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This is to certify that the
thesis entitled ?
EFFECT OF REPEATED MICROWAVE HEATING ON THE >

IMPACT RESISTANCE OF A POLYPROPYLENE CONTAINER

presented by

Ubonrat Siripatrawan

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of the requirements for

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1/98 chIRCJDamest-p.“

EFFECT OF REPEATED MICROWAVE HEATING ON THE IMPACT RESISTANCE
OF A POLYPROPYLENE CONTAINER
By

Ubonrat Siripatrawan

A THESIS

Submitted to
Michigan State University
in partial fulﬁllment of the requirements
for the degree of

MASTER OF SCIENCE

School of Packaging

1997

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ABSTRACT

EFFECT OF REPEATED MICROWAVE HEATING ON THE IMPACT RESISTANCE
OF A POLYPROPYLENE CONTAINER

By

Ubonrat Siripatrawan

The effect of microwave repeated heating on physical structure and property of a
package material was studied by evaluating drop impact resistance and degree of crystallinity.
Drop impact resistance of plastic syrup bottles was evaluated using the Bruceton Staircase
free fall drop method. Drop impact orientation (ﬂat bottom, bottom comers, and handle), ﬁll
level (ﬁill, 1%, V2, and ‘/4 ), and temperature of package material (20 i 2 °C, 42.3 i 2.2 °C,
and 8.1 i 1.6 °C) affected drop impact resistance of the bottles. Plastic OPP bottles ﬁlled
with syrup at full, 3A, ‘/2 , and ‘/4 levels were heated in a microwave oven and subjected to 12
heating treatments. Unheated bottles ﬁlled with syrup at full, 31:, ‘/2 , and ‘A levels were used
as control bottles. Drop impact resistance of unheated bottles and bottles subjected to
microwave reheating was detemrined. Degree crystallinity associated with heat of fusion of
the package material was evaluated using Modulated DSC. Degree crystallinity of the
samples taken ﬁ'om unheated and microwave reheated bottles were compared. Decreased
drop impact resistance and increased degree of crystallinity of the package material was

observed after the bottles were repeatedly heated in the microwave oven.

ACKNOWLEDGEMENTS

I would like to extend my intensity of respect and appreciation to King
Chulalongkom for his impressive contributions to the progress of modern Thai education.
I also would like to express my gratitude to my sponsors, the Royal Thai Government and
Chulalongkorn University, for giving me an opportunity to further my studies in the USA
to gain knowledge in order to help fulﬁll the betterment of Siam.

I would like to express my respect and appreciation to my major professor Dr.
Bruce Harte for his time, kind thought, and guidance throughout my research. I also
would like to express my sincere appreciation to my committee Dr. Gary Burgess, Dr.
James Steffe, and Dr. Jack Giacin for sharing their time serving as my committee and for
giving me the idea of what project I should pursue.

My appreciation also goes to Mr. Michael Kentala ﬁ'om the Pillsbury Company for
donating the plastic containers used in this research. Many thanks to Mr. Michael Rich
from the Composite Research Center for his help in MDSC analysis. Special thanks to
Ms. Beverly Underwood for her help during my study here.

My sincere thanks to all my friends, especially Manee, Nucharin, Rujida, Sunetra,
Risha, John, Jae, Muk, and Jai, for all their help, suggestion, pep talk, and for helping
make my stay here a memorable one.

Finally, I would like to express a deepest appreciation to my wonderful family for
their inspirational support.

iv

TABLE OF CONTENTS

LIST OF TABLES ................................................................................. vii

LIST OF FIGURES ................................................................................ viii

CHAPTER 1

INTRODUCTION AND OBJECTIVES .......................................................... 1

CHAPTER 2

LITERATURE REVIEW ........................................................................... 7
Microwave Heating Characteristics ...................................................... 7
Dielectric Properties ........................................................................ 8
Microwave Heating Characteristics of Foods ........................................... 9
Factors Affecting Microwave Heating .................................................. 1]
Microwave Packaging ..................................................................... 14
Plastic Package for Microwave Oven ................................................... 17
Mechanical Properties of Polymeric Packaging Materials ............................ 20
Effect of Temperature on Mechanical Properties ..................................... 23
Effect of Heat on Percent Crystallinity ................................................. 26
Mechanical Testing for Polymeric Packaging Materials .............................. 28
Drop Impact Test ........................................................................... 29
Determination of Percent Crystallinity ................................................... 31

CHAPTER 3

MATERIALS AND METHOD ................................................................... 35
Samples ...................................................................................... 35
Wall Thickness of the Bottle .............................................................. 35

Determination of the Impact Orientation of the Bottles by Free Fall Drop Test...36
Determination of Drop Impact Resistance of the Bottles Filed with syrup at

Different Levels ............................................................................. 39
Effect of Microwave Repeated Heating on the Drop Impact Resistance of

the Bottles .................................................................................. 40
Effect of Temperature on the Drop Impact Resistance of the Bottles ............... 45
Determination of the Change in Degree Crystallinity of the Packaging Material

by Thermal Analysis ....................................................................... 46

CHAPTER 4

RESULTS AND DISCUSSIONS ............................................................... 47
Wall Thickness of the Bottles ............................................................ 47
Determination of the Impact Orientation of the Bottles by Free Fall
Drop Test .................................................................................... 47
Determination of Bottle Drop Impact Resistance .................................... 49
Effect of Microwave Repeated Heating on the Drop Impact Resistance
of the Bottles ................................................................................ 53
Effect of Temperature on the Drop Impact Resistance of the Bottles ............... 64
The Change in Degree Crystallinity of the Packaging Material ....................... 71

CHAPTER 5

SUMMARY AND CONCLUSIONS ............................................................ 74
CHAPTER 6

RECOMMENDATIONS .......................................................................... 77
APPENDICES

APPENDIX A. Statistical Comparison by Cochran’s Test .................................... 79
APPENDIX B. MDSC Analysis of the Samples ................................................ 80
BIBLIOGRAPHY .................................................................................. 85

LIST OF TABLES

Table 1. Microwave Heating Treatment ........................................................ 43
Table 2. Wall Thickness of the Bottle Material ................................................ 47
Table 3. Drop Height Failure of the Bottles at Various Orientations ....................... 48

Table 4. Analysis of Variance of Drop Height of the Bottles at Various Orientations. . .48
Table 5. Weight of the Plastic Bottle Filled with Syrup at Full, V4, V2, and V4 Levels. . . ..50

Table 6. Drop Height of the Bottles Filled with Syrup at Full, V4, V2, and V4 Levels
and Dropped at the Right Bottom comer ........................................................ 52

Table 7. Analysis of Variance of Drop Height Failure of Bottles Filled with Syrup

at Full, V4, V2, and V4 Levels and Dropped at Right Bottom Comer ........................... 52
Table 8. Drop Height of the Bottles Filled with Syrup at Full, V4, V2, and V4 Levels
and Dropped onto their Handle .................................................................... 52
Table 9. Analysis of Variance of Drop Height Failure of Bottles Filled with Syrup
at Full, 3/4, V2, and V4 Levels and Dropped onto their Handle .................................. 53
Table 10. Temperature of Samples aﬁer Heating in Microwave Oven ...................... 54

Table 11. Analysis of Variance of Temperature of Samples after Heating in
Microwave Oven .................................................................................... 55

Table 12. Temperature of the Product and Package Wall ..................................... 55

Table 13. Effect of Microwave Heating and Fill Levels on Drop Impact
Resistance of the Bottles Dropped at the Right Bottom Corner .............................. 59

Table 14. Analysis of Variance of Effect of Microwave Heating and Fill Levels
on Drop Impact Resistance of the Bottles Dropped at the Right Bottom Comer. . . .......59

Table 15. Effect of Microwave Heating and Fill Levels on Drop Impact
Resistance of the Bottles Dropped onto their Handles ......................................... 6O

vii

Table 16. Analysis of Variance of Effect of Microwave Heating and Fill Levels
on Drop Impact Resistance of the Bottles Dropped onto their Handles ..................... 60

Table 17. Drop Height of the Bottles Dropped at the Right Bottom Corner at 8 °C ...... 65

Table 18. Drop Height of the Bottles Dropped onto their Handles at 8 °C ................. 67
Table 19. Comparison of Drop Height at 20 °C and 42 °C of the Repeated

Bottles Dropped onto their Handles ............................................................... 68
Table 20. Analysis of Variance of Drop Height at 20 °C and 42 °C of the

Repeated Bottles Dropped onto their Handles ................................................... 68
Table 21. Effect of Temperature of the Product on Drop

Impact Resistance ................................................................................... 69
Table 22. Effect of Microwave Reheating on the Drop Impact Resistance ................. 69

Table 23. Total Heat of Fusion Associated with Degree of Crystallinity
of Unheated and Heated Bottles in Microwave ................................................. 73

viii

LIST OF FIGURES

Figure 1. Heat Flux DSC Schematic ............................................................ 34
Figure 2. Nonreversing, Reversing, and Total Heat Flow Signals ........................... 34
Figure 3. Syrup Bottle ............................................................................. 38
Figure 4. Flow Diagram of Microwave Heating Treatment .................................. 42

Figure 5. Effect of Fill Level on Drop Height of the Bottles Dropped onto
Right Bottom comer and Handle .................................................................. 51

Figure 6. Drop Height of the Bottles After Repeated Heating in the
Microwave Oven. ................................................................................... 61

Figure 7. Effect of Microwave Reheating on Drop Impact Resistance of the
Bottles Dropped onto their Right bottom Comer. ............................................. 62

Figure 8. Effect of Microwave Reheating on Drop Impact Resistance of the
Bottles Dropped onto their Handles. ............................................................. 63

Figure 9. Effect of Temperature of the Product on Drop Impact Resistance When
Dropped onto their Handles ........................................................................ 70

Figure 10. MDSC Analysis of Sample from the Bottom of Unheated Bottle ............... 80

Figure 11. MDSC Analysis of Sample from the Upper Part Close to the Handle
of Unheated Bottle. ................................................................................. 81

Figure 12. MDSC Analysis of Sample from the Bottom of the Microwave
Reheated Bottle. ..................................................................................... 82

Figurel3. IVHDSC Analysis of Sample from the Upper Part Close to the Handle
of Microwave Reheated Bottle ................................................................... 83

Figure 14. Comparison of MDSC Analysis of Samples from the Bottom of the
Unheated Bottle ..................................................................................... 84

ix

CHAPTER 1

INTRODUCTION AND OBJECTIVES

The motivation for the development of rrricrowavable packaging materials has been
influenced by need for convenience, aesthetics, and enhanced food quality. Classically,
packaging technologists have been concerned with designing packages which: protect the
quality attributes of foods, minimize the effects of physical abuse to the product during
manufacture, storage and distribution, are relatively easy to manufacture, economical to
the consumer and simple to open and use. The increased usage of microwave ovens has a
direct effect on the design of the product and its heating performance in the microwave
oven and highlights the importance of the relationship between food formulated for
microwave heating and the appropriate packaging.

Generally, rrricrowavable containers should allow heat penetration, tolerate rapid
temperature change and preserve food quality (Rubbright, 1990). Moreover, the
appropriate packaging material for use with a rnicrowavable food should not heat
excessively or prevent efﬁcient microwave heating. The container must also be thermally
compatible with the food, i.e. it should not melt, distort or be otherwise impinged by hot
food, and it should provide shelf life properties commensurate with the food and its use.

Microwave transparent materials, such as plastics and paperboard, microwave interactive

l

2

materials, such as susceptors, and reﬂective materials, such as metals have been used for
microwavable packaging. These materials provide both opportunities and problems for
food manufacturers (Fisher, 1991). Polymeric packaging materials have been already well
established in the microwave market and are now the most popular materials used for
microwave packaging as a result of their microwave transparency, ease of processing and
forming, consumer appeal, and safety (Sacharow and Schiffman, 1992).

To design and select a plastic material for use in a microwave oven and to avoid
failure in the market place, product and package development must be thoroughly
researched. The packaging technologist must ﬁrst understand what role the package will
have in relation to the heating phenomenon, such as the effect of microwave heating on
structural properties, packaging performance, and the expected service life of the
container in order to provide an appropriate container.

During microwave heating, the highest temperature will be found at or near the
interface between food and the container. At the interface of food and container, the
temperature of the container, if it is transparent to microwaves, will usually be a result of
the food temperature and the cooling inﬂuence of the air on the outside of the container
wall (Huang, 1987). When the food contains large amounts of fat, such as on the surface
of soups, or in sauces and gravies, or a large amount of sugar, as on preserves and syrups,
then the temperature may potentially become very high and may cause problems such as
dimensional stability, discoloration, distortion, melting, and migration from plastic
packaging (Katt, 1991).

