JPH04351645A - Production of porous polytetrafluoroethylene film with asymmetrical pore diameter - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a polytetrafluoroethylene (polytetrafluoroethylene) consisting of at least two layers having different average pore sizes.
The present invention relates to a method of manufacturing a porous membrane (hereinafter referred to as PTFE), and more specifically to a method of manufacturing a porous PTFE membrane with asymmetric pore diameter in which layers are completely integrated.

0002

[Prior art and problems to be solved] PTFE is a plastic with excellent heat resistance and chemical resistance, and its porous membrane is widely used as a filtration filter for corrosive gases and liquids and as a battery membrane for electric field membranes. There is. In particular, its use as a precision filtration filter for various gases and liquids used in the semiconductor industry has become an extremely important field of application.

[0003] In order to be an excellent filtration filter, it is necessary that the pore size distribution is sharp and that the amount of permeation per unit time is large when fluid is permeated at a constant pressure. When the porosity and pore diameter are constant,
It is known that the smaller the thickness of the film, the larger it becomes. However, if the membrane thickness is reduced, the porous membrane may be deformed by the pressure during filtration, the pore size may change, or it may be torn, causing it to no longer function as a filtration filter. In addition, a thin porous membrane is extremely difficult to handle, and there are problems such as damage when processing it into a filter module or setting it in a filter holder.

[0004] In order to solve these problems, several PTFE multilayer porous membranes comprising a filtration layer having a small pore size and a support layer having a pore size larger than that of the filtration layer have been proposed. As a manufacturing method, for example, (1) a plurality of PTFE porous structures having a small pore diameter and a PTFE porous structure having a larger pore diameter are stacked and pressed together in an unfired state, and then heated and fired to a temperature equal to or higher than the melting point of PTFE. method for obtaining a PTFE multilayer porous membrane (Japanese Patent Application Laid-Open No. 54-97686), and (2)
A method of obtaining a porous film with different pore sizes on the front and back sides by simultaneously generating a temperature difference and compressive force in the thickness direction of the thin film when stretching an unfired film between a low-speed rotating roll and a high-speed rotating roll 48562) is known. In addition, although it is for separating and concentrating isotopic gas mixtures and is not intended as a precision filtration filter, as a manufacturing method for a microporous diaphragm, (3) a PTFE thin film containing a liquid pore-forming agent and a liquid pore-forming agent are used. The liquid pore-forming agent is then extracted and dissolved with a low-molecular liquid to create pores, which are formed from at least two layers with different average pore diameters. A method for obtaining a PTFE multilayer porous membrane (Japanese Patent Publication No. 55-22504) is known.

However, the method (1) requires a step of separately obtaining two or more sheet or film molded products having different porosities, and then firing them while laminating and pressing them together. Furthermore, laminating film-like molded products that are extremely thin or have low strength requires expensive equipment and advanced technology in industrial production due to problems such as wrinkles and tears during the process.

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

Furthermore, the method (3) does not involve stretching, and can be applied to PTFE with different primary particle sizes and shapes.
This is a method for obtaining layered materials with different average pore diameters due to differences in the packing density of emulsion polymerized powder and the type of pore-forming agent used. However, these pores are merely gaps between the PTFE emulsion polymer particles. To further explain this point, the unfired product produced by the paste processing method of PTFE emulsion polymer is close to the closest packing of primary particles, and the specific gravity of the primary particles is 2.1~
2.3, and the specific gravity of the entire processed product is usually 1.5 to 1.6 when molded using a petroleum solvent or the like. The difference in specific gravity is a pore, and the gaps between particles are pores. In any case, filters in this state have extremely poor fluid permeation ability, and their strength is extremely low compared to fired products, so firing them to increase their strength is difficult. The layered material is non-porous and cannot be used as a fluid filter in the semiconductor industry.

[0008] Conventionally, a method has been proposed in which rolled sheets containing a PTFE auxiliary agent are piled up, rolled further thinner, and then stretched to obtain a multilayer porous membrane (Japanese Patent Laid-Open No. 131236/1983). However, the porous body obtained by this manufacturing method has high inter-component strength, but there is no change in the porosity between the layers.

On the other hand, asymmetric membranes are made of cellulose acetate or polysulfone, which consist of an extremely thin filtration layer and a thick support layer with pores larger than the filtration layer. However, since these asymmetric membranes are made by a wet coagulation method, the membrane material must be soluble in a solvent, and this method cannot be applied to materials such as PTFE that are not soluble in solvents at all.

