here, all three materials are linear PP. Therefore,
solubility was almost same for all material.
3.3.2 DSC Results
Differential Scanning Calorimetry (DSC) tests were carried out on four different MFR PP,
PP1.9, PP5, PP40 and PP80. Figure 3.3 show cooling thermographs for four different MFR
RCPP resins in order to find the relation between the crystallization temperature and the degree
of MFR. The crystallization temperature of PP1.9, PP5, PP40 and PP80 was noted 96.08°C,
104.14°C, 118.68°C and 119.38°C. This shows that low molecular weight polypropylenes have a
faster crystallize-ability which starts to form at higher temperatures
56
3.3.2.1 Crystallization behavior on isothermal treatment in DSC and
HPDSC
As discussed in section 3.2.6, in extrusion foaming with tandem extrusion system, after making
the single phase polymer/gas solution in the first extruder, the second extruder is used for mixing
and cooling the polymer/gas solution to desired temperature but far below the melting
temperature of PP before the polymer melt exits through die. Generally, temperature profile for
the entire length of the second screw is always kept at constant desired temperature. Though the
temperature of the melt decrease gradually, the polymer melt near the wall and channels of the
extruder moves very slowly and have the large residence time; it can be assume that the polymer
melt is experiencing the isothermal condition before exiting the die. Typically, the residence time
in the second extruder is 10 to 15 minutes. Therefore, it is necessary to know how polymer and
the solution of polymer and gas behave when it is subjected to isothermal treatment. DSC studies
have been performed for PP5, PP40 and PP80 at various sets of isothermal temperatures. As
shown in Figure 3.3, the crystallization peak was at 104.14°C during non-isothermal cooling
curve for PP5 material. There was not any indication of crystal above 104.14 °C. To examine the
effect of isothermal treatment, five different isothermal temperatures were selected above its
crystallization peak temperature.
Figure 3.4 shows the isothermal treatment at various temperatures at 1 bar for PP5. There was no
peak detected above 130°C. For doing isothermal treatment at lower temperature below 105°C
the crystallization peak took place and finished while cooling from 200°C to the isothermal
temperature since the selected isothermal temperatures are below the crystallization temperature
and due to fast crystallization that takes place in this PP before reaching to the isothermal
temperature, the crystallization has taken place already completely during the cooling process.
57
But it is seen that even at 125°C, the crystal can foam through isothermal annealing although it
takes almost 4 hours to fully crystallize. At lower temperature polymer takes shorter time to
crystallize. It takes only 90 min at 120°C, 20 min. at 115 °C and 10 min. at 110 °C to crystallize
completely.
The same behavior was found for other low molecular weight PP grade. The Figure 3.5 shows
the isothermal treatment at various temperatures selected at above its crystallization temperature
for PP40. When PP40 subjected to isothermal annealing as high as 140 °C, the crystals can still
form although it tales over 2hrs to fully crystallize. At lower temperature the full crystallization
takes shorter time. At 135°C, 130°C, 125°C and 120°C take 25 min, 15 min, 7 min and 3 min,
respectively.
As shown in Figure 3.7, PP80 material shows very similar behavior to PP40. However, it
requires more time to crystallize than PP40. It take 180 min, 40min, 20min, 10min and 5 min at
140°C 135°C, 130°C, 125°C and 120°C respectively but at similar temperatures the
crystallization can occurs isothermally.
From the effect of isothermal treatment on crystallization of PP5, PP40 and PP80, it can be seen
that PP was not able to crystallize when exposed to isothermal temperatures above the certain
temperature due to too high mobility of chains. However if the polymer is kept below that certain
isothermal temperature, it starts to crystallize and during certain long time period it completely
crystallized. This period depends on the molecular weight of the material. High molecular weight
material crystallized at lower temperature. On the other hand low molecular weight PP starts to
crystallize at higher temperature.
58
In extrusion foaming experiments, gas and system high pressure also involves. Therefore it also
needs to study the effect of gas and pressure on crystallization. Since the effect of the molecular
weight on crystallization behavior has known from the above study, the PP40 was selected to
perform high pressure DSC experiments to examine the effect of gas pressure. The procedure is
explained earlier in the section 3.2.5.
Non isothermal and isothermal test on DSC test was performed at 45 bar.
Figure 3.8 shows the cooling graph of the PP sample in regular DSC at 1 bar and in high pressure
DSC at 45 bar. It is seen that crystallization amount decreases at higher CO2 pressure it takes
place at lower temperature due to the plasticization effect of gas. The isothermal results show
that although the crystallization amount decreases at higher CO2 pressure but the crystallization
takes place much faster than in atmospheric pressure and it takes place at lower temperature due
to the plasticization effect of gas. The reason for the crystallization decreases is that high
pressure might retard the crystal formation and makes the chains too mobile. In foaming, the
possible decrease of crystallization by the CO2 pressure can get compensated by the expansion
and strain induced crystallization.
After the isothermal melt crystallization investigation of the PP40 at 45 bar CO2 pressure, the
Avrami equation was used to analyze the kinetics of isothermal melt crystallization. The Avrami
equation is as follows:
𝑙𝑛�−𝑙𝑛�1 − 𝑋(𝑡)�� = 𝑛𝑙𝑛𝑡 + 𝑙𝑛𝑘 (3.7)
In this equation, 𝑋(𝑡) is the relative crystallinity at crystallization time t, k is the crystallization
kinetic constant for nucleation and the growth rate, and n is the Avrami exponent that reflects the
mechanisms of crystal nucleation and growth. By plotting 𝑙𝑛�−𝑙𝑛�1 − 𝑋(𝑡)��versus ln(t), the
59
Avrami exponent, n, and the logarithm of kinetic constant, lnk, can be determined. Figure 3.9
shows Time dependence of relative crystallinity at different isothermal temperature for PP40.
Figure 3.10 shows the plot of 𝑙𝑛�−𝑙𝑛�1 − 𝑋(𝑡)��versus ln(t). Parameter “n” derived from
Avrami plots is listed in Table 3.3.
The Avrami n value exponent reflects the mechanisms of crystal nucleation and growth: When n
is around 3 and above, the crystallization is initiated by heterogeneous nucleation, suggesting a
bulk crystallization and three-dimensional growth. However, when n is close to 2 and below, it
suggests two-dimensional homogeneous spherulitic crystallization nucleation and growth. [92].
The Avrami exponent n gradually decreases with decreasing temperature up to 115°C but at
115°C, the value of n was 2.53.
In Summary, isothermal treatment helps to start crystallized even above its crystallization
temperature. Low molecular weight material starts to crystallize at higher temperature compared
to high molecular weight and gas helps having crystallization takes place at faster rate. In
extruder polymer/gas solution experiences the system high pressure as well as high shear field,
strain rate, extensional stress. The effect of shear on crystallization behavior should be known to
simulate the conditions of extrusion system.
3.3.2.2 Effect of die temperature on the crystallization behavior of
the foamed samples
60
Figure 3.11 (a) and (b) shows DSC thermographs of the foamed samples of PP40 with 7% CO2
and PP5 with 13% CO2 respectively at various die temperature. It is noted that die temperature
did not affect on the crystallization behavior of final foamed samples. Almost for all of the
samples the crystallization amount and its melting temperature are very similar. Crystallinity (%)
and melting temperature of PP40 samples are given in Table 3.6. At the same gas content, all the
foamed samples crystallize with same trend knowing that the crystallization of PP takes place
very fast and all of them crystallize fast right after coming out of the die.
3.3.2.3 Effect of gas content on the crystallization behavior of the
foamed samples
DSC test was performed on the foamed sample at fixed die temperature with various gas
contents to investigate the effect of gas content on the crystallization behavior of the foamed
samples. Figure 3.12 (a) and (b) shows thermograph of PP40 foamed sample at 120°C and PP5
foamed samples at 115°C with various gas content respectively and
Table 3.7 shows the melting temperature and crystalinity of the PP40 foamed sample.
It is seen that when the gas content increases from 7 to 9% although the amount of crystals are
very similar but probably due to higher expansion the crystal perfection exists and causes to have
crystals with higher melting temperature. On the other hand, when increasing the gas content to
11% not only the crystal perfection can take place due to higher biaxial stretch through the larger
expansion, but also the amount of final crystallinity increases more significantly due to
additional expansion and stretch which creates more strain induced crystallization and hence
higher final crystallinity.
61
3.3.3 Effect of strain rate on shear viscosity
Figure 3.13 and Figure 3.14 illustrates the effect of shearing on the complex viscosity and
storage modulus of PP40 at three temperatures, 130°C, 135°C and 140°C. It was observed that in
the shearing mode the complex viscosity of PP40 started to increase immediately at 130°C, after
5 min at 135°C and after 20 min. at 140°C. The start of increasing viscosity credited to polymer
isothermal melt crystallization under dynamic shear action. The effect of different strain rate at
140°C was investigated. Figure 3.15 and Figure 3.16 shows the effect of shearing on shear
viscosity and storage modulus at different strain rate at 140°C respectively. Very little effect was
observed on the starting time of the increasing shear viscosity. However, in typical extrusion
process, polymer experiences lot more shear rate in the range of 100s-1 to 5000s-1. In that case,
the effect of strain rate will be more and it will start to crystallize more quickly.
