RIGID-ROD POLYMERS (ZYLON AND M5)
After the successful commercial development of Kevlar in the 1970s, significant research efforts were devoted to the development of other rigid-rod polymers. Rigid-rod polymers programs began in the 1960s at the U.S. Air Force Research Laboratory as well as in Russia. The U.S. program was accelerated in the 1970s, resulting in the development of poly-p-phenylene benzobisthiazole and polybenzoxazole (PBO) fibers.3PBO fiber was further developed initially at SRI International and later at Dow Chemical Company before being commercialized by Toyobo Company (Japan) in 1998 under the trade name Zylon. Among other applications, PBO fiber was also developed for use in fire-protective clothing as well as for ballistic protection. However, in the early 2000s it became clear that there were environmental stability issues with Zylon fiber causing decreased fiber strength over time and negatively affecting its ballistic performance. This is attributed to poor resistance to ultraviolet radiation as well as to poor hydrolytic stability.
In an attempt to improve intermolecular interactions in rigid-rod polymers with the intent of increasing the fiber compressive strength and torsional modulus, the Akzo Nobel firm in the Netherlands synthesized and processed polypyridobisimidazole (under the name M5) fiber during the 1990s.4 The fiber was further developed by Magellan Systems International, and the technology now resides with DuPont, although the fiber has not yet been commercialized.
Similar to Kevlar, both the Zylon and M5 fibers are processed from a liquid crystalline polymer solution, except in this case the solution is one of polyphosphoric acid. Depending on the polymer molecular weight, for fiber spinning, polymer concentration in solution is again typically between 5 weight percent 15 weight percent. Like the process used to make Kevlar, the nematic solution is extruded through an air gap into an acid solvent such as water. The coagulated fiber is then heat-treated under tension up to about 500°C. Structure formation mechanism in the rigid-rod chains of Zylon and M5 fibers is very similar to the structure formation mechanism in Kevlar and is quite different from that of the flexible-chain gel-spun polyethylene (Dyneema and Spectra).
Intermolecular interactions in polyethylene are only van der Waals interactions, whereas in Kevlar there is hydrogen bonding in one dimension transverse to the fiber axis, and in M5 fibers there is hydrogen bonding in two transverse directions. Ranking fibers in from greatest to least, in terms of compressive and torsional properties, shows that M5 has highest compressive and torsional properties, followed by Kevlar, then Zylon, then Spectra and Dyneema, which are approximately equal.
THERMOTROPIC LIQUID CRYSTALLINE POLYMERIC FIBERS
Thermotropic liquid crystalline polymeric fibers, developed in the 1970s, are melt processed (no solvent). These polymers exhibit liquid crystalline behavior in the melt state. Vectran, a copolyester and an example of a commercial fiber in this class, is spun at temperatures of 275°C or more. To further enhance mechanical properties, as-spun fiber may be further drawn and annealed below the polymer melting temperature. During this process, fiber may also undergo further solid state polymerization, resulting in a polymer of higher molecular weight. Unlike the liquid-crystalline-solution processing of rigid-rod polymers and the gel spinning of flexible-chain polyethylene—both of which are processed from polymer solutions containing 85 percent to 95 percent solvent (which must be removed during fiber processing)—there is no solvent to be removed in the processing of thermotropic liquid crystalline polymers. Compared to polyethylene, however, the molecular weights (and hence the chain length) of aramids, rigid-rod polymers, and thermotropic liquid crystalline polymers are much more limited. Vectran has more applications in injection-molded products than in fiber form.
The development of modern carbon fibers dates back to the 1960s with research by Shindo in Japan, Watt in England, and Bacon at Union Carbide in the United States. Early carbon fibers were made by pyrolyzing cellulose; today, carbon fibers are made starting from petroleum pitch or from polyacrylonitrile (PAN) copolymers. Pitch-based carbon fibers can have a very high tensile modulus and high electrical and thermal conductivities but exhibit relatively low tensile and compressive strength. By contrast, PAN-based carbon fibers have high tensile strength, good compressive strength, and intermediate modulus and electrical and thermal conductivities. High-purity mesophase pitch (a liquid crystalline pitch) is melted, extruded typically at about 400°C, and then carbonized in stages (Stage 1 at 600°C to 1000°C, Stage 2 at 1100°C to 1600°C, and Stage 3 at 2200°C to 2700°C) in an inert environment. Fibers carbonized at about 2700°C can exhibit up to 90 percent of the theoretical modulus. The theoretical modulus of graphite along graphene planes is 1,060 GPa, giving it a specific theoretical modulus of 469 N/tex,5 which is equivalent to 469 GPa/(g/cm3).
PAN fibers are either wet spun or dry-jet wet spun from solutions in sodium thiocyanate and water, dimethyl acetate,
3Chae, H., and S. Kumar. 2006. Rigid-rod polymeric fibers. Journal of Applied Polymer Science 100(1): 791-802.
4Sikkema, D. 1998. Design, synthesis and properties of a novel rigid rod polymer, PIPD or ‘M5’: High modulus and tenacity fibres with substantial compressive strength. Polymer 39(24): 5981-5986.
5“Tex” is the mass of a 1,000-meter length of fiber in grams.