Combining FDTD and Curing Kinetic Equations to Model the Degree of Conversion Evolution of UV-Curable Systems

Abstract

The degree of conversion (DoC) is significantly linked with many material properties of a UV-cured resin. The current curing kinetic models provide insight into how DoC evolves with depth for a pure resin subject to plane wave, light propagation. However, they do not accurately describe how DoC evolves within UV-curable composites or systems in which multidirectional ray propagation and interference are in play. This paper describes a simulation framework for predicting the spatial and temporal eand the propagation is alovolution of DoC within a UV-curable composite. The framework uses Maxwell's laws of electromagnetic theory and a series of finite-difference timedomain (FDTD) simulations to predict light intensity distribution at different reaction time steps. It also uses curing kinetic models and optical property models to predict DoC and optical properties within a volume. The framework was applied to simulate the photoinduced, free-radical polymerization of poly(ethylene glycol) diacrylate/2,2-dimethoxy-2-phenyl acetophenone (PEGDA/ DMPA)-based composites. It was used to study the influence of the resin refractive index, the filler refractive index, and the filler particle size on the spatial and temporal evolution of DoC. The predictions of DoC evolution were in agreement with both theoretical and experimental results published in the research literature. The simulations also revealed that (1) the change of the resin refractive index during polymerization has a significant impact on both light propagation and DoC within a UV-curable composite, (2) smaller fillers scatter more light than larger fillers, and (3) larger refractive index mismatch between the resin and filler leads to more light scattering and greater light attenuation. In turn, this increases the polymerization rate at shallow depths but decreases it at greater depths.