## 3 Results and Discussion
### Filtration Efficiency  
After being in storage for two days, the surface voltage of electret melt-blown fabrics was stabilized, and then the samples were tested for filtration efficiency. In Figure 5 it can be seen that either a rise in TiO\({}_{2}\) or electric field intensity has a positive influence on the filtration efficiency. The filtration efficiency and surface voltage demonstrate the same trend, which suggests that the presence of TiO\({}_{2}\) can heighten the filtration efficiency. Specifically, an electric field intensity of 2.5 kV/cm, rather than 1.5 or 2.0 kV/cm, helps to retain a relatively higher surface voltage, which contributes to higher filtration efficiency. In addition to mechanical filtration, electret also adsorbs particles with the help of electrostatic forces. Figure 6 shows that non-electret pure PP melt-blown fabrics do not perform well when filtering particles at sizes of 0.4-2.5 \(\upmu\)m. The mechanical filtration is realized by means of inertial impaction, interception, Brownian diffusion, and gravitational settling. As for particles at sizes exceeding 2.5 \(\upmu\)m, they can be effectively filtered simply by mechanical filtration. However, for particles that are smaller than 2.5 \(\upmu\)m, especially smaller than 100 nm, the employment of mechanical filtration is not satisfactory [30]. Using static electricity to adsorb particles smaller than 2.5 \(\upmu\)m has been proven effective. Particles are polarized as a result of the electric fields caused by the charged fibers in the proximity. The polarized particles are then adsorbed by the charged fibers, thereby acquiring the filtration efficiency of the static electricity adsorption. The higher the surface voltage, the better the particle adsorption performance. With an electric field intensity of 2.5 kV/cm, the melt-blown fibers exhibit good filtration efficiency of particles of 0.03-2.5 \(\upmu\)m regardless of the TiO\({}_{2}\) ratio (Figure 6). In particular, the melt-blown fibers composed of 3 wt% of TiO\({}_{2}\) have filtration efficiency higher than 92% when the particle size is between 0.03 and 2.5 \(\upmu\)m.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**TiO\({}_{2}\) Ratio (wt%)** & **Surface Resistivity (\(\Omega\)/SQ)** \\ \hline Pure PP & \(2.71\times 10^{11}\pm 4.96\times 10^{10}\) \\ \hline
1 & \(1.79\times 10^{11}\pm 7.12\times 10^{9}\) \\ \hline
2 & \(1.63\times 10^{11}\pm 5.62\times 10^{9}\) \\ \hline
3 & \(1.43\times 10^{11}\pm 1.50\times 10^{10}\) \\ \hline
4 & \(9.15\times 10^{10 Another important evaluation of filtration measurement is pressure drop performance. Figure 7 shows that the significant pressure drop is absent in all samples because they have to remain as a consistent structure. As a result, the influence of surface voltage on filtration efficiency can be measured accurately. Hence, the basis weight, thickness, fiber diameter, and air permeability of samples are identical in Table 1. Quality factor is commonly used to compare the filtration efficiency between different filters, and the equation is as follows:  
\[Q_{f}=\frac{-\ln(1-E)}{\Delta P} \tag{4}\]  
Figure 5: Filtration efficiency of electret melt-blown fabrics as related to the ratio of TiO\({}_{2}\) (1, 2, 3, and 4 wt%) and electric field intensities of (a) 1.5, (b) 2.0, and (c) 2.5 kV/cm. The filtration efficiency of non-electret pure polypropylene (PP) melt-blown fabrics is \(72.67\%\pm 1.42\%\). Figure 6: Filtration efficiency of particles at various sizes for electret melt-blown fabrics as related to ratios of 1, 2, 3, and 4 wt% and the electric field intensities of (**a**) 1.5, (**b**) 2.0, and (c) 2.5 kV/cm. where Q\({}_{\text{f}}\) is the quality factor, E is the filter efficacy, and \(\Delta\)P is the reduction voltage [31]. The quality factor is improved with a greater ratio of TiO\({}_{2}\) or higher electric field intensity (Figure 8). The pressure drop pertains to the structure of samples, and the parameters do not alter the structure. [PERSON] et al.