JPH09206568A - Material for air filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a material for an air filter having a high trapping efficiency of dust, low pressure loss and little dispersion in the performance by laminating two or more layers of polytetrafluoroethylene porous films. SOLUTION: A polytetrafluoroethylene sheet in an unburned state is stretched in the longitudinal and width directions and then the dimension of the sheet is fixed and burned to obtain polytetrafluoroethylene porous films 1 to 3 having different pressure losses and trapping efficiencies. As for a reinforcing member, nonwoven fabrics 4-7 comprising fibers having a core/sheath structure are used. The polytetrafluoroethylene porous films 1-3 are laminated with the reinforcing members 4-7 to obtain an air filter material. By this method, variance in the pressure loss can be reduced and pinholes penetrating the filter are eliminated. Thus, a leakless filter material is obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a filter material for an air filter using a polytetrafluoroethylene (hereinafter referred to as "PTFE") porous membrane, and more specifically, air and a filter material used in a clean room such as semiconductor industry. The present invention relates to a filter material suitable for capturing floating particles in a gas and used in a high-performance air filter with a small pressure loss of air and gas.

[0002]

2. Description of the Related Art Conventionally, as a material for an air filter used in a clean room, a filter medium formed by adding a binder to glass fiber and making paper has been widely used. However, such filter media have several problems. For example, the presence of attached fibrils in the filter medium, the occurrence of self-dusting at the time of bending due to processing, or the increase in the binder to prevent self-dusting increases the pressure loss (Japanese Patent Application Laid-Open No. 63-163). No. 16019). Further, the filter medium has a problem that when it comes into contact with a certain chemical such as hydrofluoric acid, the glass and the binder are deteriorated and generate dust.

In order to solve these problems, it has been proposed to use a synthetic fiber electret filter medium (see Japanese Patent Application Laid-Open No. 54-53365), but there is a problem that electret attenuation occurs.

[0004] PTFE is a clean material and also has chemical resistance. Therefore, various proposals have been made to apply the PTFE porous membrane obtained by the stretching method to a high-performance air filter. For example, international publication number WO94 / 16802,
There is JP-A-7-196831.

[0005]

However, the above-mentioned International Publication No. WO94 / 16802 and JP-A-7-196831.
The filters proposed in the publications each have a very small pore diameter (average pore diameter) of 0.5 μm or less in order to improve the collection efficiency, and thus the thickness is extremely thin in order to reduce the pressure loss. Is what you are doing. Since these have such a structure, they have the following problems. (1) Variations in thickness and hole diameter are large with respect to the average value,
As a result, the variation in pressure loss increases. (2) There are many leaks derived from pinholes or portions with extremely small pressure loss. (3) Absolute amount of pores (void marks) is small and the filter medium life is short. To describe the above (1) to (3) in detail, the variation in pressure loss in (1) is such that the wind velocity distribution becomes large when used as a filter unit, and laminar airflow cannot be obtained. In the case of (2), since particles having a particle size larger than the target particle size are transmitted, cleanliness cannot be provided. (3) is directly related to the dust holding performance of the filter, and when the above filter medium is used,
A small dust load increases pressure loss. Pressure loss is extremely important in terms of energy cost during filter operation.

In order to solve the above-mentioned conventional problems, the present invention has a high dust collection efficiency, a low pressure loss, a variation in pressure loss is alleviated, there is no pin hole penetrating, and there is no leak, and the amount of pores is small. It is an object of the present invention to provide a filter material for an air filter, which has an increased life and excellent life characteristics.

[0007]

In order to achieve the above object, the filter material for an air filter of the present invention has a structure in which two or more layers of PTFE porous membranes are laminated. With this configuration, the dust collection efficiency is high, the pressure loss is low, and a plurality of layers of porous membranes are used as compared with the conventional case of only one layer, so that the variation in pressure loss is alleviated. There are no pinholes penetrating through, and there is no leak. At the same time, the absolute porosity is increased and the life characteristics are improved.

In the above structure, it is preferable that a reinforcing material is further interposed between the laminated porous PTFE membranes.
Moreover, in the said structure, it is preferable that a reinforcing material is a nonwoven fabric.

Further, in the above structure, it is preferable that the fibers constituting the nonwoven fabric are composite fibers having a core-sheath structure, and the core component is a synthetic fiber having a relatively higher melting point than the sheath component. Further, in the above structure, it is preferable that the pressure loss when permeating air at a flow rate of 5.3 cm / sec is 65 mmH ₂ O or less. The pressure loss of the air filter medium of the present invention can be freely changed depending on the PTFE porous membrane to be constructed, other constituent materials and lamination conditions, but HEPA (Hig Effic
iency Particulate Air) filter and ULPA (Ult
In order to be used as a ra Low Penetration Air) filter, it is preferable that the pressure loss of the filter medium is 65 mmH ₂ O or less.

