Light intensity effects on resin-composite degree of conversion and shrinkage strain

The aim of this study was to investigate the relationship between the degree of conversion (DC) and shrinkage strain values of two representative resin-composites when irradiated by two light intensity levels, for different time periods.

The DC of Z100 and Tetric Ceram was measured by FTIR spectroscopy immediately post-cure and shrinkage strain values were obtained continuously for 30 min from photoinitiation with the bonded-disk technique. All samples were photopolymerized at an initial temperature of 23 degrees C with the Elipar Highlight unit under the following modes of variable light intensity (I) 40 s at 750 mW/cm2; (II) a two-step 'soft-start' irradiation mode of 10 s at 200 mW/cm2 plus 30 s at 750 mW/cm2; (III) 40 s at 200 mW/cm2; and (IV) 10 s at 200 mW/cm2. Temperature effects of the light and curing exotherm were not eliminated.

Mode II of irradiation did not reduce the DC for each material, relative to the values obtained with mode I (p > 0.05). The corresponding shrinkage strain values after 30 min were also not significantly different (p > 0.05). However, the effects of reduced light intensity levels for 10 s and 40 s (modes III and IV), were in agreement with previous findings implying reduced shrinkage strain levels. For both resin composites, data from all curing times and light-intensities gave a linear regression relationship (with r2 > 0.99) between the bonded-risk shrinkage strain versus DC values.

For the materials and light-intensities studied, the correlation between DC and shrinkage strain values means that some reductions in the problems of shrinkage may be achieved by an acceptable reduction in DC. However, this must also be consistent with generating adequate curing performance in the resin-network. Nevertheless, DC is not the exclusive parameter describing network character and a fuller understanding of the latter is an important research goal.