CONTINUOUS MICROCELLULAR FOAMING OF POLYLACTIC
ACID/NATURAL FIBER COMPOSITES
By
Carlos A. Diaz-Acosta

A DISSERTATION
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
DOCTOR OF PHILOSOPHY
Packaging
2011

ABSTRACT
CONTINUOUS MICROCELLULAR FOAMING OF POLYLACTIC
ACID/NATURAL FIBER COMPOSITES
By
Carlos A. Diaz-Acosta
Poly(lactic acid) (PLA), a biodegradable thermoplastic derived from
renewable resources, stands out as a substitute to petroleum-based plastics.

In spite

of its excellent properties, commercial applications are limited because PLA is more
expensive and more brittle than traditional petroleum-based resins.

PLA can be

blended with cellulosic fibers to reduce material cost. However, the lowered cost
comes at the expense of flexibility and impact strength, which can be enhanced
through the production of microcellular structures in the composite.
Microcellular foaming uses inert gases (e.g., carbon dioxide) as physical
blowing agents to make cellular structures with bubble sizes of less than 10 µm and
9

cell-population densities (number of bubbles per unit volume) greater than 10
3

cells/cm .

These unique characteristics result in a significant increase in toughness

and elongation at break (ductility) compared with unfoamed parts because the
presence of small bubbles can blunt the crack-tips increasing the energy needed to
propagate the crack.
batch process.

Microcellular foams have been produced through a two step

First, large amounts of gas are dissolved in the solid plastic under

high pressure (sorption process) to form a single-phase solution.

Second, a

thermodynamic instability (sudden drop in solubility) triggers cell nucleation and
growth as the gas diffuses out of the plastic.

Batch production of microcellular PLA

has addressed some of the drawbacks of PLA.

Unfortunately, the batch foaming

process is not likely to be implemented in the industrial production of foams because
it is not cost-effective.
This study investigated the continuous microcellular foaming process of PLA
and PLA/wood-fiber composites.

The effects of the processing temperature and

material compositions on the melt viscosity, pressure drop rate, and cell-population
density were examined in order to understand the nucleation mechanisms in neat and
filled PLA foams.

The results indicated that the processing temperature had a strong

effect of the rheology of the melt and cell morphology.

Processing at a lower

temperature significantly increased the cell nucleation rate of neat PLA (amorphous
and semi-crystalline) because of the fact that a high melt viscosity induced a high
pressure drop rate in the polymer/gas solution.

The presence of nanoclay did not

affect the homogeneous nucleation but increased the heterogeneous nucleation,
allowing both nucleation mechanisms to occur during the foaming process.
The effect of wood-flour (0-30 wt.%) and rheology modifier contents on the
melt viscosity and cell morphology of microcellular foamed composites was
investigated.

The viscosity of the melt increased with wood-flour content and

decreased with rheology modifier content, affecting the processing conditions (i.e.,
pressure drop and pressure drop rate) and foamability of the composites.

Matching

the viscosity of the composites with that of neat PLA resulted in the best cell
morphologies.
Physico-mechanical characterization of microcellular foamed PLA as a
function of cell morphology was performed to establish process-morphology-property
relationships.

The processing variables, i.e., amount of gas injected, flow rate, and

processing temperature affected the development of the cellular structure and
mechanical properties of the foams.

Dedicated to my parents
Aurelio Diaz and Carmen J. Acosta

iv

ACKNOWLEDGEMENTS

First, I would like to thank my advisor, Dr. Laurent Matuana, for his mentoring,
guidance, and support throughout the program. He gave me the opportunity to think
about the idea of a doctorate and push me to always higher standards. I acknowledge my
guidance committee, Dr. Maria Rubino, Dr. Pascal Nzokou, and Dr. Rafael Auras, for
their interest and valuable input in my dissertation. They were always there for advice or
a word of encouragement. I extent my gratitute to the people at the Center for Advanced
Microscopy, Dr. Stanley Flegler, Carol Flegler, Dr. Alicia Pastor, Ewa Danielewicz,
Abigail Tirrell, for their help, counsel and assistance. I want to acknowledge the
involvement of the Physical Plant Division, especially Mr. Dennis Lehman, in the setting
up and maintaining of equipment. I am thankful to the Department of Forestry, their
faculty and staff, specially: Juli Kerr, Carol Graysmith, Barb Anderson, Sue Plesko, and
Karen Ellerhorst. I extend my heartfelt gratitude to the School of Packaging, their faculty
and staff, specially the director Dr. Joseph Hotchkiss, Dr. Susan Selke, Kelby Thayer,
Colleen Wager, April Meersdom, and Sherrie Lenneman. I am also grateful to my fellow
grad students and colleagues, Dr. Omar Faruk, Dr. Shan Jin, Kojo Afrifah, the under
graduates who worked with us Deja Torrence, Thomas McKay, Anthony Weatherspoon.
I would like to acknowledge the financial support from the National Research Initiative
of the USDA Cooperative State Research, Education and Extension Service (Grant 200635504-17414).

Finally, I would like to recognize my family and friends for their

unconditional love and support.

v

TABLE OF CONTENTS

LIST OF TABLES ............................................................................................................. ix
LIST OF FIGURES .............................................................................................................x
CHAPTER 1
Introduction ..........................................................................................................................1
1.1 Introduction ........................................................................................................2
1.2 Objectives ........................................................................................................13
1.3 Hypothesis........................................................................................................14
1.4 Structure of dissertation ...................................................................................14
References ..............................................................................................................15
CHAPTER 2
Background and Literature Review ...................................................................................20
2.1 Thermoplastic foams ........................................................................................21
2.1.1 Cellular structure and foam properties ..................................21
2.1.2 Foaming agents .....................................................................22
2.1.2.1 Chemical foaming agents .......................................22
2.1.2.2 Physical foaming agents ........................................23
2.1.3 Formation of a cellular structure ...........................................24
2.1.3.1 Dissolution of the blowing agent in the polymer ...25
2.1.3.2 Cell nucleation .......................................................26
2.1.3.3 Bubble growth and stabilization ............................27
2.1.4 Microcellular plastics ............................................................28
2.1.5 Solid state microcellular foaming (the batch process) ..........30
2.1.5.1 Formation of a single-phase solution .....................32
2.1.5.2 Nucleation dynamics ..............................................34
2.1.6 Continuous microcellular extrusion process .........................36
2.1.6.1 Formation of a single-phase solution .....................39
2.1.6.2 Nucleation dynamics ..............................................40
2.2 Polylactic acid (PLA) .......................................................................................42
2.2.1 Structure and synthesis .........................................................42
2.2.2 Properties of PLA .................................................................46
2.2.2.1 Thermal properties and crystallinity ......................46
2.2.2.2 Rheological properties ...........................................47
2.2.2.3 Mechanical properties ............................................49
2.2.2.4 Barrier and sorption of CO2 into PLA ...................50
2.2.3 Property modification of PLA ..............................................51
2.2.3.1 Copolymerization ...................................................52
2.2.3.2 Plasticization ..........................................................53
2.2.3.3 Blending with other polymers ................................54
2.2.3.4 Fiber reinforcing ....................................................56
vi

2.2.3.5 Nanocomposites .....................................................58
2.2.4 Microcellular foaming of PLA..............................................60
2.2.4.1 Batch foaming of PLA ...........................................60
2.2.4.2 Continuous microcellular extrusion foaming of
PLA ....................................................................................64
2.2.4.3 Injection molded microcellular PLA .....................68
2.2.4.5 PLA foamed with chemical foaming agents ..........69
References ..............................................................................................................71
CHAPTER 3
Study of Cell Nucleation in Microcellular Poly(lactic acid) Foamed with Supercritical
CO2 through a Continuous-Extrusion Process ..................................................................82
3.1 Abstract ............................................................................................................83
3.2 Introduction ......................................................................................................84
3.2.1 Background on Cell Nucleation ........................................................88
3.3 Experimental ....................................................................................................92
3.3.1 Materials ...............................................................................92
3.3.2 Rheological Measurements ...................................................94
3.3.3 Compounding of Materials ...................................................96
3.3.4 Extrusion Foaming ................................................................97
3.3.4 Scanning Electron Microscopy (SEM) ...............................100
3.4 Results and Discussion ..................................................................................102
3.4.1 Rheological Characteristics of PLA....................................102
3.4.2 Effect of the Temperature Profile on the Morphology .......111
3.4.3 Effect of the Nucleating Agent ...........................................116
3.5 Conclusions ....................................................................................................120
3.6 Acknowledgements ........................................................................................122
References ............................................................................................................123
CHAPTER 4
Rheology Modification Approach to Continuous Microcellular Foaming of Polylactic
Acid/Wood-Flour Composites .........................................................................................128
4.1 Abstract ..........................................................................................................129
4.2 Introduction ....................................................................................................130
4.3 Experimental ..................................................................................................133
4.3.1 Materials .............................................................................133
4.3.2 Rheological Measurements .................................................133
4.3.2.1 Extensional viscosity ...........................................134
4.3.3 Compounding of Materials .................................................136
4.3.4 Extrusion Foaming ..............................................................136
4.3.5 Density measurements ........................................................139
4.3.6 Scanning Electron Microscopy ...........................................139
4.4 Results and Discussion ..................................................................................141
4.4.1 Rheological Characterization ..............................................141
4.4.2 Effect of rheology modifier content on cell morphology ...151
4.4.3 Effect of wood-flour content on cell morphology ..............156
vii

4.5 Conclusions ....................................................................................................164
4.6 Acknowledgements ........................................................................................166
4.7 References ......................................................................................................167
CHAPTER 5
Process-morphology-property relationships of microcellular poly(lactic-acid) produced
through a continuous extrusion process ...........................................................................172
5.1 Abstract ..........................................................................................................173
5.2 Introduction ....................................................................................................174
5.3 Experimental ..................................................................................................177
5.3.1 Materials .............................................................................177
5.3.2 Extrusion Foaming ..............................................................177
5.3.3 Tensile test ..........................................................................181
5.3.4 Taber stiffness test ..............................................................181
5.3.5 Density measurements ........................................................182
5.3.6 Scanning Electron Microscopy ...........................................182
5.4 Results and Discussion ..................................................................................184
5.4.1 Cell morphology analysis ...................................................184
5.4.2 Modulus of elasticity vs. relative density ...........................187
5.4.3 Tensile strength vs. relative density ....................................192
5.5 Conclusions ....................................................................................................198
References ............................................................................................................199
CHAPTER 6
Conclusions and Future Work .........................................................................................203
6.1 Conclusions ....................................................................................................204
6.2 Future work ....................................................................................................206

viii

LIST OF TABLES

Table 3.1.

Characteristics of different PLA resins used in this study .........................93

Table 3.2.

Definition of temperature profiles according to the set temperature on each
heating zone shown in Figure 3.1 ..............................................................99

Table 3.3.

Melt viscosity coefficient (K) and power law flow index (n) for
semicrystalline PLA (2002D) measured at various temperatures............104

Table 3.4.

Activation energy (Ea) and the A0 constant for viscous flow of
semicrystalline PLA (2002D) at various shear rates................................108

Table 3.5.

Effect of the processing temperature on the pressure drop rate for different
grades of PLA ..........................................................................................110

Table 3.6.

Pressure drop rate, cell-population density, and average cell size of
foamed PLA samples ...............................................................................113

Table 4.1.

Linear relationships of the effect of rheology modifier content on MFI for
each wood-flour content ..........................................................................144

Table 4.2.

Melt viscosity coefficient (K) and power law flow index (n) for neat PLA,
PLA with 20 wt. % wood-flour, and PLA with 20 wt. % wood-flour and 2
wt. % rheology modifier ..........................................................................147

Table 4.3.

Effect of rheology modifier content on cell-population density and cell
size ...........................................................................................................154

Table 4.4.

Effect of wood-flour content on cell-population density and cell size of
foamed samples ........................................................................................159

Table 5.1.

List of the processing conditions used to produce foamed PLA samples180

ix

LIST OF FIGURES

Figure 1.1.

Schematic of a batch microcellular foaming process. (a) Sample saturation
with CO2 (sorption). (b) Sample foaming (diffusion) .................................6

Figure 2.1.

Schematic of a batch microcellular foaming process. (a) Sample saturation
with CO2 (sorption). (b) Sample foaming (diffusion) ...............................31

Figure 2.2.

Typical equilibrium gas concentrations in polymers in function of
temperature and pressure ...........................................................................33

Figure 2.3.

Schematics of a continuous microcellular extrusion process ....................38

Figure 2.4.

Stereochemistry of lactic acid and lactide .................................................43

Figure 2.5.

Schematic of PLA production via prepolymer and lactide ........................45

Figure 3.1.

Schematic of the single-screw extrusion foaming system .........................98

Figure 3.2.

True viscosity of semicrystalline PLA (2002D) as a function of the true
shear rate at various temperatures (160, 175, 190, and 205 °C) ..............103

Figure 3.3.

True viscosity of semicrystalline PLA (2002D) as a function of the
reciprocal temperatures (1/T) at various shear rates (200, 300, 500, and
−1
900 s ) ....................................................................................................107

Figure 3.4.

SEM micrographs showing the effects of the processing temperature
profile levels and PLA grade on the cell morphology for amorphous PLA
processed at high (a1), intermediate (a2), and low (a3) temperatures as
well as for semicrystalline PLA processed at high (b1), intermediate (b2),
and low (b3) temperatures. All pictures were taken at the same
magnification (750×) ...............................................................................112

Figure 3.5.

Field-emission SEM micrographs (JEOL JSM 7500F) illustrating the
effect of nanoclay addition on the cell morphology of semicrystalline PLA
foams: (a and b) without nanoclay; (c and d) with nanoclay. Micrographs a
and c were taken at low magnification (750×), whereas micrographs b and
d were taken at high magnification (5000×) ............................................117

Figure 4.1.

Schematic of the single-screw extrusion foaming system .......................138

x

Figure 4.2.

Effect of wood-flour and rheology modifier contents on the MFI of PLA
2002D at 210°C. The dashed line represents the MFI of neat PLA
2002D .......................................................................................................142

Figure 4.3.

True viscosity vs. true shear rate of PLA, PLA with 20 wt. % wood-flour,
and PLA with 20 wt. % wood-flour and 2 wt. % rheology modifier .......146

Figure 4.4.

Extensional viscosity measured by (a) on-line capillary rheometry, and (b)
ARES-EVF rheometer for PLA, PLA with 20 wt. % wood-flour, and PLA
with 20 wt. % wood-flour and 2 wt. % rheology modifier ......................150

Figure 4.5.

Effect of rheology modifier content on cell morphology of foamed PLA
samples containing 20 wt.% wood-flour. (a) 2 wt.%, (b) 4 wt.%, (c) 6
wt.%. Pictures originally taken at 350X ..................................................153

Figure 4.6.

Effect of rheology modifier content on void fraction of foamed PLA
samples containing 20 wt.% wood-flour and varying amounts of rheology
modifier ....................................................................................................155

Figure 4.7.

Effect of wood-flour content on cell morphology of foamed PLA samples.
(a) 0% wood-flour, (b) 10% wood-flour, (c) 20% wood-flour, (d) 30%
wood-flour. Pictures originally taken at 350X .........................................157

Figure 4.8.

Effect of wood-flour content on void fraction fraction of foamed
PLA/wood-flour composites samples ......................................................161

Figure 4.9.

Effect of wood-flour content on cell size distribution of foamed
samples .....................................................................................................162

Figure 5.1.

Schematic of the single-screw extrusion foaming system .......................179

Figure 5.2.

Morphology of foamed samples corresponding to the different foaming
conditions in Table 5.2 .............................................................................185

Figure 5.3.

Cell-population density and cell size as a function of void fraction for
foamed PLA samples. Solid diamonds represent the cell-population
density and the clear squares represent the cell size ................................186

Figure 5.4.

Normalized modulus versus relative density of foamed PLA samples.
Solid lines show the prediction from different models: Red: constant
macro-stress, Green: square power-law, Blue: Gibson-Ashby ................189

Figure 5.5.

Normalized flexural modulus versus (a) relative density and (b)
normalized tensile modulus of foamed PLA samples..............................191

xi

Figure 5.6.

Normalized tensile strength versus relative density of foamed PLA
samples. Solid lines show the prediction from different models: Red:
Nicolais-Narkis, Blue: Gibson-Ashby .....................................................193

Figure 5.7.

Optical microscope image of the foamed filaments shape along the
length........................................................................................................195

Figure 5.8.

Energy and strain at break plotted against relative density for solid and
foamed PLA samples ...............................................................................197

xii

CHAPTER 1

Introduction

1

1.1 Introduction
The consumption of plastics per capita in the US has been estimated to reach
more than 300 pounds a year by 2010 [1]. Most polymeric materials are currently based
on non-renewable fossil resources [2]. As the petrochemical sources are depleting and
the demand is forecasted to continue to grow in the foreseeable future, prices are
expected to increase [3, 4]. Also, main petroleum suppliers are countries with unstable
political situations, which affects the price of crude oil. At the same time, environmental
concerns such as contribution to global warming and disposal of traditional plastics call
for sustainable alternatives. Recognizing these issues, research and industrial efforts
have developed plant-based bioplastics such as polylactic acid (PLA).

In general,

bioplastics refers to biodegradable plastics whose components are derived entirely from
renewable raw materials [1]. Potential exist for bioplastics to replace traditional plastics,
thus reducing our dependence on petroleum resources.
Polylactic acid (PLA) is a thermoplastic made from the polymerization of lactic
acid derived by fermentation of sugars from corn, sugarcane, tapioca, etc. This bioplastic
exhibits physical properties close to those of many synthetic plastics in addition to being
completely biodegradable to carbon dioxide, water, and humus-like matter [5].

In

addition, PLA has excellent mechanical properties and can be processed using existing
machinery with only minor adjustments [5].
Currently, there is no commercial production of PLA that is price competitive
with petroleum-based counterparts [5]. Moreover, most biodegradable plastics currently
on the market are between two and ten times more expensive than traditional plastics [5].
Despite PLA’s many properties that are equivalent to or better than many petroleum2

based plastics, there have been few commercial applications mainly in the textile industry
and in biomedical field for orthopedic surgery [5]. High brittleness and low impact
resistance of PLA are major issues for its commercialization in applications such as
packaging.
The packaging industry is one of the potential markets for PLA. However, brittle
polymers break in a thermoforming production line, which is not tolerable. To broaden
its applications, PLA is often blended with various additives such as lubricants,
plasticizers, or a second polymer [6-8]. Plasticizers such as low molecular weight esters,
oligomeric lactic acid, poly(ethylene glycol), etc., have been investigated to obtain PLA
with higher flexibility and higher impact resistance [7, 8]. Significant improvement in
flexibility (up to 180% elongation at break compared to 4% for neat PLA) and impact
resistance (over 200% increase) was obtained with the use of up to 10 wt.% low
molecular weight poly(ethy1ene-glycol) [7].

However, Increase in flexibility,

accomplished by decreasing the glass transition temperature (Tg) with the use of
plasticizers, is linked to the decrease in tensile strength and modulus.

Therefore,

performance in terms of impact resistance and toughness is achieved at the expense of
other important properties. Additionally, the cost of the final products increases due to
the cost of additives. It is important to improve the flexibility and impact resistance of
PLA while reducing the cost of manufactured parts.
To reduce the cost, compounding PLA with less expensive fillers such as wood
fibers has been reported [9-11]. Wood-flour, kenaf fibers, recycled newspaper fibers,
flax fibers, etc, have been blended with PLA in concentrations varying from 20 to 50 wt. %
[9-12]. Because of the high specific modulus of cellulosic fibers, increased flexural and
3

tensile modulus with increased fiber contents is achieved. However, tensile and flexural
strength, and impact resistance generally decrease with increasing fiber content [10, 12].
Additionally, the incorporation of brittle cellulose fibers alters the ductile mode of failure
of the matrix making the composites more brittle than the neat polymer [13].
Foaming technology, such as microcellular foaming, enhances the ductility and
impact resistance of polymer matrices while achieving a significant volume expansion
ratio and weight reduction [14]. Microcellular foaming uses inert gases (e.g., carbon
dioxide) as physical blowing agents to make cellular structures with bubble sizes of less
than 10 µm and cell-population densities (number of bubbles per unit volume) greater
9

3

than 10 cells/cm . These unique characteristics results in a significant increase in
toughness and elongation at break (ductility) compared with unfoamed parts.
Additionally, compared to conventional foaming, strength and stiffness are not
compromised [15]. By creating a microcellular structure, a considerable expansion ratio
is obtained. For example, a twofold expansion means that only half of the resin is needed
to produce the same volume, thus less material is needed, reducing the overall cost. If
microcellular structures can be created in PLA and PLA/wood-flour composites, an
expanded range of properties in terms of ductility and impact resistance would be
attained. Consequently, the market for PLA will be expanded, especially in applications
in the packaging sector.
Microcellular plastics were first produced in a two-step batch process [15, 16]. In
the first step, a sample is placed in a pressurized chamber with an inert gas atmosphere.
Under these conditions, the gas diffuses into the sample and, after enough time (from
hours to days), the sample become saturated (Figure 1.1a, sorption), forming a single4

phase solution. The second step consists on creating a thermodynamic instability by
suddenly releasing the pressure and increasing the temperature (Figure 1.1b, temperature
bath), which triggers cell nucleation and growth as the gas diffuses out of the plastic.

5

Figure 1.1. Schematic of a batch microcellular foaming process. (a) Sample saturation
with CO2 (sorption). (b) Sample foaming (diffusion).

