Volume 1, Issue 3 (March 2004)
Silyl-Terminated Polyurethanes for Construction Sealants
Construction sealants produced through the curing of silyl-terminated oligomers (STO) display properties similar to conventional polyurethanes, but they possess several distinct advantages. This paper provides a state-of-the-art review of silane-functional polymers for sealants in construction. Properties of silyl-terminated polyurethanes (STPU) are compared with traditional polyurethanes as well as silyl-terminated polyether (STPE) sealants. Urethane prepolymers, prepared from the reaction of a diisocyanate and a conventional polyol, are end-capped with an organo-functional silane. As a consequence, there are no free isocyanate molecules in the sealant. These sealants show improved adhesion to glass, UV stability, and weatherability. Sealants based on STPU chemistry are presently in use for industrial and transportation applications, which require medium to high modulus. Current work explores methods to reduce the modulus and increase elongation while retaining good cohesive properties. STPU sealants have been prepared with varying isocyanates, polyols, and silanes. They have been evaluated in accordance with several ASTM tests, including those in the ASTM Standard Specification for Elastomeric Joint Sealants (C920). The glass-transition temperatures and rheological characterizations of STO-based sealants are presented. Correlations are made between these fundamental material properties and the sealant's behavior in use, as measured by ASTM test methods. These measurements could provide a method for identifying candidate formulations that are expected to pass more extensive ASTM testing. These material characterizations as well as the results of mechanical property testing are compared for selected STPU, STPE, polyurethane, and silicone sealants.