JP7265349B2 - Method for manufacturing microporous membrane - Google Patents

Description

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

Microporous membranes are widely used as microfiltration membranes, separators for fuel cells, separators for capacitors, base materials for functional membranes that allow functional materials to be filled into the pores to develop new functions, and separators for electrical storage devices. used. Among them, polyolefin microporous membranes are suitably used as separators for lithium ion batteries, which are widely used in notebook personal computers, mobile phones, digital cameras, and the like.

Generally, polyolefin microporous membranes are manufactured using relatively low molecular weight polyethylene. On the other hand, it has also been proposed to produce a polyolefin microporous membrane that can be used as a lithium ion battery separator 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, liquid paraffin is extracted from the resulting resin composition, and a polyolefin microporous membrane is produced. A method of manufacturing has been proposed.

JP-A-06-027657 JP-A-2000-105466 JP 2007-316577 A

In recent years, along with the increase in battery capacity, polyolefin microporous membranes used in lithium-ion battery separators have improved quality (reduced gel content, suppressed deterioration of molecular weight, etc.) and improved strength (puncture resistance). improvement, improvement of tensile strength, etc.) are further required. By using a polyolefin microporous film with improved quality, it is possible to suppress the unmelted amount of resin (gel content) in the separator, thereby obtaining a separator with improved strength. . By using such a separator, the characteristics of quickly stopping the battery reaction when heated (fuse characteristics), the safety against impact (shock resistance safety), and the capacity retention characteristics when charging and discharging are repeated (cycle characteristics). It is expected that a lithium-ion battery with excellent properties such as
In view of the above situation, Patent Document 1 does not consider the viewpoint of improving the quality and strength of the polyolefin microporous membrane used in the lithium ion battery separator.

In view of the above circumstances, the present invention provides a method for producing a microporous membrane, which is used for lithium ion battery separators and which can produce a polyolefin microporous membrane having improved quality and improved strength. intended to provide

The present inventors conducted studies to solve the above problems, found that the above problems can be solved by using a power storage device separator having the following configuration, and completed the present invention. That is, the present invention is as follows.
[1]
A method for producing a microporous membrane used in 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 {wherein the swelling initiation temperature (T1) of the first polyolefin and the swelling start temperature (T2) of the second polyolefin (T1-T2) is 2°C to 50°C. };
(2) a step of melt-kneading the first component, the second component, and the plasticizer in the twin-screw extruder to produce a resin composition; and (3) the plasticizer from the resin composition Extracting to produce a polyolefin microporous membrane;
A method for producing a microporous membrane, comprising:
[2]
The step (1) is
The first component and the plasticizer using a continuous mixer under 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 sec to 60 sec. and supplying the mixed slurry and the second component to the twin-screw extruder.
[3]
The step (1) is
The first component, the second component 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 sec to 60 sec. and producing a mixed slurry from 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 pellets containing 2% by mass to 90% by mass of the second polyolefin.
[5]
The first polyolefin is
Viscosity average molecular weight (Mv) is 300,000 to 9,700,000, and the ratio of weight average molecular weight (Mw) to number average molecular weight (Mn) (Mw/Mn) is 3 to 7, in [4] A method for producing the described microporous membrane.
[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. 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 it, and processing it into a sheet-shaped molding;
A step of biaxially stretching the sheet-like molded product at an area magnification of 20 times or more and 200 times or less to form a stretched product;
a step of extracting the plasticizer from the stretched product to form a porous body; and heat-treating the porous body at a temperature below the melting point of the porous body, then 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], comprising

According to the present invention, it is used for a lithium ion battery separator, and has improved quality (reduced gel content, suppressed deterioration of molecular weight, etc.) and improved strength (improved puncture strength, improved tensile strength, etc.) It is possible to produce a polyolefin microporous membrane in which the By using a polyolefin microporous film with improved quality, it is possible to suppress the unmelted amount of resin (gel content) in the separator, thereby obtaining a separator with improved strength. .

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

EMBODIMENT OF THE INVENTION Hereinafter, the form (it abbreviates as "this embodiment" hereafter) for implementing this invention is demonstrated. However, the present invention is not limited to the following embodiments, and various modifications can be made within the scope of the gist of the present invention. In addition, unless otherwise specified, "to" in the present specification includes the numerical values before and after it as an upper limit and a lower limit.

<Method for producing microporous membrane>
The method for producing a polyolefin microporous membrane according to the present embodiment comprises 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 initiation temperature (T1) of the first polyolefin and The difference (T1-T2) from the swelling initiation 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 the plasticizer with a twin-screw extruder to produce a resin composition; and (3) Extracting the plasticizer from the resin composition, a step of manufacturing a microporous membrane;
including.
Each step will be described in order below.

