By Guo Lingyan, CNCIC

Polyimide (PI) film has good heat resistance, high mechanical strength, excellent dielectric properties, good chemical stability and low thermal expansion coefficient. It is one of the organic polymer materials, boasting the best performance and thus, being widely used in aerospace, microelectronics, liquid crystal, and separation membranes etc.
   The main application forms of polyimide include films, fibers, composite materials, engineering plastics, foams, etc. PI film is the first among them to realise commercial production. In terms of application, PI film can be divided into electrical grade for insulation and heat resistance and electronic grade with high flexibility and low expansion coefficient. Using as a special engineering material, electronic grade PI film is expensive and has high technical barriers.

Main production modes of PI film

   At present, the mainstream polyimide synthesis method is called the "two-step method", that is, the precursor polyamic acid is obtained from the dianhydride and diamine, and then the thermal imine or chemical imine method is used to form the polyimide by intramolecular dehydration and ring closure. The thermal imine method has simple process and equipment and is adopted by major Chinese manufacturers. The chemical imine method has a high single-unit production capacity and excellent physical and chemical properties of film products, but this production process and equipment are only mastered by foreign manufacturers.
   The coating method of PI film is mainly divided into dipping method, casting method and biaxial stretching method. The dipping method is the earliest film preparation method, having an extremely simple preparation process, but low production efficiency and high costs. The casting method is the mainstream manufacturing method of domestic PI film, with good product uniformity and continuous production. The biaxial stretching method is currently used to prepare high-performance PI films. This process is difficult and has high technical barriers, but the film products display the best performance.

Major PI film producers

   The PI film industry has high technical barriers. Large producers are concentrated in the US, Japan and South Korea. China’s PI film industry developed relatively late. There are around 70 domestic PI film producers, with capacities mostly at around 100 t/a and products are mainly used in the low-end market. But China’s rising demand for high-end electronic grade PI film has motivated domestic producers to foray into the high-end products market. Above-scale electronic grade PI film producers include Taimide Tech and Mortech in Taiwan and Zhuzhou Times New Materials, Danbang Technology and Rayitek in Mainland China. In 2019, domestic PI capacity totalled 16 kt/a, with the average run rate at around 60% but the output of electronic grade PI film was below 1 kt. Table 1 shows China’s major PI film producers in 2019. 

Major applications of PI film

   PI is widely used in space technology, F and H class motors, electrical insulation, flexible printed circuits (FPC), thermally conductive graphite film, TAB (pressure sensitive tape substrate), computers, electromagnetic wires, transformers, mining electronic components, automobiles, transportation, atomic energy industry and other electronic and electrical industries. Among them, electronic display, flexible printed circuit (FPC) and thermal conductive graphite film are considered to be the largest and fastest growing application areas in the future PI film market.
   As for electronic display, the development of flexible AMOLED technology drives the growth of PI film demand. In 2019, folding screens became the outlet of the electronic display industry, and Samsung's folding mobile phones Galaxy Fold and Huawei MateX were launched. Compared with quantified flexible AMOLED screens, folding screens have more demand for PI materials, including PI paste as a substrate, CPI base film as a touch panel, and CPI hardened transparent film as a cover. With the popularity of folding AMOLED, the demand for PI materials in the electronic display field will see multiple increases.
   In terms of flexible circuit printing, PI film is mainly used for insulating base film and cover film on flexible copper clad laminates (FCCL). The FPC made by FCCL is suitable for installation in three-dimensional space, saving installation space, and achieving the requirements of light and thin electronic equipment. It is currently widely applied in portable electronic equipment such as mobile phones, digital cameras, digital video cameras, and notebook computers. The rapid development of miniaturized electronic equipment is driving the growth of demand in the FCCL industry, which may further boost demand for PI film.
   Thermally conductive graphite film is an excellent heat dissipation material. It has many characteristics such as ultra-high thermal conductivity, light weight, thinness and bending resistance. Its main material is artificial graphite sheet, generated through the carbonization and graphitization of PI film. Various electronic devices are highly integrated in order to achieve high performance, instant messaging and other functions. The heat accumulation on the surface of internal materials increases. Thermally conductive graphite film is the best heat dissipation material for electronic products at this stage. In the future, with the popularization of electronic products in the 5G era, including 5G mobile phones, tablet computers, and personal computers, the demand for thermally conductive graphite films and PI films will further increase.

   At present, domestic PI film manufacturers mainly produce ordinary electrical-grade films, covering films for electronic products, reinforcing films and a small amount of high-performance PI films. Although many manufacturers have begun to deploy polyimide for flexible AMOLED, there is still a long way to go to achieve large-scale industrial applications.
   Since 2017, many domestic companies have begun to introduce advanced production equipment to deploy high-performance PI films by chemical imine process. China’s PI film capacity is expected to increase by around 30 kt/a by 2025, including the capacity additions of around 1 kt/a at Zhongke Jiuyuan, Junyou Electronics, Rayitek and Zhuzhou Times Huasheng. With the accumulation of relevant R&D and technical talents, key downstream markets such as OLED, flexible circuit boards and graphite films have shifted to the Chinese mainland. The booming development of China’s PI film industry provides the best opportunity for the substitution of domestically made high-end productions for foreign ones.