# Evaluation of UV-Curable Solid Rocket Propellants' Properties for Advanced 3D Printing Technologies
## 2 Materials and Devices  
### Materials  
The propellant studied used polybutadiene (PB) as a binder with a molecular weight of 5000 g/mol. The additives used for the polymerization process included pentaerythnitol tetrakis(3-mercaptopropionate) (PTTM), which was crucial for the thiol-ene reaction, and bis-(2,4,6-trimethylbenzoyl!) phenylphosphine oxide (BAPO), which served as the photoinitiator. Furthermore, ammonium sulfate (AS) was chosen as an inert alternative to the oxidizer component, which generally consists of ammonium perchlorate (AP) [22]. Allconstituents of the propellant were purchased from the chemical company Sigma-Aldrich (Merck KGaA, Darmstadt, Germany).  
Two different sizes were considered for the inert AS to properly mimic AP. Typically, two sizes of AP are used primarily to enhance the propulsive performance and secondarily to mitigate the excessive viscosity increase observed at high volumetric fractions of the solid component. This strategy is based on the concept of the maximum packing density, which has been applied in numerous studies in the literature [23; 24]. The fine powders used had an average size of 20 um, while the coarse powders were 200 um. Their production was entrusted to the SPLab at Politecnico di Milano. The Milan laboratory utilized the controlled recrystallization method, allowing for the precise generation of AS particles with the desired dimensions. However, the process proved to be very slow. Therefore, a process that involved a planetary milling jar was chosen, which was capable of pulverizing the coarse powders sold by Sigma-Aldrich, which were then sieved to a range close to the desired size. The swelling tests of the polymerized binder were performed using 99.8% toluene as the solvent, which was purchased from Sigma-Aldrich.  
The current formulation of the complete inert propellant is the result of extensive studies carried out through the years on various photocurable resins and their properties [21]. For 3D printing tests, the compound to be extruded was obtained by combining PB with 80 weight percent of AS powder (80% coarse + 20% fine) and manually stirring it to obtain a homogeneous mixture. Then, the addition of PTTM allowed for the photocuring thiol-ene reaction to take place, and BAPO was finally added. The quantities for PTTM and BAPO are given in Parts per Hundred Parts Resin (phr) and were, respectively, 14 phr and 4 phr. A homogeneous mixture was achieved after 5 min of manual stirring. An identical process was used to prepare samples for viscosity and pseudoplasticity tests with the exception of BAPO addition, which was purposefully left out to prevent crosslinking during testing.  
A 3D printer, the Creality Ender 3 S1 (Creality 3D Technology, Europe), was used to create the essential components of the test benches. The parts not exposed to excessive heat sources were made of polylactic acid (PLA). In contrast, components directly in contact with heat sources were made from carbon fiber-reinforced polypropylene (PPCF), a material that exhibits superior mechanical properties compared to PLA at high temperatures. The use of polyethylene terephthalate and acrylonitrile butadiene styrene was excluded due to the toxicity of their fumes. The 3D printing filaments used within the research were purchased from Filoalfa (Turin, Italy).