DT - POWDER COATING

Powder coating is a type of dry coating, which is applied as a free-flowing, dry powder. The main difference between a conventional liquid paint and a powder coating is that the powder coating does not require a solvent to keep it in a liquid form. The coating is typically applied electrostatically and is then cured under heat to allow it to flow and form a "skin." The powder may be a thermoplastic or a thermoset polymer. It is usually used to create a hard finish that is tougher than conventional paint. Powder coating is mainly used for the coating of metals, such as "white goods", aluminium extrusions, automobile and motorcycle parts.

Application of Powder

The application of powder is very simple. Filtered, compressed air, usually at 20-30 psi (137-207 kPa) pushes the powder out of the gun past the electrode which gives the powder a positive charge. The part being coated is earthed so the positive powder particles are attracted to it. When the part is completely covered, the earth is removed and the part is put into the oven.

Advantages and disadvantages of powder coating

There are several advantages of powder coating over conventional liquid coatings:

1. Powder coatings emit zero or near zero volatile organic compounds (VOC).
2. Powder coatings can produce much thicker coatings than conventional liquid coatings without running or sagging.
3. Powder coating production lines produce less hazardous waste than conventional liquid coatings.
4. Powder coated items generally have fewer appearance differences between horizontally coated surfaces and vertically coated surfaces than liquid coated items.
5. A wide range of special effects is easily accomplished which would be impossible to achieve with other coating processes.

While powder coatings have many advantages over other coating processes, there are limitations to the technology. While it is relatively easy to apply thick coatings which have smooth, texture-free surfaces, it is not as easy to apply smooth thin films. As the film thickness is reduced, the film becomes more and more orange peeled in texture due to the particle size and TG (glass transition temperature) of the powder.

For optimum material handling and ease of application, most powder coatings have a particle size in the range of 30 to 50 µm and a TG > 40°C. For such powder coatings, film build-ups of greater than 50 µm may be required to obtain an acceptably smooth film. The surface texture which is considered desirable or acceptable depends on the end product. Many manufacturers actually prefer to have a certain degree of orange peel since it helps to hide metal defects that have occurred during manufacture, and the resulting coating is less prone to show fingerprints.

Types of powder coatings

There are two main categories of powder coatings: Thermosets and thermoplastics. The thermosetting variety incorporates a cross-linker into the formulation. When the powder is baked, it reacts with other chemical groups in the powder polymer and increases the molecular weight and improves the performance properties. The thermoplastic variety does not undergo any additional reactions during the baking process, but rather only flows out into the final coating.

The most common polymers used are polyester, polyester-epoxy (known as hybrid), straight epoxy (Fusion bonded epoxy) and acrylics.

The powder coating process

The powder coating process involves three basic steps:

1. Part preparation or the Pre treatment
2. The powder application
3. Curing