Migration of plasticizer from a PVC hose into a food simulant in flowing conditions
Résumé
Tubes made in elastomer thermoplastic matrices are used in milk producing and transforming and vending machines. Chemical additives are added in the material composition in order to have flexible hoses. The contact between foods and plastics needs to control the contamination risk. The European Union edits a regulation 10/2011 which must be followed by suppliers specifying the maximum values of Overall Migration Limit (OML) and Specify Migration Limit (SML) of each species established for food in static conditions. However, for food transported through tubes, the migration of additives requires to study the mass transfer in dynamic conditions.
According the European directives, mathematical modeling is allowed to demonstrate the compliance with the EU regulation 10/2011. A numerical model, detailed in this contribution, coupling the diffusion process in plastic with the transport in food has been developed. It is decomposed in two sub-problems: i) the first describes the unsteady diffusion of a plasticizer in the tube, ii) the second is devoted to the advection and diffusion in the liquid, called Graetz problem. For the second problem, the mass transfer coefficient from the wall to the liquid can be easily determined as a function of the Graetz number which is a dimensionless group equivalent to a Péclet number. The first problem is numerically solved by a pseudo-spectral Chebyshev method. The coupling between the two sub-problems are achieved by the boundary conditions.
The temporal evolution of the plasticizer migration are computed as a function of the ratio of the additive diffusion coefficient in the plastic matrix to the diffusion coefficient in the liquid, the partition coefficient (ratio of the additive mass concentration in the liquid to the mass concentration in the plastic matrix at the plastic/liquid interface) and the Graetz number. The effect of the flow regime, laminar or turbulent, is evaluated. Conversely to the static conditions, the concentration of plasticizer at the exit of the tube decreases with time due to the renewing of the liquid showing that the compliance with the EU regulation 10/2011 can be fulfilled if the residence time of the liquid in the tube is controlled.