Powder coatings have achieved a great deal of respect in the coatings world over the years. Their positive attributes are many and the list continues to grow as new developments come about, but the most important characteristics include what the industry refers to as the “Five Es”: efficiency, economy, energy-saving, environmentally friendly and excellence of finish.
While powder coatings are also praised as being easy to use, achieving success in a powder coating operation is not nearly as easy as some might think. There are many factors to take into consideration, but if you start with the big picture, you begin with cleaning, drying, coating, and curing. Of course, zeroing in on and optimizing each of these areas is an undertaking that, as many successful coaters will tell you, requires time and effort.
Selecting the right powder formulation is just one aspect of mastering the coating portion of that big picture. Because coating selection is based on more than whether your parts are destined for an indoor or outdoor application, let’s take a look at some basics, and then work our way into some of the more advanced powder selection factors.
Building a Formulation
Thermoset powder formulations are a blend of resins (binder systems), pigments, extenders, and other additives. Resins give the finished coating its physical properties, such as flexibility, chemical resistance, outdoor durability and hardness. Each resin has different properties, so the resin is chosen according to the end use of the powder coating. For example, the resin system used for architectural aluminum coatings would be different from those used to coat domestic appliances.
Pigments not only give the coating its color, they provide the ability of the coating to hide the metal. Different pigments have different hiding powers, so the amount of pigment used can vary significantly for different colors. Pigments also come in weatherable and non-weatherable versions.
Last but not least, additives in coating formulations are used to alter the characteristics or physical performance of the coating, for example, gloss and texture. They can also affect flow and coating durability due to their ability to withstand UV exposure. Another example is using slip agents or waxes to help reduce scratching and marring.
A thorough understanding of the end use environment is critical to selecting the correct coating for your parts. Other characteristics to consider are gloss, orange peel, film thickness, salt spray resistance, heat resistance, smoothness, color, texture, substrate characteristics, and edge coverage, among others.
Although formulating for the end use application is ideal, there are times when the finisher’s systems play a role in coating selection. For example, cure ovens with short dwell times may require low-energy-cure powders. Other considerations involve part configuration—for example, parts with large variations in metal mass or parts with complex Faraday areas.
Of the infinite array of coatings that can be imagined, and of the dozens of types that have been sold during the 50-year history of powder coating manufacture, four reliable, cost-effective workhorses have emerged, especially within the North American market. These are: Epoxies (indoor), Epoxy-polyesters (indoor), TGIC Polyesters (indoor/outdoor), Urethanes (indoor/outdoor).
As mentioned earlier, each resin system will give specific properties to the coatings, making them suitable for different requirements. The chart shows the main advantages and disadvantages of each binder type.
Physical properties have defined purchasing behaviors for each coating type, but the rising cost of raw materials has also forced some changes in finishers’ buying habits. While rising costs have been pervasive, increases have been the greatest for epoxies. In the last several years, polyesters have been applied more often, even on indoor applications, due to the rising cost of epoxies.
The data reflects that switch more heavily in the use of epoxy-polyesters. Years ago, epoxy-polyesters were created to provide powders with greater overbake stability and ease of application compared to epoxies, which have very high hardness and chemical resistance. Where those characteristics are not necessary, there can be some compromise. Polyesters provide more versatility for indoor or outdoor applications, making it cost-effective to switch over to the polyesters when the epoxy-polyesters increase in price.
We are also seeing a decrease in urethane use. This is primarily due to the emission of e-caprolactam, from most urethanes during curing, which contributes to the buildup of undesirable residues.
In Europe, these numbers differ. In fact, 85% of powders used in Europe in 2006 were either epoxy-polyesters, or non-TGIC polyesters. Globally, 85% of powders purchased are epoxy-polyesters, TGIC polyesters, or non-TGIC polyesters.
Going a step beyond resin systems, let’s look at how powder coatings can also be grouped based on aesthetic or specific functional capabilities. Additional powder chemistries that serve more niche purposes include non-TGIC polyesters, super-durable polyesters, fluoropolymer powders, GMA acrylics, ultra-low bake and UV powder formulations, fusion-bonded epoxies, chrome-look powders and clear coats.
Polyesters for exterior use crosslinked with hydroxyalkylamide, better known as non-TGIC polyesters, are outdoor-durable polyesters available in a wide range of colors, textures, and glosses. Sold under the name Primid, these materials are used extensively in Europe and other parts of the world, but presently not as much in North America. We may begin to see that change.
Super-durable or premium polyesters are more resistant to UV degradation than standard polyesters. This is primarily due to the improvements in the backbone of the polyester resin. They are available in both TGIC and non-TGIC formulations. In either case, when properly formulated, they can meet stringent weathering performance requirements such as American Architectural Manufacturers Association (AAMA) 2604, a voluntary specification for application on aluminum. Common uses are window and door frames, building structures, and metal railings.
The architectural industry also now reaps the economic and environmental benefits of powder coating using fluoropolymer chemistry, and development in this area will continue. These powders meet AAMA 2605, which requires 10 years of outdoor durability.
GMA acrylics are used most often as clear coats because of their exceptional smoothness, colorless clarity, and UV resistance. They also have faster (or lower) cure profiles and improved weatherability. They are used globally, and their usage continues to grow.
