The rubber that can be used for a long time under the negative pressure of 10-1〜133X10-8Pa is called vacuum-resistant rubber, and has the characteristics of high airtightness, low gas permeability, low weight loss and the like. Vacuum-resistant rubber is generally classified into four grades according to the negative pressure (less than atmospheric pressure) of the working environment.
Vacuum-resistant rubber products are mostly used in precision fields such as spacecraft, space stations and satellites. In addition to meeting the demanding vacuum resistance requirements, that is, to achieve 6×10-9 Torr, it is also necessary to withstand the strong cosmic radiation in space (the intensity reaches 1.0 × 104 rad). In addition, the cracking of the rubber under high-energy radiation greatly exceeds the cracking under normal pressure, which may increase the damage to the properties of the rubber. In short, under high vacuum and ultra-high vacuum conditions, the rubber is subjected to conditions far more severe than under normal conditions.
(1) Performance requirements for vacuum resistant rubber
The performance requirements of vacuum-resistant rubber, in addition to the conventional properties that elastomers should have, must meet the unique requirements of the following two aspects.
1) Air tightness is specifically to achieve high airtightness, low air leakage, low gas penetration and other requirements. The main cause of failure of rubber products in vacuum systems is leakage
Gas, so ensuring the lowest possible air leak rate is critical. However, the gas leakage rate varies with the type of rubber. In addition to the gas leakage rate, the gas permeability (the ability of the gas to diffuse in the rubber) is also an indicator that must be controlled.
2) Weight loss
Rubber products working in a vacuum system, due to high temperature and high energy radiation and the action of certain media, will chemically react during use to produce low molecular weight volatiles. These volatiles sublime under reduced pressure, resulting in weight loss. Sublimation is related to the structure and formulation of rubber. Under high vacuum conditions, the highest weight loss rate is butyl rubber (39%), and the lowest is fluororubber. This is why, although butyl rubber has good airtightness, it is not suitable for use under high vacuum conditions. Because the hair is bursting from the angle of use, the weight loss rate of the rubber in vacuum should not exceed 10%.
(2) Matching points of vacuum resistant rubber
1) The choice of gas tightness and weight loss rate must be balanced and should not be neglected. The determination of the specific rubber type is also determined according to the vacuum requirements. Generally, products requiring low vacuum, such as vacuum hoses for bulbs, can also be made of natural rubber. For products with high vacuum resistance, it is necessary to use a high acrylonitrile content of butadiene rubber or fluororubber because of the weight loss. For products that are resistant to ultra-high vacuum, Viton is preferred.
In addition, it is also necessary to consider that the high-vacuum-resistant rubber is subjected to high-temperature barbecue treatment before being put into use, so the ability of the main material to withstand the high temperature of the barbecue must also be taken into consideration, that is, the selected rubber type must withstand the vacuum performance, and must withstand the barbecue. The high temperature is suitable for use at 10-6~10-7Pa or higher. Therefore, almost non-fluororubber and high acrylonitrile nitrile rubber are the only ones.
2) The compounding agent plasticizer has low volatilization point and large dosage, and it is best to use less or not. Although the amount of anti-aging agent is not large, it is volatile and should be used sparingly. It is not advisable to use white carbon black for the filler. The proper amount of carbon black can reduce the gas permeability and help to withstand the vacuum.