Permeation and Sorption of Water and Gases Through EVA Copolymers Films

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Abstract
The transport of water and gases (pure oxygen and carbon dioxide) through poly(ethylene-co-vinyl acetate) (EVA) films of different VA contents and through a low density polyethylene LDPE (used as reference with 0 wt.% VA), was analysed by permeation and sorption measurements. In the case of water, for EVA of 70 wt.% VA, a plasticization effect on the material was observed. For EVA of 19 wt.% VA, there was no plasticization, while for EVA of 4.5 wt.% VA and for LDPE, the water diffusion coefficient decreases with increasing water. An empirical law was then found to account for the reduction of the water diffusivity. A negative plasticization effect was attributed to the formation of water clusters in these rather non polar polymers. This latter result was confirmed by sorption measurements and infra-red spectroscopy. The increase in water sorption extent with the VA content leads to a steady increase in the water permeability in the EVA copolymers. In the case of gas permeation, both for O2 and CO2 and whatever the VA content of the copolymer used, the experimental curves are characterized by a constant diffusion coefficient. This is confirmed by sorption experiments. In terms of gas permeability, EVA copolymers are characterized by low coefficients especially for oxygen. In the EVA rubbery copolymers, The O2 solubility coefficient does not increase with the VA content due to the lack of polar interactions, while the CO2 one increases due to the overall interactions between CO2 induced dipoles and the polar the VA carbonyl groups. For these copolymers, the H2O/CO2 and H2O/O2 selectivities are very high. The experimental values of the selectivities indicate that these EVA copolymers appear as good candidates for applications using high perm-selectivity properties.

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