0010

[Means for Solving the Problems] An object of the present invention is to provide a method for producing a PTFE porous membrane with an asymmetric pore size that is free from such problems, has excellent permeability to various gases and liquids, and has completely integrated layers. It's about doing.

That is, the gist of the present invention is to provide a method for producing a polytetrafluoroethylene porous membrane consisting of at least two layers having different average pore diameters, in which a polytetrafluoroethylene emulsion dispersion is heated to a temperature higher than the melting point of a polytetrafluoroethylene green body. The film is coated on a polytetrafluoroethylene porous membrane base material that has never been heated before, and then heated to a temperature higher than the melting point of the polytetrafluoroethylene fired product to semi-fire it, and then stretched in at least one axial direction. The present invention relates to a method for producing a polytetrafluoroethylene porous membrane with an asymmetric pore size.

The manufacturing method of the present invention will be explained in detail below. First, a PTFE emulsion dispersion is coated on a PTFE porous membrane.

PTFE in the PTFE emulsion dispersion of the present invention
The particle diameter is at least 240 or more, preferably 300 or more in terms of turbidity diameter. Further, it is desirable to use a dispersion having a resin solid content of 60% or more, which is concentrated by stirring, standing, and removing the supernatant after adding a surfactant. When the particle size is 240 or less, and the concentration of the dispersion is 60
% or less, cracks are likely to occur in the coating film during the subsequent drying and semi-baking steps.

[0014] The dispersion liquid may contain a surfactant to improve the dispersibility of the PTFE particles, a thickener to control the film thickness during coating, and a filler to control the strength. good.

[0015] The PTFE porous membrane base material used in the present invention is:
Generally, a porous membrane is obtained by pasting extrusion of PTFE emulsion polymer powder and rolling it as necessary to obtain an unfired body or stretching a semi-fired body.In the manufacturing method of the present invention, this porous membrane It is necessary that the material has not been heated substantially above the melting point of the PTFE green body, that is, above about 347°C. It is difficult to stretch a porous membrane that has been heated above the melting point of the PTFE green body and completely fired.

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

[0017] When coating the PTFE emulsion dispersion onto the PTFE porous membrane substrate, if the PTFE porous membrane substrate is in the form of a membrane, coating equipment such as a bar coater, doctor knife, curtain coater, etc. may be used. In the case of hollow fibers, this is done by pouring a dispersion liquid onto the inner surface or by immersing the fibers in the dispersion liquid.

[0018] Next, the coating film is dried if necessary, but the method is not particularly limited as long as the water in the coating film can be removed, or in order to prevent cracks in the coating film, first air dry, then dry at 100°C or less. It is preferable to dry it in an infrared drying oven.

Next, the dried coating film is semi-baked. Semi-firing can be performed in accordance with the known method disclosed in Japanese Patent Application Laid-Open No. 59-152825.

That is, the melting point of the PTFE fired body (approximately 327
℃) or above, preferably above the melting point of the PTFE fired body PTF
E Semi-fired by heating at a temperature below the melting point of the green body (approximately 347°C).

[0021] Although it is difficult to determine the heating time unconditionally depending on the heating temperature, film thickness of the heated material, and other conditions, generally speaking, the higher the heating temperature, the shorter the heating time, and the thicker the film thickness, the shorter the heating time. The heating time may be increased and the conditions may be appropriately selected for implementation.

After semi-firing, the film is stretched in at least one axis. Usually, stretching is carried out at an appropriately selected temperature range from room temperature to below the melting point of the PTFE sintered body, and in the case of uniaxial stretching, the stretching is 1.1 to 3.0 times, and in the case of biaxial stretching, is 2.0 times in one direction and 2.0 times in the direction perpendicular to it
This can be done at a magnification of about 2x.

The porous PTFE membrane obtained by stretching may be heat set at a temperature higher than the stretching temperature, if necessary. By this treatment, the porous membrane can be brought into a state where almost no shrinkage occurs near room temperature.

[0024] The porous membrane obtained by the production method of the present invention has small pores on the coating side of the PTFE emulsion dispersion, and
The porous membrane base material side becomes an asymmetric pore diameter PTFE porous membrane with large pore diameters. Furthermore, 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, less than 8 μm, so it has excellent permeability to various fluids, and is suitable for precision filtration of various gases and liquids in the semiconductor industry, and blood cell separation in the medical field. Membranes, bacteria separation membranes, fruit juice concentration membranes in the food industry, separation and purification membranes for fermentation products,
It is useful for ski wear, rain wear, etc. in the apparel field.

