Applicability of the Cox-Merz Rule to High-Density Polyethylene Materials with Various Molecular Masses
Sprache des Titels:
Englisch
Original Kurzfassung:
The Cox-Merz rule is an empirical relationship that is commonly used in science and
industry to determine shear viscosity on the basis of an oscillatory rheometry test. However, it
does not apply to all polymer melts. Rheological data are of major importance in the design and
dimensioning of polymer-processing equipment. In this work, we investigated whether the Cox-Merz
rule is suitable for determining the shear-rate-dependent viscosity of several commercially available
high-density polyethylene (HDPE) pipe grades with various molecular masses. We compared the
results of parallel-plate oscillatory shear rheometry using the Cox-Merz empirical relation with those
of high-pressure capillary and extrusion rheometry. To assess the validity of these techniques, we
used the shear viscosities obtained by these methods to numerically simulate the pressure drop
of a pipe head and compared the results to experimental measurements. We found that, for the
HDPE grades tested, the viscosity data based on capillary pressure flow of the high molecular weight
HDPE describes the pressure drop inside the pipe head significantly better than do data based on
parallel-plate rheometry applying the Cox-Merz rule. For the lower molecular weight HDPE, both
measurement techniques are in good accordance. Hence, we conclude that, while the Cox-Merz
relationship is applicable to lower-molecular HDPE grades, it does not apply to certain HDPE grades
with high molecular weight.
Forschungs-