[Step (1)]
In step (1), a first component comprising a first polyolefin, a second component comprising a second polyolefin, and a plasticizer are fed to a twin screw extruder. The swelling initiation temperature T1 of the first polyolefin is higher than the swelling initiation temperature T2 of the second polyolefin, and the difference (T1-T2) between the swelling initiation temperature T1 and the swelling initiation temperature T2 is 2° C. to 50° C. is.
As will be described later, 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 (e.g., UHMWPE powder), the ratio of crystalline part/non-crystalline part in the first polyolefin changes due to heat generation in the system or external heating in the presence of a plasticizer, thereby , the overall motility gradually increases. Therefore, the plasticizer can be immersed in the non-crystalline portion, and can also be immersed in the space 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 such a phenomenon is hereinafter sometimes 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 the phenomenon of the plasticizer entering into the polyolefin and the subsequent diffusion of the plasticizer within the polyolefin. It is also reported that the swelling phenomenon is dominated by entropy change in the entire gel (change in osmotic pressure of the plasticizer inside and outside the gel).

On the other hand, the present embodiment is based on the new knowledge 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 measuring the solid viscoelasticity of compression-bonded resin products of various polyolefin powders, the storage elastic modulus (E') shows a broad peak in the range of 0°C to 130°C. This is called crystalline relaxation. Corresponding to the grain boundary slip (α ₁ ) and the change in the elastic modulus of the crystal itself (α ₂ ) [the corresponding temperature T is T _α2 >T _α1 ], the storage modulus (E' ), a large peak, a slip of α ₁ is observed near 60°C, and a change of α ₂ is observed above about 70°C.
Here, the present inventor made diligent efforts, based on the above-mentioned data on polyolefin and data on swelling of plasticizers, the relaxation phenomenon of the amorphous part differs depending on the type of polyolefin, and the It was found that the relaxation phenomenon of the intermediate layer is different. Furthermore, the present inventors have made diligent efforts to find out that the relaxation phenomenon differs as described above depending on the type of polyolefin, so that the swelling behavior of the plasticizer, for example, the swelling start temperature, the maximum swelling rate, and the time to complete swelling etc. are remarkably different.
At the same time, through diligent efforts, the present inventors have discovered that the swelling initiation temperature varies depending on the crystallinity of the polyolefin powder, the crystallite size, the copolymer concentration, the size of the powder, and the like.

Even if a plurality of polyethylenes, for example, UHMWPE powder and HDPE powder are mixed, it is generally recognized that it is difficult to apply the conventional technology to extrusion technology in the first place because it does not focus on the swelling behavior of the plasticizer. Met. This is because different types of polyethylene usually have different swelling behavior of their plasticizers. For example, due to differences in swelling initiation temperature, etc., the homogeneity within the system tends to be significantly reduced, and there is a tendency for a large amount of unmelted resin (gel content) to be included, resulting in the expected product performance. It was difficult to let
In contrast, the present embodiment has a viewpoint of achieving uniformity of swelling of the plasticizer. The present inventor focused on the point that the swelling behavior of the plasticizer differs depending on the type of polyolefin, and configured the first component and the second component so as to satisfy the relationship of the above difference (T1-T2). It was confirmed through diligent efforts that a desired microporous membrane can be produced by this method.

(Swelling start temperature)
In this embodiment, the difference (T1-T2) between the swelling initiation temperature T1 of the first polyolefin and the swelling initiation temperature T2 of the second polyolefin is 2°C to 50°C. From the viewpoint of uniform swelling in the extruder, the above difference (T1-T2) is more preferably 3°C to 48°C, still more 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, or 10°C or higher. may be The upper limit of the difference (T1-T2) may be 43°C or less, 40°C or less, 35°C or less, 30°C or less, or 28°C or less. It may be 25° C. or lower.

In recent years, along with the increase in battery capacity, polyolefin microporous membranes used in lithium-ion battery separators are required to have higher quality and higher strength. In order to achieve both the functions of improving quality and improving strength, a uniformly distributed higher-order crystal structure is required for the manifestation of each function.
In this embodiment, by satisfying the relationship of the difference (T1-T2), uniformity of swelling of the plasticizer is achieved. As a result, it is possible to obtain a resin composition in which the uniformly dispersed higher-order structure of the crystal structure required for manifestation of each function is suitably controlled while mixing a plurality of polyolefins. In addition, by achieving uniformity in the swelling of the plasticizer, it is possible to produce a resin composition that is uniform at the molecular level. A suppressed separator can be obtained. Such separators have high mechanical strength and excellent pore size uniformity. By using such a separator, it is possible to manufacture a lithium-ion battery that is excellent in impact resistance safety, cycle characteristics, and the like.
The swelling initiation temperature T1 of the first polyolefin and the swelling initiation temperature T2 of the second polyolefin can be measured according to the method described in 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 ) is preferably 3 to 7 polyethylene (first polyethylene). This first polyethylene is for example also called 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, still more preferably 350,000 to 8,500,000. Further, the degree of dispersion (Mw/Mn) is more preferably 3-15, still more preferably 4-14, and most preferably 4-13.

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

A single type or multiple types of UHMWPE may be used as the first polyolefin. UHMWPE is preferably poly(ethylene and/or propylene-co-α-olefin), more preferably poly(ethylene-co-propylene), poly(ethylene -co-butene), and poly(ethylene-co-propylene-co-butene). From a similar point of view, UHMWPE preferably contains 98.5 mol% or more and 100 mol% or less of structural units derived from ethylene, and more preferably contains 0.0 mol of structural units derived from an α-olefin other than ethylene. % more than 1.5 mol % or less.

