Kevlar Polymer: Properties, Applications, and Future Trends

Introduction and Historical Background

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. It is five times stronger per unit 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 provides 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.

Background and Overview of Aramid Fibers

Kevlar is an industrial textile 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 and 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 — 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 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. DuPont, however, 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, combined with a broad and deep product line, serve to lock in DuPont as the premier man-made fiber supplier. As the setter of end-product specifications and the leading developer of significant new fiber genera, DuPont has earned 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, DuPont announced plans in 2001 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 approximately 15%; however, DuPont would not disclose its total Kevlar capacity (Seewald, 2001).

The new line used DuPont's proprietary technology, which the company said would "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 metric tons per year by April 2003. Teijin had been using a similar technology at its Maydown, U.K., Kevlar plant for three years (Seewald, 2001). DuPont also debottlenecked its Richmond, Virginia, and Maydown, U.K., plants, further increasing global capacity by 15%, and produces Kevlar additionally at Tokai, Japan. DuPont reported that the new capacity was needed to meet the 5%–10% annual growth the company had been experiencing in its para-aramid fibers line.

Physical Properties and Polymerization

Kevlar is a polymer — 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 polymerization and high-polymer structure phenomena came from research carried out between 1920 and 1930 by H. Mark Staudinger and Kurt Meyer, which paved the way for subsequent work in the 1930s that 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. One problem with using them as fibers, however, was that they were of limited value in any construction other than rope, which could 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 led to the development of composite materials (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 act as a catalyst for cracks. Furthermore, while the strength and stiffness of the composite remain largely a function of the reinforcing material (the fibers), the matrix can contribute other properties, such as thermal and electrical conductivity and, most importantly, thermal stability. Additionally, the fiber-matrix combination reduces the potential for complete fracture. In a monolithic (or single) material, a crack, once initiated, will generally continue to propagate until the material fails; in a composite, however, 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 previously, but new materials introduced during the 1970s were the direct result of high-polymer research conducted largely 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, along with DuPont's aramid fibers Nomex and Kevlar, which is more resistant than steel in like volume (Aftalion, 1991).

According to Goldenberg (1992), man-made fiber production employs a few basic steps: mixing ingredients; reacting the mixture to form a monomer; polymerization, or linking the monomer into a long chain molecule; extruding the polymer as a fiber (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 — that is, the polymers flow into molds of the desired size and shape (Leal, 1999). Research into polymer flows has only recently begun to address 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 die, it does not currently have a fundamental basis to predict the shape of that die. "They have a professional die 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). Leal notes that research in this area has shifted in focus for two reasons: first, experimental techniques have improved, enabling researchers to determine what takes place at a microscopic level; and second, numerical techniques have developed to solve the equations that describe the complex behavior of polymeric liquids in flow.

The two compounds, Kevlar and Nomex, are distinguished by the structure of their polymer chains, with Kevlar containing para-oriented phenyl rings and Nomex containing meta-oriented rings. Nomex and comparable aramids marketed by other companies are generally dry-spun from the solution in which the polymer is prepared (Stevens & Kauffman, 2004). By contrast, the polymer used for Kevlar and related compounds is wet-spun from a hot, high-solids solution of concentrated sulfuric acid. As a result of the rod-like structure of the para-oriented aramids, a liquid-crystalline solution is obtained that serves to preorient the molecules even before they are spun, leading to as-spun fibers of ultrahigh strength and ultrahigh stiffness (Stevens & Kauffman, 2004).

Beyond bulletproof vests, Kevlar and its competitors are employed in belts for radial tires, cables, reinforced composites for aircraft panels and boat hulls, flame-resistant garments (especially in blends with Nomex), sports equipment such as golf club shafts and lightweight bicycles, and as asbestos replacements in clutches and brakes. Nomex-type fibers are fashioned into filter bags for hot stack gases, clothes for presses that apply permanent-press finishes to fabrics, dryer belts for papermakers, insulation paper and braid for electric motors, flame-resistant protective clothing for firefighters, military pilots, and race-car drivers, as well as V-belts and hoses (Stevens & Kauffman, 2004).

Other applications for Kevlar continue to be identified. For instance, fiber cabling jackets are being reinforced by Kevlar strands, making them less susceptible to damage during installation (Holland, 1999). A move is underway in the occupational safety industry to combine wearer comfort with increased protection. According to Donald F. Groce, technical product specialist with Best Manufacturing, employees are demanding gloves that are more flexible and wearable, while employers want better cut and puncture resistance and greater chemical protection. Jason Khayat, technical service coordinator for Perfect Fit Glove, noted that his company is working toward providing end users with more than one level of protection in a single glove. Manufacturers are now utilizing Kevlar blends for glove sleeves that provide better cut protection and comfort. "They are also making sleeves with products such as DuPont's Nomex, which offer more comfortable heat resistance" (Challenges and Emerging Issues, 2001, p. 27).

Conclusion

The research showed that Kevlar is an industrial textile 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, and new uses continue to be identified. While aramids are still not produced in as high a volume as commodity fibers such as nylon and polyester, they represent a large and growing business segment based on their higher unit price. To date, few end uses have been identified for aramids in the home — while Nomex-type fibers have been made into ironing-board covers, industrial uses are on the increase, especially for aramids of the Kevlar class, as designers learn how to exploit the unique properties offered by these unusual materials. This trend may change in the future, however, as research into consumer applications identifies additional innovative uses not currently envisioned.