Dental composite is composed of a resin matrix and filler materials. The resin filler interface is important for most physical properties. There are three causes of stress on this interface including: resin shrinkage pulls on fillers, filler modulus of elasticity is higher than resin, and filler thermo coefficient of expansion allows resin to expand more with heat. When fracture occurs, a crack propagates and strikes a filler particle. Resin pulls away from filler particle surfaces during failure. This type of failure is more difficult with larger particles as surface area is greater. A macrofill composite is stronger than a microfill composite.
Coupling agents are used to improve adherence of resin to filler surfaces. Modification of filler physical structure on the surface or aggregating filler particles create mechanical locking to improve interface strength. Coupling agents chemically coat filler surfaces and increase strength. Silanes have been used to coat fillers for over fifty years in industrial plastics and later in dental fillers. Today, they are still state of the art.
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