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

Here, the “powder” preferable as the first component has a number average particle size (d ₅₀ ) of 80 μm to 180 μm, a volume average particle size (d ₅₀ ) of 120 μm to 220 μm, and a number particle size distribution. (Nd ₈₀ /Nd ₂₀ ) is 1.1 to 4.2, more preferably 1.2 to 4.1, and volume particle size distribution (Vd ₈₀ /Vd ₂₀ ) is 1.1 to 3.3 , more preferably 1.15 to 3.2, and a crystallite size of 10.0 nm to 30.0 nm, more preferably 11.0 nm to 28.0 nm, still more preferably 12.0 nm to 23.0 nm. a crystallinity of 30% to 99%, preferably 32% to 98%, more preferably 38% to 97.5%; and containing polyethylene. Those that satisfy at least one condition. These number average particle diameter (Nd ₅₀ ), volume average particle diameter (Vd ₅₀ ), number particle diameter distribution (Nd ₈₀ /Nd ₂₀ ), volume particle diameter distribution (Vd ₈₀ /Vd ₂₀ ), crystallite size, crystallization The degree and the like can be measured according to a known technique or the 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. In addition, the crystallite size and crystallinity can be adjusted by the temperature during polymerization and the degree of annealing to the polyethylene crystal part in the drying stage after 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 distribution (Mw/Mn) of the weight average molecular weight (Mw) with respect to the number average molecular weight (Mn) of 3 to 18. (Second polyethylene) is preferred. This second polyethylene is also called high density polyethylene (HDPE), for example.

The second polyolefin may be a low density polyolefin (LDPE) such as linear low density polyethylene (LLDPE), or a high density polyolefin (HDPE), but preferably a single or multiple types of HDPE. 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. More preferably, it is 12.0 nm to 31.0 nm, and the degree of crystallinity is 30% to 93%, preferably 32% to 90%, and more preferably 38% to 89%. From the same point of view, HDPE preferably contains 98.5 mol% or more and 100 mol% or less of structural units derived from ethylene, and more preferably contains 0.0 mol of structural units derived from an α-olefin other than ethylene. % more than 1.5 mol % or less.

The second component is preferably pellets 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 characteristics peculiar to polyethylene separators, the second polyethylene contained in the second component is more preferably 2.1% by mass to 88% by mass, and 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 (e.g., HDPE powder), extruding it into a strand with an extruder, cooling it with water, and cutting it into pellets. The polyolefin powder as a raw material preferably has a viscosity average molecular weight (Mv) of 30,000 to 3,500,000 and a degree of dispersion (Mw/Mn) of 3 to 15.
Further, the viscosity average molecular weight (Mv) when the second component is formed into pellets is preferably 35,000 to 3,300,000, more preferably 30,000 to 3,200,000. . The dispersity is preferably 3-15, more preferably 4-14, and even more preferably 4-13.

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 It is a resin material with a maximum length of 10 mm or less and 1 mm or more. The shape of the pellet is not particularly limited, and may be spherical, oval, or cylindrical, for example. A raw material (for example, polyethylene powder) is melt-extruded with an extruder, and then, while being water-cooled or air-cooled, it is prepared into strands and cut continuously. The size and shape can be adjusted by stranding and cutting methods.
In this way, by melting and extruding the above "powder" to prepare the above "pellet", the swelling speed due to the plasticizer can be significantly slowed down. Inside the extruder it is important not to inhibit the swelling of the first component. Further, in the melting process after the swelling process, 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 has the above range. It refers to those having a crystallite size.
Furthermore, the "pellet" preferable as the second component refers to those having a crystallinity smaller than that of the "powder" preferable as the first component and within the above range. In the pellet manufacturing method, the temperature of extruded resin into strands, the cooling temperature at the time of cutting, or the drawing speed of strands from extrusion (melt slight drawing), etc., pellet crystallite size, crystallinity, pellet size, You can adjust the shape. 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.

Forming the HDPE into pellets can significantly improve the uniformity of swelling of the plasticizer in the twin-screw extruder when mixed with the first component (eg, UHMWPE powder). It is believed that this is because by forming HDPE into pellets, the swelling rate can be remarkably slowed compared to, for example, HDPE powder. Therefore, the HDPE pellets are preferably used in subsequent melt-kneading without excessively inhibiting the swelling of the UHMWPE powder, and the pellets themselves basically do not swell.
In particular, by applying the present embodiment, by suitably adjusting the mixed composition of polyolefins based on a preferable molecular weight or a preferable swelling start temperature difference, etc., even in the melt kneading process, it is possible to easily and uniformly reach the molecular level. It becomes possible to disperse each polyolefin.

As noted above, HDPE pellets have a significantly slower swelling rate than HDPE powder and therefore do not substantially swell. Therefore, 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 their swelling onset temperature. On the other hand, if the HDPE powder that is the raw material of the HDPE pellets, the swelling start temperature can be measured, and the swelling start temperature of the HDPE powder that is the raw material of the HDPE pellets is also measured in the examples.

