Both industry experts and market research firms agree that the biggest opportunity for expansion of the powder coating industry exists in the realm of heat-sensitive substrates. Most industrial metal parts that can fit into an oven are already being powder coated. Consequently, fabricated goods based on wood, plastic or composite substrates requiring a good durable and ecologically sound finish beckon powder coating manufacturers. But what technologies exist to meet the curing temperature thresholds, and just as importantly, the coating performance specified by the manufacturer? This edition of Technology Interchange will provide you with options to meet the critical specifications of these emerging markets.
Coating heat-sensitive substrates with powder coatings is tricky business; however, application engineers and powder coating finishers have developed techniques to apply a consistent, even coat by either preheating the part, applying a conductive solution or a combination of both. Once the powder has been deposited, it is necessary to melt, flow and cure the coating to provide a durable finish. This invariably involves applying heat with convection, or preferably, with infrared energy. After the coating has flowed and wet out the substrate, cure is accomplished with either more heat or the application of UV energy.
Thermosetting chemistries require heat to harden or crosslink the powder coating binder. UV-curable powders, on the other hand, utilize high intensity UV radiation to cure. Both can be used to coat heat-sensitive substrates. Within each of these broad curing categories are binders designed for a wide array of performance features. Let’s review options within each category.
Low-temperature Thermosetting Powder Coatings
Epoxy.
Low-temperature cure epoxies rely on formulation modifications of the more traditional epoxy powder types and deliver similar performance. Basically, resins are altered to be lower in viscosity and melt point, thus affording smooth films at relatively low curing temperatures. In addition, catalysts are incorporated to speed up cure response, but not so much as to cause pre-reaction before the powder is applied. Clever formulators have identified catalysts possessing a latency that allows rapid cure in the oven, but reasonable stability at ambient conditions.
Low-temperature cure epoxies can be crosslinked at as low as 125 degrees Celsius (257 degrees Fahrenheit). This can be accomplished using infrared curing equipment in as little as five to 10 minutes depending on part density and equipment settings.
Epoxy chemistry provides excellent chemical resistance, hardness and wear resistance. However, low-temperature types, like their more common standard-cure cousins, will fade and chalk when exposed to sunlight, thus relegating them to indoor end-uses. Typical uses for low-temperature cure epoxies include MDF shelving, composites (indoor), automotive underhood assemblies, brake parts and as primers.
Epoxy-Polyester (Hybrid).
Low-temperature cure hybrids are a variation on straight epoxy chemistry. These types are comprised of a blend of polyester and epoxy polymers and offer better yellowing resistance and somewhat lesser chemical resistance and hardness. These coatings are designed for indoor applications that require lesser durability requirements than epoxies but tighter color tolerance. Common uses include indoor MDF shelving for both general purpose and office furniture, as well as RTA (ready-to-assemble) furniture. Low temperature cure hybrids are not as reactive as epoxies and have a minimum cure temperature of around 140 degrees Celsius (284 degrees Fahrenheit).
Polyesters. TGIC (tri-glycidyl isocyanurate)-cured polyesters can be used for low-temperature applications. Just like low-temperature cure epoxies, polyesters have been modified for lower viscosity and melt point to allow film formation and cure at temperatures as low as 135 degrees Celsius (275 degrees Fahrenheit). Polyesters possess good to excellent outdoor durability and a good balance of chemical and wear resistance.
It is interesting to note that TGIC-free polyesters (aka HAA Polyesters) have only modest low cure potential. The low cure capability of this chemistry is about 160 degrees Celsius (320 degrees Fahrenheit) which helps reduce energy needed to cure the coating but doesn’t avail itself for application to most heat-sensitive substrates. Low-temperature cure polyesters open up a myriad of outdoor applications, including architectural pultrusions, composites for ACE (agricultural, construction and earth moving equipment) components, powered sports equipment and outdoor signage.
Unsaturated Polyester. About six years ago, a major resin supplier released low-temperature cure powder coating technology to a select few powder coating manufacturers. This resin binder is based on unsaturated polyester cured with a divinyl ether and emanated from technology developed in the 1990s. The resin manufacturer has now made this unique approach available on a more widespread basis. Powders based on this technology rely on latent peroxide cure and can provide cure in as little as five minutes at 130 degrees Celsius (266 degrees Fahrenheit). Using infrared heat to cure is highly recommended to rapidly melt the powder and minimize cure time. The coating of articles designed with MDF is specifically targeted for this chemistry.
