JPH078927B2 - Method for producing polytetrafluoroethylene multilayer porous membrane - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a polytetrafluoroethylene (hereinafter referred to as PTFE) multilayer porous membrane, and more specifically, to at least two different average pore diameters. The present invention relates to a method for producing a PTFE multi-layer porous body which is composed of layers and in which the layers are completely integrated.

[Conventional technology]

PTFE is a plastic with excellent heat resistance and chemical resistance,
The porous membrane is widely used as a filter for corrosive gases and liquids, an electrolytic diaphragm, and a battery diaphragm. In particular,
The use as a fine filtration filter for various gases and liquids used in the semiconductor industry has become an extremely important application field.

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 the fluid is permeated at a constant pressure.
It is known that the amount of fluid permeation increases as the film thickness decreases, when porosity and pore diameter are constant. However, if the film thickness is reduced, the porous film will be deformed due to the pressure during filtration,
The pore size may change or may be broken in some cases, and the filter may not function as a filter. In addition, a thin porous film has extremely poor handleability, and there is a problem that it is damaged when it is processed into a filter module or set in a filter holder.

In order to solve such a problem, some PTFE multi-layer porous membranes have been proposed, which include a filtration layer having a small pore size and a support layer having a pore size larger than that of the filtration layer. As the 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 stacked in a non-fired state, press-bonded, and then heated and fired at a temperature equal to or higher than the melting point of PTFE. hand
A method for obtaining a PTFE multi-layer porous membrane (Japanese Patent Laid-Open No. 54-97686), and (2) when stretching an unsintered film between a low-speed rotating roll and a high-speed rotating roll, the temperature difference and compression in the thickness direction of the thin film. Method for obtaining a porous membrane having different pore sizes on the front and back by simultaneously generating forces (Japanese Patent Publication No. 63-48562)
It has been known. Also, for the purpose of separating and concentrating isotope mixed gas, which is not intended as a microfiltration filter, as a method for producing a microporous membrane, (3) a PTFE thin film containing a liquid pore-forming agent and a liquid pore-forming material are used. At least two layers having different average pore diameters are formed by stacking a plurality of other PTFE thin films mixed with an agent and adhering them by rolling, and then extracting and dissolving the liquid pore-forming agent with a low-molecular liquid to open the pores. Consisting of PT
A method (Japanese Patent Publication No. 55-22504) for obtaining a FE multi-layer porous membrane is known. However, the above method (1) is disclosed in
As disclosed in Japanese Unexamined Patent Publication No. -30277, it is stated that a fused monolithic product can be obtained by stacking unfired stretched products and firing at a temperature equal to or higher than the melting point of the powder. Originally, an unsintered sheet or film of PTFE fine powder is wrapped and fired, and then each layer is fused and integrated into a molded product, for example, as a method for processing PTFE wrapping electric wires and PTFE wrapping tubes. It is known. Therefore, a method of stacking stretched porous bodies having different pore sizes and firing at a temperature equal to or higher than the melting point is common sense in the art. Regardless of the validity of the above judgment, this method requires a step in which two or more sheet or film-shaped molded products having different porosities are separately obtained and then they must be further laminated and pressure-bonded for firing. . Further, stacking extremely thin or low-strength film-shaped molded products requires expensive equipment and high-level technology in industrial production because of problems such as wrinkles and breakage in the process.

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

Furthermore, the method of (3) does not depend on the method of stretching,
This is a method for obtaining a layered product having different average pore diameters due to the difference in packing density of PTFE emulsion-polymerized powders having different primary particles in size and shape and the difference in type of pore-forming agent used. However, the pores are merely gaps between the PTFE emulsion-polymerized particles. To further describe this point, the unsintered product obtained by the paste processing method of the PTFE emulsion polymer is close to the closest packing of the primary particles, and the specific gravity of the primary particles is 2.1 to 2.3. Is usually 1.5 to 1.5 when molded with petroleum-based solvents, etc.
It is 1.6. The specific gravity difference is a hole, and the gap between particles is a hole. In any case, such a state has extremely poor fluid permeation ability as a filter function, and its strength is extremely smaller than that of the fired product, and if firing is performed to increase the strength, The layered product becomes non-porous and cannot be used as a fluid filter in the semiconductor industry.

