Minimization of the inevitable residual monomer in denture base acrylic

Abstract

Residual monomer ([MMA]R) in denture base acrylic continues to be of concern. The response surface of concentration vs. time and temperature for the equilibration of methyl methacrylate (MMA) and its polymer (PMMA) allows a prediction of the time to the minimum at any temperature for a closed system. It was the purpose here to determine whether this prediction applies to normal denture base processing, and whether optimum conditions could be identified. Denture bases were processed following normal laboratory procedures, including pre-cure for 3 h at 70 °C for all tests. Commercial powder and liquid were used at either 95 or 100 °C, or a plain PMMA powder and the same liquid at 95 °C, for times ranging from 5 to 192 h. Residual MMA was determined by gas chromatography. [MMA]R decreased steadily from ∼0.25% to as low as ∼0.07% with increasing time at temperature, but did not approach equilibrium. The rate of diffusive loss of MMA appears to exceed the rate of depolymerization. Residual monomer is inevitable for all PMMA-based products no matter what the curing conditions are. However, extended time at high temperature can allow low values to be attained, and the time allowed can compensate for processing temperatures somewhat lower than the ordinarily recommended 100 °C. It is suggested that overnight processing at 95 °C should be adopted to minimize [MMA]R and save energy. This result is of importance for work at high altitude. © 2005 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.