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This is to certify that the
thesis entitled

DEVELOPMENT OF BIODEGRADABLE
PLASTICS BY BLENDING A BY-PRODUCT OF
THE CORN BASED ETHANOL INDUSTRY
AND POLY (6-CAPROLACTONE)

presented by

DINESH C. AITHANI

has been accepted towards fulfillment *
of the requirements for the

MS. degree in Packaging

Nwwvéuma/fl%%eir

Major Professor’s Signature
43«¢6~ga5-

Date

MSU is an Affirmative Action/Equal Opportunity Institution

 

 

PLACE IN RETURN BOX to remove this checkout from your record.
TO AVOID FINES return on or before date due.
MAY BE RECALLED with earlier due date if requested.

 

DATE DUE

DATE DUE

DATE DUE

 

FEBZ 3 2008

 

@823fig

 

 

 

 

 

 

 

 

 

 

 

 

 

2/05 c:/ClRC/DateDue.indd—p.15‘

 

DEVELOPMENT OF BIODEGRADABLE PLASTICS BY BLENDING
A BY-PRODUCT OF THE CORN BASED ETHANOL INDUSTRY
AND POLY (c-CAPROLACTONE)

By

DINESH C. AITHANI

A THESIS

Submitted to
Michigan State University
In partial fulfillment of the requirements
For the degree of

MASTER OF SCIENCE

School of Packaging

2005

ABSTRACT
DEVELOPMENT OF BIODEGRADABLE PLASTICS BY BLENDING A
BY-PRODUCT OF THE CORN BASED ETHANOL INDUSTRY AND
POLY (C-CAPROLACTONE)
By
Dinesh C. Aithani

Development of new value-added green materials from byproducts of
corn-based ethanol industries would provide a substantial economic return to the
corn growers. Corn gluten meal (CGM), a major byproducts of com-based wet
milling ethanol industries has most of its use as livestock feed. In this research
CGM was plasticized using glycerol/ethanol mixture, destructurized by the
addition of guanidine hydrochloride (GHCI) and then blended with poly (e-
caprolactone) (PCL). Extrusion followed by injection molding was adopted to
fabricate the newly blended bioplastics/green materials. The GHCI modified corn
gluten meal was characterized by infra-red (IR) spectroscopy. The effect of
processing conditions on properties of blends was investigated. The thermo-
mechanical properties of the blends were studied using DMA (Dynamic
Mechanical Analyzer), TGA (Thermo-Gravimetric Analyzer) and UTS (Universal
Testing System). Significantly higher elongation and an almost equivalent tensile
strength was observed for GHCI modified blend in contrast to the non-GHCI
modified blend. The blends were also observed to be more thermal stable with
addition of GHCI. Scanning electron microscopy (SEM) was used to study the
surface morphology of the blends. The SEM analysis revealed better
compatibility of PCL with GHCI modified plasticized CGM in contrast to
plasticized CGM.

ACKNOWLEDGEMENTS

I would like to express my gratitude to my advisor Dr. Amar K. Mohanty for his
continuous support, inspiration and guidance. I also thank and appreciate him for
providing me valuable technical support during the project and also for his critical
review of the manuscript. My sincere appreciation and gratitude to Dr. Bruce

Harte and Dr. Janice Harte for being my committee and their support.

I am grateful to AKM research group, faculty and staff at School of Packaging
and my fellow graduate students for their kind support. I am also grateful to the
staff at Composite Materials and Structures Center (CMSC) staff for their

assistance in performing various machine operations.

I express my sincere gratitude and regards to my family and friends for their

motivation and support, which has been a great contribution in the success of

this project.

iii

TABLE OF CONTENTS

LIST OF TABLES ............................................................................... v
LIST OF FIGURES ............................................................................. vi
CHAPTER 1- INTRODUCTION ................................................................ 1
CHAPTER 2- LITERATURE REVIEW ....................................................... 15
Biodegradable Plastics... ...................................................... 15
Corn Gluten Meal and its based polymers ............................... 21
Chemical modification of Protein ........................................... 45
Polymer Blending ............................................................... 48
CHAPTER 3- MATERIAL AND METHODS ................................................ 54
Materials .......................................................................... 54
Preparation of Blends ......................................................... 55
Characterization ................................................................ 58
CHAPTER 4- RESULTS AND DISCUSSION ............................................. 65

CHAPTER 5- CONCLUSION AND RECOMMENDATIONS FOR FUTURE
WORK ........................................................................... 88

BIBLIOGRAPHY .................................................................................. 90

iv

LIST OF TABLES

Table 1. Average yield of ethanol and its byproduct per bushel of corn ......... 11
Table 2. The composition of Corn gluten meal and distiller’s dried grain ........ 12
Table 3. Various extraction techniques and recovery methods used for zein.. 26
Table 4. Composition of different blends considered for study ..................... 56
Table 5. Modulus of PCL blends with GHCI modified CGMP processed at

150 rpm and temperatures of 120°C and 150°C at the ratio of

PCL:CGMP:GHCI = 50:37.5:12.5 (wt%) ......................................... 66

Table 6. Modulus of PCL blends with GHCI modified CGMP processed at
150°C and different processing speeds of 75 and 150 rpm at the

ratio of PCL:CGMP:GHCI = 50:37.5:12.5 (wt%) .......................... 67

Table 7. Protein percentage determined by Nitrogen Content Analyzer
Instrument ......................... _ ................................................... 70
Table 8. Effect of GHCI on tensile strength of PCL blends .......................... 71
Table 9. Effect of GHCI on percent elongation of PCL blends ..................... 72

Table 10. Comparison of percent elongation and Impact strength of PCL,
PCL blend and HDPE ........................................................... 76

Table 11. Comparison of storage modulus of PCL and its blend at 30, 40
and 50°C ............................................................................ 78

Table 12. Comparison of properties between 50% PCL based blend and
60% PCL based blend ......................................................... 85

LIST OF FIGURES

Figure 1. Life cycle representation of biodegradable polymers ..................... 2
Figure 2. Representation of corn use in food and Industrial field in 2004 ....... 5
Figure 3. Structure and composition of corn kernel ................................... 6
Figure 4. Flow diagram for production of ethanol by wet milling process ........ 8
Figure 5. Flow diagram for production of ethanol by dry milling process ........ 10

Figure 6. Schematic representation of biopolymers based on their origin and

method of production ............................................................ 16
Figure 7. Schematic representation of biodegradable polyester

family ................................................................................. 20
Figure 8. Structure of Protein showing various side chains available for

Chemical modification ........ j ................................................... 24
Figure 9. Perfect linear relationship of T9 with polymer ratio ....................... 49
Figure 10. Non-perfect linear relationship of T9 with polymer ...................... 49
Figure 11. Schematic representation of miscible and immiscible regions in

polymer blend in certain proportions ......................................... 50
Figure 12. Chemical structure of Poly(e-caprolactone) .............................. 54
Figure 13. Chemical structure of Guanidine Hydrochloride ........................ 55

Figure 14. Schematic representation of sequence for mixing, blending and
preparing samples for testing .................................................. 57

Figure 15. Schematic representation of operation process of Nitrogen
Content Analyzer Instrument ................................................... 60

Figure 16. Nitrogen Content Analyzer (Model FP-428, LECO® Corporation). 61
Figure 17. Dynamic Mechanic Analyzer 0800, TA Instrument ..................... 62
Figure 18. Thermogravimetric Analyzer 2950, TA Instrument ...................... 63
Figure 19. Percentage elongation of PCL blends with GHCI modified CGMP

processed at 150’C and different processing speeds of 75 and
1 50rpm .............................................................................. 68

vi

Figure 20. IR Spectroscopy of CGMP before and after chemical modification 69

with GHCI ...........................................................................
Figure 21. Effect of GHCI on tensile strength of PCL blends ....................... 73
Figure 22. Effect of GHCI on percent elongation of PCL blends ................... 74
Figure 23. Stress-strain curves for blends with and without GHCI ................ 75
Figure 24. Comparison of percentage elongation and impact strength of

PCL, PCL blends and HDPE. ................................................ 77
Figure 25. Storage modulus of PCL and its blends .................................... 79
Figure 26. Thermal analysis of CGM and 006 by TGA ............................. 81
Figure 27. Thermal analysis of PCL and its blends with different percentage

of CGMP and GHCI .............................................................. 82
Figure 28. SEM micrographs of the blends ............................................. 84
Figure 29. Storage modulus of PCL and its blends .................................. 87
Figure 30. Chemical structure of urea. ................................................... 89

vii

Chapter 1

INTRODUCTION

Biodegradable polymers are getting increasing attention recently due to
their possible environmental benefits and societal attractions. Renewable
resource-based biodegradable materials can reduce the use of our depleting
petroleum reserves as well as can also reduce our dependence on foreign
countries for fossil fuels and thus enhancing our national security. A considerable
number of biodegradable polymers are available, derived from both natural and
synthetic sources. The growing concern regarding environment has driven an
urgent need to develop new biodegradable polymers with properties and cost
comparable to those of available synthetic polymers. The use of agricultural
products in plastic applications is considered an interesting way to utilize surplus
farm production and to develop non-food packaging applications‘. Plastics
accounts for 11.3% by weight of the total waste generation in United States2 and
the major portion of this waste comes from packaging. The use of biodegradable
polymers can reduce the waste disposal problems associated with traditional
petroleum derived plasticsa. The main interest in the use of biodegradable
materials is to develop a solution for growing waste disposal problem due to the
shortage of available landfill space and the need for environmentally responsible
use of resources‘. Biodegradable plastic can be a very good choice for
packaging of items used for a short period time and disposed as they can be

degraded in suitable environment unlike the conventional plastic, which persists

for many years after disposal. The use of biodegradable plastic can also avoid
the recycling of plastics, which are undesirable due to soiling by food or other

biological substances". The life cycle of biodegradable polymers is represented

 
    

 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

in Figure 16.
Sunfight
+ H20
Photosynthesis
Natural
Resources
Biodegradation
Innovative Technique
to Produce Polymer
Plastic Application
And Disposal
Polymer

   

Processing to
Produce Plastics

 

 

 

Figure 1: Life cycle representation of biodegradable polymers 5

Albertsson and Karlsson7 defined biodegradation as a process, which
involves an action of enzymes and/or chemical decomposition associated with
living organisms (like bacteria and fungi) and their secretion products. There can
be other abiotic reactions like photodegradation, oxidation and hydrolysis, which
may alter the biodegradation rate.

