Relationship Between Polymer Structure and ... - ACS Publications

Chapter 16

Relationship Between Polymer Structure and Performance in Food Packaging Applications

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 6, 2015 | http://pubs.acs.org Publication Date: March 9, 1988 | doi: 10.1021/bk-1988-0365.ch016

George W. Halek Food Science Department, Rutgers University, New Brunswick, NJ 08903 This paper reviews the performance of polymers as food packaging materials from the viewpoint of the chemical and physical structure of the polymers and chemical and physical interactions with food ingredients. The major factors involved in these interactions are polarity and crystalline structure of the polymer and polarity and chemical structure of the food ingredients. The effects of these factors on mechanical, barrier, and compatibility behavior of the systems are described, and the principles controlling the behaviors are applied to explain performance. Recommendations are given for further research to broaden understanding of polymer-food interactions. The performance that i s to be expected from a polymeric food packaging material i s the same as that expected from any food packaging material. It has been described elswhere ( 1 , 2 ) i n d e t a i l and can be summarized f o r our purpose as: containing the food, protecting i t from the environment, and maintaining food q u a l i t y . In the case of polymers, the a b i l i t y to perform these functions w i l l depend on their mechanical and barrier properties, to which must be added the requirement of long-term compatibility with the food. This last property w i l l depend on the nature of the food ingredients and the structure of the polymeric packaging material. Thus food packaging performance i s seen to depend on polymer mechanical, b a r r i e r , and compatibility properties. These, i n turn, w i l l depend on polymer structure and changes that can occur with time during interactions with the food ingredients. As a p r a c t i c a l matter, there are now a number of polymers that are used i n food packaging applications (3), and they have endured because they have demonstrated a defineable level of appropriate mechanical and barrier properties f o r the task. The matter of compatibility has been less easy to define and i n an increasing number of cases has been recognized as a potential source of loss in food q u a l i t y . A study of the l i t e r a t u r e has revealed only a limited amount of publication of such cases, but there are enough to permit categorization. These w i l l be described i n this paper. 0097-6156/88/0365-0195S06.00/0 © 1988 American Chemical Society

In Food and Packaging Interactions; Hotchkiss, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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FOOD AND PACKAGING INTERACTIONS

POLYMER STRUCTURE CONSIDERATIONS. The polymers that are used i n food packaging applications can be divided into several classes according to their chemical structure(Table I ) . Table I . Classes of Food Packaging Classes


Hydrocarbon Halogen Functional Vinyl Condensation Miscellaneous

Polymers

Representative Polymers Polyethylene, Polypropylene, Polystyrene. Polyvinyl chloride, Polyvinylidene chloride. Polyvinyl alcohol, Polyvinyl acetate, Polyacrylonitrile. Nylon 6,6, Polyethylene terephthalate. Cellophane, Polycarbonate, Polyurethane.

Each of these polymers can next be assigned to a series of increasing p o l a r i t y ranging for example from polyethylene (low p o l a r i t y ) to polyvinyl alcohol (high p o l a r i t y ) due mainly to the differences i n the functional groups attached to the main polymer chain. They can also be arranged according to crystalline/amorphous s o l i d state r a t i o s . F o r example they range from i s o t a c t i c polypropylene with a high crystalline/amorphous r a t i o to polyvinyl choride with a low r a t i o . Both of these factors, degree of p o l a r i t y and crystalline/amorphous r a t i o s , are major factors i n the state of matter attained i n a polymeric packaging material. That attained state of matter controls the properties of the fabricated packaging materials, and this depends on polymer structure (4). In addition there are a number of other factors that affect performance such as molecular weight and i t s d i s t r i b u t i o n , crosslinking, and additives, but these are secondary to the p o l a r i t y and c r y s t a l l i n i t y factors dealt with here. However these effects should not be ignored i n seeking information on causes of polymer performance. x

FOOD PACKAGING PERFORMANCE CONSIDERATIONS. The major performance characteristics of polymeric food packaging materials controlled by the structural considerations are shown i n Table I I . The table also includes t y p i c a l properties that are measured for those performance characteristics. Table I I . Performance Characteristics and Related Properties Performance Characteristic

Related Properties

Mechanical

Tensile Strength Compressive Strength Impact Strength Permeation Migration Sorption Desorption

