Technology Interchange—Creating Conductivity to Coat Alternative Substrates

By Andres Weber

The powder coating of nonconductive parts is still an extremely undeveloped market. If a system is developed for the coating of nonconductive substrates, the costs of powder coating can be drastically cut, making the coating process more environmentally friendly. This improved coating process would lower VOCs and HAPs, result in less exhaust in the application, provide reuse of powder overspray, and reduce the costs of waste disposal equipment.

Over the years, the powder coating industry has developed technology to lower the cure temperature of powder coatings; an environmental advantage. All composites, gypsum board, plastics, woods, and any other nonconductive substrates share advantages over metal.

Electrostatic spray deposition (ESD) is used to apply powder coatings to a substrate. The powder is applied to the part using a specialized spray gun, where air and electric current are used to charge powder coating particles and push them out of the gun. The charged powder coating particles are attracted to the conductive part. The conductive part needs to be grounded to create the attraction and provide a uniform layer of powder coating. After application, there are several ways to cure the powder. Check with your powder coating suppliers for specific curing parameters.

Before beginning the powder coating process, check both the conductivity of the substrate and the curing process of the powder. Since conductivity plays a vital role in attracting the powder particles, test your parts with a resistance meter to ensure it is conductive enough to attract the powder coating.

The coating environment, as well as the interior of a part, is ever changing. Any external variables (environment outside the part) such as the temperature, humidity, and
particles in the air, as well as the internal variables like moisture inside of wood, carbon rich areas in carbon fiber, or glass content in fiberglass, can fluctuate. When powder coating wood, looking at the external environment is important. The changes in heat and humidity during the season can affect the way the wood behaves. The internal environment is also important, as wood has moisture inside each piece. This moisture content varies between all types of wood. Maple has a different moisture content than oak, birch, MDF (medium density fiberboard), and so on. Moisture inside and outside the wood can affect the process. In the past, moisture in the powder coating process of wood was a problem. As powder coatings would cure at 350-400 degrees Fahrenheit, the wood’s internal moisture would come to a boil, outgassing and warping the part’s surface. However, modern improvements in powder curing technology have developed methods to drive curing temperatures down to as low as 180 degrees Fahrenheit. The wood’s internal moisture never reaches the water’s boiling point and remains trapped inside by the cured coating.

To powder coat wood, there are two ways to create a surface conductive enough to attract the powder. The first is to heat the part to a temperature high enough so that there is condensation of water on the surface of the part. At this optimal temperature, the water on the surface can be used as a conductor to attract the powder. For this process to work, it requires heat and energy. The second, more energy efficient method is to apply a conductive coating to the part, creating a conductive surface that attracts powder at ambient temperatures. As a reminder, after powder coating the parts using the preheating method or the greener conductive coating method, the powder still needs to be cured. See powder coating technical data for these parameters.

Currently, there are low cure powder coatings available that will cure on composites. Fiberglass and carbon fiber are composites that are a mix of glass fibers and carbon fibers mixed with resin, with a melting point of >275 degrees Fahrenheit. The possibilities of substrates will be almost endless once powder coatings are developed that cure at lower temperatures.

Like wood, composites, when heated, may outgas before melting. Be mindful of the outgassing temperature and the melting point. Methods for creating conductivity in composites vary based on the composite itself. While most composites require some kind of surface treatment, such as a conductive coating, carbon fiber is one of the few composites which possesses its own conductivity. Specifically, the carbon fiber itself is conductive, while the resin holding it is nonconductive. This creates a separate problem, where more powder is attracted to carbon rich areas in the part and less to resin rich areas, resulting in an uneven powder coating. However, technology is available that creates a barrier to cap the conductive carbon on one side of the coating and a uniformly conductive part on the outside of the coating.

Adhesion between the powder coating and substrate is another challenge, as composites and plastics can be especially difficult for powder to adhere to. Adhesion can be mechanical or chemical. Mechanical adhesion uses outside mechanical means, such as heat, pressure, and other forces, to get the powder to adhere to the substrate. Chemical adhesion is when the atoms on the surface of both the coating and the substrate form ionic, covalent, or chemical bonds. The powder coating atoms can only form a bond if the correct atoms (or functional groups) are on the surface of the substrate. If the incorrect functional groups are on the surface of the substrate, the powder will not adhere.

Mechanical adhesion is an unreliable option which can result in warping of the part due to the external temperatures and pressures. Chemical adhesion provides better ways to change the surface functional groups through such processes as heat treating, plasma treating, pretreatment coating, and many others.

When coating nonconductive parts with no functional groups, the pretreatment coating has to be conductive and has to change the functional groups to adhere to the powder coating. There are coatings and technology available that meet both objectives and would reduce the energy cost of heat pretreatment and plasma pretreatment.

As powder coating alternate substrates becomes a greater focus for the industry, we must continue to document and discuss significant issues we encounter, as well as ways to improve methodologies of incorporating the newer substrates. Continuing these discussions will allow us to exploit the advantages powder coatings have over liquid coatings,extend powder coating technology to more industries and substrates, and maintain the industry’s relevance in the world of manufacturing.

Andres Weber is laboratory director, adhesives and coatings, at Chemical Technology, Inc.