JP2022036034A - Polyolefin microporous film - Google Patents

Abstract

To provide a polyolefin microporous film that demonstrates excellent electrolyte impregnation, safety and output properties when used as a battery separator.SOLUTION: According to the inventive polyolefin microporous film, an absorbance peak between 1720 cm-1 and 1750 cm-1 is set as A and an absorbance peak between 1450 cm-1 and 1480 cm-1 is set as B as measured by infrared absorption spectroscopy, and A is divided by B to be 0.1 or more; and the difference between the maximum pore size and the average pore size is 40 nm or less.SELECTED DRAWING: None

Description

The present invention relates to a microporous polyolefin membrane.

The microporous polyolefin membrane is used as a filter, a separator for a fuel cell, a separator for a capacitor, and the like. In particular, it is suitably used as a separator for lithium-ion batteries widely used in notebook personal computers, mobile phones, digital cameras, and the like. The reason is that the microporous polyolefin membrane has excellent mechanical strength and shutdown characteristics. In particular, in recent years, lithium-ion secondary batteries have been developed with the aim of increasing energy density, size, and output, mainly for in-vehicle applications, and along with this, the required characteristics for separators have become even higher. ing.

In the lithium ion battery, a solution obtained by dissolving an electrolyte such as LiPF ₆ in an organic solvent such as propylene carbonate or dimethyl carbonate is used as the electrolytic solution. These electrolytic solutions have low permeability to the microporous polyolefin membrane, and in particular, in large batteries and batteries having a special shape, the yield in the electrolytic solution impregnation step and the unpenetrated portion of the electrolytic solution may occur. Further, in the separator having low electrolytic solution impregnation, the separator may be compressed due to expansion and contraction of the electrode during battery charging / discharging, and the electrolytic solution may be discharged, resulting in liquid withering and inferior cycle characteristics. In addition, it is expected that the use of high-viscosity electrolytes with high electrolyte concentration will spread as the energy density of lithium-ion batteries increases in the future, and the permeability to the separator will further deteriorate. Therefore, the electrolyte impregnation of the separator There is an urgent need to improve.

As a technique for improving the impregnation of the electrolytic solution, Patent Document 1 describes a method of reducing the contact angle with respect to the electrolytic solution by plasma-treating the surface of the separator. Further, Patent Document 2 describes a microporous film having improved electrolytic solution retention by containing an ethylene-ethyl acrylate copolymer resin and / or an ethylene-methyl acrylate copolymer resin.

Japanese Unexamined Patent Publication No. 11-300180 Japanese Unexamined Patent Publication No. 2011-081228

However, in the method described in Patent Document 1, the wettability is improved only on the outermost surface of the separator by plasma treatment, and it is difficult to improve the permeability of the electrolytic solution inside the separator, and the separator is improved during plasma treatment. In some cases, defects may occur and the quality may deteriorate. Further, in the method described in Patent Document 2, coarse pores and unopened pores are formed due to poor dispersion of the added ethylene-ethyl acrylate copolymer resin and ethylene-methyl acrylate copolymer resin, and safety due to the generation of dendrite is obtained. And there is concern about deterioration of output characteristics.

An object of the present invention is to solve the above-mentioned problems. That is, it is an object of the present invention to provide a polyolefin microporous membrane having excellent electrolytic solution impregnation, safety and output characteristics when used as a battery separator.

When the absorbance peak value between 1720 cm ^-1 and 1750 cm ^-1 measured by the infrared absorption spectroscopy is A and the absorbance peak value between 1450 cm ^-1 and 1480 cm ^-1 is B, A is B. The value divided by is 0.1 or more, and the difference between the maximum pore diameter and the average pore diameter (maximum pore diameter-average pore diameter) is 40 nm or less.

Since the microporous polyolefin membrane of the present invention has an excellent affinity with the electrolytic solution and has a uniform pore structure, it is safe because it can satisfactorily impregnate the highly viscous electrolytic solution when used as a battery separator. Excellent in properties and output characteristics. From this, the polyolefin microporous film of the present invention can be suitably used as a battery separator for a large secondary battery that requires high energy density, high capacity, and high output for electric vehicles and the like.

The microporous polyolefin membrane of the present invention has an absorbance peak value between 1720 cm ^-1 and 1750 cm ^-1 as measured by infrared absorption spectroscopy, and an absorbance peak value between 1450 cm ^-1 and 1480 cm ^-1 . When B is used, the value obtained by dividing A by B is 0.1 or more. It is more preferably 0.2 or more, still more preferably 0.3 or more, and particularly preferably 0.4 or more. By setting the value obtained by dividing A by B in the above range, the affinity with the electrolytic solution is improved, and the invasiveness of the electrolytic solution and the cycle characteristics are improved when used as a battery separator. From the above viewpoint, the higher the value obtained by dividing A by B is, the more preferable it is, but since it is difficult to achieve both the transparency and strength of the separator, about 2 is the upper limit. When the absorbance peak value between 1720 cm ^-1 and 1750 cm ^-1 measured by the infrared absorption spectroscopy is A and the absorbance peak value between 1450 cm ^-1 and 1480 cm ^-1 is B, A is B. In order to make the value divided by the above range, it is preferable that the raw material composition of the film is within the range described later, and the kneading conditions and stretching conditions at the time of film formation are within the range described later.

In the polyolefin microporous membrane of the present invention, the difference between the maximum pore diameter measured by the bubble point method and the average pore diameter measured by the half-dry method (maximum pore diameter-average pore diameter) is preferably 40 nm or less. It is more preferably 30 nm or less, further preferably 20 nm or less, and particularly preferably 15 nm or less. When (maximum pore diameter-average pore diameter) is 40 nm or less, the uniformity of the pore diameter is high, so that when used as a thin-film separator for high-power batteries, micro-short circuits due to dendrites are suppressed and safety and output characteristics are excellent. Since the permeation of the electrolytic solution in the membrane proceeds uniformly, the time required for the impregnation of the electrolytic solution can be shortened. From the above viewpoint, the smaller (maximum pore diameter-average pore diameter) is preferable, but from the viewpoint of compatibility with the productivity of the polyolefin microporous membrane, about 5 nm is a practical lower limit. In order to set (maximum pore size-average pore size) in the above range, it is preferable that the raw material composition of the film is within the range described later, and the extrusion conditions and stretching conditions at the time of film formation are within the range described later.

The microporous polyolefin membrane of the present invention preferably has a maximum pore size of 70 nm or less as measured by the bubble point method. It is more preferably 60 nm or less, further preferably 50 nm or less, and particularly preferably 40 nm or less. By setting the maximum hole diameter within the above range, when used as a separator for high-power batteries, it suppresses a slight short circuit due to dendrites and is excellent in safety and output characteristics. From the above viewpoint, the smaller the maximum pore diameter is, the more preferable it is, but if it is too small, the ion permeability may be insufficient and the output characteristics of the battery may be deteriorated. Therefore, the lower limit is about 15 nm. In order to set the maximum pore size in the above range, it is preferable that the raw material composition of the film is in the range described later, and the extrusion conditions and stretching conditions at the time of film formation are in the range described later.

