JP5909411B2 - Polyolefin microporous membrane and method for producing the same - Google Patents

Description

  The present invention relates to a polyolefin microporous membrane and a method for producing the same.

A polyolefin microporous membrane made of polyolefin is widely used as a separation membrane for various substances, a selective permeation separation membrane, a separation material, and the like. Examples of applications include microfiltration membranes, fuel cell separators, capacitor separators, functional membrane base materials in which functional materials are filled in the holes to develop new functions, battery separators, and the like. Among these, polyolefin microporous membranes are suitably used as separators for lithium ion batteries widely used in notebook personal computers, mobile phones, digital cameras, vehicle batteries, and the like.
In recent years, the performance of these electronic devices has been remarkably improved, and in accordance with this, a demand for particularly high capacity of batteries is rapidly increasing.
Among them, lithium ion battery separators are required to have further improved permeability, mechanical properties, and low thermal shrinkage at high temperatures.
Further, from the viewpoint of a battery drying process, a high temperature storage test, a high temperature cycle test, an oven (heating) test, and the like, a separator for a lithium ion secondary battery needs to have a low heat shrinkage rate. In particular, it is considered as one measure of safety that the battery does not generate heat or ignite above the melting point of polyethylene (PE) used in many lithium ion secondary battery separators. In the UL1642 standard, The oven test at 150 ° C. is one of the items, and it is desirable that the separator has a low heat shrinkage rate at this temperature.
Furthermore, in recent lithium ion secondary batteries, with increasing demand for higher capacity, separators are required to have higher porosity.
Further, in order to prevent a short circuit due to foreign matter in the battery when winding the separator, the puncture strength of the separator and the tensile strength in the length direction and the width direction need to have a certain level or more. When this tensile strength is large, the separator is difficult to break in safety tests such as a collision test and a crush test, and the risk of short circuit between the electrodes is reduced, so that the risk of heat generation and ignition can be avoided.
However, there is generally a contradictory relationship between increasing the strength and reducing the thermal shrinkage, increasing the porosity and closing the pores.

Therefore, in Patent Document 1, by joining two kinds of microporous membranes, a polyolefin microporous membrane having a good balance of porosity, air permeability and mechanical strength and having an excellent low heat shrinkage rate is obtained. Proposed.
Moreover, in patent document 2, in order to prevent both the thermal runaway at the time of abnormal heating and the thermal runaway when the battery is overcharged in a high capacity battery, a high air permeability is provided on the outer peripheral side of the negative electrode. There has been proposed a battery in which a separator is disposed and a separator having a low heat shrinkage is disposed on the inner peripheral side of the negative electrode.
Furthermore, in Patent Document 3, as a technique for obtaining an excellent low thermal shrinkage rate, by including a heat-resistant polymer and a ceramic filler on one side of a polymer microporous film, it is possible to suppress shrinkage even when an internal short circuit occurs. Possible microporous membranes have been proposed.

JP 2003-103625 A Patent No. 4439226 JP 2007-324073 A

However, in recent years, with the increase in capacity of batteries, Joule heat generated due to internal short-circuiting has increased, and it has become exponentially difficult to obtain a safety test result comparable to the conventional one. For example, with the increase in capacity, the inside of the battery tends to generate heat more than the set temperature, and if heat generation exceeding the melting point of the substrate occurs locally due to internal short circuit etc., melting and shrinkage may occur from there. There is.
In Patent Document 1, attention is paid to the physical property balance of porosity, strength, and heat shrinkage rate, but there remains a problem with respect to heat shrinkage rate at 150 ° C.
In addition, although the microporous membrane described in Patent Document 2 is considered for air permeability and low heat shrinkage, the high air permeability separator disposed on the outer peripheral side of the negative electrode has a large heat shrinkage. Therefore, there is a risk of causing an internal short circuit due to heat. In addition, the low heat shrinkage separator disposed on the inner peripheral side of the negative electrode has insufficient mechanical strength, so the separator may be broken by a safety test such as a collision test, and there is still room for improvement.
In Patent Document 3, thermal shrinkage at high temperature is reduced by using a ceramic filler, but there is a concern that the ceramic filler in the coating layer of the separator is likely to damage the substrate and the withstand voltage is likely to be lowered. Is done. Moreover, since the film thickness of the part damaged by the ceramic filler becomes thin and as a result, the self-discharge characteristic is expected to deteriorate, further improvement is necessary.

  The present invention has been made in view of the above circumstances. Polyolefin fine particles having a low safety rate at 150 ° C. and excellent mechanical strength, which is in contradiction to the low thermal shrinkage rate, are excellent in safety. It aims at providing a porous membrane and its manufacturing method.

  As a result of intensive studies to achieve the above-mentioned object, the present inventors have adjusted the thermal shrinkage rate at 150 ° C. in the width direction and the tensile strength in the length direction and the width direction to a specific range, In addition, it has been found that by using a resin composition containing a specific amount of polyolefin, a polyolefin microporous membrane excellent in balance between low thermal shrinkage and high strength can be obtained without impairing self-discharge characteristics.

That is, the present invention is as follows.
[1]
A polyolefin microporous membrane comprising a resin composition having a heat shrinkage rate at 150 ° C. in the width direction of less than 30%, a tensile strength in the length direction and the width direction of 30 MPa or more, and containing 90% by mass or more of polyolefin.
[2]
A polyolefin microporous membrane having a heat shrinkage at 150 ° C. in the width direction of less than 30%;
A polyolefin microporous membrane having a tensile strength in the length direction and the width direction of 30 MPa or more;
The polyolefin microporous membrane according to the above [1], which is a laminate comprising:
[3]
(A) a step of obtaining a primary film through an extrusion step of melt kneading and extruding a resin composition containing a polyolefin resin and a pore-forming material, and an extraction step of extracting the pore-forming material;
(B) preparing at least two kinds of the primary films, laminating and integrating them, and stretching at least uniaxially; and (c) heat-setting,
A process for producing a polyolefin microporous membrane.
[4]
The step (a)
A step of obtaining a primary film (1) through the extraction step after a primary stretching step of stretching in at least a uniaxial direction after the extrusion step; and
After the extrusion step, including the step of obtaining a primary membrane (2) through the extraction step while maintaining the dimensions substantially,
The step (b)
The polyolefin microporous membrane according to the above [3], comprising a secondary stretching step of preparing at least one each of the primary membrane (1) and the primary membrane (2), laminating and integrating them and stretching in at least a uniaxial direction. Production method.
[5]
The said primary film | membrane (1) is a manufacturing method of the polyolefin microporous film | membrane of said [4] description including extending | stretching at least to the width direction in the said primary extending | stretching process.
[6]
The step (a)
Obtaining a primary membrane (3) using a plasticizer as the pore-forming material; and
A step of obtaining a primary film (4) using a plasticizer and an inorganic material as the pore-forming material;
Including
The above-mentioned [3] to [5], wherein the step (b) includes a step of preparing at least one of the primary film (3) and the primary film (4), laminating and integrating them and stretching in at least a uniaxial direction. ] The manufacturing method of the polyolefin microporous film in any one of.
[7]
The method for producing a polyolefin microporous membrane according to any one of the above [3] to [6], which comprises stretching at least in the length direction in the step (a).

  According to the present invention, a polyolefin microporous membrane capable of realizing a lithium ion secondary battery excellent in safety at high temperature (low thermal shrinkage) and mechanical strength, and further excellent in self-discharge characteristics, and production thereof A method can be provided.

