Powder coating is an environmentally friendly finishing process that is rapidly expanding to more markets and wider applications due to technological advancements in both the coating materials and the application process.
Powder coating is a dry finishing process in which finely ground particles of pigment and resin are electrostatically charged and sprayed onto electrically grounded parts. The charged powder particles adhere to the surface, and are melted and fused into a smooth coating in a curing oven. This article reviews the application process, the materials and equipment, current and potential applications, and advantages of powder coating over other finishing processes.
Powder coatings are either thermoplastic or thermosetting. A thermoplastic powder coating is one that melts and flows when heat is applied, but retains the same chemical composition after it cools and solidities. Thermoplastic powders exhibit excellent chemical resistance, toughness, and flexibility. They are applied mainly by the fluidized bed application technique, in which heated parts are dipped into a vat where the powder is fluidized by air. They are suitable for many thick-film applications.
• Polyethylene thermoplastics provide excellent chemical resistance and outstanding electrical insulation properties, and often are coated onto laboratory equipment.
• Polypropylene thermoplastics produce a surface that is very inert, and they are especially appropriate where parts may be exposed to chemicals.
• Nylon thermoplastics offer excellent abrasion, wear, and impact resistance, and a low coefficient of friction. As a result, they make excellent mechanical coatings for sliding and rotating bearings in appliances, farm equipment, and textile machinery.
• Polyvinyl chloride provides good durability as well as flexibility, and is used on dishwasher racks.
• Thermoplastic polyamides provide excellent resistance to detergents, impact, and high temperature.
• Thermoplastic polyesters offer good exterior durability and weatherability, and do not usually require a primer for good adhesion. They are often applied to outdoor metal furniture.
Thermosetting powder coatings are based on lower molecular-weight solid resins, which melt when exposed to heat. However, after they flow into a uniform thin layer, molecules chemically cross-link with each other or with other reactive components to form a reaction product of much higher molecular weight. Therefore, the final coating has a different chemical structure than the basic resin.
These newly formed materials are heat stable and, unlike the thermoplastic products, do not soften back to the liquid phase when heated. Resins for thermosetting powders can be ground into the fine particles needed for spray application and thin, paint-like coatings. Because these systems produce a finish that offers properties comparable to liquid coatings, most of the technological advances in recent years have been with thermosetting powders.
Thermosetting powders are derived from three generic types of resins: epoxy, polyester, and acrylic. From these basic resin types, five coating systems are derived.
• Epoxy resin-based systems are the most common, and act as both functional and decorative coatings. They offer outstanding corrosion resistance and electrical insulation, as well as attractive finishes that are flexible, tough, and impact resistant.
• Epoxy-polyester hybrid coatings are mainly for decorative applications. They are more resistant to chalking and over-bake yellowing than pure epoxies, but have a lower surface hardness and are less resistant to solvents. Hybrids also exhibit better transfer efficiency and a greater degree of penetration into recessed areas of a part.
• Polyester-TGIC coatings contain a polyester resin cross-linked with triglycidyl isocyanurate (TGIC) as a curing agent. These powders offer very good mechanical properties, impact strength, and weather resistance. They are resistant to chalking and are often applied to patio furniture, lawn mowers, and aluminum extrusions and panels for large commercial buildings.
• Acrylic-urethane coatings are formulated with acrylic resins cross-linked with blocked isocyanates. They have excellent color, gloss, hardness, weatherability, and chemical resistance. They have an excellent thin film appearance, but are less flexible than polyesters.
• Polyester-urethane coatings are formulated with polyester hydroxyl resins combined with blocked isocyanate hardeners. Polyurethane powders exhibit outstanding thin-film appearance and toughness, as well as good weathering properties.
Whether thermoplastic or thermosetting, color selection with powder is virtually unlimited, with high and low gloss, fluorescent, metallic looks, and dear finishes available. Texture selections range from a smooth surface to a wrinkled or matte or hammer-tone finish, with rough textures designed for hiding surface imperfections. Thicknesses can vary from 0.5 mils (.0005 in.) to 10 mils, with most coatings 1.0 to 1.5 mils.
Continued advancement in formulations over the past few years has produced powders offering better resistance to high heat, as well as powders that can be cured at lower temperatures. W-curable powders have extended powder coating applications beyond the traditional metal surfaces to a growing variety of plastic and wood substrates.
