Navy leggings fail colour-fastness and recovery
A small knitwear unit in a city produces 400 pairs of 92% cotton/8% elastane leggings per week, dyed navy. The dye recipe uses reactive dyes, sodium carbonate, a 60°C dyeing temperature, 40 minutes, one warm rinse and no soaping. The local water supply has a hardness of about 380 mg/L as CaCO3. After the first bulk order, a school reports that white socks worn with the leggings turn blue, and that some leggings look patchy when compared with others from the same order. Two customers also complain that the waistbands feel looser after three washes. The unit has a winch dyeing machine, a hydro-extractor and a tumble dryer, but no spectrophotometer. The owner wants to know whether the problem is the dye, the water, the washing instructions or the fibre blend, and whether the batch can be reworked or must be sold as seconds.
1. Explain the most likely causes of the crocking and shade variation, linking them to fibre chemistry, dye class and the wash recipe.
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The crocking is most likely unfixed reactive dye, not simply poor customer washing. Reactive dyes need alkali and sufficient time to form covalent bonds with cellulose. A 40-minute cycle at 60°C with only one warm rinse and no soaping leaves hydrolysed dye - dye that has reacted with water instead of cotton - inside the fibre and on the surface. That surface dye rubs off onto white socks. The navy shade needs a high dye concentration, so the risk is greater. Hard water at about 380 mg/L consumes alkali and can precipitate calcium salts, reducing fixation and causing patchy uptake. Shade variation between pieces can come from different dye lots, uneven liquor circulation in the winch, pH drift, or uneven packing of the goods.
The elastane complaint is a separate signal. Elastane is sensitive to prolonged alkali and high temperature; loss of waistband recovery suggests fibre degradation, not just dye loss. The absence of soaping is a major process fault because soaping removes unfixed dye and improves wet-fastness. The lack of a spectrophotometer does not prevent diagnosis: a simple crock test with white cotton rubbing cloth and a wash-fastness test would show the problem.
2. Propose a revised dyeing and after-treatment procedure, including specific checks, that the unit can run without buying new machinery.
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Keep the winch, but tighten the recipe and add the missing after-treatment. First, test the water hardness and add a sequestrant if it remains above 150 mg/L. Standardise dye lot and fabric lot, and weigh every batch. Dose the alkali in stages rather than all at once, and hold at 60°C for 60 minutes rather than 40, checking pH is about 10.5–11 at the start of fixation. After dyeing, drop the bath and rinse warm. Then soap at 70°C for 10 minutes with about 1 g/L nonionic detergent; this is hot enough to remove unfixed dye but less stressful to elastane than a 90°C boil. Rinse, neutralise with dilute acetic acid to pH 6–7, and soften if required.
Run checks on every lot: a rub-fastness test with white cotton, a wash-fastness test at 40°C, and a visual shade check against a retained standard under daylight. Keep a recipe sheet recording water hardness, pH, times and chemical masses. Rework only if the crock test improves to an acceptable level; otherwise sell as seconds and state the limitation. Do not re-dye the whole batch without a lab-scale trial, because repeated alkali will further damage elastane.
3. Describe one test to confirm whether elastane degradation occurred and state what result would support your diagnosis.
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Use a recovery test on a retained sample from the same fabric lot and on a garment from the complained batch. Cut three 20 cm × 5 cm strips from each, condition them, measure the original length, stretch to 50% extension, hold for 30 minutes, release, and measure the length after 30 minutes. Calculate percentage recovery. If the complained garment shows visibly lower recovery than the retained sample - for example, 80% instead of 95% - that supports elastane degradation rather than a simple fit complaint. Repeat the test after three domestic washes at 40°C to see whether recovery falls further.
A microscope check can support the result: degraded elastane may show cracks or surface damage, but a microscope is not essential. The key is to compare like with like: same fabric, same test conditions, retained versus complained sample. If recovery is unchanged, the waistband looseness is more likely a pattern or elastic-application problem, not fibre damage.