A PROCESSING WINDOW FOR INJECTION OF MESOPHASE PITCH INTO A FIBER PREFORM

A review of viscosity data for mesophase pitches prepared by pyrolysis of petroleum or coal-tar pitches, or by chemical and thermal treatment of organic precursors (''chemical pitches''). finds that, although the apparent viscosities range widely, the temperature dependencies follow a regular pattern. Mesophase pitches prepared from petroleum or coal-tar precursors are very viscous, but can be used in a partially transformed state to obtain some reduction in viscosity; in contrast. most chemical pitches are less viscous at full transformation. The processing window for injection of mesophase pitch into a fiber preform derives from three requirements: the mesophase must be fluid enough to avoid deformation of the preform, sufficiently stable to limit viscosity increase and bubble formation during injection, but high enough in softening point to permit oxidation stabilization. Capillary flow measurements were used in selecting a mesophase pitch for injection through 2D preforms made by 45-degrees-layup of a plain-weave carbon fabric. The viscosity of a decanted mesophase pitch prepared by partial transformation of a petroleum pitch was found to lie above the processing window. However an alkylbenzene-based mesophase pitch showed a favorable combination of viscosity and thermal stability; using injection temperatures near 310-degrees-C and driving pressures of 0.1 MPa, steady penetrating flow was obtained with good filling of fiber bundles.