Biodegradable polymers are classified3' 8 as: biosynthetic, semi-
biosynthetic and chemosynthetic. The biosynthetic polymers obtained from
renewable resources are biodegradable but with varying rate and extent while
the semi-biosynthetic and chemosynthetic polymers may be biodegradable
based on their bonding, chemical structure and the degrading environment.

Biodegradable polymers like poly(lactic acid), polyhydroxybutyrate, etc.
are biopolymers made from renewable resources whereas poly(e-caprolactone),
poly(butylene succinate) etc. are biodegradable polymers made from fossil fuel
resources. Biodegradable polymers are gaining importance in recent years
because of increasing environmental consciousness 9.

An interesting source of producing thermoplastic materials are the ethanol
industries, which use corn as a source of raw material and convert the starch
present in corn into ethanol and gives corn gluten meal (CGM) and distiller’s
dried grain (DDG) as by-products. Such by-products contain substantial amount
of protein that can be converted into thermoplastic material. The use of corn in
ethanol industries has a remarkable trend. It is a good substitute to the ethanol
produced from non-renewable sources. The high production of ethanol in US.

has made ethanol the single most important and fastest growing value added

market for farmers, stimulating rural development, creating jobs and increasing
farm income. The use of ethanol can not only reduce our dependence on other
countries for oil but also can reduce ozone pollution and greenhouse gas-forming
emission”. The ethanol production from corn is done by two different processes
i.e. dry milling and wet milling. The major by-products obtained from wet milling
industries are CGM. The major by-products/co-products obtained from dry milling
industries are distiller’s dried grains with soluble (DDGS). These by-products
from corn based ethanol industries are mostly used for animal feeds. But
extensive research can find the value-added applications of such by-products in
making biodegradable plastics. In US, 26 million metric tons of plastic waste was
generated in year 2003". The development of biodegradable plastic from by-
products can help to reduce the waste generation. The high production of ethanol
and the value-added application from its by-products will benefit the corn growers
by providing new markets for corn and rural development opportunities for

grower’s communities as well as benefit the bio-products industry as a whole.

1.1 Production of Ethanol

The United States currently produces 4.2 billion gallons of ethanol in 94
different facilities with more 85% production based on corn‘°' 12. There are 28
facilities under construction and 7 under expansion, which adds up another billion
gallons to the total production capacity” ‘2. The other sources of ethanol include
milo, barley, sorgum, cheese whey, waste beer, wheat, etc.‘°. The use of ethanol

in gasoline helps to replace imported methyl tertiary butyl ether (MTBE) used as

oxygenate agent. A recent study in 2005 found that ethanol industry is boosting
the US. economy by creating more than 147,000 jobs, enhancing household
income by $4.4 billion and reducing the trade deficit by $5.1 billion”. In 2004“,
dry mill facilities accounted for 75% of US. ethanol production, and wet mills
25%. The ethanol dry mills produced approximately 7.3 million metric tons of
distiller grain. Ethanol wet mills produced 426,000 metric tons of corn gluten
meal”.
1.2 Production of corn in USA

The annual production of corn in US in 2004/05 was around 11.8 billion

bushels (1 bushel = 56 lbs), which is almost 42 percent of the global corn

Sweetners
Beverage 2.2%
1.3°/ ‘ Seed
Cereal/other , j o ‘ T 0 20/

Starch

1.8% u ' fiCorn Syrup
2.7%' ‘ ‘

5.2%

   
   

Export
18.5%

Ethanol l
11.7% 7 Feed/Residual
56.4%

Figure 2: Representation of com use in food and Industrial field in 200410
(Source: USDA, ERS, Feed Outlook, December 2004)

production and 90 percent of US feed grains production“. The corn is not only

used as a main energy ingredient in livestock feed but is also processed into a

number of food and industrial products including starch, sweeteners, corn oil and

ethanol. The proportion use of corn in food and industrial use is shown in Figure

21°. A record use of 11.8 billion bushels of corn toward the ethanol industry

indicates the importance of corn and the new value markets for corn growers‘o.

1.3 Constituents of corn

The corn kernel consists of four major components i.e. bran, gluten, starch

Figure 3: Structure and composition of corn keme

and germ. The bran is the outer covering of the corn kernel and is mostly the
fibrous part, the gluten consists of most of the protein, the starch is found at the
top, on the sides and in the middle of the kernel while the germ is at the center of

the kernel”. The composition of the corn kernel is shown in Figure 316.

 
 

 

 

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I 15

1.4 Ethanol Production Processes

The ethanol can be made from corn by using two different processes. The
two different processes are:

1. Wet milling

2. Dry milling

1.4.1 Wet Milling

This process starts with the cleaning of the corn grains and soaking or
steeping in dilute sulfur dioxide solution for a sufficient time of 30-50 hours in
order to separate the grains into its components. The steeped grains are
separated from the water and passes‘through grinding to separate the germ from
the rest of the grain, which is further used for corn oil extraction. The fibers can
be separated from the gluten and starch by the filtration and washing. These
fibers can be mixed with the steep water to get the gluten feed. The gluten and
starch is separated by centrifugation. The wet gluten can be dried to get the
gluten meal while starch can be fermented and distilled to get the ethanol“ 1°.

417,18

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The process initiates with the cleaning and grinding of the corn grains
followed by mixing with water and an enzyme and heating to high temperature to
liquefy the starch. The mixture is further cooled down and enzymes are added to
convert the starch into sugar and the sugar is fermented into ethanol and carbon
dioxide by addition of yeast. This process results in alcohol and some non-
fermented products, which are separated by distillation. The alcohol can be
removed from the top of the distillation column and the residue mash can be
received from the bottom of the column and can be centrifuged to obtain dry
distiller’s dried grain (DDG). The thin stillage is condensed by evaporation of
liquid and further mixed with DDG to get distiller’s dried grain with soluble
(DDGS). The schematic in Figure 5 17' ‘8, shows the steps of dry milling process.
The dry milling process yields in distiller’s dried grain (DDG) as its major by-

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1.5 By products

The by-product obtained from dry milling process is mainly the distiller’s
dried grain (DDG) while there is a variety of by-product obtained from the wet
milling process. The main by—products obtained from wet milling process are corn
gluten meal (CGM), corn gluten feed, corn oil (see Table 119' 2°). The by-products
like corn gluten meal and distiller’s dried grain, which contains considerable
percentage of thermoplastic protein, can be potential source of biodegradable
plastics. The thermoplastic nature of protein and its origin from renewable and
abundant resources has attracted the attention for packaging application due to
its biodegradability”.

 

 

 

 

 

 

 

Table 1: Average yield of ethanol and its by-product per bushel of com” 2°
Products Wet Milling Dry Milling
Ethanol 2.5 Gallons 2.7 Gallons
C02 18.0 lbs 18.0 lbs
Gluten Feed 13.5 lbs -
Gluten Meal 2.5 lbs -
Corn Oil 1.6 lbs -
DDG - 18.0 lbs

 

 

 

 

 

11

Table 2: The composition of corn gluten meal and distiller’s dried grain22'25

 

 

 

 

 

 

Component CGM DDG
Starch 20 -
Protein 65 27

Oil 4 13
Ash 1 4
Others* 10 56

 

 

 

 

 

*lncludes fibers, nonprotein nitrogen, soluble sugar and other by-product
of ethanol fermentation

Corn gluten meal (CGM) is a by-product obtained from the corn based
ethanol industries using a wet milling process that has been shown to exhibit
high thermoplasticityg. It mainly consists of proteins (60%) and has a high
percentage of hydrophobic amino acids (10% Ieucine) with the remaining
components mainly being moisture, fiber, and lipids”. The high demand for the
production of bio-ethanol, a better substitute for the ethanol produced from non-
renewable sources means increased generation of CGM. The current use of
CGM is mainly as animal feed but extensive research is directed towards finding
additional value-added applications for CGM. One of the growing avenues is in
the design and engineering of CGM-based biodegradable plastics to compete
with the petroleum based products on the basis of cost and performance.

Recently, several studies have been made on CGM based biodegradable

materials. Wu et al.26 have successfully extruded and compression molded

12

CGM/wood fiber composite into sheets and injection molded pots for agriculture.
CGM, glycerol, urea and an organic acid were used to prepare compression
molded thermoplastic by Nobuhiro et at”. Gioia et al.28 studied the interaction of

CGM with polar and amphiphilic plasticizers.

