Kevlar structure

The chemistry of these polymers was extended to terphthaloyl chloride and p-phenylenediamine, resulting in the commercialization of Kevlar fiber. Kevlar (structure shown below) is ideally suited for ballistic vests and high impact helmets because of its strength and the fact that it is as strong as steel but at one-fifth the... 

Fiber-reinforced plastics have been widely accepted as materials for structural and nonstructural applications in recent years. The main reasons for interest in FRPs for structural applications are their high specific modulus and strength of the reinforcing fibers. Glass, carbon, Kevlar, and boron fibers are commonly used for reinforcement. However, these are very expensive and, therefore, their use is limited to aerospace applications. 

Kevlar, a nylon polymer prepared by reaction of 1,4-benzenedicarboxylic acid (terephthalicacid) with 1,4-benzenediamine (p-phenyienediamine), is so strong that it s used to make bulletproof vests. Draw the structure of a segment of Kevlar. 

Kevlar fibers are supplied by DuPont as Kevlar-29 and 49. The former is characterized by high tensile strength and the latter by high initial modulus(4). Some Kevlar-29 end uses are in ropes and cables, which are as strong as steel at one-fifth the weight, and in ballistic vests. Some Kevlar-49 end uses are in reinforcing resins and composites for aerospace structures, boat hulls, and sport equipments. 

Figure 16 Chain structure of poly(p-phenylene terephthalamide) (Kevlar ).

Both low molecular weight materials [145] and polymers [146,147] can show liquid crystallinity. In the case of polymers, it frequently occurs in very stiff chains such as the Kevlars and other aromatic polyamides. It can also occur with flexible chains, however, and it is these flexible chains in the elastomeric state that are the focus of the present discussion. One reason such liquid-crystalline elastomers are of particular interest is the fact that (i) they can be extensively deformed (as described for elastomers throughout this chapter), (ii) the deformation produces alignment of the chains, and (iii) alignment of the chains is central to the formation of liquid-crystalline phases. Because of fascinating properties related to their novel structures, liquid-crystalline elastomers have been the subject of numerous studies, as described in several detailed reviews [148-150]. The purpose here will be to mention some typical elastomers exhibiting liquid crystallinity, to describe some of their properties, and to provide interpretations of some of these properties in molecular terms. 

Kevlar The Wonder Material. Microworlds Exploring the Structure of Materials, Lawrence Berkeley Laboratory, http //www.lbl.gov/MicroWorlds/Kevlar/... 

A particular constmction of filaments of rayon, nylon, Terylene, Kevlar or steel used as the structural reinforcement of pneumatic tyres. The cords were originally woven (cord fabric) but are now invariably used weftless. 

A) A condensation reaction between terephthalic acid and 1,6-hexamethylenediamine to form the monomer used to make KEVLAR (B) Structure of KEVLAR showing hydrogen bonds between straight chains... 

Fig. 5.19. Schematic presentations of (a) hydrogen bonding and (b) supramolecular structure of Kevlar 49...

Perhaps the most widely utilized (and studied) lyotropic LCP is poly j -phenylene terephthalamide (PPTA), more commonly known as Kevlar (see Figure 1.70). Kevlar belongs to the class of aramids that are well known for their LCP properties. Because these polymers are crystalline in solution, they are often spun into filaments, from which the solvent is subsequently removed in order to retain the aligned polymer structure. The result is a highly oriented, strong filament that can be used for a wide variety of structural applications. Most thermotropic LCPs are polyesters or copolymers that can be melted and molded into strong, durable objects. 

The most spectacular application example to date is perhaps the Aberfeldy footbridge over the river Tay in Scotland (see Fig. 11.4). This bridge is 113-m long, has a deck width of 2.2 m, and a main span of 113 m [4]. The entire deck structure, hand rails, and A-frame towers are pultruded composites, and the cable stays are Kevlar ropes. The deck structure is assembled from a modular system of pultruded 6-m long, hollow components, which consist of 70 percent by weight of E glass and 30 percent pigmented isophtalic polyester resin. 

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