Improvement of color stability of Acacia hybrid wood by TiO2 nano sol impregnation

Abstract

In this study, the TiO2-impregnation of wood from acacia hybrid (Acacia mangium x auriculiformis) was achieved by combined pressure-impregnation and hydrothermal post-treatment. The ultraviolet (UV) resistance of the obtained TiO2-impregnated wood was examined by measuring the changes in color after 960 h of UV irradiation. Results showed that the color stability against UV irradiation of the TiO2-impregnated wood was significantly improved compared to that of the untreated acacia hybrid wood. Furthermore, the prepared wood samples were characterized using a field-emission scanning electron microscope (FE-SEM), energy-dispersive X-ray (EDX) spectroscopy, and X-ray diffraction (XRD) techniques. FE-SEM and EDX analyses showed that the TiO2 micro- and nanoparticles, with rod-like shapes, were located on the inner surfaces of the wood vessels. Additionally, the crystal structure of the TiO2, with an anatase phase, was demonstrated by XRD analysis. This study suggests that the presence of an anatase TiO2 can improve the UV resistance of fabricated wood samples.

Full Article

Improvement of Color Stability of Acacia Hybrid Wood by TiO₂Nano Sol Impregnation

Vu M. Tuong* and Tran V. Chu

In this study, the TiO₂-impregnation of wood from acacia hybrid (Acacia mangium x auriculiformis) was achieved by combined pressure-impregnation and hydrothermal post-treatment. The ultraviolet (UV) resistance of the obtained TiO₂-impregnated wood was examined by measuring the changes in color after 960 h of UV irradiation. Results showed that the color stability against UV irradiation of the TiO₂-impregnated wood was significantly improved compared to that of the untreated acacia hybrid wood. Furthermore, the prepared wood samples were characterized using a field-emission scanning electron microscope (FE-SEM), energy-dispersive X-ray (EDX) spectroscopy, and X-ray diffraction (XRD) techniques. FE-SEM and EDX analyses showed that the TiO₂ micro- and nanoparticles, with rod-like shapes, were located on the inner surfaces of the wood vessels. Additionally, the crystal structure of the TiO₂, with an anatase phase, was demonstrated by XRD analysis. This study suggests that the presence of an anatase TiO₂ can improve the UV resistance of fabricated wood samples.

Keywords: Acacia hybrid wood; Color; Impregnated-wood; FE-SEM, TiO₂; UV irradiation; XRD

Contact information: Wood Industry College, Vietnam Forestry University, Ha Noi, Vietnam;

* Corresponding author: manhtuong0209@gmail.com

INTRODUCTION

Wood is an environmentally friendly material that is used extensively in indoor and outdoor applications. However, there are many unfavorable end-product properties of products that are composed of wood, such as biodeterioration, combustibility, and photodegradability. In particular, one of the drawbacks of wood for outdoor applications is a tendency for significant changes in its appearance over time (discoloration, roughening, checking), principally because of the ultraviolet (UV) light component of the sun’s rays (Rowell 2005). Thus, enhancing the UV resistance of wood is important in that it could extend the areas of use of wood.

Acacia hybrid (Acacia mangium x auriculiformis) is a low-quality wood species abundantly available in Vietnam. At present, a large proportion of the available wood of this type is used for outdoor furniture production. To help this wood meet the requirements of outdoor applications, it needs a special treatment. A previous report showed the potential of improving the dimension stability of this wood (Tuong and Li 2010). In another report of Chu et al. (2014), the wettability of this wood was improved with TiO₂ gel treatment. However, there has been a lack of research on improvement of UV resistance of acacia hybrid wood.

