Viscoelastic properties, creep behavior and degree of conversion of bulk fill composite resins.
OBJECTIVE
The aim of this study was to investigate the viscoelastic properties and creep behavior of bulk fill composites under different conditions and evaluate their degree of conversion.


METHODS
Seven bulk fill composites were examined: everX Posterior (EV), SDR (SD), SonicFill (SF), Tetric EvoCeram Bulk Fill (TE), Venus Bulk Fill (VE), x-tra base (XB) and x-tra fil (XF). Each material was tested at 21°C, 37°C and 50°C under dry and wet conditions by applying a constant torque for static and creep testing and dynamic torsional loading for dynamic testing. Degree of conversion (%DC) was measured on the top and bottom surfaces of composites with ATR-FTIR spectroscopy. Statistical analysis was performed with two-way ANOVA, Bonferroni's post hoc test and Pearson's correlation coefficient.


RESULTS
Shear modulus G ranged from 2.17GPa (VE) to 8.03GPa (XF) and flexural modulus E from 6.16GPa (VE) to 23GPa (XF) when the materials were tested dry at 21°C. The increase of temperature and the presence of water lead to a decline of these properties. Flowable materials used as base composites in restorations showed significantly lower values (p<0.05) than non-base composites, while being more prone to creep deformation. %DC ranged from 47.25% (XF) to 66.67% (SD) at the top material surface and 36.06% (XF) to 63.20% (SD) at the bottom.


CONCLUSIONS
Bulk fill composites exhibited significant differences between them with base flowable materials showing in most cases inferior mechanical properties and higher degree of conversion than restorative bulk fill materials.
Department of Biomaterials, School of Dentistry, University of Athens, Greece. Electronic address: deminho@gmail.com.
D
D
Papadogiannis
Department of Operative Dentistry, School of Dentistry, Aristotle University of Thessaloniki, Thessaloniki, Greece.
K
K
Tolidis
Department of Operative Dentistry, School of Dentistry, Aristotle University of Thessaloniki, Thessaloniki, Greece.
P
P
Gerasimou
Department of Engineering Physics, Engineering Mechanics Program and Department of Biomedical Engineering, Materials Science Program and Rheology Research Center, University of Wisconsin, Madison, WI, USA.
R
R
Lakes
Department of Operative Dentistry, School of Dentistry, Aristotle University of Thessaloniki, Thessaloniki, Greece.
Y
Y
Papadogiannis
D003188:Composite Resins / Q000737:chemistry*; D003764:Dental Materials / Q000737:chemistry*; D003799:Dental Stress Analysis; D055119:Elastic Modulus; D008422:Materials Testing; D017550:Spectroscopy, Fourier Transform Infrared; D013696:Temperature; D019415:Torque; D014783:Viscosity
D003188:Composite Resins; D003764:Dental Materials
Bulk fill; Degree of conversion; Flexural modulus; Flowable; Permanent deformation