Surface modification of polypropylene to improve its adhesion

Surface modification of a propylene homopolymer, an ethylene / propylene copolymer, and a rubber-modified polypropylene (PP) has been studied using X-ray photoelectron spectroscopy (XPS), chemical derivatisation, contact angle measurement, scanning electron microscopy (SEM), and an adhesion test. The wettability of PP surfaces was found to have been improved by solvent treatment. Mild adhesion level was obtained using solvent vapour treatment. Chromic acid etching and priming were found effective in improving the adhesion of PP. Flame treatment was found to give excellent adhesion. Optimum air-to-gas ratio and the distance from the inner cone tip of the flame to the polymer surface were found to be -11:1 and 0.5-1.0 cm respectively. The non-correspondence between oxygen concentration and contact angle at high oxygen concentrations was attributed to sub-surface oxidation and the orientation / migration of oxygen containing functional groups away from the near surface during the flame treatment. This hypothesis was supported by angle resolved XPS results. The use of simple surface composition models has shown that the oxidation depth induced by a mild flame treatment is between 40A and 60A, and that oxygen containing functional groups may have orientated or migrated a few angstroms away from the near surface for an intense flame treatment. Trifluoroacetic anhydride (TFAA) has been found to be a good derivatising reagent for hydroxyl groups. About 20% of the oxygen present on the surface of flame treated propylene homopolymer was found to be present as hydroxyl groups, while a slightly. higher percentage (-30%) was found for the two copolymers. While hydroxyl groups were found unimportant in the adhesion with an epoxy adhesive, they were found important, but not essential, in the adhesion with a polyurethane paint.