1.Development History
PBO was invented by researchers in aerodynamics from the U.S. Air Force. The basic patent for polybenzothiazole was held by Stanford Research Institute (SRI) in the United States. Later, Dow Chemical Company obtained the license and industrially developed PBO, while also improving the original monomer synthesis method. The new process produced almost no isomeric byproducts, increasing the yield of the synthesized monomers and laying the foundation for industrialization. In 1990, Japan's Toyobo Co. purchased the PBO patent technology from Dow Chemical. In 1991, Dow-Badische Textile Company developed PBO fiber on Toyobo's equipment, significantly increasing the strength and modulus of PBO fiber to twice that of PPTA fiber. In 1994, with permission from Dow-Badische Textile Company, Toyobo invested 3 billion yen to build a production line capable of producing 400 tons/year of PBO monomers and 180 tons/year of spinning. Partial mechanized production began in the spring of 1995, and by 1998, the production capacity reached 200 tons/year, with the product named Zylon. According to Toyobo's development plan for Zylon, the production capacity was expected to reach 380 tons/year by 2000, 500 tons/year by 2003, and 1000 tons/year by 2008. Currently, Toyobo remains the only company in the world capable of commercially producing PBO fiber.
2.Prospects for the Development of PBO Fibers
In recent years, high-performance fiber composite reinforcement materials have been extensively used in the construction fields such as high-rise buildings, large bridges, and marine engineering in developed countries and regions like Europe, America, and Japan. By impregnating fiber cloth with epoxy resin and adhering it to the concrete surface, the load-bearing capacity and earthquake resistance of the original structure can be significantly enhanced. Moreover, in bridge construction, steel cables cannot be used for longer bridges due to their own weight. Instead, there is a preference for lighter and stronger cables. Cables made from PBO fibers, which have high specific strength and good dimensional stability, are the best choice. PBO fibers are gradually replacing traditional asbestos materials in the field of heat-resistant materials and are currently exploring the substitution of aromatic polyamides and other flame-retardant fibers at temperatures below 350°C. At temperatures above 350°C, they are replacing stainless steel fibers or ceramic fibers and other inorganic fibers. Since inorganic fibers are quite hard and prone to causing scratches that affect their performance, PBO fibers are likely to overcome the shortcomings of inorganic fibers. Previously, the heat resistance of organic fibers was insufficient (mostly below 400°C), which limited their application development. However, PBO fibers have a decomposition temperature of up to 650°C, the highest among all organic fibers. Therefore, it is entirely possible to replace the use of organic fibers in applications above 350°C with PBO fibers, thereby broadening and developing the application of PBO fiber heat-resistant materials. International research indicates that PBO fibers have many potential applications in other areas such as electrical insulation materials, satellite detection, lightweight materials, the automotive industry, and deep-sea oil field development. As a high-speed train body material, PBO fibers not only reduce the weight of the body but also increase its strength. Utilizing the chemical resistance of PBO fibers, various corrosion-resistant protective clothing can be made. In space exploration, to reduce the limited burden, PBO fibers are suitable for making fasteners and straps used in space. In the range of cosmic space environment temperatures from -10°C to 460°C, it can also be used as a material for heat-resistant detection balloons. In the field of sports competition sailing, sails are mainly made from sheet-like thin plates made of high-strength and high-modulus fibers. To minimize the deformation of the sails when exposed to wind, the highest modulus PBO fibers must be sought for the production of racing sails. Given the excellent mechanical properties of PBO fibers, they are also the best materials for manufacturing golf clubs, tennis rackets, ski poles, ski boards, surfboards, archery bowstrings, and bicycle racing wheels. The key technology research and development and industrialization of PBO fibers can enable China to break free from the long-term control and monopoly of foreign technology and embark on a path of independent innovation, bright prospects, and broad application of domestic and large-scale development of PBO fibers. This will contribute to the development and sustainable use of high-performance PBO materials in China's aerospace, national defense, military, and civilian industries.
3.Fiber Properties
According to Toyobo reports, its high-end PBO fiber product's strength is 5.8GPa (reported as 5.2GPa in Germany), with a modulus of 180GPa, the highest among existing chemical fibers; it can withstand temperatures up to 600°C, and has a limiting oxygen index of 68, not burning or shrinking in flames, demonstrating higher heat resistance and flame retardance than any other organic fiber. It is primarily used for heat-resistant industrial textiles and fiber-reinforced materials.
Comparison of PBO with other high-performance fibers:the strength, modulus, heat resistance, and flame retardance of PBO fiber, especially its strength, not only exceed that of steel fibers but also surpass those of carbon fibers. Moreover, PBO fiber exhibits excellent impact resistance, abrasion resistance, and dimensional stability, and is light and soft, making it an extremely ideal textile raw material.
