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.
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