Emerging Technology Delivers Conductivity Plus High Heat Performance for Successful Powder Coating of Plastic

Iain Montgomery
GE Plastics
Industry Manager – Coatings
BERGEN OP ZOOM, THE NETHERLANDS – Powder coating is a fast-growing technology due mainly to its environmental benefits, particularly the elimination of solvents and related compounds that are emitted into the air in conventional liquid paint. Powder coating has traditionally been used with metal substrates; however, the need for greater design freedom has led to industry demand for technologies suitable for plastic substrates.

Lightweight, high-performance, and adaptable, plastics offer many advantages over metals in applications ranging from mobile phones and TVs to automotive body panels and tractor hoods. The challenge has been finding successful approaches to the key issues of powder coating these materials: conductivity to attract the coating, and high heat performance to withstand the cure cycle. There have been three major paths to this goal: application of a conductive primer to a heat-resistant plastic; thermal application of the coating to a heat-resistant plastic; and development of new materials that combine inherent conductivity with high-temperature properties.

Of these, the last approach provides the most advantages. New inherently conductive, high-temperature resins eliminate the cost, time, and environmental concerns of applying electrostatic primers. Further, they offer OEMs and their supply chain partners the flexibility to replace metal with a lightweight, high-performance plastic, or to run multiple materials on the same coating line for economies of scale and optimal color matching. By adopting these substrates, manufacturers can achieve greater design flexibility together with cost efficiency – while supporting environmental protection.

Challenges of Powder Coating Plastics

The coatings used on plastics have generally been liquids, which provide good adhesion and other key properties without requiring high cure temperatures. As coaters and OEMs looked to reduce costs and improve efficiency in the coating process for plastics, there were developments in areas such as conductive primers, which allow base and/or clear coats to be applied using electrostatic processes. This in turn led to an interest in the application of powder coatings to plastic materials.

Most painted plastics, such as polystyrene (PS), acrylonitrile-butadiene-styrene (ABS), and polycarbonate (PC) are amorphous and allow paint systems to key to the surface when the right solvent is used to open the surface. However, all these materials melt and deform when used with powder coatings, as they do not have sufficient resistance to the high temperatures of the powder coating cure cycle, with products like PS softening below 200 °F and ABS softening below 240 °F.

Many coaters have tried powder coating on plastic substrates given to them by end users who thought the materials could meet the challenge, but ended up with parts that came out of the cure oven with a completely different shape or size. Worse still, some parts didn’t come out of the oven at all but pooled on the floor somewhere in the process.

Similarly, semi-crystalline materials such as nylon (PA) and polypropylene (PP) will soften or melt during the cure cycle of a powder coating process. Some can, however, with help from filler technologies, meet the temperature challenge of a powder-coating process. But, these fillers can have drawbacks in terms of part weight, part wall section requirements, or even the overall finish of the coating, as they can show through coatings.

A Trio of Powder-Coating Methods

Fortunately, there are a few plastic materials, such as glass-filled nylon (GF-PA), polyetherimide (PEI), and polyphenylene ether (PPE), available today that can withstand the temperatures seen in the curing of a traditional powder coating. Focusing on the potential of these materials, manufacturers have increased their range of desirable properties to target them for applications such as office furniture and household appliances where powder coating has become the norm for coated products. Now, with these new materials available, researchers have been working to develop the best technology for applying a powder coating to them.

Technology has slowly evolved around three fundamental application methods, all of which allow some form of powder coating to be applied to a limited range of plastic substrates – those capable of sustaining cure temperature for 30 or more minutes. First, efforts were aimed at using a primer to give the surface of a non-conductive plastic substrate enough conductivity to allow powder coating to adhere. This process has some disadvantages because it requires a liquid to be applied and cured prior to the powder-coating process. Additional issues include the cost of both liquid and powder application equipment, the need for multiple layers where a single layer of powder coating could fit the requirements, and loss of some environmental advantages of a powder coating (i.e., elimination of volatile organic compound [VOC] emissions). In the powder-coating environment, there are only a few examples of this technology due to its disadvantages. It has mainly been used on glass-filled products such as polyetherimide and long glass fiber-reinforced nylon, or in liquid coating techniques for plastics.

The second technology is thermal application or flocking. In this process, the substrate is preheated and the powder is applied to the hot substrate. The heat causes the powder to stick to the surface. The part is then cured in the same way as a conventional powder coating applied at room temperature.

