Migration Polymer/food system

Poly(vinyl acetate) polymers are environmentally friendly because they easily biodegrade. Poly(vinyl acetate) may be hydrolyzed to poly(vinyl alcohol) which is then assimilated by naturally occurring organisms. In the use of emulsion polymers, the associated components (stabilizers, initiators, etc) should be scrutinized for their effect on the environment. Poly(vinyl acetate) is nontoxic and is approved by the U.S. FDA for food-packaging (qv) applications (CFR 176.170,175.105). Components in the emulsion polymer system which may migrate from the film into the food may impact the approval of the total package. In food applications the impact on odor and taste of residual low molecular weight components may be important in the selection of a product for use. 

It is possible that styrene will never reach the mass balance migration limit specified by Eq. (14-1) in certain foods because of partitioning effects. The systems most likely to have partitioning effects, i.e. when K 1, are those for styrene between aqueous foodstuffs and PS. Migration is usually highest into fats and oils since styrene is readily soluble in both the fats and polymers so that K < 1. 

The concepts discussed in Section 14.4 describe the situation that will eventuate when a multiphase packaging system reaches thermodynamic equilibrium. However, the rate of mass transfer of permeant, sorbate, and migrant molecules in the polymer is not addressed by these equilibrium considerations. For example, if we consider a potential migration process, we know that eventually the migrant will be transferred to the food and it will finally reach equilibrium, but based on the equations presented in Section 14.4 we cannot predict how long the process will take. Similarly, these relationships will not allow us to estimate the shelf life of a product in a particular package system. For this, we need to look at diffusion. 

All diffusion processes involving the transfer of a diffusantfrom or to a polymeric material asymptotically reaches a steady condition at long times. In any closed system (like a Tetra Pak carton) that has reached this condition, there is no effective mass transfer between the polymer and the surrounding phases the adjuvant concentrations are at equilibrium, and the transferred material is at the maximum possible amount In reference to Fig. 11.25 b, consider a sorption or migration process that has reached equilibrium. is the volume of contacting phase, and Vp is the volume of plastic in the container, and c y and c are the equilibrium concentrations of the adjuvant in the contacting phase and polymer, respectively. For the case of sorption of a substance from the food phase by the polymer, the mass balance at equilibrium is... 

We have known for years that the behaviour of volatile materials migrating from and through plastics can be described by Pick s Law of Diffusion. Theory and practical experience with these materials indicate that they can be monitored to ensure their safe and satisfactory use with foods. But the migration of non-volatiles from plastics is by no means so well understood and has been the subject of research, not only at PIRA, but at several other Packaging Research Laboratories over the last five years. Simple model migration systems have been used at PIRA for example, systems based on a single polymer containing one non-volatile additive, exposed to pure liquids either separately or in mixtures. The major results of this work are as follows ... 

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