JP4562074B2 - Battery separator manufacturing method - Google Patents

Description

The present invention, after the film a step to remove low molecular weight substance with a washing solvent from the porous membrane containing a low molecular weight substance method of manufacturing a separator for including batteries such as solvents.

  Polyolefin resin porous membranes are used for battery separators, electrolytic capacitor diaphragms, moisture-permeable waterproof materials, various filters, and the like. In particular, battery separators are attracting attention as important components for lithium secondary batteries that are lightweight, have high electromotive force and high energy, and have little self-discharge. Is also expected.

  Such a battery separator usually uses a microporous film having a large number of micropores in order to ensure lithium ion permeability between the positive electrode and the negative electrode. Therefore, various characteristics are required in relation to battery characteristics. Among them, it is an important required characteristic to have high strength and high porosity and to have excellent dimensional stability when the temperature rises. The microporous membrane having a high porosity is an important requirement for improving ion permeability as a separator and improving charge / discharge characteristics, particularly charge / discharge characteristics at a high current density.

  As a method for producing such a microporous membrane, conventionally, a polyolefin resin containing an ultrahigh molecular weight polyolefin resin is heated and dissolved in a solvent to obtain a kneaded product, from which a gel sheet is prepared, stretched, and desolvated. Various methods have been proposed.

  Among them, various methods have been proposed as a method for producing a microporous film having a high porosity. For example, a method for achieving a high porosity by using a saturated thermoplastic elastomer comprising a styrene block and a hydrogenated isoprene block in a polyolefin resin together with the polyolefin resin is known (see, for example, Patent Document 1). Further, a porous film having a high porosity is obtained by a polyolefin microporous film comprising an ultrahigh molecular weight polyolefin (A) having a weight average molecular weight of 500,000 or more or a composition (B) containing an ultrahigh molecular weight polyolefin having a weight average molecular weight of 500,000 or more. Has been proposed (see, for example, Patent Document 2).

  However, in these methods, the adjustment of the porosity is accompanied by a change of the material itself, so that problems such as a slight difference in the final film characteristics of the films for which the porosity has been adjusted arise. In addition, when a resin other than the polyolefin resin is used, it is possible to form a film having a very high porosity. However, the material system is significantly different, resulting in essentially different films.

  On the other hand, it has also been proposed to control the porosity by selecting a cleaning solvent used for solvent removal after film formation without significantly changing the film formation conditions. For example, a method is known in which a film-formed sheet is washed with a non-aqueous solvent, dipped and washed with a lower boiling point hydrofluorocarbon, and then dried (see, for example, Patent Document 3).

However, in this method, it has been found that even if a low boiling point solvent is used for cleaning, it takes some time for drying after cleaning, so that the film shrinks and the porosity is not sufficiently improved. In particular, assuming application to a continuous process, the concentration of impurities (film-forming solvent and non-aqueous solvent) gradually increases by immersion cleaning, so the evaporation rate of the solvent during drying decreases and the porosity is reduced. The decrease becomes noticeable. In order to prevent this problem, it is necessary to use a cleaning solvent with few impurities, and there are problems such as an increase in consumption of the cleaning solvent and an increase in purity during recycling.
JP 2000-72908 A International Publication WO00 / 49073 JP 2000-12695 A

Accordingly, an object of the present invention is to stably obtain a porous film having a high porosity by suppressing the shrinkage of the porous film because the drying time of the cleaning solvent can be shortened even if the purity of the cleaning solvent is low. it is to provide a method for producing a Ru batteries separator can.

As a result of intensive studies to solve the above-mentioned problems, the present inventors, as a washing step when manufacturing a battery separator , after immersing and washing a solvent and the like, condensing and drying using a vapor of the washing solvent The inventors have found that a high porosity can be obtained even with a solvent having a high impurity concentration and that shrinkage can be suppressed, and the present invention has been completed.

That is, the method for manufacturing a battery separator of the present invention is a method of manufacturing a battery separator comprising the step of removing the low molecular weight substance with a washing solvent from the porous membrane containing a low molecular weight substance after the film, the porous Removing a low molecular weight substance from the membrane using a liquid washing solvent, contacting the porous membrane with vapor of the same or another washing solvent to condense the washing solvent, and removing the condensed washing solvent And a step of drying from the membrane.