The microporous polyolefin membrane of the present invention preferably has an average pore diameter of 50 nm or less. It is more preferably 40 nm or less, further preferably 30 nm or less, and most preferably 25 nm or less. By setting the average pore diameter within the above range, it is possible to suppress a slight short circuit due to dendrites when used as a battery separator, and it is excellent in safety and output characteristics. From the above viewpoint, the smaller the average pore diameter is, the more preferable it is, but if it is too small, the ion permeability may be insufficient and the output characteristics of the battery may be deteriorated, so that the lower limit is about 10 nm. In order to set the average pore size in the above range, it is preferable that the raw material composition of the film is in the range described later, and the extrusion conditions and stretching conditions at the time of film formation are in the range described later.

The microporous polyolefin membrane of the present invention preferably has an endothermic peak area of 5 J / g or less due to crystal melting at 50 ° C. or higher and 110 ° C. or lower as measured by a differential scanning calorimeter, more preferably 3 J / g or lower, and further. It is preferably 2 J / g or less, and particularly preferably 1 J / g or less. By setting the endothermic peak area due to crystal melting at 50 ° C. or higher and 110 ° C. or lower measured by a differential scanning calorimeter within the above range, the uniformity of the in-plane characteristics of the polyolefin microporous film is excellent, and as a separator for high-power batteries. When used, it suppresses a slight short circuit due to dendrites and has excellent cycle characteristics, and the permeation of the electrolytic solution in the membrane proceeds uniformly, so that the time required for the infiltration of the electrolytic solution can be shortened. In order to set the endothermic peak area due to crystal melting at 50 ° C. or higher and 110 ° C. or lower measured by a differential scanning calorimeter within the above range, the raw material composition of the film should be within the range described later, and the extrusion conditions and stretching conditions during film formation should be set. Is preferably within the range described later.

The microporous polyolefin membrane of the present invention has a dropping amount of 2 μL and a contact angle with propylene carbonate 10 seconds after dropping, C, and the microporous polyolefin membrane has a dropping amount of 2 μL after press-melting and a contact angle with propylene carbonate 10 seconds after dropping. When D is, | CD | is preferably 20 ° or less, more preferably 10 ° or less, still more preferably 5 ° or less, and particularly preferably 3 ° or less. When | CD | is 20 ° or less, the affinity with the electrolytic solution is high both on the surface and inside of the polyolefin microporous membrane, and the electrolyte impregnation and cycle characteristics are improved when used as a battery separator. .. From the above viewpoint, | CD | is preferable as it is smaller, but 0.1 ° is preferable as the lower limit because it is difficult to achieve both characteristic control and productivity.

The polyolefin microporous membrane of the present invention preferably has a dropping amount of 2 μL after pressing and melting the polyolefin microporous membrane, and the contact angle D with respect to propylene carbonate 10 seconds after dropping is preferably 60 ° or less, more preferably 55 ° or less. It is more preferably 50 ° or less, and particularly preferably 45 ° or less. Since the contact angle D with respect to propylene carbonate after press melting is in the above range, it is excellent in electrolytic solution impregnation and cycle characteristics when used as a battery separator. From the above viewpoint, the smaller the contact angle D with respect to propylene carbonate after press melting is preferable, but 5 ° is preferable as the lower limit because it is difficult to achieve both output characteristics.

The microporous polyolefin membrane of the present invention preferably has a porosity of 60% or less, more preferably 50% or less, further preferably 45% or less, and particularly preferably 40% or less. .. The lower limit is preferably 20% or more, and more preferably 30% or more. When the porosity is less than 20%, when used as a battery separator, the ion permeability may be insufficient and the output characteristics of the battery may be deteriorated. If it is higher than 60%, the strength may be lowered and a short circuit may easily occur due to foreign matter in the battery during winding, or the hole diameter may become too large and a slight short circuit due to dendrite may easily occur. .. In order to set the porosity within the above range, it is preferable that the raw material composition of the film is within the range described later, and the extrusion conditions and heat fixing conditions at the time of film formation are within the range described later.

The polyolefin microporous membrane of the present invention preferably satisfies the following formula when the dropping amount is 2 μL, the contact angle with respect to propylene carbonate 10 seconds after dropping is C, and the porosity is E. Generally, when the porosity of the microporous membrane is large, the measurement solvent partially penetrates into the microporous membrane and the contact angle becomes small, and the permeability of the electrolytic solution tends to improve. However, since the microporous polyolefin membrane of the present invention can reduce the contact angle while maintaining a low porosity, it is possible to achieve both safety and permeability of the electrolytic solution by satisfying the following formula. .. In order to make the relationship between the porosity and the contact angle within the above range, it is preferable that the raw material composition of the film is within the range described later, and the extrusion conditions and heat fixing conditions at the time of film formation are within the range described later.
[Formula: C <-0.6E + 90].

The microporous polyolefin membrane of the present invention preferably has a piercing strength of 3.0 N or more in terms of a thickness of 20 μm. It is more preferably 3.5 N or more, further preferably 4.0 N or more, still more preferably 5.0 N or more. When the puncture strength is 3.0 N or more, short circuits are less likely to occur during winding or due to foreign matter in the battery, and the safety of the battery can be improved. From the above viewpoint, the higher the piercing strength is, the more preferable it is. However, in order to increase the piercing strength, the porosity is often too low and the output characteristics are deteriorated, and 20N is the upper limit. In order to set the puncture strength in the above range, it is preferable that the raw material composition of the film is in the range described later, and the extrusion conditions and stretching conditions at the time of film formation are in the range described later.

The microporous polyolefin membrane of the present invention preferably has a Gale air permeation resistance equivalent to a thickness of 20 μm of 600 seconds / 100 cm ³ or less. It is more preferably 500 seconds / 100 cm ³ or less, further preferably 400 seconds / 100 cm ³ or less, and most preferably 350 seconds / 100 cm ³ or less. By setting the galley air permeation resistance in terms of the thickness of the polyolefin microporous membrane of 20 μm within the above range, the output characteristics are excellent when used as a battery separator. From the above viewpoint, it is preferable that the Gale air permeability resistance in terms of thickness of 20 μm is low, but if the air permeability resistance is too low, the strength of the film becomes low and the handleability deteriorates, or when it is used as a separator for a high output battery, it depends on dendrites. The lower limit is about 20 seconds / 100 cm ³ because a slight short circuit may easily occur. In order to set the air permeation resistance in the above range, it is preferable that the raw material composition of the film is in the range described later, and the extrusion conditions and heat fixing conditions at the time of film formation are in the range described later.

The microporous polyolefin membrane of the present invention preferably has a propylene carbonate suction rate of 1.5 mm / 10 minutes or more after being allowed to stand at 65 ° C. and 85% RH for 48 hours. By setting the suction speed of propylene carbonate, which is generally used as an electrolytic solution in a lithium ion battery, within the above range, the affinity with the electrolytic solution can be improved. The suction speed is more preferably 2.0 mm / 10 minutes or more, further preferably 3.0 mm / 10 minutes or more, and particularly preferably 3.5 mm / 10 minutes or more. When the suction speed is 1.5 mm / 10 minutes or more, the impregnation of the electrolytic solution is good when used as a battery separator, productivity is improved, and resistance is reduced when used as a battery, which is preferable. From the above viewpoint, the faster the suction speed is, the more preferable it is, but the upper limit is substantially 10.0 mm / 10 minutes. In order to set the suction speed within the above range, it is preferable that the raw material composition of the film is within the range described later and the film forming conditions are within the range described later.

