The seal of fluid (gas, liquid) is a general technology necessary in various industrial fields, not only the industry of construction, petrochemical, shipbuilding, machinery manufacturing, energy, transportation and environmental protection can not be separated from sealing technology, aviation, aerospace and other cutting-edge industries are closely related to sealing technology. Sealing technology is widely used in the field of storage, transportation and energy conversion of fluid.

The importance of sealing technology the result of sealing failure is very serious, light "run, take, drop, leak" caused by waste of energy and resources, the heavy will make the operation failure, or even produce fire, explosion, environmental pollution and other consequences endanger personal safety! According to the preliminary statistics, more than 1/3 of the quality accidents of mechanical equipment and equipment are caused by sealing failure, which has serious consequences, such as: aircraft destruction caused by the failure of the USS USS Challenger's propeller O-ring; The Russian Soyuz spacecraft fuel tank leaked several astronauts to death. The leakage of China's Dongfanghong third spacecraft could not work properly after liftoff. This is why the machinery manufacturing enterprises at the cost of choosing high-quality sealing products.

The development of sealing materials with the development of science and technology, sealing structure of the working conditions more stringent. Due to the sealing of the fluid temperature, pressure and corrosion greatly improved, the traditional sealing materials such as blankets, hemp wire, asbestos wire, putty, etc. can not meet the requirements of the use of rubber and other synthetic materials are gradually replaced. Rubber and other synthetic materials are polymers, in the macromolecular chain with different characteristics of functional groups (such as: chlorine, fluorine, cyanide, vinyl, isocyanate, hydroxyl, carboxyl, alkane, etc.) become active cross-linking point. Under the action of Catalyst, vulcanizing agent, or high temperature and high-energy ray, Macromolecule is transformed into spatial network structure by linear structure and supporting structure, this process is called vulcanization. After vulcanization of rubber or other synthetic materials, the macromolecule loses its original fluidity, which is called elastic body with high elastic deformation. Commonly used rubber and synthetic materials are: natural rubber, SBR, neoprene, butyl cyanide rubber, ethylene propylene rubber, butyl rubber, polyurethane rubber, acrylate rubber, fluorine rubber, silicone rubber and so on.

Performance index for determining the quality of sealing materials

1. Tensile properties

Tensile properties are the first properties to be considered for sealing materials, including: tensile strength, stretching stress, elongation at break and permanent deformation of the rupture. Tensile strength is the maximum stress of specimen stretching to fracture. The constant elongation stress (the definite extension modulus) is the stress that is achieved at the specified extension. The elongation rate is the deformation caused by the specified tensile of the specimen, and the ratio of elongation to the original length. The elongation at break is the elongation at which the specimen is pulled. The permanent deformation is the residual deformation between the markings after tensile fracture of the specimen.

2. Hardness

Hardness indicates that the sealing material is resistant to external pressure and is also one of the basic properties of sealing material. The hardness of the material to some extent related to other properties, the higher the hardness relative to the greater the strength, elongation, the better wear resistance, and low temperature performance.

3. Compression performance

Rubber seals are usually in compression, due to the viscoelastic rubber materials, compression pressure will decrease with time, the compression stress relaxation, after removing the pressure can not return to the original shape, manifested as compression permanent deformation. This phenomenon is more evident in high temperature and oil medium, and the performance is directly related to the durability of sealing capability of sealing products.

4. Low temperature performance

A metric used to measure the low temperature characteristics of rubber seals, the following two methods of testing low-temperature performance: 1 Low temperature shrinking temperature: the sealing material stretching to a certain length, and then fixed, quickly cooled to the freezing temperature below, to achieve balance after release of the test, and to a certain speed, record style back shrink 10%, 30%, 50% and 70% temperature respectively in TR10, TR30, TR50, TR70 indicated. The material standard takes TR10 as the index, it is related to the brittle temperature of rubber. Low-temperature flexure: The specimen at the specified low temperature frozen to the specified time, reciprocating bending according to the specified angle, to investigate the seal at low temperature after the repeated action of the dynamic load of the sealing capacity.

5. Oil resistance or dielectric properties

In addition to contact with oil base, double ester, silicic acid oil, in the chemical industry sometimes also contact acid, alkali and other corrosive media. In these medium, in addition to corrosion, in the high temperature will also lead to swelling and strength reduction, hardness reduction, while the sealing materials in the plasticizer and soluble substances are taken out, resulting in quality reduction, volume reduction, causing leakage. Generally at a certain temperature, after soaking in the medium for some time to determine the quality, volume, strength, elongation, hardness of the changes to assess the sealing material oil resistance or resistance to media performance.

1. Aging Resistance

Sealing materials by oxygen, ozone, heat, light, moisture, mechanical stress will cause deterioration in performance, known as sealing materials aging. Aging resistance (also become weather resistance) can be used after aging style of strength, elongation, hardness changes to indicate that the smaller the rate of change, the better anti-aging performance.