3.3.4 Die Pressure during Extrusion Foaming
In this study, wide range of the MFR PP from 5, 40 and 80 g/10 min were used. During foaming
with these materials, there will be also huge difference in induced pressure drop and pressure
drop rate for each material. Therefore, three different dies were chosen to get high, medium and
large pressure drops and pressure drop rates. It was easy to generate enough die back pressure
with high viscosity material PP5 but for PP40 and PP80 due to low viscosity, die #1 and die#2
were not suitable to generate enough die back pressure so the foaming with these material was
not possible. As shown in Table 3.1, die#3 generate sufficient high die back pressure and
moderate pressure drop rate which was enough to obtain satisfactory foam structures without gas
pockets. When the die doesn’t generate enough system pressure above the solubility pressure,
gas will not dissolve in the polymer matrix and large gas pockets will be observed. The foaming
62
experiments were performed with PP5 using die#1 with an L/D ~ 8. Five gas contents from 5%
to 13% were used to check the effect of the gas. Figure 3.17 shows the die pressure at different
die temperature recorded during the foaming experiments.
It was impossible to produce foams using the same die for different viscosity materials; the
length of the die for the material with low viscosity must be longer than the length of the die
suitable for high viscosity materials to maintain the adequate die pressure for ensuring gas
dissolution. For low viscosity materials, smaller diameter and longer length die was chosen (L =
0.719” / ø = 0.021” (L/D ~ 34.238)) to get enough pressure drop and pressure drop rate to
achieve satisfactory foam structures. Later, the same die (die#3) was used for high viscosity
material PP5 and low viscosity material PP80 and the foam samples were collected for both
materials. Figure 3.18 shows the recorded die pressure values during the extrusion foaming
process. As expected, higher gas content, lower die pressure was observed due to the
plasticization effect of the gas. High viscosity materials showed high die pressure. The die
pressure and barrel pressure was always maintained higher than solubility pressure in all
experiments.
3.3.5 Expansion Behavior of PP Foam
The graphs of volume expansion versus die temperature at different gas contents for the various
foamed samples of PP5/PP40/PP80 made using die#1 and 3 are shown in Figure 3.19 and Figure
3.20. It was observed that the volume expansion ratio was a strong function of the die
temperature and gas content. All curves shows typical “mountain shape” confirming the gas loss
and crystallization phenomena. Here, it is explained only for MFR 5 with 7% CO2 gas content.
When the die temperature was high as 160°C to 150°C, the expansion ratio of most of the foam
samples was below 5 folds. When the die temperature was high, the gas diffusivity was high and
63
melt strength of the polymer was also weak. Therefore, the most of the gas easily escaped from
the hot skin of the foam. When the die temperature was in the range of 150°C to 130°C,
expansion ratio increased as die temperature decreased. This can be explained by the melt
stiffening phenomena. In other words, the gas diffusion was blocked at the surface and more gas
remained in the foam to contribute to the volume expansion as the die temperature was lowered.
When the die temperature was further decreased from 130°C to 120°C, the volume expansion
ratio started to decrease after passing the optimum temperature range as the foam structure
started to solidify and hindered the cell growth. If the melt temperature is too low, the volume
expansion ratio is governed by the crystallization behavior. The effect of gas content and
molecular weight of the material on expansion ratio are explained in the following sections.
3.3.5.1 Effect of Molecular weight
The curves of die temperature versus volume expansion ratio at three gas contents for three
materials PP5/PP40/PP80 are shown in Figure 3.19. It can be seen that Low MFR 5(high
viscosity) material has wider processing windows to achieve large volume expansion ratios than
the high MFR (low viscosity) material. For instance, the die temperature range is as wide as
20°C (from 150°C to 130°C) to produce a more than 20-time expansion ratio for 7% CO2 gas
content for PP5. On the other hand for higher gas content, die temperature range was 15°C(from
135°C to 120°C) to produce same expansion ratio but the processing window was at a lower
temperature than low gas content. One of the reason for wider processing window is increased
viscosity and high melt strength for low MFR material and low gas content. On the other hand,
for the high MFR material, the processing window for achieving high expansion more than 20-
times was only from 125°C to 120°C and for higher gas content the processing window becomes
wider and shifted to lower temperatures from 125°C to 115°C. The melt strength of high MFR
64
material might be insufficient to suppress cell coalescence at high temperatures, resulting in a
decreased expansion ratio.
One of the reasons for achieving high volume expansion ratio (>25 fold) for PP40 and PP80 is
the initiation of crystallization at the temperature above its crystallization temperature when
polymer melt experiences isothermal treatment in the second extruder. The crystallization of
polymer melt also suppresses cell coalescence by increasing melt strength from the connected
molecules through the crystal domain. As a consequence, gas loss is decreased significantly, and
thereby, a high expansion ratio can be resulted [65]. This increased expansion ratio further
promotes crystallization.
3.3.5.2 Effect of gas content on volume expansion ratio
One of the most vital parameters affecting the volume expansion of the polypropylene foam is
the amount of blowing agent injected. Figure 3.20 shows the volume expansion ratios of the
foamed samples made with die#1 with PP5 at different die temperature for various gas contents
from 5% to 13%. It was observed that the volume expansion ratio was a strong function of die
temperature and gas content. The largest volume expansion ratios obtained with 5, 7, 9, 11 and
13% CO2 was 26.5, 19.7, 21.7, 30.8 and 25.4 at the temperature between 110°C to 120°C. The
expansion ratio increases from 7 to 11% but after 13%, the volume expansion ratio decreased
slightly. One of the reasons for decrease in VER could be that 13% gas content could be excess
and hence didn’t dissolved completely. When the amount of the blowing agent increased, the
working pressure was also lowered due to the plasticizing effect of the dissolved in polymer
matrix and the pick of the volume expansion shifted towards lower temperature when blowing
agent amount increased. And Figure 3.19 shows the volume expansion of the samples made from
PP5, PP40 and PP80 using die#3. The largest expansion ratios obtained for all three gas contents
65
was between 25 to 30 fold. Both figures have typical mountain shape. It can be seen that the
largest expansion ratio achieved was a strong function of the amount of CO2 injected.
3.3.5.3 Effect of die geometry (Pressure Drop and pressure drop
rate)
Figure 3.21 shows the effect of pressure drop and pressure drop rate on volume expansion ratio.
PP5 was used for both die#1 and die#3. Both pressure drop and pressure drop rate both
significantly affect the expansion ratio. Higher pressure drop rate gives better expansion at
higher temperature. Maximum expansion was also at higher temperature than the volume
expansion ratio from the lower pressure drop rate. Lower pressure drop die improved volume
expansion ratio at lower temperature. Due to less plasticizing effect of the gas for less gas
contents, the melt strength was high and it caused wider processing window for high expansion
ratio.
3.3.6 Cell Density Characterization
The cell density was calculated for each samples of the foam made form PP5, PP40 and PP80
using 7%, 9%, and 11% CO2 gas. The SEM Images for all three PP for 7% and 11% CO2 gas at
various temperatures were shown in Figure 3.32, Figure 3.33 and Figure 3.34. Figure 3.35 and
Figure 3.36 shows the SEM images of the foamed samples produced from PP40 and PP80 using
9% CO2 gas.
3.3.6.1 Effect of Pressure Drop rate on Cell Density
The pressure profile for die#1 and 3 was shown in Figure 3.17 and Figure 3.18 respectively.
Figure 3.27 shows the effect of pressure drop rate on cell nucleation. The graph shows the cell
66
density obtained using two die#1 and die#3 which has low and high pressure drop rate
respectively as per Table 3.1. It can be seen that higher cell density can be achieved using high
pressure drop rate. Pressure drop rate is one of the most affecting parameters on cell nucleation.
3.3.6.2 Effect of Molecular weight on the cell density
Figure 3.22, Figure 3.23 and Figure 3.24 show the effect of molecular weight on the cell density
for 7%, 11% and 9% CO2 gas respectively. The foam samples of PP40 and PP80 with gas
content 7% and 9% had their respective cell densities higher than 109 cells/cm3. The maximum
cell density of 2x109cells/cm3 was achieved in case of PP80, whereas PP40 showed a cell density
of 1.5x109cells/cm3. In case of PP5 the maximum cell density achieved was 1.18X107cells/cm3.
PP40 showed better processing window than PP80 and PP5. Generally, pressure drop rate
(dp/dt) is the main governing factor for cell nucleation. High viscous material PP5 had higher
dp/dt than other two materials for the same die and same gas content. This should generate more
cell nucleation in foaming with PP5 resin as discussed in previous section. However, the foam
samples with PP40 and PP80 showed much better cell nucleation and cell growth.
