All About Powder Coating
Properties of powder coating
Because powder coating does not have a liquid carrier, it can produce thicker coatings than conventional liquid coatings without running or sagging, and powder coating produces minimal appearance differences between horizontally coated surfaces and vertically coated surfaces. The coating process emits few volatile organic compounds (VOC)because no carrier fluid evaporates away. Finally, several powder colors can be applied before curing them all together, allowing color blending and bleed special effects in a single layer.
While it is relatively easy to apply thick coatings which cure to smooth, texture-free coating, 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 glass transition temperature (Tg) of the powder.
For such powder coatings, film build-ups of greater than 50 ¼m may be required to obtain an acceptably smooth film. 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 showing fingerprints.
There are very specialized operations where powder coatings of less than 30 micrometres or with a Tg below 40 Â° C are used in order to produce smooth thin films. One variation of the dry powder coating process, the Powder Slurry process, combines the advantages of powder coatings and liquid coatings by dispersing very fine powders of 1– 5 micrometre particle size into water, which then allows very smooth, low film thickness coatings to be produced.
For garage-scale jobs, small “rattle can” spray paint are less complex and expensive than powder coating At the professional scale, the capital expense and time required for a powder coat oven, booth and gun are similar to a spray gun system. Powder coatings have a major advantage in that the overspray can be recycled. If multiple colors are being sprayed in a single spray booth, this may limit the ability to recycle the overspray.
Types of powder coating.
There are two main categories of powder coating: 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 to polymerize, improving the performance properties. The thermoplastic variety does not undergo any additional actions during the baking process, but rather only flows out into the final coating.
The most common polymers used are polyester, polyurethane, polyester-epoxy (known as hybrid), straight epoxy (fusion bonded epoxy) and acrylics.