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Properties of polytetrafluoroethylene
overview

Polytetrafluoroethylene (commonly referred to as" non-stick coating" or" easy-to-clean material" ); It is a synthetic polymer material using fluorine to replace all hydrogen atoms in polyethylene.This material has the characteristics of acid resistance, alkali resistance and resistance to various organic solvents, and is almost insoluble in all solvents.At the same time, PTFE has the characteristics of high temperature resistance, and its friction coefficient is extremely low, so it can be used as lubricating effect, and it has also become an ideal coating for easy cleaning wok and inner layer of water pipe.

Polytetrafluoroethylene (PTFE)

PTFE (abbreviated as Teflon or [PTFE, F4]) is known as/commonly known as" King of Plastics" , and its Chinese trade names are" Teflon" ," Teflon" ," Teflon" ," Teflon" ," Teflon" ," Teflon" and so on.It is a polymer compound formed by polymerization of tetrafluoroethylene, It has excellent chemical stability and corrosion resistance (it is one of the best corrosion resistance materials in the world today, In addition to molten metal sodium and liquid fluorine, can resist all other chemicals, boiling in aqua regia does not change, widely used in various needs of acid, alkali and organic solvents), sealing performance, high lubrication, non-viscosity, electrical insulation and good anti-aging endurance, excellent temperature resistance (can work at + 250 ℃ to-180 ℃ for a long time).PTFE itself is not toxic to humans, but ammonium perfluorooctanoate (PFOA), one of the raw materials used in the production process, is considered to have carcinogenic effects.
The temperature is-20 ~ 250 ℃ (-4 ~ + 482 F), allowing quenching and heating, or alternating cold and heat operation.
Pressure-0.1 ~ 6.4 mpa (full vacuum to 64kgf/cm2)
It has solved many problems in chemical, petroleum, pharmaceutical and other fields in China.Polytetrafluoroethylene seals, gaskets, gaskets.Polytetrafluoroethylene seals, gaskets, gaskets are made by molding suspension polymerized polytetrafluoroethylene resin.Compared with other plastics, polytetrafluoroethylene (PTFE) has the characteristics of chemical corrosion resistance and has been widely used as sealing materials and filling materials.
Used as engineering plastics, it can be made into PTFE tubes, rods, belts, plates, films, etc.It is generally used in corrosion-resistant pipes, containers, pumps, valves, radars, high-frequency communication equipment, radio equipment, PTFE heat exchangers and so on.Dispersion liquid can be used as insulation impregnation liquid of various materials and anticorrosion layer of metal, glass and pottery surface.All kinds of polytetrafluoride ring, polytetrafluoride gasket, polytetrafluoride packing, etc.are widely used for sealing various anticorrosive pipeline flanges.In addition, it can also be used for drawing silk, polytetrafluoroethylene fiber-fluoron (foreign trade name is Teflon).
At present, all kinds of PTFE products have played an important role in the national economic fields such as chemical industry, machinery, electronics, electrical appliances, military industry, aerospace, environmental protection and bridges.
Application conditions of polytetrafluoroethylene (PTFE) industry chemical, petrochemical, oil refining, chlor-alkali, acid making, phosphate fertilizer, pharmaceutical, pesticide, chemical fiber, dyeing, coking, gas, organic synthesis, non-ferrous smelting, iron and steel, atomic energy and high-purity product production (such as ionic membrane electrolysis), viscous material transportation and operation, food and beverage processing and production departments with highly strict hygiene requirements.

