Technical field
[0001]The present invention is a polyolefin microporous membrane, and a battery using the same.
BACKGROUND
[0002]Microporous polyolefin membrane, the separation of the material, separation membrane, microfiltration membrane used for such selective transmission, fuel cell separator, which is used as such a separator for capacitors. These other, microporous polyolefin membrane, notebook personal computers and cellular phones, are suitably used as a separator for lithium ion secondary batteries are widely used, such as a digital camera. The reason is mentioned to have a puncture strength and shutdown properties of the film having excellent microporous polyolefin membrane.
[0003]
　In the lithium ion secondary battery, it has progressed higher energy density, have high capacity and high output developed for reduction underway, even thinner separators accordingly. To equalize the battery manufacturing process and the battery safety, possible to maintain be thinned with a conventional equivalent strength molten thermal shrinkage rate is desired. This means that the strength of the hit film thickness is increased. However, the melting heat shrinkage and puncture strength in the microporous polyolefin membrane is contradictory properties, is difficult to achieve both of these.
[0004]
　The negative electrode and the positive electrode of the battery larger the maximum pore size of the separator is likely to short-circuit. Therefore, in order to reduce the self-discharge, the maximum pore size is small is desirable. However there be the maximum pore diameter decreases output density of the battery is reduced.
[0005]
　In contrast, Patent Document 1, a maximum pore size of 100nm or less, in puncture strength per 20μm or more 4N, in the TD direction maximum shrinkage on TMA external stress 2 mN (melt thermal shrinkage rate) is 0% or less method of obtaining a certain polyolefin microporous film have been proposed.
[0006]
　Further, Patent Document 2, in 100 ~ 250 nm is maximum pore size, a method of obtaining a polyolefin microporous film is 5% or less both in the length and width directions 120 ° C. Heat shrinkage has been proposed.
[0007]
　Further, Patent Document 3, a maximum pore size below 45 nm, the longitudinal direction of the tensile strength 1000 ~ 3500 kgf / cm 2 method of obtaining a microporous film have been proposed.
CITATION
Patent Document
[0008]
Patent Document 1: JP 2009-132904 Patent Publication
Patent Document 2: JP 2010-007053 Patent Publication
Patent Document 3: JP 2015-120786 JP
Summary of the Invention
Problems that the Invention is to Solve
[0009]
　However, in the embodiment of Patent Document 1, the minimum value of the maximum pore diameter was 63 nm, can not be less than this maximum pore size, the self-discharge is likely to increase. Further, the polyolefin microporous membrane described in Patent Document 1, the following as small 0% melt thermal shrinkage rate. Therefore, when used as a battery separator, since the battery starts heat runaway remains positive and negative electrodes are not minor short-circuiting when heated, is poor in energy is emitted thermal stability of the battery at a time. Further, in the above patent document 2 method, the maximum pore size of the polyolefin microporous membrane is not a 100nm or less, the output density of the battery is likely reduced. Further, in Patent Document 3, no mention in the molten thermal shrinkage rate of the microporous film did not become an issue.
[0010]
　The present invention was made to solve the above problems, the maximum pore diameter of the polyolefin microporous film, puncture strength, and controls the melt thermal shrinkage in the TD direction in the appropriate range, by taking these balance is the polyolefin microporous membrane to achieve both a high energy density and thermal stability of the battery using the separator.
Means for Solving the Problems
[0011]
　The present inventors, to solve the above problems, found that it is possible to solve the following configuration, thereby completing the present invention.
[0012]
　That is, the present invention provides puncture strength per thickness 20μm is not less than 4.5 N, comprises a polyolefin, the molecular weight distribution of the polyolefin has a weight average molecular weight 5.0 × 10 6 or more ratio 0.6-1.5 % is a polyolefin microporous membrane is.
[0013]
　Further, the microporous polyolefin membrane of the present invention preferably the maximum pore diameter of 60nm or less than 45 nm. Further, the polyolefin contained in the microporous polyolefin membrane of the present invention is preferably polyethylene exiting. Also, polyethylene, weight average molecular weight 1.0 × 10 6 preferably contains the above ultra high molecular weight polyethylene. Further, the ultra high molecular weight polyethylene with respect to 100 mass% of the whole polyolefin resin, preferably contains more than 10 wt% 20 wt% or less. The weight average molecular weight of ultra-high molecular weight polyethylene 2.0 × 10 6 is preferably less than.
[0014]
　The present invention includes a positive electrode, a negative electrode, and a separator, the separator is a battery using the polyolefin microporous membrane of the present invention.
[0015]
　The battery of the present invention is preferably a lithium ion secondary battery.
The invention's effect
[0016]
　Microporous polyolefin membrane of the present invention, the maximum pore diameter, puncture strength, and controls the melt thermal shrinkage in the TD direction in an appropriate range, by taking these balance, has a relatively large maximum pore size, and , high puncture strength, and can achieve both melting heat shrinkage small TD direction at a high level. The polyolefin microporous membrane in the battery using the separator, it is possible to achieve both high energy density and thermal stability.
DESCRIPTION OF THE INVENTION
[0017]
　It will be described in detail below microporous film according to the present invention. The present invention is not limited to the following embodiments can be modified in various ways within the scope of the invention.
[0018]
　[Polyolefin microporous film]
　In the present specification, the polyolefin microporous membrane refers to a microporous film comprising a polyolefin as a main component, for example, a microporous membrane containing 90 wt% or more of polyolefin against microporous membrane total amount Say. The following describes the physical properties of the polyolefin microporous membrane of the present invention.
[0019]
　(Maximum pore size)
　in the present specification, the term "maximum pore size", which indicates the maximum pore size among all holes distributed in the microporous membrane can be measured by a bubble point method. The lower limit of the maximum pore size of the microporous polyolefin membrane of the present invention is preferably at least 45 nm, preferably greater than 45 nm, more preferably not less than 48 nm, even more preferably 50nm or more. Preferably the upper limit of the maximum pore size is 60nm or less, and more preferably not more than 58 nm. When the maximum pore diameter is within the above range, when using the polyolefin microporous film as a battery separator, it is possible to achieve both high power density and suppression of self-discharge.