  Hereinafter, a mode for carrying out the present invention (hereinafter sometimes abbreviated as “this embodiment”) will be described in detail. The present invention is not limited to the present embodiment described below, and can be implemented with various modifications within the scope of the gist.

The polyolefin microporous membrane (hereinafter sometimes simply abbreviated as “PO microporous membrane”) in the present embodiment is a width direction (a direction perpendicular to the length direction; hereinafter abbreviated as “TD”). )) At 150 ° C. is less than 30%, and the length direction (synonymous with the raw material resin discharge direction and the machine direction; hereinafter abbreviated as “MD”) and TD tension It consists of a resin composition having a strength of 30 MPa or more and containing 90% by mass or more of polyolefin.
The PO microporous membrane in the present embodiment has the above-described configuration, and thus has an excellent heat shrinkage rate required for a high-capacity lithium ion secondary battery, resistance to collision safety, and high temperature. A lithium ion secondary battery excellent in safety can be realized.

  The polyolefin microporous membrane in the present embodiment is composed of a resin composition containing 90% by mass or more of polyolefin. The microporous film has communication holes in the film thickness direction, and has, for example, a three-dimensional network skeleton structure. When the content of the polyolefin is less than 90% by mass, the film itself is damaged by the filler and discharge becomes easy, so that the self-discharge characteristic is deteriorated.

  From the viewpoint of satisfactorily suppressing internal short circuit due to shrinkage of the PO microporous membrane at a high temperature, the TD heat shrinkage rate at 150 ° C. of the PO microporous membrane is less than 30%, preferably less than 25%, more preferably 20%. Is less than. When the TD heat shrinkage rate at 150 ° C. is less than 30%, the shrinkage rate of the PO microporous membrane is about several percent in the actual battery internal environment where the PO microporous membrane is sandwiched between the electrodes. Can prevent short circuit. Furthermore, regarding the 150 ° C. glass compression heat shrinkage that simulates the state of being wound as a battery, the TD heat shrinkage is preferably less than 20%, more preferably less than 15%, and more preferably less than 10%. More preferably.

  When the tensile strength of the PO microporous membrane is large, the battery winding property is good, and it is difficult for a short circuit to occur due to an impact test from the outside when used in a battery or foreign matter in the battery. From the above viewpoint, the tensile strength of MD and TD of the PO microporous membrane in this embodiment is 30 MPa or more, preferably 40 MPa or more, more preferably 50 MPa or more. The upper limit value of the tensile strength of the PO microporous membrane is not particularly limited, but in view of the shrinkage rate, both MD and TD are preferably 500 MPa or less, more preferably 300 MPa or less, and 250 MPa or less. More preferably.

  The PO microporous membrane in the present embodiment has an excellent low thermal shrinkage rate at high temperature and is excellent in tensile strength, so that even when wound in a state where tension is applied to the MD inside the battery, Deformation in a high temperature environment is satisfactorily suppressed. As a result, even under a high temperature such as a 150 ° C. oven test, a short circuit due to the shrinkage of the separator does not occur, so that safety can be maintained without generating heat. Moreover, since the porosity of the film due to shrinkage is suppressed, the cycle characteristics are also prevented from decreasing. In addition, when a minute internal short circuit due to a foreign object occurs, it is possible to minimize the spread of the hole, delay the rate of heat generation, and as a result, increase the external pressure necessary to suppress ignition due to the short circuit Is also considered to be expressed.

  The MD and TD tensile elongation of the PO microporous membrane in the present embodiment is preferably 10 to 500%, more preferably 10 to 450%, and still more preferably 10 to 400%. The PO microporous membrane having the MD and TD tensile elongations in the above range not only has a good battery winding property but also is less susceptible to deformation in a battery impact test or the like.

  The thickness of the PO microporous membrane in the present embodiment is preferably 1 μm or more, and more preferably 5 μm or more, from the viewpoint of membrane strength. Further, from the viewpoint of permeability, it is preferably 80 μm or less, and more preferably 40 μm or less.

  From the viewpoint of permeability, the porosity of the PO microporous membrane in the present embodiment is preferably 30% or more, more preferably 40% or more, and further preferably 50% or more. Moreover, from a viewpoint of film | membrane intensity | strength and a withstand voltage, it is preferable that it is 80% or less, and it is preferable that it is 70% or less.

  Although the air permeability of the PO microporous membrane in this embodiment is preferably as low as possible, it is preferably 1 second / 20 μm or more and more preferably 50 seconds / 20 μm or more from the viewpoint of balance with thickness and porosity. preferable. Further, from the viewpoint of permeability, it is preferably 1000 seconds / 20 μm or less, and more preferably 400 seconds / 20 μm or less. In particular, when the air permeability of the PO microporous membrane is 400 seconds / 20 μm or less, the cycle characteristics of the battery tend to be good.

  The puncture strength of the PO microporous membrane in the present embodiment is preferably 2N / 20 μm or more, and more preferably 3N / 20 μm or more. When the puncture strength is 3 N / 20 μm or more, when a PO microporous membrane is used as a battery separator, pinholes and cracks are also generated when sharp parts such as electrode materials pierce the PO microporous membrane. It can be reduced. The upper limit of the puncture strength is preferably 10 N / 20 μm or less, more preferably 8 N / 20 μm or less.

  The PO microporous membrane in this embodiment is preferably a laminate of two or more microporous membranes made of polyolefin. Such a PO microporous membrane can be obtained by, for example, stacking and integrating two or more different primary membranes by stacking and integrating at least two or more primary membranes. Thus, a good physical property balance can be imparted. The primary film as used herein refers to a film in which a polymer material and a plasticizer, or a polymer material, a plasticizer, and an inorganic material are melt-kneaded and extruded, and then the plasticizer is extracted or the plasticizer and the inorganic material are extracted. Point to. The primary membrane may be stretched (primary stretching) before extraction of the plasticizer and / or inorganic material.

  In particular, at least one sheet is subjected to at least one stretch after the extrusion process and then subjected to the extraction process, and at least one sheet is extracted after the extrusion process in a substantially maintained size. A laminated body obtained by laminating and integrating the performed primary film is preferable. Here, the “state in which dimensions are substantially maintained” means that the dimensional change is kept within ± 20%. Usually, there is a contradictory relationship between high strength and low heat shrinkage, but by bonding a primary film with a low heat shrinkage rate and a primary film with excellent strength, good physical properties can be obtained without impairing the physical properties of each primary film. A balanced PO microporous membrane can be obtained. Furthermore, since the adhesion is improved by integration while stretching, the interface of the PO microporous film becomes difficult to peel off, and as a result, the cycle characteristics of the battery tend to be improved. Further, by bonding two or more different primary films together, the reason is not clear, but the battery collision characteristics tend to be improved. This is presumably because the film having excellent strength has low heat shrinkability by adhering two or more kinds of primary films, and thus the resistance to temperature rise during the collision test is enhanced. The term “integrated” as used herein means that the peel strength is preferably 5 g or more. If the peel strength is not less than this, the characteristics of the two types of primary films tend to be fully utilized.

  As a laminate of two or more microporous membranes made of polyolefin, for example, a polyolefin microporous membrane having a thermal shrinkage rate of less than 30% at 150 ° C. in the width direction, and a tensile strength in the length direction and the width direction. And a laminate including a polyolefin microporous film of 30 MPa or more.