Regarding UV resistance of wood, many reports describe ways to improve the weathering performance of wood and wood-based products via pretreatments such as heat treatment, impregnation with preservatives (Deka et al. 2008), chemical modification (Pandey et al. 2010), and the application of a coating layer (Fjellstrom et al. 2009). In recent years, several methods of depositing thin barrier layers on wood surfaces to improve their moisture resistance (Rassam et al. 2011), dimensional stability (Sun et al. 2010), reduced wettability (Sun et al. 2011; Chu et al. 2014), and UV radiation resistance (Sun et al. 2012; Devi et al. 2013) properties were investigated. In these studies, film deposition was carried out primarily using the hydrothermal or sol-gel methods. Beside these mentioned methods, the sol-gel process also was applied to fabricate wood-inorganic composite (Saka et al. 1992; Saka and Ueno 1997; Miyafuji et al.2004). These studies mostly used tetraethoxysilane (TEOS) as a precursor to prepare the wood composites.

Titanium dioxide (TiO₂) nanoparticles are the most useful because of their excellent chemical stability and nontoxicity (Fujishima and Honda 1972; Kim et al. 2005). Nano-TiO₂ has become an interesting functional material involved in photocatalysis (Fujishima and Honda 1972; Linsebigler et al. 1995), anti-UV, antibacterial, and self-cleaning surface and material applications (Wang et al. 1999; Yamagishi et al. 2003; Jiang and Zeng 2010).

In this study, the color stability during UV irradiation of acacia hybrid wood samples following a combined impregnation and hydrothermal treatment process, was examined. Additionally, the microstructure of the TiO₂-impregnated wood and the crystal structure of the TiO₂ in the wood samples were evaluated by a field-emission scanning electron microscope (FE-SEM) and X-ray diffraction (XRD), respectively.

EXPERIMENTAL

Materials

Wood specimens free of defects were cut from an eight-year-old acacia hybrid (Acacia mangium x auriculiformis) tree collected from the Hoa Binh province, Vietnam. Approximately the same amount of heartwood at breast height diameter position in the trunk was cut to prepare each wood sample, i.e., 10 mm (T) x 20 mm (R) x 30 mm (L) in size (R: radial direction; T: tangential direction; and L: longitudinal direction), for the fabrication of the TiO₂-impregnated wood. All samples were conditioned for three weeks at 20 °C and 65% relative humidity (RH) prior to fabrication of the wood samples.

Tetrabutyl orthotitanate (TBOT, AR grade) was used as the precursor for the TiO₂ nanoparticles. Sodium dodecyl sulfate (SDS) was used as a surfactant. Ethanol (EtOH) was used as the solvent. All chemicals were supplied by Tianjin Baishi Chemical Industry Co., Ltd. and used without further purification. Distilled water was used for the preparation of all sol and solutions.

Synthesis of TiO₂ Nano Sol

The typical procedure used to prepare the aqueous nano sol was as follows. Approximately 5 mL of TBOT was dissolved into a mixture of 2 mL of acetic acid and 18 mL of anhydrous ethyl alcohol (EtOH) with magnetic stirring for 30 min until a homogenized solution was obtained. Then, 80 mL of distilled water (the pH of which was adjusted to 4 by the addition of acetic acid) was added to this solution, dropwise, under vigorous stirring for several hours at room temperature.

Preparation of TiO₂-impregnated wood

Wood samples were first impregnated with the as-prepared sol solution. The impregnation process was carried out in a vessel under 7 bar pressure for 4 h at room temperature.

In the second step, impregnated wood samples and the remaining solution were transferred into a Teflon-lined stainless-steel autoclave and heated at 110 °C for 4 h in an oven.

After the heating process, the autoclave was allowed to cool to room temperature, was opened, and a solution of 3.4 x 10^-4 M SDS at pH 6.5 was added. The autoclave was sealed again and reheated at 70 °C for 4 h in an oven. Finally, the wood samples were removed from the autoclave, ultrasonically rinsed with distilled water for 30 min, and oven-dried at 103 2 °C overnight.

UV Irradiation Test

The photostability of the TiO₂-impregnated wood was determined using a 40-W UV light source for 960 h. The color of the tangential surfaces of TiO₂-impregnated wood was measured using an NF-333 spectrophotometer (Nippon Denshoku, Japan) with measurement diameter of 8 mm before and after UV irradiation.