The thickness of the microporous polyolefin membrane of the present invention is appropriately adjusted depending on the intended use, but is preferably 20 μm or less. It is more preferably 15 μm or less, further preferably 12 μm or less, and particularly preferably 10 μm or less. If the thickness exceeds 20 μm, sufficient output characteristics and energy density may not be obtained when used as a separator. From the above viewpoint, the thinner the thickness is, the more preferable it is, but the lower limit of the thickness is about 2 μm because safety may be lowered and handling may be difficult. The thickness can be adjusted by adjusting the ejection amount of the extruder, the film forming speed, the stretching ratio, the stretching temperature, etc., within a range that does not deteriorate other physical properties.

In the present invention, specific raw materials (resin A and resin B) described later are used, and the raw material composition is within the range described later, and the extrusion conditions, stretching conditions, and heat fixing conditions at the time of film formation are within the range described later. As a result, it has excellent compatibility with the electrolytic solution and excellent pore uniformity, achieving both productivity and output characteristics when used as a battery separator. Hereinafter, the raw material of the microporous polyolefin membrane of the present invention will be described, but the present invention is not limited thereto.

The polyolefin microporous membrane of the present invention preferably contains 50 to 95% by mass of resin A and 5 to 50% by mass of resin B with respect to the total mass of the polyolefin microporous membrane. It is preferable to use polyethylene as the resin A, and the resin B is a resin obtained by copolymerizing ethylene and a vinyl compound containing a hetero atom in a chemical structural formula, for example, an ethylene-vinyl acetate copolymer or an ethylene-methyl acrylate copolymer. One or more selected from coalescing, ethylene-ethyl acrylate copolymer, ethylene-methyl methacrylate copolymer, ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer, ethylene-vinyl alcohol copolymer, etc. It is preferably made of resin.

The resin A used for the polyolefin microporous film of the present invention is, for example, low-density polyethylene, medium-density polyethylene, high-density polyethylene, linear low-density polyethylene, ultra-high molecular weight polyethylene, low-crystalline or amorphous ethylene / α. -Examples include polyethylene-based resins such as olefin copolymers. The α-olefin is not particularly limited as long as it can be copolymerized with ethylene, and for example, propylene, 1-butene, 1-hexene, 4-methyl-1-pentene, 1-octene, 1-pentene, etc. 1-Heptene can be mentioned. Among the above-mentioned resins, resin A has excellent melt extrusion characteristics and uniform stretching processing characteristics. Therefore, high-density polyethylene (density: 0.940 g / cm ³ or more and 0.970 g / cm ³ or less) or ultra-high molecular weight polyethylene is used. It is preferable to use it. Such polyethylene may be not only a homopolymer of ethylene but also a copolymer containing another α-olefin in order to lower the melting point and crystallinity of the raw material. The resin A may be used by selecting two or more types from the above-mentioned resins.

The weight average molecular weight (Mw) of the polyethylene used for the resin A is preferably 1.0 × 10 ⁴ or more, more preferably 1.0 × ¹⁰⁵ or more, and 3.0 × ¹⁰⁵ or more. Is more preferable, and ^5.0 × 105 or more is particularly preferable. Further, it is preferably less than 3.0 × ¹⁰⁶ , more preferably less than 2.5 × ¹⁰⁶ , and even more preferably less than 2.0 × ¹⁰⁶ . When the weight average molecular weight is within the above range, it is easy to control the crystal orientation due to stretching during the formation of the polyolefin microporous membrane, and the physical properties such as strength, permeability and pore size of the obtained microporous membrane are set within the appropriate range. Will be easy.

Further, the melting point of polyethylene used for the resin A is preferably 125 ° C. or higher, more preferably 130 ° C. or higher, and even more preferably 135 ° C. or higher. By setting the melting point of the resin A in the above range, it becomes easy to control the permeability of the microporous membrane within an appropriate range. From the above viewpoint, the higher the melting point of polyethylene is, the more preferable it is, but the upper limit of polyethylene that can be easily obtained as a commercially available product is substantially 138 ° C.

Further, it is possible to use a polypropylene-based resin as a part of the resin A used for the polyolefin microporous film of the present invention, and by using the polypropylene-based resin, the polyolefin microporous film was used as a separator for a secondary battery. The meltdown temperature at the time is improved. The meltdown temperature is a temperature at which the separator melts and breaks when the battery overheats abnormally, and the higher the temperature, the more preferable from the viewpoint of battery safety. As the type of polypropylene-based resin, in addition to homopolypropylene, block copolymers and random copolymers can also be used. The block copolymer and the random copolymer can contain a copolymer component with an α-olefin other than propylene, and ethylene is preferable as the α-olefin.

The resin A is preferably 50 to 95% by mass, more preferably 55 to 92% by mass, further preferably 60 to 90% by mass, and 70 to 70 to the total mass of the microporous polyolefin membrane. It is particularly preferably 85% by mass. By setting the mass ratio of the resin A used for the polyolefin microporous membrane in the above range, it becomes easy to set the strength and permeability of the polyolefin microporous membrane in an appropriate range, and it is safe when used as a battery separator. And excellent output characteristics.

As the resin B used for the polyolefin microporous film of the present invention, it is preferable to use a resin obtained by copolymerizing ethylene and a vinyl compound containing a hetero atom in a chemical structural formula, for example, an ethylene-vinyl acetate copolymer or ethylene. -Methyl acrylate copolymer, ethylene-ethyl acrylate copolymer, ethylene-methyl methacrylate copolymer, ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer, ethylene-vinyl alcohol copolymer and the like can be mentioned. .. Two or more kinds of the above-mentioned resins may be selected and used for the resin B.

The copolymerization ratio of the vinyl compound excluding ethylene in the resin used for the resin B is preferably 1% or more, more preferably 5% or more, further preferably 10% or more, and 15%. % Or more is particularly preferable. Further, it is preferably 50% or less, more preferably 40% or less, further preferably 30% or less, and particularly preferably 25% or less. By setting the copolymerization ratio of the vinyl compound containing the hetero element of the resin B in the above range, the dispersibility with the resin A becomes good, and the strength and permeability of the microporous polyolefin film and the impregnation of the electrolytic solution can be achieved at the same time. It will be easy.

The melt index (MI) of the resin B is preferably 0.01 g / min or more, more preferably 0.05 g / min or more, still more preferably 0.1 g / min or more. Further, it is preferably 30 g / min or less, more preferably 10 g / min or less, further preferably 5 g / min or less, and particularly preferably 1 g / min or less. By setting the MI of the resin B in the above range, the dispersibility with the resin A becomes good, and it becomes easy to achieve both the strength and permeability of the microporous polyolefin membrane and the impregnation of the electrolytic solution. The MI of the resin B is a value measured in accordance with JIS K7210 (1999).

