In response to innovations in military armaments, the search for improved body armor for military and law enforcement personnel has been the focus of intense research for several decades. One of the resulting products of this research, Kevlar, is the closest humans have come to matching the tensile strength of spider silk is five times stronger per weight than steel and is best known for its use in bulletproof vests (Ehrenfeld, 2000). Kevlar shares something in common with matches, COBOL, antifungal antibiotics, pulsars, vitamin A, Cepheid variable stars, radium, and mobile genes, in that all of these were discovered or invented by women (Brownlow, Jacobi & Rogers, 2000). Kevlar was developed at DuPont by a team comprised of Stephanie Kwolek, Herbert Blades, and Paul W. Morgan. In 1978, Kwolek also produced from aramids the first polymeric liquid crystals (Stevens & Kauffman, 2004). This paper will provide an overview and background of aramids in general and Kevlar in particular, including its physical properties, its advantages and disadvantages, as well as anticipated future developments and trends in its production. A summary of the research will be provided in the conclusion.
Review and Discussion
Background and Overview. Kevlar is an industrial textile that is most commonly known in the manufacture of bulletproof vests; however, it is also used in the manufacture of composites, and fiber optic and electromechanical cables (Seewald, 1991). Industrial textiles refer to the manufacture of such fabrics as asbestos, glass fibers, carbon fibers and Kevlar, which are produced for the automotive, aerospace sectors of industry (Martin, Penn & Scattergood, 1991). Following the success of nylons in such applications, aromatic nylons known as aramids were created through the condensation of a diamine and terephthalic acid (this is a carboxylic acid that contains a hexagonal benzene ring in its molecules). The close packing of the aromatic polymer chains resulted in a strong, tough, stiff, high-melting fiber that was suitable for use in radial tires, heat- or flame-resistant fabrics, bulletproof clothing, and fiber-reinforced composite materials. DuPont first began to produce Nomex (its trademark for poly-meta-phenylene isophthalamide) in 1961 and Kevlar (the trademarked name of poly-para-phenylene terephthalamide) in 1971 (Stevens & Kauffman, 2004).
Most merchant (as opposed to captive) man-made fiber producers in the United States have historically elected to pursue a narrow marketing approach, or were eventually forced to adopt such a strategy; however, DuPont chose to develop a broad approach with a remarkably wide and deep array of large volume acrylic, nylon, and polyester genera plus lines of specialty fibers, most notably spandex, Teflon and its line of Aramids such as Qiana, Nomex, and Kevlar (Goldenberg, 1992). These proprietary specialty fibers, plus a broad and deep product line, serve to "lock in" DuPont as the premier man-made fiber supplier today. As the setter of end-product specifications and the leading developer of significant new fiber genera, DuPont has earned and enjoys a uniquely powerful position in the U.S. fiber industry, which it skillfully exploits in several ways (Goldenberg, 1992).
In response to growing global demand for Kevlar, in 2001, DuPont announced plans to increase its manufacturing capacity. According to Nancy Seewald (2001), DuPont reported that it would construct a production line for its Kevlar high-strength para-aramid fiber at Richmond, Virginia, scheduled for completion by the end of 2002. The company invested $50 million in the project, which increased its worldwide Kevlar capacity by about 15%; however, DuPont would not disclose its total Kevlar capacity (Seewald, 2001).
The new line will use DuPont's proprietary technology; which the company says will "provide unique process and product capabilities," but it would not provide specifics. DuPont's only competitor in the para-aramid fiber market, Teijin, announced in June 2001 that it planned to increase capacity for its Twaron para-aramid fibers at Arnhem, the Netherlands and Matsuyama, Japan by 65%, to 20,500 m.t./year by April 2003. The new line employs DuPont's proprietary technology; which the company says will "provide unique process and product capabilities," but it would not provide specifics. Teijin has been using the technology at its Maydown, U.K. Kevlar plant for three years (Seewald, 2001).
