Powder Coating Information
Powder coating is a dry finishing process that uses finely ground particles of pigment and resin that are electrostatically charged and sprayed onto electrically grounded parts. The charged powder particles adhere to the part and are held there until melted and fused into a uniform coating in a curing oven.
Since its introduction more than 40 years ago, powder coating has grown in popularity and is now used by many manufacturers of common household and industrial products. In North America, it is estimated that more than 5,000 finishers apply powder to produce high-quality, durable finishes on a wide variety of products. Powder coated finishes resist scratches, corrosion, abrasion, chemicals and detergents, and the process can cut costs, improve efficiency and facilitate compliance with environmental regulations
Because powder coating materials contain no solvents, the process emits negligible, if any, volatile organic compounds (VOCs) into the atmosphere. It requires no venting, filtering or solvent recovery systems such as those needed for liquid finishing operations. Exhaust air from the powder booth can be safely returned to the coating room, and less oven air is exhausted to the outside, making powder coating a safe, clean finishing alternative and saving considerable energy and cost.
Theoretically, 100 percent of the powder over-spray can be recovered and reused. Even with some loss in the collection filtering systems and on part hangers, powder utilization can be very high. Over-sprayed powder can be reclaimed by a recovery unit and returned to a feed hopper for re-circulation through the system. The waste that results can typically be disposed of easily and economically.
Powder coating requires no air-drying or flash-off time. Parts can be racked closer together than some liquid coating systems and more parts can be coated automatically. It is very difficult to make powder coating run, drip or sag, resulting in significantly lower reject rates for appearance issues.
Powder coating operations require minimal operator training and supervision when compared with some other coating technologies. Employees typically prefer to work with dry powder rather than liquid paints, and housekeeping problems and clothing contamination are kept to a minimum. Also, compliance with federal and state regulations is easier, saving both time and money. In short, powder coating can provide the Five Es: economy, efficiency, energy savings, environmental compliance and an excellent finish.
There are two types of powder coatings: thermoplastic and thermosetting. Thermoplastic powders melt and flow when heat is applied but they continue to have the same chemical composition once they cool and solidify. Thermosetting powder coatings also melt when exposed to heat, but they then chemically cross-link within themselves or with other reactive components. The cured coating has a different chemical structure than the basic resin. Thermosetting coatings are heat-stable and, unlike thermoplastic powders, will not soften back to the liquid phase when re-heated. Thermoset powders can also be applied by spray application to develop thinner films with better appearance than some thermoplastic powder coatings.
The main driver in the development of powder coating materials was the pursuit of an environmentally friendly alternative to solvent-laden paints. In pursuit of a sprayable, low-VOC coating, Dr. Pieter g. de Lange of The Netherlands developed the process of hot melt compounding in a z-blade mixer. This made powder coating much more consistent. De Lange also developed the electrostatic spray application method for thermoset powder coatings in 1960. Using an addition of compressed air to the dry powder to “fluidize” the material, he was able to spray the coating and provide a decorative film. The process was introduced in the United States in the 1960s and rapid growth continued for the next 30 years.
Pretreatment for Powder
The first step in the powder coating process is to prepare or pretreat the parts. The product to be coated is exposed to cleaning and pretreatment operations to ensure that surfaces to be coated are clean and free of grease, dust, oils, rust and other contaminants. Chemical pretreatment normally takes place in a series of spray chambers. Parts are first cleaned using an alkaline, acidic or neutral cleaner. In many cases the part is surface treated with a conversion coating of iron or zinc phosphate or a transitional metal conversion coating such as a zirconium oxide product. Each stage is typically separated by a rinse stage to remove residual chemistry. Spray systems enable pretreatment of a wide variety of part sizes and configurations; dip tanks may be used instead of spray for some applications.
The specific pretreatment process selected depends on the characteristics of the coating and substrate materials, and on the end use of the product being coated. Pretreatments most often used in powder coating are iron phosphate for steel, zinc phosphate for galvanized or steel substrates and chromium phosphates for aluminum substrates. In addition to traditional phosphate processes a new group of technologies has emerged that use transition metals and organo-metallic materials or other alternatives. These alternative conversion coatings can be applied with little or no heat, and they are less prone to sludge buildup in the pretreatment bath than conventional iron or zinc phosphate formulations. The result is greater operating efficiencies in terms of lower energy costs, reduced floor-space requirements and decreased waste disposal requirements. Other advances include non-chrome seal systems, which can yield improved corrosion protection on steel, galvanized steel and aluminum alloys.
