Dyeing properties of anionic direct dyes.
The advantages of anionic direct dyes are, in that they are more economical and generally have better lightfastness compared to Basic Dyes and some Cationic Direct Dyes. The wet fastnesses are comparable.
The disadvantage of Anionic Direct dyes is their poor substantivity to pulp fibers especially containing unbleached pulp. As the pulp fiber surface in water takes an Anionic Charge, same as the Anionic charge on the Anionic Direct Dye and you get repulsion. However as the machine water contains large amounts of water hardening Ions and Alum you get a “Shelter” effect which enables the Anionic Direct Dye to attach to the fiber surface, as below. In contrast Cationic Direct Dyes have very strong Ionic Attraction to the fiber surface through Ionic Bonding and so very clean backwater.