JP2638011B2 - Electret filter manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial application field) An electret filter is an air filter made of a fiber of a dielectric substance having a semi-permanent electric polarization, compared with an air filter that does not use a conventional electrostatic force. It has the feature that high collection efficiency can be achieved with low pressure loss for fine particles. This filter can be used in fields requiring high clean air such as electronics and biotechnology, or for removing house dust and tobacco smoke in ordinary households.

(Prior Art) Numerous methods have been proposed for the production of electret filters. For example,
No. 47299 discloses a method in which a film-like dielectric material is stretched, corona-charged and split into sheets, or a 10 ¹⁴ Ωc described in JP-B-59-124.
m or more, and blown with electron particles at the outlet of the orifice, and then cooled to form a sheet.Also, a porous sheet described in JP-B-59-15167 can be used to form an insulating fiber. How to charge the corona with
JP-A-61-102476 discloses a method of corona charging a fiber sheet using a liquid electrode as an earth electrode. However, any of the above-mentioned methods has a limit in the amount of charge retained in a dielectric substance as an electret material, and is not satisfactory in filter performance. From such a background, a method of trapping electric charges in a dielectric material at a higher density has been eagerly desired.

(Problems to be Solved by the Invention) As described above, the conventional method for manufacturing an electret filter uses a corona-charged charging element and drives a charge into a dielectric substance to trap the charge. No treatment has been applied to the dielectric material. For this reason, the upper limit of the amount of trapped charge is determined by the place where the charge existing in the dielectric substance is trapped, that is, the number of crystal interfaces and defects. Therefore, no matter how long the charging element is performed or how many days the charging element is repeatedly performed, it is impossible to further increase the charge trapping amount.

(Means for Solving the Problems) In order to solve the problems of such a conventional method for manufacturing an electret filter, as a result of intensive studies on various charge treatment methods, a dielectric material was heat-treated in its crystal dispersion region,
It has been found that, when the operation of performing charge treatment after cooling is repeated, the amount of charge trapping increases dramatically, and the performance of the filter is greatly improved. That is, the present invention provides a method for manufacturing an electret filter using an electretized dielectric material, wherein the method includes a step of performing a heat treatment, a cooling, and then a charging process on the dielectric material. It is.

Generally, when electrons or ions are charged into a dielectric substance and the dielectric substance is electretized, the charge is
It is understood that they are trapped in irregular portions of the crystal such as a crystal interface and a defect of the dielectric substance and are frozen as they are. Therefore, no matter what kind of charged element is used, the upper limit of the amount of charge trapping is determined by the number of irregular portions of the crystal such as crystal interface defects. This is the reason why the charge trapping amount does not increase even if the charging process is performed for a long time or any number of times using any charging element. According to the present invention, a method of repeating a heat treatment of a dielectric substance at the temperature of its crystal dispersion region, cooling, and then performing a charge treatment is carried out by extracting the internal ions present in the dielectric substance to the dielectric surface by the heat treatment. There is a feature in the froze. The specific heat treatment temperature is 60 to 140 ° C., preferably 70 to 120 ° C., and more preferably 80 to 100 ° C., and one heat treatment can be performed for 1 to 300 seconds, preferably 2 to 120 seconds. 3 to 60 seconds. The reason why the amount of charge traps increases in the present invention will be described with reference to FIGS. 1, 2, 3, and 4. FIG. FIG. 1 is a schematic view of a dielectric fiber before a charge treatment. In the dielectric fiber, a place (b) where certain electric charges are trapped and internal ions (c) are present. The internal ion (c) is specifically an impurity ion resulting from a catalyst residue or the like. FIG. 2 shows the dielectric fiber shown in FIG. 1 subjected to a charge treatment (in this case, a positive charge treatment), and a positive charge (d) is trapped in a charge trapping location in the dielectric fiber. Be frozen. At this time, a hitting stable positive charge (e) which is excessively discharged by the charging element remains on the fiber. The charge (e) gradually disappears due to neutralization with ions in the air and the like, and does not become a stable charge. The charge trap amount is determined by the charge (d), and the charge (d) is reduced. If all the trap locations (b) are filled, the charge trap amount does not increase. So far,
This is a conventional charge processing method.

The present invention is characterized in that the dielectric fiber shown in FIG. 2 is heat-treated at the temperature of the crystal dispersion region, and then cooled and then subjected to a charge treatment, and this heat treatment and charge treatment are repeated. FIG. 3 shows a state where the dielectric fiber shown in FIG. 2 is heat-treated at a crystal dispersion temperature. The charge (e) on the unstable fiber and part of the trapped charge (d) on the unstable fiber are lost by the heat treatment. At this time, the internal ions (c) with increased mobility are charged on the verge of disappearance ( The fibers are drawn from the inside of the fiber to the surface by the electric field effect of (e) and fixed by cooling. Thereafter, when a further charge treatment is performed, the charge (d) lost by the heat treatment is trapped again, and the state shown in FIG. 4 is obtained, and as a result, the internal ions (c) contained in the dielectric fiber are emitted to the fiber surface. The amount of charge traps increases by the amount. If the above heat treatment and charge treatment are further repeated, the amount of internal ions on the fiber surface increases, and the charge trapping amount can be increased.

