JP4121846B2 - Polyolefin microporous membrane and production method and use thereof - Google Patents

Description

表１つづき

表１つづき

表１つづき

【００７０】
注：(1) 溶液中のポリオレフィン組成物濃度
(2) MD；機械方向、TD；垂直方向
(3) 膜厚変動σt ／平均膜厚×100
【００７１】
表１に示すように、本発明のポリオレフィン微多孔膜は、特に膜厚変動率が小さく、それとともに透気度、空孔率、機械的強度、寸法安定性、シャットダウン特性及びメルトダウン特性のバランスに優れている。一方比較例１のポリオレフィン微多孔膜はポリプロピレンを含まないため、実施例１〜６と比較してメルトダウン温度が低い。また比較例２〜７のポリオレフィン微多孔膜は、ポリプロピレンのMwが5×10⁵未満であるか、ポリプロピレンの融解熱が90 J／g未満であるか、又はその両方であるため、実施例１〜６と比較して膜厚変動率が大きい。
【００７２】
【発明の効果】
以上詳述したように、本発明のポリオレフィン微多孔膜は、(a) ポリエチレン、及び(b) 重量平均分子量が5×10⁵以上であり、かつ走査型示差熱量計により測定される融解熱が90 J／g以上であるポリプロピレンを含むので、膜厚の均一性、耐熱性、機械的特性、透過性、寸法安定性、シャットダウン特性及びメルトダウン特性のバランスに優れている。得られたポリオレフィン微多孔膜を電池用セパレーターとして用いることにより、容量特性、サイクル特性、低温域での放電特性等の電池特性だけでなく、耐熱性、耐圧縮性等の電池安全性及び電池生産性をも含めた全ての向上が可能になる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a microporous polyolefin membrane, a method for producing the same, and applications, and in particular, a polyolefin microporous having an excellent balance of film thickness uniformity, heat resistance, mechanical characteristics, permeability, dimensional stability, shutdown characteristics, and meltdown characteristics. The present invention relates to a porous membrane, a method for producing the same, and an application.
[0002]
[Prior art]
Polyolefin microporous membranes include battery separators used in lithium secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries, polymer batteries, etc., separators for electrolytic capacitors, reverse osmosis filtration membranes, ultrafiltration membranes, microfiltration membranes It is widely used for various filters such as moisture permeable waterproof clothing and medical materials. When the polyolefin microporous membrane is used as a battery separator, particularly a lithium ion battery separator, the performance is deeply related to battery characteristics, battery productivity, and battery safety. Therefore, the polyolefin microporous membrane is required to have excellent mechanical characteristics, heat resistance, permeability, dimensional stability, shutdown characteristics, meltdown characteristics, and the like.
[0003]
In general, a microporous membrane made of polyethylene alone has a low meltdown temperature and weak mechanical strength, while a microporous membrane made of polypropylene alone has a high shutdown temperature. Often manufactured. In order to improve mechanical strength, it is preferable to further add ultrahigh molecular weight polyolefin. For example, Japanese Patent Application Laid-Open No. 3-64334 uses a composition containing an ultrahigh molecular weight polyolefin and having a value of (weight average molecular weight / number average molecular weight) within a specific range, and a process for removing polyolefin fine particles by a process in which solvent removal treatment is not performed. A method for producing a porous membrane is proposed. In addition, Japanese Patent Laid-Open Nos. 4-126352 and 5-234578 propose separators made of a composition containing an ultrahigh molecular weight polyolefin as an essential component.
[0004]
However, a microporous membrane containing polyethylene and polypropylene, especially a microporous membrane containing ultra-high molecular weight polyolefin, is often inferior in film thickness uniformity compared to a microporous membrane made of only polyethylene. For example, Japanese Patent Laid-Open No. 6-96753 proposes a polyolefin microporous porous membrane made of high molecular weight polyethylene and high molecular weight polypropylene and not using ultrahigh molecular weight polyethylene. A separator for cylindrical electrical parts made of a polyolefin composition similar to that of 96753 is proposed. JP-A-6-96753 and JP-A-6-223802 indicate that when ultra-high molecular weight polyethylene is added to the polyolefin compositions described in these documents, the surface of the microporous porous membrane or the separator becomes crusted and cannot be formed successfully. It is described in each comparative example.
[0005]
Furthermore, the present inventors use the composition obtained by removing inorganic powder from the composition described in the above-mentioned JP-A No. 5-234578 or the composition described in the above-mentioned JP-A No. 4-126352, and When manufactured by the process described in Kaihei 3-64334, it was confirmed that the surface of the microporous film became a crusted surface and could not be formed successfully. Thus, in the case of the composition containing ultra high molecular weight polyolefin, the uniformity of film thickness may deteriorate.
[0006]
If the microporous membrane has poor film thickness uniformity, it can easily cause short circuit when used as a battery separator, and it can easily cause safety problems such as low compression resistance. Also inferior.
[0007]
On the other hand, the present inventors made a polyolefin microporous material comprising a polypropylene having an MFR of 2.0 or less and a polyethylene having a weight average molecular weight / number average molecular weight of 8 to 100, and the polypropylene content is 20 wt% or less. A film was proposed (for example, Patent Document 1).
[0008]
[Patent Document 1]
JP 2002-194132 A
[0009]
[Problems to be solved by the invention]
However, it is desired to further improve the total balance including not only the improvement in film thickness uniformity but also heat resistance, mechanical characteristics, permeability, dimensional stability, shutdown characteristics, and meltdown characteristics.
