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Article

Preparation of Hydrophobic Optically Transparent Wood via An Efficient UV-Assisted Route

by
Hang Wang
†,
Hong Yang
† and
Hongbo Mu
*
College of Science, Northeast Forestry University, Harbin 150040, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Forests 2023, 14(4), 759; https://doi.org/10.3390/f14040759
Submission received: 21 March 2023 / Revised: 2 April 2023 / Accepted: 3 April 2023 / Published: 7 April 2023
(This article belongs to the Section Wood Science and Forest Products)
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Abstract

:
In the context of the double carbon goal, the green, low-carbon and environmentally friendly modern wood construction market is promising and is expected to be further promoted and applied in the construction market. Optically transparent wood is a new building light-transmitting material with excellent performance, designed to reduce the energy consumption of buildings. An efficient and green method for the preparation of hydrophobic optically transparent wood is proposed in this study, in which its microstructure, chemical composition, surface wettability and optical properties are investigated. Hydrophobic optically transparent wood (90% light transmission, 80% haze, 130° water contact angle) with identical optical properties on the positive and negative sides was obtained by UV-assisted hydrogen peroxide treatment of natural wood to remove lignin chromogenic groups in situ, followed by dipping the epoxy resin into the wood substrate template and finally combining it with PDMS low surface energy modifications on the surface. The scanning electron microscopy and chemical composition analysis showed that the epoxy resin was successfully immersed in the internal pores of the wood and exhibited a homogeneous interface with the wood cell walls. All results confirm that this optically transparent wood preparation method is effective, and the resulting hydrophobic optically transparent wood is a new wood composite alternative material with excellent optical and hydrophobic properties, which broadens the application area of traditional wood and offers potential applications in energy-efficient buildings, smart windows and solar cells.