The melting point of the resin used for the resin B is preferably 50 ° C. or higher, more preferably 60 ° C. or higher, and even more preferably 80 ° C. or higher. Further, it is preferably 140 ° C. or lower, more preferably 120 ° C. or lower, and further preferably 110 ° C. or lower. By setting the melting point of the resin B in the above range, it becomes easy to achieve both the permeability of the microporous membrane, the pore size, and the impregnation of the electrolytic solution.

The resin B is preferably 5 to 50% by mass, more preferably 8 to 45% by mass, still more preferably 10 to 40% by mass, and further preferably 15 to 40% by mass with respect to the total mass of the microporous polyolefin membrane. It is particularly preferably 30% by mass. By setting the mass ratio of the resin B used for the polyolefin microporous membrane in the above range, it becomes easy to set the strength and permeability of the polyolefin microporous membrane and the affinity with the electrolytic solution within an appropriate range, and it becomes easy to set the battery separator. Excellent in impregnation, safety and output characteristics of the electrolytic solution when used as.

In addition, various additives such as antioxidants, heat stabilizers and antistatic agents, ultraviolet absorbers, and blocking inhibitors and fillers are added to the microporous polyolefin film of the present invention as long as the effects of the present invention are not impaired. The agent may be contained. In particular, it is preferable to add an antioxidant for the purpose of suppressing oxidative deterioration due to the thermal history of the polyethylene resin. Examples of the antioxidant include 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4) and 1,3,5-trimethyl-2,4,6-tris (3,5-di). -T-Butyl-4-hydroxybenzyl) benzene (eg, BASF's "Irganox"® 1330: molecular weight 775.2), tetrakis [methylene-3 (3,5-di-t-butyl-4-hydroxy) It is preferable to use one or more selected from [phenyl) propionate] methane (for example, "Irganox" (registered trademark) 1010: molecular weight 1177.7 manufactured by BASF). Appropriate selection of the type and amount of antioxidant and heat stabilizer is important for adjusting or enhancing the characteristics of the microporous membrane.

Next, the method for producing the microporous polyolefin membrane of the present invention will be described, but the present invention is not limited thereto. The method for producing a porous polyolefin film of the present invention preferably comprises the following steps (a) to (e).
(A) A polymer material containing resin A, resin B, a plasticizer, and an additive is kneaded and dissolved to prepare a polyolefin solution.
(B) A gel-like sheet is obtained by extruding a polyolefin solution, molding it into a sheet, and cooling and solidifying it.
(C) The gel-like sheet is stretched by a roll method or a tenter method to obtain a stretched film.
(D) The plasticizer is extracted from the stretched film by washing, and the film is dried. (E) The film is re-stretched / heat-treated.
Hereinafter, each step will be described.

(A) Preparation of polyolefin solution Prepare a polyolefin solution in which resin A and resin B are dissolved by heating in a plasticizer. The plasticizer is not particularly limited as long as it is a solvent capable of sufficiently dissolving polyethylene, but the solvent is preferably a liquid at room temperature in order to enable stretching at a relatively high magnification. Solvents include nonane, decane, decalin, paraxylene, undecane, dodecane, aliphatic hydrocarbons such as liquid paraffin, cyclic aliphatic or aromatic hydrocarbons, mineral oil distillates having corresponding boiling points, and dibutylphthalates. Examples thereof include phthalates that are liquid at room temperature, such as dioctylphthalates. In order to obtain a gel-like sheet having a stable liquid solvent content, it is preferable to use a non-volatile liquid solvent such as liquid paraffin. In the melt-kneaded state, it is miscible with polyethylene, but at room temperature, a solid solvent may be mixed with the liquid solvent. Examples of such a solid solvent include stearyl alcohol, ceryl alcohol, paraffin wax and the like. However, if only a solid solvent is used, uneven stretching may occur.

The viscosity of the liquid solvent is preferably 20 to 200 cSt at 40 ° C. When the viscosity at 40 ° C. is 20 cSt or more, the sheet obtained by extruding the polyolefin solution from the die is unlikely to become non-uniform. On the other hand, if it is 200 cSt or less, the liquid solvent can be easily removed. The viscosity of the liquid solvent is the viscosity measured at 40 ° C. using an Ubbelohde viscometer.

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

Since the polyolefin microporous film of the present invention is a single film microporous film containing polyethylene and a polyolefin other than polyethylene, it is necessary to uniformly knead and extrude a plurality of different raw materials. If the kneading state is not uniform, the pores of the microporous membrane may become coarse or the pore diameter may vary widely. In order to knead uniformly, it is preferable that the set temperature of the first half of the extruder is set to TmA + 20 ° C. or lower when the melting point of the resin A to be used is TmA, and the raw materials are uniformly mixed in the state before melting. Next, in the latter half of the extruder, the resin A and the resin B are uniformly mixed in a molten state. The set temperature in the latter half of the extruder is preferably TmA to (TmA + 50 ° C.), more preferably TmA to (TmA + 30 ° C.), still more preferably TmA to (TmA + 20 ° C.), when the melting point of the resin A to be used is TmA. ℃). Here, the melting point is a value measured by DSC based on JIS K7121 (1987), and can be measured by a method described later.

(B) Formation of Extruded Product and Formation of Gel-like Sheet Next, the melt-kneaded polyolefin solution is extruded from the T-die and cooled to obtain a gel-like sheet, but after the melt-kneading until it is extruded by the T-die. The temperature of the polyolefin solution is preferably TmA to (TmA + 70 ° C.), more preferably TmA to (TmA + 50 ° C.), and even more preferably TmA to (TmA + 30 ° C.). By setting the temperature of the polyolefin solution from the time of melt-kneading to the time of being extruded by the T-die within the above range, phase separation between the resin A and the resin B can be prevented and the dispersed state can be made uniform.

Next, by cooling the obtained extruded product, a gel-like sheet is obtained, and by cooling, the microphase of polyethylene separated by the solvent can be immobilized. It is preferable to cool to 10 to 50 ° C. in the cooling step. This is because the final cooling temperature is preferably set to be equal to or lower than the crystallization end temperature, and by making the higher-order structure finer, uniform stretching can be easily performed in the subsequent stretching. Therefore, cooling is preferably performed at a rate of 30 ° C./min or higher at least up to the gelation temperature or lower. If the cooling rate is less than 30 ° C./min, the crystallinity increases and it is difficult to obtain a gel-like sheet suitable for stretching. Generally, when the cooling rate is slow, relatively large crystals are formed, so that the higher-order structure of the gel-like sheet becomes coarse, and the gel structure forming the gel structure also becomes large. On the other hand, when the cooling rate is high, relatively small crystals are formed, so that the high-order structure of the gel-like sheet becomes dense, which leads to high strength and uniform pore size.

Examples of the cooling method include a method of directly contacting with cold air, cooling water, and other cooling media, a method of contacting with a roll cooled by a refrigerant, a method of using a casting drum, and the like.