[0020]
　For example, in Patent Document 3, by a maximum pore size of the polyolefin microporous film less 45 nm, to prevent partial short, it is described that it is possible to ensure an excellent dielectric breakdown voltage property. On the other hand, the microporous polyolefin membrane of the present invention, even the maximum pore diameter is more than 45 nm, as described later, since the high puncture strength and a small heat of fusion shrinkage is compatible at a high level, the breakdown voltage property it can be excellent or short-circuit resistance.
[0021]
　The maximum pore size, as described below, when producing the microporous polyolefin membrane, for example, the content of ultra-high molecular weight polyethylene and high-density polyethylene used as the raw material, and the weight average molecular weight Mw, the or adjusting the stretching ratio by, it can be within the above range. The maximum pore size can be measured according to ASTM F316-86 (Bubble point method).
[0022]
　(Puncture strength)
　microporous polyolefin membrane of the present invention, the puncture strength per thickness 20μm is not less than 4.5 N, preferably 4.8N or more. Piercing strength, if within the above range, when incorporated into a cell a polyolefin microporous membrane as a separator, to suppress the occurrence of short-circuiting of the electrodes, it is possible to improve the thermal stability. The upper limit of puncture strength is not particularly limited, for example, or less per thickness of 20 [mu] m 10 N.
[0023]
　Puncture strength, as described later, when manufacturing the polyolefin microporous membrane, for example, the content of ultra-high molecular weight polyethylene and high-density polyethylene used as the raw material, and the weight average molecular weight Mw, the or to adjusting the stretching ratio Accordingly, it is within the above range.
[0024]
　Puncture strength is a spherical end surface (radius of curvature R: 0.5 mm) in diameter 1mm needle, the thickness T 1 the maximum load (N when the pierce the polyolefin microporous film ([mu] m) at a speed of 2 mm / sec ) is a value was measured. Further, the piercing strength per thickness 20μm (N / 20μm) is a value which can be calculated by the following equation.
Formula: puncture strength (N / 20 [mu] m) = the measured pin puncture strength (N) × 20 / thickness T 1 ([mu] m)
　(melt thermal shrinkage rate)
　microporous polyolefin membrane of the present invention, the load 2g per thickness 20 [mu] m over the TD direction (transverse direction, the width direction) when the 25% melting heat shrinkage of 3% or more, preferably 20% or less than 5%. If fusion heat shrinkage within the above range, the battery using the polyolefin microporous membrane as a separator of the present invention, when heated, moderately since it is possible to release heat energy, moderately abnormal heating of the battery gradually shorted by contracts to release energy, it is possible to suppress the thermal runaway of the battery. Further, in order to release energy gradually shorted by appropriately contracted when an abnormality heating of the battery, it is possible to suppress the thermal runaway of the battery.
[0025]
　Melt thermal shrinkage rate, as described later, when manufacturing the polyolefin microporous membrane, the content of ultra-high molecular weight polyethylene and high-density polyethylene used as the raw material, and the weight average molecular weight Mw, the or to adjusting the stretching ratio Accordingly, it is within the above range.
[0026]
　The melt thermal shrinkage rate, TMA at (Thermo-Mechanical Analysis), width 3 mm, in the longitudinal direction of the sample length 10 mm, a load of thickness 20μm per 2g (19.6mN), 5 to 160 ° C. from 30 ° C. ° C. / when melted by heating in minutes, the initial length to (10 mm), shrinkage length becomes maximum (L 1 means a value determined as a percentage ratio of shrinkage length mm). Incidentally, the longitudinal direction of the sample was made to be TD direction of the polyolefin microporous membrane (lateral direction).
[0027]
　That is, molten thermal shrinkage rate (%), the thickness T 1 in a sample (width 3mm × length 10 mm) of the ([mu] m), can be determined by the following equation.
Formula: 100 × [10mm- length when shrunken most in the length direction (L 1 )] / 10 mm
, however, the load in the longitudinal direction [2 × (T 1 /20 [mu] m)] g, the length of the sample direction is the TD direction of the microporous polyolefin membrane (lateral direction).
[0028]
　(Film thickness)
　film thickness of the microporous polyolefin membrane of the present invention is not particularly limited, for example, a 30μm or less, preferably 4μm or more 30μm or less, more preferably 5μm or 25μm or less. The film thickness is cut out polyolefin microporous film to a size of 5 cm × 5 cm, 5 points of the four corners and the center portion, the contact thickness meter measured using, and the average was taken as the thickness ([mu] m).
[0029]
　(Air permeability)
　microporous polyolefin membrane of the present invention, the air permeability per thickness 20 [mu] m (Gurley value) is not particularly limited, for example, 100 sec / 100 cm 3 or more 600 sec / 100 cm 3 or less, preferably 400 seconds / 100Cm 3 is less than or equal to. If the air permeability is in the above range, when used as a separator for a secondary battery, excellent in ion permeability. Air permeability, by adjusting the like stretching conditions for producing the polyolefin microporous membrane, it is in the above range.
[0030]
　The air permeability per thickness 20 [mu] m P 2 (s / 100 cm 3 /20 [mu] m)) is a value that can be determined by the following equation. Formula:
P 2 = P 1 (sec / 100 cm 3 ) × 20 ([mu] m) / film thickness T 1 ([mu] m) Incidentally, the air permeability P 1 , the thickness T 1 relative to microporous film ([mu] m), in conformity with JIS P-8117, air permeability meter (Asahi Seiko Co., EGO-1T) values can be measured by (sec / 100 cm 3 a).
[0031]
　(Porosity)
　The porosity of the microporous polyolefin membrane is not particularly limited, for example, 70% more than 10% or less, preferably 60% or less than 20%, more preferably at less than 55% 25% is there. By porosity is within the above range, increase the amount retention of the electrolyte solution, it is possible to ensure a high ion permeability.
[0032]
　Porosity, the microporous membrane weight w 1 and its equivalent pore-free weight w of the polymer 2 (width, length, same polymer composition) were compared, and by the following equation, can be measured.
Formula: Porosity (%) = (w 2 -w 1 ) / w 2 × 100
　(breakdown voltage)
　microporous polyolefin membrane of the present invention, the breakdown voltage per thickness 20 [mu] m (withstand voltage) 3. it is preferably from 0kV than 4.0 kV, more preferably less than 3.05kV 3.95kV, and particularly preferably less than 3.1kV 3.9kV. Microporous polyolefin membrane of the present invention, as described above, since the both the puncture strength and the melting heat shrinkage at a high level, excellent voltage resistance.
