Dry spinning.

Solution spinning can be used to produce fibers from polymers that do not form thermally stable melts, but can be dissolved in solvents. Two important solution spinning methods are dry spinning and wet spinning. Dry spinning typically is used for polymers that ate soluble in volatile solvents. Wet spinning often is used for polymers that are dissolved in non-volatile or thermally imstable solvents. 

Control of gas flows in the column, avoiding turbulence or sticking of filaments, is important. Therefore, the hot gas flow would often be downward rather than in 

Evaporation of solvent is a relatively slow process, slower than cooling in melt spinning. It is therefore essential that the spinning filaments are thin. The common approach is to use small spinning holes (50-100 gm) and apply a low draft in the spin-line. For example, both the extrusion speed from the holes and the exit speed from the dry-spinning column would be around 300 m min . Even a draft below 1 is possible, which means that the die swell effect is not completely undone. 

Another approach is a high-draft process, spinning from large holes (500 gm) and taking up the yam at relatively high speeds. For example, the extmsion speed through the holes could be 20-50 m min and the exit speed around 500 m min which implies a draft of 10-25. The draft in the spin-line is viscous flow and only limited orientation is built up, a situation completely comparable with melt spinning. 

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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... 

Extrusion Processes. Polymer solutions are converted into fibers by extmsion. The dry-extmsion process, also called dry spinning, is primarily used for acetate and triacetate. In this operation, a solution of polymer in a volatile solvent is forced through a number of parallel orifices (spinneret) into a cabinet of warm air the fibers are formed by evaporation of the solvent. In wet extmsion, a polymer solution is forced through a spinneret into a Hquid that coagulates the filaments and removes the solvent. In melt extmsion, molten polymer is forced through a multihole die (pack) into air, which cools the strands into filaments. 

Dry Spinning. On a worldwide basis, about 90% of all spandex fibers are produced by various adaptations of dry spinning (15,16). The solution dry spinning process is illustrated in Figure 5. The polymer spinning solution is metered at a constant temperature by a precision gear pump... 

Pure PVA dissolves in water but does not fluidize by melting. Commercial production of PVA fiber is therefore carried out by wet spinning or dry spinning, utilizing aqueous PVA solution. In either case, purified PVA is dissolved in hot water and the solution is extmded through fine holes of a spinneret the extmded streams are coagulated to form continuous filaments, which are then heat-treated to have adequate mechanical properties. 

One report (13) describes the procedure for spinning dry asymmetric ceUulose acetate fiber with a bore skin. Such fibers are spun in a modified dry-spinning process in which a volatile Uquid (methyl formate) is used as the ceUulose acetate solvent. The bore coagulating Uquid is isopropyl alcohol, which is subsequentiy removed. The advantages of these dry fibers over most ceUulose acetate membranes are that they can be stored dry, they are wet-dry reversible, they can be sterilized and packed dry, and they are ready for use without removal of preservatives. 

A variation on the wet-spinning technique involves extruding into a heated gas environment. In this dry-spinning process, the temperature and composition of the gas control the extraction process. 

Copolymers of acrylonitrile are sensitive to heat, and melt spinning is not used. Solution spinning (wet or dry) is the preferred process where a polar solvent such as dimethyl form amide is used. In dry spinning the solvent is evaporated and recovered. 

Acetate A general name for processes for making cellulose acetate fibers. Cellulose is acetylated, dissolved in acetone, and spun into fibers by injecting through orifices into heated chambers. Cellulose mono-acetate is made by acetylating with a mixture of acetic acid, acetic anhydride, and sulfuric acid as the catalyst. Cellulose tri-acetate is made in a similar fashion, but using perchloric acid as the catalyst, and dry-spinning from a solution in ethanol/ methylene chloride. Cellulose tri-acetate fibers were first made commercially by Courtaulds in London in 1950. 

See also MPDI fibers commercial process for, 19 721 crystal lattice parameters of, 19 728t dry spinning of, 19 724 interfacial polymerization process for, 19 722... 

Dry spinning. The polymer, dissolved in a suitable solvent and filtered, is pressed through spinnerettes and, in an oxygen-free atmosphere, pulled by vacuum through a heated shaft, where the polymer solidifies as the solvent evaporates. The requirements of this process regarding the heat stability of pigments are much less... 

Elastomeric fibres based upon both polyester-urethane and polyether-urethane structures followed. The early work by Bayer led to the use of highly polar solvents such as dimethyl formamide. Formation of fibres by reactive spinning, where the isocyanate-ended polymer is extruded into an aqueous solution of a chain-coupling agent, was described in 1949 [75] and by dry-spinning a solution of the chain-coupled polymer in 1951 [76], However, Bayer did not immediately use their technology to produce commercial fibres [77],... 

Fibers produced by dry spinning have lower void concentrations in comparison to melt-spun fibers because the presence of solvent molecules causes voids that are often remembered by the polymer. This is reflected by greater densities and lower dyeability for the dry spun fibers. 

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