JP2012120969A - Porous multi-layer filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the particle collection rate without increasing the processing time and the filter size in a microfiltration filter.SOLUTION: A porous multi-layer filter includes: a plurality of filtration membranes (2A, 2B); and an intermediate reservoir membrane (4) interposed between the filtration membranes and comprising a porous membrane. The average pore diameters of the plurality of filtration membranes are made equal while the average pore diameter of the intermediate reservoir membrane is set to be equal to or more than 1.2 times the average pore diameters of the filtration membranes and ≤5 μm.

Description

  The present invention relates to a porous multilayer filter, and more particularly to a micropore filter capable of filtering ultrafine particles at a high flow rate.

In recent years, high-performance microfiltration filters have been demanded in various fields such as semiconductor-related fields, liquid crystal-related fields, food fields, and medical fields. Specifically, in semiconductor manufacturing, the degree of integration increases year by year, and the circuit interval is miniaturized to a region of 0.1 μm or less. Similarly, in the liquid crystal production, fine processing using a photosensitive material is performed. From the above, there is a need for a microfiltration filter that can reliably capture fine particles in a smaller region. These precision filtration filters are mainly used as clean room outdoor air treatment filters and chemical filtration filters, and the filtration performance also affects the product yield.
Further, in the food / medical field, due to the recent increase in safety awareness, there is a strong demand for the completeness of filtration (absolute removability) for minute foreign substances.

  It is necessary to reduce the pore size of the filtration layer as the particles to be collected become finer. When the pore diameter of the filtration layer is reduced, the flow rate is naturally reduced. Specifically, the flow rate of the treatment liquid that passes through the filtration layer is “(pore diameter squared × porosity) / film thickness”, and the flow rate decreases significantly as the pore size decreases. Therefore, it is effective to make the filtration membrane thin in order to suppress a decrease in the flow rate of the processing liquid.

  Therefore, in the Japanese Patent No. 4371176, the present applicant applied a fluororesin dispersion in which a fluororesin powder is dispersed in a dispersion medium on a smooth foil (or plate or film), and then dried the dispersion medium. And a microporous film obtained by stretching a fluororesin film having a film thickness of 20 μm or less and a Gurley second of 300 seconds or more, which is formed by heating substantially above the melting point of the fluororesin powder, and having substantially no defects. Yes.

Japanese Patent No. 4371176

The porous fluororesin membrane provided in Patent Document 1 can be made as thin as 20 μm or less, further 10 μm or less, and the pore size of the fluororesin membrane can be used as a filtration membrane for collecting nano-sized particles. The flow rate of the processing liquid can be suppressed by reducing the film thickness.
However, in order to increase the collection rate of fine particles and shorten the processing time, there is a limit to responding only by reducing the thickness of the filtration membrane.

  Therefore, the present invention drastically increases the collection rate while suppressing a decrease in the treatment flow rate by using a filtration membrane that is thinned as a filtration membrane capable of collecting nano-sized fine particles, and the filtration efficiency (filterability). The challenge is to provide an excellent filter.

In order to solve the above problems, the present invention comprises a plurality of filtration membranes and an intermediate storage membrane comprising a porous membrane interposed between the filtration membranes,
An average pore size of the plurality of filtration membranes is made equal, and an average pore size of the intermediate storage membrane is 1.2 times or more of the average pore size of the filtration membrane and 5 μm or less. providing.

The intermediate storage membrane of the present invention comprises a porous membrane comprising a fibrous skeleton formed by connecting flexible fibers in a mesh shape by knots, and surrounding the substantially slit-shaped hole by the fibrous skeleton.
The thickness of each filtration membrane is preferably 0.25 to 15 μm, and the average pore diameter is preferably 0.1 μm or less. In particular, the filtration membrane is preferably composed of a microporous membrane produced by stretching a fluororesin membrane having a thickness of less than 10 μm obtained by heating the fluororesin to a melting point or higher.
The lower limit of the average pore diameter of the filtration membrane is preferably 1 nm or more, more preferably 0.1 nm or more. Also. The average pore size of the intermediate storage membrane is more preferably twice or more the average pore size of the filtration membrane.

[Measurement method of average pore size]
The average pore size (also referred to as average flow rate pore size) was measured as GALWICK (propylene, 1, 1, 2, 3, 3, 3) by using a pore distribution measuring device (palm porometer: CFP-1500A: manufactured by Porous Materials, Inc.) as a liquid. Measurement was performed using trioxide hexahydrofluoric acid (manufactured by Porous Materials, Inc.).
Specifically, it was determined as follows.
First, the relationship between the differential pressure applied to the membrane and the flow rate of air passing through the membrane was measured when the membrane was dry and when the membrane was wet with liquid. Let it be a wetting curve. Let P (Pa) be the differential pressure at the intersection of the curve with the dry curve flow rate halved and the wetting curve. The average flow pore size is determined by the following formula.
Average pore diameter d (μm) = cγ / P
Here, c is a constant of 2860, and γ is the surface tension (dynes / cm) of the liquid.