According to the method for manufacturing a battery separator of the present invention, since the cleaning solvent can be supplied to the porous film with a vapor component, the impurity concentration in the vapor is lowered due to the difference in boiling point even when the cleaning solvent contains impurities. It is difficult to increase, and the evaporation rate of the cleaning solvent during drying is increased. Further, since the porous film is dried after condensing the cleaning solvent, the temperature of the cleaning solvent is close to the saturation temperature at the time of drying, so that the evaporation rate of the cleaning solvent is increased. As a result, even when the purity of the cleaning solvent is low, the drying time of the cleaning solvent can be shortened, so that the porous film having a high porosity can be stably obtained by suppressing the shrinkage of the porous film. .

  In the above, the method includes melt-kneading a resin composition containing a polyolefin-based resin and a hydrocarbon-based solvent, cooling the obtained melt-kneaded material to obtain a sheet-like material, and then extending a uniaxial direction or more. preferable. The porous film thus formed has performances suitable for battery separators such as generally excellent strength and high porosity, and the porosity can be further improved by the present invention.

  In the step of condensing the cleaning solvent, it is preferable that a part of the condensed cleaning solvent is discharged from the porous membrane to replace the cleaning solvent. Even if the cleaning solvent is condensed and dried, the effect of accelerating the evaporation of impurities can be obtained.However, by replacing such a cleaning solvent, the effect of extracting and diluting the impurities is increased, so that the cleaning efficiency is further improved. The porosity can be further improved.

  In addition, it is preferable that the step of condensing the cleaning solvent and the step of drying are continuously performed while stretching or fixing in the film width direction using the continuous porous film. While stretching or fixing in the film width direction, both processes are performed as a series of processes (more preferably, performed immediately after the liquid washing process), thereby increasing the initial drying rate and reducing the film shrinkage. It can be effectively prevented.

  Further, the cleaning solvent vapor preferably contains a fluorine-based solvent having an ozone depletion coefficient of zero. Fluorinated solvents are generally highly volatile, have no flash point or are nonflammable, have good solubility in other components, and are easy to recover. Moreover, since the ozone depletion coefficient is zero, it is also good for the global environment.

The method for producing a battery separator of the present invention includes a step of removing a low molecular weight substance from a porous film containing a low molecular weight substance after film formation using a cleaning solvent.

  Examples of the porous membrane include polyolefin resin, PVDF (polyvinylidene fluoride), PSF (polysulfone), PES (polyethersulfone), PPES (polyphenylsulfone), PVA, PTFE, cellulose resin, polyamide, polyacrylonitrile, Examples thereof include polyimide.

  The film forming method is not particularly limited as long as a low molecular weight substance remains after film formation, and is a solvent method, a non-solvent induced wet phase separation method, a heat induced wet phase separation method, a dry phase separation method, Any method such as open hole stretching may be used.

  The low molecular weight product to be removed may be any of a film-forming solvent, a plasticizer, a swelling agent, a gelation control agent, a soluble inorganic salt, a residual monomer component, and the like. The remaining low molecular weight substance may be any of those present in the pores, on the surface of the fine structure, or in the fine structure.

  In the present invention, after the porous film forming step melts and kneads a resin composition containing a polyolefin resin and a hydrocarbon solvent, the obtained melt kneaded material is cooled to obtain a sheet-like material, It is effective to include a step of stretching in a uniaxial direction or more. The polyolefin-based porous membrane obtained by these series of steps contains a hydrocarbon-based solvent such as liquid paraffin in the porous structure. Hereinafter, this film forming process will be described as an example.

As the polyolefin resin, polyolefins such as homopolymers, copolymers, and blends of olefins such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, and 1-hexene are preferable. Among these, it is desirable to use an ultrahigh molecular weight polyolefin having a weight average molecular weight of 5 × 10 ⁵ or more, preferably 5% by weight or more. Among these, ultrahigh molecular weight polyethylene is particularly preferable as a material from the viewpoint of mechanical strength of the obtained porous film.

  As the solvent that can be used in the present invention, any conventionally known solvent can be used without limitation as long as it is excellent in solubility and swelling of the resin constituting the porous membrane. For example, for polyolefin resins, nonane, decane, undecane, dodecane, decalin, tetralin, aliphatic hydrocarbons such as liquid paraffin, mineral oil fractions with boiling points corresponding to these, etc. Of these, non-volatile solvents such as liquid paraffin are preferred.

