Influence of continuous stabilization on the physical properties and microstructure of PAN‐based carbon fibers

Abstract

SYNOPSISA continuous stabilization and carbonization process was used to prepare polyacrylonitrile (PAN) -based carbon fibers. The stepwise stabilization of PAN fibers was tried at various temperatures. The effect of stepwise stabilization on the physical properties and microstructure of the final carbon fibers is reported in this article. The fixed temperature in stepwise stabilization is kept below the fusion temperature of PAN precursors to avoid overstabilization of the fibers. The optimum stepwise stabilization process not only increases the amount of ladder polymer in stabilized fiber but also improves the physical and mechanical properties of the resultant carbon fibers. The formation of closed pores from open pores in carbon fiber occurs at llOO°C, but the formation of closed pores occurs at 200°C lower for carbon fiber developed from overstabilized fiber. The effect of continuous stepwise stabilization on the properties of resulting stabilized fibers and the variation in physical properties, element composition, and microstructure of carbon fibers during the carbonization process are also reported in this article.

A review of heat treatment on polyacrylonitrile fiber

“…Sometimes, when the temperature increased up to 1300 C, the carbonized PAN fiber could achieve 96% carbon content [31]. The increase in the carbon would decrease the nitrogen, hydrogen and oxygen content [25,31,69]. Table 1 shows the percentage of nitrogen and hydrogen which was released from the fiber and the increase of carbon content when the temperature rises.…”

“…As a consequence, the air would be trapped inside the fibers and hence results in low density which could limit the tensile strength of the final carbon fiber [25]. However, Ozbek and Isaac [79] and Sauder et al [101] observed that heating temperature (HTT), which increases up to 3000 C, can eliminate the effect of open and closed pores.…”

“…High stabilization temperature promotes over absorption of oxygen in stabilized fiber and might form excessive eC]O bonds. Usually, the oxygen in these bonds escapes as water vapor [25]. It is known that the decrease of oxygen as water vapor is due to evolution of H 2 O in the early stages of carbonization in the range between 300e500 C. The evolution of H 2 O results from the crosslinking condensation reactions between two monomer units of the adjacent ladder polymeric molecular chains which is illustrated in Fig.…”

“…Through the heating process, the fiber could expel impurities as volatile by-products such as methane (CH 4 ), hydrogen (H 2 ), hydrogen cyanide (HCN), water (H 2 O), CO 2 , NH 3 and various gases [25,33,35,82]. Among that gases, HCN, NH 3 and CO are the toxic compounds that evolved during pyrolysis [83].…”

“…Recently, the stabilization process is found to play an important role in converting PAN fiber to an infusible stable ladder polymer that converts C^N bonds to C]N bonds [25] and to develop crosslink between molecules of PAN [26] which tend to operate at high temperatures, with minimum volatilization of carbonaceous material. The thermal stability of the stabilized fiber is attributed to the formation of the ladder structure due to cyclization of the nitrile groups in acrylic molecule.…”