Polyamides, aromatic Kevlar

Aromatic polyamide Kevlar Du Pont 220 — v-0 Reinforcing fibre... 

Solutions of [PcCo(CN)] and the aromatic polyamide Kevlar in concentrated H2SO4 can be wet-spun to produce flexible darkly colored fibres. Even... 

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... 

The primary driving forces behind investigation of new solvents include environmental concerns and the abiUty to form Hquid crystals in the new solvent systems. By analogy with Kevlar, a synthetic aromatic polyamide fiber, spinning from a Hquid crystalline solution should yield cellulose fibers with improved strength, as has been demonstrated in laboratory experiments. 

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). 

The reinforcing filler usually takes the form of fibres but particles (for example glass spheres) are also used. A wide range of amorphous and crystalline materials can be used as reinforcing fibres. These include glass, carbon, boron, and silica. In recent years, fibres have been produced from synthetic polymers-for example, Kevlar fibres (from aromatic polyamides) and PET fibres. The stress-strain behaviour of some typical fibres is shown in Fig. 3.2. 

Fibers in which the basic chemical units have been formed by chemical synthesis, followed by fiber formation, are called synthetic fibers. Examples include nylon, carbon, boron fibers, organic fibers, ceramic fibers, and metallic fibers. Among all commercially available fibers, Kevlar fibers exhibit high strength and modulus. (Kevlar is a DuPont trademark for poly [p-phenylene diamine terephthalamide].) It is an aromatic polyamide (aramid) in which at least 85% of the... 

An increase in rod-like arrangement of the macromolecules can also arise by stretching a polymer either in its solid state, either in the melt or even in solution (for polymers leading to lyotropic liquid crystals such as aromatic polyamides). This is the basis of the development of synthetic fibres including high modulus polyethylene Dyneema , polyamide Nylons and Kevlar , polyester Tergal or Dacron fibres. 

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. 

Working with a solution is needed for polymers which above their melting point would degrade (example aromatic polyamide fibres such as Kevlar and Twaron). For fibres the removal of the solvent is not too problematic. In e.g. injection moulding applications solvents caimot be used here thermotropic LCP s have to be used. Since these would degrade during processing, they are diluted by copolymerisation (example poly-hydroxy-benzoic acid - co - PETP)... 

Fibres are, as a result of the spinning process, molecularly oriented, and they have, therefore, a 2 to 3 times higher stiffness than the non-oriented polymer (e.g. polyamide and polyester textile fibres). With the highest attainable orientation, such as in aromatic polyamides (Twaron and Kevlar), and in the PE-fibre (Dyneema) the stiffness can be a hundred times higher than the one in the unoriented condition ... 

A major impetus was given to work, both academic and industrial, in the field of lyotropic systems by the development by duPont of commercial fibres having exceptionally high tensile strength and modulus through use of nematic anisotropic solutions of relatively rigid-chain aromatic polyamides. The earliest product to appear, Fibre B, was based upon poly (p-benzamide) (I)10), but was replaced by the fully commercial product, Kevlar, based upon poly (p-phenylene terephthalamide) (II) U). Arenka, from Akzo, also has the latter chemical repeating unit. 

Kevlar Fibers. Table IX shows results obtained with Kevlar 950 fibers. The crystallinity of this aromatic polyamide is only slightly higher than that for the aliphatic polyamide samples that we studied (Table VI). But in Kevlar, as for PET fibers, the disorder parameter k is smaller than in PA 66 fibers. Thermal treatment at 220°C for 1 hour in slack conditions does not substantially increase the average crystallinity of the Kevlar 950 fibers and produces more distortions. Consequently, we conclude that the behavior of this Aramid fiber during our annealing treatment is similar to PA 66 as far as crystallinity is concerned. Kevlar 950 is also similar to PET fiber with regard to the behavior of the disorder parameter. 

Kwolek s task at DuPont was to find a new kind of fiber that was resistant to acids and bases and that would remain stable at high temperatures. In 1964, she discovered such a product, an aromatic polyamide that was five times as strong as steel with half the density of fiberglass. The material was given the name aramid. Aramid was later marketed under the trade names of Kevlar and Nomex . Today, aramid is one of the most widely used substances in polymer matrix composites. 

In theory, almost any polymer can be used as a reinforcement in an advanced composite. By far the most common polymer fiber, however, is aramid, better known by its trade name, Kevlar. Developed originally for use in belted radial tires, aramid is an aromatic polyamide in which benzene fragments (C6H4) alternate with peptide groups (NHC = Q) in forming a polymeric structure ... 

Nylon 6 is the homopolymer of caprolactam, and Kevlar is an aromatic polyamide, poly-p-phenylene terephthalamide. 

A well-known example of a lyotropic LCP is the aramide fibre (Twaron or Kevlar), an aromatic polyamide with the structure ... 

Following the technological breakthroughs which led to the discovery of (1) the liquid crystalline behavior ofpara-oriented aramids26 and (2) a novel method for spinning anisotropic liquid crystalline polymer solutions,27 Kevlar aramid fiber was produced and commercialized by the DuPont company in 1972. Other fibers based on aromatic polyamide compositions, which were produced and commercialized by other companies, were Technora (Teijin, Japan), Teijinconex (Teijin, Japan), andTwaron (Akzo, The Netherlands). Additionally, SVM is a fiber produced in the Former Soviet Union and it was announced in 1990 that a new aramid fiber had been introduced by Hoechst, in Germany. 

Aromatic polyamides and polyesters are examples of stiff chain polymers. Poly(p-phenylene terephthalamide) (Kevlar , 1-23) can be made by reaction (4-50) in a mixture of hexamethylphosphoramide and /V-methylpyrrolidone ... 

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 ... 

Aromatic polyamides or aramids. The best known in the specialty fiber market are Nomex (poly-m-phenyleneisophthalamide) and Kevlar (poly-p-phenyleneterephthala-mide). 

Besides the aliphatic and aliphatic/aromatic nylons, aromatic polyamides have excellent capability to form fibers. Among aromatic polyamides the most common are Kevlar or poly(imino-1,4-phenyleneiminocarbonyl-1,4-phenylenecarbonyl) or poly(phenylene terephthalimide), and Nomex or poly(phenylene isophthalimide). These compounds are included in a group of polyamides known as aramids. The structures of Kevlar and Nomex are shown below ... 

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