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"Wood King" and his transparent wood have a new breakthro

Time:2021/02/02 丨 source:未知 丨 visit count:

Professor Hu Liangbing Sci. Adv.: "Wood King" and his transparent wood have a new breakthrough!

【Research Background】
Transparent wood stands out due to its unique hierarchical structure, high specific strength and good light management performance, showing a wide range of applications, including optoelectronic devices, energy-saving building materials, solar cell light management and touch panels. However, the traditional liquid-phase delignification process to make transparent wood usually consumes a lot of chemicals and energy.
[Article Introduction]
Recently, the team of Professor Liangbing Hu from the University of Maryland published the latest research progress on Science Advances under the title of "Solar-assisted fabrication of large-scale, patternable transparent wood". It reported a method of changing wood by using solar-assisted chemical brushing. Lignin structure to produce optically transparent wood method. This method retains most of the lignin as a binder and provides a strong wood support for polymer penetration, while greatly reducing chemical and energy consumption and processing time. This kind of transparent wood combines efficient, patternable and scalable production methods and is a promising candidate material for energy-efficient building applications.
[Article Interpretation]
1. Manufacturing process and technology
Figure 1A shows the manufacturing process of transparent wood through two steps: first, by brushing H2O2 on the wood surface to change the structure of lignin, and then irradiating it with ultraviolet light, natural sunlight can be used to remove the light-absorbing chromophore of lignin. Then, the epoxy resin with matching refractive index can easily penetrate into the microporous wood structure to prepare transparent wood, forming a dense microstructure and low light scattering. The layered porous structure of natural wood promotes the rapid penetration/diffusion of H2O2 solution and UV light capture, thereby effectively removing the light-absorbing chromophore of lignin and significantly reducing its light absorption rate (< 4%). Compared with the lignin-removed wood (0.4 MPa), the lignin-modified wood in this study also showed a much higher tensile strength (20.6 MPa) due to the combination of the modified lignin and the well-oriented cellulose fibrils.

Figure 1 Schematic diagram of making transparent wood and its pattern demonstration. (A) A schematic diagram of a simple, effective, environmentally friendly, scalable and low-cost transparent wood manufacturing method; (B) a large-size transparent wooden board image along the longitudinal direction; (C) a transparent wood image along the horizontal direction.
2. Structural features
Fourier transform infrared spectroscopy (Figure 2A) shows that despite the degradation of the lignin chromophore, this method still retains the lignin aromatic skeleton. In addition, the lignin content of natural wood and lignin-modified wood samples were ~23.5% and ~19.9%, respectively (Figure 2B), which further showed that most of the lignin structure was well preserved after treatment.
The preserved lignin can be used as a binder to enhance the mechanical properties of lignin-modified wood and provide a strong wood skeleton for subsequent polymer penetration. Thanks to the strong mechanical properties, this chemical brushing combined with ultraviolet light irradiation method was further used to prepare a large lignin modified wood sample with a length of about 1 m (Figure 2C).
Use this lignin to modify wood, and then use epoxy resin to infiltrate it through vacuum to obtain the final transparent wood product. Figure 2 shows the SEM images of natural wood, lignin modified wood and transparent wood. Natural wood shows a three-dimensional hierarchical and interconnected porous structure (Figures 2d and g), which is characterized by microchannels with diameters in the range of about 15 to 300 μm. The diameter of the microchannels in the lignin-modified wood was in the range of 10 to 270 μm (Figure 2e and h), indicating that the lignin-modified wood maintained a hierarchical, interconnected porous structure after treatment. Finally, the SEM image of the transparent wood shows that the epoxy resin penetrates well into the pores of the lignin-modified wood (Figure 2F and I), forming a dense structure, which helps to suppress light scattering and increase light transmittance.