Furthermore, according to the manufacturing method of the present invention, it is also possible to coat the PTFE emulsion dispersion on the inside or outside of a hollow fiber-like base material, and there is a high degree of freedom regarding the shape of the base material.

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

Turbidity diameter (average particle diameter of primary particles) Solid content approximately 0
．． It is determined from the above transmittance based on a calibration curve between the transmittance of 550 nm projected light per unit length of polymer latex diluted with water to 22% by weight and the average particle diameter determined by electron micrograph. .

Measurement of average pore size Coulter Porom
eter) [Coulter Electronics
The mean flow pore size (MEP) measured by TER Electronics (USA) was taken as the average pore diameter.

Film Thickness The film thickness was measured using a 1D-110MH type film thickness meter manufactured by Mitoyo Co., Ltd.

[0031] The weight (W) of the membrane whose pores were filled with pure water, the absolute dry weight (W0) of the membrane, and its volume (V) were measured by the porosity ethanol substitution method, and calculated using the following formula. . (W-W0)×100/V (%)

[0032] The gas flow rate porous membrane was cut into a circle with a diameter of 25 mm, and the effective permeation area was 2.
．． It was set in a 15 cm2 filter holder, pressurized with 0.639 bar nitrogen gas, and the amount of gas permeating was measured using a mass flow meter. The amount of permeation per minute (l/min) was calculated from this measured value.

[0033]

【Example】

Example 1 On a glass plate, P with an average pore diameter of 0.93 μm and a film thickness of 68 μm, which had never been heated above the melting point of the PTFE green body, was deposited.
A TFE porous membrane was laid, and a turbidity diameter of 332, a resin solid content of 60%, a nonionic content of 9 parts by weight (a resin solid content of 100%) was placed on top of the TFE porous membrane.
2 ml of PTFE emulsion dispersion (Daikin Industries, F104) (based on parts by weight) was dropped, and using a bar coater,
The bar was moved at 8 cm/s for coating. Thereafter, it was left 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.

Next, it was placed in a hot air circulation electric furnace set at 341° C. for 30 minutes to be semi-baked. At this time, the edges of the porous membrane used as the base material were fixed to prevent heat shrinkage. The thickness of the obtained semi-fired body was 73 μm.

[0035] This semi-fired body was heated at 320°C by 1.5 cm in one direction.
A PTFE porous membrane with an asymmetric pore size was obtained by stretching the film by a factor of 1.5 times in a direction perpendicular to that. The thickness of the obtained porous membrane was 48μ
m, the porosity was 92%, and the average pore diameter was 0.55 μm. Also, the nitrogen gas flow rate at 0.639 bar is 23
It was 1/min.

Example 2 A porous PTFE membrane substrate that had never been heated to a temperature higher than the melting point of the semi-fired PTFE body had an average pore diameter of 0.35 μm.
A porous PTFE membrane with an asymmetric pore diameter was obtained in the same manner as in Example 1, except that a porous PTFE membrane with a membrane thickness of 56 μm was used. The thickness of this porous membrane was 27 μm, the porosity was 80%, and the average pore diameter was 0.25 μm. Further, the nitrogen gas flow rate at 0.639 bar was 5.1 l/min.

Incidentally, an electron microscope (hereinafter referred to as SEM) photograph (70°
00 times) is shown in Figure 1, and S on the surface of the PTFE porous membrane substrate side.
An EM photograph (7000x) is shown in Figure 2. From these, P
It can be seen that the coating side of the TFE emulsion dispersion has a smaller pore size.

[Brief explanation of the drawing]

FIG. 1 shows an SEM photograph (7000 times) of the fiber shape on the coating side surface of the PTFE emulsion dispersion of the asymmetric pore diameter PTFE porous membrane obtained in Example 2.

[Figure 2] SE of fiber shape on the surface of the PTFE porous membrane substrate of the asymmetric pore size PTFE porous membrane obtained in Example 2
M photograph (7000x) is shown.

Claims (1)

[Claims] Claim 1. A method for producing a polytetrafluoroethylene porous membrane comprising at least two layers having different average pore diameters, comprising: heating a polytetrafluoroethylene emulsion dispersion to a temperature higher than the melting point of a green polytetrafluoroethylene body; Polytetrafluoroethylene with an asymmetric pore size is coated on a polytetrafluoroethylene porous membrane substrate, then heated to a temperature higher than the melting point of the polytetrafluoroethylene fired product, semi-fired, and then stretched in at least one axis direction. Method for producing ethylene porous membrane.