Barrier Polymers and Structures - American Chemical Societyhttps://pubs.acs.org/doi/pdf/10.1021/bk-1990-0423.ch010Simila...
0 downloads
98 Views
2MB Size
Chapter 10
Retortable Food Packages Containing Water-Sensitive Oxygen Barrier
Downloaded by UNIV OF ARIZONA on October 19, 2015 | http://pubs.acs.org Publication Date: May 9, 1990 | doi: 10.1021/bk-1990-0423.ch010
M. M. Alger, T. J. Stanley, and J. Day Process Engineering and Chemistry Laboratory, General Electric Company, Corporate Research and Development Center, 1 River Road, Schenectady, NY 12301
Multilayer ethylene-vinyl alcohol (EvOH) retortable food packages are steam sterilized at temperatures of 220-270°F. Water permeates through the outer package layers and into the center EvOH layer. The wet EvOH offers a reduced resistance to oxygen permeation immediately following retort. As the EvOH dries to a steady-state moisture profile, the barrier properties substantially recover, but a significant amount of oxygen can pass through the package wall during the drying stage. The long-term oxygenbarrier performance of a r e t o r t a b l e package i s a very complex function of EvOH properties and history, layer thickness, EvOH layer placement, retort time, retort temperature, and storage conditions. This study was undertaken to develop a procedure for predicting and measuring barrier performance of multilayer retortable food packages. A model that includes major variables for calculating water and oxygen permeation through the life of a retortable package is presented. Oxygen ingression tests were done on food packages to test the model. The results showed that dissolved oxygen in the package wall can make a significant contribution to oxygen gain. Package tests also suggest that there is an irreversible loss of EvOH barrier effectiveness following retort. The exact mechanism of the barrier loss is unclear; water and oxygen permeation experiments were done on t h i n coextruded films and provide some insight into mechanisms of t r a n s p o r t in m u l t i l a y e r s t r u c t u r e s . T h e r e a r e two p o l y m e r s u s e d i n a maj o r i t y o f h i g h b a r r i e r f o o d packages : copolymers c o n t a i n i n g p o l y v i n y l i d e n e c h l o r i d e ( S a r a n ) and e t h y l e n e - v i n y l a l c o h o l c o p o l y m e r ( E v a l S e l a r - O H ). Packages i n which e i t h e r p o l y v i n y l i d e n e c h l o r i d e or ethylene-vinyl a l c o h o l copolymer (EvOH) provide the b a r r i e r a r e i n v a r i a b l y m u l t i l a y e r since n e i t h e r of these materials o f f e r s the • p r o p e r t i e s r e q u i r e d t o make a good m o n o l a y e r s t r u c t u r e . T h e r e a r e advantages and d i s a d v a n t a g e s associated with each of these materials , and the f
0097-6156/90/0423-0203$06.50/0 © 1990 American Chemical Society
In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
Downloaded by UNIV OF ARIZONA on October 19, 2015 | http://pubs.acs.org Publication Date: May 9, 1990 | doi: 10.1021/bk-1990-0423.ch010
204
BARRIER POLYMERS AND STRUCTURES
p r o p e r c h o i c e d e p e n d s u p o n the a p p l i c a t i o n . For cases where EvOH i s t h e more a t t r a c t i v e a l t e r n a t i v e , a s i g n i f i c a n t concern i s the s e n s i t i v i t y of the b a r r i e r properties to moisture. Most m u l t i l a y e r b a r r i e r packages are e i t h e r h o t - f i l l e d or r e t o r t e d . In hot f i l l t h e p a c k a g e i s c h a r g e d w i t h f o o d t y p i c a l l y b e t w e e n 180 - 200°F. R e t o r t a b l e f o o d packages are steam r e t o r t e d t y p i c a l l y from 220 t o 270°F to s t e r i l i z e the contents. The l a t t e r r e t o r t operation g r e a t l y stresses the package and s u b j e c t s i t to a h i g h water a c t i v i t y . S i n c e EvOH has a v e r y h i g h water a f f i n i t y i t has been found that a s i g n i f i c a n t quantity of water can be absorbed d u r i n g the r e t o r t p r o c e s s (2-6) . I n a d d i t i o n , the g e n e r a l l y e x c e l l e n t