Figure 2 Characterization of structure and composition of transparent wood. (A) Infrared spectroscopy; (B) the retained lignin content of natural wood, lignin modified wood and transparent wood; (c) rice-level lignin modified wood pictures; natural wood (D), lignin modified wood ( E) Horizontal SEM images of transparent wood (F); (G) Longitudinal SEM images of natural wood, (H) lignin modified wood and (I) transparent wood.
3. Optical and mechanical properties of transparent wood
The transparent wood in the transverse and longitudinal directions shows excellent optical properties. Along the vertical (Figure 3A) and horizontal (Figure 3A) transparent wood image display, you can clearly see the background image. Compared with the light transmittance of natural wood (longitudinal <6%, transverse <36%), the transparent wood along the longitudinal and transverse directions has a high light transmittance of about 90% in the visible light wavelength range (400 to 800nm) (Figure 3c). Due to the removal of the light-absorbing chromophore of lignin, the absorption rate of transparent wood (close to 0%) at visible light wavelengths is also much lower than that of natural wood (longitudinal <83%) (Figure 3d), which makes Almost all visible light can pass through transparent wood. <span="">
In addition, the transparent wood retains the vertically arranged microchannels, allowing light to propagate along the channel direction. As shown in Figure 3E, a 650nm red single-mode laser is used to illuminate the transparent wood vertically along the longitudinal and lateral directions. The results show that the light beam propagates along the direction of the wood channel, indicating that the transparent wood has the light guide ability and anisotropic light transmittance.
The mechanical properties of natural wood and transparent wood in different stretching directions were measured. The tensile strength of transparent wood in the longitudinal and transverse directions are 46.2MPa and 31.4MPa, respectively, which are 1.8 times and 44.8 times higher than the tensile strength of natural wood, respectively. Compared with natural wood, the toughness of transparent wood is also significantly improved, as shown in Figure 3G.

Figure 3 Optical and mechanical properties of natural wood and transparent wood. Pictures of transparent wood along the vertical (A) and horizontal (B); light transmittance (C) and absorptivity (D) of natural wood and transparent wood; (E) light transmission of transparent wood; light transmission of natural wood and transparent wood Tensile strength (F) and toughness (G).
4. Pattern characteristics
The chemical brushing combined with the ultraviolet light irradiation method can selectively bleach the specified area of the wood sample, which makes it possible to easily prepare transparent wood with different patterns (Figure 4A and B). A transparent wood sample with a number ("4") and letter ("A") pattern is shown in Figures 4C and D. In addition, a longitudinal transparent wood sample with more complex geometric shapes was also displayed, including two transparent circles minus an opaque diamond and a Yin and Yang symbol (Figure 4E and F), indicating that the use of chemical brushing combined with UV illumination can be Transparent wood of any pattern.

Figure 4 The manufacturing process of patternable transparent wood. Schematic diagram of transparent wood production process (A) and demonstration patterning (B); transparent wood samples with (C) number 4 and (D) letters A, (E) circles and diamonds, and (F) Yin and Yang patterns.
5. Solar-assisted manufacturing of transparent wood
Use solar ultraviolet light as the driving force for wood decolorization to realize rapid production of transparent wood. The schematic diagrams in Figures 5A and B show the large-scale preparation of transparent wood based on the mature rotary wood cutting method and solar-assisted chemical brushing process in the wood industry. In the author's laboratory, after the epoxy resin was penetrated into the above-mentioned lignin-modified wood, a transparent wood with high light transmittance of 400mm×110mm×1mm was obtained (Figure 5C). Furthermore, the author evaluated the energy consumption, cost and chemical emissions of the transparent wood production process and compared it with the delignification method based on sodium chloride solution (Figure 5D and E).

Figure 5 Solar energy assists large-scale production of transparent wood. (A) Potential large-scale manufacturing of transparent wood; (B) Outdoor processing of lignin-modified wood; (C) Photograph of a large transparent wood; (D) Solar-assisted chemical scrubbing process and sodium chloride solution delignification process Energy consumption, chemical cost and waste discharge; (E) Comparison of transparent wood manufacturing process.
【in conclusion】
In summary, the research demonstrates a fast, economical, and sustainable method to make patternable transparent wood based on a scalable solar-assisted chemical brushing method. In this process, the light-absorbing chromophore of lignin is removed, which can improve the optical properties of the resulting transparent wood without completely destroying the aromatic structure of lignin. This transparent wood exhibits excellent optical properties without significantly sacrificing the mechanical strength of the material due to the high preservation of lignin. In addition, the solar-assisted chemical brushing method can selectively process the designated area of the wood sample, giving the transparent wood excellent designable patterning ability. Compared with the solution-based delignification process, the solar-assisted chemical brushing in this research has higher production efficiency, lower cost, and is more sustainable and controllable. This cheap and efficient transparent wood manufacturing can also use natural solar energy, expanding the application of this technology to large-scale industrial production.
Qinqin Xia, Chaoji Chen, Tian Li, Shuaiming He, Jinlong Gao, Xizheng Wang, Liangbing Hu, Solar-assisted fabrication of large-scale, patternable transparent wood, Science Advances,2021. DOI:10.1126/sciadv.abd7342

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