The high viscous material with high gas content should have higher cell density but interestingly,
low viscosity material with lower gas content material showed more than 109 cells/cm3. This is
mainly because high MFR material tends to crystallize more quickly at higher temperature. As
per DSC result, the MFI 40 material starts to crystallize at 140°C at 1 bar and at 130 °C at 45 bar
in HPDSC when the sample was kept isothermally for 30 mins. Due to the low velocity of
polymer near the walls of channels in the extruder and the residence time distribution of the
polymer melt in the second extruder and die, isothermal crystallization is possible. In extrusion
condition, there are many other affecting parameters such as shear stress, extension stress/strain,
gas content and high pressure. These effects will lead to decrease in the required isothermal time
67
and may create very small sized crystals which behave as a crosslinking point and polymer
molecules connected with these crystals act as a big molecule and increase the significantly melt
strength and also act as a nucleating agent. These crystals also cause strain hardening behavior
and contribute to significant increase in the extensional and shear stress in the homogeneous
mixer of polymer melt and gas. As discussed in heterogeneous nucleation section of previous
chapter, extensional and shear stress significantly promotes cell nucleation. The presences of
very small tiny crystals in the polymer melt tend to decrease the energy barrier to cell nucleation
based on CNT. A recent simulation study by Wong et al. [65] demonstrated that the presence of
fillers causes the induction of local stress variation around them in polymer/gas solutions which
significantly increases the critical bubble radius. In this case, once some bubbles are nucleated,
presence of crystals induces the local tensile stress would be generated around the crystals as a
result of induced stretching action on the surface of cells. This local pressure field decreases the
activation energy for cell nucleation, resulting in an increase in the cell density. This behavior
can also be seen in batch foaming of EPP beads.
3.3.6.3 Effect of Gas content on Cell Density
Figure 3.25 and Figure 3.26 show the effect of gas content on the final foam cell density.
Generally, increasing the concentration of CO2 gas in PP melt results in higher cell density. The
die temperature did not seem to affect the cell density. But it can be seen that when the gas
concentration increased from 7% to 9 %, the cell density increased and when it increased further
up to 11%, the cell density decreased. One of the reasons for this behavior might be the
plasticization effect of the gas that lower the melt strength of PP that led to cell coalescence and
resulted in the foam samples having cell density between 108 and 109 cells/cm3. The other reason
could be, higher gas concentration induced less pressure drop rate and that caused less cell
68
nucleation. For low MFR material, trend was very similar for all samples with the 5 to 13% gas
concentration. The effect of die temperature and gas concentration on the final foam cell density
was not so profound. It increases slightly with the decreasing of die temperature and increasing
the gas content.
3.4 Fabrication of foam and foam sheet using pilot scale
extruder
In engineering applications, pilot-scale foam extrusion systems are used to verify the design or
techniques that are optimized in a lab-scale extrusion, with a potential to develop industrial scale
extrusion system. Compared to lab-scale extrusion system, pilot-scale and industrial scale
extrusion systems have a big difference in residence time, flow rate, and temperature uniformity.
The effort has been made to produce the foam with similar result, high expansion and high cell
density (above 109cells/cm3) with large tandem extrusion system. As shown in Figure 3.28, it
consists of two single-screw extruders, a continuous gas injection pump and a foaming die. The
first 1.5” extruder has L/D (length to diameter ratio) ratio of 32:1. Mixing elements were
attached to the end of the screw. This extruder is used for melting the polymer resin and mixing
with the blowing agent. The blowing agent is injected through gas injection port and mixed in
the first extruder. The size of the second extruder is 2.5”and this extruder is used for providing
enough residence time for mixing the blowing agent with the polymer homogenously and
cooling of the melt.
Experiment with PP40 with 7% CO2 has been conducted on 1.5”-2.5” tandem extrusion system
using filamentary die(Die#4)as well as annular die to make the foam sheet, The die configuration
was different than previously used in small tandem extrusion system (0.75”-1.5”). The diameter
69
and length was 0.032” and 0.273” respectively. Therefore, the die back pressure the pressure
drop rate for the die was lower than the die#3 mentioned in
Table 3.2. Experimental procedure was explained in detail in previous section. The polymer and
gas flow rate was maintained at 31g/min and gas flow rate was maintained at 2.71ml/min to
inject 7% gas. The foam samples were collected for 160°C to 110°C and characterized for cell
density and volume expansion ratio.
Volume Expansion ratio
Figure 3.29 shows the curves of volume expansion ratio versus die temperature for large tandem
and small tandem. PP40 and 7% CO2 gas content was used for both experiments. The maximum
expansion ratios achieved were 20 fold at 125°C and 25 fold at 120°C for large tandem and small
tandem respectively. It can be observed that less expansion was achieved with large tandem
system. However the trends of the curves were almost similar. For small tandem curve, the
expansion was less than 3 fold up to 125°C but after that decreasing the temperature significantly
increased expansion up to 25 fold. The same behavior was found in large tandem but this
behavior can be seen at 5°C earlier between 130°C and 125°C. The reason for sudden rise of
expansion could be starting of the crystallization. For small tandem, the die back pressure and
pressure drop rate was high. Therefore, chances of having undissolved gas in the polymer was
very less. On the other hand, in large tandem die back pressure and pressure drop rate was not so
high and system pressure might be below solubility pressure hence gas could not dissolve
completely. Consequently, less plasticizing effect was found in large tandem extrusion
experiments and it started to crystallize more quickly than in small tandem. Moreover, less gas
was available for the growth so it was ended up with lower expansion than large extrusion.
70
Cell Density
Figure 3.37 shows the SEM images of the foamed samples made from large tandem and small
tandem. Figure 3.30 shows the graph of cell density versus die temperature for large tandem and
small tandem for the PP40 and 7% CO2 gas content. For both of the case, maximum cell density
was above 1.5x109cells/cm3. However for large tandem extrusion, the processing window for
achieving high cell density was wider (125°C -115°C) than the processing window for small
tandem (125°C -120°C ). But for higher temperature, small tandem experiments exhibits higher
cell density. This can be explained in a way that the effect of pressure drop rate at higher
temperature was significant than at lower temperature. But due to better mixing and cooling in
the large tandem extrusion processing window was higher and at lower temperature, the system
pressure exceed the solubility pressure.
Fabrication of Foam Sheet
Efforts have also been made to produce the foam sheet using MFR40 material using annular die.
The SEM images of the foam sheet are shown in Figure 3.38. The foam sheet has maximum
volume expansion ratio was only 15 fold.
Processing temperature windows to achieve high cell density and large expansion
Optimum processing window of die temperature and amount of gas content for different MFR
PP was determined to produce optimum possible foam structure particular for the specified
material. Figure 3.31 and Table 3.8 shows the processing temperature windows for achieving
optimum foam structure. More than 109 cells/cm3 cell density was achieved by PP40 and PP80
using 7% and 9% gas content. More than 25 fold expansion ratios was achieved with all three
molecular weight PP using 11% gas content, and also for PP40 and PP80 using 9% CO2, more
71
than 25 fold expansion ratio was achieved. The processing window to get both high cell density
more than 109 cells/cm3 and expansion ratio more than 25 fold was 120°C-125°C for PP40 using
7 to 9% CO2 gas content and 110°C - 120°C for PP80 using 9% CO2 gas content. 108 cells/cm3
and more than 25 fold expansion ratio was achieved by all three molecular weight material. And
processing windows are 120°C -130°C, 120°C -125°C and 110°C -120°C for PP5, PP40 and
PP80 respectively. It can be concluded that processing windows shifts toward lowered
temperature as molecular weight increases. Low molecular weight PP with high gas content is
more favorable for foaming than high molecular weight though mechanical properties may better
for high molecular weight.
