Within-tree variability of internal stress generated during drying of rubberwood lumber

The generation of drying stress within the lumber from the trunk of a rubber tree prepared from different locations (radial distance up to 110 mm and height up to 4 m) has been investigated in real-time by using a restoring force measurement on half-split specimens. Drying was performed at constant dry-bulb and wet-bulb temperatures of 90 and 60 °C, respectively. The entire restoring force profiles do not vary significantly with height. In addition, before and after the reversal of stress, the force profiles are largely similar and the maximum negative and positive forces are approximately equal regardless of wood locations within the tree trunk. However in the radial direction, the process of stress reversal consisting of two negative force maxima appears to proceed slower in the inner juvenile wood than in the outer mature wood. Upon water immersion of the specimens for 4 months, the second negative force maximum gradually disappears and the force profiles with a shorter stress reversal period become less sensitive to the wood locations. An examination of the drying curves in the second stage of drying during stress reversal also indicates a slower migration of bound water out of the lumber in the juvenile wood compared to that in the mature wood. The drying is also faster in the water-immersed specimens. It is concluded that variability of the internal stress within the trunk of a rubber tree originated from the role of cell wall amorphous constituents and cell wall extractives on creep property and the movement of bound water within the wood cell wall during drying.