Young's Modulus Correlation with Filler Size and Shape of Resin-Composites

Objective: To measure the Young's moduli (E) of a series of novel polymeric restorative materials, using two different techniques, and examine how E was influenced by differences in filler-size and shape.

Methods: Twelve model resin-composite formulations (Ivoclar/Vivadent, Schaan, Liechtenstein), with systematically-varied filler-size and shape but constant volume-fraction were investigated. Cylindrical specimens (n=6) (5mm x 6mm) were prepared from each material and were free to expand radially under loading. Compression loading was performed with a Universal Testing Machine (Zwick/Roell Z020), at 1mm/min. The Young's moduli were determined by analyzing the load/deformation plots with Sigmaplot 8.0 software. Disc specimens (12mm x 2mm) were prepared from each material and mounted in self-curing polystyrene resin. The samples were mechanically polished and tested with a nanoindenter (Nanoindenter XP, MTS Systems Co) equipped with a Berkovich diamond tip. Thirty indentations were made in each sample. Oneway ANOVA and the Bonferroni post hoc multiple comparison test at a p<0.05, were used for statistical analysis.

Results: Young's moduli ranged from 4.77 to 5.72 GPa using compression loading, and from 9.31 to 17.03 GPa under nanoindentation. Statistically-significant differences were found, depending on filler size and shape, with the nanoindentation technique but not from bulk compression.

Conclusion: In the composite micro-mechanics governing Young's moduli, filler size and shape seemed to be secondary, fine-tuning factors. Resin-composites with irregular-shape filler particles presented higher moduli than those with spherical filler particles, for constant volume-fraction.