Effect Of High Pressure Treatment On Physical And Mechanical Properties Of Poplar

With the continuous depletion of hardwood resources,industrial timber will gradually be transformed from natural forests to plantations.However,fast-growing forests have defects such as low density,poor dimensional stability and mechanical properties,which limit its further development and utilization.Therefore,it is necessary to explore a highly efficient and environment-friendly wood modification technology.In this paper,a preliminary exploration of high-pressure densification technology was studied and improved with the constraint effect.The effect of high pressure on equilibrium moisture content and deformation properties of poplar was investigated.High pressure influence on densifty profile,mechanical properties and microstructure of poplar were compared.The condition of high pressure treatment is 50-200 MPa,and the holding time is 0(pulse),30,60 and 300s.The main conclusions are as follows:1.After high pressure(HP)treatment of 150 MPa-30 s,the volume of poplar was obviously reduced,but its compression in the grain direction was extremely small(negligible).Under isostatic conditions,wood was collapsed in its weakest direction.The HP-molding compression effectively regulated its shape and greatly improved the availability of densified wood(21%enhancement).2.The compression ratio(CR)of poplar was exceeded 50%.At the same holding time,the CR increased with the increasing of pressure,and no significant change was found after 100 MPa.When the pressure is constant,the CR improved with prolonging holding time,and reached the maximum at 300s.The delayed elastic strain(DES)of poplar was observed after HP treatment,but the difference of DES among all HP-densified specimens was not significant.3.The moisture sorption isotherm of poplar was not changed after HP treatment.In the relative humidity(RH)between 33-65%,the equilibrium moisture content of densified wood was obviously lower than the control,while there was no significant difference above 76%RH.The difference of equilibrium moisture content among HP-treated wood was relatively small,and no regular change was found.4.The swelling efficiency of poplar was significantly increased after HP treatment.The irreversible recovery of compressed wood was increased with the increase of ambient RH,and reached a maximum at 95%RH.The RH thresholds of HP-densified poplar were found to be between 33-54%for beginning set-recovery and between 85-95%for rapid set-recovery,respectively.The beginning RH threshold reduced with the extension of holding time,while the rapid RH threshold was not influenced among different HP comdations.5.The N-shaped density gradient across the thickness was observed among all treated groups.The density and mechanical properties of poplar under HP treatment was effectively improved and reached the maximum between 100-150 MPa,while no significant difference was observed in hardness among HP-treated specimens.In addition,there was no obvious difference in density and mechanical properties of densified wood among different holding times.6.Scanning electron microscopy indicated that the vessels and fibers are collapsed and flattened after HP treatment,and a substantial number of tiny cracks were observed in the cell walls.What's more,the fracture of cell wall was found at 200 MPa-treated samples,which led to destroy the integrity of cell.Compared with traditional compression,the strength potential index of HP densification is relatively lower.