Powder Coating

Technology and History

Powder coating is an advanced method of applying a decorative and protective finish to a wide range of materials and products that are used by both industries and consumers. The powder used for the process is a mixture of finely ground particles of pigment and resin, which is sprayed onto a surface to be coated. The charged powder particles adhere to the electrically grounded surfaces until heated and fused into a smooth coating in a curing oven. The result is a uniform, durable, high-quality, and attractive finish. Powder coating is the fastest-growing finishing technology in North America, representing over 10% of all industrial finishing applications.

What is Powder Coating?

Powder is a dry coating. Instead of being dissolved or suspended in a liquid medium, such as solvent or water, powder is applied in a granular form. This material is finer than ground pepper but coarser than flour, and is applied directly to the surface to be coated.

The powder is created by blending the various components (binders, resins, pigments, fillers and additives) and processing them through an extruder into a continuous mass. This homo-genous mass is cooled and broken into small chips, which are then ground into the powder. Each powder particle contains within it the necessary components for reforming into the finished coating. After the powder is applied to the part, typically using an electrostatic spray process, the part passes through an oven and cures, melting into a smooth film on the surface of the part.

Powder Formulation

There are two distinct types of powder, Thermoset and Thermoplastic . The Thermoset powders are reactive, which means that under cure conditions there is a chemical "crosslinking", so that, once cured, the coating will not remelt. Epoxies, acrylics, and most hybrids are examples of Thermoset powders, making up over 90% of the current powder market.

Thermoplastic powders do not "crosslink" when cured, but simply melt and flow over the surface of the part. The film hardens on cooling, but if it is reheated it will remelt. Vinyls, nylons and fluorocarbons are examples of thermoplastic powders.

Enamel powder is a specialized formulation used in applications that previously used liquid porcelain enamel. It utilizes glass in its formulation and is cured, or fired, at a very high heat. The result is a finish that is particularly resistant to heat, scratching and harsh chemicals, and is typically used in appliances, such as washers, dryers, ovens and ranges.

How is the Powder Applied?

The application process involves applying a charge to the dry powder particles and spraying them onto a grounded substrate. The substrate, or part, is typically grounded through the conveyor or hanger holding the part. The powder, once attracted to the part, is then held on the surface until it is melted and cured into a smooth coating film in the bake oven. The spray process takes place inside a booth designed to contain the oversprayed powder and makes it possible to collect the overspray and ultimately reclaim it for reuse.

The powder is fed pneumatically out of the powder container, or hopper, into the powder applicator, or gun. As the powder exits the gun, a low amperage, high voltage charge is applied to the powder particles, causing them to be attracted to the grounded workpiece. This attraction may even cause the powder to "wrap" around the piece, coating the back side.

The oversprayed powder, suspended in the air contained inside the powder booth, is then passed through a separation process that permits the powder particles to be retrieved from the air. The clean air is fed back to the work environment, eliminating the need for air make-up. The reclaimed powder is mixed with a proportionate amount of fresh, virgin powder for reuse achieving consistent results and up to 98% material utilization.

How Will Powder Coating Benefit Me?

Reviewing the potential benefits of powder coating will give you an incentive to proceed with the quest for the best finish for your operation. So let's look at the three "E's" of powder coating in more detail.

Economy

1. Material utilization is much higher with powder, making your material costs much lower. 92% to 98% of the powder you buy will be applied to the parts you are finishing versus an average of 60% with an electrostatic liquid system (the other 40% is waste and must be disposed of!).
2. Since most of the material is used on the part, there is very little waste to be disposed of. And powder is not considered hazardous waste, so the cost of disposal is minimal compared to the high cost of toxic waste disposal.
3. Air used to exhaust the powder spray booth is returned directly to the plant, eliminating heating and cooling costs for the make-up air required when air is vented outside the plant.
4. Air loss from the curing oven is minimized as there is only a very small amount of volatile substance that must be exhausted. The cost of maintaining oven temperatures is therefore minimized also.
5. Powder is simpler to spray, so less skilled labor is needed, training is easily done and less errors are made in coating. All of which saves scrap, labor and, ultimately, operating costs.
6. Most powders require no primer, providing more savings in time and materials.

