CN117599618A - Method for producing multi-layer porous hollow fiber membrane, and porous hollow fiber membrane - Google Patents
Method for producing multi-layer porous hollow fiber membrane, and porous hollow fiber membrane Download PDFInfo
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- CN117599618A CN117599618A CN202311063668.6A CN202311063668A CN117599618A CN 117599618 A CN117599618 A CN 117599618A CN 202311063668 A CN202311063668 A CN 202311063668A CN 117599618 A CN117599618 A CN 117599618A
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Abstract
Disclosed is a method for producing a porous multilayer hollow fiber membrane having excellent filtration performance and strength. In order to achieve the above object, the present invention provides a method for producing a two-layer or more multi-layer porous hollow fiber membrane, comprising: and a step of forming a multilayer film by extruding two or more different melt-kneaded materials from a hollow fiber forming nozzle having an ejection port, wherein at least one of the melt-kneaded materials contains a thermoplastic resin having a melting point lower than 170 ℃, and the melt-kneaded materials of the layers constituting the multilayer porous hollow fiber film are joined in the ejection port, and a time t from the ejection of the joined melt-kneaded materials is t >0 seconds.
Description
[ field of technology ]
The present invention relates to a method for producing a porous membrane and a porous hollow fiber membrane.
[ background Art ]
In a process of obtaining drinking water or industrial water from natural water sources such as river water, lake water, and groundwater as suspended water, that is, in a water purification process, a process of obtaining recycled miscellaneous water by treating domestic sewage such as sewage, or a process of obtaining clean water that can be discharged, that is, in a sewage treatment, it is necessary to remove suspended substances by performing a solid-liquid separation operation (turbidity removal operation). In the water purification treatment, the turbid materials (clay, colloid, bacteria, etc.) derived from the natural source water as the suspension water are removed. In addition, in sewage treatment, suspended substances in sewage and suspended substances (sludge and the like) in treated water after biological treatment (secondary treatment) with activated sludge and the like are removed.
In the past, these turbidity removal operations have been mainly performed by precipitation, sand filtration or coagulating sedimentation sand filtration, but in recent years, membrane filtration has been becoming popular. The advantages of the membrane filtration method include, for example, the following.
(1) The obtained water quality has high and stable turbidity removal level (high safety of the obtained water).
(2) The required installation space of the filter device is small.
(3) The automatic operation is easy.
For example, in the water purification treatment, a membrane filtration method has been adopted as an alternative method to the coagulating sedimentation sand filtration method or as a method for further improving the quality of treated water after the coagulating sedimentation sand filtration provided in a subsequent stage of, for example, the coagulating sedimentation sand filtration. In the case of sewage treatment, membrane filtration has also been studied for separating sludge from sewage secondary treatment water.
In these membrane filtration-based turbidity removal operations, mainly hollow fiber-like ultrafiltration membranes or microfiltration membranes (pore diameters in the range of several nm to several hundred nm) are used. As a filtration method using a hollow fiber filtration membrane, there are two types of filtration methods, i.e., an internal pressure filtration method in which filtration is performed from the inner surface side to the outer surface side of the membrane and an external pressure filtration method in which filtration is performed from the outer surface side to the inner surface side. Of these two filtration methods, the external pressure filtration method is advantageous in that the amount of turbid matter per unit membrane surface area can be reduced in order to obtain a large membrane surface area on the side contacting the raw water in suspension. Patent documents 1 to 3 disclose hollow fibers and a method for producing the same.
As described above, the membrane filtration method has many advantages not possessed by the conventional precipitation method and sand filtration method, and therefore, the membrane filtration method has been increasingly used as an alternative technique and a complementary technique to the conventional method in water purification and sewage treatment.
However, no technology has been established that enables a membrane filtration operation that is stable over a long period of time, which has hampered the wide spread of membrane filtration methods. The reason for preventing the stability of the membrane filtration operation is mainly the deterioration of the water permeability of the membrane. The primary cause of deterioration of water permeability is clogging (fouling) of a membrane caused by a turbid substance or the like (for example, see non-patent document 1). In addition, the membrane surface may be rubbed by the abrasion of the turbid material, resulting in a decrease in water permeability.
On the other hand, as a method for producing a porous film, a thermally induced phase separation method is known. In this method, a thermoplastic resin and an organic liquid are used. As the organic liquid, a solvent which is insoluble in the thermoplastic resin at room temperature but soluble at a high temperature, that is, a latent solvent is used. The thermal phase separation method is a method of producing a porous body by kneading a thermoplastic resin and an organic liquid at a high temperature, dissolving the thermoplastic resin in the organic liquid, and cooling to room temperature to induce phase separation, thereby removing the organic liquid. This method has the following advantages.
(a) Even if a polymer such as polyethylene, which does not contain an appropriate solvent capable of dissolving the polymer at room temperature, is used, film formation can be achieved.
(b) Since the film is formed by dissolving the thermoplastic resin at a high temperature and then cooling and solidifying the resin, crystallization is promoted in the film formation process, and a high-strength film can be easily obtained, particularly when the thermoplastic resin is a crystalline resin.
Based on the advantages as described above, a thermally induced phase separation method is widely used as a method for producing a porous hollow fiber membrane (for example, see non-patent document 2 to non-patent document 5).
In addition, as a porous hollow fiber membrane, a porous multilayer hollow fiber membrane having both high blocking performance and high water permeability is known in which a blocking layer having a small pore diameter and a strength support layer having a large pore diameter are bonded.
As a method for producing a porous multilayer hollow fiber membrane, for example, patent document 1 and patent document 2 disclose techniques for melt-extruding crystalline thermoplastic resins from two annular nozzles arranged concentrically.
Although a hollow fiber membrane having a multilayer structure is known (for example, see patent document 3), it is only a method for producing a polyvinylidene fluoride material having a high melting point, and a porous hollow fiber membrane having a multilayer structure and high filtration performance in the case of a material having a low melting point is not disclosed.
[ Prior Art literature ]
[ patent literature ]
Japanese patent application laid-open No. 60-139815
Japanese patent application laid-open No. 4-065505
Japanese patent No. 5717987 (patent document 3)
[ non-patent literature ]
[ non-patent document 1 ] Y.Watanabe, R.Bian, membrane,24 (6), 1999, pages 310-318
Non-patent document 2, plausible Commission on the dictionary of plastics and functional Polymer materials, industrial survey, month 2 of 2004, pages 672 to 679
[ non-patent document 3 ] Songshanxiu (a person who is free from mountain) manufactured by a thermal induced phase separation method (TIPS method) to a porous polymer film, journal of chemistry and engineering, chemical industry Co., ltd., 1998, month 6, pages 45 to 56
[ non-patent document 4 ] Hetaozhen, film, IPC company, ping-Cheng-4 years 1 month, pages 404-406
[ non-patent document 5 ] D.R.Lloyd et al, jounal of Membrane Science, 64, 1991, pages 1-11
[ invention ]
[ problem to be solved by the invention ]
However, in the case of producing a porous multilayer hollow fiber membrane by melt-extruding crystalline thermoplastic resins from two annular nozzles arranged concentrically, there is a case where the outer layer and the inner layer of the hollow fiber membrane have different molecular weights and polymer types or even the same polymer type, thermoplastic resins having different melting points may be used depending on the polymerization method, and therefore, depending on the conditions at the time of membrane production, such as factors such as cooling rate, connectivity may be deteriorated and filtration performance may be lowered.
The purpose of the present invention is to provide a method for producing a porous multilayer hollow fiber membrane, which enables the production of a hollow fiber membrane having a three-dimensional network structure with excellent filtration performance and strength.
The present invention also aims to provide a porous multilayer hollow fiber membrane which can be suitably used in a method for removing turbidity from natural water, domestic sewage, and suspended water which is treated water thereof by a membrane filtration method.
[ means for solving the problems ]
The present inventors have made intensive studies to solve the above problems, and as a result, have found that, in at least one type of molten kneaded material, by using a thermoplastic resin having a low melting point, and by joining the molten kneaded materials constituting each layer of the multilayer porous hollow fiber membrane in the ejection port, the joined molten kneaded materials are ejected (the time t from the joining of the layers to the ejection is t > 0) and the molten kneaded materials having different compatibility are mixed in the vicinity of the interface of the layers after joining, and thus it is possible to suppress the appearance of a spherulitic structure which is a cause of a decrease in filtration performance and strength.
The present invention has been completed based on the above-described findings, and the gist thereof is as follows.
(1) A method for producing a multilayer porous hollow fiber membrane, characterized in that the method for producing a multilayer porous hollow fiber membrane comprises two or more steps of: a step of forming a film having a multilayer structure by extruding two or more different melt-kneaded materials from a nozzle for molding a hollow fiber having an ejection port,
at least one of the melt-kneaded materials contains a thermoplastic resin having a melting point of less than 170 ℃ and
and (c) merging the molten kneaded materials constituting each layer of the multilayer porous hollow fiber membrane in the ejection port, wherein a time t from the ejection of the merged molten kneaded materials is t >0 seconds.
