Mass Transfer Rate Drift in Heated Dye Pad Troughs under Variable Relative Humidity
Relative humidity drops accelerate solvent evaporation in heated pad troughs, concentrating liquor and causing head-to-tail shade drift in continuous dyeing.
Shade Tailing
Dye Graduation
Gradual changes in color intensity or hue along the length of a fabric roll indicate a loss of equilibrium in continuous dyeing or padding. Shade tailing occurs when the concentration of dye in the trough changes over time. Often this is observed in pad-batch processes where the dye has a different affinity for the fiber than the solvent.
Affinity Influence
High-affinity dyes are stripped from the padding liquor more quickly than the water. This preferential absorption means the trailing end of the fabric sees a weaker solution than the leading edge. Conversely, if the fabric absorbs more water than dye, the bath concentration increases and causes the shade to darken.
Mitigation Method
Controlling the trough volume and replenishing the bath with a more concentrated solution helps counteract these shifts. Using a small-volume pad box reduces the residence time of the liquor. Frequent lab checks during the run verify that the color remains within the tolerance ellipsoid.
Finishing Impact
Variations across the length make it difficult to cut panels for the same garment from different parts of the roll. Rejecting fabric with shade tailing prevents the assembly of clothing with mismatched components. Manufacturers must monitor feed rates to ensure the color remains stable from the first meter to the last.