EvOH oxygen b a r r i e r e f f e c t i v e n e s s i s s e r i o u s l y reduced a t h i g h h u m i d i t y (1-9). T h e r e f o r e , i m m e d i a t e l y f o l l o w i n g r e t o r t , the wet EvOH o f f e r s a reduced r e s i s t a n c e t o o x y g e n p e r m e a t i o n . As the EvOH d r i e s to a " s t e a d y - s t a t e " p r o f i l e , the b a r r i e r properties s u b s t a n t i a l l y recover, but a significant amount of o x y g e n c a n p a s s t h r o u g h the package w a l l d u r i n g the d r y - o u t stage . B e c a u s e many f o o d p a c k a g e s must be s h e l f - s t a b l e f o r many months t o y e a r s , p a c k a g e t e s t i n g becomes b o t h e x p e n s i v e and time-consuming. I t i s n e c e s s a r y t o make some e s t i m a t e o f the performance o f v a r i o u s c a n d i d a t e s t r u c t u r e s to assess t h e i r b a r r i e r p o t e n t i a l over the expected s h e l f l i f e of the p r o d u c t . There have been numerous s t u d i e s on the r e t o r t a b l e food packages (1-8) I however, a s i n g l e c o n s i s t e n t theory has yet to emerge and experimental details are limited. The l o n g r a n g e g o a l o f our work i s to d e v e l o p a c o n s i s t e n t t h e o r y f o r p r e d i c t i n g the b a r r i e r performance of any m u l t i l a y e r food package. At present q u a n t i t a t i v e models do not e x i s t f o r EvOH-containing structures due to poorly u n d e r s t o o d gas t r a n s p o r t p r o c e s s e s i n EvOH under c o n d i t i o n s found i n food package a p p l i c a t i o n s . I n t h i s paper there are two generic package structures w h i c h c o n s i d e r e d , " P E P " : F o o d / P P / E v O H / P P / a m b i e n t a n d " L E Ρ" : food/PP/EvOH/PC/ambient [PC - p o l y c a r b o n a t e , PP - p o l y p r o p y l e n e ] . T h i s paper i s d i v i d e d i n t o several sections which cover r e t o r t modelling, package t e s t i n g , and t h i n - f i l m t e s t i n g . F i r s t , the b a s i c equations are given f o r d e s c r i b i n g the performance o f a r e t o r t a b l e food package. With the model i t i s p o s s i b l e t o c a p t u r e the e s s e n t i a l features of the package's performance during and f o l l o w i n g r e t o r t . More i m p o r t a n t l y , the model provides a good basis f o r o r g a n i z i n g the many v a r i a b l e s w h i c h d i r e c t l y i n f l u e n c e the f i n a l package p e r f o r m a n c e : EvOH p r o p e r t i e s and h i s t o r y , l a y e r t h i c k n e s s e s , EvOH l a y e r placement, EvOH thickness, r e t o r t time, r e t o r t temperature, storage conditions, and package geometry. Second, we d i s c u s s r e s u l t s on package t e s t i n g that we have done t o t e s t model p r e d i c t i o n s . An oxygen headspace a n a l y s i s was used t o m e a s u r e a c c u m u l a t e d oxygen i n packages as a f u n c t i o n o f time. We show t h a t d i s s o l v e d oxygen i n v e r y t h i c k polycarbonate w a l l s can contribute to the t o t a l o x y g e n g a i n f o l l o w i n g r e t o r t . A l o s s o f EvOH s t e a d y - s t a t e oxygen b a r r i e r e f f e c t i v e n e s s f o l l o w i n g r e t o r t has been observed and c o r r e l a t e s roughly with severity of retort. I n the f i n a l s e c t i o n water and oxygen p e r m e a t i o n experiments were done on t h i n c o e x t r u d e d f i l m s to a i d i n understanding possible transport mechanisms i n m u l t i l a y e r s t r u c t u r e s . An i m p o r t a n t conclusion i s that water transport i s i m p o r t a n t and must be u n d e r s t o o d b e f o r e q u a n t i t a t i v e c a l c u l a t i o n of oxygen p e r m e a t i o n w i l l be p o s s i b l e . EvOH
Properties
EvOH i s an e x c e l l e n t gas b a r r i e r and as such i t s use i n the packaging market has i n c r e a s e d m a r k e d l y i n the l a s t s e v e r a l y e a r s . EvOH i s c o m m e r c i a l l y
In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
10.
ALGER ET AL.