  The mixing ratio of the polyolefin resin and solvent varies depending on the type of polyolefin resin, material conditions such as solubility, and kneading conditions such as kneading time and kneading temperature. The slurry-like resin mixture composition is not particularly limited as long as it can be formed into a sheet shape when melt-kneaded. For example, the blending amount of the resin component is preferably 5 to 30% by weight in the mixture, more preferably 10 to 30% by weight, and still more preferably 10 to 25% by weight. The blending amount of the resin component is preferably 5% by weight or more from the viewpoint of improving the strength of the obtained porous film, and 30% by weight from the viewpoint that the polyolefin can be sufficiently dissolved in a solvent and kneaded. The following is preferred.

  The amount of the solvent in the mixture is preferably 70 to 95% by weight, more preferably 75 to 90% by weight. The blending amount is preferably 70% by weight or more from the viewpoint of appropriate kneadability and excellent characteristics, and is preferably 95% by weight or less from the viewpoint of facilitating molding with a die during extrusion.

In addition, for the purpose of providing a shirt down function (a function of blocking the current by clogging the microporous film to prevent accidents such as ignition when the temperature in the battery film rises) One or more kinds of polyolefins having a weight average molecular weight of less than 5 × 10 ⁵ , thermoplastic elastomers, and graft copolymers may be contained.

Examples of polyolefins having a weight average molecular weight of less than 5 × 10 ⁵ include polyolefin resins such as polyethylene and polypropylene, and modified polyolefin resins such as ethylene-acrylic monomer copolymers and ethylene-vinyl acetate copolymers. Examples of the thermoplastic elastomer include thermoplastic elastomers such as polystyrene, polyolefin, polydiene, vinyl chloride, and polyester.

  Examples of the graft copolymer include graft copolymers in which the main chain is a polyolefin and the side chain is a vinyl polymer having a non-compatible group in the side chain, but polyacryls, polymethacrylates, polystyrene, polyacrylonitrile, polyoxyalkylenes. Is preferred. In addition, an incompatible group here means an incompatible group with respect to polyolefin.

The content of these polyolefins less than 5 × 10 ⁵ , the thermoplastic elastomer, and the graft copolymer is set according to the shirt down temperature required as occasion demands, but is preferably 70% by weight or less in the raw material resin mixture of the porous film. 50% by weight or less is more preferable. The content is preferably 70% by weight or less from the viewpoint of sufficiently securing the crosslinking point of the high molecular weight polyolefin and obtaining sufficient heat resistance.

  Note that additives such as an antioxidant, an antistatic agent, an ultraviolet absorber, a dye, a nucleating agent, a pigment, a flame retardant, and a filler are added to the resin composition as necessary. You may add in the range which does not impair.

  The step of melt-kneading the obtained resin composition can be performed by a commonly used known method. In this case, it is preferable to carry out by applying a sufficient shearing force to the mixture in order to obtain sufficient entanglement of polymer chains of the high molecular weight polyolefin. For example, the resin composition may be kneaded batch-wise using a Banbury mixer, a kneader, or the like, or a continuous extruder may be used. As the continuous kneader, a multi-screw kneader such as a single-screw kneader, a twin-screw extruder, or a planetary type may be used, or a process combining a plurality of these devices may be used.

  The temperature at which the mixture is dissolved and kneaded is preferably in the range of the temperature at which the solvent starts dissolving the high molecular weight polyolefin (melting start temperature) to + 60 ° C. The temperature is preferably equal to or higher than the dissolution start temperature from the viewpoint of efficiently dispersing the high molecular weight polyolefin. In order to suppress thermal decomposition and oxidative degradation of the high molecular weight polyolefin, there is no problem even if the temperature is lowered to such an extent that the film characteristics are not deteriorated during kneading after dissolution.

  The step of forming into a sheet can be performed by a commonly used known method. The method is not particularly limited, and examples thereof include a method of attaching a T-die or the like to the tip of the extruder. Further, the sheet may be formed by calendar molding or press molding.

  The obtained sheet-like extrudate is preferably sandwiched between metal plates cooled to 50 ° C. or lower, more preferably −10 ° C. or lower, and cooled to form a sheet. Although it does not specifically limit as thickness of the sheet-like molding obtained in this way, The thing of 2-25 mm is preferable from the ease of the process in a subsequent process.

