Development of mechanical properties in dental resin composite: Effect of filler size and filler aggregation state

The aim of this work was to study the effect of filler size and filler aggregation state on the mechanical properties of dental resin composites evaluated at filler loadings between 20 wt% and up to 76.5 wt%. Non-aggregated silica nanoparticles (SiNP _MPS ) (80 nm), doughnut-shaped silica nanoclusters obtained by spray drying (SDSiNP _MPS ) (3.5 μm) and amorphous barium-alumina borosilicate microparticles (BaAlBoSi _MPS ) (1.0 μm), functionalized by 3-methacryloxypropyl trimethoxysilane (MPS), were the fillers incorporated into resin matrix dental composites composed of triethylene glycoldimethacrylate (TEGDMA), urethane dimethylacrylate (UDMA), bisphenol A polyethylene glycol diether dimethacrylate (Bis EMA), and bisphenol A glycidyl methacrylate (BisGMA) (0.3:0.7:1:1 weight ratio, respectively). The mechanical properties developed in the resin composites were correlated with the formation of percolated-like particle networks, as observed by scanning electron microscopy (SEM), and volume fraction percolation thresholds (ϕ _c ) calculated from a percolation model. Resin composites with non-aggregated SiNP _MPS showed an apparent percolation threshold ϕ _c = 0.15 (i.e. 27 wt%); above this filler concentration and up to a volume fraction of particles (ϕ _P ) of 0.24 (i.e. 40 wt%) there was an increase in the flexural modulus and the compressive strength of the resin composite. However, a further increase in filler concentration diminished all its mechanical properties due to a decrease in the particle-matrix adhesion strength, demonstrated by the increase in surface roughness and fracture steps as observed by SEM images. On the other hand, a resin composite filled with doughnut-shaped silica nanoclusters (SDSiNP _MPS ) showed an apparent percolation threshold ϕ _c = 0.41 (i.e. 60 wt%); increasing filler loading over this concentration generated an improvement in its mechanical properties, except the flexural strength also due to a decrease in the particle-matrix adhesion strength. The resin composites obtained with amorphous individual BaAlBoSi _MPS microparticles (1.0 μm) and BaAlBoSi _MPS microparticle aggregates (ca. 40.0 μm) showed an apparent percolation threshold ϕ _c = 0.41 (i.e. 64 wt%) that promoted an improvement in all their mechanical properties. SEM image of BaAlBoSi _MPS resin composite at high filler loading (≥ 60 wt%) showed a decrease in fracture steps and no presence of voids, indicating a better adhesion between amorphous BaAlBoSi _MPS particles and the polymeric matrix, which explains the improvement of mechanical properties. Resin composites filled exclusively with silica doughnut-shape nanoclusters or amorphous BaAlBoSi _MPS microparticles could develop mechanical properties similar to or even better than those obtained by mixing nanofillers with spherical nanoclusters, which are commonly used in commercial resin composites.