Aging of HTPB/AP-Based Composite Solid Propellants, Depending on the NCO/OH and Triol/Diol Ratios O ¨ ZGU ¨ R HOCAOG ˘ LU, 1 TU ¨ LAY O ¨ ZBELGE, 2 FIKRET PEKEL, 1 SAIM O ¨ ZKAR 3 1 TU ¨ BITAK-SAGE, PK 16 Mamak, 06261 Ankara, Turkey 2 Department of Chemical Engineering, Middle East Technical University, 06531 Ankara, Turkey 3 Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey Received 26 January 1999; accepted 24 September 1999 ABSTRACT: Aging behavior of hydroxyl-terminated polybutadiene/ammonium per- chlorate (HTPB/AP)-based composite solid propellants was studied as a function of crosslink density, which is predominantly determined by the molar ratio of diisocyanate to total hydroxyl (NCO/OH ratio) and the molar ratio of triol to diol (triol/diol ratio). For this purpose, 16 propellant samples with different compositions were prepared by changing the NCO/OH ratio as 0.81, 0.82, 0.83, and 0.85 for each triol/diol ratio of 0.07, 0.09, 0.11, and 0.13, and subjected to an accelerated aging at 65°C. The changes in the mechanical properties were monitored throughout the aging period. In the initial part of the aging period, a sharp increase in stress, modulus, and hardness values and a sharp decrease in strain values were observed for all the propellants. At further stages of aging, only slight changes were observed in the mechanical properties. Concerning the aging criterion as reduction in the strain capability more than the half of the initial value, the propellants with respective NCO/OH-triol/diol ratios of 0.81– 0.09, 0.85– 0.09, 0.81– 0.13, 0.83– 0.13, and 0.85– 0.13 can be considered to be aged with time. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 959 –964, 2001 Key words: composite; propellant; mechanical properties; aging; HTPB; triol; diiso- cyanate INTRODUCTION Aging of the composite solid propellant is an im- portant issue in rocket motor applications, as it determines the service life of a rocket motor. In essence, aging is defined as the deterioration of the solid propellant altering the performance characteristics of the solid rocket motors. Owing to the large surface area and chemical energy potential, the composite propellants are subjected to deterioration during handling, storage, and op- eration stages leading to changes in both the bal- listic and mechanical properties. 1 The extent of crosslink density in the binder matrix significantly affects not only the mechan- ical but also the aging properties of the composite propellant. The crosslink density can be adjusted by varying the amounts of the prepolymer diol (hydroxyl terminated polybutadiene, HTPB), cur- ing agent (isophorone diisocyanate, IPDI), and the triols (triethanol amine, TEA). These are the main components making the network structure, the formation of which is practically completed at the end of curing period. In this respect, the equivalent ratios of the reacting species, namely Correspondence to: S. O ¨ zkar. Contract grant sponsor: The Turkish Academy of Sciences. Journal of Applied Polymer Science, Vol. 79, 959 –964 (2001) © 2000 John Wiley & Sons, Inc. 959