A Guide to Powder Coatings for Extreme Service Environments
Posted on Monday, June 8, 2020
By Kevin Biller
This issue of Technology Interchange will delve into the world of extreme environments and how to protect finished goods with the very best powder coating technology.
Extreme environments involve conditions that go beyond your typical incidental sunlight or office setting where an occasional wipe with a cleaner or the bump of a chair is all the durability that is needed. We’ll look into scenarios where either the prevailing conditions are acutely intense or where product performance expectations include lengthier than typical longevity.
As we all know, coatings, and in particular, powder coatings serve to protect and beautify. As requirements eclipse a typical industrial specification and venture into more aggressive and demanding performance, the options of chemistry, color, and indeed cost can change significantly.
Resins have to be more robust and pigmentation needs to be more inert and permanent. Therefore, overall costs are higher and color choice may be somewhat more limited. We’ll note these aspects along with the recommendations of coating type and processes recommended to exceed these requirements.
The outdoor durability of powder coatings fits neatly into four packages. The first are resin types that cannot be considered in any scenario requiring the maintenance of appearance in an outdoor setting. These are all epoxy containing powders, including pure epoxies, hybrids (epoxy- polyester), and acrylic hybrids (acrylic-epoxy). Although some of these products will maintain a semblance of film integrity for a year or two, the appearance will be tainted by a precipitous loss of gloss and a striking fade in color.
The second class of outdoor durable powders are what I call “industrial grade polyesters”. These are entry level powders based on low cost polyester resins (both carboxyl and hydroxyl functional) and can be cured with TGIC (triglycidyl isocyanurate), HAA (beta-hydroxy alkyl amide), or isocyanates (aka polyurethanes). Durability of these workhorses begins to wane after 12 to 18 months of continuous outdoor exposure, resulting in color fade, gloss loss, and chalking. These products can be used for lawn equipment, entry level patio furniture, children’s bicycles, and other light duty outdoor products.
The third class of outdoor durable powders are specially formulated to withstand about five years of outdoor exposure before exhibiting color and gloss change. These products are based on “superdurable” polyester resins that can be cured with TGIC, HAA, or isocyanates. Because of the composition of the polyester resin backbone, the binder of these powders resists degradation even in the demanding conditions endemic in south Florida. When properly formulated, these products meet the AAMA 2604 voluntary specification which requires at least 30 percent gloss retention and only minor color fade. These UV-durable coatings not only have to be formulated with high quality resins, they also require judicious choice of pigments. The majority of bright, organic pigments do not have the lightfastness required to withstand five years of outdoor exposure; therefore, more muted inorganic pigments are typically used in these products. Consequently, bright, clean colors in this class of powder coatings are rare and rather expensive.
If coating requirements call for 10 years or more longevity, resin choice is much more limited and considerably more expensive. There are two general markets that powder coatings serve that demand this type of extreme performance. The transportation (automotive, aviation, aerospace, etc.) and ACE (agricultural, construction, and earthmoving) markets both have finish performance specifications that call for 10+ years of durability in outdoor environments.
For transportation, the choice has been clear. Acrylic powder coatings pioneered in the 1990s were developed to meet the exacting specifications of the automotive market culminating in the commercialization of a powder clear topcoat for BMW’s finishing lines in Dingolfing, Germany. This powder clearcoat was used to finish their 5 and 7 Series product lines. These acrylic powders provided outstanding UV durability coupled with excellent acid rain and abrasion resistance. The BMW powder topcoat finishing lines were jettisoned in 2015, but the legacy of this outstanding powder technology lives on. This exceedingly durable resin chemistry continues to be used on high-end alloy wheels and automotive exterior trim parts.
For the ultimate in UV durability longevity, fluoropolymer-based powder coatings are the only choice. Their outstanding resistance to UV degradation relies upon the stability of fluorine chemistry, which is impervious to attack by solar energy. Fluoropolymer powder types come in two varieties: a thermoplastic based on Kynar™ polymer technology from Arkema, Inc., and thermoset resin technology, dubbed Lumiflon™, from AGC Chemicals. The thermoplastic fluoropolymer-based powders require specialized extrusion and grinding techniques and are typically used with a primer. The thermoset products based on Lumiflon™ are cured as polyurethanes and are processed with more conventional powder processing techniques. Both products boast 20+ years durability when formulated properly with UV-durable pigments and additives.
Highly corrosion resistant powder coating systems require the intersection of properly prepared metal, a high-quality powder formulation, and a robust curing process. The quality of the metal preparation cannot be emphasized enough. The removal of oxides, weld splatter, oils, and mill scale is essential to preparing the substrate suitably for coating. As for resin chemistry, epoxies are by far the most widely used products to thwart attack in highly corrosive environments. These include oil and gas pipelines, marine equipment, chemical processing plant equipment, and as a primer for outdoor structural steel.
