Page 157 - Advances in Textile Biotechnology

138 Advances in textile biotechnology

Fiber
Outer pigment
layer (diazonium, Outer epidermis
ruthenium red)
Colorless layer
Palisade A (acid phloroglucinol)
(ruthenium red)
Palisade B

Inner pigment
Inner
epidermis layer (vanilline)
(ruthenium red)
6.6 Diagram of cotton seed coat structure (Kim, 2001).

6.4 Conventional chemical treatments and conditions
Conventional preparation of cotton fabric for further chemical treatment
usually consists of desizing, scouring and bleaching. There are two common
textile chemical processes that use NaOH in their treatment of cotton
fabric, mercerization and scouring. The former process uses a concentrated
solution of NaOH (18–20%) to change both the crystal unit cell of cellulose

and to change the physical and chemical properties of the fiber itself. Scour-
ing (also referred to as boiling off), on the other hand, is most often done
with dilute NaOH, usually in the range of 0.5 to 4.0%, and usually at tem-
peratures approaching 90 °C or greater. Less frequently, calcium hydroxide
or sodium carbonate (soda ash) may be used (Vigo, 1994, p. 16). Chelating
agents such as ethylenediamine tetra-acetic acid (EDTA) are sometimes

added if significant concentrations of calcium and magnesium are present
in the cotton or the water source.
Scouring is done with the purpose of removing some non-cellulosic impu-

rities from the surface of the cotton fiber to give the cotton fabric adequate
water absorbance (hydrophilicity) for subsequent dyeing and fi nishing. This
requires at least partial removal of the cuticle waxes and the pectins and
proteins attached the primary wall cellulose. Other introduced impurities
that require removal before chemical processing are dirt, oils and even
defoliants applied to the cotton plant before harvesting. Alkaline scouring,
followed by hydrogen peroxide bleaching, is usually successful in both
breaking up seed coat fragments and in decolorizing them. Even if they are
not completely removed, they are no longer visible to the eye.
The conditions used for scouring depend greatly on the fabrics being
processed. Heavier fabrics may require higher concentrations of NaOH and
surfactant to improve penetration of the alkali. The surfactant may also be
needed to help emulsify those wax constituents that are not saponifi able

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