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dissertation entitled

PRODUCTION AND CHARACTERIZATION OF NOVEL
STARCH AND POLY(BUTYLENE ADIPATE-CO-
TEREPHTHALATE) - BASED MATERIALS AND THEIR
APPLICATIONS

presented by

Jacqueline Ann Stagner

has been accepted towards fulfillment
of the requirements for the

Ph.D. degree in Materials Science and
Engineering

 

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DATE DUE DATE DUE DATE DUE

IIQY, 2., 11.2011

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

5/08 KzlProVAocaPresIClRCIDateDuerindd

 

 

PRODUCTION AND CHARACTERIZATION OF NOVEL STARCH AND
POLY(BUTYLENE ADIPATE-CO-TEREPHTHALATE) — BASED MATERIALS
AND THEIR APPLICATIONS
BY

Jacqueline Ann Stagner

A DISSERTATION
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
DOCTOR OF PHILOSOPHY
Materials Science and Engineering

2009

ABSTRACT
PRODUCTION AND CHARACTERIZATION OF NOVEL STARCH AND
POLY(BUTYLENE ADIPATE-CO-TEREPHTHALATE) — BASED MATERIALS
AND THEIR APPLICATIONS
By
Jacqueline Ann Stagner
This work focuses on the production and characterization of blends of maleated

thermoplastic starch (MTPS) and poly(butylenes adipate—co-terephthalate) and

their application for use as therrnoforrned objects, films, and foams.

First, by the production and characterization of maleated thermoplastic starch
(MTPS) synthesized by reactive extrusion in a twin-screw extruder, a better
understanding of MTPS was gained. This reactive thermoplastic starch was
prepared with glycerol as the plasticizer, maleic anhydride (MA), and free-radical
initiator, 2,5-bis(tert-butylperoxy)-2,5-dimethylhexane (Luperox 101 ). Dynamic
light scattering (DLS), differential scanning calorimetry (DSC), Fourier transform
infrared spectroscopy (FTIR), soxhlet extraction in acetone, and environmental
scanning electron microscopy (ESEM) were performed to determine the effect of
maleation, extrusion temperature, initiator concentration, and maleic anhydride

concentration on the resulting MTPS.

Next, maleated thermoplastic starch (MTPS) and thermoplastic starch (T PS)
were reactively blended in a twin-screw extruder with a biodegradable polyester,

poly(butylene adipate-co-terephthalate) (PBAT). The blends were extruded to

produce therrnoforrnable sheets. The mechanical properties of the sheets were
characterized by tensile and puncture tests. Proof of grafting was determined by
soxhlet extraction in dichloromethane and Fl" IR analysis. Observations of the
thermal properties were made using 080, while the surface of the sheets was

imaged using ESEM.

Blends of MTPS and PBAT were also extruded to produce films. Mechanical
testing (tensile and puncture tests) and barrier perforrnanoe testing (carbon
dioxide, oxygen, and water vapor permeability) were performed on the films.
Transmission electron microscopy (TEM) was used to image the blends and to

view the dispersion of the various phases.

Finally, blends of MTPS and PBAT were extruded with an endothermic chemical
blowing agent to produce foams. The foams were characterized by measuring
density, expansion ratio, specific length, compressive strength, resiliency, and
moisture sorption. Also, digital light microscopy was used to image the cell

structure of the foams.

This work demonstrates that blends of starch and PBAT can be produced and
formed into thermofonned objects, films, and foams. These objects can replace
current objects made from non-biodegradable, petroleum-based plastics. By
blending the starch and PBAT together, one receives advantages over using

either component separately.

To Dan and Lindsay, for all of your love and support.

ACKNOWLEDGMENTS

There are many people who have helped me over the past four years. I would
like to thank my advisor, Dr. Ramani Narayan, and my guidance committee; Dr.
Andre Lee, Dr. Krishnamurthy Jayaraman, and Dr. Rafael Auras, for their time,

advice, guidance, and encouragement during the course of my Ph.D.

There have also been many other people who have provided their assistance. I
would like to thank the members of the Biobased Materials Research Group, past
and present; Dr. Daniel Graiver, Ken Farrniner, Dr. Vanessa Dias Alves, Leann
Lerie Matta, Laura Patrick, Dr. Alison Fowlks, Dr. Weipeng Liu, Madhusudan
Srinivasan, Mamud Dako, Maria Darsono, Rebecca Frear, Elizabeth Brown, Siva
Chalasani, and Zhiguan Yang. You provided much support, help, and valuable

input

The many other individuals who have helped me with various aspects of my
research include Mike Rich, Per Askeland, Brian Rook, Ed Drown, and Hazel-
Ann Hosein in the CMSC; Kelby Thayer, Sukeewan Detyothin, Pankaj Kumar,
lsinay Yuzay, and Siriyupa Netramai in the School of Packaging; Alicia Pastor
and Ewa Danielewicz in the Center for Advanced Microsc0py; Rui Huang in the
Department of Chemistry; Nicole Hall, Nancy Raney, John Mills, and Dan

Drazkowski.

Finally, I would like to thank my family. Dan, Lindsay, Chris, Kelly, mom and dad,
your love and support helped me more than you will ever know. Audra, your

smiles are a great pick-me-up. Finally, our little Bambino, I can’t wait to meet you.

Although I have mentioned only a few names, there are many more people who

have helped me throughout this venture. I would like to extend my appreciation to

all of the other people who I have inadvertently left out.

vi

TABLE OF CONTENTS

List of Tables ................................................................................. x

List of Figures ................................................................................ xi

Chapter 1 ...................................................................................... 1

1.1 Problem Statement .................................................................... 1

1.2 Objectives ................................................................................ 2

1.3 Specific Objectives ..................................................................... 3

1.4 Structure of Dissertation .............................................................. 5

Chapter 2 ...................................................................................... 7

2.1 Biobased and Biodegradable Plastics Rationale .............................. 7

2.2 Starch ..................................................................................... 12
2.3 Poly(butylene adipate-co-terephthalate) (PBAT) .............................. 14
2.4 Extrusion Theory ....................................................................... 15
2.5 Polymer Blends and Alloys .......................................................... 18
2.6 Review of Literature ................................................................... 20
Chapter 3 ...................................................................................... 29
3.1 Rationale ................................................................................. 29
3.2 Materials and Methods ............................................................... 30
3.2.1 Materials ............................................................................... 30
3.2.2 Equipment ............................................................................. 30
3.2.3 Procedure .............................................................................. 32
3.2.3.1 Sample Preparation .............................................................. 32
3.2.3.2 Dynamic Light Scattering (DLS) Analysis ................................... 37
3.2.3.3 Fourier Transform Infrared Spectroscopy (FTIR) Analysis ............. 38
3.2.3.4 Thermal Analysis (DSC) ......................................................... 38
3.2.3.5 Soxhlet Extraction Analysis ..................................................... 38
3.2.3.6 Environmental Scanning Electron Microscopy (ESEM) Analysis..... 39
3.2.3.7 Statistical Analysis ................................................................ 39
3.3 Results and Discussion ............................................................... 40
3.3.1 Dynamic Light Scattering (DLS) Analysis ..................................... 40
3.3.2 Fourier Transform Infrared Spectroscopy (FTIR) Analysis ............... 44
3.3.3 Thermal Analysis (DSC) ........................................................... 46
3.3.4 Soxhlet Extraction Analysis ....................................................... 50
3.3.5 Environmental Scanning Electron Microscopy (ESEM) Analysis ........ 52
3.4 Conclusions .............................................................................. 55
Chapter 4 ...................................................................................... 56
4.1 Rationale ................................................................................. 56
4.2 Materials and Methods ............................................................... 57
4.2.1 Materials ............................................................................... 57
4.2.2 Equipment ............................................................................. 57
4.2.3 Procedure .............................................................................. 58

vii

4.2.3.1 Production of Thermoplastic Starch (T PS) and Maleated

Thermoplastic Starch (MTPS) ........................................................... 58
4.2.3.2 Blend Production of TPS and MTPS with PBAT .......................... 59
4.2.3.3 Sheet Production .................................................................. 62
4.2.3.4 Therrnoforrning Process ......................................................... 63
4.2.3.5 Fourier Transform Infrared SpectrOSCOpy (FTIR) Analysis ............. 64
4.2.3.6 Tensile Tests ....................................................................... 64
4.2.3.7 Puncture Tests ..................................................................... 65
4.2.3.8 Thermal Analysis (DSC) ......................................................... 65
4.2.3.8 Environmental Scanning Electron Microscopy (ESEM) Analysis..... 66
4.2.3.10 Statistical Analysis ............................................................... 66
4.3 Results and Discussion ............................................................... 66
4.3.1 Fourier Transform Infrared Spectroscopy Analysis (FTIR) ............... 66
4.3.2 Tensile Tests ......................................................................... 69
4.3.3 Puncture Tests ....................................................................... 73
4.3.4 Thermal Analysis (DSC) ........................................................... 75
4.3.5 Environmental Scanning Electron Microscopy (ESEM) Analysis........77
4.4 Conclusions .............................................................................. 80
Chapter 5 ...................................................................................... 82
5.1 Rationale .............................. . ................................................... 82
5.2 Materials and Methods ............................................................... 83
5.2.1 Materials ............................................................................... 83
5.2.2 Equipment ............................................................................. 83
5.2.3 Procedure .............................................................................. 84
5.2.3.1 Production of Maleated Thermoplastic Starch (MTPS) .................. 84
5.2.3.2 Blend Production of MTPS with PBAT ....................................... 85
5.2.3.3 Film Production .................................................................... 86
5.2.3.4 Determining Biobased Content ................................................ 86
5.2.3.5 Mechanical Properties ........................................................... 87
5.2.3.5.1 Tensile Tests ..................................................................... 87
5.2.3.5.2 Puncture Tests .................................................................. 87
5.2.3.6 Barrier Properties ................................................................. 88
5.2.3.6.1 Carbon Dioxide Permeability ................................................ 88
5.2.3.6.2 Oxygen Permeability ........................................................... 89
5.2.3.6.3 Water Vapor Permeability .................................................... 90
5.2.3.7 Transmission Electron Microscopy (TEM) .................................. 91
5.2.3.8 Statistical Analysis ................................................................ 91
5.3 Results and Discussion ............................................................... 91
5.3.1 Determining Biobased Content ................................................... 91
5.3.2 Mechanical Properties .............................................................. 92
5.3.2.1 Tensile Tests ....................................................................... 92
5.3.2.2 Puncture Tests ..................................................................... 95
5.3.3 Barrier Properties .................................................................... 96
5.3.4 Transmission Electron Microscopy (TEM) .................................... 99
5.4 Conclusions .............................................................................. 101

viii

Chapter 6 ...................................................................................... 102

6.1 Rationale ................................................................................. 102
6.2 Materials and Methods ............................................................... 103
6.2.1 Materials ............................................................................... 103
6.2.2 Equipment ............................................................................. 103
6.2.3 Procedure .............................................................................. 104
6.2.3.1 Production of Maleated Thermoplastic Starch (MTPS) ................. 104
6.2.3.2 Blend Production of MTPS with PBAT ....................................... 104
6.2.3.3 Foam Production .................................................................. 105
6.2.3.4 Chemical Blowing Agent Thermal Decomposition ........................ 107
6.2.3.5 Extensional Viscosity ............................................................ 107
6.2.3.6 Foam Characterization .......................................................... 108
6.2.3.6.1 Density ............................................................................ 108
6.2.3.6.2 Expansion Ratio ................................................................ 108
6.2.3.6.3 Specific Length .................................................................. 109
6.2.3.6.4 Compressive Strength and Resiliency .................................... 109
6.2.3.6.5 Moisture Sorption Analysis ................................................... 110
6.2.3.6.6 Digital Light Microscopy ...................................................... 110
6.2.3.7 Statistical Analysis ................................................................ 1 11
6.3 Results and Discussion ............................................................... 111
6.3.1 Chemical Blowing Agent Thermal Decomposition .......................... 111
6.3.2 Extensional Viscosity ............................................................... 113
6.3.3 Density, Expansion Ratio, and Specific Length .............................. 116
6.3.4 Compressive Strength and Resiliency ......................................... 119
6.3.5 Moisture Sorption Analysis ........................................................ 121
6.3.6 Digital Light Microscopy ............................................................ 124
6.4 Conclusions .............................................................................. 127
Chapter 7 ...................................................................................... 128
7.1 Summary ................................................................................. 128
7.2 Conclusions .............................................................................. 129

7.2.1 Maleated Thermoplastic Starch by Reactive Extrusion Processing... 129
7.2.2 MTPS-PBAT Blends by Reactive Extrusion Processing and Their

Application and Performance for Therrnoforrned Articles ......................... 130
7.2.3 Application and Performance of MTPS-PBAT Blends for Films ......... 132
7.2.4 Application and Performance of MTPS-PBAT Blends for Foams ....... 133
Chapter 8 ...................................................................................... 135
8.1 Recommendations for Future Work ............................................... 135
References .................................................................................... 1 38

LIST OF TABLES

Table 1. Composition of the maleated and non-maleated thermoplastic
starch (MTPS and TPS) samples ....................................................... 34

Table 2. Hydrodynamic radii as measured by dynamic light scattering ...... 41

Table 3. Percentage of glycerol grafted to the starch after soxhlet
extraction of maleated thermoplastic starch in acetone for 72 hours .......... 52

Table 4. Composition of the blends of TPS and MTPS with PBAT ............ 61

Table 5. Thickness and mechanical properties (determined by tensile
tests) of sheets of blends of TPS and MTPS with PBAT .......................... 70

Table 6. Mechanical properties determined by puncture tests of the
therrnoforrned pieces of the blends of TPS and MTPS with PBAT ............. 74

Table 7. D80 parameters: glass transition temperature (T g), crystallization
temperature (T c), crystallization enthalpy (AH), and melting temperature
(Tm) of the blends of MTPS and TPS with PBAT ................................... 76
Table 8. Composition of films’of blends of MTPS with PBAT .................... 85

Table 9. Thickness and mechanical properties (determined by tensile
tests) of films of blends of MTPS with PBAT .......................................... 93

Table 10. Puncture test results of films of blends of MTPS with PBAT ....... 95

Table 11. Barrier properties of films of blends of MTPS with PBAT ............ 97
Table 12. Composition of foams of blends of MTPS and PBAT ................ 106
Table 13. Density, expansion ratio, and specific length of foams of blends

of MTPS and PBAT ......................................................................... 118
Table 14. Resiliency of foams of blends of MTPS and PBAT ................... 121

Table 15. Moisture sorption steady-state weight gain, diameter, and length
of foams of blends of MTPS and PBAT ................................................ 123

LIST OF FIGURES

Figure 1. The global carbon cycle ....................................................... 9

Figure 2. Degradation versus biodegradation of materials ........................ 11
Figure 3. Structure of amylose and amylopectin ................................... 13
Figure 4. Chemical structure of PBAT .................................................. 14
Figure 5. Schematic diagram of dispersive and distributive mixing ............ 17

Figure 6. Relationship between property values and blend composition ..... 19

Figure 7. Screw configuration of the Century CX-3O co-rotating twin-screw

extruder ........................................................................................ 31
Figure 8. Process schematic for extrusion of MTPS ............................... 35
Figure 9. Proposed reaction of starch with maleic anhydride .................... 35

Figure 10. Hydrolysis (a) and glucosidation (b) reactions occurring during
the reactive extrusion of maleated thermoplastic starch .......................... 36

Figure 11. Graph of dynamic light scattering measurements for sample 6
(MTPS high amylose starch, with 30% glycerol and 135°C) ..................... 37

Figure 12. Picture of maleated thermoplastic starch pellets ..................... 40

Figure 13. FI'IR Spectra for High Amylose Corn Starch, TPS 135°C 30%
Glycerol, and MTPS 135°C 0.1% L101 2% MA 30% Glycerol .................. 45

Figure 14. FT IR relative intensities of the carbonyl peak for the MTPS
samples ....................................................................................... 46

Figure 15. DSC therrnogram of the MTPS (at 135°C and 30% glycerol)
and TPS (at 135°C and 30% glycerol) samples ..................................... 49

xi

Figure 16. Environmental scanning electron microscope images of
extrudate samples. Images shown at 750x magnification (50 um scale bar).
a) MTPS, at 165°C, 0.1% L101, 2% MA, 20% glycerol, b) MTPS, at 165°C,
0.25% L101, 2% MA, 20% glycerol, 0) MTPS, at 165°C, 0.50% L101, 2%
MA, 20% glycerol, d) MTPS, at 165°C, 0.25% L101, 6% MA, 20%
glycerol, e) MTPS, at 135°C, 0.1% L101, 2% MA, 30% glycerol and

f) TPS, at 135°C, 70% starch and 30% glycerol .................................... 54
Figure 17. Proposed reaction scheme for MTPS and PBAT ..................... 62
Figure 18. Picture of the object formed in the Labforrn therrnoformer ........ 63

Figure 19. FTIR Spectra for HA Starch, PBAT, and MTPS 165°C 2%MA
0.1%L101 20%G-PBAT(50:50) .......................................................... 67

Figure 20. FTIR Spectra for film made after soxhlet of MTPS 165°C 0.1 %
L101 2%MA 20%G-PBAT(60:40), pellets left in thimble after soxhlet of
MTPS 165°C 0.1%L101 2%MA 20%G-PBAT (60:40), and as extruded
pellets of MTPS 165°C 0.1%L101 2%MA 20%G-PBAT (60:40) ................ 68

Figure 21. ESEM images of the samples extruded at 165°C and 20%
glycerol at 750x magnification (50 um scale bar): (a) MTPS high amylose-
PBAT (60:40), (b) MTPS high amylose-PBAT (50:50) and (c) MTPS corn
starch-PBAT (50:50) ........................................................................ 78

Figure 22. ESEM images of the samples extruded at 135°C and 30%
glycerol at 750x magnification (50 um scale bar): (a) TPS high amylose-
PBAT (50:50), (b) MTPS high amylose-PBAT (50:50), (0) TPS corn starch-
PBAT (50:50) and (d) MTPS corn starch-PBAT (50:50) .......................... 79

Figure 23. TEM images of the samples at 20,000x magnification (1 pm
scale bar): (a) PBAT, (b) MTPS:PBAT (10:90), (c) MTPS:PBAT (20:80),
(d) MTPS:PBAT (30:70), (e) MTPS:PBAT (40:60), and (f) MTPS:PBAT

(50:50). MTPS is the light phase and PBAT is the dark phase .................. 100
Figure 24. TGA graph of thermal decomposition of chemical blowing

agent ........................................................................................... 112
Figure 25. Extensional viscosity plot of PBAT at 150°C ........................... 114
Figure 26. Extensional viscosity of blends at 140°C and 1's ..................... 115

Figure 27. Logarithmic plot of compressive strength as a function of foam
density ......................................................................................... 120

xii

Figure 28. Digital light microscope images of foams of blends of MTPS
and PBAT (250 um scale bar) showing the cell structure of the foams ....... 126

xiii

CHAPTER 1

INTRODUCTION

 

1.1 PROBLEM STATEMENT

In recent years, there has been growing concern over society’s dependence on
petroleum, not only for fuel, but also as the basis for many single-use plastic
items. There are many biobased, biodegradable materials that have the potential
to be replacements for petroleum-based polymers. Biobased and biodegradable
plastics can form the basis for a sustainable and environmentally preferable
alternative to current materials based exclusively on petroleum. Recent cost
increases for petroleum are also forcing end-users to look more aggressively for
alternative products that provide the required performance characteristics, while

having a reduced carbon footprint, and an environmentally responsible design.

With this in mind, the work presented here aims to show that biodegradable
plastics that are partly biobased, can be produced to meet the needs of end-

users.

