# Evaluation of UV-Curable Solid Rocket Propellants' Properties for Advanced 3D Printing Technologies
## 3 Methodology and Procedures
### Uva and UVC Curing Test  
The mechanical properties of solid propellant grains are critical for ensuring proper rocket engine operation. Failure to meet the required minimum values can cause various problems, such as crack formation, which locally increase the burning area, leading to a possible deflagration-to-detonation transition [44]. The mechanical properties of composite solid propellants are influenced by the degree of crosslinking of the binder and the interactions between the binder and oxidizer particles. To enhance these properties, bonding agents can improve the adhesion between the resin and oxidizer particles, while other methods can be employed to increase the crosslinking of polymers, which is directly related to the mechanical characteristics of the binder [32; 40]. One purpose of the present work was to optimize the degree of polymerization of the PB by determining the binder's response not only to UVA, as has already been studied in previous work [19], but also to a combination of UVA and UVC, as suggested by previous research studies [34]. UVC LEDs are known for their significantly lower efficiency compared to UVA, which results in greater heat dissipation. This increased heat can reduce the operational lifespan of devices and necessitates an adequate cooling system. Consequently, the power transmitted to the propellant is low when compared to the input power [45]. However, these drawbacks may be offset by the higher absorbance of BAPO in the UVC range. Additionally, UVC can penetrate deeper into the propellant, aiding the curing of thicker layers. To evaluate the binder's degree of polymerization, the swelling test method was performed according to the ASTM-D2765 standard [46]. Sample preparation began by manually mixing PB, thiols, and the BAPO in a beaker for 5 min. The resulting dispersion was then poured into a rectangular silicone mold with 1 cm sides. To obtain specimens with the desired thickness, the density of the dispersion was evaluated based on the values provided in the Sigma-Aldrich datasheets. Since the surface area of the specimen remained consistently 1 cm\({}^{2}\), the required volume could be based solely on the thickness. Each sample was weighed using a precision balance to ensure compliance with the requirements, minimizing deviation. Subsequently, the silicone mold containing the dispersion was placed under the UV LED and illuminated for specified times. Afterwards, the cured specimen was extracted from the silicone mold and placed inside a container, into which toluene was poured to perform the swelling test. Due to the toxicity of toluene, this phase and the subsequent ones were carried out under a chemical hood. The swelling test involved three phases: first, the measurement of the mass, \(m_{1}\), of the sample before immersing it in toluene; second, immersing the specimen for approximately 20 h and then weighing it immediately after extraction to determine its mass, \(m_{2}\); and third, the measurement of the mass, \(m_{3}\), of the sample three days after extraction. The swelling factor (SF), required to determine the polymer's ability to absorb the solvent, was evaluated as  
\[SF=\frac{m_{2}}{m_{1}} \tag{2}\]  
while the Polymer Conversion Percentage (PCP) was evaluated as  
\[PCP=\frac{m_{3}}{m_{1}}\cdot 100 \tag{3}\]  
The principle underlying swelling tests is relatively simple. This method involves immersing the hardened sample in toluene, which induces swelling. The process causes the long and entangled polybutadiene polymer chains to expand into a larger volume,facilitating the escape of PB monomers from the specimen. Consequently, at the conclusion of the second phase, the sample will consist exclusively of the PB chains that have successfully bonded together, along with the toluene absorbed during immersion. Weighing the sample three days after extraction allows for the complete evaporation of the toluene, enabling the measurement of the weight of the polymerized chains alone.