JP3099416B2 - Method for producing polytetrafluoroethylene porous membrane with asymmetric pore size - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a polytetrafluoroethylene comprising at least two layers having different average pore sizes.
The present invention relates to a method for producing a porous membrane (hereinafter, referred to as PTFE), and more particularly, to a method for producing an asymmetric porous PTFE porous membrane in which the layers are completely integrated.

[0002]

2. Description of the Related Art PTFE is a plastic excellent in heat resistance and chemical resistance, and its porous membrane is widely used as a filter for corrosive gases and liquids, and as a battery diaphragm for electric field diaphragms. I have. In particular, its use as a microfiltration filter for various gases and liquids used in the semiconductor industry has become a very important application field.

[0003] In order to be an excellent filtration filter, it is necessary that the pore size distribution is sharp and the amount of permeation per unit time when a fluid permeates at a constant pressure is large. It is known that the amount increases when the porosity and the pore diameter are constant and the film thickness is smaller. However, when the thickness is reduced, the porous membrane is deformed by the pressure at the time of filtration, the pore diameter is changed, or in some cases, the porous membrane is broken, and the function as a filtration filter may not be achieved. In addition, a porous film having a small film thickness is extremely poor in handleability, and has a problem that it is damaged when processing into a filter module or setting in a filter holder.

In order to solve such problems, several porous PTFE membranes comprising a filtration layer having a small pore size and a support layer having a pore size larger than the filtration layer have been proposed. As its manufacturing method, for example, (1) a plurality of PTFE porous structures having a small pore size and a PTFE porous structure having a larger pore size are laminated in a non-fired state, pressed and then heated and fired to a temperature equal to or higher than the melting point of PTFE. (Japanese Unexamined Patent Publication (Kokai) No. 54-97686), and (2) when stretching an unsintered film between a low-speed rotating roll and a high-speed rotating roll, a temperature difference in the thickness direction of the thin film may occur. A method of obtaining a porous membrane having different pore sizes on the front and back sides by simultaneously generating a compressive force (Japanese Patent Publication No. 63-48562) is known.
Although it is intended for separating and concentrating isotope mixed gas and not intended for a microfiltration filter, a method for producing a microporous membrane is as follows: (3) a PTFE thin film containing a liquid pore former and a liquid pore former. A plurality of other PTFE thin films in which the agent has been mixed and rolled, so that the liquid pore-forming agent is extracted and dissolved with a low-molecular-weight liquid to form pores, and from at least two layers having different average pore diameters, (Japanese Patent Publication No. 55-22504) is known.

However, the method (1) requires a step of separately obtaining two or more sheets or film-like molded articles having different porosity, and then firing the laminated or pressed articles. Furthermore, lamination of extremely thin or low-strength film-like molded products requires expensive equipment and advanced technology in industrial production due to problems such as generation of wrinkles and tears in the process.

[0006] The method (2) is a method in which stretching is performed between rolls, and the stretching is limited to a uniaxial direction.
Biaxial stretching cannot be applied.

Further, the method (3) does not depend on the stretching method, but depends on the difference in the packing density of the PTFE emulsion polymerization powder having different primary particles in size and shape and the difference in the type of pore-forming agent to be used. This is a method for obtaining a layered material having different properties. However, these pores are merely gaps between the PTFE emulsion polymer particles. To further describe this point, the unfired product of the PTFE emulsion polymer obtained by the paste processing method is close to the closest packing of the primary particles, and the specific gravity of the primary particles is 2.1 to 2.1.
The specific gravity of the whole processed product is usually 1.5 to 1.6 when molded with a petroleum solvent or the like. The difference in specific gravity is a pore, and the gap between particles is a pore. In any case, in such a state, the filter performance is extremely poor in fluid permeation capacity, and its strength is extremely small as compared with the fired product, and when firing is performed to increase the strength, The layered material becomes nonporous and cannot be used as a fluid filter in the semiconductor industry.

Hitherto, there has been proposed a method (Japanese Patent Laid-Open No. 57-131236) in which a rolled sheet containing a PTFE auxiliary is stacked and further thinly rolled, and then stretched to obtain a multilayer porous membrane. However, the porous body obtained by this method has high strength between components, but does not change the porosity between layers at all.

On the other hand, an asymmetric membrane composed of an extremely thin filtration layer and a support layer having a larger pore size than the filtration layer is made of cellulose acetate or polysulfone. However, since these asymmetric membranes are made by a wet coagulation method, it is necessary that the membrane material be soluble in a solvent, and this method cannot be applied to a material that does not dissolve at all in a solvent such as PTFE.

[0010]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for producing a porous PTFE membrane having no such problems, having excellent permeability for various gases and liquids, and having completely integrated layers. Is to do.

