CERAMIC SURFACE-REACTIONS AND CARBON RETENTION DURING NONOXIDATIVE BINDER REMOVAL - AL2O3 POLY(METHYL METHACRYLATE) AT 20-DEGREES-700-DEGREES-C

Neat poly(methyl methacrylate) (PMMA) thermally decomposes in oxygen-free environments with negligible production of nonvolatile residue. Compacts of a-alumina powder containing PMMA that are fired in a non-oxidative ambient, however, retain appreciable amounts of char. Fourier transform infrared (FTIR) spectroscopy and evolved gas analysis utilizing mass spectral detection were employed to probe the chemical mechanism that initiates the production of organic residue from non-oxidatively fired PMMA/alumina bodies. We find that binding of organic groups to alumina particles is caused by a sponification reaction between either the ester groups of PMMA or thermally evolved PMMA fragments (including MMA) and hydroxyl groups on alumina particle surfaces. Isolated alumina surface hydroxyl groups were found to be much more reactive than mutually hydrogen-bonded alumina surface hydroxyl groups in producing surface-bound organic groups. This surface-carboxylate-forming reaction anchors organic residue to the inorganic surfaces to temperatures above the unzipping temperature of the polymer and leads to retention of organic fragments in the compacts at temperatures where neat PMMA has completely volatilized. Subsequent transformations of these surface-bound organic fragments at high temperatures produce the nonvolatile carbonaceous residues, which are retained in the fired alumina compacts. We demonstrate that unsaturation in the carboxylate side chain plays an important role in this stage of the process in increasing the char yield. We also show that the char yields are proportional to surface carboxylate coverage but that it is difficult to control surface carboxylate coverage by thermal pretreatments of the alumina powders.