Infrared microscopic analysis of multiple layers of automotive paints

A five-layer paint system for sheet molding compound (SMC), consisting of molded coating, conductive primer, primer surfacer, basecoat, and clearcoat, was analyzed by infrared microscopy. The results of three sample preparation methods were compared: polishing the paint-SMC at 5°and 90° to the top surface, and microtoming a paint chip. The quickest and easiest method was taking reflection spectra of the individual paint layers polished at 5° to the top surface. The best infrared spectra were obtained by the difficult procedure of microtoming a cross section of the paint and analyzing the individual layers by transmission spectroscopy. Experience has shown that the 5° reflection method is the preferred method and the one we most frequently use.