2.5. Characterization
For a non-destructive 3D visualizations of an internal structure of materials, X-ray computed tomography (Xµ-CT) was performed using Zeiss X-Radia 510 Versa (Zeiss, Oberkochen, Germany) (Figure S1, see Supplementary Materials) [38]. The X-ray source was operated at a voltage of 60 kV with a power of 5.0 W. The fields of view used were 800, 1200, 2000 µm, and the corresponding pixel sizes were 0.8, 1.2, 2.0 µm, respectively.
FE-SEM images of filter samples were observed by Supra 55 VP (Zeiss, Oberkochen, Germany), with prior coating with Pt at 20 mA for 120 s, using a 108auto sputter coater (Cressington Scientific Inc., Watford, UK). The thickness of respirator layers was measured using a thickness gauge under the pressure of 2.4 N. Porosity and solidity of webs were calculated based on Equations (1) and (2), where m is mass of the material, A is area of the material, t is thickness, and ρ is the material density (0.95 g/cm3 for PP was used):Porosity (%) = (1 − solidity) × 100 (%)(1)  Solidity = m/(A⋅t⋅ρ)(2)
The static contact angle (CA) of liquid was gauged as wetting property using a contact angle analyzer (SmartDrop Lab, FemtoBiomed Inc., Gyeonggi-do, Korea). A droplet of 3.0 ± 0.3 μL liquid, including distilled water (WA), ethanol (EtOH), isopropanol (IPA), chlorinated disinfectant (ClO−), and 0.1% detergent solution was dispensed on a surface of web, and CA was measured in 60 s after the droplets were settled. The chemistry of sample surface was analyzed by FTIR-ATR (TENSOR27, Bruker, Germany).