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 (
Study of novel methods for the durability enhancement of wood artifacts currently represents an important research topic [
Absorption of solar light, particularly of the ultraviolet (UV) component, is mainly responsible for wood photodegradation induced by different photolytic and/or photooxidative reactions [
Due to the general hydrophilic character of its main components (cellulose, hemicellulose, and lignin), wood can adsorb water both as vapour and as liquid, establishing an equilibrium with the air moisture. The presence of water can be merely related to shrinkage/swelling phenomena induced by absorption/desorption processes but also favors hydrolytic reactions and the action of biodeteriogens (e.g., bacteria, fungi, and xilophages), particularly when the moisture content of the wood exceeds the fiber saturation point.
Owing to the various natures of the more common decay processes affecting wood, the development of effective treatments for a whole preservation of wood artifacts still represents a challenge from a scientific and technological point of view.
Some important studies concerning the application of nanomaterials to the conservation of wood have been reported since the last decade [
In most cases, the nanoparticles enhance the performance of the protective treatments in terms of coated wood durability (resistance to photodegradation, fungal growth, staining, and discoloration) as well as of its mechanical properties (e.g., enhanced scratch resistance). [
Other studies deal with the deposition of nanometric inorganic particles on wood surfaces and show that unique properties (e.g., photostability, antifungal capability, and moisture protection) can be obtained by similar treatments [
ZnO is a semiconductor that has found wide application in the optics and electronics areas [
Several studies have also pointed out that nanostructured ZnO displays an effective antibacterial and antifungal activity that is strictly correlated with the small size of particles and the consequent large specific surface area [
Surface treatments of wooden objects (e.g., furniture, musical instruments) are often required to provide not only a general protection from environmental decay agents, but also a satisfactory aesthetical appearance.
The application of surface coatings still represents the most common practice for wood protection. Coatings for wood can be based on synthetic polymers (e.g., polyurethanes, polyesters, and polyacrylates) or on more traditional materials of natural origin (resins, oils, and waxes) [
Shellac is a resinous material of animal origin obtained from the secretions of Lac insects (e.g., Kerria lacca, Laccifer lacca), hosted as parasites by some trees growing in eastern Asia [
A few years ago, we studied the effect of ZnO nanoparticles dispersed in shellac coatings in order to improve the resistance of the coating (and of the treated wood) to the decay induced by exposition to light [
In this work, we have studied the protecting effectiveness of nanostructured ZnO when deposited on maple wood, in combination with shellac coatings. In particular, we investigated the resistance of treated wood specimens to mold growth, in comparison with untreated maple. The resistance to light-induced decay was tested by examining the surface of treated and untreated maple after accelerated ageing under UV illumination.
Analytical grade sodium hydroxide (NaOH), ethanol (95% EtOH), zinc acetate dihydrate (ZnC4H6O4·2H2O), zinc nitrate hexahydrate (Zn (NO3)2·6H2O), and hexamethylenetetramine were supplied by Sigma-Aldrich and were used as received.
Dewaxed natural shellac (food quality shellac) was purchased from Kremer Pigmente and used without any further purification. It was dissolved in ethanol solution in order to obtain a standard shellac solution (100 g/L).
Maple wood specimens (2 × 2 × 0.5 cm) were kindly provided by Civica Scuola di Liuteria (Milan, Italy). Prior to use, they were smoothed by abrasive paper sheets (progressively from 400 to 1000 mesh) similar to the real cases.
Nanostructured ZnO was “grown” on wood surface using a two-step process consisting of seed deposition and crystal growth, according to literature methods [
At first, standard solutions of NaOH (0.03 M) and of zinc acetate dihydrate (0.01 M) were prepared in ethanol. NaOH solution (133 mL) was slowly added to a Pyrex glass bottle containing zinc acetate solution (200 mL) at about 60°C, and the resulting mixture was stirred for 3 hours to form a transparent solution containing ZnO nanoparticles, which is stable at least for 20 days. Maple wood specimens were immersed in this solution at room temperature for one hour in order to obtain the deposition of ZnO (dip-coating process). Thereafter, the specimens were heated in an oven at 100°C for 3 hours. The above described process was repeated 10 times. After this treatment, specimens were then used for subsequent ZnO growth under hydrothermal conditions.
For this purpose, standard aqueous solutions of hexamethylenetetramine (HMTA, 0.02 M) and zinc nitrate hexahydrate (0.02 M) were prepared. The seed-coated wood specimens were immersed in an aqueous solution obtained by mixing an equal amount of the abovementioned standard solutions, and temperature was maintained at about 90°C (hydrothermal reaction) for 6 hours. The resulting wood specimens were properly rinsed with deionized water, and then they were dried in an oven at 60°C for 3 hours. Specimens obtained by this procedure were labeled as W+ZnO.
A group of wood specimen-coated nanostructured ZnO were treated with shellac solution (100 g/L in ethanol) by brushing and were labeled as W+ZnO+SH. Shellac was applied two times, according to perpendicular directions (the brush direction of second application was perpendicular to the first one).
