Fluorine rubber, do we really understand it?
Fluorine rubber is a synthetic polymer elastomer containing fluorine atoms on carbon atoms in the main chain or side chains. It has excellent mechanical properties and is very resistant to high temperatures, oils and chemicals. Therefore, fluororubber is widely used in various sealing products. It is also an indispensable material in cutting-edge science and technology such as modern aviation, missiles, rockets and aerospace, as well as other industries (such as automobiles).
Next, we will briefly explain the structural characteristics, application fields, main properties, products and applications of fluororubber.
1. Structural characteristics and application areas of fluororubber
The main chain of polyolefin fluororubber (such as type 26 fluororubber and type 23 fluororubber) and nitroso fluororubber does not have an unsaturated C=C bond structure. Reduces the possibility of main chain degradation and chain scission due to oxidation and pyrolysis. The methylene groups in vinylidene fluoride are crucial to the flexibility of the polymer chain. For example, fluororubber 23-21 and 23-11 are composed of ratios of 7:3 and 5:5, and the former is obviously softer than the latter.
In 1948, the American DuPont Company trial-produced fluorine rubber as poly-2-fluoro-1,3-butadiene and its copolymer with propylene, styrene and other substances. The performance of this kind of fluorine rubber is not as good as butadiene rubber and chloroprene rubber, and it is expensive and has no actual industrial value. In the late 1950s, the American company Thiokol developed a binary nitrosyl fluororubber that was resistant to strong oxidants (N2O4). It has good low temperature performance. Fluorine rubber began to be used in industry. In 1958, China also produced a variety of fluorine rubber, mainly polyolefin fluorine rubber, such as type 23, 26, 246 and nitroso fluorine rubber. Subsequently, China developed tetrapropylene fluorine rubber, perfluoroether rubber, fluorophosphorus rubber and fluorophosphorus rubber.
These fluorine rubber varieties are first based on the supporting needs of aviation, aerospace and other national defense and military industries, and are gradually promoted and applied to the civil industry sector. They have been used in modern aviation, missiles, rockets, aerospace navigation, ships, atomic energy and other cutting-edge technologies as well as automobiles, shipbuilding, chemicals, etc. , petroleum, telecommunications, instruments, machinery and other industrial fields.
Fluorine rubber has many uses in daily life, such as auto parts, aerospace, mechanical seals, pumps, reactors, agitators, compressor casings, valves, valve seats, valve stem packings, diaphragms and other equipment for various types of instruments and other equipment. Gasket. In addition, it is used in semiconductor manufacturing, food and pharmaceutical fields.
The structure and materials of fuel hoses have changed a lot, and they are used in cars such as unleaded gasoline and electronic injection devices. The inner rubber layer is composed of fluorine rubber or chlorohydrin rubber or acrylic rubber, with a thickness of approximately 0.2 to 0.7 mm. In addition, in order to reduce fuel penetration and improve heat resistance, the inner rubber layer mostly adopts a composite structure. Fuel hoses with this structure are very popular abroad. This kind of fluororubber is also used in inner rubber hoses and is used in cars such as Santana, Audi, Jetta and Fukang. In terms of automobile engines, gearboxes, and valve oil seals with high technical content, the materials selected are mainly fluorine rubber, hydrogenated nitrile rubber, etc.
As the automotive industry’s requirements for reliability and safety continue to increase, fluororubber and silicone rubber composite oil seals have become the most commonly used engine crankshaft oil seals. These oil seals work continuously for a long time in the hydraulic systems of large loading and unloading trucks and hydraulic systems of loading and unloading trucks. The oil temperature and the temperature of the machine parts rise rapidly. Ordinary rubber cannot meet these requirements. Therefore, the demand for fluororubber products in the automotive industry is growing rapidly.
In addition to being used in the automotive industry, fluororubber seals are also used in drilling machinery, oil refining equipment, natural gas and power plant desulfurization devices. They can simultaneously withstand harsh environments such as high temperature, high pressure, oil and highly corrosive media. In the petroleum and chemical industries, fluororubber seals are used in mechanical seals, pumps, reactors, agitators, compressor casings, valves, various instruments and equipment, and are usually used for valve seats, packings of valve stems, diaphragms and gaskets piece.
Fluorine rubber is one of the indispensable high-performance materials in modern aviation, missiles, rockets, aerospace, ships, atomic energy and other cutting-edge science and technology. Recently, new products of fluorine rubber have been continuously developed in the aviation and aerospace fields.
2. Main properties of fluororubber
Fluorine rubber has unique properties, and the properties of its vulcanized rubber are described below.
(1) Corrosion resistance: Fluorine rubber has excellent corrosion resistance. Generally speaking, its stability against organic liquids (fuel oil, solvents, hydraulic media, etc.), concentrated acids (nitric acid, sulfuric acid, hydrochloric acid), high-concentration hydrogen peroxide and other strong oxidants is superior to other rubbers. .
