JP5083927B2 - Method for producing polyolefin microporous membrane - Google Patents

Description

  The present invention relates to a method for producing a thermoplastic resin microporous membrane, and in particular, when removing a solvent added during film formation and / or a cleaning solvent used for removing a plasticizer, a gas phase in the production process system. A thermoplastic microporous membrane with high porosity, air permeability, dimensional stability, and appearance properties that can be removed at high speed while suppressing volatile diffusion into the inside, while suppressing shrinkage of the thermoplastic resin. It relates to a method of manufacturing.

  Thermoplastic resin microporous membranes (microporous membranes) are widely used in various applications such as battery separators, diaphragms for electrolytic capacitors, various filters, moisture permeable and waterproof clothing, reverse osmosis filtration membranes, ultrafiltration membranes, and microfiltration membranes. It has been.

  In the production of a thermoplastic resin microporous film, a solvent or a plasticizer is often added during film formation. However, since these must not remain in the final product, it is necessary to remove the added solvent and plasticizer from the gel-like molded product after film formation. When removing the solvent or plasticizer, the gel-like molded product is usually washed with a volatile washing solvent and then dried with hot air. However, since the thermoplastic resin contracts during hot air drying, there is a problem that porosity and permeability are lowered.

  For this reason, conventionally, a method has been employed in which a gel-like molded product is held in a tenter and then dried with hot air, or held in a small-diameter multistage heating roll and then dried. However, the tenter method has a problem that the film cannot be sufficiently retained because it does not have a grip force enough to withstand the contraction force of the film, particularly when drying a highly volatile washing solvent such as methylene chloride. On the other hand, when a multi-stage heating roll is used, it is necessary to reduce the diameter of the roll in order to secure the gripping force of the film. Or the film contracts in the width direction in the roll gap. Therefore, these methods are not necessarily suitable methods for drying at high speed while suppressing shrinkage of the thermoplastic resin. There is also a method of combining small-diameter multi-stage heating rolls and hot air drying, but particularly when hot air drying is performed at a high speed, a large amount of hot air is required, and the cleaning solvent is violently volatilized into the gas phase in the manufacturing process system. Since it diffuses, it is difficult to recover the washing solvent by simple cooling condensation. For this reason, when unrecovered cleaning gas is released out of the manufacturing process system, there is a problem particularly when a cleaning solvent such as methylene chloride that may cause environmental pollution is used.

  In contrast, Japanese Patent Laid-Open No. 2002-256099 (Patent Document 1) proposes a method of using a cleaning solvent having a surface tension at 25 ° C. of 24 mN / m or less as a cleaning solvent for removing the solvent and the plasticizer. Yes. By using such a cleaning solvent, it is possible to suppress the shrinkage and densification of the network structure caused by the surface tension of the gas-liquid interface that occurs inside the microporous body during drying after cleaning. As a result, the porosity / permeability of the microporous membrane can be improved.

Japanese Patent Laid-Open No. 2002-256099

  However, the method described in Patent Document 1 has a problem that the drying rate is insufficient because the film is dried by a heat drying method or an air drying method to remove the washing solvent. Further, in the method described in Patent Document 1, since the cleaning solvent volatilizes and diffuses in the gas phase in the manufacturing process system, when the unrecovered cleaning gas is released to the outside of the manufacturing process system, the cleaning solvent has a relatively flash point. There was a problem when using a lower one.

  On the other hand, Japanese Patent Application Laid-Open No. 2003-082151 proposes a method of removing the cleaning solvent from the cleaning molded article by applying a suction force with a suction roll or the like. However, when a punching roll, a slit roll, or a wire roll is used, there is a problem that the surface of the obtained film is deformed depending on the suction pressure, and indentations derived from the suction grooves or suction holes of the roll are generated. In addition, when the nonwoven fabric roll is used, there is a problem that the suction degree is lowered because clogging occurs due to continuous use, or pinholes are generated in the microporous film due to metal powder captured on the surface of the nonwoven fabric. there were.

  Therefore, the object of the present invention is to remove the cleaning solvent used for removing the solvent added during the film forming process at a high speed while suppressing volatile diffusion into the gas phase in the production process system. It is to provide a method capable of producing a thermoplastic resin microporous film having excellent porosity, air permeability, dimensional stability and appearance properties by removing while suppressing shrinkage of the thermoplastic resin. .

As a result of diligent research in view of the above problems, the present inventors have developed a cleaning solvent having a surface tension, a boiling point, and water solubility within a specific range as a cleaning solvent used for removing the solvent added during film formation. By using (A) and removing the cleaning solvent (A) remaining from the washed polyolefin microporous membrane by simply bringing the washed polyolefin microporous membrane into contact with warm water without applying a suction force, washing is performed. Cleaning solvent can be removed at high speed while suppressing volatile diffusion into the gas phase and shrinkage of the polyolefin microporous membrane in the solvent production process system, thus providing excellent porosity, air permeability, dimensional stability, and appearance. The inventors have found that a polyolefin microporous membrane can be produced and have arrived at the present invention.

That is, in the method for producing a polyolefin porous membrane of the present invention, a solution obtained by melting and kneading a polyolefin and a solvent is extruded from a die and cooled, and the solvent remaining from the gel-like molded product is cooled with a washing solvent. A method for producing a polyolefin microporous membrane comprising a step of removing the cleaning solvent remaining from the resulting washed polyolefin microporous membrane with warm water , wherein (a) the surface tension at 25 ° C. is 24 as the cleaning solvent. mN / m or less, use the (b) and a boiling point of 100 ° C. or less at atmospheric pressure, and (c) solubility in water at 16 ° C. is not more than 600 mass ppm washing solvent (a), wherein The temperature of the hot water is (the boiling point of the cleaning solvent (A) −5 ° C.) to the crystal dispersion temperature of the polyolefin, and the contact time between the cleaning polyolefin microporous membrane and the hot water is within 15 seconds. Before The washed polyolefin microporous membrane is showered with the warm water, or the washed polyolefin microporous membrane is immersed in the warm water, or both, and without applying a suction force, the washed polyolefin microporous membrane and The cleaning solvent (A) is removed from the washed polyolefin microporous membrane only by contacting with the warm water.

The lower limit of the temperature of the hot water is preferably the boiling point of the washing solvent (A) , more preferably the boiling point + 3 ° C. The upper limit of the temperature of the hot water is preferably a crystal dispersion temperature of the thermoplastic resin of −5 ° C. If showering hot water, while continuously conveying the cleaned molded product by a roll, preferably a shower of hot water in the roll winding portion of the cleaning molding. When immersed in warm water, it is preferable to immerse at least the roll-wrapped portion of the washed molded product in warm water while continuously conveying the washed molded product by a roll, or to swing the washed molded product in warm water.

The washing solvent (A) preferably satisfies the following conditions (1) to (11) so that the thermoplastic resin microporous film can obtain more excellent characteristics.
(1) The surface tension is 20 mN / m or less at 25 ° C.
(2) Boiling point under atmospheric pressure is 80 ° C or less.
(3) The solubility in water at 16 ° C. is 300 mass ppm or less.
(4) Consisting of fluorinated compounds such as hydrofluorocarbons, hydrofluoroethers, perfluorocarbons, perfluoroethers, normal paraffins having 5 to 7 carbon atoms, isoparaffins having 5 to 7 carbon atoms, and cycloparaffins having 5 to 7 carbon atoms. At least one selected from the group.
(5) The hydrofluorocarbon described in (4) above is a chain hydrofluorocarbon represented by a composition formula of C ₅ H ₂ F ₁₀ .
(6) The hydrofluoroether described in (4) above is a compound represented by a composition formula of C ₄ F ₉ OCH ₃ or C ₄ F ₉ OC ₂ H ₅ .
(7) The perfluorocarbon described in (4) above is a compound represented by a composition formula of C ₆ F ₁₄ or C ₇ F ₁₆ .
(8) The perfluoroether described in (4) above is a compound represented by a composition formula of C ₄ F ₉ OCF ₃ or C ₄ F ₉ OC ₂ F ₅ .
(9) The normal paraffin having 5 to 7 carbon atoms described in the above (4) is at least one selected from the group consisting of normal pentane, normal hexane and normal heptane.
(10) The C5-C7 isoparaffin described in the above (4) is 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylbutane, 2-methylhexane, 3-methyl Selected from the group consisting of hexane, 3-ethylpentane, 2,2-dimethylpentane, 2,3-dimethylpentane, 2,4-dimethylpentane, 3,3-dimethylpentane and 2,2,3-trimethylbutane At least one kind.
(11) The C5-C7 cycloparaffin described in (4) above is at least one selected from the group consisting of cyclopentane, cyclohexane and methylcyclopentane.

