JP2020092068A - Method for producing microporous film - Google Patents

Abstract

To provide a method for producing a microporous film allowing production of a polyolefin microporous film which is used as a separator for a lithium-ion battery, and which has improved strength and quality.SOLUTION: The method for producing a microporous film used for a lithium ion secondary battery separator is provided that includes the steps of: (1) supplying a twin-screw extruder with a first component including first polyolefin, a second component including second polyolefin, and a plasticizer (however, a difference (T1-T2) between a swelling start temperature (T1) of the first polyolefin and a swelling start temperature (T2) of the second polyolefin is 2°C to 50°C); (2) performing melt-kneading on the first component, the second component, and the plasticizer with the twin-screw extruder thereby producing a resin composition; and (3) extracting the plasticizer from the resin composition thereby producing a polyolefin microporous film.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for producing a polyolefin microporous membrane.

Microporous membranes are widely used as microfiltration membranes, fuel cell separators, condenser separators, base materials of functional membranes for filling functional materials in pores to develop new functions, and separators for electricity storage devices. It is used. Among them, the polyolefin microporous film is suitably used as a lithium ion battery separator that is widely used in notebook personal computers, mobile phones, digital cameras and the like.

Microporous polyolefin membranes are generally manufactured using relatively low molecular weight polyethylene. On the other hand, it has also been proposed to manufacture a polyolefin microporous film that can be used as a separator for lithium-ion batteries, using polyethylene having a relatively high molecular weight. For example, in Patent Document 1, a polyolefin resin having a weight average molecular weight of 500,000 or more and liquid paraffin are melt-kneaded, and liquid paraffin is extracted from the obtained resin composition to form a polyolefin microporous membrane. A method of manufacturing has been proposed.

Japanese Patent Laid-Open No. 06-027657 JP-A-2000-105466 JP 2007-16577 A

In recent years, coupled with the increase in battery capacity, the polyolefin microporous membrane used for lithium ion battery separators has improved quality (reduction of gel content, suppression of deterioration of molecular weight, etc.) and strength (puncture strength). And improvement of tensile strength) are further demanded. By using the polyolefin microporous film with improved quality, it is possible to suppress the unmelted amount (gel content) of the resin in the separator, thereby obtaining a separator with improved strength. .. By using such a separator, the characteristics of quickly stopping the battery reaction during heating (fuse characteristics), safety against impact (impact safety), and capacity maintenance characteristics after repeated charge and discharge (cycle characteristics) It is expected that a lithium-ion battery with excellent characteristics can be constructed.
In view of the above situation, Patent Document 1 has not examined the viewpoint of improving the quality and strength of the microporous polyolefin film used for the separator for lithium-ion batteries.

In view of the above circumstances, the present invention is used for a separator for a lithium-ion battery, and it is possible to produce a polyolefin microporous membrane with improved quality and strength, a method for producing a microporous membrane The purpose is to provide.

The present inventors have conducted studies to solve the above problems, and have found that the above problems can be solved by using a separator for an electricity storage device having the following configuration, and have completed the present invention. That is, the present invention is as follows.
[1]
A method for producing a microporous film used for a lithium ion secondary battery separator, comprising the following steps:
(1) A step of supplying a first component containing a first polyolefin, a second component containing a second polyolefin, and a plasticizer to a twin-screw extruder {however, the swelling start temperature (T1) of the first polyolefin. And the difference (T1-T2) between the swelling start temperature (T2) of the second polyolefin is 2°C to 50°C. };
(2) a step of producing a resin composition by melt-kneading the first component, the second component, and the plasticizer with the twin-screw extruder; and (3) the plasticizer from the resin composition. To produce a microporous polyolefin membrane;
A method for producing a microporous membrane, comprising:
[2]
The step (1) includes
The first component and the plasticizer using a continuous mixer under the conditions of a temperature of 20° C. to 70° C., a shear rate of 100 to 400,000 sec ⁻¹ , and a residence time of 1.0 second to 60 seconds. The method for producing a microporous membrane according to [1], including a step of producing a mixed slurry according to, and supplying the mixed slurry and the second component to the twin-screw extruder.
[3]
The step (1) includes
20 ° C. to 70 ° C. of temperature, 100～400,000 seconds ^- 1 shear rate, and the first component using a continuous mixer under the conditions of 1.0 seconds to 60 seconds of residence time, the second component And a step of producing a mixed slurry with the plasticizer and supplying the mixed slurry to the twin-screw extruder.
[4]
The first component is a powder containing 2% by mass to 90% by mass of the first polyolefin,
The method for producing a microporous membrane according to any one of [1] to [3], wherein the second component is a pellet containing the second polyolefin in an amount of 2% by mass to 90% by mass.
[5]
The first polyolefin is
The viscosity average molecular weight (Mv) is 300,000 to 9,700,000, and the ratio (Mw/Mn) of the weight average molecular weight (Mw) to the number average molecular weight (Mn) is 3 to 7, [4]. A method for producing the microporous membrane described.
[6]
The second polyolefin is
The viscosity average molecular weight (Mv) is 10,000 to 300,000, and the ratio (Mw/Mn) of the weight average molecular weight (Mw) to the number average molecular weight (Mn) is 3 to 18, which is described in [4]. A method for producing a microporous membrane.
[7]
The step (3) is
A step of extruding the resin composition into a sheet, cooling and solidifying the resin composition into a sheet-shaped molded body;
A step of forming a stretched product by biaxially stretching the sheet-shaped molded product at a surface magnification of 20 times or more and 200 times or less;
A step of extracting the plasticizer from the stretched product to form a porous body; and, after heat-treating the porous body at a temperature equal to or lower than the melting point of the porous body, stretching the porous body to form the microporous membrane. Manufacturing process;
The method for producing a microporous membrane according to any one of [1] to [6], which comprises:

INDUSTRIAL APPLICABILITY According to the present invention, it is used as a separator for a lithium-ion battery, and is improved in quality (reduction of gel content, suppression of deterioration of molecular weight, etc.) and strength (improvement of puncture strength, improvement of tensile strength, etc.). It is possible to produce a polyolefin microporous membrane having the above-mentioned features. By using a polyolefin microporous film with improved quality, the unmelted amount of resin (gel content) in the separator can be suppressed, and thus a separator with improved strength can be obtained. ..

The graph which shows the measuring method of the swelling start temperature in one Embodiment of this invention.

Hereinafter, modes for carrying out the present invention (hereinafter, abbreviated as “this embodiment”) will be described. However, the present invention is not limited to the following embodiments and can be variously modified and implemented within the scope of the gist. In addition, unless otherwise specified, the term “to” in the present specification means that the numerical values before and after the term are included as the upper limit value and the lower limit value.

<Method for producing microporous membrane>
The method for producing a polyolefin microporous membrane according to this embodiment has the following steps:
(1) A step of supplying the first component containing the first polyolefin, the second component containing the second polyolefin, and the plasticizer to the twin-screw extruder {however, the swelling start temperature (T1) of the first polyolefin and The difference (T1-T2) from the swelling start temperature (T2) of the second polyolefin is 2°C to 50°C. };
(2) A step of melt-kneading the first component, the second component, and a plasticizer with a twin-screw extruder to produce a resin composition; and (3) Extracting the plasticizer from the resin composition to make a polyolefin. A step of producing a microporous membrane;
including.
Hereinafter, each step will be described in order.

