# Development and Testing of Copper Filters for Efficient Application in Half-Face Masks
## 3 Results and Discussion  
Typical morphologies of the Cu powders prepared by air and water atomization are shown in Figure 2a-d. The morphology of powder particles produced by air atomization was regular in shape. The morphology of the powder particles after water atomization was mixed; the powder was mostly irregular but regular particles were present too. Both small and large powder particles were uniformly distributed in the volume of the powder mixture. Figure 2: SEM pictures of typical morphologies of copper powders. (**a**) P-Cu-A-75; (**b**) P-Cu-A-315; (**c**) P-Cu-AW-75; (**d**) P-Cu-AW-315. Successful production of a metal filter requires the optimal choice of raw metal powder in terms of its purity, particle size, and distribution. All types of the tested Cu powders met these requirements. The individual loose powders were sintered in models with a diameter of 66 mm and a height of 1.2 mm, while the effective area of the filter was 0.342 dm\({}^{2}\) (square decimeter). The mold parameters were defined by the size of the filter holder, which is part of the facial half mask [12]. Figure 3 shows a half-face mask made of 3D prints, and Figure 4 shows a filter holder with a Cu filter. An important part of a respirator is the body with a facepiece that must meet the requirement for allowable permeability (1% for FFP3 respirators). The respirator's filtration ability and a tight fit to a user's face are the two basic parameters that affect its protective function. The prototype of a filtration half-face mask (Figure 3) was created at the Faculty of Mechanical Engineering of the Technical University of Kosice and consists of the half mask, as such (a facepiece), as well as a filter-carrying part (bayonet attachment), filtration layers, and fixation components. The facepiece of the half mask was designed based on the anthropometric data obtained in the research; it was made of Santoprene 8281-65 MED, a soft, cooling, non-hydroscop thermoplastic vulcaniste (TPV) in the group of thermoplastic elastomers (TPE), which is designed for medical and healthcare applications. It does not absorb humidity or water vapor from the air; it is biocompatible and recyclable. The filtration half-face mask also comprises two inhalation filters through which the air passes directly into the area of the nose and mouth. The exhaled air passes through the filters back into the surrounding environment. The replaceable parts of the half mask can be easily and safely attached, and in case the filters are used in the environment with a low content of dust particles in the air, i.e., if they are not clogged as the pressure drop increases during breathing, the filters may be disinfected and reused. Figure 4: The half-face mask holder with a produced Cu filter. Figure 3: The 3D printed half-face mask. Filters of the required dimensions were produced by sintering, using all types of Cu powders. Typical, fractography of filters is prepared from powders by water atomization, as shown in Figures 5 and 6. In Figure 5, we can see a number of pores that are present in the Cu matrix as a result of incomplete densification of Cu powders. A fracture surface detail (Figure 6) confirms the formation of diffusion connections between the grains of Cu powder during sintering. Pressure losses were measured in filters prepared as described above using an indirect method; the pressure losses in the compressed air vessel were analyzed, and, subsequently, the total leakage flow was calculated. All measurements of filter pressure losses were performed at a dynamic air viscosity of 1.82 \(\times\) 10\({}^{-5}\) Pa\(\cdot\)s and an air density of 1.167 kg\(\cdot\)m\({}^{-3}\). The investigated cross-sectional area of the filters was 1225 mm\({}^{2}\) (attached to the measuring stand). The calculated values of permeability and resistance coefficient corresponding to the total cross-sectional area of the leak in the filters are shown in Table 2.  
Figure 5: Fractography of the filter. Figure 6: Filter fracture surface detail.