Composite fillers

The reinforcing phase in direct dental restoratives is based on glass or ceramic particles. Incorporation of these inorganic particles imparts improved strength and wear properties, decreased CTE, and reduced polymerization shrinkage. In addition, incorporation of heavy metals into the filler provides radiopacity. The initial composite fillers were limited in size because of the limited ability to grind and sieve quartz, glass, borosilicate, or ceramic particles. The particle size range was from 0.1 to 100 mm. Smaller particles have been prepared through hydrolysis or precipitation to produce what is termed fumed or pyrolitic silica. The particle sizes obtained from this process range from 0.06 to 0.1 mm 6.

The most recent process to form particles is through sol-gel chemistry, which uses silicate precursors that are polymerized to form particles ranging from nm to mm dimensions. This sol-gel process can be used to form almost mono dispersed particle sizes, which can be a significant advantage because different particle sizes can be produced and blended to optimize the packing efficiency and filler loading of the composite. In addition, the ability to produce submicron size particles allows the production of nanocomposites in which the particles approach the size of the polymer matrix molecules. Theoretically, nanocomposites have the potential to exhibit excellent mechanical and physical properties at higher filler loadings [25, 55, 80].

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