1. Introduction

On the basis of the carbon neutral background of the times, the acceleration of biomass materials as an environmentally friendly renewable material and the development of biomass energy has been a wide concern [1,2]. The application of wood by human beings to construct things, to make fire, for structure and for characteristic derivative creation has expanded from the early days of cutting wood. The understanding of the meaning of wood has also evolved from simple primary energy to the multi-functional and diversified secondary energy nowadays [3]. Wood is a natural polymer composite material, with features such as degradability, complex hierarchical structure, low density, low thermal conductivity and anisotropy, which is widely used in daily life and production [4,5,6]. As a complex polymer composite material with high porosity and three-dimensional structure composed mainly of polymers such as cellulose, hemicellulose and lignin and a small amount of low-molecular weight compounds, which is renewable and environmentally friendly, wood is regarded as a natural eco-environmental material with structural dimensions at all levels containing macroscopic wood tissue structure at the meter level, microscopic cell wall layer structure at the micron level and polymer structure at the nanometer level [7]. With low energy consumption during production and use, wood and wood products show lower levels of carbon emissions and more obvious energy-saving and carbon-reducing advantages compared to traditional building materials such as steel, glass and cement [8]. A renewable and mechanically robust natural bio-based template, wood contains a hemicellulose and lignin matrix combined with cellulose raw fibers arranged in the layers of wood. Therefore, the removal of lignin from wood cell walls without changing the hierarchical arrangement of cellulose protofibrils could bring more possibilities in the field of functional materials with aligned cellulose structures for biotemplates. Different functional materials can be developed based on the biotemplates provided by delignified wood and are used in a wide range of applications [9]. The mesoporous channels of wood allow strong light scattering of visible light, and natural wood is colored and non-transparent because it contains special chromogenic groups. Therefore, it is usually necessary to remove the chromogenic groups from the wood cell walls and immerse them in a polymer that matches the refractive index of the wood substrate to reduce the loss of light during propagation in order to obtain transparent wood composites. Based on this, transparent wood has been proposed for applications involving photovoltaic devices [10,11], energy-saving building materials [12,13], solar cell light management systems [14], and luminescent materials [15,16]. With many advantages such as high anisotropy, high light transmission, adjustable light haze, efficient heat insulation, high impact energy absorption and renewable properties, transparent wood shows great potential for multifunctionalization. As a result, it has attracted considerable scholarly attention in the fields of light, heat, electromagnetism and energy management.
Li et al. [17] prepared nanoporous structures by removing lignin from the cell walls of wood, followed by infiltration of pre-polymerized methyl methacrylate (MMA) with matching refractive indices into the cell wall lumen and nanoscale cellulose fiber network, and finally prepared transparent wood with 85% light transmission and 71% haze, which widened the scope for the selection of low-cost light-transmissive building materials. In addition, Rinky et al. [18] prepared optically transparent wood with 70.4% light transmission and 68.3% haze by a bleaching treatment and fast filling the wood with epoxy resin, which makes it more suitable for light transmission applications in the field of construction materials due to the enhanced mechanical strength of the wood by the filled epoxy resin. Qiu et al. [19] used delignified wood as a substrate and made antimony-doped tin oxide (ATO) optically transparent wood by infiltrating pre-polymerized methyl methacrylate (PMMA) with modified antimony-doped tin oxide (ATO) nanoparticles, which showed excellent NIR heat shielding properties, UV shielding properties, and high transparency and is expected to replace ordinary glass as a new NIR heat shielding glass with unbreakable, toughness, and UV protection function. However, the waste solution generated from the preparation of delignified wood in an experimental chlorate chemical environment can cause environmental pollution, and the additional costs associated with the disposal of the waste solution are high. In addition, lignin is an important structural support component in wood, and removing large amounts of lignin can damage and weaken the wood structure, which may shatter during delignification [20,21]. Epoxy resin (EP) is an environmentally friendly polymer with optical transparency [22], and the impregnation of epoxy resin can obtain wood composites with excellent light transmission, but it may cause debonding between the cell wall and the polymer under prolonged external environmental testing, which reduces the compatibility of the impregnated filled polymer with polar scaffold polysaccharides (cellulose/hemicellulose), resulting in defects such as cracks that affect their aesthetics and normal usage [23,24].
Therefore, an easy and efficient top-down method was adopted in this paper, using a chlorine-free reagent hydrogen peroxide solution treatment to quickly remove the color-generating groups from the wood under UV irradiation. In order to obtain high light transmission and haze, the decolorized treated wood template was impregnated with epoxy resin, while the surface modification with polydimethylsiloxane (PDMS) was combined to further improve the hydrophobic properties of optically transparent wood. This easy and green method greatly improves the utilization value and possibility of transparent wood for construction materials or functional materials.

2. Materials and Methods

2.1. Materials and Chemicals

The tangential veneer of balsa wood was obtained from Beijing Excelsior Models Ltd. in China. All samples were taken from wood without knots, discoloration or obvious defects, and the samples were cut to a dimensional size of 40 mm × 40 mm × 1.5 mm (L × T × R). With a view to avoiding the influence of environmental factors, the cut blocks were kept at a temperature of 20 °C and a relative humidity of 65%, thus allowing the wood samples to reach hygroscopic equilibrium. Anhydrous ethanol, hydrogen peroxide (30 wt%), ammonia (25 wt%) and deionized water were purchased from Harbin Junan Medical Chemical Glass Wholesale Station. Epoxy resin (#128 resin and # polyetheramine D230 hardener, Zhongshan Qianmo Import & Export Co., Zhongshan, China) was used to impregnate the wood. Polydimethylsiloxane (PDMS, Sylagrd 184A) and curing agent (Sylagrd 184B) were purchased from Dow Corning Corporation. Hexane (97%) was purchased from Maclean’s. All chemicals were analytically pure, and no further purification was required for use. UV irradiation lamp (model, FOVNN-CPUVLZM; wavelength, 380–390 nm; power 50 W).

