Study On Preparation And Recovery Mechanism Of Thermally Compressed Scots Pine(Pinus Sylvestris L.)

After physical compression,although the mechanical properties of light wood can be improved,its recovery is severe in the humid environment.To fix the deformation,heat treatment is applied on the compressd wood and achieved a remarkable result.However,there are problems about complicated process,environmental pollution,and increased energy consumption.Additionally,the underlying mechanism responsible for the recovery of the densified wood after the heat treatment remains unknown..Thus,Those have become a barrier to the development of functionally modified materials.Therefore,this study proposed the integration of densification,drying and heat treatment of scots pine wood by means of flat-bed hot pressing to achieve a multi-functional improvement technique,energy conservation and environmental protection,and at the same time revealed its mechanism of deformation fixation.Firstly,this paper used orthogonal test to optimize the integrated process parameters,and obtain the themerlly compressed wood.Then,the study tested its physical and mechanical properties and identified the key factors affecting the dimensional stability of the wood.The chemical composition and the basic structure of modified materials were measureed.Based on this analysis,water uptake swelling rates are analyzed,and the main reasons for the deformation recovery of water absorption of compressed wood are determined.At the same time,from the aspects of elasticity,water absorption swelling force and chemical structure of the tested material,the paper reveals the fundamental cause of the deformation fixation of thermally compressed wood,which could provide technical guidance and theoretical reference for future industrial production and research workers.The specific content is as follows:1.By using the hot-pressing method to densify,dry and heat-treated the scots pine wood,four factors and three levels orthogonal test were carried out with the factors of compaction ratio,drying temperature,heat treatment temperature and heat treatment time.And the significance and causes of the effects of the four factors on the mechanical properties of thermally compressed wood were studied respectively.The results show that:(1)The physical properties of thermally compressed wood are significantly affected by the compaction ratio,in which the equilibrium moisture content,water uptake and swelling & shrinkage rate are significantly affected by heat treatment temperature and duration.The mechanical properties of thermally compressed wood are significantly affected by the compaction ratio.(2)The density and mechanical properties of thermally compressed wood improved remarkably with the increase of compaction ratio.When the compaction ratio was 50%,the swelling & shrinkage rate of compressed wood increased obviously,and the deformation recovered seriously.After heat treatment temperature and duration,the increase value of the swelling & shrinkage rate decreased significantly.It can be seen that heat treatment temperature and heat treatment time are effective factors to restrain deformation recovery of thermally compressed wood.2.To investigate the effect of heat treatment on the chemical components and basic structure of compressed wood,its main components and functional groups were measured by using the test methods in national standard and Fourier infrared spectrometer(FTIR).At the same time,X-ray diffractometer(XRD)and mercury porosimeter were used to detect the change of crystallinity and pore microstructure of thermally compressed wood.The results show that:(1)With the increase of heat treatment temperature and duration,the content of hemicellulose decreased and the content of lignin increased.FTIR test results showed that the peak area of functional groups of sample hemicellulose decreased significantly during heat treatment,while the area ratio of lignin functional groups increased significantly,and the functional groups of cellulose did not change significantly.(2)After compression,the microfibril of cellulose in the cell wall was extruded to shorten the distance between the microfibrils and increase the crystallinity of heartwood and sapwood.High temperature heat treatment reduces the crystallinity of treated heartwood and sapwood.The relative crystallinity of heartwood is higher than that of sapwood due to maturity.Compared with the mercury injection volume of mesoporous(0.8?m?D?5?m)and macropore(D>5?m),it was found that the pore volume of untreated heartwood was mainly concentrated in the microporous area(D<0.8?m).After compression,the pore volume near 30?m and 0.18?m decreases obviously.And with the increase of heat treatment temperature and duration,the total pore volume decreases significantly with the decrease of mesoporous volume.The pore size distribution of the sapwood is more uniform than that of the heartwood,and the pore volume of the sapwood decreases rapidly after compaction,while the total pore volume of thermally compressed wood increases relatively compared with that of the compressed wood due to the increase of macropore and mesoporous pores.3.Studied on the regularity of water absorption deformation of thermally compressed wood by cyclic water absorption method,and explored moisture absorption at different relative humidity of thermally compressed wood by dynamic vapor adsorption method.Thus to reveale the intrinsic causes of heat treatment which contribute to the deformation fixation.The results show that:(1)The swelling rate and recovery of heartwood and sapwood increased with the increase of water absorption cycle times and decreased with the increase of heat treatment temperature.The moisture content of heartwood and sapwood gradually increased with the increase of relative humidity and increased with the increase of heat treated temperature.