Effects of residual ethanol on percent conversion of experimental adhesives

Objectives: To evaluate the extent of ethanol retention in five light-curing experimental methacrylate-based dental adhesives, after solvent evaporation, and the effect of residual ethanol and curing time on degree of conversion (DC) and maximum polymerization rate (DC%/s).

Methods: Resin blends tested were: (R1) 70 wt% E-BisADM, 28.75% TEGDMA; (R2) 70% bisGMA, 28.7% TEGDMA; (R3) 70% BisGMA, 28.7% HEMA; (R4) 40% BisGMA, 30% TCDM, 28.75% HEMA; (R5) 30% BisMP, 28.75% HEMA. All blends included 1% EDMAB and 0.25% CQ. Ethanol (10wt%; 20wt%; 30wt%) was added to each comonomer blend. Known weights of these mixtures were placed in glass wells and evaporated for 60s. The weight of the mixtures before and after evaporation was measured, allowing calculation of ethanol loss/retention gravimetrically. The same blends were irradiated with a halogen curing-unit (600mW/cm2) for 20, 40 and 60s. DC and DC%/s were calculated using a FTIR spectrophotometer. Both ethanol evaporation and FTIR data were analyzed with two-way ANOVA and Tuckey's post-hoc test.

Results: The percentage of ethanol retained in the comonomer mixtures significantly increased with comonomer concentration (p<0.01), varying from 12-17% in 30wt% ethanol solvated blends to 10-16% with 20wt% ethanol to 5-9% in 10wt% ethanol blends. The DC of ethanol/comonomer blends increased with ethanol concentration in 4 of the 5 comonomer blends (p<0.05) increasing by 30 to 45% when 10 or 20wt% ethanol was added to neat resins, regardless of curing time. 30wt% ethanol lowered DC at 20s but increased DC by 40-60s depending on the comonomer blends. As ethanol concentrations increased from 0 to 10, 20 or 30wt%, the maximum rate of cure of all resins decreased significantly (p<0.05).

Conclusion: Even after prolonged solvent evaporation, residual ethanol remains entrapped within the experimental adhesive blends as comonomers lower ethanol vapor pressure. This facilitates DC but lowers the DC%/s of polymers.