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Preparation of Polypropylene Foams with Micro/Nanocellular Morphology using a Sorbitol-based Nucleating Agent by Seong Soo Bae 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 Seong Soo Bae 2015
Transcript of Preparation of Polypropylene Foams with Micro/Nanocellular ... · Preparation of Polypropylene...
Preparation of Polypropylene Foams with Micro/Nanocellular Morphology using a Sorbitol-based
Nucleating Agent
by
Seong Soo Bae
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 Seong Soo Bae 2015
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Preparation of Polypropylene Foams with Micro/Nanocellular Morphology using a Sorbitol-based Nucleating Agent
Seong Soo Bae
Master of Applied Science
Department of Mechanical and Industrial Engineering
University of Toronto
2015
Abstract
The effect of a sorbitol-based nucleating agent on expanded polypropylene (EPP) bead foaming
and extrusion foaming was investigated. The main purpose of this study was to study and
develop a fundamental processing baseline for the production of nanocellular foams.
During the EPP bead foaming experiments using different concentrations of a sorbitol-based
nucleating agent, NX8000, foams having a sub-micron sized cells (<600ηm) with a very high
cell density (>1012
cells/cm3) was obtained at optimized processing conditions. The mechanism
of bubble nucleation utilizing small sized crystals induced by the addition of NX8000 was found
to be significantly affecting the foaming behaviour of PP.
The effect of NX8000 was not apparent in extrusion foaming process. High shearing force
disturbed the network structure of NX8000. Further development of extrusion system that can
preserve the nanocellular template during the process is required.
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Acknowledgments
First, I would like to express my sincere gratitude and foremost thank to my supervisor,
Professor Chul B. Park for providing his inspiring guidance, encouragement and motivation
throughout my years in Microcellular Plastics Manufacturing Laboratory (MPML). His vision
and insights have enlightened me. I feel enormously honoured to be a part of this group with
such a great mentor.
I would like to take this opportunity to thank my M.A.Sc committee members, Professor Markus
Bussmann and Prrofessor Hani Naguib.
Also I would like to thank my colleagues and friends in MPML who gave me valuable adviaces
and assiance. Without their supports, my research work would not have been successfully
completed. Special thanks goes to Dr. Saleh Amani, Dr. Changwei Zhu, Dr. Richard Lee, Dr.
Peter Jung, Dr. Anson Wong, Dr. Raymond Chu, Dr. Mohammadreza Nofar, Dr. Ali Rizvi, Jung
Hyub Lee, Eunse Chang, Dongjoo Kim, Vahid Shaayegan, Mo xu, Mehdi Sanei,Lun Howe Mark,
Nemat Nossieny, Kara Kim as well as everyone who helped me in my M.A.Sc study.
I also would like 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
machining related problems and for their work.
My special thanks go to the MIE staff members including Konstantin Kovalski, Brenda Fung,
Jho Nazal, Donna Liu, Sheila Baker, Oscar del Rio, Joe Baptista and Teresa Lai. Especially, their
advices truly helped me getting through administrative issues as an international student.
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I truly feel obligated to acknowledge Sabic and Millaken Chemical for providing me materials
for my experiments.
Last but not least, I thank my family members and friends from the bottom of my heart. I love
you all.
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Table of Contents
Acknowledgments .......................................................................................................................... iii
Table of Contents ............................................................................................................................ v
List of Tables ............................................................................................................................... viii
List of Figures ................................................................................................................................ ix
Nomenclature ................................................................................................................................ xii
Chapter 1 Introduction .................................................................................................................... 1
1.1 Polymeric Foams ................................................................................................................ 1
1.2 Research Motivation ........................................................................................................... 2
1.3 Objectives of the Thesis ...................................................................................................... 2
1.4 Overview of the Thesis ....................................................................................................... 3
Chapter 2 ......................................................................................................................................... 5
2.1 Introduction ......................................................................................................................... 5
2.2 Microcellular and Nanocellular Foaming ........................................................................... 5
2.2.1 Microcellular Plastics .............................................................................................. 5
2.2.2 Nanocellular Plastics ............................................................................................... 6
2.3 Foaming Processes .............................................................................................................. 7
2.3.1 Batch Foaming Process ........................................................................................... 7
2.3.2 Continuous Processes .............................................................................................. 9
2.4 Foaming of Semi-crystalline Polymers ............................................................................. 12
2.5 Crystallization kinetics ...................................................................................................... 14
2.5.1 Polypropylene Crystallization ............................................................................... 17
2.6 Blowing Agent .................................................................................................................. 18
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2.6.1 Chemical Blowing Agents (CBAs) ....................................................................... 18
2.6.2 Physical Blowing Agents (PBAs) ......................................................................... 19
2.7 Solubility ........................................................................................................................... 20
2.8 Cell Nucleation ................................................................................................................. 23
2.8.1 Homogenous Nucleation ....................................................................................... 23
2.8.2 Heterogeneous Nucleation .................................................................................... 28
2.8.3 Effect of Shear Stress, Extensional Stress/Strain on Bubble Nucleation .............. 32
2.9 Cell Growth and Stabilization ........................................................................................... 32
2.10 Nucleating Agents ............................................................................................................. 36
2.10.1 Sorbitol Based Nucleating-Clarifying Agent ........................................................ 37
2.11 Foam Characterization ...................................................................................................... 38
2.11.1 Foam Density ........................................................................................................ 38
2.11.2 Volume Expansion Ratio ...................................................................................... 39
2.11.3 Cell Morphology, Cell Size Distribution and Cell Density .................................. 39
Chapter 3 Bead Foaming of Polypropylene-Sorbitol Based Nucleating Agent Compound ......... 41
3.1 Introduction ....................................................................................................................... 41
3.1.1 Hypothesis ............................................................................................................. 44
3.2 Experimental ..................................................................................................................... 46
3.2.1 Materials ............................................................................................................... 46
3.2.2 Material Compounding ......................................................................................... 48
3.2.3 Thermal Analysis – Differential Scanning Calorimetry ....................................... 50
3.2.4 Thermal Analysis – High Pressure DSC ............................................................... 51
3.2.5 Rheological Measurements ................................................................................... 51
3.2.6 Experimental Set-up and Procedure ...................................................................... 52
3.2.7 Foam Characterization .......................................................................................... 55
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3.3 Results and Discussion ..................................................................................................... 57
3.3.1 Effect of Sorbitol Based Nucleating Agent Content on Crystallinity ................... 59
3.3.2 Effect of Sorbitol-Based Nucleating Agent on Complex Viscosity ..................... 60
3.3.3 HP-DSC Simulation Results ................................................................................. 62
3.3.4 EPP bead foaming results ..................................................................................... 63
Chapter 4 Extrusion Foaming of Polypropylene-Sorbitol Based Nucleating Agent Compound . 76
4.1 Introduction ....................................................................................................................... 76
4.1.1 Hypothesis ............................................................................................................. 77
4.2 Experimental ..................................................................................................................... 79
4.2.1 Material ................................................................................................................. 79
4.2.2 Experimental Set-up and Procedure ...................................................................... 79
4.3 Results and Discussion ..................................................................................................... 83
4.3.1 Effect of NX 8000 Content on Die Pressure ......................................................... 83
4.3.2 SEM Images of the Foamed Samples ................................................................... 85
4.3.3 Effect of NX 8000 Content on Volume Expansion Ratio ..................................... 92
4.3.4 Effect of NX8000 Content on Cell Density .......................................................... 94
4.3.5 Effect of NX 8000 Content on Average Cell Size ................................................ 96
Chapter 5 Conclusion & Recommendation .................................................................................. 99
5.1 Summary and Conclusion ................................................................................................. 99
5.2 Recommendations ........................................................................................................... 101
References or Bibliography (if any) ........................................................................................... 103
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List of Tables
Table 3. 1 Processing Conditions for Compounding .................................................................... 48
Table 3. 2 Processing conditions for EPP foaming ....................................................................... 54
Table 4. 1 Processing conditions .................................................................................................. 81
Table 4. 2 NX8000 and CO2 Concentrations used for each experiment ....................................... 82
Table 4. 3 SEM images of 7 wt% CO2 Samples Die temperature from 135°C to 130°C ............ 86
Table 4. 4 SEM images of 7 wt% CO2 Samples Die temperature from 125°C to 120°C ............ 87
Table 4. 5 SEM images of 9 wt% CO2 Samples Die temperature from 135°C to 130°C ............ 88
Table 4. 6 SEM images of 9 wt% CO2 Samples Die temperature from 125°C to 120°C ............ 89
Table 4. 7 SEM images of 11 wt% CO2 Samples Die temperature from 135°C to 130°C .......... 90
Table 4. 8 SEM images of 11 wt% CO2 Samples Die temperature from 125°C to 120°C .......... 91
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List of Figures
Figure 2. 1 Schematic of a laboratory-scale batch foaming system [10] ...................................... 11
Figure 2. 2 Tandem line extruder .................................................................................................. 11
Figure 2. 3 Process chain for extrusion foaming process ............................................................. 11
Figure 2. 4 SEM images of batch foamed samples ....................................................................... 13
Figure 2. 5 Optical micrograph of PLA-CO2 system ................................................................... 14
Figure 2. 6 Homogeneous and heterogeneous nucleation in a polymer-gas solution [58] ........... 26
Figure 2. 7 Critical radius and free energy barrier ........................................................................ 27
Figure 2. 8 Comparison of energy required for homogeneous and heterogeneous nucleation [71]
....................................................................................................................................................... 35
Figure 2. 9 Heterogeneous Nucleation on (a) smooth planar surface and (b) in a conical cavity
[87] ................................................................................................................................................ 35
Figure 3. 1 Multiple melting peak behaviour of EPP ................................................................... 44
Figure 3. 2 DSC thermograph of neat PP RA12MN40 after thermal history removal ................. 47
Figure 3. 3 Chemical Structure of NX8000 .................................................................................. 47
Figure 3. 4 Pictures of the Compounded Samples ........................................................................ 49
Figure 3. 5 Schematic of HP-DSC Simulation Process ................................................................ 53
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Figure 3. 6 A Schematic of Autoclave Based EPP Bead Foaming Chamber [102] ..................... 55
Figure 3. 7 EPP bead foams .......................................................................................................... 57
Figure 3. 8 Molten polymer the during process ............................................................................ 58
Figure 3. 9 DSC thermograms of PP-NX8000 compounds (Heating) .......................................... 58
Figure 3. 10 DSC thermograms of PP-NX8000 compounds (Cooling) ....................................... 60
Figure 3. 11 Complex Viscosity as a function of temperature ..................................................... 61
Figure 3. 12 DSC thermograms of PP-NX8000 (0.5wt%) after HP-DSC simulation .................. 63
Figure 3. 13 SEM images of foamed samples saturated at various Ts .......................................... 67
Figure 3. 14 Cells around NX8000 fibrils .................................................................................... 69
Figure 3. 15 Average cell size vs. NX8000 concentration ........................................................... 69
Figure 3. 16 Cell density vs. NX8000 concentration .................................................................... 70
Figure 3. 17 Volume expansion ratio vs. NX8000 concentration ................................................. 71
Figure 3. 18 DSC thermograms of EPP bead foams at various Ts................................................ 74
Figure 3. 19 Crystal melting peaks of EPP bead foams ................................................................ 75
Figure 3. 20 Total crystallinity of EPP bead foams ...................................................................... 75
Figure 4. 1 Temperature dependency of complex viscosities of PP-NX8000 compounds .......... 78
Figure 4. 2 Schematic drawing of tandem line extruder ............................................................... 81
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Figure 4. 3 Schematic drawing of filamentary die ....................................................................... 82
Figure 4. 4 Pressure vs. Temperature PP-NX8000 compounds (0 wt%, 0.5 wt%, 0.75 wt% and 1
wt% NX 8000 concentrations) processed with (a) 7 wt% CO2 (b) 9 wt% CO2 (c) 11 wt% CO2 . 85
Figure 4. 5 Volume Expansion Ratio (VER) vs. Temperature PP-NX8000 compounds (0 wt%,
0.5 wt%, 0.75 wt% and 1 wt% NX 8000 concentrations) processed with (a) 7 wt% CO2 (b) 9 wt%
CO2 (c) 11 wt% CO2 .................................................................................................................... 94
Figure 4. 6 Cell Density vs. Temperature PP-NX8000 compounds (0 wt%, 0.5 wt%, 0.75 wt%
and 1 wt% NX 8000 concentrations) processed with (a) 7 wt% CO2 (b) 9 wt% CO2 (c) 11 wt%
CO2 ............................................................................................................................................... 96
Figure 4. 7 Average cell size vs. Temperature PP-NX8000 compounds (0 wt%, 0.5 wt%, 0.75 wt%
and 1 wt% NX 8000 concentrations) processed with (a) 7 wt% CO2 (b) 9 wt% CO2 (c) 11 wt%
CO2 ............................................................................................................................................... 98
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Nomenclature
PP = Polypropylene
MFI = Melt Flow Index
SEM = Scanning Electron Microscopy
DSC = Dynamic Scanning Calorimetry
WAXS = wide-angle X-ray scattering
XRD = X-ray diffraction
NMR = Nuclear resonance
PLM = Polarized light microscopy
IR = Infrared spectroscopy
HP-DSC = High Pressure DSC
EPP = Expanded Polypropylene
PBA = Physical Blowing Agent
CBA = Chemical Blowing Agent
EOS = Equation of State
PE = Polyethylene
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PS = Polystyrene
HDPE = High density polyethylene
PLA = Polylactide or Poly (lactic acid)
PMMA = Poly(methyl methacrylate)
PET = Polyethylene terephthalate
MSB = Magnetic suspension balance
SS-EOS = Simha-Somcynsky EOS
t1/2 = The crystallization half-time
S = Solubility coefficient (cm3[STP]/g-pa)
( ) = Degree of Crystallinity (%)
= Crystal melting enthalpy (J)
= Theoretical crystal melting enthalpy of 100% crystalline polypropylene (J)
= Surface tension (N/m)
= Polymer-gas interfacial area (m)
Vb. = Bubble volume (m3)
C0 = Concentration of gas molecules (mol/m3)
Rcr = Critical radius (m)
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k = Boltzman’s constant (m2-kg/s
2-k)
T = Temperature (K)
= Gibbs free energy for heterogeneous nucleation (J)
= Rate of homogeneous nucleation (/m2-s)
VER = Volume Expansion Ratio (unitless)
Tlow
= Low crystal melting peak temperature of multiple crystal melting peak (°C)
Thigh = High crystal melting peak temperature of multiple crystal melting peak (°C)
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Chapter 1
Introduction
1.1 Polymeric Foams
Polymeric foams possess unique properties, which allow them to be suitable for our daily life
products. As the process technologies improved over the decades, manufacturers have found
ways to produce more refined products in more efficient way. Thus, the foaming industries are
experiencing a rapid growth. Polymeric foams are commonly found in many applications as
impact absorption, shock protection or thermal insulation material. Closed cell structure
polymeric foams are usually used in automotive and construction industries for their mechanical
properties. Open cell foams however, used in more various range of applications such as
packaging, sound insulation, thermal insulation and filtration [1-4].
