JP2015208894A - Polyolefin-made laminated microporous film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separator film which achieves excellent safety in a safety test, typically a nail penetration test of a battery, for further improvement of the safety of a battery.SOLUTION: A polyolefin-made laminated microporous film consists of at least three layers, has a meltdown temperature of 159-200°C, an air permeability of 50-300 s and a piercing strength of 100-550 gf and contains polypropylene in an inner layer alone, and at least one layer constituting the surface layer contains a resin of a melt flow rate of 50-150 g/10 min and a melting point of 120-130°C.

Description

  The present invention relates to a separation membrane 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, the present invention relates to a polyolefin microporous membrane suitably used as a lithium ion battery separator.

  Polyolefin microporous membranes are used as microfiltration membranes, fuel cell separators, capacitor separators, 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, digital cameras and the like. The reason is that the polyolefin microporous membrane has excellent membrane puncture strength and shutdown characteristics.

  In particular, lithium-ion battery separators are deeply involved in battery characteristics, battery productivity, and battery safety. Excellent mechanical characteristics, heat resistance, permeability, dimensional stability, pore clogging characteristics (shutdown characteristics), molten film breakage Characteristics (meltdown characteristics) are required. In recent years, development has been promoted aiming at higher energy density, higher capacity, higher output, and safety, and with this, separators also have higher strength (particularly puncture strength), higher permeability, etc. In addition to these requirements, there are increasing demands for improving the hole closing characteristics and heat resistance.

  In such a background, a method of adding a low melting point component for lowering the shutdown temperature to the polyolefin microporous membrane has been disclosed (Japanese Patent No. 5250262, Japanese Translations of PCT International Publication No. 2012-522106).

  Japanese Patent Application Publication No. 2012-521914 discloses a technique for producing a microporous film by stretching a laminate of a layer to which a low-melting-point polymer is added and a layer not containing the layer. Japanese Patent Application Laid-Open No. 2002-321323 discloses a film in which a microporous film containing polyethylene and polypropylene as essential components and a polyethylene microporous film are laminated. However, none of these patents have poor permeability and did not satisfy the requirements of low temperature shutdown, high permeability and high strength.

  Furthermore, recently, lithium ion batteries have been used as power sources for household storage batteries and electric vehicles, and the size of batteries has been increasing. With the increase in size of batteries, lithium ion batteries have been required to have higher safety than before.

  In order to increase the safety of the battery while increasing the shutdown speed, it is also important to improve the heat resistance of the separator. In such a background, a method of adding polypropylene to the microporous polyolefin membrane to increase the heat resistance, that is, to increase the meltdown temperature of the separator film is disclosed (Japanese Patent No. 4931163). Publication, Unexamined-Japanese-Patent No. 2011-63025). However, since these contain polypropylene in the film surface layer, the heat resistance is improved, but the shutdown temperature cannot be sufficiently lowered. Moreover, although the technique which has a polypropylene in the inner layer of a film is disclosed (patent No. 4940351), the technique has not achieved a sufficiently low shutdown temperature. Further, if the technology was known at that time, it was impossible to achieve both a high meltdown temperature, a low shutdown temperature, and an excellent shutdown speed.

  In other words, while maintaining separator productivity and good processability when the film is used as a lithium ion battery separator, the separator shutdown temperature is sufficiently lowered and the separator meltdown temperature is high. It was extremely difficult to achieve both.

WO2007 / 052663 publication Special table 2012-521914 gazette JP 2002-321323 A Japanese Patent No. 5250262 Special table 2012-522106 gazette Japanese Patent No. 4931163 JP 2011-63025 A Japanese Patent No. 4940351

  The subject of this invention is obtaining the separator film which achieves the outstanding safety | security in the safety test represented by the nail penetration test of a battery for the further safety | security improvement of a battery. In other words, in order to achieve both high output characteristics and safety of the battery, it has a low shutdown temperature, excellent puncture strength and air resistance, and has an excellent shutdown speed and an excellent meltdown. It is to provide a polyolefin laminated microporous membrane having a temperature. A separator film having both an excellent shutdown temperature and speed and an excellent meltdown temperature gives the battery higher safety than before.

  As a result of intensive studies in order to solve the above problems, the present inventors made the first layer containing polypropylene without containing a low melting point component as an inner layer, and containing a specific low melting point component without containing polypropylene. The inventors have found that this problem can be solved by the laminated microporous membrane having the following constitution obtained by laminating the two layers as the surface layer, and have reached the present invention. That is, the present invention is as follows.

  (1) Consisting of at least three layers, the meltdown temperature is in the range of 159 to 200 ° C., the air permeability is in the range of 50 to 300 seconds, the piercing strength is in the range of 100 to 550 gf, and only the inner layer is polypropylene. A polyolefin microporous membrane characterized by containing at least one layer containing a resin having a melt flow rate of 50 to 150 g / 10 min and a melting point of 120 to 130 ° C.

  (2) The laminated microporous membrane made of polyolefin as described in (1), wherein the shutdown temperature is in the range of 129.5 to 135.0 ° C.

  (3) The polyolefin microporous multilayer film according to (1) or (2), wherein the shrinkage ratio of TD at 130 ° C. is in the range of −10% to 10%.

(4) The multi-layer polyolefin microporous membrane according to any one of (1) to (3), wherein the shutdown speed is in the range of 1.55 × 10 ^{4 to} 3.00 × 10 ⁴ sec.

  (5) The laminated microporous membrane made of polyolefin according to any one of (1) to (4), wherein the film thickness is in the range of 3 to 25 μm.

  (6) The laminated microporous membrane made of polyolefin according to any one of (1) to (5), wherein the coefficient of static friction between the back surface and the surface of the film is in the range of 0.3 to 1.5.

  The laminated microporous membrane made of polyolefin of the present invention is excellent in puncture strength and air resistance, and by using a separator made of the present invention for a lithium ion battery, excellent safety can be imparted to the battery. Furthermore, since the surface friction coefficient can be kept in a favorable range, when the polyolefin microporous membrane of the present invention is used as a separator for a lithium ion battery, it provides excellent pin-out property in the battery manufacturing process. In addition, it can contribute to an improvement in battery production speed and an improvement in battery manufacturing yield.

It is process schematic of one aspect | mode of the method to manufacture the polyolefin laminated microporous film of this invention. It is detail drawing of the one aspect | mode (Example) of an extending process (2nd step). Temperature-air permeability curve for shutdown speed calculation

Hereinafter, the present invention will be described in detail.
The polyolefin microporous membrane of the present invention comprises at least three layers. By laminating layers having different functions, the obtained laminated microporous membrane can have various functions at the same time. Conventionally, when such different functions are mixed in one layer, these excellent functions may be canceled. By forming the multi-layer polyolefin microporous membrane of the present invention from three or more layers, it is possible to simultaneously impart excellent safety and output characteristics to a battery using the microporous membrane.

It is important that the polyolefin laminated microporous membrane of the present invention does not contain polypropylene in the surface layers on both surfaces of the microporous membrane. When polypropylene is contained in both surface layers, the shutdown temperature of the laminated microporous film tends to be high. Since it is important that the polyolefin laminated microporous membrane of the present invention has a low shutdown temperature, the surface layer does not contain polypropylene.
Here, the fact that polypropylene is not included in the surface layer can be determined by IR analysis (ATR method) or DSC measurement of the surface layer.

  On the other hand, it is important that the inner layer (the inner layer of both surface layers) comprises polypropylene. By containing polypropylene in the inner layer, the heat resistance of the entire laminated microporous membrane can be improved.

  The polyolefin microporous membrane of the present invention may have three or more layers as long as the surface layer does not contain polypropylene. It is important that the surface layer of the polyolefin microporous membrane of the present invention does not contain polypropylene, contains a low melting point component, and has polypropylene in a layer other than the surface layer.

[1] Polyolefin resin composition in the first layer (A layer) In the polyolefin microporous membrane of the present invention, the A layer is an inner layer and is a layer other than the surface layer.

