Objective:A previous study (Cho&Dickens, AADR 2001) showed significant inverse correlation between increasing initial acetone content (IAC) of experimental single-solution bonding agents (BA) and microtensile bond strengths (MTBS). This study evaluated the hypothesis that MTBS may have been related to the mechanical strength of the adhesive layer necessary to resist shrinkage stresses from the overlying polymerizing composite resin.

Methods:The flexural strength (FS), degree of conversion (DC) and remaining acetone content (RAC) of BAs containing (27, 37, 47, 57, or 67) mass fraction % acetone were evaluated using two curing modes. (1) The solvent-dried bonding agents were cured for 10 s to model the mechanical properties of the adhesive layer at the time when polymerizing the composite resin. (2) To simulate the properties when testing the bond strength, the specimens were irradiated for 10 s plus 60 s through a 1.2 mm thick layer of uncured composite resin separated by Mylar. The bonding agent was filled in a 1 mm thick mold in three increments; each increment was air-dried for 10 min. After curing, the specimens (3/group) were cut into bars (~40/group), and after 48 h subjected to a 4 point-bending test. The DC and RAC were determined on similar bulk specimens by a near infrared spectroscopic technique.

Results: After 10 s cure, neither RAC nor DC were different among the bonding agents (p>0.05), while FS decreased significantly with increasing IAC (p<0.05). Cure mode 2 gave significantly higher DC (59-62 %) and FS (59-67 MPa) than cure mode 1 (40-44 %; 18-39 MPa).

Conclusions: It is suggested that the lower FS after 10 s cure especially of BAs with higher IAC may be insufficient to resist the shrinkage stress from the polymerizing composite resin, and may, thus, contribute to the observed lower MTBS.

Support: NIDCR Grant DE13298, ADAHF and NIST.