Effect of Repeat Heat-pressing on Flexure Strength of Ceramic Materials

Objective: The purpose of this study was to evaluate the properties of heat pressable ceramic materials after repeat heat-pressing. Two commercially available heat pressable ceramic systems were selected. The materials tested were Optimal pressable ceramics (OPC^® and 3G^TM) and Empress pressable ceramics (Empress^® and Empress^® 2). Methods: Twenty disc wax patterns (14 mm X 1.5 mm) of each tested material were prepared and cast. Sprue and button parts of the as cast groups were retrieved and used for repeat heat-pressing. All the heat-pressed casting procedures were performed according to the manufacturers' instructions. A biaxial flexure strength test (ISO 6872) was used to determine the strength of ceramic disc specimens before and after repeat heat-pressing at a crosshead speed of 0.5 mm/min. Ions eluted from etching procedure were collected and examined using inductively couple plasma-Mass spectrometer. Surface characteristics were examined with electron probe microanalysis, X-ray diffraction, and secondary electron imaging (SEI). An analysis of variance and Tukey multiple comparison tests were used to evaluate results at the 0.05 level of significance. Results: The biaxial flexure strength values obtained range from 123.5MPa ± 18.5MPa to 365.9MPa ± 35.5MPa. The repeat heat-pressed group (Empress^® 2) had significantly higher biaxial flexure strength mean than the as cast group (p<0.05). The SEI micrographs of the lithium disilicate glass-ceramic material (Empress^® 2) showed a densely packed, interlocking microstructure and an increase in size of the lithium disilicate crystals after repeat heat-pressing. Conclusion: Repeat heat-pressing treatment produced statistically significant increase in the flexure strength of Empress^® 2 glass-ceramic material. Supported by National Science Council, ROC, Grant NSC 92-2314-B-010-037.