1.2 OBJECTIVES

This study focuses on the design and engineering of blends of maleated
thermoplastic starch (MTPS) with poly(butylene adipate-co-terephthalate) (PBAT)
through reactive extrusion, and the characterization of the resulting polymers.
Furthermore, the application of these polymers for products such as
thermofon'ned articles, films, and foams is studied. Specifically, the study focuses

on the following:

o Maleated thermoplastic starch by reactive extrusion processing.
0 MTPS-PBAT blends by reactive extrusion processing.
. Application and performance of MTPS-PBAT blends for therrnoforrned

articles, films, and foams.

1.3 SPECIFIC OBJECTIVES

. Maleated thermoplastic starch by reactive extrusion processing.
- Extrusion processing to graft maleic anhydride,
glycerol, and starch using free radical initiation
chemistry.
- Characterization of the resulting MTPS.
o Soxhlet extraction, Thermal Analysis,
Environmental Scanning Electron
Microscopy (ESEM), Fourier Transform
Infrared Spectroscopy (FTIR) analysis, and
Dynamic Light Scattering (DLS) analysis.
0 MTPS-PBAT blends by reactive extrusion processing.
- MTPS-PBAT blends by reactive extrusion processing
using maleic acid as a trans-esterification catalyst.
- Characterization of the resulting blends
o Soxhlet extraction, Thermal Analysis,
Environmental Scanning Electron
Microscopy (ESEM), and Fourier Transform

Infrared Spectroscopy (FTIR) analysis.

0 Application and perfomance of MTPS-PBAT blends for

thermoformed articles, films, and foams.

Extrusion of sheets of MTPS-PBAT and subsequent
therrnoforrning of the sheets.
Characterization of the resulting thermoformed
articles.
0 Tensile Testing and Puncture Testing.
Extrusion of films of MTPS-PBAT.
Characterization of the resulting films.
0 Tensile Testing, Puncture Testing,
Permeability Testing.
Extrusion of foams of MTPS-PBAT using a chemical
blowing agent.
Characterization of the resulting foams.
0 Density, Expansion Ratio (ER), Specific
Length (SL), Compressive Strength,
Resiliency, Moisture Sorption, and Digital

Light Microscopy.

1.4 STRUCTURE OF DISSERTATION

This dissertation has been divided into chapters in order to organize the

information.

Chapter 2 presents the relevant background needed to read this dissertation. It
includes information on biobased and biodegradable plastics, starch,
poly(butylene adipate—co-terephthalate) (PBAT), and extrusion theory. Chapter 2
also includes a review of literature pertinent to the subsequent chapters.
Chapters 3, 4, 5, and 6 contain the experimental details of this dissertation.
Chapter 3 deals with maleated thermoplastic starch (MTPS) production and
characterization. The characterization methodologies include soxhlet extraction,
thermal analysis, environmental scanning electron microscopy (ESEM), Fourier
transform infrared spectroscopy (FTIR) analysis, and dynamic light scattering
(DLS) analysis. Chapter 4 deals with MTPS-PBAT blends production using
reactive extrusion processing and their application and performance for
thermoformed articles. The characterization of the blends and thermoformed
sheets using DSC, FTIR, ESEM, and mechanical testing, which includes tensile
and puncture tests are discussed. Chapter 5 deals with the application and
performance of MTPS-PBAT blends for films. The methodologies used to
characterize the films are tensile and puncture testing, and carbon dioxide,
oxygen, and water vapor permeability testing. Also, transmission electron

microscopy is used to characterize the morphology of the blends. Chapter 6

deals with the application and performance of MTPS-PBAT blends for foams and
their subsequent characterization through density, expansion ratio, and specific
length measurements, compressive strength, resiliency, and moisture sorption
tests, and imagining using digital light microscopy. Summary and conclusions are
presented in Chapter 7, while recommendations for future work are discussed in

Chapter 8.

CHAPTER 2

BACKGROUND

 

2.1 BIOBASED AND BIODEGRADABLE PLASTICS RATIONALE

Topics which are receiving much attention lately are the issues of global
warming, greenhouse gases, and products’ carbon footprint. Also, petroleum
dependence and peak oil are of much concern to governments, scientists, and
policy-makers. Much work is being done to produce “environmentally-friendly"
products that are solutions to the above concerns. Using starch as a polymeric
material is a way in which these issues can be addressed. First, let us consider
the carbon cycle. Carbon in the atmosphere, in the form of carbon dioxide, is
fixed by plants through the process of photosynthesis. Over many years (>106

years), these plants become fossilized and are converted into petroleum and

natural gas. These fossil fuels are extracted from the earth’s core and used to
make fuels, polymers, and chemicals. In doing so, C02 is released back into the
atmosphere. This release of carbon dioxide back into the atmosphere happens
over a period of one to ten years.1 As one can see, the rate at which carbon
dioxide is fixed by plants and converted into petroleum is much slower than the
rate at which carbon dioxide is liberated. This leads to an accumulation of carbon
dioxide in the atmosphere. In their summary, the lntergovemmental Panel on
Climate Change concluded that global warming is occurring, that carbon dioxide
and other greenhouse gases cause global warming, and that human activity has
caused an increase in the amount of atmospheric carbon dioxide and

greenhouse gases.2

As shown in Figure 1, making polymeric materials, fuels, and chemicals from
annually renewable biomass crops instead of petroleum can balance out the rate
of carbon fixation and the rate of carbon liberation.1 Furthermore, if more plants
are grown than are harvested to produce the polymers, fuels, and chemicals, the

CO; rate equation can actually be reversed.

From the above discussion, one can assert that the benefits of using renewable
crops/biomass feedstocks to produce polymers are three-fold:

o Enabling sustainable development of biobased polymeric materials

0 Helping to meet the 002 emission standards set by the Kyoto protocol3

0 Improving the environmental profile of materials

 

The Global Carbon C cle

1-10 vea/

Polymers,
Chemicals 8:
Fuels

 

    
   

===> Biomass/Bio-organics
Bio-chemical Industry > 106 years

 
  
 
 

  
 

  

4

Chemical Industry

Fossil Resources
(petroleum, natural gas)

    

 

 

 

Figure 1. The global carbon cycle. [Adapted from Narayan‘]

' Using a holistic approach to producing materials means that products should be
designed and engineered from “conception to reincarnation”.4 Not only must the
ecological impact of the raw materials used in producing a product be
considered, but also how the product will be disposed of and where it will enter
the waste stream. Consideration must be given to whether or not a product is
biodegradable or recyclable. Many new market opportunities have opened up for
engineering biobased, biodegradable products to meet the demand for
sustainable materials that fulfill both economic and ecological requirements. This
method of designing a product with consideration for its end-of-life is not currently

given a great deal of focus, especially in the case of single-use, disposable

consumer goods and packaging. It is environmentally and ecologically sound to
design these products so that they are biodegradable, and also to ensure that the
products end up in the correct waste stream. By composting biodegradable
plastics and paper wastes, as well as food, yard, and agricultural wastes, carbon-
rich compost (humic material) can be created. This humic material can then be

placed on farmland, thereby re-initiating the carbon cycle.

When one discusses the term biodegradable, one must have a clear
understanding of the exact meaning of the term. Biodegradability means that
there exists a microbially, oxidatively or hydrolytically susceptible linkage in the
backbone of a polymer, or a catalyst that can break down the polymer chains in a
specific environment. Furthermore, the products present after the breakdown
process should not persist in the environment or be toxic, and should be used by
microorganisms in a defined time frame.” The typical time frame would be one
growing season or one year. A product that is designed to be degradable
(fragment into small pieces), and not biodegradable, can be quite harmful to the
environment. Although a degraded material may be invisible to the naked eye, it
may persist in the environment. Thompson et al.7 have reported that degraded
plastic debris from around the world is accumulating in the oceans. The plastic
debris has broken down into microscopic granular and fiber-like fragments.
Furthermore, the Algalita Marine Research Foundation° has reported that the

degraded plastics in the oceans attract and hold hydrophobic elements such as

10

PCB and DDT. The concentrations of these elements in the high surface area

plastic residues can be up to one hundred times the background levels.

As shown in Figure 2, heat, moisture, sunlight and/or enzymes can shorten
polymer chains and cause fragmentation and some cross-linking of plastic,
resulting in intractable persistent residues.9 If the degraded fragments are not
completely assimilated by microbes in the waste stream, within a short period of
time, then the material can be more harmful in the environment than if it had not
degraded at all. For this reason, biodegradation, not just degradation, is a very

important characteristic of compostable polymers.

 

   
   
 

Polymer
Fragments

    

Polymer Crosslink
Residues

    

Degradation / Fragmentation Biodegradation

 

 

 

Figure 2. Degradation versus biodegradation of materials. [Adapted from
Narayan

11

2.2 STARCH

Starch is a natural, biodegradable, polymeric material. The structure of starch is
that of a polymer of anhydroglucose units linked by alpha-1,4 linkages. Starch is
found in granules of 2 to 50 microns in diameter. The granules consist of starch
polymer in two forms. The first form of starch is amylose, a predominantly linear
q-1,4 glucan polymer. The second form of starch is amylopectin, a d-1,6
branched (II-1,4 glucan polymer. The ratio of amylose to amylopectin in starch
can vary, depending on the origin of the starch.10 In the United States, corn is the
predominant source of starch, however, starch can come from sources such as
wheat, rice, cassava, or tapioca as well. Figure 3 shows the structure of amylose
and amylopectin. Typical molecular weights for amylose and amylopectin are
approximately 1 million and 10 million, respectively. Within the granules of starch,
the amylose and amylopectin form ordered arrangements, giving crystallinity to

the granules.

12

 

 

 

 

 

 

 

 

 

 

 

 

AMYLOSE
CHZOH
O
H
OH H
O—
H H
AMYLOPECTIN
o
H—— OH H CHZOH
“°—:“./°
o
CHZOH CH2 CHZOH
o o o
H H H
OH H OH H OH H
—-—o o o o—
H OH H OH H OH

 

 

 

 

 

Figure 3. Structure of amylose and amylopectin. [Adapted from Narayan‘o]

Due to the hydroxyl groups on the surface of the starch granules, starch shows
hydrophilic properties and strong inter-molecular association in the form of
hydrogen bonding. This strong hydrogen bonding, along with the crystallinity of

the granules, causes poor thermal processing of starch. The melting temperature
13

of starch is higher than the thermal decomposition temperature. As a result,
starch begins to degrade prior to melting, during thermal processing.10 These
issues of hydrophilicity and thermal sensitivity make starch unsuitable for

thermoplastic applications.

Nevertheless, thermoplastic starch can be produced through the use of an
appropriate plasticizer. The plasticizer breaks the hydrogen bonds within the
granules and allows the starch molecules to flow like a therrnOpIastic material.
Using glycerol as a plasticizer, starch has been plasticized in a twin screw
extruder.9 However, the resultant material is brittle and does not provide

acceptable performance characteristics.

2.3 POLY(BUTYLENE ADIPATE-CO-TEREPHTHALATE) (PBAT)

Poly(butylene adipate-co-terephthalate) (PBAT) is manufactured by BASF, under
the trade name Ecoflex. The chemical structure of PBAT is shown in Figure 4. It
is an aliphatic-aromatic copolyester comprised of 1,4-butanediol, terephthalic

acid, and adipic acid.11 It is a synthetic, biodegradable polyester.

 

0

II

O—(CH2)4— O—C— (CH2)4— C O—(CH2)4— O—

:0

Ii
C

O

X

 

_<

 

 

Figure 4. Chemical structure of PBAT. [Adapted from Nabar12]

14

Ecofiex can be degraded by micro-organisms and fulfills the requirements for
biodegradable and compostable polymers set by the US standard ASTM 06400-
04, the European standard DIN EN13432, and the Japanese GreenPla

standard.13

Ecoflex is quite compatible with natural materials such as starch and cellulose12
and its low melting temperature (110-120°C)13 makes processing it with starch
very easy. By blending Ecoflex with starch, there is the opportunity to produce
materials that are much better suited for making consumer products than using
starch alone, while having the added benefit of decreasing the price of using

Ecoflex alone.

2.4 EXT RUSION THEORY

Extruders are very useful machines and have been used for many years in
industries as wide ranging as mining, petroleum, food processing, and polymers.
Extruders are used in the polymer industry (1) as plasticating pumps that convey
melts to profile dies, (2) for continuous compounding and blending, (3) as

continuous chemical reactors, and (4) as devolatilizers.14

An extruder allows the use of shear, heat, and pressure to change a solid resin

into a melt that can then be used in another process. Oftentimes, additives such

15

 

Ih
th

SI"

TI

1%?

as fillers, colors, and blending resins are also mixed in an extruder.15 The final

requirements of the products to be made dictate what additives are used.

The basic components of an extruder include a hollow tube, called the barrel,
which contains a screw. The screw has helical channels and, in general, is
divided into three sections; the solids conveying section, the compression
section, and the metering or pumping section.1° Extruders can be more
complicated, involving features such as vents, multiple feeders, multiple screws,

and dies to produce various shapes, however, the basic concept is still the same.

The raw materials, usually plastic pellets, are fed into the feed port of the
extmder from a hopper. This section then conveys the material to the
compression section. It is in this stage that the plastic material is melted. A
decrease in the depth of the screw channels forces the material against the walls
of the barrel and builds up pressure in this zone. The friction from this shearing
process increases the temperature of the plastic. As well, external heating of the
extruder also increases the temperature of the plastic. These processes cause
the material to soften and melt. Finally, in the metering section of the extruder,
the channel depth is constant and the melt is conveyed at a constant rate to the

shaping apparatus.15

The screws of an extruder are composed of screw elements. There are many

different screw elements that aid in the extrusion process. Forwarding elements

16

move material forward past various sections such as feed and vent ports. Mixing
of the polymer occurs once the material begins to melt and lasts until it reaches
the die. Proper mixing must occur before the material reaches the die. The screw
elements which aid in the mixing of the materials are called mixing elements.
Mixing can either be dispersive or distributive. Dispersive mixing elements
include fluted or splined elements, annular blister rings, wide kneaders, and
shaped lobal elements. Distributive mixing elements include pin mixer, slotted
screw flight, narrow kneaders, and sleeve mixer elements.1°'17 The difference
between dispersive and distributive mixing is shown schematically in Figure 5. In
general, mixing occurs due to shear flow and elongational flow. If the fluids that
are being mixed are compatible and do not exhibit a yield point, then distributive
mixing occurs. However, if the material is a solid and exhibits a yield point, this is

referred to as dispersive mixing.

 

Dispersive Mixing O O
O
t O O O

0
OOO

 

80 Distributive Mixing o

 

.. = 2.0
c . o.

 

 

 

Figure 5. Schematic diagram of dispersive and distributive mixing. [Adapted from
Balakrishnanw]

17

 

k5

2.5 POLYMER BLENDS AND ALLOYS

The terms blend and alloy are often used synonymously; however, they can be
distinguished based on the thermodynamic compatibility between their
components, leading to different physical and mechanical properties.18 In
general, a property value, P, of a polymer blend or alloy of two components can
be described by the following equation:

P = P1C1 + P2C2 + IP1P2
Where P1 and P2 are the property values for the two individual components, C1
and C2 are the concentrations of the two components, and l is an interaction

coefficient for the thermodynamic compatibility between the two components.

It is the value of I that determines the property values of the blend or alloy. If I is
positive (I > 0), the property values of the polymer combination are greater than
the weighted arithmetic average of the components’ properties. This is termed a
synergistic blend. An alloy is a synergistic polymer combination. If I is zero, the
property values of the polymer combination are the weighted arithmetic average
of the components’ properties. This is termed an additive blend. If I is negative (I
< 0), the property values of the polymer combination are less than the weighted
arithmetic average of the components’ properties. This is termed a
nonsynergistic blend.18 The effect of the interaction coefficient on the properties

of polymer combinations is shown in Figure 6.19

18

 

 

/ Synergistic

Property

 

Nonsynergistic

 

 

 

Polymer 1 Composition Polymer 2

 

 

 

Figure 6. Relationship between property values and blend composition. [Adapted
from Paul and Barlow19]
Thermodynamic compatibility or miscibility is determined by balancing the
entropic and enthalpic contributions of the free energy of mixing.20 The change in
the free energy upon mixing (AG) is given by the following equation:

AG = AH - TAS
Where H, S, and T are enthalpy, entropy, and temperature, respectively. Also, for
mixing to occur spontaneously, AG must be negative, thus:

AH-TAS<0

In polymers, the entropy is almost zero, thus the value of enthalpy determines

the miscibility, and if AH<0, then the mixture will spontaneously mix.20

19

2.6 REVIEW OF LITERATURE

Plastic materials are widely used for many applications due to their processability
and cost-effectiveness.21 However, plastics are almost entirely petroleum-based
and, therefore, made from non-renewable resources. Much research is being
done to produce plastics from alternative, biobased, materials. The two
requirements of such “eco-friendly" polymers are: (1) production of the polymer
from a renewable resource, and (2) biodegradability.” Polymers made from

starch can thus be considered “eco-friendly", since they meet both criteria.

Starch may be sourced from many different agricultural products. For example,
potato, com, cassava, and wheat starch have all been used to produce
thermoplastic starch.”25 There exists some mutant genotypes of maize, barley,
rice and wheat that produce starches containing an increased amylose content
(i.e. high amylose with up to 70% amylose) or an increased amylopectin content
(i.e. “waxy starch” with 99-100% amylopectin). Variations in the ratio of amylose
to amylopectin within these starches causes differences in granular structure,
physiochemical properties, and quality of end-use products.26 The crystallinity of
starch granules is affected by the amount of amylose and amylopectin within the
granules. For example, high amylose starches are approximately 15% crystalline
while waxy starches can be up to 45-50% crystalline.27 Starch is arranged into
growth rings within the granules. The concentric growth rings have alternating

crystalline and amorphous regions with higher and lower density, respectively.

