Types of Powder Coatings
There are two types of powder coatings—thermoplastic and thermosetting. A thermoplastic powder coating is one that melts and flows when heat is applied but continues to have the same chemical composition once it cools and solidifies. Thermoplastic powders exhibit excellent chemical resistance, toughness and flexibility. They are applied mainly by the fluidized bed application technique, in which heated parts are dipped into a vat where the powder is fluidized by air and are used in many thick film applications.
Thermosetting powder coatings are based on lower molecular weight solid resins and melt when exposed to heat. After they flow into a uniform thin layer, however, they chemically cross-link within themselves or with other reactive components to form a reaction product of much higher molecular weight. These newly formed materials are heat stable and, unlike the thermoplastic products after curing, will not soften back to the liquid phase when heated. Thermosetting powders are derived from three generic types of resins: epoxy, polyester and acrylic. From these resin types, several coating systems are derived. Resins used in thermosetting powders can be ground into fine particles necessary for spray application and a thin film finish. Most of the technological advancements in recent years have been with thermosetting powders.
Virtually Any Color or Gloss. Color variety is almost limitless these days, with many coating suppliers offering a pallet of off-the-shelf colors as well as custom color match services to better meet the needs of small-volume powder users.
There are also tints which can add highlight color to a substrate or base coat, such as a brass look over polished aluminum.
A range from flat to high gloss is generally available. Smooth, high gloss coatings can offer high distinctness of image creating an illusion of depth or wet looks. Matte finishes can hide surface defects or imperfections such as spot-welds, nicks and scratches on a variety of substrates.
Multi-color. Hammertones or veins give antique or distressed looks created by a black base with metallic pigments of gold, silver or copper contrasting against the black. This weathered look is popular in the furniture and display industries that demand a broad range of multi-color looks including granite, confetti, rusty and weathered appearances.
Clearcoats. Clear powders are often used as an external protective layer on many brass products such as door knobs, hinges, railings, lamps and plumbing fixtures. There are also powders available that can substitute for chrome and brass plating in certain applications, even matching the high gloss of chrome. Clearcoat primers and powder clearcoats over automotive exterior basecoats are being used on various models of several automakers.
Chrome-like Appearances. As chrome plating becomes more heavily regulated and more expensive, the powder coating industry has developed chrome replacement coatings, to replicate the chrome look for decorative chrome applications, such as wheels and automotive trim.
Fluorescent/Pearl-like. Powder coated finishes can now offer an icy or fluorescent appearance or the lustrous look of a pearl that may appear to change color depending on the angle of observation. Bicycles, wheel rims, garden tools, light fixtures, and some glass products benefit from this look.
Photoluminescent. Photoluminescent powder coatings are able to absorb the ultraviolet, fluorescent, or incandescent light, and gradually re-emit the light. These are often available in blue, yellow, and green, and are used for road signs, hazard warnings, and markings on trucks, buses, and police cars.
Metallics. Metallic coatings, using particles of aluminum, mica, copper, or brass, add sparkling highlights which can reproduce the appearance of the base metal and add sparkle to the look of the product. Metallic bonding can physically bond metal flakes to the basecoat powder, bringing greater depth to the look of the product. A variety of metallic finishes have been formulated for indoor and outdoor furniture, exercise equipment, lawn and garden tools, and other products, that can resemble the look of gold, chrome, or brass.
Textures. Textured coatings are often used to hide substrate irregularities which may show with a smooth finish, provide a non-slip surface, or hide fingerprints, while giving a distinctive feel to a product. Appearances vary from the look of fine sandpaper, a pebbly texture, or a rougher look resembling alligator skin.
Wrinkles. Wrinkle finishes are a special class of textures which offer styling variation and a consistent appearance. They exhibit excellent resistance to high wear and weatherability conditions seen with tools, exercise equipment, and shop displays.
Wood and Marble Effects. New finishing processes provide wood-grain and marble effects to aluminum, often used on indoor and outdoor architectural products such as furniture, windows, doors, vents, handles, and fixtures.
