Sandwich compression of wood: control of creating density gradient on lumber thickness and properties of compressed wood

In order to utilize the plantation poplar wood, white poplar boards were soaked, heated and compressed to improve the mechanical properties in this study. The densities and densified zones were managed inside the board by choosing various parameters of soaking time between 0.5 and 5.5 h, heating time between 20 and 200 s and pressing time between 6 and 60 s. The density gradient was generated with the higher density near surface and the lower density toward the center of board. With this method, the density near the surface can be modified to 0.89 g/cm3 with an increase of about 100 % in comparison with poplar wood without any treatment. The boundary between the high-density layer and the low-density layer was characterized by X-ray scanned images. The modulus of elasticity (MOE) of compressed white poplar wood increased linearly with the compression ratio, and the modulus of rupture (MOR) increased exponentially. At the compression ratio of 47 %, MOE and MOR of compressed poplar wood were 19.77 GPa and 153.94 MPa, respectively. They were 73.2 and 88.9 %, respectively, greater than that of untreated specimens.