[0033]
　Here, the breakdown voltage per thickness 20 [mu] m, the thickness T 1 of the breakdown voltage in the microporous film ([mu] m) V 1 when the (kV), the formula: V 2 = (V 1 × 20) / T 1 breakdown voltage V is calculated by 2 shows the average value of.
[0034]
　Incidentally, the breakdown voltage (withstand voltage) can be measured by the following method.
(1) to 150mm square on aluminum plate, the thickness T was cut into a diameter of 60 mm 1 Place the polyolefin microporous film, at the cylindrical electrodes of brass 50mm diameter thereon, Kikusui Electronics Ltd. TOS5051A connect the withstand voltage tester.
(2) the polyolefin microporous membrane, went energized at a boost rate of 0.2 kV / sec, the value V when the insulation breakdown 1 reads, conversion equation: V 2 = (V 1 × 20) / T 1 on the basis of the breakdown voltage V per thickness 20 [mu] m 2 is calculated.
(3) breakdown voltage V 2 for 15 times measurements, the mean value and the value of the breakdown voltage (withstand voltage).
[0035]
　(Composition)
　The microporous polyolefin membrane of the present invention comprises a polyolefin resin as a main component material. The molecular weight distribution of the polyolefin resin, 5.0 × 10 6 or more ratio is 0.6 to 1.5% is preferably 0.7 to 1.4%. If 0.6% or more can obtain a sufficient pin puncture strength, and it is possible to prevent the maximum pore diameter is excessively large. If 1.5% or less, it is possible to avoid that the melted thermal shrinkage rate becomes large, and it is possible to prevent the maximum pore diameter is excessively large. The molecular weight distribution is a value measured by gel permeation chromatography (GPC).
[0036]
　Polyolefin resin is preferably a main component of polyethylene. From the viewpoint of further enhancing the puncture strength, with respect to 100 wt% total polyolefin resin, preferably polyethylene is 80 mass% or more, more preferably 90 mass% or more, to be 99 wt% more preferably, it is more preferable to use polyethylene alone.
[0037]
　Polyethylene is not only an ethylene homopolymer, or a copolymer containing a small amount of other α- olefins. The α- olefin propylene, butene-1, hexene-1, pentene-1, octene, vinyl acetate, methyl methacrylate, styrene, and the like.
[0038]
　As the type of polyethylene, density of 0.94 g / cm 3 density polyethylene as weight, density 0.93 g / cm 3 or more 0.94 g / cm 3 or less of medium density polyethylene range, density 0 .93G / cm 3 less than the low-density polyethylene, linear low density polyethylene. Among them, from the viewpoint of further improving the puncture strength, it is preferred to include a high density polyethylene.
[0039]
　The weight average molecular weight of high-density polyethylene used (hereinafter, referred to as Mw) of, 1 × 10 5 is preferably at least, and more preferably 2 × 10 5 not less than. The upper limit of the Mw of high-density polyethylene is preferably Mw of 8 × 10 5 or less, more preferably Mw of 7 × 10 5 or less. If Mw of the high-density polyethylene is within the above range, it is possible to achieve both high puncture strength and melt thermal shrinkage rate. The above Mw refers to the Mw of high-density polyethylene used as the starting material. Further, Mw is a value measured by gel permeation chromatography (GPC).
[0040]
　The content of high-density polyethylene is not particularly limited, for example, the whole polyolefin resin as 100% by weight, can be 60 mass% to 95 mass% or less, be more than 80 wt% 90 wt% or less preferable.
[0041]
　Further, the polyethylene used preferably contains ultra high molecular weight polyethylene. Ultra high molecular weight polyethylene is not only an ethylene homopolymer, or a copolymer containing a small amount of other α- olefins. Other α- olefin other than ethylene may be the same as described above. When containing ultra high molecular weight polyethylene, it is possible to improve the pin puncture strength.
[0042]
　The Mw of the ultra-high molecular weight polyethylene, 1 × 10 6 or more, more preferably 1 × 10 6 2 × 10 or 6 is preferably less than. If Mw of the ultra-high molecular weight polyethylene is in the above range, the resulting microporous polyolefin membrane, a decrease in melt thermal shrinkage rate due to the decrease of the polyethylene molecules across the increase in puncture strength by refining the pores and fibrils, a plurality of lamellae preparative, it is possible to achieve both. Also, Mw of the ultra-high-molecular-weight polyethylene 2 × 10 6 is less than, for polyethylene molecules that span lamellae not too much, it is possible to prevent the molten thermal shrinkage rate increases. The 2 × 10 the Mw 6 by less than, be ensured miscibility with high density polyethylene and ultrahigh molecular weight polyethylene during melt-kneading, it is possible to avoid a reduction in pin puncture strength. The above Mw refers to the Mw of the ultra-high molecular weight polyethylene used as the starting material. Further, Mw is a value measured by gel permeation chromatography (GPC).
[0043]
　The content of ultra-high molecular weight polyethylene, with respect to 100 wt% total polyolefin resin, it is preferable that the lower limit is 10 mass% or more, it is preferable upper limit is less than 30 wt%. When the content of the ultrahigh molecular weight polyethylene is in the above range, the the puncture strength and the maximum pore diameter and the melting heat shrinkage, microporous polyolefin membrane was in the range described above can be easily obtained.
[0044]
　Further, the microporous polyolefin membrane of the present invention, low density polyethylene, linear low density polyethylene, ethylene · alpha-olefin copolymer produced by single site catalyst, such as low molecular weight polyethylene having a weight-average molecular weight of 1000 to 4000 it may also include a. If the addition of these polyethylene, can be granted a shutdown function at low temperatures, improving the properties as a battery separator. However, the low molecular weight polyethylene is large, the stretching process during manufacture, the breaking of the microporous film is likely to occur. The content of the low molecular weight polyethylene, for example, with respect to 100 wt% total polyolefin resin, 0 wt% to 10 wt% or less, may be 5 mass% or less than 0 wt%, 0 wt% or more it may be not more than 1 wt%.
