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the total transmittance and sample thickness was observed (Chen et al. 2019).
40% for ETW of thickness 0.7 mm and 3.7 mm respectively. In contrast, optical haze increases with an increase in thickness. Acetylated ETW of thickness 1.5 mm and 3 mm had an optical haze of 50% and 53%, respectively. Non -acetylated ETW of thickness 1.5 mm and 3 mm had an optical haze of 70% and 78%, respectively. The lower optical haze for acety- lated ETW was attributed to the improved compat- ibility between wood and PMMA, resulting in lower light scattering. Rao et al. (2019) reported a decrease in breath and thickness of ETW due to the simulta- neous convergence of contracted wood cells. The length, breadth and thickness of the original wood and ETW were 20×20×1 mm 3 and 20 × 16 × 0.7 mm 3 , respectively. However, the addition of propyl- ene glycol (PG) maintained the lumen and shape of ETW by reducing internal wood stresses, resulting in a sample with dimensions similar to those of origi- nal wood, i.e. 20 × 17 × 0.9 mm 3 for 50 wt% PG-ETW and 20 × 18 × 0.9 mm 3 for 100 wt% PG-RTW (Rao et al. (2019). Wood thickness also affected the dura- tion of delignification. Basswood cut in a direction perpendicular to the wood plane of thickness 20 nm and 40 nm was delignified with hydrogen peroxide and hydrogen acetate steam to a white colour after 4 and 20 h, respectively (Li et al. 2019a).
− √√
T total = exp ⎛ ⎜ ⎜ ⎜ ⎝
d ⎞ ⎟ ⎟ ⎟ ⎠
D x )
1 6
√ ( D xy
𝜇 c D xy
(3)
where T total is total transmittance, μ is the isotropic absorption, c is the speed of light in ETW samples, D xy is the diffusion coefficients perpendicular to the wood fibres, and d is the sample thickness. Lower thickness resulted in higher optical trans- mittance because a shorter light pathway low- ers light attenuation and vice versa (Montanari et al. 2019). For instance, 1 mm thick ETW had a transmittance of 85% in the radial direction, 2 mm thick ETW had a transmittance of 80% with thick- ness perpendicular to the longitudinal direction, and 3 mm thick ETW had a transmittance of 90% with thickness in the longitudinal direction (Zhu et al. 2016b; Li et al. 2018c). The transmittance of 0.7 mm and 1.5 mm thick ETW was approx. 90%, whereas haze was 10% and 30%, respectively (Li et al. 2016b). The transmittance of acetylated ETW of thickness 1.5 mm, 3 mm, 7 mm and 10 mm was 92%, 89%, 70% and 60% at a wavelength of 550 nm, respectively (Li et al. 2018c). The optical haze of acetylated ETW of thickness 1.5 mm, 3 mm, 7 mm and 10 mm was approximately 80%, 75%, 50% and 45%, respectively. Chen et al. (2019) compared the differences in transmittance for acetylated and non- acetylated ETW. They found that the transmittance of acetylated ETW of thickness 0.24 cm, 0.32 cm, 0.42 cm, 0.51 cm, and 0.65 cm was 86%, 82%, 77%, 72% and 66% at a wavelength of 550 nm, respec- tively. Non-acetylated ETW had transmittance values of 84%, 72%, 65%, 62%, 51% and 44% for samples of thickness 0.11 cm, 0.21 cm, 0.25 cm, 0.31 cm, 0.43 cm and 0.46 cm respectively. The light transmittance of 1.5 mm and 2 mm ETW was 80% and 77%, respectively (Qin et al. 2018). ETW is highly anisotropic due to its oriented fibrous structure. Correspondingly, an ETW sample will scatter light differently along two directions (trans- verse and longitudinal) depending on the light propagation direction (Vasileva et al. 2018). Li et al. (2016b) reported a light transmittance of 90% and
Type of wood
Wood species have varying densities, cellulose con- tents, cell structural morphologies, and annual ring structures, all of which affect the structural, mechani- cal, functional and optical properties of ETW (Rao et al. 2019). Examples of wood species that have been used to produce ETW are balsa, basswood, cathay poplar, pine, birch, and ash (Li et al. 2016b, 2017b, 2017a; Zhu et al. 2016a; Gan et al. 2017b). Figure 4 shows ETW produced from balsa, pine, birch, and ash by the lignin modification process and PMMA infiltration (Li et al. 2017a). Li et al. (2019a) con- ducted a comparative study on the effect of delig- nification on two wood species (i.e., basswood and pine) cut in the direction aligned either perpendicu- lar to (Ra) or along (Ro) the wood plane. Basswood had a unique multichannel structure with ultra-thin channel walls and low density. As a result, a longer time (8 h lignin modification and 18 h NaClO 2 pro- cess) was needed to achieve delignification, compared
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