UV-Cured Composite Solid Propellants: Characterization of Binder and of 3D-Printer Samples

Additive manufacturing based on extrusion deposition combined with UV light curing is an innovative technique adopted to produce composite solid propellant grains. Production of complex shapes, rapid prototyping and avoidance of toxic isocyanates are some of the advantages introduced by this new technology compared to the traditional mix-cast-cure production method. Nevertheless, the manufacturing process presents some critical aspects that need to be fully addressed in order to obtain a stable production method. Recent studies demonstrated the feasibility of this manufacturing technology, but further works are still required to optimize it. This study aims at investigating which properties of four photocurable binders based on polybutadiene backbone play a key role in the curing mechanism. The curing mechanisms are based either on a thiol-ene click reaction or on acrylate groups. The resulting polymers are tested through toluene dissolution tests, DMA, and FTIR-ATR analyses. Moreover, the influence of the crystalline oxidizer (ammonium sulfate AS) and of aluminum powder on the curing reaction is investigated through dissolution tests. Multilayer samples produced by a newly developed 3D printer are examined performing X-ray computed tomography to study the inter layer region and the printing accuracy. Results suggest that the amount of cis, trans and vinyl isomers, and the polybutadiene viscosity are the binder features that mostly influence the UV curing mechanism. Results also indicate that thiol-ene reaction-based formulations may be more suitable for this application with respect to the acrylate-based formulations, characterized by excessive stiffness. The presence of AS particles seems to improve the curing mechanism, while opaque particles (Al) hinder the process. Finally, tomography analyses demonstrate that the actual setup requires further parameter tuning to obtain geometrically accurate and homogeneous samples.