JPH0510962B2 - - Google Patents

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a novel filter.

More specifically, the present invention relates to a high-performance electret filter that smooths the increase in pressure loss during use of the filter and is excellent in maintaining dust collection performance.

[Conventional technology]

As an air filter using a conventional mechanical collection principle, for example, an air filter using ultrafine fibers is disclosed in Japanese Patent Laid-Open No. 147412/1983. Referring to the air filter conditions described in this publication, fiber diameter 1.9 μm, pore volume 92.5% (fiber filling rate 7.5%), nonwoven fabric weight 100 g/m ² , pressure loss 7
When the frilter performance was evaluated using an electret nonwoven fabric made under mmAq conditions, it was found that the following problem occurred and it could not be put to practical use.

That is, in the process of forming a dust layer, when the dust adhering to the filter surface reaches a certain amount, the dust layer collapses and a more dense dust layer is formed. At that time, the pressure loss will rise rapidly. It was also found that dust leakage occurred and the collection performance was temporarily reduced.

This phenomenon is particularly noticeable in filters using electret nonwoven fabrics, making it impossible to obtain a fully satisfactory filter.

[Problem that the invention seeks to solve]

The present inventors have arrived at the present invention as a result of extensive studies aimed at obtaining an electret filter free of the above-mentioned drawbacks.

[Means for solving problems]

The present invention has the following configuration.

The filter of the present invention that achieves the above objects is:
A filter consisting of a two-layer structure in which a coarse structure layer and a dense structure layer are arranged in parallel, wherein at least the dense structure layer is an electret nonwoven fabric, and the fiber filling rate Y% and the fiber median diameter X μm are as follows ( The present invention is a filter that satisfies formulas 1) to (4).

Y ≧ 12.5 is characterized in that the dense structure layer is an electret melt-blown nonwoven fabric.

Alternatively, the filter of the present invention is preferably characterized in that the dense structure layer has a thermally stimulated depolarization charge amount of 1.4×10 ⁻¹⁰ coulombs/m ² or more.

Alternatively, the filter of the present invention is preferably characterized in that the nonwoven fabric of the dense structure layer is electretized in the form of a sheet.

FIG. 1 is a partially cutaway schematic cross-sectional view showing an example of a filter according to the present invention.

1 is a filter, 2 is an electret fiber, 3
4 is a dense structure layer with a high fiber filling rate made of electret fibers, 5 is a coarse structure layer with a low fiber filling rate made of non-electret fibers, and arrows A and A' are the layers of air containing dust. Indicates the flow direction. The dense structure layer 4 is made of at least an electret nonwoven fabric.

The nonwoven fabric constituting the dense structure layer 4 is an electret nonwoven fabric, and the fiber filling rate is Y% and the fiber median diameter is X μm, where X and Y are as follows.
It is important that the following formulas (1) to (4) are satisfied.

Y ≧ 12.5 This is a numerical value that represents the proportion of fiber alone. The calculation method is according to the following formula.

Y = [(ω/ρ) ÷ (T x 10 ⁴ )] x 100% Y: Fiber filling rate (%) ω: Fabric weight of dense structure layer (g/m ² ) ρ: Specific gravity of fiber (g/cm ³ ) T: Thickness of the dense structure layer (cm) (The thickness of the dense structure layer is determined using a dial gauge by applying a load of 0.2 g per square centimeter.) Also, the fiber median diameter is the fiber diameter distribution. is a numerical value that represents the average characteristics of melt-blown nonwoven fabrics that do not follow a normal distribution. The method of finding is the surface of the nonwoven fabric.
More than 200 fibers were randomly selected from SEM photographs,
Accurately measure the fiber diameter and find the frequency distribution of the fiber diameter. Next, the fiber diameter at which the area surrounded by the frequency distribution curve and the horizontal axis (fiber diameter) is divided into exactly 1:1 ratio is determined using a planimeter. The determined fiber diameter represents the fiber median diameter.

On the other hand, the coarse structure layer is made to have a smaller fiber filling rate than the dense structure layer.

The material for the rough structure layer is not particularly limited. For example, it may be used in place of a non-woven fabric made of electret fibers or non-electret fibers, a non-woven fabric layer made of a mixture of both, metal or plastic wire mesh, net-like material, or porous material. .

Next, regarding the positional relationship between the dense structure layer and the coarse structure layer, it is particularly preferable that the dense structure layer side is used as the dust collection layer when used as a high performance air filter. On the other hand, when used as a high-speed excess air filter such as a medium-performance filter, it is preferable to collect from the rough structure layer side to prolong the life.

The dense structure layer will be explained in more detail.

If an electret filter is designed using the design conditions shown in Japanese Patent Application Laid-open No. 57-147142, there will be problems such as a discontinuous step increase in pressure loss during use and a decrease in collection performance due to dust leakage. be.

