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

CATIONIC DIRECT DYES

Composition: Some common features of cationic direct dyes are (a) extended conjugation, (b) planar molecular structure, and (c) an excess of positively charged over negatively charged surface groups. In the Color Index scheme, cationic direct dyes are classified along with Basic Dyes. (Note: When papermakers refer to "basic dyes" they usually are referring to the products of somewhat lower molecular size that are commonly used in coloring mechanical fibers and other high-yield fibers. Such molecules are not considered to be "direct," since they don't have strong affinity to bleached kraft fibers.) The word "conjugation" implies an alternating series of double and single bonds, e.g. -CH=CH-CH=CH-CH=. In a direct dye molecule the chain of conjugation almost always contains two or more aromatic groups. Diazo groups, -N=N-, are also present in many dyes. The function of the extended conjugation is to achieve a suitably low energy for the promotion of the outermost electrons between the bonding and antibonding states, p to p*. Depending on its wavelength (or color), the energy supplied by a photon of visible light when it is absorbed may be able to promote an electron from the bonding to the antibonding state in the dye molecule. The color also can depend on substituent groups on the molecule that tend to supply or withdraw electron density. For this reason, ionic groups on a dye molecule can have a big effect on its color, and shifts in electrolyte concentrations can affect the hue. The word "direct" implies that the dye has a sufficient molecular size so that it has a strong preference to remain on surfaces. In other words, the user can add it "directly," without a fixative or mordant, and it will stick to the surface. Cationic direct dyes have particularly high affinity for anionic surfaces due to their content of cationic amine groups.

Function: Coloration of paper, adjustment of the tint of white paper, and matching of a paper's appearance to a standard agreed upon by the customer. Cationic direct dyes can be very valuable in cases where (a) the effluent water has to be colorless, or (b) where a high frequency of production changes between different colored products provides an incentive for keeping the white water free of residual dye.

Strategies for Use: The most notable issue in the use of cationic direct dyes is their tendency to "granite." In other words, they tend to irreversible stain the first surfaces with which they come into contact. This can lead to a speckled appearance of the paper product. To avoid this, users must use adequate dilution and add the dye at a point of good agitation. Cationic direct dye never should be added to white water, since it will stain the unretained fines. Likewise, it should never be added in a chest, since it will stain the fibers that happen to be near to the point of addition. The second critical rule is to avoid direct contact between a cationic direct dye solution and streams of highly anionic additives. For example, a mixture of cationic direct dye and conventional (anionic) direct dye produces an insoluble tar. All direct dyes usually are delivered to a paper mill in the form of concentrates. These may be batch-diluted, though it is becoming more common to meter the concentrated dyes continuously with gear pumps or reciprocating positive displacement pumps. Rather than adding the concentrate directly to the process, the stream is combined with a relatively large amount of pure water (no fibers or fines). The dilution ratio is not critical. When producing deep shades of paper (highly colored) it is common to use a combination of cationic direct and (anionic) direct dyes, added at different points in the process. In such a situation the cationic dye can serve as a fixative for the anionic dyes, usually added later.

Cautions: See MSDS. Cationic direct dyes can cause objectionable and persistent stains of clothing (both yours and the paper machine's).

Molecular structure of one cationic direct dye example   Molecular structure of a typical cationic direct dye

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 .