Toward Sustainable Aesthetic Transparent Wood from a Fast-Growing Hardwood Species: Paulownia Wood Templates Infused with Epoxy Bioresin

Lignocellulosic biomass is an attractive renewable resource for the development of engineered materials in the framework of a green economy. Transparent wood (TW) products show great potential in green architecture, energy saving building, optical devices, electronics, energy storage, and conversion devices. The fabrication of TW products proceeds through the delignification of bulk wood samples followed by infiltration with a refractive index-matched polymer. This study is focused on <i>Paulownia tomentosa</i> (Thunb.) Steud., a fast-growing species rarely investigated, is characterized by low-density wood and a distinct early to late wood pattern in each growth ring. Delignification was performed by a conventional bleaching route. Aesthetic wood was obtained by infusing the delignified templates with an epoxy bioresin. The characterization was performed by nondestructive techniques: optical microscopy, scanning electron microscopy, X-ray diffraction, FT-IR spectroscopy, Raman spectroscopy, and UV-vis spectroscopy. Thermal degradation profiles were acquired by thermogravimetry, and mechanical strength was evaluated by tensile tests. The chemical treatment led to 10-15% dry mass loss, mainly due to the removal of lignin, and the efficacy of delignification was comparable for transversal and longitudinal bulk wood. The removal of a minor amount of hemicellulose, especially for axial samples, also occurred. Delignified templates preserved dimensional stability in wet and dry states and showed increased Segal crystallinity index (CI), reduced thermal stability, improved total optical transmittance, increased brightness, and loss of tensile strength. The infusion with the bioresin led to aesthetic wood characterized by increased optical transmittance (up to 60% at 800 nm) combined with fully recovered tensile strength and preserved natural wood features clearly visible to the naked eye.