KR102223395B1 - Polyolefin microporous film, separator for non-aqueous electrolyte secondary battery, wound product of polyolefin microporous film, non-aqueous electrolyte secondary battery, and method for producing polyolefin microporous film - Google Patents

Abstract

It is an object of the present invention to provide a polyolefin microporous membrane having excellent piercing strength and air permeability resistance, and excellent appearance without wrinkles or misalignment when a wound body, and a method for producing the wound body and polyolefin microporous membrane. . Polyolefin, characterized in that the piercing strength in terms of 16 µm is 400 gf or more, the air permeation resistance in terms of 16 µm is 100 to 400 sec/100 cc, and the coefficient of static friction when the front and back of the membrane is stacked is 0.5 to 1.0. Microporous membrane.

Description

Polyolefin microporous membrane, separator for non-aqueous electrolyte secondary battery, polyolefin microporous membrane winding body, non-aqueous electrolyte secondary battery and polyolefin microporous membrane manufacturing method NON-AQUEOUS ELECTROLYTE SECONDARY BATTERY, AND METHOD FOR PRODUCING POLYOLEFIN MICROPOROUS FILM}

The present invention relates to a separator used for separation of substances, selective permeation, and the like, and a microporous membrane widely used as a separator for electrochemical reaction devices such as alkali, lithium secondary batteries, fuel cells, and capacitors. In particular, it relates to a polyolefin microporous membrane suitably used as a separator for lithium ion batteries.

Polyolefin microporous membranes are used as microfiltration membranes, separators for fuel cells, separators for capacitors, and the like. In addition to these, the polyolefin microporous membrane is particularly suitably used as a separator for lithium ion batteries widely used in notebook personal computers, mobile phones, and digital cameras. The reason for this is that the polyolefin microporous membrane has excellent membrane puncture strength and shutdown properties.

Lithium ion secondary batteries are being developed with the aim of higher energy density, higher capacity, and higher output, and accordingly, demands for high strength (especially, puncture strength) and high permeability for separators are further increasing. However, in the polyolefin microporous membrane, the piercing strength and the permeability (permeability resistance) are opposite characteristics, and it has been difficult to achieve both.

In addition, in recent years, lithium ion batteries have been widely applied to automobiles and equipment used outdoors, such as lawnmowers, lawnmowers, and small ships, in addition to electric vehicles, hybrid vehicles, and electric motorcycles. For this reason, a large-sized battery is required compared to conventional small electronic devices such as mobile phones and notebook computers, and a separator with a wide width, for example, a separator having a width of 100 mm or more, is desired also for a separator incorporated in the battery. However, since polyolefin microporous membranes generally used for separators have a thickness of 30 µm or less and have very low tensile strength and stiffness, problems such as wrinkling and misalignment are likely to occur. It was difficult to obtain a wound product of the sclera. In particular, in order to improve the productivity of the microporous membrane in the future, it is expected that widening, lengthening, thinning, and high-speed production will proceed, and this trend is expected to appear more remarkable. do.

For example, a rolled product made of a polyolefin microporous membrane with less misalignment when rewound from the wound body is described in Patent Document 1 (Japanese Laid-Open Patent Publication No. 2004-99799). It has been disclosed that the winding property when rewinding is improved by setting the friction coefficient ratio of the front and back of the microporous membrane to 1.5 or less, but coexistence of high strength and high permeability is not achieved.

Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-99799 Patent Document 2: Japanese Unexamined Patent Application Publication No. Hei 8-311225 Patent Document 3: Japanese Laid-Open Patent Publication No. 2001-96614 Patent Document 4: Japanese Laid-Open Patent Publication No. 2009-132904 Patent Document 5: Japanese Laid-Open Patent Publication No. 2010-24463

The present invention makes both penetration strength and air permeation resistance, which have been difficult to achieve conventional compatibility, and controls the static friction coefficient of the front and back of the membrane, so that the width is relatively wide and the winding length is long, so that the number of stacked films is large. It is a polyolefin microporous membrane from which a wound body without wrinkles or miswraps is obtained while maintaining the necessary properties even in the case.

An object of the present invention is to provide a polyolefin microporous membrane having excellent piercing strength and air permeation resistance, and having an excellent appearance without wrinkles or misalignment when used as a wound body.

In addition, as a prior art document focusing on the coefficient of friction of the polyolefin microporous membrane, for example, Patent Document 2 (Japanese Laid-Open Patent Publication No. In pursuit of lubricity, not only the appearance of the wound body was improved, but also the balance between the punching strength and the air permeability resistance was not achieved.

In addition, there is Patent Document 3 (Japanese Laid-Open Patent Publication No. 2001-96614) as a prior art document focusing on the coefficient of friction, but it is disclosed that high molecular weight polyethylene having an intrinsic viscosity of 5.0 dl/g or more is simultaneously biaxially stretched. This is to improve the surface smoothness of the obtained high molecular weight polyethylene biaxially oriented film, and not only the appearance of the wound body was improved, but also both the piercing strength and the air permeability resistance were not achieved.

On the other hand, as a prior art document related to the stretching technology used in the production of the present invention, in Patent Document 4 (Japanese Patent Laid-Open No. 2009-132904), a polyethylene gel-like sheet having a ^{weight average molecular weight of 3.8×10 5 in the vertical direction (} Machine direction) 8.5 times in the horizontal direction and 5 times in the horizontal direction (vertical direction and perpendicular direction), the solvent is washed and dried, and then in the re-stretching process, 3.0 times in the vertical direction and in the horizontal direction A film stretched by 1.2 times is disclosed. In addition, in Patent Document 5 (Japanese Laid-Open Patent Publication No. 2010-24463), a part of the solvent was removed from a gel sheet made of ultra-high molecular weight polyethylene having a weight average molecular ^{weight of 2.0 × 10 6 and high-density polyethylene having a weight average molecular weight of 3.5 × 10 5.} Later, a film which was successively stretched 5 times in the vertical direction and 10 times in the horizontal direction is disclosed. However, there is no disclosure of the balance between the punching strength and the resistance to air permeation, and the appearance (wrinkled, twistedly wound) when the wound body is formed, and regarding the widening and lengthening of the winding product which is expected to proceed in the future. It was insufficient.

The inventors of the present invention have repeatedly intensively studied in order to solve the above problems, and as a result, have found that the solution can be solved by the following configuration, and have come to the present invention. That is, the present invention is as follows.

(1) The protrusion strength in terms of thickness 16 µm is 400 gf or more, the air permeation resistance in terms of thickness 16 µm is 100 to 400 sec/100 cc, and the coefficient of static friction when the front and back of the membrane is stacked is 0.5 to 1.0. Olefin microporous membrane characterized by the above-mentioned.

(2) The polyolefin microporous membrane according to the above (1), wherein the ratio of the piercing strength and the air permeability resistance is 1.7 to 3.0.

(3) The polyolefin microporous membrane according to the above (1) or (2), wherein the polyolefin is a polyethylene containing ultra-high molecular weight polyethylene having a weight average molecular weight of 2.0×10 ^{6 or more.}

(4) A separator for a non-aqueous electrolyte-based secondary battery comprising the polyolefin microporous membrane according to any one of (1) to (3) above.

