JPH11104417A - Filter media - Google Patents

Abstract

(57) [Summary] [PROBLEMS] The invention of the present application is capable of obtaining a high collecting ability equal to or higher than a so-called hepagrade, and does not decrease the collecting ability for a long period of time. It is an object to provide a high-performance filter medium that can be easily processed. The present invention relates to an ultrafine organic fiber having an average fiber diameter of less than 1 μm produced by a melt blow method,
The present invention relates to a filter medium in which a fibrous web mixed with heat fusible fibers having an average fiber diameter of 5 to 100 μm is bonded by the heat fusible fibers.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a filter medium, which can be used for a high-performance filter such as a hepa (HEPA) filter, and can be easily disposed of by incineration because it is composed of an organic substance. Filter media.

[0002]

2. Description of the Related Art Conventionally, glass fiber filters having a diameter of 0.1 to 1 μm have been used for high-performance filters such as hepafilters. However, the fibers are easily broken and easily fall off. there were. In addition, since glass fiber filters cannot be incinerated, there is a problem in terms of waste disposal. In addition, glass fiber filters often contain boron, which, for example, in the semiconductor industry, has had to take special measures because of the adverse effects of boron.

On the other hand, as a high-performance filter not using glass fibers, a filter using a microporous membrane of polytetrafluoroethylene is known, but this filter is not widely used because it is expensive.

Further, a filter made of polypropylene fibers having an average fiber diameter of several μm obtained by a melt blow method or the like has been studied as a high-performance filter using only organic materials in consideration of disposal processing, but the collection capacity is insufficient. However, there has been a problem that the pressure loss is too high and there is no self-holding property. As a filter medium for solving such a problem, JP-A-2-104765 discloses that an organic fiber having an average fiber diameter of several μm obtained by a melt blow method and an opened short fiber or long fiber are mixed, Electret nonwoven fabric filters collected in a DC electric field have been proposed. This filter can adsorb fine particles by the electrostatic attraction force of electretized fibers, so it can collect fine particles, and further, by mixing defibrated short fibers or long fibers. There is an effect that the pressure loss can be reduced. However, this electret nonwoven fabric filter is neutralized by the adsorbed ionic particles when used for a long period of time, or, depending on the use environment, loses the electric charge of the fiber due to heat, moisture, solvent, etc. There has been a problem that the collecting ability is reduced as time passes.

[0005]

The inventors of the present application have conducted various studies to solve these problems of the prior art, and as a result, it has been possible to obtain a high collecting ability higher than that of a so-called hepagrade, Does not decrease, and
By using organic materials as materials, we succeeded in solving these problems by developing high-performance filters that can be easily disposed of.

[0006]

Means for Solving the Problems The present invention relates to a fiber web obtained by mixing ultrafine organic fibers having an average fiber diameter of less than 1 μm produced by a melt blow method and heat-fusible fibers having an average fiber diameter of 5 to 100 μm. , A filter medium joined by heat fusible fibers.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The filter medium of the present invention contains 50 to 90% by weight of ultrafine organic fibers having an average fiber diameter of less than 1 μm and 50 to 10% by weight of heat-fusible fibers having an average fiber diameter of 5 to 100 μm. Is preferred.

[0008] The filter medium of the present invention has a thickness of 0.1 mm.
It is preferably from 1 to 1.5 mm.

Further, the filter medium of the present invention is characterized in that a fiber web in which ultrafine organic fibers and heat fusible fibers are mixed is subjected to pressure treatment at a temperature lower than the melting point of the low melting point component contained in the heat fusible fibers. It is preferred that

[0010] The filter medium of the present invention is heat-treated at a temperature not lower than the melting point of the low melting point component contained in the heat-fusible fiber and lower than the melting point of the ultrafine organic fiber, without substantially applying pressure. It is preferred that

Further, in the filter medium of the present invention, it is preferable that the ultrafine organic fibers obtained by the melt blow method have no shot or few shots.

The filter medium of the present invention has a wind speed of 5.3.
Under the condition of cm / sec, the collection efficiency of particles having a diameter of 0.3 μm is preferably 99.97% or more.