[0010]
Accordingly, an object of the present invention is to provide a polyolefin microporous membrane excellent in the balance of film thickness uniformity, heat resistance, mechanical properties, permeability, dimensional stability, shutdown properties and meltdown properties, a method for producing the same, and uses thereof. Is to provide.
[0011]
[Means for Solving the Problems]
As a result of diligent research in view of the above object, the present inventors have found that (a) polyethylene, and (b) weight average molecular weight is 5 × 10.^FiveAnd the heat of fusion measured by a scanning differential calorimeter (ΔH_mIt has been found that a polyolefin microporous film containing polypropylene having a J) of 90 J / g or more has excellent film thickness uniformity. The inventor also provides (a) polyethylene and (b) a weight average molecular weight of 5 × 10^FiveAnd the heat of fusion measured by a scanning differential calorimeter (ΔH_m) Is 90 J / g or more of polypropylene, and (c) a solvent is melt-kneaded, the resulting melt-kneaded product is extruded from a die, then cooled to form a gel-like sheet, and then the gel-like sheet is stretched After removing the solvent, stretching after removing the solvent from the gel-like sheet, or removing the solvent after stretching the gel-like sheet, and further stretching, the film thickness is uniform It has been discovered that an excellent polyolefin microporous membrane can be obtained. The present invention has been completed based on this invention.
[0012]
That is, the polyolefin microporous membrane of the present invention has (a) polyethylene, and (b) a weight average molecular weight of 5 × 10.^FiveIt is the above, It is characterized by including the polypropylene whose heat of fusion measured by a scanning differential calorimeter is 90 J / g or more.
[0013]
The method for producing a polyolefin microporous membrane of the present invention comprises (a) polyethylene and (b) a weight average molecular weight of 5 × 10.^FiveThe polypropylene having a melting heat measured by a scanning differential calorimeter of 90 J / g or more and (c) a solvent are melt-kneaded, and the resulting melt-kneaded product is extruded from a die and then cooled. After the gel sheet is stretched, the solvent is removed after stretching the gel sheet, the solvent is removed from the gel sheet and stretched, or the gel sheet is stretched and then the solvent. And is further stretched.
[0014]
In order for the polyolefin microporous membrane to obtain more excellent characteristics, the polyolefin composition preferably satisfies the following conditions (1) to (13).
(1) The heat of fusion of the polypropylene is 95 J / g or more.
(2) The polypropylene content is 20% by weight or less based on the total polyolefin composition.
(3) The weight average molecular weight of the polyethylene is 5 × 10.^FiveThat's it.
(4) Weight average molecular weight 5 × 10 as described in (3) above^FiveThe above polyethylene is ultra high molecular weight polyethylene.
(5) The ultra-high molecular weight polyethylene described in (4) above has a weight average molecular weight of 1 × 10⁶~ 15 × 10⁶It is.
(6) The weight average molecular weight of the ultrahigh molecular weight polyethylene described in (4) or (5) above is 1 × 10⁶~ 5 × 10⁶It is.
(7) The ultrahigh molecular weight polyethylene according to any one of the above (4) to (6) is an ethylene homopolymer or an ethylene / α-olefin copolymer containing a small amount of other α-olefin.
(8) The weight average molecular weight 5 × 10 according to any one of the above (3) to (7)^FiveThe Mw / Mn of the above polyethylene is 5 to 300.
(9) The polyethylene has a weight average molecular weight of 5 × 10 according to any one of the above (3) to (8).^FiveAbove polyethylene and weight average molecular weight 1 × 10^Four~ 5 × 10^FiveLess than polyethylene, weight average molecular weight 1 × 10^Four~ 4 × 10⁶Polybutene-1, weight average molecular weight 1 × 10^Three~ 1 × 10^FourThe following polyethylene wax, and weight average molecular weight 1 × 10^Four~ 4 × 10⁶A polyethylene composition comprising at least one other polyolefin selected from the group consisting of ethylene / α-olefin copolymers.
(10) The polyethylene composition according to the above (9) has a weight average molecular weight of 5 × 10.^FiveUltra high molecular weight polyethylene and weight average molecular weight 1 × 10^Four~ 5 × 10^FiveIt is a composition consisting of less polyethylene.
(11) Weight average molecular weight in the polyethylene composition as described in (10) above is 1 × 10^Four~ 5 × 10^FiveThe less polyethylene is at least one selected from the group consisting of high density polyethylene, medium density polyethylene, branched low density polyethylene, and linear low density polyethylene.
(12) The polyethylene composition according to the above (11) has a weight average molecular weight of 5 × 10.^FiveUltra high molecular weight polyethylene and weight average molecular weight 1 × 10^Four~ 5 × 10^FiveLess than high density polyethylene.
(13) The Mw / Mn of the polyethylene composition according to any one of (9) to (12) is 5 to 300.
[0015]
As for the physical properties of the polyolefin microporous membrane of the present invention, the film thickness fluctuation rate is usually 15% or less, the air permeability in terms of film thickness of 16 μm is 20 to 550 seconds / 100 cc, and the porosity is 25 to 80%, puncture strength is 2450 mN / 16μm or more, thermal shrinkage (105 ° C / 8hr) is 5% or less in both MD and TD directions, shutdown temperature is 120-140 ° C, meltdown The temperature is 165 ° C or higher, preferably 165 to 190 ° C.
