Rheology of Compounded Thermoplastic Rubber used for Processing Tyres

Thermoplastic elastomers are materials that have mechanical properties similar to those of thermoset rubber but can be processed using methods designed for standard thermoplastics, such as extrusion and injection molding. Plus, they can be recycled much more easily and efficiently. A capillary rheometer is a useful tool for determining flow characteristics of thermoplastic elastomers. Besides determining the viscosity vs. shear rate curve, it is often useful to study the extrudate swelling characteristics and this can be done with an add-on laser device for swell measurement.

A tyre manufacturer interested in studying their thermoplastic elastomer provided us with a rubbery compound consisting of an elastomer, carbon black and additives in form of flat sheets. We tested this compound according to ISO 11443 on a CEAST SR20, a twin bore Capillary Rheometer (barrel diameter 15 mm) equipped with 20 kN load cells. The test was carried out at a barrel temperature of 100°C. We cut the rubber sheets into small pieces to feed the barrel evenly. We allowed the material to preheat and compacted it with a force of 2400 N. We then subjected the molten material to shear rates between 1 to 2000 s-1 through a capillary die. We setup this test and controlled the capillary rheometer using CEASTVIEW (VISUALRHEO) software.

All specimens showed a non-Newtonian behavior with a viscosity ranging from 100,000 Pa.s at 1 s^-1 shear rate to 800 Pa.s at 1000 s^-1. We observed 1000 s^-1 to be a critical shear rate for this particular compound, above which maintaining a stable pressure plateau was quite challenging. Bagley correction was applied. Overall, the material showed good repeatability and reproducibility of results. We also performed a die swell test with the CEAST die swell laser system in order to study the influence of swell in this compound upon exit from the die. Please refer to the SQC graph on the right hand side showing repeatability of viscosity vs. shear rate and average die swell vs. shear rate curve.