Residual monomer concentrations in denture-base acrylic resin after an additional, soft-liner, heat-cure cycle J.-S. Shim * , D.C. Watts Biomaterials Science Unit, The University of Manchester Dental School, Manchester M15 6FH, UK Received 13 January 1999; received in revised form 28 April 1999; accepted 11 May 1999 Abstract Objectives: To determine the effects on methylmethacrylate (MMA) monomer concentration of a second heat-cure cycle introduced for the purpose of processing a denture soft-liner. Methods: Two denture-base resins (Lucitone199 and Trevalon; Dentsply International, York, PA, USA) were selected. Heat-cure cycles were S (73°C=90 min + 100°C=30 min); L (73°C/9 h); SM (S + 100°C=2 h); LM (L + 100°C=2 h). Cycles SM and LM thus simulated the effect of a further, soft-liner, heat-cure cycle. Groups (n  6) of the specimens of the denture-base were prepared in the form of thin polymer disks. Monomer was then obtained from the broken fragments of disks by acetone extraction. Concentrations of MMA monomer were determined by reversed-phase high performance liquid chromatography (HPLC). Two-way ANOVA and independent sample t-tests were used to compare the results. Results: The cycle S, applied to Lucitone199, gave a mean concentration of 1.95 wt% MMA (SD 0.24). The cycle L gave the highest level of residual concentration of MMA monomer in both materials. Cycles SM and LM, applied to Trevalon and Lucitone199, gave significant reductions of residual concentration of MMA monomer (p  0:05). Significance: The HPLC method was suitable for determining the amount of residual monomer in the denture-base acrylic resin. A further (soft-liner) heat-cure cycle had a statistically significant effect on reducing residual monomer concentrations, and this may have an effect upon mechanical properties.  1999 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. Keywords: Dental materials; Methyl methacrylate; HPLC; Soft-liner; Denture base; Heat-cure cycle 1. Introduction The residual concentration of methyl methacrylate (MMA) monomer has been examined widely for many reasons including polymerization conversion efficiency and its relationship to physical and mechanical properties. Residual concentration of MMA monomer is especially dependent upon the efficiency of the polymerization heat- cure cycle [1,2]. Soft linings for complete dentures have been welcomed by edentulous patients for many years [3], but are associated with many different problems during their clinical service. Early fracture is one of them. Although the rate of fracture of soft-lined complete dentures was not high, the conse- quence of the fracture created more problems for both patients and practitioners than any other failure [4,5]. Currently, some of the so-called ‘permanent soft-liners’ require a second heat-cure cycle after curing the acrylic resin denture-base material. A second heat-cure cycle may affect the concentration of MMA monomer, which may in turn affect the mechanical properties of the prosthesis. A number of methods have been developed to determine the levels of residual MMA monomer. Infrared spectro- scopy [6,7] and gas chromatography [8–10] have been most commonly reported, but high performance liquid chro- matography (HPLC) has also expanded its applications in analytical chemistry. The aim of this study was to apply the HPLC method to determine the effects on residual MMA monomer concen- tration of a second heat-cure cycle introduced for the purpose of processing a denture soft-liner. The null hypoth- esis investigated is that the mean residual MMA concentra- tion will not differ significantly after a second heat-cure cycle. 2. Materials and methods Two denture-base resins Lucitone199 (Lot No. Powder- 971201, Liquid-9712032) and Trevalon (Lot No. not Dental Materials 15 (1999) 296–300 dental materials 0109-5641/99/$20.00 + 0.00  1999 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. PII: S0109-5641(99)00048-2 www.elsevier.com/locate/dental * Corresponding author. Tel.: + 44-161-275-6749; fax: + 44-161-275- 6748. E-mail address: jfshim@fs1.den.man.ac.uk (J.-S. Shim)