Science Popularization: Understanding the Properties and Application Fields of Fluorine Rubber
Fluorine rubber refers to a synthetic polymer elastomer that contains fluorine atoms on the carbon atoms of the main or side chains. It not only has good mechanical properties, but also has high resistance to high temperature, oil, and various chemical erosion. Its comprehensive performance is particularly excellent, and it is widely used in the production of special sealing products. It is a modern aviation, missile, and rocket, Advanced science and technology such as space navigation, as well as essential materials for other industries such as automobiles.
1. Structural characteristics and application fields of fluororubber
Due to the lack of unsaturated C=C bond structure on the main chain in polyolefin fluorine rubber (26 type fluorine rubber, 23 type fluorine rubber) and nitroso fluorine rubber, the possibility of degradation and chain breakage on the main chain due to oxidation and pyrolysis is reduced. The methylene group in vinylidene fluoride plays a crucial role in the softness of polymer chains. For example, fluororubber 23-21 and fluororubber 23-11 are composed of vinylidene fluoride and trifluoroethylene in a ratio of 7:3 and 5:5, respectively. Obviously, the former is softer than the latter.
Both vinylidene fluoride and trifluoroethylene, as well as copolymers of the former with hexafluoropropene and their trimers with tetrafluoroethylene, can be predominantly crystalline or amorphous, depending on the amount of intervention of one monomer when it is the main 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 trifluoroethylene segment reaches 16%, the two copolymers still have the same homopolymer crystal structure as them. However, when the mole fraction of hexafluoropropene in the current one increases to over 15%, or the mole fraction of vinylidene fluoride in the latter increases to over 25%, the lattice is significantly destroyed, resulting in their amorphous structure dominated by rubber properties. This is due to the increase in the introduction of the second monomer, which disrupts the regularity of its original molecular chain.
Fluorine rubber can be used in combination with nitrile rubber, acrylic rubber, ethylene propylene rubber, silicone rubber, fluorosilicone rubber, etc. to reduce costs, improve physical and mechanical properties, and process performance.
The earliest fluororubber was poly (2-fluoro-1,3-butadiene) and its copolymers with styrene, propylene, etc., produced by DuPont Company in the United States in 1948. Its performance was not superior to that of chloroprene rubber and butadiene rubber, and it was expensive and had no practical industrial value.
In the late 1950s, Thiokol Company in the United States developed a binary nitroso fluororubber with good low-temperature performance and strong oxidant resistance, and fluororubber began to enter practical industrial applications.
Since 1958, China has also developed various types of fluorine rubber, mainly polyolefin fluorine rubber, such as 23, 26, 246, and nitroso fluorine rubber; Subsequently, new varieties of fluororubber, such as tetrapropylene fluororubber, perfluoroether rubber, and fluorinated phosphorus rubber, were developed. These fluororubber varieties were first developed to meet the needs of national defense and military industries such as aviation and aerospace, and gradually promoted and applied to the civilian industrial sector. They have been applied to cutting-edge technologies such as modern aviation, missiles, rockets, space navigation, ships, atomic energy, and industrial fields such as automobiles, shipbuilding, chemistry, petroleum, telecommunications, instruments, and machinery.
Fluorine rubber has a wide range of applications in daily life, such as in automotive parts, aviation and aerospace fields, mechanical seals, pumps, reactors, mixers, compressor casings, valves, various instruments, and other equipment. It is used as a filler for valve seats, valve stems, diaphragms, and gaskets, as well as in the rubber sheet industry, semiconductor manufacturing industry, and food and pharmaceutical industries.
With the use of lead-free gasoline and electronic fuel injection devices in automobiles, the structure and materials of fuel hoses have undergone significant changes. 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 mostly adopts a composite structure, which is composed of fluorine rubber, chlorohydrin rubber or acrylic rubber. Due to the high price of fluorine rubber, the fluorine rubber layer is relatively thin, with a thickness of about 0.2-0.7mm, This type of structural fuel hose has become a mainstream product abroad. China has also developed this type of fluororubber inner layer hose and used it on sedans such as Santana, Audi, Jetta, and Fukang. In terms of high-tech automotive engines, transmissions, and valve oil seals, the main materials used are fluororubber, 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 have a long continuous working time, and the oil temperature and machine component temperature rise rapidly. Ordinary rubber cannot meet its working requirements, while fluororubber products can meet various demanding technical requirements due to their excellent temperature resistance performance. With the continuous improvement of reliability, safety and other requirements in the automotive industry, the demand for fluororubber in the automotive industry is also showing a rapid growth trend.
