JPH0326090B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electric filter that effectively collects dust in the air using an electrostatic collecting mechanism.

In recent years, as it has become clear that cigarette smoke has a great effect on the human body, the importance of indoor air purification has increased, and the popularity of stationary air purifiers has been remarkable. Accordingly, from the viewpoint of noise and energy saving, there is a demand for air filters for air cleaners that have high collection efficiency and low pressure loss. As such an air filter, an electret filter made of so-called electret fibers in which electric charges are embedded in the surface and inside of the fibers is most suitable.

Since this electret filter is electrically charged and an electrostatic field is formed between the fibers, dust particles passing through it are collected by the Coulomb force and induced force in the electrostatic field, regardless of the charging state of the particles themselves. It has a high collection efficiency because of the

Such electret filters include non-woven fabrics made of randomly laminated electrified polypropylene fiber webs, felt-like fabrics in which the fibers are intertwined using a method such as needle punching, and phenol-like fabrics in which electrified polypropylene fiber webs are randomly laminated. , those impregnated with synthetic resins such as styrene are often used.

However, the high trapping efficiency of such an electret filter has the disadvantage that it is not stable for a long period of time only during the initial period of use. The reason for this is that most of the dust that air purifiers collect is cigarette smoke, and its main component, tar and other oily substances, coats the surface of the fibers that make up the filter, weakening the electrostatic field. This is thought to be because these oily substances penetrate into the fibers and neutralize the buried electric charge.

In order to prevent a significant decrease in collection efficiency due to the adhesion of oily substances contained in cigarette smoke, it is necessary to increase the web weight of the filter or use thinner fibers to increase the fiber surface area and make the fiber structure denser. I tried that. Although the collection efficiency is certainly improved, the pressure loss also increases significantly, making it impossible to withstand long-term use as an electret filter, so such densification is not desirable. Moreover, an electret filter can be obtained by passing a nonwoven fabric through a high electric field formed by applying a high DC voltage between the discharge electrode and the counter electrode, but the fibers constituting the intermediate layer are smaller than those of both surface layers. It was found that charging properties were poor and sufficient electrification was not obtained. In other words, in such conventional nonwoven fabrics, the intermediate layer makes a smaller contribution to improving the collection efficiency than both surface layers.

As a result of various studies, the inventor of the present invention overcame the drawbacks of conventional electret filters and maintained high collection efficiency over a long period of time by increasing the fiber surface area without increasing pressure loss. succeeded in doing so.

FIG. 1 is an enlarged cross-sectional view of the nonwoven fabric in the electret filter of the present invention, and FIG. 2 is a cross-sectional view of the nonwoven fabric of FIG. 1 expanded in the thickness direction. The present invention will be described with reference to the drawings. In an electret filter made of a nonwoven fabric 1 in which the fibers of a polyolefin synthetic fiber web are self-bonded, the nonwoven fabric 1 is a flat vortex oriented in the plane direction. This is an electret filter characterized in that both surface layer webs 3 and 3' are interconnected by an intermediate layer web 2 which is substantially dogleg-shaped and expandable in the thickness direction.

Non-polar polyolefin synthetic fibers such as polypropylene and polyethylene are used for the nonwoven fabric 1 to be electrified according to the present invention. Non-polar polyolefin synthetic fibers are preferred in order to maintain the long-term stability of the charges buried on the surface and inside of the fibers, while synthetic fibers such as polyamide and polyester with polar groups are preferred because the charges buried therein are stable for a long time. This is not preferable because the electric charge tends to attenuate in a short period of time due to discharge or neutralization.

As a method for forming the polyolefin synthetic fiber web of the present invention, known melt spinning methods such as jet spinning, melt blowing, and spunbonding can be used. When the spun continuous or discontinuous fibers are aggregated into a web to form a nonwoven fabric, in the present invention, when they are accumulated on two rotating bodies having a narrow gap and passed through the gap, the surface of the two rotating bodies and Both surface layer webs 3, 3' oriented in the plane direction are formed in the contact area.

On the other hand, in the gap between the two rotating bodies, an intermediate layer web 2 is formed in the shape of a bridge so as to be linked to both surface layer webs 3, 3'.

The web thus accumulated is rolled up through a presser roll, etc., and crushed into a dogleg shape.
A nonwoven fabric as shown in the figure is obtained.

Adjacent fibers of the intermediate layer web 2 are partially self-bonded to each other, but both surface layers 3 and 3' are not completely integrated, so if expanded in the thickness direction, it will form a dogleg shape. The middle layer web 2 is stretched out to form a bridge shape as shown in FIG.
It has a bulky shape.

