Hygroscopic behavior of thermo-hygro-mechanical (THM) densified oil palm sawn timber

To facilitate the drying process and enhance the properties of oil palm wood, oil palm boards were mechanically pre-dewatered and thermo-hygro-mechanically (THM) densified. The thickness of the boards was reduced with compression ratios of 40%, 60% and 75%. Since densified wood tends to recover from compression, especially under humid conditions, this study examined the set-recovery and the hygroscopic behavior of THM densified oil palm wood. The equilibrium moisture content (EMC), differential swelling and swelling coefficient, linear swelling and shrinkage as well as the differential swelling anisotropy were determined under climate conditions with different relative humidity (RH) (20°C/35% RH, 20°C/65% RH and 20°C/85% RH). The maximum swelling was measured after water soaking and the remaining set-recovery was evaluated after re-drying at 103°C. The EMC was reduced by the THM process by around 20%. In the direction of compression (thickness), the densified specimens show higher values for all analyzed swelling and shrinkage parameters than the undensified specimens from the equivalent position within the trunk. The maximum swelling in thickness of 22–38% during water soaking is mostly reversed by shrinkage during re-drying and a comparably low remaining set-recovery of 3–8% is measured at oven dry condition.