20

The crystalline regions have a lamellar structure containing crystalline and
amorphous layers that alternate. Amylopectin chains pack into an ordered
structure within the crystalline layers, while amylose, amylopectin, and
amylopectin branching points exist in a random conformation within the

amorphous layers.” 28

Starch is not a thermoplastic material, but in the presence of a plasticizer such as
water, glycerol, or sorbitol, high temperatures (90-180°C), and shear, it will
become thennoplastic.12'29'3° In the production of thermoplastic starch (TPS), the
starch loses its semi crystalline granular structure and acquires thermoplastic

behavior.“

The production of thermoplastic starch by extrusion of starch and glycerol has
been demonstrated22 and several researchers, including the Narayan group,
have successfully manufactured starch that behaves like a pure thermoplastic

material.” 33

It has been shown that thermoplastic starch can have improved processability
and reactivity if it is modified by reacting it with an organic dibasic acid or an
anhydride in the presence of a plasticizer, using a free radical initiator. This
produces a chemically modified plasticized starch (US patent 7153354)34 which
is biodegradable, reactive, and chemically modified allowing it to have a lower

viscosity and good processability. It should also be noted that during the

21

pl

lh

processing, there is no water added. This patent claims that chemically modifying
thermoplastic starch prevents problems such as high viscosity, clogging of the
thermoplastic starch melt at the die, or foaming of the thermoplastic starch melt.
Furthermore, the resulting chemically modified thermoplastic starch is highly
reactive and can form graft copolymers with polyesters. An important aspect of
these starch-polyester graft copolymers is that they are able to be formed into
films and molded products using conventional plastics processing equipment,
and the resulting product has the most preferred balance in mechanical

properties, water resistance, processability, and rate of biodegradation.34

In the present work, maleic anhydride is the anhydride used to produce modified
TPS. This maleated thermoplastic starch is thus referred to as MTPS. Since
maleated thermoplastic starch can be used in blends with other biodegradable
polymers for different applications such as films and sheets, or injection molded
and thermoformed materials, it is necessary to understand the effect that
maleation has on starch. The maleation of starch has been shown to decrease
the molecular weight of starch when no initiator was used, however, the
molecular weight increased when an initiator was used.34 The decrease in
molecular weight was attributed to the fact that starch is hydrolyzed in the
presence of maleic acid. The increase in molecular weight was attributed to

branching or cross-linking occurring with the addition of the initiator.35

22

As a packaging material, starch alone does not form films with adequate
mechanical properties such as high percentage of elongation, tensile, and
flexural strength.36 Starch also has the disadvantages of exhibiting: a) brittleness
when no plasticizers are utilized; b) hydrophilicity and poor water resistance; 0)
deterioration of mechanical properties when exposed to environmental conditions
such as humidity and d) soft and weak tendencies when plasticizers are

utilized.37

Thermoplastic starch has weaknesses that limit its utilization in packaging
applications as a film, but also for therrnoforrning applications. Therrnoforrned
objects often have low wall thickness and the drawbacks of using starch create
additional issues.38 Major drawbacks are water sensitivity, change of
mechanical properties with time, crystallization due to aging, water adsorption,

and low impact strength resistance.” 4°

In order to make starch a more feasible option for packaging applications,
thermoplastic starch (T PS) can be blended with biodegradable polyester to
improve its mechanical properties and decrease its water sensitivity.
Furthermore, commercial biodegradable polyesters are expensive (3.5 to 5.0
euro/kg),41 thus, by blending starch with biodegradable polyesters the price of the
material can be reduced. There has been extensive investigations into blending

starch with various polymers, thereby creating possible replacements for

23

conventional plastics, mainly in the area of packaging as films or injection molded

materials.

Issues arise due to the poor compatibility of high loadings of starch with
biodegradable polyesters. This results in poor physical and mechanical
properties of the materials. An effective way to improve the compatibility between
starch and the polyester matrix is to functionalize the polyester matrix by grafting
highly reactive functional groups. By functionalizing carbonyl, anhydride, epoxy,
urethane, or oxazoline groups to synthetic polymers, reactive blending with
starch has been performed. The functional groups on the synthetic polymers can
react with the hydroxyl or carboxyl groups in the starches and modified

starches.37

In order to functionalize polymers, maleic anhydride (MA) has been grafted onto

12, 42-48 and non_

many different hydrophobic polymers, both biodegradable
biodegradable“: 5° These functional polymers can then be blended with
thermoplastic starch (TPS), allowing the maleated polymers to act as

compatibilizers between the non-functional polymer and the starch.

Another option for compatibilization of starch and polymers (biodegradable or
non-biodegradable) is to functionalize the TPS and then to blend the chemically
modified starch with various polyesters.34 It has been shown that these starch-

polyester graft copolymers are readily processable into films, and molded

24

products using conventional plastics processing equipment. As well, the
mechanical and material properties of these blends are improved, due to the

compatibilization of the starch and polymer.29

There are many different biodegradable polymers. They can be categorized in
various ways, such as the way in which they are polymerized (biologically or
synthetically): a) biosynthetic (starch, cellulose, PHB, PHBV), b) semi-
biosynthetic (PLA) and (c) chemo-synthetic polymers (PCL, PBSA, PBS,
PBAT).41 Poly(butylene adipate-co-terephthalate) (PBAT), is a chemo-synthetic
biodegradable aliphatic aromatic co-polyester. It is produced in a condensation
polymerization reaction between 1,4-butanediol, terephthalic acid, and adipic
acid. It has been used in several applications, such as for producing films, in
blends with therrnoplastics starch,51 blends with biobased polymers such as

Polyaeictide).52 and in lignincellulosic-based composites.53'55

This work reports on the reactive blending of therrnoplasticized starch with the
biodegradable polyester, poly(butylene adipate—co-terephthalate), to produce
sheets of material that can be thermoformed. Both maleated thermoplastic starch
(MTPS) and thermoplastic starch (T PS) are investigated. Thermofon'ned articles
are often single-use, disposable products. Making these items biodegradable

would be environmentally responsible and preferable.

25

Starch has been considered for many years as an attractive polymer for use in
packaging film applications due to its low cost, renewability and biodegradability.
Several studies have been performed to analyze the properties of starch-based
films.4°' 5°60 Edible and/or biodegradable films will not replace all synthetic
packaging films; however they do have potential to replace conventional

packaging in some applications.

The hydrophilic nature of starch polymers makes them sensitive to environmental
humidity and the presence of glycerol in tthennoplastic starch also strengthens
this behavior.29 Blending thermoplastic starch with biodegradable polymers that
are less hydrophilic is one option to produce film materials that are more water

resistant.

In the present work, biodegradable films of blends of high amylose corn starch
with poly(butylene adipate co-terephethalate) are produced and their mechanical

and barrier properties, and microstructure are characterized.

Foam packaging, by design is lightweight but bulky. The high transportation costs
of transporting and disposing of foam articles after their useful life makes it a
suitable application in which biodegradability and compostability would be very
beneficial. The Narayan group32'61'63 has reported on the rationale, design, and

engineering of biodegradable polymer materials previously.

26

In order to impart hydrophobic characteristics to starch-based foam materials,
while maintaining the beneficial biodegradable properties of starch, many
researchers have investigated producing foam that is blended with hydrophobic
biodegradable polymers. Bastioli et al.°""37 have patented foams comprised of
starch blended with 10-30% biodegradable polymers such as PVA, PCL, and CA.
Willett and Shogren68 extruded foams of starches blended with thermoplastic
resins such as PVA and CA, and biodegradable polyesters such as PLA, PCL,
poly(hydroxyester ether) (PHEE), poly(ester amide) (PEA), poly(hydroxybutyrate-
co-valerate) (PHBV), PBAT, and others. Both regular corn starch and high
amylose corn starch, wheat starch, and potato starch were used. High levels of
starch and its poor compatibility with the various polymers, lead to poor
properties such as density, compressibility, and resilience. Fang et al.°9 showed
that the densities and expansion of the foams depended on the PBAT content.
Nabar et al.12 compatibilized starch and PBAT through the addition of maleated

PBAT and reported a reduction in density and improved resilience.

The above mentioned results have been produced through the use of water as

the physical blowing agent in the extrusion of the foam. The work presented here

I. 70

describes the use of chemical blowing agents. Work by Zirkel eta has been

performed using chemical blowing agents for the extrusion on polymeric foams at
high temperatures. Compression foaming of PVC using a chemical blowing

I. 71

agent, azodicarboxamide, has been reported by Demir eta while

27

compression foaming of biodegradable poly(propylene carbonate) using a

composite chemical blowing agent has been reported by Guan et al.72
Chemical or physical blowing agents may be used to produce foam. A chemical
blowing agent (CBA) is a chemical that will decompose at elevated temperatures
to produce gases. Physical blowing agents (PBAs) are usually gases that are
injected during the processing of foams, and they require special equipment for

their use.72

Chemical blowing agents can be divided into two categories: exothermic and
endothermic.73 During thermal decomposition, exothermic CBAs release heat
while endothermic CBAs absorb heat. Also, in general, exothermic CBAs

generate N2 gas and endothermic CBAs generate CO2 gas.

As was discussed previously, water (a PBA) has been used as the blowing agent
for many starch-based foams. In this work, CBAs have been used to produce
foams of blends of modified starch and PBAT. The properties of the foams are

characterized and presented.

28

CHAPTER 3

MALEATED THERMOPLASTIC STARCH BY REACTIVE

EXTRUSION PROCESSING

 

3.1 RATIONALE

This work was performed to better understand the effect of the amount of maleic
anhydride and initiator, and the effect of extrusion temperature on maleated
thermoplastic starch (MTPS) produced through reactive extrusion.
Characterization of the MTPS samples was performed through dynamic light
scattering (DLS), differential scanning calorimetry (DSC), and environmental
scanning electron microscopy (ESEM). Also, proof of reactive extrusion was
determined by Fourier transform infrared spectroscopy (FTIR) and soxhlet

extraction in acetone.

29

3.2 MATERIALS AND METHODS

3.2.1 Materials

High amylose corn starch was used in this work. The corn starch was purchased
from National Starch and Chemicals (Indianapolis, IN). Anhydrous glycerol,
99.9% assay, was purchased from J.T. Baker (Phillipsburg, NJ). Maleic
anhydride was purchased from Sigma-Aldrich, Inc (St. Louis, MO). The initiator,
2,5-bis(tert-butylperoxy)-2,5—dimethylhexane, also referred to as Luperox 101,
was purchased from Sigma-Aldrich, Inc (St. Louis, MO). The solvent used in the
studies was dimethyl sulfoxide, and was purchased from J.T. Baker (Phillipsburg,

NJ).

3.2.2 Equipment

A Century CX-30 co-rotating twin-screw extruder, having a length/diameter ratio
(LID) of 42 and a screw diameter of 30 mm was used to prepare the
thermoplastic starch and maleated thermoplastic starch. The screw configuration
for the Century CX-30 extruder is shown in Figure 7.16 A pelletizer was used to

cut the extrudate strands into small pellets.

30

 

20/20 (4)

28/ 14
28/28

60/60 (3)

28/28 (2)

KB40/5/14 (2)

KB40/5/20
20/20
28/28

42/42 (2)

28/28

K390/5/28 (2)

KB40/5/14 (2)
20/20
28/28
42/42

28/28

Z?\\\7\T\\T‘~~J\_. \\k\\\\\\\\\\'

I.

i

.T'i;\\?\\\\\\\\7i

' ~ 't. 'y‘tga‘r;
r c . ' 1 ‘1":
' 4‘ 2.. . l1”;“__“ ”I;

III
i

Direction of Flow

W ~

KB40/5/42 (2)

20/20 (2)
28/28 (2)

42/42 (2)

60/60 (3)

;\f“~‘;.\’.‘\3~:;.\.\\\\\\\\\\\\— ‘r.-f

28/14

 

 

 

 

Figure 7. Screw configuration of the Century CX-30 co-rotating twin-screw
extruder. [Adapted from Balakrishnan1°1

31

3.2.3 Procedure
3.2.3.1 Sample Preparation

Maleated thermoplastic starch was produced in a Century CX-30 co-rotating
twin-screw extruder, having a length/diameter ratio (L/D) of 42 and a screw
diameter of 30 mm. Maleic anhydride was ground into a fine powder using a
mortar and pestle and pre-blended with the corn starch, placed in the external
feeder of the extruder, which fed into the feed port of the extruder. Luperox 101
was pumped into the feed throat of the extruder using a peristaltic pump.
Glycerol, which was warmed in a water bath, was also pumped into the extruder
using a peristaltic pump. The extruder was electrically heated and the maximum
temperature of the extruder was set to 165°C, with a temperature profile of
25/95/125/145/160/165/165/165/150/145°C from the feed throat to the die.
Cooling water was circulated through the extruder in order to maintain the set
temperatures during the extrusion process. The feed rates of the external feeder
containing the starch and the peristaltic pump for the glycerol were set so as to
accomplish a composition of 80 wt% starch and 20 wt% glycerol and a total flow
of all material through the extruder of approximately 11 kg/h. The screw speed
was set to 125 rpm. A vacuum was pulled on the vent port of the extruder in
order to remove unreacted maleic anhydride and moisture. The extruded strand

exiting from the die was air cooled, pelletized in line, and collected.

32

Table 1 shows the composition of the samples. To produce these samples, first
the amount of maleic anhydride (as weight percentage of starch and glycerol)
was varied while the amount of initiator (Luperox 101) was held constant at 0.25
wt% of starch and glycerol. The amount of maleic anhydride in the samples was
2 wt%, 4 wt%, or 6 wt%. Next, samples were extruded with varying amounts of
initiator, while the amount of maleic anhydride was held constant at 2 wt% of
starch and glycerol. These samples contained 0.1 wt%, 0.25 wt%, or 0.5 wt% of

free-radical initiator.

To compare the effect of temperature and maleation on the properties of the
samples, TPS and MTPS containing 30 wt% glycerol were produced in the twin-
screw extruder, with a maximum temperature of 135°C

(25/115/120/125/130/135/135/135/130/130°C).

Figure 8 shows the process schematic for the extrusion of MTPS.

The proposed reaction of maleic anhydride with starch16 is shown in Figure 9.
Also, it has been shown that in the presence of maleic anhydride, starch is

hydrolyzed and glycerol and maleic anhydride are grafted to the starch

backbone. This is shown in Figure 10.74

33

Table 1. Composition of the maleated and non-maleated thermoplastic starch
(MT PS and TPS) samples.

 

 

 

 

 

 

 

 

 

 

 

Max.
Corn Maleic Luperox
Processing Glycerol
Starch Anhydride 101
Sample Temperature
(weight % of com
(°C) (weight %)
starch + glycerol)
1 165 80 20 2 0.1
2 165 80 20 2 0.25
3 165 80 20 2 0.5
4 165 80 20 4 0.25
5 165 80 20 6 0.25
6 1 35 70 30 2 0.1
7 1 35 70 30 - -

 

 

 

 

 

 

 

 

34

 

 

 

 

 

 

0 Pump
Starch

M-A- Water Bath

r/Glycerol

”Th it

 

 

 

 

 

 

 

 

Vacuum

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

|\\\\\\\\\\\\\\H
Luperox 101 Die P II t'
e e IZBI'
Figure 8. Process schematic for extrusion of MTPS.
M I '

Stare“ who” A:h:'§ride

O /

W A
——o o— 0 0 0
0H 1

II II
CHzo—C—C=C—C——OH
H H

O
OH

OH

 

 

 

Figure 9. Proposed reaction of starch with maleic anhydride. [Adapted from
Balakrishnan 61

35

 

 

 

 

 

 

 

 

° 0
I
. I
I o o o o
o I
N O
I g f I
U u o
O o
I
+ 9..
+ 5
I I
0 e
I
I o o 9..
cg 5
I I
5 o o o
I
9..
I
o 5 o
I
o
I
q.
I
~ +
9. i
I O
h
8
2.-
L9
I I
o o
I , I
o :t: _.o o I __o
I I
9. <2.
5 I5 I 5 IE I
' z
o m o g
('7) I <
' D
I >- I...
I o I o _i I o 2: ca)
0 O o O
N m N o
5 :r:I Q 5L1: 3
\ 07 I >_ 07 I-l
A A
o 3 O :9.
I I

 

 

 

 

 

 

 

 

Figure 10. Hydrolysis (a) and glucosidation (b) reactions occurring during the
reactive extrusion of maleated thermoplastic starch. [Adapted from Raquez et
aI. ]

36

3.2.3.2 Dynamic Light Scattering (DLS) Analysis

The MTPS and TPS samples were ground into a powder using a mortar and
pestle. The powder was dissolved in dimethyl sulfoxide at a concentration of 10
mg/mL. A sample of high amylose corn starch was also tested in order to
compare its hydrodynamic radius with those of the extruded samples. Dynamic
light scattering measurements were performed in a Dynapro dynamic light
scattering apparatus by Protein Solutions. This equipment has a wavelength of
827.1 nm and scatters at an angle of 90 degrees, the temperature was held

constant at 25°C and the laser intensity was set to 100% for all samples.

The dynamic light scattering equipment calculated the hydrodynamic radius of
each sample using a regularization algorithm. A distribution is calculated based
on the mass percentage of various fractions having varying hydrodynamic radii.

Figure 11 shows a graph obtained for the DLS test.

 

0.01 0.10 1.00 10.00 100.001 .00E+03.00E+04

Radius (rim)

 

 

 

Figure 11. Graph of dynamic light scattering measurements for sample 6 (MTPS
high amylose starch, with 30% glycerol and 135°C).

37

3.2.3.3 Fourier Transform Infrared Spectroscopy (FTIR) Analysis

The FTIR spectra of TPS, MTPS and starch were scanned from 650 cm'1 to 4000
cm'1 using a Perkin Elmer Spectrum One FT-IR Spectrometer. This testing was
performed in order to determine the reactions that occurred in TPS during the

reactive extrusion with MA and a free-radical initiator.

3.2.3.4 Thermal Analysis (DSC)

Differential scanning calorimetry (DSC) was performed using a high resolution
DSC 2920 from TA Instruments. The purge gas used was nitrogen. It had a flow
rate of 50 mllmin. Samples of the extruded pellets were cooled to -50°C and then
heated to 200°C at a rate of 10°C/min, cooled to -50°C and then reheated to
200°C. Analysis of the therrnograms produced shows the thermal transitions that

occurred during the test.

3.2.3.5 Soxhlet Extraction Analysis

Soxhlet extraction in acetone was used to determine the amount of glycerol that
was grafted to the starch during the extrusion process. The MTPS and TPS
samples were ground into a powder using a mortar and pestle. The ground
powder samples were dried in an oven set at 65°C for 72 hours. A piece of filter
paper, measuring 10x10cm was cut, folded, and stapled at the bottom.
Approximately 2 grams of the thermoplastic starch powder was accurately

weighed into the folded filter paper and then the top was stapled shut. The

38

packet was placed into an extraction thimble holder. Evaporated solvent
(acetone) condensed onto the packet containing the sample. The extraction was
run for 72 hours. After extraction, the packet was removed and dried to constant
weight. The top staple was then removed from the packet and the sample and
filter paper (with bottom staple) were accurately weighed to determine the
amount of material extracted during the soxhlet extraction. Since glycerol is
soluble in acetone, but starch is not, glycerol that has not reacted with the starch
will be extracted during the test; however, any glycerol that has reacted with the

starch will remain in the sample in the filter paper.

3.2.3.6 Environmental Scanning Electron Microscopy (ESEM) Analysis

An environmental scanning electron microscope (ESEM) (Phillips Electroscan
2020) was used to make observations of the fractured surface of the
thermoplastic starch. Pieces of the extnided samples were immersed in liquid
nitrogen, frozen, and then fractured. The fractured surface was viewed in the

environmental scanning electron microscope.

3.2.3.7 Statistical Analysis

Analysis of variance (ANOVA) and Tukey mean comparison tests (p s 0.05) were

performed using Statistica software (Statsoft, Oklahoma — USA).

39

3.3 RESULTS AND DISCUSSION

Observations made during the extrusion of the samples include the fact that
MTPS had improved processability over the TPS sample. The maleation process
permitted formulations with less than 30 wt% of glycerol to be extruded in the
twin—screw extruder. The resulting MTPS pellets were more transparent than the
TPS pellets. Figure 12 shows an image of MTPS pellets after extrusion and

pelletization.

 

Figure 12. Picture of maleated thermoplastic starch pellets.

3.3.1 Dynamic Light Scattering (DLS) Analysis

For each sample, the value of the hydrodynamic radius which has the largest
mass percentage has been used for comparison purposes in this paper. Table 2

shows the hydrodynamic radii that were measured for the samples.

40

Table 2. Hydrodynamic radii as measured bydynamic light scattering.

 

 

 

 

 

 

 

 

 

Hydrodynamic
Sample Composition
Radius (nm)

1 MTPS, 0.10% Luperox 101, 2% MA, 20% glycerol 4.2
2 MTPS, 0.25% Luperox 101, 2% MA, 20% glycerol 2.1
3 MTPS, 0.50% Luperox 101, 2% MA, 20% glycerol 1.2
4 MTPS, 0.25% Luperox 101, 4% MA, 20% glycerol 2.5
5 MTPS, 0.25% Luperox 101, 6% MA, 20% glycerol 2.8
6 MTPS, 0.10% Luperox 101, 2% MA, 30% glycerol 9.1
7 TPS, 30% glycerol 11.9

High amylose corn starch 12.6

 

 

 

 

 

As the percentage of maleic anhydride increases, there is negligible change in
the hydrodynamic radius; however, as the percentage of Luperox 101 increases,
the hydrodynamic radius decreases. The decrease in hydrodynamic radius as
the percentage of Luperox 101 increases could be the result of chain scission
occurring, as shown in Figure 10in Section 3.2.3.1. Chain scission would
decrease molecular weight and thus decrease the hydrodynamic radius of the

samples.

