Multiscale Modifications On Dimensional Stability Of Populus Cathayana By Alkali Lignin

Wood is a natural hygroscopic material which tends to crack and warp during processing and utilization.Especially for the fast growing wood,the problem of deformation is a major concern.Therefore,improving the dimensional stability of fast growing wood caters for the need of the market as well as the development of the wood industry.Nowadays,most of wood modification system involve volatile products.Besides,high production cost and complex process inhibit the market promotion and practical application of the modified wood.Lignin,being a regenerable aromatic network polymer,is relatively hydrophobic and has lots of active functional groups,could be served as a new type of modifier.In the present work,alkali lignin was chosen as the modifier and was combined with heat treatment or hexamethylenetetramine as the hardener,to investigate the effect of three modification methods on water resistance,dimensional stability and mechanical property of wood.Moreover,the morphological and chemical alterations were analyzed by scanning electron microscope(SEM),confocal laser scanning microscopy(CLSM)and Fourier transform infrared(FTIR)spectroscopy.The main results are listed as following:(1)SEM,CLSM and FTIR suggested that the alkali lignin existed in cell lumen and cell wall of wood fibers and vessels.The macromolecular lignin was filled in the cell cavity and deposited on the surface of the cell wall,which blocked the water pathways and obstructed the contact between moisture and hydroxyl groups in cell wall.While the small alkali lignin molecules were filled in the nanometer pores of cell wall,caused the formation of ether bonds by reacting with free hydroxyls.Therefore,multiscale modifications at cell lumen(micro-level)as well as cell wall(nano-level and molecular level)were realized.The degradation of hemicellulose and homogeneous distribution of lignin became obvious as the heat-treating temperature increased.The addition of hardener could promote the crosslink curing reaction of alkali lignin and strengthen wood cells.(2)The alkali lignin modified wood(AL)showed a great leaching resistance and surface hydrophobicity.The leaching rate and initial contact angle were 3.3%and 1220 respectively.This treatment was also capable of increasing dimensional stability of wood.Compared with the Control,the hygroscopicity of the AL dropped by 13-25%and the water absorption rate fell down by 67.0%.For swelling rate through moisture adsorption,the alkali lignin modification could lower tangential and volumetric swelling rate by 25.8%and 23.8%respectively.For swelling rate through water absorption,the alkali lignin modification could reduce tangential and volumetric swelling rate by 3.6%and 5.6%respectively.What’s more,the compression strength was also improved after treatment and the compressive strength of the AL was 42.5MPa.(3)The alkali lignin combined with heat treatment could further improve the properties of wood as mentioned above,and these changes became obvious as the heat-treating temperature increased.The leaching rate of alkali lignin combined with 180℃ heat treatment sample(AL 180℃)was only 1.4%,and initial contact angle was 1440,which is closed to the super-hydrophobic surface.Compared with the Control,the hygroscopicity of the AL 180℃ dropped by 16-28%and the water absorption rate fell down by 74.4%.For swelling rate through moisture adsorption,the alkali lignin combined with 180℃ heat treatment could lower tangential and volumetric swelling rate by 37.5%and 38.8%respectively.For swelling rate through water absorption,the alkali lignin combined with 180℃ heat treatment could decrease tangential and volumetric swelling rate by 21.5%and 23.4%respectively.The compressive strength of the AL 180℃ was 45.6MPa.(4)Compared with the alkali lignin combined with heat treatment,the alkali lignin combined with hardener could not only simplify the modification process,but also further enhance the properties of wood as mentioned above.And these changes became obvious as the content of hardener increased.The initial contact angle of the alkali lignin combined with 5%hardener sample(AL 5%)was 148°,which almost corresponds to the super-hydrophobic surface.Compared with the Control,the hygroscopicity of the AL 5%dropped by 19-26%and the water absorption rate fell down by 75.9%.For swelling rate through moisture adsorption,the alkali lignin combined with 5%hardener treatment could lower tangential and volumetric swelling rate by 48.4%and 49.8%respectively.For swelling rate through water absorption,the alkali lignin combined with 5%hardener treatment could decrease tangential and volumetric swelling rate by 24.7%and 25.3%respectively.The compressive strength of the AL 5%was 49.1MPa.