The charging element used in the charging treatment according to the present invention may be of any type, including corona charging, electric field charging, electron beam irradiation, γ-ray irradiation, X-ray irradiation, and the like, and these are applied alone or in combination. .

The electrodes used in the present invention include corona electrodes such as needles, wires, knives, and bars, and electric field electrodes such as flat plates and rolls. In charging by irradiation, the derivative substance may be directly irradiated with high energy rays, but the surface may be covered with an energy adjusting sheet to improve the charge level.

The heat treatment method used in the present invention is not limited at all, for example, hot air, hot plate, hot roll,
Anything such as infrared rays may be used.

Although various combinations are conceivable as the production method in the present invention, corona charging and treatment with hot air are particularly preferred examples. The heat treatment and the charging treatment are preferably performed alternately and repeatedly.

In the present invention, a highly insulating material is desirable as the dielectric substance, and polyethylene, olefin polymers such as polypropylene and alpha-polyolefin, polyester, polystyrene, polyvinylidene fluoride, Teflon, polycarbonate, polysulfone, polyacrylonitrile, polyvinyl chloride, and polyvinyl chloride There are synthetic resins such as vinylidene chloride, copolymers and blend compositions of two or more thereof, and non-polar glass.

Examples of the form of the dielectric substance used in the present invention include a porous sheet, a woven fabric, a nonwoven fabric, and a mixture thereof.

(Operation) In the present invention, the reason why the number of charge traps of the dielectric increases is that the ions inside the dielectric are attracted to the surface and fixed. In other words, in the conventional charge treatment, the amount of charge trapping is determined by the number of defects and the crystal interface serving as a trapping location, so that a satisfactory amount of charge trapping cannot be obtained, and a filter with satisfactory performance can be created. Did not. However, this is possible with the method according to the invention.

(Example) Example 1 A sheet having a basis weight of 30 g / m ² and a thickness of 0.3 mm made of fine polypropylene fibers (average fiber diameter 3.5 μm) was subjected to 20 kv using a needle electrode.
For 10 seconds, treated with hot air at 80 ° C. for 10 seconds, cooled to room temperature, and further subjected to corona charging at 20 kv for 10 seconds. This heat treatment and charging treatment were repeated three times.

The sheet was used as a filter, and its dust removal efficiency was evaluated using salt particles (average particle size 0.3 μ, geometric standard deviation 1.2).
The measurement was performed using a filter dust removal efficiency measuring instrument shown in FIG. The electretized sheet 10 is installed in the duct 9 shown in FIG. 5, and controls the air flow so that the flow rate of the sheet 11 is 5.3 cm / s. The number of salt particles upstream and downstream of the sheet is counted by the laser particle counter 8 ( Made by RION
KA-14). The dust removal efficiency was 99.5%. The dust removal efficiency was calculated using the following equation.

Comparative Example 1 The same polypropylene sheet as in Example 1 was used for a 20
The corona was charged for 1 minute at kg. The dust removal efficiency of the sheet is 90.0
%Met.

Comparative Example 2 The same polypropylene sheet as that of Example 1
The operation of corona charging with kg for 10 seconds was repeated three times. The dust removing effect of the sheet was 90.3%.

Example 2 A polycarbonate porous sheet having a thickness of 50 μm and a porosity of 0.5 was subjected to a charge treatment by electron beam irradiation and a heat treatment using far-infrared rays. The treatment procedure was as follows: after charging at 5 Mrad for 1 second, the sheet surface temperature was kept at 120 ° C. for 5 seconds by far infrared rays, cooled to room temperature, and charged again at 5 Mrad for 1 second. When the dust removal efficiency of the sheet was measured by the same method as in Example 1, it was 95.0%.
Met.

Comparative Example 3 The same polycarbonate porous sheet as in Example 2 was treated with 5Mra
The electron beam was irradiated for 1 second at d. The dust removal efficiency of the sheet is 56%
Met.

In the electret filter, the higher the dust removal efficiency, the larger the charge trapping amount. From the above Examples and Comparative Examples, it can be seen that the electret filter manufacturing method according to the present invention is superior.

(Effect of the Invention) The method for manufacturing an electret filter that performs the heat treatment and the charging treatment according to the present invention can provide an electret filter having a high level of charge trapping amount that cannot be obtained by a conventional charging method.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a dielectric fiber before charging, FIG. 2 is after charging, FIG. 3 is after heat treatment, and FIG. 4 is a trapped state of the dielectric fiber when further charging is performed. . FIG. 5 is a schematic view of a filter dust removal efficiency measuring device.

Claims (3)

(57) [Claims] 1. A method of manufacturing an electret filter using an electretized dielectric material, comprising a step of performing a heat treatment, a cooling treatment, and a charge treatment on the dielectric material. Production method. 2. The method for manufacturing an electret filter according to claim 1, wherein the heat treatment temperature is in a crystal dispersion region of the dielectric substance. 3. The method for manufacturing an electret filter according to claim 1, wherein the charging treatment is corona discharge, electric field discharge, electron beam irradiation, γ-ray irradiation, or X-ray irradiation.