Conventionally, there has been proposed a method (JP-A-57-131236) for obtaining a multi-layer porous film by stacking rolled sheets containing a PTFE auxiliary agent, rolling further thinly, and then stretching. However, the porous body obtained from this production method has high inter-component strength, but has no change in the inter-layer porosity. Further, Japanese Patent Publication No. 56-17216 discloses a method for producing a single-layer PTFE porous body having a strong tensile strength, and conventionally, the size of the small pores was determined by stretching and amorphous fixing.
In particular, it was controlled by the temperature, the stretch ratio per unit time, and the expansion amount.

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

[Means for Solving the Problems]

An object of the present invention is to provide a method for producing a PTFE multi-layer porous membrane having excellent permeability of various gases and liquids without such problems.

As a result of intensive studies to solve the above-mentioned problems of the prior art, the present inventors have obtained a method for obtaining a PTFE multi-layer porous membrane comprising a filtration layer having a small average pore size and a support layer having a larger average pore size. In the above, a method for producing a PTFE multi-layer porous membrane in which the interface of layers is completely integrated since an extremely thin filtration layer can be formed was established.

That is, when the present inventors surprisingly stretched the layered product of at least two or more kinds of PTFE fine powder after heat-treating and semi-firing the layered product, the stretching conditions were the same. It has been found that it is possible to easily obtain a porous film in which the pore diameters of the respective layers are different and there is no delamination.

In the manufacturing method of the present invention, first, at least two or more kinds of PTFE fine powders 1, 2, 3, ...
... is divided into a cylinder of an extrusion die, filled, extruded into a multi-layer, and then rolled as necessary to obtain a multi-layer molded body,
Then, the liquid lubricant is extracted from the multi-layer molded body and / or removed by drying to obtain a multi-layer unsintered body, and the multi-layer unsintered body is more than the melting point of the PTFE sinter, preferably the PTFE sinter. Of at least a uniaxial direction after obtaining a semi-sintered multi-layer semi-sintered body by heating at a temperature not lower than the melting point of the powder and not higher than the maximum melting point of the powder used to obtain the multi-layer unsintered body And

Hereinafter, the method of the present invention will be described in more detail.

The method for producing a PTFE multi-layer porous body of the present invention comprises the following steps.

(1) Paste Extrusion Step This step can be performed according to the paste extrusion method that has been conventionally known as a method for producing a PTFE unfired body. The feature is that the multilayer preform 7 is first obtained by the procedure shown in FIG. For example, as shown in FIG. 1 (e), this multilayer preform 7 is composed of a first layer 4, a second layer 5 and a third layer 6 made of three fine PTFE powders 1, 2 and 3. (This figure is an example of a plate-shaped three-layer structure, and is not limited to this.),
Each of these layers 4 to 6 has an average primary particle diameter of 0.2 to 0.4 μm.
It is obtained by adding a liquid lubricant such as solvent naphtha or white oil to a fine powder produced by coagulating a PTFE emulsion polymerization aqueous dispersion. The amount of the liquid lubricant used varies depending on the type, molding conditions and the like, and is usually used in the range of 20 to 35 parts by weight with respect to 100 parts by weight of fine powder. Further, a colorant or the like can be added to this. First, as shown in FIG. 1 (a), the PTFE fine powder 1 for obtaining the first layer 4 is placed in layers in the box-shaped mold 8 on the lower mold 9, and then the first mold
The upper die 10 is pressed in the direction of arrow 11 as shown in FIG. Thus, the first layer 4 is formed by being compressed.

Next, the upper mold 10 is removed, and as shown in FIG. 1 (c), the PTFE fine powder 2 is put in to form the second layer 5, and the same procedure as in FIG. 1 (b) described above is performed. Compressed using the upper mold 10 and the first layer 4 as shown in FIG. 1 (d).
The second layer 5 is formed on the top surface. Then, the PTFE fine powder 3 for the third layer 6 is further put in and pressed by the upper mold 10.

Thus, finally, in the cylinder 12 of the paste extrusion mold shown in FIG. 2, having the first layer 4, the second layer 5 and the third layer 6 as shown in FIG. 1 (e), A multi-layer preform 7 is obtained which is shaped to fit almost exactly.

Next, the preformed body 7 is housed in the cylinder portion 12 of the paste extruding device shown in FIG. The cylinder part 12 of the mold shown in FIG. 2 is, for example, a rectangle having a cross section in the direction perpendicular to the axis of 50 mm × 100 mm, and one of the cylinder parts 12 is squeezed at the outlet part 13 of the mold.
It is composed of mm x 5 mm.