advantages of use

high temperature resistance-operating temperature up to 250 ℃.
Low temperature resistance-good mechanical toughness; Even if the temperature drops to-196 ℃, the elongation of 5% can be maintained.
Corrosion resistance-Inert to most chemicals and solvents, resistant to strong acids, alkalis, water, and various organic solvents.
Weather resistance-has the best aging life in plastics.
High lubrication-the lowest coefficient of friction among solid materials.
Non-adhesion-is the smallest surface tension in a solid material and does not adhere to any substance.
Non-toxic-it has physiological inertia and has no adverse reactions when implanted into the body for a long time as artificial blood vessels and organs.
Polytetrafluoroethylene has a relatively large molecular mass, ranging from hundreds of thousands to more than 10 million, generally millions (the degree of polymerization is on the order of 104, while polyethylene is only 103).Generally, the crystallinity is 90 ~ 95%, and the melting temperature is 327 ~ 342 ℃.The CF2 units in PTFE molecule are arranged in a serrated shape.Because the radius of fluorine atoms is slightly larger than that of hydrogen, the adjacent CF2 units cannot completely cross-orient, but form a spiral twisted chain, and fluorine atoms almost cover the surface of the whole polymer chain.This molecular structure explains various properties of PTFE.When the temperature is lower than 19 ℃, 13/6 spiral is formed.At 19 ℃, the phase transition occurs, and the molecules untie slightly to form a 15/7 helix.
Although the breakage of carbon-carbon bond and carbon-fluorine bond in perfluorocarbons requires energy absorption of 346.94 kJ/mol and 484.88 kJ/mol, respectively, the depolymerization of PTFE to produce 1mol tetrafluoroethylene requires energy of only 171.38 kJ.Therefore, during pyrolysis, PTFE is mainly depolymerized into TFE.The weight loss rates (%) of PTFE at 260 ℃, 370 ℃ and 420 ℃ are 1 × 10-4, 4 × 10-3 and 9 × 10-2 per hour, respectively.It can be seen that PTFE can be used for a long time at 260 ℃.Because highly toxic by-products such as fluorophosgene and perfluoroisobutylene are also produced during pyrolysis, special attention should be paid to safety protection and PTFE should be prevented from contacting open flames.
Mechanical properties Its friction coefficient is very small, only 1/5 of that of polyethylene, which is an important feature of perfluorocarbon surface.And by The intermolecular force between fluorine-carbon chain is extremely low, so PTFE is non-sticky.
It does not melt at 250 ℃ and does not brittle at ultra-low temperatures of-260 ℃.PTFE is so smooth that even ice can't compare with it; Its insulation performance is particularly good, and a thick film of newspaper is enough to withstand the high voltage of 1500V V.
PTFE keeps excellent mechanical properties in a wide temperature range of-196 ~ 260 ℃.One of the characteristics of perfluorocarbon polymer is that it does not become brittle at low temperature.
Chemical and weatherproof PTFE is hardly corroded by any chemical agent except for molten alkali metals.For example, when boiled in concentrated sulfuric acid, nitric acid, hydrochloric acid or even aqua regia, its weight and properties have no change, and it is almost insoluble in all solvents, only slightly soluble in total alkanes (about 0.1 g/100g) above 300 ℃.Polytetrafluoroethylene is not moisture-absorbing, non-combustible, and extremely stable to oxygen and ultraviolet rays, so it has excellent weather resistance.
Electrical properties PTFE has low dielectric constant and dielectric loss in a wide frequency range, and high breakdown voltage, volume resistivity and arc resistance.
Radiation resistance PTFE has poor radiation resistance (104 rad), which causes degradation after being exposed to high energy radiation, and the electrical and mechanical properties of polymers are obviously reduced.
Polytetrafluoroethylene is formed by radical polymerization of tetrafluoroethylene.Industrial polymerization is carried out by stirring in the presence of a large amount of water to disperse the heat of reaction and facilitate temperature control.Polymerization is generally carried out at 40 ~ 80 ℃ and 3 ~ 26 kg/cm2.Inorganic persulfate and organic peroxide can be used as initiators, and redox initiator system can also be used.The heat release per mole of tetrafluoroethylene was 171.38 kJ.Dispersion polymerization requires the addition of perfluorinated surfactants, such as perfluorooctanoic acid or its salts.
The application of polytetrafluoroethylene can be molded by compression or extrusion; It can also be made into aqueous dispersion for coating, impregnation or fiber making.Polytetrafluoroethylene is widely used as high and low temperature resistant, corrosion resistant material, insulating material, anti-sticking coating and so on in atomic energy, aerospace, electronics, electricity, chemical industry, machinery, instruments, meters, construction, textile, food and other industries.

Chemical properties

Atmospheric aging resistance:Radiation resistance and low permeability:Long-term exposure to the atmosphere, the surface and properties remain unchanged.
Incombustibility:Oxygen limit index below 90.
Acid and alkali resistance:insoluble in strong acids, bases and organic solvents.
Oxidation resistance:Resistance to strong oxidant corrosion.
Acid-base:Neutral.

