Mini-Encyclopedia of Papermaking Wet-End
Additives and Ingredients, their Composition, Functions, Strategies for Use
FLUORESCENT WHITENING AGENTS (FWAs)
Composition: Members of this class of diaminostilbene sulfonate derivatives are highly conjugated molecules having planar structure and anionic charge. All of these adjectives also apply to direct dyes. What makes this particular class of direct dyes different is that they absorb ultraviolet light and re-emit light in the blue region of the visible spectrum. Another traditional term for fluorescent whitening agents (FWAs) is "optical brightening agents" (OBAs). Three general types of FWAs are widely available. The type most often used by papermakers has four sulfonate groups (tetrasulfonated). It has intermediate solubility in water and it is readily retained on fibers, especially if alum or another cationic material is present. Hexasulfonated FWAs don't retain as well when added at the wet end, but they may give higher optical efficiency when used at the size press due to less association between the molecules in the dried starch film. Disubstituted FWAs are sometimes used for specialty purposes, for instance when water bleed-fastness is critical.
Function: Increasing the white appearance of papers by absorbing invisible ultraviolet light and re-emitting it in the blue region of the visible spectrum. This strategy can compensate for a yellow tint of many types of pulps that have been bleached to moderate levels.
Strategies for Use: Some of the critical factors in FWA use include (a) retention, (b) quenching, (c) competition with other UV-absorbers, and (d) metamerism. The strategies for adding and retaining FWAs to the wet end of a paper machine are very similar to those used with direct dyes. For instance, FWA retention can be increased by sequential addition of alum to the pulp stream, either before or after the whitener. But there is a key difference; highly charged cationic polyelectrolytes easily can destroy the fluorescent character of the molecule. The effect is called "quenching." (The effect is related, but much more problematic, than the tendency of the highly cationic polymers to modify the hue of certain direct dyes.) Both lignin and titanium dioxide are potent absorbers of ultraviolet light. For this reason, internal addition of FWAs to high-yield furnish or to furnish that contains TiO2 is likely to be ineffective. This is one of the reasons why it is common for papermakers to add all or most of the FWA at the size press. The idea is that light first encounters material nearer to the surface of the paper, and this is where much of the size-press formulation ends up. Metamerism refers to the phenomenon that two objects may appear to have identical color when viewed under a certain type of illumination, but the same objects might not match under a different lighting. This is a very common occurrence in the case of samples that contain different levels of activity of fluorescent whitening agents.
Cautions: The material is not known to be particularly hazardous. It may sometimes become a target of concern for health because of the fact that its presence is less obvious than is true for "normal" dyes that absorb visible light.
|Light absorption and emission of fluorescent whitening agent as a function of wavelength|
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, email@example.com .