Polymerization efficiency affects interfacial fracture toughness of adhesives.

OBJECTIVE

To evaluate the effect of the kind and concentration of photo-initiator on the degree of conversion (DC) of adhesives on dentin/glass substrates and their mini-interfacial fracture toughness (mini-iFT) to dentin.

METHODS

We tested the adhesive Clearfil S3 Bond Plus and 4 derived experimental 'LUB' ('Leuven Univesity Bond') adhesives (all from Kuraray Noritake), namely 'LUB-CQ/amine_high', 'LUB-CQ/amine_low', 'LUB-TPO_high', and 'LUB-TPO_low', respectively containing 2.0wt% camphorquinone (CQ) and 2.0wt% EDMAB (amine), 0.35wt% CQ and 0.35wt% amine, 2.0wt% TPO, and 0.35wt% TPO. For DC, each adhesive was applied onto glass or dentin prior to being cured (Bluephase 20i; Ivoclar Vivadent: "high mode") for 10s. DC was measured at 5min, 10min, 1h, 24h and 1 week using micro-Raman spectroscopy (SENTERRA; BrukerOptik). For mini-iFT, each adhesive was bonded to 320-grit SiC-paper ground dentin and covered with composite (Z100; 3M ESPE). The restored teeth were cut in sticks (1.5×2.0×16mm), after which a single-gradient notch was prepared at the adhesive-dentin interface using a 150-μm diamond blade. The micro-specimens were loaded until failure in a 4-point bending test and the mini-iFT in term of KQvM was calculated.

RESULTS

DC was higher on dentin than on glass. All adhesives were adequately polymerized at 1 week, except for LUB-TPO_low. DC at 5min was significantly higher for LUB-TPO_high than for both CQ/amine-based adhesives. The highest and most reliable mini-iFT was measured for LUB-CQ/amine_high, despite its 5-min DC was relatively low. No correlation between DC and mini-iFT was found.

SIGNIFICANCE: Curing of TPO-based adhesives is faster, but the dark cure of the CQ/amine-containing adhesives is more efficient. The differences in curing profiles do affect the mechanical properties of the resultant interfaces at dentin.