Page 365 - Handbook of Properties of Textile and Technical Fibres
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338                             Handbook of Properties of Textile and Technical Fibres

         applied to the extruded filaments. The dope rheology profiles vary with the employed
         solvent, and the orientation improvements on stretching vary with the pliability of the
         extruded filaments to the applied tensions.
            Fiber regeneration in the viscose process results essentially from neutralization of
         the sodium cellulose xanthate with sulfuric acid which, as shown in the reaction below,
         regenerates the cellulose (Kr€ assig et al., 2008; Wilkes, 2001):

              2 CelluloseeOeCS 2 Na þ H 2 SO 4 / 2 CelluloseeOH þ 2CS 2 þ Na 2 SO 4
            The Na 2 SO 4 speeds up the regeneration due to a salting out effect (Sisson, 1960). It
         also acts as a buffer for the H 2 SO 4 and reduces the rate of cellulose degradation. If
         ZnSO 4 is present, zinc cellulose xanthate starts to form on the peripheral regions of
         the extruded filaments due to ion exchange between sodium and zinc. Further diffusion
         of the zinc ions into the filaments is retarded by the formation of their reaction products
         with CS 2 ,etc. Thus, a “skin” of zinc cellulose xanthate surrounds a “core” of sodium
         cellulose xanthate in the extruded filaments. The regeneration rate from zinc cellulose
         xanthate is slower than that from the corresponding sodium salt and that results in struc-
         tural differences between the cellulose regenerated at the skin and at the core. The skin
         regions exhibit greater orientation (M€ uller et al., 2000) and thereby greater strength. The
         proportion of the skin and core phases, and thereby the tensile properties of the regen-
         erated fibers, can be varied by changing the amount of ZnSO 4 in the regeneration bath.
            Other possible modifications include the following:

         •  Improving the extent and homogeneity of pulp xanthation, which reduces the regeneration
            rate and thereby increases degree of orientation in the extruded filaments.
         •  Adding formaldehyde to the regeneration bath, which lowers the acidity, reduces regenera-
            tion rate, and promotes an increase in the aspect ratios of crystallites. There is also the intro-
            duction of cross-links.
         •  Adding modifiers such as polyethylene glycol, primary and tertiary amines, reducing the
            temperature and/or reducing the acid concentration in the regeneration bath to reduce the
            regeneration rate, and thereby increase orientation.
         •  Increasing the H 2 SO 4 concentration to 55% or above and reducing the temperature to reduce
            the regeneration rate, and thus increase orientation (Lillienfield process).
            Regular, textile grade viscose fibers are produced as described earlier. To obtain
         high-tenacity yarns, very high quality pulp (i.e., of high a-cellulose content) is used
         in preparing the dopes, higher amounts of CS 2 are added to increase the degree of xan-
         thation, and the extruded filaments are stretched to high degrees ( 100%). To help
         achieve high stretching levels, the regeneration rates in the spin baths are reduced
         by decreasing the H 2 SO 4 levels and increasing the ZnSO 4 levels. Modifiers such as
         amines and polyethylene glycols are used in the production of cord yarns, and form-
         aldehyde is added in spin baths to produce EHM fibers. Polynosic fibers are spun with
         the viscose process from cellulose solutions of high degree of polymerization into low
         acid and low salt baths, and the solutions have greater structural viscosity regular
         viscose solutions (Asaeda, 1967).
            The morphology of viscose fibers changes with coagulation bath chemistry. During
         regeneration, the precipitation of cellulose on the surface is accompanied by deswelling
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