Page 123 - Advances in Textile Biotechnology

104 Advances in textile biotechnology

classes of nitrile-metabolizing enzymes. Nitrilases (EC 3.5.5) and amidases
(EC 3.5.1) are both hydrolases, whereas nitrile hydratase (EC 4.2.1) is a
carbon–oxygen hydrolyase, meaning it catalyses the breakage of a carbon–
oxygen bond by elimination of water. In the particular case of PAN bio-

modification the interest is on the reverse reaction. These enzymes are
described in more detail in chapter 1.

5.2.2 Side chains from cellulose acetate as enzymatic
substrates

Acetate fibres are defined as manufactured fibres by the Federal Trade

Commission of the United States, in which the fibre-forming substance is

cellulose acetate (Needles, 1986). Cellulose acetates are classified as deriv-
ative cellulose fibres, as distinct from rayon and lyocell, which are regener-

ated cellulose fibres, the chemical composition of cellulose acetate being
not cellulose but an ester of cellulose (Collier and Tortora, 2001). Each
anhydroglucose repeating unit of cellulose (Fig. 5.4) has three hydroxyl
groups located at the positions 2, 3 and 6 (La Nieve, 2007). These sites are

available for acetylation to produce the acetate fibres. The degree of acetyl-
ation or substitution (DS) is the average number of acetylated positions per
anhydroglucose unit. Commercial cellulose triacetate (CTA) has a DS of
2.91–2.96 whereas cellulose acetate has a DS of ≈2.4 (La Nieve, 2007).

Therefore, the denomination of cellulose acetate fibres is used to refer to

all commercial acetylated cellulose fibres, but it is also the common name
for the cellulose acetate fibre with a DS ≈ 2.4. To avoid misinterpretations,

throughout this text, the name cellulose acetate will be used to refer to both
types of fibres whereas cellulose diacetate (CDA) will be used to refer the

acetate with a DS of ≈2.4.
Cellulose acetates are produced from high-quality cellulose, such as
cotton linters and wood pulps, with an α-cellulose content above 95% (Saka
and Matsumura, 2004). The most commonly used commercial acetylation
process is the acetic acid system where acetic acid serves as solvent for the

6
CH OH
2
5 O O
4 OH 1
2
O
3
OH
n
5.4 Chemical structure of the anhydroglucose repeating unit of
cellulose.

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