JPS62160118A - Filter - Google Patents

Abstract

PURPOSE:To prepare a filter having a very low pressure loss and superb collecting performance compared with the existing HEPA filter by providing an electret non-woven cloth layer of dense structure on the dust collecting face and a non-woven cloth of rough structure on the downstream side. CONSTITUTION:An electret non-woven cloth 4 of dense structure is provided on the dust collecting face by the relationship of fiber filling rate Y and fiber medium diameter X which satisfies formulas I-IV. The non-woven cloth is preferably of melt-blow method having a 0.2-30mum fiber diameter distribution. Electret characteristics should be more than 1.4X10<-10> coulomb/cm<2> of ther mostimulating depolarized charge amount. Weight per unit area is around 10-180g/m<2>. Collecting performance is superbly enhanced by providing in paral lel the electret non-woven cloth layer and the non-woven cloth layer 5 of rough structure. The non-woven cloth of rough structure having a low pressure loss and high reinforcing support effect should be selected.

Description

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

In more detail, it is excellent in smoothing the increase in pressure loss when using a filter and maintaining dust collection performance.
Related to high performance electret filters.

(Prior Art) As an air filter using a conventional mechanical collection principle, for example, an air filter using ultrafine fibers is disclosed in Japanese Patent Application Laid-Open No. 147412/1983. Based on the conditions, the fiber diameter is 1.
When the filter performance was evaluated using an electret nonwoven fabric made under the conditions of 9μm pore volume 92.5% (fiber filling rate 7.5%), nonwoven fabric weight 100q/T112, and pressure loss 7mmAQ, the following problems occurred. It turned out that it could not be put to practical use.

That is, in the process of the dust adhering to the filter surface forming a dust layer, when it reaches a certain point, 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 fabric, and a fully satisfactory filter cannot be obtained.

[Problems to be Solved by the Invention] The present inventors have arrived at the present invention as a result of intensive studies on electret filters that do not have the above-mentioned drawbacks.

[Means for solving problems]

The present invention has the following configuration.

(1) A filter having a two-layer structure in which coarse and dense structure layers are arranged in parallel, and at least the dense structure layer is an electret nonwoven fabric.

(2) The dense structure layer satisfies the following formulas (1) to (4) in the relationship between the fiber filling rate (Y)% and the fiber median diameter (X>μm). filter.

Y=12.5X-15・−・−・−・<1>Y=1
2.5X+10・・・・・・・・・(2)3≦Yku25
・・・・・・・・・・・・・・・・・・・・・(3)0
．． 3≦X<2.5・・・・・・・・・・・・(4)(3
) The filter according to claim (1) or (2), wherein the dense structure layer is an electret melt-blown nonwoven fabric.

(4) The dense structure layer has a heat-stimulated deseparation charge amount of 1.4×10
-10 coulombs/Tr12 or more, the filter according to any one of claims (1) to (3).

(5) Claims (1) to (4) in which the nonwoven fabric of the dense structure layer is made into an electret in a sheet state.
) The filter described in any of the above.

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

The nonwoven fabric of the dense structure layer 4 is preferably an electret melt-blown nonwoven fabric that satisfies the following formulas (1) to (4) in terms of the fiber filling rate (Y)% and fiber median diameter (X) μm.

Y=12.5X-15 (1) Y=1
2.5X+10・・−・−・−<2)3≦Yku25・・
・・・・・・・・・・・・・・・・・・(3)0.3
≦X<2.5 (4) Fiber filling rate (Y) is a numerical value representing the proportion of only fibers in a unit volume. The calculation method is according to the following formula.

Y - [(ω/ρ) ÷ (Tx104)) x 100% Y: Fiber filling rate (%) ω: Weigh of dense structure layer (g/Tn2) ρ: Specific gravity of i1 fiber (Q/cr+f) T: Dense structure layer thickness (cm) (The thickness of a dense structure layer is determined using a dial gauge by applying a load of 0.2 C per square centimeter.) In addition, the fiber median diameter is defined as the thickness of a dense structure layer when the fiber diameter distribution is normally distributed. This value represents the average properties of melt-blown nonwoven fabrics, etc. that do not comply with the above criteria. How to find it is
More than 200 fibers are randomly selected from an SEM photograph of the surface of a nonwoven fabric, the fiber diameters are accurately measured, and the frequency distribution of fiber diameters is determined. Next, the fiber diameter in 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 coarse Itf layering is not particularly limited. For example, 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 gauze, net-like material, or porous material may also be used. .

Next, regarding the positional relationship between the dense structure layer and the coarse structure layer, it is particularly preferable to use the dense groove layer side as the dust collection layer when used as a high performance air filter. When used as a medium-performance (high-speed) filtration air filter, on the other hand, it is preferable to collect from the coarse structure layer side for a longer life.

The dense structure layer will be explained in more detail.

If an electret filter is designed under the design conditions shown in JP-A-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 study of 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 electret filters.

