Formation of brown lines in paper: characterization of cellulose degradation at the wet-dry interface.

Brown lines were generated at the wet-dry interface on Whatman paper No. 1 by suspending the sheet vertically in deionized water. Formic acid and acetic acid were quantified in three areas of the paper defined by the wet-dry boundary (above, below, and at the tideline) using capillary zone electrophoresis with indirect UV detection. Their concentration increased upon accelerated aging of the paper and was highest in the tideline. The hydroperoxides have been quantified using reverse phase high performance liquid chromatography with UV detection based on the determination of triphenylphosphine oxide produced from the reaction with triphenylphosphine, and their highest concentration was found in the tideline as well. For the first time, it was shown that various types of hydroperoxides were present, water-soluble and non-water-soluble, most probably in part hydroperoxide functionalized cellulose. After accelerated aging, a significant increase in hydroperoxide concentration was found in all the paper areas. The molar masses of cellulose determined using size-exclusion chromatography with multiangle light scattering detection showed that, upon aging, cellulose degraded significantly more in the tideline area than in the other areas of the paper. The area below the tideline was more degraded than the area above. A kinetic study of the degradation of cellulose allowed determining the constants for glycosidic bond breaking in each of the areas of the paper.