1.6 Poly (c-caprolactone) (PCL)

PCL, is an aliphatic polyester. It is one of the important biodegradable
polymers gaining interest due to its inherent biocompatibilitymz. It has
application in packaging, plant containers and medical devices“ 3‘. It is semi-
crystalline in nature with a melting point of ~60°C35 and a glass transition
temperature (T g) in the range of -65 to -60°C. PCL can be blended with a variety
of other polymers to improve their mechanical properties”. Blending a natural
polymer with polyester is one way to reduce cost and to improve the
biodegradability of the resulting polymeric blend37. Therefore, blending of a
petroleum-based biodegradable polymer (PCL) with a low cost natural
biodegradable material like CGM may provide an eco-friendly alternative
packaging material.

The uniqueness of the present investigation includes the plasticization
along with the destructurization of CGM that imparted a much improved
elongation to the blends. This newly developed green plastic shows certain
properties comparable with one specific HDPE grade and an added advantage of

biodegradability.

l3

The purpose of this research was to develop a biodegradable plastic by

blending a chemically modified and plasticized CGM with PCL. This research is

covered under the following guidelines-

Investigation of processing parameters.

Investigation of guanidine hydrochloride (GHCI) compatibility with
blending partners through characterization by Infra red spectroscopy
Investigation of therrno-mechanical properties of the blend by DMA
(Dynamic Mechanical Analyzer), TGA (Thermo-Gravimetric Analyzer)
and UTS (Universal Testing System).

Comparison of mechanical properties of the blend with a commercial
grade high density polyethylene (HDPE).

Study of blend morphology by SEM (Scanning electron microscopy)
Investigation of thermo-mechanical properties of blends with higher

PCL content

14

CHAPTER 2
Literature Review

2.1 Biodegradable Plastics

Weber et al.38

defined biobased food packaging materials as substances
derived from renewable sources that have food packaging applications. They
divided these biobased polymers into three categories based on their origin and

production (Figure 6)”.

Plastics offer enormous advantage for handling, distribution and storage of food
over other packaging materials. The plastics from renewable sources like starch
offer an alternative to the plastics from non-renewable sources. Initial hurdles
with starch like water resistance, performance and appearance were overcome
by reactive blending to give plastics with desired characteristics. The reactive
blending technique can help to convert the thermoplastic material into cast film
and other packaging containers like trays, yogurt pots, cartons and boxes using
the conventional processing methods. These materials are not just

biodegradable but totally decompostable39.

15

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The non-biodegradable plastics are considered to be strong, light weight,
inexpensive, easily processable, durable, good barriers and energy efficient in
comparable to metal or glass used for packaging but have issues with the
ultimate disposability. These plastics have irreversible nature since they cannot
be easily broken down by available natural elements such as composting to
become a part of the biological carbon cycle of our ecosystem. Therefore, it imay
be advantageous to introduce biodegradable plastics wherever possible like
single use disposable short-life packaging materials, service ware items (cups,
plates, cutlery), coating for paper and paper-board. The biodegradable materials
like PCL, Polylactic acid (PLA), poly(hydroxybutyrate—co-hydroxyvalerate),
thermoplastic starch, polyvinyl alcohOl and protein polymers, already introduced

in the market place show tremendous growth opportunity 4°.

Karlsson and Albertsson41 defined biodegradable polymers as polymers
which degrade as a result of the microbial and/or enzymatic action.
Biodegradable polymers are obtained from both natural and synthetic sources
with varying susceptibility to microbial and enzymatic degradation. The
biodegradation process may take years like for lignin to hours for proteins. The
authors mentioned the influence of structure, hydrophilicity, morphology, surface
area and additives used on the rate of biodegradation. They also claimed the
design of biodegradable polymers by choosing an appropriate molecular
architecture with predetermined degradation rates without losing important

polymeric properties. The natural polymers use either oxidation or hydrolysis as a

17

mode of degradation. The polymers that degrade by oxidation have a slower rate
than the polymers that degrades by hydrolysis. Some biodegradable polymers
have an induction time before degradation process, hence avoiding degradation

during process and use.

Polyesters lead the category of biodegradable plastics because of their
water induced degradable ester bonds“. The polyesters are derived from both
renewable and non-renewable sources and are both aromatic and aliphatic in
structure. The aromatic polyesters have excellent mechanical properties and are
resistant to microbial attack. The aliphatic polyesters are usually biodegradable.
The strength and mechanical properties of most aliphatic polyester are inferior to
the most aromatic polyesters. Aliphatic polyester can be blended with other
biodegradable materials like starch to make the process cost effective. Some of
the commercially available biodegradable polymers are“:

1) Polyhydroxyalkanoate (PHA) is aliphatic polyester synthesized by microbes
and is brittle in nature due to high crystallinity. The packaging application of PHA
is for injection molded bottles and plastic films.

2) Polylactic acid (PLA) is aliphatic polyester prepared by condensation of lactic
acid or by ring opening polymerization of the Iactide group. The applications are
thermoforrned products like food trays, containers. The material has potential to

replace polystyrene and PET in some applications.

18

3) Polycaprolactone (PCL) is synthetic aliphatic polyester prepared by ring
opening polymerization of caprolactone. The applications of PCL includes food
trays, loose fill and film bags.

4) Aliphatic-aromatic copolyester (AAC) is combination of aliphatic and aromatic
polyester to give good biodegradable and mechanical properties. The
mechanical properties of such biodegradable plastics can be compared with the
traditional non-biodegradable polymers like low density polyethylene (LDPE).

Ecoflex polymer from BASF is an example of aliphatic aromatic copolyester.

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2.2 Corn gluten meal and its based polymers

The extent and rate of biodegradation of biodegradable materials vary
from one material to another. So, the information and studies relating to
biodegradable materials are very helpful in designing the biodegradable
products. Imam et al.43 evaluated the biodegradability of corn gluten meal,
distiller’s grain, corn starch, corn zein and corn fibers under different
temperature, pH and moisture conditions. They observed that there was a
marked difference when the moisture content was raised from 30% to around
60% and there was 100% degradation observed for CGM and zein at 50-60%
moisture content in just 7 days. They observed a typical degradation behavior for
these materials under different pH’s. The CGM and zein were observed to
degrade completely under neutral conditions and the degradation was effectively
reduced to 60% under acidic and alkaline conditions while the starch degraded
40% under neutral and 60% under acidic and alkaline conditions in 20 days
period. This rate and extent of biodegradation was observed to increase with
temperature. They concluded the ideal conditions for composting of corn co-
products by degradation as 40°C temperature, 50-60% moisture content and
neutral pH. The CGM and zein were tested to have the best rate and extent of

degradation among all the co-products tested.

Jane et al.“4 investigated the properties of biodegradable plastics made

from starch, protein and mixture of starch and protein. They evaluated these

21

biodegradable plastics based on the starch from different sources, effect of
processing conditions, effect of cross-linking agent and the pH. They observed
the plastics with different kinds of starch produced plastic with different
properties; for example, plastics made from potato starch had significantly higher
water absorption in comparison to other starches. This was attributed to the
water affinity of the phosphate derivate present in higher percentage in potato
starch. They also observed that optimum moisture content is required during the
processing of starch-protein mixture to gelatinize starch (melt the starch
crystallites) and mold the plastics as the lower moisture content couldn’t
gelatinize the starch and the higher moisture content affects the physical
properties. They found a significant effect of cross-linking on the water absorption
and tensile properties. They explained the effect of pH depends on the isoelectric
points, which is the point when the protein has net electric zero charge and it
accounts to the least water solubility and highest physical strength. This

isoelectric point is characteristic to a protein type.

Proteins38 are random copolymers of amino acids and have numerous and
diverse side chains, which offer limitless opportunities to specifically design the
final polymer by suitable chemical modification. The origin of protein is from plant
and animal. The plant protein includes gluten and soy while the animal protein
includes casein, collagen and whey. The thermoplastic nature and excellent gas

barrier properties of protein makes it highly suitable as packaging material while

22

its hydrophilic nature requires some modification or protection like lamination
from moisture.

The main storage of protein in corn is gluten, which is rich is in disulfide
linkages and needs to be broken down for better processing. The gluten is
available in abundance at low price and therefore an important area of research
for edible films, adhesives and other thermoplastic applications. Zein is another
source of thermoplastic polymer obtained from purification of CGM, a by-product
from corn based wet-milling industries. It is currently used in food and
pharmaceutical coatings. Though the films made from zein are brittle, they can
be plasticized to make it flexible. These films have potential for edible and

biobased packaging 38.

23

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CGM is one of the major sources of zein. There are various extraction
techniques used to extract zein from CGM based on the required quality and
purity of zein (see Table 345'“). The other extraction sources of zein are com and
DDGS. It is a major storage of protein in corn and is insoluble in water except in
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tons per year from corn gluten meal due to their high cost. The development of
low cost manufacturing methods for zein can create new promising opportunities.
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binders, biodegradable plastics, edible moisture resistant coating for food

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29

An investigation on processing and properties of low cost corn gluten

meal/ wood fiber composite was made by Wu et al.26

. They mixed the corn gluten
meal and wood fiber in different composition and plasticized with glycerol,
distilled water and ethanol and extruded into pellets. They used these pellets to
form sheets by compression molding and injection molding to prepare low cost
seedling pots. These samples were studied for water resistance, thermal stability
and morphology. They reported that the mixture of zein and water was an
excellent plasticizer to disrupt the intermolecular interaction in zein and therefore
resulted in improvement of melt mobility. The melt viscosity increases with
increase in wood fiber and decreased water content, which led to a decrease in
melt mobility. The water content should be decreased to increase the processing
temperature and should be increased to increase the fiber content for injection
molding. Based on the thermal analysis results, they observed the optimum
processing temperature of 125-160°C. Based on the morphology results, they
reported that with higher content of wood fibers, the breaking occurred at the
fiber-matrix interface. They also reported the sheets and agricultural pots showed
medium water resistance and concluded that the biodegradable seedling pots
could have a good market potential.

