JPH06343809A - Filter material for air cleaner - Google Patents

Abstract

PURPOSE:To obtain a filter material used for an air cleaner of automobiles or the like by impregnating an outer layer composed of a low density fiber structure made mainly of a hydrophobic fiber, an intermediate layer composed of a high density fiber structure made mainly of a hydrophilic fiber and an inner layer composed of high density wet nonwoven fabric with a polyester based resin. CONSTITUTION:This filter material includes the outer layer composed of the low density fiber structure made mainly of the hydrophobic fiber, the intermediate layer composed of the high density fiber structure made mainly of the hydrophilic fiber and the inner layer composed of the high density wet nonwoven fabric and each layer is laminated and impregnated with the polyester based resin. And the layers are bonded with each other by bonding the structural fiber of each layer. Then, the filter material used for the air cleaner of automobiles or the like is obtained. As a result, not only the dust of JIS-8 kind but carbon particle are efficiently collected and for instance, in the case of using the air cleaner for automobile's engine, the engine has a longer life than the case of using a conventional air cleaner.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a filter medium for air cleaners, and more particularly to a filter medium used for air cleaners for automobiles and the like.

[0002]

2. Description of the Related Art An automobile air cleaner is used for introducing purified air into an engine room after sufficiently collecting dust in the outside air. The dust that must be sufficiently removed before it is installed in the engine compartment is
It is defined by IS D-1612 (Air cleaner test method for automobiles) and is mainly composed of sand dust (hereinafter referred to as JIS-8 type dust).

Conventionally, as an air cleaner filter medium for removing these dusts, an outer layer (coarse layer) made of a fiber web is used.
A filter material for an air cleaner having a density gradient prepared by impregnating a laminate of an intermediate layer (intermediate density layer) and an inner layer (dense layer) with an acrylic ester resin binder aqueous emulsion and drying it to impart rigidity Was known. In addition, by using a dense wet nonwoven fabric layer as the inner layer, JIS-
It has also been known that the efficiency of collecting eight kinds of dust is increased (Japanese Patent Laid-Open No. 62-279817). These conventional filter media are said to be capable of practically sufficiently collecting the JIS-8 class dust.

[0004]

The inventors of the present invention have conducted research for the purpose of further improving the performance of an air cleaner. As a result, it is necessary to remove not only the JIS-8 type dust described above but also carbon particles contained in the outside air. Found that there is. That is, conventionally, since carbon particles are also generated by combustion in the engine room, it has been considered that there is no problem even if carbon particles are included when introducing outside air into the engine room through an air cleaner. Advances will develop devices that can control the amount of fuel and air to be optimized for combustion, and will measure the amount of air using sensors in the passage from the air cleaner to the engine room. However, the problem has arisen that carbon particles in the air that have passed through the air cleaner affect the sensor and cause it to malfunction, resulting in incorrect measurement of the air amount.

Conventionally, a filter material for an air cleaner intended to collect carbon particles has not been known, but the present inventor has
By impregnating a specific resin binder into a coarse-dense laminate composed of a combination of specific fiber structures, the conventional JI
We have succeeded in developing a high-performance filter material that is capable of sufficiently collecting carbon particles while maintaining the ability to collect S-8 type dust. The present invention is based on these findings.

[0006]

SUMMARY OF THE INVENTION Therefore, the present invention provides
(A) An outer layer mainly composed of hydrophobic fibers and having a low-density (apparent density) fiber structure; and (b) an intermediate layer mainly composed of hydrophilic fibers and composed of a high-density (apparent density) fiber structure. , (C) an inner layer made of a high-density (apparent density) wet non-woven fabric, wherein each of the layers (a), (b) and (c) is impregnated with a polyester resin.
The present invention relates to a filter material for an air cleaner.

The fibrous structure composed mainly of hydrophobic fibers constituting the outer layer (a) on the outside air intake side is, for example, 50 to 100 parts by weight of hydrophobic synthetic fibers such as polypropylene fibers, polyamide fibers, preferably polyester fibers. %, Preferably 70 to 100% by weight, or a first layer obtained by subjecting the fiber web to an entanglement treatment by means such as needle punching or hydroentanglement. It was prepared by impregnating the second layer (layer for intermediate layer) and the third layer (layer for inner layer) with an aqueous emulsion of a polyester resin described later. If the content of the hydrophobic fibers is less than half, a large amount of the polyester resin adheres to the first layer (outer layer) and the thickness of the layer collapses when impregnated with the polyester resin aqueous emulsion. In this case, a dense layer having high ventilation resistance is formed as an outer layer, so that the filter material has a small amount of dust retained and a short life. Further, since the amount of fibers is relatively reduced with respect to the amount of resin, the ability to collect carbon particles is reduced when compared with a constant weight of filter medium.