Next, a high electric field obtained by applying a positive or negative DC voltage of 5 to 10 KV/cm between two electrodes consisting of a linear or plate-shaped discharge electrode and a counter electrode is applied to the first electrode. When the nonwoven fabric shown in the figure is passed through, the nonwoven fabric becomes electret and the electret filter of the present invention is obtained. At this time, since both surface layer webs are mainly oriented in the plane direction and are self-adhesive, the surface layer of the nonwoven fabric has high smoothness and high adhesion to the counter electrode.

As a result, since charges are easily deposited on the surface layer, the collection efficiency of the electret filter of the present invention is increased and its lifespan is maintained for a long time.

Moreover, as the excess air passes through the surface layer web in its thickness direction, the amount of collected air in the surface layer web increases, and even if the pressure loss increases slightly, the intermediate layer web is formed into a dogleg shape. Since the material is gradually stretched in the thickness direction, the pressure loss does not increase any further. In this way, the electret filter of the present invention forms an intermediate layer web with a special structure, focusing on the fact that the intermediate layer web has a lower charging property than the surface layer web and therefore contributes less to the collection efficiency. This made it possible to prevent an increase in pressure loss.

Moreover, even though the pressure loss is reduced, the present invention can sufficiently increase the fiber surface area by increasing the fiber weight or using thinner fibers, and furthermore, by smoothing both surface layers, it is possible to form a counter electrode. Because it has a structure that increases adhesion and allows charge to easily adhere to it, there was no decrease in collection efficiency.

In this way, the electret filter of the present invention differs from conventional electret filters in that it has high collection efficiency despite its small pressure loss.
It is capable of stably collecting dust such as indoor cigarette smoke for a long period of time, and is extremely suitable for use in air purifiers intended for indoor air purification.

Example 1 A web with an average fiber diameter of 10μ made of polypropylene resin spun by the melt jet method was accumulated on a pair of smooth steel rolls having a gap of about 15 mm, and the web passing through the gap was placed on a conveyor. A nonwoven fabric having a weight of 250 g/m ² and a thickness of 3 mm was obtained by stacking.

This nonwoven fabric was electrified by passing it through a high electric field with a positive DC voltage of 7 KV/cm applied between two electrodes to obtain an electret filter.

Comparative Example 1 A web with an average fiber diameter of 20μ spun in the same manner as in Example 1 was simply accumulated on a conveyor to produce a web with a weight of 300μ.
A nonwoven fabric having a weight of ³ mm and a weight of 3 mm was obtained.

This nonwoven fabric was electrified in the same manner as in Example 1 to obtain an electret filter.

Comparative Example 2 A web with an average fiber diameter of 10μ spun in the same manner as in Example 1 was simply accumulated on a conveyor, and a web with a weight of 250 μm was produced.
A nonwoven fabric having a weight of ^1.7 mm and a weight of 1.7 mm was obtained.

This nonwoven fabric was electrified in the same manner as in Example 1 to obtain an electret filter.

The collection efficiency (by counting method) and pressure loss of the electret filters of Example 1 and Comparative Examples 1 and 2 were measured using smoke containing tobacco particles with a diameter of 0.3 μ at a wind speed of 25 cm/sec. This is shown in FIGS. 3 and 5.

First, in Figures 3 and 4, the collection efficiency of Comparative Example 1 (B) decreases rapidly, but that of Example 1
(A) was an excellent product that decreased only gradually and maintained high collection efficiency for a long period of time.

Next, in Figures 5 and 6, Comparative Example 2 (C) has almost the same collection efficiency as Example 1 (A), but the pressure loss in Example 1 (A) is much higher. It was low and excellent.

[Brief explanation of the drawing]

FIG. 1 is an enlarged sectional view of the nonwoven fabric in the electret filter of the present invention, and FIG. 2 is a sectional view of the nonwoven fabric of FIG. 1 expanded in the thickness direction. Figures 3 and 5 show the collection efficiency of Examples and Comparative Examples,
Figures 4 and 6 show the pressure loss. 1... Nonwoven fabric, 2... Intermediate layer web, 3, 3'
...Surface layer web.

Claims (1)

[Claims] 1. In an electret filter made of a nonwoven fabric formed by self-bonding between the fibers of a polyolefin synthetic fiber web, the nonwoven fabric has smooth double-surface layer webs oriented in the plane direction in a substantially dogleg shape and thick. Electret filters characterized in that they are interconnected by intermediate layer webs that are expandable in the transverse direction.