[0045]
　Further, the microporous polyolefin membrane of the present invention may comprise polypropylene. When adding the polyethylene and polypropylene, can be further improved meltdown temperature when the polyolefin microporous membrane of the present invention is used as a battery separator. The types of polypropylene homopolymers, block copolymers, and can be used a random copolymer. Block copolymer, a random copolymer can contain a copolymer component of propylene, and other α- olefins other than propylene. In these propylene copolymers, other α- olefins, ethylene is preferable. If the addition of polypropylene, as compared with the case of polyethylene alone, because the puncture strength tends to decrease, from the viewpoint of improving the piercing strength, the content of the polypropylene is preferably small. The content of polypropylene, for example, with respect to 100 wt% total polyolefin resin, 0 to 10 mass% is preferred, may be less than 5 wt% or more 0 wt%, a 1 mass% or more 0 wt% it may be.
[0046]
　The weight average molecular weight of the polyolefin resin (total) (Mw) is 1.0 × 10 5 preferably at least. Mw of the polyolefin resin is 1.0 × 10 5 If more preferable because hardly causes breakage during stretching. Incidentally, Mw is a value measured by gel permeation chromatography (GPC).
[0047]
　Additional, the microporous polyolefin membrane of the present invention, to the extent not to impair the effects of the present invention, antioxidant, heat stabilizer and antistatic agent, an ultraviolet absorber, and further various additives such as antiblocking agent and filler agent may be contained. In particular, for the purpose of suppressing oxidation deterioration due to heat history of the polyethylene resins, it is preferable to add an antioxidant. It is important as an adjustment or enhancement of properties of the microporous membrane of selecting the type and amount of antioxidant and thermal stabilizer as appropriate.
[0048]
　Further, the microporous polyolefin membrane of the present invention is substantially free of inorganic particles. Here, "substantially free of inorganic particles", for example, when quantifying inorganic elements in fluorescent X-ray analysis, 300 ppm or less, the content preferably 100ppm or less, and most preferably equal to or less than the detection limit means. Incidentally, adhesion in the manufacturing process of the polyolefin microporous membrane, even without addition of actively particles to the polyolefin microporous membrane, and contamination components derived from foreign substances, the lines and equipment in the raw material resin or a polyolefin microporous membrane manufacturing process dirt is peeled off, because it may be mixed into the film, may contain inorganic particles traces above range.
[0049]
　[Production method of microporous polyolefin membrane]
　Production method of microporous polyolefin membrane of the present invention, as long obtained polyolefin microporous film having the above characteristics is not particularly limited, the method for producing a known microporous polyolefin film it can be used. Hereinafter will be described an example of a manufacturing method of the microporous polyolefin membrane of the present invention in detail, but is not limited to this embodiment.
[0050]
　Method of producing a microporous polyolefin membrane of the present invention preferably includes the following steps.
(A) a polyolefin resin and a plasticizer and melt-kneading the polyolefin solution preparation step by
step (b) and cooled to form extrudates extruded from the extruder the polyolefin solution to form the gel-like sheet
(c ) the gel-like sheet, the longitudinal direction (machine direction, MD direction) and lateral direction (machine direction and perpendicular to obtain a stretched film simultaneously stretched in the TD direction)
extracting plasticizer from (d) above stretching film to microporous membrane obtained process
step of drying the (e) the microporous membrane
　should be noted that the previous step (c) ~ (e), the middle, hydrophilic treatment after, charge elimination, other steps of the re-drawing, etc. it may be added. Hereinafter, each step will be described.
[0051]
　(A) Preparation of the polyolefin solution
　are first prepared polyolefin solution by melt kneading a polyolefin resin and a plasticizer. The plasticizer is not particularly limited as long as the solvent can sufficiently dissolve the polyethylene (film solvent) may be a conventionally known solvents. Among them, in order to enable relatively high draw ratio of the solvent is preferably liquid solvent is a liquid at room temperature.
[0052]
　The liquid solvent, nonane, decane, decalin, p-xylene, undecane, dodecane, aliphatic such as liquid paraffin, cycloaliphatic or aromatic hydrocarbons, and mineral oil fractions having a boiling point corresponding to these, as well as dibutyl phthalate , at room temperature, such as dioctyl phthalate phthalic acid ester of the liquid. Among them, from the viewpoint of obtaining a stable gel-like sheet, to use a non-volatile liquid solvents such as liquid paraffin preferred.
[0053]
　As the plasticizer, be a solid solvent (solid solvent) at room temperature, the melt-kneading conditions, a solvent miscible with polyethylene, it may be mixed with the liquid solvent. Such solid solvents, stearyl alcohol, ceryl alcohol, and paraffin wax. However, the use of only solid solvent, there is a possibility that the uneven stretching, etc. may occur.
[0054]
　In the polyolefin solution, the blending ratio of the polyolefin resin, per 100 parts by weight of polyolefin resin and a plasticizer, from the viewpoint of improving the moldability of the extrudate, preferably the polyolefin resin 10 parts by mass or more 50 parts by weight . If the content of the polyolefin resin is 10 parts by mass or more, in order Suueru and neck-at the exit of the die when forming into a sheet is small, the moldability and film forming property of the sheet is improved. Also, when the content of the polyolefin resin is less than 50 parts by mass, because the thickness direction shrinkage is small, it has good moldability and film forming properties.
[0055]
　The lower limit of the polyolefin solution, the content of the polyolefin resin, more preferably 20 parts by mass or more. The upper limit of the polyolefin resin, more preferably not more than 40 parts by mass, more preferably not more than 35 parts by mass. If the content of the polyolefin resin is in the above range, as described below, both the puncture strength and air permeability is easily obtained, it becomes possible to control the friction coefficient of the front and back of the membrane.
[0056]
　Incidentally, the polyolefin resin has a weight average molecular weight 1.0 × 10 as described above 6 preferably includes more ultra-high molecular weight polyolefin. Plasticizers polyolefin solution, because it is removed by extraction (washing) in a subsequent step, the composition of the polyolefin resin may be the same as the composition of the above microporous polyolefin membrane.
[0057]
　The viscosity of the plasticizer is preferably 20cSt or more 200cSt less at 40 ° C.. If the viscosity at 40 ° C. or more 20 cSt, sheet extruding the polyolefin solution through a die is less likely to become uneven. On the other hand, it is easy to remove the plasticizer if less 200 cSt.