There have been many studies concerning food/package interactions especially

with respect to the migration of packaging components into foods due to heating at high

3

temperatures in the microwave oven. One such example is the migration of volatile
compounds from a thermoformed rrricrowavable container (made from
polypropylene/Sarm/polypropylene coextruded material) into food during microwave
exposure (Dixon et a1, 1988). Plasticizer has also been known to migrate from a
laminated ﬁlm incorporating susceptor material (Bishop and Dye, 1982). Odor pick-up
and retention resulting in a change in the ﬂavor of the food has been observed (Laperle,
1988)

However, there are other problematic areas that need special attention. One
problem, known as "runaway heating" occurs in thermoplastics as they approach their heat
distortion temperature, which is the temperature at which an arbitrary deformation occurs
under arbitrary test conditions. This can cause melting or severe distortion of the
container (Korshak, 1971). Another concern with plastic containers is change in physical
property such as loss of desired packaging performance and discoloration of the container,
after repeated heating in a microwave oven (Peason, 1995; Sacharow and Schiffmann,
1992). Therefore, performance testing of microwavable packaged products is necessary.

The mechanical properties, among all the properties of polymeric packaging
materials, are often the most important because they inﬂuence virtually all service
conditions and the majority of end-use applications (Shah, 1984). Mechanical properties
of plastic materials may be affected by elevating temperature during microwave heating.
There are many interesting correlations that can be made with effect of microwave heating
on the quality of package container. Impact resistance is one of the most important
mechanical properties of the plastic materials and an increase or decrease in impact

resistance may be caused by microwave heating.

4

Physical testing of plastics can be classiﬁed as thermal, mechanical, dimensional,
transmission, and electrical (Seymour and Carraher, 1984). In this study, attention was
focused on the mechanical and thermal testing. The mechanical testing of plastic materials
is usually carried out to determine their suitability for a particular application, to control
functions, or to obtain a better understanding of their behavior under various conditions.
Most mechanical tests are carried out on molded test pieces. However, there are some
tests that can be performed on the ﬁnished containers and these will often give results
more in keeping with the end-use performance. Drop impact resistance of a blow molded
thermoplastic container (ASTM D2463) is one of the physical tests that can be used to
simulate the actual impact conditions.

There is little information available in the literature which correlates change in
physical properties of plastic containers with heating in a microwave oven. A few
technical papers address the potential effect of microwave heating on the crystallinity of
polymeric packaging materials. It is assumed that heating in a microwave at elevated
temperatures may lead to a change in percent crystallinity of the packaging materials due
to melting and recrystallization. Determining percent crystallinity is consequently
important to evaluate its effect on packaging performance. The reason which necessitates
such investigation is, in particular, the fact that the physical character of a polymer such as
its tensile strength, and impact resistance depends substantially on the percent crystallinity
of the polymer.

In this research, it was hypothesized that repeated heating of plastic containers by
microwaves may affect the degree of crystallinity which may result in a change in the

impact resistance of the package. Repeated microwave heating and free fall drop testing

5

of the containers were designed to simulate the actual use situation environment. The
microwave heating parameters were studied under comparable conditions to determine
changes in the container's crystallinity and performance. Drop impact resistance, was used
to simulate actual impact conditions and was evaluated using the ﬁ'ee fall drop test.
Differential Scanning Calorimetry (DSC), a technique measuring heat ﬂow into or out of a
material as a ﬁrnction of time and temperature, was used to assess the heat ﬂow associated
with percent crystallinity of the material.

The consequences of this investigation might be expected to provide signiﬁcant
information about the capability and limitations of the packaging and the compatibility of
package and product to repeated heating in the microwave. This information will
potentially help packaging technologists prepare appropriate container speciﬁcations for
microwave use, to develop more convenient and successful packages for microwave food
products.

The purpose of this study was to determine if there were changes occurred in the
structural and physical properties of packaging material due to repeated heating in the
microwave and to identify and quantify these changes.

The speciﬁc objectives of this research were:

1. To evaluate drop impact resistance and failure of a container when subjected to sudden
shock resulting from a free fall.

2. To compare the performance of a container to withstand the sudden shock resulting
from a free fall after repeated heating in microwave oven.

3. To determine changes in degree of crystallinity of package material due to repeated

microwave heating.

6

4. To study the effect of the change in percent crystallinity resulting from microwave

heating on packaging performance.

CHAPTER 2

LITERATURE REVIEW

Microwave Heating Characteristics

Microwaves are a form of electromagnetic energy, and are similar to visible light,
x-rays, radio wave and ultraviolet energy. Microwave generally include the frequency
spectrum from 0.3 to 300 GHz (gigaherts) or wavelength of 1 mm to 1 m.(Sacharow and
Schiﬂ‘mann, 1992). All electromagnetic waves are composed of rapidly alternating electric
and magnetic ﬁelds which oscillate at different rates. The energy frequency in industrial
ovens commonly used in plant manufacturing operations operate at 915 MHz, while in
domestic retail ovens they operate at 2450 MHz. This means that the electromagnetic
ﬁeld is alternating 2450 million times per second (Fisher, 1991).

Robertson (1992) described two main mechanisms by which microwaves produce
heat in foods including dipole rotation and ionic polarization. Ionic polarization occurs
when ions move in response to an electric ﬁeld. Ions, due to their inherent electrical
charge, are accelerated by the microwave ﬁeld, leading to multiple collisions with
nonionized molecules. Kinetic energy is given by the ﬁeld to the ions, which collide with
other ions, converting kinetic energy into heat. The more concentrated or the more dense
the solution, the greater the frequency of collision, and the more kinetic energy that is

7

8

released. At microwave frequencies, numerous collisions occur, and much heat is
generated. However, it is a less important mechanism than dipole rotation.

Microwave heating also results from interactions of the food constituents
containing polar molecules (such as water) with an electromagnetic ﬁeld. These molecules
have a random orientation and orient themselves when applied with the rapidly changing
alternating electrical ﬁeld according to the polarity of the ﬁeld. These interactions lead to
instantaneous heat generation within the product due to molecular ﬁiction primarily by the
agitation of weak hydrogen bonds associated with the dipole rotation of free water
molecules and with the electrophoretic migration of free salts in an electrical ﬁeld of
rapidly changing polarity. These effects are predominantly related to the aqueous ionic
constituents of food and their associated solid constituents. Such rotation of molecules
leads to friction within the surrounding medium, and then generate heat (Singh and
Heldman, 1993). Once the heat is generated it is then transferred to other points in the
product by conduction and convection. The speciﬁc heat, thermal conductivity, density

and viscosity all effect the rate of heat transfer (Fisher, 1991).

Dielectric Properties

The dielectric properties of a material are critical to microwave heating. The
dielectric property of a material is the physical description of how well a material can
potentially heat when it interacts with electromagnetic energy. The important electrical
properties are the relative dielectric constant (8’), and the relative dielectric loss (8”). The
dielectric loss factor for the material, which represents the quantity to which an extremely

applied electrical ﬁeld will be converted to heat, is shown by the following equation:

e = 8’ tan 6

8’ describes how much energy is reﬂected away from the product and how much
energy is transmitted into the product. 8” describes how lossy a product is or how well a
material absorbs electrical energy and converts it into heat. tan 5 or loss tangent provides
an indication of how easily the material can be penetrated by an electrical ﬁeld and how
well it dissipates electrical energy as heat (Engelder and Buﬂler, 1991).

If the lossiness of a product is large, it will effectively absorb energy passing
through it and heat rapidly. If the lossiness is small, microwave radiation will pass right
through the product without heating it. Most glass, plastic and paper packaging materials
have low dielectric constants compared to food and are transparent to microwave energy.
Many factors inﬂuence the dielectric properties of food. A few of these are; moisture
content, temperature, salt content, physical state, and chemical composition (Fisher,

1991).

Microwave Heating Characteristics of Foods

Microwave heating characteristics of foods are related not only to their dielectric
properties but also to electrical transmission properties related to dielectric heating
processes and to thermal and transport properties that affect heat and mass transfer in both
conventional and dielectric heating processes (Engelder and Bufﬂer, 1991).

Microwave heating of foods involves two phenomena: coupling of energy by the
product from an electromagnetic ﬁeld and attenuation of absorption of the coupled energy
within the product. These phenomena involve reﬂection and transmission of energy at

product surfaces and alteration of energy within the product. This results in instantaneous

10

temperature increase within the product in contrast with conventional heating process that
transfer energy from the surface with long thermal time constants and slow heat
penetration.

Microwave heating occurs as a result of the interactions between microwave and a
dielectric material. The relationship between microwave power absorbed by the material
being heated and the conversion of the microwave energy to heat can be approximated

with the following equation (Decareau, 1992, and Singh and Heldman, 1993):

P = 6E2 (watt/cm3)

where P = the power absorbed (watt/cm3)
o = the equivalent dielectric conductivity
E = the electrical ﬁeld strength (volts/cm)

The main difference between microwave heating and other heating methods is
penetration depth. The penetration depth is the distance ﬁom the surface of a dielectric
material to where the incident power is decreased to 37% (l/e) of the incident power at
the surface. Microwaves penetrate deeply into food materials and are converted to heat as
they penetrate (Decareau, 1992). The equation for converting dielectric property data

into penetration depth is :

dp = 1 / (2 or)
where dp = penetration depth
or = attenuation constant

(2 n/ 7.) {272 [(1+ tan 2 5) ”2 -1]}"2

11

Factors Affecting Microwave Heating

From the experiments conducted by Lau (1995) to determine the heating
characteristics of food materials in a microwave oven, the conclusion drawn was that the
temperature rise induced in a food and food packaging due to microwave heating depends
on two main physical mechanisms; the interaction of the food package with the microwave
ﬁeld detemrines the power deposited and the increase of temperature throughout the food
package in accordance to heat transfer mechanisms. He also observed that the power
absorbed by a food load in a microwave oven depended on the dielectric properties of the
food, the position of the food in the cavity, and the use of packaging materials.

Schiffrnann (1990) pointed out a number of factors that affect microwave heating
performance in microwave ovens:

a) Oven parameters include output wattage, output frequency, presence or absence of a
tumable and turntable materials of the, position of the food in the oven cavity, material
construction of the ﬂoor, timebase for pulsed-power control, stability of input power,
presence or absence of ﬁlament transformer, age and condition of the magnetron, and
time delay of the magnetron, i.e., cold vs warmstart.

b) Food parameters include nature of the food-single component vs. multicomponent,
dielectric properties of the foods over the range of temperatures to be encountered,
thermal properties of the food (thermal conductivity, heat transfer coefﬁcient, speciﬁc
heat, heat of fusion), evaporation, initial temperature of food, shape of the food and its

components, and regularity of food shape.

12

c) Packaging including material of construction (plastics, glass, paperboard, and metal
ﬁlms, transparency or reﬂectively), size (length, width, and depth), shape (round, oval,
rectangular, etc.),

The shape of food items is critical to microwave heating results. For many foods,
it is the food container or dish that determines the food shape and therefore affects the
heating performance. Varied different temperature proﬁles are found in round, oval, and
rectangular containers. As the size of the container changes, so will the temperature
proﬁles.

The sphere is the ideal shape as energy tends to be focused to give heating at or
toward the center of the sphere. The cylinder is the next best shape in terms of heating
performance. Round and oval are preferred over rectangular or square because
rectangular and square shape will give edge and comer heating results which may at times
be extreme. Food in the corners of a rectangular container will be overcooked before the
remainder of the food is ready. Food products heat more uniformly if formed with round
corners. Even when food product or package corners are unavoidable they should have
generous radii to minimize the overheating effect (Sacharow and Schiffrnann, 1992).

The bottom of the container should be bowed somewhat to make the food depth
thinner in the center where it receives less microwave power, than along the edges,
thereby contributing to more uniform heating. This also serves to elevate the food slightly
thus reducing heat loss to the cooler oven ﬂoor.

The side wall of the container should have generous draft angles rather than be at

90 °C angle with the bottom commensurate with stacking. On the other hand, the wall

13

should not be so shallow that overheating of a thin food proﬁle at the surface occurs
(Ohlsson and Rissman, 1978).

The composition of the food material affects how it heats in the microwave ﬁeld.
The moisture content of food directly affects the amount of microwave power absorption.
A higher amount of water in a food increases the dielectric loss factor. In the case of low
moisture content, the inﬂuence of the speciﬁc heat (the amount of energy needed to heat
1.0 g of food to 1.0 °C) on the heating process is more pronounced than that of the
dielectric loss factor. The speciﬁc heat detemrines how rapidly a given volume of food
will heat once a given amount of power is deposited within it. The lower a food’s speciﬁc
heat capacity, the more quickly it will heat. Food or components with high fat and sugar
and low water content have low speciﬁc heats. Therefore, due to their low speciﬁc heat,
some foods with low moisture content also heat at acceptable rates in microwave oven
(Singh and Heldman, 1993).

Oil has about one-half the speciﬁc heat of water, and can heat twice as fast for a
given heat input. High-solid foods, such as jellies and preserves, have low speciﬁc heats
and may not only heat fast but to very high temperatures (Fisher, 1992). This was also
observed in the study by Katt (1991) that as the sugar concentration in the food system
increased, the heating rate increased. Compared to a water control with a heating rate of
1.10 °F/second, the addition of 20 % ﬁuctose increased the heating rate to 1.20 °F/
second, and at 50 % fructose the rate was 2.10 °F/second.

Thermal conductivity is a measurement of a material’s ability to transfer heat in
response to a temperature difference. Conduction heat transfer depends on temperature

difference. Even in microwave cooking it plays an importance role in spite of the

l4

penetrating nature of microwave energy. The power absorption characteristics of a food
load changes signiﬁcantly as its temperature rises due to the change in its dielectric
properties. For food products with high conductivities, most of the power is deposited on
the surface and at sharp corners. For food product with low conductivities, standing

waves are set up within the load and the power is more evenly distributed (Lau, 1995).

