Production status and modification progress of polytetrafluoroethylene

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l Production status Due to factors such as fluorine atoms contained in the molecular structure, PTFE exhibits high chemical stability, strong high and low temperature resistance, outstanding non-stickiness, abnormal lubricity, excellent electrical insulation properties, aging resistance and radiation resistance, minimal water absorption, etc. It is called the "King of Plastics". Widely used in aerospace, petrochemical, machinery, electronic appliances, construction, textile and many other fields. Due to its unique properties, the global production and consumption of fluororesin has increased rapidly. The current global consumption of fluororesin is about 120,000 tons, of which about 70% is polytetrafluoroethylene (PTFE). Global PTFE products are divided into three categories: suspension polymer resin, dispersion polymer resin, and concentrated dispersion. The approximate composition is about 52%, 28%, and 20%. There are currently more than 20 PTFE manufacturers in the world, with more than a dozen located around the world. * * and regions, production and consumption are mainly concentrated in developed countries in the West * * and regions, the internationally renowned PTFE production companies mainly include: DuPont Company of the United States, ICI Company of the United Kingdom, Daikin Company of Japan, Dyneon Company of Germany, Ausimont Company of Italy, Kirovization Company of Russia * * Among them, the production capacity of four companies, DuPont, ICI, Dyneon and Daikin, accounts for about 80% of the world's total production capacity. It can be said that these companies determine the fate of future PTFE production and development. In the late 20th century, the overall PTFE production pattern also underwent large-scale adjustments with the wave of global mergers and reorganizations. Many companies carried out reorganizations, transfers and mergers. For example, in 1999, ICI sold its polymer business to Japan's Asahi Glass Co., Ltd. ； Hoechst sells 46% stake in Dyneon to joint venture partner 3M ； Solvay plans to acquire Italy's Ausimont, among others. From the 1980s to the 1990s, global PTFE experienced rapid development. Many companies built and expanded PTFE devices. In recent years, the development rate has slowed down. In 2002, the global PTFE production capacity was approximately 110,000 t/a. It is expected that the global production capacity will reach approximately 140,000 t/a in 2005. my country's PTFE production and research started early, but due to various factors restricting the production scale and the overall level of process technology is relatively low. The main domestic manufacturers include Shanghai Sanaifu New Materials Co., Ltd., Jinan Sanaifu Co., Ltd., Chenguang Chemical Research Institute Branch No. 2, Fuxin Chemical Plant, etc., with an annual production capacity of approximately 7,000t/a. In the 21st century, my country's PTFE industry has begun to enter a stage of rapid development. Since my country's basic raw material for PTFE production is fluorspar, which accounts for about one-third of the world's total reserves. In addition to its unique resource advantages, my country's demand for PTFE has increased rapidly. Therefore, in recent years, some domestic enterprises have planned to introduce technology to build large-scale devices. In addition, many foreign multinational companies are or plan to build fluororesin projects in China. For example, Zhejiang Juhua has introduced Russian technology to jointly build a polytetrafluoroethylene device with an annual output of thousands of tons. After the completion of Changshu International Fluorine Chemical Park, Atofina Company entered the investment field ； Daikin Corporation of Japan invested 1.33 billion yuan to build a polytetrafluoroethylene device in the park and put it into operation in 2003. ； In addition, many companies have also expressed their intention to relocate. It can be expected that my country's PTFE industry will usher in a stage of rapid development in the future. Although PTFE has good physical and chemical properties, it also has some defects, such as poor mechanical properties, large linear expansion coefficient, poor creep resistance and easy cold flow, poor wear resistance, and difficulty in molding and secondary processing. Its application is subject to certain restrictions. With the rapid increase in my country's PTFE production capacity, strengthening the research and application of PTFE modification technology and developing new and efficient PTFE composite materials have become the current research and development directions of domestic PTFE. 2 Modification of PTFE In order to improve the defects of PTFE, PTFE can be modified by various means such as reinforcement, filling, compounding and blending to make up for its own defects, so that the developed composite materials can be widely used in the preparation of parts in machinery, electronic and electrical, aerospace, automobile and other industries. The main modification methods include surface modification, filling modification and blending modification. 