What is polyimide (PI)

Polyimide (PI) refers to polymer with imide structure in the main chain of molecular structure. Polyimide is a very large family, and the main chain of high-performance PI is mostly aromatic ring and heterocyclic ring as the main structural unit.

PI has the highest flame retardant grade (UL-94), good electrical insulation properties, mechanical properties, chemical stability, aging resistance, radiation resistance, low dielectric loss, and these properties in a wide temperature range (-269℃-400℃) will not change significantly, known as "one of the most promising engineering plastics in the 21st century". Known as "problem solver", it can be said that "without polyimide, there would be no today's microelectronic technology". Its performance is at the top of the polymer material pyramid.

Polyimide material can increase the average use temperature of polymer material by more than 100℃, so it has become the polymer material with the best high temperature resistance that can be used in practice.

Application field of polyimide

The excellent properties and processing properties of polyimide enable it to be made into different products in a variety of ways and applied in various fields. Among the many polymers, polyimide is the only polymer that has a wide range of applications and shows outstanding properties in every application field.

Performance characteristics of polyimide

Polyimide materials are widely used. They are respectively used in films, fibers, photosensitive materials, foams and composite materials in flexible screens, rail transit, aerospace, fireproof materials, photoresist, electronic packaging, fan blades, warships, automobiles and other fields.

01 | polyimide film (PI film)

Among all the applications of polyimide, polyimide film (PI film) is the earliest to enter the field of commercial circulation and the largest amount of one. Polyimide film is the world's best film insulation materials, widely used in aerospace, microelectronics, atomic energy, electrical insulation, liquid crystal display, membrane separation technology and other fields. Due to its high price, high technical barriers, excellent performance, polyimide film is also known as "gold film". Polyimide film, carbon fiber and aramid fiber are considered as the three bottleneck key polymer materials which restrict the development of high-tech industry.

According to the "13th Five-Year Plan" New Materials Development Report of the National New Materials Industry Development Strategy Advisory Committee, the global market size of PI film in 2017 was $1.52 billion, and it is expected that the global PI film will reach $2.45 billion in 2022. Among them, electronic display, flexible printed circuit (FPC) and heat-conducting graphite film will become the largest and fastest growing application fields in the global polyimide film market. At present, the United States, Europe, Japan is the world's main polyimide consumption market, such as the main consumption area of the United States is plastic; Japan's main consumption areas are film and plastics; The Asia Pacific region will be the main growth market in the future, with China, India, Japan and South Korea being the main PI film market in this region.

At present, PI film high-end product localization process is accelerating, electronic grade below PI film has achieved domestic self-sufficiency, electronic grade and above PI film market is still mainly divided by overseas companies. With the gradual implementation of domestic chemical imine production lines, domestic manufacturers will participate in the high-end market share of nearly 10 billion market. It is believed that the market of high-end electronic grade PI films will be in a period of rapid expansion in the future as the flexible coppers market maintains a high growth rate and the demand for flexible substrates increases with the rapid popularity of OLED.

02 | polyimide fiber (PI) fiber

Polyimide fiber is an important high performance fiber. Its high temperature resistance polyimide fiber is currently one of the highest use temperature of organic synthetic fibers, can be used at 250~350℃, in light resistance, water absorption, heat resistance compared with aramid fiber and polyphenylene sulfide fiber are more superior, high performance polyimide fiber strength is about 1 times higher than aramid fiber, is currently one of the best mechanical properties of organic synthetic fibers.

Due to its excellent mechanical properties and high temperature resistance, polyimide fiber plays an important role in many fields, mainly including the following aspects:

(1) Aerospace: light cable sheathed, high temperature resistant special braided cable, large diameter expanded satellite antenna tension cable, etc.

(2) Environmental protection field: industrial high temperature dust removal and filtration materials.

(3) Fireproof materials: high temperature and flame retardant special protective clothing, cold clothing, racing anti-fire clothing, etc.

At present, polyimide fiber is sold at a high price. At present, it is mainly used in aerospace field for its unique low temperature applicability (suitable for the temperature environment below -100℃ in outer space). With the gradual improvement of environmental protection standards in the future, the application of polyimide fiber in industrial dust removal filter media will be gradually amplified. At present, about 80% of the flue gas of coal-fired boilers is electrostatic precipitator. Under the strict emission standards, bag precipitators will gradually become electrostatic precipitators. Polyimide fiber is the best filter material.

