Effect Of Structure Of PAN Pre-oxidized Fiber On Thermal Weight Loss Of Fiber

The infusible and non-combustible pre-oxidized structure formed by PAN fiber during heat treatment(also known as pre-oxidation)lays the structural foundation of carbon fiber.The pre-oxidized structure and its radial distribution have a critical impact on the structure and property of the final carbon fiber,because the pre-oxidized structure and its distribution affect the cracking and recombination behavior of the fiber in a high temperature environment,and the cracking behavior is directly expressed as the thermal weight loss of the fiber.In this thesis,FT-IR,NMR,XPS,optical density method,DSC,TGA and other research methods were used to study the evolution process of pre-oxidized structure and its radial distribution of PAN pre-oxidized fiber during pre-oxidation of PAN fibers.And we also stydied the effects of temperature factors and fiber diameter factors on the thermal weight loss of fibers.Research indicates:1.During the pre-oxidation process,the cyclization reaction,oxidative dehydrogenation reaction and oxidation reaction of the fiber macromolecules occur continuously with the increase of temperature,which cause the content of linear molecular structure in fiber decreases.After the macromolecule is formed into a ring,the C=N and CH=C structure is formed and the ring structure is oxidized to form a C=O structure,and the relative content of those structure is gradually increasing.Among them,the ring structure and the conjugated structure represented by C=N and CH=C functional groups have higher temperature resistance characteristics,while the oxygen-containing structure represented by C=0 is prone to thermal cracking at higher temperature.Changing the pre-oxidation temperature results in a change in the content of each functional group in the pre-oxidized fiber,thereby causing a change in the thermal weight loss of the fiber.The PAN fibers is treated in a certain temperature range,the relative contents of C=N,CH=C and C=0 structures in the fibers are increased.However,the temperature-resistance effect caused by the ring structure and the conjugate structure is much larger than that caused by the oxygen-containing structure,and the thermal weight loss phenomenon of the fiber as a whole is weakened.2.The initial temperature at which the skin-eore structure is formed has a significant effect on the skin-core structure of the fiber,as the initial temperature increases,the skin area ratio of the fiber decreases.The reason is that when the initial temperature is raised,the oxidation rate of the ring structure in the fiber is accelerated,the formation rate of the dense structure is accelerated,and the hindrance to oxygen diffusion is enhanced,so that the diffusion depth of oxygen is decreased.3.Pre-oxidation studies on PAN fibers of different diameters showed that fibers with relatively large diameter(11.8 ?m)exhibited a distinct skin-core structure,the distribution of the degree of pre-oxidation of the fiber in the radial direction exhibits inhomogeneity,and as the pre-oxidation progresses,the degree of inhomogeneity of the distribution of the degree of pre-oxidation of the fiber in the radial direction tends to be severe.For fibers with a relatively small diameter(9.2 ?m),the homogeneity of the distribution of the degree of pre-oxidation of the fiber in the radial direction is better under the same pre-oxidation conditions.It is indicated that the PAN fiber has a radial distribution characteristic of the pre-oxidized structure during the pre-oxidation process.Under the same pre-oxidation conditions(the final pre-oxidation temperature is 260 ?),the relative content of C=O and CH=C structures in small-diameter fibers is higher than that in large-diameter fibers,while the relative content of C=N structures is lower than that in large-diameter fibers.After carbonization at the same condition(the maximum processing temperature is 1350?),the small-diameter fibers exhibit higher thermal weight loss,and the thermal weight loss of the two diameter pre-oxidized fibers differs by 1.38%;when the final pre-oxidation temperature reaches 270?,the difference in thermal weight loss between the two diameter pre-oxidized fibers increases,reaching to 2.90%.4.Under the same pre-oxidation conditions(the final pre-oxidation temperature is 260?),the oxygen atom percentage(13.34%)of the surface of the small-diameter fibers is higher than that of the surface of the large-diameter fibers(10.47%),we think this is based on the oxygen diffusion mechanism in the pre-oxidation process.When the fibers of different diameters are in the same oxygen content environment,oxygen molecules obtained per unit area of fiber diffuse into the interior of the fiber along the diameter direction and undergo oxidation reaction with the fiber.For PAN fibers of the same molecular structure and fiber structure,the diffusion rate of oxygen molecules in the fiber is same.Therefore,in a certain period of time,the diffusion volume corresponding to the small-diameter fibers is small,and the relative density of the oxygen molecules is large,resulting in a high oxygen content enriched on the surface of the fiber.5.The difference in PAN fiber pre-oxidized structure leads to the difference in its final carbon fiber structure.It is found that the small-diameter carbon fiber has higher graphitization degree than the large-diameter carbon fiber,and the tensile modulus(285GPa)is also higher than that of large-diameter carbon fiber(271 GPa)at the same pre-oxidation and carbonization conditions.In addition,the tensile strength(4.25 GPa)of the small-diameter carbon fiber is higher than that of the large-diameter carbon fiber(4.13 GPa)due to the better homogeneity of its radial distribution of the pre-oxidized structure.