Patent Description:
Thermoplastic resin microporous membranes are widely used as separation membranes, selectively permeable membranes, isolation membranes, and the like of materials. Specific application examples of the microporous membrane include separators for batteries used in lithium ion secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries, and polymer batteries, separators for electric double-layer capacitors, various filters such as reverse osmosis filtration membranes, ultrafiltration membranes and microfiltration membranes, breathable waterproof clothing, medical materials, supports for fuel cells, and the like.

Especially, as separators for lithium ion secondary batteries, polyethylene microporous membranes are broadly used. In addition to their characteristics including the safety of the batteries and excellent mechanical strength which contributes greatly to the productivity, polyethylene microporous membranes also show ion permeability by an electrolyte which passes through micro pores while maintaining the electrical insulation property, and also are provided with a pore-blocking function to regulate the excessive temperature rise by blocking the penetration of ions automatically at about <NUM> to <NUM> in a case of an abnormal reaction in the outside or inside of the battery.

Lithium ion batteries have expanded their use fields to large-sized and large capacity applications such as power tools, automobiles, bicycle storage batteries, and large electrical storage equipment in addition to so-called small-sized consumer applications including conventional mobile phones and batteries for PCs. In order to meet these demands, electrodes providing a high capacity as a battery structure have been increasingly applied. The electrode materials used for these electrodes have a characteristic that expansion and shrinkage of the volume accompanied by the charge and discharge is larger than those of conventional electrodes, and the separators have been required to show small change in the performance even when the electrodes are in expansion and shrinkage.

Separators have been developed in order to meet these demands (Patent Documents <NUM> to <NUM>).

Patent Document <NUM> discloses, as a means for improving the durability of a battery, a microporous membrane film made of polypropylene and polyphenylene ether, and the microporous film shows constant values of stress and film thickness recovery rate when a flat indenter having a diameter of <NUM> is pushed <NUM> deep into the film.

In Patent Document <NUM>, as a method of improving the compression resistance, a polyolefin microporous membrane in which small protrusions of the size smaller than <NUM>% of the membrane thickness are introduced on the membrane surface has been developed. It is disclosed that the stress and membrane thickness recovery rate are maintained within a certain range after the microporous membrane cut out in a square of <NUM> was compressed at <NUM> for <NUM> seconds to the <NUM>% thickness of the initial membrane thickness.

Patent Document <NUM> discloses a polyolefin microporous membrane, wherein the maximal pore size, the elastic modulus in the longitudinal direction, the ratio of the elastic modulus in the longitudinal direction to the elastic modulus in the width direction are defined, and a <NUM>-µm microporous membrane shows excellent compression resistance in Example <NUM>. A polyolefin microporous membrane showing "good strain absorptivity" and maintaining good ion permeability after the compression is shown.

Patent Document <NUM> discloses a microporous membrane in which the injection of electrolyte is improved by regulating the polyethylene composition, and in Examples <NUM> to <NUM>, polyolefin microporous membranes show a small membrane thickness variation after the compression and have an air resistance of <NUM> seconds/<NUM> or less. Patent Document <NUM> has a characteristic of a pore size distribution curve having at least two peaks.

Patent Document <NUM> discloses a microporous membrane having a membrane thickness variation of <NUM>% or less after heating at <NUM>. <NUM> MPa and <NUM> for <NUM> minutes and an air resistance of <NUM> seconds/<NUM>/<NUM> or less after a heat compression, and such physical properties are achieved by re-stretching the membrane at least in the monoaxial direction after the removal of a membrane-forming solvent.

Patent Documents <NUM> to <NUM> disclose particular polyolefin microporous membranes. Patent Document <NUM> falls under Art. <NUM>(<NUM>) EPC.

[Patent Document <NUM>] <CIT> ; [Patent Document <NUM>] <CIT> [Patent Document <NUM>] <CIT>; [Patent Document <NUM>] <CIT>; [Patent Document <NUM>] <CIT>; [Patent Document <NUM>] <CIT>; [Patent Document <NUM>] <CIT>; [Patent Document <NUM>] <CIT>; [Patent Document <NUM>] <CIT>.

Along with the increase in the size and capacity of lithium ion batteries, electrodes suitable for higher capacity as a battery material have been increasingly applied. As a method for increasing the capacity, at least one of the following has been studied: <NUM>) adopting a new electrode material and <NUM>) increasing the electrode material density in the battery. The new electrode material used in the method of <NUM>) has a characteristic that the expansion and shrinkage accompanied by the charge and discharge is larger than those in conventional electrodes, and a problem of the deterioration of the battery performance with repeated charges and discharges has been revealed. As a measure for this problem, the separator has been required to show a small change in the performance even when the electrodes are in expansion and shrinkage. Moreover, in order to develop the method <NUM>), thinning of the separator has been developed. At the same time, as the size of batteries are increasing, it is also necessary to consider that the temperature inside the battery during the charge and discharge gets higher than before.

Patent Document <NUM> studies the compression property of a micro region and proposes a separator with a compression resistance. However, since the compression property of the micro region is studied, stress is relaxed around the region during the compression, and it was difficult to accurately simulate the compressed state inside the battery which is subject to stress in a wider area. In Patent Document <NUM>, polyphenylene ether (PPE) in polypropylene is dispersed and thus the compression resistance is improved by voids from PPE, but there still remained a problem in the strength. The applicants have disclosed an invention with improved strength (<CIT>, Example <NUM>, <NUM> N/<NUM>) as a related art, but even the improved microporous membrane still required a further improvement in the strength.

Patent Document <NUM> considers a compression behavior of a relatively large area, but the results were from the evaluation of compression resistance for a short time, and hence did not reflect a condition which can occur in a practical battery.

In Patent Document <NUM>, as a means for improving the compressibility, a membrane which is easily deformed is considered. Despite the compression process for a relatively short time, the membrane thickness changed by <NUM> or more, and even the smallest change in the air resistance showed a deterioration by <NUM> times compared to before the compression.

The applicants have so far tried to improve the compression characteristics of a separator. By studying raw material compositions, stretching methods after a solvent removal, etc., a microporous membrane and a separator which are excellent in compression resistance have been developed. As a countermeasure to the expansion and shrinkage of an electrode, following separators have been developed: a) a separator which withstands stress applied to the separator and which shows a small variation in the membrane thickness, and b) a separator in which the membrane thickness slightly decreases in response to the pressure from the electrode, but the variation in ion permeability is small.

Under the model of a), Patent Document <NUM> achieved the improvement of the injection of electrolyte and compression resistance by adjusting the polyethylene composition and regulating the pore size distribution, but the improvement of the performance was still required.

Under the model of b), Patent Document <NUM> proposes a polyolefin microporous membrane having improved compression resistance by re-stretching after the extraction of a plasticizer. The disclosed microporous membrane caused a membrane thickness variation of more than <NUM>% after a compression at <NUM> MPa and <NUM> for <NUM> minutes, but showed an improved compression resistance. However, the suppression quality of the deterioration of the air resistance needed to be improved, and the strength was also insufficient.

In recent years, as the variation in ion permeability is required to be small, the mainstream has been the development under the model of b) which shows a smaller degree of deterioration of air resistance than under the model of a). In addition, these studies have been mainly on separators which have a membrane thickness of about <NUM> and thus have room for reduction in the membrane thickness.

On the other hand, with the recent increase in capacity of batteries, thinner separators have been increasingly applied in order to increase the electrode density as much as possible, and the membranes have been thinned from the conventional thickness of about <NUM> to <NUM> or less. A large variation in the membrane thickness under the compression conditions may cause insulation failure or the possibility of a thinner separator after the compression by the electrode during the charge and discharge of the battery. Therefore, it is not possible to maximize the electrode density inside the battery at the time of the manufacturing of the battery. The prior art model based on the premise that the membrane thickness decreases to some extent (the model of the above b)) cannot offer a sufficient solution, and the deterioration of ion permeability accompanying the charge and discharge has become a major subject for development.

Patent Document <NUM> discloses a separator having excellent compression resistance at <NUM> or less, but the separator disclosed in Patent Document <NUM> shows small strength, and there is still room for improvement in the battery productivity. Moreover, the membrane thickness of the separator greatly decreases, resulting in voids in the battery after the charge and discharge. Thus, the necessity for maximizing the electrode in the battery was not satisfied.

In addition to these necessities, the safety which has been required for separators so far is still important as before. It is required to safely stop and maintain the battery function when the battery is heated abnormally, but this is a more difficult task for a thin membrane separator in which the amount of resin between electrode plates is greatly reduced.

Among the physical properties possessed by the microporous membrane as a separator, the heat shrinkage rate, shutdown, and meltdown property, for example, greatly contribute to the safety of the battery. When the battery abnormally generates heat, a short circuit may occur at the end of the electrode due to heat shrinkage of the separator. Thus, the management of the heat shrinkage rate of the separator is important. Furthermore, in the shutdown function that stops the battery function safely by closing pores of the separator in a case of an abnormal heat generation of the battery and blocking the ion conductivity, the shutdown temperature (SDT) is required to be lowered, and then the meltdown temperature (MDT) at which the ion conductivity returns again is required to be increased after the shutdown. Thus, the battery function can be safely stopped when heat is generated abnormally. Therefore, it is important not only to lower the temperature of the SDT but also to widen the temperature difference between the SDT and the MDT for the assurance of the battery safety.

After intensive studies to solve the above problems, the present inventors have found that it is possible to solve the problems by the following structure and have reached the present invention.