205
Retortable Food Packages
Downloaded by UNIV OF ARIZONA on October 19, 2015 | http://pubs.acs.org Publication Date: May 9, 1990 | doi: 10.1021/bk-1990-0423.ch010
a v a i l a b l e i n s e v e r a l grades w i t h d i f f e r e n t r a t i o s o f ethylene/vinyl a l c o h o l i n t h e c h a i n (1) . The major l i m i t a t i o n o f EvOH i s t h a t a t e l e v a t e d h u m i d i t y , >70% , i t s oxygen and water b a r r i e r effectiveness i s severely reduced because o f d i s r u p t i o n o f the polymer-polymer hydrogen bonds (10-12) . Also, because o f t h e h y d r o x y l g r o u p s on the polymer backbone, water tends t o be e x t r e m e l y s o l u b l e i n EvOH. F i g u r e 1 g i v e s t h e w a t e r s o l u b i l i t y i n EvOH-32^ as a f u n c t i o n o f h u m i d i t y a t 20°C and 120°C. We see t h a t the water s o l u b i l i t y i n c r e a s e s markedly as h u m i d i t y i s r a i s e d above 65-75%. Measurements made by Hopfenberg e t a l . (10) show t h a t the maximum water s o l u b i l i t y at 100% humidity i s a v e r y s t r o n g f u n c t i o n o f t h e v i n y l a l c o h o l c o n t e n t i n the polymer. F i g u r e 2 shows t h e oxygen p e r m e a b i l i t y c o e f f i c i e n t f o r EvOH-32 as a f u n c t i o n o f w a t e r c o n t e n t a t 20C. The oxygen b a r r i e r e f f e c t i v e n e s s of EvOH32 d e c r e a s e s markedly w i t h the absorption of water because of p l a s t i c i z a t i o n of t h e EvOH-32 m a t r i x . PRINCIPLES
o f a n EvOH R E T O R T A B L E FOOD
PACKAGE
Re t o r t a b l e f o o d packages u s i n g EvOH as the oxygen b a r r i e r are m u l t i l a y e r s t r u c t u r e s w h i c h are formed by coextrusion. The number of d i f f e r e n t layers i n a the s t r u c t u r e can v a r y ; a r e p r e s e n t a t i v e f i v e - l a y e r s t r u c t u r e i s shown i n F i g u r e 3 . O f t e n one o r more l a y e r s a r e added t o reuse s c r a p and r e g r i n d i n the package. There i s a wide range o f p o s s i b l e c o m b i n a t i o n s o f layer t h i c k n e s s e s w h i c h c a n be u s e d i n t h i s t y p e o f s t r u c t u r e . I n r e t o r t the package i s s t e r i l i z e d a t h i g h temperature, 220-270°F, f o r one t o two h o u r s . D u r i n g t h i s p e r i o d o f time the r e l a t i v e h u m i d i t y on b o t h the i n s i d e and o u t s i d e package w a l l s i s 100% . As a r e s u l t , water permeates through the o u t e r l a y e r s and i n t o the c e n t e r EvOH l a y e r . The s o l u b i l i t y of water i n EvOH a t h i g h temperatures i s r e m a r k a b l y h i g h as shown i n F i g u r e 1. S i n c e t h e EvOH l a y e r i s p r o t e c t e d from the water by the o u t e r l a y e r s , the t o t a l amount o f water absorbed i n the EvOH l a y e r d u r i n g r e t o r t i s l i m i t e d by water p e r m e a t i o n t h r o u g h these o u t e r l a y e r s . I f v e r y l o n g r e t o r t times are u s e d t h e n t h e amount o f w a t e r a b s o r b e d i n t h e EvOH l a y e r c a n be q u i t e s i g n i f i c a n t (4) . Water s o l u b i l i t y a t 120 °C i s about 35 t o 40 g/100 g EvOH (4,5). A f t e r the r e t o r t c y c l e i s complete the package i s p l a c e d i n "storage". W h i