2015 Volume 58 Issue 5 Pages 398-405
Purpose: One-step bonding adhesives now available on the market have a tendency to decrease the bond strength of resin to teeth when stored for a prolonged time or at a high temperature. In this study, in order to understand why the bond strength of resin to the teeth decreased, the hydrolysis of the functional methacrylate monomer utilized in one-step bonding adhesive was examined by using the nuclear magnetic resonance (13C NMR) technique and shear adhesive strength.
Methods: G-BOND PLUS (G-BP, GC) used as a one-step bonding material was stored at 40°C for 0, 3, 7 and 14 weeks in an incubator to serve as a test sample.
G-BP 300 mg and dimethyl sulfoxide (DMSO) 250 mg served as a test sample for 13C NMR and were weighed accurately in an NMR tube, followed by shaking and mixing before analysis by using the 13C NMR technique. Furthermore, the shear adhesive strength was also measured for the same storage period.
Results: As a result, it was found that two NMR peaks, one of which belongs to the carbonyl carbon of carboxyl methacrylate and the other to the methylene carbon of ethylene glycol, were detected when G-BP solution was stored at 40°C with their augmented NMR peak strengths when the storage duration was prolonged.
When G-BP was stored at 40°C for 14 weeks, the methacryloxy ester group of 4-MET was degraded by 13.76%; that is, the ester group of 4-MET was hydrolyzed, subsequently producing methacrylic acid, ethylene glycol and trimellitic acid.
In addition, the bond strength to enamel was 17.20 MPa when stored for 0 days (control), but the value decreased gradually with the prolonged storage period and markedly to 12.33 MPa when stored for 14 weeks. Similar to the bond strength of enamel, the value of the bond strength to dentin decreased from 15.36 to 11.61 MPa when the storage period was prolonged. The bond strength of resin to the tooth thus seemed to decrease when 4-MET in the one-step bonding adhesives was altered by hydrolysis in response to the prolonged storage period.
Conclusions: The bond strength of resin to the tooth substance was attributed to degradation of the functional monomer in one-step bonding adhesives, by the extension of the storage period.
