Detailed explanation of polyimide (PI)
1. Overview As a special engineering material, polyimide has been widely used in aviation, aerospace, microelectronics, nano, liquid crystal, separation membrane, laser and other fields. Recently, all countries are listing polyimide research, development and utilization as one of the most promising engineering plastics in the 21st century. Polyimide, because of its outstanding characteristics in terms of performance and synthesis, whether it is used as a structural material or as a functional material, its huge application prospects have been fully recognized, known as "problems to solve problems" proTIon solver), and believe that "there will be no microelectronic technology today without polyimide."
Second, the performance of polyimide
1. Fully aromatic polyimide is analyzed by thermogravimetric analysis, and its initial decomposition temperature is generally around 500 ℃. The polyimide synthesized from biphenyl dianhydride and p-phenylenediamine has a thermal decomposition temperature of 600 ° C, which is one of the most thermally stable varieties among polymers.
2. Polyimide can withstand extremely low temperatures, such as brittle cracking in liquid helium at -269 ℃.
3. Polyimide has excellent mechanical properties, the tensile strength of unfilled plastics is above 100Mpa, the film of homophenyl polyimide (Kapton) is above 170Mpa, and the biphenyl polyimide Upilex S) reached 400Mpa. As an engineering plastic, the amount of elastic film is usually 3-4Gpa, and the fiber can reach 200Gpa. According to theoretical calculations, the fiber synthesized by phthalic anhydride and p-phenylenediamine can reach 500Gpa, second only to carbon fiber.
4. Some polyimide varieties are insoluble in organic solvents, stable to dilute acids, and generally less resistant to hydrolysis. This seemingly disadvantageous performance makes polyimide different from other high-performance polymers. The characteristic is that it can utilize alkaline hydrolysis to recover raw materials dianhydride and diamine. For example, for Kapton film, the recovery rate can reach 80% -90%. Changing the structure can also get a variety that is quite resistant to hydrolysis, such as withstanding 120 ℃, boiled in 500 hours.
5. The thermal expansion coefficient of polyimide is 2 × 10-5-3 × 10-5 ℃, Guangcheng thermoplastic polyimide 3 × 10-5 ℃, biphenyl type can reach 10-6 ℃, some varieties can be It reaches 10-7 ℃.
6. Polyimide has high radiation resistance, and its film has a strength retention rate of 90% after 5 × 109rad fast electron irradiation.
7. Polyimide has good dielectric properties, with a dielectric constant of about 3.4. Introducing fluorine, or dispersing air nano-size in polyimide, the dielectric constant can be reduced to about 2.5. The dielectric loss is 10-3, the dielectric strength is 100-300KV / mm, Guangcheng thermoplastic polyimide is 300KV / mm, and the volume resistance is 1017Ω / cm. These properties can still be maintained at a high level in a wide temperature range and frequency range.
8. Polyimide is a self-extinguishing polymer with low smoke generation rate.
9. Polyimide has very little outgassing under extremely high vacuum.
10. Polyimide is non-toxic, can be used to make tableware and medical appliances, and can withstand thousands of disinfection. Some polyimides also have good biocompatibility. For example, the blood compatibility test is non-hemolytic and the in vitro cytotoxicity test is non-toxic.
3. Various ways of synthesis:
Polyimide has many varieties and forms, and there are many ways to synthesize. Therefore, it can be selected according to various application purposes. This synthetic flexibility is also difficult for other polymers to have.
1. Polyimide is mainly synthesized from dianhydrides and diamines. These two monomers are combined with many other heterocyclic polymers such as polybenzimidazole, polybenzimidazole, polybenzothiazole, and polyquinone Compared with monomers such as quinoline and polyquinoline, the raw materials are widely sourced and the synthesis is relatively easy. There are many varieties of dianhydride and diamine, and different combinations can obtain polyimides with different properties.
