Question:

Can we compare viscosity results between polymers tested at the same temperature and identical shear rates but on two different capillary rheometers?

Answer:

As an example, let’s consider two capillary rheometers with the following specifications and test parameters.

Rheometer A:

Barrel diameter = 9.55 mm

Capillary diameter = 20 mm

Capillary length = 1 mm

Test temperature = 150 C

Shear rates = 100, 200, 300, 400, 500 1/s

Rheometer B:

Barrel diameter = 15 mm

Capillary diameter = 10 mm

Capillary length = 1 mm

Test temperature = 150 C

Shear rates = 100, 200, 300, 400, 500 1/s

Results obtained from a single test performed each on Rheometer A and Rheometer B are generally not comparable.

In order to compare any number of viscosity results from two or more different capillary rheometers, it is very important to normalize the pressure data by introducing a correction called Bagley Correction post testing. This correction applied to the pressure data will generate true viscosity values which will then be comparable between different capillary rheometers. This correction is necessary because of the “entrance effects”, which occur when the molten polymer is pushed through the larger barrel diameter into the smaller capillary diameter. These “entrance effects” generally are different between Rheometers A and B. The Bagley Correction compensates for these differences, thereby making the results between the two comparable. To find out more about the Bagley Correction, and how to apply it, please refer to this blog post.