As the raw material resin of the ultrafine organic fiber obtained by the melt blow method, a polyolefin resin such as a polypropylene resin or a polyethylene resin, a polyester resin, a polyamide resin, a polycarbonate resin, a polyurethane resin or the like is used. A polypropylene resin from which fibers are easily obtained is particularly preferred. The average fiber diameter of the ultrafine organic fibers is less than 1 μm, preferably 0.05 to 1 μm, particularly preferably 0.1 to 0.6 μm. When the average fiber diameter of the ultrafine organic fibers is larger than 1 μm, it may be difficult to obtain a filter medium having high collection efficiency.

The ultrafine organic fibers used in the present invention are produced by a melt-blowing method. However, in the conventional melt-blowing method, if an attempt is made to produce fine fibers having an average fiber diameter of less than 1 μm, fiber breakage occurs during spinning and shot ( However, there is a problem that a large number of resin lumps are generated. In the invention of this application, the amount of resin discharged from the nozzle is significantly reduced compared to the normal case, and the flow rate of the heated airflow blown from the vicinity of the nozzle is increased to a range that is not normally performed because the number of shots increases. What is necessary is to produce an ultrafine organic fiber having an average fiber diameter of less than 1 μm with almost no shot. For example, using a polypropylene resin having a melt index of 500 to 1000, the amount of resin discharged from one nozzle hole is set to 0.2 c.
m ³ / min or less, preferably 0.001~0.15cm ³ /
Minute and the flow rate of the heated air flow per 1 m width is 0.5 Nm ³ /
Min., Preferably 1 to 10 Nm ³ / min, and the weight ratio (A / P) of the amount of the heated airflow per unit width to the amount of the resin is 10 Nm ³ / min.
When the average particle diameter is less than 1 μm, it is possible to produce fibers having an average fiber diameter of less than 1 μm substantially without generating shots.

By using ultra-fine organic fibers having no shot or few shots obtained by the melt blow method, large voids are formed in the filter medium due to cracks generated around the shot when the filter medium is folded or the like. A reduction in collection efficiency can be prevented. The term “shot” in the present invention refers to a lump of resin having a diameter of about 10 μm or more. In addition, the flow rate of the heated airflow at which shots do not occur and the amount of resin discharged from the nozzles are determined by the structure of the apparatus for manufacturing fibers by the melt blow method, the type of resin, and the manufacturing conditions such as the spinning temperature of the resin and the temperature of the heated airflow. It fluctuates by

The heat-fusible fiber may be any fiber that can heat-bond ultra-fine organic fibers. Examples of the heat-fusible fiber include a low-melting component all-melting fiber and a high-melting component and a low-melting component heat-fusible conjugate fiber. it can. The heat-fusible conjugate fiber is more preferable because the skeleton of the high-melting-point component remains even after bonding and can maintain the voids of the filter medium.

The heat-fusible conjugate fiber has a core-in-sheath type composite fiber in which the high melting point component is a core and a low melting point component is a sheath, an eccentric core-in-sheath type composite fiber, and a laminated structure in which a high melting point component and a low melting point component are bonded. Particularly preferred are side-by-side conjugate fibers, and sea-island conjugate fibers in which islands of a high melting point component are distributed in the sea of a low melting point component.

The low melting point component of the heat fusible fiber is lower than the melting point of the ultrafine organic fiber, and preferably lower by at least 20 ° C. When the temperature is lowered by 20 ° C. or more, the ultrafine organic fibers do not melt or form into a film when they are bonded by the low melting point component of the heat-fusible fiber. be able to.

The average fiber diameter of the heat-fusible fiber is 5 to 100 μm.
m, and particularly preferably 10 to 50 μm. If the average fiber diameter of the heat-fusible fiber is smaller than 5 μm,
It is difficult to keep the pressure loss low and the strength of the filter medium may be insufficient. On the other hand, if the thickness is more than 100 μm, it becomes difficult to uniformly mix with the ultrafine organic fiber, and the portion where the pressure loss is locally high May occur, or sufficient trapping capacity may not be obtained.

The heat-fusible fiber may be a short fiber or a long fiber, but a short fiber is preferable in consideration of the ease of mixing with the ultrafine organic fiber. It is more preferable to use a staple fiber or the like that has been subjected to a stretching treatment in a fiber production process, since sufficient strength can be obtained to maintain the pores of the filter medium.

