Effect Of Oxygen Content On The Structure Of PAN Fibers During Pre-oxidation

Pre-oxidation is an important step in the preparation of PAN-based carbon fibers.During the pre-oxidation process PAN molecules undergo cyclization,oxidation,dehydrogenation and other reactions,which transform linear PAN molecules into heat-resistant stable trapezoidal structures and generate an initial structure favorable to the formation of graphite-like structures.The diffusion behavior of oxygen in the environment from skin to core of the fiber during the pre-oxidation process can lead to the formation of radially nonuniform structure of the fiber,which affects the mechanical properties of the subsequent carbon fiber.Therefore,regulating the diffusion kinetic behavior of oxygen in fibers and improving the radial structural nonuniformity of pre-oxidized fibers will be beneficial to improve the mechanical properties of carbon fibers.In this thesis,the kinetics of oxygen diffusion in fibers was influenced by changing the content of oxygen in the ambient atmosphere,and a homemade heat treatment device was used to pre-oxidation treatment of PAN precursor at different oxygen contents,and the analytical results were combined with DSC analysis,¹³C solid-state NMR analysis,and Optical Density characterization to investigate the mechanism of the role of ambient oxygen in the pre-oxidation of PAN precursor.The influence of the coupling effect between the pre-oxidation structure and the oxygen content on the pre-oxidation behavior of PAN fibers was investigated by pre-oxidation treatment of fibers with different initial pre-oxidation structures at different oxygen contents,followed by further study of the inheritance and evolution of the coupling effect in the subsequent pre-oxidation process of the fibers.The results showed that:（1）In the pre-oxidation process of PAN fiber,under the same heat treatment conditions,the increase of ambient oxygen content can effectively improve the degree of pre-oxidation reaction of fiber.At 225℃treatment for 10min,the degree of pre-oxidation of fiber increased from 10.80%at 3%oxygen content to 14.64%at 21%,and the degree of pre-oxidation of fiber increased when the pre-oxidation time was extended to 40 min.The effect pattern of oxygen content remained the same.The effect of oxygen content on the pre-oxidation reaction of fibers was greater when the pre-oxidation temperature was increased to 235°C for a shorter treatment time（10 min）than at 225°C,but at a longer treatment time the effect of oxygen content decreased（40 min）,although the pre-oxidation degree of fibers increased.（2）At lower pre-oxidation temperatures,it takes a certain reaction time for fibers to exhibit radial structural differences,and the fibers showed obvious radial structural differences when treated at 225°C for 40 min with 3%oxygen content,as the oxygen content increased,the radial structural differences of fibers tend to slow down,the degree of radial differences decreased from 1.34%to 0.52%,the reason was that the degree of reaction in the fiber core was more influenced by the oxygen content.The pattern of the effect of oxygen content on the radial structure of fibers was basically the same when the pre-oxidation temperature was increased.（3）The coupling effect between oxygen content and fiber structure is related to the degree of pr-oxidation that the fibers already have.For the initial pre-oxidation structure fibers with a cyclization index of 24.58%,the oxygen content increased from 3%to 29%,the radial structure difference of the fibers decreased from 1.49%to 0.31%.However,after subsequent pre-oxidation at248-260℃,the radial difference of the fibers increased to 2.99%,which was less than 3.95%of the fibers obtained by traditional air atmosphere pre-oxidation.However,it also indicates that early high-oxygen treatment had limited effectiveness in improving the radial structure of fibers.（4）For mid-stage pre-oxidation structural fibers with a cyclization index of 35.87%,with the same oxygen content change,the difference in fiber radial structure decreased from 2.09%to 0.93%.After two temperature zones of 254-260℃heat treatment,the difference in fiber radial structure became 1.78%.Compared with early intervention,mid-stage oxygen content intervention had a better effect on improving fiber radial structure.After heat treatment of pre-oxidized structural fibers with a cyclization index of 45.34%,the difference in radial structure of the fibers increased from 2.35%to 2.93%.After the subsequent pre-oxidation process at 260℃,the difference in radial structure of the fibers became 2.31%.Comprehensive comparison showed that high oxygen treatment in the middle stage of pre-oxidation had the best effect on improving the radial structure of fibers.