Kinetics of cure, crosslink density and adhesion of water-reactive alkoxysilicone sealants

Abstract When an alkoxysilicone sealant is exposed to moist air, a surface skin is formed, which becomes thicker with time. It has been observed that, at constant temperature and humidity, the thickness (cure depth) is initially proportional to the square root of time, but later the gradient of such a plot increases. The location at which this occurs is called a kink. There are thus two regions of cure, each corresponding to a limb of the plot. Swelling in toluene shows that the crosslink density is greater in the shallower regions. Young's modulus and the concentration of a crosslinking agent/adhesion promoter are also greater in the shallow regions. Single lap joints in aluminium or glass have been cured in these two regions and joints from the shallow region are always stronger than corresponding ones from the deep region. This behaviour is due to the mobility of some low molar mass crosslinking agents/adhesion promoters. During the initial stages of cure they migrate into the outer region, but once cure has passed into the deeper region there is now a paucity of these compounds.