Carbon from rayon

Confirmation of these earlier methods to calculate PSDs mainly in the mesopore range has been made by Kowalczyk et al. (2003). These authors prepared activated carbons from rayon and polyacrylonitrile (PAN) fibers using carbon dioxide as the activating gas. Resultant carbons were studied by adsorption of nitrogen at 77 K and isotherms are as shown in Figure 4.62 and showing hysteresis phenomena. 

Low density, carbon fiber-carbon binder composites are fabricated from a variety of carbon fibers, including fibers derived from rayon, polyacrylonitrile (PAN), isotropic pitch, and mesophase pitch. The manufacture, structure, and properties of carbon fibers have been thoroughly reviewed elsewhere [3] and. therefore, are... 

Carbon disulfide is made by reacting carbon from natural gas, and sulfur from hydrogen iiMidc, tir sulfur dioxide. Carbon disulfide is used to make rayon and regenerated cellulose film... 

The choice of the filter medium is often the most important consideration to ensure efficient operation of a filter. Its function is generally to act as a support for the filter cake, while the initial layers of cake provide the actual filter. The filter medium should be selected primarily on the basis of its ability to retain solids without binding. It should be mechanically strong and corrosion resistant, and should offer as little resistance as possible to the flow of the filtrate. The media are made from widely different materials such as cotton, wool, linen, nylon, jute, silk, glass fiber, porous carbon, metals, rayon and other synthetics, and miscellaneous materials like porous rubber. Cotton fabrics are most commonly used because they are available in a wide variety of weaves, and are cheap. 

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. 

Figure 2. AFM image of activated carbon fiber prepared from rayon. Macropores and large mesopores can be observed. Reprinted with permission from ref. 35. Copyright 2000 Elsevier Science.

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. 

Carbon fibres are manufactured from rayon and polyacrylonitrile. Carbon fibres can be heated up to 1500°C and contains up to 95% of elemental carbon. Graphite fibres can be heated above 2500 C with 99% carbon. The formation of carbon fibres from polyacrylonitrile is outlined in Fig. 1 -34. Carbon fibres are used in the aerospace industry, in compressor blade to jet engines, helicopter rotor- blades, aircraft fuselage structures, golf-club shafts, cross-bows for archery and in high speed reciprocating parts in loom. 

Babic et al. [2] report a CIP of charging curves (25°C, 0.001-0.1 mol dm KNO3) at pH 7, for self synthesized active carbon obtained from carbonized viscose rayon cloth. Seco et al. [3] titrated commercial activated carbon at four different ionic strengths and attempted to determine the equilibrium constants of reactions (5.32) and (5.33) from these titrations. Only results obtained at extreme pH values (<3 or > 11) were used, thus the apparent surface charge densities were obtained as differences of two large and almost equal numbers. On the other hand, at pH 4-10 the titration curve of carbon suspension and the blank curve were practically identical. 

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. 

Synonyms Graphite fiber PAN carbon fiber Pitch carbon fiber Definition Made from rayon, polyacrylonitrile, or petrol, pitch avail, in various grades high tensile, high modulus, etc. 

Prepared from rayon several activated carbon fibers (ACF) with different pore structure were used to remove Cr(VI) and/or Cr(III) species from solutions. The adsorption experiments were carried out to determine the influence of ACF/solution contact time, pH, temperature, initial Cr(VI) and Cr(lll) concentration on the efficiency of chromium ions removal by ACF. It was found that for ACF with total pore volume more than 0.4cmVg the porous texture has no great importance for the amount of chromium retained. For all non-oxidized ACF the amounts of Cr species adsorbed and Cr(VI) reduced to Cr(lll) after 48 h of ACF/solution contact ate very close. At the beginning phase of ACF/solution contact the latent period of Cr(VI) to Cr(III) reduction was observed. Oxidized ACF has lower adsorption capacity to Cr(VI) species and higher to Cr(III) ions in respect to non-oxidized ACF. The increase of initial Cr(VI) concentration increases the chromium species removal but the increase of pH and temperature decreases it. 

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. 

The interest in carbon fibers started in the late 1950s with the development of a modem aerospace industry requiring stiff, strong and light structural materials. The first carbon fibers were prepared from rayon, but they had low strength, stiffness and thermal conductivity. Post treatment under tension at 2200°C yielded carbon fibers with a modulus exceeding 500 GPa. 

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 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. 

RAYON-BASED CARBON FIBERS are CARBON FIBERS made from rayon (ceUulose) precursor fibers. 

The existence of carbon liber (CFs) came into being in 1879 when Thomas Edison recorded the use of carbon fiber as a filament element in electric lamp. Fibers were first prepared from rayon fibers by the US Union Carbide Corporation and the US Air Force Materials Laboratory in 1959 [41 ]. In 1960, it was realized that carbon fiber is very usefirl as reinforcement material in many applications. Since then a great deal of improvement has been made in the process and product through research work carried out in USA, Japan and UK. In 1960s, High strength Polyacrylonitrile (PAN) based carbon fiber was first produced in Japan and UK and pitch based carbon fiber in Japan and USA. 

Fin. 2. Brittle fnictures. (a,b) Glass libres. (c.cl) Ceramic fibres, Nexlcl, Nicolon. (e) Carbon fibre from rayon precursor, (f) Elastomeric Lycra. For further explanation, see Fig. I. 

Viscose vis- k6s [obs. viscose, adj, viscous] (1896) n. (1) A solution of xanthated cellulose in dilute sodium hydroxide from which rayon fibers and cellophane films are formed. The xanthated cellulose is produced by reacting alkali cellulose, i.e., wood fibers or cotton linters treated with sodium hydroxide, with oxygen and carbon disulfide. Rayon produced by this method is known as viscose rayon. (2) Generic name for fibers from regenerated cellulose (prepared by the xanthate method). Tortora PG (ed) (1997) Fairchild s dictionary of textiles. Fairchild Books, New York. 

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... 

Carbon Fibers (qv) are made by the nonoxidative high temperature pyrolysis of fibers originally spun from either rayon, polyacrylonitrile (PAN) [25014-41-9] or mesomorphic hydrocarbon tar (MT or pitch) materials. Of these three staring materials the most work has been done with rayon because the carbon fibers produced from rayon have the best overall physical and performance characteristics (69). 

Carbon Fibers from Rayon and Other Celluloslc Precursors... 

Carbonization of rayon (and PAN) yams and fabrics was briefly Investigated by Union Carbide Corporation during World War II as a possible substitute material for control grids In vacuum tube power amplifiers [5]. However, the first conmerclal venture Into multifilament carbon fibers was about 1957 by Bamebey-Cheney, a licensee of W. F. Abbott s Carbon Wool Corporation, which briefly manufactured carbon fiber tows, mats, and batting materials from cotton and rayon precursors [6], These materials were developed for use as high temperature thermal Insulation, as particle filters for hot, corrosive gases or liquids and as activated carbon fibers. 

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