JPH10328539A - Polytetrafluoroethylene porous membrane and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a porous membrane capable of removing a fine particle with a high probability by adding a lubricant into a polytetrafluoroethylene powder to mix, stretching a sheet like material obtained by an extrusion method or a press-rolling method to a specific magnification in the uniaxial direction and using >=2 pieces of sheets formed by sintering the sheet like material at >=the m.p. SOLUTION: At the time of manufacturing a precise filtration filter composed of the polytetrafluoroethylene (hereafter called PTFE) excellent in heat resistance and chemical resistance and particularly useful as a liquid chemical filter for semiconductor, the lubricant is added into the PTFE fine powder obtained by emulsion polymerization. And the sheet like material obtained by the extrusion method or the press rolling method of uniaxially stretched to 200 times in area magnification and >=2 pieces of the sheet formed by sintering the sheet like material at >= the m.p. are laminated to produce the membrane. As the PTFE porous membrane, one satisfying an equation, (X)>=0.22×LN(Z)+1.3 in the balance of the stop-off ratio (X) to a 0.102 μm polystyrene particle with ethanol FLUX (Z) is used.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a polytetrafluoroethylene (hereinafter referred to as "PTFE") porous membrane and a method for producing the same.

[0002]

2. Description of the Related Art Polytetrafluoroethylene (hereinafter, referred to as polytetrafluoroethylene)
It is called "PTFE". The microfiltration filter comprising a porous membrane is excellent in heat resistance and chemical resistance, and is particularly useful as a chemical liquid filter for semiconductors. In recent years, in such a semiconductor manufacturing industry, high integration has progressed, and particles to be removed from a chemical liquid filter have been miniaturized due to miniaturization of the minimum pattern size of a device. For this reason, a filter that can remove more fine particles than ever has been desired. On the other hand, the permeation performance of the filter is related to the operating cost, and a filter having excellent permeability is desired from the viewpoint of energy saving. Therefore, a filter that can remove more fine particles and has excellent permeability is desired.

[0003] Generally, the particle removal performance of a PTFE porous membrane used as a microfiltration filter is such that a liquid in which polystyrene particles are dispersed in water is filtered and the upstream side (undiluted solution) is used.
Calculated from the concentration on the downstream side (filtrate) and expressed as a particle removal rate. The particle removal performance is determined substantially only by the pore diameter if the material is the same among the filter media. That is, as the pore size increases, the removal performance decreases (particle removal rate decreases), and as the pore size of the film decreases, the removal performance increases (particle removal rate increases).

[0004] The transmission performance, which is another important performance, is as follows.
The liquid (generally alcohols, acetone, etc. is used) at a constant differential pressure is tested at the permeation flow rate per hour. The permeation performance is determined by the pore diameter, the porosity, and the film thickness, if the materials are the same among the physical properties of the filter medium. In other words, the larger the pore size, the higher the porosity, and the thinner the film thickness, the higher the permeation performance (the larger the permeation flow rate). Conversely, the smaller the pore size, the lower the porosity, and the thicker the film thickness, the more the permeation performance Is low (the permeation flow rate is low).

Here, the influence of the physical properties (pore diameter, porosity, film thickness) of the filter medium on both its particle removal performance and fluid permeation performance will be considered. First, as the pore size increases, the permeation performance increases, but the particle removal performance decreases. Conversely, if the pore size is smaller, the particle removal performance is higher, but the permeation performance is lower. Next, the higher the porosity, the higher the permeation performance almost independently of the particle removal performance, and the lower the porosity, the lower the permeation performance almost independently of the particle removal performance. Further, as the film thickness decreases, the permeation performance increases substantially independently of the particle removal performance, and as the film thickness increases, the permeation performance decreases substantially regardless of the particle removal performance.

[0006] As described above, the particle removal performance and the permeation performance are contradictory properties. That is, if the removal performance of the particles is to be increased, the permeation performance tends to be low, and if the permeation performance is to be increased, the removal performance of the particles tends to be low.

In general, as shown in FIG. 1, the horizontal axis represents the flow rate, and the vertical axis represents the removal rate (rejection rate of polystyrene particles (PSL)). It can be said that the closer to (the direction of the arrow), the better the filter medium. Therefore, how to control the balance between the particle removal performance and the permeation performance, and how to improve both these performances, have become major issues.

