Optical and thermal characterization of thermochromic transparent wood as a potential smart glazing material

Abstract

Transparent wood (TW) is an emerging bio-based glazing material, and the incorporation of thermochromic pigments offers a route toward glazing that modulates visible light in response to temperature. However, the effect of thermochromic pigment concentration on the coupled optical and thermal behaviour of TW has not been quantified, and large-area thermochromic TW panels have rarely been demonstrated. In this study, thermochromic transparent wood (TTW) panels of 10 × 10 cm² and 3 mm thickness were fabricated from balsa wood by NaClO₂-based delignification followed by vacuum impregnation with an epoxy resin containing a microencapsulated leuco-dye pigment at 0.3, 0.5 and 0.7 wt%. Delignification reduced the Klason lignin content from 20.38% to 2.09%, enabling uniform polymer infiltration. Ultraviolet–visible–near-infrared (UV–Vis–NIR) spectrophotometry showed that luminous transmittance (T_lum) in the cold state decreased from 69.9% for pigment-free TW to 59.0%, 43.0% and 35.0% at 0.3, 0.5 and 0.7 wt%, while thermal activation reduced T_lum to 52.7%, 33.4% and 24.2%, corresponding to switching contrasts of 6.3, 9.6 and 10.8% points. Pigment loading therefore governs a direct trade-off between transparency and optical switching contrast. The switching time increased from 327.6 s to 377.8 s with increasing pigment content. Differential scanning calorimetry (DSC) revealed a single endothermic feature at 53.45–56.05 °C with an associated enthalpy of 6.67–10.33 J g⁻¹, which is attributed to enthalpy relaxation at the glass transition of the epoxy matrix rather than a latent-heat phase transition, thus indicating that the material has no thermal energy storage function. The pigment-free panel exhibited a thermal conductivity of 0.267 W m⁻¹ K⁻¹ at 40 °C, 3.6 times lower than the 0.958 W m⁻¹ K⁻¹ measured on the same instrument for a clear glass reference of equal thickness, indicating reduced conductive heat transfer relative to conventional glazing without qualifying the material as a thermal insulator. These results establish quantitative design limits for leuco-dye-based thermochromic transparent wood as an adaptive daylight-control glazing element for warm-climate buildings.