  Next, the obtained sheet-like molded product is stretched. The method for the stretching treatment is not particularly limited, and may be a normal tenter method, a roll method, or a combination of these methods. In addition, any method such as uniaxial stretching and biaxial stretching can be applied, and in the case of biaxial stretching, either longitudinal and transverse simultaneous stretching or sequential stretching may be used. preferable.

  The draw ratio can be set as appropriate depending on the desired porosity and strength, but is preferably in the range of usually 5 to 250 times the area before drawing.

  The temperature during the stretching treatment is preferably a temperature of the melting point of the high molecular weight polyolefin + 5 ° C. or less. If the temperature is too high, the structure may collapse and the strength may decrease. On the other hand, if the temperature is too low, the film may be broken or shrinkage may be increased after stretching.

  Next, the sheet-like molded product after the stretching treatment is washed. In the present invention, first, a low molecular weight product is used from the porous film using a liquid washing solvent (hereinafter sometimes simply referred to as “solvent”). A step of removing is performed. The washing treatment can be performed, for example, by washing the sheet-like molded product with a solvent to remove the remaining solvent.

  As the cleaning solvent, an inorganic or organic solvent can be appropriately selected according to the solvent used for preparing the resin mixture. Specific examples of organic solvents include aliphatic hydrocarbons such as pentane, hexane, heptane, octane, nonane, decane, and isononane, alicyclic hydrocarbons such as cyclopentane, cyclohexane, and methylcyclohexane, methylene chloride, and tetrachloride. Examples include volatile solvents such as chlorinated hydrocarbons such as carbon, ethers such as diethyl ether and dioxane, alcohols such as methanol and ethanol, and ketones such as acetone, methyl ethyl ketone, cyclohexanone, cyclopentanone, and methylcyclohexanone. . In addition, these can also be used individually or in mixture of 2 or more types.

  After removing the solvent with the above-mentioned solvent, multi-stage cleaning may be performed in which final cleaning is performed with a fluorine-based solvent that is quickly dried. Examples of the fluorine-based solvent include chain fluorocarbon, cyclic fluorocarbon, perfluorocarbon, perfluoroether and the like.

  The cleaning method using such a solvent is not particularly limited, for example, a method of extracting the solvent by immersing the sheet-like molded product in a bath charged with the solvent, a method of showering the solvent into the sheet-like molded product from a spray nozzle, For example, a method of removing the solvent with steam may be used. These methods can be washed alone or in combination of two or more methods, but the final step is a washing step using steam from the viewpoint of increasing the drying rate.

  That is, the production method of the present invention comprises a step of bringing the porous membrane into contact with vapor of the same or different cleaning solvent as the cleaning solvent to condense the cleaning solvent (hereinafter referred to as “vapor cleaning”), and a condensed cleaning. And a step of drying the solvent from the porous membrane.

  When performing rinsing and finishing cleaning, which are the final steps of solvent removal, the characteristics of the porous film subjected to these cleaning methods are greatly affected by the concentration of impurities in the solvent. Therefore, cleaning is performed using steam that has a low impurity concentration and a high solvent temperature due to the difference in vapor pressure, thereby increasing the evaporation rate and suppressing the shrinkage of the microporous film.

  Examples of the method for generating steam include a method in which heating of a solvent using a thermostatic bath or the like, a temperature increase of the solvent by ultrasonic waves, evaporation of the solvent by decompression, and mist formation are used in combination.

  As a method for contacting the vapor and the porous membrane, any method such as a method of introducing the porous membrane in a batch or continuous manner into an atmosphere filled with vapor, or a method of spraying the vapor with a nozzle may be used. From the viewpoint of work efficiency, a method of positively spraying steam is preferable. Further, the cleaning may be performed by immersing the porous membrane in a temperature-controlled solvent before performing the steam cleaning.

  The method of condensing the cleaning solvent simply brings the porous film into contact with the cleaning solvent vapor, and the boiling point rises due to impurities (such as the film-forming solvent and the solvent previously used). Although it can be condensed, it is preferable to use cooling together in order to increase the condensation rate. The cooling is not particularly limited, such as a method of leaving it at room temperature or a method of cooling with a refrigerant. However, it is preferable to actively cool from the outside in order to promote solvent substitution by condensing vapor on the surface of the film due to the problem of solvent loss due to natural evaporation.

