Bonding (bonding, agglutination, cementation, gluing) refers to the technology that the surfaces of homogeneous or heterogeneous objects are connected together with curing agent. It has the characteristics of continuous stress curing agent cloth, light weight, or sealing, and low temperature in most processes. Adhesive joints are especially suitable for the connection of different materials, different thicknesses, ultra-thin specifications and complex components. Gluing has the fastest development in modern times, a wide range of application industries, and has a significant impact on the progress of high and new science and technology and the improvement of people's daily life. Therefore, it is very important to research, develop and produce various curing agents.
Epoxy resin adhesive (hereinafter referred to as epoxy curing agent or epoxy adhesive) is a general term that contains two or more epoxy groups in a molecular structure, and can form three-dimensional crosslinked curing compounds under appropriate chemical reagents and conditions.
The curing agent of epoxy resin is a liquid or solid curing agent composed of epoxy resin, curing agent, plasticizer, accelerator, diluent, filler, coupling agent, flame retardant, stabilizer, etc. Among them, epoxy resin, curing agent and toughening agent are indispensable components. Others will be decided whether to add or not according to need. The adhesive process of epoxy curing agent is a complex physical and chemical process. This includes infiltration, adhesion, curing and other steps. Finally, a three-dimensional cross-linked structure of the solidified product is formed, and the adherend is combined into a whole.
There are many kinds of epoxies. Among all kinds of epoxy resins, bisphenol A epoxy resin is one of the largest and most widely used products. According to its molecular weight, it can be divided into low, medium, high and ultra-high molecular weight epoxy resin (poly phenoxy resin). The resin with low molecular weight can be cured at room temperature or high temperature, but the epoxy resin with high molecular weight can only be cured at high temperature. However, the super high molecular weight poly oxy phenol resin can form tough film at high temperature without the aid of curing agent. With the development of various adhesive theories and the development of basic research work, such as curing agent chemistry, curing agent rheology and adhesive failure mechanism, the properties, varieties and applications of curing agents have made rapid progress. Epoxy resin and its curing system are also emerging with its unique and excellent performance and new epoxy resin, new curing agents and additives. It has become a kind of important curing agent with excellent performance, many varieties and wide adaptability.
In recent years, high strength and light fiber reinforced composites are gradually used in ultra-low temperature environments. Research on the ultra-low temperature properties of epoxy resin has been strengthened day by day. Some progress has been made in the research of composite liquid hydrogen tanks, curing agents, impregnation material and fiber-reinforced composite material in the field of superconductivity. Pure epoxy resin has a high crosslinking density, and even at room temperature, it has some disadvantages such as brittle, low toughness and poor impact resistance. As the resin matrix of composite materials, it usually needs to be cured at a very high temperature. During the cooling process after curing, thermal stress will be generated in the matrix due to thermal shrinkage. When the temperature is reduced from room temperature to ultra-low temperature (- 150 ℃), the internal stress caused by thermal shrinkage in the matrix will be more significant. Once the thermal stress exceeds the strength of the resin itself, it will lead to the destruction of the resin matrix. Therefore, it is very important to improve the toughness of epoxy resin at ultra-low temperatures. At present, the main methods to improve the ultra-low temperature toughness of epoxy resin are flexible aliphatic resin, liquid rubber and flexible curing agent. Due to the low glass transition temperature and large free volume at room temperature, when the temperature drops to ultra-low temperature, the resin system will produce large thermal shrinkage, resulting in large thermal stress. This limits its application in ultra-low temperature. The blending modification of high performance thermoplastic and epoxy resin at room temperature can make the blend system have both advantages. It not only keeps the high modulus of thermosetting resin, but also has the high toughness of thermoplastic.
The bonding properties (strength, heat resistance, corrosion resistance, impermeability, etc.) of the curing agent not only depends on its structure and performance, but also on the structure and adhesive properties of the surface of the adherend, and are closely related to the joint design, the preparation of curing agent and bonding process. At the same time, it is restricted by the surrounding environment. Therefore, the application of epoxy curing agent is a system engineering. The performance of epoxy curing agent must be adapted to the above factors to obtain the best results. Using the same formula of epoxy curing agent, bonding different properties of objects, or using different bonding conditions, or in different use environment, its performance will be greatly different. Attention should be paid to the application.
Epoxy resin hardener is mainly composed of epoxy resin and curing agent. In order to improve some properties and meet different purposes, auxiliary materials such as toughening agent, diluent, accelerant and coupling agent can also be added. Due to the high adhesive strength and versatility of epoxy curing agent, it used to be called "all-purpose adhesive" and "energetical adhesive". It is widely used in aviation, aerospace, automobile, machinery, construction, chemical industry, light industry, electronics, electrical appliances and daily life.