A microporous membrane which is a polyolefin microporous membrane, wherein the average membrane thickness is <NUM> or less, the pin puncture strength corresponding to the membrane thickness of <NUM> is <NUM> mN or more, the air resistance is <NUM> seconds or less, the air resistance variation (<NUM>) after a pressurization treatment under the conditions of <NUM>, <NUM> MPa, and <NUM> minutes is <NUM>% or less, and the average pore size is <NUM> or less: wherein Air resistance variation (<NUM>) = air resistance after the pressurization treatment at <NUM> / air resistance prior to the pressurization treatment × <NUM>, and wherein the polyolefin microporous membrane comprises a polyethylene mixture containing two or more kinds of polyethylenes, the polyethylene mixture containing (I) <NUM>-<NUM>% by weight of an ultra high molecular weight polyethylene component having a weight average molecular weight of <NUM> × <NUM><NUM> or more and (II) polyethylene having a weight average molecular weight of <NUM> × <NUM><NUM> or more and <NUM> × <NUM><NUM> or less.

In some embodiments of the microporous membrane above, said air resistance variation is <NUM>% or less.

In some embodiments of the microporous membrane according to any one of the above, the air resistance variation (<NUM>) after a pressurization treatment under the conditions of <NUM>, <NUM> MPa, and <NUM> minutes is <NUM>% or less: wherein <MAT>.

In some embodiments of the microporous membrane above, said air resistance variation (<NUM>) is <NUM>% or less.

Also disclosed, not according to claim <NUM>, is a microporous membrane, wherein the average membrane thickness is <NUM> or less, the air resistance is <NUM> seconds or less, and the air resistance variation (<NUM>) after a pressurization treatment under the conditions of <NUM>, <NUM> MPa, and <NUM> minutes is <NUM>% or less: wherein <MAT>.

In the microporous membrane above, said air resistance variation may be <NUM>% or less.

Also disclosed, not according to claim <NUM>, is a microporous membrane, wherein the average membrane thickness is <NUM> or less, and the variation ratio (<NUM>/<NUM>) of the air resistance variation (<NUM>) after a pressurization treatment under the conditions of <NUM>, <NUM> MPa, and <NUM> minutes to the air resistance variation (<NUM>) after a pressurization treatment under the conditions of <NUM>, <NUM> MPa, and <NUM> minutes is <NUM>% or less: wherein <MAT> wherein, <MAT> <MAT>.

In the microporous membrane above, said variation ratio (<NUM>/<NUM>) may be <NUM>% or less.

In some embodiments of the microporous membrane according to any one of the above, the heat shrinkage rate after an exposure at <NUM> for <NUM> hours is <NUM>% or less, and the average tensile rupture elongation is <NUM>% or less.

In some embodiments of the microporous membrane according to any one of the above, wherein the shutdown temperature is <NUM> or less, or the temperature difference between the shutdown temperature and the meltdown temperature obtained by a temperature-increasing air permeability method is <NUM> or more.

Also disclosed, not according to claim <NUM>, is a microporous membrane comprising more than <NUM>% of an ultra high molecular weight polyethylene component having a weight average molecular weight of <NUM>,<NUM>,<NUM> or more, or <NUM>% or more of a molecule component having a weight average molecular weight of more than <NUM>,<NUM>,<NUM>, wherein the average membrane thickness is <NUM> or less, the air resistance is <NUM> seconds or less, the pin puncture strength corresponding to the membrane thickness of <NUM> is <NUM> mN or more, the heat shrinkage rate after an exposure at <NUM> for <NUM> hours is <NUM>% or less, the average tensile rupture elongation is <NUM>% or less, and the average pore size is <NUM> or less.

In the microporous membrane according to the above, the shutdown temperature may be <NUM> or less, or the temperature difference between the shutdown temperature and the meltdown temperature obtained by a temperature-increasing air permeability method may be <NUM> or more.

In some embodiments of the microporous membrane according to any one of the above, the formation of a hybrid structure of a ladder-like structure in a submicron region and a three-dimensional network structure in a micron region is observed on at least one surface of the microporous membrane.

Also provided is a lithium ion secondary battery, wherein the microporous membrane according to any one of the above is used.

Also provided is a method of producing a polyolefin microporous membrane of claim <NUM>, comprising:.

Although the polyolefin microporous membrane of the present invention is a thin membrane, the increase ratio of the air resistance is low under heating conditions at a high temperature that can occur in high capacity and high output batteries, and the performance deterioration as a battery is small, and thus suitable as a separator of a lithium ion secondary battery. In addition, the polyolefin microporous membrane, which is another invention, is excellent in pin puncture strength, heat shrinkage rate and processability in spite of its thin membrane.

In order to obtain a polyolefin microporous membrane which is excellent in pin puncture strength, heat shrinkage and processability while having a membrane thickness of <NUM> or less, and also shows a small performance deterioration even under heating conditions at a high temperature, after intensive studies, the present inventors have found that a polyolefin microporous membrane whose air resistance, which is an index of ion permeability, does not greatly deteriorate even by high temperature pressurization can be obtained by regulating the stretching conditions under certain conditions and forming uniform interaction of ultra high molecular weight components, and thus, the present invention has been achieved. The present invention will be explained in detail below.

Preferred examples of polyolefin resins forming the polyolefin microporous membrane are polyethylene and polypropylene. The polyolefin resin may be a single resin or a mixture of two or more different polyolefin resins, for example, a mixture of polyolefin resins selected from polyethylene, polypropylene, polybutene, and poly4-methyl-<NUM>-pentene. The polyolefin resin is not limited to a homopolymer of a single type and may be a copolymer of different olefins. Among such polyolefin resins, polyethylene is particularly preferred from the viewpoint of its excellent pore-blocking performance, and is comprised in the polyolefin microporous membrane of claim <NUM>. The melting point (softening point) of polyethylene is preferably <NUM> to <NUM> from the viewpoint of the pore-blocking performance.

The polyolefin resin used in the present invention is explained below in detail, using polyethylene as an example. Examples of polyethylene include ultra high molecular weight polyethylene, high density polyethylene, medium density polyethylene, low density polyethylene and the like. The polyethylene polymerization catalyst is not particularly limited, and a Ziegler-Natta catalyst, a Phillips catalyst, a metallocene catalyst, or the like can be used. These polyethylenes are not limited to homopolymers of ethylene but also may be copolymers containing a small amount of another α-olefin. Suitable examples of α-olefins other than ethylene include propylene, <NUM>-butene, <NUM>-pentene, <NUM>-hexene, <NUM>-methyl-<NUM>-pentene, <NUM>-octene, (meth)acrylic acid, esters of (meth)acrylic acid, styrene, and the like. The polyethylene of claim <NUM> is a polyethylene mixture containing two or more kinds of polyethylenes.

As the polyethylene mixture, a mixture of two kinds or more of ultra high molecular weight polyethylenes having different weight average molecular weights (Mw), of high density polyethylenes, of medium density polyethylenes, of low density polyethylenes, or a mixture of two kinds or more selected from ultra high molecular weight polyethylenes, high density polyethylenes, medium density polyethylenes and low density polyethylenes may be used. As the polyethylene mixture, a mixture of ultra high molecular weight polyethylene having a Mw of <NUM> × <NUM><NUM> or more and polyethylene having a Mw of <NUM> × <NUM><NUM> or more and <NUM> × <NUM><NUM> or less is used in the appended claims. The content of the ultra high molecular weight polyethylene in the polyethylene mixture is from <NUM> to <NUM>% by weight from the viewpoint of the tensile strength. When the ultra high molecular weight polyethylene is present in the polyethylene mixture in the proportion of <NUM>% by weight or more, at least one of the membrane thickness variation and the air resistance variation during the compression can be suppressed, and the membrane strength is improved compared to the case of less than <NUM>% by weight of the ultra high molecular weight polyethylene. When the content of the ultra high molecular weight polyethylene present in the polyethylene mixture is <NUM>% by weight or less, the resin extrusion productivity improves.

The molecular weight distribution of polyethylene (the weight average molecular weight (Mw) / the number average molecular weight (Mn)) is preferably within the range of <NUM> to <NUM> from the viewpoint of the mechanical strength.

<NUM>% or more of a molecule component having a weight average molecular weight of <NUM>,<NUM>,<NUM> or more is present in the polyethylene mixture. <NUM>% or more of a molecule component having a weight average molecular weight of <NUM>,<NUM>,<NUM> or more may be present in the polyethylene mixture. The polyethylene component having a weight average molecular weight of <NUM>,<NUM>,<NUM> or more is contained because the average pore size of the separator can be controlled to <NUM> or less, thereby preventing the deterioration of the separator performance under high temperature and high pressure conditions and the generation of metallic lithium dendrites accompanying the charge and discharge in the battery.

The diluent is not particularly limited as long as it is a substance which can be mixed with the polyolefin resin or a substance which can dissolve the polyolefin resin. In the melt blending state with the polyolefin resin, a solvent which is miscible with polyolefin but exists in a solid state at room temperature may be mixed with the diluent. Examples of such solid diluents include stearyl alcohol, ceryl alcohol, paraffin wax and the like. Examples of liquid diluents include aliphatic, cycloaliphatic or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, liquid paraffin and the like, mineral oil fractions which have a corresponding (same or equivalent) boiling point to that of these aliphatic, cycloaliphatic or aromatic hydrocarbons, and phthalate esters which are liquid at room temperature such as dibutyl phthalate and dioctyl phthalate, vegetal oils such as soybean oil, castor oil, sunflower oil, cotton oil, and other fatty acid esters. A nonvolatile diluent such as liquid paraffin is more preferably used in order to obtain a gel-like sheet having a stable content of the liquid diluent. For example, the viscosity of the liquid diluent is preferably from <NUM> to <NUM> cSt at <NUM>, more preferably from <NUM> to <NUM> cSt, and further preferably from <NUM> to <NUM> cSt. When the viscosity of the liquid diluent is <NUM> cSt or more, the extrudate from the die is uniform and the blending is also easy. When the viscosity of the liquid diluent is <NUM> cSt or less, the removal of the solvent (diluent) is easy.