The fibers constituting the outer layer preferably have an average fineness of 0.5-50 denier, more preferably 1-denier.
10 denier with an average fiber length of preferably 10-100
mm, more preferably 20-80 mm fibers can be used. The fibers included in the outer layer as the fibers other than the hydrophobic fibers are not particularly limited, and known synthetic fibers, semi-synthetic fibers, natural fibers, inorganic fibers and / or adhesive fibers can be used.

The basis weight of the fiber web of the first layer (outer layer) is preferably 90 to 240 g / m ² . The basis weight of the fiber structure constituting the outer layer is 100 to 250.
It is preferably g / m ² . Basis weight is 100 g / m ²
If it is less than this, the amount of carbon particles and dust retained is insufficient, and if it exceeds 250 g / m ² , the thickness becomes too thick and it becomes difficult to perform processing such as folding. When the outer layer has a two-layer structure or a structure of two or more layers, it is desirable that the outer layer as a whole including each layer satisfies the above-mentioned basis weight range.

The apparent density of the first layer before impregnated with the polyester resin is preferably 0.02 to 0.09 g / cm ³ ,
More preferably, it is 0.03 to 0.08 g / cm ³ .
Further, the apparent density of the outer layer after the polyester-based resin impregnation is preferably 0.03~0.1g / cm ^3, more preferably 0.04~0.09g / cm ^3. When the apparent density of the outer layer exceeds 0.1 g / cm ³ , the ventilation resistance increases and the amount of dust that can be collected on the filter medium decreases, and when it is less than 0.03 g / cm ³ , it is collected in this layer. Since a relatively large amount of dust that should be discharged is collected and collected in the intermediate layer, clogging in the intermediate layer becomes faster.

Further, since carbon particles and relatively large dust are mainly captured in this outer layer, it is desirable that the fiber / resin ratio is high and the fiber is rich as compared with the other layers.
Further, when the outer layer is not only composed of one layer having a uniform apparent density, but is composed of two or more layers having a coarse to dense density gradient from the outside to the inside in the apparent density, carbon particles and dust are also formed. It is preferable because the collection efficiency of the is improved and the retention amount is also increased. For example, in the case of a two-layer structure, the outer density of the first layer before impregnation with the polyester resin is set to 0.02 to 0.06 g / cm ³ and the inner density is set to 0.06 to 0.09 g. / Cm ³ , and the outer layer of the outer layer after impregnation with the polyester resin has an apparent density of 0.03 to 0.07 g / cm ³ and an inner layer of 0.07 to 0.1 g / cm ^3. it can.

The fibrous structure composed mainly of hydrophilic fibers, which constitutes the intermediate layer (b) provided between the outer layer (a) and the inner layer (c) described later, is, for example, vinylon, cotton, or preferably rayon fiber. 50 to 100% by weight, preferably 70 to 1
A fiber web contained in an amount of 00% by weight, or a second layer obtained by entanglement of the fiber web by means such as needle punching or hydroentanglement, and further the first layer (outer layer layer) and It is prepared by impregnating with a third layer (layer for inner layer) described below and an aqueous emulsion of a polyester resin described below. If the content of the hydrophilic fibers is less than half, the polyester resin is less likely to adhere to the second layer when impregnated with the aqueous emulsion of the polyester resin, and the thickness is less likely to be compressed. In such a case, a dense layer cannot be formed in the intermediate layer, and fine dust penetrates, so that the dust filtering efficiency is reduced.

The fibers constituting the intermediate layer have an average fineness of preferably 0.5 to 50 denier, more preferably 1
-10 denier, preferably 10-100 average fineness length
mm, more preferably 20-80 mm fibers can be used. The fibers other than the hydrophilic fibers are not particularly limited, and known synthetic fibers, semi-synthetic fibers, natural fibers, inorganic fibers and / or adhesive fibers can be used. The basis weight of the second layer (intermediate layer) of the fiber web is preferably 2
It is 0 to 100 g / m ² . The basis weight of the intermediate layer is preferably 40 to 120 g / m ² . Unit weight is 40g / m
^If it is less than ² , the filtration efficiency of dust is reduced to 120 g / m.
^If it exceeds ² , ventilation resistance will increase.