  The removal of the solvent with the washing solvent may be performed in two or more steps. In that case, a stage using a washing solvent other than the washing solvent (A) (hereinafter referred to as “washing solvent (B)” unless otherwise specified) may be used, but at least in the final stage, the washing solvent (A ) Is used. The washing solvent (A) may be used alone, or the washing solvent (A) and the washing solvent (B) may be used to carry out a multistage treatment of three or more stages, in this case, a three-stage to seven-stage process. It is preferable to carry out.

In order for the thermoplastic resin microporous membrane to obtain more excellent characteristics, the cleaning solvent (B) preferably satisfies the following conditions (12) to (25).
(12) At least one selected from the group consisting of methylene chloride, carbon tetrachloride, ethane trifluoride, methyl ethyl ketone, pentane, hexane, heptane, diethyl ether and dioxane.
(13) A non-aqueous solvent having a boiling point of 100 ° C or higher and a flash point of 0 ° C or higher.
(14) The non-aqueous solvent described in (13) above is a normal paraffin having 8 or more carbon atoms, a normal paraffin having 5 or more carbon atoms in which at least a part of hydrogen atoms are substituted with halogen atoms, and an isoparaffin having 8 or more carbon atoms. A cycloparaffin having 7 or more carbon atoms, a cycloparaffin having 5 or more carbon atoms in which at least some of the hydrogen atoms are substituted with halogen atoms, an aromatic hydrocarbon having 7 or more carbon atoms, or at least some of the hydrogen atoms being halogen atoms. Substituted aromatic hydrocarbons having 6 or more carbon atoms, alcohols having 5 to 10 carbon atoms in which some of the hydrogen atoms may be substituted with halogen atoms, and those in which some of the hydrogen atoms are substituted with halogen atoms A group consisting of an ester having 5 to 14 carbon atoms, an ether having 4 to 14 carbon atoms in which a part of hydrogen atoms may be substituted with a halogen atom, and a ketone having 5 to 10 carbon atoms Barre was at least one kind.
(15) The normal paraffin having 8 or more carbon atoms described in the above (14) more preferably has 8 to 12 carbon atoms, specifically from normal octane, normal nonane, normal decane, normal undecane and normal dodecane. Is at least one selected from the group consisting of
(16) The normal paraffin having 5 or more carbon atoms in which at least a part of the hydrogen atoms described in (14) is substituted with a halogen atom is 1-chloropentane, 1-chlorohexane, 1-chloroheptane, 1-chloro. At least selected from the group consisting of octane, 1-bromopentane, 1-bromohexane, 1-bromoheptane, 1-bromooctane, 1,5-dichloropentane, 1,6-dichlorohexane and 1,7-dichloroheptane It is a kind.
(17) The isoparaffin having 8 or more carbon atoms described in (14) above is 2,3,4-trimethylpentane, 2,2,3-trimethylpentane, 2,2,5-trimethylhexane, 2,3,5 -At least one selected from the group consisting of trimethylhexane, 2,3,5-trimethylheptane and 2,5,6-trimethyloctane.
(18) The cycloparaffin having 7 or more carbon atoms described in the above (14) is cycloheptane, cyclooctane, methylcyclohexane, cis- and trans-1,2-dimethylcyclohexane, cis- and trans-1,3-dimethyl. It is at least one selected from the group consisting of cyclohexane and cis- and trans-1,4-dimethylcyclohexane.
(19) The cycloparaffin having 5 or more carbon atoms in which a part of hydrogen atoms described in the above (14) is substituted with a halogen atom is at least one selected from the group consisting of chlorocyclopentane and chlorocyclohexane.
(20) The aromatic hydrocarbon having 7 or more carbon atoms described in the above (14) is at least one selected from the group consisting of toluene, orthoxylene, metaxylene and paraxylene.
(21) The aromatic hydrocarbon having 6 or more carbon atoms in which a part of the hydrogen atoms described in the above (14) is substituted with a halogen atom is chlorobenzene, 2-chlorotoluene, 3-chlorotoluene, 4-chlorotoluene, It is at least one selected from the group consisting of 3-chloroorthoxylene, 4-chloroorthoxylene, 2-chlorometaxylene, 4-chlorometaxylene, 5-chlorometaxylene and 2-chloroparaxylene.
(22) The alcohol having 5 to 10 carbon atoms in which a part of the hydrogen atoms described in the above (14) may be substituted with a halogen atom is isopentyl alcohol, tertiary pentyl alcohol, cyclopentanol, cyclohexanol, It is at least one selected from the group consisting of 3-methoxy-1-butanol, 3-methoxy-3-methyl-1-butanol, propylene glycol normal butyl ether and 5-chloro-1-pentanol.
(23) The ester having 5 to 14 carbon atoms in which part of the hydrogen atoms described in the above (14) may be substituted with a halogen atom is diethyl carbonate, diethyl maleate, normal propyl acetate, normal butyl acetate, acetic acid It is at least one selected from the group consisting of isopentyl, 3-methoxybutyl acetate, 3-methoxy-3-methylbutyl acetate, ethyl normal butyrate, ethyl normal valerate, and 2-chloroethyl acetate.
(24) The ether having 4 to 14 carbon atoms in which a part of hydrogen atoms described in (14) above may be substituted with a halogen atom is selected from the group consisting of normal butyl ether, diisobutyl ether and bischloroethyl ether. At least one kind.
(25) The ketone having 5 to 10 carbon atoms described in the above (14) is at least one selected from the group consisting of 2-pentanone, 3-pentanone, 2-hexanone, 3-hexanone, cyclopentanone, and cyclohexanone. It is.

The thermoplastic resin preferably satisfies the following conditions (26) to (35) in order for the thermoplastic resin microporous membrane to obtain more excellent characteristics.
(26) At least one selected from the group consisting of polyolefin, polyester, polyamide, polyarylene ether and polyarylene sulfide.
(27) The polyolefin described in (26) above is polyethylene or a polyethylene composition.
(28) The polyethylene according to (27) above has a mass average molecular weight of 1 × 10 ^{4 to} 5 × 10 ⁶ .
(29) The polyethylene according to (28) above has a mass average molecular weight of 1 × 10 ⁵ to 4 × 10 ⁶ .
(30) The polyethylene according to any one of the above (27) to (29) is at least one selected from the group consisting of ultrahigh molecular weight polyethylene, high density polyethylene, medium density polyethylene and low density polyethylene.
(31) The polyethylene according to any one of the above (27) to (30) is an ultrahigh molecular weight polyethylene having a mass average molecular weight of 5 × 10 ⁵ or more.
(32) The ratio Mw / Mn (molecular weight distribution) of the weight average molecular weight (Mw) to the number average molecular weight (Mn) of the polyethylene according to any one of (27) to (31) is 5 to 300.
(33) The polyethylene composition according to the above (27) essentially comprises ultrahigh molecular weight polyethylene, and further contains at least one selected from the group consisting of high density polyethylene, medium density polyethylene and low density polyethylene.
(34) The polyethylene composition according to the above (33) includes an ultrahigh molecular weight polyethylene having a mass average molecular weight of 5 × 10 ⁵ or more and a high density polyethylene having a mass average molecular weight of 1 × 10 ⁴ or more and less than 5 × 10 ⁵ .
(35) The polyethylene composition according to the above (33) or (34) is a polypropylene having a mass average molecular weight of 1 × 10 ⁴ to 4 × 10 ⁶ as an optional component, and a mass average molecular weight of 1 × 10 ⁴ to Polybutene-1 in the range of 4 × 10 ⁶ , polyethylene wax in the range of 1 × 10 ³ to 4 × 10 ⁴ in weight average molecular weight, and in the range of 1 × 10 ⁴ to 4 × 10 ⁶ in weight average molecular weight It contains at least one polyolefin selected from the group consisting of ethylene / α-olefin copolymers.

  The properties of the thermoplastic resin microporous membrane obtained by the production method of the present invention are as follows: porosity is 25-80%, air permeability is 10-2000 sec / 100 cc (film thickness 30 μm conversion), surface There is no water mark, and the heat shrinkage is 15% or less in both the machine direction (MD) and the vertical direction (TD).

  The thermoplastic resin microporous membrane obtained by the production method of the present invention is useful as a battery separator.

