8.4 Notes

Examination of Paint

Objectives

1. List the most useful examinations for performing a forensic comparison of paint

2. Describe proper collection and preservation of forensics paint evidence

Paint

One of the most prevalent types of physical evidence received by a crime laboratory.

from a hit and run from a tool during a burglary

Paint is composed of a binder and pigments dissolved or dispersed in a solvent

Vocabulary

Pigment – color and opacity Binder – support medium for pigments

and additives (polymer) Solvent – evaporates after application

leaving the hard polymeric binder and pigments

Automobile finishes

The most common types of paint examined

Manufacturers apply a variety of coating to the body

Adds diversity to automobile paint

Automotive finishing for steel Electrocoat primer

first layer consisting of epoxy-based resins provides resistance from corrosionusually black to gray

Primer surfacerapplied before base coatcompletely smooth out and hides seems and

imperfectionshighly pigmented

Basecoat layer provides color and aesthetics of the finishusually acrylic based and organic based pigmentssome have aluminum flakes and metal oxide must be able to resist UV radiation, acid rain, and

weather conditions Clearcoat

unpigmented clearcoat used for durabilitymost are acrylic based and some polyurethane appearance = glossy finish

Stereoscopic microscope

Used for comparing color, surface texture, and color layer sequence

Most specimens do not have a layer structure that allows the criminalist to individualize a chip to a single source

Chemical composition does provide points of comparison between specimensmust include a chemical analysis of either

pigments or binder composition

Pyrolysis gas chromatography Useful technique for distinguishing paint

formulations Paint chips or polymer chains are decomposed

by heat into gas products then sent through a gas chromatograph

Different polymers show a distinct patterndetails the chemical makeup of a binder or a paint

Infrared spectrophotometry Analytical technique used for binder

composition Inorganic pigments of paints can be

identified byX-ray diffraction X-ray spectroscopy Emission spectroscopy Neutron activation analysis

Collection and preservation Picked up with tweezers or scooped up

with paper Use druggist fold or Glass/plastic vials Collect whole garments or objects in

which paint chip is embedded Standard/reference samples needed of

all paint layers