Comparison on modulus and strength of PAN based and pitch based carbon fibers

In order to understand the role that carbon fibers can play, the attributes of carbon fibers must be considered:

1. Available in many grades and forms with wide-ranging properties
2. High modulus, especially pitch based fiber
3. Good strength, especially PAN based fiber
4. Low density, giving good specific properties
5. Good thermal stability in the absence of O2
6. High thermal conductivity, assisting good fatigue properties
7. Low thermal expansion coefficient
8. Excellent creep resistance
9. Good chemical resistance and does not wick
10. Low electrical resistivity
11. Biocompatibility
12. No significant inhalation problem with filament diameter down to 5 um

The disadvantages are:

1. Relatively high cost, but prices have been falling and more emphasis is now placed on using large tows
2. Low strain to failure with attendant handing problems
3. Compressive strength is lower than tensile strength and larger diameter fiber does not give improved compression properties
4. Poor impact strength of composites
5. Care required during handing carbon fiber, since it is electrically conducting and can cause havoc with electrical systems
6. Oxidation in air at temperature above 450C
7. Exhibits anisotropy in the axial and transverse directions

The first commercial carbon fibers were based on viscose rayon, a cellulosic precursor, but Polycarbon is now the only current producer of this type of carbon fiber. The properties of rayon-based carbon fibers are a difficult fiber to produce with a low yield, its main use is in existing space programs.

Oxidized PAN fiber is made by a slightly modified first stage of the PAN based carbon fiber process. Oxidized PAN fiber, with a density above 1.37 g/cm3, is non-flammable.

PAN based carbon fibers are not graphitic, this will limit the modulus attainable, but strengths are greater than pitch based fibers.

Fibers from a pitch precursor are graphitic and for a given process temperature, can attain higher moduli than PAN based fibers, approaching the value for the graphite crystal.

The properties of carbon fiber are quite dependent on the structure, in particular, the crystalline size as defined by the coherent length perpendicular and parallel to the carbon layers. These values increase as the heat treatment temperature increases and for a given process temperature, Lc is higher for a pitch based fiber, increasing with temperature at a steady rate, whereas a PAN based fiber increases sharply above 2300C.

Young’s modulus is an intrinsic property and governed by the orientation of the graphitic crystallites relative to the fiber axis. The lower this angle, greater is the modulus. La is a measure of the crystallites relative to the crystallite basal planes and increases with temperature and the modulus. The orientation for HM fiber is 25° in the core and 12-15° in the skin, with circumferential orientation.

The modulus increases steadily with temperature, but the strength peaks at about 1575C. As the quality of the PAN precursor have been improved and its diameter reduced, this has enabled carbon fibers to be produced with higher strengths, with a diameter of about 5 um. The smaller the carbon fiber diameter, the greater is the strength. The fact explains the introduction of grades of carbon fiber with improved tensile properties having filament diameters of about 5 um, whereas other earlier grades have diameters of about 7 um.

It might be expected that the properties of carbon fiber could approach those of the graphite single crystal, where the YM is of the order of 1000 GPa and the theoretical strength would be expected to about one-tenth of the YM, i.e. 100 GPa. The strengths are, however, well below this theoretical figure, but the YM of high modulus pitch based fiber types can approach 1000 GPa. Since a PAN based fiber is not graphitic, its modulus will be lower at a given production temperature as compared to a pitch based fiber, but theoretical strengths are higher.

A graphitizable carbon (pitch based carbon fiber) undergoes progressive graphitization in the range 1600C-2800C, with an increasing three-dimensional order, whereas a non-graphitizing carbon (PAN based carbon fiber) is never fully converted to graphite, even after heating for many hours at 2000C. The distance between the layer planes in the crystal structure of a true graphite is 0.3354 nm, but in a turbostratic form of carbon, the distance is always greater than crystal graphite due to the presence of sp3 bonds.