  In the present invention, in the step of condensing the cleaning solvent, it is preferable that a part of the condensed cleaning solvent is caused to flow out of the porous membrane to replace the cleaning solvent. Examples of the method for replacing the cleaning solvent include a method in which the cleaning solvent naturally flows or drops from the porous membrane, a method in which the cleaning solvent is scraped off with an elastic blade, and a method in which suction is performed using a suction roll. Above all, in a continuous process or batch process, while generating vapor from a container containing the solvent, the solvent is condensed in the porous film introduced into the space above it, and a part of the condensed solvent flows down or drops naturally. The method of returning to the container is particularly preferable. At that time, it is more preferable to promote the flow of the solvent by introducing the porous film vertically or inclined.

  The solvent used for the steam cleaning may be the same as or different from the solvent used for the liquid cleaning, and an inorganic or organic solvent is appropriately selected according to a low molecular weight material such as a solvent contained in the porous membrane. I can do it. However, it is preferable to use a solvent having a boiling point of 100 ° C. or lower, more preferably a solvent having a boiling point of 80 ° C. or lower so that evaporative cleaning can be easily performed at a low temperature. In view of safety, a nonflammable solvent is preferable.

  As the solvent used for the steam cleaning, any of the solvents exemplified as the liquid cleaning solvent can be used, and among them, the above-mentioned fluorine-based solvent is preferable. Moreover, as a fluorine-type solvent, a thing with an ozone destruction coefficient of zero is preferable. In particular, examples of fluorine solvents having an ozone depletion coefficient of 0 and a low boiling point include perfluorocarbons such as perfluoropentane, perfluorohexane, and perfluoroheptane, pentafluorobutane, heptafluorobutane, nonafluoropentane, and undecafluorohexane. And hydrofluorocarbons such as undecafluorohexane, heptafluorocyclopentane, and nonafluorocyclohexane.

  The solvent used for the steam cleaning may be used alone or in combination of two or more. Even a solvent containing a mixture of solvents having different boiling points or impurities may be used for the reason described above. In the conventional immersion cleaning (liquid cleaning), as described above, the purity control of the cleaning solvent is important, so there are problems such as an increase in the consumption of the cleaning solvent and the need to increase the purity during recycling. However, in the present invention, these problems can be avoided by cleaning with steam.

  In the present invention, it is preferable to continuously perform the step of condensing the cleaning solvent and the step of drying while stretching or fixing in the width direction of the membrane using the continuous porous membrane. In the conventional cleaning with a liquid solvent, it is necessary to provide a distance between the cleaning process and the drying process from the viewpoint of equipment design and safety, and there is a problem that natural drying occurs during that time and the initial drying speed decreases. there were. On the other hand, in the present invention, steam cleaning can be performed immediately after the liquid cleaning step, and problems due to natural drying can be eliminated. In particular, when the washing solvent condensing step and the drying step are continuously carried out while stretching or fixing in the membrane width direction, the membrane can be introduced into the drying zone without shrinking, thereby improving the drying rate. For the reason of preventing membrane shrinkage, a porous membrane having a high porosity can be obtained stably.

  In addition, you may perform the above washing process before extending | stretching. Moreover, after performing a washing process before a extending | stretching process, you may take the process of performing a washing process after an extending | stretching process again, and removing a residual solvent.

  In the present invention, after the stretching process and before and after the cleaning process, a rolling process may be further performed to improve surface properties and characteristics. For example, the porous film may be subjected to a stretching process and a cleaning process (the order of stretching and cleaning may be any order) and then subjected to a rolling process, or the porous film may be stretched and then stretched. A cleaning process may be performed. Further, the rolling treatment may be performed both after the stretching treatment and after the washing treatment.

  Next, it is preferable to perform a heat setting treatment for suppressing shrinkage or fixing the structure of the molded product having a porous structure obtained by the above-described steps.

  The heat setting treatment may be a one-stage heat treatment method in which heat treatment is performed at a time, a multi-stage heat treatment method in which heat treatment is first performed at a low temperature, and then heat treatment is performed at a higher temperature, or a temperature rising heat treatment in which heat treatment is performed while raising the temperature Although the method may be used, it is desirable to treat the porous film without impairing the original characteristics of the porous film such as the Gurley value.

