Fluoroplastics

Fluoroplastics refer to a general term for a class of plastics in which the hydrogen atoms on the side chain of aliphatic hydrocarbon macromolecular chains are partially or completely replaced by fluorine atoms, which are divided into different types according to the number of fluorine atoms and copolymer components. Among them, the homopolymers are polytetrafluoroethylene (PTFE or F4), polychlorotrifluoroethylene (PCTFE or F3), polyvinylidene fluoride (PVDF), polyvinyl fluoride (PVF); Copolymers include polyperfluoroethylene propylene (FEP or F46), tetrafluoroethylene-ethylene copolymer (ETFE or F40), chlorotrifluoroethylene copolymer (ECTFE or F30), etc., and F4, F3, F46 are the most commonly used.
(1) PTFE
1. Introduction to PTFE
Polytetrafluoroethylene is a polymer formed by polymerization of tetrafluoroethylene monomer in the presence of free radical initiators, which is referred to as PTFE or F4 in English. F4 was first industrialized by DuPont in 1948. It is widely used because of its excellent corrosion resistance, low coefficient of friction, good self-lubrication, heat resistance and electrical insulation. F4 is the largest of all fluoroplastics, and can be used for friction materials in the machinery industry, anti-corrosion materials in the chemical industry, insulating materials in the electrical industry, anti-bonding materials, separation materials and medical materials.
2. PTFE performance
(1) General performance F4 is waxy white powder in appearance, non-toxic, tasteless, odorless, non-combustible, non-bonding, non-water absorption, good gas barrier, F4 has good physiological compatibility and anti-thrombosis.
(2) Thermal performance F4 has excellent heat resistance and cold resistance, and can be used in the range of -195°C~250°C for a long time. It has a thermal conductivity of 0.25 W/(m•K), a coefficient of linear expansion that is smaller than that of most plastics, and increases with increasing temperature.
(3) mechanical properties F4 outstanding mechanical properties for low friction and self-lubrication, its friction coefficient is the lowest in plastics, the dynamic and static friction coefficients of steel are 0.04, but F4 ground wear resistance is not good, need to add molybdenum disulfide, graphite and other wear-resistant materials to modify. Other mechanical properties of F4 are average, and its tensile strength, bending strength, impact strength, rigidity, hardness, creep resistance, fatigue resistance and other properties are poor.
(4) Electrical properties F4 has very good electrical insulation, small dielectric loss tangent, basically not affected by temperature, humidity and frequency, and can be applied under humid conditions; F4 has poor corona resistance and cannot be used as a high-voltage insulating material; good arc resistance, up to 360s.
(5) Environmental performance F4 has excellent chemical stability, can resist a variety of strong polar solvents such as concentrated acid, concentrated alkali and strong oxidant, and even can resist "aqua regia"; F4 has excellent weather resistance, and its performance is basically unchanged after six years outdoors, but it is not good at radiation, and it becomes brittle after being irradiated by γ rays.
3. Modification of PTFE
Copolymerization with other monomers with F4 can improve processability, and filling with wear-resistant materials can improve wear resistance and rigidity.
4. Molding processing
(1) Processing characteristics There are fluorine atoms with strong electronegativity on both sides of the F4 macromolecular carbon chain, and the repulsion between the fluorine atoms is very large, which makes it difficult to rotate in the macromolecular chain and the molecular chain is stiff. This characteristic makes it have a melting point of up to 327°C, and when the temperature reaches the melting point, the crystals melt, but the viscosity is too high to become a flowing liquid, and even when the decomposition temperature (390°C) is reached, it cannot flow. Therefore, F4 cannot be processed by the processing method of thermoplastics, but can only be processed by special methods such as sintering molding, extrusion molding, push molding, machining and hydraulic method.
(2) Sintering molding Sintering is one of the main molding methods of F4, which is mainly suitable for the molding of thick-walled discontinuous products, such as plates, rods, casings and gaskets. Sintering molding can generally be divided into three steps: billet making, sintering, and cooling.
(3) Extrusion molding is similar to sintering molding, extrusion molding also has to go through three processes: billet making, sintering and cooling, but the billet making process does not use a press, but uses a plunger extruder or screw extruder, which can be continuously made and suitable for the production of thick-walled continuous products, such as pipes, plates, bars, etc. It is worth noting in production that, first, the diameter of the machine head should be smaller than the diameter of the barrel, and second, sintering and cooling are carried out in the machine head, and the temperature of the machine head should be divided into sintering zone and cooling zone. The extruder generally selects a screw with equal distance, equal depth and no compression ratio.
(4) Push molding, also known as extrusion molding, is essentially a combination of extrusion and calendering. It is suitable for continuous extrusion of thin-walled products such as film, but it is not sintered, so it is also called raw tape.
