Eco‐friendly chemically crosslinked solid composite propellants via catalyst‐free azide‐alkyne cycloaddition

Abstract

We have innovatively formulated solid propellants by employing a catalyst‐free azide‐alkyne cycloaddition approach, steering away from the conventional urethane curing system reliant on moisture‐sensitive isocyanate compounds. These conventional systems exhibits poor compatibility with the eco‐friendly ionic oxidizers. Azide polymers, including polycaprolactone ether (PCE), polycaprolactone (PCL), and polyethylene glycol (PEG) were incorporated, with their terminal hydroxyl groups strategically modified with azides. Additionally, glycidyl azide polymer (GAP), characterized by an abundance of azides in its side chains, was introduced. For polybutadiene‐based solid propellants, a departure from the norm was pursued. We employed polybutadiene (PB) terminated with electron‐deficient alkynes(propiolate), synthesized through a urethane reaction involving an unsymmetric divalent chain‐linker containing both isocyanate and propiolate functionalities with hydroxyl‐terminated polybutadiene (HTPB). This approach diverged from the common practice of modifying other polymers with azides at the terminal. To ensure the attainment of optical mechanical properties in azide‐terminated polymer‐based solid propellants, trivalent propiolate curatives were judiciously combined with divalent propiolate curatives in an appropriate blend ratio. A meticulously synthesized series of polymeric bonding agents, designed to establish chemical links between solid oxidizers and polymer binder, revealed the idenfication of exceptional bonding agents. These agents played a pivotal role in delivering outstanding mechanical properties in solid propellants based on ammonium perchlorate (AP) and nitramine‐typed oxidizers. GAP‐based solid propellants were meticulously prepared, incorporating both urethane moieties at the terminal and triazole moieties at the side chains. Trivalent azide‐terminal curatives were introduced for crosslinking PB terminated with propiolates. Generally, triazole‐curing system resulted in solid propellants exhibiting notably higher burning rates compared to those crosslinked through urethanes. In summary, this research presents a sophisticated approach to the formulation of solid propellants, emphasizing a departure from conventional systems, strategic polymer modifications, and the meticulous synthesis of bonding agents to achieve superior mechanical properties and burning rates.