Preparation And Structural Properties Of Lignin Based Carbon Fiber

Lignin is an important renewable resource.It is an ideal material for preparation of carbon fibers because of its carbon-rich and low-cost characteristics.But lignin is a complex three-dimensional network polymer,which is difficult to spin into fibers solely.Continuous fibers can be prepared by blending polyacrylonitrile(PAN)with lignin through using wet spinning techniques.However,this method requires the lignin to have high quality,and faces serious problems of the loss of lignin during spinning and the formation of voids in fibers.Therefore,this thesis have mainly carried out the following research works:(1)The purification process of lignin was optimized for reducing the content of inorganic impurities;the effect of purification process on the structure and properties of lignin was investigated.(2)Low-cost carbon fiber precursors were prepared by blending wheat straw lignin with the P(AN-VAc)copolymer.The interaction between lignin and P(AN-VAc)copolymer was studied.(3)The modification method for the spinning solution was studied;the dense precursor fibers were prepared by blending lignin with P(AN-IA)copolymer and using aminating modification method.(4)P(AN-IA)copolymer was used to blend with several kinds of industrial lignin;the properties of these lignin,precursor fibers and carbon fibers were studied.The main results are as follows:1.The inorganic impurities in the lignin could be effectively removed by hydrochloric acid precipitation,centrifugal separation and acid washing.When the lignin was treated with 0.6: 1 hydrochloric acid,the ash content of lignin was reduced to 0.77% after purified four times.The purified wheat straw lignin showed high reaction activity.2.Low-cost carbon fiber precursors were prepared by blending wheat straw lignin with P(AN-VAc)copolymer.It was found that the lignin content had a great influence on the viscosity of spinning solutions and the structure of precursors.The lignin could initiate the cyclization reaction of nitrile groups at lower temperature and promote the process of thermal stabilization.When lignin content increased to above 30%,void defects would be formed in the center of carbon fibers.3.Ammonium acetate was used to ammoniate the wheat straw lignin /P(AN-IA)blend spinning solution,and dense fibers with round cross-section were prepared in pure water coagulation bath.Ammonium acetate could selectively ammoniate P(AN-IA).As a result,the hydrophilicity of the spinning solution was improved,but the lignin would not be modified and lost.4.Precursor fibers were prepared by blending three types of lignin with P(AN-IA),and the properties of the lignin,precursor fibers and carbon fibers were studied.It was found that the poplar lignin had the best thermal stability and the highest yield of carbonization.The addition of lignin did not reduce the strength of precursor fibers and carbon fibers,but led to a decrease in the modulus.This thesis had made an in-depth study on the lignin purification process,spinning solution modification method,coagulation bath composition,lignin type,pre-oxidation and carbonization process.The blend precursor fibers with high lignin content and dense structure and the carbon fibers with good mechanical properties were prepared,which laid an important theoretical and technical foundation for the development of low-cost lignin based carbon fibers.