By Wu Yanni, Sublime China Information Co., Ltd.

Polyimide (PI) fiber has the properties of high strength and modulus, low dielectric, high/low temperature resistance, radiation resistance, flame retardancy and low water absorption. In addition, PI fiber has good chemical stability and can withstand strong acid corrosion. In recent years, with the progress of synthesis technology and spinning technology, the industrialization of PI in China has been accelerating. However, China's PI industry is small in scale and low in capacity, not able to meet the demand of 7 000 to 8 000 tons per year from its downstream sectors like power generation, cement, steel, and nonferrous metals. The import dependency is high.

PI fiber production technologies

The production processes of PI fiber are mainly synthesis process and fiber spinning process.
1. Synthesis process
PI is mainly synthesized from two monomers, dianhydride and diamine. The manufacturers usually apply solution spinning method to produce PI fiber, which can be divided into one-step method and two-step method according to the type of spinning solution.
The one-step method is to let dianhydride and diamine monomers react in phenols and other solvents to get a soluble PI solution, and then make PI fibers from the prepolymer. The crystallinity of the fiber is higher since the solubility of the monomer in the solvent is slightly higher. The one-step method requires the use of soluble monomers for spinning, however, the phenolic solvent pollutes the environment and is difficult to be completely removed in the subsequent processing, which limits the development and application of the one-step method hence.
The two-step method is to carry out condensation polymerization of dianhydride and diamine monomers in aprotic solvents like N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF) and N-methyl-2-pyrrolidone (NMP) to obtain PI prepolymer-polyamic acid, and then spin the polyamic acid solution into polyamic acid nascent fiber. PI fiber is obtained after thermal imidization or chemical imidization. There is a pore structure inside the fiber, which leads to lower mechanical properties of the product. However, there are many types of monomers that can be copolymerized by the two-step method, and PI fibers with different properties can be prepared through the combination of different monomers or functional groups, so it is more suitable for mass industrial production.
2. PI fiber spinning technologies
——Dry spinning
Dry spinning dominated the early stage of PI production. During dry spinning, the spinning solution is pressed out by the spinneret into the spinning tunnel, and the solvent is quickly volatilized through the hot air tunnel. After the dope is desolventized and solidified, the nascent fiber is formed by winding and stretching.
——Wet spinning
The spinning solution enters the coagulation bath through the spinneret, and precipitates in the coagulation bath to form fibers.
——Dry-wet spinning
The dry-wet spinning process has the advantages of both dry spinning and wet spinning. The stretching ratio and spinning speed of the nozzle can be increased, and the structure formation process of the fiber can be controlled more effectively.

The status quo of industrialization research in China

In 2010, Changchun Institute of Applied Chemistry and Changchun Gaoqi PI Material Co., Ltd. developed a heat-resistant PI fiber via wet spinning, which was named "Yilun", for the production of high-temperature filter materials, flame retardant protective clothing and insulation felt. In 2013, Gaoqi's capacity of PI fiber increased to 1 000 t/a, successfully realizing the industrialization of PI fiber.
In 2013, Donghua University and Jiangsu Aoshen New Materials Co., Ltd. cooperated and built a dry spinning production line with a capacity of 1 000 t/a of high-performance heat-resistant PI fiber, which is called "ASPI-TM". Its yarns and staples can be used in high temperature dust removal filtration, protective fabrics, industrial flame-retardant heat insulation and other fields. In 2016, Jiangsu Aoshen New Material Co., Ltd. successfully mass-produced black, army green, and orange-red dope-colored PI fibers. In addition, the company can also produce 70D filaments, which can be used in the aerospace field.
In 2013, Beijing University of Chemical Technology and Jiangsu Xiannuo New Material Technology Co., Ltd. cooperated to establish a high-performance PI fiber production line with an annual output of 30 tons. By using integrated continuous production technology with domestic and foreign intellectual property rights, a series of high-performance PI fiber products have been developed, especially high-strength and high-modulus PI fiber products, which will be promoted to the market. At the same time, they carry out application development in the fields of lightweight and high-strength composite materials, wave-transmitting materials, and radiation resistance and protective materials.

PI fiber applications

1. Application in paper-making industry
PI fiber is an insulating material with excellent performance. Its excellent mechanical properties and high temperature resistance make it suitable for high temperature insulating paper, meeting the needs of different industries.
2. Application in the field of filtration
PI has excellent chemical stability and mechanical properties and has been successfully used in iron furnace dust removal process, various heating equipment and energy power plant boilers, and household and medical waste incineration and other waste incineration.
3. Application in the field of special protection
PI fiber has large thermal resistance coefficient, good heat insulation performance, high temperature resistance, flame retardancy, and can be used in the manufacture of fire protective clothing. The Changchun Institute of Applied Chemistry of the Chinese Academy of Sciences has successfully developed a hybrid braided sheath whose weight is significantly lower than that of the metal braided sheath, which has a significant weight reduction effect and has application advantages in the aerospace field.
4. Application in the field of textile and clothing
The excellent thermal insulation properties of PI fiber have extended it from the field of special protection to the field of civilian clothing recently.