Dyes

Lignins are primarily derived from trees--a renewable forest resource. Lignin is the tree's natural binding agent and is released during the production of pulp. Lignosulfonates, or sulfite lignins, are hydrophilic and are products of sulfite pulping. Kraft or sulfate lignins are hydrophobic and are obtained from the kraft pulping process.

Since the early 1900's, the textile dyeing industry has known the dispersant properties of lignin. Since many dyeing processes involve the application of insoluble dyes in a dispersed form, lignsulfonates are needed to keep the dye particles in a finely divided state by preventing agglomeration. These are usually disperse dyes which are applied to polyester and acrylic fibers or vat dyes which are particularly suited for use on cotton and rayon.

Early requirements for lignin were not stringent and only simple modifications of the spent sulfite liquors were needed to manufacture acceptable dispersants. The advent of modern technology brought the development of synthetic fibers and dyes which required high temperatures during high speed dyeing operations. Various lignins have evolved and have been modified for high performance. Today not only do these products provide a strong dispersing effect but also good solubility and high temperature stability of the dispersion throughout the dyeing process.

The economical significance of dispersing agents to the textile industry is clear when we consider that 20-60% of every disperse and vat dye consists of the dispersing agent. Worldwide, 25-50 thousand tons of lignin dispersants are used each year by the dye industry.

Lignin contains two functional groups, adsorbant and hydrophilic. Each group performs a different function. The adsorbent groups attach to the dye crystals during milling operations and the hydrophilic groups interact with water molecules to form a double layer that prevents dye particles from reaglomerating.

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