Changes in polymerization stress and elastic modulus of bulk-fill resin composites for 24 hours after irradiation.

The objective of this study was to investigate changes in polymerization stress and elastic modulus for light-cured bulk-fill resin composites following irradiation. Crack analysis was applied to obtain the stresses for 24 h after irradiation, which were calculated from the lengths of cracks in a glass mold bonded to the composites. The elastic modulus was repeatedly measured by nano-indentation tests performed over 24 h. The lengths of the cracks, the interfacial stress, and the elastic modulus were significantly affected by the material and time after irradiation (p<0.01). The stress and elastic modulus continued to increase for more than 6 and 1 h, respectively (p<0.05). Strong relationships (r>0.85) were revealed between the stress and elastic modulus. The bulk-fill resin composites generated smaller stresses than a flowable resin composite reported to generate relatively low stress. Post-irradiation polymerization during the first 1 h is a major determinant for the magnitude of stress.