Dry-in-place pretreatment products, such as a seal rinse over an alkali metal phosphate, can reduce the number of stages required before powder coating application. Chrome dried-in-place treatments are effective on multi-metal substrates, and may be the sole pretreatment required for some applications. On-chrome technologies are commonly used as well. Non-chrome aluminum treatments have become very popular over time with excellent performance properties.
After the chemical pretreatment process is complete, parts are dried in a low-temperature dry-off oven. They are then ready to be coated.
For many functional applications, a mechanical pretreatment such as sand or shot blasting can be used. With this method, high-velocity air is used to drive sand, grit or steel shot toward the substrate, developing an anchor pattern on the part that improves the adhesion of the powder coating to the substrate. Mechanical cleaning is particularly useful for removal of inorganic contaminants such as rust, mill scale and laser oxide.
The most common way to apply powder coating materials requires a spray device with a powder delivery system and electrostatic spray gun. A spray booth with a powder recovery system is used to enclose the application process and collect any over-sprayed powder.
Powder delivery systems consist of a powder 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 clogging or clumping of powders prior to entry into the transport lines.
Electrostatic powder spray guns direct the flow of powder. They use nozzles that control the pattern size, shape and density of the spray as it is released from the gun. They also charge the powder being sprayed and control the deposition rate and location of powder on the target. Spray guns can be either manual (hand-held) or automatic (mounted to a fixed stand or a reciprocator or other device to provide gun movement). The charge applied to the powder particles encourages them to wrap around the part and deposit on surfaces of the product that are not directly in the path of the gun
Corona charging guns, the most commonly used, generate 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.
An alternative charging mechanism is a tribo charging spray gun. In such a gun the powder particles receive their electrostatic charge from friction which occurs when the particles rub a solid insulator or conductor inside the gun. The insulator strips electrons from the powder, producing positively charged powder particles.
Powder can also be applied by a spray device called a bell or rotary atomizer. Powder bells use a turbine that rotates in an enclosed powder bell head. Powder is delivered to the bell head and spread into a circular pattern by centrifugal force. The powder passes through an electric field between the bell head or an externally mounted electrode and either the grounded object to be coated or a counter-electrode positioned behind the bell head.
Use of oscillators, reciprocators and robots to control spray equipment reduces labor costs and provides more consistent coverage in many applications. Gun triggering (turning the gun on and off using a device that can sense when parts are properly positioned) can reduce over-spray, which results in lower material and maintenance costs.
Other Powder Application SystemsIn addition to spray application with electrostatic guns, powder coating materials can be applied by a dip method called fluidized bed. Fluidized bed powder coating was developed by Edwin Gemmer for application of thermoplastic resins and patented in 1953.
In fluidized bed coating, parts are pre-heated to 450–500°F and then dipped into a tank filled with powder material that has been “fluidized” by addition of compressed air through a porous membrane at the bottom of the tank. In some cases the powder is electrostatically charged.
Another option is flame-spray application. In flame-spray, which is used to apply thermoplastic powder materials, powder is propelled through the flame in a heat gun using compressed air. The heat of the flame melts the powder, eliminating the need for ovens.
Yet another method of application is called hot flocking. In this process, the part to be coated is preheated so that the sprayed powder will gel when it comes in contact with the hot part surface. Hot flocking is often used for functional epoxy application because it builds a thick film that will provide exceptional performance. These fusion-bond epoxy (FBE) products are often used to coat valves and pipe used in extreme conditions such as oilfield or offshore applications.
Powder Spray Booths
Powder booths are designed to safely contain the powder over-spray. Booth entrance and exit openings must be properly sized to allow clearance for the size range of parts being coated, and airflows through the booth must be sufficient to channel all over-spray to the recovery system but not so forceful that they disrupt powder deposition and retention on the part.
There are booths designed for limited production batch operations and larger booths designed for volume operations where parts are conveyed through on some type of hanger. Batch booths are used for coating individual parts or groups of parts that are handled hung on a single hanger, rack or cart. Conveyorized booths can provide continuous coating of parts hung on an overhead conveyor line in medium- to high-production operations.
Chain-on-edge booths are designed for use with an inverted conveyor featuring spindles or carriers for holding the parts. Parts are rotated on the spindle as they pass the stationary powder guns.
Flat line booths and conveyor system are used for one-sided coating of sheet metal and similar parts of minimal thickness. Flat-line booths use a horizontal conveyor that passes through the powder booth carrying the part to be coated on its surface.
Properly designed, operated and maintained powder systems can allow color changes in anywhere from 45 minutes to less than 15 minutes. Booth can include spec features that facilitate color changes such as non-conductive walls that repel rather than attract powder, curved booth walls to discourage powder accumulation and automated belts or sweepers that brush powder particles to the floor and into the recovery systems.
