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290                                                    Carraher’s Polymer Chemistry



                    Cellulose esters are used as plastics for the formation by extrusion of films and sheets and by
                 injection molding of parts. They are thermoplastics and can be fabricated employing most of the
                 usual techniques of (largely compression and injection) molding, extrusion, and casting. Cellulose
                 esters plastics are noted for their toughness, smoothness, clarity, and surface gloss.


                    Acetate fiber is the generic name of a fiber that is partially acetylated cellulose. They are also
                 known as cellulose acetate and triacetate fibers. They are nontoxic and generally nonallergic so are

                 ideal from this aspect as clothing material.
                    While acetate and triacetate differ only moderately in the degree of acetylation, this small dif-
                 ference accounts for differences in the physical and chemical behavior for these two fi ber materials.
                 Triacetate fiber is hydrophobic and application of heat can bring about a high degree of crystallinity

                 that is employed to “lock-in” desired shapes (such as permanent press). Cellulose acetate fi bers have
                 a low degree of crystallinity and orientation even after heat treatment. Both readily develop static
                 charge and thus antistatic surfaces are typically employed to clothing made from them.
                    For clothing applications, there are a number of important performance properties that depend on the
                 form of the textile. These properties include wrinkle resistance, drape, strength, and fl exibility. These
                 properties are determined using ASTM tests that often involve stress–strain behavior. Thus, the ability
                 of a textile to resist deformation under an applied tensile stress is measured in terms of its modulus of
                 elasticity or Young’s modulus. As with any area of materials, specialty tests are developed to measure
                 certain properties. Some of these are more standard tests, like the aforementioned Young’s modulus,


                 while others are specific to the desired property measured for a specific application. For instance, resis-
                 tance to slightly acidic and basic conditions is important for textiles that are to be laundered. Again,
                 these are tested employing standard test procedures. In general, triacetate materials are more resistant
                 than acetate textiles to basic conditions. Both are resistant to mild acid solutions but degrade when
                 exposed to strong mineral acids. Further, behavior to various dry cleaning agents is important. As the
                 nature of dry cleaning agents change, additional testing and modification in the fabric treatments are

                 undertaken to offer a textile that stands up well to the currently employed cleaning procedures. Again,
                 both are stable to perchloroethylene dry cleaning solvents but can soften when exposed to trichloroethy-
                 lene for extended treatment. Their stability to light is dependent upon the wavelength, humidity present,
                 and so on. In general, they offer a comparable stability to light as that offered by cotton and rayon.
                    While cellulose acetates are the most important cellulose ester, they suffer by their relatively
                 poor moisture sensitivity, limited compatibility with other synthetic resins, and a relatively high
                 processing temperature.

                 9.5.3   ORGANIC ETHERS
                 Reaction with an epoxide such as ethylene oxide under alkaline conditions gives hydroxyethylcel-
                 lulose (HEC).

                                                 O
                                                 /  \
                             Cellulose − OH, NaOH    +   H C − CH 2  Cellulose−(− O −CH  −CH  −) − OH + NaOH     (9.10)
                                                                          n
                                                2
                                                                        2
                                                                     2
                                               Ethylene  Hydroxyethylcellulose
                                                oxide
                    This is an S 2 reaction with the reaction proportional to the concentration of the epoxide and
                              N

                 alkali cellulose, but since the base is regenerated, it is first order in epoxide.
                                                  Rate = k [Epoxide]                        (9.11)
                    Industrially, HECs with DS values below 2 are used. Low DS materials (to about 0.5) are solu-
                 ble only in basic solutions while those with DS values of about 1.5 are water soluble. Concentrated
                 solutions of HEC are pseudoplastic with their apparent viscosities decreasing with increased rates of







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