# Performance Comparison among KN95-Certified Face Masks by Classical Techniques and Innovative Test
## 3 Results and Discussion
### Morphology  
The main characteristics of the face masks as well as the water contact angle measured on the external layer are summarized in Table 3.  
Face masks should own a certain level of hydrophobicity, to avoid the wetting of the layers that could decrease the filtration efficiency after prolonged exposure [36; 37], and transfer the aerosol to the face side of the mask. All the tested masks showed a great hydrophobicity, confirmed by contact angles that ranged between 123\({}^{\circ}\) and 141\({}^{\circ}\). Moreover, when leaving the water drop on the nonwoven sample for 10 min, no absorption was observed, confirming the adequate wetting resistance of the external layer of all the mask samples. The ability of the outer material not to be wet in a short time is important to avoid that the contagious droplet is then transferred to the inner layers of the mask up to the face. Regarding the external mask layer, it is a spunbonded nonwoven film, which has the main objective of maintaining a smooth surface and stopping the largest droplets from entering inside the mask's porous structure. As mentioned in \"Materials and Methods\", on the basis of experimental results discussed above, a select set of masks (i.e., samples 0, 1, 4, 6, 9) was chosen to be subjected to microscopy and BFE characterization. Figure 2 shows the nonwoven tissue structure of the four chosen samples compared with the 3M mask (sample 0), considered as the gold standard for our investigation. The fiber diameter of this external layer is about 27 um for sample 1 and approximately 20 um for samples 4, 6, and 9. The denser structure in the images corresponds to the welding points of the fiber typical of spunbonded nonwoven materials. Unlike the other masks, the  
Figure 2: Optical microscope images of gold standard (sample 0) and samples 1, 4, 6, and 9 external layers (**A**–**E**) and face side layers (**F**–**L**), respectively. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **Grammage (g/m\({}^{2}\))** & **Intermediate Layer Thickness (um)** & **Total Thickness (mm)** & **Contact Angle (\({}^{\circ}\))** \\ \hline
0 & 378 & 304 \(\pm\) 18 & 1.04 & 130 \(\pm\) 5 \\
1 & 180 & 207 \(\pm\) 17 & 0.57 & 128 \(\pm\) 3 \\
2 & 242 & 190 \(\pm\) 12 & 0.64 & 134 \(\pm\) 1 \\
3 & 179 & 117 \(\pm\) 11 & 0.71 & 141 \(\pm\) 2 \\
4 & 279 & 181 \(\pm\) 23 & 0.78 & 127 \(\pm\) 2 \\
5 & 220 & 165 \(\pm\) 11 & 0.63 & 137 \(\pm\) 7 \\
6 & 216 & 169 \(\pm\) 13 & 0.59 & 136 \(\pm\) 5 \\
7 & 258 & 241 \(\pm\) 21 & 0.75 & 129 \(\pm\) 3 \\
8 & 232 & 320 \(\pm\) 20 & 0.75 & 123 \(\pm\) 1 \\
9 & 195 & 216 \(\pm\) 13 & 0.57 & 132 \(\pm\) 3 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Morphological characteristics of the face masks and water contact angle of the external layers. structure of the external nonwoven layer of sample 0 is composed of fibers with a slightly narrower diameter (15 \(\upmu\)m) without welding points. A similar situation is on the face side of the mask. Sample 0 external layers show a thicker, porous structure than the other ones where the openings are more clearly visible. Figure 3 shows the FESEM images of the intermediate nonwoven layer of the selected face masks at 150\(\times\) (top) and 500\(\times\) (bottom) magnifications. The low magnification FESEM images (top in Figure 3) depict the uniform fibrous structure of the internal layer. At the lowest magnification level, no evident differences in the structure could be observed among the five samples. Higher magnifications (bottom in Figure 3) allowed us to better estimate the morphology of each sample. The samples were formed by a random distribution of fibers, characterized by different sizes and orientations typical of a melt-blowing production process. While the most external and face side spunbonded layers (Figure 2) clearly showed some large passages between the fibrous structure, the intermediate layers exhibited a denser fiber arrangement.