As described above, since the filtration membrane is a thin film, the average pore size is set to 0.1 μm or less so that the flow rate does not decrease even when collecting nano-sized fine particles. Particularly, the pore size of the filtration membranes arranged in multiple stages is the same. Therefore, the flow rate does not decrease even as a multilayer filter in which filtration membranes are arranged in multiple stages.
In addition, by arranging the filtration membrane in multiple stages, the particulates that could not be collected by the filtration membrane arranged on the inflow side can be collected by the filtration membrane arranged on the subsequent stage of the outflow side, thereby increasing the collection rate. it can. Simply, if the collection rate is 20% with one filtration membrane, a multilayer filter with two filtration membranes is used, and the collection rate is 20% x the number of filtration membranes. The collection rate can be dramatically increased. In addition, the collection rate is precisely the collection rate = (1−0.8 ^{number of films} ) × 100%.

In particular, in the present invention, an intermediate storage membrane is interposed between the filtration membranes, and the pore diameter of the intermediate storage membrane is 1.2 times or more, preferably 2 times or more and 5 μm or less of the filtration membrane. This allows the treatment liquid that has permeated the filtration membrane on the inflow side to function to be collected once without lowering the flow rate before flowing into the next filtration membrane, and from the collection target that the filtration membrane could not be collected here. Large particles are collected in advance to reduce clogging of the filtration membrane on the outflow side, thereby increasing the permeation rate and increasing the collection rate of the fine particles.
Further, the entire filtration membrane can be made continuous through the intermediate storage membrane, so that the whole can be thinned, and the permeation time of the treatment liquid of the multilayer filter can be shortened and the collection rate can be improved.

The filtration membrane is provided integrally with the support membrane to form a pair, the average pore diameter of the support membrane is larger than the average pore diameter of the filtration membrane and the thickness is thicker than the filtration membrane,
A first set of the support membranes interposed between the first set of filtration membranes and the second set of filtration membranes as the intermediate storage membrane;
Alternatively, the support film and the intermediate storage film may be separated and laminated with the intermediate storage film interposed between each pair.

  The filtration membrane with a reduced average pore size is made thinner to suppress a decrease in flow rate. However, when the film thickness is reduced, the strength of the filtration membrane is lowered and the handling becomes difficult. Therefore, it is easy to handle in manufacturing by assembling a pair with a support film having a large thickness. In addition, the pore size of the support membrane is made larger than the pore size of the filtration membrane so that the flow rate does not decrease in the support membrane.

The thickness of the porous membrane constituting the support membrane and the intermediate storage membrane is preferably 2 to 100 μm, and the average pore size is preferably 1.2 times or more the average pore size of the filtration membrane.
When the support membrane is separated from the intermediate reservoir membrane, the support membrane has a thickness equal to or greater than that of the intermediate reservoir membrane within the above range, and the average pore diameter is preferably 10 μm or less.

When the filtration membrane layer is the A layer, the support membrane layer is the B layer, and the intermediate storage membrane layer is the C layer, the laminated structure can be configured as follows.
A layer / B layer (also serves as C layer) / A layer / B layer, B layer / A layer / C layer / A layer / B layer, B layer / A layer / C layer / A layer / C layer / A layer / B layer etc.
It is preferable to interpose the C layer of the intermediate storage membrane between the A layers of the filtration membrane, and to use both outer layers as the B layer of the support membrane because the filtration membrane can be protected. On the other hand, if the exposed surface is the A layer, propagation of bacteria between layers can be prevented.

The porosity of the filtration membrane is preferably 30 to 90%, and the porosity of the support membrane and the intermediate storage membrane is preferably larger than the porosity of the filtration membrane, and is 50 to 90%.
When the porosity of the filtration membrane is less than 30%, the flow rate is too low, and when it exceeds 90%, the strength may be too low.
The porosity can also be estimated from Gurley seconds. You may calculate and calculate from the method of ASTM-D-792, and the volume and true specific gravity of a film | membrane. It shows that it is excellent in the permeability, so that this figure is high.

[Measurement method of air permeability (Gurley second)]
It was measured using a Wangken type air permeability measuring device (manufactured by Asahi Seiko Co., Ltd.) having the same structure as the Gurley air permeability tester specified in JIS P 8117 (Paper and paperboard air permeability test method). The measurement result is expressed in Gurley seconds.

It is preferable that the filtration membrane, the support membrane and / or the intermediate storage membrane to be laminated are closely integrated by welding or adhering the interface between adjacent porous membranes.
In the case of welding by heating, the interface is contact-fixed by heating above the melting temperature. If the thickness of the filter membrane is reduced to 1 μm or less, the pores may be closed due to deformation by heating and melting. Therefore, a solvent-soluble resin adhesive may be applied partially or entirely and bonded together. Is preferable.
It should be noted that the interfaces of the porous films to be laminated may not be bonded, and the outer periphery of the laminated body may be sandwiched and integrated with a frame. In this case, a gap may be generated at the interface of the porous film, but the gap can function as a reservoir for collecting fine particles.

The porous multilayer filter of the present invention preferably has an overall thickness of 1.6 μm to 300 μm and is accommodated in one filter cartridge.
This is because the minimum thickness when the filtration membrane is laminated on both surfaces of one intermediate storage membrane is 1.6 μm under the current manufacturing conditions, and a large number of filtration membranes such as 3 to 7 are laminated. This is because the thickness is increased to 100 μm to 300 μm. If it exceeds 300 μm, it is bulky when accommodated in a filter cartridge, and the membrane area cannot be obtained.

