Recent Research and Prospect of Deacidifying Materials for Paper and Paper-based Cultural Relics

Paper and paper-based cultural relics are of great value as the main carriers for writing and preserving information. Acid-catalyzed hydrolysis is one of the most fatal reactions for paper degradation, and deacidification would effectively slow down the rate of its degradation and prolong the life time of paper. Therefore, the design and application of deacidifying materials with safety and effectiveness is the indispensable requirement for the protection of paper and paper-based cultural relics. Generally, alkaline materials, such as amino/amine compounds, magnesium/calcium hydroxides, oxides, carbonates are the mostly used deacidifying materials for their good paper compatibility, suitable alkalinity, low cost and low toxicity, which exhibit considerable paper deacidification performance. In this review, the recent research progresses of deacidifying materials on liquid phase deacidification process were reviewed, including the categories of deacidifying materials, preparation methods and the relationship between paper deacidification performance and structures of deacidifying materials, finally, the challenges and development of deacidifying materials were also discussed. For classification purpose according to chemical structure, deacidifying materials can be divided into four categories: ionic form, molecular form, micro/nano-scale form and composite form. The deacidifying materials with molecular form can significantly improve the mechanical properties of paper and show a certain deacidification effect, it is necessary to control the improvement of mechanical properties within reasonable ranges while considering the effective deacidification performance. Micro/nano-scale materials with large surface area, good penetration and high efficiency are the widely used deacidifying materials, which can achieve better deacidification effects at lower concentrations. The controlled alkalinity, high dispersion and suspension stability in solvents and uniform distribution in paper fibers of micro/nano-scale materials are highly demanded. Additionally, ionic form materials can interact with paper fibers more evenly through homogeneous deacidification process, but their alkaline reserve is relatively insufficient. And more insight is demanded into the co-existence stability of composite deacidifying materials and its contribution to paper protection.