The apparent density of the second layer before impregnated with the polyester resin is preferably 0.06 to 0.76 g / cm ³ ,
It is more preferably 0.06 to 0.46 g / cm ³ .
The apparent density of the intermediate layer after impregnated with the polyester resin is preferably 0.1 to 0.8 g / cm ³ , more preferably 0.1 to 0.5 g / cm ³ . If the apparent density of the intermediate layer exceeds 0.8 g / cm ³ , the ventilation resistance increases, and if it is less than 0.1 g / cm ³ , the dust filtration efficiency decreases. Since the second layer is mainly composed of hydrophilic fibers, when each layer is laminated and then impregnated with an aqueous emulsion of a polyester resin, more resin is likely to adhere than other layers and the thickness of the layer is likely to be crushed. Therefore, although the apparent density of the second layer is a medium density as described above, the apparent density of the intermediate layer after impregnating the resin is high.

The inner layer (c) is prepared by impregnating a third layer composed of a wet non-woven fabric with an aqueous emulsion of a polyester resin described later. The apparent density of the third layer (inner layer) before impregnation with the polyester resin is preferably 0.07 to 0.77 g / cm ³ , more preferably 0.07 to 0.47 g / cm ³ . The apparent density of the inner layer after impregnation with the polyester resin is preferably 0.1 to 0.8 g / cm ³ , more preferably 0.1 to 0.8 g / cm ³ .
It is 0.5 g / cm ³ . Inner layer apparent density is 0.8g
If it exceeds / cm ³ , the ventilation resistance will increase to 0.1 g /
If it is less than cm ³ , the dust filtering efficiency is reduced.

The apparent density of the inner layer is preferably higher than that of the intermediate layer, but the apparent density of the inner layer may be the same as or lower than that of the intermediate layer. This is because the intermediate layer is composed of a fibrous structure mainly composed of hydrophilic fibers and has a structure in which a large amount of impregnated polyester-based resin is likely to adhere, so that the increase in weight due to the resin has a large effect. The large apparent density of the intermediate layer does not necessarily indicate that the ability to collect dust is high, and the wet non-woven fabric forming the inner layer has a large void size between fibers due to its manufacturing characteristics. Is more uniform than that of the intermediate layer, so that the dust does not pass through the large voids, and if the apparent density is the same, the dust collecting capability is excellent.

The fibers constituting the wet nonwoven fabric of the third layer are preferably crimp-free short fibers (2 to 20 m).
m, more preferably 3 to 10 mm), and the average fineness is preferably 0.1 to 2 denier, more preferably 0.2 to
Fibers of 1.5 denier can be used. The fibers can be bonded by using an adhesive fiber such as polyvinyl alcohol-based fiber or an adhesive resin such as polyvinyl alcohol resin or epoxy resin, but it is preferable to use the adhesive fiber in order to secure uniform voids between the fibers. . The basis weight of the third layer (inner layer) is preferably 20 to 90 g / m.
^{Is 2} . The weight of the inner layer is preferably 30-100.
It is g / m ² . If the basis weight is less than 30 g / m ² , the dust filtration efficiency will decrease, and if it exceeds 100 g / m ² , the ventilation resistance will increase.

The filter medium of the present invention has a structure in which the outer layer (a), the intermediate layer (b) and the inner layer (c) are laminated in three layers and further impregnated with a polyester resin. . Therefore, the filter medium of the present invention comprises, for example, a hydrophobic fiber as a main component, a first layer composed of a low-density (apparent density) fiber structure, a hydrophilic fiber as a main component, and a medium (density of the first layer). A second layer composed of a fiber structure having a density (apparent density) higher than that of the third layer and a third layer composed of a high-density (apparent density) wet non-woven fabric, the first layer, Second
It can be prepared by laminating a layer and a third layer in this order, impregnating the resulting laminate with an aqueous emulsion of a polyester resin, and further drying.