In this way, the first layer 4, the second layer 5, and the third layer 6 are completely integrated, and each layer has a uniform thickness.
Is molded. It was confirmed by a stereomicroscope that the thickness composition ratio of each layer of the paste extruded sheet 15 was the same as the thickness composition ratio of each layer of the multilayer preform. As described above, by forming the preformed body 7 in advance, a free thickness can be selected, and even an extremely thin layer having low strength can be easily formed into a multilayer.

(2) Rolling Step This step can be performed as necessary according to a usual rolling method for the paste extruded sheet.

The sheet obtained in the paste extrusion step of (1) is cut into an appropriate length and rolled with a rolling roll in the same direction as the extrusion direction or in a direction perpendicular to the extrusion direction to obtain a multilayer molded body having a thickness of 100 μm, for example. be able to. Then, the liquid lubricant is extracted and / or dried (for example, oven-heated and dried at 250 ° C. for 20 seconds) from the multi-layer molded article to be removed, and a PTFE multi-layer unsintered article is molded.

In these (1) paste extrusion step and (2) rolling step, the PTFE mixture is subjected to shearing force and partly made into a fibrous state, so that appropriate strength and elongation are obtained.

The above two steps are all about 327 ° C which is the melting point of the PTFE fired body.
Below, most commonly at room temperature.

(3) Heat treatment step In this step, the multilayer unsintered body obtained through the above (1) paste extrusion step and (2) rolling step is heated at a temperature equal to or higher than the melting point of the PTFE sintered body so that each layer is formed. A semi-baked multi-layer semi-layer having a clear peak in the range of 332 to 348 ° C on the crystal melting curve by a differential scanning calorimeter (hereinafter referred to as “DSC”) and the crystal conversion rate of each layer is 0.1 or more and 0.85 or less. This is a step of obtaining a fired body.

The multi-layer semi-sintered body in this step, the multi-layer unsintered body is above the melting point of the PTFE sintered body, preferably above the melting point of the PTFE sintered body and below the maximum melting point of the powder used to obtain the multi-layer unsintered body. It is obtained by heating at the temperature of. It can also be obtained by heating the multi-layer semi-sintered body at a temperature higher than the melting point of the PTFE unsintered body for a very short time. It is necessary to be inside. However, no matter how long the multi-layer unfired body is heated at a temperature lower than the melting point of the PTFE fired body, a multi-layer semi-fired body cannot be obtained.

It is difficult to unconditionally determine the heating time required for the heat treatment in this step depending on the heating temperature, the thickness of the heated material, and other conditions, but generally, the higher the heating temperature, the shorter the heating time. Also, the thicker the film, the longer the heating time. In this way, the treatment conditions can be experimentally determined so that the crystal conversion rate can be obtained within the above range.

In this step, whether or not each layer of the multilayer unfired body has been semi-fired has a clear melting heat peak in the range of 332 to 348 ° C. on the crystal melting curve by DSC. The crystal conversion rate defined by the heat of fusion of the body is 0.
It can be judged by having 10 to 0.85.

Crystal melting curve is DSC (DSC-7 type manufactured by Perkin-Elemer)
Record using.

First, a sample of the PTFE unfired body is placed in a DSC aluminum pan, and the heat of fusion of the unfired body and the heat of fusion of the fired body are measured by the following procedure.

Step 1 Heat a sample of PTFE unfired body (raw material of PTFE fine powder that should form each layer) to 250 ° C at a heating rate of 50 ° C / min, and then 250 ° C to 380 ° C at a heating rate of 10 ° C / min. Heat up to.

An example of the crystal melting curve recorded in this heating step is shown in FIG. The position of the melting heat curve that appears in this step is defined as the "melting point of the PTFE unsintered body" or the melting point of the PTFE fine powder.

Step 2 Immediately after heating to 380 ℃, cool the sample at a cooling rate of 10 ℃ / min.
Cool to 50 ° C.

Procedure 3 Heat the sample again to 380 ° C at a heating rate of 10 ° C / min.

An example of the crystal melting curve recorded in the procedure 3 is shown in FIG. The position of the heat of fusion curve that appears in step 3 is
It is defined as "melting point of PTFE sintered body".

The heat of fusion of the PTFE unfired body or the fired body is proportional to the area between the endothermic curve and the baseline, and Perkin-Elmer DSC
For model -7, it is automatically calculated if the analysis temperature is set.

Subsequently, a sample is taken from the semi-sintered PTFE semi-sintered body layer made of this PTFE fine powder raw material after the heat treatment in this step, and the crystal melting curve of this sample is recorded according to the procedure 1. An example of the curve in this case is shown in FIG. The PTFE semi-sintered body that was semi-sintered in this step is
It has a clear heat of fusion peak in the range of 332 to 348 ° C.

