JP3302606B2 - Polytetrafluoroethylene porous membrane and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a porous polytetrafluoroethylene (hereinafter referred to as "PTFE") membrane and a method for producing the same.
The E porous membrane has excellent performance as a filter medium for an air filter.

[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 used in many cases. However, such filter media have several problems. For example, the pressure loss increases when the binder is increased in order to prevent the presence of attached fibrils in the filter medium or to prevent self-dusting during bending due to processing (see JP-A-63-16019). 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, in recent years, PTFE porous membranes have been used in various fields as filter media for filters. In particular, it is useful as a filter medium for an air filter used in the field of semiconductor manufacturing or the like that requires a severe clean environment.

[0005] Such a useful porous PTFE membrane is prepared, for example, by preparing a sheet-like semi-sintered PTFE (Japanese Patent Application Laid-Open No. S59-5959).
JP-A-152825), which can be manufactured by biaxially stretching it to make it porous.
1541 or JP-A-5-202217). However, this manufacturing method was very difficult to realize a "semi-baked state" which was convenient for subsequent stretching. For this reason, the useful porous PTFE membrane cannot be efficiently produced. Therefore, various proposals have been made to apply a porous PTFE membrane obtained by stretching an unsintered PTFE sheet-like molded product to a filter medium of a high-performance air filter (International Publication No. WO 94/16802,
JP-A-7-196831). However, since this film is in an unfired state, its strength is low. Therefore, there are problems that durability is poor, performance deteriorates with time, dimensional stability is poor, and it is difficult to handle during processing.

In general, in order to solve the above-mentioned problem, a stretched unfired PTFE porous film is heat-treated at a melting point or higher using an oven or the like, and fired to obtain a strong PTFE porous film. There is a manufacturing method.

[0007]

However, although the strength of the PTFE porous membrane is improved by performing the heat treatment after the stretching for a long time and firing, the collection efficiency and the pressure loss, which are the most important characteristics of the filter medium, are reduced. PF (P
It has been confirmed that the performance of filter (E.f. The PF value is given by the following equation (1). A filter medium having a larger PF value has a higher dust collection efficiency and has a lower pressure loss. Note that the transmittance in the equation is obtained from the relationship of transmittance = (100−collection efficiency). PF value = {− Log (transmittance / 100) / pressure loss} × 100 (1)

[0008] Air filter media used in the field of semiconductor manufacturing and the like are required to have a low pressure loss and a high PF value in order to reduce operating costs. When the loss is less than 20 mmH ₂ O, PF
It was difficult to obtain a PTFE porous membrane having a value of 24 or more.

[0009]

Means for Solving the Problems In order to solve the above problems, a method for producing a porous PTFE membrane according to the present invention comprises:
A liquid lubricant is added to and mixed with the fine powder, and the mixture is formed into a sheet in an unfired state by at least one of an extrusion method and a rolling method, and the sheet-like formed body is stretched in at least one axial direction or more. The unfired PTFE porous membrane obtained in this way is heated at 350 to 450 ° C. for 0.01
It is characterized in that heat treatment is performed for up to 1.00 seconds. In the present invention, it has been found that by such a production method, the strength can be increased and the pressure loss can be reduced without lowering the PF value as compared with before the heat treatment, and the above object has been achieved.

Further, the porous PTFE membrane of the present invention has an endothermic peak in a temperature range of 330 ± 8 ° C. on a crystal melting curve by a differential calorimeter, and has a crystal conversion of 0.35 to 0.35.
0.80, pressure loss less than 20 mmH ₂ O, PF
The value is 24 or more. Such a PTFE porous membrane of the present invention is a membrane useful as a filter medium for an air filter.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be specifically described below. First, PTFE fine powder and a liquid lubricant are mixed, and the mixture is formed into a sheet in an unfired state by at least one of an extrusion method and a rolling method, to obtain a sheet-shaped formed body. As the PTFE fine powder, PTFE fine powder can be preferably used. The PTFE fine powder is not particularly limited, and a commercially available PTFE fine powder can be used. For example, polyflon F-104 (manufactured by Daikin Industries, Ltd.), Fluon CD-123 (manufactured by Asahi ICI Fluoropolymers), Teflon 6J (manufactured by Du Pont-Mitsui Fluorochemicals) and the like can be mentioned.

