Bases Carbonate

About half of the wodd production comes from methanol carbonylation and about one-third from acetaldehyde oxidation. Another tenth of the wodd capacity can be attributed to butane—naphtha Hquid-phase oxidation. Appreciable quantities of acetic acid are recovered from reactions involving peracetic acid. Precise statistics on acetic acid production are compHcated by recycling of acid from cellulose acetate and poly(vinyl alcohol) production. Acetic acid that is by-product from peracetic acid [79-21-0] is normally designated as virgin acid, yet acid from hydrolysis of cellulose acetate or poly(vinyl acetate) is designated recycle acid. Indeterrninate quantities of acetic acid are coproduced with acetic anhydride from coal-based carbon monoxide and unknown amounts are bartered or exchanged between corporations as a device to lessen transport costs. 

Fig. 3. Schematic illustration of PAN-based carbon fiber microstmcture based on microscopic observations (3).

Producers of PAN-based carbon fiber include Toray, Toho Beslon, Mitsubishi Rayon, and Asahi Kasai Carbon in Japan Hercules, Amoco Performance Products, BASE Stmctural Materials, Eortafil (Akzo), and Mitsubishi Rayon in the United States and Akzo, Sigri, and Soficar in Europe. Primary suppHers of high performance pitch-based carbon fibers include Amoco Performance Products, Mitsubishi Kasai, and Tonen Corp. 

Fig. 4. Process flow diagrams for (a) PAN-based and (b) pitch-based carbon fiber processes.

Fig. 6. Each of carbonization temperature on PAN-based carbon fiber strength and modulus (31). To convert GPa to psi, multiply by 145,000.

Fig. 8. Comparison of electrical and thermal conductivity of PAN- and pitch-based carbon fiber to metals, where P = pitch, T = Thornel, and...

There are two mechanisms of PAN-based carbon fiber oxidation dependent on oxidation temperature ((67,68). At temperatures below 400°C, oxygen diffuses into the fiber and attacks at pores resulting in significantly increased fiber surface area. At higher temperatures impurities catalyze the oxidation reaction. 

W. Johnson, "The Stmcture of PAN-Based Carbon Fibers and Relationship to Physical Properties," in W. Watt and B. V. Perov, eds., ELandbook of Composites, Vol. 1, Elsevier Science Pubhshers, New York, 1985. 

Carbonates undergo nucleophilic substitution reactions analogous to chloroformates except in this case, an OR group (rather than chloride) is replaced by a more basic group. Normally these reactions are cataly2ed by bases. Carbonates are sometimes preferred over chloroformates because formation of hydrogen chloride as a by-product is avoided, which simplifies handling. However, the reactivity of carbonates toward nucleophiles is considerably less than chloroformates. 

Polymers mesophase pitch polyacrylonitrile carbons" mesocarbon microbeads, carbon fibers PAN-based carbon fibers ... 

Fig. 4. Methane delivery at 298 K for active pitch-based carbon fibers as a function of weight loss after activation in steam or COj [after 95].

Alcaniz-Monge, J., Cazorla-Amoros, D., Linares-Solano, A., Yoshida, S. and Oya, A., Effect of the activating gas on tensile strength and pore structure of pitch-based carbon fibers. Carbon, 1994, 32(7), 1277 1283. 

Bohra, J. N. and Saxena, R. K., Microporosity in rayon-based carbonized and activated carbon fibers. Colloid Surf., 1991, 58(4), 375 383. 

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. 

In addition to their exceptional tensile strengths, PAN-based carbon fibers are far more resistant to compressive failure than are their pitch-based counterparts or polymeric high-performance fibers. However, because the PAN precursor is not... 

To date, there has been relatively little work reported on the mesophase pitch rheology which takes into account its liquid crystalline nature. However, several researchers have performed classical viscometric studies on pitch samples during and after their transformation to mesophase. While these results provide no information pertaining to the development of texture in mesophase pitch-based carbon fibers, this information is of empirical value in comparing pitches and predicting their spinnability, as well as predicting the approximate temperature at which an untested pitch may be melt-spun. 

The above equations have been solved to predict the commonly observed radial and line-origin textures seen in circular and non-circular mesophase pitch-based carbon fibers [39]. 

The properties of mesophase pitch-based carbon fibers can vary significantly with fiber texture. Inspection of the cross-section of a circular mesophase fiber usually shows that the graphitic structure converges toward the center of the fiber. This radial texture develops when flow is fully developed during extrusion through the spinnerette. Endo [48] has shown that this texture of mesophase pitch-based carbon fibers is a direct reflection of their underlying molecular structure. 

Since PAN-based carbon fibers tend to be fibrillar in texture, they are unable to develop any extended graphitic structure. Hence, the modulus of a PAN-based fiber is considerably less than the theoretical value (a limit which is nearly achieved by mesophase fibers), as shown in Fig. 9. On the other hand, most commercial PAN-based fibers exhibit higher tensile strengths than mesophase-based fibers. This can be attributed to the fact that the tensile strength of a brittle material is eontrolled by struetural flaws [58]. Their extended graphitic structure makes mesophase fibers more prone to this type of flaw. The impure nature of the pitch preciusor also contributes to their lower strengths. 

In contrast, there is also current interest in investigating PAN-based fibers in low thermal conductivity composites [62], Such fibers are carbonized at low temperature and offer a substitute to rayon-based carbon fibers in composites designed for solid rocket motor nozzles and exit cones. 

Further improvements in the properties of PAN-based carbon fibers are likely to emerge through improved stabilization, that is, by creating the ideally cross-linked fiber. On the other hand, as purer pitch precursors become available, further improvements in mesophase pitch-based carbon fibers are likely to arise from optimized spinnerette designs and enhanced understanding of the relationship between pitch chemistry and its flow/orientation behavior. Of course, the development of new precursors offers the potential to form carbon fibers with a balance of properties ideal for a given application. 

Fitzer, E., PAN-based carbon fibers - present state and trend of the technology from the viewpoint of possibilities and limits to influence and to control the fiber properties by the process parameters. Carbon, 1989, 27(5), 621 645. 

Liu, G. Z. and Edie, D. D., The influence of spinning conditions on the properties of pitch-based carbon fibers. In Extended Abstracts of the 2P Biennial Conference on Carbon, Buffalo, NY, 1993, pp. 322 323. 

Mochida, I., Yoon, S. H. and Korai, Y., Control of transversal texture in circular mesophase pitch-based carbon fibre using non-circular spinning nozzles, J Mat Sci, 1993,28, 2331 2336. 

Johnson, D. J., Structural studies of PAN-based carbon fibers. In Chemistry and Physics of Carbon, Vol. 20, ed. P. L. Walker. Marcel Dekker, New York, 1987, pp. I 58. 

Compared to PAN and pitch-based carbon fiber, the morphology of VGCF is unique in that the graphene planes are more preferentially oriented around the axis... 

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