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Figure 14. Noncircular capillary and resulting trilobal fiber (ref. 22).
Figure 15. Noncircular capillary and resulting trilobal fiber (ref. 30).
it seems likely that changing the microstructure will increase the compressive properties of pitch-based carbon fibers. However, recent studies indicate that changes in the macroscopic-buckling characteristics of carbon fibers may result in additional increases in their compressive properties.
PAN-Based Carbon Fibers
Recently, BASF Structural Materials, Incorporated, began producing PANbased carbon fibers using a new melt-spun precursor technology (ref. 31). In this process, the acrylonitrile is polymerized in an aqueous suspension, eliminating the need for an organic solvent. After the PAN is purified and dewatered, it is compounded, pelletized, and fed to an extruder. When melted in the extruder, the pellets, plasticized with excess water, form a homogeneous melt that can be extruded into fiber form. During extrusion the excess water flashes off, allowing the PAN precursor fiber to solidify. This process has several advantages compared with standard solution-spinning technology, including eliminating the need for solvent recovery and decoupling the polymerization step from the spinning step. Additional advantages are that, because large amounts of residual solvent do not have to diffuse out of the fiber during solidification, a more radially uniform structure should result. Also, since the fibers are essentially melt-spun, the fiber shape can be controlled.
BASF Structural Materials, Incorporated, has already used this melt-spun precursor technology to produce experimental quantities of noncircular. PAN-based carbon fibers with ribbon and tetralobal cross sections (see fig. 16). These varieties are being developed for composite applications in which improved compressive and interlaminar shear strength is critical (ref. 31).
Figure 16. Noncircular PAN-based carbon fibers produced with melt-spun technology (ref. 31).
In a combined theoretical and experimental study, Niederstadt (ref. 32) has already shown that noncircular PAN-based fibers should have a higher compressive strength than round fibers. In this study, he calculated the theoretical flexural rigidity for polymer matrix composite materials that were reinforced by both solid and hollow PAN-based carbon fibers and compared the rigidity to that predicted for the same polymer matrix reinforced by only solid carbon fibers. As figure 17 shows, if 65 percent of the fibers used to reinforce a composite are solid and merely 35 percent are hollow, the stiffness of the composite is predicted to be twice that of a composite reinforced by the solid fiber alone. In subsequent experiments, Niederstadt found that composites fabricated using a mixture of hollow and solid glass fibers had a 120 percent higher bending stiffness than composites formed using only solid glass fibers.
Figure 17. Theoretical bending stiffness for laminate reinforced by hollow carbon fiber versus solid carbon fiber (ref. 32).
Thus, the work of Niederstadt indicates that noncircular fibers, such as those being developed by BASF Structural Materials, Incorporated, should in fact exhibit improved compressive properties.
Pitch-Based Carbon Fibers
Diwan (ref. 33) investigated the effect of noncircular pitch-based carbon fibers on the performance of polymeric composites. A solvent-extracted mesophase was melt-spun into trilobal and hexalobal carbon fibers. After thermosetting and carbonization, these fibers were used to form unidirectional carbon-epoxy composite specimens. These specimens were tested in compression using a standard Illinois Institute of Technology Research Institute (IITRI) fixture. Results indicated that, although the tensile strength of the two fiber shapes was nearly identical, the compressive strength of the composites reinforced with the more highly noncircular hexalobal fibers was 16 percent higher than those reinforced with the trilobal fibers. The probable cause for this improvement in compression strength was an increased resistance to fiber buckling caused by the higher moment of inertia of the more noncircular fiber. Thus, the use of noncircular shapes may result in improved compressive properties for both PAN-based and pitch-based carbon fibers. Also, in CC composite applications, the increased surface-to-volume ratio of noncircular shapes may even yield improved interlaminar shear strength (refs. 31 and 33).
The microstructure of the polymeric precursor fibers used to form PAN-based and rayon-based carbon fibers is fibrillar, but mesophase precursor fibers are composed of extended domains of a highly oriented structure. Since, to a large extent, the final microstructure of the carbon fiber replicates that of the precursor fiber, in PAN-based and rayon-based carbon fibers the graphite crystallites are arranged into a fibrillar substructure, whereas pitch-based carbon fibers have an extended graphitic layer structure. Although these larger regions of graphitic structure allow pitch-based carbon fibers to exhibit extremely high moduli, this structure also makes pitch-based carbon fibers more flaw-sensitive, accounting for their lower tensile strengths. However, modifying the flow profile during fiber extrusion has been shown to change the microstructure significantly and even create new microstructures in pitch-based fibers, improving both the tensile and the compressive strength of these fibers. Also, it appears that noncircular fiber cross sections can increase the buckling resistance of both pitch-based and PANbased carbon fibers.
The authors thank G. P. Daumit of BASF Structural Materials, Incorporated, for generously providing the original photographs used for figure 16. In addition, Dr. D. J. Johnson and IOP Publishing Limited are to be thanked for graciously permitting figures 2, 4, and 5 to be reproduced. Thanks are also due to the Society of Plastics Engineers, Chapman and Hall, Limited, Pergamon Press, Incorporated, and Kluwer Academic Publishers for permission to reproduce figure 3, figure 7, figure 8, and figures 1 and 11, respectively. Finally, the authors thank the Materials Research Society and T. Hamada for permission to reprint figure 8.
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