Technology Interchange: The Evolution of Powder Coating Technology

By Kevin Biller

This edition of Technology Interchange will take you on a journey, exploring how our industry began and the path it took to the present day. Powder coating technology was conceived as a laboratory curiosity and blossomed a few decades later as a multi-billion dollar global industry. Many folks think powder is the latest and greatest industrial coating technology; however, its origin dates back to 1953 when a clever German scientist developed a means to apply an organic coating to a metal part without the use of a solvent carrier.

Erwin Gemmer invented the concept of dipping a heated metal object into a fluidized thermoplastic powder, which allowed the particles to adhere and fuse to the surface. This groundbreaking idea was eventually patented in 1955 and the industry was off to the races. Well, maybe not that fast, but the concept slowly evolved into a commercial reality.

From 1958 to 1965, the powder coating process was accomplished using fluidized bed techniques. Objects would be preheated then dipped into a churning vessel filled with powder, retrieved, and then finished off with a subsequent post heating process. Resultant coatings were rather thick and inconsistent, typically ranging from 6.0 to 20 mils, and were used for functional end-uses such as electrical insulation and applications requiring high corrosion resistance. The thermoplastic polymers used included Nylon 11, CAB (cellulose acetate butyrate), polyethylene and PVC (polyvinyl chloride).

The 1960s: Electrostatic Application, Thermosets Chemistry and Compounding Emerge

The 1960s brought a small revolution to the powder industry. Clever engineers adapted the electrostatic technology used to apply liquid paints to dry powder coatings, which created a more consistent means to deposit particles onto a conductive (i.e., metal) surface. Sames, a renowned paint equipment manufacturer in France, developed the first electrostatic powder spray guns. Gema in Switzerland followed closely on their heels with their version of this type of powder spray equipment.

Concurrently, creative scientists developed thermosetting cure chemistries that could be processed into fine particles capable of being applied electrostatically. The initial thermosetting powder binders were based on solid epoxy resins borrowed from liquid paint technology and were pioneered by Shell Chemical in Europe. Powder formulating expertise emanated from Wagermakers Lakfabrieken, a small paint company located in Holland and spearheaded by Pieter de Lange. Teodur was the brand assigned to this new technology

Manufacturing powder coatings in the early 1960s was still a rather primitive exercise involving relatively small batches processed in ball mills. This technique uses cylindrical vessels that rotate on a horizontal axis and typically can handle batches ranging from 10 to 1000 kilograms. Powder formulas consisting of resin flakes, crosslinkers, additives, colorant pigments and filler are introduced to the container. The milling media consists of marshmallow shaped ceramic “balls” that blend the mixture by impact. The charged container is rotated at low rotations per minute (RPM) for up to 24 hours. The resultant powder was a crude mixture yielding an ultimately imperfect finish characterized with moderate to significant orange peel.

The obvious shortcomings of the ball mill process led researchers to explore melt-mixing techniques to process powder coatings. Melt-mixing (a.k.a. compounding) provides a more intimate dispersion of dry particles such as pigments and fillers; it also distributes resinous components into a miscible blend on a near molecular level. Process engineers developed batch techniques using “Sigma” and “Z” blade mixers and two-roll mills. The blade mixers were jacketed, which allowed the inside surfaces to conduct heat to the mixture. In addition to the melt-mixing aspect intrinsic to these methods, shear was also introduced to the formula, thus augmenting pigment dispersion and more consistent color development. Two-roll mills compounded the raw materials by passing the formula repeatedly through a nip of relatively high speed rolls. One roll is heated whereas the other is cooled to avoid adherence of the molten product. The molten mixture would be milled for a few minutes then discharged as a ribbon to be flaked and pulverized.

Both blade mixing and the two-roll technique are batchtype processes that limit the manufacturing to finite batch size. Multiple batches were necessary for larger production orders. This proved to be time consuming and presented an undesirable level of inconsistency. In many cases, small “sub-batches” of finished powder had to be dry-blended to achieve a consistent final product.

The development of extrusion processing was a momentous sea change in powder coating manufacturing. Extrusion is a continuous process that takes a homogeneous dry feed of powder components and applies heat and shear through rotation of one or more screws encased in a heated barrel. The precisely controlled heat and shear melted and intimately mixed the resinous components. In addition, the shear of intermeshing screw elements deagglomerates pigments to provide consistent color development. Extrusion technology was a very common process in compounding thermoplastic polymers used in plastics and fibers and had to be modified with shorter barrels and lower temperatures to accommodate the chemically reactive thermoset powder formulations. This technique solved the problem of requiring multiple little batches to fill a production order and reduced batch process times significantly.

Late 1960s and Early 1970s: Regulations Impact the Growth of the Powder Coating Industry

During this timeframe, industrial processes encountered little regulatory oversight. Environments such as the L.A. Basin (Los Angeles) regularly experienced dreadful concentrations of smog and the result impacted the health of the general population. At one point in this era, the L.A. health commission estimated that more than 600 tons of organic solvent were emitted daily in the L.A. valley. Scientists recognized that most of these solvents are photochemically active, causing a reaction with nitrogen oxides to form ground level ozone. The medical community knew that ozone was especially harmful to infants and senior citizens, causing emphysema, bronchitis and asthma. Further research identified smog as a cancer-causing agent when experienced in a cumulative fashion.

Los Angeles County legislators enacted Rule 66 in 1966, which focused mainly on organic solvent emissions from industrial processes. Although their primary recommendations to contain these emissions involved direct-fired and catalytic after-burners, its enactment paved the way for future regulations and the emergence of powder coatings as a viable alternative to solventborne paint technology. A few years later, the Nixon administration established the Environmental Protection Agency (EPA) and its maiden action was the passage of the Clean Air Act of 1970. This legislation provided a national template for the reduction of solvent emissions from industrial finishing processes.

Lawmakers and regulators not only realized air quality was affected by solventborne paint, but aquifers and water systems were contaminated by industrial wastes. This led to the passage of the Resource Conservation and Recovery Act (RCRA) in 1976. The RCRA regulated the disposal of hazardous wastes, including the sludge generated by industrial paint overspray. The ground contamination problems experienced at Love Canal landfill in Niagara Falls, USA, had a strong influence on the passage of this legislation. In the 1970s, citizens discovered a landfill of hazardous chemicals that had been buried in the 1950s. The land was subsequently used as a site to build an elementary school and adjacent low-cost housing. With time, chemicals leached into basements and ground water causing myriad health issues in the community.

The significance of the RCRA was twofold. First, the disposal of hazardous wastes, including paint sludge, became a very expensive and time consuming task. Second, industrial finishers recognized the value of using powder coatings in a finishing system that captured and reclaimed the dry overspray to reuse.

Late 1970s & 1980s: Rapid Growth Across Many Industries

By the late 1970s, early adopters were well ensconced in the use of powder coating as an industrial finishing process. Automotive spring manufacturers, hot water heater makers and manufacturers of electrical components all had dipped their toes into the world of powder coatings. It was with their successes that more mainstream industries began converting their finishing methods from solventborne paints and porcelain enamels to this nascent technology. Major industries and enduses such as automotive parts, large appliances, bicycles, shelving and lawn furniture began replacing their paint lines with powder coating systems.

This was the era of thinner, more decorative films due to advancements in application equipment and innovations in resin technology. Formulators not only had epoxy resins to work with but also outdoor durable polyesters, chemically resistant polyurethanes and a unique marriage of epoxy and polyester polymers dubbed “hybrids.” Electrostatic charging techniques became more and more reliable and reclaim systems provided higher and higher efficiency. It was during this time that the powder coating industry coined the term “the 5 Es” for Environmental, Efficiency, Energy Savings, Economics and Excellence. This valuable combination of qualities convinced countless coating engineers to convert over to powder coatings.