[0008]

A method for producing a conventionally known porous PTFE membrane (Japanese Patent Publication No. Sho 42-13560)
In the publication, when the porosity and the pore diameter are changed, the stretching ratio is used. In order to increase the porosity, the stretching ratio must be increased, and in order to reduce the pore size, the stretching ratio must be decreased.Therefore, when attempting to realize a filter having a small pore size and a high porosity, the means are contradictory. I will. Therefore, it was difficult to change the performance of the filter medium in the direction of the arrow in FIG. 1, and the particle removal performance and the permeation performance could not be simultaneously improved.

[0009] At least two layers of a porous body of ethylene tetrafluoride resin, capable of removing 90% or more of particles of 0.109 µm, and having a flow rate (IPA flow rate) measured with isopropyl alcohol of 0.7 ml / cm. ^A porous body of ethylene tetrafluoride resin characterized by being at least ² min has been proposed (Japanese Patent Application Laid-Open No. 8-174736).
The filter performance was not sufficient.

Further, in Japanese Patent Application Laid-Open No. Hei 8-174736, a method in which a sheet obtained by stretching a paste extruded body of ethylene tetrafluoride fine powder in at least a uniaxial direction is interlayer-bonded by pressure, or thereafter, a method in which the temperature is less than the melting point. Although a method of performing heat treatment has been proposed, heat treatment at a temperature equal to or lower than the melting point has a problem of being easily deformed due to low dimensional stability and low film strength. Therefore, there is a problem in the handleability of the film and the change in the characteristics of the film when the film is used for a long time under pressure.

According to the present invention, in order to solve the above-mentioned conventional problems, it is possible to remove extremely fine particles with a high probability, and to secure a flow rate as high as possible.
An object of the present invention is to provide a porous membrane, whereby a polytetrafluoroethylene porous membrane capable of efficiently performing liquid filtration with higher precision in the semiconductor field and a method for producing the same are provided.

[0012]

Means for Solving the Problems To solve the above problems, a method for producing a porous PTFE membrane of the present invention comprises adding a lubricant to polytetrafluoroethylene fine powder obtained by emulsion polymerization, mixing, extruding, Sheet obtained by at least one method selected from the following methods and rolling methods, stretched at least 200 times in area ratio in at least one axial direction, and two or more sheets sintered at the melting point temperature or higher are laminated or laminated. It is characterized by matching. In the above description, the temperature of the melting point or higher refers to a temperature of 327 ° C. or higher. At a temperature of 400 ° C. or higher, the rate of thermal decomposition of PTFE rapidly increases, which is not preferable. However, if heat treatment is performed for a short time so as not to increase thermal decomposition, a high temperature of 400 ° C. or higher can be used.

Further, the porous PTFE membrane of the present invention has a capacity of 0.1%.
The balance between the rejection (X) of 102 μm polystyrene particles and ethanol FLUX (Z) satisfies the following formula (I). (X)> − 0.22 × LN (Z) +1.3 (I)

Further, the PTFE porous membrane of the present invention comprises:
The balance between the rejection (Y) of 0.055 μm polystyrene particles and ethanol FLUX (Z) satisfies the following expression (II). (Y)> − 0.22 × LN (Z) +0.5 (II)

As described above, if the pore size is reduced, the particle removal performance is increased (the particle removal rate is increased). If the pore size is increased, the porosity is increased, and the film thickness is reduced, the transmission performance is improved. Higher (permeation flow rate increases). Therefore, in order to increase both the particle removal performance and the permeation performance as much as possible, it is conceivable to reduce the pore diameter, increase the porosity, and reduce the film thickness. Here, analysis of each of the pore diameter, porosity, and film thickness is as follows.

First, the smaller the pore size, the higher the particle removal performance, but the lower the permeation performance. On the other hand, if the diameter of the target particles to be removed is determined, the pore diameter at which the particles can be removed by 100% is also determined. Therefore, even if the pore diameter is further reduced, the particle removal rate does not increase, and only the permeability is reduced. Therefore, the pore size must be reduced until the target removal performance is reached, but it is better to keep it to the minimum necessary in consideration of the permeation performance.

Next, when the porosity is increased, the permeation performance is increased almost independently of the particle removal performance. However, if the porosity is too high, there is a concern that problems may occur in the film strength, handleability, and the like.

Further, when the film thickness is reduced, the permeation performance increases almost independently of the particle removal performance. However, if the film thickness is too thin, problems arise in workability, film strength, handleability, and the like as a filter medium.

