Study on the Properties of Adhesive-Free Bonded Laminated Poplar Timber Fabricated Through Combined Acid and Alkali Treatment

Adhesive-free bonding technology for alkali-treated wood eliminates the need for aldehyde-based adhesives. In this study, poplar wood was treated separately with sodium hydroxide (NaOH) and citric acid. The treated poplar panels were sealed and stored for a predetermined period, followed by pretreatment processes including rinsing and air-drying. Subsequently, deionized water was applied to the surface of the citric acid-treated poplar, which was then laminated with the NaOH-treated poplar to complete the panel assembly. Finally, adhesive-free bonded poplar glulam was fabricated through a hot-pressing process. The bonding strength was evaluated, and the optimal process parameters were determined as follows: NaOH solution mass fraction of 5%, citric acid solution mass fraction of 8%, hot-pressing time of 90 min, hot-pressing temperature of 130 °C, and hot-pressing pressure of 2 MPa. Under these optimal conditions, the bonding strength reached 5.82 MPa. Fourier transform infrared spectroscopy and scanning electron microscopy analyses of the bondline were conducted. The results showed significant degradation of hemicellulose within the bondline, accompanied by slight degradation of lignin and cellulose, which promoted the formation of a molten fiber-encapsulating structure during the hot-pressing process.