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The Role of Silane Coupling Agent in Organic Adhesives


1. Performance characteristics of silane coupling agent


Silane coupling agent is a compound that can combine with polar and nonpolar substances at the same time. It is characterized by both polar and nonpolar parts in the molecule, which can be expressed as Y(CH2) nSiX3 by the general formula. Where y represents alkyl, phenyl, vinyl, epoxy, amino, sulfhydryl and other organic functional groups, which are often chemically combined with the organic functional groups in the adhesive matrix resin; X represents chlorine group, methoxy group, ethoxy group and so on. These groups are easily hydrolyzed into silanol and react with oxides or hydroxyl groups on the surface of inorganic substances (glass, silica, metal, clay and so on), and stable silica oxygen bonds. Therefore, by using silane coupling agent, a "molecular bridge" can be built between the interface of inorganic and organic substances to connect two materials with completely different properties, which can effectively improve the bonding strength of the interface layer.


2. Role of silane coupling agent in organic adhesive


Adding silane coupling agent to the adhesive can not only improve the bonding strength, but also improve the durability and moisture and heat aging resistance of the adhesive. For example, although polyurethane has high adhesion to many materials, its durability is not ideal. After adding silane coupling agent, its durability can be significantly improved.


Professional technicians add a silane coupling agent to the epoxy adhesive used when studying the damp and heat durability of titanium alloy bonding parts. In this way, the shear strength preservation rate of the bonded parts after damp heat aging can be increased from about 80% to about 97%. Silane coupling agents can even be directly used as adhesives for bonding silicon rubber, fluororubber, nitrile rubber, etc. to metals. For example, the adhesives CK-1 and Chemlock607 (USA) are silanes.


In order to improve some properties of organic adhesive (such as heat resistance, self-extinguishing, dimensional stability and so on.), or to reduce the cost of organic adhesive, some inorganic fillers are often added to the adhesive. If the filler is treated with silane coupling agent in advance, the polar group on the surface of the filler reacts with the silane coupling agent, which greatly reduces the structural effect of the filler and the resin, not only greatly improves the compatibility and dispersion of the filler to the adhesive matrix resin, but also significantly reduces the viscosity of the system, so the amount of filler can be increased. However, not all fillers treated with coupling agent are effective. Different types of fillers have different effects, and some even have no effect. The effect is best for fillers with a large number of hydroxyl groups on the surface such as silica, glass and aluminum powder, while it has no effect on fillers without hydroxyl groups on the surface such as calcium carbonate, graphite and boron.


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