(2) Swelling resistance: Fluorine rubber is the most media-resistant of all elastomers. Type 26 fluororubber is not resistant to petroleum-based oils, diester oils, silicone oils, silicic acid oils and inorganic acids. It is also resistant to most organic and inorganic solvents and pharmaceuticals. It is not resistant to low molecular weight ketones, ethers, esters, amines, ammonia, hydrofluoric acid, chlorosulfonic acid and phosphoric acid hydraulic oils. The media performance of type 23 fluorine rubber is similar to that of type 26 fluorine rubber, but it has more advantages. It is more resistant to strongly oxidizing inorganic acids than type 26 fluorine rubber, such as concentrated sulfuric acid and fuming nitric acid. Soaked in 98% HNO3 at room temperature for 27 days, its volume expansion was only 13% to 15%.
(3) Heat resistance and high temperature resistance: In terms of aging resistance, fluorine rubber is comparable to silicone rubber and better than other rubbers. Type 26 fluororubber can work at 250 ℃ for a long time and at 300 ℃ for a short time. Type 23 fluororubber still has high strength after aging at 200 ℃ × 1000 h, and can also withstand short-term high temperature of 250 ℃. The thermal decomposition temperature of tetrapropylene fluorine rubber is above 400 ℃ and can work at 230 ℃ for a long time. The performance changes of fluorine rubber at different temperatures are greater than those of silicone rubber and general-purpose butyl rubber. Its tensile strength and hardness both decrease significantly with the increase of temperature. The characteristics of the change of tensile strength are: below 150 ℃, with temperature It decreases rapidly with the increase of temperature, and decreases slowly with the increase of temperature between 150 and 260 ℃.
The high temperature resistance of fluorine rubber is the same as that of silicone rubber, and it can be said to be the best among current elastomers. 26-41 fluorine rubber can be used for long-term use at 250 ℃ and short-term use at 300 ℃; 246 fluorine rubber has better heat resistance than 26-41. The physical properties of 26-41 after air thermal aging at 300 ℃ × 100 h are equivalent to those of Type 246 after hot air aging at 300 ℃ × 100 h. Its elongation at break can be maintained at about 100%, and the hardness is 90 to 95 degrees. . Type 246 maintains good elasticity after hot air aging at 350°C for 16 hours, maintains good elasticity after hot air aging at 400°C for 110 minutes, and contains spray carbon black, thermal carbon black or carbon fiber after hot air aging at 400°C for 110 minutes. The elongation of the rubber increases by about 1/2 to 1/3, and the strength decreases by about 1/2, but still maintains good elasticity. Type 23-11 fluorine rubber can be used at 200 ℃ for a long time and at 250 ℃ for a short time.
(4) Low temperature resistance: The low temperature performance of fluororubber is not good, which is due to its own chemical structure, such as the Tg of 23-11 type > 0 ℃. The low-temperature properties of fluororubber actually used are usually expressed by brittleness temperature and compression cold resistance coefficient. The formula of the rubber compound and the shape of the product (such as thickness) have a greater impact on the brittleness temperature. If the amount of filler in the formula increases, the brittleness temperature will deteriorate sensitively. As the thickness of the product increases, the brittleness blending degree will also deteriorate sensitively. The low-temperature resistance of fluorine rubber generally allows it to maintain elasticity at a limit temperature of -15 to 20 ℃. As the temperature decreases, its tensile strength increases and appears strong at low temperatures. When used as seals, low temperature seal leakage problems often occur. Its brittleness temperature changes with the thickness of the sample. For example, the brittleness temperature of type 26 fluororubber is -45 ℃ when the thickness is 1.87 mm, -53 ℃ when the thickness is 0.63 mm, and -69 ℃ when the thickness is 0.25 mm. The brittleness temperature of its standard sample type 26 fluororubber is -25~-30 ℃, the brittleness temperature of type 246 fluororubber is -30~-40 ℃, and the brittleness temperature of type 23 fluororubber is -45~-60 ℃.
(5) Resistance to superheated water and steam: The stability of fluororubber against the action of hot water not only depends on the main body material, but also depends on the combination of the rubber. For fluorine rubber, this performance mainly depends on its vulcanization system. Peroxide vulcanization systems are better than amine and bisphenol AF vulcanization systems. The performance of type 26 fluorine rubber using an amine vulcanization system is worse than that of general synthetic rubber such as ethylene-propylene rubber and butyl rubber.