Microwave Packaging

Generally, packaging requirements for all foods must perform certain ﬁrnctions.
Chief among these are to protect and preserve the products contained, ensure the
wholesomeness of the food, allow ease of manufacturing, and provide consumer usability.
For packaging speciﬁc to microwavable foods, the three main applications are reheating,
cooking, and defrosting. The packages for nricrowavable foods must also provide
adequate venting, control the arcing within the package itself, allow for uniform
reconstitution temperatures in both multiple and single component foods, protect the user
from the potential hazards of heated products, such as discharge of viscous materials at
their boiling point, provide for safe handling and opening when hot, prevent taste and odor
from migrating out of the packaging material into the food, maintain structural integrity
under varied thermal conditions, and achieve uniform heating to provide microbiologically
safe food preparation (Rubbright, 1990; Fisher, 1997; Robertson, 1993).

Schiffmann and Sacharow (1992) deﬁned the desired properties of packaging
materials for microwave ovens, as the following; will not heat excessively, or prevent

effective microwave heating, thermally compatible with the food-that is, should not melt,

15

distort or be otherwise affected by the hot food. Therefore, the selection of materials
must be made after careful consumer use testing.

Microwave packaging can be classiﬁed as active, passive, and shielding according
to their interaction with microwaves. Active packaging is packaging constructed of
materials which respond to microwaves with the result that incident energy is converted
into heat or is focused to increase its intensity in a predetermined region of the food.
Active packaging may combine active and passive elements (Rubbright, 1990).

Passive packaging is packaging which does not modify the microwave energy ﬁeld
and does not become hot. Such materials are essentially transparent to microwave energy,
including plastics, glass, and paper products. Passive packaging includes both rigid and
ﬂexible forms. Rigid packages dominate meals in compartmented trays and for larger
serving sizes, individual lunch and vended items. Shielding packages include metallic
structure that can reﬂect microwaves. Therefore, a product encased in a metallic structure
would not exhibit any microwave heating, since no energy is able to penetrate the material
to reach the product (Robertson, 1993).

Generally, packaging materials that are used in microwave ovens include plastic,
paperboard, and glass. Plastics are well suited to microwave use and when properly
selected can be used for shelf stable, refrigerated and frozen foods. They are divided into
microwave only and dual-ovenable categories. Dual-ovenable containers are designed to
withstand the rigors of both microwave and conventional ovens. Crystalline polyester and
thermosets are good examples of dual-ovenables that are used in higher priced containers.
The problem that the package developer and food technologist must face is to ensure that

the microwavable package is not exposed to excessively high temperature. The selection

16

of materials must be made after careﬁil consumer use testing as well as in-house test
kitchen analysis (Sacharow and Schiffrnann, 1993).

Paperboard is a popular microwave packaging material because of cost
considerations. The properties of paperboard which make it attractive in microwave food
use are its transparency to microwaves, ready availability in many forms, ease of
application, mechanical strength and stiffness, printability and relative economy.
However, the deﬁciencies of paperboard are its low resistance to moisture and grease,
poor tearing resistance, low barrier to mass transfer of gasses, moisture and food
components, and lack of resealability and shapeability.

For glass, the advantages of glass as packaging container include inertness,
nonabsorbency, and a high degree of transparency to microwaves. Glass is a rigid
structure and has no migration problems associated with food and beverage products.
However, properties which mitigate against its use are its relatively high heat conductivity,
high weight and brittleness. Another possible problem with glass is that the surface may
become extremely hot and dangerous to handle. Also, the glass surface temperature may
be deceptively low while the central volume of the contents may be boiling hot.

Another category is susceptors which originally were developed to overcome the
problem of inability of products to brown and to assist in crisping microwavable pizza and
popcorn. Susceptors are materials consisting of metallised structures applied to a heat
resistant surface such as polyester ﬁlm or kraft paaperboard. They absorb microwave
power and convert it into heat. This heat is then transferred to product by conduction or
radiation, creating localized areas of high temperature on the food surface which causes

browning. However, the contact heating phenomenon found with susceptors for products

17

such as pizza and similar products makes somewhat different demands upon the susceptor.
Another problem encountered with susceptors is irregularity of heating. Moisture
condensation can also be a problem with susceptors. As the contact surface of the food
becomes very hot, evaporation of water occurs which makes the susceptor a less effective

microwave heater (Zucherman and Miltz, 1993; Sacharow and Schiffrnann, 1993).

Plastic Packaging for microwave oven

Plastic materials that have been used in microwave ovens include : polyethylene,
polypropylene (PP), acrylonitrile, butadiene styrene (ABS), polycarbonate, nylon, styrene
acrylonitrile(SAN) and mixtures of styrene and acrylic sold under the trade name Dylark
(Calto, 1978).

Polyoleﬁns including polyethylene, polypropylene, polybutylene and copolymers of
ethylene with propylene and other monomers have been developed in various forms and
are used most commonly in nricrowavable containers. Polyoleﬁns are generally remain
tough at freezer temperatures, are translucent to opaque in thick sections and are available
in an exceptionally broad range of grades with different specialized properties.
Polypropylenes, gaining favor as trayware for microwave foods such as vegetables, can be
ﬁlled with inorganic ﬁllers such as calcium carbonate or talc to raise their useﬁil
temperature limits by increasing the stiffness of formed parts.

Polypropylene has been used as a replacement for glass in maple-syrup bottles
because of the lower cost, contact clarity, and ability to withstand hot-ﬁlling of the syrup.
Savings in shipping costs can also be a major consideration (Talvitie and Gaunt, 1982).

Interest in PP has become even more intense recently, and has been quite favorable

18

compared to other commonly used plastics that can be steam sterilized (Szulczynski,
1978). Sweintekand and O’Donnell (1994) reported on a microwave ready syrup bottle.
The polypropylene bottle features several advantages for rrricrowavable packaging. Its
shape allows it to ﬁt inside microwave ovens. Bottle geometry can be created to assist in
uniform heating of the syrup.

Polypropylene ﬁlled with up to 40% by weight of an inorganic compound such as
calcium carbonate, is stiffer at temperatures reached by foods heated in microwave ovens;
it is used to provide stable handling when removing heavier food loads from the oven.
The appearance of containers made from ﬁlled resin is better than of unﬁlled PP and some
observers say that they look better than unﬁlled CPET trays. Filled PP, used for frozen
foods intended for rrricrowaving, is not dual-ovenable and is not recommended for reuse.

Polyester (PET) includes a large class of thermoplastic materials extensively used
in microwave packaging intended for single use, but also saved for reuse in the freezer.
CPET, a crystallized form of PET, exhibits heat resistance to 445 °F and is currently the
superior material for dual-ovenable plastic ware. PCTA, polycyclohexane terephthalate-
acid (modiﬁed) has a higher temperature range with approximately a 50 °F advantage over
CPET. Mica-ﬁlled nylon is an alternative to CPET or thermoset polyester containers,
which was the former preferred material for dual ovenable premium meals (Sacharow and
Schiﬂinann, 1992).

High density polyethylene, though rigid, distorts at temperatures around 170 ° F
and is brittle at low temperature. PP has a higher distortion temperature, around 230 °F,
is not rigid and has a tendency to become brittle. ABS can tolerate temperatures in the

190 to 220 °F range depending on the grade. It is somewhat subject to damage by

l9

abrasion. Polycarbonate has a distortion temperature near 250 °F and is extremely break
resistant. Nylon is strong, but has a low tolerance to temperatures of 150 to 170 °F. SAN
is strong , but limited to 190 °F. Styrene and acrylic mixtures, though tolerant to
temperatures up to 230 °F, are relatively brittle (Decareau, 1992).

Styrenics are polymers or copolymers of styrene with other monomers and may be
polymerized in the presence of toughening rubbers, forming materials such as styrene-
butadiene. Styrenics are characterized by their ease of rapid fornring into containers
shaped by injection molding, blow molding and especially thermoforming. General
purpose polystyrene (PS or GPPS) is a crystal clear plastic used extensively in food
service for takeout containers and in some microwave applications where temperatures are
limited approximately to 90-100 °C. Impact resistant grades, which contain the
aforementioned rubbers, may be translucent or opaque and withstand heat to the same
extent as GPPS. High heat resistant grades of PS are produced by copolymerizing styrene
with alpha methyl styrene or other high temperature monomers. The polymer has high
deﬂection temperatures up to the order of 120 °C (Decareau, 1992; Sacharow and
Schifﬁnann, 1992).

Expanded polystyrene, although it has good food heat retention properties, has not
been completely favorable for microwave oven usage. Although transparent to microwave
energy it is not resistant to high temperatures attained by some food products (Decareau,
1992). Monte and Landau-West (1983) tested a wide variety of frozen foods heated in
expanded polystyrene containers. The foods were heated to 180 °F and held at this
temperature for 10 minutes. Any container leakage, major distortion or softening that

might result in container failure resulted in a “not recommended” rating for that product.

20

Fatty foods (such as fried foods, gravies, certain cheese sauces, fatty meats and buttered
foods) gave high failure rates. Foods with much gravy showed breakdown of the
container at the gravy line.

Stehle (1979) compared the quality of polysulfone, a therrnoset polyester, a
thermoplastic polyester, and poly-4-methyl pentene-1 containers. A variety of food items
were used to test the reaction to high fat temperatures, high sugar temperatures, and
protein stain or residual. Bacon was used for high fat temperatures; peanut brittle for high
sugar temperatures; beef patties and meat loaf for protein residual; and pork roast for high
fat and protein residual. Appropriate utensils and accessories made from these plastics
were used. The study concluded that cookware made from polysulfone and therrnoset
polyester can be used with any food and for all microwave cooking purposes. The
cookware made from thermoplastic polyester and methyl pentene isomer warped or

became distorted in some of the shapes tested.

Mechanical Properties of Polymeric Packaging Materials

The mechanical properties, among all the properties of plastic materials, are often
the most important because virtually all service conditions and the majority of end-use
applications involve some degree of mechanical loading. Material selection is quite often
based on mechanical properties such as tensile strength, modulus, elongation, and impact
strength. Numerous factors affect various mechanical properties of polymers, including
molecular weight, processing, extent and distribution of crystallinity, composition of

polymer and use temperature.

21

Crystallinity has a number of important effects upon the mechanical properties of a
polymer. Yield stress and strength and hardness increase with an increase in crystallinity
as does elastic modulus and stiffness. Physical factor that increase crystallinity, such as
slower cooling and annealing, also tend to increase the stiffness, hardness, and modulus of
a polymeric material. Polymers with at least some degree of crystallinity are denser,
stiffer, and stronger than amorphous polymers. However, the amorphous region
contributes to the toughness and ﬂexibility of polymers. Increasing the percentage of
crystallinity decreases the impact strength and increases the probability of brittle failure. A
reduction in the average molecular weight tends to reduce the impact strength and vice
versa. (Seymour and Carraher, 1984).

Most linear polymers are hard brittle plastics at temperatures below their
characteristic glass transition temperature (Tg), leathery and rubbery at temperatures just
above the Tg, and viscous liquid at temperatures above the melting temperature (Tm).
Some polymer, including networked, highly cross-linked, and highly crystalline polymers,
are difﬁcult to melt and oﬁen undergo solid phase thermal degradation before melting
occurs (Seymour and Carraher, 1984).

The impact properties of the polymeric materials are directly related to the overall
toughness of the material. Impact strength is the amount of energy which a plastic
material can absorb before it breaks. The higher the impact strength of a material, the
higher the toughness and vice versa. Impact resistance is the ability of a material to resist
breaking under a shock loading or the ability to resist the fracture under stress applied at
high speed. Factors affecting impact properties includes temperature, orientation,

processing conditions, and degree of crystallinity.

22

Impact resistance is one of the most widely speciﬁed mechanical properties of
polymeric materials. The impact resistance of plastic materials is strongly dependent upon
temperature. At lower temperatures the impact strength is reduced drastically. The
reduction in impact is even more dramatic near the glass transition temperature.
Conversely, at higher test temperatures, the impact strength is signiﬁcantly improved
(Shah, 1984; Spath, 1957).

Processing conditions play a key role in determining the impact resistance of a
material. Inadequate processing conditions can cause the material to lose its inherent
toughness. High processing temperature can also cause thermal degradation and
therefore, reduced impact strength. Improper processing conditions also create a weak
weld line that reduces overall impact strength. Molecular orientation introduced into
drawn ﬁlms and ﬁbers may give extra strength and toughness over the isotropic material.
However, such directional orientation of polymer molecules can be very susceptible in a
molded part since the impact stresses are usually multiaxial.

Impact resistance, or resistance of brittle fracture, is also a function of the
molecular weight of a polymer. A reduction in the average molecular weight tends to
reduce the impact strength and vice versa. Impact resistance of brittle polymer is
increased by additional plasticizers. Thus, polyvinyl chloride (PVC), plasticized by
relatively large amounts of dioethyl phthalate, is much less brittle than unplasticised rigid
PVC (Seymour and Carraher, 1984).

