When considering coating plastics, the first obvious question pops up. Why bother? By simply using pigmented resin one gets a molded-in color that can’t scratch off and one that also eliminates the need for a finishing operation. The answer is two-fold. One is that, in thick, large plastic parts, it can be more economical to put color on the outside where it is needed, instead of throughout the entire thickness of the part. The other is better color matching. Everyone has had the experience of seeing an assembly of metal and plastic parts that didn’t perfectly match, and where the fading on the plastic only made the color mismatch worse over time.

Traditionally, those taking the coating route for coloring plastic parts had only one option — liquid coatings. The desire to powder coat plastic parts was but an unfulfilled dream. Even though the powder coating of metals was a proven and growing technology, the obvious challenges of putting powder on plastic restrained this approach.

Those restraints are starting to give way, though slowly. As compared to the overall powder coating market, the use of this clean, cost-effective technology to coat plastics parts is estimated to make up only about 1 percent of the market. But the use of this technique is starting to grow due to new solutions to the age-old challenges. Coaters and material suppliers are developing new ways to powder coat plastics, and with good reason. Powder coating offers a number of benefits, including design flexibility, reclaimability and reusability of material, and its overall environmental advantages.

Powder coats also provide a tough, durable, attractive finish to a part. Designers can choose from a wide range of powders that come in diverse colors, textures and gloss levels. In addition, the ability to apply powder coatings to both plastics and metals permits a finishing uniformity. This is important in applications that use multiple substrates, since it allows the matching of color and texture of both metal and plastic parts on the same appliance.

Alliance Surface Finishing can coat plastic parts in a number of colors including ASF Metallic, blue, silver, and red.

That ability to achieve a harmonious uniformity of color is perhaps the most important benefit to be gained by the ability to powder coat plastics, says Robert Langlois, President and CEO of Toronto-based Alliance Surface Finishing (ASF). The company is one of the most successful finishers of plastics using powder coating.

“On an appliance, for example, there are usually plastic surfaces and metal surfaces,” he says. “Most of the metal surfaces have been powder coated. To coat the plastic parts with a liquid would make it very difficult to duplicate the color on the metal part.” Powder coating is also environmentally friendly. The materials contain no hazardous air pollutants (HAPs) or volatile organic compounds (VOCs); both HAPs and VOCs have negative health and environmental effects.

By contrast, liquid coatings typically contain solvents that emit both HAPs and VOCs in the finishing operation, emissions that must be captured. In addition, liquid overspray creates waste material that cannot be reused. Powder coating material, on the other hand, can be reclaimed and reused.

The difference between the two methods is never more evident than when establishing a new finishing line. Liquid-based finishing lines can cost more than 10 times than would a powder-based system, says Richard Peters, secondary operations leader for GE Plastics of Pittsfield, Mass. “The infrastructure needed is dramatically different between the two processes, so it really benefits powder over liquid,” he says.

Overcoming challenges

A plastic refrigerator handle is powder coated at Alliance Surface Finishing’s R&D facility. The application is a single coat, red metallic.

Still, despite these advantages, powder coating plastic has its own unique challenges. With conventional powder coating, the finely granulated dry, solid powder materials are drawn to and held electrostatically by an electrically conductive metal part. Furthermore, after application, the part goes through a curing oven. The heat of the oven will first melt the powder coatings into a liquid layer for even dispersion over the surface. Then it initiates a chemical crosslinking process that cures and hardens the liquid layer into a solid film.

And, therein lies the two challenges. Conventional thermoplastics plastics are non-conductive and, by nature, will deform or warp when subject to high temperature.

The first challenge is addressed by making the plastic conductive, using one of two methods. One is incorporating conductive fillers in the polymer material before the part is molded. The other is to pretreat the molded part prior to coating. The second challenge is addressed by utilizing polymers that exhibit high temperature resistance, even though they are more costly.

In a liquid-coating finishing application, the coating materials cure at about 180 DegF, which is not hot enough for powder coating applications. Andrew Korzen, Noryl Americas Product Manager, GE Plastics, says that: “In most cases, “powders cure from 360 DegF to 380 DegF. There are not a lot of cost-effective plastics that can withstand those temperatures and still exhibit good physical properties.”

While high temperature plastic materials such as nylon can solve the warping problems, the extra material costs must be considered. Langlois says that the upfront material costs are greater, but the cost is recouped in lower environmental-related costs as well as improved quality of the finish.

At ASF facilities, the company’s Alliance Powder System is used to coat millions of plastic components per month. The parts are used in the appliance, automotive, and office furniture industries.

Working with its strategic partners, Nordson a coating equipment supplier in Westlake, Ohio; PPG, the Pittsburgh-based powder coatings supplier; and BASF, a supplier of nylon materials, ASF was able to develop its process for powder coating plastics over a period of two years.

