title: Pulse-delay curing: influence of initial irradiance and delay time on shrinkage stress and microhardness of restorative composites.
abstract: OBJECTIVE
  Evaluate the influence of pulse-delay curing on shrinkage stress and microhardness of 2 restorative composites (Herculite XRV and Tetric Ceram).
  
  
  METHODS
  Two pulse irradiances (500 and 100 mW/cm2) were applied for 1 or 5 seconds, respectively (radiant exposure = 0.5 J/cm2). In both cases, photoactivation was completed applying 500 mW/cm2 for 39 seconds after a delay time of 0, 1 or 3 minutes. Shrinkage stress was monitored for 10 minutes in specimens 5-mm in diameter by 1-mm in height. Knoop hardness (KHN) was used to estimate the degree of conversion 10 minutes after photoactivation and after 48 hours of storage in distilled water (37 degrees C) in specimens with similar geometry and dimensions. Additional KHN readings after 4 8 hoursof storage in ethanol (37 degrees C) were used to estimate polymer structure. The results were evaluated using ANOVA/Tukey test and Student t-test (a=0.05).
  
  
  RESULTS
  For Tetric Ceram, 3-minute delay led to stress reduction compared to continuous curing at 500 mW/cm2 (4.7+/-0.6 MPa and 7.0+/-1.3 MPa, respectively). At 100 mW/cm2, 1 minute delay was enough to cause significant stress reduction (5.2+/-0.5 MPa). For Herculite, the pulse with 3 minute delay led to stress reduction compared to no delay for both irradiances (100 mW/cm2: 6.3+/-0.5 MPa and 7.8+/-0.8 MPa, respectively; 500 mW/cm2: 6.4+/-0.3 MPa and 7.8+/-0.7 MPa, respectively). At 10 minutes, only small differences in microhardness were observed for both materials. No differences were found after water and ethanol storage (p>0.05).
  
  
  CONCLUSIONS
  The composites behaved differently when subjected to pulse curing. Stress reduction was influenced by delay time but not by pulse irradiance. KHN results suggest that similar degrees of conversion and polymer structure were achieved with the photoactivation methods tested.
authors.affiliation: Carmem Silvia Costa Pfeifer, graduate student, University of São Paulo, School of Dentistry, Department of Dental Materials, São Paulo, Brazil. cpfeifer@usp.br
authors.forename: Carmem Silvia Costa
authors.initials: CS
authors.lastname: Pfeifer
authors.forename: Roberto Ruggiero
authors.initials: RR
authors.lastname: Braga
authors.forename: Jack Liborio
authors.initials: JL
authors.lastname: Ferracane
mesh_terms: D003188:Composite Resins / Q000737:chemistry / Q000528:radiation effects*; D003764:Dental Materials / Q000737:chemistry / Q000528:radiation effects*; D000431:Ethanol / Q000737:chemistry; D006244:Hardness; D006801:Humans; D008027:Light; D008422:Materials Testing; D019279:Resin Cements / Q000737:chemistry / Q000528:radiation effects; D012997:Solvents / Q000737:chemistry; D013314:Stress, Mechanical; D013499:Surface Properties; D013696:Temperature; D013997:Time Factors; D014867:Water / Q000737:chemistry
chemical_list: D003188:Composite Resins; D003764:Dental Materials; D019279:Resin Cements; D012997:Solvents; C121818:Tetric ceram; D014867:Water; C065770:Herculite XR; D000431:Ethanol