41

By comparing the high amylose starch with the thermoplastic and maleated
thermoplastic starch, we can observe the effect of extrusion and maleation on
starch. Extruding TPS caused a decrease in the hydrodynamic radius of the
starch from 12.6 to 11.9nm. When 2% of maleic anhydride and 0.1% of Luperox
101 was used, the hydrodynamic radius decreased from 11.9 to 9.1nm. These
results could indicate that the extrusion and maleation processing degrade the

samples and one could infer that there is a decrease in the molecular weight.

When comparing the maleated samples at 165°C and 135°C, we can observe the
effect of temperature profile on the hydrodynamic radius. The values for the
samples at 135°C are much larger than for the samples at 165°C. Using 165°C
may degrade the samples and cause a drastic decrease in the hydrodynamic
radius (9.1 to 4.2 nm). Thus, temperature has a high influence on the molecular

weight of the maleated thermoplastic starch.

In comparison to other studies that have been performed to understand the effect
of Luperox 101 on the molecular weight of biobased polymers, there is not a
consistent trend. Carlson et al.75 showed that branching and chain scission
depended not only on the amount of Luperox 101 in the process, but also on the

extrusion temperature in the reactive extrusion of polylactide.

Work done by Nabar et al.12 suggests that the effect of increasing the amount of

maleic anhydride in maleated poly(butylene adipate-co-terephthalate) (PBAT) is

42

to decrease the intrinsic viscosity of the samples (ie: decrease the molecular
weight) and increasing the amount of initiator, Luperox 101, also decreases the
intrinsic viscosity of the samples. These results of the effect of maleic anhydride
on molecular weight do not show the same trends as the data presented in the
present work; however, the material that was maleated was PBAT, not high
amylose corn starch. Other work by Nabar 35 shows that when using regular corn
starch, the intrinsic viscosity of TPS is lower than the intrinsic viscosity of the
starch, and that when the starch is maleated, the intrinsic viscosity reduces even

more. This is in agreement with the work presented here.

In other work, using regular corn starch without an initiator, the intrinsic viscosity
decreased as the MA content increased.74 This trend is not consistent with the
present work; however, no initiator was used and the starch was regular corn

starch, not high amylose corn starch.

Possible explanations for the present results showing negligible change in
hydrodynamic radii as the percentage of MA increased as opposed to decreasing
hydrodynamic radii could be that as the percentage of MA increased there was
more swelling of the molecules in DMSO, or that aggregation of molecules
occurred.

I. 76

Furthermore, as was discussed by Roger eta measuring the hydrodynamic

radius of starch in DMSO can lead to some uncertainties in measurements of

43

pure amylose starch. This must be taken into consideration when viewing the
hydrodynamic radius data from the DLS measurements. Nevertheless, in this
study, all measurements were made under the same conditions so any errors
due to the solvent/starch solution will be constant for all measurements. In this
way, the DLS data gives one an understanding of how the hydrodynamic radius
has changed due to the varying amounts of MA and Luperox 101, as well as

different extrusion temperatures and glycerol contents.

3.3.2 Fourier Transform Infrared Spectroscopy (Fl’ IR) Analysis

The FTIR spectra for TPS, MTPS, and high amylose corn starch are shown in
Figure 13. The spectra for TPS and MTPS are quite similar; however, proof of
grafting of maleic anhydride to the starch backbone is exhibited by the fact that
MTPS shows a band at 1700 cm". This band corresponds to the carbonyl group
present in maleic anhydride and which is not present in either the high amylose
corn starch or the TPS samples. Also, since glycerol was used as the plasticizer,
two additional peaks at 2928 and 2886cm'1 are present in the TPS and MTPS
samples due to the formation of inter and intramolecular bonding hydroxyl group
bands between starch and glycerol. It should also be noted that the MTPS
sample showed no peak at 1787 cm". This peak would correspond to a ring
anhydride being present. Its absence suggests that the maleic anhydride ring

was fully open.

44

 

1.3

 

 

 

 

 

 

 

 

 

 

 

 

 

" 1.1
I t
1 t 3
I . l
a I
i 2‘ 0.9
g I 0.7
‘ l
I ”t l “
I :' '2,‘ g 0.5
if. I [t I
r \I 2
f .' 1 I A at
,' I. I I, “I: 0.3
.' I', .
I: t; ”I V“ a? ‘ (.4
' . um- l .w:
a :' 5 :: ‘\..~. "In r ‘F \l" .I a.
' :' ‘ “d .0 a" - ‘ I
4- v; “M. .1» .. at 4:" -- t 0.1
“I,“ ~ ' \\‘~w~. 3: “we“::{ in!“ .1;
"so“. T", “'"T '
I I I I I I ‘0.1
3650 3150 2650 2150 1650 1150 650
cm-1
------- High Amylose Corn Starch

----TPS 1350 30% Glycerol
—MTPS 1350 0.1%L101 2%MA 30% Glycerol

 

 

 

 

 

Figure 13. FTIR Spectra for High Amylose Corn Starch, TPS 135°C 30%
Glycerol, and MTPS 135°C 0.1% L101 2% MA 30% Glycerol.

From analysis of the FTIR spectra for the MTPS samples, one is able to compare
the relative intensities of the carbonyl peak in each of the MTPS samples. This is
shown in Figure 14. This peak is more intense for the samples containing 4%
and 6% maleic anhydride (Sample 4 and Sample 5, respectively), but the
samples that contain 2% maleic anhydride (Sample 1, Sample 2, Sample 3, and
Sample 6) do not show much variation in the intensity of the carbonyl peak. This
suggests that when more maleic anhydride is present, then more reacts during
the extrusion process, as one would expect. This is evidence of maleation and

45

that it depends on the amount of maleic anhydride added during the production

 

 

 

of the MTPS samples.
0.35

S

,—_, 0.30

E

g 0.25

2

o 0.20

o

S? 0.15

o

I: 0.10

% 0.05

a:

0.00
§ _ .: g :E _ .: E ‘:g C}; _
;g 3% 4% §§ Ea 4%
33 $5 33 $6 $5 3%
oo m$ 00 m$ mfi 90
o\ o N. O o\ o N O 0‘! O o\ a
:2 M «as ow o~ veg
°_N «rig °_N a)“; w”<' Sim
mg a m< l a: w<
E2 5* &2 Es 5* E2
E N 2 <1- <0 5
Sample

 

 

Figure 14. FT IR relative intensities of the carbonyl peak for the MTPS samples.

3.3.3 Thermal Analysis (DSC)

In the DSC analysis, the first heating showed endotherrn transitions while the
second heating did not. This is attributed to there being some crystallinity left in
the starch structures after extrusion or from being aged for some time after being
extruded and before running the DSC tests. This allows sufficient time for the
materials to reform into crystal structures. However, between the first and second

heatings, there is not enough time for re-crystallization to occur, and there is thus

46

no crystallization exotherm or subsequent melting endotherrn during the second

heating. Thus, all DSC observations discussed here refer to the first heating.

Figure 15 shows the therrnograms of the MTPS and TPS samples. One can
observe a broad melting endotherrn in the TPS sample, and two melting
endotherrns in the MTPS sample. This has been attributed to the fact that the
maleation of starch creates segments that have slightly different melting
characteristics.35 The first endotherrn is broad and is due to the gelatinization of
the starch, glycerol, and MA. The second endotherrn is much sharper and is due

to the fact that grafting has occurred.

Considering the effect of the amount of initiator on the DSC results, we observe
that for Sample 1 (0.1% Luperox 101), the sharp endotherrn is small
(AH=0.08397 J/g) and is located at 168.07°C. As the amount of Luperox 101
increases to 0.25% (Sample 2), the sharp endotherrn is larger (AH=2.062 J/g)
and is located at 157.30°C. Finally, when the amount of Luperox 101 is 0.5%
(Sample 3), the sharp endotherrn is larger still (AH=72.52 J/g) and is located at
145.22°C. This suggests that grafting of maleic anhydride to the starch backbone
and disruption of the crystalline starch structure has occurred during the reactive
extrusion of the MTPS. This is consistent with our FTIR and soxhlet extraction

observations.

47

Considering the effect of maleic anhydride on the melting endotherms of the
samples, one can see the same type of trend. For Sample 2 (2% MA), the sharp
endotherrn (AH=2.062 J/g) is located at 157.30°C. As the amount of MA
increases to 4% (Sample 4), the sharp endothenn is larger (AH=36.30 J/g) and is
located at 141 .17°C. Finally, when the amount of MA is 6% (Sample 5), the sharp
endotherrn is larger still (AH=83.25 Jlg) and is located at 132.60°C. This also
suggests that grafting of maleic anhydride to the starch backbone and disruption
of the crystalline starch structure has occurred during the reactive extrusion of
the MTPS. Again, this is consistent with our FTIR and soxhlet extraction

observations.

Figure 15 shows the samples that were extruded at a maximum temperature of
135°C. Comparing the two therrnograms, one can see that the sharp melting
endotherrn for the MTPS sample is at 152.69°C, while the melting endotherrn for

the TPS sample is at 112.15°C.

All of the MTPS samples exhibit melting endotherms that are at a higher
temperature than that of the TPS sample. The fact that the maleated samples
have a higher melting temperature than the TPS sample is one characteristic that
will allow it to be blended with polymers, such as PLA, that have a higher melting

temperature.

48

Another endotherm transition was observed for all of the MTPS and TPS
samples close to 0°C. The transition occurred at positive values (from 0.27 to
5.43°C) in the first heating and at negative values (from -4.56 to -0.28°C) in the
second heating. This transition resembles a first order transition (small

endotherm peak) and is most likely due to fusion of water that is present in the

 

 

 

   

 

 

 

 

samples.
-0.1
o o TPS135°C 30%G
2.43 C o
o 2' 2,59 °C (1) U MTPS135 CO.1%L101 2%MA 30%G
' :7 ‘ 429°C
03 “HEW
a ; 44.4.4“. \w
\ ‘ ,1?
5; -0.4_ \‘Wfl/EJ/ j
2 : 4445 °C 0 =
LL 75.22‘1/9 112.15 C f/
g -0.5- J
I I
'0'6‘. 147.78 °c V
84.52 J/g o
- 152.69 C
_0_7...,...,...,..-....
-50 0 50 100 150 200
Exo Up Temperature (°C)

 

Figure 15. DSC therrnogram of the MTPS (at 135°C and 30% glycerol) and TPS
(at 135°C and 30% glycerol) samples.

49

 

3.3.4 Soxhlet Extraction Analysis

The results of the soxhlet extraction in acetone of the MTPS and TPS samples
are shown in Table 3. The calculated percentage of glycerol grafted to the starch
is tabulated for each sample. The values are an average of four specimens that

were tested for each sample.

Based on the soxhlet data, some of the maleic anhydride is hydrolyzed to maleic
acid, due to the equilibrium moisture content of the starch (approximately 12%),
facilitating the reaction of glycerol with the terminal C1 functionality of the starch
backbone, chemically linking the starch and glycerol, as was shown in Figure 10,

in Section 3.2.3.1.

For the maleated thermoplastic starch extruded at 165°C, after soxhlet extraction,
28.0 to 44.1% of the original glycerol in the samples remained. These results
demonstrate that a certain amount of glycerol could not be removed after 3 days
due to the fact that the glycerol has been linked to the starch backbone. This
indicates that reactive extrusion has occurred. At this temperature, it was
observed that the Luperox 101 content has more of an effect on the amount of
glycerol extracted. When the Luperox 101 content changed from 0.1 to 0.5%, the
amount of glycerol linked to the starch changes from 38.4 to 28.0%. Comparing
samples 2 and 3, when the Luperox 101 content increased to 0.5%, less glycerol

was linked to the starch. This could be caused by the reduction in the relative

50

molecular weight of the starch which makes some fraction of the maleated starch

to be soluble in acetone.

The effect of the percentage of maleic anhydride on the soxhlet extraction is not
evident. When the MA content changed from 2 to 6%, the amount of glycerol

grafted to the starch was not significantly different.

For the maleated thermoplastic starch extruded at 135°C, after soxhlet extraction,
only 9.2% of the original glycerol in the sample remained, while the TPS at the
same temperature profile extracted almost all of the original glycerol in the
sample, leaving only 1.3%. This is an indication that in maleated samples
chemical interactions occur between starch and glycerol that hinder the

extraction of the glycerol; however, these interactions are not present in TPS.

Comparing the MTPS samples with a maximum extrusion temperature of 165°C
and 135°C, the samples extruded at the higher temperature have more glycerol
linked to the starch (from 28.0 to 44.1%) than samples extruded at 135°C (9.2%).
These results indicate that interactions between glycerol and starch, that are
promoted by the reactive extrusion using MA, are more efficient at the higher
temperature profile. Furthermore, the extrusion temperature profile also affected
the color of the MTPS samples. Samples extruded at 165°C had a green color
while the MTPS sample extruded at 135°C had a yellow color, which was more

similar to the original color of the TPS sample.

51

Table 3. Percentage of glycerol grafted to the starch after soxhlet extraction of
maleated thermoplastic starch in acetone for 72 hours.

 

Percentage of

 

 

 

 

 

 

 

 

 

 

Sample Composition Glycerol Grafted

to Starch (%)

1 MTPS, 165°C, 0.1% L101, 2% MA, 20% glycerol 38421.42“d

2 MTPS, 165°C, 0.25% L101, 2% MA, 20% glycerol 42.4:t6.49d

3 MTPS, 165°C, 0.50% L101, 2% MA, 20% glycerol 28018.320

4 MTPS, 165°C, 0.25% L101, 4% MA, 20% glycerol 44.111804“d

5 MTPS, 165°C, 0.25% L101, 6% MA, 20% glycerol 382113.00“

6 MTPS, 135°C, 0.1% L101, 2% MA, 30% glycerol 9.215.66b

7 TPS, 135°C, 30% glycerol 1.3:I:2.99a

 

Means in same column with different letters are significantly different (p50.05).

3.3.5 Environmental Scanning Electron Microscopy (ESEM) Analysis

Using the environmental scanning electron microscope (ESEM), one can

understand the topography of the samples. Areas that are higher appear brighter

in the image, while areas that are lower appear darker in the image. In this way,

variations in light and dark areas give an indication of the roughness of the

surface.

52

 

Images of the samples taken with the environmental scanning electron
microscope (ESEM) are shown in Figure 16. The samples that were processed
at 165°C (Figure 16 a,b,c and d) appear to be smoother than the MTPS and TPS
samples that were processed at 135°C (Figure 16 e and f). This could imply that

there was better plasticization of the starch at higher temperatures.

Also, since the DLS data indicates that the molecular weight of the starch is
higher when the starch is extruded at 135°C, the higher molecular weight could
make it more difficult for the starch to fully plasticize and regions of unplasticized
starch may be present in the MTPS or TPS, thus, the TPS sample appears to

have the roughest surface.

When comparing the samples extruded at 165°C, the effect of MA content is not
evident. The samples appear to have comparable regions of light and dark,
signifying that they have similar roughness. However, the effect of the content of
free-radical initiator content is evident. The sample with 0.5% Luperox (Figure 16
c) presented a more homogeneous and a smoother surface compared to the
samples with 0.1 and 0.25% Luperox (Figure 16 a and b), due to the smaller
hydrodynamic radius of the sample which facilitates the plasticization of the
starch. These observations are in agreement with the DLS data. Observations of
the material as it was extruded showed that MTPS flowed better than TPS. This
could also be an indication that there was less plasticization occurring during the

extrusion of TPS compared with MTPS.

53

 

 

 

 

 

e) ' <
Figure 16. Environmental scanning electron microscope images of extrudate
samples. Images shown at 750x magnification (50 pm scale bar). a) MTPS, at
165°C, 0.1% L101, 2% MA, 20% glycerol, b) MTPS, at 165°C, 0.25% L101, 2%
MA, 20% glycerol, c) MTPS, at 165°C, 0.50% L101, 2% MA, 20% glycerol, d)
MTPS, at 165°C, 0.25% L101, 6% MA, 20% glycerol, e) MTPS, at 135°C, 0.1%
L101, 2% MA, 30% glycerol and f) TPS, at 135°C, 70% starch and 30% glycerol.

 

54

 

3.4 CONCLUSIONS

It was observed that MTPS had improved processability over TPS. Also, the
maleation process permitted formulations with less than 30 wt% of glycerol to be
extruded in the twin—screw extruder. The resulting MTPS pellets were more
transparent than the TPS pellets. As the percentage of maleic anhydride
increased, there was negligible change in the hydrodynamic radii. However, as
the percentage of Luperox 101 increased, the hydrodynamic radii decreased. It
could be inferred that the molecular weight decreased as the percentage of free-
radical initiator increased. Using the maximum temperature during the extrusion
process of 165°C instead of 135°C caused a drastic decrease in the
hydrodynamic radius, due to the high influence of the temperature profile on the
molecular weight of the thermoplastic starch. ESEM images of the samples are in
agreement with the DLS data. FTIR of TPS and MTPS indicates that the maleic
anhydride has reacted with the thermoplastic starch during the extrusion process.
The MTPS samples presented higher melting temperatures compared to the TPS
sample. The soxhlet studies indicated that using the maximum temperature of
165°C in the extrusion temperature profile resulted in more grafting between

glycerol and starch than when the maximum temperature used was 135°C.

55

CHAPTER 4

MTPS-PBAT BLENDS BY REACTIVE EXTRUSION PROCESSING
AND THEIR APPLICATION AND PERFORMANCE FOR

THERMOFORMED ARTICLES

 

4.1 RATIONALE

This work was performed to better understand the effect of amylase content and
percentage of MTPS in MTPS-PBAT blends used to produce therrnoforrnable
sheets. The materials were characterized by mechanical properties (tensile and
puncture tests), thermal analysis (DSC), Fourier transform infrared spectroscopy
(FTIR), and microstructural analysis (ESEM). This is, to the best of the author’s

knowledge, the first report of a truly biodegradable thermoformed article.

56

4.2 MATERIALS AND METHODS

4.2.1 Materials

High amylose corn starch was purchased from National Starch and Chemicals
(Indianapolis, IN). Regular corn starch was obtained from Cargill Inc.
(Minneapolis, MN). Poly(butylene adipate-co-terephthalate) (PBAT) was
purchased from BASF Chemicals (Ludwigshafen, Germany), under the trade
name Ecoflex. Anhydrous glycerol, 99.9% assay, was purchased from J.T. Baker
(Phillipsburg, NJ). Maleic anhydride was purchased from Sigma-Aldrich, Inc (St.
Louis, MO). The initiator, 2,5-bis(tert-butylperoxy)-2,5-dimethylhexane, also
referred to as Luperox 101, was purchased from Sigma-Aldrich, Inc (St. Louis,

MO).

4.2.2 Equipment

A Century CX-30 co-rotating twin-screw extruder, having a length/diameter ratio
(LID) of 42 and a screw diameter of 30 mm was used to prepare the
thermoplastic starch and maleated thermoplastic starch, and to blend these
materials with Ecoflex. The screw configuration for the extruder is shown in
Figure 7, in Section 3.2.2. A pelletizer was used to cut the extruded strands into

small pellets.

