Appearance and Spectroscopic Characterization of Automotive Coatings for Forensic Purposes

Automotive paint is a common type of trace evidence in the forensics community due to the prevalence of vehicular collisions. Forensic examinations of these samples typically involve visual inspection for physical properties such as color. However, these observations are subject to variability due to observer and environmental factors. Therefore, objective methods of color description based on spectrophotometry using coordinate systems are now recommended by US forensics standards in addition to chemical analysis for binder and pigment information. Color meters and microspectrophotometers (MSP) are common instruments for quantifying color descriptions using opponent coordinate systems such as the International Commission on Illumination CIEL*a*b* color space (CIELAB). MSP is used on small fragments encountered in forensic cases, but requires extensive sample preparation and expensive instrumentation while color meters are frequently employed by coating manufacturers and portable for field use. The two methods were compared by calculating color difference, DE*, between fifteen automotive coating samples and standards to gauge differences and associated errors. A correlation between color meter and reflectance-mode MSP DE* was found. After physical attributes are noted in forensic analyses, samples are often characterized by molecular spectroscopy. IR and Raman spectroscopic analyses were performed on the sample set to determine their utility in binder and pigment identification. Binder chemistries of all layers in the set were classified by IR spectroscopy, and Colour Index (CI) pigments in the base coats were identified by Raman spectroscopy. Finally, as UV exposure can alter the chemistry of automotive coatings over time, controlled artificial weathering of clear coats from two major manufacturers was performed at 55°C and 75% relative humidity (RH) with 100% ultraviolet (UV) radiation using the SPHERE (Simulated Photodegradation via High Energy Radiant Exposure) at the National Institute of Standards and Technology. Appearance and spectroscopic measurements of the samples were used to track photodegradation. Color and gloss were used as physical measurements, while IR and Raman spectroscopies were applied to quantify general binder degradation, and transmission-MSP was used to track UV absorber concentration. The combination of color measurements by spectrophotometry and chemical characterization by spectroscopy should enhance the objectivity of forensic examinations of trace automotive coating evidence