2.5 Applications of Polyimide Fibers
2.5.1 Production of Polyimide Fibers
Currently， there are several companies focusing on the development of advanced PI fibers in China， including Jiangsu Aoshen Hi-tech Materials Co.， Ltd， Changchun Polyimide Materials Co.， Ltd， and Jiangsu Shino New Materials & Technology Co.， Ltd. Of these， Jiangsu Aoshen Hi-tech Materials Co.， Ltd and Donghua University jointly developed a new technology to prepare PI fiber through dry-spinning， building the first dry-spinning PI fiber production line with annual output of 1000 tons. The products， branded Suplon PI fibers， include filaments， staple fibers， chopped fibers， colorful yarn， and so on. Changchun Polyimide Materials Co.， Ltd cooperated with Changchun Institute of Applied Chemistry Chinese Academy of Sciences， and mainly used the wet-spinning technology to produce PI fibers. A series of Yilun products are commercially available， including PI filaments， staple fibers， chopped fibers， and so on. Additionally， Jiangsu Shino New Materials Technology Co.， Ltd. have been focusing on the high-performance PI fibers supported by Beijing University of Chemical Technology. A series of Shilon products have been manufactured via an integrated continuous two-step wet-spinning method， as shown in Fig. 2.24. The tensile strength and modulus of the high-performance PI fibers are measured up to 3.5 GPa and 140 GPa， respectively.
FIGURE 2.24 PI fibers with high strength and high modulus.
2.5.2 Application of Polyimide Fibers
PI fiber has been considered as one of the most important high-performance fibers， with the development of PI synthesis technology and fiber spinning technology， the process of industrialization for PI fibers with outstanding mechanical properties， good thermal stability， and radiation resistance gradually accelerated， playing a more and more important role in the fields of high-temperature filtration， special protection， aerospace， national defense， new building materials， environmentally friendly and fireproof applications， and so on. Fig. 2.25 depicts several PI fiber products， including a house-bag duster for high temperature， high-temperature roller， high-temperature- resistant fabric， etc.
FIGURE 2.25 Typical products of polyimide fibers prepared by Jiangsu Aoshen Hi-tech Materials Co.， Ltd. （A） High-temperature filtration bag； （B） high-temperature roller； （C） high-temperature-resistant batting； （D） polyimide clothing； （E） polyimide fleece jackets； （F） flame-retardant carpet.
1. High-temperature PI filter bags. PI fibers are applied in environment-protection areas mainly as high-temperature filtration materials. The total annual sales of house-bag dusters are more than 30 billion yuan， requiring around 1×108 m2 filter materials. At present， high-performance filter materials are insufficient and expensive， and product qualities are relatively low with short service life and low dedusting efficiency， which cannot meet the increasingly stringent environmental requirements. House-bag dusters produced with PI fibers can effectively catch dust grains， and PI fibers with special cross-sections can make dust gather on the surface of filtration materials， which can avoid pore block， improving the filter efficiency. In addition， PI fibers are the best high-temperature filtration materials due to their outstanding chemical resistance， acid resistance， and excellent thermal stability， and they are widely employed in the fields of thermal power generation， cement production， metal smelting， and so on， playing an important role in protecting the environmental pollution arising from PM 2.5 and PM 10.
2. Special protection. PI fibers show very low thermal conductivities. For example， the coefficient of thermal conductivity is around 0.03 W·m−1·K−1 at 300 °C. The LOI of PI fibers is in the range of 35%-50%， and PI can be self-extinguishing， incombustible， non-melting， and produces no smoke when on fire， which can significantly improve the safety and fighting ability of fire-fighters when used in the area of fire control. Meanwhile， these fibers have excellent fire-resistant properties and high thermal stabilities， thus， PI fibers can be utilized to fabricate heat-resistant workwear， fiber blankets， and flame-retardant textiles that can be used by firemen and metallurgical workers.
3. National security. Compared with other polymeric fibers， the PI fiber exhibits excellent thermal stability， radiation resistance， low water absorption， low density， and so on， making it a good candidate for application in the fields of power， national defense， and aerospace.
4. High-temperature insulting papers and structural materials. Papers made from PI fibers show a better combination performance than the Aramid papers， and they can be used as insulating materials in power generators and dry type transformers. Honeycomb-structure composite materials fabricated by PI fibers can be used to prepare radar jammers， cabin， and aviation lightweight plates.
5. PI-fiber-reinforced composites. The PI fibers with high-strength-high-modulus are used as reinforcements in the preparation of composite materials. The PI-fiber-reinforced epoxy resin composite exhibits a bending strength and modulus higher than 650 MPa and 43 GPa， respectively， promising it to be one of the preferred materials used in supersonic aircraft， aerospace equipment， nuclear facilities， high-temperature electrical insulation， bullet-proof vests， and other high-tech industries. Fig. 2.26 shows the high-strength-high-modulus fiber-reinforced composite materials.
FIGURE 2.26 PI-fiber-reinforced composite materials.
6. PI fiber ropes. The PI fiber ropes can be obtained by merging and knitting PI fibers under constant tension. The PI fiber ropes can be applied in space vehicles， nuclear industry fire-proofing materials due to their outstanding mechanical properties， lightweight， low creep， high radiation resistance， and good wear resistance.