Characterization of Universal Composite Resin Filler Particles.

Objective: Characterize the morphology and distribution of filler particles of universal composite resins using Scanning Electron Microscopy, and calculate the residual filler mass fraction (filler wt%) of each composite resin with Thermogravimetric Analysis to compare the data with the respective filler wt% reported in the literature.

Methods: Seven universal composite resins (CRs) were analyzed in this study: Essentia Universal (EU), Filtek Easy Match (EM), Filtek Universal (FU), Omnichroma (OM), Simplishade (SS), TPH Spectra ST HV (ST), and Venus Diamond One (VO). Two classic multi-shade CRs, Charisma Opal (CO) and Estelite Omega (EO) were used as controls. Two grams of each CR were dissolved in acetone, and the respective filler residue was dried and mounted on aluminum stubs. The specimens were sputter-coated with Pt and observed under a Field-Emission Scanning Electron Microscope to analyze the particle size and distribution at magnifications from ×5000 to ×80,000. For the Thermogravimetric Analysis, CR specimens were continuously weighed while heating up to 1000^o C to burn the organic contents and calculate the residual mass fraction of each material.

Results: Electron microscopy-OM showed a monomodal particle distribution of individual microparticles that contained nanoparticles. Sporadic agglomerations of microparticles were also observed for OM. EO, EU, FU, ST, VO, and EM displayed a bimodal filler distribution. The particles of CO and SS were organized under a trimodal distribution. There were similarities in particle shape and distribution for EM and FU, as well as for EO and OM. Classic particles of irregular shape larger than 2 μm were observed for CO, EU, ST, and VO. Clusters of nanoparticles up to 7.5 μm wide were observed for EM, FU, and SS. Thermogravimetric Analysis-The measured filler wt% was material specific. It ranged from 66.27% for EU to 78.28% for VO. The relative difference between the filler wt% measured for VO compared to the filler wt% reported in the literature was the smallest (-3.4%). EU resulted in the widest discrepancy (-18.2%).

Conclusion: The particle size and distribution varied for each composite resin, except for the pairs of materials EO/OM and EM/FU made by the same manufacturer. The filler wt% measured in this study was not in agreement with the description provided in the current literature for any of the CRs tested.