Therefore, optimal control of the pore diameter, porosity, and film thickness is the key to solving the problem. That is, in order to realize a filter medium having a high particle removal performance and a high fluid permeation performance, it is basically considered that the thin film should have a high porosity and a small pore size. Reached.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a method for producing a porous PTFE membrane of the present invention will be specifically described. First, a lubricant is added to and mixed with the PTFE fine powder obtained by emulsion polymerization, and an unsintered sheet is produced by at least one method selected from an extrusion method and a rolling method.

Here, PTFE obtained by emulsion polymerization
Examples of the fine powder include “Polyflon F104” (trade name) manufactured by Daikin Industries, Ltd., “CD1”, “CD4”, and “CD123” (trade name) manufactured by Asahi ICI Fluoropolymers.

Examples of the lubricant include hydrocarbons such as liquid paraffin, naphtha, white gasoline, toluene, xylene and the like, alcohols, ketones, esters, and mixtures of two or more kinds selected from these. Can be The amount of the lubricant to be added is preferably 20 to 40 parts by weight based on 100 parts by weight of the PTFE fine powder.

Next, the obtained sheet is subjected to an area magnification of at least one axis at a temperature lower than the melting point of PTFE by a factor of 20.
It is stretched 0 times or more and sintered at the melting point temperature or more. The stretching ratio is approximately 50 in area ratio in order to obtain a high porosity.
It is preferably at least 0 times. In this way, a thin PTFE porous material having sufficient film strength, dimensional stability, and high porosity is obtained. Although such a membrane has excellent fluid permeability, the pore size is often about 0.5 μm or more in many cases because the stretching ratio is large, and sufficient particle removal as a filter medium for a microfiltration filter is performed. Performance cannot be obtained.

Therefore, finally, two or more PTFE porous membranes are laminated or bonded. As a result, as shown in FIG. 2, the pore diameter of the porous PTFE membrane can be reduced in a manner inversely proportional to the number of laminated layers. Regarding the porous membrane to be laminated, the pore diameter can be reduced in both cases where films having similar characteristics (mainly, pore size) are laminated and films having different characteristics are laminated. Therefore, the pore size can be controlled by stacking two or more such PTFE porous membranes,
Along with that, the particle removal performance can also be controlled. Therefore, by such a method, the particle removal performance can be enhanced without lowering the fluid permeation performance as much as possible.

When the number of layers is increased, the particle removing performance is increased, but the pore diameter is reduced and the thickness is increased, so that the fluid permeability is reduced. Therefore, it is preferable to determine the number of laminated layers depending on how much importance is given to the fluid permeation performance and the particle rejection ratio while considering this fact.

After lamination, it is not always necessary to perform pressure bonding or bonding. However, considering the workability, strength and handling properties of the film, pressure bonding or bonding is performed so as not to reduce the porosity as much as possible and not to block the holes. Is preferred.

The method of pressure bonding includes a calender roll, a laminating roll, a flat plate press, and the like, but is not particularly limited here. The temperature at the time of pressure bonding is not particularly limited. If the pressure at the time of press bonding is increased, the peel strength is increased and the pore diameter can be reduced, so that the particle removal performance is improved, but the porosity is reduced, and the fluid permeability is deteriorated.
Therefore, it is preferable to select an appropriate pressure in consideration of this. In the case of bonding, it is preferable to use a method such as spot bonding so as not to block the holes.

The PTFE porous membrane thus obtained can have a target pore diameter and a higher porosity than a single-layer PTFE porous membrane having the same pore diameter. . Such a laminated porous PTFE membrane has a high porosity and can be easily reduced in pore size. Therefore, P having both excellent particle rejection performance and transmission performance
In addition to being able to design and manufacture a TFE porous membrane, it is easy to control the balance between particle rejection performance and permeation performance.

Specifically, according to the production method of the present invention,
The balance between the rejection (X) of 0.102 μm polystyrene particles and ethanol FLUX (Z) satisfies the following formula (I), or the rejection (Y) of 0.055 μm polystyrene particles and ethanol FLUX (Z) ), A PTFE porous membrane that satisfies the following formula (II), that is, a PTFE porous membrane having a good balance between the particle rejection performance and the permeation performance can be obtained. (X)> − 0.22 × LN (Z) +1.3 (I) (Y)> − 0.22 × LN (Z) +0.5 (II)

Further, the laminated porous PTFE membrane of the present invention
Since it is fired at a temperature equal to or higher than the melting point, it is excellent in film strength, dimensional stability, high-temperature processability, and the like. Therefore, a filter having excellent permeability can be realized, which is useful in the field of a chemical solution filter used in the semiconductor manufacturing industry and the like.