72
Table 3.1 Theoretically calculated value of pressure drop (∆P) and pressure drop rate (dp/dt)
Die 1 L = 0.413” / ø = 0.051”
(L/D ~8)
Die 2 L = 0.065” / ø = 0.0145”
(L/D ~4.5)
Die 3 L = 0.719” / ø = 0.021”
(L/D ~ 34)
ΔP
(MPa) ΔP/Δt
(MPa/s) ΔP (MPa) ΔP/Δt (MPa/s) ΔP (MPa) ΔP/Δt
(MPa/s) PP MFR5 -4 -32 -10 -6327 -49 -1335
PP MFR40 -1 -22 -4 -5054 -19 -1020
PP MFR80 -1 -17 -4 -5278 -18 -982
Table 3.2 pressure drop (∆P) and pressure drop rate (dp/dt) for Die# 4 used in large tandem
Die 4 L = 0.065” / ø = 0.0145” (L/D ~4.5)
Die 3 L = 0.719” / ø = 0.021” (L/D ~ 34)
ΔP (MPa) ΔP/Δt (MPa/s) ΔP (MPa) ΔP/Δt (MPa/s) PP MFR40 -4 -624 -19 -1020
Table 3.3 Value of parameter- n from Avrami analysis for isothermal crystallization of PP40
Die Temperature 110oC 115oC 120oC 125oC 130oC
Avrami n value 2.02 2.53 1.11 1.79 1.73
73
Table 3.4 Crystallinity and melting temperature of foamed sample of PP40 with 7% CO2
Die (oC) Crystallinity (%) Tm (oC)
120 36 144
130 36 143
140 35 142
150 36 143
Table 3.5 Crystalinity and melting temperature of foamed sample of PP40 with 7%, 9% and 11%
CO2
CO2 % Crystallinity (%) Tm (oC)
7 36 144
9 36 147
11 62 147
Table 3.6 Crystalinity and melting temperature of foamed sample of PP40 with 7% CO2
Die (oC) Crystallinity (%) Tm (oC)
120 36 144
130 36 143
140 35 142
150 36 143
74
Table 3.7 Crystalinity and melting temperature of foamed sample of PP40 with 7%, 9% and 11%
CO2
CO2 % Crystallinity (%) Tm (oC)
7 36 144
9 36 147
11 62 147
Table 3.8 Processing windows for high cell density (<108 cells/cm3, green color is for <109
cells/cm3) and high volume expansion ratio (<25 fold)
Gas Content PP80 PP40 PP5
11% 120-110°C 115-125°C 120-130°C
9% 120-110°C 120-125°C --
7% -- 120-125°C ---
75
Figure 3.1 Parameters affecting extrusion foaming process
Figure 3.2 Effect of molecular weight on solubility
0.05
0.052
0.054
0.056
0.058
0.06
0 20 40 60 80 100Solu
bilit
y (g
of g
as/g
of p
olym
er)
MFR ( g/10 min)
180 C, 1200 psi
76
30 60 90 120 150 180 210
0
2
4
6
8 Cooling rate: 10oC/min
88.48J/g119.38°C
70.97J/g118.68°C
75.45J/g104.14°C
66.36J/g96.08°C
RPP-MFI 80
RPP-MFI 40
RPP-MFI 5
RPP-MFI 1.9
Temperature (oC)
Hea
t Flo
w (J
/g)
Figure 3.3 Non isothermal DSC cooling thermographs of different MFR PP
0 20 40 600.0
0.8
1.6
2.4
0 30 60 90 120
0.01
0.02
0.03
0.04 0.000
0.005
0.010
0.015
0.020
0.025
Hea
t Flo
w (J
/g)
iso 125oC
iso 120oC
120 30024018060
Time (min)
At 1 bar
iso 105oC
iso 110oC
iso 115oC
iso 125oC
iso 120oC
0
G
Figure 3.4 Isothermal melt crystallization behavior for PP5
77
.
0 5 10 15 20 25 300.0
0.4
0.8
1.2
1.6
0 30 60 90 120 1500.000
0.005
0.010
0.015
0.020
0.025
iso 140oC
iso 140oCiso 135oC
iso 130oC
iso 125oCHe
at F
low
(J/g
)
Time (min)
iso 120oC
At 1 bar
Figure 3.5 DSC result - effect of isothermal behavior of PP40 at atmospheric pressure
0 20 40 60
0.0
0.8
1.6
2.4
0 40 80 120 1600.000
0.005
0.010
0.015
0.020
0.025
iso 140oC
At 1 bar iso 140oC
iso 135oCiso 130oC
iso 125oC
iso 120oCHeat
Flo
w (J
/g)
Time (min)
Figure 3.6 Isothermal melt crystallization behavior for PP80
78
40 60 80 100 120 140 160 1800.0
0.4
0.8
1.2
1.6
2.0
At 45 bar
At 1 bar
118oC
110oC
Temperature (oC)
Heat
Flo
w (J
/g)
Figure 3.7 Effect of gas pressure in crystallization of PP40
0 4 8 12 16 20-0.5
0.0
0.5
1.0
1.5
2.0
0 30 60 90 1200.000
0.005
0.010
0.015
0.020
0.025
Time (min)
At 45 bar iso 130oC
iso 125oCiso 120oC
iso 115oC
iso 110oC
iso 130oC
Figure 3.8 High pressure DSC Results - effect of isothermal behavior of PP40 at atmospheric
pressure
79
0 10 20 30 40 500.0
0.2
0.4
0.6
0.8
1.0
Rela
tive
crys
talli
nity
Time (min)
110C 115C 120C 125C 130C
Figure 3.9 Time dependence of relative crystallinity at different isothermal temperature for PP40
-4 -3 -2 -1 0 1 2 3 4
-6
-4
-2
0
ln (-
ln(1
-X(t)
))
ln (t)
110C 115C 120C 125C 130C
Figure 3.10 Avrami double-log plots for PP40 under different isothermal temperatures.
80
(a) (b)
Figure 3.11 DSC heating thermographs of foam sample (a) MFI 40 PP with 7% CO2 and (b)
MFI 5 PP – 13% CO2
(a) (b)
Figure 3.12 DSC heating thermographs - Effect of gas content (a) PP40 and (b) PP5
20 40 60 80 100 120 140 160 180
-1
0
1
2150oC
140oC
130oC
Temperature (oC)
Heat
Flo
w (J
/g)
120oC
70 80 90 100 110 120 130 140 150 160 170-1
0
1
2
3
4
5
Hea
t Flo
w (J
/g)
Temperature (oC)
Heating rate: 10oC/min CO2-13%RPP-MFI 5
110oC die 65.28J/g147.57°C
115oC die 64.89J/g146.81°C
120oC die 62.58J/g146.31°C
125oC die 61.07J/g145.85°C
130oC die 68.40J/g145.52°C
135oC die 65.72J/g146.27°C
69.91J/g146.50°C
140oC die
20 40 60 80 100 120 140 160 180
-1
0
1
2
Heat
Flo
w (J
/g)
Temperature (oC)
11%
9%
7%
80 90 100 110 120 130 140 150 160-1
0
1
2
3
Temperature (oC) o o
Hea
t Flo
w (J
/g)
67.80J/g145.78°C
66.34J/g146.99°C
55.55J/g145.93°C
66.25J/g145.60°C
64.30J/g146.81°C
CO2-13%
CO2-11%
CO2-9%
CO2-7%
CO2-5%
Die: 115oCRPP-MFI 5Heating rate: 10oC/min
81
0 20 40 60 80 100102
103
104
105
η* (P
a-s)
Time (Min)
130C 135C 140C
Figure 3.13 Complex viscosity of PP40 under SAOS at f10Hz measured at 135°C, 135°C and
140°C
-10 0 10 20 30 40 50 60 70 80104
105
106
f10Hz
G' (
Pa)
Time (min)
130C 135C 140C
Figure 3.14 Storage modulus (G’) versus frequency for PP40 at different temperature
82
0 20 40 60 80102
103
104
105
Time (Min)
η* (P
a-s)
140C_10Hz 140C_30Hz 140C_70Hz
Figure 3.15 Complex viscosity of PP40 under SAOS measured at 140°C at 10Hz, 30 Hz, and
70HZ
0 10 20 30 40 50 60 70 80104
105
106
G' (
Pa)
Time (min)
140C_10Hz 140C_30Hz 140C_70Hz
Figure 3.16 Storage modulus (G’) versus frequency for PP40 at different temperature
83
100 120 140 160 1800
1000
2000
3000
4000
5000
6000 MFR 5+5% CO2 with Die 1(L/D ~ 8) MFR 5+7% CO2 with Die 1(L/D ~ 8) MFR 5+9% CO2 with Die 1(L/D ~ 8) MFR 5+11% CO2 with Die 1(L/D ~ 8) MFR 5+13% CO2 with Die 1(L/D ~ 8)
Die Temperature (°C)
Die
Pres
sure
(psi
)
Figure 3.17 Die Pressure vs Die temperature for 5 to 13% gas content
100 120 140 160 180 200 2200
1000
2000
3000
4000
5000
6000
MFR 5 PP+7% CO2 MFR 5 PP+11% CO2 MFR40 PP+7% CO2 MFR 40 PP+9% CO2 MFR 40 PP+11% CO2 MFR80+7% CO2 MFR80+9% CO2 MFR80 +11% CO2 11% CO2 Solubility 9% CO2 Solubility 7% CO2 Solubility
Die Temperature (°C)
Die
Pres
sure
(psi
)
Figure 3.18 Die Pressure Vs Die Temperature for three types of PP using Die #3 (L/D~ 34)
84
100 120 140 160
0
10
20
30
Volu
me E
xpan
sion
Ratio
Die Temperature (°C)
MFR 5+7% CO2 MFR5 PP+11% CO2 MFR40 PP+ 7% CO2 MFR40 PP+ 9% CO2 MFR40 PP+ 11% CO2 MFR80+7% CO2 MFR 80+ 9% CO2 MFR80 + 11% CO2
Figure 3.19 Volume Expansion ratio versus die temperature at different gas content for the
foamed samples made from PP5/PP40/PP80 using die#3
100 105 110 115 120 125 130 135 140 145
0
5
10
15
20
25
30
35 MFR 5+13% CO2 with Die 1(L/D ~ 8) MFR 5+11% CO2 with Die 1(L/D ~ 8) MFR 5+9% CO2 with Die 1(L/D ~ 8) MFR 5+7% CO2 with Die 1(L/D ~ 8) MFR 5+5% CO2 with Die 1(L/D ~ 8)
Volu
me E
xpan
sion
Ratio
Die Temperature (°C)
Figure 3.20 Volume Expansion ratio of the foamed samples made from PP5 using die#1
85
100 110 120 130 140 150 160 170
0
5
10
15
20
25
30
35 dp/dt= -32 MPa/s, 4 MPa, 11% CO2 dp/dt= -32 MPa/s, 4MPa, 7% CO2 dp/dt=-1335 MPa/s, 49MPa, 11% CO2 dp/dt=-1335 MPa/s, 49MPa, 7% CO2
Volu
me
Expa
nsio
n Ra
tio
Die temperature (°C)
Figure 3.21 Effect of Pressure drop rate and pressure drop on volume expansion ratio
110 120 130 140 150 160 170105
106
107
108
109
Cell
Dens
ity (c
ells
/cm
3 )
Die Temperature (°C)
MFR5 PP+ 7% CO2 MFR40 PP+ 7% CO2 MFR80 PP + 7% CO2
Figure 3.22 Effect of Molecular weight on Cell Density of the foamed samples made using 7%
CO2
86
100 120 140 160106
107
108
109
Cell
Dens
ity (c
ells
/cm
3 )
Die Temperature (°C)
MFR5 PP+ 11% CO2 MFR40 PP+ 11% CO2 MFR80 PP + 11% CO2