  In the present invention, the polyolefin microporous membrane constituting the A layer contains polyethylene as a main component. Here, in order to improve permeability and puncture strength, the total polyolefin resin is 100% by weight, polyethylene is preferably 80% by weight or more, more preferably 90% by weight or more, and polyethylene It is preferable to use it alone.

The types of polyethylene, high density polyethylene such as density exceeding 0.94 g / cm ^3, density polyethylene in the range density of 0.93~0.94g / cm ^3, density of from 0.93 g / cm ³ Although low low density polyethylene, linear low density polyethylene, etc. are mentioned, it is preferable to contain a high density polyethylene and ultra high molecular weight polyethylene from a viewpoint of intensity | strength. The ultra high molecular weight polyethylene may be not only a homopolymer of ethylene but also a copolymer containing a small amount of other α-olefin. In laminated microporous membranes, it may be difficult to control uneven physical properties in the width direction due to differences in the viscosity of each layer, but the use of ultra-high molecular weight polyethylene in layer A will strengthen the molecular network of the entire membrane. Therefore, non-uniform deformation hardly occurs, and a microporous film having excellent uniformity of physical properties can be obtained.

Here, the weight average molecular weight (hereinafter referred to as Mw) of the high-density polyethylene is preferably 1 × 10 ⁵ or more, more preferably 2 × 10 ⁵ or more. The upper limit is preferably Mw of 8 × 10 ⁵ , and more preferably Mw of 7 × 10 ⁵ . If Mw is in the above range, the stability of the film formation and the finally obtained puncture strength can both be achieved. Further, the Mw of the ultrahigh molecular weight polyethylene is preferably 1 × 10 ⁶ or more and less than 4 × 10 ⁶ . By using ultra high molecular weight polyethylene having an Mw of 1 × 10 ⁶ or more and less than 4 × 10 ⁶ , the pores and fibrils can be miniaturized and the puncture strength can be increased. Further, if Mw is 4 × 10 ⁶ or more, the viscosity of the melt becomes too high, so that there may be a problem in the film forming process such that the resin cannot be extruded from the die. Further, in the present invention, since the difference in viscosity from the B layer described later becomes too high, there is a case where each layer cannot be uniformly extruded particularly in the lamination by the coextrusion method. The content of ultrahigh molecular weight polyethylene is 100% by weight based on the entire polyolefin resin, and the lower limit is preferably 2% by weight, more preferably 10% by weight, and most preferably 15% by weight. The upper limit is preferably 60% by weight, more preferably 50% by weight, and most preferably 40% by weight. Within this range, it is easy to achieve both puncture strength and air resistance, and a microporous membrane with less variation in air resistance can be obtained.

  Moreover, when polypropylene is added to polyethylene, the melt-down temperature can be further improved when the polyolefin microporous membrane is used as a battery separator. Therefore, the layer A of the polyolefin microporous membrane of the present invention contains polypropylene. It is important to. As the type of polypropylene, a block copolymer and a random copolymer can be used in addition to the homopolymer. The block copolymer and random copolymer may contain a copolymer component with α-ethylene other than propylene, and ethylene is preferable as the other α-ethylene. However, when polypropylene is added, the puncture strength is likely to be lower than when polyethylene alone is used, so 0 to 50% by weight in the polyolefin resin is preferable.

The weight average molecular weight of polypropylene is 6 × 10 ⁵ or more, ΔHm is 90 J / g or more, and Mw / Mn is 100 or less. Mw / Mn is preferably in the range of 1 to 50, more preferably 2 to 6. The weight average molecular weight of polypropylene is preferably 8 × 10 ⁵ or more, more preferably 8 × 10 ⁵ or more and 2.0 × 10 ⁶ . More preferably, it is a range of 1.1 × 10 ⁶ or more and 1.5 × 10 ⁶ or less. The ΔHm of polypropylene is 95 J / g or more, more preferably 100 J / g or more, still more preferably 110 J / g or more, and most preferably 115 J / g or more. The degree of crystallinity of polypropylene is 50% or more, more preferably 65% or more and 75% or less.
The polypropylene content contained in the A layer is in the range of 1 wt% to 100 wt% with respect to the total weight of the A layer, more preferably 20 wt% to 80 wt%, more preferably 20 wt% to 50 wt%. It is.

  The type of polypropylene is not important as long as the above conditions for Mw and ΔHm are satisfied, but polypropylene homopolymers, copolymers of propylene and other α-olefins, or mixtures thereof are preferred, among which polypropylene homopolymers Is preferred. The copolymer is a random or block copolymer. Examples of comonomers include ethylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate, styrene, and mixtures thereof. The comonomer content is 10 mol% or less with respect to 100 mol% of the copolymer.

The polypropylene used in the present invention has one or more of the following properties.
(I) Mw of polypropylene is in the range of 1 × 10 ⁴ or more and 4 × 10 ⁶ or less. More preferably, it is the range of 6 × 10 ⁵ or more and 3 × 10 ⁶ or less.
(Ii) Mw / Mn of polypropylene is in the range of 1.01 to 100, preferably in the range of 2 to 6.
(Iii) The stereoregularity of polypropylene is isotactic.
(Iv) The ΔHm of the polypropylene is at least about 95 joules / gram.
(V) The melting peak of polypropylene (at the second melting) is at least about 160 ° C.
(Vi) The trouton ratio of polypropylene is at least 15 when measured at a temperature of 230 ° C. and a strain rate of 25 seconds− ¹ .
(Vii) The elongational viscosity of the polypropylene measured at a temperature of 230 ° C. and a strain rate of 25 seconds− ¹ is at least 50000 Pa · sec.
Mw, Mn, and ΔHm of polypropylene can be determined by the method disclosed in International Patent Application WO2007 / 132294.

  Here, the low layer component is not included in the layer A of the polyolefin microporous membrane of the present invention. “No low melting point component” means that, for example, the fraction of an elution component of 90 ° C. or less extracted by a cross fractionation chromatograph or the like is 5.0% by weight or less, preferably 2.5% by weight or less. means. This is because, even if a low melting point component is not intentionally added, the polymer has a distribution in molecular weight, so that a low molecular component that can have a low melting point is included. When the low melting point component is present in all layers, the air resistance may be easily deteriorated when heated even before shutdown.

The elution component by the cross fractionation chromatograph can be obtained, for example, as follows.
・ Measuring device: Cross-fractionation chromatograph CFC2 type (manufactured by Polymer ChAR)
・ Detector: IR spectrophotometer IR4 type (manufactured by Polymer ChAR)
・ Detection wavelength: 3.42μm
Column: Shodex UT806M manufactured by Showa Denko KK
-Column temperature: 140 ° C
Solvent (mobile phase): o-dichlorobenzene Solvent flow rate: 1.0 ml / min Sample concentration: 3.0 mg / ml
・ Temperature drop time: 140 minutes (140 ° C → 0 ° C)
-Elution component amount below 90 ° C: The total extraction amount is the sum of the weights from 0 ° C to 90 ° C among the extraction amounts when fractionating from 0 ° C to 140 ° C every 10 ° C. By dividing, the amount of elution component of 90 ° C. or less is calculated.

[2] Polyolefin resin composition in the second layer (B layer) In the laminated microporous membrane made of polyolefin of the present invention, the B layer is a surface layer and is a layer exposed on the surface.

The polyolefin microporous film constituting the B layer of the present invention contains polyethylene as a main component. As a kind of polyethylene, it is preferable that a high density polyethylene is a main component from a viewpoint of intensity | strength. The high-density polyethylene preferably has a weight average molecular weight (hereinafter referred to as Mw) of 1 × 10 ⁵ or more, more preferably 2 × 10 ⁵ or more. The upper limit is preferably Mw of 8 × 10 ⁵ , and more preferably Mw of 7 × 10 ⁵ . If Mw is in the above range, the stability of the film formation and the finally obtained puncture strength can both be achieved.

