Efeito de repetidos ciclos de pré-aquecimento na microdureza e grau de conversão de resinas bulk-fil

Abstract

The preheating of composite resins is widely employed to improve their physical and chemical properties, such as reducing viscosity and increasing the degree of monomer-to-polymer conversion. However, repeated application of preheating cycles may lead to thermal degradation of the resin components, especially in bulk-fill materials. This study evaluated the effect of 10 preheating cycles at 68°C on the degree of conversion and surface microhardness of bulk-fill resins (regular and flowable). The degree of conversion (%) was analyzed by Fourier Transform Infrared Spectroscopy (FTIR), using a PerkinElmer Spectrum Two™ spectrometer with an ATR device and a ZnSe crystal. Samples were photoactivated for 20 seconds using an LED light source (1,100 mW/cm²) and analyzed in the range of 1750–1550 cm⁻¹. Calculations were based on the intensity of the C=C double bond (1636 cm⁻¹) and the aromatic ring stretch (1600 cm⁻¹ for the regular resin and 1608 cm⁻¹ for the flowable resin) as the internal standard. Vickers microhardness (HV) was evaluated using cylindrical specimens (5 mm in diameter and 2 mm in thickness), which were photoactivated and polished. Three indentations per sample were performed (50 g for 45 seconds), and values were calculated using the equation VH = P/2d². The results showed no significant differences in the degree of conversion (%) or microhardness (HV) after the preheating cycles, indicating that the tested resins have stable formulations and are resistant to multiple thermal exposures. The preheating technique appears to pose no apparent risk of degradation for the evaluated resins. However, thermal stability may vary among different formulations, highlighting the need for further studies evaluating the impact of preheating cycles on a broader range of materials and under long-term clinical conditions.