Rubber auxiliary seals are the most widely used type of auxiliary seals. Commonly used rubber seal materials are nitrile rubber, fluoroelastomer silicone rubber, neoprene rubber and so on.
(1) Nitrile rubber NBR Nitrile rubber is the most commonly used type of rubber. It is a copolymer of butadiene and acrylonitrile. According to the amount of acrylonitrile content is divided into several kinds: low acrylonitrile (NBR-18), medium acrylonitrile (NBR-26) and high acrylonitrile (NBR-40). The higher the content of acrylonitrile, the better the oil resistance, the higher the tensile strength, hardness and abrasion resistance, the higher the water resistance, and the lower the permeability. Consequently, its solubility in polar solvents increases and its corrosion resistance is affected. Elasticity and cold resistance also deteriorate. NBR is not resistant to flexing, and tear resistance is also poor. Generally the acrylonitrile content in NBR seals is 26%~50%. NBR has excellent corrosion resistance to mineral oil, animal and vegetable fats and oils, aliphatic hydrocarbons, and is widely used in equipment in contact with petrol and other oils. It can resist alkali and non-oxidising dilute acid corrosion, but not oxidising acid (e.g. nitric acid, chromic acid, etc.), aromatic hydrocarbons, grease, ketone, ether, halogenated hydrocarbons and other corrosion. Recently, hydrogenated nitrile butadiene rubber (HNBR) has been developed, and its performance is better than that of nitrile rubber. The use of temperature range of -40 ~ 150 ℃, oil resistance is better than nitrile rubber, hydrogen sulfide resistance is better than fluoroelastomer, used in 200 ℃ steam is second only to ethylene propylene rubber.
(2) Fluoroelastomer FPM fluoroelastomer has the advantages of high temperature resistance, oil resistance, chemical corrosion resistance. In concentrated sulfuric acid, nitric acid, phosphoric acid, caustic soda and other media can be used. But as the temperature rises its corrosion resistance rate decreases, the maximum operating temperature of 200 ℃. The most widely used domestic fluoroelastomers are fluorinated olefin copolymers, mainly fluorine-23 and fluorine-26. 23-type fluoroelastomers from vinylidene fluoride and chlorotrifluoroethylene at room temperature and about 3.3MPa pressure by suspension polymerisation of a kind of amorphous rubbery copolymers. 23-type fluoroelastomers are equivalent to the foreign Kel-F fluoroelastomers, which can be used in strong acids. 26-type fluoroelastomers are of two kinds: Viton-26, which is made of vinylidene fluoride and hexafluorocarbon. There are two types of fluoroelastomers: fluoroelastomer-26, an emulsion copolymer of vinylidene fluoride and hexafluoropropylene, which is equivalent to foreign Viton fluoroelastomers; fluoroelastomer-246, a terpolymer of vinylidene fluoride, hexafluoropropylene and tetrafluoroethylene. In recent years, the development of perfluoroelastomer (FFKM perfluoropolymer elastomer), equivalent to foreign Kalrez perfluoroelastomer. It has excellent anti-aging properties, no significant aging phenomenon after 112 days of use at 260°C. The tensile strength remains 90% of the original strength. The tensile strength still maintains about 90% of the original, can be used continuously at 288 ℃, 310 ℃ for a short period of time. Its oil resistance, abrasion resistance and resistance to mixed organic corrosion is good, but its coefficient of expansion is nearly twice as high as that of nitrile rubber (320*10-6/K), and it is not suitable for use below zero degrees.
(3) Silicone rubber MVQ silicone rubber high temperature resistance and low temperature resistance is very good, the safe use of temperature range of -70 ~ 200 ℃. In dilute sulfuric acid, hydrochloric acid, acetic acid, caustic soda, ethanol, mineral oil and other media, there is no significant corrosion phenomenon. Silicone rubber is made of dimethylsiloxane and other silicone monomers, in the presence of acidic or alkaline catalyst polymerised into a polar polymer. Silicone rubber has high thermal stability, but it has polarity, easy to ionic cleavage under the action of acids and alkalis, so the corrosion resistance is poor. Not suitable for petroleum-based solvents (such as benzene, toluene, etc.), acetone, ketone, ether and other organic solvents. Silicone rubber tear strength and tear elongation is small (only 1 / 3 of nitrile rubber).
Recently, fluorosilicone rubber (MFQ) can be widely used in gasoline, petroleum oils and solvents.
(4) Ethylene Propylene Rubber EPM Ethylene Propylene Rubber is polymerised from ethylene and propylene, divided into binary and terpolymer. It is particularly resistant to phosphate ester hydraulic fluids, ketones, alcohol solutions and acids and alkalis, but also resistant to high-pressure vapours, weather resistance and good ozone resistance. But in mineral oil and diester lubricants in the expansion, so can not be used in these media.
2, polytetrafluoroethylene PTFE
Polytetrafluoroethylene heat, oil and corrosion resistance than the general rubber, commonly used in mechanical seals it is made of V-shaped ring and wedge ring. Polytetrafluoroethylene plastic and rubber comparison, with greater stiffness, lower elasticity and cold flow. The coefficient of expansion of polytetrafluoroethylene is high, and with the temperature changes, especially at room temperature with a peak, which prevents it from being used in mechanical seals. However, PTFE has a wide range of use (-150 ~ 250 ℃), very low coefficient of friction (in the low speed f = 0.05 ~ 0.1) and self-lubricating, non-bonding surface, good chemical stability, resistance to chlorides, fluorine boron trifluoride, high-boiling solvents, ketones, esters, ethers, boiling nitric acid, aqua regia, and sodium hydroxide, hydrofluoric acid and so on. The only things that will attack PTFE are molten metals and fluorine under high pressure. Under the action of load, any temperature creep (i.e., cold flow), more than 8)83, it will sublimate to produce toxic fumes.
3, other materials
Used as an auxiliary sealing ring of other materials are metal, filled with PTFE, expanded graphite (flexible graphite), asbestos and rubber-plastic composite materials. These materials are mainly used in high temperature occasions.
