Study On Preparation And Performance Of Wood Adhesive Modified By Phenolic Lignin

With the increasing demands of consumers on the home environment,people expect to develop healthy and environmentally friendly wood adhesives.Biomass resources（such as starch,protein,lignin,tannin）,as one of the most important natural polymer materials,have the advantages of a wide range of sources,low prices,and no pollution.Among them,lignin is rich in functional groups and aromatic ring structures,and the preparation of adhesives with lignin as a precursor will provide new possibilities for the green and sustainable development of the field of wood-based panels.In this thesis,phenolated Kraft lignin（PKL）was prepared by ball milling-phenolization of Kraft lignin（KL）,and PKL is used as a matrix to react with formaldehyde and furfural under alkaline conditions.Prepare lignin-phenolic resin（KLPF）adhesives,phenolic lignin-phenolic resin（LPF）adhesives,phenolic lignin-furfural/formaldehyde resin（LPFF）adhesives and phenolic lignin-furfural resin（LPFu）adhesives,respectively,and its synthesis conditions and physical and chemical properties are discussed.In the study,the amount of formaldehyde was gradually reduced,and finally furfural was used to completely replace formaldehyde to realize the preparation of lignin formaldehyde-free adhesives.The main research content and results of this thesis are as follows:（1）After KL ball milling,the mass ratio of phenol to lignin is 1:1,the temperature is 80°C,and the time is 2 h to react with phenol to obtain the best modified PKL,with a total hydroxyl content of 4.8526 mmol/g,the phenolic hydroxyl content is 2.9331 mmol/g,and the alcoholic hydroxyl content is 1.9195 mmol/g.The FTIR analysis showed that part of the methoxy groups of lignin fell off,and the addition reaction of phenol and ligninα-hydroxyl groups occurred;SEM analysis showed that a large number of pores were formed on the surface of PKL particles,the specific surface area increased,and the three-dimensional network structure of lignin was destroyed;TG analysis show that the thermal stability of the modified lignin is significantly improved.KL and PKL are used as raw materials to prepare KLPF and LPF adhesives,respectively.Compared with KLPF adhesives,the dry bonding strength of LPF adhesives increased by 11.70%（from 1.88 MPa to 2.10MPa）,the wet bonding strength increased by 51.61%（from 0.62 MPa to0.94 MPa）,and the viscosity increased by 132.56%（from 86 m Pa·s to 200m Pa·s）,solid content increased by 27.19%（from 41.38%to 52.63%）,free aldehyde content decreased by 62.96%（from 0.27%to 0.10%）,free phenol content decreased by 31.85%（from 1.35%to 0.92%）.TG analysis show that the thermal stability of LPF adhesives is significantly better than that of KLPF adhesives;FTIR analysis show that PKL has higher reactivity and can effectively undergo condensation reactions with aldehydes.Therefore,the adhesive prepared after lignin modification has excellent comprehensive performance,and the modification method combining ball milling and phenolization provides an extremely broad space for the application of lignin in biomass adhesives.（2）In the case of low formaldehyde dosage,furfural is used as a crosslinking agent in the preparation of lignin adhesives.PKL,furfural and formaldehyde are used as raw materials to synthesize LPFF adhesives with high bonding strength.The results showed that the adhesive showed excellent performance when furfural was added at 15%.Its wet bonding strength was 1.30 MPa and viscosity was 124.50 m Pa·s.Compared with LPF adhesive,the wet bonding strength was increased by 38.30%and the viscosity was decreased by 37.75%.TG analysis show that the carbon residue rate of LPFF adhesive is 61.30%at 800°C,and the thermal stability is significantly better than that of LPF adhesive;The SEM analysis show that a large number of wood fiber tears are observed on the bonding section of LPFF,and the adhesive forms glue nails inside the wood,so that the LPFF adhesive has better bonding strength;FTIR,¹H-NMR and ¹³C-NMR analysis show that furfural and phenolic compounds are connected by-CHF-OH（F is furan ring）bonds to form a cross-linked network structure,which can effectively inhibit the destruction of the network structure by water molecules while promoting the degree of polycondensation.（3）Under alkaline conditions,PKL and furfural are used as raw materials to prepare formaldehyde-free LPFu adhesives.The results show that when the molar ratio of furfural to phenol is 1.8:1,the mass ratio of furfural to lignin is 0.1:1,the condensation reaction time is 3.5 h,and the amount of Na OH is 17%,the LPFu adhesive prepared has the best overall performance.The dry bonding strength is 1.90 MPa,the solid content is57.31%,the viscosity is 1597.50 m Pa·s,the free furfural content is 0.28%,and the free phenol content is 0.90%.TG and SEM analysis show that the LPFu adhesive has high thermal stability and adhesion;FTIR,¹H-NMR,¹³C-NMR and analysis show that the aldehyde group of furfural reacted with the active sites of phenolic compounds,and the small molecules were combined into a macromolecular network structure in the form of-O-,-CH₂ and-CH.The preparation of LPFu adhesive successfully solved the problem of formaldehyde pollution and provided a new possibility for the sustainable development of wood adhesives.