Review articleA review of heat treatment on polyacrylonitrile fiber
Introduction
It has been documented that the majority of all carbon fibers used today are made from PAN precursor, which is a form of acrylic fiber. PAN which is a polymer with a chain of carbon connected to one another (Fig. 1) is hard, horny, relatively insoluble, and a high-melting material [1]. It has been established that PAN-based carbon fiber is stronger than other type of precursor-based carbon fiber [2]. PAN-based fibers also have been found to be the most suitable precursors for producing high performance carbon fibers (compared to pitch, rayon, etc.) generally because of its higher melting point and greater carbon yield (>50% of the original precursor mass) [3], [4], [5], [6], [7]. Although carbon fiber can be from pitch precursor, the processing and purifying it to the fiber form is very expensive and generally, they are more expensive than PAN-based fibers [8]. PAN with molecular formula [C3H3N]n can produce carbon fiber of relatively high carbon yield giving rise to a thermally stable, extremely oriented molecular structure when subjected to a low temperature treatment [9]. PAN fiber was also preferred to be the precursor because of its fast rate in pyrolysis without changing its basic structure [9]. Optimizing the pyrolysis of PAN precursor fiber would ideally result in enhanced performance of the resulting carbon fiber.
Recent study has established that PAN fibers were used on a large scale in textile industry and one of the most suitable and widely applied for making high performance carbon fibers [10], [11], [12], [13]. Most PAN-based carbon fibers extensively applied in last two decades were used in the composite technology [14]. They are highly desirable for high performance composites for automotive and aerospace technologies due to their enhanced physical and mechanical characteristics [9]. Fitzer [15] and Chen and Harrison [16] believed that the optimization of PAN fiber would ideally result in high performance for use in aerospace application. Hence PAN-based fiber that leads to a good balance in properties can be used in structural applications and provide high strength [2].
Year by year there will be an improvement on performance as well as strength and modulus of PAN-based carbon fiber [17]. Traceski [18] stated that the total worldwide production of PAN-based carbon fiber was 19 million lbs per year for 1989 and increased up to 26 million lbs per year. In addition, the worldwide outlook for the demand of PAN carbon fibers is currently amounting to a nearly $6 billion pound per year worldwide effort [19], [20]. So, the wide availability of PAN precursor had triggered the production of carbon fiber.
Heat treatment is a process that converts the PAN fiber precursor to carbon fiber. Currently 90% of all commercial carbon or graphite fibers are produced by the thermal conversion of a PAN precursor, which is a form of acrylic fiber. The successful conversion of PAN to high strength, high modulus fibers depend in part upon the understanding of the oxidative and thermal treatment. Liu et al. [21] listed the three steps for the conversion of precursor of PAN-based fiber to carbon, which are as follows.
- i.
Oxidative stabilization, which forms ladder structure to enable them to undergo processing at higher temperatures.
- ii.
High temperature carbonization, (≤1600 °C) to keep out noncarbon atoms and yield a turbostatic structure.
- iii.
Further heat up to 2000 °C to improve the orientation of the basal planes and the stiffness of fibers, which is called graphitization.
Section snippets
Precursor stabilization
Among the conversion processes shown in Fig. 2, an essential and time-consuming step in the conversion of PAN fibers to high performance carbon fiber is the oxidative stabilization step [7]. This can be explained by chemical reactions that are involved in this process, which are cyclization, dehydrogenation, aromatization, oxidation and crosslinking which can result in the formation of the conjugated ladder structure [22], [23]. The oxidative stabilization stage is one of the most complicated
Carbonization
Carbonization was an aromatic growth and polymerization, in which the fiber would undergo heating process at a high temperature up to 800–3000 °C, typically to a 95% carbon content [31]. Carbonization at 1000 °C will produce carbon fiber in low modulus type and intermediate modulus or type II carbon fiber will produce at up to 1500 °C [13], [16], [31], [66]. Trinquecoste and group [67], also observed that heating process around 1000 °C produced high tensile strength fiber, and for high modulus
Graphitization
For further improvement on the performance, carbonized fiber must undergo graphitization process. Graphitization is the transformation of carbon structure into graphite structure by heat treatment as well as thermal decomposition at high temperature processing. Actually, the process of production of both carbon fiber and graphite fiber was essentially the same either in carbonization or in graphitization. During graphitization the temperature does not only rise until 1600 °C, but exceeds up to
Functionality gaseous
Generally, carbonized fiber can be found when the temperature reaches 1200 °C and above in inert atmosphere [76]. Through the heating process, the fiber could expel impurities as volatile by-products such as methane (CH4), hydrogen (H2), hydrogen cyanide (HCN), water (H2O), CO2, NH3 and various gases [25], [33], [35], [82]. Among that gases, HCN, NH3 and CO are the toxic compounds that evolved during pyrolysis [83]. But, HCN and NH3 are the major toxic gases that evolved from decomposition of
Effect of heating treatment on PAN-based carbon fiber properties
The characteristics of PAN-based carbon fiber could be measured through infrared spectra. The infrared spectrum would identify whether the PAN fiber was stabilized and carbonized or not. Sometimes the characteristic was measured by physical properties as well as the diameter and the density of the fiber. There was a relationship between diameter, density and performance of carbon fiber. Mittal et al. [38] observed that generally when the diameter decreased, the density would be increased. In
Acknowledgement
The authors would like to thank MOSTI for funding this project and National Science Fellowship (NSF) for developing PAN-based carbon fiber.
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