Mini-Encyclopedia of Papermaking Wet-End
Additives and Ingredients, their Composition, Functions, Strategies for Use
Composition: Dyes, as used by papermakers, are grouped into five main categories. Two of these categories are so important that they have separate listings in this document. Direct dyes are by far the most widely used colorants for paper. They are noted for having a highly conjugated, planar structure, and an anionic charge due to the presence of sulfonate groups. The word "direct" implies that the molecule has enough tendency to stick to surfaces that no fixative is required. This is true for many, but not all dyes classified as "direct." Cationic direct dyes are similar, but they have amine groups, providing a net positive charge. The combination of positive charge and relatively large, planar shape means that cationic direct dyes stick like crazy. Dyes classed as basic dyes also have a positive charge, but the molecules are not a large. They are used in highly anionic furnishes such as those prepared from mechanical pulp. Acid dyes, which have anionic charge and relative small size, are no longer very important in papermaking, due to their relatively poor affinity to fibers. Finally, insoluble colored pigments can be used in circumstances where high light-fastness is required. These have to be retained in the sheet with the help of a retention aid program, just like filler particles.
Function: Coloration of paper, shade control, white tinting.
Strategies for Use: Step one is to select suitable dyes for the grade and shade to be produced. The decision involves a process of elimination. Let's assume that the goal is to match an existing green product for which the standard has been approved by a customer. Simple color-matching is easy - if all we are trying to do is to match the color under one standard condition of illumination. But real-life customers want more; they want the color to match the standard under all types of illuminant, everything from incandescent, to fluorescent, to sunlight. Experience has shown that such complete color matching can be achieved only if the hue of each major dye in the formulation matches the hue of a dye that was used in production of the standard. The field of worthy candidate dyes then can be narrowed by choosing to focus on the class or classes of dye (see above) most suited for the type of furnish. Some of the remaining dyes in the group of candidates may have to be rejected at this point because of grade-specific or process-specific requirements for bleed-fastness, light-fastness, bleachability, or affinity for different components in the furnish. Some dyes become known as "top-sided" dyes due to their tendency to adhere to filler and fiber fines - materials that tend to be enriched near the upper surface of a Fourdrinier sheet. By contrast, "bottom-sided" dyes achieve a greater depth of shade on the fiber-rich wire-side of a Fourdrinier sheet. Regardless of dye type, it is highly recommended to dilute it well.
Cautions: Dyes can stain hands and clothing.
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, firstname.lastname@example.org .