Technique for Curing Polyurethane Coatings on Piezoelectric Transducers

Polyurethane coatings on transducers commonly had (or developed) a fishscale‐type appearance due to fissures, contained entrapped bubbles, and exhibited poor adhesion to the silvered electrodes of piezoelectric‐ceramic elements in all cases tried when the material was cured at 150°F beginning sequentially after pouring. These defects, which caused loss of watertight integrity or anomalous acoustic characteristics, were eliminated when the 150°F cure was preceded by a 24‐hr room‐temperature (75°±5°F) cure. No difficulties were encountered in potting small (112 in. diam×112 in. long) cylindrical ceramic, or in encapsulating large (6 in. diam×24 in. long) cylindrical assemblies. Use of the 2‐step curing process permitted simplification of shop practice in that the mold was not placed in the oven until after the polyurethane had achieved its initial set. Hence, mold‐handling was facilitated. All transducers that have been potted in transparent polyurethane and cured using the technique described have met the ...