Polytetrafluoroethylene (PTFE), also known as "Teflon" and "Plastic King", is widely used in various industries due to its high temperature resistance, corrosion resistance, solvent resistance, high insulation, biological inertness and other characteristics. PTFE is a branch-free polymer composed of only two elements, C and F, in which fluorine atoms replace polyethylene hydrogen atoms.
1. Structure and properties of PTFE
1. Structural characteristics
Polytetrafluoroethylene is a homopolymer of tetrafluoroethylene, which can be prepared by polymerization methods such as suspension method, dispersion method and emulsion method.
Polytetrafluoroethylene is a super strong material and the only type of fluoroplastic that can be used as engineering plastics.
2. Main properties
The relative molecular weight of PTFE is very large, so the relative molecular weight has no obvious effect on strength, but the crystallinity has a significant effect on the rigidity, toughness, elongation and strength of PTFE products.
The density of PTFE is about 2.2g/cm3, the surface is smooth and waxy, and the contact angle with water is 114°～115°. PTFE is usually milky white and opaque, but the quenched product has a certain degree of transparency, almost no water absorption, low permeability to water vapor and nitrogen, and decreases with increasing density.
The tensile strength, elongation, elastic modulus, hardness, air permeability, dielectric strength, etc. of PTFE are all related to processing conditions such as molding pressure, sintering temperature and time, and cooling rate, because the processing conditions affect the porosity and crystallinity of the product. High molding pressure, sintering in the mold and cooling under pressure can reduce the voids in the product, thereby improving its mechanical strength. PTFE has a low elastic modulus and is prone to creep. Creep is the reason why PTFE can be used for sealing in gaskets, raw tapes, elastic tapes, etc.
The hardness of PTFE is low, but it can be improved by adding fillers.
The friction coefficient of PTFE is the smallest among all solid materials and does not change with temperature. Its static friction coefficient is smaller than the dynamic friction coefficient. Therefore, PTFE bearings start smoothly and have low resistance. They can be used as low-speed high-load bearings and are noiseless when rotating at low speed.
The thermal conductivity of PTFE is low, which can be appropriately increased by adding metal fillers.
The melting point of PTFE is 327℃, the heat deformation temperature is 50-60℃ (ISO R75 A method) or 130-140℃ (B method), the operating temperature is -200-260℃, and it is non-flammable. PTFE has the highest thermal stability among thermoplastics, and there is little degradation at 204-327℃, so no heating stabilizer is required.
The relative molecular mass 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 104, while polyethylene is only 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 zigzag shape. Since the radius of the fluorine atom is slightly larger than that of hydrogen, the adjacent CF2 units cannot be completely oriented in a trans-cross orientation, but form a spiral twisted chain, and the fluorine atoms almost cover the surface of the entire polymer chain. This molecular structure explains the various properties of polytetrafluoroethylene.
2. Application fields of PTFE
PTFE has the best chemical corrosion resistance, so it is most used in anti-corrosion materials and has a wide range of applications; PTFE has excellent electrical properties, so it is used as an insulating material in the electronic and electrical industry; PTFE has a small friction coefficient and good wear resistance, so it is used to make wear-resistant materials, sliding parts and seals in the machinery industry.
PTFE is widely used in bridges and buildings as a load-bearing support. In addition, according to the selective permeability of PTFE film after treatment, it can be used as a separation material to selectively pass through gas or liquid. Its porous membrane can be used for gas-liquid separation, gas-gas separation and liquid-liquid separation, and can also be used to filter corrosive liquids. In addition, PTFE is also widely used in the medical, electronic, and construction industries. For example, PTFE membranes can be used as human organs, including artificial blood vessels, heart valves, etc.
1. Application of PTFE in the 5G field
The FR4 copper-clad laminate commonly used in the communications industry uses epoxy resin as the substrate material, but its loss is large and is not suitable for high-frequency communications.
The requirements for high-frequency copper-clad laminates in the 5G field are low dielectric constant and low dielectric loss factor, and the 5G field has its own characteristics (microwave and millimeter wave applications) and has higher requirements for copper-clad laminates.
Polytetrafluoroethylene resin is currently the polymer material with the lowest dielectric constant, and its dielectric properties and dielectric loss can meet the requirements of communication base stations in the 5G field. Therefore, PTFE is gradually used in high-frequency communications such as 5G, aerospace, and military industry, and the copper-clad laminates made of it are called high-frequency copper-clad laminates.
In addition, PTFE is also often used in the 5G field to make semi-flexible coaxial cables, RF coaxial cables, radar antenna boards, etc.
2. Application of PTFE in the hydrogen energy industry
In the field of hydrogen energy, PTFE is mainly used for sealing of alkaline electrolyzers, as well as for strengthening proton exchange membranes in PEM fuel cells and water electrolysis.
