Study On Surface Compression And Fixing Deformation Of Fast-growing Poplar Lumber

In this paper, the technology of surface compression and deformation fixation of fast-grown Poplar lumber was studied. The compression of surface layer was achieved by steaming and hot pressing treatment, the influence on different process conditions of steaming and hot pressing were determined, and the process of deformation fixation of differentiation compression was also discussed. The results showed that:(1) The compression ratio of the surface layer of Poplar lumber could be improved by steaming process. For the process condition, which the total compression ratio was 10%, the temperature of hot pressing was 170 ℃, and the time of steaming was 3 minutes, the compression ratio of the surface layer of the lumber could be as high as 21.5%, while the compression ratio of the core layer was only approximate 3.5%. For the process condition, which the total compression ratio was 20%, the temperature of hot pressing was 190℃, and the time of steaming was also 3 minutes, the compression ratio of the surface layer of the lumber could be 36.8%, at the same time the compression ratio of the core layer was only 5.6%. For the lumber which had not be treated with steaming processing, the compression ratio of the core layer was the highest, but the compression ratio of the surface layer was not obvious.(2) The density of the surface layer of the lumber could be improved by the steaming process. For the samples whose the total compression ratio were 10% by steaming process, the density of the surface layer could be as higher as 715 kg/m3, which was 31.9% higher than the average density of the untreated samples, and the surface layer density of the treated samples were 37.5% higher than the untreated samples. For the samples whose the total compression ratio were 20% by steaming process, the density of the surface layer could be as higher as 910 kg/m3, which was 47.1% higher than the average density of the untreated samples, and the surface layer density of the treated samples were 40.0% higher than the untreated samples. It is considered that the density of the lumbers could be improved by the steaming process.(3) Some defects would appear after moisture absorption for differentiation compression lumbers, such as cupping and springback deformation. These defects could be effectively alleviated by the means of the combination of conventional drying and heat treatment, which could be used as a useful means to solve the defects causing of differentiation compression and springback deformation. For the 18 mm compression Poplar lumber, which was conditioned 4h under 100℃, the differences of the moisture content between core layer and two-surface layers were 0.4 and 0.6. For the 20 mm compression Poplar lumber, which was also conditioned 4h under 100℃, the differences of the moisture content between core layer and two-surface layers were 0.9 and 0.6. That is to say the differences of moisture content in the thickness direction could be eliminated. The ratio of springback was only 0.3% and 0.8% for the samples whose compression ratio was 10% and 20% after conditioning process 30 days, meanwhile, the equilibrium moisture content of heat treated wood was lower than untreated wood and the dimensional stability of the heat treated wood was also better. The swelling and shrinkage ratio and the springback of lumber were also improved by heat treatment.(4) The mechanical properties of Poplar lumber could be remarkable improved by surface compression. For the samples whose compression ratio were 10%, the hardness of compression lumbers increased 52.8% versus untreated lumbers, the MOR and MOE were also higher 36.9% and 8.8% for the control samples. For the samples whose compression ratio were 20%, the hardness of compression lumbers increased 72.9% versus untreated lumbers, the MOR and MOE were also higher 13.1%(112.4MPa) and 28.1% for the 10% compression samples.(5) According to the anatomical structure analysis, it is showed that there were obvious deformation under the 1-1.5mm of the surface layer of the compression poplar lumber, the fiber also deformed, but there were no obvious deformation for the vessels of surface layer. With the increase of the compression ratio from 10% to 20%, there were obvious deformation for the area of vessels, the area of the vessels reduced by 36.7% versus untreated samples for the 10% compression lumber, meanwhile, the area of the vessels reduced by 50% versus untreated samples for 20% compression lumber.