2. Polyimide can be polycondensed from dianhydride and diamine in a polar solvent such as DMF, DMAC, NMP or THE / methanol mixed solvent at low temperature to obtain soluble polyamic acid, after film formation or spinning Heating to about 300 ° C dehydration to form a ring into polyimide; you can also add acetic anhydride and tertiary amine catalyst to the polyamic acid, chemical dehydration cyclization to obtain polyimide solution and powder. Diamine and dianhydride can also be polycondensed by heating in a high boiling point solvent, such as phenolic solvent, to obtain polyimide in one step. In addition, polyimide can also be obtained by reacting the dibasic ester of tetrabasic acid with diamine; it can also be converted from polyamic acid to polyisoimide first, and then to polyimide. These methods are convenient for processing. The former is called PMR method, which can obtain low viscosity and high solid solution. There is a window with low melt viscosity during processing, which is especially suitable for the manufacture of composite materials; the latter has increased Solubility, no low molecular compounds are released during the conversion process.
3. As long as the purity of dianhydride (or tetraacid) and diamine is qualified, no matter what polycondensation method is adopted, it is easy to obtain a sufficiently high molecular weight. The addition of monoanhydride or monoamine can also easily control the molecular weight.
4. Polycondensation of dianhydride (or tetraacid) and diamine, as long as it reaches an equimolar ratio, heat treatment in vacuum can greatly increase the molecular weight of the solid low molecular weight prepolymer, so as to process and powder Come convenient.
5. It is easy to introduce reactive groups at the chain ends or chains to form active oligomers, thereby obtaining thermosetting polyimides.
6. Use the carboxyl group in the polyimide to esterify or form a salt, introduce a photosensitive group or a long-chain alkyl group to obtain an amphiphilic polymer, and obtain a photoresist or be used for the preparation of an LB film.
7. The general process of synthesizing polyimide does not produce inorganic salts, which is particularly advantageous for the preparation of insulating materials.
8. The dianhydride and diamine as monomers are easily sublimated under high vacuum, so it is easy to form a polyimide film on the workpiece, especially the device with uneven surface by vapor deposition method.
Fourth, the application of polyimide:
Due to the characteristics of the above-mentioned polyimide in performance and synthetic chemistry, it is difficult to find such a wide range of applications as polyimide among many polymers, and it shows extremely outstanding performance in every aspect .
1. Film: It is one of the earliest commodities of polyimide. It is used for slot insulation of motors and cable wrapping materials. The main products are DuPont Kapton, Ube-produced Upilex series and Zhongyuan Apical. The transparent polyimide film can be used as a flexible solar cell backplane.
2. Paint: used as an insulating paint for electromagnetic wires, or as a high temperature resistant paint.
3. Advanced composite materials: used in aerospace, aircraft and rocket components. It is one of the most high-temperature structural materials. For example, the planned supersonic airliner in the United States has a designed speed of 2.4M, a surface temperature of 177 ° C during flight, and a required service life of 60,000h. It is reported that 50% of the structural materials have been determined to be thermoplastic polyimide The carbon fiber reinforced composite material of the resin is about 30 tons per aircraft.
4. Fiber: The modulus of elasticity is second only to carbon fiber. It is used as a filter material and bulletproof and fireproof fabric for high temperature media and radioactive materials.
5. Foam plastic: used as high temperature resistant heat insulation material.
6. Engineering plastics: there are thermosetting and thermoplastic types, which can be molded by injection molding, injection molding or transfer molding. Mainly used for self-lubricating, sealing, insulating and structural materials. Guangcheng polyimide material has begun to be applied to mechanical parts such as compressor rotors, piston rings and special pump seals.
7. Adhesive: used as high temperature structural adhesive. Guangcheng polyimide adhesive has been produced as a highly insulating potting compound for electronic components.
8. Separation membrane: used for the separation of various gas pairs, such as hydrogen / nitrogen, nitrogen / oxygen, carbon dioxide / nitrogen or methane, etc., to remove water from the air hydrocarbon raw material gas and alcohols. It can also be used as pervaporation membrane and ultrafiltration membrane. Because of its heat resistance and resistance to organic solvents, polyimide is of particular importance in the separation of organic gases and liquids.
9. Photoresist: there are negative and positive resists, the resolution can reach sub-micron level. It can be used for color filter with pigment or dye, which can greatly simplify the processing procedure.
10. Application in microelectronic devices: used as a dielectric layer for interlayer insulation, as a buffer layer can reduce stress and improve yield. As a protective layer, it can reduce the impact of the environment on the device, and it can also mask the a-particles to reduce or eliminate the soft error of the device.
