If someone told you that you could replace your existing organic finishing system with one that was completely environmentally compliant and run it at a lower operational cost, would you believe them? You might respond, "I'm not interested in trying new technologies that don't have a proven track record." What if this same person told you that this technology has proven itself in thousands of installations throughout North America since the mid-1960s? Would that allay your fears? You might ask, "What is the performance of this new coating?"

The advocate would reply, "In most cases, this type of coating has superior performance and appearance qualities when compared to all other organic finishes."

By now you are wondering, "What new type of miracle coating is this person talking about?"

Your friend would answer wryly, "Powder coating."

You say, "I looked at that technology years ago and determined it to be too expensive to convert, too difficult to apply, take too much time to color change and is too thick for my product." The advocate suggests that you should reevaluate this technology in light of recent improvements and cost reductions.

This article will not attempt to cover all the technical issues involved with the powder coating process. Instead, it will give the reader a general description of the materials and equipment that are unique to this process, outline the advantages and dispel the myths that have been associated with this finishing technology.

Powder Coating Fundamentals
All powder coating systems must have clean dry parts before the powder is applied. Normally, the parts are cleaned and pretreated using aqueous-based chemicals and then dried in an oven. Of course, all parts that have been powder coated must be cured using either heat or a combination of heat and UV cure
systems.

Powder Coating Materials. Powder coating is an organic finish that is divided into two basic categories: thermoplastic powders and thermoset powders. Thermoplastic powders are applied as dry particulates that melt and flow into a smooth coating when exposed to heat (approximately 300°F). After initial application and cooling, these materials will soften and flow again if exposed to heat. Examples of thermoplastic powder coatings are nylon, PTFE (Teflon®), polyethylene, vinyl, etc. Most thermoplastic powder coatings require that a liquid primer be applied to the substrate to improve the marginal adhesion characteristics of these materials. These materials are normally used as functional coatings, providing corrosion protection, slip enhancement, detergent resistance, electrical insulation, etc.

Thermosetting powder coatings are the most common materials used today. They are chosen for their excellent functional and appearance properties. Thermoset powder coatings are applied as dry particulate and will melt and flow when exposed to heat (above 250°F). However, these coatings will not soften again when exposed to heat after initial application, cure and cooling. Thermoset powder coatings come in a variety of formulations that include epoxies, polyesters, TGIC polyesters, acrylics and hybrids.

Powder coatings can be purchased in either spray or fluidized bed grades. Matching the powder coating to the application method is important in achieving the desired results. Fluidized bed formulations are normally ground coarser, resulting in a larger particle size that aids the fluidization of this material. Conversely, spray-grade powder coatings are finer, resulting in a smaller particle size that will pump better and is more easily atomized to achieve thinner film builds.

Powder Application Techniques. Powder coatings can be applied using either fluidized bed or spray techniques. The most important considerations in choosing an application method are functional vs. decorative application, film thickness control, production rate, color change, Faraday cage areas, product size and desired coating quality. Careful consideration of these issues will determine which application method makes the most sense for you.

Fluidized beds have been used to apply powder coatings to parts since the 1950s. This method can use thermal attraction or electrostatic attraction to deposit the powder particles onto a given part. If you are looking to apply thick film functional coatings (i.e. > 10 mils), then thermal attraction is your best bet. Here you preheat the part to above 350°F and dip it into a fluidized bed of powder coating. The heat attracts the particulates to the part and may partially melt them onto the surface. Final melting and full cure on thermoset powder coatings occurs when the part is further heated to the prescribed metal temperature. With thermoplastic powder coating, the part is further heated to achieve the desired surface smoothness and/or texture. Film thickness is controlled in this process by the heat of the part and the time the part is in the fluidized bed.

Electrostatic fluidized beds use ionized air to charge the powder particles, which are attracted to a grounded and cool part transported either above or placed into the bed. Varying the time in the bed and the charge on the powder particles controls the film thickness on the part. After the parts are coated, they need to be heated to melt/flow the material and, in the case of thermoset powder coatings, provide for full cure.

