Methacryloyl-Modified Silica Nanoparticles Vary the Hardness but Reduce the Flexural Strength of 3D-Printed Photopolymer Resins

Photopolymer resins were reinforced with methacryloyl-modified silica nanoparticles (MSNs) for use in stereolithography (SLA) three-dimensional (3D) printing, and the mechanical properties were evaluated. A commercially available transparent SLA acrylic-based resin was modified by incorporating MSNs, at concentrations of 5, 10, and 30 wt%. The resulting composite materials (SN-5%, SN-10%, and SN-30%) were compared with an unmodified control group (SN-0%). Specimens were fabricated using an SLA 3D printer, and their mechanical properties were evaluated by dynamic microindentation and three-point bending tests. The results revealed divergent effects: the dynamic hardness and elastic modulus were significantly higher in the 30 wt% group than in the lower concentration groups. In contrast, the flexural strength and flexural modulus decreased markedly with increasing filler content, with SN-30% exhibiting the lowest values (p < 0.05). Microscopy revealed cracks on the surface of SN-30% specimens. This is attributed to a statistically significant increase in the suspension’s viscosity at higher filler content, which impairs the printing process and leads to the formation of structural defects. These findings demonstrate that while increasing the nanoparticle concentration can enhance surface-related mechanical properties, such as hardness, it can simultaneously degrade the bulk flexural performance. Therefore, the filler content must be selected based on application requirements: higher content for wear resistance in occlusal restorations and lower content for flexibility in devices such as dentures. Overall, this study highlights the critical need to balance the reinforcing effects of nanoparticles with their detrimental impact on processing properties, such as viscosity, to preserve the structural performance of SLA-printed photopolymer composites.