Effect of high pressure treatment on density, mechanical properties and microstructure of low-density hybrid poplar

Abstract. High pressure (HP) treatment is a potential non-thermal technology for isostatic compression. In this study, Small clear hybrid poplar board samples were densified by high pressure at 50, 100, 150, 200 MPa for 30s, respectively. Properties evaluated were average density, modulus of elastic (MOE), modulus of rupture (MOR) and compressive strength parallel to grain (CSPTG). Furthermore, the morphological structures of hybrid poplar before and after HHP compression were analyzed using scanning electron microscopy (SEM). The results from the density measurement demonstrated that HHP treatment significantly increased (P < 0.05) the density of poplar wood. It should be noted that the density of 150 MPa-compressed wood specimens (0.787 g/cm3) was increased by 109% compared to the non-densified wood (0.376 g/cm3). Similar trends were found in the mechanical properties of densified wood. After 150 MPa treatment, the MOE, MOR and CSPTG were 62.2%, 44.5%, 72.6% greater than those of the control, respectively. In addition, The SEM images showed that collapsed vessels and cell walls caused by HHP treatment significantly decreased the void spaces of wood, while no significant fracture was observed in the cell wall. This reveals that the deformations in the vessels and cell walls occurred based on the viscoelastic nature of wood, which is consistent with the traditional compression. Therefore, short-term HHP treatment is a potential method for commercial use in wood densification because of good physical and mechanical properties of densified wood.