An important criteria for a long-lasting restoration of a polymer-based material, is a proper curing throughout the entire restoration. To achieve this, an effective polymerization is crucial so that the polymer network will extend throughout the material. This will ensure good mechanical properties and low amounts of residual monomers. Different methods exists that may monitor the curing or the polymerization of a polymer-based composite material for dental restorations.
Depth of cure
A standardized method for determination of the depth of cure is given in the standard ISO 4049 Polymer-based restorative materials. The layer thickness claimed by a manufacturer is usually related to this test method.
A stainless steel mould with a cylindrical hole of minimum 6 mm in height and with 4 mm in diameter is used for the preparation of test specimens of the composite material. Directly after curing, performed according to the instructions for use, the specimen is removed from the mould and the uncured material at the bottom is removed with a spatula (the scraping test). The depth of cure is defined as the height of the remaining cylinder divided by two.
Degree of conversion
Accurate determination of the extent of the polymerization reaction may be performed using spectroscopic techniques, such as FT-IR (Fourier Transform Infra-Red spectroscopy). This method calculates the percentage of converted methacrylate (reactive) double bonds in the polymerization process. The method uses ratios of peak areas of the reactive methacrylate bond and a stable chemical bond (often the aromatic double bond) found in the spectra, taken before and after curing. In order to find an acceptable layer thickness, the degree of conversion will have to be calculated at different thicknesses/depths of a material, and a minimum requirement for the degree of conversion needs to be set and generally accepted. Such a limit has not yet been established.
Different methods of hardness measurements, e.g. Knoop or Vickers, are used to evaluate the acceptable curing depth of a composite. Today micro-hardness methods are popular. In general, these methods compare a measured hardness at the top of a restoration to the hardness at the bottom (or at different depths). Suggestions have been made for acceptable ratios of bottom-to-top hardness that may represent an acceptable layer thickness or depth of cure, such as 0.8 or 0.85. If so used, a minimum requirement of the absolute hardness value should also be set. However, nor for this method have such limits been generally accepted or established in a standard.
Reference: ISO 4049:2009 – Dentistry – Polymer-based restorative materials, International Organization for Standardization, Genève.