Bronzing and Low Rub Fastness in Vat-Dyed Cotton Yarn
A handloom unit vat-dyes 50 kg of cotton yarn for denim-effect fabric. The recipe uses vat dye 3% owf, sodium hydroxide 25 g/L and sodium hydrosulphite 12 g/L at 50°C for 45 minutes. The yarn is dyed in hank form on a revolving stick, squeezed, left in air for 10 minutes to oxidise, rinsed, and soaped at 80°C. The result has a bronzy metallic surface, a weaker and duller shade than the standard, and rubbing fastness of only 2. The spent dye bath is still strongly coloured. The unit owns a redox meter but the operator has never used it. The hydrosulphite is kept in an open drum and has been there for six months. The buyer requires rubbing fastness of 3–4 and a shade close to the standard.
1. Diagnose the likely causes of the bronzy surface, weak shade and low rubbing fastness. Explain what each symptom indicates about reduction, dissolution and oxidation.
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Symptom-to-cause reasoning
- Spent bath still strongly coloured: the dye was not fully reduced and dissolved. Unreduced vat pigment remains suspended and does not enter the fibre.
- Weak, dull shade: low available hydrosulphite. The drum has been open for six months; sodium hydrosulphite oxidises in air and loses strength. 12 g/L of a weakened product may be far below the effective dose.
- Bronzy metallic surface: vat dye that is poorly reduced or insufficiently oxidised can deposit as surface pigment/aggregates; over-reduction or incomplete oxidation can also produce bronzing. The combination with low rub fastness points to surface pigment rather than dye inside the fibre.
- Alkali 25 g/L: may be marginal for keeping the leuco dye soluble; pH should be strongly alkaline, about 12.5–13.
- Air oxidation 10 minutes: may be too short for full development, especially in a thick hank; incomplete oxidation leaves a dull, unstable shade.
- Rub fastness 2: loose surface pigment is rubbed off, confirming poor penetration/fixation.
2. Recommend a corrected vat-dyeing procedure with control points for hydrosulphite, alkali, temperature and oxidation.
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Corrected procedure
- Store hydrosulphite in sealed, dry containers; discard the old drum or standardise it by iodine titration.
- Set the bath with 30–40 g/L sodium hydroxide and 15–20 g/L fresh sodium hydrosulphite; add 1–2 g/L reduction stabiliser if available.
- Heat to 50–60°C, check redox potential (about –700 to –850 mV) and pH 12.5–13.0.
- Add pre-wetted yarn; dye for 45 minutes with gentle movement; keep the bath covered to limit air oxidation.
- Check that the bath is clear/yellow-green and that a drop on filter paper is colourless (leuco).
- Squeeze uniformly, then oxidise in 1–2 g/L hydrogen peroxide with 1 g/L acetic acid at 50°C for 10 minutes, or in air until the full colour develops.
- Soap at 90°C for 15 minutes, rinse and dry.
Reasoning: fresh reductant and adequate alkali ensure a soluble leuco dye; controlled oxidation develops the final insoluble pigment inside the fibre; soaping removes surface pigment and improves rub fastness.
3. Describe how the unit could use its redox meter and a simple bath check to prevent a repeat.
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Using the redox meter and a bath check
- Calibrate the redox probe and measure the bath before adding yarn and again halfway through. Record values for each lot. A rise toward less negative values means the hydrosulphite is being consumed; add fresh reductant in small increments.
- Check pH with a meter or wide-range papers; maintain 12.5–13.0.
- Do a filter-paper spot test: a drop of bath should be pale/colourless if the dye is reduced; a strong colour indicates unreduced pigment.
- Titrate a sample of hydrosulphite solution with iodine to check strength before weighing the recipe.
- Record all values on the batch sheet so a trend can be spotted before a faulty lot is produced.