[Step (1)]
In the step (1), the first component containing the first polyolefin, the second component containing the second polyolefin, and the plasticizer are supplied to the twin-screw extruder. The swelling start temperature T1 of the first polyolefin is higher than the swelling start temperature T2 of the second polyolefin, and the difference (T1-T2) between the swelling start temperature T1 and the swelling start temperature T2 is 2°C to 50°C. Is.
As described below, examples of the first component include ultra high molecular weight polyethylene powder (UHMWPE powder), and examples of the second component include pellets (HDPE powder) made from high density polyethylene powder (HDPE powder). Pellets).

(Swelling of plasticizer)
In the first component (for example, UHMWPE powder), the ratio of crystalline part/amorphous part in the first polyolefin changes due to heat generation from the inside of the system or external heating in the presence of a plasticizer. , Gradually increase overall motility. Therefore, the plasticizer can be dipped in the non-crystalline portion, and the plasticizer can be dipped between the crystalline portion and the non-crystalline portion (intermediate layer portion). At this stage, melting and dissolution of the first component do not occur, and hereinafter such a phenomenon may be referred to as “swelling of the plasticizer”.

Conventionally, the swelling phenomenon has been studied using a water-absorbing amorphous resin (gel). Up to now, it is known that the swelling phenomenon is mainly a phenomenon of penetration of a plasticizer into a polyolefin and a subsequent phenomenon of diffusion of the plasticizer in the polyolefin. In addition, it is also reported that the swelling phenomenon is dominated by entropy change in the entire gel (change in osmotic pressure of plasticizer inside and outside the gel).

On the other hand, the present embodiment is based on a new finding that the swelling phenomenon described above is strongly related to the structural phase transition of the crystal part of the polyolefin as well as the entropy change in the entire gel.
For example, when the solid viscoelasticity of various pressure-bonded resin products of polyolefin powder is measured, the storage elastic modulus (E′) shows a broad peak at 0°C to 130°C. This is called crystal relaxation. Corresponding to the slip of the grain boundary (α ₁ ) and the change of the crystal itself into an elastic modulus (α ₂ ) [corresponding temperature T is T _α2 >T _α1 ], the storage elastic modulus (E′ ), a large peak, α ₁ slip, is observed at around 60° C., and a change of α ₂ is observed at about 70° C. or higher.
Here, the inventors of the present invention have diligently made efforts, based on the above-mentioned data regarding the polyolefin and the data regarding the swelling of the plasticizer, depending on the type of the polyolefin, the relaxation phenomenon of the non-crystalline portion is different, and It was found that the relaxation phenomenon in the intermediate layer part is different. Furthermore, since the relaxation phenomenon differs depending on the type of polyolefin as described above, the inventor of the present invention makes a swelling behavior of the plasticizer, for example, the swelling start temperature, the maximum swelling rate, and the time until the swelling is completed. It has come to be found that, etc. are remarkably different.
At the same time, the present inventor has made earnest efforts to find that the swelling initiation temperature changes depending on the crystallinity of the polyolefin powder, the crystallite size, the copolymer concentration, the size of the powder, and the like.

It is generally recognized that it is difficult to apply it to extrusion technology in the related art, which does not pay attention to the swelling behavior of the plasticizer even when trying to mix a plurality of polyethylenes such as UHMWPE powder and HDPE powder. Met. This is because the swelling behavior of the plasticizer usually differs depending on the type of polyethylene. For example, due to differences in the swelling start temperature, etc., the homogeneity in the system is significantly reduced, and the unmelted amount of the resin (gel content) tends to be large, and therefore the expected product performance is expressed. It was difficult to get him to do it.
On the other hand, the present embodiment has a viewpoint of achieving uniform swelling of the plasticizer. The present inventor pays attention to the fact that the swelling behavior of the plasticizer differs depending on the type of the polyolefin, and configures the first component and the second component so as to satisfy the above-mentioned difference (T1-T2) relationship. Thus, it was confirmed earnestly that the desired microporous membrane can be produced.

(Swelling start temperature)
In the present embodiment, the difference (T1-T2) between the swelling start temperature T1 of the first polyolefin and the swelling start temperature T2 of the second polyolefin is 2°C to 50°C. From the viewpoint of uniform swelling in the extruder, the difference (T1-T2) is more preferably 3°C to 48°C, and further preferably 3.2°C to 46°C. The lower limit of the difference (T1-T2) may be 3.5°C or higher, 4°C or higher, 5°C or higher, 8°C or higher, and 10°C or higher. May be The upper limit of the difference (T1−T2) may be 43° C. or lower, 40° C. or lower, 35° C. or lower, 30° C. or lower, 28° C. or lower. Or may be 25° C. or lower.

In recent years, coupled with the increase in battery capacity, the polyolefin microporous film used for a lithium ion battery separator is required to have improved quality and strength. In order to achieve both the functions of improving the quality and improving the strength, it is necessary to have a uniformly dispersed higher-order structure of the crystal structure necessary for manifesting the functions.
In the present embodiment, the swelling uniformity of the plasticizer is achieved by satisfying the above relationship of difference (T1-T2). Thereby, it is possible to obtain a resin composition in which a plurality of polyolefins are mixed and the uniform dispersion and higher order structure of the crystal structure necessary for the expression of the respective functions are appropriately controlled. Further, since the swelling uniformity of the plasticizer is achieved, it is possible to produce a resin composition that is uniform at the molecular level, and by using such a resin composition, the unmelted amount (gel content) of the resin is A suppressed separator can be obtained. Such a separator has high mechanical strength and excellent pore size uniformity. Then, by using such a separator, it is possible to manufacture a lithium ion battery having excellent impact resistance safety, cycle characteristics and the like.
The swelling start temperature T1 of the first polyolefin and the swelling start temperature T2 of the second polyolefin can be measured according to the method described in the examples.

(First component)
The first component includes a first polyolefin. The first polyolefin has a viscosity average molecular weight (Mv) of 300,000 to 9,700,000 and a dispersity (Mw/Mn) expressed as a ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn). Is preferably 3 to 7 polyethylene (first polyethylene). This first polyethylene is also called, for example, ultra high molecular weight polyethylene (UHMWPE).

The viscosity average molecular weight (Mv) of the first polyolefin is more preferably 260,000 to 9,000,000, further preferably 350,000 to 85,000. The dispersity (Mw/Mn) is more preferably 3 to 15, still more preferably 4 to 14, and most preferably 4 to 13.

The viscosity average molecular weight (Mv) can be measured with high accuracy and reproducibility regardless of the molecular weight. However, the number average molecular weight (Mn), the weight average molecular weight (Mw), and the dispersity (Mw/Mn) thereof, which are obtained by GPC measurement, show that the exclusion limit of the column is in the range where the viscosity average molecular weight (Mv) is 1,000,000 or more. Therefore, accurate measurement is difficult. On the other hand, in the region where the viscosity average molecular weight (Mv) is less than 1,000,000, it is possible to measure with high accuracy and reproducibility, and it becomes easy to accurately calculate the dispersity (Mw/Mn). Further, according to a verification experiment in a region where the viscosity average molecular weight (Mv) is less than 1,000,000, when the same kind of polyethylene polymerization catalyst (catalyst used when polymerizing polyethylene) is used, the number average molecular weight (Mn) is Even if the weight average molecular weight (Mw) changes, the dispersity (Mw/Mn) does not change.
Therefore, in the examples described later, it is difficult to accurately measure the number average molecular weight (Mn) and the weight average molecular weight (Mw) when analyzing the first polyolefin having a viscosity average molecular weight (Mv) of 1,000,000 or more. Further, the dispersity (Mw/Mn) was evaluated by using a value calculated in a region of 1,000,000 or less polymerized with the same type of polyethylene polymerization catalyst.

