title: Monitoring Shore A hardness of silicone facial elastomers: the effect of natural aging and silicone type after 1 year.
abstract: OBJECTIVE
  The purpose of this study was to investigate the effect of natural aging after storage in the dark for 1 year and material type on Shore A hardness of 2 silicone prosthetic elastomers.
  
  
  METHODS
  The silicone elastomers tested were low- and high-temperature vulcanizing materials, namely, Premium and Silasto 30, respectively. Ten samples, 25 × 25 × 10 mm3, from each silicone were made and stored in sealed glass containers in the dark. Shore A hardness was measured according to the American Society for Testing Materials specification D2240. Three recordings were made on each sample at baseline and then weekly for 12 months (quadruplicate per mo). Data were analyzed by general linear modeling for repeated measures and Student-Newman-Keuls test for post hoc comparisons at α = 0.05.
  
  
  RESULTS
  General linear modeling analysis indicated a significant influence of either silicone type (F = 229.5, P = 0.0001) or natural aging (F = 105.9, P = 0.0001) or their interaction (F = 27.6, P = 0.0001) on Shore A hardness values. For Premium and Silasto 30, Shore A hardness ranged from 16.9 to 26.0 and 32.0 to 36.3, respectively. The elastomers showed a trend to increase hardness over natural aging, which was significant (Premium) or not (Silasto 30), depending on the material and time intervals.
  
  
  CONCLUSIONS
  Premium silicone showed a significant hardness increase after 1 year of natural aging in the dark, reaching a convergent value approximately 6 months from the onset, whereas Silasto 30 hardness remained stable during this period. Both elastomers showed Shore A hardness values within clinical acceptable limits after aging.
authors.affiliation: From the *Department of Prosthodontics, Dental School, University of Athens, Athens, Greece; and †Department of Oral Rehabilitation, Faculty of Dentistry, University of Otago, Dunedin, New Zealand.
authors.forename: Gregory
authors.initials: G
authors.lastname: Polyzois
authors.forename: Karl
authors.initials: K
authors.lastname: Lyons
mesh_terms: D005145:Face / Q000601:surgery*; D006245:Hardness Tests; D006801:Humans; D016014:Linear Models; D019736:Prostheses and Implants; D019651:Plastic Surgery Procedures; D012826:Silicone Elastomers / Q000145:classification*
chemical_list: D012826:Silicone Elastomers