[0058]
　The method of melt kneading the polyolefin solution is not particularly limited, for example, it is possible to perform melt-kneading using an extruder. If you wish to prepare a high-concentration polyolefin solution, even in an extruder, it is particularly preferable to carry out the melt-kneaded using a twin-screw extruder. Further, the polyolefin solution may optionally various additives of antioxidants within a range not to impair the effects of the present invention may be added. It is particularly preferred to add an antioxidant to prevent oxidation of the polyethylene.
[0059]
　In the extruder, at a temperature at which the polyolefin resin is completely melted and uniformly mixed polyolefin solution. Melt-kneading temperature varies depending polyolefin resin to be used, the lower limit of the melt-kneading temperature is preferably not less than (the melting point + 10 ° C. of the polyolefin resin), more preferably (the polyolefin resin melting point + 20 ° C.) or higher. The upper limit of the melt-kneading temperature is less preferable to be (the polyolefin resin having a melting point of + 120 ° C.) or less, more preferably (the polyolefin resin melting point + 100 ° C.).
[0060]
　Here, the melting point and, based on JIS K7121 (1987), a value measured by DSC (hereinafter, the same). For example, specifically, as a main component of polyethylene, if the polyolefin resin containing with respect to the total resin 90 wt% or more (polyethylene composition), the polyethylene composition has a melting point of about 130 ~ 140 ° C., melt kneading the lower limit of the temperature is preferably 140 ° C., more preferably 160 ° C. or higher, most preferably 170 ° C. or higher. The upper limit is preferably 250 ° C. below the melt kneading temperature, 230 ° C. or less, most preferably 200 ° C. or less.
[0061]
　Also, the melt kneading temperature when containing polypropylene polyolefin solution is preferably 190 ° C. or higher 270 ° C. or less.
[0062]
　The melt-kneading temperature in view of suppressing the deterioration of the polyolefin resin is preferably low. If the melt kneading temperature is 190 ° C. or higher, non-melt extrudates extruded hardly occurs from the die, it is possible to avoid the broken film or the like in a later stretching step. Further, if the following 270 ° C. The melt-kneading temperature is, it is possible to suppress the thermal decomposition of polyolefins, the physical properties of the resultant microporous membrane, for example, it is possible to suppress the pin puncture strength, tensile strength and the like from being excessively lowered .
[0063]
　When using a twin-screw extruder, from the viewpoint ratio of the screw length (L) and diameter (D) (L / D) is to obtain a dispersion and distribution of the good processability kneading property with the resin, it is 20 to 100 inclusive preferable. The lower limit of L / D is more preferably 35 or more. The upper limit of L / D is more preferably 70 or less. If the L / D is 20 or more, it can be performed more sufficiently melted and kneaded. If L / D is 100 or less, can be the residence time of the polyolefin solution is not too long, the appropriate residence time.
[0064]
　Further, while preventing deterioration of the polyolefin resin to be kneaded from the viewpoint of obtaining good dispersibility and distribution property, preferably a cylinder inner diameter of the biaxial extruder is more than 40mm 100mm or less.
[0065]
　Twin-screw extruder having a screw rotational speed (Ns) is well disperse the polyolefin in the extrudate, from the point of view of obtaining a uniform thickness excellent microporous film is preferably not more than more than 150 rpm 600 rpm. Further, the ratio (Q / Ns) of Ns extrusion of the polyolefin solution for (rpm) Q (kg / h) is preferably below 0.6kg / h / rpm. More preferably not more than 0.35kg / h / rpm.
[0066]
　Formation of (b) the extrudate and molding the gel-like sheet
　is then shaping the gel-like sheet by cooling to form an extrudate extruded from the extruder the polyolefin solution. First, a melt-kneading polyolefin solution directly in an extruder, or even via another extruder, through a die extruding, molding to the extrudate so that the thickness of the microporous membrane of the final product is 5 ~ 100 [mu] m obtained. Die may be used a rectangular T-die. When using a T-die, a microporous membrane to control the thickness tends aspect of the final product, a slit gap of the die is preferably 0.1mm or more 5mm or less, preferably heated at 140 ° C. or higher 250 ° C. or less during extrusion .
[0067]
　Then, the gel-like sheet is obtained by cooling the extrudates obtained. Cooling, it is possible to fix the microphase polyethylene (polyolefin) separated by a plasticizer. Cooling, it is preferable to cool the extrudate obtained to below the crystallization end temperature.
[0068]
　Cooling the front and rear surfaces both of the gel-like sheet, until below the crystallization end temperature, preferably carried out at 250 ° C. / min or faster, more preferably 300 ° C. / min or faster. If the cooling rate is in the above range, in the extrudate, crystals that form gels do not coarsen, it is possible to obtain a dense conformation, roughness of the surface is less likely to become uneven. Further, when the cooling rate is in the above range, it is possible conformation obtain a fine gel-like sheet, then the molecular orientation is likely advances in the stretching, suppress the occurrence of puncture strength and air permeability of the both and curl it is possible to become.
[0069]
　Here, the crystallization end temperature is that the extrapolated crystallization end temperature as measured according to JIS K7121 (1987). Specifically, polyethylene with extrapolated crystallization end temperature of about 70 ° C. or higher 90 ° C. or less in the case of (polyethylene composition). Moreover, the cooling rate here is, can be determined by the temperature difference between the time until the resin temperature is the crystallization completion temperature, and the resin temperature of the extruder outlet and the crystallization ending temperature at the outlet of the extruder. Accordingly, in the cooling step, when cooling to below the crystallization end temperature, the difference between the respective gel-like sheet temperature sides of the resin temperature and the cooling step the outlet of the extruder outlet, divided by the time to pass through the cooling step the things.
[0070]
　As the cooling method of the extrudate, cold air, cooling water, a method of direct contact with the other cooling medium, a method of contacting the roll cooled by the refrigerant, and a method of using a casting drum or the like. Incidentally, the solution was extruded from the die is taken off during the cooling before or cooling at a predetermined take-off ratio, the lower limit of the take-off ratio of 1 or more. The upper limit is preferably 10 or less, more preferably 5 or less. If take-off ratio is 10 or less, neck-in is reduced, hardly cause rupture during stretching.
