Powder Coating

Powder coating is a relatively new surface finishing technique. Introduced in the United States in the mid 1950's powder coating is becoming a popular alternative to solvent based coating systems. Currently, it is the fastest growing surface finishing method in North America and represents about 10% of the industry. Powder coating is not without its drawbacks, but it does have many advantages over solvent-based coatings. Advantages and disadvantages of powder coating are shown in Figure 1.

Overview
Powder coating is a method used to apply a decorative or protective coating to variety of materials in both industrial and commercial applications. The powder coating process involves the dry application of finely ground particles of pigment and resin to surfaces prior to a curing process. Unlike liquid coatings, each individual powder particle contains the entire coating formulation (pigment, resin, fillers, and modifiers); there are no solvents in powder coating. The applied powder material is then heated to a temperature sufficient to melt and cure the resin. After cooling, the coating is complete; there is no cure time beyond the cool-down period.

Solvent based paints and coating systems consist of one or more primer coats, an undercoat and two or more coats of finish paint. With powder coatings, one coat is generally sufficient. Powder provides better edge protection and mechanical properties than solvent based coating systems. Powder coating adheres to itself; therefore, it does not flake, chip, or peel like liquid coatings.

Production of the powder involves combining exact amounts of resin, pigment, and other additives and heating to the resin melt temperature. The fluid melt is extruded into a thin sheet, which is cooled and flaked. The flakes are then pulverized into a fine powder.

Resins used in powder coatings are either thermoplastic or thermosetting. Application procedures and properties of these two types of powder differ greatly. Thermoplastic powder coatings were the first in the industry followed later by thermosetting powders. Figure 2 compares the two types of powder coatings.

Thermoplastic
Thermosetting
Does not react chemically upon heating.

• Powder reacts chemically during cure to create cross-linked polymer chains.
• Usually functional rather than decorative coating applications.
• Can be functional or decorative.
• Generally requires a primer.
• Adheres well to substrate without primer
• Applied in thick films.
• Can be applied in thick or thin films.

Thermoplastic Powder Coatings
Thermoplastic is a general term used to describe a polymeric material that softens when heated and hardens upon cooling. Thermoplastics used in powder coating react physically, rather than chemically, upon heating. Thermoplastics are high-molecular weight polymers that tend to be applied in thick films. Typical thermoplastic coatings are: polypropylene, nylon, vinyl, polyvinyl chloride (PVC), polyethylene, thermoplastic polyester and polyvinylidene fluoride. Thermoplastic powder coatings are well suited for high performance applications where durability is desired. Because of the thickness with which they are applied they don't generally compete in the same markets as liquid paints.

Parts to be coated with thermoplastic powder are generally preheated and then dipped in a bed of fluidized powder. Some types of thermoplastics can be sprayed on. In most thermoplastic applications, use of a primer is necessary to achieve sufficient bonding to the substrate. Primers are usually solvent-based and are applied by spraying or dipping. Primer coats must be completed cured before the powder can be applied. Thermoplastic coatings tend to be used in applications where a large film thickness and high toughness is required.

Thermosetting Powder Coatings
The term thermoset refers to polymeric materials that will not soften or melt after being cured. These types of polymers undergo a chemical cross-linking reaction when heated, resulting in a high molecular weight product. Cross-links are chemical bonds that link polymer chains together and give the material strength. The resins that are used to make thermosetting powders are epoxy, polyester, and acrylic. There are five types of powder coating systems that are derived from these three general resins: epoxy, epoxy-polyester, polyester-TGIC (Triglycidyl isocyanurate), acrylic-urethane, and polyester-urethane.

Thermosetting powders are usually applied to parts using an electrostatic gun. The powder is given an electrical charge that causes it to be attracted to the part, which is electrically grounded. Primers are generally not used with thermoset powders. Thermosetting powders can be applied in very thin film thicknesses and are used for both decorative and protective purposes.

The Powder Coating Process
One of the most important steps in the powder coating process is surface preparation. Just as in the case of liquid coatings, inadequate surface preparation leads to premature failure and poor performance of the coating system. In powder coating there are several methods of pretreatment that can be used based on the application. In each case the surface of the material to be coated must be completely dry.

Surface preparation in powder coating is generally the same as that for liquid coatings. A near white metal blast is recommended as the surface preparation method although other methods can be used just as effectively depending on the application. Chemical stripping, immersion, and multi-spray washers can be used. Immersion and multi-spray washers are popular because of their efficiency.

Powder coatings can be applied by fluidized bed, electrostatic fluidized bed, flocking, flame spray, molding, and electrostatic spray. Electrostatic spray and the fluidized bed methods are used quite often in industry and warrant further discussion. The first method that will be examined is that of the fluidized bed. This method involves the use of a large tank or hopper that holds the powder. Dry air is circulated through the powder giving it a fluid like texture. The piece to be coated is preheated to temperatures above about 500°F and then dipped into the fluidized powder. The powder melts on contact with the heated surface to form the coating.

The fluidized bed is generally used for application of very thick (10-30mils), functional coatings. Powder loss is minimal with this method. Common problems with this method are difficulty in color matching from batch to batch.

The electrostatic spray method typically involves several basic pieces of equipment. The first is a storage container to hold the powder. This container is circulated with air to fluidize the powder. A pump forces the powder and air through feed hoses to a spray gun. The gun uses one of a couple methods (corona charging or triboelectric) to give the powder a charge. The gun then sprays the charge particles onto the workpiece, which is electrically grounded through metallic hangers or hooks.

With electrostatic equipment film thicknesses of up to 5 mils are possible. To reach higher thicknesses the workpiece must be heated prior to application.

References
T.A. Misev, Powder Coatings: Chemistry and Technology, John Wiley and Sons, New York, 1991.
W.D. Lehr, Powder Coating Systems, McGraw-Hill, Inc., 1991.
"Coatings Guide." Industrial Paint and Powder Magazine. Online, (2002)