Flexural strength and modulus of a step-cured resin composite

In an attempt to reduce marginal contraction gaps, a step-curing mode of light polymerization of resin composite has been proposed. It was hypothesized that such an exposure mode, having an initial reduced curing rate, would result in composite having a lower modulus of elasticity than composite exposed in one step. The composite was initially exposed to power density levels of 50, 100, 150, or 200 mW/cm(2) for durations of 10, 20, or 40 s. The final exposure and the exposure of the control group were performed at 750 mW/cm(2) for 20 s. It was found that certain modes of two-step exposure resulted in a lower flexural modulus than did a one-step exposure of constant, high power density. Regression analysis showed, with statistical significance, that flexural modulus was relatively high following a short initial exposure at low power density or using a long initial exposure at high power density. Conversely, flexural modulus was relatively low following a long initial exposure at low power density or using a short initial exposure at high power density. The results were explained by probable differences in degree of conversion and cross-link density of the polymer network.