| Wood compression technology is a green modification method for the property improvement and high value-added utilization of plantation fast-growing wood.The influence mechanism of the temperature and moisture content distribution on the selective control of the position and thickness of compressed layerwas were investigated,and new method and mechanism of the deformation fixation of the compressed wood was studied.It can effectively reduce the loss rate of wood volume during compression and improve the quality of compressed wood products,in order to provide scientific guidance for high efficiency and high performance utilization of the plantation fast-growing wood.This has crucial meaning on theory and in practice application.In this study,white poplar wood(Populus tomentosa)fast-growing wood was used as the experimental material.The moisture content and temperature distribution of the air-dried wood in the process of preheating at 180 ℃ were analyzed and the corresponding models were established.The hydro-thermal response law of glass transition and yield stress of amorphous polymers was discussed.According to moisture content and temperature distribution law,the selective control of the thickness and position of the compressed layer was realized by controlling the immersion time,compression amount,preheating time and press closing time.Moreover,the deformation fixation of surface compressed wood was treated by 180℃ atmospheric pressure and 0.3 MPa super-heated steam and the deformation fixation effects were compared.The changes of main component and microstructure of compressed wood and its effect of deformation fixation were investigated.The physical properties and mechanical properties of the surface compressed wood before and after steam treatment were analyzed.The major conclusions of this paper were summarized as follows:(1)During the pre-heating treatment,the peak moisture content moved from the wood surface to the center layer with the preheating time extending according to the power function y=0.7616x0.3645 in the thickness direction.When the preheating time extended to 120 s,peak moisture content moved from the surface of the wood to the center layer by about 4.00 um,and it decreased from 27.16%to 15.82%.At the same time,under the action of the hot plate at 180℃,the internal temperature of the wood showd a trend of low internal and high external.With the prolongation of the preheating time,the temperature difference between the surface layer and the central layer gradually decreased from 90 0C to 60 0C.(2)Through the preheating time to control the moisture content distribution of the wood,the formation position of the compressed layer of sandwich compressed wood moved from wood surface to center with the preheating time extending according to power function y=0.0845x0.6869 in the wood thickness direction.Moreover,the peak position of moisture content in the thickness direction of wood showed a significant linear correlation with the formation position of compression layer,and the coefficient of determination was 0.99.At the same time,the average air-dried density of the compressed layer was between 0.69 and 0.89 g/cm3,and the average air-dried density of the uncompressed layer still maintained at about 0.44 g/cm3.The thickness of the compressed layer of the surface-compressed wood gradually increased by the control of the compressed thickness and the distribution of the hydro-thermal.When the compressed thickness was 2～18 mm,the thickness of the compressed layer can be controlled to be about 3.01～14.51 mm.(3)Based on the response law of wood yield stress to hydro-thermal,layered softening effect inside the wood was formed by artificially regulating the MC and temperature distribution inside the wood under the amorphous polymer in the cell wall under the dual influence of water and temperature.In the early stage of preheat treatment,the layered softening effect showd that the yield stress of the wood surface layer and the center layer are about 1.50 MPa and 5.96 MPa,respectively,and the yield stress difference was 4.46 MPa.(4)The deformation set recovery of the surface-compressed wood decreased with the compressed thickness increasing.When the compressed thickness was 18 mm,the set recovery of compressed wood declined by 7.26%and 19.8%due to moisture absorption and the water absorption than the compressed thickness of 2 mm.The steam pressure can significantly reduce the deformation set recovery of compressed wood.After 180℃ and 0.3 MPa superheated steam treatment for 2 h,the average set recovery of compressed wood decreased significantly from 12.92%and 62.16%to 1.48%and 13.06%due to moisture absorption and water absorption,respectively,and analysis of variance showd that the effect of compressed thickness on set recovery was not significant.At the same time,the difference in flexural modulus,flexural strength,compressive strength and hardness of the compressed wood before and after superheated steam treatment were not significant.Among them,the flexural modulus,flexural strength,hardness and surface hardness of compressed wood(compressed by 10 mm)after superheated steam treatment were increased by 52.59,36.44,122.03 and 129.61%,respectively,compared with the control materials.(5)Micro-cracks appeared in the cell wall after compression treatment,which provided an effective channel for the diffusion of high-temperature steam during the treatment.During the high-temperature steam treatment,the hydrophobicity of the compressed wood surface increased and the initial contact angle of distilled water increased from 103° to 136°.And the hemicellulose in the wood undergoed deacetylation reaction,and the produced acetic acid catalyzed the degradation of hemicellulose into oligosaccharides,reducing the number of strong hygroscopic groups.The relative crystallinity of cellulose increased from 53.75%to58.07%,and the rearrangement and ordering orientation of micro-fibrils in the crystallization zone,causing width of the crystallization zone increased from 2.83 nm to 3.90 nm. |
PDF Full Text Request