Strength

Barrier Behavior Compat i b i l i t y


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These performance c h a r a c t e r i s t i c s and how they are related to polymer structure and the e f f e c t s on food packaging applications are presented i n the next sections. POLYMER STRUCTURE AND MECHANICAL PERFORMANCE. The v a r i e t y of polymer classes that are used in food packaging i s shown in Table I. They represent the range of functional groups and c r y s t a l l i n e structures that provide properties that meet the needs of food packaging applications. The polymers of low p o l a r i t y for example the polyhydrocarbons, a t t a i n their mechanical strength from packing of very long chains of high molecular weight or r e g u l a r i t y so that the r e l a t i v e l y weak van der Waals interchain forces add up s u f f i c i e n t l y to meet the material requirements. Those of higher p o l a r i t y can a t t a i n that t o t a l requirement of interchain a t t r a c t i v e force at a lower molecular weight and with less s t r u c t u r a l r e g u l a r i t y because of the compensating polar a t t r a c t i v e forces. These factors are well known in polymer work (4) and w i l l not be described further. We w i l l focus on the forces involved in food-polymer interactions that can change mechanical properties. This involves t r a n s i t i o n s i n the polymer caused by migration of ingredients into the polymer in contact with food. It has been recognized for some time that there i s a strong dependence between d i f f u s i o n and relaxation in polymers (5) and that this could be explained by a coupled d i f f u s i o n - r e l a x a t i o n process (6,7). The argument i s that when an organic penetrant i s sorbed by a semicrystalline polymer, the rate of sorption i s controlled by d i f f u s i o n and a slow relaxation of the polymer (5). This further enhances the rate of d i f f u s i o n which further influences the relaxation. To the extent that t h i s occurs, changes occur in the polymer's properties. There are few published works covering the d e t a i l s of such behavior r e s u l t i n g from food-polymer interactions, but the e f f e c t s expected can be summarized from general principles involving coupled d i f f u s i o n - r e l a x a t i o n . In general, mechanical properties w i l l be changed by permeants that can affect polymer chain segmental mobility. Accordingly, food ingredients that are absorbed and p l a s t i c i z e the polymer chains w i l l permit them to s l i p past each other. Food ingredients that can displace polar a t t r a c t i v e forces (hydrogen bonds or polar bonds) in the polymer w i l l also a f f e c t chain mobility. The net e f f e c t s of these two factors would be a lowering of t e n s i l e and compressive strength and modulus, and an increase in creep properties r e s u l t i n g from continued deformation with time. Impact strength w i l l usually be increased under these conditions as the increased segmental motion and decreasing c r y s t a l l i n i t y would permit d i s s i p a t i o n of impact energy by energy-consuming contortions of the more mobile polymer chain segments. Thus, the mechanical properties are affected s i g n i f i c a n t l y by the behavior of the polymer towards permeation by the food ingredients. The structural features that affect this behavior w i l l be further considered in the next section on b a r r i e r performance. POLYMER STRUCTURE AND BARRIER PERFORMANCE. Summaries of the relationships between polymer structure and b a r r i e r performance i n food packaging materials are available (2,8,9). Much of the



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emphasis has been on permeation of H2O, 02* and CO2 which are of p a r t i c u l a r importance to food shelf l i f e . In addition, there i s a limited amount of data published on permeation by model solvents and aroma compounds. Our discussion w i l l include p r i n c i p l e s that cover a l l of these cases. In general, there are several polymer structural features that result i n good barrier performance: 1) Some p o l a r i t y but of certain types; 2) Regularity of molecular structure; 3) Close chain-to-chain packing i n the s o l i d state. The p o l a r i t y i s important from the viewpoint of functional group interactions with the polar permeants of interest to foods. The regularity of molecular structure affects the a b i l i t y of the polymer chains to approach each other during the c r y s t a l l i z a t i o n process of the packaging material f a b r i c a t i o n steps. That factor and the f i n a l close chain-to-chain packing that i t permits provides a r e s t r i c t e d pathway for the permeants. The problem i s that at times these factors can have cross-effects that must be taken into consideration when deducing the effects of structure on performance. Such effects i n the case of p o l a r i t y can be seen from the data i n Table I I I derived from the l i t e r a t u r e (2,3,8,9) for 02> C02 and H2O permeation i n a number of barrier polymers. Table I I I . Barrier Properties of Selected Polymers (2,3,8,9)

