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-f ree environments with negligible production of nonvolatile residue. Co mpacts of a-alumina powder containing PMMA that are fired in a non-oxi dative ambient, however, retain appreciable amounts of char. Fourier t ransform infrared (FTIR) spectroscopy and evolved gas analysis utilizi ng mass spectral detection were employed to probe the chemical mechani sm that initiates the production of organic residue from non-oxidative ly fired PMMA/alumina bodies. We find that binding of organic groups t o alumina particles is caused by a sponification reaction between eith er the ester groups of PMMA or thermally evolved PMMA fragments (inclu ding MMA) and hydroxyl groups on alumina particle surfaces. Isolated a lumina surface hydroxyl groups were found to be much more reactive tha n mutually hydrogen-bonded alumina surface hydroxyl groups in producin g surface-bound organic groups. This surface-carboxylate-forming react ion 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 h as 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 compact s. We demonstrate that unsaturation in the carboxylate side chain play s an important role in this stage of the process in increasing the cha r yield. We also show that the char yields are proportional to surface carboxylate coverage but that it is difficult to control surface carb oxylate coverage by thermal pretreatments of the alumina powders.