Effect of Immersion in Quaternary Ammonium Methacryloxy Silane Mixed Monomer on the Mechanical Properties and Antibacterial Activity of a 3D-Printed Urethane Dimethacrylate Denture Base Resin

Denture base resins are susceptible to microbial colonization, and current antibacterial additives often lose effectiveness and may weaken material properties. This study evaluated whether immersion in a quaternary ammonium methacryloxy silane (QAMS)-containing monomer can enhance antibacterial activity without compromising the mechanical properties of digital light processing-printed urethane dimethacrylate denture base resin. Specimens of printed denture base resin were immersed in mixtures of denture base resin and a QAMS-containing monomer at ratios of 10:0 (Control), 7:3 (K3), 5:5 (K5), 3:7 (K7), and 0:10 (K10), followed by post-curing. Flexural strength and modulus were measured by three-point bending, and surface hardness was assessed by Vickers microhardness testing. Antibacterial activity against <i>Streptococcus mutans</i> was assessed by inhibition-zone and colony-counting assays. All QAMS-treated groups preserved flexural strength, with a slight reduction in modulus in K5 (<i>p</i> < 0.05), while hardness remained unchanged. Antibacterial activity improved in all QAMS-treated groups; K5 and K7 showed the strongest results. Surface analyses using scanning electron microscopy and energy-dispersive X-ray spectroscopy verified formation of a Si-rich modified layer. QAMS immersion followed by post-curing produced a stable, contact-active antibacterial surface without reducing mechanical properties. Among the formulations, K7 (~21 wt% QAMS) provided the most favorable balance of antibacterial activity and mechanical performance.