Designing mercury-free photoelastic particles

Photoelastic polymer particles have been widely used to study the internal stress distributions in phenomena involving granular materials such as landslides, avalanches, and hopper flow. Polyurethane remains a popular choice of material for fabricating photoelastic particles owing to its cost, tunability, optical and mechanical properties, and ease in particle molding. However, the state-of-the-art grade of polyurethane used to mold photoelastic particles contains a not-insignificant amount of toxic organomercury. This poses a risk to both users and the environment. In our work, we fabricated photoelastic particles from numerous commercially available polyurethane resins that are purportedly mercury-free. We tested the mechanical and stress-optical properties of each and found an optimal candidate as a viable replacement for the state-of-the-art material. To explore whether this new polyurethane resin can be tuned to improve its requisite stress-optical properties beyond what is currently possible, we studied its photoelastic dependence on its polymer microstructure and morphology using shear rheology and differential scanning calorimetry (DSC). We altered the microstructure by creating blends of polyurethane and ethylene glycol fillers, which altered the crystalline domains of their networks. We then studied the consequent change in the photoelastic properties of these blends. These mercury-free polyurethane particles serve as a a viable replacement for the state-of-the-art materials, owing to being free of organomercury, and having similar rheological and stress-optical characteristics as the latter.