Corradini et al.37

investigated the mechanical, thermal and morphological
properties of PCL/zein blends. They prepared blends of the corn derived zein
and PCL in different proportions of 100/0, 75/25, 50/50, 25/75, 0/100. They

added 5 wt% glycerol as plasticizer and prepared sheets by compression

30

molding. These blends were compared for thermal and mechanical properties
using therogravimetric analysis (T GA), dynamic mechanical thermal analysis
(DMTA) and universal testing machine. The blends were reported to have
reduced tensile strength and elongation at break but improved Young's modulus
compared to pure PCL. The increase in PCL content in the blend resulted in
improved thermal stability, while the increase in zein content resulted in improved
storage modulus. The morphologies of these blends were also studied using
SEM and reported to have two distinct phases for PCL and zein, indicating the
immiscibility of polymers while the fractured surface of the blend at 75% zein

indicate the weak interfacial adhesion between PCL and zein.

Padua et al.66 studied the properties of biodegradable plastic derived from
corn proteins. They prepared zein based resin by plasticizing zein with palmitic,
stearic, oleic and linoleic acids in different weight ratios and melting the mixtures
in microwave oven. They rolled the resulted mixtures into sheets and
replasticized by soaking in the respective plasticizers. These prepared samples
were tested for tensile strength, elongation, Young’s modulus and toughness
using tensile testing machine and water absorption using standard water
immersion test. The saturated fatty acids plasticized sheets produced rigid
sheets good enough for food application while the unsaturated fatty acids
plasticized sheets resulted into flexible and elastic sheets. They also observed

that there was considerable decrease in water absorption after the sheets were

31

coated with linseed oil. The authors proposed the use of these sheets for

packaging application

Chemical modification of zein was made by Wu et al.67 using
polycaprolactone (PCL) and hexamethylene diissocyanate (HDI). They first
synthesized isocyanate (NCO) terminated PCL prepolymer (PCLH) by reacting
hexamethylene diissocyanate (HDI) with PCL diol and than modified the zein by
PCL prepolymer. These zein based modified polymers containing different
content of PCLH, were converted into sheets by compression molding. These
samples were tested for mechanical and thermal properties. They observed the
increase in elongation at break by 15 times and reduction in strength at break by
2 times, when modified zein based polymer contained 10% of PCLH in
comparison to commercial zein. They also observed the flexibility of modified
zein based polymer improved dramatically with negligible reduction in strength on
increasing the PCHL content. Therefore, the zein can be modified by PCL to

improve its mechanical properties.

Wu et al.68 investigated the toughness and water resistance of zein based
polymers modified by using various content of polycaprolactone (PCL) I
hexamethylene diisocyanate (HDl) prepolymer. Dibutyl L-tartrate (DBT) was used
as a plasticizer. They prepared the sheet samples by compression molding and
studied for mechanical, water resistance, thermal properties and morphology.

The toughness of the samples improved significantly in presence of small

32

amount of PCLH due to the existence of micro-phase separation structure. The
sheets were reported to exhibit better mechanical properties when modified with
20-50% PCLH followed by immersion in water for 24 hours than when stored in
75% relative humidity, which is an indication for excellent water resistance. DBT
had better interaction with zein than PCLH and therefore acted as compatibilizer

for the zein matrix and PCLH component.

Lai et al.69 investigated and compared the properties of the zein films,
plasticized by oleic acid. They prepared samples by casting and stretching the
zein resin. They observed the films produced by cast process were translucent,
had glossy surface on one side and the dull on the other and could be bended
but fractured when folded. The films prepared by stretching were translucent to
transparent, smooth surface, flexible and creased when folded. They reported
that there was phase separation of oleic acid and zein in cast film while the
stretched film remained a homogenous mixture when heated. Therefore, films
drawn from resin had higher elongation and were tougher and more flexible than
cast films. The film morphology studied by SEM revealed that the stretched films
formed fibers and ribbon-like structure and hence had better orientation in
contrast to deposition of solids without distinguishable pattern in cast films. The
low glass transition temperature of — 94°C of stretched films can make them

flexible at frozen storage temperature.

33

Rakotonirainy et al.70

evaluated corn zein films as modified atmosphere
packaging for fresh broccoli. They had plasticized the zein film with oleic acid and
coated and/or laminated with tung oil and studied for 6 days for packaging of
fresh broccoli. These finished films were reported to have improved water vapor
and gas barrier. The tensile properties were reported to increase significantly for
the films coated and/or laminated with tung oil. The broccoli florets were reported
to maintain their firmness and color after 6 days of study. The zein film coated
and laminated was found to perform best for odor control and stability. The
uncoated and unlaminated zein films had poor dimensional stability and were

found to be soft and soggy after the study due to the low 002 permeability, which

establish high C02 atmosphere and hence retard respiration process.

Wang et al.71 investigated the water interaction of extruded zein films
through moisture sorption isotherm. They used a regular grade zein powder with
90% protein content. They plasticized zein with oleic acid and emulsified with
distilled monoglycerides to form a blown film on single screw extruder while zein
sheets were prepared by slit die single screw extruder without adding plasticizer.
They reported zein powder had high moisture content due to the presence of
multiple pores and capillaries that may hold water droplets. The zein powder
possess high moisture uptake, which induces protein swelling, exposing
previously occluded polar amino acid groups to water. They reported extruded
products had less moisture adsorption and moisture content than zein powder,

which was caused by modification of zein morphology after compression of zein

34

mass into compact solid, free from pores and capillaries. They observed moisture
adsorption of zein films plasticized with oleic acid was lower than zein sheets,
indicating that the incorporation of hydrophobic oleic acid prevents moisture
sorption.

Thermal behavior of corn zein was investigated by Magoshi et al.72 using
differential scanning calorimetery (DSC), thermogravimetery analysis (TGA), X-
ray differaction (XRD) and infrared spectroscopy (IR). They had first produced
zein by grounding corn meal followed by using different separation technique
based on precipitation and distillation. The zein was dissolved in aqueous
ethanol and casted into film on a glass plate. They concluded that casted zein
film is amorphous in the random-coil conformation and this amorphous zein
crystallizes to B-crystals at about 210°C accompanied by the random coil B-form
conformational transition. They found the glass transition temperature at 165°C
and thermal degradation of zein occurring at 320°C. The steam treatment of zein
film resulted in the conformational change to the a— and B-fonns, simultaneously,
irrespective of treating temperature.

Lai et al.73

investigated the structure of granular zein, zein fibers, zein-
oleic acid resin and zein-oleic acid film by wide angle x-ray scattering (WAXS)
and small angle x-ray scattering (SAXS). They observed WAXS patterns showed
the diffused rings, which is an indication of non-crystalline structure. The

interhelix spacing in zein-oleic acid resin and films was higher than granular zein

and zein fibers, which was due to the presence of oleic acid and was related to

35

the resin preparation process. The SAXS results showed that the periodicity was
observed for oleic acid based resin and film while no periodicity was observed for
granular zein and zein fibers. The biaxially drawn resin films showed different
periodicity than the cast film, which indicate that the film morphology was
affected by forming process.

Sessa et al.74

investigated the enhancement of metal-binding properties of
distiller’s dried grain (DDG) and corn gluten meal (CGM) by generating acid
stable products on reaction with citric acid. Citric acid dehydrated at higher
temperature can formed anhydride that can interact with functional groups in the
protein of DDG and CGM to form ester and carboxyl linkages, which was
confirmed by FTIR studies. The solid state NMR analysis supported the metal
binding characteristic of citric acid reaction products and demonstrated that Al3+
was bound ionically to carboxyl group. The authors claimed that these biobased

products possess cation-exchange capability and potential biodegradability that

may be a channel to industrial wastewater treatment.

Tensile properties of the film extruded from blends of corn zein or corn
gluten meal with low density polyethylene (LDPE) was investigated by Herald et
al.75 The film samples were prepared by melt extrusion and tested for tensile
strength, percentage elongation and modulus at different composition of zein or
corn gluten meal with LDPE. The authors reported a decrease in all tensile

properties tested. They also added that the film containing zein has better tensile

36

properties than the film containing CGM, which may be contributed due to the
better compatibility of zein with PE than CGM.

Parris76 studied the characteristic properties of zein films extracted from
dry milled corn and corn gluten meal. They reported that the insoluble particles,
identified as protein aggregates were formed during extraction of zein from dry
milled corn while these insoluble aggregates were not observed in the zein
extracted from corn gluten meal. On further investigation using polyacrylamide
gel electrophoresis, they reported that B- and y—zeins were detected in dry milled
corn while only a trace of [El-zein were found in corn gluten meal. They reported
when the dry milled corn was pretreated, it resulted into higher solubility due to
the cleavage of disulphide linkages cf the B- and y-zeins and hence improvement

in tensile properties.

Gioia et al.77 investigated the effect of some polar and amphiphilic
plasticizer on the properties of corn based resins. They selected glycerol and
water as polar plasticizers and octanoic and palmitic acids, dibutyl tartrate and
phthalate, and diacetyl tartaric acid ester of mono-diglycerides (DATEM) as
amphiphilic plasticizers. The blending of corn gluten meal and plasticizers was
made by hot mixing process and the samples were prepared by compression
molding. They reported that the plasticizing efficiency at equal molar content was
proportional to the percent of hydrophilic groups of the plasticizer. They have
reported that both the family of plasticizer interact differently to different parts of

protein and hence gives different outcomes. They also mentioned that the polar

37

plasticizer interacts well with the accessible polar amino acids whereas
amphiphilic plasticizer interacts well with the non—polar zones. The octanoic acid
was reported to be a promising plasticizer for corn protein in terms of volatility,
water resistance and extrudation. The authors claim that this new low cost
biomaterial could be used for applications in the field of agriculture, packaging

and compostable short life items.

