Page 326 - Carrahers_Polymer_Chemistry,_Eighth_Edition
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Naturally Occurring Polymers—Plants 289
Varying properties are achieved by varying the amount of substitution. The melting point gener-
ally decreases with decreasing acetylation. Lower acetylation gives products with greater solubility
in polar solvents and corresponding decreased moisture resistance. Cellulose acetate is made using
heterogeneous solutions containing the cellulose, sulfuric acid as the catalyst, and acetic anhydride
in acetic acid. Reaction occurs beginning with the surface or outermost layer and continues on layer
by layer as new areas are exposed. When more homogeneous modification is desired, preswelling of
the cellulose in water, aqueous acetic acid solutions, or in glacial acetic acid is carried out.
Reaction occurs differently since there are two “types” of hydroxyl groups (as noted before), the
two ring hydroxyls and the methylene hydroxyl. In the typical formation of esters, such as cellulose
acetate, the ring hydroxyl groups are acetylated initially (9.8) before the C-6 exocyclic hydroxyl.
Under the appropriate reaction conditions, reaction continues to almost completion with all three
of the hydroxyl groups esterifi ed (9.9). In triacetate products, only small amounts (on the order of
1%) of the hydroxyls remain free and of these generally about 80% are the C-6 hydroxyl.
OH
OH OH OH
CH
H O O H O O 3 O O O O
H H H H H
OH H OH H O O H OH H H
R H H H H R (9.8)
H O O
H OH H O O H O O
CH 3 CH
3 CH 3
O CH
3 O CH O CH
3 3
O OH O O
CH
3 CH CH CH
H O O H 3 O O H 3 O O 3 O O
H H H H
O O H O O H O O H O O H H
R H H H H R (9.9)
H O O H O O H O O H O O
CH 3 CH CH CH
3 3 3
The most common commercial products are the triacetate (DS approaching 3) and the secondary
acetate (DS about 2.45).
While other organic esters are commercially available, namely cellulose butyrate and cellulose
propionate, by far the most widely used is cellulose acetate. Cellulose acetate is available as plastics,
in films, sheets, fibers, and in lacquers. Cellulose acetate is used in the manufacture of display pack-
aging and as extruded film for decorative signs, and to coat a variety of fibers. Injected molded prod-
ucts include toothbrush handles, combs, brushes. It is also used in lacquers and protective coatings
for metal, glass, and paper. Cellulose acetate films are used in reverse osmosis to purify blood, fruit
juices, and brackish water. Some eyeglass frames are made of cellulose acetate. Biodegradable fi lm,
sponges, and microencapsulation of drugs for control release also utilize cellulose acetate. Cellulose
triacetate is used for photographic film bases. Numerous continuous filament yarns, tows, staples,
and fi bers are made from cellulose acetate. The precise form of filament produced is controlled by
a number of factors, including the shape of the die.
As in all large-scale industrial processes, the formation of the cellulose esters involves recov-
ery of materials. Acetic anhydride is generally employed. After reaction, acetic acid and solvent is
recovered. The recovered acetic acid is employed in the production of additional acetic anhydride.
The recovered solvent is also reintroduced after treatment.
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