Measurements of the viscosity of polymeric fluids under high pressure pose a technical problem, because the viscometer must be constructed for high-duty service. Consequently only a few high-pressure viscometers are available and as a result the pressure effect is often neglected. A new magnetic viscometer, based on Stokes' falling sphere principle, for measurement of zero-shear viscosity has allowed development of a special pressure cell for measuring up to 70 degrees C and 1000 bar. The construction and advantages of this pressure cell are described and some results from a silicon oil are given.
The pressure dependence of viscosity of polymer melts is investigated insufficiently at present, the main reason being the requirements for high-duty viscometers. Only a few high-pressure viscometers are available and these are working with several disadvantages. The critical parts of rotating viscometers are seals and capillary types pose problems with pressure gradients and dissipation. Falling ball viscometers, according to Stokes’ law, allow measurements without problems with rotating parts, pressure gradients, and dissipation. The disadvantages of this method are very long measuring times at high viscosities. Unfortunately, polymer melts are not sufficiently stable at temperatures above the melting point, thermal degradation follows and unreliable data are the result. The magnetoviscometer, developed at the University of Linz, circumvents long measuring times by replacing the gravitational field by a much stronger magnetic field, which acts on an iron sphere in the molten polymer. As a new device for the magnetoviscometer a high-pressure cell, capable of 1000 bar and 523 K, is developed for measuring the zero-shear viscosity of polyolefine melts. The advantages and features of the high-pressure cell are described.
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