Advancements in UV-Cured Composite Solid Propellant Binders: Evaluation of Long-Term Behavior

Traditionally, composite propellant grains, composed of crystalline oxidizers, metallic fuels, and a polymeric binder, are produced through a cast-cure process, which implies the use of a forming mandrel to shape the grain. This limits complex shapes production and delays rapid prototyping. Consequently, the method negatively affects the range of available mission profiles. In addition, the curing process requires isocyanates—highly toxic substances. A 3D printing technology based on UV curing and extrusion deposition, developed jointly by Space Propulsion Laboratory of Politecnico di Milano and Politecnico di Torino, can avoid the use of the mandrel and of isocyanates, fostering a greener and safer process. The new binder formulation involves the use of polybutadiene instead of the traditionally employed hydroxyl-terminated polybutadiene. The curing occurs through a thiolene click reaction, initiated via photo activation. Recent advancements concern the investigation of the aging behavior through accelerated aging under different conditions to evaluate long-term changes in mechanical and chemical properties. The evolution of binder properties is studied performing FTIR-ATR analyses, toluene dissolution tests, and dynamic mechanical analyses. Results show a gradual increase in the storage modulus E′ and in the curing efficiency, especially for samples stored under dry air. These changes are more marked in the formulation containing the lower amount of 1,2-vinyl group. FTIR-ATR and weight variation tests suggest the possibility of water absorption for samples stored under wet air (increase in hydroxyl index). Overall, this testing campaign is essential to optimize the propellant formulation and to evaluate solid propellant in-situ, and eventually in-space, manufacturing and storage.