HIGHLY-RESOLVED TEMPOROSPATIAL SHRINKAGE KINETICS OF RESIN-BASED COMPOSITES USING LASER INTERFEROMETRY
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Photocurable resin-based composites (RBCs) are commonly used as dental restoratives due to their superior aesthetic quality. An enduring problem is that photocuring RBCs results in polymerization shrinkage that may lead to clinical failure of the restoration. A novel Michelson interferometer based approach is developed for accurately measuring shrinkage dynamics and topography of fast heterogeneously curing RBCs in the bonded disc geometry. The main components of the apparatus consist of a Helium-Neon (HeNe) laser and a CCD camera with 122 frames per second acquisition rate capable of measuring shrinkage rates up to 19.3 µm/s with a spatial resolution on the sample of 20.6 µm. The accuracy and reliability of the system were confirmed by comparison with a photodiode, profilometer, and spherical mirrors. Study on sample geometry demonstrated that coverslip rigidity affects the RBC shrinkage kinetics especially for low power inhomogeneous light-curing unit (LCU) irradiance beam profile. The inhomogeneous beam profile of a LED-based polywavelength (1 violet and 2 blue LEDs) LCU was evident in the shrinkage map at short time but obfuscated at long exposure time. Reproducibility of results and uncertainty of deflection rates are attributed to LCU power fluctuation and data acquisition rates, respectively. Autocatalytic equation fits well to experimental results and suggests a greater possible maximum shrinkage for lower LCU irradiance. A linear relationship between the degree of conversion of RBC, measured by a Fourier Transform Infrared Spectrometer, and the shrinkage was observed across the full range of measured values. Nevertheless, a difference in the reaction order parameters derived from the autocatalytic equation fits to the data for DC and shrinkage is observed.