DISCREPANCIES IN TESTING RUBBER COMPOUNDS WITH DIFFERENT COMMERCIAL CAPILLARY RHEOMETERS

Whilst capillary rheometers are expected to give similar results, prov iding that the test conditions are equal and that the appropriate corr ections (mainly for entrance pressure loss) are made, discrepancies ha ve been observed when testing rubber compounds. A typical EDPM industr ial compound was used to investigate such discrepancies as observed wh en testing with the Gottfert 2001 high pressure capillary rheometer an d the Rheoplast, a capillary rheometer with preshearing capabilities. Tests were made with two series of dies at different temperatures (i.e . 80 to 100-degrees-C), in the 20-2000 s-1 shear rate range. Results s how that the observed differences are likely to arise from either a po ssible hole pressure error with the Gottfert rheometer, or differences in die entrance geometry, or differences in preheating histories or t he possibility that, with respect to their complex structure, rubber c ompounds exhibit rheological properties which strongly depend on the t est procedure. A combination of the two latter causes is likely to be responsible for the discrepancies observed.