Carbon Fibers from Rayon

Rayon-based fibers were the first carbon fibers produced commercially. They were developed in the 1960 s specifically for the reinforcement of ablative components for rockets and missiles. However, they are difficult to process into high-strength, high-modulus fibers and have been replaced in most structural applications by PAN or pitch-based fibers. 

The conversion of rayon fibers into carbon fibers is a three-stage process  

1) Stabilization Stabilization is basically an oxidative process that involves different steps. In the first step, from 25 to 150 °C, there is physical desorption of water The next step is dehydration of the cellulose unit between 150 and 240 °C. Finally, thermal cleavage of the cydosidic linkage and sdssion of ether bonds and some C C bonds occurs via free radical reactions (240-400 °C) followed by aromatization. 

2) Carbonization Heat treatment between 400 and 700 °C converts the carbonaceous residue into graphite-hke layers. 

3) Graphitization Graphitization is carried out under strain at 700-2700 °C to obtain high modulus fibers through a longitudinal orientation of the planes. 

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Carbon Fibers from Rayon Precursors, Roger Bacon... 

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. 

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. 

Figure 6.11 Effect on the fiber modulus of stretching rayon fiber precursor up to 2800°C. Source Reprinted with permission from Bacon R, Carbon fibers from rayon precursors, Walker PL, Thrower PA eds.. Chemistry and Physics of Carbon, Marcel Dekker, New York, 1-102, 1974. Copyright 1974, CRC Press, Boca Raton, Florida.

Bacon R, Carbon fibers from rayon precursors, Walker PL, Thrower PA eds.. Chemistry and Physics of Carbon, Marcel Dekker, New York, 1-102, 1974. 

Carbon Fibers from Rayon and Other Celluloslc Precursors... 

From 1964 to 1975, spurred by the growing awareness of the potential properties of carbon fibers, high-modulus and high-strength carbon fibers from rayon, PAN, and pitch were invented, developed, and commercialized, opening up an explosive growth in the "high performance" composites Industry. 

Carbon Fibers from Rayon Precursors," by R. Bacon, in Chemistry and Physics of Carbon 9, P. L. Walker and P. A. Thrower, eds. (Marcel Dekker, New York 1973) pp." l-102. 

Industrial production of carbon fibers from rayon 1950 s... 

The higher carbon content is achieved when PAN is used as the precursor for carbon fiber production. Carbon fiber from rayon or pitch has lower carbon content. 

Reinforcing carbon fibers from reconstituted cellulose (rayon) ... 

Initial modern carbon fiber development occurred during 1944-1960 in the research and development Materials Laboratory of the Wright-Patterson Air Force Base (WPAFB), Dayton, OH, USA. The UK Royal Aircraft Establishment developed carbon fiber-RPs in the late 1950s. During this period carbon fiber produced from cotton and viscose rayon fabrics were principally used in military applications such as rocket nozzle cones and ablative surface panels on outer space vehicles (other fibers were also used). Barnaby-Cheney and National Carbon manufactured a small amount of carbon fiber from these fibers. 

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%. 

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. 

Figure 6.4 Schematic preparation of carbon fiber from a rayon precursor.

Figure 6.10 Effect of hydrogen chloride vapor on weight loss during pyrolysis of continuous filament high tenacity rayon fiber from Teijin Co. Source Adapted from Shindo A, Nakanishi Y, Soma I, Carbon Fibers from Cellulose Fibers, AppI Polym Symposia, No 9, 271-284, 1969.

Amoco Performance Products Inc., Greenville, South Carolina, USA—acquired Union Carbide s carbon fiber production at Greenville, where a range of carbon fibers from pitch, cellulosic and PAN precursors were made under the trade name of Thornel. PAN precursor is also produced there. In the early 1990s, also acquired BASF s carbon fiber production. BASF, in 1985, had acquired the Celion carbon fiber production from Celanese at Rockhill, North Carolina. Meltspun PAN was also produced at Rockhill, but this was discontinued in favour of using precursor from Toho Rayon. Became BP Amoco and was purchased by Cytec Industries Inc. 

Mitsubishi Oil Co., Kowasaki, Japan—originally produced carbon fiber from petroleum pitch. Mitsubishi Rayon, Toyohashi, Japan— Mitsubishi produce a PAN precursor in Japan, which they sell onward to Grafil Inc., which is now owned by Mitsubishi. Have also acquired Asahi Nippon s carbon fiber operation and provide an alternate PAN precursor for this activity. 

Polycarbon Inc., Charlotte, North Carolina, USA— was a subsidiary of Sigri Carbon Corp., now SGL Technik GmbH. Produces carbon fiber from a rayon based precursor mainly for the packing industry. 

Carbon fibers can be produced from a wide variety of precursors in the range from natural materials to various thermoplastic and thermosetting precursors Materials, such as Polyacrylonitrile (PAN), mesophase pitch, petroleum, coal pitches, phenolic resins, polyvinylidene chloride (PVDC), rayon (viscose), etc. [42-43], About 90% of world s total carbon fiber productions are polyacrylonitrile (PAN)-based. To make carbon fibers from PAN precursor, PAN-based fibers are generally subjected to four pyrolysis processes, namely oxidation stabilization, carbonization and graphitiza-tion or activation they will be explained in following sections later [43]. 

It has been more than 50 years that carbon fibers have been under development from rayon, polyacrylonitrile (PAN), isotropic and mesophase pitches. PAN-based technologies are most commercial production of carbon fibers and rayon-based carbon fibers are no longer in production. Pitch-based carbon fibers currently account for niche markets. Isotropic pitch-based carbon fibers have modest level of strength and modulus and are the least expensive carbon fibers. PAN and mesophase-based carbon fibers heat treated to improve modulus. Both PAN and mesophase pitch-based carbon fibers are not subject to creep or fatigue failure and set them apart from other material which is critical for application such as tension leg platforms for deep sea oil production [201]. The new generation of carbon based fibers is carbon nanotube (single and multi-walled). There are different synthesis methods for carbon... 

For nosetip materials 3-directional-reinforced (3D) carbon preforms are formed using small cell sizes for uniform ablation and small pore size. Figure 5 shows typical unit cell dimensions for two of the most common 3D nosetip materials. Carbon-carbon woven preforms have been made with a variety of cell dimensions for different appHcations (27—33). Fibers common to these composites include rayon, polyacrylonitrile, and pitch precursor carbon fibers. Strength of these fibers ranges from 1 to 5 GPa (145,000—725,000 psi) and modulus ranges from 300 to 800 GPa. 

Rayon is unique among the mass produced man-made fibers because it is the only one to use a natural polymer (cellulose) directly. Polyesters, nylons, polyolefins, and acryflcs all come indirectly from vegetation they come from the polymerization of monomers obtained from reserves of fossil fuels, which in turn were formed by the incomplete biodegradation of vegetation that grew millions of years ago. The extraction of these nonrenewable reserves and the resulting return to the atmosphere of the carbon dioxide from which they were made is one of the most important environmental issues of current times. CeUulosic fibers therefore have much to recommend them provided that the processes used to make them have minimal environmental impact. 

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. 

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