In the above structure, it is preferable that the pressure loss of the PTFE porous membrane on the upstream side is lower than the pressure loss of the PTFE porous membrane on the downstream side when the fluid flows. That is, when stacking PTFE porous membranes having different pressure losses, stacking them so that the pressure loss of the upstream PTFE porous membrane is smaller than the pressure loss of the downstream PTFE porous membrane increases the filter life. It is important to do. When a membrane with a large pressure loss is placed on the upstream side, a porous membrane with a large pressure loss generally has a high collection efficiency and traps both fine and coarse particles, and a porous membrane with a low pressure loss on the downstream side. Because the meaning of the existence of is lost.

The method for stacking the PTFE porous membranes in the present invention is not particularly limited, and a method using an adhesive,
Examples thereof include a method of laminating with heat through a thermoplastic porous material (nonwoven fabric, net, or the like). Generally PTF
Since the E porous film alone is inferior in folding workability and strength, a reinforcing material is used.

Nonwoven fabrics, woven fabrics, meshes and other porous membranes can be used as the reinforcing material. As the material for the reinforcing material, polyolefin (for example, polyethylene, polypropylene, etc.), nylon, polyester, aramid (aromatic polyamide), or a composite thereof (for example, a nonwoven fabric composed of fibers having a core / sheath structure, a low melting point) Materials and high melting point materials such as two-layer nonwoven fabric), and fluorine-based porous membranes (for example, PFA (tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer), FEP)
(Tetrafluoroethylene / hexafluoropropylene copolymer), PTFE porous membrane, etc.) can be exemplified.

Above all, a non-woven fabric composed of a composite fiber having a core-sheath structure (a sheath component is a low-melting polymer), a two-layer non-woven fabric composed of a low-melting material and a high-melting material, and the like are preferable. Such reinforcements do not shrink during lamination. In addition, such a laminated film with a reinforcing material is used for HEPA filters and ULPs.
It is easy to process as an A filter, and the folding pitch can be increased when it is used as a filter element.

The method for producing the PTFE porous membrane used in the present invention is not particularly limited, and various conventionally proposed methods can be used.

[0015]

The present invention will be specifically described below with reference to examples. In addition, each measuring method in the following examples is as follows.

(Pressure loss) Effective area of the filter medium is 100 cm ²
Set in a circular holder, and a pressure difference is applied to the inlet side and the outlet side, and the permeation velocity of the filter medium is 5.3 cm / sec with a flow meter.
The pressure loss when adjusted to 1 was measured with a pressure gauge (manometer). Samples at 50 locations were measured, and the average value was taken as the pressure loss of the filter medium. Moreover, the variation showed the maximum value and the minimum value.

(Collection efficiency) Using the same device as the pressure loss measurement, the permeation flow rate of the filter medium was set to 5.3 cm / sec, and polydisperse dioctyl phthalate (DOP) was added to the upstream side of 0.1.
Particles of μm to 0.15 μm were made to flow so as to be about 10 ⁷ / liter, the downstream concentration was measured by a particle counter, and the collection efficiency was determined by the following formula. Collection efficiency = {1- (downstream concentration / upstream concentration)} × 100% (However, the target particles are in the range of 0.1 to 0.15 μm. Further, the variation is shown by the maximum value and the minimum value. (Leak performance) In the measurement of collection efficiency, the number of samples in which particles of 0.3 μm or more were counted on the downstream side was shown.

(Increase in pressure loss) A value obtained by subtracting the initial pressure loss from the pressure loss when a load of DOP particles was applied for 5 minutes was shown under the same conditions as the measurement of the collection efficiency.

[0019]

[Example 1] 100 parts by weight of PTFE fine powder (trade name "Polyflon F-104", manufactured by Daikin Industries, Ltd.) was used as a liquid lubricant with 25 "hydrocarbon oil" trade name "ISOPER M" (manufactured by Esso Oil Co., Ltd.). The mixture obtained by uniformly mixing the parts by weight is pre-compressed at a pressure of 20 kg / cm ² , and then extruded into a rod shape, and the rod-shaped material is passed between a pair of metal rolling rolls to form a thick film. A long film having a length of 0.2 mm and a width of 150 mm was obtained.

Next, this molded product was heated to 220 ° C. to remove the liquid lubricant, and wound around a tubular core body in a roll shape. Next, this sheet is longitudinally and transversely stretched in a non-baked state, and thereafter,
The PTFE porous membranes shown in Table 1 were produced by fixing the dimensions and firing. The changes in pressure loss and collection efficiency were adjusted by changing the stretching temperature, the magnification, and the firing temperature.

