Effect of solvent type and content on monomer conversion of a model resin system as a thin film

Objective: The purpose of this study was to examine the effect of solvent concentration on the degree of conversion of a model photo-activated resin formulation when placed as a thin film in ambient air. Methods: A photo-activated co-monomer mixture (ethoxylated bis-GMA/TEGDMA) (Bisco Inc.) was diluted into six concentrations (from 1.0 to 13.0 M) with either acetone or ethanol. A controlled volume of diluted, uncured resin was placed on the horizontal surface of an attenuated reflectance unit and the infrared (IR) spectrum obtained. A light emitting diode light-curing source (Bluephase, Ivoclar/Vivadent) then immediately irradiated the specimen for 10 s (n = 5). Five minutes after exposure, IR spectra of the cured material were obtained, and monomer conversions were calculated using standard methods that monitored changes in aliphatic-to-aromatic C{double bond, long}C absorbance ratios in the uncured and cured states. Results: In the acetone/model resin system, maximum conversion occurred with 2.5-5.0 M solvent. In the ethanol/model resin system, conversion peaked at 2.5 M solvent. Above 5.0 M solvent, conversion values declined rapidly for both solvents. A 13.0-M solution resulted in near 0% conversion for both solvents. At 2.5 and 5.0 M acetone, conversions exceeded those of equivalent concentrations of the ethanol-based system. Conclusions: For both an acetone- and ethanol-solvated model resin system, conversion did not immediately decrease with addition of solvent, but instead increased over that of the model resin alone. At higher solvent content (greater than 2.5 M ethanol and 5.0 M acetone), conversion rapidly declined, with ethanol causing less conversion at equal-molar solvent concentrations.