  In the case of a one-step heat treatment, the temperature during the heat setting treatment is preferably a temperature of the melting point of the polyolefin resin of −20 ° C. or more and the melting point or less. When expressed in terms of temperature, it is preferably 40 to 140 ° C. depending on the melting point of the polyolefin resin and the composition of the porous film.

  Further, in order to complete the heat treatment in a short time without impairing various properties, a multistage type or a temperature rising type heat treatment method is also preferable. The heat treatment time in this case depends on the polyolefin resin used, but a temperature of the melting point of the polyolefin resin of −20 ° C. or more and the melting point or less is preferable. When expressed in terms of temperature, it is not generally determined by the melting point of the polyolefin-based resin or the composition of the porous film, but it is preferably 30 minutes or longer at 115 ° C., for example.

  Further, if necessary, the third and subsequent heat treatments may be performed at a higher temperature and for a shorter time.

  As specific heat treatment methods, the four corners of the porous film are fixed and put into a heat treatment furnace, wound into a roll and put into the heat treatment furnace, the area direction is fixed with a tenter and continuously passed through the heat treatment furnace, etc. A known method is used.

  The porous film thus obtained is safe when the solvent is dried, and it can be expected to improve the porosity without having to change the molding conditions significantly.

  Examples and the like specifically showing the configuration and effects of the present invention will be described below. Various characteristics were measured as follows.

[Film thickness]
Measurement was performed with a thickness gauge capable of displaying 1/10000 mm, and an average value of 25 points was used.

[Porosity]
The porous film to be measured is cut into a 5 cm square, its volume and weight are determined, and the following results are calculated from the obtained results. Porosity (volume%) = 100 × (volume (cm ³ ) −weight (g) / average density of resin and inorganic substance (g / cm ³ )) / volume (cm ³ )
[Drying speed]
A porous membrane containing a solvent is cut into a 13 cm square, placed on a balance, and the change in weight at the time of drying is about 1/2 of the total weight until complete drying. Expressed as weight loss.

"Formation of porous sheet for evaluation"
12.5 parts by weight of ultra high molecular weight polyethylene (weight average molecular weight: 10 ⁶ , melting point: about 140 ° C.), 75 parts by weight of liquid paraffin as a solvent, and 2.5 parts by weight of a thermoplastic elastomer (TPE821 manufactured by Sumitomo Chemical) 0.47 parts by weight of an antioxidant (manufactured by Ciba Specialty Chemicals, Irganox 1010) was uniformly mixed into a slurry, and the resulting resin composition was melt-kneaded at 150 ° C. using a biaxial continuous kneader. . Thereafter, the obtained kneaded material was sandwiched between metal plates and rapidly cooled to 70 degrees in a sheet form. Further, the rapidly cooled and crystallized sheet-like molded product was pressed at about 130 ° C. and a gap of 1.3 mm, and the sheet-like molded product was rolled and stretched. These quenched sheets were biaxially stretched both vertically and horizontally at a temperature of about 130 ° C. in the extrusion direction 1.5 times × width direction 3.8 times to obtain a sheet-like resin containing a solvent in the porous structure.

[Example 1]
The sheet-like resin containing the solvent was cut out and fixed to the SUS frame, and then washed in decane for 3 minutes. Further, the same washing treatment was repeated twice. After that, a sheet fixed in a frame is held in a vapor of pentafluorobutane (boiling point 40 ° C.) adjusted to 40 ° C., held for 10 minutes, and the evaporated solvent is condensed and allowed to flow down on the film surface to perform cleaning by substitution. It was. Thereafter, drying was performed under conditions of room temperature (23 ° C.) and wind speed of 0.6 m / sec. Then, for heat setting, heat treatment was performed in air at 85 ° C. × 12 h + 116 ° C. × 2 h in a state of being fixed to a metal frame to obtain a porous film.

[Example 2]
In Example 1, instead of using pentafluorobutane, a mixed solvent of pentafluorobutane (boiling point 40 ° C.) / Decane (boiling point 174 ° C.) = 90/10 (weight ratio) adjusted to 40 ° C. was used. The porous membrane was washed under the same conditions as in Example 1, dried and heat set.

Example 3
In Example 1, instead of using pentafluorobutane, a mixed solvent of pentafluorobutane (boiling point 40 ° C.) / Decane (boiling point 174 ° C.) = 80/20 (weight ratio) adjusted to 40 ° C. is used. The porous membrane was washed under the same conditions as in Example 1, dried and heat set.