(Plasticizer)
The plasticizer is a known material as long as it is liquid at a temperature of 20° C. to 70° C. and has excellent dispersibility in the first polyolefin (e.g. UHMWPE) or the second polyolefin (e.g. HDPE). can be done. The plasticizer used in step (1) is preferably a non-volatile solvent that can form a homogeneous solution above the melting point of the first polyolefin or the second polyolefin in consideration of subsequent extraction. Specific examples of nonvolatile solvents include 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 interfacial separation between the resin and the plasticizer is unlikely to occur even when the melt-kneaded product is stretched, and uniform stretching tends to be easily performed.

(Supply to twin-screw extruder)
The first component and the second component can, for example, be fed simultaneously to a twin screw extruder. For simultaneous supply, for example, the first component and the second component are dry-blended in a super mixer or the like and supplied to a twin-screw extruder; and feeding it to a twin-screw extruder.

Alternatively, the first component and the second component can be separately fed to, for example, a twin-screw extruder. To supply individually, for example, a method of supplying the first component and the second component from separate feeders to a twin-screw extruder; Method of supplying to the twin screw extruder separately from the two components; Melting the second component with another extruder (second extruder), (extruder)). In addition, when supplying a 1st component and a 2nd component separately to a twin-screw extruder, you may supply a 1st component first and may supply a 2nd component first.

Also, the plasticizer can, for example, be fed into the twin-screw extruder with the first component. After feeding the plasticizer with the first component to the twin screw extruder, additional plasticizer may be fed from the same or a different feeder.
This type of twin-screw extruder generally has an upper feed port arranged on the upstream side and a middle feed port arranged downstream of the feed port and in the middle of the melt kneading area. ing. Therefore, after supplying the mixed slurry from the upper feed port of the twin-screw extruder, the plasticizer can be additionally supplied from the middle feed port ahead of the twin-screw extruder. This makes it easier to adjust the ratio of liquid paraffin in the resin composition extruded from the twin-screw extruder to a desired ratio, and to adjust the temperature of the resin composition to a desired range. Of course, the first component or the second component can also be supplied from the middle feed port.

(mixed slurry)
Here, step (1) is performed 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 sec to 60 sec. A step of producing a mixed slurry from the first component and the plasticizer and feeding the mixed slurry and the second component to a twin-screw extruder can be included.
Further, the step (1) includes a step of producing a mixed slurry from the first component, the second component, and the plasticizer using a continuous mixer under the above conditions, and supplying the mixed slurry to the twin-screw extruder. can also contain

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 maximizing swelling of the first component, and the upper limit is set so that the first polyolefin dissolves during mixing. is preferably 68° C. or lower, more preferably 67° C. or lower, 66° C. or lower, or 65° C. or lower, from the viewpoint of obtaining a slurry by suppressing the
The shear rate of the continuous mixer is 100 to 400,000 sec-1, preferably 120 to 398,000 sec ^{- 1} , from the viewpoint of uniformly contacting the first component with the plasticizer and obtaining 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, more preferably 2.0 seconds or more, from the viewpoint of ensuring dispersion of the first polyolefin in the plasticizer. 56 seconds.

The content of the first component (e.g., UHMWPE powder) in the mixed slurry is preferably more than 0% by mass, more preferably more than 0% by mass, based on the total mass of the mixed slurry, from the viewpoint of the strength of the resulting microporous membrane. is 1% by mass or more, more preferably 2% by mass or more or 4% by mass or more. In addition, from the viewpoint of suppressing the generation of unmelted matter (gel) in the first polyolefin, this content is preferably 50% by mass or less, more preferably 40% by mass or less, and still more preferably 30% by mass or less, or It is 20% by mass or less.

When the mixed slurry contains the second component, the content 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 resulting microporous membrane. , preferably more than 0% by mass, more preferably 1% by mass or more, and still more preferably 2% by mass or more or 4% by mass or more. In addition, from the viewpoint of suppressing the generation of unmelted matter (gel) in the second polyolefin, this content is preferably 50% by mass or less, more preferably 40% by mass or less, and still more preferably 30% by mass or less, or It is 20% by mass or less.

The mixed slurry is preferably fed 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 molecular weight of the first polyolefin does not decrease while ensuring the 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 point of view, the supply temperature is more preferably 30°C to 76°C, still more preferably 30°C to 70°C.

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

Polyethylenes other than UHMWPE and polyethylenes other than HDPE include, for example, linear low density polyethylene (LLDPE), high pressure low density polyethylene, and mixtures thereof. Also, polyethylene with a narrow molecular weight distribution obtained using a metallocene catalyst may be used. These polyethylenes can be used in singular or plural and can constitute the balance of microporous membranes including UHMWPE and HDPE.

Examples of polyolefins other than polyethylene include polypropylene, polybutene, ethylene-propylene copolymers, polymethylpentene, and silane graft-modified polyolefins. These polyolefins can be used in singular or plural and can constitute the balance of microporous membranes including UHMWPE and HDPE.

