Low-Temperature-Cure Powder Coatings Offer Unique Opportunities

The concept of low-temperature-cure (LTC) powder coatings has loomed in the shadows since the dawn of powder coating technology. Standard cure technology debuted in the 1970s and 80s and typically involved cure conditions that required temperatures ranging from 375-400°F and dwell times of 20-40 min, depending on the metal mass of the parts.

Over the last 30 years, clever formulators have developed technology that can be cured at ever-lower temperatures. Some of these LTC developments have been evolutionary in nature and feature existing chemistries reformulated to cure at lower temperatures, mainly to reduce energy consumption and possibly to increase line speeds. Other breakthroughs, such as ultra-low bake (ULB), are truly revolutionary and cure at significantly lower temperatures, enabling application to heat-sensitive substrates such as plastics, composites, engineered boards, gypsum, and hardwoods.

Low-Temp-Cure Powder Coatings

LTC powder coatings have become commonplace in the industry, and most conventional powder chemistries are represented. Advantages include energy savings, improved color control due to lower oven temperatures, and cure capability across significantly differing metal masses. This technology has blossomed from the intersection of informed formulating capitalizing on the judicious selection of catalysts, along with high-flow resins developed by astute polymer manufacturers.

These advances allow powder applicators to reduce oven temperatures from the 375-400°F range to 325°F and sometimes even lower. In another scenario, high-metal-mass parts can be cured alongside thinner-gauge parts even if the denser parts achieve a lower but still sufficient temperature to develop complete cure. The cost savings of both scenarios can be significant.

The cost benefit of transitioning from a standard product to a low-curing alternative should be carefully evaluated. Table 1 depicts the potential energy savings of reducing oven temperature from 375°F to 325°F with a conveyorized finishing line running 1-lb parts using a natural gas convection oven. It is important to input the proper data to develop a meaningful estimate of savings. The worksheet requires input of several data relating to the product and oven engineering. It also accounts for the type(s) of metal being coated, the density of the conveyor and trolleys, and the surface area and insulation of the oven walls. (This worksheet was originally developed by Rodger Talbert and is available from the author upon request.)

Table 1. Oven Energy Comparison

Energy Cost Savings per Shift:   $16.35

Percent Savings:                       16.4%

Potential Annual Savings:         $4,087.50 (250 shifts/yr)

Table 2. Oven Details

*Average February 2023 (www.chooseenergy.com)

Low-cure powder coatings are available from most powder coating suppliers and may carry a modest premium. While most LTC chemistries can cure at or around 300°F, polyester-HAA (TGIC-free) powders require a minimum of around 325°F to achieve adequate cure. Table 3 compares typical bake conditions for common powder types, both as standard and low-temp-cure alternatives.

Table 3. Typical Cure Conditions for Common Powder Coating Chemistries

Another potential benefit of using low-temp-cure powder coatings is the possibility of running faster line speeds. LTC powders can cure at shorter dwell times at the higher temperatures typically required for standard-cure powder coatings. It is not uncommon to be able to increase line speed 15-20% (i.e., reduced oven dwell time) using a low-temp-cure product. Implementing this move should be undertaken with care. Consultation with the powder supplier and verification of adequate cure should be confirmed before committing valuable parts to increased line speed.

A reduction in film performance should not be experienced by switching to a lower curing product. The exterior durability, chemical resistance, and mechanical performance should be comparable to the analogous higher-temperature curing product. Nevertheless, it is always wise to thoroughly review the technical data sheet associated with every product, especially when switching to an LTC powder.

Appearance properties of the coated product may be slightly different, as the gloss and smoothness could be affected by lower curing temperatures. It is strongly recommended to run the new product through the oven with typical parts and oven loading before committing to a wholesale product change. The oven setting and line speed may need to be tweaked a little to accommodate the new product.

Ultra-Low-Bake Powder Coatings

ULB powder coatings were introduced to the industry in the 1990s and include both thermosetting and UV-curable powder coatings (see Table 4). Both types were developed specifically for application to non-traditional, heat-sensitive substrates such as plastics, composites, carbon fiber, engineered boards, hardwoods, and fully assembled parts.