2． l Surface modification Due to the extremely low surface activity and non-stickiness of PTFE, which limits its compounding with other composite materials, certain surface modifications must be performed on the PTFE material to improve its surface activity. Commonly used technologies include surface activation technology, which can use the radiation of high-energy rays to defluorinate the surface, and then fluorinate it with other materials under certain devices and conditions. ； The PTFE material is treated with low-temperature plasma of some inert gas to break the carbon-fluorine or carbon-carbon bonds and generate a large amount of free radicals to increase the surface free energy of PTFE and improve its wettability and adhesion. ； Dip PTFE into molten potassium acetate and treat it at a suitable temperature to form an activation layer with certain activity. ； The surface activity of PTFE can be improved by heating it in a certain proportion of sodium hydroxide and dipropenyl melamine mixture. ； After corona treatment of a certain intensity and time, PTFE can form an activation layer that can be bonded. Chemical corrosion modification, treating PTFE with certain chemical reagents can improve its surface activity. These chemical reagents can be ammonia solution of metallic sodium, naphthalene sodium * * Furan solution, alkali metal amalgam, pentacarbonyl iron solution, etc. Surface deposition modification, PTFE is immersed in a colloidal solution of certain metal hydroxides, so that colloidal particles are deposited on the surface of PTFE. Thereby increasing its wettability, improving its surface activity, and making it easy to compound with other materials. The above surface modification method is mainly suitable for PTFE films. Usually, after proper treatment, PTFE films can be well bonded and compounded with other materials, so they are widely used in the design and manufacturing of chemical anti-corrosion products, sealing products and lubrication devices. The main idea is to introduce polar groups and increase the interface bonding force. 2.2 Filling modification Add fillers to PTFE to improve and overcome the defects of PTFE. Currently, filled PTFE products are the most produced PTFE resin products. It is worth noting that in foreign countries, PTFE filling technology is completed by PTFE resin manufacturers, while in my country, PTFE filling technology is completed by processing and production enterprises. The pressure resistance, wear resistance and cooling properties of PTFE are improved by filling PTFE resin with different fillers such as inorganic, metallic and organic polymers. These fillers are required to withstand the sintering temperature of PTFE. ； Does not react with PTFE ； In addition, it has a certain particle size and can improve some physical and chemical properties of PTFE. Inorganic filler materials. Commonly used inorganic materials mainly include glass fiber, graphite, molybdenum disulfide and carbon fiber. The glass fiber filled with PTFE is generally alkali-free glass fiber, and the filling amount is 15%-25%. After compounding, the wear resistance of PTFE can be increased by more than 500 times, and the creep resistance and cold flow resistance are greatly improved. ； The addition of molybdenum disulfide can help increase the hardness of PTFE and reduce initial wear. ； Graphite can be used alone or in combination with glass fiber or carbon black. Graphite-filled PTFE has excellent chemical resistance, compression creep and good thermal conductivity. ； A small amount of carbon fiber filled PTFE can achieve the carbon and graphite filling effect and has extremely strong tensile properties. However, carbon fiber is expensive and its application is subject to certain limitations. Metal filling materials, in order to improve the mechanical properties, thermal conductivity and dimensional stability of PTFE, metals such as iron, copper, lead, molybdenum, tungsten, silver and their oxides are usually used to fill PTFE. Currently, there are many studies on metal filling technology, especially copper and its alloys which are most commonly used. Filling PTFE with copper powder can improve the creep resistance, compressive strength, hardness and dimensional stability of the product. Organic filling materials. The organic materials used to fill PTFE are mainly organic fibers and high molecular polymers. Organic filling can improve the heat resistance, creep resistance, pressure resistance, compression, bending and wear resistance of PTFE. In order to improve the interfacial adhesion between the filler and PTFE and prevent decomposition during sintering, the filler generally needs to be properly treated to obtain the best compatibility and surface effect. 2.3 Blending modification Blending modification mainly uses the excellent characteristics of PTFE to alloy some resins. The current research and application prospects are promising, such as PTFE/PA, PTFE/POM, PTFE/PC, PTFE/PI, PTFE/PPO, PTFE/PEEK, PTFE/PPS, PTFE/PES and other alloy products that have been continuously developed. (l) PTFE modified polyformaldehyde. Polyformaldehyde (POM) has excellent mechanical, chemical and electrical properties and is widely used in automobiles, electronics, precision machinery and building materials. In recent years, the construction of large-scale polyformaldehyde projects in Yunnan and Nantong, Jiangsu Province has promoted the industrial production and application of polyformaldehyde in my country. However, POM has defects such as poor toughness and low notched impact strength. For this reason, POM resin modification research has been carried out at home and abroad. A series of POM/PTFE blends with different PTFE contents have been developed in China using cold pressing-hot sintering processes, which can significantly improve friction and wear performance, toughness, creep resistance and appearance. ； There are also alloy pellets produced by extrusion and granulation of PTFE and POM modified by initial compatibilization at high speed, which significantly improves the friction and wear properties of modified POM. The improvement mechanism lies in the formation of PTFE transfer film. ； A variety of POM/PTFE blends are prepared abroad through mechanical blending methods, that is, POM is blended with several types of PTFE, such as PTFE, coupling agent-coated PTFE, and chemically treated PTFE. The results show that there is a strong adhesion between PTFE treated by chemical reaction and coupled with coupling agent and POM, and has very excellent performance. (2) PTFE modified polyphenylene sulfide, polyphenylene sulfide (PPS) has excellent high temperature