Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
HIGH-PERFORMANCE SYNTHETIC FIBERS FOR COMPOSITES 9 1 HIGH-PERFORMANCE SYNTHETIC FIBERS FOR COMPOSITES INTRODUCTION Fiber reinforcement of plastic, ceramic, and even metallic materials is a key technology for the automotive and aircraft industries and an enabling technology for numerous advanced systems, such as high-efficiency turbines and hypersonic aircraft. Although fiber-reinforced composite materials, such as fiberglass boats and automobile parts, and fiber-reinforced tires and drive belts, have long been articles of commerce, the development of so-called high-performance synthetic fibers has created a new generation of composite materials. This class of advanced composite materials exhibits physical properties that are vastly superior to those of the matrix material alone. These new composite materials are vital to the defense posture of the United States, but their future impact on the growth of our economy may be even more important. Over the past 20 years a large number of high-performance fibers, comprised of materials as diverse as polyethylene and boron nitride, have been fabricated and characterized. A summary of the composition, relative properties, and availability of many of these fibers is given in Table 1.1. The motivation for this proliferation of potential fiber products is a combination of government-defined needs for aerospace and/or military programs and attractive commercial opportunities limited by the performance and availability of current materials. Of the many fibers described in the literature, only a few are truly commercial products; many are developmental and the vast majority are exploratory. Arbitrarily, "commercial" may be taken to mean a production capacity of at least 10,000 pounds per year, "developmental" a capacity for about 1000 pounds per year, and "exploratory" may mean that only a few grams of the fiber have ever existed. High-performance fibers possess reinforcing properties (tensile strength, stiffness, thermal stability, etc.) that significantly exceed those of the traditional fibers that have been used for many years (textile-grade
HIGH-PERFORMANCE SYNTHETIC FIBERS FOR COMPOSITES 10 nylon or glass fibers). Currently, the principal applications of high-performance, fiber-reinforced composite materials are structural, particularly in aircraft and spacecraft where their light weight, combined with superior stiffness and strength, makes them preferable to metal. Thus, both performance and fuel economy can benefit from increased utilization of these materials. Since high-performance fibers are extensively used in aircraft structures, it is imperative to have a sound foundation in fiber technology and domestic production of these high performance fibers if the United States is to remain a world leader in aircraft production. TABLE 1.1 High-Performance Fibers Fiber Chemistry Formation State of RT Tensile Strength Use T O2 1990 Process Availability Modulus Inert Cost Aramid Solvent Spin + - 0 - - - LCpolyester Melt Spin + - 0 - - - Polyethylene Gel Spun 0 - 0 - - 0 PBO, PBZT Solvent Spin - 0 - - C Fiber, PAN Polymer + 0 - + Pyrolysis C Fiber, Pitch Meltspin 0 + 0 - + + Pyrolysis Mullite Pyrolysis + 0 0 0 0 0 Alumina Pyrolysis + 0 0 0 0 0 Si3N Polymer - 0 0 + + + Pryolysis SiC Polymer 0 0 0 + + + Pryolysis SiC CVD 0 + 0 + + + B CVD 0 + 0 + + BN Chemical - - - + + ++ Conversion B4C Chemical - 0 - 0 0 ++ Conversion ZrO2 Pyrolysis - + Alumina/ Pyrolysis - + + ++ Zirconia TiB2 CVD - + + ++ Glass Melt Spun + - 0 - - -- KEY: + 0 - Availability lb >105 >< <103 Modulus Mpsi >50 >< <20 Strength Kpsi >750 >< <250 Use T °C >1000 >< <300 Cost $lb >100 >< <15
HIGH-PERFORMANCE SYNTHETIC FIBERS FOR COMPOSITES 11 PRESENT MATERIALS: AN OVERVIEW Most materials used in structural applications are either polymers, metals, or ceramics, and in many present applications these materials perform satisfactorily in their unmodified or unreinforced form. When the thermal stability and strength of the material are not critical, low-cost polymeric materials such as acrylates, epoxies, and polycarbonates can perform acceptably. Likewise, metals such as aluminum, steel, copper, or tungsten are adequate for lightweight structural components, tooling, electrical conductors, and lamp filaments, respectively. Because cost is the controlling factor, the present performance of many ceramic structural products such as window glass, structural bricks, and cement blocks is considered satisfactory. However, in many applications where performance is the controlling factor (i.e., aerospace, transportation, underwater vessels), advanced structural materials are needed that are stronger, stiffer, lighter weight, and more resistant to hostile environments. Unreinforced, the polymer, metal, and ceramic materials available today cannot meet many of these requirements. This is especially true if the structural component must be exposed to extremely high temperatures for extended periods of time. The graph shown in Figure 1.1 provides the approximate temperature limits for the use of various structural materials. Figure 1.1 Maximum-use temperatures of various structural materials.1