l e the i n s i d e h u m i d i t y o f the package remains at 100%, the outside humidity c a n v a r y o v e r a w i d e r a n g e d e p e n d i n g upon the e x a c t s t o r a g e c o n d i t i o n s . Changes i n temperature a r e v e r y l i k e l y t o o c c u r during storage. I f we assume that there i s no i r r e v e r s i b l e loss of EvOH b a r r i e r effectiveness f o l l o w i n g retort t h e n the p r i m a r y v a r i a b l e o f importance i s the t o t a l water absorbed by the EvOH. The w a t e r c o n t e n t i s an i n i t i a l c o n d i t i o n f o r the d r y i n g p e r i o d following retort. D u r i n g s t o r a g e t h e e x c e s s w a t e r i n t h e EvOH l a y e r d e s o r b s and the p a c k a g e a p p r o a c h e s a s t e a d y - s t a t e . D u r i n g t h i s l a t t e r phase the oxygen p e r m e a b i l i t y d e c r e a s e s as water i s desorbed from the EvOH layer i n accordance w i t h the r e l a t i o n s h i p g i v e n i n F i g u r e 2 . The drying stage can l a s t from weeks t o many months depending on the p a r t i c u l a r package l a y e r d i s t r i b u t i o n and the t o t a l w a t e r a b s o r b e d d u r i n g t h e r e t o r t p r o c e s s . A f t e r excess water has permeated out o f the EvOH l a y e r , the package reaches a steady-state c o n d i t i o n
1
EvOH- 32 r e f e r s t o e t h y l e n e - v i n y l a l c o h o l copolymer w i t h 32 mol % e t h y l e n e . A l l subsequent r e f e r e n c e s t o EvOH-32 w i l l be f o r E v a l c a E v a l - F grade m a t e r i a l .
In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
206
BARRIER P O L Y M E R S AND STRUCTURES
40
+ Apicella and Hopfenberg (1982) D
this work
C\J
*
30
Ο >
120 C
LU
ο ο
20
Downloaded by UNIV OF ARIZONA on October 19, 2015 | http://pubs.acs.org Publication Date: May 9, 1990 | doi: 10.1021/bk-1990-0423.ch010
τ
ω
G5
20 C
£ 10
Ο)
20
40
60
80
100
Relative Humidity, % F i g u r e 1. Water S o l u b i l i t y i n EvOH-32 as a f u n c t i o n of humidity. Results a t 25°C w e r e t a k e n f r o m r e f 10, L L . and 26. R e s u l t s a t 120°C a r e from ref 5 .
I I
100
>>
•
Ikari and Motoishi (1984)
100% Humidity at 20 C
T = 20C π
1
4
1
1
8
— ι
12
1
1
16
~i
r
20
1
1
24
1 —
28
g water/100 g EvOH-32 F i g u r e 2. Oxygen P e r m e a b i l i t y i n EvOH-32 as a f u n c t i o n o f water r e g a i n a t 20°C (5.) .
In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
Downloaded by UNIV OF ARIZONA on October 19, 2015 | http://pubs.acs.org Publication Date: May 9, 1990 | doi: 10.1021/bk-1990-0423.ch010
10. ALGER ET AL.
207
Retortable Food Packages
EvOH polycarbonate
polypropylene
Storage, 65-75% Humidity
Food Contents, 100% Humidity
tie layers F i g u r e 3. F i v e - L a y e r S t r u c t u r e w i t h P P / t i e / E v O H / t i e / P C .
In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
208
BARRIER POLYMERS AND STRUCTURES
d u r i n g w h i c h the w a t e r and oxygen p e r m e a t i o n r a t e s r e a c h a c o n s t a n t v a l u e . W h i l e somewhat o f an i d e a l i z a t i o n , because storage conditions are not f i x e d i n r e a l i t y , the i m p o r t a n t a s p e c t s o f the performance of a r e t o r t a b l e package are c a p t u r e d u s i n g t h i s s i m p l i f i e d model.
