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compatibility between the wood template and PMMA and promoting interface bonding. For 3 mm thick acetylated and non-acetylated ETW, the optical transmittance obtained was 89% and 60%, respec- tively. In addition to improved optical properties, acetylated ETW exhibited a higher work of fracture (1.0 MJ m −3 ) compared to glass and a higher modulus of 4 GPa and stress at break of 78.9 MPa compared to the original wood and PMMA. It was concluded that the improved mechanical properties were attributed to the strong interaction between wood and PMMA facilitated by acetyl groups (Li et al. 2018c).
densities and equal thickness. The light transmit- tance of Betula alnoides with a density of 680 kg m −3 and New Zealand pine with a density of 310 kg m −3 was increased by 11.34% and 14.4%, respectively. Qin et al. (2018) confirmed that the transmittance of the ETW decreased as the original wood density increased. For instance, the light transmittance of balsa with a density of 210 kg m −3 and balsa with a density of 490 kg m −3 was 77% and 64%, respectively. Mi et al. (2020a) demonstrated successful production of tunable aesthetic wood with UV blocking prop- erties from Douglas fir softwood exhibiting annual growth ring patterns of macroscopic and microscopic scales. The two variations in patterns of Douglas fir were termed low density (284 kg m −3 ) early wood (EW) and high density (846 kg m −3 ) latewood (LW). However, the preservation of the natural wood aes- thetic in hardwood (e.g., basswood, balsa) was not possible owing to its uniform bimodal pores, large lumen diameter and uniform and thinner cell walls (approx. 1.3–2.9 μm) compared to softwood (approx. 5–10 μm for LW). Also, ETW of softwoods, with a cell wall thickness of 1.4–2.6 μm, was not as suc- cessful in preserving the natural aesthetic of natural wood upon delignification as LW owing to the faster solution diffusion in EW. Furthermore, a comparative study of wood cut by different methods (cross section and quarter slicing cutting strategies) to achieve two directions [microchannels perpendicular (R-wood) or parallel to the wood plane (L-wood)] was conducted. Delignification was done for 2 h using sodium chlo- rite in acidic conditions in order to preserve lignin which endowed ETW with UV blocking properties through its phenylpropane structures and phenolic hydroxyl groups. Epoxy was used as a polymer to produce ETW. The mechanical properties of ETW from L wood (tensile strength of 92.0 MPa, tough- ness of 2.73 MJ m −3 ) were much higher than for both R wood (tensile strength of 21.60 MPa, toughness of 0.523 MJ m −3 ) and natural wood (tensile strength of 6.24 MPa). It was concluded that the high mechani- cal properties of L-ETW were due to the improved synergy between the wood template and the polymer (Mi et al. 2020a). Li et al. (2018c) reported that acet- ylated ETW produced from wood of high density, had improved transmittance compared to lower density wood. Rao et al. (2019) reported that the density of native poplar veneer only increased from 380 kg m −3 to 910 kg m −3 after polymer impregnation due to
Wood density
The density of wood can vary significantly depending on the type of species. For instance, Balsa is the low- est density tree species (120 kg m −3 to 160 kg m −3 ), largely due to its fast growth (Qin et al. 2018; Li et al. 2019b). Populus deltoids is a porous and thin cell walled wood with large cell cavities and short fibres hence its relatively low density of 460 kg m −3 (Chen et al. 2019). After delignification, the density further decreased to 400 kg m −3 due to the enhancement of the wood cell wall porosity after the removal of lignin, hemicellulose and amorphous cellulose. After poly- mer infiltration into the pores, the ETW increased to 1198 kg m −3 . Wang et al. (2021) confirmed the den- sity of ETW (1221 kg m −3 ) to be higher than that of delignified wood (67 kg m −3 ) and natural balsa wood (121 kg m −3 ). Balsa wood of density 126 kg m −3 was delignified by sodium chlorite at pH 4.6 and reduced to a density of 65 kg m −3 . After polymer impregna- tion the density increased to 1232 kg m −3 (Wang et al. 2022). Low-density wood such as pine, ash, and pop- lar are suspectable to breakage after lignin removal (Li et al. 2017a). Lignin acts as a binder between wood cells, hence the loss in the mechanical integrity of wood after delignification (Mi et al. 2020a). Qin et al. (2018) compared the properties of low-density balsa wood (210 kg m −3 ) (Ochroma pyramidale) and higher density of basswood (490 kg m −3 Tilia tuan). The tensile strength of transparent high density bass- wood (8.1 MPa) was higher compared to low density balsa wood (6.1 MPa) with a thickness of 2 mm. Wu et al. (2020) conducted a comparative study of the optical properties of ETW produced from two differ- ent wood species ( Betula alnoides (Betula) and New Zealand pine ( Pinus radiata D. Don) of different
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