In alkaline electrolyzers, sealing gaskets are the main components, which have both sealing and insulation functions. Leakage is one of the important factors affecting the life and safety of alkaline electrolyzers. The compression resilience and creep relaxation of sealing gaskets are important indicators for measuring the performance of sealing gaskets. Domestic alkaline tank sealing materials have undergone multiple iterations and upgrades such as asbestos rubber sheets-"cloth pads in one" diaphragm gaskets-polytetrafluoroethylene (PTFE) type filling gaskets. At present, the commonly used electrolyzer sealing gaskets in China are mainly PTFE type filling gaskets. PTFE is filled and modified with reinforcing fillers such as glass fiber, alumina, and graphite, and then molded and sintered to form sealing gaskets.
In fuel cells and PEM water electrolysis, proton exchange membranes are developing towards thinness, but the life of thin homogeneous perfluorosulfonic acid membranes cannot meet the requirements of fuel cells and PEM water electrolysis. At present, the fuel cell proton membranes on the market are often composite proton exchange membranes, using ePTFE as a composite material and perfluorosulfonic acid membrane. Expanded polytetrafluoroethylene membrane (ePTFE) is a porous three-dimensional mesh microstructure with micrometer or submicrometer levels.
At present, the global ePTFE market is controlled by a few manufacturers such as Gore, Nitto Denko, and Donaldson in the United States. Domestic Pan Asia Microporous has successfully broken the overseas monopoly by continuously exploring and breaking through the ePTFE production technology. However, the overall domestic ePTFE film focuses on the mid- and low-end markets, and does not account for a large proportion of the high-end market.
3. PTFE modification technology
Polytetrafluoroethylene (PTFE) has good heat resistance, insulation, self-lubricating properties, non-flammability, non-stick and other excellent properties due to the strong fluorine-carbon bonds contained in its composition. At the same time, due to its high temperature resistance and stable chemical properties, it has the ability to resist "aqua regia" corrosion, thus gaining the reputation of "the king of plastics". It is widely used in defense, mechanical industry and medical materials, especially in the field of tribology. Therefore, in the field of engineering plastics, PTFE has become one of the materials favored by researchers.
However, due to the shortcomings of PTFE such as low hardness, easy wear and poor creep resistance, it is subject to certain restrictions in actual application and production. Therefore, researchers have been committed to finding an excellent method to improve its mechanical properties without changing the advantages of PTFE itself, thereby expanding its application field. The modification of PTFE is mainly to combine with other materials to compensate for the defects of PTFE itself, mainly including surface modification, blending modification and filling modification. Among them, blending modification and filling modification are mainly used in the preparation of composite materials, while surface chemical modification is mainly aimed at bonding problems.
1. Surface modification
PTFE has very low surface activity and outstanding non-stickiness, which reduces the degree of adhesion with other materials. Surface modification can not only improve its surface inertness and compatibility with fillers, but also improve the surface activity of the matrix material. The current chemical modification of PTFE surface is mainly plasma treatment, radiation treatment and chemical solution treatment. These methods are to remove surface fluoride ions and graft highly active functional groups on the surface to achieve the purpose of improving the activity of the matrix material.
Plasma modification bombards the surface of the sample with high-energy plasma, transfers energy to the molecules on the surface of the sample, causes thermal etching, crosslinking, degradation and oxidation reactions of the sample, and causes the C-F bond and C-C bond on the surface of the sample to break, generate a large number of free radicals or introduce certain polar groups, thereby optimizing the performance of the sample surface. The modification of the material surface by low-temperature plasma treatment can be divided into plasma surface etching, plasma bonding, plasma vapor deposition, plasma liquid deposition and plasma surface grafting.
High-energy radiation can trigger graft polymerization and give the polymer some unique properties, such as improving its hydrophilicity, biocompatibility, conductivity, etc. The radiation-treated PTFE surface can be directly grafted with hydrophilic monomers such as acrylic acid, acrylamide, styrene and styrene/maleic anhydride to form a layer of grafted polymer that is easy to bond, making the PTFE surface rough and increasing the bonding area. Commonly used radiation sources in radiation grafting include gamma rays such as cobalt-60, cesium-137 and strontium-90, as well as various types of accelerators such as X-ray tubes, linear accelerators and cyclotrons.
PTFE can be treated with chemicals to improve its surface activity. These chemicals include sodium-naphthalene tetrahydrofuran solution, ammonia solution of metallic sodium, alkali metal amalgam, pentacarbonyl iron solution, etc. The sodium-naphthalene treatment solution is obtained by dissolving or complexing equal amounts of sodium and naphthalene in active ethers such as tetrahydrofuran and ethylene glycol dimethyl ether. Sodium transfers the outermost electrons to the empty orbit of naphthalene to form anion free radicals, which then form ion pairs with sodium and release a large amount of resonance energy; then the naphthalene anions are transferred to PTFE, destroying the C-F bond and removing some fluorine atoms on the surface, thus forming a carbonized layer and some polar groups on the PT-FE surface. There are active groups such as hydroxyl, carbonyl and carboxyl on the surface of the treated PTFE, which improves the bonding properties of the PTFE surface.