57

The blended material was then extruded through a Killion single-screw extruder,
having a length/diameter ratio of 24 and a screw diameter of 1 inch. The
extrudate exited the extruder in the form of a sheet of material and was collected

on a roller.

Therrnoforming of the sheets of blended material was done using a Labforrn

therrnofonner by Hydro-Trim Corporation.

4.2.3 Procedure

4.2.3.1 Production of Thermoplastic Starch (T PS) and Maleated
Thermoplastic Starch (MTPS)

Thermoplastic starch (T PS) and maleated thermoplastic starch (MTPS) were
produced in a Century CX-30 co-rotating twin-screw extruder as described in

Section 3.2.3.1.

The samples either contained regular corn starch or high amylose corn starch.

With a temperature profile of 25/1 15/120/125/130/135/135/135/130/130°C, the

composition of the samples was 70 wt% starch and 30 wt% glycerol. With a

temperature profile of 25/95/125/145/1 60/165/165/1 65/1 50l145°C, the

composition of the samples was 80 wt% starch and 20 wt% glycerol.

58

The amount of MA and Luperox 101 in the MTPS samples was 2 wt% and 0.1

wt%, respectively, of starch and glycerol.

The extruded strand was air cooled and pelletized in line. The pellets were dried

for two days in an oven set at 65°C before being blended with Ecoflex.

The temperature profile used with the maximum temperature of 165°C was
defined in preliminary tests as the best temperature for production of MTPS of
high amylose corn starch with 20% glycerol. It is not possible to make TPS with
20% glycerol in the twin-screw extruder, due to its high viscosity. For comparison
purposes, TPS and MTPS were also produced under the same conditions; with a
temperature profile of 25/95/1 15/125/130/135/135/135/130/130°C and 30%
glycerol. With a maximum extrusion temperature of 135°C, it is possible to
produce both TPS and MTPS with 30 wt% glycerol. These samples were made
in order to compare blends of maleated and non-maleated thermoplastic starch

of high amylose corn starch and regular corn starch.

4.2.3.2 Blend Production of TPS and MTPS with PBAT

Blends of thermoplastic starch/PBAT or maleated thermoplastic starch/PBAT
were produced in a Century CX-30 co-rotating twin-screw extruder. Pellets of
TPS or MTPS were pre-mixed with PBAT in various weight of TPS (or MTPS) to
weight of PBAT ratios (60:40 and 50:50) and placed in the external feeder. The

feeder fed the pellets into the feed port of the extruder. Table 4 shows the

59

composition of each sample. The terms “HA” and “corn” in the sample names
refer to high amylose corn starch and regular corn starch, respectively. The
temperature profile of the extruder was set at
25/125/135/140/145/150/150/150/145/135°C from the feed throat to the die. The
screw speed was set to 100 rpm. The vent port was kept open to remove any
moisture. The extruded strand was air cooled and pelletized in line. The pellets

were dried for one day in an oven set at 65°C before being extruded into sheets.

In previous work done by others,16 the reaction between MTPS and PBAT shown

in Figure 17 has been proposed.

60

Table 4. Composition of the blends of TPS and MTPS with PBAT.

 

Maximum

Blend Composition

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Samples Glycerol Luperox

Tempe'atu'e TPS/MTPS PBAT

(°C) (%) (%) (%) (%)

MTPS HA 165 60 165 20 0.1 60 40

MTPS HA 165 50 165 20 0.1 50 50

MTPS com 165 50 165 20 0.1 50 50

MTPS HA 135 50 135 30 0.1 50 50

MTPS com 135 50 135 30 0.1 50 50

TPS HA 135 50 135 30 - 50 50

TPS com 135 50 135 30 - 50 50

PBAT - - - - 100

 

61

 

 

 

O O
MTPS H H
\ CH20 ----C ——-C :0 —C r---OH
o l I
H H PBAT
0H
.___.._0 0_ + WV‘LOH
H.
OH
'1 TI
CH20 -—-C --—C __.__—"'"C --—C —vax
__o I I
H H + ”20
OH
_..0 0—
0H

 

 

 

Figure 17. Proposed reaction scheme for MTPS and PBAT. [Adapted from
Balakn’shnanw]

4.2.3.3 Sheet Production

Extruded sheets of the blended material were produced in a Killion single-screw
extruder. The pellets were gravity-fed into the extruder at the feed throat. The
temperature profile of the extruder was set to 160/160/160/160/154°C from the
feed throat to the die. Cooling water was circulated through the extruder to
maintain the set temperatures. The screw speed was set to 65 rpm. The

extruded sheet was cooled on a chill roller set at 21°C and then collected on an

62

auxiliary roller. The speed of the chill roller was adjusted so as to produce a

sheet with a thickness of 500um.

4.2.3.4 Thermoforming Process

Pieces of the sheets were cut with scissors and clamped in place in the Labforrn
therrnoforrner. The upper and lower heaters of the thermoformer were both set to
154°C. The sheets were heated between the upper and lower heaters for varying
lengths of time (from 24 to 40 seconds), depending on the composition of the
material. The length of time was determined so as to ensure that the material
was adequately heated, but did not sag too much. The material was then placed
over the forming die and a vacuum of approximately 25 mmHg was pulled and
held for 20 seconds. During this process, the sheet of material was drawn by the
vacuum into the forming die, where it cooled and was formed into the desired
shape. The thermoformed object could then be removed from the thermoforrner
and the excess material around the object was trimmed away with scissors. The

shape of the object that was formed is shown in Figure 18, below.

 

Figure 18. Picture of the object formed in the Labforrn therrnoformer.

63

4.2.3.5 Fourier Transform Infrared Spectroscopy (FT IR) Analysis

The FTIR spectra of TPS, MTPS, starch, and blends of TPS or MTPS/PBAT
pellets were scanned from 650 cm'1 to 4000 cm'1 using a Perkin Elmer Spectrum

One FT-IR Spectrometer.

Soxhlet extraction in dichloromethane was performed on pellets of the “MTPS HA
165 60” sample in order to prove that grafting between the MTPS and the PBAT
had occurred. PBAT is soluble in dichloromethane but MTPS is not, therefore,
the soxhlet extraction removes the PBAT, glycerol and starch that are grafted on
the PBAT backbone, leaving in the extraction thimble the MTPS that is not
grafted to the PBAT. FTIR spectra of the pellets before soxhlet extraction and
after soxhlet extraction were obtained. Also, the solvent was collected after the

extraction and evaporated, leaving a film whose FT IR spectrum was recorded.

4.2.3.6 Tensile Tests

Tensile specimens (100mm x 25mm) of the blends of TPS and MTPS/PBAT
blends were cut from the extruded sheets of material and were conditioned as
recommended in standard method ASTM D4332," in a constant environment
room at 23 i 1°C and 50 i 2% RH for at least 40 hours before testing. The
tensile tests were measured in accordance with standard method ASTM D882.78
Specimens of each formulation were clamped between the grips of a United

Testing Systems (Huntington Beach, CA) model SFM-20 tensile test machine.

64

Force-extension curves were recorded. Tensile strength and elongation were

calculated for each of the samples. Seven specimens were tested for each blend.

4.2.3.7 Puncture Tests

Puncture tests were performed on the thermoformed specimens to determine
puncture strength (N) and deformation (mm) using a TA.TX2i Stable Micro
Systems texture analyzer (Surrey, England). Specimens measuring 40mm in
diameter were affixed with tape (3M Scotch, Brazil) onto the plate of the
equipment with a hole of 20mm diameter. A cylindrical probe of 5mm diameter
was moved perpendicularly to the sheet surface at a constant speed of 1 mm/s
until the probe passed through the film. Force-deforrnation curves were recorded
and the force and deformation at the point of rupture were determined. Each

data point represents an average of four specimens.

4.2.3.8 Thermal Analysis (DSC)

Differential scanning calorimetry (DSC) was performed using a high resolution
DSC 2920 from TA Instruments. The purge gas, nitrogen, had a flow rate of 50
mllmin. Pellets of the extruded samples were cooled to -50°C and then heated to
200°C at a rate of 10°Clmin, cooled to -50°C and then reheated to 200°C. The
thermal transition temperatures of the blends and of PBAT were determined from

analysis of the therrnograms.

65

4.2.3.8 Environmental Scanning Electron Microscopy (ESEM) Analysis

A Phillips Electroscan 2020 environmental scanning electron microscope (ESEM)
was used to make observations of the topography of the surface of the sheets of

the blends.

4.2.3.10 Statistical Analysis

The PBAT sample was used as a control and was tested to evaluate the effect of
blends of TPS and MTPS with PBAT on the mechanical and thermal properties
of the sheets and thermoformed pieces. Analysis of variance (ANOVA) and
Tukey mean comparison tests (p s 0.05) were performed using Statistica

software (Statsoft, Oklahoma — USA).

4.3 RESULTS AND DISCUSSION

4.3.1 Fourier Transform Infrared Spectroscopy Analysis (FTIR)

In Figure 19, the FT IR spectra for high amylose (HA) starch, PBAT, and a blend
of MTPS-PBAT are shown. The PBAT and the blend show a carbonyl stretch
peak at 1710 cm'1 and an ester C-O stretch peak at 1265 cm". These peaks are
not observed in the HA starch sample. The HA starch and the blend samples
exhibit a peak between 3200-3400 cm". This corresponds to —OH stretch from
the starch and glycerol in the samples. These peaks are what one would expect

to observe from the FT IR spectra.

66

 

 

 

 

 

3 v
Absorbance

 

 

~-—- -—-—-q.-
d- p-‘-—- -—

1
l

 

#9-”-.— -———-l-————__
’

I
I
J 1
o"..."....' ' Ivy.
J'e Q“.. I
T

\
._"—---—— I-———-—-l-_———_—

dfl"
‘
a ‘
b i‘
, D
c
-
f
-

--e......‘_ s. ’

u——“—‘—.:.—.o.a::JUWI-l‘fluflcon...‘.w .' ' -
I

3650 3150 2650 21 50 1650 1 1 50 650
Wavelength (cm'1)

‘ .. o
o“§~ _-0" ‘0 ,~

 

 

    

 

I ------- th Anybse Com Starch — ---FBAT MTPS 165C0.1L10120% Glycerol - PBAT(50:50fl

 

 

 

 

 

Figure 19. FT IR Spectra for HA Starch, PBAT, and MTPS 165°C 2%MA
0.1%L101 20%G-PBAT(50:50).

Figure 20 shows the FTIR spectra for as—extruded pellets of the sample “MTPS
165 60”, the pellets after soxhlet extraction in dichloromethane, and the film
produced when the solvent was evaporated. The blend and the solvent film show
a carbonyl stretch peak at 1710 cm". This is similar to what is observed for pure
PBAT. Also, the ester C-O stretch at 1265 cm'1 is exhibited in these two samples.

Again, these peaks are what one would expect to observe from the FT IR spectra.

Evidence of grafting between MTPS and PBAT is shown in Figure 20, by the fact

that the solvent film after soxhlet extraction exhibits a peak between 3200-3400

67

cm". This corresponds to —OH stretch from starch and glycerol and is an

indication that the starch, glycerol, or both were extracted in dichloromethane

with the PBAT.

 

Absorbance

's‘ .
‘43,;

“I

 

3650 3150 2650 2150 1650 1150 650
Wavelength (cm'1)

 

"""" MTPS 1656 0.1L101 20% nycerol - PBAT(60:40) (soNerltflrnafter soxhlet)

- — - - MTPS165CO.1L10120% Gycerol - FBAT(60:40) (poles after soxhlet)

— MT'PS16560.1L101 20% Gycerol - PBAT(60:40) (as extruded)

Figure 20. FTIR Spectra for film made after soxhlet of MTPS 165°C 0.1%L101
2%MA 20%G-PBAT(60:40), pellets left in thimble after soxhlet of MTPS 165°C
0.1%L101 2%MA 20%G-PBAT (60:40), and as extruded pellets of MTPS 165°C
0.1%L101 2%MA 20%G-PBAT (60:40).

 

 

 

68

4.3.2 Tensile Tests

The mechanical properties of the samples provide performance metrics for these
materials. They suggest the effect of how: a) the temperature profile used in the
extrusion of MTPS, b) the ratio of MTPS/PBAT of the blends, and c) the starch
source impact the performance of the materials. Table 5 presents the tensile

measurements of the sheets of TPS and MTPS/PBAT blends.

In examining the tensile strength of MTPS/PBAT sheets at 165°C and 20%
glycerol, one observes that increasing the MTPS content to 60% caused a
decrease in the tensile strength from 9.0 to 6.1MPa. Comparing the blends
containing MTPS samples of high amylose and regular corn starch, observations
of the effect of composition of starch on the tensile properties can be made. The
blend containing MTPS of high amylose corn starch presented a higher tensile
strength (9.0 MPa) than the blend containing MTPS of regular corn starch (6.2
MPa). This could be attributed to the linear structure of amylose chains.
According to Rindlav—Westling et al.,79 the amylose network structure is very
stable, having strong molecular orientation and forms films that are more dense
than amylopectin films. It has been observed that higher levels of amylose in
starches give stronger films, while amylopectin films generally have lower tensile

strength, due to its branched structure.“ 56' 3°

69

Table 5. Thickness and mechanical properties (determined by tensile tests) of
sheets of blends of TPS and MTPS with PBAT.

 

Tensfle
Samples Thickness Elongation
Strength
(um) (%)
(MPa)

 

MTPS HA 165 60 518.9:l:21.64 a 6.1:l:0.31a 697.0:l:50.60a

 

MTPS HA165 50 515816.153 9,010.82b 881.01139.13°'°'°

 

MTPS com165 50 498612750“ 6.210.933 6732113184“

 

MTPS HA 13550 508412456" 8.411.46" 946.8:151.35°'°'°

 

MTPS com 13550 506519.823 9.21:0.62" 9386:5227"

 

TPS HA 135 50 524712026 3 861057 b 852.5:3327 c

 

TPS com135 50 508.71: 9.81a 8010.99" 8072111120“

 

PBAT 510222450a 15912.90c 824.1:153.45°'°'°'°

 

 

 

 

 

 

Means in same column with different letters are significantly different (p50.05).

There was not a statistical difference (p50.05) between the values of tensile
strength for the samples of sheets of TPS and MTPS/PBAT at 135°C and 30%
glycerol. This is most likely due to the effect of glycerol. For blends of MTPS/T PS
with 30% glycerol, the amount of glycerol affects the tensile properties more than
the amylase/amylopectin ratio or the type of thermoplastic starch used (maleated
and non-maleated).

70

The values of the tensile strength for the blends are higher than the values
reported for blends of cassava TPS with the biodegradable polymer, PBSA
(polybutylene succinate adipate). Values of 5.2 MPa for TPS/PBSA (30:70) and
7.0 MPa for TPS/PBSA (70:30) have been reported.81 However, the values of the
tensile strength for the blends are lower than the values reported for synthetic
films such as LDPE (low density polyethylene) and HDPE (high density

polyethylene), of 16 and 30 MPa, respectively.°2' 83

There is not a statistical difference (p50.05) in the values for elongation, when
comparing all samples of sheets of TPS/PBAT and MTPS/PBAT with sheets of
PBAT.

The elongation of test specimens depends on the interactions between the
components of the blend and the elongation of the polymers used. One of the
characteristics of PBAT is its high elongation ( ~800%). In this work, the addition
of 50% of TPS or MTPS and 60% of MTPS did not change the elongation from
that of the pure PBAT. The values presented here are higher than those of PLA
blends with TPS that showed elongation at break between 5 to 20%,"’8 and
LDPE (low density polyethylene) and HDPE (high density polyethylene), which

present strain at break of 570 and 180%, respectively.82' 83

71

According to Myllarinen et al.,29

glycerol and starch interact strongly, and when
the glycerol content is low (< 20%), this interaction results in more brittle and
stronger networks. In the current work, the samples of 20% glycerol did not
show improvement in tensile strength, most likely due to the effect of the
extrusion temperature. The samples of 20 and 30% of glycerol were extruded
with different temperature profiles, with maximum temperatures of 165°C and
135°C, respectively. The lower glycerol content positively affected the tensile
strength while the higher temperature profile used negatively affected the tensile

strength, due to higher degradation of the maleated samples at the higher

temperatures.

Even though the tensile properties of TPS and MTPS blends presented similar
values, the visual characteristics of the samples were very different. The MTPS
blends have a more homogeneous surface and formed more transparent sheets
than the TPS sheets. Also, high amylose corn starch MTPS has improved
processability compared to the processability of regular corn starch MTPS. High
amylose corn starch MTPS containing 20 or 30% glycerol can be pelletized in
line; however, regular corn starch MTPS containing 20 or 30% glycerol cannot be
pelletized in line. In this case, the materials were collected and ground into a

powder before being blended with PBAT.

72

4.3.3 Puncture Tests

In packaging applications, the resistance of the package to puncture is an
important consideration if it contains products with sharp extremities or products
that could be damaged by being punctured during storage or distribution. The
shape and size of the thermoformed speciments produced in this work did not
allow for tensile tests to be performed on the thermoformed specimens; however,
puncture tests were performed on the thermoformed specimens. Table 6 shows
the thickness (um), puncture strength (N), and puncture deformation (mm) for the

samples.

The sample “MTPS com 165 50” produced a thermoformed object that was
irregular in shape and thickness, so puncture tests were not performed on
specimens of that particular blend. Also, due to the extreme flexibility of the
thermoformed PBAT specimens, puncture tests were not performed on the PBAT

specimens either.

73

Table 6. Mechanical properties determined by puncture tests of the
thermoformed pieces of the blends of TPS and MTPS with PBAT.

 

Thermoforrned Pieces

 

 

 

Samples
Puncture
Thickness Puncture
Defamation
(pm) Strength (N)
(mm)
MTPS HA 165 60 28319.6a 26010.80a 10.310441I
MTPS HA 165 50 323115b 47.712.29b 15,910.51b

 

MTPS com 165 50 - - -

 

MTPS HA 13550 333170“c 40,416.91b 19.512449d

 

 

 

 

MTPS com 135 50 380178” 27.715563 20.010.72c
TPS HA 135 50 333116b 52.312.35° 16.611 .37b
TPS com 135 50 363123° 48.412.41b 18.610.84d

PBAT - - ‘ -

 

 

 

 

 

 

Means in same column with different letters are significantly different (p50.05).

It should be noted that the thickness of the thermoformed pieces varied by more

than 5%. This could interfere with the mechanical properties of the specimens.

The puncture test results show similar behavior to the tensile test results. When

one compares the samples extruded at 165°C, it is observed that the samples

74

containing 60% MTPS have lower puncture strength and deformation than

samples containing 50% MTPS.

For the samples extruded at 135°C, the high amylose corn starch MTPS blend
presented higher puncture strength and lower puncture deformation than the
blend containing regular corn starch MTPS. This is also observed for the blend
containing high amylose TPS, compared to the blend containing regular corn

starch TPS.

4.3.4 Thermal Analysis (DSC)

The thermal transition temperatures of the MTPS/PBAT and TPS/PBAT samples
are tabulated and shown in Table 7. One can observe a melting temperature for
PBAT of 113.6°C. The MTPS and TPS blends all presented melting temperatures
that are slightly higher than that of PBAT; varying from 115.7 to 123.3°C,
measured at the second heating. The crystallization temperatures were similar
for all the MTPS/T PS blends (from 72.4 to 795°C), while the PBAT presented a

crystallization temperature of 330°C.

The fact that the crystallization temperature of the blends is higher than for neat
PBAT is a property that will allow the blends to crystallize at a higher
temperature, after being extruded and formed. This could be advantageous in a

production environment.

