title: Effects of repeated processing on the strength and microstructure of a heat-pressed dental ceramic.
abstract: BACKGROUND
  The excess material produced after heat pressing a lithium disilicate glass ceramic restoration can be either discarded or reused. The reuse of this material requires that any degradation of the material quality be investigated.
  
  
  OBJECTIVE
  The purpose of this study was to investigate the number of times that leftover lithium disilicate material can be re-pressed and to determine the effect that repeated use has on material properties.
  
  
  METHODS
  A large (6.1 g) lithium disilicate ingot (A3.5) was heat pressed to yield a ceramic disk (15 × 1.5 mm) for testing. The leftover material was reused to produce a further 3 disks, with the number of pressings increasing for each specimen. An additional unpressed group was included to investigate the properties before pressing so that, in total, 5 groups were established. Specimens were tested for biaxial flexural strength, Vickers hardness, and fracture toughness. X-ray diffraction was used to characterize the crystalline phase, scanning electron microscopy for the microstructure, and differential scanning calorimetry for the thermal properties.
  
  
  RESULTS
  No significant difference was found in the biaxial flexural strength of the groups. The hardness of the material decreased, and no significant difference was seen in fracture toughness with repeated pressings. An increase in grain size was observed with increased pressings. By using x-ray diffraction analysis, lithium disilicate was identified as the main crystal phase, and no difference in crystalline composition was found with repeated processing.
  
  
  CONCLUSIONS
  This material can be reused while maintaining good mechanical properties and without significantly altering the chemical or crystalline composition in an adverse manner.
authors.affiliation: Assistant Professor, Division of Restorative Dentistry and Periodontology, Dublin Dental University School and Hospital, Dublin, Ireland. Electronic address: catherine.gorman@dental.tcd.ie.
authors.forename: Catherine M
authors.initials: CM
authors.lastname: Gorman
authors.affiliation: Graduate student, Department of Restorative Dentistry, University Dental School and Hospital, University College Cork, Wilton, Cork, Ireland.
authors.forename: Kate
authors.initials: K
authors.lastname: Horgan
authors.affiliation: Graduate student, School of Mechanical and Materials Engineering, University College Dublin, Dublin, Ireland.
authors.forename: Ruth P
authors.initials: RP
authors.lastname: Dollard
authors.affiliation: Senior Lecturer, School of Mechanical and Materials Engineering, University College Dublin, Dublin, Ireland.
authors.forename: Kenneth T
authors.initials: KT
authors.lastname: Stanton
mesh_terms: D000465:Algorithms; D002152:Calorimetry, Differential Scanning; D002516:Ceramics / Q000737:chemistry* / Q000592:standards; D003461:Crystallography; D003735:Dental Casting Technique; D003776:Dental Porcelain / Q000737:chemistry* / Q000592:standards; D017743:Equipment Reuse; D006244:Hardness; D006358:Hot Temperature; D006801:Humans; D008422:Materials Testing; D008855:Microscopy, Electron, Scanning; D018583:Pliability; D013314:Stress, Mechanical; D013499:Surface Properties; D014961:X-Ray Diffraction
chemical_list: C508050:IPS e.max Press; C493120:lithia disilicate; D003776:Dental Porcelain; C023506:Glass ceramics