In addition to automotive industry applications, fluororubber seals are used in drilling machinery, oil refining equipment, natural gas, and desulfurization devices in power plants, and can withstand harsh conditions such as high temperature, high pressure, oil, and strong corrosive media at the same time; In chemical production, fluororubber seals are used in pumps and equipment containers to seal inorganic acids, organic substances, and other chemical substances. In the petroleum and chemical industries, fluororubber sealing products are used for mechanical seals, pumps, reactors, mixers, compressor casings, valves, various instruments, and other equipment. They are commonly used as fillers for valve seats, valve stems, diaphragms, and gaskets. Fluorine rubber is one of the indispensable high-performance materials in modern aviation, missiles, rockets, space navigation, ships, atomic energy and other cutting-edge scientific and technological fields. In recent years, 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 various properties of its vulcanized rubber are described as follows.
(1) Corrosion resistance performance
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 types of rubber.
(2) Swelling resistance
Fluoroelastomer, with high chemical stability, is one of the best media resistant elastomers at present. Type 26 fluororubber is resistant to petroleum based oils, diester oils, silicone ether oils, and silicic acid oils, as well as inorganic acids. It is resistant to most organic and inorganic solvents, drugs, etc. It is only resistant to low molecular weight ketones, ethers, and esters, and is not resistant to amine, ammonia, hydrofluoric acid, chlorosulfonic acid, and phosphate hydraulic oils. The medium performance of Type 23 fluorine adhesive is similar to that of Type 26, and it is more unique. It has better resistance to strong oxidizing inorganic acids such as fuming nitric acid and concentrated sulfuric acid than Type 26. After soaking in 98% nitric acid at room temperature for 27 days, its volume expansion is only 13% to 15%.
(3) Heat resistance and high temperature resistance
Fluorine rubber can be compared to silicone rubber in terms of aging resistance and is superior to other rubbers. Type 26 fluororubber can work for a long time at 250 ℃, and for a short time at 300 ℃. Type 23 fluororubber can work at 200 ℃ × After aging for 1000 hours, it still has high strength and can withstand short-term high temperatures of 250 ℃. The thermal decomposition temperature of tetrafluoroethylene rubber is above 400 ℃, and it can work for a long time at 230 ℃.
The performance change of fluororubber at different temperatures is greater than that of silicone rubber and general butyl rubber, and its tensile strength and hardness significantly decrease with the increase of temperature. The characteristic of the change in tensile strength is: below 150 ℃, it rapidly decreases with the increase of temperature, and between 150~260 ℃, it decreases slowly with the increase of temperature.
The high-temperature resistance of fluororubber, like that of silicone rubber, can be said to be the best among elastomers currently. 26-41 fluorine glue can be used for a long time at 250 ℃ and for a short time at 300 ℃; 246 fluorine glue has better heat resistance than 26-41. At 300 ℃ × Physical properties of 26-41 after 100 hours of air thermal aging and 300 ℃ × After 100 hours of hot air aging, the performance of the 246 type is equivalent, and its elongation at break can be maintained at around 100%, with a hardness of 90-95 degrees. The 246 type maintained good elasticity after 16 hours of hot air aging at 350 ℃, and after 110 minutes of hot air aging at 400 ℃. After 110 minutes of hot air aging at 400 ℃, the elongation of the rubber compound containing spray carbon black, hot cracking carbon black or carbon fiber increased by about 1/2 to 1/3, and the strength decreased by about 1/2, still maintaining good elasticity. 23-11 type fluorine adhesive can be used for a long time at 200 ℃ and for a short time at 250 ℃.
(4) Low temperature resistance
The low temperature performance of fluororubber is poor due to its inherent chemical structure, such as Tg>0 ℃ for 23-11 types. The low-temperature performance of fluororubber used in practice is usually represented by its brittleness temperature and compressive cold resistance coefficient. The formula of the rubber material and the shape of the product (such as thickness) have a significant impact on the brittleness temperature. If the amount of filler in the formula increases, the brittleness temperature sensitivity deteriorates, while the thickness of the product increases, and the brittleness mixing degree also sensitively deteriorates.
