Structure,Properties And Application Of Lignin/Polyacrylonitrile Blend Fibers

Lignin is a cheap renewable resource with high carbon content.Polyacrylonitrile(PAN)is a polymer with good spinnability and thermal stability.In this thesis,lignin/PAN blend fibers were prepared by wet spinning process,and their structure and properties as well as application in the development of low-cost carbon fibers were studied.Carbon fibers with relatively lower cost and higher performance were expected to be obtained by integrating the cost advantage of lignin and the performance advantages of PAN.This study is of great significance for realizing the high value utilization of lignin and the sustainable development of carbon fibers.This thesis mainly carried out the following research work:(1)A method for determining the actual content of lignin in the blended fiber was established using ultraviolet absorption spectrum: Blend solutions of different lignin content were formulated into a certain concentration solution with DMSO as a solvent and analyzed by ultraviolet-visible spectrum.A working curve was established based on the measured absorbance and lignin concentration.Then the lignin/PAN blended fibers were prepared into a dilute solution of the same concentration by the same method and the ultraviolet absorbance in the fibers was measured.The actual lignin content was calculated according to the established working curve.Compared with infrared spectroscopy,this test only needs to be performed under the condition of avoiding light,and it has lower requirements on the air in the environment.It can quickly and accurately determine the actual content of lignin in the lignin/PAN blend fiber.(2)Preparation and properties of lignin/PAN blend fibers.Effect of lignin content on the viscosity of lignin/PAN blend solutions was studied,and the appropriate lignin content for blending was confirmed.A method of testing actual lignin content was established.Effect of lignin content on the structure and properties of lignin/PAN blend fibers was studied.(3)Preparation,structure,and properties of lignin/PAN based carbon fibers.Mechanical properties,microscopic morphology and microstructure of the carbon fibers were characterized.Effects of thermal stabilization and carbonization conditions and lignin content on the properties and structure of the carbon fibers were studied.(4)Modification of lignin/PAN blend fibers for preparation of carbon fibers.Guanidine hydrochloride(GH)and boric acid(BA)were used to modify lignin/PAN blend fibers.Effect of chemical modification on the thermal properties and thermal stabilization of the fibers as well as the microscopic morphology and mechanical properties of carbon fibers were studied.The main conclusions were as follows:(1)When the content of lignin was controlled at 35% or less,uniform and stable lignin/PAN blend solutions and dense and continuous lignin/PAN blend fibers could be prepared.Introduction of appropriate amount of lignin would not adversely affect the basic physical and mechanical properties of the blend fibers.The synergistic effect of lignin and PAN endowed the blend fibers with good thermal stability.(2)The lignin/PAN-based carbon fibers had round cross section and uniform and dense structure,without obvious macrovoid defects.Increasing the carbonization temperature and the orientation degree of the lignin/PAN precursor could improve the tensile modulus of carbon fibers.The amorphous network molecular structure of lignin tended to be transformed into amorphous carbon,and a unique uniform disordered carbon structure was finally formed in the lignin / PAN based carbon fibers.(3)BA modification had little effect on the thermal properties of the blend fibers,while GH modification significantly reduced the initial exothermic temperature and promoted the cyclization reaction.At the same temperature,the highest degree of thermal stabilization was obtained by GH and BA modification,and the prepared carbon fibers had smoother surface,more uniform and dense structure,and higher tensile strength than that of the unmodified carbon fibers.