Direct dyes are the workhorses of paper colouring: anionic, water-soluble and with good affinity for cellulose.
Where to dose
Dyes are added to the thick stock or machine chest so they have time to fix before the headbox. Adding the dye before fillers and retention aids usually gives better uptake and clearer back-water.
Why liquid dyes
Liquid dyes remove dust, dissolve instantly and can be pumped and metered automatically, which keeps shade steady across reels.
Fastness requirements
- Tissue and napkins need good bleed fastness to water and milk
- Food-contact papers are tested for bleeding by EN 646
- Packaging grades may need light fastness for shelf life
Shade control
Monitor colour on the reel with a spectrophotometer and correct with small additions of shading dyes rather than large swings of the main colour.
