Mechanical and chemical properties of polytetrafluoroethylene (Teflon)

Polytetrafluoroethylene (Teflon or PTFE), has excellent chemical stability, corrosion resistance, sealing, high lubrication non-stick, electrical insulation and good anti-aging resistance. Mainly used in high performance requirements of corrosion-resistant pipes, containers, pumps, valves, as well as radar, high-frequency communication equipment, radio equipment, etc.

Material properties

Density：2.2 g/cm3

Young's modulus：0.5 Gpa

Yield strength：23 Mpa

Melting point：~327 °C

Polytetrafluoroethylene [PTFE,F4] is one of the best materials in the world today in terms of corrosion resistance, hence the name of "King of Plastics". It can be used in any kind of chemical medium for a long time, and its creation has solved many problems in the fields of chemical industry, petroleum and pharmaceuticals in China. Teflon seals, gaskets, gaskets. PTFE seals, gaskets and sealing washers are made of suspended polymerized polytetrafluoroethylene resin. PTFE has excellent chemical resistance and temperature resistance compared to other plastics, and it has been widely used as sealing and filling materials.  Polytetrafluoroethylene is a polymer of tetrafluoroethylene. Its trade name is "Teflon" (teflon). It is known as the "King of Plastics". The basic structure of polytetrafluoroethylene is. - CF2 - CF2 - CF2 - CF2 - CF2 - CF2 - CF2 - CF2 - CF2 - CF2 - CF2 - CF2 -. Teflon is widely used in a variety of applications that require resistance to acids, bases and organic solvents. It is not toxic to humans, but ammonium perfluorooctanoate (PFOA), one of the raw materials used in the production process, is thought to have a carcinogenic effect.

Polytetrafluoroethylene (Teflon or PTFE), commonly known as "plastic king", Chinese trade names "Teflon", "Teflon", "Teflon", "Teflon", "Teflon", "Teflon", "Teflon". Teflon", "Teflon", "Teflon" and so on. It is a polymer compound made of tetrafluoroethylene by polymerization, which has excellent chemical stability, corrosion resistance, sealing, high lubrication and non-stick, electrical insulation and good anti-aging resistance. It can work under the temperature from +250℃ to -180℃ for a long time, and can resist all other chemicals except molten metal sodium and liquid fluorine, and does not change when boiled in aqua regia.

Used as engineering plastics, can be made into PTFE tubes, rods, strips, plates, films, etc.. Generally used in high performance requirements of corrosion-resistant pipes, containers, pumps, valves, as well as the system of radar, high-frequency communication equipment, radio equipment. Dispersion can be used as insulation impregnation solution of various materials and anti-corrosion layer on the surface of metal, glass and pottery, etc. Various PTFE rings, PTFE gaskets, PTFE packing, etc. are widely used in various types of anti-corrosion pipeline flange sealing. In addition, it can also be used for drawing, PTFE fiber - fluorine spandex (foreign trade name Teflon).

At present, all kinds of PTFE products have played a pivotal role in the national economy such as chemical industry, machinery, electronics, electrical appliances, military industry, aerospace, environmental protection and bridges.

PTFE industry Chemical, petrochemical, oil refining, chlor-alkali, acid production, phosphate fertilizer, pharmaceutical, pesticide, chemical fiber, dyeing, coking, gas, organic synthesis, non-ferrous smelting, steel, atomic energy and high-purity products production (such as ion membrane electrolysis), viscous material conveying and operation, food, beverage and other processing production departments with highly stringent health requirements.

Media Hydrofluoric acid, phosphoric acid, sulfuric acid, nitric acid, hydrochloric acid, various organic acids, organic solvents, strong oxidizing agents and various other strong corrosive chemical media.

Temperature -20～250℃, allow sudden cold and hot, or alternate hot and cold operation.  -20～250℃（-4～＋482°F）

Pressure -0.1～6.4Mpa （Full vacuum to 64kgf/cm2） -0.1～6.4Mpa （Full vacuum to 64kgf/cm2） High temperature resistance--using working temperature up to 250℃.

Low temperature resistance - has good mechanical toughness; even if the temperature drops to -196℃, it can maintain 5% elongation.  Corrosion resistance - to most chemicals and solvents, showing inertness, can resist strong acids and bases, water and various organic solvents.  Weather resistant - has the best aging life among plastics.  High lubrication - has the lowest coefficient of friction of any solid material.

