The Influence Of Pore Structure Of Polyacrylonitrile Fiber On The Oxidative Stabilization

Developing carbon fiber from polyacrylonitrile(PAN)fiber is generally subjected to two processes,namely stabilization and carbonization.For high modulus carbon fiber,graphitization should be done under controlled conditions.The oxidative stabilization step is an essential step in the conversion of PAN fibers to high performance carbon fibers.The process of oxidation involves cyclization,oxidation and double-diffusion,in which oxygen diffuses from the surface to the core of fibers,while the small molecules and heat generated diffuse on the opposite way.Oxygen is taken up in the form of oxygen-containing functional groups on the molecular chains.With the development of the cyclization and oxygen reaction,the fiber structure gradually becomes dense to hinder the diffusion of oxygen.Finally,the gradient of the oxygen content along the fiber diameter becomes larger,which leads to serious skin-core structure.In the stage of oxidation,the diffusion of oxygen relies on the pore structure.The pores on the one hand,act as channels for the diffusion of oxygen,on the other hand eventually turn to be a kind of defects.Therefore,a dense structure in the fibers results in a serious skin-core structure,while a structure with too many pores tends to form more defects in the carbon fibers.Therefore,the pore structure is an important parameter for carbon fiber precursor and relevant to the heat treatment process conditions.In this paper,the study on the relation between the pore structure and the oxygen diffusion is carried on quantitatively and qualitatively.The qualitative statement is done through comparing the results of oxidation of the PAN fibers with different pore structure and the same chemical components prepared by wet spinning under the same conditional oxidation,where the PAN fibers with different pore structure were prepared by different stretching conditions.With the gradual increase of the stretching ratios,the preferred orientation of PAN molecules and the density of the fibers were improved.In addition,the oxygen uptake during oxidation became low,and the gradient of radial oxygen distribution became steeper,which would lead to form serious skin-core structure in the fibers.The quantitative aspect about the influence of the pore structure on oxidation is the emphasis.The PAN fibers with different chemical compositions and pore structure were selected to prepare oxidized fibers.The rate constant of oxidation for PAN fibers and the radial distribution of the oxidized fiber were characterized.The results showed that the diffusion coefficient,the oxidation temperature together with the reaction constant determined the distribution of oxygen in the fibers.A mathematical model was established to simulate the relationship between the coefficient of oxygen diffusion and the pore structure parameters.The model can be used to give an appropriate oxidation temperature for the fiber of a specific pore structure,consequently a suitable heating program can be chosen.Meanwhile,it can be used to evaluate the pore structure of PAN fibers.