[0016]
The polyolefin microporous membrane of the present invention is useful as a battery separator.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
[1] polyolefin
The polyolefin used in the polyolefin microporous membrane of the present invention is (a) polyethylene and (b) a weight average molecular weight of 5 × 10^FiveThe above is essential, and polypropylene having a heat of fusion of 90 J / g or more as measured by a scanning differential calorimeter is essential.
(a) Polyethylene
Polyethylene has a weight average molecular weight of 5 × 10^FiveThe above is preferable. Weight average molecular weight 5 × 10^FiveIf it is less than the range, breakage tends to occur at the time of stretching, and it is difficult to obtain a suitable polyolefin microporous film.
[0018]
Weight average molecular weight 5 × 10^FiveAs the above polyethylene, ultra high molecular weight polyethylene is preferable. The ultra high molecular weight polyethylene may be not only an ethylene homopolymer but also an ethylene / α-olefin copolymer containing a small amount of other α-olefin. As the α-olefin other than ethylene, propylene, butene-1, hexene-1, pentene-1,4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, and styrene are preferable. Ultra high molecular weight polyethylene has a weight average molecular weight of 1 x 10⁶ ~ 15 × 10⁶Preferably 1 × 10⁶ ~ 5 × 10⁶It is more preferable that
[0019]
Weight average molecular weight 5 × 10 as polyethylene^FiveIt is also possible to use a polyethylene composition containing the above polyethylene and another polyolefin. Weight average molecular weight 5 × 10^FiveAs the above polyethylene, the ultra high molecular weight polyethylene is preferable. Other polyolefins have a weight average molecular weight of 1 × 10^Four~ 5 × 10^FiveLess than polyethylene, weight average molecular weight 1 × 10^Four~ 4 × 10⁶Polybutene-1, weight average molecular weight 1 × 10^Three~ 1 × 10^FourThe following polyethylene wax, and weight average molecular weight 1 × 10^Four~ 4 × 10⁶At least one selected from the group consisting of ethylene / α-olefin copolymers can be used. The amount of other polyolefin added is 80 parts by weight or less based on 100 parts by weight of the entire polyethylene composition.
[0020]
When using a polyethylene composition, the ultra-high molecular weight polyethylene and the weight average molecular weight 1 × 10^Four~ 5 × 10^FiveIt is preferable to use a composition comprising less than polyethylene. This polyethylene composition can easily control the molecular weight distribution (Mw / Mn) depending on the application. Weight average molecular weight is 1 × 10^Four~ 5 × 10^FiveThe lower polyethylene is preferably at least one selected from the group consisting of high density polyethylene, medium density polyethylene, branched low density polyethylene, and linear low density polyethylene. These weight average molecular weights are 1 × 10^Four~ 5 × 10^FiveThe lower polyethylene may be not only a homopolymer of ethylene but also a copolymer containing a small amount of other α-olefins such as propylene, butene-1, and hexene-1. It is effective to use a copolymer produced with a single site catalyst. The polyethylene composition is not limited, but the weight average molecular weight is 5 × 10.^FiveUltra high molecular weight polyethylene above and weight average molecular weight 1 × 10^Four~ 5 × 10^FiveA composition comprising less than high density polyethylene is more preferred.
[0021]
The molecular weight distribution Mw / Mn of polyethylene is not limited, but is preferably 5 to 300, more preferably 10 to 100. If Mw / Mn is less than 5, the high molecular weight component is too much and melt extrusion is difficult, and if Mw / Mn exceeds 300, the low molecular weight component is too much and the strength is lowered. Mw / Mn is used as a measure of molecular weight distribution, and the larger the value, the wider the molecular weight distribution. That is, when polyethylene is an ethylene homopolymer or an ethylene / α-olefin copolymer, Mw / Mn shows an increase in the molecular weight distribution, and the larger the value, the wider the molecular weight distribution. The Mw / Mn of the ethylene homopolymer and the ethylene / α-olefin copolymer can be appropriately adjusted by preparing them by multistage polymerization. The multistage polymerization method is preferably a two-stage polymerization in which a high molecular weight component is polymerized in the first stage and then a low molecular weight component is polymerized in the second stage. When polyethylene is a polyethylene composition, the larger the Mw / Mn, the larger the difference in the weight average molecular weight of each polyolefin compounded, and the smaller the Mw / Mn, the smaller the difference in the weight average molecular weight of each polyolefin. The Mw / Mn of the polyethylene composition can be appropriately adjusted by adjusting the molecular weight and mixing ratio of each component.
[0022]
(b) Polypropylene
Polypropylene has a weight average molecular weight of 5 × 10^FiveThe heat of fusion measured by a scanning differential calorimeter (DSC) based on JIS K7122 (ΔH_m) Must be 90 J / g or more. However, the rate of temperature rise during the measurement of heat of fusion is preferably 3 to 20 ° C./min. Normally, it is 10 ° C / min. Weight average molecular weight 5 × 10^FiveOr heat of fusion (ΔH_mWhen polypropylene having a viscosity of less than 90 J / g is used, the dispersibility of the polypropylene deteriorates during sheet formation, resulting in large microscopic irregularities on the surface of the polyolefin microporous film, resulting in large variations in film thickness. Heat of fusion (ΔH_m) Is preferably 95 J / g or more. The MFR (230 ° C., 2.16 kg load) of polypropylene is preferably 2.0 or less. If the MFR exceeds 2.0, the dispersibility of polypropylene may deteriorate during sheet formation.
[0023]
The content of polypropylene is preferably 20% by weight or less of the entire polyolefin composition. If polypropylene is not included, the meltdown temperature is not improved, and if the polypropylene content exceeds 20% by weight, the uniformity of the film thickness may be deteriorated.
