JPH06256549A - Polytetrafluoroethylene porous membrane and its production - Google Patents

Abstract

PURPOSE:To obtain a polytetrafluoroethylene porous membrane high in performances such as filtration, concentration and fractionation and large in the amount of a treated substance per unit time, when used as a filtration membrane, and to provide the method for producing the same. CONSTITUTION:A tetrafluoroethylene porous membrane is characterized in that the porous membrane comprises many knots and fibers combining the knots with each other and that the orientation direction of the fibers in the surface part of the membrane is different from the orientation direction of the fibers in its inner parts. The production of the porous membrane comprises forming the mixture of polytetrafluoroethylene powder having a crystallization heat of <=18J/g with a liquid lubricant into a film, monoaxially orienting the film, calcining the film and further orientating the film in a direction orthogonal to the former orientation direction.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polytetrafluoroethylene (hereinafter referred to as "PTFE") porous membrane having a novel microstructure and a method for producing the same.

[0002]

2. Description of the Related Art PTFE porous membranes are excellent in various properties such as heat resistance, chemical resistance, lubricity, low friction and electric insulation. For example, filtration membranes, diaphragms for batteries, and electric insulation materials. It is used in a wide range of fields such as artificial blood vessels.

As a method for producing a PTFE porous membrane, the stretching method described in Japanese Patent Publication No. 42-13560 and Japanese Patent Publication No. 51-18991 is known. In this stretching method, a mixture of PTFE powder and a liquid lubricant is formed into a film, and then this film is stretched to make it porous. After stretching, it is usually heated at a temperature equal to or higher than the melting point of PTFE. By doing so, the strength of the porous film is increased by firing.

As an improved method of the method described in the above publication, a mixture of PTFE powder and a liquid lubricant is molded, and then the molded product is stretched to make it porous, and then 327 ° C. in a free state of heat shrinkage. A method of heating and firing at the above temperature, and then stretching again (Japanese Patent Publication No. 52-26786),
A method of molding a mixture of a PTFE powder having a number average molecular weight of 10 million or more and a liquid lubricant, stretching the molded article, and then heating and firing at a temperature of 327 ° C. or more in a heat shrinkage-proof state (JP JP-A-60-104319) is also known. Both of these methods were developed in order to obtain a porous membrane with the former having about 1500 Å or less and the latter having particularly fine pores of about 0.2 μm or less.

The PTFE porous membrane obtained by these stretching methods has a large number of nodules (the nodules are oriented in the direction orthogonal to the stretching direction) as disclosed in, for example, Japanese Patent Publication No. 51-18991. It is known to have a microstructure composed of a large number of fibers which are oriented along the drawing direction and which connect the nodules together. The orientation direction of the fibers in these porous membranes is the same in the surface portion and inside.

[0006]

By the way, a PTFE porous membrane is used as a filtration membrane to separate and concentrate different components.
When fractionation is performed, how to increase the processing amount is an important issue from the viewpoint of cost.

It is considered that the throughput can be achieved by increasing the pore size of the filtration membrane or by greatly increasing the number of pores per unit area of the filtration membrane. However, the former method is inevitable to filter, concentrate, and deteriorate the fractionation function, and is difficult to adopt. Although the latter method is theoretically valid, a technique capable of industrially producing a filtration membrane having a significantly increased number of pores per unit area as compared with the known PTFE porous membrane has been developed. The current situation is that there are none.

[0008]

As a result of earnest research for solving the above problems of the prior art, the present inventor has found that the conventional P
According to the membrane having a microstructure different from that of the TFE porous membrane, the functions such as filtration, concentration, and fractionation are not deteriorated,
The present inventors have completed the present invention by finding that the throughput can be increased to a high level.

That is, the PTFE porous membrane according to the present invention is a PTFE porous membrane composed of nodules and fibers that connect the nodules, and the orientation direction of the fibers at the surface portion and the orientation direction of the fibers inside are different. It is a feature.

The microstructure of the surface and inside of the PTFE porous membrane according to the present invention is a scanning electron microscope (Scanni).
ng Electron Microscope (hereinafter referred to as "SEM") using a magnification of about 2000 to 300
It can be known by observing at 00 times. PTF
The surface of the E porous membrane is already well observed by SEM, and the surface of the PTFE porous membrane according to the present invention (this surface may be one surface of the porous membrane or the opposite surface thereof). The microstructure can be observed in the same manner as in the past, and no special operation is required. On the other hand, as for the internal microstructure, for example, a notch is previously made in the porous film, an adhesive tape is attached to the surface of the porous film, and then the tape is peeled off, and at the same time, the surface portion of the porous film is simultaneously removed. It can be known by exposing the inside by peeling and observing this. The thickness of the surface portion removed by peeling is usually about 3 to 30 μm.

