Compound Modification Of Corn Stalk Lignin And Its Application In Wood Adhesive

Due to the rapid growth of energy demand and the depletion of fossil fuel reserves,biomass has become an important alternative energy source,and renewable biomass resources are an important part of it.Lignin is the second largest biomass resource in the nature.It has strong mechanical properties,environmental friendliness and biodegradability.It is expected to play an important role in the sustainable production of synthetic polymers.In this paper,lignin,a by-product of biorefinery,was used as a raw material.The lignin was firstly demethylated and then hydroxyethylated to modify the lignin to achieve an increase in the hydroxyl content of the lignin and an increase in the reactivity.The possibility of preparing formaldehyde-free wood adhesives using modified lignin was explored by compounding lignin with curing agent and polymer emulsion.In this thesis,sodium sulfite was used as the nucleophile of lignin,and its demethylation modification was carried out under alkaline mild conditions with alkali not exceeding 100?.Test methods such as acetylation titration,Fourier transform infrared spectroscopy(FTIR),ultraviolet spectroscopy(UV),thermogravimetric analysis(TGA),and differential scanning calorimetry(DSC)were used to explore the reaction time and reaction.The effect of temperature,the amount of nucleophile and the amount of NaOH on the demethylation reaction of lignin determined the optimal reaction conditions:reaction time 2h,reaction temperature 60?,mass ratio of lignin:NaOH:Na2SO3 is 10:1.5:2.After demethylation modification,the total hydroxyl content of lignin can be increased by about 60%,and the methoxyl content can be reduced mostly about 10%.After demethylation modification,the reaction activity of lignin was enhanced and the thermal stability was improved to a certain extent.Based on the demethylation modification of lignin,glyoxal is used for hydroxyethylation reaction.The preferred hydroxyethylation reaction conditions are:reaction time 4h,reaction temperature 60?,lignin and glyoxal solution.The mass ratio is 3:1.The effects of demethylation-hydroxyethylation compound modification on the lignin modification effect were studied through various analysis and testing methods.The experimental results show that the total hydroxyl content of the composite modified lignin is increased by about 90%,and the phenolic hydroxyl content is increased by about 20%.Compared with the demethylated lignin,the lignin after the composite modification has further improved the activity of the lignin,the thermal stability has continued to increase,and the residual rate has been reduced,which provides favorable conditions for the preparation of lignin-based wood adhesives.The lignin obtained by the composite modification is mixed with a certain amount of curing agent and polymer emulsion to prepare a lignin-based wood adhesive,and the three-layer plywood is pressed to characterize the performance of the adhesive.The results show that epichlorohydrin(ECH)can be used to cure composite modified lignin,and with the addition of epichlorohydrin,the mechanical properties of the adhesive also vary.When the mass fraction of ECH is 25%,the mechanical properties of the adhesive are the best.The dry strength of the plate can reach 1.68 MPa and the wet strength is 1.24 MPa.The addition of 15%by mass of polyacrylate emulsion(AE)to lignin-based wood adhesives can reduce the amount of curing agent used.At the same time,the dry strength of the board can be increased to 2.74 MPa and the wet strength can be 1.69 MPa..The composite modified lignin-based adhesive can meet the national Class ? plywood use standards.