A Patchy Reactive Dye Batch in Hard Water
A dye house processes a 200 kg batch of cotton interlock fabric on a winch machine at a liquor ratio of 1:10. The water supply is hard, about 350 ppm as CaCO3. The recipe uses a bifunctional reactive dye, 60 g/L salt added over 10 minutes, 15 g/L soda ash added all at once, dyeing at 60 °C for 30 minutes, then soaping at 90 °C for 10 minutes. The batch comes out with a patchy, ring-like shade, darker at folds and selvedges, wash fastness of 2–3 and rubbing fastness of 3. A control batch dyed in soft water with the same recipe is level and has wash fastness 4. The buyer needs 1,200 finished T-shirts and will reject any batch with wash fastness below 4. The factory has a water softener, a levelling agent, and time for one reprocess.
1. Rank the three most likely causes of the patchy shade and poor fastness, and justify each with evidence from the batch record.
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Hard water is the leading cause. At 350 ppm as CaCO3, calcium and magnesium ions can complex with reactive dye, reduce its solubility and cause precipitation or uneven uptake. The soft-water control is level with the same recipe, which strongly points to water hardness.
Salt addition over only 10 minutes is too fast for a 200 kg interlock batch. Rapid electrolyte addition drives sudden exhaustion, and thick folded fabric receives unequal liquor flow, giving darker folds and selvedges. The patchiness is therefore a migration and distribution problem, not simply a shade recipe problem.
Soda ash added all at once causes a sudden pH rise and rapid fixation before the dye has migrated evenly. The short 10-minute soaping at 90 °C is also insufficient to remove hydrolysed dye, which explains the low wash fastness of 2–3 and rubbing fastness of 3. The evidence is the combination of ring-like patchiness, fold darkening and poor fastness in a batch that otherwise matches the control recipe.
2. Write a corrected dyeing sequence with approximate times and temperatures, including water treatment. Explain why each change addresses the defect.
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Treat the water to below 50 ppm as CaCO3 before the batch. Set the bath at 40 °C at 1:10 liquor ratio, add 1–2 g/L levelling agent and the dye, and run for 10 minutes to obtain a uniform dispersion. Add the salt in three portions over 30 minutes, then raise the temperature to 60 °C at about 1 °C/min. Add the soda ash in three doses over 20–30 minutes, checking that the pH reaches the recommended range (about 10.5–11 for a reactive dye). Fix for 60 minutes at 60 °C, then drop the bath and rinse.
Soap at 95 °C for 20 minutes with about 1 g/L detergent, then rinse warm and cold. Controlled salt and alkali addition prevent local concentration and sudden fixation, giving the dye time to migrate. Soft water prevents calcium–dye complexes. Longer soaping removes unfixed hydrolysed dye, which is the direct cause of poor wash and rubbing fastness.
3. The manager wants to strip and re-dye the batch. Give a reasoned recommendation: repair, strip, or reject, and describe one fastness test to qualify the corrected batch.
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Do not strip immediately. Stripping is aggressive, can reduce tensile strength, adds cost and may still leave a patchy substrate. First try a repair: re-treat the batch at about 70 °C for 20 minutes with 1–2 g/L levelling agent, then re-soap at 95 °C for 20 minutes. If the shade is only mildly uneven, this can redistribute dye and improve fastness. If the shade is still unacceptable but the fastness is adequate, re-dye to a darker shade rather than strip.
Strip only if the shade is unsalvageable and the buyer accepts a redye; use a mild reducing-agent strip and test strength afterwards. Qualify the corrected batch with a wash-fastness test such as ISO 105-C06 at 60 °C, assessing colour change and staining against the grey scale; the batch should reach at least grade 4. A rubbing test such as ISO 105-X12 should also be checked, because the original batch failed both.