As the first polyolefin, a single type or a plurality of types of UHMWPE may be used. UHMWPE is preferably poly(ethylene and/or propylene-co-α-olefin), more preferably poly(ethylene-co-propylene), poly(ethylene) from the viewpoint of high strength of the polyolefin microporous membrane. -Co-butene), and at least one selected from the group consisting of poly(ethylene-co-propylene-co-butene). From the same viewpoint, UHMWPE preferably contains 98.5 mol% or more and 100 mol% or less of ethylene-derived structural units, and more preferably contains 0.0 mol of α-olefin-derived structural units other than ethylene. % To 1.5 mol% or less.

The first component is preferably a powder containing 2% by mass to 90% by mass of the first polyethylene, based on the total mass of the first component. From the viewpoint of ensuring the strength of the obtained microporous membrane, the content of the first polyethylene in the first component is more preferably 4% by mass or more. Further, from the viewpoint of suppressing the unmelted amount (gel content) in the first polyolefin, it is preferably 88% by mass or less.

Here, it preferred as "Powder" is used as the first component, that the number average particle size (d ₅₀₎ is 80Myuemu～180myuemu, the volume average particle size (d ₅₀₎ is 120Myuemu～220myuemu, few grains size distribution (Nd ₈₀ /Nd ₂₀ ) is 1.1 to 4.2, more preferably 1.2 to 4.1, and the volume particle size distribution (Vd ₈₀ /Vd ₂₀ ) is 1.1 to 3.3. And more preferably 1.15 to 3.2, the crystallite size is 10.0 nm to 30.0 nm, more preferably 11.0 nm to 28.0 nm, and further preferably 12.0 nm to 23.0 nm. And has a crystallinity of 30% to 99%, preferably 32% to 98%, more preferably 38% to 97.5%, and containing polyethylene. Means that satisfies at least one condition. These number average particle size (Nd ₅₀ ), volume average particle size (Vd ₅₀ ), number particle size distribution (Nd ₈₀ /Nd ₂₀ ), volume particle size distribution (Vd ₈₀ /Vd ₂₀ ), crystallite size, crystallization The degree and the like can be measured according to a known method or a method described in Examples.
The number average particle size can be adjusted by correlating with the catalyst particle size distribution of the polyolefin polymerization catalyst. Further, the crystallite size and the degree of crystallinity can be adjusted by the temperature during the polymerization and the degree of annealing treatment of the polyethylene crystal part in the drying step after the polymerization.

(Second component)
The second component includes a second polyolefin. The second polyolefin is a polyethylene having a viscosity average molecular weight (Mv) of 10,000 to 300,000 and a dispersity (Mw/Mn) of the weight average molecular weight (Mw) with respect to the number average molecular weight (Mn) of 3 to 18. It is preferably (second polyethylene). This second polyethylene is also called, for example, high density polyethylene (HDPE).

The second polyolefin may be a low-density polyolefin (LDPE) such as linear low-density polyethylene (LLDPE) or a high-density polyolefin (HDPE), but a single type or a plurality of types of HDPE are preferable. The crystallite size is 10.0 nm to 35.0 nm, more preferably 11.0 nm to 32.0 nm, from the viewpoint of not inhibiting the swelling behavior of the first polyethylene in the extruder and thereby obtaining a uniform kneaded resin product. It is more preferable that the thickness is 12.0 nm to 31.0 nm, and the crystallinity is 30% to 93%, preferably 32% to 90%, and further preferably 38% to 89%. From the same viewpoint, HDPE preferably contains 98.5 mol% or more and 100 mol% or less of ethylene-derived constituent units, and more preferably 0.0 mol of constituent units derived from an α-olefin other than ethylene. % To 1.5 mol% or less.

The second component is preferably a pellet containing 2% by mass to 90% by mass of the second polyethylene. From the viewpoint of ensuring both the mechanical strength of the membrane and the expression of the fuse characteristic peculiar to the polyethylene separator, the second polyethylene contained in the second component is more preferably 2.1% by mass to 88% by mass, further It is preferably 2.3% by mass to 82% by mass.

The second component, which is a pellet, can be obtained by drying a polymerized polyolefin powder (for example, HDPE powder), extruding it into a strand shape with an extruder, cooling with water, and then cutting into pellets. The polyolefin powder used as a raw material preferably has a viscosity average molecular weight (Mv) of 30,000 to 3,500,000 and a dispersity (Mw/Mn) of 3 to 15.
The viscosity average molecular weight (Mv) when the second component is formed into a pellet is preferably 35,000 to 3,300,000, more preferably 30,000 to 3,200,000. .. The dispersity is preferably 3 to 15, more preferably 4 to 14 and even more preferably 4 to 13.

Here, the “pellet” preferable as the second component is larger than the number average particle diameter (Nd ₅₀ ) and volume average particle diameter (Vd ₅₀ ) of the “powder” preferable as the first component, and the length of one side. Is 10 mm or less at the maximum, and is a resin material of 1 mm or more. The shape of the pellet is not particularly limited, and may be, for example, spherical, ellipsoidal, or columnar. It can be obtained by melt-extruding a raw material (for example, polyethylene powder) with an extruder, arranging it into a strand while cooling with water or air, and continuously cutting. The size and shape can be adjusted by strand formation or a cutting method.
As described above, by melt-extruding the above-mentioned "powder" and adjusting it into the above-mentioned "pellet", the swelling speed by the plasticizer can be significantly delayed. Inside the extruder, it is important not to hinder the swelling of the first component. Further, in the melting step after the swelling step, from the viewpoint of uniform melting, the “pellet” preferable as the second component is larger than the crystallite size of the “powder” preferable as the first component, and within the above range. It has a crystallite size.
Furthermore, the "pellets" preferred as the second component are those having a crystallinity lower than that of the "powder" preferred as the first component and having a crystallinity within the above range. In the production method of pellets, the temperature of the extruded resin in a strand shape, the cooling temperature at the time of cutting, or the pulling rate of the strand shape from extrusion (melt fine drawing), crystallite size of pellets, crystallinity, size of pellets, The shape can be adjusted. These number average particle diameter (Nd ₅₀ ), volume average particle diameter (Vd ₅₀ ), crystallite size, crystallinity and the like can be measured according to known methods or methods described in Examples.

By forming HDPE into a pellet shape, it is possible to significantly improve the swelling uniformity of the plasticizer in the twin-screw extruder when mixed with the first component (for example, UHMWPE powder). It is considered that this is because by forming HDPE into a pellet shape, the swelling speed can be remarkably reduced as compared with, for example, HDPE powder. Therefore, the HDPE pellets do not excessively hinder the swelling of the UHMWPE powder, and basically the pellets themselves do not swell, and are suitably used in the subsequent melt-kneading.
In particular, by applying this embodiment, under a preferable molecular weight or a preferable difference in swelling temperature, etc., by appropriately adjusting the mixed composition of the polyolefins, it is possible to easily and uniformly reach a molecular level even in the melt-kneading process. Each polyolefin can be dispersed.

As mentioned above, the HDPE pellets have a swelling rate that is significantly slower than the swelling rate of the HDPE powder, and thus do not substantially swell. That is why it is possible to prevent the swelling of the UHMWPE powder from being excessively inhibited by HDPE. Since HDPE pellets do not substantially swell, it is difficult to measure the swelling start temperature. On the other hand, the swelling start temperature can be measured with the HDPE powder that is the raw material of the HDPE pellets, and the swelling start temperature of the HDPE powder that is the raw material of the HDPE pellets is also measured in the Example section.

