PROCESSING OF HIGHLY FILLED ENERGETIC COMPOSITIONS SIMULATION OF A DOUBLE CO-ROTATING SCREW EXTRUDER AND A COMPLEX DIE DESIGN

摘要

At TNO Prins Maurits Laboratory research work has been performed on extrusion and extrusion rheometry for several years. A Theysohn co-rotating self-wiping 45 mm twin screw extruder is used for production and a Bohlin-Rosand twin bore capillary extrusion rheometer (CER) for characterisation. We started to work with thermoplastic elastomers (TPE's) because of an international trend towards the development and application of propellants using TPE's as a binder matrix for reasons of filler recovery at the end of service life. Another reason is that in extrusion processing, less solvent is needed compared to the traditional way of processing, which is beneficial from an environmental point of view. For the safe processing of energetic materials using an extruder it is important to keep pressure and temperature within safe boundaries. Although pressures and temperatures are measured during extrusion it is preferred to predict these parameters before processing. Several years ago we only used a simple model to approximate the head pressure value for ram extrusion using capillary rheometry test results as input. The result was promising. Nowadays we use commercially available simulation software. The twin screw extruder simulation software Ludovic of SCC and several Flow2000 modules of Compuplast have been used for analysing the results presented in this paper. The results of extrusion trials with a TPE bonded HE-simulant and a TPE based LOVA propellant are compared with the predictions made with the simple approximation model and with the models used in the Flow2000 and Ludovic simulations. It can be concluded that the rheology of highly filled materials may be described with a power law model and Arrhenius or exponential temperature dependency in the shear rate range of interest. Processing pressures and temperatures can be estimated using Flow2000 and Ludovic if the physical properties and rheology of the material and the processing parameters are known. Results obtained with Flow2000 correspond better with the measured pressures during ram extrusion experiments than the Benbow approach. The predictions made with Ludovic show the same trends and are in the same order of magnitude as experimental results but more experiments are needed to evaluate the software correctly.