(2) The method for producing a porous multilayer hollow-fiber membrane according to (1) above, wherein the time t to discharge the molten kneaded product after joining is t.gtoreq.0.020 seconds.
(3) The method for producing a porous multi-layer hollow fiber membrane according to (1) or (2), wherein the multi-layer porous hollow fiber membrane comprises an inner layer and an outer layer.
(4) The method for producing a porous multilayer hollow fiber membrane according to any one of the above (1) to (3), wherein when two melt-kneaded materials of the inner and outer layers are extruded and formed, the temperature difference of the melt-kneader is set to 5℃or more.
(5) The method for producing a porous multilayer hollow fiber membrane according to any one of the above (1) to (4), wherein the thermoplastic resin is a vinylidene fluoride resin.
(6) The method for producing a porous multilayer hollow-fiber membrane according to the above (3), wherein the melt-kneaded mixture constituting the inner and outer layers contains polyvinylidene fluoride, wherein,
the distance between the solubility parameter of polyvinylidene fluoride represented by the formula (1) and the solubility parameter of the solvent used in the production of the layer comprising the surface FA on the filtered liquid side of the porous multi-layer hollow fiber membrane is set to Pa, and
when the distance between the solubility parameter of polyvinylidene fluoride represented by the following formula (2) and the solubility parameter of the solvent used in the production of the layer including the surface FB, which is the surface on the filtrate side, of the porous multilayer hollow fiber membrane is Pb, pb—pa >0.05.
Pa ＝ (4 (σdm-σdp)＾2 + (σpm-σpp)＾2 + (σhm-σhp)＾2)＾0.5··· (1)
Pb ＝ (4 (σdm-σdp)＾2 + (σpm-σpp)＾2 + (σhm-σhp)＾2)＾0.5··· (2)
In the formulas (1) and (2), σdm and σdp represent the dispersion force terms of the solvent and polyvinylidene fluoride used, respectively; σpm and σpp represent dipole-coupling force terms of the solvent and polyvinylidene fluoride used, respectively; and σhm and σhp represent the hydrogen bond terms of the solvent and polyvinylidene fluoride used, respectively. ]
(7) A porous hollow fiber membrane produced by the method for producing a porous multi-layer hollow fiber membrane according to any one of (1) to (6) above.
[ Effect of the invention ]
According to the present invention, a method for producing a porous multilayer hollow fiber membrane having excellent filtration performance and strength can be provided.
Further, according to the present invention, there can be provided a porous multilayer hollow fiber membrane which can be suitably used in a method for removing turbidity from natural water, domestic sewage, and suspended water which is treated water thereof.
[ description of the drawings ]
Fig. 1 is a schematic diagram of a three-dimensional network structure.
Fig. 2 is a diagram showing the structure of an apparatus for producing a porous hollow fiber membrane.
Fig. 3 is a diagram showing an example of a two-layer hollow fiber molding nozzle.
Fig. 4A is a diagram for explaining a method of measuring a boundary of a layer, and a diagram for explaining a method of determining a line for determining a measurement position of a length of a hole used for boundary measurement.
Fig. 4B is a diagram for explaining a method for measuring the boundary of a layer, and a diagram for explaining a method for measuring the length of a hole using the line determined in fig. 3.
FIG. 5 is a diagram of a filter assembly used in the water permeability test.
[ detailed description ] of the invention
Embodiments of the present invention will be described in detail. The present invention is not limited to the following embodiments.
Hereinafter, the porous multilayer hollow fiber membrane of the present embodiment will be described.
The porous multilayer hollow fiber membrane of the present embodiment is a membrane structure having a hollow fiber membrane. Here, the hollow fiber membrane means a membrane having a hollow annular shape. Since the porous membrane has a membrane structure of a hollow fiber membrane, the membrane area per unit volume of the module can be increased as compared with a planar membrane.
However, the porous multilayer hollow fiber membrane of the present embodiment is not limited to a porous membrane having a membrane structure of a hollow fiber membrane (hollow fiber-shaped porous membrane), and may have other membrane structures such as a flat membrane and a tubular membrane.
In the porous multilayer hollow fiber membrane of the present embodiment, the melt-kneaded product constituting the membrane contains a polymer component (for example, a thermoplastic resin). Such a polymer component preferably contains, for example, a fluororesin, vinylidene fluoride-based resin, hexafluoropropylene-based resin, or chlorotrifluoroethylene-based resin as a main component. The term "containing" as a main component means containing 50 mass% or more in terms of solid content of the polymer component. The polymer component may be one kind or a combination of plural kinds.
The melt-kneaded product may be composed of two components, namely, a polymer component such as a thermoplastic resin and a solvent, or may be composed of three components, namely, a polymer component, an inorganic fine powder and a solvent.
The weight average molecular weight (Mw) of the vinylidene fluoride resin is not particularly limited, but is preferably 10 to 100 tens of thousands, more preferably 15 to 150 tens of thousands. In addition, the present invention is not limited to the single-molecular-weight vinylidene fluoride resin, and a plurality of vinylidene fluoride resins having different molecular weights may be mixed. In the present embodiment, the weight average molecular weight (Mw) can be measured by Gel Permeation Chromatography (GPC) based on a standard resin having a known molecular weight.
On the other hand, the melt-kneaded product constituting the porous multilayer hollow fiber membrane may contain other polymer components. The other polymer component is not particularly limited, and is preferably a substance compatible with a thermoplastic resin such as a vinylidene fluoride resin, and for example, a fluorine resin or the like exhibiting high chemical resistance similar to the vinylidene fluoride resin can be suitably used.
Further, as the polymerization method of the vinylidene fluoride resin, both emulsion polymerization products and suspension polymerization products can be suitably used.
The porous multilayer hollow fiber membrane of the present embodiment preferably has a three-dimensional network structure. The three-dimensional network structure is a structure schematically shown in fig. 1.
For example, the thermoplastic resin a is bonded to form a mesh, and the void b is formed. In the three-dimensional network structure, a lump of a resin of a so-called spherulitic structure is hardly observed. The void portion b of the three-dimensional network structure is preferably surrounded by the thermoplastic resin a, and the respective portions of the void portion b communicate with each other. Since the thermoplastic resin used mostly forms a three-dimensional network structure that can contribute to the strength of the porous membrane (preferably, hollow fiber membrane), a high-strength support layer can be formed. In addition, chemical resistance is also improved. Although the reason for the improvement of chemical resistance is not clear, it is believed that the greater amount of thermoplastic resin that forms the grid contributing to strength does not significantly affect the strength of the layer as a whole even if a part of the grid is eroded by the chemical.
The porous multilayer hollow fiber membrane of the present embodiment has a multilayer structure of two or more layers, and in the multilayer structure, the layer having the filtrate side surface is referred to as a layer (a), and the layer having the filtrate side surface is referred to as a layer (B).
For example, the layer (a) of the porous multilayer hollow fiber membrane and the layer (B) of the porous multilayer hollow fiber membrane thus share functions: the layer (a) is a so-called blocking layer, which functions to block the permeation of foreign matter contained in the liquid to be treated (raw water) through the membrane by using a small surface pore diameter, and the layer (B) is a so-called supporting layer, which has a function of preventing the water permeability from being reduced as much as possible while securing high mechanical strength. The sharing of the functions of the layer (a) and the layer (B) is not limited to the above.
Hereinafter, a case will be described in which the porous multilayer hollow fiber membrane of the present embodiment has a two-layer structure in which the layer (a) is a blocking layer and the layer (B) is a supporting layer.
The thickness of the layer (A) is preferably 1/100 or more and less than 40/100 of the total film thickness of the porous multilayer hollow fiber membrane. By making the thickness of the layer (a) thicker as described above, insoluble substances such as sand and coagulum can be used even if the raw water contains them. This is because the surface pore size does not change even with some wear. If the thickness is within this range, a desired balance between the blocking performance and the high water permeability can be achieved. More preferably, the thickness of the porous multilayer hollow fiber membrane is 2/100 or more and 30/100 or less of the whole thickness of the membrane. The thickness of the layer (A) is preferably 1 μm or more and 100 μm or less, more preferably 2 μm or more and 80 μm or less.
The method for producing a porous multilayer hollow fiber membrane according to the present embodiment includes a step of forming a membrane having a multilayer structure by extruding two or more different melt-kneaded materials from a nozzle for molding a hollow fiber having an ejection port.