[0021]

[Table 1]

Next, a spunbonded non-woven fabric (commercial name "Elves", manufactured by Unitika) formed of these PTFE porous membranes and a composite fiber having a core / sheath structure of polyester / polyethylene is used for thermal lamination of FIG. Such an air filter medium was prepared. In FIG. 1, 1 is Table 1
No. No. 1 in Table 1 is a porous PTFE membrane. The PTFE porous membrane of No. 2 and No. 3 of Table 1. No. 3 PTFE porous membrane. Further, 4 is a unit weight: 40 g / mm ² of the spunbonded nonwoven fabric, 5 is a basis weight: 15 g / mm ² of the spunbonded nonwoven fabric, 6 is a basis weight: 15 g / mm ² of the spunbonded nonwoven fabric, and 7 is a basis weight: 20 g / mm. ^{2 is} the spunbonded nonwoven fabric.

The characteristics of the obtained air filter medium are as follows:
It is summarized in Table 3 below.

[0024]

[Comparative Example 1] A porous membrane (a transverse stretching ratio of 40 times) composed of an example of International Publication No. WO94 / 16802 was prepared. Table 2 shows the pressure loss and the collection efficiency of this porous membrane.

[0025]

[Table 2]

An air filter medium as shown in FIG. 2 was prepared by thermal lamination using a spunbonded non-woven fabric (made by Elves Unitika) made of this PTFE porous membrane and a composite fiber having a core / sheath structure of polyester / polyethylene. In addition, the total weighing of the used nonwoven fabric was the same as that of the example. In FIG. 2, 11 is a PTFE porous membrane, 12 is a basis weight: 7
0 g / mm ² of the spunbonded non-woven fabric, 13 is a basis weight: 20
g / mm ² of the spunbond nonwoven fabric.

The air filter media of Example 1 and Comparative Example 1 were tested for pressure loss, pressure loss variation, collection efficiency, collection efficiency variation, leak performance, and pressure loss increase. Table 3 shows the results.

[0028]

[Table 3]

As is clear from Table 3, it was confirmed that the filter medium of Example 1 was excellent in the variation in pressure loss, the variation in collection efficiency, the leak performance, and the increase in pressure loss.

[0030]

As described above, according to the present invention, PT
By stacking two or more layers of FE porous membrane,
It is possible to provide a filter medium for an air filter, in which variations in pressure loss are alleviated and pinholes penetrating are eliminated as compared with the case of only layers, and which is leakless. In addition, the absolute porosity increases and the life characteristics become excellent.

In the above, according to the preferable constitution in which the reinforcing material is further interposed between the laminated porous PTFE membranes, the folding workability and the strength are improved. Further, in the above, when the reinforcing material is a fibrous nonwoven fabric, it does not shrink during lamination, and a laminated film with such a reinforcing material,
It has the advantages that it can be easily processed as a HEPA filter or ULPA filter, and that the folding pitch can be increased when forming a filter element.

Further, in the above-mentioned constitution, when the fiber constituting the non-woven fabric is a composite fiber having a core-sheath structure and the core component is a synthetic fiber having a relatively higher melting point than that of the sheath component, further shrinkage does not occur during lamination, In addition, HEPA filter and UL
It has the advantage that it can be processed more easily as a PA filter and the folding pitch can be increased when it is used as a filter element.

Further, in the above description, the pressure loss when permeating air at a flow rate of 5.3 cm / sec is 65 mmH ₂ O.
HEPA and ULP in clean rooms, etc.
It is more convenient as an A filter.

In the above, when a fluid is flown,
The pressure loss of the upstream PTFE porous membrane is the downstream PTFE
If it is lower than the pressure loss of the E porous membrane, the filter life can be further extended.

[Brief description of drawings]

FIG. 1 is a conceptual cross-sectional view of an air filter medium according to a first embodiment of the present invention.

FIG. 2 is a conceptual cross-sectional view of an air filter medium of Comparative Example 1.

[Explanation of symbols]

1,2,3 PTFE porous membrane 4,5,6,7 Spunbonded nonwoven fabric using core-sheath composite fiber 11 PTFE porous membrane 12,13 Spunbond nonwoven fabric using core-sheath composite fiber

Claims (6)

[Claims] 1. A filter material for an air filter, which comprises a stack of two or more layers of polytetrafluoroethylene porous membrane. 2. The filter material for an air filter according to claim 1, wherein a reinforcing material is further interposed between the laminated polytetrafluoroethylene porous membranes. 3. The filter material for an air filter according to claim 2, wherein the reinforcing material is a non-woven fabric. 4. The filter material for an air filter according to claim 3, wherein the fibers constituting the non-woven fabric are composite fibers having a core-sheath structure, and the core component is a synthetic fiber having a relatively higher melting point than the sheath component. 5. The filter medium for an air filter according to claim 1, wherein the pressure loss when allowing air to permeate at a flow rate of 5.3 cm / sec is 65 mmH ₂ O or less. 6. The air filter according to claim 1, wherein the pressure loss of the polytetrafluoroethylene porous membrane on the upstream side is lower than the pressure loss of the polytetrafluoroethylene porous membrane on the downstream side when flowing the fluid. Filter media.