11. Alignment aligning agent for liquid crystal display: Polyimide occupies a very important position in TN-LCD, SHN-LCD, TFT-CD and aligning agent materials for future ferroelectric liquid crystal displays.
12. Electro-optical materials: used as passive or active waveguide materials, optical switch materials, etc., fluorine-containing polyimide is transparent in the communication wavelength range, using polyimide as the matrix of the chromophore can improve the material Stability.
In summary, it is not difficult to see why polyimide can stand out from the many aromatic heterocyclic polymers that appeared in the 1960s and 1970s, and eventually became the reason for an important class of polymer materials.
V. Outlook:
Polyimide has been fully recognized as a promising polymer material, and its application in insulating materials and structural materials is continuously expanding. Functional materials are emerging and their potential is still being explored. But after 40 years of development, it has not yet become a larger variety. The main reason is that the cost is still too high compared to other polymers. Therefore, one of the main directions of polyimide research in the future should still be to find ways to reduce costs in monomer synthesis and polymerization methods.
1. Synthesis of monomers: The monomers of polyimide are dianhydride (tetraacid) and diamine. Diamine synthesis methods are relatively mature, and many diamines are also commercially available. Dianhydride is a relatively special monomer. It is mainly used for the synthesis of polyimide except for curing agent of epoxy resin. Pyromellitic dianhydride and trimellitic anhydride can be obtained by one-step oxidation of mesitylene and trimellitic acid extracted from heavy aromatic hydrocarbon oil of petroleum refining products by gas phase and liquid phase. Other important dianhydrides such as benzophenone dianhydride, biphenyl dianhydride, diphenyl ether dianhydride, and hexafluoro dianhydride have been synthesized by various methods, but the cost is very expensive. For example, hexafluoro dianhydride reaches Ten thousand yuan. High-purity 4-chlorophthalic anhydride and 3-chlorophthalic anhydride can be obtained by chlorination, oxidation and isomerization separation of o-xylene developed by Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Series dianhydrides have great potential for cost reduction and are a valuable synthetic route.
2. Polymerization process: The currently used two-step process and one-step polycondensation process all use high-boiling solvents. Aprotic polar solvents are expensive and difficult to remove. Finally, high-temperature treatment is required. The PMR method uses an inexpensive alcoholic solvent. Thermoplastic polyimide can also be polymerized and pelletized directly in the extruder with dianhydride and diamine, no solvent is needed, and the efficiency can be greatly improved. It is the most economical synthetic route to use chlorophthalic anhydride to polymerize diamine, bisphenol, sodium sulfide or elemental sulfur to obtain polyimide without passing through dianhydride.
3. Processing: The application surface of polyimide is so wide, and there are also various requirements for processing, such as high uniformity film formation, spinning, vapor deposition, sub-micron lithography, and deep straight wall Etching, large-area, large-volume molding, ion implantation, laser precision processing, nano-scale hybrid technology, etc. all open up a vast world for the application of polyimide.
As the processing technology of synthetic technology is further improved and the cost is greatly reduced, while having excellent mechanical properties and electrical insulation properties, thermoplastic polyimide will definitely show its more prominent role in the future material field. The thermoplastic polyimide is favored because of its good processability.
6. Conclusion:
Several important factors for the slow development of polyimide:
1. Preparation of raw materials for producing polyimide: pyromellitic dianhydride is not sufficiently pure.
2. The raw material for pyromellitic dianhydride, namely, mesitylene, has a limited output. International production: 60,000 tons / year, domestic production: 5000 tons / year.
3. The production cost of pyromellitic dianhydride is too high. In the world, about 1.2-1.4 tons of pyromellitic dianhydride produce 1 ton of pyromellitic dianhydride, while the best manufacturers in China currently produce about 2.0-2.25 tons of mesitylene. Tons, only Changshu Federal Chemical Co., Ltd. reached 1.6 tons / ton.
4. The production scale of polyimide is too small to form an industry. Polyimide has many side reactions and is complicated.
5. The traditional demand awareness of most domestic enterprises limits the application to a range. It is customary to use foreign products or see foreign products before seeking them in China. The needs of various enterprises come from the downstream customer needs of the enterprise, the information feedback and information source channels are not smooth, there are many intermediate links, and the amount of correct information is large.
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