Rheological characterisation of castable and extrudable energetic compositions

Castable compositions are made for over fifteen years now at TNO and there is a four year experience with extrudable compositions, including gun propellants and Plastic Bonded eXplosives. During these years rheological measurements have been carried out not only to characterise batches but also to be able to match the castable and extrudable compositions to the desired processing properties.
Castable compositions are generally characterised by their viscosity. Their flow behaviour is mostly pseudoplastic and the shear rate dependence of viscosity may be described by a power law in which the exponent is larger than 0.6. The behaviour of castable compositions is never purely viscous. The visco-elastic shear modulus is determined with a dynamic mechanical technique. Well castable compositions are characterised by a relatively low shear loss modulus (G" < 100 Pa, at 0.1 Hz) and an even smaller shear storage modulus (G').
It is observed that both loss and storage modulus rapidly increase with increasing solid load. The storage modulus, however, increases faster than the loss modulus. At a certain solid load both moduli are equal (G" ~G' 104 Pa) and then upon a further increase ofthe solid load, the storage modulus becomes even larger, indicating that the composition more and more resembles an elastic material. At too high solid loads extrusion becomes an alternative to casting because ofthe rheological behaviour.
Extrudable compositions show typical shear storage moduli G' in the range of 105 to 107 Pa.
Apart from the solid load, also the addition of low molecular weight chain-extenders like hexane diols has a significant effect on the visco-elastic properties and thus the applicability of either casting or extrusion techniques