Aaron P. R. Eberle, Donald G. Baird, Peter Wapperom,
Gregorio M. Vélez-García
Obtaining reliable transient rheological data on concentrated short
fiber suspensions using a rotational rheometer
Journal of Rheology 53, 2009, p. 1049-1068
The conventional method for obtaining transient rheological data on short
glass fiber-filled polymeric fluids is to use the parallel disk (PP) geometry
in a rotational rheometer. Using the PP geometry large transient stress
overshoot behavior was observed during the startup of flow measurements on
a 30 wt% short glass fiber-filled polybutylene terephthalate. A contributing
factor to this behavior is believed to be induced fiber collisions caused by
the inhomogeneous velocity field (radial varying velocity gradient). A novel
approach was taken in which a "donut" shaped sample was used in a
cone-and-plate device (CP-D) to maintain a sufficient gap to fiber length
ratio. The magnitude of the first normal stress difference was reduced by
70%, and the time to reach steady state was reduced by 100 strain units.
The Lipscomb model coupled with the Folgar-Tucker model for evolution of
fiber orientation was fit to the stress growth behavior measured
using both the PP geometry and CP-D resulting in different parameters.
In addition, the fitted model parameters were found to depend on the initial
fiber orientation. It is believed that the CP-D allows for an accurate
determination of the stress growth behavior and eventually will allow one
to obtain unambiguous model parameters.
Short glass fiber; glass fiber suspension; transient rheology;