Dye migration in pad-dry: working back from the fault you see

What the fault looks like coming off the dryer

A padded and dried piece opens up with a shade difference, and where that difference sits narrows the cause down to a few headings.

Four appearances come up repeatedly.

Two-sidedness. One face of the cloth is deeper than the other. It hides in a folded roll and shows up in the garment at a collar or a cuff turn.

Edge-to-centre difference. Selvedges deep and the middle pale, or the reverse depending on the range.

A band across the width. The line stopped and restarted there, so that part of the cloth dried differently.

Head and tail of the piece. The first and last metres do not match the body.

None of the four comes from the dyeing itself. Colour was even as the cloth left the padder and uneven as it left the dryer, and drying is the only step in between.

The practical way to confirm that is a wet sample taken at the padder exit. Dry it slowly at low temperature in the laboratory and compare it with the line-dried cloth under the same light. If the two differ, the problem is in drying.

Where does the dye go while the cloth dries?

Evaporation happens at the surface. As surface water leaves, liquor from inside the cloth follows by capillary action and takes dissolved or dispersed dye with it. By the end of drying the dye no longer sits evenly through the cross-section; it has collected in the outer layer.

If the two faces do not dry at the same rate, the movement runs one way. The face touching a drying cylinder dries differently from the free face, and unbalanced top and bottom nozzle flow in a stenter ends in the same place.

The same logic covers differences across the width. A zone with weak air flow dries late, liquor there stays mobile longer, and more dye travels.

Dye stacked at the surface costs more than shade. Rubbing fastness falls with it, because dye that never entered the fibre lifts off under mechanical action.

Migration or unlevelness?

Both can sit on one piece, and they are solved in different places. Three checks separate them.

1. Compare the two faces. Cut the cloth and fold two pieces back to back. A face-to-back difference points at migration; unlevelness carried out of the dyebath usually affects both faces together.
2. Look at the cross-section. On a heavy woven, a cut section shows dye concentrated at the surface and pale inside. Exhaust dyeing leaves no such profile.
3. Match the pattern to the machine. Migration follows the geometry of the dryer: cylinder contact, nozzle rows, the chamber entry. Unlevelness brought in from pre-treatment stays in the same place whatever the machine does.

Reading an unlevel pattern back to its cause runs on its own diagnostic order: unlevelness causes and cures.

What to set on the range before raising the dosage

An anti-migration agent is one of three measures, and it is not the first in the queue.

1. Pick-up. The less liquor left on the cloth, the less water there is to travel. Nip pressure, roller hardness and nip evenness across the width all belong here. A nip running slacker at one side gives an edge-to-centre difference that no chemical closes.
2. Pre-drying. Running the cloth through infrared pre-dryers before the stenter leaves a thin dry layer at the surface and cuts the flow coming from inside. On some fabrics the opposite works better: slow, even drying throughout. Which one applies is settled by trial on that cloth.
3. The first chamber of the dryer. Most of the migration happens there, because that is where the cloth is wettest. Temperature and air flow in that chamber decide the result.

The fabric is a variable too. Heavy canvas and gabardine hold more liquor, take longer to dry and keep the migration window open longer. A recipe that behaves on a light knit will not behave on a heavy woven.

Then there is air in the trough. When bubbles collect in the pad trough, the cloth takes up liquor at different rates from point to point, and that variation in pick-up turns into a shade difference after drying. Deaerating surfactants are therefore part of the migration question on a pad range; DK WET HGK and DK WET HGC ECO sit in that family.

The two ways an anti-migration agent works

The first is viscosity. Raise the flow resistance of the liquor and capillary transport slows, so the distance dye can travel within the drying time gets shorter.

The second is holding the dye. The product interacts with the dye or the fibre and lowers the free dye concentration, so water can move without the dye moving at the same rate.

What happens to the product after drying depends on the formulation. Some grades leave the cloth in the following wash or in the rinse after steaming; others stay in the finish and contribute to the handle. Which one you have needs to be known while the recipe is built, because a film left on the cloth changes rubbing fastness and adhesion.

Dosage sits in a narrow band. Too little does not stop the transport. Too much thickens the liquor, and thick liquor squeezes unevenly at the nip, builds up on the rollers and stiffens the handle. Raising the dosage to close an edge-to-centre difference recreates that same difference from another cause.

Derin Kimya's product for this job is DK COATIN MPR, listed under other auxiliary chemicals. Dosage, viscosity and compatibility figures are given in the technical data sheet, and the working point for a given cloth and range is settled on a trial sample before the recipe is released.

What changes between pigment, disperse and reactive systems?

The mechanism is the same. What travels, and how long the window stays open, is not.

Pigment printing and pigment padding. What moves is pigment and binder rather than dye. Binder collecting at the surface can look like a gain in colour yield while rubbing fastness and adhesion get worse. A thickener system is already in the liquor, so the two are checked together before an anti-migration agent goes in.

Disperse dyes and thermosol. A disperse dye is dispersed rather than dissolved, and it travels with the water film. The matter is settled in the first drying zone; by the time the cloth reaches thermosol fixation the distribution is set. On a continuous range no correction is left at the dryer exit.

Reactive pad-dry and pad-dry-pad-steam. The dye is in solution and its mobility is highest here. The distribution locks the moment drying ends, and steaming only fixes what is already there.

Pad-batch. There is no drying step, so this form of migration does not arise. The discussion there is roll rotation, dry edges on the exposed ends and dwell time. An anti-migration agent is not part of a pad-batch recipe.

How is the diagnosis verified?

Two checks come before any decision on the line. Compare the two faces under the same light, then look at the depth of dye in a cut section.

The third is a laboratory comparison: pad two samples from the same liquor, dry one slowly and one on the line profile. If the difference appears only on the second, the diagnosis is closed.

The table below is a shortcut, not evidence. Once a row matches, go and check what that row points at.

What goes wrong

1. Closing the fault by raising the dosage. The liquor thickens, the nip squeezes unevenly, and the edge-to-centre difference comes back.
2. Pushing the first chamber hotter step by step. The surface skins over quickly, but over-dried cloth loses handle and the tail of the piece goes deeper.
3. Reading two-sidedness as unlevelness and adding levelling agent. The wrong box gets the intervention; dye consumption rises and the appearance does not change.
4. Carrying a recipe proven on light cloth onto a heavy weight. The amount of water that travels changes and the same dosage no longer holds.
5. Approving a recipe on a slowly dried laboratory sample. A slowly dried sample does not show migration; the line shows it.
6. Mixing an anti-migration agent into the liquor without checking compatibility with what is already in it. An ionic clash destabilises the liquor, and what settles out marks the rollers.
7. Leaving the cloth inside the chamber when the line stops. Standing cloth keeps drying where it is, and those metres come out different.

Ask the supplier which of the two mechanisms a grade works on, and whether it washes out or stays in the finish. Both answers belong in the technical data sheet, and both change the recipe around it.