Polymer

3

0 dry

b

2

PVOH EVOH PVDC PAN PET NYLON 6,6 PP

0.02 0.05 0.08 0.03 5.00 3.00 110.00

02wet

c

7.00 7.00 0.08 0.03 5.00 15.00 110.00

C0

b 2

0.06 0.23 0.30 0.12 20.00 5.00 240.00

H 0

d

2

10.00 10.00 0.05 0.50 1.30 24.00 0.30

Functional Class Hydroxyl Hydroxyl Halogen Nitrile Ester Amide Hydrocarbon

a PVOH, polyvinyl alcohol; EVOH, ethylene v i n y l alcohol copolymer: PVDC, polyvinylidene chloride; PAN, p o l y a c r y l o n i t r i l e ; PET, polyethylene-terphthalate; Nylon 66, hexamethylenediamine adipic acid; PP, polypropylene, b cc-mil/lOOin . day . 1 atm c cc-mil/100in . day . 1 atm at 80% RH d g-mil/100in . day . at 100°F and 90% RH These data i l l u s t r a t e the cross effects when humidity i s included. For example, polyvinyl alcohol i s an outstanding barrier towards O2 and CO2 when dry. The high content of OH groups permits interactions that retard transport through the polymer. However, when water enters the polymer i t p r e f e r e n t i a l l y interacts with the OH groups and thus diminishes retardation of the permeants and also swells the structure to increase interchain distances and thus free volume. The net result i s a strong reduction i n barrier performance. The same behavior i s noted for ethylene v i n y l alcohol copolymer with i t s OH groups. A similar behavior i s seen for Nylon 6,6 with i t s amide groups, so i t i s not r e s t r i c t e d just to OH 2

2

2



16. HALEK

Polymer Structure & Performance in Packaging

199

groups. The effect i s not observed for PVDC or PAN or PET (see Table III) which also have polar groups. The indication i s that i f a permeant can p r e f e r e n t i a l l y interact with the polymer and can swell i t , the barrier performance i s reduced (PVOH, EVOH, Nylon 6,6). On the other hand, i f i t cannot, the b a r r i e r performance remains (PVDC, PAN, PET, PP). This i s also noted for the water permeation data i n Table III for these polymers. Those data show that the two polymers with OH groups and the Nylon are poor water barriers while the others are better. But, so i s the performance of the non-polar polypropylene. Thus polar goups per se do not mean good or bad barrier performance towards water nor towards O2 and C02- So long as the polymer p o l a r i t y results in strong interchain attractions that are not displaced by the permeant, the barrier performance remains good. The other factor of importance to barrier performance i s attained c r y s t a l l i n i t y . The whole topic i s complex but can be i l l u s t r a t e d by the following examples from Table III and Table IV derived from the l i t e r a t u r e (2,3,8,9). Table IV.

C r y s t a l l i n i t y Effects from Stretch Orientation

3

Polymer PP Nylon PET PAN

Oo Permeation Unoriented Oriented 414.00 2.60 8.00 0.03

240.00 1.30 4.00 0.03

b

HoO Permeation Oriented Unoriented 0.5 25.0 3.0

0.3 8.0 1.3

2

a. cc-mil/100 i n . day . atm b. g-mil/100 i n . day 90% RH at 38°C 2

The polymers i n Table III are a l l highly c r y s t a l l i z a b l e as a result of t h e i r conformational regularity which permits packing into s p e c i f i c c r y s t a l l a t t i c e s during the melting and cooling process for f i l m or container f a b r i c a t i o n . Such c r y s t a l l i n i t y reduces the volume of amorphous polymer available as a permeation medium and also increases the pathlength to be travelled through the p l a s t i c structure. The result i s an improvement i n barrier properties from attained c r y s t a l l i n i t y i n addition to the previously mentioned retardation effect resulting from functional group interaction with the permeant. The c r y s t a l l i n i t y effect can be further increased by stretch orientation of the finished structures. This i s i l l u s t r a t e d in Table IV for several polymers that can be stretch oriented. Orientation has improved the O2 and H2O barrier as a result of stretch treatment that has reduced the amount of amorphous phase and also diminished the distance between c r y s t a l l i t e s . The generalization cannot be made more s p e c i f i c here because of the numerous differences i n organizational arrangements that are described i n the l i t e r a t u r e regarding the actual structural changes occurring from stretch orientation (10, 11, 12). For example, i n some cases the movement of the c r y s t a l l i t e s actually creates voids that diminish barrier effectiveness.


