Pitch precursors properties

Further improvements in the properties of PAN-based carbon fibers are likely to emerge through improved stabilization, that is, by creating the ideally cross-linked fiber. On the other hand, as purer pitch precursors become available, further improvements in mesophase pitch-based carbon fibers are likely to arise from optimized spinnerette designs and enhanced understanding of the relationship between pitch chemistry and its flow/orientation behavior. Of course, the development of new precursors offers the potential to form carbon fibers with a balance of properties ideal for a given application. 

Pitch precursor carbon fibers have the potential of providing good mechanical properties at low cost. 

Thermogravimetry/mass spectrometry was used to determine adsorptive capacity of several commercially available activated carbons produced from coal, coconut, and petroleum pitch precursors. The range of their N2 BET surface areas was between 400 to 2000 m /g. Although, carbons with high adsorption capacity contained similar C, N, and O contents, proximate analyses, surface areas and micropore volumes, no significant correlations were found between chemical and physical properties and the NO, adsorptive capacity. One possibly important characteristic of the carbons correlated with NO, adsorption capacity was specific and narrow pore size distribution with an effective pore diameter of 0.56 nm. 

The oxidation reaction starts at a temperature that depends on composition, structure, and microtexture. For example, the oxidation of carbon fibers derived from isotropic pitch precursor fibers in flowing air starts at about 400°C whereas that of the mesopitch based UHM fibers starts at a higher temperature [64]. At a given temperature, the variation of the weight loss, AJmo, as a function of time is linear, to a first approximation, within a rather large domain of weight loss [65-66]. This property is used to characterize the oxidation rate with a kinetic constant, k, defined as ... 

Fibers from a pitch precursor are graphitic and for a given process temperature, can attain higher moduli than PAN based fibers, approaching the value for the graphite crystal ( 1000 GPa). Table 20.4 gives the properties for pitch based carbon fibers. 

Chun et al. [93] produced carbon nano fibers with diameter in the range from 100 nm to a few microns from electrospim polyacrylonitrile and me-sophase pitch precursor fibers. Wang et al. [94, 95] produced carbon nanofibers from carbonizing of electrospun PAN nanofibers and studied their structure and conductivity. Hou et al. [96] reported a method to use the carbonized electrospun PAN nanofibers as substrates for the formation of multiwall carbon Nanotubes. Kim et al. [14, 97] produced carbon nanofibers from PAN-based or pitch-based electrospim fibers and studied the electrochemical properties of carbon nanofibers web as an electrode for supercapacitor. 

Examples of polymers which form anisotropic polymer melts iaclude petroleum pitches, polyesters, polyethers, polyphosphaziaes, a-poly- -xyljlene, and polysdoxanes. Synthesis goals iaclude the iacorporation of a Hquid crystal-like entity iato the maia chaia of the polymer to iacrease the strength and thermal stabiHty of the materials that are formed from the Hquid crystal precursor, the locking ia of Hquid crystalline properties of the fluid iato the soHd phase, and the production of extended chain polymers that are soluble ia organic solvents rather than sulfuric acid. 

Carbon fibers are generally typed by precursor such as PAN, pitch, or rayon and classified by tensile modulus and strength. Tensile modulus classes range from low (<240 GPa), to standard (240 GPa), intermediate (280—300 GPa), high (350—500 GPa), and ultrahigh (500—1000 GPa). Typical mechanical and physical properties of commercially available carbon fibers are presented in Table 1. 

Because of their unique blend of properties, composites reinforced with high performance carbon fibers find use in many structural applications. However, it is possible to produce carbon fibers with very different properties, depending on the precursor used and processing conditions employed. Commercially, continuous high performance carbon fibers currently are formed from two precursor fibers, polyacrylonitrile (PAN) and mesophase pitch. The PAN-based carbon fiber dominates the ultra-high strength, high temperature fiber market (and represents about 90% of the total carbon fiber production), while the mesophase pitch fibers can achieve stiffnesses and thermal conductivities unsurpassed by any other continuous fiber. This chapter compares the processes, structures, and properties of these two classes of fibers. 

In the sixth paper of this chapter, Kierzek et al., mainly focus on modeling of pore formation vs surface area growth phenomena upon activation of coal and pitch-derived carbon precursors. These authors briefly touch on other precursor carbons as well. The properties of newly synthesized materials are being looked at from the point of view of their application as active materials in the supercapacitor electrodes. Editors thought this work by the Institute of Chemistiy and Technology of Petroleum and Coal in Poland, could be of genuine interest to the practical developers of carbon materials for the supercapacitor industry. 

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