Study On Performance And Enhancement Mechanism Of Glyoxal Compound Modified Fast-growing Poplar Wood

In order to improve the dimensional stability,hydrophobicity,and mechanical properties of the fast-growing Poplar woods and expand their application fields,glyoxal and ethylene glycol were used as a basic modifier and an enhancer,respectively,to modify the woods with the catalysis of ammonium persulfate[(NH4)2S2O8].Anti-swelling efficiency(ASE),water absorption(WU),and leachability(L)were using as evaluation indexes to optimize the treatment parameters and to explore the catalytic and modification mechanisms.In addition,urea,melamine,and alkyl ketene dimer(AKD)were used as copolycondensation monomer or compound modifier,with glyoxal together to modify the wood.In these methods,the poor mechanical and hydrophobicity of the glyoxal modified wood were improved.Fourier transform infrared spectrometer(FT-IR),differential scanning calorimetryscanning(DSC),electron microscope(SEM),thennogravimetric analyzer(TGA),thermal mechanical analyzer(DMA),and nuclear magnetic resonance(13C-NMR)were applied to investigate the structures and properties of the modified wood,and the strengthening mechanism as well.Moreover,in order to further explore the modification mechanisms of glyoxal and AKD on wood,the changes of functional groups on the microfibrillated cellulose(MFC)before and after modification were analyzed by 13C-NMR and FT-IR.The main results of this work are summarized as follows;(1)Ammonium persulfate was an efficient catalyst for fast-growing Poplar modification by glyoxal.The dimensional stability was significantly increased after this modification.Meanwhile,the incorporation of glycol could further enhance the modification performance.When the concentration of glyoxal,curing temperature,ethylene glycol/glyoxal molar ratio,and ammonium persulfate dosage were 20%,120?,1/2,and 2%,respectively,the ASE of modified wood was up to 72.5%,and the 24-hour WU was 38.7%.The high catalytic activity of ammonium persulfate could be attributed to the following two aspects:Firstly,NH4+ ions could induce the polycondensation reaction of glyoxal by protonation;Secondly,S2O82-could promote the catalytic reaction by its strong oxidation effect.The improved dimensional stability was mainly resulted from the cross-linking reaction between glyoxal and wood,and the bulking effect of glyoxal/glycol resin to the wood cell walls.(2)Urea-glyoxal(UG)resin could significantly improve the wood mechanical properties impaired by single glyoxal modification,and the introduction of nano-silicon dioxide(nano-SiO2)could further improve the mechanical properties of wood.Compared with glyoxal modified wood,UG resin modified wood possessing increased modulus of rupture(MOR)by 33.5% and modulus of elasticity(MOE)by 10%.The improved mechanical strength was mainly due to the increased cross-linking chain length between glyoxal and wood.Compared with UG resin modified wood,the MOR,MOE,CS,and hardness were increased by 25.3%,19.6%,18.6%,and 8.6%,respectively,after the introduction of 0.5%nano-SiO2.The possible mechanism was that the nano-SiO2 could disperse and transfer the stress,reducing the stress concentration,thus improving the mechanical properties of wood.(3)Melamine-glyoxal(MG)resin not only imparted wood with wonderful mechanical performance,but also excellent water resistance.Compared with the unmodified wood,the MOE,CS,and hardness of the MG modified wood(the glyoxal/melamine mole ratio was 10/1)were increased by 27.6%,34.5%,and 37.6%,respectively.Meanwhile the WU were decreased by 46.1% and the 24-hour anti-leachability was up to 95%in this condition.The improvement of mechanical properties and water resistance of wood could mainly attributed to the following two aspects:One was the filling and bulking effect of wood cell wall by the MG resin which possessed high strength,hardness,and excellent water resistance;Another one was the cross-linking effect between wood and the active hydroxyl on MG resin or the produced glyoxal oligomer.(4)AKD/glyoxal compound treatment could endow wood with excellent dimensional stability and hydrophobicity.After surface treatment with AKD,the contact angle of the glyoxal modified wood was nearly 150°.In addition,after water immersion,chloroform extraction,and acid impregnation treatment,the contact angle of the modified wood was still more than 140°,indicating that the hydrophobicity of the AKD modified wood performed excellent water and solvent corrosion resistance.(5)Glyoxal and AKD modification had little effect on the structure of MFC.During the glyoxal modification process,the aldehyde group on glyoxal and the hydroxyl on MFC underwent cross-linking reaction,leading to the formation of hemiacetal and acetal.As for AKD modification,the ring of AKD was firstly opened,and then esterified with the hydroxyl on the MFC surface.