The compounding ratio (weight ratio) of the ultrafine organic fibers and the heat-fusible fibers is preferably 90:10 to 50:50, and more preferably 90:10 to 60:40. When the amount of the fine organic fibers is less than 50,
In some cases, the collection efficiency is low. On the other hand, when the amount of the heat-fusible fiber is less than 10, the pressure loss increases,
The surface resistance and strength of the obtained filter medium may be insufficient.

As a method for forming a fibrous web by mixing ultrafine organic fibers and heat-fusible fibers, for example, a fiber web is spun into a spun fiber stream in a heated gas stream formed by a melt blow method. It is preferable to manufacture by supplying the heat-fusible fiber, mixing the both, and collecting on a collector to form a fibrous web.

It is preferable that the fiber web in which the ultrafine organic fibers and the heat-fusible fibers are mixed is subjected to pressure treatment at a temperature equal to or lower than the melting point of the low melting point component contained in the heat-fusible fibers. Specifically, for example, when the low melting point component is a polyethylene resin, it is preferable to compress the thickness by pressing with a press or a roll having a surface temperature of 80 to 120 ° C. By doing so, the fiber web can be made dense without forming any of the ultrafine organic fibers and the heat-fusible fibers into a film, so that the collection efficiency can be improved without increasing the pressure loss of the obtained filter medium.

The web produced by the pressure treatment is substantially pressed at a temperature higher than the melting point of the low melting point component contained in the heat-fusible fibers and lower than the melting point of the resin constituting the ultrafine organic fibers. It is preferred to heat-treat without heating. Heat treatment without substantial pressurization means that heat treatment is not performed in a pressurized state such as a heating calender roll or a hot press machine. This refers to a heat treatment in a non-pressure state by a method of passing through a regulated dryer or a method of passing a gas at the above temperature through a fiber web. Specifically, for example, when the low melting point component is made of a polyethylene resin and the ultrafine organic fibers are made of a polypropylene resin, it is preferable that the treatment is performed with hot air at 140 to 150 ° C. using a hot air dryer or the like.
In this manner, unlike in the case of bonding with a heating roll or the like, the bonding does not occur in the vicinity of the surface layer of the fiber web, and the low melting point component does not form a film due to the contact pressure of the roll, etc. Over time, the low melting point component of the heat-fusible fiber adheres at the point of contact with other fibers, so that a homogeneous and strong bond is obtained.

Under such conditions, the filter medium produced by the pressure treatment and the hot air treatment is homogeneously bonded to the inside of the fiber, so that it is thin, has excellent abrasion resistance on the surface, has a high strength, and is folded. And it is dense and has excellent performance of collecting fine particles. In addition, since the fibers are not formed into a film when the fibers are bonded and the gaps are not closed, there is an effect that the pressure loss does not increase so much.

[0026] The filter medium of the present invention comprises a fiber web obtained by mixing ultrafine organic fibers having an average fiber diameter of less than 1 µm and heat-fusible fibers having an average fiber diameter of 5 to 100 µm produced by a melt blow method. 0.3 μm in diameter under a wind speed of 5.3 cm / sec.
Can be adjusted so that the collection efficiency of the particles is 99.97% or more. Further, according to the invention of the present application, it is possible to realize, for the first time, a collection efficiency and a pressure loss of a grade of a hepafilter without treatment such as electretization, which is made only of an organic material.

[0027] When a higher collecting capacity is required, the filter medium or the fibers constituting the filter medium may be subjected to electretization.

The thickness of the filter medium of the present invention is 0.1
It is preferably in the range of 1.5 to 1.5 mm, particularly 0.3 to 1.0 mm. This range is particularly preferable because a unit having a large surface area can be formed by folding or the like.

An example of the method for producing the filter medium of the present invention will be described below. As shown in the manufacturing process diagram of the filter medium of FIG.
The filter medium 1 of the present invention comprises a die 5 for a melt blow device.
And the heat-fusible fiber 3 spread by the spreader 6 is mixed with the fine organic fiber, and the mixed fiber is collected on a collector 7 such as a conveyor belt. After the fiber web 4 is formed, the fiber web 4 can be manufactured by bonding the constituent fibers with a heat-fusible fiber through a heat treatment device 8 such as a dryer.

