Silane Coupling Agent: Structure, Mechanism and Types

Silane coupling agent is a kind of low molecular organosilicon compound with special structure. These groups have strong reactivity with different base resins, and X represents a group that can be hydrolyzed, such as halogen, alkoxy, acyloxy, etc. Therefore, the silane coupling agent can interact with both the hydroxyl group in the inorganic substance and the long molecular chain in the organic polymer, so that two materials with different properties can be coupled together, thereby improving various properties of biomaterials. Therefore, it is widely used in rubber, plastics, filled composite materials, epoxy encapsulation materials, elastomers, coatings, adhesives and sealants, etc.

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 group that can be hydrolyzed. Y mainly reacts with the organic polymer, while the hydrolyzable group X mainly controls the rate of hydrolysis. Under the same hydrolysis conditions, the hydrolyzable group with a large group has a slow hydrolysis rate; in an acidic environment, the hydrolyzed group with a longer alkylene group is slower, for example: a hydrolyzable alkoxy group is usually an ethoxy group 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 acidic solution is 20 times that of γ-methacryloxy-propyl-triethoxysilane.

mechanism of action

There are many explanations for the mechanism of silane coupling agent at the interface between two materials with different properties, such as chemical bond theory, reversible equilibrium theory and physical adsorption theory. However, interface phenomena are very complex, and a single theory is often difficult to fully explain. Usually, chemical bonding theory can better explain the interaction between silane coupling agent and inorganic materials. According to this theory, the coupling process of silane coupling agent at the interface of different materials is a complex physical and chemical process of liquid-solid surface. First of all, the viscosity and surface tension of silane coupling agent are low, the wettability is high, and the contact angle to glass, ceramics and metal surfaces is small, and it can spread rapidly on the surface, so that the surface of inorganic materials is wetted by silane coupling agent. Secondly, once the silane coupling agent spreads 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 with similar polarity respectively. In the thin water layer, the alkoxy group at one end will be hydrolyzed into silicon hydroxyl group, which will be oriented on the surface of the inorganic material, and at the same time undergo hydrolysis and polycondensation reaction with the hydroxyl group on the surface of the material; the organic group will be oriented on the surface of the organic material. A chemical reaction occurs between them, thus completing the coupling process between dissimilar materials. A brief equation for the chemical reaction is as follows:

type

1. Silicone peroxide coupling agent Silicone peroxide coupling agent is also a coupling agent that has been studied in recent years. It is different from traditional coupling agents in that the hydrolyzable group X is -OOR group, The characteristic is that peroxyl 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 same or different organic substances. Coupling with non-polar (such as polyolefin and silicone rubber, etc.) organic compounds. This is a good solution to the two major problems of ordinary silicone coupling agents. What is more valuable is that the curing speed of the silicone peroxide coupling agent is fast and the adhesive strength is high, thus expanding the scope of the silicone peroxide coupling agent. The scope of application of the coupling agent.

2. α-functional silane coupling agent At present, the main silane coupling agent used at home and abroad is the γ-functional silane in which the silicon atom and the organic functional group are separated by 3 methylene groups. The organosilicon 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 At present, the commonly used silane coupling agent is trialkoxy type, but trialkoxy type coupling agent may reduce the stability of matrix resin.

5. New type polymer coupling agent The synthesis of polymers with active silane 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 The surface of the adhesive forms a uniform surface, so it 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 the basic functions of a silane coupling agent on the main chain of polysiloxane and various organic functional groups. In addition to its function as an adhesion aid at the inorganic-organic interface, MMCA can also impart heat resistance, abrasion resistance, chemical resistance, impact resistance, and hydrophobicity to composite materials.

6. Modified aminosilane coupling agent Starting from A-1100 developed by UCC, bisaminosilane (A-1120) containing 1 primary amino group and 1 secondary amino group can be derived, containing 1 primary amino group and 1 secondary amino group (A-1120). Triaminosilane (A-5162) with 2 secondary amino groups, polyaminosilane (Y-5691) with 1 primary amino group and multiple secondary amino groups, etc. These silanes containing free amino groups are relatively basic and highly reactive, and with the increase of amino groups, the flexural strength of plastic products also increases accordingly.

application

1. As a surface treatment agent, it is mainly used for the surface treatment of glass fiber, which can improve the bonding performance of glass fiber and resin, and greatly improve the strength, electrical, water resistance, weather resistance and other properties of glass fiber reinforced composite materials; even in wet state, It can also significantly improve the mechanical properties of composite materials.

2. For plastics filled with inorganic fillers, the surface treatment of the fillers can be carried out in advance, and it can also be directly added to the quality. It can improve the dispersion and adhesion of fillers in resin, improve process performance and improve the mechanical, electrical and weather resistance properties of filled plastics (including rubber).

3. As a tackifier for sealants, adhesives and coatings, it can improve the bonding strength, water resistance, high temperature resistance, and weather resistance of sealants, adhesives and coatings.

4. Used as an adhesion promoter for difficult-to-adhesive materials polyolefin (such as PE, PP) and special rubber (such as silicone rubber, EPR, CR, fluororubber).

5. Used as textile auxiliaries together with silicone emulsion can improve the wearing performance of trade textiles. The test fabric has the advantages of softness, fullness, good resilience, anti-wrinkle and scratch resistance, waterproof and antistatic, washable, and comfortable to wear.

