Reactive Paste Losing Colour and Body Mid-Run
A 10 kg reactive dye paste is mixed at 7:30 with cold-water-soluble alginate, urea, soda ash and dye, then left in an open tub. The target is 80 m of pre-treated cotton. By 9:00 the paste has become noticeably thicker and the prints look lighter and slightly patchy. The workshop is at 36 °C, the mixer was left open, and the water supply tested at 480 ppm hardness. The paste pH is 11.5. The printer has already used 4 kg and wants to add thickener and more dye to bring the colour back. The order is due tomorrow and only 3 kg of dye remains in stock.
1. Diagnose the likely causes and decide whether the paste can be salvaged by adding thickener and dye.
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The symptoms point to two processes running together: water loss from the open tub is thickening the paste, and the alkaline reactive dye is hydrolysing, which lowers colour yield. The high hardness may also be consuming dye or affecting solubility. Adding thickener would make the paste even less penetrable and could cause skips; adding dye without removing hydrolysed dye may deepen shade unevenly and will not restore the lost fixation. I would first measure viscosity and pH, then print a small patch on the same fabric and compare with the 7:30 standard. If the patch is substantially lighter after fixation, the batch is not salvageable. With only 3 kg of dye left, I would not risk the whole order on a corrected batch.
2. What immediate production decision would you make, and how would you protect the remaining metres?
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I would stop using the 9:00 paste, segregate the metres printed after 9:00, and mix a fresh small batch, enough for 20 m, with softened water and a covered container. I would print and fix a test piece from the fresh batch before restarting. The earlier 4 kg batch should be recorded as suspect, and the printed metres checked after washing for shade variation. If the shade is within tolerance, they can continue; if not, they should be reprocessed or rejected. I would not top up the old batch with fresh paste unless a patch test proves the blend is uniform.
3. What mixing and storage controls would prevent recurrence?
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Mix smaller batches matched to a two-hour printing window, cover the tub when not in use, and keep the paste away from direct heat. Use soft water or treat the 480 ppm supply, and check pH and viscosity at mixing and before each refill. Record batch time, pH, viscosity and metres printed. For reactive dyes, the alkali should be added close to printing time rather than hours ahead, and leftover paste should not be carried over to the next day without a fresh patch test.