The development of polyimide fiber was first led by the United States and Japan, but for various reasons, there is no industrialization of polyimide fiber in the United States and Japan. At present, there are only P84® fiber from Evonik in Germany and nylon ® fiber from Changchun Gaoqi in China, among which Evonik's production capacity is about 1400 tons/year, and Changchun Gaoqi's production capacity is about 540 tons/year. Changchun Gaoqi's breakthrough in polyimide fiber technology has solved the problem of the demand for polyimide in the military and aerospace fields in China.

Due to its excellent heat resistance and mechanical properties, PI fiber is the core accessory material in important fields such as aerospace and military aircraft, and its application in the military market is irreplaceable. In the commercial field, PI fiber in environmental protection filter materials, fire materials and other applications are currently in the gestation period, is expected to add new vitality for PI fiber in the future.

03 | PI/PMI foam

PI/PMI foam is the most excellent structural foam core material. Besides military, it is also widely used in luxury cruise ships, speedboats and liquefied natural gas ships.

Polyimide foams can be divided into three categories: (1) Foam materials that use imide as the main chain, as with general polyimides, and use temperature of more than 300℃ (PI foam); (2) foam materials that use imide rings in the form of side groups (PMI foam); and (3) nano foam materials that are obtained by introducing heat unstable fatty chain segments into polyimide and cracking at high temperature.

Polyimide foam is an advanced functional material, which has been increasingly applied in the fields of aeronautics and astronautics, ocean transportation, national defense, microelectronics and other high-tech fields such as heat insulation, vibration and noise reduction, insulation and other key materials.

PI/PMI bubble: Benefit from the climax of warship construction, welcome the "blue Ocean" era. At present, the most important application of PI foam is heat insulation and noise reduction material for naval ships. Currently, our navy is in the third ship construction climax. As the first choice of heat insulation and noise reduction material for new warships, PI foam is rapidly increasing in demand. Polymethylacryimide foam (PMI for short) is a new polymer structural foam material with the best comprehensive performance at present. It is a kind of high-performance composite foam core material with high specific strength, high specific modulus, high closure rate and high heat resistance. It has the characteristics of light weight, high strength and high/low temperature resistance. In addition, as the most excellent structural foam core material, PMI foam is widely used in fan blades, helicopter blades, aerospace and other fields. Its replacement trend for PET foam is clear, and the market space is broad.

PI foam has strong heat resistance, good flame retardant, does not produce harmful gas, easy to install, is widely used in heat insulation and noise reduction material. Currently, the U.S. Navy uses PI foam for sound insulation on all surface ships and submarines, and INSPEC's SOLIMIDE foam has been developed for sound insulation on naval ships in more than 15 countries. It is also widely used in civilian vessels, such as luxury cruise ships, speedboats and liquefied natural gas vessels.

Similar to PI bubbles, PMI bubbles are also widely used. Typical applications of PMI bubbles include:

(1) Structural foam core material: excellent compression resistance at high temperature makes it widely used as core material in fan blades, aviation, aerospace, ships, moving equipment, medical instruments and other fields;

(2) wide-band wave-passing material: low dielectric constant and loss make it widely used in radar, antenna and other fields;

(3) Heat insulation and sound insulation materials: high-speed locomotive, tires, sound, etc.

Aluminum honeycomb or NOMEX® honeycomb, which has the advantages of high compression modulus and light weight, is commonly used in conjunction with carbon/glass prepreg. Common structures include wing leading edges, rudder, landing gear bay doors, wing body and wing tip fairing. But honeycomb sandwich material needs high maintenance and repair cost, foam core material is an ideal substitute.

After research in recent years, PMI foam has been gradually recognized as the most excellent foam core material with excellent creep resistance, heat resistance and durability. At present, PMI foam as structural foam core material has been widely used in fan blade, helicopter blade, aircraft manufacturing and other fields.

It is estimated that more than 50,000 tons of polyimide foam will be used globally by 2024, and the market capacity is expected to reach $2 billion.

For the domestic market, the downstream market is the heat insulation and noise reduction materials for ships and aircraft. At present, China is in the climax of the third warship construction, the number of warships is growing rapidly, and with the increasingly urgent demand for "blue water navy", Chinese naval warships are in urgent need of upgrading. As a new material for warships with excellent performance, polyimide foam has a broad market space, which may reach billions in the future.

PI/PMI foam in foreign countries was first researched and produced by NASA for military use, then Sordal, DuPont, Evonic, InspectFoam and other manufacturers also developed corresponding products. Evonic's Rohacell® series PMI foam and Solimide® series PI foam are the two most used varieties in the market at present.

