Novel Adhesive Film for Glyoxal-Dehydrated Lacquerware: Composite Modification of Natural Lacquer with Soy Protein Isolate and Nano-SiO2

A novel composite adhesive for lacquer film restoration was developed by modifying natural lacquer with Tween-20, soy protein isolate (SPI), and nano-SiO2 to address the bonding failure and interfacial instability of glyoxal-dehydrated lacquerware. The optimal formulation (70% lacquer, 10% Tween-20, 15% SPI, 5% nano-SiO2) achieved a shear bond strength of 3.8 ± 0.3 MPa, corresponding to a 58% increase compared with pure lacquer (2.4 ± 0.2 MPa). After 30 days of immersion in a pH 4.0 acidic solution, the adhesive retained 91 ± 3% of its initial shear strength, significantly higher than that of pure lacquer (65 ± 5%). Under accelerated aging conditions (50 °C and 95% relative humidity), the composite adhesive exhibited minimal weight gain (1.0 ± 0.2%) and no visible mold growth, whereas pure lacquer showed greater moisture uptake (3.0 ± 0.4%) accompanied by evident fungal colonization. The cured film displayed good color compatibility (ΔE ≈ 2.0) and improved flexibility (elongation at break: 12.5% vs. 4.2%). XPS and FTIR analyses suggested enhanced interfacial bonding through hydrogen-bond interactions and possible Si–O–C linkages at the wood–lacquer interface. Practical restoration of a Warring States period lacquer ear cup (China) demonstrated effective and stable reattachment of detached fragments with satisfactory visual integration and long-term durability. Overall, this work provides a compatible and durable material strategy for the conservation of glyoxal-dehydrated lacquerware.