Study On Physical And Mechanical Properties And Their Correlation With Microstructure Of Different Poplar Clones

Poplar is the main tree species of fast-growing plantation and is widely planted in China.The poplar has the characteristics of rapid growth,high yield,excellent fiber,low density,hardness and mechanical strength,and poor stability.It was mainly used in pulp and paper making,plywood manufacturing and other industries,but with the continuous development of wood science and technology,it can be used in furniture,flooring and interior decoration wood products with high added value after processing improvement.There has been no systematic study on the material characteristics of newly bred poplar clones in recent years.However,it is of great significance to explore the physical and mechanical properties of newly bred poplar clones and reveal the microstructural factors affecting its mechanical properties for the efficient processing and utilization of wood and the selection and breeding of clonal trees.In this paper,10 poplar clones of the Sect.Aegeiros Duby（including clone 50,36,Danhong,Nanyang,N179,108,Zhonglin 46,Sangju cut in Jiaozuo,Henan Province,Bofeng 1 and Zhongliao 1 in Suizhong,Liaoning Province）were selected as experimental materials.With reference to national standards,the physical and mechanical properties of 10 poplar clones,such as air-dry density,dry-shrinkage,wet swelling,bending properties,compressive properties and hardness were measured.Using the nano indentation machine,optical microscope image analysis system,X-ray diffraction and Fourier infrared spectrometer,the mechanical properties,microstructure,ultrastructure（microfibril angle,crystallinity,crystalline regions size）and relative content of lignin of cell walls that affect the macromechanical properties of wood were represented.The correction and prediction models of wood physical（air dry density）and main mechanical properties of poplar clones were established by using near-infrared spectroscopy.The main conclusions of this paper are as follows:1.Radial variation of physical and mechanical properties of poplar clonal wood（1）The air dry density of 10 poplar clones ranged from 0.388 to 0.497 g·cm^-3.The air dry shrinkage rates in radial,tangential and volume of 10 poplar clones were in 1.0%～2.8%,3.5%～5.9%,4.7%～7.7%,respectively,and the wet swelling rate from oven dry to air dry state were in 0.6%～2.2%,1.3%～3.2%,and 2.2%～4.8%,respectively.Among them,the air dry density,shrinkage rates and wet swelling rate of Nanyang was the highest,and that of Bofeng 1was the lowest.In the radial direction,the air dry density of the sapwood was greater than that of the heartwood,（2）The modulus of rupture（MOR）of the 10 poplar clones was in the range of 54.9～96.2MPa,the modulus of elasticity（MOE）was in the range of 6.8～12.4 GPa,the compressive strength parallel to grain was in the range of 26.8～47.3 MPa,and the average hardness was in the range of 2348～4547 N.Nanyang and Zhongliao 1 are clones with better mechanical properties from Jiaozuo,Henan and Suizhong,Liaoning,respectively.In the radial direction,the radial variation law of 10 poplar clones was consistent with that of air dry density.That is,except for N179 poplar,the mechanical properties of the other 9 clones increased gradually in the radial direction,and the properties of sapwood were better than that of heartwood.2.Mechanical behavior and microstructural variation of cell wall of poplar clonal wood（1）The wood cell wall hardness of four poplar clones（clone 50,clone 36,Nanyang and Zhonglin 46）ranged from 0.40 to 0.51 GPa,and the reduced elastic modulus of cell wall ranged from 11.08 to 14.17 GPa.（2）The wall-lumen ratio of the four poplar clones ranged from 0.180 to 0.267.The microfibril angle（MFA）ranges from 14.6°to 17.0°.The crystallinity ranges from 43.8%to47.1%,the width of the crystalline regions ranges from 3.22 to 3.90 nm,and the length of crystalline regions ranges from 7.30 to 9.02 nm.For the relative lignin content,the relative peak intensity ratios were in the range of 1.02～1.58,respectively.The wall thickness,wall-lumen ratio,crystallinity,crystalline regions size and lignin content of sapwood of poplar clone were higher than that of heartwood,and the mean value of lumen diameter and MFA of cell walls were lower than that of heartwood.3.Correlation between mechanical properties and microstructure of poplar clonal wood（1）Among the four poplar clones,the mechanical properties of cell wall,wall-lumen ratio,crystallinity,crystalline regions size and relative content of lignin of Nanyang were the highest,while the MFA of it was the lowest,which has highest macroscopic mechanical properties.And the macroscopic mechanical properties were positively correlated with the hardness and reduced elastic modulus of cell walls,wall thickness,wall-lumen ratio,crystallinity,crystalline regions size and the relative contents of lignin.There were negative correlations between macroscopic mechanical properties and lumen diameter/MFA.（2）The correlation coefficients between MOR,MOE,compressive strength parallel to grain,hardness and wall-lumen ratio,MFA,crystallinity,relative peak intensity ratios of I₁₅₀₉/I₁₃₇₉were in 0.60～0.82,respectively.The influence factors of cell morphology and cell wall are superimposed on each other,which jointly determine the physical and mechanical properties of poplar wood.4.NIRs prediction of physical and mechanical properties of poplar clonal wood（1）Fast prediction of near infrared spectroscopy on physical and mechanical properties of poplar clones can be achieved by using spectral information of cross section with first derivative method for air dry density,MOR and hardness,and the MSC method for MOE and compressive strength parallel to grain.（2）The correlation coefficients of the 10 prediction models for the air dry density of poplar clones ranged from 0.55 to 0.74,that of MOR ranged from 0.18 to 0.71,correlation coefficients of MOE prediction models ranged from 0.51 to 0.68,and that of compressive strength parallel to grain ranged from 0.55 to 0.76.The correlation coefficient of hardness prediction model is 0.62.（3）The prediction results of MOR and compressive strength parallel to grain were affected by different sampling methods and spot sizes.The prediction model of MOR obtained by multi-point sampling was better than that obtained by one-point sampling,and the prediction model of compressive strength parallel to grain had better effect when the spot size is 15mm and collected at one point..