Carbon fibers from Polyacrylonitrile

Cyclization is a key reaction in the production of carbon fibers from polyacrylonitrile (PAN) (acrylic fiber see Sec. 3-14d-2). The acrylic fiber used for this purpose usually contains no more than 0.5-5% comonomer (usually methyl acrylate or methacrylate or methacrylic acid). Highly drawn (oriented) fibers are subjected to successive thermal treatments—initially 200-300°C in air followed by 1200-2000°C in nitrogen [Riggs, 1985]. PAN undergoes cyclization via polymerization through the nitrile groups to form a ladder structure (XXVII). Further reaction results in aromatization to the polyquinizarine structure (XXVIII)... 

In order to produce carbon fibers from polyacrylonitrile) (PAN) and various pitches, stabilization is essential after the spinning, which consists of a chemical reaction using different oxidizing gases, such as air, oxygen, chlorine, hydrochloric acid vapor, etc. [91]. The stabilized fibers are then... 

Carbon fibers were first made by Thomas Alva Edison in 1879 from cellulose for lamp filaments. In Great Britain in 1961 the Royal Air Force produced a high-value carbon fiber from polyacrylonitrile (PAN). 

During 1961 A Shindo, of the Japanese Government Industrial Research Institute, Osaka, produced carbon fiber from polyacrylonitrile fiber, and started the development of PAN type-high performance carbon fiber. 

The controlled heating of polyacrylonitrile fibers under tension also causes an elimination of nitrogenous products to leave a carbon fiber of high tensile strength that can be considered as the end product of the line of chemical elimination reactions. Carbon fibers from cellulosic materials, lignin, and various interpolymers and blends have been developed. The structures of these products consist largely of three-dimensional carbon networks, partially crystalline and partially graphitic or amorphous. 

Polyacrylonitrile, PAN, CHj-CHCC N)-, leads on pyrolysis over an intermediate stage to complete carbonization, as depicted in Fig. 3.51. The final product is used to make strong carbon fibers. The polyacrylonitrile fibers turn yellow to red on the formation of the ladder stmcture shown in the figure. On final carbonization the fiber turns black and consists of graphite-like structures. The carbon fibers have found many applications due to their high tensile modulus in the fiber direction and their, compared to metals, low weight. They are used as fiber reinforcement in epoxy matrices. Typical applications range from aircraft and aerospace to industrial, transportation, and recreational equipment. 

Whereas carbon fibers with a low carbon content are formed predominantly from aliphatic raw materials (rayon), carbon fibers with a high carbon content are produced from aromatic feedstocks or easy-to-aromatize base materials. The most important raw materials for the manufacture of high-carbon fibers are polyacrylonitrile and mesophase pitch. 

Carbon fibers from different sources complement each other in their properties. Ktch fibers usually display higher density, higher elasticity and higher electrical conductivity, while polyacrylonitrile produces exceptionally high strength fibers. 

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 same generic dry spinning process can be used to fabricate the precursor fiber for a carbon fiber from an infusible polymer, such as polyacrylonitrile, for a polycrystalline aluminate fiber from a sol-gel or for a polycrystalline alumina fiber from a slurry. Again, a high temperature curing step is required to convert the as-spun, amorphous precursor fiber into the final functional fiber. In these cases, however, an amorphous polycrylonitrile precursor fiber changes into a carbon fiber, and an amorphous aluminate precursor fiber into a crystalline aluminate fiber. The final functional fiber is therefore directly derived from a solid and amorphous precursor fiber and only indirectly from a liquid phase, i.e., a melt or sol-gel, respectively. 

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. 

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]. 

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. 

Because of their unique blend of properties, composites reinforced with high performance carbon fibers find use in many structural applications. However, it is possible to produce carbon fibers with very different properties, depending on the precursor used and processing conditions employed. Commercially, continuous high performance carbon fibers currently are formed from two precursor fibers, polyacrylonitrile (PAN) and mesophase pitch. The PAN-based carbon fiber dominates the ultra-high strength, high temperature fiber market (and represents about 90% of the total carbon fiber production), while the mesophase pitch fibers can achieve stiffnesses and thermal conductivities unsurpassed by any other continuous fiber. This chapter compares the processes, structures, and properties of these two classes of fibers. 

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... 

Henrici-Olive, G. and Olive, S. The Chemistry of Carbon Fiber Formation from Polyacrylonitrile. Vol. 51, pp. 1—60. 

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