  In the present invention, it is important to add a low melting point component such as low density polyethylene, linear low density polyethylene, and ethylene / α-olefin copolymer produced by a single site catalyst to the B layer. By adding any of these, a shutdown function at a low temperature can be imparted, and the characteristics as a battery separator can be improved. The melt flow rate (MFR) of the low melting point component is preferably 50 g / 10 min to 150 g / 10 min. The lower limit is more preferably 75 g / 10 min, still more preferably 100 g / min. When the MFR is 50 g / 10 min or more, the fluidity is good, and thus it is difficult to create uneven thickness in the stretching process, and a uniform film thickness distribution can be obtained. In addition, because the molecular mobility is good, the residual strain is unlikely to remain, and since the molecules are sufficiently relaxed at low temperatures, the pores are hardly clogged due to the residual strain at a temperature lower than the melting point, and the increase in air permeability is suppressed. can do. When the MFR is 150 g / min or more, the viscosity of the melt is too low, so that in the coextrusion with the A layer, each layer may not be extruded uniformly. Further, in the stretching process during production, the microporous membrane may be broken due to low viscosity.

  Further, the melting point of the low melting point component is preferably 120 ° C. or higher and lower than 130 ° C. When the melting point is 120 ° C. or more, the stretchability of the laminate is good when laminated with the layer A containing ultra-high molecular weight, and when the stretching temperature is raised, the pores are hardly blocked and the air resistance is good. Easy to keep in. On the other hand, when the melting point is 130 ° C. or lower, it is easy to achieve a target low shutdown temperature.

  Here, the content of the low melting point component is preferably 100% by weight based on the entire polyolefin resin of the B layer, and the lower limit is preferably 20% by weight, more preferably 25% by weight. The upper limit is preferably 35% by weight, more preferably 30% by weight. When it is 20% by weight or more, a sufficient shutdown temperature can be lowered. Further, when the content is 30% by weight or less, breakage of the microporous film that occurs due to low viscosity can be suppressed.

From the viewpoint of strength, the B layer preferably contains an ultrahigh molecular weight polyethylene resin. The ultra high molecular weight polyethylene may be not only a homopolymer of ethylene but also a copolymer containing a small amount of other α-olefin. The puncture strength can be improved by adding ultrahigh molecular weight polyethylene. The Mw of the ultra high molecular weight polyethylene is preferably 1 × 10 ⁶ or more and less than 4 × 10 ⁶ . By using ultra high molecular weight polyethylene having an Mw of 1 × 10 ⁶ or more and less than 4 × 10 ⁶ , the pores and fibrils can be miniaturized. For this reason, the pores are quickly closed by the low melting point component, and the shutdown temperature can be lowered more effectively. The content of ultrahigh molecular weight polyethylene is 100% by weight of the entire polyolefin resin, and the lower limit is preferably 2% by weight, more preferably 10% by weight, and even more preferably 15% by weight. The upper limit is preferably 60% by weight, more preferably 50% by weight, and most preferably 40% by weight. The ultrahigh molecular weight polyethylene resin has a large difference in molecular mobility from the low melting point component, so if it exceeds 60% by weight, separation from the low melting point component tends to proceed at the time of melt-kneading. May cause poor appearance.

  In both the A layer and the B layer, the polyolefin microporous membrane of the present invention includes an antioxidant, a heat stabilizer, an antistatic agent, an ultraviolet absorber, an anti-blocking agent and a filler as long as the effects of the present invention are not impaired. Various additives such as materials may be included. In particular, it is preferable to add an antioxidant for the purpose of suppressing oxidative deterioration due to the thermal history of the polyethylene resin. Appropriate selection of the type and amount of antioxidants and heat stabilizers is important for adjusting or enhancing the properties of the microporous membrane.

  Moreover, it is preferable that the polyolefin microporous film of the present invention does not substantially contain inorganic particles. “Substantially free of inorganic particles” means, for example, a content of 50 ppm or less, preferably 10 ppm or less, most preferably the detection limit or less when inorganic elements are quantified by fluorescent X-ray analysis. Even if particles are not positively added to the polyolefin microporous membrane, contaminants derived from foreign substances and raw material resin or dirt attached to the line and equipment in the polyolefin microporous membrane manufacturing process are peeled off. It is because it may be mixed in.

[3] Polyolefin resin composition in the third layer (C layer) In the polyolefin microporous membrane of the present invention, the C layer is a layer other than the A layer and the B layer, and is an arbitrary layer. In the polyolefin microporous membrane of the present invention, it is essential that the A layer is an inner layer and the B layer is a surface layer, but in addition, a C layer different from the A layer and the B layer may be included. The polyolefin microporous film constituting the C layer of the present invention preferably contains polyethylene as a main component, and examples thereof include high-density polyethylene and ultrahigh molecular weight polyethylene. However, when C layer becomes the other surface layer (one is B layer), it is necessary not to contain polypropylene.

[4] Method for Producing Polyolefin Microporous Membrane Next, the method for producing a polyolefin microporous membrane of the present invention will be specifically described, but the invention is not limited to this embodiment.

The method for producing a polyolefin microporous membrane of the present invention includes the following steps.
(A) The step of preparing a polyolefin solution A by melt-kneading the polyolefin resin composition constituting the A layer and the plasticizer, and (b) melt-kneading the polyolefin resin composition constituting the B layer and the plasticizer. (C) Preparation of polyolefin solution B (c) Polyolefin solutions A and B obtained in steps (a) and (b) are coextruded from a multilayer die, and the resulting extrudate is cooled to form a gel sheet. Step of molding (d) Step of extracting the plasticizer from the stretched film obtained in step (d) of step (e) and step (d) of stretching the sheet obtained in step (c) (f) ) Other steps such as a hydrophilization treatment and a charge removal treatment can be added before, during and after the steps (c) to (f) of drying the microporous membrane obtained in the step (e). Moreover, a redrawing process can also be provided after the (e) process.

(A), (b) Preparation of polyolefin solutions A and B A polyolefin solution in which a polyolefin resin is dissolved in a plasticizer by heating is prepared. The plasticizer is not particularly limited as long as it is a solvent that can sufficiently dissolve polyethylene. In order to enable stretching at a relatively high magnification, the solvent is preferably a liquid at room temperature. Liquid solvents include nonane, decane, decalin, paraxylene, undecane, dodecane, liquid paraffins and other aliphatic, cycloaliphatic or aromatic hydrocarbons, and mineral oil fractions with boiling points corresponding to these, and dibutyl phthalate And phthalic acid esters which are liquid at room temperature such as dioctyl phthalate. In order to obtain a gel-like sheet having a stable content of the liquid solvent, it is preferable to use a non-volatile liquid solvent such as liquid paraffin. In the melt-kneaded state, it is miscible with polyethylene, but a solid solvent may be mixed with the liquid solvent at room temperature. Examples of such a solid solvent include stearyl alcohol, seryl alcohol, and paraffin wax. However, if only a solid solvent is used, stretching unevenness and the like may occur.

  The blending ratio of the polyolefin resin and the plasticizer is 100% by weight of the total of the polyolefin resin and the plasticizer, and from the viewpoint of improving the moldability of the extrudate, the content of the polyolefin resin is 10 to 50% by weight. Is preferred. The lower limit of the content of the polyolefin resin is preferably 15% by weight, more preferably 20% by weight. The upper limit is preferably 40% by weight, more preferably 30% by weight. When the content of the polyolefin resin is 10% by weight or more, since the swell and neck-in are small at the exit of the die when forming into a sheet shape, the sheet formability and film formability are good. Further, when the content of the polyolefin resin is 40% by weight or less, since the shrinkage in the thickness direction is small, the moldability and the film forming property are good.

  The viscosity of the liquid solvent is preferably 20 to 200 cSt at 40 ° C. If the viscosity at 40 ° C. is 20 cSt or more, the sheet obtained by extruding the polyolefin solution from the die is less likely to be non-uniform. On the other hand, if it is 200 cSt or less, removal of the liquid solvent is easy.

