Mechanism of moisture-cure of isocyanate reactive hot melt adhesives

The depth of moisture cure (z) has been measured for lap joints in aluminium with two isocyanate moisture curing hot melt adhesives. With one adhesive, z is proportional to the square root of time, but with the other there is a time-lag before cure commences. Skins form when the adhesives are exposed to moist air, and their thicknesses (sigma) are proportional to root of time of exposure. The depth of cure is given by z = sigma = (2PV pt)(1/2), where P is the permeability coefficient of water in cured sealant, V is the volume of sealant which reacts with 1 mole of water, p the vapour pressure of water and t the time; this is after correction for the time-lag. The final state of cure is independent of relative humidity. One adhesive cures more rapidly than the other, and the main factor responsible for this is its higher water permeability. (C) 1998 Elsevier Science Ltd. All rights reserved.