Scalable aesthetic transparent wood for energy efficient buildings

The increasing global energy consumption and environmental pollution necessitates the development of energy-saving building materials. Natural materials, such as wood and its derivatives, are attractive alternatives due to their abundance, renewability, low cost, and sustainability. Recent advancements have focused on transparent wood composites, which combine the anisotropic hierarchical structure of wood with improved optical, mechanical, and thermal properties. These composites offer advantages like lightweight, high optical transmittance, tunable haze, low thermal conductivity compared to glass, and excellent mechanical robustness. Furthermore, the light-guiding effect in transparent wood facilitates effective sunlight harvesting, contributing to energy savings and improved indoor lighting. However, current fabrication methods often involve complete delignification, which can damage the wood's structure and reduce the visibility of natural patterns. This study aims to address these limitations by developing an aesthetic transparent wood that retains the natural aesthetics of wood while maintaining its desirable functional properties.

Existing transparent wood composites are generally fabricated through a complete or near-complete delignification process, removing lignin and extractives to enhance transparency. However, this intensive chemical treatment can significantly alter the wood's original structure, leading to a loss of natural patterns and potentially reduced mechanical strength. While previous research has focused on the optical, mechanical, and thermal properties of transparent wood, the preservation of aesthetic features such as natural growth ring patterns and scalable manufacturing processes have received less attention. This work builds upon previous studies by emphasizing the preservation of aesthetic features while achieving high optical transparency, UV-blocking, thermal insulation, and mechanical strength.

The researchers developed an aesthetic transparent wood using spatially selective delignification of Douglas fir, chosen for its pronounced structural contrast between earlywood (EW) and latewood (LW). A short 2-hour chemical treatment (using an acidic NaClO2 method) selectively removes lignin from the EW while preserving it in the LW, maintaining the growth ring patterns. Refractive-index-matched epoxy is then infiltrated into the nanoscale framework, creating the transparent wood with preserved patterns. Two types of aesthetic wood were fabricated: aesthetic wood-R (channels perpendicular to the wood plane) and aesthetic wood-L (channels parallel to the wood plane). The fabrication process was evaluated via scanning electron microscopy (SEM), Raman spectroscopy imaging with vertex component analysis (VCA), and other characterization techniques to assess the morphology, chemical composition, optical properties, mechanical properties, and scalability. The optical properties (transmittance, haze, reflectance, UV-blocking), mechanical properties (tensile strength, toughness), and thermal conductivity were measured using standard techniques. The scalability of the process was demonstrated by fabricating a large sample (320 mm × 170 mm × 0.6 mm). Additionally, the weathering stability of the aesthetic wood was assessed by outdoor exposure testing, analyzing changes in optical and mechanical properties.

The spatially selective delignification process successfully preserved the natural wood patterns while achieving high optical transparency. Aesthetic wood-R demonstrated an average transparency of 80% at 600 nm, while aesthetic wood-L achieved 87% transmittance and 65% haze at 600 nm. The material exhibited excellent UV-blocking capabilities, effectively shielding UVC and UVB, and a significant portion of UVA radiation. The 2-hour delignification treatment resulted in a weight loss of 13.5 wt%, significantly less than the 35 wt% loss observed after 10 hours, demonstrating the efficiency of the selective delignification process. The aesthetic wood also displayed improved mechanical properties compared to natural wood. Aesthetic wood-L exhibited a high longitudinal tensile strength of 91.95 MPa and a toughness of 2.73 MJ m⁻³. Furthermore, the process demonstrated excellent scalability, allowing for the creation of large-sized samples. Weathering stability tests showed minimal degradation in optical and mechanical properties after 3 weeks of outdoor exposure, indicating promising short-term durability. The low thermal conductivity of 0.24 W m⁻¹K⁻¹ was also observed, indicating good thermal insulation properties.

The results demonstrate the successful fabrication of a novel aesthetic transparent wood with a combination of desirable properties for energy-efficient building applications. The spatially selective delignification process enables the preservation of natural wood aesthetics without compromising optical transparency, UV-blocking, or mechanical strength. The high scalability and promising durability make this material a strong candidate for replacing traditional materials such as glass in various applications, like glass ceilings, rooftops, transparent decorations, and indoor panels. The combination of aesthetic appeal and functional performance holds significant promise for sustainable construction. The findings address the need for energy-efficient and aesthetically pleasing building materials, offering a viable and sustainable alternative to current options.

This study successfully developed a scalable aesthetic transparent wood that retains the natural beauty of wood while exhibiting excellent optical, mechanical, and thermal properties. The spatially selective delignification method is efficient and allows for the creation of large-scale samples. The material shows promise as a sustainable alternative for energy-efficient building applications. Future research could focus on improving long-term durability and exploring the use of different wood species to expand the range of aesthetic patterns achievable.

The study primarily focused on short-term weathering stability, with only 3 weeks of outdoor exposure testing. Longer-term studies are needed to fully assess the durability of the aesthetic wood in various environmental conditions. Further investigation into the effects of different epoxy types and concentrations on the properties of the transparent wood could lead to further optimization of the material.