Mini-Encyclopedia of Papermaking Wet-End Chemistry
Additives and Ingredients, their Composition, Functions, Strategies for Use

DIRECT DYES

Composition: The "direct dye" classification in the Color Index system refers to various planar, highly conjugated molecular structures that also contain one or more anionic sulfonate group. It is because of these sulfonate groups that the molecules are soluble in water. Though most direct dyes still can be obtained in powder form, it is increasingly popular to receive them as liquid concentrates. The advantage of concentrates is that they are easy to handle and meter. The disadvantage is that the surfactants and co-solvents needed to keep the dye concentrates stable may interfere with retention and sizing in the case of very deeply colored grades.

Functions: Coloration of paper, tint and shade control, correction of two-sided paper color.

Strategies for Use: Where you add the dye depends on such factors as the tintural strength of the desired paper color, the dye fixation strategy, and the color control strategy. Those responsible for process control will always advocate adding either all or some of the dye to the thin stock, within seconds upstream of where the stock comes out of the headbox. This practice can eliminate most of the time delay that is always a headache for online control strategies. But thin-stock addition might not give as high retention as addition to the thick stock. In some cases, especially in the past, it has been common to produce very deeply dyed products by adding the dye in a chest, giving the dye a long time to penetrate into the fiber cell walls. Other ways to achieve high retention of dyes include the use of highly cationic polyelectrolytes, alum, or polyaluminum chloride. The best efficiency of the colorant usually is achieved by adding the fixative ahead of the dye. However, the reverse order may give better rub-fastness or water-fastness. Another promising strategy is to add some cationic direct dye at a separate addition point upstream of the addition point from the (anionic) direct dye.

Cautions: The main hazards in handling of dyes are aesthetic; it is easy to stain hands and clothing. Consult the MSDS for other hazards.

Direct dye example, Direct Red 81   Molecular structure of a typical direct dye (anionic)

PLEASE NOTE: Users of the information contained on these pages assume complete responsibility to make sure that their practices are safe and do not infringe upon an existing patent. There has been no attempt here to give full safety instructions or to make note of all relevant patents governing the use of additives. Please send corrections if you find errors or points that need better clarification.


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This page is maintained by Martin Hubbe, Associate Professor of Wood and Paper Science, NC State University, m_hubbe@ncsu.edu .