Although the market demand for polymeric foam is still large, the trend of the polymeric
foaming industry is now shifting from the conventional foaming techniques to advanced
polymeric foaming techniques such as microcellular and nanocellular polymeric foaming.
Polymeric foams can be characterized by its structural parameters such as cell density, expansion
ratio, cell size and open-cell content. Also foams can be categorized into four main domains
based on their cell sizes and cell densities: Conventional, Fine-celled, Microcellular and
Nanocellular. The conventional foams have an average cell size typically larger than 300μm with
a cell density less than 106 cells/cm
3. The fine-celled foams refer the polymeric foams having an
average cell size between 10μm to 300μm. The cell density of the fine-celled foams does not
exceed 109cells/cm
3. The microcellular foams typically have cell size in order of 10μm and cell
2
densities between 109cells/cm
3 to 10
11cells/cm
3. Finally, the nanocellular foams are the most
recently developed group of foams that have an average cell in sub-micron level.
1.2 Research Motivation
In past decades the extrusion foaming technology has been well established and utilized in
industries. However, polymeric foam markets still demand foams with better mechanical and
thermal properties. In order to fulfill the market’s demand, developing a technique for the
production of foams having great surface quality, smaller cell size and larger expansion ratio is
necessary. Continuous microcellular foaming technique was developed to accommodate such
needs. Nowadays, the microcellular foaming technique has been well defined by numerous
researchers and they produced microcellular foams with different types of polymers, physical or
chemical blowing agents and with different additives. The interest is now shifting towards to
reducing the cell size further down to have nanocellular foams. Nanocellular foaming was
already done using batch foaming process. However, processes involved with batch foaming
process have limitation in production rate, so the need of development of a continuous
nanocellular foaming technique arises.
1.3 Objectives of the Thesis
The main objective of this study is to develop a fundamental processing baseline for nanocellular
foaming. This baseline will used in the development of a solid-state nanocellular foaming
process. The solid-state extrusion foaming processes will be designed to have a feature that melts
the polymer beads partially. By partial melting of the polymer, crystals in the polymer matrix
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will be unobstructed. By utilizing the crystals in cell nucleation stage, nanocellular foams can be
produced.
To achieve a successful design of solid-state nanocellular extrusion foaming system, two studies
were conducted in this research:
First, expanded polypropylene foams with large cell density and small cell size were produced
using a lab-scale autoclave-based bead foaming chamber. This study emphasizes the effect of
sorbitol-based nucleating agent, NX8000 which alters the crystal structure of polypropylene
suitable for nanocellular foaming. Double melting crystal melting peak generation of expanded
bead foams was also investigated by evaluating DSC thermographs, since the double crystal
melting peak is necessary for partial melting during the solid-state extrusion process.
Second, the extrusion foaming behaviour of polypropylene and sorbitol based nucleating agent
compound was investigated. The effect of NX8000 on cell density, cell size, and expansion ratio
was investigated.
1.4 Overview of the Thesis
This thesis is divided into five chapters:
Chapter 1 has a brief introduction of polymeric foams. The motivation, objective and overview
of the research are included in the chapter.
Chapter 2 includes the literature reviews on different topics including foaming fundamentals,
introduction to microcellular and nanocellular foaming, polymer crystallization kinetics, foaming
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processes, blowing agent, solubility, heterogeneous and homogenous nucleation, cell growth,
nucleating agent and methods for foam characterization.
Chapter 3 includes the overall experimental procedure and experimental results of expanded
polypropylene bead foaming using a lab-scale autoclave-based bead foaming chamber. The
foaming simulating using HP-DSC is also included in this section. The effect of NX8000 on
foaming behaviour and cell structure is studied and explained in this chapter. The DSC
thermographs of the foamed samples are also presented to explain the effect of NX8000 on
double crystal meting peak generation.
Chapter 4 includes the experiments performed using a tandem line extrusion foaming system.
The foaming behaviour of polypropylene – NX8000 compound is studied and explained.
Chapter 5 provides a summary of contributions of this research thesis. It also includes
recommendations for future works.
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Chapter 2
Theoretical Background and Literature Review
2.1 Introduction
This chapter includes the information of microcellular/nanocellular foaming fundamentals,
foaming techniques and characterization techniques.
2.2 Microcellular and Nanocellular Foaming
2.2.1 Microcellular Plastics
Microcellular plastic refers to the polymeric foams having closed cell foams with a cell density
in excess of 100 million per cm3 and cell size under 10µm. By significantly reducing the cell size,
microcellular plastics can be used to reduce the material usage while maintaining optimum
mechanical and thermal properties. In most cases, reduced material usage means cost reduction
in mass production of the plastic items. If the bubbles in foam are smaller than the critical flaws
that already exists in plastics, and introduced in sufficient numbers, then the material density
could be reduced while maintaining the essential mechanical properties [5]. One of the most
basic processes that have been developed to produce microcellular plastic incorporates with two
main steps, where the polymer sample is first saturated by a physical blowing agent in a pressure
vessel, and then the supersaturated polymer sample is removed from the vessel and heated to the
foaming temperature. However, this processing technique is a non-continuous process which can
be a critical drawback for most of the industrial applications. To overcome this downside, Park
and Suh applied the theory originally developed by Kumar to ordinary extruders. Kumar showed
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that the abrupt reduction of the solubility of the polymer can be a significant driving force for
microcell nucleation without creating supersaturation condition [6]. Park and Suh implemented a
filament die at the end of the extruder to introduce thermal instability to reduce the solubility of
the polymer. This methodology is now broadly used since the processing time is dramatically
reduced when compared to the conventional technique. Both techniques can be applied to
majority of the semi-crystalline polymer groups including: Polypropylene, poly carbonate, poly
vinyl carbonate, polystyrene, polyethylene terephthalate and poly lactic acid.
Mechanical properties can be one of the most important design factors when choosing material
for a new product design. Microcellular plastics have several improvements over unfoamed
plastics. Waldaman proved that the energy absorbed before fracture in uniaxial tensile tests for
microcellular polystyrene foams is increased by five to seven-fold than unfoamed polystyrene [7].
This fact proves that microcellular plastics are perfectly suitable for energy absorbing material,
such as car bumpers. Moreover, in Kumar and Seeler’s research, the fatigue behaviour of high
density microcellular plastic exceeds the unfoamed polymer up to a factor of 17. Thus,
replacement of the conventional plastic parts experiencing fatigue, with a microcellular plastic
can improve fatigue life of the parts dramatically.
2.2.2 Nanocellular Plastics
In recent years, nanocellular plastics have been drawing attention from researchers and
manufacturers due to their superior mechanical properties and versatile application. Nucleating
nano-scaled cells in polymer matrix is relatively new innovation. Its formation is complicated.
Where conventional techniques enabled to fabricate microcellular structures are not applicable
for nanocellular plastic production, subsequent studies have made to bring down the cell size to
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the nano-scale (~100ηm). Kim et al. found that classical nucleation theory does not yield an
accurate estimation of the critical radius and nucleation barrier. They claimed that the failure of
the classical cell nucleation theory proclaims that a new technology/theory must be introduced in
order to control the nanocellular structure during the nanocellular foaming [8].
Ohshima et al. proposed a new method for nanocellular foam preparation. They produced
nanocellular plastics using nanoscale-ordered spherical morphology of polymer blends or alloys
as a template for cell nucleation and growth. By doing this, the foaming agent’s solubility is
increased while reducing the visco-elastic behaviour of the disperse domain with respect to the
matrix polymer conventionally used for localization of cell nucleation [9].
2.3 Foaming Processes
A foaming process can either be batch or continuous.
2.3.1 Batch Foaming Process
In a batch foaming process, pre-shaped thermoplastic parts are impregnated with a blowing agent
at elevated pressure and temperature in a confined pressurized foaming chamber. Typically, the
time for gas impregnation is several hours long and it is predetermined based on the type of the
polymer and the blowing agent. Figure 2.1 shows the schematic diagram of a typical laboratory
batch foaming system [10]. In the batch foaming process, the thermodynamic instability is
induced by the rapid release of pressure in the foaming chamber using the release valve. The
thermodynamic instability will cause a reduction in solubility of the gas. If the chamber
temperature is above the glass transition temperature of the polymer, small bubbles of saturated
gas in the polymer matrix will nucleate and start to grow, creating the cellular structure [11].
8
Despite the whole process is fairly simple and commonly used in experimental practices, it takes
a long period of time to saturate the polymer with gas. In terms of commercial value, batch
foaming processes are not cost-effective. Continuous foaming processes such as an extrusion
foaming process were developed to improve cost-effectiveness.
2.3.1.1 Expanded Polypropylene Bead Foam
Expanded Polypropylene (EPP) beads refer pre-foamed polymer pallets which are readily
available for steam chest molding processes. These highly-expanded PP beads tend to have
higher manufacturing and transporting cost than expanded polystyrene beads. Although it has
disadvantages in price wise, EPP beads are very compelling material for many industrial
applications since it has excellent physical and chemical properties such as; water and chemical
resistance, high impact resistance, high energy absorption, thermal insulation and great
recyclability [12]. It is no surprise that EPP beads are widely spread in industries. EPP bead
foams are commonly found in automotive parts, toys and packaging. With EPP bead foams, it is
possible to maintain desirable resilience with low density while forming. Manufacturing of EPP
beads is mostly done with batch foaming processes. First, if some specific additives are required,
PP resin is compounded using an extruder with the required additives such as nucleating agents.
Then the compound is palletized using palletizer. Then the compounded pallets are put into an
autoclave with a heat transfer medium (typically water), surfactants and a dispersing agent. The
autoclave has a stirring mechanism so that the submerged pallets won’t agglomerate. After
putting the pallets and additives, the autoclave chamber is pressurized with a physical blowing
agent to saturate the polymer pallets. The saturation process is done at elevated temperatures,
near polymers’ melting temperatures. After appropriate processing time, the polymer pallets are
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released. A sudden drop of pressure let the pallets to expand. The whole process takes about an
hour. The expanded PP pallets are then sent to steam-chest molding manufactures to have final
shapes.
2.3.2 Continuous Processes
Continuous processes are more cost-effective and have higher productivity when compared to
batch foaming processes. Injection mold foaming and extrusion foaming are the two major
foaming techniques of continues processes. In this section, only the extrusion foaming technique
will focused. Among the common continuous processing techniques, an extrusion process is one
of the most common processing techniques because of its efficiency and versatility. Since the
screw within the extruder has a large contacting surface with a polymer, the extrusion process is
capable of generating the most efficient heat transfer [13]. Therefore, the overall processing time
would be dramatically reduced since the molten state of the polymer is achieved quickly without
any thermal degradation. Moreover, the efficiency of the extrusion process can be improved by
modifying the geometry of the extruder. Shaft dimensions, flight angles and barrel diameter to
barrel length ratio can be varied to maximize the efficiency. Modifying the geometries or the
processing conditions to achieve high production performance for different polymer is easy for
extrusion foaming process.
Figure 2.2 represents the schematic diagram of a tandem line extruder. As shown in the diagram,
tandem line extruder consists of two ordinary extruders. The first extruder is in charge of melting
the material since the material feeder is located at the very beginning of the extruder. The band
heater temperature is usually set to just above the melting temperature of the material used to
prevent overheating of the plastic. Continuous syringe gas pumps are usually used to inject
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physical blowing agents. The main objective of the second extruder is to cool the single phase
solution of the polymer. Because polymers have such low thermal conductivity, cooling process
cannot be finished in a short time interval when the production rate is high. Thus, the second
extruder is installed just for cooling of the single phase solution of the polymer melt and the
physical blowing agent. The mass flow rate of the two extruders must be matched by controlling
the RPMs of the extruders to prevent the leakage at the junction of two extruders. The die can be
in any form (filament, sheet or round) as long as it can produce a large pressure drop to introduce
a huge solubility drop for foaming.
Figure 2.3 shows the process chain for an extrusion process. An extrusion process basically has 5
main steps. The first step is polymer melting. Melting of the polymer is done by both band
heaters and screw motion of the extruder screw. After melting is done, a physical blowing agent
is injected by a syringe pump. The motion of the screw in the extruder uniformly mixes two
solutions and creates a single phase solution of the physical blowing agent and the polymer melt.
While cooling, cells are nucleated and develop their size. And finally, at the exit of the die, the
cells grow their size.
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Figure 2. 1 Schematic of a laboratory-scale batch foaming system [10]
Figure 2. 3 Process chain for extrusion foaming process
Figure 2. 2 Tandem line extruder
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2.4 Foaming of Semi-crystalline Polymers
In semi-crystalline polymers, crystallites are dispersed in an amorphous region. The fraction of
fully crystalline part of the polymer matrix is known as the crystallinity. Crystallinity in
polymers is one of the main deciding factors when determining the processing conditions during
the extrusion foaming process. It has a great effect on cell growth and melt-strength of the
polymer during the foaming process. Achieving high crystallinity does not always guarantee a
fine cell structure with a reasonable expansion ratio. Increasing the crystallinity in polymers will
eventually decrease the amount of gas dissolved into the polymer matrix, since the gas will not
dissolve into the rigid (close packed crystals) section [14]. Colton [15] also addressed the
difficulties in foaming of semi-crystalline polymers. In his investigation of polypropylene
microcellular foaming, five types of polypropylene homopolymers and copolymers were used.
