Development of Mg-Al LDH and LDO as novel protective materials for deacidification of paper-based relics

Acidification of paper-based relics is a common problem, leading to their degradation and eventual loss. Paper deacidification is highly dependent on a limited variety of alkaline materials, and the development of new materials that are safe, efficient and easy-to-prepare is highly demanded to ensure a high level of safety and effective protection of paper-based relic. This study proposes the introduction of layered double hydroxide (LDH) and its calcined product, mixed metal oxide (layered double oxide (LDO)), as innovative protective materials for the deacidification of paper with varying levels of acidity. The results demonstrate that treatment with Mg-Al LDH/LDO can effectively modify the pH of acidic paper (e.g., pH ∼ 4.0–6.4) to a neutral or weakly basic state, maintaining this desirable pH range even under long-term accelerated aging condition. Remarkably, LDH proves to be well-suited for the protection of slightly acidified paper (e.g., pH > 5.5), while LDO serves as an especially option for the deacidification of severely acidified paper (e.g., pH ≤ 5.5). During aqueous deacidification, due to the memory effect of the LDH-based materials, LDO is converted to rehydrated LDH, which creates a mild and appropriate alkaline retention in the paper, avoiding damage caused by strong alkalinity such as cellulose degradation and pigment fading during subsequent long-term natural preservation of the paper. Furthermore, Mg-Al LDH/LDO materials also exhibit flame-retardant and bacteriostatic properties. This opens up opportunities for the safe, efficient and multifunctional protection of acidified paper-based relics.