FIG. 1 is an SEM photograph showing an example of the microstructure of the surface portion of the PTFE porous membrane according to the present invention, and FIG. 2 is an SEM photograph showing an example of the internal microstructure.
1 and 2, the PTFE porous membrane according to the present invention is composed of a large number of nodules on both the surface portion and the inside and a large number of fibers that connect adjacent or nearby nodules. It can be seen that the spaces between have microstructures that are micropores.

From the comparison between FIG. 1 and FIG. 2, it can be seen that the orientation direction of the fibers in the surface portion is different from the orientation direction of the fibers in the inside. In this example, the orientation direction of the fibers in the surface portion and the orientation direction of the fibers in the inside are substantially orthogonal to each other (the orientation directions of the fibers differ by about 90 °).
Thus, the greatest feature of the porous membrane according to the present invention is that the orientation direction of the fibers in the surface portion and inside is different,
Such a microstructure is a novel one which is completely different from the conventional PTFE porous membrane. It is also found that in the porous membrane shown in this example, the size of the nodule on the surface portion is larger than the size of the nodule inside.

The reason why the PTFE porous membrane having such a microstructure can increase the throughput by maintaining the functions of filtration, concentration, fractionation, etc. has not yet been elucidated. The effect was confirmed as shown in FIG.

Next, a method for producing the PTFE porous membrane according to the present invention will be described. This method has a heat of crystallization of 18 J / g
The following mixture of PTFE powder and liquid lubricant was formed into a film, and then this film was uniaxially stretched, and then the dimension in the stretching direction was regulated, or the film was shrunk to the extent that it did not cause total shrinkage. It is characterized in that it is heated to a temperature and fired, and then stretched in a direction orthogonal to the stretching direction.

The method according to the invention has a heat of crystallization of 18 J /
It is necessary to use less than g PTFE powder. If a PTFE powder having a heat of crystallization of more than 18 J / g is used, it is impossible to obtain a porous film in which the orientation directions of the fibers in the surface portion and the inside are different.

The heat of crystallization of the PTFE powder is measured by a differential scanning calorimeter (Differential Scanning Ca).
It is already known that it can be measured by a lorimeter (hereinafter referred to as "DSC"). Therefore, also in the present invention, the heat of crystallization of the PTFE powder can be measured by this method. For example, about 10 mg of PTFE powder is sampled and precisely weighed, and using this as a sample, an initial setting temperature of 200 ° C., a temperature rising rate of 10 ° C./min, a maximum attainable temperature of 380 ° C., and a temperature lowering rate of 1
Figure 3 under the temperature conditions of 0 ℃ / min and the final temperature drop of 200 ℃
The DSC curve as shown in is taken on a chart paper. And
The heat of crystallization (J / g) can be determined by calculating the peak area (J) from this DSC curve according to a conventional method and then dividing the peak area by the sample weight (g).

In the present invention, it is preferable to use secondary particles obtained from primary particles having a particle size of 0.25 μm or more as the PTFE powder. The PTFE powder formed by mixing with a liquid lubricant is also called "fine powder". This fine powder produces primary particles of PTFE by emulsion polymerization of tetrafluoroethylene in water, and then the primary particles are Secondary particles are formed by coagulation or high-speed stirring, then separated from water, and dried to produce. In the present invention, as described above, it is preferable to use, as the fine powder, primary particles having a particle size of 0.25 μm or more, which are made into secondary particles.

The particle size of the primary particles used to obtain the fine powder as the secondary particles can be determined, for example, by observing the secondary particles at a magnification of about 10,000 times with an SEM. The major axis and minor axis of the PTFE primary particles (the particles are spherical or ellipsoidal) are determined, and the total value of the major axis and the minor axis is divided by "2" to obtain the primary particle diameter.

In the method of the present invention, first, PTFE is used.
A mixture of powder and liquid lubricant is formed into a film.
The mixture is formed by extrusion and / or rolling, but other forming methods such as compression may be additionally performed. What is important in this step is to use the PTFE powder having a heat of crystallization of 18 J / g or less as described above, and other points may be the same as the conventional PTFE paste molding. .

Therefore, as the liquid lubricant, one which can wet the PTFE powder and can be removed by extraction or evaporation at an appropriate stage after molding can be used. Specific examples thereof include liquid paraffin, naphtha, white oil and the like. Examples thereof include hydrocarbon oils, aromatic hydrocarbons such as toluene and xylene, alcohols, ketones, esters, silicone oil, fluorochlorocarbon oil, water containing a surfactant, and the like.

