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Effect of repeated microwave heating on the impact resistance of a polypropylene container

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E F F E C T O F R E PE A T E D M IC R O W A V E H E A T IN G O N T H E IM PA CT R E S IS T A N C E
OF A P O L Y P R O P Y L E N E C O N T A IN E R
By
U bonrat S iripatraw an
A T H E S IS
Subm itted to
M ichigan State U niversity
in partial fulfillment o f th e requirem ents
for th e d eg ree o f
M A S T E R O F S C IE N C E
School o f P ackaging
1997
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UMI Number:
1388580
UMI Microform 1388580
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A B STR A C T
E F F E C T O F R E PEA TE D M IC R O W A V E H E A T IN G O N T H E IM PA C T R E S IS T A N C E
OF A P O L Y P R O P Y L E N E C O N T A IN E R
By
U bonrat Siripatraw an
T he effect o f m icrowave repeated heating on physical stru ctu re and p roperty o f a
package m aterial w as studied by evaluating drop im pact resistance and degree o f crystallinity.
D rop im pact resistance o f plastic syrup bottles w as evaluated using the Bruceton Staircase
free fall drop m ethod. Drop im pact orientation (flat bottom , bottom com ers, and handle), fill
level (full, 3A, !/2 , and 'A ), and tem p eratu re o f package m aterial (20 + 2 °C, 42.3 + 2.2 °C ,
and 8.1 + 1.6 °C ) affected drop im pact resistance o f the bottles. Plastic OPP b ottles filled
w ith syrup at full, 3A, 'A , and V* levels w ere heated in a m icrow ave oven and subjected to 12
heating treatm ents. U nheated bottles filled w ith syrup at full, 3A, X
A , and 'A levels w ere used
as control bottles. D rop im pact resistance o f unheated b ottles and bottles subjected to
m icrow ave reheating w as determ ined. D egree crystallinity associated w ith heat o f fusion o f
the package material w as evaluated using M odulated D SC. D egree crystallinity o f the
sam ples taken from unheated and m icrow ave reheated b ottles w ere com pared. D ecreased
drop im pact resistance and increased d eg ree o f crystallinity o f th e package material w as
observed after th e bottles w ere repeatedly heated in the m icrow ave oven.
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Dedicated to my mother, Amnouy Younyong, for her inspirational support.
iii
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ACKNOWLEDGEMENTS
I w ould like to extend my intensity o f resp ect and appreciation to King
C hulalongkom for his im pressive contributions to th e progress o f m o d em Thai education.
I also w ould like to ex p ress my gratitude to my sponsors, th e Royal Thai G overnm ent and
C hulalongkom U niversity, for giving me an o p p o rtu n ity to fu rther my studies in th e U SA
to gain know ledge in o rd er to help fulfill the b etterm en t o f Siam.
I w ould like to express my respect and ap preciation to my m ajor professor Dr.
B ruce H arte for his tim e, kind thought, and g u id an ce th ro u g h o u t my research. I also
w ould like to express my sincere appreciation to m y com m ittee Dr. G ary Burgess, Dr.
Jam es Steffe, and Dr. Jack G iacin for sharing th e ir tim e serving as my com m ittee and for
giving m e the idea o f w hat project I should pursue.
M y appreciation also goes to M r. M ichael K entala from th e Pillsbury C om pany for
donating the plastic containers used in this research. M any thanks to M r Michael Rich
from the C om posite R esearch C enter for his help in M D SC analysis. Special thanks to
Ms. B everly U nderw o o d fo r her help during my study here.
M y sincere th an k s to all my friends, especially M anee, N ucharin, Rujida, Sunetra,
Risha, John, Jae, M uk, and Jai, fo r all their help, suggestion, pep talk, and for helping
m ake my stay here a m em orable one.
Finally, I w o u ld like to express a deepest appreciation to my wonderful family for
their inspirational support.
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TABLE OF CONTENTS
LIST OF T A B L E S ................................................................................................................................ vii
LIST O F F IG U R E S ...............................................................................................................................viii
CHAPTER I
IN TR O D U C TIO N AND O B JE C T IV E S ............................................................................................ I
C H A PT E R 2
LITER A TU R E R E V IE W ....................................................................................................................... 7
M icrow ave H eating C h a ra c te ris tic s ..................................................................................... 7
Dielectric P ro p e rtie s ...................................................................................................................8
M icrow ave H eating C haracteristics o f F o o d s ....................................................................9
Factors A ffecting M icrow ave H e a tin g ............................................................................... 11
M icrow ave P a c k a g in g ..............................................................................................................14
Plastic P ackage for M icrow ave O v e n ................................................................................ 17
M echanical P roperties o f Polym eric Packaging M a te ria ls .......................................... 20
Effect o f T em perature on M echanical P ro p e rtie s ..........................................................23
Effect o f H eat on Percent C ry sta llin ity ............................................................................. 26
M echanical Testing for Polym eric Packaging M a te ria ls ..............................................28
Drop Im pact T e s t...................................................................................................................... 29
Determ ination o f Percent C rystallinity................................................................................31
C H A PT E R 3
M A TERIA LS A N D M E T H O D .......................................................................................................... 35
Sam ples.........................................................................................................................................35
Wall Thickness o f the B o ttle .................................................................................................. 35
D eterm ination o f th e Im pact O rientation o f th e B ottles by Free Fall D rop T est..3 6
D eterm ination o f D rop Im pact R esistance o f th e B o ttles Filed w ith syrup at
Different L e v e ls ......................................................................................................................... 39
Effect o f M icrow ave R epeated H eating on th e D rop Im pact R esistance o f
the B o ttle s .................................................................................................................................. 40
Effect o f T em perature on th e D rop Im pact R esistance o f the B o ttles....................... 45
D eterm ination o f th e C hange in D egree C rystallinity o f the Packaging M aterial
by Thermal A nalysis................................................................................................................ 46
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CH APTER 4
R ESU LTS A N D D IS C U S S IO N S .....................................................................................................47
Wall Thickness o f the B o t t l e s .............................................................................................. 47
D eterm ination o f the Im p act O rientation o f the B ottles by Free Fall
D rop T e s t..................................................................................................................................... 47
D eterm ination o f B ottle D ro p Im pact Resistance ..........................................................49
Effect o f M icrow ave R ep eated H eating on th e D rop Im pact R esistance
o f th e B o ttles............................................................................................................................... 53
Effect o f T em perature on th e D rop Impact R esistance o f th e B o ttle s .......................64
T he C hange in D egree C rystallinity o f the Packaging M aterial.................................... 71
CHAPTER 5
SU M M A R Y A ND C O N C L U S IO N S ................................................................................................74
CHAPTER 6
R E C O M M E N D A T IO N S ......................................................................................................................77
A PPE N D IC E S
A PP E N D IX A. Statistical C om parison by C ochran’s T e st.........................................................79
A PP E N D IX B. M D SC Analysis o f the Sam ples............................................................................80
B IB L IO G R A P H Y ...................................................................................................................................85
VI
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LIST OF TABLES
Table I.
M icrow ave H eating T reatm en t......................................................................................... 43
Table 2.
W all Thickness o f th e B ottle M aterial............................................................................47
Table 3.
D rop Height Failure o f the B ottles at V arious O rien tatio n s................................... 48
Table 4.
Analysis o f Variance o f D rop H eight o f th e B ottles at V ario u s O rientations. . 48
Table 5.
W eight o f the Plastic B ottle Filled w ith S yrup at Full, %, '/2, and lA L ev els......50
T able 6. D rop H eight o f the B ottles Filled w ith Syrup at Full, %, VS, and VS Levels
and D ropped at th e Right B ottom c o m e r....................................................................................... 52
T able 7. Analysis o f Variance o f D rop H eight F ailure o f B ottles Filled w ith Syrup
at Full, 3A, VS, and VS Levels and D ropped at R ight B o tto m C o m e r.......................................... 52
T able 8. D rop Height o f the B ottles Filled w ith Syrup at Full, 3A , VS, and VS Levels
and D ropped o n to their H andle............................................................................................................. 52
Table 9. Analysis o f Variance o f D rop H eight Failure o f B ottles Filled w ith Syrup
at Full, 3/S, VS, and VS Levels and D ropped o n to th e ir H an d le.......................................................53
Table 10. T em perature o f Samples after H eatin g in M icrow ave O v e n ...................................54
T able 11. Analysis o f V ariance o f T em p eratu re o f Sam ples after H eatin g in
M icrow ave O v en ....................................................................................................................................... 55
T able 12. T em perature o f the P roduct and
P ack ag e W all.........................................................55
T able 13. Effect o f M icrow ave H eating and Fill Levels on D rop Im p act
R esistance o f th e Bottles D ropped at the R ight B o tto m C o m e r................................................ 59
T able 14. Analysis o f V ariance o f Effect o f M icro w av e H eating and Fill L evels
on D rop Im pact Resistance o f th e B ottles D ro p p e d at the Right B o tto m C o m e r ............ 59
Table 15. E ffect o f M icrow ave H eating and Fill L evels on D rop Im p act
R esistance o f th e Bottles D ropped o n to their H a n d le s ................................................................. 60
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Table 16. Analysis o f V ariance o f E ffect o f M icrow ave H eatin g and Fill Levels
on D rop Im pact R esistance o f th e B o ttles D ropped onto th e ir H an d les............................... 60
Table 17. D rop H eight o f the B ottles D ro p p ed at the Right B o tto m C o m e r at 8 °C
65
Table 18. D rop H eight o f the B o ttles D ro p p ed onto their H an d les at 8 ° C ........................ 67
Table 19. C om parison o f Drop H eight at 20 °C and 42 °C o f th e R epeated
B ottles D ropped o n to their H andles....................................................................................................68
Table 20. Analysis o f Variance o f D rop H eight at 20 °C and 4 2 °C o f the
Repeated B ottles D ro p p ed onto their H an d les................................................................................68
Table 21. Effect o f T em perature o f th e P ro d u ct on Drop
Im pact R esistance.................................................................................................................................... 69
Table 22. Effect o f M icrow ave R eheating on the D rop Im pact R esistan ce.........................69
Table 23. Total H eat o f Fusion A ssociated w ith D egree o f C rystallinity
o f Unheated and H eated Bottles in M icro w av e............................................................................. 73
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LIST OF FIGURES
Figure 1. H eat Flux D SC Schem atic............................................................................................... 34
Figure 2.
N onreversing, Reversing, and T otal H eat Flow Signals......................................... 34
Figure 3.
Syrup B o ttle ..........................................................................................................................38
Figure 4.
Flow D iagram o f M icrow ave H eating T reatm en t..................................................... 42
Figure 5. Effect o f Fill Level on D rop H eight o f the B ottles D ropped o n to
Right B ottom c o m e r and H andle........................................................................................................51
Figure 6. D rop H eight o f the B ottles A fter R epeated H eating in the
M icrow ave O ven...................................................................................................................................... 61
Figure 7. E ffect o f M icrow ave R eheating on D rop Im pact R esistance o f the
B ottles D ropped o n to th eir Right b o tto m C o m e r..........................................................................62
Figure 8. Effect o f M icrow ave R eheating on D rop Im pact R esistance o f the
B ottles D ropped o n to th eir H andles................................................................................................... 63
Figure 9. Effect o f T em perature o f th e P roduct on D rop Im pact R esistance W hen
D ropped onto th eir H andles................................................................................................................. 70
Figure 10. M D S C Analysis o f Sample from the B o tto m o f U nheated B o ttle ...................... 80
Figure 11. M D S C Analysis o f Sample from th e U pper Part C lose to the H andle
o f Unheated B o ttle ...................................................................................................................................81
Figure 12. M D S C Analysis o f Sample from the B ottom o f the M icrow ave
Reheated B o ttle......................................................................................................................................... 82
Figure 13. M D S C A nalysis o f Sample from the U pper Part C lose to the H andle
o f M icrow ave R eh eated B o ttle.......................................................................................................... 83
Figure 14. C om parison o f M DSC Analysis o f Sam ples from the B o tto m o f the
U nheated B o ttle ....................................................................................................................................... 84
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CHAPTER 1
INTRODUCTION AND OBJECTIVES
The m otivation for th e developm ent o f m icrow avable packaging m aterials has been
influenced by need fo r convenience, aesthetics, and enhanced food quality. Classically,
packaging tech nologists have been concerned w ith designing packages w hich: protect the
quality attributes o f foods, minimize th e effects o f physical abuse to th e p ro d u ct during
m anufacture, sto rag e and distribution, are relatively easy to m anufacture, econom ical to
the consum er and sim ple to open and use. T he increased usage o f m icro w av e ovens has a
direct effect on th e design o f the product and its heating perform ance in th e microwave
oven and highlights th e im portance o f the relationship betw een food form ulated for
m icrowave heating and th e appropriate packaging.
Generally, m icrow avable containers should allow heat p enetration, tolerate rapid
tem perature ch an g e and preserve food quality (R ubbright, 1990). M o reo v er, the
appropriate packaging m aterial fo r use w ith a m icrow avable food sh o u ld not heat
excessively o r p rev en t efficient m icrow ave heating. The container m ust also be thermally
com patible w ith th e food, i.e. it should not melt, distort or be o therw ise im pinged by hot
food, and it should provide sh elf life pro p erties com m ensurate w ith th e fo o d and its use.
M icrow ave tran sp aren t m aterials, such as plastics and paperboard, m icro w av e interactive
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m aterials, such as susceptors, and reflective m aterials, such as metals h ave been used for
m icrow avable packaging. T h ese m aterials provide b o th opportunities and problem s for
fo o d m anufacturers (Fisher, 1991). Polymeric packaging materials have b een already well
established in th e m icrow ave m ark et and are now th e m ost popular m aterials used fo r
m icrow ave packaging as a resu lt o f their m icrow ave transparency, ease o f p rocessing and
form ing, consum er appeal, an d safety (S acharow and Schiffinan, 1992).
T o design and select a plastic material for u se in a m icrow ave o ven and to avoid
failure in th e m arket place, p ro d u ct and package developm ent m ust be th o ro u g h ly
researched. T he packaging technologist m ust first understand w hat role th e p ack ag e will
have in relation to the h eating phenom enon, such as the effect o f m icrow ave heating on
structural properties, packaging perform ance, and th e expected service life o f the
container in o rd er to provide an appropriate container.
D uring m icrow ave heating, th e highest tem p eratu re will be found at o r near the
interface betw een food and th e container. At the interface o f food and container, th e
tem p eratu re o f the container, if it is transparent to m icrow aves, will usually be a result o f
the food tem perature and th e cooling influence o f th e air on the outside o f th e co ntainer
wall (H uang, 1987). W hen th e food contains large am ounts o f fat, such as on the surface
o f soups, o r in sauces and gravies, o r a large am o u n t o f sugar, as on p reserv es and syrups,
then the tem perature may p o tentially becom e very high and may cause pro b lem s such as
dim ensional stability, discoloration, distortion, m elting, and m igration from plastic
packaging (K att, 1991).
T here have been m any studies concerning fo o d /p ack ag e interactions especially
w ith respect to the m igration o f packaging com ponents into foods due to h eating at high
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tem peratures in th e m icrow ave oven. O ne such exam ple is th e m igration o f volatile
com pounds from a therm oform ed m icrow avable container (m ade from
polypropylene/Saran/polypropylene co ex tru d ed m aterial) into food durin g m icrow ave
exposure (D ixon et al, 1988). P lasticizer has also been know n to m igrate from a
laminated film incorporating su scep to r m aterial (B ishop and Dye, 1982). O d o r pick-up
and retention resulting in a change in th e flavor o f the food has been o bserved (Laperle,
1988).
H ow ever, there are other problem atic areas th at need special attention. O ne
problem, know n as "runaway heating" occurs in therm oplastics as they ap p ro ach their heat
distortion tem perature, which is th e tem p eratu re at which an arbitrary deform ation occurs
under arbitrary test conditions. This can cause melting o r severe distortion o f the
container (K orshak, 1971). A nother concern w ith plastic containers is change in physical
property such as loss o f desired packaging perform ance and discoloration o f the container,
after repeated heating in a m icrow ave oven (Peason, 1995; Sacharow and Schiffmann,
1992). Therefore, perform ance testin g o f m icrow avable packaged p ro d u cts is necessary.
The mechanical properties, am ong all th e properties o f polymeric packaging
m aterials, are often the m ost im portant because they influence virtually all service
conditions and th e majority o f end-use applications (Shah, 1984). M echanical properties
o f plastic m aterials may be affected by elevating tem perature during m icrow ave heating.
T here are many interesting correlations th at can be m ade w ith effect o f m icrow ave heating
on the quality o f package container. Im pact resistance is one o f the m ost im portant
mechanical pro p erties o f the plastic m aterials and an increase o r decrease in im pact
resistance may be caused by m icrow ave heating.
