title: Calculation of contraction rates due to shrinkage in light-cured composites.
abstract: OBJECTIVE
  To calculate the contraction rate that results from polymerization shrinkage in photo-cured resins.
  
  
  METHODS
  Fourteen materials were irradiated in a previously developed instrument. This instrument uses measurements of deflection using a 'bonded disk' method. Six measurements were made on each material at 20 +/- 2 degrees C and 70 +/- 10% RH. Means and standard deviations were analyzed.
  
  
  RESULTS
  Shrinkage-strain and contraction rate are reported. Total shrinkage-strain for photo-polymerized resins (packable and flowable composites) varies between 1.65 and 4.16%. Both are ormocers. The contraction rate for photo-polymerized resins varies between 55.71 and 167.00 microm/min. Packable resins present a lower contraction rate than flowable resins.
  
  
  CONCLUSIONS
  The distance-time graph is linear. The slope of this line is the average velocity. This concept was used to calculate the average contraction rate. The monomer percentage affects the contraction rate, because higher contraction rate means higher percentage of monomer. We can infer that contraction rate bears some relation to polymerization shrinkage.
authors.affiliation: Laboratorio de Investigación de Materiales Dentales, División de Estudios de Posgradoe Investigación, Facultad de Odontología, UNAM, Circuito de la Investigación Cientifica, Delegación Coyoacán, Ciudad Universitaria, Mexico. calvarezg@infosel.net.mx
authors.forename: Carlos
authors.initials: C
authors.lastname: Alvarez-Gayosso
authors.forename: Federico
authors.initials: F
authors.lastname: Barceló-Santana
authors.forename: Jorge
authors.initials: J
authors.lastname: Guerrero-Ibarra
authors.forename: Gabriel
authors.initials: G
authors.lastname: Sáez-Espínola
authors.forename: Miguel A
authors.initials: MA
authors.lastname: Canseco-Martínez
mesh_terms: D000704:Analysis of Variance; D003188:Composite Resins / Q000737:chemistry / Q000528:radiation effects*; D003799:Dental Stress Analysis; D008027:Light; D008422:Materials Testing; D015394:Molecular Structure; D044367:Phase Transition; D011108:Polymers / Q000737:chemistry; D018709:Statistics, Nonparametric; D014159:Transducers
chemical_list: D003188:Composite Resins; D011108:Polymers