(5) Machining with sintered bars, cutting on the machine tool, can produce F4 cooked product film, the thickness can reach about 0.04mm. If necessary, a thinner (0.005mm) product can be obtained by calendering.
(6) The hydraulic method, also known as the rubber bag method, is suitable for large and complex products that are not easy to be produced by the pressing method. The specific method is to evenly place the loose F4 powder between the rubber bag and the mold wall, apply hydraulic pressure of 12~13Mpa in the rubber bag, and obtain the parison sintering after 20~30min pressure holding.
5. Scope of application
Due to its excellent performance, F4 is widely used in sealing materials, sliding materials, anti-corrosion materials, insulating materials and medical materials. Specifically:
(1) Corrosion resistance: Pure F4 resin is used to manufacture various chemical containers and parts, such as pipes, storage tanks, reactors, equipment linings, valves, pump bodies, joints, filter materials, separation materials and other products in contact with corrosive media.
(2) Wear-resistant F4 is filled with wear-resistant modification treatment, which can be used for wear-resistant materials, such as dynamic and static sealing rings, piston rings, gaskets, valve discs, valve seats, bearings, bearings, sliders, guide rails, etc.
(3) Insulating materials: Pure F4 resin is used to manufacture high-temperature, high-frequency electrical insulation products, such as high-frequency insulators, high-frequency cables, moisture-resistant cables, capacitor coils, motor slot insulation and integrated circuit boards.
(4) Medical materials Pure F4 resin can be used to manufacture substitute blood vessels, artificial hearts, artificial esophagus, artificial peritoneums, artificial heart-lung devices, various intubations and catheters, etc.
(5) Non-stick materials are mainly used for various coated products such as non-stick pans, food processing machinery, anti-stick rollers of machines, etc.
(2) Polychlorotrifluoroethylene
1. Introduction to polychlorotrifluoroethylene
Polychlorotrifluoroethylene is made by polymerization of chlorotrifluoroethylene monomer through different polymerization methods, and its English abbreviation is PCTFE or F3. F3 is the first fluoroplastic variety to be developed, and industrial production was achieved in 1946.
F3 is not as good as F4 in terms of chemical resistance and heat resistance, but it can be processed using conventional processing methods for thermoplastics. For some products with low requirements for corrosion resistance and F4 that cannot be molded and processed, F3 can be selected. As a result, F3 is the second largest fluoroplastic material after F4.
2. Structural performance
(1) Structure Compared with F4, F3 macromolecule replaces a fluorine yard by a chlorine atom, which destroys the symmetry of the macromolecule, reduces its regularity, and makes its crystallinity lower than that of F4. Since the C-Cl bond is not as stable as the C-F bond, its heat resistance is not as high as that of F4. Due to the introduction of chlorine atoms, the intermolecular forces increase, so that the hardness and strength of F3 are better than those of F4.
(2) environmental performance F3 chemical corrosion resistance is second only to F4, soluble in some highly halogenated solvents such as carbon tetrachloride, benzene, toluene, p-xylene, cycloethane, cycloethylketone and 2,5-dichlorotrifluorobenzene at high temperature, melted alkali metal, elemental fluorine, ammonia, chlorine, hydrofluoric acid, concentrated sulfuric acid and concentrated nitric acid can be corroded; F3 has good weather resistance and radiation resistance is the best among fluoroplastics.
(3) Thermal properties The glass transition temperature of F3 is 58 °C, the crystallization melting point is 215 °C, and the decomposition temperature is 260 °C. It has good cold resistance, can work at a temperature of -195°C, and has a long-term heat resistance of less than 120°C. The linear expansion coefficient of F3 is low, only (4.5~7.0)×10-5/K.
(4) Mechanical properties The mechanical properties of F3 are better than those of F4, and the mechanical properties are affected by the magnitude of its crystallinity, with the increase of crystallinity, the hardness, tensile strength, bending strength, etc. are improved, while the impact strength and elongation at break decrease.
(5) Electrical properties F3 has the properties of non-hygroscopic, non-carbon, non-combustion, etc., so it has excellent electrical insulation, and its volume resistivity and dielectric strength are very high. However, due to the destruction of symmetry by chlorine atoms, the dielectric constant and dielectric loss increase, and increase with the increase of frequency and temperature, which limits its application under high temperature and high frequency conditions.
(6) Air permeability F3 is one of the varieties with good barrier property in plastic materials, and has impermeability to many gases and liquids, such as the permeability coefficient of water is 0.16g•mm/m2•d•Mpa, which is lower than PVDC, and is the best water-blocking material in plastics. The F3 can be used to produce gas blocking films, packaging materials for brine and industrial gases, as well as sealing materials for high vacuum systems.
