Abstract:

Objective: In order to provide reliable methods and scientific basis for the modification of poplar wood surface and the optimization manufacturing process of wood scrimber, the directional recombination technology was applied to manufacture the wood scrimber of poplar with high-hardness, and then the effect of compression ratios (CR) on the surface hardness of wood scrimber was studied. Method: The wood scrimber with compression ratios of 50%, 55% and 59% were prepared by the directional recombination technology, and the poplar, beech and Russian oak were taken as the control samples. The values of Janka hardness was measured by the universal mechanical testing machine. And the macroscopic and microscopic morphology of samples, as well as the damaged surface for samples during the testing process of Janka hardness were observed by ultra-depth-of-field three-dimensional microscope (UDTM) and field emission scanning electron microscope (SEM). Additionally, the porosity of the samples was tested by mercury injection porosimetry (MIP) and distribution of phenol-formaldehyde resin (PF) within the wood scrimber was characterized by laser scanning confocal microscopy (LSCM) and transmission electron microscopy (TEM). Result: The Janka hardness values of wood scrimber increased with increasing compression ratios. The Janka hardness value of wood scrimber with compression ratio of 59% increased by 12.63 times compared with that of poplar wood. Furthermore, the Janka hardness value of wood scrimber was much higher than those of Russian oak and beech. In practical applications, the appropriate compression ratio of the wood scrimber can be determined according to the needs in different area. The analysis of stress-strain curve of Janka hardness showed that the proportional ultimate stress of wood scrimber increased with increasing compression ratio. The observation of SEM and UDTM revealed that the failure surface of poplar wood was ductile fracture, but changed to brittle fracture for the wood scrimber. The porosity of the samples tested by MIP showed that directed recombination technology significantly reduced the porosity of poplar wood, and the porosity of the wood scrimber decreased with increasing compression ratio. The observation of LSCM and TEM showed that the width and depth of PF resin distributed along the glue layer band gradually increased with increasing compression ratio and penetrated into the vessel, wood rays and fiber cells. The cell wall mechanical properties of the wood scrimber were tested by nanoindentation. When the compression ratio of the wood scrimber was 55% and the resin content was 15%, the hardness and elastic modulus of the cell wall were increased by 54.65% and 20.14% compared with the poplar wood, respectively. The reasons for the increase in surface hardness of wood scrimber were as following: In the manufacturing process, the vessels and fiber cells of poplar wood were compressed, and the porosity of wood scrimber was reduced. Therefore, the parenchymal density of wood cells in per unit volume was increased. Additionally, the binding area between the wood cells is increased and the connections are tighter. Meanwhile, the PF not only fixed the compressed cell structure, but also increases the strength of the connection between the wood cells through the formed glue nails, the PF resin immersed in the cell wall also enhanced its hardness and elastic modulus, which effectively improved the surface hardness of the wood scrimber. Conclusion: The directional recombination technology can be applied to solve the soft problem and low surface hardness of poplar wood, and the surface hardness of the wood scrimber can be adjusted through the compression ratio, so that the products prepared by the directional recombination technology can meet the needs of different applications.

Key words: fast-growing poplar wood, directional reorganization, compression ratio, hardness, wood scrimber