To investigate the influence of the specimen preparation and of the experimental method on the rheological properties of polyamide.
Rheological characterisation results on polyamide melts strongly depend on sample preparation (e.g. humidity control) and on details of the experimental procedure. As a consequence, data from various laboratories do not agree. Aim of the project is to develop a widely accepted guideline for sample pre-treatment and measurement, both for rotational and capillary rheometry. These guidelines should be in particular be pertinent for industrial laboratories.
The purpose of the present investigation is to find conditions to make reproducible and comparable rheological measurements. The molecular structure of the polyamide shouldn’t change much within the time scale of these rheological experiments. The result should be a guideline or even standard method to perform rheological measurements on polyamide in the melt. These conditions for rheological measurements should be independent from processing conditions, i.e. these drying conditions may be impractical and inefficient for processing larger amounts of polyamide.
A commercial Polyamide 6 (Durethan B30S) has been distributed and the members of the task group performed rheological measurement with different type of equipment at various temperatures, resulting in (as expected) non comparable results. Measurements of the complex viscosity at different temperatures as a function of the measurement time showed, that in capillary flow the viscosity can increase or decrease, depending on the humidity content of the sample. In oscillatory flow the viscosity is always increasing with time. The effects are most pronounced at temperatures above 250°C. Above 280°C excessive foaming makes a reasonable measurement impossible.
Experiments on controlled predried samples showed, that it is very difficult to achieve exactly the same amount of humidity in the samples at every participating laboratory. The same drying time in different types of ovens yielded different humidity values. Even the exact procedure how to measure the water content was found not to be straightforward too and we didn’t want to come up with a guideline with predescribed type of oven and Karl-Fischer-equipment.
Oscillatory experiments on relatively good dried samples gave very reproducible results if the measured parameters were extrapolated to measurement zero. As described above, the viscosity is increasing with measurement time. A procedure was developed to measure several frequency sweeps at different times (e.g. after 5, 10 20 minutes) and the values of storage and loss modulus at each frequency was extrapolated to measurement time zero. Samples with different drying times were found to yield the same rheological behaviour at this extrapolated time.
The same extrapolation method is now being explored for other kinds of equipment (e.g. capillary rheometers)
> see also <www.iupac.org/divisions/IV/IV.2.1> for more recent updates
Feb 2009 - Project completed – IUPAC Report published in Pure Appl. Chem., 2009, Vol. 81, No. 2, pp. 339-349