Lignin-Retaining Transparent Wood

Abstract

Optically transparent wood, combining optical and mechanical performance, is an emerging new material for light-transmitting structures in buildings with the aim of reducing energy consumption. One of the main obstacles for transparent wood fabrication is delignification, where around 30 wt % of wood tissue is removed to reduce light absorption and refractive index mismatch. This step is time consuming and not environmentally benign. Moreover, lignin removal weakens the wood structure, limiting the fabrication of large structures. A green and industrially feasible method has now been developed to prepare transparent wood. Up to 80 wt % of lignin is preserved, leading to a stronger wood template compared to the delignified alternative. After polymer infiltration, a high-lignin-content transparent wood with transmittance of 83 %, haze of 75 %, thermal conductivity of 0.23 W mK^-1 , and work-tofracture of 1.2 MJ m^-3 (a magnitude higher than glass) was obtained. This transparent wood preparation method is efficient and applicable to various wood species. The transparent wood obtained shows potential for application in energy-saving buildings.

Keywords: building materials; delignification; energy saving; lignin; wood.

Scheme 1

Preparation of lignin‐retaining transparent wood. Lignin was mainly preserved in the wood structure after lignin‐retaining treatment using alkaline H₂O₂. By filling the wood structure with PMMA, a piece of transparent wood was obtained. The dimension of the transparent wood in the picture is 100 mm×100 mm×1.5 mm.

Scheme 2

Representative lignin reactions and structures contributing to wood color, as well as the main products of the two routes (NaClO₂‐based delignification and alkaline H₂O₂‐based lignin modification).

Figure 1

a) Wood brightness before and after lignin modification and delignification; inset images are the photographs of the original wood (left), delignified wood template (middle), and lignin‐modified wood template (right). b) FTIR spectra for original wood, lignin‐modified wood template, and delignified wood template. c) Photoluminescence spectra of original wood, lignin‐modified wood template, and delignified wood template. d–f) SEM images of cell wall structures of original wood (d), lignin‐modified wood template (e), and delignified wood template (f). The inset images are low‐magnification SEM images with scale bars of 100 μm. Red arrows point to the lignin‐rich middle lamella, which is almost empty and open in (f). g) Pine templates with dimensions of 20 mm×20 mm×1.5 mm obtained through delignification (top) and lignin modification (bottom).

Figure 2

a) SEM image of TW‐lignin, showing the distribution of PMMA in wood lumen space. b) Optical transmittance of TW‐lignin; the inset is a photograph of transparent wood with thickness of 1.5 mm. c) Optical haze of TW‐lignin; the inset is a picture of TW‐lignin with a 5 mm gap between the sample and the underlying paper. d) Three‐point bending calculated stress–strain curves of TW‐lignin and glass; inset images are the fractured samples after test. e) SEM image of TW‐lignin after fracture, demonstrating the ductile fracture. f) Images of lignin‐retaining transparent wood made from balsa, pine, birch, and ash.

Figure 3

a) Design of model house with transparent wood roof. b) Model house with original wood roof, where indoor is dark. c) Model house with transparent wood roof, where indoor is bright. d) Design of transparent wood stool. e) Photograph of a transparent wood stool model. f) Photograph of a luminescent transparent wood stool.