The CIELAB system used in this study is characterized by three parameters: L*, a*, and b*. The L* axis represents the lightness, and +a* is red, –a* is green, +b* is yellow, and –b* is blue. L* varies from 100 (white) to 0 (black). The L*, a*, and b* color coordinates of each sample before and after UV irradiation were used to calculate the total color change, ∆E, according to the following equations,

∆L* = L*_UV – L*_o (1)

∆a* = a*_UV – a*_o (2)

∆b* = b*_UV – b*_o (3)

∆E = (∆L*² + ∆a*² + ∆b*²)^1/2 (4)

where ∆L*, ∆a*, and ∆b* are the changes between before- and after-UV irradiation color values. Each parameter L*, a*, and b*contributes to the total color change, ∆E. A low ∆E value corresponds to a small color change.

Characterization Methods

The microstructure morphologies of the TiO₂ formed in the TiO₂-impregnated wood were characterized by a field-emission scanning electron microscope (FE-SEM, 7600F, JEOL, Japan) combined with energy-dispersive X-ray (EDX, Oxford Instruments) spectroscopy at the voltage of 10kV. The crystal structures of the TiO₂-impregnated wood were analyzed by X-ray diffraction with a Siemens (Germany) D5000 X-ray diffractometer, operating with Cu K_α radiation with a weighted average wavelength of 0.154184 nm, and the scan angle (2) ranging from 5^o to 70^o.

RESULTS AND DISCUSSION

Morphological Studies

Scanning electron microscopy study

FE-SEM images of the TiO₂-impregnated wood are presented in Fig. 1. The apparently continuous layer of TiO₂ gels was located on the inner surface of the wood vessels (Fig. 1a), and some intervessel pits were blocked by TiO₂ gels (Fig. 1b and 1d). In addition, a number of evenly distributed, rod-like TiO₂ particles were observed on the thin coating (Fig. 1c). The morphology of the TiO₂ in this impregnated wood was similar to that observed by Sun et al. (2011), who hydrothermally treated the sol at pH 6.72.

Fig. 1. FE-SEM images of TiO₂-impregnated wood samples at various magnifications: (a) the inner surface of the wood vessels (magnification: 1000X, scale bar: 10 µm); (b, c, and d) the intervessel pits in the tangential section: (b) magnification: 3000X, scale bar: 1 µm; (c) magnification: 10,000X, scale bar: 1 µm; (d) magnification: 20,000X, scale bar: 1 µm

Energy-dispersive X-ray analysis (EDX)

The presence of TiO₂ inside the wood, as observed by FE-SEM, was also investigated with energy-dispersive X-ray analysis (EDX). Figure 2 shows the EDX spectroscopy spectra of untreated wood and TiO₂-impregnated wood samples. There was a significant titanium (Ti) peak in the spectra of the TiO₂-impregnated wood samples (Fig. 2b) compared to those of untreated wood samples (Fig. 2a). This suggests that TiO₂ was a constituent element present in the samples after impregnation and hydrothermal treatment.

Fig. 2. EDX spectra of (a) untreated wood and (b) TiO₂-impregnated wood samples

X-Ray Diffraction Analysis

The XRD patterns of the untreated and TiO₂-impregnated wood samples were collected across a 2θ range of 10° to 70° to characterize their crystal structures. As shown in the diffraction patterns of the samples (Fig. 3), untreated and impregnated wood samples exhibited distinctly different characteristic peaks. In the pattern of the untreated wood sample (Fig. 3a), the only diffraction peaks, located at 2θ of 16° and 22.6°, were assigned to peaks of the cellulose crystal planes 101 and 002, respectively (Li 2003). However, in the pattern of the impregnated wood (Fig. 3b), other strong diffraction peaks were observed. The diffraction peaks at 2 of 25°, 38°, 48°, and 55° were assigned to peaks related to anatase TiO₂. This result indicates that anatase TiO₂ was present in the impregnated wood (Chen and Mao 2007; Yang et al. 2009; Sun et al. 2012).