The porous membrane constituting the filtration membrane, the intermediate storage membrane, and the support membrane to be laminated is preferably made of a fluororesin membrane. This is because the fluororesin thin film has characteristics such as excellent chemical resistance and heat resistance.
Fluororesin includes PTFE, tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene / hexafluoropropylene copolymer, polychloro / trifluoroethylene, tetrafluoroethylene / ethylene copolymer, chlorotri One type selected from the group consisting of fluoroethylene / ethylene copolymer, polyvinylidene fluoride, and polyvinyl fluoride, or a mixture of two or more types.

Among the fluororesin thin films, it is preferable to use a porous PTFE sheet mainly composed of PTFE as the filtration membrane, the intermediate storage membrane and / or the support membrane.
Here, “mainly” means containing at least 50% by volume or more. In addition, the fluororesin mainly composed of PTFE includes a resin composed entirely of PTFE.
The porous PTFE (polytetrafluoroethylene) sheet is porous in addition to the properties of PTFE itself such as high heat resistance, chemical stability, weather resistance, nonflammability, high strength, non-adhesiveness, and low coefficient of friction. It has characteristics such as flexibility, liquid permeability, particle trapping, and low dielectric constant of the body.
The PTFE porous filter is suitably used as a liquid / gas microfiltration filter (membrane filter), particularly in semiconductor-related fields, liquid crystal-related fields, and food / medical fields, due to its excellent chemical stability and other characteristics. .

  The filtration membrane comprising the porous PTFE sheet used in the present invention was formed into a film by applying a fluororesin dispersion in which a fluororesin powder mainly composed of PTFE was dispersed in a dispersion medium on a smooth foil or film. Thereafter, the dispersion medium is dried, and a fluororesin film obtained by heating and sintering the fluororesin at a temperature equal to or higher than the melting point is preferably stretched to be made porous.

That is, the filtration membrane made of the porous PTFE sheet is a fluororesin membrane disclosed in Patent Document 1 relating to the application of the present applicant. When the fluororesin membrane is used, both the fine pore size and the thin membrane of the filtration membrane can be achieved. That is, a step of forming a fluororesin thin film, and, after the formation of the fluororesin thin film, the filtration membrane on one side or both sides of a porous substrate to be the fluororesin thin film and the support film or / and the intermediate storage film Can be easily adhered.
As described above, it is preferable to use an adhesive for adhesion between the filtration membrane made of the fluororesin thin film and the intermediate storage membrane or / and the support membrane made of the porous substrate. As the adhesive, use of a solvent-soluble or thermoplastic fluororesin or fluororubber is more preferable because it can be used for applications that make use of the heat resistance and chemical resistance of the fluororesin thin film itself.

  Among the fluororesins mainly composed of PTFE, it is more preferable to include a thermoplastic fluororesin or / and to include a nonionic water-soluble polymer having a molecular weight of 10,000 or more such as polyethylene oxide and polyvinyl alcohol. Since these do not affect the dispersion of the fluororesin dispersion, and gelate when dried with water to form a film, a fluororesin thin film with fewer defects and a longer Gurley second can be obtained.

  As the thermoplastic fluororesin, a resin having a low surface tension at the time of melting and a low melt viscosity is preferable because it has a large effect of suppressing the defects. Specifically, the melt flow rate is preferably 5 g / 10 min or more, more preferably 10 g / 10 min or more, and further preferably 20 g / 10 min or more.

  More specifically, as thermoplastic fluororesin used in combination with PTFE, PFA, tetrafluoroethylene / hexafluoropropylene copolymer (FEP), tetrafluoroethylene / hexafluoropropylene / perfluoroalkyl vinyl ether copolymer (EPA) ), Tetrafluoroethylene / ethylene copolymer (ETFE), and the like, and a plurality of these may be used in combination. Among them, PFA is preferable because it has a melting point (327 ° C.) or higher of PTFE and the thermal decomposition hardly proceeds. In particular, it is preferable that the thermoplastic fluororesin is PFA and the volume ratio (solid content) of PFA is less than 20% with respect to the volume of the mixture of PTFE and PFA because defects are particularly reduced.

  The molecular weight of PTFE used for the fluororesin dispersion is preferably 1 million to 5 million, more preferably 1 million to 3.5 million, and even more preferably 1.2 million to 1.8 million. When the molecular weight is too high, the porosity tends to decrease, and when the molecular weight is too low, there is a tendency that pinholes are generated or the film is easily broken during stretching. The molecular weight of PTFE may be adjusted during the polymerization of PTFE, or high molecular weight PTFE is used as a raw material, and ionizing radiation irradiation or heating is performed on the raw material or a molded product obtained from the raw material. Alternatively, the polymer chain may be adjusted by a method of decomposing.

  Further, the heat of fusion of the fluororesin is preferably 32 J / g or more. Here, the measurement of the heat of fusion is performed using a heat flux differential scanning calorimeter (Heat flux differential scanning calorimeter DSC-50 manufactured by Shimadzu Corporation).

The present invention provides a separation membrane element characterized by using the porous multilayer filter as a separation membrane.
The laminated filter including the intermediate storage membrane of the present invention between the filtration membranes is very flexible and is easy to handle because it is excellent in mechanical strength and the like due to the porous substrate. Therefore, it can be used for production of various known separation membrane elements. For example, a pleated module type separation membrane element, a spiral module type separation membrane element, or a tubular module in which a porous fluororesin composite thin film is wound around a porous support and formed into a tubular shape after being processed into a flat membrane. A separation membrane element (module) can be manufactured by loading a large membrane area such as a mold separation membrane element into a compact container.
If the porous multilayer filter of the present invention is used as a filter for a separation membrane element, it is possible to provide a separation membrane element that has high filtration efficiency and maintainability and little reduction in filtration efficiency due to clogging. And the filtration system comprised with this separation membrane element is used suitably for gas-liquid absorption, deaeration, and filtration in a semiconductor, a foodstuff, another field | area, etc.

