Effect of Storage Conditions on Mechanical Properties of Commercial Adhesives

Purpose: To evaluate the effects of wet versus dry storage on the ultimate tensile strength (UTS) and modulus of elasticity (E) of commercial dental adhesives with different hydrophilicity.

Methods: Since the exact composition of commercial adhesives cannot be fully disclosed, a water sorption protocol (Malacarne et al., 2006) was used to estimate the hydrophilicity of commercial adhesives based on their apparent Hoy's solubility parameter. The neat bonding resins of Clearfil SE Bond (SE) and Scotchbond Multi-Purpose (MP) represented the least hydrophilic materials; while the two-step solvated systems, Single Bond (SB) and Excite (EX), represented the most hydrophilic. Halter-shaped specimens were prepared by pouring the uncured adhesives into a brass mould. Solvated adhesives were gently blown with oil/water-free air. Adhesives were light-cured (120 s) and stored in a desiccator at 37°C. After dry storage for 24h, twelve specimens of each adhesive were tested under tension (0.5 mm/min), and their UTS and E calculated. For the remaining specimens, the UTS and E were determined after 24h, 7, 30 and 180 days of storage in distilled water. Data were analyzed by ANOVA/Tukey's tests (a=0.05).

Results: Significant reductions of UTS and E (p<0.05) were observed for all tested adhesives within the first 24h of storage in water. The highest percentage of reduction in UTS and E were observed for the most hydrophilic, solvated adhesives, SB and EX. No statistical differences in the UTS and E were observed after the 7^th and 30^th days of water storage, regardless of the material (p>0.05). The UTS and E of SB and EX decreased significantly after 180 days of water storage (p<0.05).

Conclusion: Premature exposure of hydrophilic adhesives to water caused abrupt reduction of their mechanical properties. Efforts must be made to make possible the use of hydrophobic resins in the formation of durable bonds to dentin.