(5) Polyolefin having a piercing strength of 400 gf or more in terms of thickness of 16 µm, an air permeation resistance of 100 to 400 sec/100 cc in terms of thickness of 16 µm, and a coefficient of static friction of 0.5 to 1.0 when the front and back of the membrane are stacked The first microporous membrane is wound around a core, has a width of 300 mm or more, the number of stacked polyolefin microporous membranes wound on the core is 1,500 or more, and the cross section in the width direction of the polyolefin microporous membrane is displaced. A rolled body of a polyolefin microporous membrane, characterized in that it is 0 to 3 mm in both left and right in the lamination direction of the polyolefin microporous membrane.

(6) The polyolefin microporous membrane wound body according to (5), wherein the polyolefin microporous membrane is a separator for a non-aqueous electrolyte-based secondary battery.

(7) A nonaqueous electrolyte secondary battery comprising the separator for a nonaqueous electrolyte secondary battery according to the above (4).

(8) (a) A process of melt-kneading a polyolefin resin containing an ultra-high molecular weight polyolefin having a weight average molecular weight of 2×10 ⁶ or more and less than 4×10 ^{6 and a plasticizer to prepare a polyolefin solution}

(b) The molten mixture obtained in step (a) is extruded from an extruder to form an extrudate, and the cooling rate of both the front and the back of the extrudate is 250°C/min or more, and the cooling rate difference between the front and the back is 15°C/sec or more. To form a gel-like sheet

(c) Step of stretching the sheet obtained in step (b) in the longitudinal direction (machine direction)

(d) The step of stretching the sheet obtained in step (c) in the transverse direction (the direction perpendicular to the machine direction)

(e) Step of extracting plasticizer from the stretched film obtained in step (d)

(f) including the step of drying the microporous membrane obtained in step (e),

The method for producing a polyolefin microporous membrane according to any one of the above (1) to (3), wherein the step (c) and the step (d) are each performed continuously.

The polyolefin microporous membrane of the present invention is excellent in piercing strength and air permeation resistance, and has an excellent appearance when used as a wound body, and is suitable as a separator for a lithium ion secondary battery.

The present invention is clearly different from the adjustment of the coefficient of friction by adding a lubricant such as inorganic particles to a polyolefin resin used as a raw material for a polyolefin microporous membrane. This is because when a lubricant such as inorganic particles is added to the polyolefin resin, the lubricant is eliminated in a post process, contaminating the process, and consequently causing a serious defect in the polyolefin microporous membrane.

Hereinafter, the present invention will be described in detail.

[1] polyolefin resins

It is preferable that the polyolefin resin used for the polyolefin microporous membrane of the present invention contains polyethylene as a main component. In order to improve the permeability and piercing strength, the total polyolefin resin is 100% by mass, and the proportion of the polyolefin is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferable to use a polyolefin alone. .

Polyethylene may be a homopolymer of ethylene as well as a copolymer containing a small amount of other α-olefins. Examples of the α-olefin include propylene, butene-1, hexene-1, pentene-1,4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, and styrene.

Here, as the type of polyethylene, high-density polyethylene having a density exceeding 0.94 g/cm3, medium-density polyethylene having a density in the range of 0.93 to 0.94 g/cm3, low-density polyethylene having a density less than 0.93 g/cm3, linear low-density polyethylene, etc. However, in order to increase the piercing strength, it is preferable to contain high-density polyethylene. It is preferable that the weight average molecular weight (hereinafter referred to as Mw) of the high-density polyethylene is 1×10 ⁵ or more, more preferably 2×10 ⁵ or more. The upper limit of Mw of the high-density polyethylene is preferably 8×10 ⁵ Mw, more preferably 7×10 ⁵ Mw. When Mw is in the above range, the stability of film formation and the finally obtained piercing strength can be both achieved.

In the present invention, it is important to contain ultra-high molecular weight polyethylene in polyethylene. The ultra-high molecular weight polyethylene may be a copolymer containing not only a homopolymer of ethylene but also a small amount of other α-olefins. Other α-olefins other than ethylene may be the same as above. By adding ultra-high molecular weight polyethylene, it is possible to improve the piercing strength. As Mw of the ultra-high molecular weight polyethylene, it is preferable that it is ^{2×10 6} or more and less than 4×10 ^6. By using ultra-high molecular weight polyethylene having an Mw of 2×10 ⁶ or more and less than 4×10 ⁶ , it is possible to make pores and fibrils fine, so that the surface of the film becomes densely rough, and the strength of the protrusions can be increased. Further, since the film surface becomes densely rough, it is possible to control the coefficient of friction by combining it with a manufacturing method described later. Here, "densely coarse" means that fine crystals exist densely, as will be described later. In addition, if the Mw of the ultra-high molecular weight polyethylene is 4×10 ⁶ or more, the viscosity of the melt becomes too high, so that a problem may occur in the film-forming process, such as the inability to extrude the resin from the detention (die), or the thermal contraction rate may deteriorate. There is a concern. In addition, when the Mw of the ultra-high molecular weight polyethylene is 4×10 ⁶ or more, it is easy to separate from the polyethylene as the main component, so that the surface of the microporous membrane is too rough, and the friction coefficient may be too low. The content of the ultra-high molecular weight polyethylene is 100% by mass as the whole polyolefin resin, and the lower limit is preferably 10% by mass, more preferably 20% by mass, and still more preferably 30% by mass. It is preferable that the upper limit of the content of ultra-high molecular weight polyethylene is 40 mass%. When the content of the ultra-high molecular weight polyethylene is within this range, it becomes easy to obtain both the piercing strength and the air permeability resistance by the film forming method described later. In addition, when the content of the ultra-high molecular weight polyethylene is within the range described above, since the ultra-high molecular weight polyethylene is sufficiently dispersed, it is easy to control the crystallinity of the surface, and the friction coefficient can be appropriately controlled by the film forming method described later.

Low-density polyethylene, linear low-density polyethylene, ethylene-α-olefin copolymer produced by a single-site catalyst, and low molecular weight polyethylene having a weight average molecular weight of 1000 to 4000 are added to provide a shutdown function at low temperature. As a result, the characteristics as a battery separator can be improved. However, when there is a large amount of low molecular weight polyethylene, the microporous membrane is liable to be fractured in the stretching step at the time of production, and thus 0 to 10% by mass is preferable in the polyolefin resin.

Further, when polypropylene is added to polyethylene, the meltdown temperature can be improved when the polyolefin microporous membrane of the present invention is used as a battery separator. As for the kind of polypropylene, in addition to the homopolymer, block copolymers and random copolymers can also be used. The block copolymer and the random copolymer may contain a copolymer component with α-ethylene other than propylene, and as the other α-ethylene, ethylene is preferable. However, when polypropylene is added, the piercing strength tends to decrease compared to the use of polyethylene alone, and thus 0 to 10% by mass of the polyolefin resin is preferable.

The weight average molecular weight (hereinafter referred to as Mw) of the polyolefin resin ^{is preferably 1×10 5} or more. When Mw is ^{less than 1×10 5} , there is a concern that fracture tends to occur during stretching.

In addition, the polyolefin microporous membrane of the present invention can contain various additives such as antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, and antiblocking agents and fillers, as long as the effects of the present invention are not impaired. In particular, it is preferable to add an antioxidant for the purpose of suppressing oxidative deterioration due to thermal history of the polyethylene resin. It is important to adjust or enhance the properties of the microporous membrane to appropriately select the type and amount of the antioxidant or heat stabilizer.

