Physical Phenomenon
The movement of suspended colourants from the interior of a wet fabric to its surface during drying causes uneven pigment distribution. In textile finishing, dye particle migration occurs when water evaporates from the outer layers of the substrate, drawing dissolved or dispersed dye molecules along with it. This activity ceases once the fabric moisture content drops below a critical threshold.
System Mechanism
Capillary action drives the liquid flow through the interstitial spaces between yarns toward the heated evaporation front. When the fabric surface heats up, water vaporises first from the outermost fibres. Liquid water from the damp core flows outward to replace the lost moisture, carrying the suspended dye particles to the outer boundaries.
If unchecked, this movement concentrates the dye on the face and back of the material, leaving the core lighter. The resulting shade variation becomes visible after the thermal fixation step.
Operational Control
Adding specialised chemical polymeric antimigrants to the pad bath increases the viscosity of the liquid to restrict movement. During thermal processing, continuous drying must happen gradually in the infrared pre-dryer to avoid rapid moisture flashing. Thermocouples measure the fabric temperature to ensure the drying rate matches the wet pick-up level.
Controlling the air velocity in the hot-air dryer also prevents uneven evaporation.
Process Constraint
High concentration of thickening agents can cause fabric stiffness or alter the hand feel of the finished apparel. This phenomenon occurs more severely in polyester and cotton blends because the different fibres absorb water and dye differently. Maintaining a balance between drying speed and moisture control is essential to prevent this defect.