The ASF process does not rely on using inherently conductive plastics. Instead, the company pretreats the surface of the substrate to accept the specially formulated powders. While not revealing details, Langlois says that the process is analogous to procedures used in the electroless plating of plastic.

He also points to the plastic materials developed by BASF that can withstand the temperatures that are achieved during the curing process. To cure the powders requires temperatures to reach up to 375 DegF. “At that level,” Langlois says, “a typical PC/ABS substrate would melt away.”

In developing the process, Langlois didn’t want to have to “reinvent” any materials. Because one of the first applications was for the automotive industry, he wanted to use technology that had been tested in the marketplace and which had already met automotive standards.

At ASF, the predominant materials used are nylons (6, 66, glass and mineral filled) because of their high heat resistance properties. But the company coats a variety of other plastic materials, including polyethylene terephthalate (PET), PCT, other engineering thermoplastics, and also thermosets such as SMC/BMC materials.

In addition to plastics, the company can also coat aluminum, stainless steel and other metals, and die cast alloys. In applications where multiple substrates are used, the company can coat the entire group of components with the same color and texture of powder to provide a consistent finish over the entire assembly.

Using PPG powders, ASF can use more than 400 different colors, textures and special effects. Paul Olejniczak, North American Sales Director, PPG Industrial Powder Coatings, says that PPG began working with Alliance Surface Finishing four and a half years ago. PPG had to refine its existing technology to ensure it met ASF’s requirements for appearance, smoothness and color.

Olejniczak says that the company had several challenges to overcome in developing the powder for coating plastics. “The biggest challenge that we really had to overcome was in obtaining the appearance that one would expect from a metal substrate, but get that look over plastic,” he says. “Specifically, the challenges were in combating surface finish defects, distinction of image, and gloss.”

Powder formulas in use at ASF can give the appearance of a number of different materials, including a metallic, chrome look. Achieving that look with powder coating offers a huge advantage over actual chrome plating. According to Langlois, electroplated hexavelent chrome, the predominant and still successful method of chrome plating, has environmental problems and is one of the materials restricted by the European Community’s RoHS Directive. The company’s CRT, or chrome replacement technology program, uses polyamide (PA) and polyamide blending materials that undergo the company’s pretreatment and application process to produce the desired chrome look.

New conductive material

Powder coatings produced by PPG for Alliance Surface Finishing.

While ASF utilizes substrate pretreatment to make the plastic part conductive, another method is to use parts molded with a conductive polymer. The material is made conductive by the incorporation of conductive fillers. GE Plastics, Pittsfield, Mass., is one supplier of such materials.

GE Plastics developed a special resin for powder-coating applications. Its Noryl GTX conductive resins are a blend of PA with modified polyphenylene ether (MPPE). It is reinforced with conductive filler packages to ensure electrostatic adhesion of the powder coating. GE Plastics’ Peters says that the polyphenylene ether has a glass transition temperature (Tg), or heat resistance, of about 415 DegF, when added to the high temperature polyamide. Combined, the material is able to withstand the curing temperatures of typical powder coating systems.

One of the technical issues that needed to be addressed was the conductivity balance of the material, according to Korzen. Metal parts will conduct the charge consistently throughout a part. With plastics, when adding these conductive fillers, the challenge is dispersing the fillers throughout the entire part so there is an equal attraction of the powder. Otherwise, the coating can build up in areas with higher concentrations of the filler.

Peters added that the blend is balanced so that the conductive fillers do not create any loss in terms of physical properties such as impact performance. “Adding these conductive fillers can tend to make other thermoplastics brittle, “ he says. “We can do it in such a fashion that we do not make the material brittle and we also can increase the impact resistance through other modifiers.”

Noryl GTX resins can be powder-coated simultaneously with metal parts. Prior to coating, metal parts must go through a wash cycle, dried and then coated. Plastic parts can be integrated into production prior to the drying step or be placed along side the metal parts and go through the same wash cycle, says Peters. “Its chemical resistance is such that it will withstand typical soaps and detergents that are used in a wash cycle,” he says.

The ASF process and GE’s product both allow for the integration of plastics alongside metal, a key advantage of the new technologies on the market. Whether using ASF’s method, or another process with GE’s materials, powder-coating plastics is a production technology that seems ready to grow.

But, even with the advancements, each application is different, says PPG’s Olejniczak. Coating materials have to tested and honed to suit the unique requirements of each application whether it is for ASF or another company. “Most likely we couldn’t supply the same material composition that we supply to ASF because everybody painting plastics has different requirements based on the individual application and the company wouldn’t get the same results.”

Editor’s note: After this article was written, GE Plastics was acquired. The company’s new name is SABIC Innovative Plastics.

For more information, email:
Alliance Surface Finishing: rlanglois@asf-powder.com
BASF: Robert.hutchings@basf.com
GE Plastics: gelit@ge.com
PPG: olejniczak@ppg.com