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...]

The last post introduced the concept of using fumed silica as a rheology modifier to tailor the dispensing of many types of adhesives used in electronic applications.  In the image in the top right in the figure above shows a die attach adhesives dispensed on a PBGA substrate.  After dispensing, the adhesives retain the “snowflake” or “asterisk” dispensing pattern.  The ... [Click to Continue...]

The last post described how to measure the viscosity-shear rate relationship using either a cone-and-plate or a parallel plate rheometer using a shear rate sweep.  An example is shown above.  This post covers how the viscosity profile influences the dispensing process and provides a short introduction to viscosity modifiers used to control the shear thinning and yield ... [Click to Continue...]