5 456
Cellulose (2023) 30:5447–5471
Fig. 2 Production process of ETW. Reproduced with permission Copyright 2018, John Wiley and Sons (Bisht et al. 2021)
L (0.93 MJ m −3 ) and T (1.64 MJ m −3 ) was produced (Xia et al. 2021). Bi et al. (2018) used a low-cost green eutectic solvent to break the bonds between lignin and carbohydrate in 1 mm thick balsa wood under a microwave assistant treatment. The ETW pro- duced by acrylic acid infiltration had light transmit- tance and haze of 85% and 85%, respectively. Li et al. (2019a) delignified basswood with 30 wt% hydrogen peroxide steam, which penetrated the wood cells and removed lignin together with other dissolved degra- dation products. The epoxy resin infiltrated ETW of 0.5 mm thickness and had a light transmittance and haze of 87% and 90% respectively. Various types of polymers used in the produc- tion of ETW include epoxy resin polyethylene oxide (PEO), polyvinylpyrrolidone (PVP), polydimethyl- siloxane (PDMS), styrene, vinyl carbazole, iso- bornyl methacrylate and poly(methyl methacrylate) (PMMA) (Bi et al. 2018). Although PMMA has high optical transparency, its main drawback is the lack of compatibility between the polymer and the cell wall of the delignified wood and a refractive index (RI) mismatch with wood (RI is 1.49 for PMMA and 1.53 for holocellulose) leading to decreased transparency and mechanical properties (Li et al. 2018c). Qiu et al. (2019) used a combination of PMMA and antimony tin trioxide (ATO), resulting in improved mechani- cal properties, although the transparency of polar wood was reduced (Qiu et al. 2019). ATO was com- posed of hydrophilic groups in the silane coupling, which improved interfacial bonding between PMMA and the delignified wood. Li et al. (2018c) achieved 1.5 mm thick ETW with an optical transmittance of 92% (in the visible wavelength range) by surface application of acetylation on delignified wood sub- strates followed by PMMA infiltrations. Acetylation reduced the hydrophilicity of wood, thus improving
acetate buffer solution, and the delignification time was varied from 30 to 150 min at 30 min intervals. The maximum optical transmittance achieved was 61% at 800 nm for ETW with 9% lignin content at 150 min delignification time and with methyl meth- acrylate (MMA) impregnation and polymerisation. The drawbacks of using sodium chlorite were the production of toxic gases and yellowing when lignin reached equilibrium point (Qin et al. 2018). Qin et al. (2018) obtained transparent ETW with 82% light transmittance from 2 mm thick balsa wood deligni- fied by a two-step partial delignification process using 1 wt% sodium chlorite followed by treatment with 5 mol L −1 hydrogen peroxide to prevent yellowing. Li et al. (2017a) modified lignin by only removing the chromophores using hydrogen peroxide from balsa wood. The process was less toxic because hydrogen peroxide is an oxidant that produces only water as a by-product, notably reducing waste liquid produc- tion. Eliminating chromophores resulted in highly stable, rigid, well-preserved bleached wood with 80% lignin retention (Li et al. 2017a; Bisht et al. 2021). However, on the downside, high amounts of chemi- cal, water, and energy consumption were noted. Par- tial delignification was reported to endow the ETW with color and texture for decoration, where absolute transparency was not a priority (Wu et al. 2020). In order to minimise chemical consumption during delignification, Xia et al. (2021) performed a brush- ing technique on 0.6 mm balsa wood with hydrogen peroxide which preserved the aromatic backbone of lignin but diminished its chromophore content. This was followed by a clear epoxy infiltration into pores using vacuum. ETW with a transmittance > 90%, high haze > 60%, excellent light-guiding over visible wave- length, patternable surface, excellent tensile strength (46.2 MPa for L and 31.4 MPa for T), toughness of
1 3 Vol:. (1234567890)
Made with FlippingBook Digital Publishing Software