(C) Stretching step The obtained gel-like sheet is stretched. As the stretching method used, MD uniaxial stretching by a roll stretching machine, TD uniaxial stretching by a tenter, sequential biaxial stretching by a roll stretching machine and a tenter, or a combination of a tenter and a tenter, simultaneous biaxial stretching by a simultaneous biaxial tenter, etc. Can be mentioned. The draw ratio varies depending on the thickness of the gel-like sheet from the viewpoint of uniformity of film thickness, but it is preferable to stretch 5 times or more in any direction. The area magnification is preferably 25 times or more, more preferably 36 times or more, still more preferably 49 times, and particularly preferably 64 times or more. If the area magnification is less than 25 times, the stretching is insufficient and the uniformity of the film is likely to be impaired, and an excellent microporous film cannot be obtained from the viewpoint of the uniformity of the pore diameter and the strength. The area magnification is preferably 200 times or less. When the area magnification is increased, tearing is likely to occur frequently during the production of the microporous film, the productivity is lowered, and when the orientation is advanced and the crystallinity is high, the melting point and strength of the porous substrate are improved. However, increasing the crystallinity means that the amorphous portion is reduced, the melting point of the film is increased, and when used as a separator, it may be difficult to obtain the safety function due to the shutdown of the film.

The stretching temperature is preferably in the range of (melting point of the gel-like sheet + 10 ° C. or lower) to (crystal dispersion temperature Tcd of the polyolefin resin) to (melting point of the gel-like sheet + 5 ° C.). Specifically, since the polyethylene composition has a crystal dispersion temperature of about 90 to 100 ° C., the stretching temperature is preferably 90 to 125 ° C., more preferably 90 to 120 ° C. The crystal dispersion temperature Tcd is obtained from the temperature characteristics of dynamic viscoelasticity measured according to ASTM D 4065. Alternatively, it may be obtained from NMR. If the temperature is lower than 90 ° C., the pores are insufficiently opened due to low temperature stretching, it is difficult to obtain uniformity in film thickness, and the porosity is low. If the temperature is higher than 125 ° C., the sheet melts and the pores are likely to be closed.

Due to the above stretching, cleavage occurs in the higher-order structure formed on the gel sheet, the crystal phase becomes finer, and a large number of fibrils are formed. Fibrils form a three-dimensionally irregularly connected network structure. Stretching improves the mechanical strength and expands the pores, making it suitable as a battery separator. Further, by stretching before removing the plasticizer, the polyolefin is in a sufficiently plasticized and softened state, so that the cleavage of the higher-order structure becomes smooth and the crystal phase can be uniformly miniaturized. .. Further, since the cleavage is easy, strain at the time of stretching is less likely to remain, and the heat shrinkage rate can be lowered as compared with the case of stretching after removing the plasticizer.

(D) Cleaning / Drying Step Next, the solvent remaining in the gel-like sheet is removed using a cleaning solvent. Since the polyethylene phase and the solvent phase are separated, a microporous membrane can be obtained by removing the solvent. Examples of the cleaning solvent include saturated hydrocarbons such as pentane, hexane and heptane, chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, ethers such as diethyl ether and dioxane, ketones such as methyl ethyl ketone, and ethane trifluoride. Chain fluorocarbon and the like can be mentioned. These cleaning solvents have low surface tension (eg, 24 mN / m or less at 25 ° C.). By using a cleaning solvent having a low surface tension, shrinkage is suppressed by the surface tension of the gas-liquid interface when the network structure forming microporous structure is dried after cleaning, and a microporous membrane having porosity and permeability can be obtained. .. These cleaning solvents are appropriately selected according to the plasticizer and used alone or in combination.

The cleaning method can be carried out by a method of immersing the gel-like sheet in a cleaning solvent and extracting it, a method of showering the gel-like sheet with the cleaning solvent, a method using a combination thereof, or the like. The amount of the cleaning solvent used varies depending on the cleaning method, but is generally preferably 300 parts by mass or more with respect to 100 parts by mass of the gel-like sheet. The washing temperature may be 15 to 30 ° C., and if necessary, heat to 80 ° C. or lower. At this time, from the viewpoint of enhancing the cleaning effect of the solvent, from the viewpoint of preventing the physical properties of the obtained microporous membrane from becoming non-uniform in the TD direction and / or the MD direction, the mechanical properties and electricity of the microporous membrane. From the viewpoint of improving the physical characteristics, the longer the gel-like sheet is immersed in the cleaning solvent, the better. The above-mentioned washing is preferably carried out until the residual solvent in the gel-like sheet after washing, that is, the microporous membrane becomes less than 1% by mass.

Then, in the drying step, the solvent in the microporous membrane is dried and removed. The drying method is not particularly limited, and a method using a metal heating roll, a method using hot air, or the like can be selected. The drying temperature is preferably 40 to 100 ° C, more preferably 40 to 80 ° C. If the drying is insufficient, the porosity of the microporous membrane will decrease in the subsequent heat treatment, and the permeability will deteriorate.

(E) Heat treatment / re-stretching step The dried microporous membrane may be stretched (re-stretched) at least in the uniaxial direction. The re-stretching can be performed by the tenter method or the like in the same manner as the above-mentioned stretching while heating the microporous membrane. The re-stretching may be uniaxial stretching or biaxial stretching. In the case of multi-stage stretching, simultaneous biaxial and / and sequential stretching are combined.

The temperature of the re-stretching is preferably not higher than the melting point of the polyethylene composition, and more preferably in the range of (Tcd-20 ° C.) to the melting point. Specifically, 70 to 135 ° C. is preferable, and 110 to 132 ° C. is more preferable. Most preferably, it is 120 to 130 ° C.

In the case of uniaxial stretching, the re-stretching ratio is preferably 1.01 to 1.6 times, particularly preferably 1.1 to 1.6 times in the TD direction, and more preferably 1.2 to 1.4 times. In the case of biaxial stretching, it is preferable to increase the ratio by 1.01 to 1.6 times in the MD direction and the TD direction, respectively. The re-stretching magnification may be different in the MD direction and the TD direction. By stretching within the above range, the porosity and permeability can be increased, but when stretching at a magnification of 1.6 or more, the orientation progresses, the melting point of the film rises, and the film is used as a separator. Sometimes it is difficult to obtain the safety function by shutting down the film. From the viewpoint of heat shrinkage and wrinkles and sagging, the relaxation rate from the maximum restretching ratio is preferably 0.9 or less, more preferably 0.8 or less.

(F) Other Steps Further, depending on other uses, the microporous membrane can be hydrophilized. The hydrophilization treatment can be performed by monomer grafting, surfactant treatment, corona discharge and the like. The monomer graft is preferably performed after the cross-linking treatment. It is preferable to carry out a cross-linking treatment on the microporous polyolefin film by irradiation with ionizing radiation such as α-rays, β-rays, γ-rays and electron beams. In the case of electron beam irradiation, an electron dose of 0.1 to 100 Mrad is preferable, and an acceleration voltage of 100 to 300 kV is preferable. The cross-linking treatment raises the meltdown temperature of the microporous polyolefin membrane.

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

The microporous polyolefin membrane is made of a porous fluororesin such as polyvinylidene fluoride and polytetrafluoroethylene, polyimide, polyphenylene sulfide, etc. for the purpose of improving meltdown characteristics and heat resistance when used as a battery separator. A surface coating such as a porous body or an inorganic coating such as ceramic may be applied.

