A Practical Approach to Rheology and Rheometry

Shear stress τ
Rheology
Flow curve Viscosity curve
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0
8
Shear rate
Log shear rate
I: First Newtonian range → viscosity η0 not
yet dependent on shear rate.
II: In this range viscosity drops as a result
of molecular or particle orientation.
III: Second Newtonian range → viscosity η∞
stays constant, independent of further increases
of shear rate.
Fig. 8 The shear rate dependence of pseudoplastic liquids
24
Log Viscosityη
η 0
η
8
I II
III
B. Liquids which show a dilatant flow behaviour – (Curves 3 in Fig. 6)
There is one other type of a material characterized by a shear rate dependent
viscosity: “dilatant” substances – or liquids which under certain conditions of
stress or shear rate show a dilatant flow behaviour – increase their viscosity
whenever shear rates increase. In other words, the more one tries to increase
the coating speed for some PVC-plastisols to coat a fabric substrate, the stiffer
the coating material becomes. The resistance to flow may become so high that
the fabric is torn or even the rolls of the coating calender are broken.
Dilatant flow behavior is found for example in highly concentrated suspensions
in which solid particles such as emulsion-PVC are mixed with liquids such as
plasticizers to form plastisols. The particles are densely packed and the
amount of plasticizer added is just sufficient to fill the void between the particles.
At rest or at low coating speeds the plasticizer fully lubricates the particle
surfaces and thus allows an easy positional change of particles when forces
are applied: this suspension behaves as a liquid at low shear rates. At higher
shear rates, particles will wedge others apart causing general volume increases.
At this stage the plasticizer’s share of the total plastisol volume decreases.
Since the plasticizer is no longer sufficient to fill all voids and to keep
the PVC-particle surfaces fully lubricated, the plastisol becomes more viscous.
Dilatancy in liquids is rare. Inasmuch as this flow behavior most likely complicates
production conditions, it is often wise to reformulate the recipe in order
to reduce dilatancy.
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