THE INFLUENCE OF FREE-RADICAL CONCENTRATION ON THE SHEAR BOND STRENGTH OF DENTAL COMPOSITES

The aim of this study was to evaluate the influence of the free-radical concentration on the shear bond strength between a composite resin and self-adhesive composite cement. Filtek ultimate flowable restorative composite was used for the thin layer samples, which were analyzed using electron paramagnetic resonance (EPR). Rely-X-Unicem cement was applied to the disc-shaped samples and the shear bond strength and failure modes were analysed at weekly intervals. The concentration of the free radicals dropped to 10 % after 24 h. A value for the degree of conversion equal to 74 +/- 2 % was achieved after 2 d. The shear bond values were in the range from 25.5 +/- 3.7 MPa to 9.8 +/- 4.3 MPa for four groups of samples, measured in four different time intervals. A high concentration of free radicals ensures reliable chemical bonding with a freshly applied cement in group 1, while in the samples of group 2 (7 d), group 3 (14 d) and group 4 (21 d of recombination in water), the potential of the free radicals was diminished.