Abstract
Modification of Pinus yunnanensis using SiO2–TiO2 was carried out via the sol–gel method. The aim was to improve the hydrophobicity, aging resistance, and photocatalysis of the wood surface via the formation of new chemical bonds with penetrated SiO2 and TiO2. The air-dried P. yunnanensis wood underwent penetration, gelation, aging, and drying. The wood was exposed to high temperatures for modification, and its microstructure, composition, photodegradability, resistance to aging, dimensional stability, and hydrophobicity were then determined. The results indicated that during modification, SiO2–TiO2 gel was generated in the wood, and the content of the gel increased as penetration time was extended. No structural change in the wood was observed. Meanwhile, chemical bonds were formed among SiO2, TiO2, and wood. The contact angle of the modified wood increased noticeably relative to that of unmodified wood. This increase indicated a noticeable increase in the hydrophobicity of the wood surface. The modified wood exhibited high photocatalytic degradation; however, its durability was not evident. The water absorption and thickness swelling of the modified wood markedly increased. After ultraviolet-aging resistance testing, the color change in the surface of the modified wood was noticeably less than that of the unmodified wood.
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Properties of Pinus Modified with Silicon–Titanium Binary Oxides
Xiaoling Liu,a Songwu Chen,a,* and Yunlin Fu b,*
Modification of Pinus yunnanensis using SiO2–TiO2 was carried out via the sol–gel method. The aim was to improve the hydrophobicity, aging resistance, and photocatalysis of the wood surface via the formation of new chemical bonds with penetrated SiO2 and TiO2. The air-dried P. yunnanensis wood underwent penetration, gelation, aging, and drying. The wood was exposed to high temperatures for modification, and its microstructure, composition, photodegradability, resistance to aging, dimensional stability, and hydrophobicity were then determined. The results indicated that during modification, SiO2–TiO2 gel was generated in the wood, and the content of the gel increased as penetration time was extended. No structural change in the wood was observed. Meanwhile, chemical bonds were formed among SiO2, TiO2, and wood. The contact angle of the modified wood increased noticeably relative to that of unmodified wood. This increase indicated a noticeable increase in the hydrophobicity of the wood surface. The modified wood exhibited high photocatalytic degradation; however, its durability was not evident. The water absorption and thickness swelling of the modified wood markedly increased. After ultraviolet-aging resistance testing, the color change in the surface of the modified wood was noticeably less than that of the unmodified wood.
Keywords: Pinus yunnanensis wood; Sol–gel method; Silicon dioxide; Titanium dioxide; Photocatalysis
Contact information: a: Guangxi Forestry Research Institute, Nanning 530002, China; b: Forestry College of Guangxi University, Nanning 530005, China; *Corresponding author: Fylin@126.com
INTRODUCTION
With the development and utilization of natural resources and the emergence of novel materials, functional wood has received increasing attention. Modification technologies have recently been used to improve wood properties such as magnetism (Jin et al. 2015; Pan et al. 2015; Guo 2017; Lou et al. 2017; Muff et al. 2018), photocatalysis (Mo et al. 2012; Jin et al. 2015; Li et al. 2015; Lin et al. 2015; Rao et al. 2016; Dong et al. 2017; Chang et al. 2018), and aging resistance (Guo et al. 2016; Li et al. 2018), among others (Liu et al. 2013; Guo et al. 2015; Zhang et al. 2015; Huang et al. 2016). To summarize, there are many methods of inorganic modification of wood, but there are some drawbacks. Most of the inorganic substances belong to the classification of refractory material. Such treatments are done with the help of external pressure, which is needed so that the sol will enter the wood cell cavity or cell walls. The resulting effects are mainly due to physical filling, not chemical changes. However, the inorganic modified wood prepared in previous studies still has not very well met the needs for surface hydrophobicity, aging resistance, and photocatalysis improvement effects, especially hydrophobicity. However, most wood modification technologies are complex and costly, impeding their broad applications. Therefore, simple, feasible, efficient, and low-cost wood modification technologies need to be developed.
