On the eve of MMXX, the year which marks the 65th anniversary of the original powder coating patent authored by Erwin Gemmer, we offer a review of the most recent breakthroughs in powder coating technology. Gemmer, if you recall, patented a novel process to coat preheated hand tools with a thick film by dipping them into a fluidized bed of thermoplastic particles. Over the ensuing decades, wise formulators and engineers advanced powder coating technology beyond Gemmer’s wildest dreams to create the thin-film world of thermosetting chemistries possessing remarkable durability, performance and, economics.
As requirements for improved performance in extreme working environments have emerged and the ever-present need for exacting process control and reasonable economics continues, technologists have stepped up to the plate to deliver the goods.
The concept of “high durability” covers a variety of notions. On one hand, functional coatings are being pushed to perform in harsher environments, including more aggressive chemicals and higher operating temperatures; on the other hand, architects are looking to the powder coating industry for a finish that can withstand 20 to 30 years of sunlight, precipitation and, various other environmental attacks. These are two of the most prominent areas that have recently experienced breakthrough advancements. We will focus on the primary resin technology that has made these advancements possible.
Functional coatings are typically used in extreme environments where the ultimate in corrosion and chemical resistance are required. Good examples are powders designed to coat pipelines and rebar. Energy companies are accessing crude oil and natural gas in harsher environments and using more aggressive means of extraction. The drilling equipment and corresponding pipelines need to withstand temperatures as high as 177 degrees Celsius (350 degrees Fahrenheit). Conventional functional powders, often described as fusion-bonded epoxies, because of the manner in which they are applied and cured, can withstand temperatures in the 120 to 135 degrees Celsius (250 to 275 degrees Fahrenheit) range. Recent advances in epoxy resin technology and powder formulating have pushed that boundary.
Functional powder coatings with very high Tgs (glass transition temperature) can be formulated thanks to innovations in epoxy resins by the Kukdo Chemical Co. Ltd. Polymer scientists there have developed a unique powder coating grade resin, KD-2012H, that yields coatings with a Tg of over 160 degrees Celsius (320 degrees Fahrenheit). (The significance of cured film Tg is that it marks thermal performance that allows the coating to maintain integrity at or just below that temperature.) Olin Chemical (formerly Dow Chemical product line) has developed an epoxy resin, DER 6510HT, that can be used to create cured powder coatings with a Tg as high as 170 degrees Celsius (338 degrees Fahrenheit). Both of these unique resins are commercially available.
SABIC, a relative newcomer to the powder coating industry, has developed epoxy resins that have the ability to produce cured film Tgs up to 200 degrees Celsius (392 degrees Fahrenheit). This unique chemistry is dubbed PPPBP-Epoxy and was presented last year at a major powder coating technology event. SABIC’s PPPBP-Epoxy can be used as a modifier resin with more conventional epoxy resins to dial in a specific Tg or as a sole resin crosslinked with either a phenolic or DICY-based curing agent.
Not only do functional coatings require durability at high operating temperatures, they also need excellent chemical resistance to handle fracking chemicals and proppants (e.g. sand, bauxite or, ceramics). Kukdo has developed an epoxy resin, KD-9211, which is based on Bis-Phenol F with a long molecular chain. They recommended combining it with a multi-functional epoxy novolac and cure with a phenolic hardener. The resulting coating provides a highly chemical resistant surface capable of withstanding a barrage of aggressive compounds.
Another key property of functional coatings is resistance to moisture. Hydraulic fracking requires the use of aqueous media. In addition, pipelines can also be used for the transport of hot water and steam. Kukdo has developed a high humidity resistant hardener for epoxy-based powder coatings. Laboratory tests have shown that powder coatings based on this phenolic hardener, KD-420A, maintain adhesion after nine days of immersion in 90 degrees Celsius (194 degrees Fahrenheit) water.
Adhesion to stainless steel and non-ferrous metals such as aluminum, magnesium and, titanium alloys is always a challenge for the powder coating formulator. Kukdo has developed an epoxy resin, KSR-950, that incorporates a silane pendant group which provides a significant improvement in adhesion compared to conventional epoxies. Test results show a powder based on KSR-950 maintains adhesion to un-pretreated aluminum substrates after 24 hours submersion in 90 degrees Celsius (194 degrees Fahrenheit) water.
Low-Temperature Cure Polyesters
Stepan Company has developed a new LTPC carboxyl polyester resin, Rucote 9900, designed for cure with TGIC. This resin exhibits excellent cure performance and appearance with bakes as low as 130 degrees Celsius (266 degrees Fahrenheit). Targeted end-uses include assemblies that are comprised of multi-components that have some parts that cannot endure typical high bake temperatures, finishing lines seeking lower temperature oven economics and, higher speed coating lines with short oven residence time.
Allnex has developed a comprehensive range of low-temperature cure polyesters designed to be cured with epoxy resins (hybrids), TGIC,
β-hydroxyalkyl amide (HAA) and, Araldite™ PT-910. Hybrid and PT-910 cure is claimed to be as low as 150 degrees Celsius, whereas these specialized polyesters can cure as low as 140 degrees Celsius with TGIC. Low-temperature curing polyesters designed for cure with HAA can be processed as low at 160 degrees Celsius. This sales range of polyesters is focused on meeting the needs of heavy substrates, pre-assembled goods and, heat-sensitive substrates such as plastics and engineered boards.
