Scalable aesthetic transparent wood for energy efficient buildings

The study addresses the need for green, energy-saving building materials that reduce energy consumption and environmental pollution. Transparent wood composites have emerged as promising materials due to their light weight, high transmittance, tunable haze, low thermal conductivity relative to glass, and good mechanical robustness. However, conventional fabrication relies on complete or near-complete delignification, which can damage wood’s hierarchical structure and erase natural growth ring aesthetics while also lengthening processing time. The research aims to create a transparent wood that preserves natural wood patterns (aesthetics) while achieving high optical transmittance, UV-blocking, low thermal conductivity, good mechanical strength, and scalability via a rapid, spatially selective delignification and polymer infiltration process.

Prior work on transparent wood largely used extensive delignification to remove lignin and chromophores to achieve optical clarity, sometimes retaining partial lignin (~80%). These approaches often degrade cell walls, diminish growth ring visibility, and require long processing times. Studies focused on anisotropy and properties (optical, mechanical, thermal) with limited attention to preserving natural aesthetics or enabling efficient scalable manufacturing. Some efforts achieved blurred patterns by tuning processing but struggled to deliver clear, designable patterns with simultaneously high transparency, UV-blocking, low thermal conductivity, and high mechanical strength. Softwoods and hardwoods differ structurally; diffuse-porous hardwoods (e.g., basswood, balsa) show bimodal pore distributions that lead to synchronous delignification and loss of distinctive patterns, whereas softwoods (e.g., Douglas fir, pine) present pronounced earlywood/latewood contrasts favorable for pattern preservation.

Materials: Douglas fir (primary), pine (additional), basswood and balsa (comparative). Chemicals: sodium chlorite (NaClO2, ~80%) and acetic acid; solvents: deionized water, ethanol (95%). Polymer: Aero-Marine epoxy resin (#300 and #21). Fabrication concept: Two orientations were defined—wood-R (aligned microchannels perpendicular to the surface) and wood-L (channels parallel), exploiting softwood’s annual growth ring contrast between earlywood (EW) and latewood (LW). Delignification: Prepare NaClO2 solution in DI water; adjust pH to ~4.6 with acetic acid. Immerse wood and maintain at boiling for ~2 h until EW turns white, indicating spatially selective delignification (EW largely delignified; LW retains some lignin to preserve patterns). Rinse with DI water three times and store in ethanol. Polymer infiltration and curing: Infiltrate delignified scaffolds thoroughly with the prepared epoxy. Cure for 24 h to produce dense polymer-filled structures with refractive index matching for transparency. Cutting strategies: Cross-section cutting for wood-R to highlight ring patterns; quarter-slicing for wood-L to create straight-line patterns. Demonstrated scalability to 320 mm × 170 mm × 0.6 mm panels. Characterization: SEM (Tescan XEIA FEG) for morphology before/after infiltration; Raman mapping with confocal Raman microscope (785 nm laser, 60x water immersion objective; 2 s integration, 600 nm step) and vertex component analysis to map lignin (bands near 1598, 1656, 1269 cm−1) and epoxy distribution (640, 1001, 1608 cm−1). Optical properties (transmittance, haze, reflectance) measured with UV–vis spectrometer (PerkinElmer Lambda 35, integrating sphere); absorbance A = 1 − T − R. Mechanical properties measured by tensile testing (Instron; specimens 90 mm × 60 mm; n=3). Thermal conductivity measured with a steady-state laser–infrared camera system using a sandwich setup (sample 1 cm × 1 cm × 2 mm between two Al blocks). Weathering stability assessed via 3-week outdoor exposure followed by optical and mechanical tests. Light guiding tested using a house model with a xenon lamp solar simulator (illumination area 5 cm diameter).