[0024]
The type of polypropylene is not particularly limited, and may be either a propylene homopolymer or a copolymer of propylene and another α-olefin, but a propylene homopolymer is preferred. As the copolymer, either a random copolymer or a block copolymer can be used, and as the α-olefin, ethylene, butene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methacryl Examples include methyl acid and styrene. These polypropylenes have a weight average molecular weight of 5 × 10.^FiveAnd the heat of fusion measured by a scanning differential calorimeter (ΔH_m) Is 90 J / g or more, two or more types of propylene homopolymers, random copolymers or block copolymers having different weight average molecular weights may be used. Two or more selected from a coalescence, a random copolymer and a block copolymer may be used.
[0025]
[2] Method for producing polyolefin microporous membrane
The method for producing a polyolefin microporous membrane of the present invention includes (a) a step of melt-kneading polyethylene, polypropylene and a solvent to prepare a polyolefin solution, and (b) extruding the polyolefin solution from a die and cooling to form a gel-like sheet. (C) a stretching / solvent removing step, and (d) a step of drying the obtained film. Further, after the steps (a) to (d), (e) heat treatment, (f) cross-linking treatment with ionizing radiation, (g) hydrophilization treatment, and the like may be performed as necessary.
[0026]
(a) Preparation process of polyolefin solution
First, polyethylene, polypropylene and a solvent are melt-kneaded to prepare a polyolefin solution. Various additives such as an antioxidant, an ultraviolet absorber, an anti-blocking agent, a pigment, a dye, and an inorganic filler can be added to the polyolefin solution as necessary as long as the object of the present invention is not impaired. For example, finely divided silicic acid can be added as a pore forming agent.
[0027]
As the solvent, a liquid solvent which is liquid at room temperature is preferably used. By using a liquid solvent, it is possible to stretch at a relatively high magnification. Liquid solvents include nonane, decane, decalin, paraxylene, undecane, dodecane, liquid hydrocarbons and other aliphatic or cyclic hydrocarbons, mineral oil fractions with boiling points corresponding to these, and room temperature such as dibutyl phthalate and dioctyl phthalate Then, a liquid phthalate ester can be used. In order to obtain a gel-like sheet having a stable liquid solvent content, a non-volatile liquid solvent such as liquid paraffin is preferably used. In the heated melt kneaded state, it is mixed with polyethylene, but at room temperature, a solid solid solvent may be mixed with the liquid solvent. As such a solid solvent, stearyl alcohol, seryl alcohol, paraffin wax and the like can be used. If only a solid solvent is used, stretching unevenness may occur.
[0028]
The viscosity of the liquid solvent is preferably 30 to 500 cSt at 25 ° C., more preferably 50 to 200 cSt. When the viscosity at 25 ° C. is less than 30 cSt, foaming tends to occur and kneading is difficult. On the other hand, if it exceeds 500 cSt, it is difficult to remove the liquid solvent.
[0029]
Although the method of melt kneading is not particularly limited, it is usually carried out by uniformly kneading in a twin screw extruder. This method is suitable for preparing highly concentrated solutions of polyolefins. The melting temperature is preferably the melting point of polyolefin + 10 ° C. to + 100 ° C. Usually, it is 160-230 degreeC, and it is preferable that it is 180-220 degreeC. Here, the melting point is a value obtained by differential scanning calorimetry (DSC) based on JIS K7121 (the same applies hereinafter). The solvent may be added before the start of kneading or may be added from the middle of the extruder during the kneading, but it is preferable to add the solvent before starting the kneading and to make a solution in advance. In melt kneading, an antioxidant is preferably added to prevent oxidation of the polyolefin.
[0030]
The blending ratio of the polyolefin and the liquid solvent in the polyolefin solution is 1 to 50% by weight, preferably 20 to 40% by weight, with the total amount being 100% by weight. When the polyolefin is less than 1% by weight, swell and neck-in are increased at the die outlet when forming a gel-like molded product, and the moldability and self-supporting property of the gel-like molded product are lowered. On the other hand, if it exceeds 50% by weight, the moldability of the gel-like molded product is lowered.
[0031]
(b) Gel sheet formation process
The melt-kneaded polyolefin solution is directly or via another extruder, or once cooled and pelletized, and then extruded from the die again via the extruder. As the die, a sheet die having a rectangular base shape is usually used, but a double cylindrical hollow die, an inflation die, or the like can also be used. In the case of a sheet die, the die gap is usually 0.1 to 5 mm, and this is heated to 140 to 250 ° C. during extrusion. The extrusion rate of the heated solution is preferably 0.2 to 15 m / min.
[0032]
A gel-like molded product is formed by cooling the solution thus extruded from the die. Cooling is preferably performed at a rate of 50 ° C./min or more at least up to the gelation temperature or less. It is preferable to cool to 25 ° C. or lower. In this way, the phase separation structure in which the polyolefin phase is microphase-separated by the solvent can be fixed. In general, when the cooling rate is low, the higher order structure of the gel-like molded product becomes coarse, and the pseudo cell unit forming it becomes large, but when the cooling rate is high, it becomes a dense cell unit. When the cooling rate is less than 50 ° C./min, the degree of crystallinity increases and it is difficult to obtain a gel-like product suitable for stretching. As a cooling method, a method of directly contacting cold air, cooling water, or other cooling medium, a method of contacting a roll cooled by a refrigerant, or the like can be used.