Wood modified with inorganic substances has the potential not only to ensure structural integrity; it also can be environment-friendly, less expensive, and result in significant improvements in the physical, mechanical, and surface properties of wood. Wood that has been modified using inorganic substances has recently drawn considerable attention, particularly for the functional improvement of wood. Common methods of wood modification using inorganic substances include the sol–gel method, the penetration method, in situ synthesis, and the hydrothermal method. The reaction precursor for the sol-gel method is generally a metal alcohol salt, but also it may consist of some salts, hydroxides, esters, etc., with water as an initiator or solvent. The reaction process generally involves the precursor being added to the solvent, stirring, and forming a uniform solution, hydrolysis reaction at a certain reaction temperature, and the formation of sol. The sol is generated by the reaction, and in the course of drying, condensation makes the structure change, such that the gel forms a network structure. Because the reaction working temperature required by sol-gel method is relatively low and the operation is not complicated. The conditions are favorable for retaining the characteristics of wood itself. As a result of the treatment, the wood is endowed with new functions, which has been widely used in the research of ceramics, glass, wood, biological materials, and many functional materials.
Saka et al. (1992) produced wood/inorganic composites using the sol–gel method and found that the composites exhibited dimensional stability and flame retardancy. Miyafuji et al. (1998) produced SiO2–P2O5–B2O3 wood/inorganic composites via the sol–gel method and observed an increase in leaching resistance in the composites. Ogiso and Saka (1994) produced wood/inorganic composites loaded with SiO2 gel via the sol–gel method and demonstrated that the heat resistance and dimensional stability of wood were improved significantly. Sun et al. (1998) prepared wood/SiO2 nanocomposites by pre-treating wood with conventional or coupling agents and found that the properties of wood/SiO2 nanocomposites were improved. Liao et al. (2001) prepared wood/SiO2 composites and wood/methyl methacrylate (PMMA)/SiO2 composites using ethyl silicate and PMMA as precursors and modifiers, respectively. Wu and Zhang (2011) used the sol-gel method to prepare nano-TiO2/wood composites with tetrabutyl titanate as the titanium source. Anatase crystalline TiO2 composites improve heat resistance and flame retardancy. SiO2 and TiO2 are commonly used as inorganic substances in wood modification and have produced desirable effects. However, SiO2 or TiO2 have merely been shown to improve the surface of wood (Wang et al. 2004; Qiu and Li 2005; Mo et al. 2009; Jiang et al. 2010; Ye and Li 2012; Zhang et al. 2012) and have not been reported to endow wood with hydrophobicity and photocatalytic properties. In particular, this problem was observed in Pinus yunnanensis, which is widely planted in Guangxi but used in a limited capacity due to its soft texture and poor hydrophobicity.
Titanium dioxide is a type of amphoteric oxide, which can have the form of a white solid or powder. It is generally available as either the anatase or rutile crystalline type. At room temperature, TiO2 in a thin layer form can be generated on a wood surface, and such treatment improves moisture absorption stability and aging resistance, etc. Studies show that TiO2 not only physically fills wood cell walls in the form of gel particles, but it also forms stable chemical bonds and combines with modified timber, which endows the prepared modified timber with good flame retardation and dimensional stability. The layered deposition method was adopted to generate TiO2 particles on the surface of cellulose fiber, so that the fiber has a self-cleaning surface and reinforcing body. This was used to prepare a polymer material formed by cellulose matrix composite material and polymer, with the goal of achieving good hydrophobicity and light stability. The wood/TiO2 composite material with photocatalytic degradation was prepared by taking advantage of the photocatalytic action of TiO2, which could degrade organic matter. If the wood was first treated with silicate, then the prepared wood/SiO2/ TiO2 composite material not only could maintain the original characteristics of the wood, but also it could achieve the degradation of organic matter. In the work described in this paper, titanium dioxide was introduced into wood by sol-gel method, which improves the moisture absorption and aging resistance of wood, makes wood fiber have self-cleaning surface and reinforcing body, has good hydrophobicity and photocatalysis, and improves the surface properties of wood.
