Coagulation spinning

Velocity of spinning solution through spinneret Type and velocity of internal coagulant through the inner tube External coagulant Spinning temperature Distance of the air gap... 

Widely used industrial approaches for the produetion of polymer fibres and yarns, such as coagulation spinning and electrospinning, have also been utilised for the fabrication of polymer nanotube eomposites. In coagulation spinning, for example, composite fibres ean be produced by an injection of surfactant-stabilised nanotube dispersion in water into a rotating bath of polymer e.g. PVA) dissolved in water sueh that nanotube and polymer dispersions flowed in the same direction at the point of injection. In this case, polymer molecules replace surfactant molecules on the nanotube surface, thus destabilising the nanotubes dispersion whieh eollapses to form a fibre. These fibres can then be retrieved from the bath, rinsed and dried. 

Common traditional approaches for the fabrication of CNT-polymer composites include solution processing of composites, melt spinning, melt processing, in situ polymerization, processing of composites based on thermosets, electrospinning and coagulation spinning for composite fibers and yam production [92]. 

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. 

To produce a spandex fiber by reaction spinning, a 1000—3500 molecular weight polyester or polyether glycol reacts with a diisocyanate at a molar ratio of about 1 2. The viscosity of this isocyanate-terrninated prepolymer may be adjusted by adding small amounts of an inert solvent, and then extmded into a coagulating bath that contains a diamine so that filament and polymer formation occur simultaneously. Reactions are completed as the filaments are cured and solvent evaporated on a belt dryer. After appHcation of a finish, the fibers are wound on tubes or bobbins and rewound if necessary to reduce interfiber cohesion. 

Water is continuously added to the last extraction bath and flows countercurrenfly to filament travel from bath to bath. Maximum solvent concentration of 15—30% is reached in the coagulation bath and maintained constant by continuously removing the solvent—water mixture for solvent recovery. Spinning solvent is generally recovered by a two-stage process in which the excess water is initially removed by distillation followed by transfer of cmde solvent to a second column where it is distilled and transferred for reuse in polymer manufacture. 

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. 

Fig. 2. Pliotogiapli of the cross sections of PVA fiber manufactured by wet spinning with a coagulating bath of sodium sulfate (a) and sodium hydroxide...

The correct viscose age or ripeness for spinning varies according to the type of fiber being made. Ripeness can be assessed by estabHshing the salt concentration necessary to just coagulate the viscose dope. The preferred test uses sodium chloride (salt figure) although ammonium chloride is the basis of the alternative method (Hottenroth number). 

Aqueous dispersions are used for spinning PTEE fibers. The dispersion is mixed with a matrix-forming medium (116,117) and forced through a spinneret into a coagulating bath. The matrix material is removed by heating and the fibers are sintered and drawn molten to develop their full strength. 

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