Oxidative Degradation Mechanism and Antidegradation Strategy of Paper-Based Relics with Transition Metal Pigments: from the Cellobiose Model to Cellulose Paper

Paper-based cultural relics often experience various deterioration by acid hydrolysis, alkaline degradation, and oxidative degradation during their long-term preservation. Some pigments containing transition metal species would damage the paper substrates seriously due to their possible catalytic effects on the in situ formation of hydroxyl radical (center dot OH) and the simultaneous occurrence of Fenton-like oxidation. By using cellobiose (the smallest unit of cellulose macromolecule) as the model, the oxidative degradation mechanism of cellulose paper is explored in the Cu2+-H2O2 system. The center dot OH radical that originated from H2O2 will attack the alpha-H atom dominantly at the C1 position in the glucopyranose ring and promote the cleavage of the adjacent beta-1,4-glycosidic bond, thus leading to the cracking of cellobiose. In addition to Cu species, the transition metal pigments containing Fe, Cr, and Hg species also exhibit relatively high catalytic activity for the degradation of cellobiose or cellulose paper via the Fenton-like oxidation. To protect paper from being oxidatively deteriorated, a series of scavengers for center dot OH radicals are investigated since they will not react with cellulose paper directly and have no specificity to metal ions or pigments. Remarkably, 2,5-hexanediol (2,5-HDO) not only retards the cleavage of cellulose chains but also inhibits the generation of reducible carbonyl and carboxyl groups by effectively capturing center dot OH generated in the Fenton-like system. Such a good performance combining its low volatility, high water solubility, and quite low impact on the paper substrates makes 2,5-HDO a novel, safe, and sustainable antidegradation agent for the conservation of pigment-containing paper relics.