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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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