The microporous polyolefin membrane obtained as described above can be used for various purposes such as filters, separators for fuel cells, and separators for capacitors, but it has excellent compatibility with electrolytic solutions and has a uniform pore structure. Therefore, as a separator for a battery, the productivity of the battery and the cycle characteristics of the battery are improved, and it can be preferably used.

Hereinafter, the present invention will be described in detail with reference to Examples. The characteristics were measured and evaluated by the following methods.

Hereinafter, the present invention will be described in more detail with reference to Examples. Unless otherwise specified, the measurements in the present application are evaluated in an environment with a temperature of 23 ° C. and a humidity of 65%. Moreover, the present invention is not limited to these examples.

〔Measuring method〕
[Infrared absorption spectrum measurement]
Measurement was performed by the transmission method using FT-720 manufactured by HORIBA, Ltd. The number of scans was 30 times, the measurement range was 600 to 4000 cm ^-1 , and the resolution was 4 cm ^-1 . In the obtained absorption spectrum, the peak absorbance value between 1720 cm ^-1 and 1750 cm ^-1 is A, the peak absorbance value between 1450 cm ^-1 and 1480 cm ^-1 is B, and the value A is the value B. The value of A / B was calculated by dividing by.

[Differential Scanning Calorimetry (DSC)]
The measurement of the crystal melting peak of the polyolefin microporous membrane was measured by the differential scanning calorimetry (DSC) method. A 6.0 mg sample was enclosed in an aluminum pan, and the temperature was raised from 30 ° C. to 230 ° C. at 10 ° C./min using a PYRIS Diamond DSC manufactured by Parking Elmer for measurement. For the crystal melting peak of the obtained polyolefin microporous membrane, the peak area in the range of 30 ° C. to 110 ° C. was calculated.

[Contact angle]
The measurement was performed using DM-501Hi manufactured by Kyowa Interface Science. A sample obtained by pressing and melting a polyolefin microporous film cut out in a 5 cm square or a polyolefin microporous film by the method described later is fixed to a paper frame having a thickness of 2 mm, an outer dimension of 7 cm square, and an inner dimension of 5 cm × 3.5 cm with tape. , The surface on which the microporous polyolefin membrane was not attached was set face down, and the test solution was dropped into the area inside the inner dimension of the paper frame to measure the contact angle. Propylene carbonate was used as the test solution, the dropping amount was 2 μL, and the contact angle 10 seconds after the dropping was measured. Five points at arbitrary positions were measured in one sample fixed to the paper frame, and the average value at N = 5 was calculated. Further, the contact angle with respect to the microporous polyolefin membrane evaluated by the above method was C, the porosity of the microporous polyolefin membrane was E, and the evaluation was performed based on the following criteria.
〇: E <-0.6C + 90
X: E ≧ −0.6C + 90.

[Press melting of microporous membrane]
0.1 g of a microporous membrane was collected, folded, sandwiched between Teflon sheets, set in a heating press machine whose temperature was adjusted to 200 ° C., pressed at 2 MPa, and held for 1 minute. Next, the sample was taken out, sandwiched between a Teflon sheet and a stainless steel plate at room temperature, and the sample was cooled and solidified to obtain a sheet sample in which a microporous membrane was press-melted.

[Film thickness]
The film thickness of 5 points within the range of 50 mm × 50 mm of the polyolefin microporous film was measured with a contact thickness meter, Mitutoyo Co., Ltd. Lightmatic VL-50 (10.5 mmφ super hard spherical gauge, measuring load 0.01 N). The average value was defined as the film thickness (μm).

[Porosity]
A sample was cut into a square of 50 mm × 50 mm square from the microporous polyolefin membrane, and the film thickness T ₁ (cm) and mass (g) were measured. The film thickness was measured according to the method described in the above-mentioned measurement method. From these values and the membrane density (g / cm ³ ), the porosity of the polyolefin microporous membrane was calculated by the following formula.
Porosity (%) = (T ₁ x 25-mass / film density) / (T ₁ x 25) x 100
The film density of the microporous polyolefin membrane was calculated on the assumption that it was a constant value of 0.99 g / cm ³ . When calculating the porosity of the polyolefin microporous film containing the coat layer, the film thickness T ₁ of the polyolefin microporous film and the thickness T ₂ of the coat layer are used, and the value of the film density calculated by the following formula is used. Using.
Membrane density of polyolefin coated layer (g / cm ³ ) = 0.99 × T ₁ / (T ₁ + T ₂ ) + 4.0 × T ₂ / (T ₁ + T ₂ ).

[Puncture strength]
The puncture strength was measured according to JIS Z 1707 (2019), except that the test speed was 2 mm / sec. When a microporous polyolefin film is pierced in an atmosphere of 25 ° C with a needle with a spherical tip (radius of curvature R: 0.5 mm) and a diameter of 1.0 mm using a force gauge (DS2-20N manufactured by Imada Co., Ltd.). The maximum load (N) of was measured, and the puncture strength when the film thickness was 20 μm was calculated from the following formula.

Formula: Puncture strength (20 μm conversion) (N) = maximum load (N) × 20 (μm) / film thickness (μm) of polyolefin microporous membrane.

[Air permeability]
For a microporous polyolefin membrane with a film thickness of T ₁ (μm), in accordance with JIS P-8117, use a Wangken air permeability meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.) in an atmosphere of 25 ° C. The air permeability (seconds / 100 cm ³ ) was measured. In addition, the air permeability (20 μm conversion) (seconds / 100 cm ³ ) when the film thickness was 20 μm was calculated by the following formula.

Formula: Air permeability (20 μm conversion) (seconds / 100 cm ³ ) = air permeability (seconds / 100 cm ³ ) × 20 (μm) / film thickness of polyolefin microporous membrane (μm).

[Average hole diameter, maximum hole diameter]
Using a palm poromometer (CFP-1500A, manufactured by PMI), the average pore diameter and the maximum pore diameter of the polyolefin microporous membrane were determined. GALWICK (surface tension: 15.9 days / cm) was used as the impregnating liquid for the microporous polyolefin membrane, and the measurement was performed in the order of Dry-up and Wet-up. The average pore size (nm) is measured based on ASTM E1294-89 (1999) (half-dry method), and the dry-up measurement shows a half slope of the pressure and flow rate curves, and the Wet-up measurement. The pore diameter was converted from the pressure (KPa) at the point where the curves of. For the maximum pore diameter (nm), the maximum pore diameter was calculated from the bubble point pressure (KPa) measured based on the bubble point method (JIS K 3832 (1990)). For both the average pore diameter and the maximum pore diameter, the following formula was used to convert the pressure and the pore diameter.

d = C · γ / P
(In the above formula, "d (nm)" is the average pore diameter or the maximum pore diameter of the microporous membrane, "γ (dynes / cm)" is the surface tension of the impregnated liquid, "P (KPa)" is the pressure, and "C" is. It is a constant and is set to 2860.)
[Weight average molecular weight of polyolefin resin]
The weight average molecular weight (Mw) and the molecular weight distribution (MwD) of the polyolefin resin were determined by the gel permeation chromatography (GPC) method under the following conditions.

