Physical Property Characteristics of Pitch Materials
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PHYSICAL PROPERTY CHARACTERISTICS OF PITCH MATERIALS
DAVID P. ANDERSON*, PHILIP G. WAPNER", AND DAVID B. CURLISS***
*University of Dayton Research Institute, 300 College Park Avenue, Dayton, OH 45469-0168 "**University of Dayton Research Institute, Phillips Lab, Edwards AFB, CA ***Wright Laboratory, WL/MLBC Wright-Patterson AFB, OH 45433-6533 ABSTRACT The two most important material properties for foam process modeling are liquid viscosity and surface tension. Several different pitches were examined including Ashland petroleum pitches and a synthetic mesophase pitch (Mitsubishi AR). Viscosities of isotropic pitch were measured at various temperatures above their softening points using an oscillating disk rheometer and found to be independent of shear rate and closely match the manufacturer's data. Only a very weak dependence of surface tension on temperature was found from calculations made from pendent drop measurements. The density of these materials, which was needed for the surface tension calculations, was measured using an elevated temperature mercury dilatometer. Transitions indicated by the volumetric results were also investigated by DSC. INTRODUCTION Petroleum and coal-tar pitches are currently being used for carbon matrix precursors for carbon-carbon composites and precursors for carbon fibers. Synthetic pitches, such as the Mitsubishi AR resin, are also beginning to enter these areas. Carbon pitches are also being used in new roles such as starting materials for graphitic foams [1,2]. In order to adequately model the behavior of these materials during processing, their rheological and surface energy behavior must be known in the processing temperature ranges. The temperature dependence of density is needed to accurately calculate the surface tension; the density results produced additional questions about their thermodynamic behavior. This study was initiated to generate the required data such that the melt flow behavior of several pitches could be understood in the context of the graphite foam production. The materials examined included the Ashland A-240; Aerocarb 60, 70, and 80; Reilly coal tar pitch; and Mitsubishi AR synthetic pitch. VISCOSITY MEASUREMENTS A Rheometerics RDSII rheometer using oscillating parallel plates (25 mm diameter) at several frequencies was used to measure the viscosity behavior of the pitches as a function of temperature. The data closely matches the manufacturer's data for the Ashland pitches although our data extends beyond the temperature and viscosity limits reported [3]. Figure 1 shows a semi-log plot of measured and reported viscosity vs. temperature for the A-240 pitch. Ourdata differs from Ashland's by a factor of 1.7. This is considered quite good considering the four decade viscosity range and lack of information about how Ashland generated their data. The viscosity was also measured as a function of frequency or shear rate; for isotropic pitches the viscosity is independent of shear rate in the range examined as shown in Figure 2. Measurements at constant shear r
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