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One further point of interest i s that of laminates. Current technology permits formation of composites either by adhesive coatings or coextrusions to combine a desired set of properties i n one structure. For example the outstanding O2 and CO2 b a r r i e r of PVOH can be combined with polypropylene which i s four magnitudes poorer i n that respect but which can provide the water b a r r i e r that the PVOH lacks. Guidance i n interpreting the results to be expected from such combinations can be obtained by employing the basic p r i n c i p l e s described above. POLYMER STRUCTURE AND COMPATIBILITY. In this section we are taking compatibility to mean the a b i l i t y of the polymer and the food to coexist without s i g n i f i c a n t changes occurring i n either one as a result of their contact. There i s a close relationship between this subject and the discussions in the section on barriers because the factors for interactions in the polymer are the same: p o l a r i t y and c r y s t a l l i n i t y . However, here we w i l l put more emphasis on the nature of the migrants. Further, we w i l l l i m i t our discussion to migration of ingredients from the food into the polymer. Migration in the other d i r e c t i o n has been covered extensively by the large volume of publications involving regulatory concerns connected with migration of toxic ingredients or adulterants into foods (13,14). Even within those studies a body of t h e o r e t i c a l considerations of migration behavior at low concentrations of migrants has been formed and the reader i s directed to two a r t i c l e s (15,16) that discuss the main viewpoints involved. The whole area i s quite extensive and w i l l not f i t into this presentation. There are two main ways in which food can interact with polymers namely, 1) The food can react with or complex or in some way form a bond with the packaging material surface; 2) The food can be absorbed into the packaging material. In both cases the interactions depend on the nature of the food ingredients and on the p o l a r i t y and state of matter of the polymer. A current search of l i t e r a t u r e on this subject reveals only a limited amount of publication involving measurement of food-polymer interactions. A sampling of recent publications i s l i s t e d i n Table V. These examples, while r e s t r i c t e d to p o l y o l e f i n s , indicate the kind of behavior observed for the sorption of food ingredients by polymers. Reference 17 showed that limonene was strongly sorbed by polyethylene while other oxygenated aroma compounds were sorbed to a lesser extent. The other papers also report equilibrium p a r t i t i o n i n g that i s quite favorable to the p l a s t i c . For example the partition coefficient reported for limonene between polyethylene and water was 3,400 (19). For the other ingredients from that work, the c o e f f i c i e n t s ranged from 2-38. In another work (18) the sorption isotherms for limonene i n water into polyethylene showed a biphasic sorption behavior. Reaction rates indicated that adsorption was occurring very rapidly while absorption was slower. Carvone behaved s i m i l a r l y but had a much lower p a r t i t i o n c o e f f i c i e n t (18). S i m i l a r l y , differences were found for the sorption rates of terpenes, l i n a l o o l , and c i t r o n e l l a with polyethylene and polypropylene (20). These reports confirm that the hydrocarbon model permeants are sorbed to a greater extent by polyolefins than are the more polar ingredients.


16. HALEK

Table V.


201

Polymer Structure & Performance in Packaging Food Ingredient Sorption by Polymers

Ingredients

Polymers

Limonene Limonene Carvone Limonene Citral Benzaldehyde Ethyl Butyrate Terpenes Linalool Citronella Fats i n the form of ten foods including cheeses, margarine, chocolate, cream, milk and yogurt

