Structure Optimization And Surface-layer Heat Treatment Process Of Unilaterally Surface-densified Wood

Surface-densified wood has been extensively utilized in non-structural applications such as flooring,tabletop,chair panel,and wallboard due to its excellent properties of high strength-to-weight ratio,environmentally friendly features,and high surface hardness.Based on the previous researches,however,it was not comprehensive research for the surface-densified wood with respect to structural characteristics,mechanical properties,and dimensional stability,which significantly limit its large-scale applications.In this paper,the control mechanism of the position and thickness of the effective dense layer were studied firstly.The influence of the position and thickness of the effective dense layer on mechanical properties were discussed,which provide a scientific basis for the structural optimization of the surface-densified wood.Finally,the surface-densified wood was thermally modified by the surface-layer heat treatment process to improve its dimensional stability.The main contents and results are as follows:(1)A series of surface-densified wood with different structures were prepared by controlling the preheating and closing time during the hot-pressing process.The vertical density profile and mechanical properties of these densified wood were tested,which further revealed the control mechanism of the position and thickness of the effective dense layer.The results showed that the closing time significantly affected the density peak and thickness of the effective dense layer,which further affected the surface hardness of the surfacedensified wood.The preheating time had a notable effect on the position of the effective dense layer,which further affected the surface hardness and impact resistance.In the case of the DLU,the position of the effective dense layer had no influence on the MOR,while the effective dense layer thickness was a major contributor to the strength of surface-densified wood.However,the MOR values were not significantly different among all the groups in the case of the DLD.The failure mode of asymmetric surfaced-densified wood under the bending load was dependent on the loading surface.(2)To reveal the mechanism of dimensional stability improvement for surface-layer heat-treated wood,we prepared the surface-layer heat-treated wood by controlling the treatment temperature and duration during the hot-pressing process.The indicator of the mass loss,moisture absorption and desorption,thickness swelling,and cupping deformation of surface-layer heat-treated wood were tested.The results showed that the mass loss of the surface-layer heat-treated wood increased with the increase of treatment duration and temperature in a linear manner.The equilibrium moisture content of moisture absorption and desorption of the uncompressed heat-treated layer and the densified heat-treated layer was lower than that of the core layer.The core layer of the surface-layer heat-treated wood had comparable hygroscopic hysteresis coefficient to the uncompressed layer of the only densified wood.Set-recovery and thickness swelling of the surface-layer heat-treated wood immersed in water and placed in the humidity chamber were significantly decreased with increasing treatment temperature and duration.When the treatment temperature was 220 ℃and duration was above 20 min,the surface-layer heat-treated wood showed a low cupping deformation.(3)To further optimize the process parameters of the surface-layer heat treatment,the vertical density profile,mechanical properties,surface color characteristics,and wettability of surface-layer heat-treated wood were tested and discussed.The results showed that the average density of the densified heat-treated layer and uncompressed heat-treated layer decreased with increasing treatment temperature and duration.The surface hardness,MOR,and MOE of the surface-layer heat-treated wood also significantly decreased.The bending properties of surface-layer heat-treated wood were not dependent on the loading surface because the uncompressed heat-treated layer had a low hygroscopicity and high stiffness.The surface color characteristics and wettability of surface-layer heat-treated wood were also significantly affected by the treatment temperature and duration.The water contact angle of the surface-layer heat-treated wood was lower than that of the un-densified wood and the only densified wood.However,for the diiodomethane contact angle,no significant change among different treatment groups was observed.This result showed that the nonpolar of the surface-layer heat-treated wood was not changed significantly.Based on the results above,the optimal treatment temperature and duration were 220 ℃ and 20 min,respectively.