Study On Modification Of Fast-growing Poplar Wood With Furfuryl Alcohol Resin And Its Mechanism

As primary timber supply in China,fast-growing plantation wood was limited in usage as furniture and engineering materials,due to its intrinsic properties,such as low density and dimensional instability.In order to improve its properties,wood modifications with physical method,chemical method and others were of great significance to develop wood industry.Furfurylation involved in a green wood modification method to improve properties of fast-growing wood and endow the wood with high added-value applications.Although wood modified with monomer of furfuryl alcohol has been extensively studied,its mechanisms have not been clearly clarified.Simultaneously,monomer of furfuryl alcohol could be easily polymerized into prepolymer of furfuryl alcohol.It was instructive for industrial production to study modified wood with prepolymer of furfuryl alcohol.Fast-growing poplar as a material was modified with monomer of furfuryl alcohol,prepolymer of furfuryl alcohol and Si O₂ sols-monomer of furfuryl alcohol.Physical properties,mechanical properties,etc.of treated woods with these modifiers were investigated.Furthermore,mechanisms of modified wood with monomer of furfuryl alcohol were explored,including distributions of furfuryl alcohol resin in anatomical structure,hierarchical porous structures,physical properties and mechanical properties of cell walls,accessible OH groups,and reaction mechanisms between furfuryl alcohol resin and cell wall components.Main conclusions and innovations were shown as follows:（1）Physical and mechanical properties significantly improved by modification with monomer of furfuryl alcoholModified woods with 23%weight gain percent（WPG）and 69%WPG were obtained by modified woods with solutions of 10 wt%and 30 wt%monomer of furfuryl alcohol.Density of modified wood with high WPG was 46.58%higher than that of control sample.Compared with control sample,anti-swelling efficiency at high WPG after three times soaking water showed about 50%.Water uptake at high WPG was less 48.09%than that of control sample after immersion for 492 hour.Reduced equilibrium moisture content（EMC_R）of modified woods was slightly smaller than that of untreated wood at low RH and slightly higher than that of untreated wood at high RH.Fiber saturation point of modified wood with different WPGs was higher than those of control sample.Compared with control sample,compressive strength parallel to grain increased by 61.73%at low WPG and 98.95%at high WPG.No significant difference in bending strength was found between control sample and modified woods with different WPGs.Modulus of rupture for modified woods with both WPGs was about 25%greater than that of control sample.（2）Physical properties,mechanical properties and flame retardancy of wood improved by modification with monomer of furfuryl alcohol-Si O₂ solutionOrganic-inorgainc composites were prepared by homogeneous solutions of inorganic Si O₂sols and organic monomer of furfuryl alcohol.WPG and density of the modified woods increased with increased Si O₂content（0～8%）.Dimensional stability of the modified woods with different Si O₂contents was less than that of the modified wood with monomer.Water uptake at 5%Si O₂content was 13.76%lower than modified wood with the monomer.Compressive strength parallel to grain of the modified woods with different Si O₂contents was higher than that of modified wod with monomer.Bending strength of the modified woods was comparable with that of modified wood with monomer.Modulus of rupture of the modified wood at 8%Si O₂content was greater than that of modified wood with the monomer.Fire retardancy of the modified wood at 8%Si O₂content was improved,compared with that of modified wood with the monomer.Total heat release,peaks of heat release rate,total smoke release and peaks of smoke release of the modified wood at 8%Si O₂content were less than those of modified wood with the monomer.Residual mass percentage of the modified wood at 8%Si O₂content was 32%greater than that of modified wood with the monomer.