Comparative investigation of the effect of pretreatment type and densification conditions on physical and mechanical properties of paulownia and poplar woods

Due to the lack of forest resources, the use of fast-growing wood species has been considered by designers and engineers. Low density and mechanical strength are the main challenges of using some of these wood species, especially in the manufacturing of construction products and their final properties. Using different methods such as hydrothermal and hygrothermal treatments combined with compaction techniques can be a solution to this problem. This study aimed to investigate the effect of pretreatment type and densification conditions on the physical and mechanical properties of paulownia (Paulownia fortune) and poplar (Populus deltoides) wood. For this purpose, pretreatment with water at 25˚C and compression in hot press at 160˚C, and also pretreatment with steam at 160˚C and compression in cold press at 60C˚ were used. Compression was performed at two levels of 25 and 50 percent for 20 minutes in a press with a pressure of 5MPa to reach a final thickness of 2cm. Then, the equilibration of the samples in the cold press was performed to reduce the spring back and reach the temperature and equilibrium moisture content. To investigate the physical and mechanical properties of the samples, tests including density, water absorption, and thickness swelling (after 96 hours of immersion in water), modulus of rupture, modulus of elasticity, impact strength, and hardness were performed. The results showed that the poplar wood has better physical and mechanical properties than the paulownia wood due to its higher density. Except for thickness swelling, desirable physical properties and higher mechanical strengths were obtained in wood samples modified by the steam-cold pressing method. The greater impact of the wood modification process by steam-cold pressing on the properties was well observed. Water absorption and thickness swelling values of wood samples increased with increasing compaction percentage. The mechanical properties of wood had shown a decreasing trend, with increasing compaction and consequently increasing the density of wood samples, except for impact resistance