## 3 Results  
Figure 5 shows the morphological changes in the oyster shell particle in response to being heated for plastic working. Compared with the untreated particles in Figure 5a, plastic working reduces the distance between the particles in Figure 5b,c, which correspond to temperatures of 600 \({}^{\circ}\)C and 1200 \({}^{\circ}\)C, respectively. The inter-particle spacing affects the density of the shielding material, thereby directly affecting its shielding efficiency [32].  
Table 1 lists the thicknesses, weights, and densities of the three oyster shell sample variations. Through plastic working, the particle size and number of voids are reduced, thus increasing the density of the sheet.  
In addition, the particle distribution within the oyster shell-based samples was analyzed by performing SEM imaging of sheet cross-sections (Figure 6). Oyster shell powder processed at high temperature mixed excellently with polymer materials, and the particle distribution was extensive when compared at the same magnification. In addition, mixing the processed oyster shell powder with HDPE changed the distribution state of the powder particles, as shown in Figure 6, but the visual properties remained intact.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Thickness (mm)** & **Weight (g)** & **Density (g/cm\({}^{3}\))** \\ \hline Oyster shell sheet without & & & \\ heat treatment & 0.32 & 23.39 & 8.12 \\ Oyster shell sheet with plastic & & & \\ working at 600 \({}^{\circ}\)C & 0.31 & 28.12 & 10.07 \\ Oyster shell sheet with plastic & & & \\ working at 1200 \({}^{\circ}\)C & 0.32 & 31.25 & 10.85 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Physical properties of the fabricated oyster shell-based shielding sheets.  
Figure 5: SEM images providing a comparative analysis of oyster shell particle morphology and spacing in response to plastic working: (**a**) untreated oyster shell particles, (**b**) particles after plastic working at 600 \({}^{\circ}\)C, and (**c**) particles after plastic working at 1200 \({}^{\circ}\)C.  
Tables 2 and 3 present the experimental results for the shielding efficiency of the oyster shell sheet presented in this study. First, the shielding rate of the oyster shell sheet based on the plastic working temperature was compared to that of the shielding sheet manufactured through the same process as that of the eco-friendly shielding material, and evaluated.  
Table 2 shows the shielding efficiency of the oyster shell sheet based on the temperature during plastic working, and the shielding rate of the shielding sheet processed at 1200 \({}^{\circ}\)C was the highest. The experimental data reveal that the oyster shell powder without plastic working provides a low shielding efficiency of approximately 18% in the low-energy X-ray region; however, this increases to approximately 37% for the sheet fabricated using plastic working at 1200 \({}^{\circ}\)C. Table 3 shows the results of comparing the shielding sheets made of barium sulfate, bismuth oxide, and tungsten, which are the most used main materials for eco-friendly shielding sheets. As a result, the oyster shell sheet processed at 1200 \({}^{\circ}\)C showed an average shielding efficiency that was 7.2% lower than that of the barium sulfate shielding sheet, which had the lowest shielding efficiency. In addition, a lead sheet of an equivalent of 0.3 mm, with the same thickness, is almost shielded, and the oyster shell sheet can be predicted to have a shielding efficiency that is approximately 40% of the lead efficiency.  
Figure 6: SEM images of sheet cross-sections providing a comparative analysis of the oyster shell particle morphology in the plastic working state: (**a**) untreated oyster shell particles, (**b**) particles after plastic working at 600 \({}^{\circ}\)C, and (**c**) particles after plastic working at 1200 \({}^{\circ}\)C.