(Plasticizer)
The plasticizer is a known material as long as it is liquid at a temperature of 20° C. to 70° C. and is excellent in dispersibility of the first polyolefin (eg UHMWPE) or the second polyolefin (eg HDPE). You can The plasticizer used in the step (1) is preferably a non-volatile solvent capable of forming a homogeneous solution at a temperature equal to or higher than the melting point of the first polyolefin or the second polyolefin in consideration of the subsequent extraction. Specific examples of the non-volatile solvent include, for example, hydrocarbons such as liquid paraffin, paraffin wax, decane, and decalin; esters such as dioctyl phthalate and dibutyl phthalate; higher alcohols such as oleyl alcohol and stearyl alcohol. Etc. Among them, liquid paraffin is preferable because it has high compatibility with polyethylene or polypropylene, and even if the melt-kneaded product is stretched, interfacial peeling between the resin and the plasticizer is unlikely to occur, and uniform stretching tends to be easily performed.

(Supply to twin screw extruder)
The first component and the second component can be simultaneously supplied to, for example, a twin-screw extruder. In order to supply them at the same time, for example, a method in which the first component and the second component are dry-blended with a super mixer or the like to be supplied to the twin-screw extruder; However, the method of supplying this to a twin-screw extruder is mentioned.

Also, the first component and the second component can be individually supplied to, for example, a twin-screw extruder. To supply separately, for example, a method of supplying the first component and the second component from a separate feeder to a twin-screw extruder; a mixed slurry described below is prepared from the first component and the plasticizer, A method of supplying the twin screw extruder separately from the two components; the second component is melted by another extruder (second extruder), and the second component is melted and separated from the first component by the twin screw extruder (first Extruder). In addition, when supplying a 1st component and a 2nd component separately to a twin-screw extruder, a 1st component may be supplied first and a 2nd component may be supplied first.

Further, the plasticizer can be supplied to the twin-screw extruder together with the first component, for example. After the plasticizer is supplied to the twin-screw extruder together with the first component, the plasticizer may be additionally supplied from the same or different feeder.
This type of twin-screw extruder generally has an upper feed port arranged on the upstream side and an intermediate feed port arranged on the downstream side of the feed port and in the middle of the melting and kneading area. ing. Therefore, after the mixed slurry is supplied from the upper feed port of the twin screw extruder, the plasticizer can be additionally supplied from the middle feed port of the twin screw extruder. This makes it easy to adjust the liquid paraffin amount ratio in the resin composition extruded from the twin-screw extruder to a desired ratio, and also to easily adjust the temperature of the resin composition to a desired range. Of course, the first component or the second component can be supplied from the middle-stage feed port.

(Mixed slurry)
Here, step (1) uses a continuous mixer under the conditions of a temperature of 20° C. to 70° C., a shear rate of 100 to 400,000 sec ⁻¹ , and a residence time of 1.0 second to 60 seconds. The method may include a step of producing a mixed slurry with the first component and the plasticizer, and supplying the mixed slurry and the second component to the twin-screw extruder.
In addition, the step (1) is a step of producing a mixed slurry from the first component, the second component, and the plasticizer using the continuous mixer under the above conditions, and supplying the mixed slurry to the twin-screw extruder. It can also be included.

The lower limit of the set temperature of the continuous mixer is preferably 25°C or higher, more preferably 30°C or higher from the viewpoint of maximally swelling the first component, and the upper limit thereof is the dissolution of the first polyolefin during mixing. From the viewpoint of suppressing the temperature and obtaining a slurry, the temperature is preferably 68°C or lower, more preferably 67°C or lower, 66°C or lower, or 65°C or lower.
The shear rate of the continuous mixer is 100 to 400,000 sec ⁻¹ , and preferably 120 to 398,000 sec ⁻¹ , from the viewpoint of uniformly contacting the first component with the plasticizer to obtain a dispersion. It is preferably 1,000 to 100,000 sec ^-1 .
The residence time of the continuous mixer is 1.0 to 60 seconds, preferably 2.0 to 58 seconds, and more preferably 2.0 seconds to, from the viewpoint of ensuring the dispersion of the first polyolefin in the plasticizer. 56 seconds.

The content of the first component (for example, UHMWPE powder) in the mixed slurry is preferably more than 0% by mass, more preferably from the viewpoint of the strength of the microporous membrane obtained, based on the total mass of the mixed slurry. Is 1% by mass or more, more preferably 2% by mass or more or 4% by mass or more. Further, this content is preferably 50% by mass or less, more preferably 40% by mass or less, further preferably 30% by mass or less, from the viewpoint of suppressing the generation of unmelted material (gel) in the first polyolefin. It is 20 mass% or less.

When the mixed slurry contains the second component, the content rate of the second component (for example, HDPE pellets) in the mixed slurry is based on the total mass of the mixed slurry, from the viewpoint of the strength of the obtained microporous membrane. %, preferably more than 0 mass%, more preferably 1 mass% or more, still more preferably 2 mass% or more, or 4 mass% or more. Further, this content is preferably 50% by mass or less, more preferably 40% by mass or less, further preferably 30% by mass or less, from the viewpoint of suppressing generation of unmelted material (gel) in the second polyolefin. It is 20 mass% or less.

The mixed slurry is preferably supplied to the twin-screw extruder at a temperature of 25°C to 80°C. From the viewpoint of ensuring the entanglement of the polymer chains to the extent that the decrease in the molecular weight of the first polyolefin does not occur while ensuring an appropriate viscosity of the mixed slurry, the supply temperature can be adjusted within the range of 25°C to 80°C. preferable. From the same viewpoint, the supply temperature is more preferably 30°C to 76°C, further preferably 30°C to 70°C.

(Other resins or additives)
The above-mentioned first component, second component, and mixed slurry may optionally contain polyethylene other than UHMWPE; polyethylene other than HDPE; resins other than polyethylene.

Examples of polyethylene other than UHMWPE and polyethylene other than HDPE include linear low density polyethylene (LLDPE), high pressure low density polyethylene, or a mixture thereof. Further, polyethylene having a narrow molecular weight distribution obtained by using a metallocene catalyst may be used. These polyethylenes can be used in one or more and can make up the balance of the microporous membrane, including UHMWPE and HDPE.

Examples of polyolefins other than polyethylene include polypropylene, polybutene, ethylene-propylene copolymer, polymethylpentene, and silane-grafted modified polyolefin. These polyolefins can be used in one or more and can make up the balance of the microporous membrane, including UHMWPE and HDPE.

Examples of resins other than polyolefins include engineering plastic resins such as polyphenylene ether; polyamide resins such as nylon 6, nylon 6-12 and aramid resin; polyimide resins; polyester resins such as polyethylene terephthalate (PET) and polybutene terephthalate (PBT). resins; polycarbonate resins; polyvinylidene fluoride (PVDF), fluorinated resin such as polytetrafluoroethylene, ethylene and copolymers of vinyl alcohol, copolymers of α- olefin and carbon monoxide C ₂ -C ₁₂ and Hydrogenated product thereof; hydrogenated product of styrene-based polymer; copolymer of styrene and α-olefin and hydrogenated product thereof; copolymer of styrene and aliphatic monounsaturated fatty acid; (meth)acrylic acid, Polymers of (meth)acrylate and/or derivatives thereof; thermoplastic resins selected from copolymers of styrene and conjugated diene unsaturated monomers and hydrogenated products thereof; polysulfones, polyether sulfones, polyketones Etc. These resins can be used in one or more and can constitute the balance of the microporous membrane, including UHMWPE and HDPE.