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Physical testing o f plastics can be classified as therm al, m echanical, dimensional,
transm ission, and electrical (Seym our and C arraher, 1984). In this study, attention was
focused on th e m echanical and therm al testing. T he mechanical testing o f plastic materials
is usually carried o u t to determ ine their suitability for a particular application, to control
functions, o r to obtain a better understanding o f their behavior under v arious conditions.
M ost m echanical tests are carried o ut on molded test pieces. H ow ever, th ere are som e
tests that can be perform ed on th e finished containers and these will often give results
m ore in keeping w ith th e end-use performance. D rop im pact resistance o f a blow molded
therm oplastic con tain er (A ST M D 2463) is one o f the physical tests that can be used to
simulate th e actual im pact conditions.
T here is little inform ation available in the literature which co rrelates change in
physical p ro p erties o f plastic containers with heating in a m icrow ave oven. A few
technical p ap ers address the potential effect o f m icrow ave heating on th e crystallinity o f
polymeric packaging materials. It is assumed th at heating in a m icrow ave at elevated
tem peratures m ay lead to a change in percent crystallinity o f the packaging m aterials due
to melting and recrystallization. Determ ining percent crystallinity is consequently
im portant to ev alu ate its effect on packaging perform ance. The reason w hich necessitates
such investigation is, in particular, the fact that th e physical character o f a polym er such as
its tensile strength, and im pact resistance depends substantially on the percen t crystallinity
o f the polym er.
In this research, it w as hypothesized that repeated heating o f plastic containers by
m icrow aves m ay affect th e degree o f crystallinity w hich may result in a change in the
im pact resistance o f th e package. R epeated m icrow ave heating and free fall drop testing
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o f the containers w ere designed to sim ulate the actual use situation environm ent. The
m icrow ave heating param eters w ere studied u n d er com parable conditions to determ ine
changes in th e con tain er’s crystallinity and perform ance. D rop im pact resistance, w as used
to sim ulate actual im pact conditions and w as ev aluated using the free fall d ro p test.
Differential Scanning Calorim etry (D SC ), a technique m easuring heat flow into o r out o f a
material as a function o f time and tem perature, w as used to assess th e heat flow associated
with percent crystallinity o f the material.
T he consequences o f this investigation m ight be expected to provide significant
inform ation ab o u t the capability and limitations o f th e packaging and th e com patibility o f
package and p ro d u ct to repeated heating in the m icrow ave. This inform ation will
potentially help packaging technologists prepare ap p ro p riate container specifications for
m icrow ave use, to develop m ore convenient and successful packages fo r m icrow ave food
products.
The p u rp o se o f this study w as to determ ine if th ere w ere changes o ccurred in the
structural and physical properties o f packaging m aterial due to rep eated heating in the
m icrow ave and to identify and quantify these changes.
T he specific objectives o f this research w ere:
1. To evaluate d rop im pact resistance and failure o f a container w hen subjected to sudden
shock resulting from a free fall.
2. To com pare th e perform ance o f a container to w ithstand the sudden shock resulting
from a free fall after repeated heating in m icrow ave oven.
3. T o determ ine changes in degree o f crystallinity o f package m aterial d u e to repeated
m icrow ave heating.
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4. T o study the effect o f th e change in p ercen t crystallinity resulting fro m m icrow ave
heating on packaging perform ance.
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CHAPTER 2
LITERATURE REVTEW
Microwave Heating Characteristics
M icrow aves are a form o f electrom agnetic energy, and are similar to visible light,
x-rays, radio w ave and ultraviolet energy. M icrow ave generally include th e frequency
spectrum from 0.3 to 300 G H z (gigaherts) o r w avelength o f 1 mm to 1 m .(S ach aro w and
Schiffm ann, 1992). All electrom agnetic w aves are com posed o f rapidly alternating electric
and m agnetic fields which oscillate at different rates. The energy frequency in industrial
ovens com m only used in plant m anufacturing operations operate at 915 M H z, w hile in
dom estic retail ovens they operate at 2450 M H z. This m eans that the electrom agnetic
field is alternating 2450 million times per second (Fisher, 1991).
R obertson (1992) described tw o main m echanism s by which m icrow aves produce
heat in foods including dipole rotation and ionic polarization. Ionic polarization occurs
w hen ions move in response to an electric field. Ions, due to their inherent electrical
charge, are accelerated by th e m icrow ave field, leading to multiple collisions w ith
nonionized molecules. K inetic energy is given by th e field to the ions, w hich collide with
o th e r ions, converting kinetic energy into heat. The m ore concentrated o r th e m ore dense
th e solution, the g reater th e frequency o f collision, and the m ore kinetic energy th a t is
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released. At m icrow ave frequencies, num erous collisions occur, and m uch heat is
generated. H ow ever, it is a less im portant mechanism than dipole rotation.
M icrow ave heating also results from interactions o f th e food co n stitu en ts
containing p o lar m olecules (such as w ater) with an electrom agnetic field. T h ese m olecules
have a random orientation and orient them selves w hen applied w ith th e rapidly changing
alternating electrical field according to the polarity o f th e field. T hese interactions lead to
instantaneous h eat generation w ithin the product due to m olecular friction prim arily by the
agitation o f w eak hydrogen bonds associated with the dipole rotation o f free w ater
m olecules and w ith the electrophoretic migration o f free salts in an electrical field o f
rapidly changing polarity. T hese effects are predom inantly related to the aq u eo u s ionic
constituents o f fo o d and their associated solid constituents. Such rotation o f m olecules
leads to friction w ithin the surrounding medium, and then g en erate heat (Singh and
Heldm an, 1993). O nce the heat is generated it is then transferred to o th er p oints in the
p roduct by co n duction and convection. T he specific heat, therm al conductivity, density
and viscosity all effect the rate o f heat transfer (Fisher, 1991).
Dielectric Properties
The dielectric properties o f a material are critical to m icrow ave heating. The
dielectric p ro p erty o f a material is th e physical description o f how well a m aterial can
potentially heat w hen it interacts w ith electrom agnetic energy. T he im portant electrical
properties are th e relative dielectric constant (e ’), and th e relative dielectric loss ( s ” ). The
dielectric loss facto r for the material, which represents th e quantity to w hich an extrem ely
applied electrical field will be converted to heat, is show n by th e follow ing equation:
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e”
=
e ’ tan 5
e ’ describes how much energy is reflected aw ay from th e product and h ow much
energy is transm itted into the product, e ” describes h o w lossy a product is o r h o w well a
material absorbs electrical energy and converts it into heat, tan 5 o r loss tangent provides
an indication o f how easily the m aterial can be p en etrated by an electrical field and how
well it dissipates electrical energy as heat (E n g eld er and Buffler, 1991).
If th e lossiness o f a product is large, it will effectively absorb energy passing
through it and h eat rapidly. I f the lossiness is small, m icrow ave radiation will pass right
through the p ro d u ct w ithout heating it. M ost glass, plastic and p ap er packaging m aterials
have low dielectric constants com pared to food and are tran sp aren t to m icrow ave energy
M any factors influence the dielectric p roperties o f food. A few o f these are; m oisture
content, tem perature, salt content, physical state, and chem ical com position (Fisher,
1991).
Microwave Heating Characteristics of Foods
M icrow ave heating characteristics o f fo o d s are related not only to their dielectric
properties b u t also to electrical transm ission pro p erties related to dielectric heating
processes and to therm al and tran sp ort p roperties th at affect heat and mass transfer in both
conventional and dielectric heating p rocesses (E n g eld er and Buffler, 1991).
M icrow ave heating o f foods involves tw o phenom ena: coupling o f energy by the
product from an electrom agnetic field and atten u atio n o f ab so rp tio n o f the coupled energy
within the product. These phenom ena involve reflection and transm ission o f energy at
product surfaces and alteration o f energy w ithin th e pro d u ct. This results in instantaneous
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10
tem perature increase within the product in co n tra st w ith conventional heating p ro cess that
transfer energy from th e surface w ith long therm al tim e co n stan ts and slow heat
penetration.
M icro w av e heating occurs as a result o f th e interactions betw een m icrow ave and a
dielectric m aterial. The relationship betw een m icrow ave p o w er absorbed by the m aterial
being heated and th e conversion o f the m icrow ave energy to heat can be approxim ated
w ith the follow ing equation (D ecareau, 1992, and Singh and Heldman, 1993).
w here
P
=
ctE2
(w att/cm J)
P
=
the pow er absorbed (w att/cm 3)
c
=
the equivalent dielectric conductivity
E
=
the electrical field stren g th (volts/cm )
T he main difference betw een m icrow ave heating and o th er heating m ethods is
penetration depth. The penetration depth is th e distance from th e surface o f a dielectric
material to w h ere th e incident p o w er is decreased to 37% ( 1/e) o f the incident p o w er at
the surface. M icrow aves penetrate deeply into food m aterials and are converted to heat as
they p en etrate (D ecareau, 1992). The equation for converting dielectric property d ata
into penetration depth is :
dp
w here
dp
a
=
1 / (2 a )
=
penetration depth
=
attenuation constant
(2 7t/ X) {e72 [(1+ tan 2 8) 1/2 -1 ]} I/2
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Factors Affecting Microwave Heating
From th e experim ents conducted by Lau (1995) to determ ine the heating
characteristics o f food m aterials in a m icrow ave oven, the conclusion draw n w a s that the
tem perature rise induced in a food and food packaging due to m icrow ave h eating depends
on tw o main physical mechanisms; th e interaction o f th e food p ack ag e with th e m icrow ave
field determ ines th e pow er deposited and th e increase o f tem p eratu re th ro u g h o u t th e food
package in acco rd an ce to heat transfer mechanisms. H e also observed that th e p o w er
absorbed by a food load in a m icrow ave oven depended on th e dielectric p ro p erties o f the
food, the position o f the food in th e cavity, and the use o f packaging materials.
Schiffinann (1990) pointed o u t a num ber o f factors th a t affect m icrow ave heating
perform ance in m icrow ave ovens:
a) O ven param eters include o u tp u t w attage, output frequency, presence o r ab sen ce o f a
tum able and turntable m aterials o f the, position o f th e food in th e oven cavity, material
construction o f th e floor, tim ebase for pulsed-pow er control, stability o f in p u t pow er,
presence o r absence o f filament transform er, age and condition o f the m agnetron, and
tim e delay o f the m agnetron, i.e., cold vs warmstart.
b) Food param eters include nature o f th e food-single com ponent vs. m ulticom ponent,
dielectric properties o f the foods over th e range o f tem p eratu res to be en co u n tered ,
therm al properties o f the food (therm al conductivity, heat transfer coefficient, specific
heat, heat o f fusion), evaporation, initial tem perature o f food, shape o f th e fo o d and its
com ponents, and regularity o f food shape.
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c) Packaging including material o f construction (plastics, glass, paperboard, and m etal
films, transparency o r reflectively), size (length, w idth, and depth), shape (round, oval,
rectangular, etc.).
T he shape o f food items is critical to m icrow ave heating results. F or m any foods,
it is the food container o r dish that determ ines the food shape and therefore affects the
heating perform ance. Varied different tem perature profiles are found in round, oval, and
rectangular containers. As the size o f th e container changes, so will th e tem p eratu re
profiles.
T he sphere is the ideal shape as energy tends to be focused to give heating at o r
tow ard the center o f the sphere. T h e cylinder is th e next best shape in term s o f heating
perform ance. Round and oval are preferred over rectan g u lar o r square because
rectangular and square shape will give edge and c o m e r heating results which may at times
be extrem e. Food in the com ers o f a rectangular container will be overcooked befo re the
rem ainder o f th e food is ready. F ood products heat m ore uniformly if form ed w ith round
com ers. Even w hen food product o r package co m ers are unavoidable they should have
g enerous radii to minimize the overheating effect (S ach aro w and Schiffinann, 1992).
T he bottom o f the container should be bow ed som ew hat to m ake the food depth
thinner in the center w here it receives less m icrow ave pow er, than along the edges,
thereby contributing to m ore uniform heating. This also serves to elevate the food slightly
thus reducing heat loss to th e cooler oven floor.
T he side wall o f the container should have gen ero u s draft angles rather than be at
90 °C angle w ith the bottom com m ensurate w ith stacking. O n the other hand, th e wall
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13
should not be so shallow th at overheating o f a thin fo o d pro file at the su rface o ccu rs
(O hlsson and R issm an, 1978).
T he co m p o sitio n o f the food material affects h o w it heats in the m icro w av e field.
The m oisture co n ten t o f food directly affects the am ount o f m icrow ave p o w e r absorption.
A higher am o u n t o f w ater in a food increases the dielectric loss factor. In th e case o f low
m oisture co ntent, th e influence o f the specific heat (th e am o u n t o f energy n eeded to heat
1.0 g o f food to 1.0 °C) on the heating process is m ore p ro n o u n ced than th a t o f the
dielectric loss factor. T he specific heat determ ines how rapidly a given volum e o f food
will heat once a given am ount o f p o w er is deposited w ithin it. T h e low er a fo o d ’s specific
heat capacity, th e m ore quickly it will heat. Food o r co m p o n en ts w ith high fat and sugar
and low w ater co n ten t have low specific heats. T herefore, d u e to their low specific heat,
some foods w ith low m oisture content also heat at accep tab le rates in m icrow ave oven
(Singh and H eldm an, 1993).
Oil has ab o u t o n e-h alf th e specific heat o f w ater, an d can heat tw ice as fast for a
given heat input. H igh-solid foods, such as jellies and p reserv es, have low specific heats
and may not only heat fast but to very high tem peratures (F isher, 1992). This w as also
observed in the stu d y by K att (1991) that as the sugar co n ce n tratio n in the fo o d system
increased, the heating rate increased. C om pared to a w a te r co n tro l w ith a heating rate o f
1.10 °F/second, th e addition o f 20 % fructose increased th e h eatin g rate to 1.20 °F/
second, and at 50 % fru cto se the rate w as 2.10 °F/second.
Therm al conductivity is a m easurem ent o f a m aterial’s ability to tran sfer heat in
response to a te m p eratu re difference. C onduction h eat tra n sfe r depends on te m p eratu re
difference. Even in m icrow ave cooking it plays an im p o rtan ce ro le in spite o f th e
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penetrating n atu re o f m icrow ave energy. T he pow er abso rp tio n characteristics o f a food
load changes significantly as its tem p eratu re rises due to th e change in its dielectric
properties. F o r fo o d p ro d u cts w ith high conductivities, m ost o f the p o w er is d ep o sited on
the surface and a t sharp com ers. F o r food product w ith low conductivities, standing
w aves are set u p within the load and the p o w er is m ore evenly distributed (Lau, 1995).
Microwave Packaging
G enerally, packaging requirem ents for all foods m ust perform certain functions.
C hief am ong th ese are to p rotect and preserve the p ro d u cts contained, ensure th e
w holesom eness o f the food, allow ease o f m anufacturing, and provide consum er usability.
For packaging specific to m icrow avabie foods, the th ree main applications are reheating,
cooking, and defrosting. The packages for m icrow avabie foods must also provide
adequate venting, control the arcing w ithin the package itself, allow for uniform
reconstitution tem p eratu res in both m ultiple and single com ponent foods, p ro tec t th e user
from the potential hazards o f heated products, such as discharge o f viscous m aterials at
their boiling point, provide fo r safe handling and opening w hen hot, prevent taste and o d o r
from m igrating o u t o f th e packaging m aterial into the food, maintain structural integrity
under varied therm al conditions, and achieve uniform heating to provide micro'oiologically
safe food preparation (R ubbright, 1990; Fisher, 1997; R obertson, 1993).
Schiffm ann and S acharow (1992) defined the desired properties o f packaging
materials for m icrow ave ovens, as th e following; will n o t heat excessively, o r prevent
effective m icrow ave heating, therm ally com patible w ith th e food-that is, should n o t melt,
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distort o r be otherw ise affected by th e hot food. T herefore, th e selection o f m aterials
m ust be m ade after careful co n su m er use testing.
M icrow ave packaging can be classified as active, passive, and shielding according
to their interaction with m icrow aves. Active packaging is packaging co n stru cted o f
m aterials which respond to m icrow aves with the result th at incident energy is converted
into heat o r is focused to increase its intensity in a predeterm ined region o f th e food.
A ctive packaging may com bine active and passive elem ents (R ubbright, 1990).
Passive packaging is packaging which does n o t m odify th e m icrow ave energy field
and does not becom e hot. S uch m aterials are essentially transparent to m icrow ave energy,
including plastics, glass, and p ap er products. Passive packaging includes b o th rigid and
flexible forms. Rigid packages dom inate meals in com partm ented trays and fo r larger
serving sizes, individual lunch and vended items. Shielding packages include metallic
structure that can reflect m icrow aves. Therefore, a p ro d u ct encased in a m etallic structure
w ould not exhibit any m icrow ave heating, since no energy is able to p en etrate th e material
to reach the p roduct (R obertson, 1993).
