Comparison of oxidative treatments of polypropylene films and fibers to promote zinc oxide nanoparticle adhesion

Ozone, heating-in-air, and chromic acid treatments of polypropylene (PP) films and fibers were evaluated for their ability to promote ZnO nanoparticle (NP) adhesion. X-ray photoelectron spectroscopy (XPS) demonstrated that the oxygen content of the surface of PP films increased due to all treatments, with high binding energy O1s components emerging consistent with carbonyl, carboxylic acid, ester and hydroperoxide groups. The latter two were dominant for chromic acid treatment. The XPS measurements were complemented by attenuated total reflectance FTIR spectroscopy studies. Pull-off measurements from PP films, using a ZnO particle attached to the cantilever of an atomic force microscope, indicated that ozone and chromic acid treatments were equally effective at increasing adhesive forces. However, scanning electron microscopy studies of PP fibers treated with either ozone or chromic acid, and subsequent immersion in an ethanolic ZnO NP suspension, showed that ozone treatment was more effective than chromic acid treatment at promoting adhesion. This likely results from the poorer ability of ester and hydroperoxide groups, compared to carboxylic acid ones, to bind to hydroxylated ZnO NPs. It was further shown that excessive ozone treatment leads to brittleness of the fibers, as evidenced by cracking when the fibers are stirred. These results indicate that moderate ozone treatment of PP films and fibers may be a convenient method to attach metal oxide particles to PP surfaces.