Infrared Spectra of U.S. Automobile Original Topcoats (1974–1989): III. In Situ Identification of Some Organic Pigments Used in Yellow, Orange, Red, and Brown Nonmetallic and Brown Metallic Finishes—Benzimidazolones

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Abstract A number of unidentified absorptions, which appeared to be due to specific color-imparting pigments, were observed in infrared spectra of some U.S. automobile original topcoats (1974–1989) from the Reference Collection of Automotive Paints. In previous work of this study, several inorganic pigments were identified which were responsible for some of these. Unlike the inorganic pigment absorptions, which were usually broad and few in number, the remaining unidentified absorptions were sharp and numerous, indicative of organic compounds. Because these peaks are narrower than most binder absorptions, spectral subtraction can be a useful means to help delineate pigment peaks obscured by binder features. Using this technique, several organic pigments have been identified. In cases in which the pigment peaks are prominent, the specific pigment responsible can usually be identified based on the peaks observed between binder absorptions or superimposed upon them, without having to use spectral subtraction. This is possible even when absorptions of two or more pigments are present. This paper discusses the analysis of benzimidazolone pigments, which were identified in some yellow, orange, red, and brown nonmetallic single layer Reference Collection topcoats, and a few brown metallic ones. Absorptions of these particular pigments were not observed in spectra of any Reference Collection topcoats produced before 1977. Use of the benzimidazolones—especially Benzimidazolone Orange—increased in the 1980s because they were common replacements for lead chromate pigments, which were phased out of use in U.S. automobile original finishes during this period.