Deacidification of paper-based relics by calcium hydroxide nanoparticle suspension

Abstract Historic paper relics are crucial for preserving cultural heritage, but acid-catalyzed cellulose hydrolysis causes severe deterioration. Traditional alkaline deacidification struggles with long-term protection, especially for highly acidified papers. This study developed stable Ca(OH) 2 nanoparticles suspensions (hydrodynamic diameter 100–200 nm, polydispersity index 0.17) by reacting calcium methoxide with water in 95 % ethanol under ultrasonic treatment. The acquired nanoparticle suspensions remained stable for 70 days. When applied to deacidified acidic paper, it raised the pH from 4.2 to 9.0, and the pH maintained at ∼7.3 after one-week artificial aging under the constant conditions (80 °C, and 65 % relative humidity). X-ray diffraction revealed the transformation of calcium compounds during deacidification, while SEM-EDS/AFM confirmed uniform calcium distribution forming protective surface layers. Cross-sectional analysis via SEM showed nanoparticles filling paper microfiber gaps without altering structural topography. This nanoparticle-based approach effectively neutralizes acidity while preserving paper integrity, offering a promising solution for long-term conservation of historical documents.