Bleaching of textile materials is, undeniably, an important step in the fabric preparation. Under-bleaching
of fabrics result in poor colour value after dyeing of the fabrics, while over-bleaching causes tendering of the fibres with drastic reductions in the tenacity and degrees of polymerisation. Conventional hazardous bleaching agents are replaced by many safe chemicals and enzymes; however, the conventional bleaching agents still find a predominance in fabric preparation.
Natural colour of cotton fibres
Various reasons of natural colour present in the raw cotton fibres are classified into four groups namely, dirt, fibre geometry and morphology, intrinsic and extrinsic colours50. Change in reflectance values due to removal of dirt is not great, but plays a significant role in the lower grades. Fibre geometry or morphology refers to degree of convolutions, cross sectional shape that has proven effects on reflectance of fabrics34.
Postulations of intrinsic colour, inherent and genetic colour, attributed to the variety of seeds and amorphous mixtures of nitrogen containing compounds, soluble and insoluble in alcohol, were subsequently replaced, by two theories. First concept is attributed to flavones, orange-yellow pigments that usually occur in the form of glucosides and, second possible reason colouring matter is chlorogenic acid, usually isolated from coffee bean that acts as a precursor of the pigment. After opening of the cotton boll in the plant, reflectance value gradually decreases because of discolouration due to fungi growth during weathering of cotton. Alternaria (blackish), Cladosporium (dark brown), and Fusarium moniliforms (near white), isolated from weathered fibres, flourish under correct weather conditions and appear to be responsible for discolouration. Suberin, a unique plant cell wall biopolymer, has two distinct chemical domains namely, a poly (phenolic) domain and poly (aliphatic) domain, chemically linked to each other, of which poly (phenolic) domain is associated with lignin present in the plant tissues43, 3.
About the Author
D Saravanan holds MTech Fibre Science & Technology from I Delhi. At present, he is Lectu at Bannari Amman Institute Technology, Sathyamangalam, Tamil Nadu. He has published many rese papers, organised seminars conferences.