Paints and Coatings

The Case of the Careening Motorcycle n A motorcyclist is charged with drunk driving (motorcycling?) and with damaging a police car. He claimed that he was driving his motorcycle on Woodward Ave. in Detroit and was sideswiped by a white police car which caused him to hit another white police car that was parked on the street n There was another police car in the area but the driver claimed that his car was not involved in the incident.

Careening Motorcycle (2) n White paint smears and chips were found embedded in parts of the blue motorcycle and some flecks of blue paint were found in the white paint of the parked motorcycle. n No visible damage was found on the other police car.

Careening Motorcycle (3) n Of what value would paint analysis be in this case? Could it determine who is telling the truth? Is there any other physical evidence that could determine ground truth in this case?

Introduction n Chemistry of paint very complex n Forensic role is to compare paint evidence from scene to known source n Individual evidence only in cases where large enough paint samples exist to physically match

Definitions n PAINT: A suspension of pigment in a film former (also called vehicle)  Usually not a solution. Pigment is suspended in film former  Pigment is usually mixture of inorganic metal salts. Purpose is to impart color  Film former is usually organic polymer(s). Purposes are to protect surface and hold pigments

What are some of the uses for paint? n Art works n Decoration (residential paints) n Protection of a surface (rust inhibiting auto paints) n Warnings (reflective paint, fluorescent paint)

Paint Cross Section

Other Coatings n Varnish: A film former, commonly polyurethane, dissolved in a solvent, which normally doesn’t contain a stain. Purpose is to protect wood n Stain: A mixture of organic dyes dissolved in solvent. Made to penetrate wood and stain in. Does not protect n Enamel: originally a glossy, thermosetting paint. Now any paint which dries glossy

How does paint dry? n When paint dries, a thin polymer film is formed that adheres to the surface and suspends the pigments. Paint dries by a number of different mechanisms: n Solvent evaporation  Rust proof paints n Heating (thermosetting)  Automotive n Oxidation  Drying oils, artistic paints  Linseed oil

Paint As Evidence n Paint chips: Pieces of paint which have come off surface. They usually contain all of the layers of the paint n Paint smears: The top layer of paint which has loosened owing to being wet or air oxidation. Smears onto another surface after brushing contact. Layer structure is not present

Analysis of Paint n Color layer analysis  Requires cross section of paint chip  Cannot be done on paint smears  Same number and order of layers  Relative thickness must be same  Is a class test (but see case at end of lecture)

Paint Cross Section

Analysis of Paint n Solubility  Use solvents such as acetone, dichloromethane, pyridine. Acrylic lacquers are soluble in acetone n Pyrolysis GC  Analysis of film formers  Bulk technique, all layers analysed together

Pyrolyzing Unit

Analysis of Paint n Pyrolysis GC

Analysis of Paint n FTIR  Will determine type of film former  Does not show pigments unless they are organic  Is a bulk method: if more than one layer is present, IR will be a composite

FTIR of Automobile Paint

Analysis of Paint n Electron microscopy  Inorganic pigments  Excellent for single, top layer analysis and paint smears n UV-visible spectrophotometry  Must be able to separate pigment from film former and dissolve it

The Case of the 11 Layer Paint Chip n Hit and run case n Paint chips left at crime scene on victim n Chips had 11 distinct layers of various colors n Samples taken from suspect auto showed 11 layers with matching color- layer sequencing

The Case of the 11 Layer Paint Chip n Paint examiner testified that paint found at scene CAME FROM THE SUSPECT CAR (individualized chip to car) n Defense attorney asked examiner the following hypothetical question: “What would your conclusion have been if there had been only 10 matching layers?”

The Case of the 11 Layer Paint Chip n Where is the defense attorney going with this line of questioning? n Why does this question present a problem for the examiner? n How would you answer defense attorney’s question?

The Case of the 11 Layer Paint Chip n One possible answer to question: “I cannot give you a definitive answer. I take each case on an individual basis and reach a conclusion based on the evidence in that case. It would not be possible to generalize about an optimal number of layers of paint that are present.”