Characterization of the acid–base properties of NH3 or O2 plasma-treated polypropylene films and the effects of ageing

Abstract

The acid–base properties of polypropylene (PP) films treated by O₂ and NH₃ plasmas have been studied using contactangle measurements. The work of adhesion was determined by using test liquids with different pH values. Bromonaphthalene as a non-polar liquid was chosen to calculate the dispersive component of the solid surface energies. X-Ray photoelectron spectroscopy (XPS) was employed as a complementary surface diagnostic technique to characterize the different moieties grafted to the surface. The molecular orbital (MO) version of the Lewis definitions has been applied to acid-base interactions at the surface. The NH₃-treated PP showed a basic character for treatment times of the order of 0.7–1 s. With ageing, the latter was no longer clearly observed and the surface seemed to exhibit an amphoteric character. In contrast, the O₂-treated PP showed an acidic character for short and long treatment times. These properties remained dominant even for aged samples. It has been clearly shown that in the case of plasmatreated surfaces, the dispersion component of the total work of adhesion, W_sl^d, obtained by the Hüttinger method (K. J. Hüttinger et al., J. Adhesion Sci. Technol., 1992, 6, 317; Carbon, 1991, 29, 449) differed greatly from that calculated by using bromonaphthalene (F. M. Fowkes et al., Langmuir, 1991, 7, 2464). These results allow the acid-base component of the total work of adhesion W_sl^ab to be separated in two types of interaction: the first one, W_sl^abl(type I), should include the strong interactions and should reveal the dominant character of the surface. The second, W_sl^abll(type II), expresses the weaker contribution of the acid–base interactions, its importance depending on the hydrophilic character of the surface.