JP5112251B2 - Method for producing filter medium for air filter - Google Patents

Description

  The present invention relates to a filter medium for an air filter using a polytetrafluoroethylene (hereinafter referred to as “PTFE”) porous membrane, and more particularly suitable for capturing suspended particles in a gas used in a clean room such as the semiconductor industry or the pharmaceutical industry. Further, the present invention relates to a filter medium for an air filter using a PTFE porous membrane as a collection layer.

  Conventionally, as a filter medium for an air filter used in a clean room, paper made by adding a binder to glass fiber has been used in many cases. However, such filter media have problems such as the presence of adhering fibrils in the filter media and self-dusting during bending by processing. Further, this filter medium has a problem that it deteriorates and generates dust when it comes into contact with certain chemicals such as hydrofluoric acid.

PTFE is a clean material and has excellent chemical resistance. Therefore, in recent years, a PTFE porous membrane has been used as a filter medium for a high-performance air filter used in a clean room of the semiconductor industry. One example is the PTFE porous membrane described in JP-A-5-202217. In general, a porous PTFE membrane is laminated with a support material having air permeability for reinforcement to form a filter medium. This filter medium is fold-folded into a continuous W shape (hereinafter also referred to as “pleating”), and is attached to a frame to be used as an air filter.
Japanese Patent Laid-Open No. 5-202217

  As a pleating method, a reciprocating method or a rotary method is usually used. The rotary method is advantageous in terms of productivity, while the reciprocating method is advantageous in terms of the cleanliness of pleating. By the way, conventionally, when a pleating process is performed on a filter medium in which a PTFE porous membrane is reinforced with a breathable support material, the filter medium is damaged and pinholes are generated, resulting in a leak in the obtained filter. It was.

  The present invention has been made to solve the above-described conventional problems, and the PTFE porous membrane is not damaged even when pleated by pleating, and is suitable for providing a clean space required for a clean room. An object is to provide a filter medium for a filter.

  In order to achieve the above object, an air filter medium of the present invention is an air filter medium including a laminate of a PTFE porous membrane and a breathable support material, and at least one exposed surface has a maximum frictional resistance. It is comprised by the surface of the said air-permeable support material which made 25 gf or less.

Here, the maximum frictional resistance is determined by the maximum load measured using a Bowden-Leben type reciprocating friction tester and setting the conditions as follows.
-Opposite material: Steel ball with a diameter of 10 mm (steel ball specified in JIS B 1501)
・ Sliding speed: 700mm / min ・ Load: 100gf
・ Movement distance: 40mm
In this way, the static frictional resistance and the dynamic frictional resistance are measured, and the maximum value is determined as the maximum frictional resistance.

It is thought that the pinhole of the filter medium which became a problem conventionally occurs by the mechanism as shown in the following 1-3.
1. A blade that partially presses the surface of the filter medium in order to form peaks and valleys in the filter medium rubs the filter medium surface.
2. A shearing stress is generated inside the filter medium due to the frictional force between the filter medium and the blade.
3. This shear stress concentrates on the adhesive portion between the PTFE porous membrane and the air-permeable support material inside the filter medium, and the porous structure of the PTFE porous membrane having a relatively low strength is destroyed.

  However, according to the air filter medium of the present invention, the maximum frictional resistance of at least one exposed surface is 25 gf or less, more preferably 20 gf or less. The frictional resistance between the filter media is not excessive. As a result, shear stress acting on the filter medium is reduced, and pinholes are less likely to occur in the PTFE porous membrane. The lower limit of the maximum frictional resistance is not particularly limited, but is usually 2 gf.

  In order to achieve the above object, the method for producing an air filter medium of the present invention is a method for producing a filter medium for an air filter including a laminate of a PTFE porous membrane and a breathable support material, wherein the breathability The method includes a step of heating the support material, and a step of pressing the heated air-permeable support material against the smoothing member.

  According to the production method of the present invention, it is possible to provide a filter medium for an air filter in which the surface is smoothed and the PTFE porous membrane is hardly damaged even when pleated by pleating. The degree of smoothing of the surface of the breathable support material is 25 gf or less, more preferably 20 gf or less, expressed by the maximum frictional resistance value. The surface of the air-permeable support material is disposed on the exposed surface of the air filter medium in the lamination step with the PTFE porous membrane, which is performed before or after the smoothing step.