Note: Weather resistance refers to the plastic products due to sunlight, temperature changes, rain, and other external conditions, and the appearance of fading, discoloration, cracking, powder and strength decline, such as a series of aging phenomenon. The ultraviolet irradiation is the key factor to promote the aging of plastics.

Valve commonly used seals material introduction:

1, Butyl cyanide rubber (NBR) is butadiene and propylene cyanide monomer by emulsion polymerization synthesis of the irregular copolymer, the molecular structure of the formula:-(CH2-CH=CH) m (ch2-ch2-ch) N-CN

Butyl cyanide rubber was first developed in Germany in 1930, is butadiene and 25% propylene cyanide copolymer, because of its anti-aging, heat, wear resistance is superior to natural rubber, so the rubber industry attention. During the Second World War, with the rapid development of weaponry and equipment, the demand for heat-resistant, oil-butadiene rubber as war materiel increased dramatically. So far more than 20 countries to produce butyl cyanide rubber, annual output of 560,000 tons, accounting for the world's total synthetic rubber 4.1%. Due to excellent heat resistance, oil and mechanical properties, has now become the main oil-resistant rubber products, accounting for all oil-resistant rubber demand for about 80%.

Butyl cyanide rubber in the 1950s made a great development, so far more than 300 brands, in accordance with the propylene Cyanide content division, in the 18%~50% propylene cyanide content can be divided into: Propylene cyanide content >42% for very high cyanide grade, 36%~41% high cyanide grade, 31%~35% for medium and high cyanide grade, 25% ~30% for the cyanide grade, below 24% for low cyanide grade. The largest industrial use is the low cyanide grade of 18 (combined with the propylene cyanide content 17%~20%), cyanide-grade 26 (combined with propylene cyanide content 27%~30%), high cyanide-grade cyanide-40 (combined with cyanide content 36%~40%). The increase of propylene cyanide content can significantly improve the oil and heat resistance of butadiene rubber, but it is not the better, because the increase of propylene cyanide content will reduce the low temperature performance of rubber.

Butyl cyanide rubber is mainly used in the manufacture of petroleum based hydraulic oil, lubricants, kerosene and gasoline work of rubber products, its operating temperature of 50-100 degrees; short-term work can be used for 150 degrees, in the air and ethanol glycerin antifreeze solution operating temperature of 45-100 degrees. Butyl cyanide anti-aging resistance, when the high concentration of ozone will rapidly aging cracking, not suitable for long-term work in high temperature air, also can not in the phosphate ester fire-resistant hydraulic oil work.

General physical properties of NBR Rubber: 1 butyl cyanide rubber is generally black, color can be adjusted according to customer needs, but must increase some costs, and may affect the use of rubber effect. 2 Butyl cyanide rubber has a slight smelly egg to ask. 3 According to the oil resistance of butyl cyanide rubber and the use of temperature range to determine whether the sealing material is butyl cyanide rubber.

2. Silicone Rubber (SI or vmq) is a linear polymer with a silicon-oxygen bond Unit (-SI-O-SI) as the main chain and an organic base as the side base.

Due to the development of aviation, aerospace and other cutting-edge industries, there is an urgent need for high-temperature and low-temperature-resistant rubber sealing materials. Early use of natural, butyl cyanide, neoprene and other general rubber can not meet the needs of industrial development, so in the early 1940s the United States two companies began to put into production two methyl silicone rubber, is the earliest silicone rubber. Our country also studied successfully in the early 1960s and put into production. After several decades of development, silicone varieties, performance and production have been greatly developed.

Main characteristics of Silica gel:

1 Heat-resistant silica gel high temperature stability performance. Can be used for a long time under 150 ℃, the performance will not be significantly changed, under 200 ℃ can work for more than 10,000 hours, or even at 350 ℃ high temperature for short periods.

2 cold resistance and low phenyl silica gel and the -70℃-60 ℃ and the cold coefficient are more than 0.65 with good low-temperature elasticity. And the general use of silica gel temperature is 50 ℃.

3 oil resistance and chemical resistance of silicone to ethanol, acetone and other polar solvents and food oil, such as tolerance is very good, only cause a very small expansion, mechanical properties will not be reduced; silica gel for low concentrations of acid, alkali, salt tolerance is also better, in 10% sulfuric acid solution placed 7 days, volume change rate is less than 1%, The mechanical properties are basically unchanged. But silicone is not resistant to concentrated sulfuric acid, concentrated alkali, carbon tetrachloride and toluene and other non-polar solvents.

4 Aging Resistance, silica gel has obvious ozone resistance and anti-radiation ability is not comparable to ordinary rubber.

5 Dielectric Properties Silica Gel has a high volume resistivity (1014~1016ω cm) and its resistance values remain stable in a wide range. Suitable for insulating materials under high pressure conditions.