The impact resistance of bottles can be improved by biaxial drawing which has
contributed to the success that PET has experienced due to stretch blow molding.

However, there are several processing techniques, such as extrusion blowing that cannot

23

create this orientation effect. Therefore, other approaches such as impact modiﬁer must
be provided to improve the property. Impact modiﬁers primarily used for packaging
purpose especially for the production of sheets and bottles, improve the impact resistance
of any type of plastic while maintaining transparency (Meyer and Leblanc, 1995)

Troy, Shortridge, and Fozey (1985) studied the impact modiﬁer performance in 16
ounce PVC bottles using a drop impact resistance test. Impact modiﬁer (methacrylate-
butadiene-styrene) are added to PVC blow molding compounds. PVC bottles with impact
modiﬁer can resist the higher drop heights before failing than those of the bottles without
impact modiﬁer. This indicates that impact modiﬁer can be used to overcome the brittle
characteristics of unmodiﬁed PVC resins. Besides methacrylate-butadiene-styrene,
chlorinated polyethylene, EVA, acrylic resin derivatives, and ABS are also used as an

impact modiﬁers (Briston, 1994).

Effect of Temperature on Mechanical Properties

Polymers pass through different physical states according to their temperature; at
low temperature, since the internal mobility of the macromolecules is “frozen”, they
solidify and present a glassy, amorphous appearance. As the contact temperature rises,
the material passes through a phase of relaxation, characterized by an erratic behavior on
the path of Young’s modulus (the ratio of stress to corresponding strain below the
proportional limit of a material, or a measure of material’s stiffness) and other physical
properties.

Generally, both crystalline and amorphous polymers are brittle at low temperature

and both have relatively low impact strengths. As the temperature of a material decreases,

24

its ultimate strength and young’s modulus increase; this applies to tension, compression,

and bending. At the same time elongation (the increase in the length of a test specimen

produced by a tensile load) decreases (Seymour and Carraher, 1984; Spath, 1957).

At low temperatures, the atoms vibrate with small amplitudes. With increasing
temperature, the vibrations increase in magnitude and eventually become coordinated to
the degree that translational chain motions are produced, which involve many chain atoms
at elevated temperatures (Calister, 1994).

When a polymer is heated to a certain temperature, changes of a physical nature,
can occur. These may consist of various transitions, which are accompanied by changes in
the physical properties of the material (brittleness, elasticity, devitriﬁcation, softening,
melting, etc.).

Kemp and Kennedy (1987) indicated that the effect of heat on polymers can
manifest itself in 2 ways:

a) the polymer softens or melts. The kinetic energy of the chains becomes large enough
to overcome the intermolecular forces, and the plastic ﬂows easily;

b) the structure is degraded. Some macromolecular compounds undergo scission to
lower molecular weight products or even to monomer without changing chemical
composition-i.e. are depolymerized, others release low-molecular weight fragments
with simultaneous change in chemical composition-i.e. are decomposed. Both
processes are called degradation.

Korshak (1971) pointed out that polymers exposed to high temperature undergo
chemical transformations of three main kinds: purely thermal conversions, which include

thermal degradation and cross-linking of the polymer; oxidative degradation and cross-

 

25

linking; and hydrolytic degradation. The course of each of these reactions depends to a
great extent on the structure of the polymer.

Degradation is a rupture of the macromolecular structure, which results in a
progressive decrease in the molecular weight of the polymer, and hence in the
deterioration of its mechanical properties. Cross-linking consists of the formation of
bonds between the macromolecular chains, with consequent increase in the molecular
weight of the polymer; this means that the physical and mechanical characteristics of the
polymer and its heat resistance may improve to a certain extent as a result (Korshack,
1971)

Mechanical stress due to impact arises usually during distribution. Mechanical
stress due to pressure changes within the package is normal during production processes
involving heat treatments of foodstuﬁ‘s. In these cases the stress results due to thermal
expansion of the content. The volume increases due to heating of the contents from an
initial temperature to the high temperature. Ifthe free volume in the package is smaller
than the volume increase of the incompressible aqueous phase, the inner pressure would
cause rupture of the package or of the closure. The pressure is also aﬁ‘ected by gaseous
components of the product and by expansion of the package itself (Kemp and Kennedy,

1987)

Effect of Heat on Degree of Crystallinity
Polymer properties depend on the phase state (amorphous or crystalline) of the
polymers as well as their molecular weight, and chemical composition. Polymers are

closely packed, having conformation of the lowest possible energies. If at a certain

26

temperature the cohesion energy of the chain is larger than the kinetic energy, then
conditions are suitable for parts of the macromolecule to exhibit close packaging and
incorporation into a crystalline arrangement. When cohesive energy exceeds the kinetic
energy of the chains, provided that there is not too large of a steric hindrance,
crystallization take place (even in polymers which do not crystallize under normal
condition) (Miller, 1966).

Crystallization is temperature dependent to a considerable extent. Primary
crystallization comprises formation of the crystal nuclei and grth of the spherulites.

The secondary crystallization has great practical importance. It is this latter stage which is
responsible for the undesirable volume and other physical changes which take place
usually after processing. Its course and extent are affected markedly by the thermal
history of the polymer (Kemp and Kennedy, 1987).

Crystallites in polymers reinforce their structure and improve their mechanical
properties and resistance to elevated temperatures. Therefore, an important consideration
of the polymer change is the change in crystallinity of the polymer because crystallinity can
play a signiﬁcant role in the physical character of a polymer such as clarity, tensile
strength, and impact strength (Seymour and Carraher, 1984).

Zucherman and Miltz (1993) studied the changes in degree of crystallinity of thin-
layer susceptors during microwave heating. The structure used was a metallized
polyethylene terephthalate ﬁlm laminated to paperboard with a polyurethane-based
adhesive. The sample was heated in a 700 watt microwave oven. The degree of
crystallinity of the PET ﬁlm before and after heating in the microwave was measured using

DSC. The results showed that microwave heating affected the crystallinity of the

27
metallized PET. They concluded that the degree of crystallinity of the ﬁlm was reduced

during heating in the microwave oven caused by relaxation of the oriented PET ﬁlm. In
addition, differences in thermal diffusivity between aluminum and PET, resulting in
differences in heating and expansion rates may have added to this eﬂ‘ect.

Brennen (1978) investigated the effect of thermal conditioning on percent
crystallinity of low density polyethylene ﬁlm used for food storage. Specimens were
analyzed without thermal conditioning and after having annealing at 100 °C for 12 hours.
The results showed that the annealing increased percent crystallinity by boosting the high
temperature crystallinity of the material.

In pharmaceutical packaging, various types of sterilization processes are employed.
One of these is gamma irradiation. This process may not only sterilize, but may also affect
material properties and thermal characteristics of the polymeric packaging material
(Breakey and Cassel, 1979). This is in agreement with the study of Trice and Goolsby
(1990). They determined the physical and thermal property changes of polypropylene
packaging subjected to gamma irradiation using DSC. Results showed that the gamma-
irradiated sample, when compared to the nonirradiated material, had melting point
depression, and broadening of the endotherm with a much smaller peak amplitude in the
isothermal crystallization region. This was due to crosslinking which occurred during the
irradiation process.

Christie, Gregory, and Wood (1993) determined the effects of polypropylene
crystallization on ﬁlm forming by investigating the effects of operating parameters
(quenching temperatures) on the ﬁnished product properties of a ﬁlm grade PP

homopolymer. This study showed correlations between operating parameters and

28

crystallinity in PP ﬁlms. Processing conditions had a dramatic impact on ﬁnal crystallinity
and corresponding ﬁlm properties. Increases in density because of aging due to secondary
crystallization also affect ﬁnal ﬁlm properties. These results were supported by Shah
(1984) who found that increases in percent crystallinity decreased the impact strength and
increased the probability of brittle failure.

Nicastro, et al (1993) studied the effect of heat sealing on the seal strength and
crystallinity of unoriented cast polypropylene ﬁlm with thickness of 3 mil. PP samples
were heat sealed at 135, 137.8, and 140.6 C, dwell time 0.5, 1, and 10 second, and
pressure 3, 15, and 30 psi. The results indicated that sealing temperature had a signiﬁcant
effect on the increase in percent crystallinity of the ﬁlm seal region, while heat sealing
dwell time and pressure had little effect on the increase in crystallinity. This increase in
crystallinity increased the seal strength due to the larger amount of interdiﬁ‘using polymer
chain segments. In contrast, Selikhova (1989) observed that a greater crystallinity may
actually increase the brittleness of the polymer in the seal region and cause the seal layer of

ﬁlm laminate to be more susceptible to delarrrination.

Mechanical Testing for Polymeric Packaging Materials

After being in use for a period of time, materials undergo many changes due to
mechanical stress and various other inﬂuences such as humidity, temperature, radiation,
and chemical radiation. The impact test, among other methods, can be used to investigate
such changes. Impact tests can be divided into six major classes and subdivided into many

different types having slight variation as follow: Pendulum impact tests, High rate tension

29
tests, Falling weight impact tests, Instrument impact tests, High rate impact tests, and

miscellaneous tests (Shah, 1984).

Most physical test are carried out on a molded test piece or, in the case of
permeability, on ﬁlm or sheet. For the test that can be carried out on the ﬁnished
container, impact tests on plastic bottles are useful when change in material and /or bottle
design is contemplated. Performance tests will quickly show any fundamental weakness
that may lead to leakage of the contents during distribution. Drop impact resistance
evaluation of a blow molded thermoplastic container can be carried out using a standard
test (ASTM-D 2463) which practically simulates end-use environmental conditions. It is
also useful for comparing different materials as well as evaluating the inﬂuence of

processing conditions on the impact properties of bottles (Briston, 1994; Shah, 1984).

Drop Impact Test

Drop impact resistance of blow molded thermoplastic containers can be
determined by three conventional tests: the Static Drop Height, the Bruceton Staircase,
and the Cumulative Drop methods. These standard test method are used to evaluate the
effect of construction, materials and processing conditions on the impact resistance of
blown containers (ASTM-D 2463, 1996).

In the Static drop height, the container is dropped from a ﬁxed height and the
percent failure report. For the Bruceton staircase methods, a set of containers are
dropped from various heights. The drop height is raised or lowered depending on the
result obtained from the previous test, if the previous test container did not fail, the next

bottle will be tested at a higher height. If the previous container failed, the drop height of

30

the next bottle will be lowered. The mean failure height and standard deviation are then
calculated from the data obtained. In the cumulative drop test, each of 20 bottles is
subjected to successively higher drop heights until breakage occurs, and the number of
broken bottles at each drop height is recorded (Briston, 1994).

Troy, Shortridge, and Fazey (1985) used the drop ten test protocol for testing
PVC bottles in groups rather than one at a time to ﬁnd drop heights that span the range
from no breaks to all break and to reduce testing times to just slightly longer than those of
the Bruceton test in which bottles are dropped individually. If more than half the bottles
break, the drop height is lowered. Likewise, if breakage is very low the drop height is
raised. They concluded that values derived ﬁ'om the Drop ten test are highly reproducible.
In addition, this test generated more data, since results are based on a lot more bottles
than used in the three most common tests (Bruceton, Static, and Cumulative tests).

Meyer and Labrant (1995) performed drop testing to determine the effect of an
impact modiﬁer on the performance of 750 ml. extrusion blown PETG containers with and
without impact modiﬁer by comparing the drop heights and the percent of failures at
speciﬁc drop heights. The result showed that breakage levels of both unmodiﬁed and
impact modiﬁed bottles increased with increasing drop heights. The impact modiﬁed
bottles exhibited little breakage compared with unmodiﬁed bottles. Successive increases of
impact modiﬁer improved the bottle failure drop height.

Besides impact modiﬁcation, test conditions such as impact angle, ﬁll level and
wall thickness can inﬂuence the results of a drop impact resistance. Bottles dropped onto

a ﬂat surface have higher drop height resistance than those dropped at an angle of 10

31

degrees because stress is concentrated in angle drops at a single point instead of being

dispersed over the entire bottom of the bottle.

Determination of Percent Crystallinity

Measurements which can be related to the crystalline state of a polymer are : x-ray
difﬁ'action, density, nuclear magnetic resonance absorption, heat capacity, infrared
absorption of low molecular weight compounds and deuterium exchange. Each of these
measurements describes something about the polymer, not necessarily the percentage
crystallinity, but something about the regularity of packing, the freedom of motion of the
molecules or the extent of intermolecular hydrogen bonding of the polymer (Sperling,
1986; Miller, 1966).

Percent crystallinity can be determined by quantifying the heat associated with
melting (fusion) of the polymer. Differential scanning calorimetry (DSC) is a thermal
analysis technique to measure the temperatures and heat ﬂows associated with transitions
in materials as a function of time and temperature. Such measurements provide
quantitative and qualitative information about physical and chemical changes that involve
endothermic or exothermic processes, or changes in heat capacity. Percent crystallinity of
polymer can be determined by quantifying the heat associated with melting endotherm
(ﬁrsion) of the polymer by developing a ratio against the heat of ﬁision for a 100 %
crystalline sample of the same material, or more commonly by rationing against a polymer
of known crystallinity to obtain relative value (Thomas, 1995).