(2)When the compressed wood absorbed water,the hydrophilic chemical substance of the cell wall absorbed moisture in the main and secondary adsorption sites in the cell wall in a hygroscopic manner until the condensation phenomenon in the capillary,and then too much free water gone into the big capillary,the intercellular space and the lumen.The tension of water caused the closed intercellular space and the volume of the cell cavity to expand,which resulted in the expansion of the cell itself and the release of residual stress,and then caused the deformation recovery of wood.After high temperature treatment,The hydrophilic substances of the sample were significantly degraded,and resulted in decrease in effective adsorption sites,in sorption properties and in water uptake,and led to a decrease of water uptake and recovery.4.Measured the variation of absorption and expansion force of heartwood and sapwood of scots pine wood by water absorption expansion device and studied the viscoelastic changes of heartwood and sapwood before and after modification by dynamic thermo-mechanical analyzer.Based on the analysis of the reasons for the change of water absorption and expansion force of wood,this paper explains from the mechanical point of view the intrinsic reasons that high temperature heat treatment is helpful to the deformation fixation of compacted wood.The results showd that:(1)The water swelling forces of compressed heartwood and sapwood were 277.38 N and 313.13 N respectively,which decreased obviously with the increase of heat treatment temperature.The storage modulus and loss modulus of heartwood and sapwood decreased gradually with the increase of temperature in the temperature spectrum,and the storage modulus and loss modulus of compacted heartwood and sapwood decreased obviously after high temperature heat treatment.It can be seen that high temperature heat treatment can significantly decrease the elasticity of compacted materials.(2)The high temperature heat treatment can reduce the elasticity of wood by degrading the chemical composition of wood,resulting in the reduction of elastic residual stress,which will lead to the decrease of water absorption expansion and water absorption recovery.Finally,the deformation fixation of compressed wood can be realized.5.Continuous thermal analyzer and ion chromatograph were used to test the pyrolysis law and polysaccharide content of thermally compressed wood,and the effects of heat treatment on the whole cell wall structure of wood were analyzed by means of 2D HSQC NMR instrument.The molecular structure reveals the fundamental cause of deformation fixation of thermally compressed wood.The results show that:(1)Hemicellulose in heartwood and sapwood degraded first,followed by cellulose and lignin.The materials of the two untreated wood,compressed wood and thermally compressed wood,both experienced two obvious thermo-gravimetric losses,the first time was water loss,the second time was chemical degradation,while the thermal stability of thermally compressed wood was higher than that of compressed wood and untreated wood.And sapwood is easier to degrade than heartwood.(2)The content of glucose in heartwood and sapwood was the most,about 3 times of mannose and 10 times of xylose,and the content of three monosaccharide in heartwood was significantly higher than that in sapwood.With the increase of heat treatment temperature,the concentration of glucose in thermally compressed wood increased gradually,and the contents of mannose and xylose decreased obviously.The deformation recovery of thermally compressed wood was mainly affected by mannose content,while sapwood was affected by xylose content as well as mannose content.(3)Side chain galactose,4-O-methy-?-D-glucuronic acid and ?-L-Araf of galactoglucomannans and 4-O-methyglucuronoarabinoxylans of hemi-cellulose in heartwood and sapwood degraded,and whereafter by cellulose and lignin.2-O-Ac-?-D-Manp and 2-O-Ac-?-D-Manp on the main chain deacetylated,and mannan and xylan began to depolymerize.The ?-O-4 bonds of heartwood and sapwood lignin,the G2 and G6 bonds in the wood-based structure of callus and the B? and B? bonds in phenylcoumaran structure are broken at high temperature.However,the content of methoxy group increased with the decrease of lignin cross linking degree.(4)Combined with NMR analysis of whole cell wall,holocellulose and cellulose,hemicellulose of heartwood and sapwood mainly consisted of mannan and a small amount of xylan.After high temperature treatment,both mannose and xylan in sapwood were obviously degraded,but mannan in heartwood was easy to degrade,but xylan was not easy to be pyrolyzed.Thus,from the molecular structure,the paper explains why the deformation recovery of compressed sapwood is affected significantly by both of them,while the heartwood is mainly affected by mannose.