Effect of Moist and Dry Heat Weathering Conditions on Cellulose Degradation of Historical Manuscripts exposed to Accelerated Ageing: ^13C NMR and FTIR Spectroscopy as a non-Invasive Monitoring Approach

Over time, paper undergoes unavoidable ageing process that results mainly in the degradation of cellulose. Paper endurance depends on its intrinsic properties, which are related to the environmental conditions and the manufacturing process. This work focused on evaluating the state of conservation of three restored ancient Moroccan manuscript papers (16ᵗʰ, 19ᵗʰ and 20ᵗʰ centuries) before and after artificial ageing over a period of 28 days at 90 °C and under two weathering conditions (dry-heat, moist-heat). In order to correctly discriminate samples, to inform on the conservation state as a preliminary step for the conservation process, and to elucidate well the structural changes in the chemical structure of cellulose polymers, a combined analytical approach has been applied for characterization, using solid state ¹³C NMR-CP/MAS and FTIR spectroscopy. The analysis was performed to carefully study the behavior of papers under extreme conditions of storage and to monitor artificial ageing effects. The IR crystalline phase is more dominant than the amorphous one (1475-1300 cm⁻¹), confirmed by ¹³C NMR evaluation at C4 (88.7 ppm) and C6 (64.9 ppm). The phenomena of oxidation were shown after extended ageing 28 days in moist heat weathered conditions, and the cellulose alteration is more affected and pronounced in the 16ᵗʰ century samples. After ageing, the FTIR region (1800-1520 cm⁻¹) revealed the presence of oxidized groups corresponding to the C=O former. The cellulose degradation confirmed by the presence of intense and strong IR absorption bands of C=O at 1740 cm⁻¹ (free –COOH; –COOR of normal and/or cyclic ester group: σ-valerolactone). Morever, the appearance of new polar, strong and relative intense IR bands at 1575 and 1539 cm⁻¹ were attributed to C=O of carbonyls, originating from the contribution of oxidized low phenolic content in the lignin. The ¹³C NMR signals with a weak and broad hump between 176-167 ppm justified the occurred oxidation and confirmed the deterioration process and the IR purpose.