In addition, the polyolefin microporous membrane of the present invention contains substantially no inorganic particles. The term ``substantially free of inorganic particles'' means a content that is less than or equal to 300 ppm, preferably less than 100 ppm, and most preferably less than the detection limit when the inorganic elements are quantified by, for example, fluorescence X-ray analysis. do. This is because even if the particles are not actively added to the polyolefin microporous membrane, contaminants derived from foreign matters or dirt adhered to the raw material resin or lines or devices in the polyolefin microporous membrane manufacturing process may be peeled off and mixed into the membrane. to be.

[2] Method for producing a polyolefin microporous membrane

Next, although the manufacturing method of the polyolefin microporous membrane of this invention is demonstrated concretely, it is not limited to this aspect.

The method for producing a polyolefin microporous membrane of the present invention includes the following steps (a) to (f).

(a) A process of melt-kneading a polyolefin resin containing an ultra-high molecular weight polyolefin having a weight average molecular weight of 2×10 ⁶ or more and less than 4×10 ^{6 and a plasticizer to prepare a polyolefin solution}

(b) The polyolefin solution obtained in step (a) is extruded from an extruder to form an extruded product, and the cooling rate of both the front and the back of the extrudate is 250°C/min or more, and the cooling rate difference between the front and the back is 15°C/sec or more. To form a gel-like sheet

(c) Step of stretching the sheet obtained in step (b) in the longitudinal direction (machine direction)

(d) The step of stretching the sheet obtained in step (c) in the transverse direction (the direction perpendicular to the machine direction)

(e) Step of extracting plasticizer from the stretched film obtained in step (d)

(f) Step of drying the microporous membrane obtained in step (e).

Here, the process (c) and the process (d) are performed continuously, respectively. That is, in the present invention, it is not a so-called batch method (a manufacturing method in which a microporous membrane of a specific size is manufactured using a certain amount of resin, and then the previous series of steps is repeated using another raw material), but a raw material. From the preparation process of the microporous membrane to the winding process of the microporous membrane, a production method that is carried out continuously and normally is adopted.

Other steps, such as hydrophilization treatment and antistatic treatment, may be added before, during, and after steps (c) to (f).

(a) Preparation of polyolefin solution

A polyolefin solution obtained by heating and dissolving a polyolefin resin in a plasticizer is prepared. The plasticizer is not particularly limited as long as it is a solvent capable of sufficiently dissolving polyethylene. In order to enable a relatively high magnification of stretching, the solvent is preferably liquid at room temperature. Examples of the liquid solvent include aliphatic, cyclic aliphatic or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, liquid paraffin, and mineral oil fractions having a boiling point corresponding to them, and dibutyl phthalate, dioctyl phthalate, and the like. At room temperature, a liquid phthalic acid ester is mentioned. In order to obtain a gel-like sheet in which the content of the liquid solvent is stable, it is preferable to use a nonvolatile liquid solvent such as liquid paraffin. In the melt-kneaded state, it is mixed with polyethylene, but at room temperature, a solid solvent can be mixed with a liquid solvent. As such a solid solvent, stearyl alcohol, ceryl alcohol, paraffin wax, etc. are mentioned. However, if only a solid solvent is used, there is a concern that uneven stretching or the like may occur.

As for the blending ratio of the polyolefin resin and the plasticizer, the total of the polyolefin resin and the plasticizer is 100% by weight, and from the viewpoint of improving the moldability of the extruded product, 10 to 50% by weight of the polyolefin resin is preferable. The lower limit of the content of the polyolefin resin is more preferably 20% by weight. The upper limit of the content of the polyolefin resin is more preferably 40% by weight, more preferably 35% by weight. When the content of the polyolefin resin is 10% by weight or more, since the swell or neck in at the exit of the die is small when forming into a sheet shape, the formability and film forming property of the sheet are improved. In addition, when the content of the polyolefin resin is 50% by weight or less, the shrinkage in the thickness direction is small, so that the molding processability and film forming property are improved. When the content of the polyolefin resin is within this range, it becomes easy to obtain both the piercing strength and the air permeation resistance by the film forming method described later. In addition, if the content of the polyolefin resin is in the range already described, since the progress of crystallization due to the plasticizing effect becomes favorable, it becomes easy to control the crystal structure of the front and back of the film, and thus the friction between the front and back of the film by the film forming method described later The coefficient can also be controlled.

The viscosity of the liquid solvent (plasticizer) is preferably 20 to 200 cSt at 40°C. When the viscosity of the liquid solvent at 40°C is 20 cSt or more, the thickness of the sheet obtained by extruding the polyolefin solution from the die is unlikely to be uneven. On the other hand, when the viscosity of the liquid solvent is 200 cSt or less, it is easy to remove the liquid solvent.

The homogeneous melt-kneading of the polyolefin solution is not particularly limited, but when it is desired to prepare a high-concentration polyolefin solution, it is preferably performed in an extruder, particularly a twin screw extruder. If necessary, various additives such as antioxidants may be added to the polyolefin solution within a range that does not impair the effects of the present invention. In particular, it is preferable to add an antioxidant to prevent oxidation of the polyethylene.

In the extruder, the polyolefin solution is uniformly mixed at a temperature at which the polyolefin resin is completely melted. The melt-kneading temperature varies depending on the polyolefin resin to be used, but the lower limit is preferably (melting point of the polyolefin resin + 10°C), and more preferably (melting point of the polyolefin resin + 20°C). The upper limit of the melt-kneading temperature is preferably (the melting point of the polyolefin resin + 120°C), more preferably (the melting point of the polyolefin resin + 100°C). Here, the melting point refers to a value measured by DSC based on JIS K7121 (1987) (hereinafter the same). For example, specifically, since the polyethylene composition has a melting point of about 130 to 140°C, the lower limit of the melt-kneading temperature is preferably 140°C, more preferably 160°C, and most preferably 170°C. The upper limit of the melt-kneading temperature of the polyethylene composition is preferably 250°C, 230°C, and most preferably 200°C.

Further, the melt-kneading temperature in the case of containing polypropylene in the polyolefin solution is preferably 190 to 270°C.

From the viewpoint of suppressing deterioration of the resin, it is preferable that the melt-kneading temperature is lower, but if it is lower than the above temperature, an unmelted substance is generated in the extruded product extruded from the die, causing fractures, etc. in the later stretching process. In some cases, when the temperature is higher than the above temperature, the thermal decomposition of the polyolefin becomes violent, and the physical properties of the resulting microporous membrane, such as piercing strength and tensile strength, may be inferior.

The ratio (L/D) of the screw length (L) and the diameter (D) of the twin-screw extruder is preferably 20 to 100 from the viewpoint of obtaining good process kneading properties and dispersion component composition of the resin. The lower limit of the ratio is more preferably 35. The upper limit of the ratio is more preferably 70. When L/D is 20 or more, melt-kneading becomes sufficient. When L/D is 100 or less, the residence time of the polyolefin solution does not increase too much. From the viewpoint of preventing deterioration of the resin to be kneaded and obtaining good dispersion component composition, the inner diameter of the cylinder of the twin-screw extruder is preferably 40 to 100 mm.