That is, the gist of the present invention is to provide a method for producing a polytetrafluoroethylene porous membrane comprising at least two layers having different average pore diameters, wherein the emulsified dispersion of polytetrafluoroethylene is melted at a temperature not lower than the melting point of the unfired polytetrafluoroethylene. Coated on a polytetrafluoroethylene porous membrane base material that has not been heated to a temperature of at least the melting point of the fired body of polytetrafluoroethylene, semi-baked, and then stretched in at least one axial direction. The present invention relates to a method for producing an asymmetric pore diameter polytetrafluoroethylene porous membrane.

Hereinafter, the production method of the present invention will be described in detail. First, a PTFE emulsified dispersion is coated on a PTFE porous membrane.

PTFE in the PTFE emulsified dispersion of the present invention
The particle size is at least 240 or more in turbidity size, preferably 300 or more. It is preferable that the dispersion is concentrated by adding a surfactant, stirring, standing, and removing the supernatant to obtain a resin having a solid content of 60% or more. When the number of particles is 240 or less, and when the concentration of the dispersion is 6
If it is 0% or less, cracks are likely to occur in the coating film in the subsequent drying and semi-baking steps.

The dispersion may contain a surfactant for improving the dispersibility of the PTFE particles, a thickener for controlling the film thickness at the time of coating, and a filler for controlling the strength. Good.

The PTFE porous membrane substrate used in the present invention is:
Generally, it is a porous film obtained by stretching a green body or a semi-baked body obtained by paste extrusion of a PTFE emulsion polymerization powder and, if necessary, rolling. In the production method of the present invention, the porous film is It must be substantially not heated above the melting point of the unfired PTFE, ie, above about 347 ° C. It is difficult to stretch a porous film that has been heated to a temperature equal to or higher than the melting point of the unfired PTFE and completely fired.

The thickness of the PTFE porous membrane substrate is not particularly limited, but is preferably from 10 μm to 100 μm, and the average pore diameter is preferably from 0.3 μm to 0.9 μm.
The PTFE porous membrane substrate may be in the form of a membrane or a hollow fiber.

In coating the PTFE emulsified dispersion on the PTFE porous membrane substrate, when the PTFE porous membrane substrate is in the form of a film, the coating is performed using a coating apparatus exemplified by a bar coater, a doctor knife, a curtain coater or the like. In the case of a hollow fiber, the dispersion is poured into the inner surface or immersed in the dispersion.

Next, the coating film is dried if necessary. The method is not particularly limited as long as the moisture in the coating film can be removed, or it is air-dried first to prevent cracking of the coating film, and then 100 ° C. or lower. And drying in an infrared drying oven.

Next, the dried coating film is semi-baked. Semi-firing can be performed according to a known method disclosed in JP-A-59-152825.

That is, the melting point of the fired PTFE (about 327)
C) or higher, preferably higher than the melting point of the fired PTFE.
E The semi-sintered body is heated at a temperature lower than the melting point of the unsintered body (about 347 ° C.).

It is difficult to determine the heating time based on the heating temperature, the film thickness of the heated object, and other conditions. However, generally, the heating time is shorter as the heating temperature is higher, and the heating time is shorter as the film thickness is larger. What is necessary is just to lengthen a heating time and to select conditions suitably for implementation.

After the semi-firing, the film is stretched in at least one direction. Usually, the stretching is carried out by appropriately selecting the temperature in the range of room temperature to the melting point of the PTFE fired body or lower. In the case of stretching in the uniaxial direction, the stretching time is 1.1 to 3.0 times, and in the case of stretching in the biaxial direction. Can be performed at a magnification of about 2.0 times in one direction and about 2.0 times in a direction perpendicular thereto.

The PTFE porous membrane obtained by stretching may be heat-set at a stretching temperature or higher, if necessary. By this treatment, it is possible to make the porous film hardly shrink near room temperature.

The porous membrane obtained by the production method of the present invention has a small pore diameter on the coating side of the PTFE emulsified dispersion,
The porous membrane substrate becomes an asymmetric pore-diameter PTFE porous membrane having a large pore diameter. Further, since the coating film is entangled with the porous substrate at the layer interface, delamination does not occur.

The thickness of the layer on the small pore side of this asymmetric porous membrane is extremely thin, 8 μm or less, so that it is excellent in permeability of various fluids, precision filtration filters for various gases and liquids in the semiconductor industry, and blood cell separation in the medical field. Membrane, bacteria separation membrane, fruit juice concentration membrane in the food industry field, separation / purification membrane of fermentation products,
It is useful for applications such as ski wear and rain wear in the apparel field.

Further, according to the production method of the present invention, it is possible to coat the inside or outside of the hollow fiber-shaped base material with the PTFE emulsified dispersion, so that the degree of freedom with respect to the shape of the base material is high.

Examples are shown below, and various physical properties in the examples are measured by the following methods.

The turbidity diameter (average particle diameter of primary particles) is determined by the transmittance of 550 nm projected light per unit length of a polymer latex diluted with water to a solid content of about 0.22% by weight and an electron micrograph. It is determined from the above transmittance based on a calibration curve with the average particle diameter obtained.