For a number of years now, the market has pursued and made progress in designing formulations for heat-sensitive substrates. Ultra-low bake and UV powders have significantly lower temperature requirements and shorter cure cycles than conventional formulations, resulting in less heat migration into the substrate and therefore fewer defects. In most cases, these products cure at 240°F for just a few minutes to melt and flow the powder, then a flash with UV light completes the cure. The materials continue to show great potential for plastics, engineered and natural woods and pre-assembled components.
Fusion-bonded epoxies (FBE) have been around for more than 40 years and continue to be the coating of choice for corrosion protection of oil, gas, and water pipelines, valves, and rebar. FBEs are nearly 100% epoxy for the maximum in hardness and corrosion-resistant properties. Fillers should be carefully selected to minimize interference with corrosion properties. FBEs are designed for thicker application (10–30 mils) than regular epoxies, which are typically applied at thicknesses of 1.5–3 mils.
Chrome replacement technologies using powder coatings and vacuum metallization continue to grow. Using this three-step process—the combination of a powder primer, vacuum metallization and acrylic topcoat—the OEM wheel market can meet some of the automotive industry’s toughest testing requirements. Price of entry into this market runs high, mainly due to the cost of the physical vapor deposition (PVD) chambers needed for metallization. However, the process eliminates both the environmental impact and the typical labor-intensive aspects of chrome plating, such as sanding, buffing and polishing.
Clear topcoats are typically needed for added protection and/or improved aesthetics. Several interesting examples are available today, including:
- Acrylic-modified polyesters in both clear and tinted clears, to give a clean, anodized look.
- Medium-gloss clear with great clarity, ideal for high-end stainless steel appliances. The very smooth yet lower-gloss
finish not only shows less fingerprinting because of the reduced gloss but also allows for ease in cleaning.
Polyester clear, originally formulated for diamond-plate aluminum tool boxes. It is easy to apply, has excellent weatherability (>3000 hr of salt spray) and gloss retention, and has good, even flow for consistent film build.
As discussed earlier, there are many additives available that can change the characteristics of the powder coating. These include antimicrobials, metallics and additives to improve outgas resistance.
Powder coatings are now available that contain special silver ion technology. This silver technology provides a safe, natural antimicrobial solution to those cleanliness issues that concern us all. The silver antimicrobial compound, while just one option available today, protects surfaces against the growth of bacteria, fungi, yeast, mold, and mildew.
The market continues to see more and more unique and vibrant finishes using metallics. Metallic or special effect (interference) pigments can be added to powders by dry-blending, bonding, or extruding. Best consistency and reclaimability is achieve through the bonding process, which assures that the effect pigments are physically attached to the base powder coating during the manufacturing process. Aluminum, brass, and copper are encapsulated for better chemical and weathering resistance. The interference pigments refract and reflect incident light to create selected colors. Any change in direction can show off new colors.
Brass and copper pigments oxidize and darken upon prolonged or elevated- temperature exposure. Exterior exposure or high traffic and handling can cause the soft metallic-effect pigments to erode and discolor. A clear topcoat is recommended, especially for oxidizing atmospheres, harsh environments, or high-wear uses.
Outgassing presents many problems for coaters, and is common when coating porous castings. Coatings can be formulated using additives designed to reduce the effect on appearance caused by gases or moisture released from the cast substrate during curing.
There is some truth to the saying that every coating formulation is a compromise. For example, because designing to maximize coating hardness will typically lead to brittleness, it is usually necessary to settle for less than maximum hardness to gain adequate flexibility.
The flexibility versus hardness trade-off is only one of the many compromises formulators must make. Other common trade-offs include: smoothness versus edge coverage, cost versus weather resistance, and smoothness versus storage stability and ease of application. Another problem that often leads to compromise is that coatings with almost any unusual property, such as high temperature resistance or extreme smoothness, become difficult to manufacture.
Not only is the problem of formulation complex and the trade-offs sometimes non-intuitive, but the difficulties are compounded by the fact that a formulator must select the half a dozen components of the coating from the thousands of raw materials used in powder coatings. It has become even more difficult to play the balancing game when formulating for today’s marketplace to achieve a final cost that is acceptable and competitive in the industry while still offering a quality product. The cost of many raw materials has almost doubled, and coating manufacturers have not been able to pass on all the increased costs to users. This has focused unprecedented attention on formulating practices.
The coatings a company manufactures are in a sense a fossil record of its formulating practice over the years. There are many business-related and technical reasons to retain the original formula of a successful product. Even though a formulating approach may become expensive, these products, and the raw materials which produce them, may be retained for decades. Some may fill a niche in the marketplace or have been specified and validated over decades of successful performance history.
But all coating formulations come under intense scrutiny, mainly to reduce raw material complexity and leverage volumes for cost advantages. Meanwhile, new coatings are being designed within ever more tightly controlled sets of raw materials.
We’ve given you a lot to think about here, but the most important thing to remember is that there are a lot of options available. Powders are not all the same, so it is important to understand the end use environment and work with your powder coatings manufacturer to design a specific coating for your needs.