Why must a dye resist washing, heat, and light after it is applied to fabric?

A dye must resist washing, heat, and light to ensure the coloration is permanent and durable, a quality known as fastness. For a process to be considered true dyeing rather than mere staining, the color must not be readily removed by water or normal washing procedures, nor should it fade rapidly when exposed to light.12

These requirements are essential because textiles encounter various environmental conditions during their lifespan, including exposure to moisture, detergents, and sunlight. Achieving this resistance involves anchoring dye molecules securely within the microstructure of the fiber through chemical and physical interactions, such as covalent bonds, hydrogen bonding, ionic forces, or van der Waals forces.12

Ultimately, the dyer must select a dye that can withstand the specific conditions the finished article will encounter. Because higher standards of fastness often increase production costs, the final choice of dye is frequently a compromise between the required performance standards and the overall cost of the dyeing process.23