2. Blending modification
The basic principle of blending is the principle of like dissolves like, so the solubility value and surface tension of the blended materials must be similar. Blending PTFE with other engineering plastics can achieve the purpose of complementary advantages while integrating the strengths of each component, thereby expanding the application field to a certain extent. In blending modification, PTFE can be used as both a matrix material and a filler to reinforce other polymers. Here we mainly introduce polyphenylene ester (POB), polyphenylene sulfide (PPS) and polyetheretherketone (PEEK).
POB has excellent compressive creep resistance and high hardness. Blending with PTFE can make up for the shortcomings of PTFE and improve the mechanical and tribological properties of PTFE.
Unlike POB, PPS has excellent wear resistance, solvent resistance, heat resistance and easy manufacturing. It is widely used in aerospace and other fields. It can also be used as a matrix for super hydrophobic coatings. PTFE has the advantages of potential biodensity, high thermal stability, high chemical inertness, low surface energy and good self-lubricating ability. Blending PPS with PTFE is an ideal choice for improving the tribological properties of hydrophobic coatings.
PEEK and PTFE are both common matrix materials in solid lubricating composites. Cai Zhenjie et al. prepared PTFE-modified PEEK composites and studied the mechanical properties and wear mechanism. When the mass fraction of PTFE micropowder is 5%, the friction coefficient is reduced from 0.35 to about 0.3, and the volume wear is minimized. This composite material can be used not only in the mechanical field, but also in the medical field.
Blending modification is simpler and pollution-free than surface chemical modification, but it is generally only modified with polymers, which limits the addition of inorganic fillers such as metals, ceramics, and fibers, resulting in limited performance in improving the strength, hardness, and thermal conductivity of composite materials. In addition, the high inertness of PTFE makes it less compatible with other polymers. The surface needs to be treated before modification or some specific components need to be added during the modification process to improve compatibility.
3. Filling modification
Filling modified PTFE is a simple and effective method. Adding fillers and additives can significantly improve the mechanical properties of PTFE, especially creep and wear rate. Commonly used fillers include glass fiber, carbon fiber, graphite, molybdenum disulfide, bronze, steel, etc.
Graphite is a good solid lubricant. Filling graphite in PTFE can not only greatly reduce the wear of PTFE composites, but also improve the thermal conductivity and poor compression creep of PTFE.
Molybdenum disulfide (MoS2) has a lower friction coefficient than graphite and is stable in nature, so it is widely used. However, the price of MoS2 is very high. The performance of tungsten disulfide (WS2) is not much different from that of MoS2, but WS2 has better dry friction performance. MoS2 and WS2 can both improve the friction stability and wear resistance of composite materials while improving mechanical properties. Compared with pure PTFE, the friction stability of WS2 filling can be improved by about 33.3%. If the composite filling is used, the wear resistance can be improved by 2.3% compared with the single filling.
Carbon fiber (CF) has high specific strength, high modulus, low density, excellent wear resistance and creep properties. Carbon fiber is essential for reducing creep, increasing hardness, increasing flexibility and compression modulus. Polytetrafluoroethylene mixed with carbon fiber compounds has high thermal conductivity and low thermal expansion coefficient. Carbon fiber is inert to strong alkali and hydrofluoric acid (glass fiber can tolerate these two acids). These parts are very suitable for manufacturing automotive parts such as shock absorbers.
GF has always been favored in the production of industrial friction materials due to its high strength, high modulus and relatively low price, and is more widely used than CF in the field of polymer filling and modification.
Potassium titanate whisker (PTW) has much better mechanical properties than commonly used GF, CF, etc. due to its unique highly ordered crystal structure. The addition of PTW can greatly improve the strength and wear resistance of the composite material, while improving the stiffness and toughness of the composite material, both strengthening and toughening, changing the previous phenomenon of improving one property while sacrificing another when modifying GF and CF, and has stable chemical properties, good thermal insulation and wear resistance. Although the filling and modification effect of PTW is better than GF and CF, the compatibility between PTW and the matrix material needs to be further improved.
PTFE filled with bronze, this compound has excellent thermal conductivity and electrical conductivity, making it very suitable for applications with extreme loads and temperatures.
IV. Summary of PTFE
PTFE has excellent comprehensive properties and is the most widely used variety among fluoroplastics, playing an increasingly important role. With the advancement of science and technology, pure PTFE can no longer meet market demand, so it has become an inevitable trend to modify PTFE, mainly surface modification, blending modification and filling modification. At present, China has become a major producer of PTFE, and has basically mastered the molding and processing methods of modified PTFE. However, compared with foreign countries, there is still a big gap in technology and product quality. Therefore, the research, processing and application of modified PTFE and the tribological behavior and mechanism under different working conditions need to be further studied.
