Aromatic polyamide Fiber spinning

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. 

Aromatic polyamide fibers are produced by spinning liquid crystalline polymer solutions of PPTA-sulfuric acid dopes into a water coagulation bath [414], resulting in the formation of a crystalline fiber with a surface skin. Variations in the structure produced by annealing at elevated temperature are known to increase the fiber modulus due to a more perfect alignment of the molecules [472]. The chemistry and physics of the aromatic polyamide fibers have been reviewed [419]. 

Aromatic polyamide fibers are produced by spinning liquid crystalline polymer solutions of PPTA-sulfuric acid dopes into a water coagulation bath [337], resulting in the formation of a crystalline fiber with an exterior surface skin. 

Fiber spinning, 11 174, 175, 170-171 carbon-nanotube, 13 385-386 methods of, 16 8 models of, 11 171-172 of polyester fibers, 20 12-15 Fiber structure, of aromatic polyamides, 19 727... 

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. 

It was, however, observed that such systems under appropriate conditions of concentration, solvent, molecular weight, temperature, etc. form a liquid crystalline solution. Perhaps a little digression is in order here to say a few words about liquid crystals. A liquid crystal has a structure intermediate between a three-dimensionally ordered crystal and a disordered isotropic liquid. There are two main classes of liquid crystals lyotropic and thermotropic. Lyotropic liquid crystals are obtained from low viscosity polymer solutions in a critical concentration range while thermotropic liquid crystals are obtained from polymer melts where a low viscosity phase forms over a certain temperature range. Aromatic polyamides and aramid type fibers are lyotropic liquid crystal polymers. These polymers have a melting point that is high and close to their decomposition temperature. One must therefore spin these from a solution in an appropriate solvent such as sulfuric acid. Aromatic polyesters, on the other hand, are thermotropic liquid crystal polymers. These can be injection molded, extruded or melt spun. 

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. 

Conio et al. studied the fiber formation of PpBA from an organic solvent, viz. JV,JV -dimethylacetamide containing 3% LiCl, but in these experiments an air gap was hardly used [86]. This may be the reason that they arrived at somewhat different conclusions concerning the spinning of para-aromatic polyamide solutions. 

Polycondensation of p-phenylenediamine with terephthaloyl dichloride produces aromatic polyamides, which display liquid-crystal characteristics. The spinning of these polymers from hexamethylphosphoramide or sulfuric acid solution yields high-value aramid fibers (Kevlar Du Pont), Twaron Akzo) which are distinguished by their high temperature resistance (see Chapter 7.3.1). 

Among the aromatic polyamides, poly(l,4-benzamide) (PBA) and poly(/)-phenylene terephthalamide) (PPTA) were the first cotmnerdal aramid fibers spinned from lyptropic LC solutions (Chapter 5.19). 

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. 

Swierenga and co-workers [79,80] in two articles described in detail their development of the calibration model used for PET measurements. Van Wijk et al. [72] summarized from their work that Raman spectroscopy can be used for determining the dye uptake or measuring one or more structural parameters or mechanical properties of polymeric fibers. Based on the results of their work, the authors stated that Raman spectroscopy was useful for studying melt-spinning thermoplastics, such as polyester, polyamide, polyolefins, and alternating copolymers of carbon monoxide and olefins so-called polyketones and, in addition, for polymers which are spun from solution such as cellulose, aromatic polyamides, polyketones, aromatic polyesters, and polyolefins. 

Preparation of spinning solutions. Aromatic polyamides are soluble in a limited number of solvents. The low solubility makes it possible to precipitate them from solution during synthesis in organic solvents. The precipitated polymer with the solvent residues washed off is then usually dissolved in sulfuric acid for processing into fibers. Sulfuric acid is a proton-donor solvent, and the amides tehave like strong bases toward it, adding a proton according to the scheme... 

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