Optimizing, Polymerization, Rheology and Adhesion in Experimental Self-Adhesive Composites

Objectives: To evaluate the effects of varying silica nanoparticle concentrations on the polymerization kinetics, rheological behavior, and bond strength of experimental self-adhesive flowable resin composites (SAFRCs), in order to develop next-generation self-adhesives with enhanced clinical performance.
Methods: Five experimental SAFRCs were formulated by varying concentrations of nanosilica (NanoSi: 0%, 2.5%, 5%, 7.5%, and 10%) in a UDMA-PPGDMA organic matrix, containing 10-MDP as a functional monomer and 1% camphorquinone. These components were homogenized using a centrifugal force mixer (Flacktek Speed Mixer, USA) at 1250 rpm for 45 s, at a 2:5 powder-liquid ratio. Polymerization kinetics were analyzed using ATR-FTIR spectroscopy (Spectrum 65, Perkin-Elmer, MA, USA; n=3), comparing 20 s and 40 s light-curing durations, with a high-intensity LED unit (Bluephase PowerCure, Ivoclar Vivadent, Schaan, Liechtenstein; 1200 mW/cm2) on 2 mm-thick discs over 20 min. Kinetics parameters included the time to reach half conversion (t0.5), maximum polymerization rate (%/s), and degree of conversion (Dc). Rheological behavior was assessed with a cone-plate rheometer (MCR 92, Anton Paar, USA), under varying shear rates at room temperature to evaluate viscosity profiles. Finally, microshear bond strength tests were carried out in 2x2 mm SAFRC cylinders bonded to dentin, at 1 mm/min (Shimadzu Autograph AG-IS, Tokyo, Japan), thus comparing adhesive performance across formulations.
Results: Regarding the tested formulations, NanoSi_2.5 showed optimal polymerization efficiency, combining balanced filler content and light diffusion. Higher nanosilica concentrations (≥5%) delayed curing and displayed an increased viscosity at low shear rates, while NanoSi_2.5 displayed smoother flow behavior. Bond strength decreased with rising silica content, with NanoSi_10 showing partial recovery, suggesting excessive filler can hinder adhesive performance.
Conclusions: In this screening assay, NanoSi_2.5 achieved the best balance of polymerization, viscosity, and bond strength. This demonstrates the importance of optimizing nanosilica content for self-adhesive viscous composites.