Temperature influence on moisture transfer through synthetic films

Water transfer through films was investigated as a function of the temperature, the physical state of water and the relative humidity (RH) difference. The films were two synthetic packagings: a hydrophobic one, the polyethylene, a hydrophilic one, the cellophane. Some phenomena which could occur at low temperatures were discussed, such as polymer structure change due to mechanical relaxation or network plasticisation and modification of the penetrant diffusion at its melting temperature. But it seems that the comparison of water permeabilities (WPs) with temperature, especially at subzero temperatures, requires the correction of their calculation with the difference of water chemical potential (Δμ) instead of the water vapour pressure difference (Δp). The predominant factor of the water transfer is the kinetic factor, i.e. the water diffusion; nevertheless, the thermodynamic parameter, i.e. the water sorption, linked to the physical state of water, affects the barrier efficiency of water-sensitive films inducing a water liquid permeability (WLP) higher than that of water vapour (WVP). For hydrophobic polymers, such as polyethylene, the water transfer depends only on the RH difference of both sides of the film.