The low temperature resistance of fluororubber is generally limited to -15-20 ℃, which can maintain elasticity. As the temperature decreases, its tensile strength increases and appears strong and tough at low temperatures. When used as a sealing element, there is often a problem of low-temperature sealing leakage. The brittleness temperature varies with the thickness of the sample. For example, when the thickness of Type 26 fluororubber is 1.87mm, its brittleness temperature is -45 ℃, when the thickness is 0.63mm, it is -53 ℃, and when the thickness is 0.25mm, it is -69 ℃. Its standard sample, Type 26 fluororubber, has a brittleness temperature of -25~-30 ℃, Type 246 fluororubber has a brittleness temperature of -30~-40 ℃, and Type 23 fluororubber has a brittleness temperature of -45~-60 ℃.
(5) Resistance to hot water and steam
The stability of fluororubber against hot water not only depends on the material itself, but also on the combination of the rubber material. For fluororubber, this performance mainly depends on its vulcanization system. The peroxide vulcanization system is better than amine and bisphenol AF vulcanization systems. The performance of the 26 type fluorine rubber compound using amine vulcanization system is worse than that of general synthetic rubber such as ethylene propylene rubber and butyl rubber.
(6) Compression set performance
The compression deformation of fluororubber used for sealing at high temperatures is its key performance. The widespread application of Viton fluororubber is closely related to its improved compression deformation. It is an important performance that must be controlled as a sealed product. The compression permanent deformation performance of Type 26 fluororubber is better than other fluororubbers, which is one of the reasons why it has been widely used. The compression permanent deformation appears to be significant within the temperature range of 200 to 300 ℃. But in the 1970s, DuPont Company in the United States improved it and developed a low compression permanent deformation rubber material (Viton E-60C), which was improved from the raw rubber variety (Viton A was improved to Viton E-60) and the vulcanization system selection (from amine vulcanization to bisphenol AF vulcanization). This made the compression permanent deformation of fluororubber better when sealed for a long time at high temperatures of 200 ℃, and fluororubber was stored for a long time at 149 ℃, Its sealing retention rate is in a leading position among various types of rubber.
(7) Climate aging resistance and ozone resistance performance
Fluorine rubber has excellent weather aging 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 significant cracking after 45 days of exposure in air with an ozone volume fraction of 0.01%. The weather aging resistance and ozone resistance of Type 23 fluororubber are also excellent.
(8) Mechanical properties
Fluorine rubber generally has high tensile strength and hardness, but poor elasticity. The strength of type 26 fluororubber is generally between 10-20MPa, the elongation at break is between 150-350%, and the tear strength is between 3-4 kN/m. The strength of Type 23 fluororubber is between 15.0 and 25MPa, the elongation is between 200% and 600%, and the tear resistance is between 2 and 7MPa. Generally speaking, fluorine rubber has a large compression set at high temperatures, but if compared under the same conditions, such as from the same compression set at 150 ℃ for the same time, both butadiene and chloroprene rubber have a larger compression set than 26 type fluorine rubber, which is at 200 ℃ × The compression deformation under 24 hours is equivalent to that of butadiene rubber at 150 ℃ × Compression deformation for 24 hours.
(9) Electrical performance
The electrical insulation performance of fluororubber is not very good and is only suitable for use under low frequency and low voltage. The temperature has a significant impact on its electrical performance, and when it rises from 24 ℃ to 184 ℃, its insulation resistance decreases by 35000 times. The electrical insulation performance of Type 26 fluororubber is not very good and is only suitable for low-frequency and low-voltage applications. Temperature has a significant impact on its electrical properties, that is, as the temperature increases, the insulation resistance significantly decreases. Therefore, fluororubber cannot be used as an insulation material at high temperatures. The type and dosage of fillers have a significant impact on the electrical properties. Precipitated calcium carbonate endows the vulcanizate with higher electrical properties, while other fillers are slightly worse. As the dosage of fillers increases, the electrical properties decrease accordingly.
