Silane coupling agent is a compound that can simultaneously form certain binding forces with polar and non-polar substances. Its characteristic is that the molecule has both polar and non-polar parts, which can be represented by the general formula Y (CH2) nSiX3, where Y represents alkyl, phenyl, and organic functional groups such as vinyl, epoxy, amino, thiol, etc. It often chemically binds with organic functional groups in the adhesive matrix resin; X represents chlorine, methoxy, ethoxy, etc. These groups are easily hydrolyzed into silanol and react with oxides or hydroxyl groups on the surface of inorganic substances (glass, silica, metal, clay, etc.), forming naturally stable silicon oxygen bonds.
Therefore, by using silane coupling agents, a "molecular bridge" can be built between the interface of inorganic and organic substances, connecting two materials with completely different properties together, effectively improving the bonding strength of the interface layer.
Adding silane coupling agents to adhesives can not only improve bonding strength, but also enhance the durability and resistance to moisture and heat aging of the adhesive. For example, although polyurethane has high adhesion to many materials, its durability is not ideal. After adding silane coupling agents, its durability can be significantly improved. When studying the wet heat durability of titanium alloy bonded parts, Chen Ruizhu et al. added silane coupling agent to the epoxy adhesive used, which improved the shear strength preservation rate of the bonded parts after wet heat aging from about 80% to about 97%. Silane coupling agents can even be directly used as adhesives for bonding silicone rubber, fluororubber, nitrile rubber, and other metals.
To improve certain properties of organic adhesives (such as heat resistance, self extinguishing, dimensional stability, etc.), or to reduce the cost of organic adhesives, inorganic fillers are often added to the adhesive. If the filler is treated with silane coupling agent in advance, the polar groups on the surface of the filler react with the silane coupling agent, greatly reducing the structural interaction between the filler and the resin. This not only greatly improves the compatibility and dispersibility of the filler with the adhesive matrix resin, but also significantly reduces the viscosity of the system, thus increasing the amount of filler used. However, not all fillers can be effectively treated with coupling agents. The effect varies depending on the type of filler, and some may even be completely ineffective. For fillers with a large number of hydroxyl groups on the surface such as silica, glass, and aluminum powder, the effect is the best, while for fillers without hydroxyl groups on the surface such as calcium carbonate, graphite, and boron, it has no effect.