More specifically, the method is a step of molding a hollow fiber-shaped melt-kneaded material by discharging a melt-kneaded material containing two or more kinds of thermoplastic resin, organic liquid, inorganic fine powder, etc. from a spinneret having annular discharge ports, and then preferably further comprises a step of solidifying the discharged hollow fiber-shaped melt-kneaded material, and then extracting and removing the organic liquid and the inorganic fine powder to produce a porous multilayer hollow fiber membrane.
The thermoplastic resin used in the method for producing a porous multilayer hollow fiber membrane according to the present embodiment is a resin that has elasticity at normal temperature and does not exhibit plasticity, but exhibits plasticity by appropriate heating, thereby being capable of molding. The thermoplastic resin is a resin which returns to its original elastomer again when cooled and the temperature is lowered, and which does not undergo chemical changes such as molecular structure during this period (for example, see "chemical dictionary 6 printing plate" edited by the chemical dictionary edition Commission, co-pending publications, pages 860 and 867, 1963).
Examples of the thermoplastic resin include resins described in thermoplastic articles (pages 829-882) of chemical products (chemical industry journal of the society, 1995) of 12695 and resins described in pages 809-810 of revised 3 rd edition of chemical toilet-use application (Japanese chemical society, mars, 1980).
Specific examples of the thermoplastic resin include polyolefin such as polyethylene and polypropylene, fluororesin such as polyvinylidene fluoride, ethylene-vinyl alcohol copolymer, polyamide, polyetherimide, polystyrene, polysulfone, polyvinyl alcohol, polyphenylene ether, polyphenylene sulfide, cellulose acetate, and polyacrylonitrile. Among them, polyolefin having crystallinity, fluororesin such as polyvinylidene fluoride, crystalline thermoplastic resin such as ethylene-vinyl alcohol copolymer and polyvinyl alcohol can be suitably used in view of the strength expression. Further preferably, a fluorinated resin such as polyolefin or polyvinylidene fluoride is used, and since it is hydrophobic, it has high water resistance, and it is expected to have durability in filtration of a usual aqueous liquid.
More specifically, the aforementioned fluororesin preferably contains, as a main component, two or more of vinylidene fluoride resin (PVDF), chlorotrifluoroethylene resin, tetrafluoroethylene resin, ethylene-tetrafluoroethylene copolymer (ETFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), hexafluoropropylene resin, and a mixture of these resins alone or in combination, and more preferably consists of only a resin in which one or two or more of them are combined. In particular, polyvinylidene fluoride having excellent chemical durability such as chemical resistance can be preferably used as the fluorine-based resin.
Examples of the polyvinylidene fluoride include a vinylidene fluoride homopolymer and a vinylidene fluoride copolymer having a vinylidene fluoride ratio of 50 mol% or more. Examples of the vinylidene fluoride copolymer include a copolymer of vinylidene fluoride and one or more monomers selected from the group consisting of tetrafluoroethylene, hexafluoropropylene, chlorotrifluoroethylene and ethylene. As the polyvinylidene fluoride, a vinylidene fluoride homopolymer is particularly preferable.
Further, in the melt-kneaded product used in the method for producing a porous multilayer hollow fiber membrane of the present embodiment, at least one of the melt-kneaded products contains a thermoplastic resin having a melting point of 170 ℃ or less. This is because, when the melting point of the thermoplastic resin is low, the strength may be low, but the thermoplastic resin is easily melted during heating processing, and the processability is excellent. In addition, when the polyvinylidene fluoride is used as the thermoplastic resin, it is presumed that the deterioration due to chemical substances is caused by an amorphous part exhibiting flexibility, and therefore, in general, the lower the melting point is, the lower the crystallinity is, and it is desirable to be 170 ℃ or lower.
In the method for producing a porous multilayer hollow fiber membrane according to the present embodiment, at least one of the melt-kneaded products contains a thermoplastic resin having a melting point of less than 170 ℃. In this case, the workability can be further improved.
The concentration (content) of the thermoplastic resin in the melt-kneaded mixture is preferably 30 to 48 mass%, more preferably 32 to 45 mass%. If the content of the thermoplastic resin is 30 mass% or more, the mechanical strength of the porous multilayer hollow fiber membrane of the present embodiment can be easily ensured; if the content of the thermoplastic resin is 48 mass% or less, the porous multilayer hollow fiber membrane of the present embodiment does not have a decrease in water permeability.
Further, in the case where the porous multilayer hollow fiber membrane of the present embodiment is a membrane having a two-layer structure, the concentration (content) of the thermoplastic resin in the melt-kneaded mixture of the layer (B) is preferably 34 to 48 mass%, more preferably 35 to 45 mass%. The concentration (content) of the thermoplastic resin in the melt-kneaded product of the layer (a) is preferably 10 to 35% by mass, more preferably 12 to less than 35% by mass. If the content of the thermoplastic resin is 10 mass% or more, both the pore diameter and the mechanical strength of the surface can be achieved in the porous multilayer hollow fiber membrane of the present embodiment, and if the content of the thermoplastic resin is 135 mass% or less, the water permeability of the porous multilayer hollow fiber membrane of the present embodiment does not decrease.
The organic liquid contained in the melt-kneaded product is an organic liquid which is a latent solvent for the thermoplastic resin used in the present embodiment. Here, the latent solvent means a solvent in which the thermoplastic resin is hardly dissolved at room temperature (25 ℃) but is soluble at a temperature higher than room temperature. The thermoplastic resin may be in a liquid state at a temperature at which the thermoplastic resin is melt kneaded, and is not necessarily in a liquid state at ordinary temperature.
Examples of the organic liquid include phthalic acid esters such as dibutyl phthalate, diheptyl phthalate, dioctyl phthalate, di (2-ethylhexyl) phthalate, diisodecyl phthalate, and ditridecyl phthalate when the thermoplastic resin is polyethylene; sebacates such as dibutyl sebacate; adipates such as dioctyl adipate; trimellitic anhydride esters such as trioctyl trimellitate; phosphate esters such as tributyl phosphate and trioctyl phosphate; glycerol esters such as propylene glycol dicaprate and propylene glycol dioleate; paraffin such as liquid paraffin; and mixtures thereof, and the like.
When the thermoplastic resin is polyvinylidene fluoride, examples of the organic liquid include phthalic acid esters such as dimethyl phthalate, diethyl phthalate, dibutyl phthalate, dicyclohexyl phthalate, diheptyl phthalate, dioctyl phthalate, and di (2-ethylhexyl) phthalate; sebacates such as dibutyl sebacate; adipates such as dioctyl adipate; benzoates such as methyl benzoate and ethyl benzoate; phosphate esters such as triphenyl phosphate, tributyl phosphate, and tricresyl phosphate; ketones such as gamma-butyrolactone, ethylene carbonate, propylene carbonate, cyclohexanone, acetophenone, isophorone, and the like; and mixtures thereof, and the like.
Examples of the inorganic fine powder contained in the melt-kneaded product include silica, alumina, titania, zirconia, calcium carbonate, and the like, and silica fine powder having an average primary particle diameter of 3nm or more and 500nm or less is particularly preferable. More preferably 5nm or more and 100nm or less. More preferably, the hydrophobic silica fine powder is not easily aggregated and has good dispersibility, and still more preferably, the hydrophobic silica having a MW (methanol wettability) value of 30% by volume or more. The MW value as used herein refers to the value of the volume% of methanol that completely wets the powder. Specifically, it is determined by: when silica was added to pure water and methanol was added under stirring to the liquid surface, the volume% of methanol in the aqueous solution at the time of sedimentation of 50 mass% of silica was obtained. The "average primary particle diameter of the inorganic fine powder" means a value obtained by analysis of an electron micrograph. That is, first, a group of inorganic fine powders is pretreated by the method of ASTM D3849. Then, the average primary particle diameter of the inorganic fine powder was calculated by measuring 3000 to 5000 particle diameters photographed on a transmission electron microscope photograph and arithmetically averaging the values.
The amount of the inorganic fine powder to be added is preferably 5 to 50% by mass, more preferably 10 to 40% by mass, based on the mass ratio of the inorganic fine powder to the melt-kneaded product. When the proportion of the inorganic fine powder in the melt-kneaded product is 5 mass% or more, the effect of kneading the inorganic fine powder can be sufficiently exhibited; if the content is 40% by mass or less, stable spinning can be achieved.
The mixture of polyvinylidene fluoride and an organic liquid material, or the mixture of polyvinylidene fluoride, an organic liquid material, and an inorganic fine powder can be obtained by mixing using a Henschel mixer (Henschel mixer), a Banbury mixer (Banbury mixer), a coulter mixer (Ploughshare mixer), or the like.
