Monday, August 17, 2009

Properties of Polyurethane Adhesives

Adhesives interest has been mainly in aromatic polysocyanates although aliphatic in particular hexamethylene diisocyanate, and cycloaliphatic isocyanates have been used. A large number of different polyisocyanates can be synthesized, and an even larger number of compound can be made for reaction with them, as a result an enormous number of plyurethane systems are possible, but only a few are of commercial interest.

In adhesive applications the two most used isocyanates are tolylene diisocyanate and triphenylmethane-p, p’,p” –triisocyanate. The former is of more interest in the gluing of metal, wood and certain plastics, and the latter in applications where one adherend is rubber, either natural or synthetic. A third but less important isocyanate adhesive is dipheniylmethane p,p’-diisocyanate foties, the polyurethanes received considerable attention as wood glues and were in some instances preferred formaldehyde resins in the construction of wooden aero planes. As wood glues the best examples have a strength, after either cold or hot curing, almost comparable with the urea and phenolic glues, and superior to the former but markedly inferior to the latter in resistance to hot water. Being less brittle than either urea or phenolic glues they were probably used in wooden aircraft construction in the belief that they would have higher impact strength, apart from this there would not appear to be a strong case for using polyurethanes for gluing wood. Although little is known of their durability, an unspecified polyurethane adhesive performed creditably in a series of test on wood adhesives, being grades as “sound” after twelve and a half years exposure to English weather.