This is the Conclusions section of the paper "Heat-Induced Yellowing of TCF-Bleached Sulphite Pulps--Mechanistic Aspects and Factors that Influence the Process," originally published in the Journal of Pulp and Paper Science, 25(2):47-51, Feb. 1999. It is reproduced here with permission, as an example of the research on "color reversion" or darkening of paper with age or as a result of specific chemical interactions.
TCF means "totally chlorine-free." Sulphite pulp mills are common in Europe, but not in the U.S. The trend away from use of chlorine in bleaching of pulps has been fueled by serious environmental concerns.
The heat-induced yellowing of TCF bleached pulps is an extraordinarily complex process. A high number of interacting factors influence the brightness stability of the materials. The yellowing process proceeds predominantly in the carbohydrate part of the pulps. Oxidation processes seem to play a role in the formation of chromophores only in cases of relatively high lignin content. Except for the temperature, the most important factors are humidity and intrinsic pH of the pulps, indicating that the hydrolytic processes are mainly responsible for discoloration. The yellowing tendency of the pulp is stronger the higher the oxidation state of the carbohydrate matrix. During heating, the carbohydrates undergo reactions which lead to the formation of furan-type heteroaromatics. The formation of furan derivatives indicates that hemicelluloses may also be involved in these processes.
A weak influence of the carbonyl content on the brightness stability was found. Ketone and aldehyde groups could be analytically separated. Carboxyl groups act as proton donors which catalyze hydrolytic reactions and the dehydration of the carbohydrates, thus accelerating the formation of chromophores.
Heavy-metal ions seem to act in the same manner as carboxylic protons. They can be masked by complexing agents. In this case, yellowing is less marked.
These facts are like pieces of a puzzle ready to be assembled into a more complete picture of the yellowing process. Although the results of the experiments shown here support the mechanism proposed in this scheme, some of the pathways remain hypothetical. However, they seem quite plausible, and nevertheless, might serve as a starting point for further investigation.