An empirical model for the viscosity buildup of hydroxy terminated polybutadiene based solid propellant slurry

Abstract

IndiaThe Polar Satellite Launch Vehicle (PSLV) makes use of hydroxy terminated polybutadiene (HTPB) based solid propellant in the booster stages, strap-ons, and the third stage. An empirical model that describes the change in viscosity of various formulations of propellant slurry with time and temperature has been developed as an extension of the Andrade equation. The model incorporates constants that account for the viscosity of the slurry caused by the combined effect of the continuous binder phase, dispersed particulate solids, and progress of crosslinking between reactive species of the binder and curators. The correspondence between predicted viscosity buildup and measured values obtained during casting of motors is verified and found satisfactory. A method of tailoring some constants, so as to make them batch-specific, is explained. Several practical applications are described. The model serves as a predictive tool for evaluating the potlife and rheological behavior of the slurry during production of large solid motors. The experimental method and approach used for data analysis can be used for other slurries that are similar in composition to propellant slurry. XNTRODUCTIOIWior of rigid particles as well as deformable fillers such olid propellant slurry consists of a mixture of liq-S uids such as hydroxy terminated polybutadiene (HTPB) resin, toluene di-isocyanate (TDI), a trifunctional crosslinking agent and a plasticizer, which form the continuous or binder phase and particles of ammonium perchlorate, aluminum powder bum-rate modifier, and other solid ingredients forming the disperse phase. The cure reaction between the hydroxyl groups of HTPB and crosslinker and the isocyanate groups of TDI causes the viscosity of slurry to increase with time. The polymeric polyurethane matrix is thereby formed at the end of cure. Rheologid research on slurries and suspensions has been carried out in four basic ways: i) as a twophase mixture of liquids and solids: the pioneering study in this direction was by Einstein (l), who developed a model to describe the behavior of very dilute suspensions of rigid spheres in a Newtonian liquid. The work was extended by other researchers (2-13) to study the behavior of concentrated suspensions of various kinds of fillers-rigid spherical, rigid nonspherical, in monodisperse and polydisperse distribution in Newtonian liquids. On similar lines, the behav-as fibers in non-Newtonian mediums such as polymer melts has been studied (14-17). ii) Some researchers (1 8-2 1) considered the slurry a homogenous material and described the bulk stress generated in it when it was subjected to a bulk velocity gradient. iii) Slurries made of concentrated suspensions of rigid particles in viscoelastic fluids have been characterized by dynamic testing methods (22). iv) Rheological parameters such as viscosity have been applied to study the cure kinetics of reactive liquid or slurry systems (23-29).However, work that is specific to propellant slurry is rather limited. Chen and Hsu (30) have developed a...

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