These two processes can only be used with plastics that have high heat deflection temperatures (a measurement of a plastic’s resistance to deform at higher temperature), or plastics filled with glass or minerals that can increase stiffness at higher temperatures. However, the choice of substrate here is still somewhat limited and does require the end user to have a good understanding of the performance of the plastic substrate during the curing process for the powder coating.

The third alternative is not really a process but rather the result of new developments in conductive plastics. Specialized plastic substrates have been developed to permit the direct application of powder coatings to the surface with no pre-treatments or pre-processing, ultimately giving the coater the option to hang plastic substrates next to metal substrates on a powder-coating line. This technology has been under development for a number of years at GE Plastics, leading to a product line called Noryl GTX* resins.

This resin family was originally designed to meet the requirements of the automotive market, where it is used to injection-mold body panels and trim parts that are then assembled onto the chassis before going through electrostatic paint lines. Automotive applications called for a material that could be molded into large panels or small intricate parts and that could withstand the temperatures seen in the bake ovens of an automotive manufacturer. When these properties were combined with fillers to give the substrate conductivity to attract electrostatic coatings, similar to the attraction of metallic parts, the world’s automotive manufacturers quickly moved to adopt conductive resins.

Auto manufacturers also saw the potential to use these new materials in the powder-coating process and a new set of requirements was born. GE Plastics has developed a range of grades that will work with both liquid and powder coating processes, as well as grades that are designed purely for powder coating. These conductive materials deliver the benefits of powder coatings and plastic molded parts or extrusions in the same product. They have been seen as a breakthrough in many industries.

Conductive resins are now making inroads into the powder coated plastics markets. Their ability to be coated with the same powder coating as metallic substrates in the same assembly has been shown to reduce or eliminate the following issues: difficulties in color matching different coating systems; the cost of coating inventories for end users applying their own coatings; and the costs and environmental impact of liquid-painted plastic components. At the same time, conductive resins enable end users to leverage part consolidation and other benefits of plastic molding processes.

A Tough, Versatile Plastic Alloy

An alloy of polyamide (PA) and polyphenylene ether (PPE) by GE offers an outstanding balance of properties, including mechanical performance, chemical resistance, and heat resistance, which make it an excellent candidate for many industries and applications. The ability to be modified for use in electrostatic coating processes and to withstand cure temperatures up to 410 °F (210 °C) enables the material to be powder coated using most powder chemistries found in the marketplace, including polyesters, epoxies, and hybrids.

The material also meets the requirements for mechanical performance of automotive body panels and the structural requirements of housings in office furniture and enclosures found in the telecommunications market.

As an alloy, this resin derives strengths from both materials that comprise it. These include the excellent chemical resistance and good flow properties of PA combined with the stiffness, high temperature resistance, and excellent dimensional stability of PPE resin. Although there are some PA grades that can be liquid or powder coated directly, the addition of PPE resin allows the resin to solve drawbacks of PA: it reduces moisture uptake that would normally require significant drying prior to coating; it reduces post-shrinkage and reorientation, which in turn reduces warpage; and it lowers density for weight savings of up to 25 percent over glass-filled PA, where the glass is required in PA for heat resistance in the powder-coating process.

While many conductive resins used in the liquid coating environments are suitable for powder-coating applications, some grades have been customized to give them a wider processing window in powder-coating applications. Table 1 shows key properties of these grades.

Coating Plastic and Metal Substrates Together

The benefits of powder-coating metal substrates are well-known and include minimum VOC emissions, excellent adhesion, high transfer efficiency through electrostatic attraction, good edge coverage, paint wraparound, and a more uniform coating thickness¹.

Through the use of a well-developed powder coating process and by ensuring proper grounding and racking of the metal substrate, all of these benefits can be achieved at ambient conditions. However, proper chemical preparation of the metal surface is required; for example, cleaning off any surface contaminants; ensuring a consistent chemical makeup of the surface; or removal of any preexisting corrosion. This generally means a typical coating process can also involve several pre-treatment steps prior to the spraying of the powder and high-temperature cure.² Conductive resins not only provide performance advantages already discussed, but also fits into the existing infrastructure developed for metals.

