Characterization Of Lignin And Synthesis Of Environmentally Friendly Phenol-formaldehyde Resin Adhesive

Integrated valorization of lignocellulosic biomass has received intensive attention over the past decade.As a major component of lignocellulosic biomass,the value-added application of lignin naturally becomes an unavoidable issue.The phenolic nature of lignin makes it an ideal material for synthesizing lignin-phenol-formaldehyde resin adhesive(LPF).However,the complex compositions and structural features of various technical lignins,which are caused by the diversity of materials and refining processes,significantly hinder the commercialization of LPF.In order to overcome these obstacles,some work has been done in this thesis.Milled wood lignin(MWL),lignin-carbohydrate complex(LCC),cellulolytic enzyme lignin(CEL),and enzymatic hydrolysis residual enzyme lignin(EHREL)fractions were isolated from plant cell walls through neutral solvent and moderate method.The total pure yield of the four lignin fractions could reach up to 92.6%based on Klason lignin content of the material.The results of quantitative structural analysis indicated that the main substructures in the four lignin samples were ?-O-4'and ?-?' linkages.The lignin and xylan in LCC fraction were mainly connected by benzyl ether bonds.In addition,some low-molecular-weight lignin fractions were isolated from the secondary wall.An adequate knowledge of the structural features of the native lignin in plant cell walls could provide reasonable guidance for the integrated valorization of lignocellulosic biomass.The active site contents of com cob alkaline lignin(L1),poplar wood hydrolysate lignin(L2,obtained from hydrothermal treatment of wood),poplar wood kraft lignin(L3),and wheat straw alkaline lignin(L4)were analyzed through 31P-NMR technique.The suitability of the four technical lignins for partial incorporation into a phenol-formaldehyde resin adhesive was also evaluated.A phenolation treatment under alkaline conditions was conducted to enhance the reactivity of the technical lignins.The high purity of the four technical lignins(>88%)made them ideal materials for the preparation of LPF.The active site contents of L1,L2,L3,and L4 were 1.72,0.81,1.39 and 0.99 mmol/g,respectively.The active site contents of the phenolated lignins were 1.58?3.20 times as much as those of the untreated lignins,and the treatment method was easily included in the synthesis process of LPF.An alkaline lignin was modified through hydrothermal degradation without catalyst.The products were divided into five fractions including gas,volatile organic compounds,water-soluble oil,heavy oil,and solid residue.This 'five lump system' was used to investigate the effect of reaction temperature and retention time on the degradation of lignin.Results showed that the conversion rate of the alkaline lignin during the the hydrothermal degradation process was lower than 63%,and the composition of the products was complex.A large amount of low molecular weight phenolic compounds was obtained through this method.The reaction temperature had more effect on the products distribution than the retention time.The heavy oil had more active sites than the untreated lignin,and the reactivity of the heavy oil increased with the elevated temperatures.The heavy oil obtained from the hydrothermal degradation of lignin was an ideal material for the synthesis of LPF.The formulation for the synthesis of LPF was designed based on the active site content of technical lignin.LPFs with various substitution rates(10?60%)were successfully synthesized according to the designed formulation.Results indicated that the designed formulation for the synthesis of LPF could effectively control formaldehyde emissions without significantly depleting the bonding strength of the bonded plywoods.In addition,the four technical lignins from different biomass refining processes were used to synthesize LPF based on the designed formulation.The properties of the LPFs and the corresponding plywoods prepared were tested.Results showed that the proposed formulation exhibited favorable adaptability to all four of these technical lignins for the synthesis of LPFs.In an effort to provide a fundamental understanding of lignin structure-property relations for LPF synthesis and application,two distinct technical lignins were examined as-obtained from an acidic(La)and an alkaline(Lb)organosolv bamboo pulping processes.Two LPFs were successfully synthesized using La and purified Lb.The high content of extractives in Lb,particularly long-chain hydrocarbon compounds,severely affected the synthesis of LPF.The performance of the LPFs were affected by many factors.The co-condensation mechanism of lignin-phenol-urea-formaldehyde resins synthesized through different reaction processes under alkaline conditions(pH>10)was investigated.The structural features and curing behavior of corresponding co-polymer resins were also studied.Results indicated that the co-condensation was easily achieved in the process starting from urea-formaldehyde resin synthesis.The co-polymer resins obtained from the process starting from phenol-formaldehyde resin synthesis provided better physical performance than those obtained from the process starting from urea-formaldehyde resin synthesis.However,the formaldehyde emissions of the wood products prepared with the co-polymer resins obtained from the process starting from phenol-formaldehyde resin synthesis were relatively high.The lignin could be successfully embedded into co-polymer resin system under alkaline conditions,and the formaldehyde emissions of the co-polymer resins could be significantly reduced after the addition of lignin.A moderate alkaline-ethanol post-treatment was applied as a supplement of acid steam explosion treatment to promote the conversion of cellulose in corn stover.It was found that high NaOH concentrations were beneficial for removing the lignin remained in the acid-steam-exploded corn stover.An expansion of cellulose I lattice also occurred under high NaOH concentration conditions.Both the elimination of lignin and expansion of cellulose I lattice improved the digestibility of the cellulose in the post-treated corn stover.In addition,the lignin(AL)released during the alkaline-ethanol post-treatment and the lignin-rich residue(EHR)obtained after the enzymatic hydrolysis of the corn stover were successfully used to synthesize LPFs.The bonding strength and formaldehyde emissions of the plywoods prepared with the synthesized LPFs met the requirements of corresponding industrial standards.The integrated valorization of corn stover was realized through this route.