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2-hydroxyethyl methacrylate treated) than lignin-oxi- dized ETWs (12.97 for untreated, 3.93 for acetylated, 5.47 for methacrylated, 46.9 for 2-hydroxyethyl meth- acrylate treated) at 4 °C due to its high lignin con- tent that was water resistant (Foster et al. 2021). The diffusivity increased with increase in temperature for all ETWs produced because increased thermal energy facilitated faster transport of water into the ETW composite (Foster et al. 2021).
strength = 13.3 MPa, modulus = 260 MPa (Zhu et al. 2016b; Yu et al. 2017; Rao et al. 2019; Wang et al. 2019b). Correspondingly, pure PMMA (tensile strength = 55.4 MPa, modulus = 2900 MPa) followed by epoxy (tensile strength = 50.2 MPa, Young modu- lus = 2400 MPa) have the highest strength properties, whereas PVA (tensile strength = 48.4 MPa, Young modulus = 707.9 MPa) has the lowest values (Ibrahim and Hassan 2011; Gong et al. 2016; Yusof et al. 2016; Jain et al. 2017; Seghir and Pierron 2018). It can be concluded that the strength of the polymer positively contributes to the strength of ETW produced. Yue et al. (2021) reported transmittance values (wave- length = 600 nm) of 76.6%, 73.4%, and 64.6% for ETW composed of polyvinyl alcohol (PVOH), PVP and PMMA, respectively. A study to determine the extent to which transmittance can be improved when delignified wood is acetylated before polymer infil- tration was conducted (Li et al. 2018c). Acetylated ETW had a high transmittance of 92% at a thickness of 1.5 mm, while non-acetylated ETW had a lower transmittance of 83%. In this case, the wood texture of acetylated EWT was less pronounced. This indi- cated improved compatibility between the wood and the PMMA polymer. It was observed that with an increase in the thickness of ETW, the transmittance decreased. However, the transmittance of acetylated ETW remained higher than non-acetylated ETW of the same thickness (Li et al. 2018c). In addition, an increase in the difference in transmittance between the acetylated and non-acetylated ETW of the same thickness was noted. Zhu et al. (2016a) opted for a biodegradable polymer, PVP, over epoxy resins which are made of toxic monomers such as bisphe- nol-A and epichlorohydrin. However, the resulting ETW lacked flexibility. In order to improve the flex- ibility of the PVP infiltrated ETW, a plasticing agent such as propylene glycol (PG) was used (Rao et al. 2019). Besides biodegradability, PVP is desirable due to its low cost, water solubility, low viscosity in water, excellent film forming property, toughness and transparency, with a refractive index of 1.48 (Wu et al. 2012; Mi et al. 2020b). The hydrophilic nature of PVP alleviated the need to use dehydrating chemi- cals such as ethanol and acetone in order to increase its compatibility with delignified wood. The thermal conductivities of ETW composed of different poly- mers had different conductivities, which were far less than the thermal conductivities of glass. For instance,
Type of infiltration polymer and compatibility with wood components
The infiltration polymer plays a major role in deter- mining the structural, optical and functional proper- ties of the ETW (Wang et al. 2021). A good polymer is one that has a refractive index that matches that of the wood components, which are cellulose (RI = 1.525), hemicellulose (RI = 1.532), and lignin ( RI = 1.610) (Vasileva et al. 2018). Besides, the refractive index, the viscosity, compatibility and shrinkage of the poly- mer are important selection criteria (Li et al. 2018a). The polymer’s ability to reduce the optical scattering of the wood may also lead to a desirable combination of high transmittance and low haze. In most studies, PMMA, epoxy resin, polyvinylpyrrolidone, n -butyl methacrylate, polystyrene, dibutylphthalate, iso-bor- nyl methacrylate, diallyl phthalate and poly vinylcar- bazole and poly(acrylic acid) with refractive indices of approximately 1.49, 1.5, 1.53, 1.50, 1.59, 1.52, 1.48, 1.50, 1.68 and 1.45, respectively, are used (Fink 1992; Vasileva et al. 2018; Chen et al. 2020). Li et al. (2018c) reported that polymers had a higher transmit- tance of 95% compared to transparent wood despite the full infiltration of polymer into wood cell lumens. The decreased transmittance was due to interface bonding which resulted in interface gaps and created optical heterogeneity. The low refractive index of air (1.000) in the interface gaps resulted in light scatter- ing hence decreased transmittance. The air gaps were caused by low compatibility between hydrophilic wood cells and hydrophobic polymers and the shrink- age of the polymer during polymerisation (Li et al. 2018c; Vasileva et al. 2018; Chen et al. 2019). The mechanical properties (in ascending order) of ETW composed of different polymers were: PMMA-ETW; fracture strength = 59.8 MPa and modulus 2720 MPa, epoxy-ETW; fracture strength = 23.4 MPa, modulus=1220 MPa PVA-ETW-PG; fracture
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