Further, the mixed slurry contains known additives, for example, dehydration condensation catalyst, metal soaps such as calcium stearate or zinc stearate, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, color pigments and the like. Good.

[Step (2)]
In the step (2), the first component, the second component, and the plasticizer are melt-kneaded with a twin-screw extruder to produce a resin composition. In the present embodiment, since the swelling uniformity of the plasticizer with respect to the first component and the second component is ensured in the above step (1), the gist of the present invention is not deviated in the step (2). The conditions such as the type of twin-screw extruder, the supply of each raw material to the twin-screw extruder, the extrusion time, the extrusion rate, the shear rate, and the shear force are not limited as long as they are within the range.

[Step (3)]
In step (3), a plasticizer is extracted from the resin composition produced in step (2) to produce a polyolefin microporous membrane.
Particularly, the step (3) according to this embodiment includes a sheet processing step (3-1), a stretching step (3-2), an extraction step (3-3), and a heat treatment step (3-4).

(Sheet processing step (3-1))
In the step (3-1), the resin composition obtained in the above step (2) is extruded into a sheet, cooled and solidified to be processed into a sheet-shaped molded body. The resin composition may include a polyolefin resin containing UHMWPE or HDPE, a plasticizer, and the like.

From the viewpoint of sheet moldability, the proportion of the polyolefin resin in the sheet-shaped molded product is preferably 10 to 80% by mass, more preferably 20 to 60% by mass, and most preferably 30, based on the mass of the sheet-shaped molded product. Is about 50% by mass.

As a method for producing a sheet-shaped molded article, for example, the resin composition obtained in the above step (2) is extruded into a sheet through a T-die or the like, and brought into contact with a heat conductor to crystallize the resin component. A method of cooling to a temperature lower than the solidification temperature and solidifying can be mentioned. Examples of the heat conductor used for cooling and solidification include metal, water, air, and a plasticizer. Among these, it is preferable to use a metal roll because the heat conduction efficiency is high. Further, when the extruded resin composition is brought into contact with a metal roll, it may be sandwiched between the rolls to further increase the efficiency of heat conduction, orient the sheet to increase the film strength, and improve the surface smoothness of the sheet. It is preferable because it tends to occur. When the resin composition is extruded from a T-die into a sheet, the die lip interval is preferably 200 μm or more and 3,000 μm or less, and more preferably 500 μm or more and 2,500 μm or less. When the die lip interval is 200 μm or more, dents and the like are reduced, streak, defects and the like are less affected on the film quality, and the risk of film breakage in the subsequent stretching step can be reduced. On the other hand, when the die lip interval is 3,000 μm or less, the cooling rate is high and uneven cooling can be prevented, and the thickness stability of the sheet can be maintained.
Moreover, you may roll the extruded sheet-shaped molded object during a process (3-1).

[Stretching step (3-2)]
In the step (3-2), the sheet-shaped molded product obtained in the above step (3-1) is biaxially stretched at a surface magnification of 20 times or more and 200 times or less to form a stretched product.

As the stretching treatment, biaxial stretching is preferable to uniaxial stretching from the viewpoint that the film thickness distribution and the air permeability distribution in the width direction can be reduced. By stretching the sheet biaxially at the same time, the number of times the sheet-shaped molded body is repeatedly cooled and heated during the film forming process is reduced, and the distribution in the width direction is improved. Examples of the biaxial stretching method include simultaneous biaxial stretching, sequential biaxial stretching, multistage stretching, and multiple stretching. From the viewpoint of improving the puncture strength and the uniformity of stretching, simultaneous biaxial stretching is preferable, and from the viewpoint of easy control of plane orientation, sequential biaxial stretching is preferable.

In the present specification, the simultaneous biaxial stretching means a stretching in which MD (machine direction of continuous forming of microporous membrane) stretching and TD (direction crossing MD of microporous membrane at an angle of 90°) are simultaneously performed. Method, and the draw ratio in each direction may be different. Sequential biaxial stretching refers to a stretching method in which MD and TD stretching is performed independently. When stretching is performed in MD or TD, the other direction is fixed in an unrestrained state or fixed length. To be in a state of being

The stretch ratio is preferably in the range of 20 times or more and 200 times or less in terms of surface magnification, more preferably in the range of 25 times or more and 170 times or less, and further preferably 30 times or more and 150 times or less. The draw ratio in each axial direction is preferably in the range of 2 to 15 times in MD, 2 to 15 times in TD, and 3 to 12 times in MD and 3 to 12 times in TD. Is more preferable, and MD is more preferably 5 times or more and 10 times or less, and TD is more preferably 5 times or more and 10 times or less. When the total area ratio is 20 times or more, sufficient strength can be imparted to the obtained microporous membrane, while when the total area ratio is 200 times or less, the film rupture in the step (3-2) is prevented. There is a tendency for prevention and high productivity to be obtained.

The stretching temperature is preferably 90 to 150° C., more preferably 100 to 140° C., and further preferably 110 to 130° C. from the viewpoint of the meltability and film forming property of the polyolefin resin.

[Extraction step (3-3)]
In step (3-3), the plasticizer is extracted from the stretched product obtained in step (3-2) to form a porous body. Examples of the method for extracting the plasticizer include a method in which the stretched product is immersed in an extraction solvent to extract the plasticizer, and then dried. The extraction method may be either a batch method or a continuous method. In order to suppress the shrinkage of the porous body, it is preferable to restrain the end portion of the sheet-shaped molded product during a series of dipping and drying steps. Further, the residual amount of the plasticizer in the porous body is preferably less than 1% by mass with respect to the mass of the entire porous film.
Note that after the step (3-3), the plasticizer may be recovered and reused by an operation such as distillation.

As the extraction solvent, it is preferable to use a solvent that is a poor solvent for the polyolefin resin, a good solvent for the plasticizer, and a boiling point lower than the melting point of the polyolefin resin. Examples of such an extraction solvent include hydrocarbons such as n-hexane and cyclohexane; halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane; non-chlorine-based solvents such as hydrofluoroether and hydrofluorocarbon. Examples thereof include halogenated solvents; alcohols such as ethanol and isopropanol; ethers such as diethyl ether and tetrahydrofuran; ketones such as acetone and methyl ethyl ketone. In addition, these extraction solvents may be recovered and reused by an operation such as distillation.

[Heat treatment step (3-4)]
In step (3-4), after heat-treating the porous body at a temperature equal to or lower than the melting point of the porous body obtained in step (3-3), the porous body is stretched to form a microporous membrane. To manufacture.

The porous body is subjected to heat treatment for the purpose of heat fixation from the viewpoint of suppressing shrinkage. As the method of heat treatment, for the purpose of adjusting the physical properties, a predetermined atmosphere, a predetermined temperature and a predetermined temperature and a predetermined operation at a predetermined temperature and a predetermined stretching ratio, and/or for the purpose of reducing the stretching stress. The relaxation operation is performed at the relaxation rate of. A relaxation operation may be performed after the stretching operation. These heat treatments can be performed using a tenter or a roll stretching machine.

From the viewpoint of increasing the strength and porosity of the microporous membrane, the stretching operation is preferably 1.1 times or more, more preferably 1.2 times or more, the MD and/or TD of the membrane.
The relaxation operation is a reduction operation of the membrane to MD and/or TD. The relaxation rate is a value obtained by dividing the dimension of the membrane after the relaxation operation by the dimension of the membrane before the relaxation operation. When both MD and TD are relaxed, it is a value obtained by multiplying the MD relaxation rate and the TD relaxation rate. The relaxation rate is preferably 1.0 or less, more preferably 0.97 or less, and further preferably 0.95 or less. The relaxation rate is preferably 0.5 or more from the viewpoint of film quality. The relaxation operation may be performed in both MD and TD, or in only one of MD and TD.

