Application of Raman and Infrared Microscopy for the Forensic Examination of Automotive Clear Coats and Paint Smears

As submitted by the proposer:

Modern automotive paints typically use thin undercoat and color coat layers protected by a thicker clear coat layer. All too often, a clear coat is the only layer of automotive paint left at the crime scene. Current approaches used by PDQ, the largest forensic automotive paint database in the world, to identify clear coats have been unsuccessful because the FTIR spectra of clear coats are too similar to generate accurate hit lists by searching clear coat FTIR spectra alone.

Recently published studies of pattern recognition methods applied to FTIR spectra of clear coats have shown that information about the line and model of the vehicle can be obtained from these spectra. To further enhance the general discrimination power of clear coats, Raman spectroscopy and pattern recognition techniques will be investigated as a potentially better solution to the problem of extracting investigative lead information from clear coats. The chemical formulation of modem automotive clear coats is either acrylic melamine styrene or acrylic melamine styrene polyurethane.

Both Raman and FTIR spectra of clear coats contain distinct and recognizable peaks for melamine, styrene, and urethane. As for the acrylate component, there are eight distinct acrylate monomers present in a typical clear coat formulation. Although FTIR spectra only exhibit the carbonyl stretch characteristic of all acrylates, Raman spectra contain acrylate bands not only of the carbonyl but also of the C-0-C symmetric stretch which is characteristic of specific acrylate monomers.

The general discrimination power of Raman for automotive paint comparisons involving 500 General Motors clear coat samples (spanning 25 assembly plants for the production years 2000-2006) will be compared to results previously obtained using FTIR. A procedure to simulate the type of paint smear generated in vehicle-vehicle or vehicle-pedestrian hit-and-run collisions will also be developed as part of this proposed research. Currently, paint smears encountered in a vehicle collision cannot be created in a laboratory. To this end, the researchers will develop three alternative testing accessories to be adapted using a standard Charpy impact tester. This will allow the researchers the capability to acquire and analyze a large number of paint smears on metal, plastic, and fabric surfaces.

The proposed technique can simulate an automobile collision with controllable collision speed and direction as well as momentum/energy transfer. The long-term goal of the proposed paint transfer effort is the development of forensic standards for automotive paint transfer and paint smear analysis.

Note: This project contains a research and/or development component, as defined in applicable law, and complies with Part 200 Uniform Requirements - 2 CFR 200.210(a)(14).

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