6

Batch production of microcellular PLA has been shown to overcome some of the
drawbacks of PLA [5, 17, 18]. Results of microcellular foamed neat PLA in the batch
process showed significant improvements in the volume expansion ratio (a twofold
expansion over unfoamed PLA), impact resistance (up to fourfold increase over unfoamed
PLA), strain at break (up to twofold increase over unfoamed PLA), and toughness (up to
fourfold increase over unfoamed PLA) [5]. Unfortunately, the batch foaming process
used to generate microcellular foamed structures is not likely to be implemented in the
industrial production of foams because it is not cost-effective. Multiple steps involved in
the microcellular batch foaming process and the long saturation times required (from
hours to days) limit this technique to be scaled up to industrial level. It is important to
develop the technology to manufacture microcellular PLA foams in a continuous process,
such as extrusion, to facilitate its mass production.
Compared to the batch foaming process, relatively few publications feature
continuous microcellular foaming of PLA in extrusion. Reignier and co-workers reported
a thorough investigation of the continuous extrusion foaming of amorphous PLA, using
CO2 as a blowing agent [19]. They produced low density foams under various injection
pressures (2.5-9.0 MPa), CO2 contents (1.8-9.3 wt.%), processing temperatures (90-100
o

C), and by adding 0.5 wt.% talc in the resin as a nucleating agent. Neat PLA foamed
3

samples achieved densities down to 21 kg/m with cell sizes of 60 to 420 μm and the
number of nucleated cells (or cell-population densities) in the range of 10
8

3

4

to

10 cells/cm . Other investigators reported microcellular extrusion of semi-crystalline

7

PLA and starch (75% amylopectin and 25% amylose) blends using a CO 2 concentration
3

of 8 wt.%. They created low density neat PLA foams (25 kg/m ) [20] but the cell
morphology was not discussed in detail. The micrographs suggest the cells have sizes of
80 μm and up. Lee and coworkers produced microcellular foams from amorphous PLA
resin (6300D from NatureWorks) intended for fiber melt spinning with CO2 to study the
expansion and cell-population density dependence on the amount of blowing agent (2-9
3

wt.%) [21]. Densities in the 20 to 700 kg/m range were produced with cell sizes of
5

3

around 100 μm and cell-population density of 10 cells/cm . Pilla and coworkers used
epoxy-functionalized chain extender to improve rheological properties of the melt of
semi-crystalline PLA (3001D, NatureWorks) and investigated its effects on the
foamability of the PLA. The addition of 1.0% chain extender doubled the volume
expansion ratio of the foams compared to the foamed neat PLA. Densities down to 300
3

7

3

kg/m and cell densities in the 10 cells/cm range were produced, averaging 30 m and
up in cell size.
These previous studies clearly demonstrate the continuous extrusion process of
PLA foamed with supercritical CO2. However, distinction between the cell morphology
achieved and microcellular foams persist due to lower cell-population density (less than
9

3

10 cells/cm ) with cells being larger than 60 m on average [22]. Continuous extrusion
of microcellular foamed PLA with fine cell size (< 10 m) and high cell-population
9

3

density (>10 cells/cm ) will make possible not only the production of foamed materials

8

with superior mechanical properties compared to their solid counterparts, but also the
production of foamed structures in thin film [22].
The basis for the continuous microcellular process lies in the concept of
thermodynamic instability to promote high cell nucleation rates in a flowing polymer
matrix [22]. The nucleation phenomena has been clearly described by Matuana and Diaz
[22]. In this paper the authors claim that achieving these high nucleation rates involves
first forming a polymer/gas solution under high pressure, followed by rapidly decreasing
the solubility of gas in the polymer by a rapid pressure drop and/or a rapid temperature
increase, which induces a greater thermodynamic instability in the system. The classical
nucleation theory (Equation 1.1) predicts this phenomenon [22, 23]:

  G 
N nucl  Co  f o exp 

 kT 

(Equation 1.1)

with Nnucl as the cell nucleation rate, Co the concentration of gas molecules, fo the
frequency factor for gas molecules joining the nucleus, k Boltzmann’s constant, T the
system temperature, and ΔG the free energy barrier to initiate homogeneous (ΔGhom) or
heterogeneous (ΔGhet) cell nucleation, given by the equations

Ghom 

3
16     bp

3p 2
9

(Equation 1.2)

Ghet 

3
16     bp

3p 2

f  

(Equation 1.3)

with γbp as the surface energy of the polymer-bubble interface, Δp the pressure drop of
the gas/polymer solution [22, 24], and f(θ) depending on the contact angle at the polymernucleating agent interface. Heterogeneous nucleation occurs at the interface between the
polymer and another phase.
The classical nucleation theory (Equation1.1) predicts that minimization of the
free energy barrier for nucleation (ΔG) will promote high cell nucleation rates [22].
Although many relevant factors can affect the cell nucleation rate (Equation1.1), a higher
pressure drop of the gas/polymer solution lowers the free energy barrier to initiate
nucleation (equations 1.2 and 1.3), resulting in a higher nucleation rate. Park and coworkers demonstrated that the pressure drop rate in the die (-dp/dt) given by Equation1.4
correlates with the thermodynamic instability necessary to nucleate high numbers of cells;
i.e., the higher the pressure drop rates, the higher the cell-population densities [22, 25-28]:

 dp  p  p  q


2
dt
t
  ro  L

10

(Equation 1.4)

with Δt as the average residence time of the flowing polymer/gas solution in the nozzle, L
the length of the nozzle, q the volumetric flow rate of the polymer through the nozzle,
and ro the radius of the nozzle.
Matuana and Diaz explained that because the pressure drop rate depends on the
nozzle geometry (ro and L) and polymer flow properties (Δp and q), the potential in using
these parameters as effective process and material variables for controlling the cell
nucleation in continuous microcellular processing exists [22].
The processing temperature influences the density and cell morphology of foams
because of its effect on the melt viscosity [22], as described by the Arrhenius relation

 Ea 

 RT 

  Ao exp 

(Equation 1.5)

with η as the melt viscosity, Ao a constant, R the universal gas constant, T the absolute
temperature, and Ea the activation energy for melt flow [29].
Generally, as the processing temperature is varied, the melt viscosity changes
inversely; i.e., the lower the processing temperature, the higher the melt viscosity, and
vice versa, subsequently leading to a change in the pressure generated during the
extrusion process. The melt viscosity affects the pressure built into the system, thus
affecting the cell nucleation rate. Additionally, the melt viscosity plays an important role
during the growth and stabilization of nucleated cells [30].
11

Melt viscosity can also be tailored by varying the material formulation, e.g.
additives and fillers contents [30, 31]. Incorporation of wood-flour in the polymer matrix
is known to increase the viscosity of the resin [31-33].

Also, additives such as

plasticizers, acrylic modifiers, compatibilizers, lubricants, and elastomers, significantly
affect the melt viscosity of filled and unfilled matrices [31, 34, 35]. Li and co-workers
studied the effects of wood-flour and internal lubricant contents on the rheological
properties of wood-flour filled polypropylene. The results showed that increasing the
wood-flour content increased the apparent viscosity of the melt because the solid wood
particles in the molten polymer increased the resistance against the flow. By contrast, the
addition of internal lubricant significantly decreased the melt viscosity of the composites
by reducing the friction between polymer chains and wood particles [36]. Nearly a 50%
reduction in apparent viscosity was observed by adding 5 wt.% of an ester based internal
lubricant.

Consequently, the effects of the processing temperature and material

compositions on the melt viscosity, pressure drop rate, and cell-population density must
be examined to understand the nucleation mechanisms in neat and filled PLA foams.

12

1.2 Objectives
The goal of the project is to develop the fundamental and engineering basis
needed to manufacture microcellular foamed PLA/wood-flour composites through a
continuous extrusion process using supercritical CO2. To achieve this goal, the following
specific objectives were proposed:

1.

Examine the effect of processing temperature on melt viscosity, pressure drop
rate and cell-population density to gain in-depth understanding of the nucleation
mechanisms in neat PLA to generate microcellular foams.

2.

Investigate the effect of the addition of nanoclay as nucleating agent in the
formulation on the cell morphology to understand the heterogeneous nucleation
mechanism in the continuous extrusion process.

3.

Study the effects of wood-flour and rheology modifier contents on the melt
viscosity and cell morphology of foamed samples to assess the influence of
incorporation of wood-flour on the foamability of PLA.

4.

Characterize the physico-mechanical properties of foamed samples as a function
of cell morphology to establish process-morphology-property relationships and
develop predictive models for the foamed materials.

13

1.3 Hypotheses
This research is intended to test the hypotheses that: (i) the control of the melt
rheology through processing temperature and material composition can lead to the
production of microcellular foams in PLA and PLA/natural fiber composites in a
continuous extrusion process using physical blowing agents; and (ii) creating cellular
structures in PLA products is an efficient way to improve flexibility and impact strength.

1.4 Structure of Dissertation
The first chapter introduced the rationale of this research. A background on foam
technology and polylactic acid is reviewed in the second chapter. The influence of
processing temperature on melt rheology and cell morphology of PLA (amorphous vs.
semicrystalline), and the effect of addition of nanoclay as nucleating agent are discussed
in Chapter 3. The effect of material composition, i.e. wood-flour content and rheology
modifier contents, on melt rheology, and the effect of wood-flour content on cell
morphology are discussed in Chapter 4. The mechanical characterization of foamed PLA
and as well as process-morphology and morphology property relations are discussed in
chapter 5.

14

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15

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Snow, N. (2006) EIA: World energy forecast reflects higher oil prices. Oil & Gas
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Houlton, G. (2010) Crude demand to increase, feed-quality changes in store. Oil
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Matuana, L. M. (2008) Solid state microcellular foamed poly(lactic acid):
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Semba, T., Kitagawa, K., Ishiaku, U. S. and Hamada, H. (2006) The effect of
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Jacobsen, S. and Fritz, H. G. (1999) Plasticizing polylactide - The effect of
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Baiardo, M., Frisoni, G., Scandola, M., Rimelen, M., Lips, D., Ruffieux, K. and
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Huda, M. S., Drzal, L. T., Misra, M., Mohanty, A. K., Williams, K. and
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Oksman, K., Skrifvars, M. and Selin, J. F. (2003) Natural fibres as reinforcement
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Matuana, L. M., Faruk, O. and Diaz, C. A. (2009) Cell morphology of extrusion
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Martini, J., Waldman, F. and Suh, N. P. (1982) Production and analysis of
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Matuana, L. M., Park, C. B. and Balatinecz, J. J. (1997) Processing and cell
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Ema, Y., Ikeya, M. and Okamoto, M. (2006) Foam processing and cellular
structure of polylactide-based nanocomposites. Polymer. 47, 5350-5359

18

Wang, X. X., Kumar, V. and Li, W. (2007) Low density sub-critical CO2-blown
solid-state PLA foams. Cellular Polymers. 26, 11-35

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Reignier, J., Gendron, R. and Champagne, M. F. (2007) Extrusion foaming of
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Mihai, M., Huneault, M. A., Favis, B. D. and Li, H. B. (2007) Extrusion foaming
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Lee, S. T., Kareko, L. and Jun, J. (2008) Study of thermoplastic PLA foam
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17

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Matuana, L. M. and Diaz, C. A. (2010) Study of cell nucleation in microcellular
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Colton, J. S. and Suh, N. P. (1987) The nucleation of microcellular thermoplastic
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24

Park, C. B., Baldwin, D. F. and Suh, N. P. (1995) Effect of the pressure drop rate
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Park, C. B. and Suh, N. P. (1996) Filamentary extrusion of microcellular
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Lee, J. W. S., Wang, K. Y. and Park, C. B. (2005) Challenge to extrusion of lowdensity microcellular polycarbonate foams using supercritical carbon dioxide.
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Wong, A., Leung, S. N., Li, G. Y. G. and Park, C. B. (2007) Role of processing
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Muksing, N., Nithitanakul, M., Grady, B. P. and Magaraphan, R. (2008) Melt
rheology and extrudate swell of organobentonite-filled polypropylene
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Diaz, C. A. and Matuana, L. M. (2009) Continuous extrusion production of
microcellular rigid PVC. Journal of Vinyl & Additive Technology. 15, 211-218

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Shah, B. L. and Matuana, L. M. (2004) Online measurement of rheological
properties of PVC/wood-flour composites. Journal of Vinyl & Additive
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32

Guo, G., Lee, Y. H., Rizvi, G. M. and Park, C. B. (2008) Influence of wood fiber
size on extrusion foaming of wood fiber/HDPE composites. Journal of Applied
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Li, T. Q. and Wolcott, M. P. (2005) Rheology of wood plastics melt. Part 1.
Capillary rheometry of HDPE filled with maple. Polymer Engineering and
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34

Huang, H. X. and Zhang, J. J. (2009) Effects of filler-filler and polymer-filler
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35

Münstedt, H. (1981) Rheology of rubber-modified polymer melts. Polymer
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36

Li, H. J., Law, S. and Sain, M. (2004) Process rheology and mechanical property
correlationship of wood flour-polypropylene composites. Journal of Reinforced
Plastics and Composites. 23, 1153-1158

19

CHAPTER 2

Background and Literature Review

20

2.1 Thermoplastic foams
As a general definition, a foam consists of gas bubbles immersed in a continuous
solid matrix [1]. Thermoplastic foams are those made of thermoplastic polymers such as
polyethylene, polypropylene, polystyrene, polyvinyl chloride, polylactic acid, etc.
Foamed thermoplastics are used in a variety of applications such as insulation,
cushioning, flotation devices, packaging, shock and sound attenuation, gaskets, sport
goods, etc. Moreover, highly porous biodegradable plastics have been used as tissue
engineering scaffolds, facilitating tissue regeneration (i.e., cell growth and attachment) [2,
3].

2.1.1 Cellular structure and foam properties
The properties of plastic foams are dictated by the type of polymer matrix and the
cellular structure. In general flexible polymers will produce flexible foams and rigid
polymers will produce rigid foams. The cellular structure or cell morphology refers to
the parameters that characterize the foam such as density, number of bubbles per unit
volume (cell-population density), cell size distribution, open-cell content, etc.

The

performance of the foam is correlated with the cell structure and strongly depends on the
technology and polymer matrix used to produce the foam [4, 5]. For instance, foams with
interconnected cells (open-cell structure) provide higher absorptive capacity and
cushioning [6] whereas foams with isolated cells and a well defined cell wall (closed-cell
structure) result in better thermal and acoustic isolation properties [7].

21

2.1.2 Foaming agents
The gaseous phase of thermoplastic foams derives from the use of a blowing or
foaming agent. There are two types of foaming agents: chemical and physical foaming
agents. A chemical foaming agent (CFA) is a solid compound in the form of powder or
pellets which undergoes a chemical reaction in the foaming process to produce blowing
gas. The reaction occurs because of a heat-induced decomposition. On the other hand, a
physical foaming agent (PFA) does not change chemically. Instead, the PFA is dissolved
directly into the polymer under pressure. Its blowing effect comes as a result of a change
in physical state, e.g., liquid to vapor or supercritical fluid to vapor. The foaming agent is
added to the polymer and the quantity is usually expressed as a fraction of the weight of
the resin, e.g., 1 to 5 wt.%.

2.1.2.1 Chemical foaming agents
Chemical foaming agents can be divided into two families, i.e., exothermic and
endothermic foaming agents, depending on their enthalpy of reaction.

Exothermic

foaming agents release heat as they decompose, mainly into nitrogen. An example of an
exothermic CFA is azodicarbonamide (ADC). In general exothermic CFA have high
yield (large volume of gas per unit weight) and have the ability to customize the
decomposition temperature through the use of activators [8]. Nitrogen possesses a low
diffusion rate, which is beneficial for bubble growth, maximizing the blowing efficiency
of the CFA. Endothermic foaming agents, such as sodium bicarbonate, absorb heat
during the decomposition reaction and decompose mainly into carbon dioxide (CO2).

22

The endothermic reaction produces a cooling effect that has a stabilizing effect on the
melt. In bubble formation, the CO2 exerts lower pressure than nitrogen resulting in
smaller cell sizes [8].

2.1.2.2 Physical foaming agents
Physical foaming agents include volatile hydrocarbons such as propane, butane,
pentane, etc., low boiling points liquids such as chlorofluorocarbons (CFC's),
hydrofluorocarbons (HFC's) and hydrochlorofluorocarbons (HCFC's), as well as inert
gases such as carbon dioxide or nitrogen. Hydrocarbons are used in the production of
low density polystyrene. However, hydrocarbons are flammable which can lead to
handling and shipping hazards [9]. Chlorinated hydrocarbons (e.g., methyl chloride and
methylene chloride) have low flammability but are considered toxic and have been used
to a limited extent. For a long time, chlorofluorocarbon (R12) was the preferred foaming
agent because of its nonflammability, low thermal conductivity and high solubility in the
plastic matrix [9]. Unfortunately, the use of CFCs has been discontinued because they
react with the ozone in the atmosphere depleting the ozone layer. Hydrofluorocarbons
(HFC's) and hydrochlorofluorocarbons (HCFC's) were developed as replacements for
CFCs with lower ozone depletion potential, but are still subjected to phaseout. Inert
gases such as nitrogen and carbon dioxide have been gaining a lot of attention as blowing
agents for they can replace conventional organic blowing agents since they are not
associated with environmental issues.

Also, inert gases have been used to produce

microcellular plastics where uniform cellular structures with cell sizes of less than 10 μm

23

render unique material properties, superior to conventional foams.

A detailed

explanation is provided in section 2.1.4.

2.1.3 Formation of a cellular structure
Cellular plastics are produced by different methods and equipment in continuous
and batch processes. In the continuous process, single and twin screw extruders are
extensively used to produce foams with CFA. The heat and shear provided by the
extruder melt the plastic, mix the ingredients and supply heat for thermal decomposition
of the foaming agent generating nitrogen or carbon dioxide. On the other hand, physical
foaming agents are directly injected into the extrusion system. The gas and the polymer
mix and dissolve forming a single phase solution, which is forced through a shaping die
to produce the desired profile. As the polymer exits the extruder it experiences a high
pressure drop as it goes from high pressure inside the extruder to atmospheric pressure
outside the extruder. This pressure drop lowers the solubility of the gas in the polymer
forcing the gas to come out of the solution forming bubbles [4]. Foams are also produced
through injection and blow molding following the same principles [9].
The batch or solid state foaming process is another method which consists in
placing a solid sample in a high pressure chamber under an inert gas atmosphere. The
gas dissolves in the polymer until saturation is reached. Through pressure drop and
temperature increase, the solubility is lowered and the gas comes out of the matrix
forming bubbles. Irrespective of the blowing agent or production method, three steps are
required in the manufacture of plastic foams:

24

1. Dissolution of the blowing agent in the polymer;
2. Cell nucleation;
3. Bubble growth and stabilization.

Irrespective of the nature of the foaming agent, chemical or physical, the foaming
process follows these steps which correspond to thermodynamic states of the polymer/gas
system.

2.1.3.1 Dissolution of the blowing agent in the polymer
Quality foams begin with blowing agent being thoroughly dissolved in the plastic
melt. Once the chemical foaming agent has decomposed into CO2 or N2, or the physical
foaming agent has been in contact with the polymer they should mix and dissolve to form
a single phase solution. This process in controlled by the solubility of the blowing gas in
the polymer. Solubility is the ability of the polymer to accept and retain small molecules,
which reside in the free volume. The solubility limit is the maximum amount of gas or
liquid that can be dissolved in a polymer at a given temperature and pressure. For
instance in extrusion foaming, undissolved foaming gas reside in the melt as
microdroplets or microbubbles which grow nearly explosively when the pressure is
release [9]. As a result, large voids and a deteriorated cellular structure are developed,
thus affecting the mechanical properties. For this reason, dissolution of the blowing
agent into a homogeneous solution is a prerequisite to produce uniform cellular structures.
25

Another important factor in the formation of the polymer/gas solution is the time
require for the dissolution process which is controlled by the diffusivity. The driving
forces are chemical potential, gas concentration gradient, differential partial pressure, or
temperature gradient [9]. The diffusivity parameter is affected by temperature, pressure,
and shear mixing. Processing at higher temperatures and the use of mixing screws
accelerates the sorption process.

2.1.3.2 Cell nucleation
Nucleation is the physical process by which a new stable phase is created. In the
context of foaming gas bubbles nucleate from the polymer/gas solution. The nucleation
is driven by a drop in solubility which can be attained by a sudden pressure drop or
temperature increase. A sudden drop in solubility will make the polymer/gas solution
supersaturated forcing the gas to form bubbles or escape to the atmosphere through the
surface.

Gas concentration, degree of supersaturation and pressure drop rate are

important factors controlling the nucleation process.
The nucleation in the polymer can be homogeneous, heterogeneous, or a mixedmode nucleation (both). In homogeneous nucleation, nucleation occurs right through the
mass of the polymer when a critical amount of gas precipitates to form a stable second
phase. In contrast, heterogeneous nucleation occurs at the interface between the polymer
and another phase, such as a particle. Generally, the addition of external nucleating
agents (e.g., nanoparticles, talc, stearates, TiO2, etc.) reduces the polymer-bubble
interfacial energy and provides sites for the heterogeneous nucleation of cells [10-13].

26

2.1.3.3 Bubble growth and stabilization
Once cells are nucleated they will continue to grow given the little resistance
encountered. Cell growth rate is controlled by the gas pressure inside the bubble, the
diffusion rate of gas from the polymer/gas solution to the growing bubble, and the
viscoelastic properties of the melt.

The viscoelastic properties of the melt and the

processing temperature are of vital importance during cell growth and stabilization [1416]. While melts with relatively low viscosity are needed to facilitate bubble growth,
viscosity must increase, by cooling or strain hardening, to a level adequate for bubble
stabilization [17, 18]. Consequently, melt viscosity, melt strength, and melt elasticity,
which control the cell growth and stabilization, should be tailored to provide the
conditions for a proper expansion and stabilization of the cellular structure.
Melt strength/elasticity is the capability of the melt to undergo stretching without
rupture [18] and should be high to resist the membrane tearing. At the same time
moderately high elongational viscosity allows biaxial stretching. These characteristics
are especially important for the production of closed-cell foam morphologies. Also, if
the polymer can crystallize, high crystallization rates will improve the stabilization
process.
Polymers with poor melt strength/elasticity hinder controlled cell expansion and
the stabilization of growing bubbles. In this case, changes in molecular weight by
grafting, chain extension, branching or crosslinking enhance the viscoelastic properties of
the polymers [18, 19].

27

2.1.4 Microcellular plastics
Industrially, most thermoplastic foams are manufactured through an extrusion
process using either CFA or low boiling point PFA [1].

Conventional foams are

characterized by average cell sizes ranging from 100 μm to 10 mm with cell-population
4

6

3

densities of around 10 to 10 cells/cm .

Large and nonuniform cells, typical of

conventional foaming, decrease mechanical properties and necessitate relatively thick
cross sections to ensure that the cells are contained within the part. In addition to this,
foam manufactures are looking for alternatives to CFA and volatile organic compounds
due to environmental the environmental issues associated with them.
Supercritial fluids such as N2 and CO2 are environmentally friendly and offer
many advantageous properties including inertness, nonflamablity and low cost.
Supercritical fluids are compressed gases that show properties of both liquids and gases
[20]. They combine gas-like viscosity and diffusivity, and liquid-like density, making
them excellent candidates for continuous extrusion foaming. Foaming technology makes
use of supercritical fluids in order to accelerate diffusion processes and completely
dissolve the gas into the polymer in a short timeframe [21].