Examples of resins other than polyolefin include engineering plastic resins such as polyphenylene ether; polyamide resins such as nylon 6, nylon 6-12 and aramid resins; polyimide resins; polyester resins such as polyethylene terephthalate (PET) and polybutene terephthalate (PBT). Resin; Polycarbonate resin; Fluorine resin such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene; copolymer of ethylene and vinyl alcohol, copolymer of C ₂ to C ₁₂ α-olefin and carbon monoxide hydrogenated products thereof; hydrogenated products of styrene polymers; copolymers of styrene and α-olefins and hydrogenated products thereof; copolymers of styrene and aliphatic monounsaturated fatty acids; Polymers of (meth)acrylates and/or derivatives thereof; thermoplastic resins selected from copolymers of styrene and conjugated diene-based unsaturated monomers and hydrogenated products thereof; polysulfones, polyethersulfones, polyketones etc. These resins can be used in singular or plural and can constitute the balance of microporous membranes including UHMWPE and HDPE.

In addition, the mixed slurry contains known additives such as dehydration condensation catalysts, metal soaps such as calcium stearate or zinc stearate, ultraviolet absorbers, light stabilizers, antistatic agents, antifog agents, color pigments, and the like. OK.

[Step (2)]
In step (2), the first component, the second component, and the plasticizer are melt-kneaded using a twin-screw extruder to produce a resin composition. In the present embodiment, since the uniformity of swelling of the plasticizer with respect to the first component and the second component is ensured in the above step (1), step (2) does not depart from the gist of the present invention. Conditions such as the type of twin-screw extruder, supply of raw materials to the twin-screw extruder, extrusion time, extrusion speed, shear rate, and 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) above to produce a polyolefin microporous membrane.
In particular, step (3) according to the present embodiment includes sheet processing step (3-1), stretching step (3-2), extraction step (3-3), and heat treatment step (3-4).

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

From the viewpoint of sheet moldability, the proportion of the polyolefin resin in the sheet-shaped molding 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 molding. ~50% by mass.

As a method for producing a sheet-shaped molding, 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 obtain crystals of the resin component. A method of solidifying by cooling to a temperature lower than the solidification temperature is exemplified. Heat conductors used for solidification by cooling include metals, water, air, and plasticizers. Among these, it is preferable to use a metal roll because the efficiency of heat conduction is high. In addition, when the extruded resin composition is brought into contact with metal rolls, sandwiching it between the rolls further increases the efficiency of heat conduction, aligns the sheet, increases the film strength, and improves the surface smoothness of the sheet. preferred because it tends to The die lip distance when the resin composition is extruded into a sheet from a T-die is preferably 200 μm or more and 3,000 μm or less, more preferably 500 μm or more and 2,500 μm or less. When the die lip interval is 200 μm or more, die buildup and the like are reduced, the influence of streaks and defects on film quality is small, 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.
Further, the extruded sheet-like compact may be rolled during the step (3-1).

[Stretching step (3-2)]
In the step (3-2), the sheet-like molded product obtained in the step (3-1) is biaxially stretched at an area 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 air permeability distribution in the width direction can be reduced. By simultaneously stretching the sheet in the biaxial directions, the number of times the sheet-like molding 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. Simultaneous biaxial stretching is preferable from the viewpoint of improvement of puncture strength and uniformity of stretching, and sequential biaxial stretching is preferable from the viewpoint of ease of control of plane orientation.

In this specification, simultaneous biaxial stretching means stretching in which MD (machine direction of continuous molding of microporous membrane) and TD (direction transverse to 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 are performed independently. be in a state where

The draw ratio is preferably in the range of 20 times or more and 200 times or less, more preferably in the range of 25 times or more and 170 times or less, and still more preferably 30 times or more and 150 times or less. The stretching ratio in each axial direction is preferably in the range of 2 to 15 times in MD, 2 to 15 times in TD, 3 to 12 times in MD, and 3 to 12 times in TD. is more preferably in the range of 5 to 10 times the MD, and more preferably in the range of 5 to 10 times the TD. When the total area magnification is 20 times or more, the obtained microporous membrane tends to have sufficient strength. It tends to prevent and obtain high productivity.

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

[Extraction step (3-3)]
In step (3-3), a plasticizer is extracted from the stretched product obtained in step (3-2) to form a porous body. As a method for extracting the plasticizer, for example, a method of immersing the stretched product in an extraction solvent to extract the plasticizer and drying the product can be used. The extraction method may be either batch type or continuous type. In order to suppress the shrinkage of the porous material, it is preferable to restrain the ends of the sheet-like molding during the series of steps of immersion and drying. Moreover, 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 membrane.
After 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 one that is a poor solvent for the polyolefin resin, a good solvent for the plasticizer, and has a boiling point lower than the melting point of the polyolefin resin. Examples of such extraction solvents include hydrocarbons such as n-hexane and cyclohexane; halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane; non-chlorinated solvents such as hydrofluoroethers and hydrofluorocarbons; halogenated solvents; alcohols such as ethanol and isopropanol; ethers such as diethyl ether and tetrahydrofuran; and ketones such as acetone and methyl ethyl ketone. These extraction solvents may be recovered by an operation such as distillation and reused.

[Heat treatment step (3-4)]
In step (3-4), the porous body obtained in step (3-3) is heat-treated at a temperature below the melting point of the porous body, and then the porous body is stretched to form a microporous membrane. manufacture.