Physical properties
PTFE has soft mechanical properties.Has a very low surface energy.
Polytetrafluoroethylene (F4, PTFE) has a series of excellent service properties:high temperature resistance-long-term service temperature 200 ~ 260 degrees, low temperature resistance-soft at-100 degrees; Corrosion resistance-capable of aqua regia and all organic solvents; Weather resistance-the best aging life in plastics; High lubrication-has the smallest friction coefficient in plastics (0.04); Non-viscosity-having the smallest surface tension in solid materials without adhering to any substance; Non-toxic-with physiological inertia; Excellent electrical performance, it is an ideal C-class insulating material.Polytetrafluoroethylene materials are widely used in national defense, military industry, atomic energy, petroleum, radio, electric machinery, chemical industry and other important departments.Products:Polytetrafluorotetrafluorotetrafluorotetrafluorotetraethylene bar, pipe material, plate and turning plate.Polytetrafluoroethylene is a polymer of tetrafluoroethylene.English abbreviation is PTFE.The structural formula is.It was discovered in the late 1930s and put into industrial production in the 1940s.The relative molecular weight of polytetrafluoroethylene is relatively large, ranging from hundreds of thousands to more than 10 million, generally millions (the degree of polymerization is in the order of magnitude of 104, while polyethylene is only 103).Generally, the crystallinity is 90 ~ 95%, and the melting temperature is 327 ~ 342 ℃.The CF2 units in PTFE molecule are arranged in a serrated shape.Because the radius of fluorine atoms is slightly larger than that of hydrogen, the adjacent CF2 units cannot completely cross-orient, but form a spiral twisted chain, and fluorine atoms almost cover the surface of the whole polymer chain.This molecular structure explains various properties of PTFE.When the temperature is lower than 19 ℃, 13/6 spiral is formed.At 19 ℃, the phase transition occurs, and the molecules untie slightly to form a 15/7 helix.
Although the breakage of carbon-carbon bond and carbon-fluorine bond in perfluorocarbons requires energy absorption of 346.94 kJ/mol and 484.88 kJ/mol, respectively, the depolymerization of PTFE to produce 1mol tetrafluoroethylene requires energy of only 171.38 kJ.Therefore, during pyrolysis, PTFE is mainly depolymerized into TFE.The weight loss rates (%) of PTFE at 260 ℃, 370 ℃ and 420 ℃ are 1 × 10-4, 4 × 10-3 and 9 × 10-2 per hour, respectively.It can be seen that PTFE can be used for a long time at 260 ℃.Because highly toxic by-products such as fluorophosgene and perfluoroisobutylene are also produced during pyrolysis, special attention should be paid to safety protection and PTFE should be prevented from contacting open flames.
Mechanical properties Its friction coefficient is very small, only 1/5 of that of polyethylene, which is an important feature of perfluorocarbon surface.Because the intermolecular force between fluorine and carbon chains is extremely low, PTFE is non-sticky.
PTFE keeps excellent mechanical properties in a wide temperature range of-196 ~ 260 ℃.One of the characteristics of perfluorocarbon polymer is that it does not become brittle at low temperature.
Chemical and weatherproof PTFE is hardly corroded by any chemical agent except for molten alkali metals.For example, when boiled in concentrated sulfuric acid, nitric acid, hydrochloric acid and even in aqua regia, its weight and properties have no change, and it is almost insoluble in all solvents, only slightly soluble in total alkanes (about 0.1 g) above 300 ℃/100g).Polytetrafluoroethylene is not moisture-absorbing, non-combustible, and extremely stable to oxygen and ultraviolet rays, so it has excellent weather resistance.
Electrical properties PTFE has low dielectric constant and dielectric loss in a wide frequency range, and high breakdown voltage, volume resistivity and arc resistance.
Radiation resistance PTFE has poor radiation resistance (104 rad), which causes degradation after being exposed to high energy radiation, and the electrical and mechanical properties of polymers are obviously reduced.
Polytetrafluoroethylene is formed by radical polymerization of tetrafluoroethylene.Industrial polymerization is carried out by stirring in the presence of a large amount of water to disperse the heat of reaction and facilitate temperature control.Polymerization is generally carried out at 40 ~ 80 ℃ and 3 ~ 26 kg/cm2.Inorganic persulfate and organic peroxide can be used as initiators, and redox initiator system can also be used.The heat release per mole of tetrafluoroethylene was 171.38 kJ.Dispersion polymerization requires the addition of perfluorinated surfactants, such as perfluorooctanoic acid or its salts.
Coefficient of expansion (25 ~ 250 ℃) 10 ~ 12 × 10-5 ℃