Next, an example of a method for producing the PO microporous membrane in the present embodiment will be described. In the following production method, if the obtained PO microporous membrane satisfies the characteristics defined in this embodiment, the polymer species, solvent species, extrusion method, stretching method, extraction method, pore opening method, heat setting and There is no limitation in the heat treatment method or the like. Examples of the method for producing the PO microporous membrane include a method including the following steps (A) to (G).
(A) A kneading step of kneading a polyolefin, a plasticizer, and, if necessary, an inorganic material.
(B) An extrusion process for extruding the kneaded material obtained through the kneading process (whether it is a single layer or a laminated layer).
(C) A sheet forming step in which the extrudate obtained through the extrusion step is formed into a sheet shape and cooled and solidified.
(D) A stretching process (primary stretching process) for stretching the sheet-like molded product obtained through the sheet molding process in a direction of uniaxial or more as necessary.
(E) An extraction step of extracting a plasticizer and, if necessary, an inorganic material from the sheet-like molded product obtained in the step (C) or the stretched film obtained in the step (D).
(F) A step (secondary stretching step) in which two or more primary films that have undergone the step (E) are simultaneously drawn and stretched in a direction of one axis or more while being laminated and integrated.
(G) A post-processing step in which the laminated film stretched in (F) is heated and heat-set.

Hereinafter, each step will be described.
(A) A process is a kneading | mixing process of kneading | mixing polyolefin, a plasticizer, and an inorganic material as needed.
The polyolefin used in the step (A) is not particularly limited, and for example, at least selected from the group consisting of ethylene or propylene homopolymer, ethylene, propylene, 1-butene , 1 -hexene, 1-octene and norbornene. Examples include copolymers obtained by polymerizing two or more monomers. These polyolefins are used alone or as a mixture of two or more. The use of a mixture of two or more types is preferred because it tends to facilitate control of the fuse temperature and short-circuit temperature of the PO microporous film. In particular, for example, a mixture of an ultrahigh molecular weight polyolefin having a viscosity average molecular weight (hereinafter sometimes abbreviated as “Mv”) of 500,000 or more and a polyolefin having an Mv of less than 500,000 is microporous due to its moderate molecular weight distribution. This is preferable because it tends to give isotropic properties to the strength of the film. In addition, in this specification, Mv means the value measured based on ASTM-D4020, as described in the Example mentioned later.

  When polyethylene is used as the polyolefin, polyethylene is a high-density homopolymer from the viewpoint that heat fixing can be performed at a higher temperature while suppressing pore clogging of the PO microporous membrane, and the heat shrinkage rate is reduced. It is preferable.

  The Mv of the entire PO microporous membrane is preferably 100,000 to 1,200,000, more preferably 300,000 to 800,000. If the Mv is 100,000 or more, it tends to easily exhibit film resistance during heat generation due to a short circuit caused by foreign matter or the like, and if it is 1,200,000 or less, the orientation to MD in the extrusion process is suppressed, There is a tendency to develop isotropic properties.

  The polyolefin preferably includes polypropylene, particularly a plurality of types of polypropylene having different Mv. As a result, when heat is locally generated due to a short circuit inside the battery, an endothermic action inside the battery due to melting of the polypropylene occurs near the melting point of polypropylene of 160 ° C., and the safety tends to increase.

  (A) It is preferable that content of polypropylene with respect to the whole polyolefin used in a process is 1-80 mass%, More preferably, it is 2-50 mass%, More preferably, it is 3-20 mass%, Most preferably, it is 5 -10 mass%. When the content of polypropylene is 1% by mass or more, the effect associated with the inclusion of the above-mentioned polypropylene tends to be easily developed, and when it is 80% by mass or less, the transparency tends to be easily secured.

  In addition, the compounding ratio of polyolefin is preferably 1 to 60% by mass, more preferably 10 to 40% by mass with respect to the total mass of the polyolefin, the plasticizer, and the inorganic material blended as necessary.

  The polyolefin used in the step (A) further includes known additives such as metal soaps such as calcium stearate and zinc stearate, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, and coloring pigments. It may be included.

  The plasticizer is not particularly limited, and examples thereof include organic compounds that can form a uniform solution with polyolefin at a temperature equal to or lower than the boiling point. Specific examples include decalin, xylene, dioctyl phthalate, dibutyl phthalate, stearyl alcohol, oleyl alcohol, decyl alcohol, nonyl alcohol, diphenyl ether, n-decane, n-dodecane, and paraffin oil (liquid paraffin). Of these, paraffin oil and dioctyl phthalate are preferred.

  The blending ratio of the plasticizer is not particularly limited, but from the viewpoint of the porosity of the PO microporous membrane, it is 20% by mass or more based on the total mass of the polyolefin, the plasticizer, and the inorganic material blended as necessary. It is preferable that it is 90 mass% or less from a viewpoint of a viscosity.

  The inorganic material is not particularly limited. For example, oxide ceramics such as alumina, silica (silicon oxide), titania, zirconia, magnesia, ceria, yttria, zinc oxide, iron oxide; silicon nitride, titanium nitride, nitride Nitride ceramics such as boron; silicon carbide, calcium carbonate, aluminum sulfate, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amicite, bentonite Ceramics such as asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; glass fibers. These may be used alone or in combination of two or more. Among these, silica, alumina, and titania are preferable from the viewpoint of electrochemical stability.

  The blending ratio of the inorganic material is preferably 5% by mass or more, more preferably 10% by mass or more, from the viewpoint of obtaining good separability with respect to the total mass of the polyolefin and the inorganic material. From the viewpoint of ensuring strength, the content is preferably 99% by mass or less, and more preferably 95% by mass or less.

  As a kneading method in the step (A), for example, first, a part or all of the raw materials are premixed using a Henschel mixer, a ribbon blender, a tumbler blender or the like as necessary, and then all the raw materials are uniaxially mixed. Examples thereof include a melt kneading method using a screw extruder such as an extruder and a twin screw extruder, a kneader, a mixer, and the like.

  At the time of kneading, it is preferable to mix an antioxidant with a raw material polyolefin at a predetermined concentration, then replace the periphery of the mixture with a nitrogen atmosphere, and perform melt kneading while maintaining the nitrogen atmosphere. The temperature at the time of melt-kneading varies depending on the composition of the raw material and the polyolefin concentration, but is preferably a temperature range in which the MD orientation of the PO microporous film is reduced. From the above viewpoint, the temperature during melt kneading is preferably 160 ° C. or higher, and more preferably 180 ° C. or higher. Moreover, as an upper limit, it is preferable that it is less than 300 degreeC.

(B) A process is an extrusion process (it does not ask whether it is a single layer and lamination | stacking) which extrudes the kneaded material obtained through the kneading | mixing process.
In the step (B), the kneaded product obtained through the kneading step is extruded by an extruder such as a T-die or an annular die. At this time, it may be single layer extrusion or laminated extrusion. The same conditions as before can be adopted for the various conditions during extrusion.

Step (C) is a sheet forming step in which the extrudate obtained through the extrusion step is formed into a sheet shape and cooled and solidified.
In the step (C), the extrudate obtained through the steps (A) and (B) is formed into a sheet and cooled and solidified. Examples of the sheet forming method include a method of solidifying the extrudate by compression cooling. Examples of the cooling method include a method in which the extrudate is brought into direct contact with a cooling medium such as cold air or cooling water, and a method in which the extrudate is brought into contact with a roll or press machine cooled with a refrigerant. Especially, the method of making an extrudate contact the roll cooled with the refrigerant | coolant and the press machine is preferable at the point which is excellent in film thickness control.