FIG. 2(a) is a schematic diagram showing the initial state of dust adhesion on the electret nonwoven fabric layer. Moreover, FIG. 2(b) shows a state in which the amount of dust adhesion increases as shown in FIG. 2(a). Also the third
Figure (a) is a schematic diagram showing the initial state of dust adhesion on the non-electret nonwoven fabric layer.Furthermore, Fig. 3(b) shows the state in which the amount of dust adhesion has increased as shown in Fig. 3(a).

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, the air flow starts to adhere directly to the top surface of the fibers, as shown in FIG. It accumulates almost concentrated on the upper surface.

On the other hand, in the case of an electret filter, as shown in FIG. 2(a), since the adsorption force due to electrostatic force is not working, the particles adhere not only to the upper surface of the fibers but also to the lower surface thereof. If the supply of dust continues, as shown in Figure 2 (b), the dust will adhere to the entire periphery of the fibers, and the dendrites of the dust will extend, even into the spaces where there is no supporting material between the fibers. A coarse dust layer forms. This is presumed to be due to the surface charge of the electret fibers reaching the outside through 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. be. 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 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 and the use of electret filters. The characteristics of the filter will be lost.

In the present invention, as a result of intensive studies to solve these problems, an appropriate amount of densely structured electret nonwoven fabric layer is placed on the dust collection surface, and a nonwoven fabric layer with a coarser structure is placed on the downstream side of the air flow. It was confirmed that the configuration in which the layers are arranged in parallel is extremely excellent in terms of dust retention, continuous pressure Jfi loss, initial pressure drop reduction, and maintenance of dust collection performance.

That is, as shown in Figure 4, the requirements for the dense structure layer are as follows (in the relationship between fiber filling rate (Y) and fiber median diameter (X>):
It is preferable that formulas 1) to (4) are satisfied.

Y=12.5X-15 (1) Y=1
2.5X+10・・・・・・・・・(2)3≦Yku25
・・・・・・・・・・・・・・・・・・・・・(3〉0
．． 3≦X<2.5・・・・・・・・・・・・(4) Fourth
The range of conditions indicated by the diagonal line A in the figure is a preferable condition that satisfies the initial pressure loss and collection performance and allows the pressure loss to be continuous. 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 dust being collected near the surface of the dense structure layer,
Since dust collects dust, the collection efficiency is further improved. Further, since the dust hardly reaches the downstream side, the life of the electret filter becomes 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, within the range C in FIG. 4, the collection performance is not satisfactory as a high-performance filter, and the pressure loss increases step by step, which is not preferable.

Therefore, a range of A in FIG. 4 is appropriate. or,
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=12.5X-2,5...(5) Y=12.
5X+5・・・・・・・・・・・・(6) 3≦Y<22
．． 5・・・・・・・・・・・・・・・(7) 0.4≦X
<2...(8) For the dense structure layer that satisfies the above requirements, an electret melt-blown nonwoven fabric made by one melt-blowing method is optimal.

The reason is that nonwoven fabrics produced by one-method meltblowing generally have a wide fiber diameter.For example, nonwoven fabrics with a wide range of fiber diameter distributions from 0.2 to 30tim can be manufactured, so the retention of five types of nonwoven fabrics is This is because it has excellent properties and is also suitable for collecting atmospheric dust consisting of a wide range of particle sizes.

In addition, for nonwoven fabric manufacturing methods other than the melt blow one-way method, spunpond nonwoven fabrics made of split fibers and nonwoven fabrics using one burst fiber method are also suitable.

Further, the electret characteristic of the dense structure layer has a charge direction in which the thermally stimulated depolarization charge amount is 1.4×10 −10 coulombs/− 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 Cl/m2.

The method of converting the nonwoven fabric used for the dense structure layer into electret is as follows:
As shown in FIG.
A semiconductor sheet or dielectric sheet 10 having a diameter of
An application electrode 1 connected to a DC high voltage generator 12
This can be carried out by a method of performing corona discharge treatment according to No. 3, a method of using a grid electrode in combination with this method, or a method of applying a DC high voltage with both sides of the melt-blown nonwoven fabric in contact between flat electrodes.

The electret nonwoven fabric made in this way has different polarities on the front and back sides, and the electric field is oriented in one direction (thickness direction).
are available. This type of nonwoven fabric created by the electric field @
It has higher collection performance than non-woven fabrics with random electric field structure.

Next, the coarse structure layer will be explained.

In the present invention, the rough structure layer is provided for the following two purposes.

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

(2) Use as a support and reinforcement material.

Since the dense structure layer has a high ali fiber filling rate, the pressure loss is high.

Therefore, it is conceivable to design the dense structure layer to reduce the weight loss 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, and the dense structure layer may be deformed by wind pressure.

Therefore, the pressure loss is low, and the balance between collection performance and pressure loss is adjusted by a coarse structure layer that has a reinforcing and supporting effect, and it is made to be 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 life can be extended.

To determine the amount of heat-stimulated depolarization charge, place the electret sheet in a TSC device manufactured by Toyo Seiki Co., Ltd., and calculate the heat-stimulated depolarization current when the temperature is raised at 5°C/min. The area surrounded by the depolarizing current (depolarizing charge amount) is divided by the area of the sample to calculate the thermally stimulated depolarizing charge amount (coulombs/mouse).
seek.

