Electrostatic spraying process

The volume of thermosetting powders sold exceeds that of thermoplastics by a wide margin. Thermoplastic resins are almost synonymous with fluidized-bed appHed thick-film functional coatings whereas thermosetting powders are used almost exclusively in electrostatic spray processes and appHed as thin-film decorative coatings. 

Airless Spraying the process of atomisation of paint by forcing it through an orifice at high pressure. This effect is often aided by the vaporisation of the solvents especially if the paint has been previously heated. The term is not generally applied to those electrostatic spraying processes which do not use air for atomisation. 

Half of the normally oversprayed material is thus captured as useful coating. To the average spray finishing operation a reduction of 25% in the amount of material required to do the job is already a sizable saving. Because of this possible saving, this electrostatic spray process has been used by industry to coat many types of items from automobiles to bicycles. 

The airless electrostatic spray process uses airless spray guns with the addition of a DC power source that electrostatically charges the coating droplets. Its advantage over airless spray is the increase in transfer efficiency due to electrostatic attraction of charged droplets to the product. 

The process is versatile with low capital investment. In the electrostatic spraying process, the particles released from the spray gun are electrostatically charged and propelled at low velocity by air or revolving spray head. Too much air pressure is to be avoided. This procedure produces a good wrap around without the need to positioning the workpiece. The process is suitable for tubular articles. 

Almost all PVC powder coatings are appHed by the fluidized-bed coating process. Although some electrostatic spray-grade formulations are available, they are very erratic in their appHcation characteristics. The resistivity of plasticized PVC powders is low compared to other powder coating materials and the appHed powder quickly loses its electrostatic charge. For the same reason, PVC powders show poor appHcation characteristics in an electrostatic fluidized bed and are seldom used in this process. 

Electrostatic spraying PVC can be applied by the electrostatic process to continuous galvanised wire mesh. 

Nylon 11 is applied using a fluidised bed process to a wide variety of substrates including metal chair frames, door furniture and wire dishwasher baskets. It can also be applied by electrostatic spraying, but generally only where the application is decorative and where the metal work is thin, i.e. less than 0.2 mm. 

Powder coating is a solventless coating system that is not dependent upon a sacrificial medium such as a solvent, but is based on the performance constituents of solid TP or TS plastics. It can be a homogeneous blend of the plastic with fillers and additives in the form of a dry, fine-particle-size compound similar to flour. The three basic methods are the fluidized bed, electrostatic spray, and electrostatic fluidized bed processes (9). 

Electrostatic spray Charged particles are sprayed on electronically conductive parts process gives high paint utilization more expensive than conventional spray. Spray gun, high-voltage power supply pumps dryers Pretreating station for parts (coated or preheated to make conductive). 

Figure 1.3. Schematic of aerosol spray process electrostatic aircraft spray.

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