Effect of Hydroxyl‐Terminated Polybutadiene Grade on Rheology and Settling of High‐Solids Mixtures of Mock Propellants

ABSTRACT The most common binder used in composite rocket propellants is based on the pre‐polymer hydroxyl‐terminated polybutadiene (HTPB). It has been shown to have excellent mechanical properties and a long life once crosslinked, and studies have examined the kinetics of crosslinking and pot life of energetic formulations with HTPB. What is less well‐understood, however, is how the HTPB pre‐polymer and its formulations with particles behave prior to curing, an important aspect for storage, handling, and processing. To fill this gap, we specifically target understanding how the HTPB functionality impacts settling and rheological properties. We compared model similar molar mass pre‐polymers, specifically the linear grades (LBH‐2000 and LBH‐3000) with a functionality of approximately 1.95, to the standard Poly bd HTLO R45 with a higher functionality of approximately 2.4‐2.6, leading to more opportunities for hydrogen bonding and a more globular conformation. We show that the stability of the pastes against settling is higher for the linear LBH pre‐polymers due to the higher binder and paste viscosity compared to the HTLO R45. Using three‐interval thixotropy and small amplitude oscillatory shear testing, we demonstrate, however, that HLTO R45 recovers more readily after shear compared to the LBH pre‐polymers and that viscous yielding is important for this stability. These insights into the role of HTPB structure in processing can provide an understanding of processing challenges currently experienced in the field and lay the groundwork for molecular design of pre‐polymers that are more readily processible through both traditional methods and emerging processes like additive manufacturing .