Generally, packaging m aterials that are used in m icrow ave ovens include plastic,
paperboard, and glass. Plastics are well suited to m icrow ave use and w hen properly
selected can be used for sh elf stable, refrigerated and frozen foods. They a re divided into
m icrow ave only and dual-ovenable categories. D ual-ovenable containers a re designed to
w ithstand the rigors o f both m icrow ave and conventional ovens. Crystalline polyester and
therm osets are g o o d exam ples o f dual-ovenables that are used in higher p riced containers.
T he problem that the package developer and food technologist m ust face is to ensure that
the m icrow avabie package is n o t exposed to excessively high tem perature. T h e selection
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o f m aterials m ust be m ade after careful consum er u se testing as well as in-house test
kitchen analysis (Sacharow and Schiffm ann, 1993).
Paperboard is a popular m icrow ave packaging m aterial because o f cost
considerations. The properties o f p ap erb o ard w hich m ake it attractive in m icrow ave food
use are its transparency to m icrow aves, ready availability in m any forms, ease o f
application, mechanical strength and stiffness, printability and relative econom y.
H ow ever, the deficiencies o f pap erb o ard are its low resistance to m oisture and grease,
p o o r tearing resistance, low barrier to m ass tran sfer o f gasses, m oisture and food
com ponents, and lack o f resealability and shapeability.
F or glass, the advantages o f glass as packaging container include inertness,
nonabsorbency, and a high degree o f transparency to m icrow aves. Glass is a rigid
stru ctu re and has no m igration problem s associated w ith food and beverage products.
H ow ever, properties which m itigate against its u se are its relatively high heat conductivity,
high w eight and brittleness. A nother possible problem w ith glass is that the su rface may
becom e extrem ely hot and d angerous to handle. A lso, the glass surface tem p eratu re may
be deceptively low while the central volum e o f th e co n ten ts may be boiling hot.
A nother category is su scep to rs w hich originally w ere developed to ov erco m e the
problem o f inability o f products to b ro w n and to assist in crisping m icrow avabie pizza and
popcorn. Susceptors are m aterials consisting o f m etallised structures applied to a heat
resistant surface such as polyester film o r kraft p aaperboard. T hey absorb m icrow ave
p o w er and convert it into heat. This heat is then transferred to product by con d u ctio n o r
radiation, creating localized areas o f high tem p eratu re on the fo o d surface w hich causes
brow ning. H ow ever, the con tact heating phenom enon found w ith susceptors fo r p ro d u cts
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such as pizza and sim ilar p ro d u cts m akes som ew hat different dem ands upon the susceptor.
A nother problem en co u n tered w ith susceptors is irregularity o f heating. M oisture
condensation can also b e a problem w ith susceptors. As th e co n tact surface o f th e food
becom es very hot, ev ap o ratio n o f w ater occurs which m akes th e su scep to r a less effective
m icrow ave h eater (Z ucherm an and M iltz, 1993; S acharow and Schiffmann, 1993).
Plastic Packaging for microwave oven
Plastic m aterials th at have been used in m icrow ave ovens include : polyethylene,
polypropylene (PP), acrylonitrile, butadiene styrene (A B S), polycarbonate, nylon, styrene
acrylonitrile(S A N ) and m ixtures o f styrene and acrylic sold u n d er th e trade name Dylark
(C alto, 1978).
Polyolefins including polyethylene, polypropylene, polybutylene and copolym ers o f
ethyiene w ith propylene and o th e r m onom ers have been d ev elo p ed in various form s and
are used m ost com m only in m icrow avabie containers. Polyolefins are generally remain
tough at freezer tem peratures, are translucent to op aq u e in thick sections and are available
in an exceptionally b ro ad range o f grad es w ith different specialized properties.
Polypropylenes, gaining favor as trayw are fo r m icrow ave fo o d s such as vegetables, can be
filled w ith inorganic fillers such as calcium carbonate o r talc to raise their useful
tem p eratu re limits by increasing th e stiffness o f form ed parts.
P olypropylene has been used as a replacem ent fo r g lass in m aple-syrup bottles
because o f th e low er co st, co n tact clarity, and ability to w ithstand hot-filling o f the syrup.
Savings in shipping co sts can also be a m ajor consideration (T alvitie and Gaunt, 1982).
Interest in PP has b ecom e even m ore intense recently, and has been quite favorable
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com pared to o th e r com m only used plastics th at can be steam sterilized (Szulczynski,
1978).
S w eintekand and O ’Donnell (1 9 9 4 ) reported on a m icrow ave ready syrup bottle.
The polypropylene b o ttle features several advantages fo r m icrow avabie packaging. Its
shape allow s it to fit inside m icrow ave ovens. B ottle geom etry can be created to assist in
uniform heating o f th e syrup.
P olypropylene filled w ith up to 40% by weight o f an inorganic com pound such as
calcium carbonate, is stiffer at tem peratures reached by foods heated in m icrow ave ovens;
it is used to provide stab le handling w hen rem oving heavier food loads from th e oven.
The appearance o f co n tain ers m ade from filled resin is b etter than o f unfilled PP and som e
observers say th at they look b etter than unfilled C PET trays. Filled PP, used fo r frozen
foods intended fo r m icrow aving, is not dual-ovenable and is not recom m ended fo r reuse.
Polyester (P E T ) includes a large class o f therm oplastic m aterials extensively used
in m icrow ave packaging intended for single use, but also saved fo r reuse in th e freezer.
CPET, a crystallized form o f PET, exhibits heat resistance to 445 °F and is currently the
superior material for dual-ovenable plastic w are. PCTA, polycyclohexane terephthalateacid (m odified) has a higher tem perature range with approxim ately a 50 °F advantage over
CPET. M ica-filled nylon is an alternative to C PET o r therm oset polyester containers,
which w as th e fo rm er p referred m aterial for dual ovenable prem ium meals (S ach aro w and
Schiffmann, 1992).
High density polyethylene, though rigid, distorts at tem peratures around 170 ° F
and is brittle at low tem p eratu re. PP has a higher distortion tem perature, around 230 °F,
is not rigid and has a ten d en cy to becom e brittle. ABS can to lerate tem peratures in the
190 to 220 °F range depending on the grade. It is som ew hat subject to dam age by
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abrasion.
P olycarbonate has a distortion tem perature n ear 250 °F and is extrem ely break
resistant. Nylon is strong, but has a low tolerance to tem p eratu res o f 150 to 170 °F. SAN
is strong , but lim ited to 190 °F. Styrene and acrylic m ixtures, th o u g h tolerant to
tem peratures up to 230 °F, are relatively brittle (D ecareau, 1992).
Styrenics are polym ers o r copolym ers o f styrene w ith o th e r m onom ers and may be
polym erized in the presence o f toughening rubbers, form ing m aterials such as styrenebutadiene. Styrenics are characterized by their ease o f rapid form ing into containers
shaped by injection molding, blow m olding and especially therm oform ing. General
purpose polystyrene (PS o r G PPS) is a crystal clear plastic used extensively in food
service for tak eo u t containers and in som e m icrow ave applications w here tem peratures are
limited approxim ately to 90-100 °C. Im pact resistant grades, w hich contain the
aforem entioned rubbers, may be translucent o r opaque and w ithstand heat to the same
extent as G PPS. H igh heat resistant grades o f PS are p ro d u ced by copolym erizing styrene
with alpha methyl styrene o r other high tem perature m onom ers. T he polym er has high
deflection tem peratures up to the o rd er o f 120 °C (D ecareau, 1992; Sacharow and
Schiffmann, 1992).
E xpanded polystyrene, although it has good food heat retention properties, has not
been com pletely favorable for m icrow ave oven usage. A lthough transparent to m icrowave
energy it is not resistant to high tem peratures attained by som e food products (D ecareau,
1992). M onte and L andau-W est (1983) tested a w ide variety o f frozen foods heated in
expanded polystyrene containers. The foods w ere heated to 180 °F and held at this
tem perature fo r 10 minutes. Any container leakage, m ajor disto rtio n o r softening that
might result in container failure resulted in a “ not recom m ended” rating for that product.
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Fatty foods (such as fried foods, gravies, certain cheese sauces, fatty m eats and b u ttered
foods) gave high failure rates. F oods with much gravy show ed breakdow n o f th e
container at th e gravy line.
Stehle (1979) co m p ared th e quality o f polysulfone, a therm oset polyester, a
therm oplastic polyester, and poly-4-m ethyl pentene-1 containers. A variety o f fo o d items
w ere used to test the reactio n to high fat tem peratures, high sugar tem peratures, and
protein stain o r residual. B aco n w as used for high fat tem peratures; peanut b rittle fo r high
sugar tem peratures; b eef p atties and m eat lo a f for protein residual; and pork ro ast fo r high
fat and protein residual. A p p ro p riate utensils and accessories m ade from th ese plastics
w ere used. T he study concluded th at cookw are m ade from polysulfone and th erm o set
polyester can be used w ith any food and for all m icrow ave cooking purposes. T he
cookw are m ade from therm oplastic polyester and methyl pentene isom er w arp ed o r
becam e distorted in som e o f th e shapes tested.
Mechanical Properties of Polymeric Packaging Materials
T he mechanical properties, am ong all the pro p erties o f plastic materials, are often
the m ost im portant because virtually all service conditions and th e majority o f en d -u se
applications involve som e d eg ree o f mechanical loading. M aterial selection is q u ite often
based on mechanical p ro p erties such as tensile strength, m odulus, elongation, and im pact
strength. N um erous factors affect various mechanical p ro p erties o f polymers, including
m olecular w eight, processing, extent and distribution o f crystallinity, com position o f
polym er and use tem perature.
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Crystallinity has a number o f im p o rtan t effects upon th e m echanical properties o f a
polym er. Yield stress and strength, and hardness increase w ith an increase in crystallinity
as does elastic m odulus and stiffness. Physical facto r that in crease crystallinity, such as
slow er cooling and annealing, also tend to increase the stiffness, hardness, and modulus o f
a polym eric material. Polymers w ith at least som e degree o f crystallinity are denser,
stiffer, and stronger than am orphous polym ers. H ow ever, th e am o rp h o u s region
contributes to the toughness and flexibility o f polym ers. Increasing th e percentage o f
crystallinity decreases th e impact stren g th and increases th e probability o f brittle failure. A
reduction in the average m olecular w eight tends to reduce th e im pact strength and vice
versa. (Seym our and C arraher, 1984).
M ost linear polym ers are hard brittle plastics at tem p eratu res below their
characteristic glass transition tem p eratu re (Tg), leathery and rubbery at tem peratures ju st
above the Tg, and viscous liquid at tem p eratu res above th e m elting tem p eratu re (Tm).
Som e polymer, including netw orked, highly cross-linked, and highly crystalline polymers,
are difficult to melt and often undergo solid phase therm al d eg rad atio n before melting
occurs (Seym our and Carraher, 1984).
The im pact properties o f the polym eric materials are directly related to the overall
toughness o f the m aterial. Impact stren g th is th e am ount o f en erg y w hich a plastic
m aterial can absorb befo re it breaks. T h e higher the im pact stren g th o f a material, the
higher the toughness and vice versa. Im p act resistance is th e ability o f a m aterial to resist
breaking under a shock loading o r th e ability to resist the fractu re u n d er stress applied at
high speed. F actors affecting im pact p ro p erties includes tem p eratu re, orientation,
processing conditions, and degree o f crystallinity.
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Im pact resistance is one o f th e m ost widely specified m echanical properties o f
polymeric m aterials. The im pact resistance o f plastic materials is strongly dependent upon
tem perature. A t low er tem peratures th e im pact strength is reduced drastically. The
reduction in im pact is even m ore dram atic near the glass transition tem perature.
Conversely, at higher test tem peratures, the im pact strength is significantly im proved
(Shah, 1984; Spath, 1957).
P rocessing conditions play a key role in determ ining the im pact resistance o f a
material. Inadequate processing conditions can cause the material to lose its inherent
toughness. H igh processing tem p eratu re can also cause therm al d egradation and
therefore, reduced im pact strength. Im proper processing conditions also create a weak
weld line th at red u ces overall im pact strength. M olecular orientation introduced into
drawn films and fibers may give ex tra strength and toughness over th e isotropic material.
However, such directional orientation o f polym er molecules can be very susceptible in a
molded part since the impact stresses are usually multiaxial.
Im pact resistance, o r resistance o f brittle fracture, is also a function o f the
molecular w eight o f a polymer. A reduction in the average m olecular w eight tends to
reduce the im pact strength and vice versa. Im pact resistance o f brittle polym er is
increased by additional plasticizers. Thus, polyvinyl chloride (PV C ), plasticized by
relatively large am ounts o f dioethyl phthalate, is much less brittle than unplasticised rigid
PVC (S eym our and Carraher, 1984).
T he im pact resistance o f b o ttles can be im proved by biaxial draw ing which has
contributed to th e success that P E T has experienced due to stretch b lo w molding.
However, th e re are several processing techniques, such as extrusion blow ing that cannot
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23
create this orientation effect. T herefore, o ther approaches such as im pact modifier m ust
be provided to im prove the property. Im pact modifiers prim arily used fo r packaging
purpose especially for the pro d u ctio n o f sheets and bottles, im prove th e impact resistance
o f any ty p e o f plastic while m aintaining transparency (M ey er and Leblanc, 1995)
T roy, Shortridge, and F ozey (1985) studied the im pact m odifier perform ance in 16
ounce PV C bottles using a d rop im pact resistance test. Im p act m odifier (m ethacrylatebutadiene-styrene) are added to PV C blow molding com pounds. PV C bottles with im pact
m odifier can resist the higher d rop heights before failing th an th o se o f th e bottles w ithout
im pact m odifier. This indicates th at im pact modifier can be used to overcom e the brittle
characteristics o f unmodified PV C resins. Besides m ethacryiate-butadiene-styrene,
chlorinated polyethylene, EVA, acrylic resin derivatives, and A B S are also used as an
im pact m odifiers (Briston, 1994).
Effect of Temperature on Mechanical Properties
Polym ers pass through different physical states acco rd in g to their tem perature; at
low tem perature, since the internal mobility o f the m acrom olecules is “frozen”, they
solidify and present a glassy, am o rphous appearance. As th e c o nta ct tem perature rises,
the m aterial passes through a phase o f relaxation, characterized by an erratic behavior on
the path o f Y o u n g ’s modulus (th e ratio o f stress to corresp o n d in g strain below the
proportional limit o f a material, o r a m easure o f m aterial’s stiffness) and other physical
properties.
G enerally, both crystalline and am orphous polym ers are brittle at low tem perature
and both have relatively low im pact strengths. As the te m p eratu re o f a material decreases.
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24
its ultim ate strength and y o u n g ’s m odulus increase; this applies to tension, com pression,
and bending. At the sam e tim e elongation (th e increase in the length o f a te st specim en
produced by a tensile load) decreases (S ey m o u r and Carraher, 1984; S p ath , 1957).
At low tem peratures, th e atom s vibrate w ith small am plitudes. W ith increasing
tem perature, the vibrations increase in m agnitude and eventually b ecom e co o rd in ated to
the degree that translational chain m otions are produced, which involve m any chain atom s
at elevated tem peratures (C alister, 1994).
W hen a polym er is heated to a certain tem perature, changes o f a physical nature,
can occur. These may co n sist o f various transitions, which are accom panied by changes in
the physical properties o f th e material (brittleness, elasticity, devitrification, softening,
melting, etc.).
K em p and K ennedy (1 9 8 7 ) indicated th a t th e effect o f heat on polym ers can
m anifest itself in 2 ways:
a)
the polym er softens o r melts. The kinetic energy o f the chains becom es large enough
to overcom e the interm olecular forces, and th e plastic flows easily;
b) the structure is degraded. Som e m acrom olecular com pounds u n d erg o scission to
low er m olecular w eight p ro d u cts o r even to m onom er w ithout changing chem ical
com position-i.e. are depolym erized, o thers release low -m olecular w eig h t fragm ents
w ith sim ultaneous change in chemical com position-i.e. are decom posed. B o th
processes are called degradation.
K orshak (1971) p ointed o u t that polym ers exposed to high te m p eratu re undergo
chem ical transform ations o f th ree main kinds: purely therm al conversions, w hich include
therm al degradation and cross-linking o f the polym er; oxidative d eg rad atio n and cross­
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25
linking; and hydrolytic degradation. T h e co u rse o f each o f these reactio n s depends to a
great exten t on th e structure o f th e polym er.
D egradation is a rupture o f th e m acrom olecular structure, w hich results in a
progressive decrease in the m olecular w eight o f th e polym er, and h en ce in the
deterioration o f its mechanical pro p erties. C ross-linking consists o f th e form ation o f
bonds betw een the m acrom olecular chains, w ith consequent increase in the m olecular
weight o f th e polymer; this m eans th a t th e physical and mechanical characteristics o f the
polym er and its heat resistance m ay im prove to a certain extent as a result (K orshack,
1971).
