DISPERSE DYE DISPERSIONS

Dispersion of disperse dyes:

Disperse dyes are available as powders, grains, pastes or aqueous dispersions. These all contain micro-fine dye particles, many with diameters below 1 _m, as well as varying amounts of dispersing agents. Powders may cause dusting but this is not as bad as with some ionic dye powders. Granular forms pour easily, dust less and facilitate weighing. Solid forms of the dyes contain much more dispersant than pastes and liquid forms to prevent particle aggregation during drying. Liquid disperse dyes are useful for continuous dyeing. Settling and aggregation of dye particles in pastes and liquids can lead to coloured specks on the dyed fabric.

Powdered or granular disperse dyes rarely contain more than 50% dye, the rest being dispersant plus smaller amounts of diluents, oils and shading colours. The role of the dispersing agent is to coat the surface of each dye particle with a monomolecular layer of adsorbed dispersant. Since these chemicals are invariably anionic polymers, the more hydrophobic sections of the polymer chain adsorb onto the hydrophobic dye particle surface with the anionic groups of the polymer exposed to the surrounding water. 

The original disperse dye dispersing agent, sulphated ricinoleic acid (1); a modern dispersant, sodium oleyl-p-anisidide sulphonate

The overall negative charge on the surface of each particle prevents their coalescence and aggregation. The original dispersant was sulphated ricinoleic acid (1, in Figure). Modern dispersants are often either lignosulphonates from paper pulping, sulphonates of alkylnaphthaleneformaldehyde condensates or sodium oleyl-p-anisidide sulphonate (2). Many commercial dispersants for dyeing are mixtures of non-ionic and anionic compounds. The dye particles dissolve in the micelles of the non-ionic surfactant and the anionic component raises the cloud point of the former  to values above the dyeing temperature so that the dispersion is stable.

The presence of a dispersing agent in the dyebath increases the apparent water solubility of the dye and therefore decreases the dyeing exhaustion. This is because dispersant micelles in solution dissolve an appreciable number of dye molecules. The number of dye molecules per solid particle is much larger than the number per surfactant micelle. The number present as single soluble molecules in true solution is even less. All these dye species appear to be in rapid equilibrium in the dyebath. Therefore, dye transferred from the aqueous solution to the fibre is rapidly replenished by dye from a micelle or from a particle surface.

To ensure that the rate of solution of the dye does not control the overall rate of dyeing, the dyes consist of very small particles to give the maximum specific surface area. Therefore, during dyeing the average particle size will gradually increase because the smaller particles dissolve more rapidly. The very fine state of division avoids dye specks on the goods. In fact, the quality of the fine dispersion is often evaluated by running it through coarse filter paper and examining for any coloured residue. Dispersions may not always be stable under the dyeing conditions, particularly those involving high temperatures and high shear forces as in jet and package machines.

Partial chemical structures of lignosulphonate (a) and alkylnaphthalene sulphonate – formaldehyde condensate (b) dispersing agents

The dyebath usually contains additional dispersant to maintain the dispersion of the dye and to promote levelling. This is more important for pale shades when the amount of dispersant added from the dye powder or liquid is low. No electrolyte is added to the dyebath since it will not influence the dyebath exhaustion. Disperse dyes are non-ionic and the negative water–fibre boundary charge does not influence adsorption. Salt addition often affects the layer of anionic dispersant on the surface of the dye particles, resulting in undesirable particle aggregation.

As always in dyeing, good quality water is essential. Some dyes react with traces of metal ions such as iron or copper and give shade changes. Calcium and magnesium ions may react with anionic dispersants and wetting agents rendering them much less effective. Addition of a sequestering agent is valuable, especially with metal-sensitive dyes.