Effect on hygroscopic characteristics of n-ZnO additions to resin composite

This study investigates the effect of adding different amounts of n-ZnO to model resin-based composites on their hygroscopic characteristics. Six groups (n = 5) were formulated using Bis-GMA, TEGDMA, UDMA, inert barium glass powder, silica nanoparticles, and varying amounts of n-ZnO (0-5 wt.%). The photoinitiator system included camphorquinone, diphenyliodonium hexafluorophosphate, and ethyl 4-(dimethylamino) benzoate. Sorption, solubility, and hygroscopic expansion specimens were prepared following ISO 4049:2019 and immersed in water for 168 days. Sorption and solubility were assessed with an analytical balance, while hygroscopic expansion was measured using a laser scan micrometer. Zn²⁺ release was analyzed using ICP-MS (n = 3). Increasing n-ZnO concentrations significantly reduced sorption and solubility, with the control (0 wt.% n-ZnO) showing the highest values (28.7 and 2.1 µg/mm³) and the 2 wt.% n-ZnO group showing the lowest (27 µg/mm³, -0.4 µg/mm³). However, increasing the concentrations of n-ZnO significantly increased the hygroscopic expansion. The volumetric expansion for the lowest (control) and highest (n-ZnO at 3 wt.%) was 1.63% and 1.87%, respectively. ICP-MS revealed progressively higher Zn²⁺ release with increasing n-ZnO concentration, peaking at 675.1 ppb in the 5 wt.% group. Overall, n-ZnO reduced sorption and solubility while increasing expansion and Zn²⁺ release, with all formulations meeting ISO 4049 standards.