Study On The Influence Of Heat Treatment On Wood Structure And Mechanical Properties

Wood is widely used in many fields,but its dehiscence and deformation can be caused by the change of environmental temperature,in this thesis,Pinus sylvestris var.Mongolica and Poplar alba were treated by heat treatment at 160,180,200,220°C for2,4,6 hours to explore the influence of temperature on the content change,structure,macro-mechanical properties and micro-mechanical properties of wood.The main research conclusions are as follows:1.During the extraction treatment,not only the extract was removed,but also the main chemical components of the wood were degraded to different degrees.The area of amorphous zone decreased,and the relative crystallinity increased.In addition,wood cell wall and intercellular layer appeared pores,pit membrane cracks,but also the main chemical components caused by degradation.The degradation of chemical components also leads to the increase of wood porosity and changes of wood density and mechanical properties.The density of Pinus sylvestris var.Mongolica decreased by0.047 g/cm~3,the flexural strength decreased by 5.50 MPa,the tensile strength decreased by 4.48 MPa,and the impact toughness decreased by 6.97 KJ/m~2.The density of poplar increased by 0.118 g/cm~3under the action of dry shrinkage,the flexural strength,tensile strength and impact toughness increased by 36.09 MPa,9.77MPa and 3.02 KJ/m~2respectively.2.Chemical composition analysis showed that during heat treatment,the content of cellulose and cellulose decreased,while the relative content of lignin increased.The intensity of the characteristic absorption peaks of cellulose and hemicellulose in the infrared image decreases,while that of lignin increases,which also confirms the change rule.It was found that the content of the three major elements changed significantly at a critical point of 200°C,when the heat treatment temperature was below 200°C,the content of the composite cellulose and cellulose decreased more slowly,when the temperature was above 200°C,the relative content of cellulose and cellulose decreased sharply,and the relative content of lignin also increased greatly.3.At the early stage of heat treatment,the cellulose molecular chains in the amorphous region rearrange,which results in the increase of the area of the crystalline region and the relative crystallinity of the wood.However,with the increase of temperature,the degradation of wood cellulose becomes more serious and the relative crystallinity of wood tends to decrease.4.When the strength of heat treatment is lower,the bending strength and tensile strength of wood show an upward trend,which may be due to,on the one hand,the rearrangement of molecular chains in the amorphous region of cellulose,forming new hydrogen bonds,on the other hand,the cross-linking reaction of lignin results in stronger cell wall structure,so that the mechanical properties are improved.With the increase of the heat treatment intensity,the mechanical properties of wood decreased,mainly because the pyrolysis reaction of the main chemical components of wood was intensified,and the cellulose macromolecular chain was broken to form the small molecular chain,the skeletal structure of the wood is destroyed and therefore the mechanical strength is reduced.5.Nano-indentation test of Pinus sylvestris var.mongolica wood after heat treatment showed that the longitudinal elastic modulus and hardness of cell wall of Pinus sylvestris var.mongolica increased first and then decreased with the increase of temperature.6.The content change models of comprehensive cellulose,cellulose and lignin of Pinus sylvestris var.Mongolica and Poplar were established under different heat treatment conditions.