75

Another endotherm transition was observed for all of the samples close to 0°C.
The transition occurred at positive values (from 0.3 to 51°C) in the first heating
and at negative values (from -6.3 to -0.7°C) in the second heating. This
transition is a first order transition (small endotherm peak) and is most likely due

to fusion of water that is present in the samples.

Table 7. DSC parameters: glass transition temperature (T g), crystallization
temperature (T c), crystallization enthalpy (AH), and melting temperature (T m) of
the blends of MTPS and TPS with PBAT.

 

 

 

 

 

 

 

 

 

 

 

First
Sample Cooling Second Heating
Heating

T9 (°C) Tc (°C) AHc (J19) T9 (°C) Tm (°C)
MTPS HA 165 60 3.2 78.9 6.2 —0.7 123.1
MTPS HA 165 50 5.1 79.5 7.7 -1.0 123.3
MTPS com 165 50 0.9 72.4 8.4 -6.3 115.7
MTPS HA 135 50 1.2 75.8 6.9 -2.8 117.8
MTPS com 135 50 3.0 74.5 9.3 -4.9 116.8
TPS HA 135 50 2.3 79.2 7.8 -2.8 121.5
TPS com 135 50 0.3 72.6 8.5 -4.7 121.5
PBAT 2.3 33.0 20.1 -2.4 113.6

 

 

 

 

 

 

76

 

4.3.5 Environmental Scanning Electron Microscopy (ESEM) Analysis

Environmental scanning electron microscopy (ESEM) was used to observe the
surface of the sheets of the blends. Figure 21 shows the samples containing
MTPS that were extruded with a maximum temperature of 165°C and Figure 22
shows the samples containing MTPS and TPS that were extruded with a
maximum temperature of 135°C. In general, the blends containing maleated
thermoplastic starch extruded with a maximum temperature of 165°C are more
homogeneous and smoother than the blends containing MTPS and TPS
extruded with a maximum temperature of 135°C. This could imply that there was
better plasticization of the starch with higher temperatures. According to Mali et
al.,84 a homogeneous matrix of films is a good indicator of their structural integrity
and consequently good mechanical properties would be expected. However, the
samples containing MTPS extruded with a maximum temperature of 165°C did
not exhibit better mechanical properties. This is most likely due to the
therrnoplasticization process. Although there was better plasticization of the
MTPS, there was also more degradation of the starch during the reactive

extrusion to produce the MTPS at the higher temperature profile.

77

 

 

 

 

(C)
Figure 21. ESEM images of the samples extruded at 165°C and 20% glycerol at
750x magnification (50 pm scale bar): (a) MTPS high amylose-PBAT (60:40), (b)
MTPS high amylose-PBAT (50:50) and (c) MTPS corn starch-PBAT (50:50).

78

 

 

 

 

Figure 22. ESEM images of the samples extruded at 135°C and 30%glycerol at
750x magnification (50 pm scale bar): (a) TPS high amylose-PBAT (50:50), (b)
MTPS high amylose-PBAT (50:50), (0) TPS corn starch-PBAT (50:50) and (d)
MTPS corn starch-PBAT (50:50).

79

 

4.4 CONCLUSIONS

The processability of high amylose corn starch MTPS is much better than the
processability of regular corn starch MTPS. Blends of plasticized starch, with or
without maleation, with PBAT can form sheets which can be thermoformed. To
the best of the author’s knowledge, this is the first report of a 100%
biodegradable thermoformed article. FTIR results show that starch, glycerol, or
both have been grafted to the PBAT. The tensile properties of the blends of
PBAT with TPS and MTPS extruded with a maximum temperature of 135°C are
not statistically different. However, the maleated thermoplastic samples formed
sheets and thermoformed pieces that were more transparent and had a more
homogeneous surface. The effect of the amylose content of the starches used
did not present significant differences in the tensile properties when the glycerol
content was 30% and the ratio of thermoplastic starcthBAT used was 50:50.
However, when the MTPS contained 20% glycerol, the ratio of MTPS:PBAT was
50:50, and the maximum processing temperature during the extrusion of the
MTPS was 165°C, the blend containing high amylose corn starch MTPS showed
better tensile properties than the blend containing regular corn starch MTPS.
Increasing the MTPS (high amylose corn starch) content to 60% decreased the
tensile strength and elongation of the sheets. The addition of TPS or MTPS
decreased the tensile strength, but did not cause a significant difference in the
elongation, compared to the elongation of PBAT. The puncture test results show

similar behavior to the tensile test results. DSC analysis shows that the blends all

80

have higher melting and crystallization temperatures than neat PBAT. Finally,
ESEM shows that the blends containing MTPS extruded with a maximum
temperature of 165°C are smoother and more homogeneous than the blends

containing MTPS and TPS extruded with a maximum temperature of 135°C.

81

CHAPTER 5

APPLICATION AND PERFORMANCE OF MTPS-PBAT BLENDS

FOR FILMS

 

5.1 RATIONALE

This work was performed to better understand the effect of the percentage of
MTPS in MTPS-PBAT blends used to produce extruded films. The materials
were characterized by mechanical properties (tensile and puncture tests), barrier
properties (carbon dioxide, oxygen, and water vapor permeability), and

microstructural analysis (transmission electron microscopy).

82

5.2 MATERIALS AND METHODS

5.2.1 Materials

High amylose corn starch was purchased from National Starch and Chemicals
(Indianapolis, IN). Poly(butylene adipate-co-terephthalate) (PBAT) was
purchased from BASF Chemicals (Ludwigshafen, Germany), under the trade
name Ecoflex. Anhydrous glycerol, 99.9% assay, was purchased from J.T. Baker
(Phillipsburg, NJ). Maleic anhydride was purchased from Sigma-Aldrich, Inc (St.
Louis, MO). The initiator, 2,5-bis(tert-butylperoxy)—2,5-dimethylhexane, also
referred to as Luperox 101, was purchased from Sigma-Aldrich, Inc (St. Louis,

MO).

5.2.2 Equipment

A Century CX-30 co-rotating twin-screw extruder, having a length/diameter ratio
(L/D) of 42 and a screw diameter of 30 mm was used to prepare the maleated
thermoplastic starch (MTPS) and to blend the MTPS with PBAT. Figure 7, in
Section 3.2.2 shows the screw configuration for the twin-screw extruder. A

pelletizer was used to cut the extrudate strands into small pellets.

The blended material was then extruded through a Killion single-screw extruder,

having a length/diameter ratio of 24 and a screw diameter of 1 inch. The

83

extrudate exited the extruder in the form of a film of material and was collected

on a roller.

5.2.3 Procedure
5.2.3.1 Production of Maleated Thermoplastic Starch (MTPS)

Maleated thermoplastic starch (MTPS) was produced in a Century CX—30 co-
rotating twin-screw extruder as described in Section 3.2.3.1. The concentration of
maleic anhydride used was 2 wt% with respect to the total mass. The
concentration of Luperox 101 used was 0.1 wt% with respect to the total mass.
The feed rates of the external feeder and the peristaltic pump for the glycerol
were set so as to accomplish a composition of 80 wt% starch and 20 wt%
glycerol. The temperature profile of the extruder for producing maleated
thermoplastic starch was 25/95/125/145l160/165/165/165/150/145°C from the
feed throat to the die. The extruded strand was air cooled and pelletized in line.
The pellets were dried for two days in an oven at 65°C bef0re being blended with

PBAT.

84

5.2.3.2 Blend Production of MTPS with PBAT

Blends of maleated thermoplastic starch/PBAT were produced in a Century CX-
30 co-rotating twin-screw extruder. Pellets of MTPS were pre-mixed with PBAT in
a 10:90, 20:80, 30:70, 40:60, or 50:50 weight of MTPS: weight of PBAT ratio
(Table 8) and placed in the external feeder. The feeder fed the pellets into the
feed port of the extruder. The temperature profile of the extruder was set at
25/125/135/140/145l150/150/150/145/135°C from the feed throat to the die. The
screw speed was set to 100 rpm. The vent port was kept open to remove any
moisture. The extruded strand was air cooled and pelletized in line. The pellets

were dried for one day in an oven set at 65°C before being extruded into films.

Table 8. Composition of films of blends of MTPS with PBAT.

 

 

 

 

 

 

 

 

Samples Glycerol Maleic Luperox Blend Composition
Anhydride ‘
(%) (%) (%) MTPS (%) PBAT (%)
1 ' ' ' - 100
2 20 2 0.1 10 90
3 20 2 0.1 20 30
4 20 2 0.1 30 70
5 20 2 0.1 40 60
5 20 2 0.1 50 50

 

 

 

 

 

 

 

 

85

5.2.3.3 Film Production

Extruded films of the blended material as well as neat PBAT were produced in a
Killion single-screw extruder. The pellets were gravity-fed into the extruder at the
feed throat. The temperature profile of the extruder was set to
160/160/160/160/154°C from the feed throat to the die. Cooling water was
circulated to maintain the set temperatures. The screw speed was set to 65 rpm.
The extruded sheet was cooled on a chill roller set at 21°C and then collected on
an auxiliary roller. The speed of the chill roller was adjusted so as to produce a

film with a thickness of 150nm.

5.2.3.4 Determining Biobased Content

Samples of MTPS and the 50:50 MTPS:PBAT blend were sent to Beta Analytic
Inc. (Miami, FL) for biobased content analysis. This analysis was run according
to standard method ASTM D6866 Method 3.85 In this testing, the amount of
radiocarbon (14C) in a test sample is compared to that of a modem reference
sample. The ratio of the unknown to the standard is reported as a percentage,
having units of percent modem carbon (pMC). A sample that is a mixture of fossil
carbon (ie: contains no radiocarbon) and present day radiocarbon will have a
pMC value that correlates directly to the amount of biobased content in the

sample.

86

It should be noted that the results of the biobased content analysis give a value
for the percentage of carbon that is radiocarbon, out of the total amount of

carbon in the sample, not the total weight of the sample.

5.2.3.5 Mechanical Properties
5.2.3.5.1 Tensile Tests

Tensile specimens (100mm x 25mm) of the MTPS/PBAT blends were cut from
the extruded sheets of material and were conditioned as recommended in
standard method ASTM D4332,77 in a constant environment room at 2311°C and
5012% RH for at least 40 hours before testing. The tensile tests were measured
in accordance with standard method ASTM D882.78 Specimens of each
formulation, cut along the machine direction and along the transverse direction,
were clamped between the grips of a United Testing Systems (Huntington
Beach, CA) model SFM-20 tensile test machine. Force-extension curves were
recorded. Values for the tensile strength and elongation were determined for
each sample, in both the machine and transverse direction. Seven specimens

were tested for each blend and direction.

5.2.3.5.2 Puncture Tests

Puncture tests were performed on the film samples to determine puncture
strength (N) and deformation (mm) using a TA.TX2i Stable Micro Systems

texture analyzer (Surrey, England). Specimens (40mm x 40mm) were affixed

87

with tape (3M Scotch, Brazil) onto the plate of the equipment with a hole of
20mm diameter. A cylindrical probe of 5mm diameter was moved perpendicularly
to the film surface at a constant speed of 1 mm/s until the probe passed through
the film. Force-deformation curves were recorded and the force and defamation
at the point of rupture were determined. Each data point represents an average

of ten specimens.

5.2.3.6 Barrier Properties
5.2.3.6.1 Carbon Dioxide Permeability

Carbon dioxide (00;) permeability tests were performed using Mocon
Permatran-C Model 4/41 carbon dioxide permeability equipment. Testing was run
per ASTM standard F2476.86 For the 002 permeability tests, specimens of each
formulation were cut from the films that were extruded. Aluminum masks (4 mil
thick) were used to ensure that the area of the specimen tested was 1.41 cmz.
The carrier gas used was nitrogen. Permeability tests were run using a flow rate
of 50 sccm for carbon dioxide and nitrogen. All tests were run at 23°C and 0%
humidity. Once the specimens were loaded into the Permatran-C, they were
conditioned for 2 hours. Permeability data was then collected for 30 minutes for
each specimen (one cycle each). The Permatran-C has two cells and can test
two specimens at once. After every two cycles, the test module was rezeroed.
Each test was run until steady-state conditions were achieved. Values for

permeability of CO; for each specimen were determined by multiplying the

88

average thickness of the specimen area tested by the average steady-state
transmission rate. The Permatran-C compensates the transmission data to 1 atm
barometric pressure. The values reported are an average of 4 specimens for

each sample.

5.2.3.6.2 Oxygen Permeability

Oxygen (02) permeability tests were performed using Illinois Instruments Model
8001 oxygen permeability equipment. Testing was run per ASTM standard
D3985.87 For the 02 permeability tests, the specimens that were used to test
carbon dioxide permeability were tested. Aluminum masks (4 mil thick) were
used to ensure that the area of the specimen tested was 1.41 cmz. The carrier
gas used was nitrogen. Permeability tests were run using a flow rate of 10 sccm
for oxygen and nitrogen. All tests were run at 23°C and 0% humidity. Once the
specimens were loaded into the test equipment, the gas lines and cells were
purged. Permeability data was then collected for 5 minutes for each sample (one
cycle each). The Illinois Instruments equipment has two cells and can test two
specimens at once. Each test was run until steady-state conditions were
achieved. Values for permeability of 02 for each specimen were determined by
multiplying the average thickness of the specimen area tested by the average
steady-state transmission rate. The Illinois Instruments equipment acquires the
data at atmospheric pressure. The permeability data presented has been
compensated to 1 atm barometric pressure. The values reported are an average

of 4 specimens for each sample.

89

5.2.3.6.3 Water Vapor Permeability

Water vapor (H2O) permeability tests were performed using Mocon Permatran-W
Model 3/33 water vapor permeability equipment. Testing was run per ASTM
standard F1249.88 For the H20 permeability tests, the specimens that were used
to test carbon dioxide and oxygen permeability were tested. Aluminum masks (4
mil thick) were used to ensure that the area of the specimen tested was 1.41
cm2. The carrier gas used was nitrogen. Permeability tests were run using a flow
rate of 100 sccm. All tests were run at 378°C and 100% humidity. Once the
specimens were loaded into the Permatran-W, they were conditioned for 2 hours.
Permeability data was then collected for 15 minutes for each specimen (one
cycle each). The Permatran-W has two cells and can test two specimens at
once. After every two cycles, the test module was rezeroed. Each test was run
until steady-state conditions were achieved. Values for permeability of H20 for
each specimen were determined by multiplying the average thickness of the
specimen area tested by the average steady-state transmission rate. The
Permatran-W acquires the data at atmospheric pressure. The permeability data
presented has been compensated to 1 atm barometric pressure. The values

reported are an average of 4 specimens for each sample.

90

5.2.3.7 Transmission Electron Microscopy (T EM)

A transmission electron microscope (T EM) (JEOL-100CX) was used to make
observations of the morphology of the blends. Samples of PBAT and the
extruded blends were cryogenically frozen with liquid nitrogen and microtomed.
The samples were mounted on TEM grids and then viewed with the transmission

electron microscope.

5.2.3.8 Statistical Analysis

One control sample, PBAT, was tested to evaluate the effect of blends of MTPS
with PBAT on mechanical and barrier properties of the films. Analysis of variance
(ANOVA) and Tukey mean comparison tests (p s 0.05) were performed using

Minitab 15 software (Minitab Inc., State College, PA).

5.3 RESULTS AND DISCUSSION

5.3.1 Determining Biobased Content

The results of the biobased content analysis show that MTPS has a mean
biobased result of 96 i 3%, while the 50:50 blend has a mean biobased result of
32 :l: 3%. That is to say that 96 i 3% of the carbon in the MTPS is biobased

carbon, while 32 :I: 3% of the carbon in the 50:50 blend is biobased carbon.

91

It should be noted that PBAT is a synthetic biodegradable polyester that is not

produced with any biobased content.

One can see that blending PBAT with MTPS significantly reduces the biobased
content of the material. In order to maintain the greatest amount of biocontent in
a material, the least amount of PBAT should be used that will provide the

required material properties for a specific application.

5.3.2 Mechanical Properties

5.3.2.1 Tensile Tests

The tensile tests that were performed give an indication of the effect of the ratio
of MTPS:PBAT of the blends and the effect of the direction from which the
specimens have been cut on the mechanical properties of the materials. These
effects will be discussed here. Table 9 presents the tensile strength (MPa) and

break elongation (%) for the samples.

92

Table 9. Thickness and mechanical properties (determined by tensile tests) of
films of blends of MTPS with PBAT.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Tensile Elongation at
Sample Direction Thickness (um)
Strength (MPa) Break (%)

PBAT Machine 148.8 1 6.51“ 19.7 1 3.27“ 789.0 1 179.08“
PBAT Transverse 146.4 1 3.45“ 15.3 1 0.90B 709.6 1 35.79“'3
10:90 Machine 150.4 1 5.63“ 18.9 1 1.59“ 937.5 1 90.11“B
10:90 Transverse 154.7 1 7.79“B 14.3 1 2.14'3-C 857.5 1 141.60“B
20:80 Machine 152.1 1 9.96“ 17.6 1 1.26”“3-D 991.8 1 121.00“C
20:80 Transverse 170.9 1 3.14C 14.3 1 0.903"):E 877.4 1 34.00“?”C
30:70 Machine 155.8 1 4.59“0 13.5 1 2.50” 908.6 1 192.09”-C
30:70 Transverse 154.8 1 6.95“E 11.5 1 2.798“-G 781.3 1 196.92“°-°
40:60 Machine 152.8 1 5.97“" 12.2 1 1.22ELC 940.0 1 104.64“B-°
40:60 Transverse 163.3 1 5.263'C'D'E'F'G 8.9 1 1.25‘5-H 776.0 1 102.23”-C
50:50 Machine 144.9 1 4.52“ 8.6 1 1.12‘“ 736.9 1 99.54“B
50:50 Transverse 157.1 1 6.93“G 7.1 1 1.32HJ 709.0 1 138.81“8

 

93

Means in same column with different letters are significantly different (p50.05).

 

The tensile strength values for samples tested in the machine direction are
greater than samples tested in the transverse direction. Also, as the amount of
MTPS in the samples increases, the tensile strength decreases. Adding MTPS to

the material decreases the tensile strength of the resulting films.

Observing the elongation at break of the samples, there is not a statistical
difference between the samples when comparing the composition and the
direction. Adding MTPS to PBAT does not significantly change the elongation
exhibited. One also observes that all samples show elongations greater than

700%. These materials can deform greatly prior to breaking.

The values for tensile strength and elongation of films containing 20% MTPS are
greater than those of Parulekar and Mohanty,89 in which extruded cast films of 50
wt% PHBV, 30 wt% PBAT, 12 wt% high amylose corn starch, and 8 wt% glycerol
exhibited tensile strength and elongation values of 15 MPa and 368%,
respectively. However, Otey and Westhoff90 reported blown films consisting of
20% corn starch and 80% poly(ethylene-co-acrylic acid) having tensile strength

and elongation of 28.5 MPa and 120%, respectively.
From the data presented in this work, one would expect the tensile strength of

starch films to be lower than the MTPS-PBAT blends. This has been shown by

Zamudio-Flores et al.91 who reported values for tensile strength of native and

94

acetylated banana starch films of approximately 2.5 MPa and 3 MPa,

respectively.

5.3.2.2 Puncture Tests

The results of the puncture tests on the film specimens are shown in Table 10,

below. The thickness, force, and defamation of the samples are given. Each

entry is an average of 10 specimens.

Table 10. Puncture test results of films of blends of MTPS with PBAT.

