The Response Mechanism Of Wood Cell Wall Chemical Interface Structure To Humidity

In order to analyze the cracking and deformation of wood products caused by changes in environmental humidity during use at the molecular level,this article mainly uses Pinus sylvestris(Pinus sylvestris L.var.mongholica Litv.)and poplar(Populus L.)as the research objects.Firstly,the sample is extracted and processed,and then the moisture content of the sample is adjusted.With the help of modern testing methods and material analysis software such as XRD,FTIR,SEM,nanoindenter,etc.,observe the effects of moisture content on the microstructure of wood and the macromechanical properties of wood.The effect of the mechanical properties of the cell wall is used to judge the influence of moisture on the structure of the cell wall during the moisture absorption process of wood.The research results show that:(1)Taking the extracts to dissolve the most and the cellulose,hemicellulose and lignin degradation as the standard,the variance and range analysis of the extraction results are carried out,and the best extraction process of Pinus sylvestris var.mongolica is obtained at a temperature of 80?and a time of 6 h,The solid-liquid ratio is 1:20;the best extraction process of poplar wood is temperature 70?,time 6 h,solid-liquid ratio 1:20.By investigating the changes in the structure and mechanical properties of the samples after the advancement of extraction,it was found that the main chemical components of Pinus sylvestris and poplar were degraded to varying degrees during the extraction process,of which hemicellulose was mostly degraded by 2.84%,3.56%,followed by lignin degradation at 2.7%and 2%,and the main degradation areas are concentrated in the fiber cell wall and composite intercellular layer.The cellulose degradation is at least 1.04%and1.4%,respectively.The degradation of hemicellulose and lignin caused the thermal degradation temperature of cellulose to shift to low temperature,the thermal stability decreased and the crystallinity increased(the average relative crystallinity of Pinus sylvestris and poplar increased by 3.7%and respectively after extraction.12.4%).(2)The degradation of the main chemical components of the wood caused by the extraction caused more pores in the cell wall of the wood.After the extraction,the number of pores in the pore size of 3.06 nm and greater than 5.6 nm of Pinus sylvestris increased significantly;there are a large number of pore structures with a pore size of 3.05 nm,and the overall number of mesopores in poplar wood after extraction is significantly reduced due to the drying shrinkage.The above-mentioned structural changes caused the density of Pinus sylvestris to decrease by 0.108 g/cm~3 after extraction,while the density of poplar wood increased by 0.199 g/cm~3 after extraction.This change in density causes a change in the mechanical properties of the wood.After extraction,the bending strength,tensile strength and impact toughness of Pinus sylvestris var.sylvestris decreased by 2.55 N·m/Kg,14.38 N·m/Kg,12.28 KJ/m~2,respectively;the bending strength of poplar wood after extraction And the tensile strength increased by 42.61 N·m/Kg and 5.3 N·m/Kg,respectively,and the impact toughness decreased by 0.97 KJ/m~2.(3)Moisture absorption treatment on the sample after extraction found that when the relative humidity is less than 57.57%,the water molecules of Pinus sylvestris are mainly adsorbed by monolayers,and vice versa.Poplar is similar,with a relative humidity cut-off point of 45.2%.During the formation of monolayer adsorption,the speed of multi-molecular layer adsorption gradually increases.In addition,the entry of water molecules will cause the molecular chains in the amorphous region of cellulose to rearrange,so that the area of the crystalline region in the wood increases,and the degree of crystallinity increases.At the same time,the entry of water molecules will increase the plasticity of the wood,resulting in a decrease in the bending strength and tensile strength of the test material,and an increase in the impact toughness.(4)Through nanoindentation test on samples of Pinus sylvestris with different moisture content after extraction,it is found that the longitudinal elastic modulus and hardness are 12.79 GPa and 0.62 GPa,respectively.When the water content of the cell wall increased from 0%to 24.54%,the longitudinal elastic modulus and hardness values decreased to 5.83 GPa and 0.24 GPa,respectively,with a decrease of 54.42%and 61.29%.Due to the restriction of the poplar wood sample after extraction,the cell wall mechanical property test was not performed on it.(5)Taking the extracted Pinus sylvestris and poplar wood as objects,a calculation model for the volume of the microfibril amorphous zone under different moisture contents was established.The simulation calculation found that there is a certain error between the average calculated value and the measured value in the amorphous area,and the maximum error does not exceed 24%,indicating that the established calculation model is feasible.