A study on the Stabilization Process Affecting on Performance of PAN-based Carbon fiber

Abstract

1. Introduction

For making Polyacrylonitrile (PAN)-based carbon fiber having high-performance, there are three-step heat treatment processes including stabilization, carbonization, and graphitization. The stabilization is the one of the most important steps and it is essential to optimize the heat-treating conditions. This stabilization is typically conducted in the temperature range of 200 and 300 ℃, and the nitrile functional groups in PAN precursor change to ladder structure and become high thermal stability after stabilization process [1]. During the stabilization, the residence time is important considerations influencing the internal structure development for manufacturing carbon fiber. In this study, the relationship between the residence time in the stabilization process and the mechanical properties of the carbon fiber was analyzed by tensile test and wide angle X-ray diffraction to optimize the high-performance carbon fiber manufacturing process.

2. Experimental

2.1 Experimental methods The PAN precursor fiber was heat-treated using a continuous heat treatment facility in UNIST. The stabilization process was carried out under air atmospheric up to 280 °C for the controlled residence times of 90, 120, 150, and 180 min. The carbonization process consists of low-temperature and high-temperature carbonization, which was carried out at 700 °C and 1400 °C under N_2 conditions, respectively.

2.2 Characterization The structural characteristics of the fibers were analyzed by wide angle X-ray diffraction (WAXD) and the Herman’s orientation factor (f_002) was calculated to define the degree of orientation at (002) plane from azimuthal curves of (002) diffraction peak [2]. Also, the tensile properties of the precursor fiber, stabilized and carbonized fibers were tested using a single filament tensile testing machine (FAVIMAT+) at a gauge length of 1 inch. The cross-sectional surfaces of the stabilized and carbonized fibers were imaged using scanning electron microscope (SEM). Fig. 2. WAXD equatorial scans of precursor and stabilized fibers. (a) precursor, (b) 90 min, (c) 120 min, (d) 150 min and (e)180 min stabilized fiber.

3. Results and Discussion

In generally, precursor fibers occur the thermal shrinkage during the stabilization due to chain structural change. Here, to prevent shrinkage and allow the graphite sheets to align well, each side of rollers in continuous heating facility (Fig.1) prevent the thermal shrinkage of PAN molecular chains which can lower mechanical properties by inducing the amorphous or turbostratic structure. The WAXD results indicate that the peak intensity ratio ((002)/(200,110)) and orientation factor (f_002) according to the stabilization residence time. As the stabilization time increase, the intensity ratio and orientation factor increase, suggesting that the (002) plane structure is well developed. It can be confirmed that our system can control the shrinkage well. It is resulting a higher orientation factor than short residence time. Since then, in carbonization process, the orientation factor of carbon fiber tends to be proportional to the stabilized fiber’s orientation factor at same carbonization conditions, and the tensile modulus of carbon fiber is dependent on the orientation of (002) plane [3].

4. Conclusions and Future works

In this study, it has been confirmed that the stabilization time affected to the (002) plane orientation of stabilized fiber. In addition, the graphite sheet orientation in carbon fiber is one of the important factors affecting the tensile modulus, and it was confirmed that the orientation of carbon fibers is proportional to the orientation coefficient of stabilized fibers. In future research, our group plan to study how to optimize the stabilization residence time to produce high-quality carbon fibers with high tensile properties.