  As is clear from the above description, the present invention has a configuration in which a plurality of filtration membranes having the same pore diameter are stacked with an intermediate storage membrane interposed, and particles are collected using a multistage sieving mechanism. In addition, the collection rate of fine particles can be dramatically increased. In addition, since each filtration membrane and the intermediate storage membrane are thin, the treatment liquid can permeate through a plurality of filtration membranes without lowering the permeate flow rate and flow velocity, and the particulate collection rate can be dramatically increased.

It is a schematic sectional drawing which shows the porous multilayer filter of 1st Embodiment of this invention. (A)-(E) are schematic which shows a manufacturing process. (A) (B) (C) is the schematic which shows the porous multilayer filter of other embodiment.

Hereinafter, embodiments of the porous multilayer filter of the present invention will be described with reference to the drawings.
1 and 2 show a porous multilayer filter 1 (hereinafter abbreviated as filter 1) of the first embodiment.
In the filter 1 of the first embodiment, two sets (first set I and second set II) in which the filtration membrane 2 and the support membrane 3 are integrated are provided, and the first set I and the second set II are intermediate. They are stacked via the storage film 4. That is, as shown in FIG. 1, from the processing liquid inflow side A to the outflow side B, the first set I support membrane 3A, the filtration membrane 2A, the intermediate storage membrane 4, the second set II filtration membrane 2B, and the support membrane 3B. The supporting membranes 3A and 3B are disposed on the outer surfaces on both sides, and the intermediate storage membrane 4 is interposed between the filtration membranes 2A and 2B.

  The filtration membrane 2, the support membrane 3, and the intermediate storage membrane 4 are all made of a porous expanded PTFE sheet, and have a fibrous skeleton in which flexible fibers are connected in a network shape by knots. It consists of a porous membrane surrounding the slit-shaped holes 2h, 3h, 4h, and the average pore diameter and thickness are set as follows.

  The filtration membrane 2 (2A, 2B) is made of the same microporous membrane. The filtration membrane 2 has an average pore size of 0.1 nm to 0.1 μm, preferably 1 nm to 70 nm, more preferably 10 nm to 50 nm. The thickness is 0.25 μm to 3 μm, preferably 1.6 to 2.0 μm, and the porosity is 20 to 90%, preferably 50 to 80%.

  The support membrane 3 (3A, 3B) is also formed of the same porous membrane, and the average pore diameter is preferably 5 to 1000 times the average pore diameter of the filtration membrane 2 and is about 0.1 μm to 10 μm. The thickness is 3 μm to 100 μm, and the porosity is 20 to 90%, preferably 50 to 80%.

  The intermediate reservoir membrane 4 has an average pore size of 1.2 times to 5 μm, preferably 2 times or more, more preferably 2 times or more the average pore size of the filtration membrane 2, specifically an average pore size of 0.1 μm to 5 μm, preferably 0.1 ˜1 μm. The thickness is approximately the middle between the filtration membrane 2 and the support membrane 3, and the specific thickness is 3 μm to 100 μm.

  The average pore diameters of the filtration membrane 2, the support membrane 3, and the intermediate storage membrane 4 are measured by the pore distribution measuring instrument (palm porometer: CFP-1500A) described above.

  As described above, in the filter 1 of the present invention, the first set I filtration membrane 2A arranged on the inflow side of the processing liquid and the second set II filtration membrane 2B arranged on the outflow side have the same pore diameter. The pore diameter of the filtration membrane 2B on the outflow side is not substantially smaller than the pore diameter of the filtration membrane 2A on the inflow side, and the average pore size between the filtration membranes 2A and 2B is larger than that of the filtration membranes 2A and 2B. It is important that the intermediate storage film 4 is interposed.

When the intermediate storage membrane 4 is interposed between the filtration membranes 2A and 2B, fine particles larger than the target size fine particles that could not be collected by the filtration membrane 2A are collected by the intermediate storage membrane 4 and collected in the filtration membrane 2B. The occurrence of clogging can be suppressed. In addition, the collection rate of the entire filter is obtained by multiplying the collection rate of fine particles in each filtration membrane by the number of filtration membranes (2 in this embodiment) using the mechanism of multistage sieving. = 1− (collection rate of 1-1 filtration membranes) The ^{number of filtration membranes} .

The filter 1 of this embodiment has two sets of filtration membranes and support membranes, and the intermediate storage membrane and the five-layer filter 1 has an overall thickness of 100 to 250 μm.
In the case of a three-layer structure in which the filtration membranes 2A and 2B are laminated on both surfaces of the intermediate storage membrane 4, the thickness can be reduced to 1.6 μm, and in the seven-layer structure having three filtration membranes, the thickness can be increased to about 300 μm.

  The first set and the second set of filtration membranes 2 and the support membrane 3 are formed in an integrated manner in advance, as will be described later, and thereafter, the first set of filtration membranes 2A and the second set of filtration membranes are formed. The intermediate storage membrane 4 is sandwiched between the filtration membranes 2B and bonded with an adhesive to form the filter 1.

