Inasmuch as fast room temperature curing, single component sealants and adhesives are desirable and useful, particularly in original equipment manufacturing, it is not surprising that a number of one component elastomeric sealants are now available in the marketplace. Such sealants include various polymer bases such as polysulfides, mercaptan terminated polyethers, polysiloxames and polyurethanes.
Certain industries need elastomeric adhesives or sealants which cure by exposure to ambient conditions and which will develop a high tensile strength. Applications of this type include sealing automobile windshields which are often intended as structural components in design. For these applications the elastomeric sealant or adhesive must not only have high tensile strength but should achieve such strength in a matter of a few hours so that the automobile may be safely driven shortly after installation of the windshield.
Of the various liquid elastomers available today, cured polyurethanes, in general, have the highest mechanical strength and therefore are the polymers of choice as a windshield sealant or adhesive provided that the adhesive or sealant can cure rapidly under ambient conditions without exhibiting other problems such as foaming, storage instability, depolymerization, etc..
An example of a one component, room temperature, moisture curing polyurethane sealant is disclosed in U.S. Pat. No. 3,779,794 wherein a polyurethane sealant in combination with a particular type of silane primer is disclosed. In that patent, the polyurethane sealant is an isocyanate terminated polyethylene ether diol-polypropylene ether triol combination having from 1.2 to 1.5% free isocyanate terminals. These terminals are blocked with a volatile blocking agent which, when exposed to air, evaporates and the moisture in the air cures the polyurethane. The polyurethane sealant disclosed in this patent in combination with the silane primer cures, according to the patent, to a tensile strength of 40-60 pounds per square inch (psi) after 6 hours exposure at 77.degree. F. and 30% relative humidity. Although such sealants are satisfactory in terms of ultimate elongation characteristics and the like, nevertheless, there is a need for the development of a sealant having higher early strength, i.e. 100 psi or more in a six hour period of time, with equivalent ultimate elongation.