Influence of curing modes on thermal stability, hardness development and network integrity of dual-cure resin cements

To explore the effect of different curing modes of conventional and self-adhesive dual-cure resin cements on their rates of thermal decomposition, hardness development and network integrity.

Five self-adhesive (PANAVIA SA, RelyX Universal Resin, RelyX Unicem 2, Bifix SE and SpeedCEM Plus) and three conventional (PANAVIA V5, Nexus Third Generation and RelyX Ultimate Universal) dual-cure resin cements were investigated. Thermal decomposition stages, initial onset temperatures, the maximum rate of mass-loss and the filler mass-fraction of each resin cement were analysed by thermogravimetric analysis (TGA). Surface hardness was measured at 1h post-cure and after 24h of dry storage at 37°C. The relative network integrities were estimated from reductions in hardness after 168h of water storage. Data were analysed via one-way ANOVA, Tukey post-hoc tests and paired/independent sample t-tests (a=0.05).

No difference was apparent between TGA data for self-cured and light-cured specimens. Numerical differentiation of mass-loss versus temperature showed either single or multiple peaks. For the set of 8 cements, the maximum rate of mass-loss (%/°C) correlated negatively with residual mass at 600°C. All dry-stored cements increased in hardness from 1 to 24h, ranging from 20.4% to 52.6% for light-cure mode and from 41.3% to 112.6% for self-cure. After 168h water storage, the hardness of cements decreased: by 18.5%-36.2% for light-cured and by 9.8%-17.9% for self-cured. Overall, surface hardness was greater for light-cured cements. The initial onset temperature (IOT) of thermal decomposition correlated negatively with the hardness decrease produced by water-storage: r²=0.77 for light-cure and r²=0.88 for self-cure. This provided the basis for a relative scale of composite network integrity, probably reflecting differences in cross-link density.

Light-curing, where possible, remains beneficial to the hardness and related properties of dual-cure resin cements. Combination of TG analysis and solvent softening experiments give an indication of relative network integrity - between materials - and their relative cross-link densities.