Below, a manufacturing process is demonstrated with reference to FIG.
In the first step, the fluororesin dispersion is adjusted, and the prepared fluororesin dispersion is dropped on the smooth foil 10 shown in FIG. 2A and stretched uniformly on the foil 10. Then, after drying, it sinters and forms the fluororesin thin film 20 fixed on the foil 10. In the second step, the PFA dispersion used as an adhesive is adjusted, and the adhesive 23 is dropped on the surface of the fluororesin thin film 20 fixed to the foil 10 and stretched uniformly. A stretched PTFE porous body 30 (manufactured by Sumitomo Electric Fine Polymer Co., Ltd., trade name: PORFLON) is placed over the membrane.
In this state, heating is performed at a required temperature for a required time, and then natural cooling is performed. As a result, the fluororesin thin film 20 is bonded to the expanded PTFE porous body 30 with the adhesive 23 made of PFA, and a composite in which the foil 10 is fixed to the fluororesin thin film 20 is obtained.
In the third step, the foil 10 is dissolved and removed with hydrochloric acid to obtain a fluororesin composite 25 in which the nonporous fluororesin thin film 20 and the expanded PTFE porous body 30 are integrally bonded with an adhesive 23 of PFA. .
In the fourth step, the fluororesin composite 25 is stretched at a required magnification (3 times in the present embodiment) with a stretching machine, and the nonporous fluororesin thin film 20 is formed as shown in FIG. The filter membrane 2 is made porous by making pores, and the pores of the expanded PTFE porous body 30 are widened to form a support membrane 3, and a set of composites 5 in which the filter membrane 2 and the support membrane 3 are integrated. obtain.
In the fifth step, as shown in FIG. 2 (E), the second set of composite 5B composed of the obtained filtration membrane 2 (2B) and the support membrane 3 (3B) is the filtration membrane 2B as an upper layer, A first expanded PTFE porous membrane (trade name: Porefluorone) serving as the intermediate storage membrane 4 is laminated on the upper surface, and a first set comprising a filtration membrane 2 (2A) and a support membrane 3 (3A) on the surface. The composite 5A is laminated.
In the sixth step, the entire laminate is made translucent by wetting with IPA, and after confirming that there are no voids at the interface with the naked eye, it is dried, and the porous composite having an intermediate storage membrane between the filtration membranes of the above embodiment is used. The layer filter 1 is obtained.

As the dispersion medium constituting the fluororesin dispersion used in the first step, an aqueous medium such as water is usually used. The content of the fluororesin powder in the fluororesin dispersion is preferably in the range of 20% by weight to 70% by weight.
For the application of the fluororesin dispersion to the smooth foil, a coater such as a capillary method, a gravure method, a roll method, a die (lip) method, a slit method or a bar method can be used as a coating device. In particular, for forming a thin film, a capillary method, a die method, a slit method and a bar method are preferable.

Depending on the method of applying the fluororesin and the variation or deflection of the smooth foil film thickness, the coating jig of the applicator directly touches the surface of the foil and damages the foil surface, resulting in damage to the fluororesin thin film. When transferred, surface irregularities may be generated, and when severe, defects such as pinholes may occur. Therefore, if an anionic surfactant is added as a lubricant at 0.5 mg / ml or more, more preferably 2.5 mg / ml or more, the friction coefficient can be lowered, thereby suppressing the occurrence of defects such as surface irregularities and pinholes. Can do. Further, the addition of the anionic surfactant is preferably 30 mg / ml or less, more preferably 10 mg / ml or less. When the amount of the anionic surfactant exceeds this upper limit, problems such as viscosity becoming too high and resin aggregation tend to occur are likely to occur. In addition, decomposition residue remains and discoloration easily occurs. Therefore, it is preferable to use a fluororesin dispersion containing 0.5 to 30 mg / ml of an anionic surfactant.
Examples of the anionic surfactant include carboxylic acid types such as polyoxyethylene alkyl ether carboxylic acid ester salts, sulfuric acid ester types such as polyoxyethylene alkyl ether sulfonic acid ester salts, and polyoxyethylene alkyl ether phosphoric acid ester salts. And surfactants such as phosphate ester type. However, the dispersibility of the fluororesin powder decreases due to the addition of an anionic surfactant, so if you add a surfactant, you can always complete the production within a time that does not cause sedimentation, separation, etc. A production method is preferred while continuing to add stirring such as ultrasonic waves.
After the application, the dispersion medium is dried. Drying can be performed by heating to a temperature close to or higher than the boiling point of the dispersion medium. A film made of fluororesin powder is formed by drying, and a fluororesin thin film can be obtained by heating and sintering the film to a temperature equal to or higher than the melting point of the fluororesin.

The effect of reducing defects such as voids and cracks in the obtained fluororesin thin film can also be obtained by adding a water-soluble polymer that gels under high concentration conditions to a fluororesin powder mainly composed of PTFE. By adding both the thermoplastic fluororesin and the water-soluble polymer, the effect becomes more remarkable.
If this water-soluble polymer is nonionic, there is little or no effect on the dispersibility of the fluororesin. Therefore, the water-soluble polymer is preferably nonionic rather than anionic or cationic. The molecular weight of the nonionic water-soluble polymer is preferably 10,000 or more. By setting the molecular weight to 10,000 or more, during drying, the film is formed by gelation before water is completely removed, so that the generation of cracks due to the surface tension of water can be suppressed.
Specific examples of the water-soluble polymer include polyethylene oxide, polypropylene oxide, polyvinyl alcohol, starch, and agarose.
The water-soluble polymer content in the fluororesin dispersion is preferably 1 mg / ml or more and 34 mg / ml or less. When the amount is less than 1 mg / ml, the effect of containing the water-soluble polymer may not be sufficiently exhibited. On the other hand, when the amount exceeds 34 mg / ml, the viscosity of the fluororesin dispersion may become too high and handling may be difficult.

