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

Abstract

Literature is strongly contradictory about the molecular reasons for yellowing and brightness reversion of pure (lignin-and hemicellulose-free) celluloses, such as in highly bleached pulps, bacterial cellulose, or cotton linters. While oxidized groupscarbonyls (CO) and carboxyls (COOH)-have been recognized as the initiators of yellowing, they are generally always found together; thus, their effects are permanently superimposed in real-world cellulose. For this reason, their individual contributions could not be reliably determined. To tackle this conundrum, we have used a two-stage study: the employment of glucopyranose-derived model compounds and the use of special cellulosic pulps. Both substrates had either only carbonyl functions, only carboxyl functions, or defined ratios of both functionalities present at the same time. The model compounds alone already provided strong indications of the CO-related and COOH-related effects, and further confirmation was obtained by the pulp study. Here, in regard to the polymer case, the carbonyl groups are the minimum functional unit in cellulose responsible for chromophore generation (termed as the ''CO effect''). The carbonyl groups are the precursors for the chromophores that are formed later upon yellowing/ aging. Chromophore formation increases strictly linearly with the carbonyl content at a constant given carboxyl content. Carboxyl groups alone (i.e., in the absence of carbonyl groups) are fully innocent regarding the color generation. However, they have a

Insights into modification of lignocellulosic fillers with isophorone diisocyanate: structure, thermal stability and volatile organic compounds emission assessment

“…Such a phenomenon is commonly known and widely described in the literature by various researchers (Ahmed et al 2014(Ahmed et al , 2015(Ahmed et al , 2016. Rise of b* parameter is related to the yellowing of cellulose, as a result of mechanical and chemical treatment (Ahn et al 2019). However, overall, it can be seen that the performed modifications of fillers did not cause significant differences in their color, which was expressed by relatively low values of ΔE* parameter.…”

Gellan Gum Microgels as Effective Agents for a Rapid Cleaning of Paper

“…This trend is common for ancient paper, being due to oxidation products such as carbonyls in the glucosidic rings of cellulose. 52 For Office paper, other small spectroscopic features due to lignin (at about 285 nm) and optical brighteners (at about 375 and 400 nm) are superimposed to those due to cellulose carbonyls groups. 53 Indeed, it has been demonstrated that these groups are UV–vis active (chromophores) and therefore responsible for the changes of the optical properties of cellulose artifacts during aging, 27 , 40 , 41 , 54 in particular for the yellowing evident in paper aging and degradation.…”

Stability of Alum-Containing Paper under Alkaline Conditions

“…Regarding the deacidification and yellowing of the paper, at least two questions waiting to be answered should be mentioned. The first concerns yellowing, where the literature is still contradictory about the molecular reasons for yellowing and brightness reversion of pure (lignin- and hemicellulose-free) celluloses [ 24 , 25 ]. The second relates to the distribution of the deacidifying agent in the paper volume.…”