Page 333 - Advances in Textile Biotechnology

314 Advances in textile biotechnology

Therefore, the effects of cellulases observed on a certain substrate are
dependent on several factors which will be discussed in more detail below.

This interlinkage between treatment conditions, fibre properties and history
of preceding treatments explains the sometimes unexpectedly high weight
loss and strength loss obtained in practice under similar treatment condi-
tions and using similar cellulosic substrates, when the effect of treatments
preceding enzymatic hydrolysis had not been considered.

13.2 Regenerated cellulose fi bres

Regenerated cellulose fibres offer a high variability in fibre structure and
numerous options to modify the material, thus differences in observed cel-
lulase reactivity can be more distinct than in cotton fi bres. Regenerated

cellulose fibres commercially available in the textile fibre market at present,
can be roughly divided into two major groups with regard to their produc-
tion process. The first group of fibres is produced by the viscose process and

the second group are lyocell-type fi bres.

Viscose fibres and modal fibres are produced by dissolution of cellulose

xanthogenate in alkaline solution and precipitation/coagulation, and cel-
lulose regeneration in acid baths, respectively. The combination of precipi-
tation, coagulation and cellulose regeneration leads to the core skin structure
known for viscose fi bres (Abu-Rous et al., 2007). During the coagulation/
regeneration of the cellulose considerable shrinkage of the coagulated
structure occurs and the fibre obtains its characteristic multi-lobal form.

Modal fibres are produced following the xanthogenate process, but special
conditions are applied, such as addition of modifiers to the spin-dope and

Zn salts in the spin bath. Cellulose polymers with a higher degree of poly-
merisation then can be regenerated to high wet modulus fi bres which
exhibit an all-skin structure.

Lyocell fibres are formed from NMMO solution (N-methylmorpholine-

N-oxide monohydrate) by a wet-spinning process. The fibre shows high
crystallinity, high longitudinal orientation of crystallites, high amorphous
orientation, low lateral cohesion and relatively large void volumes (Öztürk
et al., 2006, 2009; Schurz, 1994; Schurz and Lenz, 1994). The fi brillar structure
of the lyocell fibres explains their high longitudinal stability in the swollen

state, but it also causes signifi cant weakness of the fi bre against fi brillation.
Penetration of swelling agents into amorphous regions of the fi bre weakens
the interfibrillar bonding and, as a result, the long macrofibrils can be split

off when shear stress is applied on swollen fi bres.

Typical fibre properties of viscose, modal, lyocell fibres and cotton fi bres,
which are related to swelling in water and sorption of water molecules are
summarised in Table 13.1. In addition to the water retention value deter-
mined by the centrifugation method, the porosity V p characterised by

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