Infrared spectra of North American automobile original finishes. XIV: Identification of Naphthol Red (C.I. Pigment Red 170) in finish layers and in color-coordinated primers

The naphthol reds are a family of intermediate performance monoazo organic pigments and three of its members, C.I. Pigment Red 148, C.I. Pigment Red 170 (Naphthol Red), and C.I. Pigment Red 188, have been cited as having original automotive paint applications. This study sought to determine which were used in North American vehicle original finishes (1974 to 2019) and how they can be differentiated and identified in situ in paint infrared spectra. Naphthol Red was the only such pigment found and based on a comprehensive survey, absorptions of this pigment were identified in spectra of 12 of the 373 (3%) red or maroon nonmetallic original finishes examined. Identification of Naphthol Red in an unknown finish can thus serve to limit the number of possible source vehicles and spectra of the twelve are presented to facilitate this examination, along with a list of the vehicles that the twelve were used on. For a comparative analysis, identification of this pigment in questioned and known specimens may provide some means of assessing the significance of the finding. Two Inmont red nonmetallic acrylic melamine basecoats, one of which contains Naphthol Red, were found to have quite unusual infrared spectra that are readily distinguished from those of the other 371 finishes examined, and they are discussed. Naphthol Red was also identified in some color-coordinated automotive paint primers and its use appears to be more common in these than in the finish layers.