Methods and impact of reducing salt in dyeing cellulose: a case study with CI Direct Blue 90

The addition of large amounts of salt is one of the most important environmental problems in the dyeing of cellulosic fibres with direct or reactive dyes. This study was undertaken to determine an accurate and effective method for measuring the impact of salt on the dyeing process. This method has been applied to the case of CI Direct Blue 90 by utilising real-time measurements of dye exhaustion for different salt and dye concentrations. Reductions in salt by a factor of 4 were achieved with no effect on the final shade. It is proposed that this method could be used to reduce the amount of salt for many other direct dyes, or to enable the dyer to quantify the trade-off between time to final exhaustion and salt addition.