Spray application techniques for powder coatings can be broken down into several categories: Corona guns, Tribo guns, flame spray guns, Corona bells and Tribo discs. All of these methods require powder to be pumped from a vibrating box feeder, fluidized hopper or gravity hopper. The pump uses compressed air to draw powder from the box or hopper using the venturi principle and propel it to the spray device via a powder feed hose. Both powder volume and transport speed can be adjusted at the spray apparatus control panel. Atomization of the powder particles occurs at the spray device where the particles are deflected, spun, directed, or air atomized into a well-dispersed cloud. The powder within this cloud is either electrostatically charged or melted in the case of the flame spray gun. The powder particles are then attracted to a grounded part to form a continuous film between 1.0 and 10.0 mils thick, depending upon the desired application constraints. Film thickness tolerances can be controlled to ± 0.20 mil in a well-designed and closely controlled process.

Corona guns and bells use an electrostatic generator that creates an electrostatic field between the gun and the grounded part. The powder particles accept the electrostatic charge as they penetrate this field and are attracted to the grounded part. Electrostatic voltages are adjustable at the control panel up to 100 kV. The current within this field can approach 80 microamps. Some units have automatic feedback systems that vary voltage to maintain a constant current within the electrostatic field. This leads to more consistent film thickness and makes it easier to powder coat complex shapes. Corona guns are most commonly used today to coat a variety of products.

The powder pattern with guns is achieved by using conical deflectors, fan spray tips or pneumatic atomizers. Selecting the right pattern control device will allow for large, well-dispersed patterns useful in coating flat areas or narrow and focused patterns designed to penetrate recessed areas.

Corona bells are used in applications where large flat areas need to be coated at a high rate of speed. If you have to spray a lot of powder in a short period of time onto a relatively flat surface, then these are the preferred application devices. Typical bell applications are automotive car bodies and appliance outer shells. The charging technique is the same as is used in Corona guns, with similar effect.

Tribo guns and discs use frictional charging techniques to charge the powder particles. Powder particles develop this charge by the rubbing action caused by charging/transport channels molded into the gun/disc body. These channels are normally much longer than those used in Corona equipment to ensure that each powder particle has had an opportunity to accept a Tribo charge caused by the rubbing action between these channel surfaces and the particle. The amount of charge imparted on the powder particles is directly related to the materials sprayed and the composition of the channels along with the duration of frictional contact between the particle and the channel surface. These powder application devices are more suited to overcoming Faraday cage problems since they do not use electrostatic fields to charge the powder particles.

Flame spray powder coating equipment is typically used to coat large objects using thermoplastic powders where using an oven is impossible. This equipment ignites LPG, or other combustible gasses, at the gun tip. The resultant heat combines with thermoplastic powder that has been pumped by compressed air from a feed hopper. The heat melts the powder as it is propelled to the part's surface, resulting in a continuous protective coating (8-10 mils thick) that is both durable and highly corrosion-resistant. These devices are used to apply protective thermoplastic powder coatings to very large parts, such as water tanks, tanker rail cars, bridges, etc.

Powder Spray Booths and Recovery Systems
Powder spray booths are different from liquid spray booths because of their filtration techniques and fans. In liquid systems, simple and relatively coarse filters stop droplets of paint while the solvent fumes are exhausted from the plant airspace. Powder booths use much finer filters to stop small powder particles and in most cases return this air back into the plant airspace. Since the filters are finer and hold some of the waste powder onto their surface, the fans used in powder booths are typically rated at a higher static pressure.

All powder-coating booths are designed to accomplish the same goals: containment of the powder particulate within the spray booth and separation of this particulate from the air stream using filters for reuse or disposal. Some spray booth designs, which use contained and separate collection systems, require that enough air be introduced into the spray area to ensure a safe condition. Lastly, all powder booths that return the booth air back to the plant's airspace must use final filters capable of removing particulates down to 0.3 micron. This fine filtration will ensure that all safety codes for personnel areas are enforced, while the returned air will eliminate any air make-up requirements.

Powder booths come in three configurations: cartridge booths, cyclone booths and filter belt booths. Cartridge booths use paper or fabric cartridges in a collection system to separate the powder particles from the containment air stream. Cyclone booths use cyclone(s) to remove most of the powder particulates before the air is further cleaned by a cartridge collector. Filter belt booths use a rotating internal fabric belt to separate the particulates from the air stream while a separate collection device vacuums the belt clean.

Selecting which booth is best suited to your particular application is accomplished after careful consideration of many factors: number of colors to be sprayed or collected, color change time requirements, floor space considerations and the necessity of additional safety devices (i.e. segregation dampers, explosion vents and explosion ductwork).

