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Exploring the Limits of Polymer Properties: Structural Components From Rigid- and Flexible-Clain Polymers ROGER S. PORTER It has been seven decades since Baekeland commercialized the first plastic produced from small molecules. Since then, synthetic polymers have come to fill an array of needs, many generated by the very avail- ability of these extraordinary materials. ~ The increase in production and use of polymers has been spectacular. In the United States alone pro- duction in 1981 exceeded 24 million metric tons. In volume this exceeds the production of steel. Polymer production represents more than $100 billion of value added by manufacture and involves the employment of 3.3 million people. Polymers thus represent a large, rapidly expanding, and significant class of materials of importance to both the economy and national security. The rapid growth in use of polymers and their substitution for other materials has led to the design and evaluation of new polymer products. Short-term polymer tests designed to accelerate degradation have been commonly used. Frequently these tests are not adequate for more than the ranking of performance under a single set of conditions. As a result, polymers may fail prematurely and unexpectedly in use. If the advan- tages of polymers are to be fully exploited through innovative use, meaningful service-life prediction and nondestructive characterization methods must be further developed. This need has been recognized in The introduction to this paper is based on "Opportunities and Needs for Research on the Performance of Polymers" by R. K. Eby, Chief, Polymer Science and Standards Division, National Bureau of Standards, Washington, D.C. 109
110 ADVANCES IN STRUCTURAL MATERIALS at least six studies. i-6 For example, a National Research Council reports lists research opportunities in polymer science, and one-half of these deal with accelerated tests for long-term behavior, viz, failure and deg- radation of polymers. From a positive perspective, this paper first illustrates several recent polymer applications, particularly those in construction and transpor- tation. Among advanced uses for bulk polymer is the Bethlehem Steel Corporation process for making biaxially oriented polypropylene that is tough enough to stop a bullet. More generally, the performance of polymers is being enhanced with increasing sophistication by reinforce- ment with glass, or, better, by reinforcement with polymer fibers, and finally by the ultimate generation of self-reinforcing polymer. The paper thus concludes with a brief description of major and recent advances that can provide the next generation of high-performance polymers. The major opportunities are still in the future for polymer applications and for substitution of other materials. This is well recognized abroad, as is seen in the section below on "The Japanese Challenge." RECENT APPLICATIONS The lighter weight of reinforced plastics, with consequent savings in fuel, is a major factor in the switchover from metals to reinforced plastics in automobiles and aircraft. New fabricating techniques that make pos- sible the production of composite parts at a lower cost than for all-metal parts are also playing a growing role in this change. Among the new and demanding applications is a pultruded graphite-reinforced helicop- ter windshield post that is part of a Department of Defense contract for a prototype helicopter. The publicized Lear Fan Turboprop makes such use of carbon/epoxy composites that only items such as the engine, landing gear, and wing-attach fittings are metal. Wing-tip fuel tanks for the F-18 fighter plane are a complex composite that withstands fire, impact, and even bullets that destroy all-aluminum tanks. (With ref- erence to recent applications, see Society of the Plastics Industry, News of 1983.) In automobiles polymers have been used for many years for deco- rative, nonstructural purposes. Present considerations are to use plastics in more stress-critical components, such as hood, trunk lid, and struc- tural frame' for weight saving. Plastics will, in such evolutionary fashion, find their way more and more into critical automobile applications re- quiring strength and stiffness. The pace at which this occurs will depend on factors such as the development of a data base for engineering design and on the ability of engineers to use the data. While it may seem that