HIGH-PERFORMANCE SYNTHETIC FIBERS FOR COMPOSITES 12 CHARACTERISTICS OF MATERIALS IN FIBER FORM Natural fibers such as cotton and wool are some of the oldest materials. These fibers were used by early man when strength and light weight were critical. However, only in the past 50 years, with the development of analytical techniques such as X-ray diffraction, has the reason for the unusual properties of materials in fiber form been understood. Scientists now know that the molecules within fibers tend to align along the fiber axis. This preferred alignment makes the strength and modulus (stiffness) of both natural fibers and synthetic fibers superior to the same material in a randomly oriented bulk form. As an example, Table 1.2 shows the strength and modulus of a typical polymer in various forms. While the strength of an injection-molded polyamide plate in only 0.08 GPa, the tensile strength of the same polymer is over five times greater when it is extruded into a textile-grade fiber. If this same textile-grade fiber is stretched in an extensive drawing process, an industrial-grade tire cord fiber can be produced that is 10 times stronger and nearly twice as stiff as the injection-molded polymer. Chemically, all of these materials are identical, differing only in the orientation and structure of the solid polymer. When both natural and synthetic polymers are extruded and/or drawn into fiber form, the processes of extrusion and extension orient the structure along the fiber axis. This results in high strength and increased stiffness for much the same reason that an oriented mass of strings (a rope) is stronger and stiffer than the same mass of strings with no orientation. TABLE 1.2. Properties of Polyamid in Various Forms Form Tensile Strength (GPa) Tensile Modulus (GPa) Orientation Injection molded 0.08 2.5 Textile-grade fiber 0.43 2.5 Industrial-grade fiber 0.92 4.5 Kevlar®â 3.50 186.0 â Trademark of E.I. DuPont de Nemours and Co Rigid, liquid-crystal-forming polymers (e.g., aramid fibers) can develop nearly perfect orientation and alignment during fiber formation. This allows
HIGH-PERFORMANCE SYNTHETIC FIBERS FOR COMPOSITES 13 a kilogram of fibers formed from this rigid polyaramide molecule to be five times stronger than a kilogram of steel and still be five times as stiff. Since the density of aramide fiber is only one-fifth that of steel, this new class of synthetic high-performance fibers already is an obvious replacement for metal in many applications. Brittle materials, like carbon, also have a higher strength and stiffness when formed into fibers. High- performance carbon fibers formed from pitch are now available commercially with a tensile strength of 3.9 GPa. This is approximately 1000 times greater than the strength of unoriented carbon in bulk form. In the case of brittle materials, the higher strength of fibers is caused by two factors. First, like polymeric fibers, the molecular structure and orientation are improved by the fiber-formation process. Second, since the failure of brittle objects is dominated by flaws, the small size of fibers limits the size of the flaws that can exist. Thus, in addition to forming a more perfect structure, brittle materials in fiber form contain smaller flaws, further enhancing the tensile strength.2 Unfortunately, the increased tensile strength of fibers does not come without a penalty. Fibers, like rope, display this increased strength only when the load is applied parallel to the fiber axis. Even though the tensile strength parallel to the fiber axis increases as the orientation and structure become more perfect in the fiber direction, this same increase causes a decrease in strength perpendicular to the fiber axis. For example, the strength of a carbon fiber perpendicular to the fiber axis is 10 times less than the strength parallel to the axis. Also, as the orientation of a fiber increases, it often becomes brittle, making it more susceptible to damage by abrasion. Thus, to take advantage of the