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Adhesives are now used in virtually every sector of the national economy. In many situations, adhesives can effectively replace welding, riveting, bolting and other mechanical connections, providing an effective way for the production front line to simplify processes, save energy, reduce costs and improve economic efficiency. With the rapid development of science and technology, ever-higher demands are placed on the performance of adhesives and bonding technology. Because different materials have different interfacial properties, coupling agents containing reactive groups are often used to form chemical bonds with the solid surface of the adhered material, so that a strong interfacial layer is obtained between the adhesive and the surface of the adhered material. Substances that can be used as coupling agents include organosilanes, titanates, phosphates and certain organic acids, among which silane coupling agents are the best known and most widely used.

1. The Role of Silane Coupling Agents in Organic Adhesives

Silane coupling agents are compounds that can bond simultaneously with polar and non-polar substances. Their characteristic feature is that the molecule contains both polar and non-polar parts — for example, organic functional groups such as amino, epoxy, alkyl and phenyl groups, which often combine chemically with the organic functional groups of the adhesive matrix resin. Groups such as methoxy and ethoxy hydrolyse readily to silanols, which react with the oxides and hydroxyl groups on the surface of inorganic substances (glass, silica, metals, clay, etc.) to form stable siloxane bonds. Thus, by using a silane coupling agent, a “molecular bridge” can be built at the interface between inorganic and organic materials, joining two materials of completely different natures, effectively improving the bonding strength of the interfacial layer.

Adding a silane coupling agent to an adhesive not only increases bonding strength but also improves the durability of the adhesive and its resistance to wet-heat ageing. Polyurethane, for example, bonds well to many materials but its durability is not ideal; after adding a silane coupling agent, its durability is markedly improved. In studies of the wet-heat durability of titanium alloy bonded joints, adding a silane coupling agent to the epoxy adhesive used raised the retention of shear strength after wet-heat ageing from about 80% to about 97%. Silane coupling agents can even be used directly as adhesives, for bonding silicone rubber and fluororubber to metals.