Wet spinning.

The route from SD into the separation region will determine how the coagulation will proceed. (Note that a path on the line SP represents dry spinning solvent is removed from the fiber and the polymer concentration is increased there is no coagulation but there is gelation of the system.) If the amount of nonsolvent enter- 

In any case, solvent diffusion and coagulation will be more rapid in the skin than in the core of filaments. Skin formation is an inherent problem for wet spinning (and also for dry spinning). The main effect is that the filaments collapse the original round cross-section is transferred into a dog bone shape. Further information is available in Ref. 6, Chapter 4. 

Based on recent reports, CHT is a good dispersing agent for SWNT. Scientists have also proposed several methods for preparing a SWNT/CHT macroscopic structure 

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Both thermodynamic and kinetic aspects of mixed systems (e.g., the precipitation step in wet spinning) involve the properties of the other components (solvent and nonsolvent in wet spinning) as well as the polymer. 

Drying promoters >Dry-j et wet spinning Dry-laid nonwovens Dry milk... 

Reaction spinning equipment is quite similar to that of solution wet spinning. It differs principally in the use of fewer wash baths and in the use of belt-type dryers instead of heated cans. 

The People s RepubHc of China introduced Kuraray technology and started production of PVA fiber by a wet spinning process in 1965. Its annual capacity reached 165,000 tons in 1986 (9). The Democratic People s RepubHc of Korea produce PVA and reportedly have an annual production capacity of 50,000 tons (9). 

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. 

Tows and staples, as well as short cut chips, are being manufactured by wet spinning and filament yams by both dry and wet spinning. 

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...

Asahi s innovations have done much to transform the cuprammonium process from an uneconomic competitor for viscose and synthetics into the fastest wet-spinning system in the world. They now cl aim it to be competitive both economically and environmentally with the viscose filament process. 

In a typical commercial dry jet-wet spinning process, PPT polymer of inherent viscosity 6.0 dL/g is added to 99.7% sulfuric acid in a water-jacketed commercial mixer in a ratio of 46 g of polymer to 100 mL of acid. The mixture is sealed in a vacuum of 68.5—76 mL of mercury. Mixing takes place for 2 h... 

Fig. 6. Schematic of dry-jet wet spinning employing tube-in-orifice spinneret A, bore injection medium (liquid, gas, or suspended soHds) B, pump C, spinneret D, polymer spinning solution E, micrometer ( -lm) "dope" filter F, coagulation or cooling bath G, quench bath and H, collection spool.

HoUow-fiber fabrication methods can be divided into two classes (61). The most common is solution spinning, in which a 20—30% polymer solution is extmded and precipitated into a bath of a nonsolvent, generally water. Solution spinning allows fibers with the asymmetric Loeb-Soufirajan stmcture to be made. An alternative technique is melt spinning, in which a hot polymer melt is extmded from an appropriate die and is then cooled and sohdified in air or a quench tank. Melt-spun fibers are usually relatively dense and have lower fluxes than solution-spun fibers, but because the fiber can be stretched after it leaves the die, very fine fibers can be made. Melt spinning can also be used with polymers such as poly(trimethylpentene), which are not soluble in convenient solvents and are difficult to form by wet spinning. 

Solution (Wet) Spinning. In the most widely used solution spinnerette system (60) the spinnerette consists of two concentric capillaries, the outer capillary having a diameter of approximately 400 ]Am and the central capillary having an outer diameter of approximately 200 ]lni and an inner diameter of 100 ]lni. Polymer solution is forced through the outer capillary while air or Hquid is forced through the inner one. The rate at which the core fluid is injected into the fibers relative to the flow of polymer solution governs the ultimate wall thickness of the fiber. Figure 19 shows a cross section of this type of spinnerette. 

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. 

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