As the order of mixing the above three components of polyvinylidene fluoride, organic liquid material and inorganic fine powder, it is advantageous in terms of melt moldability and improvement of the porosity and mechanical strength of the obtained porous film that the inorganic fine powder and the organic liquid material are mixed first, and the organic liquid material is sufficiently adsorbed to the inorganic fine powder, and then polyvinylidene fluoride is mixed.
Instead of the preliminary kneading by a henschel mixer or the like, polyvinylidene fluoride and an organic liquid material may be directly supplied to a melt kneading extruder such as a twin screw extruder. In order to improve the kneading property, the pellets may be fed to a melt kneading extruder after the mixing, subjected to melt kneading once, extruded into a hollow fiber shape, and cooled and solidified into a hollow fiber.
The melt kneading of the mixture may be performed by a usual melt kneading method, for example, by using an extruder. The case of using an extruder is described below, but the method of melt kneading is not limited to an extruder.
Fig. 2 schematically shows an example of a manufacturing apparatus used in the manufacturing method of the present embodiment.
The apparatus for producing a porous multilayer hollow fiber membrane shown in fig. 2 comprises: an extruder 10, a hollow fiber molding nozzle 20, a coagulation bath 30 for storing a solution for coagulating a film-forming stock solution, and a plurality of rolls 50 for transporting and winding up the porous hollow fiber film 40. Further, a suction machine (not shown) and a high-temperature container (not shown) may be included. The space S shown in fig. 2 is an air moving part through which the film-forming raw liquid discharged from the hollow fiber-forming nozzle 20 passes before reaching the solution in the coagulation bath 30.
In the apparatus for producing a porous multilayer hollow fiber membrane shown in fig. 2, a hollow fiber molding nozzle 20 having one or more annular ejection ports arranged concentrically is attached to the tip of an extruder 10, and the molten kneaded product is extruded by the extruder 10 and ejected from the hollow fiber molding nozzle 20.
In the case of producing a porous multilayer hollow fiber membrane, there are the following methods: as shown in fig. 3, a method of attaching a hollow fiber molding nozzle 20 having two or more annular ejection ports to the tip of an extruder 10, and supplying and extruding a molten kneaded material to each annular ejection port by using a different extruder 10, or a method of coating the remaining layers after one of the layers is produced.
The method for producing a porous multilayer hollow fiber membrane according to the present embodiment is a method for producing a hollow fiber molding nozzle 20 having two or more annular ejection ports, and for example, as shown in fig. 3, by joining and stacking the melt kneaded materials supplied separately to the ejection ports, a hollow fiber-like extrudate having a multilayer structure can be obtained. In this case, each of the melt-kneaded materials passes through the spaces 5 and 6 through which the melt-kneaded materials having different compositions pass, and is then extruded from the annular ejection ports adjacent to each other, whereby a film having a multilayer structure in which the pore diameters of the adjacent layers are different can be obtained. The composition of the melt-kneaded materials different from each other means that the melt-kneaded materials have different compositions when the constituent materials of the melt-kneaded materials are different or when the constituent materials are the same but the constituent ratios are different. Further, even in the case of the same type of thermoplastic resin, the constituent materials (compositions) are considered to be different in the present invention when the molecular weight and the molecular weight distribution are significantly different.
As described above, there is also a method of coating the remaining layers after one of the layers is produced, but it is difficult to exhibit high filtration performance because the process is complicated due to the increase in number of steps and the interface is densified due to dissolution by a solvent.
In the method for producing a porous multilayer hollow fiber membrane according to the present embodiment, the joining positions of the melt-kneaded materials having different compositions are located on the ejection front side of the ejection port. The "position of the joining of the molten kneaded materials" located on the ejection front side of the ejection port "means that the molten kneaded materials constituting the respective layers in the ejection port of the hollow fiber forming nozzle 20 are joined and then ejected, that is, the time t from the joining of the molten kneaded materials constituting the respective layers of the multilayer porous hollow fiber membrane in the ejection port to the ejection of the joined molten kneaded materials is t >0 seconds.
By securing the time t (t > 0) from the ejection of the molten kneaded material after the joining, the time after the joining can be secured, and the membrane structure can be made to be a structure suitable for filtration. For example, in the case of producing a porous film having a two-layer structure using a thermoplastic resin having a melting point lower than 170 ℃, the time from ejection to solidification is longer than that of a raw material having a high melting point due to a low melting point. Therefore, the layer in the thickness direction of the film among the layers gradually cools, and a part of the layer is likely to have a spherulitic structure. In general, spherulitic structures are more readily developed using well-compatible organic liquids. In order to form each layer into a porous film having a different pore diameter, organic liquids having different compatibility are generally used. Therefore, in this embodiment, by setting the time t to discharge the melt-kneaded product after the confluence to t >0, the organic liquids of the respective layers having different compatibility are mixed in the vicinity of the interface of the layers after the confluence, and it is possible to suppress the appearance of the spherulitic structure in the vicinity of the interface on the layer side having low compatibility particularly with the thermoplastic resin. If the spherulitic structure is developed, the connectivity of pores is poor, and the filtration performance tends to be lowered, and a three-dimensional network structure is desirable.
From the same viewpoint, the time t from the ejection of the molten kneaded material to the confluence is preferably not less than 0.020 seconds.
When the polyvinylidene fluoride is used as the thermoplastic resin, a solvent for the polyvinylidene fluoride needs to be appropriately selected in order to achieve both a high open cell content and a high compression strength. First, as a method for increasing the aperture ratio, there are a method for reducing the concentration of polyvinylidene fluoride and a method for increasing the temperature of the hollow portion forming fluid as described above. In the case of using a method of producing a film by reducing the concentration of polyvinylidene fluoride, since the pore size is also increased, it is necessary to select a solvent capable of realizing a high open cell ratio and a small pore size.
The following parameter P is a relational expression between the three-dimensional solubility parameter of polyvinylidene fluoride and the three-dimensional solubility parameter of solvent, and the solubility of polyvinylidene fluoride and solvent is evaluated. The right side of the relationship shows the dissolution range of Hansen (Hansen) solubility parameters in three dimensions and quantitatively represents the distance from PVDF three-dimensional solubility parameters (σdp, σpp, σhp) to solvent three-dimensional solubility parameters (σdm, σpm, σhm).
P＝(4×(σdm-σdp)2+(σpm-σpp)2+(σhm-σhp)2)×0.5
[ wherein σdm and σdp represent the dispersion force terms of the solvent and polyvinylidene fluoride, respectively; σpm and σpp represent dipole-coupling force terms of the solvent and polyvinylidene fluoride, respectively; σhm and σhp represent hydrogen bond terms of the solvent and polyvinylidene fluoride, respectively. ]
The above-described idea is not limited to polyvinylidene fluoride, and may be applied to other thermoplastic resins.
In the case of a porous membrane having a two-layer structure, the parameter Pb between the solvent used in the preparation of the melt-kneaded product of the layer (B) forming the surface FB, which is the surface on the filtrate side, of the porous multi-layer hollow fiber membrane and polyvinylidene fluoride is preferably greater than 7.95, more preferably 7.98 to 10, still more preferably 7.8 to 9.0. If the value is more than 7.95, the decrease in water permeability can be suppressed.
In the preparation of the melt-kneaded product a of the layer (a) including the surface FA on the liquid-to-be-filtered side, which is the surface of the porous multilayer hollow fiber membrane, the parameter Pa between the solvent and polyvinylidene fluoride used is preferably 7.95 or less, more preferably 0 to 7.95, and still more preferably 3.0 to 7.89. If the value is 7.95 or less, a high aperture ratio and a small aperture diameter can be achieved.
Here, the difference Pb-Pa between the solubility parameters of the aforementioned layer (A) and the aforementioned layer (B) is preferably Pb-Pa >0.05. The layer (A) has good compatibility with the solvent of polyvinylidene fluoride, so that a spherulitic structure is easily produced, but if Pb-Pa >0.05, the compatibility is moderately lowered after the solvent of the layer (B) is mixed with the solvent of the layer (A) at the interface, so that the formation of a spherulitic structure can be suppressed.
In the method for producing a porous multilayer hollow fiber membrane of the present embodiment, the temperature difference between the melt-kneading machine for extrusion-forming the melt-kneaded product of the layer (a) and the layer (B) is preferably 5 ℃ or higher. When the temperature reaches 5℃or more, the mechanical strength can be improved. It is further preferable to raise the temperature of the melt kneader of the layer (B) by 5℃or more relative to the temperature of the melt kneader of the layer (A). Although the reason is not yet clear, it is assumed that this is because the temperature of the layer (B) having lower compatibility is increased from the standpoint of compatibility between polyvinylidene fluoride and the solvent, so that the dissolved state can be made closer to the layer (a) and the layer (B), and entanglement of polyvinylidene fluoride with each other increases when the two melt-kneaded materials merge.