From the viewpoint of the melting point of the polyolefin resin (hereinafter, also referred to as “Tm”), the temperature of the heat treatment including the stretching or relaxation operation is preferably in the range of 100 to 170°C. When the temperature of the stretching and relaxation operations is in the above range, it is preferable from the viewpoint of the balance between the reduction of heat shrinkage and the porosity. The lower limit of the heat treatment temperature is more preferably 110°C or higher, further preferably 120°C or higher, even more preferably 125°C or higher, and the upper limit is more preferably 160°C or lower, further preferably 150°C or lower, and further It is preferably 140° C. or lower.

During the step (3-4) or after the step (3-4), the microporous membrane may be subjected to a post-treatment such as a hydrophilic treatment with a surfactant or the like and a cross-linking treatment with an ionizing radiation or the like. The order of the steps (3-2), (3-3) and (3-4) described above may be rearranged or these steps may be performed simultaneously, but from the viewpoint of film formability. It is preferable to perform these steps in the order of steps (3-2), (3-3) and (3-4) using a biaxial stretching machine.
The obtained microporous membrane can be wound with a winder to form a roll or cut with a slitter from the viewpoints of handleability and storage stability.

<Microporous polyolefin film>
The polyolefin microporous membrane can be produced by the method for producing a polyolefin microporous membrane including steps (1) to (3). A microporous polyolefin membrane has a porous structure in which a large number of very small pores are gathered to form dense communication pores, so it is extremely superior in ion conductivity and at the same time has good withstand voltage characteristics and high It has the characteristic of being strong.

The characteristics of the microporous polyolefin membrane will be described below. These characteristics are the case where the polyolefin microporous film as the separator for the electricity storage device is a flat film, but when the separator for the electricity storage device is in the form of a laminated film, the layers other than the microporous film are excluded from the laminated film. Can be measured afterwards.

The polyolefin microporous membrane has the following formula from the viewpoint of high quality and strength so that it can be used as a separator for electricity storage devices:
10 μm conversion permeability coefficient (J)=air permeability (Pe)÷porosity (Po)
The converted transmission coefficient per 10 μm of film thickness is preferably 0.5 to 14, more preferably 1 to 13.

The porosity of the polyolefin microporous membrane is preferably 20% or more, more preferably 30% or more, still more preferably 32% or more or 35% or more. When the porosity of the microporous membrane is 20% or more, the followability to rapid movement of lithium ions tends to be further improved. On the other hand, the porosity of the microporous membrane is preferably 90% or less, more preferably 80% or less, still more preferably 50% or less. When the porosity of the microporous film is 90% or less, the film strength is further improved, and self-discharge tends to be further suppressed. The porosity of the microporous membrane can be measured by the method described in Examples.

The air permeability of the polyolefin microporous membrane is preferably 1 second or more, more preferably 50 seconds or more, further preferably 55 seconds or more, still more preferably 100 seconds or more, 120 seconds or more per 100 cm ^3. , 140 seconds or more or 150 seconds or more. When the air permeability of the microporous membrane is 1 second or more, the balance among the film thickness, the porosity, and the average pore diameter tends to be further improved. The air permeability of the microporous membrane is preferably 400 seconds or less, more preferably 300 seconds or less, and further preferably 270 seconds or less. When the air permeability of the microporous membrane is 400 seconds or less, the ion permeability tends to be further improved. The air permeability of the microporous membrane can be measured by the method described in Examples.

The tensile strength of the polyolefin microporous membrane is preferably 1000 kgf/cm ² or more, more preferably 1050 kgf/cm ² or more, further preferably 1100 kgf/cm ² or more, in both MD and TD directions. .. When the tensile strength is 1000 kgf/cm ² or more, breakage during slitting or winding of the electricity storage device is more suppressed, or short circuit due to foreign matter or the like in the electricity storage device is more likely to be suppressed. On the other hand, the tensile strength of the microporous membrane is preferably 5000 kgf/cm ² or less, more preferably 4500 kgf/cm ² or less, and further preferably 4000 kgf/cm ² or less. When the tensile strength of the microporous film is 5000 kgf/cm ² or less, the microporous film is relaxed early in the heating test, the contraction force is weakened, and as a result, the safety tends to be improved.

The film thickness of the polyolefin microporous film is preferably 1.0 μm or more, more preferably 2.0 μm or more, still more preferably 3.0 μm or more, 4.0 μm or more, or 5.0 μm or more. When the film thickness of the microporous film is 1.0 μm or more, the film strength tends to be further improved. The film thickness of the microporous film is preferably 500 μm or less, more preferably 100 μm or less, and further preferably 80 μm or less, 22 μm or less or 19 μm or less. When the film thickness of the microporous film is 500 μm or less, the ion permeability tends to be further improved. The film thickness of the microporous film can be measured by the method described in Examples.

When the polyolefin microporous membrane is a separator used in recent relatively high capacity lithium ion secondary batteries, the thickness of the microporous membrane is preferably 25 μm or less, more preferably 22 μm or less or 20 μm or less. And more preferably 18 μm or less, particularly preferably 16 μm or less. In this case, when the thickness of the microporous film is 25 μm or less, the permeability tends to be further improved. In this case, the lower limit of the film thickness of the microporous film may be 1.0 μm or more, 3.0 μm or more, 4.0 μm or more, or 5.0 μm or more.

The above-mentioned measured values of various physical properties are values measured according to the measuring methods in Examples described later, unless otherwise specified.

<Lithium-ion secondary battery separator>
The polyolefin microporous membrane according to this embodiment can be used as a separator for a lithium ion secondary battery. Since a separator for a lithium ion secondary battery is required to have insulation and lithium ion permeability, a polyolefin microporous film capable of constructing a dense and uniform porous structure and oxidation-reduction resistance deterioration of the separator is excellent.

The separator is a flat membrane (for example, formed of one microporous membrane), a laminated membrane (for example, a laminated body of a plurality of polyolefin microporous membranes, a laminated body of a polyolefin microporous membrane and another membrane), It may be in the form of a coating film (for example, when a functional substance is coated on at least one side of a microporous polyolefin film).

<Lithium-ion secondary battery>
The lithium-ion secondary battery contains a lithium transition metal oxide such as lithium cobalt oxide or a lithium cobalt composite oxide as a positive electrode, a carbon material such as graphite or graphite as a negative electrode, and a lithium salt such as LiPF ₆ as an electrolytic solution. It is a storage battery using an organic solvent. During charge/discharge of the lithium-ion secondary battery, ionized lithium reciprocates between the electrodes. Further, since the ionized lithium needs to move between the electrodes at a relatively high speed while suppressing contact between the electrodes, a separator is arranged between the electrodes.

Next, the present embodiment will be described more specifically with reference to examples and comparative examples. However, the present invention is not limited to the following examples as long as the gist of the present invention is not exceeded. The physical properties in the examples were measured by the following methods.

