The Fractionation Of Lignin And Effect On Strength Properties Of Derived Carbon Fibers

With the exhaustion of mineral fuel resources and the increase in environmental pollution,conventional nonrenewable resources are being replaced by renewable resources.Lignin has received increasing attention due to its being an abundant renewable resource,which is only next to cellulose on a global scale and arising many people's attention.Annually,the pulp and paper industry can produce approximately 70 million tons of lignin.However,only about 2%of the lignin is utilized effectively,and most of the lignin is discharged into rivers or directly burned as fuel,thereby causing serious environmental pollution and a grievous waste of renewable resources.Lignin is a unique biomacromolecule with an exciting chemical structure,including large molecular weight(Mw),high carbon content,and aromatic monomers.Its unique chemical characteristics make it a potential precursor to produce carbon fibers(CFs).CFs are industrially important fiber materials because of their excellent performances,such as low density,high creep resistance,good tensile strength,and corrosion resistance.Currently,almost 90%of commercial CFs are mostly made of polyacrylonitrile(PAN),which is nonrenewable and expensive.The utilization of lignin not only reduces the cost of CFs production but also enhances the sustainability and cost-effectiveness of pulp and paper industries.However,due to the complexity of lignin structure and heterogeneous of molecular weight hinder the research and application of lignin.In this paper,lignin was fractionated from corn stalk refining residues or lignin was modified then dialyzed with different dialysis tube.In addition,we also discuss the chemical structures and spatial configuration of different fractionated lignin and used lignin as precursor material to prepare high-performance lignin-based CFs by electrospinning.Through the analysis of the results,the mechanical strength and electrochemical properties of lignin-based carbon fibers were studied.This study elucidates the high-value utilization of resources is realized and may thus guide the electrochemical applications of lignin-based CFs for the development as supercapacitor.In this paper,lignin was extracted from corn stalk refining residues by alkali solution and acid precipitation.The fractionated lignin precipitation carried out by dropping HCl to adjust pH value and obtain different pH levels(10,8,6,4,and 2).Carbon fibers were prepared by different fractionated lignin and PAN through electrospinning after pre-oxidation and carbonization.In addition,the mechanical properties of CFs were studied.The structures and properties of different fractionated lignin were studied by elemental analysis,FTIR,GPC,HSQC,TG and DSC.CFs were measurement by SEM,XRD,Raman spectroscopy and tensile properties.Results suggest that a fractionated lignin sample with a large molecular weight,low PDI,more linear molecular configuration and strong thermal stability are the necessary conditions to produce carbon fibers with excellent performance.After acid precipitation,the mechanical properties of lignin-based carbon fibers were greatly improved.The tensile strength of different fractionated lignin-based carbon fibers increased from 13.19MPa(pH2)to 21.21 MPa(pH10).Young's modulus increased from 2.64GPa(pH2)to 4.54GPa(pH10).Four kinds of representative lignin such as poplar lignin,pine lignin,reed lignin,and residue lignin are used to prepare lignin-based CFs.The effects of their spatial configurations(linear or spherical)on the properties of lignin-based carbon fibers were discussed through the measurement and analysis of lignin from different sources.The result shows that softwood lignin displays a more linear molecular configuration compared with hardwood lignin and grass lignin.The pine lignin has the most linear molecular configuration and best morphologies than others.The specific surface area of p ine CFs is 689.13 m~2/g,and the maximum adsorption amount of methylene blue is 249.80 mg/g,.The contact angle is120.39°.The electrochemical performance of pine CFs is also excellent,the electrochemical impedance is the smallest and the specific capacitance is 149 F/g.Corn straw cellulose ethanol residue lignin was modified with NCO,and improve molecular weight.The modified lignin was fractionated by dialysis bag of 3500Mw and7000Mw,and several different fractionated lignin were named as Non-ML,ML-3.5 K,ML-3.5-7 K and ML-7 K,then characterization and analysis of different fractionated lignin.Activated carbon fibers(ACFs)were prepared by different fractionated lignin and PAN through electrospinning after pre-oxidation,carbonization and activation.The mechanical properties and electrochemical properties of ACFs with different fractionated of lignin were studied.The results showed that the mechanical strength of CFs and the electrochemical performance of ACFs were greatly improved by lignin modification and classification.The molecular weight of ML-7 K is largest which can reach 11316 g/mol,the molecular space configuration is more linear,the morphologies of the CFs is smoothest and the diameter is the smallest.The specific surface area of the ML-7 K ACFs is 2042.86 m~2/g,and the tensile strength of CFs is up to 29.97 MPa,the Young's modulus can reach 6.01GPa.In addition,the electrochemical performance of ACFs is the best,the electrochemical impedance is the smallest,and the specific capacitance is 136.46 F/g.Using corn straw cellulose ethanol residue as raw material,The lignin fraction of pH2and pH10 were obtained by acid precipitation and named F1 and F2,respectively.Then the pH10 lignin was modified with NCO in order to increase relative molecular weight of the modified lignin.The modified lignin was classified by dialysis bag of 5000Mw.After that the modified lignin was cutoff by dialysis bag were named F3(dialyzed fraction)and F4(dialyzed non-dialyzed fraction),respectively.CFs were prepared by different fractionated lignin and PAN through electrospinning after pre-oxidation and carbonization and the mechanical properties of CFs with different fractionated of modified lignin were studied.The results showed that the mechanical strength of CFs were greatly improved by lignin modification and fractionation.The molecular weight of F4 is largest which can reach 35305 g/mol,the molecular space configuration is more linear,the morphologies of the CFs is smoothest and the diameter is the smallest.The tensile strength of F4 carbon fibers is up to 34.25 MPa and the Young's modulus can reach 7.21GPa.