Aging of HTPB/AP-based composite solid propellants, depending on the NCO/OH and triol/diol ratios

Aging behavior of hydroxyl-terminated polybutadiene/ammonium perchlorate (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.