<Weight average molecular weight (Mw), number average molecular weight (Mn)>
A calibration curve was prepared by measuring standard polystyrene under the following conditions using ALC/GPC 150C type (trademark) manufactured by Waters. Further, a chromatogram was measured for each of the following polymers under the same conditions, and the weight average molecular weight and the number average molecular weight of each polymer were calculated by the following method based on the calibration curve.
Column: Tosoh GMH ₆ -HT (trademark) 2 +GMH ₆ -HTL (trademark) 2 Mobile phase: o-dichlorobenzene Detector: Differential refractometer Flow rate: 1.0 ml/min
Column temperature: 140℃
Sample concentration: 0.1wt%
(Weight average molecular weight and number average molecular weight of polyethylene)
The molecular weight distribution curve in terms of polyethylene was obtained by multiplying each molecular weight component in the obtained calibration curve by 0.43 (Q factor of polyethylene/Q factor of polystyrene=17.7/41.3), and the weight average was obtained. The molecular weight and the number average molecular weight were calculated. In terms of chromatogram performance, it is difficult to accurately measure the molecular weight distribution in the region where the molecular weight is 1,000,000 or more.

<Viscosity average molecular weight (Mv)>
Based on ASTM-D4020, the intrinsic viscosity [η] at 135°C in the decalin solvent was determined. The Mv of polyethylene was calculated by the following formula.
[Η]=6.77×10 ⁻⁴ Mv ^0.67

<Thickness of each layer (μm)>
The film thickness was measured at an atmospheric temperature of 23±2° C. using a micro thickness gauge (type KBN, terminal diameter Φ5 mm) manufactured by Toyo Seiki. When measuring the thickness, 10 microporous membranes are stacked and measured, and the value obtained by dividing the total thickness by 10 is taken as one thickness.

<Porosity (%)>
A 10 cm×10 cm square sample was cut from the microporous membrane, its volume (cm ³ ) and mass (g) were determined, and the porosity was calculated from these and the density (g/cm ³ ) by the following formula. As the density of the mixed composition, a value calculated by calculating the density of each raw material used and the mixing ratio was used.
Porosity (%)=(volume-mass/density of mixed composition)/volume×100

<Air permeability (sec/100 cm ³ )>
According to JIS P-8117 (2009), the air permeability of the sample was measured by G-B2 (trademark), a Gurley type air permeability meter manufactured by Toyo Seiki Co., Ltd.

<Puncture strength (gf)>
Using a handy compression tester “KES-G5” (trade name, manufactured by Kato Tech), the puncture strength of the sample film was determined by performing a puncture test on the sample film under the conditions of a radius of curvature of the needle tip of 0.5 mm and a puncture speed of 2 mm/sec. It was

<Tensile breaking strength (kgf/cm ² )>
The tensile strength of the sample film was measured under the conditions of a load cell load of 5 kgf and a chuck distance of 50 mm using a sample film cut into a size of 10 mm×100 mm and a tensile tester.

<Quantification of resin aggregates in separator>
The resin aggregate (gel-containing material) in the separator has an area of 100 μm in length × 100 μm in width or more when the separator obtained through the film forming steps of Examples and Comparative Examples described later is observed with a transmission optical microscope. It is quantified by the areas that have and do not transmit light. The number of resin aggregates per 1000 m ^{2 of} separator area was measured by observation with a transmission optical microscope.

<Quantification of molecular weight deterioration>
A fixed amount of the mixed raw material before being put into the twin-screw extruder was measured, and the viscosity [η] was measured based on the viscosity average molecular weight measuring method described in the above item <Viscosity average molecular weight (Mv)>. η ₀ ]. Further, the film finally obtained was similarly measured in a fixed amount and the viscosity was measured, which was defined as [η _a ]. The molecular weight deterioration rate (%) was calculated according to the following formula.
Molecular weight deterioration rate=[η _a ]/[η ₀ ]

<Swelling measurement>
Slurry liquid obtained by immersing polyethylene powder in liquid paraffin (“Smoyl P-350P” (trademark, manufactured by Matsumura Oil Research Co., Ltd.) is applied (brushed) to a hot stage with a camera capable of recording, and specific heat capacity and heating uniformity are obtained. In consideration of the above, the change in the particle size of the powder was recorded while heating the slurry liquid at a rate of 9° C./min.The image obtained by the recording was processed by the Partial Analysis processing function in IgorPro. The size of the particle shape was statistically measured, and the slurry liquid was adjusted to a volume concentration so that individual contours could always be observed during the measurement without the powder particles covering each other. Taking into account the error in resolution and powder particle size distribution, we used about 81 powders each time and used the data obtained by averaging the results of 3 measurements.The swelling degree (swelling rate) is based on the powder at 25°C. The ratio of the increase in powder volume converted to the projected area circular phase with the increase in temperature was taken as the percentage value, and the change in swelling rate with respect to temperature change was measured and used as the swelling curve.
FIG. 1 shows changes in swelling ratio (swelling curve) of the two types of polyolefin powders used in Example 1. The two kinds of polyolefin powders here are UHMWPE powder which is the first component and HDPE powder which is used to form the second component (HDPE pellet).
The swelling start temperature can be obtained as the temperature corresponding to the inflection point before reaching the maximum swelling rate by second-order differentiating the swelling curve. That is, a tangent line was drawn before and after the inflection point, and the temperature at the intersection of the tangent lines was taken as the swelling start temperature. The horizontal axis of FIG. 1 is shown based on the maximum swelling ratio of the UHMWPE powder, which is the first polyolefin used in Example 1.

<Particle size measurement>
It was measured using a flow type image analysis particle diameter/shape measuring device Particle Insight manufactured by Micromeritics. The total count number was 3,000 to 4,000, and the volume distribution was calculated using the number average distribution, the projected area circular phase conversion and the like. Before the measurement, it was calibrated with standard particle size substances (80 μm, 150 μm).
<Measurement of pellet size>
The size of the pellet was measured by measuring the length of one side with a caliper being calibrated.

<Measurement of crystallite size and crystallinity>
<Polyethylene crystallite size (110)>
(Measurement)
XRD measurement was performed using Rigaku's X-ray diffractometer Ultima-IV. A Cu-Kα ray was incident on the sample, and diffracted light was detected by a detector D/tex Ultra manufactured by Rigaku Corporation. KRD measurement was performed under the conditions of a sample-detector distance of 285 mm, an excitation voltage of 40 kV, and a current of 40 mA. A concentrated optical system was adopted as the optical system, and the measurement was performed under the slit conditions of DS=1/2°, SS=opening and vertical slit=10 mm.
(analysis)
The range from 2θ=9.7° to 2θ=29.0° of the obtained XRD profile was divided into three orthorhombic (110) plane diffraction peaks, orthorhombic (200) plane diffraction peaks and amorphous peaks. The crystals were separated and the crystallite size was calculated from the full width at half maximum of the (110) plane diffraction peak according to Scherrer's formula (Formula 1). The (110) plane diffraction peak and the (200) plane diffraction peak were approximated by the Voigt function, and the amorphous peak was approximated by the Gauss function. The peak position of the amorphous peak was fixed at 2θ=19.6° and the full width at half maximum was fixed at 6.3°, and the peak position and the full width at half maximum of the crystal peak were not fixed and the peaks were separated. From the full width at half maximum of the (110) plane diffraction peak calculated by peak separation, the crystallite size was calculated according to Scherrer's formula (the following formula). The crystallinity is a percentage value obtained by dividing the sum of separated crystals and the amorphous peak by the area of the crystal peak.
D(110)=Kλ/(β cos θ)
D(110): crystallite size (nm)
K: 0.9 (constant)
λ: X-ray wavelength (nm)
β: (β ₁ ² −β ₂ ² ) ^0.5
β ₁ : full width at half maximum (rad) of peak calculated as a result of peak separation (hkl)
β ₂ : full width at half maximum (rad) of spread of incident beam
θ: Bragg angle