[0071]
　The lower limit of the thickness of the gel-like sheet is preferably at least 0.5 mm, more preferably 0.7mm or more. The upper limit is less than 3mm, and more preferably 2mm or less. When the thickness of the gel sheet is 3mm or less, in the cooling process, hardly could uneven structure in the thickness direction, conformation throughout the thickness direction can be in close, and both close the front and back of the structure be able to. Also, if the 3mm or less the thickness of the gel-like sheet, the cooling rate of the gel-like sheet tends to the preferred ranges described above.
[0072]
　Above, laminated although microporous polyolefin film has been described the case of a single-layer, microporous polyolefin membrane of the present invention is not limited to a single layer, formed by laminating two or more microporous film (layer) it may be a body. Each microporous membrane to form a laminate (the layer) may be a polyolefin microporous membrane of the same composition, or may be a polyolefin microporous membrane having different compositions. Microporous polyolefin membrane to form a laminated body, other polyethylene as described above, may include a respective desired resin to an extent not to impair the effects of the present invention. As a method of forming a laminate by laminating a polyolefin microporous membrane, it is possible to use a conventional method. Laminate, for example, prepared as necessary the desired polyolefin resin, at a desired temperature by supplying these resins separately to the extruder, the solvent and allowed to melt-kneading, by merging with a polymer tube or a die can be like performing extrusion through a slit die with each laminated layer thickness of interest is formed.
[0073]
　(C) simultaneous biaxial stretching
　then the gel-like sheet, the longitudinal direction (machine direction, MD direction) and lateral direction (machine direction perpendicular to the direction, TD direction) at the same time stretching (simultaneous biaxial stretching) to stretch film obtained. By thus performing longitudinal and transverse stretching simultaneously, it becomes possible to reduce the melting shrinkage ratio in the transverse direction. Stretching the gel-like sheet is heated is carried out at a predetermined magnification by a typical tenter method.
[0074]
　Stretch ratio varies depending on the thickness of the gel-like sheet, either 5 times or more in the direction, in area magnification is preferably stretched to more than 25 times. The stretching ratio of the longitudinal and transverse direction may be different.
[0075]
　Stretching temperature is preferably equal to or less than the polyolefin resin melting point, more preferably in the range of (the polyolefin crystal dispersion temperature Tcd of the resin) or more (the melting point of the polyolefin resin). When the stretching temperature is below the melting point of the gel-like sheet, the molten polyolefin resin is prevented, it is possible allowed to orient the molecular chains efficiently by stretching. Further, if the stretching temperature is more than the crystal dispersion temperature of the polyolefin resin, softening of the polyolefin resin is it is sufficient, due to the low draw tension, film formability becomes good, stretching at high magnification hardly ruptured during stretching it is possible.
[0076]
　Specifically, in the case of the polyethylene resin because it has a crystal dispersion temperature of about 90 ° C. or higher 100 ° C. or less, the stretching temperature is preferably 80 ° C. or higher. The upper limit of the stretching temperature is preferably 130 ° C. or less, and more is preferably 125 ° C. or less, most preferably 120 ° C. or less. Crystal dispersion temperature Tcd is obtained from the temperature characteristics of dynamic viscoelasticity was measured according to ASTM D 4065. Or may be obtained from the NMR.
[0077]
　Causes cleavage conformation formed into a gel in the sheet by stretching as described above, the crystalline phase is finer, numerous fibrils are formed. Fibrils form a three-dimensionally and irregularly connected network structure. The mechanical strength is improved by stretching, the pores are enlarged, it is possible to obtain a polyolefin microporous membrane can be suitably used as a battery separator.
[0078]
　In the production method of the microporous polyolefin membrane of the present invention, simultaneous biaxial stretching it is preferably carried out before removing the plasticizer in the gel-like sheet. For the plasticizer is a state when included in a sufficiently gelatinous sheet polyolefin is sufficiently plasticized softened by stretching prior to removal of the plasticizer, cleavage of conformation becomes smooth, crystal phase fine reduction can be uniformly perform. Thus, in the polyolefin microporous film obtained is more uniformly miniaturized pores and fibrils, and improvement in puncture strength, as described above, the compatibility between the molten thermal shrinkage rate can be achieved more stably .
[0079]
　(D) extraction of the plasticizer from the stretched film (cleaning)
　Next, the solvent remaining in the gel-like sheet, the extraction and removal, i.e. washing with cleaning solvent. Since separate from the polyolefin phase and the solvent phase, the microporous membrane is obtained by removing the solvent. The cleaning solvent, such as pentane, hexane, saturated hydrocarbons such as heptane, dichloromethane, tetrachlorinated hydrocarbons carbon tetrachloride, etc., ethers such as diethyl ether, dioxane, ketones such as methyl ethyl ketone, trifluoride ethane, C 6 F 14 , C 7 F 16 linear fluorocarbons such as, C 5 H 3 F 7 cyclic hydrofluorocarbons such as, C 4 F 9 OCH 3 , C 4 F 9 OC 2 H 5 hydrofluoroether such as, C 4 F 9 OCF 3 , C 4 F 9 OC 2 F 5 volatile solvents perfluoro ether of the like. These washing solvents have a low surface tension (e.g., below 24 mN / m at 25 ° C.). The use of low surface tension cleaning solvent, a microporous network which forms the cares during drying after washing - that shrinks by the surface tension of the liquid interface is suppressed, microporous having a high porosity and permeability have film can be obtained. These cleaning solvents suitably selected according to the solvent used to dissolve the polyolefin resin, used alone or in combination.
[0080]
　Washing process can be carried out a method of extracting by immersing the gel-like sheet in the cleaning solvent, a method for shower cleaning solvent into a gel sheet or by a method such as by a combination thereof. Although the amount of cleaning solvent varies depending cleaning method, generally to be gel-like sheet 300 parts by mass or more with respect to 100 parts by weight preferred. Washing temperature may at 15 ~ 30 ° C., heated to 80 ° C. or less as required. In this case, the viewpoint of enhancing the cleaning effect of the solvent, in view of the microporous film properties in the transverse and / or longitudinal direction of the physical properties of the microporous membrane obtained are prevented from becoming uneven, mechanical properties and electrical of the microporous membrane physical properties from the viewpoint of improving the time to gel-like sheet is immersed in the cleaning solvent it is longer the better the longer.
