The Research Of Dimensional-Effect On The Thermal Reaction Proceading Of Pan Fibers’

In the process of preparing PAN-based carbon fibers, one of the key ultimate decision steps is the pre-oxidation process. In this stage, the elementary structure of carbon fiber is obtained and the anomalistic graphite structure of carbon fibers mostly depends on it. Along with appearance ceaselessly of PAN based carbon fibers with high performance, reducing diameter of fibers was attached importance to gradually by carbon fiber researcher. To improve the performance of carbon fiber the diameter of PAN fibers is reduced. This text manily studied the dimensional effect and pre-oxidation process which play an important role in preparing carbon fibers and affect of dimensional effect on thermal reaction process of PAN fibers. Also the fundamental reasons of the dimensional effect on carbon fiber properties had been explored.With different fineness of PAN fibers pre-oxidation experiments were done under certain conditions, by using FTIR, DSC, XRD, Raman, NMR and other testing methods, initially acquiring the difference of PAN fibers structure with different fineness after heating treatment. And the concept of "aromatic carbon without linking hydrogen" and naphthyridine-like structure had been proposed and these structures are called pre-oxidation characteristic structure in the paper. Main tasks are as follows:1、Through the structural analysis, thermal analysis, and analysis of skin-core structure in different stages, the evolution of the structure of PAN fibers in the pre-oxidation process had been analyzed. We find out:with the increasing of treating temperature, the degree of oxidation has been significantly improved. Parameters characterized of pre-oxidation such as relative cyclization rate, degree of cyclization, aromatization index and density are changed according to certain rules. Cyclization is main reaction in pre-oxidation process, dehydrogenation reaction and oxidation reaction severly occur in late reaction stage. The concept "aromatic carbon without linking hydrogen", and the relative content of "aromatic carbon without linking hydrogen" in the C=C which represent the mesh size of the structure by intramolecular and intermolecular cyclization in pre-oxidation process had been proposed, which will affect graphite sheet size through the cross-linking.2、To the different fineness of PAN fiber, morphology、aggregation structure and chemical structure were influenced by dimensional effect among the heat treatment process. The results showed that the dimensional effect of fiber influences core-shell structure obviously after heating treatment, the fiber which fineness is0.478dtex don’t have core-shell structure and the homogeneity of the fiber is good. But, there is obvious core-shell when fineness is0.758dtex and1.210dtex. Dimensional effect affected the cyclizing level of PAN fiber among heat treatment process; the lower fineness, the higher cyclizing level. The density of fibers sample treated at different temperature is indiscriminate. The dimensional effect also obviously influenced the molecular structure of fiber, the lower fineness, the higher relative amount of " aromatic carbon without linking hydrogen" in pre-oxidation structure, which means that the degree of intermolecular cross-linking is higher. The lower fineness is, the more the class naphthyridine structure is. Besides, the influence mechanism of dimensional effect on "aromatic carbon without linking hydrogen" and class naphthyridine structure had been provided in this paper. Owing to the fiber with low fineness has preponderance on thermal transferring and mass transfer substances, the aggregation structure of fibers with different fineness are unlike.3^The formation mechanism of pre-oxidation characteristic structure in the process of heating treatment of PAN fibers from the view of time effect, temperature effect and temperature range effect had been explored, further validation of the dimensional effect on pre-oxidation structure was got. The analyzing of extension of heat treatment time will be helpful in increasing the relative content of "aromatic carbon without linking hydrogen" and class naphthyridine structure. The formation of two pre-oxidation characteristic structures is greatly relative with temperature. Considering the factors of heating treatment time and temperature, reasonable temperature spectrums also will be helpful for the formation of pre-oxidation characteristic structure.4、Starting with the evolution of the pre-oxidation structure to carbon fiber structure during the preparation of carbon fiber, the relationship of the evolution of the pre-oxidation characteristic structure to carbon fiber structure effect on carbon fibers’structure and contribution to the modulus had been provided. From the analysis results, the more the number of heterocyclic aromatic nitrogen of pre-oxidation characteristic structures ("aromatic carbon without linking hydrogen" and class naphthyridine structure), the better the degree of graphitization of carbon fibers, which can increase the size of the conductive graphite layers. Thus, the contribution to the increase of the modulus was improved.5、This paper has four main innovations, firstly, the pre-oxidation characteristic structures are found which affect modulus of carbon fiber, which are called "aromatic carbon without linking hydrogen" and class naphthyridine structure. Secondly, the dimensional effects on the pre-oxidation characteristic structures have been presented. Nextly, the studying of the time-temperature effect in the process of the formation of pre-oxidation characteristic structures has been carried on. Finally, the relationship between pre-oxidation characteristic structures and modulus and a mechanism that pre-oxidation characteristic structures change to the structure of carbon fibers has been obtained.