Progress:
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
Last update: 2 November 2007
<project announcement to be published
in Chem. Int.>