[0012] As a liquid lubricant, it can wet the surface of PTFE fine powder, and after obtaining the sheet-like molded body,
There is no particular limitation as long as it can be removed by a technique such as evaporation or extraction. For example, liquid paraffin,
In addition to hydrocarbon oils such as naphtha, white oil, toluene, and xylene, alcohols, ketones, esters, and mixtures of two or more of these are included.

The amount of the liquid lubricant to be added to the PTFE fine powder is adjusted by the type of the PTFE fine powder and the liquid lubricant, and the molding method for obtaining a sheet-like molded product.
It is about 5 to 50 parts by weight based on 100 parts by weight of the PTFE fine powder.

When a mixture of the PTFE fine powder and the liquid lubricant is formed into a sheet by at least one of an extrusion method and a rolling method, a specific example of the forming method is a mixture of the PTFE fine powder and the liquid lubricant. After extruding into a rod shape, a method of forming a rolled sheet from a pair of rolls, a method of extruding a plate and forming a sheet, and the like can be given. The thickness of the sheet-like molded body thus obtained is
Usually, it is set to 0.05 to 0.5 mm.

Next, the liquid lubricant contained in the sheet-shaped molded product is removed. The removal of the liquid lubricant is performed by a heating method, an extraction method, or a combination thereof.

Further, the sheet-like molded body from which the liquid lubricant has been removed is stretched in at least one axial direction to be porous. The stretching temperature at the time of stretching is preferably 327 ° C. or lower.

The PTFE porous membrane obtained as described above is in an unfired state and has a low strength. This heat treatment is performed with the dimensions fixed by, for example, grasping both ends of the PTFE porous membrane with a chuck or the like. The heat treatment is usually P
Perform at a temperature equal to or higher than the melting point of the TFE fired body,
The firing time is preferably 0.01 to 1.
It is preferably 00 seconds. The reason is that if the temperature is low and the firing time is short, the PTFE porous membrane will have uneven firing and the performance will vary, while if the temperature is high and the firing time is long, the PF value which is the most important property of the filter medium will be low. This is because it is significantly reduced. The heating method used at this time is not particularly limited as long as it can heat-treat the PTFE porous film, such as a method of blowing hot air, a method using an infrared heater, and a method of passing through a high-temperature furnace. In order to heat-treat in a short time, it is particularly preferable to use a method of blowing hot air.

By the above heat treatment, an endothermic peak is present in a temperature range of 330 ± 8 ° C. on a crystal melting curve by a differential calorimeter, the crystal conversion is 0.35 to 0.80, and the pressure loss is 20 mmH. _A PTFE porous membrane having less than ₂ O and a PF value of 24 or more is obtained. Such a PTFE porous membrane has excellent durability and dimensional stability. This PT
Although the FE porous membrane can be used as it is, it may be laminated with another reinforcing material to improve the folding workability and the strength.

As the reinforcing material, non-woven fabric, woven fabric, mesh, and other porous membranes can be used. As the material of the reinforcing material, polyolefin (polyethylene, polypropylene, etc.), nylon, polyester, aramid, or a composite thereof (for example, non-woven fabric composed of core / sheath fibers, two layers of a low melting material and a high melting material) Non-woven fabric, etc.), and a fluorine-based porous film (for example, PFA (tetrafluoroethylene / perfluoroalkylvinyl ether copolymer), FEP (tetrafluoroethylene / hexafluoropropylene copolymer), PTFE porous film, etc.). . In particular, a nonwoven fabric made of a synthetic fiber having a core component and a melting point higher than that of the sheath component, or a two-layer nonwoven fabric of a low-melting material and a high-melting material is preferable. This is because a nonwoven fabric made of a core-sheath composite fiber or a two-layer nonwoven fabric made of a low-melting material and a high-melting material does not shrink during lamination, and a film obtained by laminating these materials is a HEPA filter or an ULPA filter. This is because it is easy to process as a filter element, and the folding pitch can be increased when forming a filter element.

