Biaxial extensional viscosity, measurement techniques

A method for measuring the uniaxial extensional viscosity of polymer soHds and melts uses a tensile tester in a Hquid oil bath to remove effects of gravity and provide temperature control cylindrical rods are used as specimens (218,219). The rod extmder may be part of the apparatus and may be combined with a device for clamping the extmded material (220). However, most of the mote recent versions use prepared rods, which are placed in the apparatus and heated to soften or melt the polymer (103,111,221—223). A constant stress or a constant strain rate is appHed, and the resultant extensional strain rate or stress, respectively, is measured. Similar techniques are used to study biaxial extension (101). 

All three types of extensional viscosity can be measured (119,121) imiaxial, biaxial, and pure shear. Only a few commercial instruments are available, however, and most measurements are made with improvised equipment. Extensional viscosity of pol5mier melts can be estimated from converging flow (entrance pressure) or from a melt strength drawdown test (237) as well as measured with techniques described below. 

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