Above the critical

temperature and pressure (critical point) a gas is considered to be in its supercritical state.
Carbon dioxide is ideal for extrusion foaming because of is high solubility in most
polymer matrices and because its supercritical conditions (Tc = 304.1 K, Pc = 7.38 MPa)
can be easily attained by the use of a syringe pump of a high pressure gas delivery system
[22].
28

Supercritical fluids can overcome the drawbacks of conventional foaming and
produce more uniform cell morphologies with higher cell-population densities and
smaller cell sizes. High amounts of small cells uniformly distributed in the matrix would
improve the mechanical properties of the foam.
conventional or microcellular.

Classified by size, foams can be

Microcellular foams are characterized by cell sizes

smaller than 10 μm with a narrow cell size distribution and a large cell-population density
9

3

(greater than 10 cells/cm ). The production of foamed structures in thinner parts and
foamed films is conceivable in microcellular foams.
The concept of microcellular plastics was developed in the 80s by researchers at
Massachusetts Institute of Technology with the dual goal of reducing material usage and
increase toughness. The rationale was that by creating microscopic cells smaller than
preexisting flaws in the material in large amounts, density reduction can be attained while
maintaining mechanical properties [23, 24]. Studies have reported improvements in
impact strength and toughness [23, 25], fatigue life [26], and thermal stability [27] in
microcellular plastics compared to unfoamed specimens. Small bubbles inhibit crack
propagation by blunting the crack tip and increasing the amount of energy to propagate
the crack [23]. Density reductions of 75% or more can be attained in microcellular
plastics due to the presence of microcells [25]. The unique properties of microcellular
foams can be used in a large number of innovative applications such as packaging,
automotive, sports equipment, etc. [28]
Microcellular plastics can be manufactured through noncontinuous [12, 25, 29,
30], semi-continuous [31], and continuous processes [17, 28, 32, 33]. Microcellular
structures have been produced in amorphous and semi-crystalline polymers. Recently,
29

microcellular structures have also been produced in composite materials with cellulosic
fibers [30] and plastics filled with inorganic particles [34]. The next section discusses the
two main processing methods used to produce microcellular foams, the batch process and
the continuous extrusion process.

2.1.5 Solid state microcellular foaming (the batch process)
The microcellular foaming process dissolves an inert gas into a plastic to create a
swarm of bubbles. Microcellular plastics, produced first in a batch process, involve a
solid sample placed into a chamber under high pressure in an inert gas atmosphere (e.g.,
CO2, N2, etc.) where gas molecules diffuse into the plastic, forming a single-phase
solution (Figure 2.1a, sorption).

Subsequently, nucleated and grown cells form by

creating a thermodynamic instability (i.e., sudden pressure drop and/or temperature
increase, Figure 2.1b, temperature bath).

30

Figure 2.1. Schematic of a batch microcellular foaming process. (a) Sample saturation
with CO2 (sorption). (b) Sample foaming (diffusion).

31

Cell morphology in the batch process is controlled by the amount of gas absorbed,
which is influenced by the pressure in the vessel and time allowed to saturate (Figure
2.1a); and by the viscosity of the melt in the foaming stage, which is dictated by the
temperature of the bath (Figure 2.1b). Cell stabilization is accomplished by freezing the
matrix by dipping the sample in cold water. This additional step is required to stop the
bubble growth and stabilize the cell structure. Longer times in the temperature bath may
result in cell coalescence and bubble collapse; whereas very short temperature bath may
result in insufficient time to develop the cellular structure. Optimum processing windows
are usually determined experimentally and with the help of statistical packages for
experiment design.

2.1.5.1 Formation of a single-phase solution
In solid state microcellular foaming the amount of gas to be dissolved in the
polymer is determined by temperature, pressure, and time the samples stay in the
chamber. These conditions are usually room temperature and high pressure or high
temperature and pressure (autoclave). In the former case a sample may take days to reach
equilibrium while in the latter case a few hours may be enough. The dependence of
solubility on temperature and pressure is illustrated in Figure 2.

In general higher

pressures induce higher solubilities whereas temperature has the opposite effect.

32

Equilibrium
Ceq
Gas Concentration

C

B

A

O

Pressure

Temp

Figure 2.2. Typical equilibrium gas concentrations in polymers in function of temperature
and pressure [35].

33

Sorption isotherms show the dependence of solubility on pressure. If the relation
is linear, the polymer gas system is said to follow Henry’s law expressed by Eq. 2.1:

C  kD  P

(Equation 2.1)

where C is the volumetric concentration of gas (standard temperature and pressure, STP)
per volume of polymer, KD is the Henry’s law constant and P is the pressure [36].
Henry’s law is a good fit at low pressures. At higher pressure ranges an isotherm may
differ from a linear form having some curvature due to interactions between the gas and
the polymer. In this case other models such as the dual sorption model, the Flory–
Huggings based models and the equation-of-state-based models should be considered
[37].

2.1.5.2 Nucleation dynamics
Colton and Suh modeled the nucleation of batch microcellular foams in
polystyrene. Classical nucleation theory was used as the basis to describe the nucleation
phenomena. Equation 2.2 predicts that the rate of nucleation is proportional to the gas
concentration and exponential with respect to the activation energy barrier (G) [10-12,
29]:

34

  G 
N nucl  Co  f o exp 

 kT 

(Equation 2.2)

with Nnucl as the cell nucleation rate, Co the concentration of gas molecules, fo the
frequency factor for gas molecules joining the nucleus, k Boltzmann’s constant, T the
system temperature, and G the free energy barrier. The activation energy barrier is
derived from the Gibbs free energy balance between interfacial free energy and volume
free energy [10]. Clusters of gas need to be greater than a critical size to overcome the
activation energy and create a stable bubble.

Activation energy for homogeneous

nucleation is given by Equation 2.3:

Ghom 

3
16     bp

3p 2

(Equation 2.3)

with γbp as the surface energy of the polymer-bubble interface, and p the pressure drop
of the gas/polymer solution. Based on equation 2.3, minimizing the free energy calls for
reducing the surface energy or increasing the pressure drop in the system.

In a

polystyrene/nitrogen system Colton and Suh [11] showed an increase in the number of
cells nucleated of three orders of magnitude by increasing the saturation pressure from 2
to 10 MPa. Furthermore, adding zinc stearate below the solubility limit increased the
bubbles nucleated for the same saturation pressure because of the reduced surface energy.
Above the solubility limit, additives precipitate providing sites for heterogeneous
35

nucleation almost eliminating the dependence of nucleation on saturation pressure [11].
Goel and Beckman [29] used classical nucleation theory to predict homogeneous
nucleation in a poly(methyl methacrylate) (PMMA)/CO2 system with a good agreement
between experimental and calculated nucleation rates [29].
Heterogeneous nucleation occurs at the interface between the polymer and a
particle and effectively reduces the activation energy. Equation 2.4 describe the
heterogeneous nucleation activation energy:

Ghet 

16  
3p

2

3
  bp  f ( )

(Equation 2.4)

with f() depending on the wetting angle (θ) at the polymer-nucleating agent interface. A
-3

typical wetting angle of 20° will result in a f() in the order of 10 [10].

2.1.6 Continuous microcellular extrusion process
Continuous microcellular foaming follows the same principles as the batch
process.

A single-phase solution of plastic and gas needs to be created, and a

thermodynamic instability is required to nucleate and grow bubbles. Inert gases in their
supercritical state are injected into the extrusion barrel. The gas will mix with and diffuse
into the molten polymer and form a single-phase solution. Thermodynamic instability
will occur as the polymer/gas solution leaves the extruder through the use of a nozzle die

36

(Figure 2.3). As the melt passes through the nozzle, the solution is taken from high
pressure in the extrusion system to atmospheric pressure [28, 38]. Nucleation, cell
growth and stabilization occur while the polymer leaves the extruder. In contrast to the
batch process, all stages of foaming in the continuous process have to take place in a
matter of seconds.

37

CO2
supply

Nozzle die

Figure 2.3. Schematics of a continuous microcellular extrusion process.

38

The continuous microcellular foaming implies that the polymer/gas mixing
requires a shorter time frame, from hours/days in the batch process to seconds in the
continuous process. Using gas in supercritical conditions and the polymer in the molten
state enhances the diffusion. The independent variables in the continuous extrusion
process are the amount of gas injected, and the processing conditions, i.e., rotational
screw speed and temperature profile.

2.1.6.1 Formation of a single-phase solution
In continuous microcellular foaming the amount of gas to be dissolved in the
polymer is set by injecting a metered amount of gas in the extrusion barrel. The gas will
mix and diffuse in the polymer given that the high pressure in the system is sufficient to
solubilize the gas. In other words, the metered amount of gas injected in the barrel
should be lower than the solubility of the gas in the polymer at the pressure and
temperature in the system; otherwise a single phase solution cannot be attained. By using
gas in supercritical conditions and the polymer in the molten state, the diffusion is
enhanced [17]. By comparing reported values of diffusivity from the literature [22, 39]
going from room temperature to 180°C will increase the diffusivity by two orders of
magnitude.

The dependence on temperature of the diffusion coefficient obeys an

Arrhenius relationship as follows:

 E 
D  Do exp   a 
 RT 

39

(Equation 2.5)

where Do is the pre-exponential factor; Ea is the activation energy for the gas diffusion; R
is the universal gas constant, and T is the temperature in the absolute scale [40].
Although a high diffusion is preferred in the continuous microcellular foaming process to
ensure the creation of a single-phase solution, it can also be detrimental during nucleation
and cell growth because gas molecules can quickly move to the surface and escape to the
atmosphere reducing the efficiency of the foaming agent [41].

2.1.6.2 Nucleation dynamics
Nucleation in the continuous system is also explained using classical nucleation
theory [41]. However, the time period over which the thermodynamic instability occurs
plays an important role in the number of bubbles nucleated.

Park and coworkers

identified a link between pressure drop rate and nucleation [42]. Higher pressure drop
rates result in nucleation rates (higher number of bubbles nucleated).

Basically,

depending on the time allowed for the pressure drop, cells can prematurely nucleate in
the nozzle creating a region around them with low gas concentration where nucleation
does not occur and the dissolved gas is used to grow the bubble. This competition
between nucleation and cell growth mechanisms limits the nucleation ability. Therefore,
higher pressure drop rates translate into more gas used to nucleate rather than grow
bubbles, resulting in higher nucleation rates [41, 43]. Equation 2.6 shows the relationship
of the pressure drop rate with the flow characteristics and nozzle geometry.

40

 dp  p  p  q


2
dt
t
  ro  L

(Equation 2.6)

with Δt as the average residence time of the flowing polymer/gas solution in the nozzle, L
the length of the nozzle, q the volumetric flow rate of the polymer through the nozzle,
and ro the radius of the nozzle. Based on Equation 2.6, the pressure drop rate depends on
the nozzle geometry (ro and L) and the polymer flow properties (Δp and q). Therefore,
the geometry of the nozzle or the flow properties of the material are variables for
controlling the cell nucleation in continuous microcellular processing [44].

41

2.2 Polylactic acid (PLA)
Poly(lactic acid) (PLA) is a biodegradable thermoplastic obtained from annually
renewable resources such as corn, beets, sugar cane, etc. The monomer lactic acid can be
obtained from starch fermentation (biological process), or by chemical processes. PLA
has excellent mechanical properties, good biocompatibility and low toxicity. It has been
used in biomedical and packaging applications and has the potential to replace petroleumbased plastics.

2.2.1 Structure and synthesis
Lactic acid is the chemical repeat unit (constitutional unit) of PLA. Because of its
chiral nature, lactic acid exists as two enantiomers, L- and D-lactic acid, giving place to
three types of the cyclic dimmers of lactic acid (lactide monomer), i.e., L-lactide, Dlactide, and meso-lactide (see Figure 2.4). Because of this, PLA can be systhetize into a
variety of primary structures.

Examples of this can be the isotactic homopolymers

poly(L-lactide) (PLLA) or poly(D-lactide) (PDLA) where all of the monomers in the
chain are of the same optical composition [45], or random copolymers of L-, D- and
meso lactide. Most commercial PLAs are copolymers of L-lactide and lesser amounts of
D- and meso-lactide.

The effect of stereochemistry on crystallinity and thermal

properties is discussed in the next section.

42

D-lactic acid

L-lactic acid

L-lactide

meso-lactide

Figure 2.4. Stereochemistry of lactic acid and lactide [46].

43

D-lactide

Figure 2.5 shows a schematic of the PLA production. First, anaerobic bacteria
converts plant sugars into lactic acid. Second, condensation reaction of aqueous lactic
acid produces PLA with low molecular weight, which is then depolymerized into the
cyclic dimer of lactic acid or lactide monomer. Third, ring-open polymerization produces
PLA with high molecular weight [47, 48]. PLA possesses physical and mechanical
properties similar to those of polystyrene or poly ethylene terephthalate (PET) and can be
processed with existing machinery such as extrusion, injection molding, etc., without
major adjustments [36].

44

OH

HO
O

Lactic acid

High molecular weight PLA

Condensation
– H2O

Ring opening
polymerization
O
O

Depolymerization
O

Prepolymer
Mn ~ 5000

O

Figure 2.5. Schematic of PLA production via prepolymer and lactide [48].

45

2.2.2 Properties of PLA
2.2.2.1 Thermal properties and crystallinity
Thermal properties such as glass transition temperature (Tg) and melting
temperature (Tm), and degree of crystallinity of PLA depend on molecular weight,
relative amounts of L- and D-lactic stereoisomers, and thermal history. Poly(L-lactic
acid) (PLLA), made entirely from L-lactide, is a semi-crystalline polymer due to
stereoregularity. Similarly, Poly(D-lactic acid) (PDLA), made entirely from D-lactide is
also semi-crystalline. Most commercial grades of PLA are made from L-lactide with
lesser amount of Meso- and D-lactide. These last two components induce twists in the
regular poly(L-lactic acid) molecular architecture resulting in a decreased rate and extent
of crystallinity [49].
The extent of crystallinity can be controlled by tuning the relative amounts of Land D-lactic stereoisomers in the polymer. In general, PLA made from more than 93%
L-lactic acid is semicrystalline. Lesser relative amounts of L-lactic acid, between 50 and
93%, will result in an amorphous PLA [49].

One drawback of PLA is its low

crystallization rate which result in products with low heat resistance [50]. For instance,
the low heat deflection of PLA, between 50 to 60°C, can cause packages to deform
during storage [51]. So far PLA commercialization has been limited to applications that
do not require high temperatures such as refrigerated food packaging and cold beverages
cups. Researchers are looking for ways to increase the heat resistance of PLA. This
could be achieved through increasing the crystallinity. Inducing crystallization can be
accomplished by different methods including annealing, addition of nucleating agents or
46

biaxial orientation [52].

It is known that PLLA and PDLA mixtures can form

stereocomplex crystallites with increased crystallization rates [53].
Foaming of PLA can influence the material crystallinity. A significant increase in
crystallinity has been reported when comparing foamed and unfoamed PLA samples [54],
because of biaxial stretching during bubble growth. Additionally, dissolved CO2 into
PLA contributes to the development of crystallinity [55], because its plasticizing effect
enhances chain mobility.

Increase of cristallinity, induced by foaming, helps the

stabilization of the cellular structure and increases the heat resistance of the foam.
Thermal properties can vary among different PLA types.

PLA has a glass

transition temperature between 40 and 70°C, and a melting temperature between 130 and
180°C [56]. Thermal properties are also affected by the L-lactic content; higher L-lactic
content increases the Tm and Tg [57].

2.2.2.2 Rheological properties
Causing the melt to flow is the basis of converting plastics into useful articles.
Consequently plastics rheology is intimately related to its processability, particularly the
shear viscosity. Viscosity of PLA depends on the molecular weight ( M w ). The zero
shear viscosity ( o ) follows the relationship presented in equation 2.7.

logo   14.26  3.4 logM w 

47

(Equation 2.7)

Equation 2.7 is representative of PLA with molecular weight in the 10-100
Kg/mol range at 180°C regardless of stereochemical composition [58]. PLA shows shear
thinning behavior at common processing shear rates [59], between 10 to 1000 1/s, and the
viscosity (η) versus shear rate (  ) curve follows a power law as shown in equation 2.8.


  K   n 1

(Equation 2.8)

where K and n represent the melt viscosity coefficient and power law index, respectively.
Capillary rheometry results showed a power law index (n) of 0.36 for PLA with M w
=100 Kg/mol at 180°C [59]. Branched PLA is known to have stronger shear thinning
behavior and higher zero-shear viscosity. Control of the rheology is possible through
blending on PLA with different molecular architectures without affecting thermal and
mechanical properties [60].
One drawback when processing PLA is its susceptibility to thermal and hydrolytic
degradation, which affect its viscosity measurements and processing. Special attention
should be placed in drying the material prior to processing and in some cases the use of
stabilizers may be necessary to prevent polymer degradation.
Another important rheological parameter is the melt strength, which is related to
the ability of the melt to stretch. Melt strength is especially important in blown film,
deep draw thermoforming, and foaming. Lower melt viscosity and melt strength is one
of the deficiencies of PLA which limits its blow molding and foaming processability [19,
61]. In foam processing, lower melt strength will result in the shear thinning of the cell48

wall during bubble growth which facilitates cell coalescence and the overall cell structure
deterioration. Strategies to overcome this issue should be considered. Branching and
crosslinking of PLA has been attempted to improve rheological properties [19, 61].
Some results will be discussed in the next section.

2.2.2.3 Mechanical properties
Polylactic acid possesses good mechanical properties in terms of tensile strength
and modulus superior to many traditional plastics, e.g., polyethylene and polypropylene.
It fractures through a crazing mechanism and has low impact resistance and low strain at
break when compared to traditional petroleum-based plastics. Polylactic acid modulus of
elasticity is between 3 to 4 GPa. It has a tensile strength of 50 to 70 MPa. It is a hard
polymer with hardness measured in Rockwell H scale of 70 to 90 [62, 63].
2

Notched izod impact resistance of PLA can vary from 2 to 7 KJ/m depending
mostly on degree of crystallinity and is highly sensitive to notching. Unnotched PLA
2

impact resistance is in the range of 13 to 35 KJ/m . Samples with higher degree of
crystallinity possess higher impact resistance [62]. Impact modification is indispensable
for PLA to compete in automotive and packaging markets.

Increasing the impact

resistance is important for both end-use and manufacturing. For instance, in a packaging
production line the brittle polymer will break during thermoforming and hampers
production speed [51].

Approaches to overcome brittleness and improve impact

resistance are discussed in a separate section.

49

2.2.2.4 Barrier and sorption of CO2 into PLA
The permeation of small molecules in a polymer, characterized by parameters
such as permeability, diffusivity, and solubility, is important in many applications such as
packaging. The ability of a molecule to easily permeate through a membrane or be
obstructed by it is critical in designing a package system. PLA is a good barrier for CO2
and aroma compounds similar to PET. However, it is a poor barrier to O2 and water
vapor [64]. Research efforts are being directed towards manipulation and control of PLA
barrier properties to expand its applications.
In the context of foaming, sorption is the process by which gas molecules are
absorbed by the polymer. The gas diffuses from a higher chemical potential to a lower
chemical potential until equilibrium is reached. Solubility is the amount of gas absorbed
at thermodynamic equilibrium and is important because it determines the amount of gas
available for foaming.
The solubility of CO2 in PLA at room temperature was examined by Matuana in
the 1.38 to 5.52 MPa pressure range [36]. A maximum solubility of 16.4% [36] was
observed and the Henry’s law constant kD was estimated to be 0.47 MPa. The data
slightly differ from the Henry’s law showing an upward curvature, attributed to the
affinity between CO2 and PLA. Using a magnetic suspension balance, Aionicesei and
coworkers, measured solubility of poly(L-lactide) (PLLA) at supercritical conditions and
room temperature. A solubility up to 43% was observed in the 10 to 30 MPa range. The

50

shape of the sorption isotherm showed a downward curvature opposite to the observed in
the subcritical region [22, 65].

Li et al., measured the solubility and diffusivity of CO2 in PLA at high
temperature (i.e., 180 and 200°C) and high pressure (i.e., up to 28MPa) [39], similar to
the conditions experienced during extrusion foaming. The results were corrected to
account for swelling, and a solubility up to 20% was measured in the PLA/CO2 system
(180°C and 27 MPa) [39]. In extrusion foaming, efficiently dissolving the gas into the
polymer requires the pressure in the system to be well above the equilibrium pressure.
According to their data, the equilibrium pressure to dissolve 10% of CO2 in molten PLA
at 180°C is 2000 psi (14 MPa). That means that if 10% CO2 is injected in the extrusion
barrel, the pressure in the system should be at least 2000 psi for the single phase solution
to be created. Lower temperatures will make this pressure requirement lower since the
solubility increases as temperature decreases, lowering the equilibrium pressure for the
same amount of gas.

2.2.3 Property modification of PLA
Property modification of PLA can be accomplished by different means including
copolymerization, plasticization, blending with other resins, fiber reinforcement,
compounding with nanoparticles, foaming, etc. Modification is performed to obtain

51

improved material properties compared to the neat PLA in order to make the material
suitable for a specific application.

2.2.3.1 Copolymerization
Copolymerization allows two or more monomers to be combined into one
polymeric chain to provide materials with a specific set of properties. A wide range of
mechanical properties can be attained by this method. However, the process is not
economically viable unless produced in a large industrial scale [66]. PLA has been
synthesized into random copolymers, block copolymers, and graft copolymers to modify
its properties [67]. Research in PLA copolymers has been mostly directed towards
biomedical applications.
Poly(lactic-co-glycolic acid) is a random or block copolymer made by ringopening polymerization of the cyclic dimers of lactic acid and glycolic acid.

This

material, as well as block copolymers comprising poly(ethylene glycol) (PEG) and PLA,
are used in medical implants due to their biocompatibility and because the biodegradation
rate of the copolymer can be modulated by adjusting the ratio of the constituents [68, 69].
Other co-monomers that have been combined with PLA include glycolide derivatives,
lactones, cyclic amide esters, cyclic ether esters, cyclic carbonates, etc [67].

One

restriction to form copolymers of PLA is that the other co-monomer must participate in
ring-opening polymerization.

