Impact of Zinc Oxide Nanoparticles on Model Resin Composite Properties and Strategies for Optimisation

Abstract

Zinc oxide nanoparticles (n-ZnO) have been widely used in various fields, including dentistry, due to their potent antibacterial properties. Additionally, their white colour closely resembles that of natural teeth, making them an attractive candidate for aesthetic restorative applications. However, despite these advantages, the incorporation of n-ZnO may adversely affect certain material properties, which require further investigation or optimisation. This thesis investigates the effects of n-ZnO additions into model resin-based composites (RBCs), with a focus on properties during polymerisation, optical and mechanical performance, and hygroscopic characteristics; aiming to evaluate their drawbacks and find strategies to improve them. Model RBCs were formulated using a trimodal dimethacrylate resin monomer system (Bis-GMA, TEGDMA, and UDMA) and inert barium glass powder, silica nanoparticles and systematically varied amounts of n-ZnO (0-5 wt.%). The photoinitiator systems consisted of camphorquinone (CQ), ethyl 4-(dimethylamino)benzoate (EDMAB), and diphenyliodonium hexafluorophosphate (DPI). The concentrations of each component were varied depending on the experiment. Depth of cure (DoC) and degree of conversion (DC) were measured using Vickers hardness and FTIR spectroscopy, respectively. Increasing n-ZnO concentrations significantly reduced DoC. However, it was found that the binary initiator system (CQ/DPI/EDMAB) enhanced DoC and the maximum rate of polymerisation (RPmax) compared to the unary system (CQ/EDMAB). Reducing photoinitiator concentrations also improved DoC. Light transmission (LT%) and shrinkage strain (SS) were measured under different irradiance conditions: 1200 versus 2000 mW/cm². n-ZnO significantly reduced LT% from 48–53.9% at 0 wt.% to 11.9–14.6% at 4 wt.%, depending on irradiance and time. Shrinkage decreased with increasing n-ZnO and glass powder loading, ranging from 6.9% to 5.1%. The irradiance mode significantly affected transmission but not material shrinkage. Water sorption, solubility, hygroscopic expansion and Zn2+ ion release were evaluated using multiple techniques. Long-term (168 days) water immersion experiments showed that n-ZnO reduced water sorption and solubility, although increasing hygroscopic expansion. The magnitudes of sorption and solubility remained within ISO 4049:2019 limits. Volumetric expansion ranged from 1.6% to 1.9%. ICP-MS analysis showed a gradual increase in Zn²⁺ release with increasing n-ZnO concentration, up to 675.1 ppb with 5 wt.% n-ZnO. Martens hardness (HM), indentation modulus (EIT), and indentation creep (CIT) were measured after 0 (baseline), 7 and 28 d water storage. At 1200 mW/cm², increasing n-ZnO up to 4 wt.%, increased HM to 355 N/mm² for baseline readings. In contrast, with 4 wt.% n-ZnO cured at 2000 mW/cm², HM significantly decreased to 207 N/mm². Water aging negatively affected HM and EIT, while CIT remained unaffected. These findings provide new scientific insights into resin-composite systems incorporating n-ZnO. They reveal the potential for fine-tuning property outcomes through choice of the photoinitiator system and light-irradiation conditions. These composites are essentially of the ‘flowable’ type and have depths-of-cure appropriate for base layers in deep cavities. But that is exactly where incorporation of powerful antibacterial agents such as n-ZnO are most needed.