Powder coatings consist of solid paint particles that melt and cure during film formation at high temperatures, generally 150°C to 220°C, on substrates that can withstand these temperatures. Substrates that are commonly coated with powder include metals such as steel and aluminium.¹
A wide range of important substrates, such as wood, MDF (Medium Density Fibreboard) and plastics, cannot withstand the high temperatures needed to melt and cure conventional powder coatings. Because of the clear advantages of powder coatings (like having excellent protective properties and being solvent-free and storage stable), industry is keen on developing powder coatings that can be applied on these so-called heat-sensitive substrates (HSS). Powder coatings for these applications are often called low temperature cure (LTC) powder coatings.
The key problem in developing and using LTC powder coatings is that two opposite demands must be met in one system.
First, the solid particles must melt and coalesce at temperatures that will not damage the substrate. For the heat-sensitive substrates mentioned, this is in the range of 110°C to 130°C. Also, the molten system must flow, level and fully cure at these mild temperatures.
Secondly, the powder paint must be storage-stable. The solid particles must not fuse together, and virtually no chemical reaction should take place within the particles at temperatures up to 40°C. The glass transition temperature (Tg) of the binder system of the powder paint must at least be 50°C to assure storage stability.
The Core of LTC Powder Coating
It became clear that breakthroughs had to be found with respect to the binder system of powder coatings—consisting of resins, crosslinkers and additives—to be able to meet the challenge of developing storage-stable systems that form a strong film at mild temperatures. One option is to use radiation-curing powder coatings, systems that cure under the influence of ultraviolet (UV) or electron beam (EB) radiation. Another approach is to use heat to obtain good film formation.
Unsaturated Polyester Resins
Unsaturated polyesters contain carbon-carbon double bonds (C=C). The double bonds can crosslink during film formation. There are several ways to initiate this curing mechanism, called free radical polymerisation. The first option is to use photoinitiators that decompose under the influence of UV or EB radiation. Another option is to use thermal peroxides that decompose under the influence of heat.
At present, it is possible to use powder coating on heat-sensitive substrates like MDF and wood. The Uralac® Ultra technology was developed by DSM Coating Resins, a leading company in resins for powder coatings.2 The binder system, which cures thermally in three minutes at only 130°C under infrared radiation, is based on unsaturated polyester and a peroxide initiator. Powder coatings based on this breakthrough technology are used on a commercial scale for furniture, kitchen cabinets and bathroom cabinets.
Kitchen cabinet coated with LTC powder coating. (Courtesy of DSM Coating Resins)
- Beetsma J.The Basics of Powder Coatings. 2015 Sept 4. Overland Park, KS: UL Prospector; [accessed 2016 June 20]. (https://knowledge.ulprospector.com/3008/pc-basics-of-powder-coatings).
- Verlaak J. Sustainable Powder Coatings for Wooden Furniture. September 2013. Milano, Italy: World Furniture Online; [accessed 2016 June 20]. www.worldfurnitureonline.com. Page 39-40. Membership required.
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