-Measuring device: GPC-150C manufactured by Waters Corporation
-Column: Showa Denko Corporation Shodex UT806M
・ Column temperature: 135 ℃
-Solvent (mobile phase): o-dichlorobenzene-Solvent flow rate: 1.0 ml / min-Sample concentration: 0.1% by mass (dissolution condition: 135 ° C / 1h)
-Injection amount: 500 μl
-Detector: Waters Corporation differential refractometer (RI detector)
-Calibration curve: Prepared using a polyethylene conversion coefficient (0.46) from a calibration curve obtained using a monodisperse polystyrene standard sample.

[Evaluation of electrolyte impregnation]
The microporous membrane cut into a rectangle of 25 mm × 50 mm was allowed to stand in a high temperature and humidity chamber at 65 ° C. and 85% RH for 48 hours. Next, the above-mentioned sample was placed so that the long side was in the vertical direction, the upper end 1 cm was overlapped with a glass plate and attached with tape, and a metal clip of about 3 g was attached as a weight within the range from the lower end to 5 mm. Subsequently, the film sample was fixed in a glass tank containing propylene carbonate with the lower end 2 cm immersed in it, the liquid level at this time was set to the reference position (0 mm), and after standing for 10 minutes, the microporous membrane sucked up the liquid. The height of the position where the film was transparent was read.

[Evaluation of battery safety and output characteristics (cycle test)]
From the cycle characteristics of the produced lithium-ion battery, the safety and output characteristics when the polyolefin microporous membrane was used as the battery separator were evaluated. The battery materials and evaluation conditions used for the evaluation are as follows. The test was performed at N = 3, and the average value of the capacity retention rate after 500 cycles was calculated.
-Positive electrode current collector: Al foil-Positive electrode: NCM523 + conductive material + PVDF binder (mixing ratio 92% by mass: 5% by mass: 3% by mass)
・ Negative electrode current collector: Copper foil ・ Negative electrode: Natural graphite + binder + CMC (mixing ratio 98% by mass: 1% by mass: 1% by mass)
-Electrolytic solution: 1.1 mol / L LiPF6 EC: MEC: DEC = 3: 5: 2 + VC (0.5% by mass)
-Exterior material: coin battery (CR2032)
・ Test temperature: 27 ° C
-Charging conditions: CCCV 2mA 4.2V 0.2mA Cut
・ Discharge condition: CC 2mA 2.5V Cut
-Number of cycles: 500 times Regarding the evaluation result of the cycle characteristics, the safety and pass / fail of the output characteristics when used as a separator were judged according to the following criteria.
Capacity retention rate after 500 cycles is 75% or more: 〇 Capacity retention rate after 500 cycles is less than 75%: ×.

[Example 1]
As resin A, 60% by mass of high-density polyethylene having a weight average molecular weight (Mw) of 3.0 × ¹⁰⁵ and a melting point of 135 ° C., a weight average molecular weight (Mw) of 2.0 × ¹⁰⁶ , and a melting point of more than 133 ° C. 40% by mass of high molecular weight polyethylene was used, and 100% by mass of Ultrasen 526 (ethylene-vinyl acetate copolymer, vinyl acetate copolymer ratio 7%, MI 25 g / 10 minutes) manufactured by Toso Co., Ltd. was used as the resin B. Resin A was charged into the twin-screw extruder at a ratio of 22.5% by mass, resin B at 2.5% by mass, and liquid paraffin at a ratio of 75% by mass from different feeders. Further, when the total mass of the resin A and the resin B is 100 parts by mass, 0.5 parts by mass of 2,6-di-t-butyl-p-cresol and 0.7 parts by mass of tetrakis [methylene-3-3]. (3,5-di-t-butyl-4-hydroxylphenyl) -propionate] Methane was dry-blended with resin A in advance as an antioxidant and charged into a twin-screw extruder. After kneading with a twin-screw extruder with the extruder temperature set to 150 ° C, the line temperature until being supplied to the T die was controlled to be 180 ° C, and the polyolefin solution was supplied to the T die and extruded into a sheet. After that, the extrude was cooled with a cooling roll controlled to 30 ° C. to form a gel-like sheet.

The obtained gel-like sheet was cut into a quadrangle of 80 mm square, and simultaneously biaxially stretched at a stretching temperature of 115 ° C. and a stretching speed of 1000 mm / min so as to be 7 times in the MD direction and 7 times in the TD direction. rice field. The stretched membrane is washed in a methylene chloride washing tank to remove liquid paraffin, the washed membrane is dried in a drying oven adjusted to 20 ° C, and heated in an electric oven at 120 ° C for 10 minutes. A microporous polyolefin membrane was obtained by immobilization.

[Example 2]
As resin A, 100% by mass of high-density polyethylene having a weight average molecular weight (Mw) of 3.0 × ¹⁰⁵ and a melting point of 135 ° C., and as resin B, Ultrasen 710 (ethylene-vinyl acetate copolymer,) manufactured by Toso Co., Ltd. Using 100% by mass of vinyl acetate copolymerization ratio (28%, MI 18g / 10 minutes), resin A was 24% by mass, resin B was 6% by mass, and liquid paraffin was 70% by mass. It was carried out in the same manner as in Example 1 except that it was put into an extruder and the washed film was heat-fixed in an electric oven at 100 ° C. for 10 minutes.

[Example 3]
Except that 100% by mass of Rexpearl A4200 (ethylene-ethyl acrylate copolymer, ethyl acrylate copolymer ratio 20%, MI 5 g / min) manufactured by Japan Polyethylene Corporation was used as the resin B, as in Example 2. It was carried out in the same manner.

[Example 4]
Examples except that 100% by mass of Ultrasen YX-11 (ethylene-vinyl acetate copolymer, vinyl acetate copolymer ratio 32%, MI 0.2 g / 10 minutes) manufactured by Tosoh Corporation was used as the resin B. It was carried out in the same manner as in 2.

[Example 5]
Except for the fact that 100% by mass of Rexpearl EB240H (ethylene-methyl acrylate copolymer, methyl acrylate copolymer ratio 20%, MI 6 g / min) manufactured by Japan Polyethylene Corporation was used as the resin B, as in Example 2. It was carried out in the same manner.

[Example 6]
The same procedure as in Example 3 was carried out except that the gel-like sheet was simultaneously biaxially stretched 9 times in the MD direction and 9 times in the TD direction.

[Example 7]
As resin A, ultra-high molecular weight polyethylene having a weight average molecular weight (Mw) of 1.5 × ¹⁰⁶ and a melting point of 135 ° C. is 100% by mass, and as resin B, Ultrasen 625 (ethylene-vinyl acetate copolymer) manufactured by Toso Co., Ltd. , Vinyl acetate copolymerization ratio 15%, MI 14 g / 10 minutes) was used in the same manner as in Example 2 except that 100% by mass was used.

[Comparative Example 1]
The same procedure as in Example 1 was carried out except that the resin A was charged in a proportion of 24% by mass, the resin B in an amount of 1% by mass, and the liquid paraffin was charged in a proportion of 75% by mass from different feeders to the twin-screw extruder.

