Reactive Dye Shade Variation Across a Batch
A knit-dyeing unit exhaust-dyes 100% cotton single jersey, 220 gsm, in a 400 kg winch machine using reactive dyes. The customer requires a shade match with a colour difference (DE) of 1.0 maximum under D65. A recent lot shows DE of 2.4, with the lab dip approved at DE 0.6. The dyeing recipe used 40 g/L Glauber's salt and 12 g/L soda ash, with alkali added in one dose at 60 degrees C. Water hardness is 180 ppm as CaCO3. The machine's liquor ratio is 1:10. The lot was dyed after a machine cleaning that used a caustic scour, but the rinse before dyeing was shortened to save time, and the first rinse water showed pH 10.5.
1. Analyse why the production shade differs from the approved lab dip despite using the same recipe.
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Several variables differ from a controlled lab dip. The shortened rinse left the bath at pH 10.5 before dyeing, so the fibre was not at the neutral starting condition the recipe assumes; reactive dye exhaustion and fixation then start from a higher pH, causing uneven and deeper strike. Alkali added in a single dose at 60 degrees C causes a rapid, uncontrolled fixation - the dye strikes fast and unevenly rather than migrating, which raises DE and can cause patchiness. Water hardness of 180 ppm is high; calcium and magnesium can complex with dyes and auxiliaries, reducing colour yield and shifting shade. The lab dip, run in soft water with controlled alkali addition, would not reproduce these conditions. The DE of 2.4 versus 0.6 is consistent with a fixation-rate and water-chemistry mismatch rather than a wrong recipe.
2. Recommend corrective actions for the next lot and a verification protocol.
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Corrective actions: (1) Restore full rinsing after caustic scour and confirm rinse water pH is at or below 7.5 before dyeing. (2) Add alkali in split doses over a controlled time (for example, one-third at start, remainder over 20-30 minutes) rather than a single dose, to control fixation rate. (3) Install or verify a water-softening or sequestering step so hardness is reduced or chelated; confirm with a hardness test. (4) Verify liquor ratio and salt concentration against the recipe, since 40 g/L at 1:10 must be checked for the actual bath volume.
Verification: dye a 20 kg pilot lot under corrected conditions, measure DE against the standard under D65, and require DE of 1.0 or below before releasing the full 400 kg. Record pH at each stage and retain a sample of the rinse water. If the pilot still exceeds DE 1.0, run a shade-addition correction rather than re-dyeing from scratch.