52

2.2.3.2 Plasticization
Plastic films, e.g. polyolefin films, are extensively used in multiple applications
such as flexible packaging. However the non-biodegradability of the resins traditionally
used is a drawback in view of sustainability. PLA has excellent characteristics in terms
of transparency and barrier but low flexibility and brittleness, which prevent its
processability. Packaging is one of the potential markets for PLA. However, brittleness
causes tearing in the plastic film on a production line, which is not tolerable [36, 70].
Plasticization of PLA is then required in order to produce flexible films.
Flexibility and toughness of PLA can be improved with the use of plasticizers
[66]. However, attention in choosing the right plasticizer is essential to maintain nontoxicity, miscibility, and ultimate biodegradability [70]. Migration of the plasticizer may
cause contamination of products in contact with PLA. Oligomeric lactic acid, glycerol,
and low-molecular weight esters are among the studied plasticizers for PLA [70].
Generally the introduction of a plasticizer is accompanied by a depression in the
glass transition temperature (Tg) [71], which in turn results in lower tensile strength and
modulus. Ljungber and Wesslèn studied triacetine, tributyl citrate, acetyl tributyl citrate,
triethyl citrate, and acetyl triethyl citrate and their plasticizing effect on PLA through
dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC).
Significant reduction in Tg was observed when using triacetine or tributyl citrate in
concentrations up to 30%. However, the plasticizer was only partially miscible with PLA
and phase separation was observed [70]. Piorkowska and coworkers, plasticized semicrystalline PLA with poly(propylene glycol), which decreased Tg and caused a sensitive
53

drop in tensile strength and phase separation [72]. the major drawbacks of most of the
available plasticizers seems to be the drop in tensile strength, large amount of plasticizer
needed, and poor miscibility (evidenced by phase separation).
Increase in flexibility, accomplished by decreasing the Tg with the use of
plasticizers, is linked to the decrease in tensile strength and modulus.

Therefore,

performance in terms of impact resistance and toughness is achieved at the expense of
other important properties. Additionally, the cost of the final products increases due to
the cost of additives. It is important to improve the flexibility and impact resistance of
PLA while reducing the cost of manufactured parts.

2.2.3.3 Blending with other polymers
Polymer blending consists on the creation of a new material by mixing two or
more polymers. It is a way to improve the properties of PLA and/or improve the cost
effectiveness of the material. Studies on blending PLA with other polymers have focused
on maintaining its biodegradability.
Due to its natural origin and abundant availability starch is the main candidate to
blend with PLA. Incorporation of hydrophilic starch into hydrophobic PLA has been a
subject of research with the goal of reducing cost while maintaining biodegradability.
Starch usually adds rigidity to PLA and is miscible only at low concentrations, thus using
compatibilizers and plasticizers is commonly observed in the blends [73]. Drawbacks of
starch deal with its moisture sensitivity which limits its applications [74].

54

Blends of PLA with other biodegradable polymers such as polyhydroxyalkanoates
(PHAs), poly(ε-caprolactone) (PCL) [75], and poly(butylenes succinate) (PBS) [76], have
been reported. Also, to improve the toughness, PLA has been blended with polyethylene
[77] and rubber [78].
In order to improve the brittle behavior of PLA, Semba and coworkers studied the
effect of its blending with PCL and the addition of peroxide to induce crosslinking on the
mechanical properties of the blends. Low concentrations of dicumyl peroxide, i.e., 0.3
phr, showed best results with a PLA/PLC blend ratio of 70/30. Ductile behavior was
observed along with an increase in impact strength of 250% over neat PLA for the
samples prepared with the optimum formulation. The values compared with those of
acrylonitrile butadiene styrene (ABS) and high impact polystyrene (HIPS) [75].
Immiscibility of PLA blends is a common observation among different studies
and the use of compatibilizers is a necessary condition to achieve property improvement.
Chen and coworkers used funtionalized organoclay to compatibilize the immiscible blend
of poly(l-lactide) (PLLA) and poly(butylene succinate) (PBS). The organocaly was
treated with (glycidoxypropyl) trimethoxy silane to improve the dispersion and provide
chemical interaction with both PLA and PBS. Improvement in both the elastic modulus
(from 1.1 to 1.0 GPa) and elongation at break (from 4.4 to 118.1%) of the PLLA-PBS
blends (75:25) with the addition the funtionalized organoclay in concentrations from 5 to
10 wt.% was observed[76]. Although the blending of PLA can render an increase in
properties, the use of specialized polymers and compatibilizers may significantly increase
the cost of the final products.

55

Blending of PLA with polyolefins to increase the toughness has been reported
[77]. The study by Anderson and coworkers reported the production of super-tough PLA
by blending a 80:20 mixture of PLA and linear low density polyethylene (LLDPE) with
the addition of PLLA-PE block copolymer (5%) as compatibilizer. However, blends with
non-biodegradable plastics may have a negative perception in view of sustainability.

2.2.3.4 Fiber reinforcing
Using wood-flour, or other natural fibers to reinforce polymer is a common
practice that combines the properties of wood and plastics to form new materials with
improved moisture sensitivity over wood and greater stiffness over neat plastics. Wood
fiber content influences the processing and properties of Polymers. Generally, properties
such as modulus of elasticity, are somewhere in between those of the polymer and the
fiber depending on the relative amount of the constituents. In other words, they follow a
rule of mixtures. Other properties such as tensile strength depend on the stress transfer
from the matrix to the load bearing fiber [79]. Generally hydrophilic cellulosic fiber and
hydrophobic polymer matrices result in poor interfacial interactions which reflect in
lower mechanical properties such as tensile strength, which can be improved by adding
coupling agents.

Coupling agents are chemical compounds with functional groups

capable of reacting with both the reinforcement and the matrix. A variety of coupling
agents have been developed with over forty types available [80].
Most coupling agents are organic compounds with bi-functional groups in their
molecular structure such as isocyanates, maleic anhydrides (MA), silanes, etc. They can
modify the polymer matrix by grafting or even crosslinking at the interface. Modified
56

polymer matrices with grafted MA are common types of coupling agents. For instance,
the maleated polyolefin reacts with the hydroxyl groups of the wood component through
esterification or hydrogen bonding [81].
With the aim of reducing the cost of final products while retaining
biodegradability, blending of PLA with low cost cellulosic fibers has been investigated
[82-84].

By applying different manufacturing techniques such as compression and

injection molding, the effects of wood fiber content as well as fiber’s pretreatment have
been studied.
Huda and coworkers studied the thermo-mechanical properties of PLA
composites reinforced with recycled newspaper cellulosic fibers. For the composites
containing 30% fibers, tensile and flexural modulus were significantly higher than for
neat PLA and the values compared with those of polypropylene/talc composites. On the
other hand, tensile and flexural strength decreased.

Thermal properties remained

unaffected by the inclusion of fibers while notched Izod impact strength drastically
decreased [83]. Pilla and coworkers used pine wood-flour as filler in PLA. Samples
were produced by injection molding with wood loadings of up to 40%. Wood-flour
content increased the modulus of elasticity but decreased toughness and elongation
mainly due to decreased deformability of the matrix because of restriction imposed by the
brittle cellulosic fibers. Additionally, the use of a silane coupling agent proved to be
ineffective to improve the strength [84].
Oksman and coworkers produced PLA/flax composites with fiber contents of 30
and 40%. The components were mixed in a twin extruder and compression molded into
testing samples.

Tensile modulus increased but the impact strength decreased.
57

Recognizing the need to improve impact strength the study incorporated glycerol
triacetate ester as a plasticizer; unfortunately, no significant improvement was observed
[85].

In a different study from the same group, PLA composites with up to 25%

microcrystalline cellulose were prepared [86]. Tensile modulus increased and tensile
strength decreased with increased filler content. Wood-flour and wood pulp composites
were used as reference materials and mechanically outperformed microcrystalline
cellulose composites, mainly because of their higher aspect ratio which resulted in a
better stress transfer across the interphase [86].
Based on the reviewed literature, filling PLA with cellulosic fibers can increase
the tensile modulus and reduce the overall cost. However, while cost reduction can be
achieved, toughness and impact resistance are deteriorated. Two major drawbacks of
PLA include brittleness and low impact resistance; this approach worsens these two
properties.

Therefore, different approaches should be considered to increase the

toughness while reducing the material cost.

2.2.3.5 Nanocomposites
The term nanocomposite is commonly referred to polymer matrices reinforced
with nano-sized fillers (at least one dimension <100 nm) [87]. Some common fillers for
this purpose include nanoclay (layered silicates), carbon nanotubes, cellulose
nanocrystals, etc [88].

Nanocomposites have generated a lot of interest because

remarkable improvement in properties (e.g., mechanical and thermal properties) can be
obtained at low filler content, usually less than 5 wt.%, without affecting processability.

58

The high surface area inherent of the filler size allows for more filler-matrix interactions,
thus the property enhancement.
Preparation

of

PLA

nanocomposites

with

modified

and

unmodified

montmorillonites (type of nanoclay) has been reported. The preparation methods include
in-situ polymerization, melt-mixing, and solvent-casting. Significant mechanical and
thermal properties improvement have been reported [89]. In their pure state, clays come
as a stack of parallel silicate layers. Upon processing, the layers can be slightly separated
(intercalated) or randomly dispersed in the matrix (exfoliated), depending on its affinity
for the polymer. Increase in mechanical properties such as tensile strength and modulus,
depends on the degree of exfoliation of the nanoclay. Agglomeration of the nanoclay is a
common problem in nanocomposites preparation and lead to poor mechanical properties.
A good dispersion of intercalated or exfoliated nanoclay is important to make use of their
reinforcing potential. Krikorian and Pochan obtained Poly(L-lactic acid) nanocomposites
with a spectrum from intercalated to fully exfoliated morphologies by altering the
hydrophobicity of the organoclay. In this study, nano composites were prepared by
dispersion of organoclay in solvent and subsequent mixing with PLLA [90]. Jiang and
co-workers compared the performance of two nano-sized fillers, i.e., nano-sized
precipitated calcium carbonate (NPCC) and organically modified montmorillonite clay,
mixed with commercial PLA [91]. Clay nanocomposites showed best improvement in
strain at break (3 fold increase) at low concentrations, i.e., 2.5 wt.%, and an overall
higher modulus and tensile strain at similar concentrations compared to NPCC (in the 0
to 5 wt.% range) because of the larger aspect ratio and intercalation of the nanoclay.

59

Based on the above discussion, nanoclay modification is required in order to
enhance the interaction of the filler with the matrix, and promote dispersion and
intercalation. On the other hand, lack of toughness of the PLA nanocomposites is usually
observed [92]. Platicization followed by nanocomposite preparation have been proposed
to overcome some of this drawbacks [93].
In foam processing, solid particles have the potential to induce heterogeneous
nucleation. Therefore, the nucleation potential of nanocomposites in foaming has been
studied [24, 34, 94]. Okamoto and coworkers reported that the number of cells nucleated
in a batch foamed polypropylene increased by 100 times by adding 7.5 wt.% of nanoclay
[94]. Foamed PLA nanocompostites are discussed in the next section.

2.2.4 Microcellular foaming of PLA
2.2.4.1 Batch foaming of PLA
Recently, production of microcellular-foamed structures in PLA through batch
foaming process has been used to address some limitations of the polymer [3, 24, 34, 36,
95]. Matuana reported significant improvements in the volume expansion ratio (a 2-fold
expansion), impact resistance (up to a 4-fold increase), strain at break (up to a 2-fold
increase), and toughness (up to a 4-fold increase) of foamed PLA over that of the
unfoamed counterpart by creating microcells in the samples. This implies that foamed
PLA exhibits enhanced ductility and impact resistance owing to the presence of
microbubbles [36].

60

A wide range of morphologies can be produced in foamed PLA through batch
microcellular foaming. Wang et al. 2007 produced low density PLA foams, down to 5%
relative density compared to the unfoamed counterpart, using subcritical CO2 as foaming
agent. Unfortunately the morphology associated with the low density foams had large
5

cell sizes of several hundreds of microns and low cell population density, i.e., ~10
3

cells/cm . Increasing the saturation pressure had a tremendous effect on the morphology
lowering the cell size below 30 μm. The change in cell morphology with saturation
pressure was attributed to changes in both degree of crystallinity and diffusivity
parameter. An increase in the extent of crystallization, from 5 to 25%, was observed
when increasing the saturation pressure from 2 to 3 MPa [95].

Similarly, higher

saturation pressures induced a higher diffusivity of CO2 in the PLA (10 times increase
from 1 to 5 MPa). Similar results of crystallinity induced by CO2 and the increased
diffusivity with increased saturation pressure in PLA have been reported [55]. Foams
7

with cell sizes in the 30 to 40 μm range and cell population densities, around 8 x 10
3

cells/cm , were produced at a 2.8 MPa saturation pressure showing a uniform cellular
structure [55]. Higher saturation pressures, i.e., 4.1 MPa, resulted in smaller cells (1 to
10

3

10 μm) and high cell population densities (>10 cells/cm ). However, in the later case
the cellular structure was less uniform.

It was speculated that the higher diffusion

coefficient may have deteriorated the uniformity of the cellular structure [55]. Matuana
and Faruk manufactured microcellular foamed PLA and PLA/wood-flour composites
using CO2 in the batch process [96]. The effect of saturation pressure on the morphology
61

and volume expansion ratio of neat PLA was investigated.

The highest volume

expansion (10 times) compared to unfoamed PLA was associated with lower saturation
pressures up to 2.76 MPa, which corresponds to a critical gas concentration of 9.4%.
Higher saturation pressures resulted in deteriorated morphologies due to the higher
diffusion rates [96].
Since nanoparticles can act as heterogeneous nucleating agents, they have the
potential to control the cellular structure.

With the objective of understanding the

correlation between cell morphology and the dispersed silicate particles, Fujimoto and
coworkers studied the incorporation of nanoclay in PLA foamed with CO2 in the batch
process.

The cellular structure showed closed cells sizing from several microns to

fractions of a micron, i.e., 360 nm, . Achieving smaller cell sizes correlated with the
degree of dispersion of the nanoclay [34]. Similar results were reported by Ema and
coworkers focusing on the nucleating effect of the clay particles. Nanoclay induced
heterogeneous nucleation lowering the activation energy barrier as compared to
homogeneous nucleation, resulting in controlled cellular structures with cell sizes from
microcellular to nanocellular [24].
A different approach to controlling the cellular structure of microcellular PLA is
through modification of the rheology. Di and coworkers used chain extenders to increase
the molecular weight of PLA and improve the viscoelastic properties. Lower cell sizes,
5

from 227 to 24 μm, and an increase in cell-population density from 7.7 × 10 to 6.7 × 10

8

3

cells/cm was achieved as a result of the increased viscosity and elasticity. In this case, a
20:80 mixture of CO2 and N2 was used as the physical foaming agent [19]. Zhu and
62

coworkers investigated the effect of chain branching and the addition of lubricant on the
morphology of PLA foamed with CO2 [61]. Branched PLA showed superior viscoelastic
properties and the occurrence of strain hardening, which produced better cell
morphologies by preventing cell coalescence. Branched PLA produced smaller cell sizes
compared to linear PLA (~250 μm vs. ~50 μm) because of the higher melt strength. Also,
addition of lubricant resulted in an increase in cell-population density [61].
Microcellular foaming of wood fiber filled PLA has been investigated to reduce
weight and improve performance of the material. Incorporation of wood-flour into the
PLA matrix significantly affects the morphology and the expansion ratio of foamed
composites [96]. In the study by Matuana and Faruk, cell size and expansion ratio
decreased with increasing wood-flour content because the fibers make the matrix stiffer,
which provided high resistance to the bubble growth. The wood fibers were pretreated
with 1% silane coupling agent to enhance adhesion between fibers and the matrix. It is
known that surface modification of wood fibers with coupling agent has strong effects on
both the solubility of gas in the matrix and the cell morphology [30, 97]. Higher cellpopulation densities and void fractions due to fiber modification has been reported [97].
Microcellular foaming can also be used as means of overcoming the brittleness and low
impact resistance of wood filled composites.

Microcellular foamed PVC/wood

composites produced in the batch process showed that the creation of a microcellular
structure increased the impact resistance to values higher than the unfoamed and unfilled
polymer [25].
The above mentioned studies give valuable insights in the production of
microcellular PLA foams.

However, the industrial implementation of microcellular
63

foams remains unlikely in the batch foaming process because of its time-consuming
nature and multiple steps involved in the production of foamed samples, i.e., manufacture
of solid samples, saturation of solid samples with gas (sorption experiments), and
nucleation of cells and cell growth [98-100]. It is important to develop the technology for
the continuous production of microcellular foamed PLA and PLA wood fiber composites.

2.2.4.2 Continuous microcellular extrusion foaming of PLA
Commercial applications for microcellular foamed PLA and PLA composites
necessitate the development of a continuous process. Research initiatives in extrusion
foaming [41, 54, 101] and injection-molding processes [102] of PLA using supercritical
CO2 have already shown initial results.
Reignier and coworkers investigated the continuous-extrusion foaming of
amorphous PLA, using CO2 as a blowing agent [101]. They pointed out the importance
of the plasticization effect of CO2, which allows the process to be carried out at lower
temperatures.

The plasticization was measured in terms of Tg reduction and was

established to be between 6 to 8°C/wt. % CO2. They produced foams under various
injection pressures (2.5-9.0 MPa), CO2 contents (1.8-9.3 wt %), and processing
temperatures (90-100°C). Neat PLA foamed samples achieved densities down to 21
3

kg/m with cell sizes of 60-420 μm and the number of nucleated cells (or cell-population
4

8

3

densities) in the range of 10 -10 cells/cm . The processing window associated with the
64

3

7

8

3

low density foams (21 kg/m ) and high nucleation rate (10 -10 cells/cm ) involved an
injection pressure (or an equivalent gas concentration of ∼7 wt %) that corresponds to the
critical pressure of CO2 (7.38 MPa). Talc affected the cell morphology of PLA foams at
lower CO2 content (below ∼5 wt %) because its addition into the matrix led to foams
8

3

with much finer cells (∼50 μm) and a higher cell-population density (10 cells/cm )
4

3

compared to neat PLA foams (∼400 μm and 10 cells/cm ). This observation indicates
that talc particles act as heterogeneous nucleating sites for cell nucleation. Nevertheless,
the effect of talc on the cell morphology disappeared at higher CO2 contents because the
samples processed with or without talc had similar porous morphologies (∼120 μm and
8

3

10 cells/cm ).
Foaming of semicrystalline PLA has been demonstrated with results similar to
those of amorphous PLA. Mihai and coworkers foamed semicrystalline PLA (2002D
from NatureWorks) using a CO2 concentration of 8 wt.% and also created low-density
3

open-cell foams (25 kg/m ) [54]. The foamed samples showed crystallinities of around
15% induced by CO2 and the stretching during bubble growth. Unfoamed samples
usually show little to no-crystallinity due to low crystallization rates of PLA. The use of
0.5 wt % talc did not modify the foam structure or change the degree of crystallinity.
Microcellular extrusion of an amorphous PLA resin (6300D from NatureWorks)
intended for fiber melt spinning has also been produced with CO2. The dependency of

65

the expansion on the amount of blowing agent (2-9 wt %) has been study [41]. Densities
3

in the 20-700 kg/m range were produced with cell sizes of around 100 μm and a cell5

3

population density of 10 cells/cm . Low density foams were associated with higher
concentrations of CO2, i.e., >6 wt.%.
Pilla and coworkers investigated the use of epoxy-functionalized chain extender
on the foamability of semi-crystalline PLA (3001D, NatureWorks) to improve
rheological properties of the melt.

Increasing the molecular weight through chain

extenders is known to increase the melt strength of the resin. When both talc and the
chain extender were added in the formulation, foamed samples with densities down to
3

7

3

300 kg/m and cell-population densities of approximately 10 cells/cm were produced.
An average cell size of around 30 m was observed [103]. The use of talc as nucleating
agent (i.e., 0.5 wt.%) was a necessary condition to achieve uniform morphologies with
increased cell-population density. The addition of 1.0% chain extender doubled the
volume expansion of the foams compared to the foamed neat PLA at the lowest die
temperature, i.e., 130°C. Higher die temperatures led to lower expansion ratios, i.e., 140,
150, and 160°C.
Matuana and Diaz investigated the effect of wood-flour content on the cell
morphology of extrusion foamed PLA with CO2 [104]. Amorphous PLA with 20 wt.%
wood-flour was foamed in a continuous extrusion process and compared with foamed
neat PLA. Wood fibers were pretreated with a 1 wt.% silane coupling agent to enhance
adhesion between the matrix and the filler [96] and 5 wt.% nanoclay was added as

66

nucleating agent. Micrographs of the foamed samples showed a cell morphology with
fine cells of 2 to 10 μm in size in PLA/wood-flour composite foams. Nevertheless, the
samples also contained larger cells (>60 μm) and cracks making the morphology nonuniform. The high temperature profile used in the processing may have played an
adverse role in the foaming process [104]. The processing temperature for the composite
was 20°C above the processing temperature to foam the neat polymer. Since the addition
of wood particles into a polymer significantly increases the viscosity of the melt [105],
the higher processing temperature was required to make the composite more flowable in
the microcellular extrusion system.

Processing at lower temperature profiles led to

overpressure in the system due to the increased viscosity of the composite melt. It has
been shown that high processing temperatures in the extrusion barrel contribute to the
wood component degradation by releasing water vapor and volatiles in the system during
foaming [106], with damaging effects on the morphology. Guo and coworkers suggested
a temperature of 170°C or less to suppress adverse effects from the volatiles generated by
wood-flour during foaming. Above this temperature, these residues are detrimental to the
foam morphology and should be kept to a minimum [106].

Additionally, high

temperatures can also affect the consistency of the PLA which is sensitive to thermal
degradation [63].

In a different study, the effect of wood-flour mesh size on the

foamability of HDPE/wood-flour composites was investigated. The results showed that
finer sizes of wood fibers lead to more uniform morphologies with smaller cell size
because of their enhanced nucleating effect [107] .
These previous studies offered valuable insights, demonstrating the continuousextrusion process of PLA foamed with supercritical CO2. However, the morphology
67

achieved in foamed PLA failed to satisfy the definition of microcellular foam because of
9

3

a lower cell-population density (less than 10 cells/cm ), with cells being larger than 60
μm in average size. Distinctions between these features and microcellular foam persist,
9

3

where the cell-population density ranges from 10 cells/cm and up, with fully grown
cells being smaller than 10 μm in average size. Continuous extrusion of microcellular
9

3

PLA with fine cell size (<10 μm) and high cell-population density (10 cells/cm and up)
will enable not only the production of foamed materials with superior mechanical
properties (compared to their solid counterparts) but also the production of foamed
structures in a thin film.

2.2.4.3 Injection molded microcellular PLA
Through a microcellular injection molding process, Pilla and coworkers
manufactured foamed PLA and PLA/cellulose fiber composites [33]. The addition of
cellulosic fibers increased the specific modulus of both foamed and unfoamed samples.
By creating a cellular structure the storage modulus was increased but the specific
strength and strain decreased. Adding cellulosic fibers reduces the cell size and increases
the cell-population density because of the heterogeneous nucleation effect of the fibers.
However, the morphology of neat PLA showed larger cells (>20 μm) and cell-population
densities in the 10

7

cells/cm

3

range.