Parts to be powder coated are first exposed to a pretreatment operation to ensure that the surface is dean and free of grease, dust, rust, and other contaminants. Parts are normally pretreated in a series of spray chambers, where alkali cleaners, iron or zinc conversion coatings, and rinses are applied. However, dip tanks may be used instead of spray for some applications. Powder coating lines usually incorporate a phosphate application step that adds corrosion protection and improves the adhesion of the coating to the substrate.
The most commonly applied pretreatments are iron phosphate for steel, zinc phosphate for galvanized or steel substrates, and chrome phosphate for aluminum substrates. After the parts have passed through all of the pretreatment steps, they are normally dried in a low-temperature dry-off oven.
The powder coating application process involves four types of equipment: the powder delivery system, the electrostatic spray gun system, the spray booth, and the powder recovery system.
• The delivery system consists of a storage container or feed hopper, and a pumping device that transports a mixture of powder and air into hoses or feed tubes. Some feed hoppers vibrate to help prevent dogging or dumping of powders prior to entry into the transport lines.
• Electrostatic powder spray guns direct the flow of powder; control the pattern size, shape, and density of the spray as it is released from the gun; charge the powder being sprayed; and control the deposition rate and location of powder on the target. Spray guns may be either manual (hand-held) or automatic (mounted to a fixed stand or gun mover).
The most common type is the “corona charging” gun, which generates a high-voltage, low-amperage electrostatic field between the electrode and the product being coated. Powder particles that pass through the ionized electrostatic field at the tip of the electrode become charged and are deposited on the electrically grounded surface of the part. A drawback to this type of gun is the relative difficulty of coating irregularly shaped parts that have recessed areas or cavities into which the electrostatic field lines cannot reach.
The powder particles in a “tribocharging” electric gun receive an electrostatic charge as a result of friction caused by powder particles brushing against the solid insulator or conductor inside the gun. Outer electrons are stripped from the particles, leaving them positively charged. Because no overall electrostatic field is developed, the charged particles migrate toward the grounded part and are deposited in an even layer over the entire surface, including into recesses.
Oscillators, reciprocators, and robots help to control spray equipment and reduce labor costs. Gun triggering, or triggering the gun on and off by means of a device that can sense when the target is properly positioned, reduces oversprayed material, which translates into lower material and maintenance costs.
• The powder spray booth is designed to safely contain the powder so that overspray cannot migrate into other areas. The airflows through the booth must be sufficient to channel all overspray to the recovery system, but not so forceful that it disrupts the powder deposition and retention on the part.
• Powder recovery systems include either cyclones or cartridge filter modules that are dedicated to each color and can be easily removed and replaced when a color change is needed. Equipment manufacturers have made significant design improvements in powder spray booths that both allow color changes to be made with a minimal downtime and allow the recovery of a high percentage of the overspray, which can raise powder utilization to nearly 100%.
Three basic oven types normally cure powder coated parts: convection, infrared, or a combination of the two. In convection ovens, air is heated and circulated inside the oven around the powder coated parts. As a result, the parts are heated to oven temperature.
Infrared (IR) ovens emit radiation in the IR wavelength band. This radiated energy is absorbed by the powder and the substrate surface, but the entire part need not be heated to cure temperature. This allows a relatively rapid heat rise, which causes the powder to flow and cure when exposed for a sufficient time.
In combination ovens, an IR zone first melts the powder quickly. In the following convection zone, high-velocity hot air currents may be circulated, because the melted powder is in no danger of being disturbed. These higher velocities permit faster heat transfer and a shorter cure time.
The powder coating process prevents or minimizes many of the problems and issues inherent in liquid finishing. Liquid processes require solvents, and these solvents necessitate venting, filtering, and solvent recovery systems to control the emission of volatile organic compounds (VOCs). Powder coating contains no solvents and therefore emits negligible, if any, VOCs into the atmosphere. Exhaust air from the coating booth can be safely returned to the coating room, and less oven air is exhausted to the outside, making the process a safer and cleaner finishing alternative, besides saving considerable energy.
