Any introduction to thermosetting powder coating should include a warning: "For users with imagination, powder coatings can be addictive."
The "addiction" begins when users of powder coatings realize the benefits they are accruing. It's not only that tougher federal and state regulations are making it hard to
ignore the environmental benefits of powders. It's the excitement brought on by new powder formulations, which have increased the range of potential applications ranging from automotive clear coats to coil coating.
What's more, custom powders formulated for large-volume users can now meet rigorous, individualized specifications. Those powders can provide a distinct, competitive advantage when it comes to lower reject rates, improved product appearance and performance, and resulting profits. For those who are beginning their investigation into thermosetting powder coatings, this overview gives a conversational knowledge of the types of thermosetting powders now being used in a variety of applications.
Today, six types of thermosetting powders are used: epoxy, hybrid, polyester-urethane, polyester-TGIC, polyester amide and acrylic.
Epoxy powders are largely functional in nature, providing excellent protection from chemicals and physical abuse. Rebar stock and well pipes are typical applications for epoxy powders, along with electrical junction boxes, office furniture and automotive underbody parts. This powder's ability to resist abrasion and its fast cure time make it a favorite for these uses. However, its low resistance to ultraviolet rays precludes its use on objects that will experience long periods of direct sunlight.
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| For a smooth, attractive finish that resists wear, Cosco coats these bunkbeds with hybrid powders. |
Hybrid powders tend to be lower in cost than other types, and offer some advantages over their epoxy cousins. Better behavior in tribolelectric charging applications and good Faraday Cage penetration make hybrids an excellent choice for many coaters.
Another advantage of hybrids is their ability to withstand over-baking and resist yellowing when compared to epoxies. While hybrids can be applied smoothly in a medium to high gloss, they don't have quite the chemical resistance and hardness of epoxy powders. Typical hybrid applications are file cabinets, drawer slides, shelving and hospital furniture.
Polyester-urethane powders provide some of the most attractive finishes. These coatings can be formulated in a variety of gloss levels, from a high-gloss, high-distinctiveness-of-image coating, to very low gloss.
An excellent choice for outdoor products, polyester-urethane powders can be found on lawn and garden equipment, patio furniture and automobile exterior trim. Indoors, appliances such as ranges and refrigerators are likely candidates.
Among the drawbacks of polyester-urethane powders are their restriction to low film builds (less than three mils) and lower corrosion resistance than other types of powder coatings, particularly over marginally pretreated substrates.
Polyester-TGIC powders have won nearly 24 pct of the powder coating market in the U.S. by providing good exterior durability and the reputation of good performance when applied to marginally pretreated substrates. In addition, this type offers good edge coverage and impressive chemical resistance.
Polyester-TGIC powder is used on air conditioning unit housings and other HVAC components, garden equipment, as clear coatings on products such as die cast aluminum wheels, and on outdoor electrical transformers, where 15 mils of coating are required for dielectric strength. A manufacturer is not likely to turn to polyester-TGIC powders for applications that calls for low-gloss properties, as it is difficult to formulate a low-gloss system in this chemistry.
Polyester amide powder coatings are a new type of chemistry available to end users. This chemistry is an alternative to polyester TGIC or polyester urethane coatings.
Polyester amide powders have found interesting lawn and garden, patio furniture, and other applications where outdoor durability is needed. This product has a little more orange peel than its polyester urethane cousin; however, it will provide good edge coverage. Water is produced as a byproduct during the curing process. Therefore, film thickness needs to be controlled to prevent outgassing.
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| Kohler faucets are coated with acrylic powders that provide them with a high-gloss, Class-A finish. |
Acrylic powders are available in two types: urethane and GMA. Overall, acrylics offer the best exterior durability of any thermoset powder coating, characterized by very good hardness and high resistance to stains, detergents and moisture. Applications include exterior light fixtures, appliances, bathroom fixtures and electrical junction boxes. Acrylic urethane powders are known to produce a high degree of orange peel. Another drawback of acrylic powders is relatively poor flexibility.
Future Developments. Can coating is a market of interest to powder coating manufacturers. Application equipment is now available to not only apply but also recover and recycle the very small particle size powder required to maintain thin films and to run at line speeds common in this industry. Food and Drug Administration approval must be obtained.
Lower-temperature cures are being developed that will allow more types of products to be coated with powder, including plastics and pre-assembled products that contain heat-sensitive fluids or gaskets.
Polymer and formulating chemists are working to develop powders that incorporate superior weathering capabilities, such as the new aliphatic polyester powders, new crosslinking technologies and other weatherable additives.
Powder manufacturers are continually working to develop powders that can form thinner films than previously attainable, resulting in savings of material and thus money.
New applications being developed for thermosetting powder coatings include plating replacements such as simulated brass.
As replacements for special-effect liquid coatings, veining and hammertone powder finishes already exist. Under development are such novelties as flecking, crackle, candy-apple, fluorescents, and even phosphorescents.
Demands for environmentally friendlier processes can be met by powder coatings, and powder manufacturers can deliver coatings to support product quality that consumers want and will pay for. PFD
