Polyimide is a typical heat-resistant polymer and the most valuable high-temperature-resistant engineering plastic. It also has high mechanical, dielectric, low-temperature and radiation resistance properties. PI can maintain physical properties in short term at 550℃, and can be used at close to 330℃ for a long time. It is widely used in aviation, aerospace, electronics, electrical and automotive functional parts, etc.
The molecular structure of polyimide contains a very stable aromatic heterocyclic structure, which makes it with excellent properties unmatched by other polymers. The specific structure and properties of PI are as below.
(1) Molecular structure
The macromolecular main chain of PI contains a large number of nitrogen-containing five-membered heterocyclic rings and aromatic rings. It is a semi-ladder ring chain polymer. Meanwhile, the main chain contains a certain number of ether bonds. The overall result is that the molecule is relatively rigid. Due to the conjugation effect of aromatic heterocycles, it has excellent stability and heat resistance; Due to the molecular symmetry of different varieties, PI has two types: crystalline and amorphous.
(2) General performance
PI is a flame-retardant variety, and its oxygen index can reach 36%. PI has good gas barrier properties and low water absorption.
(3) Mechanical properties
PI has good mechanical properties, especially tensile strength, creep resistance, wear resistance and friction, and does not change much by temperature. For example, the tensile strength of the Polypyromellitimide film is as high as 170MPa, and the tensile strength of the biphenyl type PI film is as high as 400MPa, and basically not affected by temperature.
(4) thermal properties
Outstanding high temperature resistance, the thermal decomposition temperature of benzene-type PI is as high as 600℃, which is the polymer variety with the highest thermal stability so far; The service temperature can reach 300℃under anaerobic conditions; PI has excellent low temperature resistance, and the homobenzene type PI will not become brittle at a low temperature of -269℃; the linear expansion coefficient of PI is low, and the linear expansion coefficient of general PI is (2~3)X10-5K-1, of which the biphenyl type is 10-6K-1, and individual varieties can reach 10-7K-1, so the dimensional stability is good.
(5) Electrical properties
PI molecules contain polar groups, but due to the symmetrical structure and high rigidity, the activities of polar groups are affected, so PI still has very good electrical properties; it has outstanding arc corona resistance and is one of the few corona resistant materials, which has high dielectric strength, little influence on electrical properties with frequency. The specific values of its electrical properties are: dielectric constant 3.4, dielectric loss tangent 0.003, dielectric strength 100~300kV/mm, volume resistivity 1017Ω cm.
(6) Environmental performance
PI has excellent oil resistance and solvent resistance, but not alkali resistance; PI has good radiation resistance, and after being irradiated by 4.28X107 Gy drill rays, the strength decreases very little, and the retention rate is 90%.
(7) PI is non-toxic and has good physiological compatibility and other properties.
The performance comparison of commonly used PI is shown in the table below
