E02. Identify three monomers often used in dental composites, and discuss their advantages and drawbacks.

The most commonly used monomer in dental composites and sealants is bisGMA. As mentioned earlier, bisGMA has a very high viscosity, making it difficult to incorporate filler particles into this monomer.

To reduce the viscosity of bisGMA, Bowen suggested that the bisGMA should be replaced with another dimethacrylate, a dimethacrylate called triethyleneglycoldimethcarylate (TEGDMA). The advantage with the TEGDMA is its lower viscosity and higher molecular flexibility. The higher flexibility increased the probability of increased interaction during cure, which meant that the addition of TEGDMA to the bisGMA resin resulted in a more complete consumption of the double bonds of the methacrylate groups. However, the lower molecular weight of the TEGDMA as well as its contribution to higher double bond consumption resulted in increased shrinkage.  

To manage the problems with the high viscosity of the bisGMA and the increased shrinkage caused by the addition of TEGDMA, the British company ICI explored the use of a urethane (UEDMA). The advantage with this monomer was that it, like bisGMA and TEGDMA, was a dimethacrylate. Its molecular weight was also close to that of bisGMA, which meant that it did not shrink as much as TEGDMA. Compared to bisGMA, the UEDMA chain was also more flexible, which resulted in more efficient cure than the bisGMA. A potential drawback with this monomer, though, is that the urethane groups have a tendency to hydrogen bond over time, which can change the handling properties to some extent. To minimize the latter, UEDMA is often mixed with other dimethacrylates such as TEGDMA.