Fast color change can also be facilitated using blow-off nozzles set up at each gun barrel and easily changed connections at the back of the gun outside the booth. Guns can have the outside of the barrels blown off automatically and also use an automated purge system for the interior of the hoses and gun barrels.
systems use either cyclones or cartridge filter modules that can be dedicated to each color and removed and replaced when a color change is needed. Equipment suppliers have made significant design improvements in spray booths that can allow both fast color changes with minimal downtime and recovery of a high percentage of the over-spray. The use of the right powder recovery technology can increase powder utilization.
Curing of Powder-Coated Parts
Thermoset powder materials require a certain amount of thermal energy applied for a certain time to produce the chemical reaction needed to crosslink the power into a film. The powder material will melt when exposed to heat, flow into a level film and then begin to chemically crosslink before ultimately reaching full cure. Various methods can be used to supply the energy needed for cure.
Convection ovens that use a heat source, usually natural gas, a fan and air distribution duct to circulate air inside the oven and heat the part, are the most common type of cure oven used for powder. As the part reaches peak temperature it will conduct heat into the coating and cause the powder to cure.
Infrared (IR) ovens, using either gas or electricity as their energy source, emit radiation in the IR wavelength band. This radiated energy is absorbed by the powder and substrate immediately below the powder without heating the entire part to cure temperature. This allows a relatively rapid heat rise, causing the powder to flow and cure when exposed for a sufficient time. Parts can be cured in less time in an IR oven but the shape and density of the part can affect curing uniformity.
Combination ovens generally use IR in the first zone to melt the powder quickly. The following convection zone can then use relatively higher airflows without disturbing the powder. These higher flows permit faster heat transfer and a shorter cure time.
A variety of radiation curing technologies are available, including near-infrared, ultraviolet (UV) and electron beam (EB). These processes have the potential to open up new applications for powder coating of heat-sensitive substrates such as wood, plastic parts and assembled components with heat-sensitive details.
UV curing requires specially formulated powders that can be cured by exposure to ultraviolet light. The powder first needs to be exposed to enough heat so it is molten when exposed to UV energy; the initial heat source is typically infrared but convection heating can also be used. The coating is then exposed to a UV lamp. A photo-initiator in the coating material absorbs the UV energy and converts the molten film to a solid cured finish in a matter of seconds.
Near-infrared curing also uses specially formulated powders coupled with high-energy light sources and high-focusing reflector systems to complete the powder coating and curing process within several seconds. Heat-sensitive assembled parts such as internal gaskets, hydraulic cylinders, and air bag canisters can benefit from this technology.
Induction ovens are normally used to pre-heat parts before powder coating to help accelerate film build. Induction ovens are often used in fusion-bonded epoxy coating applications such as concrete rebar and coating of pipe used for gas transmission. Such systems operate at high line speeds, and film builds of >10 mils are common.
Powder Technology Advances
Recent developments in several areas of powder application equipment and processing have significantly increased productivity and quality throughout the process and expanded applications for powder coated parts. These include application on Medium Density Fiberboard (MDF), pultrusions, glass and other unique substrates. Lower temperature cure products have been developed to accommodate heat sensitive substrates.
An in-mold powder coating process for plastic parts 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, resulting in a product with a coating that is chip and impact resistant.
Multi-layer processes have been developed to provide exceptional performance combined with a very high quality appearance. Primers, base-coats and color coats are being combined with clear-coats on automotive products, boats and other products that demand exceptional quality.
Advances in microprocessors and robotics are also facilitating increased production in powder coating facilities. Robots are typically used where repeatability and high production of a limited variety of components are factors. When combined with analog powder output and voltage controls, robots can adjust powder delivery settings during coating, maneuvers too difficult to be accomplished manually.
Powder Coating Markets and Uses
Today, powder coating materials are available in virtually every color and a variety of textures and glosses. Powder coatings are used on hundreds of types of parts and products, including almost all metal patio furniture and the majority of metal display racks, store shelving and shop fixtures. Wire-formed products such as springs and storage baskets for the home and office are often powder coated.
For thermosetting powders, the appliance industry is the largest single market sector. Thermosetting powder materials provide even, thin films with high levels of resistance to chips, impact, detergents and chemicals, which are critical to the appliance industry. Applications include refrigerators, washer tops and lids, dryer drums, range housings, dishwashers, microwave oven cavities, freezer cabinets and external air conditioning units.
Automotive applications for powder coated parts include wheels, grills, bumpers, hubcaps, door handles, decorative trim, radiators, air bag components, engine blocks and numerous under-hood components, along with trailers and trailer hitches. Several automakers are now applying powder clearcoats over liquid exterior basecoats, and there are some automobiles that are powder color coated. Clear acrylic topcoats have been used on BMW and Mercedes vehicles.
