Problems originating from Cotton Fiber -II

Problems Caused by Dyeability Variation in Cotton

The results of research confirm the dyeability variations in cotton obtained from different sources. It has been suggested that the substrate should be obtained from a single source, wherever possible, in order to keep the dyeability variations to a minimum. Since some dyestuffs are more sensitive to dyeability variations than others; those dyes should be selected for dyeing which are less sensitive to dyeability variation.

 Problems Caused by Contaminants in Cotton

While cotton fibre may be as much as 96 % cellulose, there are other components present which must be removed in preparation for a successful dyeing. Table 1 gives a summary of naturally occurring impurities in cotton. The level of contamination in cotton is affected by: geology of cultivation area; soil constitution; weather conditions during the maturing period; cultivation techniques;

chemicals, pesticides and fertilizers; as well as harvesting techniques [20]. For the dyer, the elements that pose the greatest threat are alkaline earth and heavy metal contaminants such as calcium, magnesium, manganese, and iron. Depending on its origin, raw cotton can exhibit widely different contents of alkaline earth and heavy metal ions.

Levels of fats, oils and waxes present in cotton can be reduced to acceptable limits by the action of alkali and surface-active products. In extreme cases, the use of solvent and surface active mixtures may be necessary. Pectins and the related substances can be rendered soluble by the action of alkali, usually caustic soda, which also acts as a swelling agent. Amino acids are also rendered soluble in the presence of alkali by producing the corresponding sodium salts. Metals, however, cannot be adequately removed by conventional alkaline processes since, in an alkaline medium, sequestering agents cannot quantitatively separate the minerals of a complex structure containing heavy metals. Moreover, in the alkaline pH region, cellulose swells rapidly and strongly, thus impairing the transport of crystalline minerals from the core to the periphery of the fiber. Demineralization with organic or inorganic acid is more effective as compared to the alkaline treatment process. However, regardless of the efficacy of an acid treatment, the use of organic or inorganic acids for the demineralization of cellulosic fibers involves a number of disadvantages such as corrosion of machine parts, difficulties in handling, and risk of fiber damage with strong inorganic acids, while organic acids give lower demineralization and are more volatile. Specialty products based upon strongly acidic sequestering agents or a mixture of sequestering agents with organic buffer systems are recently being used for demineralization of cotton. These products offer numerous advantages over conventional acids such as hydrochloric acid or sulphuric acid. Some of the advantages are given as follows:

• No corrosion

• No steam volatility

• No unpleasant odor

• Prevention of dissolved metal ions from re-precipitating

• Synergy with surfactants, improving the washing effect, dispersion power and soil suspension capacity

• Lower ash content

• Improved degree of whiteness

• No fiber damage

However, with such an intensive demineralization treatment, care must be taken that magnesium ions are added in subsequent peroxide bleaches, in order to avoid fiber damage in the bleach owing to insufficient stabilization of hydrogen peroxide.

Effect of Cotton Color Grade on the Color Yield of Dyed Goods

The difference in the color yield of cotton of different original color grades, when dyed after scouring and bleaching, is so small as to be explicable by experimental variation.