High modulus fibers aromatic polyamides

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). 

More recently, high modulus fibers, such as aromatic polyamides, that are difficult to prepare by other methods, were fragmented into fibrillar fine structures [20] by high wattage ultrasonic irradiation in water. This is extremely useful for producing very finely divided material for transnussion methods such as lattice imaging and electron diffraction. The major drawback is... 

High modulus fibers from lyotropic aromatic polyamides, poly(p-phenylene terephthalamide) (PPTA), were first conunercialized imder the Kevlar trademark by DuPont [414]. The aromatic polyamides, or aramids, are produced by a dry jet-wet spinning process where the nematic structure in solution is responsible for the high modulus fiber performance [415-419]. Another class of lyotropic fibers, also produced by dry jet-wet spinning, are the rigid rod polymers developed as part of the U.S. Air Force Ordered Polymers Program [420-424]. The most conunon of these ordered polymers, poly(p-phenylene benzobisthiazole) (PBZT), is difficult to process, but it exhibits the highest tensile properties of all the LCP fibers produced to date. 

Most often the lyotropic LC polymers form nematic mesophases. Most of the polymers in this class are aromatic polyamides with aromatic ring structures, as shown in Thble 7.1 (7). Several of the polymers in Table 7.1 form very high-modulus fibers see Chapter 11. The fibers are crystalline after formation. 

Aromatic polyhydrazides are well known from the work of Frazer, Wallenberger and Sweeny (1,2) as precursors to poly-1,3,4-oxadiazoles. Poly-amide-hydrazides have been described in detail by Black, Preston, and coworkers (3,4,5) and by Culbertson and Murphy (6). High tenacity, high modulus fibers have been made from poly(terephthalie hydrazide)(7) and from polyamide-hydrazides with ordered structures (5). 

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... 

Hence, the tremendously high equilibrium rigidity and the ordered structure of para aromatic polyamides favouring the formation of mesophases in the concentrated polymer solutions and permitting their use for the manufacture of ultrahigh-modulus fibers is ensured by both the frans-structure of the amide groups and the para position of the phenyl rings. 

Most micromechanical theories treat composites where the thermoelastic properties of the matrix and of each filler particle are assumed to be homogeneous and isotropic within each phase domain. Under this simplifying assumption, the elastic properties of the matrix phase and of the filler particles are each described by two independent quantities, usually the Young s modulus E and Poisson s ratio v. The thermal expansion behavior of each constituent of the composite is described by its linear thermal expansion coefficient (3. It is far more complicated to treat composites where the properties of some of the individual components (such as high-modulus aromatic polyamide fibers) are themselves inhomogeneous and/or anisotropic within the individual phase domains, at a level of theory that accounts for the internal inhomogeneities and/or anisotropies of these phase domains. Consequently, there are very few analytical models that can treat such very complicated but not uncommon systems truly adequately. 

Among the commercially available aramid fibers are Du Font s Nomex (I) and Kelvar (II) in fact these trade names are commonly used in lieu of the generic name. Kelvar 49 is a high-modulus aramid fiber and is the most widely used reinforcing aramid fiber. Kevlar 29 has a lower modulus and Kevlar 149 has a higher modulus than Kevlar 49. Aromatic polyamides are described in greater detail in Chapter 4. 

For a variety of technical reasons the development of aromatic polyamides was much slower in comparison. Commercially introduced in 1961, the aromatic polyamides have expanded the maximum temperature well above 200°C. High-tenacity, high-modulus polyamide fibers (aramid fibers) have provided new levels of properties ideally suited for tire reinforcement. More recently there has been considerable interest in some new aromatic glassy polymers, in thermoplastic polyamide elastomers, and in a variety of other novel materials. 

Aromatic polyamides are, for example, wet spun from hot 100% sulfuric acid into cold water. The lateral self-association of the macromolecules in the nematic mesophase is retained after precipitation of the filaments and leads to the very high moduli of elasticity and good tensile strengths of such fibers. In these cases, the properties essentiaUy depend on the draw ratio during spinning, that is, on the ratio of the filament diameter at the spinneret to that at the first winding roller. The modulus of elasticity increases and the elongation at break decreases with increase in this ratio. 

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