Classification of dyeing and selection of dyes.
The dyeing process can be divided into wet-end dyeing and surface dyeing. Normally dyes used in wet-end dyeing are Anionic Direct Dyes(ADD), Cationic Direct Dyes(CDD) and Basic dyes. In Size Press and surface dyeing Cationic Direct Dyes and Basic Dyes-Anionic Direct dyes are normally not used as get tailing.
Types of paper dyes.Direct dyes (divided to anionic direct dyes and cationic direct dyes), basic dyes and pigments.
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.
Dyeing properties of cationic direct dyes.
The advantages of cationic direct dyes is their strong Cationic Charge enabling them to bond Ionically to Pulp Fibers especially Unbleached Pulp and Recycled Pulp which results in strong bright dyeings with little color in backwater.
Dyeing properties of basic dyes.
The advantages of basic dyes are: Bright Shades and Low cost BUT very poor fastness and bleachability.
Pigment application in papermaking industry.
Pigments can be used in the wet-end or coating especially for shading P&W and Coated Papers. Poor substantivity so very rarely used for dyeing papers.
Are liquid dyes made from powder dyes mixed with water?
No, they aren’t. Liquid dyes are produced from direct synthesis during production, whereas Powder dyes are salted out from this reaction mass and have to be dried.
Why liquid dyes?
1) Stable strong solutions.
2) Easily metered with pumps into the pulp stream enabling Closed Loop Color Control.