Modeling of Composite Propellant Properties based on Polymer Rheology

Abstract

This paper describes the theoretical analysis and experimental research work on composite propellants for their mechanical and rheological properties. The theory of the polymers rheology has been described to model the polymer properties. The theory is extended to the composite propellants comprising of a polymer binder filled with solid oxidizer and metallic fuel. The variations in the oxidizer particle size and its distribution have been studied experimentally to analyze the effect on the performance properties. The results of propellant viscosity and mechanical properties have been measured and discussed. It was concluded that fine AP particles act as active fillers and enhance the mechanical properties of composite propellants. Viscosity decreases with increase in the amount of fine AP particles and then starts increasing when fine proportion increases beyond 20~25 percent. Similarly elongation increases with increase in fine AP particle size and maximum value is obtained at fine AP contents of 25~35 percent. Size of the AP mean diameter also affects the mechanical properties. Both mechanical strength and elongation substantially decrease as the mean AP particle size increases. I. Introduction HE composite solid propellants are composed of a polymeric binder with high solid loads of oxidizer and a metallic fuel with some additives. The relationship between the formulation variables and different properties of composite propellants are highly complex and interrelated. The knowledge on different dependencies and availability of modeling tools can be of significant help for propellant developers and analysts. Much research work has been carried out on these propellants since their use in the middle of last century. In spite of many achievements thereafter, design of a propellant formulation is yet considered as an art and almost all of the practical formulations are developed through working knowledge and practical experience. The propellant performance can be characterized with respect to specific impulse, density, gas composition, flame temperature and a number of thermochemical properties using the computer programs 1 . However, the variation of important performance parameters such as burning rate, mechanical and bonding properties, and processing characteristics with variation in formulation cannot yet be presented by mathematical models. These properties are changed with raw materials, solids shape, particle size and its distribution, and even changing the source of raw materials. Many studies have been reported previously to correlate the individual performance properties with the formulation parameters 2~7 . Most of these efforts are based on empirical relationships. Rheology is the science and technology of deformation of polymeric or plastic materials. The polymeric binder imparts main physical and chemical properties to the propellant. The composite propellant properties can be modeled by applying the concept of polymer rheology. Conventional polymers and rubbers filled with solid particles have previously been studies based on this theory 8 , while no such work has previously been reported on the composite propellants at least in the open literature. In the present study the theory and concepts of physical chemistry of polymer rheology are applied to understand and model the propellant properties such as viscosity and mechanical properties. The paper presents both the theoretical and experimental work on the effect of solid fillers in the propellant on the performance properties. In the experimental work, the results of mechanical properties (tensile strength, elongation, and modulus) and viscosity were measured for different formulations. These results have been analyzed and the discussed based on the theory of polymer rheology for the highly filled rubbers and polymers.