Although great progress has been made in the improvement of inorganic wood, there are still many shortcomings. First of all, when wood is modified by inorganic salts, most of the inorganic salts only play a physical filling role. Secondly, most of the inorganic wood to improve the whole wood needs to use external pressure to enter the wood cell cavity and cell wall. The cost of impregnation under pressure is high and the operation has a certain difficulty. Again, by means of the surface improvement of wood, the surface properties of wood (hydrophobicity, anti-aging, photocatalysis, etc.) can be greatly improved and the function can be increased. Through hydrolysis of ethyl n-silicate alone, the free hydroxyl group on the wood surface is replaced by a silicon dioxide film, which reduces the moisture absorption effect of wood. However, the generated silicon dioxide film contains hydroxyl group itself, which is hygroscopic and cannot achieve the expected hydrophobicity effect. Existing studies have basically improved wood surface through SiO2 or TiO2 alone, which cannot make wood have both hydrophobicity and photocatalytic ability. Therefore, this study proposes to improve P. yunnanensis using the sol–gel method. SiO2 and TiO2 were simultaneously loaded, forming a binary sol inside the wood. The microstructure, composition, surface hydrophobicity, durability, and photocatalytic properties of the modified wood were evaluated. Wood surface by simultaneously load SiO2, TiO2 binary oxide film modified, using the sol-gel method modifies the wood surface by means of generation of a dual sol. Through the high temperature effect the dual oxide film undergoes sintering, by which the non-crystalline structure into a crystalline structure, strengthen the wood’s hydrophobicity, aging resistance, and light catalytic role.
EXPERIMENTAL
Materials
Pinus yunnanensis was collected from the Yachang Forests in Baise City, Guangxi Province, China. After the sample was collected, the sample was first dried to the moisture content of about 15%. After planing and 120 mesh sanding, the wood was cut into specimens measuring 95 mm × 65 mm × 5 mm and 20 mm × 20 mm × 20 mm. The sample 95 mm × 65 mm × 5 mm was used for the aging resistance test (tangential section was the test surface), and 20 mm × 20 mm × 20 mm was used for dimensional stability, microscopic performance, photocatalysis and other performance test. Ethyl orthosilicate, tetrabutyl titanate, ethanol, nitric acid, and the azo dye AO7 were purchased from Songyuan Chemical and Glass Instrument Co., Ltd. in Nanning City, China.
Treatment of wood
The prepared silicon and titanium unit sol were mixed proportionally via the sol–gel method (n1(Si):n2(Ti) = 1:2), and the wood was immersed (immersion time = 2, 4, 6, and 8 h) to allow penetration. After aging for 5 d and air drying, the wood specimen was subjected to high-temperature treatment at 250 °C for 20 min. Then, the wood surface was loaded with silicon–titanium dual oxide film.
Methods
Microscopic and morphological characterization of the wood surface
Changes in the wood structure were observed using a stereomicroscope (Discovery V20; Carl Zeiss Micro Imaging, Jena, Germany). The morphology of each section of the wood was analyzed using an S-3400N scanning electron microscope (SEM, Hitachi Ltd., Tokyo, Japan); and the distribution of SiO2 and TiO2 in the wood was observed using a PV8200 energy-dispersive spectroscope (EDAX; Hitachi Ltd., Tokyo, Japan). The chemical structures of SiO2 and TiO2 in wood and their interaction with wood components were analyzed using a Nicolet 6700 Fourier-transform infrared spectroscope (FTIR, Nicolet 6700; Thermo Fisher, Waltham, MA, USA) under the following conditions: optical resolution of 4 cm-1; wavenumber accuracy of 0.01 cm-1; scanning range of 4000 to 400 cm– 1; and number of scans, 32. The crystallization of SiO2 and TiO2 in the wood was analyzed by X-ray diffraction (XRD) (Rigaku SmartLab, Tokyo, Japan), and the scanning range was 10° to 70°.