In a subsequent study, the addition of

hyperbranched polyester (HBP) was evaluated as a means to improve toughness and cell
morphology in the microcellular PLA [108].

68

Hyperbrached polyester increased

toughness and strain at break in both solid and microcellular foamed specimens while
reducing the specific strength. The cell size decreased with the addition of HBP and the
cell-population density increased. Addition of 2% nanoclay in the formulation further
decreased the cell size and increased the cell-population density, i.e. ~12 μm and 1.6 ×
7

3

10 cells/cm , respectively [108].

2.2.4.5 PLA foamed with chemical foaming agents
Although most research efforts to foam PLA have focused on inert gases as
physical foaming agent, chemical foaming agents are a viable alternative to produce
expanded PLA products. Utilization of existing machinery without adjustments (does not
required a gas delivery system) is one of the advantages of this process. The main
variables are type and amount of chemical foaming agent (endothermic or exothermic)
along with the processing conditions, which influence the cellular structure of extruded
foamed plastics.
A recent investigation showed that fine homogeneous cell morphologies (cell
sizes ~100 μm) could be successfully achieved in extrusion foamed PLA using
endothermic foaming agent. The effects of amount of CFA, the melt flow index (MFI) of
the PLA and processing speed on the cellular structure were investigated.

Results

showed that the melt-flow index of PLA must lie within a range that provides a melt
viscosity low enough to allow cell formation and growth but high enough to prevent cell
coalescence [109]. The amount of CFA affected the cell-population density and the void
fraction in the range studied (0-2%). Both cell-population density and void fraction

69

increased as the CFA amount increased, given the more availability of gas for both
nucleation and cell growth. However, at concentrations above 1.5% cell coalescence
became dominant producing the opposite effect.

The effect of processing speed

(rotational screw speed) was analogous to the CFA content. Higher processing speeds
result in lower residence times in the extruder and less gas for foaming because of
incomplete decomposition of the CFA. Moderate speeds, i.e. ~40 rpm showed best
results [109].
Lee and coworkers investigated the effect of exothermic foaming agent,
azodicarbonamide (ADC), on the expansion of amorphous PLA [41]. The expansion
increased with increased CFA content, reaching a maximum (2.5 times) at around 1%
content of ADC. Higher CFA concentrations led to lower expansion because of the
occurrence of premature foaming which reduce the efficiency of the foaming agent [41].

70

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Wang, Y. B. and Hillmyer, M. A. (2000) Synthesis of polybutadiene-polylactide
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Keener, T. J., Stuart, R. K. and Brown, T. K. (2004) Maleated coupling agents for
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Lu, J. Z., Wu, Q. L. and McNabb, H. S. (2000) Chemical coupling in wood fiber
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Chowdhury, J. and Wolcott, M. (2007) Compatibilizer selection to improve
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Suryanegara, L., Nakagaito, A. N. and Yano, H. (2010) Thermo-mechanical
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Huda, M. S., Drzal, L. T., Misra, M., Mohanty, A. K., Williams, K. and
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in polylactic acid (PLA) composites. Composites Science and Technology. 63,
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81

CHAPTER 3

Study of Cell Nucleation in Microcellular Poly(lactic acid) Foamed with
Supercritical CO2 through a Continuous-Extrusion Process

This chapter is slightly modified from a research paper that was published in Industrial &
Engineering Chemistry Research, 49 (5): 2186–2193 (2010). It is co-authored by L.M.
Matuana and C.A. Diaz.

82

3.1 Abstract
Using a continuous-extrusion foaming process, this study examined the effect of
the processing temperature on the melt viscosity, pressure drop rate, and cell-population
density of poly(lactic acid) (PLA) foams. The study was intended to understand the
nucleation mechanisms in order to generate microcellular structures in neat PLA in a
continuous-extrusion process using CO2 as the blowing agent.

Not only did the

temperature affect the melt viscosity but also the cell nucleation rate owing to its effect
on the pressure drop rate.

When processed at high temperature, the melt viscosity

decreased and PLA nucleated fewer bubbles, irrespective of the PLA type (amorphous vs
semicrystalline), because of the lower pressure drop rate in the system. Conversely,
processing at a lower temperature significantly increased the cell nucleation rate for both
PLA types owing to the fact that a high melt viscosity induced a high pressure drop rate.
The addition of nanoclay allowed both homogeneous and heterogeneous nucleation to
occur during the foaming process. The results showed that the processing temperature is
a sensible parameter to produce microcellular PLA with cell densities on the order of 10
3

cells/cm and cell sizes of around 10 μm.

83

9

3.2 Introduction
Poly(lactic acid) or PLA, a plant-based biodegrable plastic, exhibits many
properties equivalent or superior to those of many petroleum-based plastics. However,
few commercial applications exist because of its low impact resistance and higher cost
over synthetic plastics and also its narrow processing window because of its sensitivity to
hydrolytic and thermal degradation [1, 2].
Blending PLA with various additives such as lubricants, impact modifiers,
plasticizers, or a second polymer broadens its applications [3, 4]. Given the additional
cost associated with additives, unfortunately, this approach could increase the
manufacturing cost [5]. Additionally, plasticizers and impact modifiers reduce the glass
transition temperature of the resin, making the material more flexible and tougher while
simultaneously reducing properties such as tensile strength and modulus.

Previous

efforts to overcome the cost of PLA included its blending with fillers such as cellulosic
fibers [6]. However, lowered cost comes at the expense of other properties such as
ductility (lower elongation at break) and lower impact resistance because the
incorporated brittle cellulosic fibers alter the ductile mode of failure of the matrix,
making the composites more brittle than neat polymer [7-9]. Efforts are continuously
made to further reduce the cost of PLA while enhancing its flexibility and toughness.
Foaming technology, such as microcellular foaming, enhances the ductility and
impact resistance of polymer matrices while also providing a significant expansion ratio
and weight reduction in plastic parts. The high expansion ratio induced by foaming
generally reduces the material cost and consumption in mass-produced plastic parts
without a major compromise to the required properties [10, 11].
84

The microcellular foaming process dissolves an inert gas into a plastic to create a
swarm of bubbles. Microcellular plastics, produced first in a batch process, involve a
solid sample placed into a chamber under high pressure in an inert gas atmosphere (e.g.,
CO2, N2, etc.), wherein gas molecules diffuse into the plastic, forming a single-phase
solution. Subsequently, nucleated and grown cells form by creating a thermodynamic
instability (i.e., sudden pressure drop and/or temperature increase). Characterization of
the morphology of microcellular foams includes cell-population densities of 10

9

3

cells/cm and up and cell sizes smaller than 10 μm.
Recently, production of microcellular-foamed structures in PLA through a batch
foaming addressed some limitations of the polymer [5, 12-15]. A previous study showed
significant improvements in the volume expansion ratio (a 2-fold expansion over that of
unfoamed PLA), impact resistance (up to a 4-fold increase over that of unfoamed PLA),
strain at break (up to a 2-fold increase over that of unfoamed PLA), and toughness (up to
a 4-fold increase over that of unfoamed PLA) by creating microcells in PLA samples,
implying that foamed PLA exhibits enhanced ductility and impact resistance owing to the
presence of microbubbles [5]. Nevertheless, implementation in the industrial production
of foams remains unlikely in the batch foaming process used to generate cellular foamed
structures in PLA because of its time-consuming nature, with multiple steps involved in
the production of foamed samples, i.e., manufacture of solid samples, saturation of solid
samples with gas (sorption experiments), and nucleation of cells and cell growth [16-18].
Limitations of a batch-microcellular foaming process have recently redirected
attention to the manufacturing PLA foams through continuous-extrusion [19-21] and
85

injection-molding processes [22] using supercritical physical blowing agents. The
continuous microcellular foaming implies that the polymer/gas mixing requires a shorter
time frame, from hours/days in the batch process to seconds in the continuous process.
Using gas in supercritical conditions and the polymer in the molten state enhances the
diffusion. The creation of the single-phase solution takes place in the extrusion barrel
with thermodynamic instability produced by a high pressure drop at the exit of the
extruder with the use of a nozzle. Nucleation, cell growth, and bubble stabilization occur
while the polymer exits the die [23, 24]. Compared with the batch foaming process,
relatively few publications feature continuous microcellular foaming of PLA in extrusion.
Reignier and co-workers recently reported a thorough investigation of the
continuous-extrusion foaming of amorphous PLA, using CO2 as a blowing agent [20].
They produced low-density foams under various injection pressures (2.5−9.0 MPa), CO2
contents (1.8−9.3 wt %), and processing temperatures (90−100 °C) and by the addition of
0.5 wt % talc in the resin as a nucleating agent. Neat PLA foamed samples achieved
3

densities down to 21 kg/m with cell sizes of 60−420 μm and the number of nucleated
4

8

3

cells (or cell densities) in the range of 10 −10 cells/cm . The processing window
3

7

8

associated with the low-density foams (21 kg/m ) and high nucleation rate (10 −10
3

cells/cm ) involved an injection pressure (or an equivalent gas concentration of 7 wt %)
that corresponds to the critical pressure of CO2 (7.38 MPa). Talc affected the cell
morphology of PLA foams at lower CO2 content (below 5 wt %) because its addition
into the matrix led to foams with much finer cells ( 50 μm) and a higher cell-population
86

8

3

4

3

density (10 cells/cm ) compared to neat PLA foams ( 400 μm and 10 cells/cm ). This
observation indicates that talc particles act as heterogeneous nucleating sites for cell
nucleation. Nevertheless, the effect of talc on the cell morphology disappeared at higher
CO2 contents because the samples processed with or without talc had similar porous
8

3

morphologies ( 120 μm and 10 cells/cm ).
Microcellular extrusion of semicrystalline PLA (2002D from NatureWorks) and
starch (75% amylopectin and 25% amylose) blends using a CO2 concentration of 8 wt %
3

also created low-density open-cell foams (25 kg/m ) [19]. Obtaining fine cell structure
and low-density foam required interfacial modification of the PLA/starch blends using
maleated PLA. The use of 0.5 wt % talc did not modify the foam structure, and the paper
neglected to include the number of cells nucleated in PLA.
Microcellular extrusion of an amorphous PLA resin (6300D from NatureWorks)
intended for fiber melt spinning has also been produced with CO2 to study the expansion
and cell-population density dependency on the amount of blowing agent (2−9 wt %) [21].
3

Densities in the 20−700 kg/m range were produced with cell sizes of around 100 μm and
5

3

a cell-population density of 10 cells/cm .
While these previous studies offered valuable insights, demonstrating the
continuous-extrusion process of PLA foamed with supercritical CO2, the morphology
achieved in foamed PLA failed to satisfy the definition of microcellular foam because of
9

3

a lower cell-population density (less than 10 cells/cm ), with cells being larger than 60
87

μm in average size. Distinctions between these features and microcellular foam persist,
9

3

where the cell-population density ranges from 10 cells/cm and up, with fully grown
cells being smaller than 10 μm in average size. Continuous extrusion of microcellular
9

3

PLA with fine cell size (<10 μm) and high cell density (10 cells/cm and up) will enable
not only the production of foamed materials with superior mechanical properties
compared to their solid counterparts but also the production of foamed structures in a thin
film.
Because the nucleation process, which governs the cell morphology of materials,
is one of the critical steps in producing microcellular plastics, the aim of the present study
was to understand the nucleation mechanisms in neat PLA in order to generate
microcellular structures in a continuous-extrusion process.

3.2.1 Background on Cell Nucleation
The basis for the continuous microcellular process lies in the concept of
thermodynamic instability to promote high cell nucleation rates in a flowing polymer
matrix. As pointed out by Park and co-workers [25], achieving these high nucleation
rates involves first forming a polymer/gas solution under high pressure, followed by
rapidly decreasing the solubility of gas in the polymer by a rapid pressure drop and/or a
rapid temperature increase, which induces a greater thermodynamic instability in the
system.

Moreover, the classical nucleation theory (equation 3.1) predicts this

phenomenon [26]:

88

  G 
N nucl  Co  f o exp 

 kT 

(Equation 3.1)

with Nnucl as the cell nucleation rate, Co the concentration of gas molecules, fo the
frequency factor for gas molecules joining the nucleus, k Boltzmann’s constant, T the
system temperature, and ΔG the free energy barrier to initiate homogeneous (ΔGhom) or
heterogeneous (ΔGhet) cell nucleation, given by the equations

Ghom 

Ghet 

3
16     bp

3p 2

3
16     bp

3p 2

f  

(Equation 3.2)

(Equation 3.3)

with γbp as the surface energy of the polymer-bubble interface, Δp the pressure drop of
the gas/polymer solution [25], and f(θ) depending on the contact angle at the polymernucleating agent interface. Heterogeneous nucleation occurs at the interface between the
polymer and another phase.
The classical nucleation theory (equation 3.1) predicts that minimization of the
free energy barrier for nucleation (ΔG) will promote high cell nucleation rates. Although
89

many relevant factors can affect the cell nucleation rate (equation 3.1), a higher pressure
drop of the gas/polymer solution lowers the free energy barrier to initiate nucleation (eqs
1.2 and 1.3), resulting in a higher nucleation rate. Park and co-workers demonstrated that
the pressure drop rate in the die (-dp/dt) given by equation 3.4 correlates with the
thermodynamic instability necessary to nucleate high numbers of cells; i.e., the higher the
pressure drop rates, the higher the cell-population densities [25, 27-29]:

 dp  p  p  q


2
dt
t
  ro  L

(Equation 3.4)

with Δt as the average residence time of the flowing polymer/gas solution in the nozzle, L
the length of the nozzle, q the volumetric flow rate of the polymer through the nozzle,
and ro the radius of the nozzle.
Because the pressure drop rate depends on the nozzle geometry (ro and L) and
polymer flow properties (Δp and q), the potential in using these parameters as effective
process and material variables for controlling the cell nucleation in continuous
microcellular processing exists.
Selection in this study designated control of the melt rheology through processing
temperatures as a strategy to alter the rate of pressure drop during foaming. The
processing temperature has a great influence on the density and porous morphology of
foams because of its effect on the melt viscosity, as described by the Arrhenius relation

90

 Ea 

 RT 

  Ao exp 

(Equation 3.5)

with η as the melt viscosity, Ao a constant, R the universal gas constant, T the absolute
temperature, and Ea the activation energy of melt flow [30].
Generally, as the processing temperature is varied, the melt viscosity changes
inversely; i.e., the higher the processing temperature, the lower the melt viscosity, and
vice versa, subsequently leading to a change in the pressure generated during the
extrusion process. The melt viscosity, an important parameter in foaming, affects the
pressure built into the system, thus affecting the cell nucleation rate. Additionally, the
melt viscosity plays an important role during the growth and stabilization of nucleated
cells. Consequently, the effect of the processing temperature on the melt viscosity,
pressure drop rate, and cell-population density must be examined to understand the
nucleation mechanisms in neat and filled PLA foams.

91

3.3 Experimental
3.3.1 Materials
Two different grades of PLA from NatureWorks were used as matrices. The
semicrystalline (PLA 2002D) and amorphous (PLA 8302D) grades had d-lactic contents
of approximately 4% and 10%, respectively. Table 3.1 lists the properties of these resins
measured by the authors [31]. Montmorillonite clay modified with methyl, tallow ( 65%
C18,

30% C16, and

5% C14), and quaternary bis(2-hydroxyethyl)ammonium salt

(Cloisite 30B) purchased from Southern Clay Products (Gonzales, TX) was used as the
nucleating agent, and CO2 with a purity of 99.5% supplied by Airgas was the physical
foaming agent.

92

Table 3.1. Characteristics of different PLA resins used in this study [31].
Melt properties

Thermal properties

Density of the
3
solid (g/cm )

MFR

ρm

(g/10 min)

(g/cm )

χc (%)

Tm (°C)

2002D
semicrystalline

1.25

3.4

1.142

15

149

8302D
amorphous

1.25

6.5

1.149

1.2

PLA grade

a

3a

Value used in the calculation of the volumetric flow rate of the polymer melt (q) in
equation 3.4.

93

3.3.2 Rheological Measurements
The alterations in the melt viscosity induced by the processing temperature
remain vitally important for cell nucleation as well as during cell growth and stabilization.
While facilitating bubble growth requires melts with relatively low viscosity, the
viscosity must increase by cooling or strain hardening to a level adequate for bubble
stabilization. Consequently, understanding the conditions for a proper cell nucleation as
well as expansion and stabilization of the cellular structure entails examination of the
dependence of the melt viscosity on the temperature.
The viscosities of semicrystalline PLA (PLA 2002D) were measured as a function
of the temperature, in the range of 160−205 °C, with 15 °C increments. Measurements
were performed online on an Intelli-Torque Plasticorder Torque rheometer (C. W.
Brabender Instruments Inc., South Hackensack, NJ) equipped with a 19.1 mm singlescrew extruder (L/D ratio 30:1; C. W. Brabender Instruments Inc., South Hackensack,
NJ). The polymer was fed into the extruder and pushed through a capillary die with
different L/D ratios (10:1, 15:1, and 20:1) of capillary inserts. The extruder’s barrel
temperature profile and die temperature were kept constant to maintain a constant melt
temperature for accurate viscosity measurements. However, the rotational screw speeds
varied from 10 to 50 rpm to generate different shear rates during experiments [32].
Experiments followed ASTM standard D5422 without injection of CO2 into the system.
The apparent shear stress (τa), which measures the resistance to flow through a
capillary die, is given by

94

a 

P
4( L / D)

(Equation 3.6)

with ΔP as the pressure drop across the capillary and L/D the length-to-diameter ratio of
the capillary insert. The apparent shear rate at the wall of the capillary for a Newtonian
fluid uses the following formula:

a 

32  Q

  D3

(Equation 3.7)

with Q as the volumetric flow rate of the polymer melt and D the diameter of the
capillary.
Obtaining the true melt viscosity calls for correction of the data to account for an
excess of pressure drop at the capillary entry (Bagley correction) and the shear-thinning
effect (Rabinowitsch correction) [32, 33].
First, the Bagley correction was applied to the shear-stress values as an additional
length of the capillary “end effect” (e). Measurements of the viscosity at the same shear
rate with at least two different capillaries were performed to apply the Bagley correction,
calculated as follows:

w 

P
4( L / D  e)
95

(Equation 3.8)

Second, the apparent shear rate, which gives only Newtonian behavior (constant
viscosity), required a correction for shear-thinning fluids (pseudoplastic fluids). The
Rabinowitsch correction was done to calculate the true shear rate at the wall ( w ) of the
capillary by using the following equation:

w 

3n  1
a
4n

(Equation 3.9)

with n as the flow behavior index obtained as the slope of the linear plot of log τw versus
log a . The true melt viscosity (η) was then calculated, from the corrected shear stress
and shear rate, as follows:



w
w

(Equation 3.10)

3.3.3 Compounding of Materials
PLA in pellet or powder form used in the formulations was oven-dried at 55 °C
for 8 h prior use. Two different foamed formulations were made including (i) neat PLA
and (ii) PLA filled with nanoclay.

96

Neat PLA (formulation i) was foamed by feeding dried pellets directly into the
extruder. In the case of PLA/nanoclay (formulation ii), the resin was first ground to
enhance the dispersion of nanoclay. Powdered PLA was then mixed with 5 wt %
nanoclay (based on the amount of PLA) in a kitchen mixer (Blender MX1050XTS from
Waring Commercial Xtreme, Torrington, CT) at 22 000 rpm for 30 s. The blended
material was oven-dried at 55 °C for 8 h prior to foaming experiments.

3.3.4 Extrusion Foaming
Figure 3.1 shows a schematic of the foaming system used in this study, similar to
the one developed by Park and associates [23, 24]. The process used a 19.1 mm singlescrew extruder (L/D ratio 30:1; C. W. Brabender Instruments Inc., South Hackensack, NJ)
with a mixing screw powered by a 5 hp driver with a speed-control gearbox (Brabender
Prep Center). A positive-displacement syringe pump (model 260D; Teledyne Isco,
Lincoln, NE) delivered the gas into the extrusion barrel. A diffusion-enhancing device
(static mixer Omega FMX8441S) attached to the extruder ensured the creation of a
polymer/gas solution with a rapid pressure drop induced by a nozzle die with 0.5 mm
diameter. All foaming experiments were performed with a fixed amount of gas injected
(5 wt %) and a screw rotational speed of 80 rpm. Table 3.2 summarizes the processing
temperatures used for each PLA grade. Samples were collected after the process reached
steady-state conditions and were cooled by air.

97

Gas
Injection port

Syringe
pump

Pressure
transducers

Hopper

Drive motor

Static
mixer

Filament die
Heating zones

1

2

3

4

5

Figure 3.1. Schematic of the single-screw extrusion foaming system.

98

6

Table 3.2. Definition of temperature profiles according to the set temperature on each
heating zone shown in Figure 3.1.
Set temperature for each heating zone (°C)
PLA grade

Temperature
profile level

1

2

3

4

Static mixer

Nozzle

high

180

180

175

165

165

160

intermediate

170

170

165

155

155

150

low

160

160

155

145

145

140

semicrystalline(2002D) high

210

210

205

195

195

190

intermediate

190

190

185

185

175

170

low

170

170

165

155

155

150

amorphous(8302D)

99

Foamed extrudates were collected to measure the mass output (or the volumetric
flow rate of the polymer through the nozzle q) needed in the calculation of the average
residence time of the flowing polymer/gas solution in the nozzle (equation 3.4), while a
pressure transducer installed in the filamentary die (Figure 3.1) measured the pressure
drop of the polymer/gas solution at the steady state of each processing temperature. These
two parameters are needed to estimate the pressure drop rate according to equation 3.4.

3.3.4 Scanning Electron Microscopy (SEM)
The morphology of the foamed samples was analyzed by SEM. SEM images were
taken using either a JEOL JSM-6400 or a JEOL JSM 7500F (field emission) scanning
electron microscope with accompanying analiSYS software. The accelerating voltage was
set to either 12 kV for JSM-6400 or 5 kV for JSM-7500F. Samples were fractured in
liquid nitrogen and coated with gold prior to the test.
Images obtained by the SEM micrographs allowed for characterization of the
number of cells nucleated per unit volume of the original unfoamed polymer or cellpopulation density (N0) [17]:

3

 nM 2  2  1 
 
No  

 A  1  V f 


 

100

(Equation 3.11)

with n as the number of cells in the micrographs and A and M the area and magnification
factor of the micrograph, respectively. Vf, the void fraction, was measured following the
approach described in our previous work [5]. The densities of both unfoamed and
foamed PLA samples were measured according to ASTM standard D792 (Buoyancy
method) to calculate the void fraction.
SEM images made possible the measurement of the cell size by averaging the
diameters of at least 100 randomly selected cells using well-established procedures [7, 16,
17, 34-36].