EXPLORING THE LIMITS OF POLYMER PROPERTIES 111 we are in the "Plastic Age," we are just beginning to see the options. In 1977 Ford planned for a prototype car with body, chassis, and power- train components made of graphite-fiber composites. This project was undertaken to demonstrate the potential of graphite-fiber-composite technology for construction of a light-weight car with good fuel economy, yet retaining the performance, interior space, and comfort of larger vehicles. The completed experimental vehicle weighed 2,504 pounds, some 1,246 pounds less than a 1979 production Ford LTD equipped with a 351, 5.0-liter CID engine. Only the power train, trim, and some chassis components were not converted. Even most of these (e.g., en- gine, brakes, and transmission) could be downsized for secondary weight reductions. THE JAPANESE CHALLENGE The world position of nations is influenced by technology. In contem- porary competition, military materiel has become the science and en- gineering of materials. This has been dramatically illustrated by the delivery of steel from Japan to Pittsburgh at a favorable price and qual- ity. Our polymer developments are also being challenged in Japan. The following stark example is from a translation of Nikkei Sangyo Shimbrun (July 27, 1982~: The Ministry of International Trade and Industry (MITI), the synthetic fiber industry, and academic institutions are to engage in the joint development and practical application of "the third generation fiber" which will have more than twice the high tenacity, high modulus, low elongation of the present fibers. With government subsidy, MITI has designated, effective fiscal year 1983, this next generation research/development program. The industrial infrastructure plans to allocate 3 billion-5 billion yen in funds, with practical application targeted five years ahead. The new fiber is expected to replace nylon and carbon fiber and expand the area of fiber demand. MITI and Japanese industry look forward to this development project as a conclusive factor for the revitalization of the fiber industry, now suffering from recession, and for increasing the value-added for polymer products. At present the closest fiber to this third-generation fiber is Du Pont's Aramid (a grade known as "Kevlar-49"), with a high tenacity of ~28 grams per denier, ~3.6 gigapascals (GPa), the world's most tenacious commercial fiber, but with a maximum modulus inferior to that of carbon fiber and ultradrawn polyethylene. The Japanese project may be the world's first development project in the area of new materials that is the equal in importance with advances in electronics and biotechnology. In typically Japanese fashion, this research is to be conducted jointly
112 ADVANCES IN STRUCTURAL MATERIALS within overlapped government, industry, and academic circles, and under government subsidy. RECENT RESEARCH TOWARD HIGH-MODULUS POLYMERS Polymer researchers have approached the problem of making the strongest possible polymers in two diverse ways: (1) by chemically con- structing polymers with rigid and linear backbone chains and (2) by processing conventional flexible-chain polymers in ways that result in a transformation of the internal structure and properties. Chemical con- struction of rigid macromolecules has been approached by syntheses leading to parasubstituted aromatic rings in the polymer backbone. In general, these polymers cannot be processed by means of conventional polymer techniques; however, some industrial examples, viz, Du Pont Kevlar and Monsanto X-500, have been solution-processed into fibers of very high strength. In the second category, flexible-chain polymers are converted into highly oriented and chain-extended conformation, with substantially in- creased tensile moduli, by drawing from dilute flowing solution or from a gel state or by extruding a supercooled melt by solid-state extruding or by drawing below the polymer melting point under controlled con- ditions. FLEXIBLE-CHAIN POLYMERS New and successful drawing techniques for flexible-chain polymers have been recently developed by workers in several countriesin the United States at the University of Massachusetts and elsewhere, and in Japan. It has been found possible, for example, to ultradraw single- crystal mats of ultrahigh-molecular-weight polyethylene (UHMWPE). By the principal deformation technique of solid-state coextrusion, draw has been achieved even at room temperature and at up to 130°C, i.e., just below the melting point. Moreover, the resulting stable extrudate exhibits extreme orientation. Multiple drawing by repeated coextrusion at 110°C produces an extrudate of UHMWPE with a draw ratio (DR) of 110 and a tensile modulus of 100 GPa. An even higher DR has been achieved by a combination of solid-state coextrusion followed by tensile drawing at controlled rate and temperature. The maximum achieved for the present by this drawing combination is a DR of 250. This superdrawn sample has a tensile modulus of 222 GPa, which is about twice the highest previously reported room-temperature experimental value (110 GPa) for polyethylene. Figure 1 summarizes some of these new results.
