The last several posts have provided details of the rheological properties of highly filled thermoset resins.  Fumed silica was shown to be a very effective rheology modifier when a yield point and shear thinning are required or to control sagging, such as for die attach or coil bond adhesive.  Silica fillers are used extensively to reduce the coefficient of thermal expansion ... [Click to Continue...]

In previous posts, the characterization and rheological properties of highly filled systems was discussed.  In particular, the rheological response of epoxies filled with the common thixotrope, fumed silica filler was covered in detail.  Recall that the particle-particle interactions during shearing control to a large extend the viscosity shear rate relationship.  When using ... [Click to Continue...]

In a previous post, the concept of thixotropy or the shear-rate and time dependency was introduced.  Thixotropy develops due to the time required to re-build the particle-particle interactions (i.e. re-form the three dimensional particle network).  Unfortunately, for many years, the terms thixotropy and shear thinning were used interchangeably.  Recently, a distinction has been ... [Click to Continue...]

Figure above courtesy of Cabot [1]. The last post the role of fumed silica in developing thixotropy in filled thermosets.  This post will expand on that discussion and provide more insights into the shear thinning and viscosity recovery process. The interesting observation in the recovery experiment is the relatively rapid recovery of structure after shearing.  For the two ... [Click to Continue...]

The last post provided an introduction to the mechanism of rheology modification using fumed silica.  This post will expand on that discussion and provide more insights into the shear thinning and viscosity recovery process. Figure 1. Viscosity as a function of shear stress for a fumed silica filled epoxy resin.  Figure 1 clearly shows the viscosity decreases ... [Click to Continue...]