Chemistry Reference
In-Depth Information
and again aged for 2-5 days. During the aging period, some xanthate groups decompose. The solution
is then spun into dilute sulfuric acid to regenerate the cellulose and form fibers:
OH
OH
HO
10 - 15% H
2
SO
4
10 - 20% Na
2
SO
4
O
O
HO
O
O
n
n
OH
O
S
S
a
The rayon fibers are washed, bleached, and submitted to other various treatments, like dyeing, etc.,
depending upon intended use.
When cellulose xanthate is extruded through narrow slits into acid baths, cellophane films form.
These films are usually plasticized by washing in baths containing some glycerin.
8.2.3.2 Derivatives of Cellulose
Many
have been synthesized over the years [
12
-
14
]. These include esters of
both organic and inorganic acids, ethers, and various graft copolymers. Only some of them, however,
achieved commercial importance.
One of the earliest commercial esters of cellulose was
cellulose nitrate
. It was originally prepared
as an explosive (
guncotton
) in the middle of the nineteenth century, and later as a medical aid
(
derivatives o f cellulose
, for covering wounds). Later films from cellulose nitrate were used in photography, called
celluloid
. Nitrocellulose was also probably the first successful commercial plastic, used to form many
articles. Today it is generally displaced by other materials. Cellulose nitrate, however, is still being
used in some surface finishes, though here too it is gradually being displaced.
Cellulose is nitrated by mixtures of nitric and sulfuric acids. The type of acid mixture used depends
on the intended products. For the preparation of plastic grade materials, 25% of nitric acid is
combined with 55% of sulfuric acid and 20% water. The dried cellulose is soaked for 20-60 min at
30-40
C. There is little change in appearance as the structure of the cellulose is maintained. The bulk
of the acid is then removed, usually by spinning in a centrifuge and the remaining acid washed out
with copious amounts of water. The product is often bleached with sodium hypochlorite and washed.
The degree of nitration is controlled by reaction conditions and particularly by the amount of water
in the nitrating bath. Products with 1.9-2.0 nitrate groups per each glucose unit are used in plastics
and lacquers. Some materials, however, with a nitrate content as high as 2.0-2.4 groups per each
glucose have been used in some lacquers. The higher nitrate content of 2.4-2.8 groups per
each glucose is in materials intended for use as explosives. The esterification reaction can be
illustrated as follows:
collodion
OH
ONO
2
HO
HO
HNO
3
/H
2
SO
4
O
O
O
O
n
n
CH
2
OH
CH
2
ONO
2
