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Exposure to outdoor conditions can cause chemical components on wood surfaces to deteriorate. To develop effective wood protection methods, a deep understanding of the mechanisms of weathering-induced wood surface degradation is required. However, the effects of outdoor exposure on wood at the cellular level have not been investigated in detail. Additionally, a comparative study of the degradation behavior of sapwood and heartwood has also not been conducted. This paper investigated the chemical changes in cell walls in Japanese cedar (Cryptomeria japonica D. Don) sapwood and heartwood during natural weathering using confocal Raman microscopy. Spectral and chemical mapping revealed that heartwood had higher weather durability than sapwood due to the large extractives content in heartwood. Although there were differences in the rates of lignin reduction between sapwood and heartwood, the molecular structures and lignin-degraded sites of the weathered samples were almost identical. The degradation patterns of naturally weathered wood were also similar to that of artificially weathered wood. The knowledge revealed here will help in the development of more effective wood protection methods.

Since wood chemical components can be depolymerized and modified by weathering, a better understanding of the mechanisms governing these processes in needed to develop effective protection methods for wood surfaces. Unfortunately, very little has been reported about the micro-scale chemical changes in wood, particularly hardwood, during weathering. The purpose of the present work is to determine the degradation behavior of Japanese beech (Fagus crenata Blume) under artificial weathering at the cellular-level. Herein, the structural and micro-distributional changes in wood components during weathering were investigated using micro-Raman spectral and chemical mapping analyses. The Raman spectra showed that weathering facilitated lignin degradation and modification. The degradation behavior of lignin differed depending on the type of wood tissue. The rate of lignin reduction followed a descending order: vessel element > axial parenchyma cell > wood fiber. Raman mapping determined that cellular-level lignin reduction on the surface layers differed for wood species. Although lignin degradation of cedar tracheids proceeded from both the surface and the cell lumen, the lignin in beech fibers degraded according to the depth.

Weathering leads to rapid depolymerization and modification of wood chemical components. The present study aims to assess the structural and distributional changes in the main wood polymers, such as lignin and polysaccharides, located in the surface layers during weathering exposure. A confocal Raman microscopic technique, which is useful for evaluating the structure of molecules with a high spatial resolution, was utilized to examine the effects of weathering on the chemical composition of wood surfaces at the cellular level. Raman spectra showed that lignin degradation during weathering proceeds with the formation of o- and p-quinones, carbonyl groups, and certain types of CC double bonds such as stilbene derivatives. Comparing the weathering conditions between light only and light plus water exposure, it was found that weathering in the presence of water significantly enhances the degradation of lignin. The Raman images exhibited that the degree of lignin degradation by light only exposure is according to the depth. However, in the case of light plus water exposure, lignin degradation in the outermost cell walls proceeded from both the exposed surface side and the lumen side of the cell walls. This study is expected to potentially promote development of more effective and efficient methods to protect wood surfaces against weathering.

Raman microscope has been applied for the various studies on wood tissues because it can analyze without any pretreatments. In this study, therefore, we have applied Raman microscope to quantify the lignin contents of various wood tissues in various wood species. Lignin contents can be calculated from Raman spectra for each wood tissue by using calibration curve from standard samples. Lignin contents in various wood tissues coincide with the values obtained by conventional lignin quantification methods. Consequently, Raman microscopy, which is very simple without any pretreatments, is applicable and a powerful method to determine the lignin contents of wood tissues.

Tension wood that is an abnormal part formed in angiosperms has been barely used for wood industry. In this study, to utilize the tension wood effectively by means of liquefaction using ionic liquid, we performed morphological and topochemical determination of the changes in tension wood of Japanese beech (Fagus crenata) during ionic liquid treatment at the cellular level using light microscopy, scanning electron microscopy and confocal Raman microscopy. Ionic liquid treatment induced cell wall swelling in tension wood. Changes in the tissue morphology treated with ionic liquids were different between normal wood and tension wood, moreover the types of ionic liquids. The ionic liquid 1-ethyl-3-methylimidazolium chloride liquefied gelatinous layers rapidly, whereas 1-ethylpyridinium bromide liquefied slowly but delignified selectively. These novel insights into the deconstruction behavior of tension wood cell walls during ionic liquid treatment provide better understanding of the liquefaction mechanism. The obtained knowledge will contribute to development of an effective chemical processing of tension wood using ionic liquids and lead to efficient use of wood resources.

The anatomical characteristics of opposite wood of Japanese cedar (Cryptomeria japonica) treated with ionic liquids were investigated by light microscopy and scanning electron microscopy at the cellular level. The morphological changes in the cell walls differed for normal wood and opposite wood and for the types of ionic liquids. In addition, the ionic liquids, 1-ethyl-3-methylimidazolium chloride and 1-ethylpyridinium bromide, liquefied cellulose- and lignin-rich regions of the opposite wood cell walls, respectively.

Cryptomeria japonica was treated with 90% pyridinium chloride ([Py]Cl) and 10% water w/w solution at 80 and 120 °C. Most hemicellulose in C. japonica was liquefied and over half the lignin in C. japonica was solubilized after treatments at 80 and 120 °C. However, cellulose was mostly insoluble at 80 °C and partially soluble at 120 °C. The crystal structure of cellulose in the cell walls was retained after treatment at 80 °C for 48 h. The degradation products from the polysaccharides were obtained in different yields. The 90% [Py]Cl and 10% water w/w solution is effective for the treatment of lignocellulosics, such as liquefaction of lignocellulosics and the production of useful low molecular weight compounds.

The chemical changes in various morphological regions of Japanese beech (Fagus crenata) upon treatment with the ionic liquid 1-ethyl-3-methylimidazolium chloride ([C2mim][Cl]), which preferentially dissolves cellulose, have been investigated by Raman microscopy. The reactivity of polysaccharides and lignin with [C2mim][Cl] differed depending on the types of wood tissues. Chemical components in parenchyma cells showed higher resistance to degradation upon [C2mim][Cl] treatment than wood fiber and vessel. In addition, the lignin structure was partially degraded or modified by [C2mim][Cl] treatment, and its binding mode was complicated; nevertheless, lignin was stable in [C2mim][Cl].

The morphological and topochemical changes in wood tissues in compression wood of Japanese cedar (Cryptomeria japonica) upon treated with two types of ionic liquids, 1-ethyl-3-methylimidazolium chloride ([C2mim][Cl]) and 1-ethylpyridinium bromide ([EtPy][Br]) were investigated. Compression wood tracheids were swollen by both ionic liquids but their swelling behaviors were different in the types of ionic liquids used. Under the polarized light, we confirmed that crystalline cellulose in compression wood is amorphized by [C2mim][Cl] treatment whereas it changes slightly by [EtPy][Br] treatment. Raman microscopic analyses revealed that [C2mim][Cl] can preferentially liquefy polysaccharides in compression wood whereas [EtPy][Br] liquefy lignin. In addition, the interaction of compression wood with ionic liquids is different for the morphological regions. These results will assist in the use of ionic liquid treatment of woody biomass to produce valuable chemicals, bio-fuels, bio-based composites and other products.