Preparation And Properties Regulation Of Corn Straw Lignin-based Micro/nano Carbon Fibers

With the fast development of cellulosic fuel ethanol industry,how to make good use of cellulosic ethanol enzymatic hydrolysis residue has become an urgent problem that to be solved.High value utilization of corn straw lignin in cellulosic ethanol enzymatic hydrolysis residue has practical significance as well as strategic significance as it is an effort to implement the policy of long-term utilization of biomass resources.On the other hand,the development of carbon fiber(CF)is restricted by the price of costly and non-renewable petroleum-based precursor materials(mainly polyacrylonitrile,PAN).In addition,the toxic waste gas containing cyanide is released in the process of PAN-CF carbonization.Accordingly,the exploitation of low-cost,renewable and green biomass-based CF precursors is becoming a hot issue highly concerned worldwide.In this thesis,corn straw lignin-based micro/nano carbon fibers were prepared through three steps,including electrospinning,thermo-stabilization and carbonization.Based on the selection of lignin structure specific,the mechanical properties of lignin-based CF were improved,and the electrochemical properties of lignin-based CF were studied,so as to regulate and control the former and latter properties.In this research,high value utilization of corn straw lignin was studied to develop the industrial chain of cellulosic fuel ethanol.It was aimed to provide technical reference and theoretical basis for the exploitation of low-cost and sustainable CF precursors,the preparation of high-strength lignin-based micro/nano CF and self-activated lignin-based micro/nano CF.In the first part of this thesis,three methods were used to extract lignin from cellulosic ethanol enzymatic hydrolysis residue,and the adaptability of the three types of lignin in preparing high-strength lignin-based micro/nano CFs by electrospinning was compared.The results showed that the molecular weight of ethanol Lignin(EL)extracted by organic solvent method is small and its thermal stability is inferior,as a result,its precursor(EL-P)is difficult to maintain fiber morphology in the process of thermo-stabilization and carbonization,therefore,EL is not suitable for the preparation of high-strength micro/nano CFs.The molecular weight of the acid extracted formic/acetic acid lignin(FAL)is in the middle,and its molecular structure linearity is relatively low because of many side chains in molecular structure.There are a large number of holes in surface of the CF from FAL(FAL-CF),therefore,FAL is not suitable for the preparation of high-strength micro/nano CF.The molecular weight of alkali lignin(AL)extracted by alkali is large,and the molecular configuration linearity and thermal stability are better.The mechanical properties of the CF from AL(AL-CF)are relatively excellent,so AL is suitable for the preparation of high-strength micro/nano CF.However,the tensile strength and Young's modulus of AL-CF are 18.0±4.1 MPa and 2.1±0.4 GPa,respectively,which are still at a low level due to the serious polydispersity of AL molecular weight.In the second part of this thesis,a simple and efficient way of lignin fractionation was designed,named"stepwise fractionation extraction(SFE)method".The step-1 ethanol Lignin(F₁),step-2 formic/acetic acid lignin(F₂)and step-3 alkali lignin(F₃)were obtained from the residue by using SFE method.Through fractionation,the molecular weight polydispersity of F₂improved as compared with that of FAL,however,the degree of molecular configuration linearity remained low and the prepared CF(F₂-CF)was still rich in pores.Therefore,the CF based on formic/acetic acid lignin had natural self-activated characteristic.The molecular weight polydispersity and the molecular configuration linearity of F₃greatly improved.As compared with AL-CF,the graphitization degree and mechanical properties of F₃-CFsignificantly improved.The tensile strength and Young's modulus were 40.3±3.9 MPa and 6.1±0.3 GPa,respectively.As compared with AL-CF,the corresponding strength increased by 124%and 190%respectively,reflecting a better level among the same types of lignin-based CFs.In the third part of this thesis,based on the difference of thermal stability between F₁ and F₃,the interconnected lignin-based micro/nano CF mat was prepared by adjusting the component ratio and the preparation conditions.Moreover,the electrochemical properties of the three types of CFs were investigated.When the component ratio was F₁:F₃=0.5:9.5,thermo-stabilization rate was 2?/min,thermo-stabilization temperature was 250?,carbonation rate was 3?/min,the mechanical properties of the CF mat(F₁/F₃-CF)were observed to be the best,that is,tensile strength was 50.3±4.3 MPa and Young's modulus was7.1±0.5 GPa,which were 25%and 16%higher than that of F₃-CF respectively.That is attributed to the synergistic effect of the"carbon welding force"caused by the slight inter-connect between CFs and the excellent thermal stability of the main components of the fiber on the retention force of the fiber rigidity.The adsorption and electrochemical properties of the three types of carbon fibers were tested.There were a large number of micropores and mesopores on the surface and inside of F₂-CF due to its self-activated characteristics.The specific surface area of FAL was 1067 m²/g,the pore volume was 0.82 cm³/g,and the average pore diameter was 7.1 nm.When the current density was 0.1 A/g,the specific capacitance of F₂-CF can reach to 201 F/g and the electronic storage capacity is promising.In the fourth part of this thesis,the application effect of the CF in composite preparation field was investigated.The micro/nano multiphase lignin-based CFs reinforced composite(MPRC)was prepared with epoxy resin(EP)as matrix phase,ultra-short cut F₃-CF as nano dispersion reinforcement phase and CF mat F₁/F₃-CF as continuous fiber reinforcement phase.With the aspect ratio of 14 and the additive amount of 0.4%,F₃-CF dispersed well in the matrix,and the strength of the reinforced matrix was the highest.The continuous fiber reinforced phase(F₁/F₃-CF)significantly increased the toughness of MPRC,and the elongation at break of MPRC-0.4%containing continuous fiber reinforced phase and 0.4%ultra-short cut F₃-CF was nearly six times that of pure EP.The mechanical properties of MPRC were improved due to the combination of the crack weakening mechanism of nano dispersion reinforcement and the bridging toughening mechanism of continuous fiber reinforced phase.The tensile strength,bending strength,impact strength and hardness of MPRC-0.4%increased by 72%,107%,63%and 26%respectively as compared with those of pure EP.It was showed that the CF prepared in this paper has positive reinforcement effect.Furthermore,the addition of reinforcement(ultra-short cut F₃-CF and CF mat F₁/F₃-CF)contributes MPRC preferable wear resistance and certain conductivity,and widens its application field.