Thiourethane oligomers and functionalized TiO₂ nanostructures in experimental resin composites

Titanium dioxide (TiO₂) nanostructures and thiourethane oligomers (TU-Oli) have each been shown to improve the properties of resin composites. This study evaluated their combined effect on clinically-relevant properties of experimental composites.

BisGMA/UDMA/TEGDMA was used as the resin matrix (CQ/EDMAB/BHT photoinitiator) and 70 wt% silanized filler, 1 µm average size was used as the main filler system. In phase 1, methacrylate-based TMSP-silanized TiO₂ nanostructures (nanotubes or nanoparticles) were incorporated at 0, 0.3, or 0.9 wt% into the matrix with or without 20 wt% thiourethane oligomer (TU-Oli). Polymerization kinetics, degree of conversion, light transmission, and depth of cure were measured. This allowed for the evaluation of the presence of TU, the type and concentration of nanostructures affected conversion in depth. Based on results from phase 1, in phase 2, 0.3 wt% TiO₂ nanostructures were added to a TU-Oli-free matrix. Phase 2 focused on mechanical properties of a selected number of groups. Flexural strength/modulus (FS/E), polymerization stress (PS), and glass transition temperature (Tg) were measured. Data were analyzed by two-way ANOVA/Tukey or Kruskal-Wallis/Dwass-Steel-Critchlow-Fligner tests (α = 0.05).

All TU-Oli significantly increased conversion (p < 0.001) and decreased rate of polymerization (RP_max; p < 0.001). Nanostructure type and content significantly influenced RP_max (p = 0.002, p = 0.023). Composites containing 0.3 wt% TiO₂ had greater depth of cure and light transmission than those with 0.9 wt% (p < 0.001). Flexural strength (p = 0.256) and toughness (p = 0.260) were similar for all groups; modulus was significantly higher for the control (p = 0.006). Polymerization stress (p = 0.094) was similar for all groups. Tg was significantly lower for all composites containing TiO₂ nanotubes (p < 0.05), while nanoparticle-containing groups were similar to the control.

Tu-Oli significantly improved properties of experimental composites, while silanization of TiO₂ nanostructures did not affect properties. 0.3 wt% TiO₂ offers the best balance between optical and physical performance for this composite system.

Judicious design of TiO₂-containing materials, in conjunction with TU-Oli is a promising strategy to improve restorative composite performance.