Shrinkage stress kinetics of Bulk Fill resin-based composites at tooth temperature and long time.

OBJECTIVE: To determine the shrinkage stress kinetics at up to 12h after light exposure and at tooth temperature during placement of selected Bulk Fill resin-based composites (RBCs).

METHODS: Five representative Bulk Fill RBCs from four companies were chosen with a wide range of viscosity and filler volume content. The shrinkage stress kinetics at T=33°C was measured continuously over a period of 12h using a modified tensometer with the ability to measure the cantilever beam deflection to better than 40nm accuracy at a sampling rate of up to 200 samples/s, and thermally stable resulting in a measurement accuracy better than 0.05MPa at 12h. The tensometer compliance was 0.105μm/N. A custom made heater was used to control the RBC sample temperature at T=33°C with a temperature gradient across the sample of less than 1°C. The samples were irradiated for 20s with irradiance of 1.1W/cm2 and total energy density of 22J/cm2 . Three samples (n=3) were used for each RBCs.

RESULTS: The shrinkage stress at 12h for the five Bulk Fill RBCs ranged from 2.21 to 3.05MPa, maximum stress rate ((dS/dt)M ) varied from 0.18 to 0.41MPa/s, time at which the maximum stress rate occurred (tMax ) were between 1.42 to 3.24s and effective gel time (tgel ) varied from 50 to 770ms. Correlations were observed between (dS/dt)M and tMax (r=-0.946), tMax and filler volume fraction (r=-0.999), and between the shrinkage stress at 12h and tgel (r=0.994). However, no correlation was observed between the stress at 12h and filler volume fraction.

SIGNIFICANCE: The shrinkage stress for four of the five Bulk Fill RBCs were not significantly different (p<0.05) at 6h and beyond after photo-curing and that fully developed stress induced by photo-cured RBCs may only be reached at times longer than 12h.