The porous body is subjected to heat treatment for the purpose of heat setting from the viewpoint of suppressing shrinkage. As a method of heat treatment, for the purpose of adjusting physical properties, a drawing operation is performed in a predetermined atmosphere, at a predetermined temperature, and at a predetermined draw ratio, and/or for the purpose of reducing drawing stress, a predetermined atmosphere, a predetermined temperature, and a predetermined A relaxation operation performed with a relaxation rate of The stretching operation may be followed by a relaxing 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.
Also, the relaxation operation is a contraction operation to the MD and/or TD of the membrane. The relaxation rate is the 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 relaxation rate of MD and the relaxation rate of TD. The relaxation rate is preferably 1.0 or less, more preferably 0.97 or less, and even more preferably 0.95 or less. The relaxation rate is preferably 0.5 or more from the viewpoint of film quality. Relaxation operations may be performed in both MD and TD or only in MD and TD.

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

During step (3-4) or after step (3-4), the microporous membrane may be subjected to post-treatment such as hydrophilization treatment with a surfactant or the like, cross-linking treatment with ionizing radiation or the like. The order of steps (3-2), (3-3) and (3-4) described above may be rearranged, or these steps may be performed simultaneously. 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 up by a winding machine to form a roll or cut by a slitter from the viewpoint of handling and storage stability.

<Polyolefin microporous membrane>
A polyolefin microporous membrane can be produced by the method for producing a polyolefin microporous membrane including steps (1) to (3). Polyolefin microporous membranes have a porous structure in which a large number of extremely small pores are gathered to form dense, interconnected pores. It has the characteristic of being strong.

The properties of the polyolefin microporous membrane are described below. These characteristics are obtained when the polyolefin microporous membrane as the separator for the electricity storage device is a flat membrane, but when the separator for the electricity storage device is in the form of a laminated membrane, the layers other than the microporous membrane are removed from the laminated membrane. can be measured from

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

The polyolefin microporous membrane preferably has a porosity of 20% or more, more preferably 30% or more, and still more preferably 32% or more or 35% or more. When the porosity of the microporous membrane is 20% or more, the ability to follow the 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, even more preferably 50% or less. When the porosity of the microporous membrane is 90% or less, the membrane strength tends to be further improved and self-discharge is 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, still more preferably 55 seconds or more, still more preferably 100 seconds or more, or 120 seconds or more per 100 cm ³ . , 140 seconds or longer, or 150 seconds or longer. When the air permeability of the microporous membrane is 1 second or more, the balance between the membrane thickness, the porosity and the average pore size 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 even more 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, and still more 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 at the time of slitting or winding of the electricity storage device tends to be more suppressed, or short-circuiting due to foreign matter or the like in the electricity storage device tends to be more 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, still more preferably 4000 kgf/cm ² or less. When the tensile strength of the microporous membrane is 5000 kgf/cm ² or less, the microporous membrane relaxes early during the heat test, weakening the shrinkage force, and as a result, the safety tends to be enhanced.

The thickness of the polyolefin microporous membrane is preferably 1.0 μm or more, more preferably 2.0 μm or more, and 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 thickness of the microporous membrane is preferably 500 µm or less, more preferably 100 µm or less, and still more preferably 80 µm or less, 22 µm or less, or 19 µm or less. When the film thickness of the microporous membrane is 500 μm or less, the ion permeability tends to be further improved. The film thickness of the microporous membrane 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. , more preferably 18 μm or less, particularly preferably 16 μm or less. In this case, when the film thickness of the microporous membrane 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 membrane may be 1.0 μm or more, 3.0 μm or more, 4.0 μm or more, or 5.0 μm or more.

In addition, the measured values of various physical properties described above are values measured according to the measurement methods in the examples described later, unless otherwise specified.

<Separator for lithium ion secondary battery>
The polyolefin microporous membrane according to this embodiment can be used as a separator for lithium ion secondary batteries. Separators for lithium-ion secondary batteries require insulation and lithium-ion permeability, so polyolefin microporous membranes are excellent because they are resistant to oxidation-reduction deterioration and can build a dense and uniform porous structure.

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

<Lithium ion secondary battery>
Lithium ion secondary batteries contain lithium transition metal oxides such as lithium cobaltate and lithium cobalt composite oxides as positive electrodes, carbon materials such as graphite and black lead as negative electrodes, and lithium salts such as _LiPF6 as electrolytes. It is a storage battery using an organic solvent. During charging/discharging of a lithium-ion secondary battery, ionized lithium reciprocates between electrodes. In addition, a separator is placed between the electrodes because it is necessary for the ionized lithium to move between the electrodes at a relatively high speed while suppressing contact 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. Physical properties in the examples were measured by the following methods.

<Weight average molecular weight (Mw), number average molecular weight (Mn)>
Using ALC/GPC Model 150C (trademark) manufactured by Waters, standard polystyrene was measured under the following conditions to create a calibration curve. In addition, the chromatogram was measured under the same conditions for each of the polymers below, and the weight average molecular weight and number average molecular weight of each polymer were calculated by the following method based on the calibration curve.
Column: 2 GMH ₆ -HT (trademark) + 2 GMH ₆ -HTL (trademark) manufactured by Tosoh Mobile phase: o-dichlorobenzene Detector: Differential refractometer Flow rate: 1.0 ml/min
Column temperature: 140°C
Sample concentration: 0.1 wt%
(Weight average molecular weight and number average molecular weight of polyethylene)
Each molecular weight component in the obtained calibration curve is multiplied by 0.43 (Q factor of polyethylene/Q factor of polystyrene = 17.7/41.3) to obtain a molecular weight distribution curve in terms of polyethylene, and the weight average Molecular weight and number average molecular weight were calculated. Due to the performance of the chromatogram, 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] in decalin solvent at 135°C was determined. 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 manufactured by Toyo Seiki (type KBN, terminal diameter Φ5 mm). In addition, 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 the thickness of one sheet.

