Influencing factors for dyeing with direct dyes Direct dyes have a high affinity for cellulose fibers and can be dyed in neutral or weak alkaline media. Dyeing protein fibers is generally carried out in a neutral or weak acid medium.
Direct dyes are very sensitive to acid, adding acid can make it precipitate. After direct dyeing, if it is treated with fixing agent, organic acid, etc., the fastness can be improved.

The dye is absorbed by the fiber to the surface in the solution, and then continuously diffuses to the amorphous area of the fiber, forming a hydrogen bond and a combination of van der Waals force with the fiber macromolecule. Its derived dyes include direct lightfast dyes and direct copper salt dyes.
Influencing factors of direct dyeing (1) The influence of temperature on direct dye dyeing: natural pigments are greatly affected by temperature. For example, beet red is unstable above 60℃, so dyeing should not be carried out at high temperature. Factors affecting dyeing with environmentally friendly direct dyes
(2) The influence of the pH value of the dyeing bath: Only by controlling the dyeing bath to dye under proper pH conditions can a higher equilibrium adsorption capacity be obtained. For example, Betarubin is unstable under alkaline conditions, so dyeing should be carried out under acidic conditions. One COO- in the pigment molecule can be combined with -H3N+ on the wool as a salt bond. The lower the pH of the dye bath, the more positive charges the wool has, the faster the dyeing rate and the higher the dye exhaustion rate.
(3) The role of dispersant and stabilizer: some natural pigments are unstable and easy to change color, and stabilizers need to be added. Most natural dyes have relatively large relative molecular weights, which tend to reunite and hinder dyeing.
However, anionic or non-ionic surfactants can be used to disperse the dye particles in the dye liquor to form a more stable dispersion system, which will increase the chance of contact between the fabric and the dye and increase the dyeing speed. The lower the water solubility of the dye, the more obvious the effect of dyeing with dispersant.