Objective: It has been reported about incompatibilities between light-cured adhesives and chemically polymerized resin composite materials. One hypothesis for this is the protonation of tertiary amines from the peroxide amine initiator system by acid monomers. If sufficient quantities of amine are consumed by the acid-base reaction, the formation of free radicals is diminished, resulting in inadequate polymerization. This study determined the effect of pH-value and initiator concentration on the polymerization behaviour of an experimental, self curing, model system.
Methods: Investigations at the adhesive composite junction are quite difficult and sometimes questionable; therefore we used a model for our investigations. Variation of the pH value (pH: 4.5-5.0) (A): 68.9% TEGDMA, 3.3% DEPT, 13.9% EtOH and 13.9% MA154 (mixtures from MA 154 and MA 154 Na-salt). Variation of the amine concentration (5 steps from 3.3-9.0% DEPT) (B): 71.3% TEGDMA, 14.4% EtOH and 1.4% MA154, 12.9% MA154 Na-salt and various amounts of DEPT. After initiation by addition of 100 µL BPO 20% in EtOH the polymerization was observed for both experiments by exotherm time measurements according to ISO 4049.
Results: The investigations demonstrate a strong dependence of the self curing process from the pH-value. PH 4.93 of the adhesive composition corresponds to 452 s whereas pH 4.55 corresponds to an exotherm time of 682s (A). The self curing amine peroxide system will be disturbed by the reaction between amine accelerator and acidic monomers. Related to this the polymerization process also depends on the amine concentration. Using 9wt.% DEPT instead of 3.3% (pH:4.55), the exotherm time shortens down to 396s (B).
Conclusion: For a reliable polymerization of self-etching, self-curing systems the neutralization of the acidic monomers and a high amount of amine accelerator is essential.