Impact of thermal modification on color and chemical changes of African padauk, merbau, mahogany, and iroko wood species

Abstract Thermal modification is an environment-friendly technology for improving various wood properties, especially the dimensional stability, decay resistance, and color homogeneity. In this work, four tropical wood species (African padauk, merbau, mahogany, and iroko) were thermally modified by the ThermoWood process. The influence of heat treatment on the color and chemical changes of wood was studied by spectrophotometry, Fourier transform infrared (FTIR) spectroscopy, and wet chemistry methods. As the temperature increased, a decrease in lightness (L*) and a simultaneous decrease in chromatic values (a*, b*) were observed, indicating darkening and browning of the wood surface. As a result of the heat treatment, the relative content of hemicelluloses decreased the most in merbau and mahogany, while the thermal stability of iroko and African padauk was higher. All examined wood species showed a strong correlation between the lightness difference value (ΔL*) and the content of hemicelluloses (r = 0.88–0.96). The FTIR spectroscopy showed that the breakdown of C═O and C═C bonds in hemicelluloses and lignin plays an important role in the formation of chromophoric structures responsible for the color changes in the wood.