[0081]
　Washing as described above, the gel-like sheet after washing, i.e. preferably carried out until the residual solvent in the microporous membrane is less than 1 wt%.
[0082]
　(E) drying the microporous membrane
　after washing, the washed solvent was dried and removed. The method of drying is not particularly limited, heat drying method, drying by air-drying method or the like. Drying temperature is preferably from the crystal dispersion temperature Tcd of the polyethylene composition, in particular, it is preferably not more than (Tcd-5 ℃). Drying, the dry weight of the microporous membrane 100 mass%, is preferably carried out to a residual cleaning solvent is 5 wt% or less, and more preferably carried out until the 3 wt% or less. Insufficient drying undesirably porosity of the microporous membrane in subsequent heat treatment is lowered, the permeability deteriorates.
[0083]
　(F) Other Steps
　In addition vertically after washing drying in order to improve the mechanical strength of the puncture strength and the like or crosswise or both directions of about 5% and 20% stretch, (hereinafter referred to as re-stretched) may be performed.
[0084]
　On the other hand, in the manufacturing method of the microporous polyolefin membrane of the present invention, a stretched film or microporous membrane after stretching may be treated heat treatment and / or thermal relaxation. Heat-treatment, the crystal by heat relaxation treatment is stabilized, lamellar layer is made uniform, pore size is large, can be fabricated microporous film excellent in strength. Heat treatment is performed within a temperature range of the crystal dispersion temperature or higher-melting point of the polyolefin resin constituting the polyolefin microporous membrane. Heat treatment is conducted by a tenter method, a roll method or a rolling method. Heat time is, for example, 60 seconds or less 20 seconds or more.
[0085]
　As the annealing method can utilize a method disclosed in, for example, JP-2002-256099.
[0086]
　Further, according to other applications, it may be subjected to a hydrophilic treatment to the stretching film or microporous membrane. The hydrophilic treatment can be conducted monomer grafting, surfactant treatment, a corona discharge or the like. Monomer-grafting is preferably carried out after the crosslinking treatment.
[0087]
　For detergent treatment, a nonionic surfactant, cationic surfactant, but any of anionic surfactants and amphoteric surfactants can be used, nonionic surfactants are preferred. Surfactant water or methanol, ethanol, or immersing the microporous membrane in a solution of a lower alcohol such as isopropyl alcohol, or coated with the solution by a doctor blade method microporous membrane.
[0088]
　If necessary, in a mixed atmosphere of stretch film or at least one side air or nitrogen or carbon dioxide and nitrogen of the microporous membrane, it may be corona discharge treatment.
[0089]
　[Applications]
　The microporous polyolefin membrane of the present invention can be suitably used as a separator (separation member) of the electrochemical reactor, such as a battery or a capacitor. Cell includes at least a positive electrode, a negative electrode, and a separator. Among them, the non-aqueous electrolyte secondary battery can be suitably used particularly as a separator for lithium ion secondary battery. Further, the microporous polyolefin membrane of the present invention can be suitably used as it is but the separator may be laminated coating layer having a non-woven fabric and heat resistance. In this case, it can be suitably used as a separator.
[0090]
　Microporous polyolefin membrane of the present invention, when used as a separator for lithium ion secondary batteries, since 4.5N or higher and puncture strength is large, the thermal stability of the sufficient battery be thinned for high energy density it can be ensured. The molecular weight distribution of the polyolefin resin, 5.0 × 10 6 or more ratios can be secured enough puncture strength for 0.6 to 1.5% and it is possible to suppress the melt heat shrink small.
Example
[0091]
　[Measurement of Physical Properties Method
　explaining a measuring method of the properties below.
[0092]
　(1) Thickness (average thickness)
　cut a polyolefin microporous film to a size of 5 cm × 5 cm, measured 5 points at the four corners and the center, and the average was taken as the thickness ([mu] m). Using contact thickness meter for measurement.
[0093]
　(2) the microporous polyolefin membrane of puncture strength
　spherical end surface (radius of curvature R: 0.5 mm) diameter 1mm in needle, the thickness T1 ([mu] m) microporous membrane when the puncture at a speed of 2 mm / sec the maximum load was measured. The maximum load measurements La, the formula: by Lb = (La × 20) / T1, converting the film thickness to the maximum load Lb when formed into a 20 [mu] m, and a penetration strength (gf / 20μm).
[0094]
　(3) Melt thermal shrinkage rate
　in TMA (Thermo-Mechanical Analysis), width 3 mm, applying a 2g load in the longitudinal direction of the sample length 10 mm, when the temperature was raised at 5 ° C. / min up to 160 ° C. from 30 ° C. the / 10 mm (length when shrunken 10mm- most), and a heat of fusion shrinkage.
[0095]
　(4) the maximum pore size
　maximum pore diameter of the microporous membrane, a method in accordance with ASTM F316-86 (Bubble point method
was measured by). Incidentally, PMI Co. PERMPOROMETER (model
number: CF is used as a measuring instrument P-1500A), and the Galwick as measurement liquid, were used, respectively.
[0096]
　(5) Weight average molecular weight (Mw), molecular weight distribution
　Mw of UHMWPE and HDPE was used as a material was determined by gel permeation chromatography (GPC) method under the following conditions.
Measurement apparatus: Waters Corporation Ltd. GPC-150C
-Column: Showa Denko KK Shodex UT806M
· Column temperature: 135 ° C.
Solvent (mobile phase): o-dichlorobenzene
-solvent flow rate: 1.0 ml / min
Sample concentration : 0.1 wt% (dissolution conditions: 135 ° C. / 1h)
- injection volume: 500 [mu] l
- detector: Waters Corporation Ltd. differential Refractometer
- calibration curve: Produced from a calibration curve obtained using a monodisperse polystyrene standard samples, It was prepared by using a predetermined conversion constant.
