START-UP PRESSURE TRANSIENTS IN A CAPILLARY RHEOMITER

The ''rise time'' required to achieve a steady pressure reading in a capillary rheometer operated at constant piston speed can be very long, up to several hours under certain circumstances. This phenomenon can pose a serious problem in the measurement of melt viscosity, and it would be useful to be able to estimate the rise time in the planning of experiments. Based on experiments involving several types of polyethylene, we found that the rise time increases with L/D and the amount of polymer initially in the reservoir and decreases with diameter and piston speed. When the rise time is short, melt viscoelasticity contributes to the rise time, but when it is long, melt compressibility is the dominant factor. A model was developed for the latter case, and this was found to give an accurate prediction of the rise time, given the viscosity and compressibility. The model can also be used to determine the power-law parameters from the start-up pressure trace, P(t), for a single experiment. Alternatively, if the viscosity is known, the compressibility can be inferred from a single pressure trace.