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New to thermoset powder coating? Here is a breakdown.
According to Thomas Net:
“Thermoset materials are used for the majority of powder coating processes because they can provide a surface layer that is both durable and decorative. Most thermoset powders have a molecular weight lower than that of thermoplastic materials and higher than that of liquid coatings. Thermosets are solid resins that, when heated, melt, flow, and—unlike thermoplastics—can crosslink with one another or other reactive substances to form compounds with higher molecular weights. After curing, a thermoset coating remains thermally stable, meaning it cannot be melted back into a liquid from further heating.
“In a powder coating system, the more brittle thermoset resins can be broken up into a very fine powder that can then be fabricated into an exceptionally thin, paint-like film. This film exhibits chemical and physical properties comparable to those of liquid-based coatings. In addition, advancements in thermoset technology have continuously increased the versatility of this material group, allowing for more customization options. Many standard thermoset materials can now be chemically altered to provide the specific characteristics required for a given application.
Thermoset Powder Coating Properties
“The most commonly used thermoset powder coating resins derive from the epoxy, hydroxyl or carboxyl, acrylic, and silicone groups. They generally require lower curing temperatures than thermoplastics, and manufacturers often introduce additives to accelerate or delay the curing reaction. It is important to subject a powder-coated product to thorough heating and allow the thermoset to flow evenly over the targeted surface because once the thermoset powder has completed crosslinking, it cannot be reflowed to correct any flaws in the coating. To achieve successful coating results, it also necessary to match the thermoset formulation with the product’s intended corrosion resistance, curing cycle, texture, and aesthetic properties.
“One of the most significant developments in thermoset powder coating involves the capacity to engineer resin types with variable properties designed to complement metal finishing treatments. This broader range of characteristics has led to an increase in specialized roles for thermoset powders, with polyesters and acrylics finding greater use in the automotive and appliance industries despite the traditional reliance on epoxy-based coatings. Most thermoset powders can provide a high level of corrosion, temperature, and impact resistance. They can also be designed with a wide range of colors, glosses, and surface finishes. Coating texture can be wrinkled, smooth, or matte, while film thickness is highly adjustable.
Epoxy-Based Powder Coatings
“Epoxies are the most common thermoset resins used in industrial powder coating, and they have a wide range of formulation options. Different types of epoxy can be fabricated into functional thick film or more decorative thin film, while their crosslinking properties are similar to those of epoxy adhesives and paints. Most epoxy-based materials are crafted to be thermally stable at room temperature. Some standard epoxy powder coatings include:
“Functional Film: Thick, functional film epoxies are often employed for electrical insulation and corrosion resistance applications. As insulation, epoxy powder bonds to a surface and follows its contours with relatively few voids or other defects. This is useful for devices such as electrical motors, switch gears, and automotive alternators. Anti-corrosion epoxy provides low cost and long-lasting protection for products in chemically hazardous environments, such as underground gas and oil pipes.
Thin Film: Thin film coatings are typically designed to provide specific decorative results while preserving the durability and resistance characteristics of other epoxy groups. They are generally restricted to interior coatings, as de-glossing and chalking are frequent risks for thin exterior layers. Thin epoxy film coatings are commonly found in fire extinguishers, furniture, hospital equipment, and a wide number of household appliances.
Epoxy-Polyester Blends: An epoxy resin crosslinked with a reactive acid polyester will form a blend, or hybrid, material that has flexibility and impact resistance qualities similar to those of other epoxies, but provides a higher level of ultraviolet light protection. These blends are effective when applied as an electrostatic spray and have a range of applications similar to those of thin film epoxies.
Acrylic Powder Coatings
“Acrylic powder coatings are beneficial for their high level of exterior durability and relative ease of application. Acrylics require curing temperatures close to those of hydroxyl polyesters, and they combine high quality surface aesthetics with flexibility and impact resistance. They also exhibit excellent alkali resistance, making them well-suited for use on appliances, such as ovens and washing machines. Acrylic powder coatings can be effectively applied through electrostatic spraying and have adjustable thin film characteristics. However, acrylics are more responsive to substrate attributes than most other powder coatings, making them incompatible with certain chemical compounds. Aside from their use in appliances, acrylic powder coatings can also be found on aluminum extrusions, automotive trim components, and tractors.
Hydroxyl and Carboxyl Polyester Powder Coatings
“Hydroxyl polyester powder, also known as urethane, provides both a high quality surface finish and resistance to wear. It is usually fabricated as thin film because its thick film form exhibits lower impact resistance and flexibility. A hydroxyl polyester coating has aesthetic qualities comparable to those of liquid paint, and it is commonly used for coating light fixtures, furniture, automotive components, wheels, and appliances. During curing, some hydroxyl polyesters release an emission that needs to be vented away from the cure oven to prevent defects or contamination.
“Carboxyl polyesters have durability characteristics similar to those of epoxy-polyester blends and weathering resistance comparable to urethane. They have consistent mechanical properties across a range of standard coating thicknesses and provide a high level of flow, glossiness, and material strength. However, their resistance to chemical solvents can be lower than that of hydroxyl coatings. Carboxyl polyester powder coating is commonly used on irrigation pipes, outdoor furniture, fences, aluminum extrusions, and steel wheels.”