M echanical stress due to im pact arises usually during distribution. M echanical
stress due to pressure changes w ithin th e package is normal during p ro d u ctio n processes
involving h eat treatm ents o f foodstuffs. In these cases the stress resu lts due to thermal
expansion o f th e content. T he v olum e increases d u e to heating o f th e co n ten ts from an
initial tem p eratu re to the high tem perature. I f th e free volume in th e p ack ag e is smaller
than the volum e increase o f the incom pressible aq u eo u s phase, the inner pressure would
cause a ip tu re o f the package o r o f th e closure. T he pressure is also affected by gaseous
com ponents o f the product and by expansion o f th e package itself (K em p and Kennedy,
1987).
Effect of Beat on Degree of Crystallinity
Polym er properties depend on th e phase state (am orphous o r crystalline) o f the
polym ers as well as their m olecular w eight, and chem ical com position. Polym ers are
closely packed, having conform ation o f th e low est possible energies. I f at a certain
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26
tem perature th e cohesion energy o f the chain is larger than the kinetic energy, then
conditions a re suitable for parts o f the m acrom olecule to exhibit clo se packaging and
incorporation into a crystalline arrangem ent. W hen cohesive energy ex ceed s the kinetic
energy o f th e chains, provided that there is n o t to o large o f a steric hindrance,
crystallization tak e place (even in polym ers which do not crystallize u n d er normal
condition) (M iller, 1966).
C rystallization is tem perature dependent to a considerable extent. Prim ary
crystallization com prises form ation o f the crystal nuclei and gro w th o f the spherulites.
The secondary crystallization has great practical im portance. It is this latter stage which is
responsible fo r th e undesirable volume and o th e r physical changes w hich tak e place
usually after processing. Its course and extent are affected m arkedly by th e therm al
history o f th e polym er (Kem p and Kennedy, 1987).
C rystallites in polym ers reinforce their structure and im prove th eir mechanical
properties an d resistance to elevated tem peratures. Therefore, an im p o rtan t consideration
o f the polym er change is the change in crystallinity o f the polym er b ecau se crystallinity can
play a significant role in the physical character o f a polymer such as clarity, tensile
strength, and im pact strength (Seym our and C arraher, 1984).
Z ucherm an and M iltz (1993) studied th e changes in degree o f crystallinity o f thinlayer suscep to rs during m icrow ave heating. T he structure used w as a m etallized
polyethylene terephthalate film laminated to paperboard with a polyurethane-based
adhesive. T h e sam ple w as heated in a 700 w a tt m icrow ave oven. T h e degree o f
crystallinity o f th e PE T film before and after heating in the m icrow ave w as m easured using
DSC. The results show ed that m icrowave heating affected the crystallinity o f the
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27
m etallized PET. They concluded that the degree o f crystallinity o f the film w as reduced
during heating in the m icrow ave oven caused by relaxation o f th e o riented P E T film. In
addition, differences in therm al difTusivity betw een aluminum and PET, resulting in
differences in heating and expansion rates may have added to this effect.
B rennen (1978) investigated the effect o f therm al conditioning on percent
crystallinity o f low density polyethylene film used fo r food storage. Specim ens w ere
analyzed w ithout therm al conditioning and after having annealing at 100 °C for 12 hours.
The results show ed that th e annealing increased p ercen t crystallinity by b o o sting the high
tem perature crystallinity o f the material.
In pharm aceutical packaging, various ty p es o f sterilization p ro cesses are employed.
O ne o f these is gam m a irradiation. This process m ay n o t only sterilize, b u t may also affect
material properties and therm al characteristics o f th e polym eric packaging m aterial
(B reakey and Cassel, 1979). This is in agreem ent w ith the study o f T rice and Goolsby
(1990). They determ ined th e physical and therm al property changes o f polypropylene
packaging subjected to g am m a irradiation using D SC . Results show ed that the gam mairradiated sample, w hen com pared to the nonirradiated material, had m elting point
depression, and broadening o f the endotherm w ith a m uch smaller peak am plitude in the
isotherm al crystallization region. This was due to crosslinking w hich o ccu rred during the
irradiation process.
Christie, G regory, and W ood (1993) determ ined the effects o f polypropylene
crystallization on film form ing by investigating th e effects o f operating p aram eters
(quenching tem peratures) on th e finished product properties o f a film g rad e PP
hom opolym er. This study show ed correlations betw een operating param eters and
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28
crystallinity in PP films. Processing conditions had a dram atic im pact on final crystallinity
and corresponding film properties. Increases in density b ecau se o f aging d u e to secondary
crystallization also affect final film properties. These results w ere su p p o rted by Shah
(1984) w ho found that increases in p ercen t crystallinity d ecreased th e im pact stren g th and
increased th e probability o f brittle failure.
N icastro, et al (1993) studied th e effect o f heat sealing on th e seal stren g th and
crystallinity o f unoriented cast polypropylene film with thickness o f 3 mil. PP sam ples
w ere 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 resu lts indicated that sealing tem p eratu re had a significant
effect on the increase in percent crystallinity o f the film seal region, w hile h eat sealing
dwell tim e and pressure had little effect on th e increase in crystallinity. T his increase in
crystallinity increased the seal stren g th due to the larger am o u n t o f interdiffusing polym er
chain segm ents. In contrast, S elikhova (1989) observed th a t a g re a te r crystallinity may
actually increase the brittleness o f th e polym er in the seal reg io n and cau se th e seal layer o f
film lam inate to be m ore susceptible to delamination.
Mechanical Testing for Polymeric Packaging Materials
A fter being in use for a period o f tim e, materials u n d erg o m any ch an g es due to
mechanical stress and various o th e r influences such as hum idity, tem p eratu re, radiation,
and chem ical radiation. The im pact test, am ong other m ethods, can be u sed to investigate
such changes. Im pact tests can be divided into six major classes and subdivided into many
different types having slight variation as follow: Pendulum im pact tests, H igh rate tension
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29
tests, Falling w eight im pact tests, Instrum ent im pact tests. H igh rate im pact tests, and
m iscellaneous te sts (Shah, 1984).
M ost physical test are carried o u t on a m olded test piece or, in the case o f
permeability, on film o r sheet. F or th e test th at can be carried out on the finished
container, im pact tests on plastic bottles are useful w hen change in material and /o r bottle
design is contem plated. Perform ance tests will quickly sh o w any fundamental w eakness
that may lead to leakage o f the contents during distribution. D rop im pact resistance
evaluation o f a b lo w m olded therm oplastic co ntainer can be carried o ut using a standard
test (A ST M -D 2 463) w hich practically sim ulates end-use environm ental conditions. It is
also useful for com paring different m aterials as well as evaluating the influence o f
processing conditions on the impact properties o f bottles (B riston, 1994; Shah, 1984).
Drop Impact Test
D rop im pact resistance o f blow m olded therm oplastic containers can be
determ ined by th ree conventional tests; th e Static D rop H eight, the B ruceton Staircase,
and the C um ulative D rop m ethods. T hese standard test m ethod are used to evaluate the
effect o f construction, materials and processing conditions on th e impact resistance o f
blow n containers (A S T M -D 2463, 1996).
In the S tatic d rop height, th e container is dropped from a fixed height and the
percent failure report. F or the B ruceton staircase m ethods, a set o f containers are
dropped from various heights. The drop height is raised o r low ered depending on the
result obtained from th e previous test, if th e previous test container did not fail, the next
bottle will be tested at a higher height. I f th e previous co ntainer failed, the d rop height o f
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30
the next b o ttle will be low ered. T h e mean failure height and stan d ard deviation are then
calculated from th e d ata obtained. In the cum ulative d ro p test, each o f 20 bottles is
subjected to successively higher d rop heights until b reakage occurs, and the num ber o f
broken b o ttles at each drop height is recorded (B riston, 1994).
T roy, S hortridge, and Fazey (1985) used th e drop ten test p ro to co l for testing
PVC bottles in g ro u p s rather than one at a time to find d rop heights th at span the range
from no break s to all break and to reduce testing tim es to ju st slightly longer than those o f
the B ruceton test in which bottles are dropped individually. I f m ore than h alf th e bottles
break, the dro p height is low ered. Likewise, if breakage is very low th e drop height is
raised. T hey concluded that values derived from the D rop ten test a re highly reproducible.
In addition, this test generated m ore data, since results are based on a lot m ore bottles
than used in th e three m ost com m on tests (B ruceton, Static, and C um ulative tests).
M eyer and Labrant (1995) perform ed drop testing to d eterm ine th e effect o f an
impact m odifier on th e perform ance o f 750 ml. extrusion blow n P E T G containers w ith and
w ithout im pact m odifier by com paring th e drop heights and the p ercen t o f failures at
specific d ro p heights. The result show ed that breakage levels o f b o th unm odified and
impact m odified bottles increased w ith increasing drop heights. T h e im pact modified
bottles exhibited little breakage com pared with unm odified bottles. Successive increases o f
impact m odifier im proved the bottle failure drop height.
B esides im pact m odification, test conditions such as im pact angle, fill level and
wall thickness can influence the results o f a drop im pact resistance. B o ttles dro p p ed onto
a flat surface have higher drop height resistance than th o se dro p p ed a t an angle o f 10
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31
degrees because stress is concentrated in angle drops at a single point instead o f being
dispersed over th e entire b o tto m o f the bottle.
Determination of Percent Crystallinity
M easurem ents w hich can be related to th e crystalline state o f a p o ly m er are : x-ray
diffraction, density, nuclear m agnetic resonance absorption, heat capacity, infrared
absorption o f low m olecular w eight com pounds and d euterium exchange. E ach o f these
m easurem ents describes som ething about th e polym er, not necessarily th e p ercen tag e
crystallinity, but som ething about the regularity o f packing, th e freedom o f m otion o f the
m olecules o r the ex ten t o f interm olecular hydrogen bonding o f the polym er (Sperling,
1986; Miller, 1966).
Percent crystallinity can be determined by quantifying the heat asso ciated with
m elting (fusion) o f th e polymer. Differential scanning calorim etry (D SC ) is a therm al
analysis technique to m easure th e tem peratures and heat flow s associated w ith transitions
in materials as a function o f tim e and tem perature. Such m easurem ents p rovide
quantitative and qualitative inform ation about physical and chem ical changes th at involve
endotherm ic or exotherm ic processes, or changes in heat capacity. P ercent crystallinity o f
polym er can be determ ined by quantifying th e heat associated w ith m elting endotherm
(fusion) o f the polym er by developing a ratio against th e heat o f fusion fo r a 100 %
crystalline sample o f th e sam e material, or m o re com m only by rationing ag ain st a polym er
o f know n crystallinity to obtain relative value (T hom as, 1995).
The conventional instrum ent used for m aking D SC m easurem ents is th e heat flux
design show n in F igure 1. In this design, a m etallic disks is th e prim ary m eans o f heat
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32
transfer to and from th e sam ple and reference. T he sam ple, w hich is placed in a metal pan,
and the reference (an em p ty pan) sit on raised platform s form ed in the co n stan tan disc. As
heat is transferred th ro u g h th e disc, the differential heat flow to the sam ple and reference
is m easured by area th erm o co u p les form ed by th e ju n ctio n o f the constantan disc and
chromel w afers w hich c o v e r th e underside o f th e platform s (A nonym ous, 1997).
M odulated D S C (M D S C ) is a relatively new therm al analysis technique which
provides the same in form ation as conventional D SC , but in addition has th e unique ability
to m easure heat cap acity continuously (M ele e t al, 1995).
The general eq u atio n which describes th e resultant heat flow at any point in a DSC
experim ent is:
where:
d Q / dt
=
C p (3 + f(T ,t)
dQ / d t
=
Cp
=
h eat capacity
P
=
heating rate
f (T, t)
=
h eat flow from kinetic (ab so lu te tem perature
to tal heat flow
and tim e dependent) process
DSC can m easu re only heat flow w hich is co m p o sed o f tw o com ponents. O ne
com ponent is a function o f th e sam ple’s heat cap acity and rate o f tem p eratu re change, and
the o th er is a function o f ab solute tem p eratu re and tim e (T hom as, 1995).
M D SC determ ines th e total, as well as heat capacity (reversing) heat flow
com ponent and kinetic (nonreversing) co m p o n en t o f to tal h eat flow, to p ro v id e increased
understanding o f co m p lex transitions in m aterials (F ig u re 2). M D SC is able to do this
based on the tw o h eatin g rate s seen by th e m aterial; th e av erag e heating rate w hich
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33
provides total h eat flow inform ation (dQ / dt) and th e sinusoidal heatin g ra te which
provides heat capacity (reversing heat flow, Cp{3 ). The kinetic (n o n rev ersin g ) heat flow is
determ ined as th e arithm etic difference between the total heat flow and th e heat capacity
com ponent: N o n rev ersin g heat flow
=
Total heat flow - R eversing heat flow
(A nonym ous, 1997; T hom as, 1995).
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34
GAS PURGE
INLET
7ZZZZZZZZZZZZZ25L
7ZZZZZZZZZZZ
SAMPLE
PAN
reference
PAN
CHROMEL
DISC
THERMOELECTRIC
DISC (CONSTANTAN)
ALUM EL
WIRE
THERMOCOUPLE
JUNCTION
HEATING BLOCK
CHROMEL WIRE
Figure I . H eat Flux D SC S chem atic (A nonym ous, 1997)
Heat Flow (W/g)
o.i 4
NO NREVERSING
/
\
----------------------
0.2
0.1
ra
03
* - 0 .1 O)
o-Q.2]
0.3
-
o
n: o.o-
z
-
0.1
5
REVERSING
0.1
0.2
s.0.1-
\
TOTAL
o .o
0.2
Heat Flow (W/g)
/\
-
-
-
/
-
0.0
\ Rev
0.3
0.2
5.5mg sample
helium purge
-
03
.
-
2°C/minutc heating rate. i l ° C amplitude, 100 second period
50
100
150
'
200
250
300
"
°
- 3
Temperature (°C)
Figure 2. Nonreversible, Reversible, and T o tal H eat Flux Signal (Thom as, 1995)
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CHAPTER 3
MATERIALS AND METHOD
3.1 Samples
Plastic containers: 46 cases o f th e 24 o unce plastic bottles w ere provided by a
p roduct m anufacturer. T he containers w ere extrusion blow m olded from polypropylene
into a squat ju g shape. T he dim ensions o f th e b o ttle w ere; height 7 inches w ith an eclipse
bo tto m and w ere pinched at 2 points o f th e b o ttle ’s handle. T he b o ttles w ere filled
com m ercially w ith su g ar syrup and closed in th e norm al m anner (F igure 3). All bottles
w ere placed in an environm entally controlled ro o m m aintained at 20 + 2 °C , and 50 + 5 %
relative humidity.
3.2 Wall Thickness of Materials Used
Six b o ttles w ere selected random ly so th at they w ere rep resen tativ e o f th e lots
being tested. T he thickness o f the m aterial w as m easured at flat bo tto m , body wall, right
bo tto m com er, left b o tto m com er, and side seam o f each bottle. T he wall thickness
m easurem ent w as carried o u t using a M agna-M ike instrum ent (M odel 800, Param etics,
M A ) com posing o f a steel target ball, m agnetic probe, and control unit. T o m easure the
wall thickness, a 3/4 inch diam eter targ et ball w as d ro p p ed into th e b o ttle and a m agnetic
35
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36
probe tip w as applied to th e o u tsid e o f th e bottle. The ball w as a ttra c te d to the probe
through the n onm agnetic plastic container, th e container w as m oved o v e r the tip o f the
probe to lo cate th e p o ints a t w hich m easurem ents w ere to be m ade. T h e field strength
varies as the d istance b etw een th e ball and th e probe changes, w hich is con v erted with a
thickness m easurem ent.
3.3 Determination of the Impact Orientation of the Bottles By Free Fall Drop Test
A dro p te st w as designed to establish th e critical im pact o rien tatio n using a free fall
drop. The hypothesis w as th a t im pact o f a particular orientation(s) can lead to bottle
breakage.
A ST M D -2463 (1 9 9 7 ), Standard T est M ethod for D eterm ining D ro p Impact
Resistance o f B lo w -m o ld ed T herm oplastic C ontainers, w as used fo r th is purpose. This
standard consists o f d ro p p in g containers filled w ith tap w ater. Since d a ta developed w ith
a water-filled co n tain er m ay not be representative o f w hat m ight be e x p ec ted with a
product o f high specific gravity, th e containers filled with syrup as received, w ere used as
test samples.
Lansm ont M odel P D T 56 E D rop T ester w as used to sim ulate th e free fall drop.