In the automotive aftermarket, high-heat resistant powder coatings are used to finish mufflers to resist corrosion, protect against nicks and prolong the life of the muffler. Light truck and SUV owners can purchase powder coated running boards, bed rails, luggage racks and toolboxes as dealer add-ons or from aftermarket suppliers. Powder manufacturers are also working with the automotive industry to perfect powder coating on plastic items such as wheel covers, rear-view mirrors, door handles and air conditioning vents.
As more powder coaters are able to accommodate large parts, off-road vehicle frames such as those used in agricultural and construction equipment are being powder coated, with good UV and weather protection, and high resistance to salt spray and fertilizer.
Manufacturers of architectural components and building supplies powder coat aluminum extrusions used on windows, doorframes, storefronts and shelters.
vertical lines for powder coating aluminum extrusions, commonplace in Europe for many years, have improved speed of production as well as finish quality. Many highway and building projects use powder coating on light poles, stadium seating, guard rails, posts and fencing.
Many lawn and garden implements, including wheelbarrows, lawn mowers, lawn sprinklers, snow blowers, snow shovels, barbecue grills, propane tanks and garden tools, are powder coated, as are everyday products as lighting fixtures, antennas, and electrical components. Sporting goods applications include powder coated bicycles, camping equipment, exercise equipment and golf clubs.
Powder coating is widely used for office furniture and equipment including file drawers, computer cabinets and desks. Parents use powder coated baby strollers, cribs, playpens, car seats and toys; consumers also own electronic components, bathroom scales, toolboxes, laptop computers, cell phones and fire extinguishers with powder coated components.
Functional powder applications are an ever-growing market where powders are applied to rebar used to strengthen bridges, buildings, retaining walls and roads. Fusion-bonded epoxy powder coatings are applied to protect both the inside (ID) and outside (OD) diameter of gas and oil transmission pipe, valves, potable water applications and springs.
Applications for powder coating are expanding. More applications continue to develop in the areas of powder on plastics and powder on wood, specifically medium-density fiberboard. Ongoing development in powder coating materials and new methods of applying powder promise even more uses that may be unimaginable today.
The Manufacture of Powder Coatings
The stages numbered in the diagram are as follows:
- (…12) Raw materials from the store are selected and accurately weighed according to a recipe card.
- Mixing. The dry raw material aggregate is loaded into a pot and mechanically mixed to a homogenous blend.
- Extrusion. The raw material aggregate is heated and intimately kneaded, compounded and mixed and fed into a:
- Chiller. The hot homogenised mixture is cooled on a cooling belt and then broken down into 5 – 10 mm flakes, called extrudate.
- The flake is fed into a grinding chamber and broken down to an average particle size of 40 microns. The ground powder is cyclone classified into:
- Sub-10 micron powder, which is bag filtered, and the remainder:
- Powder for sale is filled into bags.
- Oversized particles are collected for regrinding (stage 8).
Powder Coating Over Hot Dip Galvanizing
The following is a suggested recommended procedure and grit blasting should be investigated separately.
- Hot dip galvanize and do not water or chromate quench
- Remove all drainage spikes and surface defects
- Powdercoat within 12 hours of galvanizing. Do not get surfaces wet. Do not leave outside
- Keep the surface clean. Do not transport uncovered loads. Diesel fumes will contaminate surface
- If surface contamination has occurred or is suspected, clean surface with proprietary solvent/detergent designed for pre-cleaning prior to powdercoating
- Use zinc phosphate pretreatment if highest adhesion is required. Surface must be perfectly clean. Zinc phosphate has no detergent action and will not remove oil or soil.
- Use iron phosphate if standard performance is required. Iron phosphate has a slight detergent action and will remove small amounts of surface contamination. Best used for pre-galvanized products
- Pre-heat work prior to powder application
- Use ‘degassing’ grade polyester powder only
- Check for correct curing by solvent testing. Adjust pre-heat and line speed to ensure full cure.
