Mechanism of Silane Coupling Agent

Jun 28, 2021

Overview
Silane coupling agent is a kind of low molecular organosilicon compound with special structure. Its general formula is RSiX3, where R represents amino, mercapto, vinyl, epoxy, cyano and methyl propionyloxy groups. Groups, these groups and different matrix resins have strong reactivity, X represents a group that can be hydrolyzed, such as halogen, alkoxy, acyloxy and so on. Therefore, the silane coupling agent can not only interact with the hydroxyl in the inorganic substance but also with the long molecular chain in the organic polymer, so that two materials with different properties can be coupled, thereby improving the various properties of biological materials. Therefore, it is widely used in rubber, plastics, filled composite materials, epoxy packaging materials, elastomers, coatings, adhesives and sealants.

Structure
The general structural formula of the silane coupling agent is Y-R-Si-X3, Y represents an organic functional group, R represents an alkylene group, and X represents a hydrolyzable group. Y mainly reacts with organic polymers, and the hydrolyzable group X mainly controls the rate of hydrolysis. Under the same hydrolysis conditions, the hydrolysis rate of the hydrolyzable groups of large groups is slow; in acidic environments, the hydrolysis of longer alkylene groups is slower. For example, the hydrolyzable alkoxy groups are usually ethoxy or Methoxy, under the same hydrolysis conditions, the hydrolysis rate of trimethoxysilane is faster than that of triethoxysilane. The hydrolysis rate of α-methacryloxy-methyl-triethoxysilane in acid solution is 20 times that of γ-methacryloxy-propyl-triethoxysilane.

Mechanism of Action
There are many explanations for the interface mechanism of silane coupling agent between two materials with different properties, such as chemical bond theory, reversible equilibrium theory and physical adsorption theory. However, the interface phenomenon is very complicated, and a single theory is often difficult to fully explain. Under normal circumstances, chemical bonding theory can better explain the interaction between silane coupling agents and inorganic materials. According to this theory, the coupling process of silane coupling agent at the interface of different materials is a complex liquid-solid surface physical and chemical process. First of all, the viscosity and surface tension of the silane coupling agent is low, the wetting ability is high, and the contact angle to the glass, ceramic and metal surface is small, and it can be spread quickly on the surface, so that the surface of the inorganic material is wetted by the silane coupling agent. Secondly, once the silane coupling agent spreads out on its surface and the surface of the material is infiltrated, the two groups on the silane coupling agent molecule will diffuse to the surface of similar polarity, because the surface of the material in the atmosphere always adsorbs thin In a thin water layer, the alkoxy group at one end is hydrolyzed into silanol groups, which are oriented on the surface of inorganic materials, and at the same time undergo hydrolysis and condensation reaction with the hydroxyl groups on the surface of the material; the organic groups are oriented on the surface of organic materials. The chemical reaction takes place, thus completing the coupling process between dissimilar materials. The brief equation of the chemical reaction is as follows:

Mechanism of silane coupling agent
Figure 1 shows the mechanism of action of silane coupling agent
species
1. Silicone peroxide coupling agent Silicone peroxide coupling agent is also a coupling agent that has been studied in recent years. The difference from traditional coupling agents is that the hydrolyzable group X is -OOR group. The characteristic is that the peroxy group is easily decomposed into free radicals with high reactivity after being heated. It can not only be used as a coupling agent between organic and inorganic substances, but also can couple two identical or different organic substances. Coupling with non-polar organics (such as polyolefin and silicone rubber, etc.). This solves the two major problems of common silicone coupling agents. The more valuable thing is that the curing speed of silicone peroxide coupling agent is fast and the adhesive strength is high, thus expanding the silicone peroxide. The scope of application of the coupling agent.
2. α-functional group silane coupling agent At present, the main silane coupling agent used at home and abroad is γ-functional group silane with silicon atom and organic functional group separated by 3 methylene groups. The organic silicon compound with this structure has better stability.
3. Long-chain alkyl silane coupling agent Long-chain alkyl alkoxy silane is a new type of organosilicon compound, the general structural formula is YRnSiX3-n, where n=0～3, X is usually -OCH3 and -OC2H5 Etc., Y is a long chain alkyl group. The representative product is DH-109, the chemical name is methyldodecyldimethoxysilane, and the structural formula is C12H25CH3Si(OCH3)2.
4. Two-functional silane coupling agent The commonly used silane coupling agent is trialkoxy type, but the trialkoxy type coupling agent may reduce the stability of the matrix resin.
5. The synthesis of new polymer coupling agents with active silyl groups is also one of the development directions of silane coupling agents. This coupling agent has better compatibility with the resin in the adhesive and can be used in The surface of the sticky object forms a uniform surface, which has a better bonding effect. NUC Corporation of Japan has newly developed a new type of polymer coupling agent (MMCA), which is a polymer compound with a hydrolyzed group and various organic functional groups on the main chain of polysiloxane that has the basic functions of a silane coupling agent. In addition to its function as an adhesion promoter at the inorganic-organic interface, MMCA can also impart heat resistance, abrasion resistance, chemical resistance, impact resistance and hydrophobicity to composite materials.
6. Modified amino silane coupling agent starting from A-1100 developed by UCC, bisamino silane (A-1120) containing 1 primary amino group and 1 secondary amino group can be derived, containing 1 11 primary amino groups and Triaminosilane with two secondary amino groups (A-5162), and polyaminosilane with one primary amino group and multiple secondary amino groups (Y-5691), etc. These free amino-containing silanes are more basic and have higher reactivity, and with the increase of amino groups, the flexural strength of plastic products also increases accordingly.