【文章內(nèi)容簡介】 s processing operation unstable and poses a safety hreat in continuous processing of energetic suspensions (16). The extrudate merging from the die can be resinrich or poor in a cyclic fashion. At relatively igh screw speeds the pressure before the die can rapidly as affected by dramatic emixing and pulverization of the energetic filler in the continuous processor. Capillary flow experiments can be used to provide a better understanding ofthe flow instabilities of concentrated suspensions occurring in continuous processing operations (16, 31). Under unstable flow conditions, the average pressurenecessary to extrude the suspension grows unbounded with time, with generallyincreasing amplitude of oscillations. The time periodic oscillations in extrusionpressure are related to the time periodic mechanism of formation of a mat ofsolids at the capillary or die and its breakup. This occurs conitant with thefiltration of the binder. During the most severe conditions of filtration, the binder, containing visible air pockets, emerges from the capillary to form drops thatmomentarily cling to the bottom of the die. The apparent shear rate range in which the flow of the highly filled suspensionis stable can be broadened by increasing the viscosity of the matrix (lower temperature), increasing the shear stress at the wall, and through the use of dies with reater diameters. For die flow of an energetic suspension where the suspension lows like a plug with a shear stress dependent slip velocity at the wall, U, the low bees unstable when the filtration velocity Vm bees greater than the lip velocity of the plug (I 6, 31). The critical apparent shear rate value for each ie at which the flow instabilities are onset can be predicted on the basis of the omparison of filtration and wall slip rates (16). Obviously in the processing of propellants and explosives the filtration of thematrix and thus the unstable region should be avoided. Otherwise, the additionalloss of the binder, due to axial migration at solid loading levels that are veryclose to the maximum packing fraction, will render the suspension unprocessable. The unstable region in converging flows can be avoided by the properselection of the production rate and the geometry of the channel through whichthe suspension is flowing. For example the experimental data and the results ofthe theoretical approach used during the analysis of the propellant used in theAdvanced Shuttle program (ASRM) suggested that the critical apparent shearrate, below which flow instabilities prevail, decreases with increasing channeldiameter. The migration of the particles in the transverse to the flow directionneeds to be assessed also (12, 13, 32).Air Entrainment Effects Since the formulations of propellants and explosives involve volume fractionsof solids which approach the maximum packing fraction of the solid phase, theincorporation of even small concentrations of air, makes a significant differencein the rheological behavior and hence the processability of highly filled suspend ons (19, 33). In general the suspension samples processed without vacuum can ontain an additional few percent by volume air under ambient conditions. The hear viscosity of the suspension decreases and wall slip velocity values increase ith the incorporation of air into the suspension. The air entrained into an nergetic suspension sample can be studied using xray radioscopy and magn tic resonance imaging micrographs (19). The amount of air entrained into the uspension increases at partially full regions in the continuous processor in omparison to suspension samples collected from pletely full sections of thecontinuous processor (20). For example, the density values of samples collectedfrom the reversely configured screw sections of a twin screw extruder (whichnecessitate pletely full mixing volume) are greater than those of the samplescollected from the partially full sections of the same extruder ., at forwardlyconfigured screw sections (20). The air entrained into the suspension also affects the development and natureof the slip layer (binder rich region found adjacent to the wall) of the suspensionduring die flows. Under moderate die pressures, pockets of air cling to the wallmomentarily and are spread out to be later draggedon by the bulk of thesuspension. The wall of the die appears to be covered partially with films of airduring extrusion, with the air film continuously being removed and replenished(Figure 3).MICROSTRUCTURAL ANALYSIS OF PROPELLANTS ANDEXPLOSIVES DEGREE OF MIXINGBackground One of the most challenging aspects of any mixing operation, where two ormore identifiable ponents are brought together, is the characterization of thestate of the mixture ., the degree of mixing or the goodness of mixing. In the ondiffusive mixing of a viscous polymeric binder with solid ponents, the omplete description of the state of the mixture would require the specification f the sizes, shapes, orientations and the positions of