For the mixing ratio of the polyolefin resin and the diluent, when the total of the polyolefin resin and the diluent is considered as <NUM>% by mass, <NUM> to <NUM>% by mass of the polyolefin resin is preferred from the viewpoint of good sheet formability of the extrudate. The proportion of the polyolefin resin to the mixture of the polyolefin resin and the diluent is more preferably <NUM> to <NUM>% by weight, and further preferably <NUM> to <NUM>% by weight. When the ratio of the polyolefin resin to the mixture of the polyolefin resin and the diluent is <NUM>% by weight or more, the die swell and neck-in at the die exit during the extrusion can be prevented, resulting in improved membrane formability of a gel-like sheet. On the other hand, the ratio of the polyolefin resin to the mixture of the polyolefin resin and the diluent of <NUM>% by weight or less can maintain a small differential pressure at the die portion, resulting in the stable production of a gel-like sheet. The uniform melt blending step of the polyolefin solution is not particularly limited and include, in addition to a calendar and various mixers, an extruder equipped with a screw and the like.

A production method by the wet method includes, for example, a method in which a microporous membrane is obtained by heating and melt blending polyethylene (polyolefin resin) and a solvent for sheet forming, extruding and cooling the obtained resin solution from a die to form an unstretched gel-like sheet, stretching the obtained unstretched gel-like sheet at least in a monoaxial direction, removing the solvent for sheet forming and drying the resulting.

The polyethylene microporous membrane may be a single layer membrane or have a layer structure composed of two or more layers having different molecular weights or different average pore sizes. In the case of a layer structure composed of two or more layers, the molecular weight and the molecular weight distribution of at least one outermost layer of the polyethylene resin preferably satisfy the above ranges.

A multi-layer polyethylene microporous membrane composed of two or more layers can be produced by any of the following examples: a method in which each polyethylene (polyolefin resin) constituting the layer A and the layer B is heated and melt blended with a solvent for sheet forming, and each of the obtained resin solutions is introduced from respective extruders into one die and combined by co-extrusion, a method in which gel-like sheets constituting each layer are overlaid on each other and thermally fused, a method in which the gel-like sheets are individually fused thermally after stretching, a method in which gel-like sheets are thermally fused after the solvent is removed. The co-extrusion method is preferred because adhesion strength between layers can be easily obtained, and easy formation of communication holes between the layers allows the permeability to be kept high, and the productivity is excellent.

The preferred range of the temperature of the polyolefin solution in the extruder depends on the resin. For example, for the polyethylene composition, a temperature of <NUM> to <NUM> is preferred, and when polypropylene is contained, the preferred range is <NUM> to <NUM>. The temperature of the polyolefin solution in the extruder is indirectly measured by installing a thermometer inside the extruder or at a cylinder portion, and the heater temperature, rotation speed and extrudate amount of the cylinder portion are appropriately adjusted so that the target temperature is achieved. The diluent may be added before the blending or added during the blending. In melt blending, an antioxidant is preferably added in order to prevent oxidation of the polyolefin resin.

The polyolefin solution melt blended in the extruder is cooled to form a diluent-containing resin composition. In this case, the extrusion from a die having a slit-like opening is preferred in order to prepare a sheet-like resin composition, but a so-called inflation method in which a resin composition is solidified by extrusion from a die for a blown film having a circular opening can be also applied. The extrusion temperature is preferably <NUM> to <NUM>, more preferably <NUM> to <NUM>, and further preferably <NUM> to <NUM>. The extrusion temperature of <NUM> or more can prevent the excessive increase in pressure at the die portion, and the extrusion temperature of <NUM> or less can prevent the deterioration of materials. The extrusion rate (the sheet membrane-forming rate) is preferably <NUM> to <NUM>/min. A gel-like sheet is formed by cooling the polyolefin resin solution extruded in a sheet form. Cooling methods include a method of bringing the polyolefin resin solution into contact with a coolant such as cold air, cooling water or the like, a method of bringing the polyolefin resin solution into contact with a cooling roll, or the like, and a method of bringing the polyolefin resin solution into contact with a roll cooled with a coolant is preferred. For example, an unstretched gel-like sheet can be formed by bringing the polyethylene resin solution extruded in a sheet form into contact with a rotary cooling roll having a surface temperature set at <NUM> to <NUM> with a coolant. The extruded polyethylene resin solution is preferably cooled to <NUM> or lower. In this case, the cooling rate is preferably <NUM>/min or more. Cooling methods as above can lead to a microphase separation of the polyolefin phase from the solvent. This way, the unstretched gel-like sheet is likely to have a dense structure, and the excessive increase of crystallinity can be suppressed, and thus, the unstretched gel-like sheet obtains a structure suitable for stretching. As a method of cooling, in order to improve sheet cooling efficiency and sheet planarity, a method of cooling the polyolefin resin solution by placing two or more types of rolls close to each other and holding the resin solution extruded onto one roll with one or more rolls may be applied. Further, in order to form a gel-like sheet in the high-speed membrane production, a chamber for attaching the sheet with the roll may be used. The membrane thickness can be controlled by adjusting each extrusion amount of the polyolefin solution. As the extrusion method, for example, the methods disclosed in <CIT> and <CIT> can be used.

The first stretching step is stretching in two or more stages with the diluent contained. Preferred examples also include performing simultaneous biaxial stretching after monoaxial stretching, and performing monoaxial stretching after simultaneous biaxial stretching. Considering the productivity and the investment cost, an example of another monoaxial stretching after monoaxial stretching is also preferred. As for the stretching directions of the sheet transport direction (MD) and the sheet width direction (TD), for example, the TD stretching may be performed after the MD stretching, or the MD stretching may be performed after the TD stretching. After heating, a gel-like sheet can be stretched by a tenter method, a rolling method, a flatting method, or a combination thereof.

As an example, stepwise biaxial stretching in which roll-stretching is performed in the MD direction and then stretching by a tenter method is carried out in the TD direction is explained.

The stretching magnification before the extraction of the diluent varies depending on the thickness of the gel-like sheet, but the MD stretching (MDO) by <NUM> times to <NUM> times is preferred. The MD stretching magnification before the extraction of the solvent is more preferably <NUM> times to <NUM> times, further preferably <NUM> times or more and <NUM> times or less, and still further preferably <NUM> times or more and <NUM> times or less. The MD stretching by <NUM> times or more before the extraction of the solvent allows for a uniform stretching. Therefore, during the TD stretching following the MD stretching, a formation of an uneven structure in the MD direction can be avoided. The MD stretching by <NUM> times or more before the extraction of the solvent results in a uniform membrane thickness distribution in the MD direction and is preferred in order to control the membrane quality (wrinkles, sagging) which will be important in the post-processing. Further, the MD stretching can be performed in two or more stages. In the MD stretching, the region where the MD stretching is applied is formed by a preheating part, a stretching part, and a thermal fixation part, and the temperature of the gel-like sheet (or the film being stretched) is controlled by heating and cooling with rolls in the above region. In the stretching part, a peripheral speed difference between the rolls and a stretching section divided into a plurality of stages can be used to carry out the stretching. That is, in the stretching part, the peripheral speed of the downstream side (winding side) roll adjacent to the roll on the most upstream side (the die side) is increased, and the peripheral speed difference between these two rolls is used to stretch the gel-like sheet. In this manner, the MD stretching in two stages or more (multistage) is performed as rolls with a peripheral speed higher than that of the upstream rolls are arranged sequentially on the downstream stage side. Specifically, when two pairs of rolls having different peripheral speeds from each other (two pairs of rolls which are set such that the peripheral speed of the downstream side roll is faster than that of the upstream side roll) are arranged in the stretching part, the MD stretching is carried out in two stages, and when three pairs of the rolls are arranged in the stretching part, the MD stretching is carried out in three stages. Among these pairs of rolls, the roll on the downstream side in any pair and the roll on the upstream side in a pair of rolls which is adjacent to and on the downstream side of the above pair may be made common. For example, a two-stage stretching section may be formed by three rolls.

The stretching magnification in each pair of rolls can be either equal or different. The stretching magnification in each stage is more preferably different in a way that the stretching magnification increases on the more downstream side, thereby preventing further the increase in the air resistance during the heat compression. Although the reason is uncertain, at an equal magnification, stretching is carried out at a relatively high magnification at the initial stage of stretching. When the stretching magnification is increased at different magnifications, it is presumed that the micro structure formed by the MD stretching tends to be uniform and the compression resistance is improved.

In order to improve the strength in the sheet width direction and the productivity, the stretching in the TD direction after the MD stretching is preferably by <NUM> to <NUM> times, more preferably <NUM> times to <NUM> times, and further preferably <NUM> times to <NUM> times. In order to form a uniform membrane structure in the TD direction (in order to form a uniform formation of holes), the stretching magnification in the TD direction is desirably <NUM> times or more. In order to obtain more uniform physical properties (air resistance, strength (pin puncture, tensile), heat shrinkage rate) in the TD direction, the stretching magnification in the TD direction of <NUM> times or more is more desired. The stretching magnification in the TD direction of <NUM> times or less can prevent the variation of physical properties due to a high stretching magnification. From the viewpoint of the production stability (in order to obtain uniform physical properties in the TD direction while stabilizing the productivity), the stretching magnification in the TD direction is further preferably <NUM> times or less.

