PAN Precursors

The carbon content of acrylonitrile (CH2 = CHCN) is 67.9% and it is not surprising PAN precursors have a carbon yield of some 50-55%, coupled with the ability to produce high modulus fibers. 

An acrylic fiber is defined as having acrylonitrile (AN) monomer content greater than 85%. Fibers with AN content less than 85% are termed modacrylics and are not suitable for use as carbon fiber precursors. 

A cellulosic precursor (C6Hio05) has a carbon content of 44.4% but, unfortunately, in practice, the reaction is more complicated than just simple dehydration and the carbon yield is only of the order of 25-30%. 

Pitch based carbon fibers, however, do have a higher yield of 85% with a high resultant modulus but, due to their more graphitic nature, they will have poorer compression and transverse properties as compared to PAN based carbon fibers. 

Other forms of precursor such as vinylidene chloride and phenolic resins have been investigated and have not been found to be commercially viable. 

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Itaconic acid is a specialty monomer that affords performance advantages to certain polymeric coatings (qv) (see Polyesters, unsaturated). Emulsion stabihty, flow properties of the formulated coating, and adhesion to substrates are improved by the acid. Acrylonitrile fibers with low levels of the acid comonomer exhibit improved dye receptivity which allows mote efficient dyeing to deeper shades (see Acrylonitrile polymers Fibers, acrylic) (10,11). Itaconic acid has also been incorporated in PAN precursors of carbon and graphite fibers (qv) and into ethylene ionomers (qv) (12). 

Fig. 5. Chemical reactions occurring during stabilization of PAN precursor (13).

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... 

Secondly, the drop in Ni(II) concentration is even more pronounced in the presence of the PAN fibres compared with the fact that equivalent amounts of PAN precursors are dissolved. By equivalent amounts, it is meant that an equivalent amount of functional groups are dissolved in solution. This difference can be explained by two effects ... 

Using precursors, Ni(II) concentration drops because of reduction by rongalite and complex formation with the functional groups of the PAN precursors, followed by reduction. 

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. 

Polyacrylonitrile (PAN) is the most common precursor used to make carbon fibers. A flow diagram showing the steps involved in making PAN-based carbon fiber is shown in Fig. 8.3. The PAN precursor has a flexible polymer chain structure like any other polymer, but it has an all carbon backbone chain that contains polar nitrile groups, as shown in Fig. 8.4. During the stabilization treatment, the PAN precursor fiber is heated to 200-220 C, under tension. When this is done oxygen is absorbed, and it serves to cross-link the chains the fibers turn black, and a stable ladder structure is formed. A ladder polymer is a rigid... 

The major reaction products from the atmospheric reactions of the ketones are aldehydes and PAN precursors, although bifunctional oxygen-containing compounds will probably be formed in small yield. 

The stabilization of the PAN precursor involves preoxidation by heating the fiber in an air oven at 200°C-300°C (392°F-5722°F) for approximately one hour while controlling the shrinkage/tension of the fiber so that the PAN polymer is converted into a thermally infusible aromatic ladder-Uke structure. 

FIGURE 2.52 Typical PAN precursor manufacturing steps based on solution polymerization and wet spinning. 

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