Because the filtration techniques used in powder coating are so efficient, collection of oversprayed powder material for reuse is a definite advantage. Sometimes the amount and subsequent value ofthe overspray powder material is so small that collection for reuse is not prudent. In these cases, powder-coating materials can be disposed of in most cases as non-hazardous materials. Those systems that reuse oversprayed powder material incorporate sieves and fresh powder replenishment devices to ensure properly conditioned powder material necessary for consistent product quality.

Powder booths come in all shapes and sizes. Batchtype booths, where products are manually transported into and out of the booth, can be small bench top designs or large walk-in booths capable of containing 20-ft or larger parts. Conveyorized booths, where products are automatically transported through the booth, are available in standard sizes or custom designs. These booths can be designed to be compatible with chain-on-edge, powered overhead or power-and-free conveyor systems. Equipment suppliers offer limitless booth designs that can accommodate high- or low-volume production requirements for just about any size product.

Advantages of Powder Coating
This article started with several sweeping statements about this finishing technology. Now is the time when I will provide testament to these statements. I will discuss most of the misunderstandings and myths that have surrounded this technology since it has gained prominence in the last three decades.

Environmental and Safety Issues
All powder coatings are VOC-free materials, and most of them are considered landfill (non-hazardous) materials. Powder coatings contain no solvents and in most cases contain no heavy metals. In fact, only zinc-rich epoxy primers are considered hazardous because of their heavy metal content. Pigments have long been heavy-metal free to ensure easy disposal of powder coating formulations. Imagine taking your waste coating materials, spray booth filters and gun parts and simply throwing them away with your normal plant garbage. Powder coating end users do just that every day in just about every municipality in North America. They may have to melt the powder to eliminate dust problems, but that normally is all that the municipality requires.

Powder coatings also are remarkably safer than normal solvent-borne coatings. Solvent fumes readily catch fire and can be a health threat to plant personnel. Powder coating materials are not flammable, but may combust in a very narrow concentration of powder and air. Insurance underwriters rate powder systems much safer than liquid systems, resulting in lower premiums. Sprayers only have to wear personal protection categorized for nuisance dust environments as opposed to heavy respirators required for most liquid paints.

Coating Performance Issues
Powder coatings have remarkably better mechanical properties, corrosion resistance and chemical resistance properties than all other organic finishes. This is mainly due to the powder coating's molecular weight and dense crosslinking when fully cured. Further, because of the absence of solvent, powder coatings have less porosity than liquid paint. All these conditions make powder coatings more durable (i.e. harder, more impact resistant, more flexible, etc.), more corrosion-resistant (up to 5,000 hours salt spray resistance on aluminum substrates), more chemical-resistant and more weather-resistant. Superior coating performance properties have made powder coatings the preferred choice by designers looking for a "bullet-proof" organic coating.

Operational Cost Issues
The most significant cost savings is realized in lower coating material cost. Powder coating materials are less expensive and can cover more area than just about all other organic finishes. With powder, coating systems can be designed to accommodate denser product hang patterns, allowing for higher productivity for each minute of run time. Powder coatings lend themselves well to automation, reducing operational labor costs. Since most powder coatings are non-hazardous and do not contain any solvents, disposal costs are dramatically reduced and spray booth make-up air is eliminated. With no make-up air requirements and lower cure oven exhaust requirements, the powder coating process requires less energy to operate. Lower reject rates (typically under 4%) experienced with powder coating systems mean you spend less time and money reworking bad products.

Proven Track Record
Powder coatings have been used in the North American market since the late 1960's and in Europe even longer. Although no one knows for sure how many powder coating systems there are in North America, some say the number is in the tens of thousands. Powder coating is the fastest growing segment of all finishing technologies.

It's too expensive to convert to powder coating! Complete systems, including pretreatment, ovens, spray booths and guns, cost nearly the same for both liquid and powder coating equipment. However, when you are converting an existing liquid system to powder coating, it is often more expensive since the spray booth needs to be replaced or retrofitted. Also, in some cases, the existing liquid system's pretreatment equipment or cure oven cannot support the demands of powder coating.

The best way to determine if the existing pretreatment equipment can be used for a powder coating process is to perform a "water-break-free" test on parts as they exit the pretreat equipment. If the parts pass this test, then the equipment can be used. Most times a change in pretreatment chemical strength or formulation will improve this process sufficiently to accommodate the cleanliness requirements of the powder coating process.

