Measuring uniaxial extensional viscosity using a modified rotational rheometer

Knowledge of the extensional behaviour of polymer melts is extremely important due to the industrial relevance of extensional flows in common processing techniques and sequences such as blow moulding, film blowing, fibre spinning, melt flow through extrusion dies and injection mould filling. One of the main problems both researchers and industrialists come across is the fact that, unlike shear flows, steady-state extensional flows are not easy to generate and maintain experimentally. This 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, easy to set-up, flexible extensional rheometer. This is based on the well-known Meissner-type extensional rheometer and makes use of the accurate velocity control and torque measurement possibilities of the rotational apparatus. In this case, the adaptation was performed on a TA Instruments Weissenberg Rheogoniometer, but the idea is applicable to most other similar devices. The feasibility of the modification will be discussed and confirmed, results being presented for two materials at different temperatures. These include the calculation of transient uniaxial extensional viscosity and a study of rupture conditions.