PBO, as a super-performance fiber of the 21st century, possesses outstanding physical, mechanical, and chemical properties. Its strength and modulus are twice that of Kevlar fibers and also feature the heat resistance and flame retardancy of meta-aramid fibers, with overall physical and chemical properties that completely surpass those of Kevlar fibers, which have been leading in the field of high-performance fibers. A single PBO filament with a diameter of 1 millimeter can lift a weight of 450 kilograms, which is more than ten times the strength of steel wire fibers.
4.Surface modification of PBO fibers.
The interfacial shear strength (IFSS) between PBO fibers and the resin matrix can be enhanced, but an excessive amount of coupling agent may lead to a thick crosslinking layer of the coupling agent, which in turn reduces the IFSS. Plasma etching on the fiber surface primarily affects the coupling agent, forming a grafted crosslinking layer that provides certain protection for the fibers, thus the decline in the σ of PBO fibers is not significant. Analysis shows that the optimal conditions for the combined process of coupling agent and plasma modification are: A-187 coupling agent content at 2%, argon low-temperature plasma treatment time of 2 min, pressure at 50 Pa, and power at 30 W. Among the selected coupling agents, the A-187 type has the best effect on improving the IFSS between PBO fibers and epoxy resin, with an optimal content of 2%. (1) When the A-187 content is 2%, and the argon low-temperature plasma treatment conditions are 2 min, 30 W, and 50 Pa, the modified PBO fibers' IFSS can reach up to 10.44 MPa, which is a 52% increase compared to using only the A-187 coupling agent for modification, and a 78% increase compared to the original fibers' IFSS. The wettability of PBO fibers has also been significantly improved. (2) For PBO fibers modified by argon low-temperature plasma combined with a coupling agent, the decline in IFSS over time is not significant; the increase in the contact angle is also not significant, showing a tendency towards stability, and there is even a slight downward trend. The degradation effect of PBO fibers modified by argon low-temperature plasma combined with a coupling agent is not pronounced.
5.Preparation
PBO is synthesized by the solution polycondensation of 4,6-diaminoresorcinol dihydrochloride (also known as DAR·2HCl) with terephthalic acid in a solvent of polyphosphoric acid (PPA), or through dehydration using P2O5. PPA serves both as a solvent and as a catalyst for the polycondensation. The synthesis of the monomer DAR·2HCl was successfully developed by Dow Chemical Company in the United States, starting from trichlorobenzene as the raw material. This method avoids the formation of isomers during synthesis, yielding high yields and playing a significant role in the industrial production of PBO. The polymer dope is spun using a dry-wet spinning process, followed by washing and drying. When dissolved to liquid crystalline properties, the use of liquid crystal spinning can form an extended chain structure, with the initial spun fiber (AS fiber - standard type) possessing a strength of over 3.53 N/tex and an elastic modulus of over 10.84 N/tex. To improve the modulus, heat treatment can be performed at about 600°C, resulting in a high modulus fiber (HM fiber - high modulus type) with a modulus of up to 176.4 N/tex while maintaining the same strength.
6.Application
PBO fibers are characterized by their excellent heat resistance, high strength, and high modulus, making them widely applicable.
(1) Applications of filament include reinforcing materials for rubber products such as tires, conveyor belts, and hoses; reinforcing materials for various plastics and concrete; strengthening components for ballistic missiles and composite materials; tension members and protective membranes for fiber optic cables; reinforcing fibers for electric wires, headphone wires, and other flexible wires; high-tensile materials for ropes and cables; heat-resistant filter materials for high-temperature filtration; protective equipment for missiles and bullets, bulletproof vests, bulletproof helmets, and high-performance flight suits; sports equipment for tennis, speedboats, racing yachts; high-grade speaker diaphragms, new communication materials; aerospace materials, etc.
(2) Applications of chopped fibers and pulp include reinforcing fibers for friction materials and sealing gaskets; enhancing materials for various resins and plastics, etc.
(3) Applications of yarn include firefighting clothing; heat-resistant workwear for molten metal handling, such as foundry and welding apparel; protective clothing for cut resistance, safety gloves, and safety shoes; race car driver suits, jockey outfits; various sportswear and active sports gear; Carrace pilot suits; anti-cut equipment, etc.
(4) Applications of short fibers mainly include heat-resistant buffer felt pads for aluminum extrusion processing; heat-resistant filter materials for high-temperature filtration; thermal protection belts, etc.