Study On Processing, Physical And Chemical Behavior During Carbonization Of PAN-Based Stabilized Fiber

Carbonization of the stabilized fiber is one of the essential processes which determine the quality of the resulting carbon fiber during the preparation of the polyacrylonitrile (PAN) -based carbon fiber. In order to obtain high quality carbon fibers, it is important to carry out deeply study on the carbonization of the stabilized fiber. The researches on the physical and chemical changes, structure evolution, as well as the relation between processing, structure and properties during carbonization can provide academic help for the preparation of the high-quality carbon fiber. In this work, a series of stabilization and carbonization experiments were performed on a pilot line by using home precursor fiber as the raw material fiber. The stabilized fibers which can be used for carbonization study were prepared by exploring the stabilization processing parameters. Then, the thermal behavior and structure evolution of the stabilized fiber during carbonization were investigated in detail. Several technologies, such as differential scanning calorimetry (DSC), thermal gravimetry (TG), Fourier transform infrared spectroscopy (FTIR), elemental analyzer (EA), X-ray diffraction (XRD), electronic spinning resonance spectrom (EPR) and high resolution transmission electron microscopy (HRTEM) were used to systemically characterize the structure and feature of the samples. At the same time, the influences of processing conditions on the changes of the properties of carbon fibers were also discussed.The stabilization processing parameters, such as the initial temperature, the highest temperature, the temperature gradient distribution, the running speed and the stretching ratio were confirmed by consider systematically the various stabilization indices and factors, like exothermic property of the PAN fiber, the oxygen content increment of the stabilized fiber with raising stabilization temperature, the density, aromatization extent and skin-core structure of the stabilized fiber, and so on. The stabilized fiber which can produce the T300-level carbon fiber were prepared by using the optimal stabilization processing parameters. These stabilized fiber were carbonized further for carrying out the carbonization study.The investigation on the thermal behavior of the stabilized fiber during carbonization shows that the exothermal reactions happen when the stailized fiber was carbonized at low temperature, while endothermal reactions appear at high-termperature carbonization stage. The heat liberation at low-temperature carbonization stage is the result of conjugation reaction, aromatization reaction and cross-linking reaction which are facilitated by the oxygen element of the stabilized fiber. These exothermic reactions further stabilize the strcutre of the stabilized fiber. Additionally, the stabilization extent of the stabilized fiber, the heat treatment atmosphere and the heating rate all have effects on the exotherm of the fiber. The endothermal peak and endothermic quantity during high-temperature carbonization, which are reduced by the pyrolysis of the unstable structure, are closely related to the oxygen content and stabilization extent of the stabilized fiber. The further studies indicate that the heating treatment in oxygen-free atomosphere is better than that in air; the heating treatment at low velocity during low-temperature carbonization is beneficial to the increasing of the yield of the carbon fiber; the oxygen element in stabilized fiber is important to the preparation of the high-quality carbon fiber, because it not only can increase the stability of the structure and avoid the hard endothermic reactions, but also can facilitate the pyrolysis reaction to start as early as possible.The structure of the stabilized fiber was further studied by FTIR, XRD and LRS. From the view of the chemical structure, the macromolecules of the stabilized fiber are composed of some aromatized ladder segments which are connected by linear segments, accompanying the intro- and inter-molecular cross-linking; the aromatized ladder segment is composed of 2-4 aromatization cycles. From the view of the phase structure, the stabilized fiber is constitutive of the most amorphous structure, a few linear segments order regions and a few aromatized ladder segments order regions.The study on the structure evolution of the stabilized fiber during two-step carbonization by FTIR, EA, XRD, EPR and LRS indicates that the total carbonization process can be divided into three stages. The first is the further stability stage of the fiber structure at 300～450℃. At this stage, the exothermal reactions like cross-linking reaction and aromatization reaction happen, which can improve the stability of the fiber structure and result in the increase of the aromatization extent and compactability. The structure of the fiber still consists of linear segments and ladder polymer segments; but the former gradually reduces and the latter increases. The