At this time, the mixing ratio of the PTFE powder and the liquid lubricant may be changed depending on the presence or absence of other additives.
Usually, liquid lubricant 5 is added to 100 parts by weight of PTFE powder.
˜50 parts by weight.

In addition to the PTFE powder and the liquid lubricant,
Various additives such as pigments for coloring, carbon black, graphite, silica powder, glass powder, metal powder, metal oxide powder, metal sulfide for improving strength against compression, wear resistance, etc. It is also possible to mix powders and the like.

After the mixture is formed into a film in this manner, it is uniaxially stretched. The stretching is not limited as long as it is lower than the melting point of PTFE, but from the viewpoint of workability, it is preferably about 50 to 250 ° C. Since this molded product contains a liquid lubricant, prior to stretching, the extraction method,
The lubricant can be removed by a heating method or a combination thereof. Of course, when the stretching is performed under heating conditions, the liquid lubricant may be removed by evaporation during the stretching, or may be removed after the stretching.

The uniaxially stretching of the film-shaped molded product is preferably 30 to 300%, preferably 50 to 50%, in order to obtain a porous film having different fiber orientation directions at the surface portion and inside.
~ 250% has been found to be more preferred. This uniaxial stretching makes the film porous, but its microstructure is not unique at this stage. That is, this uniaxially stretched film is composed of a large number of nodules and a large number of fibers that connect these nodules (the voids between the fibers are micropores) like the conventional stretched PTFE porous membrane, and the fibers are stretched. The fibers are oriented along the same direction (oriented in the same direction as the drawing direction), and the orientation direction of this fiber is the same in the surface portion and the inside without any difference.

The film-like material thus made porous is then baked by heating. The heating temperature may be equal to or higher than the melting point of PTFE, but is preferably 340 to 410 ° C. in order to prevent alteration of the film-like material. The firing time may vary depending on the temperature, film thickness, heating method, etc., but is usually about 1 to 10 minutes.

The stretched film has a property that the dimension in the stretching direction shrinks to the dimension before stretching by heating, but this firing is regulated so as not to change the dimension in the stretching direction of the stretched film material, or Alternatively, it is performed while shrinking so as not to completely shrink (shrink to the size before stretching). The degree of shrinkage in the case of performing shrinkage may vary depending on the stretch ratio during uniaxial stretching, the target pore diameter of the porous membrane, and the like, but is preferably 60% or less. This shrinkage (%) is based on the following dimension ₁ based on the dimension L ₀ before firing (the dimension in the stretching direction during uniaxial stretching performed to make the film-like material porous) and the dimension L ₁ after firing. It is a calculated value.

[0027]

[Equation 1]

When firing is performed with the dimension in the stretching direction being restricted or not being completely shrunk as described above, the surface portion has a substantially non-porous structure in which both nodules and fibers disappear due to melting, while the inside has a fiber length. Although it tends to be slightly shorter and the pore size of the micropores tends to be smaller, it is preserved without changing the basic microstructure consisting of nodules and fibers connecting the nodules, and the fibers are oriented along the stretching direction. To be done. However, in the case where the shrinkage ratio exceeds 60% during firing, the nodules and fibers are drastically reduced or disappeared even in the interior, and even if stretching is performed in the next step, the orientation of the fibers in the surface portion and the interior is Since it is difficult to obtain different porous membranes, it is necessary to pay attention to this point.

It can be confirmed by observing the surface of the film with an SEM that the surface of the fired film-like material has a substantially non-porous structure as described above.
It can also be confirmed from the fact that the flux (liquid permeation amount) is substantially “0” when a liquid permeation test is performed using the film-like material as a filtration membrane.

In the method of the present invention, the fired film is then stretched in the direction orthogonal to the stretching direction during the uniaxial stretching. This stretching is also below the melting point of PTFE, preferably 80-
It is carried out under the temperature condition of 300 ° C. The stretching rate is not particularly limited, but is usually about 50 to 500%.

By this stretching, a large number of nodules and a large number of fibers connecting these nodules are reformed on the surface portion, and the surface portion has a porous structure again (the voids between the fibers are fine). It is a hole). The fibers of this surface portion are oriented along the stretching direction (that is, the stretching direction at the time of stretching after firing). On the other hand, the internal microstructure hardly changes except that the diameter of the micropores increases.
As a result, the orientation direction of the fibers in the surface portion is different from that in the inside, and the desired porous membrane is obtained.

