Influence of thermal pretreatments on dimensional change and humidity sensitivity of densified spruce and poplar wood

Abstract

Densification modification is an effective method to improve many properties of wood. However, densified wood is sensitive to humidity and is not dimensionally stable. The effect of thermal pretreatments on the dimensional change and humidity sensitivity of densified Picea orientalis (spruce) and Populus nigra (poplar) wood were investigated. A thermal pre-treatment was applied on the wood specimens at 140 degrees C, 160 degrees C, 180 degrees C, and 200 degrees C for 7 h and 9 h. Wood specimens were then compressed at ratios of 20 % and 40 % at a temperature of 150 degrees C. The results showed that spring-back and thickness swelling increased in all specimens (thermally pre-treated and untreated) depending on the increase in compression ratio. However, set-recovery wasdetermined higher at20 % compression ratio.Theequilibrium moisturecontentvaluesof untreated specimens and thermally pre-treated specimens at low temperatures (140 degrees C and 160 degrees C) were found lower than uncompressed specimens. The impact of compression ratio on equilibrium moisture content was not clear. Thermal pretreatments significantly affected the dimensional stability and hygroscopicity of densified specimens (especially poplar wood). Depending on the increase in thermal pre-treatment temperature and duration, spring-back, set-recovery and thickness swelling in wood specimens decreased up to 31 %, 67 % and 62 %, respectively. In addition, equilibrium moisture content and water absorption decreased with the increase in thermal pre-treatment temperature and duration. Moreover, the thermal treatment temperature was more important than duration on the investigated properties.