1. Development History and Current Status of Polyimide Fibers

n In the 1960s, American companies discovered the liquid crystal behavior of aromatic polyimides, which facilitated the development and industrial production of polyimide fibers.

n During the same period, high-performance fibers made from polymers such as polyamide-imide and polyamide-azomethine were also successfully developed.

n In the mid-1980s, Austrian companies developed and promoted P84 fibers.

n In the 1990s, the United States conducted extensive research on high-performance polyimide fibers from different systems, improving the preparation process and fiber properties.

n In recent years, Russia has developed a type of polyimide fiber with excellent mechanical properties, with a strength of 5.8 GPa and a modulus of 285 GPa, marking a new stage in the development of polyimide fibers.

2. Main Properties and Applications of Polyimide Fibers

n Main Properties:

u High strength and high modulus.

u Excellent resistance to high temperatures, chemical corrosion, radiation, and flame.

u Good thermal oxidative stability and mechanical properties in superheated steam.

u Strong resistance to acid and alkali corrosion and light radiation, with a strength retention rate of 90% after exposure to 1 x 10¹⁰ rad fast electron dose.

u Limiting oxygen index between 35-75, with low smoke emission and self-extinguishing properties.

u High thermal decomposition temperature, with the initial decomposition temperature of fully aromatic polyimide fibers around 500°C, and a thermal shrinkage rate of less than 1% at 250°C for 10 minutes.

u Polyimides synthesized from biphenyl dianhydride and p-phenylenediamine have a thermal decomposition temperature of 600°C, a maximum working temperature of 300°C, and can be used continuously at 260°C.

u Dielectric constant of about 3.4, with good dielectric properties and low-temperature resistance, not easily brittle in liquid helium at -269°C.

n Applications:

u Used for high-temperature filtration nets for radioactive, organic gases, and liquids, fire blankets, protective clothing, racing flame retardant suits, flight suits, and other fire retardant clothing.

u As a reinforcing agent for advanced composites, used in lightweight cable sheaths, high-temperature insulation appliances, and engine nozzles for aerospace and aviation.

u Used in protective covers and special fireproof materials for military and aerospace applications, as well as structural materials in atomic energy facilities.

u The market prospects are broad, and China has technical and price advantages in the production technology of polyimide raw materials, which is conducive to international market development.

3. Preparation Processes of Polyimide Fibers

n Dry Spinning:

u In the early stages, most spinning of polyamic acid solutions was done using dry spinning.

u In 1966, it was first reported that polyamic acid was obtained from pyromellitic dianhydride (PMDA), 4,4'-oxydianiline (CODA), and 4,4'-diaminodiphenyl sulfide (SDA) in dimethylacetamide (DMAc), and then spun into fibers. After imidization under certain temperature and tension, the fibers were stretched at 550°C to obtain polyimide fibers.

n Wet Spinning:

u Pyromellitic dianhydride (PMDA) and 4,4'-oxydianiline (4,4'-ODA) are condensed in NMP to form a polyamic acid solution, which is then wet-spun, thermally or chemically cyclized, and hot-stretched at 290°C to obtain PMDA/4,4'-ODA polyimide fibers. However, the strength of the fibers is small, with an initial modulus of 6.6~7.2 GPa.

n Dry/Wet Spinning:

u Dry/wet spinning, also known as dry-jet wet spinning, involves the spinning solution being extruded from the spinneret under pressure, not directly entering the coagulation bath but first forming a fiber state in the air under the action of traction, and then entering the coagulation bath to remove the solvent and solidify.

u The two-step spinning method is suitable for polyimide systems with a precursor of polyamic acid. The spinning process is divided into two steps: the first step produces polyamic acid fibers, and the second step cyclizes the polyamic acid fibers into polyimide fibers.

u The one-step spinning method uses a polyimide solution as the original solution, and the resulting nascent fiber is a polyimide fiber without a cyclization process.

n Melt Spinning:

u Typically, a system of dianhydride and diamine is used, with non-proton strong polar solvents (such as DMAc, DMF, NMF, etc.) used as solvents. The molecular weight of the polyimide is controlled by adjusting the monomer ratio and adding end-capping agents during low-temperature condensation.

u The product is precipitated from polyimide in a methanol/water solution, and after washing, filtering, and drying, heat treatment is performed to fully cyclize the macromolecules. The processing temperature is generally between 340-360°C, and after the spinning process, the resulting product is further heat-traction treated at high temperatures to obtain polyimide fibers.

n Electrospinning:

u Mainly used for the spinning of polyimide hollow fibers, the polymer solution is spun under high voltage to obtain nascent fibers, which are then washed and heat-treated to produce nano-polyimide fibers.

u Ultrafine polyimide fibers are spun from a specific device with a spinning voltage of 10-25 kV and a distance of 8 cm between the spinneret and the coagulation bath. The fibers are stretched during the process of entering the coagulation bath and then solidified in the coagulation bath in a non-woven form.

u Studies have found that polyimide fibers maintain good thermal stability above 500°C and begin to thermally degrade at 530°C.

n Liquid Crystal Spinning:

u Christian et al. used a polymerization approach to first polymerize into a PAE slurry with an intrinsic viscosity of 5.89 dL/g, and the weight average molecular weight was measured by LS to reach 520008 mol-1.

u The fully aromatic PAE forms a liquid crystal solution in 40% (wt) NMP at 80C, and is spun into acetone through dry jet wet spinning. In the acetone bath, the newly formed fibers can be easily stretched, and the chain repeating units and intermolecular distances of the fibers are shortened before and after thermal cyclization, which helps to improve the mechanical properties of the fibers.

u The strength and modulus of the polyimide fibers formed by thermal cyclization reached 700MPa and 68GPa, respectively.

4. Comprehensive Evaluation and Comparison of Spinning Processes

n The dry/wet spinning method combines the main design ideas of dry and wet methods and improves the quality of spun fibers, with the two-step method being the best.

n In the two-step process, most of the polar solvents are removed in the coagulation bath and washing process, so the environmental pollution is small, and because the solvent residue is very little, the processing properties of polyimide fibers are improved.

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