As a result of a detailed investigation into the cause of this problem, it was found that the mechanisms of dust accumulation are different between air filters that use only conventional mechanical collection principles and electric filters.

FIG. 2a is a schematic diagram showing the initial state of dust adhesion to the electret nonwoven fabric layer. Moreover, FIG. 2b shows a state in which the amount of dust adhesion increases as shown in FIG. 2a. FIG. 3a is a schematic diagram showing the initial state of dust adhesion to the non-electret nonwoven fabric layer. Moreover, FIG. 3b shows a state in which the amount of dust adhesion increases as shown in FIG. 3a.

Reference numeral 6 indicates electret fibers, 7 indicates attached dust, 8 indicates non-electret fibers, and arrows A and A' indicate the flow direction of air containing dust.

That is, in a filter that uses only the mechanical collection principle, as shown in Figure 3a, the airflow starts to adhere from the top of the fibers that are directly hit, and as shown in Figure 3b, it is almost concentrated on the top of the fibers. It continues to accumulate.

On the other hand, in the case of an electric filter, Fig. 2a
As shown in Figure 2, because of the adsorption force due to electrostatic force, it adheres not only to the upper surface of the fiber but also to the lower surface. If the supply of dust continues, as shown in Figure 2b,
The dust begins to adhere to the entire periphery of the fibers, and furthermore, the dendrites of the dust extend, and a rough dust layer is formed even in the spaces where there is no support material between the fibers. This is presumed to be because the surface charge of the electret fibers is transferred to the outside through the dust.

When the coarse dust layer that extends like a tree eventually reaches a certain amount of dust, it collapses due to wind pressure, forming a slightly denser dust layer that closes the gaps between the fibers, causing a sudden increase in pressure loss. Further, at this time, dust may temporarily leak, reducing the collection performance. Furthermore, as a result of dust adhering to the electret fibers in the inner layer, the surface charge may decrease and the life of the filter may be shortened.

In order to prevent these troubles from occurring, designing with extremely thin fibers such as HEPA filters and ULPA filters made of ultra-fine glass fibers can provide a preventive effect, but on the other hand, this can lead to an increase in pressure loss. The characteristics of the electric filter will be lost.

In the present invention, as a result of intensive studies to solve these problems, an appropriate amount of a densely structured electret nonwoven fabric layer is placed on the dust collection surface, and a coarser structure is placed on the downstream side of the air flow. By arranging non-woven fabric layers in parallel, dust retention and
It was confirmed that the system is extremely superior in terms of continuous increase in pressure loss, reduction in initial pressure loss, and maintenance of dust collection performance.

In other words, the requirements for the dense structure layer are as shown in Figure 4.
In the relationship between the maintenance filling rate Y and the fiber median diameter X,
It is necessary to satisfy the following formulas (1) to (4).

Y ≧ 12.5 The conditions in which the initial pressure loss and collection performance are satisfied and the pressure loss can be made continuous are preferable conditions. When a dense structure layer of nonwoven fabric made within this range of conditions is used as a dust collection surface, the collected dust forms a dust layer on the extreme surface layer of the dense structure layer.

A dendritic dust layer is also formed between the fibers, but because the fiber filling rate and fiber diameter are designed in a well-balanced manner, sudden collapse of the dust layer does not occur.

As a result, the increase in pressure drop becomes favorable and continuous. Furthermore, as a result of the dust being collected near the surface of the dense structure layer, dust collects dust, so that the collection efficiency is further improved. or,
Since almost no dust reaches the downstream side, the life of the electret filter is extremely long.

On the other hand, within the range B in FIG. 4, the initial pressure loss of the filter becomes too high and is not suitable as an air filter. On the other hand, C in Figure 4
Within this range, the collection performance is not satisfactory as a high-performance filter, and the pressure loss increases step by step, which is not preferable.

Therefore, the range A in FIG. 4 is appropriate. Further, a more preferable range is within the condition range D in FIG. 4 and is within a range that satisfies the following formulas (5) to (8).

Y ≧ 12.5 Electret melt-blown nonwoven fabric made using the melt-blown method is most suitable.

Nonwoven fabrics made using the melt-blown method generally have a wide fiber diameter distribution. This is because it is possible to manufacture nonwoven fabrics with a wide range of fiber diameter distributions, for example from 0.2 to 30 μm, which provides excellent retention of the nonwoven fabric form and is also suitable for collecting atmospheric dust having a wide range of particle diameters.

In addition, nonwoven fabric manufacturing methods other than the melt blow method include spunbond nonwoven fabrics made of split fibers,
Non-woven fabrics using the burst fiber method are also suitable.

Further, the electret characteristic of the dense structure layer has a thermally stimulated depolarization charge amount of 1.4×10 ⁻¹⁰ coulombs/cm ² or more.

