Kevlar mechanical properties

Table II Effect of Ar Plasmas on Kevlar Mechanical Properties...

Similady, hquid-crystal polymers exhibit considerable order in the hquid state, either in solution (lyotropic) or melt (thermotropic). When crystallized from solution or melt, they have a high degree of extended-chain crystallinity, and thus have superior mechanical properties. Kevlar (Du Pont) is an aromatic polyamide (atamid) with the repeating unit designated as (2). It is spun into... 

Aramid Fibers. Aromatic polyamide fibers exhibiting a range of mechanical properties are available from several manufacturers, perhaps the best known being Du Pont s proprietary fiber Kevlar. These fibers possess many unique properties, such as high specific tensile strength and modulus (see Fig. 4). Aramid fibers have good chemical resistance to water, hydrocarbons, and solvents. They also show excellent flame retardant characteristics (see High PERFORMANCE fibers Polyamdes). 

As an illustration of the results of the measurements just described, the mechanical properties for four unidirectionally reinforced composite materials, glass-epoxy, boron-epoxy, graphite-epoxy, and Kevlar 49 -... 

Fluoroelastomers Novikova et al. [32] reported unproved physico-mechanical properties of fluoro mbbers by reinforcement with chopped polyamide fibers. Other fiber reinforcements are covered by Grinblat et al. [33]. Watson and Francis [34] described the use of aramid (Kevlar) as short fiber reinforcement for vulcanized fluoroelastomer along with polychloroprene mbber and a co-polyester TPE in terms of improvement in the wear properties of the composites. Rubber diaphragms, made up of fluorosilicone mbbers, can be reinforced using aramid fiber in order to impart better mechanical properties to the composite, though surface modification of the fiber is needed to improve the adhesion between fluorosUicone mbber and the fiber [35]. Bhattacharya et al. [36] studied the crack growth resistance of fluoroelastomer vulcanizates filled with Kevlar fiber. 

Mechanical properties of carbon fiber-epoxy matrix and Kevlar fiber-epoxy matrix composites with and without PVAL coaling at room temperature". 

Due to the fact that the mechanical properties of unidirectional, continuous-fiber-reinforced composites are highly anisotropic, maximum effectiveness is often achieved by making laminate composites of multiple layers. This is particularly true of carbon and Kevlar -reinforced polymers, which will be described in Section 5.4.3. 

Commercial applications have been identified primarily in the electronics industry where requirements for dimensional stability, mechanical properties, and high temperature resistance make these systems attractive in advanced circuit board technology. Other commercial applications include high temperature membranes and filters where these materials offer performance improvements over glass, Kevlar, and graphite composites. Industrial development of these types of materials will most likely be dependent on monomer cost and advances in various product properties requirements. 

In most of the previous work with polyimide fibers, the fibers were spun from poly(amic acid) precursors, which were thermally imidized in the fiber form. However, high degrees of imidization were not achieved. Thus, tensile properties of these polymers were not as good as those of high-performance fibers. Work in our laboratories has shown that when the fibers are spun directly from preimidized polymers, it is possible to achieve tensile properties that are as good or even better than those of poly(p-phenyleneterephthalamide) (PPTA or Kevlar ) fibers. For example, fibers have been prepared from m-cresol solutions of BPDA-PFMB using a dry-jet wet-spinning method. The as-spun fibers were then extensively drawn and annealed above 400°C to achieve excellent mechanical properties. 

High performance polymer fibers (HPPF) have excellent mechanical properties compared to traditional textile fibers such as nylon. The typical HPPFs are aramid and polyethylene fibers (6). Aramid is a generic name for a class of aromatic polyamide fibers, most of which are varieties of poly(p-pheny-lene terephthalamide). Kevlar is the trade name of the varieties of aramid polymers introduced conunercially by Dupont. The molecules in the fibers of these materials are oriented in the axial direction. Poly(p-phenylene terephthalamide) is a rigid molecule with the following structure ... 

Quartz fibers and Kevlar have also been coated. A uniform adherent, conducting, and electroactive coating is produced using this simple approach. The mechanical properties of the base textile are not significantly affected. The authors suggest that the adherent coating arises from the fact that it is not the monomer or oxidant that is adsorbed onto the textile, but rather the radical and/or oligomers. 

The most important development in lyotropic liquid crystalline polymers after Kevlar is probably the synthesis of poly [benzo(l,2-d 4,5-d ) bisthiazole-2,6-diyl]-l,4-phenylene, or for short, poly(p-phenylene benzo-bisthiazole) ( PBT Wolfe and Loo, 1980 Wolfe et al., 1981), and the closely related poly [benzo(l,2-d 5,4-d )bisoxazole-2,6-diyl]-l,4-phenylene or poly(p-phenylene benzobisoxazole) ( PBO Helminiak and Arnold, 1977 Wolfe and Arnold, 1981). Both PBT and PBO are lyotropic liquid crystalline and can be spun into fibers with mechanical properties even superior to that of Kevlar fibers. The molecular structures of these polymers are shown in Figure 5.2. 

In this section, we consider the case of solutions of rigid or semi-flexible polymers which display one or several liquid crystalline phases in a given range of concentration. The main control parameter is not flie temperature as is the case for thermotropic LCPs but rather the concentration of polymer in the solvent. There are many different kinds of lyotropic LCPs. Some are synthetic like Kevlar which has become a very important structural material with mechanical properties comparable to those of steel. Some are natural like the Tobacco Mosaic Virus and like DNA which shows a nematic and a hexagonal phase. Some are mineral like the vanadium pentoxide ribbons. In the next section, we shall first describe the lyotropic system which is probably best known, namely the tobacco mosaic virus. 

Kevlar is a lyotropic liquid crystal which can be obtained from a sulfuric acid solution when the concentration reaches a critical value, e.g., 10%. However, this polymer like other LCPs is also anisotropic and its mechanical properties is directional, but less so in fibers than in the extruded plaques. The fiber properties in Table I are compared with other organic fibers and steel ( 15). On an equal weight basis, Kevlar has a strength several times that of steel. Perhaps, Kevlar is the first polymer at least comparable to metals. 

Fabrics woven from Kevlar 49 aramid fiber are often used as composite reinforcement, since fabrics offer biaxial strength and stiffness in a single ply. The mechanical properties of Kevlar 49 aramid are dependent on the fabric constmction. The composite properties are functions of the fabric weave and the fiber volume fraction (typically 50%-55% with ply thickness 5-10 mils, depending on fabric constmction). In 1987, Du Pont introduced high-modulus Kevlar 149. Compared to Kevlar 49 it has higher performance (47% modulus increase) and lower dielectric properties (65% decrease in moisture regain). 

Other fibers may be used in POs, though some of these may be chosen more for their special properties. Basalt mineral fibers or Kevlar can provide extreme reinforcement for ballistics applications other fibers include metal fibers for electromagnetic shielding purposes (discussed in Chapter 6). However, as in the case of stainless steel fibers, just because an additive is in fiber form does not necessarily mean it provides meaningful mechanical property reinforcement, but it may provide the opposite of what is wanted [6-4, 7-57]. 

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