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286                                                    Carraher’s Polymer Chemistry

                 9.4   CELLULOSE-REGENERATING PROCESSES

                 Cellulose is sometimes used in its original or native form as fibers for textile and paper, but often it is

                 modified through dissolving and reprecipitation or through chemical reaction. The xanthate viscose

                 process, which is used for the production of rayon and cellophane, is the most widely used regenera-
                 tion process. The cellulose obtained by the removal of lignin from wood pulp is converted to alkali

                 cellulose. The addition of carbon disulfide to the latter produces cellulose xanthate.
                    While terminal hydroxyl and aldehyde groups, such as present in cellobiose, are also present in
                 cellulose, they are not significant because they are only present at the ends of very long chains.

                    The hydroxyl groups are not equivalent. For instance, the pKa values of the two ring hydroxyl
                 groups are about 10 and 12, which is about the same as the hydroxyl groups on hydroquinone and

                 the first value about the same as the hydroxyl on phenol. The pKa value of the nonring or methylene
                 hydroxyl group is about 14, same as found for typical aliphatic hydroxyl groups.

                                             OH
                                                                       OH







                                             OH
                                      Hydroquinone (9.4)           Phenol (9.5)

                    In the cellulose-regenerating process, sodium hydroxide is initially added such that approxi-
                 mately one hydrogen, believed to be predominately a mixture of the hydroxyl groups on carbons
                 2 and 3, is replaced by the sodium ion. This is followed by treatment with carbon disulfi de form-
                 ing cellulose xanthate, which is eventually rechanged back again, regenerated, to cellulose. This
                 sequence is depicted below:
                              OH                 OH                OH                 OH

                      R  H      O O       R  H     O O R   R         O O      R  H      O O
                            H       R                         H                    H
                          OH   H             OH H  H           OH H  H   R       OH   H    R
                                  H
                                                    H                  H                 H
                          H    OH                 −
                                             H   O             H   O             H    OH
                                                    Na +
                                                                         −
                                                                        S                    (9.6)
                                                                   S     Na +
                          Cellulose     Sodium salt     Cellulose xanthate  Regenerated
                                                                            cellulose−rayon or
                                                                            cellophane
                    The orange-colored xanthate solution, or viscose, is allowed to age and is then extruded as a fi l-
                 ament through holes in a spinneret. The filament is converted to cellulose when it is immersed in a

                 solution of sodium bisulfite, zinc II sulfate, and dilute sulfuric acid. The tenacity, or tensile strength,

                 of this regenerated cellulose is increased by a stretching process that reorients the molecules so
                 that the amorphous polymer becomes more crystalline. Cellophane is produced by passing the vis-

                 cose solution through a slit die into an acid bath. Important noncellulosic textile fibers are given in
                 Table 9.2, and a listing of important cellulosic textile fibers is given in Table 9.3.

                    Since an average of only one hydroxyl group in each repeating glucose unit in cellulose reacts
                 with carbon disulfi de, the xanthate product is said to have a degree of substitution (DS) of 1 out of
                 a potential DS of 3.







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