Abstract
The color changes corresponding to chromophore structures in lignin caused by exposure of Eucalyptus (Eucalyptus grandis and E. urophylla) to heat were investigated. Eucalyptus wood powders were heat treated under saturated steam atmospheres for 10 h at 110 °C, 130 °C and 150 °C. The lignin was isolated before and after heat treatment. The physicochemical properties of the lignin and changes in chromophore structures during heat treatment was evaluated through wet chemical analysis, Fourier transform infrared spectroscopy (FTIR), diffuse reflectance ultraviolet-visible spectroscopy (DRUV-Vis), gel permeation chromatography (GPC), X-ray photoelectron spectroscopy (XPS) and 13C Cross polarization magic angle spinning nuclear magnetic resonance (13C CPMAS NMR). Wood color darkened and reddened with the increase in pressure and temperature. Depolymerization and dehydration reactions occurred via demethoxylation with heat treatment in saturated steam at 110 °C or 130 °C. Lignin condensed to form insoluble compounds after heat treatment in saturated steam at 150 °C. G units increased and S units decreased through demethylation during heat treatment, as revealed by FTIR and 13C-NMR analysis.
Highlights
Wood is a popular decorative material due to the structure and texture, especially its natural color
Thermal treatment under hot air conditions ranging from 180 ◦ C to 220 ◦ C usually causes degradation of hemicellulose and amorphous areas
The Klason lignin content direction with the dioxane lignin, which increased for 110 ◦ C and 130 ◦ C conditions and decreased variation showed the same direction with the dioxane lignin, which increased for 110 °C and 130 °C
Summary
Wood is a popular decorative material due to the structure and texture, especially its natural color. Discoloration during wood heat treatment depends on heating conditions that include heat medium, temperature, moisture content, humidity, and air pressure. Chen et al used saturated steam at 130 ◦ C to induce discoloration, with no change in the structure of cellulose and hemicellulose [2,17,18], demonstrating that higher humidity can cause obvious discoloration under saturated steam treated especially at lower temperature, with no effect on polysaccharide components. Conjugated structures formed and generally lead to color changing in the process of lignin degradation under heat treatment [20]. We here report our study of the effects of steam heat treatment on lignin (especially chromophore structure and content) and its relationship to the discoloration mechanism, and the role of the corresponding changes of the chromophores in the color of wood
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