The effects of repeated heat-pressing on properties of pressable glass-ceramics

Abstract

The aim of this study was to evaluate the properties of four heat-pressed glass-ceramic materials after repeated heat-pressing. Two commercially available heat-pressed glass-ceramic systems [Optimal pressable glass-ceramics (OPC ® and 3G™) and Empress pressable glass-ceramics (Empress® and Empress®2)] were selected. Disc samples (14 mm × 1·4 mm) of each tested material were heat-pressed and used as controls. Sprue and button parts of the pressed groups were retrieved and used for repeated heat-pressing to construct specimens of re-pressed group. All the heat-pressed casting procedures were performed according to the manufacturers' instructions. A biaxial flexural strength (BFS) test (ISO 6872) was performed to determine the strength of pressed and re-pressed glass-ceramic disc specimens (n = 10) at a crosshead speed of 0·5 mm min-1. Ions eluted from etching procedure were collected and examined using inductively coupled plasma mass spectrometer. Surface characteristics were examined with electron probe microanalysis, X-ray diffraction and secondary electron imaging (SEI). The data were analysed statistically (anova + Tukey's HSD post hoc test, P < 0·05). The BFS values obtained ranged from 123·5 ± 18·5 to 365·9 ± 35·5 MPa. The re-pressed Empress®2 group had a statistically significant higher BFS mean than the pressed control group (P < 0·05). The SEI micrographs of the lithium disilicate-reinforced glass-ceramic material (Empress®2) showed a densely packed, interlocking microstructure and an increase in size with preferred orientation of the lithium disilicate crystals after repeated heat-pressing. Repeated heat-pressing treatment produced a statistically significant increase in the flexural strength of Empress®2 glass-ceramic material. © 2008 Blackwell Publishing Ltd.