Downloaded by UNIV OF ARIZONA on October 19, 2015 | http://pubs.acs.org Publication Date: May 9, 1990 | doi: 10.1021/bk-1990-0423.ch010
MODEL o f a R E T O R T A B L E
PACKAGE
To p r e d i c t t h e p e r f o r m a n c e o f a r e t o r t a b l e package b o t h w a t e r and oxygen p e r m e a t i o n must be i n c l u d e d i n t h e m o d e l . To i l l u s t r a t e a s i m p l i f i e d c a l c u l a t i o n o f t h e p e r f o r m a n c e o f a r e t o r t a b l e package we c o n s i d e r t h e i d e a l i z e d f i v e - l a y e r structure shown i n Figure 3. There are two d i s t i n c t parts of the c a l c u l a t i o n : r e t o r t and s t o r a g e . To a f i r s t approximation the r e t o r t c a l c u l a t i o n d e t e r m i n e s the t o t a l water absorbed by the EvOH l a y e r during the r e t o r t c y c l e and can be thought o f as an i n i t i a l c o n d i t i o n f o r the s t o r a g e calculation. RETORT CALCULATION D u r i n g t h e r e t o r t p r o c e s s we assume t h a t no oxygen i s p r e s e n t so t h a t o n l y water t r a n s p o r t i s considered. At r e t o r t temperatures the d i f f u s i o n c o e f f i c i e n t of water i n materials of i n t e r e s t i s s u f f i c i e n t l y h i g h so t h a t a p s e u d o - s t e a d y - s t a t e model, i n w h i c h l i n e a r p r o f i l e s are assumed a c r o s s a l l s t r u c t u r a l l a y e r s , can be used. The EvOH layer i s assumed to be at a u n i f o r m w a t e r a c t i v i t y a t a g i v e n time which i s good a s s u m p t i o n based on w a t e r p e r m e a b i l i t y m e a s u r e m e n t s i n EvOH a t h u m i d i t i e s found i n package applications (13). W r i t i n g a mass b a l a n c e f o r water i n the s t r u c t u r e g i v e s the f o l l o w i n g e q u a t i o n f o r r e g a i n , Φ [g water/100 g EvOH]: d - " d t
P^T) - - - - P
W (*V
5
EvOH EvOH
*Η
Ε ν 0 Η
1
) - /> (RH w
Ev0H
- RH )>
(1)
2
2
We use the term " r e g a i n " t o r e f e r t o an amount o f water i n the EvOH l a y e r a t a g i v e n c o n d i t i o n and " s o l u b i l i t y " a t a g i v e n water a c t i v i t y to r e f e r to the a m o u n t o f w a t e r a b s o r b e d w h e n t h e EvOH i s i n e q u i l i b r i u m w i t h t h e surrounding water a c t i v i t y . I n the water mass b a l a n c e , e f f e c t i v e water t r a n s m i s s i o n rates, /3 , are d e f i n e d f o r t h e l a y e r s b e t w e e n t h e f o o d a n d EvOH l a y e r W
k _ 1
β„
-
( Σ
. 1=1
1
and
δ c--))"
Ί
f o r the layers
< Σ
. , . i=k+l
1
(2)
w. ι
between N
"2
Ρ
ambient
a n d t h e EvOH
s. ( Ρ w.
—
(
3
)
ι
For the g e n e r a l c a s e , the k ^ l a y e r i s EvOH and i n t h i s example k=3 and N=5. δι i s the t h i c k n e s s and Ρ i s the water permeability of l a y e r i , r e s p e c t i v e l y .
In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
Downloaded by UNIV OF ARIZONA on October 19, 2015 | http://pubs.acs.org Publication Date: May 9, 1990 | doi: 10.1021/bk-1990-0423.ch010
10.
Retortable FoodPackages
ALGER ET AL.
209
The temperature p r o f i l e d u r i n g r e t o r t can be s p e c i f i e d as any general f u n c t i o n of t i m e ; however, we have used o n l y c o n s t a n t r i s e and f a l l r a t e s and f i x e d r e t o r t t e m p e r a t u r e . By so d o i n g we assume t h a t h e a t t r a n s f e r through the package and r a t e o f h e a t i n g the c o n t e n t s i s v e r y r a p i d . T h i s assumption i s r e a s o n a b l e ; t o r e l a x i t w o u l d s e r v e o n l y t o c o m p l i c a t e the c a l c u l a t i o n . An i m p o r t a n t p a r t o f the r e t o r t s i m u l a t i o n f o r LEP and PEP structures i s t h e t e m p e r a t u r e dependence o f the