Ind. Eng. Chem. Res. 2007, 46, 2907-2910
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Analysis of Oxidative Stabilization of Mesophase Pitch Matrix in Carbon-Carbon Composites, with Respect to Oxygen Permeability and Crosslinking Sreedevi Upadhyayula* Department of Chemical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
Salma Saddawi and William Strieder Department of Chemical and Biomolecular Engineering, UniVersity of Notre Dame, Notre Dame, Indiana 46556
In the resin transfer molding (RTM) densification process, a liquid synthetic pitch is injected at high pressure into the void spaces of the carbonized porous carbon fiber-carbon matrix (C-C) composite and allowed to solidify. To prevent the expulsion of this pitch during the subsequent high-temperature carbonization, this thermoplastic pitch must be stabilized (thermoset) by crosslinking with oxygen and exposing the composite to air at a fixed temperature of 160-220 °C. The distributed layer of reacted oxygen is strongly dependent on the permeability of the gaseous oxygen across the solid mesophase pitch. The shape and penetration of the reacted oxygen profile, obtained from Auger spectroscopy by ion etching, is used to estimate the permeability of oxygen as 1.86 × 10-12 cm2/s. Furthermore, Photoacoustic sampling-Fourier transform infrared spectroscopy (PAS-FTIR) was used to characterize the matrix of the C-C samples to determine the functional group changes during oxidation, qualitatively. Introduction Carbon fiber-carbon matrix (C-C) composites have found numerous applications in the manufacture of high-performance parts, such as rocket nozzles, nose cones, and friction materials for commercial and military aircraft, as well as in the aerospace field and also in automobile industry. The process of manufacture of these composites constitutes molding chopped carbon fiber with phenolic resin, charring these preforms, and then performing densification using resin transfer molding (RTM) infiltration and chemical vapor deposition (CVD), followed by high-temperature carbonization. In the RTM densification process, a liquid synthetic pitch is injected at high pressure into the void spaces of the composite and allowed to solidify. Oxidation stabilization of this thermoplastic pitch is necessary to convert it to a thermoset and prevent its expulsion during the subsequent high-temperature carbonization.1-4 This is accomplished by exposing the solid composite to air at a fixed temperature in the range of 160-270 °C. During oxidative stabilization, the gaseous oxygen enters the composite from the edge surface, diffuses into the pitch, and reacts with the solid. The reaction mechanism can be written as k1
gas1 + b(solid1) {\ } c(gas2) + d(solid2) k
(1)
2
The constants b, c, and d are either true chemical stoichiometric coefficients or molar balance empirical values to support a reasonable lumped reaction. For the oxidation thermosetting stabilization of mesophase carbon, “gas1” is molecular oxygen. The reacting “solid1” is mesophase carbon discotic liquid crystal, with a chemical formula of CHn (for 0.3 < n < 0.6). The model chemical reaction 1 with O2 and CHn has already * To whom all correspondence should be addressed. Tel.: +91-1126591083. Fax: +91-11-26581120. E-mail:
[email protected].
been used as a successful basis for the analysis of the combustion of hydrocarbons by Szekely et al.5 The direct oxidation of hydrocarbons will usually produce water vapor (“gas2”). These reactions result in oxygen-containing functional groups bonding onto the large mesophase pitch molecules with subsequent oxygen crosslinking. The permeability of the matrix to oxygen and the oxygen crosslinking determines the time required for the thermosetting process and the final properties of the thermoset composite crucial in the stabilization step of C-C composite manufacture. The oxygen permeability, reported here, was determined experimentally using the Auger ion etching technique, which was not attempted earlier by other researchers in the field. The accuracy of the permeability measurement using this technique is high, because the sample that is placed in the evacuated sample holding chamber of the Auger spectrometer minimizes the interference of the oxygen environment during measurement. The nature of the oxygen functional groups and oxygen crosslinking during this stabilization process was studied using Fourier transform infrared (FTIR) spectra collected using the photoacoustic cell technique. This photoacoustic cell technique is attempted for the first time on C-C composite samples. This technique yielded very prominent FTIR spectra of these black C-C samples, compared to spectra collected by previous workers with fabricated potassium bromide (KBr) pellets of such samples.6 These clear spectra give an insight into the chemical reactivity in the stabilization step. This information can be used to optimize the reaction variables to maximize yield of oxygen functional groups that will contribute to the oxygen mass gain required in this stabilization step of the manufacturing process. Experimental Section Rectangular (4 mm × 4 mm × 2 mm) samples of a C-C composite RTM piece that contained unstabilized mesophase synthetic pitch as carbon matrix were lapped to 0.1 µm, using
10.1021/ie061640z CCC: $37.00 © 2007 American Chemical Society Published on Web 03/31/2007
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Ind. Eng. Chem. Res., Vol. 46, No. 9, 2007
Figure 3. Molecular ratio of oxygen to carbon (O/C) versus the penetration depth (in micrometers) into the RTM piece.
Results and Discussions
Figure 1. Auger spectroscopic image of unreacted RTM piece, showing the analysis points in the matrix.
Experimental Determination of Oxygen Permeability. The experimental curve in Figure 3 is the result of combining the Auger spectrometer readings from several locations on the surface and then averaging them for the single slab. Auger measurements of
