16.1: Paint

• Paint –  a suspension of pigments and additives intended to color or protect a surface.

• Pigment – a fine powder that is insoluble in the medium in which it is dispersed; are e intended to color and/or cover a surface; they may be organic, inorganic, or a mixture.

• Binder – that portion of the coating, other than the pigment, that allows the pigment to be distributed across the surface.

• Vehicle – refers to the solvents, resins, and other additives that form a continuous film, binding the pigment to the surface.

• Solvents – dissolve the binder and give the paint a suitable consistency for application.

Four Categories of Paints:

1. Architectural Paints – household paints, and are those coatings most often found in residences and businesses.

2. Product Coatings – applied in the process of manufacturing products including automobiles.

3. Special-Purpose Coatings – fulfill some specific need beyond protection or aesthetic improvements, such as skid resistance, waterproofing, or luminescence.

4. Art Paints –  are occasionally encountered in forgery cases. It is somehow the same as Architectural Paints, but many artists formulate their paints, leading to potentially unique sources.

Other Coating Definitions

• Enamel – A pigmented coating that has a high gloss when it dries.

• Lacquer – Clear or pigmented coatings that dry quickly through evaporation of the solvent.

• Latex – A suspension of a pigment in a water-based emulsion of several resins.

• Shellac – A solution of melted lac, a resinous excretion of the Lac insect dissolved in alcohol used as a sealant, adhesive, or insulating varnish.

• Stain – A solution of dye or a suspension of a pigment designed to color, but not protect, a wood surface.

• Varnish – A clear solution of oils and organic or synthetic resins in an organic solvent.

16.2: Paint Manufacturing

The automotive finishing process for vehicles consists of at least four separate coatings.

1. Pretreatment –  applied to the steel body of the vehicle to inhibit rust.

2. Primer – usually an epoxy resin with corrosion-resistant pigments; the color of the primer is coordinated with the final vehicle color to minimize contrast and “bleed-through.”

3. Topcoat – the form of a single-color-layer coat, a multilayer coat, or a metallic color coat; this is the layer that most people think of when they think of a vehicle’s color.

4. Clear coats – unpigmented coatings applied to improve the gloss and durability of a vehicle’s coating.

16.3: Paint Collection

• The collection of paint samples should proceed with caution.

• Nearly any object or surface may retain a paint transfer and may include things as varied as tools, architectural structures and elements (floors, wainscoting), glass fragments, fabrics, hairs, fingernails, roadways and signs, and, of course, vehicles.

• Evidentiary items with paint transfers should be packaged and submitted to the laboratory in their entirety, if possible.

• It is also important to remember that cross-transfer could have occurred.

• Known and questioned samples should be collected from both surfaces.

• Paint evidence should be first photographed and then removed manually with nonmetallic tools, such as small wooden sticks, toothpicks, or plastic forceps.

• If tape lifts are to be used, the paint evidence should be collected first

• Lifting or prying out loose flakes is one of the ways to remove flakes of paint from a surface.

• When a painted object strikes a glancing blow to another object, it can transfer paint in the form of a smear.

• It is important when collecting known paint samples, therefore, that they are collected from areas as close as possible to, but not within, the point(s) of damage or transfer.

• All paint samples should be clearly labeled as per origin, with drawings or photographs as documentation.

16.4: Analysis of Paint Samples

• A combination of microscopes (stereo, transmitted light, and polarized light) at magnifications of 2× to 100× is used to examine the layers in paint.

• Microtome – a minivice that holds a sample in place while a heavy and very sharp glass- or diamond-edged knife slices off sections of a few tens of microns thick.

• Polarized light microscopy (PLM) is appropriate for the examination of a layer structure as well as the comparison and/or identification of particles present in a paint film including, but not limited to, pigments, extenders, additives, and contaminants.

• Solvent and microchemical tests have been used to discriminate between paint layers of different pigment and binder compositions that are otherwise visually similar.

• Infrared (IR) spectroscopy can identify binders, pigments, and additives used in paints and coatings.

• FT-IR –  measures the absorption of IR energy, over a range of wavelengths, as different bonds in the molecule vibrate and move in characteristic fashions.

  • The analysis of paints by FT-IR can be done in transmittance and reflectance.

• Raman Spectroscopy – it is based on light scattering rather than absorption. It provides complementary information to that obtained from IR spectroscopy.

• Absorption spectroscopy, using a microspectrophotometer (or MSP for short), has been used to categorize and discriminate between otherwise visually similar paints.

  • Metamerism – the condition in which two colors appear similar under one set of conditions but different under others.

• Scanning electron microscope – can be used to characterize the structure and elemental composition of paint layers.

A consensus of forensic paint examiners is that the following factors strengthen an association between two analytically indistinguishable paint samples:

• The number of layers;

• The sequence of layers;

• The color of each layer;

• Cross-Transfer of paint between items.

• Paint Data Query (PDQ) – run by the Royal Canadian Mounted Police (RCMP) and is the largest international automotive paint database.

  • It contains nearly 20,000 samples of paint systems, which represent over 74,000 individual paint layers used on most domestic and foreign vehicles being sold in North America, Australia, and Asia.