Preparation And Performance Study Of Lignin-based Carbon Fiber By Chain Extension Method

Lignin is the second largest natural polymer material in nature.Currently,it is mainly discharged directly as a by-product of industrial pulping,which is not only a waste of resources but also causes serious environmental pollution.In recent years,lignin has been widely used as an ideal raw material for the preparation of carbon fibers due to low cost,high carbon content and wide sources,which not only improves the utilization value of lignin,but also reduces the production cost of carbon fibers.However,lignin was degraded during the extraction and separation process,the molecular weight was reduced,which caused the strength of the lignin-based carbon fibers was low.Therefore,increasing the molecular weight of lignin is one of the ways to increase the strength of lignin-based carbon fibers.In this paper,1,6-dibromohexane as a chain extender was used to react with the phenolic hydroxyl groups of lignin in an alkaline solution to increase the molecular weight of the lignin,and obtained lignin chain extension product.The spinning material was prepared by melt modifying.Then the lignin chain extension product was modified to prepare fiber by the melt spinning,and then the lignin chain extension product carbon fibers were prepared through pre-oxidation and carbonization.The specific research contents are as follows:FTIR,XPS,GPC,TG and Folin-Ciocalteua were used to characterize the structure,molecular weight,phenolic hydroxyl content of lignin chain extension products and their performance.The results showed that 1,6-dibromohexane mainly reacted with the phenolic hydroxyl group of lignin.With the increase of1,6-dibromohexane content,the content of phenolic hydroxyl groups gradually decreased,the molecular weight of the lignin chain extension product gradually increased,and the thermal stability gradually improved.The solubility of lignin chain extension products was investigated,and the results showed that the lignin chain extension products which prepared the mass ratios of lignin and 1,6-dibromohexane were 1:0.1,1:0.2,and 1:0.3 all solubled in most organic solvents.The lignin chain extension product which prepared the mass ratio of lignin to 1,6-dibromohexane was1:0.4 had poor solubility.When the mass ratio of lignin and 1,6-dibromohexane was1:0.3,the molecular weight of the lignin chain extension product was the highest,which was 6,500 Da.It was selected as the raw material for the preparation of carbon fibers.The lignin chain extension product was melt-modified by phenol liquefactionfollowed by hexamethylenetetramine.The effects of the ratio of lignin chain extension product to phenol,the amount of hexamethylenetetramine and the reaction temperature on the spinning materials were discussed.FTIR,XPS,melting point and rheological test methods were used to characterize the structure and properties of the spinning materials under different conditions.The results showed that the lignin chain extension products reacted under different reaction conditions and the spinning materials of phenolic compounds were prepared.Spinning materials all had melting properties.The reaction material had a stable rheological property and good spinnability when the ratio of lignin chain extension product to phenol was 1:6,the amount of hexamethylenetetramine was 13% and the reaction temperature was130 ??The primary fiber of the lignin chain extension product was prepared by melt spinning,and the carbon fiber of the lignin chain extension product was prepared through pre-oxidation and carbonization.The TG curve was used to simulate the process of pre-oxidation and carbonization,which determined the temperature of pre-oxidation and carbonization.FTIR,XRD,SEM and elemental analysis were used to characterize the fibers at different stages and to test the mechanical properties of the lignin chain extension product carbon fibers.The results showed that the lignin chain extension product carbon fibers with a carbon content of 92.85% and a strength of79.86 MPa were successfully prepared.