Improving the Mechanical Properties of Composite Rocket Propellants

Abstract: Solid composite rocket propellants usually contain ammoniumperchlorate embedded in an elastic polymer binder. The bindercan be based on a liquid prepolymer such as hydroxyl-terminatedpolybutadiene, HTPB, or poly(3-nitratomethyl-3-methyl oxetane,PolyNIMMO. HTPB is today widely used for this purpose whereasPolyNIMMO has not yet found its way to an application. Bothprepolymers can be cured with diisocyanates to formpolyurethane rubber, yielding solid and elastic rocketpropellants. It is essential that the solid propellant has goodmechanical properties to ensure that the rocket will perform asintended. The propellant must also retain its elasticproperties down to the minimum service temperature and thus alow glass transition temperature is important. In fact, themajor cause of failure of solid rocket motors is linked to themechanical properties of the propellants. HTPB has a very lowglass transition temperature but in some applications itstensile strength is insufficient. PolyNIMMO, on the other hand,has too high a glass transition temperature and a suitableplasticizer is needed. The purpose of this work is to increasethe knowledge of the mechanical properties of polymers bystudying how to increase the tensile strength of HTPB and howto decrease the glass transition temperature of PolyNIMMO.The tensile strength of HTPB was studied by increasing thehard segment content, 1,4-butanediol and 1,4-cyclohexanedimethanol being used as chain extenders. The materials werecrosslinked with either isophorone diisocyanate,1,6-hexamethylene diisocyanate or dicyclohexylmethane4,4'-diisocyanate. The results show that the tensile strengthincrease strongly with the addition of up to two moles of diolper mole HTPB. The highest tensile strength was obtained byusing dicyclohexylmethane 4,4'-diisocyanate and1,4-butanediol.The depression of the glass transition temperature ofPolyNIMMO was studied by using a new energetic plasticizer,2,2-dinitro-1,3-bis-nitrooxy-propane. Two commercial energeticplasticizers, namely bis(2,2-dinitropropyl) acetal/formal andN-N-butyl-N-(2-nitroxy-etyl)nitramine were used for comparison.2,2-Dinitro-1,3-bis-nitrooxy-propane andN-N-butyl-N-(2-nitroxy-etyl)nitramine were found to interactstrongly with PolyNIMMOand they were thus very effective inlowering the glass transition temperature.Bis(2,2-dinitropropyl) acetal/formal on the other hand was noteffective, and the depression of the glass transitiontemperature in this case was due only to dilution of thesample.