Can you imagine a window pane made just of wood? Qi Zhou, KTH Royal Institute of Technology, Stockholm, Sweden, and colleagues have developed a method that could make the production of optically transparent wood composites much simpler [1]. The cornerstone of this method is a compressible wood xerogel that is readily impregnated with acrylic resin, producing a transparent wood composite with a high wood content and high optical transmittance. The stability of the new material and the simple production method offer new perspectives for the large-scale production of biobased and biodegradable functional materials.

 

The Making of Transparent Wood

Transparent wood is made from delignified wood and acrylic resin. In principle, the resin, which hardens to a clear solid, fills the voids in the cellular structure of the wood that had been occupied by lignin. Lignin is a major component of plant cell walls and is the light-absorbing component of wood. Pulping and bleaching processes remove the lignin, leaving a porous cellulose and hemicellulose matrix that can be impregnated with the resin. The result is a lightweight, optically transparent material with low thermal conductivity but beneficial light-guiding properties and strength.

The material gets its unique properties from the cellulose fiber content and the geometric orientation of the tubular fiber cells. This structure allows sunlight to pass but in a directed manner. What’s more, the material is more thermally insulating than normal glass, pointing to possible applications for energy-saving glazing designs. It has potential for use in solar panels, decorative elements in buildings, the medical field, transparent electronics, and renewable packaging.

However, at present, the technology is still limited to the laboratory and the experimental production of thin and small-scale materials. One hindrance to production is the impregnation step, which requires solvents and vacuum technology. In addition, the wood content in the final material is usually low, reducing its sustainability. “Transparent wood usually contains less than 10 % wood mass, so the major component is the polymer matrix,” says Zhou.

 

 

Aerogels and Xerogels

Looking for ways to improve the impregnation process, the team turned to aerogel technology. Aerogels are extremely lightweight, thermally insulating mesoporous networks made by freeze-drying gels. Their highly porous structure is readily infiltrated by liquid, making them a promising starting point for finding improved chemical structures. However, their production is energy-intensive.

To increase the porosity of the wood samples, the team used a chemical fibrillation technique. Treating the delignified wood with an oxidizing agent, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), caused cellulose microfibrils to separate from the cellulose fibers in the cell wall. After removing the solvent and crosslinking with aluminum ions, the researchers obtained a stable wood xerogel—a mesoporous, lightweight, dried gel with a large surface area, which, in contrast to aerogels, can be produced at ambient conditions.

This wood xerogel was easily impregnated by soaking in acrylic resin. After UV curing, the researchers obtained a transparent wood composite with excellent performance. In contrast, without TEMPO treatment, the delignified wood had a small surface area, was insufficiently impregnated, and yielded an opaque composite.

 

Compression

The researchers also highlighted another property: The wood xerogel is compressible. As its framework is the structure of the tubular cellulose fiber cells, it could be compressed perpendicularly to the direction of these fibers. The scientists highlight that the compressed xerogel, at only a quarter of the thickness of the original, retained its mesopores and could be impregnated with the same ease as an uncompressed block. This means an increase in the wood content in the resulting composite from 10 % to 50 %.

“The optical properties of the transparent wood composites are also significant,” says Zhou. “The high optical transmittance of the wood xerogel composites remained at high wood volume fractions, suggesting that aggregation of cellulose microfibrils was not induced by compression of the wood xerogel.” This means that not only has the production of transparent wood been simplified, but also that the resulting material has a higher wood content for better sustainability and performance.

The new wood xerogel could be the basis for a new generation of wood composites, potentially including large functional window panes and new wood-based optically transparent insulators. A 50 % biobased, possibly biodegradable window made of wood? Not a bad idea.

 

Reference

[1] Shennan Wang, Lengwan Li, Li Zha, Salla Koskela, Lars A. Berglund, Qi Zhou, Wood xerogel for fabrication of high-performance transparent wood, Nat. Commun. 2023, 14, 2827. https://doi.org/10.1038/s41467-023-38481-x

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