Hydroxyl-terminated polybutadienes (HTPB) and glycidyl azide polymer (GAP) as solid rocket propellant binders: A review of synthesis and properties

Hydroxyl-terminated polybutadienes (HTPB) are the most extensively used binder in solid rocket propellant formulations owing to its exceptional mechanical properties when crosslinked, low glass transition, and ease of processing. Given its endothermic decomposition during rocket launch, however, research focus has also been on more energetic alternatives, e.g. glycidyl azide polymer (GAP), to enhance the specific impulse of the binder, therefore extending ballistic missile range. Here, we report three conventional syntheses of HTPB: free radical, anionic, and ring-opening metathesis polymerisation, the choice of which is demonstrated to tune the chemical composition, crosslinking potential and mechanical properties of the final binder material. It is routine for HTPB when applied as a binder to be crosslinked/chain extended using diisocyanates to form thermoset crosslinked polyurethanes. There is often the addition of antioxidants to increase stability and plasticisers to modify the thermal properties and performance especially at ambient and sub ambient temperatures. The synthesis of GAP, including indirect polymerisation of epichlorohydrin with subsequent azidation, and direct anionic ring-opening polymerisation initiated by the ate complex are also discussed. Crosslinking strategies for GAP include the use of diisocyanates, alkynes, or a combination of both. This review provides a concise introduction to binders for solid rocket propellants.