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Flexible transparent conductive films or substrates prepared from plastics or cellulose are widely used in optoelectronic devices. However, all of these films or substrates are fabricated by complex and expensive methods, which consume much energy and time. In this work, we report for the first time a remarkably facile and effective approach for fabricating flexible transparent films directly from wood. The resulting films exhibit an array of exceptional optical and mechanical properties. The well-aligned cell structures in natural wood are maintained during delignification, leading to anisotropic films with high transparency (≈90% transmittance). These anisotropic films with well-aligned cell structures show mechanical tensile strengths higher than those of the original wood, and can be used as screen protection films for cellphones. Furthermore, ultrathin, highly transparent, and outstandingly conductive films have been prepared from such films and silver nanowires (AgNWs) using the Meyer technique. A conductive film with an optimal area density (341 mg m) of AgNWs showed outstanding synergistic properties, with a transmittance of 80% and a sheet resistance of 11 Ω sq, equal to the conductivity of ITO. Of importance here is that the low-cost anisotropic transparent wood film shows promising potential for electronics applications in solar cells, flexible displays, and other products.

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Density reduction has become a topical issue in wood composite materials for application in building and furniture. In this study, lightweight wood-polyurethane (W-PU) composite foams with the addition of 30 wt% wood particles were prepared. Industrial kraft lignin was used as bio-polyol to substitute partial petroleum-based diethylene glycol (DEG) to synthesize rigid W-PU foams. The effect of varying lignin contents (5, 10, 15 and 20 wt% based on DEG mass) on the reactivity, morphology, density, compressive properties, water absorption and thermal stability of the foams was evaluated. Fourier transform infrared (FTIR) analysis confirmed the formation of characteristic urethane linkages in all the foam samples. With the incorporation of lignin, the foam cellular shape became irregular with formation of large cells. W-PU foams exhibited poor cellular structures with a larger number of open cells. The density of W-PU foams increased from 47 to 96 kg/m 3 as the lignin content increased from 0 to 20%. Although the foam reactivity was decreased by the incorporation of lignin, both the compressive strength and modulus were increased upon the incorporation of lignin. Furthermore, the specific compressive strength and modulus of W-PU foams increased by 55% and 48% with lignin content increasing from 0 to 20%, and the 20-day water absorption decreased by 38%. Thermal gravimetric analysis showed that the incorporation of lignin did not significantly affect the thermal degradation behaviour of foam, but it rather increased the mass of char residue. This study provides a promising method for value-added utilization of technical lignin in W-PU lightweight composites.

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Developing a high-strength, flame-resistant structural material derived from wood is desirable for advanced applications in the transportation, construction, automotive, and aerospace sectors. Here, a low-cost and high-efficiency top-down method was designed for processing natural wood veneers into lightweight yet strong bulk structural wood-based composites. The composites were produced using delignified wood veneers as a continuous reinforcing material and epoxy resin as the adhesive, followed by hot pressing. Densification of the veneers and good adhesion among the veneers played a pivotal role in the mechanical performance of the composites. A record-high flexural strength of 436.1 MPa and a toughness of 12.3 MJ•m −3 were achieved. Furthermore, the low density of the wood composite led to a specific flexural strength of 323.0 MPa•cm 3 •g −1 , which is significantly greater than those reported for other wood-based composites and some metal materials (e.g., steels and alloys). We also explored the feasibility of using the composite in unmanned drones. In addition, the delignified wood veneers were infiltrated with 1.5% ammonium polyphosphate (APP), which improved the fireretardant and self-extinguishing properties of the composites. This work demonstrates that high-strength wood products exhibit significant potential in many applications.

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