title: Competitive light absorbers in photoactive dental resin-based materials.
abstract: OBJECTIVE
  The absorbance profile of photoinitiators prior to, during and following polymerization of light curable resin-based materials will have a significant effect on the cure and color properties of the final material. So-called "colorless" photoinitiators are used in some light-activated resin-based composite restorative materials to lessen the yellowing effect of camphoroquinone (CQ) in order to improve the esthetic quality of dental restorations. This work characterizes absorption properties of commonly used photoinitiators, an acylphosphine oxide (TPO) and CQ, and assesses their influence on material discoloration.
  
  
  METHODS
  Dimethacrylate resin formulations contained low (0.0134 mol/dm(3)), intermediate (0.0405 mol/dm(3)) or high (0.0678 mol/dm(3)) concentrations of the photoinitiators and the inhibitor, butylated hydroxytoluene (BHT) at 0, 0.1 or 0.2% by mass. Disc shaped specimens (n = 3) of each resin were polymerized for 60s using a halogen light curing unit. Dynamic measurements of photoinitiator absorption, polymer conversion and reaction temperature were performed. A spectrophotometer was used to measure the color change before and after cure.
  
  
  RESULTS
  GLM three-way analysis of variance revealed significant differences (p<0.001), where photoinitiator concentration (df = 2; F = 618.83)>photoinitiator type (df = 1; F = 176.12)>% BHT (df = 2, F = 13.17). BHT concentration affected the rate of polymerization and produced lower conversion in some of the CQ-based resins. Significant differences between photoinitiator type and concentrations were seen in color (where TPO resins became yellower and camphoroquinone resins became less yellow upon irradiation). Reaction temperature, kinetics and conversion also differed significantly for both initiators (p<0.001). Despite TPO-based resins producing a visually perceptible color change upon polymerization, the color change was significantly less than that produced with CQ-based resins.
  
  
  CONCLUSIONS
  Although some photoinitiators such as TPO may be a more esthetic alternative to CQ, they may actually cause significant color contamination when used in high concentrations and therefore manufacturers should limit its concentration in order to improve its esthetic quality.
authors.affiliation: Biomaterials Unit, University of Birmingham, School of Dentistry, College of Medical and Dental Sciences, St Chads Queensway, Birmingham B4 6NN, United Kingdom. m.hadis@bham.ac.uk
authors.forename: Mohammed A
authors.initials: MA
authors.lastname: Hadis
authors.forename: Adrian C
authors.initials: AC
authors.lastname: Shortall
authors.forename: William M
authors.initials: WM
authors.lastname: Palin
mesh_terms: D000042:Absorption; D000975:Antioxidants / Q000737:chemistry; D002084:Butylated Hydroxytoluene / Q000737:chemistry; D003116:Color; D003124:Colorimetry; D003188:Composite Resins; D004955:Esthetics, Dental; D055115:Light-Curing of Dental Adhesives; D008422:Materials Testing; D010720:Phosphines / Q000737:chemistry; D055668:Photochemical Processes; D058567:Photoinitiators, Dental / Q000737:chemistry; D013729:Terpenes / Q000737:chemistry
chemical_list: C520678:(2,4,6-trimethylbenzoyl) diphenylphosphine oxide; D000975:Antioxidants; D003188:Composite Resins; D010720:Phosphines; D058567:Photoinitiators, Dental; D013729:Terpenes; D002084:Butylated Hydroxytoluene; C048339:camphoroquinone