Chemical Formulation
A concentrated pad bath contains thickening polymers that increase resistance to flow to stabilize the wet pick-up on fabric surfaces. Mills formulate a high-viscosity dye liquor to suppress capillary movement of moisture during the pre-drying phase of continuous dyeing. This formulation holds the dye particles in place until the polymer chains lock them in during thermal fixation.
System Mechanism
The presence of long-chain polymers raises the internal friction of the liquid, which opposes the capillary forces that draw water to the evaporating surface. As the wet fabric enters the pre-dryer, the increased viscosity limits the travel of the dye particles within the yarn structure. This resistance prevents the particles from concentrating at the fabric surface, ensuring a uniform distribution of colour through the cross-section.
The liquor maintains its high viscosity until the heat begins to break the polymeric structure.
Operational Control
Viscometers measure the flow behaviour of the mixture before it is pumped into the padder. Using a high-viscosity dye liquor requires precise control of the pad mangle pressure to achieve the target wet pick-up. Technicians monitor the temperature of the storage tank, as heating reduces the viscosity of the polymer-thickened liquor.
Regular addition of fresh thickener maintains the rheological properties of the bath.
Process Constraint
High liquid thickness reduces the penetration of dye into dense, heavy fabrics, which can leave the inner core of the yarn undyed. This mixture also requires intensive washing to remove the residual polymer after the dyeing process is complete.