This is the amount of charge necessary to maintain the collection performance as a high-performance filter over a long period of time.

Further, the basis weight of the dense structure layer can be freely selected so as to satisfy the collection performance and pressure loss required for the product, but is preferably about 10 to 180 g/m ² .

As shown in FIG ^. 5, the method of electretizing the nonwoven fabric ^used for the dense structure layer is as shown in FIG ^.
Semiconductor sheet or dielectric sheet in the 10 ¹⁸ Ωcm range 1
A method in which a fiber sheet, for example, a melt-blown nonwoven fabric 11 is placed on top of the fiber sheet 0, and a corona discharge treatment is performed using an application electrode 13 connected to a DC high voltage generator 12, or a method in which a grid electrode is used in combination with this method, or This can be carried out by applying a DC high voltage while both sides of the melt-blown nonwoven fabric are in contact between flat plate electrodes.

The electret nonwoven fabric made in this way has different polarities on the front and back sides, and the electric field is aligned in one direction (thickness direction). A nonwoven fabric with such a nonwoven electric field structure has a higher trapping performance than a nonwoven fabric with a random electric field structure.

Next, the rough structure layer will be explained.

In the present invention, the purpose of providing the rough structure layer is mainly for the following two reasons.

(1) To collect dust leaking from the dense structure layer.

(2) Use as a support and reinforcement material.

The dense structure layer has a high fiber filling rate and therefore has a high pressure loss. Therefore, it is conceivable to design the dense structure layer to reduce the area weight to the extent that the required collection performance can be obtained to lower the pressure loss.

In such a case, a small amount of dust may leak,
Also, the dense structure layer may be deformed by wind pressure.

For this purpose, the balance between collection performance and pressure loss is adjusted by the coarse structure layer, which has low pressure loss and has a reinforcing and supporting effect, and is able to withstand wind pressure.

Structurally, the dense structure layer and the coarse structure layer are arranged in parallel.

Therefore, the coarse structure layer has a lower filling factor than the dense structure layer.

Note that the filter of the present invention has a dense structure layer and a coarse structure layer arranged in parallel, but when designing for the purpose of ultra-high collection performance, the dense structure layer and the coarse structure layer are arranged in parallel. By providing an electret melt blown nonwoven fabric layer with a higher fiber filling rate than the structural layer, the collection performance can be further improved and the product life can be extended.

To determine the heat-stimulated depolarization charge, place the electret sheet in a TSC device manufactured by Toyo Seiki Co., Ltd.
Calculate the heat-stimulated depolarization current when the temperature is raised at 5°C/min, and divide the area surrounded by the temperature and heat-stimulated depolarization current (depolarization charge amount) by the area of the sample to determine the heat-stimulated depolarization. Find the amount of charge (coulombs/cm ² ).

[Effect]

The problem of discontinuous increase in pressure loss arises because the collection mechanisms of electret fibers and non-electret fibers are different.

In an electret filter, dust dendrites are formed at an extremely early stage even in spaces where there is no supporting material between the fibers. Further, since dust adheres to the entire periphery of the fibers, a rough dust layer is formed.

When this dust layer collapses, the pressure loss increases rapidly and the collection performance temporarily decreases.

Therefore, in the present invention, in order to form a stable and coarse dust layer, lower the initial pressure loss, and satisfy the collection performance, we have developed a dense structure layer with a well-balanced fiber filling rate and fiber median diameter. was used as the collection surface. As a result, the distance between fibers and dust particles was better balanced, helping to form a stable and coarse dust layer, and successfully solving the problem.

Furthermore, since the dust started to accumulate near the surface of the dense structure layer, the effect of dust trapping dust became remarkable, and this also worked to improve the trapping performance and extend the life.

Also, since the coarse structure layer is arranged in parallel to the dense structure layer,
It became possible to reduce the initial pressure loss. Also, due to the reinforcement and support effect of the coarse structure layer, it has become a long-life filter that can withstand increased pressure loss.

〔Example〕

The present invention will be further explained in detail with reference to Examples, but is not limited to these Examples.

Example 1 Dense structure layer with fiber median diameter of 0.8 μm and fiber filling rate of 12
%, sheet weight 40g/m ² , initial pressure loss 2.5mmAq
(air overspeed 2.5 cm/sec) and heat-stimulated depolarization charge amount 4.0×10 ⁻¹⁰ coulombs/cm ² electret polypropylene melt-blown nonwoven fabric was used.

In addition, the coarse structure layer has a fiber median diameter of 1.5 μm, a fiber filling rate of 10%, a sheet weight of 60 g/m ² , and an initial pressure loss.
A polypropylene melt-blown nonwoven fabric with a heat-induced depolarization charge of 2.0 mmAq (air overspeed 2.5 cm/sec) and a heat-stimulated depolarization charge of 2.0×10 ⁻¹⁰ coulombs/cm ² was used. The filter of the present invention was formed by laminating these two layers, and air was passed from the dense structure layer side to examine the collection performance and pressure loss.

