Polyimide (PI) is a high temperature resistant polymer, which can maintain the main physical properties in a short time at 550 ℃, and can be used at close to 330 ℃ for a long time. Among high-temperature resistant engineering plastics, it is one of the most valuable varieties. It has excellent size and oxidation stability, chemical resistance and radiation resistance, as well as good toughness and softness. It can be widely used in fields such as aviation/aerospace, electrical/electronics, locomotives, automobiles, precision machinery, and automatic office machinery.
Due to the extremely stable aromatic heterocyclic structure in the polyimide molecule, it reflects the excellent performance that other polymer materials cannot match: temperature resistance and low temperature resistance. The thermal decomposition temperature of PI synthesized from biphenyl dianhydride and p-phenylenediamine can reach 600 ℃, which is one of the varieties with high thermal stability in polymers so far. At this temperature, the original physical properties can be basically maintained for a short period of time. It can be used for a long time below 333 ℃, and will not crack at -269 ℃; The mechanical strength is high, and the tensile strength of the homogeneous benzene type PI film can reach 170MPa, while the biphenyl type can reach 400MPa, with little change as the temperature increases; Good radiation resistance; Excellent dielectric performance; Stable chemical properties, very stable to acids and bases; In addition, PI has strong creep resistance and excellent friction performance.
In 2003, the global consumption of polyimide was about 35000 tons, and there were about 50 major manufacturers. At present, the consumption of the United States, Western Europe and Japan has exceeded 20000 tons/year, including 13000 tons/year in the United States. It is estimated that the consumption of the United States, Western Europe and Japan will reach 27000 tons/year in 2004. There are 14 production factories in the United States, 11 in Western Europe, and 13 in Japan that produce PI. In addition, Russia, China, India, South Korea, Malaysia, and Taiwan all produce and consume a small amount of PI. Polyether imide accounts for about 70% of the demand for PI in the United States. Because of its good performance and processing characteristics, it tends to compete with polyethersulfone, polyphenylene sulfide and other aromatic PIs. It is expected that the global demand for polyimide will increase at an annual rate of about 10% in the next few years.
Our country almost started the research and development of polyimide at the same time as the world. Now, the development and research pattern has basically formed. Several major varieties of PI have been developed, such as homobenzene, trimeric anhydride, biphenyl dianhydride, bisphenol A dianhydride, monoether anhydride, ketone anhydride, etc., and have been initially applied. There are currently nearly 20 manufacturers with a production capacity of over 800 tons per year. In 2002, the PI film market strengthened, with a production capacity of 750 tons per year. In 2003, six production plants added a total of 400 tons per year, and the number of manufacturers expanded to 26. Four manufacturers have exported films, and with the addition of copper foil exports, the national export volume is nearly 70 tons/year. The main manufacturers currently include Shanghai Synthetic Resin Research Institute, Changshu Aerospace Insulation Material Factory, Baoying Yabao Insulation Material Factory, Northwest Chemical Research Institute, Sichuan University Experimental Factory, etc. In 2001, the domestic production was about 400 tons. China has also developed rapidly in PI molding compounds in recent years. Shanghai Synthetic Resin Research Institute has developed a variety of products, Northwest Research Institute of Chemical Industry produces bismaleimide, and Jilin Changchun Yinghua Institute and Jilin High tech Company produce biphenyl PI molding compounds.
Compared with foreign products in many varieties, China has a significant competitive advantage in terms of price. Therefore, the electric insulation film market in China is basically occupied by domestic products, and domestic products have begun to be exported to foreign countries. For example, Northwest Chemical Research Institute exports 30 tons of bismaleimide to Europe every year, and Shanghai Synthetic Resin Research Institute also exports a small amount of products every year.
At present, thermoplastic polyimide is mainly used in automobile engine parts, oil pump and air pump covers, high temperature sockets for electronic/electrical instruments, connectors, printed circuit boards and computer hard disks, integrated circuit chip carriers, aircraft internal cargo systems, etc.
