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Appendix F High-Performance Fibers ARAMID FIBERS (KEVLAR AND TECHNORA) fibers and films is semicrystalline, consisting of 60 to 70 vol percent crystals; the remainder consists of amorphous, Poly(p-phenylene terephthalamide), Kevlar, was first entangled polymer chains. Interestingly, melt-processed synthesized by Kwolek at DuPont in the 1960s. Kevlar is polyethylene contains chain-folded crystals with a modulus processed from sulfuric acid, with the polymer concentration in the 1 GPa range. Trash bags and milk jugs, having typical at about 20 wt percent. Surprisingly, there is a decrease in molecular weights of 50,000 to 200,000 g/mole, are common viscosity with increased polymer concentration due to local examples of such polyethylene products. But if PE molecules alignment of polymer molecules in the solution to form a could be extended into straight chains, the carbon-carbon nematic phase. Thus the solution becomes liquid crystal- backbone would give outstanding properties. Indeed, after line, a feature that had earlier been predicted by Flory.1 The nearly half a century of process development in the field of solution is extruded through an air gap into an acid solvent, polyethylene, a new type of spinning was invented by Smith such as water, where it coagulates. Removal of the approxi- and Lemstra in the Netherlands in early 1980s.2 Known mately 80 percent acid from solution during fiber drying and as gel spinning, this process is able to extend the macro- tension heat treatment (500°C) leads to the formation of a molecules to nearly their full length and results in a highly highly aligned, extended chain fiber. However, the coagula- crystalline extended-chain polyethylene fiber exhibiting high tion process also creates undesirable defects. The number of strength and high modulus characteristics that show ballistic defects can be estimated from the deviation of the actual fiber protection capability. Because the molecules are processed density from the theoretical crystal density of 100 percent from a dilute solution, the molecular weight of the polyeth- (approximately 1.45 g/cm3 versus 1.50 g/cm3). Kevlar fiber ylene used in gel spinning can be in excess of 3 million g/ was developed and commercialized at DuPont, originally for mole or higher, much higher than that in any other synthetic completely different applications than for body armor (for polymer. Fiber is processed from a decalin solution that typi- example, it was used for reinforcing tires). The potential of cally contains less than 5 wt percent polymer. The polymer Kevlar for use in ballistic protection was realized only when solution is extruded at between 130°C to 150°C or so into the National Institute of Justice conducted ballistic testing on a cold coagulant such as water. This resulting gel-like fiber, Kevlar fabric. Other polyaramids followed, including Tech- which contains more than 95 percent solvent, is typically nora, an aramid copolymer fiber that is produced in the Neth- then drawn at between 90°C and 130°C to draw ratios of erlands and Japan from terephthaloyl chloride and a mixture 50 to 100. The macromolecules become extended and form of p-phenylenediamine and 3,4’-diaminodiphenylether. near-single-crystal fibers. The theoretical density of polyethylene is 1.00 g/cm3, POLYETHYLENE (SPECTRA, DYNEEMA) while the density of Spectra and Dyneema fibers is about 0.97 g/cm3. This underscores the fact that even today’s highly Unlike the extended rigid-rod molecular structure of extended-chain polyethylene fibers contain a significant Kevlar, polyethylene (PE) is one of the most flexible poly- number of defects and suggests an opportunity for even more mers. Since the 1930s, fibers and films have been manufac- significant gains in future development of this material. tured from PE by melt processing. The morphology of these 1Flory, P. 1956. Phase equilibria in solutions of rod-like particles. Pro - 2Smith, P., and P. Lemstra. 1980. Ultra-high-strength polyethylene fila - ceedings of the Royal Society of London Series A—Mathematical and ments by solution spinning/drawing. Journal of Materials Science 15(2): Physical Sciences 234(1196): 73-89. 505-514. 136

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

