The chemical principles of ancient papermaking are mainly reflected in a series of chemical changes that occur during the production processes, including raw material processing, cooking, pulping, papermaking, and drying.
In the raw material processing stage, papermaking raw materials are mostly plant fibers, such as hemp, bark, and bamboo. The raw materials are soaked in alkaline lime water, which reacts chemically with impurities in the plant fibers. Lignin, contained in the plant fibers, undergoes hydrolysis under the influence of lime water, its structure is destroyed, and it separates from the fiber. Simultaneously, lime water also decomposes pectin, removing impurities from the fiber and making it purer.
In the cooking stage, the soaked raw materials are placed in a pot for cooking. Under high temperature and alkaline conditions, the chemical composition of the fibers further changes. Cellulose undergoes partial degradation, its molecular structure changes, its degree of polymerization decreases, and the molecular chains shorten. This improves the fiber's flexibility and plasticity, creating conditions for subsequent papermaking.
The pulping process aims to further refine and disperse the fibers, a process that triggers chemical reactions. The fibers are subjected to mechanical forces, causing changes in the surface functional groups. For example, hydroxyl groups on the fiber surface undergo a slow oxidation reaction with oxygen in the air, generating new oxygen-containing functional groups. These new functional groups increase the hydrogen bonding force between fibers, giving the paper better strength and toughness.
In the papermaking process, pulp is spread on a bamboo screen or mold, and the water is drained to form paper. As the water evaporates, the distance between the fibers decreases, and the interaction forces increase. Simultaneously, small molecules on the fiber surface concentrate with the evaporation of water, forming chemical bonds between the fibers and enhancing the structural stability of the paper.
