Influence of Aluminosilicate Clay Nanofiller Reinforcement on the Flexural Strength, Surface Hardness, Surface Roughness and Microstructure of Auto-Polymerized PMMA Denture Base Resin: An In Vitro Study

Background/Objectives: Auto-polymerized polymethyl methacrylate (PMMA) is widely used in denture repair, relining, and base-plate fabrication but is susceptible to fracture due to limited flexural strength and fatigue resistance. This in vitro study investigated the concentration-dependent effects of synthetic aluminosilicate clay (ASC) nanoparticles on the flexural strength, Vickers hardness, surface roughness, and microstructure of auto-polymerized PMMA. Methods: A total of 120 specimens of SR Triplex Cold were prepared with 0 (control), 1, 3, or 5 wt.% ASC (50–100 nm; n = 30/group). ASC was dispersed in the monomer by probe sonication for 10 min. After pressure curing and storage in distilled water at 37 °C for 7 days, flexural strength, Vickers hardness, and surface roughness (Ra) were measured (n = 10/test/group). Microstructural characterization employed SEM, EDX, and XRD. Data were analyzed using ANOVA with appropriate post hoc tests (α = 0.05). Results: ASC concentration significantly affected all three properties (p < 0.001). At 1 wt.% ASC, flexural strength increased from 61.50 ± 3.37 to 79.70 ± 2.11 MPa (+29.6%), and hardness increased from 84.39 ± 3.81 to 97.84 ± 2.56 VHN. At 3 and 5 wt.%, these properties declined to 52.60 ± 3.75 and 31.70 ± 2.83 MPa, and to 63.97 ± 2.94 and 27.73 ± 2.62 VHN, respectively. Surface roughness increased with ASC concentration, from 0.102 ± 0.012 µm (control) to 0.195 ± 0.013, 0.245 ± 0.022, and 0.327 ± 0.035 µm at 1, 3, and 5 wt.%, respectively. SEM revealed favorable particle dispersion, crack deflection, and particle bridging at 1 wt.%, whereas higher concentrations promoted agglomeration, microvoid formation, and interfacial debonding. XRD confirmed the coexistence of the amorphous PMMA phase and crystalline aluminosilicate reflections. Conclusions: Under the conditions tested, 1 wt.% ASC provided the most favorable balance of mechanical and surface properties, improving flexural strength and hardness while maintaining Ra below 0.2 µm. Higher concentrations compromised mechanical performance, with the 3 and 5 wt.% groups falling below the minimum flexural strength for Type 2 polymers. The findings suggest that reinforcement depends primarily on particle dispersion and interfacial stress transfer rather than chemical modification of the PMMA backbone.