Therefore, the crystal conversion rate is calculated by the following formula: Crystal conversion rate = ( _{ΔH 1 −ΔH 3} ) / ( _{ΔH 1 −ΔH 2} ) where _{ΔH 1} is the melting of the PTFE unsintered body. Heat (see FIG. 3), _Δ H ₂ is the heat of fusion of the PTFE fired body (see FIG. 4), and _Δ H ₃ is the heat of fusion of the PTFE semi-fired body (see FIG. 5). Therefore, when the crystal conversion rate of each layer measured after the heat treatment in this step is 0, for example, it remains a non-fired body, and when the crystal conversion rate is 1, it can be said to be a completely fired body.

The crystal conversion rate of the semi-sintered PTFE body of the layer heat-treated in this step must be 0.10 to 0.85, preferably 0.15 to 0.75.

(4) Stretching process The above (1) paste extruding process, (2) rolling process,
(3) The multi-layer semi-baked body obtained through the heat treatment step is stretched in at least a uniaxial direction.

Stretching is generally performed using a tenter device between rolls having different rotation speeds or in an oven. The stretching temperature is
It is suitable to carry out at a temperature below the melting point of the PTFE fired body.
The stretching ratio can be determined according to the purpose and can be uniaxial or biaxial. Usually, for industrial production, stretching is performed according to the following procedure.

(A) In the case of uniaxial stretching, the multi-layer semi-baked body is stretched in the same direction as the extrusion direction or in the direction perpendicular thereto.

(B) In the case of stretching in the biaxial direction, the multilayer semi-sintered body is first stretched in the uniaxial direction in the same manner as in (a), and then stretched in the direction perpendicular thereto.

By this stretching, each layer of the multi-layer semi-baked body became a porous structure in which micropores were uniformly distributed throughout, and finally a PTFE multi-layer porous membrane having micropores in each layer was obtained.

Further, the obtained multi-layer porous membrane is heated at a temperature not lower than the melting point of the PTFE fired body, or is heated at a temperature not lower than the stretching temperature, if necessary. By this heating, the dimensional change of the multilayer porous body disappears and the mechanical strength also increases.

Here, the average pore diameter of each layer of the multilayer porous membrane is determined by the type and composition of the PTFE fine powders 1, 2, 3, ... That is, in the present invention, as a means for obtaining a multilayer porous membrane composed of at least two layers having different average pore sizes, each layer is made of at least two types of PTFE.
It is important that it consists of fine powders 1, 2, 3 ...

As a condition that PTFE fine powders 1, 2, 3, ... differ from two or more kinds, the difference in melting heat peak on the crystal melting curve by DSC can be mentioned first.

The crystal melting curves of PTFE fine powder by DSC can be classified into various types depending on the manufacturing conditions, but it is generally difficult to classify them, but they are generally classified into the following two types. That is, there is a type I (shown as an example of this type in FIG. 6) that has a sharp peak of heat of fusion on the high temperature side at 341 to 348 ° C. and has no clear peak of heat of fusion below that temperature. In addition, the heat of fusion peak on the high temperature side at 337-348 ℃ and 333-342
There is a type II with a low temperature side peak at ℃ (Fig. 9 shows an example of this type). However, in the case of type II, one of the two heats of fusion peak does not show a clear peak and some peaks are observed as shoulders. (1 in Fig. 12
In general, type I PTFE fine powder and type II
When a multilayer unfired body composed of PTFE fine powder is heat-treated at a temperature higher than the melting point of the PTFE fired body, the layer made of type I PTFE fine howder has a small crystal conversion rate, and type II PTFE fine powder. A layer consisting of a multi-layer semi-baked product having a large crystal conversion rate is obtained, and when this multi-layer semi-baked product has a small crystal conversion rate when stretched in at least one axial direction, that is, the type I layer has a large average pore size And a large crystal conversion rate, that is, the type II layer is a multi-layer porous membrane having a small average pore size. Therefore, the PTFE that should constitute each layer of the multi-layer semi-sintered body
In selecting the fine powder, it suffices that the crystal conversion rates of the respective layers of the multi-layer semi-sintered body are different. In addition to the combination of the above type I and type II PTFE fine powders, for example, among the type I And combinations from Type II are also practically possible.

When combining the PTFE fine powder so that the crystal conversion rate of each layer of the multi-layer semi-baked material is different, the difference between the maximum value and the minimum value of the crystal conversion rate of each layer of the multi-layer semi-baked material should be 0.1 or more. Is preferred.

