EQUIPMENT FOR APPLYING
100% SOLIDS POLYURETHANE COATINGS

 

100% solids polyurethane coatings can be designed with a variety of viscosity and setting times. They can be applied through brush, roller, conventional single component spray guns, and other types of equipment. They are not in any difference from those regular application equipment that people have used for decades.

However, application equipment for fast setting plural component polyurethanes is somewhat more complex than that for conventional solvent-borne systems. The following outlines the configuration of a set up as an example. For more information, please refer to my paper: Assuring Quality When Applying 100 Percent Solids Polyurethanes

 

 

The standard airless spray system can be divided into 3 parts: the low pressure part, the high pressure part, and the spray gun. The low pressure part provides clean product from individual drums of isocyanate and polyol components. A desiccant drier should be affixed to the air intake of each drum to avoid moisture contamination. The material is gravity fed or pumped by using a transfer pump, from the supply through an intake hose and "Y" strainer (low pressure filter) to the bottom of the displacement pump. Electric band heaters are strapped around the drums to warm and maintain the component feed temperatures at the required values.

The high pressure part supplies the spray gun with material in a 1:1 mixing ratio. The material is supplied evenly and consistently at the required temperature and sufficient pressure. The individual component is drawn in at the bottom of the displacement pump and pushed out at the top. It then passes through the secondary heater, high pressure relief valve, high pressure filter, and heated discharge hose leading to the gun. The secondary heaters further increase the temperatures of the separate materials, and the heated hoses maintain these temperatures to control fluid viscosity. The individual components are thoroughly combined into a mixing device located within the spray gun or directly before the spray tip and are then sprayed to the substrate. Materials remaining in the mixing device are purged by solvent flushing or mechanical motion, depending on the type of spray gun used. A solvent pump is used for a solvent flushing spray gun.

Additives are normally included in the coating formula to ensure homogeneity of the isocyanate-rich and polyol-rich components. Nevertheless, phase separation or settling may occur. It is essential that materials within the individual drums be recirculated or agitated before spray application to ensure a homogeneous mix. This can be achieved using drum mixers, but often a better solution is to use a recirculation kit as shown in the figure. The kit is a by-pass system that allows the product to be pumped out of the individual drums through the plural component pump and back to the original drums. An hour or 2 of recirculation will provide adequate mixing in all but the oldest product. During recirculation, the materials are also preheated and filtered.

 


Copyright © by Dr. Shiwei William Guan
E-mail: sguan@canada.com

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