He suggested that the foaming process of semi-crystalline polymers should be done near the
crystal melting temperature. Typically, semi-crystalline polymers with high crystallinity have
higher melt-strength and rigidity which hinders from achieving high expansion ratio during the
foaming process. Thus, optimizing the crystallinity is critical in foaming processes. Doroudiani
et al [16] also investigated the effect of crystallinity on microcellular foam structure of semi-
crystalline polymers. They used three cooling rates to differentiate the crystallinity: the cold-
water quenching, 85°C/min and 0.75°C/min. The crystallinities of the samples were increased by
decreasing the cooling rates. For the cold-water quenched sample, the total crystallinity was
45.1%, for the cooling rate of 85°C/min got 47.2% and the 0.75°C/min sample showed 69.2%.
Figure 2.4 shows the SEM images of batch foamed of each cooling rate. As shown in the figure,
the sample with less crystallinity with a less crystal nuclei density (the cold-water quenched
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sample) produced foams having better a cell distribution with finer cells. They concluded that the
existing crystals in the polymer matrix decreased the gas solubility which resulted in non-
uniform foam structure.
However, some researchers claimed that the crystalline phase can induce the bubble nucleation
during the foaming process. Baldwin et al. [17, 18] suggested in their studies of foaming of PET,
that the interfacial area of crystalline and amorphous regions can be the preferential bubble
nucleation sites during the foaming process. Taki et al [19] studied the effect of growing
crystalline phase on bubble nucleation in a PLA-CO2 batch foaming process. They observed that
the growing crystal spherulites expelled CO2 from the amorphous region and induced bubble
accumulation/nucleation at the interfaces of spherulite and amorphous regions. Figure 2.5 shows
the optical micrograph of PLA-CO2 system. As shown in the figure, the expelled CO2 and
accumulated around the spherulites.
The previous researches by various researchers confirm that the morphology of semi-crystalline
polymers have a great impact on the foam structure.
Figure 2. 4 SEM images of batch foamed samples
14
2.5 Crystallization kinetics
As mentioned above, crystallization plays a significant role in deciding the processing conditions
during foaming and the mechanical properties of end products. The crystallization process of
polymers takes happen upon cooling. Crystallites nucleate from the molten polymer where the
polymer chains are randomly oriented. The nucleation of crystallites occurs at a specific
temperature range depending on the polymer types. After the nucleation, crystallites grow and
from three-dimensional crystal conglomerations called spherulites. The growth rate of spherulite
is usually quicker than the nucleation rate of crystallites, since the free energy barrier for
Figure 2. 5 Optical micrograph of PLA-CO2 system
15
spherulite growth is much lower than that of crystal nucleation. Application of stress can
increase the rate of crystallization because the applied stress makes polymer chains more
packable.
Nucleation mechanisms for polymers can be identified into two categories: homogenous and
heterogeneous. In homogeneous nucleation, chains of polymer molecules in the matrix align and
fold to form lamella fibrils. Heterogeneous nucleation occurs when the lamella fibrils start to
form around the impurities, fillers or additives [20-22]. Gibbs had developed the classical
nucleation theory which stated that the nucleation energy barrier of the crystal surfaces can be
overcome by the energy variations in the super-cooled phase. Lauritzen and Hoffman developed
a crystal growth theory which suggested a relationship between the free energy barrier and the
average crystal thickness and growth rate. They also proposed a liner pattern between the growth
rate and degree of undercooling [23].
Avrami model has been commonly used for analyzing the isothermal nucleation and crystal
growth. The Avrami equation is the following:
[ ( ( ))] ( ) ( ) (2.1)
Where X(t) is the relative crystallinity at a specific crystallization time t, n is the Avrami
exponent, which describes the mechanism of crystal nucleation and growth and k is the
crystallization kinetic constant. Determining the Avrami exponent n and crystallization kinetic
constant k can be done by plotting ln[-ln(1-X(t)) ] versus ln(t). The slope of the graph will give
the n value and the intercept will give the k value [24]. Although there is no clear physical
meaning for Avrami constants n and k, attempts to interpret the constants has been made by
researchers. If the n value is lying between 1 and 2, it tells us that 2-dimensional homogenous
16
nucleation and crystallization have taken place. When n is close to 3, it means that heterogeneous
nucleation is dominant with 3-dimensional crystal growth. The measure of time duration when
the crystallization is 50% complete can be described by the crystallization half-time (t1/2). The
reciprocal of the crystallization half-time can be taken as the crystal growth rate G, which has a
great importance in explaining the crystallization kinetics. The crystallization half-time (t1/2) can
be calculated by the following equation [25]:
(
)
(2.2)
The degree of crystallinity can be estimated by various methods including: Differential scanning
calorimetry (DSC), wide-angle X-ray scattering (WAXS), X-ray diffraction (XRD), density
measurement, nuclear resonance (NMR), electron microscopy, polarized light microscopy (PLM)
and infrared spectroscopy (IR). Among the methods, DSC is commonly used among the
researchers because of its simplicity in measuring the degree of crystallinity. DSC simply
measures the heat flow of a semi-crystalline polymer during the thermal transition. After
measuring the amount of heat given by the sample, the degree of crystallinity can be calculated
by dividing the specific melting enthalpy of the sample by the melt enthalpy of the same type of
polymer having 100% crystallinity. Thus the equation for the degree of crystallinity can be
described as the following:
( )
(2.3)
Where, ( ) is the degree of crystallinity, is the crystal melting enthalpy, is the
crystal melting enthalpy of the sample having 100% crystallinity and w is the polymer weight
fraction.
17
2.5.1 Polypropylene Crystallization
Polypropylene is commonly used in thermoplastic industries. The popularity of PP is based on its
desirable mechanical and thermal properties with reasonable price. Its unique properties are from
the highly packed polymer chain structure which is called crystallites. These highly packed
stacks of polymer chains in the polymer matrix govern the melting temperature of the semi-
crystalline polymers. The morphology and tacticity of stacked polymer chains depends on the
molecular structure of the mer unit. In case of isotactic polypropylene (i-PP), four types of
crystalline forms are reported [26]. The formation of different crystal modifications depends on
the applied external (heat, pressure and additives) conditions. The α-form (monoclinic) is the
primary and most thermodynamically stable from of all crystal structure. It can be easily
obtained from the melt of solution after the crystallization process. By differentiating the thermal
treatment, two modifications of α-form, α1 and α2, can be obtained. The α1-from can be obtained
upon a rapid cooling of polymer melt and α2-form is obtainable from a slow cooling or annealing.
It is reported that the α2 -form is more ordered than α1-from [27]. The trigonal β-form of PP
crystals can be formed by adding some special kind of additives or by thermal treatments [28].
The transformation from unstable β-form to more stable α-form may occur with a certain heating
rate. It has to occur at temperature above the melting temperature of β-form crystal, but the
mechanisms of crystal transformation are not fully understood. The triclinic γ-modification can
be observed under high pressure crystallization especially in PP random copolymers [29]. The
presence of β-modification of PP crystal improves some of the mechanical properties. Adding β-
nucleating agent is one of the most famous methods to induce the formation of β-form crystals in
PP.
18
2.6 Blowing Agent
Selecting the right type and amount of blowing agent is critical in thermoplastic foaming
processes. Excessive amount of blowing agent will cause deterioration in the cell density because
the undissolved gas creates large voids in the molten polymer. The selection of processing
conditions and the blowing agent must be done carefully, because they are interrelated [30].
There are two types of blowing agents: Chemical and Physical. Chemical blowing agents use
chemical reactions to generate gases. Usually, physical blowing agents are directly injected to
extruder barrel and mixed by the rotation of the screw.
2.6.1 Chemical Blowing Agents (CBAs)
CBAs are chemical mixtures that release gas like CO2 and/or N2 a specific temperature range
upon thermal decomposition. CBAs are generally used to produce high to medium density plastic
and rubber foams. Since the price of CBAs is relatively high, they are rarely used to make foams
with densities below 400kg/m3. With CBAs, it is about 10 times more expansive to produce the
same amount of gas in a cylinder. However, unlike the physical blowing agents it does not
require any modifications on the extruder and is relatively easier to control the amount of gas to
be generated. Typically the amount of CBA needed for extrusion foaming processes is low and is
around 2 wt%. Depending on the type of chemicals the chemical reaction can either be
endothermic or exothermic. Endothermic CBAs absorb the heat energy during the decomposition
process and release mainly carbon dioxide gas and water vapor after the reaction. They tend to
have wider decomposition temperature ranges. Sodium bicarbonates and its derivatives fall into
the category of endothermic-grade CBAs [1, 2, 3, 30].
19
Exothermic CBAs on the other hand, release heat during the thermal decomposition. The release
of heat energy is spontaneous once the reaction takes the place. Exothermic CBAs including Azo
compounds like Azodicarbonamide and 4,4’-oxybis(benzenesulfonylhydrazide) are commonly
used in LDPE and EVA foaming. The compounds mentioned mainly release nitrogen gas after
the thermal decomposition. The decomposition temperature, rate, type of gas they liberate,
amount of gas they liberated in cm3/g of CBA, and the pressure generated after the reaction must
be considered in the selection of CBA [3, 31].
2.6.2 Physical Blowing Agents (PBAs)
As mentioned previously, physical blowing agents (PBAs) are directly injected into the polymer
melt in either a liquid or gas phase. Pentane and Isopropyl alcohol are good examples of PBAs
injected in a liquid phase and they remain in liquid phase in after the injection into the polymer
melt (which is under high pressure) because of their low boiling point [2]. Until 1987, before the
Montreal Protocol (An international agreement on the discontinuation of the manufacturing of
halogenated hydrocarbons to minimize the ozone layer damage) had signed, CFC had been
mainly used as a physical blowing agent because of its nontoxic nature, high solubility, low
thermal conductivity, and volatility. Researchers quickly found the alternatives, such as butane
and pentane. They are relatively cheap and can be easily injected with a moderate modification
of the foaming equipment. One of the major drawback is they are flammable and may cause
hazards during the production, shipping and handling. Because of the safety issues, flammable
PBAs were replaced with inert gases like CO2 and N2 [1, 3]. Syringe pumps are commonly used
for injecting PBAs into the foaming equipment.
20
2.7 Solubility
Before explaining the formation of homogenous gas-polymer solution, the gas solubility of
polymers must be covered. The solubility of polymer is a measure of gas intake of polymers at
certain temperature and pressure. The physical properties of polymer change upon gas
dissolution. The dissolved gas in the molten polymer affects isothermal compressibility, swollen
volume, thermal expansion coefficient, viscosity, surface tension and many other properties.
Thus, having knowledge about gas solubility and the effects of gas in the polymer is critical in
the polymer foaming industries [32-35]. Many efforts have been made to understand the
solubility of gases in polymers since the 1950s. Different approaches have been made by
researchers including the experimental measurements and theoretical thermodynamic
calculations. The most common measurement methods are the volumetric and gravimetric
methods. One of the main drawback of these methods is the measurement of gas solubility
depends on the swollen volume of polymer/gas mixtures particularly at elevated temperature and
pressure. Therefore, the swollen volume due to gas dissolution or the density of the polymer-gas
solution must be determined beforehand to improve the accuracy of data. The swollen volume or
the density of polymer-gas solution can be estimated by either using any Equation of State (EOS)
or by experimental data [36 -41].
One other method to determine the solubility of gases in polymer is pressure decay method. Sato
et al. used this method to determine the solubility of N2 and CO2 in Polypropylene (PP),
Polystyrene (PS) and High Density Polyethylene (HDPE). The pressure decay method is popular
for its simplicity in experimental apparatus preparation and operation. It simply measures the
pressure changes in a gas chamber as the polymer specimen uptakes the gas. However, this
method is not suitable for measuring the solubility of gas in molten polymers because it requires
21
a high resolution pressure sensor that can operate under elevated temperatures. Also, it requires
longer period of time to measure the solubility since the method needs a larger polymer sample.
To shorten the measurement time, an electro-balance is introduced to measure the mass uptake
during the gas sorption. This method yields high accuracy data in relatively short time of period
but only works at low temperatures. To measure the solubility at elevated temperatures,
researchers designed a system that has a separate temperature control for the chamber and the
electro-balance. This method does not account the effect of convention-induced gas density
variation on the measurement, which affects the accuracy of the measurement. The problem was
solved by introducing a Magnetic Suspension Balance (MSB) which is a gravimetric method
developed by Kleinrahm and Wanger [45]. MSB gravimetric method weights the samples in the
compartment which is separated from an outer chamber, which makes it possible to measure the
gas solubility and diffusivity at high temperatures and pressures. This method spread widely
among the researchers [42- 45].
Researchers later found that the mass reading of the dissolved gas in the MSB shows lower
solubility values than the actual solubility, because of the buoyancy effect of a swollen polymer-
gas mixture during the gas dissolution process. In the absence of an accurate pressure-volume-
temperature (PVT) data of the polymer-gas mixtures, various EOS were used to compensate the
buoyancy effect. [46, 47] Rodgers et al. extensively tested several theoretical EOSs including
Simha-Somcynsky (SS) EOS and Sanchez-Lacombe EOS on polymers and oligomers. [48-50] It
was proven in his researches that SS-EOS has an excellent capability to describe the PVT data of
molten polymers over a wide range of temperature and pressure. Also, Rodgers et al. verified the
prediction of polymer swelling by gas dissolution using SS-EOS [40, 41]. To further improve the
22
accuracy in measuring solubility data of polymers, visualization systems have been developed to
measure PVT data directly.
The processing temperature and pressure determines the solubility limit of a polymer. The
solubility limit can be estimated by Henry’s law [51].