[0020]

The present invention will be specifically described below with reference to examples. The measurement of the pressure loss, the collection efficiency, the PF value, the endothermic curve, the crystal conversion ratio, and the delamination strength in the examples was performed by the following methods.

(Pressure loss) A sample (porous PTFE membrane, filter medium) was set in a circular holder having an effective area of 100 cm ² , a pressure difference was applied between the inlet side and the outlet side, and the permeation flow rate of the sample was measured by a calorimeter. The pressure loss at 0.3 cm / sec was measured with a pressure gauge.

(Collection efficiency) Using the same apparatus as that for measuring the pressure loss, setting the permeation flow rate of the sample (porous PTFE membrane, filter medium) to 5.3 cm / sec, and setting polydisperse dioctyl phthalate (DOP) ) Is flowed so that particles of 0.1 to 0.15 μm are about 10 ⁷ particles / liter, and the sample is passed. The concentration of the downstream side of the sample is measured by a laser particle counter (LPC). The collection efficiency (%) was determined in 2). Collection efficiency (%) = {1− (downstream concentration / upstream concentration)} × 100 (2) (However, the target particles are in the range of 0.1 to 0.15 μm.)

(PF value) The collection efficiency and pressure loss were calculated by applying the above formula (1).

(Endothermic peak, crystal conversion ratio)
Similarly to the publication No. 152825, a differential scanning calorimeter (DS
C) It is defined by the peak temperature measured and the heat of crystal fusion. These endothermic peaks and crystallization conversion rates are described in
It measures as follows similarly to 9-152825.

First, from the PTFE porous membrane of the present invention, 1
0.0 ± 0.1 mg is weighed, cut out, and used as a sample.
Since the heat denaturation of the PTFE proceeds from the surface of the sheet to the inside, when the sample is collected, the denaturation degree in the sheet thickness direction should be included in average. In the same manner as above, 10.0 ± 0.1 mg of a sample of the unfired PTFE porous membrane before the heat treatment is prepared. Then, using these samples, a crystal melting curve is obtained as follows.

That is, the sample of the unfired PTFE porous membrane was charged into a DSC aluminum pan, and the heat of fusion of the unfired PTFE porous membrane and the fired PTFE porous membrane were measured.
The heat of fusion of the FE porous membrane is calculated by the following steps (1) to (3).
Measure according to the following procedure.

(1) The sample was heated at a heating rate of 160 ° C./min for 2 hours.
Heat to 77 ° C, then 277 at a heating rate of 10 ° C / min.
Heat from ° C to 360 ° C. The position of the endothermic peak appearing in the crystal melting curve recorded in this heating step is represented by “P
It is defined as "melting point of unfired TFE" or "melting point of PTFE fine powder".

(2) Immediately after heating to 360 ° C.,
Cool to 277 ° C at a cooling rate of 0 ° C / min.

(3) The sample is heated again to 360 ° C. at a heating rate of 10 ° C./min. The position of the endothermic peak appearing in the crystal melting curve recorded in this heating step is referred to as “PTFE”.
The melting point of the fired body is defined.

The heat of fusion of the unfired PTFE body and the fired PTFE body is proportional to the area between the endothermic curve and the baseline. The base line is a straight line drawn from the point of 307 ° C. on the DSC chart so as to be in contact with the base at the right end of the endothermic curve.

Next, the crystal melting curve of the PTFE porous membrane of the present invention is recorded in accordance with the above step (1).

Then, the crystal conversion rate is calculated by the following equation (3). Crystal conversion rate = (S ₁ −S ₃ ) / (S ₁ −S ₂ ) (3) In the above equation (3), S ₁ is the area of the endothermic curve of the unfired PTFE, and S ₂ is the endothermic curve of the fired PTFE. The area of the curve, S _3, is the area of the endothermic curve of the PTFE porous membrane of the present invention.