DSM has continued to optimize their Uralac® Ultra line of low-temperature cure polyester-based powder binders. This system is comprised of an unsaturated polyester and a divinyl ether curing agent and is said to cure as low as 120 degrees Celsius (add Fahrenheit?). The target market for this technology is replacing liquid paint and/or laminates on MDF (medium-density fiberboard). Infrared curing is recommended to speed up the cure speed of this unique technology. Cure times in as little as three minutes are achievable according to DSM.
High Durability Pigments
Formulators now have more choices in high-performance colorant pigments. Shepherd Color has partnered with Oregon State University to bring YInMn (yttrium-indium-manganese) blue to the coatings and allied industries. This dark blue shade inorganic pigment based on mixed metal oxides has high solar reflectivity, which is conducive to cool surfaces on roofs and other architectural surfaces. In addition, it has excellent UV durability, chemical resistance and, IR properties.
Shepherd Color has also introduced micro-mirror technology based on silver-coated glass flakes. These special effect pigments, dubbed StarLight, offer remarkable reflectance and sparkle and are designed for the transportation, architectural, electronics and, consumer goods markets.
Poly Group, LLC has developed a novel anti-microbial technology based on positively charged compounds that attack and destroy negatively charged pathogens. This technology, named Nouvex™, creates no chemical binding or linking issues, unlike other quaternary ammonium compounds, and has a large molecular structure providing limited toxicity. Nouvex™ has proven performance through licensed laboratory testing, so the EPA preservative and health claims will be accessible to the end-use product manufacturers. This makes Nouvex™ a potential replacement for silver ions, triclosan, copper, and other quaternary substances with little or no process modification.
Advancements have been made in how powder coatings are manufactured. Specifically, extrusion equipment companies have listened to powder operations managers and have designed feed systems that can accommodate difficult to feed premixes and materials. Some powder formulations are based on lightweight, low-density materials and have a tendency to build in extruder feed ports causing unwanted surging and variable torque. Material builds up in the feed zone and seriously compromises throughput and therefore, processing economics.
Baker Perkins’ Max3 feed system is based on a re-designed feed port and screw design which significantly improves the consistent flow of material into the extruder. Pilot plant studies show outputs of difficult-to-feed materials increased as much 34 percent. This innovative feed system is available on all new machines and as a retrofit option on existing machines ranging from the MPX24 up to MPX80 model.
Film Thickness Measurement
Evaluating film thickness on a coated (and cured) part is easy due to magnetic and eddy current instrumentation technology developed decades ago. Film thickness gauges are compact, easy to use and, relatively inexpensive. Every powder coating finishing line and laboratory should have one or more of them. Measuring the film thickness of an uncured powder layer is not so easy. Enterprising engineers have developed ultrasonic devices that can get the job done but are sometimes cumbersome when switching powder coating products and substrates. Recent advances have offered an alternative to these ultrasonic devices.
Winterthur Instruments AG, and their flagship product, the Coatmaster, have developed a non-contact means to measure the film thickness of both uncured and cured powder coatings. This technology is called ATO (Advanced Thermal Optics) and is based on the thermodynamics of a layer of powder and how it responds to a light pulse that warms the powder. Following the light pulse, high-speed infrared sensors detect the rate at which the material cools. Using proprietary algorithms, the Coatmaster is able to translate this rate of cooling to a highly accurate film thickness measurement. The time to take a reading varies by application, but is typically 100-300 milliseconds, and individual point readings can occur as quickly as once every 1.5 seconds.
The Coatmaster device can be mounted at distances from two to 48 inches and can read parts traveling as fast as 350 feet per minute. Data can then be captured and processed by quality control software that can emit alarms and analyze trends.
PhotoFusion Technologies, Inc. has developed a novel technique to cure powder coatings. Everyone is quite familiar with convection ovens and perhaps to a lesser extent the varieties of infrared curing technology available to heat and cure thermosetting powder coatings. PhotoFusion Technologies, Inc. takes powder coating curing to a whole different level with their laser technology. Basically, the way the technology works is by applying powder to a surface and then melting and curing it with a moving patch of laser directed light. The laser in this case is diffused and forms a large pattern that is traversed over the powder coated surface. Within seconds the powder layer melts, flows and, cures to a hard, durable finish.
PhotoFusion Technologies, Inc. sees opportunity to revolutionarily change the heat-sensitive substrate market including composites and a myriad of plastics. These processes are expected to impact the automotive graphics and aerospace industries, among others. In addition, the ability to deliver high-intensity energy to a surface could open up opportunities in the petrochemical industry where heavy-duty assets may be able to be coated and cured in the field.
Powder coating technology is not sitting still. Clever scientists, formulators and, engineers are solving age-old problems with technological breakthroughs and energetic entrepreneurs are finding new opportunities to expand the reach of the highest quality, most environmentally compliant finishing technology. The increasingly more demanding needs of the pipe coatings industry are now being addressed by innovative work at the resin manufacturers. Technologists continue to chip away at expansion into new markets by developing extraordinary new curing technology with lasers. Their breakthroughs may be able to open untapped markets that utilize heat-sensitive substrates. And the age-old powder coating processing issue regarding low-density materials has been tackled by the pioneering engineers at extruder companies.
Author: Kevin Biller,
the technical editor of Powder Coated Tough and president of The Powder Coating Research Group