Aromatic polyamide fibres

Aromatic polyamide (aramid) fibres are produced by spinning from liquid crystaUine solutions using solvents such as sulphuric acid [28,29]. The properties 

Penn and Milanovich [31] looked at the effect of deformation upon the Raman spectrum of Kevlar 49 and, although they found that stress caused a change in the relative band intensities and depolarization ratios, they did not find any measurable shift in the band wavenumbers with stress [31]. This finding is completely at odds with the findings of Young and coworkers [32,33], who have shown that significant and measurable frequency shifts of several Raman bands in Kevlar take place on the application of stress or strain. 

It has been shown [37] that the Raman technique follows only the crystal and molecular stretching and, moreover, that the change in the peak position dAv is a measure of the molecular stress rather than the molecular strain [37]. The rate of shift per unit strain in the higher modulus fibres is a reflection of the higher levels of molecular stress in such fibres. 

Penn and Milanovich [31] expressed surprise that they did not find any frequency shifts during the deformation of aramid fibres, luring in mind the earlier polymer deformation studies using infrared [42] and Raman spectroscopy [8] and wide-angle X-ray scattering (WAXS) studies of deformed aramid fibres [39]. The situation was confused further by the publication by Edwards and Hadiki [34], who reported that no Raman band shifts were found when Kevlar 

In recent years there has been considerable interest in aromatic polyamide fibres, better known as aramid fibres. These are defined by the US Federal Trade Commission as a manufactured fibre in which the fibre-forming substance is a long chain synthetic polyamide in which at least 85% of the amide linkages are attached directly to two aromatic rings.  

The Kevlar polymer may be regarded as a liquid crystal polymer (see Chapter 25) and the fibres have exceptional strength. They are thus competitive with glass, steel and carbon fibres. 

Originally developed for tyre cords, Kevlar-type materials have also become widely used in composites. Uses include filament-wound rocket motors and pressure vessels, metal-lined Kevlar-overwrapped vessels in the space shuttle, boat and kayak hulls, Kevlar-epoxy helmets for the US military, and as one of the reinforcements in composite lorry cabs. 

Polymers have also been prepared from cyclic amines such as piperazine and bis-(p-aminocyclohexyl)methane. An early copolymer, Igamid 1C, was based on the latter amine. This amine is also condensed with decanedioic acid, HOOC (CH2)iqCOOH, to produce to silk-like fibre Quiana (Du Pont). 

In addition to the commercial aromatic polyamides described above many others have been prepared but these have not achieved commercial viability. There are, however, a number of other commercial polymers that contain amide groups such as the polyamide-imides. The latter materials are discussed in Section 18.14. 

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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)... 

Nylon 6T melts at 370 C, Nomex is practically flameproof. Kevlar, poly (p-phynyleneterephthalamide) represents a break-through in high modulus aromatic polyamide fibre and mainly used as reinforcement tyres, conveyers belt etc., PBI is... 

In 1973 Du Pont commenced production of another aromatic polyamide fibre, a poly-(p-phenyleneterephthalamide) marketed as Kevlar. It is produced by the fourth method of polyamide production listed in the introductory section of this chapter, namely the reaction of a diamine with a diacid chloride. Specifically, p-phenylenediamine is treated with terephthalyl chloride in a mixture of hexamethylphosphoramide and A-methylpyrrolidone (2 1) at -10°C Figure 18.32). 

Some three decades ago, scientists from the Du Pont company developed polycondensation methods which allowed the preparation of high molecular weight wholly aromatic polyamides. The first commercially produced wholly aromatic polyamide fibre was poly(m-phenyleneisophthalamide) (Nomex, Du Pont, 1967) [la, c]. Some years later, development of the preparation and processing of poly(p-phenyleneterephthalamide) (PPTA) led to the commercialization of the para product Kevlar (Du Pont) in the early seventies [lb, c]. While Nomex shows excellent thermal stability and flame-retardance, and indeed is referred to as a heat and flame resistant aramid fibre, Kevlar fibre also has similar properties, but in addition it has exceptional tensile strength and modulus, and is referred to as an ultra-high strength, high modulus aramid fibre. 

EFFECT OF DOPE ANISOTROPY ON AS-SPUN PROPERTIES OF AROMATIC POLYAMIDE FIBRES... 

A distinctive structural feature of PpPTA and other para-aromatic polyamide fibres is the pleated sheet, which is shown in Fig. 13c, 13d and 13e. It was for the first time observed by Ballou in dark-field images taken from meridional reflections of the ED pattern [132]. This phenomenon has been extensively studied by Dobb et al. [125]. The pleated sheet consists of parallel-oriented chains, which are hydrogen bonded along the direction of the crystallographic b-axis. They are usually oriented perpendicular to the surface of the filament, i.e., in the circular cross-section the b-axes are directed preferably along the radius, as shown in Fig. 14. The angle between adjacent planes of the pleat is about 170° and the distanee between two pleats is about 250 nm, but both values may vary along the radius of the fibre cross-section. 

A high-strength, high-stiffness, aromatic polyamide fibre. 

The production of aramid fibre, an aromatic polyamide fibre, is by an extrusion and spinning process (Hollaway, 2008). There are two grades of stiffness available ... 

The polyamide fibre (Kevlar 49) has been studied by electron diffraction and electron microscopy. The supramolecular structure has been shown to be a system of sheets regularly pleated along their long axes and airan d radially. The effect of stress on the fluorescence of Kevlar fibre has also been investigated. Structural observations have also been reported on aromatic polyamide fibres based on phenylene diamine and teiephthalic acid. ... 

The aromatic polyamide fibre formed from p-phenylene diamine and terephthalic acid (Kelvar) has similar liquid crystal properties and can only be spun from concentrated H2SO4 solutions. Further developments in these types of polymers will certainly be forthcoming. 

There are two types of aramid (fully aromatic polyamide) fibre those with high thermal stability and flame resistant properties but mechanical properties similar to those of standard textile fibres such as polyester, sometimes referred to as meta-aramid fibres because the main commercial products are of this chemical structure the others with additionally exceptionally high tensile strength and modulus, sometimes referred to as para-aramid fibres on similar grounds. The former type is not widely used in composite structures, but some of the most important uses of the latter lie in this area. 

This result suggested that cotton fibres in this blend accelerated the thermal degradation of aromatic polyamide fibres. On the other hand, compared to the calculated activation energies, an important decrease took place during the thermal degradation process, showing the formation of intermediate degradation structures with lower thermal stability. 

Goto, T, Maeda, M. and Hibi, S. (1989), High-strength and high toughness aromatic polyamide fibre. I. Preparation and properties of block poly(para-phenylene and 4,4 -diphenyl ether terephthalamide) fibre. Journal of Applied Polymer Science. 37(4) jp. 867-875. 

M.G. Dobb, D.J. Johnson and B.P. Seville, Structural aspects of high modulus aromatic polyamide fibres , Phiios. Trans. R. Soc. London, Sen A. 294,1980, 483-485. 

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