Non-adhesive - has the lowest surface tension among solid materials and does not adhere to any substance.

Non-toxic - physiologically inert, no adverse reaction to long-term implantation as artificial blood vessels and organs.

Polytetrafluoroethylene relative molecular mass is large, low hundreds of thousands, high up to more than 10 million, generally millions (degree of polymerization in the order of 104, while polyethylene only in 103). The general crystallinity is 90-95%, and the melting temperature is 327-342℃. The CF2 units in the polytetrafluoroethylene molecule are arranged in a sawtooth shape. Since the radius of the fluorine atom is slightly larger than that of the hydrogen, the adjacent CF2 units cannot be fully cross-orientated in trans, but form a helical twisted chain, with the fluorine atom covering almost the entire surface of the polymer chain. This molecular structure explains the various properties of polytetrafluoroethylene. At temperatures below 19°C, a 13/6 helix is formed; at 19°C a phase transition occurs and the molecule unravels slightly, forming a 15/7 helix.

Although the breakage of carbon-carbon and carbon-fluorine bonds in perfluorocarbons requires the absorption of energy 346.94 and 484.88 kJ/mol, respectively, the depolymerization of polytetrafluoroethylene to generate 1 mol of tetrafluoroethylene requires only 171.38 kJ of energy. so at high temperature cracking, polytetrafluoroethylene mainly depolymerizes to tetrafluoroethylene. The weight loss rates (%) of PTFE at 260, 370 and 420°C are 1×10-4, 4×10-3 and 9×10-2 per hour, respectively. it can be seen that PTFE can be used at 260°C for a long time. Since high temperature cracking also produces highly toxic by-products such as fluorophosgene and perfluorinated isobutene, special attention should be paid to safety protection and to prevent PTFE from contact with open flame.

Mechanical properties Its coefficient of friction is extremely small, only 1/5 of that of polyethylene, which is an important feature of perfluorocarbon surfaces. Because the intermolecular force between fluorine and carbon chains is extremely low, PTFE has non-stick properties.

It does not melt at a temperature of 250°C and does not become brittle in the ultra-low temperature of -260°C. Teflon is so smooth that even ice can't compare with it; its insulating property is especially good, and a film of newspaper is thick enough to resist the high voltage of 1500V.

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

Chemical corrosion and weathering resistance In addition to molten alkali metals, PTFE is not subject to corrosion by almost any chemical reagents. For example, in concentrated sulfuric acid, nitric acid, hydrochloric acid, and even boiling in aqua regia, its weight and performance are not changed, and almost insoluble in all solvents, only slightly soluble in full alkane (about 0.1g / 100g) above 300 ℃. Polytetrafluoroethylene does not absorb moisture, non-combustible, extremely stable to oxygen, ultraviolet light, so it has excellent weather resistance.  Electrical properties PTFE has low dielectric constant and dielectric loss in a wide range of frequencies, and high breakdown voltage, volume resistivity and arc resistance.

Radiation resistance The radiation resistance of polytetrafluoroethylene is poor (104 rads), degradation caused by high-energy radiation, the electrical and mechanical properties of the polymer are significantly reduced.

Polymerization Polytetrafluoroethylene is produced by the free radical polymerization of tetrafluoroethylene. The industrial polymerization reaction is carried out in the presence of large amounts of water with stirring to disperse the heat of reaction and to facilitate temperature control. Polymerization is generally carried out at 40 to 80°C, 3 to 26 kg force/cm2 pressure, and can be initiated by inorganic peroxides and organic peroxides, or by redox initiation systems. The polymerization of tetrafluoroethylene exerts 171.38 kJ of heat per mole, and the dispersion polymerization is subject to the addition of perfluorinated surfactants, such as perfluorooctanoic acid or its salts.

Applications Teflon can be molded by compression or extrusion; it can also be made into aqueous dispersions for coating, impregnation or fiber making. PTFE is widely used in the industries of atomic energy, aerospace, electronics, electrical, chemical, machinery, instruments, meters, construction, textile, food, etc. as high and low temperature resistant, corrosion resistant materials, insulating materials, anti-adhesive coating, etc.

Atmospheric aging resistance: irradiation resistance and low permeability: long-term exposure to the atmosphere, the surface and performance remains unchanged.  Non-combustibility: limited oxygen index below 90.

