A dye may be defined as a colored substance, which has an affinity to the substrate to which it is being applied and then imparts a permanent color to that which is not removed by rubbing or washing with water, soap or on exposure to sunlight.
Properties of dyes:
01. It must have a suitable color.
02. It must have the capability of being fixed to the fabric directly or with the help of certain reagents called ‘mordant’.
03. It must be easy to apply on fabrics.
04. The affinity of the dye is connected with the linearity of the molecular structure of the dye, so that they can lie with their long axis parallel to the axis of cellulose molecule, which is also linear.
05. For a direct dye to have affinity for cellulosic fibers, the aromatic rings present in the dye molecule must be coplanar.
06. Resistant to dilute acid and alkali.
07. After dyeing, the dye must not loosen the strength of the fiber.