The common assumption of considering external mass transfer resistance as negligible can lead to a systematic bias that underestimates the actual diffusivity value. In this context, the suitability of two methods and the possible experimental setups (global or local concentration measurements) for determination of both diffusivity and mass transfer coefficient are discussed. The assessment was based on the experimental results of the desorption of Uvitex OB from low density polyethylene into the food simulant Miglyol 829. A practical identifiability analysis performed for each of the methods and experimental setups demonstrated that the increased amount of information provided by local measurement methods allows a better identifiability of the parameters, thereby reducing the experimental work and providing an unbiased method for determination of diffusivity.

A reliable prediction of migration levels of plastic additives into food requires a robust estimation of diffusivity. Predictive modelling of diffusivity as recommended by the EU commission is carried out using a semi-empirical equation that relies on two polymer-dependent parameters. These parameters were determined for the polymers most used by packaging industry (LLDPE, HDPE, PP, PET, PS, HIPS) from the diffusivity data available at that time. In the specific case of general purpose polystyrene, the diffusivity data published since then shows that the use of the equation with the original parameters results in systematic underestimation of diffusivity. The goal of this study was therefore, to propose an update of the aforementioned parameters for PS on the basis of up to date diffusivity data, so the equation can be used for a reasoned overestimation of diffusivity.

A fast, nondestructive method based on the determination of local concentration profiles in the polymer thickness with Raman microspectroscopy is presented here. It was used to assess the diffusivity of a model molecule (p‐terphenyl) in amorphous polystyrene films at 95°C (2.38 ± 1.08 × 10−17 m2/s). This methodology was validated by comparison with a more classical destructive approach based on the monitoring of the concentration evolution in the whole of the film with gas chromatography (89.4 × 10−17 m2/s). These values were in agreement with data available in the literature for molecules of the same molecular weight and temperature range determined with local measurement and were significantly lower than those determined by global measurement. Raman microspectroscopy was found to be adapted to slow diffusion speeds typically found in high‐barrier polymers; this allowed diffusivity to be obtained long before the equilibrium was reached and, thus, without the need for the partition coefficient. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40958.