stability, flame retardancy and electrical properties. It is widely used in automobiles, electrical electronics and machinery and other fields, becoming the largest variety of special engineering plastics. At present, domestic units such as Sichuan University have successfully developed industrialized devices with an annual output of hundreds of tons. The country needs to import a considerable amount every year to meet domestic demand. However, the disadvantages of PPS are poor impact resistance and difficulty in processing and molding. Since the inert surface of PTFE is difficult to bond with PPS, Japan uses solubilizers to reduce the two-phase interfacial tension from the perspective of improving surface affinity, and uses mixing technology at high shear rates to alloy this incompatible system. ； In China, PPS powder is mixed with a mixture and then PTFE powder is added to make a coating, which makes the coating have excellent friction and wear properties, adhesion, flexibility and anti-sticking properties. The mixture generally uses a system of ethanol, water, and sodium dioxane dodecyl sulfonate. PTFE/PPS alloy solves the problem of high melt flow rate and difficulty in direct molding of PPS. It can still maintain high mechanical properties above 300°C and is mainly used in corrosion-resistant pumps, valves, gaskets, dynamic seals, bushings, automobile engine valve covers, chromatograph sliding seals and guides, etc. (3) PTFE modified polyamide. Polyamide (PA) ranks first in terms of production capacity of engineering plastics. It is especially suitable for filling, reinforcing and modifying glass fiber and other materials. It is widely used in many fields such as automobiles, electronic appliances, packaging, machinery, daily consumer goods, etc. The current domestic annual consumption is more than 80,000 t/a. The research and application of modified PA started early in China. Currently, a large number of modified PAs are used in many industries. PA added with PTF E is mainly to improve its sliding properties. According to the data, when the PTFE filling amount is greater than 10%, the friction reduction and wear resistance of PA are significantly improved. For example, 5% linear low-density polyethylene/propylene-styrene copolymer and 10% PTFE, which are partially compatible with it, are added to the PA system at the same time. The synergistic effect of the two is very good. It is an ideal modification method in terms of improving the performance of composite materials and reducing costs. (4) PTFE modified polyimide. Polyimide (PI), as a new type of engineering plastic, has super heat resistance, radiation resistance and wear resistance. It is mainly used in the aerospace industry. In recent years, its application has expanded to electronics, automobiles and other fields. my country's research and production started early. Currently, more than ten scientific research units have been formed, and some products are exported. Foreign composite materials composed of 33% PTFE, 2% carbon black and 65% soluble PI are oil-free lubricating materials with excellent friction and wear properties. For example, foreign RTP companies have developed RTP4200 series products using the technology of blending thermoplastic polyimide and PTFE or adding other abrasive agents and fillers, which can be used for under-hood parts of automobiles, aerospace equipment and office electronic equipment. (5) PTFE modified polyether ether ester, polyether ether ether ester (PEEK) is a new type of engineering plastics with high strength, heat resistance and flame retardancy. PEEK composite materials are widely used in aerospace, electronic and electrical fields. Domestic research units take advantage of the good mechanical properties and high heat resistance of PEEK, the low friction coefficient of PTFE, and improve the processing technology with additives. They prepare PEEK/PTFE blends through melt blending and reinforce them with glass fiber/carbon fiber mixed fibers to improve their mechanical properties. They develop an oil-free, high-temperature-resistant, low-friction material with good process performance and injection molding for use as high-temperature engine parts. (6) PTFE modified poly(m-phenylene isophthalamide). Poly(m-phenylene isophthalamide) (PMIA) is a polyamide with much higher mechanical properties and high temperature resistance than other aliphatic materials. It is an ideal basic resin for self-lubricating materials. It is mainly used as ball bearings, rotating gears, piston rings, etc. In order to further improve the tribological properties of the material, lubricating fillers need to be used to improve the friction and wear properties. In China, high-speed mixing devices are used to fully mix PMIA powder and PTFE, and samples are obtained through compression casting. Experiments show that when the PTFE content is 20%, the blend has the lowest friction coefficient. (7) PTFE modified linear low-density polyethylene. Linear low-density polyethylene (LLDPE) is the most commonly used general plastic variety. It is easily eroded by ultraviolet light, heat and oxygen and becomes brittle. Therefore, LLDPE is modified to extend its life. However, while improving the ultraviolet stability of LLDPE, its mechanical properties often decrease. In order to take into account both, PTFE is used to modify it. These problems can be effectively solved. If foreign reports use gamma rays to irradiate powdered PTFE and treat it with a silane coupling agent at the same time. After filling the modified LLDPE with surface-treated PTFE, it can not only improve the adhesion between PTFE and LLDPE, but also improve the mechanical properties of the blend. The processability and UV stability of LLDPE have been significantly improved by testing. (8) Others, in addition to the above introduction, a lot of research has been done at home and abroad on the blending of PTFE and various other