[Comparative Example 2]
Resin A was added to the twin-screw extruder at a ratio of 24% by mass, resin B was charged to 6% by mass, and liquid paraffin was charged to the twin-screw extruder at a ratio of 70% by mass. It was carried out in the same manner as in Example 1 except that the smelting was carried out.

[Comparative Example 3]
The same procedure as in Example 4 was carried out except that the line temperature from kneading with a twin-screw extruder to being supplied to the T-die was controlled to 210 ° C., but the appearance was madara-like due to poor dispersion. Defects occurred and a uniform film could not be obtained.

[Comparative Example 4]
As resin A, 60% by mass of high-density polyethylene having a weight average molecular weight (Mw) of 3.0 × ¹⁰⁵ and a melting point of 135 ° C., a weight average molecular weight (Mw) of 2.0 × ¹⁰⁶ , and a melting point of more than 133 ° C. 40% by mass of high molecular weight polyethylene was used, 25% by mass of resin A and 75% by mass of liquid paraffin were charged into a twin-screw extruder from different feeders. Further, when the mass of the resin A is 100 parts by mass, 0.5 parts by mass of 2,6-di-t-butyl-p-cresol and 0.7 parts by mass of tetrakis [methylene-3- (3,5) -Di-t-butyl-4-hydroxylphenyl) -propionate] Methane was used as an antioxidant in advance by dry blending with resin A and charged into a twin-screw extruder. After kneading with a twin-screw extruder with the extruder temperature set to 150 ° C, the line temperature until being supplied to the T die was controlled to be 180 ° C, and the polyolefin solution was supplied to the T die and extruded into a sheet. After that, the extruded product was cooled with a cooling roll controlled to 30 ° C. to form a gel-like sheet.

The obtained gel-like sheet was cut into a quadrangle of 80 mm square, and simultaneously biaxially stretched at a stretching temperature of 115 ° C. and a stretching speed of 1000 mm / min so as to be 7 times in the MD direction and 7 times in the TD direction. rice field. The stretched membrane is washed in a methylene chloride washing tank to remove liquid paraffin, the washed membrane is dried in a drying oven adjusted to 20 ° C, and heated in an electric oven at 120 ° C for 10 minutes. A microporous polyolefin membrane was obtained by immobilization.

Both sides of the microporous polyolefin membrane obtained by the above method were subjected to surface treatment by corona discharge at a discharge power of 1000 W, a discharge length of 0.2 m, and a film feed rate of 100 m / min.

[Comparative Example 5]
44% by mass of alumina particles with an average particle size of 0.5 μm, 10% by mass of Polysol EVA P-3N (ethylene-vinyl acetate copolymer resin emulsion) manufactured by Showa Denko KK, and carboxymethyl cellulose manufactured by Daicel FineChem Co., Ltd. 1 part by mass of CMC Daicel 1220 ”and 45% by mass of ion-exchanged water were mixed to adjust the coating liquid. This coating liquid is applied to one side of the polyolefin microporous film produced by the procedure described in Comparative Example 4 using a metabar, and dried in an oven at 60 ° C. for 1 minute to obtain a coat layer-containing polyolefin microporous film. Got After drying the microporous polyolefin membrane containing the coat layer, the thickness of the coat layer was set to 4 μm.

[Comparative Example 6]
The same procedure as in Example 1 was carried out except that the gel-like sheet was simultaneously biaxially stretched so as to be quadrupled in the MD direction and quadrupled in the TD direction.

(evaluation)
The microporous polyolefin membranes of Examples 1 to 5 have a good affinity with the electrolytic solution and have a uniform and fine pore structure, so that they are highly invasive to the electrolytic solution when used as a separator for a secondary battery. It is possible to impart excellent safety and output characteristics. On the other hand, the microporous polyolefin film of Comparative Example 1 is inferior in affinity with the electrolytic solution, and the microporous polyolefin films of Comparative Examples 2 and 3 have unfavorable dispersibility between the resin A and the resin B, and therefore have a pore size. Inferior in uniformity and quality as a film. The microporous polyolefin membranes of Comparative Examples 4 and 5 have insufficient affinity for the electrolytic solution inside the film. The microporous polyolefin membrane of Comparative Example 6 has insufficient pore opening and lacks uniformity in pore diameter.

The polyolefin microporous membrane of the present invention can provide a polyolefin microporous membrane having excellent safety and output characteristics at the same time as being excellent in impregnation of an electrolytic solution when used as a battery separator. In particular, it can be suitably used for a secondary battery that is required to have a large size and a high capacity.

Claims (10)

When the absorbance peak value between 1720 cm ^-1 and 1750 cm ^-1 measured by the infrared absorption spectroscopy is A and the absorbance peak value between 1450 cm ^-1 and 1480 cm ^-1 is B, A is B. A polyolefin microporous film having a value divided by 0.1 or more and a difference between the maximum pore size measured by the bubble point method and the average pore size measured by the half-dry method of 40 nm or less. The microporous polyolefin membrane according to claim 1, wherein the maximum pore size measured by the bubble point method is 70 nm or less. The microporous polyolefin membrane according to claim 1 or 2, wherein the endothermic peak area due to crystal melting at 50 ° C. or higher and 110 ° C. or lower as measured by a differential scanning calorimeter is 5 J / g or less. When the dropping amount is 2 μL and the contact angle with propylene carbonate 10 seconds after dropping is C, and the contact angle with the propylene carbonate 10 seconds after dropping is 2 μL after pressing and thawing the polyolefin microporous membrane | The microporous polyolefin membrane according to any one of claims 1 to 3, wherein CD | is 20 ° or less. The polyolefin microporous membrane according to any one of claims 1 to 4, wherein the dropping amount of the polyolefin microporous membrane after press melting is 2 μL, and the contact angle D with respect to propylene carbonate 10 seconds after the dropping is 60 ° or less. The microporous polyolefin membrane according to any one of claims 1 to 5, which has a porosity of 60% or less. The microporous polyolefin membrane according to any one of claims 1 to 6, wherein the porosity E, the dropping amount of 2 μL, and the contact angle C with respect to propylene carbonate 10 seconds after dropping satisfy the following formula 3.
[Equation 3: E <-0.6C + 90] The polyolefin microporous membrane according to any one of claims 1 to 7, wherein the puncture strength in terms of thickness of 20 μm is 3.0 N or more. The microporous polyolefin membrane according to any one of claims 1 to 8, wherein the Gale air permeability in terms of thickness 20 μ is 600 seconds / 100 cm ³ or less. The resin A contains 50 to 95% by mass and the resin B contains 5 to 50% by mass, the resin A is made of polyethylene, and the resin B is an ethylene-vinyl acetate copolymer, an ethylene-methyl acrylate copolymer, or an ethylene-ethyl acrylate. The polyolefin according to any one of claims 1 to 9, which comprises one or more resins selected from a copolymer, an ethylene-methylmethacrylate copolymer, an ethylene-acrylic acid copolymer, and an ethylene-methacrylic acid copolymer. Microporous membrane.