E21. Explain why the repair strength of dental composites is
substantially lower than the unrepaired composite.
How Different
Treatments Affect the Flexural Repair Strength.
Only acid = cut surface was acid etched and coated with an unfilled resin before the composite was rebuilt
Toluene = cut surface was coated with a toluene-silane solution before it was coated with an unfilled resin and repaired.
Scotchbond = acid etched and coated with Scotchbond (phosphate based dentin bonding agent) before composite was rebuilt.
Scotchprime = acid etched and coated with Scotchprime (silane containing bonding system) before composite was rebuilt.
Unrepaired = original composite build up
The above graph shows that strength decreases after storage in water. This decrease is, at least to some extent, due to the plasticizing effect of water.
Explanations
If a composite has set, most of the double bonds have been consumed. The cross-linked structure makes it more difficult for monomer molecules to diffuse into the cured resin matrix within a clinically acceptable time.
If a set composite material is cut, filler particles are exposed and ground flat. During this step, all silane present on the cut surface is removed, which means that no chemical bonds can be formed between the filler and the matrix in this region.
The above considerations means that when an uncured composite is placed in contact with cut composite surface, only a few covalent bonds can be formed between the new and old composite. By making the surface rougher and treating the exposed filler particles with a silane makes it possible to improve the repair strength, but rarely to strength values exceeding 50% of the strength of the unrepaired composite.