  Before the stretching step, the fluororesin thin film 20 preferably has a Gurley second of 1000 seconds or more. When the Gurley second is 1000 seconds or more, there are fewer defects such as voids and cracks, and since the film thickness is 20 μm or less, a high processing speed can be obtained as a filter. In particular, when the Gurley second is 5000 seconds or more, the number of defects is further reduced, which is more preferable.

Here, the Gurley second is a numerical value representing the air permeability (air permeation amount) described in JIS-P8117, and specifically, the time (second) for 100 ml of air to pass through an area of 645 cm ^2. Represents. When the thin film has a defect, since air permeates through the defect, the Gurley second becomes small, but as the defect decreases, the air becomes difficult to permeate and the Gurley second increases.

  Regarding the stretching conditions in the stretching step of the fourth step, the temperature at which the fluororesin composite 25 is stretched is preferably 100 ° C. or lower, more preferably 50 ° C. or lower, which is lower than the melting point of the fluororesin. The obtained porous fluororesin composite is formed of a porous fluororesin thin film having a uniform pore size and a high porosity and few defects. Since this membrane is thin, it has a high processing speed and excellent filterability. In addition, since it is a composite of a porous fluororesin thin film and a porous substrate, it has excellent chemical resistance, heat resistance and mechanical strength. Therefore, the porous fluororesin composite of the present invention exhibits excellent filterability and is excellent in chemical resistance, heat resistance and mechanical strength, and is suitable as a filter.

  Hereinafter, the membranes of Examples 1 and 2, Comparative Examples 1 and 2, and Reference Examples 1 and 2 of the present invention were prototyped, and the average pore diameter and air permeability (Gurley second) were measured by the measurement method described above. The collection rate was measured by the following method.

[Measurement method of collection rate]
A spherical polystyrene particle latex (Bangs Laboratories, Inc. catalog code: DS02R) having an outer diameter of 0.030 μm is diluted 50 times with a 0.1% aqueous solution of polyoxyethylene (10) octylphenyl ether, and this solution is used as a test solution. To do. By punching a sample was fabricated into a disk of 47 mm, after impregnated with isopropanol, was fixed to the filtration holder (effective area 9.61cm ^2), it was filtered test liquid 5ml differential pressure 0.42kgf / cm ^2. The standard particle concentrations of the test solution and the filtrate were determined by measuring the particle concentration from the absorbance at 300 nm using a spectrophotometer (UV-160, manufactured by Shimadzu Corporation), and obtaining the collection rate from the following equation.
Collection rate = <1- (standard particle concentration of filtrate) / (standard particle concentration of test solution)> × 100 [%]

"Reference Example 1"
PTFE dispersion 34JR (Mitsui / Dubon Fluoro Chemical Co.) having a heat of fusion of 50 J / g, MFA Latex D5010 (Solvay Solexis), and PFA Dispersion 920HP (Mitsui / Dubon Fluoro Chemical Co.) were used. A fluororesin dispersion in which MFA / (PTFE + MFA + PFA) (volume ratio) and PFA / (PTFE + MFA + PFA) (volume ratio) were 2% each was prepared. Furthermore, a fluororesin dispersion was prepared by adding polyethylene oxide having a molecular weight of 2 million to a concentration of 0.003 g / ml and polyoxyethylene alkyl ether sulfate triethanolamine (20T manufactured by Kao Corporation) to a concentration of 10 mg / ml. .
Next, an aluminum foil having a thickness of 50 nm is spread and fixed on a glass plate so that there are no chicks. The fluororesin dispersion adjusted as described above is dropped, and then a stainless steel slide shaft (stainless steel by Nippon Bearing Co., Ltd.). A fine shaft SNSF type, outer diameter 20 mm) was slid so that the fluororesin dispersion was evenly spread over the entire surface of the aluminum foil. The foil was dried at 80 ° C. for 60 minutes, heated at 250 ° C. for 1 hour, and heated at 340 ° C. for 1 hour, and then naturally cooled to form a nationally defined fluororesin thin film on the aluminum foil.
The average thickness of the fluororesin thin film calculated from the weight difference per unit area of the aluminum foil before and after the fluororesin thin film was formed and the true specific gravity (2.25 g / cm ³ ) of the fluororesin was about 1.6 μm. .
Next, 920HP was diluted with distilled water to a volume 4 times that of PFA, and polyethylene oxide having a molecular weight of 2 million was added at a concentration of 0.003 g / ml, polyoxyethylene alkyl ether sulfate triethanolamine (manufactured by Kao Corporation). 20 T) was added so as to be 10 mg / ml, and a 4-fold diluted PFA dispersion was prepared.
The fluororesin thin film fixed on the aluminum foil was spread and fixed on a glass flat plate so as not to be wrinkled, and a 4-fold diluted PFA dispersion was dropped. After that, while sliding the same stainless steel slide shaft made by Nihon Bearing Co., Ltd. as described above and extending the PFA dispersion diluted 4 times uniformly over the fluororesin thin film, while the moisture did not dry, the nominal pore size Expanded PTFE porous body having a thickness of 0.45 μm and a thickness of 80 μm (manufactured by Sumitomo Electric Fine Polymer Co., Ltd., trade name: Poreflon FP-045-80 (average flow pore size 0.173 μm, porosity: 74%, Gurley seconds = 10. 7 seconds).
Then, it dried at 80 degreeC for 60 minutes, heated at 250 degreeC for 1 hour, heated at 320 degreeC for 1 hour, and heated at 317.5 degreeC for 3 hours. After passing through each of these steps, it was naturally cooled, and a composite in which a fluororesin thin film was bonded to a stretched PTFE porous body with a thermoplastic PFA having a melting point lower than that of PTFE, and an aluminum foil was further fixed thereon. Obtained.
Next, the aluminum foil was dissolved and removed with hydrochloric acid to obtain a test specimen. The Gurley second of this test body was 5000 seconds or more, and ethanol was contacted from the fluororesin thin film side at room temperature, but there was no hole that penetrated. It was shown to be a fluororesin composite containing a substantially nonporous fluororesin thin film that does not penetrate ethanol.
Next, using a specially made horizontal axis stretching machine, stretching was performed 3 times at an inlet chuck width of 230 mm, an outlet of 690 mm, a stretching zone length of 1 m, a line speed of 6 m / min, and a temperature of 25 ° C. to obtain a prototype film. .