Fig. 3. XRD patterns of (a) untreated wood and (b) TiO₂-impregnated wood samples

Color Changes by UV Irradiation

Sun et al. (2011) reported that during hydrothermal treatment, anatase TiO₂ was chemically bonded to the wood surface via hydrogen groups. This finding was confirmed by many characterization methods, including SEM, EDX, XRD, and FTIR. It was inferred that the adherence of TiO₂ to the wood was strong when no significant loss of titanium occurred, even after vigorous washing of the specimens (Sun et al. 2010). The remaining hydrophobicity of the TiO₂-impregnated wood, even after immersion in boiling water for 20 h (Chu et al. 2014), is further evidence supporting this point.

To determine the effect of TiO₂ on the UV resistance of the fabricated wood samples, the color changes of the TiO₂-impregnated wood surfaces after UV irradiation for 960 h were examined. The experimental results are presented in Fig. 4. As shown in Fig. 4a, the ∆L* value of the untreated wood decreased from 0 to -8.68 with increasing irradiation time, indicating darkening. However, the ∆L*value of the TiO₂-impregnated wood increased, from 0 to 0.98, indicating slight brightening. The ∆L* value of the TiO₂-impregnated wood had only a slight change compared to that of the untreated wood. As shown in Fig. 4b, the ∆a* value of both untreated wood and TiO₂-impregnated wood indicated reddening that increased with UV irradiation time. However, the ∆a* value of the TiO₂-impregnated wood was smaller than that of the untreated wood. As shown in Fig. 4c, the ∆b* value of both untreated wood and the wood-TiO₂composites exhibited a tendency opposite to that of their respective ∆L* values. The total color change (∆E*) is shown in Fig. 4d. The ∆E* value of both untreated wood and TiO₂-impregnated wood increased with increasing irradiation duration. Moreover, the ∆E*value of the TiO₂-impregnated wood was smaller than that of the untreated wood, suggesting that the TiO₂-impregnated wood had greater UV resistance than untreated acacia hybrid wood, thereby preventing damage to the samples.

Fig. 4. Color changes of untreated wood and TiO₂-impregnated wood versus irradiation time

It is well known that under UV irradiation, lignin, one of the components of wood, is subjected to photo-oxidation by the formation of carbonyl-based chromophoric compounds. This induces the color change of irradiated wood surfaces (George et al. 2005). TiO₂ particles absorbed UV radiation and thus decreased the UV intensity when contact to wood components, slowing the oxidation of the TiO₂-impregnated wood. The increase in the samples’ ability to resist UV irradiation with the incorporation of TiO₂ was reported by Sun et al. (2012) and (Devi et al. 2013). Hence, the UV resistance of the TiO₂-impregnated wood was improved compared to that of untreated wood, perhaps because of the high UV radiation absorption capability of anatase TiO₂.

CONCLUSIONS

1. TiO₂-impregnated wood made from acacia hybrid wood and TiO₂ were successfully fabricated by a combined impregnation and hydrothermal post-treatment process. The existence of anatase micro- and nano-sized TiO₂ particles with rod-like shapes on the wood surfaces was verified by FE-SEM, EDX, and XRD analyses.
2. The UV resistance of the TiO₂-impregnated wood, expressed as their color change during UV irradiation, was significantly higher than that of untreated wood, indicating that the impregnation and hydrothermal post-treatment process improves the ability of acacia hybrid wood to resist UV irradiation.

ACKNOWLEDGMENTS

This research was funded by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.99-2012.18.

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Article submitted: March 3, 2015; Peer review completed: May 15, 2015; Revised version received: July 3, 2015; Further revised version received and accepted: July 7, 2015; Published: July 16, 2015.

DOI: 10.15376/biores.10.3.5417-5425