(2.4)
Where S is the solubility constant of Henry’s law constant (cm3[STP]/g-Pa), C is the
concentration of absorbed gas per unit mass of polymer or solubility of gas (cm3/g) and lastly, p
is the saturation pressure of gas in Pa [51].
The solubility coefficient S can be described by a following equation:
(
) (2.5)
Where S0 is the pre-exponential factor or solubility coefficient constant (cm3[STP]/g-Pa), ∆Hs is
the molar heat of sorption (J), R is the gas constant (J/K), and T is the temperature (K). The
solubility of gas in a polymer can be estimated by using the two equations (2.1) and (2.2).
It is important in extrusion foaming process to control the polymer flow rate and gas injection
rate to keep the concentration of the gas below the solubility limit of the polymer.
23
2.8 Cell Nucleation
The definition of cell nucleation can be expressed as the aggregation of small group of gas
molecules to form larger and energetically stable gas pockets. Introducing a thermodynamic
instability to the polymer-gas mixture will cause the cell nucleation. The thermodynamic
instability can be achieved either by a rapid cooling or a pressure drop. After the saturation of
polymer with a gas, the polymer-gas solution becomes supersaturated once the solubility limit of
the polymer is lowered by introducing the thermodynamic instability aforementioned. The result
is gas molecules in the polymer-gas solution form bubbles because of their tendency stay in low-
energy stable state. The classical nucleation theory [52, 53] is widely accepted to explain the
nucleation process of polymer-gas mixture. There are two different nucleation types according to
the theory: homogeneous nucleation and heterogeneous nucleation. In the case of homogenous
nucleation, the bubble nucleation occurs randomly throughout the pure polymer-gas solution. It
is a phase separation process which the dissolved gas or the physical blowing agent forms a
second phase (bubbles in this case) in a primary phase (polymer matrix). Heterogeneous
nucleation on the other hand, requires preferable bubble nucleation sites such as impurities in
polymer matrix, phase boundaries or sites provided by additives like nucleating agents. In most
of the cases, the heterogeneous nucleation requires less energy than the homogeneous nucleation.
Figure 2.6 shows a schematic of the two nucleation types.
2.8.1 Homogenous Nucleation
According to the classical nucleation theory, the work needed to generate a single bubble can be
estimated by taking a difference between the work required to create a bubble with surface
24
tension and the work done by expansion of gas inside of a bubble. This can be described into a
following equation:
(2.6)
Where, is the surface tension and is the polymer-gas interfacial area. The multiplication
of the two gives the work required to generate a bubble, which is the first term of the equation
(2.6). The second term of the equation accounts the work done by the expansion of gas inside of
a bubble of volume Vb. Colton and Suh [54] modeled the nucleation behaviour in microcellular
foaming based on the equation above. The surface area and the volume of a bubble Ab, Vb can be
substituted with the appropriate geometric equations of a sphere. After the substitution, the
equation becomes the following:
(2.7)
Note that the interfacial energy term is always positive. The volume free energy can be
positive or negative depending on the system temperature. In this case, we assume that the
system is an undercooled liquid where the contribution of the second term is negative. Figure 2.4
(a) and (b) show the graphical representation of Equation 2.7. On figure 2.8 (a), it shows the
interfacial energy, volume free energy and the overall free energy change as a function of r.
Since the contribution of interfacial energy goes as r2
and that of volume free energy goes as r3,
the contribution of interfacial energy always dominates at smaller r. As mentioned above, the
interfacial energy term is always positive and it suppresses the formation of bubble. Unless the
size of the bubble grows larger than a certain size to overcome the predominant interfacial
energy, the bubble will collapse and it is defined as the critical radius. Thus, it can be said that
25
the free energy barrier must be overcome in order for the bubble to grow spontaneously. The
critical radius can be calculated by differentiating W with respect to r:
[
]
(2.8)
Wich gives,
(2.9)
And by subsituting rc into Equation 2.5 gives the following eqautiuon:
( ) (2.10)
Which is the equation for Gibb’s free energy of forming a critical nucleus. [37, 36]
The rate of homogeneous nucleation can be estimated by the following equation:
(
) (2.11)
Where Co is the concentration of gas molecules, f0 is the frequency factor of gas molecules
joining the nucleus, K is the Boltzmann’s constant and T is the temperature in kelvin. The
homogenous nucleation rate accelerates as the temperature increases. Yet, this behaviour needs
further investigation for extrusion foaming processes. Researchers found different relationship
between the homogenous nucleation rate and temperature. Goel and Beckman did not agree with
Equation (2.11) and claimed that the nucleation rate decreased with increased temperature for a
PMMA-CO2 mixture. [55] On the other hand, Ramesh et al. verified the acceleration effect of
26
temperature on the homogenous nucleation rate with a PS-CO2 system. [56] Baldwin et al. also
examined this effect with amorphous and semi crystalline polymers. They found that the cell
densities of amorphous PET and CPET increased by increasing the temperature. There was no
significant effect on cell densities for the case of semi-crystalline PET and CPET. [57]
Figure 2. 6 Homogeneous and heterogeneous nucleation in a polymer-gas solution [58]
27
(a)
Figure 2. 7 Critical radius and free energy barrier
(b)
28
2.8.2 Heterogeneous Nucleation
Unlike the homogenous cell nucleation, heterogeneous nucleation emanates from preferable
nucleation sites including phase boundaries or sites provided by additives such as nucleating
agents. Heterogeneous nucleation is generally occurs before and faster than homogeneous
nucleation because of its lower activation energy barrier [54]. Figure 2.8 shows the graphical
comparison between the activation energies of the two nucleation mechanisms. As shown in
figure, the free energy barrier of heterogeneous nucleation is much lower than that of
homogenous nucleation. Due to its theoretical and technical complexities, the base mechanism of
heterogeneous nucleation has not been investigated deeply. However, it has been proved that
adding additives or nucleating agents, which promotes the heterogonous nucleation, improves
the cell densities while foaming [59].
The mechanism of heterogeneous nucleation using additives was investigated by Chen et al.
from Trexel Inc. [60]. They claimed that the dissolved gas inside of the polymer-additive mixture
forms bubbles around the interfacial area of additive and polymer upon sudden drop of pressure
during the foaming process. Recalling the classical nucleation theory, agglomerates of
undissolved gas on certain sites in polymer-gas solution may grow and become cells under a
specific condition: The radius of the spot has to be larger than the critical radius, rc.
The micro-voids formed by gas agglomerates near the polymer-additive boundaries can act as
nucleation sites for homogenous nucleation. During the formation of homogeneous solution of
polymer and gas, these micro-voids may not be filled because of the surface tension between the
gas and the additive. The experimental verification was done by Chen et al. confirming that
29
when the gas agglomerates around the polymer-additive boundaries are larger than the critical
radius, they become cells and produce a fine-cell structure with less gas content [60].
The mathematical expression of Gibbs free energy for heterogeneous nucleation is identical to
that obtained in the homogenous nucleation case in Equation 2.10 expect for the energy
reduction term. On a smooth, planar surface, the Gibbs free energy term for heterogeneous
nucleation can be expressed as the following:
( ) ( ) (2.12)
where is the surface energy of the bubble, is the pressure difference between the gas
bubble and the polymer matrix, and ( ) is the reduction of energy due to heterogeneous
nucleation sites. The energy reduction term, ( ) on a flat, smooth surface can be expressed as
the following:
( ) (
) ( )( ) (2.13)
Where, is the contact angle of the additive and gas interface and is between 0 and 1 in value.
In most of cases, the surface geometry of nucleation sites is not flat and smooth. It depends on
the type of nucleating agent, the presence of impurities within the polymer matrix. Taking
account that the surface geometry of nucleation sites can vary from one to another, it is necessary
to add the semi-conical angle assuming the cell nucleation can occur in conical cavities as
shown in Figure 2.9. The semi-conical angle is randomly distributed between 0 to 90 °
30
depending on the site geometry. Therefore, to be more precise, the energy reduction term can be
expressed as the following [61, 62]:
( )
[ ( )
( )
] (2.14)
The mathematical expression for the heterogeneous nucleation is very similar to that of
homogenous nucleation:
(
) (2.15)
Where, is the frequency factor of gas molecules joining the nucleus, is the concentration of
gas molecules, is the Boltzman’s constant and is the temperature in Kelvin.
The total bubble nucleation rate can be calculated by combining the heterogeneous and
homogenous nucleation rates:
(2.16)
As mentioned previously, heterogeneous nucleation is more favourable than homogenous
nucleation because of lower energy barrier. It means that the gas concentration for the
homogeneous nucleation case is going to be lowered after the homogenous nucleation.
term in Equation (2.16) accounts the reduction in gas concentration. The modified homogenous
nucleation rate equation can be expressed as the following:
(
) (2.17)
Where, is the concentration of gas molecules after the occurrence of heterogeneous nucleation.
31
It is experimentally proven that preferential bubble nucleation sites provided by additives
enhance the overall cell structure of the foam. By controlling the amount of additive, desired cell
density can be achieved. Xu et al. claimed that the cell density of PS foam was increased by
adding talc. Another experimental focus was to find out the effect of talc on nucleation with
different die geometries. They found that the addition of talc increased cell nucleation more
significantly with lower pressure drop dies [63]. Han et al. observed that the addition of
intercalated or exfoliated nano-clay enhanced cell nucleation during the foaming process. They
found improvements on cell densities and reduction in cell sizes on local spots where the
additive is accumulated. Difficulties in dispersing the additive hindered them from getting a
uniform cell structure [64-66]. They also found that after a certain concentration, the addition of
the additive hardly affect the cell density. Lee et al. agreed that the heterogeneous nucleation
promotes cell nucleation during the foaming process, by investigating the gas absorption
behaviour of polymers mixed with mineral fillers such as talc and CaCO3 [67]. A model for
heterogeneous nucleation was prepared by Ramesh et al. with a blend of high impact PS (HIPS)
and PS with the presence of micro-voids [68]. A demonstration of heterogeneous nucleation was
done by Leung et al. using PS-CO2 system. Accelerated cell nucleation behaviour due to
heterogeneous nucleation was observed, which agreed with their theoretical hypothesis [69, 70].
However, explaining the cell nucleation behaviour solely by the classical nucleation theory is
arguable, since the previous findings lack a good experimental quantitative agreement with the
theoretical predictions without using fitting parameters.
32
2.8.3 Effect of Shear Stress, Extensional Stress/Strain on Bubble
Nucleation
Recently, it was found that the shear stress can induce cell nucleation during the foaming process.
Different researchers had tried to investigate the effects of shear stress by in-situ observing
extrusion foaming process through slit dies with transparent sections. In these studies, they had
concluded that the effect of shear stress promoted cell nucleation [53, 71-73].
The studies done by using extrusion foaming processes had a highly coupled environment.
Complexities in understanding thermodynamic, fluid mechanic and rheological behaviour
underlying the extrusion process made the studies difficult to be thoroughly understood. Some
researchers have developed a visualization system for batch foaming processes [72, 74]. Chen et
al. proposed that proposed that the effect of shear stress is more apparent particularly in extrusion
foaming processes, when the saturation pressure or the amount of gas in the polymer matrix is
lower [75]. They concluded that the rate of heterogeneous nucleation was increased by shear
stress.
Albalak et al. proposed that the bubble expansion can generate local tensile stress near the bubble
[76]. The local tensile stress depresses the local pressure around the bubble and it increases the
chance of superheat that causes secondary bubble nucleation near the bubble. Wang et al.
conducted a numerical analysis and suggested that growing cell would induce a pressure
fluctuation around the bubbles which promote the secondary cell nucleation [77]. As an
extension of this research, Leung et al. modified the expressions for energy barrier and critical
radius as the following [78]:
33
( ) (2.18)
( ( )) ( ) (2.19)
( ( )) (2.20)
Wong et al. developed a visualization system for batch foaming process to study the effect of
extensional stress/strain on foaming behaviours of PS and PS-Talc compound. They concluded
that cell density increased with the application of extensional strain. The effect of extensional
strain was more apparent at low processing temperature and with the addition of talc [79].
2.9 Cell Growth and Stabilization
The expansion of nucleated cells follows naturally once they overcome the free energy barrier.
The pressure difference between Pbubble and Psystem is the main driving force of cell expansion.
Cells continue to grow until the polymer-gas system reaches equilibrium. The mechanism of cell
growth is governed by several parameters including: diffusion coefficient, viscosity, gas
concentration, allocation of time for cells to grow, number of cells growing simultaneously and
hydrostatic pressure applied to the polymer matrix.
The temperature dependency of melt strength of polymer and diffusivity of dissolved gas is very
high, thus the cell growth can be controlled by changing the system temperature. For example,
decreasing the temperature will prevent cells to grow fully, due to the decreased melt viscosity
and diffusivity of gas at lower temperature. A precise temperature control is essential to keep the
gas in the polymer matrix for achieving desirable cell growth and high volume expansion during
34
the microcellular foaming process [80-82]. The growth rate of microcellular foams is much
higher than that of conventional foams, which leads microcellular foams to have finer cell size
with high cell density. However, this may also induce undesirable cell coalescence since the cell
walls are very thin [83]. If the cell coalescence occur during the cell growth stage causes a drop
in initial cell density of the foam. As cells grow, cell wall ripening will cautiously happen since
the adjacent cells tend to fuse together due to lowered free energy caused by reduction in the
surface area [84]. The shear force generated around the growing bubble is also one of the reasons
for cell coalescence [85]. The deteriorated cell density caused by cell coalescence will affect the
mechanical and thermal properties of the foam. It is very hard to prevent cell wall ripening
during the foaming process. Baldwin et al. proposed a method to prevent cell coalescence using a
die that induce high back pressure. They concluded that cell coalescence is almost unavoidable
in a larger die which produce foams with a larger cross-section. They added that it may not be
possible to prevent cell wall ripening just by controlling the backpressure [86]. Park et al.
proposed a different way to prevent coalescence; control the processing temperature to increase
the melt strength of polymer during the microcellular extrusion foaming process [87].