(10) High vacuum resistance
Fluorine rubber has excellent vacuum resistance. This is because fluororubber has a small gas release rate and minimal gas volatilization under high temperature and vacuum conditions. Type 26 and 246 fluororubber can be applied to 133 × l0-9～133 × The ultra-high vacuum environment of 10-10 Pa is an important rubber material in spacecraft. The air permeability of fluororubber is relatively low among rubbers, similar to that of butyl rubber and nitrile rubber. The addition of fillers can reduce the gas permeability of the vulcanizate, and the effect of barium sulfate is more significant than that of medium particle hot cracking method carbon black (MT). The gas permeability of fluororubber increases with temperature, and the solubility of gas in fluororubber is relatively high, but the diffusion rate is very small, which is beneficial for application under vacuum conditions. Fluorine rubber has a relatively high solubility in gases, but its diffusion rate is relatively small, so its overall permeability is also low. According to reports, the permeability of 26 type fluororubber to oxygen, nitrogen, helium, and carbon dioxide gases at 30 ℃ is comparable to that of butyl rubber and butadiene rubber, and better than that of chloroprene rubber and natural rubber. In fluororubber, the addition of fillers fills the gaps inside the rubber, thereby reducing the air permeability of the vulcanizate, which is beneficial for vacuum sealing.
(11) Flame resistance
The flame resistance of rubber depends on the content of halogen in its molecular structure. The higher the halogen content, the better the flame resistance. Fluorine rubber can burn when in contact with flames, but it automatically extinguishes after leaving the flame, so it belongs to the self extinguishing type of 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 the cracking and structuring of fluororubber. The radiation resistance of fluororubber is a relatively poor type of elastomer. Type 26 rubber exhibits crosslinking effect after radiation, while type 23 fluororubber exhibits cracking effect. 246 type fluororubber radiates at room temperature in the air at 5 × At a dose of 107 rem, the performance undergoes significant changes at 1 × Under the condition of 107 coulombs, the hardness increases by 1-3%, the strength decreases by less than 20%, and the elongation decreases by 30-50%. So, it is generally believed that 246 fluororubber can withstand 1 × 107 Luns, with a limit of 5 × 107 lun.
4. Application examples
(1) Oil resistant FKM mixture
Viton seals are particularly suitable for environments requiring high heat temperatures and high chemical stability. A dedicated FKM mixture has been developed for special environments such as high additives and lubricants. When selecting the suitable material for the sealing element, in addition to considering the temperature range of the sealing element, it is also necessary to consider the properties of the liquid or gas in contact with it. The expansion or contraction of elastomer and chemical stability are important factors affecting the stability of seals.
These materials can withstand temperatures up to 200 ℃, depending on the polymer structure and crosslinking system. Diamines, bisphenols, or peroxides can cause cross-linking. The fluorine content determines the chemical stability. The higher the fluorine content, the more resistant FKM materials will be to highly aggressive environments.
For public vehicles and mobile machinery, sealing materials need to meet extremely harsh application conditions for temperature and surrounding media. Due to the high content of oil additives and the increasing working temperature, the requirements for temperature and chemical stability are becoming more and more stringent. The new generation of long-life oils for engines and drivers can extend their service life by over a thousand hours. This type of oil is mainly mineral oil with a high content of amine additives.
(2) Alkali resistant FKM material
Due to the excellent technical performance of fluororubber, its application fields are constantly expanding, and there are more and more types of products. The main products include adhesive tape, tape, hose, film, and impregnation products. Various rubber pipes and composite rubber pipes are made of fluororubber, which are used for oil pipelines, high-temperature and high-pressure hydraulic rubber pipes, air ducts, and hot liquid ducts, as well as various sealing materials. The diaphragm in pumps and valves made of fluororubber as a corrosion-resistant medium is widely used in special fields. Applying fluororubber slurry to glass fiber cloth, polyester fiber cloth, and other textiles can be made into fire-resistant containers, high-temperature gaskets, non combustible adhesive tapes, protective clothing, and protective gloves. Insulation materials are mainly used as sheaths for cables and wires that are resistant to high temperature, oil, and pressure. The fluorine rubber used to manufacture asbestos paper board can replace other rubbers to manufacture asbestos paper board. It has high temperature and high pressure resistance properties and can also be used as flange gaskets for crop material pipes. It is widely used in fields such as chemical engineering, light industry, and machine manufacturing.
