Fiber rayon precursor processing

Two categories of pitch-based fiber exist isotropic carbon fiber produced from an isotropic pitch precursor, and an oriented, anisotropic fiber produced from a mesophase pitch precursor. Isotropic fibers were developed from low melting point isotropic pitches The precursor was melt-spun into fibers, which were oxidized to render them infusible, and then carbonized. Their low strengths and moduli make these fibers unsuitable for use in advanced composites. Orientation was accomplished by a hot-stretching process (>2200°C), but it is accompanied by the same processing difficulties encountered in the rayon precursor process. A different approach was suggested by the discovery of carbonaceous mesophase. ... 

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%. 

Several books and reviews have been published which detail the conversion of viscose rayon to carbon fibers [1-7], Chapter 3 has described how carbon fiber first came onto the scene, way back in 1880, with the introduction of Thomas Edison s electric lamp filaments made from cellulosic precursors. Almost 80 years later, in 1959, the National Carbon Company (a division of Union Carbide) introduced a carbon cloth from a rayon precursor, to be followed by a carbon yarn in 1961. These products are described by Cranch [8]. The best grade on offer was WYB cloth, which was processed at 2200°C and although called graphite, was a form of carbon that was non-graphitizing. 

Little information is available on the production process and a schematic layout of the preparation of carbon fiber from a rayon precursor is shown in Figure 6.4 and each stage will be considered separately. 

Carbon fibers can be made by pyrolysis of a hydrocarbon precursor. Rayon was one of the first precursors used to make carbon fibers. During the processing of Rayon fibers into carbon fibers, only 25% of the fiber mass is retained. This made carbon fibers manufactured from Rayon precursors very expensive. Another precursor that has proved to be economical is the polyacrylontrile... 

Each of the three major precursor processes has its own special place in history and will be discussed in turn. Rayon-based fibers were first in commercial production (in 1959) and led the way to earliest applications, which were primarily military. PAN-based fibers have proven to be superior to rayon-based fibers in several respects, notably in tensile strength, and have largely dominated the explosive growth of the industry since 1970. Pitch-based fibers, however, are uniquely capable of achieving extremely high axial Young s modulus and thermal conductivity and, therefore, have an assured place in critical military and space applications. 

Japan. Shindo recognized the importance of an oxidative heat treatment step prior to carbonization in reducing the processing time and improving the carbon yield from PAN. He demonstrated good tensile strengths and moduli over 20 million psl, about three times those available from rayon precursor carbon fibers of that time. Shlndo s work was quickly taken up... 

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. 

The pyrolysis of organic fibers used as graphite precursors is a multistage process. The three principal graphite precursors are PAN, pitch, and rayon, with PAN as the predominant product. 

Soltes and Abbot of USA during 1955 developed processes for converting both natural cellulose and rayon into fibrous carbon. Essentially the carbon fibers were produced by heat-treating the precursors to temperatures about 1,000C (1,832F) in inert atmosphere. Fiber tensile strengths were as high as 40 ksi (275 MPa). 

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