Preparation And Application Of Bagasse Kraft Lignin Based Phenolic Resin Adhesive

In this paper,a method for countercurrent extraction of sugarcane bagasse kraft lignin was established,compared the traditional method to study the change in countercurrent acid consumption.Purification and fractionation of extracted lignin,chemical composition analysis,the purified lignin(KL)and its fractional components(F1,F2,F3 and F4)were characterized by chemical composition analysis and gel permeation chromatography(GPC)to study the uniformity of fractional composition.Further use of Fourier transform infrared(FT-IR),elemental analysis(EA),methoxy determination,nitrobenzene oxidation(NBO),nuclear magnetic resonance(including 31P NMR,1H-13C HSQC NMR)and thermogravimetry(TGA)comprehensive analysis of the structure and properties of lignin and its fractionated components,molecular structure,reactivity,functional groups and other characteristics of structural lignin fractionation components.The phenolation process of KL was optimized by orthogonal experimental design under acidic conditions and the structural changes of KL induced by phenolation were analyzed by FT-IR,GPC,and 1H-13C HSQC NMR.The fractions including F1,F2 and F3 as raw materials were respectively phenolated under the optimal condition and the changes of the active sites were analyzed.KL,F1,F2 and F3 were used as raw materials to replace 10,20,30 and 40%phenol to synthesize lignin-based phenolic resin adhesive(LPF)respectively,and the properties of LPF and LPF as adhesives for the preparation of plywood were determined,studying the effect of different lignin substitution rates on LPF performance.Eventually,PLPF with a phenol substitution of 40%were prepared from the phenolated lignin(PKL,PF1,PF2 and PF3),and the properties of PLPF and PLPF as adhesives for the preparation of plywood were determined.The effect of phenolic modification on the performance of LPF was described.(1)Compared with the classic acid extraction,when the extraction yield of lignin was the same,about 25.6%of sulfuric acid was saved by the countercurrent acid extraction.The kraft lignin(KL)was successively fractionated into four fractions.The yields of F1,F2,F3,and F4 were 59.6,28.4,10.8,and 1.2%based on KL,respectively.The chemical composition and GPC analysis showed that the lignin content of all the samples exceeded 95%.The molecular weight decreased from F1 to F4.All fractions had a lower polydispersity than KL.The homogenization of lignin was significantly improved by the sequential organic solvent extraction.(2)The comprehensive analysis indicated that the organic solvents fractionation hardly altered the lignin structure and only improved the homogeneity of lignin.Demethylation of lignin during the kraft pulping process produces catechol.The lower the molecular weight of the fractions,the higher the contents of methoxy,catechol and phenolic hydroxyl groups.The thermal stability of lignin was related not only to its molecular weight but also to its polydispersity.(3)The optimal conditions of phenolation of KL was that the mass ratio of phenol to lignin was 4:1,t=2.5 h,T=110 ?,the catalyst content(98 wt%sulphuric acid)was of 4%.Compared with KL,the molecular structure of phenolated lignin(PKL)was degraded,and the active site content was increased.In addition,PKL has a reduced molecular weight and tends to be more uniform.(4)The active site contents of F1,F2 and F3 after phenolation were 9.8 times,8.5 times and 5.3 times of the untreated samples,respectively.With the increased of lignin substitution rate of phenol,the content of free formaldehyde in LPF increased,and the bonding strength of plywood prepared with LPF as adhesive decreased.In addition,the lignin reactivity was low,the free formaldehyde content in LPF(0.42?1.24%)was exceeded(?0.3 mg/L),and the bonding strength(0.78-1.18 MPa)of LPF(Except for the LPF synthesized by replacing 40%phenol with KL)as adhesive to prepare plywood met the standard,and the formaldehyde emission of plywood was high(0.81?1.46 mg/L),far exceeding the standard limit requirement of plywood(?0.5 mg/L).The results showed that compared with LPF,the free formaldehyde in PLPF prepared by phenolated lignin was lower,the bonding strength(>0.7 MPa)was higher,and the formaldehyde emission was significantly lower(<0.5 mg/L).Compared with LPF,PLPF had less free fonnaldehyde content(0.16-0.27%),which met the standard limit requirement(<0.5 mg/L);the bonding strength of plywood prepared with PLPF as adhesive was large(1.15-1.36 MPa),and the formaldehyde emission was low(0.31-0.43 mg/L).In summary,LPF had excellent performance and low free formaldehyde,which can be used in environmentally friendly plywood production to achieve high value utilization of pulping waste lignin.