Tailing in a deep olive reactive dye lot
A processing house in Solapur dyes 1,200 kg of cotton sheeting in a soft-flow machine at a liquor ratio of 1:6. The approved lab dip is a deep olive shade. The bulk recipe uses a total of 3% owf reactive dye, 70 g/L salt added over 10 minutes, and 20 g/L soda ash added over 15 minutes at 60 °C, followed by 45 minutes fixation. The first batch shows a lighter tail end with ΔE 1.2 against the middle, and a pale band at a seam. The second batch is 1.0 ΔE darker than the standard. The dyeing master proposes adding 0.5% dye at the end of each future batch to 'push' the shade, and the manager wants to dispatch the 1,200 kg after soaping because the buyer's tolerance is ΔE 1.0.
1. Diagnose the likely causes of the tailing, the pale seam band and the batch-to-batch variation.
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Tailing is a rate-of-addition problem. Salt at 70 g/L added over only 10 minutes, followed by soda ash over 15 minutes, gives the first fabric to enter the machine a much higher effective electrolyte and alkali concentration than the tail. Dye strikes quickly at the front, exhausts, and leaves a lighter tail. A short liquor ratio of 1:6 makes concentration gradients stronger, not weaker.
The pale seam band is a penetration or air-pocket fault. A seam that is thick, poorly permeable or trapped against the fabric can resist dye liquor flow. Check seam construction, machine loading and whether the seam is positioned where liquor short-circuits.
Batch variation is a separate control issue. A 1.0 ΔE darker batch suggests differences in substrate moisture regain, actual dye strength, salt purity, liquor ratio or weighing. Do not assume the dye lot is standard; check the dye certificate and the actual batch weight. The lab dip must also be compared on the same substrate and by the same wash-off route.
2. Evaluate the proposal to add 0.5% dye at the end of each future batch. What process changes would you recommend instead?
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Adding dye at the end is not a levelling correction. By the end of fixation, the fibre sites are largely occupied and the dye has little chance to migrate. Extra dye added then tends to sit on the surface, increasing unlevelness, crocking and wash-fastness risk. A 0.5% addition is large for a deep olive shade and may overshoot the standard rather than correct a small tail.
Correct the addition profile. Add salt in stages, for example a small portion at the start and the remainder over 30–40 minutes, and add soda ash gradually with pH control rather than in one short dose. Use an approved levelling or migration agent if the recipe allows it, ensure uniform wet-out before dye addition, and confirm that circulation reverses correctly. Control the temperature ramp and the actual liquor ratio.
Verify on bulk substrate. Recheck the lab dip using the actual production fabric and water, because lab-to-bulk differences in salt, alkali and substrate can shift a deep olive shade. The aim is a reproducible exhaustion profile, not a final shade adjustment.
3. The buyer's tolerance is ΔE 1.0. What should be done with the 1,200 kg: dispatch, reprocess, or sell as second quality?
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Measure before deciding. Take head, middle and tail samples plus the seam area, measure ΔE against the approved standard, and test wash and rub fastness after soaping. The tail is 1.2 and the second batch is 1.0 darker, so the lot is at or beyond the tolerance and should not be certified as a single conforming shade.
Do not simply add dye. If the tail is only slightly light and the middle is acceptable, segregate the tail rather than reprocessing the whole 1,200 kg. A deep olive reactive shade is difficult and costly to strip, and strong stripping can reduce strength and cause shade drift.
Choose the least-cost compliant route. Options are: level and re-dye the off-shade portion if the shade can be brought within tolerance; use it for a darker approved shade where variation is less visible; or offer the buyer a documented deviation or second-quality price. Dispatch without disclosure is the worst option because it risks a claim and future business. Compare reprocessing cost with the value of 1,200 kg and record the decision.