2.2. Preparation of Transparent Wood

The decolorization solution was prepared by mixing hydrogen peroxide, deionized water and ammonia in the ratio of 7:3:1 (volume ratio), and then the cut natural balsa wood samples were irradiated under UV light for 4–6 h to accelerate the decolorization process of the wood. The prepared decolorized wood templates were washed three times using flowing deionized water to remove the residual chemical solution from the decolorized wood templates. In order to ensure that the original morphology of the decolorized wood is maintained after dehydration and drying, and that the pores do not shrink, the residual water in the wood template is replaced by anhydrous ethanol, and the procedure is repeated three times to obtain the lignin-modified decolorized wood template.
The impregnating solution was prepared by mixing epoxy resin and corresponding hardener in the ratio of 3:1 and stirred magnetically at room temperature for 15 min to obtain a homogeneous impregnating solution, and the impregnating solution was treated under ultrasonic waves for 5 min to remove the air bubbles generated during the stirring process. The impregnating solution and the decolorized wood samples after UV irradiation treatment were placed in the mold and covered with a glass sheet to prevent the wood from being suspended during the vacuum treatment and not being able to complete the impregnation. The whole of the above is transferred to a vacuum atmosphere and evacuated at a pressure of −0.1 Mpa to remove the gases and residual organic solvents etc. from the wood template. The vacuum is released every 30 min and left to stand for 10 min at atmospheric pressure so that the impregnating solution is impregnated into the wood apertures under atmospheric pressure, and this was repeated 3–4 times.
The impregnated wood is removed and cured by drying on tin paper at room temperature for 24 h to produce a highly translucent transparent wood. The dried optically transparent wood was immersed in 1 wt% PDMS hexane solution, and the samples were removed after 10 min and cured in a vacuum drying oven at 80 °C for 3–4 h. Finally, a hydrophobic coating was obtained on the transparent wood and hydrophobic transparent wood was prepared.

2.3. Characterization

The cold field emission scanning electron microscopy (FE-SEM, JSM-7500F, JEOL, Tokyo, Japan) was used to characterize the microscopic-like features and morphology of the wood samples. Changes in wood chemical composition were assessed by Fourier transform infrared spectroscopy (FTIR, Nicolet iS10, Thermo Fisher Scientific, Waltham, MA, USA) with 32 scans and a resolution of 4 cm−1. An X-ray diffractometer (XRD, XRD-6100, Shimadzu, Kyoto, Japan) was used to analyze the crystal structure changes of the samples, scanning at a rate of 8°/min in the 2θ range of 5–60°. The transmission spectra and haze were measured with a color haze meter (CS-700, Color Spectrum, Shanghai, China) with an acquisition wavelength range of 400–700 nm. Wettability was measured by a contact angle measurement system (DSA, SDC-350, China), and water contact angle (WCA) was measured by a contact angle measurement system with approximately 5 μL of water droplets. The final reported water contact angle results for each group of samples were obtained by taking five measurements at different locations on the sample surface and calculating the average value.

2.4. Preparation of Optically Transparent Wood

A detailed flow chart for the preparation of UV-assisted hydrophobic optically transparent wood is shown in Figure 1. Briefly, in this experiment, pristine balsa wood (PW) was rapidly modified and decolorized with lignin in situ under UV irradiation to obtain decolorized lignin-modified wood (LMW), which was subsequently treated with epoxy resin impregnation to obtain optically transparent wood (OTW), and then combined with low surface energy modification treatment with polydimethylsiloxane to obtain hydrophobic transparent wood (HTW). Each step in the above process has an effect on the microscopic morphology and physicochemical properties of balsa wood.

