Pale Cores in Reactive-Dyed Cotton Packages
A small dyehouse receives an order for 20 kg of cotton hank yarn in a single reactive turquoise shade. The yarn is wound into packages at 0.34 g/cm³ and loaded into a package dyeing machine at a liquor ratio of 1:8. The water supply is hard, about 450 mg/L as CaCO₃, and no sequestrant is used. The recipe is reactive dye 2% owf, sodium chloride 60 g/L and soda ash 15 g/L. Salt is added over 10 minutes at 40°C, then the bath is raised to 60°C at 2°C/min. Soda ash is added in one lot at 60°C and dyeing continues for 60 minutes. After draining, the yarn is soaped at 90°C for 15 minutes. When the packages are unwound, three of twelve show pale inner layers; the shade is otherwise close to standard. Wash fastness is 3, and some colour bleeds in the soaping bath. The customer needs a level shade and wash fastness of at least 4.
1. Diagnose the most likely causes of the pale inner layers and the low wash fastness. Rank the causes and explain the mechanism of each.
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Ranking and mechanism
- Poor flow through tight packages. At 0.34 g/cm³ the package is dense; if pump pressure is marginal, liquor channels around the package and the core is starved of fresh dye and alkali. Pale inner layers that appear only after unwinding fit this mechanism.
- One-lot alkali addition. Adding 15 g/L soda ash at once raises pH rapidly. Reactive dye strikes the outer yarn first; the core sees a depleted, lower-pH bath. This produces tailing and pale cores even when the total dye is correct.
- Hard water without sequestrant. At 450 mg/L as CaCO₃, calcium and magnesium bind anionic reactive dye and carbonate, lowering colour yield and forming precipitates that can block packages.
- Incomplete fixation/soaping. Wash fastness 3 and bleeding show unfixed dye, but this is mainly a consequence of uneven alkali/dye distribution and inadequate soaping, not the primary cause of the core/outer difference.
A useful check is to compare the colour of the outer and inner layers before and after strong soaping. If the core stays pale, penetration is the problem; if it equalises, the problem was unfixed surface dye.
2. Write a corrected package-dyeing procedure, including water treatment, package density/flow, and the addition sequence for salt and alkali.
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Corrected procedure
- Treat water with 1–2 g/L sequestrant (polyacrylate or EDTA) and confirm hardness below 50 mg/L; otherwise use softened water.
- Reduce package density to about 0.28–0.30 g/cm³, or increase pump pressure and reverse flow every 10 minutes. Confirm flow is uniform across the spindle.
- Set liquor ratio 1:10. Enter yarn at 40°C and circulate for 10 minutes.
- Add salt in three portions over 20 minutes (20 g/L each) at 40°C.
- Raise to 60°C at 1°C/min.
- Add soda ash in three portions of 5 g/L at 10-minute intervals; check pH 10.5–11.0.
- Hold 45–60 minutes, then drain and rinse.
- Soap at 90°C for 15 minutes with 1 g/L nonionic detergent, rinse warm, then cool.
Reasoning: slow, staged salt and alkali addition keeps the dye mobile until it has penetrated; sequestrant protects both shade and package flow.
3. Describe a practical works test that would show whether the inner-layer paleness is due to poor penetration or incomplete fixation.
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Practical discrimination test
Unwind one pale package and cut a radial cross-section. Keep one half as a control. Treat the other half in a fresh bath containing 1 g/L nonionic detergent and 1 g/L soda ash at 90°C for 15 minutes, then rinse. Compare inner, middle and outer layers visually and, if available, by spectrophotometer (K/S).
- If the pale core persists after strong soaping, the dye never penetrated: the fault is package density/flow or dye strike.
- If the core darkens or matches the outer layer after soaping, the core was under-fixed or covered with unfixed dye: the fault is alkali distribution/fixation.
A solvent extract of each layer can also be compared; more extractable dye in the pale layer indicates unfixed dye rather than missing dye.