Effect of heat treatment on dimensional stability of phenolic resin impregnated poplar wood.

The poplar wood and phenol formaldehyde (PF) impregnated wood were subjected to heat treatment at 160, 180, 200 and 220 ℃ for 1 to 4 hrs. Hydroscopicity, hygroscopicity, linear (radial and tangential) and volume swelling, and surface wettability of the control and PF impregnated wood after heat treatment were measured. The results showed that the dimensional stability can be improved considerably after heat treatment. The water absorption of PF impregnated wood was reduced by 17.37% compared with that of control group after soaking in water for eight days, and the maximum decline could reach up to 63.8% and 74.7% for heat-treated wood and PF impregnated heat-treated wood, respectively. The swelling ratio of PF impregnated wood was reduced by 14.71%, 36.93%, and 30.19% for radial, tangential, and volume, respectively, as compared to that of untreated wood, whereas the maximum reduction could reach up to 64.99%, 74.94%, and 72.33% for heat-treated wood and 94.4%, 90.61%, and 91.37% for PF impregnated heat-treated wood. The adsorption moisture content decreased by 11.14% for PF impregnated wood compared with that of control group, while 55.57% and 60.62% reduction was observed for heat-treated wood and PF impregnated heat-treated wood, respectively. The contact angle of PF impregnated was close to that of control group, while the maximum increase could reach up to 143.7% and 139.4% for heat-treated wood and PF impregnated heat-treated wood, respectively, indicating that the surface wettability of heat-treated wood and PF impregnated heat-treated wood were decreased significantly. After heat treatment, significant changes can be found in infrared spectrogram. The improved dimensional stability of PF impregnated heat-treated wood was attributed to the reduction of hydroxyl, carboxyl and carbonyl which was mainly caused by the degradation of hemicellulose and the formation of chemical bonds between PF resin components and the cell wall polymers. PF resin impregnation combined with heat treatment was one of effective methods to improve the dimensional stability of poplar wood.