35
Figure 2. 8 Comparison of energy required for homogeneous and heterogeneous nucleation [71]
Figure 2. 9 Heterogeneous Nucleation on (a) smooth planar surface and (b) in a conical cavity [87]
(a) (b)
36
2.10 Nucleating Agents
The properties of semi-crystalline polymers are highly dependent on the degree of crystallinity,
which is governed by the crystallization process of polymers. The crystallization process can be
easily manipulated by introducing foreign particles into the polymer matrix. As discussed in
previous sections, impurities in the polymer matrix will act as nucleation sites. Nucleation sites
can be provided by adding nucleating agents in to the polymer matrix. Nucleating agents are
generally inorganic materials having a small average particle size. Conventional nucleating
agents have high melting point to remain solid during the entire processing procedures. With the
addition of nucleating agents, the nucleation process of the polymer – nucleating agent mixture
will occur at a higher temperature because the heterogeneous nucleation is predominant at
elevated temperatures. Also, the nucleation process will significantly be shortened since the
molten polymer does not have to align its chains to form nuclei (homogenous nucleation) for the
crystallization process. Thus, nucleating agents are commonly used in industries to reduce the
cycle time of the process [88]. Nucleating agents can be used in polymer foaming processes to
promote the bubble nucleation. Nucleation agents including talc, nano-clay and nanotubes have
been extensively studied by many research groups. Choosing a right nucleating agent can be
challenging. One should carefully consider the factors like; nucleating efficiency and affinity. An
ideal nucleating agent should have the following characteristics: first, the surface geometry of the
nucleating agent should be shaped in such way that the free energy for nucleation is lower than
the homogenous nucleation; secondly, it should be easy to disperse and lastly, the size of the
particles should be uniform.
Some researchers have been investigating the effect of nucleating agents on foaming behaviour
of semi-crystalline polymers. W. Kaemesri et al. investigated the effect of talc content on
37
extrusion foaming behaviour of PP. They claimed that the addition of talc increased the volume
expansion of extruded PP foams. However, they observed a depression in volume expansion of
PP foams beyond certain concentration of talc. The result showed that there was no further
improvement on cell structures and cell densities of the foams after 3 wt% of talc. They claimed
that the excessive amount of talc will negative impact on the cell structure and cell geometries of
the foams because of the following reason: Higher talc concentration promoted higher cell
density while increasing the chance of cell ripening [89].
2.10.1 Sorbitol Based Nucleating-Clarifying Agent
Sorbitol and its derivatives are widely used in polymer processing industries to improve the
clarity of PP. 1,2,3,4,-dibenzylidene sorbitol (DBS) and 1,2,3,4-bis(p-methoxybenzylidene
sorbitol) (DOS) represents the first generation of such clarifying agents. The second generation
of clarifying agents can be accounted as ones that containing alkyl and haloderivatives. 1,2,3,4-
bis(p-methylbenzylidene sorbitol) (MDBS), 1,2,3,4,-p-chloro-p’-methyldibenzylidene sorbitol
and 1,2,3,4-bis(p-ethylbenzylidene sorbitol) (EDBS) are good examples of the second generation
sorbitol derivatives. Recently, the third generation of clarifying agent, 1,2,3,4-bis(3,4-
dimethylbenzylidene sorbitol) was developed. Unlike the conventional nucleating agents,
sorbitol derivatives are designed to dissolve into the polymer melt to form a homogenous
solution. Upon cooling, the sorbitol derivatives crystallize and form a 3-dimensional nano-
fibrillar network in the matrix [90-94]. The size of the resulting nano-fibrillar structure has a
diameter varies from 10nm to 100nm, depending on the processing condition [95], meaning that
it is almost the same size as the lamellar thickness of the polymer crystals. This fibrillar structure
provides a large amount of surface to promote heterogeneous nucleation of the polymer,
resulting in a large number of very fine crystallites. The unique clarifying effect of sorbitol
38
derivatives is pertaining to the large number of very fine sized polymer crystals, which allows
light can pass through the polymer. The formation of nano-fibrillar structure of the sorbitol
derivatives happens prior to the crystallization process of the polymer itself, causing a jump in
the apparent viscosity of the compound near the crystallization temperature of the clarifying
agent. This is the reason why sometimes sorbitol derivatives are referred as gelling agent.
In most cases, sorbitol based clarifying agents are used in the low additive concentration range,
not exceeding 0.5wt%. Below this concentration, the polymer and the additive form a
homogenous solution above the melting temperature of the clarifying agent. Upon cooling, the
additive will crystallize before the polymer and form the nano-fibrillar structure without a phase
separation. Thus, using below 0.5wt% concentration range is recommended for maximized
clarity and nucleating effect [94].
2.11 Foam Characterization
The structure of thermoplastic foams can be identified by foam density, volume expansion ratio
and cell morphology. These parameters are highly related to the processing conditions and
materials, which are deciding factors for the final performance of the finished product. They also
are good indication of the degree of cell nucleation and expansion during the foaming process.
2.11.1 Foam Density
Foam density is the structural parameter that represents the reduction in density of the polymer
after the foaming process. It can be calculated by the following equation:
(2.21)
39
Where is the foam density, M is the mass of the foamed sample and V is the volume of the
foamed sample in cm3.
Water submerging and displacement method is commonly used for determining the bulk density
of closed-cell structure foam samples.
2.11.2 Volume Expansion Ratio
Volume expansion ratio is an indication of the material savings which is the result of the void
volume that replaces the original material. The volume expansion ratio can be calculated by
taking the ratio of the bulk density of un-foamed material to the bulk density of the foamed
sample. The equation looks like the following:
( )
(2.22)
Some of the researchers use void fraction (Vf) instead of volume expansion ratio to describe the
amount of void in the foamed sample. The equation for void fraction is the following:
(2.23)
2.11.3 Cell Morphology, Cell Size Distribution and Cell Density
Verification of the cell size and cell density is as important as foaming process. The cell
morphology can be characterized by its cell density, size distribution and cell size. It can be done
using a scanning electron microscope (SEM). The SEM uses electrons instead of light to form an
image. It is now broadly used to verify the cell size and cell density of polymeric foams,
especially for microcellular and nanocellular foams. The SEM has much higher resolution than
40
conventional microscopes. It also has much more control in the degree of magnification. Cell
density represents the number of cell per cubic centimeter volume. Cell densities of foams can be
calculated by the following equation:
(
)
(2.24)
The number of cells can be counted from a SEM image with the aid of image utility software.
Defined area (cm3) also can be measured from the SEM image using the scale given in the image.
Cell size distribution of the foamed sample can be measured in three ways: Measure from the
SEM micrographs, with the mercury porosimeter and using the mercury immersion technique.
41
Chapter 3
Bead Foaming of Polypropylene-Sorbitol Based Nucleating Agent Compound
3.1 Introduction
Sorbitol and its derivatives are widely used in industries to reduce the haziness and improve the
clarity of PP. They can be used as a nucleating agent by utilizing their unique property: Unlike
the conventional nucleating agents, sorbitol derivatives can be dissolved in the molten polymer
matrix. Upon cooling, the homogenous solution of PP and sorbitol form a three dimensional
nano-fibrillar structure [89-94]. This nano-fibrillar structure provides a large amount of surface
area for heterogeneous nucleation of PP during the crystallization process. Since the surface area
provided by the network structure is abundant, the resulting crystal structure tends to have a large
number of very fine crystallites. This unique feature of sorbitol derivatives can be incorporated
with many existing foaming techniques to improve the overall cell structure of the foams.
Expanded polymeric foams are widely used in many applications, especially when the products
require complicated geometries [4]. These pre-foamed polypropylene pallets can utilize the
steam chest molding technology acquire a complex 3-D shape. To utilize the steam chest
molding technique, the Expanded Polypropylene (EPP) foams should have double or multiple
crystal melting peaks. Thus, the processing temperature of steam chest molding process can be
selected near the lower crystal melting temperature for sintering purposes while preserving the
crystallites having higher melting peak. Figure 3.1 shows the typical double crystal melting peak
behaviour of EPP foams and the desirable processing temperature range for steam chest molding
42
processes. This allows to have better sintering among the bead foams and to maintain desirable
mechanical properties at the same time.
Batch foaming process is the main manufacturing method for EPP bead foaming. Meaning that
the manufacturing of EPP bead foams tends to be more expansive than other continuous foaming
methods; it requires longer processing time. Despite the fact it is more costly, it has several
advantages over the continuous foaming process methods. EPP bead foams produced using the
batch foaming process have better cell structure and higher cell density. Moreover, the batch
foaming process has better control on processing temperature and pressure which is critical for
double crystal melting peak generation.
The double melting peak generation of semi-crystalline polymers has been studied by numerous
researchers. The double melting peak structure is commonly observed when semi-crystalline
polymers undergo isothermal heat treatment near the melting temperature. The generation of new
melting peak at higher temperature range can be caused by one of the following reasons:
different crystal sizes among the crystal spherulites, various crystal structures and their
rearrangement to have more closely packed crystal structure during the isothermal treatment [96-
99]. It is reported that there are at least three types of crystal form for PP crystalline structure.
Among the three forms, α form is reported as the primary reason for the multiple melting peak
structure of PP. Upon isothermal treatment, the α crystal form can have two limiting
modifications α_1 and α_2 [100]. The α_1form is the less packed form which can be obtained by
a rapid crystallization process, while the α_2 form can be obtained by annealing at an elevated
temperatures blow the melting temperature of the polymer. The α_2 form crystals have more
closely-packed structure when compared the α_1, due to the crystal perfection. The α_2crystal
form has a melting temperature little above the original melting temperate of the polymer.
43
EPP bead foams are gaining more popularity than other polymeric bead foams since they have
desirable mechanical and chemical properties including: heat resistance, chemical resistance, oil
and water resistance and impact resistance. Considering that EPP foams are more favourable
than other polymeric bead foams in terms of their properties, researchers have been trying to find
ways to improve the cell structure and foamability of EPP by optimizing the processing
conditions.
The manufacturing method of EPP and the double melting peak generation technique are already
well-developed by previous research. Y. Guo et al. had developed a lab-scale autoclave-based
EPP foaming process to investigate the mechanism of the formation of cellular morphology and
evolution of the crystal melting peaks of the expanded beads. For better heat distribution and
transfer, they used water as the heat transfer medium. A propeller guided chamber was
incorporated to prevent from the polymers aggregates inside the chamber [101]. Later they
conducted an extensive study to investigate the critical parameters in processing of EPP bead
foaming. They observed that as the saturation pressure during the annealing process was
increased, the high melting peak structure of the EPP beads diminished. High expansion ratios
were obtained when the saturation pressure was high. They also found that if the die at the exit
has higher L/D ratio, the high melting peak crystallinity increases as a result of shear-enhanced
crystallization [102].
More recently, Barzegari et al. also conducted a systematic investigation of the correlations
between the processing conditions and the cell structure of EPP foams using a lab-scale
autoclave-based foaming chamber. They also agreed that the saturation pressure is the most
critical parameter in EPP bead foaming. The change in saturation pressure influences on both the
foam expansion ratio and cell densities of the EPP foams. It was also observed that the double
44
melting peak structure was successfully achieved when the samples were saturated with
pressurized CO2 in 550 to 800 psi at the saturation temperature of 130°C [103].
3.1.1 Hypothesis
The manufacturing method for EPP and its critical parameters are already well defined in
previous studies conducted by numerous researchers. Also the crystallization kinetics of PP-
sorbitol compound is well defined. However, the foaming behaviour of PP-sorbitol compound
has not been covered extensively in previous studies. Sorbitol derivatives can act as a highly
efficient nucleating agent in specific conditions. It is known that the nucleating efficiency of the
sorbitol derivatives is higher than that of the conventional nucleating agents like talc.
Before conducting the extrusion foaming trials, a batch foaming process incorporating an
autoclave system with a propeller guided chamber is used in this chapter. It is important to
Figure 3. 1 Multiple melting peak behaviour of EPP
45
conduct a series of experiments before the extrusion process since during the extrusion foaming
process, the material will experience high shear force form the extruder screw. This might
potentially destroy the 3 dimensional network structure of sorbitol nucleating agent and lower
the overall nucleating efficiency. Also the rotating motion of the screw might cause the
entanglement of the polymer chains. The batch foaming process conducted in this study
provided a good base understanding of the foaming behaviour of the PP-sorbitol compound. It
will also allow observing the change in crystallization kinetics after the foaming process since
the crystal structure will be unobstructed sine there is no shearing force involved like in the
extrusion processes.
The hypothesis behind this study can be described as the following. Well dispersed fine sized
crystals caused by the addition of sorbitol based nucleating agent will act as the heterogeneous
nucleation sites during the bubble nucleation sites. The well-dispersed sorbitol based nucleating
agent will alter the crystal structure in favourable way for the foaming process. It is expected that
the addition of sorbitol based nucleating agent suppresses the expansion of the foam while
increase the cell density significantly.
The sorbitol based nucleating agents have not been extensively tested for any type of foaming
techniques. This study has done in both qualitative and quantitative manner to verify that the
very fine sized crystal structure caused by the sorbitol based additive facilitates the foaming with
extremely high cell densities due to the abundant bubble nucleating sites provided by the crystals.
This chapter includes the method to produce foams with very fine cell structure having relatively
large expansion ratio with very high cell density (more than 1011
cells/cm3) using the lab-scale
autoclave based bead foaming chamber. Different concentrations of sorbitol based nucleating
46
agent are used to study the effect of sorbitol based nucleating agent on the foaming behaviour
and the crystallization kinetics. The effects of processing temperature on the generation of
double melting peak behaviour and the cell structure were investigated too.
