A comparison of extensional viscosity measurements from various RME rheometers

The transient uniaxial extensional viscosity eta (e) of linear low density polyethylene (LLDPE) has been measured using the commercial Rheometric Scie ntific RME and the Munstedt Tensile Rheometer in an effort to compare the p erformance of available extensional rheometers. The RME indicated a signifi cant strain hardening of the LLDPE, especially at a strain rate of 1 s(-1). In contrast, the Munstedt rheometer showed the LLDPE to be only slightly s train hardening. This artificial strain hardening effect in the RME resulte d from the strain rate applied to the sample, determined from the sample de formation, being up to 20% less than the set strain rate. These results ini tiated a round-robin experiment in which the same LLDPE was tested on sever al RMEs in various locations around the world. All but one of the RMEs indi cated a deviation between set and applied strain rates of at least 10%, esp ecially at strain rates above 0.1 s(-1). The strain rate deviation was foun d to depend strongly on the value of the basis length L-0, and may result f rom the upper pair of belts not properly gripping the sample during extensi on. Thus visual inspection of the sample deformation is necessary to determ ine the applied strain rate. The most accurate measurements of eta (e) with respect to the strain rate deviation were obtained when the correct L-0 va lue and belt arrangement were used. A list of recommendations for running a n RME test is provided. Future work focusing on the fluid mechanics during the test may identify fully the cause of the strain rate deviation, but fro m a practical point of view the problem can be corrected for in the determi nation of eta (e).