Bonding (bonding, bonding, cementation, gluing) refers to the technology of connecting the surfaces of homogeneous or heterogeneous objects with adhesives, which has the characteristics of continuous stress distribution adhesive cloth, light weight, or sealing, and low temperature in most processes. Adhesive bonding is especially suitable for the connection of different materials, different thicknesses, ultra-thin specifications and complex components. Glue has the fastest development and a wide range of application industries, and has a great impact on the progress of high-tech science and technology and the improvement of people's daily life. Therefore, it is very important to research, develop and produce all kinds of adhesives. Epoxy resin adhesive (abbreviated as epoxy adhesive or epoxy adhesive) refers to a general term for curing compounds that contain two or more epoxy groups in a molecular structure and can form three-dimensional cross-linking under appropriate chemical reagents and conditions. Epoxy resin adhesive is a liquid or solid adhesive composed of epoxy resin, curing agent, plasticizer, accelerator, diluent, filler, coupling agent, flame retardant and stabilizer. Among them, epoxy resin, curing agent and toughening agent are indispensable components, and others are added according to needs. The gluing process of epoxy adhesive is a complicated physical and chemical process, which includes steps such as soaking, adhesion and curing. Finally, the cured product with three-dimensional cross-linked structure is generated, and the adherend is combined into a whole. There are many kinds of epoxy adhesives. Among all kinds of epoxy resins, bisphenol A epoxy resin is the one with the largest output and the widest application. According to its molecular weight, it can be divided into low, medium, high and ultra-high molecular weight epoxy resins (poly-phenoxy resin). Low molecular weight resin can be cured at room temperature or high temperature, but high molecular weight epoxy resin can only be cured at high temperature, while ultra-high molecular weight poly-phenoxy resin can form a tough film at high temperature without the help of curing agent. With the successive introduction of various adhesive theories and the in-depth progress of basic research work such as adhesive chemistry, adhesive rheology and adhesive failure mechanism, the properties, varieties and applications of adhesives have developed by leaps and bounds. Epoxy resin and its curing system have become an important kind of adhesive with excellent performance, many varieties and wide adaptability because of its unique and excellent performance and the continuous emergence of new epoxy resins, new curing agents and additives.

2. Molecular structure and classification of epoxy adhesives Epoxy resin is a polymer compound with two or more epoxy groups in its molecule and low molecular weight. 1. Classification There are many varieties and brands of epoxy resin, but bisphenol A glycidyl ether epoxy resin is usually called bisphenol A epoxy resin, which is the most important one. It accounts for 90% of the total output of epoxy resin. Bisphenol A epoxy resin bisphenol A epoxy resin, also known as general epoxy resin and standard epoxy resin, is obtained by polycondensation of bisphenol (BPA or DPP) and epichlorohydrin (ECH) under sodium hydroxide. Depending on the ratio of raw materials, reaction conditions and methods adopted, viscous liquids with low relative molecular weight and solids with high relative molecular weight and high softening point with different polymerization degrees can be prepared. The average relative molecular mass is 300 ~ 7000. The appearance is almost colorless or yellowish transparent viscous liquid or flaky brittle solid. Epoxy resin itself is a thermoplastic linear polymer. When heated, the viscosity of liquid resin becomes lower and solid resin softens or melts. Soluble in acetone, methyl ethyl ketone, cyclohexanone, ethyl acetate, benzene, toluene, xylene, anhydrous ethanol, ethylene glycol and other organic solvents. Hydrogenated bisphenol A epoxy resin The chemical name of hydrogenated bisphenol A epoxy resin is hydrogenated bisphenol A diglycidyl ether, which is obtained by polycondensation of hexahydrobisphenol A obtained by hydrogenation of bisphenol A and epichlorohydrin catalyzed by sodium hydroxide. It is an epoxy resin with very low viscosity, long gel time and good weather resistance.

3. Performance characteristics of epoxy adhesives Generally, the epoxy resin structure contains hydroxyl and ether bonds, which makes it have high adhesion. Because of these polar groups, it can generate electromagnetic force at the adjacent interface, which is less in the curing process, accompanied by chemical interaction with curing agent, and can further generate warp and ether bonds, which not only has high cohesion, but also produces strong adhesion, so epoxy adhesives have many materials such as metals, plastics, glass, wood and fibers.

Epoxy resin adhesive has been widely used because of its excellent adhesion and chemical stability. The development of modern industry requires EP adhesive to have both room temperature curing and high temperature resistance. The matrix material of epoxy resin adhesive is widely available, which is suitable for bonding various substrates. It has the characteristics of high strength, low curing shrinkage, corrosion resistance, good thermal stability and electrical insulation, and has the advantages of simple construction technology, high efficiency, low energy consumption and low cost, which can greatly reduce labor intensity and save labor costs, and has huge market potential and broad application prospects.