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

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

<Puncture strength (gf)>
Using a handy compression tester "KES-G5" (manufactured by Kato Tech, trademark), a puncture strength was obtained by performing a puncture test on the sample film under conditions of a radius of curvature of the needle tip of 0.5 mm and a puncture speed of 2 mm/sec. rice field.

<Tensile breaking strength (kgf/cm ² )>
Using a sample film cut out to 10 mm×100 mm and a tensile tester, the tensile strength of the sample film was measured under the conditions of a load cell load of 5 kgf and a distance between chucks of 50 mm.

<Quantification of resin aggregates in separator>
The resin agglomerate (gel content) 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 process of Examples and Comparative Examples described later is observed with a transmission optical microscope. quantified by the area that has and does 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 certain amount of the mixed raw material before feeding into the twin-screw extruder is measured, and the viscosity [η] is measured based on the viscosity-average molecular weight measurement method described in the above item <Viscosity-average molecular weight (Mv)>. η ₀ ]. Also, a certain amount of the finally obtained film was measured in the same manner and the viscosity was measured, and this was defined as [η _a ]. A molecular weight deterioration rate (%) was calculated according to the following formula.
Molecular weight deterioration rate = [η _a ]/[η ₀ ]

<Swelling measurement>
A slurry liquid in which polyethylene powder is immersed in liquid paraffin (“Smoil P-350P” (trademark, manufactured by Matsumura Oil Research Institute) is applied to a hot stage with a recordable camera, and the specific heat capacity and heating uniformity are measured. Considering, while heating the slurry liquid at a rate of 9 ° C./min, the change in the size of the particle size of the powder was recorded.The image obtained by recording was processed using the Partical Analysis processing function in IgorPro. In addition, the volume concentration of the slurry solution was adjusted so that individual contours could always be observed during the measurement without covering the powder with each other. Considering the error in resolution and powder particle size distribution, about 81 pieces of powder were used for each measurement, and the data obtained by averaging the results of three measurements was used.The degree of swelling (swelling rate) is based on the powder at 25 ° C. The rate at which the powder volume increased in terms of the projected area equivalent to a circle was defined as a percentage value, and the swelling rate change with temperature change was measured and used as a swelling curve.
FIG. 1 shows the change in swelling rate (swelling curve) of the two polyolefin powders used in Example 1. As shown in FIG. The two types of polyolefin powders herein are the UHMWPE powder, which is the first component, and the HDPE powder used to form the second component (HDPE pellets).
The swelling initiation temperature can be obtained as the temperature corresponding to the point of inflection before the maximum swelling rate is reached by secondary differentiation of the swelling curve. That is, a tangent line was drawn from before and after the above 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 rate of UHMWPE powder, which is the first polyolefin used in Example 1.

<Particle size measurement>
It was measured using a flow-type image analysis particle size/shape measuring device Particle Insight manufactured by Micromeritics. The total number of counts was 3,000 to 4,000, and the volume distribution was calculated using the number average distribution and the projection area circular phase conversion. Before measurement, calibration was performed 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 calibrated vernier caliper.

<Crystallite size, crystallinity measurement>
<Polyethylene crystallite size (110)>
(measurement)
XRD measurement was performed using an X-ray diffractometer Ultima-IV manufactured by Rigaku. A Cu-Kα ray was incident on the sample, and the diffracted light was detected by a detector D/tex Ultra manufactured by Rigaku. KRD measurement was performed under conditions of a sample-detector distance of 285 mm, an excitation voltage of 40 kV, and a current of 40 mA. A converging optical system was adopted as the optical system, and the measurement was performed under the slit conditions of DS=1/2°, SS=open 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 peaks: an orthorhombic (110) plane diffraction peak, an orthorhombic (200) plane diffraction peak, and an amorphous peak. 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 a Voigt function, and the amorphous peak was approximated by a Gaussian 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 full width at half maximum of the crystalline peak were not particularly fixed, and the peak separation was performed. 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). Crystallinity is the percentage of the sum of the separated crystalline and amorphous peaks divided by the area of the crystalline 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 (hkl) calculated as a result of peak separation
β ₂ : Full width at half maximum (rad) of divergence of incident beam
θ: Bragg angle