This machine w as u sed in com pliance w ith A ST M D-2463 standard te s t m ethod. It is
equipped w ith a d ro p le a f pneum atic actuation system which prevents p ack a g e rotation
and assures rep ro d u cib le results. T he container w as dropped on to a 4 6 ” x 36” x 0.5” steel
plate m ounted in concrete.
Prior to th e test, 50 b o ttles filled w ith syrup to the frill level, w e re conditioned for
24 hours at 20 + 2 °C an d 50 + 5 % relative hum idity to bring th e b o ttle s into equilibrium
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37
w ith an average ro o m conditions. F or any one o f th e test orientations (including face, flat
bottom , handle, right b o tto m com er, and left bo tto m com er), a set o f 7-10 bottles was
used. The b o ttles w ere then subjected to the free fall drop test using th e Free Fall D rop
T ester (Lansm ont M odel P D T -5 6 E ). D rop im pact resistance w as determ ined using the
B ruceton Staircase o r “ U p and D ow n” m ethod (A ST M standard D -2463, 1997). It
consists o f dropping a set o f containers from various heights, th e drop height being raised
o r lowered depending on th e result o f the previous test, that is, if the previous container
failed, the drop height is low ered; if the previous container did not fail, the drop height is
raised.
The dro p s w ere co n d u cted on several package orientations: face, flat bottom , right
bottom com er, left b o tto m com er, and handle (F igure 3), from a d rop height o f 24 inches
at increments o r d ecrem en ts o f 3 inches. A positioning jig w as used to hold a test
container at the desired orientation. A new container w as used for each drop. Following
each drop, the co n tain er w as visually observed. W hen a container in a set failed, the
following rule w as applied, if th e first container dropped did not fail, th e second container
w as dropped from a height 3 inches higher. I f the first container failed, drop the
second container w as d ro p p ed from a height 3 inches lower.
R esult Evaluation
All specim ens w e re exam ined for failure after dropping them at th e particular
height. A failure w as defined as any fracture visible to the observer. Any m ark o f
contained liquid on th e ex terio r o f th e bottle em erging from any ap ertu re o th er than the
m olded opening w as also considered a failure. B ottles w ere squeezed gently after impact
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38
F ig u re 3.
S y r u p B o ttle : (1 ) left b o tto m co m er. (2) rig h t b o tto m co m er.
(3) handle. (4) flat b o tto m . (5 ) face.
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39
to determ ine any pinhole type failure. Since not all bottles failed at th e sam e drop height,
a m ean failure height and stan d ard deviation w ere calculated from the data.
S tatistical Analysis
S tatistical analysis w as co n d u cted using a Com pletely R andom ized D esign with 5
im pact orientations. SAS so ftw are version 6.12 (SA S Institute, Inc.) w as used to run a
one-w ay analysis o f variance in association w ith D uncan’s M ultiple R ange Test. The
A N O V A and D u n can ’s M ultiple R ange T est w ere used to establish significant difference
using p-value < 0.05 (Steel, T o rrie, and D ickey, 1997).
3.4 Determination of Drop Impact Resistance of the Bottles Filled with Syrup at
Different Levels
T he prelim inary test established th at im pact orientations at th e right bottom com er
and handle had potential to cause failure o f th e container. An experim ent w as then
designed to establish the failure d ro p height o f bottles filled w ith syrup at full, V*, half, and
‘A levels. T he bottles were d ro p p ed o n to th eir right bottom co m er and handle using the
B ruceton Staircase free fall drop m ethod (A ST M Standard-D 2463, 1997). T he first bottle
w as d ropped from 21 inches o n to its handle and from 36 inches o nto its right bottom
co m er w ith increm ents o r decrem ents o f 3 inches, thereafter.
Statistical Analysis
Statistical analysis w as co n d u cted using a C om pletely R andom ized D esign o f the 4
fill levels. SAS softw are version 6.1 2 (SA S Institute, Inc.) w as used to ru n a one-w ay
analysis o f variance in association w ith D u n can ’s M ultiple R ange Test. T h e A N O V A and
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
40
D uncan’s M ultiple R an g e T est w ere used to establish significant difference using p-value <
0.05 (S teel, T orrie, and D ickey, 1997).
3.5 Effect of Microwave Repeated Heating on the Drop Impact Resistance of the
Bottles
It w as hypothesized th at repeated m icrow ave heating o f a plastic container may
affect its m echanical p ro p erties such as d rop im pact resistance. R ep eated microwave
heating and free fall d ro p s w ere conducted to sim ulate the actual situation likely to occur
in end-use.
3.5.1 Microwave exposure
Em pty plastic syrup bottles were filled with sugar syrup at full, 3A '/>, and 'A levels,
respectively. H eating tim e w as varied (2.00, 1.45, 1.30 and 1.00 minutes, respectively) in
proportion to the level o f syrup. The heating times used, followed the directions indicated on
the label to reach a specific end temperature. Each container w as placed upright in the
m icrowave oven (G old Star: 2450 MHz, max. microwave output 800 W. w ith a cavity
measuring 13 lA " x 8”x 14 Vi") at exactly the same location (center o f the glass plate) for every
run so as to maintain the sam e electric field. Each container w as heated at the same starting
tem perature (20 ° C) to reach the specific end tem perature (58-60 ° C ) and then cooled to
room tem perature before the next reheating in the microwave oven.
A m icrowave heat treatm ent scheme (Figure 4) w as developed to simulate the
m icrowave heating directions as found on the product’s label. F o r each experiment, one set o f
bottles (approxim ately 84 bottles, each bottle contains 710 ml sugar syrup) w as subjected to 12
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41
different microwave heating variables. Heating time was dependent on product fill level. The
heating and reheating regim e as described above is presented in m ore detail in T able 1.
3.5.2 Temperature of the Sample After Reheating in Microwave Oven
Sixteen bottles (uniform initial tem perature 20 °C ) filled w ith syrup at full, 3/>, '/:*,
and 'A levels w ere placed on a fixed location in the center o f a household m icrow ave oven
(G old Star, 2450 Hz, max. 800 W ) operating at full pow er for 2.00, 1.45, 1.30, and 1.00
min., respectively. A fter heating in the m icrow ave oven, the tem perature in syrup and on
the inside and outside bottle surfaces w ere m easured using a p o cket-probe digital
therm ocouple (Electronic D evelopm ent Laboratories Inc., NY).
Statistical Analysis
A statistical analysis to determ ine com bination effects o f three different p ro d u ct areas and
4 fill levels on product and package m aterial tem perature was perform ed by Factorial
Design. SAS softw are version 6.12 (SA S Institute, Inc.) was used to run a m ultiple
analysis o f variance. W hen the effect o f product area and/or fill level w as significant,
statistical difference o f m eans w as perform ed using D uncan’s M ultiple R ange T est using
p-value < 0.05 (Steel, Torrie, and D ickey, 1997).
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42
Figure 4. Flow Diagram of Microwave Heat Treatments
Step 1: H eat each bottle in th e set (120 b o ttles, each bottle contains 710 ml. sugar syrup)
fo r 2 min. and cool to ro o m tem perature ( - 2 0 ° C)
I'
—> take 10 bottles for d ro p testing
Step 2: H eat each o f 110 bottles for 2 min. and cool to room tem p eratu re ( - 2 0 °C )
•I
—> take 10 bottles for d ro p testing
Step 3: H eat each o f 100 bottles for 2 min. and cool to room tem p eratu re ( - 2 0 °C )
—> tak e 10 bottles for d ro p testing
Step 4: R em ove 177.5 ml. o f syrup from each bo ttle to leave 532.5 ml (3/4 level) in the
b o ttle
H eat each o f 90 bottles for 1.75 min. and cool to room tem p eratu re ( - 2 0 °C )
1
—> tak e 10 bottles for d ro p testing
Step 5: H eat each o f 80 bottles for 1.75 min. and cool to room tem p eratu re (-20 °C)
1
—> take 10 bottles for d ro p testing
Step 6: H eat each o f 70 bottles for 1.75 min. and cool to room tem p eratu re ( - 2 0 °C )
4-
—> tak e 10 bottles for d ro p testing
Step 7: R em ove 177.5 ml. o f syrup from each bottle to leave 355 ml (1/2 level) in the
bo ttle
H eat each o f 60 bottles for 1.5 min. and cool to room tem p eratu re ( - 2 0 °C )
■I
—> tak e 10 bottles for d ro p testing
Step 8: H eat each o f 50 bottles for 1.5 min. and cool to room tem p eratu re ( - 2 0 °C )
i
—> tak e 10 bottles for d ro p testing
Step 9: H eat each o f 40 bottles for 1.5 min. and cool to room tem p eratu re (-20 °C)
4-
—> take 10 bottles for d rop testing
Step 10: Remove 177.5 ml. o f syrup from each bottle to leave 177.5 ml (1/4 level) in the bottle
H eat each o f 30 bottles for 1 min. an d cool to room tem perature ( - 2 0 °C )
■I
—> tak e 10 bottles for d rop testing
Step 11: H eat each o f 20 bottles for I min. and cool to room tem p eratu re ( - 2 0 °C )
—> tak e 10 bottles for d rop testing
Step 12: H eat each o f 10 bottles for 1 min. and cool to room tem p eratu re ( - 2 0 °C )
I
tak e 10 b o ttles fo r drop testing
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43
Table 1. Microwave Heating Treatment
Microwave
heating
treatment
1
Full level heat for 2.00 min / cool , 1 time
2
Full level heat fo r 2.00 min / cool , 2 times
3
Full level heat for 2.00 min / cool , 3 times
4
From treatm en t 3, rem ove syrup to V* level in the bottle
Condition of Heating
heat fo r 1.45 min/ cool, 1 tim e
5
From treatm en t 3, rem ove syrup to % level in th e bottle
heat for 1.45 min/ cool, 2 tim es
6
From treatm en t 3, rem ove syrup to % level in the bottle
heat for 1.45 m in/ cool, 3 tim es
7
From treatm en t 6, rem ove syrup to V2 level in the bottle
heat for 1.30 min/ cool, 1 tim e
8
From treatm en t 6, rem ove syrup to Vi level in the bottle
heat fo r 1.30 min/ cool, 2 tim es
9
From treatm en t 6, rem ove syrup to V2 level in the bottle
heat for 1.30 min/ cool, 3 tim es
10
From treatm en t 7, rem ove syrup to 'A level in the bottle
heat for 1.00 m in/ cool, 1 tim e
11
From treatm en t 7, rem ove syrup to V* level in the bottle
heat fo r 1.00 m in/ cool, 2 tim es
12
From treatm en t 7, rem ove syrup to V* level in the b o ttle
heat for 1.00 min/ cool, 3 tim es
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44
3.5.3 Drop Impact Resistance of Microwave Reheated Bottles
A fter h eated in th e m icrow ave oven (Figure 4), the bo ttles w ere cooled to
20 + 2 ° C and co n d itio n ed fo r 24 hours at the testing room conditions to bring th e bottles
into equilibrium w ith average room conditions (20 + 2 °C and 50 + 5 % relative
humidity). T h e bottles w ere then subjected to free fall drop testin g using th e Free Fall
D rop T ester (L an sm o n t M odel PD T-56E ) to determ ine drop im pact resistance using the
B ruceton S taircase M ethod.
3.5.4 Effect of Microwave Reheating on the Drop Impact Resistance of the
Bottles
U nheated b o ttles filled w ith syrup at full, 3A, Vz and 'A levels w ere used as control
bottles for co m p ariso n to m icrow ave reheated bottles from treatm en ts 1-3, 4-6, 7-9, and
10-12, respectively. F o r drop height evaluation, the B ruceton S taircase M ethod w as used
to determ ine effect o f m icrow ave heating on dro p im pact resistance. B efore drop testing,
all bottles w ere sto red at 20 + 2 °C , 50 + 5 % relative hum idity at least 24 hours.
Statistical A nalysis
Statistical analysis w as done using a Com pletely R andom ized Design. SAS
softw are version 6.12 (SA S Institute, Inc.) w as used to run a o ne-w ay analysis o f variance
to assess the influence o f repeated heating on th e drop im pact resistance o f the bottles.
W hen the effect o f m icrow ave heating was significant, statistical difference o f m eans was
perform ed using D u n can ’s M ultiple Range Test using p-value < 0.05(S teeI, T orrie, and
Dickey, 1997).
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45
3.6 Effect of Temperature on the Drop Impact Resistance of the Bottles
The effect o f the package m aterial tem perature on the drop im pact resistance w as
determined by com paring drop im pact resistance o f the bottle w hen d ro p p ed at room
tem perature (20 ± 2 °C), at elevated tem p eratu re following heating in th e m icrow ave, and
at low tem p eratu re after cooling in a refrigerator. A fter heating at full p o w er in the
m icrowave oven, all bottles w ere then im m ediately subjected to th e free fall drop test to
determine th e d ro p im pact resistance at handle and right bottom c o m e r using th e B ruceton
Staircase M ethod.
A fter heating in th e m icrow ave oven, the bottles w ere refrigerated for 24 hours at
a tem perature o f 5-9 °C . T he bottles w ere then subjected to the free fall drop test to
determine their d rop im pact resistance at right bottom co m er and handle using B ruceton
Staircase M ethod. This test w as developed to identify influence o f tem p eratu re on im pact
resistance. This test w as also used sim ulate the actual p ro d u ct’s end u se requirem ent.
Statistical Analysis
Statistical analysis w as done using Factorial Design. C o ch ran ’s T est w as used to
determine significant influence o f the three tem peratures w hen data w ere qualitative. SAS
softw are version 6.12 (SA S Institute, Inc.) w as used to run a m ultiple analysis o f variance
to assess the influence o f th e effect o f tem peratures and repeated heating on the drop
impact resistance o f the bottles. D uncan’s M ultiple R ange Test w as th e n used to establish
statistical difference o f m eans using p-value < 0.05(Steel, Torrie, and D ickey, 1997).
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46
3.7 Determination of the Change in Percent Crystallinity of the Packaging Material
by Thermal Analysis
H eat o f fusion w hich co rrelates to percent crystallinity o f th e container can be
determ ined by M odulated D ifferential Scanning C alo rim eter (T A Inst. 2200, TA
Instrum ents Inc., DE). H eat o f fusion o f the bottle ex p o sed to m icrow ave heating was
com pared to that o f the co n tro l (unheated) bottle to determ ine th e effect o f repeated
heating on the percent crystallinity o f the packaging m aterial.
T o m easure p ercent crystallinity, tw o unheated b o ttles and tw o bottles w hich w ere
subjected to m icrow aves w ere selected and 4 material specim ens w ere cut from the
bottom and upper part n ear th e handle o f each bottle and w eighed in a tared aluminum
pan, and then placed in th e M D SC cell to measure th e endotherm ic processes o r changes
in heat capacity. The tem p eratu re o f the specimen w as program m ed through its melting
point while recording th e therm al curve o f tem perature and heat flow associated with
transitions in the samples as a function o f time and tem perature.
Experim ental conditions included:
H eating rate : 2.5 ° C / min
Reference: em pty alum inum M D SC pan
S tarting Tem perature: -50 °C
Limit Tem perature: 175 ° C
Polym er crystallinity can be determ ined using M o d u lated D SC by quantifying the
heat associated with fusion o f th e polymer. Heat o f fusion w as rep o rted as relative %
crystallinity by com paring u n heated polym er against heated polym er to obtain relative
values.
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CHAPTER 4
RESULTS AND DISCUSSIONS
4 . 1 Wall Thickness of the Bottles
The wall thickness o f th e b o ttles used w ere m easured at different positions: body
wall, bottom face, right b o tto m corner, left b o tto m com er, and side seam. T h e thickness
o f the bottle m easurem ents (T able 2) rep resen t 6 replications. Thickness varied at th e
different positions.
Table 2. Wall Thickness of the Bottle Material.
Area
Thickness (inch)a
B ody wall
0.055 + 0.001
B o tto m face
0.181 ± 0 .0 0 9
Right bo tto m c o m e r
0.106 + 0.006
Left bo tto m c o m e r
0.188 + 0.008
Side seam
0.057 + 0.002
a mean with standard deviation
4.2 Determination of the Impact Orientation of the Bottle by Free Fall Drop Test
This experim ent w as perform ed in o rd e r to establish the particular o rien tatio n
which was th e m ost susceptible to b reak ag e d u e to a free fall drop. B ottles filled w ith
syrup w ere tested fo r fracture, leakage, o r b reak ag e by dropping them from v ario u s
47
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48
heights using the B ruceton Staircase M ethod. Since n o t all bottles failed a t th e sam e drop
height, test results w ere statistically evaluated (T ab le 3). A ccording to A S T M Standard
T est M ethod for Drop Im pact R esistance o f T h erm o p lastic C ontainers (1 9 9 6 ), drop height
from a free fall drop provides a m easure o f the d ro p im pact resistance o f plastic bottles.