（3）Reduction in the volatilization loss of prepolymer of furfuryl alcohol and improvement in the curing efficiency of furfuryl alcohol resin by modified wood with prepolymer of furfuryl alcoholThe majority of molecular weight regarding prepolymer of furfuryl alcohol was 298～397.Leachability,dimensional stability and mechanical properties of modified wood with the prepolymer of furfuryl alcohol were comparable with those of modified wood with monomer of furfuryl alchol at comparable WPG.Distributions of furfuryl alcohol resin in the modified wood with the prepolymer were similar with those of furfuryl alcohol resin in modified wood with the monomer.Besides,alterations of functional groups for modified wood with the prepolymer were consistent with those with the monomer.However,at comparable WPG,the mass of furfuryl alcohol in modified solution of treated wood with the prepolymer was less than 12%of that with the monomer,which were attributtable to reduce the volatilization loss of the prepolymer and improve the curing efficiency of the prepolymer,compared with that of the monomer.（4）Analyses of cell wall structures and cell wall properties of modified wood with the monomer at cellular scaleFurfuryl alcohol resin were distributed at fibre cells,vessels,ray cells,perforations and pits,and existed at secondary cell walls,lumina,middle lamella and cell corners.Decreased lumina diameters and increased cell wall thickness of various cells,occluded perforations,and occluded pits were shown in modified wood.Hierarchical porous structures of modified wood were influenced by furfuryl alcohl resin in the wood.Compared with that of untreated wood,porosity of modified wood with high WPG significantly decreased.Pore volume of both furfurylated woods was reduced at most of the pore diameters.Besides,physical properties and mechanical properties of cell walls were affected by furfuryl alcohol resin in cell walls.Modulus of rupture of cell walls for modified woods at low WPG and high WPG was improved by 6.16%and38.82%,respectively.Hardness of cell walls for modified wood at low WPG and high WPG increased by 13.25%and 24.10%,respectively.Cell walls were significantly bulked by furfuryl alcohol resin.Bulking ratios of modified woods with 23%WPG and 69%WPG were 54%and72%,respectively.（5）Explanation of mechanisms on wood modification with the monomer at molecular scaleChemical reactions occurred between uncrowded ring positions and side chains of lignin and furfuryl alcohl resin,which resulted in intermolecular crosslinkings and intramolecular crosslinkings between neighboring lignin chains.There were no chemical bonds of furfuryl alcohol resin with both cellulose and hemicellulose.However,Compared with control sample,accessible OH groups of modified woods with low WPG and high WPG decreased by 33%and46%,respectively,determined by deuterium exchange method.（6）Elucidating mechanisms regarding influences of modified wood with the monomer on hygroscopicity,water uptake and dimensional stabilityThe decrease in accessible OH groups of modified woods contributed to a reduction in sites of hygroscopicity and water uptake.Chemical interactions between lignin and furfuryl alcohol resin were possibly responsible for enhanced cell wall stiffness.Chemical interactions and increased modulus of rupture were likely to decrease flexibility of cell wall polymers.Capillary condensation possibly happened at lumina with decreased lumina diameter of modified woods.Decreased EMC_R of modified wood at low RH was probably caused by both decreased water hygroscopic ability due to reduced hygroscopic sites and reduced cell wall swelling and reduced new sorption sites during cell wall swelling due to decrease flexibility of cell wall polymers.Increased EMC_R of modified wood at high RH was presumably due to interactions of both sides.On one hand,the reason was consistent with that of decreased EMC_R at low RH,which resulted in decreased EMC_R at high RH.On the other hand,capillary condensation water in lumina at high RH leaded to increased EMC_R.Decreased water uptake of the modified woods was probably caused by the interactions of three reasons:decreased accommodation possibility for water in hierarchical pores of modified wood was exhibited,owing to reduced porosity of modified wood;the decrease in water absorbed sites brought about a decrease in water uptake ability;reduced cell wall swelling and reduced new sorption sites during cell wall swelling were showed due to decrease flexibility of cell wall polymers.Improved dimensional stability of the modified woods was attributable to the interactions of three reasons:additional swelling of modified wood during water soaking was reduced due to permanent cell wall bulking of modified wood;reduced water absorbed ability was exhibited due to decreased absorbed sites;decrease flexibility of cell wall polymers resulted in reduced cell wall swelling and reduced new sorption sites during cell wall swelling.