Analysis of the effect of thickness and irradiance on polymerization delay and initial rate in light-cured resin filling materials

OBJECTIVE: Due to improvements in technology, it has been confirmed that there is a depth-dependent delay before polymerization starts in light-cured resin-based composite filling materials (RBCs) due to the presence of inhibitors. It has therefore become necessary to explore further the early kinetics of such systems and the effect of irradiance. METHODS: Degree of conversion (DC) was determined using Fourier-transform infrared spectroscopy (FT-IR) at a rate of 13 measurements per second. Six commercial RBCs were tested at three thicknesses (0.2, 2 and 4 mm) and four incident irradiance values (472-3985 mW/cm²). The attenuation coefficient was measured directly on both uncured and cured materials for comparison purposes. A first-principles model was developed to determine the effective ('fictive') start time for polymerization and the initial polymerization rate to interpret the DC data. RESULTS: Both the fictive start time and initial rate of polymerization were found to fit the behaviour expected from the Beer-Lambert Law, however the irradiance-dependent reaction rate exponent for polymerization did not in general agree with the value of 0.5 expected from classical theory. The effective attenuation coefficients extracted from the DC data using the model were in overall good agreement with those calculated from the transmission data. SIGNIFICANCE: Polymerization onset delay is a fundamental expectation when inhibitors are present in light-curing polymer systems. This has a bearing on the completeness of the cure obtained under some exposure circumstances. Photo-polymerization theory needs to be revisited to accommodate products such as those tested here that show a non-classical reaction rate exponent. Numerical models of DC development with exposure time must necessarily account for the onset delay of polymerization, while also accommodating the effects of variation in attenuation coefficient during curing and over depth. Data must be collected at a high rate when using high-irradiance and thin test piece conditions.