Hygroscopicity of thermally modified poplar (Populus spp.) wood.

Boards of poplar wood were subjected to thermal treatment at 180 o C and 200 o C, for durations of 2, 4, 6, 8 and 10 hours, in order to investigate the effect of heat treatment on the hygroscopic properties. Specifically, the change of equilibrium moisture content, density, swelling and absorption of wood were examined, in order to determine the significance of treatment temperature and duration in the enhancement of wood hygroscopicity. Generally, swelling in tangential direction proved to be quite higher than swelling in radial direction, while thermal treatment enhanced in great extent swelling in tangential and radial direction and the absorption of the specimens as well. Swelling in tangential direction decreased within the range of 8.17% and 33.3% for specimens treated at 180 o C and the range of 43.85% and 65.09% for specimens treated at 200 o C for 2-10 hours, referring to the final measurement after 72 hours of water immersion. In radial direction, the swelling decrease ranged between 2.47% and 28.45% for specimens treated at 180 o C and between 19.62% and 60.87% for specimens treated at 200 o C for 2-10 hours, respectively, after 72 hours immersion. Most of the heat treatments resulted also in absorption decrease, attesting the improvement of the poplar wood hygroscopic behavior. Finally, as it was proved, the higher the treatment temperature that is applied, the shorter is the duration, needed for the same swelling and absorption decrease level to be achieved.