Effect of light exposure on fracture toughness and flexural strength of light-cured composites

This study was conducted to investigate the curing characteristics of light-cured composites and their related mechanical properties.

Single-edge notch specimens [25 mm x 2.5 mm x 5 mm with a 5 mm notch (a/W = 0.5)] were prepared for fracture toughness measurements. For flexural strength testing, a stainless steel mold (25 mm x 2 mm x 2 mm) was used. Light-cured composites were condensed into the mold, and the middle third of the specimen was first activated for 30 s with 400 mW/cm2, for 60 s with 200 mW/cm2, or for 120 s with 100 mW/cm2. Then the remaining thirds were activated at the same intensity and curing time as the middle third. After 24 h storage in 37 degrees C water, three-point bending tests were performed with a span length of 20 mm at a crosshead speed of 0.5 mm/min. A one-way ANOVA, followed by a Newman-Keuls test (p < 0.05), were used to compare the data obtained from each group to test the effect of the curing conditions.

Fracture toughness, flexural strength, and flexural modulus varied with resin composites. Among the three curing conditions for each material, there were no significant differences in fracture toughness, flexural strength, or flexural modulus.

The fracture toughness and the flexural strength were the same when irradiations with the same amount of energy (light intensity multiplied by curing time) were used. It was found that, at lower light intensity, longer curing was required to provide comparable mechanical properties. An accumulated irradiation energy obtained through a product of the light intensity and curing time may serve as a guideline to produce samples exhibiting equivalent fracture toughness as well as flexural strengths.