(3) Fluorine rubber products and their applications
Fluorine rubber can be used as a sealing material to make various types of gaskets, valve sealing gaskets, O-shaped sealing rings, V-shaped sealing rings, leather cups, oil seals, and corrugated connecting pipes. These products can withstand temperatures above 200 ℃ and do not deform in various oil media environments. Fluoroelastomer sealing materials are mainly used in the fields of automobile and aerospace in China. At present, the main products of fluororubber materials used for automobile parts in China include engine crankshaft front oil seal, crankshaft rear oil seal, valve cylinder oil seal, engine diaphragm, engine cylinder liner waterstop, refueling hose, fuel hose, oil filter one-way valve, filler cap O-ring, gearbox and reducer oil seal, etc. Fluorine rubber, silicone rubber, acrylic rubber, and heat-resistant elastomers have become the development trend and mainstream of rubber materials for future automobiles. Many automotive components use better performance fluororubber to replace traditional materials.
When fluororubber seals are used for sealing automotive engines, they can operate for a long time at temperatures ranging from 200 ℃ to 250 ℃, and their working life can be the same as that of engine repair; When used in the chemical industry, it can seal inorganic acids (such as 67% sulfuric acid at 140 ℃, 70% concentrated hydrochloric acid at 70 ℃, and 30% nitric acid at 90 ℃), organic solvents (such as chlorinated hydrocarbons, benzene, high aromatic gasoline), and other organic compounds (such as butadiene, styrene, propylene, phenol, fatty acids at 275 ℃, etc.); When used for deep well oil recovery, it can withstand harsh working conditions of 149 ℃ and 420 atmospheres; When used for superheated steam seals, it can work for a long time in a steam medium of 160~170 ℃. In the production of monocrystalline silicon, fluorine rubber seals are commonly used to seal special media such as trichlorosilane, silicon tetrachloride, gallium arsenide, phosphorus trichloride, trichloroethylene, and 120 ℃ hydrochloric acid at high temperatures (300 ℃).
The rubber hose made of fluororubber is suitable for high-temperature, oil-resistant, and special medium resistant situations. Wires and cables made of fluororubber have good flexibility and insulation. Glass fiber adhesive made of fluororubber can withstand high temperatures of 300 ℃ and chemical corrosion. After being coated with fluorine adhesive, aramid cloth can be used to make connecting expansion pipes between high-temperature, acid and alkali resistant storage tanks in petrochemical plants. It can withstand high pressure, high temperature, and medium corrosion, and has a buffering and shock-absorbing effect on the deformation and expansion of the two tanks. The sealing bag made of nylon cloth coated with fluorine glue is used as a soft seal for internal floating roof storage tanks in refineries, playing a role in sealing and reducing the evaporation loss of oil level.
Type 23 and tetrapropyl fluororubber are mainly used for acid and special chemical resistant corrosive sealing occasions. Hydroxy nitroso fluororubber is mainly used as protective and sealing products, as a non combustible coating in solution form, and is applied to fire-resistant electronic components and components working in pure oxygen. Its solution and liquid rubber can be sprayed, poured, and other methods to manufacture many products, such as spacesuits, gloves, pipe straps, balls, etc. It can also be used as an adhesive for glass, metal, and fabrics, and to manufacture various sealing components such as sponges and gaskets, O-rings, capsules, and valves that come into contact with rocket propellants.
The seals made of G-type series fluororubber have high temperature vapor resistance, methanol gasoline resistance, or high aromatic gasoline resistance that cannot be achieved using Viton A, B, E and other fluororubbers;
GLT fluororubber, chlorinated phosphorus rubber, perfluoroether rubber, etc. have a wide temperature range, low tenderness, softness, and elastic sealing properties. Perfluoroether rubber also has outstanding resistance to medium corrosion and is widely used in cutting-edge military technology.
The sealant made of fluororubber - putty, with outstanding fuel oil resistance, can be used in oil at around 200 ℃ and is used as a sealing material for the overall fuel tank of aircraft.
The closed cell sponge made of fluororubber has acid resistance, oil resistance, a wide temperature range of use, and good insulation. It can be used as a sealing material for rocket fuel, solvents, hydraulic oil, lubricants, and ointments, as well as a shock absorber material for rockets and missiles, with a temperature resistance of up to 204 ℃.
Asbestos fiber cloth impregnated with fluorine rubber lotion can be made into asbestos rubber sheet, which is used in occasions with high temperature resistance, combustion resistance and chemical corrosion resistance.
