What is substantivity?
It is define as an attraction tha a dye has for a fibre under given dyeing conditions.
whereby, the dye is selectively extracted from an application medium by the fibre.
Direct dye have high substantivity for cellulose fibres. because direct dye molecules contains some group such as (-NH2) amono group which can get attached to cell -OH Macromolecules at the hydroxy group of the weak hydrogen bonds.
When a cellulose fibre is immersed in solution of a direct dye in water, the dye molecules are immediately absorbed by the cellulose Macromolecules present on the surface of the fibre.
The dye stuff molecules then diffuse into the interior of the fibre through the pores in the amorphous region of the cellulose structure.