Properties of densified poplar wood through partial delignification with alkali and acid pretreatment

Poplar trees can be grown on marginal land to conserve water and recycle nutrients, but the wood has poor mechanical properties for structural applications. This study evaluated the feasibility of densifying poplar wood through delignification to improve mechanical properties. Delignification was conducted using a commercial soda pulping process under various NaOH concentrations of 0–6% with a reduced liquor-to-wood ratio of 3, along with a novel hydrotropic fractionation process using maleic acid. Delignified wood was pressed with or without stop bars at a low pressure of 1 MPa at 150 °C for 15 min or 100 °C for 2 or 10 h. Results indicate that wood density is the key variable dictating wood strength properties rather than extent of delignification. Pressing with stop bars resulted in similar wood densities for samples with different levels of delignification. Wood modulus of rupture (MOR), modulus of elasticity (MOE), and Brinell hardness (H_B) all increased linearly with wood density, in agreement with literature results from pure hydrothermal compression. When removing stop bars in pressing, MOR, MOE, and H_B were increased to 150 MPa, 20.7 GPa, and 43.2 MPa, respectively, from 88.5 MPa, 6.5 GPa, and 14.8 MPa for undensified poplar when wood density was increased to 0.81 g/cm³ from 0.46 g/cm³. Low-pressure densification with delignification is advantageous to significantly improve wood properties.