Materials Science Effect of Filler Content and Size on Properties of Composites

Two series of dental composites, along with the unfilled resin matrix, were examined to determine the effects of filler level and size on selected properties. Both series were prepared by incorporating a silanated barium borosilicate filler into a visible-light-activated polyphenylene polymethacrylate resin matrix. One series had a filler particle size of 2 microns, with filler levels of 20, 40, 45, 50, and 53% (vol). The second series contained a 15-microns filler in amounts of 20, 40, 50, 60, and 65% (vol). Tests conducted included: depth of cure as evaluated by hardness, water sorption, compressive strength, stress-strain behavior under slow compression, toothbrush abrasion, and wear by hydroxyapatite. Analysis of the data indicated that increased filler levels resulted in increased hardness, compressive strength and stiffness, and decreased water sorption. Also, there was a slight trend toward improved depth of cure. Incorporation of the 2-microns filler decreased the abrasion resistance of the resins to toothbrushing as compared with the unfilled resin, while addition of the 15-microns filler improved resistance. All filled resins exhibited a significant improvement in resistance to wear by hydroxyapatite as compared with the unfilled resin. There was a trend for increased wear with increased filler level. The particle size of the filler appeared to have a moderate influence on the properties. When compared with 15-microns filled resins of the same filler levels, the 2-micron filled series appeared to have inferior properties in terms of depth of cure, compressive strength, water sorption, and resistance to toothbrush abrasion. Properties which were less affected by particle size were hardness, stiffness, and wear resistance to hydroxyapatite.