In the production of the polyolefin microporous membrane by the solvent method of the present invention, as a cleaning solvent used for removing the solvent, (a) the surface tension at 25 ° C. is 24 mN / m or less, and (b) at atmospheric pressure. A cleaning polyolefin microporous membrane having a boiling point of 100 ° C. or less and (c) a cleaning solvent (A) having a solubility in water at 16 ° C. of 600 mass ppm or less and without applying a suction force The remaining cleaning solvent (A) is removed from the cleaned polyolefin microporous membrane by simply bringing it into contact with warm water at a temperature not lower than (boiling point of cleaning solvent (A) −5 ° C.) and not higher than (polyolefin dispersion temperature). Therefore, it is possible to remove the washing solvent at high speed while suppressing the volatile diffusion into the gas phase and the shrinkage of the polyolefin microporous membrane in the manufacturing process system, and thus the porosity, air permeability, dimensional stability and appearance properties. A microporous polyolefin membrane It is possible. The obtained microporous membrane is useful for battery separators, filters and the like.

[1] Thermoplastic resin Examples of the thermoplastic resin that can be used in the production of the thermoplastic resin microporous membrane of the present invention include polyolefin, polyester, polyamide, polyarylene ether, and polyarylene sulfide. Of these, polyolefin is preferred. The polyolefin may be either a single material or a composition comprising two or more types of polyolefin.

As the polyolefin, any of homopolymers or copolymers such as ethylene, propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, styrene, etc. is used. be able to. Among these, polyethylene is preferable as the polyolefin. Although there is no restriction | limiting in particular in the mass mean molecular weight of polyethylene, Usually, it is 1 * 10 < ⁴ > -1 * 10 < ⁷ >, Preferably it is 1 * 10 < ⁴ > -5 * 10 < ⁶ >, More preferably, it is 1 * 10 < ⁵ > -4 *. 10 is a ^6.

Examples of polyethylene include ultra high molecular weight polyethylene, high density polyethylene, medium density polyethylene, and low density polyethylene. These polyethylenes may be copolymers containing a small amount of other α-olefins. As α-olefin other than ethylene, propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, styrene and the like can be used. Among these, ultra high molecular weight polyethylene is preferable as the polyethylene. The mass average molecular weight of the ultra high molecular weight polyethylene is preferably 5 × 10 ⁵ or more, more preferably in the range of 1 × 10 ⁶ to 15 × 10 ⁶ , and in the range of 1 × 10 ^{6 to} 5 × 10 ⁶ . It is particularly preferred that

  In the case of a polyethylene or a composition comprising two or more types of polyethylene, the ratio Mw / Mn (molecular weight distribution) of the mass average molecular weight (Mw) and the number average molecular weight (Mn) is not limited. , Preferably in the range of 5 to 300, more preferably in the range of 10 to 100. In order to adjust the molecular weight distribution, polyethylene may be prepared by multistage polymerization. Of course, one-stage polymerized polyethylene can be used.

As a polyolefin composition, the composition which requires polyethylene is preferable, and the composition which requires the said ultra high molecular weight polyethylene is more preferable. The polyolefin composition essentially including the ultra high molecular weight polyethylene preferably contains at least one selected from the group consisting of high density polyethylene, medium density polyethylene and low density polyethylene, and more preferably contains high density polyethylene. These high density polyethylene, medium density polyethylene and low density polyethylene preferably have a mass average molecular weight of 1 × 10 ⁴ or more and less than 5 × 10 ⁵ .

Above ultra high molecular weight polyethylene polyolefin composition as essential, polypropylene, weight-average molecular weight of 1 × 10 ⁴ ~4 × 10 having a mass average molecular weight as an optional component in the range of ^{1 × 10 4 ~4 × 10 6} 6 Polybutene-1 in the range of 1, polyethylene wax in the range of 1 × 10 ³ to 4 × 10 ⁴ , and ethylene α- in the range of 1 × 10 ⁴ to 4 × 10 ⁶ At least one polyolefin selected from the group consisting of olefin copolymers can be added. The amount of these optionally added polyolefins is preferably 80 parts by mass or less, based on 100 parts by mass of the entire polyolefin composition.

[2] Manufacturing method of thermoplastic resin microporous membrane The manufacturing method of the thermoplastic resin microporous membrane of the present invention is as follows. (1) After adding a solvent to the thermoplastic resin, melt-kneading to prepare a thermoplastic resin solution (2) After extruding the thermoplastic resin solution from the die lip, cooling to form a gel-like molded product, (3) Solvent removal step with a washing solvent, (4) Removing the washing solvent from the obtained film A step of removing, and (5) a step of drying the obtained film. If necessary, a (6) stretching step may be provided before and / or after the step (3). Further, after the steps (1) to (5) or (1) to (6), if necessary, (7) a crosslinking treatment step by ionizing radiation, (8) a heat treatment step, (9) a hydrophilization treatment step and ( 10) A surface coating treatment step or the like may be provided.

(1) Preparation process of thermoplastic resin solution First, an appropriate solvent is added to the thermoplastic resin, and then melt-kneaded to prepare a thermoplastic resin solution. Various additives such as an antioxidant, an ultraviolet absorber, an anti-blocking agent, a pigment, a dye, and an inorganic filler can be added to the thermoplastic resin solution as necessary within a range not impairing the effects of the present invention. For example, finely divided silicic acid can be added as a pore forming agent.

  Either a liquid solvent or a solid solvent can be used as the solvent. Examples of the liquid solvent include nonane, decane, decalin, paraxylene, undecane, dodecane, aliphatic hydrocarbons such as liquid paraffin, and mineral oil fractions having boiling points corresponding to these. In order to obtain a gel-like molded product having a stable solvent content, it is preferable to use a non-volatile liquid solvent such as liquid paraffin. The solid solvent preferably has a melting point of 80 ° C. or lower. Examples of such a solid solvent include paraffin wax, ceryl alcohol, stearyl alcohol, dicyclohexyl phthalate, and the like. You may use it, mixing a liquid solvent and a solid solvent suitably.

  The viscosity of the liquid solvent is preferably in the range of 30 to 500 cSt at 25 ° C., and more preferably in the range of 50 to 200 cSt. When the viscosity of the liquid solvent at 25 ° C. is less than 30 cSt, the thermoplastic resin solution is not uniformly discharged from the die lip and is difficult to knead. On the other hand, if it exceeds 500 cSt, it is difficult to remove the solvent.

  The method of melt kneading is not particularly limited, but it is usually carried out by uniformly kneading in an extruder. This method is suitable for preparing a high concentration solution of a thermoplastic resin. The melting temperature is preferably in the range of the melting point of the thermoplastic resin + 10 ° C. to + 100 ° C. Specifically, the melting temperature is preferably in the range of 140 to 230 ° C, more preferably in the range of 170 to 200 ° C. Here, the melting point refers to a value obtained by differential scanning calorimetry (DSC) based on JIS K7121. The solvent may be added before the start of kneading, or may be added from the middle of the extruder during the kneading, but it is preferable to add the solvent before starting the kneading to make a solution in advance. In melt kneading, it is preferable to add an antioxidant in order to prevent oxidation of the thermoplastic resin.

  In the thermoplastic resin solution, the blending ratio of the thermoplastic resin and the solvent is 1 to 50% by mass, preferably 20 to 40% by mass, with the total being 100% by mass. When the ratio of the thermoplastic resin is less than 1% by mass, swell and neck-in are increased at the die outlet when extruding the thermoplastic resin solution, and the moldability and self-supporting property of the gel-like molded product are lowered. On the other hand, when the ratio of the thermoplastic resin exceeds 50% by mass, the moldability of the gel-like molded product is lowered.

(2) Formation process of gel-like molded product The melt-kneaded thermoplastic resin solution is extruded from the die directly from the extruder or through another extruder, or once cooled and pelletized, and then again through the extruder. And extrude from the die. As the die lip, a sheet die lip having a rectangular base shape is usually used, but a double cylindrical hollow die lip, an inflation die lip, or the like can also be used. In the case of a sheet die lip, the gap of the die lip is usually in the range of 0.1 to 5 mm, and is heated to a temperature of 140 to 250 ° C. during extrusion. The extrusion rate of the heated solution is preferably in the range of 0.2 to 15 m / min.

  A gel-like molded product is formed by cooling the solution extruded from the die lip in this manner. Cooling is preferably performed at a rate of 50 ° C./min or more at least up to the gelation temperature or less. Cooling is preferably performed to 25 ° C. or lower. By performing such cooling, it is possible to fix a phase separation structure in which the thermoplastic resin phase is microphase-separated by a solvent. In general, when the cooling rate is low, the higher order structure of the gel-like molded product becomes coarse, and the pseudo cell unit forming it becomes large, but when the cooling rate is high, it becomes a dense cell unit. When the cooling rate is less than 50 ° C./min, the degree of crystallinity increases and it is difficult to obtain a gel-like product suitable for stretching. As a cooling method, a method of directly contacting cold air, cooling water, or other cooling medium, a method of contacting a roll cooled by a refrigerant, or the like can be used.

