**Particle size in dispersion of the dyes**

The aqueous solubility of disperse dye particles in a dispersion increases with decreasing particle size (Kenneth & Skelly, 1973). Thus an increase in the severity of milling that accompanies a reduction in the particle size of the dye enhances the solubility and adsorption of disperse dye.

### **pH of dyebath**

Generally, in the commercial dyeing of polyester fibres with disperse dyes, dyeing is carried out within the pH range 5.5 to 6.5. Strongly alkaline or acidic conditions, such as higher than pH 9 and lower than pH 4, induce hydrolysis of the fibre as well as decomposition of azo disperse dyes (Nunn, 1979). In the case of high temperature dyeing, this degradation of polyester fibre and dye is accelerated.

#### **Substantivity of disperse dyes**

The substantivity of disperse dyes towards polyester fibres is one of the most critical factors in determining dyeing behaviour and there have been many studies carried out to evaluate the substantivity of disperse dyes towards hydrophobic fibres, including polyester, in order to select suitable dyes. These attempts include the 'Solubility Parameter Concept' (Ingamells, 1990) and 'IOR (Inorganic/Organic) values' (Kim et al, 2003).