Measurement of average pore size Coulter Porometer (Coulter Electronics)
(USA)]
P) was taken as the average pore size.

[0030] Using the film thickness Corporation Mittoyo made 1D-110MH type film thickness meter, was measured.

The porosity Ethanol displacement method was used to measure the weight (W) of the membrane filled with pure water in the pores, the absolute dry weight (W ₀ ) and the volume (V) of the membrane, and calculated using the following equation. did. (W−W ₀ ) × 100 / V (%)

Gas flow rate The porous membrane was cut into a circular shape having a diameter of 25 mm, set in a filter holder having a permeation effective area of 2.15 cm ² , pressurized with 0.639 bar of nitrogen gas, and the amount of permeated gas was measured with a mass flow meter. did. From this measured value, the permeation amount per minute (l / min) was calculated.

[0033]

【Example】

Example 1 A P with an average pore diameter of 0.93 μm and a film thickness of 68 μm, which was not heated above the melting point of the unfired PTFE, was placed on a glass plate.
A TFE porous membrane was laid, and a turbidity diameter of 332, a resin solid content of 60%, and a nonionic content of 9 parts by weight (resin solid content of 100%) were placed thereon.
2 parts by weight of PTFE emulsified dispersion (by weight part) (Daikin Industries, F104) using a bar coater.
The bar was moved at 8 cm / s for coating. Thereafter, the film was allowed to stand in a room at about 25 ° C. for 1 hour, and then placed in an infrared drying oven at 80 ° C. for 10 minutes to remove moisture from the coating film.

Then, it was placed in a hot-air circulation type electric furnace set at 341 ° C. for 30 minutes and semi-baked. At this time, the ends of the porous film used as the base material were fixed so that the porous film did not shrink.
The thickness of the obtained semi-baked body was 73 μm.

The semi-sintered body was heated at 320 ° C. for 1.5 times in one direction.
The film was stretched 1.5 times in the direction perpendicular to that of the film to obtain an asymmetric porous PTFE porous membrane. The thickness of the obtained porous membrane is 48μ.
m, the porosity was 92%, and the average pore size was 0.55 μm. The nitrogen gas flow rate at 0.639 bar is 23 l.
/ Min.

Example 2 As a PTFE porous membrane substrate which has not been heated to a temperature equal to or higher than the melting point of the semi-baked PTFE, an average pore diameter of 0.35 μm was used.
m, an asymmetric pore diameter PTFE porous membrane was obtained in the same manner as in Example 1 except that a PTFE porous membrane having a thickness of 56 μm was used.
This porous membrane had a thickness of 27 μm, a porosity of 80%, and an average pore diameter of 0.25 μm. The nitrogen gas flow rate at 0.639 bar was 5.1 l / min.

Incidentally, an electron microscope (hereinafter, SEM) photograph (700) of the surface of the coating film side of the PTFE emulsified dispersion of the obtained porous film.
0 times) is shown in FIG. 1 and SE on the PTFE porous membrane substrate side surface
An M photograph (at a magnification of 7000) is shown in FIG. From these, PTF
It can be seen that the coating film side of the E-emulsified dispersion has a smaller pore size.

[Brief description of the drawings]

FIG. 1 shows a SEM photograph (magnification: 7000) of a fiber shape on a coating film side surface of a PTFE emulsified dispersion of an asymmetric pore diameter PTFE porous membrane obtained in Example 2.

FIG. 2 is an SEM of a fiber shape on the surface of a PTFE porous membrane substrate of the asymmetric pore diameter PTFE porous membrane obtained in Example 2.
A photograph (7000 times) is shown.

──────────────────────────────────────────────────続 き Continuing from the front page (72) Osamu Tanaka, Inventor 1-1, Nishiichitsuya, Settsu-shi, Osaka Daikin Industries, Ltd. Yodogawa Works (72) Inventor Osamu Inoue 1-1, Nishiichitsuya, Settsu-shi, Osaka Daiki (56) References JP-A-53-134065 (JP, A) JP-A-64-34408 (JP, A) JP-B-48-532 (JP, B1) (58) Field (Int.Cl. ⁷ , DB name) C08J 9/00-9/42 B01D 71/36 B32B 27/30

Claims (1)

(57) [Claims] 1. A method for producing a polytetrafluoroethylene porous membrane comprising at least two layers having different average pore diameters, wherein a polytetrafluoroethylene emulsified dispersion is heated to a melting point of a polytetrafluoroethylene green body or higher. Asymmetrical pore diameter polytetrafluoroethylene characterized in that it is coated on a polytetrafluoroethylene porous membrane base material, then heated to a melting point of the polytetrafluoroethylene fired body or more and semi-baked, and then stretched in at least one axial direction. A method for producing an ethylene porous membrane.