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High temperature sealant in vacuum system
Sealant is a type of adhesive used to connect and seal various components in a vacuum system. It not only has good bonding strength, but also has good vacuum sealing and pressure resistance performance. It is an indispensable material for the connection of various parts of the vacuum system. With the wide application of vacuum technology in scientific research and production, the rapid development of electronics, radio, aerospace and aerospace industries has put forward more requirements for the sealing connection of vacuum components and promoted the study of vacuum sealants. . Permeability refers to the ability of various gases to pass through the sealant when there is a pressure difference, and the permeability of the material is generally expressed by the air permeability. The amount of gas released refers to the amount of gas released by the sealant itself at the operating temperature. Only when both are low can high vacuum be achieved.
In a vacuum system, it is usually necessary to remove most of the gas adsorbed on the inner wall surface of the vacuum system by heating to 400 to 500 and other methods (ion or electron bombardment); some vacuum systems also need to work at a low temperature Therefore, the selected sealant should be able to adapt to changes in temperature under vacuum conditions, not only to withstand high temperatures, but also to withstand the test of alternating high and low temperature impact.
There is a pressure difference between the vacuum system and the outside world (such as the atmosphere). The sealant must have the ability to withstand differential pressure, and it needs a certain degree of toughness and sufficient mechanical strength.
The sealant should have good wettability to the adherent in order to facilitate the sealing of the material and obtain a high adhesive strength.
Due to the special needs of vacuum system cleaning, leak detection, etc., vacuum sealant is required to have certain anti-solvent properties, such as acetone resistance and absolute ethanol.
When preparing the vacuum adhesive, it is necessary to comprehensively consider these special requirements and adjust the molecular structure of the sealant components and its main resin to meet the requirements for use.
The choice of the matrix of the sealant The main component of the commonly used organic vacuum sealant is the organic polymer. This kind of adhesive has good adhesiveness and convenient operation. It can connect various materials, but its heat resistance is not good enough. Use at lower temperatures. And can be used in 200 ~ 500 * C or higher temperature without losing the original adhesive performance of high temperature sealant, currently mainly epoxy, modified phenolic, silicone, polyimides, Other nitrogen-containing heterocyclics and inorganic adhesives.
Epoxy resin is a vacuum adhesive commonly used in vacuum technology. Its chemical structure makes it not emit any small-molecule compound when it is cured, so it has a very low vapor pressure, but its working temperature is only *C long-term use. , The maximum use temperature can reach 315 ° 3. Silicone vacuum sealant has high heat resistance, but the silicone polymer as a vacuum material, has its own shortcomings, the silicone resin emits small molecules of water in the curing, heat Small molecules and cyclic compounds are also released during the cracking, which leads to the porosity of the adhesive layer and reduces the airtightness. The use temperature is about 200~250°C. The epoxy modified silicone vacuum adhesive Working temperature can reach 300 °C. Polyimide (PI) vacuum sealant has excellent heat resistance and vacuum performance, small thermal expansion coefficient, has good solvent resistance, can work long-term in the 370 ~ 390 * C , Short-term use temperature can exceed 5ro * C This type of adhesive high temperature strength, good electrical properties, excellent resistance to radiation, low temperature performance, excellent overall performance, is the most important type of high temperature material / Ti2 (mass fraction It is 1. 5%) The composite film was baked at 400*C, and the weight loss was about 23.2 after 10 hours. Other properties The prepared PAA/Ti2 sealant also has some other excellent properties, as shown in Table 2: Table 2 Other properties of the sealant film Test results Solvent-resistant acetone, 48h film intact, no bubbles, no peeling, no change in color, absolute ethanol, 48h film intact, no bubbles, no peeling, no color change Adhesion> Secondary impact strength shear strength ceramic/ceramic stainless steel / Stainless steel vacuum performance and heat resistance can be heated at 400C450C high temperature exhaust, 10-5 106Pa no leakage phenomenon 3 Conclusion High temperature vacuum sealant is a special field in the military, defense, aviation, aerospace, electronics and other aspects All of them are indispensable materials. The PAA/Ti2 sealant prepared in this experiment has good storage stability. When the mass fraction of Ti2 is 1.5%, it can be stored at 10*C for 1 month; it can be obtained by gradient temperature curing. The highly imidized PI/Ti2 film has good thermal stability, and its thermal weight loss is 4.7% at 400C at 20h; the film at the temperature of 41 and 500° at 450° remains intact without cracking and shedding; and has good resistance Solvent performance, Junction performance and vacuum performance.
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