3.2 Experimental
3.2.1 Materials
A random PP co-polymer (RA12MN40) from Saudi Basic Industries Corporation (SABIC) was
selected to investigate the foaming behaviour and double crystal melting peak generation. PP
random co-polymers are more viable option in foaming than PP homopolymers. Also they have
more industrial applications since they have better impact strength in low temperatures. The melt
flow rate of the polymer is 40g/10mins (tested method: ASTM D1238 230°C and 2.16kg) and
the density is 0.905g/cm3
(test method: ASTM D792) respectively. The crystallinity of the
pallets was checked by a regular differential scanning calorimetry (DSC) (DSC2000 TA
Instruments, New Castle, DE). First the sample pallet was heated with the heating rate of 10°C
/min to 200°C and held for 10 minutes to remove the thermal history of the pallet. The pallet
showed a single melting point at 150.99°C with a shoulder at around 131°C. The total
crystallinity of the pallet was 38%. Figure 3.2 shows the DSC thermogram of the neat
RA12MN40 Pallet.
Millard NX8000 (Bis(4-Propylbenzylidene)Propyl Sorbitol), a sorbitol based nucleating agent
from Milliken Chemical was used as a nucleating agent in this study. Figure 3.3 shows the
chemical structure of NX8000. The melting temperature of NX8000 is much lower than that of
DMDBS. This gives two advantages: first, he processing time for material compounding can be
47
dramatically shortened by using NX8000. And second, the compounding of those two materials
can be done without the degradation of the polymer.
CO2 with 99.5% purity was provided from The Linde Group and used as received.
Figure 3. 2 DSC thermograph of neat PP RA12MN40 after thermal history removal
Figure 3. 3 Chemical Structure of NX8000
48
3.2.2 Material Compounding
In order to ensure the good dispersive and distributive mixing, a lab scale counter rotating twin-
screw extruder, ZSE 27HP (27mm screw diameter, Max rpm of 1200, Max drive power of
41.0KW) from Leistritz is used to compound PP and sorbitol nucleating agent. 5 different
NX8000 contents (0.3wt%, 0.5wt%, 0.75wt%, 1wt% and 3wt %) were compounded using the
lab scale twin screw compounder. The direct melt compounding method was used for all 5
sorbitol nucleating agent contents, to preserve the unique network structure of the additive in
every pallet. The masterbatch dilution method was not used since it could cause the uneven
distribution of the additive network among the pallets. The processing temperature profile of the
twin screw extruder was set just above the melting temperature of the sorbitol nucleating agent.
It accounted both the degradation temperature of RA12MN40 and the melting temperature of
NX8000. The processing conditions of the twin screw extruder are shown in Table 3.1.
Right after compounding, the compound was palletized into 2-3mm micro pallets using
Brabender underwater palletizer
Screw RPM: 50
Section T10 T9 T8 T7 T6 T5 T4 T3 T2 T1
Temperature
(°C)
190 190 210 230 240 235 235 230 200 170
Table 3. 1 Processing Conditions for Compounding
Material flow direction
49
Figure 3.4 shows the image of the PP-sorbitol compounds. As shown in the pictures, the clarity
of the samples increases up to 1wt% of sorbitol content level. After 1wt%, the compounds
showed transparent light blue color and increased haziness.
Figure 3. 4 Pictures of the Compounded Samples
50
3.2.3 Thermal Analysis – Differential Scanning Calorimetry
A differential scanning calorimetry (DCS) was used to conduct the thermal analysis. The
crystallinities of the samples were measured using DSC2000 (A Instruments, New Castle, DE).
The measurement procedure was as follows: First, each sample pallet was put and sealed in a
DSC aluminum sample pan. Each sample should have 10mg to 15mg in weight. Second, the
samples were heated from room temperature to 200°C at a rate of 10°C/min. After the system
reached the target temperature, the system maintained the isothermal condition for 10 minutes.
The isothermal heat treatment was done to remove the thermal and stress history of the samples.
After the heat treatment, the samples were cooled with a cooling rate of 30°C/min to room
temperature. Then the samples were subjected to the second heat cycle. The samples were heated
to 200°C at a rate of 10°C/min and recorded to construct the DSC thermographs. The DSC
thermographs were used to measure the percentage total crystallinity of the samples. The area
under the second heating curve shows the melting enthalpy, ; and dividing this value with
the melting enthalpy of 100% crystalline PP ( ) gives the fractional value of total
crystallinity of the samples. The theoretical value of for PP is 207.1 J/g [104]. Thus, the
crystallinity of the PP samples can be calculated by the following equation:
( )
(3.1)
The melting enthalpy of each foamed samples was also measured. The heating and cooling
procedure for constructing DSC thermograph is the same. Only one heating/cooling cycle is used
to measure the melting enthalpy of the foamed samples.
51
3.2.4 Thermal Analysis – High Pressure DSC
A high pressure DSC (NETZSCH DSC 204 HP, Germany) was utilized to study the changes in
crystallinities of the pp samples under the pressured CO2 conditions. This study was conducted
as a means of simulating the batch foaming process. Having the HP-DSC simulation experiments
suggested a good guideline for the design of experiment before conducting the actual bead
foaming. Rather than selecting a random temperature for isothermal treatment, a series of HP-
DSC simulation was done for checking the validity of each isothermal treatment temperatures for
double crystal melting peak generation. The isothermal heat treatment temperatures were
selected near the melting temperature of neat PP RA12MN40 which was obtained from the DSC
thermograph. In order to calibrate the HP-DSC, the heat of fusion and melting point of Indium
was measure in various temperature and pressure conditions.
3.2.5 Rheological Measurements
The rheological properties of the neat PP RA12MN40 and its compounds were measured by the
Small Amplitude oscillatory Shearing (SOAS) method to observe the changes in complex
viscosities of the PP-NX8000 compound. According to Kristiansen et al., A shoulder-like initial
increase in the complex-viscosity is the evidence of the formation of a nano-fibrillar network due
to the crystallization of the additive [105]. First, samples with 25mm in diameter and 1mm in
thickness were prepared using the compression molding machine. Then the specimen was tested
using an ARES Rheometer (TA Instruments, New Castle, DE); the parallel-parallel plate
geometry was used during the test. A temperature sweep test method was employed. The test
was started from 250°C and decreased to 130°C at a rate of 2°C /min with 1rad/sec and 10%
stain.
52
3.2.6 Experimental Set-up and Procedure
3.2.6.1 HP DSC simulation
As mentioned in the previous section, the HP-DSC simulations were conducted to identify the
optimal isothermal heat treatment temperature range for double melting peak generation. This
simulation was only done to obtain a suggestive guideline for setting the effective processing
conditions before the actual bead foaming trials. The method of study is based on the previous
study by M. Reza et al. [106]. They suggested a method to simulate the EPP bead foaming using
the HP-DSC. They also suggested that the effective isothermal treatment temperatures are
located near the melting temperature where crystals have higher mobility and can rearrange
themselves to form highly packed crystal structures (i.e. crystal perfection). In this study, the
isothermal treatment temperature range was based on the melting temperature of the neat PP
RA12MN40 specimen. 135°C, 137°C, 140°C, 143°C, 145°C and 148°C were selected as the
isothermal heat treatment temperatures. The saturation pressure of 55bar was selected since the
same pressure was used in the actual bead foaming practices. Only one concentration (0.5 wt%)
from the sorbitol compounds was used in this study. The procedure for simulation was as follows:
First, the DSC chamber was pressurized to 55bar. Then the samples were heated from room
temperature to the target heat treatment temperature at a rate of 50°C/min and held at the target
temperature for 40 minutes. After the isothermal heat treatment and gas saturation, the samples
were cooled at a rate of 15°C/min and the chamber was depressurized simultaneously at a rate of
55bar/min. The samples were then took out of the chamber and degassed for 72 hours at
atmospheric pressure and room temperature. Regular DSC was employed after degassing to
study the changes in crystal melting peaks. Figure 3.5 shows the schematic of processing
procedure for the simulation of bead foaming process.
53
Figure 3. 5 Schematic of HP-DSC Simulation Process
54
3.2.6.2 Lab-scale Autoclave-based Bead Foaming Chamber Set-up
A lab-scale autoclave-based bead foaming chamber which was developed previously in
Microcellular Plastics Manufacturing Laboratory was implemented in this study [101]. Figure
3.6 represents a schematic of the bead foaming chamber. The chamber was first filled with 800cc
of water. Then 10 grams of polymer pallets were added into the system. Water was used as a heat
transfer medium to disperse heat uniformly. To avoid the problems of polymer agglomeration,
the chamber was constantly stirred up during the whole process. The propeller rpm was set to
500 rpm. The chamber was then pressurized with CO2 at 800 psi (55.158 bar). Only one pressure
was used for polymer saturation. Then the chamber was heated to the target saturation
temperature (Ts) to create the second crystal melting peak. The temperature range for Ts was
based on the HP-DSC simulation results.137°C, 140°C, 143°C and 145°C were used as Ts. The
saturation time of 40 minutes was selected based on the previous studies by Barzegari et al [103].
After 40 minutes, the whole content in the chamber was discharged to a water filled collecting
bucket by opening the release valve. The pressure change caused the thermodynamic instability
among the saturated polymer pallets and foaming occurred in consequence. Table 3.2 shows the
processing conditions used.
Neat PP 0.3 wt% 0.5 wt% 0.75 wt% 1 wt% 3 wt%
Ts = 145°C EXP 1 EXP 2 EXP 3 EXP 4 EXP 5 EXP 6
Ts = 143°C EXP 7 EXP 8 EXP 9 EXP 10 EXP 11 EXP 12
Ts = 140°C EXP 13 EXP 14 EXP 15 EXP 16 EXP 17 EXP 18
Ts = 137°C EXP 19 EXP 20 EXP 21 EXP 22 EXP 23 EXP 24
Table 3. 2 Processing conditions for EPP foaming
55
3.2.7 Foam Characterization
The obtained samples from the bead foaming experiments were characterized to examine their
volume expansion ratios, cell densities and cell sizes. Three random foamed samples from each
processing condition were collected and characterized.
Motor
Temperature
Controller
Pressure
Gauge
Pressure
Regulator
CO2
Tank
Release
Valve
Band
Heaters
EPP Bead
Foams
Figure 3. 6 A Schematic of Autoclave Based EPP Bead Foaming Chamber [102]
56
3.2.7.1 Volume Expansion Ratio
The volume expansion ratios (VER) of the foamed samples were calculated after the bead
foaming experiment. The volume expansion ratio is the ratio between the bulk density of solid
un-foamed sample and the bulk density of the foamed sample The foam densities of EPP bead
foams were measured in order to calculate the volume expansion ratios. VER can be calculated
using the following simple equation:
(3.2)
The densities of the foamed and un-foamed polymer samples were measured using the
standardized method ASTM D792-13 “Standard Test Methods for Density and Specific Gravity
(Relative Density) of Plastics by Displacement”.
3.2.7.2 Cell Density and Cell Structure
Studying the cell structures and morphologies of the samples were imperative in this study. The
Scanning Electron Microscope (SEM) images of the samples were taken using Hitachi SEM 510.
As a preparation for the SEM, the samples were first dipped into the liquid nitrogen for several
minutes then cut into two pieces using a razor knife to observe the foamed cross section. In this
study, cryogenic fracturing method was not used since the foamed samples were too small. Then
the samples were sputter coated for SEM observation. After taking the SEM images, the number
of cells per area for each sample was counted using an image processing software Image-J
(National Institutes of Health). The cell density was calculated using the following equation:
(
)
(3.3)
57
3.3 Results and Discussion
Figure 3.7 shows the picture of the foamed EPP samples using the lab-scale autoclave-based
bead foaming chamber in various saturation temperatures at a CO2 saturation pressure of 800 psi.
Above the saturation temperature of 147°C the polymer pallets got melt and could not be foamed.
Figure 3.8 shows the molten pallets during the foaming process.
137°C 140°C
143°C 145°C
Figure 3. 7 EPP bead foams
58
Figure 3. 8 Molten polymer the during process
Figure 3. 9 DSC thermograms of PP-NX8000 compounds (Heating)
59
3.3.1 Effect of Sorbitol Based Nucleating Agent Content on Crystallinity
Figure 3.9 shows the DSC thermograms (heating) of the PP-NX8000 compounds with
concentrations from 0wt% to 3wt%. All of the thermograms were recorded on the second heating
cycle after removal of the thermal and stress history. All specimens showed the shoulder-like
low melting plateaus before the actual crystal melting peak. The total crystallinity did not show
any changes with the addition of NX8000. The crystal melting peak temperatures did not show
any significant changes.
The crystallization peak temperatures, however, showed some changes. With the compound
containing 0.5 wt% concentration, the crystallization temperature decreased to 117.44 °C, which
is 2°C lower than the neat polymer. For PP-NX8000 compositions having higher concentrations,
the crystallization temperature increased back to 119.47°C with the 1 wt% sample, then
increased further to 121.71°C with the 3 wt% sample. Other researchers observed this kind of
behaviour (depressed crystallization temperatures at lower concentrations) in their researches
using i-PP compounded with DMBDS as a nucleating/clarifying agent. They also claimed that
DMBDS is not effective at very low concentrations [91,105-107]. However, they observed the
depression in crystallization temperatures occurred at very low concentration, around 0.1 wt% to
0.2 wt%. In their findings, the crystallization temperatures significantly increased after 0.3 wt%
content level. This does not match with the findings in this study and further investigation is
required for the NX8000 case.
With PP-NX8000 compounds, small secondary crystallization peaks were observed at higher
temperatures. These peaks possibly represent the crystallization peaks of the additive. It means
that from the homogenous solution of PP-NX8000, the additive crystallizes prior to the polymer
60
creating the fibrillar structure, without separating into two liquids. Figure 3.10 shows the DSC
thermograms (cooling) of the PP-NX8000 compounds.
3.3.2 Effect of Sorbitol-Based Nucleating Agent on Complex Viscosity
Figure 3.11 shows the complex viscosity as a function of temperature. A very distinctive initial
increase in complex viscosity was observed for all NX8000 concentrations. These jumps in
complex viscosity occurred before the crystallization process of PP since the Neat PP sample
showed a relatively linear behaviour during the whole test. Also the temperature point when the
complex viscosity experiences the sharp change was depending on the concentration of the
Figure 3. 10 DSC thermograms of PP-NX8000 compounds (Cooling)
61
additive. It was observed that the complex viscosity values for the PP-NX8000 compounds ere
30~35 times higher than that of neat PP.. This concentration-dependent increase in complex
viscosity was caused by the formation of nano-fibrillar structure during the crystallization of the
additive. Kristiansen et al. also observed the same behaviour in their studies with i-PP and
DMBDS [105]. However, further investigations using X-ray Scattering methods (WAXS, SAXS)
and Transmitting Electron Microscopy (TEM) are required to confirm the formation of nano-
fibrillar structure of NX8000 in later studies.