[0033]
(c) Stretching / solvent removal process
Next, the obtained gel-like sheet is stretched and then the liquid solvent is removed, the liquid solvent is removed from the gel-like sheet and then stretched, or the gel-like sheet is stretched and then the liquid solvent is removed and further stretched.
[0034]
Stretching is performed at a predetermined magnification by heating the gel-like sheet and then using a normal tenter method, roll method, inflation method, rolling method, or a combination of these methods. The stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferred. In the case of biaxial stretching, any of simultaneous biaxial stretching, sequential stretching or multi-stage stretching (for example, a combination of simultaneous biaxial stretching and sequential stretching) may be used, but simultaneous biaxial stretching is particularly preferable. The mechanical strength is improved by stretching.
[0035]
Although a draw ratio changes with thickness of a gel-like sheet | seat, when performing uniaxial stretching, it is preferable to set it as 2 times or more, and it is more preferable to set it as 3-30 times. In biaxial stretching, it is preferably at least 3 times in any direction, preferably 9 times or more in terms of surface magnification, and more preferably 25 times or more in terms of surface magnification. By setting the surface magnification to 9 times or more, the puncture strength is improved. On the other hand, when the surface magnification is more than 400 times, there are restrictions in terms of the stretching apparatus, stretching operation, and the like.
[0036]
When the polyethylene is a homopolymer or an ethylene / α-olefin copolymer, the stretching temperature is preferably the melting point + 10 ° C. or less, and more preferably within the range from the crystal dispersion temperature to less than the crystal melting point. When the stretching temperature exceeds the melting point + 10 ° C., the polyethylene melts and the molecular chain cannot be oriented by stretching. If the stretching temperature is lower than the crystal dispersion temperature, the softening of polyethylene is insufficient, the film is easily broken during stretching, and high-stretching cannot be performed. However, when performing sequential stretching or multistage stretching, primary stretching may be performed at a temperature lower than the crystal dispersion temperature. Here, the crystal dispersion temperature refers to a value obtained by measuring temperature characteristics of dynamic viscoelasticity based on ASTM D 4065. The crystal dispersion temperature of polyethylene is generally 90 ° C.
[0037]
When the polyethylene is a polyethylene composition containing other polyolefin, the stretching temperature is preferably in the range from the crystal dispersion temperature of the polyethylene contained in the composition to the crystal melting point + 10 ° C. or less. When such a polyethylene composition is used, in the present invention, the stretching temperature is usually 100 to 140 ° C, preferably 110 to 120 ° C.
[0038]
Depending on the desired physical properties, the film can be stretched by providing a temperature distribution in the film thickness direction, or can be subjected to sequential stretching or multi-stage stretching in which the film is first stretched at a relatively low temperature and then secondarily stretched at a higher temperature. A polyolefin microporous film generally excellent in mechanical strength can be obtained by providing a temperature distribution in the film thickness direction and stretching. As the method, for example, the method disclosed in JP-A-7-188440 can be applied.
[0039]
A cleaning solvent is used for removing (cleaning) the liquid solvent. Since the polyethylene phase is phase-separated from the solvent, a porous membrane can be obtained by removing the liquid solvent. The removal (washing) of the liquid solvent can be performed using a known washing solvent. Known cleaning solvents include, for example, chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, hydrocarbons such as pentane, hexane and heptane, fluorinated hydrocarbons such as ethane trifluoride, ethers such as diethyl ether and dioxane, Examples include readily volatile solvents such as methyl ethyl ketone. As the cleaning solvent, in addition to the known cleaning solvent, a cleaning solvent having a surface tension at 25 ° C. of 24 mN / m or less disclosed in JP-A No. 2002-256099 can be used. By using a cleaning solvent having such a surface tension, the shrinkage and densification of the network structure caused by the surface tension of the gas-liquid interface that occurs inside the microporous layer during drying after cleaning can be suppressed. The porosity and permeability of the microporous membrane are further improved.
[0040]
The washing method can be carried out by a method of immersing the stretched membrane or gel-like sheet in a washing solvent, a method of showering the membrane or gel-like sheet after stretching with a washing solvent, or a combination thereof. The washing solvent is preferably used in an amount of 300 to 30,000 parts by weight with respect to 100 parts by weight of the gel sheet. The washing with the washing solvent is preferably carried out until the remaining liquid solvent is less than 1% by weight based on the amount added.
[0041]
(d) Membrane drying process
The film obtained by stretching and solvent removal can be dried by a heat drying method or an air drying method. The drying temperature is preferably a temperature not higher than the crystal dispersion temperature of polyethylene, and particularly preferably a temperature lower by 5 ° C. or more than the crystal dispersion temperature.
[0042]
The content of the cleaning solvent remaining in the polyolefin microporous membrane is preferably 5% by weight or less by drying treatment (the weight of the membrane after drying is 100% by weight), and more preferably 3% by weight or less. preferable. If the drying is insufficient and a large amount of the washing solvent remains in the film, it is not preferable because the porosity decreases and the permeability deteriorates in the subsequent heat treatment.
[0043]
(e) Heat treatment process
It is preferable to perform heat treatment after removing the washing solvent. The crystal is stabilized by the heat treatment, and the lamellar layer is made uniform. As a heat treatment method, any one of a heat stretching treatment, a heat setting treatment, and a heat shrinkage treatment may be used, and these are appropriately selected according to physical properties required for the polyolefin microporous membrane. These heat treatments are performed below the melting point of the polyolefin microporous membrane, preferably 60 ° C. or higher and melting point −10 ° C. or lower.