Specifically, this conductive resin family has been developed to withstand the rigors of pre-treatment processes used for metallic substrates, such as the high temperatures of electro-coating baths used for corrosion protection and aggressive chemical cleaning solutions. Therefore, metal and plastic components can be assembled prior to any coating or pre-treatment applications, thus reducing damage to coated surfaces during assembly and ensuring no aesthetic mismatching.

Conductive resins have been shown to be easily painted on electrostatic coating lines at ambient temperatures, resistant to the chemicals used in the pretreatment steps, and able to withstand high-temperature curing. Shown in Figure 1 is a typical process for a powder-coating line. Conductive plastic products to be coated can be introduced into the process at the same point as a metal substrate. Of course, chemical preparation is not required for plastic surfaces, so conductive resin parts can also be inserted further downstream in the process, generally at the pre-treatment drying oven. A drying step prior to powder coating is normally required to remove any water that has been absorbed into the PA part of the alloy.³

However, if the parts are powder coated within hours of molding, they will not require this drying step, as water absorption is reduced from PA levels; they can literally go from the molding machine into the coating line at the application stage.

In most cases involving small to medium-sized parts, the grounding contacts and racking used for powder coating of metals can be also used for conductive plastics. For larger components it is general practice to have additional support to properly orient and support the plastic parts through powder spray and cure steps.

Coating Performance of Conductive Plastics

Table 2 compares electrostatic efficiency, aesthetics, and adhesion of conductive plastic panels with those of metal panels that have been powder coated using the same process settings. This table was created using typical polyester and epoxy/polyester hybrid powder coatings. The conductive resin panels were dried at 380 °F (193 °C) for 30 minutes, allowed to cool, and then powder coated together with metal panels at room temperature, and cured according to the powder supplier’s recommended cure conditions. Powder transfer efficiency can be calculated by measuring the weight gained during the coating application and the film build or coating thickness. The data indicates that the conductive plastic panels exhibited weight gains and film builds comparable to those for metals, which shows that similar coating efficiencies are possible under these conditions. This finding also shows that the conductive resin eliminates the need for a conductive primer, which would otherwise be required for a non-conductive plastic to work directly in the powder-coating process.

Having demonstrated the ability of these innovative resins to be powder coated, we addressed the questions of whether acceptable coating aesthetics and adhesion can be achieved. First, a visual inspection could not distinguish differences between the coatings applied to the conductive plastic and the metal panels. Second, measurements by colorimeter and gloss meter revealed no discernable color or gloss differences (gloss levels are shown in Table 2). This indicates that an excellent aesthetic match is possible when simultaneously powder coating conductive plastics and metals at the same process settings. Finally, a crosshatch adhesion test showed that for the three powder-coating systems used in this study, excellent adhesion was obtained both on the conductive plastic and metal panels.

Additional powder-coating trials were carried out to show results when a non-conductive plastic is run through the powder coating process. Table 3 compares the representative powder efficiency and aesthetics using an epoxy/polyester hybrid powder coating over three substrate types. At room temperature, the non-conductive plastic did not work in the powder-coating process at all. There was minimal or no powder adhesion to the surface, which resulted in no weight gain and zero film build. When all the panels were preheated to 200 °F (93 °C), the powder transfer efficiency for the metal and conductive plastic panels did not change significantly, though some adhesion to the non-conductive materials was seen. Through a thermal softening mechanism, the hot surface enabled the powder particles to stick. The powder particles became tacky at the boundary layer with the substrate, which allowed the non-conductive plastic to show some weight gain and film build. However, figures show that the film build and weight gain were considerably lower than those seen on the conductive plastic or the metal substrates, resulting in lower powder transfer efficiency.

When the three panels were examined visually it was observed that the non-conductive plastic had significant orange peel not seen on the other two, and this resulted in a lower gloss measurement.

It may be possible to optimize this type of powder-coating process further and improve coating performance for a non-conductive plastic. Although several companies are working in this area, non-conductive plastic is not a direct fit into the existing powder coating infrastructure used for metal substrates. It may require added energy input and proper heat management to control the film build and coating uniformity for a non-conductive plastic substrate.

Benefits of Conductive Plastics for Powder Coating

Customers who are directly applying powder coatings to a conductive plastic have shown that this can translate into cost savings in the order of 20 - 40 percent vs. multi-layered liquid coating systems used over traditional lower-temperature plastics, as well as providing environmental benefits through the reduction of VOC emissions.

