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410 kg m 3 was delignified, acetylated and infiltrated with PMMA. Two-ply ETW was arranged with one ply in the same direction as the fibre alignment and the other ply positioned 90° transverse to the fibre alignment. The optical properties of acetylated ETW (transmittance = 46%, haze 89%) were superior to those of non-acetylated ETW (transmittance = 38%, haze = 90%). On the other hand, the elastic modulus of acetylated ETW (7 GPa) was slightly greater than non-acetylated, whereas the tensile strength of non- acetylated ETW (75 MPa) was much greater than acetylated ETW (40 MPa). However, for single-ply ETW, the mechanical properties of acetylated ETW (elastic modulus = 4 GPa, tensile strength = 15 MPa) were much higher than for non-acetylated ETW elas- tic modulus = 3.5 GPa, tensile strength = 10 MPa) positioned 90°/transverse to fibre alignment.
Fig. 4 Images of ETW produced from balsa, pine, birch, and ash. Reproduced with permission Copyright 2017, John Wiley and Sons (Li et al. 2017a)
to pine which required 4 h. Wu et al. (2021) fabri- cated multi-layered ETW from two different wood types, i.e., New Zealand pine and Basswood, of den- sities 330 kg m −3 and 420 kg m −3 , respectively and thickness of 0.5 mm. The three-layered ETW were arranged either in the same direction or alternating directions (the first layer and the third layer were in the same direction) according to fibre directions for each type of wood studied. Each type of wood stud- ied was partially delignified using NaClO 2 followed by PMMA infiltration into the three layered deligni- fied wood samples. The light transmittance (wave- length = 550 nm) for the three-layered New Zealand pine and basswood ETW arranged in the same direc- tion according to wood fibres was 8% and 6%, respec- tively. On the other hand, the light transmittance of the three-layered New Zealand pine and basswood ETW arranged in alternating directions was 4% and 3%, respectively. The tensile strength of the three layered ETWs in the same direction and in different directions was approximately 50 MPa and 40 MPa, respectively, for each of the wood species studied. Original New Zealand pine and Basswood had tensile strengths of approx. 37 MPa and 39 MPa for the same direction and approx. 30 MPa and 32 MPa for differ- ent directions, respectively. In another study, Foster et al. (2019) compared the optical and mechanical properties of acetylated and non-acetylated single ply and double-ply ETW in an effort to improve mechani- cal properties and alleviate thickness constraints on ETW. Basswood of thickness 1.6 mm and density
Wood direction
The optical properties of wood are direction depend- ant due to its anisotropic nature. The way in which wood is sawn affects its cellulose volume fraction, which in turn affects the optical transmittance of ETW. Radially sawn wood and longitudinally sawn wood have a low and high-volume fraction of cel- lulose, respectively (Kanócz et al. 2020). Low and high volume fraction wood with a thickness of 2 mm exhibits an optical transmittance of 90% and 80%, respectively (Li et al. 2016b). Zhu et al. (2016b) com- pared the mechanical properties of transparent radi- ally and longitudinally sawn wood to that of their nat- ural counterparts. The fracture strength of transparent longitudinally cut wood strength (45 MPa) was almost twice that of transparent radially cut wood (23 MPa). The fracture strength of natural radially and natural longitudinally cut wood was 4.5 MPa and 42 MPa, respectively (Zhu et al. 2016b). These results were similar to those obtained by Wang et al. (2021), where the mechanical properties of longitudinal ETW (frac- ture strength = 23 MPa, elastic modulus = 666 MPa, and toughness = 728 kJ m −3 ) were greater than transverse ETW (fracture strength = 2.6 MPa, elas- tic modulus = 46 MPa, and toughness = 320 kJ m −3 ). The scanning electron microscope (SEM) analy- sis revealed that in longitudinal ETW, a 3D net- work was formed due to the interaction of cellulose nanofibers in delignified wood (DW) and the poly- mer, hence the high mechanical strength compared
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