Melt-sprayed coatings

Melt-sprayed coatings have ethyl cellulose, acetyl cellulose or cellulose ace-tobut3Tate as a film-forming base. Mineral oil, which dissolves a number of Cl of ferrous and nonferrous metals and is perfectly compatible with the majority of cellulose plasticizers, is often introduced into the molten composition to lower adhesion of the coating to the goods being protected. 

Flame spraying is no longer the most widely used melt-spraying process. In the power-feed method, powders of relatively uniform size (<44 fim (325 mesh)) are fed at a controlled rate into the flame. The torch, which can be held by hand, is aimed a few cm from the surface. The particles remain in the flame envelope until impingement. Particle velocity is typically 46 m/s, and the particles become at least partially molten. Upon impingement, the particles cool rapidly and soHdify to form a relatively porous, but coherent, polycrystalline layer. In the rod-feed system, the flame impinges on the tip of a rod made of the material to be sprayed. As the rod becomes molten, droplets of material leave the rod with the flame. The rod is fed into the flame at a rate commensurate with melt removal. The torch is held at a distance of ca 8 cm from the object to be coated particle velocities are ca 185 m/s. 

Low viscosity cellulose propionate butyrate esters containing 3—5% butyryl, 40—50% propionyl, and 2—3% hydroxyl groups have excellent compatibihty with oil-modified alkyd resins (qv) and are used in wood furniture coatings (155). Acetate butyrate esters have been used in such varied apphcations as hot-melt adhesive formulations (156), electrostatically spray-coated powders for fusible, non-cratering coatings on metal surfaces (157—159), contact lenses (qv) with improved oxygen permeabiUty and excellent wear characteristics (160—162), and as reverse-osmosis membranes for desalination of water (163). 

Flame spray coating involves blowing a plastic powder through a flame that partially melts the powder and fuses it as it contacts the substrate. The... 

The supply chain for this process can be seen in Fig. 8.11. Electrostatic spray coating or powder coating is a way of applying a dry paint to a surface. Most people have used liquid paints with a brush, a spray can, or even with their fingers. Powder coatings are dry there are no liquid solvents. The dry powder is applied to an item (substrate), then the powder is turned to liquid, usually by melting. The dry powder in its molten state can then flow out to cover the substrate, coalesce, and crosslink. The end result is a painted surface. 

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