1. Silane coupling agents can effectively improve the bonding strength
Silane coupling agent is a kind of compound which can combine with polar substance and nonpolar substance at the same time. There are both polar and nonpolar parts in the molecule. The general formula is Y(CH2)nSiX3. 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 chloro, 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.) to form stable silica oxygen bonds.
Therefore, through the use of silane coupling agent, a "molecular bridge" can be set up between the interface of inorganic and organic materials, and two materials with completely different properties can be connected together, thus effectively improving the bonding strength of the interface layer.
2. Effect of silane coupling agent on properties of adhesive
Adding silane coupling agent into the adhesive can not only improve the adhesive strength, but also improve the durability and hygrothermal aging resistance of the adhesive.
For example, although polyurethane has high adhesion to many materials, its durability is not ideal. The durability can be improved by adding silane coupling agent.
Silane coupling agent can even be directly used as adhesive for bonding metal with silicone rubber, fluororubber, nitrile rubber, etc.
In order to improve some properties of organic adhesives (such as heat resistance, self extinguishing, dimensional stability), or to reduce the cost of organic adhesives, some inorganic fillers are often added to the adhesives.
If silane coupling agent is used to treat the filler in advance, the structural interaction between the filler and the resin is greatly reduced because the polar groups on the surface of the filler react with silane coupling agent. It not only improves the compatibility and dispersibility of the filler to the adhesive matrix resin greatly, but also reduces the viscosity of the system obviously, thus increasing the amount of filler.
However, not all the fillers treated with silane coupling agent are effective. Different fillers have different effects, and some even have no effect.
For silica, glass, aluminum powder and other fillers with a large number of hydroxyl groups on the surface, the effect is the best. For calcium carbonate, graphite, boron and other fillers without hydroxyl groups, it has no effect.