Powder coating is a type of coating that 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 the binder and filler parts in a liquid suspension form. The coating is typically applied electro statically and is then cured under heat to allow it to flow and form a skin . The powder may be thermoplastic or a thermostat polymer. It is usually used to create a hard finish that is tougher than conventional paint. Powder coating is mainly used for coating of metals, such as whiteware , aluminum extrusions, and automobile and bicycle parts.
Advantages and Disadvantages
There are several advantages of powder coating over conventional liquid coatings:
- Powder coatings emit zero or near zero volatile organic compounds (VOC).
- Powder coatings can produce much thicker coatings than conventional liquid coatings without running or sagging.
- Powder coating overspray can be recycled and thus it is possible to achieve nearly 100% use of the coating.
- Powder coating production lines produce less hazardous waste than conventional liquid coatings.
- Capital equipment and operating costs for a powder line are generally less than for conventional liquid lines.
- Powder coated items generally have fewer appearance differences between horizontally coated surfaces and vertically coated surfaces then liquid items.
- A wide range of specialty effects is easily accomplished which would be impossible to achieve with other coating processes.
Powder Application Process
The most common way of applying the powder coating to metal objects is to spray the powder using an electrostatic gun, or corona gun. The gun imparts a positive electric charge on the powder, which is then sprayed towards the grounded object by mechanical or compressed air spraying and then accelerated toward the work piece by the powerful electrostatic charge. There are a wide variety of spray nozzles available for use in electrostatic coating. The type of nozzle used will depend on the shape of the work piece to be painted and the consistency of the paint. The object is then heated, and the powder melts into a uniform film, and is then cooled to form a hard coating. It is also common to heat the metal first and spray the powder onto the hot substrate. Preheating can help to achieve a more uniform finish but can also create other problems, such as runs caused by excess powder.
Curing
When a thermostat powder is exposed to elevated temperature, it begins to melt, flows out, and then chemically reacts to form a higher molecular weight polymer in a network-like structure. This cure process, called cross linking, requires a certain temperature for a certain length of time in order to reach full cure and establish the full film properties for which the material was designed. Normally the powders cure at 200 degrees Celsius (390 degrees Fahrenheit) for 10 minutes. The curing schedule could vary according to the manufacturer s specifications. The application of energy to the product to be cured can be accomplished by convection cure ovens or infrared cure ovens.
July 2010
To Whom It May Concern:
Re: VOC Emissions of Powder Coatings
While powder coatings have many performance benefits, it should not be overlooked that it is also a very environmentally friendly technology and little to no volatile organic compounds are released during cure of most powder coatings.
PCI technical brief # 7 discusses the presence of moisture in powder coatings; typically that level is at 0.5% or less. Depending on the chemistry of the powder coating, various substances can be released during cure and emitted into the environment. So called non emissive systems such as Polyester/TGIC and hybrids generally have very low levels of such volatile matter. Usually low molecular weight residue from the manufacturing process of the polymeric binder and possibly small concentrations of some additives that are added to the formulation such as Benzoin will evaparate.
Typical levels of weight loss during cure from organic substances are usually in the range of 0.1-0.5%. So called Polyester Urethanes or Polyurethanes frequently have a blocking agent E – Caprolactam evaporate during cure, depending on the formulation this alone may amount to several % of the weight of the coating, in addition to the other items mentioned above.
Karl Rijkse
VP of Technical Sales