Evaporation of low-volatility components in polymeric dental resins

This study provides measurement of the volatility of selected photoinitiators and monomers used in dental adhesive resins. A detailed determination of the spatial and temporal character of camphorquinone (CQ) volatilization with respect to air flow conditions as well as media viscosity is assessed to gauge the effect of evaporative loss on the photopolymerization process and the photopolymers formed.

Vapor pressures of materials are measured by thermogravimetric analysis. A quantitative model assuming one-dimensional Fickian diffusion with surface evaporation is presented and compared with measured photoinitiator volatilization from viscous and non-viscous resin samples, obtained by spectrophotometry and confocal microscopy. Model resins are prepared and subject to airthinning followed by photocuring, monitored in real-time by Fourier transform infrared spectrometry.

Vapor pressure measurements of the individual components of the adhesive resin span nearly four orders of magnitude, with the photoinitiator CQ near the middle (0.6Pa) and the monomer HEMA at the upper end (10Pa). We see depth-averaged CQ loss from non-viscous open films, while depthresolved measurements of viscous droplets show strong surface-localized CQ depletion. Good agreement is observed between measurements and the model. Finally, air-thinning of samples prepared with more-volatile photoinitiator and monomer is shown to cause longer induction times, slower early-stage polymerization rates and lower late-stage degree of conversion.

Widely used compounds with vapor pressures as low as 0.6Pa (0.001Torr) undergo significant volatilization from samples ventilated under conditions generally representative to clinically used air-thinning procedures, with the potential to adversely affect the photopolymerization of both viscous and non-viscous resins. The inverse relationship between air-thinning and adhesive bond strength, observed elsewhere, may be partially caused by this same effect.