Degradation of single bottle type self-etching primer effectuated by the primer's storage period

Purpose: To examine the hydrolytic stability of the functional methacrylate monomer utilized in single bottle type self-etching primers, by using the ¹³C nuclear magnetic resonance (NMR) technique in order to understand why the bond strength of resin to the dentin decreased, when the primer's storage duration was prolonged. Methods: After deuterium oxide was added to the Mega Bond Primer (MB), or Unifil Bond Primer (UB), the MB or UB solutions were then stored at 37°C for 0, 3, and 10 weeks, respectively. At the end of the storage periods, ¹³C NMR observations of the MB or UB solution were performed at 25°C by using an EX-270 spectrometer. Results: The acidic and/or hydrophilic methacrylate monomer utilized in single bottle type self-etching primers became altered during the primer's storage period. Alteration of the functional monomer was possible since the acidic monomer, utilized in the self-etching primer, could supply sufficient amounts of hydronium ions, which were the result of the dissociation of the acidic group in the acidic monomer. The degradation of the primer, caused by the alteration of the functional monomer, would result in the degradation of the etching and/or priming efficacy on the tooth. The bond strength, as a result, would become decreased as the storage period was prolonged.