Modulus and Chemical Mapping of Multi-layer Coatings
Aaron M. Forster, Chris A. Michaels, Li Piin Sung, Justin Lucas
Thermoplastic olefins (TPO) are polymeric materials utilized for interior and exterior automotive parts. These materials are often painted to enhance appearance and protect the soft TPO substrate. TPO materials posses a low surface energy due to the aliphatic backbone and adhesion promoters, such as chlorinated polyolefins (CPO), are used to improve coating adhesion. The penetration of the CPO into the TPO substrate is critical for achieving a strong adhesive bond, but determining the width of the penetration zone is challenging. Depth sensing indentation and confocal Raman microscopy are two surface sensitive techniques that can provide localized information on mechanical properties and chemical composition at the CPO-TPO interface, respectively. The cross-section of a painted TPO coupon was investigated using depth sensing indentation and confocal Raman microscopy. The impact of CPO structure and processing conditions on the degree of CPO interpenetration was investigated for the same TPO and acrylic melamine base/clear coat system (paint). Three CPO materials that varied in chlorine content and molecular weight were exposed to two different processing conditions. The interfaces, CPO/base coat or CPO/TPO, were chemically and mechanically sharp at the 1 m lateral resolution of both techniques. In addition, depth sensing indentation measured a gradient in the modulus of the cross-section of the clear coat coating that is related to processing. The combination of solvent evaporation and crosslinking reaction within the melamine coating leads to a modulus gradient through the thickness of the coating. The free air surface of the clear coat had a higher modulus than the interior of the clear coat.