  Uniform melt-kneading of the polyolefin solution is not particularly limited. However, when a high-concentration polyolefin solution is desired to be prepared, it is preferably performed in an extruder, particularly a twin-screw extruder. As needed, you may add various additives, such as antioxidant, in the range which does not impair the effect of this invention. In particular, it is preferable to add an antioxidant in order to prevent oxidation of 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 polyolefin resin + 10 ° C.), more preferably (melting point of polyolefin resin + 20 ° C.). The upper limit is preferably (melting point of polyolefin resin + 120 ° C.), more preferably (melting point of 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 is preferably 250 ° C, 230 ° C, and most preferably 200 ° C. The melt kneading temperature when polypropylene is contained in the polyolefin solution is preferably 190 to 270 ° C.

  From the viewpoint of suppressing the deterioration of the resin, the melt kneading temperature is preferably low, but if it is lower than the above-mentioned temperature, an unmelted product is generated in the extrudate extruded from the die, causing film breakage or the like in the subsequent stretching step. When the temperature is higher than the above-described temperature, the thermal decomposition of the polyolefin becomes violent, and the physical properties of the resulting microporous film, such as puncture strength and tensile strength, may be inferior.

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

  In order to disperse polyethylene well in the extrudate and to obtain excellent thickness uniformity of the microporous membrane, it is preferable to set the screw rotation speed (Ns) of the twin screw extruder to 150 rpm or more. Furthermore, the ratio of the extrusion rate Q (kg / h) of the polyolefin solution to Ns (rpm), Q / Ns is preferably 0.64 kg / h / rpm or less. More preferably, it is 0.35 kg / h / rpm or less.

(C) Extrudate formation and gel-like sheet molding
Polyolefin solutions A and B melt-kneaded by an extruder are coextruded directly from a multilayer die through another extruder or a laminated extrudate is obtained. As a general lamination method, a microporous film to be laminated is prepared separately and then bonded while being hot-drawn through a calender roll or the like, or a polyolefin solution is separately fed to an extruder and melted at a desired temperature. There are methods such as polymer tube or die merging and co-extrusion and lamination, and from the viewpoint of interlayer adhesion, it is preferable to use a co-extrusion method. Further, from the viewpoint of the balance between physical properties such as shutdown characteristics and strength and permeability, at least two layers of A layer and B layer are preferable. From the viewpoint of the front and back balance of the final laminated microporous membrane, a three-layer configuration of B layer / A layer / B layer is more preferable. In particular, the B layer added with the low melting point component has an excellent shutdown speed by forming the surface of the present invention, and the A layer added with polypropylene forms the inner layer of the present invention, resulting in an excellent meltdown. Can have combined temperatures. Moreover, it can also be set as 3 types, 3 layers of C layers other than A layer and B layer, and it can also be set as the structure of 4 layers or more, either of the 2 layers (at least one is B layer) used as a surface layer It is important that no polypropylene is contained.

  Here, the ratio of the polyolefin solution B is preferably 30% by weight or more and 80% by weight or less with respect to the mass of the entire solution. The lower limit is more preferably 40% by weight, and the upper limit is more preferably 70% by weight. If the ratio of the solution B is in the above range, the balance between the low shutdown characteristics due to the low melting point component, the stability of the permeability in the usage range of the separator, and the puncture strength can be made good.

  The obtained extrudate is cooled to obtain a gel-like sheet, and the polyethylene microphase separated by the solvent can be fixed by cooling. In the cooling step, it is preferable to cool to a temperature lower than the crystallization end temperature of the resin constituting each layer. Here, the crystallization end temperature is an extrapolated crystallization end temperature measured according to JIS K7121 (1987). For example, high-density polyethylene has an extrapolation crystallization end temperature of about 70 to 90 ° C.

  Cooling is preferably performed at a rate of 30 ° C./min or more at least up to the gelation temperature or less. Furthermore, a speed of 50 ° C./min or more is preferable. When the cooling rate is less than 30 ° C./min, the degree of crystallinity increases and it is difficult to obtain a gel-like sheet suitable for stretching. In general, when the cooling rate is slow, relatively large polyethylene crystals are formed, so the higher-order structure of the gel-like sheet becomes rough, and the pseudo cell unit forming it becomes large, but when the cooling rate is fast, Since relatively small polyethylene crystals are formed, the cell structure becomes a dense cell unit in which the higher-order structure of the gel-like sheet is dense.

  As a method for cooling the extrudate, there are a method of directly contacting cold air, cooling water, and other cooling media, a method of contacting a roll cooled with a refrigerant, a method using a casting drum, and the like. The solution pushed out from the die is taken up at a predetermined take-up ratio before or during cooling, and 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. When the take-up ratio is 10 or less, the neck-in becomes small, and breakage hardly occurs during stretching.

  When the polyethylene solution is cooled with a cooling roll, it is important that the polyethylene solution is uniformly adhered to the cooling roll. As an adhesion method to the cooling roll, any of an electrostatic application (pinning) method, an adhesion method using the surface tension of water, an air knife method, a press roll method, an underwater casting method, etc. may be used. As a method for obtaining the laminated microporous membrane of the present invention, it is preferable to use an air knife method or an electrostatic application method that has good thickness controllability and can control the cooling rate by the temperature of the blowing air. Here, in the air knife method, air is blown from the non-cooling roll surface, and the temperature is preferably 1 to 90 ° C., and the surface crystallization is controlled by controlling the surface cooling rate, and thus The surface porosity can be controlled, that is, the permeability and pore diameter of the resulting laminated microporous membrane can be controlled.

  The lower limit of the thickness of the gel sheet is preferably 0.5 mm, more preferably 1.0 mm. The upper limit is 3.0 mm, more preferably 2.0 mm. When the thickness of the gel-like sheet is 3.0 mm or less, it is difficult for the structure to be uneven in the thickness direction during the cooling process, the higher-order structure can be made dense throughout the thickness direction, and both the front and back structures are made dense. It can be. Moreover, if the thickness of a gel-like sheet is 3.0 mm or less, it is easy to make the cooling rate of a gel-like sheet into the above-mentioned preferable range.

(D) Stretching The obtained gel-like sheet is stretched in the longitudinal direction (machine direction) and the transverse direction (direction perpendicular to the machine direction). It may be simultaneous stretching in which longitudinal stretching and transverse stretching are simultaneously performed, or sequential stretching in which transverse stretching is performed after longitudinal stretching. Stretching is performed at a predetermined magnification by heating the gel-like sheet and using a normal tenter method, a roll method, or a combination of these methods.

  Here, as the oven of the stretching machine, it is preferable to use an oven composed of a plurality of zones divided at equal intervals in the longitudinal direction. The number of zones is 3 to 6, and 6 zones are most preferable. It is preferable to raise the temperature stepwise from the zone on the twin-screw extruder side in the previous stage toward the zone on the winder side in the subsequent stage.

  The first stage stretching speed is preferably 3% / second or more in the stretching axis direction. For example, in the case of uniaxial stretching, it is set to 3% / second or more in the longitudinal direction (machine direction; MD direction) or the width direction (transverse direction; TD direction). In the case of biaxial stretching, it is 3% / second or more in the MD direction and TD direction, respectively. Biaxial stretching may be simultaneous biaxial stretching, sequential stretching, or multistage stretching (for example, a combination of simultaneous biaxial stretching and sequential stretching). The stretching speed (% / second) in the stretching axis direction is the length stretched per second with the length in the stretching axis direction before secondary stretching as 100% in the region where the film (sheet) is secondarily stretched. The ratio of By stretching under the above conditions, the obtained laminated microporous film can have a uniform and dense pore structure (holes having a small pore diameter are uniformly present in the surface direction). This uniform and dense pore structure can easily control the shutdown temperature of the laminated microporous membrane of the present invention within a preferable range.

  Although a draw ratio changes with the thickness of a gel-like sheet | seat, it is preferable to extend | stretch 4 times or more in any direction, More preferably, it is 5 times or more. If it is 4 times or more, the stretching orientation proceeds in each direction, and high strength can be imparted. The upper limit is preferably 10 times, more preferably 7 times. If it is 10 times or less, tearing due to stretching hardly occurs. The total area ratio of the longitudinal stretching and the lateral stretching is preferably 16 times or more, more preferably 25 times or more, and most preferably 42 times or more.