EXPLORING THE LIMITS OF POLYMER PROPERTIES 113 _ _ 200 - ~n 150 J o ~ 100 An LL _ O _ )~0 ~ 50 o __ _ / 0d - ,~ / 1 1 1 ~ 1 50 100 150 200 250 TOTAL DRAW RATIO (EDR x OR) FIGURE 1 The tensile modulus of high-density polyethylene increases markedly with draw attained by linear extension. A draw ratio (DR) of 10 means extending 10 times by solid-state extrusion (EDR) and followed with tensile pulling. RIGID-ROD POLYMERS Carbon and graphitic fibers produced from polymeric precursors ex- hibit some of the highest performance characteristics of materials avail- able to date. Indeed, such fibers have been extensively investigated over the last two decades owing to their high-temperature stability and ex- ceptional mechanical properties. Commercially available fibers possess tensile moduli of up to 690 GPa along with tensile strengths of 2.2 GPa.7 Such fibers, however, are quite brittle, which may limit their use in certain applications. Also, to produce carbon and graphitic fibers, ex- treme processing conditions are required, leading to high production and product costs. The electrical conductivity of these fibers is also not always desirable in application. Thus, there still exists a need for ad- ditional high-performance polymers. Indeed, research continues in re- lated areas, and a sizable activity concerns extended-chain and rigid-rod polymers possessing high-performance characteristics.
114 ADVANCES IN STRUCTURAL MATERIALS Fibers produced from lyotropic liquid crystalline solutions of ex- tended-chain polymers have not only achieved desirable high-perform- ance characteristics but have become successful engineering materials through the development of conventional wet spinning techniques for their manufacture. Both Monsanto and Du Pont9 have had success in developing high-modulus/high-strength fibers based on wholly aromatic polymers that possess a rodlike character derived from steric effects; however, only Du Pont has pursued commercial development (Kevlar). Even here there are interesting but disconcerting limitations in compres- sion and shear (see Figures 2 and 34. Remarkably, stretching after compression produces a virtually restored high-modulus Kevlar-49. Initial success in producing fibers from extended-chain macromole- cules has encouraged further investigation of rigid-rod polymers. A siz- able research effort sponsored by the U.S. Air Force Wright-Patterson Materials Laboratory and the U.S. Air Force Office of Scientific Re- search (Ordered Polymer Research Programed is currently evaluating the nature of novel rigid-rod macromolecules. The University of Mas- sachusetts is playing a major part in this activity. To our knowledge, it is the only research of this type in the Western world; it is described briefly below. The goals of the Air Force Ordered Polymer Research Program have focused on extended-chain, aromatic heterocyclic molecular structures. Three of the polymers synthesized as part of this program are a poly- 1 ' ' ' ' ' ' ' 1 KEVLAR49 FIBER 3.2 - CL - cn U) LL cr 1.6 In / as-received /~ // 1 / compressed 0.0 ~ ' 0.0 1.0 2.0 STRAIN (%) 3.0 4.0 FIGURE 2 Tensile properties of as-received and compressed Kevlar- 49 fiber.
EXPLORING THE LIMITS OF POLYMER PROPERTIES FIGURE 3 Micrographs comparing Kevlar-49 fibers before (left) and after (right) axial compressive failure, showing helical kink bands. 115 benzimidazole, PDIABii; poly-(p-phenylene benzobisoxazole), PBOi2; and poly-(p-phenylene benzobisthiazole), PBT.~3 i4 Of these structures the PBT polymer offers the best thermal and oxidative stability. For these reasons, since 1978 the emphasis has centered on its development. PBT is soluble only in strong acids.~4 i5 The viscosity of such solutions passes through a maximum with increasing polymer concentration, in- dicating formation of a lyotropic liquid crystal phase. The ability of PBT solutions to be spun from this mesophase with formation of high-mod- ulus/high-strength fibers has been demonstrated. Heat-treated fibers with moduli of 300 GPa and strength of 3 GPa have been routinely produced. These fibers are highly anisotropic and, like Kevlar, are considerably weaker in shear and compression than their graphite or glass competitors, as illustrated above. However, this characteristic also gives these fibers the amazing flaw insensitivity and non-brittle-type behavior in compression and shear that permits their use in applications such as bulletproof vests. Thus we now have seen by at least two diverse routes flexible and stiff chains" that crystallizable polymers have been developed into structures of both extraordinary tensile and impact prop- erties. This is leading to a range of applications well beyond the inno- vations described above. The potential for polymer applications remains unbounded.
