Term Paper: Polymer Analysis (Kevlar) Brief History

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[. . .] 6). 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 as shown in Figure 1 below:

Figure 1. Nomex and Kevlar Polymer Chains [Source: Stevens & Kauffman, 2004].

Nomex and the comparable aramids being marketed by other companies today 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 rodlike 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 the above-mentioned 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; and V belts and hoses (Stevens & Kauffman, 2004).

Other applications for Kevlar continue to be identified as well. 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 do more to combine wearer comfort with increased protection, according to Donald F. Groce, technical product specialist with Best Manufacturing, a hand protection manufacturer. Today, employees are demanding gloves that are more flexible and wearable, while employers want better cut and puncture resistance and more chemical protection.

According to Jason Khayat, technical service coordinator for Perfect Fit Glove, his company, like many others, is working toward providing end users with more than one level of protection in one glove. However, until recently, the choices for arm protection were limited; manufacturers are now utilizing Kevlar blends for glove sleeves that provide better cut-protection and comfort though. "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).

Advantages and Disadvantages. While there have been some distinct advantages to using Kevlar in certain applications, there are some problems associated with the material as well. For instance, when the Canadians decided to use Kevlar as a roofing material for their Olympic Stadium, the results were less than impressive. "The saga of the roof would be almost comical, if it weren't such a fiscal catastrophe. It took 11 years after the Olympics for a putatively functioning roof to be affixed to the tower (at a cost of $75 million); by 1992, the Kevlar roof had ripped eight times, costing Quebec taxpayers $55 million" (Levine, 2003, p. 102). Further, the manufacturing process required to produce Kevlar are hazardous and result in potentially dangerous emissions.

For instance, in her 1997 book, Biomimicry, Janine Benyus pointed out that spiders convert digested crickets and flies into silk that is as strong as Kevlar without the need for boiling sulfuric acid and high-temperature extruders (Hawken, Lovins & Lovins, 1999). According to Ehrenfeld (2000), "The closest material humans have yet invented to match the properties of spider silk is Kevlar, used in bulletproof vests. But Kevlar comes from petroleum derivatives, subjected to high temperatures in a bath of concentrated sulfuric acid, producing hazardous by-products" (p. 35). Further, Kevlar's unique physical structure appears to lend itself to some applications more than others (Brandenburg, Merlo, Wickens & Yeh, 2003).

The bulletproof vests made from Kevlar are not cheap either. One estimate provided by Cook and Ludwig was derived from multiplying the total number of police protection employees (856,000) times one-fifth (on average the proportion whose vests are replaced in any given year) times $550, the midpoint of the price range for bulletproof Kevlar vests. This figure assumes that each police-protection employee is already assigned their own vest and that each police department rotates vests once every five years. "While the actual annual expenditure level may be slightly different, our estimate of $100 million is probably in the right ballpark" (Cook & Ludwig, 2000, p. 208). While bulletproof vests are perhaps the best-known application of Kevlar today, there are numerous others that continue to be identified.

Future Developments and Trends. The consumer market for aramids is currently limited, but this may change radically in the future as additional applications are identified, and the useful properties of this polymer are exploited. For instance, at first glance, there appear to be just six man-made fibers that compete for tire business today. "Rayon, Kevlar Aramid, nylon, polyester, fiberglass and steel seemingly are rivals in this market. But things are not this simple. Not all rayons, Aramids or nylons are acceptable tire ingredients" (Goldenberg, 1992, p. 14). Compared with the other subvarieties of rayon, only the high-tenacity versions of viscose filament rayon can produce commercially viable tires by today's standards; for example, some types of rayon and all acetate fibers lack the tenacity, heat resistance, and flex strength despite their low cost. It also explains the tire industry's rejection of all but one kind of nylon (Type 6, 6), although other types may be much lower priced (Goldenberg, 1992).

Rather than competing against each other, each of the six man-made fibers tends to serve an isolated segment of the market. According to Goldenberg, Kevlar fiber's incredibly high resistance to being broken by force exerted on its long axis (in other words, its tenacity) and other technical properties make it well suited for the needs of specialty tires for racing and other applications where the price does not matter when compared with performance.

Protecting law enforcement officials on the streets and soldiers' lives on the battlefield are other applications where price is deemed less important than its intended purpose. According to Vause and Grantham (1999), military helmets are designed to protect soldiers from potential head injury during training and combat operations; unfortunately, such protective headgear may also tend to restrict the ability of a soldier to localize sounds. Although previous research cited by Vause and Grantham reported that earlier helmet designs did not disrupt localization, there had been no similar research conducted on the efficacy of the Kevlar helmets currently in use in the U.S. Army.

According to a study of Kevlar's effectiveness as a helmet by Vause and Grantham (1999), when the Kevlar protective helmet was worn alone, there was a small (about 25%) increase in horizontal-plane localization errors compared with the bare-head conditions in both the frontal and lateral orientations; however, these differences were not statistically significant. In the final analysis, these researchers reported that the combination of the Kevlar helmet and certain types of earplug degraded localization performance when compared with the Kevlar helmet alone (Grantham & Vause, 1999).

Haworth (1999) reports that the real dangers to tank crews involve shaped charge penetrations, secondary fires and explosions from fuel and ammunition. Some of the methods being developed to palliate them included "spall liners" of various substance, with DuPont's Kevlar polyaramid fiber being the newest and most promising-inside the main armored envelope to stop impinging fragments.

These liners were designed to catch the secondary fragments produced by penetrating shaped-charge jets and, to a lesser extent, other projectiles. According to Haworth, these liners were a distinct improvement and were already in use on the upgraded M113A3s (APCs) entering service in 1999. "The APC's straightforward, boxy internal geometry made this a relatively simple addition" (Haworth, 1999, p. 138). The implications of these findings remain unclear, though, in view of the ongoing research into improved body armament for military and law enforcement applications that may result in Kevlar's obsolescence or further refinements in its physical properties.

Man-made fibers such as Kevlar have facilitated significant technical advances in at least two medical areas (surgery and nephrology); however, other life-saving devices depend completely on such man-made fibers. For instance, vehicle safety belts and air bags for automobiles and stunt work are but two obvious nonmilitary examples, while parachutes and light, flexible Kevlar body armor are more combat-related life-saving devices that require man-made fibers (Goldenberg, 1992).

Conclusion.

The research showed that 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 and other uses continue to be identified. While aramids are still not produced in as high a volume as the commodity fibers such as nylon and polyester; based on their higher unit price, they still represent a large and growing business segment. To date, there have been few… [END OF PREVIEW]

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