(D) A process is an extending | stretching process which extends | stretches the sheet-like molded product obtained through the sheet forming process in the direction of more than one axis as needed (primary extending process).
As a method for stretching a sheet-like molded product, MD uniaxial stretching by a roll stretching machine, TD uniaxial stretching by a tenter, sequential biaxial stretching by a roll stretching machine and a tenter, or a combination of a plurality of tenters, simultaneous biaxial tenter or inflation molding Examples include simultaneous biaxial stretching. In addition, when the PO microporous film in the present embodiment has a laminated structure, at least one sheet-shaped molding is performed from the viewpoint of the uniformity of the film thickness uniformity, tensile strength, porosity, and heat shrinkage rate of the PO microporous film. The stretch ratio of the primary stretching of the product is preferably at least 3 times, more preferably at least 5 times in at least uniaxial direction. By first stretching the sheet-like molded product, the mechanical strength of the obtained primary film tends to be improved. On the other hand, since the primary film that has been subjected to the extraction process with the dimensions substantially maintained after the extrusion process is excellent in heat shrinkage, including at least one primary film improves battery safety at high temperatures. Preferred above.

The step (E) is an extraction step for extracting a plasticizer and, if necessary, an inorganic material from the sheet-like molded product obtained in the step (C) or the stretched film obtained in the step (D).
Examples of the extraction method include a method of immersing a sheet-shaped molded product or a stretched film in an extraction solvent, and a method of bringing the extraction solvent into contact with them by spraying such as a shower. The extraction solvent is preferably a poor solvent for polyolefin, a good solvent for plasticizers and inorganic materials, and a boiling point lower than the melting point of polyolefin. Examples of the extraction solvent include hydrocarbons such as n-hexane and cyclohexane; halogenated hydrocarbons such as methylene chloride, 1,1,1-trichloroethane, and fluorocarbon compounds; alcohols such as ethanol and isopropanol. Ketones such as acetone and 2-butanone; and alkaline water. An extraction solvent is used individually by 1 type or in combination of 2 or more types.

  (E) There is no restriction | limiting in particular about the order of extraction in a process, a method, and the frequency | count. During extraction, film removal to TD may occur due to solvent removal, and the orientation to MD and TD may change. Therefore, it is preferable to control the orientation by tension control.

(F) A process is a process of extending | stretching in the direction of 1 axis or more, paying out simultaneously the two or more primary films which passed through the (E) process, and laminating and integrating.
In the step (F), two or more primary films that have undergone the step (E) are prepared, simultaneously fed out and overlapped, and laminated and integrated while stretching in at least a uniaxial direction. Thereby, the layers are bonded with good adhesion, and the characteristics of the various primary films can be fully utilized. Moreover, since each layer becomes difficult to peel at the interface, the cycle characteristics of the battery can be improved as a result.

  The stretching method in the step (F) is not particularly limited as long as the layers can be joined, and examples thereof include roll stretching and stretching using a tenter. From the viewpoint of being bonded while uniformly stretching, roll stretching is performed. preferable. Any number of stretches may be stacked, but from the viewpoint of contrast, it is preferable to stretch the primary film A and the primary film B by stacking three sheets in the order of A-B-A. It becomes difficult and handling becomes good.

  The temperature at the time of stretching in the step (F) is preferably 110 ° C. or more, and the stretching ratio is preferably 2 times or more from the viewpoint of film uniformity and strength, and is 5 times or more. Is more preferable.

Step (G) is a post-processing step in which the laminated film stretched in (F) is heated and heat-set.
In step (G), the laminated film that has undergone step (F) is heated and fixed at a predetermined temperature. Examples of the heat setting method include a method of performing stretching and relaxation operations using a tenter or a roll stretching machine. The relaxation operation refers to a reduction operation performed at a predetermined relaxation rate on the MD and / or TD of the film. The relaxation rate is a value obtained by dividing the MD dimension of the film after the relaxation operation by the MD dimension of the film before the operation, or a value obtained by dividing the TD dimension of the film after the relaxation operation by the TD dimension of the film before the operation, or When the relaxation is performed in both directions MD, TD, the value is obtained by multiplying the relaxation rate of the MD film and the relaxation rate of the TD film.

  The temperature at which the heat setting is carried out is preferably 100 ° C. or higher from the viewpoint of thermal shrinkage, and is preferably less than 140 ° C. from the viewpoint of porosity and permeability. The relaxation rate is preferably 0.9 times or less and more preferably 0.8 times or less from the viewpoint of heat shrinkage rate. Moreover, it is preferable that a relaxation rate is 0.6 times or more from a viewpoint of preventing generation | occurrence | production of a wrinkle, and a porosity and a permeability viewpoint. The relaxation operation may be performed in both the MD and TD directions, or the relaxation operation may be performed only in one of the MD and TD directions, whereby not only in the operation direction but also in the direction orthogonal to the operation. It is possible to reduce the shrinkage rate.

  The manufacturing method of the PO microporous membrane in this embodiment is a surface on which the surface of the obtained PO microporous membrane is subjected to surface treatment such as electron beam irradiation, plasma irradiation, surfactant coating, chemical modification, etc. A processing step may be included.

  Further, after the above processing step, the master roll wound with the PO microporous film can be subjected to an aging treatment at a predetermined temperature, and then the master roll can be rewound. Thereby, the orientation of the microporous film in the master roll that is usually strongly oriented in the MD is easily relaxed.

  The temperature at the time of aging treatment of the master roll is preferably 35 ° C or higher, more preferably 45 ° C or higher, and further preferably 60 ° C or higher. Further, from the viewpoint of maintaining the permeability of the PO microporous membrane, the temperature is preferably the melting point or lower, and when polyethylene is used as the polyolefin, it is preferably 120 ° C. or lower. The time required for the aging treatment is not particularly limited, but is preferably 24 hours or longer because the above-mentioned effect tends to be exhibited.

Moreover, as a manufacturing method of the polyolefin microporous membrane in this embodiment, the following aspects are also included.
(A) a step of obtaining a primary film through an extrusion step of melt kneading and extruding a resin composition containing a polyolefin resin and a pore-forming material, and an extraction step of extracting the pore-forming material;
(B) preparing at least two kinds of the primary films, laminating and integrating them, and stretching in at least a uniaxial direction; and (c) heat-setting.
A manufacturing method comprising:
Here, the pore forming material refers to the above-described plasticizer and inorganic material.

The step (a)
A step of obtaining a primary film (1) through the extraction step after a primary stretching step of stretching in at least a uniaxial direction after the extrusion step; and
After the extrusion step, including the step of obtaining a primary membrane (2) through the extraction step while maintaining the dimensions substantially,
The step (b)
The method for producing a polyolefin microporous membrane as described above, comprising a secondary stretching step in which at least one of the primary membrane (1) and the primary membrane (2) is prepared, laminated and integrated, and stretched in at least a uniaxial direction.

  The said primary film | membrane (1) is a manufacturing method of the polyolefin microporous film | membrane of the said description including extending | stretching at least to the width direction in the said primary extending | stretching process.

The step (a)
Obtaining a primary membrane (3) using a plasticizer as the pore-forming material; and
A step of obtaining a primary film (4) using a plasticizer and an inorganic material as the pore-forming material;
Including
The polyolefin microporous membrane according to the above, wherein the step (b) includes a step of preparing at least one each of the primary membrane (3) and the primary membrane (4), laminating and integrating them and stretching at least in a uniaxial direction. Manufacturing method.

  In the step (a), the method for producing a polyolefin microporous membrane as described above, comprising stretching at least in the length direction.