The simultaneous powder coating of metal and plastic parts not only provides an excellent aesthetic match, but can also be the most cost-effective solution, as it eliminates the need for an in-house dedicated plastics powder coating line or an outsourcing strategy using complicated logistics.

In addition to potential cost savings, greater design freedom, and lower weight, plastic designs can offer better ductility and impact performance. These properties may result in better chip resistance of an aesthetic or functional coating over plastic substrates. To show this we took metal and conductive resin panels coated with a white epoxy/polyester hybrid and subjected them to the ASTM D3170 / SAE J400 gravelometer test at room temperature. Figure 2 shows pictures of the resulting coating surface for both substrates. The metal panel, representative of typical sheet metal applications, exhibits dents and deformations around the chips, more paint chips on the panel, and a greater number of exposed substrate areas. Exposure of the metal under the coating could lead to corrosion and accelerated coating failure. In contrast, the conductive plastic panel does not show any dents or deformations, and has fewer paint chips and a much lower number of exposed substrate areas. Not only does the coated plastic have a better chance of retaining its aesthetic appeal in this test but also is not susceptible to corrosion. As a result, the overall coating life may be significantly longer.

Applications

The ability to powder coat conductive resin has attracted interest in several markets and applications: in automotive where primer/surfacer coatings are moving to powder coatings from liquid applications; in applications such as office furniture where plastics and metals are combined in a single component; in marine environments where a plastic substrate completely removes corrosion issues; in heavy vehicles where weight savings are important; and in products like household awnings where plastic components can now be coated with the same weatherable coatings as the metal structures.

Manufacturers are reaping the benefits of using a conductive plastic substrate in powder coating; these include shorter logistic chains, reduced coating inventories, in-house rather than sub-contracted coating operations, process step reductions, and part consolidation and weight reduction. GE Plastics has continued developing the conductive resin range with the addition of resins with flame-retardant properties, glass and mineral fillers for structural components, and grades suitable for extrusion. These new grades, along with developments in the processing of conductive resins with different powder chemistries, have led to multiple opportunities in wider industries and applications.

Summary

Plastics are rapidly becoming materials of choice in a broad range of global industries. From consumer electronics, aerospace, and automotive applications to end products in healthcare, furniture, and telecommunications, the high-performance attributes of plastics are helping to address an array of OEM requirements. These include lower weight, excellent impact strength, and resistance to chemicals and ultraviolet (UV) light; the ability to consolidate parts; ways to meet growing environmental legislation with “green” plastic resin technologies; and greater overall design flexibility, improved manufacturability, and reduced systems costs. As plastics continue to displace metals and other traditional materials worldwide, many critical applications are calling for a coating or finish to achieve aesthetics and/or increase functional properties.

The powder coating of plastic materials can be accomplished in several ways. However, using a conductive plastic substrate, which can fit directly into the existing powder coating infrastructure for metals, provides the simplest and the most cost-effective approach. With the emergence of plastics such as conductive Noryl GTX resins that are optimized for powder coating, this solution is already delivering successful applications.

* Noryl GTX is a trademark of General Electric Company.

ALTHOUGH ANY INFORMATION, RECOMMENDATIONS, OR ADVICE CONTAINED HEREIN IS GIVEN IN GOOD FAITH, GE’S PLASTICS BUSINESS MAKES NO WARRANTY OR GUARANTEE, EXPRESS OR IMPLIED, (i) THAT THE RESULTS DESCRIBED HEREIN WILL BE OBTAINED UNDER END-USE CONDITIONS, OR (ii) AS TO THE EFFECTIVENESS OR SAFETY OF ANY DESIGN INCORPORATING ITS PRODUCTS. Each user bears full responsibility for making its own determination as to the suitability of GE’s Plastics business’ products, materials, services, recommendations, or advice for its own particular use.

References

¹ Roobol, Norman R., “Industrial Painting & Powder Coating: Principles and Practices – 3rd Edition,” Hanser Gardner Publications, Cincinnati, OH (2003).

² “The Complete Finisher’s Handbook for Powder Coating – 3rd Edition,” The Powder Coating Institute, Alexandria, VA (2004).

³ Rubin, Irvin I., “Handbook of Plastic Materials and Technology,” John Wiley & Sons, Inc., New York, NY (1990).

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