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(Wu et al. 2019a). Chemical delignification methods are time consuming and operated at high tempera- tures. For instance, Yaddanapudi et al. (2017) deligni- fied beech wood with thickness 0.1 mm using 5 wt % sodium chlorite in an acetate buffer solution at 95 °C for 12 h. Qin et al. (2018) opted for a 2 step partial delignification and an optimum time of 3 h NaClO 2 and 1 h H 2 O 2 treatment for 1 mm thick balsa wood. However, the delignification time required increased with increase in wood thickness. NaClO 2 treatment times of 4 h, 5 h and 12 h were recorded for balsa woods of thickness 1.5 mm, 2 mm, and 5 mm, respec- tively. Xia et al. (2021) avoided using large amounts of chemicals and producing liquid waste streams by using H 2 O 2 brushing instead of delignification by immersion. The ETW produced had a transmit- tance > 90%, high haze > 60%, and an excellent light guiding over the visible wavelength. Furthermore, the strength of the ETW was 50 times more superior than ETW obtained by delignification by immersion. Wu et al. (2019a) studied the effect of the extent of delig- nification of basswood on the morphological, optical and mechanical properties of transparent wood pro- duction. The lightness of transparent wood increased with delignification, while the redness and yellowness decreased (Wu et al. 2019a). The surface colour was the most obvious observation of ETW samples of different lignin contents (viz ., 16%, 15%, 13%, 12% and 9%). Unmodified wood (with 24% lignin content) had a yellowness value of 52.5, while the yellowness values of the ETW samples decreased to approxi- mately 25, 21, 19, 18, and 15, respectively. In con- trast, the lightness increased from 69.5 for unmodi- fied wood to 80, 85, 87, 88, and 90 for the ETW samples, respectively. The optical transmittance of the ETW samples increased with an increase in the degree of delignification (Qin et al. 2018; Wu et al. 2019a). For instance, unmodified wood and the most delignified wood (lignin content of 9%) had optical transmittance values of 0% and 61% at a wavelength of 800 nm, respectively. Mechanical properties of the ETW were negatively affected by increased levels of delignification. The tensile strength decreased with a decrease in lignin content in ETW samples. The highest tensile strength was reported for the ETW sample with a low degree of delignification (15% lignin content). Additionally, delignified wood was reported to have lower tensile strength value com- pared to their respective ETW samples, indicating
that polymer impregnation of MMA contributed to the final improvement of mechanical strength of ETW samples. It was concluded that the pores cre- ated by the removal of lignin were responsible for the improvement of light transmittance in samples (Wu et al. 2020). Li et al. (2017a) compared the wet strength measured perpendicular to the fibre direc- tion of various wood species (balsa, birch, pine and ash) with thickness 1.5 mm as a measure of the stabil- ity that were delignified by either lignin removal or lignin modification. The wet strength of balsa with a lignin content of 21.3% (lignin modified) and 2.5% (lignin removed) was 7.9 MPa and 6.9 MPa, respec- tively. Lignin modified (lignin = 20.1%) and deligni- fied Birch wood (lignin = 3.3%) had a wet strength of 14.4 MPa and 1.4 MPa respectively. Lignin modified (lignin = 22.4%) and delignified ash wood (lignin = 5.3%) had a wet strength of 13.9 MPa and 0.8 MPa respectively. Lignin modified pine had a wet strength of 14.4 MPa. Although ash, birch, and pine showed a significant difference between the wet strength of delignified and lignin modified wood samples, the wet strength difference for balsa wood was marginal. Delignified pine wood was too weak for strength testing. Höglund et al. (2020) compared bleaching to delignification in the production of ETW. While the transmittance showed little difference (90% and 85%), the haze of bleached ETW wood and delig- nified ETW wood varied greatly (36% and 63%). Zhu et al. (2016a) illustrated that the optical transmittance of delignified wood samples increased with increase in degree of delignification. For example, 33%, 50% and 100% delignified wood had transmittances of approx. 2%, 5% and 15%, respectively. These results were in agreement with Khalili et al. (2017), where the light transmittance values of 33%, 35%, 47%, 51%, and 64% delignified ETW was approx. 25%, 35%, 43%,47% and 50%, respectively. The tensile strength of 33%, 35%, 47%, 51% and 64% delignified ETW was 166 MPa, 171 MPa, 161 MPa, 159 MPa and 152 MPa, respectively. Foster et al. (2021) com- pared the water transport property in chemically mod- ified ETW produced by peroxide-based lignin modifi- cation and chlorite-based lignin oxidation. The lignin modified and oxidised wood was further treated via acetylation, 2-hydroxyethyl methacrylate treated treatment and methacrylation. Lignin-modified ETW had lower diffusion coefficients (10.6 for untreated, 2.47 for acetylated, 1.88 methacrylated, 18.91 for
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