In the extrusion of the molten kneaded material from the annular nozzle of the hollow fiber forming nozzle 20, it is preferable that the extrusion is performed so that the nozzle extrusion parameter R (1/second) is 10 or more and 1000 or less, since a film having high productivity and spinning stability and further high strength can be obtained. Here, the spin-port ejection parameter R is a value obtained by dividing the ejection linear velocity V (m/sec) by the slit width d (m) of the ejection port. The discharge line speed V (m/sec) is the discharge capacity (m) per unit time of the molten kneaded material 3 Per second) divided by the ejectionCross-sectional area of the mouth (m 2 ) The obtained values. If R is 10 or more, the filament diameter of the hollow extrudate does not have the problem of pulse-like fluctuation, and can be spun stably with good productivity. In addition, if R is 1000 or less, the elongation at break, which is one of the important strengths of the obtained porous hollow fiber membrane, can be kept sufficiently high. Elongation at break is the elongation (%) relative to the original length when stretched in the film length direction.
In the method for producing a porous multilayer hollow fiber membrane according to the present embodiment, the spinning nozzle ejection parameter (R) is preferably 50 or more and 1000 or less, which is a value obtained by dividing the ejection linear velocity V of the molten kneaded material obtained by joining and stacking the molten kneaded materials by the slit width d of the ejection port.
The hollow fiber-shaped melt-kneaded material discharged from the discharge port of the hollow fiber molding nozzle 20 is coagulated by a refrigerant such as air or water, but depending on the porous hollow fiber membrane to be used, the hollow fiber-shaped melt-kneaded material is passed through the air moving section S composed of an air layer and then passed through a coagulation bath 30 in which water or the like is placed. That is, the air moving portion S is a portion from the ejection port of the hollow fiber forming nozzle 20 to the water surface of the coagulation bath 30. As needed, a container such as a tube may be used for the air moving portion S from the ejection port. After passing through the coagulation bath 30, the material is wound around a reel or the like as necessary.
The plastic resin may be in the form of pellets or granules. As means for pulverizing the pellets or the granular thermoplastic resin, there are a multi-stage pulverizing method in which the pellets are coarsely pulverized and then finely pulverized, a method in which single stage is used until fine pulverization is achieved, and the like, and the method is not limited. Even if the fine powder does not reach a predetermined particle size by the micronizer, the fine powder is pulverized by the ultrafine pulverizer capable of further micronizing. Specific pulverizing means include pulverizing means using a hammer mill, a Turbo mill (Turbo mill), a jet mill, a pin mill, a centrifugal mill, a Rotoplex, a pulverizer, wet pulverization, a chopper mill, a super rotor (Ultra rotor), etc., and normal temperature or freeze pulverizing means may be used. For example, vinylidene fluoride resins having glass transition temperatures as low as about-35℃are suitably freeze-pulverized.
Classification is performed using an appropriate classifier to obtain fine powder having a predetermined particle size range. When fine powder having a predetermined particle diameter range is obtained from the fine powder after classification, fine powder having a particle diameter equal to or smaller than the predetermined particle diameter may be removed after further classification by another classifier, and the remaining fine powder (intermediate powder) may be used as a product. After classification, the particles in a range larger than the target particle size range may be crushed again to obtain particles in a predetermined particle size range. Examples of the device for classification include a vibrating screen, an inertial air classifier, and a rotary blade classifier, but are not particularly limited thereto.
In the case of mixing two polymer components, each polymer component may be pulverized and then mixed by a mixer. The classification may be performed after pulverization or after mixing, and is not particularly limited.
The particle size distribution after pulverization can be measured by using a laser diffraction/scattering particle size distribution measuring apparatus.
The median particle diameter (D50 particle diameter) based on the volume obtained from the particle diameter distribution of the plastic resin is preferably in the range of 50 μm or more and 500 μm or less. If the particle size is 50 μm or more, for example, in melt kneading by an extruder or the like, the kneading into the screw is not likely to occur, and the kneading can be stably carried out. If the particle diameter is 500 μm or less, dissolution failure or the like does not occur, and a porous film can be stably produced.
Similarly, the D10 particle size and the D90 particle size can be obtained from the particle size distribution of the plastic resin. When the resulting D10, D50, D90 particle diameters are defined as the particle diameter dispersity V= (D90-D10)/D50, V is preferably more than 0.8. More preferably 1.3 or more.
In the coagulated hollow fiber-like material, the polymer-rich fraction phase and the organic liquid-rich fraction phase are finely separated. For example, when the inorganic fine powder is added, and when the inorganic fine powder is a silica fine powder, the silica fine powder is unevenly present in the organic liquid dense phase. The organic liquid and the inorganic fine powder are extracted from the hollow fiber, and the dense phase portion of the organic liquid becomes a void. Thus, a porous hollow fiber membrane can be obtained.
When the particle diameter dispersity is large, the fine powder of the thermoplastic resin is introduced into the aggregated inorganic fine powder, and between the inorganic fine powder and the inorganic fine powder, so that the miscibility of the thermoplastic resin and the inorganic fine powder in the polymer portion and the hollow portion forming the porous film is improved, a more uniform porous film structure can be obtained, and the variation in film performance can be reduced. The uniformity of the porous membrane can be evaluated by the deviation of the membrane properties.
The ratio of the D50 particle diameter after pulverization to the primary particle diameter of the inorganic fine powder is preferably 3200 to 35000. If 3200 or more, the time for which the temperature of the extruder is stable becomes faster when the three components of polyvinylidene fluoride, organic liquid material and inorganic fine powder are mixed and fed into the extruder. After mixing, it is assumed that silica adheres to the polymer wall surface, but if 3200 or more, the surface of the polymer wall surface exposed without being covered with silica becomes large, and therefore, it is estimated that heat is easily transferred to the polymer, and the temperature stability becomes fast. In particular, there is no reason, but if it is 35000 or less, stable mixing can be achieved.
The particle shape after pulverization is preferably 0.5 or less in roundness. The smaller the roundness, the more likely the bulk density becomes high, and the more stable the input to the extruder. The roundness is represented by formula (1), and a shape close to 1.0 is represented by a shape close to a circle.
[ number 1 ]
The linearity is preferably 1.8 or more. When three components, i.e., a particle having high linearity, an organic liquid material and an inorganic fine powder are mixed, the gaps between particles increase before the bulk density stabilizes, and thus the miscibility with the inorganic fine powder improves. The linearity is expressed by the formula (B), and the closer to the circle, the closer to 1.0 the value. The longer the length the greater the value.
[ number 2 ]
The extraction and removal of the organic liquid and the extraction and removal of the inorganic fine powder may be performed simultaneously if the same solvent can be used for the extraction and removal. Typically by separate extraction.
For the extraction removal of the aforementioned organic liquid, such a liquid is used: the thermoplastic resin used is insoluble or modified and is suitable for mixing and extraction with organic liquids. Specifically, the contact may be performed by a method such as dipping. The liquid is preferably volatile for ease of removal from the hollow fiber membranes after extraction. Examples of the liquid include alcohols and methylene chloride. If the organic liquid is water-soluble, water may also be used as the extraction liquid.
The extraction and removal of the inorganic fine powder is usually performed using an aqueous liquid. For example, when the inorganic fine powder is silica, the silica may be converted into silicate by first bringing it into contact with an alkaline solution, and then bringing it into contact with water to extract and remove the silicate.
The extraction and removal of the organic liquid and the extraction and removal of the inorganic fine powder may be performed in advance. When the organic liquid and water are non-miscible, it is preferable to perform extraction and removal of the organic liquid first and then extraction and removal of the inorganic fine powder. Since the organic liquid and the inorganic fine powder are mixed and coexist in the concentrated organic liquid phase, extraction and removal of the inorganic fine powder can be performed smoothly, which is advantageous.
In this way, the organic liquid and the inorganic fine powder are extracted and removed from the solidified porous hollow fiber membrane, thereby obtaining a porous hollow fiber membrane.
The hollow fiber membrane after solidification may be stretched in the longitudinal direction of the porous hollow fiber membrane within a range of 3 times the stretching ratio at any of the following stages: (i) before the extraction to remove the organic liquid and the inorganic micro powder; (ii) After the organic liquid is removed by extraction, the inorganic micro powder is removed by extraction; (iii) After the inorganic micro powder is removed by extraction, the organic liquid is removed by extraction; and (iv) extracting to remove the organic liquid and the inorganic micro powder. In general, if a hollow fiber membrane is stretched in the longitudinal direction, the water permeability is improved, but the pressure resistance (rupture strength and compression strength) is lowered, so that the stretched membrane cannot be practically used in many cases. However, the porous hollow fiber membrane obtained by the production method of the present embodiment has high mechanical strength. Thus, stretching can be performed at a stretching ratio of 1.1 times or more and 3.0 times or less. By stretching, the water permeability of the porous hollow fiber membrane is improved. The draw ratio as used herein refers to the value obtained by dividing the length of the hollow fiber after drawing by the length of the hollow fiber before drawing.
