Fluorescent Whitening Effect (High, Low, Metamerism, Variable)

Fluorescent whitening can be a critical attribute of any paper product that is likely to be in view of the customer or end-user. As noted elsewhere in this website, fluorescent whitening agents (FWAs or OBAs) work by absorbing ultraviolet light energy, dissipating some of the energy as heat, and then releasing visible light, mainly in the blue region of the spectrum. Often the simplest way to assess levels of whitening effect in paper is to measure the brightness with and without a UV filter in the path of the incident light.

If paper contains a degree of whitening effect that differs from that of the standard for the grade being produced, then one can expect color-matching problems. These problems can arise even if an instrumental measurement or a light-booth observation shows the paper to be an exact color match to the standard. The reason is that the customer may view the sample under different conditions of illumination. See metamerism for further discussion of this issue.

LOW WHITENING EFFECT is the easiest problem to deal with, since the answer may be to add a higher level of FWA. However, the whitening effect provided by this kind of chemical is adversely affected by anything else that absorbs ultraviolet light. This means that wet-end addition of FWA can be expected to be less effective in the presence of high levels or titanium dioxide or mechanical fiber (due to the lignin). In such cases it can make sense to add all or most of the FWA at a size press or to a coating formulation. Another factor that can hurt the performance of FWAs is high levels of highly cationic polymers. Such polymers can quench the whitening effect by interacting with the negatively charged FWA molecules. Finally, it is worth keeping in mind that increasing amounts of FWA tend to approach a level of diminishing returns - or they even can turn the paper green, depending on the purity, composition, and amount of the additive used.

TOO HIGH WHITENER LEVELS often result when the furnish contains either deinked office waste paper or paper machine broke. Depending on the customer needs, it probably will not be possible to fully compensate for the excess whitener effect just by addition of dye (see metamerism). Ways to get rid of unwanted whitener effect in pulp include (a) bleaching of the pulp, and (b) addition of highly charged cationic polymers to partially quench the effect.

VARIABLE WHITENER LEVELS can be expected if one is using either office waste fiber or broke from a variety of different white paper grades. If problems are severe or common, an online control system is worth considering. The amount of fresh FWA added to the system can be controlled to keep the whitener effect within certain specified limits. Alternatively, one can identify which of the incoming furnish streams has an excessively high whitener level and bleach that stock or meter it into the furnish gradually rather than intermittently.

References

Crouse, B. W., and Snow, G. H., "Fluorescent Whitening Agents in the Paper Industry," Tappi 64 (7): 87 (1981).

Muller, F., Loewe, H.-D., and Hunke, B., "Fluorescent Whiteners - New Discoveries Regarding their Properties and Behavior in Paper," Paper S. Africa 13 (2): 4 (1993).

Roltsch, C. C., "The Efficient Use of Fluorescent Whitening Agents in the Paper Industry," Proc. TAPPI 1987 Papermakers Conf., 87 (1987).

PLEASE NOTE: The information in this Guide is provided as a public service by Dr. Martin A. Hubbe of the Department of Wood and Paper Science at North Carolina State University (m_hubbe@ncsu.edu). 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. Go to top of this page.


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