  The PO microporous membrane obtained as described above is used, for example, as a separator for a lithium ion secondary battery after being processed into a desired shape. The separator made of the PO microporous membrane in the present embodiment has higher safety at high temperatures and better resistance to foreign matters, especially when used as a separator for high-capacity batteries, compared to conventional separators. In addition, a lithium ion secondary battery having excellent high-temperature cycle characteristics can be realized.

  The lithium ion secondary battery includes a separator made of the PO microporous membrane according to the present embodiment, a positive electrode, a negative electrode, and an electrolytic solution. The lithium ion secondary battery may have the same members as those of the known lithium ion secondary battery except that the separator includes the PO microporous film in the present embodiment. Moreover, the lithium ion secondary battery can have the same structure as a Kochi lithium ion secondary battery, and can be manufactured by a well-known method.

  The separator made of the PO microporous membrane in the present embodiment is suitable for applications that need to have a high capacity, such as for notebook computers, electric tools, electric vehicles, and hybrid vehicles, among lithium ion secondary batteries. By using it for the above application, the effect of the present application becomes more remarkable. In addition, the separator made of the PO microporous membrane in the present embodiment has safety against high heat, so it has a high capacity type that is likely to generate Joule heat, and a square type that is susceptible to the thermal contraction rate of the separator. And it is suitable as a separator for a laminate type lithium ion secondary battery.

  Various characteristics (parameters) in the present embodiment are measured according to the measurement methods in the examples described later unless otherwise specified.

  Next, the present embodiment will be described more specifically with reference to examples and comparative examples. However, the present embodiment is not limited to the following examples as long as the gist thereof is not exceeded. Various characteristics in the examples were measured by the following methods.

(1) Viscosity average molecular weight (Mv)
Based on ASTM-D4020, the intrinsic viscosity [η] at 135 ° C. in a decalin solvent was determined. From the intrinsic viscosity [η], Mv of polyethylene was calculated by the following formula.
[Η] = 6.77 × 10 ⁻⁴ Mv ^0.67
Similarly, Mv of polypropylene was calculated from the intrinsic viscosity [η] by the following formula.
[Η] = 1.10 × 10 ⁻⁴ Mv ^0.80

(2) Film thickness (μm)
The film thickness of the PO microporous membrane was measured at room temperature 23 ± 2 ° C. using a micro-thickness meter KBM (trademark) manufactured by Toyo Seiki Co., Ltd.

(3) Porosity (%)
A 10 cm × 10 cm square sample was cut from the PO microporous membrane to obtain a sample, and its volume (cm ³ ) and mass (g) were determined. From these and the film density (g / cm ³ ), the porosity of the PO microporous film was calculated by the following equation.
Porosity = (volume−mass / film density) / volume × 100
The film density was calculated at a constant 0.95 g / cm ³ .

(4) Air permeability (second)
The air resistance according to JIS P-8117 was defined as the air permeability. The air permeability of the PO microporous membrane was measured using a Gurley type air permeability meter (G-B2 (trademark), manufactured by Toyo Seiki Co., Ltd.).

(5) Puncture strength (N)
Using a KES-G5 (trademark), a handy compression tester manufactured by Kato Tech, a PO microporous membrane was punctured under the following conditions. The maximum piercing load (N) at that time was measured and used as the piercing strength.
Diameter of sample holder opening: 11.3 mm
Curvature radius of needle tip: 0.5mm
Piercing speed: 2mm / sec
Atmospheric temperature: 23 ± 2 ° C

(6) Tensile strength (MPa) and tensile elongation (%)
Based on JIS K7127, it measured about MD and TD sample (shape; width 10mm x length 100mm) using the Shimadzu Corporation tensile tester and autograph AG-A type (trademark). Moreover, the sample used the thing which stuck the cellophane tape (Nitto Denko Packaging System Co., Ltd. make, brand name: N.29) on the single side | surface of the both ends (each 25mm) of the sample for the distance between chuck | zippers 50mm. Furthermore, in order to prevent sample slippage during the test, a fluororubber having a thickness of 1 mm was attached to the inside of the chuck of the tensile tester.
The tensile elongation (%) was determined by dividing the amount of elongation (mm) up to fracture by the distance between chucks (50 mm) and multiplying by 100. The tensile strength at break (MPa) was determined by dividing the strength at break by the cross-sectional area of the sample before the test. The measurement was performed under conditions of a temperature of 23 ± 2 ° C., a chuck pressure of 0.30 MPa, and a tensile speed of 200 mm / min.

(7) Thermal contraction rate of TD at 150 ° C. (%)
A sample obtained by cutting a PO microporous membrane 100 mm in MD and 100 mm in TD was allowed to stand in an oven at 150 ° C. for 1 hour. At this time, the sample was sandwiched between two sheets of paper so that the hot air did not directly hit the sample. After taking the sample from the oven and cooling, the length (mm) of the TD is measured, and the thermal shrinkage calculated by the following formula is the thermal shrinkage of TD at 150 ° C. (150 ° C. TD thermal shrinkage). did. In addition, about what cannot secure sample length, the longest possible sample was used in the range which enters into 100 mm x 100 mm.
Thermal contraction rate (%) = (100 (mm) −TD length after heating (mm)) / 100 (mm) × 100

(8) 150 ° C. glass compression heat shrinkage A sample of MD 60 mm × TD 40 mm was cut out from the PO microporous membrane, and this was sandwiched between two glass plates of length 80 mm × width 50 mm × thickness 5 mm and weight 44 g, and lightly from above Pressed and deflated. Place it horizontally in an oven at 150 ° C. and let it stand for 1 hour, then air-cool and measure the shortest length of TD width (mm) while sandwiched between glass plates. Shrinkage (150 ° C. TD glass compression thermal shrinkage) was calculated.
Shrinkage rate (%) = (1-shortest TD width (mm) / 40 (mm)) × 100

(9) Peel strength Measured with a tensile tester (trade name: AG-100A) manufactured by Shimadzu Corporation. The PO microporous membrane was sampled in a strip shape of MD 200 mm and TD 10 mm, and one end A thereof was peeled off with a tape or the like. The two peeled edges were fixed to the chuck of the tensile tester (distance between grips 50 mm), and the average load when the two edges were peeled in the direction of 180 degrees at a speed of 200 mm / min was read. At this time, the load up to 5 mm after the measurement of the peel strength was not included in the average load, and the average of the load from 5 mm to 200 mm after the start of the measurement was read as the peel strength.

(10) Oven test, crash test, self-discharge characteristics a. Production of Positive Electrode A lithium cobalt composite oxide LiCoO ₂ as a positive electrode active material, graphite and acetylene black as conductive materials were dispersed in polyvinylidene fluoride (PVDF) and N-methylpyrrolidone (NMP) as binders to prepare a slurry. This slurry was applied to a 15 μm-thick aluminum foil serving as a positive electrode current collector with a die coater, dried at 130 ° C. for 3 minutes, and then compression molded with a roll press. The obtained molded body was slit to a width of 57.0 mm to obtain a positive electrode.

b. Manufacture of Negative Electrode An artificial graphite as a negative electrode active material, an ammonium salt of carboxymethyl cellulose and a styrene-butadiene copolymer latex as a binder were dispersed in purified water to prepare a slurry. This slurry was applied to a copper foil serving as a negative electrode current collector with a die coater, dried at 120 ° C. for 3 minutes, and then compression molded with a roll press. The obtained molded body was slit to a width of 58.5 mm to obtain a negative electrode.

