Silicones have special properties compared to other adhesives based on organic polymers because silicones have a different chemical backbone. They remain highly elastic at low temperatures, -100oF (-75°C), and also have very good temperature stability; up to 390oF (200°C) continuous exposure and up to 575oF (300°C) for short periods. The properties of silicones remain virtually unchanged over this temperature range. Silicones are nearly inert to chemicals and have excellent resistance to moisture and weathering. Bonds made with silicones can, however, only be subjected to relatively small mechanical loads. That is why they are chiefly used as sealants. Due to their low surface tension they cannot be painted. They are used for bonding metal when the low bond strength is offset by the higher flexibility and resistance to low temperatures.
Single component silicone adhesives require a humidity of 5% to 95% to cure. Besides the presence of humidity, a temperature of between 40 and 100oF (5 and 40°C) is required to cure the adhesive. Complete curing depends on the thickness of the adhesive film and can take several days. The onset of curing is indicated by formation of a skin. For an adhesive film thickness of a few millimeters the adhesive generally fully cures in 24 hours. Several types of moisture curing silicones are available depending on the bonding conditions and substrates. Typical applications include bonding glass and ceramics, metals, plastics, concrete, and brickwork. Special formulations are used in aircraft and aerospace applications.
Two component silicone adhesives are available with a range of properties and cure rates. The initial strength and rate of strength build-up is typically higher than moisture cured silicones. The curing reaction can take up to 24 hours. Meter mix equipment is used to pump the two components from pails or drums in a metered amount through a mixing element. The mixed adhesive is then dispensed in bead form. Two component silicone adhesives are used extensively in the assembly of windows.