[Example 1]
As shown in Table 1, a powder containing the first polyolefin (ultra-high molecular weight polyethylene having a viscosity average molecular weight of 1,000,000; UHMWPE) whose weight average molecular weight and number average molecular weight are difficult to measure, and a second polyolefin (weight average molecular weight 12.6). , A high-density polyethylene having a number-average molecular weight of 20,000 and a viscosity-average molecular weight of 150,000 (HDPE) (a cylindrical body having a diameter of 5 to 6 mm and a height of 3 to 2 mm), and other modifiers described in Table 1 ( In the table, "StCa" means calcium stearate) was dry-blended with a super mixer, supplied to a twin-screw extruder, and melt-mixed. Then, while melt-mixing, liquid paraffin (kinematic viscosity at 37.78° C. 7.59×10 ⁻⁵ m ² /s) was injected into a twin-screw extrusion equipped with a manifold (T die) having a die lip interval of 1500 μm with an injection nozzle. The resin composition was extruded by supplying it to a machine and further kneading. At this time, the middle extruding part (the middle feeding port of the twin screw extruder is adjusted so that the ratio of the amount of liquid paraffin in the resin composition extruded from the twin screw extruder is 70% and the temperature of the resin composition is 220° C. ) Was further injected with liquid paraffin. Subsequently, the extruded resin composition was extruded onto a cooling roll whose surface temperature was controlled to 25° C. and cast to obtain a sheet-shaped molded body.
Next, it was guided to a simultaneous biaxial tenter stretching machine and biaxially stretched to obtain a stretched product. The stretching conditions were a stretching surface magnification of 50 to 180 times, and the porosity, air permeability, thickness, and puncture strength were adjusted by adjusting the stretching temperature, the amount of heated air, the magnification, and the like. In Example 1, the biaxial stretching temperature was 125°C.
Next, the porous body was introduced into a TD tenter to perform heat setting, and heat setting (HS) was performed at 128° C., and then a relaxation operation of 0.5 times in the TD direction (that is, the HS relaxation rate was 0.5 times). ) Was done. Then, the above-mentioned evaluation was performed on the obtained microporous membrane. The evaluation results are shown in Table 1.

[Example 2]
Powder and pellets are provided in separate feeders and are supplied from the same charging port of the twin-screw extruder (upper feed port of the twin-screw extruder), and the raw material composition and process conditions are changed as shown in Table 1, except that A polyolefin microporous membrane was produced in the same manner as in Example 1. The evaluation results of the obtained film are shown in Table 1.

[Examples 3 and 4]
A shearing rate of 40,000 ^-1 and a residence time of 20 seconds were used, except that a mixed slurry whose temperature was adjusted to 65°C was prepared and the raw material composition and process conditions were changed as shown in Table 1. A polyolefin microporous membrane was produced in the same manner as in Example 1. The evaluation results of the obtained film are shown in Table 1. The liquid paraffin content in the mixed slurry was adjusted to 70% by mass. Further, the obtained mixed slurry was supplied to a twin-screw extruder equipped with a manifold (T die) having a die lip interval of 1500 μm by a feeder at 60° C. under a nitrogen atmosphere.

"Example 5"
The pellets are melted by another extruder (second extruder) and supplied to the above twin-screw extruder (first extruder), and the raw material composition, process conditions, etc. are shown in Table 1. An attempt was made to prepare a microporous polyolefin film by the same method as in Example 1 except for the change, and the physical properties of the obtained film were evaluated. The evaluation results are also shown in Table 1.

[Examples 6 to 7 and Comparative Examples 1 to 3]
Starting from the above Example 1, the raw material composition, process conditions, etc. were changed as shown in Table 1 to produce a polyolefin microporous membrane. The evaluation results of the obtained film are shown in Table 1.

[Examples 8 to 18]
Starting from the above Example 1, the raw material composition, the process conditions, etc. were changed as shown in Table 2 to prepare a polyolefin microporous membrane. The evaluation results of the obtained film are shown in Table 2.

As can be seen from Table 1, by the method of Examples 1 to 7 using UHMWPE powder and HDPE pellets, the method of Comparative Example 1 in which HDPE powder is directly mixed with UHMWPE powder, and the swelling start temperature (T1) and the swelling start temperature. Compared with the method of Comparative Examples 2 to 3 in which the difference (T1−T2) with (T2) is outside the range of 2° C. to 50° C., the content of gel is reduced and deterioration of the molecular weight is suppressed. It was confirmed that a microporous membrane can be manufactured. Further, it was confirmed that the method of Example 1 can produce a microporous polyolefin membrane having improved puncture strength and tensile strength as compared with the methods of Comparative Examples 1 to 3 described above.
Therefore, it was confirmed that the method of Example 1 can produce a microporous polyolefin membrane having improved quality and strength as compared with the methods of Comparative Examples 1 to 3. The polyolefin microporous membrane obtained by the method of Examples 1 to 7 can be suitably used for a lithium ion battery separator.

T1 Swelling start temperature of first polyolefin T2 Swelling start temperature of second polyolefin

Claims (7)

A method for producing a microporous film used for a lithium ion secondary battery separator, comprising the following steps:
(1) A step of supplying a first component containing a first polyolefin, a second component containing a second polyolefin, and a plasticizer to a twin-screw extruder {however, the swelling start temperature (T1) of the first polyolefin. And the difference (T1-T2) between the swelling start temperature (T2) of the second polyolefin is 2°C to 50°C. };
(2) a step of producing a resin composition by melt-kneading the first component, the second component, and the plasticizer with the twin-screw extruder; and (3) the plasticizer from the resin composition. To produce a microporous polyolefin membrane;
A method for producing a microporous membrane, comprising: The step (1) includes
The first component and the plasticizer using a continuous mixer under the conditions of a temperature of 20° C. to 70° C., a shear rate of 100 to 400,000 sec ⁻¹ , and a residence time of 1.0 second to 60 seconds. The method for producing a microporous membrane according to claim 1, comprising a step of producing a mixed slurry according to, and supplying the mixed slurry and the second component to the twin-screw extruder. The step (1) includes
20 ° C. to 70 ° C. of temperature, 100～400,000 seconds ^- 1 shear rate, and the first component using a continuous mixer under the conditions of 1.0 seconds to 60 seconds of residence time, the second component The method for producing a microporous membrane according to claim 1, further comprising: producing a mixed slurry with the plasticizer and supplying the mixed slurry to the twin-screw extruder. The first component is a powder containing 2% by mass to 90% by mass of the first polyolefin,
The method for producing a microporous membrane according to claim 1, wherein the second component is a pellet containing 2% by mass to 90% by mass of the second polyolefin. The first polyolefin is
The viscosity average molecular weight (Mv) is 300,000 to 9,700,000, and the ratio (Mw/Mn) of the weight average molecular weight (Mw) to the number average molecular weight (Mn) is 3 to 7. A method for producing the microporous membrane described. The second polyolefin is
The viscosity average molecular weight (Mv) is 10,000 to 300,000, and the ratio (Mw/Mn) of the weight average molecular weight (Mw) to the number average molecular weight (Mn) is 3 to 18. A method for producing a microporous membrane. The step (3) is
A step of extruding the resin composition into a sheet, cooling and solidifying the resin composition into a sheet-shaped molded body;
A step of forming a stretched product by biaxially stretching the sheet-shaped molded product at a surface magnification of 20 times or more and 200 times or less;
A step of extracting the plasticizer from the stretched product to form a porous body; and, after heat-treating the porous body at a temperature equal to or lower than the melting point of the porous body, stretching the porous body to form the microporous membrane. Manufacturing process;
The method for producing a microporous membrane according to any one of claims 1 to 6, which comprises: