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Powder coatings grace the surface of many things we touch. From appliances to lawn furniture, from functional engine parts under a hood to flashy wheels on vehicles, and from electrical bus bars in industrial environments to rebar for concrete reinforcement, the ubiquitous presence of powder coatings is attributable to their appearance, durability and environmentally responsible application process.

1) What are Powder Coatings?

Powder coatings are made from polymeric resins that are blended with curatives, pigments, leveling agents, flow modifiers and other additives. They are 100 percent solid systems devoid of solvents or water in their composition. Their deposition creates a high-quality surface finish on metallic substrates that is durable, anti-corrosive and abrasion resistant. Not only are powder coatings decorative and attractive, they are also rugged and deliver excellent performance properties.

2) What is the Coating Process

There are two primary powder coating methods and the choice is typically governed by part size and geometry. Large workpieces require the use of a controlled, factory-applied process called electrostatic spray deposition (ESD). The coating system is comprised of a micronized powder released by a spray gun, whereby the powder is attracted to a grounded part via an induced electrostatic charge. When coatings are decorative in nature, acrylics and polyesters are used. For industrial applications, epoxies are the standard, with hybrid resins attracting interest.

In the fluidized-bed process, preheated parts are dipped in a trough containing coating powder with air that is blown in from beneath the particles. The part is instantaneously dipped in this floating bath and removed, creating a discrete film.

Whether sprayed or dipped, the coating is uniform, the process is efficient and environmental exposure is minimized. After the powder coating is applied, parts enter a curing oven where the polymer resin chemically reacts, producing long molecular chains with high cross-link density, resulting in a uniform, homogeneous coating. The part can then be placed into service.

3) Applications: From Delicate…Figure 1: Powder-coated electronic components. Source: Evonik Industries

A myriad of applications exists for powder coatings use, ranging from small and delicate to large and rugged.

Electronic components populate a wide variety of devices from smartphones, computers and tablets to TVs, stereos and more. Flexible and rigid printed circuits are used throughout these devices, carrying resistors, capacitors and sophisticated semiconductors. No matter where these delicate components reside, powder coatings safeguard their operation and provide insulation.

4) …to Rugged

Pipelines used in oil and gas exploration and delivery are powder coated inside and out to provide protection from the environment, heat and chemical exposure.

Figure 2: Pipelines require coatings inside and outside to address the harsh environments of both.

Epoxy powder coatings improve the abrasion and corrosion resistance of underground pipe coatings that are exposed to bacteria, soil acids and alkalis, fungus and soil stress as well as underwater pipe coatings that are exposed to corrosive gases, solvents, wastewater, petrochemicals and salt water.

Within this industrial setting, epoxy powder coatings are also known as fusion-bonded epoxies (FBE), cured with anhydrides, phenolics or dicyandiamides, depending on the level of performance required. In all cases, anhydrides provide an ultimate level of protection.

5) What are Anhydrides?

Anhydrides are ideal epoxy thermal curatives, delivering sustainable physical, mechanical and electrical properties under adverse thermal conditions. There are two classes of anhydrides used as epoxy curatives: monoanhydrides and dianhydrides. Monoanhydrides tend to be liquids, while dianhydrides are solids.

Whether safeguarding circuit boards in a cramped computer or protecting oil and gas pipelines, dianhydrides are preferred curatives as they deliver the highest obtainable levels of heat resistance. This is typically measured by glass transition temperature (Tg), which represents the onset of molecular motion within the polymer as it is exposed to heat.

Heat resistance is delivered via extensive crosslinking attributed to the tetrafunctional structure of dianhydrides. The polymer network formed by this reaction resists chemicals and abrasion; insulates; and improves dielectric behavior. While these properties are desirable, dianhydride-cured epoxies may in some cases be too rigid, requiring tougheners to improve impact and bending resilience. Skillful formulators achieve the correct balance for each application.

6) The BTDA® Edge

A common dianhydride used for epoxy coating powders is 3,3’, 4,4’-benzophenone tetracarboxylic dianhydride (BTDA). This workhorse curative, in use since the 1970s, addresses the increased design complexity of electronics and the demands of in-service thermal and dielectric properties. The tetrafunctional structure of BTDA allows for highly crosslinked epoxies, capable of retaining their properties when exposed to soldering, hot crude oil and other harsh environments. BTDA is recognized for Tg improvements as high as +30° C in select epoxy resins, particularly those based on DGEBA. Available in powder and micronized forms, BTDA is perfectly suited for dry blending processes typical of powder coatings. In cases where rigidity is too high, toughening may be introduced to balance properties.

Summary

Although powder coatings have been around for many decades, the evolution of performance requirements across several applications is rapidly advancing their use. Powder-coated articles in a variety of colors and textures surround us in the workplace, in our vehicles and public transportation and throughout our homes. Today’s modern manufacturing processes ensure uniform film thickness and improved quality results, while being environmentally friendly. In comparison to conventional deposition methods, overspray is eliminated and waste is dramatically minimized.

For more complex industrial applications, greater demands are placed on curing agents that improve the durability of epoxy resins in powder coatings. Anhydrides, in general, and dianhydrides, in particular, are the basis of greater and more sustainable coating performance.

In spite of BTDA’s 50-year existence, this dianhydride continues to be widely recognized as the flagship epoxy thermal curative. High purity, the availability of an assortment of particle sizes, supply chain security and high-temperature performance, provide a competitive edge to powder coating formulators seeking to excel in today’s and tomorrow’s marketplace.

BTDA is a registered trademark of Evonik Industries AG