At present, the main manufacturers of polyimide foam in China are Ningbo Materials Institute of Chinese Academy of Sciences, Qingdao Ocean, Kangda New Materials and Tiansheng New Materials, etc. Among them, Ningbo Institute of Materials of Chinese Academy of Sciences has built a pilot equipment for polyimide micro-foaming particles; Qingdao Ocean and Kangda Xincai polyimide product performance has been in the forefront of domestic counterparts.

04 | PI matrix composites

Fiber reinforced composite is a new generation of lightweight materials after magnesium aluminum alloy. Polyimide as a resin based composite has excellent high temperature resistance and tensile properties, and is widely used. Polyimide resin matrix composite has the characteristics of high heat resistance, excellent mechanical properties, dielectric properties, solvent resistance and so on. It is the resin matrix composite with the highest temperature in use at present, and has been widely used in aviation (especially aero-engine), aerospace and other fields.

After nearly 40 years of development, four generations of polyimide high temperature resistant resin matrix composites have been developed, and the use temperature has been continuously improved. At present, the most advanced fourth generation polyimide resin matrix composites can be used at 450℃ for a long time.

At present, the application and research and development of polyimide composites in China is still catching up. Companies such as AVIC Composites Corporation have been able to produce third-generation resin products.

In addition, with the gradual maturity of the carbon fiber industry, the demand for carbon fiber reinforced composites has increased significantly. As one of the most excellent composite material combinations, the combination of polyimide and carbon fiber has obvious advantages in occupying the high-end market.

05 | PSPI (photosensitive polyimide)

Core technology abroad control, domestic market demand depends on imports, localization process is a long way to go.

Photosensitive polyimide (PSPI) is a kind of organic material with imine ring and photosensitive gene in polymer chain, which has excellent thermal stability, good mechanical properties, chemical and photosensitive properties.

Photosensitive polyimide has two main functions in the field of electronics: photoresist and electronic packaging. Adding sensitizer and stabilizer to photosensitive polyimide can get "polyimide photoresist".

Compared with traditional photoresist, PSPI photoresist does not need to be coated with light blocking agent and can greatly reduce the processing process. At the same time, PSPI is also an important electronic packaging adhesive. Photosensitive polyimide as packaging material can be used in buffer coating, passivation layer, alpha ray shielding material, interlayer insulation material, chip packaging material, etc. At the same time, it is also widely used in the microelectronics industry, including integrated circuit and multi-chip packaging.

Another role of photosensitive polyimide is as an advanced electronic packaging resin. With the miniaturization, thin-film, high performance, multi-function, high reliability and low cost of electronic products, the market demand increases the density of integrated circuit packaging, and the requirements for IC packaging materials are also increasing. With the advent of the era of large chips, the original metal and ceramic materials packaging technology has been greatly impacted. Electronic packaging materials represented by epoxy resin and polyimide resin are gradually emerging as important IC packaging materials.

Photosensitive polyimide as packaging materials can be used in buffer coating, passivation layer, alpha ray shielding material, interlayer insulation material, wafer packaging material, and also widely used in the microelectronics industry, including integrated circuits (ICs) and multi-chip packaging (MCPs).

The domestic PSPI industry started late. At present, the research and development, production and application of PSPI are far behind Japan, the United States and other countries, and most of our PSPI market demand depends on imports. At present, in the global market, the PSPI market is mainly controlled by HDM Company and Toray Company, a joint venture between the United States and Japan. Toray is one of the most successful enterprises in the global market of positive PSPI products. Its positive products are applied in many fields such as microelectronics packaging and optoelectronic packaging.

At present, domestic enterprises continue to deepen PSPI, and some enterprises have mastered the production technology. At present, the layout of PSPI research and development, production of local enterprises include Ruihuatai thin film technology, Time New materials, Aoshen new materials, Huicheng Technology, Guofeng Plastic Industry, etc.

Concluding remarks

The threshold of polyimide industry is high and there are high technical barriers. As polyimide related materials play an irreplaceable role in aerospace, military, high-end electronics and other sensitive fields, most foreign polyimide raw materials, technologies and products are strictly blocked from China. Although domestic enterprises have been trying to catch up, but our domestic and mass production of high-end products and foreign advanced level is still a large gap. Therefore, to vigorously develop polyimide related products is very urgent, a long way to go!