In order to disperse polyethylene well in the extrudate and obtain excellent uniformity in thickness of the microporous membrane, it is preferable to set the screw rotation speed (Ns) of the twin-screw extruder to 150 to 600 rpm. In addition, it is preferable that the ratio of the extrusion amount (Q) (kg/h) of the polyolefin solution to Ns (rpm) and Q/Ns be 0.6 kg/h/rpm or less. Q/Ns is more preferably 0.35 kg/h/rpm or less.

(b) Formation of extruded product and molding of gel-like sheet

The polyolefin solution melt-kneaded with an extruder is extruded from a die directly or through another extruder, and molded so that the microporous membrane of the final product has a thickness of 5 to 100 µm to obtain an extruded product. As for the die, a rectangular T die can be used. In the case of using the T-die, from the viewpoint of easy control of the thickness of the microporous film of the final product, the slit spacing of the die is preferably 0.1 to 5 mm, and heating at 140 to 250° C. during extrusion is preferable.

A gel-like sheet is obtained by cooling the obtained extrudate, and the microphase of polyethylene separated by a solvent can be fixed by cooling. In the cooling step, it is preferable to cool the gel-like sheet to the crystallization end temperature or lower. Cooling is preferably performed at a rate of 250° C./min or higher, and more preferably at a rate of 300° C./min or higher until both the front and back sides of the gel-like sheet become equal to or lower than the crystallization end temperature. When the cooling rate is within the above range, the crystals forming the gel are not coarsened, and a dense high-order structure can be obtained, so that the roughness of the surface is unlikely to be uneven. In addition, when the cooling rate is within the above range, since the higher-order structure is fine, molecular orientation tends to proceed in subsequent stretching, so that both protrusion strength and air permeability resistance can be achieved. In addition, when the cooling rate is within the above range, the presence of fine crystals densely makes it possible to finely roughen the surface of the finally obtained microporous membrane, so that the target friction coefficient can be controlled. When the cooling rate is too low, the crystals become too coarse, so that it becomes difficult to obtain the target coefficient of friction. Here, the crystallization end temperature means the extrapolated crystallization end temperature measured according to JIS K7121 (1987). Specifically, polyethylene has an extrapolated crystallization end temperature of about 70 to 90°C. In addition, the cooling rate here can be calculated|required by the time until the temperature of the resin discharged|emitted from the exit of the extruder reaches the crystallization completion temperature, and the temperature difference between the resin temperature and the crystallization completion temperature at the exit of the extruder. Therefore, in the case of cooling down to the crystallization end temperature or lower in the cooling process, the cooling rate of each of the front and back sides of the gel-like sheet is the difference between the resin temperature at the outlet of the extruder and the temperature of the gel-like sheet at the front and back of the cooling process outlet, It is obtained by dividing the cooling process by the time that a part at a certain arbitrary position in the gel-like sheet passes through. In addition, it is preferable that the difference between the cooling rate of one side (surface) of the gel-like sheet and the cooling rate of the other side (back side) is 15°C/sec or more. By respectively controlling the cooling rates at the front and back of the gel-like sheet so that the difference in cooling rate is 15°C/sec or more, a microporous membrane having a static friction coefficient of 0.5 to 1.0 when the front and back of the membrane are stacked can be obtained.

As a cooling method of the extrudate, there are methods of direct contact with cold air, cooling water, or other cooling medium, a method of contacting a roll cooled with a refrigerant, a method of using a casting drum, and the like. In order to obtain, a method using a casting drum is preferred. In addition, when using the casting drum, cold air, cooling water or other cooling medium, a roll cooled with a refrigerant, or the like can also be used together. Further, the solution extruded from the die is withdrawn at a predetermined take-up rate before or during cooling, but the lower limit of the take-up ratio is preferably 1 or more. The upper limit is preferably 10 or less, more preferably 5 or less. If the take-up ratio is 10 or less, the neck-in becomes small, and it becomes difficult to cause breakage during stretching.

The lower limit of the thickness of the gel-like sheet is preferably 0.5 mm, more preferably 0.7 mm. The upper limit of the thickness of the gel-like sheet is 3 mm, more preferably 2 mm. When the thickness of the gel-like sheet is 3 mm or less, in the cooling process, it is difficult to cause unevenness of the structure from the outermost layer to the inner layer of the film, so that a high-order structure can be made dense throughout the thickness direction. In addition, when the thickness of the gel-like sheet is 3 mm or less, it is easy to set the cooling rate of the gel-like sheet to the above preferable range.

Until now, the case where the microporous membrane is a single layer has been described, but the polyolefin microporous membrane of the present invention is not limited to a single layer, and may be a laminate in which several microporous membranes (layers) are laminated. In the layer to be further laminated, as described above, in addition to polyethylene, each desired resin may be contained so as not to impair the effects of the present invention. As a method of forming a polyolefin microporous membrane as a laminate, a conventional method can be used, but for example, a desired resin is prepared as necessary, and these resins are individually supplied to an extruder to melt at a desired temperature, and a polymer tube Alternatively, there is a method of forming a laminate, such as joining in a die and extruding from a slit-shaped die at respective desired laminate thicknesses.

(c) and (d) stretching

In the present invention, the obtained gel sheet is stretched in the longitudinal direction (machine direction) (step (c)), and then successively stretched in the transverse direction (direction perpendicular to the machine direction) (step (d)). . In this way, by performing the longitudinal stretching and the transverse stretching individually and continuously, it becomes easy to obtain both the piercing strength and the air permeation resistance, and to obtain a predetermined coefficient of friction. Stretching is performed by heating a gel-like sheet and performing a conventional tenter method, a roll method, or a combination of these methods at a predetermined magnification. In addition, such stretching is arranged so as to be adjacent to each other in the manufacturing direction of the microporous membrane (direction from the extruder side to the winding side of the microporous membrane) with the vertical stretching machine stretching the gel-like sheet in the vertical direction and the horizontal stretching machine stretching in the horizontal direction. It is carried out continuously using a vertical stretching machine and a horizontal stretching machine.

In the stretching method of the present invention, since the longitudinal stretching and the lateral stretching are separately performed, the molecular orientation tends to proceed by applying a stretching tension only in each direction in each stretching step. For this reason, molecular orientation can be made high even at the same area magnification compared with simultaneous stretching, and high piercing strength can be achieved.

Although the draw ratio varies depending on the thickness of the gel-like sheet, it is preferable to draw at least 5 times in any direction. The stretching in the longitudinal direction is preferably 5 times or more, more preferably 7 times or more. In addition, the upper limit of the stretching in the longitudinal direction is preferably 12 times, more preferably 10 times. When the stretching in the longitudinal direction is 5 times or more, high strength can be imparted by the stretching orientation. Further, if the stretching in the longitudinal direction is 12 times or less, tearing due to stretching is unlikely to occur.

The stretching in the transverse direction is preferably 4 times or more, more preferably 6 times or more. The upper limit of stretching in the transverse direction is preferably 10 times, more preferably 8 times. If the draw ratio in the transverse direction is 4 times or more, even higher strength can be provided depending on the draw orientation. In addition, if the draw ratio in the transverse direction is 10 times or less, tearing due to stretching is difficult to occur, and it is possible to prevent the surface from becoming smooth due to crushing of irregularities on the surface of the film by stretching, so that the desired coefficient of friction is obtained. It becomes easy to lose.

