Oscillatory shearing

This test method is used to determine the dynamic and mechanical properties of polymer melts. By determining the precise complex viscosity of a polymer melt, it is possible to calculate the limit of thermal stability.

A polymer melt is exposed to a harmonic shear deformation in a plate-plate rheometer and the material response detected in the form of a harmonic shear stress. This results in the storage modulus G' as well as the loss modulus G" and the loss factor tan δ . The actual measurand, known as the complex viscosity, can then be calculated from the storage and loss moduli.

Oscillatory rheology using a plate-plate rheometer Oscillatory rheology using a plate-plate rheometer

An oscillatory shear in the small amplitude range performs the function of a sampling probe during the measurement, ensuring a significantly more precise result than with tests that rely on stationary shear flow (capillary rheometer, MFR). This method can be used to quantify such variables as zero shear viscosity, the linear viscoelastic range or the thermal stability of melts with high resolution.

Results of the test method

Representation of the complex viscosity of a polymer melt in a so-called “time sweep”. A 5% deviation in complex viscosity based on a melt’s initial value indicates that the thermal stability limit has been reached.

Test specimen

Round sheet 25 mm, thickness d = 1,7 mm