Polyethylene Polyethylene

17 18

Polyethylene

19

Polyethylene terephthalate/ Foil/Polyethylene or polypropylene Polyethylene

20

Reference

14

The data reported on fat sorption from dairy products (14) were obtained by placing the foods, including s o l i d structures, i n direct contact with the polyethylene f i l m . The results showed a wide range of sorption of the fats by the p l a s t i c that was dependent on the structure of the foodstuffs. From these reports there i s not enough information to permit comment on the effects of polarity or crystallinity on the polymer-food ingredient interactions. Nor do they allow comment on d i f f e r e n t i a t i n g surface adsorption from internal absorption. However, they do show that the general p r i n c i p l e s of sorption behavior established with non-food ingredients are being followed. FUTURE RESEARCH NEEDS. From our b r i e f review i t can be seen that there i s a need for broadening the scope of research on the interactions between food and packaging polymers. The broadening i s needed both i n the nature of the food ingredients and the structure of the polymers. The need extends through a l l three of the performance c h a r a c t e r i s t i c s -mechanical, barrier and compatibility. A l l three of those areas are closely t i e d together by coupled diffusion-relaxation process considerations once the permeant enters the polymer. Thus, future researchers should be aware of a l l three characteristics when they carry out t h e i r p a r t i c u l a r experiments i n polymer-food interactions and to the extent that i s possible could c o l l e c t the additional data that w i l l further the understanding of the relationships between polymer structure and performance i n food packaging a p p l i c a t i o n s . F i n a l l y , i t would be helpful i f publications of food-polymer interactions would include more detailed data on the composition and properties of the polymers used. Most useful would be information about the composition (comonomers, additives) stereoregularity, type and degree of branching, molecular weight and d i s t r i b u t i o n , and state of attained c r y s t a l l i n i t y and morphology, including d i s t r i b u t i o n of


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spherulites for the description they were scientists but can not

and defects in the specimens actually used. S i m i l a r l y , food ingredients there i s need for more complete of the chemical composition and treatments to which exposed. Such information would be of great use to who wish to help unravel the food-polymer interactions find the observations that would be h e l p f u l .


LITERATURE CITED 1. Halek, G. Proceedings of the F a l l 1984 Meeting of the R & D Associates for M i l i t a r y Food and Packaging Systems, U.S. Army Natick R & D Center. A c t i v i t i e s Report. V o l . 37, No.1, 1985, p. 28-30. 2. Ashley, R. In Polymer Permeability; Comyn, J . , Ed.; E l s e v i e r : New York, 1985; Chapter 7. 3. Packaging Encyclopedia 1985; Cahners Publishing: Boston; p. 64 F. 4. Deanin, R. Polymer Structure, Properties of Polymer Films: Cahners Books Boston, 1972. 5. Rogers, C. In Structure and Properties of Polymer Films; Lenz, R. and Stein, R. Eds.; Plenum: New York, 1973; p. 297. 6. F u j i t a , H. and Kishimoto, A. J . Polymer S c i . , (1958), 28:547, 569. 7. Machin, D. and Rogers, C. J . Polymer S c i . A2(1972), 10:887. 8. Nemphos, S., Salame, M., and Steingiser, S. In Kirk-Othmer Encyclopedia of Chemical Technology; Wiley: New York, 1978; Vol. 3, p. 480-502. 9. Combellick, W. In Encyclopedia of Polymer Science and Engineering; Mark, H., Bikales, N., Overberger, C., Menges, G.; Eds.; Wiley: New York, 1985, p. 176-192. 10. Lenz, R. and Stein, S. Eds. Structure and Properties of Polymer Films; Plenum: New York, 1973. 11. Samuels, R. Structured Polymer Properties. Wiley: New York, 1974. 12. A l l e n , G. and P e t r i e , S. Physical Structure of the Amorphous State; Marcell Dekker: New York, 1977. 13. Crompton, T. Additive Migration From P l a s t i c s Into Food. Pergamon: New York, 1979. 14. Figge, K. In Progress in Polymer Science; Pergamon: New York, 1980; V o l . 6, p. 187-252. 15. G i l b e r t , S., M i l t z , J . , Giacin, J . J . Food Processing and Perservation. 1980, 4:27. 16. Niebergall, V., Kutzki, R. Deutsche Lebensmittel-Rundschau. 1982, 78:82. 17. Durr, P., Schobinger, U., Waldvogel, R. Alimenta. 1981, 20:91. 18. Meyers, Μ., Halek, G. Abstracts of 46th Annual Meeting of Institute of Food Technologists, Dallas, June 1986, Paper 161. 19. Kwapong, O., Hotchkiss, J . Abstracts of 46th Annual Meeting of Institute of Food Technologists, Dallas, June 1986, Paper 162. 20. Shimoda, M., Nitanda, T., Kadota, N. Ohta, Η., Suetsuna, Κ., Osajima, Y. J . Jap. Soc. Food S c i . and Tech. 1984, 31:n.11, 697. RECEIVED September 24, 1987


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