[Example 1]
Powder containing the first polyolefin (weight average molecular weight and number average molecular weight are difficult to measure, ultra high molecular weight polyethylene with a viscosity average molecular weight of 1 million; UHMWPE) shown in Table 1, second polyolefin (weight average molecular weight 12.6 10,000, number average molecular weight of 20,000, viscosity average molecular weight of 150,000 high density polyethylene; HDPE) containing pellets (cylindrical body with a diameter of 5 to 6 mm and a height of 3 to 2 mm), and other modifiers listed in Table 1 ( In the table, "StCa" means calcium stearate.) was dry-blended in 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) is twin-screw extruded with an injection nozzle equipped with a manifold (T-die) with a die lip interval of 1500 μm. The mixture was supplied to a machine, further kneaded, and the resin composition was extruded. At this time, the liquid paraffin amount ratio in the resin composition extruded from the twin-screw extruder is 70% by mass, and the temperature of the resin composition is 220 ° C. The extrusion middle section (middle feed port of the twin-screw extruder ) was further injected with liquid paraffin. Subsequently, the extruded resin composition was extruded onto a cooling roll controlled to have a surface temperature of 25° C. and cast to obtain a sheet-like molded product.
Next, it was led to a simultaneous biaxial tenter stretching machine and biaxially stretched to obtain a stretched product. The stretching conditions were a stretched area ratio of 50 to 180 times, and the porosity, air permeability, thickness, and puncture strength were adjusted by adjusting the stretching temperature, the amount of heating air, the stretching ratio, and the like. In addition, in Example 1, the biaxial stretching temperature was 125°C.
Next, the porous body is introduced into a TD tenter for heat setting, heat setting (HS) is performed at 128 ° C., and then a 0.5-fold relaxation operation in the TD direction (that is, the HS relaxation rate is 0.5 times ) was performed. After that, the obtained microporous membrane was evaluated as described above. Table 1 shows the evaluation results.

[Example 2]
Separate feeders were provided for the powder and pellets, and supplied from the same input port of the twin-screw extruder (upper feed port of the twin-screw extruder). A polyolefin microporous membrane was produced in the same manner as in Example 1. Table 1 shows the evaluation results of the obtained film.

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

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

[Examples 6-7 and Comparative Examples 1-3]
A polyolefin microporous membrane was produced by changing the raw material composition, process conditions, etc. from Example 1 as shown in Table 1. Table 1 shows the evaluation results of the obtained film.

[Examples 8 to 18]
A polyolefin microporous membrane was produced by changing the raw material composition, process conditions, etc. from Example 1 as shown in Table 2. Table 2 shows the evaluation results of the obtained film.

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

T1 The swelling initiation temperature of the first polyolefin T2 The swelling initiation temperature of the second polyolefin

Claims (9)

A method for producing a microporous membrane used in 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 {wherein the swelling initiation temperature (T1) of the first polyolefin and the swelling start temperature (T2) of the second polyolefin (T1-T2) is 2°C to 50°C. };
(2) a step of melt-kneading the first component, the second component, and the plasticizer in the twin-screw extruder to produce a resin composition; and (3) the plasticizer from the resin composition Extracting to produce a polyolefin microporous membrane;
including
The step (1) is
The first component and the plasticizer using a continuous mixer under 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 sec to 60 sec. and supplying the mixed slurry and the second component to the twin-screw extruder . A method for producing a microporous membrane used in a lithium ion secondary battery separator, comprising the following steps:
(1) A step of supplying a powder containing the first polyolefin, pellets containing the second polyolefin, and a plasticizer to a twin-screw extruder {wherein the swelling initiation temperature (T1) of the first polyolefin and the second The difference (T1-T2) from the polyolefin swelling start temperature (T2) is 2°C to 50°C. };
(2) a step of melt-kneading the powder, the pellets, and the plasticizer in the twin-screw extruder to produce a resin composition;
(3) extracting the plasticizer from the resin composition to produce a polyolefin microporous membrane;
including
The step (1) is
Mixing with the powder 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 sec to 60 sec. 2. The method for producing a microporous membrane according to claim 1, comprising the steps of producing a slurry and feeding the mixed slurry and the pellets to the twin-screw extruder. The step (1) is
The first component, the second component 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 sec to 60 sec. and the plasticizer to produce a mixed slurry, and supplying the mixed slurry to the twin-screw extruder. The step (1) is
Temperature from 20°C to 70°C, 100 to 400,000 seconds ^- A mixed slurry is produced from the powder, the pellets, and the plasticizer using a continuous mixer under conditions of a shear rate of 1 and a residence time of 1.0 to 60 seconds, and the mixed slurry is passed through the biaxial 3. The method for producing a microporous membrane according to claim 2, comprising the step of feeding to an 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 or 3 , wherein the second component is pellets containing 2% to 90% by mass of the second polyolefin. The powder contains 2% to 90% by mass of the first polyolefin,
The method for producing a microporous membrane according to claim 2 or 4, wherein the pellets contain 2% by mass to 90% by mass of the second polyolefin. The first polyolefin is
Claims 1 to 3, wherein 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. 7. The method for producing a microporous membrane according to any one of 6 . The second polyolefin is
Claims 1 to 6, wherein 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 according to any one of the items. The step (3) is
a step of extruding the resin composition into a sheet, cooling and solidifying it, and processing it into a sheet-shaped molding;
A step of biaxially stretching the sheet-like molded product at an area magnification of 20 times or more and 200 times or less to form a stretched product;
a step of extracting the plasticizer from the stretched product to form a porous body; and heat-treating the porous body at a temperature below the melting point of the porous body, then 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 8 , comprising

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