Particle packing models to determine time-dependent slip flow properties of highly filled polyurethane-based propellant

Hydroxyl-terminated polybutadiene (HTPB) is a reliable binder to build a strong polyurethane network for materials with more than 85 wt% of fillers, such as composite solid propellant (CSP), a fuel for space launch vehicles. However, a careful composition design is still challenging as high filler contents intricate the process, and it is responsible for both the perfection of a solid structure of CSP and the safety of the vehicle. The PU-based liquid contents (LC) CSP consists of HTPB, diisocyanate compound, amine compound, and plasticiser. The fillers include aluminium and ammonium perchlorate particles. The tendency of aluminium to agglomerate in PU-based LC and to be dispersed in the plasticiser, combined with the formation of a polyurethane network, causes time-dependency in viscosity of the CSP slurry. Thus, models to predict initial viscosity and minimum viscosity are highly demanded. In this work, particle packing models for both types of viscosity are proposed considering the wall slip phenomenon and the nature of the constituents. The models were tested on CSP slurry with 87.5–90 wt% fillers, including 15–20 wt% micro-aluminium. Based on viscosity measurements, high correlation levels ( r = 0.7–0.99) were found in the correlation between particle packing parameters and the viscosities. This convinces the reliability of the models for predicting the initial and minimum viscosities of CSP. Moreover, phenomena revealed by a deep analysis of the start-up of shear flow, such as increasing shear modulus and residual stress with increasing particle packing parameters and the existence of positive and negative thixotropic behaviours, can be clearly explained by the models.