JP7547844B2 - Porous Polyolefin Film - Google Patents

Description

The present invention relates to a porous polyolefin film.

Porous polyolefin films are used as filters, fuel cell separators, and capacitor separators. They are particularly well suited for use as separators for lithium-ion batteries, which are widely used in notebook personal computers, mobile phones, digital cameras, and other devices. This is because microporous polyolefin membranes have excellent mechanical strength, shutdown properties, and lithium ion permeability.

As lithium-ion secondary batteries become more widespread and their applications expand, the quality standards required for separators are improving year by year, and high levels of various properties such as strength and permeability are now required. For example, since the separator is wound under tension, it is preferable that the tensile strength and tensile modulus are high to prevent film rupture during winding. In addition, since the separator is usually wound in the MD direction, it is preferable that the tensile modulus and tensile strength in the MD direction (hereinafter sometimes simply referred to as "MD tensile modulus" and "MD tensile strength") are moderately higher than the tensile modulus and tensile strength in the TD direction (hereinafter sometimes simply referred to as "TD tensile modulus" and "TD tensile strength").

On the other hand, as secondary batteries, such as those for automobiles, become larger, ensuring safety is becoming more important. Therefore, from the perspective of preventing membrane rupture due to foreign objects or impacts inside the battery, it is preferable that the puncture strength is high as well as the tensile modulus and tensile strength.

In particular, in recent years, lithium-ion secondary batteries have come to be used in vehicles, so there is a need to shorten charging times and improve acceleration, and battery characteristics such as rapid charging (high current charging) and increased power consumption (high current discharging) are required. Accordingly, there is an even higher demand for separators to improve output characteristics.

In addition, as the driving range of automobiles increases, the capacity of batteries is increasing, and thinner separators are being required. However, generally, making the membrane thinner to increase permeability reduces mechanical strength. Also, increasing the porosity to increase permeability reduces mechanical strength. If the mechanical strength of the separator decreases, it becomes more susceptible to short circuits caused by electrodes or foreign objects (foreign object resistance) and membrane rupture when the battery is subjected to impact (impact resistance), reducing the safety of the battery. For this reason, even higher strength than before is required.

Methods for improving the strength of porous films include stretching a sheet and laminating porous sheets. However, with conventional technology, it has been difficult to obtain a porous film with an excellent balance between strength and permeability.

For example, Patent Document 1 describes a porous film produced by melt-kneading ultra-high molecular weight polyethylene and high density polyethylene, simultaneously biaxially stretching the mixture, removing the solvent from the resulting stretched product, and then stretching the film again in the TD direction. However, although this porous film has a high porosity and high pin puncture strength, its MD tensile modulus is not necessarily sufficient, and there is room for improvement in process transportability. In addition, due to the laminated structure, there is a problem in that it is difficult to make the film thinner.

Patent Document 2 describes a ceramic composite that is produced by adding ceramic particles to a polyolefin resin, melt-kneading the mixture, performing simultaneous biaxial stretching, removing the solvent from the resulting stretched product, and then performing stretching again in the TD direction. This ceramic composite has a high porosity and high pin puncture strength, but has problems with its MD tensile modulus not being necessarily sufficient and its transportability being poor. In addition, because it contains ceramic particles, it is difficult to form a thin film.

As described above, there is room for improvement in the development of separators with excellent output characteristics, safety, and process transportability to meet the diversifying needs of customers associated with higher energy density, higher capacity, higher output, and faster charging.

Patent No. 4794098 Patent No. 5288736

The object of the present invention is to solve the above problems. That is, to provide a porous polyolefin film that has excellent output characteristics, safety, and process transportability when used as a battery separator.

That is, the present invention relates to a porous polyolefin film having a porosity of 50% or more, a pin puncture strength of 3.7 N or more when measured at a thickness of 10 μm, and a tensile modulus of elasticity in the longitudinal direction E _MD of 980 MPa or more.

The present invention also relates to a battery separator that uses the porous polyolefin film of the present invention.

The present invention also relates to a secondary battery that uses the battery separator of the present invention.

