Spray-coating transfer efficiency

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. 

As originally introduced. Electrostatic Bell Process 2 was used primarily with solvent-based coating materials. The bells were rotated at the lowest possible rate consistent with obtaining good atomization of these materials because it was desired that the mechanical forces acting on the sprayed particles be held to a minimum. Rotation rates were on the order of 1800-3600 rpm. With the recent advent of high solids and waterborne materials that are much harder to atomize, it has become necessary to increase the rotation speed of the bells. Most bells used with this system today are rotated at 20,000-30,000 rpm. This Increase does increase the size of the spray pattern emitted from the bell but does not materially decrease the overall transfer efficiency on those applications where the arrangement of the equipment has been made with this fact in mind. 

Continuing efforts to lower the VOC of lacquers and improvements in application equipment caused both coating manufacturers and users of lacquers to evaluate electrostatic spray application, On the basis of experience with other coated products, the EPA requested a 50% transfer efficiency from users of coatings. Their studies showed that conventional spray application results in 30% transfer efficiency. Thus, they feel that 50% is a reasonable, significant, and attainable improvement. Some studies of electrostatic application of lacquers suggest that this unsupported judgment by EPA may be achievable, and some literature (21) tends to support this judgment. 

Environmental Protection. On account of the high paint transfer efficiency and low solvent content, environmental pollution in electrodeposition coating is much lower than with spray coating. The solvent contents of electrodeposition paints relative to the solids content are 5-10% in comparison with solventborne spray paints with a solvent content of 50-100%. The most important sources of emission are the electrodeposition tank, the rinsing zone, the stoving oven, and the drain of the ultrafiltrate [3.125]. Developments in new electrodeposition paints aim to reduce the organic volatile solvent content, to have fewer condensation products in the stoving ovens, and to have lead-free formulations [3.126]. 

Air electrostatic spray equipment has the advantage of being able to handle high-solids materials. This is overshadowed by the fact that it has the lowest transfer efficiency of the electrostatic spray coating methods. 

Multicomponent spray coating. Multicomponent spray coating equipment (30 to 70% TE) is used for applying fast-curing coating system components simultaneously. Since they can be either hydraulic or air-atomizing, their transfer efficiencies vary from low to medium. They have two or more sets of supply and metering pumps to transport components to a common spray head. 

The advantage of the air-atomized spray gun is that a skilled operator can adjust fluid flow, air pressure, and coating viscosity to apply a high-quality finish on most products. The disadvantages are its low transfer efficiency and ability to spray only low-viscosity coatings, which emit great quantities of VOCs to the atmosphere. 

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. 

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