Polyimide (PI) is a high molecular polymer containing an imide structure in the main chain of the molecular structure. The main chain of high-performance PI is mostly based on aromatic rings and heterocyclic rings as the main structural units. PI has the highest flame retardant grade, good electrical insulation performance, mechanical properties, chemical stability, aging resistance, radiation resistance, dielectric constant of 4.0 at 103 Hz, and dielectric loss of only 0.004-0.007, belonging to F to H grade insulation, and these properties will not change significantly in a wide temperature range (-269℃ to 400℃). Its performance is at the top of the pyramid of polymer materials and is the best performing film insulation material.
PI has the characteristics of high insulation strength, high and low temperature resistance, low thermal expansion coefficient, radiation resistance, flame retardant self-extinguishing, and high stability. Thanks to its excellent comprehensive performance and outstanding processing performance, PI can be made into various forms of products other than rubber, including PI film, PI fiber, PI foam, PI resin, PI-based composite materials, photosensitive PI (PSPI), etc. The diversity of product types ranks first among polymer materials and is widely used in various fields such as electronic communications, aerospace, new energy, electrical insulation, and automotive industry. Its importance to high-tech industries is prominent.
PI film products generally use PMDA and ODA as the main monomers, polymerize in polar solvents DMAc or DMF to form PAA resin solution, and then spit out into gel film, which is made through directional stretching, imidization, and post-treatment processes. Except for resin synthesis, other processes are carried out in clean workshops above Class 10,000.
The production of PI film mainly includes the following processes:
(1) PAA resin synthesis. PAA resin synthesis is the initial solution resin for preparing PI film. The resin formula design and synthesis process directly affect the performance of PI film and the film making process setting. Generally, PMDA monomer and ODA monomer are used to synthesize high-viscosity PAA resin in DMAc polar solvent, and other fillers or monomers are added according to the performance requirements and process characteristics of different products.
(2) Casting. Casting is the process of realizing resin film formation. The high-viscosity PAA resin material is uniformly extruded onto a uniformly running circular steel belt through an automatic extrusion die with a set thickness. Through precise hot air drying, a self-supporting PAA gel film is made and introduced into the directional stretching process.
(3) Directional stretching and imidization. Directional stretching and imidization are the processes of further drying the PAA gel film and performing directional stretching and imidization to make PI film. Directional stretching can be biaxial stretching or uniaxial stretching according to process requirements. During stretching, the polymer chains of the PAA gel film are oriented along the directional direction through multi-level precise temperature setting, stretching ratio setting and speed control. During the stretching and running process, the PAA gel film is fully dehydrated and cyclized and imidized to form a PI film.
(4) Post-treatment. The imidized PI film needs to undergo post-treatment processes such as high temperature treatment, surface treatment and slitting and winding to prepare a PI film product.
According to different application categories, PI films can be divided into electrical PI films, electronic PI films, thermal control PI films, PI films for aerospace, CPI films for flexible displays, etc.
(1) Electrical PI films: Mainly used in the field of electrical insulation, including high-grade insulation systems for motors, transformers, etc. Key characteristics include temperature resistance and insulation strength. Products with corona resistance can also be used in insulation systems in high-speed rail transportation and wind power generation.
(2) Electronic PI films: Mainly used in the field of electronic substrates, as an insulating base film and copper foil bonded to form the substrate part of FCCL, it can also be covered on the surface of FPC for protection. Products that meet the requirements of high-frequency and high-speed transmission can also be used in the field of 5G communications.
(3) Thermal control PI films: Mainly used in the field of electrical thermal control systems, such as high thermal conductivity graphite film precursor PI films that are processed through carbonization, graphitization and other processing steps to make high thermal conductivity graphite films for heat dissipation and heat conduction. The specially designed PI film structure has the characteristics of easy graphitization and suitable for whole roll firing.
(4) PI film for aerospace: mainly used for thermal control or protective materials of spacecraft, etc., and must have excellent properties such as high and low temperature resistance, radiation resistance, low vacuum mass loss and low condensable volatiles.
(5) CPI film for flexible display: used in the field of device optical cover, mainly used as OLED screen cover, touch sensor panel, etc., and must have high light transmittance and bending resistance.
