Surface stability of polypropylene compounds and paint adhesion

Abstract

Thermoplastic olefins based on polypropylene compounds are being increasingly used for the production of painted automotive parts. The poor adhesion properties of these compounds are improved with flaming, which results in good adhesion for waterborne paints. The surface stability and adhesion properties of two commercial injection-molded compounds and their base polymer blends (polypropylene /ethylene-propylene rubber) were investigated after they were flamed with various parameters under typical vapor jet conditions. The compounds and the corresponding base blends showed mainly the same behavior. Thus, filler particles and additives seemed to have only a minor influence on the adhesion properties of these compounds. For the characterization of the surface itself and the near-surface region, scanning and transmission electron microscopy, XPS, and microthermal analysis were used. The utmost surface layer for all specimens consisted of a skin of pure polypropylene. The oxygen concentrations at the surface after flaming were rather similar for both compounds. Differences could be found in the surface roughness, the oxygen diffusion from the interior to the surface, and, probably most importantly, the viscosity and elasticity of the impact modifier. © 2005 Wiley Periodicals, Inc.