(3) Solvent removal step from the gel-like molded product The solvent is removed from the gel-like molded product obtained by cooling the extruded solution. As will be described later, stretching is performed before and / or after solvent removal, as necessary. For solvent removal (washing), (a) the surface tension at 25 ° C is 24 mN / m or less, (b) the boiling point at atmospheric pressure is 100 ° C or less, and (c) water at 16 ° C. It is necessary to use a cleaning solvent (A) whose solubility is 600 ppm by mass or less. When the surface tension of the cleaning solvent (A) is 24 mN / m or less at 25 ° C., the shrinkage of the film caused by the extraction of the cleaning solvent can be suppressed in the subsequent step of removing the cleaning solvent with hot water. This improves the porosity and air permeability. The reason for this is that the surface tension at 25 ° C is 24 mN / m or less, so that the network is caused by the tension at the interface between the cleaning solvent (A) and the microporous membrane that occurs inside the microporous membrane during extraction of the cleaning solvent (A). This is thought to be because the shrinkage and densification of the tissue can be suppressed. However, even if the surface tension at 25 ° C is 24 mN / m or less, if a cleaning solvent with a solubility in water at 16 ° C of more than 600 ppm by mass is used, the reason is not clear, but the cleaning solvent is removed with warm water at the latter stage. Water marks may be formed in the process, and the surface properties of the film may deteriorate. The surface tension of the cleaning solvent (A) at 25 ° C. is preferably 20 mN / m or less. In the present specification, “surface tension” means the tension generated at the interface between gas and liquid, and is measured based on JIS K 3362. The surface tension of the cleaning solvent (A) decreases as the use temperature increases, but the usable temperature range is limited to the boiling point or less. The boiling point of the cleaning solvent (A) under atmospheric pressure is preferably 80 ° C. or lower.

  When the boiling point of the cleaning solvent (A) under atmospheric pressure is 100 ° C. or lower, it can be quickly removed in the subsequent step of removing the cleaning solvent with warm water. When a cleaning solvent having a boiling point of more than 100 ° C. under atmospheric pressure is used, it takes a long time to remove the cleaning solvent with warm water, resulting in a decrease in production efficiency. Also, if it takes a long time to remove the cleaning solvent, even if the surface tension of the cleaning solvent is 24 mN / m or less at 25 ° C, it will be heated for a long time with hot water, so the porosity and air permeability will not be improved. It will be enough.

  Preventing the formation of water marks (water bubble marks) in the cleaning solvent removal step with warm water in the latter stage, because the solubility of the cleaning solvent (A) in water at 16 ° C. is 600 mass ppm or less. Can do. The solubility of the washing solvent (A) in water is preferably 300 ppm by mass or less at 16 ° C. The solubility of the washing solvent (A) in water increases as the use temperature rises, but as long as the value at 16 ° C. is 600 mass ppm or less, the washing solvent (A) is removed in the washing solvent removal step with warm water at the latter stage. Water marks are not formed even when the cleaning molded product is contacted with warm water.

  As the washing solvent (A), a solvent that is incompatible with the thermoplastic resin is used. Specific examples of the cleaning solvent (A) include, for example, fluorine compounds such as hydrofluorocarbons, hydrofluoroethers, perfluorocarbons, perfluoroethers, normal paraffins having 5 to 7 carbon atoms, isoparaffins having 5 to 7 carbon atoms, and carbon numbers. Examples include 5 to 7 cycloparaffins.

As the fluorine-based compound, for example, a chain hydrofluorocarbon represented by a composition formula of C ₅ H ₂ F ₁₀ , for example, a hydrofluoroether represented by a composition formula of C ₄ F ₉ OCH ₃ and C ₄ F ₉ OC ₂ H ₅ , For example, selected from the group consisting of perfluorocarbons represented by the composition formulas of C ₆ F ₁₄ and C ₇ F ₁₆ and perfluoroethers represented by the composition formulas of C ₄ F ₉ OCF ₃ and C ₄ F ₉ OC ₂ F ₅ , for example. At least one of these is preferred. Since these fluorine-based compounds are not ozone destructive, even if they diffuse outside the manufacturing process system, the environmental load can be reduced, and the flash point is 40 ° C or higher (some compounds have a flash point) The risk of flammable explosion is low.

  Examples of the normal paraffin having 5 to 7 carbon atoms include normal pentane, normal hexane and normal heptane, and normal pentane is preferable. Examples of the isoparaffin having 5 to 7 carbon atoms include 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylbutane, 2-methylhexane, 3-methylhexane, 3-ethylpentane, 2, Examples include 2-dimethylpentane, 2,3-dimethylpentane, 2,4-dimethylpentane, 3,3-dimethylpentane, 2,2,3-trimethylbutane, and the like. Examples of the cycloparaffin having 5 to 7 carbon atoms include cyclopentane, cyclohexane and methylcyclopentane. Cyclopentane having a surface tension of 24 mN / m or less at 20 ° C. is preferable.

  The cleaning solvent (A) described above may be a single composition or a mixed composition of the cleaning solvents (A). Table 1 shows the surface tension, boiling point, and water solubility of representative compounds listed as the washing solvent (A).

The cleaning solvent (A) described above has (a) a surface tension at 25 ° C of 24 mN / m or less, (b) a boiling point at atmospheric pressure of 100 ° C or less, and (c) water at 16 ° C. As long as the solubility of is not more than 600 mass ppm, it may be a mixture with other solvents. As such a mixture, for example, at least one selected from the group consisting of the fluorine-based compound, the normal paraffin having 5 to 7 carbon atoms, the isoparaffin having 5 to 7 carbon atoms, and the cycloparaffin having 5 to 7 carbon atoms. For example, use a mixture containing a small amount of a cyclic hydrofluorocarbon represented by the composition formula of C ₅ H ₃ F ₇ , an aliphatic ether having a boiling point of 100 ° C. or less, an aliphatic ketone, an aliphatic alcohol, an aliphatic ester, etc. Can do.

  The washing using the washing solvent (A) may be performed by two or more steps, and at this time, a stage using a washing solvent other than the washing solvent (A) [washing solvent (B)] may be included. When a step using the washing solvent (B) is entered, the washing solvent (A) may be used at least in the final step. As the washing solvent (B), those not compatible with the thermoplastic resin are used. For example, chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride known as washing solvents; Hydrocarbon; diethyl ether; ether such as dioxane; methyl ethyl ketone and the like can be used. A non-aqueous solvent having a boiling point of 100 ° C. or higher and a flash point of 0 ° C. or higher can also be used as the cleaning solvent (B). The washing solvent (B) is appropriately selected according to the solvent used for dissolving the thermoplastic resin composition, and used alone or as a mixture of the washing solvents (B).

  The cleaning effect is further improved by two or more cleaning steps using the cleaning solvents (A) and (B). By treating with the washing solvent (A) at the final stage of the washing process, the washing solvent (B) used in the previous stage can be removed (hereinafter referred to as “rinsing treatment”), which occurs when the washing solvent is removed with warm water after washing. The network can be prevented from shrinking and densifying. When processing with the cleaning solvent (A) in the final stage of the cleaning process, the use of the above-mentioned fluorine-based compound can reduce the burden on the environment even if the cleaning solvent (A) diffuses outside the manufacturing process system. There is also.

Non-aqueous solvents that have a boiling point of 100 ° C or higher and a flash point of 0 ° C or higher that can be used as cleaning solvent (B) are less volatile, have a low environmental impact, and have a low risk of flammable explosion. is there. In addition, since such non-aqueous solvents have a high boiling point, they are easy to condense, recover easily, and are easy to recycle. Regarding the cleaning solvent (B), “boiling point” refers to the boiling point at 1.01 × 10 ⁵ Pa, and “flash point” refers to a value measured based on JIS K 2265. The flash point of the non-aqueous solvent is preferably 5 ° C. or higher, and more preferably 40 ° C. or higher.

  Examples of the non-aqueous solvent include paraffinic compounds having a boiling point of 100 ° C. or higher and a flash point of 0 ° C. or higher, aromatics, alcohols, esters, ethers, ketones, and the like. Among these, as the non-aqueous solvent, normal paraffins having 8 or more carbon atoms, normal paraffins having 5 or more carbon atoms in which at least some of the hydrogen atoms are substituted with halogen atoms, isoparaffins having 8 or more carbon atoms, or cycloparaffins having 7 or more carbon atoms. , A cycloparaffin having 5 or more carbon atoms in which at least some of the hydrogen atoms are substituted with halogen atoms, an aromatic hydrocarbon having 7 or more carbon atoms, or 6 or more carbon atoms in which at least some of the hydrogen atoms are substituted with halogen atoms Aromatic hydrocarbons, alcohols having 5 to 10 carbon atoms in which some hydrogen atoms may be substituted with halogen atoms, esters having 5 to 14 carbon atoms in which some hydrogen atoms may be substituted with halogen atoms , Selected from the group consisting of ethers having 4 to 14 carbon atoms in which some of the hydrogen atoms may be substituted with halogen atoms, and ketones having 5 to 10 carbon atoms Even without a kind it is preferable.

  As the normal paraffin having 8 or more carbon atoms, normal octane, normal nonane, normal decane, normal undecane and normal dodecane are preferable, and normal octane, normal nonane and normal decane are more preferable.

  Examples of normal paraffins having 5 or more carbon atoms in which at least some of the hydrogen atoms are substituted with halogen atoms include 1-chloropentane, 1-chlorohexane, 1-chloroheptane, 1-chlorooctane, 1-bromopentane, 1-bromopentane, Bromohexane, 1-bromoheptane, 1-bromooctane, 1,5-dichloropentane, 1,6-dichlorohexane and 1,7-dichloroheptane are preferred, 1-chloropentane, 1-chlorohexane, 1-bromopentane And 1-bromohexane is more preferred.

  Isoparaffins having 8 or more carbon atoms include 2,3,4-trimethylpentane, 2,2,3-trimethylpentane, 2,2,5-trimethylhexane, 2,3,5-trimethylhexane, 2,3,5- Trimethylheptane and 2,5,6-trimethyloctane are preferred, and 2,3,4-trimethylpentane, 2,2,3-trimethylpentane, 2,2,5-trimethylhexane and 2,3,5-trimethylhexane are preferred. More preferred.

  Cycloparaffins having 7 or more carbon atoms include cycloheptane, cyclooctane, methylcyclohexane, cis- and trans-1,2-dimethylcyclohexane, cis- and trans-1,3-dimethylcyclohexane, and cis- and trans-1 1,4-dimethylcyclohexane is preferred, and methylcyclohexane is more preferred.

  As the cycloparaffin having 5 or more carbon atoms in which at least a part of hydrogen atoms are substituted with halogen atoms, chlorocyclopentane and chlorocyclohexane are preferable, and chlorocyclopentane is more preferable.

  As the aromatic hydrocarbon having 7 or more carbon atoms, toluene, orthoxylene, metaxylene and paraxylene are preferable, and toluene is more preferable.

  Aromatic hydrocarbons having 6 or more carbon atoms in which at least a part of hydrogen atoms are substituted with halogen atoms include chlorobenzene, 2-chlorotoluene, 3-chlorotoluene, 4-chlorotoluene, 3-chloroorthoxylene, 4-chloro Ortho-xylene, 2-chlorometaxylene, 4-chlorometaxylene, 5-chlorometaxylene and 2-chloroparaxylene are preferred, and chlorobenzene, 2-chlorotoluene, 3-chlorotoluene and 4-chlorotoluene are more preferred.

  Examples of the alcohol having 5 to 10 carbon atoms in which a part of hydrogen atoms may be substituted with a halogen atom include isopentyl alcohol, tertiary pentyl alcohol, cyclopentanol, cyclohexanol, 3-methoxy-1-butanol, 3 -Methoxy-3-methyl-1-butanol, propylene glycol normal butyl ether and 5-chloro-1-pentanol are preferred, 3-methoxy-1-butanol, 3-methoxy-3-methyl-1-butanol, propylene glycol normal Butyl ether and 5-chloro-1-pentanol are more preferred.

  Examples of the ester having 5 to 14 carbon atoms in which a part of hydrogen atoms may be substituted with a halogen atom include diethyl carbonate, diethyl maleate, normal propyl acetate, normal butyl acetate, isopentyl acetate, 3-methoxybutyl acetate, and acetic acid 3 -Methoxy-3-methylbutyl, ethyl normal butyrate, ethyl normal valerate, 2-chloroethyl acetate are preferred, and isopentyl acetate, 3-methoxybutyl acetate, 3-methoxy-3-methylbutyl acetate, ethyl normal butyrate and 2-chloroethyl acetate are preferred. More preferred.

  Dipropylene glycol dimethyl ether, normal butyl ether, diisobutyl ether, and bischloroethyl ether are preferable as the ether having 4 to 14 carbon atoms in which a part of hydrogen atoms may be substituted with a halogen atom. Dipropylene glycol dimethyl ether and bischloroethyl Ether is more preferred.

  As the ketone having 5 to 10 carbon atoms, 2-pentanone, 3-pentanone, 2-hexanone, 3-hexanone, cyclopentanone and cyclohexanone are preferable, and 2-pentanone and 3-pentanone are more preferable.

The cleaning solvent (B) as described above may be used as a mixture of the cleaning solvents (B), but as an optional component (C) in the cleaning solvent (B), for example, a composition formula of C ₅ H ₂ F ₁₀ Chain hydrofluorocarbons represented, for example, hydrofluoroethers represented by the composition formula of C ₄ F ₉ OCH ₃ and C ₄ F ₉ OC ₂ H ₅ , for example, cyclic hydrofluorocarbons represented by the composition formula of C ₅ H ₃ F ₇ , For example, a perfluorocarbon represented by a composition formula of C ₆ F ₁₄ or C ₇ F ₁₆ and at least a perfluoroether represented by a composition formula of C ₄ F ₉ OCF ₃ and C ₄ F ₉ OC ₂ F ₅ , for example. You may use what mixed the solvent chosen 1 type. The cleaning solvent (B) and the optional component (C) are preferably mixed at a rate that the surface tension is 24 mN / m or less at any temperature of 20 to 80 ° C. Generally, the optional component (C) is 2 to 98 parts by mass, preferably 5 to 50 parts by mass in 100 parts by mass of the mixed solvent. By containing 2 to 98 parts by mass of the optional component (C), sufficient cleaning can be performed while suppressing shrinkage densification of the network caused by the surface tension of the cleaning solvent.

  As the cleaning solvent (B), a solvent having a surface tension of 24 mN / m or less at any temperature of 20 to 80 ° C. is preferably used. Examples of such a washing solvent (B) include normal pentane, hexane, heptane, trifluoroethane, diethyl ether, 2-methylpentane, 3-methylpentane, cyclohexane, cyclopentane, acetone, methyl ethyl ketone, and the like.

Preferred combinations of the cleaning solvent (B) used in the first stage of the cleaning process and the cleaning solvent (A) used in the second stage are shown below. However, as will be described later, the cleaning using the cleaning solvent (A) and the cleaning solvent (B) can be performed in three or more stages. Therefore, these are not intended to be performed in two stages. For example, washing solvent (B) / washing solvent (A) = methylene chloride / C ₄ F ₉ OCH ₃ , methylene chloride / C ₄ F ₉ OC ₂ H ₅ , methylene chloride / C ₆ F ₁₄ , methylene chloride / C ₇ F ₁₆ , methylene chloride / normal heptane, methylene chloride / normal hexane, ether / hydrofluoroether, normal paraffin / hydrofluoroether, isoparaffin / hydrofluoroether, cycloparaffin / hydrofluoroether, and ketone / hydrofluoroether. Preferred among the above combinations are washing solvent (B) / washing solvent (A) = methylene chloride / C ₄ F ₉ OCH ₃ , methylene chloride / C ₄ F ₉ OC ₂ H ₅ , methylene chloride / C ₆ F ₁₄ , Methylene chloride / C ₇ F ₁₆ , methylene chloride / normal heptane, methylene chloride / normal hexane, normal heptane / C ₄ F ₉ OCF ₃ , and normal heptane / C ₆ F ₁₄ . By using such a combination, the porosity and permeability of the microporous membrane can be improved while effectively removing the solvent.

  Washing with the washing solvent (A) alone or with the washing solvent (A) and the washing solvent (B) may be performed in three or more steps. Three or more steps are effective when the solvent cannot be sufficiently removed by washing in one or two steps, and the physical properties of the resulting thermoplastic resin microporous film are lowered. In the case of washing in three or more steps using the washing solvent (A) and the washing solvent (B), the washing solvent (A) may be treated at least in the final stage, and the number of washing is not particularly limited. Usually, it is 3 steps to 7 steps, preferably 3 to 4 steps.