The total area magnification in the MD stretching and the TD stretching before the extraction of the solvent is preferably <NUM> times or more, more preferably <NUM> times or more, and the most preferably <NUM> times or more. In order to improve the strength, the area magnification of the stretching before the extraction of the solvent is preferably <NUM> times or more. The average tensile elongation is determined by the square root of the product of the elongation in the MD direction and the elongation in the TD direction, and represented by the formula, ((the elongation in the MD direction) × (the elongation in the TD direction))<NUM>. This average tensile elongation is preferably <NUM>% or less. In order to obtain the average tensile elongation of <NUM>% or less, the stretching area magnification before the extraction of the solvent is preferably set at <NUM> times or more. On the other hand, the area magnification of the stretching before the extraction of the solvent is preferably <NUM> times or less, more preferably <NUM> times or less, and the most preferably <NUM> times or less. When the stretching before the extraction of the solvent is carried out at the area magnification of <NUM> times or less, the stability is obtained during the membrane formation and is preferred from the viewpoint of the production of the microporous membrane.

The TD stretching magnification and the MD stretching magnification before the extraction of the solvent preferably satisfy the following Equation (<NUM>-<NUM>). <MAT> The α in the Equation (<NUM>) represents the degree of orientation in the unstretched sheet as described later, and is a value derived empirically.

By satisfying such a relationship, a microporous membrane excellent in high temperature compression characteristics (high temperature compression resistance) can be obtained. The reason can be assumed as follows, although it is not clear. In the present technique, since the structure is likely to be limited in the MD direction at the stage of the formation of the unstretched sheet, the stretching magnification in the subsequent MD stretching and the structure (orientation) of the unstretched sheet are combined to obtain a structure oriented in the MD direction. After the MD stretching, the structure in TD direction perpendicular to the MD direction is developed further by the TD stretching of a certain magnification or more. Thus, a more uniform membrane is easily obtained. The degree of orientation resulting from the orientation during the formation of an unstretched sheet is considered as α. In consideration of this, when the ratio of the MD stretching magnification to the TD magnification is adjusted to be higher in the TD direction than in the MD direction, it is believed that a membrane in which fibrils are developed uniformly in the surface direction can be obtained. In summary, when the TD stretching magnification and the MD stretching magnification satisfy the Equation (<NUM>-<NUM>), the pore-forming of the fibrils is likely to occur in both in the MD direction and in the TD direction. When the TD stretching is performed after the MD stretching, a higher stretching magnification in the TD direction during the pore-forming of the fibrils can facilitate the interaction between the fibrils. The development of the interaction of the fibrils can form a structure whose physical properties at the time of the compression are not likely to change. Therefore, the TD stretching magnification and the MD stretching magnification need to be regulated. The α is <NUM>, more preferably <NUM>, further preferably <NUM>, still further preferably <NUM>, and the most preferably <NUM>.

The stretching temperature in both of the MD stretching (MDO) and the TD stretching (TDO) is preferably the melting point of the polyolefin resin or less, and more preferably in the range of (the crystal dispersion temperature Tcd of the polyolefin resin) to (the melting point of the polyolefin resin -<NUM>), and further preferably (the crystal dispersion temperature Tcd of the polyolefin resin + <NUM>) to (the melting point of the polyolefin resin -<NUM>). For example, the stretching temperature in the case of the polyethylene resin is about <NUM> to <NUM>, and more preferably <NUM> to <NUM>. The stretching temperature of the polyethylene resin is further preferably <NUM> to <NUM>. When the stretching temperature of the polyethylene resin is higher than or equal to the crystal dispersion temperature of the polyolefin resin, the occurrence of micro cracks at the time of stretching can be suppressed, thereby preventing a bigger pore size (particularly the maximal pore size, BP) in the end. As a result, the permeation of ions becomes uniform, and Li dendrites are less likely to occur. Thus, a good battery performance is maintained. In addition, when the stretching temperature of the polyethylene resin is less than or equal to the melting point of the polyolefin resin, a uniform stretching occurs, which prevents wrinkles and sagging and thus assures the productivity of the separator.

Considering that stretching is performed in the MD direction by a rolling method after the stretching in the TD direction by a tenter method, a relationship of each stretching temperature in the case of performing the stretching in at least two or more stages is described in detail.

In the film production, the stretching is generally performed in multi stages. Even the monoaxial stretching is performed in multi stages in order to regulate the temperature and the magnification, thereby producing a more uniform membrane. In the biaxial stretching in different directions, a method of performing the stretching in the MD direction and in the TD direction, for example, is used widely as a method of obtaining a membrane with little anisotropy. In so-called "wet-process stretching" in which a solvent is used as a plasticizer used for separator production, a simultaneous biaxial stretching process has been used especially when polyethylene is used as the resin. Such a simultaneous biaxial stretching process can provide uniform physical properties of the film such as the membrane thickness, the air resistance and the strength and has been suitable for production of a thin membrane of <NUM> or less. However, in the simultaneous biaxial stretching process, since the stretching with a solvent, in other words, "wet-process stretching" is performed in one stage (once), the stretching temperature has been regulated only for a small region. In other words, in the conventional wet-method simultaneous biaxial stretching process, a long heating oven was required to prevent the uneven temperatures in the heating oven and to expand the temperature regulation possible range during the stretching, which increased the initial introduction cost. Thus, the expansion of the stretching temperature was difficult. Therefore, in the conventional wet-method simultaneous biaxial stretching process, the membrane production was limited in a relatively narrow range of temperatures in the heating oven.

Furthermore, wet-process stretching in multi stages, in other words, a method of performing separately the stretching in one axial direction of two axes (two directions) of the MD direction and the TD direction and the stretching in the other axial direction was also considered. However, such a consideration was mainly about the technique which focused on the MD stretching conditions, and few technological developments have been made which focus on the influence of each temperature in the separate stretching processes in the two axial directions. As described in detail with respect to the stretching magnification, the stretching magnification in each of the MD direction and the TD direction is <NUM> times or less, and at such a stretching magnification, the orientation is mainly in the stretching direction at the completion stage of the first axial stretching. Therefore, in this method, it is considered that the uniformity as a membrane can reach a level usable as a separator by adjusting the total area stretching magnification to <NUM> times or more in the second axial or subsequent stretching. As the total area stretching magnification becomes higher, the pin puncture strength and the tensile strength are more improved, but the shutdown temperature which affects the safety of the battery tends to rise. The shutdown temperature herein refers to the temperature at which the pores of the separator are blocked to block the permeation of lithium ions and thus, the battery function is stopped. In producing a thin membrane of <NUM> or less, particularly of <NUM> or less, it is necessary to increase the strength of the separator compared to conventional ones for a suitable battery production. One method of increasing the strength is to increase the total area stretching magnification. However, when the shutdown temperature increases and especially when the total area stretching magnification exceeds <NUM> times, it is more difficult to keep the balance between mechanical properties and the safety of the separator. As one method for decreasing the shutdown temperature, the technique of using a resin having a low-melting point has been developed, but the balance among the air resistance, the strength and the heat shrinkage rate were not sufficient.

In the present invention, for a separator of <NUM> or less, particularly of <NUM> or less having a good balance among the air resistance, the strength and the heat shrinkage rate, in order to improve the safety as well, the stretching temperature conditions were considered for when the stretching is performed separately and in order in one axial direction of two axes (two directions) of the MD direction and the TD direction (the first axial stretching) and then in the other axial direction (the second axial stretching). Generally, the shutdown temperature is affected greatly by the resin mobility and the pore distribution. Mainly, the meltdown temperature is affected greatly by the melting point and the viscosity of the resin, and when the resin composition is fixed under arbitrary conditions, the meltdown temperature stays constant even if other production conditions change. The increase in the shutdown temperature results in a smaller temperature difference between the meltdown temperature and the shutdown temperature. Even if the shutdown function shuts off the ion conduction in the case of an abnormal heat generation in the battery, when the temperature continues to rise due to the heat accumulation in the battery, the ion conduction is resumed by the meltdown of the separator. Thus, the risk of an ignition or the like increases. Therefore, it is necessary to lower the shutdown temperature as much as possible, or widen the difference between the shutdown temperature and the meltdown temperature.

Since it is important to lower the shutdown temperature for these reasons, a method of lowering the shutdown temperature was studied. The reason why the shutdown temperature rises with the increase of the total area stretching magnification is assumed that the increase in the total stretching magnification makes the mobility of the resin constituting the separator more restrictive. Accordingly, in the stretching process in which the first axial stretching and the second axial stretching are sequentially carried out (alternatively, the second axial stretching is further followed by another stretching at least once in an axial direction other than the first and second axes), an attempt was made to improve the mobility of the resin by increasing the stretching temperature of the second axial or subsequent stretching with respect to the that of the first axial stretching; however, contrary to the expectation, no improvement in the shutdown temperature could be obtained.

Therefore, the influence of the stretching temperature was further studied, and the following facts were found out.

That is, it was discovered that the shutdown temperature could be decreased when the relationship between the first axial stretching temperature (T1) and the maximal stretching temperature (T2) after the second axial stretching is maintained as in the Equation (<NUM>-<NUM>). By maintaining the relationship of the Equation (<NUM>-<NUM>), the shutdown temperature can be adjusted to <NUM> or less, or the temperature difference between the shutdown temperature (SDT) and the meltdown temperature (MDT) can be adjusted to <NUM> or more. Alternatively, when the relationship between the first axial stretching temperature (T1) and the maximal stretching temperature (T2) after the second axial stretching is maintained as in the Equation (<NUM>-<NUM>), the temperature difference between the shutdown temperature and the meltdown temperature can be adjusted to <NUM> or more while the shutdown temperature is kept under <NUM> or less. <MAT> In Equation (<NUM>-<NUM>), it was discovered that when β is preferably <NUM>, more preferably <NUM>, further preferably <NUM>, and still further preferably <NUM>, at least one of the shutdown temperature of <NUM> or less and the temperature difference of <NUM> or more between the meltdown temperature and the shutdown temperature could be achieved.