[0031]

Next, embodiments of the present invention will be described, but the present invention is not limited to these embodiments.

[0032]

Example 1 An admixture containing 100 parts by weight of PTFE fine powder (CD-123, manufactured by Asahi ICI) and 30 parts by weight of liquid paraffin as a lubricant was preformed, extruded, rolled, and subjected to solvent The lubricant is removed through the layers to give a sheet of about 0.3 mm thickness. This is stretched 2,000% in the length direction in an atmosphere of 300 ° C.
Subsequently, it is stretched 3000% in the width direction in an atmosphere of 90 ° C.
A heat treatment at 350 ° C. is performed with the dimensions fixed. Eight PTFE porous sheets made in this way,
Crimping was performed at room temperature at 4.5 kg / cm ² . Using the sheet thus obtained, the PSL rejection (0.102 μ
m) and ethanol FLUX were measured and the results were as follows.

PSL rejection (0.102 μm): 100%, ethanol FLUX: 11.1 ml / cm ² · min PSL rejection (0.102 μm): 90%, ethanol FLUX: 14.8 ml / cm ² · min

The above values were calculated using the PSL rejection and ethanol FL.
Calculation using the expression (I) representing the balance with the UX satisfies the following expression and satisfies the expression (I). 1.0> −0.22 × LN (11.1) + 1.3 =
0.77 0.9> −0.22 × LN (14.8) + 1.3 =
0.71

[0035]

Example 2 A mixture of 100 parts by weight of PTFE fine powder (manufactured by Asahi ICI, CD-123) and 30 parts by weight of liquid paraffin as a lubricant was preformed, extruded, rolled, and subjected to a solvent treatment. The lubricant is removed through the layers to give a sheet of about 0.3 mm thickness. This is stretched 2,000% in the length direction in an atmosphere of 300 ° C.
Subsequently, it is stretched 3000% in the width direction in an atmosphere of 90 ° C.
A heat treatment at 350 ° C. is performed with the dimensions fixed. Eighty PTFE porous sheets thus produced were stacked and pressed at room temperature at 4.5 kg / cm ² . Using the sheet thus obtained, the PSL rejection (0.05
5 μm) and ethanol FLUX were measured.
L rejection (0.055 μm) is 96%, ethanol FL
UX was 0.3 ml / cm ² · min.

The above values were used as PSL rejection and ethanol FL.
Calculation using the expression (II) representing the balance with UX results in the following, which satisfies the expression (II). 0.961.0> −0.22 × LN (0.3) +0.5
= 0.76

[0037]

Comparative Example 1 A mixture of 100 parts by weight of PTFE fine powder (manufactured by Asahi ICI, CD-123) and 30 parts by weight of liquid paraffin as a lubricant was preformed and extruded into a sheet. After further rolling, the lubricant was removed by heating to produce a 0.3 mm thick dry sheet. Next, this sheet was stretched by 200% at 280 ° C. in the extrusion direction of the sheet, and then stretched in a direction perpendicular to the extrusion direction by 1500% at a stretching zone temperature of 70 ° C. and a heat setting zone temperature of 300 ° C. The two stretched sheets thus produced are stacked and rolled again at room temperature at a roll temperature of 60% of the original film thickness.
A laminate was obtained. The PSL rejection (0.102 μm and 0.055 μm) of this porous PTFE laminate and the measurement of ethanol FLUX were as follows.

PSL rejection (0.102 μm): 100%, ethanol FLUX: 0.6 ml / cm ² · min PSL rejection (0.055 μm): 35%, ethanol FLUX: 0.6 ml / cm ² · min

The above values were used as the PSL rejection and ethanol FL.
Calculations using formulas (I) and (II) representing the balance with UX result in the following, and neither formula (I) nor formula (II) is satisfied. 1.0> −0.22 × LN (0.6) + 1.3 =
1.41 0.35> −0.22 × LN (0.6) + 0.5 =
0.61

[0040]

Comparative Example 2 A mixture of 100 parts by weight of PTFE fine powder (F104, manufactured by Daikin Co.) and 30 parts by weight of liquid paraffin as a lubricant was preformed, extruded into a sheet, and further rolled. Thereafter, the lubricant was removed by heating to produce a dry sheet having a thickness of 0.3 mm. Next, this sheet was stretched at 280 ° C in the extrusion direction of the sheet by 250%, and then stretched in a direction perpendicular to the extrusion direction at a stretching zone temperature of 70 ° C and a heat setting zone temperature of 300 ° C by 1500%. The two stretched sheets thus produced are stacked and rolled again at room temperature at a roll temperature of 60% of the original film thickness.
A laminate was obtained. The PSL rejection (0.102 μm and 0.055 μm) of this porous PTFE laminate and the measurement of ethanol FLUX were as follows.

