Migrants, Migration diffusion

In order to use the migration equations, especially the generally accepted equation (7-51), values for the partition coefficient K of the migrant between P and L and the diffusion coefficient DP of the migrant in P are needed. For migrants with a high solubility in the foodstuff or simulant, the value K = 1 can be used and a worst case estimation is obtained in this way. 

Here the migration rate of i in the food is determined by the value of the diffusion coefficient in the food as well as by the partition coefficient. The concentration cL,t of migrants that are poorly dissolved in the food (K > 1) increases more slowly than when they are more easily dissolved. 

The considerable influence of the food simulant can be observed in many cases for non-polyolefins. For example, the migration of an additive with Mr = 549 from IPS into 50 % ethanol in water in Table 15-4 shows a decrease of the migration amount measured at 49 °C after an initial contact temperature of 66 °C This phenomenon cannot be explained by changes in diffusion. The decrease in migration must be a consequence of a strong increase of the partition coefficient, KPR with decreasing temperature that shifts the equilibrium concentration of the migrant to the plastic phase. 

The temperatures to which the polymers are exposed must be below certain limits because above these (which are unique for each type of polymer) the diffusion behaviour of the migrant changes and cannot be calculated using the migration models. 

One of the most important migration problems occurs if a liquid food or food simulant F with the volume Vp and density pp comes in contact with a plastic layer P of thickness dp and density pp. The mass transfer takes place across an interface with area A between two different media with different characteristics, e.g., with different diffusion coefficients Dp and Z>p of the migrant. If the value of a quantity is desired, for example, the concentration of the substance transported across the interface in one of the two media, then a mass balance must be considered that takes into account the ratio of the contact surface area and the volume of the corresponding medium. The model describing this process is based on the following assumptions ... 

The fraction of migrant diffused from F into P up to time t, from mpo = VpCpo and the fraction migrated from P into F up to time t, from mpo = FpCpo are ... 

In order to use eqn 8.21 in practical cases the availability of data for two fundamental constants is needed (i) the partition coefficient, Kpp, of the migrating compound between the polymer P and the foodstuff or simulating liquid F and (ii) the diffusion coefficient. Dp, of the migrant in P. So far upper limits for migration amounts are needed from regulatory standpoints, predictions of worst case scenarios can start with the assumption of good solubility of the migrant in F and consequently A pp = 1 can be used. Much... 

Migration of unreacted polyol components through the polyethylene for the polyether-based laminate was observed. Although an excess of isocyanate was present, diffusion of polyol components into the polyethylene prior to reaction with isocyanate was postulated to explain the migration. Cyclic oligomers from the polyol starting materials were identified as the main migrants from the polyester-based adhesive. 

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. 

This section covers the relations between the chemical structure of the migrants and the main properties controlling the migration of these substances through polymers partition coefficients and diffusion coefficients. For brevity, only the food-packaging partition coefficients are presented, but the same principles can be apphed to polymer 1/polymer 2 partition coefficients. We can refer to data and additional models in [VIT 09]. 

Unlike the case in liquids and gases, where atoms and molecules are continuously moving, in a solid the motion is usually much more restricted. However weakly the migrant species is bound to neighboring atoms, it cannot readily move to a new position in the solid unless there is a space there that it can occupy. If this should be the case, the position vacated by the migrant species becomes available for a similar jump by a second species. When diffusion through a crystal lattice occurs by such a series of migrational transitions, it is said to occur by a vacancy mechanism. 

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