Brookfield DV2+Pro Viscometer Manual de usuario descargar pdf (Pagina 62)

Brookeld Engineering Labs., Inc. Page 62 Manual No. M03-165-F0612

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ow.Thismodelismostsuitable

foruidsthatexhibitPseudoplasticorshearthinning,owbehaviorafteryielding.

Theseuidshaveanon-linearowcurve.Thepointatwhichitcrossesthey-axisistheproduct’s

yield stress (

). 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

proleandapplytheCassonmodel.Fromthe

results, shown in Figure VI-4, they learn that

their ointment has a higher yield stress,

, and

lower plastic viscosity, n, than they originally

intended.Asaresultitisdifcult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difcult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 (

h) = 329.8 cP

Yield Stress (

) = 325.8 dynes/cm

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Brookfield DV2+Pro Viscometer Manual de usuario Pagina 62