Hybrid poplar (Populus deltoides × Populus trichocarpa) specimens were densified by mechanical compression at high temperature (150 °C, 160 °C, and 170 °C) and three conditions of steam pressure - saturated, transient and saturated steam. The modulus of rupture (MOR) and the modulus of elasticity (MOE) of the control (undensified) wood and of the densified wood specimens were determined.

Wood densification altered wood properties. Microscopic observation revealed that the deformations in all densified wood specimens were mostly the result of the viscous buckling of cell walls without fracture. Temperature and conditions of steam environment during densification affected the properties of the resulting densified material. While the temperature significantly affected the oven-dry density of specimens compressed under transient steam conditions, temperature had little effect under saturated steam conditions. The highest density of the compressed specimens was obtained under transient conditions at 170 °C and under saturated steam at all examined temperatures (150 °C, 160 °C, and 170 °C). The temperature of the treatment significantly affected the equilibrium moisture content after conditioning in a constant environment, with higher temperature resulting in lower equilibrium moisture content. In specimens compressed under saturated steam the MOR and MOE were increased proportionally to the increase in density. Compression under transient conditions at 170 °C produced densified wood material with the greatest MOE and MOR, which were increased more than the proportional increase of density.

Therefore, the results presented here suggest that wood densification should be applied in transient conditions at 170 °C, where movement of moisture and temperature at the point of wood compression resulted in a densified wood material with the greatest increase of mechanical performance in relation to the increase of density.