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By Gebhard Schramm

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If a sample is visco-elastic then some percentage of any shear energy input during unsteady processes is stored elastically and temporarily and only the unknown rest will induce flow. Under these conditions the viscosity-related direct correlation between the known amount of energy input and resulting flow is no longer valid. Simple viscometry will provide very erroneous results for this class of fluid. Very many samples such as polymer melts or polymer solutions and also very many dispersions show both a viscous and an elastic response to shear stresses or shear rates.

45 Shear stress [Pa] Rheology Shear rate [1/s] Fig. 20 Flow curves of 3 PMMA melts at 220°C Fig. 20 compares the resolution of the 3 PMMA-flow curves at a constant shear rate of gS =193 (1/s) and at a constant shear stress of τ = 216 000 Pa. 4 = 45 Constant Stress rheometers are much better – here, more than tenfold – at differentiating between the three melts than Constant Rate rheometers and this differentiation becomes even better the more the slope α of the flow curves decreases – the more the samples deviate from Newtonian flow behavior.

45 48 Viscosity [Pas] Rheology Shear rate [1/s] Fig. 23 CS-comparison: viscosity curves of 3 PMMA melts at 220°C Constant shear stress comparisons have one additional important effect Fig. e. of equal exponents n. Although the different grades of polymer melts have different viscosity curves a CS-analysis will allow a good comparison since the polymers are equally non-Newtonian and their flow pattern in the capillaries or shearing gaps is alike. 49 Rheology Viscosity [Pas] CR-data comparisons (see Fig.

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