There are many varieties of polyimides, but the main varieties are polyether imide (PEI), polyamide imide (PAI) and bismaleic imide (BMI). The current development trend of PEI is to introduce para phenylenediamine structure or alloy it with other engineering plastics to improve its heat resistance, or alloy it with engineering plastics such as polycarbonate and polyamide to improve its mechanical strength; PAI is a kind of high strength polyimide. At present, the development trend is to strengthen and modify it, and to alloy it with other plastics; BMI is a bifunctional compound with maleimide imide as the active group. It has the same fluidity and moldability as typical thermosetting resins, and is basically the same as the processing and molding of epoxy resins. It is currently a hot spot in domestic research and development. In order to improve the application performance of polyimide, the development and application of polyimide modification and composite materials have become the research and development focus of polyimide for a long time in the future. For example, polyhomophenylimide filled with graphite or polytetrafluoroethylene can significantly reduce the friction coefficient, and can be used for automotive thrust washers, high-temperature brake pads, windshield wipers, bearings, bushings and gaskets for aircraft gas turbine engines; Polyimide is called super strong engineering plastics after adding glass fiber or boron fiber, which is used to prepare structural parts of jet engine; Silicon containing polyimide can give good solubility, air permeability, impact resistance, weather resistance and adhesion to materials; In addition, there are fluorinated polyimides, etc.
Nissan Chemical Company of Japan plans to double its production of polyimide resin for LCD manufacturing. The company invested about 2 billion yen (US $18.4 million) to expand the production capacity of its polyimide resin plant in Chiba to 460 tons/year in the first or second quarter of 2005. In addition, the company plans to spend 1 billion yen to increase the production of microchip materials at its production facility in Toyama Prefecture by 60%. Nissan's goal is to increase the sales of these products by approximately 50% from 16 billion yen in 2003 by 2007. Polyimide resin is a heat-resistant and super strong material, which helps to display high-resolution images on LCDs.
A material thermoplastic polyimide developed by Nanjing University of Technology and supported by research funds provided by Changzhou Guangcheng New Plastic Co., Ltd. was successfully developed. The emergence of this special engineering plastic has ended China's long-term dependence on imports for this type of product, filling the domestic gap. This thermoplastic polyimide not only has excellent mechanical properties, dielectric properties, heat resistance, corrosion resistance, wear resistance and other characteristics, but also overcomes the defects of most polyimides that are insoluble and difficult to process, showing good hot working characteristics. It can be molded by hot molding, extrusion and injection methods, and can be compounded with carbon fiber, glass fiber, polytetrafluoroethylene, graphite and other materials, further improving the tensile, self-lubricating, wear resistance and other properties of the materials. This material can not only be made into films, but also molded into pipes, bars, and precision parts with complex structures. Therefore, it is an ideal material that can replace metals, ceramics, thermosetting resins, low-temperature thermoplastics, and difficult to machine polyimides under specific conditions. At present, this technology has been fully industrialized, and has formed a 200 ton/year device capacity. The thermoplastic polyimide products produced can be used in automobile and aircraft engines, communication instruments, construction machinery, industrial machinery, commercial equipment, electronic appliances and microelectronics, analysis and medical equipment, transmission and textile equipment and other fields, which will promote the upgrading of China's chemical products and improve market competitiveness.
Polyimide in China has developed rapidly with the development of national defense industry, and its research and application in some fields have reached the world level. The gap with advanced countries abroad is mainly reflected in its small scale. Currently, the larger production equipment in China is 100 tons per year, while developed countries abroad have a scale of basically 1000 tons; Our country's products have poor quality and unstable performance, which affects their application; The products are not refined enough and have few varieties. At present, they are mainly poly (pyromellitic imide) films, and their application fields are also mainly films and molding compounds. However, there are many foreign products, such as films, molding parts, coatings, adhesives, enamels, foam and fibers. While developing the existing varieties, China should vigorously develop the main resin products with large amount, such as polyether imide, polyphenyltetramethylene imide, thermoplastic polyimide and polybismaleimide.