Preparation Of Modified Lignin And Its Application And Research In Epoxy Resin

With the exhaustion of petroleum resources,people pay more and more attention to the renewable biomass resources.In recent years,lignin,as the only macromolecule with aromatic structure in nature,has been used to replace some petrochemical products or as a modified lignin used in polymer materials.Because lignin contains a variety of active functional groups(hydroxyl,methoxy,carbonyl,etc.),chemical reactions can be carried out directly or indirectly.As a common coating in daily life,epoxy resin does not have application properties,only after curing with a curing agent can exhibit good chemical stability,mechanical properties,and so on.The disadvantage of epoxy resin is poor heat resistance,poor toughness,poor weather resistance,etc.In view of the more difficult to change the nature of its own,it is generally to change the nature of the curing agent to change the epoxy resin.Because lignin has the same active functional group as epoxy curing agent,this paper aims to prepare the extracted cotton stalk lignin as a non-toxic and environmentally friendly additive and apply it to epoxy resin.The research contents of this paper are as follows:1)Cotton stalk lignin was reacted with formaldehyde in alkaline condition to synthesize hydroxymethyl lignin(HL)as an auxiliary curing agent for epoxy resin.Taking the mass fraction of free formaldehyde as the objective function,the optimal reaction conditions for HL preparation were as follows:reaction temperature 85?,reaction time 4.5 h,p H=11.5,m_lignin:m_formaldehyde=3.5:1.The successful preparation of HL was proved by Fourier Infrared Spectrometer(FT-IR).HL was used as an auxiliary curing agent in epoxy resin.It was found by Thermogravimetric Analyzer(TGA)that the thermal stability of epoxy resin would not be affected only when the addition amount of HL was controlled.The curing kinetic theory of epoxy resin with the aid of HL was studied by Differential Scanning Calorimeter(DSC),and it was found that its apparent activation energy was 43.78 k J/mol,which was significantly lower than the apparent activation energy of the system without HL(54.94 k J/mol).The faster curing rate of epoxy resin assisted by HL was found by Dynamic Mechanical Analysis(DMA)and Advanced Rotary Rheology.The influence of different mass fraction of HL on the tensile strength and water resistance of epoxy resin was investigated,and the optimal addition amount of HL was determined.2)Under the catalysis of anhydrous AlCl₃,lignin was degraded at high temperature and reacted with Na I to prepare polyphenol lignin product(PL).Taking the content of phenolic hydroxyl group as the target product,the optimal experimental conditions for the preparation of PL were as follows:anhydrous AlCl₃ accounted for 24%of the mass fraction of lignin,m_lignin:m _{Na I}=5:3,reaction time was 5 h,reaction temperature was 80?,and the content of phenolic hydroxyl group increased from 1.6967 mmol/g to 3.6625mmol/g.The polyphenol lignin was successfully prepared by FT-IR characterization.Through TGA characterization,it was found that the addition amount of PL was less than 20%,the thermal stability of epoxy resin would not be affected,so the addition amount of PL was less than 20%for subsequent studies.After studying the curing kinetic theory of epoxy resin with PL assistance by DSC,it was found that the apparent activation energy of epoxy resin system with PL assistance was 51.09 k J/mol,which was lower than the apparent activation energy of the system without PL.The faster curing rate of epoxy resin assisted by PL was found by DMA and Advanced Rotary Rheology.The influence of different mass fraction of PL on the tensile strength and water resistance of epoxy resin was investigated,and the best addition amount of PL was determined.3)Aminated lignin(ML)was prepared by Mannich reaction using melamine as the modified monomer of lignin and formaldehyde as the graft functional group of the two compounds.Taking the yield of ML as the objective function,the optimal reaction conditions for ML were determined as follows:reaction temperature 90?,reaction time4 h,p H=11.5,and the mass fraction of lignin accounted for by melamine was 60%.The grafting of melamine to lignin was proved to be successful by elemental analysis and FT-IR characterization.Through TGA characterization,it was found that the thermal stability of epoxy resin would not be affected if the addition amount of ML was less than 20%.The apparent activation energy of the epoxy resin system with the aid of ML was studied by DSC to be 52.60 k J/mol,which was lower than the apparent activation energy of the system without ML.The slower curing rate of epoxy resin assisted by ML was found by DMA and Advanced Rotary Rheology.The influence of different mass fraction of ML on the tensile strength and water resistance of epoxy resin was investigated,and the best addition amount of ML was determined.