[0032]

EXAMPLES The present invention will be described in more detail below with reference to examples.

Example 1 PTFE obtained by converting primary particles having a particle size of 0.51 μm into secondary particles
Powder (heat of crystallization 12.6 J / g, average particle size 450 μm)
33 parts by weight of liquid paraffin were uniformly mixed with 100 parts by weight, and the mixture was compression preformed under a pressure of 20 kg / cm ² and then extruded to a diameter of 20 mm.
The rod-shaped rod is rolled into a long film having a thickness of 150 μm by rolling with a metal rolling roll, and immersed in trichlorethylene to extract and remove liquid paraffin.

This film-like material is uniaxially stretched at a temperature of 200 ° C. in the longitudinal direction so that the stretching ratio is 100%. Next, firing is performed by regulating the lengthwise direction so as not to change and heating at a temperature of 355 ° C. for 5 minutes. Then, the film is drawn at a temperature of 150 ° C. in the width direction (direction orthogonal to the lengthwise direction) so that the draw ratio is 200%, and the thickness is about 80.
A porous film of μm was obtained.

Photographs of the microstructures on the surface and inside of this porous film observed by SEM are shown in FIGS. 1 and 2 (magnification: 10,000 times). The inside is observed by adhesive tape (made by Nitto Denko Corporation, trade name on one surface of the porous film.
After adhering No. 375), the pressure-sensitive adhesive tape was peeled off, and when the tape was peeled off, the surface portion of the porous film was simultaneously peeled off (peeling thickness of about 10 μm) to expose the inside. Since the notch was made on the surface of the porous film on which the adhesive tape was adhered by a knife prior to adhering the tape, the surface portion could be easily peeled off.

From FIGS. 1 and 2, the microstructure of this porous membrane is composed of a large number of nodules and fibers connecting these nodules, and in the surface portion, the fibers are stretched in the stretching direction (arrow X in the drawings). Direction), and inside is oriented along the stretching direction (arrow Y in the figure) before firing, and it can be seen that the orientation direction of the fiber in the surface part and the inside is almost orthogonal. .

This porous membrane is used as a filtration membrane and has a particle size of 0.10.
Water containing 5 μm of styrene latex at a concentration of 100 ppm was permeated at a differential pressure of 380 mmHg, and the latex removal rate was measured to be 92%. Also, AST
When the ethanol flux was measured by the method specified in M-F317-72, it was 3.8 ml / cm ² · min.
Met.

Example 2 PTFE obtained by converting primary particles having a particle size of 0.42 μm into secondary particles
Powder (heat of crystallization 15.2 J / g, average particle size 480 μm)
A porous membrane (thickness: about 80 μm) was obtained in the same manner as in Example 1 except that was used.

The microstructure of this porous membrane is also composed of a large number of nodules and fibers that connect these nodules to each other, as in the case shown in FIGS. Oriented along the direction of the arrow X in the figure) and internally oriented along the direction of stretching before firing (arrow Y in the figure), and the orientation of the fibers on the surface and inside is substantially orthogonal. Met.

When the same test as in Example 1 was conducted on this porous film, the latex removal rate was 61% and the flux was 4.0 ml / cm ² · min.

Example 3 PTFE obtained by converting primary particles having a particle size of 0.30 μm into secondary particles
Powder (heat of crystallization 17.0 J / g, average particle size 480 μm)
A porous membrane (thickness: about 80 μm) was obtained in the same manner as in Example 1 except that was used.

Similarly to the microstructure of this porous membrane, as shown in FIGS. 1 and 2, a large number of nodules and fibers connecting these nodules to each other are formed. Oriented along the arrow X) in the figure,
In the inside, the fibers were oriented along the stretching direction (arrow Y in the figure) before firing, and the orientation of the fibers in the surface portion and inside were substantially orthogonal.

When the same test as in Example 1 was conducted on this porous membrane, the latex removal rate was 50% and the flux was 4.2 ml / cm ² · min.

Example 4 The same procedure as in Example 1 was carried out except that the stretching ratio during uniaxial stretching was 180%, and firing was performed while shrinking the dimension in the stretching direction during uniaxial stretching by 30%. A porous film (thickness: about 80 μm) was obtained.

This porous membrane is also composed of a large number of nodules and fibers that connect the nodules, as in the case shown in FIGS. 1 and 2, and in the surface portion, the fibers are stretched in the stretching direction (arrows in the drawings). X), the fibers were oriented along the stretching direction (arrow Y in the figure) before firing inside, and the orientation direction of the fibers on the surface portion and inside was substantially orthogonal. .