Tilekeratne et al.78 had modified corn zein film by incorporating plasticizer
of two different molar masses. (i.e. 400 and 1000 g/mole) and investigated the
thermal and mechanical properties using differential scanning calorimetry (DSC),
dynamic mechanical analysis (DMA), tensile strength, moisture absorption
isotherms and water vapor transmission rate measurements. They had
concluded that the polyethylene glycol (PEG) is an effective plasticizer for the
corn zein film. The incorporation of PEG up to 30 wt% substantially enhances the
tensile strength and its resistance to water vapor transmission into the protein
film. The PEG400 had higher hydrophilicity group density than PEG1000, which
result in higher rate of water absorption. They concluded that the PEG acts by
reducing the intermolecular forces with in the protein chains. The higher
molecular mass PEG was found to be a more effective plasticizer than the lower

molecular mass PEG.

Effect of gamma-irradiation on the physiochemical properties of zein film

was investigated by Lee et al.79. In their study they irradiated 10% zein solution in

38

ethanol to gamma radiation and reported the change in the structure of zein
molecules and also reported the cross-linking, degradation and aggregation of
the polypeptide chains. They also reported that gamma irradiation also causes
the increase in solubility of zein and decrease in viscosity of zein solution due to
the cleavage of peptide chains. They also observed the decrease in water vapor

permeability and increase in elongation.

Ha et al.8° had proposed the use of zein sheets plasticized by fatty acids
for various food industry and food service packaging applications by use of
extrusion technology. They had plasticized the zein by fatty acids mixture
comprising linoleic, linolenic, streaic, oleic and palmitic acid in aqueous ethanol
solution. The solid zein resin was recovered and melt extruded in a twin screw
extruder and rolled into sheets. The effect of extrusion temperature and ethanol
added during extrusion were studied by evaluating their mechanical and water
resistance properties. The high extrusion temperature resulted in the increase in
tensile strength and decrease in elongation. The plasticized zein sheets showed

good storage stability for 3 months of study.

Takahashi et al.81 designed a process for preparing zein based water
resistant film. They dispersed the zein in 65-70 volume% aqueous solution of
acetone and converted them into film by drying at humidity less than 85% and
initial drying temperature less than 55°C. The authors reported that these

dispersions can also impart water resistance to the articles with poor water

39

resistance by coating them and drying them at humidity less than 85% and initial
drying temperature less than 55°C. The authors claimed that these film or coating
neither dissolve nor becomes weak when in contact with water.

Trezza et al.82

studied the water vapor and oxygen barrier properties of
corn zein coated paper. They reported the water vapor transmission rates
(WVT R) were 70-131 g/100 inzlday for paper coated with 5-15 lblream corn zein
but the addition of plasticizer increased the WVTR for a 10 lblream paper
coating. The oxygen transmission rates for coating levels of 5, 10 and 15 lblream
were observed to be 95,000, 35,000, and 16,000 cm3/100 inzlday respectively.
This improved oxygen barrier may be attributed to good film flowing properties of

solution into the paper matrix. They claimed this paper has potential to be used in

packaging for improved barrier properties.

Lawton 83 investigated different plasticizers for zein and their effects on
tensile properties and water absorption. It is reported that cast zein films are
brittle at room temperature so the plasticizers are added to make them flexible.
The cast films were prepared using various plasticizers as glycerol, triethylene
glycol, dibutyl tartarate, levulinic acid, polyethylene glycol and oleic acid. These
film samples were prepared and stored for one week at different RH. He
observed that the tensile strength and young modulus decreased with increased
RH. The films with glycerol and oleic acid as plasticizer did not show any effect

on elongation with increase in RH while the other plasticizers showed the

40

increase in elongation with increase in RH. The author added that film with
glycerol absorbed the most amount of water while the film with oleic acid
absorbed the least therefore the film with hygroscopic plasticizer are weak and
the film with hydrophobic plasticizer have good mechanical properties at high

RH.

Yoshino et al.84 investigated the influence of preparation conditions on the
physical properties of zein films. They made the zein film under various
controlled drying conditions and tested for tensile strength, puncture strength,
gas and water vapor permeability. The author reported a variation in the results
in the tensile properties and concluded that these properties are very much
dependent on the drying conditions of the film during preparation. The water
vapor permeability was also reported to be different on both sides of zein film at

high humidity.

Dejing et al85 studied the properties, preparations and applications of zein,
a water insoluble protein, obtained from corn. As reported, zein has excellent
antioxidant properties for lipids and good film forming properties therefore it is a
potential material for biodegradable and edible packaging. The authors reviewed
the different characteristics and properties of zein and their possible end uses.
They concluded that although zein has unique characteristics but its expensive

cost prohibits its viable use in competition with petroleum plastics.

41

Lawton et al.86 explained that the corn gluten meal is a co-product of corn
wet milling and contains 60-70% protein and the remainder is starch and fibers.
The zein is a prolamine protein and had shown the thermoplastic properties
under perfect conditions. They concluded that research is being conducted to
identify the appropriate conditions, which may result into a thermoplastic melt.
They also added that water is an important plasticizer for corn gluten meal when

used in combination with other plasticizer.

Effect of UV light on the physical properties of corn zein was investigated
by Rhim et al.87. They exposed the films to 51.7 .l/m2 uv light for 24 hours and
evaluated tensile strength, water vapor permeability, soluble matter and color.
They observed that the tensile strength increased on UV exposure due to cross-
Iinking while there was no effect on water vapor permeability. The UV light
causes the destruction of zein pigments, resulting in fading out the yellow color of
zein film. They concluded that the UV light could be used to modify zein film

properties.

Esposito88 reported that scientists at Illinois University claimed that a by-
product from corn based ethanol industry could help to develop environmental
friendly plastics, which decays naturally in the soil. They said the zein extracted
from corn gluten meal can be plasticized with fatty acid to improve its properties.
They observed that these plastics could be further coated with hot flax oil to

make strong and waterproof sheets, which can be molded to sandwich boxes,

42

plates and food trays. But to make it commercially viable, cheaper zein extraction

techniques need to be developed in order to lower the cost of zein.

Wang et al.89

investigated the effect of processing methods on the thermal
behavior of zein film. They prepared a solution by dissolving the zein in 75%
ethanol at 60°C and gradually adding 41% by weight of oleic acid. The resin was
prepared by precipitating the zein-oleic solution, which was further extruded in
single screw and twin screw to form films. The cast films were prepared by
pouring the solution on flat surface and cooling at room temperature. Non-
extruded resin films were also prepared by stretching the resin for testing. The
DSC thermograms showed large melting peak for cast films while there was no
peak for the film samples prepared by extrusion or heat treatment. They
concluded an enhanced interaction between oleic acid and zein under heat
treatment diminishes the peak. A low glass transition temperature T9 of -80"C
was observed for the films due the plasticization effect of oleic acid.

Wang et al.90

plasticized the zein with oleic acid and studied the tensile
properties of extruded zein sheets and blown films. The authors analyzed the
effect of hot rolling on the properties of the extrudate. They also studied the
influence of moisture content and the barrel temperature on the extruded films.
They observed that both single and twin screw extruded sheets showed higher

elongation, lower tensile strength and lower modulus than the non-extruded

samples. They also observed a difference in the tensile strength and elongation

43

in the samples prepared from single and twin screw extruder, which was
attributed to the voids created in the single screw extruded samples as observed
by SEM. The blown film was observed to be affected by feed moisture content
and the barrel temperatures. The optimum moisture content was determined to
be 14-15% while the temperature was 20-25, 20-25 and 35°C for three extruders

zones respectively and the temperature of the blowing head was 45°C.

44

2.3 Chemical modification of protein

Kumar et al.91 studied the effect of guanidine hydrochloride (GHCI) as a
denaturant to protein structure. GHCI cross link in different parts of protein due to
the noncovalent interaction between protein groups and itself. This results into a
stabilized form, which is dynamically constrained and lower in entropy. But if the
concentration of GHCI is increased beyond certain limit, it leads to the unfolding
of protein structure. The cross-linking is supposed to restrict the freedom of the
molecules and hence producing stiffening in the protein structure.

Dunbar et al.92

studied the effect of denaturant on protein. They claimed
all the protein folding reactions are either aided by heat, acid or chemical
denaturant. High concentration of denaturants is required in order to alter the
protein structure and the surrounding solvent. These changes in the structure
can be studied by X-ray crystallography. The denaturants like guanidine
hydrochloride and urea form hydrogen bonding with the protein structure
therefore they act as non-covalent cross-linking agent. These denaturants
interact with both the protein side chain and the backbone. The actual position of

these non-covalent cross-linking is not known specifically. Guanidine

hydrochloride and urea have similar effects on the native structure of protein.

Stability of protein at different concentration of guanidine hydrochloride

and urea was investigated by Bhuyanga. They added that the guanidine

45

hydrochloride and urea stabilize the protein at low concentration and this stability
increases with the increase in concentration of denaturant to a limit of its
subdenaturing effect. The further increase in concentration of denaturant causes
the protein unfolding effect. The author explained that the stability at lower
concentration of the denaturant is entropic due to the non-covalent interaction

between the denaturant molecules and the protein structure.