Figure 3. 11 Complex Viscosity as a function of temperature
62
3.3.3 HP-DSC Simulation Results
As previously mentioned, the samples were undergone isothermal treatment under pressurized
CO2 condition in the HP-DSC chamber for 40 minutes to simulate the EPP bead foaming
experiment. Only one concentration of NX8000 (0.5 wt%) was used. The main goal for this
study was to investigate the temperature range that yields double crystal melting peak under
pressurized CO2 condition. Figure 3.12 shows the DSC thermograms of PP-NX8000 compounds
after the HP-DSC bead foaming simulation. At the Ts of 135°C, No significant change in the
total crystallinity was observed. However, the crystal melting peak temperature (Tm) increased
from 150.99°C to 154.31°C. When Ts increased, the Tm also shifted to higher temperatures. The
crystal melting peak temperatures of the samples kept on increasing until when Ts of 148°C was
used. The shift in Tm can be explained with the crystal perfection phenomena mentioned in the
earlier section. At the Ts of 140°C the PP-NX8000 compound started to show the multiple crystal
melting peaks. The lower melting peak for the sample which saturated at 140°C appeared at
138.85°C and for the Ts = 143°C sample, the lower melting peak was observed at 139.27°C. It is
noticeable that when Ts of 145°C was used, three crystal melting peaks were observed. The third
melting peak was located at a significantly higher temperature, which was 163.41°C. The triple
melting peak structure disappeared when the sample was saturated at 148°C. Samples with the
saturation temperature of 143°C, 145°C and 148°C showed a decrease in total crystallinity (~3%
decrease).
The simulation with HP-DSC showed that the multiple melting peak structure generation starts
around the temperature range of 140°C to 145°C. Although the results showed that there is a
decrease in overall crystallinity, it suggested a guideline to use the specific temperature range for
the actual bead foaming trials.
63
3.3.4 EPP bead foaming results
3.3.4.1 SEM Images of Foamed Samples
Figure 3.13 shows the SEM images of the foamed samples at various Ts. The CO2 saturation
pressure and time were 55MPa and 40 minutes. All images have the same magnification (X250).
Figure 3. 12 DSC thermograms of PP-NX8000 (0.5wt%) after HP-DSC
64
Ts = 145°C
Neat PP 0.3 wt%
0.5 wt% 0.75 wt%
1 wt% 3 wt%
65
Ts = 143°C
Neat PP 0.3 wt%
0.5 wt% 0.75 wt%
1 wt% 3 wt%
X 1500
X 1500 X 1500
66
Ts = 140°C
Neat PP 0.3 wt%
0.5 wt% 0.75 wt%
1 wt% 3 wt%
X 1500
X 1500 X 1500
X 1500
X 1500
67
Ts = 137°C
Figure 3. 13 SEM images of foamed samples saturated at various Ts
Neat PP 0.3 wt%
0.5 wt% 0.75 wt%
1 wt% 3 wt%
X 2500
X 2500 X 2500
X 4000 X 4000
X 1500
68
3.3.4.2 Effect of NX8000 Content on Cell Size
It was observed from the SEM images, there were large discrepancies in the cell sizes at high
NX8000 concentrations. Cell size discrepancy is more apparent at higher saturation temperatures.
For instance, with the 3wt% samples saturated at 137°C, 140°C and 143°C showed bimodal
distribution of cell sizes. The size variation of the bubbles was because of the fibrillar structure
of NX8000. During the isothermal treatment process, crystals rearrange to have more closely
packed structure. The rearrangement of crystals occurs at energetically favourable locations;
around the fibrils of NX8000 in this case. Thus the gas molecules tend to aggregate around the
fibrils and form larger bubbles during the foaming process. However, further investigation is
required. Figure 3.14 shows the cells around the NX8000 fibrils.
Figure 3.15 shows the relationship between the NX8000 concentrations and the average cell
sizes of EPP bead foams. At lower saturation temperatures and NX8000 concentration ranges,
the cell sizes were significantly reduced. The average cell sizes tend to decrease until the
NX8000 concentration of 0.75 wt% for most of the samples. For the sample saturated at 140°C
containing 0.75wt% NX8000, the average cell size was 3.57um and this value was 8 times
smaller than the neat PP sample. It is notable that EPP foam samples with submicron bubbles
were obtained with 1 wt% NX8000 saturated at 137°C. The average cell size was 600ηm, while
the small cells have less than 200ηm in diameter.
69
Figure 3. 15 Average cell size vs. NX8000 concentration
1 wt%
135°C
3 wt%
135°C
Figure 3. 14 Cells around NX8000 fibrils
70
3.3.4.3 Effect of NX8000 Content on Cell Density
The cell densities of the EPP bead foams saturated at various Ts are shown in Figure 3.16. The
highest cell density was obtained with 1wt% concentration compound saturated at 137°C
(1.245E12 cells/cm3) which was
220 times higher than the neat PP sample. The cell density
increased until 0.75 wt% concentration and decreased after it reached the maximum point. The
graph shows that between 0.3wt% to 0.75 wt% is the sweet spot where the nucleating effect of
NX8000 is the most effective.
Figure 3. 16 Cell density vs. NX8000 concentration
71
3.3.4.4 Effect of NX8000 Content on Expansion Ratio
Figure 3.17 shows the volume expansion ratio of the foamed samples. The highest expansion
ration (VER = 26.59) was obtained with 0.3 wt% concentration sample saturated at 145°C. It
was observed that the expansion ratio was actually decreased with the addition of NX8000. This
was due to the enhanced melt strength of the PP-NX8000 compound. The fibrillar network
structure in the polymer matrix impeded the expansion of the foam during the foaming process.
Thus, the expansion ratio was much lower at higher concentration (10.34 vs. 26.56 with 3wt%
and 0.3 wt% concentration respectively, at Ts = 145°C).
The expansion ratio was increased by increasing Ts, as expected.
Figure 3. 17 Volume expansion ratio vs. NX8000 concentration
72
3.3.4.5 Crystallization Behaviour of EPP Bead Foams
Figure 3.18 shows the DSC thermograms of EPP bead foams. It is categorized into the saturation
temperature to observe the effect of NX8000 on the crystallization behaviour of bead foams.
Even though the saturation temperatures were selected and used based on the HP-DSC
simulation, the results were quite different. The DSC thermograms of bead foam samples
showed that the double crystal melting structure was not obtained in most of the samples. Only
the samples underwent high saturation temperature showed small melting peaks at significantly
lower temperatures. When Ts of 140°C was used, samples with the concentration of 0 wt%, 0.3
wt%, 0.5 wt% and 0.75 wt% showed double crystal melting peak structure. However, at higher
concentrations (1 wt% and 3 wt%) the double melting peak behaviour was not observed.
Figure 3.19 shows the crystal melting temperatures of EPP bead foams. It shows that the Tm_high
of the samples shifted to higher temperatures after the foaming process. For instance, unfoamed
0.5 wt% pallet had Tm of 149.61°C. After foaming, Tm then shifted to 151.32°C, 155.8°C and
157.49°C at the saturation temperature of 140°C, 143°C and 145°C respectively. This proves that
the crystal perfection had occurred during the foaming experiment. One of the possible reasons
that the double melting structure was not obtained even though the crystal perfection had
occurred is the crystal spherulites became too small because of the addition of NX8000. Smaller
crystal spherulites granted higher mobility during the crystal perfection process thus the lower
crystal melting peak disappeared.
Figure 3.20 shows the changes in total crystallites after foaming. The behaviour seems
disordered and the correlations could not be found. Further investigation is required
73
74
Figure 3. 18 DSC thermograms of EPP bead foams at various Ts
75
Figure 3. 19 Crystal melting peaks of EPP bead foams
Figure 3. 20 Total crystallinity of EPP bead foams
76
Chapter 4
Extrusion Foaming of Polypropylene-Sorbitol Based Nucleating Agent Compound
4.1 Introduction
The extrusion foaming process of semi-crystalline polymer is much more complicated than the
batch foaming process. Recalling the process chain for an extrusion foaming process, it usually
goes through 5 main steps: polymer melting, injection of PBA, formation of the homogenous
solution of the PBA and polymer, cell nucleation due to a sudden thermodynamic instability
(usually caused by a rapid drop of the pressure at a die) and cell growth. The main reason what
makes the extrusion process is more complex than the batch forming process is; it has more
variables than the batch foaming process. Each independent variable (die temperature, melting
temperature, CO2 concentration, screw rpm, additive concentration, etc.) can influence the
dependent variables (die pressure, material flow rate, etc.) significantly, where the dependent
variables govern the characteristics of finished products. One of the distinctive differences
between the extrusion foaming process and the batch foaming process is the polymer melt
experiences the extensive shear force from the rotational motion of the screw. The shear force in
fact, can enhance the crystallization kinetics of the polymer [108-111]. Alireza et al. developed
an in-situ visualization system for a tandem-line extruder to visualize the crystallization process
of PP flowing through the extruder. They verified that after the complete melting of polymer in
the first extruder, the crystallization process can be induced by controlling the processing
temperatures of the second extruder [112].
77
Since manipulating the crystallization process of molten polymer by controlling the processing
conditions during the extrusion process is complicated, it arouse interest of the development of a
new extrusion foaming system which can preserve the crystalline structure of the semi-
crystalline polymer during the whole process. Preserving the crystalline structure of the polymer
would be beneficial for producing high-cell density foams, since the crystals provide bubble
nucleation sites. The crystalline structure of polymer pallets can be tailored by pre-processing.
For example, polymer pallets can be foamed prior to the extrusion foaming process using
expanded bead foaming technology. It will allow beads to have the double crystal melting peak
structure with exceptionally high cell densities. Then the processing temperature of the extrusion
foaming process can be set in the lower crystal melting peak range to sinter the beads and
preserve the high-melting point crystals. This special type of extrusion process is called solid-
state extrusion. It is mainly developed for shape forming and structural modification of highly
oriented polymer materials by utilizing the polymer deformation just below the melting
temperature of the polymer [113-115].
The development of such system requires an extensive knowledge about the extrusion systems
and the foaming behaviour of semi-crystalline polymer during the extrusion process.
4.1.1 Hypothesis
In the previous study in chapter 3, using the supramolecular nucleating agent was proved to
improve the overall cell density and cell structure. Especially, at the foaming temperature of
127°C with 1 wt% concentration the cell density increased to 1.245E12 cells/cm3. A large
number of small crystals caused by the network structure of NX8000 will enhance the
78
foamability and cell density. Similar foaming behaviour is expected with the extrusion foaming
process.
Figure 4.1 recalls the measurements of the complex viscosities of PP-NX8000 compounds using
the temperature sweep method. The red line on the bottom of the graph shows the temperature
range where the actual foaming starts. As shown in the graph, the complex viscosity of the PP-
NX8000 compounds is 30~35 times higher than that of the neat PP. The increased viscosities
around the foaming temperature due to the unique network structure of the sorbitol derivative
will enhance the melt strength of the polymer and improve the overall cell morphology.
This chapter includes the method to produce PP foams using a lab-scale tandem-line extruder.
Three different concentrations of sorbitol based nucleating agent are used to study the effect of
supramolecular nucleating agent on the foaming behaviour. The effects of processing
temperature on the barrel back pressure and cell morphology were investigated too. The results
from this study will be used as a guideline for the development of the solid state extrusion
system for nanocellular foaming in later studies.
Figure 4. 1 Temperature dependency of complex viscosities of
PP-NX8000 compounds
Foaming Starts
79
4.2 Experimental
4.2.1 Material
A random PP co-polymer (RA12MN40) from Saudi Basic Industries Corporation (SABIC) was
used. The melt flow rate of the polymer is 40g/10mins (tested method: ASTM D1238 230°C and
2.16kg) and the density is 0.905g/cm3
(test method: ASTM D792) respectively. Millard NX8000
(Bis(4-Propylbenzylidene)Propyl Sorbitol), a sorbitol based nucleating agent from Milliken
Chemical was used as a nucleating agent in this study. Three concentrations (0.5 wt%, 0.75 wt%
and 1 wt%) were compounded using a twin screw extruder and used in this study.
CO2 with 99.5% purity was provided from The Linde Group and used as received.
4.2.2 Experimental Set-up and Procedure
Figure 4.2 shows the schematic drawing of the tandem line extruder used in this study. The
system is built with two single-screw extruders: the first one is from Brabender 3/4’’ system with
5-hp motor, for melting and mixing of the polymer and PBA. The second one is a 1½’’extruder
from Killion, (Killion KN-150) with a built-in 15 hp drive unit for mixing and cooling.
Table 4.1 shows the sample processing temperatures. The processing temperature profile was set
carefully to preserve the fibril network structure of NX8000. The melt temperatures of the first
extruder were set below the melting temperature of NX 8000, which is 230°C. Since the
temperatures were well below the melting temperature of NX8000, the fibril network structure of
NX8000 was not harmed during the whole process. The temperature profile of the second
extruder was set for cooling purposes. Various die temperatures were tested during the
80
experiment. The die temperature was cooled down by 3°C from 140°C to 115°C. Samples were
collected at each die temperature after waiting 10~15 minutes when the system was fully
stabilized (no fluctuation in temperature or pressure).
Figure 4.3 shows the schematic drawing of the filamentary die used in this study. Prior to the
study, a couple of dies were tested with Neat PP RA12MN40 to check the pressure drop during
foaming. The die with the L/D ratio of 36 (Length = 0.76’’ / Diameter = 0.021’’) was used in
this study.
