Viscous melt spinning processes

Wet spinning. This technique is characterized by spinning a filtered viscous polymer mass, dissolved in a suitable solvent, into contact with a precipitation or coagulation bath. Polyacrylonitrile, polyvinyl acetate, cellulose acetate, and other materials are processed by this method. Thermal requirements for pigments are less stringent than for melt spinning but pigments are expected to be fast to the solvents and chemicals used. 

If the polyurethane molecule is allowed to form a three-dimensional structure, it becomes insoluble and neither wet nor dry spinning processes can be used. A chemical spinning process must be used in this case. The trick is to spin the isocyanate-terminated prepolymer at a stage where it forms a viscous melt, and with the jets produced in an environment that contains a chain extender. This chain extender diffuses into the fibers and reacts to couple the prepolymer molecules (Couper, 1985). 

The one step process for the fabrication of pure silica sliver from sodium silicate [1-3] is an adaptation oil the generic dry spinning process that has been practiced for over 50 years in the fabrication of polymer organic textile fibers. Acrylic polymers, unlike polyesters or nylons, are infusible and cannot be melt spun. They can be dry or wet spun from viscous solutions. The model for inorganic dry spinning processes is the dry spinning process by means of which acrylic fibers such as Orion, are spun using dimethyl formamide as the solvent (Table I). 

Surface tension-driven breakup into droplets is rarely important in melt spinning, where the large viscous and elastic forces overwhelm the surface tension forces, ft is an important mechanism in the formation of the dispersed phase in polymer blends, and it is important in solution processing. The surface tension-driven breakup of a viscoelastic filament has been analyzed using both thin filament equations and a transient finite element analysis, but we will not pursue the topic here because it is not relevant to our present discussion. 

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. 

Most often, fibers are spun from the molten state in a process called melt spinning. The material to be spun is first heated until it forms a relatively viscous liquid. Next, it is... 

One distinct advantage of oxide fibers in terms of processing is that several alumina precursors suitable for forming fibers are available. The aqueous chemistry of aluminum allows for the formation of viscous basic aluminum salt solutions that can be made into fibers by dry-spinning. Polymeric aluminoxanc precursors have also been used to produce alumina-based fibers. Recently, aluminoxanes that can be melt-spun into fibers have been produced. Details of both types of chemical precursors arc given below. 

---