Chitosan-zinc oxide (CS-ZnO) ambient-temperature anchored coating enhancing lightfastness of natural dyeing wood

Objective Natural plant dyes have gained attention in wood coloration due to their environmental benefits; however, their poor lightfastness limits outdoor applications. While traditional incorporation of nano-ZnO can enhance light aging resistance, issues such as particle aggregation and weak bonding at the wood interface often lead to failure. This study focuses on the approach of acid-base stepwise control at room temperature and pressure to develop a chitosan-ZnO (CS-ZnO) composite coating, aiming to address the dual technical challenges of poor dispersion stability of nanoparticles and weak adhesion of the coating to wood, providing an effective and durable strategy for light protection of naturally dyed wood.

Method We employed a three-step pH control process of alkaline-acid-alkaline to indigenously extend CS molecular chains at room temperature and coordinate them with Zn²⁺, forming a uniformly dispersed cylindrical ZnO network. Optimization through coupling precursor concentration and reaction temperature facilitated the chemical bonding and physical anchoring of CS-ZnO on the surface of gardenia yellow dyed poplar. We utilized FTIR, XPS, and XRD to analyze the coordination structure, while SEM-EDS assessed the coating morphology and elemental distribution. The change in surface wettability was evaluated through contact angle measurements, and accelerated aging tests were conducted under conditions of 50 °C, 55% relative humidity, and an irradiation intensity of 550 W/m² for 50 h, with a focus on color difference evolution.

Result CS served as a molecular bridge, securely anchoring cylindrical ZnO on the wood surface, resulting in a continuous, dense coating without aggregation. After 50 h of aging, the ΔE of treated wood decreased by 89.76% compared with the untreated dyed wood, with shifts in a^* (green-red axis) and b^* (blue-yellow axis) dropping by 81.31% and 91.02%, respectively, significantly suppressing surface darkening, greening, and bluing. Mechanistically, the tubular ZnO array extended the path for photothermal diffusion and enhanced UV reflection, while the CS-Zn coordination network inhibited free radical generation, together creating a dual light resistance effect through physical shielding and chemical stability.

Conclusion The stepwise control strategy of acid and base allows for the construction of stable and robust CS-ZnO coatings at room temperature and pressure, overcoming the technical challenges of nanoparticle aggregation and weak bonding. This significantly enhances the lightfastness of gardenia yellow dyed poplar, providing a feasible and eco-friendly technical route for the large-scale application of natural plant dyes in high-value wooden products.