title: Influence of radiant exposure on contraction stress, degree of conversion and mechanical properties of resin composites.
abstract: OBJECTIVE
  To verify the influence of radiant exposure (H) on contraction stress (CS), degree of conversion (DC) and mechanical properties of two restorative composites.
  
  
  METHODS
  Filtek Z250 (3M ESPE) and Heliomolar (Ivoclar) were photoactivated with 6, 12, 24, or 36 J/cm2 at continuous irradiance of 600 mW/cm2. CS at 10 min was determined in a low compliance testing system. DC, flexural strength (FS), flexural modulus (FM) and Knoop microhardness (KHN) were measured after 24 h storage at 37 degrees C. KHN and DC measurements were conducted on the irradiated surface of 1mm thick disk-shaped specimens. Bar-shaped specimens were submitted to three-point bending to determine FS and FM. Data were analyzed by one-way ANOVA/Tukey's test (alpha = 0.05) separately for each composite.
  
  
  RESULTS
  For Filtek Z250, no significant increase in CS was observed above 12 J/cm2. DC and FM were similar at all H values, while FS increased significantly between 6 and 24 J/cm2. KHN was significantly different among all H levels, except between 12 and 24 J/cm2. For Heliomolar, CS and KHN increased significantly with H, except between 24 and 36 J/cm2. DC, FM and FS did not vary, regardless of the radiant exposure.
  
  
  CONCLUSIONS
  Variables tested behaved differently. CS and KHN were more sensitive to increasing radiant exposures than the other properties evaluated. FS varied only for Filtek Z250, while, for both composites, DC and FM were not affected by different H values.
authors.affiliation: Department of Dental Materials, University of São Paulo, School of Dentistry, Av. Professor Lineu Prestes, 2227, São Paulo, SP 05508-900, Brazil. fcalabro@usp.br
authors.forename: Fernanda C
authors.initials: FC
authors.lastname: Calheiros
authors.forename: Yoshio
authors.initials: Y
authors.lastname: Kawano
authors.forename: Jeffrey W
authors.initials: JW
authors.lastname: Stansbury
authors.forename: Roberto R
authors.initials: RR
authors.lastname: Braga
mesh_terms: D000180:Acrylic Resins / Q000528:radiation effects*; D000704:Analysis of Variance; D003188:Composite Resins / Q000528:radiation effects*; D003793:Dental Restoration, Permanent; D003799:Dental Stress Analysis; D004548:Elasticity; D006244:Hardness; D008027:Light; D008422:Materials Testing; D044367:Phase Transition; D018583:Pliability; D011140:Polyurethanes / Q000528:radiation effects*; D018709:Statistics, Nonparametric
chemical_list: D000180:Acrylic Resins; D003188:Composite Resins; C444701:Filtek Z250; C513708:Heliomolar; D011140:Polyurethanes