The conventional instrument used for making DSC measurements is the heat ﬂux

design shown in Figure 1. In this design, a metallic disks is the primary means of heat

32

transfer to and from the sample and reference. The sample, which is placed in a metal pan,
and the reference (an empty pan) sit on raised platforms formed in the constantan disc. As
heat is transferred through the disc, the differential heat ﬂow to the sample and reference
is measured by area thermocouples formed by the junction of the constantan disc and
chromel wafers which cover the underside of the platforms (Anonymous, 1997).

Modulated DSC (MDSC) is a relatively new thermal analysis technique which
provides the same information as conventional DSC, but in addition has the unique ability
to measure heat capacity continuously (Mele et al, 1995).

The general equation which describes the resultant heat ﬂow at any point in a DSC

experiment is:

dQ / dt = Cp [3 + flit)
where: dQ / dt = total heat ﬂow
Cp = heat capacity
[3 = heating rate
f (T, t) = heat ﬂow from kinetic (absolute temperature

and time dependent) process
DSC can measure only heat ﬂow which is composed of two components. One
component is a ﬁrnction of the sample’s heat capacity and rate of temperature change, and
the other is a function of absolute temperature and time (Thomas, 1995).
MDSC determines the total, as well as heat capacity (reversing) heat ﬂow
component and kinetic (nonreversing) component of total heat ﬂow, to provide increased
understanding of complex transitions in materials (Figure 2). MDSC is able to do this

based on the two heating rates seen by the material; the average heating rate which

33
provides total heat ﬂow information (dQ / dt) and the sinusoidal heating rate which

provides heat capacity (reversing heat ﬂow, CpB ). The kinetic (nonreversing) heat ﬂow is
determined as the arithmetic difference between the total heat ﬂow and the heat capacity
component: Nonreversing heat ﬂow = Total heat ﬂow - Reversing heat ﬂow

(Anonymous, 1997; Thomas, 1995).

 

34

 

 

 

 

 

 

 

 

 

 

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Figure 2. Nonreversible, Reversible, and Total Heat Flux Signal (Thomas, 1995).

CHAPTER 3

MATERIALS AND METHOD

3.1 Samples

Plastic containers: 46 cases of the 24 ounce plastic bottles were provided by a
product manufacturer. The containers were extrusion blowmolded from polypropylene
into a squat jug shape. The dimensions of the bottle were; height 7 inches with an eclipse
bottom and were pinched at 2 points of the bottle’s handle. The bottles were ﬁlled
commercially with sugar syrup and closed in the normal manner (Figure 3). All bottles
were placed in an environmentally controlled room maintained at 20 i 2 °C, and 50 i 5 %

relative humidity.

3.2 Wall Thickness of Materials Used

Six bottles were selected randomly so that they were representative of the lots
being tested. The thickness of the material was measured at ﬂat bottom, body wall, right
bottom comer, left bottom corner, and side seam of each bottle. The wall thickness
measurement was carried out using a Magna-Mike instrument (Model 800, Parametics,
MA) composing of a steel target ball, magnetic probe, and control unit. To measure the
wall thickness, a 3/4 inch diameter target ball was dropped into the bottle and a magnetic

35

36
probe tip was applied to the outside of the bottle. The ball was attracted to the probe

through the nonmagnetic plastic container, the container was moved over the tip of the
probe to locate the points at which measurements were to be made. The ﬁeld strength
varies as the distance between the ball and the probe changes, which is converted with a

thickness measurement.

3.3 Determination of the Impact Orientation of the Bottles By Free Fall Drop Test

A drop test was designed to establish the critical impact orientation using a ﬁee fall
drop. The hypothesis was that impact of a particular orientation(s) can lead to bottle
breakage.

ASTM D-2463 (1997), Standard Test Method for Determining Drop Impact
Resistance of Blow-molded Thermoplastic Containers, was used for this purpose. This
standard consists of dropping containers ﬁlled with tap water. Since data developed with
a water-ﬁlled container may not be representative of what might be expected with a
product of high speciﬁc gravity, the containers ﬁlled with syrup as received, were used as
test samples.

Lansmont Model PDT 56 E Drop Tester was used to simulate the free fall drop.
This machine was used in compliance with ASTM D-2463 standard test method. It is
equipped with a drop leaf pneumatic actuation system which prevents package rotation
and assures reproducible results. The container was dropped on to a 46”x 36” x 0.5” steel
plate mounted in concrete.

Prior to the test, 50 bottles ﬁlled with syrup to the ﬁrll level, were conditioned for

24 hours at 20 j: 2 °C and 50 J_r 5 % relative humidity to bring the bottles into equilibrium

37

with an average room conditions. For any one of the test orientations (including face, ﬂat
bottom, handle, right bottom comer, and left bottom comer), a set of 7-10 bottles was
used. The bottles were then subjected to the free fall drop test using the Free Fall Drop
Tester (Lansmont Model PDT-56E ). Drop impact resistance was determined using the
Bruceton Staircase or “Up and Down” method (ASTM standard D-2463, 1997). It
consists of dropping a set of containers from various heights, the drop height being raised
or lowered depending on the result of the previous test, that is, if the previous container
failed, the drop height is lowered; if the previous container did not fail, the drop height is
raised.

The drops were conducted on several package orientations: face, ﬂat bottom, right
bottom comer, left bottom corner, and handle (Figure 3), from a dr0p height of 24 inches
at increments or decrements of 3 inches. A positioning jig was used to hold a test
container at the desired orientation. A new container was used for each drop. Following
each drop, the container was visually observed. When a container in a set failed, the
following rule was applied, if the ﬁrst container dropped did not fail, the second container
was dropped from a height 3 inches higher. If the ﬁrst container failed, drop the
second container was dropped from a height 3 inches lower.

Result Evaluation

All specimens were examined for failure after dropping them at the particular
height. A failure was deﬁned as any fracture visible to the observer. Any mark of
contained liquid on the exterior of the bottle emerging from any aperture other than the

molded opening was also considered a failure. Bottles were squeezed gently after impact

38

 

Figure 3. Syrup Bottle: (1) left bottom corner, (2) right bottom corner,
(3) handle, (4) ﬂat bottom, (5) face.

39
to determine any pinhole type failure. Since not all bottles failed at the same drop height,

a mean failure height and standard deviation were calculated from the data.
Statistical Aﬂlysis

Statistical analysis was conducted using a Completely Randomized Design with 5
impact orientations. SAS software version 6.12 (SAS Institute, Inc.) was used to run a
one-way analysis of variance in association with Duncan’s Multiple Range Test. The
ANOVA and Duncan’s Multiple Range Test were used to establish signiﬁcant difference

using p-value 5 0.05 (Steel, Torrie, and Dickey, 1997).

3.4 Determination of Drop Impact Resistance of the Bottles Filled with Syrup at
Different Levels

The preliminary test established that impact orientations at the right bottom corner
and handle had potential to cause failure of the container. An experiment was then
designed to establish the failure drop height of bottles ﬁlled with syrup at full, %, half, and
V4 levels. The bottles were dropped onto their right bottom comer and handle using the
Bruceton Staircase free fall drop method (ASTM Standard-D2463, 1997). The ﬁrst bottle
was dropped from 21 inches onto its handle and from 36 inches onto its right bottom
corner with increments or decrements of 3 inches, thereafter.
StiltisticaIAﬂlysis

Statistical analysis was conducted using a Completely Randomized Design of the 4
ﬁll levels. SAS software version 6.12 (SAS Institute, Inc.) was used to run a one-way

analysis of variance in association with Duncan’s Multiple Range Test. The ANOVA and

40
Duncan’s Multiple Range Test were used to establish signiﬁcant difference using p-value 5

0.05 (Steel, Torrie, and Dickey, 1997).

3.5 Effect of Microwave Repeated Heating on the Drop Impact Resistance of the
Bottles

It was hypothesized that repeated microwave heating of a plastic container may
affect its mechanical properties such as drop impact resistance. Repeated microwave
heating and free fall drops were conducted to simulate the actual situation likely to occur

in end-use.

3.5.1 Microwave exposure

Empty plastic syrup bottles were ﬁlled with sugar syrup at full, V4, V2, and V4 levels,
respectively. Heating time was varied (2.00, 1.45, 1.30 and 1.00 minutes, respectively) in
proportion to the level of syrup. The heating times used, followed the directions indicated on
the label to reach a speciﬁc end temperature. Each container was placed upright in the
microwave oven (Gold Star: 2450 MHz, max. microwave output 800 W. with a cavity
measuring 13 V4” x 8”x 14 V2”) at exactly the same location (center of the glass plate) for every
run so as to maintain the same electric ﬁeld. Each container was heated at the same starting
temperature (20 ° C) to reach the speciﬁc end temperature (58-60 ° C) and then cooled to
room temperature before the next reheating in the microwave oven.

A microwave heat treatment scheme (Figure 4) was developed to simulate the
microwave heating directions as found on the product’s label. For each experiment, one set of

bottles (approximately 84 bottles, each bottle contains 710 ml sugar syrup) was subjected to 12

41

different microwave heating variables. Heating time was dependent on product ﬁll level. The

heating and reheating regime as described above is presented in more detail in Table 1.

3.5.2 Temperature of the Sample After Reheating in Microwave Oven

Sixteen bottles (uniform initial temperature 20 °C) ﬁlled with syrup at full, V4, V2,
and V4 levels were placed on a ﬁxed location in the center of a household microwave oven
(Gold Star, 2450 Hz, max. 800 W) operating at full power for 2.00, 1.45, 1.30, and 1.00
min., respectively. After heating in the microwave oven, the temperature in syrup and on
the inside and outside bottle surfaces were measured using a pocket-probe digital
thermocouple (Electronic Development Laboratories Inc., NY).
mtistical Analysis
A statistical analysis to detemrine combination effects of three different product areas and
4 ﬁll levels on product and package material temperature was performed by Factorial
Design. SAS software version 6.12 (SAS Institute, Inc.) was used to run a multiple
analysis of variance. When the effect of product area and/or ﬁll level was signiﬁcant,
statistical difference of means was performed using Duncan’s Multiple Range Test using

p—value 5 0.05 (Steel, Torrie, and Dickey, 1997).

42

Figure 4. Flow Diagram of Microwave Heat Treatments

Step 1: Heat each bottle in the set (120 bottles, each bottle contains 710 ml. sugar syrup)
for 2 min. and cool to room temperature (~20 °C)
~L —> take 10 bottles for drop testing

Step 2: Heat each of 110 bottles for 2 min. and cool to room temperature (~20 °C)
Jr —) take 10 bottles for drop testing

Step 3: Heat each of 100 bottles for 2 min. and cool to room temperature (~20 ° C)
Jr —) take 10 bottles for drop testing

Step 4: Remove 177.5 ml. of syrup from each bottle to leave 532.5 ml (3/4 level) in the
bottle
Heat each of 90 bottles for 1.75 min. and cool to room temperature (~20 ° C)
I —> take 10 bottles for drop testing

Step 5: Heat each of 80 bottles for 1.75 min. and cool to room temperature (~20 °C)
.1. —> take 10 bottles for drop testing

Step 6: Heat each of 70 bottles for 1.75 min. and cool to room temperature (~20 °C)
~L —) take 10 bottles for drop testing

Step 7: Remove 177 .5 ml. of syrup from each bottle to leave 355 ml (1/2 level) in the
bottle
Heat each of 60 bottles for 1.5 min. and cool to room temperature (~20 °C)
iv ——) take 10 bottles for drop testing

Step 8: Heat each of 50 bottles for 1.5 min. and cool to room temperature (~20 ° C)
.1. —> take 10 bottles for drop testing

Step 9: Heat each of 40 bottles for 1.5 min. and cool to room temperature (~20 ° C)
Jr —) take 10 bottles for drop testing

Step 10: Remove 177 .5 ml. of syrup from each bottle to leave 177.5 mi (l/4 level) in the bottle
Heat each of 30 bottles for l min. and cool to room temperature (~20 ° C)

I —-> take 10 bottles for drop testing
Step 11: Heat each of 20 bottles for 1 min. and cool to room temperature (~20 ° C)
Jr —> take 10 bottles for drop testing

Step 12: Heat each of 10 bottles for 1 min. and cool to room temperature (~20 °C)

i

take 10 bottles for drop testing

43

Table 1. Microwave Heating Treatment

 

 

 

 

 

 

 

 

 

 

 

 

 

Microwave
heating Condition of Heating
treatment

1 Full level heat for 2.00 nrin / cool , 1 time

2 Full level heat for 2.00 min / cool , 2 times

3 Full level heat for 2.00 min / cool , 3 times

4 From treatment 3, remove syrup to V4 level in the bottle
heat for 1.45 min/ cool, 1 time

5 From treatment 3, remove syrup to V4 level in the bottle
heat for 1.45 min/ cool, 2 times

6 From treatment 3, remove syrup to 3/4 level in the bottle
heat for 1.45 min/ cool, 3 times

7 From treatment 6, remove syrup to V2 level in the bottle
heat for 1.30 min/ cool, 1 time

8 From treatment 6, remove syrup to V2 level in the bottle
heat for 1.30 min/ cool, 2 times

9 From treatment 6, remove syrup to V2 level in the bottle
heat for 1.30 min/ cool, 3 times

10 From treatment 7, remove syrup to V4 level in the bottle
heat for 1.00 min/ cool, 1 time

11 From treatment 7, remove syrup to V4 level in the bottle
heat for 1.00 min/ cool, 2 times

12 From treatment 7, remove syrup to V4 level in the bottle

heat for 1.00 min/ cool, 3 times

 

44
3.5.3 Drop Impact Resistance of Microwave Reheated Bottles

After heated in the microwave oven (Figure 4), the bottles were cooled to
20 i 2 ° C and conditioned for 24 hours at the testing room conditions to bring the bottles
into equilibrium with average room conditions (20 i 2 °C and 50 i 5 % relative
humidity). The bottles were then subjected to free fall drop testing using the Free Fall
Drop Tester (Lansmont Model PDT-56E ) to determine drop impact resistance using the

Bruceton Staircase Method.