Epoxy-based powder coatings achieve superior corrosion resistance by a combination of exceptional adhesion to metal substrates and a tough, nearly impervious barrier provided by the polymeric matrix and functional fillers in the formula. For the ultimate in corrosion resistance, thick films up to 20 mils (500 microns) enhance the barrier performance. Epoxy-based powder coatings have been used on gas and oil pipelines since the early 1960s and some of them are still in service today.
Some finishing requirements call for both UV durability and corrosion resistance, which depending upon the specification, can be mutually exclusive. In these cases, the use of a tough epoxy-based primer combined with an outdoor-durable powder topcoat can usually fit the bill. The need for meticulous metal preparation is essential, including a high-quality chemical pretreatment. These systems call for a carefully controlled curing process. Best performance is achieved by undercuring the epoxy primer before applying and curing the UV-durable topcoat. An epoxy primer followed by a “superdurable” polyester topcoat is a very good system to use as an outdoor-durable finish in a highly corrosive environment.
Another primer-topcoat combination that provides excellent overall performance utilizes an electrocoat primer with a UV-durable powder topcoat. This system has been used historically in both the automotive and laundry appliance industries. The electrocoat is typically an epoxy- based cathodic type and is applied at around 1.2 mils (30 microns). The advantage of an e-coat primer is that this technology, by nature, coats all the edges, nooks, and crannies of a complex part. E-coat requires a dedicated coating bath and separate drying/curing process and is most commonly used in OEM facilities.
Another option for high performance corrosion resistance is thermoplastic powder coatings. This sub-class of powder coating technology includes polyethylene, polyamide (Nylon), and PVC (polyvinyl chloride) chemistries. These highly durable and corrosion resistant coatings can be applied with electrostatic spray or fluidized bed to film thicknesses of 10 to over 50 mils (250 to 1250 microns). In some cases, a primer is recommended to achieve optimum adhesion to difficult substrates.
Chemical resistance covers a multitude of environmental situations. Powder coatings can be used as a lining for a vessel, as part of a fluid transport system (pipelines, valve, etc.), as a finish in chemical plants (e.g. structural steel for staircases, etc.), for laboratory furniture, kitchen appliances (pesky food stains), and many more situations requiring resistance to chemical attack. Formulators create powder coatings that meet these challenges by selecting tough polymers and densely cross- linked binder systems.
For indoor and underground environments, epoxy-based powders offer best the protection against aggressive compounds. Not all epoxies are created equally, so you must coordinate the needs of the finish with your powder coating supplier. In addition, for the most extreme chemical resistance (e.g. strong acids, strong alkalis, aggressive solvents, hydraulic fluids, etc.) the epoxy variety may be darker in color and therefore, not conducive to clean bright colors.
For the intermediate chemical resistance that is required for major appliances, polyurethanes, acrylic-epoxies, and some high-performance hybrids (epoxy-polyester) meet these specifications. Polyurethanes are typically more expensive than the other two and therefore, have limited use. Acrylic-epoxies possess a desirable balance of decent light stability and abrasion resistance coupled with good resistance to detergents, bleach, and laundry additives. Hybrids offer the lowest price option for applications that are cost sensitive.
If your finishing specification calls for outdoor durability combined with high chemical resistance, you have two options. As a single coat, polyurethanes offer the best balance of UV durability and chemical resistance. To meet these requirements, the powder has to be specially formulated with superdurable resins and high urethane content. This provides the outdoor durability with a tough finish that can withstand attack from most harsh chemicals. Another option is to use a two-coat system. For the highest longevity, a fluoropolymer topcoat with an epoxy-based primer will resist nearly all environmental attack, including acid rain, industrial emissions, and industrial cleaners.
The small, specific class of silicone- modified powder coatings provide the heat resistance required for recreational cooking appliances and exhaust parts for automotive, power sports, and lawn care equipment. These coatings are typically formulated with a precise blend of silicone and organic polymers (e.g. acrylic, epoxy, or polyester). The organic component provides the initial film formation, while the silicone gives the ultimate heat resistance, usually hardening after the part is placed in use and experiences temperatures above 300 degrees Celsius.
Silicone-based powder coatings provide unique heat performance; however, because of the silicone content, they have a tendency to contaminate conventional powder coatings such as epoxy, hybrid, polyester, and polyurethane types. In addition, most pigments degrade at temperatures above 200 degrees Celsius, so high heat resistant powder coatings have a limited color palette.
Powder coatings offer incredible options for finishes destined for extreme environments. As we all know, powders are not created equally so it is vitally important to carefully discuss your coatings needs with your powder supplier. They can provide the best options to meet your exacting needs.
Kevin Biller is technical editor of Powder Coated Tough and president of The Powder Coating Research Group.