There are four recognised methods of surface pretreatment that produce a sound substrate for paint coating:
- T-Wash (or its proprietary equivalent)
Despite the fact that this preparation process has been available for some considerable time, T-Wash is still generally considered to be the best pretreatment method for painting galvanized steel. T-Wash is a modified zinc phosphate solution which contains a small amount of copper salts. When applied, a dark grey or black discolouration of the zinc surface will result. T-Wash must not be allowed to pool on horizontal surfaces or this will prevent maximum paint adhesion. Any excess should be removed by water.T-wash is most suitable for application to new galvanizing and should not be used on weathered galvanizing (see etch primers). Sufficient time must be allowed for the T-Wash to react and dry thoroughly before the first coat of paint is applied. (Suppliers’ information will give recommended time intervals). While research has shown that T-Washed surfaces can be left for up to 30 days before painting and good paint adhesion can still result, it is advisable to minimise the time between pre-treatment and paint application. Any white salt formed by the exposure of the T-Washed surface to moisture must be removed before painting, using a stiff brush. If the T-Washed surface has become contaminated it must be cleaned in accordance with the suppliers’ recommendations.
- Constituents of T-Wash
The constituents of T-Wash are phosphoric acid (9.0%), ethyl cellusolve (16.5%), methylated spirit (16.5%), water (57.0%) and copper carbonate (1.0%). Variations to this composition may exist and so it is wise to consult the supplier if a successful result is to be achieved.
- Constituents of T-Wash
- Etch primers
Etch primers have also been used successfully. Their major disadvantage is the absence of any visible colour change as is the case with T-Wash. Therefore, there can never be complete confidence that all surfaces have reacted with the primer. Etch primers are most suited to application on older, weathered galvanizing.
- Sweep blasting
A mechanical method of pre-treatment is sweep blasting using fine copper slag, J blast or carborundum powder with a blast pressure of no greater than 40psi (2.7 bar). This will ensure that only the minimum amount of oxide is removed and the zinc surface is left in a slightly roughened condition. Care should be taken when carrying out sweep blasting on very thick galvanized coatings to avoid damage to the coating. The optimum nozzle-to-work piece distance and angle of blasting needs to be identified for all surfaces on the galvanized steelwork if optimum results are to be achieved. Angular iron blasting grit must not be used under any circumstances. Sweep blasting is often used in addition to the chemical preparation stage.
This process only becomes fully effective after a galvanized surface has been exposed to the atmosphere for a period of at least six months. The surface is prepared using either abrasive pads or a stiff brush to remove all loose adherent materials and making sure that the bright zinc surface is not restored.
This is followed by a hot detergent wash and rinsing with fresh clean water. The surface must be fully dry before any paint is applied. Weathering should not be used as a method of surface preparation in marine environments with high chloride levels.
FAQ ON POWDER COATING
Q. What is Blooming? What is the white dusty material that sometimes forms when powder coatings are taken from the oven?
During the production of polyester resins some monomers (starting reagents) do not fully react. These remain in the resin as supplied to the powder coating manufacturer. At temperatures above 160 Celsius, these monomers are free to migrate to the surface of the powder coating. At normal baking temperatures these monomers will sublime off the powder coating surface, however where full baking temperatures have not been achieved, or where the substrate is very thick, resulting in a prolonged cooling cycle, the monomers will stay on the surface. The result is a dusty effect that is easily wiped off the coating.
Whilst this can be a mere nuisance for small runs, where large production runs consistently result in this effect it is worth investigating whether a ‘blooming-free’ system is available, which incorporates different base polyester resins.
Q. What is Bonding?
Bonding is a process solely applicable to metallic and pearl effect powder coatings. Unlike standard coatings where the ingredients are homogenously and intimately mixed during the extrusion process, effect powders are manufactured from a blend of a base powder coating and an effect pigment. As the two constituents will have very different charging characteristics they can give spraying problems, including ‘spitting’; they can also separate during the spray process and the recovery system will not return powder in the same base:effect ratio as the virgin powder, resulting in colour shift.
The bonding process takes the powder and effect pigment and very gently heats the mix until the powder just starts to soften. At this point the effect pigment will stick (bond) to the powder coating particles. This creates an intimate system that has both spraying and overspray recovery benefits.
Q. How can I tell if the powder is cured?
Without the use of specialised lab equipment it can be difficult to ascertain whether a powder coating is fully cured using only one method.
Different powder chemistries result in coatings with different finished physical and chemical properties. For instance fully cured standard architectural polyesters will pass the falling weight test, however so-called superdurable architectural polyesters will crack. This does not mean that the superdurable systems are less well cured.
However, for polyesters and epoxy-polyester hybrids a very useful and quick test is the solvent rub test. Here, a rag dipped in methyl ethyl ketone (MEK) is rubbed back and forth across a small area (3-4 inches) 20-50 times. A cured system will show little or no staining on the rag and the surface, whilst softened, will not scratch down to the substrate surface.
Q. Is there a British Standard for Coating Aluminium Substrates?
BS 12206-1 (supersedes BS 6496)
Q. Is there a British Standard for Coating Galvanised Substrates?
BS 13438 (supersedes BS 6497)