Indigo Ring Dyeing of Cotton Warp Yarns for Denim Fabric

Abstract

Denim is the worldwide much popular form of fabric. A faded stylish look can be achieved due to ring dyeing of warp yarns of denim garments. This effect occurs naturally in the process of indigo dyeing. Stonewashed cotton fabrics with finest washed appearance are generally woven from ring spun yarns which are ring dyed. This paper primarily focuses on the concept of ring dyeing, its depth and significance as well as the important ionic forms of leucoindigo derivatives suitable for uniform ring dyeing. The measurement and control of the dyebath pH is very important in case of achieving desired uniform ring dyeing effect. It has been discussed when the pH of the dye bath is decreased from 13 to 11, the denim yarn progressively becomes more ring-dyed. The paper also focuses on the continuous process of indigo dyeing of denim fabric. Again, the paper emphasizes on the appropriate concentration of dyebath and the dipping time to ensure an even and optimal ring effect by dyeing in several passages. Increasing dye concentration assists in building up shade depth, but the use of too concentrated a dye bath is not effective for deep shades as it results in poor rubbing fastness and more reddish, duller shade. Later, the paper discusses about the typical oxidation-reduction potential (ORP) range, its importance and how it is changed dependent upon the desired final shade. Next, the paper shows how the mechanical parameters such as roller numbers and diameter, squeezing, dyeing speed, flow profile etc. influence on the dyestuff exchange and the dye bath stability. Finally, the paper highlights the indigo dyeing process in a nitrogen atmosphere to reduce the consumption of sodium hydrosulphite in dyeing as it is oxidized by consuming NaOH when atmospheric oxygen is present in the alkaline medium. As shade variation is frequently occurring problems in ring dyeing, this atmosphere is essential to eliminate shade variation throughout the dyeing set and to be able to reproduce the same shade and dye quality on subsequent dye lots.