Diffusion coefficients and activation energies of diffusion of low molecular weight migrants in Poly(ethylene terephthalate) bottles

Poly(ethylene terephthalate) (PET) is nowadays the packaging material of choice for beverages. Therefore knowledge about the diffusion coefficients at a certain temperature or activation energies of diffusion of potential migrants in the polymer is of interest, especially for the definition of the basic parameter set for migration modelling of PET. In this study, the diffusion coefficients of acetaldehyde, benzene and tetrahydrofuran in PET bottle materials were determined from kinetic migration experiments at four different temperatures. The activation energies for tetrahydrofuran and benzene were determined to be 106.8 kJ mol⁻¹ and 101.4 kJ mol⁻¹, respectively. The activation energy for acetaldehyde is significantly lower (75.7 kJ mol⁻¹) which is due to the lower molecular weight of this molecule compared to benzene and tetrahydrofuran. The results were compared with literature data of diffusion coefficients of other low molecular weight molecules in PET. From the results it is evident that the current migration model with the default modelling parameters for PET does not describe realistically the diffusion coefficients in PET. The migration of small molecules like acetaldehyde will be underestimated whereas higher molecular weight compounds will be overestimated by the current migration model. Whereas the overestimation is useful for compliance evaluation of PET bottles, for more realistic migration calculations, e.g. for exposure estimations, such overestimation is not desirable. Therefore, more accurate modelling parameters should be established. The key parameters for more realistic migration modelling are the activation energies of diffusion in the polymer.