Reactive Dyeing of Cotton: The Control Points That Decide Right-First-Time

A composite knit mill running 72 per cent right-first-time on reactive shades is losing roughly twice the cost of the reprocessed batches once you count machine hours, water, steam and the delivery risk. The recovery path is not better dyes. It is control of five variables that mills routinely treat as constants.

1. Preparation decides the ceiling

Nothing downstream fixes uneven preparation. Scouring and bleaching must deliver consistent absorbency and a controlled degree of polymerisation. Two field checks matter:

• Drop test: a water droplet should sink within 1 to 2 seconds, and within the same time everywhere on the batch. Variation across the roll is the leading cause of listing and ending.
• Whiteness consistency: for pastels, a CIE whiteness spread above 3 units within a lot will show as shade variation no dyeing recipe can correct.

Residual peroxide is the silent killer. Anything above about 5 ppm entering the dyebath oxidises reactive dye and shifts the shade unpredictably. Peroxide killer dosing must be verified with a test strip on every batch, not assumed from the recipe.

2. Salt is a dosing profile, not a quantity

Electrolyte drives exhaustion by suppressing the negative charge on cellulose. Dumping the full salt charge at the start pushes dye onto the fibre faster than it can migrate, which produces unlevel dyeing that fixation then locks in permanently.

Typical linear dosing for a 4 per cent shade, 1:8 liquor ratio:

  0 min   : 20 per cent of total salt
  0-20 min: remaining 80 per cent, linear dose
  20-30   : circulate, no addition
  30 min  : begin alkali dosing

Salt requirement scales with depth of shade, roughly 20 g/l for pastels up to 80 g/l for deep shades, and it scales inversely with liquor ratio. A machine running 1:5 needs materially less salt than one at 1:10 for the same result — which is one of the reasons low liquor ratio machines pay back.

3. Alkali dosing controls fixation, and fixation is irreversible

Once pH rises, the dye-fibre covalent bond forms and migration effectively stops. Linear or progressive alkali dosing over 20 to 30 minutes gives the dye time to level before it fixes. Shock dosing soda ash is the single most common cause of unlevel deep shades.

Target pH is 10.8 to 11.2 for most bifunctional reactives. Above 11.5, hydrolysis accelerates and you lose fixation efficiency — the dye reacts with water instead of cotton, and washes down the drain.

4. Temperature ramp, not temperature

Specifying "60 degrees" is incomplete. The ramp rate from 40 to 60 determines whether strike is controlled. A rate of 1 to 1.5 degrees per minute is a reasonable default for medium-energy reactives; faster ramps on dark shades cause listing.

5. Liquor ratio must match the recipe

Recipes are calculated at a stated liquor ratio. Running the same recipe at a different ratio changes the effective concentration of both dye and electrolyte. If your machine loads vary between 60 and 100 per cent of nominal capacity, and you use one recipe for all of them, you have built shade variation into the process by design.

Measuring RFT honestly

Counting a shading addition as "right first time" because the batch eventually passed is the most common way mills convince themselves the problem is smaller than it is.

Where to start on Monday

Instrument the three cheapest things first: residual peroxide on every batch, actual versus recipe liquor ratio on every load, and a logged temperature ramp. In most mills those three alone move RFT by eight to twelve points before a single recipe is changed.