6. For biochemical and environmental protection, silane coupling agent is an important raw material for the preparation of silicone resin solid trypsin carrier. Furthermore, the immobilized enzyme can be insoluble in water, and the non-inactivated solid-phase enzyme can be continued to be used after being filtered, which not only improves the utilization rate of the biological enzyme, but also avoids pollution and waste.

7. Used for “denture” molding “dentures” in domestic dentistry are generally made of modified methacrylate polymers and fillers. In order to improve the bonding force between them, and to increase the strength and rigidity of the “denture”, before it is cured, add KH-570 silane coupling agent to the formula. In addition, silane coupling agents are also widely used as waterproofing agents, crosslinking agents, metal preservatives, glass and ceramic protective agents, fiber and leather finishing agents, and auxiliary agents for petroleum development and transportation. Certain coupling agents (such as -aminopropyltriethoxysilane, -chloropropyltriethoxysilane) can also be used as a combustion aid if exposed to an open flame.

apply effects

1. Application effect of coupling agent in rubber

(1) Effect on rubber-filler interaction When nanomaterials are used as rubber reinforcing agents, since nanoparticles are prepared under non-equilibrium and harsh conditions, the surface atoms are in a highly activated state, and the surface energy is very large. Particles are easy to agglomerate, coupled with the surface characteristics of nanoparticles and their low dispersion energy, resulting in poor compatibility with rubber. After being treated with coupling agent, nano-silica has low surface energy and is easily infiltrated by rubber macromolecules, which improves the dispersion degree of silica filler. At the same time, because the coupling agent acts as a bridge between the rubber and the filler, the interface bonding between the nano-silica particles and the rubber matrix is ​​enhanced, and its reinforcing ability to the rubber matrix is ​​improved.

(2) Influence on rubber vulcanization characteristics Adding coupling agent during rubber vulcanization can improve the vulcanization characteristics of rubber materials, and significantly improve the processing performance and mechanical properties of rubber products.

(3) Influence on the physical and mechanical properties of the rubber compound. The silane coupling agent plays a coupling and filling role in the system. When the amount of the coupling agent is small, the rubber macromolecules are less bound, easy to slide and orientation, and the stress distribution is uniform. Therefore, the tensile strength is higher; as the amount of coupling agent increases, the number of molecules of the coupling agent increases, the rubber macromolecules are greatly bound, it is not easy to slide, and the stress distribution is uneven, so the tensile strength decreases; the amount of coupling agent continues to decrease. Increase, the excess coupling agent is filled in the system, so that the macromolecular chains are easy to slide and align, the stress distribution is uniform, and the tensile strength increases instead. Silane coupling agents can also improve the physical processing properties of filled rubber. Since the silane coupling agent improves the compatibility and dispersibility between the filler and the rubber base, the viscosity of the rubber is reduced, the mixing time is shortened, the extrusion processing performance is improved, and the product quality is improved.

2. Application of silane coupling agent in plastics

(1) Used as raw materials to synthesize silicone plastics, methyl or phenyl silane is used as a monomer to form silicone resin through hydrolysis and condensation, and then mixed with mica, asbestos, glass fiber or glass cloth and other fillers, through compression molding or lamination It is a thermosetting silicone plastic, which has high heat resistance, excellent electrical insulation and arc resistance, and waterproof, moisture-proof and other properties.

(2) Water-crosslinked polyolefins, moisture-curable acrylic acid-organic silicon coatings, etc., which are used as modified plastics and polymers on the market, use trialkoxymethane to decompose and self-condense when encountering water. Properties that form siloxane bonds. In addition, Nippon Suso Co., Ltd. modified 4-aminobenzenetrimethoxysilane to obtain low thermal expansion and adhesive polyimide. American Dow Corning Corporation used aminophenoxypropyltrimethoxysilane to obtain alkoxysilane-containing polyimide. Vinyl acetate adhesive.

3. Used as a coupling agent for composite materials

1) Application in glass fiber reinforced plastics (FRP) FRP is obtained by laminating and curing glass fibers or glass cloth coated with thermosetting resins such as unsaturated polyester or phenolic formaldehyde. The glass fiber or glass cloth used generally requires It is treated with silane coupling agent to improve the wet mechanical and electrical properties of FRP, reduce water absorption and improve appearance.

2) The application of silane coupling agents in mineral-filled thermosetting plastics can improve the dispersion and adhesion of inorganic fillers in resins. The silane coupling agent is used as the surface treatment agent of the filler, which fully improves the dispersion and adhesion of the filler in the phenolic resin, and improves the rigidity, bending resistance and tensile strength of the grinding wheel.

3) In the application of thermoplastics such as polyethylene and polypropylene with non-reactive groups in mineral-filled thermoplastics, the effect of using coupling agents is generally not significant, but some additives that can generate free radicals are added to the composite materials, so that The reaction of silane coupling agent with resin can improve the physical properties of most thermoplastics containing inorganic fillers and the dispersibility of pigments in plastics, and protect plastics from water erosion when soaked in water. At the same time, for glass fiber reinforced thermoplastic resins, the properties endowed by silane coupling agents far exceed that of glass, and when the mixture is under harmful conditions, it can maintain good physical and electrical properties. Silane coupling agents mixed with mineral fillers can improve the physical and wet electrical properties of filled systems, making them approach or in some cases exceed unfilled resins.