Spray Application of Urethane Elastomers

Prepolymers are soluble in several solvents, including acetone, methyl ethyl ketone, ethyl acetate and toluene, where with their curatives they react much more slowly in solvent than as solventless simple mixtures, and the solutions can be used for spraying, brushing, knife- or roller-coating and dipping. Whichever technique is used, an oven cure for 1 h at 100°C is 

Since no solvent is used, the film thickness can be built up very rapidly. Thicknesses of up to 12 mm or more can be achieved by continuously spraying successive coatings. No hot-air cure is necessary, but it is recommended that the sprayed coating is matured at room temperature for several days before being put into service. 

This sprayable system is of use where relatively thick, resilient polyurethane coatings are required and where it is impracticable to apply prepolymer itself This criterion is well illustrated in the covering of a ship s propeller. 

The polyols used for solid urethane elastomers must contain very low water contents, otherwise porosity would result in a reaction with the diisocyanate as a result of CO2 generation. Many polyesters are supplied in flake form but the flaking process and the flake form are both susceptible to absorption of atmospheric moisture. By filling the containers with the liquid polyester taken directly from the still and cooling under dried air, the uptake of atmospheric moisture is minimized. When required the polyester can then be melted using electric drum heaters and passed through a heat exchanger to obtain the required temperature. 

It may be necessary, on occasion, to dry the polyol for solid elastomer production and this can be achieved in several ways  

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