Effect of norlignan components in black heartwood of Japanese cedar on the self-bonding performance of binderless boards

Abstract Most Japanese cedar heartwood is reddish in color, but some of the wood is blackened. This phenomenon is related to the high potassium concentration, weak alkalinity, and denaturation of norlignans. Owing to its dark color and high moisture content, blackened wood has a low timber price. In this study, binderless boards were produced from black heartwood, and the mechanical performance and water resistance were compared to those of normal (red) heartwood. The changes in the main wood components and extractives caused by heat and pressing were also analyzed. The results showed that the mechanical performance and water resistance of black heartwood boards were higher than those of red heartwood boards. The optimal production conditions were 220 °C for 20 min for red and black heartwood, or 240 °C for 10 min for black heartwood. Norlignan analysis revealed that more sequirin-C was lost during the hot- pressing of black heartwood. In the infrared spectra, a peak around 1717 cm –1 appeared only after the black heartwood samples were hot-pressed. These results indicate that the hydroxy group–rich sequirin-C may have formed new ester bonds with the carboxyl group of hemicellulose during hot-pressing, thereby increasing the internal bonding strength.