p o l y p r o p y l e n e and p o l y c a r b o n a t e water p e r m e a b i l i t i e s . Measurements o f p o l y p r o p y l e n e and p o l y c a r b o n a t e water p e r m e a b i l i t i e s were made u s i n g a MOCON Permatran-W™ and i t was found t h a t p o l y c a r b o n a t e has a water p e r m e a b i l i t y which decreases s l i g h t l y with temperature whereas p o l y p r o p y l e n e i n c r e a s e s w i t h temperature. Near r e t o r t temperature the water p e r m e a b i l i t i e s of both materials are s i m i l a r ; at storage p o l y c a r b o n a t e has a water p e r m e a b i l i t y about t e n times g r e a t e r than p o l y p r o p y l e n e . Measurements shown i n F i g u r e 4 are i n good agreement w i t h previously reported r e s u l t s f o r polycarbonate (2,14-18) and polypropylene (5,1921) . The r e t o r t s i m u l a t i o n c a l c u l a t i o n was performed by i n t e g r a t i n g eqn 1 u s i n g the i s o t h e r m f o r EvOH-water i n F i g u r e 1 and the water p e r m e a b i l i t i t y f o r p o l y p r o p y l e n e and p o l y c a r b o n a t e shown i n F i g u r e 4. S i n c e v o l u m e t r i c measurements are n o t a v a i l a b l e , the EVOH-32 was assumed t o have a constant d e n s i t y o f 1.19 g/cm^ i n t h e c a l c u l a t i o n . STORAGE CALCULATION D u r i n g s t o r a g e the temperature and o u t s i d e package h u m i d i t y are b o t h lowered. Storage temperatures i n the range o f 10 to 40°C and h u m i d i t i e s from 50 t o 85% RH are common. A g a i n , w h i l e i t i s p o s s i b l e to use a g e n e r a l i z e d time-dependent s t o r a g e h u m i d i t y and temperature, most o f the i n t e r e s t i n g performance c h a r a c t e r i s t i c s are r e a l i z e d w i t h constant values. The i n s i d e package h u m i d i t y was s e t t o 100% f o r a l l c o n d i t i o n s we considered f o r wet f o o d a p p l i c a t i o n s . Modelling of v a r i a b l e package humidity coupled w i t h o x y g e n p e r m e a t i o n h a s b e e n d i s c u s s e d b y Hows mon a n d P e p p a s (2_2) . The water r e g a i n i n the EvOH l a y e r i s c a l c u l a t e d as a f u n c t i o n o f time u s i n g eqn 1. The i n i t i a l l o a d i n g o f water i s t a k e n d i r e c t l y from the r e t o r t c a l c u l a t i o n ; o u t s i d e h u m i d i t y i s imposed o n l y on the outside of the package. The f o o d c o n t e n t s a r e a s s u m e d t o r e m a i n a t 100% h u m i d i t y a t a l l t i m e s . With the water c o n t e n t o f the EvOH a v a i l a b l e from i n t e g r a t i o n of eqn 1, t h e o x y g e n t r a n s m i s s i o n r a t e o f t h e package, OxTr, i s c a l c u l a t e d from:
OxTr -
( I
i=l
(— i - ) f
ρ 0
1
(4)
2
i s
β
where P Q ^ i - ^ oxygen p e r m e a b i l i t y o f l a y e r i . Normally a l l of the oxygen b a r r i e r i s s u p p l i e d by the EvOH l a y e r . The OxTr can then be i n t e g r a t e d w i t h time t o c a l c u l a t e accumulated oxygen: 2
.t 3 cm ( s t p ) = 0.21 A
OxTr d t 0
In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
(5)
Downloaded by UNIV OF ARIZONA on October 19, 2015 | http://pubs.acs.org Publication Date: May 9, 1990 | doi: 10.1021/bk-1990-0423.ch010
210
BARRIER POLYMERS AND STRUCTURES
polycarbonate
20 C
0.0025
0.0029
0.0033
temperature, 1/Kelvin F i g u r e 4. Water P e r m e a b i l i t y for Polypropylene and Polycarbonate. P o l y p r o p y l e n e measurements were made on 9.5 m i l extruded sheet ; polycarbonate measurements were made on 10 and 20 mil extruded sheet. Note that permeability has water pressure i n the denominator which is d i s t i n c t l y d i f f e r e n t from a WVTR (water vapor transmission rate) .
