Painting Plastics - American Coatings Association™

Those of you who paint plastics or formulate paints for them know that plastics are very different from metals and can be considerably more difficult to paint. Paints over plastics suffer many of the same problems as those applied to metals, but have a number of their own defects. One of the difficulties with painting plastics is that there are so many different plastics in use, each with its own surface properties and paintability. Due to having lower surface energies and polarities, many plastics tend to experience more wetting and adhesion problems than do pretreated metals. Substrates must be clean to ensure good coating adhesion and minimize defects. This is particularly true for plastics, which may have mold release agents on their surfaces and seem to have great affinities for oils, sanding dust, and other kinds of dirt. In addition, plasticizers or other components may come to the surface over time. These surface contaminants must be removed. However, not every bump or irregularity in a coated surface is due to dirt. Occasionally, what looks like dirt really stems from surface damage caused by molding problems or bad part handling practices. Plastics are commonly cleaned by a power wash followed by a solvent wipe. This process normally ensures paintability, but it must be audited periodically to determine whether parts really are clean. I have been in a number of plants where they were not. The cleaning process itself can cause problems. Detergent residues can hurt adhesion, result in water spotting and contribute to other appearance problems. Checking parts by eye after cleaning is useful, but testing wettability by one of the several available methods is even better. The standard technique involves the measurement of contact angles of paint, water, or solvent on the surface of parts (see ASTM standards D7334 and D7490), but it requires specialized equipment and is not practical in the field. There are several simpler wetting/dewetting tests in ASTM D7541 that can show if cleaning is effective. This document describes cotton swab, marking pen and drawdown techniques that simulate the application of a wet film. The swab and commercially available marking pen techniques are simple and rapid and are particularly useful for testing in the field or on curved, irregular, or porous surfaces where contact angles cannot be measured. Application techniques for coating plastics are similar to those used for metals (i.e., conventional air spray, airless, electrostatic (guns or bells), flow coat or even dip coating). However, conductive primers are needed if topcoats are to be applied electrostatically. Application can affect appearance in ways ranging from smoothness to color to aluminum or mica flake orientation. Often, these problems are blamed on the paint, but may be possible to solve by adjusting atomization pressures and flow rates and optimizing the ratio between them. Guns and bells can produce dirt-like spits and drops, especially if they are worn or damaged (not unusual in plastic painting facilities). Defects such as cratering, crawling, telegraphing, popping, and delamination occur over virtually all substrates. These have been covered by articles in JCT CoatingsTech and other publications. Additional defects found with the coating of plastics include fiber read-through, bondline readout, gassing, and micropopping. Fiber read-through in coatings over fiber-filled composites is a special case of telegraphing. The glass fibers in the composite are amplified instead of being hidden. This may be a fault in the composite in that the fibers are too close to the surface, but usually occurs when solvents in the paint penetrate and swell gloss paint for metal the surface producing an effect similar to grain raising in wood. It is important to use a primer that does not itself cause the problem and works well as a barrier to prevent topcoat solvents from getting to the composite. Bondline readout is a peculiar defect that occurs on baking of plastic parts with reinforcements. The bonded backing acts as a heat sink and the heat-up and cool-down behavior of backed and non-backed areas are different. This results in significant temperature gradients in these areas, which, in turn, cause surface tension gradients that drive flow. The result is the formation of noticeable steps and/or color differences along outlines of the reinforcements. Longer flash times and higher bake temperatures have helped to reduce readout. Gassing is a defect similar to popping in appearance, but the volatiles come from below the organic coating. There also are forms of gassing that occur over galvanized steel, but I have seen a lot more over plastic substrates. The defect usually looks like a crater or solvent pop, but careful examination of a cross section with a microscope often shows a small hole that goes all the way to the substrate. Any plastic that has bubbles or pores near the surface has potential for this defect. Prevention usually depends on using a primer that seals the surface well. Micropops are very small bubbles, bumps, or pinholes that appear in the coating, often late in the bake. To the eye, the result may be fuzzy reflections, or a surface that just does not look right. Keeping the film open longer by using slightly slower solvents may help.