Page 124 - Advances in Textile Biotechnology

Enzymatic modifi cation of polyacrylonitrile and cellulose acetate fi bres 105

cellulose acetylation and acetic anhydride and sulfuric acid as catalysts (La
Nieve, 2007).

Acetate fibres are soft and cool, have silk-like aesthetics and good drape,

and they can be easily blended with other fibres like silk, rayon, nylon,
cotton and polyester (Law, 2004). The moisture regains for CDA and CTA
are 6.5% and 3.5%, respectively (Steinmann, 1998; La Nieve, 2007). The
CDA has a moisture regain close to the value 7% of natural cotton yarn,
whereas the CTA has a lower value but still higher than the commercial
synthetic fibres. Their unique attributes remain desirable and they are

responsible for the survival of acetate production in the competitive market

of man-made fibres. Another attribute that is gaining importance is the fact
that cellulose acetate fibres are environmental friendly compared with the

major synthetic fi bres.
Whereas cellulose, either from cotton linters or wood pulp, is highly
crystalline, dry-spun cellulose acetates show very low crystalline order
owing to the substitution of the hydroxyl groups by acetyl groups and con-
sequent disruption of the original cellulose structure (La Nieve, 2007). In
both CTA and CDA, hydrogen bonding between cellulose chains is sub-
stantially decreased and the bulky acetyl group prevents the close packing
of cellulose chains (Needles, 1986). The van der Waals forces are the major
associative forces between the polymer chains, and their lower magnitude
is the reason for cellulose acetate being considerably weaker than cellulose
fibres. Both CDA and CTA have a very low strength and their chemical

stability is poor (Steinmann, 1998; Collier and Tortora, 2001). They are
attacked by a number of organic solvents capable of dissolving esters, strong
acids and bases, which result in saponification of acetyl groups. For these

reasons, the physical and/or chemical modification of these fibres is of very
limited use.
Some methods were developed to improve the strength, abrasion resis-
tance and dimension stability of acetate fibres, in particular of CDA

(Steinmann, 1998). One approach was to apply polymer additives to the
CDA spin dope. Several were tested but, unless their concentration was
below 5%, the phase compatibility was poor (Steinmann, 1998). To improve
the compatibility, some polymers were grafted onto CDA. In the case of
acrylonitrile, the graft copolymer increased the compatibility of PAN and

cellulose acetate and the resulting fibres had improved thermal and chem-
ical stabilities (Steinmann, 1998). The effect of crosslinking agents was also
investigated on CDA, though the improved properties were still not equal
to those of heat-treated CTA (Steinmann, 1998).
Work on the modification of cellulose acetate with enzymes has been

done in the context of its biodegradation (Puls et al., 2004). Figure 5.5 sum-
marizes the main reactions expected to occur during the biodegradation of
cellulose acetate with special emphasis on the deacetylation reaction. The

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