Transparent wood represents a major breakthrough in materials science, blending the inherent qualities of wood with improved optical transparency. This is accomplished by removing lignin-the component responsible for light scattering-through specialized processing techniques.
Glycerol infiltration
One effective technique for producing transparent wood involves the infiltration of glycerol into wood specimens. In a notable study, poplar wood was cut to precise dimensions and treated with glycerol, followed by the application of a transparent epoxy resin coating. This method not only improves optical transparency but also preserves the mechanical strength of the wood, making it well-suited for interior design and construction applications (Şahin & Bingöl, 2024).
The glycerol infiltration process enhances the physical and chemical properties of wood through a combination of chemical transformation and thermal modification. The procedure typically begins with impregnating the wood using a glycerol-water solution (ranging from 5 to 60 wt.%), which causes the wood to swell and facilitates deeper penetration (Georges & Stephane, 2012). To further improve infiltration, the wood is often compressed to 50-90% of its original thickness before immersion in the glycerol solution (Hiroyuki et al., 1992).
Following impregnation, the wood is subjected to thermal treatment at temperatures between 120 and 200 °C. This step accelerates lignin degradation and alters cellulose crystallinity, thereby reducing the wood’s hygroscopicity and increasing its resistance to biological decay (Shuang et al., 2021). Glycerol also promotes the hydrolysis of wood polymers, resulting in smaller molecular fragments and the formation of novel compounds through a variety of chemical reactions (Demirbas, 2009). Additionally, esterification reactions between glycerol and acids-such as citric acid-during heat treatment further enhance the material’s durability and optical performance (Stephane & Georges, 2014).
Ethanol-aqueous solution
Another promising approach to transparent wood fabrication utilizes an ethanol-aqueous solution for delignification. This method involves drying wood veneers, selectively removing lignin at controlled temperatures, and bleaching with hydrogen peroxide. It preserves the wood’s native cellular structure while achieving high optical transmittance, offering an environmentally friendly alternative to more aggressive chemical treatments (Jing et al., 2019).
The ethanol-aqueous treatment process leverages a mixture of ethanol and water to improve various properties of wood, including pest resistance, flavor enhancement, and structural modification. This technique has diverse applications in wood preservation, food contact materials, and materials engineering.
In the context of delignification, the ethanol-water solution acts as a solvent for lignin and wood preservatives such as alkyl dimethyl benzyl ammonium saccharinate. The wood is first vacuum-treated and then pressurized to ensure deep impregnation of the solution, offering protection against biological agents like fungi and insects (Jones, 2004). When applied to wood particles, the solution is often heated, which enhances the extraction of flavor-related compounds-an effect particularly valued in food-related wood products (Gross & Barbier, 1997; Charles & F, 2002).
For delignification, research shows that using a 70% ethanol solution at elevated temperatures (around 220 °C) yields optimal results by efficiently breaking down lignin while maintaining cellulose integrity (Alexey et al., 2018; Puech et al., 1990). The addition of hydrogen peroxide in later stages serves as a bleaching agent, improving the wood’s final transparency and appearance.
Interface manipulation
Recent advancements in transparent wood technology have centered on manipulating the internal interfaces of wood structures to enhance optical performance. One such innovation involves acetylating wood templates, a process that introduces acetyl groups to the wood structure. This treatment significantly reduces interfacial debonding between the wood and polymer matrix, resulting in an impressive light transmittance of up to 92%-comparable to that of pure polymethyl methacrylate (PMMA). This technique is particularly effective for producing thicker transparent wood suitable for smart building applications (Li et al., 2018).
Several interface manipulation techniques have emerged to optimize transparency and durability. In the acetylation method, the wood is chemically modified to reduce the mismatch at the polymer-cell wall interface, thereby minimizing light scattering and improving optical clarity (Li et al., 2018). Another method combines ultraviolet (UV) light with hydrogen peroxide to eliminate lignin’s chromogenic groups, producing a hydrophobic transparent wood that achieves 90% light transmission (Wang et al., 2023).
Moreover, the integration of nanoparticles such as nanosilver and nano-vanadium oxide into the wood matrix has been shown to further improve optical properties. These nanoparticles facilitate better light propagation and control, expanding the functional potential of transparent wood in applications requiring tunable light management (Yanfeng & Ziya, 2018).
Solar-Assisted Chemical Brushing
A novel method for producing transparent wood involves a solar-assisted chemical brushing technique, which modifies the lignin structure while retaining its natural binding properties. This approach significantly reduces both chemical usage and energy consumption, making it a more sustainable alternative to conventional delignification methods. The resulting transparent wood exhibits high optical transmittance and customizable surface patternability, making it particularly well-suited for energy-efficient engineering and architectural applications (Xia et al., 2021).
The process harnesses solar energy to accelerate the removal of chromogenic groups within the lignin, thereby enhancing the wood’s optical clarity without fully removing the lignin. By preserving much of the lignin structure, the mechanical integrity and original framework of the wood are maintained-critical for successful polymer infiltration in later stages (Xia et al., 2021). The final material can achieve over 90% light transmittance while maintaining a high degree of haze, offering both functional efficiency and visual appeal. The technique also enables the formation of diverse surface patterns, expanding its aesthetic and practical uses in modern architecture (Xia et al., 2021).
Bibilography
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- Li, Y., Yang, X., Fu, Q., Rojas, R., Yan, M., & Berglund, L. (2018). Towards centimeter thick transparent wood through interface manipulation. Journal of Materials Chemistry, 6(3), 1094-1101. https://doi.org/10.1039/C7TA09973H
- Li, J., Ding, M., Liao, C., & Hui, Z. (2019). Method of making transparent wood with organic solvent, prepared transparent wood and application thereof.
- Şahin, S., & BİNGÖL, Ş. (2024). Production of Transparent Wood Using Glycerine Extracted from Transverse and Longitudinal Sections of Poplar Wood. https://doi.org/10.21203/rs.3.rs-4274544/v1
- Stephane, A. C., & Georges, C. (2014). Method for treating substrate in wood material utilized for e.g. door, involves performing heat treatment for esterification of glycerol and citric acid, where material includes wood brought upto temperature between specific ranges.
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- Chen, S., Liu, R., Lei, Y., Morrell, J. J., & Yan, L. (2021). Accelerating thermal decomposition of wood cell wall with glycerol. Journal of Materials Research and Technology, 11, 1637-1644. https://doi.org/10.1016/J.JMRT.2021.02.011
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- Hu, L., Zhu, M., Li, T., Gong, A., & Song, J. (2017). Transparent wood composite, systems and method of fabrication. https://patents.google.com/patent/WO2017136714A1/en
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- Xia, Q., Chen, C., Li, T., He, S., Gao, J., Wang, X., & Hu, L. (2021). Solar-assisted fabrication of large-scale, patternable transparent wood. Science Advances, 7(5). https://doi.org/10.1126/SCIADV.ABD7342
- Wang, H., Yang, H., & Mu, H. B. (2023). Preparation of Hydrophobic Optically Transparent Wood via An Efficient UV-Assisted Route. Forests, 14(4), 759. https://doi.org/10.3390/f14040759
