Influence of irradiant energy on degree of conversion, polymerization rate and shrinkage stress in an experimental resin composite system

Objective: This study evaluated the degree of conversion (DC), maximum rate of cure (R_p^max), and polymerization stress (PS) developed by an experimental dental composite subjected to different irradiant energies (3, 6, 12, 24, or 48 J/cm²) under constant irradiance (500 mW/cm²). Methods: DC and R_p^max were monitored for 10 min on the bottom surface of 2-mm thick disks and on 150-μm thick films (representing the top of the specimen) using ATR-FTIR. PS was monitored for 10 min in 2-mm thick disks bonded to two glass rods (Ø = 5 mm) attached to a universal testing machine. One-way ANOVA/Tukey tests were used and differences in DC and R_p^max between top and bottom surfaces were examined using Student's t-test. Statistical testing was performed at a pre-set alpha of 0.05. Results: For a given surface, DC showed differences among all groups, except at the top between 24 and 48 J/cm². R_p^max was similar among all groups at the same surface and statistically higher at the top surface. PS also showed significant differences among all groups. Data for 48 J/cm² were not obtained due to specimen failure at the glass/composite interface. Significance: Increases in irradiant exposure led to significant increases in DC and PS, but had no effect on R_p^max.