Overview of silicone coupling agent

1.Overview of coupling agent  

Coupling agent is a kind of organic silicon compound with special structure.  In its molecules, there are reactive groups that can be combined with inorganic materials (such as glass, cement, metal, etc.) and organic materials (such as synthetic resin, etc.).  The commonly used theories are chemical bond theory, surface infiltration theory, deformation layer theory, constraint layer theory and so on.  Coupling agent can be used as surface modifier to improve the dispersion and adhesion of inorganic fillers.  

2. coupling agent type  

Coupling agent mainly includes organic chromium coupling agent, organic silicon coupling agent and titanate coupling agent.  Silicone coupling agent is often used in adhesives, the general formula is RSiX3, in which R is the organic group, such as -C6H5, -CH=CH2, etc., can be combined with resin;  X is the group that can be hydrolyzed, such as -OCH3, -OC2H5, -Cl, etc.  

3.coupling agent action process  

B•Arkles proposed a 4-step reaction model based on the coupling process of the coupling agent, namely:  

① The SiX group connected with silicon atom hydrolyzes to form SiOH;  

(2) Dehydration condensation between Si-OH to form si-OH containing polysiloxane;  

(3) The SiOH in the polysiloxane forms hydrogen bonds with the OH on the substrate surface;  

(4) In the process of heating and curing, covalent bond is formed with the substrate with dehydration reaction.  

It is generally believed that only one of the three silane hydroxyl groups generated by the hydrolysis of silane coupling agent is bonded to the surface of the substrate.  The remaining two Si-Oh compounds are either condensed with or in a free state from other silanes.  Therefore, by using silane coupling agent, the interface of two materials with very different properties can be coupled, so as to improve the performance of the composite and increase the bonding strength, and obtain a new type of composite with excellent performance and reliability.  Silane coupling agent is widely used in rubber, plastics, adhesives, sealants, coatings, glass, ceramics, metal anticorrosion and other fields.  Now silane coupling agent has become one of the indispensable auxiliaries in material industry.  

The effects and effects of silane coupling agents have been recognized and affirmed by people, but there is no complete coupling mechanism to explain why a very small amount of coupling agents at the interface can have such a significant impact on the properties of composites.  There have been many studies on the mechanism of coupling agent on the interface between two materials with different properties, and some explanations such as chemical bonding and physical adsorption have been proposed.  Among them, chemical bonding theory is the oldest but one which is considered to be more successful so far.