A New Prepolymer: Lactone-Terminated Polybutadiene (LTPB) for High-Energy Solid Propellants

N composite propellants, polybutadiene prepolymers are considered as high-energy fuels. Poly(butadiene-acrylic acid) (PBAA) is the earliest prepolymer in this category wherein the functional COOH groups are randomly distributed along the chain. The uneven spacing of the carboxyl groups, which gives rise to poor mechanical properties of the PBAA-based propellant, is improved by the interjection of acrylonitrile groups between the polybutadiene and acrylic acid molecules. The resulting terpolymer, viz. poly(butadiene-acrylic acid-acrylo-nitrile) (PBAN), has more reproducible mechanical characteristics and is widely used as a fuel prepolymer in high-energy solid propellants. However, for better low-temperature properties and for improved performance, carboxyl-terminated polybutadiene (CTPB) prepolymer made by various methods1;3 is preferred to PBAN. Hydroxyl-terminated polybutadiene (HTPB) is the most recent entrant; it permits solid loadings of up to 90% and still retains mechanical properties within acceptable limits. An extensive review of the work on polybutadiene based solid propellants is given by Mastrolia and Klager.4 In the present paper we report the synthesis of a new prepolymer, lactone-terminated polybutadiene (LTPB), which has terminal functional groups different from those in any of the polybutadiene prepolymers reported so far. The characteristics of the propellant based on the LTPB prepolymer are also presented here. The prepolymer is designated as HEF-20, short for high-energy fuel of series 20.