Research and Application of High Temperature Resistant Adhesive (I)

Adhesives are divided into two categories: organic adhesives and inorganic adhesives. It is a very important fine chemical product, and its application has penetrated into various fields of the national economy. Among them, high-temperature adhesives are widely used in aerospace, electronics, automotive, machinery manufacturing and other high-tech fields. With the rapid development of science and technology, the requirements for the heat resistance, medium resistance, and other properties of adhesives in special environments are becoming increasingly demanding. For this reason, a large number of research institutes and companies at home and abroad are constantly developing resistance to new uses and new properties. High temperature adhesive.

The definition, classification, and evaluation criteria for the high temperature resistance of adhesives have not been unified at home and abroad. In general, the high temperature resistance should be evaluated according to the specific strength, time and medium required to maintain the bond strength required by the design or have a certain strength retention. In other words, in addition to meeting certain temperature requirements, high-temperature adhesives must also meet the following comprehensive properties: good thermal physics and thermochemical properties; good processability; at higher temperatures and working conditions, Higher bond strength and better physical and mechanical properties maintain this performance over a specified period of time.

1 Organic adhesive

There are many types of organic high-temperature adhesives. At present, there are many epoxy compounds, phenolic resins, silicones, and nitrogen-containing heterocyclic polymers that are widely studied and widely used. Other types of high temperature resistant adhesives are under development. In recent years, researchers have made great progress in the study of the thermal stability, oxidation stability, and adhesive properties of the above adhesive series.

1.1 High Temperature Epoxy Adhesive

Epoxy resin (EP) is a general term for high molecular compounds containing two or more epoxy groups, and is a thermosetting resin in a variety of forms ranging from liquid to viscous and solid. This kind of material can not be used alone. It can only be cured and cross-linked to form a bonding effect. High-temperature epoxy adhesives have the advantages of high bonding strength, good overall performance, and simple use process. The outstanding advantage is that there are few volatiles in the curing process and the shrinkage is small. It can work for a long time at -50-232°C, and the maximum use temperature can reach 500-1000°C. However, the unmodified epoxy adhesive is relatively brittle after being cured and has poor mechanical properties and heat resistance. The molecular structure of epoxy resin and curing agent and their mutual reactivity determine the thermal deformation temperature and thermal oxidation stability of the cured product. Therefore, the curing agent with excellent high temperature resistance and thermal stability and the addition of a curing agent in the rubber system are selected. Tougheners, anti-oxidants and other functional fillers are effective ways to improve epoxy adhesives.

Bai Zongwu et al. used a carboxyl-terminated carboxyl fluid rubber (CTBN) to toughen the epoxy resin to obtain a high-temperature, good toughness adhesive. The adhesive can still maintain the bonding of 12 to 13 MPa at 200 to 250°C. Strength, suitable for high temperature operation. It has been reported that modification of epoxy adhesives with nano-POSS has found that mechanical properties such as shear strength and peel strength have been significantly improved. Zhao Shilin et al. used a high melting point bismaleimide modified epoxy resin to produce a high temperature resistant adhesive with a strength of about 30 MPa. Chenguang Chemical Research Institute, the epoxy resin and phenolic resin copolymer produced F-44, F-48 epoxy-phenolic adhesive maintains good adhesion and high temperature resistance, can be used at 260 °C long-term use, the maximum temperature can be used 315 °C. Zhang Datai reported that FB resin and F, B, H, and HE series curing agents can make epoxy resin have excellent flame-retardant properties, and the temperature resistance reaches 500-1000°C.

1.2 High temperature phenolic resin adhesive

Phenolic resin is a thermoplastic or thermosetting resin obtained by polycondensation of phenol (or phenolic organics) and formaldehyde in the presence of an acid or alkali catalyst. It is the earliest developed high-temperature resin. In the field of adhesives, both the amount and the yield are large.