 

 

 

 

 

 

 

 

 

 

 

Sample Thickness (um) Farce (N) Defamation (mm)
PBAT 155 1 6.6“ 42.5 1 3.22“ 17.9 1 1.74“
10:90 150 1 8.4““ 42.1 1 3.21“ 24.2 1 1.78“
20:80 152 1 7.4““ 40.9 1 2.93“ 23.5 1 1.32“
30:70 148 1 10.1““ 38.2 1 6.04A 25.2 1 3.09“
40:60 151 1 8.5““ 32.2 1 1.18“ 24.8 1 2.14“
50:50 143 1 3.9“ 30.3 1 1.72“ 20.3 1 2.04“

 

Means in same column with different letters are significantly different (p50.05).

One can see that as the amount of MTPS in the samples increases, the

maximum force achieved prior to rupture decreases. This is the same

phenomenon as was observed for the tensile tests. Adding MTPS to PBAT

 

decreases the amount of force that can be withstood both in a tensile type
loading and puncture type loading. One would expect starch films to exhibit lower

1.92 observed that

puncture strength than MTPS-PBAT films. Aguilar-Mendez eta
gelatine - corn starch films had, on average, lower puncture strength and

defamation than what has been reported here.

With regard to the defamation, it is observed that the blends containing 10, 20,
30, and 40% MTPS have the highest defamation prior to rupture, but the PBAT
sample and the 50:50 blend sample have lower values for defamation. However,

all samples exhibited puncture defamation values between 17.9 and 25.2 mm.

5.3.3 Barrier Properties

A main function of packaging, especially food packaging, is often to avoid or at
least decrease the transfer of gases such as oxygen, carbon dioxide, and water
vapor between the contents of the package and the surrounding environment.
Thus, the pemeability of the packaging material to oxygen, carbon dioxide, and
water vapor should be as low as possible.93 Values for carbon dioxide, oxygen,

and water vapor pemeability of the film samples are shown in Table 11.

96

Table 11 . Barrier properties of films of blends of MTPS with PBAT.

 

 

 

 

 

 

 

 

 

 

 

Carbon Dioxide Oxygen Water Vapor
Pemeability Pemeability Pemeability
Sample 2 1 1
(kg.m.m-2.s-1.Pa-1) x (kgmm .5 'Pa ) X (kg.m.m-2.s-1.Pa-1) x
1016 1018 1015

PBAT 3.0 1 0.33“ 16.8 1 1.77“ 5.0 1 0.50“

10:90 2.5 1 0.10“B 14.5 1 1.27“B 5.9 1 0.24“

20:80 2.1 1 0.17““ 12.2 1 0.70““ 6.7 1 0.12“
30:70 1.8 1 0.31““ 10.0 1 1.79““ 8.1 1 1.10““
40:60 1.3 1 0.10“:E 8.0 1 0.67“E 10.4 1 0.74““
50:50 1.1 1 0.12E 6.0 1 0.39E 10.4 1 0.80D

 

Means in same column with different letters are significantly different (p50.05).

One can see that by increasing the amount of MTPS in the sample, the carbon

dioxide and oxygen pemeability values decrease. For carbon dioxide, the

pemeability value decreases from 3.0 x 10'16 kg'm'm'z's'I'Pa'1 for PBAT to 1.1 x

10'16 kg'm'm‘z's'I'Pa'1for the 50:50 blend. While, for oxygen pemeability, the

values decrease from 1.68 x 10'17 kg'm'm'z's'1'Pa'1for PBAT to 6.0 x 10'18 kg‘m'm'

2's'1'Pa'1 for the 50:50 blend. This could be due to the fact that a second phase

has been added to the material, causing the transport of the gases through the

films to be hindered.

97

 

However, for water vapor pemeability, one can see that the values increase from
5.0 x 10'15 kg°m'm'2's'1'Pa'1for PBAT to 1.04 x 10'“ kg'm'm'z's'I'Pa‘1 for the 50:50
blend. Starch is a hygroscopic material, and thus has a greater affinity for water
than PBAT. This causes an increase in the pemeability of water vapor through

films with higher starch content.

One would expect starch films to have carbon dioxide and oxygen pemeability
values that are lower than the Values for the 50:50 blends reported in this work.
In fact, this is the case. Biliaderis et al.94 observed carbon dioxide and oxygen
pemeability values of 1.60 x 10'18 kg'm‘m'z's'1'Pa'1 and 5.46 x 10'19 kg'm‘m'z's'
1"Pa‘1, respectively, for polyol-plasticized pullulan-starch blend‘films. Bae et al.95
reported an oxygen pemeability value for mungbean starch films of 9.22 x 10'20
kg‘m'm'z's'I‘Pa‘I. Thirathumthavom and Charoenrein9° reported oxygen
pemeability values of less than 4.38 x 10‘19 kg'm'm'z's'I'Pa'1 for films of tapioca

starch plasticized with sorbitol.

One would also expect starch films to have water vapor pemeability values that
are higher than the value for the 50:50 blends reported in this work. This is also
the case. Bae et al.“:’5 reported a water vapor pemeability value of 1.8 x 10'12
kg'm'm‘z‘s'1'Pa'1 for mungbean starch films, while Thirathumthavom and
Charoenrein96 reported a value of 3.08 x 10'13 kg'm'm'z's'I'Pa'1 for films of tapioca

starch plasticized with sorbitol.

98

 

With regard to other polymers, Avella et al.97 reported carbon dioxide and oxygen
pemeabilities for isotactic polypropylene (iPP) of approximately 7.4 x 10'17
kg'm'm'z's‘1'Pa'1 and 1.7 x 10'17 kg'm'm'z's'1'Pa'1, respectively. These values are
lower and slightly higher, respectively, when compared to the pemeabilities of
the blends reported in this work. Also, the blends reported in this work exhibit
oxygen pemeability values that are higher than that reported by Frounchi and

Dourbash"8 (3.4 x 10'19 kg'm'm'z's'1'Pa'1) for films of poly(ethylene terephthalate)

(PET). Shellhammer and Krochtag9 reported values for the water vapor
pemeability for low-density polyethylene (LDPE), cellophane, and whey protein
plasticized with glycerol of 3.6 x 10'16 kg'm'm'z's'1'Pa'1, 8.4 x 10'14 kg'm'm'z's'1'Pa'
1, and 5.3 x 10‘13 kg'm'm'2's‘1'Pa'1, respectively. In the current work, the water
vapor pemeability values for the MTPS/PBAT blends are lower than cellophane

and whey protein plasticized with glycerol, but higher than for LDPE.

5.3.4 Transmission Electron Microscopy (TEM)

Images of the samples taken with the transmission electron microscope are
shown in Figure 23. The PBAT is the continuous phase, while the MTPS is the
dispersed phase. One can see that with an increasing amount of MTPS in the

samples, the dispersed phase becomes finer and more regularly spaced.

For all blends, the MTPS phase is dispersed and has not significantly

agglomerated. This demonstrates that the extrusion process of blending MTPS

99

and PBAT has resulted in a well mixed material. This is most likely due to the fact
that maleating themaplastic starch makes it more compatible with the PBAT,
thus resulting in a more consistent blend. This regularity will help to provide

consistent properties for films produced using these materials.

 

 

 

 

 

 

(e)

 

 

 

 

Figure 23. TEM images of the samples at 20,000x magnification (1pm scale
bar): (a) PBAT, (b) MTPS:PBAT (10:90), (c) MTPS:PBAT (20:80), (d)
MTPS:PBAT (30:70), (e) MTPS:PBAT (40:60), and (f) MTPS:PBAT (50:50).
MTPS is the light phase and PBAT is the dark phase.

100

5.4 CONCLUSIONS

Blends of MTPS and PBAT can be produced and famed into films. Mechanical
tests of the films show that the tensile strength decreases with increasing MTPS
in the samples; however, the elongation of the samples is not significantly
affected by the addition of MTPS to the PBAT. Also, puncture force decreases as
the amount of MTPS in the samples increases. As a barrier to carbon dioxide
and oxygen, the addition of MTPS to PBAT improves the perfomance of the
films. The pemeability of carbon dioxide and oxygen decreases as the amount of
MTPS in the materials increases. However, for water vapor pemeability, the
opposite is true. With increasing MTPS in the samples, the pemeability of water
vapor increases due to the hygroscopic nature of starch. Images of the samples,
using transmission electron microscopy, show that MTPS is the dispersed phase
while PBAT is the continuous phase. Also, as the amount of MTPS in the
samples increases, the dispersed phase becomes finer and more regularly
spaced. In order to use these materials for specific film applications, one must
detemine the necessary trade-off between mechanical properties and barrier
properties. One should also keep in mind that by increasing the amount of PBAT
in the material, the percent biocontent decreases. To maintain the maximum
amount of biocontent in the material, the minimum amount of PBAT should be

used that will still give the required material properties for the specific application.

101

 

CHAPTER 6

APPLICATION AND PERFORMANCE OF MTPS-PBAT BLENDS

FOR FOAMS

 

6.1 RATIONALE

This work was performed to better understand the effect of the percentage of
MTPS in MTPS-PBAT blends and the percentage of chemical blowing agent
used on various properties of the foams produced. The foam samples were
characterized by measurements of density, expansion ratio, and specific length,
compressive strength, resiliency, and moisture sorption tests, and imaging using

digital light microscopy.

102

6.2 MATERIALS AND METHODS

6.2.1 Materials

High amylase corn starch was purchased from National Starch and Chemicals
(Indianapolis, IN). Poly(butylene adipate-co-terephthalate) (PBAT) was
purchased from BASF Chemicals (Ludwigshafen, Germany), under the trade
name Ecoflex. Anhydrous glycerol, 99.9% assay, was purchased from J.T. Baker
(Phillipsburg, NJ). Maleic anhydride was purchased from Sigma-Aldrich, Inc (St.
Louis, MO). The initiator, 2,5-bis(tert-butylperoxy)—2,5-dimethylhexane, also
referred to as Luperox 101, was purchased from Sigma-Aldrich, Inc (St. Louis,
MO). An endathemic chemical blowing agent, SAFTEC® UBA-60, was supplied
by Reedy lntemational Corporation (Keyport, NJ). The chemical blowing agent
consists of sodium salts of carbonic and polycarboxylic acids in an ethylene-

methyl acrylate copolymer carrier.100

6.2.2 Equipment

A Century CX-30 co-rotating twin-screw extruder, having a length/diameter ratio
(LID) of 42 and a screw diameter of 30 mm was used to prepare the maleated
themaplastic starch, and to blend these materials with PBAT. A pelletizer was

used to cut the extruded strands into pellets.

103

Foaming of the material was performed using a Killian single-screw extruder,

having a length/diameter ratio of 24 and a screw diameter of 1 inch.

6.2.3 Procedure

6.2.3.1 Production of Maleated Themaplastic Starch (MTPS)

Maleated themaplastic starch (MTPS) was produced in a Century CX-30 co-

rotating twin-screw extruder in the same manner as described in Section 5.2.3.1.

6.2.3.2 Blend Production of MTPS with PBAT

Blends of maleated themaplastic starch/PBAT were produced in a Century OX-
30 ca-rotating twin-screw extruder. Pellets of MTPS were pre-mixed with PBAT in
a 50:50 and 30:70 weight of MTPS:weight of PBAT ratio and placed in the
external feeder. The feeder fed the pellets into the feed port of the extruder. The
temperature profile of the extruder was set at
25/125/135/140/145/150/150/150/145/135°C from the feed throat to the die. The
screw speed was set to 100 rpm. The vent port was kept open to remove any
moisture. The extruded strand was air cooled and pelletized in line. The pellets

were dried for one day in an oven set at 65°C before being extruded into foams.

104

 

6.2.3.3 Foam Production

Extruded foams of the blended material were produced in a Killian single-screw
extruder. Pellets of the MTPS/PBAT blends were pre-mixed with the chemical
blowing agent. The chemical blowing agent content was either 3 wt%, 5 wt%, or
7 wt% (of the blends) for production of the foams (Table 12). The pellets were
gravity-fed into the extruder at the feed throat. The temperature profile of the
extruder was set at 150/160/160/150°C from the feed throat to the die. Cooling
water was circulated to maintain the set temperatures. The screw speed was set

to 65 rpm. The extruded strands exited through a 4.72mm cylindrical die.

105

Table 12. Composition of foams of blends of MTPS and PBAT.

 

 

 

 

 

 

 

 

 

 

 

Blowing Agent Blend Composition
Sample
(%) MTPS (%) PBAT (%)

1 3 50 50
2 5 50 50
3 7 50 50
4 3 30 70
5 5 30 70
6 7 30 70
7 3 - 100
8 5 - 100
9 7 - 100

 

 

 

 

 

 

106

6.2.3.4 Chemical Blowing Agent Themal Decomposition

The chemical blowing agent used in the foaming of the material had a reported
process range of 100°C to 180°C. To ensure that themal decomposition
occurred within this range, themal gravimetric analysis (T GA) was used to test a
sample of the blowing agent. TGA was perfomed using a high resolution TGA
2950 from TA Instruments. The purge gas, nitrogen, had a flow rate of 50 mllmin.
A pellet of the blowing agent was heated from room temperature to 600°C at a

rate of 20°C/min.

6.2.3.5 Extensional Viscosity

Extensional viscosity measurements of PBAT and the 30:70 and 50:50 blends
was performed by the Jayaraman group at Michigan State University. The PBAT
was tested using an extensional viscosity fixture (EVF) mounted on an ARES
melt rheometer at 150°C and tested to a strain of 3. The blends were tested
using a Rheometrics Melt Elongational Rheometer (RME) at 140°C and tested to
a strain of 3. If the samples broke before a strain of 3 was achieved, then that

data was discarded.

107

 

6.2.3.6 Foam Characterization

Foam specimens of the MTPS/PBAT blends were cut to approximately 25mm in
length using a razor blade and were conditioned per standard method ASTM
D4332,77 in a constant environment room at 23:1:1°C and 50:I:2% RH for at least

72 hours before testing.

6.2.3.6.1 Density

The density of the foam samples was calculated according to ASTM test method
D3575101 (Method A), using the mass and volume of the specimens.12"°""1°3 Ten
specimens were measured for each sample. The density reported for each

sample is the average of the ten specimens.

6.2.3.6.2 Expansion Ratio

The expansion ratio of the foam was calculated as the ratio of the cross-sectional
area of the foam specimen to the cross-sectional area of the die.12 Ten
specimens were measured for each sample. The expansion ratio reported for

each sample is the average of the ten specimens.

108

 

6.2.3.6.3 Specific Length

The specific length of the foam was calculated as the ratio of the length of each
specimen to its mass.12 The values used were those used in the density
calculation. The units of specific length are centimeters per gram. Ten specimens
were measured for each sample. The specific length reported for each sample is

the average of the ten specimens.

6.2.3.6.4 Compressive Strength and Resiliency

Compressive strength of foam specimens were measured on a Stable Micro
Systems texture analyzer (Surrey, England). Foam specimens were positioned
lengthwise and compressed by a steel probe (6.35mm diameter) with a spherical
end cap. The probe was lowered to the foam surface and an initial load of 0.5N
was applied to the specimen. The probe was then lowered at a rate of 30
mm/min for a distance of 3mm. The probe was held at the distance of 3mm for
60 seconds. Load-deflection curves were recorded. Compressive strength was
calculated by dividing the maximum load achieved by the cross-sectional area of
the probe. Resiliency was calculated as the ratio of the compressive force at the
end of the 60s hold period divided by the maximum force necessary to compress
the foam specimen by 3mm, and is expressed as a percentage.12 The
compressive strength and resiliency reported for each sample are the average of

five specimens.

109

6.2.3.6.5 Moisture Sorption Analysis

Five foam specimens of each sample were placed in a Blue M Electric Model
WP-4800 (Watertown, WI) environmental humidity chamber at 3815°C and
9515% RH.12 The weight and the dimensions (length and diameter) of the
specimens were measured prior to entering the chamber. The weight of the
specimens was monitored until steady state values were reached (approximately
60 days). The weight and dimensions were measured again at steady state. The
initial weight, diameter, and length of the specimens were designated W0, 00,
and Lo, respectively. The steady state weight, diameter, and length of the
specimens were designated W, D, and L, respectively. Nomalized values for
steady state weight gain, steady state diameter, and steady state length were
calculated using the initial and steady state values that were recorded. The
values for nomalized steady state weight gain, nomalized steady state
diameter, and nomalized steady state length reported for each sample are an

average of five specimens.

6.2.3.6.6 Digital Light Microscopy

A digital light microscope (Keyence Corporation of America, Woodcliff Lake, NJ)
was used to make observations of the morphology of the cross-sectional area of

the foams. A razor blade was used to section the foam samples.

110

6.2.3.7 Statistical Analysis

One control sample, PBAT, was tested to evaluate the effect of blends of MTPS
with PBAT on the properties of the foams produced. Analysis of variance
(ANOVA) and Tukey mean comparison tests (p s 0.05) were performed using

Minitab 15 software (Minitab Inc., State College, PA).

"i.

6.3 RESULTS AND DISCUSSION

6.3.1 Chemical Blowing Agent Themal Decomposition

The results of the TGA testing of the blowing agent are shown in Figure 24. The
maximum rate of themal decomposition of the blowing agent occurs at 150°C.
This is towards the upper processing temperature range reported by the
manufacturer. The temperature profile chosen for foaming of the materials,
150/160/160/150°C, is higher than the themal decomposition temperature of the
blowing agent, ensuring that decomposition of the blowing agent occurs during

the foaming process.

111

 

 

 

 

 

 

 

 

105 0.8

100— 0.6
8
o\
3 95 0.43}
9., .c
.. .9
.1: o
.3 E
E .2'
90— 0.2 a
o

85“ 0.0

80 . . . , , . . . , . . . . . . . -0.2

0 50 100 150 200 250
Temperature (°C)

 

 

Figure 24. TGA graph of themal decomposition of chemical blowing agent.

112

6.3.2 Extensional Viscosity

Successful foaming of polymers requires a certain amount of strain hardening
during extensional flow.104 Figures 25 and 26 display the results of the
extensional viscosity measurements of PBAT and the blends, respectively. One
can see that the PBAT sample, the nonlinear viscoelastic response in
extensional viscosity (strain hardening) increases as the strain rate increases.
However, for the blends, the extensional viscosity rapidly decreases after it has

begun to stretch (strain softening). This strain softening behavior is not conducive

 

to foaming of the material. In the foam characterization data presented here, the
differences between the melt properties of PBAT and the blends are apparent in

their effect on the resulting foam properties.

113

 

 

 

 

, 150°C
PBAT -1
- 2s
0.1s'1
10000-
’a'? 2
I
m 4
c‘L’ .
+ w
r
1000 . . ......, . . .....,
0.1 1 10
t(S)

 

Figure 25. Extensional viscosity plot of PBAT at 150°C.

114

 

 

 

 

    
    

PBAT:MTPS (50:50)
100000

PBAT:MTPS (70:30)

"5 10000

n; (Pa

1000

 

 

100 . . . .

t (s)

 

 

 

Figure 26. Extensional viscosity of blends at 140°C and 1'“.

115

 

6.3.3 Density, Expansion Ratio, and Specific Length

Table 13 displays the data for the density, expansion ratio, and specific length of

the foam samples.

It should be noted that the densities of extruded, unfoamed PBAT, and 30:70 and
50:50 blends are 1.22 :t 0.01, 1.30 :I: 0.01, and 1.37 :t 0.03 g/cm3, respectively.
All foams showed a reduction in their densities, compared to the unfoamed

material.

From Table 13, one observes that the foam samples that showed the best
characteristics of lowest density, highest expansion ratio, and highest specific
length are the PBAT samples. This is most likely due to the fact that the blends
containing MTPS go through an additional heat cycle during the foaming
process, causing the starch to degrade. This shortens the length of the starch
polymer chains and decreases the ability of the blends to support the expanding
cells during the foaming process. During the foaming process, it was observed
that the foamed blends shrunk more than the foamed PBAT upon exiting the

extruder and cooling.