For example, when a porous hollow fiber membrane having a hollow fiber length of 10cm is stretched to a hollow fiber length of 20cm, the stretching ratio is 2 times according to the following formula.
20cm÷10cm＝2
The stretching of the porous hollow fiber membrane is desirably performed at a space temperature of 0 ℃ or higher and 160 ℃ or lower. When the temperature is higher than 160 ℃, the elongation at break is reduced and the water permeability is reduced, which is not preferable, and when the temperature is 0 ℃ or lower, the possibility of tensile fracture is high, which is not practical. The space temperature in the stretching step is more preferably 10 ℃ or more and 140 ℃ or less, and still more preferably 20 ℃ or more and 100 ℃ or less.
In the production method of the present embodiment, the porous hollow fiber membrane containing the organic liquid material is preferably stretched. The hollow fiber membrane containing the organic liquid material has less breakage during stretching than the hollow fiber membrane containing no organic liquid material. Further, in the hollow fiber membrane containing the organic liquid material, the shrinkage of the hollow fiber membrane after stretching can be increased, and thus the degree of freedom in setting the shrinkage rate after stretching increases.
Further, it is preferable to stretch a porous hollow fiber membrane containing inorganic fine powder. In the hollow fiber membrane containing the inorganic fine powder, the hollow fiber membrane has hardness due to the presence of the inorganic fine powder contained in the hollow fiber membrane, and therefore the hollow fiber membrane becomes difficult to collapse at the time of stretching. In addition, the pore diameter of the finally obtained hollow fiber membrane can be prevented from becoming too small or the filament diameter from becoming too small.
In the present invention, it is more desirable to stretch a hollow fiber membrane containing both an organic liquid material and an inorganic fine powder.
For the above reasons, it is more preferable to stretch a hollow fiber membrane containing any one of an organic liquid material and an inorganic fine powder than to stretch a hollow fiber membrane after the completion of extraction, and it is more preferable to stretch a hollow fiber membrane containing both of an organic liquid material and an inorganic fine powder than to stretch a hollow fiber membrane containing any one of an organic liquid material and an inorganic fine powder.
In addition, the method of extracting the hollow fiber membrane after stretching has an advantage that the surface and internal voids of the hollow fiber membrane increase due to stretching, so that the extraction solvent easily permeates into the hollow fiber membrane. In addition, since the method of extraction after the steps of stretching and then shrinking is a hollow fiber membrane having a low tensile elastic modulus and being easily bendable, as described later, when extraction is performed in a liquid stream, the hollow fiber membrane is easily shaken by the liquid stream, and the stirring effect is increased, and therefore, there is an advantage that efficient extraction can be performed in a short time.
In the production method of the present embodiment, since the step of stretching the hollow fiber membrane and then shrinking the hollow fiber membrane is included, a hollow fiber membrane having a low tensile elastic modulus can be finally obtained. The term "low tensile elastic modulus" as used herein means that the fiber is easily elongated with a small force and returns to its original state when the force is removed. If the tensile elastic modulus is low, the hollow fiber membrane is not flattened, is easily bent, and is easily shaken by water flow during filtration. Since the pollutant layer deposited on the surface of the membrane cannot grow and is easily peeled off as the fiber is not fixedly shaken by the bending of the water flow, a high level of filtered water can be maintained. Further, in the case where the fiber is forcibly shaken by washing or air washing, shaking becomes large, and the washing recovery effect becomes high.
Regarding the degree of shrinkage of the fiber length when shrinkage is performed after stretching, it is desirable to set the fiber length shrinkage ratio with respect to the fiber length increment caused by stretching to a range of 0.3 or more and 0.9 or less. For example, when 10cm of the fiber is stretched to 20cm and then contracted back to 14cm, the fiber is stretched according to the following formula
Fiber length shrinkage = { (maximum fiber length at stretching) - (fiber length after shrinkage) }/[ (maximum fiber length at stretching) - (fiber starting length) ] = (20-14)/(20-10) =0.6 fiber length shrinkage was 0.6.
When the fiber length shrinkage is 0.9 or more, the water permeability tends to be low; below 0.3, the tensile elastic modulus tends to increase, and is therefore not preferable. In the present invention, the fiber length shrinkage is more preferably in the range of 0.50 and more and 0.85 and less.
In addition, by adopting a step of stretching the hollow fiber membrane to a maximum fiber length at the time of stretching and then shrinking the same, the finally obtained hollow fiber membrane is not broken even when stretched to the maximum fiber length at the time of stretching in use.
When the draw ratio is X and the fiber length shrinkage rate with respect to the fiber length increase caused by drawing is Y, the rate Z indicating the degree of assurance of the elongation at break can be defined by the following formula.
Z= (maximum fiber length at stretching-fiber length after shrinkage)/fiber length after shrinkage= (XY-Y)/(x+y-XY)
Among them, Z is preferably 0.2 or more and 1.5 or less, more preferably 0.3 or more and 1.0 or less. If Z is too small, the degree of assurance of elongation at break decreases, and if Z is too large, the water permeability becomes lower than the possibility of breakage which is high at the time of stretching.
In addition, in the manufacturing method of the present embodiment, since the steps of stretching and then shrinking are included, the stretch-break elongation is extremely small at low elongation, and the distribution of the stretch-break elongation can be narrowed.
From the viewpoint of time and physical properties of shrinkage, it is desirable that the space temperature in the step of stretching and then shrinking is in the range of 0 ℃ to 160 ℃. If it is below 0 ℃, shrinkage takes time, and thus it is not practical, and if it exceeds 160 ℃, elongation at break and water permeability are lowered, and thus it is not preferable.
In the production method of the present embodiment, it is preferable to crimp the hollow fiber membrane in the shrinking step. Thus, a hollow fiber membrane having a high degree of crimp can be obtained without crushing or damaging.
In general, since the porous hollow fiber membrane has a straight tubular shape without bending, when the porous hollow fiber membrane is bundled into a filtration module, gaps between the hollow fibers are eliminated, and there is a high possibility that the porous hollow fiber membrane becomes a fiber bundle with a low void fraction. In contrast, if a hollow fiber membrane having a high degree of crimping is used, the hollow fiber membrane spacing is evenly enlarged due to the bending of the individual fibers, and a fiber bundle having a high degree of void can be formed. In addition, in the filter module comprising hollow fiber membranes having low crimp, particularly when used under external pressure, the voids of the fiber bundles are reduced, the flow resistance is increased, and the filtration pressure cannot be effectively transmitted to the central portion of the fiber bundles. Further, when the filtration deposit is peeled off from the hollow fiber membrane by back flushing or rinsing, the cleaning effect of the inside of the fiber bundle is also deteriorated. The fiber bundle composed of porous multi-layer hollow fiber membranes with high crimping degree has large void volume, can maintain the gaps of the hollow fiber membranes even if external pressure filtration is performed, and is not easy to generate bias.
In the production method of the present embodiment, the degree of curling is preferably in the range of 1.5 or more and 2.5 or less. When 1.5 or more is used, the above-mentioned reasons are preferable, and when less than 2.5, the decrease in the filtration area per unit volume can be suppressed.
As a crimping method of the hollow fiber membrane, there is mentioned a method in which the hollow fiber membrane is placed between, for example, a pair of gear rolls with periodic irregularities or a pair of sponge-like belts with irregularities in a step of stretching and then shrinking, and they are taken out therefrom during the shrinking.
In the manufacturing method of the present embodiment, it is preferable to stretch the sheet using a take-out apparatus composed of a pair of opposed endless track belts. In this case, removal apparatuses are used on the upstream side and the downstream side of the stretching, in which the hollow fiber membranes are placed between opposite belts, and the carrying of the fibers is performed by moving both belts in the same direction at the same speed. In this case, it is preferable to draw the fiber at a higher downstream fiber carrying speed than the upstream fiber carrying speed. If the stretching is performed in this manner, the stretching tension is not generated and the slip is not caused during the stretching, and the fiber can be prevented from being flattened.
Here, the endless track belt is preferably made of a high elastic belt such as a fiber reinforced belt on the inner side in contact with the drive roller, and an elastic body on the outer side in contact with the hollow fiber membrane. The elastic modulus of the elastic body in the thickness direction is 0.1MPa or more and 2MPa or less, and more preferably the thickness of the elastic body is 2mm or more and 20mm or more. From the viewpoints of chemical resistance and heat resistance, it is particularly preferable that the elastomer of the outer surface is silicone rubber.