Next, as another condition for making the PTFE fine powders 1, 2, 3 ... Different from two or more kinds, at least one of the PTFE fine powders 1, 2, 3 ... contains a non-fibrous substance. There are cases.

Generally, PTFE fine powder has a property that fibers are easily formed from powder particles in a process such as a paste extrusion process, a rolling process, and a drawing process in which an object to be processed receives a shearing force.
On the other hand, particles of polymer such as PTFE low molecular weight polymer, PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer), and FEP (tetrafluoroethylene-hexafluoropropylene copolymer) are produced by the above-mentioned processing step. Does not form. Therefore, the fine powder layer containing a non-fibrous substance such as polymer particles that do not form fibers has a small number of fibers formed in each of the above steps, resulting in a large average pore size, and a layer consisting of only fine powder. Will have a smaller average pore size. Therefore, the non-fibrous material is preferably contained in the layer having a small crystal conversion rate. The polymer particles that do not form fibers do not fall off easily because they are incorporated in the fiber entanglement of the fine powder, but in order to completely eliminate the fallout, polymer particles that do not form fibers. It is effective to weld the fibers by heating at a temperature equal to or higher than the melting point of.

The mixing ratio of the non-fibrous polymer particles is 5 to 120 parts by weight with respect to 100 parts by weight of fine powder, preferably
20 to 100 parts by weight. If it is 5 parts by weight or less, there is no effect of mixing, and if it is 120 parts by weight or more, there is a problem that the strength of the multilayer porous membrane becomes weak.

Further, these non-fibrous substances are not limited to the above-mentioned fluororesins, but include inorganic substances such as carbon, graphite, titanium oxide, iron oxide, silica, glass fibers, glass beads and the like, and organic compounds. These objects can be achieved by mixing particles such as polyimide, polyamideimide, polyphenylene sulfide, aromatic polyester, and polyether ether ketone as molecules.

As described above, the production method of the present invention is
It is possible to obtain a multilayer porous membrane having at least two layers having different average pore sizes only by the steps of paste extrusion, rolling and stretching, and the respective layers are integrally bonded, and as a means for obtaining this, at least two layers are used. It is characterized by the fact that it is composed of the above PTFE fine powders 1, 2, 3 ... and that it does not require the troublesome process of laminating films.

According to the production method of the present invention, the filtration layer having the minimum average pore size that becomes the rate-determining amount of gas and liquid can be formed into a thin layer, so that the obtained multilayer porous membrane is a microfiltration filter having high permeability. Since the multi-layer interface is completely integrated, there is no fear of peeling at the multi-layer interface during use.

The multi-layer porous body obtained by the production method of the present invention, when in a flat plate shape, is useful as a liquid or gas microfiltration filter, a battery diaphragm, an electrolytic diaphragm, an electric insulating material and the like. Further, the tubular membrane is useful as a hollow fiber filter for liquid or gas, a material for artificial organs such as artificial blood vessels and artificial lungs, a tube for endoscopes, and the like.

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

(1) Film thickness It was measured using a film thickness meter manufactured by Mitutoyo Corporation (1D-110MH type).

(2) Porosity The weight (W, g) and absolute dry weight (Wo, g) and the volume (V, cm ³ ) of the membrane in which the pores were filled with pure water were measured using the ethanol substitution method, It was calculated using the following formula.

(W-Wo) x 100 / V (%) (3) Gas flow rate The porous membrane is cut into a circle with a diameter of 25 mm, and the effective permeation area is 2.15 cm ^2.
It was set in the filter holder of No. 3, and this was pressurized with 0.639 bar of nitrogen gas, and the amount of permeating nitrogen gas was measured with a mass flow meter.

From this measured value, the effective transmission area is 1 square centimeter (cm
² ) and the permeation amount per hour (unit: l / cm ² · hour) was calculated.

(4) Average pore size Coulter Porometer [Coulter Electronics (USA)
The mean flow pore size (MFP) measured by the above method was used as the average pore size. It was confirmed by the following model experiment that the actually measured average pore diameter of the multilayer porous membrane of the present invention substantially matches the pore diameter of the layer having the smallest pore diameter of the porous membrane.

(Model experiment) Average pore size measured by Coulter Poromer 0.2
0 μm, thickness 47 μm (porous membrane A) and average pore size 0.98 μm,
A single-layer PTFE porous membrane having a thickness of 69 μm (porous membrane B) was prepared. Next, the average pore diameter of a porous membrane consisting of two layers obtained by simply superposing the porous membrane A and the porous membrane B and a porous membrane consisting of three layers sandwiched by the porous membrane B as an intermediate layer was measured by a Coulter Porometer. Is 0.19μm, the latter is 0.18μ
The average pore size was m, and the average pore size was almost the same as the average pore size of the porous membrane A.