(Delamination strength) A PTFE porous film cut to a width of 25 mm with a double-sided tape (No. 500, manufactured by Nitto Denko Corporation) was lightly bonded to a SUS plate, and a tensile compression tester (manufactured by Imada Seisakusho) was used. The PTFE porous membrane was delaminated at a rate of 50 mm / min. Five points were taken from the maximum value in the chart at this time, and the average value was taken as the delamination strength of the PTFE porous film.

Example 1 30 parts by weight of a liquid lubricant (liquid paraffin) was uniformly mixed with 100 parts by weight of PTFE fine powder (Fluon CD-123, manufactured by Asahi ICI Fluoropolymers Co., Ltd.), and the mixture was mixed at 20 kg / cm. It is preformed under the conditions of ² and then extruded into a rod, and the rod is passed between a pair of metal rolling rolls to form a long sheet-like molded body having a thickness of 0.2 mm. Obtained. Next, the liquid lubricant was removed from this sheet-like molded body by an extraction method using trichloroethylene (trichlene S, manufactured by Toagosei Co., Ltd.), and then wound around a tubular core in a roll.

This sheet-like molded body was stretched 20 times in the longitudinal direction at 320 ° C. by a roll stretching method. next,
The above-mentioned sheet-like PTFE molded body is stretched in the width direction at 100 ° C. by 20 times using a tenter, and the unfired PTFE is formed.
A porous membrane was obtained (No. 1). This unfired PTF
The endothermic peak, crystal conversion rate, pressure loss, collection efficiency, and PF value of the E porous membrane were examined. Table 1 shows the results.

[0036]

[Table 1]

Example 2 The unfired PTFE porous membrane obtained in Example 1 (No.
The four sides of 1) are fixed with a metal frame, and heat treatment is performed by changing the temperature using the hot air sintering apparatus shown in FIG.
TFE porous membranes were prepared (Nos. 2 to 8). In this heat treatment, the PTFE porous membrane was passed through the hot air outlet having a width of 5 mm at 4.2 m / min, so that the heat treatment time of the porous PTFE membrane was 0.07 seconds. The endothermic peak, crystal conversion ratio, pressure loss, collection efficiency, and PF value of the PTFE porous film after the heat treatment were examined. Table 2 shows the results.

[0038]

[Table 2]

From the results, it can be seen that The PTFE porous membrane of No. 2 has an endothermic peak at 344.1 ° C. and is in an unfired state.
No. 3 has an endothermic peak of 337.
It exists at two points of 8 ° C. and 343.2 ° C., in which the unfired part and the baked part are mixed, and it is understood that heat treatment at a temperature higher than 345 ° C. is required to stabilize the performance. . In addition, No. 8 is P
When the F value is No. It is lower than 1. No. 4-7 P
The TFE film is obtained by the production method of the present invention, and is heat-treated without lowering the PF value.
It has an endothermic peak in a temperature range of 330 ± 8 ° C., a crystal conversion ratio of 0.35 to 0.80, and a pressure loss of 20 m.
mH less than ₂ O, PF value is understood to have specific properties of the present invention that are 24 or more.

Example 3 The unfired PTFE porous membrane obtained in Example 1 (No.
The four sides of 1) were fixed with a metal frame, and heat treatment was performed by changing the sintering time with the hot-air sintering apparatus in FIG. 1 to produce four types of PTFE porous membranes (Nos. 9 to 12). The temperature of this heat treatment was 350 ° C. The endothermic peak, crystal conversion ratio, pressure loss, collection efficiency, and PF value of the PTFE porous film after the heat treatment were examined. Table 3 shows the results.

[0041]

[Table 3]

From the results, it is found that No. 9-11 PTFE
The porous film is heat-treated without lowering the PF value, and has the specific physical properties of the present invention. No. 12 has a PF value of No. 1, which indicates that the firing time needs to be 1.00 seconds or less when heat treatment is performed at 350 ° C.