In the area magnification obtained by combining the longitudinal stretching and transverse stretching, 25 times or more is preferable, more preferably 30 times or more, and most preferably 42 times or more.

The stretching temperature is preferably equal to or lower than the melting point of the polyolefin resin, and more preferably in the range of (crystal dispersion temperature (Tcd) of the polyolefin resin) to (melting point of the polyolefin resin). When the stretching temperature is less than or equal to the melting point of the gel-like sheet, melting of the polyolefin resin is prevented, and it becomes possible to efficiently orient molecular chains by stretching. In addition, when the stretching temperature is equal to or higher than the crystal dispersion temperature of the polyolefin resin, the polyolefin resin is sufficiently softened and the stretching tension is low, so that the film forming property is improved, and the stretching at a high magnification that is unlikely to be broken at the time of stretching becomes possible.

Specifically, since the polyethylene resin has a crystal dispersion temperature of about 90 to 100°C, the longitudinal stretching temperature is preferably 80°C or higher. In the case of using a polyethylene resin, the upper limit of the longitudinal stretching temperature is preferably 130°C, more preferably 125°C, and most preferably 120°C. The crystal dispersion temperature (Tcd) is obtained from the dynamic viscoelastic temperature characteristics measured according to ASTM D 4065. Alternatively, the crystal dispersion temperature (Tcd) may be determined from NMR.

Cleavage occurs in the higher order structure formed in the gel-like sheet by the above stretching, and the crystal phase is refined to form a large number of fibrils. Fibrils form a three-dimensional, irregularly connected network structure. Since the mechanical strength is improved by stretching and the pores are enlarged, it is suitable for a battery separator.

In addition, in the present invention, it is important to perform successive stretching before removing the plasticizer in the gel-like sheet. When the plasticizer is sufficiently contained in the gel-like sheet, the polyolefin is sufficiently plasticized and softened, so that the cleavage of the higher-order structure becomes smooth by stretching before removal of the plasticizer, and the crystal phase can be refined uniformly.

(e) Extraction (cleaning) of plasticizer from the stretched film

Next, the solvent remaining in the gel-like sheet is extracted and removed, that is, washed using a cleaning solvent. Since the polyolefin phase and the solvent phase are separated, a microporous membrane is obtained by removal of the solvent. Examples of the cleaning solvent include saturated hydrocarbons such as pentane, hexane, and heptane, chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, ethers such as diethyl ether and dioxane, ketones such as methyl ethyl ketone, ethane trifluoride, C ₆ Chain fluorocarbons such as F ₁₄ , C ₇ F ₁₆ , cyclic hydrofluorocarbons such as C ₅ H ₃ F ₇ , hydrofluoroethers such as C ₄ F ₉ OCH ₃ , C ₄ F ₉ OC ₂ H ₅ , Nonvolatile solvents, such as perfluoroethers, such as C ₄ F ₉ OCF ₃ and C ₄ F ₉ OC ₂ F _{5, are mentioned.} These cleaning solvents have a low surface tension (eg, 24 mN/m or less at 25°C). By using a cleaning solvent having a low surface tension, the network structure forming the microporous is suppressed from shrinking due to the surface tension of the gas-liquid interface upon drying after cleaning, thereby obtaining a microporous membrane having high porosity and permeability. Lose. These cleaning solvents are appropriately selected depending on the solvent used to dissolve the polyolefin resin, and are used alone or in combination.

The washing method can be performed by a method of immersing the gel-like sheet in a cleaning solvent for extraction, a method of showering a cleaning solvent in the gel-like sheet, or a combination thereof. The amount of the cleaning solvent to be used varies depending on the cleaning method, but is generally preferably 300 parts by weight or more with respect to 100 parts by weight of the gel sheet. The washing temperature may be 15 to 30°C, and heating to 80°C or less as necessary. At this time, from the viewpoint of enhancing the cleaning effect of the solvent, from the viewpoint of preventing non-uniform properties of the microporous membrane in the transverse direction and/or the longitudinal direction of the physical properties of the resulting microporous membrane, and from the viewpoint of improving the mechanical and electrical properties of the microporous membrane, the gel-like sheet The longer the time is immersed in the cleaning solvent, the better.

It is preferable to perform washing as described above until the residual solvent in the gel-like sheet after washing, that is, the microporous membrane becomes less than 1% by weight.

(f) drying of the microporous membrane

After washing, the cleaning solvent is dried and removed. The drying method is not particularly limited, but is dried by a heat drying method, a wind drying method, or the like. The drying temperature is preferably less than or equal to the crystal dispersion temperature (Tcd) of the polyethylene composition, and particularly preferably less than or equal to (Tcd-5°C). Drying is preferably performed until the dry weight of the microporous membrane is 100% by weight, the remaining cleaning solvent is 5% by weight or less, and more preferably 3% by weight or less. If drying is insufficient, the porosity of the microporous membrane decreases in the subsequent heat treatment, and the permeability deteriorates.

(g) other processes

Here, in general, in order to improve the mechanical strength, such as the piercing strength, after washing and drying, stretching of about 5% to 20% (hereinafter referred to as re-stretching) is additionally performed vertically, horizontally, or in both directions. have. However, when re-stretching is performed, the irregularities formed on both the front and back sides of the microporous membrane are elongated, and it becomes difficult to obtain a desired coefficient of friction. In other words, in the stretched film before removing the plasticizer, the plasticizer is contained not only in the interior but also in the front and rear surfaces as described above. Therefore, by going through the process of extracting the plasticizer from the stretched film, voids are formed in the interior of the microporous membrane, and irregularities are formed well in the front and rear surfaces of the microporous membrane as much as the portion of the space from which the plasticizer was removed. For this reason, when the microporous membrane is restretched after extraction of the plasticizer, the irregularities on the front and back surfaces of the microporous membrane are lengthened to increase the smoothness, so that the friction coefficient on the front and back surfaces of the microporous membrane decreases. On the other hand, when such re-stretching is performed, mechanical strength such as piercing strength increases. Therefore, in the present invention, in order to obtain a microporous membrane having as high mechanical strength as possible, it is preferable to perform the re-stretching previously described, but in order to obtain a microporous membrane in which the friction coefficient is set within the range described later, the drying step of the microporous membrane ( After f), it is preferable to wind up the microporous membrane around the core without performing re-stretching.

On the other hand, in the present invention, the stretched film or microporous film after stretching may be subjected to heat setting treatment and/or heat relaxation treatment. The crystal is stabilized by the heat setting treatment and the heat relaxation treatment, the lamellar layer is made uniform, and a microporous membrane having a large pore diameter and excellent strength can be produced. The heat setting treatment is performed within a temperature range of not less than the crystal dispersion temperature to not more than the melting point of the polyolefin resin constituting the polyolefin microporous film. The heat setting treatment is performed by a tenter method, a roll method, or a rolling method.

As the heat relaxation treatment method, for example, a method disclosed in Japanese Laid-Open Patent Publication No. 2002-256099 can be used.

In addition, depending on other uses, the stretched film or the microporous film can be subjected to a hydrophilic treatment. Hydrophilization treatment can be performed by monomer graft, surfactant treatment, corona discharge, or the like. It is preferable to perform the monomer graft after crosslinking treatment.