  In the method for producing a filter medium for an air filter, the breathable support material is pressed against the smoothing member while being heated to a temperature at which the surface of the breathable support material is deformed by being pressed. It is preferable. Therefore, specifically, it is preferable to press the breathable support material against the smoothing member in a state of being heated to at least the softening point of at least one material constituting the breathable support material. Furthermore, the present invention provides a breathable support in which at least one surface has a maximum frictional resistance of 25 gf or less (preferably 20 gf or less) as a breathable support material laminated with a PTFE porous membrane and used as a filter medium for an air filter. Providing materials. As will be described later, the air-permeable support material may be smoothed by laminating the PTFE porous membrane and then pressing the surface against the smoothing member, and smoothing the surface in advance as described above. It may be laminated with a film. The lower limit of the maximum frictional resistance on at least one surface of the breathable support material is not particularly limited, but is usually 2 gf.

  According to the present invention, it is possible to provide a filter medium for an air filter that is less likely to cause pinholes even when pleated, and that is suitable for maintaining a clean space required for a clean room.

  The filter material for an air filter of the present invention includes a laminate of a PTFE porous membrane and a breathable support material for protecting and reinforcing the membrane. The breathable support material gives the filter medium the stiffness necessary for pleating.

  The PTFE porous membrane can be obtained by a conventionally used method. As an example of this method, a mixture of unsintered PTFE powder and a liquid lubricant (such as naphtha) is formed into a film by extrusion and / or rolling, the liquid lubricant is removed from the unsintered film, The method of extending | stretching and making it porous can be mentioned. Usually, after stretching, the strength is improved by heating to a temperature equal to or higher than the melting point of PTFE and firing.

As the PTFE porous membrane, any pore diameter, thickness, porosity and the like can be used without particular limitation as long as the collection performance corresponding to the intended use is exhibited. However, in order to obtain a filter medium used for a filter such as a semiconductor clean room, it is preferable that the PF value (Performance of Filter) indicating the collection performance is larger than 20. The PF value is expressed by the following formula.
PF value = {(− log permeability) / pressure loss} × 100

  On the other hand, the material, structure, and form of the breathable support material are not particularly limited. However, the breathable support material is more excellent in breathability than the PTFE porous membrane, for example, nonwoven fabric, woven fabric, mesh (mesh-like sheet), other Porous materials can be used. However, a nonwoven fabric is preferable from the viewpoint of strength, flexibility, and workability. Furthermore, when the maximum frictional resistance on the surface of the filter medium is reduced by an operation described later, some or all of the fibers constituting the nonwoven fabric are core-sheath structure composite fibers, and the core component has a melting point relatively higher than that of the sheath component. High synthetic fibers are preferred. The material of the breathable support material is not particularly limited, but polyolefin (polyethylene (PE), polypropylene (PP), etc.), polyamide, polyester (polyethylene terephthalate (PET), etc.), aromatic polyamide, or What consists of these composite materials etc. can be used.

  Further, in the laminate, the PTFE porous membrane and the air-permeable support material may be alternately laminated, or one or both of the PTFE porous membrane and the air-permeable support material are continuously laminated. There may be some parts. Further, a material for adhesion may be interposed at the interface of the layers.

  Examples of air filter media comprising such a laminate are shown in FIGS. As shown in the figure, it is preferable that the air-permeable support material 1 is disposed on the outermost layer of the air filter medium. Further, in order to effectively suppress the occurrence of pinholes, it is preferable to include two or more layers of PTFE porous membrane 2 as shown in FIGS. 3 to 5, and in particular, as shown in FIG. It is preferable to include a layer structure in which the breathable support material 1 is sandwiched between the PTFE porous membrane 2. This is to relieve the shear stress during pleating. The laminate may have a two-layer structure in which a breathable support material is disposed on one side of a PTFE porous membrane. The maximum frictional resistance of at least one of the outermost layers of these laminates, preferably both outer surfaces (exposed surfaces of the laminate), is 25 gf or less.

  The method for producing such a laminate is not particularly limited, and for example, methods such as adhesive lamination and thermal lamination can be applied. For example, in the case of laminating by heat lamination, a part of a breathable support material such as a nonwoven fabric may be melted by heating and laminated. Moreover, you may adhere | attach by interposing a fusing agent like hot melt powder. On the other hand, as a method of laminating PTFE porous membranes, there are a method of pressure laminating at the time of film formation, a method of heat fusion, and the like.