Monera et al.9“ studied whether guanidine hydrochloride and urea have
the same effect on stability of a particular protein. The process of folding and the
resulting structure has been long and difficult task due to the complex structure of
native protein and its denaturation process. They designed four coiled-coil
analogs and studied the intra-chain and inter-chain electrostatic interactions.
They concluded that guanidine hydrochloride does not contribute to the
electrostatic interaction at low concentration due to its ionic property, which might
mask the positively and negatively charged amino acid side chains. But at the
higher concentrations, guanidine hydrochloride becomes denaturant irrespective
of the types of the electrostatic interactions present in the protein while urea,
being an uncharged molecule, has no significant electrostatic effect and the
denaturing action of urea is based on the ability to bind the protein. Therefore,
guanidine hydrochloride and urea may give totally different estimate of protein
stability.

L95

Mayr et a studied the effect of guanidinium chloride on the stability of

protein structure. They have addressed that the guanidinium chloride has a dual

46

effect on the stability and folding of protein structure. At low concentration it acts
as stabilizer and stabilizes the protein structure while at high concentration the
denaturant character dominates and they unfold the protein structure. This is
unlike urea where only unfolding takes place. They suggested this behavior of
guanidinium chloride at low concentration being due to the binding of guanidine

ions with other cationic sites in the native protein structure.

47

2.4 Polymer blending

Polymer blending is an economical technique to modify the properties of
the polymers. Blending of polymers is very complex and unlike the low molecular
liquids where the compatibility is only controlled by cohesive energy density and
polarity. Blending of polymer has many other additional factors controlling the
nature of compatibility because of the large molecules. Blending has great
importance in using the scrap or defective material produced during
manufacturing and hence producing new material without adversely affecting the
properties of material. Blending resulting in immiscible blends also have great
relevance. For example the immiscibility of polybutene with LLDPE or LDPE may

help to produce peel heat seal Iayersge.

Blending is really useful to incorporate the properties from two different
polymers into the resulting polymer. If the polymer blends are miscible than the
resulting blend will have the properties somewhere in between the unblended
polymers. In miscible blends the properties of resulting blend is generally linear
but may be sometimes higher or lower depending on the degree of binding forces
between the polymer is higher or lower than the polymer themselves97 (See

Figures 9 and 10)”.

48

Tg I , Tg

of polymer B
Tg ,

of polymer A

 

 

 

 

V

% polymer B

Figure 9: Perfect linear relationship of T9 with polymer ratio”

T8 I ..................... . : Tg
"""" of polymer B

T3 ,5 .........................
of polymer A

 

 

 

 

V

% polymer B

Figure 10: Non-perfect linear relationship of T9 with polymer ratio”

49

Some polymers form miscible blends only in certain proportions and
immiscible in the other proportions, which is due to the lower free energy in the
miscible regions than the immiscible regions (see Figure 1197). These miscibility
regions may change with temperature. If they disappear at some higher
temperature, then the temperature is called as upper critical solution temperature
and if disappears at lower temperature then called as lower critical solution

temperature”.

 

lllliscible Immlscible Misclble

 

 

 

 

 

0 30 70 100
% polymer B ‘

 

V

Figure 1 1: Schematic representation of miscible and immiscible
regions in polymer blend in certain proportions97

In the immiscible blends the polymers do not have a continuous phase
and tend to separate from each other. lmmiscible blends sometimes result in
enhancement of properties. For example, the immiscible blend of polystyrene
and polybutadiene results into HIPS, a tougher and more ductile material.
Compatibilizers are sometimes used to enhance the properties of the immiscible
blends. They help in binding the two immiscible phases to give a synergistic

effects".

50

Poly(a-caprolactone) (PCL), cellulose acetate (CA) and their blends were
analyzed by Braganca et al.98 for their thermal, mechanical and morphological
properties. They studied the compatibility of the blend by using optical
microscope, which showed the separate phases indicating the incompatibility of
the blend. The characteristic melting temperature peaks for PCL and CA also
indicated the incompatibility in the blend. Tensile properties decreased on
blending at different composition of PCL and CA except for the blend 20/80
(PCL/CA), where the tensile strength was higher than PCL. This was supported
by the prevalent nature of CA at this blending ratio. The authors claimed that the
lower cost of this polymer blend than PCL makes it more attractive along with its

favorable biodegradation properties due to the weak interactions.

The moisture sensitivity, critical ageing, dimensional stability and
resilience of thermoplastic starch can be improved by blending it with PCL
maintaining its biodegradable properties. Averous et al."9 studied the different
compositions of PCL and thermoplastic starch with different proportion of
glycerol. They extruded the samples by melt blending and injection molding to
study the tensile, thermal and therrno-mechanical and hydrophobic properties.
Although the phase separation was observed but these blends have improved

properties in contrast to thermoplastic starch.

The biodegradability of poly(e-caprolactone)lstarch blends and composites

L100

was studied by Singh et a under composting and culture environment. They

51

prepared four types of PCL-starch compositions as “PCL-granular starch blends”,
“hydrophobic coating of starch granules and melt blending with PCL”, “PCL-
grafted dextran copolymer and used as a compatibilizer in PCL-granular starch
blends” and “in-situ PCL grafting onto starch granules and melt blending with
PCL”. They evaluated the samples based on the weight loss on composting and
surface morphology by SEM. They concluded that the biodegradability of PCL
was increased with addition of the “PCL-grafted dextran copolymer” as
compatibilizing agent and further increased with the addition of starch content.
They also added that the grafting of PCL onto starch granules affected the

degradation process.

Voigt et al.101 designed a biodegradable thermoplastic material by
blending starch esters and polyalkylene glycols with addition of polybasic
aliphatic carboxylic acid. Blending with aliphatic polycarbonates modified the
mechanical and barrier properties. These thermoplastic materials were
processed by premixing and extruding into granules followed by injection molding
or extrusion into films as required. These thermoplastic biodegradable materials
can be used in packaging of food, cosmetics, detergent, paper tissues and toilet
articles. These thermoplastic materials were claimed to feed into special
recycling process or may be conventional recycling process without adversely

affecting their properties.

52

Lepoittevin et al.102

studied the mechanical, thermal and rheological
properties of PCL nanocomposites, prepared by melt blending with natural and
modified montmorillonite clay. They prepared the blends with varied percentage
of clay and tested for tensile, Izod impact and thermal properties. The stiffness

and the thermal properties were observed to increase up to 5% clay. The further

addition of clay resulted in decrease of these properties.

53

CHAPTER 3

MATERIAL AND METHODS

3.1 Materials

Commercial corn gluten meal (CGM), a by—product from corn based
ethanol industries with a protein content of (~60%) was supplied by Cargill lnc.,
MN. Distiller’s dried grain with solubles (DDGS) was supplied by Michigan
ethanol, Caro,MI.

A commercial grade PCL (TONE® 787) was obtained from Union Carbide
Corporation. The chemical structure of PCL is represented in Figure 12. It is a
homopolymer made by ring opening polymerization of s-Caprolactone, a seven
member ring compound. It has density of 1.145 glcc. It is a semi crystalline
polymer with low sharp melting temperature (T m) of 60°C and has the glass
transition temperature (T9) range of — 65 to — 60°C. It has melt flow index (MFI) of
0.5 - 4 (g/10min). The pellets were sufficiently dried in the vacuum oven prior to
process. The important features of this grade are its biodegradability, low and
sharp melting point, outstanding adhesion to a broad spectrum of substrates,
broad miscibility with many polymers, pigments and fillers, mechanical
. ii ,
—— [CH2 ]— C —O__

5
K Jn

 

 

Figure 12 : Chemical structure of Poly(£-caprolactone)

54

compatibility with many polymers and its non-toxic naturem.

Glycerol (99.9 % purity) ACS grade from J.T.Baker and ethanol 190 proof
ACS grade from Phannco was used. Guanidine hydrochloride was supplied by
Acros Organic, NJ. The chemical structure of guanidine hydrochloride is shown

in Figure 13.

NH

ll
NHz—C —NH2 .HCI

Figure 13: Chemical structure of Guanidine Hydrochloride

3.2 Preparation of blends

A micro scale extruder with injection molder (DSM) (DSM Research,
Netherlands) was used for blending. The blending was accomplished by
plasticizing CGM with glycerol and ethanol (denoted as plasticized CGM or
CGMP) followed by chemical modification with guanidine hydrochloride (GHCI).
The CGMP was made by weighing the CGM and glycerol/ethanol separately and
the adding glycerol/ethanol slowly into the CGM while mixing to ensure a uniform
distribution. The specified ratio of CGMP and GHCI were weighed and melt
blended using the DSM. This GHCI modified CGMP was dried at 80°C for 3 hrs
in a vacuum oven before blending with PCL. Injection molded test samples were

prepared for dynamic mechanical analysis (DMA) and tensile testing. The

55

schematic representation for mixing, blending and sample preparation is shown
in Figure 14. The different blending compositions were made as shown in Table

4.

Table 4: Composition of different blends considered for study.

 

 

 

 

 

 

 

 

 

 

 

 

Blends CGMP GHCI PCL
1 45 5 50
2 42.5 7.5 50
3 37.5 12.5 50
4 50 0 50
5 34 ‘ 6 60
Note:

CGMP- Plasticized corn gluten meal
GHCl- Guanidine hydrochloride

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3.2.1 DSM

DSM is a micro compounding instrument consists of twin vertical screws,
with a length of 150 mm, L/D of 18 and a maximum capacity of 15 cc. It has co-
rotating screws, which blend and circulate the material inside the barrel and then
the material is extruded out from the bottom. The extruded material from the
blend can be injection molded at a specific temperature and pressure into molds
designed to prepare samples for DMA, tensile and izod testing. The samples
were sealed in airtight bags to avoid any moisture contact and were stored at

room temperature for at least 24 hours for conditioning before testing.

