E05. Discuss the bonding mechanism between a coupling agent and a filler surface and between a coupling agent and a resin matrix.

In order to make a strong polymer-ceramic composite (polymer matrix containing silica and/or glass particles), the filler particles must be bonded to the resin. This bonding is achieved with a so-called "coupling agent." The most efficient coupling agent between methacrylates (bisGMA, TEGDMA or UEDMA) is methacryloxypropyltrimethoxy silane. The methacryloxypropyltrimethoxy silane molecule can at one end bond to the silica surface and at the other end bond to the matrix resin.  

However, before the silane can form a bond, the methoxy groups must be replaced with OH groups (see above). This reaction occurs when water is present, and sufficient water is present on most filler surfaces. It is important to emphasize here that also silane used for ceramic inlay and veneer coating is sensitive for water. Water present in the air can convert the silane to the silanol. If this happens, the silane may star to autopolymerize via condensation reactions, and the silane becomes useless for treating inlays and veneers. Practically, though, one can detect such a change by looking at the translucency of the silane solution. A transparent silane solution is OK, while a milky looking solution should not be used.  

Filler particles lacking silane are shown above. The resin matrix cannot transfer stess to the filler in this case. 

Filler particles with silane transfer stress between filler and matrix.

From the above it should be clear that by treating a filler surface, veneer or an inlay/onlay with a coupling agent such as silane,, it is possible to form a chemical bond between the resin and the ceramic surface (see above).

The slides below show how the silane forms filler coating, and how the dimethacrylate resin later bonds to the filler. The coupling agent makes it possible to transfer stresses from the weaker resin matrix to the filler. Without a coupling agent, the composite would become weaker in tension and filler particles would easily be plucked out from the surface during mastication. The latter may explain why the first composites had poor wear resistance, since the first composites had not been optimized regarding filler silanization.