  The stretching temperature is preferably lower than the melting point of the low melting point component polyolefin resin added to the B layer, and more preferably (the crystal dispersion temperature Tcd of the polyolefin resin constituting the A layer) to (the low melting point added to the B layer). The melting point of the component). When the melting point is lower than the melting point of the low melting point component added to the layer B, poor cracking of the hole due to melting of the low melting point component is prevented, and it becomes possible to efficiently orient the molecular chain by stretching. Can be obtained. Further, if the stretching temperature is equal to or higher than the crystal dispersion temperature of the polyolefin resin constituting the A layer, the polyolefin resin is sufficiently softened even in the A layer, and thus uneven deformation that may occur due to insufficient softening is suppressed. Further, since uniform opening by a molecular network formed by the ultrahigh molecular weight polyethylene added to the A layer is possible, in addition to good permeability, the uniformity of physical properties is also good. Further, since the stretching tension of the entire laminate can be sufficiently lowered, the film-forming property is improved, and it is difficult to break the film during stretching, and stretching at a high magnification is possible. In the case of a polyethylene resin, since it has a crystal dispersion temperature of about 90 to 100 ° C., the longitudinal stretching temperature is preferably 80 ° C. or higher. Since the melting point of the low melting point component in the present invention is 120 ° C. or more and less than 130 ° C., the upper limit is less than 130 ° C. The crystal dispersion temperature Tcd is determined from the temperature characteristics of dynamic viscoelasticity measured according to ASTM D 4065. Or it may obtain | require from NMR.

  Cleavage occurs in the higher order structure formed in the gel-like sheet by stretching as described above, the crystal phase is refined, and a large number of fibrils are formed. Fibrils form a three-dimensional irregularly connected network structure. Stretching improves the mechanical strength and enlarges the pores, making it suitable for battery separators.

(E) Extraction (washing) of plasticizer from the stretched membrane
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 can be obtained by removing 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, Chain fluorocarbons such as C ₆ F ₁₄ and C ₇ F ₁₆ , cyclic hydrofluorocarbons such as C ₅ H ₃ F _7, hydrofluoroethers such as C ₄ F ₉ OCH ₃ and C ₄ F ₉ OC ₂ H ₅ , C ₄ Examples include readily volatile solvents such as perfluoroethers such as F ₉ OCF ₃ and C ₄ F ₉ OC ₂ F ₅ . These cleaning solvents have a low surface tension (eg, 24 mN / m or less at 25 ° C.). By using a low surface tension cleaning solvent, the network structure forming the micropores is prevented from shrinking due to the surface tension at the gas-liquid interface during drying after cleaning, and thus the microporous material having high porosity and permeability. A membrane is obtained. These washing solvents are appropriately selected according to the solvent used for dissolving the polyolefin resin, and used alone or in combination.

  The cleaning method can be performed by a method of immersing and extracting the gel-like sheet in a cleaning solvent, a method of showering the gel-like sheet with the cleaning solvent, or a method using a combination thereof. Although the usage-amount of a washing | cleaning solvent changes with washing | cleaning methods, generally it is preferable that it is 300 weight part or more with respect to 100 weight part of gel-like sheets. The washing temperature may be 15 to 30 ° C., and it is heated 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 the physical properties of the obtained microporous film from becoming uneven in the horizontal and / or vertical direction, the mechanical properties and electrical properties of the microporous film From the viewpoint of improving the physical properties, the longer the time during which the gel-like sheet is immersed in the cleaning solvent, the better.

  The washing as described above is preferably performed until the gel-like sheet after washing, that is, the residual solvent in the microporous membrane is less than 1% by weight.

(F) Drying of microporous membrane After washing, the washing solvent is dried and removed. The drying method is not particularly limited, but drying is performed by a heat drying method, an air drying method, or the like. Drying is preferably performed until the dry weight of the microporous membrane is 100% by weight and the residual cleaning solvent is 5% by weight or less, more preferably 3% by weight or less. If the drying is insufficient, the porosity of the microporous membrane is lowered by the subsequent heat treatment, and the permeability is deteriorated.

(G) Re-stretching (second-stage stretching)
As shown in FIG. 2, the obtained laminated microporous membrane made of polyolefin was preheated and stretched in the width direction by a stretching machine, and then the width of the microporous membrane was fixed in the stretching machine as it was, and at a constant temperature for a certain time. Heat setting treatment (stretching treatment step (second stage)). Here, an oven having a plurality of zones divided in the longitudinal direction is used as the oven of the stretching machine. As temperature setting, gradually increase the temperature from the zone on the twin screw extruder side to the center zone, and then decrease the temperature stepwise to the zone on the subsequent winder side. Is preferred. Also, the temperature of the last zone is preferably lower than the temperature of the first zone. By such temperature setting, the residual stress in the microporous film can be reduced.

  The stretching speed of the second stage is preferably 2% / second or more, and preferably 7% / second or less. By setting each stretching speed in the TD direction to 5% / second or less, the obtained laminated microporous film can have a uniform and dense pore structure by stretching under the above conditions (pores with small pore diameters). Exists uniformly in the surface direction). This uniform and dense pore structure can easily control the shutdown temperature of the laminated microporous membrane of the present invention within a preferable range.

  The stretching ratio of the second stage is preferably 1.1 to 2.5 times. If this magnification is less than 1.1 times, the strength of the resulting microporous membrane may be insufficient. On the other hand, if the magnification is more than 2.5, there is a risk that the film will be easily broken or the thermal dimensional stability may be lowered. The stretching ratio is more preferably 1.1 to 2.0 times.

(H) Heat setting After extending | stretching, it heat-processes, fixing a width | variety in the state which hold | gripped the polyolefin microporous membrane with the clip (width direction heat setting process process). The temperature at this time is preferably 115 ° C to 135 ° C. By heat-setting at 115 to 135 ° C., the shrinkage ratio in the width direction at 90 ° C. to 135 ° C. of the polyolefin microporous membrane of the present invention can be reduced.

  Further, after that, it is preferable to perform a relaxation treatment that relaxes the polyolefin laminated microporous membrane by about several percent in the width direction in a state where the polyolefin microporous membrane is held by a clip (width direction relaxation treatment step). The temperature at this time is preferably lower than the temperature of the above-described stretching or width direction heat setting treatment, and specifically, a temperature of 100 ° C. or lower, more preferably 90 ° C. or lower is preferable.

  In addition, after the width direction relaxation treatment, it is also preferable to perform heat treatment while fixing the width in a state where the polyolefin laminated microporous film is held by a clip (width direction heat treatment step). The temperature at this time is preferably lower than that in the width direction relaxation treatment. This is because when the polyolefin laminated microporous membrane is taken out from the oven, the unevenness of the residual stress in the width direction can be reduced as the temperature change of the laminated microporous membrane is smaller. The smaller the unevenness of the residual stress in the width direction, the smaller the time-lapse of the wound microporous film and the microporous film after slitting, especially after a long period of time, Generation can be eliminated.

  Furthermore, the stretched membrane or the microporous membrane may be subjected to a hydrophilic treatment depending on other applications. The hydrophilic treatment can be performed by monomer grafting, surfactant treatment, corona discharge or the like. Monomer grafting is preferably performed after the crosslinking treatment.

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

  If necessary, at least one surface of the stretched membrane or the microporous membrane can be subjected to corona discharge treatment in air, nitrogen, or a mixed atmosphere of carbon dioxide and nitrogen.

[5] Structure and physical properties of polyolefin microporous membrane The polyolefin microporous membrane of the present invention has the following properties.

(1) Film thickness It is preferable that the thickness of the polyolefin laminated microporous film used for this invention is 3-25 micrometers. When the film thickness of the polyolefin microporous film of the present invention is 3 to 25 μm, the battery capacity is improved while maintaining the strength of the separator when the microporous film is used as a separator of a lithium ion battery. be able to. Within this range, the battery has a practical puncture strength and is suitable for increasing the capacity of a battery that will be advanced in the future.