The reason why the shutdown temperature is improved under these conditions is not clear but can be assumed as follows. As described above, it is known that the shutdown temperature is influenced by the mobility of the resin and the pore size distribution, and therefore, it can be understood that the addition of a resin with a low melting point also increases the mobility of the resin and thus decreases the shutdown temperature. The reason for the improvement of the shutdown temperature by lowering the temperature after the second axial stretching in the multistage stretching process is assumed that, when the second axial or subsequent stretching is performed while the skeleton of the structure formed by the first axial stretching is maintained, a micro structure starting from the structure formed by the first axial stretching is formed, thereby lowering the shutdown temperature. It is also assumed that, in the case where the second axial stretching is performed at a higher temperature than in the first axial stretching, the pore-formation and the formation of a new structure by the stretching progress simultaneously, resulting in a broader structure distribution and thus the formation of a non-uniform micropore structure.

At this time, by selecting the stretching conditions that satisfy the relationship of Equation (<NUM>-<NUM>), not only the shutdown temperature and the meltdown temperature can be maintained as described above, but also a fine structure is formed. As a result, a separator which has a small variation in permeability after the compression and which is excellent in strength and heat shrinkage rate can be produced. In order to prevent the decrease in the air resistance after a treatment at a high temperature (<NUM>), β is <NUM>, further preferably <NUM>, and still further preferably <NUM>. In such a condition, the increase in the air resistance after the compression can be prevented.

The deformation rate in the TD stretching (TDO) can be determined from a membrane production rate and the clip position (site where the film is held) in the width direction (TD direction). By controlling the rail position in the heating oven in the TD direction, it is possible to control the width-widening rate in the TD direction, that is, the deformation rate. The deformation rate is desirably controlled at a constant rate in a region of preferably <NUM>% or more, more preferably <NUM>% or more, further preferably <NUM>% or more of the entire stretching stages in the TD stretching. The desired deformation rate of the TD stretching is preferably <NUM>%/second or less, more preferably <NUM>%/second or less, and further preferably <NUM>%/second or less. By setting the deformation rate of the TD stretching to <NUM>%/second, it is possible to suppress the residual stress in the separator, and to achieve a stable production with a low possibility of discontinuation of the production due to a membrane rupture or the like. The desired deformation rate of the TD stretching is preferably <NUM>%/seconds or more, more preferably <NUM>%/seconds or more, and further preferably <NUM>%/seconds or more. By setting the deformation rate of the TD stretching to <NUM>%/second, the investment for the equipment can be suppressed, and the production of economically useful separators is possible. The deviation in the deformation rate of the TD stretching (maximal deformation rate - minimal deformation rate) is preferably <NUM>%/second or less, more preferably <NUM>%/second or less, further preferably <NUM>%/second or less, and the most preferably <NUM>%/second or less. By controlling the deviation of the deformation rate of the TDO stretching below a certain value, the interaction of ultra high molecular weight components uniformly develops, and a microporous membrane which does not greatly deteriorate the air resistance when subjected to high temperature compression can be obtained.

Due to stretching as described above, cleavage occurs in the higher order structure formed in the gel-like sheet, resulting in a refined crystal phase and the formation of numerous fibrils. Fibrils form a three-dimensionally connected network structure. In addition to the improvement of the mechanical strength by stretching, the degree of interaction between fibrils can be controlled by regulating at least one of the stretching magnification and the stretching temperature. Therefore, even if a pressure is applied at a temperature of <NUM> or less, a structure whose performance hardly changes is obtained. Further, by performing the above stretching, since the shape is retained even at a high temperature, the insulation property can be easily maintained. Thus, the resulting microporous membrane is suitable for a separator of a battery, for example.

The stretched sheet thus obtained is subjected to a conventional technique, for example, a method described in <CIT> in order to wash and remove the diluent, followed by drying, and thus a dried microporous plastic film is obtained. In obtaining a microporous plastic film, the stretched sheet may be reheated in the dry-process stretching step (second stretching step) after the washing step and re-stretched. The re-stretching step may be either by a roller type or a tenter type. It is also possible to adjust physical properties and remove residual strain by performing a heat treatment in the same step. By using the temperature condition according to the following Equation (<NUM>) as the dry-process stretching temperature D (T), the solid heat shrinkage rate is further improved. Specifically, the use of the condition of the Equation (<NUM>) can decrease the heat shrinkage rate at <NUM>, and a desired heat shrinkage rate is preferably <NUM>% or less, more preferably <NUM>% or less, and further preferably <NUM>% or less in both the MD direction and the TD direction. <MAT> "SDT" refers to "the shutdown temperature" and the measurement method thereof is explained later. When γ is <NUM>, the solid heat shrinkage rate can be suppressed.

That is, various experiments are carried out in advance and data are obtained in order to figure out what kind of correlation exists between the shutdown temperature and the dry-process stretching temperature, and based on the acquired data, the dry-process stretching temperature is roughly set so that the shutdown temperature is, for example, around <NUM>. While the membrane formation is continued, the shutdown temperature of the microporous membrane which has been already wound is measured, and the dry-process stretching temperature is adjusted to satisfy the Equation (<NUM>) as described above. A microporous membrane with an excellent heat shrinkage rate can be thus obtained.

In order to produce a microporous membrane which shows a small deterioration of the air resistance after the compression, among Equations (<NUM>-<NUM>), (<NUM>-<NUM>) and (<NUM>) explained above, when the Equation (<NUM>-<NUM>) is satisfied the resulting microporous membrane shows a small air resistance variation after a pressurization at <NUM> and under <NUM> MPa for <NUM> minutes, after a pressurization at <NUM> and under <NUM> MPa for <NUM> minutes, and after the increase of the heating temperature from <NUM> to <NUM>. When the Equation (<NUM>-<NUM>) is satisfied in addition to the Equation (<NUM>-<NUM>), the deterioration of the air resistance after the compression is suppressed more than when only the Equation (<NUM>-<NUM>) is satisfied, and it is also possible to decrease the shutdown temperature (or increase the temperature difference between the shutdown temperature and the meltdown temperature).

Moreover, the Equation (<NUM>) is satisfied simultaneously with the achievement of <NUM>% or more of a polyethylene component having a molecular weight of <NUM>,<NUM>,<NUM> or more in accordance with the claims, optionally also of <NUM>% or more of an ultra high molecular weight polyethylene, because the microporous membrane thus obtained has at least one of the shutdown temperature of <NUM> or less or the difference of <NUM> or less between the shutdown temperature and the meltdown temperature.

In order to produce a microporous membrane with a high strength, the condition of containing er5% or more of a polyethylene component having a molecular weight of <NUM>,<NUM>,<NUM> or more mentioned above and the condition of the total surface stretching magnification of <NUM> times or more before the extraction of the solvent are preferably satisfied at the same time because the resulting microporous membrane shows improved pin puncture strength and tensile strength, such as the pin puncture strength of <NUM> mN (<NUM> conversion). In order to control the tensile rupture elongation and improve the processability for the case where a battery is assembled using the microporous membrane or the surface of the microporous membrane is coated, the total surface stretching magnification before the solvent extraction of <NUM> times or more is preferred because it is possible to obtain a microporous membrane having an average tensile rupture elongation (the square root of the product of the tensile rupture elongation in the MD direction and the tensile rupture elongation in the TD directions) of <NUM>% or less.

When Equation (<NUM>) is satisfied it is possible to obtain a microporous membrane having a solid heat shrinkage rate (for example, heat shrinkage rate after a heat treatment at <NUM> for <NUM> hours) of <NUM>% or less for each of MD and TD.

As a method of controlling the average pore size of the microporous membrane, when an ultra high molecular weight polyethylene is contained in an amount of <NUM>% or more mentioned above, a microporous membrane in which the maximal pore size is preferably <NUM> or less and the average pore size is <NUM> or less, and more preferably, the maximal pore size is <NUM> or less and the average pore size is <NUM> or less, can be obtained.

Preferably, the microporous membrane obtained herein has a hybrid structure in which a ladder-like structure is dominant in the submicron region and a three-dimensional network structure is dominant in the micron region. The "submicron region" refers to a structure size that can be confirmed under the observation with a <NUM> square field of view (<NUM> × <NUM>) by AFM (atomic force microscope) or the like. On the other hand, the "micron region" refers to a structure size that can be confirmed under the observation with a field of view of <NUM> square (<NUM> × <NUM>) or more by AFM or the like. The "ladder-like structure" defines a case where structures between adjacent fibrils (arrangement structure between adjacent fibrils) are arranged in a positional relationship which is close to orthogonal to each other. The "three-dimensional network structure" defines the case where fibrils take a three-dimensional network-like structure. "Dominant" indicates a case where a corresponding structure is observed in <NUM>% or more of the area in the observation field. Therefore, under the observation of a fibril at an arbitrary position, when fibrils orthogonal to (intersecting) this fibril can be dominantly confirmed, they are referred to as the aforementioned "ladder-like structure". When fibrils branching from the fibril at the arbitrary position are dominantly confirmed, they are called "three-dimensional network structure". The "micron region" is formed by "submicron regions", but when the "micron region" is observed, it is thought that the features of a larger and thicker structure become dominant and are observed as the structure difference as described above.

