Castor oil as a sustainable alternative to HTPB binder in polyurethane matrices for composite solid propellants: viscosity, curing kinetics, and hardness evaluation

Increasing environmental regulations are driving the development of green solid propellants with reduced toxicity and lower life-cycle environmental impact. The binder plays a critical role in solid propellants, with hydroxyl-terminated polybutadiene (HTPB) being the most widely used despite its petroleum origin, environmental concerns, and restricted availability. This study evaluates castor oil (CO), a renewable, biodegradable, and low-toxicity resource, as a sustainable alternative binder. A CO-based polyurethane prepolymer containing 30% (w/w) toluene diisocyanate was synthesized and characterized by Fourier transform infrared spectroscopy (FT-IR), rheology, particle distribution, hardness tests, and scanning electron microscopy (SEM). The results showed 20% faster curing and hardness comparable to HTPB, highlighting the potential of CO as a viable and sustainable binder for solid propellant applications. Additionally, SEM analysis comparing the top and bottom regions of two CO-based samples produced by different synthesis using ammonium perchlorate (AP) as filler demonstrated a more homogeneous AP distribution in the sample #2 (a prepolymer it was first synthesized by reacting CO with 30% of the TDI needed for complete curing).