Spray coatings applications

D. C. Busby and co-workers. Supercritical Fluid Spray Application Technology A Pollution Prevention Technologyfor the Futures PP- 218—239 Proceedings of the 17th Water-Borne and High-Solid Coating Symposiums New Orleans, La., 1990. 

Sprayed, vacuum-deposited and plated coatings can be applied to most metals and to many non-metals, e.g. vacuum deposition is applied to many substrates including plastics spray application can be used for coating fabric, plastic and paper. Hot dipping and other diffusion processes are dependent on the nature of the substrate for the properties of the coating. Most commercial applications of aluminium coatings are on iron and steel with smaller quantities applied to aluminium alloys and plastics. 

Metal Spraying application of a metal coating to a surface (metallic or non-metallic) by means of a spray of metal particles. The metal particles may be produced by atomising a metal wire in a flame-gun or by introducing metal powder into a similar gun. 

Most metals and many refractory compounds can be thermal-sprayed. Applications include coating of gas-turbine components for aircraft and industrial use, components of steam turbines and diesel engines, components for the oil and gas industry, paper and pulp industry, and chemical processing industry. 

The use of roll and coil coating systems leads to pollution prevention over traditional spray application systems due to their higher transfer efficiency (>95%) and lower volatilization of organic solvents. 

Wire is coated by being passed through a plastic extruder, but most materials are coated with solutions, emulsions, or hot powders. The classic brushing process has been replaced by roll coating, spraying, and hot powder coating. The application of polymers from water dispersions to large objects, such as automobile frames, has been improved by electrode-positon of the polymer onto the metal surface. 

Other. Numerous other supercritical fluid processes and techniques have been and are continuing to be developed. The novel spray application UNICARB uses supercritical C02 to replace volatile diluents in coating formulations, thereby reducing the volatile oiganic compound emissions by up to 80% (128). Rapid expansion of C02 in the spray can also improve the quality of coatings, thus performance benefits are obtained in a process developed for environmental advantages. 

Material produced during start-up and shutdown of the process may have to be either scrapped or reworked, since it is likely to have received less than the targeted levels of coating as a result of reduced exposure to the spray application process. 

Deposition of the coating is much more precise than is typically achieved using current spray application processes, leading to substantial improvements in coating uniformity both from tablet to tablet and across the surface of each individual tablet. 

Chromium carbide is important in powder preparations designed for thermal spray applications of corrosion and wear-resistant coatings on tool and machine parts. Lower carbon carbides of chromium are important in hardfacing rods and electrodes for weld-applied overlays on machine wear surfaces. However, these carbides are usually formed in situ from Cr and C in the rod and not added as preformed carbides. The properties of Cr3C2 are listed in Table 2. 

A primer surfacer, composed most often of a polyester to be cross-linked with a blocked isocyanate or melamine cross-linker, is applied next at a dry film thickness of approximately 1.0 mil by conventional spray application. The function of this coating is to provide filling and leveling properties as well as some stone-chip resistance. Cure conditions range from 250 to 325°F for 20 min. 

The clear-coat is referred to as a two-component (2K) clear-coat when isocyanate is used as the cross-linker. The isocyanate must be added to the hydroxy functional acrylic just prior to spray application to prevent premature cross-linking (or gelation). For this reason, the two-part mixing is referred to as 2K. Cure conditions for isocyanate-cured clear-coat range from room temperature (car refinish) to 250°F for 30 min. 

The rise time for airless-sprayed urethane foam is about 30 s. It can be refoamed for additional thickness after this period. The foam can be walked on after 3 to 4 minutes and reaches its full properties in 24 h. The airless spray gun is held about 30 in. (76 cm) from the surface and moved steadily over it. Dispensing rates of 4 to 6 lb (1.8 to 3.6 kg) per minute are generally considered optimum for most spray applications. The surface on which the foam is sprayed must be free of loose scale or grease. The adhesion of urethane foam to steel is essentially equal to the tensile strength of the foam, provided the surface is clean. Aluminum surfaces, on the other hand, do not provide a good bond unless a primer coat, such as vinyl wash, is used prior to spray foaming. 

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