  The prototype membrane of Reference Example 1 had an average flow pore size of 69 nm and a Gurley second of 24 seconds as measured with the reagent GALWICK (propylene, 1,1,2,3,3,3 oxide hexafluoric acid: manufactured by Porous Materials). It was. The 30 nm particle collection rate was 25%.

"Reference Example 2"
Reference Example 2 was an expanded PTFE porous material (trade name: Poreflon HP-010-30) manufactured by Sumitomo Electric Fine Polymer Co., Ltd. having a nominal pore size of 0.10 μm.
The membrane of Reference Example 2 had an average flow pore size of 0.114 μm, a thickness of 30 μm, a porosity of 63%, a Gurley second of 17 seconds, and a 30 nm particle collection rate of 0%.

"Example 1"
A nominal pore size of 0.10 μm manufactured by Sumitomo Electric Fine Polymer Co., Ltd. was applied so that the filtration membrane (1.6 μm thin film) of Reference Example 1 was spread on a crystallized glass plate so as to be on top and adhered to the filtration membrane. One stretched PTFE porous membrane (trade name: Poreflon HP-010-30) was laminated. Furthermore, it laminated | stacked so that the filtration membrane (1.6 micrometer thin film) side of the reference example 1 may contact | connect HP-010-30, and may contact | adhere on it.
Next, the entire laminate was wetted with IPA to make it translucent, and after confirming that there were no voids at the interface with the naked eye, it was dried to obtain a laminate film having an intermediate storage membrane between the filtration membranes.
The average pore size of the membrane of Example 1 was 69 nm, the Gurley second was 55 seconds, and the 30 nm particle collection rate was 40%.

"Example 2"
It was spread and fixed on a heat-resistant crystallized glass plate so that the filtration membrane (1.6 μm thin film) of Reference Example 1 was on top, and the same 4-fold diluted PFA dispersion was dropped. Then, while sliding the same stainless steel slide shaft made by Nippon Bearing Co., Ltd. as described above, while extending the PFA dispersion diluted 4-fold uniformly over the entire surface of the filtration membrane, while the moisture did not dry, the filtration membrane One piece of expanded PTFE porous membrane having a nominal pore size of 0.10 μm manufactured by Sumitomo Electric Fine Polymer Co., Ltd., which serves as a storage membrane, was laminated. Furthermore, after dropping the 4-fold diluted PFA dispersion on the same, slide the same stainless steel slide shaft made by Nihon Bearing Co., Ltd. to uniformly distribute the 4-fold diluted PFA dispersion over the entire surface of the storage membrane. While stretching so that the moisture did not dry, the layers were laminated so that the filtration membrane (1.6 μm thin film) side of Reference Example 1 was in contact with and in close contact with HP-010-30. Then, after drying at 80 ° C. for 60 minutes, heating at 250 ° C. for 1 hour, heating at 320 ° C. for 1 hour, and heating at 317.5 ° C. for 3 hours, it is naturally cooled, and PTFE is applied onto the expanded PTFE porous body. After the fluororesin thin film was bonded with thermoplastic PFA having a low melting point, it was removed from the crystallized glass plate to obtain an integral laminate having an intermediate storage membrane between the filtration membranes.
The average pore size of the membrane of Example 2 was 69 nm, the Gurley second was 58 seconds, and the 30 nm particle collection rate was 45%.

“Comparative Example 1”
It was spread and fixed on a heat-resistant crystallized glass plate so that the filtration membrane (1.6 μm thin film) of Reference Example 1 was on top. On top of that, the filtration membrane (1.6 μm thin film) side of Reference Example 1 was turned down and laminated so that the filtration membranes were in contact with each other. Next, the whole laminate was wetted with IPA to make it translucent. After confirming that there was no void at the interface with the naked eye, it was dried and removed from the crystallized glass.
The average pore size of the film of Comparative Example 1 was 69 nm, the Gurley second was 50 seconds, and the 30 nm particle collection rate was 23%.

