Abstract
In this study, the maple wood surface was coated with nanostructured zinc oxide, grown on the surface by using a hydrothermal process, and furtherly treated with shellac varnish. Samples obtained both after ZnO treatment and after the final varnish application were characterized by different techniques, i.e., X-ray diffraction (XRD), scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (SEM-EDS), micro-FTIR with attenuated total reflectance (μ-ATR-FTIR), chromatic variation measurements, and contact angle determinations. Analytical results showed that the wood surface was covered by quite a homogeneous array of inorganic nanoparticles and that the natural resin forms a regular film over the ZnO nanostructures. An accelerated aging test was used to evaluate the protecting effectiveness of the treatments towards UV-induced decay of wood material. After the test, wood treated with ZnO and with the shellac/ZnO combination underwent a considerably lower chromatic change if compared to the untreated wood, suggesting an enhanced resistance of the treated maple to the decay due to light exposition. The presence of nanostructured ZnO protects from decay not only the wood substrate but also the shellac film. A microbiology test showed that growth of fungal species, e.g., common mold, is prevented on the wood surface treated with ZnO or with shellac/ZnO, indicating that the nanostructured zinc oxide also provides an effective protection from biodeterioration. The coating obtained by consecutive application of nanosized ZnO particles and shellac varnish combines the excellent aesthetical features and water repellence of the traditional finish with the protecting effectiveness of the nanostructured inorganic component.
Highlights
Study of novel methods for the durability enhancement of wood artifacts currently represents an important research topic [1]
Samples obtained both after ZnO treatment and after the final varnish application were characterized by different techniques, i.e., X-ray diffraction (XRD), scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (SEM-energydispersive X-ray (EDS)), micro-FTIR with attenuated total reflectance (μ-ATR-FTIR), chromatic variation measurements, and contact angle determinations
Further studies will be needed in order to better clarify the antifungal mechanism of nanostructured ZnO, which is active against fungi even when covered by an organic coating film. These results indicate that consecutive application of nanostructured ZnO and shellac on wood provides a “combined” protecting effect from fungi proliferation: organic coating acts as a barrier that slows down the growth of mold by avoiding its contact with wood surface; zinc oxide nanoparticles behave as toxic agent against microbial cells [67] and prevent fungal growth even if shellac coating should fail
Summary
Study of novel methods for the durability enhancement of wood artifacts currently represents an important research topic [1]. Wood is an organic material that undergoes different degradation processes due to environmental agents (e.g., solar radiation, water, and biodeteriogens) [2]. Absorption of solar light, of the ultraviolet (UV) component, is mainly responsible for wood photodegradation induced by different photolytic and/or photooxidative reactions [3,4,5]. When exposed to solar light, wood undergoes a range of decay processes from the simple change of esthetic appearance (e.g., surface discoloration) to a severe decrease in the mechanical properties [5,6,7,8]. The presence of water can be merely related to shrinkage/swelling phenomena induced by absorption/desorption processes and favors hydrolytic reactions and the action of biodeteriogens (e.g., bacteria, fungi, and xilophages), when the moisture content of the wood exceeds the fiber saturation point
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