3.5.4 Effect of Microwave Reheating on the Drop Impact Resistance of the
Bottles

Unheated bottles ﬁlled with syrup at ﬁrll, 3A, V2 and V4 levels were used as control
bottles for comparison to microwave reheated bottles from treatments 1-3, 4-6, 7-9, and
10-12, respectively. For drop height evaluation, the Bruceton Staircase Method was used
to determine effect of microwave heating on drop impact resistance. Before drop testing,
all bottles were stored at 20 i 2 ° C, 50 i 5 % relative humidity at least 24 hours.
St_atistica1 An_alysis

Statistical analysis was done using a Completely Randomized Design. SAS
software version 6.12 (SAS Institute, Inc.) was used to run a one-way analysis of variance
to assess the inﬂuence of repeated heating on the drop impact resistance of the bottles.
When the effect of microwave heating was signiﬁcant, statistical difference of means was
performed using Duncan’s Multiple Range Test using p-value 5 0.05(Steel, Torrie, and

Dickey, 1997).

45
3.6 Effect of Temperature on the Drop Impact Resistance of the Bottles

The effect of the package material temperature on the drop impact resistance was
determined by comparing drop impact resistance of the bottle when dropped at room
temperature (20 i 2 °C), at elevated temperature following heating in the microwave, and
at low temperature after cooling in a reﬁigerator. After heating at ﬁill power in the
microwave oven, all bottles were then immediately subjected to the free fall drop test to
determine the drop impact resistance at handle and right bottom comer using the Bruceton
Staircase Method.

After heating in the microwave oven, the bottles were refrigerated for 24 hours at
a temperature of 5-9 °C. The bottles were then subjected to the free fall drop test to
determine their drop impact resistance at right bottom corner and handle using Bruceton
Staircase Method. This test was developed to identify inﬂuence of temperature on impact
resistance. This test was also used simulate the actual product’s end use requirement.
ﬂatistical Anahysis

Statistical analysis was done using Factorial Design. Cochran’s Test was used to
determine signiﬁcant inﬂuence of the three temperatures when data were qualitative. SAS
software version 6.12 (SAS Institute, Inc.) was used to run a multiple analysis of variance
to assess the inﬂuence of the effect of temperatures and repeated heating on the drop
impact resistance of the bottles. Duncan’s Multiple Range Test was then used to establish

statistical difference of means using p-value 5 0.05(Steel, Torrie, and Dickey, 1997).

46
3.7 Determination of the Change in Percent Crystallinity of the Packaging Material

by Thermal Analysis

Heat of fusion which Correlates to percent crystallinity of the container can be
determined by Modulated Differential Scanning Calorimeter (TA Inst. 2200, TA
Instruments Inc., DE). Heat of fusion of the bottle exposed to microwave heating was
compared to that of the control (unheated) bottle to determine the effect of repeated
heating on the percent crystallinity of the packaging material.

To measure percent crystallinity, two unheated bottles and two bottles which were
subjected to microwaves were selected and 4 material specimens were cut from the
bottom and upper part near the handle of each bottle and weighed in a tared aluminum
pan, and then placed in the MDSC cell to measure the endothermic processes or changes
in heat capacity. The temperature of the specimen was programmed through its melting
point while recording the thermal curve of temperature and heat ﬂow associated with
transitions in the samples as a ﬁrnction of time and temperature.

Experimental conditions included:
Heating rate : 2.5 0C/ min

Reference: empty aluminum MDSC pan
Starting Temperature: -50 °C

Limit Temperature: 175 °C

Polymer crystallinity can be determined using Modulated DSC by quantifying the
heat associated with ﬁision of the polymer. Heat of fusion was reported as relative %
crystallinity by comparing unheated polymer against heated polymer to obtain relative

values.

CHAPTER 4

RESULTS AND DISCUSSIONS

4.1 Wall Thickness of the Bottles
The wall thickness of the bottles used were measured at different positions: body
wall, bottom face, right bottom comer, left bottom corner, and side seam. The thickness

of the bottle measurements (Table 2) represent 6 replications. Thickness varied at the

different positions.

Table 2. Wall Thickness of the Bottle Material.

 

 

 

Area Thickness (inch)'I
Body wall 0.055 : 0.001
Bottom face 0.181 : 0.009
Right bottom comer 0.106 i 0.006
Left bottom comer 0.188 j: 0.008
Side seam 0.057 i 0.002
a mean with standard deviation

4.2 Determination of the Impact Orientation of the Bottle by Free Fall Drop Test
This experiment was performed in order to establish the particular orientation
which was the most susceptible to breakage due to a free fall drop. Bottles ﬁlled with

syrup were tested for fracture, leakage, or breakage by dropping them from various

47

48
heights using the Bruceton Staircase Method. Since not all bottles failed at the same drop

height, test results were statistically evaluated (Table 3). According to ASTM Standard
Test Method for Drop Impact Resistance of Thermoplastic Containers (1996), drop height
from a free fall drop provides a measure of the drop impact resistance of plastic bottles.
Results of the one-way analysis of variance (Table 4) for drop heights of the
bottles dropped onto their handles, face, ﬂat bottom, right bottom comer, and left bottom
corner orientations showed that bottles dropped with different drop orientations exhibited

signiﬁcantly different drop impact resistance (p50.05).

Table 3. Drop Height Failure of the Bottles at Various Orientations.

 

 

Orientations Drop Height '
(inch)
Handle 22.6 i 07'
Right bottom comer 38.6 i 1.3 b
Left bottom corner 43.8 i 0.7 c
Face 480:1.5d
Flat bottom 51.0 : 1.0°

 

"° means with same superscript alphabets are not signiﬁcantly different (p50.05).
comparison are made only within the same column.
f mean with standard deviation

Table 4. Analysis of Variance of Drop Height of the Bottles at Various Orientations

 

 

(at 20 °C).
Source of Variation d.f. Mean Square ‘ F
Orientations 4 516.060 13 6. 524*
Error 20 3.780

 

* denotes a statistically signiﬁcant difference (p50.05).

49

Bottles falling onto their handles, right bottom corner, and left bottom comer
were more susceptible than those falling onto their face or ﬂat bottom. Drop impact
resistance was low at the bottom comers because stress was concentrated at a single
point (angle drop) instead of being dispersed over the entire bottom or surface of the
bottle. The low drop impact resistance at the handle was in good agreement with that
reported by Briston (1995) who stated that the weak positions on the bottles were
generally at mold parting lines and pinch-off regions. Hence, bottles that were impacted
on their handles corresponds to where the resin was pinched off. These bottles were prone
to breakage.

It was also evident that the right bottom comer was less resistant to impact than
that of the left bottom comer, even though these two positions cause local areas of stress
concentration. One possibility is that the material was thinner at the right bottom comer
(Table 2). As the thickness increases, the impact resistance increases, and the energy
required to fracture the specimen also increases (Shah, 1984). It is also possible that being
on the same side as where the handle was pinched off resulted in decreased impact
resistance at the right bottom comer. The right bottom corner and handle were the

weakest points and therefore, were use as targets for ﬁrrther experiment.

4.3 Determination of Bottle Drop Impact Resistance

Generally, drop impact resistance is determined by dropping blow-molded
containers ﬁlled with water. Data developed from water-ﬁlled containers may not be
representative of a product having high speciﬁc gravity such as syrup. Thus, in this study,

the bottles were ﬁlled with syrup which is the actual product contained. To establish a

50

correlation between breakage, drop height, and ﬁll level and to simulate speciﬁc end-use,
drop impact resistance of the bottles was determined at full, three quarters, half, and
quarter ﬁll levels of syrup. Product weight was proportional to ﬁll levels (Table 5). The

higher the ﬁll level, the higher the weight.

Table 5. Weight of the Plastic Bottle Filled with Syrup at full, V4, V2, and V4 Levels.

 

 

Fill Level Product Weight (g)'
Full 907.5 gt 2.4
Three quarter 694.9 3: 3.1
Half 483.9 1 2.9
Quarter 271.4 j; 2.0

 

" mean with standard deviation

The weak points of the bottle had been established as right bottom comer and
handle. Hence, this experiment was performed by dropping the bottles at these two points.

Tables 6 and 8 show drop impact resistance of bottles dropped at right bottom
comers and handles, respectively. One-way analysis of variance of drop heights at the
right bottom comer (Table 7) and on the handle (Table 9), found that ﬁll level of the
syrup signiﬁcantly affected drop impact resistance of the bottles (p50.05). The inﬂuence
of ﬁll levels on drop impact resistance both at right bottom comer and handle found that
the higher the ﬁll level, the higher the weight of the sample, and , therefore, the lower the

drop impact resistance (Figure 5).

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52

Table 6. Drop Heights of the Bottles Filled with Syrup at full, V4, V2, and V4 Levels
and Dropped at the Right Bottom Corner.

 

 

Level of Syrup Drop Heights
(inch)
Full 39.0 i 2.1 ‘
Three quarter 45.6 i 2.5 b
Half 54.0 i 1.3 °
Quarter 57.0 : 2.1d

 

"1"“ means with superscript alphabets are not signiﬁcantly different (pSO-05)-

comparison are made only within the same column.
° mean with standard deviation

Table 7. Analysis of Variance of Drop Height Failure of Bottles Filled with Syrup at
full, V4, V2, and V4 Levels and Dropped at the Right Bottom Corner.

 

 

Source of Variation d.f. Mean Square F
Level of Syrup 3 334.20 67.515*
Error 16 4.950

 

* denotes a statistically signiﬁcant difference (p50.05).

Table 8. Drop Height of the Bottles Filled with Syrup at full, V4, V2, and V4 Levels
and Dropped onto their Handles.

 

 

Level of Syrup Drop Heights
(inch)
Full 22.8 11.5 '
Three quarter 25.8 i 1.5 b
Half 33.8 i 13"
Quarter 40.5 i 1.5d

 

" b’ °' d means with superscript alphabets are not signiﬁcantly different (p50.05).
comparison are made only within the same column.
° mean with standard deviation

53

Table 9. Analysis of Variance of Drop Height Failure of Bottles Filled with Syrup at
full, V4, V2, and V4 Levels and Dropped onto their Handles.

 

 

Source of Variation d.f. Mean Square F
Level of Syrup 3 281.55 105.534*
Error 14 2.67

 

* denotes a statistically signiﬁcant difference (p50.05).

4.4 Effect of Microwave Repeated Heating on the Drop Impact Resistance of the
Bottles
4.4.1 Temperature of the Sample after Heating in Microwave Oven

Bottles ﬁlled to full, three quarter, half, and at the quarter level were heated in a
microwave oven, at ﬁill power for 2.00, 1.45, 1.30 and 1.00 min, respectively Gigure 4),
which simulated the directions for reheating indicated on the product label. During
microwave heating, the closure allowed steam to escape, while causing a slight pressure
within the package. The temperature of the syrup, and inside and outside surface of the
bottles was measured at several positions using a pocket-probe digital thermocouple
(Electronic Development Laboratories Inc. NY).

Heating time was increased with ﬁll levels, in order to raise the food product to the
speciﬁc end temperature (58-60 ° C). The mean temperatures at each product package
area resent 4 replications (Table 10). The temperatures measured at each product area
were quite uniform (low standard deviation). Syrup, which is high in sugar and hence has
low speciﬁc heat, heated quickly in the microwave. Bufﬂer and Stanford (1991), reported
that the lower a food’s speciﬁc heat capacity, the more quickly it will heat. Furthermore,

bottle geometry (squat shape with round comer) also promotes uniform heating of the

54

product since food products heat more uniformly in a microwave oven if packaged in a
container with round comers. Keeping the closure on during heating also promotes more
uniform temperature.

The effect of ﬁll level and product area (syrup, inside, and outside bottle surface)
on measured temperatures was examined. Using factorial analysis (Table 11), only the
product area was signiﬁcantly inﬂuenced by the temperature measured, whereas level of
syrup and the interaction factors of product area and level of syrup were insigniﬁcant
(p_<_0.05). The effect of level of syrup on the temperature was the same for all product
areas. Since only effect of product area was signiﬁcant, the product area effect was the

best estimate of the difference of the sample’s temperature and is presented in Table 12.

Table 10. Temperature (°C )‘ of Samples after Heating in Microwave Oven.