  The method for washing the gel-like molded article is a method of showering the washing solvent (A) and / or the washing solvent (B), a method of immersing in the washing solvent (A) and / or the washing solvent (B), and a combination thereof. Etc. The process of showering the washing solvent (A) and / or the washing solvent (B) on the gel-like molded article or immersing the gel-like molded article in the washing solvent (A) and / or the washing solvent (B) is a gel It is preferable to carry out the shaped molded product while conveying it continuously or intermittently. A roll is usually used as a means for conveying the gel-like molded product. When the gel-like molded product is immersed in the washing solvent (A) and / or the washing solvent (B) while being continuously conveyed, the gel-like molded product is allowed to pass through a washing solvent bath. When immersing the gel-shaped product in the cleaning solvent (A) and / or the cleaning solvent (B) while intermittently transporting the gel-shaped product, immerse the cleaning part of the gel-shaped product in the cleaning solvent bath and shake at about 100 rpm. It is preferable to move. At this time, it is preferable to fix the periphery of the washed portion of the gel-like molded product to a frame plate or the like.

  The washing solvent (A) and the washing solvent (B) are preferably used in an amount of 300 to 30000 parts by mass with respect to 100 parts by mass of the gel-like molded product. When washing is performed in two or more steps using the washing solvent (A) and the washing solvent (B), the amount of the washing solvent (A) used is 50 to 100 parts by mass with the amount of the washing solvent (B) used being 100 parts by mass. The amount is preferably 200 parts by mass. Washing with the washing solvent (A) alone or with the washing solvent (A) and the washing solvent (B) is preferably performed until the amount of the solvent remaining in the gel-like molded product is less than 1% by mass.

  The washing temperature when the washing solvent (B) is used depends on the boiling point of the washing solvent (B). When the boiling point of the cleaning solvent (B) is 150 ° C. or lower, it can be cleaned at room temperature, and may be heated and cleaned as necessary, and it is generally preferable to wash at a temperature of 20 to 80 ° C. When the boiling point of the cleaning solvent (B) is 150 ° C. or higher, the permeability of the cleaning solvent (B) into the membrane is poor at room temperature.

  The cleaning temperature and / or rinsing temperature when the cleaning solvent (A) is used depends on the surface tension of the cleaning solvent (A). Specifically, it is preferable to perform the cleaning and / or rinsing treatment at a temperature at which the surface tension of the cleaning solvent (A) is 24 mN / m or less. Since the surface tension of the cleaning solvent (A) is 24 mN / m or less at 25 ° C. at most, heating is not necessary in most cases, and cleaning and / or rinsing can be performed at room temperature. If the temperature of the cleaning solvent (A) is less than the temperature at which the surface tension is 24 mN / m or less, the cleaning solvent (A) is reduced to a temperature at which the surface tension is 24 mN / m or less as necessary. Heat.

(4) Cleaning solvent removal step The remaining cleaning solvent (A) is removed using warm water from the washed molded product obtained by stretching and solvent removal without applying a suction force . By using warm water as the removal medium for the washing solvent (A), the washed molded product can be heated much faster than when warm air is used. For this reason, the outflow and / or volatilization of the cleaning solvent (A) from the cleaning molded product occurs rapidly, and the speed of removing the cleaning solvent (A) can be increased. Since the cleaning solvent (A) extracted from the cleaning product is mainly diffused in warm water, the volatile diffusion of the cleaning solvent (A) is suppressed by replacing dirty warm water with unused warm water at an appropriate frequency. Can do. The cleaning solvent (A) can be removed from the washed molded product by a method of showering warm water in the washed molded product, a method of immersing the washed molded product in warm water, a method using a combination thereof, or the like.

  The lower limit of the temperature of the hot water is preferably the boiling point of the cleaning solvent (A) to be removed is −5 ° C. or higher, more preferably the boiling point or higher, and particularly preferably the boiling point + 3 ° C. or higher. By setting the lower limit of the temperature of the warm water to the boiling point of the washing solvent (A) to be removed at −5 ° C. or higher, the washing solvent (A) can be removed quickly. The upper limit of the temperature of the hot water is preferably not higher than the crystal dispersion temperature of the thermoplastic resin, and more preferably not higher than the crystal dispersion temperature −5 ° C. If the upper limit of the hot water temperature exceeds the crystal dispersion temperature, the resin may be softened. As described above, for example, the crystal dispersion temperature of polyethylene is generally 90 ° C. Even when the crystal dispersion temperature of the thermoplastic resin is 95 ° C. or higher, the temperature of the hot water is preferably 95 ° C. or lower, more preferably 85 ° C. or lower in order to suppress the amount of water vapor generated from the hot water.

The treatment of showering warm water on the washed molded product or immersing the washed molded product in warm water is preferably performed while the washed molded product is conveyed continuously or intermittently. Is preferably heated amount when showering hot water is 50 to 5000 ml / m ² in washing the molded product, and more preferably 100~2000 ml / m ^2. If the amount of hot water in the shower is less than 50 ml / m ² , the hot water cannot be uniformly showered on the surface to be removed. On the other hand, if it exceeds 5000 ml / m ² , control for circulating the hot water becomes difficult. When the washed molded product is immersed in warm water, it is preferable to jet the warm water into the washed molded product in a jet in a warm water bath. As a result, the extraction rate of the cleaning solvent (A) is further increased, and the extracted cleaning solvent (A) is easily diffused in warm water. As means for injecting hot water in a jet form, for example, a hot water nozzle can be mentioned.

  When showering hot water while continuously transporting the washed molded product, it is preferable to use a roll for removing the solvent and shower the roll-wrapped portion of the washed molded product. As a result, the roll is heated with warm water, and the washed molded product can be heated quickly, so that the removal rate of the washing solvent (A) is further increased.

  When the washed molded product is immersed in warm water while being continuously conveyed, it is preferable to use a roll for removing the solvent and immerse at least the roll-wrapped portion of the washed molded product in the warm water. Of course, you may immerse the whole roll in warm water.

  If hot water is showered on the roll wrapping part of the washed molded product or at least a part of the roll is immersed in warm water, the solvent removal roll is heated with warm water. The roll may be heated from within. The temperature for heating the roll for solvent removal is preferably not higher than the crystal dispersion temperature of the thermoplastic resin, and more preferably not higher than the crystal dispersion temperature -5 ° C.

  The diameter of the solvent removal roll is preferably 3 to 100 cm, more preferably 5 to 30 cm. When the diameter is less than 3 cm, the contact area between the cleaning molded product and the roll is small, and the heat of the solvent removing roll is not sufficiently conducted to the cleaning molded product. On the other hand, if the diameter exceeds 100 cm, the equipment becomes too large. The conveyance speed when removing the cleaning solvent (A) while continuously conveying the washed molded product is preferably 0.5 to 80 m / min from the viewpoint of production efficiency, and preferably 1 to 50 m / min. Is more preferable. Usually, one roll for removing the solvent may be provided, but a plurality of rolls may be provided if necessary.

  When the washed molded product is immersed in warm water while being transported intermittently, it is preferable to swing the solvent removal site of the washed molded product in the warm water. At this time, it is preferable that the solvent removal site of the washed molded product is fixed with a frame plate or the like and oscillated at a frequency of about 100 rpm.

In any of the process of immersion in hot shower or hot water, the contact time between the cleaning molding and hot water you within 15 seconds. Since the boiling point of the washing solvent (A) is 100 ° C. or less, the contact time is usually 15 seconds or less. By using warm water as the removal medium for the cleaning solvent (A), the time required for removing the cleaning solvent (A) can be reduced to less than half compared to when warm air is used, and the production efficiency is improved.

  The content of the cleaning solvent remaining in the thermoplastic microporous membrane is preferably 5% by mass or less by treatment with warm water (the mass of the dried film is 100% by mass), and 3% by mass or less. More preferably. If the removal is insufficient and a large amount of the washing solvent remains in the film, it is not preferable because the porosity decreases and the permeability deteriorates in the subsequent heat treatment.

(5) Membrane drying step The membrane from which the washing solvent has been removed can be dried by a heat drying method, an air drying method or the like. Since water has low affinity with the microporous membrane, it can be easily and quickly removed by blowing hot air on the membrane after the hot water treatment. The drying temperature is preferably a temperature not higher than the crystal dispersion temperature of polyolefin, and particularly preferably a temperature lower by 5 ° C. or more than the crystal dispersion temperature.

