Single component moisture curing adhesives cure on exposure to moisture either in the substrate or atmosphere. A small quantity of carbon dioxide is released during cure that, usually, has no effect on the bonding process. However, this can lead to foaming of the bond line in very high humidity or in very thick bond lines. The reaction takes place from 40 to 100oF (5 to 40°C) with a relative humidity of 40 to 70% being required. For very low humidity environments or where the substrates are impermeable to moisture, moisture can be added to the bond line during the assembly process to facilitate cure. Curing of the adhesive film takes place from outside to inside at a rate of a few millimeters per day. When processing adhesives, the so-called “skinning time” must be heeded, namely the time after which the adhesive solidifies on its surface (forms a “skin”) and wetting of the second substrate is no longer possible. In its cured state, the adhesive is elastic and flexible. This is why moisture curing, single component polyurethane systems are used where materials with widely differing elongation under loads and differing thermal coefficients of expansion must be joined such as the bonding of glass-fiber reinforced plastics (FRP) to steel, and aluminum wall and floor plates to steel supports.
Moisture curing urethanes are also available in hot melt form (curing hot melts). These adhesives are formulated from urethane prepolymers that are solid at room temperature and melt when heated for dispensing. Curing hot melts combine the fast setting and high initial strength of hot melts with the improved creep and heat resistance of traditional moisture curing urethane adhesives. As a result, components can be rapidly joined for further processing. Final strength is reached later. Reactive polyurethane hot melts yield rubbery and plastic cured films. Curing hot melts have the advantage that they can be applied at very low temperatures, 125 to 250oF (50 to 120°C). In contrast, standard hot melts are applied at higher temperatures 250 to 470oF (120 to 240°C).
Heat curing urethane adhesives are formulated using components (blocking agents) that temporarily react with the urethane prepolymer. On heating to specified temperatures, the blocking agent “unblocks” allowing the urethane prepolymers to react. Heat curing urethanes require a temperature of 200 to 390oF (100 to 200°C) to cure with the cure time varying from a few minutes to several hours depending on the actual temperature employed. Bonds formed using heat curing urethane adhesives are generally tough and hard and of high strength, but still elastic. The heat employed for curing these adhesives may liberate isocyanate compounds from the system. It is important to follow manufacturer’s directions for safe handling and proper ventilation.
London’s City Hall, home of the Greater London Authority. Made possible by window glazing adhesives and an advanced synthetic rubber membrane system. Courtesy
of Sika Corporation.