PI is currently the most suitable material for flexible displays. Flexible OLED displays are manufactured based on polymer flexible substrates, and currently flexible OLED display technology is developing from curved to foldable and rollable directions. In this development process, not only the display unit in OLED displays needs to be flexible, but also other functional layers including conductive electrodes and touch screen units need to be flexible, and the key material to achieve flexibility is polyimide film. When future display screens adopt foldable or rollable forms, other hard material layers in the screen will also be gradually replaced by polyimide (PI) or transparent polyimide films.
Polyimide has six major characteristics:
(1) High temperature resistance: It can withstand high temperatures of over 400 ℃, with a long-term use temperature range of -200~300 ℃ and no obvious melting point.
(2) High insulation performance: dielectric constant of 4.0 at 103Hz, dielectric loss of only 0.004~0.007, belonging to F to H level insulation materials.
(3) Excellent mechanical properties: The tensile strength of unfilled plastics is above 100Mpa.
(4) Self extinguishing: Polyimide is a self extinguishing polymer with a low smoke emission rate.
(5) Non toxic: Polyimide is non-toxic and can withstand thousands of cycles.
(6) Stability: Some polyimide varieties are insoluble in organic solvents and stable to dilute acids, while general varieties are not very resistant to hydrolysis.
In terms of flexible display, compared with several commonly used substrate materials, PI material has significant advantages. The deformation of PET is relatively large, but it will undergo plastic deformation under long-term bending. CPI has a large deformation and excellent performance. As an optical transparent film for flexible electrodes, touch substrates, display substrates, or cover plates, it needs to be able to adapt to high-temperature processing and have bending resistance of more than 100000 times. It also requires high transparency and low thermal expansion coefficient. At present, transparent polyimide film is one of the few organic film materials that can meet these high-performance requirements. Therefore, the flexible materials used in the layout of foldable screens for smartphones often adopt CPI.
In the field of polymers, PI also demonstrates significant advantages. In the manufacturing process of AMOLED display devices, in order to produce high-quality TFT backplates, processes such as LTPS are required, and the process temperature often exceeds 350 ℃. This requires polymer optical thin film materials as flexible substrates to maintain good performance at this process temperature. Traditional polymer optical films, such as PET (Tg~78 ℃), PEN (Tg~122 ℃), etc., cannot meet the application requirements. PI can withstand high temperatures and has good mechanical properties and chemical stability, making it an ideal flexible OLED material.
Flexible display brings billions of market for PI materials
During the transition from rigid OLED to flexible OLED, some materials are no longer suitable for flexible requirements due to their inherent physical and chemical limitations. The inherent brittleness of materials such as glass and ITO makes it difficult for them to meet a wide range of emerging application scenarios, and they need to switch to PI with better performance.
There are mainly the following changes:
(1) Switching from glass substrate to PI substrate;
(2) Switching from ITO+PET based film for touch control to nano silver wire+PI based film;
(3) Switch from glass or metal cover to PI cover. Except for the substrate that can use yellow PI, both touch and cover plates require CPI, which has high requirements for optical properties.
According to IHS data, the amount of PI used for flexible substrates reached 2687 tons in 2018, and it is expected that the amount of PI used for flexible substrates will reach 3507 tons in 2019, a year-on-year increase of 31%. In the long run, taking the 6th generation production line of BOE with a capacity of 48k/month as an example, considering that the cover plate, touch screen, and substrate materials are all switched to PI materials, based on a PI consumption of 3 kilograms per square meter, the PI material demand for BOE's single line alone will reach 4800 tons. The theoretical production capacity of flexible OLED production lines worldwide is 37.22 million square meters, and the demand for PI materials at full capacity is as high as 111700 tons. The market price of PI film reaches 600000 to 3 million yuan per ton, among which the market price of biaxially stretched electronic film is above 1 million yuan per ton, while the value of CPI film used for flexible display is even higher. Based on an average price of 1.8 million yuan per ton, the market size of PI materials will be over 100 billion yuan.
High end PI film relies on imports, with international giants monopolizing the market
PI film belongs to the high-tech barrier industry. Currently, the demand for PI film in the domestic market is growing rapidly and has enormous potential. However, most of the products produced by domestic PI film manufacturers are used for electrical insulation, and high-end electronic grade PI heavily relies on imports, which is in a state of supply shortage. The synthesis of PI is extremely difficult, and downstream product processing is also challenging. In terms of monomer synthesis, although there are corresponding monomers commercialized, the cost is very expensive, for example, hexafluorodianhydride can reach tens of thousands of yuan per kilogram; In terms of polymerization methods, both two-step and one-step methods currently use high boiling point solvents, which are expensive and difficult to handle in the later stage; In terms of production process, the impregnation method and biaxial stretching method have high production costs, while the casting method has relatively low costs but high pollution.
PI is in an oligopolistic monopoly worldwide, with strict technological blockades and global production capacity mainly monopolized by a few foreign companies. PI's research and development, as well as core technologies, are mainly focused abroad, such as DuPont in the United States, Ube Industries, Chung Yuan Chemical, Toray Group, Mitsui Dongya in Japan, Cologne and SKC in South Korea, etc. Among them, DuPont, Dongli, Zhongyuan, and Ube account for about 70% of the global PI market sales.
The research on PI in China started relatively late, starting in 1962. After unremitting efforts, various PI varieties such as biphenyl type, homobenzene type, ketone anhydride type, soluble, colorless and transparent have been developed. Domestic R&D and key technologies are mainly concentrated in the Chinese Academy of Sciences, Changchun Institute of Applied Chemistry and relevant universities, as well as Guilin Electric Power Academy, Shenzhen Ruihuatai, Changchun Gaoqi, Shandong Wanda, Jiangsu Yabao, Liyang Huajing, Ningbo Jinshan Electronics, Jiangyin Tianhua, Wuxi Gaotuo, Jiangsu Xiannuo and other companies.
The PI industry is developing rapidly, and leading enterprises are expected to enjoy the dividend of substitution
CPI overcomes the shortcomings of traditional polyimide films with light or dark yellow colors and is a relatively new type of polyimide. Its excellent thermal properties meet the needs of new developments in optoelectronic materials and play an important role in optoelectronic devices such as flexible device substrate films.
The industrialization of CPI film is carried out by Mitsubishi Gas, DuPont, Mitsui Chemicals SKC、KOLON、 Dongyang Textile and others are the main producers. Mitsubishi Gas was the earliest company to commercially produce CPI film, and SKC and KOLON are also actively researching and developing CPI film.
China is relatively backward in the field of optical grade transparent polyimide films, and no enterprise has been able to achieve large-scale production yet. But with the rapid development of the entire industry, many enterprises are actively laying out, improving manufacturing technology and product quality through continuous research and development investment, technology introduction and absorption, achieving a reasonable layout of technology and products, and are expected to gain a place in the high-end polyimide market in the future. The listed companies in China that have laid out PI films mainly include Danbang Technology, Times New Materials, Guofeng Plastic Industry, Xinlun Technology, Dinglong Shares, etc. With the continuous breakthrough of technology and the gradual landing of production capacity, the quality of domestic polyimide products is gradually catching up with the international level. In the future, domestic dragon head enterprises are expected to achieve import substitution and enjoy market dividends.
Editor: Sichuan Jinzhongde Science and Technology Research Institute