(6) Stretching step If necessary depending on the intended use of the microporous membrane, stretching is performed before and / or after the solvent removal step (3) above. Stretching is preferably performed before the solvent removal step. Stretching can be performed at a predetermined magnification by heating the gel-like molded product and then using a normal tenter method, roll method, inflation method, rolling method, or a combination of these methods. The stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferred. In the case of biaxial stretching, either simultaneous biaxial stretching or sequential stretching may be used, but simultaneous biaxial stretching is preferred. The mechanical strength is improved by stretching.

  Although a draw ratio changes with thickness of a gel-like molded object, it is preferable to set it as 2 times or more by uniaxial stretching, and it is more preferable to set it as 3-30 times. In biaxial stretching, it is preferable to set at least 3 times or more in any direction, that is, to make the surface magnification 9 times or more because the puncture strength can be improved. If the surface magnification is less than 9, the stretching is insufficient and a highly elastic and high strength thermoplastic resin microporous film cannot be obtained. On the other hand, when the surface magnification exceeds 400 times, there are restrictions in terms of stretching apparatus, stretching operation, and the like.

  The stretching temperature is preferably the melting point of the thermoplastic resin + 10 ° C. or less, and more preferably within the range from the crystal dispersion temperature to less than the crystal melting point. When the stretching temperature exceeds the melting point + 10 ° C., the resin melts and the molecular chain cannot be oriented by stretching. Further, if the stretching temperature is lower than the crystal dispersion temperature, the resin is not sufficiently softened, the film is easily broken during stretching, and high-stretching cannot be performed. For example, the crystal dispersion temperature of polyethylene is generally 90 ° C. In the present invention, the stretching temperature is usually in the range of 100 to 140 ° C, preferably in the range of 110 to 120 ° C. Here, the crystal dispersion temperature refers to a value obtained by measuring temperature characteristics of dynamic viscoelasticity based on ASTM D 4065.

(7) Film cross-linking treatment step It is preferable to remove water by a heat drying method, an air drying method or the like, and subject the dried microporous film to a cross-linking treatment by ionizing radiation. As the ionizing radiation, α rays, β rays, γ rays, electron beams and the like can be used. The crosslinking treatment by ionizing radiation can be performed with an electron dose of 0.1 to 100 Mrad and an acceleration voltage of 100 to 300 kV. The meltdown temperature can be improved by the crosslinking treatment.

(8) Heat treatment step The film from which the cleaning solvent has been removed is preferably heat treated. The crystal of the microporous membrane is stabilized by the heat treatment, and the lamellar layer is made uniform. As a heat treatment method, any one of a heat stretching treatment, a heat setting treatment, and a heat shrinkage treatment may be used, and these may be appropriately selected according to physical properties required for the microporous membrane. These heat treatments are performed at a temperature below the melting point of the polyolefin microporous membrane, preferably within a temperature range of 60 ° C. or higher and melting point −10 ° C. or lower.

  The heat stretching treatment is performed by a commonly used tenter method, roll method, or rolling method, and is preferably performed at a stretching ratio of 1.01 to 2.0 times in at least one direction, and more preferably at a stretching ratio of 1.01 to 1.5 times.

  The heat setting treatment is performed by a tenter method, a roll method or a rolling method. The heat shrink treatment may be performed by a tenter method, a roll method or a rolling method, or may be performed using a belt conveyor or a floating method. The heat shrink treatment is preferably performed in a range of 50% or less in at least one direction, and more preferably performed in a range of 30% or less.

  You may perform combining the above-mentioned heat | fever extending process, heat setting process, and heat shrink process many. In particular, it is preferable to perform a heat shrinking treatment after the heat stretching treatment because a microporous film having a low shrinkage rate and a high strength can be obtained.

(9) Hydrophilization treatment step The membrane from which the washing solvent has been removed may be subjected to a hydrophilic treatment. As the hydrophilic treatment, monomer graft treatment, surfactant treatment, corona discharge treatment or the like is used. The monomer grafting treatment is preferably performed after ionizing radiation.

  As the surfactant, any of a nonionic surfactant, a cationic surfactant, an anionic surfactant or a zwitterionic surfactant can be used, but a nonionic surfactant is preferably used. preferable. When using a surfactant, the surfactant is made into an aqueous solution or a solution of a lower alcohol such as methanol, ethanol, isopropyl alcohol and dipped, or the microporous membrane is hydrophilized by a method using a doctor blade. .

  The obtained hydrophilic microporous membrane is dried. In order to improve permeability at the time of drying, it is preferable to perform heat treatment while preventing shrinkage at a temperature below the melting point of the polyolefin microporous membrane. Examples of the method of performing heat treatment while preventing shrinkage include a method of performing heat treatment while stretching.

(10) Surface coating treatment step The microporous membrane obtained by removing the washing solvent has a fluororesin porous body such as polyvinylidene fluoride and polytetrafluoroethylene, or a porous body such as polyimide and polyphenylene sulfide on the surface. By coating, the melt-down characteristic when used as a battery separator is improved. Further, the microporous membrane obtained by removing the washing solvent improves the high temperature characteristics when used as a battery separator by forming a polypropylene thin film having a racemic diad content of 0.12 to 0.88 on the surface.

[3] Thermoplastic resin microporous membrane The physical properties of the microporous membrane produced as described above are usually 25 to 80% porosity and 10 to 2000 sec / 100 cc (membrane) The thickness is 30 μm), there is no water mark on the surface, and the thermal shrinkage rate is 15% or less in both the machine direction (MD) and the vertical direction (TD). The film thickness of the thermoplastic resin microporous film can be appropriately selected depending on the application, but for example, when used as a battery separator, it is preferably 5 to 200 μm. Thus, since the thermoplastic resin microporous film obtained by the production method of the present invention exhibits excellent permeability, it can be suitably used as a battery separator, filter, or the like.

  The present invention will be described in more detail with reference to the following examples, but the present invention is not limited thereto.

Example 1
A composition comprising 25% by mass of ultra high molecular weight polyethylene (UHMWPE) having a mass average molecular weight of 2.0 × 10 ⁶ and 75% by mass of high density polyethylene (HDPE) having a mass average molecular weight of 3.5 × 10 ⁵ and having Mw / Mn = 16 Product (melting point 135 ° C., crystal dispersion temperature 90 ° C.) and tetrakis [methylene-3- (3,5-ditertiarybutyl-4-hydroxyphenyl) -propionate] methane as an antioxidant at 0.375 per 100 parts by mass of the composition A polyethylene composition added with parts by mass was obtained. 25 parts by mass of the obtained polyethylene composition was put into a twin screw extruder (inner diameter 58 mm, L / D = 42, strong kneading type), and 75 parts by mass of liquid paraffin was supplied from the side feeder of this twin screw extruder. The mixture was melt-kneaded at 200 ° C. and 200 rpm to prepare a polyethylene solution in an extruder. Subsequently, this polyethylene solution was extruded from a T-die installed at the tip of the extruder to a thickness of about 40 μm when biaxially stretched, and then taken up with a cooling roll adjusted to a temperature of 50 ° C. Formed. The resulting gel-like sheet was biaxially stretched at 116 ° C. so as to be 5 × 5 times using a continuous stretching machine, and a stretched film was obtained. The obtained membrane was fixed to a frame plate [size: 20 cm × 20 cm, made of aluminum (hereinafter the same)], and the temperature was adjusted to 23 ° C. Normal pentane [surface tension: 15.5 mN / m (25 ° C.), boiling point: 36 ° C., solubility in water: 225 mass ppm (16 ° C.)], and washed with rocking at 100 rpm for 30 seconds. The above series of washing operations was repeated three more times while replacing normal pentane with a new solution each time. Next, the translucent membrane is immersed in a washing tank containing warm water adjusted to 50 ° C. while being fixed to the frame plate, and the warm water is swung until the normal pentane in the membrane is extracted and the membrane is whitened. Processed. The time required for removing the solvent was 5 seconds. Next, the water adhering to the membrane was blown off with an air spray and further heat fixed at 122 ° C. for 60 seconds to produce a polyethylene microporous membrane.