3.3 Characterization

3.3.1 Infrared Spectroscopy: The infrared spectrum was measured using a
FT-IR spectrophotometer (Spectrum 2000, PerkinElmer, MA). An ATR
attachment was used to measure the lR-spectrum for the CGMP before and after
chemical modification in order to observe the chemical changes that occurred in
the structure. The samples were analyzed in the range of 4000-650 cm". The IR-

spectrum of GHCI was also obtained.

3.3.2 Moisture Content : The moisture content of CGM and DDG was measured
by using a standard AACC 44-15A, where a 2-3g sample was placed in oven at

103 i 1 °C for 72 hours. The samples were placed in the desicator to cool down

58

after heating to prevent absorption of any moisture. The difference in weight was

calculated for moisture content (wet basis) determination.

3.3.3 Nitrogen Content Analyzer: The nitrogen content was measured by using
Nitogen Content Analyzer (Model 428, LECO Corp., MI). This instrument detects
the nitrogen percent by measuring the thermal conductivity of the evolved
nitrogen gas for a given weight of sample. A blank run was made initially made

by purging 02 (99.9%) in order to remove nitrogen from the furnace space. The

schematic representation of operation process of nitrogen content analyzer is in
Figure 15. The instrument is than calibrated before starting the actual samples.
A standard factor104 of 5.7 used to cOnvert the percentage of nitrogen into

percentage of protein. The picture of the instrument is given in Figure 16.

59

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3.3.4 Dynamic Mechanical Analyzer (DMA): The storage modulus of the blend
was measured using a Model Q800 DMA from TA Instruments (TA, Wilmington,
DE), using a single cantilever clamp and multi frequency strain mode. The
samples were brought to equilibrium temperature at 30° for 15 minutes. The
measurement was made at 1Hz and 15pm amplitude over the temperature range
of 30 to 60°C at the heating rate of 2°C/min. The picture of the instrument is

given in Figure 17.

 

Figure 17: Dynamic Mechanic Analyzer, Q800, TA Instruments
(Courtsey: AKM Research Group)

3.3.5 Tensile Testing: The tensile testing was performed at Universal testing
machine (UTS, Model SFM-20) according to ASTM D638. The tensile samples
were made by injection molding immediately after extrusion from the DSM. A
load cell of 1000 lbs capacity was used for the testing, with a cross-head speed

of 2 in/min for tensile tests. Five samples were tested for evaluation.

62

3.3.6 Thermal Gravimetric Analyzer: The thermal stability was measured by

using thermal gravimetric analyzer 2950 series from TA Instruments (TA,

 

Figure 18: Thermogravimetric Analyzer 2950, TA Instruments.
Coursesy: AKM Research Group

Wilmington, DE). The measurement was done in nitrogen environment. An auto
sampler was used to test a series of samples and an equilibrium temperature
was achieved before the test of every sample. The samples were subjected to a
temperature ramp of 10°Cl minutes to 400°C. The picture of the instrument is

given in Figure 18.
3.3.7 IZOD Impact Tester: The samples were notched using a TMI‘m notching
cutter (model TMI 22-05, Testing Machine Inc., Amityville, NY) and the impact

strength was measured using a TM|® IZOD impact strength tester (model 43-02).

63

A 5 lb pendulum was used for impact measurement. ASTM D 256 was followed
for this testing. The samples were notched and conditioned for 48 hrs at 23°C

and 50% relative humidity. Five samples were tested for evaluation.

3.3.8 Scanning Electron Microscopy (SEM): The fractured surfaces were

coated with gold thin films and examined in a JEOL 6300 Field Emission

Scanning Electron Microscope (FESEM) at an accelerating voltage of 10 kV.

64

CHAPTER 4

Results and Discussion

Distiller’s dried grain (DDG) and corn gluten meal (CGM) are the by-
products obtained from different types of ethanol industries. CGM, which
contains a higher percentage of protein than DDG, is an important source for
development of biodegradable polymers. Zein, an alcohol soluble protein can be
extracted from CGM through solvent extraction. Polymers derived purely from
zein are very expensive and have found limited applications”? Therefore, this
research examined the use of CGM without any purification in order to make a
cost effective CGM based plastic.

The brittle nature of the corn protein based materials necessitates the use
of plasticizers. Glycerol has been extensively studied as a plasticizerze' "’6. The
plasticization effect of glycerol is attributed to its small size which helps in its

insertion and positioning within the protein network‘°7' 1°“

and therby reducing the
intermolecular forces and increasing the mobility of protein chains mg. The use of
ethanol aids the processing of CGMZS. The glycerol/ethanol (3:1) mixture was
used for better plasticization of CGM system. The GHCI was used to breakdown
the protein structure (destructurize) in order to make it more flexible and
compatible with the other components of the polymer system.

Various studies have been done on the interaction of GHCI with proteins,

which revealed the induction of denaturation and unfolding of the protein

structure“‘“12. Denaturation is the alteration of secondary, tertiary or quaternary

65

structures in the protein molecules. The GHCI forms cross-link at different
locations in the side chains and on the backbone of the protein molecule, by
means of hydrogen bondingm’. This phenomenon is likely responsible for the
improvement of mechanical properties of the protein blends.

The processing conditions, i.e. the temperature and processing speed
play an important role in determining the final properties of the plasticized
cellulose acetate as explained by Mohanty et al.‘“. To analyze the effect of
processing conditions, a blend containing PCL:CGMP:GHCl at the ratio of
50:37.5:12.5 on a weight basis were processed at 120°C and 150°C. The
mechanical properties (modulus) of the resulting blends were evaluated using
DMA. Although the average modulii of the blends was comparable, the material
processed at 120°C had a much larger standard deviations versus the material
processed at 150°C (Table 5).

Table 5: Modulus of PCL blends with GHCI modified CGMP processed at 150

rpm and temperatures of 120°C and 150°C at the ratio of PCL:CGMP:GHCI =
50:37. 5:125 (wt%)

 

 

 

 

Processing Storage Modulus (MPa) at 30, 40 8: 50°C
Temperature, °C
30°C 40°C 50°C
120 88 1 10.77 75 :l: 9.43 49 :l: 5.52
150 87 :l: 3.17 74 a: 2.57 48 :t 1.73

 

 

 

 

 

 

Note: GHCI is guanidine hydrochloride and CGMP is the plasticized corn gluten meal

The consistent modulus obtained at the higher processing temperature is

based on the fact that a processing temperature allowed better melt mixing of the

66

components. Based on these results, a processing temperature of 150°C was

used versus 120°C in the subsequent studies.

In order to study the effect of extruder screw speeds on blend properties,
the blends were prepared at 150°C using two different screw speeds. Speeds of
75 and 150 rotations per minute (rpm) were used during processing. The
modulus of the blend was reduced by about 50% with an increase in processing
speed (Table 6). The material blended at 75 rpm had large standard deviations in
its modulus. The material blended at 150 rpm exhibited a much improved percent
elongation (Figure 19) in contrast to their counterparts blended at 75 rpm. Based
on such results a processing speed of 150 rpm was chosen for further studies.

Such observations were attributed to inconsistent mixing at lower screw speeds.

Table 6: Modulus of PCL blends with GHCI modified CGMP processed at 150°C
and different processing speeds of 75 and 150 rpm at the ratio of
PCL:CGMP:GHCI = 50:37. 5:125 (wt%)

 

 

 

 

Processing Storage Modulus (MPa) at 30, 40 8: 50°C
Speed' '9'" 30°C 40°C 50°C
75 170 :l: 31.64 148 1: 29.27 105 :l: 22.28
150 87 t 3.17 74 :l: 2.57 48 11.73

 

 

 

 

 

 

Note: GHCI is guanidine hydrochloride and CGMP is the plasticized corn gluten meal

67

 

450 1

% elongation

 

 

rotation per minute

 

 

 

Figure 19: Percentage elongation of PCL blends with modified CGMP
processed at 150°C and different processing speeds of 75
and 150 rpm.
Note: GHCI is guanidine hydrochloride and CGMP is the plasticized corn gluten
meal
Through IR spectra, the GHCI modified corn gluten meal based bioplastics
were characterized. A peak shift was observed in GHCI modified CGMP
bioplastic at 1651 cm'1 (Figure 20) in contrast to CGMP bioplastic (e.g.
unmodified CGMP) at 1624 cm“. This peak shift is attributed to hydrogen

bondingm. Research by Dunbar et al112 also supports the formation of hydrogen

bonds during the interaction of GHCI with protein.

68

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The protein percentage was determined for CGM and DDG. A standard
factor 5.7 was used to convert the nitrogen percentage to protein percentage.

The protein percentage in CGM and DDG were 60 and 25.4% respectively (see

Table 7)

Table 7: Protein percentage determined by Nitrogen Content Analyzer

 

 

 

Instrument
% % % 0/
Material Nitrogen Nitrogen Nitrogen Protein
sample# 1 sample# 2 (Average)
CGM 10.55 10.51 10.53 60.00
06 4.51 4.40 4.45 25.40

 

 

 

 

 

 

 

The water also acts as a plasticizer for CGM during the processing”. So
the presence of moisture will assist in the processing. The moisture content in
CGM and DDG measured by AACC 44-15A method was calculated as 7.9 and

11.75% respectively (wet basis).