The ultrafine organic fibers 2 are formed by a melt blow method using a die 5 for a melt blow device. FIG.
As shown in FIG.
1 and a hot air stream outlet 52 for blowing a heated air stream from the vicinity of the nozzle are provided, and the molten resin extruded from the nozzle is thinned by the heated air stream to form ultrafine organic fibers. Usually, a plurality of nozzles 51 are arranged in a straight line at a predetermined interval, and hot air outlets 52 are provided on both sides in a continuous slit shape. Further, in the invention of this application, it is possible to supply ultrafine organic fibers having an average fiber diameter of less than 1 μm and having almost no shots by greatly restricting the discharge amount of the molten resin and increasing the flow rate of the heated airflow.

The heat-fusible fibers 3 are supplied by using the fiber opening device 6 to the above-mentioned fiber stream of ultrafine organic fibers and mixed. As the spreader 6, a card machine, a garnet machine, or the like can be used, but as shown in FIG. 3, a spreader in which a plurality of spread cylinders 61 are housed in a housing 62 is preferable. In this fiber opening machine, fibers are opened by causing fibers to collide with the inner wall of a housing by centrifugal force of a cylinder. Therefore, the fiber can be opened without crimping the fiber as in a card machine. In addition, the fiber opening machine is less susceptible to restrictions such as the length and thickness of the fiber as compared with the card machine.

When the opened heat-fusible fiber is supplied to the fiber stream of the ultrafine organic fibers, it is preferable to supply the fiber from the direction perpendicular to the fiber stream of the ultrafine organic fibers as much as possible. It is preferable because it is easy to perform. When the fiber flow of the ultrafine organic fibers is formed in the horizontal direction by the melt blow method, the heat-fusible fiber may be supplied by being dropped from the upper part. However, as shown in FIG. Is formed in the vertical direction, it is preferable to provide an air nozzle 63 or the like and supply the heat-fusible fiber in the horizontal direction (perpendicular to the fiber flow), as in the fiber spreader 6 in FIG.

If necessary, the angle of supply of the heat-fusible fibers may be adjusted to change the distribution of the heat-fusible fibers in the thickness direction so that a dense structure can be formed in the thickness direction. .

The mixed ultrafine organic fibers 2 and heat-fusible fibers 3 are collected on a collector 7 such as a conveyor belt to form a fiber web 4. As a collector, a roll, a net, or the like can be used. The collecting body is preferably air permeable so that the fiber web is not disturbed or scattered by the collision of the gas flow, and it is further preferable that the gas is sucked to the opposite side of the collecting surface of the collecting body. .

Next, the fibrous web 4 is heat-treated by the heat treatment device 8 and the constituent fibers are bonded by the heat-fusible fibers 3 to produce the filter medium 1. It is preferable to use a drier, a hot air drier, a drier with suction, and the like as the heat treatment device 8, and it is preferable to perform the heat treatment in a state where pressure is not substantially applied (under no pressure). The heating condition is preferably a temperature below the melting point at which the ultrafine organic fibers do not melt, and a temperature above the melting point of the low melting point component to which the heat-fusible fibers adhere. When the heat-fusible fibers are heat-bonded under such conditions, the fibers can be bonded uniformly in the thickness direction of the fiber web, and the fine void structure formed by the ultrafine organic fibers does not collapse due to the heat treatment. good.

In the case where the filter medium 1 has a more dense structure and a smaller thickness, it is preferable to perform a pressure treatment by the pressure treatment device 9 before the heat treatment. As the pressure processing device 9, a pressure roll, a press, or the like can be used, but a heating device 91 as shown in FIG.
A device for performing pressure treatment between a pair of endless tracks 92 arranged inside is particularly preferable. In this device 9, since the pressing time is longer than that of a pressure roll or the like, a strong shearing force is less likely to be applied to the fibrous web, and the pressure loss of the obtained filter medium is hardly increased. The pressure treatment is preferably performed at a temperature lower than the melting point at which the low melting point component of the heat fusible fiber 3 does not melt so that the heat fusible fiber 3 does not form a film and close the fine voids of the filter medium.

[0037]

The present invention will be specifically described below with reference to examples.

