| Compression and densification of wood can improve its mechanical properties,but it will produce greater elastic recovery in hot and humid environment,so it is necessary to fix its compression deformation.Previous studies often focused on how to improve the deformation and fixation effect of compressed wood,but little attention was paid to the influence of the preparation process of compressed wood on the residual stress and fixation effect of wood.This study systematically studied the effect of hot pressing on the properties of Chinese fir,starting from the process of compaction and compaction.Mastered the evolution law of wood physical and mechanical properties in the process of compression and fixation in order to achieve the maximum control of compression deformation and the minimum loss of mechanical properties.At the same time,attention was paid to changes in surface quality such as wood color to improve its performance.Finally,the mechanism of cell wall chemical composition and micromechanical analysis were explained.The study found that:1.The wood compaction technology was studied using high precision hot press and Press MAN monitoring system.The changes in the interior of slab during wood compaction under different moisture content,hot pressing temperature and time,press loading rate and compression rate were discussed respectively.Emphasis was laid on the study of slab thickness direction temperature and internal steam pressure during compression,and the press pressure curve and thickness curve were also concerned.(1)During the hot pressing process,the internal temperature of the wood can be divided into three stages: rapid heating,slow heating,and heat preservation.When the temperature raised to about 100?,the bound water of the cell wall absorbed the heat,which caused the temperature rate to have an inflection point and entered a slow temperature rising phase.The steam pressure inside the wood can be divided into three stages: rapid rise,pressure holding,and pressure reduction.(2)It can be inferred from the pressure curve of the press that the water content of the slab was 12%,which was the preferred moisture content.At this time,the pressure of the press was smooth,and there was no undulating curve during the pressure holding stage.It indicated that.under the action of high temperature and pressure,the cell wall can be fully softened by the bound water in the cell wall,and excessive gas pressure was not easily generated inside the slab.(3)Under the premise that the hot pressing temperature reached the softening point of the wood cell wall component,the lower the temperature,the better.If the temperature was too high,the vapor pressure inside the slab was large and the the duration was long,which was not conducive to the tight binding of the wood cell wall material under the conditions of wet heat compression,and the defects were easily generated when the pressure was released.The hot pressing temperature can be selected from 160 to 180?.(4)The higher the loading rate of the press,the larger the vapor pressure inside the slab,and the cell wall may not be fully softened and crushed;the smaller the loading rate,the longer the time required for the core layer to reach the target temperature.It was advisable to choose2mm/min for the press loading rate.(5)The larger the compression ratio,the larger the vapor pressure inside the slab.When the compression ratio was 50%,the pressure curve of the press fluctuated greatly,which was presumed to be caused by the weakening of the weak tissue of the sheet or the excessive vapor pressure inside the sheet,or the combination of the two.The compression ratio should be less than50%.2.Taking air-dried wood as compression object,the loading rate of fixed press is 2 mm/min,and the compression test of hot pressing temperature,time and compression rate is three factors and three levels.The fixed effect of heat treatment and steam treatment on compressed wood is compared by the full factor test of temperature and time at two factors and three levels.The residual stress,water boiling resilience and suction of compressed wood are systematically discussed by various factors and levels.The influence of wet resilience and mechanical properties on wet resilience and its significance.The results showed:(1)The hot pressing temperature,time and compression ratio had significant effects on the residual stress of the test material and the boil recovery rate.In general,the three compression effect observation indicators decreased with the increase of temperature and time,and with the decrease of the compression ratio.The four mechanical properties increased with the increase of temperature,the extension of time and the increase of compression ratio.The order of each indexaffected by each factor was not completely the same.The comprehensive evaluation method was that the compression process had a hot pressing temperature of 180?,a pressurizing time of 40 min,and a compression ratio of 40%.(2)The steam treatment reduced the residual stress of the compressed material,the boiled rebound rate and the moisture resilience rate much more than the heat treatment,and the influence on the mechanical properties of the material was less than the heat treatment.The preferred compression deformation was fixed to steam treatment at 180? for 2h.Under this process condition,the hardness,flexural modulus,flexural strength and tensile strength of the treated material were 1433 N,11206Mpa,98.6 Mpa,101.1 Mpa,and the relative compression material was reduced by 4.6%,3.1%,and 8 %,6.7%,compared with the Chinese fir material increased by37%,100.1%,60%,28.7%;while boiled recovery rate,moisture absorption recovery rate,residual stress were 9.8%,7.66%,0.49%,respectively,relative to the compression material,reduced by 69.9%,70.3%,68%.3.In order to understand the variation of wood surface quality such as color with hot pressing and compression deformation,the effects of treatment conditions on wood properties were studied by single factor experiments.The main characteristics were as follows: with the increase of hot pressing temperature,compression time and compression rate,the wear value and surface roughness of the compressed wood decrease,the contact angle of the surface increases,and the color of the wood deepened gradually.After vapor compression deformation and fixation,the wear value increased,the surface roughness increases slightly,the surface contact angle increased greatly,and the wood color deepened further.Among them,the control of wood color can be carried out by heat-sensitive luminosity value because of the large number of color evaluation indicators4.Combining Fourier transform infrared microscopy,XRD,XPS,Raman microscopy and nanoindentation technology,the response of cell wall to hot pressing and compression deformation fixation was analyzed comprehensively,so as to analyze the changes of physical and mechanical properties of specimens under different treatment conditions from the micro level,and explain its mechanism,and provide theoretical reference for compression deformation fixation and controllable physical and mechanical properties.(1)According to the analysis of correlation maps,the effect of steam heat treatment on the deformation of compressed wood was mainly due to the significant degradation of hemi-fibers at high temperature;on the other hand,the structure of another kind of amorphous lignin in cell wall becomes more compact and inelastic during steam treatment,and it becomes a rigid mesh filler to compress wood stress.The restraint was fixed and the compression deformation is fixed.This phenomenon also occured in the process of wood hot pressing,which can explain why the deformation of wood was temporarily fixed after short compression.(2)The mechanical response of cell wall of compressed wood and deformed fixed wood to heat,time and pressure has a strong correlation with the macro-mechanical properties of wood at the level of various factors,and the change law is basically the same.Therefore,the mechanical properties of compressed wood can be controlled from the perspective of micro-mechanics under certain premises. |
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