[0044]
The heat stretching treatment is performed by a commonly used tenter method, roll method or rolling method, and is preferably performed at least in one direction at a draw ratio of 1.01 to 2.0 times, more preferably 1.01 to 1.5 times.
[0045]
The heat setting treatment is performed by a tenter method, a roll method or a rolling method. Moreover, you may perform a heat shrink process by a tenter system, a roll system, or a rolling system, or may be performed using a belt conveyor or a floating. The heat shrink treatment is preferably performed in a range of 50% or less in at least one direction, and more preferably performed in a range of 30% or less.
[0046]
In addition, you may perform combining many above-mentioned heat | fever extending processes, heat setting processes, and heat shrink processes. In particular, when a heat stretching treatment is performed after the heat setting treatment, the permeability of the resulting polyolefin microporous membrane is improved and the pore diameter is enlarged. Further, it is preferable to perform a heat shrinkage treatment after the heat stretching treatment because a polyolefin microporous membrane having a low shrinkage rate and high strength can be obtained.
[0047]
(f) Film cross-linking process
It is preferable to subject the polyolefin microporous film obtained by stretching / solvent removal to a polyolefin microporous film dried by a heat drying method, an air drying method, or the like, by ionizing radiation. As the ionizing radiation, α rays, β rays, γ rays, electron beams and the like are used, and can be performed with an electron dose of 0.1 to 100 Mrad and an acceleration voltage of 100 to 300 kV. Thereby, meltdown temperature can be improved.
[0048]
(g) Hydrophilization treatment process
The polyolefin microporous membrane obtained by stretching and solvent removal may be subjected to a hydrophilic treatment. As the hydrophilic treatment, monomer grafting, surfactant treatment, corona discharge treatment, or the like is used. The monomer grafting treatment is preferably performed after ionizing radiation.
[0049]
When using a surfactant, any of a nonionic surfactant, a cationic surfactant, an anionic surfactant or an amphoteric surfactant can be used, but a nonionic surfactant is used. Is preferred. In the case of using a surfactant, the surfactant is made into an aqueous solution or a solution of a lower alcohol such as methanol, ethanol, isopropyl alcohol, and dipped or hydrophilized by a method using a doctor blade.
[0050]
The obtained hydrophilic microporous membrane is dried. At this time, in order to improve permeability, it is preferable to perform heat treatment while preventing shrinkage at a temperature below the melting point of the polyolefin microporous membrane. Examples of the method of performing heat treatment while preventing shrinkage include a method of performing heat treatment while stretching.
[0051]
[3] Polyolefin microporous membrane
The polyolefin microporous membrane according to a preferred embodiment of the present invention has the following physical properties.
(1) The film thickness fluctuation rate is 15% or less. When the film thickness fluctuation rate exceeds 15%, there is a possibility that a short circuit may occur when used as a battery separator, or battery productivity may decrease due to a decrease in yield.
(2) The porosity is 25-80%. If the porosity is less than 25%, good air permeability cannot be obtained. On the other hand, if it exceeds 80%, battery safety and impedance cannot be balanced.
(3) Air permeability is 20 to 550 seconds / 100 cc (converted to a film thickness of 16 μm). When the air permeability is 20 to 550 seconds / 100 cc, the battery capacity is increased and the cycle characteristics of the battery are also improved. When the air permeability exceeds 550 seconds / 100 cc, the battery capacity decreases when the polyolefin microporous membrane is used as a battery separator. On the other hand, if the air permeability is less than 20 seconds / 100 cc, shutdown will not be performed sufficiently when the temperature inside the battery rises.
(4) The puncture strength is 2450 mN / 16μm or more. If the pin puncture strength is less than 2450 mN / 16 μm, a short circuit may occur when the polyolefin microporous membrane is incorporated in a battery as a battery separator.
(5) Thermal shrinkage after exposure at 105 ° C for 8 hours is 5% or less in both machine direction (MD) and vertical direction (TD). When the thermal shrinkage rate exceeds 5%, when the polyolefin microporous membrane is used as a lithium battery separator, when the heat is generated, the end of the separator shrinks, and the possibility of occurrence of a short circuit increases.
(6) The shutdown temperature is 120 ~ 140 ℃.
(7) The meltdown temperature is 165 ° C or higher, preferably 165 to 190 ° C.
[0052]
Thus, since the polyolefin microporous film of the present invention is excellent in film thickness uniformity, it can be suitably used as a battery separator, filter or the like. The film thickness of the polyolefin microporous membrane can be appropriately selected depending on the application, but is preferably 5 to 200 μm, for example, when used as a battery separator.
[0053]
【Example】
The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.
[0054]
[The heat of fusion of polypropylene (ΔH_m)Measuring method]
Determined according to JIS K7122. About 10 mg of the sample was placed in a pre-weighed aluminum sample pan, and then the mass of the sample pan containing the sample was precisely weighed, and the difference from the sample pan mass was taken as the sample mass. The sample pan containing the sample was left in the sample holder of a scanning differential calorimeter (manufactured by Perkin Elmer, Inc., DSC-System7), and after heat treatment at 190 ° C./10 minutes in a nitrogen atmosphere, Cooled to 40 ° C at 10 ° C / min. Subsequently, after holding at 40 ° C./2 minutes, the temperature was raised to 190 ° C. at a rate of 10 ° C./min. For the DSC curve (melting curve) obtained during the heating process, a linear baseline is set in the range of 85 to 175 ° C, and the amount of heat is calculated from the area surrounded by the linear baseline and the DSC curve. Was converted per sample mass.
