Influence of Densification on Mechanical Properties of Thermally Pretreated Spruce and Poplar Wood

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The effects of mechanical densification on density, Brinell hardness, bending strength (MOR), modulus of elasticity (MOE), and compression strength (CS) of thermally pretreated spruce (Picea orientalis) and poplar (Populus nigra) wood samples were investigated. Thermal treatment was applied on the wood samples at four different temperatures (140 °C, 160 °C, 180 °C, and 200 °C) and two different durations (7 h and 9 h) under atmospheric pressure. Wood samples were then densified by compression at a temperature of 150 °C to two degrees (20% and 40%) of compression. The results indicated that the density, hardness, and MOR values of both compressed and non-compressed thermally pretreated spruce and poplar samples decreased with increasing treatment temperature and duration. At temperatures below 200 °C, the MOE was generally increased in thermally pretreated samples. However, the MOE was reduced in thermally pretreated samples at 200 °C compared to the untreated samples. Additionally, all thermal pretreatments increased CS values in compressed and non-compressed wood samples. The CS tended to decrease in thermally pretreated samples (especially 40% compressed) at 200 ºC. After the densification, all of the strength properties tested in thermally pretreated samples increased significantly depending on the increase in compression ratio.