At present, the application of adhesives has penetrated into various fields of the national economy. In many cases, adhesives can effectively replace welding, riveting, screwing and other mechanical connections, simplifying processes for various production fronts, saving energy, reducing costs, and improving Economic benefits provide an effective way.
With the rapid development of science and technology, higher and higher requirements are put forward for the performance of adhesives and bonding technology. Because different materials have different interface properties, in order to obtain a strong adhesive interface layer between the adhesive and the surface of the adherend during the bonding process, coupling agents containing reactive groups and the solid surface of the adherend are often used This is achieved by forming chemical bonds. Substances that can be used as coupling agents include organosilanes, titanates, phosphates, and certain organic acids. Among them, the application of silane coupling agents is the most mature.
⒈The role of silane coupling agent in organic adhesives
The silane coupling agent is a compound that can produce certain synergy with polar 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 as Y(CH2)nSiX3, where Y represents alkyl, phenyl and vinyl, epoxy, amino, mercapto and other organic functional groups, often chemically combined with the organic functional groups in the adhesive matrix resin; X represents chlorine, methoxy, ethoxy, etc. The group is easily hydrolyzed into silanol and reacts with oxides or hydroxyl groups on the surface of inorganic substances (glass, silica, metal, clay, etc.) to generate stable silicon-oxygen bonds. Therefore, by using a silane coupling agent, a "molecular bridge" can be built between the interface of the inorganic substance and the organic substance, and two completely different materials can be connected together, which effectively improves the bonding strength of the interface layer. .
Adding silane coupling agent to the adhesive can not only improve the adhesive strength, but also improve the durability and aging resistance of the adhesive. For example, polyurethane has high adhesion to many materials, but its durability is not ideal. After adding silane coupling agent, its durability can be significantly improved. Chen Ruizhu, etc., when studying the topic of wet heat durability of titanium alloy bonding parts, added a silane coupling agent to the epoxy adhesive used, so that the shear strength preservation rate of the adhesive parts after humid heat aging improved from about 80% To about 97%. The silane coupling agent can even be used directly as an adhesive for the bonding of silicone rubber, fluororubber, nitrile rubber, etc. to metals, such as the adhesives CK-1 and Chemlock 607 (USA) are silanes.
In order to improve some properties of organic adhesives (such as heat resistance, self-extinguishing, dimensional stability, etc.), or to reduce the cost of organic adhesives, it is often necessary to add some inorganic fillers to the adhesives. If the filler is treated with a silane coupling agent in advance, the polar group on the surface of the filler reacts with the silane coupling agent, thereby greatly reducing the structuring effect of the filler and the resin. Not only does the filler affect the adhesive matrix resin Compatibility and dispersion are greatly improved, and the viscosity of the system is significantly reduced, so the amount of filler can be increased. However, it is not effective to treat all fillers with coupling agents. Different types of fillers have different effects, and some even have no effect. For fillers such as silica, glass, and aluminum powder with a large number of hydroxyl groups on the surface, the effect is the best, but for calcium carbonate, graphite, boron and other fillers without a hydroxyl group on the surface, it has no effect.
⒉How to use silane coupling agent
⑴Surface pretreatment method The silane coupling agent is formulated into a 0.5-1% concentration dilute solution. When used, only a thin layer needs to be coated on the clean adhered surface, and it can be glued after drying. The solvent used is mostly water, alcohol, or a mixture of water and alcohol, and water without fluoride ions and inexpensive, non-toxic ethanol and isopropanol are suitable. In addition to aminohydrocarbyl silanes, solutions prepared from other silane coupling agents need to add acetic acid as a hydrolysis catalyst, and adjust the pH to 3.5~5.5. Long-chain alkyl and phenylsilane are not suitable for use in aqueous solution due to their poor stability. The hydrolysis process of chlorosilane and acetoxysilane is accompanied by severe condensation reaction, and it is not suitable to be used as an aqueous solution or hydroalcoholic solution, but as an alcohol solution. The silane coupling agent with poor water solubility can be added with 0.1~0.2% (mass fraction) of non-ionic surfactant, and then added with water to be processed into an aqueous emulsion.
⑵Migration method The silane coupling agent is directly added to the adhesive component, generally the amount is 1~5% of the amount of matrix resin. After the glue is applied, the coupling agent molecules migrate to the bonding interface to generate coupling effect due to the diffusion of molecules. For adhesives that need to be cured, they need to be placed for a period of time after curing to be cured, so that the coupling agent can complete the migration process to obtain better results.
In actual use, the coupling agent often forms a deposition layer on the surface, but the only thing that works is the monomolecular layer. Therefore, the amount of coupling agent need not be excessive.
⒊ Selection of silane coupling agent
Among the two types of groups with different properties of the silane coupling agent, the Y group is the most important, which 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. Generally, the Y group is required to be compatible with the resin and can initiate 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 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., pay attention to its reactivity and compatibility with the resin used. Aminosilane coupling agent is a general-purpose type, which can almost play a coupling role with various resins, except polyester resin. Commonly used silane coupling agents in adhesives (see Table 1).
4. Research trends of silane coupling agents
At present, the commonly used silane coupling agent is the trialkoxy type, but the trialkoxy type coupling agent may reduce the stability of the matrix resin, so the research and application of the dialkoxy type coupling agent have been paid attention in recent years. 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 form a uniform surface on the surface of the adherend. Has better bonding effect. Peroxysilane is also a kind of coupling agent that has been researched in recent years. Its characteristic is that under the action of heat, the coupling agent decomposes natural free radicals and can crosslink with olefinic polymers, thereby promoting olefinic polymers. Of bonding.
