Dual-slit rheo-optical die for the characterization of process sensitive melts

Abstract

Instrumented slit dies coupled to single or twin screw extruders have been used by many researchers to measure shear viscosity and normal-stress differences and perform rheo-optical experiments, namely small angle light scattering, birefringence, polarized optical microscopy, and turbidityturbidimetry. Generally, generation of a range of shear rates implies operating the extruder with varying screw speeds or feed rates. However, this changes the thermomechanical environment inside the extruder and, consequently, may affect the characteristics of the material at the inlet of the die, thus putting at risk the validity of the measurementsrheo-optical characterization. To avoid this problem, the authors developed a dual-slit die, which is an adaptation of the twin-slit concept proposed earlier [1,2]. It consists of two flow channels, each fitted with an output control valve at its inlet. By handling simultaneously the two valves, shear rate in the measuring channel can be changed within a range while maintaining constant the total pressure drop. A few constructive and measurement issues are discussed. Experimental data for a few representative polymer systems are presented.

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