101

3.4 Results and Discussion
3.4.1 Rheological Characteristics of PLA
The melt viscosity of PLA, an important parameter in foaming, affects the
pressure built into the system (the cell nucleation rate) and plays a key role during the
growth of the newly formed nuclei and the subsequent stabilization of the foamed cellular
structure.
Figure 3.2 illustrates a double-logarithmic plot of the true viscosity versus the true
shear rate of semicrystalline PLA at a temperature range of 160−205 °C. Irrespective of
the test temperature, PLA melts showed shear-thinning (i.e., pseudoplastic) behavior
because their viscosities decreased linearly with increasing shear rate and with n values
of less than 1 (Table 3.3). The results indicate that the melts obeyed the power law

  K  w n 1

(Equation 3.12)

where K and n, listed in Table 3.3 and obtained from a linear regression of the curves in
Figure 3.2, represent the melt viscosity coefficient and flow index, respectively.

102

Log viscosity (Pa.s)

3.0

2.5

160°C
175°C
190°C
205°C
2.0
2.0

2.4

2.8

3.2

3.6

Log shear rate (s-1 )

Figure 3.2. True viscosity of semicrystalline PLA (2002D) as a function of the true shear
rate at various temperatures (160, 175, 190, and 205 °C).

103

Table 3.3. Melt viscosity coefficient (K) and power law flow index (n) for semicrystalline
PLA (2002D) measured at various temperatures.
regression parameters η = K
n

n−1
2

temperature (°C)

K (kPa·s )

n

R

160

104.11

0.169

0.9932

175

20.26

0.399

0.9674

190

11.38

0.438

0.9999

205

3.71

0.586

0.6966

104

This decreased viscosity of the melts with increasing shear rate was expected
because shearing better orients the molecular chains of the polymer and reduces the
number of entanglements, which results in decreased interactions between chain
segments [30].
Increasing the test temperature decreased the melt viscosity of PLA (Figure 3.2)
because of the greater free space available for molecular chain motion at higher
temperature. In other terms, the ability of macromolecular chain motion is enhanced and
the resistance between the melt layers decreases by increasing the processing temperature,
leading to a reduction in the melt viscosity. With a rise in the test temperature, the melt
viscosity coefficient (K) decreased, whereas the flow index (n) increased significantly
(Table 3.3), suggesting that the melts became less viscous (lower K values) and less
pseudoplastic (higher n values). However, it is worth mentioning that the obtained
viscosity data at a higher processing temperature of 205 °C (Figure 3.2) had a significant
2

scatter (R = 0.6966 in Table 3.3) probably because of the sensitivity of PLA to
hydrolytic and thermal degradation when processed at higher temperature, particularly at
lower shear rate or equivalently longer residence time [1, 2]. Moreover, the reduction in
the melt viscosity of PLA as a function of the test temperature followed the Arrheniustype relationship given in equation 3.5 because the logarithmic viscosity of PLA varied
linearly with the reciprocal temperature (Figure 3.3). The activation energy of viscous
flow (Ea) obtained from the slopes of the curves shown in Figure 3.3 decreased
significantly with increasing shear rate (Table 3.4), implying that the effect of the test
temperature on the melt viscosity became less pronounced at higher shear rate.
Nevertheless, it should be pointed out that, because the shear rate encountered in the
105

2

3 −1

extruder during processing is in the range of 10 −10 s

[37], the temperature profile

used during the extrusion foaming process will significantly affect the melt viscosity
(Figure 3.2).

106

Ln viscosity (Pa.s)

8

200
300
500
900

7

s
s
s
s

ˡ
ˡ
ˡ
ˡ

6

5
2

2.1

2.2

2.3

2.4

1000/T (K-1 )

Figure 3.3. True viscosity of semicrystalline PLA (2002D) as a function of the reciprocal
−1

temperatures (1/T) at various shear rates (200, 300, 500, and 900 s ).

107

Table 3.4. Activation energy (Ea) and the A0 constant for viscous flow of semicrystalline
PLA (2002D) at various shear rates.
shear rate
200 s

−1

300 s

−1

500 s

−1

900 s

−1

Ea (kJ/mol)

43.02

36.99

29.4

20.66

A0 (Pa·s)

0.0082

0.0316

0.173

1.2229

0.999

0.999

0.989

0.938

R

2

108

Not only did the temperature affect the melt viscosity but also the pressure
developed in the system (or pressure drop Δp) during the extrusion of microcellular PLA.
Table 3.5 summarizes the effect of the different processing temperatures on the pressure
drop, average residence time of the flowing PLA/CO2 solution, and calculated pressure
drop rate. Processing at lower temperature developed higher pressure in the system,
regardless of the PLA grade. With insensitivity of the average residence time of flow
through the nozzle to the processing temperatures, increasing the pressure in the system
resulted in higher pressure drop rates.

109

Table 3.5. Effect of the processing temperature on the pressure drop rate for different
grades of PLA.

Δp
(MPa)

flow rate
3
(cm /s)

residence
−4
time (10 s)

pressure
drop rate
(GPa/s)

PLA grade

temperature
profile level

amorphous(8302D)

high

12.4

0.772

3.814

32.8

intermediate

14.1

0.798

3.692

38.3

low

16.6

0.81

3.634

45.5

semicrystalline(2002D) high

12.1

0.832

3.542

34.1

intermediate

14.5

0.809

3.639

39.8

low

17.9

0.836

3.524

50.9

110

It should be pointed out that the viscosity of PLA illustrated in Figures 3.2 and 3.3
was measured without injection of CO2 into the extrusion system. The dissolution of
supercritical fluids in the polymer significantly reduces the viscosity of the polymer
because of the plasticizing effect of the dissolved gas and the increased free volume [28,
29, 36, 38, 39].

Similarly, the viscosity of the gas/polymer solutions (e.g.,

polystyrene/CO2 and polycarbonate/CO2) decreases with increasing temperature [29]. In
the same manner, it is expected that the viscosity of the PLA/CO2 solution will also
decrease with increasing temperature. Although the viscosity of PLA reported in this
study was measured without the addition of any CO2 in the system, it is believed that the
PLA/CO2 solution processed at a lower temperature profile would have higher melt
viscosity than the PLA/CO2 solution processed at a higher temperature profile. Moreover,
because the pressure drop (Δp) is related to the shear stress (equation 3.6), i.e., viscosity,
the data summarized in Table 3.5 clearly support this assertion; processing at lower
temperature developed a higher pressure drop of the PLA/CO2 solution in the system as a
result of the viscosity increase.

3.4.2 Effect of the Temperature Profile on the Morphology
Figure 3.4 shows the effects of the processing temperature and PLA grade on the
cell morphology of microcellular foamed samples. Table 3.6 lists the cell-population
density and average cell size of foamed PLA.

111

a1

a2

a3

50 μm

50 μm

50 μm

b1

b2

50 μm

50 μm

b3

50 μm

Figure 3.4. SEM micrographs showing the effects of the processing temperature profile levels and PLA grade on the cell morphology
for amorphous PLA processed at high (a1), intermediate (a2), and low (a3) temperatures as well as for semicrystalline PLA processed
at high (b1), intermediate (b2), and low (b3) temperatures. All pictures were taken at the same magnification (750X).
112

Table 3.6. Pressure drop rate, cell-population density, and average cell size of foamed
PLA samples.
PLA grade
cell-population
9

average cell
3

size (μm)

density (10 cells/cm )
temperature
profile level

amorphous

semicrystalline

amorphous

semicrystalline

high

0.01

0.04

18.3

16.3

intermediate

0.2

0.1

12.1

15.2

1

1.5

7.9

6.9

low

113

When processed at high temperature, PLA nucleated fewer bubbles, irrespective
of the PLA type (parts a1 and b1 of Figure 3.4). Processing at high temperature lowered
the melt viscosity, thus preventing sufficient pressure from building in the system to form
a one-phase PLA/CO2 solution and, consequently, resulting in a lower pressure drop rate.
The diffusivity (D) of CO2 could also account for the lower nucleation rate observed at
high processing temperature because the diffusivity of CO2 is a function of the
temperature, as given by the following Arrhenius-type relation [29]:

 ED 
D  Do exp  

 RT 

(Equation 3.13)

with D0 as the preexponential factor (diffusion coefficient constant), ΔED the activation
energy for diffusion of gas in a polymer, R the universal gas constant, and T the
processing temperature in the absolute scale.
As the processing temperature was increased, the viscosity of the PLA/CO2
solution decreased (according to equation 3.5) and the gas diffusion rate into the polymer
increased (according to equation 3.13) because of the mobility of gas molecules at high
temperature. Therefore, it is believed that the increased gas diffusivity (higher gasdepletion rate), which promoted gas loss to the environment, also accounted for a smaller
amount of CO2 available for cell nucleation. When the processing temperature was too
high, the surface of the extrudate was not effective in blocking gas escape. Very similar
114

results were obtained by Park et al. in a study on low-density microcellular foam
processing of high-impact polystyrene in extrusion using CO2 as a blowing agent [24].
Conversely, processing at a lower temperature significantly increased the cell
nucleation rate (Figures 3.4a3 and 3.4b3) for both PLA types because of the fact that a
high melt viscosity induced a high pressure drop rate. This trend was expected because
the higher pressure drop rate of the polymer/gas solution lowers the free energy barrier to
initiate nucleation, resulting in a higher nucleation rate. These results agree with those
reported by other investigators showing that increasing the pressure drop rate
significantly increased the number of nucleated cells [25, 27-29]. Additionally, the
nucleation of bubbles appropriately used most of the gas injected into the system due to
the lower gas diffusion (lower gas-depletion rate) at low processing temperature
(equation 3.13). Higher melt viscosity also prevented gas depletion in the PLA/CO2
solution when the processing temperature was lower.
The data presented in Table 3.6 also indicate the importance of a larger pressure
drop rate occurring at low processing temperature on the number of nucleated cells. The
cell-population density, by reduction of the processing temperature from a high to low
profile, marked a 2 orders of magnitude increase. Microcellular morphology was
successfully produced in PLA through a continuous-extrusion process. Irrespective of the
9

3

PLA type, the number of nucleated bubbles, with an order of 10 cells/cm and an
average cell size of less than 10 μm, satisfies the definition of microcellular plastics. As
the melt viscosity increased with a decrease in the temperature, the cell coalescence
decreased, and, consequently, most nucleated cells survived during the cell growth step.
115

3.4.3 Effect of the Nucleating Agent
As was previously mentioned, heterogeneous nucleation occurs at the interface
between the polymer and another phase. Generally, the addition of external nucleating
agents (e.g., solid particles such as talc, nanoclay, etc.) provides sites for heterogeneous
nucleation of cells because of their nature in reducing the surface energy at the
polymer−particle interface [40]. The free energy barrier for nucleation (ΔGhet), reduced
by lowering the surface energy of the polymer−particle interface (equation 3.3), promotes
high cell nucleation rates (equation 3.1).
The effect of nanoclay addition on the cell morphology of semicrystalline PLA
foams was assessed by filling semicrystalline PLA with 5 wt % nanoclay. This
formulation was microcellular foamed using the low-temperature profile for
semicrystalline PLA (Table 3.2), which provided the best conditions for microcellular
PLA foamed in a continuous-extrusion process. Figure 3.5 presents SEM micrographs
illustrating this effect.

116

a

b

30 μm

5 μm

c

d

30 μm

5 μm

Figure 3.5. Field-emission SEM micrographs (JEOL JSM 7500F) illustrating the effect of
nanoclay addition on the cell morphology of semicrystalline PLA foams: (a and b)
without nanoclay; (c and d) with nanoclay. Micrographs a and c were taken at low
magnification (750X), whereas micrographs b and d were taken at high magnification
(5000X).

117

The SEM images taken at low magnification revealed no apparent differences in
morphology between the neat PLA (Figure 3.5a) and PLA filled with nanoclay (Figure
3.5c), suggesting that the addition of a nucleating agent had no effect on the
homogeneous nucleation. By contrast, SEM images taken at high magnification revealed
noticeable differences between the neat and filled PLA. For the neat PLA, observations
indicate a solid membrane between grown bubbles (Figure 3.5b), whereas this membrane
contained a large number of fine cells in PLA filled with nanoclay (Figure 3.5d), with the
fine cells measured in nanometers. Increased cell nucleation accounts for this behavior
because of the presence of the heterogeneous nucleation sites. This bicellular structure
was also observed in our previous work on microcellular foaming of rigid PVC using
CO2 as the foaming agent in both batch and continuous-extrusion processes [7, 41] and
may be indicative of two nucleation stages (homogeneous and heterogeneous) occurring
during foaming.
It is believed that larger cells were nucleated earlier from the rapid pressure drop
in the nozzle (homogeneous nucleation), while the finer cells resulted later from the
presence of solid particles (nanoclay) in PLA, which provided sites for heterogeneous
nucleation because of their nature in reducing the surface energy at the polymer−particle
interface. Cell nucleation and cell growth are competing processes for gas consumption
during foaming [36]. The final cell size is determined by the amount of gas used for the
cell growth in the end [7]. Because the available amount of gas was shared by both
homogeneously and heterogeneously nucleated cells growing simultaneously, cells
nucleated earlier (homogeneous) had a longer time for growth, resulting in cells of larger

118

size compared to cells nucleated later (heterogeneous), which had a shorter time for
growth, leading to cells of smaller size.

119

3.5 Conclusions
The aim of the present study was to understand the nucleation mechanisms in
PLA in order to generate microcellular structures in a continuous-extrusion process using
CO2 as the blowing agent. Particular emphasis also focused on assessing the effect of
nanoclay as a nucleating agent. Examination of the effect of the processing temperature
on the melt viscosity, pressure drop rate, and cell-population density aided in
understanding the nucleation mechanisms in PLA foams.
The processing temperature had a strong effect of the rheology of the melt.
Increasing the test temperature significantly decreased the melt viscosity of PLA, and this
reduction in the melt viscosity of PLA as a function of the test temperature followed the
Arrhenius-type relationship.
Not only did the temperature affect the melt viscosity but also the cell nucleation
rate because of its effect on the pressure developed in the system during the extrusion of
microcellular PLA. Microcellular morphology was successfully produced in PLA through
a continuous-extrusion process. Irrespective of the PLA type, the number of nucleated
9

3

bubbles, with an order of 10 cells/cm and an average cell size of less than 10 μm,
satisfies the definition of microcellular plastics.
When processed at high temperature, PLA nucleated fewer bubbles, irrespective
of the PLA type. Processing at high temperature lowered the melt viscosity, thus
preventing sufficient pressure from building into the system and, consequently, resulting
in a lower pressure drop rate. The increased gas diffusivity, which promoted gas loss to
the environment, also accounted for a smaller amount of injected CO2 available for cell
120

nucleation. Conversely, processing at a lower temperature significantly increased the cell
nucleation rate for both PLA types because of the fact that a high melt viscosity induced a
high pressure drop rate.
The presence of the nanoclay did not affect the homogeneous nucleation but
increased the heterogeneous nucleation, with the fine cells measured in nanometers. The
addition of nanoclay allowed both homogeneous and heterogeneous nucleation stages to
occur during the foaming process.

121

3.6 Acknowledgment
This project was supported by the National Research Initiative of the USDA
Cooperative State Research, Education and Extension Service (Grant 2006-35504-17414).
The authors gratefully acknowledge Valerie Edge, an undergraduate student at Michigan
State University, for her thoughtful and constructive comments on the paper.

122

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123

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127

CHAPTER 4

Rheology Modification Approach to Continuous Microcellular Foaming
of Polylactic Acid/Wood-Flour Composites

128

4.1 Abstract
This study investigated the effects of wood-flour and rheology modifier contents
on the melt viscosity and cell morphology of microcellular foamed polylactic acid
(PLA)/wood-flour composites produced through a continuous extrusion process. The
melt viscosity of PLA increased as the relative amount of wood-flour increased in the
composites. In contrast, increasing the rheology modifier content produced the opposite
effect reducing the viscosity of the composites. A linear relationship was identified for
the effect of rheology modifier on melt flow index (MFI) for the PLA/wood-flour
composites and was used to formulate composites with lower, similar, and higher
viscosities than the neat PLA. Matching the viscosity of the composites with that of neat
PLA resulted in the best cell morphologies. Lower concentrations of rheology modifier
resulted in unprossesable conditions because of the increased viscosity which generated
elevated pressures in the system above operational limits. Higher concentrations of
rheology modifier resulted in foams with poor cell morphologies. By matching the
viscosity of the composites with that of the neat PLA, foamed composites with woodflour contents up to 30 wt.% were manufactured.

The results confirmed that

microcellular structures, with cell sizes of around 10 μm and cell-population densities in
the order of 10

9

3

cells/cm , were successfully produced in the PLA/wood-flour

composites.

129

4.2 Introduction
Polylactic acid (PLA) is a thermoplastic made from lactic acid derived from
renewable resources such as corn and sugar beets. Recently, large scale production and
commercialization of PLA have prompted this material to lead the market of
biodegradable plastics as an environmentally sustainable substitute to petroleum-based
plastics. Despite the good publicity and environmental benefits of green materials, the
shift to bio-plastics is still limited by the higher cost of the resins and the better
performance of traditional plastics. PLA is more brittle and has a lower impact resistance
than traditional plastics such as polystyrene and polyethylene.

Also, it has narrow

processing windows due to its sensitivity to thermal and hydrolytic degradation [1].
With the aim of reducing the cost of final products while retaining
biodegradability, blending of PLA with low cost cellulosic fibers has been investigated
[2-4]. Filling PLA with cellulosic fibers can increase the tensile modulus and reduce the
overall cost. However, while cost reduction can be achieved, toughness and impact
resistance are deteriorated [3]. To overcome these shortcomings, one alternative is to
create microcellular structures in PLA/wood-flour composites because the presence of
micro-bubbles can inhibit crack propagation by blunting the crack tip increasing the
energy for crack propagation [5]. Microcellular foamed PVC/wood-flour composites
produced in the batch process showed that the creation of a microcellular structure
increased the impact resistance to values higher than the unfoamed and unfilled polymer
[6]. The basic approach is to saturate the solid samples with large amounts of an inert gas
under high pressure and induce a thermodynamic instability by a sudden drop in
solubility. Unfortunately, the batch process is not appealing for industrialization because
130

of its long cycle times. Continuous processes, such as extrusion foaming, have been
developed based on the concept of thermodynamic instability created by a rapid pressure
drop at extruders exit with the use of a nozzle die [7].

This process has been

demonstrated on neat PLA. However, there are no studies on microcellular extrusion
foaming of PLA/wood-flour composites.
Diaz and Matuana studied the effect of processing temperature on the cell
morphology of microcellular PLA foams in a continuous extrusion process using CO2 as
foaming agent [9]. The viscosity of the melt proved to be of paramount importance to
successfully produce microcellular foams in PLA because it controlled the pressure built
in the system and the pressure drop rate, necessary to nucleate a swarm of bubbles [9, 10].
Nevertheless, the addition wood-flour affects the microcellular extrusion process because
of their effect on the rheology of the melt [11, 12], heterogeneous cell nucleation [13],
and foamability [14]. In the molten state, wood-flour increases the viscosity of the resin
[11, 15] affecting the operating conditions in a foam extrusion system because of its
effect on melt viscosity, thus affecting the cell nucleation rate [9].
Reported studies on extrusion foaming of wood fiber composites have focused on
assessing the effects of extrusion parameters, wood (content, size and moisture), blowing
agent (type and concentration), etc., on the cell morphology [5, 11, 14, 17, 18]. However,
there are no studies considering the melt viscosity as a key parameter in the foaming
process. It is important to develop approaches to control the melt viscosity to produce
microcellular structures in the composites.

131

One way to alter the viscosity of the composites is through changes in the
processing temperature. Increasing the melt temperature is known to reduce the viscosity
of the melt, as described by an Arrhenius relationship [9, 19]. Unfortunately, since
increasing the temperature can accelerate the degradation of the fibers releasing water
vapor and volatiles detrimental for the foaming process [17] and initiate the degradation
of PLA [1], a different approach should be considered.
An alternative means to alter the viscosity of the composites is through changes in
the material formulation.

Li and co-workers studied the effects of wood-flour and

internal lubricant contents on the rheological properties of wood-flour filled
polypropylene. The results showed that increasing the wood-flour content increased the
apparent viscosity of the melt because the solid wood particles in the molten polymer
increased the resistance against the flow. By contrast, the addition of internal lubricant
significantly decreased the melt viscosity of the composites by reducing the friction
between polymer chains and wood particles [20]. Nearly a 50% reduction in apparent
viscosity was observed by adding 5 wt.% of an ester based internal lubricant [20]. Low
molecular weight additives are known for their low viscosity and plasticizing effects, a
desirable characteristic to control the viscosity of a blend. By controlling the viscosity of
the melt through changes in the formulation, the foaming process can be carried out at
same conditions as for the neat PLA without raising the processing temperature. Also,
processing at lower temperatures would favor the foamability of the composites [17].
In this study the effects of wood-flour and rheology modifier contents on the melt
viscosity and cell morphology of foamed samples were investigated to identify melt
properties suitable for microcellular foam processing of PLA/wood-flour composites.
132

4.3 Experimental
4.3.1 Materials
A semi-crystalline grade of polylactic acid (PLA 2002D) from NatureWorksTM
was used as matrix with an approximately D-lactic content of 4% and a melt flow index
between 6.9 g/10min (210°C/2.16 kg). The wood-flour used was softwood-flour (pine)
grade 12020, supplied by American Wood-flour Fibers (Schofield, WI). To promote
heterogeneous nucleation, talc (Mistron Vapor-R) from Luzenac Corporation, with a
2

median particle size of 2 μm and a specific surface area of 13.4 m /g, was used. The
rheology modifier used in the study was Epolene E-43, low molecular weight maleic
anhydride-modified polypropylene (MAPP), supplied by Eastman Chemicals (Rochester,
NY). CO2 with a purity of 99.5%, supplied by Airgas, was used as physical foaming
agent.