〔Example〕

In the examples and comparative examples of the present invention, the following PTFE fine powder was used.

The above PTFE fine powders 1 to 4 have an average primary particle size of about
It is a coagulated powder of 0.2 to 0.4 μm PTFE emulsion polymerization aqueous dispersion.

* Preparation method for PTFE fine powder 4 Average primary particle size with heat of fusion peak shown in Fig. 6 is approx.
2 to 0.4 μm of PTFE emulsion polymerization aqueous dispersion, and 100 parts by weight of each particle of the aqueous dispersion of PTFE low molecular weight polymer particles (manufactured by Daikin Industries, Ltd., trade name Lubron L-5) as a non-fibrous substance. Make a mixture. When this mixture is stirred in a stirring tank, the respective primary particles are uniformly mixed to form secondary agglomerated particles having a size of about 200 μm to 1000 μm. The secondary agglomerated particles were dried at 150 ° C. to remove water, and PTFE fine powder 3 was obtained.

Example 1 PTFE fine powder 1 (having a heat of fusion peak shown in FIG. 6) and 2 (having a heat of fusion peak shown in FIG. 9)
23 parts by weight of a liquid lubricant (trade name: Isopar M, manufactured by Exxon Co., Ltd.) were mixed with each other, and the thickness composition ratio of each layer was 1: 1 according to the procedure shown in FIG. A molded body was produced. Next, this multilayer preform was housed in a cylinder 12 of a paste extrusion mold shown in FIG. 2 and extruded by a ram 14 to obtain a sheet. further,
The obtained sheet is cut into a length of about 100 mm, rolled in a direction perpendicular to the extrusion direction, and then heated and dried in an oven at 250 ° C for 20 seconds to remove the liquid lubricant, and a thickness of 100
A multilayer unfired body of μm was obtained.

Here, the same multilayer unfired body as the above-mentioned multilayer unfired body was prepared by using one layer of powder colored in advance with a pigment, and the thickness cross section was observed with a stereomicroscope. It was confirmed that the composition ratio was 1: 1 like the thickness composition ratio of each layer of the multilayer preform.

Next, this multilayer unsintered body was heat-treated in an oven at 338 ° C. for 320 seconds to obtain a multilayer semi-sintered body.

The surface of the layer made of fine powder 1 of this multi-layer semi-baked product was shaved to collect a sample, and the melting heat peak on the crystal melting curve by DSC was measured. FIG. 10 shows the result of measurement of the heat of fusion peak of the layer.

Further, the result of measurement of the heat of fusion peak on the crystal melting curve by DSC of the calcined product of fine powder 1 is shown in FIG. 8, and the result of the measurement of the heat of fusion peak of the calcined product of fine powder 2 is shown in FIG.

From FIG. 6, FIG. 7 and FIG. 8, the crystal conversion ratio of the layer of fine powder 1 of the obtained semi-sintered body was 0.58.
From FIG. 9, FIG. 10 and FIG. 11, the crystal conversion ratio of the layer of fine powder 2 in the obtained multi-layer semi-baked product was 0.75.

Next, this multi-layer semi-baked body was stretched 3 times in the same direction as the rolling direction at 400% / sec in an oven at about 300 ° C., and further stretched 5 times in the direction perpendicular to the rolling direction to give a thickness of 59 μm. A multi-layer porous membrane of

A scanning electron micrograph (3000 times, hereinafter referred to as SEM photograph) of the layer-side surface made of fine powder 1 of this multilayer porous membrane is shown in Fig. 13, and an SEM photograph of the layer-side surface made of fine powder 2 is shown in Fig. 13. Shown in Figure 14. From these, it can be seen that the layer made of fine powder 1 has a large average pore diameter, and the layer made of fine powder 2 forms a multilayer porous membrane having a small average pore diameter.

The porosity of this multilayer porous membrane is 83%, the average pore diameter is 0.24 μm,
The gas permeation rate was 280 l / cm ² · hour.

Example 2 In the same manner as in Example 1 except that the PTFE fine powders 1 and 2 used in Example 1 were used and the thickness composition ratio of the layer made of fine powder 1 and the layer made of fine powder 2 was set to 4: 1. In accordance with the above, pressing, rolling and heat treatment were performed to obtain a multi-layer semi-baked body having a thickness of 100 μm.

