A green steam-modified delignification method to prepare low-lignin delignified wood for thick, large highly transparent
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A green steam-modified delignification method to prepare low-lignin delignified wood for thick, large highly transparent wood composites Huayang Li1
, Xuelian Guo2, Yuming He1, Rongbo Zheng1,a)
1
Yunnan Province Key Laboratory of Wood Adhesives and Glued Products, College of Chemical Engineering, Southwest Forestry University, Kunming 650224, People’s Republic of China 2 Wetland College, Southwest Forestry University, Kunming 650224, People’s Republic of China a) Address all correspondence to this author. e-mail: [email protected] Received: 26 August 2018; accepted: 12 November 2018
To suppress the interface gap between the cell walls of wood and filled epoxy resin, a green and universal H2O2 or H2O2/HAc steam-modified delignification approach is developed to remove more lignin, thereby generating more pores to be more conveniently backfilled by epoxy resin for highly transparent wood composites. Utilizing the excellent penetration ability of steam, not only different wood species, such as basswood and pine, with different cutting directions but also the thickest (40 mm) and largest (210 190 mm) wood samples can be successfully delignified. Compared with the 1.9% lignin content (which is the normal content of delignified wood prepared by solution-based methods) of delignified wood, the as-prepared delignified wood has the lowest lignin content of 0.84% to date. After the infiltration of epoxy resin, not only did the mechanical strength of the 5-mm transparent wood composite increase from 12.5 to 20.6 MPa, but the transmittance (the wavelength was 550 nm) also increased from 80 to 87% due to the lower absorbance of visible light by lignin and the suppression of the interface debonding gap between the cell walls and the backfilled epoxy resin.
Introduction Transparent wood composites [1, 2, 3, 4, 5, 6], have received great attention as energy-efficient building materials [4, 7], functional (fluorescent, magnetic, heat-shielding) transparent wood composites [3, 5, 8], and broadband light management layers in solar cells [9] because of their high anisotropy, high transparency, high haze, excellent thermal insulation, and high impact absorption capability [7]. However, natural wood is completely nontransparent. The reasons are the large refractive index (RI) differences between the cellulose (approximately 1.53) and air (approximately 1.00), microsized channels in natural wood that scatter light in the visible range [6], and the fact that wood is comprised of 20–30 wt% lignin, which absorbs visible light, resulting in an opaque appearance [1, 11]. Great efforts have been devoted to letting light through wood by the delignification of wood and backfilling the RI matching polymer [2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 13]. According to the literature, lignin can be removed by soaking wood samples in the following bleaching solutions (Table S1): solutions containing NaOH and
ª Materials Research Society 2019
Na2SO3, and subsequently in a H2O2 solution [2, 7, 8, 9]; H2O2/ HAc solutions [10]; a peracetic acid solution [14]; NaClO2 with
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