In vacuum sealing, it is very common to use PTFE material as a sealing ring, which is mainly determined by its physical and chemical properties. Polytetrafluoroethylene is a polymer of tetrafluoroethylene.

Polytetrafluoroethylene is widely used in various acid, alkali and organic solvents. It is non-toxic to the human body. However, perfluorooctanoate (PFOA), a raw material used in the production process, is believed to be carcinogenic.

The relative molecular mass of polytetrafluoroethylene is relatively large, with a low of several hundred thousand, a high of more than 10 million, and generally several million (the degree of polymerization is on the order of 104, while that of polyethylene is only 103). Generally, the crystallinity is 90-95%, and the melting point temperature is 327-342°C. The CF2 units in the polytetrafluoroethylene molecule are arranged in a zigzag pattern. Since the radius of the fluorine atom is slightly larger than the radius of the hydrogen atom, adjacent CF2 units cannot completely cross in trans, but form a helical twisted chain, which is almost covered by fluorine atoms. The surface of the entire polymer chain. This molecular structure explains the various properties of polytetrafluoroethylene. When the temperature is lower than 19°C, a 13/6 helix is formed; at 19°C, a phase change occurs, and the molecules spread out slightly to form a 15/7 helix.

Although the rupture of the carbon-carbon bond and the carbon-fluorine bond in the perfluorocarbon compound needs to absorb energy of 346.94 and 484.88kJ/mol, respectively, the depolymerization of PTFE to produce 1mol of tetrafluoroethylene only needs 171.38kJ of energy. Therefore, in the high-temperature cracking process, polytetrafluoroethylene is mainly depolymerized into tetrafluoroethylene. At 260℃, 370℃ and 420℃, the weight loss rate (%) of PTFE is 1×10-4, 4×10-3 and 9×10-2 /h, respectively. It can be seen that PTFE can be used for a long time at 260°C. Because high-temperature cracking will produce highly toxic by-products such as fluorophosgene and perfluoroisobutylene, special attention must be paid to safety protection to prevent contact between PTFE and open flames.

Its friction coefficient is small, only 1/5 of polyethylene, which is an important feature of perfluorocarbon surface. In addition, due to the low intermolecular forces of the fluorocarbon chain, PTFE is not sticky.

PTFE maintains excellent mechanical properties in a wide temperature range of -196 to 260°C. One of the characteristics of perfluorocarbon polymers is that they are not brittle at low temperatures.

Chemical resistance and weather resistance Except for molten alkali metals, PTFE is hardly corroded by any chemical agents. For example, in concentrated sulfuric acid, nitric acid, and hydrochloric acid, even if boiled in aqua regia, its weight and performance have no change, it is almost insoluble in all solvents, and only slightly soluble in all alkanes (about 0.1g/100g) above 300°C. PTFE does not absorb moisture, is not flammable, and is very stable to oxygen and ultraviolet rays, so it has excellent weather resistance.

PTFE has a lower dielectric constant and dielectric loss in a wide frequency range, and has a higher breakdown voltage, volume resistivity and arc resistance.

The radiation resistance of PTFE is poor (104 rad), it will degrade under high-energy radiation, and the electrical and mechanical properties of the polymer will be significantly reduced.

Polytetrafluoroethylene is formed by radical polymerization of tetrafluoroethylene. Industrial polymerization is agitated in the presence of a large amount of water to disperse the heat of reaction and facilitate temperature control. The polymerization is generally carried out at 40-80°C and a pressure of 3-26 kgf/cm2. Inorganic persulfates and organic peroxides can be used as initiators, and redox initiation systems can also be used. Per mole of tetrafluoroethylene polymerization releases 171.38kJ of heat. Dispersion polymerization must add perfluorosurfactant, such as perfluorooctanoic acid or its salts.

Polytetrafluoroethylene can be formed by compression or extrusion; it can also be made into water dispersant for coating, impregnation or fiber making. PTFE is widely used in nuclear energy, aerospace, electronics, electrical, chemical, machinery, instrumentation, construction, textile, food and other industries as high and low temperature resistant, corrosion resistant materials, insulation materials, anti-stick coatings, etc.

Chemical nature

Corrosion resistance: can withstand the action of all strong acids (including aqua regia), strong oxidants, reducing agents and various organic solvents, except for molten alkali metals, fluorinated media and sodium hydroxide above 300°C.

Insulation: Not affected by the environment and frequency, the volume resistance can reach 1018 ohm•cm, the dielectric loss is small, and the breakdown voltage is high.

High and low temperature resistance: small influence on temperature, wide temperature range, applicable temperature -190~260℃.

Self-lubricating: The coefficient of friction between plastics is small, making it an ideal oil-free lubricating material.

Surface non-stickiness: It is known that solid materials cannot adhere to the surface and are solid materials with the smallest surface energy. Weather resistance, radiation resistance and low air permeability: long-term exposure to the atmosphere, the surface and performance remain unchanged.

Non-combustibility: The oxygen limit index is lower than 90. The mechanical properties of PTFE are relatively soft. The surface energy is very low.

Polytetrafluoroethylene sealing ring PTFE (polytetrafluoroethylene) composite material is a kind of composite material, which contains two or more different properties or contains PTFE materials in different stages. Polymerized or combined together, its performance is better than its constituent materials. New composite materials. Through material compounding, it not only has the excellent characteristics of PTFE, but also overcomes the shortcomings of cold flow creep and the resistance of non-wear PTFE, making it anti-friction, wear-resistant, self-lubricating, corrosion-resistant, and resistant. High temperature, anti-aging and abrasion resistance. Compression resistance, creep resistance, dimensional stability, low linear expansion coefficient, etc., and improve thermal conductivity and hardness, reduce costs. The new composite material has been used for sealing under special working conditions such as high pressure, high temperature, high speed, high vacuum, flammable, explosive, toxic, and highly corrosive media at home and abroad. These seals are widely used in petrochemicals, fertilizers, pesticides, aviation, aerospace and nuclear industries, and are often used as seals for reciprocating axial movement, such as seals for pistons and piston rods. It can be used for dry friction and oil-free lubrication occasions, and it can also be used for oil lubrication when conditions permit.

In order to enhance the elasticity of the PTFE composite seal, it must be structurally combined with elastomers, compression rings, convex valve seats, concave valve seats and various forms of metal springs to form an elastic drive seal. Commonly used is a metal spring drive, which is to place a spiral plane spring or a round wire spring in the annular space processed or formed in the sealing ring to ensure that the sealing ring can produce a high degree of elasticity and wear. Automatic compensation to form an excellent dynamic seal.

2.1 Type J and Type L PTFE seals

J-type and l-type sealing rings often add graphite, glass fiber, bronze, molybdenum disulfide and various metal oxides to PTFE to improve and enhance the various properties of PTFE. It is characterized by low friction and can withstand dry friction; low wear rate of lip surface and shaft surface; applicable temperature range -50~200℃; oil and corrosion resistance; good sealing performance. For example, hydraulic pistons and pneumatic pistons use l-shaped sealing rings made of PTFE and glass fiber. If 15% glass fiber is added, the coefficient of thermal expansion can be reduced by 1/2. L-shaped sealing ring can be used as one-way or two-way piston seal.

The j-shaped seal ring is used as a seal for the piston rod, and is generally installed on the housing as a fixed seal to seal the reciprocating shaft. The bottom of the sealing ring is stuck in the groove of the housing and is supported by the gland. The end of the lip always faces the positive pressure side, the inner diameter of the lip is in contact with the shaft, and the seal is realized by the principle of self-tightening, and the lip can move freely along the shaft surface.