What is powder coating?

Powder coating is by far the youngest of the surface finishing techniques in common use today. It was first used in Australia about 1967.

Powder coating is the technique of applying dry paint to a part. The final cured coating is the same as a 2-pack wet paint. In normal wet painting such as house paints, the solids are in suspension in a liquid carrier, which must evaporate before the solid paint coating is produced.

In powder coating, the powdered paint may be applied by either of two techniques.

• The item is lowered into a fluidised bed of the powder, which may or may not be electrostatically charged, or
• The powdered paint is electrostatically charged and sprayed onto the part.

The part is then placed in an oven and the powder particles melt and coalesce to form a continuous film.

There are two main types of powder available to the surface finisher:

• Thermoplastic powders that will remelt when heated, and
• Thermosetting powders that will not remelt upon reheating. During the curing process (in the oven) a chemical cross-linking reaction is triggered at the curing temperature and it is this chemical reaction which gives the powder coating many of its desirable properties.

Preparation

The basis of any good coating is preparation. The vast majority of powder coating failures can be traced to a lack of a suitable preparation.

The preparation treatment is different for different materials.

In general, for all applications the preparation treatment for aluminium is as follows:

Oils and greases are removed in weak alkali or neutral detergent solutions and the surface is etched to remove heavy oxides. After rinsing, the aluminium is dipped into a chromate or phosphate solution to form a conversion coating on the aluminium. This film is chemically attached to the aluminium. After rinsing the aluminium is finally rinsed in demineralised water. Some non-chrome, dried in place pretreatment is beginning to come onto the market; currently, these are not recommended for exterior applications.

The conversion coating has two functions:

• It presents a surface to the powder which favours adhesion more than the oxides that form very readily on aluminium surfaces, and
• It reduces the incidence of under film corrosion, which may occur at holidays in the coating.

The use of demineralised water reduces the presence of chemical salts on the aluminium surface. These salts have been found to cause filiform corrosion in humid conditions.

For steel the preparation for interior applications may be:

For exterior applications:

The grain refiner is used after acid cleaning of steel surfaces and before zinc phosphating, otherwise the zinc phosphate coatings produced will be very coarse with low adhesion. The powder coating applied to a coarse phosphate will produce rough coatings (a little like "sandpaper") and possess low adhesion.

For hot dipped galvanized coatings, which have been stored for more than about 4 hours before powder coating, the following process is necessary for exterior applications.

The etch is required to remove the zinc corrosion products which begin to form almost immediately the zinc is removed from the galvanizing kettle. The grain refiner ensures a fine phosphate is produced.

How is it done -- electrostatic