 

 

 

Fill Time Product Area of
Level Interest
(min.) Syrup Bottle Bottle

(inside) (outside)
Full 2.00 57.4 i 2.0 56.5 i 2.6 43.0 i 2.6
Three quarter 1.75 57.0 i 1.4 56.3 i 2.9 43.0 i 2.1
Half 1.50 58.8 i 2.2 56.0 i 1.8 44.5 :13
Quarter 1.00 60.5 i 1.3 57.3 _+_ 2.5 44.5 i 2.6

 

mean with standard deviation

55

Table 11. Analysis of Variance of Temperatures of Samples after Heating in
Microwave oven.

 

 

Main effect d.f. Mean Square F

A: Product Area 2 1009.64 197539“
B: Level of syrup 3 12.05 2.359
Interaction

AxB 6 8.20 1.605
Residual Error 36

 

*denotes a statistically signiﬁcant difference (p50.05).

Table 12. Temperature of the Product and Package Wall.

 

 

Product Area Temperature ‘1
(° C)
Syrup 58.2 '
Inside wall 56.3 "
Outside wall 43.8 c

 

" 5’ ° means with superscript alphabets are not signiﬁcantly different (p50.05).
comparison are made only within the same column.
6 mean with standard deviation

Temperature of the product (Table 12) from the highest to the lowest were as
follows: syrup, inside bottle wall, and outside bottle wall. The temperature of the inside
bottle surface was slightly lower than that of the syrup, but higher than that of the outside
bottle surface. As the plastic bottle itself does not absorb microwave energy, rise in
temperature depends on the presence of a microwave absorber (water, dipolar ion, etc.) in
the contained food. At the interface of a food and plastic container which is reasonably

transparent to microwaves, the temperature of the container will be a result of the food

temperature (Sacharow and Schiffmann, 1992). Thermal conduction plays a part in any

56

microwave heating process when a temperature gradient exits, i.e. high temperature region
to the low temperature region (Hallstrom, Skjolderbrand, and Tragardh, 1988). Heat
energy was transferred from the syrup (which had high energy density) to the bottle which
had low energy density. The temperature of the inside wall (56.3 ° C) was, therefore,
somewhat close to that of the syrup (58.2 °C) than the outside surface.

Also, the surface of microwave cooked foods, or the outside of the container wall
will be cooler than the inside because the surrounding air does not heat (Decareau, 1992).
Heat was lost from the surface to the cool oven, which also reduced the surface
temperature. Subsequently, temperature at the outside wall surface (43 .8 °C) was lower

than the syrup and inside wall (56.3 °C) of the bottle.

4.4.2 Effect of Microwave Reheating on Drop Impact Resistance of the Bottles

This test was done to determine the inﬂuence of microwave reheating on the
impact resistance of the bottles. A three steps approach was used. First, drop height of
unheated bottles was measured using a free fall drop test. Second, another set of bottles
was heated in the microwave oven, as described in Figure 4, and then were subjected to
the free fall drop test. Before drop testing, all bottles were conditioned for 24 hours at
room temperature (20 j: 2 °C, 50 i 5 % relative humidity). Third, drop heights of
unheated and microwave reheated bottles were compared. The magnitude of change in
drop heights of the bottles was used as an indicator of microwave heating inﬂuence on the
drop impact resistance.

Drop heights of the bottles after reheating in the microwave oven as dropped onto

their right bottom comer or handle are shown in Figure 6. Figure 6 shows that there was

57

correlation between drop height and microwave heating treatments (right bottom comer,
R2 = 0.9232; handle, R2 =0.8672). Figure 6 also shows that the bottles dropped at right
bottom corner exhibited lower drop heights than those dropped at the handle.

Unheated bottles ﬁlled at full, V4, V2, and V4 syrup levels were used as control
bottles and compared to microwave reheated bottle, treatments 1-3, 4-6, 7-9, and 10-12,
respectively. Comparisons of drop impact resistance between unheated and heated bottles
when dropped at right bottom corner and handle are shown in Table 13 and 15,
respectively. In Tables 14 and 16 are shown the results of one-way analysis of variance in
association with Duncan’s Multiple Range Test of effect of microwave reheating on drop
impact resistance of the bottles dropped onto their right bottom corner and handles.

From one-way analysis of variance in association with Duncan’s Multiple Range
Test (Table 14 and 16), microwave heating was found to have signiﬁcantly affected the
drop impact resistance. A statistically signiﬁcant difference in drop height was observed
between unheated bottles (control) and the bottles reheated 9 times when dropped at right
bottom corner (Figure 7), and for 7 times when dropped onto their handle (Figure 8).
Further reheating caused the drop impact resistance to decrease considerably. There are
actually two issues including practical and statistical signiﬁcance to address with respect to
the difference observed. Drop impact resistance of the microwave reheated bottles was
statistically different from unheated bottles, however, the decrease in drop impact
resistance was probably not large enough to be of practical importance for the actual end-

use performance of the bottle.

58

Degree of crystallinity can have a number of important effects on the properties of
a polymeric material (Seymour and Carraher, 1984). The decrease in drop impact
resistance, after repeated heating in the microwave oven may result from changes in
physical character of the polymer.

Kemp and Kennedy (1987) found that when a polymer was heated to a speciﬁc
temperature, change in physical structure occured. Change in polymer structure may
result from various molecular transitions, which can lead to change in the physical
properties of the material such as brittleness.

Impact properties of polymers are often modiﬁed by adding an impact modiﬁer or
plasticizer (Meyer and Leblanc, 1995). Polymers modiﬁed with low molecular additives
become hard and brittle with loss and/or change in properties due to loss of these additives
by evaporation from the polymer surface, or through migration during contact with
solvents, water, oil, etc., (Kemp and Kennedy, 1987). Therefore, it could be possible that
repeated heating of the bottles in the microwave may induce the loss of additives or
impact modiﬁers, causing the plastic to become brittle.

In this work, the effect of additive loss on impact resistance of the bottles was not

studied.

59

Table 13. Effect of Microwave Reheating on Drop Impact Resistance of the Bottles
Dropped at the Right Bottom Corner.

 

 

 

 

 

Microwave Level of Syrup Drop Height f
Reheating (inch)

Treatment 8
- Full 39.0 i 2.1'
1 Full 38.4 i 1.2'
2 Full 37.5 i 1.7‘
3 Full 397 i 1.5'
- 34 45.6 i 2.5 "
4 34 42.7 i 1.5 b
5 34 43.8 i 1.5"
6 34 44.4 i 2.5"
- '/2 54.0 :13 d
7 14 49.2 : 15°‘1
8 V2 49.8 : 1.5“l
9 v. 48.7 : 15°
- % 570:2.1f
10 v. 54.0 i 19‘
11 V4 54.0 1 17°
12 '4 51.7 : 1.3dc

 

a-e

means with superscript alphabets are not signiﬁcantly diﬁ‘erent (p50.05).
comparisons are made only within the same column.

f mean with standard deviation

3 refer to microwave heat scheme in Figure 4

Table 14. Analysis of Variance of Effect of Microwave Reheating on Drop Impact
Resistance of the Bottles Dropped at the Right Bottom Corner.

 

 

Source of Variation d.f. Mean Square F
Treatments 15 185.318 49.762*
Error 58 3.72

 

* denotes a statistically signiﬁcant difference @5005).

60

Table 15. Effect of Microwave Reheating on Drop Impact Resistance of the Bottles
Dropped onto their Handles.

 

 

 

 

 

Microwave Level of Syrup Drop Heightf
Reheating (inch)
Treatment 3
- Full 22.8 : 15°”
1 Full 22.5 : 15°
2 Full 22.2 : 15°
3 Full 22.0 : 1.4‘
34 25.8 : 15°
4 :2. 25.5 : 1.5bc
5 3/4 26.3 : 25°
6 34 26.3 : 13°
- V2 33.7 : 13°
7 1/2 30.0 : 2.1“
8 '/2 28.5 : 15°“
9 V2 28.0 : 14°“
- V4 40.5 : 1.5 f
10 14 36.7 : 1.3 °
11 % 368:2?
12 v. 36.0 : 21°

 

3'6

means with superscript alphabets are not signiﬁcantly different (p50.05).
comparisons are made only within the same column.

f mean with standard deviation

3 refer to microwave heat scheme in Figure 4

Table 16. Analysis of Variance of Effect of Microwave Reheating on Drop Impact
Resistance of the Bottles Dropped onto their Handles.

 

 

Source of Variation d.f. Mean Square F
Treatments 15 145.630 3 8. 802*
Error 48 3 .75

 

* denotes a statistically signiﬁcant difference (p50.05).

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4.5 Effect of Temperature on the Impact Resistance of the Bottles

To investigate the effect of temperature on the bottle impact resistance, the drop
height impact resistance of the bottles at room temperature (20 i 2 ° C), after heating in

microwave oven (42.3 i 2.2 °C), and at reﬁigeration (8.1 i 1.6 °C) was compared.

4.5.1 Drop Impact Resistance at Right Bottom Comer

Bottles from the microwave heat scheme (Figure 4) were kept in a refrigerator for
24 hr, before subjected to the drop test. The bottles were dropped immediately upon
taking them from refrigeration. The measured temperature of outside surface of the bottle
was at 8.1 i 1.6 °C. The results are shown in Table 17. At quarter and half-ﬁlled levels,
the mean drop heights of the bottle was 20.2 and 17.5 inches, respectively. Bottles ﬁlled
to the three quarter and full levels broke at 16 inches, which was the lowest height setting
of the drop testing machine. It was possible that the bottles would actually break at less
than 16 inches, therefore a drop height of 16 inches cannot be reported as the actual drop
height.

Bottles from microwave heating treatment (Figure 4) were dropped immediately
aﬁer heated in microwaves at the measured temperature of 42.3 i 2.2 ° C (outside
surface). For bottles that were dropped at right bottom comer, number of bottle failures
was very small, even at the maximum drop height (66 inches) of the drop testing machine.
Mean failure drop height could not be established. Therefore, the result was reported as

nonfailure across all treatments.

65
Table 17. Drop Height Failure of the Bottles Dropped at the Right Bottom Corner

 

 

at 8 °C.

Microwave Level of Syrup Drop Heightal
Reheating (inch)
Treatmentb

l Full $16.0

2 Full 516.0

3 Full _<_ 16.0

4 3/4 316.0

5 3/4 $16.0

6 3/4 $16.0

7 1/2 16.8 i 1.5

8 1/2 17.5 i 1.7

9 1/2 17.5 i 1.7

10 1/4 20.2 i 1.6

11 1/4 19.8 i 2.1

12 1/4 19.8 i 1.5

 

‘ mean with standard deviation
b refer to microwave heat scheme in Figure 4

Since data from dropping the bottles at 8 ° C (Table 17) and 42 ° C were not
quantitative, the results for bottles dropped onto their right bottom corner were
transferred to two nominal possible outcomes as “failure” or “nonfailure”. The statistical
signiﬁcance of effect of temperature of the bottles on the drop heights was determined
using Cochran’s test. Statistical comparison of drop impact resistance of the bottles at 20,
42, and 8 ° C was tabulated in Appendix A. The analysis showed that temperatures of the

bottles signiﬁcantly inﬂuenced the drop impact resistance (pSODS).

66
4.5.2 Drop Impact Resistance onto their Handles

For bottles dropped at the handle, it was found that they broke at 16 inches which
was the lowest height of the machine. Since the bottles may actually break at less than 16
inches, the actual drop height could not be determined.

Drop height failure of drops at the handle at low temperature (8 ° C) was not
quantitative (Table 18). Therefore, only drop height data of the bottles at room
temperature (20 ° C), and after heating in the microwave (42 ° C) were compared to
determine the effect of temperature on drop impact resistance (Table 19).

Using factorial analysis, individual parameters (temperature and microwave
reheating treatments) had a signiﬁcant inﬂuence on the bottle’s drop impact resistance
(Table 20). Whereas, interaction factors of temperature and microwave heating were
nonsigniﬁcant. Thus, it could be concluded that these factors acted independently of each
other. Therefore, the results for the individual parameters of temperature and microwave
heat treatments are shown in Table 21 and 22, respectively.

At a higher product temperature, the bottle had increased drop impact resistance.
Lower product temperature decreased the drop impact resistance (Figure 9).

The impact resistance of the bottles was strongly dependent upon the product
temperature. At low temperature, the impact resistance was reduced and all plastics tend
to become rigid and brittle. Conversely, at higher test temperature, the impact resistance

was improved (Calister, 1994).

67

Table 18. Drop Height of Bottles Dropped onto their Handles at 8 °C.

 

 

Microwave Level of Syrup Drop Height‘
Reheating (inch)
Treatment”
1 Full S 16.0
2 Full S 16.0
3 Full S 16.0
4 3A S 16.0
5 % S 16.0
6 3%: S 16.0
7 V2 S 16.0
8 l/2 S 16.0
9 ‘/2 S160
10 ‘A S 16.0
11 ‘A S 16.0
12 ‘A S 16.0

 

‘ mean with standard deviation
b refer to microwave heat scheme in Figure 4

68

Table 19. Comparison of Drop Height at 20 °C and 42 °C of the Reheated Bottles
Dropped onto their Handles.