Hydrophobicity of wood samples
The contact angles of distilled water on samples before and after aging were determined using a contact angle instrument (Krüss DSA100S; Krüss GmbH, Hamburg, Germany). The droplet volume was set to 2 μL, and the contact angle of the 30th droplet was determined. Measurements were conducted on both tangential and radial sections. Three measurements were performed on each surface, and their means were assigned as the contact angles of the samples.
Photocatalysis of wood samples
The degradation of the azo dye AO7 (target pollutant) by the modified wood samples was determined. The photocatalytic activity was determined using two methods. One method involved evenly coating the surfaces of the samples with the AO7 solution at 100 mg/L. The samples were left to sit horizontally to allow complete absorption of the AO7 solution, and then the specimens were exposed to ultraviolet (UV) light. The chromatic aberration of the samples was determined hourly; subsequently, the photocatalytic effect of the modified material on the azo dye AO7 was evaluated. The other method involved soaking the wood samples in the AO7 solution at 40 mg/L and then placing the solution under UV light. The absorbance of the solution at λ = 484 nm was measured using a UV spectrophotometer (Ultrospec 2100 pro; Biochrom Ltd., Cambridge, England), and the concentration was calculated based on the standard curve. Changes in AO7 concentration after photocatalysis were determined.
Aging resistance of the wood samples
Aging of the wood samples under outdoor light was simulated using a spray-type QUV/SPRAY UV aging instrument (Q-Lab Corporation, Shanghai, China); the lamp type was UVA-340, and the light intensity was 0.89 w/m2. To characterize the aging resistance of the samples, the surface chromaticity coordinate values L* (lightness), a* (red/green), and b* (yellow/blue) of each sample were measured using an ADCI-60-C automatic colorimeter (Chentaike Instrument Technology Co., Ltd., Beijing, China). The difference in the total color difference (ΔE) of the sample surface before and after aging was calculated according to Eq. 1,
(1)
where △L*, △a*, and △b* represent the changes in L, a, and b, respectively.
Dimensional stability of wood samples
The modified wood samples were dried in an oven. The oven-dried wood samples were stored in the dryer until use. After cooling down, the samples were placed on a paper board with small holes on it, and the bottom of the dryer was filled with an appropriate amount of distilled water. The weight and dimensions of the wood samples were determined every 6 h. The dimensional stability of the wood samples was indicated by their water absorption and thickness swelling, which were calculated according to Eqs. 2 and 3,
(2)
where △m was the water absorption rate (%), m0 was the oven-dry weight (g) of the samples, and m1 was the weight (g) of the samples at the time of measurement. Equation 3 was as follows,
(3)
where △L was the thickness swelling (%) of the wood samples, L0 was the dimension (mm) of the oven-dried samples, and L1 was the dimension (mm) of the samples at the time of measurement.
RESULTS AND DISCUSSION
Microscopic and Structural Morphology of the Wood Surface
Microscopic image analysis
The transverse, tangential, and radial sections of the modified wood are presented in Fig. 1.
Fig. 1. Scanning electron microscopy images showing the (a) transverse, (b) tangential, and (c) radial sections of the modified wood
Noticeable alterations in structures were apparent in the modified wood. As a softwood, P. yunnanensis features regularly arranged tracheids, soft texture, and evident differences between earlywood and latewood. The tracheids in the modified wood were evenly filled with gel. The SEM images of the wood before and after modification are shown in Fig. 2. In the modified wood, solid precipitation along the cell wall and cell lumen was clearly observed, which likely consisted of silicon–titanium oxides formed in situ by using the sol–gel method. Blocks of SiO2 and TiO2 filled in the vessels or covered the surface of the wood and were attached to the cell wall via chemical bonds and/or physical absorption, a basis for improving the hydrophobicity of the wood.
