Polyimides are usually divided into two main categories:
Thermoplastic polyimides, such as imide films, coatings, fibers and modern microelectronics with polyimide.
Thermosetting polyimide, mainly including bismaleimide (BMI) type and monomer reactant polymerization (PMR) type polyimide and their respective modified products.BMI easy to process but brittle. (1) excellent heat resistance. Polyimide decomposition temperature is generally more than 500 ℃, sometimes even higher, is currently known to be one of the highest thermal stability of organic polymers, mainly because the molecular chain contains a large number of aromatic rings.
(2) excellent mechanical properties. The tensile strength of the unreinforced matrix material is above 100MPa. The tensile strength of Kapton film prepared with homoanhydride is 170 MPa, while biphenyl polyimide (Upilex S) can reach 400 MPa. The modulus of elasticity of polyimide fibers can reach 500 MPa, second only to carbon fibers.
(3) good chemical stability and resistance to heat and humidity. Polyimide materials are generally insoluble in organic solvents, corrosion resistance, hydrolysis resistance. Change the molecular design can be obtained from different structural varieties. Some varieties can withstand 2 atmospheric pressure, 120 ℃, 500h of boiling.
(4) good radiation resistance. Polyimide film in 5 × 109rad dose radiation, the strength is still maintained 86%; some polyimide fiber by 1 × 1010rad fast electron radiation, its strength retention rate of 90%.
(5) good dielectric properties. The dielectric constant is less than 3.5, if the introduction of fluorine atoms in the molecular chain, the dielectric constant can be reduced to 2.5 or so, dielectric loss of 10, dielectric strength of 100 to 300kV/mm, volume resistance of 1015-17Ω-cm. Therefore, the synthesis of fluorine-containing polyimide materials is currently a more popular research areas.
The above properties are stable over a wide temperature range and frequency range. In addition, polyimide also has low temperature resistance, low coefficient of expansion, flame retardant and good biocompatibility and other characteristics. Polyimide's excellent comprehensive performance and synthesis of chemical diversity, can be widely used in a variety of fields. (1) film: polyimide is one of the earliest commodities, used for motor slot insulation and cable winding materials. The main products are DuPont's Kapton, Japan's Upilex series of UBE and Apical of Zhong Yuan. Transparent polyimide film can be used as a soft solar cell substrate;
(2) coatings: as an insulating varnish used for electromagnetic wires, or as a high-temperature resistant coatings used;
(3) advanced composite materials matrix resin: used in aerospace, aviation vehicle structure or functional components and rockets, missiles and other parts, is one of the most high-temperature resistant structural materials;
(4) fiber: polyimide fiber modulus of elasticity second only to carbon fiber, can be used as a high-temperature media and radioactive material filtering materials and bullet-proof fireproof fabrics;
(5) foam: can be used to do high-temperature insulation materials;
(6) engineering plastics: there are thermosetting and thermoplastic, can be molded or available for injection molding or transfer molding (RTM), is mainly used for Self-lubricating, sealing, insulation and structural materials. In addition, polyimide can also be used as adhesives in high-temperature environments, separation membranes, photoresists, dielectric buffer layer, liquid crystal orientation agent, electric - optical materials, etc. PI film is divided into two categories according to the use of the general insulation and heat for the purpose of electrical grade and electronic grade with the requirements of flexibility. Electrotechnical grade PI film due to lower requirements of the domestic has been able to mass production and performance and foreign products are not significantly different; electronic grade PI film is produced with the development of FCCL, is the largest application areas of PI film, which in addition to maintaining the excellent physical and mechanical properties of the electrical class PI film, the coefficient of thermal expansion of the film, the face of the anisotropy (thickness uniformity) put forward more stringent requirements. The future still need to import a large number of electronic grade PI film, the reason is that the domestic PI film in the performance of the imported PI film there is a certain gap, can not meet the requirements of FCCL high-end products. In predicting future market prices, for a long time the pricing of electronic grade PI film has been controlled by DuPont, Zhong Yuan company, but with the South Korean SKC and KOLON two companies in recent years to join the reorganization of the two companies respectively, as well as the economic crisis on the impact of foreign sales of electronic products, the product price has also been reduced, but the electronic grade PI film is still there is a higher profit margins.