Brookfield DV-III Ultra Rheometer - Manual (Page 62)

Brookfield Engineering Laboratories, Inc. Page 62 Manual No. M/98-211-B0104
Brookfield Engineering Laboratories, Inc. Page 62 Manual No. M98-211-E0912
When should you use it?
The Casson model should be used with non-Newtonian materials that have a yield stress and that
do not exhibit a “Newtonian-like” behavior once they begin to flow. This model is most suitable
for fluids that exhibit Pseudoplastic or shear thinning, flow behavior after yielding.
These fluids have a non-linear flow curve. The point at which it crosses the y-axis is the product’s
yield stress (
€
t
o
). To protect the point at which the curve will intersect with the y-axis, the Casson
model linearizes or straightens the plot by taking the square root of the data. To ensure accurate
extrapolation to yield stress it is best to take some data at low shear rates.
An Example of the Casson Model at Work
Before releasing a new over the counter gel, a
pharmaceutical company needs to learn how
it will behave which it is being used by the
end consumer. They perform a full viscosity
profile and apply the Casson model. From the
results, shown in Figure VI-4, they learn that
their ointment has a higher yield stress,
€
t
o
, and
lower plastic viscosity,
η
, than they originally
intended. As a result it is difficult or dispense
from its container (due to the high yield stress) and it does not hold it shape very well (due
to the low plastic viscosity), making it difficult to apply a small amount to the affected area
of the skin. Based on this data, formulators are able to modify the ingredients accordingly.
Once a formulation is established, multi-point tests and the Casson model are performed as a
QC tool to check batches before and after processing.
Pharmaceutical Gel
Plastic Viscosity (
η
) = 329.8 cP
Yield Stress (
€
t
o
) = 325.8 dynes/cm 2
IV.7.5 Other Rheological Models
The NCA/CMA Casson Model
(1 + a)
€
t =
€
2 t
o
+
(1 + a)
€
ηg
(
€
t
= shear stress
,
€
t
o
= yield stress, η = plastic viscosity, and = shear rate)
The NCA/CMA Casson model is designed by the National Confectioners Association and the
Chocolate Manufacturers Association as the standard rheological model for the industry. This
model determines yield and flow properties under specified conditions and closely approximates
the plastic behavior of chocolate before final processing.
.