Figure 3.23 Effect of Molecular weight on Cell Density of the foamed samples made using 11%
CO2
100 120 140 160105
106
107
108
109
Cell
Dens
ity (c
ells
/cm
3 )
Die Temperature (°C)
MFR40 PP+ 9% CO2 MFR80 PP + 9% CO2
Figure 3.24 Effect of Molecular weight on Cell Density of the foamed samples made using 9%
CO2
87
100 110 120 130 140 150
104
105
106
107
MFR 5+13% CO2 with Die 1(L/D ~ 8) MFR 5+11% CO2 with Die 1(L/D ~ 8) MFR 5+9% CO2 with Die 1(L/D ~ 8) MFR 5+7% CO2 with Die 1(L/D ~ 8) MFR 5+5% CO2 with Die 1(L/D ~ 8)
Cell
Dens
ity (c
ell/c
m3 )
Die Temperature (°C )
Figure 3.25 Cell Density of the foamed samples made from PP5 using die#1
110 120 130 140 150 160
104
105
106
107
108
109
Cell
Dens
ity (c
ells
/cm
3 )
Die Temperature (°C) MFR5 PP+ 7% CO2 MFR40 PP+ 7% CO2 MFR80 PP + 7% CO2 MFR5 PP+ 11% CO2 MFR40 PP+ 9% CO2 MFR80 PP + 9% CO2
MFR40 PP+ 11% CO2 MFR80 PP + 11% CO2
Figure 3.26 Effect of molecular weight and gas contents on the cell nucleation
88
100 120 140 160104
105
106
107
PP5 + 11%CO2 with Die#3(L/d~34) PP5+11%CO2 with Die#1(L/d~8)
(Cell
Den
sity
cells
/cm
3 )
Die Temperature (°C)
Figure 3.27 Effect of Pressure drop rate on cell nucleation using two different dies
Figure 3.28 Tandem Extrusion System
89
100 110 120 130 140 150
0
5
10
15
20
25
30
Volu
me
Expa
nsio
n Ra
tio
Die temperature (°C)
MFR40 PP+ 7% CO2 (Small Tandem) MFR40 PP+ 7% CO2(Large Tandem)
Figure 3.29 Volume Expansion Ratio versus die temperature of PP40 for large tandem (1.5”-
2.5”) and small tandem (0.75”-1.5”)
100 110 120 130 140 150 160105
106
107
108
109
Cell
Dens
ity (c
ells
/cm
3 )
Die Temperature (°C)
MFR40 PP+ 7% CO2 (Small Tandem) MFR40 PP+ 7% CO2 (LargeTandem)
Figure 3.30 Cell density versus die temperature for large and small tandem
90
Figure 3.31 Processing windows for high cell density (<109 cells/cm3) and high volume
expansion ratio (<25 fold) – (blue for <108 cells/cm3 and red for <109 cells/cm3)
91
7% CO2 content 11% CO2 content
PP5
PP40
PP80
Figure 3.32 SEM images, for 7% and 11% CO2 gas content at 120°C for PP5, PP40 and PP80
92
7% CO2 content 11% CO2 content
PP5
PP40
PP80
Figure 3.33 SEM images, for 7% and 11% CO2 gas content at 125°C for PP5, PP40 and PP80
93
7% CO2 content 11% CO2 content
PP5
PP40
PP80
Figure 3.34 SEM images, for 7% and 11% CO2 gas content at 130°C for PP5, PP40 and PP80
94
PP40 PP80
120°C
125°C
130°C
Figure 3.35 SEM images, for 9% CO2 gas content at 120°C, 125°C and 130°C for PP40 and
PP80
95
PP40 PP80
105°C
110°C
115°C
Figure 3.36 SEM images, for 11% CO2 gas content at 105°C, 110°C and 115°C for PP40 and
PP80
96
Large Tandem (PP40+7% Gas) Small Tandem (PP40+7% Gas)
115°C
120°C
125°C
Figure 3.37 SEM images, for 7% CO2 gas content at 115°C, 120°C and 125°C for PP40 made
from large tandem and small tandem
97
Figure 3.38 SEM Images of Foam sheet produced from MFR40
98
Chapter 4 Effect of Nano-clay on Polypropylene Foaming
4
4.1 Introduction
As discussed earlier, due to the excellent functional characteristics and low material cost, PP has
dominated in the polyolefin group as a commodity polymer. It is expected that it will take up to
26.5% of the market share in 2015 in the world. However, the usage of linear PP in polymeric
field is very limited due to its weak melt strength. It is found that the cell walls are not able to
withstand the extensional force that is generated during bubble growth and bubbles tend to
coalesce during the foaming process. As a result, the foamed PP products are usually having high
open-cell content and non-uniform cell distribution which is not desirable for many applications.
Various methods have been developed to improve the foamability of linear PP to overcome its
limitation of weak melt strength such as long chain branching, cross-linking and polymer
blending and compounding. Several grade of HMS PP have been developed for foaming but
these materials are 1.8 times expensive compared to the linear PP and prices are going up and
that reflecting the final price of the products. The effect of blending was also not so significant to
improve the cell morphology.
The usage of nano-sized additive has been found to be a novel method to improve rheological
properties and melt strength. In addition, improving rheological properties, with well dispersed
nano-additives can play an important role as a cell nucleation agent which reduces the energy
barrier of cell nucleation by generating negative pressure and that helps to enhance the cell
99
nucleation and to increase the cell density. Nano-clays have drawn attention due to their
properties. Inclusion of nanoclay in polymer improves mechanical, thermal, gas barrier and
flammability properties due to their platelet shape composed of layers that are almost 1 mm in
thickness, which has high aspect ratio and large surface area. Addition of well dispersed nano-
clay also causes the strain hardening of PP melt. The main issue for using clay in the polymer is
how to disperse it within the polymer. Well-dispersed and exfoliated nanoclays are very
important to get better mechanical and physical properties of the nanocomposites.
The most important factors that affect the dispersion of the nano-clays are the properties of the
selected coupling agent and the polymer, processing methods and conditions. The nanoclays are
hydrophilic and the polyolefin polymers are hydrophobic. Therefore, it is very difficult to get a
good disperse in the polymer caused due to poor compatibility of between the two materials. To
overcome this issue, two methods were developed for the dispersion and exfoliation of clay
within polymer matrix. The first method involves the alteration of nano-silicates by organo-
intercalants to improve the interaction. The other method is to use the hydrophilic coupling agent
such as Maleicanhydride grafted polymer, as a polyolefin modifier to increase the compatibility
with nano-silicates. This coupling agent has been used successfully for polymer foam processing
to overcome the compatibility problem between two materials. The dispersion of nano clay in the
polymer depends on the MFR of these materials. Lee et al. [117] investigated the factors
affecting the dispersion of the clay within PP with a melt flow rate of 2.8g/10min. He used three
types of PP-g-Man with three different MFR and found that better result could be obtained with
higher viscosity coupling agents with lower MFR PP which causes to generate more shear stress
to exfoliate the nanoclay lamellar and increases interlayer spaces. Fornes et al. [118] showed that
higher molecular weight and melt viscosity generates better exfoliation degrees of nanoclay
layers within the polymer. Therefore, it is very essential to know which range of the MFR for the
100
polymer and coupling agent has the most influences on foaming. In another work done by Zheng
and Zhai et al.[119, 120], the foaming properties have been investigated by adding different
content of nano-clay to LPP and coupling agent (MD511D) with an MFR of 12g/10min and
24g/10min, respectively. They achieved 108 - 109cells/cm3 cell density and 3 to 4 times
expansion by adding 1 wt% nanoclay. [121]
4.2 Statement of the Project Scope
As discussed in previous chapter, large expansion and high cell density were achieved using high
MFR RPP. The introduction of the nanoclay in the PP may help to get uniform cell structure and
higher cell density. The foaming behavior of MFR40 PP with nanoclay particles was
investigated using a tandem extrusion line. The effects of nanoclay content, gas content and die
temperature on the foam expansion and cell morphology were verified with these experiments.
