Low-Temperature-Cure Powder Coating Technology Concepts

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 Use Low-Temperature-Cure Powder Coating?

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 vs. 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) vs. the more complex proposition of “ultra-low-cure” (ULC) powder technology designed for 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 vs. low bake oven conditions.

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.

(*Natural gas $/100 cu ft as of May 2022, https://www.eia.gov/dnav/ng/hist/n3035us3M.htm)

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/year operation, this equates to an annual savings of $3,325. Overall savings will vary depending on 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 comprises 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.

When using a UV-curable powder, the coating is formed into a film with infrared heat, which usually takes 60 seconds. The molten powder coating is exposed to high-intensity UV light. The unsaturated chemical groups react almost instantaneously, and the coating is cured. After a brief cooldown, the parts can be handled.

Other heat-sensitive substrates include plastics and composites. Practically speaking, the integrity of the part must be maintained at a minimum of around 203°F (95°C). Plastics possess a characteristic heat deflection temperature at which the plastic distorts or deflects under a modest load (see ASTM D-648). Table 3 lists the most common plastics used in the fabrication of parts typically coated for eventual assembly, along with their characteristic heat deflection temperature.

Preassembled items offer another heat-sensitive-substrate (UVPC) scenario. The first commercial UVPCs were applied to fully assembled electric motors at the Baldor Motors plant in Westville, Okla. UV-curable powder was applied to rotating motors that traversed on a chain-on-edge spindle conveyor. The coated motors were exposed to infrared for 3 minutes, followed by exposure to UV lamps. UV-curable powders have been evaluated as a finish for automotive shock absorbers, car radiators, and hydraulic door closers.

Non-Conductive Substrates

The conventional powder coating process is accomplished by generating an electrostatic charge and moving a cloud of particles through the field created by this charge. The charge is produced by a high-voltage corona at the tip of the powder gun discharge. The particles acquire a negative charge as they pass through the field. These charged particles seek a conductive object earthed to a ground. The negatively charged particles are deposited, and the excess electrons are discharged through the ground.

Non-conductive substrates pose a challenge with electrostatic spray processes. Powder deposition is predicated on charged particle attracted to a ground. Hence, conductivity must be introduced into the non-conductive part. This can be accomplished by the addition of conductive species. Carbon black or other materials—such as carbon nanotubes or graphene—can be incorporated into plastic substrates to impart conductivity. However, these techniques are cumbersome and expensive.

Another option is a thermal spray process, which entails specialized spray equipment that introduces a flame (or plasma) at the exit of the gun. The energy melts the powder cloud and deposits a molten mass unto the substrate. Film thickness is difficult to control using this technique, and low-temperature powders can pre-react before forming a reasonably continuous film.

Non-conductive parts can be preheated and sprayed “hot” with conventional powder application techniques. However, thermal losses can be inconsistent throughout the part. This adversely affects deposition efficiency and film thickness control.

The in-mold coating process is a unique approach to powder coating non-conductive substrates. In this scenario, powder is applied to a conductive mold and melted; the part is then built with the mold using either successive layers of resin and lathe or with polymeric sheets. The powder is cured along with the part inside the mold during the fabrication process. This process requires dedicated, relatively complex equipment, but it may be appropriate for specific applications.

The application of a simple conductive solution onto a non-conductive part generally imparts adequate conductivity to electrostatically deposit a powder coating. These conductive solutions are based on quaternary ammonium compounds (see U.S. Patent 3,236,679) and can be water or alcohol based. The solution is applied to the part, and the carrier is flashed off, leaving a conductive surface.

If the coating used is based on UV chemistry, the melted coating is quickly exposed to a few seconds of intense UV energy. At this point, the coating is completely cured and can be cooled to afford handling to the next manufacturing step.

ULC Powder Coatings – Polymeric Considerations

Practical approaches to lower temperature curing rely on highly reactive solid polymers and oligomers coupled with specialized catalysis schemes. However, achieving a smooth, aesthetically pleasing film requires the judicious manipulation of polymer properties to provide excellent melt viscosity at targeted cure temperatures—while maintaining an acceptably high melting point—to ensure adequate storage stability. Hence, polymer chemists and physicists must craft lower-molecular-weight resins that exhibit low melt viscosity while maintaining relatively high glass transition temperatures (T_g) to ensure physical storage stability.

Another technique used to develop smooth films with LTC and ULC powder coatings is the incorporation of reactive and non-reactive crystalline and semi-crystalline materials. These products must be compatible with the base polymer and can only be used in relatively low concentrations (< 10% of binder) because their presence can create compounding issues in the extrusion process. When crystalline materials melt during extrusion, the overall melt viscosity may decrease to the point of significantly reducing shear. This loss of shear must be avoided, as it can interfere with the mixing and dispersion of the powder coating, causing phase separation of binder components and incomplete dispersion of additives and pigments.

LTC Powder Coating Chemistries

Polyester powder coatings cured with tri-glycidyl isocyanurate (TGIC) are well-established as an LTC option. Polyester resins designed for LTC have been modified for lower viscosity and melting point to allow film formation and curing at temperatures as low as 275°F (135°C). Powders based on this chemistry possess good to excellent outdoor durability and a good balance of chemical and wear resistance.

