Nanoparticle-modified shellac for protection of wood surface

Shellac has been widely used as a natural protecting material for wooden furniture and musical instruments, due to its excellent film-forming behaviour and aestethical effect, ease of application, high adhesion to the wood surface and protecting properties along with its nonpoisonous nature [1]. However, its application displays some limitations, such as the softness of the coating, photodegradation, and sensitivity to to pH variations [2-3]. In the present study, dewaxed natural shellac was modified by introducing different inorganic nanoparticles. Silicates nanoparticles as well as zirconium oxide nanoparticles were added with the aim of improving the film hardness, while zinc oxide nanoparticles were expected to improve the decay resistance and antifungal performances of the shellac matrix. Different shellac-based materials containing different amount (1, 2, or 3%) of silica, montmorillonite, zinc oxide, or zirconium oxide were prepared by dispersing nanoparticles in ethanolic solutions of shellac. Materials were characterised by performing different experimental analyses both on modified shellac films and on maple wood specimens treated by brushing. Properties of shellac-based materials were investigated by performing hardness and adhesion tests, moisture adsorption mesurements, thermogravimetric analysis, differential scanning calorimetry, UV and solar test, contact angle measurements, evaluation of chromatic variation, solubility and alkali resistance tests. X-ray, FTIR and SEM-EDS analyses were also carried out on films and treated wood specimens. Quality and thickness of the coating are important parameters when products are developed for the real case applications. Both native and modified shellac treatments on wood specimens provided good quality coatings whose thickness were very close to real applications (about 25 μm). Hardness measurements performed by international standard test indicated that the presence of zirconium oxide, silica and montmorillonite nanoparticles in shellac improves the hardness of the resulting coating. In addition, SiO2-shellac applied to wood specimens afforded an improvement of hydrophobic properties, while montmorillonite-shellac increased the base resistance property with respect to the native shellac. ZnO-shellac treatment preserved almost unaltered the chromatic properties (L*, a* and b* coordinates of the CIELAB space) with respect to native shellac, even after the solar ageing (1000h). [1] J. Wang, L. Chen, Y. He, Progress in Organic Coatings (62), 2008, 307–312. [2] W. H. Gardner, W. F. Whitmore, Industrial and Engineering Chemistry (21), 1929, 226-229. [3] M. Licchelli, M. Malagodi, M. Somaini, M. Weththimuni, C. Zanchi, Surface Engineering (29), 2013, 121-127.