Page 93 - Basic Principles of Textile Coloration

82 NATURAL CELLULOSIC FIBRES

water. It can even be recovered from the desizing liquor and re-used. This has the
additional advantage of a low load of organic chemicals in the washing effluent.
Bacteria readily metabolise starch products, rapidly consuming the dissolved
oxygen in the water. Starch-based products therefore dramatically increase the
biological oxygen demand (BOD) of the effluent (Section 8.5.1).

5.4.3 Alkali boiling of cotton [8]
Treatment of cotton with boiling dilute NaOH solution hydrolyses the natural fats
in the fibre, producing soaps that then aid in emulsifying the cotton wax. The
latter is resistant to hydrolysis. The classical method of kier boiling involves
heating cotton with about 2% aqueous NaOH solution under pressure. Besides
NaOH, the solution may also contain soaps and solvents such as pine oil, which
aid wax removal from the fibre cuticle. Uniform plaiting of the rope of fabric into
the kier boiler avoids channelling of the circulating liquor and non-uniform
treatment. Before the boiler is closed to develop pressure, the steam from the
boiling solution is vented, thus removing air from the system. This avoids any
oxidation of the alkaline cotton. For the same reason, after several hours of boiling
under pressure, oxygen must not enter the hot boiler. After the initial cooling, the
dirty alkaline solution is slowly drained off but replaced with water to completely
cover the hot, alkaline goods and avoid direct contact with the air. Provided
oxygen is absent, the cellulose is unaffected by the hot alkaline solution.

Continuous alkali treatment involves impregnating the cotton fabric with
NaOH solution and then heating it in a sealed pressurised steamer. Because
continuous processes have a shorter treatment time, a more concentrated solution
of NaOH is used.

The final step, after either batch or continuous alkaline boiling, is good rinsing
with much hot water to remove the soluble and emulsified impurities. The process
results in a 5–8% loss of weight from emulsification of the cotton wax, hydrolysis
of fats, and the solubilisation of pectates, organic acids, cellulose oligomers and
mineral salts, as well as any residues of sizing material. Again, the effect of these
chemicals on the BOD of the effluent is of concern.

5.4.4 Cotton bleaching [9]
The objective of cotton bleaching is to eliminate the yellowish natural colouring
matter in the fibres. This is only present in very small amounts and its chemical