<FIG> show AFM photos obtained from the microporous membrane of the present invention showing such a structure. <FIG> shows a photo taken in the micron region as described above, and <FIG> shows a photo taken in the submicron region. Hereinafter, the longitudinal direction in Figures is the MD direction and the transverse direction is the TD direction. As described in detail above, in the present invention, fine fibrils branch out from a middle portion of a specific fibril and extend to form a three-dimensional structure in the micron region while, in the submicron region, each fibril is arranged in a way that other fibrils intersect with the specific fibril to form a ladder-like structure. As a preferred microporous membrane of the present invention, any one of the surfaces has the hybrid structure described above, and more preferably, the both sides have the above hybrid structure. The ratio of regions having characteristic structures may be different between both the surface layers.

The structure of the microporous membrane produced by a "simultaneous stretching" method in which an unstretched sheet is stretched in the MD direction and in the TD direction simultaneously is different from the microporous membrane of the present invention because the "three-dimensional network structure" is formed in both of the submicron and micron regions. Specifically, the AFM photos of a microporous membrane produced by such a simultaneous stretching method are shown in <FIG> (the micron region) and <FIG> (the submicron region), and certain fibrils branch off in a net-like shape to form a three-dimensional structure in both Figures.

In a conventional stepwise biaxial stretching method in which an unstretched sheet is stretched gradually in the MD and TD directions, although the submicron region shows a ladder-like structure, the micron region shows a structure in which the fibrils are arranged selectively in the MD direction or in the TD direction (ladder-like structure). The AFM photos of a microporous membrane produced by a conventional stepwise biaxial stretching method are shown in <FIG> (micron region) and <FIG> (submicron region). The fibrils are arranged in a ladder form in both pictures (the fibrils are not arranged randomly, but oriented in one direction). Therefore, the microporous membrane of the present invention has a structure which is different not only from that of microporous membranes produced by the simultaneous biaxial stretching method but also from that of microporous membranes obtained by the stepwise biaxial stretching process which has been known so far.

In summary, the microporous membrane of the present invention forms a three-dimensional network structure formed by a large number of ladder-like structures. That is, the microporous membrane of the present invention shows a ladder-like structure from a microscopic view, but as the ladder-like structures accumulate, each ladder-like structure is arranged so that a three-dimensional network structure is formed. On the other hand, the microporous membranes produced by the conventional stepwise biaxial stretching method has a ladder-like structure as a result of the accumulation of ladder-like structures while the microporous membranes produced by the conventional simultaneous biaxial stretching method has a three-dimensional network structure as a result of the accumulation of three-dimensional network structures. Therefore, the microporous membrane of the present invention can possess a highly uniform and fine structure inherent in the microporous membranes produced by the simultaneous stretching method and a structure capable of achieving high permeability and strength, which is inherent in the microporous membranes produced by the stepwise biaxial stretching method. With such a structure, while high permeability, high strength and low heat shrinkage are exhibited, the safety function (low-temperature SDT and high-temperature MDT) can be achieved without deteriorating the ion conductivity even under high temperature and compression conditions in the battery. Therefore, as can be seen from Examples, compared with the comparative examples, remarkable characteristics in terms of physical properties can be obtained.

Further, depending on the application of the microporous membrane thus obtained, a surface treatment such as corona discharge or a functional coating such as heat resistant particles may be applied to the surface of the microporous plastic film.

The polyolefin microporous membrane according to a preferred embodiment of the present invention has the following physical properties.

The membrane thickness of the polyolefin microporous membrane is preferably <NUM> to <NUM>, more preferably <NUM> to <NUM>, and further preferably <NUM> to <NUM> as the density and capacity of batteries have been increasing in recent years. The membrane thickness of <NUM> or more can result in a separator which assures the insulation property.

The polyolefin microporous membrane has a maximal pore size of preferably <NUM> or less, more preferably of <NUM> or less, further preferably of <NUM> or less, and still further preferably of <NUM> or less. The maximal pore size is determined from bubble points (BP) obtained by a palm porometer. The average pore size determined with a palm porometer is <NUM> or less, more preferably of <NUM> or less, further preferably of <NUM> or less, still further preferably of <NUM> or less, and still further preferably of <NUM> or less. A smaller pore size of the whole membrane makes the pores more resistant to crashes and reduces the variation in the membrane thickness and the air resistance.

The air resistance (Gurley value) is <NUM> sec/<NUM><NUM> or less. With the air resistance of <NUM> sec/<NUM><NUM> or less, when the microporous membrane is used in a battery, good ion conductivity is obtained. The air resistance can be adjusted by the stretching temperature and magnification before the extraction of the solvent, the dry-process stretching temperature and magnification after washing, and the resin composition.

The porosity is preferably <NUM> to <NUM>%. When the porosity is <NUM>% or more, good air resistance can be obtained. When the porosity is <NUM>% or less, the strength in the battery in which the microporous membrane is used as a separator is sufficient, and the short circuit can be prevented. The porosity is more preferably <NUM> to <NUM>%, and further preferably <NUM> to <NUM>%. The porosity in such a range is preferred because the pores of the separator are unlikely to be crashed during the compression.

The pin puncture strength corresponding to <NUM> is <NUM> mN (<NUM> gf) or more, preferably <NUM> mN, and more preferably <NUM> mN or more. With the pin puncture strength corresponding to <NUM> is <NUM> mN or more, when the microporous membrane is incorporated in a battery as a separator for batteries, especially in the case of a thin membrane of <NUM> or less, the short circuit between electrodes can be prevented.

The tensile rupture strength is preferably <NUM> MPa or more either in the MD direction and in the TD direction. The tensile rupture strength in this range can reduce the risk of the rupture of the membrane. The tensile rupture strength in the MD direction is preferably <NUM> MPa or more, more preferably <NUM> MPa or more, and further preferably <NUM> MPa or more. The tensile rupture strength in the TD direction is preferably <NUM> MPa or more, more preferably <NUM> MPa or more, and further preferably <NUM> MPa or more. When the tensile rupture strength is in the above preferred range, the membrane is more resistant to rupture even when pressed thermally under a high pressure in the production step of the battery, and thus the pores are unlikely to be crashed.

The tensile rupture elongation is <NUM>% or more either in the MD direction and in the TD direction. This reduces the possibility of the rupture of the membrane of the separator during the battery production and when external force acts on the battery. The average tensile rupture elongation determined by the equation explained later is preferably <NUM>% or less, more preferably <NUM>% or less, further preferably <NUM>% or less, and in this range, the residual strain after the winding can be lowered, and excellent processability is obtained.

The heat shrinkage rate after the exposure to a temperature of <NUM> for <NUM> hours is <NUM>% or less in both of the MD direction and TD direction. When the heat shrinkage rate is <NUM>% or less, even in the case of the use of the microporous membrane as a separator for a large lithium battery, a short circuit between the electrodes can be prevented since the end portions of the separator retracts toward the center at the time of heat generation. Therefore, in order to prevent the short circuit between the electrodes even when the battery generates heat, the heat shrinkage rate is preferably <NUM>% or less, more preferably <NUM>% or less, and further preferably <NUM>% or less in both of the MD direction and TD direction. The heat shrinkage rate is still further preferably less than <NUM>%. The heat shrinkage rate is preferably <NUM>% or less in both of the MD direction and TD direction, and especially in the MD direction.

The shutdown temperature is preferably <NUM> or less, more preferably <NUM> or less, and further preferably <NUM> or less. With the shutdown temperature in this range, pores close at a lower temperature, thereby blocking the mobility of lithium ions, and thus the battery function can be safely stopped.

The meltdown temperature is preferably <NUM> or more, more preferably <NUM> or more, and further preferably <NUM> or more. A larger difference between the meltdown temperature and the shutdown temperature is preferred. With a larger difference between the meltdown temperature and the shutdown temperature, when the flow of lithium ions stops during the abnormal heat generation in the battery, the battery function can stop without resuming the lithium ion conduction even in the case of overheating (overshoot) of the temperature in the battery. The difference between the meltdown temperature and the shutdown temperature is preferably <NUM> or more, more preferably <NUM> or more, and further preferably <NUM> or more.

Physical properties associated with the heat compression test are described below. The upper limit of the internal pressure of the battery that can occur when high capacity electrodes are applied was set at <NUM> MPa. The heating temperatures were <NUM> and <NUM>. Between these, <NUM> is assumed to be the temperature range inside the battery when the battery is used at a low rate (the use at less than <NUM> C, wherein <NUM> C is the discharge rate for the exhaustion within <NUM> hour), and <NUM> is assumed to be the maximal temperature that can occur inside the battery when a rapid charge and discharge state of <NUM> C or more is performed.

As an example, a pressurization test was performed assuming the conditions of <NUM> and <NUM> MPa as the upper limit of temperature and pressure that can occur (reach) at a high output in a large battery of <NUM> Ah or more. The membrane thickness variation after the heat compression under a pressure of <NUM> MPa at <NUM> for <NUM> minutes (see the equation below) is, when the membrane thickness before the compression is considered as <NUM>%, preferably <NUM>% or more, more preferably <NUM>% or more, and further preferably <NUM>% or more. With the membrane thickness variation of <NUM>% or more, when the microporous membrane is used as a battery separator, the volume of the separator after the charge and discharge shows a small variation. Therefore, the number of electrodes to be used can be increased at during the battery assembly, leading to the maximization of the battery capacity.

The membrane thickness variation after the heat compression under a pressure of <NUM> MPa at <NUM> for <NUM> minutes (the same equation as above) is, when the membrane thickness before the compression is considered as <NUM>%, preferably <NUM>% or more, more preferably <NUM>% or more, and further preferably <NUM>% or more.

