Theoretical analysis and experimental characterization of the TPO/adhesionpromoter/paint interface of painted thermoplastic polyolefins (TPO)

Exterior and interior automotive applications of TPO (thermoplastic polyole fin) resins, which are, often, composed of a paint coating over an injectio n-molded TPO, have increased interest in the surface chemistry and physics of TPOs. Specifically, the interface system composed of base-coat paint/adh esion promoter/TPO is of primary importance in controlling the paint adhesi on to the TPO. The major, active component in the adhesion promoter is a ch lorinated polypropylene (CPO). A theoretical model based on phase thermodyn amics and diffusion kinetics resulted in a prediction that the TPO/CPO inte rface should have a lower bound thickness of about 11 nm and an upper bound of about 400 nm. A battery of experimental strategies to characterize this interface system was discussed. Techniques used were transmission electron microscopy (TEM). atomic force microscopy (AEM) and scanning transmission X-ray microscopy (STXM). The near-surface morphology of both unpainted and painted, injection molded TPO plaques exhibited ethylene-propylene rubber p articles close to the surface, i.e. within the first 0.1-0.8 micrometer of the TPO surface, and no "overlayer" of transcrystalline polypropylene at th e surface of the TPO. Each of these microscopic methods shaved that the adh esion promoter/TPO interface was very sharp. The thickness of this interfac e was measured with respect to the interdiffusion of the CPO and TPO by STX M. The STXM measurements yielded an apparent interface thickness between th e adhesion promoter and TPO of 340 +/- 80 nm. This was in good agreement wi th the theoretical predictions.