High Performance, Fully Bio-Based, and Optically Transparent Wood Biocomposites

Abstract

The sustainable development of engineering biocomposites has been limited due to a lack of bio-based monomers combining favorable processing with high performance. Here, the authors report a novel and fully bio-based transparent wood biocomposite based on green synthesis of a new limonene acrylate monomer from renewable resources. The monomer is impregnated and readily polymerized in a delignified, succinylated wood substrate to form optically transparent biocomposites. The chemical structure of the limonene acrylate enables diffusion into the cell wall, and the polymer phase is both refractive index-matched and covalently linked to the wood substrate. This results in nanostructured biocomposites combining an excellent optical transmittance of 90% at 1.2 mm thickness and a remarkably low haze of 30%, with a high mechanical performance (strength 174 MPa, Young's modulus 17 GPa). Bio-based transparent wood holds great potential towards the development of sustainable wood nanotechnologies for structural applications, where transparency and mechanical performance are combined.

Keywords: biocomposite; bio‐based polymers; nanotechnology; sustainable; transparent wood.

Figure 1

a) Chemical route for the synthesis of limonene acrylate (LIMA) monomer. b) Optical transmittance and haze of PLIMA. c) Mechanical properties of PLIMA bio‐based polymer compared with common thermosets. d) Schematic illustration showing the structure of wood and the various preparation steps of the bio‐based transparent wood (TW) and TW‐SA. First, a green peracetic acid (PAA) delignification is performed to remove the lignin, followed by succinylation for improved LIMA impregnation, using bio‐based succinic anhydride (SA) under neat conditions. Finally, LIMA monomer is infiltrated in the wood substrate and polymerized. e) Photograph of PLIMA, bio‐based TW, and TW‐SA.

Figure 2

Photographs and SEM images of balsa showing a) native wood (NW), b) delignified wood (DW), c) succinylated‐delignified wood (DW‐SA), and d) succinylated transparent wood (TW‐SA) with the bio‐based PLIMA polymer as a matrix, where the high‐resolution cross‐sectional SEM micrographs at the bottom show the interior of the cell wall.

Figure 3

TEM images from a cross‐section of a) DW and b) DW‐SA cell wall. The top region corresponds to the compound middle lamella (CML), followed by the secondary cell wall layers S1 (outer), S2, and S3 (inner). Cross‐sections of the c) TW and d) TW‐SA biocomposites, where the arrows show the interface between the cell wall and the lumen filled by PLIMA. The scale bars correspond to 500 nm.

Figure 4

Optical properties of the sustainable transparent wood composites showing the a) transmittance and b) haze of 1.2 mm thick TW and TW‐SA prepared from balsa with a wood volume fraction (V _f) of 12%. c) SEM micrograph of TW‐SA showing the interphase region between PLIMA and cell wall. d) Photographs taken at a distance of about 20 cm from the flower before (left) and after (right) placing a 1.2 mm thick TW‐SA sample in front of the camera lens. e) Photographs of TW‐SA composites prepared from balsa of various thicknesses. Optical haze versus transmittance of 1.2, 2, and 3 mm thick f) TW‐SA and g) TW biocomposites prepared from balsa (V _f = 12%) at a wavelength of 550 nm. h) Illustration showing the interactions between wood substrate and PLIMA in TW and TW‐SA. i) Haze versus transmittance for documented TW composites of similar thickness, compared to this study (1.2 mm thick TW‐SA based on balsa, V _f = 6% and 12%).^{[ 5} , ⁶ , ⁸ , ⁹ , ¹⁷ , ⁴⁹ , ⁵⁰ , ⁵¹ , ⁵² , ⁵³ , ^{54 ]} Numbers in parentheses represent the thickness of the sample.

Figure 5

a) SEM micrographs of alder, birch, and beech wood species. b) Mechanical properties of neat PLIMA, native birch, and TW and TW‐SA biocomposites prepared from birch. c) Specific strength versus specific stiffness for documented TW composites, compared to this study (TW‐SA prepared from birch).^{[ 5} , ⁶ , ¹¹ , ¹² , ¹³ , ¹⁴ , ¹⁷ , ¹⁸ , ⁵⁰ , ⁵² , ⁵⁵ , ⁵⁶ , ⁵⁷ , ⁵⁸ , ⁵⁹ , ⁶⁰ , ^{61 ]} d) Transmittance of TW‐SA biocomposites prepared from various wood species as a function of wood volume fraction (V _f). The transmittance of 0.7 mm thick balsa (V _f = 12%) corresponds to the predicted value. e) Haze of TW‐SA biocomposites prepared from various wood species as a function of V _f.