DuPont recently debottlenecked its Richmond, Virginia and Maydown, U.K. Kevlar plants, thereby increasing its global capacity by 15%; DuPont also produces Kevlar at Tokai, Japan (Seewald, 2001). DuPont reported that the new capacity is needed to meet 5%-10% annual growth the company has enjoyed with its para-aramid fibers line.
Physical Properties. Kevlar is a polymer; a polymer is a chain made of many similar molecular groups, known as monomers, that are bonded together. Each Kevlar monomer is a chemical unit comprised of 14 carbon atoms, 2 nitrogen atoms, 2 oxygen atoms and 10 hydrogen atoms (Kevlar, 2003). A better understanding of the polymerization and high-polymer structure phenomena were due to research carried out between 1920 and 1930 by H. Mark Staudinger and Kurt Meyer, which paved the way for the subsequent work in the thirties which would revolutionize the chemistry of plastics, fibers and synthetic rubbers (Aftalion, 1991). By the late 1950s, a revolution in materials development took place in response to the increasing need for lightweight, thermally stable materials. As a result, boron-tungsten filaments, carbon-graphite fibers, and organic aramid fibers were introduced which proved to be strong, stiff, and light; however, one problem with using them as fibers was that they were of limited value in any construction other than rope, which was able to bear loads in only one direction. Consequently, materials scientists needed to identify a method whereby these compounds could be made useful under all loading conditions, which resulted in the development of composites (Stevens & Kauffman, 2004).
Although the structural value of a bundle of fibers is low, the strength of individual fibers can be harnessed if they are embedded in a matrix that acts as an adhesive, binding the fibers and lending solidity to the material. The matrix also serves to protect the fibers from environmental stress and physical damage, which can be the catalyst for cracks. Furthermore, while the strength and stiffness of the composite remain largely a function of the reinforcing material (in other words, the fibers), the matrix can contribute other properties, such as thermal and electrical conductivity and, most important, thermal stability. Finally, fiber-matrix combination reduces the potential for complete fracture. In a monolithic (or single) material, a crack, once begun, will generally continue to propagate until the material fails; however, in a composite, if one fiber in an assemblage fails, the crack may not extend to the other fibers, so the damage is restricted (Stevens & Kauffman, 2004).
Research after 1960 was not as fruitful or as eventful as the leaps that had been made to date, but the new materials during the 1970s were the direct result of research in high polymers that had been essentially conducted within industry itself (Aftalion, 1991). For instance, it was through such research that ICI's "PEEK" (polyether ether ketone), one of the first high-performance aromatic polymers, was placed on the market, together with DuPont's aramide fibers Nomex and Kevlar, which is more resistant than steel in like volume (Aftalion, 1991).
According to Goldenberg (1992), man-made fiber production employs but a few basic steps according to recognized authorities. Those basic steps are: mixing ingredients; reacting the mixture to form a monomer, the basic compound from which a polymer is formed; polymerization, or linking the monomer into a long chain molecule; extruding the polymer as a fiber or fiber spinning; and winding the fiber onto a package. "These steps become complex in practice. Over ten variables have to be controlled and coordinated. There usually is at least one alternative to any selected fiber-spinning process, with often subtle, hard to evaluate but important economic and technical trade-offs" (Goldenberg, 1992, p. 27). Making products from these materials involves processing them in liquid form; in other words, the polymers flow into molds of the desired size and shape (Leal, 1999). The research into polymer flows has only recently assumed the complicated geometries that are normal in manufacturing. According to Leal, when a tire company wants to change the cross-sectional shape of a tire by extruding from a dye, they do not currently have a fundamental basis to predict the shape of the dye. "They have a professional dye maker who, after a few tries, gets it right. It costs tens of thousands of dollars, but that's the way they do it. If instead they could use a computer to design prototypes, it would be far less expensive" (Leal, 1999, p. 5). According to his essay, "Ubiquitous Polymers" (1999), Leal notes that research in this area has shifted in focus in recent years for two reasons: 1) On the experimental front, techniques have improved thereby enabling researchers to determine what takes place on a microscopic level; and 2) numerical techniques have developed to solve the equations that describe the complex behavior of polymeric liquids in flow. "Materials are going to be a major force in the American economy in the future," notes…