Special Properties of Paper Treated With Flourescent Whitening Agents and the Effects of Agent Application, 1996 Coating Conference Proceedings

Absorption and fluorescence are the key properties of fluorescent whitening agents (FWAs). We determined these two entities from the difference in the reflectance of untreated paper and paper treated with FWAs. The spectral distribution of the light so observed revealed a significant difference in the “yellowing” tendencies of different FWAs. This yellowing is a result of broadening of the absorption band, with its maximum at 360 nm, through increased FWA concentrations resulting in absorption far into the visible part of the spectrum i.e. up to 440 nm. Distyrylbiphenyl (DSBP), a new FWA for application on the paper surface, shows distinct advantages over conventional stilbenes in this respect. With DSBP the absorption remains constant at about 410 nm for comparable degrees of whiteness. We also correlated the maximum absorption with fluorescence. These correlations were established for application in the wet end, but particular attention was paid to the coating application which is the preferred method for using DSBP. Many variants were explored: LWC (light weight coated) or free base sheets, carbonate and china clay as pigments and differing amounts of co-binders. We were able to show that under ideal conditions, i.e. for unsized wet end applications, the correlation between absorption and fluorescence is quite independent of the kind of FWA chosen.