"Comparative Example 2"
The filter membrane (1.6 μm thin film) of Reference Example 1 was spread and fixed on a glass plate so that it was on top. After dropping the same 4-fold diluted PFA dispersion as above, slide the same stainless steel slide shaft made by Nihon Bearing Co., Ltd. to uniformly distribute the 4-fold diluted PFA dispersion over the entire membrane surface. While the water was not dried, the filter membrane (1.6 μm thin film) side of Reference Example 1 was placed on top of the filter membrane so that the filter membranes were in contact with each other. Then, after drying at 80 ° C. for 60 minutes, heating at 250 ° C. for 1 hour, heating at 320 ° C. for 1 hour, and heating at 317.5 ° C. for 3 hours, it is naturally cooled, and PTFE is applied onto the expanded PTFE porous body. After the fluororesin thin film was bonded with thermoplastic PFA having a low melting point, it was removed from the crystallized glass flat plate to obtain an integrally laminated product.
The average pore size of the film of Comparative Example 2 was 69 nm, the Gurley second was 55 seconds, and the 30 nm particle collection rate was 20%.

  As shown in Table 1, the effect of multi-stage sieving was exhibited when the intermediate storage film was present. Moreover, when the permeability was observed in Garley seconds, it was confirmed that only a pressure loss corresponding to the overlap occurred.

The porous multilayer filter 1 of the present invention is not limited to the first embodiment, and may have a laminated structure of the second, third, and fourth embodiments shown in FIGS. 3 (A), (B), and (C).
In the second embodiment shown in FIG. 3A, a three-layer structure in which the filtration membranes 2A and 2B are bonded to both surfaces of the intermediate storage membrane 4 with an adhesive is employed. Since other configurations are the same as those of the first embodiment, the same reference numerals are given and description thereof is omitted.
In the third embodiment of FIG. 3B, the support membrane 3 manufactured integrally with the filtration membrane 2 is used as an intermediate storage membrane. That is, the first set of filtration membranes 2A, the support membrane 3A, the second set of filtration membranes 2B, and the support membrane 3B are laminated, and the first membrane support membrane 3A and the second set of filtration membranes 2B are bonded with an adhesive. is doing.
In the fourth embodiment of FIG. 3C, the first set of support membrane 3A, filtration membrane 2A, intermediate storage membrane 4, filtration membrane 2, intermediate storage membrane 4, second set of filtration membrane 2B, support membrane 3B and Laminated.
When three layers of filtration membranes having the same average pore diameter are arranged as in the fourth embodiment, the collection rate is about three times that of a single layer, and the collection rate is reduced while suppressing an increase in processing time. It can be improved dramatically.

DESCRIPTION OF SYMBOLS 1 Porous multilayer filter 2 (2A, 2B) Filtration membrane 3 (3A, 3B) Support membrane 4 Intermediate storage membrane

Claims (10)

An intermediate storage membrane comprising a plurality of filtration membranes and a porous membrane interposed between the filtration membranes,
The porous multilayer filter is characterized in that the average pore diameter of the plurality of filtration membranes is equal, and the average pore diameter of the intermediate storage membrane is 1.2 times or more and 5 μm or less of the average pore diameter of the filtration membrane. The intermediate storage membrane includes a fibrous skeleton formed by connecting flexible fibers in a network shape by knots, and is composed of a porous membrane that surrounds a substantially slit-shaped hole with the fibrous skeleton,
The porous multilayer filter according to claim 1, wherein each of the filtration membranes has a thickness of 0.25 to 15 µm and an average pore diameter of 0.1 µm or less. The filtration membrane is provided integrally with the support membrane to form a pair, the average pore diameter of the support membrane is larger than the average pore diameter of the filtration membrane and the thickness is thicker than the filtration membrane,
A first set of the support membranes interposed between the first set of filtration membranes and the second set of filtration membranes as the intermediate storage membrane;
3. The porous multilayer filter according to claim 1 or 2, wherein the support film and the intermediate storage film are separated from each other, and are laminated with the intermediate storage film interposed between each pair.   The support membrane and the intermediate storage membrane have a thickness of 2 to 100 μm, the average pore diameter is more than 0.1 μm and 10 μm or less, and the support membrane as a separate body from the intermediate storage membrane has a thickness and average within the above range. The porous multilayer filter according to claim 3, wherein the pore diameter is large.   The porous multilayer filter according to any one of claims 1 to 4, wherein adjacent boundary surfaces of the filtration membrane, the intermediate storage membrane and / or the support membrane to be laminated are in close contact or bonded together. .   The porous multilayer filter according to any one of claims 1 to 5, wherein an overall thickness is 1.6 µm to 300 µm.   The porous multilayer filter according to any one of claims 1 to 6, wherein the porous membrane used as the filtration membrane, the intermediate storage membrane and / or the support membrane is made of a fluororesin.   The porous multilayer filter according to claim 7, wherein the porous membrane is made of porous expanded PTFE.   9. The porous expanded PTFE filtration membrane is formed as a porous membrane by stretching a fluororesin membrane obtained by forming a membrane and then heating and sintering the fluororesin at a melting point or higher. 2. A porous multilayer filter according to 1.   A separation membrane element using the porous multilayer filter according to any one of claims 1 to 9 as a separation membrane.

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