second stage is at 450～750℃, during which the aromatization plane grows and converts to the carbon basal plane. The linear segments pyrolyzes inregularly in large scale to produce much free radicals. The pyrolysis reaction happens in short time and produce a great deal of free radicals. After pyrolysis, the fiber is mainly compose of the ladder aromatization structures which convert gradually into carbon basal planes with raise of the temperature. The third stage is at 100-1400℃, during which turbo graphite micro-cystal forms and grows. The non-carbon elements are taken off from the fiber and the turbo graphite structure form resulted from the growth of the carbon basal planes. Till 1400℃, the crystallinity of the carbon fiber is nearly up to 50%. However, there are mass structure defects lying in the edge of the graphite planes or between graphite planes, indicating that the graphite crystalline region is not perfect.The microstructure of the stabilized fiber and the carbonized fiber at different temperature were observed and the effects of the carbonization temperature on the fine structure of the fiber were explored. The experimental results show that the HRTEM images of the stabilized fiber present typical amorphous disorder maze-like clusters, which reflects the amorphous structure of the fiber. After carbonized at 600℃, the HRTEM images of the skin of the fiber are the complex of the line section with preferred orientation and amorphous. The line section is produced by the diffraction of the tiny poly-cyclic plane or the carbon basal plane. However, the line section in the core of the fiber is without preferred orientation. There are some bend ribbon-like fringes appearing in the HRTEM image of the skin of the carbonized fiber after treated at 1000℃. The ribbon-like fringes are more than 10 nm in length and include 2-5 fringes in width, producing by the diffraction of the turbo graphite micro-crystall planes. Whereas, the ribbon-like fringes in the core of the fiber is anisotropic, without preferred orientation. The ribbon-like fringes become longer and wider with the increase of treatment temperature from 1000℃to 1400℃. There are the overlap, cross-linking and intertwist between ribbon-like fringes. The fringes in the core of the fiber are tiny, without forming the ribbon.The formation process of the turbo graphite ribbon structure during carbonization is reflected by the HRTEM images. After the amorphous stabilized fiber was carbonized at medium-temperatuer carbonization temperature, the ladder segments condense into the poly-cyclic planes or carbon basal plane. Their reorganization bring the formation of the relative order region with axial orientation. After carbonized at 1000℃, the adjacent poly-cyclic planes or carbon basal plane connect together head-to-head to form longer carbon planes in the axial direction and more planes stack in the radial direction. So, the turbo graphite micro-cystals form. When the fiber was carbonized at 140Q℃, the crystal regions grow by three styles, which can be speculated from the morphorlogy of the ribbon-like fringes in the HRTEM images. One is two adjacent and parallel crystal regions combine together after reorganization in the direction of the axis; The second is the adjacent but unparallel crystal regions combine partially. And the third is the adjacent crystal ribbons connect together head-to-head to form longer ribbon in the radial direction, which brings the quick growth of the crystal region.The tensile strength of the carbon fiber can be greatly improved by adjusting the carbonization gradient temperature. The temperature gradient experiments were carried out in the three stages of structure evolution, respectively. The experimental results shows that the pre-carbonization treatment at 500℃is benefit to the improvement of the quality of the carbon fiber. However, the pre-treatment at 400℃is harmful to the preparation of the carbon fiber because the cross-linking reaction lows the the order of the fiber structure and aggravate the skin-core structure. In the experimatal range from 500℃to 700℃, the increase of the temperature can improve the tensile strength of the carbon fiber. In the high-temperature carbonization stage, the formation speed, quantity and crystal size of the turbo graphite structure quickly increase with raising temperature below 1200℃and then slowly grow above 1200℃. Accordingly, the tensile strength of the carbon fiber increase quickly at first below 1200℃and then slowly above 1200℃. The proper stretching ratio during low-temperature carbonization can restrain the thermal shrinkage, which can retain and perfect the orientation of the ladder polymer. The appropriate relax also effect on the strength of the carbon fiber. The on-line tension values can be used to judge whether the current stretching ratio applied is proper. The high-quality carbon fiber can be obtained when the on-line tension at pre-carbonization stage lies 150 cN/dtex or so, while the on-line tension at high-temperature carbonization stage lies 200-300 cN/dtex.