JPH04358510A - Manufacture of electret filter - Google Patents

Abstract

PURPOSE:To provide the method of turning a fiber laminate into an electret efficiently in a short time under the normal temperature. CONSTITUTION:A non-woven cloth of polyolefin rigid fiber web of 1.5 intrinsic viscosity is made to adhere to an earth electrode with smooth surface, and an electret filter is manufactured of said non-woven cloth having an electret property by means of high pressure electrical field under the normal temperature.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing an electret filter that effectively collects dust in dust-containing air using an electrostatic collecting mechanism.

0002

2. Description of the Related Art Conventionally, many methods for manufacturing electret filters have been taught in literature. (1) A method in which a fiber laminate or an electrode is coated on at least one side with a dielectric film, heated and cooled in a high electric field, and a more or less semi-permanent charge is introduced into the fiber (US
P3591679, Special Publication No. 49-4433) (2)+
A film heated to near the melting point in a high electric field having charge-generating poles of and - is stretched and cooled to inject a semi-permanent charge into the film, and then the film is made into fibrils with a needle roller and then made into fibers. Method of forming laminates (U.S.
P3998916) (3) A method of forming an electret fiber laminate by applying electrically charged particles to the polypropylene fibers extruded from an orifice into high-speed hot air by the melt-blowing method before solidifying the fiber stream (Japanese Patent Application Laid-Open No. Showa 54-113
(No. 900) (4) At least one pair of corona charges adjacent to both surfaces of a composite sheet obtained by laminating conductive fibrous sheets made of cellulose fibers on the front and back sides of a filter material made of thermoplastic fibers such as polypropylene fibers. A method (U
SP4375718) etc.

0003

However, all of the above-mentioned methods for manufacturing electret filters have the problem that the fibers must be heated to near their melting point to be charged with an electric charge.

Furthermore, the method (1) of coating an electrode or fiber laminate has the problem of requiring a long time to achieve a desired charge state, and (2) the method of coating an electret in the form of a film The method of forming a fiber laminate after oxidation is superior in that it forms fibers with a sufficiently semi-permanent electric charge, but the production process is complicated, time-consuming, and expensive, and the formed fibers are The problem is that the diameter is limited.

Furthermore, in the case of (3), in the method of converting the fiber flow extruded from the nozzle orifice into electret, the charge is injected into the fiber in a molten state, so that the charge is partially neutralized by the time the fiber is completely solidified. However, there is a problem in that it is difficult to fix sufficient charge.

[0006] Case (4) is excellent in that the fibrous sheet can be turned into an electret in a short time at room temperature, but the introduction of charge depends on the close contact between the filter material and the conductive sheet, and moreover, the charge Since it remains on the fiber surface of the filter material and is not necessarily introduced into the fiber, the filtration efficiency is unstable. Furthermore, during the production of filter materials and conductive sheets, there is a drawback that the transfer of charged particles due to corona discharge is influenced by the state of generation of static charges caused by friction with rolls and the like.

[0007] Therefore, as a result of extensive research into a method for solving the problems of the prior art, that is, a method for effectively converting fiber laminates into electrets at room temperature and in a short time, the present invention was arrived at.

[0008]

[Means for Solving the Problems] The present invention has an intrinsic viscosity of 1
．． A nonwoven fabric made of a polyolefin synthetic fiber web of 5 or less is brought into close contact with a ground electrode having a smooth surface,
This is a method for producing an electret filter that is made into an electret at room temperature using a high-voltage electric field.

[0009] The polyolefin synthetic fiber in the present invention is a nonpolar fiber such as polyethylene or polypropylene, which is formed by a melt spinning method such as a jet spinning method, a melt blowing method, or a spunbond method, alone or in a mixture or in combination. In other words, non-polar polyolefin synthetic fibers are preferred in order to maintain long-term stability of the charges introduced into the surface and inside of the fibers, and polyester, polyamide,
In the case of polar synthetic fibers such as polyvinyl chloride, the charge is not stable for a long time, and the charge decays significantly due to discharge or neutralization.

Furthermore, in the present invention, the intrinsic viscosity η of the polyolefin fiber is 1.5 or less, preferably 1.0 or less. In other words, it is a polyolefin fiber that has been thermally degraded to have an η of 1.5 or less, preferably 1.0 or less in a melt-spinning extruder and nozzle orifice.

Note that η is measured using a tetralin solution at 135°C. Any product having η of 1.5 or less can be made depending on the manufacturing method or the shape of the extruder nozzle orifice, and the heating temperature conditions and the like are not particularly limited. If η exceeds 1.5, an effective electret filter cannot be obtained unless the fiber is heated to near its melting point and charged with electric charge for a long period of time, as in the conventional method.

In the present invention, η is 1.5 or less, preferably 1
．． By using a material of 0 or less, it can be heated to 10
Charges are effectively buried inside and on the surface of the fibers by a short load of less than a second, and the buried charges remain stable for a long time, with almost no decrease in collection efficiency even after storage for half a year. An effective electret filter was obtained.

