MEASURING UNIAXIAL EXTENSIONAL VISCOSITY USING A MODIFIED ROTATIONAL RHEOMETER

Knowledge of the extensional behaviour of polymer melts is extremely i mportant due to the industrial relevance of extensional flows in commo n processing techniques and sequences such as blow moulding, film blow ing, fibre spinning, melt flow through extrusion dies and injection mo uld filling. One of the main problems both researchers and industriali sts come across is the fact that, unlike shear flows, steady-state ext ensional flows are not easy to generate and maintain experimentally. T his fact limits the extent to which one can characterise the materials and, therefore, the degree of optimisation of the productive process. In this paper, a modification to a commercially available controlled rate rotational rheometer is proposed in order to produce a cheap, eas y to set-up, flexible extensional rheometer. This is based on the well -known Meissner-type extensional rheometer and makes use of the accura te velocity control and torque measurement possibilities of the rotati onal apparatus. In this case, the adaptation was performed on a TA Ins truments Weissenberg Rheogoniometer, but the idea is applicable to mos t other similar devices. The feasibility of the modification will be d iscussed and confirmed, results being presented for two materials at d ifferent temperatures. These include the calculation of transient unia xial extensional viscosity and a study of rupture conditions. (C) 1999 Elsevier Science B.V. All rights reserved.