Phenolic resin adhesive has strong adhesion, good chemical resistance and electrical insulation properties, high temperature resistance (very high adhesive strength at 300°C), but pure phenolic resin has high brittleness, low peel strength, and hardness High, poor toughness, and easy to decompose at high temperatures, sizing process is also more complex, so in some areas the application of the glue is limited. With the development of aeronautical and aerospace cutting-edge businesses, adhesives modified with rubber or other resins for phenolic resins have attracted attention and formed an important branch of high-temperature adhesives.

Yin Hongru reported that using polyvinyl butyral as a toughening agent modified phenolic resin, still has a shear strength of 8.85MPa at 300°C. The use of self-made tetramaleimide (TMl) modified phenolic resin has been reported, heat resistance 300 °C, to meet the spacecraft fairing adhesion. The FB resin developed and reported by Zhang Duotai is a boron-modified thermosetting phenolic resin. The adhesive made of this resin has high temperature resistance, flame resistance, and high adhesion performance. It still has 2 MPa at 800°C for 1 hour. strength. Hao Hui et al. used a molybdenum-modified phenolic resin to synthesize an adhesive with good heat resistance and a thermal decomposition temperature of 522°C. Zhang Min et al. also synthesized a 75% solid residue boric phenolic thermosetting resin at 700°C.

1.3 High Temperature Silicone Adhesive

Silicone adhesives can be divided into silicone adhesives and silicone rubber adhesives. Their polymers contain Si-0 bonds in inorganic structures, and some of the properties of inorganic and organic materials can be in a wide range of temperatures (-60 ~ 1200 °C) Keep physical and chemical properties unchanged.

Silicone resin is a kind of high-temperature adhesive with polyorganosiloxane and its modified body as the main raw material. It is often used in high temperature protective layer. Pure silicone adhesive has excellent heat resistance, can be used at -60 ~ 400 °C long-term use, short-term use to 450 ~ 550 °C, instantaneous use up to 1000 ~ 1200 °C; but the main drawback is brittle, adhesive strength Low, cure temperature is too high. In order to obtain better high-temperature physicochemical properties, phenolic resin, epoxy resin, and polyurethane resin are commonly used to modify the resin, and the requirements of good adhesion, room temperature curing, and high temperature resistance can be achieved. Introduce various aromatic heterocycles or other heat-resistant cyclic structures and heteroatoms into the siloxane backbone to improve its overall performance without reducing its heat resistance; while introducing phenylene and diphenyl in the main chain Silicone ladder polymers are formed from ether subunits, biphenyls and other aromatic subunits, and the high temperature resistance can reach 300-500°C. The trapezoidal polymer with silicon as the main chain can withstand high temperature of 1300°C and still have certain strength at 1250°C. Xi'an Jiaotong University uses methyltrichlorosilane as a raw material, and through the reaction with n-butylamine, the product is obtained by the hydrolysis and polycondensation reaction of a ladder-shaped polymethylsilsesquioxane, which is excellent in heat resistance, and the weight loss rate at 700°C is 4 %, can be used as high temperature materials and adhesives.

Silicone rubber adhesives are divided into hot vulcanized silicone rubber adhesives and room temperature vulcanized silicone rubber (RTV) adhesives. Because the latter has a simple vulcanization process, at the same time, it has oxidation resistance, high temperature resistance, cold resistance, ozone resistance, and excellent insulation properties. It has attracted increasing attention from people. It has developed rapidly and is widely used in the aerospace industry; its main material is Hydroxyl-terminated polysiloxanes. However, the RTV adhesive itself has low strength and low adhesion strength to various materials and is not used as a structural adhesive. The modification method is mainly to add a toughening agent and a special adhesion promoter to blend, copolymerize and graft to form a mixed glue system to improve the bonding strength: adding reinforcing fillers and increasing the crosslinking density can improve silicon to some extent Thermal stability of rubber. Polymethyl trifluoropropyl siloxane prepared by the Institute of Chemistry, Chinese Academy of Sciences, its main chain contains tetraphenyl tetramethylcyclodisilazane, has excellent thermal stability, nitrogen closed system at 300 °C With a weight loss of only 2.3% in 144h, it is the best silicone rubber at home and abroad.

(to be continued)

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