Infrared and Raman spectroscopy of automotive paints for forensic identification of natural weathering

Abstract

Natural weathering of automotive paint modifies the original chemical composition of this multi-layered material. As a result, the reliability of spectral forensic analysis decreases depending on the degradation level of the sample. In this work, impounded vehicles with different colors (white, red and silver) containing a similar clearcoat composition (acrylic urethane styrene) were analyzed by Infrared and Raman spectroscopy. After being seized by the Brazilian Police, these vehicles were left exposed to the outdoor environment for up to seven years. Paint samples were extracted from two vehicle panels of different degradation levels for chemical comparison. For rapid and full determination of changes in the composition of paint layers, we collected in situ IR and Raman spectra from the surface of the paint chips. We also recorded Raman images of cross-sections to show an effect of alternation in stratigraphy and the composition of paint layers on the routinely used in situ analysis. Both modes of IR and Raman measurements are suitable to reveal degradation processes of vehicle paints and their analysis revealed that the deterioration process is strongly dependent on several factors such as the exposure time to natural weathering, the color of the vehicle and the degradation degree of the protective layer – clearcoat. This case report not only describes valuable data of natural weathering of original manufacturer paints but also shows the importance of proper sample collection and analysis of different vehicle panels that forensic scientists shall take into consideration at the crime scene.