Factors Contributing to the Incompatibility Between Simplified-step Adhesives and Chemically-cured or Dual-cured Composites. Part. III. Effect of Acidic Resin Monomers

This study examined the polymerization kinetics of acid-contaminated light- and chemically-cured resins with the use of differential scanning calorimetry. Light-cured and chemically-cured versions of an experimental bis-GMA/TEG- DMA resin at the ratio (w:w) of 62:38 were prepared. Four acidic resin monomers with either carboxylic (DSDM and MAA) or phosphoric functional groups (MP and 2MP) were added at a 10 wt% concentration to these resins to simulate the intermixing of resin composites and unpolymerized acidic monomers along the adhesive-composite interface of simplified-step dentin adhesives. Different concentrations of the most acidic monomer 2MP (10 to 50 wt% for the light-cured resin, and 2 to 4 wt% for the chemically-cured resin) were also added to examine their contribution to reducing the extent and rate of polymerization of these resin mixtures. The effect of sodium benzene sulphinate on the polymerization of the chemically-cured resin that was contaminated with 10 wt% 2MP was also investigated. At 10 wt% concentration, all acidic monomers had a limited effect on the polymerization of the light-cured resin. The rate and extent of polymerization in the chemically-cured resin was substantially reduced by the carboxylic acid monomers DSDM and MAA, while polymerization was completely inhibited in the presence of the organophosphate monomers MP and 2MP. Substantial reductions in the rate and extent of polymerization of the light-cured resin occurred only in the presence of high concentrations (30 to 50 wt%) of 2MP. More acute polymerization inhibition was observed when the chemically-cured resin was contaminated with 2 to 3 wt% 2MP, with polymerization completely inhibited at 4 wt%. Addition of sodium benzene sulphinate to 2MP-contaminated chemically-cured resin revived the previously uncured resins, but their rate and extent of polymerization were still inferior to that of uncontaminated chemically-cured resin. When the influence of adhesive permeability was excluded with the use of neat, water-free resins, deactivation of the tertiary amine utilized in the chemically-cured resin by even very low concentrations of acidic resin monomers accounts for the reported incompatibility between simplified-step dentin adhesives and chemically-cured composites. The polymerization of light-cured resin is only affected by much higher concentrations of acidic resin monomers.