Page 310 - Advances in Textile Biotechnology

Chitin, chitosan and bacterial cellulose for textiles 291

12.2.2 Chitosan structure and properties
Chitosan (Fig. 12.1b) is a chitin derivative, produced by chitin deacetylation
with concentrated alkali solutions at elevated temperatures (Agboh and
Qin, 1997). By controlling temperature in chitin processing, it is possible to
obtain the desired degree of deacetylation and molecular weight of the
chitosan derivative, varying from 30 kDa to well above 1000 kDa. The rigid
d-glucosamine structures, high crystallinity and capacity for hydrogen bond
formation between the chitosan molecules lead to the poor solubility of
chitosan in common organic solvents and complete insolubility in aqueous
solutions above pH 7. Reducing the molecular weight and lowering the
crystallinity by random deacetylation improves its solubility in dilute acids
where the free amino groups are protonated and the molecule becomes
fully soluble below pH 5. Chitosan is an interesting polymer because of its
physicochemical properties, including its solid-state structure and the dis-
solved state conformation (Pa and Yu, 2001). In the solid state, relatively
rigid crystallites form because of the regularly arranged hydroxyl and amino
groups, whereas in solution, hydrogen bonding drives the formation of

microfibrils, depending on chitosan concentration (Ohkawa et al., 2004).

A number of unique properties were identified for both chitin and chi-
tosan, such as their biocompatibility, biodegradability, non-toxicity and the
ability to chelate heavy metals. Such characteristics make chitin and chito-
san among some of the most interesting biopolymers for exploitation in the

field of fi bre preparation.

12.2.3 Bacterial cellulose structure and properties

Amongst the cellulose-forming bacteria, Gluconacetobacter (formerly
Aceto bacter), Agrobacterium, Aerobacter, Achromobacter, Azotobacter,
Rhizobium, Sarcina, Salmonella and Escherichia (Deinema and Zevehvi-
zen, 1971; Römling, 2002; Ross et al., 1991), Acetobacter strains have been
reported as the most effi cient producers of cellulose. These Gram-negative
and strictly aerobic bacteria are not pathogenic and are commonly found
in naturally grown fruits and in fruit products. The production of cellulose

by Acetobacter xylinum was reported for the first time by A. J. Brown
(1886). Strains of the Acetobacter xylinum species produce extracellular
cellulose that is easily isolated as a fi bre material. The primary structure of
bacterial cellulose is similar to that of plant cellulose: an unbranched
polymer of β-1,4-linked glucopyranose residues (Fig. 12.1c). However, its
degree of polymerization (DP) ranges from 2000 to 8000, and its crystallin-
ity is between 60 and 90%. The major advantage of bacterial cellulose is
that it is obtained free of lignin, hemicellulose or pectin in a 3-D network

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