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to transverse ETW which exhibited weak bonding interfaces between the wood tissue and the polymer. However, R-ETW had an higher optical transmittance of 73.3% than L-ETW with 68.2% (Zhu et al. 2016b; Zhang et al. 2020). It was concluded that the poly- mer had better compatibility with R-wood due to the small depth of the lumina (Zhu et al. 2016b). Wang et al. (2022) reported thermal conductivities of 0.3 mW m −1 K −1 and 0.29 mW m −1 for transverse and longitudinal editable shape-memory ETW, respec- tively. Zhang et al. (2020) confirmed the difference in thermal conductivities by producing radial ETW and longitudinal ETW with thermal conductivities of 0.32 W m −1 K −1 and 0.20 W m −1 K −1 , respectively. The difference in thermal conductivity values is due to well preserved wood cells even after the delignifi- cation process resulting in anisotropic thermal prop- erties. (Li et al. 2016a). Mi et al. (2020a) reported anisotropic thermal properties for axial-ETW and radial-ETW. Radial-ETW and axial ETW had ther- mal conductivities of 0.24 W m −1 K −1 and 0.41 W m −1 K −1 . It was concluded that restrained heat trans- fer in the radial direction results in greater light scat- tering than in the axial direction. Further comparison was made between epoxy polymer, ETW and glass. The thermal conductivity of epoxy (0.29 W m −1 K −1 ) was slightly less than that of ETW (0.3 W m −1 K −1 ) and glass (0.86 W m −1 K −1 ). The low thermal con- ductivity of ETW was due to the low transmission of photon in the wood fibre (Li et al. 2016a). The study of the effect of temperature on the thermal conduc- tivities of transverse ETW showed an increase in thermal conductivity from 0.29 W m −1 K −1 , 0.3 W m −1 K −1 to 0.32 W m −1 K −1 at 0 °C, 25 °C and 50 °C, respectively. However, the conductivities remained far less than that of glass. It was concluded that ETW is a more efficient thermal insulator than glass, there- fore, making it a suitable alternative in energy effi- cient buildings (Wang et al. 2022). The same trend was reported by Zou et al. (2022), where the pectin- PMMA ETW had a thermal conductivity of approxi- mately 0.1 Wm −1 K −1 far less than that of glass (~ 0.9 Wm −1 K −1 ). However, the thermal conductivity of PMMA (approx. 0.2 Wm −1 K −1 ) was slightly higher than that of pectin-PMMA-ETW (Zou et al. 2022). Longitudinal ETW is characterised by the propaga- tion of light perpendicular to the wood longitudinal direction, high interface density (number of interfaces per length), high transmittance, low haze, isoptropic
light scattering effect and low density of polymer/ cellulose interfaces due to the hollow cylinder shape of wood cells. In contrast, transverse wood is charac- terised by propagation of light in the transverse plane and with low interface density. Longitudinal wood exhibits isotropic light scattering whereas transverse wood exhibits anisotropic light scattering at the wood cell wall and polymer interface. Moreover, light scat- tering of longitudinal wood has a balanced angular distribution in both the x and y direction, whereas a larger scattering angle in the y direction compared to the x direction was observed for transverse wood (Li et al. 2018a).
Degree and type of delignification process
A fast, environmentally friendly, lignin preserving and mechanical strength preserving delignification/ modification method is the most favourable in the pro- duction of ETW (Li et al. 2017a). In addition, the del- ignified wood should be strong enough to allow easy handling during polymer infiltration (Li et al. 2018a). The delignification of wood creates nano and micro- scale voids in the cell wall, which leads to increased specific surface area from 1.2 m 2 g −1 to 20 m 2 g −1 in natural wood (Li et al. 2018a). Li et al. (2019a) reported the complete removal of lignin using hot hydrogen peroxide and hydrogen acetate solution to a concentration as low as 0.84%, resulting in increased pores in the cell wall corners, which allowed for more efficient epoxy resin infiltration, high cellulose reten- tion, suppressed interface debonding gaps, high trans- mittance (87%), high haze (90%), and mechanical strength of ETW. Delignification mainly affects the colour of the natural wood. The methods of deligni- fication include bleaching by total immersion with 1 wt% of sodium chlorite in acetate buffer solution (pH 4.6) at 80 °C (Li et al. 2018c), two step delignifica- tion with 1 wt% of sodium chlorite in acetate buffer solution (pH 4.6) at 80 °C sodium chlorite followed by 5 mol L −1 hydrogen peroxide at 90 °C (Qin et al. 2018), 30 wt% H 2 O 2 brushing followed by solar illumination (Xia et al. 2021), lignin chromophore removal using alkaline H 2 O 2 hydrothermal solution (Li et al. 2017a), 2 step delignification with NaOH and sodium sulphate followed by hydrogen peroxide and hydrogen peroxide and hydrogen acetate steaming (Wang et al. 2019a). The measures of delignification are lignin content, lightness, redness and yellowness
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