(6) Compression permanent deformation performance: Compression deformation is its key performance when fluororubber is used for sealing at high temperatures. The reason why Viton type fluororubber has been widely used is inseparable from its improvement in compression deformation. It is an important performance that must be controlled as a sealing product. Type 26 fluororubber has better compression set performance than other fluororubbers, which is one of the reasons why it is widely used. Its compression permanent deformation appears to be very large in the temperature range of 200 to 300 ℃. However, in the 1970s, the American DuPont Company improved it and developed a low compression permanent deformation compound (Viton E-60C), which was improved from raw rubber (Viton A to Viton E-60) and vulcanized The system selection (from amine vulcanization to bisphenol AF vulcanization) has been improved, which makes the fluorine rubber have better compression permanent deformation when sealed for a long time at a high temperature of 200 ℃, and the fluorine rubber can be stored for a long time at 149 ℃. , its seal retention rate is in a leading position among all types of rubber.
(7) Weathering resistance and ozone resistance: Fluorine rubber has excellent weathering resistance and ozone resistance. According to reports, the performance of VitonA developed by DuPont is still satisfactory after 10 years of natural storage, and there is no obvious cracking after 45 days in air with an ozone volume fraction of 0.01%. Type 23 fluororubber also has excellent weather aging resistance and ozone resistance.
(8) Mechanical properties: Fluorine rubber generally has high tensile strength and hardness, but poor elasticity. The general strength of type 26 fluorine rubber is between 10 and 20 MPa, the elongation at break is between 150 and 350%, and the tear resistance is between 3 and 4 kN/m. The strength of type 23 fluorine rubber is between 15.0 and 25 MPa, the elongation is between 200% and 600%, and the tear resistance is between 2 and 7 MPa. Generally speaking, the compression permanent deformation of fluorine rubber at high temperature is large, but if compared under the same conditions, such as the compression permanent deformation of the same time at 150 ℃, both butyl and chloroprene rubber are larger than type 26 fluorine rubber. The compression deformation of type 26 fluorine rubber at 200 ℃ × 24 h is equivalent to the compression deformation of butadiene rubber at 150 ℃ × 24 h.
(9) Electrical properties: The electrical insulation properties of fluororubber are not very good and are only suitable for use under low frequency and low voltage. Temperature has a great influence on its electrical properties. When it rises from 24 ℃ to 184 ℃, its insulation resistance drops 35,000 times. The electrical insulation properties of type 26 fluororubber are not very good and are only suitable for low-frequency and low-voltage applications. Temperature has a great influence on its electrical properties, that is, as the temperature increases, the insulation resistance decreases significantly. Therefore, fluororubber cannot be used as an insulating material at high temperatures. The type and amount of fillers have a great influence on the electrical properties. Precipitated calcium carbonate gives the vulcanized rubber higher electrical properties, while other fillers have slightly worse electrical properties. As the amount of fillers increases, the electrical properties decrease.
(10) High vacuum resistance: Fluorine rubber has excellent vacuum resistance. This is because fluororubber has a small outgassing rate and a very small amount of gas volatilization under high temperature and high vacuum conditions. Type 26 and 246 fluororubber can be used in ultra-high vacuum situations of 133×10-9 ~ 133×10-10 Pa, and are important rubber materials in spacecraft. The air permeability of fluorine rubber is the lowest among rubbers, similar to butyl rubber and nitrile rubber. The addition of fillers can reduce the air permeability of vulcanized rubber, and the effect of barium sulfate is more significant than that of medium particle thermal carbon black (MT). The gas permeability of fluororubber increases as the temperature increases. The solubility of gas in fluororubber is relatively large, but the diffusion rate is very small, which is beneficial to application under vacuum conditions. The solubility of fluorine rubber to gas is relatively large, but the diffusion rate is relatively small, so the overall breathability is also small. According to reports, the breathability of type 26 fluororubber to oxygen, nitrogen, helium, and carbon dioxide gases at 30°C is equivalent to butyl rubber and butyl rubber, and better than chloroprene and natural rubber. In fluorine rubber, the addition of fillers fills the gaps inside the rubber, thereby reducing the air permeability of the vulcanized rubber, which is very beneficial for vacuum sealing.
(11) Flame resistance: The flame resistance of rubber depends on the halogen content in the molecular structure. The more halogen content, the better the flame resistance. Fluorine rubber can burn when in contact with flame, but will automatically extinguish after leaving the flame, so fluorine rubber is a self-extinguishing rubber.
(12) Radiation resistance: Fluorine rubber is a material that is resistant to moderate doses of radiation. The radiation effect of high-energy rays can cause cracking and structuring of fluorine rubber. The radiation resistance of fluororubber is relatively poor among elastomers. Type 26 rubber shows a cross-linking effect after radiation, and type 23 fluororubber shows a cracking effect. The performance of type 246 fluorine rubber changes drastically under normal temperature radiation in the air at a dose of 5×107 Lun. Under the condition of 1×107 Lun, the hardness increases by 1 to 3, the strength decreases by less than 20%, and the elongation decreases by 30% to 50%. Therefore, it is generally believed that type 246 fluororubber can withstand 1×107 liters, and the limit is 5×107 liters.
