Carbon yield from pitches

The carbon yield from pitches can be understood by studying the yields of the generic components of pitch using the method of corbett (1,2). The amounts of saturates, napthene-aromatics, polar aromatics asphalentenes can be determined for different pitches for various... 

There are also large differences in yields the yield of carbon fibers from rayon is between 20 and 25%, from polyacrylonitrile 45 to 50% and from pitch around 75 to 85 %. The high yield from pitch is a fundamental reason for the great efforts which are being made worldwide to bring about wider use of pitch as a precursor for carbon fibers. 

In the early 1960s, polyacrylonitrile (PAN) fibers afforded a total carbon yield after pyrolysis that was higher, and high strength carbon fibers were obtained by stretching PAN fibers in steam and oxidizing them under stress before carbonization. Carbon fibers from pitch precursors are a more recent development. Pitches are low value residues of the petroleum industry. 

Pitch as a precursor material is cheaper than PAN as a precursor fiber, but the conversion of pitch into mesophase pitch and subsequent fiber formation is complex and costly. When a pitch is not transformed into a mesophase and is spun as an isotropic liquid, the resulting carbon fibers have extremely poor mechanical properties. These considerations explain why more than 90% of today s carbon fibers are fabricated from PAN based precursors. Processes for fabricating carbon fibers from PAN or pitch based precursor fibers differ in important aspects, but also share important commonalties (Figure 2). Finally, the carbon yield from PAN based precursor fibers is 50%, that from mesophase pitch is 70-80%, and that from rayon is 25%. 

Figure 14.13 Effect of carbonization pressure on carbon yield from petroleum pitch. Source Reprinted with permission from Lachmann WL, Crawford SA, McAllister LE, Proc Int Conf on Composite Mats, Met Soc of AIME, New York, 1978. Copyright 1978, The Metallurgical Society of AMIE, now TMS (The Minerals, Metals and Materials Society).

Table 4 summarizes the yield of soluble pitch for the hydrogenation experiments. Hydrogenation of WVGS 13407 at 350°C increased the pilch yield from about 66 to 84 wt% Although the incremental yield between untreated and hydrogenated coal is only 18 wt%, there were significant differences m the properties of the pitches in terms of their carbonization behavior. 

The first high-strength carbon fibres were produced in the 1950s (see Donnet and Bansal, 1984). The early carbonized products were rayon-based, but it was soon found that the mechanical properties and the carbon yield could be improved by the use of polyacrylonitrile (PAN) as the precursor. Also, less expensive fibres of somewhat lower strength and modulus could be made from various other precursors including petroleum pitch and lignin. However, cotton and other forms of natural cellulose fibres possess discontinuous filaments and the resulting mechanical properties were consequently found to be inferior to those of the rayon-based fibres. 

Polyacrylonitrile (PAN) precursor fibers are more expensive than rayon. Nevertheless, PAN is more commonly used because the carbon fiber yield is about double that from rayon. Pitch-based carbon fibers are also important, because, potentially pitch is perhaps the cheapest raw material. Table 8.2 shows that carbon yield is highest from the mesophase pitch. The reader is cautioned that this is true only if we exclude the losses during the mesophase conversion step. If, however, one compares the overall carbon fiber yield from raw pitch to that from PAN, then the yield from PAN is higher. In any event, the carbon fiber yield or precursor weight loss is a very important factor in the economics of processing. 

The overwhelming success of PAN-based carbon fibers over rayon and pitch can be attributed to several key aspects.f Structurally, PAN has a faster rate of pyrolysis without much disturbance to its basic structure and to the preferred orientation of the molecular chains along the fiber axis present in the original fiber. By contrast, carbon fibers from rayon suffer from extremely low carbon yield (20-25%) due to chain fragmentation, which eliminates the orientation of the precursor fiber. While improved properties can be achieved by stretch graphitization, this process is expensive and does not compensate for the low yields. 

Fibers spun from polyvinyl alcohol, polybenzimidazoles, polyamides, and aromatic polyamides have been used as carbon fiber precursors. However, at present, the most attractive precursors are made from acrylonitrile copolymers and pitch, and a small amount from rayon. Today more than 95% of the carbon fibers produced for advanced composite applications are based on acrylic precursors. Pitch-based precursors are generally the least expensive, but do not yield carbon fibers with an attractive combination of tenacity (breaking strength, modulus, and elongation as those made from a acrylic precursor fiber). The acrylic precursors provide a much higher carbon yield where compared to rayon, typically 55% versus 20% for rayon, and this translates directly into increased productivity. 

Pitches can be derived from coal tar or petroleum and have been discussed as precursor materials for making pitch based carbon fibers (Chapter 4, Section 4.4). Pitches are oligomers and the composition will depend on the exact source and method of processing. A pitch with a high carbon yield and the ability to flow under high pressure should be selected. 

Carbon fibers made from the spinning of molten pitches are of interest because of the carbon yield approaching 99% and the relative low cost of the starting materials. 

The carbon fibre produced from the PITCH precursor is relatively low in cost and high in carbon yield compared to the PAN fibre, but from batch to batch the fibres tend to be non-uniform in their final cross-section. This does not generally cause a problem in the civil engineering industry but they are not used in the aerospace industry. 

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