Chapter 11: Forensic Examination of Paint

1. Chapter 11: Forensic Examination of Paint By: Taylor Miguelino Period 3

2. Composition of Paint • ELECTROCOAT PRIMER • Location: applies to the steel body of the car • Composition: epoxy based resins • Purpose: provide corrosion resistance • Characteristics: uniform in appearance/ thickness, range from black to gray

3. Composition of Paint • PRIMER SURFACER • Location: applied before base boat • Composition: epoxy-modified polyesters/ urethanes • Purpose: smooth seams/ imperfections for basecoat application • Characteristics: highly pigmented= pigments minimize color contrast between primer and topcoats (ex: light gray primer under pastel blue car)

4. Composition of Paint • BASECOAT • Location: over basecoat • Composition: acrylic-based polymer with automotive pigments (composition determined by toxicity/ environmental impact= tend to be natural pigments like pearl) • Purpose: provide aesthetic appeal through color, and resist weather, UV rays and acid rain • Characteristics: besides pigments, aluminum flakes may be used to add a metallic appearance

5. Composition of Paint • CLEAR COAT • Location: outermost layer • Composition: acrylic-based, but polyurethane clear coats are becoming popular • Purpose: improve gloss, durability, and appearance • Characteristics: provide etch resistance

6. Paint Under the Microscope • Traditionally, the microscope was the most important instrument for location and comparing samples, and still is today • Samples best compared side by side under a stereoscopic microscope for color, texture, and color layer sequence

7. Paint Under the Microscope • Why is color layer sequence so important? • Matches in number of layers and sequence of colors allow relation to a common origin • No accepted number of matches must be obtained to make a connection • Depends on the uniqueness of each layer’s color an texture and the frequency the color combination is under study

8. Paint Under the Microscope • Most samples do not have a layer structure specific enough to individualize it and piecing together chips is unrealistic • Chemical analysis is used for comparison of pigments, binders, or both

9. Analytical Techniques in Comparison • Paint manufacturers make finishes in hundreds of varieties • Useful when trying to associate a sample from thousands of similar models produced in any one year • Although a standard of quality must be maintained, batch formula of any paint binder can vary depending on availability and cost of ingredients= variations year to year

10. Characterization of Paint Binders • Process of pyrolysis gas chromatography is used (refers to the decomposition of organic matter by heat) • Paint cannot be readily dissolved in a solvent to be used in gas chromatography= must be pyrolyzedat temperatures 500-1000 C to promote decomposition • Pyrolyzed material is characterized by a graph called a pyrogram

11. Characterization of Paint Binders • Disadvantage of pyrolysis= invaluable for distinguishing most formulations (most ingredients common) • Results on pyrogram reflect the chemical make up of the binder

12. Characterization of Paint Binders • Alternative to pyrolysis= infrared spectrophotometry • Binders selectively absorb infrared radiation to yield a spectrum specific to the sample

13. Characterization of Paint Pigments • Inorganic pigments of paints can be identified by: • Emission spectroscopy= detects elements based on selective absorption of UV, visible, or infrared radiation • Can detect 15-20 elements based on this fact- some are very common and thus have little forensic value, but rarer ones serve as excellent points of comparison

14. Characterization of Paint Pigments • Two types of spectrums: • Continuous= type of emission showing a continuous band of colors all blending into one another (ex: incandescent bulb) • Line= type of emission showing a series of lines separated by black areas. Each line represents a specific wavelength (ex: sodium lamp)

15. Characterization of Paint Pigments • How emission spectroscopy is used with paint: • When a solid or liquid is vaporized and “excited” by heat each element emits a light composed of select frequencies • Spectrum= “fingerprint” for comparison and identification • Other methods of characterization: neuron activation analysis and x-ray spectroscopy (which are less common)

16. Inductively Coupled Plasma Emission Spectrometry (ICP) • Identifies and measures elements through light energy emitted by excited atoms • Instead of using electrical arc, atoms excited by hot plasma torch (torch is 3 concentric quartz tubes with argon gas and radio-frequency coil that carries a current wrapped around the tubes to create a magnetic field)

17. Inductively Coupled Plasma Emission Spectrometry (ICP) • Process of ICP: • Spark causes electrons to be stripped from their argon atoms, creating a chain reaction resulting in the gas being broken down into argon atoms, argon ions, and electrons (inductively coupled plasma discharge) • Plasma discharge acts like a continuous flame • Sample in aerosol form is introduced to the plasma and colliding electrons produce ions= ions emit light with characteristic wavelengths

18. Significance of Paint Evidence • Comparison of surface colors is never enough • Paint layers offer valuable points of comparison • Subtle differences in paint binder • Differences in the elemental composition of paint • Statistics show significance: • If a suspect sample and a reference sample are compared, the odds against the crime-scene paint originating from another randomly chosen vehicle are approximately 1 to 33,000= strong link

19. Significance of Paint Evidence • Evidence most often used in identifying unknown vehicles in hit-and-runs • Not usually possible to state exact make and model of car in question because any one paint could be used on many cars

20. Significance of Paint Evidence • Color charts available from paint manufacturers and refinishers • Since 1975 Royal Canadian Mounted Police Forensic Laboratories have been collecting data on paint in the Paint Data Query (PDQ) • PDQ allows analysts to access information on paint in relation to make, model, and year • Also information on layer colors, primer colors, and binder composition on PDQ

21. Significance of Paint Evidence • Some labs maintain in-house collections of paint information

22. Collection and Preservation of Paint Evidence • Evidence usually found on or near people/ objects involved in hit-and-run incidents • Collection with tweezers/ scooped up with a piece of paper common (bindles and plastic viles good for collection) • If paint is smeared/ embedded in objects the sample should not be removed- the whole object must be examined

23. Collection and Preservation of Paint Evidence • Standard samples must be collected from an undamaged area of a vehicle in suspect and close to the area suspected to have contacted the victim • Sample must be removed to include all layers down to the bare metal= scalpel or blade used to obtain ¼ inch square • Cross transfer from contact of 2 vehicles requires similar procedure where sample is obtained near point of contact

24. Works Cited • Richard Saferstein Forensic Science: An Introduction Textbook • All photos from Google Images