Page 158 - Advances in Textile Biotechnology

Enzymatic versus chemical processing of cotton 139

and have not been made water soluble. Another variable that can affect the
amount of alkali and surfactant used is the depth of shade intended for the
fabric in subsequent dyeing; darker shades can actually require less vigor-
ous scouring. If cotton/polyester fabrics are being scoured, the treatment
may need to be milder and at a lower temperature, since polyester is subject
to alkaline degradation. Although it is not often used, solvent scouring of
cotton and cotton/polyester blends has been done with trichloroethylene
(Hull, 1980). This will only remove the cotton wax and subsequent alkali
treatment would be necessary if hydrophilicity needs improvement.

6.4.1 The effect of properties of cotton on
conventional treatments

Alkaline scouring is a process that has been used successfully for centuries.

It generally does not unduly damage the cotton fiber and fabric unless the
fabric is allowed to be in contact with air during the process, or if trace
metals such as copper or iron are either in the scouring bath or come into
contact with the fabric while it is at elevated temperature and under highly
alkaline conditions. In that instance, oxidative damage can occur from the
formation of oxycellulose. The highly alkaline conditions can cause depo-
lymerization of the cellulose by reducing end elimination through the
β-alkoxy-carbonyl mechanism. Loss of fabric strength can then result.
Another question that has been raised with regard to conventional alka-
line scouring is the non-specific nature of the removal of cotton fi ber surface

materials. It may well be that many alkaline scouring processes remove
more wax from the surface than is necessary for achieving the necessary
hydrophilicity, or for successful and problem-free spinning of cotton yarns.
A side issue regarding the removal of wax during alkaline scouring is the
creation of a harsh hand (feel of the fabric) as a result of complete wax
removal.
The weight losses for alkaline scouring are typically in the range of 5.6
(0.5% NaOH) to 6.5% (4.0%) when treated for 1 h at 95 °C with a 0.05%
non-ionic surfactant (Dhandapani et al., 2009), indicating that almost all of
the non-cellulosic materials are removed in most scouring operations.

6.4.2 Environmental problems resulting from
conventional treatments
The desizing operations and the use of NaOH in scouring and bleaching
create high chemical (COD) and biological (BOD) oxygen demand of effl u-

ent from the starch removal process, and significant salt concentration, high
alkalinity and high pH in effluents from scouring and bleaching operations.

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