4.3 Experimental Procedure
Materials: Random copolymer of PP (RA12MN40, MFR - 40 g/10 min, ASTM D1238,
230°C/2.16 kg) was supplied by SABIC, CO2 supplied by BOC Gas (The Linde Group),
Masterbatch made with 80% random COPP with MFR 1.9., 15wt% of coupling agent (DuPont™
Fusabond P613, anhydride modified polypropylene, Melt Index 120 g/10 min 190°C/2.16kg,
ASTM D1238) and 5 wt% Cloisite 20A Nanoclay (Montmorillonite modified with a quaternary
ammonium salt) was supplied by Southern Clay Products, Inc. 0.5,1 and 2% nanoclay contents
were used for foaming experiments by diluting the master batch of 5 wt%
101
4.3.1 Extrusion Foaming
A single screw tandem extrusion system was used in this study and the details of the machine
and processing have been described in previous chapter. A filament dies with length/diameter
ratio L/D ~ 34 (L = 0.719” / ø = 0.021”) was used for foaming experiments. 7% and 11% CO2
content was injected into the barrel which was accurately adjusted and regulated by controlling
both the gas flow rate of syringe pump and mass flow rate of the polymer exited through the die.
The screw RPM, temperature profile were maintained same as used in without additives
experiments. The foamed samples at different die temperature were collected for the
characterization.
4.4 Result and Discussion
4.4.1 Effect of nanoclay content on cell nucleation
Figure 4.1 and Figure 4.2 show the cell structure of the foam sample made with PP40 and
PP/nano clay composite at different die temperatures using 7% and 11% CO2 gas content
respectively. It was found that using nanoclay led to poor foam morphology. Large cell sizes, a
small number of cells and nonuniform cell distributions were observed in the foam with using
nanoclay. The foam with the nano clay had inferior property than pure PP foam. One of the
reasons could be the masterbatch used for this experiment. This masterbatch made from MFR 1.9
RCPP, coupling agent MFR120 and nanoclay Cloisite 20A. There is a huge difference in MFR
of base polymer and carrier coupling agent. The coupling agent and PP40 moved fast compared
to PP1.9. Therefore, uniform dispersion of nanoclay could not be achieved and that resulted in
poor cell density and non uniform bimodal cell structure. However the foam morphology was
better for higher nano clay content. And also decreasing die temperature, the foam morphology
102
somewhat improved. This is due to the increase of the melt strength at lower temperature. At
120 °C, the foam with the 2% NC content with 7% gas content showed higher cell density of
9.6X108, which is very near to the cell density of foam without additives. For 11% CO2 gas
content cell density almost remain same from 140°C to lower for all the samples. Die
temperature or nanoclay content didn’t improve so much in cell density.
Figure 4.5 shows the expansion ratio of PP40 and PPNC nanocomposite foams at various die
temperature with the presence of 11% gas content. The foam with pure PP without any additives
exhibited a more than 25 fold expansion ratio between 120-125°C which is higher than foam
sample with any nanoclay content. However increasing the amount of nanoclay from 0.5 to 1,
maximum expansion ratio peak increased from 5 to 12 and further increasing of the clay content
from 1% to 2 increased from 12 to 20 for 7% CO2 gas content. Figure 4.6 shows the expansion
behavior in the presence of 11% gas content. Similar trend was observed; pure PP40 exhibited
higher expansion ratio than the nanocomposite. The reason could be mixing of coupling agent(
MFR 120) and RCPP(MFR 1.9) having vast difference of MFR. Due to this reason coupling
agent and PP40 moved very fast in the extruder on other hand PP1.9 moved very slow. Due to
this reason, uniform dispersion of nanoclay could not be obtained. The reason for increasing
volume expansion ratio by increasing nanoclay content was that it increased melt strength of the
polymer due to the addition of nanoclay also causes the strain hardening of PP melt.
4.4.2 Effect of blending
PP40 foam samples made without any additives exhibited superior cell morphology than the
foam sample with additives. The masterbatch was made from 80 wt% PP1.9, 15 wt% coupling
agent and 5 wt% nanoclay. This masterbatch diluted from 5 wt% of nanoclay to 0.5%, 1% and
2% nanoclay. The proportion of the two material PP40 and PP1.9 per each 1kg material, for
103
0.5% - 900g/80g, for 1% 800g/160g, for 2% 600/320g. The proportion of PP40 was 91.83%,
83.33% and 65.21% for 0.5, 1 and 2% nanoclay clay content respectively. In the first two batch,
the proportion of PP40 was dominated and the effect of other material PP1.9 can be ignored but
the effect of 2wt% nanoclay samples cannot be ignored as it has a proportion of 65.21% of low
viscosity PP40 and 34.79% of high viscosity PP1.9. This might be the reason of achieving better
cell density and expansion ratio than lower nanoclay content despite selecting the two different
MFR- coupling agent and the base polymer of the masterbatch. From the Figure 4.3 to Figure 4.2
it can be concluded that blending of high viscosity and low viscosity material could achieve
better results due to different behavior of these two materials for solubility, diffusivity,
crystallization, and shear thinning. For instance, as shown in Figure 3.3, PP1.9 which has large
molecular weight took long time to crystallize and its crystallization temperature is 96.08°C. On
the other hand PP40 which has small molecular weight, crystallized at 118.68°C.
104
Die Temperature = 120°C Die Temperature = 125°C
PP40
PP40+0.5NC
PP40+1NC
PP40+2NC
Figure 4.1 SEM images for various nano-clay content at 7% CO2 content
105
Die Temperature = 120°C Die Temperature = 125°C
PP40
PP40+0.5NC
PP40+1NC
PP40+2NC
Figure 4.2 SEM images for various nano-clay content at 11% CO2 content
106
110 120 130 140 150 160
105
106
107
108
109
Cell
Dens
ity (c
ells
/cm
3 )
Die Temperature (°C)
MFR40 PP+ 11% CO2 MFR40+0.5NC+11% CO2 MFR40+1NC+11% CO2 MFR40+2 NC +11% CO2
Figure 4.3 Effect of Nano clay content on the cell density for PP40 +11% CO2
110 120 130 140 150 160
106
107
108
109
Cell
Dens
ity (c
ells
/cm
3 )
Die Temperature (°C)
MFR40 PP+ 7% CO2 MFR40+0.5NC+7% CO2 MFR40+1NC+7% CO2 MFR40+2NC + 7% CO2
Figure 4.4 Effect of Nano clay content on the cell density for PP40 +11% CO2
107
100 110 120 130 140 150 160
0
5
10
15
20
25
30
Volu
me
Expa
nsio
n Ra
tio
Die Temperature
MFR40 PP+ 7% CO2 MFR40 PP+0.5NC+7% CO2 MFR40 PP+1NC+7% CO2 MFR40 PP+2NC +7% CO2
Figure 4.5 Effect of nanoclay content on the expansion ratio of the foamed sample with 7% CO2
100 120 140 160
0
10
20
30 MFR40 PP+ 11% CO2 MFR40 PP+0.5NC+11% CO2 MFR40 PP+1NC+11% CO2 MFR40 PP+2NC+11% CO2
Volu
me E
xpan
sion
Ratio
Die Temperature (°C)
Figure 4.6 Effect of nanoclay content on the expansion ratio of the foamed sample with 11%
CO2
108
Chapter 5 Acoustic Behavior of Perforated Expanded Polypropylene Foam
5
5.1 Introduction
The bead foam process is the only technology that can manufacture three dimensional polymer
foam products with ultra low densities foam beads such as expanded polypropylene (EPP),
expanded polystyrene (EPS), expanded polyethylene (EPE) etc. EPP bead foam has higher
strength to weight ratio, excellent impact resistance, thermal insulation, and chemical and water
resistance. Due to these properties, EPP can be found in many everyday products, from
automobiles to packaging, from construction products to consumer goods, and more [122,123,
124]. The production process consists of two steps- 1) Producing EPP beads 2) Manufacturing
EPP bead foam. After producing EPP beads, the beads are fed into a steam chest molding
machine. In the steam chest molding process, high temperature steam works as a heating
medium, which heats up beads and fuses them together inside a 3-dimensional mold cavity. In
addition, steam also acts as a blowing agent which diffuses into beads and later expands the
softened cellular structure further [125]. This EPP bead foam contains high closed-cell content
that makes EPP bead foams poor sound absorbers.
In general closed-cell foams are poor sound absorbers compare to open-cell foams but they have
better mechanical properties and a lower production cost than open-celled foams. The cell walls
of the closed-cell foams can be ruptured through the mechanical perforation, roller crushing and
109
vacuum rupture. In this study, the mechanical perforation method was used to make open-cell
EPP foam [126-127].
