Wet-spun fibers

Fiber cross sections are also deterrnined by the coagulation conditions or, in the case of dry spinning, by the solvent evaporation process. The skin that forms early in the solvent removal process may remain intact as the interior of the filament deflates from solvent removal. Wet spun fibers from organic solvents are often bean shaped, while those from inorganic solvent systems are often round. Dry spun fibers, such as Du Font s Odon, are... 

The crystal stmcture of PPT is pseudo-orthorhombic (essentially monoclinic) with a = 0.785/nm b = 0.515/nm c (fiber axis) = 1.28/nm and d = 90°. The molecules are arranged in parallel hydrogen-bonded sheets. There are two chains in a unit cell and the theoretical crystal density is 1.48 g/cm. The observed fiber density is 1.45 g/cm. An interesting property of the dry jet-wet spun fibers is the lateral crystalline order. Based on electron microscopy studies of peeled sections of Kevlar-49, the supramolecular stmcture consists of radially oriented crystaUites. The fiber contains a pleated stmcture along the fiber axis, with a periodicity of 500—600 nm. 

Wet spinning of this type of hoUow fiber is a weU-developed technology, especiaUy in the preparation of dialysis membranes for use in artificial kidneys. Systems that spin more than 100 fibers simultaneously on an around-the-clock basis are in operation. Wet-spun fibers are also used widely in ultrafiltration appUcations, in which the feed solution is forced down the bore of the fiber. Nitto, Asahi, Microgon, and Romicon aU produce this type of fiber, generaUy with diameters of 1—3 mm. 

In all cases the anisotropic polymerization mixtures (10% by weight) could be used directly in the formation of dry-jet wet-spun fibers. Monofilament fibers were obtained by coagulation in water, tension dried at 150 °C and heat treated at 500-600 °C with a 30s residence time. The best fibers were obtained from the high molecular weight PBZT polymer (VII) which exhibited modulus values that ranged between 172 GPa and 207 GPa and tenacity values up to 2.4 GPa. Unfortunately, the compressive property as measured by the tensile recoil test was only 380 MPa, showing only a slight improvement over PBZT. 

Hot-stretched wet-spun fibers HipCo SWNT, MWNT, (SDS) 30 wt% 195000 6.5 Miaudet 05... 

Figure 11.7. (a) Stress-strain curve of a raw wet-spun fiber (the inset focuses on the elastic regime), (b) stress-strain curves of wet-spun fibers that have been hot-stretched at 180°C, at various draw-ratios, from 0 (raw) to 800%. 

Table 11.2. Mechanical properties of wet-spun fibers, hot-stretched at different draw-ratios. The fibers are drawn at 180°C and contain a fraction of 25wt% nanotubes. The PVA is 195000 g/mol and 99% hydrolized...

In dry spinning, the dope is extruded through spinnerets located at the top of a tower. As the uncoagulated filaments flow down the tower, they are brought into contact with an inert gas heated above the boiling point of the dope solvent. The solvent evaporates from the filaments as they pass down the column and solidify. Conceptually, dry spinning can be considered to be a special case of melt spinning, in which the polymer solidification or crystallization temperature has been depressed by the solvent. The solidification temperature of the filaments will continuously increase as the solvent evaporates until solid filaments are formed. The filaments are continuously removed from the bottom of the tower, washed free of solvent, and then for the most part processed like the wet-spun fibers. 

The relationship between fiber structure, cross-sectional shape, and bending modulus (stiffness) was discussed in Section 12.5.2 and is also described by Morton and Hearle [353]. The influence of the cross-sectional shape is particularly important for acrylics. Fibers that have high aspect ratio cross sections, such as the dry-spun fibers with a dog-bone-shaped cross section, tend to be more compliant and softer in comparison with the wet-spun fibers, which have a more rounded cross section. 

A number of methods have been reported in the literature for improving the thermal stability of acrylic fibers. For example, impregnating wet-spun fibers in the gel state with salts of metals from group II in the periodic table of the elements [496] or sodium sulfide solution [487] have been reported. Also, dried fiber can be impregnated with sulfuric acid solutions of formamide [488], phosphates or borax [489], or organotin salts [490]. 

J.P. Knudsen and W.E. Fitzgerald, The Influence of Gel Network Mechanics on the Tensile Properties of Wet-Spun Fibers, presented at the Gordon Res. Conf. Textiles (1965). 

Since there are many process steps involved in the formation of wet-spun fibers, the effect of these process parameters must be thoroughly investigated to determine the optimal set of conditions to maximize the desired mechanical properties of the as-spun polyaniline fibers. In particular, for spiiming... 

Dry spun fibers have lower void concentrations than wet spun fibers. This is reflected in greater densities and lower dyeability for the dry spun fibers. 

Dry spinning generates a fiber that initially appears different from typical wet-spun fibers as there is no opportunity for the spin bath to diffuse into the fiber. However, when the unoriented dry-spun fiber is stretched, an oriented fibrillar structure develops, indistinguishable from a stretched wet-spun fiber. Hence, all acrylic fibers, whether dry- or wet-spun are fundamentally similar. 

Dyeing, gel n. Passing a wet-spun fiber that is in the gel state (not yet at full crystallinity or orientation) through a dyebath containing dye with affinity for the fiber. This process provides good accessibility of the dye sites. 

In dry spinning, the solvent evaporation occurs by airflow. In wet spinning, the polymer flber precipitates out in a coagulation bath. Examples of wet spun fibers are Artelon, polyurethanes, polyacrylonitrile. Fibers of diameter 10 -100 pm are typically obtained by the solvent spinning method. 

Wet-spun fibers are formed by extruding a highly viscous polymer solution through a spinneret into an appropriate liquid bath, where it is solidified. The solidification is brought about by a diffusional interchange between the extruded polymer filaments and the bath. In this process, called coagulation, one or more components from the bath diffuse into the fiber while, in turn, solvent leaves the forming filaments. The net result is a solid fiber. In some fiber systems, such as viscose rayon, there is also a chemical reaction superimposed on the diffusional process [60-62]. In others, the situation is strictly diffusional. Only the diffusional process will be considered in this treatment. 

Published research in the area of reaction spinning has been somewhat limited. The principal papers to date are the work of Pohl [78] and Fok et al. [79]. Mention should also be made of the work done on postspinning treatment of fibers. This type of process, widely used for high-temperature fibers [80-83], consists of subjecting dry- or wet-spun fibers of a given polymer to time and temperature to bring about a reaction that takes the material to another polymer compound. This process is used because the final high-temperature polymer is either infusible or insoluble or both. 

Although the foregoing was developed for dry- or wet-spun fibers, the importance of type 3 orientation also holds for melt-spun fibers. It therefore be-... 

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