Thermoplastic coatings electrostatic spraying

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. 

Thermoplastic resins, such as vinyl chlorides, vinyl acetates and polyamides are employed, particularly in the water industry, on buried pipes and fittings. To provide both internal and external coating, application may be by one of these principle techniques dipping in a plastisol, fluidised beds or electrostatic spray. 

In powder coatings the coating material is applied to the workpiece in the form of dry (i.e., solvent-free) thermoplastic or thermosetting powder. The powder particles are heated and melt to form a film. The thermoplastic powders melt and fuse on heating whereas the thermosetting powders also become chemically cross-linked. Two main application processes are used electrostatic spraying and fluidized-bed coating. 

Powder coatings are applied by thermoplastics powder flame spraying or electrostatic powder spraying. In most cases, electrostatic powder spray systems are more versatile. An electrostatic charge is appUed on the substrate and an opposite charge is appHed to the powder in the spray head (gun). A coating film forms after heat-fusion. 

Both thermosetting and thermoplastic powders can be applied by powder coating processes. (See Table P.9.) Elaborate reclaiming systems to collect and reuse oversprayed material in electrostatic spray powder systems boost flansfer efficiency. Since the enhancement of the air quality standards this method has grown markedly. 

Thermoplastic coatings are often fabricated from powders. To coat a metallic object it is either dipped into a fluidized bed from which the powder deposits or one uses a spray gun to project the powder onto the metal surface. In electrostatic powder coating the polymer powder is projected in the presence of an electric field. This permits coating of recesses and hidden faces that are not in line of sight with the spray gun. After application of the powder, the object is annealed, and the powder transforms into a compact polymer coating. 

Coatings 50-75 microns thick can be apphed electrostatically to cold objects and coatings up to 250 microns thick to hot objects. The polymers used in spraying of powders are the same as those used in fluidized beds. The key characteristic for any polymer, thermoplastic or thermoset, applied as a powder is low melt viscosity, which enables polymer particles to flow together and form a continuous coating. 

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