c. Preparation of Nonaqueous Electrolytic Solution LiPF ₆ as a solute was dissolved in a mixed solvent of ethylene carbonate: dimethyl carbonate: ethyl methyl carbonate: = 1: 1: 2 (volume ratio) to a concentration of 1 mol / liter, and non-aqueous electrolyte was used. An electrolyte solution was prepared.

d. Battery Assembly After laminating the positive electrode, the PO microporous film described later, and the negative electrode, a wound electrode body was produced by a conventional method. The number of windings was adjusted according to the thickness of the PO microporous film. The outermost peripheral end portion of the obtained wound electrode body was fixed by applying an insulating tape. The negative electrode lead was welded to the battery can, the positive electrode lead was welded to the safety valve, and the wound electrode body was inserted into the battery can. Thereafter, 5 g of the non-aqueous electrolyte was poured into the battery can, and the lid was caulked to the battery can via a gasket, to obtain a cylindrical secondary battery having an outer diameter of 18 mm and a height of 65 mm. This cylindrical secondary battery was charged to a battery voltage of 4.2 V at a current value of 0.2 C (current that is 0.2 times the hourly rate (1 C) of the rated electric capacity) in an atmosphere of 25 ° C. The battery was charged for a total of 3 hours by a method of starting to reduce the current value so as to hold 2 V. Subsequently, the battery was discharged to a battery voltage of 3.0 V at a current value of 0.2 C, and the battery capacity at that time was set to X mAh.

e. Self-discharge characteristics The battery assembled in d was charged to a battery voltage of 4.2 V at a current value of 0.2 C and left for 24 hours. This operation was carried out with a total of 50 cells. Thereafter, the ratio (%) of cells that maintained a capacity of 90% or more of X out of 50 cells was calculated as self-discharge characteristics.

f. Cycle test Using the battery assembled in d, (i) current amount 0.5C, upper limit voltage 4.2V, constant current constant voltage charge for 8 hours in total, (ii) 10 minutes rest, (iii) current Charging / discharging was carried out 50 times for convenience under a cycle condition of an amount of 0.5 C, a constant current discharge with a final voltage of 2.5 V, and (iv) a pause of 10 minutes. All the charge / discharge treatments were performed in an atmosphere of 25 ° C. Thereafter, the ratio of the discharge capacity at the 50th cycle to the battery capacity XmAh was multiplied by 100 to obtain the capacity retention rate (%).

g. Oven Test Using the battery assembled in d, the battery after charging was heated from room temperature to 150 ° C. at a rate of 5 ° C./min and left at 150 ° C. Those that did not ignite for 1 hour or more were marked as ◎, those that did not ignite for 30 minutes to 1 hour were marked as ◯, those that did not ignite for 10 to 30 minutes were marked as Δ, and those that ignited in less than 10 minutes were marked as ×.

h. Collision test Using the battery assembled in d, a SUS rod with a diameter of 15.8 mmφ was placed perpendicular to the length of the battery, and a weight of 9.1 kg was freely placed from a position of 61 cm in height. The battery was dropped and collided, and the battery temperature was increased and the ignition status was confirmed in 5 cells. After the collision, the battery surface temperature rise is 30 ° C or less for all 5 cells. ◎ One cell temperature rise is 80 ° C or less. ○ The cell temperature rise is 80 ° C or less for 3 cells or more. The resulting product was evaluated as Δ, and even one cell had a heat generation of 80 ° C. or higher.

[Example 1]
Using a tumbler blender, 47% by mass of homopolymer polyethylene with an Mv of 700,000, 46% by mass of homopolymer polyethylene with an Mv of 300,000, 7% by mass of polypropylene with an Mv of 400,000 (7% by mass of PP blend) Blended. To 99 parts by mass of the obtained pure polymer mixture, 1 part by mass of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant was added, and the tumbler was added again. A dry blend was performed using a blender to obtain a polymer mixture. The obtained mixture of polymers and the like was substituted with nitrogen and then fed to the twin-screw extruder with a feeder under a nitrogen atmosphere. Further, liquid paraffin (kinematic viscosity at 37.78 ° C .: 7.59 × 10 ⁻⁵ m ² / s) was injected into the extruder cylinder by a plunger pump.
The obtained mixture is melt-kneaded, and the liquid paraffin content ratio in the total mixture to be extruded is 68% by mass (that is, the polymer concentration (sometimes abbreviated as “PC”) is 32% by mass. The feeder and pump were adjusted. The melt-kneading conditions were a set temperature of 200 ° C. and a discharge rate of 12 kg / h.
Subsequently, the melt-kneaded product was extruded and cast onto a cooling roll controlled at a surface temperature of 60 ° C. through a T-die, to obtain a gel sheet having a thickness of 100 μm.
Next, it led to the TD tenter stretching machine and performed TD stretching (primary stretching). The stretching conditions were a TD magnification of 4.0 times and a set temperature of 120 ° C.
Next, the stretched film that has been TD-stretched is introduced into a methylene chloride bath, sufficiently immersed in methylene chloride to extract and remove liquid paraffin, and then the methylene chloride is removed by drying to obtain a primary membrane. Membrane A was designated.

  Separately, 45 parts by mass of liquid paraffin, 45 parts by mass of liquid paraffin, fine silica (Mv 700,000 homopolymer polyethylene 47%, Mv 300,000 homopolymer polyethylene 46% by mass, Mv 400,000 polypropylene 7% by mass polymer) Granulated by mixing 21 parts by mass of Tosoh Silica Co., Ltd., 0.3 parts by mass of BHT (dibutylhydroxytoluene) as an antioxidant, and 0.3 parts by mass of DLTP (dilauryl thiodipropionate) with a Henschel mixer. did. Thereafter, a gel sheet having a thickness of 30 μm was formed with a calender roll kneaded at 200 ° C. with a twin-screw extruder equipped with a T die and cooled to 150 ° C. DOP is extracted by immersing the molded product in methylene chloride, then fine silica is extracted by immersing in sodium hydroxide, and then a solvent is removed by drying to obtain a primary membrane, which is obtained as membrane B. It was.

The film A and the film B obtained above are stacked in the order of film B-film A-film B, and stretched 3.0 times to MD with a stretching roll heated to 120 ° C. (secondary stretching), then 130 ° C. In a tenter, a PO microporous film was obtained by stretching 1.2 times in the TD direction. Table 1 shows the physical properties of the obtained PO microporous membrane.
As a result of battery evaluation, all showed good results.

[Example 2]
When the film A was formed, a gel sheet having a thickness of 170 μm was prepared with a discharge rate of 20 kg / h. Further, when the film B was formed, a gel sheet having a thickness of 50 μm was prepared. Furthermore, MD stretch (secondary stretch) magnification was set to 5.0 times. Except for the above, a PO microporous membrane was obtained in the same manner as in Example 1. Table 1 shows the physical properties of the obtained PO microporous membrane.
As a result of battery evaluation, all showed good results.

[Example 3]
When forming the film A, a gel sheet having a thickness of 230 μm was prepared with a discharge amount of 28 kg / h. Further, when the film B was formed, a gel sheet having a thickness of 70 μm was prepared. Furthermore, MD stretch (secondary stretch) magnification was set to 7.0 times. Except for the above, a PO microporous membrane was obtained in the same manner as in Example 1. Table 1 shows the physical properties of the obtained PO microporous membrane.
As a result of battery evaluation, all showed good results.