(2) Air permeability resistance It is important that the air permeability resistance of the polyolefin microporous membrane of the present invention is 50 to 300 seconds / 100 cc. When the polyolefin laminated microporous membrane of the present invention is used as a separator for a lithium ion battery, the battery can be provided with excellent self-discharge characteristics and excellent long-term storage characteristics. When the air permeability resistance is greater than 300 seconds / 100 cc, the ion permeability is low, and when the polyolefin laminated microporous membrane of the present invention is used as a separator for a lithium ion battery, the rate characteristics excellent for the battery are lowered. . Moreover, the speed of charging / discharging falls and the time concerning charging / discharging becomes long time. When the air permeability resistance is less than 50 seconds / 100 cc, the strength of the polyolefin microporous membrane is substantially lowered, and sufficient strength as a separator of a lithium ion battery cannot be maintained. The upper limit of the air resistance is more preferably 250 seconds / 100 cc. More preferably, it is 200 seconds / 100 cc or less. The lower limit is preferably 70 seconds / 100 cc, more preferably 100 seconds / 100 cc.

(3) Puncture strength The puncture strength of the polyolefin laminated microporous membrane of the present invention is 100 to 550 gf. When a polyolefin laminated microporous membrane is incorporated in a battery as a separator, the electrode is not short-circuited and the safety of the battery is increased. When the puncture strength is less than 100 gf, the laminated microporous membrane made of polyolefin of the present invention is insufficient as a separator and decreases the safety of the battery. On the other hand, when the puncture strength is greater than 550 gf, the heat shrinkage rate is increased. If the heat shrinkage rate is high, battery safety at high temperatures may be impaired. The lower limit of the puncture strength is preferably 150 gf or more, and more preferably 200 gf or more. The higher the puncture strength, the better. However, the polyolefin microporous membrane of the present invention has a puncture strength of less than 550 gf because of the relationship with the heat shrinkage rate.

(4) Shutdown temperature,
The shutdown temperature of the polyolefin microporous membrane of the present invention is preferably 129.5 to 135.0 ° C. When the shutdown temperature is within the above range, the pores are blocked when the battery heats up when an abnormality occurs, and the battery reaction is likely to stop before the battery reaches a high temperature. It is easy to give high safety to the battery. From the viewpoint of providing excellent output characteristics, charge / discharge characteristics, and excellent safety (depending on the shutdown temperature of the separator) when the laminated microporous membrane of the present invention is used as a separator of a lithium ion battery. The film of the present invention preferably has a shutdown temperature of 129.5 to 135.0 ° C.

(5) Shutdown speed The shutdown speed of the polyolefin microporous membrane of the present invention is preferably 1.55 × 10 ^{4 to} 3.00 × 10 ⁴ sec. When the shutdown speed is within the above range, the pores are blocked quickly when the battery is abnormally heated, and thermal runaway can be stopped at the initial stage. Therefore, it is easy to give high safety to the battery. When the polyolefin laminated microporous membrane of the present invention is used as a separator for a lithium ion battery, the film according to the present invention has a shutdown / shutdown speed of 1.55 × 10 ^{4 to} 3 in order to provide excellent safety of the battery. It is preferably 0.000 × 10 ⁴ sec.

(6) Shrinkage rate at 130 ° C. The polyolefin microporous membrane of the present invention preferably has a thermal shrinkage rate of −10 to 10% at 130 ° C. By being the said range, when the porous film of this invention is used as a separator of a lithium ion battery, the high temperature safety | security excellent in the said battery can be provided. When the shrinkage amount of the polyolefin laminated microporous membrane of the present invention is in the above range, when the battery reaches a high temperature, the shrinkage of the polyolefin laminated microporous membrane inside the battery is sufficiently small. Contact between the electrodes can be prevented. Since the internal short circuit of the battery at a high temperature can be prevented, the laminated microporous membrane made of polyolefin of the present invention can impart excellent safety to the battery.

(7) Coefficient of Friction Preferably, the laminated microporous membrane made of polyolefin of the present invention has a coefficient of static friction between the front and back surfaces of the film of 0.3 to 1.5. More preferably, it is 0.5 or more. One surface (front surface) of the laminated microporous membrane made of polyolefin and the opposite surface (back surface) are overlapped to measure the static friction coefficient. When the coefficient of static friction is within the above range, when the polyolefin microporous membrane of the present invention is used as a separator for a lithium ion battery, excellent pin-out performance is exhibited in the battery manufacturing process. The excellent pin pull-out property can contribute to an improvement in battery productivity and a reduction in battery production cost, such as an increase in production speed during battery manufacture and an improvement in yield. Furthermore, when the laminated microporous membrane made of polyolefin of the present invention is produced, slipping in the form of a transport roll is difficult even when transported at high speed when slitting the laminated microporous membrane of polyolefin of the present invention after film formation. , It becomes easy to suppress the occurrence of slit defects and film wrinkles due to meandering.

(8) Porosity The upper limit of the porosity of the multilayer microporous membrane made of polyolefin of the present invention is preferably 70%, more preferably 60%, and most preferably 55%. The lower limit is preferably 30%, more preferably 35%, and most preferably 40%. If the porosity is 70% or less, sufficient mechanical strength and insulating properties can be easily obtained, and short-circuiting is unlikely to occur during charging and discharging. Moreover, if the porosity is 30% or more, the ion permeability is good, and good charge / discharge characteristics of the battery can be obtained.

(9) Meltdown temperature It is important that the meltdown temperature of the polyolefin laminated microporous membrane of the present invention is in the range of 159 to 200 ° C. When the meltdown temperature is 159 ° C. or lower, safety may be reduced when the polyolefin microporous membrane of the present invention is used as a separator of a lithium ion battery. On the other hand, it is substantially difficult to make the meltdown temperature 200 ° C. or higher while maintaining good permeability and strength in the laminated microporous membrane of the present invention mainly composed of polyolefin resin.

[6] Use The polyolefin laminated microporous membrane of the present invention is suitable as a separator (separator) for electrochemical reaction devices such as batteries and capacitors. Especially, it can be used conveniently as a separator of a nonaqueous electrolyte system secondary battery, especially a lithium secondary battery.

[7] Measuring method of physical properties The measuring method of each physical property is described below.

(1) Thickness (average film thickness)
A test piece was prepared by cutting out from an arbitrary position of the polyolefin laminated microporous membrane into a square of 5 cm in the longitudinal direction and 5 cm in the width direction. Any five points of this test piece were measured with a thickness contact thickness meter and averaged to obtain the thickness of the test piece. Ten test pieces were prepared and measured for the same polyolefin laminated microporous membrane. The average value of all 10 test pieces was taken as the thickness of the polyolefin microporous membrane.
As a thickness measuring machine, a lightmatic VL-50A manufactured by Mitsutoyo was used.

(2) Air permeability resistance (average)
Using a digital type Oken air permeability tester EGO1 manufactured by Asahi Seiko Co., Ltd., the polyolefin microporous membrane of the present invention is fixed so that wrinkles do not enter the measurement part. JIS P-8117 (2009). The sample was 5 cm square, the measurement point was one point at the center of the sample, and the measured value was the air permeability [second] of the sample. The same measurement was performed on 10 test pieces collected from arbitrary film positions, and the average of the 10 measured values was defined as the air permeability of the polyolefin microporous membrane (sec / 100 ml).

(3) the puncture strength puncture strength of the polyolefin microporous membrane, the polyolefin multilayer microporous film having a thickness of T _1, a spherical tip surface (radius of curvature R: 0.5 mm) using a stylus having a diameter of 1mm with Is defined as the maximum load measured (in gram force or “gf”) when pierced at a rate of 2 mm / sec.

