Chemistry Reference
In-Depth Information
where
represents several thousand units. Hydrolysis of cellulose yields 95-96%
D
-glucose.
This establishes its structure. Acetolysis of cellulose, however, yields cellobiose, a disaccharide,
4-O-
n
-
D
-glucopyranosyl-
D
-glucopyranose:
b
OH
HO
HO
HO
HO
O
O
O
OH
CH
2
OH
CH
2
OH
The structure of cellulose is, therefore, officially based on cellobiose units. Careful molecular
weight measurements by many [
12
] established that the DP of cellulose ranges from 2,000 to 6,700,
depending upon the source. The polymer is highly crystalline and is characterized by a very high
degree of intermolecular and intramolecular hydrogen bonding. This prevents it from being thermo-
plastic as it decomposes upon heating without melting.
8.2.3.1 Regenerated Cellulose
Cellulose is used in many forms. Often it is modified chemically to render it soluble in organic
solvents. In other modifications, it is treated in a manner that allows forming it into desired shapes,
like films or fibers, followed by restoration of its chemically insoluble form. The material is then
called regenerated cellulose.
Several processes evolved for preparation of regenerated cellulose. One, developed as far back as
1884, converts it first to a nitrate ester. The nitrated material is dissolved in a mixture of ethyl alcohol
and diethyl ether and extruded into fibers. The fibers are then denitrated by treatment with ammonium
hydrogen sulfide at about 40
C. The product is called
Chardonnet silk
. It appears that this process is
no longer practiced anywhere.
In another process cellulose is dissolved in ammoniacal cupric hydroxide (Cu(NH
3
)
4
(OH)
2
).
The solution is then spun as a fiber into a dilute sulfuric acid solution to regenerate the cellulose.
The product is called
Cuprammonium rayon.
The material may still be manufactured on a limited scale.
The third, probably major commercial process used today, forms a material that is known as
Viscose rayon
. The regenerated cellulose is prepared and sold as a fiber as well as a film, known as
cellophane
. The viscose, or more properly referred to as the
xanthate
process, consists of forming
cellulose xanthate by reacting alkali cellulose with carbon disulfide:
OH
OH
OH
HO
HO
HO
NaOH
CS
2
O
O
O
O
O
O
CH
2
OH
CH
2
ONa
CH
2
S
O
SNa
In a typical procedure, cellulose is steeped in an approximately 20% aqueous sodium hydroxide
solution at room temperature for anywhere from 20 min to a whole hour. It is believed that this
treatment results in formation of sodium alcoholate at every hydroxymethyl group. The resultant
material is pressed out to remove excess liquid, shredded, and aged for 2-3 days. The aging is known
to cause some molecular weight reduction. After aging, the alkali cellulose is treated with carbon
disulfide for 2-4 h to form cellulose xanthate. The amount of xanthate groups in the product average
out to one per every two glucose units. The material is dissolved in a dilute sodium hydroxide solution