The process also has other environmental advantages. Since it is a dry powder, up to 98% of overspray can be readily retrieved and reused. The unused powder is reclaimed by a recovery unit and returned to a feed hopper for recirculating through the system. The waste that results is negligible, and can be disposed of easily and economically.
Greater efficiency is achieved because powder coating requires no drying or flash-off time, parts can be racked closer together on a conveyor, and more parts can be coated automatically. Powder coating does not run, drip, or sag, and resulting in significantly lower reject rates.
The appliance industry is the largest single market sector for thermosetting powders. Current applications include refrigerators, washer tops and lids, dryer drums, range housings, dishwashers, microwave oven cavities, and freezer cabinets. As porcelain-replacement powders become further developed, the appliance market will continue to grow.
The automotive industry currently coats powder on wheels, bumpers, hubcaps, door handles, decorative trim, radiators, engine blocks, and numerous under-the-hood parts and components. Powder is being selected by some manufacturers for the exterior body intermediate coat — the primer-surfacer. However, the most dramatic development in the auto industry in recent years is dear powder coatings over automotive exterior basecoats. These powder clearcoats are now applied by some European auto manufacturers, and are being tested in a joint project by the Big Thee automakers in Detroit.
The architectural and building market powder coats aluminum extrusions for frames on windows and doors, and in modular furniture. Because of the excellent durability of powder coatings, many highway and building projects coat powder onto light poles, guard rails, posts, and fencing.
Other powder-coated products include lighting fixtures, antennas, and electrical components, office furniture and cabinets, tractors and farm equipment, lawn mowers and snow blowers, barbecue grills, garden tools and tool boxes, fire extinguishers, baby strollers, pens and desk accessories, metal toys and wagons, golf dubs, ski poles, and exercise equipment.
Recent and ongoing developments in powder coating equipment have significantly reduced the time, effort, and capital cost required for color changes. Properly designed powder systems can change colors in minutes. High-production powder systems apply more than 20 different colors, with several color changes per day.
Technology continues to be developed for powder coil coating, in which one or both sides of flat metal sheets or strips are coated on a continuous production line basis.
An in-mold process has been developed in which powder coating material is sprayed onto a heated mold cavity before the molding cycle begins. During the molding operation, the powder coating chemically bonds to the molding compound and produces a product with a coating that is chip and impact resistant.
Powder coating of pre-cut metal blanks, which are then post-formed prior to final assembly, remains a strong growth area, particularly in the appliance market. This process allows for high transfer efficiency and uniform film thickness.
Advances in microprocessors, robotics, and infrared curing technology are allowing increased production in powder coating facilities. All of these advances, plus the inherent advantages of working with powder, ensure that powder coatings will have a permanent and ever-increasing share of the finishing market.
For more information: Greg Bocchi is the executive director of the Powder Coating Institute, 2121 Eisenhower Avenue, Suite 401, Alexandria, VA 22314; tel: 703/684-1770; fax: 703/684-1771; Web site: www.powdercoating.org.
Legend for Chart:
A – Properties
B – Epoxy
C – Epoxy/polyester hybrid
D – TGIC polyester
E – Polyester urethane
F – Acrylic urethane
A B C
Application thickness 0.5-20 mils(a) 0.5-10 mils
۰٫۵-۱۰ mils 0.5-10 mils
(metal temperatures)(b) 450°F – 3 min; 450°F – 3 min;
۲۵۰°F – 30 mm 325°F – 25 min
۴۰۰°F – 7 min; 400°F – 7 min;
۳۱۰°F- 20 min 350°F – 17 min
۴۰۰°F – 7 min;
۳۶۰°F – 25 min
Outdoor weatherability Poor Poor
Very good Very good
Pencil hardness HB-5H HB-2H
Direct impact resistance,
in.⋅lb(c) 80-160 80-160
Adhesion Excellent Excellent
Chemical resistance Excellent Very good
(a): Thickness of up to 150 mils can be applied via multiple
coats in a fluidized bed. (b): Time and temperature can be
reduced, by utilizing accelerated curing mechanisms,
while maintaining the same general properties. (c): Tested
at a coating thickness of 2.0 mils.
Electrostatic powder spray guns may be either hand-held or mounted to a stand.
By Greg Bocchi, The Powder Coating Institute, Alexandria, Va.