  The smoothing of the surface of the air-permeable support material includes, for example, a step of heating the air-permeable support material to a temperature at which the unevenness present on the surface of the air-permeable support material can be deformed by pressing, and the air-permeable support material. And a step of smoothing the surface by pressing against the smoothing member.

  In the step of heating the air-permeable support material, the air-permeable support material is heated to a temperature above the softening point (preferably above the melting point) of at least one material constituting the air-permeable support material. Although the upper limit of heating temperature is not specifically limited, Practically it is preferable to set it as a temperature 100 degree | times higher than melting | fusing point of a breathable support material. The heating temperature of the breathable support material may be appropriately determined according to the material of the breathable support material, the time from heating to smoothing, and the like. For example, when the breathable support material includes a composite fiber having a core-sheath structure in which the core component has a melting point higher than that of the sheath component, the heating temperature of the breathable support material is higher than the softening point of the sheath component (preferably higher than the melting point). It is preferable to set it lower than the melting point of the component.

  The breathable support material can be heated, for example, by contact with a heating member such as a roll that has heated the breathable support material to a predetermined temperature. In particular, the method by contact with a heating roll is preferable because the heating operation can be carried out continuously.

  After heating, the breathable support material is pressed against the smoothing member. By this pressing, the contact surface of the breathable support material with the smoothing member is smoothed by the pressing force. The smoothing member is not particularly limited as long as it has a surface capable of smoothing the surface of the air-permeable support material. For example, a silicone member can be given as a specific example. If the smoothing member has a roll shape, the smoothing operation can be performed continuously. As such a roll, for example, a silicone roll can be used. The smoothing member may be provided with a temperature adjustment mechanism.

  It is preferable that the temperature of the support material when the breathable support material is in contact with the smoothing member is equal to or higher than the softening point of at least one material constituting the breathable support material. For example, when the breathable support material includes a composite fiber having a core-sheath structure in which the core component has a higher melting point than the sheath component, the temperature of the breathable support material during smoothing is preferably equal to or higher than the softening point of the sheath component. . In order to suppress a temperature drop from heating to smoothing, the time from heating to smoothing is preferably as short as possible.

  The method described above is preferably used in combination with a method for producing a laminate by thermal lamination. After laminating the PTFE porous membrane and the breathable support material by a process involving heating, if the surface of the breathable support material to be the exposed surface is pressed against the smoothing member and smoothed, the frictional resistance is effectively low. A filter medium for an air filter having an exposed surface can be manufactured.

  The method described above can be efficiently performed by an apparatus including a heating roll and a smoothing roll (preferably a silicon roll) disposed adjacent to the heating roll. Moreover, when manufacturing the laminated body with a PTFE porous membrane by thermal lamination, the roll for crimping | bonding a laminated body to the position facing a heating roll is prepared, and air permeability support material and this roll and a heating roll are used. It is preferable to heat laminate the PTFE porous membrane.

  An example of an apparatus for carrying out such a method is shown in FIG. FIG. 1 shows an example in which a filter medium having the laminated structure (breathable support material / PTFE porous membrane / breathable support material) shown in FIG. 2 is continuously produced. As shown in the figure, the breathable support material 1 and the PTFE porous membrane 2 are first supplied to the guide roll 5 so that the breathable support material 1 sandwiches the PTFE porous membrane 2. These materials are heated to a temperature equal to or higher than the melting point of the breathable support material 1 while traveling on the outer periphery of the heating roll 3 in an overlapping state, and pass between the pinch roll 6 and the heating roll 3. Crimped to Furthermore, this laminated body is sent out through the outer periphery of the silicone roll 4 as a smoothing member disposed in the vicinity of the heating roll 3. The surface of the silicone roll 4 is smooth, and the exposed surface of the laminate is pressed and smoothed by contact with the smooth surface.

  The smoothing of the surface of the breathable support material can also be carried out by a method using a calendar roll, for example. Alternatively, the surface of the breathable support material may be smoothed in advance, and the breathable support material and the PTFE porous membrane may be laminated.

  When the air filter medium as described above is used, the frictional resistance with the blade can be kept small even when processed with a reciprocating type pleating machine, and the shear stress acting on the filter medium can be reduced. Therefore, damage to the PTFE porous membrane, which is the collection layer, can be mitigated, and an air filter free from leakage can be obtained.

Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples.

Example 1
Baked PTFE porous membrane (thickness: 10 μm, porosity 93%, average pore diameter 0.7 μm, pressure loss: 35 mmH ₂ O, collection efficiency: 99.999995%, PF value: 21) and thickness 150 μm, PET / PE core-sheath nonwoven fabric with a basis weight of 30 g / m ² ("Elves TO303WDO" manufactured by Unitika Ltd., melting point of sheath PE: 129 ° C, softening point of PE: 74 ° C, melting point of core PET: 261 ° C) 1 was laminated by a heat laminating method (heating roll temperature: 160 ° C.) using the same apparatus as in FIG. In addition, the temperature of the nonwoven fabric was 124 ° C. when the three-layer product in which the porous film and the nonwoven fabric were pressed through the pinch roll and the heating roll was in contact with the silicone roll as the smoothing member.

Moreover, the softening point of the said nonwoven fabric was measured in the penetration mode (Penetration Mode) by TMA (Thermomechanical measurement apparatus; Thermo Mechanical Analysis). As the apparatus, TMA / SS120C manufactured by Seiko Instruments Inc. was used, and the conditions were set as follows.
Probe needle diameter: 1mmφ
Load: 4g
Temperature increase rate: 5 ° C / min

  The maximum frictional resistance of the obtained filter medium on the silicone roll contact surface and the silicone roll non-contact surface was measured by the above method (using AFT-15B manufactured by A & D Co., Ltd. as a Bowden-Leben type reciprocating friction tester). However, they were 9.9 gf and 27.0 gf, respectively.

(Example 2)
A filter medium was obtained in the same manner as in Example 1 except that the temperature of the heating roll was 150 ° C. The temperature of the nonwoven fabric was 115 ° C. when the three-layer product obtained by pressure-bonding the porous membrane and the nonwoven fabric was in contact with the silicone roll as the smoothing member. When the maximum frictional resistance of the obtained filter medium on the silicone roll contact surface and the silicone roll non-contact surface was measured by the above method, it was 16.3 gf and 28.6 gf, respectively.

(Example 3)
A filter medium was obtained in the same manner as in Example 1 except that the temperature of the heating roll was 140 ° C. The temperature of the nonwoven fabric was 104 ° C. when the three-layer product obtained by pressure-bonding the porous membrane and the nonwoven fabric was in contact with the silicone roll as the smoothing member. When the maximum frictional resistance of the obtained filter medium on the silicone roll contact surface and the silicone roll non-contact surface was measured by the above method, they were 22.3 gf and 28.1 gf, respectively.

Example 4
The front and back of the filter medium obtained in Example 1 are reversed. And it passed again through the roll (roll temperature: 160 degreeC) of the apparatus similar to FIG. 1, and then was made to contact with a silicone roll, and the filter medium in which the two exposed surfaces were smoothed was obtained. In addition, the temperature of the nonwoven fabric when it contacts with the silicone roll which is a smoothing member was 122 degreeC. When the maximum frictional resistance of both sides of the obtained filter medium was measured by the above method, it was 9.9 gf (surface first contacted with the silicone roll) and 11.1 gf (second surface contacted with the silicone roll), respectively. there were.

(Example 5)
The same PTFE porous membrane as used in Example 1 and one each of PET / PE core-sheath nonwoven fabric were laminated through a pair of rolls. The roll temperature was set to 140 ° C. Next, in place of the breathable support materials 1 and 1 in FIG. 1, this two-layer product is disposed so that the nonwoven fabric is on the outside, and the PET / PE core-sheath nonwoven fabric (Example) instead of the PTFE porous membrane 2 1). These were laminated by a heat laminating method (roll temperature: 160 ° C.) to obtain a filter medium having a five-layer structure of nonwoven fabric / PTFE porous membrane / nonwoven fabric / PTFE porous membrane / nonwoven fabric. In addition, the temperature of the nonwoven fabric when contacting with the silicone roll was 121 ° C. When the maximum frictional resistance of the obtained filter medium on the silicone roll contact surface and the silicone roll non-contact surface was measured by the above method, they were 12.5 gf and 28.0 gf, respectively.