(Operation) The cause of the trouble in which the pressure loss increases discontinuously occurs because the collecting mechanisms of electret fibers and non-electret fibers are different.

In an electret filter, dust dendrites are formed extremely quickly even in the spaces without 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 began to accumulate near the surface of the dense structure layer, the effect of collecting dust became remarkable, and this also worked to improve the collection performance and make the line longer.

In addition, since the coarse structure layer is arranged in parallel with the dense structure layer, it is possible to reduce the initial pressure loss, and the reinforcement and support effect of the coarse structure layer creates a long-life filter that can withstand the upper limit of pressure loss. .

(Examples) The present invention will be roughly explained in detail by examples, but it is not limited to these examples only.

Example 1 Dense structure layer with fiber median diameter of 0.8 μm and fiber filling rate of 12%
, sheet weight 40Q/m2, initial pressure loss 2.5n+m
An electret polypropylene melt-blown nonwoven fabric having an AQ (air filtration rate of 2.5 cm/5 ec) and a heat-stimulated depolarization charge amount of 4.0×10 −10 coulombs/d 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/Tl12, an initial pressure loss of 2.0 rrIrnAQ (air-overspeed 2.5C)
m/5ec), heat-stimulated depolarization charge amount 2.0X10-1
A polypropylene melt-blown nonwoven fabric having a coefficient of 0 coulomb/i was used. The filter of the present invention was formed by laminating these two layers, and air was passed through from the dense structure layer side to examine the collection performance and pressure loss.

Initial pressure loss 4.7mmAQ (air filtration rate 2°5c)
m/sec>, the collection performance was 99.99% or more. (The collection performance was measured using a Hitachi Aerosol Monitor AN'105 model for particles larger than 0.3μ in atmospheric air.) As a result of a 2000-hour continuous collection test, the pressure drop continued to rise, and no step increase was observed. Ta.

In addition, the collection performance was 99.99% or more, which was 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 were thermally bonded to form a laminated filter having a sheet weight of 700/ln2, a fiber filling rate of 12%, and an initial pressure loss of 0.5 mm/^q.

A 2000 hour air collection test was conducted.

Initial pressure loss: 3.1 mmAq, initial collection performance: 99°9
It was 8% or more. (The collection performance was measured in the atmosphere [with particles of 0.3 μm or larger)] The pressure loss increased continuously, and no stepwise increase was observed.

Moreover, the collection performance was improved to 99.99% or more.

A further 4,000 hours of supplementary testing was conducted, but no problems occurred.

Comparison 1 Example 1 An electret polypropylene melt-blown nonwoven fabric having a median fiber diameter of 1.9 μm, @fiber filling rate of 7.5%, and a sheet basis weight of 20 Cl/m2 was placed on a grounded metal ground plate and subjected to DC corona discharge treatment. The electric field strength +7K
It was performed at V/Cm. The thermally stimulated depolarization charge amount of this sheet was 4×10 −10 coulombs/d. Seven of these sheets were stacked and used as an air filter for a continuous collection test for 4000 hours.

Initial pressure loss (9.5mmAQ, air-overspeed 2°5
m/SeC), the initial collection performance was 99.99% or more for atmospheric dust of 0.3 μm or more.

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

(Effects 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 electret property 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 schematic cross-sectional view of an example of a filter according to the present invention, and FIG. Figure (b) is a schematic diagram showing the state in which the amount of dust adhesion in Figure 2 (a) has increased; Figure 3 (a) is a schematic diagram showing the initial state of dust adhesion on the non-electret nonwoven fabric layer; Figure 3 (b) is a schematic diagram showing the state of Figure 3 (a) where the amount of dust adhesion has increased, and Figure 4 shows the relationship between fiber filling rate (Y) and fiber median diameter (X>). The graph shown in FIG. 5 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 fiber 3.8: Non-electret fiber 4: From electret fiber Dense structure layer 5: Rough structure layer 7 made of non-electret fibers: Dust 9: Grounded metal electrode plate 10: Semiconductor sheet or dielectric sheet 11: Melt-blown non-woven fabric 12: DC high voltage generator 13: Electrode patent applicant Azuma Shi Co., Ltd. Oda1ko Y 4 years old

Claims (1)

Claims: (1) A filter consisting of a two-layer structure in which coarse and dense structure layers are arranged in parallel, wherein at least the dense structure layer is an electret nonwoven fabric. (2) The dense structure layer satisfies the following formulas (1) to (4) in the relationship between the fiber filling rate (Y)% and the fiber median diameter (X) μm. filter. Y=12.5X-15......(1) Y=12.5X+10......(2) 3≦Y<25......(3) 0.3≦X<2.5... ...(4) The filter according to claim (1) or (2), wherein the (3) dense structure layer is an electret melt-blown nonwoven fabric. (4) The dense structure layer has a heat-stimulated depolarization charge amount of 1.4 × 10^-
The filter according to any one of claims (1) to (3), which is ^1^0 coulomb/m^2 or more. (5) The filter according to any one of claims (1) to (4), wherein the nonwoven fabric of the dense structure layer is made into an electret in a sheet state.