[Example 4]
When forming the film A, a gel sheet having a thickness of 250 μm was prepared at a discharge rate of 30 kg / h, and stretched by setting the TD stretching (primary stretching) ratio to 6 times. Further, when the film B was formed, a gel sheet having a thickness of 50 μm was prepared. Furthermore, MD stretch (secondary stretch) magnification was set to 5.0 times. Except for the above, a PO microporous membrane was obtained in the same manner as in Example 1. Table 1 shows the physical properties of the obtained PO microporous membrane.
As a result of battery evaluation, all showed good results.

[Example 5]
When the film A was formed, a gel sheet having a thickness of 330 μm was prepared with a discharge rate of 40 kg / h, and stretching was performed with a TD stretching (primary stretching) ratio set to 8 times. Further, when the film B was formed, a gel sheet having a thickness of 50 μm was prepared. Furthermore, MD stretch (secondary stretch) magnification was set to 5.0 times. Except for the above, a PO microporous membrane was obtained in the same manner as in Example 1. Table 1 shows the physical properties of the obtained PO microporous membrane. The peel strength of this microporous film was 70 g, and the capacity retention rate in the cycle test was 95% or more.
As a result of battery evaluation, all showed good results.

[Example 6]
When forming the film A, a gel sheet having a thickness of 240 μm was prepared at a discharge rate of 30 Kg / h, and the film was stretched with the TD stretching (primary stretching) ratio set to 4 times. Membrane C was obtained by the method. Further, when forming the film B, a film D was obtained in the same manner as in Example 1 except that a gel sheet having a thickness of 70 μm was prepared. Further, secondary stretching was performed by setting the MD stretching ratio to 5.0 times in two layers of film C and film D. Except for the above, a PO microporous membrane was obtained in the same manner as in Example 1. Table 1 shows the physical properties of the obtained PO microporous membrane.
As a result of battery evaluation, all showed good results.

[Example 7]
Using a tumbler blender, 47% by mass of homopolymer polyethylene with an Mv of 700,000, 46% by mass of homopolymer polyethylene with an Mv of 300,000, 7% by mass of polypropylene with an Mv of 400,000 (7% by mass of PP blend) Blended. To 99 parts by mass of the obtained pure polymer mixture, 1 part by mass of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant was added, and the tumbler was added again. A dry blend was performed using a blender to obtain a polymer mixture. The obtained mixture of polymers and the like was substituted with nitrogen and then fed to the twin-screw extruder with a feeder under a nitrogen atmosphere. Further, liquid paraffin (kinematic viscosity at 37.78 ° C .: 7.59 × 10 ⁻⁵ m ² / s) was injected into the extruder cylinder by a plunger pump.
The obtained mixture was melt-kneaded, and the feeder and the pump were adjusted so that the liquid paraffin content ratio in the total mixture to be extruded was 68% by mass (that is, PC was 32% by mass). The melt-kneading conditions were a set temperature of 200 ° C. and a discharge rate of 20 kg / h.
Subsequently, the melt-kneaded product was extruded and cast onto a cooling roll controlled at a surface temperature of 60 ° C. through a T-die, thereby obtaining a gel sheet having a thickness of 170 μm.
Next, it led to MD tenter stretching machine and MD stretching was performed (primary stretching). The set stretching conditions were an MD magnification of 5.0 times and a set temperature of 120 ° C. Thereafter, the film was introduced into a methylene chloride bath, sufficiently immersed in methylene chloride to extract and remove liquid paraffin, and then the methylene chloride was dried and removed to obtain a primary membrane.

  When forming the membrane B, a membrane F was obtained in the same manner as in Example 1 except that a 50 μm thick gel sheet was prepared.

The film E and the film F obtained above are stacked in the order of film F-film E-film F, and bonded together with a hot roll heated to 115 ° C., and led to a TD tenter stretching machine to perform TD stretching (secondary Stretching). The set stretching conditions were a TD magnification of 4.0 times and a set temperature of 120 ° C. Then, a PO microporous film was obtained by stretching 1.2 times in the TD direction in a 130 ° C. tenter. Table 1 shows the physical properties of the obtained PO microporous membrane.
As a result of battery evaluation, all showed good results.

[Example 8]
When the film A was formed, a film G was obtained in the same manner as in Example 1 except that a gel sheet having a thickness of 170 μm was produced with a discharge rate of 20 kg / h. In addition, when forming the film A, a gel sheet having a thickness of 65 μm was prepared with a discharge rate of 10 g / h, and the film was stretched with the TD stretching (primary stretching) ratio set to 2 times. A film H was obtained by the method described above. Film H-film G-film H are stacked in this order, and stretched 5.0 times MD (secondary stretching) with a stretching roll heated to 120 ° C (secondary stretching), and then stretched 1.2 times in the TD direction in a 130 ° C tenter. As a result, a PO microporous membrane was obtained. Table 1 shows the physical properties of the obtained PO microporous membrane.
As a result of the battery evaluation, it was ignited 20 minutes after reaching 150 ° C. in a 150 ° C. oven test.

[Comparative Example 1]
A PO microporous membrane was obtained in the same manner as in Example 4 except that two membranes C obtained in Example 6 were prepared and layered on two layers of membrane C and membrane C. The physical properties of the obtained PO microporous membrane are shown in Table 2.
As a result of the battery evaluation, it ignited 8 minutes after reaching 150 ° C. in a 150 ° C. oven test.

[Comparative Example 2]
After preparing the film C and the film D obtained in Example 6 one by one, each film was secondarily stretched by setting the MD stretch ratio to 5.0 times with a stretching roll heated to 120 ° C. The film was stretched 1.2 times in the TD direction in a 130 ° C. tenter. Subsequently, these two sheets were superposed and thermally bonded with a calendar roll at 110 ° C. to obtain a PO microporous film. The physical properties of the obtained PO microporous membrane are shown in Table 2. The peel strength of this microporous film was 2 g, and the capacity retention rate in the cycle test was less than 95%.
As a result of battery evaluation, in the oven test at 150 ° C., it ignited 5 minutes after reaching 150 ° C. Further, as a result of the collision test, there was an exothermic reaction at 80 ° C. or more in one cell. When the battery was disassembled, it was confirmed that the microporous membrane was torn and peeling of the laminated separator was also found.

[Comparative Example 3]
Using a tumbler blender, 47% by mass of homopolymer polyethylene with an Mv of 700,000, 46% by mass of homopolymer polyethylene with an Mv of 300,000, 7% by mass of polypropylene with an Mv of 400,000 (7% by mass of PP blend) Blended. To 99 parts by mass of the obtained pure polymer mixture, 1 part by mass of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant was added, and the tumbler was added again. A dry blend was performed using a blender to obtain a polymer mixture. The obtained mixture of polymers and the like was substituted with nitrogen and then fed to the twin-screw extruder with a feeder under a nitrogen atmosphere. Further, liquid paraffin (kinematic viscosity at 37.78 ° C .: 7.59 × 10 ⁻⁵ m ² / s) was injected into the extruder cylinder by a plunger pump.
The feeder and pump were adjusted so that the liquid paraffin content ratio in the total mixture melt-kneaded and extruded was 68 mass% (that is, PC 32 mass%). The melt-kneading conditions were a set temperature of 200 ° C. and a discharge rate of 110 kg / h.
Subsequently, the obtained melt-kneaded product was extruded and cast onto a cooling roll controlled at a surface temperature of 60 ° C. through a T-die, thereby obtaining a gel sheet having a thickness of 1000 μm. Next, the obtained gel sheet was guided to a simultaneous biaxial tenter stretching machine, and a stretched sheet was obtained by biaxial stretching. The stretching conditions were an MD magnification of 7.0 times, a TD magnification of 7.0 times, and a stretching temperature of 120 ° C. (primary stretching).
Next, the film was introduced into a methylene chloride bath and sufficiently immersed in methylene chloride to extract and remove liquid paraffin, and then the methylene chloride was removed by drying to obtain a primary membrane.
Thereafter, a PO microporous membrane was obtained by stretching 1.2 times in the TD direction in a tenter having a maximum heating temperature of 130 ° C. The physical properties of the obtained PO microporous membrane are shown in Table 2.
As a result of the battery evaluation, it was ignited 4 minutes after reaching 150 ° C. in a 150 ° C. oven test.

