title: The viscoelastic behavior of dental adhesives: a nanoindentation study.
abstract: OBJECTIVE
  In order to predict the long-term performance of dental adhesives, it is necessary to understand their mechanical properties. The objective of this study was to use a new nanoindentation technique to characterize the in-plane linear viscoelastic properties of dental adhesive layers.
  
  
  METHODS
  The dental adhesives used were Clearfil SE Bond (CSE) and Clearfil Tri-S Bond (CTS) by Kuraray Medical and Single Bond (SIB) and Single Bond Plus (SBP) by 3M ESPE. A thin film of each adhesive was made on a micro-glass slide, and was then tested on a nanoindenter system (ENT 1100, Elionix) with a Berkovich indenter at a constant loading rate of 0.1 mN/s up to a maximum load of 1.8 mN. The load-displacement data of the loading segment were fitted to a curve to find best fit parameters for a generalized Kelvin viscoelastic model, from which creep compliance and Young's modulus were calculated. The modulus results were compared to the values calculated by the nanoindentation device.
  
  
  RESULTS
  The experimental data fitted well to the viscoelastic model for all materials (R>0.9999). SIB and CTS showed higher creep compliance compared to SBP and CSE. The modulus values obtained using the model were 4.0, 2.6, 2.4 and 4.2GPa for CSE, CTS, SIB and SBP, respectively. The nanoindentation default software designed for time-independent materials significantly overestimated the modulus values up to 2.5 times.
  
  
  CONCLUSIONS
  As generally expected for polymer materials, the adhesives tested showed time-dependent viscoelastic behavior. The mechanical evaluation techniques developed for time-independent materials ignore this behavior and may not be appropriate for dental adhesives.
authors.affiliation: Department of Restorative Sciences, Graduate School, Tokyo Medical and Dental University, Tokyo, Japan. alireza.ope@tmd.ac.jp
authors.forename: Alireza
authors.initials: A
authors.lastname: Sadr
authors.forename: Yasushi
authors.initials: Y
authors.lastname: Shimada
authors.forename: Hongbing
authors.initials: H
authors.lastname: Lu
authors.forename: Junji
authors.initials: J
authors.lastname: Tagami
mesh_terms: D001696:Biomechanical Phenomena; D017438:Bisphenol A-Glycidyl Methacrylate / Q000737:chemistry; D003738:Dental Cements / Q000737:chemistry*; D055119:Elastic Modulus; D004548:Elasticity; D006244:Hardness; D006801:Humans; D008422:Materials Testing; D008956:Models, Chemical; D036103:Nanotechnology; D019279:Resin Cements / Q000737:chemistry; D013314:Stress, Mechanical; D014783:Viscosity
chemical_list: C438313:Clearfil SE Bond; C515604:Clearfil Tri-S Bond; D003738:Dental Cements; D019279:Resin Cements; C511238:Single Bond Plus; C115612:single bond; D017438:Bisphenol A-Glycidyl Methacrylate