3. Fluorine rubber products and applications
Seal materials made of fluorine rubber can be made into various types of gaskets, valve sealing gaskets, ○-shaped sealing rings, V-shaped sealing rings, leather cups, oil seals and corrugated connecting pipes, etc. These products can withstand temperatures above 200°C and will not deform in various oil media environments. Fluorine rubber sealing materials are mainly used in the automotive and aerospace fields in China. At present, the main products of fluorine rubber materials for domestic automobile parts include engine crankshaft front oil seal, crankshaft rear oil seal, valve cylinder oil seal, engine diaphragm, and engine cylinder liner resistance. Water rings, refueling hoses, fuel hoses, oil filter check valves, fuel filler cap O-rings, oil seals of gearboxes and reducers, etc. Fluorocarbon rubber, silicone rubber, acrylic rubber and heat-resistant elastomers have become the development trend and mainstream of automotive rubber materials in the future. Many auto parts use fluorocarbon rubber with better performance to replace traditional materials.
When fluorine rubber seals are used to seal automobile engines, they can work for a long time at 200 ℃ ~ 250 ℃, and the working life can be the same as the engine repair life; when used in the chemical industry, they can seal inorganic acids (such as 67% at 140 ℃ sulfuric acid, concentrated hydrochloric acid at 70 ℃, 30% nitric acid at 90 ℃), organic solvents (such as chlorinated hydrocarbons, benzene, highly aromatic gasoline) and other organic substances (such as butadiene, styrene, propylene, phenol, 275 ℃ fatty acids, etc.); when used in deep well oil production, it can withstand the harsh working conditions of 149 ℃ and 420 atmospheric pressure; when used in superheated steam seals, it can work in the steam medium of 160~170 ℃ for a long time. In the production of single crystal silicon, fluorine rubber seals are commonly used to seal special media at high temperatures (300 ℃) – trichlorosilane, silicon tetrachloride, gallium arsenide, phosphorus trichloride, trichlorethylene and 120 ℃ of hydrochloric acid, etc.
Hose made of fluororubber are suitable for high temperature resistance, oil resistance and special media resistance. Wires and cables made of fluororubber have good flexibility and good insulation. Glass fiber tape made of fluorine rubber can withstand high temperatures of 300°C and chemical corrosion. After aramid cloth is coated with fluorine rubber, it can be used to make telescopic pipes for connection between high-temperature and acid-alkali-resistant storage tanks in petrochemical plants. It can withstand high pressure, high temperature and medium corrosion, and can buffer and absorb the deformation and expansion of the two tanks. connection effect. The tape sealing bag made of nylon cloth coated with fluorine rubber is used as a soft seal for the internal floating roof storage tank of the refinery to seal and reduce the evaporation loss of the oil level.
Type 23 and tetrapropylene fluororubber are mainly used for corrosive sealing applications that are resistant to acid and special chemicals. Hydroxy-nitroso fluoroelastomer is mainly used as protective products and sealing products. It is used as a non-flammable coating in solution form and is used in fireproof electronic components and components working in pure oxygen. Its solution and liquid rubber can be used to manufacture many products by spraying, pouring and other methods, such as space suits, gloves, pipe belts, balls, etc. It can also be used as an adhesive for glass, metal, and fabrics to make sponges and gaskets, ○-rings, capsules, valve seals, etc. that come into contact with rocket propellant (N2O4). G-type series fluorine rubber production The seals have the performance of high temperature steam resistance, methanol gasoline or gasoline containing high aromatic hydrocarbons that cannot be achieved by using VitonA, B, E and other fluorine rubbers; GLT type fluorine rubber, chlorinated phosphorus rubber, perfluoroether rubber, etc. are even more Wide operating temperature range, low-temperature softness, elastic sealing, etc. Perfluoroether rubber also has outstanding resistance to media corrosion and is widely used in sealants made of fluorine rubber, such as putty. It has outstanding resistance to fuel oil and can be used in oil at around 200°C. It is used as a sealing material for aircraft integral fuel tanks. A closed-cell sponge made of fluorine rubber is acid-resistant, oil-resistant, has a wide operating temperature range and has good insulation properties. , can be used as a seal for rocket fuel, solvent, hydraulic oil, lubricating oil and grease, and as a shock-absorbing material for rockets and missiles. It has a temperature resistance of up to 204°C. Asbestos fiber cloth impregnated with fluorine rubber emulsion can be made into asbestos rubber sheets. Used in high temperature, combustion and chemical resistant applications.
Content source: Part of the content is reproduced from Rubber Technology Network(Contact us )
