Settling behavior of specific acrylic particles in bis-GMA based reactive slurries

Sedimentation is a limiting factor for the shelf life of filled paints and adhesive resins, and requires consideration when optimizing a resin's formulation for an end application. Static settling experiments have been performed using a bis-GMA-based pliotocurable resin system by varying some characteristics of the filler particles (size and composition) and of the resin (viscosity and composition). The recorded batch settling curves, mudline height vs. time, displayed the standard sedimentation shape. In particular, the mudline movement rate was constant in the initial period of sedimentation. The variations of this constant mudline velocity as a function of the settling conditions has been compared to sedimentation model predictions. A close correspondence has been obtained between the data and a model describing the sedimentation behavior of non-aggregated rigid spheres at high concentration in the Stokes regime. In particular, the constant mudline velocity is proportional to the square of the particle diameter. This result can be used as a way to limit the settling phenomenon in filled resin systems, by choosing a particle diameter as small as possible. As a validation of this result, no visually observable settling was seen in 50%-filled resin systems for a period more than 4 months at room temperature using 5.1-mum poly(methyl methacrylate-co-ethylene glycol dimethacrylate) particles as filler.