In the case of surfactant treatment, any of a nonionic surfactant, a cationic surfactant, an anionic surfactant, and an amphoteric surfactant can be used, but a nonionic surfactant is preferred. A microporous membrane is immersed in a solution obtained by dissolving a surfactant in water or a lower alcohol such as methanol, ethanol, or isopropyl alcohol, or the solution is applied to the microporous membrane by a doctor blade method.

If necessary, corona discharge treatment may be performed on at least one surface of the stretched film or microporous film in air or nitrogen or in a mixed atmosphere of carbon dioxide and nitrogen.

After each step described above is completed, a microporous membrane is wound around the core to obtain a wound body.

[3] Structure and properties of polyolefin microporous membranes

As a preferred embodiment of the polyolefin microporous membrane of the present invention, there are the following physical properties.

(1) coefficient of friction

In the polyolefin microporous membrane of the present invention, the coefficient of static friction between the membranes is 0.5 to 1.0, more preferably 0.7 or more. Here, the coefficient of static friction between the membranes refers to the coefficient of static friction measured by making one surface (surface) of the polyolefin microporous membrane and the surface of the opposite side (rear surface) face to face (overlapping). By setting the coefficient of static friction in the above range, it is possible to provide a polyolefin microporous membrane having a good wound shape without wrinkles or protrusions in the cross section when a rolled body of the polyolefin microporous membrane is produced. If the static friction coefficient is 0.5 or more, the grip strength of the front and back of the polyolefin microporous membrane can be secured, and even if the microporous membrane after film formation is conveyed at high speed when slit, it is difficult to cause slipping in the unwinding part of the wound body. You can restrain your progress. In addition, when the coefficient of static friction is 1.0 or less, since the sliding property of the front and back of the membrane is good, blocking in the wound body of the microporous membrane after film formation can be suppressed, and when the microporous membrane is slit, the unwinding portion of the wound body Since the tension fluctuation in is less likely to occur, the occurrence of wrinkles in the winding body (after the slit) can be suppressed even when winding is performed at high speed. For this reason, particularly in a wound body having a long winding length and a large number of laminated films, the microporous membrane having a coefficient of static friction of the present invention has a remarkable effect. In addition, when the microporous membrane winding body finally obtained is used by setting the static friction coefficient in the above range, the unwinding behavior of the microporous membrane is stabilized as in the case of slit of the microporous membrane, and in subsequent battery creation, deviation from the electrode In addition, it is possible to suppress the occurrence of wrinkles in the microporous membrane. In addition, the static friction coefficient refers to a value measured by a measurement method described later.

(2) Speculation resistance

The upper limit of the air permeation resistance of the polyolefin microporous membrane of the present invention is 400 sec/100 cc Air/16 µm, more preferably 300 sec/100 cc Air/16 µm, more preferably when the film thickness is 16 µm. 200 sec/100 cc Air/16 µm, and the lower limit of the air permeability resistance is 50 sec/100 cc Air/16 µm, preferably 70 sec/100 cc Air/16 µm, more preferably 100 sec/100 cc Air /16 μm. When the air permeability resistance is 400 sec/100 cc Air/16 µm or less, ion permeability is good, and charging and discharging can be performed at high speed. In addition, when the air permeation resistance is 50 sec/100 cc Air/16 µm or more, deterioration of the battery can be prevented.

(3) Breakthrough strength

The piercing strength of the polyolefin microporous membrane of the present invention is 400 gf/16 µm or more, and preferably 450 gf/16 µm. If the piercing strength is 450 gf/16 µm or more, when the polyolefin microporous membrane is incorporated in the battery as a separator, a short circuit of the electrode does not occur, and the safety of the battery is improved.

(4) The ratio of penetration strength and speculative resistance

The lower limit of the ratio of the piercing strength and the air permeation resistance of the polyolefin microporous membrane of the present invention, (piercing strength [gf]/permeation resistance [sec/100 cc Air]: both in terms of film thickness of 16 μm) is preferably 1.7, More preferably, it is 2.0. It is preferable that the lower limit of the ratio of the penetration strength and the air permeation resistance is 3.0. Since the ratio of the piercing strength and the air permeability resistance is 1.7 or more and 3.0 or less, the balance between safety and ion permeability is excellent when a polyolefin microporous membrane is used as a separator and incorporated into a battery.

(5) public ratio

With respect to the porosity of the polyolefin microporous membrane of the present invention, the upper limit is preferably 70%, more preferably 60%, and most preferably 55%. The lower limit of the porosity is preferably 30%, more preferably 35%, and most preferably 40%. When the porosity is 70% or less, sufficient mechanical strength and insulating properties are easily obtained, and short circuit hardly occurs during charging and discharging. In addition, when the porosity is 30% or more, ion permeability is good, and good charge/discharge characteristics of a battery can be obtained.

(6) The thickness of the polyolefin microporous membrane

The upper limit of the thickness of the polyolefin microporous membrane used in the present invention is preferably 30 µm. Further, the upper limit of the thickness of the polyolefin microporous membrane is preferably 16 µm, most preferably 12 µm. The lower limit of the thickness of the polyolefin microporous membrane is 5 µm, preferably 6 µm. When the thickness of the polyolefin microporous membrane is within the above range, practical protrusion strength and hole blocking function can be maintained, and thus it is suitable for higher capacity of the battery to be advanced in the future.

(7) Polyolefin microporous membrane winding body

It is preferable that the microporous membrane wound body obtained in the present invention has a width of 300 mm or more and a diameter of 150 mm or more. Further, the core (core) for winding the microporous membrane preferably has an inner diameter of 76 mm or more, and more preferably 152 mm or more. The difference between the inner diameter and the outer diameter of the core is preferably 5 mm or more and 50 mm or less, and is adjusted according to the strength of the material to be used. The tolerance between the inner diameter and the outer diameter of the core is preferably ± 0.5 mm or less, more preferably ± 0.3 mm or less. Moreover, as a material of a core, paper, plastic, fiber-reinforced composite material, etc. are mentioned. That is, the microporous membrane winding body of the present invention is formed by winding the microporous membrane a plurality of turns along the outer circumferential surface of the approximately cylindrical core. Therefore, the "width" of the winding body refers to the distance between two circular surfaces facing each other in parallel with the circumferential surface interposed therebetween among the outer surfaces of the winding body. In addition, the "diameter" of the wound body is synonymous with the diameter of the circular surface. In addition, as for the microporous membrane wound body obtained in this invention, it is preferable that the number of lamination|stacking sheets of the film (microporous membrane) wound around a core is 1500 or more. If the dimension of the width of the winding body is within the above range, it can be suitably used for the enlargement of the battery to be carried out in the future. In addition, when the winding body is coated with a heat-resistant resin, etc. on the microporous, if the diameter of the winding body is within the above range, since the winding body has a sufficient winding length, the frequency of replacement of the microporous membrane winding body during coating can be reduced, and the width Because of this wide area, the proportion of the portion lost by trimming in the slit after coating can be reduced, and thus the cost is excellent. In addition, in the present specification, coating means forming a heat-resistant resin or the like on a microporous membrane, and is different from adding a lubricant such as inorganic particles to a polyolefin resin used as a raw material for the microporous membrane. In addition, the diameter of a winding object is the diameter of the whole microporous membrane winding object including the diameter of a core.

