Changes in the properties of light-irradiated wood with heat treatment

This study investigated the changes in the color of wood with heat treatment after exposure to light at various conditions. The change with exposure is attributed to the decomposition of lignin, and the process follows a first-order reaction law. The degree of color change in softwood was greater than in hardwood, while there was no significant difference between the reaction rates for softwood and hardwood. This is attributed to differences in the lignin content. The change in the color of light-irradiated wood with heat treatment increased with light-irradiation time, heating temperature, while the changes in the color of wood irradiated for more than 30 to 40 h were similar. With the investigation of the effect of wavelength, the change in the lightness, ΔL*, of all the woods tested decreased with wavelength. ΔL* became positive for light-irradiated Japanese cypress and hackberry; however, this effect disappeared with heat treatment. The chroma coordinates Δa* and Δb* of all species were minimal when irradiated with wavelengths longer than 500 and 440 nm, respectively. However, this effect disappeared with heat treatment. In softwood, Δb* was negative when exposed to light longer than 440 nm, while in hardwood, it turned negative when exposed to 390, 440, and 500 nm. However, the minimum value observed with light-irradiation disappeared with heat treatment after light-irradiation. Therefore, it is thought that chemical changes involved in the color changes due to light-irradiation differ form those involved in the color changes due to heat treatment.