Since the polyester resin is impregnated in the laminated body composed of the first layer, the second layer and the third layer, the respective layers may be simply laminated, but Bond each layer with a suitable adhesive, or
When the constituent fibers of each layer are entangled and the layers are bonded to each other by applying mechanical entanglement means such as needle punching after the lamination, a filter medium free from problems such as delamination can be obtained, which is preferable. However, since the constituent fibers of the wet non-woven fabric constituting the third layer (layer for the inner layer) are short and are bonded in advance by the adhesive fiber or the like, there is a risk of breaking if the needle punching process is performed too strongly. After laminating the fiber structure forming the (outer layer layer) and the second layer (intermediate layer layer) and subjecting to needle punching, a wet non-woven fabric is laminated on the second layer (intermediate layer) side. It is desirable to further perform needle punching treatment from the first layer (outer layer layer) side to integrate the layers.

In the filter medium of the present invention, a polyester resin is impregnated in order to impart rigidity to the three-layer laminate. The polyester resin is impregnated as an aqueous emulsion and dried in the same manner as the conventional acrylic ester resin. The polyester resin is impregnated in an amount of solid content attached to the weight of the filter medium, preferably 5 to 25% by weight, more preferably 10 to 20% by weight. If the amount is less than 5% by weight, the rigidity cannot be obtained, and if the amount exceeds 25% by weight, the amount of fibers is relatively reduced, so that the carbon particle collecting ability is deteriorated.

[0021]

In the filter medium of the present invention, the carbon particles are mainly collected in the outer layer, and the JIS-8 type dust is collected in the outer layer, the intermediate layer and the inner layer according to the particle size. The present inventor has not clarified the reason, but in the fibrous structure layer constituting the filter medium, when the ratio of fibers to resin increases and the amount of fibers increases, the ability to collect carbon particles improves accordingly. I found that. It was also found that when a fiber structure mainly composed of hydrophobic fibers (particularly polyester fibers) is impregnated with a polyester resin, sufficient rigidity can be obtained with a relatively small amount of resin. For example, while the conventional acrylic acid ester-based resin required about 30% by weight (weight ratio to the filter medium), when the polyester-based resin was used,
It can be reduced to about 10 to 25% by weight (weight ratio to filter medium). Therefore, when the polyester-based resin is used, the fiber / resin ratio can be increased, and the fiber amount can be relatively increased, so that a filter medium having a high carbon particle trapping ability can be prepared.

However, if the fiber / resin ratio becomes high, J
On the contrary, the ability to collect IS-8 type dust is lowered. JI
The decrease in the ability to collect S-8 type dust is a fatal defect for a filter material for an air cleaner. This is because, in general, the invasion of carbon particles does not immediately adversely affect the engine, whereas the intrusion of sand or the like immediately leads to a fatal engine failure.

On the other hand, in order to obtain sufficient rigidity with the acrylic ester type resin used in the conventional filter media, a relatively large amount of resin is required, so that the fiber / resin ratio becomes low and carbon particles are trapped. There was a limit to the improvement of ability. However, if acrylic ester resin is used,
Since the JIS-8 type dust with a large particle size is easily collected because the resin film is formed in the fiber gap, in the fiber structure using the polyester resin, the resin collects at the fiber intersection and the resin film is formed. Since no voids are formed, dust easily escapes through the fiber gap.

Surprisingly, however, the present inventor has adopted JIS-8 by adopting a wet non-woven fabric as the high density inner layer even when the polyester resin is used.
It has been found that fine dust of seed dust can be collected like the conventional filter media. As described above, the present invention was completed for the first time by overcoming many new technical problems in order to solve the new problem of collecting carbon particles and combining them appropriately. INDUSTRIAL APPLICABILITY The filter medium of the present invention can be suitably used for an air cleaner, particularly for an air cleaner of various automobiles.

[0025]

The present invention will be described in detail below with reference to examples, but these do not limit the scope of the present invention. In the following examples and comparative examples, the performance of the filter media was measured by the following method. (1) Rigidity: The stiffness was measured by measuring the waist strength, and the larger the value, the higher the rigidity. The waist strength is measured by the following method. Prepare a test piece of 53 mm x 200 mm,
This test piece is placed on a horizontal table so that the short side becomes the height, and the upper end is pulled by the chuck of the tensile tester so that the width is 10
Insert so that it becomes mm. Then, chuck 500mm
At a speed of / min, the specimen is lowered until it is compressed from 53 mm to 46 mm. The maximum buckling load during this descent is measured and this is the waist strength. Two types of test pieces are prepared, one in which the compression direction is the vertical direction of the filter medium and the other in which the compression direction is the horizontal direction, and the vertical and horizontal waist strengths of the filter medium are measured.

