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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         K10478.indb   289                                                                    9/14/2010   3:40:41 PM
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