Photo-polymerization for additive manufacturing of composite solid propellants

The established state-of-the-art for composite solid propellant grain manufacture consists in mix-cast-cure process using hazardous chemicals and specific molds for propellant forming. In most of the cases, polyaddition of oligomers involves isocyanate functional groups. Construction constraints limit the feasibility of propellant geometries, confining the pressure-time history of rocket motors to some established configurations. Composition pot-life becomes one of the most important parameters in the definition of correlated industrial processes. An additive manufacturing process for propellant grain production based on UV curing has been recently proposed for patenting. This technique enables more complex grain geometries, paving the way for new propulsive missions, thanks to customized thrust-time profiles or local composition fine tuning. The new curative method makes innovative use of pre-polymers, replacing isocyanates with UV-sensitive components characterized by lower chemical hazard for operators. The paper illustrates the experimental results obtained during the preliminary test campaign on propellant inert simulators, produced as lab-scale proof of concept. The activity targeted mono-layer samples, focusing on binder properties. Dynamic-mechanical thermal analysis, thermal-gravimetric analysis, and stress–strain tests have been carried out to measure mechanical and physical characteristics of different formulations. Both hydroxyl-terminated polybutadiene (HTPB) and polybutadiene diacrylate (PBDDA) binders have been considered, using ammonium sulfate as substitute of ammonium perchlorate. Aluminized and non-aluminized samples have been manufactured to evaluate the impact of opaque-reflective materials during the UV curing process. The reported analyses show comparable results for both the new UV-cured materials and demonstrate the feasibility of propellants based on isocyanate-free process.