In addition, the stretched film may be heat-treated as needed to improve the compression strength. The heat treatment is desirably performed at 80 ℃ and above and 160 ℃ and below. If the temperature is 160 ℃ or lower, the decrease in elongation at break and the water permeability can be suppressed; when the temperature is 100 ℃ or higher, the compressive strength can be improved. In addition, from the viewpoint of reducing the variation in fiber diameter, porosity, pore diameter, and water permeability, it is desirable to heat treat the hollow fiber membrane after the completion of extraction.
[ example ]
Hereinafter, the present embodiment will be described in more detail with reference to examples and comparative examples, but the present embodiment is not limited to these examples.
< raw Material >
The raw materials used in examples and comparative examples are shown below.
Thermoplastic resin
Vinylidene fluoride homopolymers
Trade name manufactured by sorvin company: solef6010
Tradename manufactured by ARKEMA Co., ltd: kynar740, kynar720
Organic liquid
Di (2-ethylhexyl) phthalate (DEHP) (CG escer company)
Dibutyl phthalate (DBP) (CG ESTER Co., ltd.)
Inorganic micropowder
Silica fine powder (trade name: R972 primary particle diameter 16nm, manufactured by AEROSIL Co., ltd.)
< preparation of samples of examples and comparative examples >
Example 1
Vinylidene fluoride homopolymer (Kynar 740, manufactured by ARKEMA corporation) was used as the thermoplastic resin. The pellet Kynar740 was pulverized by freeze-pulverizing, and pulverized by a Linrex mill (manufactured by HOSOKAWAMICRON Co., ltd.). Classification was performed using a vibrating screen to remove 355 μm or more sieve holes and 53 μm or more sieve holes were used as products. The D50 particle diameter after pulverization was 160 μm and the particle diameter dispersity V was 1.1. The roundness was 0.43 and the linearity was 2.5.
A porous hollow fiber membrane having a two-layer structure was produced by using the above-mentioned vinylidene fluoride homopolymer, a mixture of di (2-ethylhexyl) phthalate and dibutyl phthalate as an organic liquid, and silica fine powder as an inorganic fine powder, and the layer (A) was set to the outer surface side of the hollow fiber membrane and the layer (B) was set to the inner surface side of the hollow fiber membrane. The composition of the melt-kneaded product of the layer (a) was defined as a vinylidene fluoride homopolymer, bis (2-ethylhexyl) phthalate, dibutyl phthalate, silica micropowder=34.0:34.4:6.2:25.4 (mass ratio), and the composition of the melt-kneaded product of the layer (B) was defined as a vinylidene fluoride homopolymer, bis (2-ethylhexyl) phthalate, dibutyl phthalate, silica micropowder=40.0:35.8:4.8:23.0 (mass ratio).
The melt-kneaded product was extruded from a hollow fiber molding nozzle having an outer diameter of 2.00mm and an inner diameter of 0.92mm, the temperature difference between the melt-kneader for the layer (A) and the layer (B) being 5℃and the discharge temperature being 240 ℃. The joining time of the layer (A) and the layer (B) in the nozzle was 0.080 seconds.
Then, the hollow fiber-like molten kneaded material extruded at an ejection temperature of 240℃was moved in the air for 0.60 seconds, and then introduced into a coagulation bath to which water at 30℃was added. Taken out at a speed of 30 m/min, clamped on a belt, stretched at a speed of 60 m/min, contracted at a speed of 45 m/min while blowing hot air with the apparatus set at 140 ℃, and wound up on a reel.
The obtained hollow fiber was immersed in isopropyl alcohol to extract and remove di (2-ethylhexyl) phthalate and dibutyl phthalate, and then dried. Then, the porous hollow fiber membrane was immersed in a 50 mass% aqueous ethanol solution for 30 minutes, immersed in water for 30 minutes, immersed in a 20 mass% aqueous sodium hydroxide solution for 1 hour at 70 ℃, and further washed with water repeatedly to extract and remove silica fine powder.
Table 1 shows the detailed compositions and conditions.
Example 2
A porous hollow fiber membrane was produced under the same conditions as in example 1, except that the composition of the melt-kneaded product of layer (a) was a vinylidene fluoride homopolymer, bis (2-ethylhexyl) phthalate, dibutyl phthalate, silica micropowder=34.0:32.5:8.1:25.4 (mass ratio), and the composition of the melt-kneaded product of layer (B) was a vinylidene fluoride homopolymer, bis (2-ethylhexyl) phthalate, dibutyl phthalate, silica micropowder=40.0:31.7:5.3:23.0 (mass ratio).
Example 3
A porous hollow fiber membrane was obtained under the same conditions as in example 1, except that the vinylidene fluoride homopolymer of layer (B) was used as Solef6010 manufactured by sorvi corporation in pellet form.
Example 4
A porous hollow fiber membrane was obtained under the same conditions as in example 1, except that the vinylidene fluoride homopolymer of layer (a) was used as Solef6010 manufactured by sorvi corporation in pellet form.
Example 5
A porous hollow fiber membrane was obtained under the same conditions as in example 2, except that the joining time of each layer was set to 0.020 seconds.
Example 6
A porous hollow fiber membrane was produced under the same conditions as in example 1, except that the composition of the melt-kneaded product of layer (a) was a vinylidene fluoride homopolymer, bis (2-ethylhexyl) phthalate, dibutyl phthalate, silica micropowder=34.0:34.4:6.2:25.4 (mass ratio), and the composition of the melt-kneaded product of layer (B) was a vinylidene fluoride homopolymer, bis (2-ethylhexyl) phthalate, dibutyl phthalate, silica micropowder=40.0:31.7:5.3:23.0 (mass ratio).
Example 7
Vinylidene fluoride homopolymer (Kynar 720, manufactured by ARKEMA corporation) was used as the thermoplastic resin. The pellet Kynar720 was pulverized by freeze-pulverizing, and was pulverized by using a Linrex mill (manufactured by HOSOKAWAMICRON Co., ltd.). Classification was performed using a vibrating screen to remove openings of 425 μm and above. The D50 particle diameter after pulverization was 100. Mu.m, the particle diameter dispersity V was 2.3, the roundness was 0.45, and the linearity was 2.5. The composition of the melt-kneaded matter of the layer (a) was set to vinylidene fluoride homopolymer bis (2-ethylhexyl) phthalate dibutyl phthalate silica micropowder=34.0:32.5:8.1:25.4 (mass ratio), the composition of the melt-kneaded matter of the layer (B) was set to vinylidene fluoride homopolymer bis (2-ethylhexyl) phthalate dibutyl phthalate silica micropowder=40.0:32.4:4.6:23.0 (mass ratio), and the air movement time was set to 0.20 seconds. Except for this, a porous hollow fiber membrane was obtained under the same conditions as in example 1.
Comparative example 1
A porous hollow fiber membrane was obtained under the same conditions as in example 2, except that the joining time of each layer was set to 0 seconds.
Example 8
A porous hollow fiber membrane was obtained under the same conditions as in example 1, except that the temperature difference between the melt-kneading machine for the layer (A) and the melt-kneading machine for the layer (B) was 0 ℃.
< evaluation >
The physical properties of each sample of the porous hollow fiber membrane obtained in the examples and comparative examples were measured and evaluated as follows.
The measurement below was performed at 25℃in all cases unless otherwise specified.
(1) Measurement of outer diameter, inner diameter, film thickness (mm)
The hollow fiber membrane was slit at 15cm intervals in a direction perpendicular to the longitudinal direction of the membrane by a razor or the like, the major and minor diameters of the inner diameter and the major and minor diameters of the outer diameter of the cross section were measured by a microscope, the inner diameter and the outer diameter were calculated by the following formulas (2) and (3), the inner diameter was subtracted from the calculated outer diameter, and the value obtained by dividing by 2 was calculated as the film thickness. The average value of 20 points was measured and used as the inner diameter, outer diameter and film thickness under the above conditions.
[ number 3 ]
[ number 4 ] the method comprises
(2) Pure water permeability (L/m) 2 /hr)
The hollow fiber membrane was immersed in a 50 mass% aqueous ethanol solution for 30 minutes, and then immersed in water for 30 minutes, so that the hollow fiber membrane became wet. One end of a wet hollow fiber membrane having a length of about 10cm was sealed, an injection needle was inserted into the hollow portion at the other end, pure water at 25℃was injected into the hollow portion from the injection needle at a pressure of 0.1MPa, and the amount of water permeated through the outer surface was measured to determine the pure water permeation flux by the following formula. Here, the effective membrane length means a net membrane length excluding a portion into which the injection needle is inserted. The number of measurements was 10, and the average value was defined as the pure water permeability under each condition.