[0055]
Example 1
Weight average molecular weight (Mw) is 2.0 × 10⁶Ultra high molecular weight polyethylene (UHMWPE) 20% by weight, Mw 3.5 × 10^FiveHigh-density polyethylene (HDPE) 70% by weight, and Mw 5.9 × 10^FiveAnd a polyolefin composition comprising 10% by weight of a propylene homopolymer having a heat of fusion of 95.7 J / g [MFR (230 ° C., 2.16 kg load): 0.5] (for a polyethylene composition comprising UHMWPE and HDPE, Mw / Mn 16), melting point 135 ° C., crystal dispersion temperature 90 ° C.) and tetrakis [methylene-3- (3,5-ditertiarybutyl-4-hydroxyphenyl) -propionate] as an antioxidant A polyolefin composition obtained by dry blending 0.25 parts by weight of methane per 100 parts by weight of the polyolefin composition was obtained. 30 parts by weight of the obtained hot polyolefin composition was put into a twin screw extruder (inner diameter 58 mm, L / D = 42, strong kneading type), and liquid paraffin [35 cst (40 c ° C)] 70 parts by weight were supplied, melt-kneaded under the conditions of 210 ° C and 200 rpm, and a polyolefin solution was prepared in an extruder. Subsequently, this polyolefin solution was extruded from a T-die installed at the tip of the extruder, and cooled while being drawn by a cooling roll adjusted to 0 ° C. to form a gel-like sheet. The obtained gel-like sheet was simultaneously biaxially stretched at 118 ° C. using a tenter stretching machine so that both the machine direction (MD) and the vertical direction (TD) were 5 times, thereby obtaining a stretched film. The obtained stretched membrane is fixed to a 20 cm × 20 cm aluminum frame and contains a methylene chloride [surface tension 27.3 mN / m (25 ° C.), boiling point 40.0 ° C.] adjusted to 25 ° C. It was immersed in and washed with rocking at 100 rpm for 3 minutes. The obtained membrane was air-dried at room temperature, and then subjected to heat setting treatment at 125 ° C. for 30 seconds while holding the membrane in a tenter to produce a polyolefin microporous membrane.
[0056]
Example 2
As polypropylene, Mw is 6.1 × 10^FiveA polyolefin microporous membrane was prepared in the same manner as in Example 1 except that a propylene homopolymer [MFR (230 ° C., 2.16 kg load): 0.5] having a heat of fusion of 98.0 J / g was used.
[0057]
Example 3
As polypropylene, Mw is 8.6 × 10^FiveA polyolefin microporous membrane was prepared in the same manner as in Example 1 except that a propylene homopolymer [MFR (230 ° C., 2.16 kg load): 0.4] having a heat of fusion of 104.5 J / g was used.
[0058]
Example 4
As polypropylene, Mw is 5.7 × 10^FiveAnd a polyolefin microporous membrane was prepared in the same manner as in Example 1 except that a propylene homopolymer [MFR (230 ° C., 2.16 kg load): 0.5] having a heat of fusion of 99.3 J / g was used.
[0059]
Example 5
As polypropylene, Mw is 6.0 × 10^FiveAnd a polyolefin microporous membrane was prepared in the same manner as in Example 1 except that a propylene homopolymer [MFR (230 ° C., 2.16 kg load): 0.5] having a heat of fusion of 91.6 J / g was used.
[0060]
Example 6
As polypropylene, Mw is 5.9 × 10^FiveA polyolefin microporous membrane was prepared in the same manner as in Example 1 except that a propylene homopolymer [MFR (230 ° C., 2.16 kg load): 0.5] having a heat of fusion of 92.4 J / g was used.
[0061]
Comparative Example 1
As a polyolefin composition, UHMWPE (Mw: 2.0 × 10⁶) 20% by weight and HDPE (Mw: 3.5 × 10^FiveA polyolefin microporous membrane was prepared in the same manner as in Example 1 except that a polyethylene composition comprising 80% by weight was used.
[0062]
Comparative Example 2
As polypropylene, Mw is 6.8 × 10^FiveA polyolefin microporous membrane was prepared in the same manner as in Example 1 except that a propylene homopolymer [MFR (230 ° C., 2.16 kg load): 0.4] having a heat of fusion of 83.0 J / g was used.
[0063]
Comparative Example 3
As polypropylene, Mw is 3.8 × 10^FiveA polyolefin microporous membrane was prepared in the same manner as in Example 1 except that a propylene homopolymer [MFR (230 ° C., 2.16 kg load): 1.4] having a heat of fusion of 97.6 J / g was used.
[0064]
Comparative Example 4
As polypropylene, Mw is 3.5 × 10^FiveA polyolefin microporous membrane was prepared in the same manner as in Example 1 except that a propylene homopolymer [MFR (230 ° C., 2.16 kg load): 1.3] having a heat of fusion of 71.3 J / g was used.
[0065]
Comparative Example 5
As polypropylene, Mw is 2.3 × 10^FiveAnd a polyolefin microporous membrane was prepared in the same manner as in Example 1 except that a propylene homopolymer [MFR (230 ° C., 2.16 kg load): 7.0] having a heat of fusion of 72.0 J / g was used.
[0066]
Comparative Example 6
As polypropylene, Mw is 2.4 × 10^FiveA polyolefin microporous membrane was prepared in the same manner as in Example 1 except that a propylene homopolymer [MFR (230 ° C., 2.16 kg load): 7.0] having a heat of fusion of 104.0 J / g was used.