Polyurethanes. Polyurethanes are not known for lowtemperature capability; however, a few niche products can be cured at around 145 degrees Celsius (293 degrees Fahrenheit), albeit for only about 25 minutes. Polyurethanes offer unique performance, including excellent chemical resistance, one shot matte finishes and very good UV resistance. This low-temperature approach finds use on composites and some SMC (sheet molding compound) applications that require outdoor durability.
Acrylics. Acrylics are a very interesting but oftentimes overlooked powder coating type. Formulators have developed low-temperature cure variations of this highly weatherable powder chemistry. Resultant finishes exhibit excellent smoothness, high gloss, scratch resistance and distinctness of image. Exterior durability can exceed 10 years of Florida exposure, making them a good choice for automotive trim, architectural components and farm equipment end-uses.
Bio-based Polyester Amides. A powder coating technology based on soybean derivatives is emerging and has been presented at major coating conferences over the past couple years. This unique polymer technology is comprised of 84 percent true bio-based components and offers cure as low as 135 degrees Celsius. What’s more, it exhibits excellent accelerated UV durability (4000 hours QUV-B) and coating flexibility. This technology is currently being scaled up to pilot-sized quantities and is available to the powder coating industry at large. Targeted end-uses include MDF components, architectural pultrusions and composite-based items. Coatings are being placed in Florida exposure testing and are expected to provide excellent outdoor durability.
UV-Curable Powder Coatings
The concept of UV-curable powder coating is not new. The first patents were issued in the 1970s and commercialization debuted in 1997. A small number of UV-cure powder finishing lines were installed in the ensuing years. End uses span everything from preassembled electric motors to automotive radiators and MDF shelving and furniture. Some finishing lines fell by the wayside, whereas a few enterprising stalwarts continued using this unique technology and have built long-lasting businesses.
The UV powder cure process entails three basic process steps:
- Application of powder coating to the substrate.
- Melt and flow, typically using infrared heat.
- Cure using high intensity UV.
Because the cure is accomplished by UV radiation, the entire finishing process can be quite compact. Application can take seconds, melt and flow 60 to 120 seconds and UV cure less than five seconds. Consequently, heat exposure to the substrate is limited to less than five minutes and capital equipment needs are modest.
UV-curable powder coating technology is not a specific chemistry, but a cure mechanism that can be utilized with a variety of resin types. Polyester resins are most common; however, epoxy and polyurethane types are available. Formulators often use blends of these resins to balance film performance and raw material cost.
UV-curable powders are emerging as viable alternatives to 2K urethane liquid coatings. Not only do they offer an ecological, high efficiency finishing process; they also require a shorter conveyor and process time. In addition, they eliminate issues with capturing and reporting VOCs. This is opening a variety of markets, including automotive parts and trim, ACE components, personal electronics and office furniture.
Handling Issues
Powder coatings capable of low temperature cure are based on resins possessing relatively low melt points and low viscosity. In addition, the cure chemistry is highly catalyzed. Therefore, it is necessary to exercise caution when transporting, storing and handling these products. With most types it is highly recommended for the powders to never experience ambient temperatures above 25 degrees Celsius (77 degrees Fahrenheit). This may require the use of refrigerated transportation and specific timing of when and how long shipments are on the road. For example, shipping over weekends is to be avoided to ensure expeditious delivery and to minimize exposure to inclement conditions. In addition, it is recommended to “pre-condition” powder that has been in cool storage. Powders relocated to the finishing shop should acclimate to the shop environment for 24 to 48 hours before introducing it to the application system.
Converting your finishing system over to low-temperature cure powder coatings requires doing your homework. Careful investigation of processing equipment is necessary as well as verification of coating performance to meet the requirements of your customers. Nevertheless, powder coating technology is commercially available for application to a myriad of heat sensitive substrates including MDF, hardwood, injection molded plastics, composites and SMC. Contact your powder coating suppliers and become part of the evolving expansion of powder into new frontiers.
- by Kevin Biller, technical editor of Powder Coated Tough and president of The Powder Coating Research Group.