Excellence

1. The cured powder finish is less susceptible to damage than a liquid finish. There is less need for repair work on the finished item, and packaging is less elaborate, saving time and cost on rework and packaging.
2. Epoxy, acrylic and hybrid powders provide excellent adhesion and hard-ness for improved resistance to chipping, abrasion, corrosion, and chemicals; and it's flexible enough to be formable without cracking.
3. Polyester powders provide additional advantages in ultraviolet and weathering resistance.

Ecology

Powder is the overwhelming preference of the EPA, eliminating:

1. Solvent fumes and VOCs from spray booth and oven exhausts that pollute the air.
2. Potentially toxic sludge and water that can contaminate the earth and must be disposed of as hazardous waste.

What Are Its Advantages?

More Durable

Powder coating gives consumers, businesses, and industry one of the most economical, longest-lasting, and most color-durable quality finishes available.

Powder coated surfaces are more resistant to chipping, scratching, fading, and wearing than other finishes. Color selection is virtually unlimited with high and low gloss, metallic, and clear finishes available. And colors stay bright and vibrant longer. Texture selections range from smooth surfaces to a wrinkled or matte finish, and rough textures designed for hiding surface imperfections.

Protects the Environment

Powder coating is also highly protective of our environment. While liquid finishes contain solvents which have pollutants known as volatile organic compounds (VOCs), powder coating contains no solvents and releases negligible amounts, if any, of VOCs into the atmosphere. Thus, there is no longer a need for finishers to buy costly pollution control equipment. In addition, most powder coating overspray that does not adhere to the part can be retrieved and reused, virtually eliminating the waste commonly found in liquid finishing processes.

Saves Money

Elimination of VOCs and reduction of wastes saves money and helps companies comply more easily and economically with the regulations of the U.S. Environmental Protection Agency. In fact, one of the major elements in expanding the market for powder coating has been the implementation over the past 30 years of stringent air pollution control legislation.

"Things to consider about Powders and your powder coating projects"

Powder is basically dry paint consisting of 4 or 5 components:

1. Resin
2. Crosslinker (not used in Thermo plastic powders)
3. Pigments
4. Flow Aids
5. Degassing Solvent (Solid)

Two basic types of powders:

1. Thermoset Powders - Curing or crosslinking takes place so that once cured, the coating will not remelt. The majority of powder coatings today are thermosetting type powder. Typical Thermosetting type powders are:
• Epoxies
• Acrylics
• Most polyesters
2. Thermoplastic Powders - Do not crosslink and can remelt after curing when exposed to sufficient heat. Typical Thermoplastic powders are:
• Vinyls
• Nylons
• Fluorocarbons

Functional vs. Decorative Powders

Most Decorative Coatings are applied as relatively thin (1-4 mils) thermoset powder coatings and typically used for:

• Various automobile parts
• Appliances
• Clear coatings
• Functional powders are typically used for protective purposes.

Kinds of Powder:

Epoxy Polyester Hybrid Powders

Not suitable for outdoor applications. Resistant to over-baking. Typical Applications:

• Shelving
• Hot water heaters
• Power tools
• Fire extinguishers

Polyester TGIC Powders

Maintains excellent mechanical properties at film build above 3 mils. Good edge coverage. Typical Applications:

• Aluminum extrusions
• Automotive wheels
• Air conditioners
• Lawn furniture

Polyester Urethane Powder

Combine very smooth, thin film, excellent mar and chip resistance, and good weathering properties. Applications include:

• Light fixtures
• Lawn and garden equipment
• Electrical enclosures
• Air conditioners
• Automotive trim

Acrylic Urethane

Offers better weathering, and improved chemical resistance compared to polyester urethanes. Flexibility and impact resistance is poor. Typical Applications:

• Plumbing fixtures
• Automotive wheels
• Vending machines

GMA Acrylic Powders

Faster cure and improved weatherability than acrylic urethanes. Well suited for clear top coats. Typical Applications:

• Plumbing fixtures
• Automotive wheels
• Microwave cavities

Acrylic Hybrid Powders

Offer good weatherability, mechanical properties, and flexibility, but not outdoor durable. Typical Applications:

• Metal furniture
• Lighting fixtures