During capillary rheometer tests, pressure is measured above the die inlet. The true pressure drop along the capillary is therefore ‘hidden’ by an additional pressure drop at the entrance of the die, where the flowing material goes from a wide reservoir (the main cylinder or barrel) to a narrow capillary, possibly also creating turbulences. Assuming that the same additional pressure drop takes place with different capillary lengths (but keeping constant barrel and capillary diameter, and inlet shape), it’s possible to correct the pressure reading and estimate much more accurately the true pressure drop. This is called the Bagley correction (named after Edward B. Bagley, American scientist).
The simple pressure reading is used to calculate what is called an apparent shear stress, giving only an apparent viscosity value. Bagley correction gives a true shear stress, and the true viscosity can be calculated. True viscosity is a property of the material (varying with test temperature), and is comparable even if obtained with different rheometers and with different capillary dies. The correction is strongly recommended when the data is to be used for design, flow simulation or fundamental studies. On the contrary, apparent viscosity can only be used to compare different samples tested with the same instrument and die, and is typically used for quality control.
The Bagley correction requires at least two sets of data obtained with the same sample, same temperature, same barrel and capillary diameter, same capillary inlet, and different capillary lengths. The use of ‘zero-length’ dies is not recommended by international standards.
With a twin-bore capillary rheometer it is possible to get Bagley-corrected viscosity right at the end of a single test run, while the same result requires two tests with a single-bore machine.
References
- ISO 11443: 2005 “Determination of the fluidity of plastics using capillary and slit-die rheometers”
- ASTM D3835-09 “Standard Test Method for Determination of Properties of Polymeric Materials by Means of a Capillary Rheometer”