Industrial Paint for Plastic
OEMs everywhere can attest that industrial paint for plastic surfaces can be very different from painting metal. Overall, plastics see some of the main problems you see with covering metal surfaces, but each plastic surface can be affected by paint differently. The problem with painting plastics is that there are so many different plastics in use, each with its own surface properties and paintability issues. When it comes to plastic vs. metal coatings, plastic sees lower surface energies and polarities often generating more wetting and adhesion problems than pretreated metal surfaces do.
To begin with, OEMs must start with clean substrates to ensure good coating adhesion. They work constantly to minimize defects in the uncoated surfaces to start the coating process in a precautionary manner. Problems can develop when working with plastics as they may have small amounts of mold release agents on their surfaces possibly contaminating the final product. The same goes for oils, sanding dust, and other kinds of surface dirt. These contaminants must be removed while priming the surface. While dirt can be a culprit, bumps and irregularities can also be caused by other factors. This can come from surface damage including molding problems and also from bad part-handling practices.
To ensure paintability, most plastics are cleaned by power wash and a solvent wipe, but this process has to be monitored. Cleaning in this manner can cause problems for OEMs. Adhesion can be affected by detergent residues resulting in water spots and other appearance issues. While this can be inspected physically, testing wettability is even better for ROI. While standard techniques can be chosen, there are several simpler wetting/dewetting tests that can show if cleaning is effective. The swab and commercially available marking pen techniques are simple and rapid and are particularly useful for testing surfaces that are curved, irregular, or porous where contact angles can’t be measured.
Conventional air spray, airless, electrostatic, flow coat, or even dip coating are application techniques used for plastic as well as metal surfaces, but conductive primers are needed if the topcoat is to be applied electrostatically. All of these application methods can affect appearance readily and the problems that arise can be blamed on the paint, but adjustments can be made to the atomization pressures and flow rates to counter this. Cratering, crawling, telegraphing, popping, and delamination occur over virtually all substrates, but when it comes to industrial paint for plastic, OEMs also experience fiber read-through, bondline readout, gassing, and micropopping.
There are three ways that painting can affect the mechanical strength of plastics. First, paint solvents may attack the plastic. In fact, it is often desirable for the paint solvents to attack, or etch, the plastic to a certain extent since this improves adhesion. However, solvents can also cause surface crazing or even environmental stress cracking. In addition, paint solvents may cause swelling, softening, or dissolution of the polymer. Once again, a certain amount of this may help to promote adhesion, but taken to an extreme, it can degrade the properties of the plastic to unacceptable levels.
There are two additional ways that painting can affect plastics—thermal and mechanical.
High temperatures can have a number of effects on industrial paint for plastic surfaces. First, they promote solvent diffusion into the plastic, potentially accelerating any solvent-related effects. Second, high temperatures may relieve molded-in stresses in the plastic part, potentially causing warpage. In addition, shrinkage of the paint during the curing process may produce residual stresses. Third, high temperatures may cause thermal degradation of the polymer.
There are also mechanical effects: The strength and stiffness of the paint are different from those of the plastic, but they have to move together as a unit. Often, the paint is more brittle than the plastic substrate, which means that the paint will crack under loads that would not normally crack the plastic. Once started, a crack in the paint can do either of three things: The crack can continue through into the substrate, it can cause the paint to delaminate from the substrate, or it can “channel” through the paint without affecting the substrate.
The effects of painting on plastics are not straightforward and are difficult to predict. Using the wrong paint system can cause plastic parts to fail prematurely. On the other hand, picking the right paint system can help to ensure a world-class durable product. That’s what you get with U.S. Paint. Paint systems that work for your unique needs in the OEM sector. Don’t just take us on our word – come visit our facilities, meet our staff, and learn firsthand that U.S. Paint should be your high-quality specialty paint and coatings partner!