3. Results and Discussion

Characterization of Transparent Wood

3.1.1. Micromorphology and Microstructure

SEM images of pristine wood (PW), lignin-modified decolorized wood (LMW), and optically transparent wood (OTW) were analyzed to investigate the microscopic morphological characteristics of the samples and the effects of UV-assisted chemical treatment and polymer immersion on wood cell lumen production. Figure 2a,b provide cross-sectional images of the original balsa wood (1.5 mm thick), showing typical thin-walled fibers with an irregular, honeycomb shape and cell cavities in a hollow state with large filling spaces, the microscopic shape of the original wood can be clearly observed at the red arrows in Figure 2b, the layered porous wood structure is conducive to the rapid penetration of H2O2 solution and efficient capture of UV light by microchannels, which can effectively remove the light-absorbing chromogenic groups from the wood cell walls. The lignin-modified decolorized wood opened up more new pores in the original porous structure after treatment (Figure 2c,d), with visible stomata in the cell walls, but without significant damage and still retaining the intact skeletal structure. The presence of stomata can be clearly observed at the red arrows in Figure 2d, and the resulting micropores provide more sites for filling and attachment by polymer impregnation in the next step. Furthermore, the macroscopic optical picture shows that the wood changes from tan to white, which strongly suggests that the color-emitting groups as well as a small amount of lignin are removed from the wood after UV-assisted hydrogen peroxide solution treatment and that the original microscopic layered porous structure of the natural wood is better preserved. Finally, SEM images of transparent wood indicate that the epoxy resin penetrates well into the wood and that it fills both the natural pores of the wood and the pores formed after the lignin modification decolorization (Figure 2e). Due to the refractive index mismatch between air and wood templates, the difference between the two is too great causing scattering of incident light, combined with the fact that lignin absorption causes light attenuation, both pristine wood and lignin-modified decolorized wood are opaque. After modifying the lignin and filling it with an epoxy resin with matching refractive index, the transparent epoxy polymer filled the pores to achieve conditions for favorable light transmission, so the optically transparent wood is transparent. PDMS, on the other hand, is a surface modification modifier with very good light transmission and is present in only a relatively small layer on optically transparent wood, so hydrophobically transparent wood is also transparent. Although it was mentioned above that some of the original micro-grooves and pore structures were filled and covered after the impregnation filling of the wood, it is clear from the red arrows in Figure 2f that there is a certain amount of void space between the polymer and the wood, which is the main reason for the transparency of the samples and the high haze it exhibits [23].

3.1.2. Chemical Composition

The crystallographic properties of PW, LMW, OTW, HTW, and EP samples was investigated by X-ray diffraction mapping. As shown in Figure 3a, there are three peaks in the diffraction pattern of the pristine wood, located at 2θ = 15.3°, 2θ = 21.9° and 2θ = 34.4°, which can be attributed to the 101 crystal plane, 002 crystal plane and 040 crystal plane of cellulose [25]. The height of the diffraction peak indicates the degree of the presence of the phase, and the less the phase, the lower the diffraction peak. The X diffraction peak of the lignin-modified decolorized wood shows that the in situ modification of lignin by UV-assisted hydrogen peroxide treatment to remove the chromophore does not affect the crystalline properties of cellulose too much, and also changes the coexistence of crystalline and non-crystalline regions in wood cellulose, which greatly preserves the cellulose-based skeletal structure of interest to us. After filling the lignin-modified decolorized wood with epoxy resin impregnation, the peak shape of OTW was changed more compared to PW and LMW, which can be mainly explained by the filling and impregnation of epoxy resin. The main components of OTW are the wood template framework and epoxy resin as the main component, so the typical amorphous material scattered peaks of epoxy resin mask the crystalline nature of wood cellulose diffraction peaks, which further indicates that the epoxy resin has been successfully impregnated into the wood structure. The peaks in the XRD spectra of OTW, HTW and epoxy resin were consistent, and they all showed the diffuse peaks (bun peaks) characteristic of amorphous substances, which indicated that the crystalline structure of the substances did not change significantly before and after the modification by PDMS.
To further verify this conclusion, FTIR analysis was performed. As shown in Figure 3b, 1592 cm−1, 1503 cm−1 and 1453 cm−1, represent the aromatic fraction of lignin in the IR spectrum of the wood. Peaks 1592 cm−1 and 1503 cm−1 represent aromatic skeleton vibrations and C=O stretching; peak height 1453 cm−1 belongs to C-H deformation and asymmetry in -CH3 and -CH2- [26,27]. The lignin characteristic peaks of lignin-modified decolorized wood templates did not disappear compared to pristine wood, which means that most of the lignin is preserved and only the color-emitting regions are affected. The peak height of 1732 cm−1 represents the ester bond of the carboxyl or hemicellulose carboxyl group, which decreases during lignin modification, indicating the degradation of hemicellulose [28]. The FT-IR spectral images of OTW show that the benzene skeleton has benzene disubstitution absorption peaks near 826 cm−1 and 1592 cm−1, an epoxy group symmetric stretching vibration absorption peak near 1243 cm−1, and a C-O bond of the ethylene oxide group at 915 cm−1 [29]. All of these results indicate that the epoxy was successfully introduced, and all of them mainly show the characteristic peaks of the epoxy resin and no new characteristic peaks were formed. The methyl stretching vibration peak at 2924 cm−1 indicates that PDMS has been effectively coated on the transparent wood. The XRD and FT-IR characterization results together explain that the UV-coordinated hydrogen peroxide chemical treatment proposed in this work is able to retain most of the lignin and remove the color-emitting groups from the wood, and the epoxy resin is successfully impregnated and perfused inside the wood cell cavity, and PDMS is effectively coated on the transparent wood surface.