(2) Ventilation resistance and "filtration efficiency" and "dust holding capacity (DHC: dust holdi) of JIS-8 type dust.
ng capacity) ”: These measurements are based on JIS D-16
12 (automobile air cleaner test method). However, a flat filter medium having an effective filtration area of 1000 cm ³ was used as the air cleaner element for the test. The test conditions for "filtration efficiency" and "dust retention capacity" of JIS-8 type dust were a dust concentration of 1 g / m ³ and a wind speed of 30 cm / sec.

(3) "Filtration efficiency" and "treatment amount (DFC: dust feeding capacity)" of carbon particles: The "treatment amount" was measured except that the test dust was changed to carbon particles composed of light oil burning carbon. , The above JIS-
The amount of carbon particles collected in the filter medium is measured in the same manner as the measurement of the "dust holding capacity" of the 8th type dust, and the amount of carbon particles collected by the absolute filter after passing through the filter medium is measured. And add. The test conditions for this test and the test for measuring the "filtration efficiency" were a carbon particle concentration of 0.04 g / m ³ and a wind speed of 30 cm / sec.

Example 1 (1) A first layer (outer layer) having a two-layer structure of a rough outer layer and a dense inner layer was prepared. That is, a web with a basis weight of 70 g / m ² consisting of 100% polyester fiber (6 d × 51 mm) was used for the outer layer, and 30% polyester fiber (2 d × 51 mm) and polyester fiber (3 d
A web having a basis weight of 120 g / m ² composed of 70% x 64 mm) was used. (2) As the second layer (layer for intermediate layer), a web having a basis weight of 50 g / m ² composed of 80% rayon fibers (1.5 d × 51 mm) and 20% polyethylene terephthalate fibers (1.5 d × 38 mm). Using. (3) As the third layer (layer for inner layer), 50% of polyester fiber (0.5 d × 5 mm) and polyester fiber (1.
A wet non-woven fabric having a basis weight of 45 g / m ² of 5 d × 10 mm) 50% was used. (4) Of the above layers, first, the fiber web forming the first layer and the fiber web forming the second layer are laminated,
The constituent fibers were entangled and bonded by needle punching. Then, after laminating the wet non-woven fabric constituting the third layer on the second layer side, needle punching treatment is performed from the first layer side, and 3
A laminate having a basis weight of 285 g / m ² was obtained by combining the layers. This laminate was impregnated with a polyester resin binder aqueous emulsion and dried with a hot air dryer at 130 ° C. to contain the resin in an amount of 40 g / m ² (weight after drying; weight ratio to filter medium 12.3 wt%), Is 3.6 mm,
A filter material of the present invention having a basis weight of 325 g / m ² was obtained. The apparent density of each layer of the obtained filter medium was 0.07 g / c for the outer layer.
m ³ , middle layer is 0.23 g / cm ³ , inner layer is 0.13 g
/ Cm ³ .

Comparative Example 1 (1) A first layer (outer layer) having a two-layer structure of a rough outer layer and a dense inner layer was prepared. That is, the outer layer has 15% rayon fiber (3d × 51 mm), 20% polyester fiber (3d × 64 mm), and polyester fiber (4d).
X 51mm) 15% and polyester fiber (6d x 51m)
m) 50% web with a basis weight of 45 g / m ² , 40% rayon fiber (1.5 d × 51 mm), 20% polyester fiber (1.5 d × 38 mm) and 20% polyester fiber (3 d × 64 mm) in the inner layer. ) 20% and polyester fiber (6d × 51 mm) 20%, areal weight 65 g /
A m ² web was used. (2) The second layer (layer for intermediate layer) has a basis weight of 65 g / 100% of rayon fiber (1.5 d × 51 mm).
A m ² web was used. (3) After the above two layers were laminated in order, the layers were combined by needle punching to form a laminate with a basis weight of 175 g / m ² , impregnated with an acrylic ester resin binder aqueous emulsion, and dried with a hot air dryer at 130 ° C. As a result, the resin was contained in an amount of 75 g / m ² (weight after drying; weight ratio to filter medium 30% by weight), and the thickness was 2.9 mm.
Thus, a comparative filter medium 1 having a basis weight of 250 g / m ² was obtained. The apparent density of each layer of the obtained filter medium was 0.0 for the outer layer.
It was 5 g / cm ³ and the intermediate layer was 0.38 g / cm ³ .

