Effect of alkaline pretreatment and surface impregnation on the set recovery of densified poplar wood

Delignification and subsequently densification of wood is one of the newest approaches leads to a considerable improvement in mechanical properties of wood. However, because removal of the hydrophobic lignin, the problem of set recovery is still remained. In the present study, the poplar wood samples were chemically treated with sodium hydroxide and sodium sulfite solution and then were soaked in the solution of glycerol-citric acid; then densification was conducted in a pilot hydraulic press at temperature of 100oC for duration of 24 hours. Set recovery of the densified samples was decreased considerably, however removal of the hydrophobic lignin led to increase of the thickness swelling and anti-swelling efficiency became negative. Results of infrared spectroscopy on the delignified densified samples proved the formation hydrogen bonding between cellulosic units in compressed state which is a good reason for decreasing the set recovery. Surface impregnation with glycerol-citric acid combination led to a considerable reduction of set recovery. The infrared spectroscopy and also the microscopic study showed the presence of polyesters in the microstructure of the densified-impregnated samples.