Cure ovens used in a powder coating process must be capable of providing the time at metal temperature needed to properly cure the powder coating. Each coating formulation has its own curing requirements but most powder
cure requirements are both longer and hotter than typical liquid paints. Verification of your specific oven should be made by the powder-coating supplier. If they approve the oven, you're in luck. If they do not approve the oven for use
with your specific powder coating(s), an infrared booster can be a quick way to gain the metal temperature in the reduced dwell time compatible with your existing oven. Furthermore, low-temperature-cure powder coating formulations can be an additional method of accommodating your existing cure oven. These materials can be fully cured at lower oven temperatures or in shorter dwell times than standard powder coatings. However, they may not have the high performance properties your product may need, and they must be stored and used in an environmentally controlled room.

Even if the cost to convert to powder coating is higher initially, payback and return on investment favors powder coating because of its lower operational cost. Some companies have experienced two years or less payback (50% ROI).

Powder coatings are difficult to apply. Powder coatings require a slightly different manual application technique than liquid paints. With liquid paints often the gun is triggered on and off with each pass to prevent runs. Each pass by the sprayer must be made fast to reduce the incidences of runs or sags.

Powder coatings on the other hand, should be applied in slow, deliberate gun motions without triggering on/off. This ensures that the powder is properly charged and deposited evenly on the surface. Powder coatings, due to their formulation properties, are extremely resistant to runs and sags at film buildup to eight mils thick. This brings us to our next myth.

Powder coatings are applied too thick. Powder coatings have a reputation for being thick coatings. True, powder coatings are normally applied in the 1.5–3.0 mils range as compared to liquids that are applied in the 0.5–1.5 mils range. But this is normally due to operator error more than anything else. Powder coatings have been successfully controlled in many applications to 1.5 mils ±0.2 mil. Not many finishers maintain the equipment to ensure this level of control. And since powder coatings are less expensive, they would rather write-off the additional coating expense than implement the controls required to maintain a tighter film thickness. This fact is further reinforced because of the powder coating's resistance to runs/sags. Heavily coated parts are always acceptable (except when the coating is so thick that you can't assemble the components) and light coverage sometimes requires re-coating. So, most people err on the heavy side.

Color change is difficult. People look at minutes to change liquid colors, which normally require flushing the guns with solvent and connecting a new pressure pot. Well, powder guns can be cleaned just as easily by flushing with compressed air and changing powder hoppers (or the powder box when using box-feeder equipment). The real problem occurs when the powder coating is reclaimed for reuse. In these systems, the reclaim equipment must either be cleaned or changed to get ready for the next color (reclaim is obviously a benefit for powder coating) taking 15 minutes to 1.5 hours, depending upon the size and amount of reclaim equipment. This may seem ridiculously long for a color change, but if you reclaim sufficient overspray powder to justify the effort, then you gain the benefit of reduced operating cost. Most people have difficulty in determining which colors they should reclaim and often reclaim colors that are not economically justified. It is then that they complain that it takes one hour to color change for a color they are spraying for only 10 minutes. Powder should be reclaimed only if it is economically justified. If you follow this rule, the time and cost for color change will be offset by the value of the reclaimed powder material. If it doesn't, then scrap the powder overspray as you would with liquid paints.

Color matching is impossible. Maybe at one time getting powder coatings in all colors was impossible, but that's no longer the case today. Some powder coating formulators boast more than 300 stock colors and textures. Other powder formulators will specially match your color in five-pound quantities or more. This means that if you need a special color or texture in powder coating that you will most likely find a stock material or have it custom formulated for you. Deliveries of special materials may take a bit longer, but is normally worth waiting for. True, you can't just go to your local paint store for a color-matched powder coating (at least not yet). But getting custom powder coatings in a day or two isn't so bad.

Why Convert to Powder Coating?
Although it may seem there are infinite reasons that people convert to powder coatings, I've learned to fit them into the following categories:

New System. Powder coating is often the choice when either the old paint system is worn out or a first-time system is installed. Since the cost for new equipment for either powder or liquid is nearly the same, and then you look at the other benefits of powder coating, the decision normally favors powder
coating.

Superior Coating Performance. Many OEM manufacturers look to powder coatings to improve their product performance in the field. Powder coatings are well known for their improved durability and long life.

Environmental Conformance. Powder coatings are one of the few finishing technologies that meet all EPA requirements for air and water pollution control. If you are looking to gain EPA compliance, then powder coatings are a good solution.

Reduced Coating Cost. The applied material costs for powder coatings are typically 50% less expensive than conventional liquid paints. Energy and manpower costs are also less expensive with powder coatings. Overall, this means it can be 50–70% less expensive to run a powder coating line than a liquid paint line.