Comparative Example 2 (1) A first layer (outer layer) having a two-layer structure of a rough outer layer and a dense inner layer was prepared. That is, a web having a basis weight of 60 g / m ^{2 made} of 100% polyester fiber (3d × 64 mm) is used for the outer layer, and a basis weight of 100% made of 100% polyester fiber (1.5 d × 38 mm) for the inner layer.
A web of g / m ² was used. (2) As the second layer (layer for the intermediate layer), 100 g of areal weight consisting of 70% rayon fiber (1.5 d × 51 mm) and 30% polyester fiber (1.5 d × 38 mm)
A web of / m ² was used. (3) After the above two layers are laminated in order, the layers are combined by needle punching to form a laminate having a basis weight of 270 g / m ² , impregnated with a polyester resin binder aqueous emulsion, and dried with a hot air dryer at 130 ° C. it allows the resin 30 g / m ^2; in an amount of (dried weight to the filter medium weight 10% by weight) having a thickness of 3.2 mm, weight per unit area to obtain a comparative material 2 of 300 g / m ^2. The apparent density of each layer of the obtained filter medium was 0.06 g / c for the outer layer.
m ³ and the intermediate layer was 0.30 g / cm ³ .

Comparative Example 3 (1) As the first layer (outer layer), the same web as in Comparative Example 1 was used. (2) As the inner layer (third layer), a basis weight of 60 g / consisting of 50% polyester fiber (0.5 d × 5 mm) and 50% polyester fiber (1.5 d × 10 mm)
A wet nonwoven fabric of m ² was used. (3) After the above two layers were laminated in order, the layers were combined by needle punching to form a laminate having a basis weight of 170 g / m ² , impregnated with an acrylic acid ester resin binder aqueous emulsion, and dried with a hot air dryer at 130 ° C. As a result, the resin was contained in an amount of 70 g / m ² (weight after drying; weight ratio to filter medium was 29% by weight), and the thickness was 3.2 mm.
Thus, a comparative filter medium 3 having a basis weight of 240 g / m ² was obtained. The apparent density of each layer of the obtained filter medium was 0.0 for the outer layer.
It was 6 g / cm ³ and the inner layer was 0.16 g / cm ³ . The results of examining the performance of each of the filter media prepared in the above Examples and Comparative Examples are shown in Tables 1 and 2 below.

[0032]

[Table 1] Filter material Waist strength (vertical) Waist strength (horizontal) Airflow resistance Inventive filter material 7.2 5.5 8.0 Comparative filter material 1 4.0 3.6 6.0 Comparative filter material 2 3.1 2.6 7.0 Comparative filter material 3 3.0 2.5 8.0

[0033]

[Table 2] Filter medium JIS-8 type carbon filtration efficiency DHC filtration efficiency DFC Filter medium of the present invention 99.2 9.9 92.0 0.22 Comparative filter medium 1 99.4 9.2 87.4 0.18 Comparative filter medium 2 98.5 6.8 85.4 0.27 Comparative filter medium 3 99.3 6.5 90.5 0.11

In Tables 1 and 2, the unit of "waist strength" is kgf, the unit of ventilation resistance is mmAq, and the unit of filtration efficiency of JIS-8 type dust and carbon particles is%, respectively. The unit of DHC and DFC is g /
It is 100 cm ² .

[0035]

INDUSTRIAL APPLICABILITY The filter material of the present invention can efficiently collect not only JIS-8 type dust but also carbon particles. Can last longer than a cleaner.

Claims (3)

[Claims] 1. An outer layer mainly composed of hydrophobic fibers and composed of a low-density fiber structure, an intermediate layer mainly composed of hydrophilic fibers and composed of a high-density fiber structure, and an inner layer composed of a high-density wet non-woven fabric. And a filter material for an air cleaner, wherein each of the above layers is impregnated with a polyester resin. 2. The filter medium for an air cleaner according to claim 1, wherein the layers are bonded by intertwining the constituent fibers of each layer. 3. A high-density wet non-woven fabric comprising a first layer composed mainly of hydrophobic fibers and composed of a low-density fiber web, a second layer composed mainly of hydrophilic fibers and composed of a medium-density fiber web. A method for producing a filter medium for an air cleaner, comprising laminating a third layer consisting of the above in order, and impregnating the obtained laminate with an aqueous emulsion of a polyester resin.