high strength of fibrous materials in a structure, the fibers must be oriented in the direction of the applied load and separated to prevent damage by abrasion. Mechanical reinforcement of matrices can also be accomplished by short, randomly oriented fibers, by crystal ''whiskers,'' or by particulates. These types of reinforcement offer directionally independent (isotropic) reinforcement, but the degree of reinforcement is not as great as that obtainable from longer continuous filamentary fibers. This report is concerned principally with high-performance continuous fibers. ADVANCES IN FIBER TECHNOLOGY Advances in the performance of fibers have come about because of continuity of effort on fiber materials and fiber-processing research and development. This is illustrated by Figure 1.2 for the case of organic polymer fibers; and similar illustrations apply to other types of fibers. In the early 1920s the first synthetic fibers were produced from cellulose. Because this natural polymer degrades before it ever melts, this early synthetic fiber was precipitated from a concentrated polymer solution. After precipitation the cellulose fibers had to be drawn in order to orient the polymer molecules and improve the mechanical properties. Nylon, one of the
HIGH-PERFORMANCE SYNTHETIC FIBERS FOR COMPOSITES 14 earliest synthetically produced polymers, was introduced as a fiber in the 1940s. Since this polymer would melt before it degraded, a melt spinning process was developed. This new fiber-formation process allowed some limited orientation to be introduced during extrusion, further increasing the fiber strength. With the recent discovery of liquid crystal polymers, fully oriented fibers can now be spun. Thus, as Figure 1.2 indicates, the mechanical properties of fibers have improved dramatically over the past 50 years, and the major breakthroughs have been due to the development of new materials and processing techniques. However, as with other technologies, these breakthroughs were the direct result of continuing research and development. Without continuity in effort, breakthroughs in any technology are unlikely. Figure 1.2. Strength of typical commercial organic fibers. During the period shown in Figure 1.2, the size of the U.S. synthetic fibers industry also grew dramatically. In 1988 the industry produced over 9 billion pounds of synthetic fibers for industrial and textile end uses,3 accounting for about 24 percent of the total world production. Because of this high production volume, the U.S. synthetic fibers industry is one of the most efficient in the world, in terms of pounds produced per worker, and is a vital part of the domestic economy. Even though the total world usage of high-performance fibers was only about 11 million pounds in 1988, it has consistently grown at a rate of 10 to 20 percent per year.
HIGH-PERFORMANCE SYNTHETIC FIBERS FOR COMPOSITES 15 High-performance fibers represent a major area of growth for the synthetic fibers industry, and a number of these fibers are already available commercially. The potential market for a low-cost, high-performance fiber is enormous. In fact, a large market already exists for a low-cost reinforcement fiber. During 1988, 3.6 billion pounds of glass fiber, a textile-grade nonsynthetic fiber used in low-strength structural composites, was produced worldwide, an indication of the potential market that could be available, at least in part, to a low-cost, high performance fiber. Projections of world demand for advanced composites indicate that it will reach approximately 500 million metric tons by the early decades of the twenty-first century, of which industrial and other applications will grow to 55 percent of market share while the aircraft/aerospace market share will drop to 45 percent. At a 60 percent loading factor, this amounts to a demand for approximately 300 million metric tons of high-performance fibers in the above-mentioned time period. It is important to note that this is a conservative estimate based on the current price of advanced fibersâ$15 to $20 per pound. It is forecast that if the cost of these fibers could be reduced to a few dollars per pound, the demand would be a factor of 10 higher. To increase productivity and reduce manufacturing and processing costs, synthetic fibers are normally produced and sold in multifilament bundles. As Figure 1.3 shows, there is often a trade-off between the size of this bundle of fibers, final fiber properties, and fiber cost, even for current high-performance fibers. Figure 1.3 Fiber price versus bundle size and fiber physical properties.