[ number 5 ] the method comprises
(3) Particle size distribution
The thermoplastic resins used for the layers (a) and (B) were measured using MS3000 (manufactured by MALVERN PANALYTICAL corporation) as a particle size distribution measuring apparatus, water as a dispersion medium, and 1.330 as a dispersion medium refractive index and 1.420 as a particle refractive index. D10, D50, D90 are calculated on the basis of the particle volume.
(4) Roundness and linearity of particles
For the thermoplastic resin used for the layer (A) and the layer (B), polymer particles were observed under an acceleration voltage of 3kV using a Hitachi electron microscope SU8000 series. When observed, photographing was performed so as to achieve a magnification at which 20 or more polymer particles could be confirmed. In order not to overlap the polymer particles with each other, the sample is arranged on a stage at the time of manufacturing the observation sample so as to be as thin and flat as possible. Using the captured image, a transparent sheet is superimposed on a copy of the image, and the particle portion is blackened using a black pen or the like, and the transparent sheet is copied to white paper, whereby the particle portion is blackened and the portion is white. Binarization was performed by discriminant analysis using windof 2018 ver 4.23.1. And calculating roundness and linearity according to the shape characteristic value analysis of the obtained binarized image.
(5)DSC
As a device, using a DisoveryDSC 2500 manufactured by TA instruments Co., ltd., a sample amount of about 5mg was put into an aluminum pan at 50 mL/min under a nitrogen atmosphere, and the measurement was performed at a temperature ranging from-20℃to 200℃and a rise-fall rate of 10℃per minute. The melting point is the melting peak temperature after the first temperature rise and fall and at the second temperature rise.
(6) Pore diameter and aperture ratio of inner and outer surfaces
The surface of the filtrate was photographed by the same electron microscope as that used for the measurement of the roundness and linearity of the particles (4). Shooting is performed at a magnification at which the shape of20 or more holes can be confirmed, and in the present example and comparative example, shooting is performed at 10000 times.
Using the captured image, for example, as described in japanese unexamined patent publication No. 2001/53213, a transparent sheet is superimposed on a copy of the image, and a hole portion is blackened by a black pen or the like, and the transparent sheet is copied to a white paper, whereby the hole portion is clearly distinguished from the non-hole portion to be white. The binarization was then performed by discriminant analysis using commercially available image analysis software windof 2018 ver 4.23.1. The area occupied by the binarized image thus obtained is obtained, whereby the aperture ratios of the surfaces FA and FB are obtained.
(7) Method for determining layer-to-layer boundaries in the case of a multilayer structure
The cross section of the film was observed using a Hitachi electron microscope SU8000 series at an acceleration voltage of 3 kV. In the present example and comparative example, the vicinity of the layer-to-layer boundary was photographed at 1000 times. When a boundary line is distinguished from layer to layer from a captured image, the boundary line can be defined as a layer-to-layer boundary. In the porous hollow fiber membranes of the present example and comparative example, the boundary line was also defined as a layer-to-layer boundary because the boundary line can be distinguished.
In the porous hollow fiber membranes obtained in the examples and comparative examples, the boundary line between layers can be identified by the above method, but when the boundary between layers cannot be identified by the above method, the boundary can be determined by the following method. For example, a method for determining the boundary between the layer (a) and the layer (B) in the case of a porous hollow fiber membrane having a two-layer structure will be described. The following is a method in the case where the layer (a) is a blocking layer and the layer (B) is a supporting layer.
The cross section of the hollow fiber membrane was photographed by the electron microscope, and a photograph was taken, in which the shape of 20 or more holes could be confirmed. For the total observation of the cross section, the image is a plurality of images. For example, measurement is performed at 5000 times, and a cross-sectional electron microscope sample is obtained by cutting a film sample frozen in ethanol into a wafer.
Using commercially available image analysis software windof 2018 ver4.23.1, an image was drawn with 100 lines L (i.e., lines connecting points of the same film thickness) having the same distance from the surface FA at intervals of 101 equally dividing the entire film thickness as shown in fig. 4A, and the length Lh of the portion corresponding to the void h in the image was measured as shown in fig. 4B. The average value of the cross-cut length Lh is calculated by arithmetic average, and the cross-sectional aperture of each film thickness portion is obtained. When the magnification of the scanning electron microscope photograph is sufficiently high, the line having the same distance to the surface FA may be approximated by a straight line. By normalizing the cross-sectional pore diameter of each film thickness portion using the maximum value of the obtained cross-sectional pore diameter, the point from the surface FA where the normalized value reaches closest to 0.7 can be regarded as the boundary layer of the layer.
(8) Breaking strength (MPa)
The load and displacement at the time of stretching and breaking were measured under the following conditions.
Sample: wet hollow fiber membrane produced by the method of (2)
Measuring instrument: instron tensile tester (manufactured by Shimadzu corporation: AGS-X) distance between chucks: 5cm
Stretching speed: 20 cm/min
The breaking strength was determined by the following formula.
[ number 4 ] the method comprises
The membrane cross-sectional area was determined by the following formula.
[ number 5 ] the method comprises
(9) Water permeability test
The resulting porous hollow fiber membrane was used to produce a filter module 11 shown in fig. 5. The filter assembly 11 was composed of 300 hollow fibers having an effective membrane length of 1m, and the hollow fibers at both ends were sealed with an epoxy-based sealing material 13. At the upper end of the module, the hollow of the hollow fiber membranes 12 is open, and at the lower end, the hollow of the hollow fiber membranes is sealed. River water having a turbidity of 2 to 4 degrees is filtered from the outer surface side of the hollow fiber through the inlet 14 for raw water and air, and filtered water is obtained from the inner surface side of the upper end. The set Flux (m/day)) was gradually increased to the filtered flow (m 3 Per day) divided by the surface area of the membrane (m 2 ) And the value obtained), flux immediately before the onset of a sharp rise in transmembrane pressure difference was taken as critical Flux (m/day). The abrupt increase in the transmembrane pressure difference was evaluated by using an increase rate of about 50kPa/5 days as an index.
[ Table 1 ]
[ Utility ] A method for manufacturing a semiconductor device
According to the present invention, a method for producing a porous multilayer hollow fiber membrane having excellent filtration performance and strength can be provided.
Further, according to the present invention, there can be provided a porous multilayer hollow fiber membrane which can be suitably used in a method for removing turbidity from natural water, domestic sewage, and suspended water which is treated water thereof.
Claims (6)
1. A method for producing a multilayer porous hollow fiber membrane, characterized in that the method for producing a multilayer porous hollow fiber membrane comprises two or more steps of: a step of forming a film having a multilayer structure by extruding two or more different melt-kneaded materials from a nozzle for molding a hollow fiber having an ejection port,
at least one of the melt-kneaded materials contains a thermoplastic resin having a melting point of less than 170 ℃ and
and (c) merging the molten kneaded materials constituting each layer of the multilayer porous hollow fiber membrane in the ejection port, wherein a time t from the ejection of the merged molten kneaded materials is t >0 seconds.
2. The method for producing a porous multilayer hollow fiber membrane according to claim 1, wherein the time t of the melt-kneaded mixture after the ejection and the joining is t+.0.020 seconds.
3. The method for producing a porous multi-layer hollow fiber membrane according to claim 1 or 2, wherein the multi-layer porous hollow fiber membrane is composed of an inner layer and an outer layer.
4. The method for producing a porous multilayer hollow fiber membrane according to claim 1 or 2, wherein the thermoplastic resin is a vinylidene fluoride resin.
5. The method for producing a porous multilayer hollow fiber membrane according to claim 3, wherein the melt-kneaded materials constituting the inner and outer layers each contain polyvinylidene fluoride, wherein,
the distance between the solubility parameter of polyvinylidene fluoride represented by the formula (1) and the solubility parameter of the solvent used in the production of the layer comprising the surface FA on the filtered liquid side of the porous multi-layer hollow fiber membrane is set to Pa, and
when the distance between the solubility parameter of polyvinylidene fluoride represented by the following formula (2) and the solubility parameter of the solvent used in the production of the layer including the surface FB, which is the surface on the filtrate side, of the porous multilayer hollow fiber membrane is Pb, pb—pa >0.05.
Pa ＝ (4 (σdm-σdp)＾2 + (σpm-σpp)＾2 + (σhm-σhp)＾2)＾0.5··· (1)
Pb ＝ (4 (σdm-σdp)＾2 + (σpm-σpp)＾2 + (σhm-σhp)＾2)＾0.5··· (2)
In the formulas (1) and (2), σdm and σdp represent the dispersion force terms of the solvent and polyvinylidene fluoride used, respectively; σpm and σpp represent dipole-coupling force terms of the solvent and polyvinylidene fluoride used, respectively; and σhm and σhp represent the solvent used and the hydrogen bonding term of polyvinylidene fluoride, respectively ].
6. A porous hollow fiber membrane produced by the method for producing a porous multi-layer hollow fiber membrane according to claim 1 or 2.
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