Impact of the Combination of Densification and Thermal Modification on Dimensional Stability and Hardness of Poplar Lumber

With a view to evaluating the impact of the combination of densification and thermal treatment on dimensional stability and surface hardness, poplar (Populus lasiocarpa Oliv.) lumber was densified under three compression ratios (10, 18, and 25%), three press temperatures (130, 150, and 170°C), and two pressure holding times (15 and 35 min). It was subsequently thermally treated at three temperatures (180, 190, and 200°C) and three exposure times (1.5, 2.5, and 3.5 h). Density, density profiles, water-soaked radial (thickness) swelling, and surface hardness were examined. The results showed that the density of poplar was increased by 8–24% due to densification. Compared to the densified specimens, the average water-soaked radial swelling was reduced by 43% through thermal treatments. Compared to the nondensified and non-thermally treated specimens, the maximum water-soaked radial swelling was reduced by 38% after thermal treatment at 200°C for 3.5 h. When the compression ratio of 10, 18, or 25% was applied for the densification, the average surface hardness was increased from 18.9 to 27.1, 34.6, or 43.9 MPa, respectively. These hardness values are similar to or greater than that of birch, which is commonly used for hardwood flooring. It was concluded that the combination of densification and thermal treatment could convert the low-grade poplar into high-value products with improved dimensional stability and surface hardness.