When the same test as in Example 1 was conducted on this porous membrane, the latex removal rate was 71% and the flux was 4.4 ml / cm ² · min.

Example 5 The same operation as in Example 1 was carried out except that the stretching ratio during uniaxial stretching was 230%, and firing was performed while shrinking the dimension in the stretching direction during uniaxial stretching by 50%. A porous film (thickness: about 90 μm) was obtained.

This porous membrane is also composed of a large number of nodules and fibers connecting the nodules, as in the case shown in FIGS. 1 and 2, and in the surface portion, the fibers are drawn in the direction of stretching after firing (the arrow in the figure). X), the fibers were oriented along the stretching direction (arrow Y in the figure) before firing inside, and the orientation direction of the fibers on the surface portion and inside was substantially orthogonal. .

When the same test as in Example 1 was conducted on this porous membrane, the latex removal rate was 81% and the flux was 3.2 ml / cm ² · min.

Comparative Example 1 A porous film was obtained in the same manner as in Example 1 except that PTFE powder having a crystallization heat of 22 J / g and an average particle size of 500 μm was used. When the microstructure of this porous film was observed in the same manner as in Example, fibers oriented along the stretching direction before firing were observed inside, but almost no fibers were observed on the surface, and formation The holes made were also coarse.

When the same test as in Example 1 was conducted on this porous membrane, the latex removal rate was 2% and the flux was 2.1 ml / cm ² · min.

Comparative Example 2 In the same manner as in Example 1, mixing of PTFE powder and liquid paraffin, compression preforming, extrusion molding, rolling, extraction and removal of liquid paraffin, and uniaxial stretching in the longitudinal direction are performed. Then, it is stretched at a temperature of 150 ° C. so as to have a stretching ratio of 200% in the width direction, then regulated so that the dimensions in both directions are not changed, and heated at a temperature of 350 ° C. for 5 minutes for firing.
A porous film having a thickness of about 90 μm was obtained.

When the microstructure of this porous membrane was observed in the same manner as in the example, it was the same on the surface and in the inside, and consisted of a large number of nodules and a large number of fibers connecting the nodules, and the fibers were It was confirmed that it was extending radially.

When the same test as in Example 1 was conducted on this porous film, the latex removal rate was 38% and the flux was 4.7 ml / cm ² · min.

[0055]

EFFECTS OF THE INVENTION The present invention is constructed as described above, and has a microstructure in which the orientation directions of the fibers in the surface portion and inside are different, so that not only is the filtration function excellent, but the throughput per unit time is also high. Also has the advantage of being large. Further, according to the manufacturing method of the present invention, the porous film having this novel microstructure can be easily manufactured.

[Brief description of drawings]

FIG. 1 is an SEM photograph showing the orientation of fibers in the surface portion of a PTFE porous membrane according to the present invention.

FIG. 2 is an SEM photograph showing the orientation of fibers inside the PTFE porous membrane according to the present invention.

FIG. 3 is a DSC curve for determining the heat of crystallization of the PTFE powder used in the present invention.

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[Procedure amendment]

[Submission date] March 4, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Brief description of the drawing

[Correction method] Change

[Correction content]

[Brief description of drawings]

FIG. 1 is an SEM photograph showing a shape of oriented fibers in a surface portion of a PTFE porous membrane according to the present invention.

FIG. 2 is a SEM photograph showing the shape of oriented fibers inside a PTFE porous membrane according to the present invention.

FIG. 3 is a DSC curve for determining the heat of crystallization of the PTFE powder used in the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masahiro Uebayashi 1-2-1, Shimohozumi, Ibaraki City, Osaka Prefecture Nitto Denko Corporation

Claims (3)

[Claims] 1. A polytetrafluoroethylene porous membrane comprising nodules and fibers connecting the nodules to each other, wherein the orientation of the fibers in the surface portion and the orientation of the fibers in the interior are different. Porous membrane. 2. A mixture of a polytetrafluoroethylene powder having a heat of crystallization of 18 J / g or less and a liquid lubricant is formed into a film, the film is uniaxially stretched, and then the dimension in the stretching direction is regulated. Alternatively, the polytetrafluoroethylene porous membrane is characterized in that it is heated to a temperature equal to or higher than the melting point of polytetrafluoroethylene while being shrunk in a range where it is not totally shrunk, is baked, and is then stretched in a direction orthogonal to the stretching direction. Manufacturing method. 3. The method for producing a polytetrafluoroethylene porous membrane according to claim 2, wherein a secondary particle powder of polytetrafluoroethylene formed from primary particles having a particle size of 0.25 μm or more is used.