Results o f th e o n e-w ay analysis o f variance (T ab le 4) for drop heights o f the
bottles dropped onto their handles, face, flat b o tto m , right bottom co m er, and left bottom
c o m e r orientations show ed th at bottles dropped w ith different drop o rientations exhibited
significantly different d rop im pact resistance (p< 0.05).
Table 3. Drop Height Failure of the Bottles at Various Orientations.
Orientations
Drop H eightr
(inch)
H andle
22.6 + 0 .7 a
Right bottom c o m e r
38.6 ± 1.3 b
Left bottom c o m e r
43.8 + 0 .7 C
Face
4 8 .0 + 1.5 d
Flat b o tto m
51.0 + 1 .0 C
" m e a n s w ith same superscript alphabets are not significantly different (p< 0.05).
com parison are m ade only w ithin the same column.
r mean w ith 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
O rientations
4
516.060
136.524*
E rro r
20
3 .780
* denotes a statistically significant difference (p< 0.05).
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49
B ottles falling onto th eir handles, right b o tto m com er, and left b o tto m co m er
w ere more susceptible than th o se falling onto their face o r flat bottom . D rop im pact
resistance w as low at the b o tto m com ers because stress w as co ncentrated at a single
point (angle drop) instead o f being dispersed o v er th e entire b o tto m or surface o f the
bottle. The low drop im pact resistance at the handle w as in g o o d agreem ent w ith that
reported by B riston (1995) w ho stated that th e w eak positions on the bottles w ere
generally at m old parting lines and pinch-off regions. H ence, b o ttles that w ere im pacted
on their handles co rresp o n d s to w here the resin w as pinched o f f These bo ttles w ere prone
to breakage.
It w as also evident th at the right b o tto m c o m e r w as less resistant to im pact than
that o f the left b o tto m co m er, even though these tw o positions cause local areas o f stress
concentration. O ne possibility is that the m aterial w as thinner at th e right b o tto m co m er
(Table 2). As the thickness increases, the im pact resistance increases, and the energy
required to fracture the specim en also increases (Shah, 1984). It is also possible that being
on the same side as w here th e handle w as pinched o ff resulted in decreased im pact
resistance at th e right b o tto m com er. The right b o tto m co m er and handle w ere the
w eakest points and therefore, w ere use as ta rg e ts fo r further experiment.
4.3
Determination of Bottle Drop Impact Resistance
Generally, dro p im pact resistance is determ ined by dropping blow -m olded
containers filled w ith w ater. D ata developed from w ater-filled containers may n o t be
representative o f a p ro d u ct having high specific gravity such as syrup. Thus, in this study,
the bottles w ere filled w ith syrup which is the actual p ro d u ct contained. T o establish a
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50
correlation betw een breakage, drop height, and fill level and to sim ulate specific end-use,
drop im pact resistance o f the bottles was determ ined at full, th ree quarters, half, and
quarter fill levels o f syrup. P ro d u ct w eight w as p ro portional to fill levels (Table 5). The
higher the fill level, th e higher th e weight.
Table 5. Weight of the Plastic Bottle Filled with Syrup at full,
!4, and lA Levels.
Fill Level
Product Weight (g)*
Full
907.5 + 2.4
T h ree q u arter
69 4 .9 + 3.1
H alf
4 8 3 .9 ± 2 .9
Q u arter
2 7 1 .4 + 2.0
1 mean with stan d ard deviation
The w eak points o f th e bottle had been established as right b o tto m co m er and
handle. Hence, this experim ent w as perform ed by dropping th e bottles at these tw o points
Tables 6 an d 8 show drop impact resistance o f bottles dro p p ed at right bottom
com ers and handles, respectively. O ne-w ay analysis o f variance o f drop heights at the
right bottom c o m e r (T able 7) and on the handle (T able 9), found that fill level o f the
syrup significantly affected dro p im pact resistance o f th e bo ttles (p< 0.05). The influence
o f fill levels on d ro p im pact resistance both at right b o tto m c o m e r and handle found that
the higher the fill level, th e higher the w eight o f th e sample, and , therefore, the low er the
drop impact resistance (F igure 5).
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
60
' 1Right Bottom Comer
50
HHandle
40
nn
XI
if ( h i
u
a
JS
jot
'5
X
a
ou
O
H i i ;1
!;;! i i
30
20
illpi
l , i ;'
ifi :
in
Iji i l il i :
illii
‘»i ■
K i i ill
! 1*1 }i!
it's Hi;
i Hjijli
10
n iim ?
l| hi;
i ; ;■ f f j 1• ?
!
m i !)
if*
f! ;pI •; :
.‘ i f I Si
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ijfh !iI■
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ini(!
hi!!;
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i’ffilj
ilH:
:t .
iiliij
liilil
SlS’t l i i H
nhiiiiFull
3/4
1/2
1/4
Fill Level
Figure 5. Effect of Fill Level on Drop Height of the Bottles Dropped onto their Right Bottom
Corner and Handles.
52
Table 6. Drop Heights of the Bottles Filled with Syrup at full, 3A , '/z, and lA Levels
and Dropped at the Right Bottom Corner.
Level of Syrup
Drop Heights
(inch)
Full
39.0 + 2.1 a
T hree quarter
4 5 .6 ± 2 .5 b
H alf
54.0 ± 1.3 c
Q uarter
57.0 ± 2 . 1 d
■
, b ':"d m eans w ith superscript alphabets are not significantly different (p<0.05).
com parison are made only w ithin th e sam e column.
c m ean w ith standard deviation
Table 7. Analysis of Variance of Drop Height Failure of Bottles Filled with Syrup at
full, %, Vi, and lA Levels and Dropped at the Right Bottom Corner.
Source of Variation
d.f.
Mean Square
F
Level o f Syrup
3
3 34.20
67.515*
E rro r
16
4 .950
* denotes a statistically significant difference (p<0.05).
Table 8. Drop Height of the Bottles Filled with Syrup at full, %, '/z, and lA Levels
and Dropped onto their Handles.
Level of Syrup
Drop Heights
(inch)
Full
22.8 + 1.5 a
T hree quarter
25.8 + 1.5b
H alf
33.8 + 1.3c
Q uarter
40.5 + 1 .5 d
a' b,c d m eans w ith superscript alphabets are not significantly different (p<0.05).
com parison are made only w ithin th e sam e column.
c m ean w ith standard deviation
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53
Table 9. Analysis of Variance of Drop Height Failure of Bottles Filled with Syrup at
full,
Vz, and V* Levels and Dropped onto their Handles.
Source of Variation
d.f.
Mean Square
F
Level o f S yrup
3
281.55
105.534*
E rro r
14
2.67
* denotes a statistically significant difference (p<0.05).
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
B ottles filled to full, three quarter, half, and at the q u arter level were heated in a
m icrow ave oven, a t full p o w er for 2.00, 1.45, 1.30 and 1.00 min, respectively (Figure 4),
w hich simulated th e directions for reheating indicated on the p ro d u ct label. During
m icrow ave heating, th e closure allow ed steam to escape, while causing a slight pressure
within the package. T he tem perature o f the syrup, and inside and outside surface o f the
bottles was m easured at several positions using a p o ck et-p ro b e digital therm ocouple
(Electronic D evelopm ent L aboratories Inc. NY).
H eating tim e w as increased w ith fill levels, in o rd er to raise th e food product to the
specific end tem p eratu re (58-60 °C ). The mean tem p eratu res at each product package
area resent 4 replications (Table 10). The tem p eratu res m easured at each product area
w ere quite uniform (lo w standard deviation). Syrup, w hich is high in sugar and hence has
low specific heat, heated quickly in the m icrowave. Buffler and S tanford (1991), reported
that the low er a fo o d ’s specific heat capacity, th e m ore quickly it will heat. Furtherm ore,
bottle geom etry (sq u at shape w ith round com er) also p rom otes uniform heating o f the
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54
product since food p ro d u cts heat m ore uniformly in a m icrow ave oven i f packaged in a
container with round corners. K eeping the closure on during heating also p ro m o tes m ore
uniform tem perature.
T he effect o f fill level and p ro d u ct area (syrup, inside, and o u tsid e bo ttle surface)
on m easured tem p eratu res w as exam ined. Using factorial analysis (T ab le 11), only the
product area was significantly influenced by the tem perature m easured, w h ereas level o f
syrup and the interaction factors o f p roduct area and level o f syrup w ere insignificant
(p<0.05). The effect o f level o f syrup on the tem perature w as the sam e fo r all product
areas. Since only effect o f p ro d u ct area w as significant, th e p ro d u ct area effect w as the
best estim ate o f th e difference o f th e sam ple’s tem perature and is p resen ted in Table 12.
Table 10. Temperature (°C ) a of Samples after Heating in Microwave Oven.
Fill
Level
Product Area of
Interest
Time
(min.)
Syrup
Bottle
(inside)
Bottle
(outside)
Full
2.0 0
57.4 ± 2 .0
56.5 + 2.6
43 .0 ± 2 .6
T hree quarter
1.75
5 7 .0 + 1.4
56.3 ± 2 .9
4 3 .0 ± 2 .1
H alf
1.50
58.8 ± 2 .2
56.0 ± 1.8
44 .5 ± 1.3
Q uarter
1.00
60.5 ± 1.3
57.3 ± 2 .5
44.5 ± 2 .6
a mean with standard deviation
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55
Table 11. Analysis of Variance of Temperatures of Samples after Heating in
Microwave oven.
d.f.
Mean Square
F
A: P ro d u ct A rea
2
1009.64
197.539*
B: Level o f syrup
3
12.05
2.359
AxB
6
8.20
1.605
Residual E rro r
36
Main effect
Interaction
*denotes a statistically significant difference (p < 0 .0 5 ).
Table 12. Temperature of the Product and Package Wall.
Product Area
Temperatured
Syrup
(°C )
58.2 1
Inside wall
56.3 b
O utside wall
43.8 c
a b c m eans w ith superscript alphabets are not significantly different (p£0.05).
com parison are made only within th e sam e column.
d m ean w ith standard deviation
T em perature o f the p roduct (T able 12) from th e highest to the low est w ere as
follow s: syrup, inside bottle wall, and outside b o ttle wall. T he tem p eratu re o f the inside
bottle surface w as slightly low er th an th at o f the syrup, but higher than th at o f th e ou tside
b o ttle surface. As the plastic bo ttle itse lf does n o t ab so rb m icrow ave energy, rise in
tem p eratu re depends on th e presence o f a m icrow ave ab so rb er (w ater, dipolar ion, etc.) in
the contained food. At the interface o f a food and plastic co ntainer which is reasonably
transparent to m icrowaves, the tem p eratu re o f th e co n tain er will be a result o f th e food
tem p eratu re (S acharow and Schiffm ann, 1992). T herm al conduction plays a p art in any
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56
m icrow ave heating p ro cess w hen a tem perature gradient exits, i.e. high tem p eratu re region
to the low tem perature region (H allstrom , Skjolderbrand, and Tragardh, 1988). H eat
energy w as transferred from the syrup (which had high energy density) to the bottle w hich
had low energy density. T he tem perature o f the inside wall (56.3 °C ) was, therefore,
som ew hat close to th at o f th e syrup (58.2 °C ) than th e outside surface.
Also, the surface o f m icrow ave cooked foods, o r th e outside o f the co ntainer wall
will be cooler than the inside because the surrounding air does not heat (D ecareau, 1992).
H eat w as lost from th e surface to th e cool oven, w hich also reduced the surface
tem perature. Subsequently, tem perature at the outside wall surface (43.8 °C ) w as low er
than the syrup and inside wall (56.3 °C ) o f the bottle.
4.4.2 Effect of Microwave Reheating on Drop Impact Resistance of the Bottles
This test w as do n e to determ ine the influence o f m icrow ave reheating on the
im pact resistance o f th e bottles. A three steps approach w as used. First, drop height o f
unheated bottles w as m easured using a free fall drop test. Second, another set o f b o ttles
w as heated in the m icrow ave oven, as described in Figure 4, and then were subjected to
the free fall drop test. B efore d rop testing, all bottles w ere conditioned for 24 hours at
room tem perature (20 + 2 °C , 50 + 5 % relative humidity). Third, drop heights o f
unheated and m icrow ave reheated bottles w ere com pared. T he m agnitude o f change in
drop heights o f the b o ttles w as used as an indicator o f m icrow ave heating influence o n the
drop im pact resistance.
D rop heights o f th e bottles after reheating in th e m icrow ave oven as d ro p p ed onto
their right bottom c o m er o r handle are shown in F igure 6. Figure 6 shows that th ere w as
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57
correlation betw een drop height and m icrow ave heating treatm en ts (right bottom com er,
R2 = 0.9232; handle, R2 = 0.8672). Figure 6 also show s th at th e bottles dropped at right
bottom c o m e r exhibited lo w er drop heights than th o se d ro p p ed at the handle.
U nheated bottles filled at full, %, Vi, and '/* syrup levels w ere used as control
bottles and com pared to m icrow ave reheated bottle, treatm en ts 1-3, 4-6, 7-9, and 10-12,
respectively. C om parisons o f d rop impact resistance betw een unheated and heated bottles
w hen d ropp ed at right bottom co m er and handle are show n in Table 13 and 15,
respectively. In Tables 14 and 16 are shown the results o f one-w ay analysis o f variance in
association w ith D uncan’s M ultiple Range T est o f effect o f m icrow ave reheating on d rop
im pact resistance o f the b o ttles dropped onto their right b o tto m co m er and handles.
From one-w ay analysis o f variance in association w ith D uncan’s M ultiple Range
T est (T able 14 and 16), m icrow ave heating w as found to have significantly affected the
drop im pact resistance. A statistically significant difference in drop height w as observed
betw een unheated bottles (co n tro l) and the bottles reh eated 9 tim es when dropped at right
b o tto m c o m er (Figure 7), and for 7 times w hen d ro p p ed o n to their handle (Figure 8).
F urther reheating caused the d rop impact resistance to decrease considerably. T here are
actually tw o issues including practical and statistical significance to address w ith respect to
the difference observed. D rop im pact resistance o f th e m icrow ave reheated bottles w as
statistically different from u nheated bottles, how ever, th e decrease in drop im pact
resistance w as probably not large enough to be o f practical im portance for the actual enduse perform ance o f the bottle.
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58
D egree o f crystallinity can have a num ber o f im p o rtan t effects on th e properties o f
a polym eric material (S ey m o u r and Carraher, 1984). T h e d ecrease in d ro p im pact
resistance, after repeated heating in the m icrowave oven m ay result from changes in
physical character o f the polym er.
Kemp and K ennedy (1 9 8 7 ) found that w hen a polym er w as heated to a specific
tem perature, change in physical structure occured. C hange in polym er stru ctu re may
result from various m olecular transitions, which can lead to change in th e physical
properties o f the material such as brittleness.
Im pact properties o f polym ers are often m odified by adding an im pact modifier or
plasticizer (M eyer and Leblanc, 1995). Polymers m odified w ith low m olecular additives
becom e hard and brittle w ith loss and/or change in p ro p erties due to loss o f these additives
by evaporation from th e po ly m er surface, o r through m igration during co n tact w ith
solvents, water, oil, etc., (K em p and Kennedy, 1987). T herefore, it could be possible that
repeated heating o f the b o ttles in the microwave may induce th e loss o f additives o r
im pact modifiers, causing th e plastic to becom e brittle.
In this w ork, the effect o f additive loss on im pact resistance o f th e b ottles w as not
studied.
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59
Table 13. Effect of Microwave Reheating on Drop Impact Resistance of the Bottles
Dropped at the Right Bottom Corner.
Microwave
Reheating
Treatment 8
Level of Syrup
Drop H eightf
(inch)
-
Full
39.0 + 2. l a
1
Full
38.4 ± 1 2a
2
Full
37.5 ± 1.7“
J
Full
39.7 ± 1.5*
-
3/4
45.6 + 2.5 b
4
3/4
42.7 ± 1.5 b
5
3/4
43.8 ± l.5 b
6
3/4
44.4 ± 2.5b
-
'/2
54.0 +1.3 d
7
'/2
4 9 .2 + 1.5cd
8
'/2
49.8 ± l.5 cd
9
'/ 2
48.7 ± l.5 c
-
'/ 4
57.0 ± 2. l f
10
'/ 4
5 4 .0 + 1 .9 e
11
'/ 4
5 4 .0 + 1.7*
12
Va
51.7 ± 1.3dc
m eans with superscript alphabets are not significantly different (p < 0 .0 5 ).
com parisons are m ade only w ithin th e sam e colum n.
f m ean with standard deviation
B refer to m icrow ave heat schem e in Figure 4
Table 14. Analysis of Variance of Effect of Microwave Reheating on Drop lmpa<
Resistance of the Bottles Dropped at the Right Bottom Corner.
Source of Variation
d.f.
Mean Square
F
T reatm ents
15
185.318
49.762*
E rro r
58
3.72
* denotes a statistically significant difference (p<0.05).
