Yellowing and brightness reversion of celluloses: CO or COOH, who is the culprit?

Yellowing and brightness reversion of celluloses, two easily recognizable aspects of the more general term “cellulose aging,” have always been linked to oxidation phenomena. In the case of real-world pulps, the underlying chemistry is, in principle, rather simple. There are only two relevant reactions in cellulose which—in a most simplifying way—can be conceived as a polyol with multiple primary and secondary hydroxyl groups: the oxidation of secondary alcohols (at C2 and C3) to keto groups and the oxidation of primary alcohols (C6) to aldehyde functions and further to carboxylic acids. The oxidation of the only reducing chain end of cellulose (a hemiacetal) to the carboxylic acid is of little importance but might become more significant upon cellulose hydrolysis, which generates the new reducing ends of the shortened cellulose chains. The factors that cause the yellowing and aging of pulp and pure cellulose have been well-studied. These include (naturally) oxidative conditions, such as contact with gases (e.g., O 2 , O 3 , and halogens), aggressive chemicals from the surroundings, light and irradiation, or increased temperatures. Commonly, the yellowing of celluloses is thought to be caused by three stress factors: chemical (oxidative and/or hydrolytic) stress, photostress (light and irradiation), and thermal stress (high temperatures). These factors cause the yellowing individually or in combination. As a consequence of these physical and chemical influences, molecular changes arise in the cellulose material, which are reflected by losses in molecular weight (and often crystallinity) (Wilson and Parks 1979 ; Ahn et al. 2013 ; Area and Cheradame 2011 ; Lewin 1965 ) and by a more or less pronounced discoloration, that is, the yellowing of the material, originating from the formation of chromophores (Rosenau et al. 2011 ). Considering the role of the pulp and paper industries as a mainstay in many economies worldwide, the importance of bleaching processes in the pulp and paper industries, and the customer’s notion that bright white materials are of high quality and purity, it is evident that yellowing and brightness reversion of celluloses have been a topic of great interest (Suess 2010 ).