PSL rejection (0.102 μm): 90%, ethanol FLUX: 2.0 ml / cm ² · min PSL rejection (0.055 μm): 10%, ethanol FLUX: 2.0 ml / cm ² · min

The above values were used as PSL rejection and ethanol FL.
Calculations using formulas (I) and (II) representing the balance with UX result in the following, and neither formula (I) nor formula (II) is satisfied. 0.9> −0.22 × LN (2.0) + 1.3 =
1.15 0.1> −0.22 × LN (2.0) + 0.5 =
0.35

[0043]

Comparative Example 3 An admixture of 100 parts by weight of PTFE fine powder (manufactured by Asahi ICI, CD-123) and 30 parts by weight of liquid paraffin as a lubricant was preformed and extruded into a sheet. This was further rolled, and then the lubricant was removed by heating to produce a 0.3 mm thick dry sheet. The sheet is then stretched 200% at 280 ° C. in the sheet extrusion direction,
The film was stretched 1500% in a direction perpendicular to the extrusion direction at a stretching zone temperature of 70 ° C and a heat setting zone temperature of 300 ° C. Two sheets of the stretched sheet thus produced are stacked and rolled again at room temperature at a roll temperature of 60% of the original film thickness.
It heat-processed at ℃, and obtained the porous PTFE laminated body. The PSL rejection (0.102 μm and 0.055 μm) of this porous PTFE laminate and the measurement of ethanol FLUX were as follows.

PSL rejection (0.102 μm): 100%, ethanol FLUX: 0.9 ml / cm ² · min PSL rejection (0.055 μm): 30%, ethanol FLUX: 0.9 ml / cm ² · min

The above values were used as the PSL rejection and ethanol FL.
Calculations using formulas (I) and (II) representing the balance with UX result in the following, and neither formula (I) nor formula (II) is satisfied. 1.0> −0.22 × LN (0.9) + 1.3 =
1.32 0.3> −0.22 × LN (0.9) + 0.5 =
0.52

Examples 1 and 2 of the present invention and Comparative Example 1
PSL rejection of the porous PTFE laminates (0.10 to 0.10)
2 and 0.055 μm) and the values calculated by the formulas (I) and (II), which represent the balance between ethanol and FLUX, are summarized in FIG. 3 for the formula (I) and FIG. 4 for the formula (II). It became as shown in.

[0047]

According to the production method of the present invention, it is possible to obtain a PTFE porous membrane capable of removing extremely minute particles with a high probability and securing a flow rate as high as possible. Thereby, more accurate liquid filtration can be efficiently performed in the semiconductor field.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between particle removal performance and permeation performance in a PTFE porous membrane.

FIG. 2 is a graph showing the relationship between the pore size and the number of stacked layers in a PTFE porous membrane.

FIG. 3 is a graph showing the balance between PSL rejection (0.102 μm and 0.055 μm) and ethanol FLUX of the porous PTFE laminates of Examples 1 to 2 and Comparative Examples 1 to 3 of the present invention. A graph summarizing the values calculated.

FIG. 4 is a graph showing the balance between the PSL rejection (0.102 μm and 0.055 μm) and the ethanol FLUX of the porous PTFE laminates of Examples 1 to 2 and Comparative Examples 1 to 3 of the present invention. A graph summarizing the values calculated.

Claims (3)

[Claims] 1. A polytetrafluoroethylene fine powder obtained by emulsion polymerization, a lubricant is added and mixed, and a sheet-like material obtained by at least one method selected from an extrusion method and a rolling method is subjected to at least one axial direction. 200 in area magnification
A method for producing a porous polytetrafluoroethylene membrane, comprising laminating or laminating two or more sheets stretched twice or more and sintered at a melting point or higher. 2. A polytetrafluoroethylene porous membrane in which the balance between the rejection (X) of 0.102 μm polystyrene particles and ethanol FLUX (Z) satisfies the following formula (I). (X)> − 0.22 × LN (Z) +1.3 (I) 3. A polytetrafluoroethylene porous membrane wherein the balance between the rejection (Y) of 0.055 μm polystyrene particles and ethanol FLUX (Z) satisfies the following formula (II). (Y)> − 0.22 × LN (Z) +0.5 (II)