Mechanism of silane coupling agent
The role and effect of silane coupling agents are recognized and affirmed, but why a very small amount of coupling agents at the interface will have such a significant effect on the properties of composite materials, there is no complete set of coupling mechanisms to explain. The coupling mechanism of the coupling agent at the interface between two different materials has been studied a lot, and explanations such as chemical bonding and physical adsorption have been proposed. Among them, the chemical bonding theory is the oldest, but it is considered to be a relatively successful theory so far.
1.Chemical combination theory
The theory is that the coupling agent contains a chemical functional group that can form covalent bonds with the silanol groups on the surface of the glass fiber or molecules on the surface of other inorganic fillers. In addition, the coupling agent also contains a different functional group and Polymeric molecules are bonded to obtain a good interfacial bond, and the coupling agent acts like a bridge that interconnects the inorganic and organic phases.
The silane coupling agent is taken as an example to explain the chemical bond theory. For example, aminopropyltriethoxysilane, when it is first treated with inorganic fillers (such as glass fiber, etc.), the silane is first hydrolyzed into silanol, and then the silanol group undergoes a dehydration reaction with the surface of the inorganic filler to perform chemical bonding and reaction. The formula is as follows:
The groups in silane are hydrolyzed—the hydroxyl group reacts with the inorganic filler after hydrolysis—the Y group in the coupling agent will interact with the organic polymer when the inorganic material filled with the coupling agent is filled to prepare the composite material. In the end, the bridge between the inorganic filler and the organic matter is set up.
There are many types of silane coupling agents. The Y groups in the formula are different, and the types of polymers suitable for the coupling agents are also different. This is because the group Y has a selective reaction to the polymer, such as containing vinyl and methyl group. Acryloyloxy-based silane coupling agents are particularly effective for unsaturated polyester resins and acrylic resins. The reason is that the unsaturated double bond in the coupling agent and the unsaturated double bond in the resin are the result of a chemical reaction under the action of the initiator and the accelerator. However, the coupling agent containing these two groups is not effective when used in epoxy resin and phenolic resin, because the double bond in the coupling agent does not participate in the curing reaction of epoxy resin and phenolic resin. However, epoxy group-based silane coupling agents are particularly effective for epoxy resins, and because epoxy groups can react with hydroxyl groups in unsaturated polyesters, epoxy-containing silanes are also suitable for unsaturated polyesters; Amine-based silane coupling agents are effective for resins such as epoxy, phenolic, melamine, and polyurethane. -SH-containing silane coupling agents are widely used in the rubber industry.
Through the above two reactions, the silane coupling agent improves the adhesion between the polymer and the inorganic filler in the composite material through chemical bonding, so that its performance is greatly improved. What is the treatment effect of the coupling agent? It can be characterized by the calculation of the theoretical adhesive force. According to the bonding theory of interfacial chemistry, the dispersive force is mainly considered as the adhesion force of the subvalent bond per unit area between the adhesive and the adherend.