Page 357 - Handbook of Properties of Textile and Technical Fibres
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330                             Handbook of Properties of Textile and Technical Fibres

            The solubility of cellulose in N-methylmorpholine N-oxide (NMMO) was discov-
         ered in 1939 by Charles Graenacher and Richard Sallman, but its potential in cellulose
         fiber production was demonstrated in the latter part of the 1970s, and the first commer-
         cial samples were produced in 1984. At the current time, a majority of artificial cellu-
         losics are produced through the viscose and lyocell routes.
            The general process of producing regenerated cellulosic fibers may be briefly
         described as follows. Wood pulp is dissolved in a solvent; the solution (or spinning
         dope) is filtered to remove any particulates and is then pumped through a spinneret
         into a regeneration bath containing a nonsolvent. The spinneret is a shower headelike
         apparatus with an end plate containing a series of orifices, and the cellulose solution is
         streamed out in filamentous jets. The extruded jets are collected and transported
         through the regeneration bath by being wound around a series of rotating rollers. At
         this stage, a stretch is applied while the polymer is being precipitated by maintaining
         differential rotation speeds between the rollers (the latter rollers turn faster than the
         preceding ones). Finally, the precipitated filaments are washed, optionally bleached;
         antistatic agents are applied, dried, and, if necessary, cut to size.
            An exception to the above description is the Fortisan method, where first cellulose
         acetate fibers are produced from wood pulp and then saponified with sodium hydrox-
         ide or sodium acetate to regenerate the cellulose (Northolt et al., 2001). But by far, the
         majority of regenerated cellulosics are produced through the dissolution and regener-
         ation route.
            There exist a number of solvents for cellulose, and they may be classified, as per
         Liebert (2010), Heinze and Koschella (2005), into three categories:

         •  Where dissolution proceeds after chemical modification of cellulose, e.g., the xanthate and
            carbamate processes
         •  Aqueous solvents that require no chemical modification of cellulose prior to dissolution, e.g.,
            alkaline complexes of transition metal ions and NaOH with polyethylene glycol
         •  Nonaqueous solvents that require no chemical modification of the cellulose prior to dissolu-
            tion, e.g., ionic liquids and amine oxides

            Only a few of the available solvent systems are currently used in industrial-scale
         fiber production, and they are the xanthate (or viscose) process, the transition metal
         complex cuprammonium hydroxide (or cuoxam) process, and the amine oxide
         NMMO (lyocell process). Of these, fibers from the cuoxam process find use mainly
         in products for hygiene and medical applications (Kamide and Nishiyama, 2001)
         and constitute only a small proportion of the commercially available regenerated cel-
         lulose fibers.
            In the viscose process, pulp of molecular weight typically in the range of 750e850
         is first swollen in an NaOH solution, shredded, aged to reduce the polymer molecular
         weight to around 270e350, and reacted with carbon disulfide to yield sodium cellulose
         xanthate (Wilkes, 2001). The derivative is then dissolved in NaOH to produce a dope
         containing 7%e12% cellulose, which is allowed to rest for a period (aging), and then
         extruded into a regeneration bath typically containing 7%e12% H 2 SO 4 , 12%e24%
         Na 2 SO 4 , and 0.5%e3% ZnSO 4 to produce the fibers (Kr€ assig et al., 2008). A
         schematic illustration is shown in Fig. 10.1.
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