# Evaluation of UV-Curable Solid Rocket Propellants' Properties for Advanced 3D Printing Technologies
## 4 Results
### Uva and UVC Curing Tests  
Table 2 shows the series of tests carried out to analyze the response of the photocurable propellant to different UV light frequencies. These tests were performed using the delta UV curing system described in Section 2.3.  
The first section of Table 2 presents the results of tests conducted using only UVA LEDs for a layer of about 1 mm and different exposure times. These serve as a reference, demonstrating that increasing the UVA exposure time leads to higher PCP values. The second section examines samples exposed for a total time of 270 s, with three combinations of UVA and UVC times. The results suggest that the PCP is primarily driven by UVA radiation, while longer UVC exposure does not provide additional benefits. In the third section, the results are shown for different UVA exposure times tested while keeping the UVC exposure constant at 180 s. The PCPs obtained were comparable to those obtained with UVA alone, indicating that the tested UVC LEDs were not useful even as a postcuring process, as occurs in industrial stereolithography [37].  
For samples of a 1 mm layer height, multifrequency tests for fixed UVC exposure (180s) combined with a varying UVA exposure duration (90 s, 180 s, 270 s) consistently showed a \(p\)-value \(\leq\) 0.001, indicating with strong statistical significance that the PCP increases with longer UVA exposure. When comparing monofrequency-cured samples (UVA only, as shown in the first section of Table 2: 90 s, 'low-cured'; 180 s,'mid-cured'; 270 s, 'high-cured') to their multifrequency counterparts (same UVA exposure duration plus 180 s of UVC), the trend was less distinct. Specifically, reference series for 'low-cured' and 'high-cured' samples exhibited a \(p\)-value \(\leq\) 0.05, i.e., the addition of UVC had moderate statistical signifance. On the other hand, the'mid-cured' samples displayed a \(p\)-value \(\approx\) 0.30, necessitating further testing to be conducted in the future.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Temperature** & **UVA Time** & **UVC Time** & **Expected** & **Mean Layer** & **PCP [\%]** \\
**[\({}^{\circ}\)C]** & **[s]** & **[s]** & **Layer Size** & **Size [mm]** & **Size [mm]** \\ \hline
25 & 90 & 0 & & & 75.1 \(\pm\) 5.5 \\
25 & 180 & 0 & 1.00 & 1.09 \(\pm\) 0.06  
The fourth and fifth sections show the results from the analysis of thicker layers (2 mm and 3 mm) while maintaining a total exposure time of 270 s. Since UVC wavelengths are shorter, they were hypothesized to penetrate deeper into the resin and aid in curing thicker layers. However, the results, though only preliminary, contradict this assumption. For samples cured only with UVA, the PCP difference between 2 mm (91.6%) and 3 mm (89.7%) was minimal. In contrast, when using 180 s UVA + 90s UVC, the PCP dropped significantly from 91.6% (2 mm) to 80.7% (3 mm), indicating that UVC does not improve deeper-layer curing These findings suggest that the selected UVC LEDs, in combination with the photoinitiator system of PB, BAPO, and PPTM (identified as highly effective across a broad spectrum of resins [19]), do not offer any advantages in the current formulation.  
Finally, the last section examines the impact of preheating at 90 \({}^{\circ}\)C. The reduction in the PCP for the preheated samples aligns with the degradation of tensile mechanical properties reported in [20]. As shown in Table 2, this PCP reduction can be adequately mitigated by extending the exposure time. Additionally, the PCP can be finely adjusted to enhance interlayer adhesion in 3D printing applications. Further investigations, including swelling tests and microtomography, will be conducted to refine the current analysis.