Looking at the effect of the amount of blowing agent on the density of the foams
shows that for PBAT samples, increasing the blowing agent from 3% to 5%
decreases the density, however, increasing the blowing agent to 7% does not

affect the density of the foam. For the 30:70 blend, the density of the foams is not

116

 

affected by the amount of blowing agent added. As will be discussed in the digital
light microscopy results, this is most likely due to the fact that with only 3%
blowing agent, there is not enough gas liberated during the blowing agent
decomposition; however, when 5% and 7% blowing agent are used, the melt
properties of the blends are inadequate to‘support the expanding cells, leading to
cell collapse and the fomation of voids in the center of the samples. These same
observations are made for the 50:50 blend samples, however, with 7% blowing

agent, the cell collapse actually causes the density to increase slightly.

The data reported here shows values for density, expansion ratio, and specific
length of the foams that are quite high, low, and low, respectively, when
compared with foam extrusion of starch, using water as the blowing agent.“ 68
When one compares the densities to studies using chemical blowing agents to
foam HDPE,73 they are more closely related. Using chemical blowing agents with
PBAT and blends of MTPS:PBAT show a comparable reduction in density to the
work by Li and Matuana73 foaming HDPE and HDPE composites using chemical

blowing agents.

117

 

Table 13. Density, expansion ratio, and specific length of foams of blends of

MTPS and PBAT.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Specific Length
Sample Density (glcm3) Expansion Ratio
(cm/9)
PBAT 3% 0.73 1 0.06“ 2.49 1 0.48“ 3.28 1 0.70““
PBAT 5% 0.49 1 0.02“ 2.95 1 0.28“ 4.00 1 0.45““
PBAT 7% 0.50 1 0.05“ 2.72 1 0.30““ 4.29 1 0.46“
30:70 3% 0.84 1 0.03“ 1.92 1 0.09“ 3.54 1 0.19““
30:70 5% 0.91 1 0.08““ 1.87 1 0.15C 3.40 1 0.31“
30:70 7% 0.83 1 0.05C 1.91 1 0.13“ 3.63 1 0.12““
50:50 3% 0.89 1 0.07““ 2.60 1 0.26““ 2.50 1 0.10E
50:50 5% 0.94 1 0.10“:E 2.22 1 0.24“““ 2.78 1 0.16E
50:50 7% 1.081 0.05“ 1.86 1 0.10““ 2.85 1 0.10“:E

 

Means in same column with different letters are significantly different (p50.05).

118

 

 

 

6.3.4 Compressive Strength and Resiliency

Foams can better resist defamation during loading if they are denser and have
thicker cell walls, as opposed to less dense foams that have thinner walls. A
power-law relationship has been observed between compressive strength, ac,
and foam density, p, where 06 ~p". In Figure 27 below, one observes a strong
relationship between compressive strength and foam density. The slope of the
regression line was calculated to be n=0.94. A value of n=0.92 was reported by
Nabar et al.12 and Willet et al.,68 however, Hutchinson eta/.105 reported values of
n=1 .5-1 .6 for foams made from maize grits. In the current work, the value of n is
in agreement with the values calculated by Nabar et al. and Willett et al. It should
be noted; however, that this value is dependent on the extrusion system and the

materials used.12
In Figure 27, the point at (6.59, 14.81) is an outlier (Dixon’s Q-test for outliers,

with 95% confidence interval). This point has not been used to calculate the

regression line.

119

_. -1

 

 

 

14.9
14.8 I .
O
o
14.7 /
14.6 K

14 5 y = 0.9442x + 8.2454 /,
14': R2 = 0949/

14.3 /

14.2 /

14.1 9/

O

 

 

 

 

 

 

 

LN (Compressibility) (Pa)

 

 

 

14
6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 7 7.1

 

LN (Foam Density) (kg/m3)

 

 

 

Figure 27. Logarithmic plot of compressive strength as a function of foam
density.

The resiliency values of the foam samples are shown in Table 14. From the
values of resiliency listed, one observes that resiliency improves when there is
more PBAT present in the samples. The foamed samples of PBAT have the
greatest resiliency, 82.14% - 80.82%, while the foamed samples of the blends
have a much lower resiliency; 57.50%-61.58% for the 30:70 blend, and 54.73%-
51.33% for the 50:50 blend. The effect of the amount of blowing agent added is

not evident.

When one compares the densities and the resiliencies, one observes that foams

of PBAT have lower densities and higher resiliency values than foams of blends

of MTPS and PBAT.

120

Table 14. Resiliency of foams of blends of MTPS and PBAT.

 

 

 

 

 

 

 

 

 

 

Sample Resiliency (%)
PBAT 3% 82.14 1 1.23“
PBAT 5% 81.51 1 0.35A
PBAT 7% 80.82 1 1.07“
30:70 3% 59.57 1 5.16““
30:70 5% 57.50 1 6.51“““
30:70 7% 61.58 1 2.05““
50:50 3% 54.73 1 0.89““
50:50 5% 53.46 1 1.34““
50:50 7% 51.33 1 2.01““

 

 

 

 

Means in same column with different letters are significantly different (p50.05).

6.3.5 Moisture Sorption Analysis

The results of the moisture sorption tests are shown in Table 15. In other work
that has been perfomed previously,12 foams consisting of starch shrink during
moisture sorption tests. Arif et al.106 also observed the shrinkage of starch foam
when it was subjected to conditions of 30°C and 89% relative humidity. This was
not observed in the current work. This unexpected result is due to the fact that

PBAT foms the matrix for the materials that were foamed. The PBAT is the

121

continuous phase, while the MTPS is the dispersed phase. PBAT expands when
it sorbs water. Since the PBAT is the continuous phase in these samples, they

also expand when they sorb water.

Starch is a hygroscopic material, so adding MTPS to PBAT (30:70 blend and
50:50 blend) causes an increase in the nomalized steady state weight gain. The
“50:50 7%” sample shows a much lower steady state weight gain value than the
other two 50:50 blend samples. This is most likely due to the fact that the density
is highest in this sample, which does not allow moisture to be sorbed well

throughout the sample.

The perfomance of starch foam (cushioning and insulation) is negatively
affected by the shrinkage that usually occurs during moisture sorption of starch-
based foams.12 From the results of this study, it would appear that foams which
contain higher amounts of PBAT would not see this adverse affect. Starch can be
added to the PBAT (which would decrease the cost of the material), and the

resulting foam still does not shrink during moisture sorption.

122

 

 

Table 15. Moisture sorption steady-state weight gain, diameter, and length of
foams of blends of MTPS and PBAT.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Nomalized Nomalized Nomalized
Sample Steady State Steady State Steady State
Weight Gain“I Diameter” Length"
PBAT 3% 3.54 1 0.47“ 203.94 1 11.95““ 100.38 1 0.46““
PBAT 5% 7.13 1 0.24““ 182.49 1 5.54“ 100.88 1 0.56“
PBAT 7% 8.67 1 0.87““ 186.99 1 8.00“ 100.87 1 0.52“
30:70 3% 24.08 1 0.57“” 214.62 1 2.50“ 103.19 1 0.56“
30:70 5% 26.98 1 0.44“»F 217.30 1 6.95“ 103.43 1 0.22“
30:70 7% 32.01 1 1.19“ 217.16 1 6.53“ 103.34 1 0.48“
50:50 3% 22.92 1 5.33““ 188.69 1 4.57“ 99.03 1 0.26“
50:50 5% 20.03 1 6.38“ 195.36 1 7.49““ 99.45 1 0.16“
50:50 7% 9.56 1 1.18““ 214.06 1 2.82“ 99.74 1 0.28““

 

°[(W-WO)NVo]x100%, °[D/Do]x100%, °[L/Lo]x100%
Means in same column with different uppercase letters are significantly different

(p50.05).

123

 

 

6.3.6 Digital Light Microscopy

Images of the foam samples are shown in Figure 28. One can see that the foams
of PBAT have a more regular cell structure. For the sample with 3% blowing
agent, there are some areas of unfoamed PBAT. However, for the samples
containing 5% and 7% blowing agent, the cells appear to be throughout the

matrix.

For the foams of 30:70 blends, one can see that with 3% blowing agent there are
some areas of unfoamed 30:70 material. However, for the samples containing
5% and 7% blowing agent, the cells appear to become larger and more of an

open cell structure is observed.

For the foams of the 50:50 blends, one can see that with 3% blowing agent there
are some areas of unfoamed 50:50 material (as was seen with the PBAT and
30:70 blend). Also, like the 30:70 blend, the samples containing 5% and 7%
blowing agent show cells that appear to be an opened cell structure. The
samples also show evidence of cell collapse and aggregation to large voids in

the center of the sample.

These images suggest that PBAT has better melt properties than the blends
which allow the cells to grow but not collapse. The blends; however, show large
irregular cells that have joined to fom an open cell structure. This is observed in

the blends with greater than 3% blowing agent in them; however, with 3%

124

 

 

blowing agent there are still areas that have not been foamed. This is not an

optimum condition.

The results of the density measurements agree with the images that have been
shown above. For the PBAT, one can see that with 5% and 7% blowing agent,
we see similar densities and they are less than the density of PBAT with 3%
blowing agent. With 3% blowing agent, there is not enough gas liberated by the
decomposition of the blowing agent to fully foam all of the material. With 5% and
7% blowing agent, there is enough gas present, and foaming occurs throughout

the material.

For both the 30:70 and 50:50 blends, one notices that the densities are similar,
independent of the amount of blowing agent added. This is due to the fact that
with only 3% blowing agent, there is not enough gas liberated during blowing
agent decomposition; however, when 5% and 7% blowing agent are used, the
melt properties of the blends are inadequate to support the expanding cells,

leading to cell collapse and the fomation of large voids.

125

 

g) PBAT 7%

 

 

h) 30:70 7%

 

i) 50:50 7%

 

Figure 28. Digital light microscope images of foams of blends of MTPS and
PBAT (250 pm scale bar) showing the cell structure of the foams.

126

 

6.4 CONCLUSIONS

Blends of MTPS and PBAT can be produced and foamed using a chemical
blowing agent. Measurement of the resulting foam samples shows that the best
characteristics of lowest density, highest expansion ratio, and highest specific
length are observed in the PBAT samples. The compressive strength of the foam
samples follows a power-law relationship, with a slope of the regression line of
the logarithmic plot of compressive strength versus foam density of n=0.94.
Samples of foamed PBAT have the greatest resiliency, while samples of foamed
30:70 blends have lower resiliency, and samples of foamed 50:50 blends have
the lowest resiliency. Moisture sorption tests of the foam samples show an
increase in the diameter of the samples as moisture is sorbed. All samples gain
weight during the moisture sorption testing, with the blends gaining more weight
that the PBAT samples, due to the hygroscopic nature of MTPS. Images of the
samples using a digital light microscope show that PBAT foams have a more
regular cell structure than the foams of blends of MTPS and PBAT. With only 3%
blowing agent, there appears to be regions of unfoamed material in all of the
samples. For the blended materials, with 5% and 7% blowing agent, the cells
become larger and show more of an open cell structure. Blending MTPS with
PBAT decreases the cost of using only PBAT, however, one can see that this
affects the properties of the foam. In order to use these materials for specific
applications, one must detemine the necessary trade-off between required

properties and cost of the material.

127

CHAPTER 7

SUMMARY AND CONCLUSIONS

 

7.1 SUMMARY

The engineering significance of the work that has been perfomed is that it shows
that blends of starch and the biodegradable polyester, PBAT, can be produced
and famed into themoformed objects, films, and foams. These objects can be
used to replace current objects made from petroleum-based plastics that are not
biodegradable. By blending the starch and PBAT together, one receives
advantages over using either component separately; namely, improved
mechanical properties through the addition of the PBAT to thestarch, and lower

cost through the addition of the starch to the PBAT. As well, improvements in

128

some barrier properties of films have also been observed by blending MTPS with

PBAT.

The materials produced in this work have the potential to decrease the carbon
footprint of the manufacturers who produce the products, as well as the
consumers who use the products. Also, since the products are biodegradable,

they can be composted and will not add to the garbage that is in landfills.

7.2 CONCLUSIONS

Upon completion of this work, one can make the following conclusions:

7.2.1 Maleated Themaplastic Starch by Reactive Extrusion Processing

0 MTPS had improved processability over TPS.

o The maleation process pemitted fomulations with less than 30% of
glycerol to be extruded in the twin-screw extruder, resulting in pellets that
were more transparent than themaplastic starches not modified with
maleic anhydride.

0 As the percentage of maleic anhydride increased, there was negligible
change in the hydrodynamic radii. However, as the percentage of Luperox
101 increased, the hydrodynamic radii decreased.

0 Using the maximum temperature during the extrusion process of 165°C

instead of 135°C caused a drastic decrease in the hydrodynamic radius,

129

 

 

due to the high influence of the temperature profile on the molecular
weight of the themaplastic starch.

The MTPS samples presented higher melting temperatures compared to
the TPS sample.

The soxhlet studies indicated that using the maximum temperature of
165°C in the extrusion temperature profile resulted in more grafting
between glycerol and starch than when the maximum temperature used
was 135°C.

There is an increased understanding of maleation of high amylase corn

starch and the effect of maleic anhydride and initiator content.

7.2.2 MTPS-PBAT Blends by Reactive Extrusion Processing and Their
Application and Perfamance far Themafamed Articles

The processability of high amylase corn starch MTPS is much better than
the processability of regular corn starch MTPS.

Blends of plasticized starch, with or without maleation, with PBAT can
fom sheets which can be themofomed.

To the best of the author’s knowledge, this is the first report of
themofoming of 100% biodegradable material.

The tensile properties are similar for blends of PBAT with TPS and MTPS
that have been extruded at a maximum temperature of 135°C. However,
the maleated themaplastic samples famed sheets and thermoformed

pieces that were more transparent and had a more homogeneous surface.

130

 

 

The effect of amylase content of the different starches used did not
present significant differences in the tensile properties when the glycerol
content was 30% and the ratio of themaplastic starch/PBAT used was
50:50.

When the MTPS contained 20% glycerol and the maximum processing
temperature was 165°C, the blend containing MTPS of high amylase corn
starch showed better tensile properties than the blend containing MTPS of
regular corn starch, when the ratio of themaplastic starch/PBAT used was
50:50.

Increasing the MTPS (high amylase corn starch) content to 60%
decreased the tensile strength and elongation of the sheets.

The addition of TPS or MTPS decreased the tensile strength, but did not
cause a significant difference in the elongation, compared to the
elongation of PBAT.

The puncture test results show similar behavior to the tensile test results.
DSC analysis shows that the blends all have higher melting and
crystallization temperatures than neat PBAT.

ESEM shows that blends containing MTPS extruded with a maximum
temperature of 165°C are smoother and more homogeneous than the
blends containing MTPS and TPS extruded with a maximum temperature

of 135°C.

131

7.2.3 Application and Perfamance of MTPS-PBAT Blends for Films

Blends of MTPS and PBAT can be produced and famed into films.
Increasing the amount of PBAT in the material causes a decrease in the
percent biocontent.

Mechanical tests of the films show that the tensile strength decreases with
increasing MTPS in the samples, however, the elongation of the samples
is not significantly affected by the addition of MTPS to the PBAT.

The puncture force of the films decreases as the amount of MTPS in the
samples increases.

As a barrier to carbon dioxide and oxygen, the addition of MTPS to PBAT
improves the perfomance of the films. The pemeability of carbon dioxide
and oxygen decreases as the amount of MTPS in the materials increases.
With increasing MTPS in the samples, the pemeability of water vapor
increases due to the hygroscopic nature of starch.

Images of the samples, using transmission electron microscopy, show that
MTPS is the dispersed phase while PBAT is the continuous phase.

As the amount of MTPS in the samples increases, the dispersed phase

becomes finer and more regularly spaced.

132

7.2.4 Application and Perfamance of MTPS-PBAT Blends far Foams

o Blends of MTPS and PBAT can be produced and foamed using a
chemical blowing agent.

0 Measurement of the resulting foam samples shows that the best
characteristics of lowest density, highest expansion ratio, and highest
specific length are observed in the PBAT samples.

0 The compressive strength of the foam samples follows a power-law
relationship, with aslope of the regression line of the logarithmic plot of
compressive strength versus foam density of n=0.94.

0 Samples of foamed PBAT have the greatest resiliency, while samples of
foamed 30:70 blends have lower resiliency, and samples of foamed 50:50
blends have the lowest resiliency.

0 Moisture sorption tests of the foam samples show an increase in the
diameter of the samples as moisture is sorbed.

o All samples gain weight during the moisture sorption testing, with the
blends gaining more weight that the PBAT samples, due to the
hygroscopic nature of MTPS.

0 Images of the samples using a digital light microscope show that PBAT
foams have a more regular cell structure than the foams of blends of
MTPS and PBAT.

0 With only 3% blowing agent, there appears to be regions of unfoamed

material in all of the samples.

133

For the blended materials, with 5% and 7% blowing agent, the cells
become larger and show more of an open cell structure.

Blending MTPS with PBAT decreases the cost of using only PBAT;
however, one can see that this affects the properties of the foam.

In order to use these materials for specific applications, one must
detemine the necessary trade-off between required properties and cost of

the material.

134

CHAPTER 8

RECOMMENDATIONS FOR FUTURE WORK

 

8.1 RECOMMENDATIONS FOR FUTURE WORK

The work presented here describes much of the materials testing performed on
MTPS and MTPS-PBAT blends. These materials have the potential to be used to
produce themofomed objects, films, and foams. Future work could focus on
testing the materials for specific applications to ensure that they meet the
requirements for each application. This testing could include product durability,

product dimensional, and product environmental testing.

Heat has been shown to decrease the molecular weight of starch during the
extrusion process. In the procedures described in this work, starch was extruded

135

to produce MTPS, then it was blended with PBAT in another extrusion process,
and finally, it was famed into sheets, films, or foams in a third extrusion process.
A procedure could be developed in which the starch, glycerol, maleic anhydride,
Luperox 101, and PBAT are reactively extmded at once, and famed into sheets,

films, or foams upon exit from the extruder.

Polymeric materials are often injection molded to fam various products. Injection
molding of the MTPS-PBAT blends described in this work could be performed.
Mechanical and themal testing of the resulting injection molded products would
give an indication of the usefulness of these materials for injection molded
applications. Furthermore, adding fibers to the materials during the extrusion
process could be investigated to detemine if composites of MTPS-PBAT blends
have improved characteristics for injection molding, versus MTPS-PBAT blends

without fibers.

Biodegradability studies were not perfomed on these materials. Although it is

known that the materials are biodegradable, testing should be perfomed to

better understand the length of time required for biodegradation to occur.

Biodegradability can be measured according to ASTM standard test method

05338, “Standard Test Method for Detemining Aerobic Biodegradation of Plastic
1: 107

Materials Under Controlled Composting Conditions , and in combination with

ASTM standard test method 06400, “Standard Specification for Compostable

136

Plastics”.108 In this way, the biodegradability perfomance of these materials can

be compared to other biodegradable products.

A life cycle analysis of the MTPS and MTPS-PBAT blends was not performed. A
recommendation for future work would be to perform a life cycle analysis for the
materials produced in this work. ASTM standard test method D7075, “Standard
Practice for Evaluating and Reporting Environmental Perfomance of Biobased
Products”,109 should be used to perform this evaluation. Having a life cycle
analysis of the materials would allow potential manufacturers to better
understand the impact of these materials and how the impacts compare to

conventional polymeric materials.

137

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