To inject the precise amount of CO2 during the process, a positive displacement syringe pump
was employed. The flow rate of the polymer was recorded when they system is stabilized after
the die temperature change. In most cases at a fixed RPM rate (10.1RPM on the first extruder
and 3 RPM on the second one) the material flow rate was typically 11g/min. after measuring the
material flow rate, the CO2 flow rate was changed in accordingly to match the desired CO2
concentration.
Total numbers of 12 experiments were conducted in this study. Table 4.2 shows the NX8000
concentration and CO2 concentration used for each experiment.
81
Extruder #1
(RPM: 10.1)
Section T5 T4 T3 T2 T1
Temperature
(°C) 180 190 190 170 120
Extruder #2
(RPM: 3)
Section Die T4 T3 T2 T1
Temperature
(°C)
140~
115
140 145 150 155
Table 4. 1 Processing conditions
Material flow direction
Material flow direction
Figure 4. 2 Schematic drawing of tandem line extruder
82
7 wt% CO2 9 wt% CO2 11 wt% CO2
Neat PP
EXP #1 EXP #2 EXP #3
0.5 wt%
NX 8000
EXP #4 EXP #5 EXP #6
0.75 wt%
NX 8000
EXP #7 EXP #8 EXP #9
1 wt%
NX 8000
EXP #10 EXP #11 EXP #12
Table 4. 2 NX8000 and CO2 Concentrations used for each experiment
Figure 4. 3 Schematic drawing of filamentary die
83
4.3 Results and Discussion
4.3.1 Effect of NX 8000 Content on Die Pressure
The system pressure or the back pressure of the extrusion foaming system is very important
because it governs the bubble nucleation caused by the thermal instability initiated by a rapid
pressure drop at a die exit. Figures through 4.5 (a) to (c) shows the relationship between the
system pressure and the die temperature. The graphs show that the compound with 1 wt%
concentration of NX8000 showed the highest processing pressure in all temperature ranges. Both
compounds with 0.75 wt% and 1 wt% of NX8000 showed higher system pressure in all
temperature ranges than the neat PP. This was due to the increased complex viscosity due to the
addition of NX8000. Recalling the complex viscosity graph shown in Figure 4.1, the complex
viscosities of the 0.5 wt% and 1 wt% compounds jumps at 157°C and 170°C respectively. The
foaming practices in this study were conducted well below those temperature ranges where the
compounds experience the jumps in complex viscosity. However, for the 0.5wt % NX8000
compound showed lower system pressure except at the temperature ranges between 135°C and
130°C. The reason behind this is unknown at this point of study and requires further
investigation. With higher NX8000 concentrations the foamability of PP went down, meaning
that the foaming was only could be done in narrower range of die temperature. For example, the
system pressure went too high with 0.75 wt% and 1 wt% samples after the die temperature of
120°C and the system had to be shut down. It was also observed that with higher CO2
concentrations, the system pressure went down for all NX8000 compounds. It was due to the
plasticizing effect of the CO2 .
84
(a)
(b)
85
4.3.2 SEM Images of the Foamed Samples
The tables from Table 4.3 to Table 4.8 contain the SEM images of the collected samples from
the extrusion foaming experiments. All images have the same magnification (250X). The tables
were categorized into the CO2 concentration used during the study. Table 4.3 and 4.4 show the
samples processed with 7 wt% CO2, the 9 wt% CO2 samples are shown in Table 4.5 and 4.6 and
lastly the samples processed with 11 wt% CO2 are shown in Table 4.7 and 4.8. The SEM images
for the processing temperatures above 135°C and below 120°C have omitted in this report
because of the space constraint.
Figure 4. 4 Pressure vs. Temperature PP-NX8000 compounds (0 wt%, 0.5 wt%, 0.75 wt%
and 1 wt% NX 8000 concentrations) processed with (a) 7 wt% CO2 (b) 9 wt% CO2 (c) 11
wt% CO2
(c)
86
7 wt% CO2
Tdie = 135°C Tdie = 130°C
Neat PP
0.5 wt%
0.75 wt%
1 wt%
Table 4. 3 SEM images of 7 wt% CO2 Samples Die temperature from 135°C to 130°C
87
7 wt% CO2
Tdie =125°C Tdie = 120°C
Neat PP
0.5 wt%
0.75 wt%
1 wt%
Table 4. 4 SEM images of 7 wt% CO2 Samples Die temperature from 125°C to 120°C
88
9 wt% CO2
Tdie = 135°C Tdie = 130°C
Neat PP
0.5 wt%
0.75 wt%
1 wt%
Table 4. 5 SEM images of 9 wt% CO2 Samples Die temperature from 135°C to 130°C
89
9 wt% CO2
Tdie =125°C Tdie = 120°C
Neat PP
0.5 wt%
0.75 wt%
1 wt%
Table 4. 6 SEM images of 9 wt% CO2 Samples Die temperature from 125°C to 120°C
90
11 wt% CO2
Tdie = 135°C Tdie = 130°C
Neat PP
0.5 wt%
0.75 wt%
1 wt%
Table 4. 7 SEM images of 11 wt% CO2 Samples Die temperature from 135°C to 130°C
91
11 wt% CO2
Tdie =125°C Tdie = 120°C
Neat PP
0.5 wt%
0.75 wt%
1 wt%
Table 4. 8 SEM images of 11 wt% CO2 Samples Die temperature from 125°C to 120°C
92
4.3.3 Effect of NX 8000 Content on Volume Expansion Ratio
Figure 4.5 shows the expansion ratios of the samples collected at each die temperature. When the
CO2 concentration of 7 wt% was used, the maximum volume expansion ratio (VER) was
achieved with the Neat RA12MN40 PP. The compounds containing NX8000 showed lower VER
at all temperature range. This behaviour can be explained with the increased melt viscosity of the
PP-NX8000 compound. The increased melt viscosities of the PP-NX8000 compounds result in
increment in melt strength. The increased melt strength of the PP-NX8000 compound hindered
the expansion of the extrudate and the neat PP resin showed the highest expansion ratio. When
the CO2 concentration was increased to 9 wt%, all resins showed similar values. This is because
of the plasticization effect of dissolved CO2 and the melt strength of the compound balanced out.
The plasticization effect will soften the polymer extrudate and the extrudate will have lower
VER as a result. The neat PP samples foamed with 9 wt% CO2 concentration for instance, the
VER at lower die temperature range were significantly lower than the samples foamed with 7 wt%
CO2. However, the PP-NX8000 compounds showed little increment in the VER values, showing
that the increased melt strength caused by the addition of NX8000 predominantly affected the
VER. This behaviour is more apparent with the 11 wt% CO2 samples. For all the samples except
the 1 wt% of NX8000 showed similar VER in all foaming temperature range. However, the 1 wt%
NX8000 compound showed significantly higher VER where at the die temperature of 120°C, it
showed the highest VER of 10.685. This proves that the increased melt strength had a
predominant effect and helped to retain the cell structure. The SEM images of the samples (11 wt%
CO2 with 1wt% NX8000 foamed at 125°C and 120°C) in Table 4.8 show that the cell structures
were well preserved without cell ripening or collapsing.
93
(a)
(b)
94
4.3.4 Effect of NX8000 Content on Cell Density
Figure 4.6 shows the cell density versus die temperature graphs. The highest cell density
(1.79E09 cells/cm3) was achieved with 0.75 wt% NX8000 compound using 9 wt% CO2
processed at the die temperature of 120°C. For 7 wt% and 9 wt% CO2 concentrations, the 1 wt%
NX8000 compound showed higher cell density than the other compounds. However, there was
no significant trend or effect of the nucleating agent observed. During the extrusion foaming
process, the polymer melt experiences high shear force from the rotating motion of the screw.
The shear force may have disturbed the fibrillar structure of NX8000 thus the bubble nucleating
effect was less effective than the EPP foaming case. Also the cell densities of the samples were
significantly lower than the EPP bead foaming samples.
(c)
Figure 4. 5 Volume Expansion Ratio (VER) vs. Temperature PP-NX8000 compounds (0 wt%, 0.5
wt%, 0.75 wt% and 1 wt% NX 8000 concentrations) processed with (a) 7 wt% CO2 (b) 9 wt%
CO2 (c) 11 wt% CO2
95
(a)
(b)
96
4.3.5 Effect of NX 8000 Content on Average Cell Size
Figure 4.7 (a) to (c) show the graphs of average cell size versus die temperature. The average cell
sizes of the samples vary from 10μm to 30μm. The smallest cell size was obtained with 1 wt%
NX8000 concentration sample processed with 7 wt% CO2 and the die temperature of 135°C. In
most cases, the 1 wt% NX8000 samples showed smaller cell sizes over the foaming temperature
range. However, no particular correlation between the NX8000 concentration and cell size was
observed. This observation was opposite from the hypothesis. In the hypothesis, it was believed
that the addition of sorbitol based nucleating agent would help to increase the cell density and
decrease the cell size. However, with the extrusion foaming practice, the sorbitol based
nucleating agent did not improve both cell density and cell size.
(c)
Figure 4. 6 Cell Density vs. Temperature PP-NX8000 compounds (0 wt%, 0.5 wt%, 0.75 wt%
and 1 wt% NX 8000 concentrations) processed with (a) 7 wt% CO2 (b) 9 wt% CO2 (c) 11 wt%
CO2
97
(a)
(b)
98
(c)
Figure 4. 7 Average cell size vs. Temperature PP-NX8000 compounds (0 wt%, 0.5 wt%,
0.75 wt% and 1 wt% NX 8000 concentrations) processed with (a) 7 wt% CO2 (b) 9 wt%
CO2 (c) 11 wt% CO2
99
Chapter 5
Conclusion & Recommendation
5.1 Summary and Conclusion
In this study, the effect of sorbitol based nucleating agent on EPP bead foaming and PP extrusion
foaming was investigated using the latest generation of sorbitol based nucleating agent, Millard
NX8000 (Bis(4-Propylbenzylidene)Propyl Sorbitol. NX8000, developed by Milliken Chemical
was originally designed to improve the clarity of PP products. However, the three dimensional
nano-fibrillar structure formation of NX8000 and its unique crystal nucleating effect was focused
in this study. The main objective of this study was to prepare foams having sub-micron sized
cells with high cell density. The foaming practices conducted in this study suggested a guideline
in development of a system that can produce nanocellular plastics in continuous fashion.
The foaming practices of PP-NX8000 compounds were first done using batch foaming system;
then the tandem-line extrusion foaming system was used. Prior to the batch foaming practice, the
HP-DSC EPP simulation was conducted to determine the suitable processing temperature for
double crystal melting peak generation. It was found that the most adequate processing
temperature range was in between 137°C to 145°C. The batch foaming process was done with
the lab-scale autoclave-based EPP foaming chamber. The operating parameters such as propeller
RPM, PBA saturation pressure and the amount of pallets used were based on the previous studies
done by other researchers.
100
During the EPP bead foaming experiments the cell densities of PP-NX8000 compounds had
significant improvement on cell densities. The highest cell density (1.245E12 cells/cm3) was
obtained with the 1 wt% NX8000 processed at 137°C. Also, by adding NX8000, the average
cell size was significantly reduced. Again, with the 1 wt% NX8000 sample processed at 137°C,
it showed the lowest average cell size (600ηm). The results showed that the nano-fibrillar
structure of NX8000 in the polymer matrix triggered heterogeneous nucleation along the fibrils,
leaving abundant small sized crystals. This ample amount of small sized crystals acted as bubble
nucleation sites during the bubble nucleation stage, resulting high cell density with small cells.
The heterogeneous nucleation along the NX8000 fibrils was shown with the SEM images of the
samples having high NX8000 concentration. Larger bubbles were foamed alongside the web-like
structure of NX8000. The expansion ratio however, was decreased by the addition of sorbitol
based nucleating agent. This was believed because of the increased melt strength of the PP-
NX8000 compounds caused by the fibrillar structure of NX8000 in the polymer matrix impeded
expansion during the foaming stage. The double crystal melting peak structure was also observed
in EPP bead foams processed at the saturation temperature of 145°C. However, the double
melting peak structure of EPP bead foams were not distinctive and showed shoulder-like peaks
in most cases.
Foaming experiments with an extrusion foaming process on the 0.75’’ – 1.5’’ tandem line
extrusion system were also conducted. PP foams having the average cell size of 20 μm with high
cell density over 109cells/cm
3 were achieved. However, there was no particular improvement in
cell densities and average cell sizes with the addition of NX8000. It’s because of the high shear
force induced by the rotating motion of the screw during the extrusion process may have
destroyed or altered the network structure of the NX8000.
101
5.2 Recommendations
The following suggestions can be made for further development of the continuous PP
nanocellular foaming system.
1) The phenomena of creation of nano-sized crystals induced by the addition of the sorbitol
based nucleating agent should be verified with in-situ visualization system.
2) The study will be used as a guideline for the development of solid-state nanocellular
extrusion foaming system. The steps for producing PP nanocellular foams in continuous
fashion can be listed as the following:
A. Compound PP with the sorbitol based nucleating agent. The previous study suggests
the optimal concentration is in between 0.5 wt% to 1 wt%.
B. Pre-process the compounds to have double crystal melting peak structure. This can
be done with the autoclave based bead foaming chamber with or without using a
PBA.
C. Design a solid-state extrusion foaming system. This step requires extensive research
on the screw design, extrusion foaming process, and crystallization kinetics during
the extrusion foaming process.
D. Set all the processing temperature of the system at or just slightly above the lower
melting crystal melting peak of the compound. This will allow the polymer to melt
partially, leaving some desirable nano-sized crystals within the plastics in the
extrusion barrel.
102
E. Because the polymer is not completely molten, it has to be plasticized in order to
flow. This can be done by using a polymer plasticizer.
F. A new design of screw is required. During the extrusion foaming study, the fibrillar
structure of NX8000 was damaged. To preserve the network structure, the screw has
to be specially designed to accommodate the partial melting of the polymer beads
while maintaining a desirable flow rate.
G. The first prototype of the screw will have no mixing or metering section and large
channel depth.
103
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