Chemistry Reference
In-Depth Information
The molecular weight of cellulose nitrates used in plastics and lacquers is usually reduced. This is
done by heating the slurry of the polymer in water at about 130-160
C for up to 30 min under
pressure.
Cellulose acetate
was also prepared originally in the nineteenth century. Commercial develop-
ment, however, started early in twentieth century. In the 1920s acetate rayon and acetate fibers were
developed and cellulose acetate became an important molding material. At about the same time
cellulose lacquers were also developed. Today, however, many of these materials have been replaced
by other polymers.
The acetylation reaction of cellulose is often prepared by forming a solution in a mixture of acetic
anhydride and sulfuric acid. This results in formation of a triacetate. When a lower degree of
esterification is desired, the triacetate is partially hydrolyzed. A two-step procedure is needed because
it is not possible to control the degree of esterification in the reaction with acetic anhydride and
sulfuric acid. In a typical process, dry cellulose is pretreated with 300 parts acetic anhydride, 1 part
sulfuric acid, and 400 parts methylene chloride. The reaction mixture is agitated while the tempera-
ture is maintained at 25-35
C for 5-8 h. By the end of that period, all the cellulose is dissolved and
the cellulose triacetate has formed in the solution.
Partial hydrolysis is accomplished by adding to the methylene chloride solution aqueous acetic
acid (50%). The solution is then allowed to stand to reach the desired degree of hydrolysis.
This usually takes about 72 h at room temperature. Sulfuric acid, still present from acetylation, is
then neutralized by addition of sodium acetate and most of the methylene chloride is distilled off.
The partially hydrolyzed cellulose acetate is then precipitated by addition of water and washed
thoroughly. The washing also includes a 2-h wash with very dilute sulfuric acid to remove hydrogen
sulfate esters that cause polymer instability.
The process can be illustrated as follows:
O
O
O
OH
O
HO
(CH
3
CO
2
)O
O
O
O
O
H
2
SO
4
n
n
CH
2
OH
O
O
O
H
O
O
H
2
O
O
O
H
n
O
O
Cellulose triacetate is also prepared by a heterogeneous process in the presence of benzene,
a non-solvent. The triacetate that forms in both processes is hard to mold, but it can be used in
films and fibers. The diacetate is more suited for plasticization and molding.