[0097]
　(6) Evaluation of breakdown voltage performance
　over 150mm square aluminum plate, the thickness T was cut into a diameter of 60 mm 1 Place the microporous film, at the cylindrical electrodes of brass 50mm diameter thereon, was connected Kikusui made TOS5051A withstand voltage tester. 0.2 kV / sec went energized at a rate of rise of the value V when the insulation breakdown 1 reads, conversion equation: V 2 = (V 1 × 20) / T 1 on the basis of the per thickness 20μm the breakdown voltage V 2 was calculated. Breakdown voltage V 2 measurements was carried out 15 times, the maximum value, to obtain a mean value and a minimum value.
[0098]
　Further illustrate the present invention by the following experimental examples. The present invention unless departing from the gist of the present invention is not limited to the description of the experimental examples below.
[0099]
　[Example 1]
　
　weight-average molecular weight (Mw) of 1.0 × 10 6 of the 10 wt% ultrahigh molecular weight polyethylene (UHMWPE), Mw is 2.8 × 10 5 high density polyethylene ( HDPE) polyethylene (PE) composition 100 parts by mass consisting of 90% by weight of tetrakis [methylene-3- (3,5-di - tert-butyl-4-hydroxyphenyl) - propionate] methane 0.375 parts by It was dry-blended to obtain a mixture.
[0100]
　The resulting mixture 28.5 parts by weight was charged into a strong-blending double-screw extruder, supplying liquid paraffin 71.5 parts by mass from the side feeder of the twin-screw extruder, and melt-kneaded at a temperature of 180 ° C. to prepare a polyethylene solution. Incidentally, L / D in the range of 20 to 100, a cylinder inner diameter of the biaxial extruder 54 mm, Q / Ns is a condition equal to or less than 0.6 kg / h / rpm, were melt-kneaded.
[0101]
　The resulting polyethylene solution was supplied to a T-die from the twin-screw extruder, extruded as a sheet-like formed body. The extrudate was until below the crystallization end temperature while drawing by a cooling roll controlled at 25 ° C., then cooled at 250 ° C. / min or faster, to form a gel-like sheet. The resulting gel-like sheet by a tenter method to be 5 × 5 times at a stretching temperature 115 ° C. was simultaneously biaxially stretched. The film after stretching was washed with the cleaning tank dichloromethane controlled at 25 ° C., to remove the liquid paraffin. The washed membranes were dried in a drying oven adjusted to 60 ° C., to obtain a microporous film having a thickness of about 20μm by fixing for 40 seconds heat at 125 ° C. in the tenter.
[0102]
　Examples 2-3, Comparative Examples 1-5
　except that the resin composition of the polyolefin microporous film was changed as shown in Table 1 in the same manner as in Example 1 to prepare a polyolefin microporous membrane.
[0103]
　Table 1 shows the physical properties of the resulting microporous polyolefin membrane in Examples 1-3 and Comparative Examples 1-3.
[0104]
[Table 1]

[0105]
　(Evaluation Results)
　from Table 1, the polyolefin of Example microporous membrane, the lateral direction of the molten thermal shrinkage rate of 20μm conversion is 3 to 25% puncture strength of 20μm conversion was more than 4.5 N. Microporous polyolefin membrane of these embodiments, withstand voltage, and a 3 kV / 20 [mu] m or more, excellent withstand voltage characteristics.
[0106]
　In contrast, in the molecular weight distribution of the polyolefin resin of the separator, 5.0 × 10 6 Comparative Example 1 or a ratio of 0.3%, the transverse direction of the molten thermal shrinkage rate of 20μm conversion is less than 3%. As a result, when used as a battery separator, since the battery begins micro short circuit between the positive and negative electrode insufficient remains thermal runaway when heated, the energy of the battery is inferior in thermal stability because they are released at a time. Further, 5.0 × 10 6 or more of the comparative example of the ratio of 0.5% 2, the maximum pore diameter is larger than 60 nm. As a result, when used as a battery separator, self-discharge increases. Further, 5.0 × 10 6 or more ratio Comparative Example 3 1.6% of the transverse direction of the molten thermal shrinkage rate of 20μm conversion is greater than 25%. As a result, when used as a battery separator, occurs abruptly short circuit between the positive and negative electrode when the battery is melted is heated separator, the energy of the battery is poor are released thermal stability at a time.
[0107]
　Further, 5.0 × 10 6 or more ratio Comparative Example 4 1.6%, the puncture strength of 20μm converted is smaller than 4.5 N. As a result, when used as a battery separator, comprising the separator is likely to occur smoke or fire was broken when shocked. The 5.0 × 10 6 Comparative Example of the above ratio is 3.2% 5, the horizontal direction of the molten thermal shrinkage rate of 20μm conversion is greater than 25%. As a result, when used as a battery separator, occurs abruptly short circuit between the positive and negative electrode when the battery is melted is heated separator, the energy of the battery is poor are released thermal stability at a time.

The scope of the claims

[Requested item 1]Puncture strength per thickness 20μm is not less than 4.5 N, comprises a polyolefin, the molecular weight distribution of the polyolefin has a weight average molecular weight 5.0 × 10 6 that more ratio is 0.6 to 1.5% microporous polyolefin membrane which is characterized.
[Requested item 2]
　TD direction of the molten thermal shrinkage ratio when a load is applied 2g per thickness 20μm is 25% less than 3%, the microporous polyolefin membrane according to claim 1.
[Requested item 3]
　Maximum pore diameter is 60nm or less than 45 nm, the polyolefin microporous membrane according to claim 1 or claim 2.
[Requested item 4]
　The polyolefin is polyethylene, microporous polyolefin membrane according to any one of claims 1 to 3.
[Requested item 5]
　The polyethylene, the weight-average molecular weight 1.0 × 10 6 containing more ultra-high molecular weight polyethylene, microporous polyolefin membrane according to claim 4.
[Requested item 6]
　The ultra-high molecular weight polyethylene with respect to 100 mass% of the whole polyolefin resin, containing less than 10 wt% to 30 wt%, the microporous polyolefin membrane according to claim 5.
[Requested item 7]
　The weight average molecular weight of ultra-high molecular weight polyethylene 2.0 × 10 6 less than, the polyolefin microporous membrane according to claim 5 or 6.
[Requested item 8]
　A positive electrode, a negative electrode, and includes a separator, the separator, a battery using the polyolefin microporous membrane according to any one of claims 1 to 7.
[Requested item 9]
　It is a lithium ion secondary battery, battery of claim 8.