[4] Use

The polyolefin microporous membrane of the present invention is suitable as a separator (isolator) for an electrochemical reaction device such as a battery or a capacitor. Among these, it can be suitably used as a separator for a non-aqueous electrolyte-based secondary battery, particularly a lithium secondary battery.

[5] Method of measuring physical properties

Hereinafter, a method of measuring each property will be described.

(1) Thickness (average film thickness)

The polyolefin microporous membrane was cut out to a size of 10 cm x 10 cm, and 16 points were measured at intervals of 3 cm in width and length, and the average value was taken as the thickness (µm). For the measurement, a contact type thickness measuring system was used.

(2) Speculation resistance

Measurement was carried out in accordance with JIS P8117 using an Oken type air permeation resistance meter (EGO-1T, manufactured by Asahi Seko Co., Ltd.).

(3) Penetration strength of the polyolefin microporous membrane

The maximum load was measured when a microporous membrane having a film thickness (T1) (µm) was traversed at a speed of 2 mm/sec with a needle having a diameter of 1 mm having a spherical tip (curvature radius (R): 0.5 mm). The measured value (La) of the maximum load is converted into the maximum load (Lb) when the film thickness is 16 µm by the formula: Lb = (La x 16)/T1, and the piercing strength (gf/16 µm) It was made into.

(4) The coefficient of friction of the polyolefin microporous membrane

According to JIS K7125 (1999), the test direction was made parallel to the longitudinal direction of the polyolefin microporous, and the front and back of a polyolefin microporous membrane were combined, and it measured. However, the relative speed of the sliding piece was 100 mm/min, the mass of the auxiliary plate was 5 g, and the total mass of the sliding piece was 200 g.

(5) Determination of the appearance of the rolled object

The obtained polyolefin microporous membrane was evaluated according to the state of wrinkles and the degree of winding misalignment when slit and wound at a running speed of 150 m/min and a tension of 32 N/m using a slitter FN335E manufactured by Nishimura Seisakusho Co., Ltd. . The criteria for determination were as described below. In addition, "shift in the cross section of the winding object" or "winding out of position" refers to a value obtained by the following measurement. Specifically, after creating the winding body, for each of the left and right ends of the winding body, among the plurality of cross-sections of the microporous membrane laminated on the core, the cross-section most protruding to the outside of the winding body and the inner side of the winding body most entered. The distance between the cross-sections that are present is measured in the width direction of the wound body, and this measurement result is evaluated as “shift in the cross section of the wound body” or “displaced winding” described above.

◎(Excellent): The deviation in the cross section of the wound body is in the range of 0 to 1 mm in both the left and right sides, and there is no occurrence of wrinkles on the surface layer of the winding body.

○ (Good): The deviation in the cross section of the winding body is in the range of 0 to 3 mm in both the left and right sides, and there is no occurrence of wrinkles on the surface layer of the winding body.

X (defect): The deviation in the cross section of the wound body is greater than 3 mm in at least one of the left and right, or wrinkles have occurred in the surface layer of the winding body.

(6) Weight average molecular weight (Mw)

Mw of UHMWPE and HDPE was determined by gel permeation chromatography (GPC) under the following conditions.

-Measuring device: GPC-150C manufactured by Waters Corporation

-Column: Showa Denko Corporation Shodex UT806M

-Column temperature: 135 °C

-Solvent (mobile phase): o-dichlorobenzene

-Solvent flow rate: 1.0 ml/min

-Sample concentration: 0.1% by mass (dissolution condition: 135 ℃/1h)

-Injection volume: 500 µl

-Detector: Differential refractometer manufactured by Waters Corporation

-Calibration curve: A calibration curve obtained using a monodisperse polystyrene standard sample was prepared using a predetermined conversion constant.

(7) Public ratio (%)

The polyolefin microporous membrane was cut out to a size of 5 cm×5 cm, and the volume (cm 3) and mass (g) were calculated, and calculated from these and the membrane density (g/cm 3) using the following equation.

Porosity = ((volume-mass/film density)/volume)×100

Here, the film density was set to 0.99. In addition, for the calculation of the volume, the thickness measured in (1) above was used.

(8) Number of stacked polyolefin microporous membranes

_{The number of stacked polyolefin microporous membranes (X) was measured by measuring the outer diameter (R 1} ) (mm) of the laminated portion of the microporous membrane of the polyolefin microporous membrane winding body, and the outer diameter of _{the core used (R 2} ) (mm) and the polyolefin microporous membrane. It was calculated using the following equation from the thickness (T) (µm) and the air bite rate (AD) of the wound body.

X=((R ₁ -R ₂ )/2)/(AD+1))/(T/1000)

Here, the air entrainment rate (AD) is the theoretical diameter when there is no air entrainment, and the side cross-sectional area of the roll (St) (㎡) calculated from the thickness (T) (㎛) and the length of the winding body (L) (m) And the roll side cross-sectional area (Sr) (m ₂ ) calculated from each of the actual diameters (R ₁ ) and diameters (R 2 ), by the following equation.

Sr=3.14×(R ₁ /2) ² -3.14×(R ₂ /2) ²

St=(T/1000)×L

AD=Sr/St-1

Example

Hereinafter, examples are shown and described in detail, but the present invention is not limited at all by these examples.

Example 1

<Polyolefin microporous membrane>

Tetrakis in 100 parts by mass of a polyethylene (PE) composition comprising 40 mass% of ultra-high molecular weight polyethylene (UHMWPE) having a mass average molecular weight (Mw) of 2.5×10 ^{6 and 60 mass% of a high-density polyethylene (HDPE) having a Mw of 2.8×10 5} 0.375 parts by mass of [methylene-3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate]methane was dry-blended to obtain a mixture.

25 parts by mass of the obtained mixture was put into a twin-screw extruder of a strong kneading type (amount of polyethylene composition (Q): 54 kg/h), and 75 parts by mass of liquid paraffin was supplied from the side feeder of the twin-screw extruder, and screw rotation speed (Ns) Was melt-kneaded at a temperature of 210°C while maintaining at 180 rpm (Q/Ns: 0.3 kg/h/rpm) to prepare a polyethylene solution.

The obtained polyethylene solution was supplied from a twin-screw extruder to a T-die, and extruded to form a sheet-shaped molded body. The extruded molded body was cooled while being taken up with a cooling roll controlled at 35°C to form a gel-like sheet. Here, the cooling roll contact surface was the front surface and the non-contact surface was the rear surface, and the cooling rate of the surface was 399°C/min, and the cooling rate of the back surface was 380°C/min. The obtained gel-like sheet was vertically stretched by a roll method so as to increase to 9 times at a stretching temperature of 115°C, and subsequently guided by a tenter, followed by transverse stretching at a stretching ratio of 6 times and a stretching temperature of 115°C. The film after stretching was washed in a washing tank of methylene chloride temperature-controlled to 25°C to remove liquid paraffin. The washed membrane was dried in a drying furnace adjusted to 60° C., and heat-fixed at 125° C. for 40 seconds in a tenter to obtain a microporous membrane having a thickness of 16 μm. The obtained microporous membrane was slit to 300 mm in width and 2000 m in length, and wound around an ABS core (inner diameter 152.4 mm, outer diameter 200.0 mm) to prepare a polyolefin microporous membrane wound body.