engineering plastics. For example, blending PTFE with the amorphous polymer polyethersulfone (PES) can significantly improve the lubrication performance of PES. The British ICI Company and Japan's Sumitomo Chemical have successively developed a series of new wear-resistant PES products modified by PTFE. ； PTFE and polyphenylene ether (PPO) blends combine the heat resistance, mechanical properties and dimensional stability of PPO with the wear resistance and lubricity of PTFE. This blend is particularly suitable for making integral and large bearing parts. ； Poly(phthalate-diphenolpropane) resin is an amorphous transparent polymer with many excellent properties, but its chemical resistance and self-lubricating properties are poor. After modification with PTFE, the chemical resistance and self-lubricating properties are significantly improved. ； The PTFE and polycarbonate (PC) blend developed by Teijin Kasei of Japan is particularly suitable for the production of gear cams, bearings and other products for machinery, vehicles, electrical appliances and other equipment. In addition to the above modification methods, a small amount of non-fluorine groups are introduced into the linear PTFE chain, and block grafting is performed to destroy its symmetry, thereby obtaining a modified PTFE that can be processed by thermoplastic processing methods. The processing performance is greatly improved, and it is increasingly valued by the industry. In addition, PTFE dispersions, PTFE micropowders and expanded PTFE have received much attention due to their excellent processing performance. 3 Processing and Application PTFE has a high melting point, a large melt viscosity, and is very sensitive to shear in the amorphous state, easily causing melt rupture. Therefore, conventional thermoplastic plastic molding processes such as melt extrusion and injection molding cannot be used, and sintering molding can only be carried out using methods similar to powder metallurgy. The manufacturing of filled PTFE is the same as the molding of PTFE. It can be preformed, free sintering, or plunger extrusion. The above processing technology is generally suitable for products with a certain wall thickness, but not suitable for the processing of PTFE films. In recent years, a lot of research has been carried out in China. The more mature processing technologies are: Calendering process for dispersible PTFE resin ； Utilizing the low cohesion and fibrosis characteristics of this resin, PTFE aqueous emulsion is added to the powdered system, stirred for a certain period of time, the PTFE fibrils entangle the powder, and then compress it to turn the powder into a solid. By calendering the solid, a PTFE-filled modified film can be obtained. Utilize the characteristics of low hardness and good toughness of PTFE sintered products for metal turning processing. The thickness of the PTFE film processed by turning can reach about 0.04mm. This kind of film has good mechanical properties. No substances are added during processing. It has low impurity content and is non-toxic. It can be used in the medical field. There is also an extrusion process that is a synthesis method of conventional extrusion and calendering. The resin is directly processed into raw materials without melting and plasticizing. This method uses the residual deformation characteristics of PTFE under the action of compression force, and by adding a certain additive, the residual deformation can be increased. Usually, the PTFE material added with an extrusion aid is pressed into a ring of a certain density, and is placed in an extruder for proper heating. After extruding thin strips, it is finally sent to a calendering roller to be rolled into a film. In addition to the primary processing technology introduced above, there are also some newly developed secondary processing technologies worthy of attention in PTFE processing, such as PTFE vacuum forming technology, hot press molding and hot blow molding technology, isobaric molding processing technology, etc. PTFE is the preferred corrosion-resistant lining material for corrosion-resistant pipes, pipe fittings, bellows, pump bodies, valves, kettles, tanks, towers and various types of standard equipment. ； As a sealing material, it is widely used in the machinery, petrochemical, transportation, textile and construction industries. ； PTFE can also be used as valves, bearings, piston rings, guide rails, etc. that require corrosion resistance and low friction. ； The PTFE membrane has selective air permeability after treatment and can be used as a separation material to selectively transmit gas or liquid, especially to filter corrosive liquids. ； In recent years, PTFE membranes have been developed abroad for use as artificial organs, such as artificial blood vessels, heart valves, etc. Moreover, many new application technologies and fields are constantly being developed, and PTFE and its modified materials show good development prospects. According to the current research, production and application status at home and abroad, our country should vigorously develop modified PTFE materials to make them commercialized, serialized, and high-performance. It is especially important to prioritize the development of PTFE filled with glass fiber, carbon fiber, graphite, and copper powder that are in high demand and have good fluidity. Making full use of my country's rare earth resources, developing new varieties of rare earth filled PTFE and nano-modified PTFE will be our country's key research direction in the future. References I Kaufman H. Introduction Polymer Science and Technology, An SPE TEX Book, 1997. 2 Wu Jinshen. Fluorine plastic alloy. Plastics, 1998, 27(2): 17－18． 3 Wu Guoan, Xu Guoyao, Ye Changming. Research and development of polyoxymethylene and polytetrachlorethylene polymer alloys China Plastics, 2001, 15(6): 42-45． 4 Zou Jilin. 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