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Silane Coupling Agent: A Guide to Its Applications and Mechanisms

The silane coupling agent has a functional group in which organic materials and inorganic materials are combined with each other in the molecule, and plays a mediating role in the collection of organic materials and inorganic materials. It can improve the mechanical strength of the composite material, improve the bonding performance, change the resin and Its surface properties and other functions.

Silane coupling agent was originally a chemical agent developed by Union Carbide Corporation, mainly used in glass fiber reinforced plastics. The molecular structure of the silane coupling agent is generally YR-Si(OR)3 (where YR or YC is an organic functional group, and Si(OR)3 or Si-X is a siloxyl group).

R : alkyl or aryl;
X : methoxy, ethoxy, chlorine, etc.;
Y : organic reactive group (vinyl, epoxy, amino, mercapto, etc.).

The hydrolyzable group represented by X can chemically combine with organic materials, so the silane coupling agent acts as a bridge at the interface between inorganic materials and organic materials, so it is widely used in the modification of composite materials.

The application of silane coupling agent can be roughly summarized into three aspects:

1) Surface treatment

It can improve the bonding performance of glass fiber and resin, and greatly improve the strength, electrical, water resistance, and weather resistance of glass fiber reinforced composite materials. Even in wet state, it can improve the mechanical properties of composite materials, and the effect is very obvious. with. The use of silane coupling agents in glass fibers is quite common, and the silane coupling agents used in this area account for about 50% of the total consumption. Among them, vinyl silane, amino silane, methyl Acryloxysilane, etc.

2) filled with plastic

The filler can be surface treated in advance, or added directly to the resin. It can improve the dispersion and adhesion of fillers in resins, improve the compatibility between inorganic fillers and resins, improve process performance and improve the mechanical, electrical and weather resistance properties of filled plastics (including rubber).

3) Used as a tackifier for sealants, adhesives and coatings

It can improve their bonding strength, water resistance, weather resistance and other properties. Silane coupling agents can often solve the problem that certain materials cannot be bonded for a long time. The principle of the silane coupling agent as a tackifier is that it has two groups; one group can be combined with the bonded skeleton material; while the other group can be combined with polymer materials or adhesives , so as to form a strong chemical bond at the bonding interface, which greatly improves the bonding strength.

4 ) Other applications

Other applications of silane coupling agents include:

① Attach the immobilized enzyme to the surface of the glass substrate,

②Sand control in oil well drilling,

③ Make the masonry surface hydrophobic,

④ Through the anti-moisture absorption effect, the fluorescent lamp coating has a high surface resistance;

⑤ Improve the hygroscopic performance of the organic relative glass surface in the liquid chromatographic column.

The mechanism of action of silane coupling agent

People have done quite a lot of research on its mechanism of action, and put forward various theories, but so far there is no complete and unified understanding. Siloxyl groups are reactive towards inorganics and organofunctional groups are reactive or compatible with organics. Therefore, when the silane coupling agent is interposed between the inorganic and organic interfaces, a bonding layer of organic matrix-silane coupling agent-inorganic matrix can be formed.

1) Chemical bond theory

According to the theory, the X group in silane can react with the hydroxyl group on the surface of the inorganic material to form a chemical bond, and the Y group can react with the resin to form a chemical bond. These two materials with very different chemical properties are “coupled” by chemical bonds to obtain a good connection, which is why this type of compound is called a coupling agent. The chemical bond theory has been widely used to explain the role of coupling agents, especially how to choose a coupling agent has certain practical significance.

2) Wetting effect and surface effect

In the manufacture of composite materials, good wetting of the liquid resin and the adherend is of paramount importance. If complete wetting can be obtained, the bond strength of the physical adsorption of the resin to the high-energy surface will be much higher than the cohesive strength of the organic resin. Treating the surface of glass fiber (or other inorganic materials) with a suitable silane coupling agent will increase its surface tension, thereby promoting the infiltration and spreading of organic resin on the surface of inorganic materials.

3) Morphological Theory

Silane treatments on inorganic materials modify the morphology of adjacent organic polymers in a way that improves bonding. The deformable layer theory suggests that a flexible resin layer can be created to relieve interfacial stress; while the constrained layer theory suggests that the silane can “tighten” the polymer structure in the interphase region.

4 ) Other theories

The coupling agent on the interface may play a variety of other functions, such as may produce a kind of lubrication, thereby protecting the inorganic material from the stress corrosion of water. In addition, there are acid-base reaction theory, reversible hydrolysis bond theory, reversible hydrolysis mechanism, etc.

Selection principle of silane coupling agent

It is known that the hydrolysis rate of the silane coupling agent is taken from the silicon functional group Si-X , while the reactivity with organic polymers is taken from the carbon functional group CY . Therefore, for different substrates or processing objects, it is very important to choose a suitable silane coupling agent. The selected method is mainly pre-selected through experiments and should be carried out on the basis of existing experience or rules.

For example, in general,
• Most unsaturated polyesters use silane coupling agents containing CH2=CMeCOO, Vi and CH2-CHOCH2O-;
• Most epoxy resins use CH2-CHCH2O and H2N-silane coupling agents;
• Most phenolic resins use H2N- and H2NCONH-silane coupling agents;
• Vinyl silane is selected for polyolefin;
• For rubber vulcanized with sulfur, hydrocarbon-based silanes are often used.

The bonding strength between dissimilar materials is affected by a series of factors, such as wetting, surface energy, interface layer and polar adsorption, the effect of acid and alkali, interpenetrating network and covalent bond reaction, etc. Therefore, pre-selection by test alone is sometimes not accurate enough, and the composition of the material and its sensitivity to the reaction of the silane coupling agent need to be considered comprehensively. In order to improve the hydrolytic stability and reduce the cost of modification, trihydrocarbyl silane can be mixed into the silane coupling agent; for difficult-to-adhesive materials, the cross-linked polymer of the silane coupling agent can also be shared.

When the silane coupling agent is used as a tackifier, it is mainly achieved by forming chemical bonds and hydrogen bonds with the polymer; wetting and surface energy effects; improving polymer crystallinity, acid-base reaction, and formation of interpenetrating polymer networks. of. Viscosity mainly revolves around three systems: namely ( 1 ) inorganic material to organic material; ( 2 ) inorganic material to inorganic material; ( 3 ) organic material to organic material. For the first type of bonding, it is usually required to bond inorganic materials to polymers, so the reactivity between Y in the silane coupling agent and the functional groups contained in the polymer should be given priority; the latter two belong to the bonding between materials of the same type , so the anti-hydrophilic polymer of the silane coupling agent itself and the silane coupling agent selected when the inorganic material requires viscosity increase.

How to use silane coupling agent

Whether the use method of silane coupling agent is correct or not will directly affect its use effect. In practice, there are often cases where the silane coupling agent fails or fails to achieve the desired effect due to improper use.

( 1 ) Direct mixing method, also known as overall mixing method, migration method, etc.;

( 2 ) Silane coupling agent organic solvent solution treatment filler;

( 3 ) Silane coupling agent aqueous solution (or alcohol – aqueous solution) treatment filler;

( 4 ) Dry mixing method for fillers;

( 5 ) Primer ( paint ) treatment method, using silane, silane solution, silane and liquid organic polymer mixture as the primer material;

( 6 ) Two or more silane coupling agents are used at the same time;

( 7 ) While using the silane coupling agent, add a catalyst that promotes the silane coupling agent .

From the perspective of giving full play to its effectiveness and reducing costs, the direct mixing method (dry mixing method for fillers) and adding them to the adhesive system are the most widely used methods, but the effect will be compromised.