116 ADVANCES IN STRUCTURAL MATERIALS NOTES 1. Polymer Science and Engineering: Challenges, Needs and Opportunities, Report of the Ad Hoc Panel on Polymer Science and Engineering, National Research Council, Washington, D.C., 1981. 2. Organic Polymer Characterization, NMAB 332, National Materials Advisory Board, National Research Council, Washington, D.C., 1977. 3. Polymer Materials: Basic Research Needs for Energy Applications, CONF-780643, U.S. Department of Energy, Washington, D.C., 1978. 4. Morphology of Polyethylene and Cross-linked Polyethylene, Workshop proceedings; EL-2134-LD, Electric Power Research Institute, 1981. 5. Organic Matrix Structure Composites: Quality Assurance and Reproducibility, NMAB- 365, National Materials Advisory Board, National Research Council, Washington, D.C., 1981. 6. Materials for Lightweight Military Combat Vehicles, NMAB-396, National Materials Advisory Board, National Research Council, Washington, D.C., 1982. . W. Bruce Black, "High Modulus/High Strength Organic Fibers," Annul Rev. Mater. Sci., 10:311, 1980. 8. W.B. Black and J. Preston, eds., High Modulus Wholly Aromatic Fibers, Marcel Dekker, New York, 1973. 9. H. Blades, U.S. Patent 3,869,430, "High Modulus, High Tenacity Poly~p-Phenylene Terephthalamide) Fiber," assigned to Du Pont, 1975. 10. T.E. Helminiak, "The Air Force Ordered Polymers Research Program: An Over- view," Am. Chem. Soc. Org. Coat. Plast. Prepr., 4:475, 1979. 11. R.F. Kovar and F.E. Arnold, "Pare-Ordered Polybenzimidazole," J. Polym. Sci., Polym. Chem. Ed., 14:2807, 1976. 12. T.E. Helminiak, F.E. Arnold, and C.L. Benner, "Potential Approach to Non-Rein- forced Composites," Polym. Prepr., Am. Chem. Soc., Div. Polym. Chem., 16~2~:659, 1975. 13. J.F. Wolfe, B.H. Loo, and F.E. Arnold, "Thermally Stable Rod-like Polymers: Synthesis of an All-Para Poly(Benzobisthiazole)," Polym. Prepr., Am. Chem. Soc., Div. Polym. Chem., 19~2~: 1, 1978. 14. J.F. Wolfe, B.H. Loo, and F.E. Arnold, "Rigid Rod Polymers. 2. Synthesis and Thermal Properties of Para-Aromatic Polyamides with 2,6 Benzobisthiazole Units in the Main Chain," Macromolecules, 14:915, 1981. 15. E.W. Choe and S.N. Kim, "Synthesis, Spinning and Fiber Mechanical Properties of Poly~p-Phenylene Benzobisthiazole)," Macromolecules, 14:920, 1981. 16. S. Allen, "Mechanical and Morphological Correlations in Poly~p-Phenylene Benzo- bisthiazole) Fibers," Ph.D. thesis, University of Massachusetts/Amherst, 1983. ACKNOWLEDGMENT This review was prepared initially with Richard J. Farris for presentation at the Workshop on Substituting Non-Metallic Materials for Vulnerable Minerals sponsored by the National Science Foundation, Washington, D.C., June 27-28, 1983.