Fig. 2. Scanning electron microscopy images showing the (a) transverse, (b) tangential, and (c) radial sections of the unmodified wood and the (d) transverse, (e) tangential, and (f) radial sections of the modified wood
Compositional analysis of the modified wood with different penetration times
Figure 3 presents plots showing the Si and Ti contents of the wood samples treated with different penetration times, as revealed by EDAX analysis. Compared with those in the unmodified samples, the Si and Ti contents in the modified samples were increased noticeably, and both the Si and Ti contents increased with penetration time. When the penetration time was set to 8 h, the maximum Si and Ti contents were achieved—that is, 0.48%, and 1.54% respectively.
Fig. 3. Energy dispersive x-ray analysis diagram of the modified and unmodified wood; (a) unmodified wood, (b) wood with 2 h penetration, (c) wood with 4 h penetration, (d) wood with 6 h penetration, and (e) wood with 8 h penetration
The FTIR spectra of the unmodified and modified wood with different penetration times are shown in Fig. 4. The characteristic peaks of major wood components—lignin (1730 cm-1), cellulose (1630 cm-1), and hemicellulose (1025 cm-1)— showed no changes before and after modification, suggesting that the modification did not alter the wood’s chemical structure. Compared with the unmodified wood, the modified wood showed several new peaks. The peaks at 1025 and 789 cm-1 were attributed to the stretching vibration of Ti–O–Si and absorption vibration of Ti–O–C , this is consistent with the research of Zhong et al. (2002), suggesting that after modification, TiO2 successfully entered the inside of wood and chemically bonded with the wood.
Fig. 4. Fourier-transform infrared spectra of the unmodified and modified wood with different penetration times
The samples were further analyzed by XRD (Fig. 5).
Fig. 5. XRD diagram of the modified and unmodified wood
The position of the diffraction peak 2θ = 23.6° was not changed after modification, suggesting that the unit cell parameters of the wood remained the same. This finding further revealed that modification did not alter the wood chemical structure. Meanwhile, the intensity of the diffraction peak changed noticeably, suggesting large amounts of SiO2 and TiO2 were formed using the sol–gel method. But with relatively unmodified wood, the intensity of the diffraction peak differed from the modified wood. Because SiO2 has lower crystallinity, for specimens after high temperature processing, a small diffraction peak of the modified wood was apparent at about 25°; this corresponded to the TiO2 anatase form, which is consistent with the research of Wu et al. (2011). However, with the increase of the impregnation time, the content of SiO2 and TiO2 generated on the surface and inside of the wood increased, the relative content of the wood decreased, and the diffraction peak strength decreased.
Analysis of the hydrophobicity of the wood surface
The major chemical components of wood include cellulose, hemicellulose, and lignin, all of which are rich in hydrophilic groups that impart relatively high hydrophilicity to wood. The contact angles of the modified and unmodified wood samples are presented in Fig. 6, and the measurements are listed in Table 1. The contact angles of the modified wood were larger than those of the unmodified wood samples. The contact angles of the tangential and radial sections were almost similar and larger than those of the cross-section. The modification increased the contact angles from 61.3° to 121.1°, suggesting a noticeably increased hydrophobicity. This effect was attributable to the wood surface being coated with SiO2–TiO2 or bonds with the hydroxyl groups on the wood surface, which greatly reduced the exposure of hydrophilic groups on the wood surface, limiting moisture absorption. The modification also led to the formation of a hydrophobic film on the wood surface, further enhancing its hydrophilicity. In addition, the nanoscale titanium dioxide that formed on the wood surface during modification noticeably increased the intrinsic roughness of the wood, which further increased the hydrophilicity of the wood. The contact angle of the modified wood before and after aging changed noticeably, with the maximum difference reaching 62°. This result suggested the relatively low durability of the modified wood, which greatly depended on the intrinsic structure of the wood.