4.3.2 Rheological Measurements
Since wood-flour and additives concentrations change the rheology of PLA
composites [12], the effect of wood-flour and rheology modifier contents on the viscosity
of the melt was measured via melt flow index (MFI), on-line capillary rheometry, and
rotational rheometry with extensional viscosity fixture.
MFI was measured according to the procedure outlined in ASTM D1238
(Procedure A) using a Melt Indexer (model LMI 4000) by Dynisco Polymer Testing Inc.
The melt temperature was set at 210°C and an applied dead load of 2.16 kg (including the

133

piston) was employed. A melt time of 2 min was set in order to avoid PLA thermal
degradation.
On-line capillary rheometry measurements were performed on an Intelli-Torque
Plasticorder® Torque Rheometer (C.W. Brabender Instruments Inc.) equipped with a
19.1 mm single-screw extruder (L/D ratio 30:1). A capillary die with three different
inserts (L/D ratios: 10:1, 15:1, 20:1) were use for the experiment. The temperature
profile in the extruder and die was kept constant to maintain a constant melt temperature
for accurate viscosity measurements. All of the heating zones were set to 170°C. The
rotational screw speeds varied from 10 to 80 rpm to generate different shear rates during
experiments [12]. Experiments followed ASTM standard D5422 without injecting CO2
in the system as described in our previous study [9]. The melt density used in the
viscosity calculations was measured as described in a previous work [21].
Both Bagley and Rabinowitsch corrections were performed account for the excess
of pressure drop due to entrance flow and the shear thinning effect of pseudoplastic fluids
respectively. A detailed description of the calculations can be found elsewhere [9, 22].

4.3.2.1 Extensional viscosity
Two methods were used to characterize the effect of wood-flour and rheology
modifier contents on the extensional viscosity of PLA: on-line capillary rheometry and
dynamic rotational rheometer. From the entrance pressure drop in the on-line capillary
measurements, apparent extensional viscosity was calculated using the Cogswell’s
equations [22-24].
134


The extensional viscosity is the ratio of the extensional strain rate (  A ) and the
extensional shear stress (  E ) calculated as follows:

2
4 a a
3n  1Pe

(Equation 4.1)

3
n  1Pe
8

(Equation 4.2)


A 

E 

where ( a ) is the apparent shear rate, (  a ) the apparent shear viscosity and ( Pe ) the
pressure drop at the capillary entrance, calculated from the end effect as:

Pe  4  e   w

(Equation 4.3)

Finally, the apparent extensional viscosity can be calculated as:

 EC 

 E 9n  12 Pe 2


A
2
32 a a

135

(Equation 4.4)

Direct measurement of the extensional viscosity was done using the extensional
viscosity fixture (EVF) on the ARES extensional rheometer from TA instruments (New
Castle, DE). Rectangular samples with 18 ×10 ×0.78 mm dimensions were molded at
180°C. Extensional viscosity was measured at a fixed Henky strain rate (0.1 s-1) as a
function of time at 160°C. Further discussion of the method can be found elsewhere [25,
26].

4.3.3 Compounding of Materials
PLA in powder form (ground from pellets) and wood-flour used in the
formulations were oven dried at 55 oC for 8 hrs and 105°C for 48 hrs prior to use,
respectively. PLA, wood-flour, rheology modifier and 0.5 wt. % of talc were mixed in a
kitchen mixer (Blender MX1050XTS from Warning Commercial Xtreme) at 22,000 rpm
for approximately 45 sec.

4.3.4 Extrusion Foaming
Figure 4.1 shows a schematic of the foaming system used in this study. The
process used a 19.1 mm single screw extruder (L/D ratio 30:1, C.W. Brabender
Instruments Inc.) with a mixing screw powered by a 5 hp driver with speed-control
gearbox (Brabender Prep Center). A positive displacement syringe-pump (Teledyne Isco.,
Model 260D) delivered the gas into the extrusion barrel. A diffusion enhancing device
(static mixer Omega FMX8441S) attached to the extruder ensured the creation of a
polymer/gas solution with a rapid-pressure drop induced by a nozzle die with a 0.5 mm
136

diameter. All foaming experiments were performed with gas injected at a constant
volume of 5 mL/min (approximately 5 wt. %) and screw rotational speed of 80 rpm. The
processing temperatures was set in the extruder barrel 170-170-165°C from hopper to
extruder’s exit, 165°C in the static mixer and 150°C in the nozzle die. Samples were
collected after the process reached steady state conditions and cooled by air.

137

Gas
Injection port

Syringe
pump

Pressure
transducers

Hopper

Drive motor

Static
mixer

Filament die
Heating zones

1

2

3

4

5

Figure 4.1. Schematic of the single-screw extrusion foaming system.

138

6

4.3.5 Density measurements
Densities measurements were carried out according to ASTM standard D792
(buoyancy method), for unfoamed (ρu) and foamed (ρf) samples. By measuring the
weights of at least five samples in the air (m1) and in an immersion liquid (m2), i.e., nhexane, the density was calculated as:



m1   FL
m1  m2 

(Equation 4.5)

3

where  FL is the density of n-hexane (0.6594 g/cm ). The void fraction (Vf) was then
determined using the following equation:

Vf 

u   f   100
u

(Equation 4.6)

4.3.6 Scanning Electron Microscopy
Morphology of foamed samples was analyzed via scanning electron microscopy
(SEM JEOL JSM-6400, 10 kV). Samples were fractured in liquid nitrogen and coated
with gold prior to the test. The images obtained aided the characterization of cell
population density using Eq. 7 [27]:

139

3

 nM 2  2  1 

No  
 A  1  V f 


 



(Equation 4.7)

with n as the number of cells in the micrographs, and A and M as the area and the
magnification factor of the micrograph, respectively. Vf is the void fraction described in
Eq. 6.
Cell sizes were measured using image analysis software, UTHSCSA Image Tool.
After a proper spatial calibration of the micrograph, the diameter of at least 100 randomly
selected cells was manually measured and the average cell size calculated. Breaking the
cell size values into classes allowed the making of histograms and cumulative percentage
plots.

140

4.4 Results and Discussion
4.4.1 Rheological Characterization
The effects of wood-flour and rheology modifier contents on melt viscosity were
characterized via melt flow index (MFI). MFI denotes the rate of extrusion of molten
resin through a die under specific temperature and load in g/10 min and is an indirect
measurement of the viscosity. Higher MFI values correspond to lower viscosity. Figure
4.2 is the plot of MFI as a function of rheology modifier contents for varying amounts of
wood-flour, i.e., 10, 20 and 30 wt.%. Without rheology modifier (0% E-43), increasing
the amount of wood-flour reduced the MFI, making the melt more viscous. This is
attributed to the interaction between wood-flour particles and the matrix [11]. Solid
wood-flour particles embedded in the polymer matrix increase the friction towards the
flow of the melt resulting in higher viscosity [28].

141

Figure 4.2. Effect of wood-flour and rheology modifier contents on the MFI of PLA
2002D at 210°C. The dashed line represents the MFI of neat PLA 2002D.

142

The addition of rheology modifier increased the MFI as its relative amount
increased irrespective of the wood-flour concentration, easing the flowability of the
composite. Low molecular weight additives can produce a plasticizing effect. Generally,
the additive separates the polymer chains, increasing their flexibility [29]. Also, the
additive can produce a lubricating effect reducing the friction between the polymer and
the wood-flour [20].
In the range studied, the effect of adding rheology modifier on MFI was linear for
all three wood-flour contents (Figure 4.2). Drawing a line parallel to the X-axis at a MFI
value of 6.9 g/10 min (MIF of neat PLA, dashed line in Figure 4.2), intercepts with the
MFI versus rheology modifier lines at rheology modifier concentrations of 0.3, 1.7, and
4.7 wt. %, for the composites containing 10, 20, and 30 wt. % wood respectively (see
Table 4.1). Above these concentrations, the formulations have a viscosity lower than that
of neat PLA (higher MFI) while below the formulations showed higher viscosities (lower
MFI) than neat PLA. It is hypothesized that formulations with viscosities similar to that
of neat PLA will yield the best morphologies. Table 4.1 shows the linear regression
equation for the effect of rheology modifier content on MFI for each wood-flour content
and the interception with the MFI of neat PLA, matching point.

143

Table 4.1. Linear relationships of the effect of rheology modifier content on MFI for each
wood-flour content.
Wood-flour

Linear equation

R

2

content (%)

Predicted E-43
concentration (%)

10

2.495x + 6.256

0.966

0.253

20

2.006x + 3.504

0.982

1.688

30

1.279x + 0.959

0.996

4.637

144

To corroborate the results from MFI, and to gain a deeper understanding on the
role of wood-flour and rheology modifier on the melt rheology, shear viscosity as a
function of shear rate was measured in selected formulations (i.e., neat PLA, PLA with
20 wt. % wood-flour, and PLA with 20 wt. % wood-flour and 2 wt. % rheology modifier).
Although MFI is related to the viscosity of the melt it does not provide the actual flow
properties during processing operations. Viscosity characterization at typical extrusion
-1

shear rates (100 to 1000 s ) [30] is important to understand the flow of the material
during foaming. Figure 4.3 shows the double-logarithmic plots of the true viscosity
versus the true shear rate of the three formulations. Irrespective of the formulation the
melts showed shear thinning behavior and obeyed the power law as described by Eq. 8:

  K  w n 1

(Equation 4.8)

where K and n are constants representing the melt viscosity coefficient and power law
flow index, respectively [9]. The values for K and n, obtained from the curve fitting of
the power law equation, are listed in Table 4.2.

145

Figure 4.3. True viscosity vs. true shear rate of PLA, PLA with 20 wt. % wood-flour, and
PLA with 20 wt. % wood-flour and 2 wt. % rheology modifier.

146

Table 4.2. Melt viscosity coefficient (K) and power law flow index (n) for neat PLA,
PLA with 20 wt. % wood-flour, and PLA with 20 wt. % wood-flour and 2 wt. %
rheology modifier.
Regression parameters

  K n 1

Material composition

n

R

2

K (kPa·s )

n

Neat PLA

30.6

0.318

0.9951

PLA+20%WF

51.7

0.257

0.9999

PLA+20%WF+2%E-43

11.4

0.456

0.9999

147

When comparing the neat PLA with the composite with 20 wt. % wood-flour, a
shift of the curve upwards was observed as well as a higher viscosity coefficient (Table
4.2), indicating that the composite melt was more viscous. A change in the flow index,
from 0.32 to 0.26, indicates a stronger shear thinning effect when the wood-flour fibers
are present.

Shear thinning responds to the alignment and disentanglement of the

polymeric chains when subjected to shear; adding of wood-flour fibers is known to
accentuate this behavior because the fibers also align and induce higher local shear rates
in the thin polymeric layer between wood-flour particles, thus the observed reduction in n
[31]. Adding the rheology modifier (E-43 in table 4.2) lowered the viscosity of the melt
to similar or lower values than neat PLA and also changed the flow index, from 0.26 to
0.46, making the melt less pseudoplastic (n=1 means the flow is Newtonian). This
behavior is attributed to the plasticizing effect of the additive, which increases the
mobility of the polymer chains and the flow of wood-flour particles. Similar results have
been observed in polypropylene/wood-flour composites [20].
Beside shear viscosity, elastic properties of the melt such as extensional viscosity
also play an important role in foam processing. Extensional viscosity is responsible for
entrance pressures when the polymer flows though contracting channels or tubes [32],
affecting the pressures drop during on-line capillary rheometry measurements and the
pressures built in the extrusion system when foaming. Also, as bubbles grow during
foam processing, the polymer is stretched making the elastic properties of the melt a
decisive factor in controlling the cell morphology [16].

The polymer resistance to

stretching is proportional to the stretching rate, namely extensional viscosity [33]. Figure
4.4a shows the double logarithmic plot of apparent extensional viscosity versus apparent

148

strain rate calculated from the end effects using the Cogswell’s equations, Eqs. 1 to 4.
Irrespective of the formulation, as the extensional strain rate increased the extensional
viscosity decreased in a linear manner, thus a power law relation persists, similar to that
of the shear viscosity (Figure 4.3). Neat PLA showed the lowest extensional viscosity.
When wood-flour is added in the formulation the extensional viscosity increases,
confirming the toughening effect of wood-flour.

Strong dependence of extensional

viscosity on wood-flour content has been shown in HDPE/Maple wood-flour flour
composites with extensional viscosity increasing with increasing wood-flour loading [15].
Adding 2 wt. % rheology modifier to the PLA/wood-flour composite did not produce a
significant change in the extensional viscosity and the curve overlapped the one without
the additive.

149

a

b

Figure 4.4. Extensional viscosity measured by (a) on-line capillary rheometry, and (b)
ARES-EVF rheometer for PLA, PLA with 20 wt. % wood-flour, and PLA with 20 wt. %
wood-flour and 2 wt. % rheology modifier.

150

Extensional viscosity measurements in a rotational rheometer were performed to
corroborate the results from capillary rheometry (Figure 4.4b). Neat PLA showed the
lowest extensional viscosity whereas PLA/20%wood-flour showed the highest.

The

addition of rheology modifier (E-43) slightly reduced the extensional viscosity of the
composite. As expected from the chain architecture of PLA, none of the formulation
showed strain hardening. Similar results were reported by other investigators [26].

4.4.2 Effect of rheology modifier content on cell morphology
Foaming was perform on PLA/20 wt. % wood-flour composites with different
amounts of rheology modifier (below the matching point, close to the matching point,
and above the matching point) to understand the effect of rheology modifier on the
foamability of the composites. Below the matching point (i.e., 1.3 wt. %, MFI=6) the
foaming process was unstable and the pressure increased to a point the system had to be
shut down. During the foaming process, the melt experience contraction flow through
the static mixer and the nozzle (Figure 4.1), generating entrance pressures which led to a
rise in pressure above the operational limits in the extrusion system when the melt was
too stiff. Also, the nozzle used for these experiments was particularly small, i.e., 0.5 mm
of diameter, and without the proper lubrication, provided by the rheology modifier, the
wood-flour particles may agglomerate leading to the nozzle being blocked. Foaming was
successful in formulations with rheology modifier amounts close and higher to the
matching point (i.e., 2, 4, and 6 wt. %, MFI=7.5, 11.5, and 15.5 respectively). However,
the cell morphology significantly varied.

151

Figure 4.5 shows the effect of rheology modifier content on the cell morphology.
For a concentration close to the matching point, i.e., 2 wt. % (Figure 4.5a), the
morphology was uniform with cell sizes in the order of 10 μm (see Table 4.3). As the
amount of E-43 increased from 2 to 6 wt. %, the morphology was deteriorated showing
big cracks and cell coalescence (Figures 4.5b and c) also reflected in the measured cellpopulation density and average cell size (Table 4.3). It is believed that the softer melt at
higher rheology modifier concentrations did not provide sufficient melt strength to trap
the growing bubbles leading to cell coalescence [16]. The void fraction decreased as the
amount of E-43 (see Figure 4.6) increased because the melt became too soft favoring cell
coalescence which may have led to channels for the gas to escape to the environment
during foaming. The results clearly showed that the composite with rheology modifier
amount close to the matching point produced the best morphology and confirmed the
adequacy of the strategy proposed to set the amount of E-43 based on the MFI.

152

a

b

c

100 μm

100 μm

100 μm

Figure 4.5. Effect of rheology modifier content on cell morphology of foamed PLA
samples containing 20 wt.% wood-flour. (a) 2 wt.%, (b) 4 wt.%, (c) 6 wt.%. Pictures
originally taken at 350X.

153

Table 4.3. Effect of rheology modifier content on cell-population density and cell size.
E-43 content
(%)

2
4
6
a

a

Cell-population density
9

Average cell size

3

(μm)

A

9.2

B

15.5

C

18.3

(10 cells/cm )
0.67 ± 0.106

0.28 ± 0.094
0.08 ± 0.043

The superscript letters represent the ANOVA results. The means with different letters

are significantly different.

154

40

Void fraction (%)

30

A

B

20

C
10

0
0

2

4

6

8

E-43 content (%)

Figure 4.6. Effect of rheology modifier content on void fraction of foamed PLA samples
containing 20 wt.% wood-flour and varying amounts of rheology modifier. The letters in
the graph represent the ANOVA results. The means with different letters are significantly
different (p<0.05).

155

4.4.3 Effect of wood-flour content on cell morphology
Foaming of PLA composites with wood-flour contents varying from 0 to 30 wt. %
was performed and the morphology is shown in Figure 4.7. Rheology modifier contents
of 0.3, 2 and 5 wt. % E-43 were used for the composites containing 10, 20 and 30 wt. %
wood-flour respectively, matching the MFI of PLA as previously discuss. The amounts
of rheology modifier used were slightly higher than those predicted in Table 4.1 to ensure
processability of the composites. The micrographs clearly show a foamed matrix with
embedded wood-flour particles, corroborating the idea that the gas solubilize only in the
wood component [8]. Foamed composites with 10 and 20 wt. % wood-flour rendered
fine celled uniform morphologies as seen in Figure 4.7b and c. At the highest wood-flour
content (i.e., 30 wt.%), the cellular structure showed large cells with irregular shape.

156

a

b

100 μm

100 μm

c

d

100 μm

100 μm

Figure 4.7. Effect of wood-flour content on cell morphology of foamed PLA samples. (a)
0% wood-flour, (b) 10% wood-flour, (c) 20% wood-flour, (d) 30% wood-flour. Pictures
originally taken at 350X.

157

The effects of wood-flour content on cell-population density and average cell size
of foamed composite are presented in Table 4.4. The cell-population density decreased
gradually with increasing amounts of wood-flour. Several factors affected the cellpopulation density. Since the plastic component available to dissolve the gas and form
bubbles is reduced as the amount of wood-flour increased, the number of cells nucleated
per unit volume was reduced at higher wood-flour contents. Without wood-flour (Figure
4.7a) the bubble nucleation is mainly homogeneous [9]. For small amount of wood-flour,
i.e., 10 wt.% (Figure 4.7b), this nucleation mechanism remained the dominant one.
However, at higher wood-flour concentrations, i.e., 30 wt. %, bubbles may have
nucleated at the interface of the wood-flour and the plastic reducing the overall
nucleation rate, thus the lower cell-population density.

158

Table 4.4. Effect of wood-flour content on cell-population density and cell size of foamed
samples.
Wood-flour
content (%)

a

Cell-population density
3

(μm)

A

7.4

AB

8.9

B

9.3

C

11.3

9

(10 cells/cm )

0

1.46 ± 0.54

10

1.13 ± 0.33

20

0.83 ± 0.13

30
a

Average cell size

0.36 ± 0.12

The superscript letters represent the ANOVA results. The means with different letters

are significantly different (log transformation applied).

159

The void fraction of the foamed composites is plotted in Figure 4.8. The addition
of wood-flour did not significantly affect the void fraction which remained around 20%
irrespective of wood content. Figure 4.9 shows the effect of wood-flour content on the
cell size distribution of the foamed samples. For Neat PLA more than 90% of the cells
have cell sizes below 10 μm, with high cell uniformity given by the steeper curve. At 10
and 20 wt.% wood-flour contents, samples showed similar cell size distributions with
about 60% of the cells being smaller than 10 μm and 90% smaller than 15 μm. At 30 wt.%
wood, the cell size distribution becomes broader and less uniform, less steeper curve,
showing that big cells are present in the samples. High wood-flour concentration may
have provided sites for undissolved gas pockets during processing leading to uncontrolled
cell growth and overall deterioration of the morphology [34]. Additionally, wood-flour
can release vapor and volatiles which may have play a deleterious effect on the cell
structure, especially at the highest wood-flour content [17].

160

Figure 4.8. Effect of wood-flour content on void fraction fraction of foamed PLA/woodflour composites samples. The differences in means were not statistically significant.

161

Figure 4.9. Effect of wood-flour content on cell size distribution of foamed samples.

162

Microcellular structures with cell sizes in the order of 10 μm and cell-population
9

3

densities of 10 cell/cm were produced in PLA containing up to 20 wt. % wood. The
method proposed for matching the viscosity of the composites to that of the neat PLA by
means of low molecular weight rheology modifier proved to be a suitable approach to
facilitate

the

microcellular

foaming

163

of

PLA/wood-flour

composites.

4.5 Conclusions
This study investigated the effect of wood-flour and rheology modifier contents
on the melt viscosity and cell morphology of microcellular foamed polylactic acid/woodflour composites produced through a continuous extrusion process with supercritical
carbon dioxide as foaming agent. The viscosity of the melt was of key importance in the
production of microcellular foams as they affected the processability and foamability of
the composites.
The melt viscosity of PLA increased as the relative amount of wood-flour
increased in the composites.

In contrast, increasing amounts of rheology modifier

produced the opposite effect reducing the viscosity of the composites.

A linear

relationship was identified for the effect of rheology modifier on melt flow index (MFI)
for the PLA/wood-flour composites and was used to formulate composites with lower,
similar, and higher viscosities than the neat PLA.

Matching the viscosity of the

composites with that of neat PLA resulted in the best cell morphologies.

Lower

concentrations of rheology modifier resulted in unprossesable conditions because of the
increased viscosity which generated elevated pressures in the system above operational
limits. Higher concentrations of rheology modifier resulted in foams with poor cell
morphologies. The low viscosity of composites with high amounts of rheology modifier
facilitated the gas escape and cell coalescence resulting in lower void fractions.
By matching the viscosity of the composites with that of the neat PLA, foamed
composites with wood-flour contents up to 30 wt.% were manufactured. The results
confirmed that microcellular structures, with cell sizes of around 10 μm and cell densities

164

in the order of 10

9

3

cells/cm , were successfully produced in the PLA/wood-flour

composites. The results show the feasibility of using low molecular weight additives to
control the viscosity of the composites in order to tailor the melt viscosity of PLA/woodflour composites.

165

4.6 Acknowledgements
This project was supported by the National Research Initiative of the USDA
Cooperative State Research, Education and Extension Service, grant number 200635504-17414.

166

REFERENCES

167

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171

CHAPTER 5

Process-morphology-property relationships of microcellular poly(lacticacid) produced through a continuous extrusion process

172

5.1 Abstract
High density microcellular polylactic acid foams were produced by a continuous
microcellular extrusion process using the Mucell technology with supercritical carbon
dioxide as foaming agent. In this study, characterization of the physico-mechanical
properties of foamed samples as a function of cell morphology was done to establish
process-morphology-property relationships.

Tensile and Taber stiffness tests were

performed on microcellular polylactic acid filaments prepared under different processing
conditions. Processing variables such as amount of gas injected, extruders speed, gear
pump speed, and processing temperature affected the development of the cellular
structure. The results show the dependence of tensile strength and modulus on foam
relative density.

