Research On Liquid Phase Stabilization And Carbonization Of Polypropylene Fiber

PAN is a commonly-used but high-cost precursor for high-performance carbon fiber.Polyolefin fiber?polyethylene,polypropylene,etc.?,on the other hand,is a highly competitive precursor fiber due to its high carbon content,low cost,melt-spinnability properties.Many reports have been presented that PE fibers can be used as a new carbon source for high-performance carbon fiber production.Usually,linear molecules are cross-linked by liquid-phase sulfonation and carbonized at high temperatures,respectively.Compared with PE,the production process of PP fibers is more mature,thus the cost and the yields of PP fibers are better.Isotactic polypropylene?i-PP?,with highly aggregated crystalline and uniformly distributed short branched molecular structure,is similar to that of high-density PE and linear low-density PE.Thus it is worth developing PP fibers as a new carbon source of carbon fiber for both academy research and potential usage.However,little work on the structural evolution of PP fibers during sulfonation and carbonization has been reported.In this thesis,concentrated sulfuric acid was used to stablized PP fibers before carbonization.Structural transformation of PP fibers during sulfonation and carbonization was characterized by XRD,XPS,Raman and SEM.Study on sulfonation and carbonization reaction kinetics of PP fibers was carried out.The influence of swelling pre-treatment and doping post-treatment on the structure evolution of PP based carbon fibers was clarified further to provided necessary support for the preparation of low-cost carbon fibers.PP fibers were sulfonated with different time before further carbonization.Linear molecular chains of i-PP were cross-linked under the sulfonation and strong oxidation of concentrated sulfuric acid during sulfonation reaction.The crosslinked i-PP molecules grew into graphite-like structure with the removal of sulfur and other atoms and carbonizated at higher temperature.Mesogenic crystal structure of PP was destroyed with the growth of graphite microcrystalline structure during sulfonation process.When sulfonation time was short,the hollow structure appeared in carbonized fibers,indicating that the PP molecules in the core were not fully infusible.The hollow structure in carbonized fiber disappeared,as well as d-spacing(d₀₀₂)and crystalline size?L_c?of sulfonated and carbonized fibers increased,with prolonging sulfonation time.PP fibers were sulfonated at different temperatures before further carbonization.By introducing relative high temperature,the original mesogenic crystalline of the PP fibers was mostly remained,large size of the graphite microcrystalline was generated and both sulfonation and oxidation reaction rate was obviously accelerated.Hydroxyl and carboxyl groups successively appeared on the surface of sulfonated fibers before finally elimination,indicating that sulfonation and strong oxidation coexisted in sulfonation reaction.Therefore,relative content of functional groups on surface of carbon fibers could be changed by reasonable control on sulfonation time and temperature,which is conducive to the further functionalization of the sulfonated fibers.The effect of carbonization temperature on the structural evolution of sulfonated fibers was studied.It was found that as carbonization temperature increased,the diameter of the carbonized PP fiber gradually decreased,indicating that the removal of heteroatoms is closely related to carbonization temperature.When carbonization temperature did not exceed 800?,the order degree of carbon structure of the sheath of carbon fibers increased,while that of the core weakened,resulted in the increase of skin-core difference.When the infusible fibers were carbonized at 1000-1200?,the order degree of the carbon structure in carbon fibers was greatly reduced and the skin-core structure difference was slightly reduced.When carbonization temperature reached 1400?,I_D/I_G was approximately 1.3 meaning that the order degree of carbon structure in carbon fibers was significantly increased,and the skin-core difference of carbon fibers was basically eliminated.A strong dispersion diffraction peak appeared again in the XRD trace,which indicates that a large amount of graphite-like microcrystalline structures were generated.The mechanism of the effect of temperature on the structure evolution of PP based carbon fiber is discussed.The graphite-like structure of PP based carbon fiber was destroyed due to the transfer and elimination of non-carbon heteroatoms at the core of the fiber,with medium carbonization temperature?800-1200^oC?,After the mostly removal of heteroatoms,highly order graphite microcrystalline structure was formed in the carbon fiber,as well as the reduction of the difference of skin-core structure,with higher carbonization temperature?1400^oC?.PP fibers were swelled before sulfonation in order to regulate the aggregation structure of PP fibers.Sulfonation reaction was benefited by long time swelling,due to the destroy of the original mesogenic structure of PP.The increase of d-spacing in graphite crystalline of carbon fiber was observed as increase of swelling time and temperature.The grain size of graphite crystalline of carbon fiber increased with swelling time under low swelling temperature.The degree of graphitization was effectively increased and structural difference between skin and core of carbon fibers was gradually eliminated by swelling process.The sulfonated fibers were doped with boric acid by liquid-phase impregnation.It was observed that,d₀₀₂ of carbonized fibers decreased while L_c increased,by introduction of boron.The order degree of the carbonized fibers was also increased,and the structural difference between skin and core of carbon fibers is greatly reduced.Thus the treatment of sulfonated PP fiber with boric acid is beneficial to the development of graphite structure of PP-based carbon fiber.