Study on the pre-consolidation mechanism of barium phosphotungstate before mounting-repairing of degraded paper historical relics

The restoration of deteriorated paper-based cultural relics often requires water-soluble binders, but water-induced swelling can disintegrate fibers, hindering restoration and long-term preservation. Heteropoly acids can pre-strengthen such before restoration to mitigate this risk. This study reported that in situ formation of barium phosphotungstate (BPW) can suppress fiber disintegration in water-saturated deteriorated paper and investigates the mechanism. Using carboxylate nanocellulose (CNF) as a model, interactions among CNF and phosphotungstic acid (PTA) and barium hydroxide (Ba(OH)2) were analyzed. Results show PTA crosslinks CNF via hydrogen bonds between its O = W and CNF’s C-OH and COOH. Subsequently, Ba(OH)₂ reacts with PTA at the cross-linking sites to form BPW. The BPW precipitation reduces water associated with the fibers, promoting cellulose aggregation and densification. In situ BPW formation fills capillary pores and inhibits swelling by binding to fibers, thereby significantly enhancing the wet strength of the deteriorated paper.