The rheology of curing composite materials is important to many manufacturing processes. At Sandia, we work with particle-filled epoxy systems for a variety of applications. One particular system, designated as “459,” exhibits complex, counter-intuitive rheological dependence on temperature, flow history and particle-concentration. Despite the particles being relatively large (10 μm), 459 exhibits shear-thinning hysteretic behavior reminiscent of colloidal systems. It is hypothesized that the thixotropy arises from reaction of the 459 curative with the surface of the particulates. Under certain conditions, the addition of filler offsets the effect of epoxy polymerization on the viscosity, resulting in a viscosity that is constant with time. For this reason, we have developed an experimental model system that can be used to separate the effects of particle aggregation from the effects of cure on the viscosity. We have done experiments with this model suspension to determine its time-dependent response in step shear. In addition, we are working on a computational model that can be used to predict the behavior of the aggregating suspension. Preliminary modeling efforts focus on using a generalized Newtonian constitutive equation that relates the viscosity to the local aggregate concentration, defined by a dimensionless structure factor, to capture the time-dependence. Results from this constitutive equation are presented and compared to the experimental data from step-shear experiments in a Couette viscometer. Modeling results are also presented for a transient fiber spinning problem where the free surface evolves as the viscosity develops over time.

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