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60
Table 15. Effect of Microwave Reheating on Drop Impact Resistance of the Bottles
Dropped onto their Handles.
Microwave
Reheating
Treatment 8
Level of Syrup
Drop Heightf
(inch)
-
Full
2 2 .8 + l.5 ab
1
Full
2 2 .5 + 1.5*
9
Full
22.2 ± 1.5*
J
Full
22.0 ± 1.4*
-
3/4
2 5 .8 + I.5C
4
%
25.5 + 1.5bc
5
3/4
26.3 ± 2 .5 C
6
3/4
26.3 ± 1.3C
-
'/2
33.7 ± 1.3C
7
'/2
30.0 + 2. l d
8
*/2
28.5 + 1.5cd
9
V2
28.0 ± 1.4cd
-
'/ 4
40.5 + 1.5f
10
'A
3 6 .7 + 1.3 c
11
'/ 4
36.8 ± 2 .5 C
12
'/ 4
36.0 ± 2.1c
■>
^ m eans with superscript alphabets are not significantly different (p<0.05).
com parisons are m ade only within the sam e colum n.
r m ean w ith standard deviation
s refer to m icrow ave heat schem e in Figure 4
Table 16. Analysis of Variance of Effect of Microwave Reheating on Drop Impai
Resistance of the Bottles Dropped onto their Handles.
Source of Variation
d.f.
Mean Square
F
T reatm ents
15
145.630
38.802*
E rror
48
3.75
* denotes a statistically significant difference (p<0.05).
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
60
y = 1.5661x + 35.995
R2 - 0.923
50
40
.Sf> 30
m
On
— A— Right Bottom Comer
— ■ — Handle
Linear (Right Bottom Comer)
— ■ — - Linear (Handle)
y = 1 .4 1 8 5 x + 19.171
R2 = 0.8672
10
0
0
6
8
10
12
Microwave Repeated Heating (Time)
Figure 6. Drop Heights of the Bottles After Repeated Heating in Microwave Oven (Right Bottom
Corner and Handle).
14
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
70
□ 0 = No reheating in micorwave
1111-12 = Number o f reheating in microwave
60
10
7
50
5 40
°
1
2
8
051
3
ja
M
’3
H
a. 30
ou
ON
©
51
20
115
10
U®
0
Full
3/4
Fill Level
1/2
1/4
Figure 7. Effect of Microwve Reheating on Drop Impact Resistance of the Bottle Dropped
onto their Right Bottom Corner.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
50
□ 0 = N o reheating in microwave
(D 1- 12 = Number o f reheating in microwave
40
10
£
11
12
30
0
\\V s V
.SI)
‘S
H
o
P
N
\ .\\\s\i\\\
20
0\
U>
Q
10
^ss/^
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0
Full
3/4
1/2
1/4
Fill Level
Figure 8. Effect of Microwave Reheating on Drop Impact Resistance of the Bottles
Dropped onto their Handles.
64
4.5 Effect of Temperature on the Impact Resistance of the Bottles
To investigate th e effect o f tem perature on th e bo ttle im pact resistance, the drop
height impact resistance o f th e bottles at room tem p eratu re (20 + 2 °C ), after heating in
m icrowave oven (42.3 ± 2.2 °C ), and at refrigeration (8.1 + 1.6 °C ) w as com pared.
4.5.1 Drop Impact Resistance at Right Bottom Corner
Bottles from the m icrow ave heat scheme (F igure 4) w ere kept in a refrigerator for
24 hr, before subjected to the drop test. The bottles w ere dro p p ed im m ediately upon
taking them from refrigeration. T he m easured tem perature o f ou tsid e surface o f the bottle
w as at 8.1 + 1.6 °C . T he results are shown in Table 17. At q u arte r and half-filled levels,
the mean drop heights o f th e bo ttle w as 20.2 and 17.5 inches, respectively. Bottles filled
to the three quarter and full levels broke at 16 inches, w hich w as th e low est height setting
o f the drop testing machine. It w as possible that th e b o ttles w ould actually break at less
than 16 inches, therefore a drop height o f 16 inches cannot be rep o rted as the actual drop
height.
Bottles from m icrow ave heating treatm ent (F igure 4) w ere dropped immediately
after heated in m icrow aves at th e m easured tem perature o f 42.3 + 2.2 °C (outside
surface). For bottles that w ere dropped at right bo tto m com er, num ber o f bottle failures
w as very small, even at th e m axim um drop height (66 inches) o f th e d rop testing machine.
M ean failure drop height could n o t be established. T herefore, th e result w as reported as
nonfailure across all treatm ents.
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65
Table 17. Drop Height Failure of the Bottles Dropped at the Right Bottom Corner
at 8 °C.
Microwave
Reheating
Treatment15
Level of Syrup
Drop Heighta
(inch)
1
Full
<16.0
2
Full
<16.0
3
Full
<16.0
4
3/4
<16.0
5
3/4
<16.0
6
3/4
<16.0
7
1/2
16.8 ± 1.5
8
1/2
17.5 ± 1.7
9
1/2
17.5 ± 1.7
10
1/4
20.2 ± 1.6
11
1/4
19.8 ± 2.1
12
1/4
19.8 + 1.5
a m ean w ith standard deviation
b refer to m icrowave heat schem e in Figure 4
Since data from dropping the bottles at 8 °C (T able 17) and 42 °C w ere not
quantitative, the results for b o ttles dropped o n to th eir right bo tto m co m er w ere
transferred to tw o nominal possible outcom es as “failure” o r “ nonfailure” . The statistical
significance o f effect o f tem p eratu re o f the b o ttles on th e d ro p heights w as determ ined
using C ochran ’s test. Statistical com parison o f d ro p im pact resistance o f the bottles at 20,
42, and 8 ° C was tabulated in A ppendix A. T h e analysis show ed that tem peratures o f the
bottles significantly influenced th e drop im pact resistance (p<0.05).
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66
4.5.2 Drop Impact Resistance onto their Handles
F or bottles d ro p p e d at th e handle, it was found th at they b ro k e at 16 inches which
w as the low est height o f th e m achine. Since the bottles m ay actually b reak at less than 16
inches, the actual d ro p height could n o t be determ ined.
D rop height failure o f dro p s at th e handle at low te m p eratu re (8 ° C ) w as not
quantitative (T able 18). T herefore, only drop height d ata o f th e b o ttles at room
tem perature (20 °C ), an d after heating in the m icrow ave (42 ° C ) w ere com pared to
determ ine the effect o f tem p eratu re on drop im pact resistance (T ab le 19).
Using factorial analysis, individual param eters (tem p eratu re and m icrow ave
reheating treatm ents) had a significant influence on th e b o ttle ’s d ro p im pact resistance
(T able 20). W hereas, interaction factors o f tem perature and m icrow ave heating w ere
nonsignificant. T hus, it could be concluded that these factors acted independently o f each
other. Therefore, th e resu lts for th e individual param eters o f te m p eratu re and m icrow ave
heat treatm ents are sh o w n in Table 21 and 22, respectively.
A t a higher p ro d u c t tem perature, the bottle had increased d ro p im pact resistance.
L ow er product te m p e ra tu re decreased th e drop im pact resistance (F ig u re 9).
The im pact resistan ce o f th e b o ttles was strongly dep en d en t u p o n th e p roduct
tem perature. A t lo w tem p eratu re, th e im pact resistance w as red u ced and all plastics tend
to becom e rigid and brittle. C onversely, at higher test tem p eratu re, th e im pact resistance
w as im proved (C alister, 1994).
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67
Table 18. Drop Height of Bottles Dropped onto their Handles at 8 °C.
Microwave
Level of Syrup
Reheating
Drop Height3
(inch)
Treatmentb
1
Full
<16.0
2
Full
<16.0
3
Full
<16.0
4
3/4
<16.0
5
3/4
<16.0
6
3/4
<16.0
7
'/2
<16.0
8
Vi
<16.0
9
'/ 2
<16.0
10
%
<16.0
11
'/ 4
<16.0
12
'/ 4
<16.0
3 m ean w ith standard deviation
b refer to m icrow ave heat schem e in Figure 4
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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
Reheating
Drop Height
at 20 °C 3
(inch)
Drop Height
at 42 ° C a
(inch)
Treatment11
1
Full
22.2 ± 1.5
33 .0 ± 2.1
2
Full
22.5 ± 1.5
3 3 .0 ± 1.9
3
Full
22.0 ± 1.4
33.6 ± 2 .2
4
3/4
25.5 ± 1.5
39 .0 ± 2 .1
5
3/4
26.3 ± 2 .8
39.0 ± 2 .4
6
3/4
26.3 ± 1.3
3 8 .3 ± 1.3
7
1/2
30.0 ± 2 .1
42 .0 ± 2.1
8
'/2
28.5 ± 1.5
42.6 ± 1.2
9
Vz
28.0 ± 1.4
4 2 .0 ± 1.9
10
'/4
36.8 ± 1.3
45.8 ± 2 .5
11
Va
36.8 ± 2.5
46.5 ± 1.5
12
Va
3 6 .0 + 2.1
4 6 .2 + 1.5
a mean with stan d ard deviation
b refer to m icrow ave heat schem e 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.
d.f.
M e a n S q u a re
F
A: Tem perature
1
3294.24
754.135*
B : M icrow ave R eheating
11
219.71
50.298*
AxB
11
6.05
1.386
Residual E rro r
74
4.368
M a in effect
Interaction
'‘denotes a statistically significant difference (p<0.05).
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69
Table 21. Effect of Temperature of the Product on Drop Impact Resistance.
Temperature
Drop Height (inch)
20 °C
28.2 a
42 °C
40.1 b
a' b m eans w ith superscript alphabets are not significantly different (p<0.05).
com parison a re m ade only within th e same column.
Table 22. Effect of Microwave Reheating on the Drop Impact Resistance.
Microwave
Level
Reheating
of Syrup
Drop Height0
(inch)
Treatmentf
1
Full
27.7 a
2
Full
27.6 a
3
Full
27.3 a
4
3/4
32.2 b
5
3/4
32.6 b
6
3/4
32.2 b
7
'/2
35.6 c
8
'/2
35.5 c
9
'/2
35.0 c
10
'/ 4
41.2 d
11
'/ 4
41.6 d
12
'/ 4
41.1 d
a_d m eans w ith superscript alphabets are not significantly different (p<0.05).
com parisons are m ade only within th e sam e column.
c m ean w ith stan d ard deviation
f refer to m icrow ave heat scheme in Figure 4
R ep ro d u ced with p erm ission o f th e copyright ow ner. Further reproduction prohibited w ithout perm ission.
■T
^72598361610
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<u
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999999999999999^1
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^11077677971581100
O U
O
CN
3
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o
m
o
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(qaui) tqSiaq dojQ
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Figure 9. Effect of Temperature of the Product on Drop Impact Resistance When Dropped
onto their Handles.
70
71
G enerally, m o st polym ers are hard and brittle at tem p eratu re below their Tg,
change from leath ery to rubbery increase as the tem p eratu re is increased above Tg. At
low tem perature (8 °C ), th e tem perature o f the PP bottle w as close to its Tg ( -8 °C),
therefore m ake th e polym er m ore brittle than at 25 °C and 42 °C.
At low tem p eratu re, th e internal mobility o f th e m olecules o f the polym er is less
than at higher tem p eratu re. Additionally, at low tem perature, the m olecules o f polym er
are so sluggish th a t they cannot absorb and dissipate th e energy o f a sudden shock. As
tem perature rises, th e m aterial passes through a phase o f relaxation, and the m odulus is
low (Shah, 1984; Sperling, 1986). Thus at low er tem perature, th e polym er is m ore
susceptible to b rittlen ess than at higher tem perature.
4.6 The Change in Percent of Crystallinity of the Packaging Material
Crystallinity affects m any im portant polym er physical properties, such as strength,
stiffness, and brittleness. In this study an attem pt w as m ade to determ ine the effect o f
repeated m icrow ave heating on the crystallinity changes in th e polym eric packaging
material. H ow ever, to understand the effect o f rep eated heating on drop im pact resistance
due to change in physical stru ctu re o f the polym er required M odulated DSC evaluation o f
degree o f crystallinity o f th e polymer.
It w as h ypothesized th at after repeated heating in m icrow ave oven, percent
crystallinity o f th e b o ttle m ay increase which may result in change o f the im pact resistance
o f the plastic container. Crystallinity changes w ere investigated using M odulated DSC.
In Figures 10 and 11 (A ppendix B), are show n therm al curves o f the M odulated
D SC heat capacity and nonreversing heat flow. T est specim ens w ere taken from the
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72
b o tto m and from the u p p er part o f th e bottle close to the handle o f unheated bottles. In
Figures 12 and 13 (A ppendix B), therm al curves are show n o f sam ples from the b ottom
and upper part o f m icrow ave repeatedly heated bottles (12 tim es). Furtherm ore, in
Figure 14, it is show n that after reheating in the microwave, th e polym er show ed a higher
heat o f fusion than th a t o f the unheated polymer.
This heat o f fusion is rep o rted as percent crystallinity by ratioing against th e heat
o f fusion for a 100 % crystalline sam ple o f the same material. T h e d egree o f crystallinities
o f the unheated and m icrow ave reheated bottles w ere com pared by using 209 J/g, the
heat o f fusion o f theoretically 100 % crystalline polypropylene (M iller, 1966). T he total
heat o f fusion associated w ith the therm al curves and percent crystallinity o f the sam ples
are sum m arized in T able 23. D egree o f crystallinity o f the b o ttle exposed to m icrow ave
repeated heating w as approxim ately 2.7 % higher than that from the control bottle.
During heating in the m icrow ave, the tem perature o f th e b o ttle will increase via
heat conduction from th e product. A num ber o f im portance transitions occur in
polypropylene at high tem peratures. These may result from torsional and rotational
m otions in both am orphous and crystalline regions. These transitions can m ake polym ers
consist o f unit configurations sufficiently alike to pack into a lattice (S ym our and
C arraher, 1984). M iller (1966), reported that thermal m otion o f a polym er increases with
increasing tem perature, and w hen cohesion energy exceeds the kinetic energy o f the
chains, crystallization m ay take place (even in polymers which d o n o t crystallize under
norm al condition). N icastro, et al (1993) reported that increased mobility in the
am orphous segment and m elting o f the semi-crystalline region resu lted in the form ation o f
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73
a m o re crystalline structure. T h ese m echanism s may have caused crystalline inducem ent
o f th e sam ple after reheating in th e m icrow ave.
Table 23. Total Beat of Fusion Associated with Degree of Crystallinity of Unheated
and Heated Bottles in Microwave.
Melt Peak
Temperature
(°C)
Enthalpy
(J/g)
Degree of Crystallinity
U p p er part
153.16 + 0.44
93.9 ± 0 .2
4 5 .0 ± 0 .1
B o tto m
153.45 + 0.11
93.6 ± 0 .6
44 .8 ± 0 .2
U pper part
153.19 + 0.40
98.8 ± 0 .3
47.3 ± 0 .1
B o tto m
152.96 ± 0 .1 6
99.0 ± 0 .0
4 7 .4 ± 0 .0
Sample
(% )a
Unheated
Heated
1 % crystallinity based on the rep o rted theoretical value o f 272 J/g for 100 %
crystalline polypropylene (M iller, 1966).
C hristie, G regory, and W o o d (1993) show ed th at processing co n d itio n s have a
dram atic im pact on final crystallinity and corresponding polym er properties. In addition,
N icastro, et al (1993) provided an excellent exam ple o f change in crystallinity d u e to high
tem p eratu re exposure. They found that high tem p eratu re achieved during heat sealing
increased percent crystallinity o f cast polypropylene film. This resulted in an increase in
brittleness o f th e polymer. A dditionally, Shah (1984) and Kail (1991) rep o rte d that
increases in percent crystallinity decreased the im pact strength and increased the
probability o f brittle failure.
T he results from this stu d y sug g est that repeated m icrow ave heating induced
crystallinity form ation, w hich m ay probably decreased th e im pact resistance o f the
polym er.
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CHAPTER 5
SUMMARY AND CONCLUSIONS
D rop height m easurem ent using a free fall drop instrum ent and the B ruceton
Staircase M ethod provided a m easure o f th e d ro p im pact resistance o f plastic syrup
bottles. B ottles dropped o nto th eir handles o r right bottom c o m e r w ere m ore susceptible
to breakage then w hen dropped o n to a flat b o tto m o r face d rop because the stress was
concentrated in bo tto m co m er (angle d ro p ) a t a single point instead o f being dispersed
o v er the entire b o tto m o r surface o f th e bottle. T h e low d rop im pact resistance at the
handle resulted from the w eak p oints on th e b o ttles due to the presen ce o f mold parting
lines and pinch-off regions. D rop im pact resistance o f the b o ttles w as determ ined at full,
3/4. 1/2, and 1/4 syrup levels to establish a correlation betw een breakage, d rop height,
and fill level. T h e higher th e fill level, th e higher th e w eight o f th e sample, and the low er
the drop im pact resistance.