Next, this multi-layer semi-baked body was stretched 6 times at 100% / sec in the same direction as the rolling direction in an oven at about 300 ° C to give a thickness of 59μ.
A multi-layer porous membrane of m was obtained.

As in Example 1, observation of the SEM photograph revealed that the layer made of fine powder 1 had a large average pore diameter and the layer made of fine powder 2 had a small average pore diameter. The porosity of this multilayer porous membrane was 63%, the average pore diameter was 0.08 μm, and the gas permeation amount was 13.5 l / cm ² · hour.

Example 3 The multilayer semi-baked body was stretched 3 times in the same direction as the rolling direction at 400% / sec in an oven at about 300 ° C., and further stretched 5 times in the direction perpendicular to the rolling direction. As well as the thickness
A 43 μm multilayer porous membrane was obtained.

From the SEM photograph, it was found that the layer made of fine powder 1 had a large average pore diameter and the layer made of fine powder 2 had a small average pore diameter. This multi-layer porous membrane has a porosity of 82%, an average pore diameter of 0.25 μm, and a gas permeation amount of 34%.
It was 5 l / cm ² · hour.

Example 4 Both melting heat peaks on the crystal melting curve by DSC are type I
As an example of a combination of fine powders belonging to I, first, fine powder 2 containing 23 parts by weight of a liquid lubricant.
Liquid lubricant (having the peak of heat of fusion shown in FIG. 9) 22
Extruding and rolling according to Example 2 using fine powder 3 (having a heat of fusion peak shown in FIG. 12 and also a copolymer with 0.01 wt% of perfluorovinyl ether) blended in parts by weight. However, the thickness composition ratio of the layer made of fine powder 2 and the layer made of fine powder 3 is 4
A multilayer unfired body having a thickness of 100 μm, which is a pair 1, was obtained. Next, this multilayer unsintered body was heat-treated in an oven at 338 ° C. for 150 seconds to obtain a multilayer semi-sintered body. The crystal conversion rates of the layer made of fine powder 2 and the crystal conversion rate of the layer made of fine powder 3 of the obtained multi-layer semi-baked product were measured in the same manner as in Example 1, and were 0.55 and 0.73, respectively. It was
Next, the multi-layer semi-baked body was biaxially stretched in the same manner as in Example 1 to obtain a multi-layer porous membrane having a thickness of 42 μm.

As a result of observing the SEM photograph, it was found that the layer made of fine powder 2 had a large average pore diameter and the layer made of fine powder 3 had a small average pore diameter. The porosity of this multilayer porous membrane was 80%, the average pore diameter was 0.19 μm, and the gas permeation amount was 129 l / cm ² · hour.

Example 5 A mixture of 100 parts by weight of the PTFE fine powder 1 used in Example 1 and 100 parts by weight of PTFE low molecular weight polymer particles was designated as PTFE fine powder 4, which was used in this PTFE fine powder 4 and Example 1. Using the PTFE fine powder 2 prepared above, the thickness composition ratio of the layer composed of the fine powder 4 and the layer composed of 2 was set to 4: 1, and extrusion, rolling, heat treatment, and stretching were carried out in accordance with Example 1 to obtain the thickness. A multi-layer porous membrane having a size of 49 μm was obtained.

FIG. 15 shows a SEM photograph of the layer side surface of fine powder 4 of this multilayer porous membrane, and FIG. 16 shows a SEM photograph of the layer side surface of fine powder 2. From these, the layer made of fine powder 4 has a large average pore size,
It can be seen that the layer made of fine powder 2 forms a multilayer porous membrane having a small average pore size.

The porosity of this multilayer porous membrane is 84%, the average pore diameter is 0.25 μm,
The gas permeation rate was 523 l / cm ² · hour.

Example 6 PTFE fine powder 4 used in Example 4 and Example 1
Using the PTFE fine powder 2 used in step 1, a multi-layer preform having a thickness composition ratio of 2: 1: 1 consisting of 3 layers in which a layer made of fine powder 2 is sandwiched by a layer made of fine powder 4 is prepared, Extrusion, rolling, heat treatment and stretching were carried out according to Example 1 to obtain a multilayer porous membrane having a thickness of 48 μm.

The pore diameter of this multilayer porous membrane is 83%, the average pore diameter is 0.24 μm,
The gas permeation rate was 435 l / cm ² · hour.