HIGH-PERFORMANCE SYNTHETIC FIBERS FOR COMPOSITES 16 TYPES OF HIGH PERFORMANCE FIBERS Today, numerous types of high-performance fibers are commercially available. These fibers range from polymeric fibers, such as aramid and extended-chain polyethylene, to carbon fiber, boron fiber, and ceramic fibers such as silicon carbide, and alumina. As Figure 1.4 shows, when a load is applied parallel to the fiber axis, all of these fibers are much stronger and more rigid (per given mass of a material) than traditional metals such as steel or aluminum. However, each of these high-performance fibers has certain additional advantages. For example, in oxygen-free environments, carbon fibers can retain their strength at extremely high temperatures. Polymeric fibers are much lighter than carbon and ceramic fibers and transparent to radar. Ceramic fibers, on the other hand, are resistant to oxidation but lose strength at high temperatures. The advantages and deficiencies of these and other high-performance fibers are detailed in later sections of this report. Figure 1.4. Specific strength and modulus of high-performance fibers and other materials "specific property" means the property divided by the density. FIBER-REINFORCED COMPOSITES In a composite material the fibers are surrounded by a thin layer of matrix material that holds the fibers permanently in the desired orientation
HIGH-PERFORMANCE SYNTHETIC FIBERS FOR COMPOSITES 17 and distributes an applied load among all the fibers. The matrix also plays a strong role in determining the environmental stability of the composite article as well as mechanical factors such as toughness and shear strength. Since the reinforcing fibers can be oriented during fabrication of an item, composites can be tailored to meet increased load demands in specific directions. The combined fiber-matrix system is an engineered material designed to maximize mechanical and environmental performance. There is an important, but not generally well understood, difference between the development time for traditional materials compared to that for high-performance fibers. Because a composite material is a complex system of two components coupled at an interface, the time required to develop and optimize new high- performance fibers for a particular application is much longer than that needed for the development of traditional materials. For composite applications it normally takes from 5 to 10 years to develop a new high-performance reinforcing fiber. This development time differs very markedly from the 1- to 3-year development period common to many government projects. Composite materials containing fibers (whether they be short staple fibers, whiskers, or fibers in continuous filament form such as roving or textiles) provide considerable flexibility in the design of structures. Because of this, composites of inexpensive glass fibers embedded in a plastic-matrix material have been used extensively in medium-to-high-volume applications by the transportation, construction, and recreation industries for over 40 years in applications such as auto body panels, boat hulls, and chemical tanks. However, high performance fibers dramatically expand the opportunities for composite materials. When high-performance fibers, such as carbon or polyaramide, at fiber loadings typically greater than 45 percent, are surrounded by the same plastic matrix, the material becomes an "advanced composite." Thus, in many ways, advanced composite materials represent a major breakthrough for a composite material technology that has been extensively utilized for many years. It is the added strength and stiffness of these new reinforcing fibers that allows the new advanced composites to out perform current metal and metal alloy structures. By dispersing fibers or particles of one material in a matrix of another material, today's designer can obtain structural properties that neither material exhibits on its own.4 For example, a metal alloy selected for its resistance to high temperature but having low resistance to creep at use temperature can be reinforced with fibrous inorganic oxide fibers to provide enhanced creep resistance and still be stable at high temperatures. A ceramic matrix, brittle and sensitive to impact or fracture induced by thermal stresses, may be reinforced with ceramic fibers to increase its resistance to crack propagation, providing greater toughness and protecting against catastrophic failure. The addition of reinforcing fibers to provide equal mechanical properties at a greatly reduced weight is often an important reason for choosing composites over traditional structural materials. Another vital consideration is the substitution of readily available materials for critical elements in short supply or those available only from foreign sources. Composite materials made from abundant, domestically available materials such as carbon, polymers, ceramics, or common metals can often outperform these
HIGH-PERFORMANCE SYNTHETIC FIBERS FOR COMPOSITES 18 imported strategic materials. In defense applications this substitution could be vital since even the nickel, chromium, and cobalt needed for common steel alloys are now imported.5 Even advanced ceramic composites, toughened and less likely to fail catastrophically, may provide a viable alternative to some of these imported materials.6 Reinforced polymers have been making inroads in the metal alloy market for years. In fiber-reinforced polymers, even fatigue resistance is improved significantly. According to B. R. Norton,7 the improved strength and stiffness properties of fiber-reinforced polymers allow rotational components fabricated from these materials to operate much faster than those of steel alloys. Finally, unlike conventional materials, it is relatively simple to incorporate sensors and optical fibers into fiber-reinforced composites to monitor applied loads and detect damage, creating "smart" structures. In summary, some of the specific advantages of fiber-reinforced composites are as follows: ⢠Higher specific modulus and strength values ⢠Lighter weight ⢠Controllable ("tailorable") properties Toughness Electrical and thermal conductivities Thermal expansion Stiffness ⢠Resistance to corrosion ⢠Better resistance to creep at high temperatures ⢠Substitution for critical or strategic materials ⢠Creation of multipurpose or "smart" structures The recently published Handbook on Composites8 is an important reference for current information on composites and basic materials used in composites, including fibers and fiber properties useful in the design or development of composites. Also, an especially pertinent broad review of present materials and needs for the future was published in the October, 1986, issue of Scientific American.9 Projections for High Performance Fibers The fiber research needed will depend on both the application and the fiber in question. For example, even though carbon fibers are commercially available, research directed toward product improvement and cost reduction will still yield significant payback. Research already being funded by the federal government is yielding progress in ceramic fibers and whisker technology, but new applications, such as the hypersonic transport vehicle, may require significant advances in this art. Present metals and superalloys cannot withstand the operating temperatures predicted for the National Aerospace Plane (NASP), and the use of fiber-reinforced composites is the likely solution. As existing composite materials are exposed to ever-increasing temperatures, it appears that present fiber reinforcement materials will not meet many of these requirements and that new fibers and whiskers are probably needed. At elevated temperatures, present high-performance fibers
HIGH-PERFORMANCE SYNTHETIC FIBERS FOR COMPOSITES 19 exhibit excessive grain growth or oxidize, resulting in deficiencies such as low modulus or strength properties, excessive creep rates, thermal expansion mismatch, or reaction with matrices. Other potential applications for high-performance fibers include applications such as electronic or weapons systems, where the ability to match the thermal expansion coefficients of adjacent components and to dissipate heat is critical. This may be the major future market for pitch-based carbon fibers, which can develop a thermal conductivity that is at least three times greater than copper. The application of these fibers to dissipate heat could revolutionize both the size and operating speed of computer and electronic systems. In all these projected areas for high-performance fiber development, it is critical that support be continuous and that the longer time required to develop these fibers for composite applications be recognized. Unfortunately, development of new fibers with promising properties is costly. Thus, federal funding may be necessary both to ensure a domestic source for these fibers and to support the research and development needed to improve manufacturability and reduce costs. When this is the case, it is especially important for the government sponsor and the funded development group to establish a relationship based on cooperation, rather than on strict oversight, to accelerate fiber development and minimize management costs. Present performance-driven applications provide the opportunity and the need to develop a strong domestic technology base in the high-performance fibers required for composite materials. Future high-volume markets such as automotive and construction applications will be cost-driven, and it is vital that the domestic fiber industry be prepared to aggressively compete in these markets. It is this potential for tremendous future growth, coupled with the fact that high-performance fibers are citical for many present high-technology products, that makes basic fiber research and the health of our domestic fiber industry vital to both the U.S. economy and our national security. Because the composites industry is highly international, extensive fiber science/technology bases also exist in Western Europe and Japan, where there are strong commitments to support the development of high-performance fibers. Unless steps are taken to strengthen our domestic fiber science/technology base, to facilitate its industrial application, and to broaden the industrial base for high-performance fibers, the United States might lose its present competitive position in this key industry. REFERENCES 1. "Guide to Selected Engineering Materials," special issue of Advanced Materials and Processes, 2(1), 1987. 2. Reinhart, Theodore J., "Introduction to Composites," Composites, Engineered Materials Handbook, 1, ASM INTERNATIONAL Metals Park, Ohio, pp. 27-39, 1987.
HIGH-PERFORMANCE SYNTHETIC FIBERS FOR COMPOSITES 20 3. "Fiber Roundup," America's Textiles International, p. 12, April, 1989. 4. Chou, Tsu-Wei, Roy L. McCullough, and R. Byron Pipes, "Composites," Scientific American, pp. 193-203, October, 1986. 5. Hurlich, A., "Strategic MaterialsâTechnological Trends," Mech. Eng., 104(7), p. 44, 1982. 6. Katz, R. Nathan, "Substitution TechnologyâAdvanced Ceramics," Ceramic Engineering and Science Proceedings, pp. 475-484, July-Aug., 1983. 7. Norton, Bryan R., "General Use Considerations," Composites, Engineered Materials Handbook, 1, ASM INTERNATIONAL, Metals Park, Ohio, pp. 35-37, 1987. 8. Composites, Engineered Materials Handbook, 1, ASM INTERNATIONAL, Metals Park, Ohio, 1987. 9. "Materials for Economic Growth," Scientific American, October, 1986. Other Selected References Rice, Roy W., "Fundamentals Needs to Improve Ceramic-Fiber Composites," Ceramic Engineering and Science Proceedings, pp. 485-491, July-Aug., 1983. "Lightweight Composites are Displacing Metals," Business Week, July 30, 1979. Young, John D., "The Future of Man-Made Engineering Materials," Automotive Engineering, pp. 55-59, March, 1980. Wachtman, John B. Jr., "Starting Materials for Advanced CeramicsâNeeds and Trends," Ceramic Engineering and Science Proceedings, pp. 1191-1220, Sept.-Oct., 1985. Presentations at Federal Sector Briefing, "Advanced Composites 89; An Industry Overview," Suppliers of Advanced Composite Materials Assn. (SACMA), April 29, 1989.