PTFE products forming methods summary

1, molding method
2, pushing method
3, skin bag method
4, spraying method
5, braiding method
6, winding method
7, rolling method
8, extrusion method
9, bonding method
10, welding method
11, heat setting method
12, machining method

PTFE sintering process

Shaping shrinkage:3.1-5.0%
Molding temperature:330-380 ℃
Sintering conditions:The best temperature should not exceed 385 ℃, otherwise the molecules will die and affect the quality.

Material properties
Long-term service temperature-200-260 degrees, excellent chemical corrosion resistance, corrosion resistance to all chemicals, the lowest friction coefficient in plastics, and good electrical properties, its electrical insulation is not affected by temperature, and it is known as" the king of plastics" .
is transparent or translucent.The higher the crystallinity, the worse the transparency.Most of the raw materials are powdered resin or concentrated dispersion, which have extremely high molecular weight and high crystallinity thermoplastic polymer.
Suitable for making corrosion-resistant parts, wear-resistant parts, seals, insulating parts and medical instrument parts 1 K)

Formability
1.Crystalline material with small moisture absorption.
2.Poor fluidity, easy decomposition, and corrosive gas is produced when decomposing.The molding temperature should be strictly controlled, the mold should be heated, and the resistance of gating system to material flow should be small.
3.Powder resins are usually formed by powder metallurgy and sintering.The sintering temperature is 360-375 degrees and cannot exceed 410 degrees.Emulsion resin is usually processed by cold extrusion and sintering, which can form anticorrosive coating on the surface of articles.If the transparency and toughness of products are required, rapid cooling should be adopted.Extrusion molding can also be adopted, and tubes, rods and profiles can be extruded.
4, the viscosity of PTFE melt is very high, and the volume viscosity decreases with the increase of shear stress, showing the characteristics of non-Newtonian fluid.
5.Secondary processing, which can be hot-pressed, welded, bonded, reinforced, machined, etc.to make the final product.
6.Preferably sintered with curves the first step is dried at 120 degrees
The second step, such as graphite filling or two-fluidized ink, is temperature treated at 250 degrees.
3, the third step is treated once at 345 degrees
4, the fourth step is treated at 375 degrees
5, the fifth step is not too fast

PTFE application

PTFE can be molded by compression or extrusion; It can also be made into aqueous dispersion for coating, impregnation or fiber making.
Polytetrafluoroethylene is widely used as high and low temperature resistant, corrosion resistant material, insulating material, anti-sticking coating, etc.in atomic energy, national defense, aerospace, electronics, electricity, chemical industry, machinery, instruments, meters, construction, textile, metal surface treatment, pharmacy, medical treatment, textile, food, metallurgy and smelting industries, making it an irreplaceable product.
PTFE has excellent comprehensive properties, high temperature resistance, corrosion resistance, non-stick, self-lubricating, excellent dielectric properties, very low coefficient of friction.Used as engineering plastics, it can be made into PTFE pipes, rods, belts, plates, films, etc.It is generally used in corrosion-resistant pipes, containers, pumps, valves, radars, high-frequency communication equipment, radio equipment and so on.The mechanical properties of PTFE can be greatly improved by adding any filler which can withstand the sintering temperature of PTFE.At the same time, other excellent properties of PTFE are maintained.Filled varieties are glass fiber, metal, metallized oxide, graphite, molybdenum disulfide, carbon fiber, polyimide, EKONOL …, etc., wear resistance, limit PV value can be increased 1000 times.
Teflon PTFE pipe is made of suspension polymerized PTFE resin by plunger extrusion.PTFE has the best chemical corrosion resistance and dielectric properties among known plastics.PTFE braided packing is a good dynamic sealing material.It is woven from expanded polytetrachloroethylene strips.PTFE braided packing is a good dynamic sealing material.It is woven from expanded polytetrachloroethylene strips.It has excellent properties such as low friction coefficient, wear resistance, chemical corrosion resistance, good sealing performance, no hydrolysis and no hardening.Gasket seals and lubricating materials for working in various media And electrical insulators for use at various frequencies.Capacitor dielectric, track insulation, electrical instrument insulation, etc.Polytetrafluoroethylene film is suitable for capacitor dielectric, insulation layer of special cable, wire insulation, electrical instrument insulation and sealing gasket, and can also be used as non-adhesive tape, sealing tape and demoulding.

