Theory of leather dye fixation

The dye woods, being closely related to the vegetable tannins, are similar to the vegetable tannins in this theory of fixation. The dyewoods are loss colloidal in nature and will penetrate more easily than the vegetable tanning but the same general principles are involved. Fixation is by hydrogen bonding and the lower the pH, the greater the fixation. Direct dyes are attracted to the leather fiber and will be absorbed on the surface of the leather by physical forces rather than by strong chemical and physical bonds. Direct dyes are not penetrating when used on chrome tanned leather; rather they will be absorbed on the surface. Acid dyes behave as weak acid, being absorbed by the hide the fixation and penetration of an acid dye follow the same general laws that apply in the acid syntans. The acid dyes are attracted to the leather through the positively charged groups of the hide; therefore the acid group will be attracted to the amino group on the hide and will be fixed by hydrogen bonding. At low pH the acid dyes will be fixed more readily than at high pH, but they will penetrate deeper into the skin as the pH values increases. Since dyestuffs vary in molecular size, in their degree of solubility and in their acid –base characteristic, fixation of the dye by the leather will depend on all these factors under given conditions the dye will vary in these characteristic from one dye to another.
The base dyes are attracted by negatively charged groups on the leather under acid condition leather, like untanned protein will absorb hydrogen ions and assume a positive charge the basic dye also positively charged will have little affinity for the leather. As the pH is raised the leather become more negatively charged and fixation of the basic dye is acided in addition to the pH factor in the fixation of dyes; the presence of the other materials on the leather fiber is important to the behavior of the dye in relation to the leather chrome tanned leather carries an additional positive charge due to the presence of cation reaction with the hide. The acid dyes, therefore will be more strongly attracted to chrome leather and will fix directly to it. Basic dyes on the other hand have little affinity for chrome leather. In order to fix a basic dye onto chrome leather this is done by mordanting chrome tanned leather by adding a vegetable tanning material or an anionic syntan. The basic dye than will very strongly attracted to the “mordanted” fiber and stable fixation will take place