5 464
Cellulose (2023) 30:5447–5471
the thermal conductivities of PVA-ETW, PMMA- ETW, and Epoxy-ETW were reported to be 0.19 W m −1 K −1 , 0.23 W m −1 K −1 and 0.24 W m −1 K −1 , respectively (Li et al. 2017a, 2019b; Mi et al. 2020a, 2020b). Wachter et al. (2021) compared the extent of photo degradation of ETWs fabricated using two different polymers (methyl methacrylate, 2-hydroxy- ethyl methacrylate) over a period of 35 days exposed to monochromatic UV-C (λ—250 nm) radiation. The optical transmittance of methyl methacrylate- ETW reduced from 58 to 43%, whereas 2-hydroxy- ethyl methacrylate-ETW reduced from 69 to 57% within 7 days of exposure to UV radiation. Further increase in time resulted in a negligible decrease in optical transmittance (Wachter et al. 2021). It was concluded that decrease in optical properties of both types of ETW was caused by the degradation of both the acrylic polymers and the wood templates. The disadvantages of PVA-ETW were that it had a high solubility in water resulting in weight gain of ETW samples exposed to high humidity conditions. On the other hand, the solubility of PVA favours easy recy- cling of ETW and recovery of PVA (Rao et al. 2019). Höglund et al. (2020) infiltrated the bleached wood pores with a thiol-ene thermoset to prepare ETW with an optical transmittance of 90% and a haze of 36%. The thiol-ene thermoset network was prepared from thiol PETMP and trifunctional ene TATATO monomers by polymerisation. It was concluded that the adhesion of the thiol-ene polymer significantly contributed to the low haze and high transmittance instead of the wood-polymer matching refractive indices (Höglund et al. 2020).
electromagnetic shielding and magnetocaloric (Li et al. 2017b; Zhang et al. 2020). Bisht et al. (2021) fabricated UV resistant ETW by combing a UV absorber (2-(2H-Benzotriazol-2-yl)-4, 6- di-tert- pentylphenol) with an epoxy resin before infiltra- tion into bleached wood. This led to photostable ETW that did not photodegrade nor change colour when exposed to UV light. Also, the UV absorber had no effect on the optical properties of the ETW. A comparative study showed that ETW without a UV absorber had a change in yellowness value of approximately 10, 20, and 25 after 25 h, 100 h and 250 h of UV exposure, respectively. In com- parison, ETW containing 1.75% UV absorber con- tent had a change in yellowness value of approxi- mately 2, 5 and 10 after 25 h, 100 h and 250 h of UV exposure, respectively. Increasing UV exposure time resulted in increased photodegradation which was indicated by a decrease in light transmittance. For example, at a wavelength of 550 nm and 250 h UV exposure time, the percentage transmittance loss was 27.5%, 1.43%, and 0.96% for ETW with UV absorber concentrations of 0%, 1% and 1.75%, respectively (Bisht et al. 2021). Wang et al. (2019a) reported the fabrication of photochromic ETW with photo-switching of transmittance in the visible light region and colour tuning properties. The deligni- fied wood was filled with a mixture of photochro- mic material 30,30-dimethyl-6-nitro-spiro[2H- 1-benzopyran-2,20-indoline]-10-ethanol (DNSE) and pre-polymerized methyl methacrylate (MMA). The components of DNSE, spiropyran and mero- cyanine responded to UV light or green light. A similar study involved the fabrication of photochro- mic and fluorescent ETW with color switching properties from lignin modified basswood, methyl methacrylate (MMA) and a photoluminescent lanthanide-doped aluminum strontium oxide (Al- Qahtani et al. 2021). Chen et al. (2020) produced a flame-retardant transparent wood composite from pristine natural wood by delignification followed by polyimide infiltration and chemical imidisation. The transparent wood composite showed high mechani- cal strength (stress and modulus of 169 MPa and 2.11 GPa, respectively) in addition to outstand- ing flame retardancy characterised by the physical integrity of the composite during combustion. Com- pared to natural wood and polyimide film, where a shrinkage, twisting and formation of soft and loose
Modification/functionalisation of ETW
The disadvantages of ETW, such as low transmit- tance caused by light scattering at the cellulose- polymer interface, and the loss of optical and mechanical integrity due to exposure to adverse weather conditions (e.g., rain, temperature etc.), can be overcome through functionalisation. The poten- tial for modification and multi functionalities of wood and ETW is due to micro-scale channels and nanopores in the cell wall. Furthermore, function- alisation extends the applications of ETW. Various additives provide the wood composites functionali- ties, such as luminescence, conductivity, UV stabil- ity, aesthetics, fluorescence, stimuli responsiveness,
1 3 Vol:. (1234567890)
Made with FlippingBook Digital Publishing Software