Polytetrafluoroethylene classification and its filling products

1.General purpose materials
Various rods, tubes, plates, films, belts, cords, packing, gaskets, and graphite, molybdenum disulfide, aluminum oxide, glass fiber and carbon fiber as fillers to improve the mechanical properties of pure polytetrafluoroethylene.
2.Anticorrosion class
1.Pipes and fittings:pure polytetrafluoroethylene pipes; Polytetrafluoroethylene lining pipe; Wrapped glass steel pipe; Steel composite flange;
2.Chemical container lining:PTFE lined kettle; Polytetrafluoroethylene lining groove; Polytetrafluoroethylene lining tower;
3.Heat exchanger;
4.Corrugated telescopic tube;
5.Main components of valves and pumps;
6.Steel wire reinforced full pressure hose;
7.Filter material.
Polytetrafluoroethylene film undergoes longitudinal and transverse biaxial stretching with a large number of pores in it, It is a new material, which can be compounded with other fabrics to make solid-phase anti-corrosion filter bags for smoke and dust, rain gear, sportswear, winter clothes, special protective clothing and light tents, compressed air for pharmaceutical use, aseptic filtration of various solvents and filtration of high-purity gas in electronic industry.
3.Seals
1.Static seals:Sandwich gaskets; Seating belt; Elastic sealing belt;
2.Dynamic seals (packing, annular seals):V-type seals-for shafts, piston rods, valves; Turbopump inner seal; Composite sealing ring of polytetrafluoroethylene and rubber; Telescopic mechanical seal with bellows.
4.Load bearing
1.Filled PTFE bearings for food, chemical, paper, textile machinery;
2.Porous copper impregnated fluoroplastic metal bearing can be used normally under high temperature and high pressure dry friction and vacuum conditions;
3.Bearing liners made of composite fabrics of polytetrafluoroethylene fibres blended with glass fibres or other fibres for use at low speeds and high loads;
4.Filled PTFE piston rings, guide rings, machine guide rails and bridge sliders;
5.Insulation category:
1.Class C insulating materials for wires and cables;
2.Jackets for stator and rotor diversion pipes and thermocouples of dual-water internally cooled turbo-generators;
3.Microwave insulating materials for HF and UHF communication equipment and radar;
4.Printed circuit substrates and insulation materials for motors and transformers (including gas transformers);
5.Insulation materials for air conditioners, electronic furnaces, various heaters and sulfur hexafluoride circuit breakers;
6.Anti-sticking category:
1.PTFE glass cloth coating on the hot roller of sizing machine-it can avoid the phenomenon of sticking roller formed by chemical size and greatly improve the production rate and grey cloth quality;
2.Microwave drying conveyor belts for food industry-compared with conveyor belts made of other materials, they have the advantages of non-absorption of microwave energy and non-stickiness, thus saving electricity and cleaning;
3.Anti-sticking material of heat-closing sleeve for polyethylene bag sealing;
4.Anti-sticking coatings-for kitchen pans, baking molds for baking bread, frozen food storage trays, electric iron supports, copier clamps;
7.Temperature resistance category:
1.Drive transmission device of microwave oven, such as coupling and roller of microwave oven;
2.Temperature-resistant fittings for various refrigerators, air conditioners, oxygen generators and compressors;
8.Other categories:
1.Human substitute arteries, venous vessels and cardiac membranes;
2.Endoscope, clamp catheter, trachea;
3.Other medical equipment such as tubes, bottles, filter cloths, etc.

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