(4) Shutdown temperature / meltdown temperature The shutdown temperature (T _SD ) was measured by heating the polyethylene microporous membrane at a rate of temperature increase of 5 ° C./min. −1T), the air permeability was measured, the temperature at which the air permeability reached the detection limit of 1 × 10 ⁵ sec / 100 cm ³ was determined, and this was taken as the shutdown temperature.
After reaching the shutdown temperature, heating is continued at a rate of temperature increase of 5 ° C./min, the temperature at which the air permeability becomes 1 × 10 ⁵ sec / 100 cm ³ is obtained again, and the meltdown temperature ( _TMD ) did.

(5) Shutdown speed (permeability change rate)
A temperature-air permeability curve as shown in FIG. 3 is created from the data of the air permeability P with respect to the temperature T obtained in the measurement of the shutdown temperature, and the air permeability becomes 1 × 10 ⁴ sec / 100 cm ^3. The slope of the curve at a certain temperature (the slope Δp / ΔT of the tangent L5 shown in FIG. 3) was determined and used as the shutdown speed.

(6) Shrinkage at 130 ° C. Using a thermomechanical analyzer (manufactured by Seiko Instruments Inc., TMA / SS6000), a test piece of 10 mm (TD) × 3 mm (MD) with a load of 2 g While pulling in the direction, the temperature was raised from room temperature at a rate of 5 ° C./min, the dimensional change rate with respect to the size at 23 ° C. was measured three times at 130 ° C., and the shrinkage rate was obtained by averaging.

(7) Friction coefficient In accordance with JIS K7125 (1999), the test direction is parallel to the longitudinal direction (MD) or width (TD) of the laminated microporous polyolefin, and measured by combining the front and back of the laminated microporous membrane made of polyolefin. did. 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.

(8) A porous polyolefin porous microporous membrane is cut into a size of 5 cm × 5 cm, its volume (cm ³ ) and mass (g) are determined, and from these and the membrane density (g 2 / cm ³ ), Calculated using
Porosity = ((Volume−Mass / Membrane density) / Volume) × 100
Here, the film density is 0. 99. Moreover, the average film thickness measured by above-mentioned (1) was used for calculation of a volume.

(9) Weight average molecular weight (Mw)
Mw of UHMWPE and HDPE was determined by gel permeation chromatography (GPC) method under the following conditions.
-Measuring device: GPC-150C manufactured by Waters Corporation
Column: Shodex UT806M manufactured by Showa Denko KK
-Column temperature: 135 ° C
Solvent (mobile phase): o-dichlorobenzene Solvent flow rate: 1.0 ml / min Sample concentration: 0.1% by weight (dissolution condition: 135 ° C./1 h)
・ Injection volume: 500μl
-Detector: Differential refractometer manufactured by Waters Corporation-Calibration curve: Prepared from a calibration curve obtained using a monodisperse polystyrene standard sample using a predetermined conversion constant.

(10) Melting point Measured according to JIS-K7121.

(11) Melt flow rate (MFR)
It measured according to JIS-K7210.

(12) Film forming property When a polyethylene laminated microporous film was formed at a casting speed of 2 m / min for 5 hours, it was determined according to the following criteria.
• A: No tearing occurs • B: One tear occurs • C: Two tears occur • D: Three or more tears occur

(13) Battery preparation A square battery was prepared according to the following procedure.
In a lithium ion secondary battery, a positive electrode and a negative electrode are laminated via a separator, and the separator contains an electrolytic solution (electrolyte). Lithium cobalt composite oxide LiCoO ₂ was used as the positive electrode active material, graphite was used as the negative electrode active material, and 1 mol / L LiPF ₆ prepared in a DC / DMC mixed solvent was used as the electrolyte. The battery is assembled by stacking a positive electrode, a separator made of a multilayer microporous membrane, and a negative electrode, then preparing a wound electrode body by a conventional method, inserting it into a battery can, impregnating with an electrolytic solution, and then a positive electrode equipped with a safety valve Crimp the battery lid that also serves as the terminal through a gasket.

(14) Battery test Nail penetration test The battery assembled in (13) above was fully charged to 4.2V at 0.5C, and then a nail with a 3mm diameter nail was inserted into the center of the battery at a speed of 150mm / sec. Go. The same test was performed on any five batteries, and the case where no ignition and explosion occurred was passed (O), and the case where either ignition or smoke was observed was rejected (X).

(15) Battery test Hot box test The battery assembled in (13) above was charged at a constant current to a voltage of 4.2 V at a current value of 1 C, then charged at a constant voltage and a constant pressure of 4.2 V, and then 3 at a current of 0.2 C. Discharge was performed to a final voltage of 0V. Next, after constant current charging to 4.2 V with a current value of 0.2 C, 4.2 V constant voltage charging was performed as a pretreatment. The pretreated battery was put into an oven, heated from room temperature at 5 ° C./min, and then left at 150 ° C. for 30 minutes. Those that smoked and ignited within 15 minutes after reaching 150 ° C. were evaluated as x, those that ignited within 30 minutes exceeding 15 minutes were evaluated as Δ, and those that did not ignite and exceeded 30 minutes were evaluated as ◯.

  EXAMPLES Hereinafter, although an Example is shown and demonstrated concretely, this invention is not restrict | limited at all by these Examples.

Example 1
As raw materials, ultra high molecular weight polyethylene (UHMWPE) having a Mw of 2.0 × 10 ⁶ , an Mw / Mn of 5, a melting point of 135 ° C., and a viscoelastic relaxation phenomenon observed near 90 ° C., The weight average molecular weight (Mw) is 5.6 × 10 ⁵ , the molecular weight distribution (Mw / Mn) is 4.1, the melting point is 135 ° C., and the viscoelastic relaxation phenomenon is observed near 90 ° C., Polyethylene compositions A and B were prepared using as a main component 0.1 high density polyethylene (HDPE) having 10,000 terminal vinyl groups per 10,000 carbon atoms. The blending ratio is shown in Table 1.

In addition, for the polyethylene composition A forming the inner layer (A layer), a polypropylene resin having Mw of 1.6 × 10 ⁶ , Mw / Mn of 5.2 and ΔHm of 114.0 J / g was used. .
Further, in the polyethylene composition B forming the surface layer (B layer), the content of the ethylene / 1-hexene copolymer having the melting point and MFR shown in Table 1 as the low melting point component is shown in Table 1. Used for.

  Further, 100 parts by mass of polyethylene compositions A and B were dry blended with 0.375 parts by mass of tetrakis [methylene-3- (3,5-di-tertiarybutyl-4-hydroxyphenyl) -propionate] methane to obtain a polyethylene composition. Products A and B were obtained.

  The obtained polyethylene compositions A and B were each fed into a strong kneading type twin screw extruder (polyethylene composition charge Q: 54 kg / h), and liquid paraffin was fed from the side feeder of the twin screw extruder. While maintaining the screw rotation speed Ns at 180 rpm, melt kneading was performed (Q / Ns: 0.3 kg / h / rpm) to prepare polyethylene solutions A and B. The melting temperature was 210 ° C.

  The obtained polyethylene solution was supplied from a twin-screw extruder to a multilayer die and extruded at a ratio shown in Table 1 so as to form a sheet-like molded body. The extruded molded body was cooled while being drawn with a cooling roll adjusted to 35 ° C. to form a gel-like sheet.

  When the polyethylene solution is cooled by a cooling roll, the compressed air blown from the slit nozzle is uniformly applied in the width direction of the sheet at the point where the polyethylene solution contacts the metal roll, and the polyethylene solution adheres uniformly to the cooling roll. I did it.

Here, the draft ratio when the molten polymer extruded from the T die was brought into contact with the cooling roll was 2.5.
The obtained gel-like sheet was simultaneously biaxially stretched five times in the longitudinal direction and the width direction at 110 ° C. by a tenter stretching machine, and the sheet width was fixed in the tenter stretching machine as it was and heated at a temperature of 115 ° C. for 10 seconds. Fixing treatment was performed (stretching treatment step (first stage)).