Moreover, nine specimens of untreated wood were treated only with shellac in order to compare the performances with the combined treatment and were labeled as W+SH.
Optical microscope observations of wood specimens were performed by a light-polarized microscope Olympus BX51TF, equipped with visible (Olympus TH4-200) and UV (Olympus U-RFL-T) lamps or by a Dino-Lite AM 413TFVW-A digital microscope.
Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDS) analyses were performed at the Arvedi Laboratory, CISRiC-Università di Pavia, by using a Tescan FE-SEM (MIRA3 XMU series) and EDAX instruments. The SEM apparatus was equipped with a Schottky field emission source, operating in both low and high vacuum. Samples were previously gold-sputtered using a Cressington sputter coater 208HR.
X-ray powder diffraction (XRD) measurements were taken using a Bruker D5005 diffractometer with the CuK
Micro-FTIR spectra were measured at Arvedi Laboratory, CISRiC-Università di Pavia, by a Nicolet iN10 Thermo Fischer
Colour measurements were taken by using a Konica Minolta CM-2600d spectrophotometer:
Surface wettability was evaluated by water contact angle measurements (by a CAM 200 apparatus, KSV Instruments) in order to analyze the hydrophobic behavior of coated wood samples (W+SH, W+ZnO, and W+ZnO+SH), and results were compared with untreated wood.
Five measurements were taken on each specimen area for colorimetric and contact angle determinations, and all the given results are average values.
The examined wood samples (UW, W+SH, W+ZnO, and W+ZnO+SH) were artificially weathered by using an irradiation system (Helios Italquartz, Milan) equipped with a couple of mercury lamps (15 W) for 520 hours. During the irradiation, temperature and relative humidity in the chamber were controlled at 25°C and 30%, respectively. For all experiments, the specimens were removed from the chamber after 280 h and analyzed by measuring chromatic variations.
Antifungal performance of the surface treatments was evaluated by applying a procedure reported in literature [
The production of ZnO nanostructures (e.g., nanorods, nanobelts, and nanorings), by hydrothermal process on different substrates has been widely investigated [
In the present work, the growth of ZnO nanostructures on the wood surface was obtained by a two-step process, adapted from a well-known literature procedure [
In the second step, the hydrothermal growth of ZnO on the deposited seeds was carried out by immersion of maple specimens in aqueous solution containing zinc(II) nitrate and hexamethylenetetramine (HMTA). The experimental conditions were properly set in order to favor a slow and controlled growth of nano-objects, as described in literature [
Shellac was applied on the surface of wood specimens (both plain and ZnO-treated maple) by brush according to a conventional procedure [
Treated wood specimens were examined by different experimental techniques in order to investigate the properties of wood surface after the zinc oxide deposition (specimens W+ZnO) and after the combined treatment (W+ZnO+SH), as well as the composition of the resultant coatings. All the determinations were performed by properly comparing the results obtained from treated specimens with the untreated wood (UW) or with wood varnished with plain shellac (W+SH).
Table
Changes of chromatic properties determined for treated wood specimens after different treatments. Contact angle values for all the considered specimens are also reported. Standard deviation values are given in brackets.
Samples | |||||
---|---|---|---|---|---|
UW | — | — | — | — | 56 (±3) |
W+ZnO | -9.9 (±0.4) | 0.5 (±0.3) | 1.7 (±1.2) | 10 (±1) | 68 (±6) |
W+SH | -4.1 (±1.9) | 1.6 (±0.8) | 6.5 (±0.6) | 8 (±2) | 88 (±5) |
W+ZnO+SH | -11.6 (±1.6) | 0.9 (±0.2) | 1.8 (±1.3) | 12 (±1) | 95 (±4) |
All treatments induce a considerable variation of the chromatic properties of maple surface with
Shellac-based coatings are expected to provide a moderate protection from water and environmental humidity. The hydrophobic behavior of the investigated treatments was evaluated by carrying out static contact angle (
The X-ray diffraction patterns of the analyzed samples are shown in Figure
XRD patterns determined on untreated and coated wood specimens.
Surfaces of treated wood specimens were observed by scanning electron microscopy experiments in order to have a better insight on the microscopic properties (i.e., particle size, morphology, and homogeneity) of the coating layers, while information about their elemental composition was gained by taking energy-dispersive X-ray spectroscopy (EDS) measurements.
SEM micrographs taken on all the treated wood samples (W+ZnO, W+SH, and W+ZnO+SH) are reported in Figure
SEM-EDS experiments performed on treated and untreated wood surface: SEM micrographs of (a) untreated maple; (b) and (c) W+ZnO at different magnification; (d) EDS spectrum measured on the ZnO-coated wood surface; (e), (f), and (g) ZnO particles displaying different shapes and sizes at increasing magnification; (h) W+SH; (i) W+ZnO+SH.
The chemical properties of the wood surface after the different treatments were investigated by micro-ATR-FTIR analyses. Spectra obtained from measurements on all the considered specimens, including UW, are reported in Figure
Micro-ATR-FTIR spectra of untreated and treated wood specimens.
