Polymerization spinning

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. 

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. 

Dimensional Stability. The wet heat resistance of PVA fiber is indicated by the wet softening temperature (WTS) at which the fiber shrinks to a specified ratio. At one time, the WTS was not more than 95°C for nonacetalized PVA fiber, but improvement of WTS has been achieved by improvement in heat-drawing and -treating techniques other methods proposed include suppression of polymerization temperature of vinyl acetate (36) and employment of alkafi spinning (37). 

On the other hand, water-soluble PVA fibers are available on the market. They are stable in cool water but shrink in warm water and dissolve at 40 to 90°C. The dissolution temperature is controlled by the degree of polymerization and hydrolysis of PVA, he at-treatment conditions after spinning, etc. 

Synthetic Fiber and Plastics Industries. In the synthetic fibers and plastics industries, the substrate itself serves as the solvent, and the whitener is not appHed from solutions as in textiles. Table 6 Hsts the types of FWAs used in the synthetic fibers and plastic industries. In the case of synthetic fibers, such as polyamide and polyester produced by the melt-spinning process, FWAs can be added at the start or during the course of polymerization or polycondensation. However, FWAs can also be powdered onto the polymer chips prior to spinning. The above types of appHcation place severe thermal and chemical demands on FWAs. They must not interfere with the polymerization reaction and must remain stable under spinning conditions. 

Technora. In 1985, Teijin Ltd. introduced Technora fiber, previously known as HM-50, into the high performance fiber market. Technora is based on the 1 1 copolyterephthalamide of 3,4 -diaminodiphenyl ether and/ -phenylenediamine (8). Technora is a whoUy aromatic copolyamide of PPT, modified with a crankshaft-shaped comonomer, which results in the formation of isotropic solutions that then become anisotropic during the shear alignment during spinning. The polymer is synthesized by the low temperature polymerization of/ -phenylenediamine, 3,4 -diaminophenyl ether, and terephthaloyl chloride in an amide solvent containing a small amount of an alkaU salt. Calcium chloride or lithium chloride is used as the alkaU salt. The solvents used are hexamethylphosphoramide (HMPA), A/-methyl-2-pyrrohdinone (NMP), and dimethyl acetamide (DMAc). The stmcture of Technora is as follows ... 

Tris(2,4-pentanedionato)iron(III) [14024-18-1], Fe(C H202)3 or Fe(acac)3, forms mby red rhombic crystals that melt at 184°C. This high spin complex is obtained by reaction of iron(III) hydroxide and excess ligand. It is only slightly soluble in water, but is soluble in alcohol, acetone, chloroform, or benzene. The stmcture has a near-octahedral arrangement of the six oxygen atoms. Related complexes can be formed with other P-diketones by either direct synthesis or exchange of the diketone into Fe(acac)3. The complex is used as a catalyst in oxidation and polymerization reactions. 

Depending on the final polymerization conditions, an equilibrium concentration of monomers (ca 8%) and short-chain oligomers (ca 2%) remains (72). Prior to fiber spinning, most of the residual monomer is removed. In the conventional process, the molten polymer is extmded as a strand, solidified, cut into chip, washed to remove residual monomer, and dried. In some newer continuous processes, the excess monomer is removed from the molten polymer by vacuum stripping. 

Polymerization and Spinning Solvent. Dimethyl sulfoxide is used as a solvent for the polymerization of acrylonitrile and other vinyl monomers, eg, methyl methacrylate and styrene (82,83). The low incidence of transfer from the growing chain to DMSO leads to high molecular weights. Copolymerization reactions of acrylonitrile with other vinyl monomers are also mn in DMSO. Monomer mixtures of acrylonitrile, styrene, vinyUdene chloride, methallylsulfonic acid, styrenesulfonic acid, etc, are polymerized in DMSO—water (84). In some cases, the fibers are spun from the reaction solutions into DMSO—water baths. 

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