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138 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS dimethylsulfoxide, or zinc chloride and water.6 Depending individual tubular shells slipping past one another, whereas on the molecular weight, solvent, and the copolymer compo- single-wall CNTs are essentially the ultimate for a high- sition, the polymer concentration in solution is typically 5 to strength polymer molecule, having a theoretical strength as 25 wt percent. After spinning, fibers are successively drawn high as 150 GPa and modulus values as high as 1,050 GPa, at several different temperatures (typically between room respectively. The theoretical modulus of carbon nanotubes temperature and 175°C). Drawn fibers are oxidized under is dependent on their diameter since their central portion is tension typically between 200°C and 350°C for approxi- empty; however, their specific theoretical modulus is 469 N/ mately 2 hours. Oxidized fibers are then carbonized under tex irrespective of the diameter. tension in stages, similar to the carbonization of pitch-based fiber. Fibers with the highest tensile strength are typically ALUMINA, BORON, SILICON CARBIDE, GLASS, AND obtained at about 1300°C to 1500°C. ALUMINA BOROSILICATE CERAMIC FIBERS Boron fiber is processed using chemical vapor deposi- CARBON NANOTUBE FIBERS tion on substrates such as tungsten or carbon, whereas silicon Carbon nanotube (CNT) fibers to date have been pro- carbide fibers can be processed either by chemical vapor cessed primarily by one of the following two techniques: (1) deposition or by a precursor method similar to the processing CNT smoke drawn directly from the chemical vapor deposi- of carbon fibers. Alumina and alumina borosilicate fibers are tion reactor in the form of aerogel fibers7 and (2) fiber pro- typically processed using a sol-gel precursor followed by sin- cessed from aqueous8 or acidic9 dispersions of CNTs. In both tering. Nextel fibers (from 3M Company) are ceramic oxide cases, it is important that the CNTs be as long as possible fibers that belong to the category of alumina-boro-silicate. and as perfect as possible, and they should be free of catalyst Compared to polymeric and carbon fibers, these fibers retain and other foreign impurities, including amorphous carbon. their mechanical properties to much higher temperatures. Al- The tube-to-tube diameter variation should be minimized and though the tensile strength of these fibers is not quite as high the diameter should be relatively small. Nanotube orientation as that of some of the polymeric fibers, their compressive also plays a critical role with respect to mechanical proper- strength can be comparable to or higher than that of carbon ties.10 Multiwall CNTs tend to undergo telescoping, with the fiber having the best compressive strength. Owing to ionic- covalent bonds in all directions, these fibers are much more 6Gupta, isotropic than are carbon and polymer fibers, which exhibit V., and V. Kothari. 1997. Manufactured Fibre Technology. New York, N.Y.: Chapman and Hall. a very high degree of anisotropy.11,12,13 7Koziol, K., J. Vilatela, A. Moisala, M. Motta, P. Cunniff, M. Sennett, Glass is melt-extruded and drawn into fibers typically and A. Windle. 2007. High-performance carbon nanotube fiber. Science at 1000°C to 1200°C. Fiber tensile strength is limited by 318(5858): 1892-1895. defects, residual stresses, and structural inhomogeneities in 8Vigolo, B., A. Penicaud, C. Coulon, C. Sauder, R. Pailler, C. Journet, the fibers. P. Bernier, and P. Poulin. 2000. Macroscopic fibers and ribbons of oriented carbon nanotubes. Science 290(5495): 1331-1334. 9Ericson, L., H. Fan, H. Peng, V. Davis, W. Zhou, J. Sulpizio, Y. Wang, 11Chawla, K. 1998. Fibrous Materials. Cambridge, U.K.: Cambridge R. Booker, J. Vavro, C. Guthy, A. Parra-Vasquez, M. Kim, S. Ramesh, R. University Press. Saini, C. Kittrell, G. Lavin, H. Schmidt, W. Adams, W. Billups, M. Pasquali, 12Elices, M., and J. Llorca. 2002. Fiber Fracture. Oxford, U.K.: Elsevier W-F. Hwang, R. Hauge, J. Fisher, and R. Smalley. 2004. Macroscopic, Science. neat, single-walled carbon nanotube fibers. Science 305(5689): 1447-1450. 13Watt, W., and B. Perov, eds. 1986. Strong Fibers. North-Holland: 10Liu, T., and S. Kumar. 2003. Effect of orientation on the modulus of Elsevier Science. SWNT films and fibers. Nano Letters 3 (5): 647-650.