Control Of The Radial Structure Of PAN Fibers During Pre-oxidation Under Positive Pressure

The pre-oxidation process is a key link in the preparation of PAN-based carbon fibers,but it’s easy to form a radially uneven skin-core structure due to the competitive mechanism of the capture by cyclization structure and oxygen diffusion on the surface of the fiber.This radial structure,which will increase the structural defects of carbon fibers and limit the improvement of mechanical properties,will be inherited in the following carbonization treatment.It is of great significance for the development of new high-performance carbon fibers to explore how to effectively control the radial structure of PAN pre-oxidized fibers.In this paper,the effect of pressure on the chemical structure of PAN pre-oxidized fibers was investigated by preparing PAN pre-oxidized fibers under different ambient pressures.The evolution of the radial structure under different pressures was extracted and summarized.On this basis,time was introduced to study the synergistic effect mechanism of these two process parameters.Then we used thermal gravimetric analyzers to simulate the carbonization process and studied the relationship between the radial structure and thermal stability of pre-oxidized fibers.The main research results show that:（1）In the pre-oxidation process,compared with samples prepared under atmospheric pressure,PAN pre-oxidized fibers prepared under positive pressure have a large skin area,the skin area increased from 88.11%under normal pressure to 95.35%under 22k Pa.The reaction area expands to the core and the radially uneven skin-core structure is effectively improved.When the pressure exceeds 22k Pa,the level of PAN macromolecular reaction in the core increases rapidly,and the skin area increases from 95.35%under 22k Pa to 98.67%under30k Pa,and the radial structure changes from the skin-core structure to a homogeneous structure.With the help of solid ¹³C-NMR,we find that the level of cyclization greatly increases from 0.318 to 0.458 as pressure increased from14k Pa to 22k Pa,indicating that the pressure promoted the oxygen diffusion process,broke the barrier of the surface layer of the fiber,and promoted the level of cyclization in the core to be greatly improved.When the pressure continues to increase to 30k Pa,the level of cyclization tends to be flattened from0.458 to 0.496,and the level of dehydrogenation and oxidation is significantly enhanced.It indicates that pressure promotes chemical reactions in the core of the fiber,moving towards oxidation and dehydrogenation in a positive direction.（2）Under a treatment temperature of 225℃,the skin-core optical density difference of PAN precursor fiber pre-oxidized under 14k Pa after treatment times of 20min,40min and 60min is 0.0319,0.0375,0.0375.It shows that the phenomenon of skin-core inhomogeneity of pre-oxidized fiber under low pressure is obvious,and the radial heterogeneity of pre-oxidized fiber is increased within a certain range of time,as the ring structure captures oxygen dominantly.The skin-core optical density difference of PAN precursor fiber pre-oxidized under 30k Pa after treatment times of 20min,40min and 60min is0.0197,0.0196,0.0189.It shows that oxygen diffusion dominates under high pressure,and the radial structure of PAN pre-oxidized fiber is more uniform.（3）Under a treatment temperature of 225℃and treatment time of 40min,the carbonization yield at 800℃of pre-oxidized fiber prepared increases from55.30%to 65.58%with the increase of pressure from 14k Pa to 30k Pa,which is higher than the carbonization yield of 52.48%of the pre-oxidized fiber under 0k Pa.It indicates that the pre-oxidized fiber prepared with positive pressure has a higher carbonization yield.The carbonization yield at 500℃of pre-oxidized fiber increases from 62.31%to 74.69%,which is due to the improvement of radial structure uniformity with positive pressure,the pre-oxidized fiber generates more ring structures and improves thermal stability.（4）Under a treatment temperature of 225℃,the weight loss at 800℃of PAN pre-oxidized fiber under 14k Pa decreases from 49.53%to 44.70%,and then to 38.92%as the treatment time increases from 20min to 40min and then to 60min.The weight loss at 800℃of PAN pre-oxidized fiber under 30k Pa decreases from 40.21%to 34.42%,and then to 34.22%as the treatment time increases from 20min to 40min and then to 60min.The results indicate that with the increase in treatment time,the content of characteristic structure in PAN pre-oxidized fiber increases,and the thermal stability is improved.The radial structure of PAN pre-oxidized fiber is relatively uniform at high pressure.As treatment time increases,the change of thermal weight loss is small,and the time-dependent effect of the increase of PAN pre-oxidized fiber characteristic structures is not obvious.It shows that the improvement of radial uniformity is more conducive to the improvement of thermal stability.