1.Development History
PBO was invented by researchers in aerodynamics development from the United States Air Force. The basic patent for polybenzoxazole was initially owned by Stanford Research Institute (SRI) at Stanford University in the United States. Later, Dow Chemical Company obtained authorization and industrially developed PBO, while also improving the original synthesis method of the monomer. The new process produced almost no isomeric by-products, increasing the yield of the synthesized monomer and laying the foundation for industrialization. In 1990, Toyobo Co., Ltd. of Japan purchased the PBO patent technology from Dow Chemical Company. In 1991, Dow-Badische Fibers Inc. developed PBO fiber on the equipment of Toyobo Co., Ltd., significantly increasing the strength and modulus of PBO fiber to twice that of PPTA fiber. In 1994, with permission from Dow-Badische Fibers Inc., Toyobo Co., Ltd. invested 3 billion Japanese yen to build a production line with an annual output of 400 tons of PBO monomers and 180 tons of spinning. In the spring of 1995, it began partial mechanized production, and by 1998, the production capacity reached 200 tons/year, with the commercial name Zylon. According to Toyobo's development plan for Zylon, the production capacity was expected to reach 380 tons/year in 2000, 500 tons/year in 2003, and 1000 tons/year in 2008. Currently, Toyobo Co., Ltd. remains the only company in the world capable of commercially producing PBO fiber.
2.The Prospects of PBO Fiber Development
In recent years, developed countries and regions such as Europe, America, and Japan have widely used high-performance fiber-reinforced composite materials in the construction fields of high-rise buildings, large bridges, and marine engineering. 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 improved. 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 strength, 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 applications below 350°C to replace flame-retardant fibers such as aromatic polyamides. Above 350°C, they are replacing inorganic fibers like stainless steel or ceramic fibers. Since inorganic fibers are harder and prone to scratches that affect their performance, PBO fibers have the potential to overcome these shortcomings. 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 650°C, the highest among all organic fibers. Therefore, it is entirely possible to replace organic fibers with PBO fibers in applications above 350°C where organic fibers were previously difficult to use, thus broadening and developing the application of PBO fiber heat-resistant materials.
International research indicates that PBO fibers have many applications in other fields such as electrical insulation materials, satellite detection, lightweight materials, the automotive industry, and deep-sea oil field development. PBO fibers used in high-speed train bodies not only reduce the weight of the vehicle but also increase its strength. Utilizing the chemical resistance of PBO fibers, various corrosion-resistant protective clothing can be made. In aerospace, to reduce the limited burden, PBO fibers are suitable for making fasteners and straps used in space. In the range of cosmic temperatures from -10°C to 460°C, they can also be used as materials for heat-resistant detection balloons. In sports competition sailing, sails are mainly made from high-strength, high-modulus fiber-made plate-like thin materials. To minimize deformation when the sails are blown by the wind, the highest modulus PBO fibers must be sought for making competitive sailing 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 racing bicycles.
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, 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, their high-end PBO fiber product has a strength of 5.8 GPa (reported as 5.2 GPa in Germany), a modulus of 180 GPa, which is the highest among existing chemical fibers; it can withstand temperatures up to 600°C, with a limiting oxygen index of 68, and does not burn or shrink in flames, exhibiting higher heat resistance and flame retardancy than any other organic fiber. It is mainly used for heat-resistant industrial textiles and fiber-reinforced materials.
Performance comparison of PBO with other high-performance fibers:
As can be seen from the table, PBO fibers exhibit superior strength, modulus, heat resistance, and flame retardancy. Notably, the strength of PBO fibers not only surpasses that of steel fibers but also exceeds that of carbon fibers. Additionally, PBO fibers excel in impact resistance, abrasion resistance, and dimensional stability. They are also lightweight and flexible, making them an ideal textile raw material.
