Effect of superheated steam pressure on the physical and mechanical properties of sandwich-densified wood

To improve the undesirable properties of fast-growing wood, an environmentally friendly modification technology that combined controllable densification and superheated steam treatment was used to process fast-growing wood into a sandwich-densified structural material. The effects of the position of the densified layer(s) and superheated steam pressure on the density, microstructure, color, dimensional stability, and mechanical properties of poplar wood were investigated. The result showed that sandwich-densified wood with two structural modes, namely surface-densified wood and central-densified wood can be formed depending on preheating time. Compared with the control wood, the average density of the densified layer, surface hardness, modulus of rupture, and modulus of elasticity (MOE) of sandwich-densified wood with a compression rate of 20% were increased by more than 87.63%, 52.96%, 24.50%, and 36.44%, respectively. The cell lumen volume in densified layer was significantly reduced compared to that of the transitional layer and un-densified layer. When the superheated steam pressure increased by 0.2 MPa, the moisture excluding efficiency and anti-swelling efficiency of sandwich-densified wood were significantly increased (P < 0.05), but the density, MOE, and surface hardness were not significantly decreased (P ≥ 0.05). The sandwich-densified wood treated with superheated steam showed substantially higher mechanical properties and better dimensional stability, providing a promising method for processing fast-growing wood into solid wood flooring.