Volume 43, Issue 3 (May 1998)
Infrared Spectra of U.S. Automobile Original Topcoats (1974–1989): IV. Identification of Some Organic Pigments Used in Red and Brown Nonmetallic and Metallic Monocoats—Quinacridones
A previous study in this series demonstrated the feasibility of identifying organic pigments in situ based on their absorptions in infrared spectra of automobile topcoats. To isolate pigment peaks, spectra of closely matched topcoats lacking pigment features are subtracted from spectra of topcoats containing the pigment of interest. Once identified by this means, these pigments can often be determined based on the more limited number of pigment peaks observed between the broader binder absorptions, or superimposed on them. Spectra of closely related pigments are used to provide some indications of how distinct a particular set of pigment absorptions is. Using these techniques, several different quinacridone pigments have been identified in single-layer U.S. automobile original topcoats (1974–1989) from the Reference Collection of Automotive Paints. Two of these pigments, Quinacridone Red Y and Quinacridone Violet, were found to be common in red nonmetallic topcoats. Quinacridones were also identified in brown nonmetallic and red and brown metallic finishes. A number of different pigment combinations of quinacridones with both inorganic and organic pigments have been identified, and examples of these are presented. As a prerequisite to the analysis of pigments using this in situ method, analysts should be very familiar with the absorptions of automotive topcoat binders.