Polyacrylonitrile , carbon fibres

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. 

The commercial appearance of phenolic resins fibres in 1969 is, at first consideration, one of the more unlikelier developments in polymer technology. By their very nature the phenolic resins are amorphous whilst the capability of crystallisation is commonly taken as a prerequisite of an organic polymer. Crystallisability is not, however, essential with all fibres. Glass fibre, carbon fibre and even polyacrylonitrile fibres do not show conventional crystallinity. Strength is obtained via other mechanisms. In the case of phenolic resins it is obtained by cross-linking. 

Moehida, L, Kuroda, K., Kawano, S., Matsumura, Y., Yoshikawa, M., Grulkc, E. and Andrews, R., Kinetic study of the continuous removal of SO, using polyacrylonitrile-baaed activated carbon fibres. 2. Kinetic model, Fuel, 1997, 76(6), 537 541. 

Figure 11.5. Model of structure of polyacrylonitrile-based carbon fibre (after Johnson 1994).

The final important form of carbon is the carbon fibre formed from polyacrylonitrile (PAN), cellulose or pitch and which is finding increasing ase in fibre-teinfbrced Composites. The corrosion of carbon fibril in composites designed for use under high temperature conditions is currently a severe limitation on their use. 

The production of carbon fibres is based on the pyrolysis of organic fibres or precursors. The main starting materials are polyacrylonitrile (PAN) and pitch (coal tar or petroleum asphalt). They can be classified according to their mechanical performances ... 

A growing specialty application for acrylonitrile is in the manufacture of carbon fibres. These are produced by pyrolysis of oriented polyacrylonitrile fibres and are used to reinforce composites for high-performance applications in the aircraft, defence and aerospace industries. Other minor specialty applications of acrylonitrile are in the production of fatty amines, ion exchange resins and fatty amine amides used in cosmetics, adhesives, corrosion inhibitors and water-treatment resins (Brazdil, 1991). 

Carbon fibres are made by carbonizing or pyrolyzing polymer fibres. I came across the chemical aspects of this process in the book Principles of Polymerization by G. Odian in which the author describes how these fibres are made of polyacrylonitrile (PAN) a polymer which is represented in figure 14.4 (see also chapter 3). 

A study of the hydrophilic sites on the surface of activated carbon fibres has been made recently by Kaneko et al. (1995) with the aid of X-ray photoelectron spectroscopy (XPS). In this work cellulose (CEL)- and polyacrylonitrile (PAN)-based activated carbon fibres were used and samples were either chemically treated with H202 or heated in H2 at 1000°C. As expected, surface oxidation by the H202 treatment increased the initial uptake of water, while the H2 reduction caused a marked decrease in the amount of water adsorbed at low p/p°. Measurement of the peak areas of the XPS spectra provided a means of determining the fractional surface coverage by the hydrophilic sites. In this way a linear relationship was found between the low-pressure adsorption of water vapour and the number of hydrophilic sites (mainly —COOH). 

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. 

In recent years, extensive studies have been undertaken by Kaneko and his coworkers of the properties of activated carbon fibres (ACFs) produced from cellulose, polyacrylonitrile (PAN) and pitch. X-ray diffraction and electron microscopy revealed that the PAN-based and pitch-based fibres had a more homogeneous pore structure than that of the cellulose-based material, although the latter had the largest surface area and pore volume (Kakei et al., 1990). 

Three high-strength PAN (polyacrylonitrile)-based carbon fibres (supplied -by Elf Aquitaine France), corresponding to three different stages of manufacturing, were used in this study ... 

Pakalapati et al [115] investigated some carbon/thermoplastic laminates. The materials were pultruded and they consisted of 50 v/o unidirectional continuous polyacrylonitrile-based carbon fibres in DuPont J-2 aromatic polyamide-based thermoplastic matrix. They were subjected to anodic and cathodic currents in sea water. Dynamic mechanical analysis was carried out in situ to measure the shear storage modulus (G ) and shear loss modulus (G") of 1.27mm diameter rod shaped samples, subjected to small amplitude torsional oscillations. The moduli were constant with time in air. 

In the case of thermosets, deliberate and extensive orientation is virtually unknown. This appears to be the result of the practical difficulties involved, rather than from any theoretical obstacle. For example, it is possible that the fibre Kynol produced by the Carborundum Corporation is oriented to some extent. This is produced from a melt-spun Novolak phenol-formaldehyde resin, which is later further cross-linked with formaldehyde. It is, of course, legitimate to consider carbon fibres as extreme examples of thermosets. Formed by the cyclisation and subsequent graphitisation of polyacrylonitrile (or other suitable precursors), they are highly oriented. 

Carbon Fibres—Fibres produced by the pyrolysis of organic precursor fibres such as rayon, polyacrylonitrile (PAN), or pitch in an inert atmosphere. The term is often used interchangeably with graphite . However, carbon fibres and graphite fibres differ in the temperature at which the fibres are made and heat-treated, and the carbon content. 

R. Moreton and W. Watt, The spinning of polyacrylonitrile fibres In clean room conditions for the production of carbon fibres, Carbon, 12, 543-554 (1974). 

Watt W, Johnson W, Carbon fibres from 3 denier polyacrylonitrile textile fibres, Paper presented to 3 Conference on Industrial Carbons and Graphite, London, 1970. 

Moreton R, The removal of sodium from polyacrylonitrile precursor fibre and its effect on the mechanical properties of RAE carbon fibre, RAE Tech Memo MAT 78, Feb 1970. 

Moreton R, Spinning of polyacrylonitrile precursor fibres with reference to the properties of carbon fibres, 3" Conference, Industrial Carbons and Graphite, SCI, London, 1970. 

Hughes JDH, Morley H, The production of high strength carbon fibre from polyacrylonitrile, AERE-R9328, Jul 1979. 

Johnson JW, Marjoram JR, Rose PG, Stress graphitization of polyacrylonitrile based carbon fibre. Nature, 221, 357-358, 25 Jan 1969. 

Olive GH, Olive S, The chemistry of carbon fibre formation from polyacrylonitrile, Adv Polym Sci, 51, 1, 1983. 

Watt W, Chemistry and physics of the conversion of polyacrylonitrile fibres into high modulus carbon fibres. Watt W and Perov BV eds., Vol, Strong Fibres, Elsevier, Amsterdam, 327-388,1985. Johnson W, The structure of PAN-based carbon fibres and relationship to physical properties. Watt W and Perov BV eds., Vol 1, Strong Fibres, Elsevier, Amsterdam, 389-444, 1985. 

Bromley J, Jackson EE, Robinson PS, The carbonization stage of carbon fibre manufacture Part 1 Gas evolution. United Kingdom Atomic Energy Authority Report, AERE R6297 Harwell, 1970. Hughes JDH, Morley H, An experimental rig for continuous production of high strength carbon fibre from polyacrylonitrile, AERE Harwell Report, M 3036, Feb 1981. 

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