PBO, as a super-performance fiber of the 21st century, possesses exceptionally excellent physical and mechanical properties as well as chemical properties. Its strength and modulus are twice that of Kevlar fibers and it also shares the thermal resistance and flame retardancy of meta-aramid fibers. Moreover, its physical and chemical properties completely outperform Kevlar fibers, which have hitherto led 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 fibers.
4.Surface modification of PBO fibers
The enhancement of TIFSS (Interfacial Shear Strength) between PBO fibers and the resin matrix improves, but an excess of coupling agents can lead to a thicker crosslinking layer of the coupling agent, which in turn reduces TIFSS. The etching effect of plasma on the fiber surface primarily acts on the coupling agent, enabling the formation of a grafted crosslinking layer. This coupling agent layer provides certain protection to the fibers, thus the decline in the σ (strength) of PBO fibers is not significant.
It can be analyzed that the optimal conditions for the combined process of modifying with coupling agents and plasma are: the content of A-187 coupling agent at 2%, argon low-temperature plasma treatment time for 2 min, pressure at 50Pa, and power at 30W. Among the selected coupling agents, A-187 has the best effect on improving the IFSS between PBO fibers and epoxy resin, with an optimal content of 2%.
(1) When the content of A-187 is 2%, and the argon low-temperature plasma treatment conditions are 2min, 30W, and 50Pa, the modified PBO fiber's ΓIFSS (Interfacial Shear Strength) can reach as high as 10.44MPa. This represents a 52% increase compared to using only the A-187 coupling agent for modification and a 78% increase compared to the original fiber's ΓIFSS. The wettability of PBO fibers has also been greatly improved.
(2) For PBO fibers modified by argon low-temperature plasma combined with a coupling agent, the decline in ΓIFSS over time is not significant; the increase in the contact angle is also not substantial, showing a tendency towards stability with a slight downward trend. Therefore, the degradation effect of PBO fibers modified by argon low-temperature plasma combined with a coupling agent is not pronounced.
5.Preparation
PBO is prepared by solution polycondensation of 4,6-diaminoresorcinol hydrochloride (DAR·HCl) with terephthalic acid using polyphosphoric acid (PPA) as a solvent. Alternatively, it can be synthesized using P2O5 dehydration for polycondensation. PPA serves both as a solvent and as a catalyst for polycondensation.
The synthesis of monomer diamino resorcinol has been successfully developed by the American Dow Chemical Company, starting with trichlorobenzene as the raw material. This method avoids the generation of isomers during the synthesis process, yielding a high recovery rate, which plays a significant role in the industrial production of PBO.
Polymer dope is spun using the dry-wet spinning method, followed by washing and drying. When the spinning solution is dissolved to form liquid crystals and liquid crystal spinning is used, it can form an extended chain structure. The initial spun fiber (AS fiber-standard type) already possesses a strength of over 3.53N/tex and an elastic modulus of over 10.84N/tex. To increase the modulus, heat treatment can be performed at around 600℃, resulting in a high modulus fiber (HM fiber-high modulus type) with a modulus reaching 176.4N/tex while maintaining the same strength.
6.Applications
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; enhancement components for ballistic missiles and composite materials; tension members and protective films for fiber optic cables; reinforcing fibers for electric heating wires, headphone cables, 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 boats, etc.; high-grade speaker diaphragms, novel communication materials; aerospace materials, etc.
(2) Applications of chopped fibers and pulp include reinforcing fibers for friction materials and sealing gaskets; enhancement materials for various resins and plastics, etc.
(3) Applications of yarn include firefighting clothing; heat-resistant workwear for furnace front and welding operations; protective clothing for cut resistance, safety gloves, and safety shoes; race car driver suits, jockey suits; various sportswear and active sports equipment; Carrace pilot suits; anti-cut equipment, etc.
(4) Applications of short fibers are mainly for heat-resistant buffer pad felt used in aluminum extrusion processing; heat-resistant filter materials for high-temperature filtration; thermal protection belts, etc.