Powder coating is a type of coating that is applied as a free-flowing, dry powder. Unlike conventional liquid paint, which is delivered via an evaporating solvent, powder coating is typically applied electrostatically and then cured under heat or with ultraviolet light. The powder may be a thermoplastic or a thermosetting polymer. It is usually used to create a thick, tough finish that is more durable than conventional paint. Powder coating is mainly used for coating of metal objects, particularly those subject to rough use. Advancements in powder coating technology like UV-curable powder coatings allow for other materials such as plastics, composites, carbon fiber, medium-density fibreboard (MDF), and solid wood to be powder coated, as little heat or oven dwell time is required to process them.
History, properties, and uses of powder coating
The powder coating process was invented around 1945 by Daniel Gustin and received US Patent 2538562 in 1945. This process coats an object electrostatically and then cures it with heat, creating a finish harder and tougher than conventional paint. Originally used on metal manufactures, such as household appliances, aluminium extrusions, drum hardware, automobile parts, and bicycle frames, the practice of powder coating has been expanded to allow finishing of other materials. 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. Further, because no carrier fluid evaporates away, the coating process emits few volatile organic compounds (VOC). Finally, several powder colors can be applied before all are cured together, allowing color blending and special bleed effects in a single layer. While it is relatively easy to apply thick coatings that 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 because of the particle size and glass transition temperature (Tg) of the powder.
Most powder coatings have a particle size in the range of 2 to 50 μm, a softening temperature Tg around 80 °C, and a melting temperature around 150 °C, and are cured at around 200 °C for a minimum of 10 minutes to 15 minutes (exact temperatures and times may depend on the thickness of the item being coated). 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 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 that apply powder coatings of less than 30 μm or with a Tg below 40 °C 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 μm sized particles into water, which then allows very smooth, low-film-thickness coatings to be produced. For small-scale jobs, "rattle can" spray paint is less expensive and complex than powder coating. At the professional scale, the capital expense and time required for a powder coat gun, booth and oven are similar to those for a spray gun system. Powder coatings do have a major advantage in that the overspray can be recycled. However, if multiple colors are being sprayed in a single spray booth, this may limit the ability to recycle the overspray.
Advantages over other coating processes Powder coatings contain no solvents and release little or no amount of volatile organic compounds (VOC) into the atmosphere. Thus, there is no need for finishers to buy costly pollution control equipment. Companies can comply more easily and economically with environmental regulations, such as those issued by the U.S. Environmental Protection Agency. Powder coatings can produce much thicker coatings than conventional liquid coatings without running or sagging. Powder coated items generally have fewer appearance differences than liquid coated items between horizontally coated surfaces and vertically coated surfaces. A wide range of speciality effects are easily accomplished using powder coatings that would be impossible to achieve with other coating processes. Curing time is significantly faster with powder coatings compared to liquid coatings especially when using ultraviolet cured powder coatings or advanced low bake thermosetting powders.
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