Example 2
A biaxially stretched membrane produced in the same manner as in Example 1 was fixed to a frame plate, and methylene chloride temperature-controlled at 23 ° C. [surface tension: 27.3 mN / m (25 ° C.), boiling point: 40.0 ° C., solubility in water : 20000 ppm by mass (20 ° C.)] and washed while rocking at 100 rpm for 30 seconds. The above series of washing operations was repeated twice more while exchanging methylene chloride with a new solution each time. Next, methyl perfluorobutyl ether whose temperature was adjusted to 23 ° C. while being fixed to the frame plate [composition formula: C ₄ F ₉ OCH ₃ , product name: Novec HFE-7100, manufactured by Sumitomo 3M Limited, surface tension: 13.6 mN / m (25 ° C), boiling point: 61 ° C, water solubility: 12 mass ppm (25 ° C), no flash point] Rinsing was performed while rocking. The methyl perfluorobutyl ether was replaced with a new solution, and the above series of rinsing operations was repeated once more. Next, the translucent film is immersed in a washing tank containing warm water adjusted to 70 ° C. while being fixed to the frame plate, and is shaken until methyl perfluorobutyl ether in the film is extracted and the film turns white. It was treated with warm water. The time required for removing the solvent was 10 seconds. Next, the water adhering to the membrane was blown off with an air spray and further heat fixed at 122 ° C. for 60 seconds to produce a polyethylene microporous membrane.

Example 3
Cleaning in the first cleaning tank is performed using normal decane (surface tension: 23.4 mN / m (25 ° C), boiling point: 173 ° C, solubility in water: 50 ppm by mass (20 ° C)) adjusted to 60 ° C. A microporous polyethylene film was prepared in the same manner as in Example 2 except that the heating temperature was 80 ° C. (the number of washings was 3 times in total). The time required for solvent removal was 2 seconds.

Example 4
In the same manner as in Example 1, a biaxially stretched film having a length of about 600 m and a width of 0.4 m was produced. The obtained stretched membrane was washed by passing it through a continuous washing apparatus at a speed of 2 m / min. The continuous cleaning device has three first cleaning tanks containing methylene chloride temperature-controlled at 23 ° C. and two second cleaning tanks (rinsing tanks) containing methyl perfluorobutyl ether temperature-controlled at 23 ° C. A thing was used. The residence time in each of the three first washing tanks was 30 seconds, and the residence time in each of the two second washing tanks was 20 seconds. After washing, the washing solvent contained in the membrane was removed by passing through a solvent removal treatment tank containing 70 ° C. warm water. In the solvent removal tank, place a roll with a diameter of 10 cm so that the hot water level is 2 cm below the roll axis, and bring the membrane into contact with the semicircular circle at the bottom of the roll. It was made to contact warm water in the state which contacted the roll as much as possible. Hot water was continuously supplied and withdrawn so as to maintain 70 ° C. and not be excessively contaminated by the washing solvent. The residence time in the solvent removal tank of the membrane (contact time between the membrane and hot water) was set to 4 seconds. After removing the solvent, water adhering to the membrane was blown off by air spray, and further heat-fixed at 122 ° C. for 60 seconds to produce a polyethylene microporous membrane.

Example 5
In the same manner as in Example 1, a biaxially stretched film having a length of about 600 m and a width of 0.4 m was produced. The obtained stretched membrane was washed by passing it through a continuous washing apparatus at a speed of 2 m / min. The continuous cleaning device includes three first cleaning tanks containing methylene chloride adjusted to 23 ° C. and perfluorohexane adjusted to 23 ° C. [Composition formula: C ₆ F ₁₄ , product name: Florinato HC-72, Two second washing tanks (rinse tanks) containing Sumitomo 3M Limited, surface tension: 12.0 mN / m (25 ° C), boiling point: 56 ° C, solubility in water: 100 mass ppm or less (25 ° C)] ) Was used. The residence time in each of the three first washing tanks was 30 seconds, and the residence time in each of the two second washing tanks was 20 seconds. The washing solvent contained in the washed membrane was removed by showering with 75 ° C. warm water. A roll with a diameter of 10 cm is installed in the solvent removal treatment tank, and the film is run from above the roll by a plurality of nozzles arranged in the axial direction of the roll in a state where the film is in contact with a substantially semicircular circle on the upper part of the roll. Showed 5 L / min of warm water. The contact time between the membrane and hot water was 4 seconds. Next, the water adhering to the membrane was blown off with an air spray and further heat fixed at 122 ° C. for 60 seconds to produce a polyethylene microporous membrane.

Comparative Example 1
The washing in the washing tank was carried out with normal decane adjusted to 60 ° C. (the number of washings: 4 times in total), and the temperature of the warm water was changed to 80 ° C. A porous membrane was produced. The time required for solvent removal was 600 to 900 seconds.

Comparative Example 2
The washing tank was washed with methylene chloride adjusted to 23 ° C. (number of washings: 4 times in total), and the temperature of the warm water was changed to 70 ° C. A porous membrane was produced. The time required for removing the solvent was 3 seconds.

Comparative Example 3
Cleaning in the washing tank is performed with diethyl ether (surface tension: 16.4 mN / m (25 ° C), boiling point: 35 ° C, solubility in water: 65000 mass ppm (20 ° C)) adjusted to 23 ° C ( A microporous membrane of polyethylene was prepared in the same manner as in Example 1 except that the number of washings was 4 times in total and the warming temperature of the hot water was set to 70 ° C. The time required for solvent removal was 2 seconds.

Comparative Example 4
A polyethylene microporous film was prepared in the same manner as in Example 2 except that the washing solvent was removed by a method of blowing hot air at 70 ° C. The time required for removing the solvent was 40 seconds.

Comparative Example 5
A microporous polyethylene film was prepared in the same manner as in Example 4 except that the washing solvent for the rinsing tank was changed to methylene chloride. The time required for removing the solvent was 8 seconds.

  The photograph which image | photographed the surface of the microporous film of Example 1 and Comparative Example 2 is shown in FIG. 1 (Example 1) and FIG. 2 (Comparative Example 2). As shown in FIGS. 1 and 2, the surface of the microporous film of Example 1 is homogeneous without water marks, but water marks are generated on the surface of the microporous film of Comparative Example 1.

The physical properties of the thermoplastic resin microporous membranes obtained in Examples 1 to 5 and Comparative Examples 1 to 5 were measured by the following methods.
-Appearance property: Visually confirmed.
Good: No water marks are observed.
Poor: Water marks are observed.
Film thickness: measured with a contact thickness meter (manufactured by Mitutoyo Corporation).
Air permeability: Measured according to JIS P8117 (converted to a film thickness of 30 μm).
-Porosity: Measured by mass method.
Heat shrinkage rate: MD and TD shrinkage rates were measured 3 times each when the microporous membrane was exposed at 105 ° C. for 8 hours, and an average value was calculated.

Table 2 continued
Table 2 continued

  As shown in Table 2, the thermoplastic resin microporous membranes of Examples 1 to 5 produced by the method of the present invention are excellent in appearance properties, porosity and air permeability. On the other hand, in Comparative Example 1, since the rinsing process is performed with a cleaning solvent having a boiling point exceeding 100 ° C., it takes a long time to remove the cleaning solvent, and the porosity and the air permeability are inferior. In Comparative Examples 2 and 5, since the surface tension at 25 ° C. is more than 24 mN / m and the solubility in water at 16 ° C. is more than 600 ppm by mass, the appearance properties are washed and / or rinsed. Poor porosity and air permeability. In Comparative Example 3, the appearance properties, the porosity, and the air permeability are inferior because the washing treatment with water having a solubility in water at 16 ° C. exceeds 600 mass ppm. In Comparative Example 4, since the cleaning solvent is removed with warm air, it takes a long time to remove the cleaning solvent.

2 is a photograph showing the appearance properties of the microporous membrane of Example 1. 4 is a photograph showing the appearance properties of a microporous membrane of Comparative Example 2.

Claims (1)

A solution obtained by melt-kneading polyolefin and a solvent is extruded from a die and cooled, and the remaining solvent is removed from the gel-like molded product by a washing solvent, and the remaining from the obtained washed polyolefin microporous membrane. A method for producing a polyolefin microporous membrane comprising the step of removing the washing solvent with warm water , wherein (a) the surface tension at 25 ° C. is 24 mN / m or less as the washing solvent, and (b) under atmospheric pressure. And (c) using a washing solvent (A) having a solubility in water at 600 ° C. of 600 mass ppm or less, and setting the temperature of the hot water (the boiling point of the washing solvent (A)) −5 ° C.) to (the crystal dispersion temperature of the polyolefin) or less, and the hot water is showered on the washed polyolefin microporous membrane so that the contact time between the washed polyolefin microporous membrane and warm water is within 15 seconds. Do Alternatively, the washed polyolefin microporous membrane is immersed in the warm water, or both of them are performed, and the washed polyolefin microporous membrane is simply brought into contact with the warm water without acting a suction force. A method for producing a polyolefin microporous membrane, comprising removing the washing solvent (A) from the porous membrane.