Table 4. Thermoset vs. UV-Cure ULB Powder Coatings

Electrostatic application of powders to nonconductive substrates requires the incorporation of conductivity to the surface prior to powder coating deposition. This can be accomplished by applying a conductive solution to the surface before introducing the part to the powder spray system. Conductive solutions are commercially available and relatively easy to use.

Novel thermosetting binders have been developed based on unsaturated polyester resins cured with vinyl-ether functional crosslinkers. Powders based on this chemistry rely on a rather latent cure driven with peroxide-based catalysts. The crosslinkers are semicrystalline in nature, which enhances flow and leveling at relatively low temperatures.

Since the debut of this technology, coating chemists have modified formulations to provide better processing and cure consistency. In addition, curing conditions have evolved to ensure thorough crosslinking without damaging heat-sensitive substrates. Specifically, curing schemes that combine infrared and convection heating have been optimized. Infrared curing is mainly directional and is delivered to the surface of the coated item, thereby concentrating the heat on the surface rather than the bulk of parts being coated. This allows the coating to be fully cured with less risk of damage to the substrate from excess heat.

The development of UV-curable (UVC) powder coatings combined radiation-cure chemistry with the solid form of powder coating. The technological magic of this marriage of technologies is the segregation of thermal cure from the powder manufacturing process. This allows UV-cure formulations to be manufactured without concern for pre-reaction in the extrusion process of making a powder coating.

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 energy, the entire finishing process can be quite compact. Application can take seconds, melt and flow 60-120 seconds, and UV cure < 5 seconds. Consequently, heat exposure to the substrate is limited to less than 5 minutes, and capital equipment needs are modest.

UVC powders are applied electrostatically (just like conventional powders) and melted by infrared or another heat source to form a continuous film. The still-warm coating is then exposed to high-energy UV energy to activate cure. The advantage of this approach is two-fold: the entire curing process can be accomplished in 1-5 min, and it requires quite low temperatures (220-255°F). A word of caution, however, as this technology requires all coated surfaces to be exposed to the required dosage of UV energy. If any coated areas are masked or otherwise not accessible, the coating can experience catastrophic failure due to an absence of adequate cure.

Handling Issues

ULB powder coatings are based on resins possessing relatively low melt points and low melt viscosity. In addition, the cure chemistry of thermosetting types may be highly catalyzed. Therefore, it is necessary to exercise care when transporting, storing, and handling these products.

ULB powders should not be exposed to ambient temperatures above 80°F. This may require the use of refrigerated transportation and specific timing for when and how long shipments are on the road. For example, shipping over weekends is to be avoided to minimize transportation time and extended exposure to elevated temperatures.

In addition, after receiving the powder, 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 24-48 hours before introducing it to the application system. Otherwise, condensation may damage the powder, causing clumping and handling issues.

Applications for ULB Powders   

Whether thermosetting or UV-curable, ULB powders offer an option to coat myriad heat-sensitive substrates. The ubiquitous end uses of medium-density fiberboard (e.g., cabinetry, office furniture, point-of-sale displays, hospital carts, stepstools, and children’s furniture) represent just a few examples of applicable products. Certain tight-grained hardwoods (e.g., birch, maple, mahogany, and cherry) and high-density fiberboards can also be coated with ULB powders, opening up applications such as architectural trim, moldings, doors, furniture, and shelving.

ULB powder can also be used to finish items constructed with plastics and/or composites, including sporting goods, automotive trim, engine covers, headlight lenses, ATVs, jet skis, pultrusions (window/door trim, ladders), motorcycle components, bicycle frames, chair pedestals, appliance handles, personal electronics, signage, flooring, medical instruments, military hardware, and a vast array of consumer products.

As with most organic coatings, plastics may require a surface pretreatment (chemical or mechanical) to ensure acceptable coating adhesion. In addition, atmospheric plasma has proven to be a viable process to enhance the adhesion of organic coatings (including powders) to a variety of plastic substrates.

Explore the Possibilities

Converting a finishing system over to low-temperature-cure or ultra-low-bake powder coatings requires some homework. Careful investigation of processing equipment is necessary, as well as verification of coating performance to meet customer requirements. Nevertheless, powder coating technology is commercially available for application to a host of heat-sensitive substrates. Companies looking to make the switch can contact their powder coating suppliers and become part of the evolving expansion of powder into new frontiers.