Study On The Effect Of High Pressure Densification On Deformation Of Poplar Wood

High pressure densification(HPD)is a kind of environment-friendly and efficient fast-growing wood densification technology,which could reduce the volume of pores inside the wood quickly,and improve the density and mechanical properties of fast-growing wood.However,the densified wood had irregular shapes and low availability after HPD.Also,the densified wood was sensitivity to moisture and the dimensional stability of the densified wood was poor.Hence,the deformation of poplar wood was investigated after HPD and the difference of compression deformation among axial,radial and tangential directions were analyzed in this study.A new measuring device was used to detect the elastic strain,plastic strain and total strain of densified poplar wood in this study.In addition,the effect of the cutting method on deformation and availability of densified poplar wood were explored in this study.Thermal modification(TM)was used as the post-treatment of HPD to improve the dimensional stability of the densified wood.The main results were listed below:1.The anisotropy of compression deformation of densified wood was discovered and compression ratio(CR)in tangential direction was much more CR in radial direction.However,there was no significant deformation in axial direction.No significant difference in CR of densified poplar has been found between sapwood and heartwood.The cutting method has an effect on the compression deformation of the wood sample.The CR of the SW wood sample is slightly higher than the JN wood sample,whether CR is in radial or tangential directions.2.Significant difference in elastic strain of densified poplar wood was found between radial and tangential directions,so is the plastic strain and total strain of densified poplar wood.In addition,the plastic strain and total strain of wood sample were improved with increasing pressure,but there was a sudden drop in elastic strain.When the treatment pressure was low,the holding time had an effect on the elastic and plastic strain of poplar wood.In other words,extending the holding time could increase the plastic strain of the sample by reducing the elastic strain,which could improve the efficiency of HPD.However,extending the holding time cannot change the total strain of the poplar wood sample.3.With increasing pressure,poplar wood density after pressure was improved.Also,wood density after pressure increased with increasing holding time when the treated pressure was less than 60 MPa.There was significant difference in wood density between under pressure and after pressure.Wood density under pressure was much more than and it after pressure.The oven-dry wood density under pressure is about1495.5 kg/m~3 when the treated pressure was 150 MPa,which is close to the density of wood cell walls.4.The cutting method had a significant impact on the availability of densified poplar wood.The availability of JN wood samples was higher than that of SW wood samples after HPD,especially when the treated pressure is high.However,the availability of the samples increased by about 10%after modification of the cutting method of SW wood samples.In addition,among the common cutting methods of wood,true quarter-sawn was the most suitable for HPD,while plain-sawn was the opposite.However,the availability of reverse pair plain-sawn wood sample and forward pair plain-sawn wood sample increased significantly,comparing with plain-sawn wood sample without pair.5.The thickness swelling(TS)of the densified poplar wood increased with the increasing immersion time,and the final TS of the densified poplar wood also increased with the treated pressure.A large amount of recovery occurred in the radial and tangential directions of the densified wood sample after the wet-dry cycle treatment,and the tracheid and wood fiber cells return to a complete and full shape.Also,the treated pressure had no effect on set-recovery of the densified wood.6.The TS and set-recovery of the densified poplar wood decreased with the increase of the time of TM,and the recovery of cell wall deformation decreased after the wet-dry cycle treatment.However,the volume swelling(VS)of densified poplar wood increased with the increase of treated pressure.Therefore,wood samples with a higher degree of densification need a longer time of TM to increase the anti-swelling efficiency(ASE)and dimensional stability of it.