Influence of stretching ratio and salt concentration on the porosity of polypropylene films containing sodium chloride particles

The incorporation of salt in a polymer matrix could potentially achieve a humidity-regulating film system of high porosity for packaging applications. In this study, sodium chloride (NaCl) powder was used as a model substance for humidity-regulating fillers in polypropylene films. A polypropylene homopolymer was extrusion-blended with NaCl at concentrations of 0.03, 0.06, 0.12, and 0.24 g NaCl·g film-1. Films were produced and were subsequently oriented mono and biaxially. At stretching ratios (SRs) of between 2 and 25 a porosity of 20% v/v to 50% v/v was achieved. The porosity positively correlated with the SR. An empirical model based on ellipsoids that approximated the correlation between the SR and the porosity was developed. This correlation offers a basis for further studies on other salt-polymer systems that have undergone differing orientation processes.