Paints and Coatings

1
Paints and Coatings
2
The Case of the Careening Motorcycle

• 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
• There was another police car in the area but the
  driver claimed that his car was not involved in
  the incident.

3
Careening Motorcycle (2)

• 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.
• No visible damage was found on the other police
  car.

4
Careening Motorcycle (3)

• 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?

5
Introduction

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

6
Definitions

• 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

7
What are some of the uses for paint?

• Art works
• Decoration (residential paints)
• Protection of a surface (rust inhibiting auto
  paints)
• Warnings (reflective paint, fluorescent paint)

8
Paint Cross Section
9
Other Coatings

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

10
How does paint dry?

• 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
• Solvent evaporation
• Rust proof paints
• Heating (thermosetting)
• Automotive
• Oxidation
• Drying oils, artistic paints
• Linseed oil

11
Paint As Evidence

• Paint chips Pieces of paint which have come off
  surface. They usually contain all of the layers
  of the paint
• 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

12
Analysis of Paint

• 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)

13
Paint Cross Section
14
Analysis of Paint

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

15
Pyrolyzing Unit
16
Analysis of Paint

• Pyrolysis GC

17
Analysis of Paint

• 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

18
FTIR of Automobile Paint
19
Analysis of Paint

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

20
The Case of the 11 Layer Paint Chip

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

21
The Case of the 11 Layer Paint Chip

• Paint examiner testified that paint found at
  scene CAME FROM THE SUSPECT CAR (individualized
  chip to car)
• Defense attorney asked examiner the following
  hypothetical question What would your
  conclusion have been if there had been only 10
  matching layers?

22
The Case of the 11 Layer Paint Chip

• Where is the defense attorney going with this
  line of questioning?
• Why does this question present a problem for the
  examiner?
• How would you answer defense attorneys question?

23
The Case of the 11 Layer Paint Chip

• 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.