[Comparative Example 4]
47 parts by mass of polyethylene of Mv 700,000 homopolymer, 46% by mass of polyethylene of Mv 300,000 homopolymer, and 7 parts by mass of polypropylene of Mv 400,000. 21 parts by mass), 0.3 parts by mass of BHT (dibutylhydroxytoluene) as an antioxidant, and 0.3 parts by mass of DLTP (dilauryl thiodipropionate) were mixed with a Henschel mixer and granulated. Then, it knead | mixed and extruded at 200 degreeC with the twin-screw extruder equipped with T dice | dies, and it shape | molded in the 130-micrometer-thick sheet form with the calender roll cooled at 150 degreeC. The molded product was immersed in methylene chloride to extract liquid paraffin, and then immersed in sodium hydroxide to extract finely divided silica, and then the solvent was removed by drying to obtain a primary membrane.
Two layers of the obtained primary film were stacked, and after stretching 5.0 times (secondary stretching) to MD with a stretching roll heated to 120 ° C. (secondary stretching), TD was placed in a tenter with a maximum heating temperature of 130 ° C. A PO microporous membrane was obtained by stretching 1.2 times in the direction. The physical properties of the obtained PO microporous membrane are shown in Table 2. As a result of the collision test, there was an exothermic reaction at 80 ° C. or more in the two cells. When the battery was disassembled, it was confirmed that the microporous membrane had a tear.

[Comparative Example 5]
After performing the corona discharge treatment (discharge amount 50 W) on the surface of the PO microporous film obtained in Comparative Example 2, 96 parts by mass of alumina particles, 8 parts by mass of SB latex, and an aqueous solution of ammonium polycarboxylate on the treated surface side Using a bar coater, an aqueous solution in which 1 part by mass (San Nopco SN Dispersant 5468) and 1 part by mass of a polyoxyalkylene surfactant (San Nopco SN wet 980) are uniformly dispersed in 150 parts by mass of water, respectively. Applied. Subsequently, drying was performed at 60 ° C. to remove water, thereby obtaining a multilayer porous film having a total film thickness of 20 μm in which a porous layer having a thickness of 4 μm was formed on the microporous film. The physical properties of the obtained multilayer porous membrane are shown in Table 2.
As a result of battery evaluation, it was suggested that the self-discharge characteristic was 80%, the film was locally thin, and discharge was easy.

[Comparative Example 6]
Production of PO microporous membrane 1: A PO microporous membrane 1 was obtained in the same manner as in Comparative Example 2 except that a gel sheet having a thickness of 1250 μm was produced at a discharge rate of 140 kg / h.
Production of PO microporous membrane 2: A PO microporous membrane 2 was obtained in the same manner as in Comparative Example 3 except that a 150 μm thick gel sheet was formed with a calender roll cooled to 150 ° C.
The physical properties of each obtained PO microporous membrane are shown in Table 2.
When assembling the battery, the positive electrode, the PO microporous film 1, the negative electrode, and the PO microporous film 2 were laminated in this order, and wound so that the PO microporous film 2 was located on the inner peripheral side of the negative electrode.
As a result of battery evaluation, in the 150 ° C. oven test, ignition occurred in 5 minutes after reaching 150 ° C., and in the collision test, there was an exothermic reaction of 80 ° C. or more in three cells. When the battery after the test was disassembled and the microporous membrane was confirmed, it was confirmed that the PO microporous membrane 1 was torn.

[Comparative Example 7]
Production of PO microporous membrane 3: One membrane C obtained in Example 6 was secondarily stretched at an MD stretch ratio of 5.0 times, and stretched 1.2 times in the TD direction in a 130 ° C. tenter. As a result, a PO microporous membrane 3 was obtained.
Production of PO microporous membrane 4: One membrane D obtained in Example 6 was set at a MD stretch ratio of 5.0 times and second stretched, and 1.2 times in the TD direction in a 130 ° C. tenter. The PO microporous membrane 4 was obtained by stretching.
The physical properties of each obtained PO microporous membrane are shown in Table 2.
When assembling the battery, the PO microporous membrane 3 and the PO microporous membrane 4 were used in an overlapping manner. As a result of the collision test, there was an exothermic reaction at 80 ° C. or more in the two cells.

  From the results shown in Table 1, the polyolefin microporous membranes of the present embodiment (Examples 1 to 8) are excellent in safety at a high temperature (low heat shrinkage) and mechanical strength, and also excellent in self-discharge characteristics. It can be seen that a lithium ion secondary battery can be realized.

  According to the present invention, a polyolefin microporous membrane capable of realizing a lithium ion secondary battery excellent in safety at high temperature (low thermal shrinkage) and mechanical strength, and further excellent in self-discharge characteristics, and production thereof A method can be provided.

Claims (7)

A resin composition having a heat shrinkage rate at 150 ° C. in the width direction of less than 30%, a tensile strength in the length direction and the width direction of 30 MPa or more and 90% by mass or more of polyolefin (provided that 4-methyl- except 1-pentene polymer resin.) laminated microporous polyolefin membrane made of. The laminated polyolefin microporous membrane according to claim 1, wherein the polyolefin comprises a mixture of an ultrahigh molecular weight polyolefin having a viscosity average molecular weight of 500,000 or more and a polyolefin having a viscosity average molecular weight of less than 500,000, including polyethylene and polypropylene. The laminated polyolefin microporous membrane according to claim 1 or 2, wherein the entire polyolefin microporous membrane has a viscosity average molecular weight of 100,000 to 1,200,000. (A) a step of obtaining a primary film through an extrusion step of melt kneading and extruding a resin composition containing a polyolefin resin and a pore-forming material, and an extraction step of extracting the pore-forming material;
(B) preparing at least two kinds of the primary films, laminating and integrating them, and stretching at least uniaxially; and (c) heat-setting,
A method for producing a polyolefin microporous membrane , comprising:
The step (a)
A step of obtaining a primary film (1) through the extraction step after a primary stretching step of stretching in at least a uniaxial direction after the extrusion step; and
A step of obtaining a primary membrane (2) through the extraction step with the dimensions substantially maintained after the extrusion step.
Including
The step (b)
A production method comprising a secondary stretching step of preparing at least one primary membrane (1) and one primary membrane (2), laminating and integrating them and stretching in at least a uniaxial direction.   The said primary membrane (1) is a manufacturing method of the polyolefin microporous membrane of Claim 4 including extending | stretching at least to the width direction in the said primary extending process. The step (a)
Obtaining a primary membrane (3) using a plasticizer as the pore-forming material; and
A step of obtaining a primary film (4) using a plasticizer and an inorganic material as the pore-forming material;
Including
The step (b) is, the prepared one by at least one primary membrane (3) and the primary membrane (4), they are laminated integrally comprising the step of stretching in at least one direction, of claim 4 or 5, wherein A method for producing a polyolefin microporous membrane. The method for producing a polyolefin microporous membrane according to any one of claims 4 to 6, comprising stretching at least in the length direction in the primary stretching step of the step (a).