includes two categories: homophenyl-type polyimide film and biphenyl polyimide film. The former is a product of DuPont Company in the United States, the trade name Kapton, which is prepared by diphenyltetracarboxylic anhydride and diphenyl ether diamine. The latter is produced by Ube Corporation of Japan under the trade name Upilex and is prepared from diphenyl tetracarboxylic anhydride with diphenyl ether diamine (type R) or m-phenylenediamine (type S).

Advantages of polyimide

(1) Excellent heat resistance. Polyimide decomposition temperature generally exceeds 500 ° C, and sometimes even higher, is one of the most thermally stable varieties of known 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 homogeneous anhydride is 170MPa, while biphenyl polyimide (Upilex S) can reach 400MPa. The elastic modulus of polyimide fiber can reach 500MPa, second only to carbon fiber.

(3) Good chemical stability and resistance to moisture and heat. Polyimide materials are generally insoluble in organic solvents and resistant to corrosion and hydrolysis. Variations in molecular design can result in varieties with different structures. Some varieties can withstand 2 atmospheres of pressure, 120℃,500h of boiling.

(4) Good radiation resistance. The strength of polyimide film remained 86% after 5×109rad radiation dose; The strength retention rate of some polyimide fibers after 1×1010rad fast electron radiation is 90%.

(5) Good dielectric properties. The dielectric constant is less than 3.5, and if fluorine atoms are introduced into the molecular chain, the dielectric constant can be reduced to about 2.5, the dielectric loss is 10, the dielectric strength is 100 to 300kV/mm, and the volume resistance is 1015-17Ω·cm. Therefore, the synthesis of fluorinated polyimide materials is a hot research field.

The above properties are stable over a wide range of temperatures and frequencies. In addition, polyimide also has the characteristics of low temperature resistance, low expansion coefficient, flame retardant and good biocompatibility. Polyimide can be widely used in many fields due to its excellent comprehensive properties and diversity in synthetic chemistry.