[0067]
Comparative Example 7
As polypropylene, Mw is 1.1 × 10^FiveA polyolefin microporous membrane was prepared in the same manner as in Example 1 except that a propylene homopolymer [MFR (230 ° C., 2.16 kg load): 50.0] having a heat of fusion of 99.4 J / g was used.
[0068]
The physical properties of the polyolefin microporous membranes obtained in Examples 1 to 6 and Comparative Examples 1 to 7 were measured by the following methods.
Average film thickness: A value measured by a contact thickness meter at an interval of 5 mm over a length of 30 cm in the vertical direction (TD) of the polyolefin microporous film was averaged.
・ Thickness variation σ_t: Standard deviation calculated from data when measuring average film thickness.
-Film thickness fluctuation rate: Film thickness fluctuation σ_tThe value obtained by dividing by the average film thickness was expressed as a percentage.
Air permeability: measured in accordance with JIS P8117 (film thickness 16 μm conversion).
-Porosity: measured by gravimetric method.
Puncture strength: The maximum load when a polyolefin microporous membrane was punctured at a speed of 2 mm / sec using a needle having a diameter of 1 mm (0.5 mm R) was measured (converted to a film thickness of 16 μm).
Heat shrinkage rate: The shrinkage rate in the machine direction (MD) and the vertical direction (TD) when the polyolefin microporous membrane was exposed at 105 ° C. for 8 hours was measured.
Shutdown temperature: Measured as the temperature at which the air permeability becomes 100,000 seconds / 100 cc or more by heating to a predetermined temperature.
Meltdown temperature: measured as the temperature at which the film melts and breaks when heated to a predetermined temperature.
[0069]
[Table 1]

Table 1 continued

Table 1 continued

Table 1 continued

[0070]
Note: (1) Polyolefin composition concentration in solution
(2) MD: Machine direction, TD: Vertical direction
(3) Film thickness fluctuation σt / average film thickness x 100
[0071]
As shown in Table 1, the polyolefin microporous membrane of the present invention has a particularly small thickness fluctuation rate, and at the same time, balance of air permeability, porosity, mechanical strength, dimensional stability, shutdown characteristics and meltdown characteristics. Is excellent. On the other hand, since the polyolefin microporous film of Comparative Example 1 does not contain polypropylene, it has a lower meltdown temperature than Examples 1-6. The polyolefin microporous membranes of Comparative Examples 2 to 7 have a polypropylene Mw of 5 × 10^FiveSince the heat of fusion of polypropylene is less than 90 J / g, or both, the film thickness variation rate is large as compared with Examples 1-6.
[0072]
【The invention's effect】
As described above in detail, the polyolefin microporous membrane of the present invention comprises (a) polyethylene and (b) a weight average molecular weight of 5 × 10.^FiveIncluding polypropylene with a melting heat of 90 J / g or more as measured by a scanning differential calorimeter, so film thickness uniformity, heat resistance, mechanical properties, permeability, dimensional stability, shutdown Excellent balance between properties and meltdown characteristics. By using the obtained polyolefin microporous membrane as a battery separator, not only battery characteristics such as capacity characteristics, cycle characteristics, and discharge characteristics at low temperatures, but also battery safety such as heat resistance and compression resistance and battery production All improvements including sex are possible.

Claims (6)

(a) polyethylene, and
(b) A polyolefin microporous membrane comprising polypropylene having a weight average molecular weight of 5 × 10 ⁵ or more and a heat of fusion measured by a scanning differential calorimeter of 90 J / g or more. The polyolefin microporous membrane according to claim 1, wherein the polyethylene has a weight average molecular weight. Five × Ten ^Five Above polyethylene and weight average molecular weight 1 × Ten ^Four more than~ Five × Ten ^Five Less than polyethylene, weight average molecular weight 1 × Ten ^Four ~ 4x Ten ⁶ The polybutene -1 , Weight average molecular weight 1 × Ten ^Three ~ 1 × Ten ^Four The following polyethylene waxes and weight average molecular weights 1 × Ten ^Four ~ 4x Ten ⁶ Of ethylene - Polyethylene composition comprising at least one other polyolefin selected from the group consisting of olefin copolymers, or weight average molecular weight Five × Ten ^Five A polyolefin microporous membrane characterized by being the above ultrahigh molecular weight polyethylene. The polyolefin microporous membrane according to claim 1 or 2, wherein the film thickness variation rate is 15 % Or less of the polyolefin microporous membrane. The polyolefin microporous film according to any one of claims 1 to 3, wherein the film thickness is 16 μ m The converted air permeability is 20 ~ 550 Sec / 100 cc And the porosity is twenty five ~ 80 And the puncture strength is 2450 mN / 16 μ m That's it, 105 The thermal shrinkage after exposure for 8 hours at ℃ is 5% or less in both the machine direction and vertical direction, and the shutdown temperature is 120 ~ 140 ℃, meltdown temperature is 165 A polyolefin microporous membrane having a temperature of at least ° C. (a) polyethylene,
(b) a polypropylene having a weight average molecular weight of 5 × 10 ⁵ or more and a heat of fusion measured by a scanning differential calorimeter of 90 J / g or more;
(c) Solvent and knead the solvent, the obtained melt-kneaded product is extruded from a die and then cooled to form a gel sheet, and then the gel sheet is stretched and then the solvent is removed or the gel A method for producing a polyolefin microporous membrane, comprising stretching after removing the solvent from the sheet, or stretching the gel-like sheet and then stretching the gel-like sheet. A battery separator comprising the polyolefin microporous membrane according to any one of claims 1 to 4 .