Carbon fiber properties

In the initial stage of carbonization, it is critical to control the off-gases that evolve from the fiber as it is heated. Excessive heating rates result in void formation, stmctural dismption, and lower carbon fiber properties. 

High molecular orientation is generally considered to be an indispensable prerequisite for good carbon fibers. As mentioned in Section 3.3, there is evidence in the literature that molecular order in the precursor fiber may take effect on carbon fiber properties. 

Numerous copolymer compositions have been screened with regard to ultimate carbon fiber properties, under comparable conditions of spinning, stabilization and carbonization. Although it is evident that different compositions will require different conditions for optimal properties, it was felt that a standard screening procedure, taking care of complete stabilization (match test), and providing for a minimum of fiber breaks, should help to select potential candidates. 

Table 5 shows the results from screening of fibers spun from binary copolymers as precursors. The fibers spun from AN/VBr copolymers are clearly the best with regard to carbon fiber properties. 

Although in some of the cases moderate improvements over the simple binary compositions may be discerned from the Tables 6 and 7, none of the compositions was superior to the binary AN/VBr copolymers, at least not under the screening conditions. It should be noted that VBr containing compositions are again at the top of each of the Tables. The 1 1 blend of AN/MMA + AN/VBr (first in Table 7) is particularly interesting because it shows carbon fiber properties comparable to the binary compositions AN/VBr (4-6 % VBr) with an overall content of VBr of only 2%. 

Compounds like dimeric sugars (e.g. sucrose) or polyacids (e.g. H3BO3), are of the hydrogen-bonding type. In the case of fibers from AN/VA (3.1 %), the addition of H3BO3 or sucrose to the polymer, prior to spinning, improved the carbon fiber properties substantially fibers from AN/VBr, however, did not show this positive response (see Table 8). 

A polymer solution containing AN/MMA copolymer plus 5% (referred to polymer) of maleic anhydride gelled completely, indicating that the expected crosslinking actually took place. As the maleic anhydride concentration was reduced to 0.5%, fiber spinning was no longer a problem. However, at this low concentration the beneficial effect on carbon fiber properties was only minor (see Table 8). 

Table 9. Influence of stretching on carbon fiber properties...

Correlation between Presursor and Carbon Fiber Properties... 

Table 12. Influence of precursor molecular weight (specific viscosity, r, p) on carbon fiber properties comonomer concentration 4.CM.2% standard screening conditions"...

From this point of view, improved carbon fiber properties are to be expected from the fibers with highest apparent shrinkage force, where the loss of attained molecular order by relaxation is the lowest. This may be an additional reason for the good performance of the AN/VBr precursor. In fact, the carbon fiber properties of this precursor are the best of those compared in Table 13. However, there is no clear trend relating properties to shrinkage force. As discussed in the preceding sections, the whole process of carbon fiber making is too complex to expect a simple relationship. 

The effect of the molecular weight (specific viscosity) on the carbon fiber properties was discussed in Section 4.4.2, and the data available for AN/VBr precursors are included in Table 12. A range of Psp of 0.17-0.18 appears to be the most recommendable. 

Table 15 shows the effect of stretching during spinning, on the carbon fiber properties, in the particular case of an AN/VBr precursor with 4.2% of VBr. It can be seen that increased spinning stretch results in a small, but significant increase of the tensile strength CT of the carbon fiber, despite the fact that the higher stretched precursor fibers cannot take the same amount of stretch in stabilization. 

TaWe 15. Influence of stretching during spinning, on carbon fiber properties. Precursor AN/VBr (4.2%),Usp = 0.173 stabilization 50min, profile B (cf. Fig. 16)... 

Table 16, Effect of stretching during stabilization on carbon fiber properties AN/VBr precursors...

Table 20. Influence of carbonization rate on carbon fiber properties. Precursor fiber An/VBr (4.2 %). spinning stretch 13X stabilization 40min, profile A,. 15% stretching carbonization 1400 C, furnace length 75 cm...

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