TGIC-free polyesters, which are hydroxy alkyl amide (HAA) polyesters, possess only modest LTC potential. This lowest feasible cure temperature for this chemistry is at least 325°F (163°C), which may reduce the energy needed to cure the coating but doesn’t avail itself for application to most heat-sensitive substrates.

Epoxy- and epoxy-polyester (hybrid)-based powder coatings are capable of curing at around 255-285°F (124-141°C). These chemistries offer good flow and leveling to produce aesthetically pleasing finishes. In addition, they offer good to excellent hardness, scratch, and chemical resistance. Both types possess limited UV durability and should not be considered for outdoor applications.

A unique binder chemistry developed by DSM (now Covestro) employs the reaction of an unsaturated polyester with a crystalline vinyl ether crosslinker. Powders based on this technology offer good overall film performance (hardness, chemical resistance, smoothness) with a relatively short bake time at 265°F (129°C).

GMA acrylic technology provides excellent Class A automotive appearance coupled with outstanding UV durability (10 years, Florida) and chemical resistance. These are priced relatively highly and are incompatible with other powder chemistries, causing craters with cross contamination.

LTC polyurethanes utilize a unique blocking agent to achieve cure below 300°F (572°C). The normal blocking agent, є-caprolactam is replaced with a pyrazole compound that is liberated around 290°F (143°C), allowing for lower temperature curing. Powders based on this binder chemistry have excellent chemical resistance and can be formulated into attractive matte finishes.

UV-Curable Powder Coatings

Radiation-curable powder coatings are a separate class of ULC powder coatings. This unique technology originated as a laboratory curiosity in the 1970s by Vince McGinniss at Glidden Coatings (see U.S. Patent 4,163,810). It was not until the 1990s that a concerted effort at commercialization was made by resin companies—UCB (now allnex), Hoechst (now allnex), DSM (now Covestro), and Degussa (now Evonik)—and by powder formulators—Herberts (now Axalta), Morton (now AkzoNobel), and ProTech. In addition, the upstart powder applicator, DVUV, formed its own UV powder formulation enterprise, Keyland Polymer.

These pioneers established the foundation for the UV-curable powder coating (UVPC) technology still available today. UVPCs comprise a delicate blend of unsaturated polymer binders, photoinitiators, film formation additives, and, if required, colorant pigments and fillers. The powder coatings are compounded using conventional processing techniques, although the low melt point of most UVPCs requires cooler grinding conditions and storage of the finished powder coating.

The finishing process for UVPCs begins with application to the substrate, which can be either conductive or non-conductive. The deposited powder is fused and melted to form a film with heat, typically applied by infrared emitters and/or convection. The molten film is exposed to high-intensity UV radiation, which causes the coating to cure and harden. The UV energy source can be delivered by medium-pressure sealed lamps that rely on mercury vapor to emit EMR wavelengths capable of exciting photoinitiators to affect free radical cure of the unsaturated polymers. In recent years, UV emitters based on LED technology have developed that show promise in curing some types of UVPCs.

The advantages of UV-curable powders are manifold. Processing temperatures can be as low as 203°F (95°C) for as little as 60 seconds, with infrared as a preferred heating method. The footprint of the finishing system can be fraction of that of a conventional powder coating system. In addition, most UVPCs provide excellent hardness and chemical resistance. The formulator’s toolkit of UV-curable powder resins is still limited, but there are many chemistries available, including polyester, epoxy, and urethane varieties. Formulating for performance typically requires as blend of two or more chemistries to meet coating requirements.

The Future of LTC and ULC Powder Coatings

The paradigm has shifted regarding “standard-cure” powder coatings. Traditional powder coatings once considered standard, which cured at 375-400°F (191-204°C), have been replaced in many applications with 325°F (163°C) bake powders. Moreover, ULC powder technology is pushing the frontiers of cure technology to sub-300°F (149°C) curing conditions, expanding powder’s reach into a universe of new applications. Powder coatings are being evaluated for coating-injection molded plastics, sheet molding compounds, composites, and hardwoods. In addition, powder coatings are being tested as a finish for assembled items that possess heat-sensitive components, such as wiring, seals, and electronic boards.

The success of new applications reaching commercial status will spawn additional research and development in materials and processes, which will in turn create innovation to expand LTC and ULC powders into even more novel end uses. Resin companies have renewed their efforts to develop ULC powder resins and revived their ULC product portfolios to support formulators pursuing new opportunities.

Certain universities have established programs to develop the next generation of UV-curable powder coatings. The University of Akron, under the tutelage of Professor Mark Soucek, has recently hosted a steady stream of graduate projects with a concentration on UVC powders. In addition, some breakthrough work has been published by a Ph.D. candidate, Dominika Czachor-Jadacka, under the direction of Professor Barbara Pilch-Pitera at Rzeszow University of Technology in Poland.

The powder coating industry is poised to create the next generation of technology to conquer a vast array of non-traditional and heat-sensitive substrates.

For more information, contact the author at kbiller@chemquest.com or visit https://chemquest.com/cqpcr.

See the article in Coatings World.