3.1.3. Wettability

Wood is particularly sensitive to moisture and moist environments due to its composition of mainly cellulose, which contains a large number of hydrophilic hydroxyl groups in its structure [30,31]. When droplets fall on the surface of a wood material, the contact angle can then be used to assess the ability of the liquid to wet the surface of the wood solid material. The surface wettability and water contact angle of pristine wood, optically transparent wood and hydrophobic transparent wood were studied by the water contact angle system as shown in Figure 4. The initial water contact angle of pristine wood was measured to be 76° at the instantaneous moment (0 s) when a water drop was dropped on the wood surface, and the contact angle changed to 0° within 60 s. The water drop was completely absorbed and disappeared within a short time, which is due to the high hygroscopicity of pristine wood due to its porous structure and the presence of a large number of hydrophilic hydroxyl groups. The initial contact angle (0 s) of the OTW samples was 105° compared to pristine wood, but the water droplets on the sample surface were still being absorbed with time, and the water contact angle decreased to 82° after 120 s. The improved surface wettability of optically transparent wood can be attributed to the reduced accessibility of absorbent hydroxyl groups due to the filling of cell wall pores by epoxy resin impregnation, which can improve the hydrophobicity of optically transparent wood samples. On the other hand, since the epoxy resin also contains some hydroxyl groups, the optically transparent wood also possesses a certain degree of water absorption, which explains the phenomenon that the water contact angle of the OTW surface starts to decrease with time. After the low surface energy modification treatment with PDMS coating, HTW achieved the highest initial water contact angle (131.6°) and more stable hydrophobicity (only 0.8° reduction in water contact angle in 120 s) compared to the uncoated OTW. This phenomenon can be explained by the fact that PDMS provides a lower surface energy for OTW, combined with some micro–nano structures on the surface of pristine wood and some micro-layers formed after epoxy filling, which can trap a larger portion of air and thus reduce the contact of liquid droplets with the surface of HTW, which helps to improve the moisture absorption and dimensional stability of both pristine wood and optically transparent wood, offering the possibility of optically transparent wood for outdoor applications such as windows and glass.
Meanwhile, the adhesion behavior of the surface of the HTW sample was investigated by moving a 5 μL water droplet up and down on its surface. As shown in Figure 5a–f, the static continuum of water droplets approaching, touching and leaving the coating was recorded. It can be seen that the water droplets leave the coating easily, which indicates that the low surface energy modification of PDMS can significantly reduce the adhesion of water droplets on the transparent wood surface.

