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shrinkage of polyvinyl alcohol (PVA) and deforma- tion of the lignin modified wood template during PVA drying. Mi et al. (2020a) reported that deligni- fication of high-density hardwood takes a longer time than low density softwood.
from birch wood with 30% cellulose by volume, had a transmittance of 64%, while acetylated ETW had a transmittance of 90%. A lower volume fraction of wood of 5% had a transmittance of 83% and 92% for non-acetylated and acetylated ETW, respectively. It was concluded that a high-volume fraction of cellu- lose resulted in light-scattering originating from the cellulosic cell wall (Li et al. 2016b). An increase in cellulose volume fraction from 5 to 65% resulted in a decrease in transmittance from 85 to 34.6% (Li et al. 2019b). The cellulose volume fraction and cell wall porosity were increased by compression of delignified wood. For instance, Li et al. (2016b) increased the cellulose volume from 5 to 19% by compression of delignified balsa wood. Compression is also impor- tant in tailoring wood to the desired thickness.
Cellulose volume fraction
The cellulose volume fraction of wood was calculated according to Eqs. 1 and 2 (Li et al. 2016b).
W f × 𝜌
c
(1)
V f =
𝜌 f
W f
W m 𝜌 m )
𝜌 c = 1 ∕ (
(2)
+
𝜌 f
where V f is the volume fraction of cellulose, ρ c is the density of ETW, ρ f is the density of holocellulose (1500 kg m −3 ), ρ m is the density of the polymer, W m is the weight fraction of the polymer, and W f is the weight fraction of cellulose. The cellulose volume fraction in wood is a tun- able property that has an effect on the properties of ETW. Fu et al. (2018) reported a linear relationship between the strength of ETW with cellulose vol- ume fraction. For instance, the ultimate strength of ETW with a cellulose volume fraction of 5%, 12.5% and 20% was approximately 40 MPa, 62 MPa and 90 MPa, respectively. Jia et al. (2019) studied the effect of thickness and cellulose volume fraction on the optical properties of ETW. They found that the transmittance of ETW with cellulose volume frac- tions of 2.89%, 5.86% and 8.71% was approx. 90%, whereas the haze gradually increased to 5%, 10% and 41.6%, respectively. Li et al. (2018c) reported that non-acetylated ETW of a thickness of 1.5 mm, made
Wood thickness
Wood thickness and its direction has a direct effect on optical transmittance and haze (Li et al. 2018c). The thinner the wood sample, the higher its light trans- mittance and the lower its haze (Li et al. 2018a; Qin et al. 2018; Kanócz et al. 2020). Zou et al. (2022) confirmed that an increase in thickness resulted in an increase in haze due to longer light pathways and increased scattering centres. Figure 3 illustrates the decrease in transparency of ETW with an increase in thickness. Kanócz et al. (2020) reported a dependence of light transmittance on both the thickness and vol- ume fraction of cellulose. Chen et al. (2019) stud- ied the dependence of the optical transmittance of acetylated and non-acetylated ETW on thickness using the anisotropic photon diffusion equation and an equation that quantifies the total transmittance (see Eq. 3). An exponential relationship between
Fig. 3 Photographs of ETW of varying thickness. Reproduced with permission Copyright 2018, John Wiley and Sons (Vasileva et al. 2018)
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