Uneven Bleaching and Crease Marks in Cotton Knit Dyeing
A dyehouse processes 1,200 kg/day of 180 gsm cotton single jersey in rope form on two soft-flow machines. The bleaching recipe per litre is 4 g H2O2 (50%), 3 g NaOH, 0.5 g stabilizer, 0.3 g wetting agent, at 98°C for 45 min, liquor ratio 1:8. The bleached fabric has 72 Berger whiteness against a target of 80, absorbency of 4 seconds against a target below 2 seconds, and strength loss of 12% against a target below 8%. Subsequent reactive dyeings show dark crease marks and shade variation between machines. The dyehouse proposes doubling H2O2 to 8 g/L and adding 2 g/L NaOH to reach whiteness. The effluent plant has a permit limit of 1,500 mg/L COD and the current bleach bath COD is 1,100 mg/L. Production has 8,000 kg of export fabric due in 12 days. The machines have a maximum liquor ratio of 1:8 and a rope speed of 250 m/min.
1. Evaluate the proposed increase in H2O2 and NaOH. Will it solve the problem? Support your answer with process reasoning and the given quality data.
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No. The combination of low whiteness, poor absorbency and 12% strength loss points to uneven or over-oxidative bleaching, not simply too little peroxide. Doubling H2O2 to 8 g/L will increase radical attack on the cellulose, raise strength loss further, create oxycellulose and increase the risk of shade variation in reactive dyeing. Adding 2 g/L NaOH may push the pH too high; peroxide decomposition becomes very rapid above about pH 11, so the bleach can be exhausted before it penetrates the fabric. If the pH is currently too low, the correct action is to control pH to about 10.5–11, not to add alkali blindly. The crease marks are a separate rope-form problem: likely causes are overloading, low liquor ratio, rapid heating or cooling, and insufficient anti-crease lubrication. The 0.5 g/L stabilizer is low for 4 g/L H2O2 at 1:8, so the peroxide is probably decomposing unevenly. Increasing chemicals will also raise effluent COD and may breach the 1,500 mg/L limit.
2. Propose a corrected bleaching and dyeing plan for the 8,000 kg order, including two control tests before dyeing.
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Correct the bleach first: load each machine to no more than about 70% of capacity; use 3–4 g/L H2O2, 1.0–1.2 g/L stabilizer, 0.3–0.5 g/L wetting agent and NaOH to pH 10.5–11 at 1:8 liquor ratio. Heat at about 1°C/min to 98°C, hold for 45 min, then cool at about 1.5°C/min. Rinse and neutralize to pH 6.5–7.5. Add an anti-crease lubricant and control the rope speed. Before dyeing, test whiteness and absorbency on each batch; whiteness should be at least 78–80 Berger and absorbency below 2 seconds. Also test for residual peroxide, because residual H2O2 destroys reactive dye and causes pale or uneven shades. A simple peroxide test strip or a spot test on the fabric is enough. For dyeing, standardize the recipe on both machines, control salt and alkali addition, and ramp the temperature slowly. Run one 600 kg trial batch, check shade and crease marks, then process the remaining order in matched batches.
3. Recommend how to manage the effluent and production sequencing to meet the 12-day deadline without exceeding the COD permit.
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Do not double the peroxide; that would raise both cost and effluent load. Quench residual peroxide before draining, using sodium bisulphite or catalase, and segregate the bleach bath from dyehouse wastewater so it can be equalized and treated. Use counter-current rinsing to reduce water volume. Check COD before discharge; if the bleach bath is near 1,100 mg/L, any extra chemical load could push the combined stream towards the 1,500 mg/L limit. For production, divide the 8,000 kg into batches of about 600 kg, run a trial batch on each machine, and test whiteness, absorbency, residual peroxide and shade before bulk processing. Standardize both machines and do not mix machines within one dye lot unless the shades are proven. With 1,200 kg/day capacity, 8,000 kg needs about seven working days, leaving five days for testing, rework and packing, provided the trial is accepted quickly.