As one major player swapped an aging finishing line to initiate plans to replace their systems in like fashion. Consequently, industry sectors converted quickly to this emerging finishing technique like dominoes as they upgraded their finish quality and the economics of their operations. The appliance manufacturers began with more functional applications such as dryer drums and refrigeration shelving but soon advanced to more cosmetic appearance end-uses such as range cabinetry and washer tops & lids. Similar trends were seen in the automotive parts sector. Initial applications included coil springs and suspension parts but evolved into high visibility components like valve covers, alloy wheels and window trim.

The 1990s: The Automotive Era

The widespread use of powder coatings in the general industrial and appliance markets encouraged automakers to explore the use of powder as a body coat. The first body coats emerged as primer-surfacers for small trucks and later across the passenger vehicle spectrum. Primer-surfacers are applied to an electrocoat primer and before the basecoat/topcoat layers. This intermediate layer functions as a smooth surface to enhance topcoat appearance and to provide a chip-resistant bond to protect against stones and road debris.

Primer-surfacer technology pushed the envelope on smoothness. Earlier industrial powder applications could exhibit a certain amount of orange peel as they mimicked finishes they were replacing such as porcelain enamel and some solventborne paints. The exacting standards of the automotive world wouldn’t compromise the aesthetic expectations of their market-inspired formulators to achieve advancements in smoothness and appearance.

Arguably the most demanding automotive finish requirement is the clear topcoat on luxury cars. During the 1990s, BMW challenged the powder coating community to develop a coating to meet its rigorous specification. This finish required perfect smoothness and clarity, exceptional chemical resistance and UV durability, and needed to be cured at a relatively low temperature to avoid damaging the coating layers beneath. Powder coating producers, specifically PPG and Herberts Powder Coatings, and equipment engineers, specifically Nordson, responded with the most advanced powder coating technology of this era. Formulators, in concert with their resin suppliers, created a product with incredible smoothness and distinctness of image coupled with resistance to acid rain, cleaners and years of exposure in the relentless sun. Application engineers developed a finishing system that kept the spray and reclaim equipment immaculately clean whilst depositing a thin, even coat of acrylic powder. Some experts believe that this was the pinnacle of powder coating materials and engineering technology.

The 1990s were a time for innumerable innovations in powder technology. Research and development (R&D) budgets were still fairly robust, and creative technologists pushed the envelope in all sorts of directions. UV-curable powders made their debut on a variety of products such as assembled motors, automotive radiators, medium density fiberboard (MDF) and vinyl flooring. High-heat-resistant powders based on silicone resins were developed for the gas grill and exhaust parts industries. In addition, fluoropolymers were being formulated into powders to meet hyperdurable specifications of 10 and more years of outdoor exposure.

The 2000s and Beyond: Monumental Change and a Resurgence of Technology

The 2000s ushered in a strong dose of reality across many aspects of the industry. The rapid growth of the 1990s created overcapacity for European and American powder producers. Overcapacity led to lower profit margins and a consolidation of companies through mergers and acquisitions. Economic recessions caused a shrinkage in R&D and marketing budgets and a focus on manufacturing efficiency. Lean manufacturing, Kaizen and Six Sigma became regular terms in our lexicon. Globalization shifted investments to Asia and other parts of the world with a corresponding slowdown in innovation as emerging markets struggled to keep up with burgeoning economic growth.

Throughout the 2000s, the mature powder markets of Europe and North America actually shrank and have only recently recovered. Growth in Asian markets have recently cooled off and some industry is reshoring back to the United States. Labor costs have increased in China and automation in U.S. factories continues to advance, thus bringing some manufacturing back home. Powder manufacturers are starting to reinvest in new technology and have been seeking expansion into new end uses and markets.

Powder coatings for engineered boards and natural wood are experiencing a rebirth. Innovative applications such as powder on glass bottles, composites and injection-molded plastics have become a reality. UV-curable powders are back in vogue. Powder coil coating is being looked at once again and novel curing techniques using infrared (IR) are being commercialized.

They say that history repeats itself. Powder coatings grew rapidly in the 1980s and 1990s, and then they experienced stagnation in the 2000s. It appears that the pendulum is beginning to swing back to strong, sustainable growth.