Structural characteristics and application fields of fluororubber, and various properties of fluororubber vulcanizate
Fluorine rubber refers to a synthetic polymer elastomer containing fluorine atoms on the carbon atoms of the main chain or side chain. It not only has good mechanical properties, but also has high resistance to high temperature, oil and various chemicals. Its characteristics and comprehensive performance are particularly excellent, so it has a wide range of applications, especially in the production of special sealing products. It is indispensable in modern aviation, missiles, rockets, aerospace and other cutting-edge science and technology and other industries (such as automobiles) Material.
1. Structural characteristics and application fields of fluororubber
Because there is no unsaturated C=C bond structure in the main chain of polyolefin fluororubber (type 26 fluororubber, type 23 fluororubber) and nitroso fluororubber. Reduces the possibility of degradation and chain scission on the main chain due to oxidation and pyrolysis. The methylene group in vinylidene fluoride plays a very important role in the flexibility of the polymer chain. For example, fluorine rubber 23-21 and fluorine rubber 23-11 are composed of vinylidene fluoride and chlorotrifluoroethylene in a ratio of 7:3 respectively. And the ratio of 5:5, obviously, the former is softer than the latter.
Whether it is vinylidene fluoride and chlorotrifluoroethylene, or the copolymers of the former and hexafluoropropylene, and their terpolymers with tetrafluoroethylene, they can be mainly crystalline or amorphous. This depends on how much of one monomer is involved when the other monomer is the main chain segment of the copolymer. Electron diffraction studies indicate that when the mole fraction of hexafluoropropylene in the vinylidene fluoride segment reaches 7%, or the mole fraction of vinylidene fluoride in the chlorotrifluoroethylene segment reaches 16%, the two copolymers still have and Its crystal structure is comparable to that of a homopolymer. However, when the mole fraction of hexafluoropropylene in the former increases to more than 15%, or the mole fraction of vinylidene fluoride in the latter increases to more than 25%, the crystal lattice is greatly destroyed, causing them to have an amorphous shape with mainly rubber properties. structure. This is due to the increase in the amount of the second monomer introduced, which destroys the regularity of its original molecular chain. Fluorine rubber can be used together with nitrile rubber, acrylic rubber, ethylene-propylene rubber, silicone rubber, fluorosilicone rubber, etc. to reduce costs and improve physical and mechanical properties and process performance.
The earliest fluorine rubber was poly-2-fluoro-1,3-butadiene and its copolymer with styrene, propylene, etc., which was trial-produced by DuPont Company in the United States in 1948. Its performance is no better than that of chloroprene rubber and butadiene rubber. , and it is expensive and has no actual industrial value. In the late 1950s, the American Thiokol Company developed a binary nitroso fluororubber with good low-temperature performance and resistance to strong oxidants (N2O4). Fluorine rubber began to enter practical industrial applications. China has also developed a variety of fluororubbers since 1958, mainly polyolefin fluororubbers, such as type 23, 26, 246 and nitroso fluororubber; later, a newer variety of tetrapropylene fluororubber was developed , perfluoroether rubber, 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 is widely used in daily life, such as automotive parts, aviation and aerospace fields, mechanical seals, pumps, reactors, agitators, compressor casings, valves, various instruments and other equipment as valve seats. , valve stem packing, diaphragms and gaskets, as well as in the rubber sheet industry, semiconductor manufacturing industry and food and pharmaceutical industries.
With the use of unleaded gasoline and electronic injection devices in automobiles, the structure and materials of fuel hoses have changed greatly. The inner rubber layer has been replaced by fluorine rubber instead of nitrile rubber. In order to reduce fuel penetration and further improve heat resistance, the inner rubber layer has been The rubber layer mostly adopts a composite structure, which is composed of fluorine rubber and chlorohydrin rubber or acrylate rubber. Since fluorine rubber is relatively expensive, the fluorine rubber layer is relatively thin, with a thickness of about 0.2 to 0.7 mm. This kind of structure fuel hose has become a mainstream product abroad. Our country has also developed this kind of hose with fluororubber as the inner layer, and it is used in Santana, Audi, Jetta, Fukang and other models of cars. 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.
Fluorine rubber and silicone rubber composite oil seals have become the most commonly used engine crankshaft oil seals. The hydraulic system of loading and unloading trucks and the hydraulic system of large loading and unloading trucks work continuously for a long time, and the oil temperature and machine parts temperature rise rapidly. Ordinary rubber cannot meet their working requirements, and fluorine rubber products can meet various demanding requirements due to their excellent temperature resistance. technical requirements. As the automotive industry’s requirements for reliability and safety continue to increase, the demand for fluororubber in the automotive industry has also shown a rapid growth trend.
In addition to automotive industry applications, fluororubber seals are used in drilling machinery, oil refining equipment, natural gas and power plant desulfurization devices. They can withstand harsh conditions such as high temperature, high pressure, oil and strong corrosive media; in chemical production, fluororubber seals Parts are used in pumps and equipment containers to seal chemical substances such as inorganic acids and organic substances. In the petroleum and chemical industries, fluorine rubber sealing products are used in mechanical seals, pumps, reactors, agitators, compressor casings, valves, various instruments and other equipment. They are usually used as packing for valve seats, valve stems, and diaphragms. and gaskets. Fluorine rubber is one of the indispensable high-performance materials for modern aviation, missiles, rockets, aerospace navigation, ships, atomic energy and other cutting-edge science and technology. In recent years, new fluorine rubber products have been continuously developed in the aviation and aerospace fields.
2. Main properties of fluorine rubber
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 has a high degree of chemical stability and has the best media resistance among all elastomers currently. Type 26 fluorine rubber is resistant to petroleum-based oils, diester oils, silicone oils, silicic acid oils, inorganic acids, most organic and inorganic solvents, drugs, etc., and is only not resistant to low-molecular ketones, ethers, Ester, not resistant to amines, ammonia, hydrofluoric acid, chlorosulfonic acid, and phosphoric acid hydraulic oils. The dielectric performance of type 23 fluorine rubber is similar to that of type 26, and it is more unique. Its resistance to strong oxidizing inorganic acids such as fuming nitric acid and concentrated sulfuric acid is better than that of type 26. It can be immersed in 98% HNO3 at room temperature. 27 Its volume expansion is 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 brittle 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 body material, but also 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 the key performance of fluorine rubber when 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 very large in the temperature range of 200 ~ 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 is large at high temperatures. However, if compared under the same conditions, such as the compression permanent deformation of the same time at 150 ℃, both butadiene 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 with temperature. 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 fluororubber 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.