The air resistance variation (<NUM>) after the heat compression under a pressure of <NUM> MPa at <NUM> for <NUM> minutes (the variation of Gurley values (sec/<NUM><NUM>) before and after the heat compression, see the equation below) is <NUM>% or less, more preferably <NUM>% or less, and further preferably <NUM>% or less. When the air resistance variation after the heat compression under a pressure of <NUM> MPa at <NUM> for <NUM> minutes is in this range, a battery which suppresses the deterioration of the ion permeability in the temperature range inside the battery (about <NUM>) which is assumed when the battery is used at a low rate (the use at less than <NUM> C, wherein <NUM> C is the discharge rate for the exhaustion within <NUM> hour) can be formed. Depending on the battery system to be used, when the air resistance variation after the heat compression at <NUM> for <NUM> minutes is <NUM>% or less (<NUM> sec/<NUM><NUM> or less in the air resistance), the deterioration of the cycle property, which is the battery life, is suppressed.

On the other hand, the air resistance variation (<NUM>) after the heat compression under a pressure of <NUM> MPa at <NUM> for <NUM> minutes (the variation of Gurley values (sec/<NUM><NUM>) before and after the heat compression) is preferably <NUM>% or less, more preferably <NUM>% or less, and further preferably <NUM>% or less. When the air resistance variation after the heat compression at <NUM> for <NUM> minutes is <NUM>% or less, especially in the case of use as a separator for a large battery, the deterioration of the battery performance can be suppressed in the temperature range which can occur partially for a short time inside the battery during the rapid charge and discharge.

The variation ratio of the air resistance variation (<NUM>) to the air resistance variation (<NUM>) (<NUM>/<NUM>), which is defined by the following equation, is preferably <NUM>% or less, more preferably <NUM>% or less, and further preferably <NUM>% or less. This ratio is further preferably <NUM>% or less. <MAT> The smaller this ratio is, the less deterioration of the separator performance occurs when the rapid charge and discharge is sporadically performed during normal use. That is, if the ratio is <NUM>% or less, the deterioration of the cycle property can be suppressed even after the rapid charge and discharge, and thus, the irreversible deterioration of the impedance of the separator, which is a measure of flowability of lithium ions, can be prevented.

The membrane thickness variation (%) after the heat compression and the air resistance variation (%) after the heat compression are easily affected by the crystal orientation, the structure of pores of the membrane, the heat shrinkage rate, and the like. In particular, a microporous membrane with a small variation in the air resistance can be obtained when the interaction formed by the polyethylene molecular weight component having a molecular weight of <NUM>,<NUM>,<NUM> or more occurs uniformly in the microporous membrane. As a means for achieving this, in addition to the control of the stretching magnification and the stretching temperature at the stretching stage under certain conditions, the deformation rate during stretching can be maintained within a constant rate fluctuation range to control uniformly the interaction structure formed by the ultra high molecular weight polyethylene component.

The present invention will be explained in more detail by way of the following Examples, but the present invention is not limited to these Examples. The physical properties of the polyolefin microporous membrane were measured by the following methods.

The membrane thickness of <NUM> points within the range of <NUM> × <NUM> of the microporous membrane was measured with a contact thickness meter (Litematic manufactured by Mitutoyo Corporation), and the average value of the membrane thickness was obtained.

The average pore size (average flow pore size) and maximal (bubble point (BP)) pore size (nm) of the polyolefin microporous membrane were measured as follows.

A palm porometer manufactured by Porous Materials Inc. (trade name, model: CFP-1500A) was used to carry out the measurement in the order of Dry-up and Wet-up. For Wet-up, a pressure was applied to the polyolefin microporous membrane sufficiently soaked with Galwick (trade name) with a known surface tension, and the pore size converted from the pressure at which air started penetrating was used as the maximal pore size. For the average flow rate size, the pore size was converted from the pressure at the point where the pressure and flow rate curve showing the slope of <NUM>/<NUM> from the Dry-up measurement and the curve from the Wet-up measurement intersect. The following equation was used to convert the pressure to the pore size. <MAT> In the equation, d (µm) is the pore size of the microporous membrane, γ (dynes/cm) is a surface tension of the liquid, P (Pa)is the pressure, and C is the pressure constant (<NUM>).

The air resistance (Gurley value) was measured in accordance with JIS P8117.

The porosity was calculated from the following equation, using the mass w1 of the microporous membrane and the mass w2 of a pore-free membrane of the same size made of the same polyethylene composition as the microporous membrane.

The maximal load value (P1) when a polyolefin microporous membrane was punctured with a needle having a diameter of <NUM> (tip: <NUM> mmR) at a speed of <NUM>/sec was measured as the pin puncture strength. The <NUM>-corresponding pin puncture strength (P2) was converted to the corresponding value in the membrane thickness T1(µm), using the following equation.

The tensile rupture strength was measured according to ASTM D882 using a strip test piece having a width of <NUM>.

The tensile rupture elongation was determined by taking <NUM> strips with a width of <NUM> from the central portion of the polyolefin microporous membrane in the width direction and calculating the average value of the measurement results by ASTM D882 for each. The average tensile rupture elongation (LA; %) obtained from the tensile rupture elongation in the MD (LM) and TD (LT) directions was determined by the following equation.

For the heat shrinkage rate, the shrinkage rate in the MD direction and the TD direction after the exposure of the microporous membrane at <NUM> for <NUM> hours was individually measured three times, and the average value was calculated.

The shutdown temperature and the meltdown temperature were measured by the method disclosed in <CIT>. According to this method, the microporous membrane is exposed to an atmosphere at <NUM>, and the temperature is raised at <NUM>/min, during which the air resistance of the membrane is measured. The temperature at which the air resistance (Gurley value) of the microporous membrane first exceeds <NUM>,<NUM> seconds/<NUM><NUM> was defined as the shutdown temperature of the microporous membrane. The meltdown temperature was defined as the temperature at which the air resistance which was measured while the temperature continued to rise after reaching the shutdown temperature reached again <NUM>,<NUM> seconds/<NUM><NUM>. The air resistance of the microporous membrane was measured in accordance with JIS P8117 using an air resistance meter (EGO-1T, manufactured by Asahi Seiko Co.

The membrane thickness was measured with a contact thickness meter (manufactured by Mitutoyo Corporation). A polyolefin microporous membrane is sandwiched between a pair of press plates having a highly smooth surface and subjected to heat compression by a press machine under a pressure of <NUM> MPa at <NUM> (or <NUM>) for <NUM> minutes. A value obtained by dividing the membrane thickness after the compression by the membrane thickness before the compression is expressed in percentage as a membrane thickness variation (%) (see the equation below). Three points from the central portion of the polyolefin microporous membrane in the width direction were measured for the membrane thickness, and the average value of the measurement results were calculated. <MAT> The "treatment" in the above equation means a treatment of pressurization and compression under the above-described conditions.

A polyolefin microporous membrane was subjected to heat compression under the same conditions as in the above (<NUM>), and the value obtained by dividing the air resistance after the heat compression by the air resistance before the heat compression was expressed in percentage as the air resistance variation (%) (see the equation below). Three points from the central portion of the polyolefin microporous membrane in the width direction were measured for the air resistance, and the average value was calculated. <MAT> <MAT> The variation ratio of the air resistance variation (<NUM>) to the air resistance variation (<NUM>) (<NUM>/<NUM>) was determined from the following equation.

An impedance measuring device (SI <NUM>, SI <NUM>, manufactured by Solartron Metrology Ltd) was used to measure the impedance. Between electrodes in which a Ni foil (<NUM> × <NUM>) was provided on a glass plate (<NUM> (W) × <NUM> (L) × <NUM> (T)), a microporous membrane (<NUM> (W) × <NUM> (L)) and about <NUM> of a typical electrolyte consisting of, using LiPF<NUM> of <NUM> mol/L, a lithium salt, ethylene carbonate (EC) and ethylmethyl carbonate (EMC) (EC: EMC = <NUM>: <NUM> VOL%) were placed, and <NUM> kV was applied (constant voltage), and the value after <NUM> seconds was defined as electric resistance (Ω·cm<NUM>). The measured value at room temperature was considered as <NUM>%, and the impedance value after the heat compression treatment was obtained as a relative value.

Using the device described in (<NUM>) above, the above cell is placed in an oven. The temperature inside the oven was increased from a room temperature (<NUM>) to <NUM> in <NUM> minutes, and the impedance was continuously measured. As the cell temperature was measured, the temperature at which the impedance first reached <NUM> × <NUM><NUM> (Ω·cm<NUM>) was considered as the shutdown temperature, and while the temperature increase was continued, the duration during which <NUM> × <NUM><NUM>(Ω·cm<NUM>) was maintained was measured. The evaluation was as follows: <NUM>) remarkably excellent for <NUM> seconds or more (⊚), <NUM>) excellent for <NUM> seconds or more (∘), <NUM>) standard for <NUM> seconds or more (△). On the other hand, it was evaluated as <NUM>) inappropriate (×) for less than <NUM> seconds, and as <NUM>) not able to contribute to the battery safety when the shutdown state was not reached. Such a test was carried out twice for each Example and each Comparative Example (limited to the samples judged to be testable) to obtain an average. In Tables <NUM> and <NUM> described later, these impedance evaluation results were expressed as "impedance retention test result".

The Mw and Mw/Mn of UHMWPE and HDPE each were measured by gel permeation chromatography (GPC) under the following conditions.