5.2 Theoretical Background In porous materials sound is absorbed by viscous, thermal and structural losses. The main
mechanism of absorption is viscous losses induced by boundary layer effects. Air is a viscous
fluid that passes through cell walls and dispels sound energy via friction. Thermal losses take
place due to the time lag between compression and heat flow. Structure losses happen in
poroelastic materials in which the structure of the foam is deformed and sound energy is
converted into internal vibrations. The absorption mechanisms identified above are only
effective in open cell foams with interconnected cell networks [128,129].
Open-cell foams are extensively used for sound absorption applications and extensive literature
can be found for the modeling of the sound propagation in porous material. Two main categories
can be found. The first one considers the porous media as an equivalent fluid with effective
density and bulk modulus and this class of modeling applies to the materials having either a rigid
skeleton or a limp Skeleton. In these materials, wave propagation can be described by a unique
compression wave. The second category considers the elasticity of the frame. The Biot theory is
based on this consideration. The porous medium is modeled as two superimposed phases that are
fluid and solid and describes wave propagation in terms of three waves propagating
simultaneously in the solid and fluid phases: two compression waves and one shear wave [130].
One widely used model from the first category is the Johnson-Champoux-Allard model. This
model considers the rigid foam frame as solid and the air-saturated in the porous medium as fluid
having an effective density (ρ) and an effective bulk modulus (K). The values of these two
110
quantities are found from the Equations (1) and (2) by five macroscopic quantities: the open
porosity (Φ), the static airflow resistivity (σ), the tortuosity (α∞), viscous characteristic length
(Λ) and the thermal characteristic length (Λ') [131].
∞
+∞= )(
0
10 ωαωρ
σφραρ JG
j (5.1)
( )
−
∞+−−=
1
)2('0
2'
110 ωαωρ
φσγγγ BJG
jBPK
(5.2)
where 0P is the atmospheric pressure, 0ρ is the density of air, ω is the angular frequency, γ is
the adiabatic constant, B is Prandtl number, σ' ≈ с'σ where с' is a coefficient. )(ωJG and )(' ωJG
are the functions of the angular frequency and defined by the Equations (3) and (4) .[129-131]
2/1
2220
241)(
Λ
∞+=
φσ
ωηραω
jJG (5.3)
2/1
22'2'
20
241)2('
Λ
∞+=
φσ
ωηραω
BjBJG
(5.4)
The value of Λ and Λ' is given by Equations (5) and (6).
'8'
φσ
ηα∞=Λ (5.5)
φσ
ηα∞=Λ81
c (5.6)
111
where, с is a constant that defines the cell structure shape. The viscous characteristic length Λ
corresponds to the dimension of the narrow sections (small pores) in the pore network where
viscous loss is dominant due to the boundary layer effect. While the thermal characteristic length
Λ' refers to the dimension of the sections with larger surface areas within the pore network where
thermal loss is dominant. As per this definition, Λ' will be larger than or equal to Λ depending
upon the value of the constant c which depends on the geometry of the pore structure [129-131].
Once the effective density and bulk modulus have been determined, the surface impedance ( sZ )
can be calculated by Equation (7). [129-131]:
( )dkgcZjsZ .cot..
φ−= (5.7)
where ρ.kcZ = and Kk
ρω.=
and d is thickness of the porous material. Finally, the
reflection coefficient (R) and the absorption coefficient (α) of the material can be estimated by
Equations (8) and (9), respectively. [129-131]:
00
00csZ
csZR
ρ
ρ
+
−= (5.8)
21 R−=α
(5.9)
112
5.3 Experimental Procedure
5.3.1 Materials and Sample Preparation
EPP beads with 15 fold and 30 fold expansion ratios were supplied by JSP (Grade: ARPRO 5446
and ARPRO 5425 respectively, Bulk Density: 60.9 g/L and 31.3 g/L respectively and average
bead diameter: 2-3 mm) were used to manufacture the EPP bead foam samples. A lab-scale
steam chest molding machine (DABO Precision, Korea) was used to manufacture the EPP bead
foam. The dimension of the mold cavity was 30 cm x 30 cm x 10 cm. The 10 mm, 15mm and
20mm thickness foam sheets were cut by a saw machine. The samples for testing sound
absorption were cut into 30 mm cylinders by using a circular saw machine. The samples were
perforated on a drilling machine using 0.75 mm, 1.2 mm, 1.75 mm and 2mm carbide tipped
high-speed steel drill bits. Holes were drilled at 2mm and 3mm spacing as shown in Figure 5.1.
The design of experiments for this study is shown in Table 5.1.
5.3.2 Characterization
Absorption coefficient
The absorption coefficients of the samples were measured with a BSWA impedance tube in
accordance with the ASTM 1050 standards. BSWA small impedance tube measures the sound
absorption coefficient over a frequency range of 800Hz to 6300 Hz. It requires a 30 mm diameter
sample. Before starting the measurement, both microphones were calibrated using a sound
calibrator supplied by BSWA. The block diagram of the impedance tube setup is shown in
Figure 5.2. A sound source (generator with amplifier) was attached at one end of the impedance
tube and a 30 mm sample of the material was placed at the other end of the tube directly against
113
a rigid wall. Teflon tape was used on the rim of the circular sample to ensure a tight seal between
the sample and the tube would form when placed in the impedance tube. The sound source
generates broadband, stationary random sound waves, and propagates as plane waves in the tube.
These plane waves strike on the sample and reflect back. The reflection results in a standing-
wave interference pattern due to the superposition of forward and backward travelling waves
inside the tube. The two 1/4” microphones in the tube measure the sound pressures at two fixed
locations to calculate the complex transfer function. The Transfer Function Method decomposed
the incident and reflected the sound pressure from the measured transfer function and estimated
the sound absorption and complex reflection coefficients and the normal acoustic impedance of
the material located at the end of the tube [132, 133].
The surface acoustic impedance (Z) at the front surface of the poroelastic plate is calculated from
measurement by using Equation (10). [12-13]
( ) ( )cRcRZ −+= 11 (5.10)
cR is the complex reflection coefficient , is given by,
)(2 slkjeHjkse
jkseHcR +
−
−−= (5.11)
where l is the distance from the test sample to the centre of the nearest microphone, s is the
centre to centre spacing between microphones, and H is the measured transfer function of the
two microphone signals corrected for the microphone response mismatch [132-133].
The corresponding normal incidence absorption coefficient is obtained from Equation (12). [12-
13]
114
21 cR−=α (5.12)
5.4 Results and Discussion
To analyze the acoustic behavior of the perforated EPP foam, samples were perforated having
0.75, 1.2 mm, 1.75 mm and 2 mm diameter holes at the spacing of 2 mm and 3 mm with the
three different thicknesses of 10 mm, 15mm and 20 mm. The perforated foam samples were
characterized for sound absorption.
5.4.1 Effect of Perforation on Sound Absorption
The absorption coefficient was measured at high frequencies from 800 Hz to 6300 Hz for the
samples with different perforation ratios. The sample thicknesses were the same for all samples
but the perforation ratios were varied by changing the perforated hole diameter and spacing
between them. Table 5.2 shows the sample size and perforation ratio for all samples. The
perforation ration is calculated by the following equation for uniform perforation [134].
Perforation Ratio 2
4
=s
dx
π (5.13)
where d is a diameter of the hole and s is spacing between two adjacent holes.
The results were compared with the foam without perforation in order to know the effect of
perforation on sound absorption. The experiments were repeated three times for the validity of
the result. As shown in Figure 5.3, higher perforation ratios showed better sound absorption up to
a certain sound frequency but after certain sound frequency the peak remains at the same height
but it starts to shift towards higher frequency.
115
In Figure 5.3, the samples with the perforation ratio 0.11 and 0.13 shows similar trend of the
absorption coefficient which shows that absorption coefficient peak at 2200 Hz reaches at 70%
absorption. For the sample with perforation ratio 0.27 to 0.34, the peak of the absorption
coefficients remain at approximately the same height between 90% and 95% absorption but shift
towards higher frequency at 2800 Hz. The sound absorption can be optimized by selecting
proper perforation size and spacing between them to get the proper perforation ratio to get the
better sound absorption at lower frequencies.
5.4.2 Effect of sample thickness on Sound Absorption
Figure 5.4 shows the effect of sample thickness on the sound absorption behavior for samples
with hole diameter=1.2 mm and spacing = 2 mm. Sample thickness affects the location of the
maximum sound absorption peak. As thickness increases, peak shifts towards lower frequency.
For sample thickness 20mm, at frequency 2300 Hz to 2500Hz range it absorbs more than 90% of
the sound. For sample thickness 15mm and 10mm, absorption coefficient peak shifts towards
higher frequency. For sample thickness 10mm, the wide frequency ranges from 3500 Hz to 5500
Hz, it absorbs the more than 90% of the sound.
5.4.3 Effect of Expansion Ratio on Sound Absorption
The middle part (without top skin) of the 15 fold and 30 fold EPP beads foams were used to
make the 10mm thickness samples to check the effect of expansion ratio. The samples were
perforated with 1.75 mm drill bits with 3 mm spacing between two holes. Figure 5 shows the
effect of expansion ratio on the sound absorption behavior of the EPP. The results were the same
for both expansion ratio beads. It did not have any significant effect on the sound absorption
behavior. It can be inferred that sound absorption is more governed by the perforation ratio
116
which can be varied by hole size and spacing between them. But the mechanical strength and
weight of the bead foam will be different for different expansion ratio beads.
