Sandwich compression of wood: effects of preheating time and moisture distribution on the formation of compressed layer(s)

Wood sandwich compression technology can on the one hand effectively improve the physical and mechanical properties of low-density fast-growing wood, on the other hand it saves at least 25 vol% of raw materials (compared to the traditional wood compression technology) by controlling the position of compressed layer(s). In this study, white poplar (Populus tomentosa) lumbers were firstly soaked in water for 2 h, then preheated at 180 °C for 0–600 s. After the preheating process, radial compression was applied to obtain sandwich-compressed wood, and the compression was fixed by superheated steam treatment. Moisture distribution along the wood thickness and density distribution in the compressed wood were characterized. Furthermore, effects of preheating time on moisture distribution, position and thickness of the compressed layer(s) were investigated. Results indicated that the position of the compressed layer(s) moved from the wood surface to the center as a result of preheating time extension. The initial moving speed of the compressed layer(s) was 0.040 mm/s, which was sharply reduced to 0.014 mm/s with the extension of preheating time to 120 s. Further extension of preheating time above 120 s did not extensively reduce the compressed layer(s) moving speed. When the preheating time was 480 s, two compressed layers in the compressed wood converged to one at the center along the thickness in the compressed wood. More importantly, it was found that the positions of the compressed layer(s) highly matched up with high moisture content (MC) regions in the preheating process. A significant linear correlation between positions of the MC peak in high MC region of the preheated lumbers and density peak in the compressed layer(s) was built. In addition, moisture distribution along the thickness of wood can be controlled by adjusting the preheating time to eventually control the position and thickness of the compressed layer(s) in the sandwich-compressed wood. Superheated steam treatment after wood sandwich compression contributed to the reduced set recovery percentage of 1.58% after conditioning at 40 °C and relative humidity of 90%.