In the liquid contact charging method, the liquid with which the fiber aggregate is brought into contact or collision is not particularly limited as long as characteristics are obtained as desired. In view of handleability and performance, the liquid is preferably water. The electric conductivity and pH of water vary according to, for example, a content of calcium ions or magnesium ions. Instead of water, liquid obtained by adding a sub-component (component other than water) to water may be used, and the electric conductivity and pH of the liquid can be adjusted by, for example, a kind or an amount of the sub-component to be added.

The liquid to be contacted or impinged in the liquid contact charging method preferably has a pH of 1 to 11, more preferably 3 to 9, and further preferably 5 to 7. Furthermore, the liquid to be contacted or impinged in the liquid contact charging method preferably has an electric conductivity of 100 μS/cm or less, more preferably 10 μS/cm or less, and further preferably 3 μS/cm or less.

As the electret of the present invention, the layer can be used in combination with another constituent member if necessary. That is, the electret of the present invention can be used in combination with a pre-filter layer, a fiber protection layer, a reinforcing member, or a functional fiber layer and the like is also preferable.

Examples of the pre-filter layer and the fiber protection layer may include spun bond nonwoven fabrics, thermal bond nonwoven fabrics, foamed urethane, and the like, and examples of the reinforcing member may include thermal bond nonwoven fabrics, various kinds of nets, and the like. Further, examples of the functional fiber layer may include colored fiber layers for antibacterial purposes, antivirus purposes, and purposes of identification or design.

The electret of the present invention and a filter including the electret can be used widely owing to the functions obtained by the present invention such as dust collection, protection, ventilation, anti-fouling, water-proofness, and the like, and especially preferably usable as dust respirators, various types of air conditioning elements, air cleaners, cabin filters, and filters aimed at protection of various types of apparatuses.

This application claims priority to Japanese Patent Application No. 2019-151213, filed on Aug. 21, 2019. The entire contents of the specifications of Japanese Patent Application No. 2019-151213, filed on Aug. 21, 2019 are hereby incorporated by reference.

EXAMPLES

Hereinafter, embodiments of the present invention will be described. The test method is shown below.

(1) Concentration of Magnesium in Sample

0.5 g of a sample was weighed into a platinum crucible, and preliminarily carbonized to 400° C. on a hot plate. Thereafter, a type FO610 electric furnace manufactured by Yamato Scientific Co., Ltd. was used to perform incineration at 550° C. for eight hours. After the incineration, 3 mL of 6.0 N hydrochloric acid was added, acid-decomposition was performed on the hot plate at 100° C., and heating was performed until the hydrochloric acid was completely volatilized. After the acid-decomposition ended, 20 mL of 1.2 N hydrochloric acid was added, and the obtained product was used as a test liquid to be measured. An amount of magnesium in the test liquid was quantified by using a SPECTROBLUE-type ICP emission spectrometer manufactured by Hitachi High-Tech Science Corporation according to a calibration curve generated with standard solution of a target element, and the concentration of magnesium in the sample was obtained according to the following equation.

- Concentration of magnesium in sample: A (ppm (=mg/kg))
- Concentration of magnesium in the test liquid: B (mg/L)

Magnesium concentration measured in the same method as described above except that a sample was not used (sample was not weighed into a platinum crucible): C (mg/L)

A=(B−C)×(20÷1000)(L)/(0.5÷1000)(kg)

(2) Ventilation Resistance

Each sample punched into 72 mm φ was attached to an adapter having an effective ventilation diameter of 50 mm φ, piping having an inner diameter of 50 mm where a microdifferential pressure gauge was connected was coupled vertically, ventilation was carried out at 10 cm/s, and ventilation resistance (pressure loss) was measured without throttling.

(3) Particle Penetration

Each sample punched into 72 mm φ was attached to an adapter having an effective ventilation diameter of 50 mm φ, and particle penetration was carried out with a filter by the following method using a light scattering type particle counter KC-01E manufactured by Rion Co., Ltd.

- Evaluation particles: Airborne dust
- Air velocity: 10 cm/s
- Calculation of efficiency: The number of particles having particle diameters of 0.3 to 0.5 μm was measured by a light scattering calculation.
- Particle penetration (%)=(the number of particles having particle diameters of 0.3 to 0.5 μm through the electret/the number of particles having particle diameters of 0.3 to 0.5 μm before through the electret)
  
  (4) Filter Material Quality Factor (QF)

The initial QF value was calculated by the following equation using values of the ventilation resistance measured in the above (2) and the particle permeability measured in the above (3).

QF [mmAq⁻¹]=—[ln(particle penetration(%)/100)]/[ventilation resistance (mmAq)]

Example 1

A product obtained by adding 0.00025 parts by mass of magnesium stearate (melting point: 88° C.) and 1 part by mass of Chimassorb (registered trademark) 944 manufactured by BASF as a hindered-amine-based compound to 100 parts by mass of polypropylene homopolymer having a magnesium concentration of not larger than 0.1 ppm and a melt flow rate (MFR) of 1200 g/10 minutes, was spun by using a meltblowing device at a resin temperature of 260° C. and an air temperature of 260° C., to obtain a fiber sheet having a weight per unit area of 20 g/m². Water having an electric conductivity of 0.7 μS/cm and a pH of 6.8 was caused to collide with the obtained fiber sheet to perform charging, and the obtained product was then left as it was at 100° C. for 30 minutes and thus dried, thereby obtaining an electret filter. The ventilation resistance was 3.60 mmAq, a QF value was 1.13 mmAq⁻¹, and a magnesium concentration was 7.8 ppm.

Examples 2 to 8 and Comparative Examples 1 to 4

Electret filters were each produced in the same manner as in example 1 except that an amount of magnesium stearate to be added was as indicated in Table 1.

Comparative Example 5

An electret filter was produced in the same manner as in example 5 except that a hindered-amine-based compound was not added.

Comparative Example 6