Application Of Gelation Mechanism To The Preparation Of Carbon Fiber Precursors

Carbon fiber(CF) is an advanced material which is widely used in varieties of fields due to its high performance,i.e.high strength,high modulus,heat resistance,erosion resistance,etc.Nowadays,there are large gaps in the properties and production of polyacrylonitrile(PAN)-based carbon fiber by domestic and foreign countries.PAN precursors play a key role in producing high-performance PAN carbon fibers.At present,wet spinning and dry-jet wet-spinning are the main methods for manufacturing PAN precursors.However,two vital defects can't be avoided in both of these two methods,i.e.,skin-core structure and large pores.Gel spinning used to produce high strength and high modulus polyethylene(PE) fiber has been introduced to producing PAN precursors.In our study,PAN copolymer with molecular weight of 70～80 thousand and 23%(wt) PAN spinning were used,where a small amount of nonsolvent was added the solution.The gelation of the spinning solution occurred before the double diffusion in the coagulation bath.Based on thermal-induced gelation,gel spinning was used to produce high-performance PAN precursor fibers with little skin-core difference, homogeneous microstructure and uniformly distributed micropores.The gelation temperature is an important processing parameter for the gel spinning of PAN precursor fiber.In this thesis,dynamic rheological experiments were used to determine the gelation temperature of PAN/DMSO solution.The gelation temperatures of the PAN/DMSO solutions with different amounts of water were determined through measuring the evolution of dynamic rheological parameters(loss tangent tanS).The dynamic rheological parameters(dynamic moduli G'andG",loss tangent tan8 and complex viscosityη~*) were used to characterize the gelation behavior of PAN solution.The effect of water content on the gelation behavior and gel structure was also analyzed.The results of rheological experiments were the basis of the gel spinning.In our work,20 wt%and 23 wt%PAN/DMSO solutions containing different amounts of water were used.With different coagulating bath temperatures and drawing conditions,gel spinning was applied to prepare PAN precursor fibers.The structure and performance of the PAN precursor fibers were characterized by mechanical properties testing,swelling capacity testing,X-ray Diffraction(XRD) and Scanning Electron Microscopy(SEM).The results showed that,with the coagulating bath temperature decreased,the big pore in fiber also decreased.When coagulating bath temperature was set at-9℃,large pores disappeared.With the PAN concentration increased,the swelling capacity decreased and mechanical properties increased.Since the forementioned gel spinning method can't eliminate the skin-core structure and the large pores in the PAN fiber,an improved spinning method was proposed.In this improved spinning method,an aging process of the spinning solution was introduced to change the solution into a steady three dimensional network structure before it entered the coagulating bath.23 wt%PAN/DMSO solution containing 4%water was used to prepare PAN precursor fiber at different spinning temperatures.The structure and performance of the PAN precursor fibers were characterized by mechanical properties testing,swelling capacity testing,X-ray Diffraction(XRD),Scanning Electron Microscopy(SEM), optical microscopy and skin-core structure testing.The experimental results showed that the PAN precursor fibers obtained from the improved gel spinning possess better physical properties with less skin-core difference and large pores as well as circular cross section.