The surface of the microporous membrane in the submicron region or the micron region can be observed with a commercially available scanning probe microscope. As an example of such a scanning probe microscope, a measurement example using SPA-<NUM> manufactured by Hitachi High-Technologies will be described below. Specifically, the surface of the microporous membrane can be observed in the DFM mode of such a device. A sample (microporous membrane) is fixed to the sample stage with a carbon tape, and the SI-DF <NUM> for DMF can be used as the cantilever. Then, the amplitude attenuation rate is set to -<NUM> to -<NUM>, the scanning frequency is set to <NUM> to <NUM>, and the I gain and the P gain are individually adjusted to observe the surface. A typical field size to be used can be <NUM> square and <NUM> square. The "ladder-like structure" refers to a structure where structures formed between adjacent fibrils (arrangement structure between adjacent fibrils) are arranged in a positional relationship which is close to orthogonal to each other. The "three-dimensional network structure" defines the case where the fibrils take a three-dimensionally network-like structure. "Dominant" indicates a case where a corresponding structure is observed in <NUM>% or more of the area in the observation field. Therefore, under the observation of a fibril at an arbitrary position, when fibrils orthogonal to (intersecting) this fibril could be dominantly confirmed, they were referred to as the aforementioned "ladder-like structure". When fibrils branching from the fibril at the arbitrary position were dominantly confirmed, they were called "three-dimensional network structure".

To <NUM> parts by mass of a composition consisting of <NUM>% by mass of ultra high molecular weight polyethylene having a mass average molecular weight of <NUM> x <NUM><NUM> and <NUM>% by mass of high density polyethylene having a mass average molecular weight of <NUM> x <NUM><NUM>, <NUM> parts by mass of tetrakis[methylene-<NUM>-(<NUM>,<NUM>-ditertiarybutyl-<NUM>-hydroxyphenyl)-propionate]methane was dry-blended to prepare a polyethylene composition. The obtained polyethylene composition was charged in an amount of <NUM> parts by weight into a twin-screw extruder. Further, <NUM> parts by weight of liquid paraffin was fed from the side feeder of the twin-screw extruder and subjected to melt blending. Thus, a polyethylene resin solution was prepared in the extruder. Subsequently, a polyethylene resin solution was extruded at <NUM> from a die installed at the tip of the extruder, and taken out by a cooling roll with the internal cooling water temperature kept at <NUM> to form an unstretched gel-like sheet.

The obtained unstretched gel-like sheet was passed through a group of four preheating rolls to raise the temperature of the sheet surface to <NUM>. For a longitudinal stretching (MDO) roll, a metal roll with a width of <NUM>, a diameter of <NUM>, and a hard chromium plating (the surface roughness: <NUM>) was used. The surface temperature of each longitudinal stretching roll was <NUM>, and the range of temperature fluctuation was ± <NUM> or less. By controlling the peripheral speed ratio of each roll such that the rotation speed of each stretching roll of the longitudinal stretching apparatus was faster downward, the gel-like sheet was stretched in the machine direction by <NUM>/<NUM>/<NUM> times in three divided stages with a total magnification of <NUM> times. Then, the sheet was passed through four cooling rolls and the sheet temperature was cooled to <NUM>. Thus, a longitudinally stretched gel-like sheet was formed.

Both ends of the resulting longitudinally stretched gel-like sheet were gripped with clips, and the sheet temperature was raised at a preheating temperature of <NUM> in a tenter divided into <NUM> zones, and then the sheet was stretched in the transverse direction at <NUM> by <NUM> times and treated at a thermal fixation temperature of <NUM>. Thus, a biaxially stretched gel-like sheet was formed. The interval between the clips in the direction of the traveling sheet was set to <NUM> from the entrance of the tenter to the exit. The wind speed fluctuation width of hot air in the width direction in the tenter was adjusted to be <NUM>/sec or less. The average deformation rate in the transverse stretching (TDO) region was <NUM>%/second, and the deviation between the maximal and minimal deformation rate was <NUM>%/second. The obtained biaxially stretched gel-like sheet was cooled to <NUM>, and then liquid paraffin was removed in a methylene chloride washing tank controlled at <NUM>, and dried in a drying oven adjusted to <NUM>.

The obtained dried sheet was heated to <NUM> in a re-stretching device, re-stretched to have a lateral magnification of <NUM> times with respect to the entrance width of the re-stretching device, and then subjected to a heat treatment such that the transverse magnification would be <NUM> times with respect to the entrance width of the re-stretching device. Then, the obtained sheet was subjected to a heat treatment for <NUM> seconds to obtain a polyolefin microporous membrane having a thickness of <NUM>.

The production conditions of the microporous membrane in Example <NUM> are shown in Tables <NUM> and <NUM>, and the physical properties obtained from this microporous membrane are shown in Tables <NUM> and <NUM>.

Microporous membranes were obtained in the same way as in Example <NUM> except that the composition of the resins, and the like were changed according to the conditions described in Tables <NUM> and <NUM>.

The obtained unstretched gel-like sheet was introduced into a simultaneous biaxial stretching machine and stretched by 5x5 times in the sheet transport direction (MD) and in the sheet width direction (TD). The temperature of the preheating/stretching/thermal fixation was adjusted to <NUM>/<NUM>/<NUM>. The obtained biaxially stretched gel-like sheet was cooled to <NUM>, and then liquid paraffin was removed in a methylene chloride washing tank controlled at <NUM>, and dried in a drying oven adjusted to <NUM>.

The obtained dried sheet was heated to <NUM> in a re-stretching device, re-stretched to have a lateral magnification of <NUM> times with respect to the entrance width of the re-stretching device, and then subjected to a heat treatment such that the transverse magnification would be <NUM> times with respect to the entrance width of the re-stretching device. Then, the obtained film was subjected to a heat treatment for <NUM> seconds to obtain a polyolefin microporous membrane having a thickness of <NUM>.

A microporous membrane was obtained in the same way as in Comparative Example <NUM> except that the conditions were changed as described in Tables <NUM> and <NUM>.

A microporous membrane was obtained from the same polyethylene composition as in Example <NUM> under the conditions described in Tables <NUM> and <NUM>. The heat shrinkage rate at <NUM> was high both in the MD direction and TD direction. The evaluation was canceled because of the inferior safety.

The membrane production experiment was attempted by changing the longitudinal stretching temperature of Example <NUM> to <NUM>/<NUM> in preheating/stretching steps, respectively. In the longitudinal stretching step, a behavior of the sheet shifting in the width direction of the stretching roll over time (generally referred to as "meandering") was observed, and thus, a stable membrane production could not be performed.

The membrane production experiment was attempted by changing the longitudinal stretching temperature of Example <NUM> to <NUM>/<NUM> in preheating/stretching steps, respectively. A phenomenon in which liquid paraffin which was a solvent flew out from the sheet was observed during the preheating stage, and meandering occurred before the stretching roll. A stable membrane production was difficult.

A membrane was produced under the same conditions as in Example <NUM> except that TDO preheating/stretching temperatures were changed to <NUM>/<NUM>, respectively. Since the stretching temperature was low, the stretching tension was increased, and a phenomenon that the film was detached from the holding clips during the TDO stretching. As a result, a microporous membrane could not be obtained.

A membrane was produced under the same conditions as in Example <NUM> except that TDO preheating/stretching temperatures were changed to <NUM>/<NUM>, respectively. The TDO stretching temperature was so high that the stretching behavior varied in the TD direction, resulting in an uneven film thickness, and a stable membrane production was impossible.

A membrane was produced under the conditions in Tables <NUM> and <NUM>. The resulting separator has an SDT or more than <NUM>, and the difference from the meltdown temperature was less than <NUM>. Therefore, the safety was considered inferior, and the evaluation was canceled.

A membrane was produced under the conditions in Tables <NUM> and <NUM>. After the longitudinal stretching roll, the sheet thickness varied over time in the MD direction after the MDO stretching. The resulting separator had uneven thickness in the MD direction, and wrinkles and sagging were observed. The resulting separator was unsuitable for use as a separator.

A membrane was produced under the same conditions as in Example <NUM> except that the TDO magnification was changed to <NUM> times. Since the distribution of the thickness in the TD direction was unlikely to be even, and the air resistance, the strength and the heat shrinkage rate varied in the TD direction, the evaluation as a separator was difficult.

A membrane was stretched under the same conditions as in Example <NUM> except that the TDO stretching magnification was changed to <NUM> times. Due to the high stretching magnification, the membrane was ruptured during the TDO stretching process, and it was impossible to obtain stably a separator.

A membrane was produced under the conditions in Tables <NUM> and <NUM>. The separator obtained by increasing the MDO stretching magnification was excellent in the strength and the heat shrinkage rate, but the compression resistance turned out to be inferior.

Based on Tables <NUM> and <NUM> in which the above results are described, Examples <NUM> to <NUM> show that the obtained separator showed excellent permeability and a small membrane thickness variation even after the exposure to a high temperature and a high pressure, and were excellent in strength and heat shrinkage rate required as a separator. Furthermore, by controlling SDT and MDT, which are a measure of safety, a high insulation state after the shutdown (impedance maintained at <NUM> x <NUM><NUM> (Ω·cm<NUM>)) could be achieved over a long period of time.

Claim 1:
A polyolefin microporous membrane, wherein the average membrane thickness is <NUM> or less, the pin puncture strength corresponding to the membrane thickness of <NUM> is <NUM> mN or more, the air resistance is <NUM> seconds or less, the air resistance variation (<NUM>) after a pressurization treatment under the conditions of <NUM>, <NUM> MPa, and <NUM> minutes is <NUM>% or less, and the average pore size, determined with a palm parameter, as described herein, is <NUM> or less: wherein <MAT> wherein the air resistance is measured in accordance with JIS P8117, and the pin puncture strength corresponding to the membrane thickness of <NUM> and the average pore size are measured by the methods described in the description, wherein the polyolefin microporous membrane is obtainable from a polyethylene mixture containing two or more kinds of polyethylenes, the polyethylene mixture containing (I) <NUM>-<NUM>% by weight of an ultra high molecular weight polyethylene component having a weight average molecular weight of <NUM> x <NUM><NUM> or more and (II) polyethylene having a weight average molecular weight of <NUM> x <NUM><NUM> or more and <NUM> x <NUM><NUM> or less.