Blends of PCL and zein have shown poor mechanical properties in
previous research”. These results were attributed to incompatibility between the
components of the system. In this present study, plasticized CGM as well as
GHCI modified plasticized CGM were used as the blending partners for PCL.
This was done in order to study the effectiveness of GHCI in designing new

biodegradable plastics.

7O

The tensile strength of PCL was reduced upon blending with CGMP (see
Table 8 & Figure 21) as expected. The percent elongation of PCL decreased
when blended with CGMP but showed significant improvement when blended
with GHCI modified CGMP. The comparative results with different proportions of
GHCI into the blends are shown in Figure 22 (also see Table 9). As apparent
from the Figure 22, the percent elongation was not proportional to the percentage
of GHCI in the blend. The elongation of the blend first increased with increasing
GHCI content from 5 to 7.5% then decreased with further addition of GHCI

content to 12.5%.

Table 8: Effect of GHCI on tensile strength of PCL blends

 

 

 

 

 

 

migraines. Whiting:
100:0:0 37.0 4.4
50:37.5:12.5 8.4 1.5
50:42.5:7.5 10.2 1.6
50:45:05 8.7 0.6
50:50:0 9.5 0.6

 

 

 

 

 

71

Table 9: Effect of GHCI on percent elongation of PCL blends

 

 

 

 

 

 

(Bfigkdccivngirifiin) % elongation Std Dev
100:00:00 657 57.9
50:37.5:12.5 323 84.4
50:42.5:7.5 463 33.3
50:45:05 418 19.8
50:50:00 10 2.1

 

 

 

 

 

Therefore, 7.5% GHCI was considered as the optimum under the present
experimental conditions. The stress-strain curve in Figure 23 shows the effect of
GHCI in the blend. The higher area under the curve for the blend that was
modified with GHCI indicates higher plastic character as contrast to its

counterpart not modified with GHCI.

72

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Table 10: Comparison of percent elongation and Impact strength of PCL, PCL
blend and HDPE

 

 

 

 

Sample Ratio %elongation stdev Impact (Jim) stdev
A PCL-100% 677 51.5 283 12.0
PCL:CGMP:GHCI=
B 50:42.5:7.5 463 33.3 55 3.1
C HDPE*- 100% 320 NA 47 NA

 

 

 

 

 

 

 

 

* The data obtained from Technical Data Sheet of Dow Polyethylene 25455N (Injection
molding resin), Dow Plastics, Published 08/01.

Among the blends prepared and tested, the PCL-CGMP blend with 7.5%
GHCI (sample B) was found to be better in comparison to the other composition
containing 0, 5 and 12.5% GHCI. This blend composition containing 7.5% GHCI
was compared to HDPE115 (sample C), Figure 24, Table 10). The percent
elongation of this blend was 463% while HDPE has a percent elongation of 320%.
So, the higher percent elongation of blend can allow the addition of some filler in
order to enhance the strength/stiffness. This blend was also found to have higher
impact strength versus HDPE"5 (Figure 24). The comparison of 7.5% GHCI

containing PCL-CGMP blend and HDPE is presented in Table 10 and Figure 24.

The storage modulii of PCL and its blends with CGMP as well as with
GHCI modified (with varying content of GHCI) CGMP were measured using
Dynamic Mechanical Analyzer (DMA) at 30, 40 and 50°C. The comparison of
storage modulus of PCL-CGMP blend with different percentage of GHCL is

presented in Table 11 and Figure 25. The PCL as well as all of the blends

76

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77

exhibited decreasing modulus values with an increase of temperature from 30 to
50°C. As expected, the modulus of the PCL decreased on blending with CGMP
as well as GHCI modified CGMP bioplastics. The main thrust of this investigation
is to find green biodegradable plastics for plastic packaging applications where
high strength and modulus are not required through blending an inexpensive
proteineous by-product, like CGM, with a commercial biodegradable plastic like
PCL. While studying the effect of the amount GHCI in the GHCI modified CGMP
it was observed that 7.5% of GHCI resulted an optimized modulus and percent

elongation values (Figures 21, 22 and 25) of the resulting biodegradable blends.

Table 11: Comparison of storage modulus of PCL and its blend at 30, 40 and

 

 

 

 

 

 

 

50 'C.
PCL:CGMP:GHCI Storage Modulus (MPa) at 30, 40 & 50'C
(Blend Composition) 30 40 50

100:0:0 459 (25) 427 (28) 339 (31)

50:37.5:12.5 87 (3) 74 (3) 48 (2)

50:42.5:7.5 140 (18) 116 (16) 75 (12)

50:45:05 171 (23) 140 (17) 86 (11)
50:50:0 265 (31) 227 (27) 145 (19)

 

 

 

 

 

 

Note: The values in ( ) represents the standard deviation.

78

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The thermal stability for CGM and DDGS was analyzed with Thermal
Gravimetric Analyzer (TGA) (Figure 26). There was an initial loss in weight in
both CGM and DDGS, which represents the loss of the moisture. There were two
different weight loss peaks observed for DDGS at 159 and 247°C, which may
represent mixture of different material mixtures like hull, fibers, etc. The
processing temperature for DDGS should be less than the lower degradation
temperature. The CGM was observed to be quite stable until 254°C. Therefore,
CGM was found to have higher processing temperature range and was more

stable.

PCL and its blends with CGM were also analyzed for thermal stability. The
blends considered were PCL: CGMP: GHCL (50: 37.52125), PCL: CGMP: GHCL
(50: 42.5: 7.5), PCL: CGMP: GHCL (50: 45: 5), PCL: CGMP (50: 50). It was
observed that the blends containing higher percentage of GHCI were found to be
more thermal stable than the blends with lower percentage of GHCI. The blend

with no GHCL was found to be least stable (See Figure 27)

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The blend morphology is closely related to its mechanical properties. In
our investigations of blend compositions and their mechanical properties
evaluations we found that GHCI modified CGMP was superior to the unmodified
counterpart in properties like percent elongation and impact. Blends with
PCLzCGMPzGHCl in the ratio of 50:42.5:7.5 and 50:50:0 were selected and the
fractured surfaces of these compositions were examined using SEM in order to
reveal the morphology of the blend with and without GHCI. The morphological
analysis revealed that the blend without GHCI contained cavities, which were
likely to be created in the fracture region during the spontaneous separation of
the PCL and CGMP regions due to the weak interfacial adhesion between PCL
and CGMP. The presence of GHCI in the blend exhibited quite a high degree of
homogeneity, which was a sign of increased compatibility between PCL and

CGMP phases (Figure 28).

83

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84

The main motivation of the present investigations was to design novel
biodegradable plastics with the maximum permissible content of CGM, thereby
finding value-added application of the by-products of corn-based ethanol industry.
The use of unique chemical modification of CGMP with GHCI followed by
blending with PCL has resulted in a new biodegradable plastic composition
having a tensile strength of ~10 MPa and a percent elongation of 462%. The
newly developed biodegradable plastic shows immense potential for plastic film
applications. One can manipulate the properties by changing the ratio of the
blend’s components. In separate investigation, the content of PCL was increased
to 60% from 50% in the blend system so that it can be compared with some other
synthetic plastics. A comparison of some properties of the two blends is shown in

Table 12.

Table 12: Comparison of properties between 50% PCL based blend and 60%
PCL based blend

 

 

 

Blend Composition Tensile Strength % elongation
(MPa)

50:50

PCL: GHCI modified 10 :l: 1.61 462 :I: 33.3
CGMP
60:40

PCL: GHCI modified 17 :t: 1.13 598 :I: 15.6
CGMP

 

 

 

 

 

Note: GHCI is guanidine hydrochloride and CGMP is the plasticized corn gluten meal.
All compositions are in wt%.

The blend with 7.5% GHCI was found to be the best in the overall
properties under the present experimental conditions. The resulting blend with

60% PCL showed improvement in its mechanical properties as shown in Table

85

12 and Figure 29. The tensile strength, percent elongation and modulus

improved by 70%, 29% and 28% respectively.

86

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87

Chapter 5

CONCLUSION AND RECOMMENDATIONS FOR FUTURE WORK

We have successfully developed injection molded biodegradable plastic
formulations from the blends of corn gluten meal based bioplastic and PCL. The
choice of the ratio of components, the melt processing temperature and the
screw speed of the extruder all affected the overall properties of the resulting
biodegradable polymer blends. The uniqueness of the present investigation is
the chemical modification with GHCI, which has resulted in a biodegradable
plastic having a high percent elongation that show potential applications for
biodegradable packaging. Among the different blends tested, the blend with
7.5% GHCI produced the best results under present experimental conditions.
Further increase in the PCL content, i.e. from 50 to 60% in the final blend
improved the overall mechanical properties of the blend. We were successful in
making a new class of biodegradable plastic from PCL-based blend with the
maximum permissible content of a renewable resource as well as inexpensive
by-product like corn gluten meal. The result shows the immense potential for
designing and engineering new value-added and eco—friendly bio-based
materials from the by-products of corn based industries. The morphological
analysis revealed increased homogeneity between PCL and CGM phases in the
presence of GHCI. Urea (see Figure 30), which has some similarity in structure to
GHCI (see Figure 13) can be evaluated as a replacement for GHCI in order to

reduce the overall cost of the blends.

88

0
II
NHz— C _ NHz

Figure 30: Chemical structure of Urea

89

1)

2)

3)

4)

5)

5)

7)

8)

9)

10)

11)

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