HYDROPHOBIC FILMS AND THEIR EFFICIENCY AGAINST MOISTURE TRANSFER .1. INFLUENCE OF THE FILM PREPARATION TECHNIQUE

The barrier properties of two hydrophobic materials, oil and wax paraffins, were studied as a function of the characteristics of the technique employed to prepare hydrophobic films with a constant content of paraffin. Three cellulose derivatives with different polarity and porosity were used as supports to prepare films by several techniques: emulsion, emulsion plus coating (methylcellulose), and dipping (porous filter paper and nonporous cellophane). Results show that the ability of the hydrophobic substances to retard moisture transfer depends on the homogeneity of its final repartition in the matrix and/or on the surface. The least efficient films to retard the movement of water correspond to the highly heterogeneous systems (emulsion and dipped filter paper) independent of the nature of the substance as seen by scanning electron microscopy. The most efficient difference, a 100-fold in water vapor permeability, is obtained in the cellophane films with homogeneous repartition of paraffin wax independent of thickness and relative humidity. This behavior is in contrast to that of the paraffin oil.