Volumetric shrinkage and conversion of composites at different polymerization times

Objectives: To measure the effect of polymerization time intervals on volumetric shrinkage (VS) and degree of conversion (DC) of core composites. Methods: PhotoCore (PH, light-cured composite - Kuraray America) and LuxaCore (LU, dual-cured composite - DMG) were light polymerized for 40 s at different delay times (0, 30, 60, 90, 120 s) and tested without light polymerization at the same time intervals. DC (n=3) was measured using a Nexus 670 FT-NIR spectrometer (Thermo-Nicolet) between 4500-7400 cm-1 with 30 scans and 8 cm-1 resolution and calculated using the area of absorbance peak around 6163 cm-1. VS (n=3) was measured using AccuVol (BISCO) in single-view mode. Original volume was recorded 5 s after sample placement and VS recorded at 5 min after light cure. Data was analyzed using One Way ANOVA. Results: Immediate light-cured samples of LU and PH attained a DC of 65.2% (3.2%) and 76.2% (0.31%) respectively. Delayed curing did not significantly affect (p>0.05) DC. For samples not light polymerized, LU had a mean DC of 3.0% (0.1%) while PH had a DC of 2.0% (1.4%) after 120 s. Mean VS for LU was lowest when cured after 90 s (4.5%), and not significantly different (p>0.05) from VS obtained curing after 30 s (4.7%). Values were significantly lower (p<0.05) than VS obtained by immediate curing or curing after 120 s. VS of PH was lowest when cured after 30 s (2.6%) and not different (p>0.05) from shrinkage obtained after immediate cure (2.8%). These values were significantly lower than VS obtained curing after 60 s (3.2%), 90 s (3.2 %) and 120 s (3.7%). Conclusion: Delayed light polymerization of LU by 30 to 90 s significantly reduced VS. LU performed more as a light-cured composite without any significant self cure. Delayed light polymerization had no effect on DC of either composite.