3.1.4. Optical Characteristics

Optical photographs of the pristine wood, lignin-modified decolorized wood, optically transparent wood, and hydrophobically transparent wood are shown in Figure 6. It can be seen that after the UV-assisted chemical decolorization with hydrogen peroxide, the wood chips change from their natural yellow-brown color to white, and although the lignin-absorbing chromophores are removed, the pristine wood and the lignin-modified decolorized wood are still optically opaque because the layered porous structure of the wood remains (Figure 6a,b). When epoxy resin is impregnated and filled into LMW, OTW can show good optical transparency because the matrix framework structure formed by epoxy resin and cellulose and hemicellulose in wood has comparable refractive index, epoxy resin fills the pores of wood, and the cured epoxy resin has high light transmission (90.82%~91.06%). As shown in Figure 6c, when the sample is placed on the printed text surface, the background can be clearly observed through the optically transparent wood, and the text in the background is clearly readable. Moreover, because the PDMS material has excellent light transmission, HTW still maintains good optical transparency (89.76%~90.16%) after the hydrophobic modification treatment, and the improvement of the hydrophobic properties of the sample surface does not seem to affect the optical transparency of TW by human eye observation.
To further verify whether there is a significant change in the light transmission of the optically transparent wood after hydrophobic modification, the A and B sides (i.e., the front and back sides of the transparent wood along the longitudinal direction) of Figure 6c,d were further tested and analyzed using a color haze meter. The transmittance curves for OTW and HTW in the wavelength range of 400–700 nm about the A and B sides are shown in Figure 7.
From the curves, it can be seen that the optical properties of the positive and negative sides of both OTW and HTW are almost the same, but there are slight differences in some wavelength ranges. This phenomenon can be explained by the anisotropy and natural non-uniformity of wood, considering the structural and morphological characteristics of natural wood, there are slight differences in the properties at different locations of the wood, which is what causes the differences in the optical properties of the samples after the lignin-modified decolorization treatment and the epoxy resin impregnation filling. On the other hand, the roughness condition of the sample surface also has an influence on the optical performance, considering that the roughness of different micro–nano levels causes different degrees of light scattering and refraction. Compared with OTW, the optical transmittance of HTW is almost the same, but the haze increases by nearly 7% to about 80% (Table 1), and this high haze result is very favorable for practical applications in photovoltaic fields such as solar cells. Meanwhile, it is obvious from the data in Table 1 that both achieve a light transmission of about 90%, which further confirms the feasibility of the UV-assisted in situ modification of lignin to remove the color-emitting groups to retain most of the lignin in the treatment.

4. Conclusions

In this paper, a green and efficient top-down method was adopted to rapidly remove raw chromophores from wood and retain most of the lignin using a chlorine-free reagent hydrogen peroxide treatment and irradiation under UV light. To obtain both high transmittance and haze, we applied epoxy resin immersed in a lignin in situ modified decolorized wood template, followed by low surface energy modification by PDMS to obtain an optically transparent wood with good hydrophobic effect, with optical transmittance up to 91% and haze up to 80%, and the optical properties of both sides of the material are almost the same. The SEM images and chemical composition analysis showed that delignification treatment was not a necessary step in the manufacture of transparent wood, and in situ modification of lignin was able to remove most of the color-emitting groups while retaining the wood framework structure, and the epoxy resin was successfully impregnated and infused into the wood pores and cell cavities, and PDMS was effectively coated on the transparent wood surface, without significant effects of chemical treatment and PDMS modification on the crystalline properties of the substance. The surface wettability analysis demonstrates that the hydrophobic transparent wood holds a water contact angle of up to 131° compared to the hydrophilic nature of the pristine wood, which proves that its surface is hydrophobic, which will better overcome harsh external environments such as rain, snow, frost and fog, making it ideal for future materials in the construction industry. In conclusion, we have demonstrated an optically transparent wood with hydrophobic properties via UV assistance, and the prepared hydrophobic transparent wood has a wide application potential in areas such as smart windows, windshields and solar cell substrates.