Example 1 A filter medium was manufactured according to the manufacturing process shown in FIG. The die shown in FIG. 2 was used as a die for a melt blow device. The die is provided with 900 nozzles having a diameter of 0.2 mm in a straight line at an interval of 0.8 mm, and slit-shaped outlets of a heated airflow are formed on both sides thereof. The temperature in the vicinity of the nozzle is adjusted to 330 ° C., and a molten polypropylene resin (melt index = about 600) is discharged at a resin amount of 0.033 cm ³ / min per nozzle. The flow rate of the heated air flow is 2 Nm ³ / min per 1 m width.
This forms a fiber stream substantially free of shots of ultrafine organic fibers having an average fiber diameter of 0.5 μm melt-blown from the die. On the other hand, a heat-fusible fiber having an average fiber diameter of 16 μm and a length of 51 mm having a core of a polypropylene resin and a sheath of a polyethylene resin is opened by an opening machine shown in FIG. Was supplied from a substantially right angle direction and mixed. The mixed fibers were collected on a conveyor belt to form a fibrous web.
In addition, the belt was a mesh body, and was sucked in the thickness direction from the collection surface of the belt to the back surface to prevent the fibers of the fiber web from being disturbed. The resulting fibrous web contains 8 fine organic fibers.
5 g / m ^2, heat-fusible fibers are contained 30 g / m ^2,
The total weight was 115 g / m ² . The fiber web is heated at a temperature of 120 ° C. lower than the melting point of the polyethylene resin,
After pressure treatment for 0 seconds, the air flow is passed through the thickness direction of the fiber web with a dryer at an atmosphere temperature of 145 ° C. higher than the melting point of the polyethylene resin and lower than the melting point of the polypropylene resin, and the heat treatment is performed. The fibers were combined with each other to obtain a filter medium having a thickness of 0.9 mm. The initial pressure loss of this filter media is
The collection efficiency was 350 Pa, and the collection efficiency was 99.99%. The measurement was performed under the condition of a wind speed of 5.3 cm / sec, and polystyrene particles having a particle diameter of 0.3 μm were used as test particles. This collection efficiency is three times higher than normal air.
It was the same after months of ventilation and showed stable performance. Further, this filter medium was excellent in strength, and had no problem even when subjected to folding such as pleating, and neither fiber was broken nor unentangled. Furthermore, since it is composed only of organic fibers, incineration is possible and there is no problem of disposal.

Comparative Example 1 A filter medium was prepared in the same manner as in Example 1, except that the average fiber diameter of the ultrafine organic fibers was 2 μm, and the blending amount of the ultrafine organic fibers and the heat-fusible fiber, the apparatus, and the production process were the same as in Example 1. The initial pressure loss of this filter medium was 210 Pa, the collection efficiency was 87.9%, and the collection capacity was clearly insufficient.

Comparative Example 2 The filter material obtained in Comparative Example 1 was electretized by a corona discharge method. The initial pressure loss of the obtained filter medium is 2
Although the collection efficiency was 10 Pa and the collection efficiency was 99%, the collection efficiency of the filter medium dropped to 95% after passing through the ordinary atmosphere for 3 months, and a stable collection ability was not obtained.

Example 2 50 g / m ² of ultrafine organic fibers having an average fiber diameter of 0.3 μm melt-blown, an average fiber diameter of 16 μm, a core made of a polypropylene resin and a sheath made of a polyethylene resin, and a length of 51 μm
A filter medium having a thickness of 0.6 mm and an areal density of 80 g / m ² was prepared in the same manner as in Example 1 by using 30 g / m ² of the heat-fusible fiber of 30 mm / mm. The initial pressure loss of this filter medium is 350P
a, The collection efficiency was 99.99%, indicating the ability of hepagrade. Further, the collection efficiency was the same even after the normal atmosphere was ventilated for three months, indicating a stable capacity. Further, this filter medium was excellent in strength, and had no problem even when subjected to folding such as pleating, and neither fiber was broken nor deentangled. Furthermore, since it is composed only of organic fibers, incineration is possible and there is no problem of disposal.

Example 3 45 g / m ^{2 of} an ultrafine organic fiber having an average fiber diameter of 0.5 μm melt-blown, an average fiber diameter of 16 μm having a core of a polypropylene resin and a sheath of a polyethylene resin, and a length of 51 μm
The mm heat-fusible fibers using a 60 g / m ^2, a thickness of 0.8mm in the same manner as in Example 1, the surface density of 105 g / m ²
Was prepared. The initial pressure loss of this filter medium is 107
Pa and the collection efficiency were 99%. Further, the collection efficiency was the same even after the normal atmosphere was ventilated for three months, indicating a stable capacity. Further, this filter medium was excellent in strength, and had no problem even when subjected to folding such as pleating, and neither fiber was broken nor deentangled. Furthermore, since it is composed only of organic fibers, incineration is possible and there is no problem of disposal.

