Degradation of Chinese handmade papers with different fiber raw materials on molecular and supramolecular structures

As the carriers of paper-based cultural relics, ancient handmade papers have greatly promoted the development of human culture, knowledge and civilization. Chinese papermakers wisely selected a broad variety of plants to make papers with better properties and longer lifetime. However, the correlation between the types of fiber raw materials and the durability of handmade papers is still unclear. Herein, six handmade papers classified as bark paper, bamboo paper and grass paper were selected to study their aging behavior and degradation mechanism. Optical microscope and fiber quality analyzer were used to characterize the fiber morphology and length distribution of papers. The degradation kinetics of six handmade papers were systematically studied under accelerated aging condition. On molecular scale, the degradation rate, variation of pH, oxidation degree, chromaticity, and content of reducing carbonyl groups were discussed for bark, bamboo and grass papers. On supramolecular scale, the crystallinity, energy and distance of hydrogen bonds for paper cellulose were quantitatively calculated. For six handmade papers, most of the degradation occurs in the amorphous region of cellulose, leading to the cleavage of glycosidic bonds, the generation of oxidized groups, and the variation of H-bonds arrangement. Owing to the specific molecular and supramolecular structures of cellulose in bast fiber, bark papers have longer lifetime and better durability than bamboo and grass papers. The results of this work will improve the understanding of paper degradation mechanism at microscopic multiscale structures, and provide the scientific foundation for the preparation of handmade papers with better durability.