Powder Coatings Industry Overview

Powder coating, the technique of applying dry paint to a part, is by far the youngest of the surface finishing techniques in common use today. In standard wet paints, such as house paints, the solids are in suspension in a liquid carrier, which must evaporate before the solid paint coating is produced. Powder coatings avoid this issue and the associated use of solvents, yet produce a final cured coating with similar properties to that achieved with a two-pack wet paint.
Powder coatings are relatively hard, abrasion-resistant (depending on the specification) and tough. Thin powder coatings can be flexible but this is not recommended for exterior applications. The choice of colors and finishes is almost limitless. Color matching is quite acceptable batch to batch. When powder coated items are installed without damage, and maintained regularly (appropriately washed at regular intervals), the coating should be relatively permanent. A correctly applied coating, although not metallurgically bonded to the metal, will not crack, chip or peel as happens with conventional paint films.
Powder coating technology has already successfully displaced liquid based alternatives for a variety of applications and its use continues to grow. Demand for powder coatings in the US is forecast to rise by 7.9% per year to 505 million lbs in 2005, valued at $1.6 billion, according to a study by the Freedonia Group. The company reports that powders will continue to find the bulk of their use in durable goods markets but in addition the motor vehicle market offers excellent long term prospects. Although the appliance and houseware market is more mature and will offer below-average growth, powder demand is expected to post gains that are well in excess of that industry as a whole, as manufacturers continue the trend of shifting coating lines from liquids to powders. The furniture market is also relatively mature but powders are experiencing expanded opportunities as a result of the development of low temperature and UV-cured products that can be used on wood substrates. Continued improvements in powder products will further expand applications into other heat sensitive substrates, such as plastics. In addition to furniture, the lawn and garden equipment, and sporting goods markets would benefit from this development.

* Source: The Freedonia Group
What are powder coatings?
Coating powders consist of solid resin, pigments, and additives which are compounded together to form a coating powder.
There are three main categories of coating powder:
• Thermoset coatings: These are coatings which do not re-melt upon heating; during the curing process irreversible crosslinking reactions occur which give these coatings many of their desirable properties. Typically particle sizes for these powders are 30-50µm.
Typical size distribution of thermoset powder coating:
Min = 0.7µm, Max = 125µm, Dv(50) = 38-40µm

• Thermoplastic coatings: These materials will re-melt upon heating. Typically particle sizes for these powders are around 100µm. Some powders (especially some thermoplastic powders) have mostly large particles and have no potential to emit particulate matter to the air. Powders with more than 95% by weight above 75µm may justify exemption from LAPC / LAPPC.
• Thermoplastic toners: Typically these particles are in the size range 5-20µm.
Powder coatings production
Powder coatings are made by mixing the basic polymer with charge agents and other additives and then heating until they form a melt. This liquid is then extruded into chips or pellets which are fed into a grinder (most commonly an air classifier) to give a first cut of the product. The powder then passes through a single or double cyclone separator in which the very fine particles are removed. The coarse product is removed from the bottom of the cyclone and bagged directly. The fines are collected separately to be recycled. In many cases, the grinding is done in batch runs, with the powder coating manufacturer switching frequently between a variety of coating products.
Color is added to powder coatings during the manufacturing process, ie before the powder reaches the powder coater. There is little that can be done to change the color consistently, once the powder leaves the manufacturing plant.
Find out more about Powder Coating production



Powder coating application
In powder coating, the powdered paint is applied by either lowering the part into a fluidized bed of powder, which may or may not be electrostatically charged, or spraying with electrostatically charged powder. In both cases the part is then placed in an oven, where the powder particles melt, coalesce and bond to the metallic surface forming a continuous film. Powder coatings give a clean finish to goods and are hard wearing and corrosion resistant.
Of primary concern to the end-user is consistent finish and color. The critical factors which determine how well the surface will be coated are the chemistry of the powder coating, the powder and metallic surface charges, the heat applied and the particle size of the product. Obviously to ensure a uniform coating, particle size must also be uniform. Different grades of finish can be made by applying various grades of coarseness of powder.
Powder coatings can be applied over a wide range of thickness. The new Australian Standard, "AS/NZS 4506 - Thermoset powder coatings", will recommend 25 micron minimum for mild interior applications and up to 60 micron minimum for exterior applications. Care must be exercised when quoting minimum thickness because some powder will not give "coverage" below 60 or even 80 micron. "Coverage" is the ability to cover the color of the metal with the powder. Some of the white colors require about 75 micron to give full "coverage". One of the orange colors must be applied at 80 micron.
A key part of the coating process is surface preparation. The vast majority of powder coating failures can be traced to a lack of a suitable preparation. The required preparation treatment is different for different materials. For aluminum surfaces, preparation typically involves the removal of oils and greases with an alkali or neutral detergent solution, etching to remove heavy oxides, and chromate or phosphate dipping to form a conversion coating. This conversion coating has two functions in that it presents a surface to the powder which favors adhesion and it reduces the incidence of under film corrosion.
A final rinse in demineralized water completes the preparation process. For steel, the preparation method also typically includes a grain refining step which ensures that the conversion coating produced is relatively fine with good adhesion properties.