 

 

Microwave Level of Syrup Drop Height Drop Height
Rehea ting at 20 °C' at 42 °C‘
(inch) (mch)

Treatmentb
1 Full 22.2 :15 33.0 i 2.1
2 Full 22.5 11.5 33.0 i 1.9
3 Full 22.0 i 1.4 33.6 i 2.2
4 3/4 25.5 :15 39.0 i 2.1
5 3/4 26.3 i 2.8 39.0 i 2.4
6 3/4 26.3 :13 38311.3
7 1/2 30.0 i 2.1 42.0 i 2.1
8 ‘/2 28.5 :15 42.6 i 1.2
9 '/2 28.0 i 1.4 42.0 i 1.9
10 ‘A 36.8 11.3 45.8 i 2.5
11 1A 36.8 i 2.5 46.5 i 1.5
12 ‘/4 36.0 i 2.1 46.2 :15

 

‘ mean with standard deviation
b refer to microwave heat scheme in Figure 4

Table 20. Analysis of Variance of Drop Height at 20 °C and 42 °C of the Reheated

Bottles Dropped onto their Handles.

 

 

Main effect d.f. Mean Square F

A: Temperature 1 3294.24 754135“
B: Microwave Reheating 11 219.71 50.298*
Interaction

AxB 11 6.05 1.386
Residual Error 74 4.368

 

*denotes a statistically signiﬁcant difference (p50.05).

69

Table 21. Effect of Temperature of the Product on Drop Impact Resistance.

 

 

Temperature Drop Height (inch)
20 °C 28.2 '
42 °C 40.1 "

 

1hrmeans with superscript alphabets are not Signiﬁcantly different (1350-05)-

comparison are made only within the same column.

Table 22. Effect of Microwave Reheating on the Drop Impact Resistance.

 

 

Microwave Level Drop Height °

Reheating of Syrup (inch)

Treatmentf
1 Full 27.7 '
2 Full 27.6 '
3 Full 27.3 '
4 34 32.2 b
5 1%. 32.6 "
6 3A 32.2 b
7 1/2 35.6 °
8 1/2 35.5 °
9 1/2 35.0 °
10 V. 41.2 d
11 M. 41.6 d
12 14 41.1 d

 

“1 means with superscript alphabets are not signiﬁcantly different (p50.05).
comparisons are made only within the same column.

° mean with standard deviation

f refer to microwave heat scheme in Figure 4

70

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Generally, most polymers are hard and brittle at temperature below their Tg,

change ﬁ'om leathery to rubbery increase as the temperature is increased above Tg. At
low temperature (8 0C), the temperature of the PP bottle was close to its Tg ( -8 °C),
therefore make the polymer more brittle than at 25 °C and 42 °C.

At low temperature, the internal mobility of the molecules of the polymer is less
than at higher temperature. Additionally, at low temperature, the molecules of polymer
are so sluggish that they cannot absorb and dissipate the energy of a sudden shock. As
temperature rises, the material passes through a phase of relaxation, and the modulus is
low (Shah, 1984; Sperling, 1986). Thus at lower temperature, the polymer is more

susceptible to brittleness than at higher temperature.

4.6 The Change in Percent of Crystallinity of the Packaging Material

Crystallinity affects many important polymer physical properties, such as strength,
stiffness, and brittleness. In this study an attempt was made to determine the effect of
repeated microwave heating on the crystallinity changes in the polymeric packaging
material. However, to understand the effect of repeated heating on drop impact resistance
due to change in physical structure of the polymer required Modulated DSC evaluation of
degree of crystallinity of the polymer.

It was hypothesized that after repeated heating in microwave oven, percent
crystallinity of the bottle may increase which may result in change of the impact resistance
of the plastic container. Crystallinity changes were investigated using Modulated DSC.

In Figures 10 and 11 (Appendix B), are shown thermal curves of the Modulated

DSC heat capacity and nonreversing heat ﬂow. Test specimens were taken from the

72
bottom and from the upper part of the bottle close to the handle of unheated bottles. In

Figures 12 and 13 (Appendix B), thermal curves are shown of samples from the bottom
and upper part of microwave repeatedly heated bottles (12 times). Furthermore, in
Figure 14, it is shown that after reheating in the microwave, the polymer showed a higher
heat of ﬁision than that of the unheated polymer.

This heat of ﬁrsion is reported as percent crystallinity by ratioing against the heat
of fusion for a 100 % crystalline sample of the same material. The degree of crystallinities
of the unheated and microwave reheated bottles were compared by using 209 J/g, the
heat of fUSiOl'I of theoretically 100 % crystalline polypropylene (Miller, 1966). The total
heat of fusion associated with the thermal curves and percent crystallinity of the samples
are summarized in Table 23. Degree of crystallinity of the bottle exposed to microwave
repeated heating was approximately 2.7 % higher than that from the control bottle.

During heating in the microwave, the temperature of the bottle will increase via
heat conduction from the product. A number of importance transitions occur in
polypropylene at high temperatures. These may result from torsional and rotational
motions in both amorphous and crystalline regions. These transitions can make polymers
consist of unit conﬁgurations sufficiently alike to pack into a lattice (Symour and
Carraher, 1984). Miller (1966), reported that thermal motion of a polymer increases with
increasing temperature, and when cohesion energy exceeds the kinetic energy of the
chains, crystallization may take place (even in polymers which do not crystallize under
normal condition). Nicastro, et al (1993) reported that increased mobility in the

amorphous segment and melting of the semi-crystalline region resulted in the formation of

73

a more crystalline structure. These mechanisms may have caused crystalline inducement

of the sample after reheating in the microwave.

Table 23. Total Heat of Fusion Associated with Degree of Crystallinity of Unheated
and Heated Bottles in Microwave.

 

 

 

Sample Melt Peak Enthalpy Degree of Crystallinity
Temperature (J/g) (%)‘I
(°C )

Unheated

Upper part 153.16 : 0.44 93.9 i 0.2 45.0 i 0.1
Bottom 153.45 : 0.11 93.6 i 0.6 44.8 i 0.2
Heated

Upper part 153.19 1 0.40 98.8 i 0.3 47.3 i 0.1
Bottom 152.96 i 0.16 99.0 i 0.0 47.4 i 0.0

 

‘ % crystallinity based on the reported theoretical value of 272 J/g for 100 %
crystalline polypropylene (Miller, 1966).

Christie, Gregory, and Wood (1993) showed that processing conditions have a
dramatic impact on ﬁnal crystallinity and corresponding polymer properties. In addition,
Nicastro, et al (1993) provided an excellent example of change in crystallinity due to high
temperature exposure. They found that high temperature achieved during heat sealing
increased percent crystallinity of cast polypropylene ﬁlm. This resulted in an increase in
brittleness of the polymer. Additionally, Shah (1984) and Kai] (1991) reported that
increases in percent crystallinity decreased the impact strength and increased the
probability of brittle failure.

The results from this study suggest that repeated microwave heating induced

crystallinity formation, which may probably decreased the impact resistance of the

polymer.

CHAPTER 5

SUMMARY AND CONCLUSIONS

Drop height measurement using a ﬁ'ee fall drop instrument and the Bruceton
Staircase Method provided a measure of the drop impact resistance of plastic syrup
bottles. Bottles dropped onto their handles or right bottom corner were more susceptible
to breakage then when dropped onto a ﬂat bottom or face drop because the stress was
concentrated in bottom comer (angle drop) at a single point instead of being dispersed
over the entire bottom or surface of the bottle. The low drop impact resistance at the
handle resulted from the weak points on the bottles due to the presence of mold parting
lines and pinch-off regions. Drop impact resistance of the bottles was determined at 1111],
3/4, 1/2, and 1/4 syrup levels to establish a correlation between breakage, drop height,
and ﬁll level. The higher the ﬁll level, the higher the weight of the sample, and the lower
the drop impact resistance.

Bottles ﬁlled at the, 3/4, 1/2, and 1/4 levels were heated in a microwave oven, at
full power for 2.00, 1.45, 1.30 and 1.00 min, respectively. These times were used to
simulate directions on the product label. Heating time was correlated to ﬁll level in order

to raise the food product to a speciﬁc temperature (60° C). Temperature of the product

74

75
and container surfaces were as follows: syrup (58.2 ° C), inside bottle wall (56.3 ° C), and

outside bottle wall (42.3 ° C). Since the plastic bottle does not absorb microwave energy,
rise in material temperature depended on the presence of a microwave absorber (water,
dipolar ion, etc.) in the contained food. The temperature of the container was, therefore, a
result of heat transfer from the high temperature region (syrup) to the low temperature
region (bottle’s wall). Temperature at the outside of the container wall is cooler than at
the inside surface because the surrounding air does not heat. Heat was lost from the
surface to the oven which also reduced the surface temperature (Decareau, 1992).

Drop heights of unheated and microwave reheated bottles were compared. The
magnitude of change in drop heights of the bottles was used as an indicator to determine
microwave heating inﬂuence on the drop impact resistance. A signiﬁcant difference in
drop height was observed after the bottles experienced reheating for 9 times when
dropped at the right bottom comer, and 7 times when dropped at the handle. Heating
times greater than this caused the impact resistance to decrease considerably.

The impact resistance of the bottles was strongly dependent upon the temperature
of the product. At higher test temperature, the impact resistance was signiﬁcantly greater.
Conversely, at lower temperature, the impact resistance was reduced drastically. This is
most likely because the internal mobility of the polymer molecules of the polymer is less
than at higher temperature and the polymer molecules are so sluggish that they cannot
absorb and dissipate the energy of a sudden shock ﬁom free a fall drop (Shah, 1984;
Sperling, 1986).

Thermal curves of (Modulated DSC) the heat capacity and nonreversing heat ﬂow

of test specimens taken from unheated bottles and repeatedly heated bottles were

76
compared. Degree of crystallinity associated with heat of fusion of the specimens taken

from the bottle exposed to microwave repeated heating was approximately 2 % higher
than those from the unheated bottle, indicating that repeated heating in the microwave can
induce crystallinity.

A possible mechanism for the inducement of crystallinity within the bottles is as
follows; during repeated heating in the microwave oven, the temperature of the bottle is
raised as well as the temperature of the product inside due to heat conduction, thus
resulting in change of the physical structure. Thermal mobility of a polymer increases with
increasing temperature. A number of important transitions occur in polypropylene at high
temperature. These may result from torsional and rotational motions in both amorphous
and crystalline regions. These transitions can make polymers consist of unit having
conﬁgurations sufﬁciently alike to pack into a lattice and result in the formation of a more
crystalline structure (Seymour and Carraher, 1984; Nicastro, et al,1993).

Results suggest that repeated microwave heating induced crystallinity formation

which may probably decrease the impact resistance of the syrup bottle.

CHAPTER 6

RECOMMENDATIONS

The recommended ﬁirther studies are as follow:

1. Change in mechanical properties of the package material due to microwave
reheating can be determined by measuring dynamic mechanical behavior of the polymer
using Dynamic Mechanical Analysis (DMA). DMA measures the change in the modulus
(normalized stiffness) and damping (energy dissipation) of viscoelastic materials with
temperature, as these materials are subjected to oscillatory stresses (forces) and resultant
strains (displacements).

2. Determination of the effect of additive loss on impact resistance of the bottles
since it could be possible that repeated heating of the bottle in the microwave may induce

loss of additives or impact modiﬁer, causing the plastic to become brittle.

The following recommendations are for achieving higher impact resistance and
duration of protection of the product packaged.
l. Copolymerization of propylene with other oleﬁns, such as ethylene can

improve its impact resistance.

77

78
2. Decrease percent crystallinity of the package material since high crystallinity

makes polymers harder and more easily ﬁ'actured. The brittleness depends on the percent
crystallization.

3. Addition of additive such as impact modiﬁer to improve impact resistance.

4. Avoid using this product at low temperatures, especially at a temperature close
to its Tg since therrnoplastics above their Tg are rubbery in nature, while below Tg they
are hard and brittle. '

5. Increase the thickness of the bottle at its weak point including handle and
bottom comers.

6. Bottle design can also be made to minimize wall collapse by strengthening the

container by changing its geometry.

 

APPENDICES

79
APPENDIX A

Cochran’s Test for Related Frequencies

Statistical Comparison of Drop Impact Resistance of the Bottles Dropped at the
Right Bottom Comer at 20, 42, and 8 °C (01 = 0.05).
Ho 3 F 20°C = F 42°C = F8°C

H1 3 F 20°C i F 42°C 3* F8°C

 

 

 

 

Treatment 20°C 42°C 8°C Row Total (Fr)
1 1 1 1 3
2 1 1 1 3
3 1 1 1 3
4 1 0 1 2
5 1 0 1 2
6 1 0 1 2
7 1 0 1 2
8 1 0 1 2
9 1 0 l 2
10 l 0 l 2
1 1 1 0 1 2
12 1 0 1 2
Column Total (Fe) 12 3 12 27
1 = failure, 0 = nonfailure
22Fc = 27
2F,2 = 297
SF. = 27
2F} = 63

x2... ., = (a-1)[a(>:1=.2) - (211.) 2] / (a 2F. - 2F} )
X2(a-1= 2, a. = 0.05) = 18

X294, 0.) Table = 5.99

X2(a-1=2.a=o.05) = 18 > X2(a-l,a)'l‘able = 5.99
Reject hypothesis that Ho : F 20cc = F 42°C = Fgoc

80

APPENDIXI!

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