In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
10. ALGER ET AL.
Retortable Food Packages
211
Downloaded by UNIV OF ARIZONA on October 19, 2015 | http://pubs.acs.org Publication Date: May 9, 1990 | doi: 10.1021/bk-1990-0423.ch010
where A is the surface area of the package and the factor of 0.21 is added for ambient oxygen pressure. Oxygen concentration i s calculated from t o t a l cm^(stp) found from eqn (5) and the volume of the package. An expression for the EvOH oxygen permeability as a function of humidity and temperature is r e q u i r e d to i n t e g r a t e eqn ( 5 ) . Using any standard numerical routine, the above set of coupled equations, eqns 1- 5 , can be solved to simulate both the retort and storage of a multilayer package. RESULTS o f SIMULATED PACKAGE PERFORMANCE F i g u r e 5 shows the c a l c u l a t e d ppm' s of oxygen vs time for symmetrical LEP and PEP structures s t o r e d both at 65 and 75% RH. Figure 6 gives the water desorption i n the EvOH l a y e r as a f u n c t i o n of time corresponding to the ppm' s of oxygen i n F i g u r e 5. From s o l u t i o n of the retort model equations we find that the use of a p o l y c a r b o n a t e outs ide l a y e r decreases the amount of oxygen which w i l l permeate into a package for a fixed shelf l i f e . The reason for this is the following. At r e t o r t conditions , the water permeability of polycarbonate and p o l y p r o p y l e n e are comparable, as shown i n Figure 4 , so that the amount of water allowed into the EvOH during r e t o r t is s i m i l a r i n the two structures. However , under t y p i c a l storage c o n d i t i o n s , polycarbonate has a water p e r m e a b i l i t y w h i c h i s a p p r o x i m a t e l y 10 times g r e a t e r than that of p o l y p r o p y l e n e ; thus, the EvOH dried out faster i n the LEP structure and, t h e r e f o r e , l e s s oxygen can diffuse through the LEP during dry out. Some early studies suggested that the ratio of polycarbonate to polypropylene water permeabilities was the same under retort conditions as under storage conditions w h i c h w o u l d r e s u l t i n a " f l o o d " of water p a s s i n g through the outer polycarbonate skin into the EvOH layer during r e t o r t . As seen i n Figure 4 , t h i s a s s u m p t i o n i s a p o o r one. While the above model c e r t a i n l y captures the performance of a retortable package, package tests have shown that complications arise from the retort process which lead to an i r r e v e r s i b l e change i n EvOH barrier properties ( 4 ) . Since the exact nature of t h i s change i s very p o o r l y understood, i t is necessary to develop a test methodology to provide insight into mechanisms o p e r a t i n g i n the p a c k a g e to make a t r u l y q u a n t i t a t i v e m o d e l . OXYGEN INGRESSION EXPERIMENTS
on FOOD PACKAGES
An oxygen head-space analysis was used to measure oxygen ingression into packages as a function of time following r e t o r t . Packages were placed in an oxygen-free glove box, f i l l e d with 1-2 cc of oxygen-free water, and then sealed. The packages were retorted under predetermined conditions and then placed i n storage at a fixed humidity. At various time intervals individual packages were s e l e c t e d and destructively tested using a M0C0N/T0RAY LC7OOF™ headspace analyzer to measure oxygen concentration i n the package. The procedure can be modified by f i r s t r e t o r t i n g the package, flushing with nitrogen, and placing i t in storage (8). While usually not a problem, this l a t t e r approach w i l l 'miss ' any oxygen which permeates into the package during the r e t o r t process . We have found that dissolved oxygen i n the package material can diffuse i n during r e t o r t . This is illustrated by Figure 7 which shows ppm oxygen vs time following r e t o r t for a LEP package. In Figure 7 a 2 ppm oxygen offset was observed immediately following r e t o r t . The offset was presumed to be a r e s u l t of the very thick polycarbonate outer wall in which there was dissolved oxygen available for d i f f u s i o n into the package during r e t o r t .
In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
212
BARRIER POLYMERS AND STRUCTURES
18.0 "PEP"
16.0 Storage Humidity = 75%
14.0 c Φ
12.0
CD
._ _
Ε
8.0
ίΓ 100 ο
Downloaded by UNIV OF ARIZONA on October 19, 2015 | http://pubs.acs.org Publication Date: May 9, 1990 | doi: 10.1021/bk-1990-0423.ch010
Ω α.
6.0
= 75%
"LEP"
4.0 2.0
= 65%
0.0
1
— ι
1
0
1
r
1
— ι
40
1
1
1
1
1
80 120 time, days
1
1
1
1
r~
160
200
F i g u r e 5 . C a l c u l a t e d ppm' s oxygen vs time f o r s y m m e t r i c a l LEP and PEP s t r u c t u r e s ( 10 m i l PP/2 EvOH/10 PP o r PC ; R e t o r t 90 m i n u t e s , 120°C) .
12
C\J CO I X
10
Ο >
LU Ο) Ο ο CO
