Homogeneity of automotive paint system: Evaluation of chemical variability and degradation

Automotive paint traces are common due to their ease of transfer during collisions between vehicles. These traces are frequently found in hit-and-run cases, and the use of databases can provide a list of potential makes, models, and years of production. It is well known that automotive paintwork is not homogeneous and can sometimes show variations in the number of layers (repairs, repaints), thickness and composition. Degradation due to external environmental conditions can also introduce differences in homogeneity within a vehicle. While it is generally known that a paint comparison sample should be taken in an area close to the damaged area, due to the heterogeneity of the paint application, the homogeneity of automotive paint over the entire body of a car is a subject that has received little attention in the literature. Therefore, the homogeneity of the paint system of eight vehicles was evaluated using microscopic examinations, spectroscopic analysis (infrared and Raman) and principal component analysis (PCA). Variability in the number and thickness of paint layers is observed in all of the different vehicles sampled. PCA of Raman and FTIR spectra reveals the presence of discrimination according to location, degradation and aftermarket refinishes. Degradation is evaluated by calculating the photo-oxidation index (POI). The results provide additional information when comparing references and paint traces taken from real cases and help to understand possible differences depending on the sampling location.