Page 84 - Basic Principles of Textile Coloration

COTTON 73

hydrophilic, water-absorbent fibre. Its porous structure allows ready penetration of
water molecules between the fibrils and into the amorphous regions of the polymer
where they can easily form hydrogen bonds with free cellulose hydroxyl groups.
The standard regain is about 8% and rises to around 25–30% water at 100%
relative humidity, at room temperature. Water absorption causes swelling of the
secondary wall but little fibre elongation. The wet fibres become softer and more
pliable. Cotton is unusual in that water absorption increases the fibre strength.
Although absorbed water acts as a plasticiser, it also cements the cellulose chains
and fibrils together by hydrogen bonding.

Cellulose shows remarkable stability towards strongly alkaline solutions in the
absence of oxygen, even at high temperatures. This is significant in the alkali
boiling process mentioned above. Cellulose is, however, very sensitive to mineral
acids. These catalyse hydrolysis of the bonds between the glucose units and cause
depolymerisation eventually leading to the monomer, glucose (Section 5.3.2).

Cellulose is a carbohydrate and, despite its alkali stability, it is a sensitive
biopolymer. On heating, cotton’s behaviour is like that of sugar. Browning and
hardening of the fibres becomes more pronounced at temperatures above 150 °C.
If processing requires higher temperatures, shorter treatment times avoid thermal
damage. Unlike synthetic fibres, damp cotton readily supports the growth of
bacteria and moulds. These first generate an unpleasant odour and leave coloured
marks on the fabric that are often difficult to remove. In bad cases, the fibres may
become substantially weakened. Warm, damp conditions foster the growth of
mildew, but application of an appropriate anti-fungal agent to the cotton material
prevents this problem.

Cotton is a natural vegetable fibre and exhibits considerable variability in
properties, even among fibres from the same seed. Cotton is available in a variety
of qualities based on the fibre length or staple, and the colour. The longest fibres
are usually the finest. Cotton fibre fineness is often evaluated by measuring the
pressure drop across a pad of cotton fibres of given weight that is required to
produce a given air flow rate. The pressure drop is related to the total fibre surface:
the finer the fibres, the greater the pressure drop [4]. Cottons from different
geographical regions are quite distinctive. The top quality cottons, such as Sea
Island, are 25–65 mm long (12 mm diameter). The bulk of the cotton produced
consists of medium quality fibres 12–33 mm long (15 mm diameter). The courser
Asiatic grades are only 10–25 mm long (18 mm diameter). The major producing
countries are the USA, Russia, China and India.

Cotton fibres from the same harvest do not all have the same degree of