Optimizing Dyeing Vessel Liquor Ratios for Fabric Quality
Optimizing liquor ratios requires balancing steam and salt savings against pump turnover rates to prevent severe shade unlevelness and mechanical creasing.
Salting Out
Solubility Reduction
The precipitation of dissolved dye molecules or organic compounds from an aqueous solution occurs when the concentration of electrolyte is increased. This action, referred to as salting out, happens because the added salt ions attract the water molecules more strongly than the dye molecules do. This action reduces the number of free water molecules available to hydrate the dye, forcing it to aggregate.
Chemical Mechanism
Sodium chloride or sodium sulfate added to the bath releases sodium ions that shield the negative charges on both the dye molecules and the fibre surface. This shielding reduces the electrostatic repulsion between them, allowing the dye to migrate from the liquid phase onto the solid textile. In extreme cases of excess electrolyte, the dye will precipitate fully and form a sludge in the machine.
Dyeing Application
Controlled addition of salt is the primary method for driving direct and reactive dyes onto cellulosic fibres during the exhaustion phase. The salt is added in portions to prevent a sudden jump in exhaustion that would lead to uneven or patchy shades.
Process Constraint
The solubility limit of both the salt and the dye restricts the maximum amount of electrolyte that can be added without causing filtration issues.