Figure 3. Flow diagram for BASF melt-assisted spinning process (ref. 3).

Heat Treatment of PAN Precursor Fibers

Oxidation of PAN Precursor Fibers

The molecular orientation produced as the PAN precursor is drawn and stretched must be locked into place if the final mechanical properties of the carbon fiber are to be maximized. Commercially, this is accomplished by heating the PAN precursor fiber in air under tension at 220°C to 270°C for a period that varies from 30 min to as much as 7 hr (ref. 4). The exact temperature and time required for this thermosetting process depend upon the exact composition and diameter of the PAN precursor fiber. The primary reactions that occur during this step (called oxidation) are cyclization of the nitrile groups, dehydration of saturated carboncarbon bonds, and of course oxidation. These reactions convert the PAN molecules into an infusible, cyclized network of hexagonal carbon-nitrogen rings. Although many structures result, figure 4 (ref. 5) summarizes the functional groups most often observed.

Commercially, the oxidation step is carried out in a large furnace, such as that shown in figure 5 (ref. 6). Drive rolls are used to slowly pull the PAN precursor fiber through the oven under a controlled tension. If tension were not applied during the thermoset process, the polymer would relax to a poorly aligned state. The oven air temperature and flow rate are accurately controlled to regulate the rate of oxidation. Because the PAN stabilization reactions generate HCN, as well as CO2 and water, the exit air from these furnaces must be properly exhausted.

Figure 4. Stabilization PAN precursor summarizing the most frequently observed functional groups (ref. 5).

Figure 5. Continuous process for oxidizing PAN precursor fibers (ref. 6).

Carbonization and Graphitization

After being stabilized, the fiber is finally carbonized and sometimes graphitized by slowly heating it in an inert atmosphere to temperatures ranging from 1000°C to 2800°C. By definition, carbonization implies heat treatment at temperatures of 1700°C or less, whereas graphitization means heat treating to higher temperatures (often approaching 3000°C). During this final heat treatment, almost all noncarbon

elements are driven from the fiber. In fact, the carbon content of the final fiber can range from 80 percent to in excess of 99 percent, depending upon the final carbonization temperature. Often, a carbon resistance furnace, similar to that shown in figure 6, is used for this process step. Thus, the inert atmosphere not only prevents oxygen from pitting the fiber at these high temperatures, but also it protects the carbon heating elements of the furnace from oxygen attack.

Figure 6. Schematic of carbon resistance furnace used to continuously carbonize stabilized precursor fiber.

Because gases such as CH4, H2O, NH3, N2, HCN, CO2, and CO are evolved at a rapid rate as the stabilized precursor fiber is heated to 1000°C, relatively slow heating rates are used initially (ref. 7). However, above 1000°C, only smaller molecules such as H2 and N2 are given off. Thus, carbonization often is conducted in two steps: precarbonization (heat treatment up to 1100°C) and carbonization (heat treatment at temperatures ranging from 1600°C to 1800°C). Even though the carbon content of PAN is 54 percent, carbon loss during the heat treatment steps makes the overall yield for converting PAN precursor fiber to carbon fiber approximately 40 to 45 percent (ref. 7).

Both the final heat treatment temperature and the degree of molecular orientation of the molecules in the thermoset precursor fiber govern the modulus of the final carbon fiber product. As in all brittle materials, structural flaws limit the strength of the final carbon fiber. Thus, the purity in the precursor fiber, the final carbonization conditions, and even the void content of the precursor fiber can influence the strength of the final carbon fiber.

After final heat treatment, most PAN-based carbon fibers are given a surface treatment to improve their bonding with polymeric matrix materials. Although surface treatment results in some roughening of the surface, its primary effect is to increase the concentration of oxygenated groups on the fiber surface. Various

techniques can accomplish this: exposing the carbon fiber to gases (such as air or carbon dioxide) at elevated temperatures, submerging the fiber in sodium hypochlorite or nitric acid solutions, or electrolytically etching the fiber. The principal goal of this process step is to increase the interfacial bond strength between the fiber and the matrix material and, thus, improve the interlaminar shear strength of the composite.

After being surface-treated, a small amount of size (approximately 1 percent by weight) is added to improve the wettability of the carbon fiber. Normally, this size is a low molecular weight form of the anticipated matrix polymer. In other words, epoxy-sized fiber is coated with a low molecular weight epoxy. It should be mentioned that the primary objective of this size is to improve the wettability of the fiber, not to improve its handleability. Therefore, even sized fibers can be difficult to handle during preform weaving and composite fabrication.

Manufacture of Rayon-Based Carbon Fibers

Rayon (or cellulose) precursor fibers were pyrolyzed to form the first highstrength carbon fibers. However, currently less than 1 percent of all carbon fibers are produced this way. The molecular structure of cellulose, a naturally occurring polymer found in wood pulp and cotton, is shown in figure 7.

A wet-spinning process produces these cellulose precursor fibers. To form the solution needed for wet spinning, raw cellulose is dissolved in a basic solution and then treated with CS2 to form cellulose xanthate. This soluble derivative of cellulose then is dissolved in NaOH and extruded through a spinnerette into a coagulation bath containing 10 to 15 percent sulfuric acid. As the cellulose xanthate enters the acidic bath, it is hydrolyzed, and cellulose filaments precipitate. The surface of these precipitated cellulose filaments is crenulated, a characteristic of wet-spun fibers.

Unlike PAN precursor fiber, cellulose fiber does not need to be oxidized in order to render it infusible. Nevertheless, because oxidation significantly improves its carbon yield, the cellulose precursor fiber is oxidized by heating it in air to temperatures as high as 400°C. Initially, as the fiber is heated, the physically absorbed water is desorbed. As heating continues, additional water is evolved because of the reaction of hydroxyl groups in the cellulose. Finally, as the cellulose begins to decompose, CO2, CO, and water are given off, and aromatization of the structure begins (refs. 7 and 8). Because the cellulose polymer decomposes as it is stabilized, prestretching or stabilizing under tension (useful for PAN precursor fibers) is ineffective (ref. 9).

After being stabilized, cellulose precursor fibers are carbonized and graphitized in an inert atmosphere at temperatures similar to those used for PAN. However, because the cellulose molecules in the precursor fiber lose most of their axial

Figure 7. Molecular structure of cellulose and approximate structure during thermal degradation to carbon (ref. 8).

orientation during pyrolysis, the fibers are strained at this high temperature to increase the preferred orientation and improve the final mechanical properties. The filaments are quite plastic at high temperatures and can be stretched as much as 150 percent. When stretched 100 percent during graphitization at 2800°C, fibers with a modulus approaching 720 GPa can be formed (ref. 9). However, if the same fibers are graphitized (but not stretched) at the same temperature, they attain a modulus of only 72 GPa.

The overall yield for converting the cellulose precursor fiber to carbon fiber ranges from 10 to 30 percent, compared with 40 to 50 percent for the PAN precursor. This low yield is the direct result of the low carbon content of cellulose (44 percent) and the extensive decomposition that occurs during stabilization. This low conversion, especially when coupled with the expense of the stretchgraphitization, accounts for the high cost of rayon-based carbon fibers.

Manufacture of Pitch-Based Carbon Fibers

Mesophase pitch-based carbon fibers are an attractive precursor candidate for carbon fibers because of the high availability of low-cost raw pitch. There are several reasons why the mesophase pitch process should produce a lower cost,