117
Table 5.1 Design of Experiments
Experiments Drill Size
(mm)
Spacing
Between two
Holes (mm)
Thickness (mm) Expansion ratio
1 0.75 2 20 15
2 1.2 2 10/15/20 15
3 1.2 3 20 15
4 1.75 3 20 15
5 2 3 20 15
6 1.75 3 10 15/30
Table 5.2. Perforation ratio for various samples
Sample No.
Thickness of
the sample
(mm)
Diameter of
the Hole
(mm)
Spacing
between two
holes (mm)
No. of
Holes Perforation ratio
1 20 0.75 2 169 0.11
2 20 1.2 3 69 0.13
3 20 1.2 2 169 0.29
4 20 1.75 3 69 0.27
5 20 2 3 69 0.34
118
Figure 5.1 Samples with Perforation
Figure 5.2 Impedance Tube Set-up
119
Figure 5.3. Effect of Perforation on sound absorption
Figure 5.4. Effect of sample thickness on sound Absorption for samples with hole diameter: 1.2
mm and spacing = 2 mm
120
Figure 5.5. Effect of Expansion ratio on Sound Absorption for the samples with hole diameter
=1.75 mm and spacing= 3 mm, thickness= 10 mm
121
Chapter 6 SUMMARY, CONCLUSION & RECOMMANDATION
6
6.1 Summary
In this study, the effect of molecular weight on PP extrusion foaming was investigated using
three different molecular weight PP (MFR 5, 40 and 80 g/10min ASTM D1238, 230°C/2.16 kg).
The main objective behind this study was to achieve soft touch, largely expanded , high cell
density non crosslinked PP foam, which can be 100% recyclable for sheet foaming application.
The foams will be processed using environment friendly CO2 gas as the physical blowing agent.
In previous research, largely expanded and high cell density could not be obtained even with the
high melt strength. Material physical properties such as viscosity, surface tension, Crystallization
temperature, CO2 solubility and diffusivity; operating parameters screw RPM, Barrel
temperature and pressure, die temperature, die geometry diameter and length which governs
pressure drop and pressure drop rate, gas flow rate and pressure, significantly affect the final
foam structure. By selecting parameters, the optimum foam structure with high cell density and
large expansion ratio was achieved using lab scale 0.75”-1.5” single screw tandem extrusion
system. The foam sample and material is characterized by SEM, DSC, HPDSC, shear viscosity
and solubility measurements. The effects of processing parameters, such as processing
temperature and blowing agent content on the volume expansion ratio, cell density, and cell
morphology were also investigated and analyzed. The experiment was extended with 1.5” -2.5”
large tandem single screw extrusion system with filamentary die and achieved similar result.
Effort was also made to make the foam sheet using annular die.
122
6.2 Conclusion
The above stated experimental study described in this thesis contributes to the following
conclusions.
The foaming experiments were performed with three different molecular weight PPs using the
same die with three different gas contents. The characterization results compared on the basis of
final foam structure obtained at different temperature. Optimum processing window of die
temperature and amount of gas content for different MFR PP was determined to produce
optimum possible foam structure particular for the specified material. More than 109 cells/cm3
cell density was achieved by PP40 and PP80 using 7% and 9% gas content. More than 25 fold
expansion ratios was achieved with all three molecular weight PP using 11% gas content and
PP40 and PP80 also achieved more than 25 fold expansion ratio with 9% CO2. The processing
window to get both high cell density more than 109 cells/cm3 and expansion ratio more than 25
fold was 120°C-125°C for PP40 using 7 to 9% CO2 gas content and 110°C - 120°C for PP80
using 9% CO2 gas content. 108 cells/cm3 and more than 25 fold expansion ratio was achieved by
all three molecular weight material. And processing windows are 120°C -130°C, 120°C -125°C
and 110°C -120°C for PP5, PP40 and PP80 respectively. It can be concluded that processing
windows shifts toward lowered temperature as molecular weight increases. . Low molecular
weight PP with high gas content is more favorable for foaming than high molecular weight
though mechanical properties may better for high molecular weight.
Generally, high pressure drop promotes more cell nucleation. In this case PP5 was subjected
higher pressure drop but cell density was less compare to PP40 and PP80 while using same die,
gas contents and extruder RPM. This result leads to conclude that there might be something else
123
that also governing cell nucleation. DSC and HPDSC results revealed that when the melt was
kept at isothermal temperature for long time at above its crystallization temperature, crystals
started to create at higher temperature than crystallization temperature. The crystals start to
create more quickly if the polymer subjected to shear stress/strain, extensional stress, high
pressure and amount of gas content. In extrusion foaming process polymer melt experiences high
shear stress/strain, extensional stress which significantly reduced the crystallization time. In
extrusion system, it was tried to keep the temperature profile same for though out of second,
extruder, adapter and die so that isothermal temperature can be maintained. Residence time was
very less in the extrusion system compared to the time needs to allow the crystal formation. But
due to shear stress, extensional stress, high pressure and high gas content decrease the time
required to create the crystals at higher temperature than crystallization temperature. Nano sized
crystal formation act as a crosslinking point with other polymer molecules and dramatically
increase the melt strength that helps to avoid the cell coalescence and to get the high volume
expansion ratio as well as crystals provide heterogeneous nucleation sites to promote more cells
nucleation. DSC and HPDSC result showed the possibility to create the nano scale crystals
however the result can be verified by developing the in-situ visualization system which can
capture the cloud of the small crystals.
This foaming experiment was also done on pilot scale tandem extrusion system (1.5”- 2.5”)
using PP40. High expansion about 20 fold and high cell density over 109 cells/cm3 was achieved
using the large tandem extrusion system even with low pressure drop rate compared to that was
achieved with small tandem extrusion with die#3.
Typically, nano particles in the polymer during the foaming process improve the foaming
behavior but in this study nano particles didn’t improve the foam structure. The foam structure
124
was better without any additives. The one of the reasons could be the difference in the MFR of
the material used in the masterbatch. High viscous base polymer and low viscous coupling agent
caused poor dispersion of nanoclay. The cells were bimodal. Increasing nanoclay content
improves the expansion ratio as well as cell density.
The effect of blending high and low viscous material with nano particles was also investigated. It
improved the cell morphology. More addition of the amount of the low viscous material
improved expansion ratio and cell density.
Research efforts have been made to check the potential of EPP foam as a sound absorber and
how it can be optimized as a better sound absorber. The 15 fold and 30 fold EPP foam with three
thicknesses perforated with different perforation ratios. The results show that increasing the
perforation ratio improves the acoustic behavior of EPP foam up to a certain limit and after that
peak of the absorption coefficient shifts towards higher frequency. The absorption coefficient
was less than 0.1 for without perforation but it was increased by more than 0.9 by perforation.
By increasing the EPP sample thickness, the absorption peak shifts towards lower frequency.
The sample thickness can be varied to move the peak of the absorption coefficient to the lower
frequency. The expansion ratio does not affect so much on the absorption behavior but weight
and strength of the sound absorption material to be able to be varied as per the application of the
material. Therefore, the perforation ratio is a dominant factor to improve the sound absorption
behavior of the EPP foam, which can be optimized by selecting the proper pore size and spacing
between two adjacent pores.
125
6.3 Recommendations
The following suggestions can be made for the direction of future research on polypropylene
foam sheets.
1) This study reported the method to produce foams having high cell density, large
expansion ratio using low molecular weight material. This result was also verified with
large tandem extrusion line using the filamentary die. Effort was also made to make the
foam sheet using annular die with larger tandem extrusion system. But material was not
enough to do some more experiments to achieve expected properties of the foam. It is
recommended to perform more experiments using PP40 and PP80 to produce foam sheets
with desired properties.
2) The phenomena of creation of nano sized crystal should be verified with in-situ
visualization system.
3) The ideal choice of a mastrbatch is one that matches the MFI of the masterbatch as
closely as possible with the MFI of the base resin for the proper dispersion of the nano
particles.
4) The crosslinked foam sheets have high elasticity, high toughness, impact strength and
compressive strength. The mechanical properties of the foam sheet made with low
molecular weight material were not good enough to replace the ideal crosslinked foam
sheet. Some of the strategies are recommended in this section that will be helpful in
future research work.
• To make the PP Foams with High Elasticity, it is recommended
To use random copolymer PP or terpolymer PP
To add rubber or LDPE
To produce high cell density and large expansion ratio
• To produce PP Foams with High Toughness and High Impact Strength, it is
recommended
126
To use nanocomposite
To increase cell density while avoiding cell opening
To use large MW PP
• To make the PP Foams with High Compressive Strength in Thickness Direction,
it is recommended
To induce the orientation of cells in the thickness direction
To reduce stretching from the tension in the machine direction
To reduce stretching from cooling mandrel in the transverse direction
127
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