Carbon from acrylic precursor fibers

One of the most significant steps in the preparation of carbon fibers from acrylic precursor fibers is the oligomerization of the nitrile groups. This reaction has originally been studied in context with the problem of thermal discoloration of PAN (e.g. McCarthney Grassie and McNeill Grassie and Hay It was supposed to lead to a so-called ladder structure ... 

Peebles, L. H., Carbon fibers from acrylic precursors. In Carbon Fibers Formation, Structure, and Properties. CRC Press, Boca Raton, FL, 1995, pp. 7 26. 

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. 

Figure 5.42 Density of carbon fibers as a function of the density of stabilized fibers obtained from an AN/MA acrylic precursor fiber. Source Reprinted with permission from Takaku A, Hashimoto T, Miyoshi T, JAppI Polym Sci, 30, 1565, 1985. Copyright 1985, John Wiley Sons Ltd.

The rate controlling step in the production of carbon fiber from an acrylic precursor is the oxidation stage and G Gould and his research team looked at ways of speeding up this reaction. Various techniques could be used to catalyze the cyclization of PAN, but because the SAF already contained a catalyst comonomer (itaconic acid), the effects were much smaller than those reported in the literature for other acrylic fibers. One of the most promising was treatment with a Lewis acid, SnCU, which when applied as a solution in diphenyl ether, reduced the residual exotherm of SAF to less than 50 cal in only 6 min, which would normally have taken some 3 h of air oxidation at 220°C to have produced the... 

Acrylic precursors for the carbon fiber industry originated from companies that were established commercial scale producers of textile grade acrylic fibers. Hence, the manufacturers that could most readily adapt their existing technology to create a precursor grade material have been most successful (Table 4.2). However, some aspects such as dyeability and a tendency to yellow are not important parameters for a carbon fiber precursor but, because that particular polymer formulation was initially used for other textile end uses, the polymer composition could not be changed. As carbon fibers have developed, the market requirement for suitable precursors has increased and new polymers have been developed specifically for the manufacture of carbon fibers. 

Carbon fibers are not directly spun but are the product of a complicated aftertreatment. Nowadays, most carbon fibers (90%) are produced from an acrylic precursor. Cellulose rayon is no longer applied as a precursor. Production from pitch has been developed, but is still a small-volume business. 

PAN currently is the most used precursor for carbon fibers. Figure 11.1 shows the main processes for producing carbon fibers by using the PAN route. PAN can be synthesized from acrylonitrile through a radical polymerization process. Dirring the synthesis of PAN, co-monomers, such as methyl acrylate (up to 5%) can be added to improve the processability of PAN-based precursor fibers. The addition of co-monomers also can improve the mecharrical properties of the final carbon fibers by increasing the molecular orientation. 

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