Examples 2 to 14, Comparative Examples 1, 2, 8 to 10

A polyolefin microporous membrane wound body was produced in the same manner as in Example 1 except that the Mw of the UHMWPE used, the resin composition, the film forming conditions, the slit width, the length, and the number of laminated films wound on the core were changed as shown in Tables 1 and 2.

Comparative Example 3

Only HDPE having an Mw of 3.8×10 ^{5 was} used, and a gel-like sheet was produced under the same extrusion conditions as in Example 1. The produced gel-like sheet was vertically stretched at a stretching temperature of 115°C so as to be 9 times, and then horizontally stretched at a stretching temperature of 120°C so that the draw ratio was 6 times. The film after stretching was washed in a washing tank of methylene chloride controlled at a temperature of 25°C, and in the extraction process of extracting liquid paraffin, tension was applied in the vertical direction to stretch 3%, and shrink about 12% in the horizontal direction. The washed membrane was dried in a drying furnace adjusted to 60° C., re-stretched to 120% in the horizontal direction at 125° C. in a tenter, contracted 16.7%, and heat-set for 40 seconds to obtain a microporous membrane having a thickness of 16 μm. The obtained microporous membrane was slit to a width of 300 mm and a length of 2000 m, and was wound around an ABS core (inner diameter of 152.4 mm, outer diameter of 200.0 mm) to prepare a polyolefin microporous membrane wound body.

Comparative Example 4

A gel-like sheet was produced under the same extrusion conditions as in Example 1, except that 35 mass% of UHMWPE having a Mw of 2.5×10 ⁶ and 65 mass% of HDPE having a Mw of 3.1×10 ^{5 were used, and 84 parts by mass of liquid paraffin were used.} The produced gel-like sheet was simultaneously biaxially stretched at a stretching temperature of 115°C so as to be 5 times each in the longitudinal direction and transverse direction. After stretching, the membrane was washed, air-dried, and heat-fixed in the same manner as in Example 1 to obtain a microporous membrane having a thickness of 16 μm. The obtained microporous membrane was slit to a width of 300 mm and a length of 2000 m, and was wound around an ABS core (inner diameter of 152.4 mm, outer diameter of 200.0 mm) to prepare a polyolefin microporous membrane wound body.

Comparative Example 5

In the same manner as in Comparative Example 3, except that 30% by mass of UHMWPE having a Mw of 2.5×10 ⁶ and 70% by mass of HDPE having a Mw of 2.8×10 ⁵ were used, and the vertical draw ratio was set to 5 times and the lateral draw ratio was set to 6 times, A microporous membrane having a thickness of 16 µm was obtained. The obtained microporous membrane was slit to a width of 300 mm and a length of 2000 m, and was wound around an ABS core (inner diameter of 152.4 mm, outer diameter of 200.0 mm) to prepare a polyolefin microporous membrane wound body.

Comparative Example 6

After air drying, a microporous membrane having a thickness of 16 µm was obtained in the same manner as in Comparative Example 4 except that re-stretching was performed 1.2 times in the horizontal direction at 128°C. The obtained microporous membrane was slit to 300 mm in width and 2000 m in length, and wound around an ABS core (inner diameter 152.4 mm, outer diameter 200.0 mm) to prepare a polyolefin microporous membrane wound body.

Comparative Example 7

A gel-like sheet was produced under the same extrusion conditions as in Example 1 except that only UHMWPE having a Mw of 2.5×10 ^{6 was used and 70 parts by mass of liquid paraffin was used.} The resulting gel sheet was washed in a methylene chloride washing tank temperature-controlled at 25°C, and liquid paraffin was extracted, and then the washed film was air-dried under reduced pressure at room temperature. The obtained unstretched sheet was simultaneously biaxially stretched at a temperature of 120 deg. C so as to be 6 times each in the longitudinal direction and transverse direction, and then heat-fixed at 140 deg. C for 1 minute in a tenter to obtain a microporous membrane having a thickness of 16 mu m. The obtained microporous membrane was slit to a width of 300 mm and a length of 2000 m, and wound around an ABS core (inner diameter of 152.4 mm, outer diameter of 200.0 mm) to produce a polyolefin microporous membrane wound body.

The resin composition and film forming conditions of the polyolefin microporous membranes obtained in Examples 1 to 14 and Comparative Examples 1 to 10 are shown in Table 1, and physical properties are shown in Table 2.

<Table 1>

<Table 2>

From Table 1, the polyolefin microporous membranes of Examples 1 to 14 are relatively wide, and even in a state where the number of stacked membranes is large, the penetration strength and the air permeation resistance are both balanced, and the coefficient of static friction of the front and back of the membrane can be controlled. In addition, it is possible to obtain a winding body having an excellent appearance without windings out of the way.

Claims (8)

Characterized in that the piercing strength in terms of thickness of 16 µm is 400 gf or more, the air permeation resistance in terms of thickness of 16 µm is 100 to 400 sec/100 cc, and the coefficient of static friction when the front and back of the membrane is stacked is 0.61 to 1.0. Polyolefin microporous membrane. The method of claim 1,
A polyolefin microporous membrane having a ratio of piercing strength and air permeability resistance of 1.7 [(gf)/(sec/100cc)] to 3.0[(gf)/(sec/100cc)]. The method according to claim 1 or 2,
A polyolefin microporous membrane in which the polyolefin is polyethylene containing ultra-high molecular weight polyethylene having a weight average molecular weight of 2.0 to 10 ^{6 or more.} A separator for a non-aqueous electrolyte-based secondary battery comprising the polyolefin microporous membrane according to claim 1 or 2. A polyolefin microporous membrane having a piercing strength of 400 gf or more in terms of thickness of 16 μm, an air permeability resistance of 100 to 400 sec/100 cc in terms of thickness of 16 μm, and a static friction coefficient of 0.61 to 1.0 when the front and back of the membrane are stacked. The number of stacked polyolefin microporous membranes wound on the core and having a width of 300 mm or more, and the number of stacked polyolefin microporous membranes wound on the core is 1,500 or more. In this case, the polyolefin microporous membrane wound body is 0 to 3 mm in both left and right sides. The method of claim 5,
A polyolefin microporous membrane wound body in which the polyolefin microporous membrane is a separator for a non-aqueous electrolyte-based secondary battery. A nonaqueous electrolyte secondary battery comprising the separator for a nonaqueous electrolyte secondary battery according to claim 4. (a) a step of melt-kneading a polyolefin resin containing an ultra-high molecular weight polyolefin having a weight average molecular weight of ² to 10 ^{6 and less than 4 to 10 6 and a plasticizer to prepare a polyolefin solution,}
(b) The polyolefin solution obtained in step (a) is extruded from an extruder to form an extruded product, and the cooling rate of both the front and the back of the extrudate is 250°C/min or more, and the cooling rate difference between the front and the back is 15°C/sec or more. To form a gel-like sheet,
(c) a step of stretching the sheet obtained in step (b) in the longitudinal direction (machine direction),
(d) a step of stretching the sheet obtained in step (c) in a transverse direction (a direction perpendicular to the machine direction),
(e) a step of extracting a plasticizer from the stretched film obtained in step (d),
(f) including the step of drying the film obtained in step (e),
The method for producing a polyolefin microporous membrane according to claim 1 or 2, wherein the step (c) and the step (d) are each performed continuously.