Initial pressure loss 4.7mmAq (air overspeed 2.5cm/
sec), and the collection performance was 99.99% or higher. (Catching performance was measured using Hitachi Aerosol Monitor Model AN105 for atmospheric dust particles larger than 0.3μ).

As a result of a 2000 hour continuous collection test, the pressure drop increased continuously and no step increase was observed. or,
The collection performance was 99.99% or higher, which is completely unchanged from the initial stage. The desired effects of the present invention were observed.

Example 2 The same electret melt-blown nonwoven fabric as in Example 1 was used for the dense structure layer.

A polyester long fiber nonwoven fabric was used for the rough structure layer. Both of these are thermally bonded and the sheet weight is 70g/
m ² , a fiber filling rate of 12%, and an initial pressure loss of 0.5 mm/Aq were laminated to form a filter.

A 2000 hour air collection test was conducted.

Initial pressure loss: 3.1 mmAq, initial collection performance:
It was over 99.98%. (Catching performance is 0.3 μm of atmospheric dust.
(The pressure drop was continuously increased, and no stepwise increase was observed. In addition, the collection performance was improved to over 99.99%.

A further 4,000 hours of collection tests were conducted, and no problems occurred.

Comparative Example 1 An electret polypropylene melt-blown nonwoven fabric having a median fiber diameter of 1.9 μm, a fiber filling rate of 7.5%, and a sheet weight of 20 g/m ² was placed on a grounded metal ground plate, and subjected to DC corona discharge treatment at an electric field strength of +7 KV. /cm was used. The heat-stimulated depolarization charge amount of this sheet was 4×10 ⁻¹⁰ coulombs/cm ² .
Seven of these sheets were stacked and used as an air filter for a continuous collection test of 4,000 hours.

Initial pressure loss 9.5mmAq (air overspeed 2.5cm/
sec), the initial collection performance is for atmospheric dust of 0.3 μm or more.
It was over 99.99%.

As a result of the test, the pressure drop increased step by step, and the collection performance also temporarily decreased.

〔Effect of the invention〕

In the filter of the present invention, the fiber filling rate and fiber diameter are selected in a well-balanced manner.

Therefore, it has extremely low pressure loss and excellent collection performance compared to conventional HEPA filters.

The increase in pressure loss is also continuous and slow, so when used in a clean room, etc., the change in the airflow velocity in the room is small, making it easy to adjust the airflow velocity. Therefore, it is optimal for maintaining indoor cleanliness.

Furthermore, since a stable dust layer is formed and grows mainly on the surface layer of the filter, hardly any dust adheres to the electret fibers in the inner layer. Therefore, the electrification is maintained for a long period of time, and the life of the filter is extremely long.

[Brief explanation of drawings]

FIG. 1 is a partially cutaway cross-sectional schematic diagram showing an example of a filter according to the present invention, FIG. 2 a is a schematic diagram showing the initial state of dust adhesion on the electret nonwoven fabric layer, and FIG. Schematic diagram showing the state where the amount of dust adhesion has increased in Figure 2a, Figure 3a
is a schematic diagram showing the initial state of dust adhesion on the non-electret nonwoven fabric layer, FIG. 3b is a schematic diagram showing the state in which the amount of dust adhesion in FIG. FIG. 5, a graph showing the relationship with the fiber median diameter X, is a schematic explanatory diagram showing the method for converting the electret nonwoven fabric used in the filter of the present invention into electret. 1: filter, 2, 6: electret fibers, 3, 8: non-electret fibers, 4: dense structure layer consisting of electret fibers, 5: coarse structure layer consisting of non-electret fibers, 7: dust, 9:
Grounded metallic electrode plate, 10: Semiconductor sheet or dielectric sheet, 11: Melt-blown nonwoven fabric, 1
2: DC high voltage generator, 13: Electrode.

Claims (1)

[Scope of Claims] 1. A filter having a two-layer structure in which a coarse structure layer and a dense structure layer are arranged in parallel, wherein at least the dense structure layer is an electret nonwoven fabric and has a fiber filling rate of Y% in the fibers. The diameter Xμm is as shown in (1) to (4) below.
A filter characterized by comprising more than one that satisfies the formula. Y ≧ 12.5 Claim 1 characterized in that
Filter as described in section. 3 The dense structure layer has a heat-stimulated depolarization charge amount of 1.4×10 ^-10
3. The filter according to claim 1 or 2, wherein the filter has a coulomb/m ² or more. 4. The filter according to claim 1, 2, or 3, wherein the nonwoven fabric of the dense structure layer is electrified in a sheet state.