(7) Optical properties With the decrease of crystallinity, the transparency of F3 increases, such as the sheet with low crystallization δ=3mm can still be transparent. The refractive index of F3 varies in the range of 1.429~1.435 with different crystallinity. The 4~7μm F3 film has an infrared transmittance of 80%, which can be used on the infrared window of missiles.
3. Molding processing
Unlike F4, F3 can be melted and flowed when heated, and can be processed by thermoplastic processing methods, such as injection molding, extrusion, molding and coating. F3 is a non-Newtonian fluid, and its apparent viscosity decreases with the increase of shear rate. The viscosity of F3 is high, up to 0.5~5MPa•s at 230°C, and it must have sufficient fluidity at high temperature. The processing temperature is 250~300 °C, the decomposition temperature is 315 °C, and the molding temperature range is narrow. The thermal conductivity of F3 is small, the heat transfer is slow, and the heating and cooling rate during processing should not be too fast. F3 In the molding process, the processing conditions have a great influence on the crystallinity, and the molding shrinkage is 1%~2.5%. Due to the high viscosity, F3 products are prone to internal stress, which can be improved by adding a small amount of vinylidene fluoride/chlorotrifluoroethylene copolymer. F3 is highly corrosive to the processing equipment, and the part of the processing equipment that contacts the melt should be plated with hard chrome.
4. Scope of application
(1) Use corrosion resistance to manufacture corrosion-resistant products with complex shapes and difficult to form F4, such as corrosion-resistant high-pressure seals, discs of corrosion-resistant high-pressure valves, linings, parts, diaphragms and sight glasses of pumps and pipelines.
(2) Using opticality to manufacture optical windows, such as the infrared window of missiles
(3) Using the barrier property to make sealing materials for high-vacuum devices.
(4) It is used for the coating of various chemical equipment by using the coatability.
(3) Polyperfluoroethylene propylene
1. Introduction to polyperfluoroethylene propylene
Polyperfluoroethylene propylene is a copolymer polymerized by tetrafluoroethylene monomer and hexafluoropropylene monomer under certain conditions, which is called tetrafluoroethylene/hexafluoropropylene copolymer, or FEP or F46 in English.
The corrosion resistance of F46 is slightly worse than that of F4, and other properties are similar to F4, and its outstanding advantage is that it has good processability and can be processed by conventional methods of thermoplastics, so it has become the third largest fluoroplastic material. The F46 was first developed by DuPont in the United States in 1960.
2. Structural performance
(1) Structure F46 can be regarded as the product of the substitution of a fluorine atom in the F4 molecule by trifluoromethyl, which increases the steric hindrance and reduces the symmetry and integrity of the molecule. As a result, the corrosion resistance is not as good as that of F4, but the processing fluidity is improved and can be processed by conventional methods. F46 belongs to the random copolymerization of tetrafluoroethylene and hexafluoropropylene.
(2) General properties The hydrophobicity and non-viscosity of F46 are similar to those of F4.
(3) Thermal performance The heat resistance of F46 is lower than that of F4 but better than that of F3, the long-term use temperature is -85~205 °C, and the linear expansion coefficient increases with the increase of temperature.
(4) Environmental performance The corrosion resistance of F46 is close to that of F4, but slightly inferior to F4. It is resistant to inorganic acids, alkalis, alcohols, ketones, aromatics, halogenated hydrocarbons, detergents and greases, etc., and only alkali metals and chlorine trifluoride at high temperature can interact with it. F46, like F4 and F3, has good weather resistance, but its radiation resistance is not good.
(5) Electrical properties F46 has excellent electrical properties, the dielectric constant and dielectric loss tangent are relatively small, and it is less affected by frequency, temperature and humidity.
(6) Mechanical properties The outstanding mechanical properties of F46 are high impact strength, even with notches, and continuous punching at room temperature. Tensile strength, flexural strength, impact strength, and elongation at break decrease with increasing temperature. F46 has better creep resistance than F4 at room temperature, but is not as good as F4 at high temperatures.
3. Molding processing
Processing characteristics The fluid properties of F46 are close to those of F3, which is a non-Newtonian fluid, the viscosity is sensitive to the shear rate, and can be processed by thermoplastic processing methods, and its melt viscosity is relatively high, in the range of 343~393 °C, the melt viscosity is 103~104Pa•s, which is 10~100 times larger than that of general thermoplastics, and the processing temperature range is narrow, and the thermal conductivity is small. The molding shrinkage rate of F46 is 3%~6%, easy to electrostatic adsorption, and antistatic agent needs to be added to keep it clean during processing.
The processing methods can be injection molding, extrusion molding, and compression molding.
4. Scope of application
F46 can be used to make corrosion-resistant products with complex shapes, which are used in cable sheath materials, mechanical seals, and bearings. The film made of F46 can be applied to the equipment that needs to be non-stick to make printed circuit boards, hoses, pipes, etc.