lecture 10: paint analysis

what cases involved paint most frequently?

• hit and run cases

• burglary

what is the composition of paint?

• pigment

• additives (dissolved in solvent)

• binders

What is the electrocoat primer?

• epoxy based resins

• provide resistant from corrosion

• black to gray

What is the primer surfacer?

• epoxy modified polyesters or urethanes

• highly pigmented and hence minimize contrast between primer and topcoat

• smooth out imperfections

What is the basecoat/colorcoat?

• acrylic based polymer

• provide color and aesthetic of the car

• resist weather/UV radiation/Acid rain.

What is the clear coat?

• acrylic based or polyurethane based

• unpigmented layer for gloss durability and appearance

Microscopic examination of paint

• color

• color layer sequence

• surface texture

What are the analytical techniques for examination of paint?

• X-ray spectroscopy for elemental analysis of paints

• pyrolysis gas chromatography (PGC) for comparison of paint binders

• infrared spectrophotometry for comparison of paint binders

X-ray spectroscopy: for elemental analysis of paints

uses SEM, some of these elements are relatively common to all paints and have little forensic value; others are less frequently encountered and provide excellent points of comparison between paint specimens.

Inductively coupled plasma (ICP) for elemental analysis of paints

the atoms are excited by a plasma torch, which is surrounded by a radio frequency (RF) coil that carries a current that creates a magnetic field. Argon gas flows through the torchm when a high-voltage spark is applied some electrons are stripped off. The argon atoms and ions and the eelctrons collide with one-another, continuing the chain reaction. The sample is introduced in the form of an aerosol, and when it collides with argon electrons it generates charged particles which can then be identified.

What is the paint data query?

• since 1975, royal canadian mounted police forensic laboratories have maintaned PDQ databses

• include information on primer colors, paint layer colors, and binder composition of automotive paints

• in many cases it is not possible to state the exact make and model of a car just based on paint color

How should intact paint chips be collected?

with tweezers or by a piece of paper from crime scene

How should a paint sample of approx 0.25 inches squared be collected?

using a clean scalpel or knife blade from an undamaged area

What should be used for storage of paint evidence?

Druggist fold on paper or glass/plastic vial. Location of each paint sample should be labeled on each package.

What happens to paint attached to garments?

should go to the laboratory undisturbed in a separate paper bag for examination.