Although the reason is not clear, by thermally degrading, thermally decomposing, and oxidatively decomposing polypropylene resin,
As the molecular ends of the resin increase, charges are more likely to be buried.
And it is thought that it will be in a state where it is easy to be retained. The polypropylene fiber of the present invention may be in an unstretched state with randomly oriented molecules or oriented by stretching, and is not particularly limited, and the fiber diameter can be arbitrarily determined depending on the purpose and use. can.

[0014] In the present invention, η is 1.5 or less, preferably 1.0.
A nonwoven fabric obtained by laminating the following webs made of polyolefin fibers such as polypropylene and polyethylene is connected to a discharge electrode made of stainless steel wire or tungsten wire with a diameter of 0.2 to 0.5 mm and a steel drum or plate with a smooth surface. 5 to 10 KV between the two poles consisting of the earth electrode
Electretization is performed in a high electric field in which a positive or negative DC voltage of /cm is applied to generate a corona discharge.

In the present invention, in particular, if the web is made of polyolefin synthetic fibers with an intrinsic viscosity of 1.0 or less, electric charges can be injected into the fibers at room temperature, so a stable and long-lasting electret effect can be obtained. More preferred. In the present invention, it is preferable to make the surface of the earth electrode made of metal such as a steel drum into a mirror-like circle or arc shape, since this improves adhesion and improves charge introduction. Furthermore, it is even better if pretreatment with ions of either the same polarity or different polarity as the high-pressure corona discharge element is performed before performing the corona discharge treatment at high voltage. Such pretreatment methods include lower voltage corona discharge treatment or ion wind irradiation treatment. By performing pretreatment in this manner, it is possible to eliminate charge unevenness that occurs during the production of nonwoven fabrics. As a result, sudden arcing below the breakdown voltage can be prevented and stable production can be achieved. Moreover, since the electret formation is performed uniformly, there is an advantage that the collection efficiency can be made uniform.

In the present invention, unlike conventional methods, this electret formation can be carried out at room temperature, and the time required for the nonwoven fabric to pass through the high electric field between two electrodes is about 2 to 10 seconds, which is 1/1/2 that of conventional methods. It is extremely short, ranging from 20 to 1/50.

Note that the electrode or nonwoven fabric may be covered with a dielectric film, but depending on the thickness of the dielectric film, it is necessary to set the DC voltage to 10 KV/cm or more.

[0017]

【Example】

Example 1 Using polypropylene fibers with η of 0.8, a web with a fiber diameter of 12 μm was formed by melt blowing and laminated to a weight of 60 μm.
g/m2, 0.7mm thick non-woven fabric (untreated filter)
I made it. 15 between two poles consisting of a discharge electrode with a diameter of 0.2 mm and a ground electrode made of a steel drum with a smooth surface.
mm, the nonwoven fabric was passed through a high electric field with a DC voltage of 10 KV/cm applied thereto for 5 seconds at room temperature while being in close contact with a ground electrode to form an electret.

Example 2 A nonwoven fabric was prepared in the same manner as in Example 1 using polypropylene fibers with η of 1.2, and was made into an electret at room temperature in the same manner as in Example 1.

Example 3 Polypropylene fiber with η of 0.5 and fiber diameter of 0.5 to 3μ
A web in which the majority of Similarly, it was made into an electret at room temperature. There was no arcing and stable production was achieved. Example 1
Similarly, when comparing the collection efficiency and its retention rate, the initial collection efficiency was 98%, but after 6 months it was still 96.7% and had a retention rate of 98.7%. Note that the untreated sample had a collection efficiency of 15%.

Comparative Example 1 Comparative Example 1 is the untreated nonwoven fabric of Example 1 that is not converted into electret.

Comparative Example 2 A nonwoven fabric was made in the same manner as in Example 1 using polypropylene fibers with η of 1.7, and was made into an electret at room temperature.

Comparative Example 3 The nonwoven fabric of Comparative Example 1 was made into an electret at a high temperature of 130°C. Table 1 shows the results of comparing the collection efficiency and retention rate of the Examples and Comparative Examples at the initial stage and after storage for 6 months. Note that the measurement of the collection efficiency was performed using cigarette smoke with a diameter of 0.3 μm.

[0023]

[Table 1]

[0024]

Effects of the Invention As described above, the present invention does not perform electretization at high temperatures as in the prior art, so charges are not neutralized and has extremely high collection efficiency.
The present invention is extremely useful because it allows the electret filter to be manufactured inexpensively and efficiently without decreasing its collection efficiency over a long period of time.

Claims (1)

[Claims] 1. A method for producing an electret filter, in which a nonwoven fabric made of a web of polyolefin synthetic fibers having an intrinsic viscosity of 1.5 or less is brought into close contact with a ground electrode having a smooth surface, and then converted into an electret at room temperature using a high-voltage electric field.