The role of silane coupling agents in organic adhesives

Currently,The application of adhesiveshas penetrated into various fields of the national economy. In many cases, adhesives can effectively replace welding, riveting, screw joints and other mechanical connections, simplifying processes, saving energy, and It provides an effective way to reduce costs and improve economic benefits. With the rapid development of science and technology, higher and higher requirements have been placed on the performance of adhesives and bonding technology. Because different materials have different interface properties, in order to obtain a strong bonding interface layer between the adhesive and the surface of the adherend during the bonding process, coupling agents containing reactive groups are often used to interact with the solid surface of the adherend. To achieve this by forming chemical bonds. Substances that can be used as coupling agents include organosilanes, titanates, phosphates, and certain organic acids. Among them, silane coupling agents are the most mature ones.

⒈. The role of silane coupling agents in organic adhesives

Silane coupling agentis a compound that can produce a certain binding force with polar substances and non-polar substances at the same time. It is characterized by having both polar and non-polar parts in the molecule. It can be expressed by the general formula Y(CH2 )nSiX3, where Y represents alkyl, phenyl and vinyl, epoxy, amino, mercapto and other organic functional groups, often chemically combined with organic functional groups in the adhesive matrix resin; X represents chlorine, methoxy, ethoxy These groups are easily hydrolyzed into silanol and react with oxides or hydroxyl groups on the surface of inorganic substances (glass, silica, metal, clay, etc.) to form stable silicon-oxygen bonds.

Therefore, by using silane coupling agents, a “molecular bridge” can be built between the interface of inorganic substances and organic substances, and the two materials with completely different properties can be Connected together, this effectively improves the bonding strength of the interface layer. Adding silane coupling agents to adhesives can not only increase the bonding strength, but also improve the durability and heat-humidity aging resistance of the adhesive. For example, although polyurethane has high adhesion to many materials, its durability is not ideal. Its durability can be significantly improved after adding a silane coupling agent.

When Chen Ruizhu et al. studied the wet-heat durability of titanium alloy bonded parts, they added a silane coupling agent to the epoxy adhesive used to make the bonded parts survive wet-heat aging. The shear strength retention rate is increased from about 80% to about 97%. Silane coupling agents can even be directly used as adhesives for bonding silicone rubber, fluorine rubber, nitrile rubber, etc. to metals. Adhesives such as CK-1 and Chemlock607 (USA) are silanes. In order to improve certain properties of organic adhesives (such as heat resistance, self-extinguishing properties, dimensional stability, etc.), or to reduce the cost of organic adhesives, some inorganic fillers are often added to the adhesive. If the filler is treated with a silane coupling agent in advance, the polar groups on the surface of the filler react with the silane coupling agent, thereby greatly reducing the structuring effect of the filler and the resin. The compatibility and dispersion are greatly improved, and the viscosity of the system is significantly reduced, so the amount of filler can be increased. However, not all fillers are effectively treated with coupling agents. Different types of fillers have different effects, and some even have no effect at all. For fillers with a large number of hydroxyl groups on the surface, such as silica, glass, and aluminum powder, the effect is best, but for fillers without hydroxyl groups on the surface, such as calcium carbonate, graphite, and boron, it has no effect.

⒉. How to use silane coupling agent

⑴Surface Preparation The treatment method is to prepare the silane coupling agent into a dilute solution with a concentration of 0.5 to 1%. When using it, you only need to apply a thin layer on the clean adhered surface, and the glue can be applied after drying. The solvents used are mostly water, alcohol, or water-alcohol mixtures, and water without fluoride ions and cheap and non-toxic ethanol and isopropyl alcohol are suitable. Except for aminoalkyl silane, solutions prepared from other silane coupling agents need to add acetic acid as a hydrolysis catalyst and adjust the pH value to 3.5 to 5.5. Long-chain alkyl and phenyl silanes are not suitable for use in aqueous solutions due to their poor stability. The hydrolysis process of chlorosilanes and acetoxysilane is accompanied by severe condensation reactions, and it is not suitable to be used in an aqueous solution or a hydroalcoholic solution. Instead, it is mostly used in an alcoholic solution. For silane coupling agents with poor water solubility, 0.1 to 0.2% (mass fraction) of non-ionic surfactant can be added first, and then added with water to form a water emulsion.

⑵ Migration method Add silane coupling agent directly to the adhesive component, generally in an amount of 1 to 5% of the base resin. After gluing, relying on the diffusion of molecules, the coupling agent molecules migrate to the bonding interface to produce coupling. For adhesives that need to be cured, they need to be left for a period of time before curing, so that the coupling agent can complete the migration process and achieve better results. In actual use, the coupling agent often forms a deposition layer on the surface, but what really works is only a single molecular layer, so the amount of coupling agent does not need to be too much.

⒊. Selection of silane coupling agent

In silane even Among the two groups of coupling agents with different properties, the Y group is the most important, and it directly determines the application effect of the silane coupling agent. Only when the Y group can react with the corresponding matrix resin can the bonding strength of the organic adhesive be improved. It is generally required that the Y group can be miscible with the resin and be able to undergo a coupling reaction, so for different resins, a silane coupling agent containing an appropriate Y group must be selected. When Y is a non-reactive alkyl group or aryl group, it has no effect on polar resins, but can be used in the bonding of non-polar resins, such as silicone rubber, polystyrene, etc. When Y is a reactive functional group, such as vinyl, epoxy, mercapto, etc., attention should be paid to its reactivity and compatibility with the resin used. Aminosilane coupling agents are universal and can couple with almost all kinds of resins, except polyester resins. Silane coupling agents commonly used in adhesives