173

5.2 Introduction
Microcellular plastics refer to any polymeric materials with small bubbles of 10
μm or less in diameter and cell densities (number of bubbles per unit volume) of 10

9

3

cells/cm or more [1]. It is made by subjecting the plastic with a large amount of
dissolved gas to a thermodynamic instability to nucleate a large number of cells [2].
Microcellular foams have the potential of improving material properties such as impact
strength, toughness, and fatigue life while reducing plastic consumption [2, 3], thus
reducing cost. These unique characteristics of microcellular plastics make the technology
ideal for brittle polymers such as polylactic acid (PLA), since the presence of small
bubbles can blunt the crack-tips and act as crazing initiation sites making the material
tougher [2, 4].
Polylactic acid (PLA), a plastic derived from starch of corn, sugar beets, etc., has
attracted much attention as replacement for traditional petroleum-based thermoplastics
because it exhibits physical and mechanical properties close to those of many synthetic
plastics in addition to being biodegradable [5, 6]. However, it is more brittle and more
expensive than traditional oil-based plastics [7, 8], which have limited its large-scale
applications. Consequently, improving the flexibility and impact resistance of PLA while
reducing the cost of manufactured parts is an important factor for its market expansion.
Production of microcellular foamed structures in polylactic acid has addressed
some of the limitations of the material such as low impact resistance and brittleness.
Using a batch microcellular process, improvements in impact resistance (up to a 4-fold
increase over that of unfoamed PLA), strain at break (up to a 2-fold increase over that of
174

unfoamed PLA), and toughness (up to a 4-fold increase over that of unfoamed PLA) have
been reported [5]. However, the industrial implementation of the batch process remains
unlikely because of the multiple steps involved in the production of foamed samples and
long saturation times [9].

Interest in industrialization of this technology calls for

continuous processes, such as extrusion to produce the foams.

The potential for

industrialization has prompted researchers in exploring avenues for the continuous
production of microcellular foams.
Different studies have been reported on continuous foaming of PLA in extrusion
systems [9-12].

Matuana and Diaz studied the nucleation mechanisms to produce

microcellular PLA using supercritical CO2. Observations made included the critical role
of the processing temperature on the cell morphology since lower temperatures increased
the viscosity of the melt generating adequate pressure drop and pressure drop rates to
9

3

induce high nucleation rates, i.e. higher than 10 cells/cm [9]. Pilla et al., investigated
the effects of adding epoxy-functionalized chain extender on the foaming ability of PLA
processed via the microcellular extrusion process [11]. Wang and coworkers compared
the expansion ratio and cell-population density of linear versus branched PLA. The
expansion was more than doubled (from 17.6 to 41) when foaming the branched resin
because of its higher melt strength [12]. Most of the studies found focused on the cell
morphology of the foamed material without consideration of its mechanical properties.
One of the reasons to avoid testing of mechanical properties may be the shape of the
extrudate. When produced through a nozzle die, in many cases the extrudate is a filament
with irregular cross section [9, 12], making the testing a cumbersome task. Although the

175

mechanical properties have been evaluated in the batch process it remains important to
test them in samples made by the continuous process.
Reignier and coworkers investigated the foam behavior of linear PLA in a twin
extrusion system using carbon dioxide. They produced low density foams (>30 times
expansion ratio) and evaluated compression modulus and strength. The foams were of
similar density from 20.8 to 24.2 kg/m

3

and a correlation between density and

mechanical properties could not be established. Interestingly, both the modulus and
strength correlated with the gas content which varied from 7.3 to 9.4 wt. %. Higher CO 2
contents yielded lower modulus and strength. This behavior was attributed to a higher
level of plasticization induced by residual CO2. Mechanical property evaluation is scarce
in continuous microcellular foaming of PLA. There is the need for a systematic study of
mechanical properties in a wide range of morphologies in order to allow the
determination of morphology-property relationships.
The aim of this study is to characterize the physico-mechanical properties of
foamed PLA samples as a function of cell morphology to establish process-morphologyproperty relationships and develop predictive models for the foamed materials in the
continuous extrusion process.

176

5.3 Experimental
5.3.1 Materials
A semi-crystalline grade of polylactic acid (PLA 2002D) from NatureWorksTM
was used as matrix with an approximately D-lactic content of 4% and a melt flow index
between 5 to 7 g/10min (210°C/2.16 kg). CO2 with a purity of 99.5%, supplied by
Airgas, was used as physical foaming agent.

5.3.2 Extrusion Foaming
Figure 5.1 shows a schematic of the foaming system used in this study. The
process used a 19.1 mm single screw extruder (L/D ratio 30:1, C.W. Brabender
Instruments Inc.) with a mixing screw powered by a 5 hp driver with speed-control
gearbox (Brabender Prep Center).

Supercritical carbon dioxide was injected to the

extruder at approximately 2/3 of the distance down the barrel length using a MuCell®
supercritical fluid system (Trexel Inc.).

A diffusion enhancing device (static mixer

Omega FMX8441S) attached to the extruder ensured the creation of a polymer/gas
solution. The resulting single phase PLA/gas solution went through the gear pump
(Zenith, PEP-II 1.2 cc/rev), where the volumetric flow was controlled by the motor. The
gear pump controls the polymer melt volumetric flow rate, independent of temperature
and pressure changes [13]. Subsequently the PLA/gas solution entered the die where a
rapid-pressure drop was induced by a nozzle with a 0.5 mm diameter. Cell nucleation
and growth occurred at the die exit. The processing temperature profile in the extruder
was kept constant at 170-170-165-155 °C from hoper to extruder’s exit and varied for the
177

static mixer, gear pump and die. Samples were collected after the process reached steady
state conditions and cooled by air.
This study has two main experiments. The first set of experiments consisted of
producing an array of foamed samples at different processing conditions with the purpose
of generating different morphologies. Table 5.1 list the processing conditions used to
produce foamed samples with different void fractions. Cell morphology and mechanical
characterization were performed to establish morphology-property relationships. The
second set of experiments was aimed at elucidating process-morphology relationships
through a design of experiments.

178

Mucell supercritical
fluid delivery system

Injection port
Pressure
transducers

Drive motor

Hopper

Filament die

Static
mixer

Gear pump

Figure 5.1. Schematic of the single-screw extrusion foaming system.

179

Table 5.1. List of the processing conditions used to produce foamed PLA samples.

Sa
mp
le

Static
mixer
(°C)

Gear
pump
temp.
(°C)

Die
temperature
(°C)

Screw
speed
(rpm)

Gear
pump
speed
(cc/min)

Gas flow
rate
(Kg/hr)

Void
fraction
(%)

a

155

160

150

60

63

0.16

9 ± 1.5

b

155

150

150

40

64

0.15

16.8 ± 2.4

c

155

145

140

60

59

0.18

18.9 ± 3.1

d

150

140

135

40

64

0.15

39.7 ± 3.7

180

5.3.3 Tensile test
Tensile properties were measured with an Instron 5585H testing machine using
the Instron Bluehill 2.14 software and a 5 kN load cell.

There is no standardized

procedure to test the obtained specimens which are foamed PLA filaments of around 0.5
mm in diameter with irregular cross section. For this reason a method was developed
based on the ASTM standards C1557 and D3822 intended for tensile testing of ceramic
fibers (glass, carbon, etc.) and man-made single textile fibers, respectively. The samples
were mounted on cardboard tabs and glued with Loctite Super Glue at a gage length of
1.0 in. To improve the gripping, two tabs were used to hold the foamed specimen in a
sandwich like manner. Drying of the glue for at least 24 hours was allowed before the
test. Each specimen was centrally placed in the clamps of the tensile tester and the
cardboard was carefully cut at both sides of the foamed filament. Tensile test was carried
out using a crosshead speed of 3.81 mm/min.

The diameter of the filaments was

measured using optical microscopy prior to the test.

5.3.4 Taber stiffness test
Flexural modulus of elasticity was measured by means of Taber stiffness. The
Taber units can be converted to modulus of elasticity by taking into account the shape,
allowing the comparison between samples. Diaz et al., demonstrated the feasibility of
using the Taber unit to calculate flexural modulus of elasticity [14].

The stiffness

measurements on the Taber unit followed the procedures outlined in the ASTM standard
D5342. The study used a Taber V-5 stiffness tester model 150-D. Foamed filaments
were directly mounted in the unit.
181

5.3.5 Density measurements
Density measurements were carried out according to ASTM standard D792
(buoyancy method), for unfoamed (ρu) and foamed (ρf) samples. By measuring the
weights of at least five samples in the air (m1) and in an immersion liquid (m2), i.e., nhexane, the density was calculated as:

m1   FL
m1  m2 



(Equation 5.1)

3

where  FL is the density of n-hexane (0.6594 g/cm ). The void fraction (Vf) was then
determined using the following equation:

u   f 

Vf 

u

(Equation 5.2)

5.3.6 Scanning Electron Microscopy
Morphology of foamed samples was analyzed via scanning electron microscopy
(SEM JEOL JSM-6400, 10 kV). Samples were fractured in liquid nitrogen and coated
with gold prior to the test. The images obtained aided the characterization of cellpopulation density using Equation 5.3 [15]:

182

3

 nM 2  2  1 

No  
 A  1  V f 




 

(Equation 5.3)

with n as the number of cells in the micrographs, and A and M as the area and the
magnification factor of the micrograph, respectively. Vf is the void fraction described in
Equation 5.2.
Cell sizes were measured using image analysis software, UTHSCSA Image Tool.
After a proper spatial calibration of the micrograph, the diameter of at least 100 randomly
selected cells was manually measured and the average cell size calculated.

183

5.4 Results and Discussion
5.4.1 Cell morphology analysis
Varying the processing parameters allowed for the production of foamed samples
with different cell morphologies. Figure 5.2 shows the morphologies obtained ranging
from lower (a) to higher (d) void fractions. Noticeably, irrespective of the void fraction
the cells have similar sizes while the number of bubbles increased with void fraction. To
confirm this observation a quantitative analysis of the cell size and cell-population
density as a function of void fraction was performed and plotted in Figure 5.3. The
average cell size did not significantly change with void fraction whereas the cellpopulation density increased with increasing void fractions. The void fraction depends
on both the number of bubbles (cell-population density) and the size of the bubbles.
Since there is no significant difference in the cell size among samples, the cell-population
density is the main parameter determining the density reduction (void fraction). The
results suggest that nucleation is the dominant phenomena in the development of the
cellular structure because the number of cells correlated with the void fraction. The
invariant cell size may be explained by the viscoselastic properties of the material, i.e.,
the melt strength. The melt strength plays a key role during foam expansion controlling
the cell growth [16]. It is known that linear PLA, such as the one used in this study, have
poor melt strength [12], thus the cell growth may have been limited to a certain extent
resulting in the invariant cell size.

184

a

b

200 μm

c

200 μm

d

200 μm
200 μm
Figure 5.2. Morphology of foamed samples corresponding to the different foaming
conditions in Table 5.2.

185

20

1.2

16

0.9

12

0.6

8

0.3

4

0

0
0

10

20

30

Cell size (μm)

Cell-population density 10 9 cells/cm3

1.5

40

Void Fraction (%)

Figure 5.3. Cell-population density and cell size as a function of void fraction for foamed
PLA samples. Solid diamonds represent the cell-population density and the clear squares
represent the cell size.

186

5.4.2 Modulus of elasticity vs. relative density
Normalized tensile modulus of elasticity (From Instron) and normalized flexural
modulus of elasticity (from Taber) were plotted against relative density (Figures 5.4 and
5.5a). The normalization consisted of dividing the modulus of the sample by the modulus
of the matrix (unfoamed material) [17, 18]. Models in the literature to predict properties
of cellular materials are usually expressed in terms of the normalized property as a
function of relative density [19, 20], where the relative density is defined as:

Relative Density 

f


 1Vf

(Equation 5.4)

where  f is the density of the foam,  is the density of the matrix or unfoamed material
and V f if the void fraction.
The lines in Figure 5.4 represent available models for the relationship between
relative density and modulus. Different models have been proposed in the literature
comprising theoretical and empirical approaches [17]. The empirical square power-law
has been widely used [17, 18, 21] and is valid for most high density foams [17]. The
square power-law is defined in Equation 5.5 and plotted in Figure 5.4.





E f Em   f  m 2

187

(Equation 5.5)

The other two models plotted in Figure 5.4 are the constant macro-stress model and the
Gibson-Ashby model.

Constant macro-stress assumes spherical micro-inclusions

embedded in an unbounded matrix without considering interactions between the
inclusions and the matrix [17]. The Gibson-Ashby model assumes a cubic array of cells
on which the cell struts, cell walls, and the internal gas pressure contribute to the modulus
[17].

188

Normalized Modulus

1.0
0.8
0.6
0.4
0.2
0.0
0.6

0.7

0.8

0.9

1

Relative Density
Figure 5.4. Normalized modulus versus relative density of foamed PLA samples. Solid
lines show the prediction from different models: (········): constant macro-stress, (−−−−):
square power-law, (−·−·−·−): Gibson-Ashby.

189

The three models plotted in Figure 5.4 overestimate the modulus of elasticity of
the foamed samples. The Gibson-Ashby model showed the lowest deviations from the
experimental data. Noticeably, the normalized modulus does not reach unity when the
relative density is 1 because the model was developed for low density foams. Similar
observations were reported by Xiao and coworkers who observed normalized modulus
evidently smaller than the square power-law model in polycarbonate foams prepared by
compression molding, in a similar range of void fractions [22]. The dependency of the
modulus of elasticity on void fraction is also due to the fact that the cell-population
density varies as shown in Figure 5.3.
Normalized flexural modulus of elasticity measured by means of Taber stiffness
was plotted against relative density in Figure 5.5a; the solid line represents the square
power-law model. Similar to the tensile modulus, the experimental values are lower than
the predicted ones by the square power-law relationship.

The comparison between

normalized flexural and tensile modulus is shown in Figure 5.5b. A linear relationship is
observed between the two variables where the normalized flexural modulus was slightly
higher than the normalized tensile modulus since the values fell above the solid line (the
solid line represents a 1:1 relationship).

190

1

Normalized Flexural Modulus

Normalized Flexural Modulus

1

a

0.8
0.6
0.4

0.2
0
0.6

0.7
0.8
0.9
Relative Density

1

b

0.8
0.6
0.4

0.2
0
0

0.2

0.4

0.6

0.8

1

Normalized Tensile Modulus

Figure 5.5. Normalized flexural modulus versus (a) relative density and (b) normalized
tensile modulus of foamed PLA samples.

191

5.4.3 Tensile strength vs. relative density
In Figure 5.6 the normalized tensile strength is plotted against relative density.
Again, the lines represent available models. The tensile stress decreases with decreasing
relative density. However this trend is due to the fact that the cell-population density also
varies.
The Gibson-Ashby model and the Nicolais-Narkis model were used to compare
with the experimental results due to their reported good representation of experimental
data [19]. In this case the Nicolais-Narkis model showed the lowest deviations from the
experimental points. However, high variability was observed in the experimental data.
The high standard deviation showed by the error bars may be due to the irregularity of the
foamed rods. The irregular cross section area of the foamed rods may have induced
stress concentrators decreasing the overall strength of the material.

192

Normalized Tensile Strength

1.0
0.8
0.6
0.4
0.2
0.0
0.6

0.7
0.8
0.9
Relative Density

1

Figure 5.6. Normalized tensile strength versus relative density of foamed PLA samples.
Solid lines show the prediction from different models: (−−−−): Nicolais-Narkis,
(−·−·−·−): Gibson-Ashby.

193

The foamed filaments have irregular shape, and are not uniform along the length.
The calculation of the cross-section area was done assuming that the filament is perfectly
round. However, the foamed filaments differ from the ideal uniform shape (see Figure
5.2). Figure 5.7 presents an optical microscope image of a characteristic shape of the
foamed PLA filaments and show the irregularity of the shape along the length making the
determination of the cross sectional area a complex task. The inaccuracy in determining
the cross-section diameters may be one of the reasons for the observed high standard
deviations in strength and modulus as they are inversely proportional to the square of the
diameter. For a cylindrical geometry the tensile stress (  ) is defined as:

  4F D 2

(Equation 5.6)

where F is the tensile load and D is the cross-section diameter. A foamed filament is
expected to fail at the smallest cross section. However, significant defects can make the
sample fail at a different location having a high stress concentration, affecting the
calculation of tensile strength and modulus. A similar scenario on which the irregularity
of the samples affects the determination of mechanical properties is the single fiber
tensile test of natural fibers where standard deviations over 30% are common [23]. It is
important to develop strategies to improve the uniformity of the samples and the
consistency of the cellular structure for an accurate measurement of mechanical
properties.

194

1 mm
Figure 5.7. Optical microscope image of the foamed filaments shape along the length. For
interpretation of the references to color in this and all other figures, the reader is referred
to the electronic version of this dissertation

195

Figure 5.8 shows the data points for energy at break and strain at break for
unfoamed (relative density=1) and foamed PLA plotted against the relative density. The
data is scattered and a trend cannot be identified. Correlating the energy at break and
strain at break with the relative density showed low R

2

values of 0.32 and 0.13

respectively. An increase in toughness was expected for the foamed samples because the
inclusion of micro-bubbles can inhibit the crack propagation by blunting the crack tip
increasing the energy needed to propagate the crack [4]. However, this claim cannot be
corroborated with the experimental data. Noticeably, the highest value of energy at break
was from a foamed sample. Inconsistent cellular structures and the irregularity of the
filaments may have caused the high variation observed.

196

25

a

25

b
Strain at break (%)

Energy at break (mJ)

30

20

15
10
5

20
15
10
5

0

0
0.6

0.7

0.8

0.9

1

0.6

1.1

Relative density

0.7

0.8
0.9
1
Relative density

1.1

Figure 5.8. Energy and strain at break plotted against relative density for solid and
foamed PLA samples.

197

5.5 Conclusions
This study examined the physico-mechanical properties of microcellular foamed
polylactic acid (PLA) as a function of cell morphology to establish process-morphologyproperty relationships. Tensile and Taber stiffness tests were performed on microcellular
PLA filaments prepared under different processing conditions. The processing variables
evaluated, i.e., amount of gas injected, extruders speed, gear pump speed, and processing
temperature affected the development of the cellular structure. The results show the
dependency of tensile strength and modulus on foam relative density. However, this trend
is due to the fact that the cell-population density also varies.

Inconsistent cellular

structures and the irregularity of the filaments may have caused the high variation in
mechanical properties. The irregular shape along the length makes the calculation of the
cross-section area difficult and induces stress concentrators which affected the failure
mode of the foamed filaments.

198

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199

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CHAPTER 6

Conclusions and Future Work

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6.1 Conclusions
Poly(lactic acid) (PLA), a biodegradable thermoplastic derived from renewable
resources, stands out as a substitute to petroleum-based plastics. In spite of its excellent
properties, commercial application is limited because PLA is more expensive and more
brittle than petroleum-based resins. PLA can be blended with cellulosic fibers to
reduce material cost. However, the lowered cost comes at the expense of flexibility
and impact strength, which can be enhanced through the production of microcellular
structures in the composite. Characterization of the morphology of microcellular
9

3

foams includes cell-population densities of 10 cells/cm and up and cell sizes smaller
than 10 μm. Continuous microcellular foaming has been developed based on the
concept of thermodynamic instability created by a rapid pressure drop of a polymer/gas
single phase solution.

Understanding of the nucleation dynamics is the key to

achieving high nucleation rates.

Consequently, the effects of the processing

temperature and material compositions on the melt viscosity, pressure drop rate, and
cell-population density were examined to understand the nucleation mechanisms in
neat and filled PLA foams.

The following conclusions were drawn from the

experimental results.
1) The processing temperature had a strong effect of the rheology of the melt.
Increasing the test temperature significantly decreased the melt viscosity of PLA
affecting the cell nucleation rate because of its effect on the pressure generated in the
system during the extrusion of microcellular PLA. Processing at a lower temperature
significantly increased the cell nucleation rate of PLA (Anorphous and semi-crystalline)
because of the fact that a high melt viscosity induced a high pressure drop rate.
2) The presence of the nanoclay did not affect the homogeneous nucleation but

204

increased the heterogeneous nucleation, with the fine cells nucleated at the membrane
of homogeneously nucleated bubbles. The addition of nanoclay allowed both
homogeneous and heterogeneous nucleation stages to occur during the foaming
process.
3) The melt viscosity of PLA increased as the relative amount of wood-flour
increased in the composites because of the interaction between wood-flour and the
matrix increasing the friction towards the flow. In contrast, increasing amounts of
rheology modifier produced the opposite effect reducing the viscosity of the
composites because of the lubrication effect of the low molecular weight additive.
4) Matching the viscosity of the PLA/wood-flour composites with that of neat
PLA resulted in adequate processing conditions for microcellular foaming and resulted
in the best cell morphologies. Lower concentrations of rheology modifier resulted in
unprossesable conditions because of the increased viscosity which generated elevated
pressures in the system above operational limits. Higher concentrations of rheology
modifier resulted in foams with poor cell morphologies because of the low viscosity
which facilitated the gas escape and cell coalescence.
5) The physico-mechanical properties of microcellular foamed polylactic acid
(PLA)

as

a

function

of

cell

morphology

were

studied

to

establish

process-morphology-property relationships. The processing variables, i.e., amount of
gas injected, extruders speed, gear pump speed, and processing temperature affected
the development of the cellular structure. Tensile strength and modulus modulus of
elasticity of foamed samples correlated with the relative density.

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6.2 Future work
Production of microcellular structures in polylactic acid (PLA) and
PLA/wood-flour composites was demonstrated in the continuous extrusion process
injecting carbon dioxide as foaming agent. The filamentary extrusion was used as
means to produce microcellular foams by a rapid pressure drop at the nozzle die.
Control over the rheology of the melt proved to be of key importance to effectively
nucleate a swarm of bubbles. However, it is important to extrapolate the results
achieved in the filament to other shapes such as films. For this purpose the use of
annular dies and cast film dies should be investigated. Successful production of
microcellular foamed films and other shapes will not only contribute to the scale-up of
the technology to an industrial level but also allow for a more precise characterization
and modeling of process-morphology-property relationships.
This study investigated the effects of the processing temperature and material
compositions on the melt viscosity, pressure drop rate, and cell-population density of
microcellular PLA and PLA/wood-flour composites. However, other approaches such
as nozzle design (nozzle geometry) have the potential to modify the processing
conditions (i.e., pressure drop and pressure drop rate) and expand the processing
windows to produce microcellular PLA and PLA/wood-flour composites.
This study investigated the effect of extensional viscosity on the foamability of
PLA to a limited extend. However, it is known of the importance of extensional
properties, such as melt strength, in the foaming process. PLA grades intended for
foaming applications are now commercially available as well as branching agents
specially designed for PLA. Assessment of the performance of such materials should
be investigated in view of expanding the range of properties and density reduction in

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foamed PLA.

207