Resin-based materials: Degree of conversion

Resin-based materials, such as resin-based composites, adhesives, pit & fissure sealants and resin cements, undergo monomer to polymer conversion during both light-activated or chemically-activated polymerisation. Whilst conversion is an inherent property of resin-based materials, the degree of conversion (DC) depends on material chemical composition and curing conditions. The DC affects mechanical properties of resin-based materials, such as wear, fracture toughness, hardness, flexural modulus and fatigue. Less than optimal conversion may compromise mechanical properties but also result in leaching of monomers from restorations.

Traditionally, manufacturers' technical and scientific data did not incorporate results of internal or external tests for the DC. Even most recent materials often lack these data but there seems to be a growing understanding of the importance of this property.

The DC of resin-based materials in dental studies has been determined using various methods for more than two decades, but recently, the most widely accepted and used methods are infrared and Raman spectroscopy. These are based on measuring the changes in either the absorbance or scattering effect of those molecular groups which take part in polymerisation of resin-based materials. The DC is determined as the ratio of absorbance or scattering of these groups and a certain internal standard in uncured and cured material. An internal standard is another molecular group which does not take part in polymerisation and, thus, its infrared absorbance or Raman scattering remains constant before and after polymerisation.

It is very important to point out that the DC indicates the number of unreacted methacrylate or other polymerisable groups and not the amount of unreacted monomers in the polymer. The DC of e.g. 70% indicates that there is 30% of uncreacted groups and not 30% of free, unreacted monomers trapped within the polymer network that could theoretically leach out. Cross-linking monomers in resin-based materials most often contain more than one polymerisable group which means that cross-linking may occur via some but not all such groups. Furthermore, unreacted polymerisable groups always exist at the ends of polymer chains. There have been some estimates that in resin-based composites with the DC of around 70%, the amount of unreacted monomers is actually less than 10%. This depends on material chemical composition and may vary significantly in different resin-based materials.