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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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