Modelling and numerical simulation of water vapour sorption kinetics in humidity regulating polypropylene films containing sodium chloride

This study proposes a mathematical time-dependent model to examine the humidity sorption kinetics and the storage capacity of a porous packaging polymer material (PP) containing NaCl as hygroscopic substance. Water vapour pressure gradient was chosen as the principal driving force for the transport of water vapour in the film. Solving the diffusion equation numerically by the finite element method resulted in the evolution of water vapour contents calculated inside humidity regulating film specimens. The influence of the diffusion and the sorption coefficients of the polymer, and of the thickness and the NaCl content of the film on kinetics and capacity of the humidity regulating films could be determined. By comparison of the calculated amount of absorbed water vapour with experimental data, a good agreement between the predicted tendencies of the numerical and experimental results was found. The model was validated in a region of the salt's sorption isotherm above the deliquescence point of NaCl and significantly below the humidity at which the holes in PP are completely filled with water. The results permit to develop optimized humidity regulating films for individual food requirements. Application of this model could facilitate the development of these active films using different polymers and hygroscopic substances.