Acid and alkali resistance: insoluble in strong acids, strong alkalis and organic solvents.  Oxidation resistance: Resistant to corrosion by strong oxidizing agents.  Acid-base: neutral.

Polytetrafluoroethylene has soft mechanical properties. Has a very low surface energy.

Polytetrafluoroethylene (F4, PTFE) has a series of excellent performance: high temperature resistance - long-term use temperature 200 ~ 260 degrees, low temperature resistance - still soft at -100 degrees; corrosion resistance - can resist aqua regia and all organic solvents; weather resistance - The best aging life in plastic; high lubrication - with the smallest coefficient of friction in plastic (0.04); non-stick - with the smallest surface tension in solid materials without adhering to any material; non-toxic - with physiological inertia; excellent electrical properties, is the ideal C-class insulation materials.

Uses: PTFE material, widely used in national defense military, atomic energy, petroleum, radio, electric machinery, chemical industry and other important sectors. Products: Teflon rods, tubes, sheets, and turned sheets. Polytetrafluoroethylene is a polymer of tetrafluoroethylene. The structural formula is PTFE, which was discovered in the late 1930s and put into industrial production in the 1940s.

Properties The relative molecular mass of polytetrafluoroethylene is large, low hundreds of thousands, high up to more than 10 million, generally millions (the degree of polymerization in 104 orders of magnitude, while polyethylene only in 103). The general crystallinity is 90-95%, and the melting temperature is 327-342℃. The CF2 units in the polytetrafluoroethylene molecule are arranged in a sawtooth shape. Since the radius of the fluorine atom is slightly larger than that of the hydrogen, the adjacent CF2 units cannot be fully cross-orientated in trans, but form a helical twisted chain, with the fluorine atom covering almost the entire surface of the polymer chain. This molecular structure explains the various properties of polytetrafluoroethylene. At temperatures below 19°C, a 13/6 helix is formed; at 19°C a phase transition occurs and the molecule unravels slightly, forming a 15/7 helix.

Although the breakage of carbon-carbon and carbon-fluorine bonds in perfluorocarbons requires the absorption of energy 346.94 and 484.88 kJ/mol, respectively, the depolymerization of polytetrafluoroethylene to generate 1 mol of tetrafluoroethylene requires only 171.38 kJ of energy. so at high temperature cracking, polytetrafluoroethylene mainly depolymerizes to tetrafluoroethylene. The weight loss rates (%) of PTFE at 260, 370 and 420°C are 1×10-4, 4×10-3 and 9×10-2 per hour, respectively. it can be seen that PTFE can be used at 260°C for a long time. Since high temperature cracking also produces highly toxic by-products such as fluorophosgene and perfluorinated isobutene, special attention should be paid to safety protection and to prevent PTFE from contact with open flame.  Mechanical properties Its coefficient of friction is extremely small, only 1/5 of that of polyethylene, which is an important feature of perfluorocarbon surfaces. Because the intermolecular force between fluorine and carbon chains is extremely low, PTFE has non-stick properties.

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

Chemical corrosion and weathering resistance In addition to molten alkali metals, PTFE is not subject to corrosion by almost any chemical reagents. For example, in concentrated sulfuric acid, nitric acid, hydrochloric acid, and even boiling in aqua regia, its weight and performance are not changed, and almost insoluble in all solvents, only slightly soluble in full alkane (about 0.1g / 100g) above 300 ℃. Polytetrafluoroethylene does not absorb moisture, non-combustible, extremely stable to oxygen and ultraviolet light, so it has excellent weather resistance.  Electrical properties PTFE has low dielectric constant and dielectric loss in a wide range of frequencies, and high breakdown voltage, volume resistivity and arc resistance.

Radiation resistance The radiation resistance of polytetrafluoroethylene is poor (104 rads), degradation caused by high-energy radiation, the electrical and mechanical properties of the polymer are significantly reduced.

Polymerization Polytetrafluoroethylene is produced by the free radical polymerization of tetrafluoroethylene. The industrial polymerization reaction is carried out in the presence of large amounts of water with stirring to disperse the heat of reaction and to facilitate temperature control. Polymerization is generally carried out at 40 to 80°C, 3 to 26 kg force/cm2 pressure, and can be initiated by inorganic peroxides and organic peroxides, or by redox initiation systems. Dispersion polymerization requires the addition of perfluorinated surfactants, such as perfluorooctanoic acid or its salts.  Expansion coefficient (25-250℃) 10-12×10-5/℃.