[Comparative Example 1]
The sheet-like resin formed in the same manner as in Example 1 was washed in decane (boiling point 174 ° C.) for 3 minutes. Further, the same washing treatment was repeated twice. Thereafter, drying and heat setting were performed in the same manner as in Example 1.

[Comparative Example 2]
The sheet-like resin formed in the same manner as in Example 1 was washed in heptane (boiling point 98 ° C.) for 3 minutes. Further, the same washing treatment was repeated twice. Thereafter, drying and heat setting were performed in the same manner as in Example 1.

[Comparative Example 3]
The sheet-like resin formed in the same manner as in Example 1 was washed in decane for 3 minutes. Further, the same washing treatment was repeated twice. Then, it was immersed in pentafluorobutane for 10 minutes. Thereafter, drying and heat setting were performed in the same manner as in Example 1.

[Comparative Example 4]
The sheet-like resin formed in the same manner as in Example 1 was washed in decane for 3 minutes. Further, the same washing treatment was repeated twice. Thereafter, it was immersed in a mixed solvent of pentafluorobutane / decane = 90/10 (weight ratio) for 10 minutes. Thereafter, drying and heat setting were performed in the same manner as in Example 1.

表１の結果のうち、実施例１と比較例３とを対比することで、ペンタフルオロブタンを用いた蒸気洗浄と浸漬洗浄との比較を行うことができるが、蒸気洗浄を行う実施例１では乾燥速度の向上による空孔率の改善が見られる。また、実施例１〜３を対比することで、ペンタフルオロブタンにデカン（浸漬溶媒）が混合して純度が低下する場合の結果が分かるが、いずれも十分な空孔率を維持している。特に、デカン濃度が１０重量％の実施例２と比較例４とを対比すると、空孔率が顕著に増加しており、不純物を含む場合でも蒸気洗浄が有効なことがわかる。更に、従来法であるデカンやヘプタンを用いて浸漬洗浄を行った比較例１〜２と実施例１〜３とを対比すると、低沸点溶剤を用いた蒸気洗浄が特に有効であることが分かる。

By comparing Example 1 and Comparative Example 3 among the results in Table 1, it is possible to compare the steam cleaning using pentafluorobutane with the immersion cleaning, but in Example 1 in which steam cleaning is performed, The porosity is improved by increasing the drying speed. Moreover, although the result when a decane (immersion solvent) mixes with pentafluorobutane and purity falls by comparing Examples 1-3 is understood, all maintain sufficient porosity. In particular, when Example 2 having a decane concentration of 10% by weight is compared with Comparative Example 4, the porosity is remarkably increased, and it can be seen that steam cleaning is effective even when impurities are included. Further, when Comparative Examples 1-2 and Examples 1-3, which were subjected to immersion cleaning using decane or heptane, which are conventional methods, are compared, it can be seen that steam cleaning using a low boiling point solvent is particularly effective.

Claims (5)

In the method for producing a battery separator including a step of removing a low molecular weight substance from a porous film containing a low molecular weight substance after film formation using a cleaning solvent,
A step of removing low molecular weight substances from the porous membrane using a liquid washing solvent; a step of bringing the porous membrane into contact with vapor of the same or another washing solvent to condense the washing solvent; and a condensed washing solvent method for producing a battery separator, wherein a and a step of drying the porous membrane. 2. The method according to claim 1, comprising melt-kneading a resin composition containing a polyolefin-based resin and a hydrocarbon-based solvent, cooling the obtained melt-kneaded material to obtain a sheet-like material, and then stretching the resin composition in a uniaxial direction or more. Manufacturing method for battery separator . The method for producing a battery separator according to claim 1 or 2, wherein in the step of condensing the cleaning solvent, a part of the condensed cleaning solvent flows out of the porous membrane to replace the cleaning solvent. 4. The method according to claim 1, wherein the step of condensing the cleaning solvent and the step of drying are continuously performed while stretching or fixing in the membrane width direction using the continuous porous membrane. Manufacturing method of battery separator . The method for producing a battery separator according to any one of claims 1 to 4, wherein the cleaning solvent vapor contains a fluorine-based solvent having an ozone depletion coefficient of zero.