The FTIR spectrum of untreated maple displays the typical set of bands due to the absorption of both carbohydrate components (cellulose and hemicellulose) and lignin of wood [
The FTIR spectrum of W+SH exhibits the characteristic bands due to the natural resin (Figure
Peaks observed in the 1250-1000 cm-1 region can be mainly ascribed to different vibration modes of C-O and C-C bonds [
As the spectrum measured on the surface of W+ZnO+SH specimen displays the same absorption bands of the wood varnished with plain shellac (Figure
As mentioned above, exposition of wood to light, particularly to UV irradiation, may induce even drastic decomposition of its components (i.e., lignin and cellulose) as well as of possible coatings (e.g., coatings) that are applied to its surface. An accelerated ageing test was performed in order to assess the resistance of ZnO-treated specimens to the UV-induced degradation, with respect to the untreated wood and to the wood treated with plain shellac. Therefore, specimens labeled as UW, W+SH, W+ZnO, and W+ZnO+SH were irradiated with a UV lamp up to 520 h and their surface examined in the middle and at the end of the artificial weathering process. Taking into account that colour changes can be considered as direct and visible expression of decay occurred to wood components, chromatic coordinates were measured on the surface of the examined specimens in the course and at the end of the irradiation time.
Variations of
Variations of chromatic coordinates of different wood specimens after UV ageing. The values obtained by comparing
Samples | 280 h | 520 h | |||||
---|---|---|---|---|---|---|---|
UW | -3.9 (±2.1) | 1.4 (±0.9) | 7.9 (±0.3) | -6.8 (±2.4) | 3.5 (±0.8) | 12.6 (±0.3) | — |
W+ZnO | -0.4 (±0.2) | 0.4 (±0.1) | -0.4 (±0.8) | -0.6 (±0.4) | 0.9 (±0.3) | 0.5 (±0.1) | 70 (±5) |
W+SH | -5.3 (±1.4) | 4.2 (±1.1) | 7.9 (±2.1) | -6.0 (±2.6) | 5.7 (±1.6) | 11.8 (±1.6) | 74 (±2) |
W+ZnO+SH | -3.4 (±0.2) | 2.3 (±0.1) | 2.3 (±0.2) | -2.3 (±0.3) | 2.5 (±0.6) | 2.8 (±0.6) | 100 (±7) |
Overall chromatic variation of different wood specimens during and at the end of UV irradiation.
After artificial ageing, both untreated wood and wood treated with plain shellac underwent considerable chromatic changes (
Contact angles were also measured on the surface of wood specimens after UV aging (see Table
Among the fungi occurring on trees, wood decay fungi are considered an important source of damage to both living trees and wood artifacts. The growth of common fungal species, i.e., mold, on the surface of untreated and treated maple specimens was investigated in order to assess the possible contribution of nanostructured ZnO to increasing the wood resistance toward this biodegradation agent.
Mold growth was investigated by exposing water-saturated specimens (UW, W+ZnO, W+SH, and W+ZnO+SH) to controlled ambient conditions (constant relative humidity and temperature, alternate light and dark; see experimental section) and checking for the presence of mold on their surface for 40 days. Optical observations by a digital microscope were performed on a daily basis, while scanning electron microscopy experiments were carried out at the end of the experiment to better characterize the fungal microorganisms. Appearance of mold was observed on some areas of untreated maple and, to a lesser extent, of W+SH after just two days (Figure
Digital microscope images of untreated and coated wood during the fungi test.
SEM micrographs taken on the surface of the investigated specimens after 40 days are reported in Figure
SEM micrographs of wood specimens at the end of the fungi test (after 40 days) with two different magnifications: (a) and (b) UW; (c) and (d) W+ZnO; (e) and (f) W+SH; (g) and (h) W+ZnO+SH.
At any rate, 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 [
In this work, the protective effects induced by the consecutive applications of nanostructured ZnO and shellac varnish on the surface of maple wood have been investigated. A quite homogeneous layer of nanosized crystalline ZnO can be produced on the wood surface by a two-step hydrothermal procedure. This nanostructured material relevantly increases the resistance of maple wood to UV-induced ageing, as expected on the basis of previous literature works. An enhancement of the resistance to the growth of common fungal agents, i.e. mold, is also provided by this treatment.
The increased protecting ability from photodegradation and fungi biodeterioration exhibited by nanostructured ZnO is preserved after covering it with shellac coating, which is largely used as a finish for wooden artifacts.
A previous study [
In conclusion, the consecutive application of nanostructured zinc oxide and shellac varnish on wood surface can be considered an interesting and promising protecting treatment for wooden materials, since it combines the aesthetical and water-repellant features of the traditional shellac finish with the excellent properties of inorganic nanoparticles, which enhance the resistance towards decay induced by light and fungal agents.
The data used to support the findings of this study are included within the article.
The authors declare that they have no conflicts of interest.