6 Flame Retardant Performance silicone fire will not immediately burn, and its combustion produces less toxic gas, after the combustion of the product will form insulating ceramics have been, so the silica gel is also an excellent flame retardant materials.

Combined with the above characteristics, silica gel is widely used in the household appliances industry used in seals or rubber parts, such as electric kettle, scald bucket, the rubber parts in the microwave oven; seals or rubber parts in the electronics industry, such as mobile phone buttons, shock absorbers in DVD, seals in cable connectors, etc. seals on all kinds of supplies that are in contact with the human body , such as kettle, water dispenser and so on.

3. Fluorine Rubber (FKM or vtion), also known as Fluorine elastomer, is a polymer containing fluorine atoms on the main and side-chain carbon atoms.

Since the early 1950s, the United States and the former Soviet Union began to develop fluorine-containing elastomers. First put into production is the United States DuPont and 3M Company's Vtiona and Kel-f after half a century of development, fluorine elastomer in heat, Medium, low-temperature and technology and other aspects of the rapid development, and form a series of products.

Fluorine adhesive has excellent heat resistance, ozone and a variety of hydraulic oil performance. Its operating temperature in the air is-40~250℃, the operating temperature in the hydraulic oil is-40~180℃. As a result of fluorine adhesive processing, bonding and low-temperature performance than general rubber, the price is also more expensive, so much for general rubber can not be competent in high-temperature media, but not suitable for certain phosphate ester solution.

4. EPDM is a ternary copolymer of ethylene, propylene and a small number of non conjugated diene olefins.

1957 Italy to achieve ethylene, propylene two Yuan copolymer rubber (two yuan ethylene-propylene rubber) of the industrialized production. 1963 DuPont in the United States on the basis of two Yuan EPDM added a small number of non-conjugated annular diene as the third monomer, the molecular chain with a double bond of the low unsaturated EPDM. Because the molecular main chain is still saturated, EPDM has maintained the outstanding performance of two-C, and achieved the purpose of vulcanization.

EPDM rubber has excellent ozone resistance, in the ozone concentration of 1*10-6 environment 2,430 hours still not cracked; good corrosion resistance: good stability for alcohols, acids, alkalis, oxidants, detergents, animal and vegetable oils, ketones, and certain lipids (but in oil-based fuels, hydraulic fluids are swollen, Can not be in contact with mineral oil in the environment to work), heat-resistant properties of the good, 60~120℃ temperature in the long-term use, with good water resistance and electrical insulation capability.

EPDM rubber is beige, excellent elasticity.

5. Polyurethane Elastomer (PU) is made of poly (isocyanate) and polyether polyol or polyester polyol or/and small molecular polyols, polyamine or water, such as chain-extender or cross-linking agent, such as polymer. In 1937, Professor Otto Bayer of Germany first discovered that poly (isocyanate) and polyol compounds could be used to make polyurethane, and the basis of which was to enter into industrialized applications. Polyurethane elastomer using temperature range in -45℃-110℃, can be in a wide range of hardness with high elasticity and strength, excellent wear resistance, oil-resistant, fatigue resistance and vibration resistance, especially for lubricants and fuel oil have good anti swelling performance, with "wear-resistant rubber" said.

Polyurethane Elastomer has excellent comprehensive performance, has been widely used in metallurgy, petroleum, automotive, mineral processing, water conservancy, textile, printing, medical, sports, food processing, construction and other industrial sectors.

6. PTFE (PTFE)

PTFE (abbreviated as Teflon or [Ptfe,f4]), praised by the United States as/commonly known as "Plastic King", the Chinese product name "Teflon", "Teflon" (Teflon), "Teflon", "Special Fu Long", "Teflon" and so on. It is a polymer made of tetrafluoroethylene by polymerization, with excellent chemical stability, corrosion resistance (is one of the best corrosion-resistant materials in the world today, in addition to molten metal sodium and liquid fluorine, the ability to all other chemicals, in aqua regia, boiling also does not change, widely used in all kinds of need to resist acid-base and organic solvents), Sealing, high lubrication viscosity, electrical insulation and good anti-aging endurance, excellent temperature (can be +250℃ to -180℃ temperature of long-term work). PTFE it itself is not toxic to humans, but one of the raw materials used in the production process (PFOA) is believed to have carcinogenic effects.

Temperature -20~250℃ ( -4~+482°f) allows for sudden cold or hot or cold alternating operation.

Pressure -0.1~6.4mpa (full negative pressure to 64kgf/cm2) (fully vacuum to 64kgf/cm2)

Advantages：

High temperature-use operating temperature up to 250 ℃.

Low temperature resistance-good mechanical toughness, even if the temperature drops to 196 ℃, can also maintain 5% elongation.

Corrosion resistance-for most chemicals and solvents, it shows inertia, strong strong acid strong alkali, water and various organic solvents.

Weather resistant-The best aging life in plastics.

High lubrication-is the lowest friction coefficient in solid materials.

Disadvantage: poor elasticity.