Modification Of Lignin And Its Application In Lignin-based Resins

One of the foremost challenges in the 21 st century is the replacement of fossil resources by more sustainable alternatives.A viable substitute should at least be renewable,CO2-neutral,widely available,and should not compete with food production.Inedible plant biomass in the form of lignocellulose is one of the few resources that meets these essential criteria.Over the years,different biorefinery strategies have been proposed,each with their own advantages and limitations.Lignin is a by-product of chemical processing of lignocellulosic biomass.whereas the lignin portion is mostly regarded as a low-value by-product or a cheap energy source,thus,the conversion of lignin to high-value polymeric materials becomes urgently needed.HBr and HI were used as reagents to demethylate an alkali lignin(AL)to increase its hydroxyl content and thereby enhance its reactivity for the preparation of phenolic resins.Analyses by FT-IR,1H-NMR and 2D-NMR(HSQC)demonstrated both a decrease in methoxyl groups and an increase in hydroxyl groups for each demethylated lignin(DL).In addition,the molar amounts of phenolic hydroxyls,determined by 1H-NMR,increased to 0.67 mmol/g for the HI-DL,and 0.64 mmol/g for the HBr-DL,from 0.52 mmol/g for the AL.These results showed that HI,a stronger nucleophilic reagent than HBr,provided a higher degree of AL demethylation.Lignin-containing resins,prepared by copolymerization,met the bonding strength standard for exterior plywood with DL used to replace as much as 50 wt.% of phenol.The increased hydroxyl contents resulting from the lignin demethylations also imparted faster cure times for the lignin-containing resins and lower formaldehyde emissions.We extracted the organic solvent lignin(OSL)from poplar wood,and OSL and succinic anhydride were catalyzed by N-methylimidazole to prepare carboxylated lignin(L-COOH).A cocured lignin-based epoxy resin with carboxylated lignin and Succinic anhydride was prepared.The carboxyl group concentration of lignin increased by 2.28 mmol / g after carboxylization modification.And the mechanical properties of epoxy resin have also improved,not only the tensile strength has improved,but also the bending strength has improved.Among them,when the lignin content was 10 wt%,the tensile strength of the epoxy resin increased by 61.8%,and the elongation at break increased by 119%.Moreover,as the mass fraction of lignin increased from 0% to 20%,the flexural strength of the lignin epoxy resin increased from 23.3 MPa to 101 MPa.The control sample Lep10-control,which did not use the one-pot method,was significantly different from the epoxy resin prepared by the one-pot method due to its heterogeneity.It is proved that the addition of lignin improves the mechanical properties of epoxy resin,and the one-pot method makes the distribution of lignin in the material more uniform,and improves the uniformity and integrity of lignin epoxy resin.