The presence of moisture in polyurethanes is the greatest cause of failure or performance loss in coatings and sealants. Moisture can react with isocyanate to form gaseous carbon dioxide, leading in turn to bubbling, pinholing and other defects.

 

Moisture contamination can occur either from within the formulation or as result of excessive atmospheric humidity. Solvents may contain traces of water which can lead to precipitation of urea crystals. Similarly, pigments, fillers and flatting agents can contain absorbed moisture which forms carbon dioxide gas on reaction with isocyanate. This carbon dioxide is then entrapped within the film surface – particularly so in high solids formulations – causing pinholes or bubbles. As well as detracting from the surface appearance of the coating, these defects can also produce significant reductions in performance characteristics such as chemical and abrasion resistance.