(Example 6)
First, a PET nonwoven fabric having a thickness of 170 μm and a basis weight of 32 g / m ² (“Syntex MY R-200” manufactured by Mitsui Petrochemical Industries, Ltd., melting point: 256 ° C.) was passed through a calender roll to smooth both sides of the nonwoven fabric ( The roll temperature at this time was 100 ° C.). Low density polyethylene powder (melting point 97.5 ° C., 30 mesh pass) was uniformly sprayed on one side of the nonwoven fabric at a rate of 10 g / m ² , and this was heated to 120 ° C. to heat-fuse the powder.

  Next, a non-woven fabric having a smoothed surface is disposed on both sides of the PTFE porous membrane (the same as that used in Example 1) (placed so that the powder spreading surface is on the PTFE porous membrane side), and FIG. Was subjected to heat laminating (roll temperature: 140 ° C.) using the same apparatus as above (however, the silicone roll 4 was not disposed) to obtain a three-layer structure filter medium having a PTFE porous membrane as an intermediate layer. When the maximum frictional resistance of both sides of the obtained filter medium was measured by the above method, they were 13.5 gf and 14.6 gf, respectively.

(Comparative Example 1)
A filter medium was obtained in the same manner as in Example 1 except that it was not brought into contact with the silicone roll. When the maximum frictional resistance of both sides of the obtained filter medium was measured by the above method, they were 26.9 gf and 28.2 gf, respectively.

(Comparative Example 2)
A three-layer filter medium was obtained in the same manner as in Example 6 except that the surface of the nonwoven fabric was not smoothed with a calender roll. When the maximum frictional resistance of both sides of the obtained filter medium was measured by the above method, they were 29.0 gf and 27.6 gf, respectively.

Air filter media obtained in Examples 1 to 6 and Comparative Examples 1 and 2 were pleated by a reciprocating type pleating machine, and the collection performance was measured by the following method for each filter media by 30 points or less. The presence or absence of was investigated.
1 Using an air filter medium (measurement area 100 cm ² ) as a partition, separate the upstream side and the downstream side.
2 Introduce cold DOP (dioctyl phthalate), which is a test particle, on the upstream side, and suck it from the downstream side at a surface speed of 5.3 cm / sec.
3 Measure the upstream particle concentration and the downstream particle concentration leaked from the filter medium with a laser particle counter (LPC).
4 When the particle transmittance of _0.1 to 0.2 μm was P _0.1 and the particle transmittance of _0.2 to 0.3 μm was P _0.2 , it was judged that there was a leak when the following conditions were satisfied. The frequency of P _0.2 / P _0.1 > 0.1 leak is shown in the table below.

  From Table 1, it can be seen that the air filter medium having a maximum frictional resistance of 25 gf or less on at least one exposed surface is less likely to generate pinholes even when pleated by a reciprocating method that easily damages the filter medium.

It is a figure which shows the outline of the example of the apparatus for manufacturing the filter medium for air filters of this invention. It is sectional drawing which shows the structure of the example of the filter medium for air filters of this invention. It is sectional drawing which shows the structure of another example of the filter medium for air filters of this invention. It is sectional drawing which shows the structure of another example of the filter medium for air filters of this invention. It is sectional drawing which shows the structure of another example of the filter medium for air filters of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Breathable support material 2 PTFE porous membrane 3 Heating roll 4 Silicone roll 5 Guide roll 6 Pinch roll

Claims (4)

A pleated air filter filter medium manufacturing method comprising a laminate of a polytetrafluoroethylene porous membrane and a breathable support material, both exposed surfaces being constituted by the surface of the breathable support material. There,
Before or after laminating the air permeable support material with the polytetrafluoroethylene porous membrane, the air permeable support material is heated, and the surface of the air permeable support material is pressed against a smoothing member, thereby the laminate. A smoothing step of making the maximum frictional resistance of the surface of the breathable support material constituting at least one exposed surface of 25 gf or less,
A pleat processing step of fold-folding the laminated body , the exposed surface of which has been smoothed, into a continuous W-shape using a reciprocating pleating machine. Method. The manufacturing method of the filter material for air filters of Claim 1 in which the said air permeable support material which comprises both the exposed surfaces of the said laminated body is smoothed in the said smoothing process.   The method for producing a filter medium for an air filter according to claim 1 or 2, wherein the breathable support material is a nonwoven fabric. The said air-permeable support material is a manufacturing method of the filter material for air filters in any one of Claims 1-3 which protects and reinforces the said polytetrafluoroethylene porous membrane.

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