Powder coatings have been around for decades. Early commercial versions emerged in the 1960s and 1970s; however, the industry did not gain momentum until the late 1980s and 1990s. This technology is dominated by thermosets, but thermoplastics represent an important niche with an approximate 5% market share. Radiation curable varieties emerged in the mid-1990s as a unique intersection of solid powder with ultraviolet curing binders capable of curing at temperatures as low as 212°F (100°C). Powder coatings represent about 17% of the global industrial coatings market with nearly $14 billion in annual sales and an estimated CAGR of 4.3%.
Historically, most powder coating chemistries required relatively high curing temperatures ranging from 350–400°F (177–204°C) for 5–20 minutes (substrate temperature). Lower-temperature curing chemistries emerged in the 1990s and are becoming commonplace. The following will present compelling reasons for the use of low-temperature-cure powders and identify which end uses can benefit with their use. State-of-the-art binder technologies will be presented, along with the unique application and curing techniques required to utilize them. And finally, we’ll glimpse into the future to glean where the latest low-temperature-cure powders will emerge commercially.
Why Low-Temperature Curing?
Finishers may use low-temperature-cure powder coatings for a variety of reasons, such as energy savings and opportunities to coat heat-sensitive substrates. Energy savings scenarios typically use powder coatings capable of curing at around 325°F (163°C) instead of a more conventional 375°F (191°C) cure. Heat sensitive substrates require powder coatings capable of curing below 300°F (149°C) and preferably around 250–260°F (121–127°C).
Low-Temperature Curing versus Ultra-Low Temperature Curing
It is important to differentiate between coatings technologies designed for relatively straightforward energy reduction (i.e., “Low Temp Cure” or LTC) versus the more complex proposition of “Ultra Low Cure” (ULC) powder technology designed for the application to heat sensitive substrates. Ideally an LTC powder will reduce needed oven operating temperatures with minimal alteration to an existing application process. ULC technology, by its nature, usually involves specialized transport, handling, storage, and application/curing equipment.
Energy Savings with LTC Powder Coating Technology
Reducing oven curing temperatures can provide quantifiable savings in operating costs. LTC powder coatings are available from most powder coating suppliers and usually carry a modest premium. The energy savings to replace a standard curing product with an LTC alternative can be determined by comparing the fuel costs to cure coated parts at standard versus low bake
Table 2 demonstrates the potential energy savings realized by switching from a 375°F (191°C) curing product to a 325°F (163°C) bake powder. It depicts a hypothetical comparison of a natural-gas-heated curing oven conveying 1 lb. steel parts at 3 feet per minute. The following worksheet requires data input for the product being used and the oven engineering. It also accounts for the type of metal being coated, the density of the conveyor/trolleys, and the insulation of the oven walls. It is important to factor the typically higher material costs for LTC powders into the analysis.
This comparison demonstrates the achievable cost savings by reducing a powder curing oven from 375°F (191°C) to 325°F (163°C). Cost savings realized per 9 hour shift are $13.30 or 16.4% (based on the cost of natural gas in May 2022). For a one-shift, 250 day per year operation, this equates to an annual savings of $3,325. Overall savings will vary depending upon oven capacity, burner type, part weight, metal type, line speed, and relative costs of the powder coatings.
Heat Sensitive Substrates
Traditionally powder coatings have been used as a finish for metal-based parts, generally in a factory setting. Powder is applied electrostatically with automatic and/or manual spray guns, and the ware is placed in an oven until the parts are heated to the recommended conditions (time and part temperature) required to cure the powder coating. The old adage, “if it’s metal and can fit into an oven, it can be powder coated” applies here.
The development of LTC and ULC powders has opened the door to a vast array of non-traditional substrates that can be finished with powder coatings. One of the most prominent substrates is medium density fiberboard (MDF), which is commonly used for kitchen cabinetry, hospital carts, point of sales displays, ready-to-assemble (RTA) furniture and shelving. Both ULC powders and UV-curable powder coatings are used to finish MDF components.
The powder finishing process for MDF is comprised of four steps. After machining and sanding, the MDF surface is preheated with infrared to approximately 250°F (121°C). This step takes 30–60 seconds and enhances surface conductivity, which allows the powder to be applied with a conventional electrostatic process. The heated board is transferred into a powder application booth, and powder is typically applied with automatic spray guns. Applied film thickness is normally 2.5–4.0 mils, depending on the end-use and coating requirements. Film thickness control is more critical with UV-curable powder, which is usually limited to 2.2–2.8 mils.
The coated boards are then exposed to another set of infrared emitters that melt the powder layer into a continuous film. If a thermosetting powder is used, this is followed by more heat, either by infrared or a combination of infrared and convection. The thermoset powder usually requires a cure of 5–10 minutes at 250–275°F (121–135°C) depending on the formulation. Parts are cooled to allow handling.