Polyimide (PI) is a high temperature resistant polymer, which can maintain its main physical properties at 550 (?) C for a short time and can be used at close to 330 (?) C for a long time. It is one of the most valuable varieties of engineering plastics with high temperature resistance. 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 aviation/aerospace, electrical/electronic, locomotive, automobile, precision machinery and automatic office machinery.
Because of the stable aromatic heterocyclic structure in polyimide molecule, it shows the excellent properties that other polymer materials can not compare with: temperature resistance and low temperature resistance. PI synthesized from biphenylene dianhydride and p-phenylenediamine can be decomposed at 600 C, which is one of the varieties with high thermal stability in polymer so far. At such a temperature, the physical properties can be basically maintained for a short time. It can be used for a long time below 333 C, and still not brittle at - 269 C. The tensile strength of the PI film with pyromelline type can reach 170 MPa, while that of the PI film with biphenyl type can reach 400 MPa, which changes little with the increase of temperature. It has good radiation resistance, excellent dielectric property, stable chemical property, and very stable to acid and alkali. In addition, PI has good creep resistance and friction performance.
In 2003, about 35,000 tons of polyimide were consumed globally, with more than 50 major manufacturers. At present, the consumption of the United States, Western Europe and Japan has exceeded 20,000 tons/year, of which the United States has 13,000 tons/year. It is expected that the consumption of the United States, Western Europe and Japan will reach 27,000 tons/year in 2004. There are 14 production plants in the United States, 11 in Western Europe and 13 in Japan. In addition, Russia, China, India, Korea, Malaysia and Taiwan all produce and consume a small amount of PI. Polyetherimide (Ulterm) accounts for about 70% of PI demand in the United States. Because of its good properties and processing properties, it has a tendency to compete with polyethersulfone, polyphenylene sulfide and other aromatic PIs. It is expected that the global demand for polyimide will increase at an annual average rate of about 10% in the next few years.
The research and development of polyimide in China started almost at the same time as that in the world. Now it has basically formed a pattern of development and research. Several PI varieties have been developed, such as pyromelline type, polar anhydride type, biphenylene dianhydride type, bisphenol A dianhydride type, monoether anhydride type, ketone anhydride type and so on, and have been initially applied. At present, there are nearly 20 manufacturers with production capacity exceeding 800 tons per year. In 2002, the PI film market strengthened, with production capacity reaching 750 tons per year. In 2003, six factories added 400 tons per year and 26 factories expanded. Four manufacturers export films, together with copper foil export, the national export volume is nearly 70 tons per year. The main manufacturers are Shanghai Institute of Synthetic Resin, Changshu Aerospace Insulation Material Factory, Baoying Yabao Insulation Material Factory, Northwest Institute of Chemical Industry, Sichuan University Experimental Factory, etc. In 2001, the domestic output was about 400 tons. In recent years, PI moulding plastics have also developed rapidly in China. Shanghai Research Institute of Synthetic Resins has developed a variety of products. Northwest Research Institute of Chemical Industry produces bismaleimide. Changchun Yinghua Institute of Jilin Province and Jilin Hi-Tech Company produce biphenyl PI moulding plastics.
Compared with foreign products, China has great competitive advantage in price in many varieties. Therefore, the market of electrical insulating film in China is basically occupied by domestic products, and domestic products have begun to be exported abroad, such as 30 tons of Bismaleimide exported to Europe by Northwest Chemical Research Institute every year, and Shanghai synthetic resin research. A small number of products are exported each year.
At present, thermoplastic polyimide is mainly used in automobile engine parts, oil pump and gas pump cover, high temperature socket for electronic/electrical instruments, connectors, printed circuit boards and computer hard disk, integrated circuit chip carriers, aircraft internal cargo system, etc.
There are many varieties of polyimides, but at present the main varieties are polyetherimide (PEI), polyamide-imide (PAI) and bismaleimide (BMI). At present, the development trend of PEI is to introduce p-phenylenediamine structure or alloy with other engineering plastics to improve its heat resistance, or to make alloys with polycarbonate, polyamide and other engineering plastics to improve its mechanical strength; PAI is a kind of high strength polyimide, and the current development trend is to strengthen modification and alloying with other plastics; BMI is a bifunctional compound with maleimide as active group. It has similar fluidity and mouldability with typical thermosetting resins. It is basically the same as the processing and moulding of epoxy resins. It is a hotspot 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 hotspot of polyimide for a long time in the future. For example, when polyimide is filled with graphite or polytetrafluoroethylene, the friction coefficient can be obviously reduced, which can be used in automobile thrust washers, high temperature brake pads, etc. Windshield wipers, bearings, bushings and gaskets for aircraft gas turbine engines; polyimides, when added with glass or boron fibers, are called super engineering plastics for the preparation of jet engine structural components; silicon-containing polyimides, which can be endowed with good solubility, air permeability, impact resistance and weather resistance of materials In addition, there are fluorinated polyimides and so on.
Nissan Chemical Co. of Japan plans to double its production of polyimide resins for liquid crystal displays. The company invested about 2 billion yen ($18.4 million) and expanded its production capacity in Chiba polyimide resin plant to 460 tons per year in the first or second quarter of 2005. In addition, the company plans to spend 1 billion yen to increase the output of its microchip material production plant in Fushan County by 60%. Nissan aims to increase sales of these products by about 50% from 16 billion yen in 2003 by 2007. Polyimide resin is a heat-resistant and super-strong material, which helps to display high-definition images on LCDs.
The development of thermoplastic polyimide, a cutting-edge material developed by Nanjing University of Technology and supported by Changzhou Guangcheng New Plastics Co., Ltd., has been successful. The emergence of this special engineering plastics has ended the long-term dependence on imports of this kind of products in China and filled the domestic gap. The thermoplastic polyimide not only has excellent mechanical properties, dielectric properties, heat resistance, corrosion resistance and abrasion resistance, but also overcomes most of the defects of insoluble polyimide and difficult to process. It shows good thermal processing characteristics. It can be moulded by hot pressing, extrusion and injection methods, and can be used with carbon fibers, glass fibers, etc. Polytetrafluoroethylene (PTFE) and graphite are compounded to further improve the tensile properties, self-lubrication and wear resistance of the material. The material can be made into thin film, pipe, bar and precise parts with complex structure. Therefore, it is an ideal material to replace metal, ceramics, thermosetting resin, low temperature thermoplastic plastics and refractory polyimide under specific conditions. At present, the technology has been fully industrialized, and has formed 200 tons/year capacity. The thermoplastic polyimide products can be used in automotive and aircraft engines, communication instruments, construction machinery, industrial machinery, commercial equipment, electronic appliances and microelectronics, analysis and medical equipment, as well as transmission and textiles. In the field of equipment and other fields, it will promote the upgrading and upgrading of chemical products in China, so as to improve the market competitiveness.
With the development of national defense industry, polyimide in China has developed rapidly, and its research and application in some fields have reached the world level. The gap between China and foreign advanced countries is mainly reflected in the small scale. At present, the largest domestic production facility is 100 tons per year. The developed countries are basically 1000 tons in scale. The quality of products in China is poor, the performance is unstable, and the application is affected. The products are not refined enough and the varieties are few. At present, the main products are poly (pyromethylenetetramide) films. The main application areas are thin films and moulded plastics, and foreign products are various, such as thin films, mouldings, paints, adhesives, enamels, foams and fibers. While developing the existing varieties, China should vigorously develop the main resin products with large dosage, such as polyetherimide, polybiphenylenetetriamide, thermoplastic polyimide and polybismaleimide.