[0043]

According to the filter medium of the present invention, a fibrous web obtained by mixing ultrafine organic fibers having an average fiber diameter of less than 1 μm produced by a melt blow method and heat-fusible fibers having an average fiber diameter of 5 to 100 μm is formed by heat. Since they are bonded by the fusible fibers, high collection efficiency can be obtained while maintaining a low pressure loss. For this reason, the filter medium of the present invention can also provide a filter medium having a hepagrade-collecting ability without performing electretization treatment or the like. In addition, this filter medium has a stable collection ability over a long period of time without the collection ability decreasing with use time unlike a filter medium subjected to electretization treatment. Furthermore, since it is composed only of organic materials, it can be incinerated and is easy to dispose of.
Since it does not contain boron or the like, it can be used without problem as a filter medium for the semiconductor industry.

In particular, when the ultrafine organic fibers having an average fiber diameter of less than 1 μm are contained in a proportion of 50 to 90% by weight and the heat-fusible fibers having an average fiber diameter of 5 to 100 μm are contained in a proportion of 50 to 10% by weight, the hepagrade is high. Collection efficiency can be realized.

Further, the filter medium of the present invention is heat-treated at a temperature higher than the melting point of the low melting point component contained in the heat-fusible fiber and lower than the melting point of the ultrafine organic fiber without substantially applying pressure. In this case, the fibers are evenly bonded to the inside,
The abrasion resistance of the surface is excellent, and since it is strong, it can be folded, etc., and because it is dense, it has excellent performance of collecting fine particles. In addition, since the fibers do not form a film when the fibers are bonded and do not close the voids, there is an excellent effect that the initial pressure loss does not become too large.

Further, the thickness of the nonwoven fabric is 0.1 to 1.5 m
In the case of m, a unit having a large surface area can be formed by folding or the like.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a manufacturing process of a filter medium.

FIG. 2 is a schematic cross-sectional view of an example of a die for a melt blow device.

FIG. 3 is a cross-sectional model diagram of an example of an opening machine.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Filter medium 2 Ultrafine organic fiber 3 Heat-fusible fiber 4 Fiber web 5 Die for melt blow apparatus 51 Nozzle 52 Hot air flow outlet 6 Spreader 61 Spreading cylinder 62 Housing 63 Air nozzle 7 Collector 8 Heat treatment device 9 Heat treatment Equipment 91 Heating equipment 92 Endless track

Claims (7)

[Claims] 1. An ultrafine organic fiber having an average fiber diameter of less than 1 μm produced by a melt blowing method, and an average fiber diameter of 5 to 1
A filter medium, wherein a fibrous web mixed with a heat-fusible fiber of 00 μm is bound by the heat-fusible fiber. 2. The method according to claim 1, wherein 50 to 90% by weight of ultrafine organic fibers having an average fiber diameter of less than 1 μm and 50 to 10% by weight of heat-fusible fibers having an average fiber diameter of 5 to 100 μm. Filter media. 3. The filter medium according to claim 1, wherein the thickness is 0.1 to 1.5 mm. 4. A fibrous web in which ultrafine organic fibers and heat-fusible fibers are mixed is subjected to a pressure treatment at a temperature equal to or lower than the melting point of the low-melting component contained in the heat-fusible fibers. The filter medium according to claim 1. 5. A heat treatment is performed at a temperature not lower than the melting point of the low melting point component contained in the heat-fusible fiber and lower than the melting point of the ultrafine organic fiber without substantially applying pressure. The filter medium according to any one of claims 1 to 4. 6. An ultrafine organic fiber obtained by a melt blow method,
The filter medium according to any one of claims 1 to 5, wherein the filter medium has no shot or has few shots. 7. The filter medium according to claim 1, wherein a collection efficiency of particles having a diameter of 0.3 μm is 99.97% or more under a condition of a wind speed of 5.3 cm / sec. 3. The filter medium according to item 1.