Example 4 The unfired PTFE porous membrane obtained in Example 1 (No.
The four sides of 1) were fixed with a metal frame, and heat treatment was performed by changing the firing time with the hot air firing apparatus of FIG. 1 to produce four types of porous PTFE membranes (Nos. 13 to 16).
The temperature of this heat treatment was 450 ° C. The endothermic peak, crystal conversion ratio, pressure loss, collection efficiency, and PF value of the PTFE porous film after the heat treatment were examined. Table 4 shows the results.

[0044]

[Table 4]

From these results, it was found that No. 1 obtained by the production method of the present invention. The PTFE porous membranes of Nos. 13 to 16 were heat-treated without lowering the PF value, and have the specific physical properties of the present invention.

Example 5 The unfired PTFE porous membrane obtained in Example 1 (No.
Table 5 shows the results of measuring the delamination strength of No. 1). 17
Shown in Further, N having the physical properties of the present invention obtained in Example 4
o. The results of measuring the delamination strength of No. 14 are shown in Table 5 as No. 14.
18.

[0047]

[Table 5]

From these results, it can be seen that by performing the production method of the present invention, a PTFE porous membrane having improved strength can be obtained as compared with an unfired PTFE porous membrane.

Example 6 The unfired PTFE porous membrane obtained in Example 1 (No.
The four sides of 1) were fixed with a metal frame, and the ambient temperature was 36
Heat treatment in a 5 ° C. oven for 10 seconds, PTF
An E porous membrane was prepared. The endothermic peak, crystal conversion ratio, pressure loss, collection efficiency, and PF value of the PTFE porous film after the heat treatment were examined. Table 6 shows the results.

[0050]

[Table 6]

From these results, it can be seen that when the heat treatment is performed for a long time, P
It can be seen that the F value decreases.

[0052]

As described above, after the PTFE molded article is stretched in at least one axial direction, it is heated at 350 to 450 ° C.
By performing the production method of the present invention, wherein the heat treatment is performed for 0.01 to 1.00 seconds at a temperature of 330 ± 8 ° C. on a crystal melting curve by a differential calorimeter, and crystal conversion is 0.35 to 0.80, the pressure loss is 20mmH less than ₂ O, PTFE porous film PF value has a particular physical property of the present invention which is a more than 24 can be obtained. Therefore, according to the present invention, a PTFE porous membrane having excellent performance as a filter medium for an air filter can be provided.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a configuration of a hot-air sintering apparatus used in Examples 2 to 4 of the present invention.

[Explanation of symbols]

 Reference Signs List 1 PTFE porous membrane 2 PTFE porous membrane fixing metal frame 3 Hot air generator 4 Wire 5 Winding reel 6 Nozzle

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification code FI C08L 27:18 C08L 27:18 (72) Inventor Osamu Tabei 1-2-1 Shimohozumi, Ibaraki-shi, Osaka Nitto Denko Corporation (56) References JP-A-60-104319 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C08J 9/00

Claims (2)

(57) [Claims] 1. The crystal melting curve of 33 by a differential calorimeter.
It has an endothermic peak in a temperature range of 0 ± 8 ° C., a crystal conversion ratio of 0.35 to 0.80, and a pressure loss of 20 mmH
_A polytetrafluoroethylene porous membrane characterized by having a PF value of less than ₂ O and 24 or more. 2. A liquid lubricant is added to and mixed with fine powder of polytetrafluoroethylene, and the mixture is formed into a sheet in an unfired state by at least one of an extrusion method and a rolling method. Is stretched in at least one axial direction, and then the unfired polytetrafluoroethylene porous membrane obtained by stretching the sheet-like molded body of polytetrafluoroethylene in at least one axial direction is heated at 350 to 450 ° C. A polytetrafluoroethylene porous membrane, wherein the heat treatment is performed for 0.01 to 1.00 seconds.