FILLERS
Industrial composites use glass, kevlar, graphite, mica, wood, hollow glass spheres, or the like to modify resin. Dental composites require materials that match tooth color and translucency so an optical index of 1.5 is required. Materials such as strontium glass, barium glass, quartz, borosilicate glass, ceramic, silica, prepolymerized resin, or the like are used.
Fillers are placed in dental composites to reduce shrinkage upon curing. Physical properties of composite are improved by fillers, however, composite characteristics change based on filler material, surface, size, load, shape, surface modifiers, optical index, filler load and size distribution.
Fillers are classified by material, shape and size. Fillers are irregular or spherical in shape depending on the mode of manufacture. Spherical particles are easier to incorporate into a resin mix and to fill more space leaving less resin. One size spherical particle occupies a certain space. Adding smaller particles fills the space between the larger particles to take up more space. There is less resin remaining and therefore, less shrinkage on curing the more size particles used in proper distribution.
One micron is a critical filler size. Fillers greater than one micron are visible to the human eye. As resin matrix around filler particles wears, the filler becomes prominent and visible so the composite surface looks rough. Fillers less than one micron do not produce a rough appearing surface with aging. Fillers greater than one micron are referred to as macrofills and fillers less than one micron are referred to as microfills. A new classification of filler is the nano particles. The nano particles fill between all other particles to further reduce shrinkage. A mixture of different particle sizes is referred to as a hybrid.
Distribution of filler particles can be uniform or distributed over a bell curve so a microfill composite might contain many particles greater than one micron but the predominance of particles are one micron or less.
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