Blotchy Acid Dyeing and Felting of Woollen Shawl Yarn
A cottage unit dyes 30 kg of woollen hank yarn for shawls with a leveling acid dye. The bath is set at pH 4.0 with acetic acid, 8% owf Glauber's salt is added, and the yarn is entered at 40°C. The temperature is raised to 98°C over 25 minutes and held for 60 minutes while the hanks turn continuously. The water is hard, about 350 mg/L as CaCO₃. After cooling, several hanks are blotchy, the fibre feels harsh, and contact points show felting. The spent bath is still noticeably coloured. The unit has no pH meter and no leveling agent in stock. The operator proposes adding more acetic acid and boiling for another 30 minutes to exhaust the dye, and the buyer has rejected the previous lot for patchiness.
1. Explain how pH, the heating rate, hard water and prolonged boiling each contributed to the blotchy shade, harsh handle and felting. State why adding more acid and boiling longer is likely to make matters worse.
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Effect of each factor
- pH 4.0: wool is amphoteric. At pH 4 many amino groups are protonated, so acid dye anions are strongly attracted. That increases exhaustion but also increases the risk of rapid, uneven strike, especially at the start.
- Heating at about 2.3°C/min to 98°C: the dye strikes the outside of the hank before it can diffuse to the core. Fast heating locks in a shell-to-core difference.
- Hard water (350 mg/L): calcium and magnesium form complexes with acid dyes and with soap residues, dulling shades and producing blotches; lime deposits also make the fibre harsh.
- Boiling 60 minutes with continuous turning: wool scales interlock under heat, moisture and mechanical action, causing felting at contact points. Prolonged boiling also damages the fibre, giving a harsh handle.
- Spent bath still coloured: the bath is not exhausted, but adding more acid and boiling another 30 minutes would increase the strike rate and the mechanical/thermal damage. The result would be worse patchiness and more felting, not level shade.
2. Give a revised acid-dyeing procedure for the next 30 kg lot, including pH target, electrolyte, temperature programme and mechanical handling.
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Revised procedure
- Use soft water or add 1–2 g/L sequestrant; check hardness.
- Set pH 4.5–5.0 with acetic acid and sodium acetate buffer; measure with pH papers or a meter, not by guesswork.
- Add 5–10% owf Glauber's salt as a retarder and, if available, 0.5–1% nonionic leveling agent.
- Enter the hanks at 40°C and run for 10 minutes.
- Raise to 90–95°C at 1°C/min, not to a hard boil; hold for 45 minutes with gentle turning only.
- Cool slowly to 50°C before rinsing; avoid cold shock.
- Rinse and dry under shade.
Reasoning: the buffer and salt slow the strike; the slower ramp lets dye diffuse before it fixes; lower top temperature and gentler action reduce felting while still giving good exhaustion.
3. Name and briefly describe one test that could confirm fibre damage and one check that would assess levelness.
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Tests
- Fibre damage: alkali solubility. Treat a dried wool sample in 0.1 M sodium hydroxide at 65°C for 1 hour, filter, dry and weigh the residue. Increased solubility compared with an undyed control indicates damaged wool. Urea–bisulphite solubility is an alternative.
- Levelness: compare several points on each hank under standard lighting, or measure reflectance/K/S at 5–10 positions. A visual check should include the inside and outside of the hank, since the outside can look level while the core is pale.