Dye Sublimation. The process of transferring three-dimensional images onto parts and products, known as dye sublimation, has been successfully applied to powder coated products. Now high-resolution images, designs, and wood-grain looks can be transferred to a variety of powder coated products made of steel, aluminum, glass, and medium density fiberboard. Bottles, multi-function knives, bicycle frames, and writing utensils are among the products benefiting from this vibrant new look.
Advanced Durability. Recent research and testing has developed specially formulated polyester powders that provide even greater weatherability than achievable in previous years, and showing greater success in long-term Florida exposure testing. TGIC-based powders are being used on outdoor stadium seating and other exterior applications that were previously susceptible to degradation from ultraviolet rays. Light poles, lawn furniture, shopping carts, and shelving often benefit from powder with polytetrafluoroethylene (Teflon) additives for increased resistance to abrasion.
Electroconductive & Electrodissipative. Makers of electronic components sometimes use electroconductive and electrodissipative powder coatings, which provide electrostatic discharge protection of the components, critical during the manufacture, testing and transport of electronic goods. The most common colors for these special powders are black and charcoal, but lighter colors are also available.
Heat-Sensitive Substrates. In recent years, ongoing research and development has been devoted to formulating powders that can cure at lower temperatures as low as 200°F, without compromising durability or quality. These powders can be used on temperature-sensitive materials, as well as on massive parts that require enormous amounts of energy with other curing systems. More wood materials such as particle board and fiberboard can now benefit from a powder coated finish. This technology has helped to penetrate markets for office furniture, kitchen cabinets, and ready-to-assemble furniture for homeowners. In-roads have also been made in perfecting powder coating for hardwoods.
In addition to successfully penetrating the market for wood products, powder has proven to be an effective finishing choice for other heat sensitive substrates such as some plastic parts and products, using nylon, polycarbonates, polyethylene, and other materials. Plastic products now powder coated include office furniture, handles and knobs for major appliances, automotive parts and trim, medical equipment, and seating for buses, trains, and stadiums. The coating is effective in covering imperfections in the plastic molding process.
Assembled or partially assembled products such as appliances, electric motors, automotive radiators, shock absorbers, and foam core doors, and other products that may have plastics, laminations, electrical wires, or rubber seals, can now also receive a powder coated finish. In addition, heat-sensitive alloys such as magnesium can now be powder coated. Powder coating is also used to color or frost glass products such as bottles or light bulbs.
Hygienic Powder Coatings. Ancient Egyptians and Phoenicians used silver particles to repel microorganisms and germs from spoiling food and water. Now an antimicrobial silver additive is being used in powder coating finishes on germ-gathering surfaces in hospitals and doctor’s offices, including medical implants. In addition, grain silos, restaurants, water treatment plants, laboratories, senior facilities, food packaging plants, kitchens, bathrooms, and children’s rooms now benefit from antimicrobial and antibacterial powder coatings.
High-Temperature Resistant Powders. Silicone-based powder coatings are often used on products that are subjected to prolonged periods of high heat, up to 1,000°F, and hold their color, gloss, and adhesion. Some of these products include gas and charcoal grills, heaters, fireplace inserts, engine exhaust components, and lighting fixtures.
Thin-Film Powders. Powder manufacturers have developed more powders for use in thin-film applications, at 1 mil or less. These powders offer more with less—better powder penetration into recesses, more film thickness control, and more effective first-pass transfer efficiency, with darker colors most effective in hiding the substrate underneath.
UV-Curable Powders. Specially formulated powders can cure by ultraviolet radiation in a matter of seconds, which is one way to cure products with heat sensitive-substrates or components. Many wood products have enough moisture content to provide sufficient conductivity, and electrostatic attraction can be increased by pretreating and pre-heating the wood to promote adhesion.
Near-Infrared-Curable Powders. Other specially formulated powders can be cured in a matter of seconds through near-infrared technology, due to the extremely high heat-up rate. These powders can even work on some heat sensitive materials, because the irradiated energy is almost entirely absorbed by the powder coating layer.
Just as powder coatings can now be used in applications once thought impossible just a few years ago, continued advancements in powder coating formulation will ensure greater powder penetration in markets and on substrates in the near future that are currently thought to be unattainable.