Effect of partitioning on mass balance

If there are significant partitioning effects occurring between the PS and product, then the amount of styrene migrating may reach a thermodynamic upper limit (Chapter 4). The partition coefficient is the ratio of the concentration of the migrant in the polymer cp.oc to the concentration of migrant in the food cFoc at equilibrium (long times)  

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. 

A guide for estimating the general behavior of partition coefficients is like dissolves like . Thus styrene, a relatively nonpolar hydrocarbon, will tend to remain in a nonpolar polystyrene polymer if the package contains a polar aqueous food (Chapter 9). 

Incorporating the effect of the partition coefficient into a mass balance Eq. (14-1) gets (Chapter 7)  

Note that as K becomes very small (the styrene partitions very readily in the food phase as opposed to the polymer phase, e.g. fatty foods) then Eq. (14-3) simplifies to Eq. (14-1) which assumes complete migration into the food. For this reason migration into foods with high fat contents is generally best estimated using Eq. (14-1). 

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