Author Contributions

Conceptualization, H.W. and H.Y.; methodology, H.W.; software, H.W.; validation, H.Y., H.W. and H.M.; formal analysis, H.W.; investigation, H.W.; resources, H.Y.; data curation, H.W.; writing—original draft preparation, H.W.; writing—review and editing, H.Y. and H.M.; visualization, H.W.; supervision, H.M. and H.Y.; project administration, H.M.; funding acquisition, H.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Fundamental Research Funds for the Central Universities (No. 2572020BC07).

Data Availability Statement

Data are available upon request due to privacy or ethical restrictions. Data from this study are available from the corresponding authors upon request. Because of the privacy implications of the data in this study, these data are not publicly available.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic diagram for the preparation of UV-assisted, hydrophobic, optically transparent wood.
Figure 1. Schematic diagram for the preparation of UV-assisted, hydrophobic, optically transparent wood.
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Figure 2. SEM images at different magnifications: (a,b) pristine wood sample, (c,d) lignin-modified decolorized wood sample, (e,f) optically transparent wood sample. The microscopic shape of the sample can be clearly seen at the red arrow in the figure.
Figure 2. SEM images at different magnifications: (a,b) pristine wood sample, (c,d) lignin-modified decolorized wood sample, (e,f) optically transparent wood sample. The microscopic shape of the sample can be clearly seen at the red arrow in the figure.
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Figure 3. The XRD (a) and FT-IR (b) patterns of several groups of wood samples.
Figure 3. The XRD (a) and FT-IR (b) patterns of several groups of wood samples.
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Figure 4. Results of water contact angle for three groups of samples at different times.
Figure 4. Results of water contact angle for three groups of samples at different times.
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Figure 5. Continuous images of water droplets on a syringe suspended on the HTW surface: (a–f). The arrows indicate the direction of motion of the syringe.
Figure 5. Continuous images of water droplets on a syringe suspended on the HTW surface: (a–f). The arrows indicate the direction of motion of the syringe.
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Figure 6. Figure 6. Pictures of wood samples during experimental treatment: (a) Pristine wood, (b) Lignin-modified wood, (c) Optically transparent wood, (d) Hydrophobic transparent wood. The photo was taken by placing the sample on the emblem of Northeast Forestry University.
Figure 6. Figure 6. Pictures of wood samples during experimental treatment: (a) Pristine wood, (b) Lignin-modified wood, (c) Optically transparent wood, (d) Hydrophobic transparent wood. The photo was taken by placing the sample on the emblem of Northeast Forestry University.
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Figure 7. Transmittance curves of OTW and HTW A and B sides.
Figure 7. Transmittance curves of OTW and HTW A and B sides.
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Table
Table 1. Average transmittance and haze of OTW and HTW A and B sides.
Table 1. Average transmittance and haze of OTW and HTW A and B sides.
Optical PropertiesOTW-AOTW-BHTW-AHTW-B
Transmittance (%)91.0690.8289.7690.16
Haze (%)72.4573.2080.1979.69
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Wang, H.; Yang, H.; Mu, H. Preparation of Hydrophobic Optically Transparent Wood via An Efficient UV-Assisted Route. Forests 2023, 14, 759. https://doi.org/10.3390/f14040759

AMA Style

Wang H, Yang H, Mu H. Preparation of Hydrophobic Optically Transparent Wood via An Efficient UV-Assisted Route. Forests. 2023; 14(4):759. https://doi.org/10.3390/f14040759

Chicago/Turabian Style

Wang, Hang, Hong Yang, and Hongbo Mu. 2023. "Preparation of Hydrophobic Optically Transparent Wood via An Efficient UV-Assisted Route" Forests 14, no. 4: 759. https://doi.org/10.3390/f14040759

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