PAINT MANUFACTURING AUTOMOTIVE FINISHES
One of the most commonly encountered kinds of paint evidence is automotive paint. Cars, trucks, and similar vehicles are so deeply integrated into our daily lives; it’s easy to see why this is so. Automotive paints are also a good example of how manufacturing styles and variation contribute to the significance of forensic evidence. The automotive finishing process for vehicles consists of at least four separate coatings. The first is a pretreatment, typically zinc electroplating, applied to the steel body of the vehicle to inhibit rust. The steel is then washed with a detergent, rinsed, treated a second time, and then washed again. The significance of this coat ing is for the forensic paint analyst to be aware that any zinc found during elemental analysis may come from this coating and not necessarily the paint itself. The second coating is a primer, usually an epoxy resin with corrosion-resistant pigments; the color of the primer is coordinated with the final vehicle color to mini mize contrast and “bleed-through.” The steel body of the vehicle is dipped in a large bath of the liquid primer and plated on by electrical conduction. The primer coating is finished with a powder “primer surfacer” that smooths the surface of the metal and provides better adhesion for the next coating. The topcoat is the third coating applied to the vehicle and may be in 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, shown in Figure 1. Topcoat chemistry is moving toward water-based chemistries to provide a healthier atmosphere for factory workers and the environment. For example, heavy metals, such as lead or chrome, are no longer used in the
formulation of topcoats. Metallic or pearlescent coatings, growing in preference for new model vehicles, have small metal or mica flakes incorporated to provide a shimmering, changing color effect. Metallic pigments, including zinc, nickel, steel, and gold-bronze, give a glittering finish to a vehicle’s color, while pearles cent pigments, mica chips coated with titanium dioxide and ferric oxide try to replicate the glowing luster of pearls. The topcoat is often applied and flashed, or partially cured, and then finished with the next and final coating, the clearcoat. Clearcoats are unpigmented coatings applied to improve gloss and durability of a vehicle’s coating. Historically, clearcoats were acrylic-based in their chemistry, but nearly half of the automotive manufacturers have moved to two-component urethanes. It is important for forensic paint analysts to keep up-to-date with the latest trends and techniques in the paint industry. It is equally important, however, for them to be aware of the previously used formulations and manufacturing techniques because they constitute the bulk of vehicles currently on the road. A three- to five-year-old pickup truck is far more likely to be encountered in a forensic case than the newest model sports car, so forensic analysts must not be surprised by “history.” Repaired and repainted vehicles are an additional consideration because they may have been coated with virtually anything, including spray paint! A final note on vehicle coloration is that of the newer plastic substrates. Vehicle bodies are no longer made exclusively of steel; various plastics are now commonly used. For example, fenders may be nylon, polymer blends, or polyurethane resins; door panels and hoods may be of thermosetting polymers; front grills and bumper strips have long been plastic or polymer but now may be colored to match the vehicle. Braking systems, chassis, and even entire cars (BASF unveiled an entirely plastic car in 1999, as an extreme example) are now constructed from plastics. It wouldn’t be unusual for forensic paint examiners to encounter steel, aluminum, and polymer parts on the same vehicle, each colored by a very different coating system.
FIGURE 1 A green vehicle paint chip, showing the layer structure common to most automotive paints.
