Most composites for dental restoration are based on acrylate or methacrylate and ceramic filler, often silica or silicate glasses. The two phases are bonded together with a difunctional coupling agent capable of binding both the filler surface and the polymer matrix by copolymerization. The strength of this bond is of great importance for the mechanical properties of a composite. The hardness and the diamentral tensile strength are higher in composites containing a silanized filler compared to a composite containing an unsilanized filler. The most commonly used coupling agent in dental resin based composites is gamma-methacryloxypropyl trimethoxy silane, which not only increases the strength properties of the material but also limits the hydrolytic degradation.
In dental resin based composite materials zirconia particles are sometimes added to make the material radio-opaque or to improve the mechanical properties. Normally the effect of surface modifying the zirconia with a surfactant and its effect on the water resistance of the interface strength in dental materials is determined by the decrease in flexural strength after water aging by a three point bending. The test method is often defective due to air bobbles, insufficient mixing and other defects in the test specimens. The specimen preparation is a time consuming procedure and the results often have large deviations. We have developed an easier test method, which is not affected by the topology of the filler or the polymerization shrinkage.