  Here, an oven consisting of 6 zones divided at equal intervals in the longitudinal direction was used as the oven of the tenter stretching machine. Two zones (zone No. 1, 2 / preheating) on the front twin screw extruder side are 100 ° C., the next two zones (zone No. 3, 4 / stretching) are 110 ° C., and 2 zones on the subsequent winder side. The zone (zone No. 5, 6 / fixed) was set to 115 ° C. Stretching in the longitudinal and width directions was performed in zones 3-4.

  Next, the stretched gel-like sheet was immersed in a methylene chloride bath in a washing tank to remove liquid paraffin and washed to obtain a polyethylene laminated microporous membrane.

  The obtained polyethylene microporous membrane was preheated at 115 ° C. and then stretched 1.0 times in the width direction by a tenter stretching machine, and then the width was fixed in the tenter stretching machine as it was, at a temperature of 125 ° C. The film was heat-set for 2 seconds (stretching process step (second stage)).

Here, an oven composed of a plurality of zones divided in the longitudinal direction was used as the oven of the tenter stretching machine. FIG. 2 shows an outline of the stretching process (second stage). One zone on the front side of the twin screw extruder was 115 ° C. (preheating zone), the next zone was 125 ° C. (stretching zone), and the next zone (fixed zone) was 80 ° C.
Next, the polyethylene laminated microporous membrane was cooled to room temperature, and wound around a paper tube with a tension of 10.0 N / m by a winder to obtain a polyethylene laminated microporous membrane roll. The porous film had a length of 1000 m, a width of 2000 mm, and a thickness of 14 μm. And the roll of the polyethylene laminated microporous film after winding was left still for 24 hours in the thermostat heated at 60 degreeC.

  Various physical properties of the obtained polyethylene laminated microporous membrane are shown in Table 3, and it was found that a lithium ion battery using the porous film obtained as shown in Table 3 as a separator has excellent battery characteristics and safety. It was.

Examples 2-6
A polyolefin laminated microporous membrane was produced in the same manner as in Example 1 except that the resin composition and the film forming conditions of the polyolefin laminated microthick membrane were changed as shown in Tables 1-2. Table 3 shows the physical properties of the resulting microporous polyolefin membrane.

Examples 7-8
In addition to the polyethylene composition A and polyethylene composition B described above, the polyethylene composition C for the layered microporous membrane made of polyolefin uses the polyethylene composition C for the C layer, and at the time of creating the gel sheet, A multi-layer polyolefin made in the same manner as in Example 1 except that a multi-layer die forming a three-layer / three-layer structure is used to form an A / B / C layer structure, based on the conditions shown in Tables 1-2. A microporous membrane was prepared. Table 3 shows the physical properties of the obtained polyolefin laminated microporous membrane.

Examples 9-11
A polyolefin laminated microporous membrane was produced in the same manner as in Example 1 except that the resin composition and the film forming conditions of the polyolefin laminated microthick membrane were changed as shown in Tables 1-2. Table 3 shows the physical properties of the obtained polyolefin laminated microporous membrane.

Comparative Example 1
When a polyethylene resin is measured by differential scanning calorimetry (DSC) analysis at a heating rate of 10 ° C./min, the ratio of the endothermic amount up to 125 ° C. of the crystal melting heat amount ΔHm is expressed as ΔHm (≦ 125 ° C.). . The temperature when the endothermic amount reaches 50% of the crystal melting heat amount ΔHm is expressed as T (50%) ”.

Adjustment of first layer (A layer) First layer (A layer) 63.7% of HDPE with (a) Mw of 5.6 × 10 ⁵ and Mw / Mn of 4.05 based on the weight of the polyolefin composition. (B) 1.3% by weight of UHMWPE with Mw of 1.9 × 10 ⁶ and Mw / Mn of 5.09, and (c) Mw of 1.6 × 10 ⁶ and Mw / Mn of 5. 2 and a polyolefin composition containing 35% by weight of a polypropylene resin having a ΔHm of 114.0 J / g was prepared by dry blending. The melting point of the polyethylene resin in the polyolefin composition A was 135 ° C., and the crystal dispersion temperature was 100 ° C. 30 parts by weight of the obtained polyolefin composition A was supplied to a strong kneading twin-screw extruder having an inner diameter of 58 mm and an L / D of 42, and 70 parts by mass of liquid paraffin (50 cSt at 40 ° C.) was biaxially extruded from the side feeder. Supplied to the machine. A polyethylene solution A of the first layer (A layer) was prepared by melt-kneading at 210 ° C. and 200 rpm.

Adjustment of the second layer (B layer) The weight of the second layer (B layer) polyolefin composition is (a) 82 wt.% HDPE with Mw of 5.6 × 10 ⁵ and Mw / Mn of 4.05. (B) Polyolefin composition B containing 18% by weight of UHMWPE with Mw of 1.9 × 10 ⁶ and Mw / Mn of 5.09 was prepared by dry blending. The melting point of the polyethylene resin in the polyolefin composition was 135 ° C., and the crystal dispersion temperature was 100 ° C.

  25 parts by weight of polyolefin composition B was supplied to a strong kneading twin screw extruder having an inner diameter of 58 mm and an L / D of 42, and 75 parts by weight of liquid paraffin (50 cSt at 40 ° C.) was fed from the side feeder to the twin screw extruder. Supplied to. The second layer (B layer) polyethylene solution B was prepared by melt-kneading at 210 ° C. and 200 rpm.

Manufacture of microporous membrane A polyethylene solution A of layer A and polyethylene solution B of layer B are fed from the respective twin-screw extruders to a three-layer T-die, the layer structure is B / A / B, and the layer thickness ratio is The extrudate was formed to 46.45 / 7.1 / 46.45.

  The extrudate is cooled by passing through a cooling roll controlled at 20 ° C. to form a three-layered gel sheet, and at 119.3 ° C., the machine direction (longitudinal direction) and the transverse direction are simultaneously 5 times at 119.3 ° C. The film was stretched at a stretch ratio of.

  Next, the stretched gel-like sheet was immersed in a methylene chloride bath in a washing tank to remove liquid paraffin and washed to obtain a polyethylene laminated microporous membrane.

  The dried laminated microporous membrane was re-stretched at a stretching ratio of 1.5 times in the TD direction at 127.3 ° C. by a batch stretching machine, and then the stretching ratio in the TD direction was relaxed to 1.3 times at the same temperature. . The draw ratio is based on the width in the TD direction of the microporous film before dry stretching before re-stretching. The re-stretched microporous membrane was heat-treated at 127.3 ° C. for 10 minutes with the batch stretcher attached, to produce a three-layer microporous membrane. The obtained polyethylene laminated microporous membrane was designated as Comparative Example 1.

1: Laminated microporous membrane (or gel sheet)
21: Kneading (melt kneading) step 22: Sheet forming step 23: Stretching treatment step (first stage)
24: Cleaning process 25: Stretching process (second stage)
26: Winding process

Claims (6)

It consists of at least three layers, the meltdown temperature is in the range of 159 to 200 ° C., the air permeability is in the range of 50 to 300 seconds, the piercing strength is in the range of 100 to 550 gf, and contains polypropylene only in the inner layer. A polyolefin microporous membrane characterized in that at least one layer forming a surface layer contains a resin having a melt flow rate of 50 to 150 g / 10 min and a melting point of 120 to 130 ° C. 2. The polyolefin laminated microporous membrane according to claim 1, wherein the shutdown temperature is in the range of 129.5 to 135.0 ° C. 3. 3. The polyolefin microporous membrane according to claim 1, wherein a shrinkage ratio of TD at 130 ° C. is in a range of −10% to 10%. ^4. The polyolefin microporous membrane according to claim 1, wherein a shutdown speed is in a range of 1.55 × 10 ^{4 to} 3.00 × 10 ⁴ sec. The polyolefin microporous membrane according to any one of claims 1 to 4, wherein the film thickness is in the range of 3 to 25 µm. The laminated microporous membrane made of polyolefin according to any one of claims 1 to 5, wherein the coefficient of static friction between the back surface and the surface of the film is in the range of 0.3 to 1.5.

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