Spinning dope

Acrylonitrile and its comonomers can be polymerized by any of the weU-known free-radical methods. Bulk polymerization is the most fundamental of these, but its commercial use is limited by its autocatalytic nature. Aqueous dispersion polymerization is the most common commercial method, whereas solution polymerization is used ia cases where the spinning dope can be prepared directly from the polymerization reaction product. Emulsion polymerization is used primarily for modacryhc compositions where a high level of a water-iasoluble monomer is used or where the monomer mixture is relatively slow reacting. 

Fibers spun by this method may be isotropic or asymmetric, with dense or porous walls, depending on the dope composition. An isotropic porous membrane results from spinning solutions at the point of incipient gelation. The dope mixture comprises a polymer, a solvent, and a nonsolvent, which are spun into an evaporative column. Because of the rapid evaporation of the solvent component, the spinning dope solidifies almost immediately upon emergence from the spinneret in contact with the gas phase. The amount of time between the solution s exit from the spinneret and its entrance into the coagulation bath has been found to be a critical variable. Asymmetric fibers result from an inherently more compatible solvent/nonsolvent composition, ie, a composition containing lower nonsolvent concentrations. The nature of the exterior skin (dense or porous) of the fiber is also controlled by the dope composition. 

A fiber spinning dope is prepared by mixing the polysulfone, solvent and a nonionic surfactant. A preferred solvent is di-methylformamide (DMF) a preferred surfactant is an alkylaryl polyether alcohol. The dope is prepared such that its bulk viscosity is typically between 9000 and 12000 poises. Successful dopes demand control of the mode of addition, degree of polymer comminution, the temperature protocol and agitation rate. The dope is typically filtered through a 10 micron filter under pressure prior to spinning. 

Research effort at Albany International Research Co. has developed unit processes necessary for pilot scale production of several species of reverse osmosis hollow fiber composite membranes. These processes include spin-dope preparation, a proprietary apparatus for dry-jet wet-spinning of microporous polysul-fone hollow fibers, coating of these fibers with a variety of permselective materials, bundle winding using multifilament yarns and module assembly. Modules of the membrane identified as Quantro II are in field trial against brackish and seawater feeds. Brackish water rejections of 94+% at a flux of 5-7 gfd at 400 psi have been measured. Seawater rejections of 99+% at 1-2 gfd at 1000 psi have been measured. Membrane use requires sealing of some portion of the fiber bundle for installation in a pressure shell. Much effort has been devoted to identification of potting materials which exhibit satisfactory adhesion to the fiber while... 

Hollow fiber spinning dopes and preparation procedures vary over a wider range than their flat-sheet equivalents, but some representative dopes and spinning conditions taken from the patent literature [98,103,104] are given in Table 3.4. 

As NMMO is a solid at room temperature, dissolution and processing of the spinning dope require elevated temperatures of about 100 °C. The dope is spun into an air gap and water, where cellulose is regenerated and NMMO is washed out. After purification and evaporation of the water, the amine N-oxide is reintroduced into the system and used again for cellulose dissolution. 

The overall performance difference between the artificial fibroin silk and natural silk is induced by many factors. Composition of the spinning dope is critical but not the only factor. Important to understand is that the spinning process which determines the condensed structure of silk is crucial. It suggest that knowing the spinning process details it should be feasible to produce high-performance silk artificially and "design" silk. 

Regenerated spidroin and fibroin dissolved in various solvents are used as spinning dope while the coagulation baths are mainly alcohol (Table 4). In addition certain fiber post-treatments, such as drawing, are used as well. 

Pioneering work in fibroin wet spinning can be traced back to 1930s. After that, little work has been done until the late 1980s, when more research was done to investigate the spinning dope systems, and structure and properties of the artificial fibroin silk. The composition of the dope is very important to the properties of the final fiber. Several kinds of solvents, such as LiBr—EtOH, Ca(NOo,)2—MeOH, formic acid, HFIP, hexafluoro acetone (HFA), and so on, are used to prepare the spinning dope (Table 4). Very recently, an ionic liquid was used as dope solvent (Phillips et al., 2005). 

From a scientific perspective, the artificial silk experiments have provided insight into the morphology of reconstituted silk. In the spinning dope, fibroin molecules adapt a random coil or other less extended conformations. 

Nevertheless, silk spinning remains a very complex process. Spiders and silkworms not only have a set of well-developed spinning glands but also have a set of well-defined and controlled chemical boundary conditions. Besides the composition of the spinning dope, the spinning techniques and the combination of chemical parameters (pH and metallic ions) must be considered and optimized. 

Zarkoob et al. (1998, 2004) were the first to report on the electrospinning of silkworm silk and Nephila clavipes dragline protein. They used an HFIP solution of protein as the spinning dope. The resulting fibers had a wide distribution in diameter and the continuity during spinning could be significantly improved. 

Viscose or rayon A well-known inherently FR viscose fiber is Viscose FR, marketed by Lenzing. The fiber is produced by adding Sandoz 5060 (Clariant 5060)-bis(2-thio-5,5-dimethyl-l,3,2-dioxa-phosphorinyl)oxide in the spinning dope before extrusion. As this additive is phosphorus based, it is similar to other phosphorus-based FRs in terms of mode of action (condensed phase). 

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