B ottles filled at the, 3/4, 1/2, and 1/4 levels w ere heated in a m icrow ave oven, at
full pow er for 2.00, 1.45, 1.30 and 1.00 min, respectively. T hese tim es w ere used to
sim ulate directions on the p ro d u ct label. H eating tim e w as co rrelated to fill level in order
to raise the food p ro d u ct to a specific tem p eratu re (6 0 ° C). T em p eratu re o f the product
74
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75
and container surfaces w ere as follow s: syrup (58.2 °C ), inside bo ttle w all (56.3 °C ), and
outside bottle wall (42.3 °C ). Since the plastic bottle does not absorb m icrow ave energy,
rise in m aterial tem perature d ep ended on th e presence o f a m icrow ave ab so rb er (w ater,
dipolar ion, etc.) in the contained food. T he tem perature o f the container was, therefore, a
result o f heat transfer from th e high tem perature region (syrup) to the low tem perature
region (b o ttle’s wall). T em p eratu re at the outside o f the container wall is co o ler than at
the inside surface because th e surrounding air does not heat. H eat w as lost from the
surface to th e oven which also reduced the surface tem perature (D ecareau, 1992).
D rop heights o f unheated and m icrow ave reheated bottles w ere com pared. The
m agnitude o f change in drop heights o f the bottles w as used as an indicator to determ ine
m icrow ave heating influence on th e drop im pact resistance. A significant difference in
drop height w as observed after th e bottles experienced reheating for 9 tim es w hen
dropped at the right bottom co m er, and 7 tim es w hen dropped at the handle. H eating
times g reater than this caused th e im pact resistance to decrease considerably.
T he im pact resistance o f th e bottles w as strongly dependent upon the tem perature
o f the product. A t higher test tem perature, the im pact resistance w as significantly greater.
Conversely, at low er tem perature, the im pact resistance was reduced drastically. This is
m ost likely because the internal m obility o f th e polym er molecules o f th e polym er is less
than at higher tem perature and th e polym er m olecules are so sluggish th at they cannot
absorb and dissipate the energy o f a sudden shock from free a fall drop (Shah, 1984;
Sperling, 1986).
Therm al curves o f (M o d u lated D SC ) th e heat capacity and nonreversing heat flow
o f test specim ens taken from unheated bottles and repeatedly heated b o ttles w ere
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76
com pared. D egree o f crystallinity associated with heat o f fusion o f the specim ens taken
from the bo ttle exposed to m icrow ave repeated heating w as approxim ately 2 % higher
than those from th e unheated b o ttle, indicating that repeated heating in th e m icrow ave can
induce crystallinity.
A possible mechanism fo r th e inducem ent o f crystallinity within th e b o ttles is as
follows; during repeated heating in th e m icrow ave oven, th e tem p eratu re o f th e bottle is
raised as well as th e tem perature o f th e product inside due to heat conduction, thus
resulting in change o f th e physical structure. Thermal mobility o f a polym er increases with
increasing tem perature. A num ber o f im portant transitions o ccu r in polypropylene at high
tem perature. T hese may result fro m torsional and rotational m otions in b oth am orphous
and crystalline regions. These transitions can make polym ers consist o f unit having
configurations sufficiently alike to pack into a lattice and result in the form ation o f a m ore
crystalline stru ctu re (Seym our and C arraher, 1984; N icastro, et al,1993).
R esults suggest that rep eated m icrow ave heating induced crystallinity form ation
which may probably decrease th e im pact resistance o f the syrup bottle.
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CHAPTER 6
RECOMMENDATIONS
T he recom m ended further studies are as follow:
1. C hange in mechanical p ro p erties o f the package m aterial d u e to m icrow ave
reheating can be determ ined by m easuring dynamic mechanical behavior o f th e polym er
using Dynamic M echanical Analysis (D M A ). DM A m easures the ch an g e in th e m odulus
(normalized stiffness) and dam ping (energy dissipation) o f viscoelastic m aterials w ith
tem perature, as th ese materials are subjected to oscillatory stresses (fo rces) and resultant
strains (displacem ents).
2.
D eterm ination o f the effect o f additive loss on im pact resistan ce o f th e b o ttles
since it could be possible that rep eated heating o f the bottle in the m icro w av e m ay induce
loss o f additives o r im pact m odifier, causing th e plastic to becom e brittle.
T he follow ing recom m endations are fo r achieving higher im pact resistan ce and
duration o f p ro tectio n o f the p ro d u ct packaged.
l.
C opolym erization o f p ro p y len e w ith other olefins, such as ethylene can
improve its im pact resistance.
77
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78
2.
D ecrease percent crystallinity o f th e p ack ag e m aterial since high crystallinity
m akes polym ers harder and m ore easily fractured. T he brittleness depends on the p ercent
crystallization.
3.
A ddition o f additive such as im pact m odifier to im prove im pact resistance.
4.
A void using this product at low tem p eratu res, especially at a tem perature close
to its T g since therm oplastics above their T g are rubbery in nature, while below T g they
are hard and brittle.
5.
Increase the thickness o f the b ottle at its w eak point including handle and
bottom com ers.
6.
B o ttle design can also be m ade to m inim ize wall collapse by strengthening the
container by changing its geom etry.
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APPENDICES
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
79
APPENDIX A
Cochran’s Test for Related Frequencies
S tatistical C om parison o f D rop Im pact R esistance o f the B o ttles D ro p p ed at the
Right B otto m C o m e r at 20, 42, and 8 °C ( a = 0.05).
Ho : F 2o°c = F 42°c = Fg°c
H i : F 2o°c ^ F 42°c ^ Fs°c
Treatment
1
2
3
4
5
6
7
8
9
10
11
12
Column Total (Fc)
20°C
1
1
I
1
1
1
1
1
1
1
1
1
12
42°C
I
I
1
0
0
0
0
0
0
0
0
0
3
8°C
1
1
1
1
1
1
1
1
1
1
1
1
12
Row Total (Fr)
j->
j
j
2
2
2
2
2
2
2
2
2
27
-»
1 = failure, 0 = nonfailure
IF C
= 27
EFC2 =
297
IF r
= 27
ZFr2
= 63
X2(a-i, a, = ( a - 1)[a(ZFc2) - ( I F e) 2] / (a EFr - I F r2 )
18
x 2 ( a - l =2. <z = 0.05)
=
X^la-l. a) Tabic
— 5.99
X^la-l -
2,
a =0.05) = 1 8
> X^la-l. a) Tabic
~
5.99
R eject hypothesis that Ho : F 2o-c = F 42«c = Fg«c
R ep ro d u ced with p erm ission o f th e copyright ow ner. Further reproduction prohibited w ithout perm ission.
. 02
0 . 02-
- -
.04
-0.05-
-
0 . 10-
0 . 0 0 -----
.06
0.02
.08
\y
-
ro - 0 . 1 5 - 0 . 0 4 ----- 0 . 10
-
0 . 20-
-0.06---
-0.25
-50
0
50
150
100
Temperature*
200
(°C)
Figure 10. MDSC Analysis of Sample from the Bottom of Unheated Bottle.
250
.
12
. 14
2200
APPENDIX B
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
0.00
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
0.00
0.00
141.11 °C
14 0.44 °C
94.12 J /g
0 .02-
-0 . 0 5 cn
•--0.05
o.oocn
3:
*
s
o
-0 . 0 2
0
.
10
+j
ra
cu
I
CO
- - - 0 . 15
-0 . 2 0 -0.06-
153.37 °C
- 0 .25
-5 0
0
50
100
Temperature*
150
r-------------.------------- b - 0 . 2 0
200
250
M0SC V I . 1A TA I n s t 2 2 0 0
Figure 11. MDSC Analysis of Sample from the Upper Part close to Handle of Unheated Bottle.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
0. 0 0
0 .0 0
0.04-0.05
OJ
\
2
a i —'
— 0.05
,—.
138.61 °C
99 .00 J / g
\
3:
*
2
a
* r-1
2 U.
O
i—i ■M
li. ID
QJ
+J X
/o
QJ >
X QJ
C
'> C
QJ o
cr Z
-0. 15-
0 .00-
0.10
00
S)
R
CD
-0.04- - 0 . 15
-0 .
20-0 . 0 6 -
1 52. 84 °C
-0.25-50
50
i----------- 1----------- —
100
Temperature*
(°C)
-
150
200
MOSC V I . 1 A
250
TA I n s t
Figure 12. MDSC Analysis of Sample from the Bottom of Microwave Reheated Bottle.
0.20
220
139.58 °C
99 . 00 J / g
142.07 °C
0 .06-0
cn
05-
O)
N
2
--0.05
0.04-
1 - 0 . 10-
—
-
0 . 10
Ll .
0 .02-
ro
QJ
X
- - 0 . 15
-0. 15-
0 .00-
-
0.20
-
-50
0
50
100
Temperature*
lon,
( C)
150
200
MDSC V I . 1A
250
TA I n s t
Figure 13. MDSC Analysis of Sample from the Upper Part close to Handle of Microwave Reheated Bottle.
0.20
220
APPENDIX B
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
0 . 00
0.00
-
0.02
-0 .04
-0.05
--0.05
0. OB
0 . 10
-0 . 2 0 -
-
-
0.12
Unheated bottle
M icrowave reheated bottle
-0.25-50
0
50
100
Temperaturex
I On,
( C)
150
200
MDSC V I . 1A
250
TA I n s t
2200
Figure 14. Comparison of MDSC Analysis of Sample from (he Bottom o f Unheated and M icrowave Reheated Bottles.
APPENDIX B
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
0.00
BIBLIOGRAPHY
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BIBLIOGRAPHY
A nonym ous, 1997. M odulated D SC ™ C om pendium : Basic Theory and Experim ental
Consideration. Therm al Analysis Instrum ents Co., Delaware.
A ST M D esignation : D 2463-95. 1997. S tandard test m ethod for drop im pact resistance
o f blow -m olded therm oplastic containers. Annual b o o k o f A ST M standards
volum e 8.02, American Society fo r T esting and M aterials.
B randrup, J. and Im m ergut, E.H. 1989. Polym er handbook 2nd Edition. Jo h n Wiley &
Sons, Inc. N ew York.
Breakey, D. W. and Cassel, R. B. 1979. U se o f therm al analysis m ethods in foam
research and developm ent. Therm al Analysis A pplication Study N o .29, The
Perkin-Elm er C orporation, C onnecticut.
Brennan, W . P. 1978. C haracterization and quality control o f engineering therm oplastics.
Perkin-Elm er Therm al Analysis A pplication Study N o .22, The P erkin-E lm er
C o rp o ra tio n , C onnecticut.
B riston, J. H. 1994. Rigid and sem i-rigid plastic containers. John Wiley & Sons, Inc. New
Y ork.
Buffler, C. R. and Stanford, M. A. 1991. Effects o f dielectric and therm al properties on
the m icrow ave heating o f foods. M icrow ave World. 12 (4) : 15-22.
C alato, A. E. 1978. M icrow ave properties o f m aterials for m icrow ave cookery.
M icrow ave E n erg y Appl. New sl. 11 (6): 3-6, 13.
Calister, W. D, Jr. 1994. M aterials science and engineering. John Willey & Sons, Inc. New
Y ork.
Christie, A. W ., W ood, C. A., and G regory, R. B. 1993. Polypropylene crystallization
effects on film forming. Polym ers, Lam inations & C oatings C onference. 39-41.
Engelder, D. S. and Buffler, C. R. 1991. M easuring dielectric properties o f food products
at m icrow ave frequencies. M icro w a ve W orld. 12 ( 2 ) : 6-15.
85
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
86
H allstrom , B., Skjoldebrand, C. and T ragardh, C. 1988. H eat transfer and food products.
E lsevier Science Publishing C o., Inc. N ew Y ork.
H uang, H.F. 1987. N ew p ro d u ct co n cep ts in m icrow avable food packaging.
M icrow ave w o rld .8(6): 5-7.
Fisher, R. W. 1991. Interaction o f packaging & foods to provide superior quality
m icrow aveable food products. Session 5 : D esign fo r Plastics Packaging. 8th
Annual Food Plas C onference. M arch 5-7. O rlando Florida.
K att, J. L. 1991. T he effect o f starches and sugars on m icrow ave cooking.
M icrow ave world. 12(2): 19-23.
K orshak, V.V. 1971. The chem ical stru ctu re and therm al characteristics o f
polymers. Israel program fo r scientific translations, W iener B indery Ltd.,
Jerusalem .
Lau, R. 1995. M odeling and m icrow ave heating characteristics o f food and food packaging
system s, using co m p u ter sim ulation. T h e 8th M icrow ave A ssociation C onference,
Packed, W rapped and M icrozapped. 14-15 Sep.
M ele, B. V., et al. 1995. M odulated D SC evaluation o f isotherm al cure & vitrification for
therm osetting system s. T herm ochim ica Acta, 268. 121-142.
M eyer, J. P. and Leblanc, D. 1995. Polyethylen terephthalat schlagzah m odifizieren.
K unststoffe. 85 (4) : 452-456.
Miller, M. L. 1966. T h e stru ctu re o f polym ers. R einhold Publishing C orporation.
N ew York.
M onte, W. C. and L andau-W est, D. 1983. E xpanded polystyrene containers in m icrow ave
cookery. J. Am . D ietet. A ssoc. 83 (3) : 323-327.
N icastro, et al. 1993. C hange in crystallinity during heat sealing o f cast polypropylene film.
P la stic Film & Sheeting. 9 (4): 159-167.
O hlsson, T. and Risman, P. O. 1978. T em p eratu re distribution o f m icrow ave heatingspheres and cylinders. J o u rn a l o f M icro w a ve Pow er. 13 (4): 303-310.
Peason, R. 1995. Plastics play vital role in the food revolution. Pac/cag. week.
11(1): 30-32.
R ubbright, H .A. 1990. Packaging fo r m icrow avable foods. C ereal fo o d s world.
35(9): 927-930.
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
87
Sacharow , S. and Schiffm ann, R.F. 1992.
Leatherhead, England.
M icrow ave
packaging. P ira international,
Selikhova, V. I., et al. 1989. E ffect o f orientation and annealing on m elting and
recrystallization processes in polypropylene. P olym er Science U .S.S.R .
3 1(2):804-808.
Shah, Vishu. 1984. H an d b o o k o f plastics testing technology. John W iley & Sons, Inc.
N ew York.
Singh, R. H. and H eldm an, D .R . 1993. Introduction to food engineering. A cadem ic Press,
Inc. New Y ork.
Spath, W. 1961. Im pact testing o f m aterials. G o rd o n and B reach S cien ce Publisher.
N ew York.
Sperling, L. H. 1986. Intro d u ctio n to Physical Polym er Science. John W iley & Son, Inc.
N ew York.
Stehle, A. P. 1979. Packaging and utensil perspective : shape o f things to com e.
M icrow ave E n erg y A ppl. N ew sl. 12(4) : 13-15.
Steel, R. G .D ., Torrie, J. H. and Dickey, D. A. 1997. Principles and p ro c e d u re s o f
statistics: A biom etrical approach. M cG raw -H ill Inc. N ew Y ork.
Sw ientek, B. 1994. Form ulating and packaging m icrow aveable foods. P re p a r e d Foods.
6(4): 36-38.
Szulczynski, J. Z. 1978. Polypropylene is w ining the bo ttle battle. P la stic E ngineering.
3(6); 47-49.
Talvitie, J. P. and G aunt N. M . 1982. Polyethylene for thin-wall packaging. C an a d ia n
Packaging. 2 (8) : 26-33.
T hom as, L. E. 1995. C h aracterization o f m elting phenom ena in linear lo w density
polyethylene by M o dulated D SC . TA . Instrum ents Publication N o. T A 227.
Trice, R. W. and G oolsby, R. D. 1990. Effects o f irradiation on th e m echanical properties
o f polyethylene sulfide sulfone film. S A M P E Journal. 26 (5) 2 6 -2 8 .
Troy, E .J., Shortridge, T.J., and Fazey, A.C. 1985. A test plastic-bottle d esigners can
count on. P la stic E n g in eerin g . 11(8): 33-36, 38.
Zuckerm an, H. and M iltz, J 1994. C hanges in thin-layer susceptors d u rin g m icrow ave
heating. P a cka g in g T ech n o lo g y a n d Science. 7(4): 21-26.
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
IMAGE EVALUATION
TEST TARGET ( Q A - 3 )
/
sy,
W
&
&
'/
/
V
1.0
HIM
HIM
i*o III 2.0
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A P P L IE D A IIW1GE . Inc
——
- = -■
- = = ~—
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Phone: 716/482-0300
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