- Optical transparency and haze: Aesthetic wood-R achieved ~80% average transmittance at 600 nm with ~93% haze, enabling anti-glare and diffuse lighting. Aesthetic wood-L (0.6 mm thick) showed 87% total transmittance and 65% haze at 600 nm. - UV-blocking: 2-mm-thick samples treated for 2 h blocked nearly 100% of UVC and UVB and most UVA (200–400 nm), while maintaining high visible transmittance (~80% at 600 nm) and low reflectance. Longer treatments (9 h) reduced UVA blocking but increased transmittance at 600 nm from 47% to 85%. - Thermal insulation: Thermal conductivity measured at 0.24 W m−1 K−1, lower than standard glass, indicating better insulation. Energy modeling suggests that replacing single-pane glass ceilings with aesthetic wood-L increases indoor temperature (for a baseline ΔT = 30 °C and constant heating power) by approximately +2.43 °C for 6-mm thickness and +0.81 °C for 2-mm thickness; similarly, it would decrease indoor temperature under cooling conditions. - Mechanical performance: Aesthetic wood-L exhibited high tensile strength of 91.95 MPa and toughness of 2.73 MJ m−3. Aesthetic wood-R showed tensile strength of 21.56 MPa (vs natural R-wood 6.24 MPa) and toughness 0.523 MJ m−3. Fractography showed polymer bridging and full lumen filling, with anisotropy attributed to aligned cellulose nanofibers. - Scalability: Successfully fabricated large samples up to 320 mm × 170 mm × 0.6 mm with preserved patterns and dense polymer infiltration, demonstrating potential for scale-up with short processing times. - Pattern control: Clear, designable patterns retained due to selective delignification (EW vs LW). Transmittance varied spatially: EW averaged ~86%, LW ~68%. Layer stacking allowed creation of lattice patterns. - Weathering stability: After 3 weeks outdoor exposure, transmittance decreased slightly and haze increased (e.g., aesthetic wood-R haze ~93% to ~98% over 400–800 nm). Mechanical strength showed no significant degradation. - Species dependence: Softwoods (Douglas fir, pine) enabled pattern preservation; diffuse-porous hardwoods (basswood, balsa) did not retain patterns due to synchronous delignification and bimodal pore structures.

The work demonstrates that spatially selective delignification leveraging intrinsic softwood heterogeneity (EW versus LW) can preserve natural growth ring aesthetics while delivering the optical clarity and functional performance expected of transparent wood. Index-matched epoxy infiltration yields high transmittance with high haze, enabling anti-glare diffuse illumination and light guiding superior to glass. The retained lignin in LW confers UV-blocking while minimizing processing time and maintaining structural integrity, addressing prior challenges of structural degradation from exhaustive delignification. The resulting composite also exhibits low thermal conductivity and robust mechanical properties, and can be manufactured at larger sizes than previously reported delignified-wood-based transparent woods. Model-based analysis indicates real potential for improved indoor thermal management when replacing glass with aesthetic wood panels. Collectively, these results show that the approach simultaneously addresses aesthetics, energy efficiency, and scalability for building applications.

This study introduces an aesthetic transparent wood that preserves native wood patterns via a rapid, spatially selective NaClO2-based delignification followed by epoxy infiltration. The composite achieves high visible transmittance (~80–87%), high haze (65–93%), strong UV-blocking (200–400 nm), low thermal conductivity (0.24 W m−1 K−1), and excellent mechanical performance (up to 91.95 MPa tensile strength; 2.73 MJ m−3 toughness). The process is compatible with large-area fabrication (up to 320 mm × 170 mm × 0.6 mm) and supports pattern design by layering. These attributes highlight aesthetic wood’s potential for energy-efficient, glare-free, patternable building components such as ceilings, rooftops, windows, and interior panels. Future work should focus on increasing thickness without compromising aesthetics to enhance load-bearing capacity, broadening softwood species utilization for diverse patterns, and assessing long-term outdoor durability and performance under varied environmental conditions.

- Long-term durability: While short-term outdoor exposure (3 weeks) showed minimal degradation, long-term weathering, UV exposure, and environmental cycling effects remain unquantified. - Thickness and load-bearing: Current large-scale demonstrations are relatively thin (e.g., 0.6 mm); thicker panels are desired for structural applications without sacrificing transparency or patterns. - Anisotropy: Mechanical properties are direction-dependent due to wood’s anisotropic structure, which may constrain certain load-bearing designs. - Species constraints: Diffuse-porous hardwoods (e.g., basswood, balsa) did not retain aesthetic patterns under this protocol, limiting pattern-preserving fabrication largely to selected softwoods. - UV-blocking versus transparency trade-off: Extended delignification increases visible transmittance but reduces UVA blocking, requiring optimization per application.