title: Biaxial flexural strength and microstructure changes of two recycled pressable glass ceramics.
abstract: OBJECTIVE
  This study evaluated the biaxial flexural strength and identified the crystalline phases and the microstructural features of pressed and repressed materials of the glass ceramics, Empress 1 and Empress 2.
  
  
  METHODS
  Twenty pressed and 20 repressed disc specimens measuring 14 mm x 1 mm per material were prepared following the manufacturers' recommendations. Biaxial flexure (piston on 3-ball method) was used to assess strength. X-ray diffraction was performed to identify the crystalline phases, and a scanning electron microscope was used to disclose microstructural features.
  
  
  RESULTS
  Biaxial flexural strength, for the pressed and repressed specimens, respectively, were E1 [148 (SD 18) and 149 (SD 35)] and E2 [340 (SD 40), 325 (SD 60)] MPa. There was no significant difference in strength between the pressed and the repressed groups of either material, Empress 1 and Empress 2 (p > 0.05). Weibull modulus values results were E1: (8, 4.7) and E2: (9, 5.8) for the same groups, respectively. X-ray diffraction revealed that leucite was the main crystalline phase for Empress 1 groups, and lithium disilicate for Empress 2 groups. No further peaks were observed in the X-ray diffraction patterns of either material after repressing. Dispersed leucite crystals and cracks within the leucite crystals and glass matrix were features observed in Empress 1 for pressed and repressed samples. Similar microstructure features--dense lithium disilicate crystals within a glass matrix--were observed in Empress 2 pressed and repressed materials. However, the repressed material showed larger lithium disilicate crystals than the singly pressed material.
  
  
  CONCLUSIONS
  Second pressing had no significant effect on the biaxial flexural strength of Empress 1 or Empress 2; however, higher strength variations among the repressed samples of the materials may indicate less reliability of these materials after second pressing.
authors.affiliation: Biomaterials Science Research Unit, Faculty of Dentistry, University of Sydney, United Dental Hospital, Surry Hills, NSW 2010, Australia.
authors.forename: Mohammad
authors.initials: M
authors.lastname: Albakry
authors.forename: Massimiliano
authors.initials: M
authors.lastname: Guazzato
authors.forename: Michael Vincent
authors.initials: MV
authors.lastname: Swain
mesh_terms: D000704:Analysis of Variance; D002516:Ceramics; D003764:Dental Materials; D008855:Microscopy, Electron, Scanning; D013718:Tensile Strength; D014961:X-Ray Diffraction
chemical_list: D003764:Dental Materials; C023506:Glass ceramics