Coupling Monte Carlo Light Propagation Method and Curing Kinetic Equations to Model the Degree of Conversion Evolution of UV-Curable Composites

The degree of conversion (DoC) is significantly linked with many material properties of a UV-cured resin. Current curing kinetic models provide insight into how DoC evolves with depth for a pure resin cured by the collimated light beam. However, they do not accurately describe how DoC evolves within UV-curable composites or systems in which multidirectional ray propagation is in play. This paper describes a simulation framework for predicting the spatial and temporal evolution of DoC within a UV-curable composite. The framework uses the Monte Carlo method to predict light propagation and absorbed energy density 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/phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide-based composites. It was used to study the influence of filler volume fraction and filler particle on the curing depth and curing width. The results show that larger filler volume fraction or smaller filler leads to smaller curing depth and width, which is in qualitative agreement with experimental results published in the research literature. The framework was also applied to simulate the formation of adhesive joints in a photoactivated adhesive workholding fixture application. The influence of a light source, a filler, and a workpiece surface on the final DoC distribution was investigated. The results show that (1) both the beam spread from a light-emitting diode (LED) light and the usage of a filler have large influences on the curing width of an adhesive joint. However, such influences will not be aggregated. (2) The light scattering by the fillers leads to a higher DoC in the secondary curing zone at shallow depth ranges, while the beam spread of the LED light beam results in a higher DoC in the secondary curing zone at deep depth ranges. (3) If the adhesive is not thick, then a highly reflective workpiece surface can promote the DoC at deep depth ranges. (4) Inside the primary curing zone, diffuse reflection caused by the workpiece surface greatly promotes the polymerization rate near the workpiece surface.