The thickness of the intermediate layer of this multilayer porous membrane was measured and found to be about 9 μm. When the inventors of the present invention made a trial production of this multilayer porous membrane by the conventional laminating method, it was difficult to form the membrane, and a membrane having a uniform thickness of about 9 μm was not obtained. Further, a physical breakage test was performed on the multilayer porous membranes obtained in Examples 1 to 6, but no peeling from the laminated interface was observed.

Comparative Example 1 Using only PTFE fine powder 2, extrusion, rolling, heat treatment and stretching were carried out according to Example 1 to obtain a porous film having a thickness of 35 μm.

The porosity of this porous membrane was 82%, the average pore diameter was 0.24 μm, and the gas permeation rate was 207 l / cm ² · hour.

Comparative Example 2 Using only PTFE fine powder 2, extrusion, rolling, heat treatment and stretching were carried out according to Example 2 to obtain a porous film having a thickness of 56 μm.

The porosity of this porous membrane was 62%, the average pore diameter was 0.08 μm, and the gas permeation amount was 5.6 l / cm ² · hour.

Comparative Example 3 Using only PTFE fine powder 3, extrusion, rolling, heat treatment and stretching were carried out according to Example 4 to obtain a porous film having a thickness of 40 μm.

The porosity of this porous membrane was 77%, the average pore diameter was 0.19 μm, and the gas permeation rate was 111 / cm ² · hour.

The above results are shown in Table 1.

[Brief description of drawings]

FIG. 1 is an explanatory view showing a procedure for producing a multilayer preform. FIG. 2 is a cross-sectional view showing a state where paste extrusion molding is performed. FIG. 3, FIG. 4 and FIG. 5 show examples of the crystal melting curves of the PTFE unfired body, the fired body and the semi-fired body, respectively, obtained by a differential scanning calorimeter. FIG. 6, FIG. 7 and FIG. 8 show the crystal melting curves of the PTFE fine powder 1 used in Example 1, respectively, for the unfired body, semi-fired body and fired body by a differential scanning calorimeter. FIG. 9, FIG. 10 and FIG. 11 show crystal melting curves of the unsintered, semi-sintered and sintered PTFE fine powder 2 used in Example 1 by a differential scanning calorimeter. FIG. 12 shows PTFE fine powder 3 used in Example 4.
3 shows a crystal melting curve of the unfired body of Example 1 by a differential scanning calorimeter. 13 to 16 are scanning electron micrographs (3000 times) of the following portions of the multilayer porous membranes obtained in Example 1 and Example 5, respectively. 13: Surface of layer composed of PTFE fine powder 1 of multi-layer porous membrane of Example 1 FIG. 14: Surface of layer of PTFE fine powder 2 of multi-layer porous membrane of Example 1 FIG. 15 ... Surface of layer composed of PTFE fine powder 4 of multi-layer porous membrane of Example 5 FIG. 16 Surface of layer composed of PTFE fine powder 2 of multi-layer porous membrane of Example 5 8 ... Mold, 9 ... … Lower mold, 10 …… Upper mold 12 …… Cylinder, 13 …… Exit part, 14 …… Ram

Claims (4)

[Claims] 1. A method for obtaining a polytetrafluoroethylene multi-layer porous membrane comprising at least two layers having different average pore diameters, in which at least 2 liquid lubricants are mixed.
Multiple types of polytetrafluoroethylene fine powders 1, 2, 3, ... Are separately filled in a cylinder of an extrusion mold, paste-extruded into multiple layers, and then rolled as necessary to form multiple layers. Body, and then the liquid lubricant is removed from the multi-layer molded body to obtain a multi-layer unsintered body, and the multi-layer unsintered body is heated at a temperature not lower than the melting point of the polytetrafluoroethylene fired body. A method for producing a polytetrafluoroethylene multilayer porous membrane, comprising: obtaining a semi-baked multilayer semi-baked body, and then stretching it in at least uniaxial direction. 2. At least one of polytetrafluoroethylene fine powders 1, 2, 3, ... Has a melting heat peak on a crystal melting curve by a differential scanning calorimeter different from the others. The method for producing a polytetrafluoroethylene multilayer porous membrane according to claim 1. 3. In a multi-layer semi-baked body composed of polytetrafluoroethylene fine powders 1, 2, 3 ..., The difference between the maximum value and the minimum value of the crystal conversion rate of each layer is 0.1 or more. The method for producing a polytetrafluoroethylene multilayer porous membrane according to claim 1, wherein. 4. The polytetrafluoroethylene multilayer porous membrane according to claim 1, wherein at least one of the polytetrafluoroethylene fine powders 1, 2, 3 ... Production method.