Modification Of Lignin By Microwave Heating And Curing Kinetics Of Lignin-phenol-formaldehyde Resins

The conversion of renewable resources into useful materals has considerable economic and social value,particularly in the context of environmental pollution and limited fossil fuel resources.Lignin is the most abundant natural renewable polyphenol compound in nature.Lignin with similar structure to phenyl is widely used in the preparation of lignin based phenol formaldehyde resin.However,the complex structure and low reaction site of lignin molecules seriously limit its application in the lignin-phenolic resin materials.It is necessary to activate lignin in order to increase its reactivity and improve performance of lignin based materials.The idea of this paper is to modify lignin from byproduct of cellulose ethanol using three methods.Besides,lignin and modified lignin replacing part of phenol are used to prepare lignin based phenol-formaldehyde?PF?resin.The aim of research on curing kinetics is to provide reference for the application of lignin based phenolic resin.The main contents and conclusions are summarized as follows:1.The demethylation modification was carried out in the hydrobromic acid?HBr?-hexadecyltributylphosphonium bromide?TBHDPB?system assisted by microwave heating.The effects of HBr dosage,temperature,time and dosage of catalyst TBHDPB on reaction were investigated.Structural analysis of starting purified lignin and modified lignin was characterized by ultraviolet spectrum,nuclear magnetic resonance hydrogen spectrum?^<sup>1H NMR?,Fourier transform infrared spectroscopy?FTIR?,gel permeation chromatograph?GPC?and elemental analysis.The results showed that the content of phenolic hydroxyl group and reactive activity of modified lignin increased by 32.71% and 18.15%,respectively;the content of methoxyl group decreased by 20.44% when the acid reacted with the purified lignin at the dosage of 20mmol/g at 95 ? for 1 h with TBHDTB?2% to lignin?under microwave irradiation.2.The hydroxymethylate modification was carried out under microwave heating.The best reaction conditions were 80? and 10.5 of pH value for 30 min by measuring the amount of formaldehyde consumption under different reaction temperature,pH value and reaction time.The reaction time was shortened from 3 h to 30 min by comparing the conventional method and microwave heating,which significantly improved the efficiency of the hydroxymethylate modification of lignin.The lignin and modified lignin was characterized by FTIR,¹H NMR and gel permeation chromatograph?GPC?.The results showed that the content of hydroxyl group and reactive activity of modified lignin increased.The number-average molecular weight?M_n?and weight-average molecular weight?M_w?of acetylated hydroxymethylation lignin increased,as well as the molecular weight distribution?M_w/M_n?tended to be wider.3.The phenolation was carried out by taking sulfuric acid as catalyst and phenol as solvent.The phenolic hydroxyl content in lignin and molecular weight changes were measured by ultraviolet spectrum?UV?and gel permeation chromatograph?GPC?.The best reaction conditions were 2% dosage of sulfuric acid,under 90? for 60 min.Structural analysis and thermal stability of modified lignin was characterized by nuclear magnetic resonance hydrogen spectrum?¹H NMR?,FTIR,thermogravimetric analysis?TGA?,elemental analysis and Pyrolysis gas chromatography-mass spectrometry?Py-GC-MS?.The absorbance peak of FTIR showed that new absorption peaks appeared in 753 cm^-1 and 692 cm^-1 and absorption peak intensity increased significantly at 831 cm^-1,which indicated that there was a condensation reaction between ?-hydroxyl at side chains of lignin and orthopara of phenol,accompanied by cleavage of ether bond and the degradation of lignin.¹H NMR spectra further confirmed that lignin wasphenolated because phenolic hydroxyl increased while alcohol hydroxyl decreased.Thermal stability of modified lignin decreased according to TGA.There was a distinct thermal degradation peaks at 382.8 oC.,while two obvious peak period of thermal degradation appeared between 187.2 and 386.6 ?.,The pyrolysis residues of lignin and phenolated lignin at 900 ? were 43.23% and 37.16% respectively.Py-GC-MS results showed that the ratio of H/G/S of the lignin and phenolated lignin were 9/78/13 and 80/18/2,and this verified the guaiacyl units dominated in lignin and hydroxy phenyl structure mainly in phenolated lignin.A lot of phenol generated in cleavage products also proved that lignin was phenolated.4.The expandable lignin-based PF resin was prepared using lignin and three kinds of modified lignin replacing 30% phenol.The curing kinetics of four kinds of lignin-based phenolic resins were analyzed by non-isothermal DSC.Three kinds of modified lignin had higher curing temperature and faster curing rate than lignin had from activation energy and pre-exponential factor.