The porous polyolefin film of the present invention has excellent output characteristics, safety, and process transportability when used as a battery separator, and can therefore be suitably used as a battery separator for secondary batteries that require high energy density, high capacity, and high output, such as those used in electric vehicles.

The porous polyolefin film of the present invention has a porosity of 50% or more. The porosity is more preferably 52% or more, even more preferably 55% or more, and most preferably 58% or more. By making the porosity 50% or more, the ion permeability is improved, and the output characteristics of the battery can be improved, especially when used as a battery separator for rapid charging applications that will be required in the future. The higher the porosity, the better from the viewpoint of output characteristics, but if it is too high, the strength may decrease, so the upper limit is about 70%. In order to make the porosity in the above range, it is preferable that the raw material composition of the film is in the range described later, and that the stretching conditions during film formation are in the range described later.

The porous polyolefin film of the present invention has a puncture strength of 3.7 N or more when converted to a film thickness of 10 μm. More preferably, it is 4.0 N or more, even more preferably, 4.4 N or more, and even more preferably, 4.7 N or more. By making the puncture strength 3.7 N or more, when the film is made into a thin film, the occurrence of a short circuit due to winding or foreign matter inside the battery can be suppressed, and the safety of the battery can be improved. From the viewpoint of improving safety, the higher the puncture strength, the more preferable it is, but there is often a trade-off between improving the porosity and improving the puncture strength, and the upper limit is about 10 N. In order to make the puncture strength in the above range, it is preferable that the raw material composition of the film is in the range described below, and that the stretching conditions during film formation are in the range described below.

The porous polyolefin film of the present invention has a tensile modulus E _MD (unit, MPa) in the longitudinal direction of the film of 980 MPa or more. The tensile modulus is more preferably 1100 MPa or more, even more preferably 1300 MPa or more, and even more preferably 1500 MPa or more. By setting the E _MD to 980 MPa or more, the transportability of the film can be improved in the coating layer application process and the battery winding process. From the viewpoint of improving the film transportability in the coating layer application process and the battery winding process, the higher the E _MD , the more preferable it is, but there is often a trade-off between improving the porosity and improving the E _MD , and the upper limit is about 2500 MPa. In order to set the E _MD in the above range, it is preferable that the raw material composition of the film is set to the range described later, and that the stretching conditions during film formation are set to the range described later.

In the present invention, the direction parallel to the film production direction is referred to as the film production direction, longitudinal direction, or MD direction, and the direction perpendicular to the film production direction within the film plane is referred to as the width direction or TD direction.

To improve the output characteristics of a battery, it is necessary to increase the porosity of the porous film. However, when the porosity is increased, the amount of resin in the porous film decreases, which can reduce the puncture strength and decrease the safety of the battery. As a result, there is a trade-off between porosity and puncture strength, and there have been challenges in achieving both output characteristics and safety.

In the present invention, it has been found that it is possible to achieve both high porosity and high pin puncture strength by setting the ultra-high molecular weight polyethylene content of the porous polyolefin film to the range described below, setting the film-forming conditions to the range described below, and in particular by subjecting the porous film after solvent extraction to dry re-stretching in the MD and TD directions under the conditions described below. That is, by adding ultra-high molecular weight polyethylene, a porous film with excellent pin puncture strength and tensile modulus can be obtained. Furthermore, it has been found that dry re-stretching promotes fibril cleavage, increasing the porosity, and at the same time promoting the orientation of polyolefin crystals, increasing the pin puncture strength and tensile modulus, so that the porosity, pin puncture strength, and tensile modulus in the MD direction satisfy the above-mentioned ranges, and a porous film with excellent output characteristics, safety, and process transportability can be obtained.

In the porous polyolefin film of the present invention, when the tensile modulus of elasticity in the longitudinal direction of the film is E _MD and the tensile modulus of elasticity in the width direction is E _TD (unit, MPa), the ratio of the tensile modulus of elasticity in the MD direction to the TD direction E _MD / E _TD is preferably 1.0 or more. E _MD / E _TD is more preferably 1.0 or more and 6.0 or less, even more preferably 1.0 or more and 3.0 or less, and even more preferably 1.0 or more and 2.0 or less. By making E _MD / E _TD 1.0 or more, the transportability of the film can be improved in the coating layer application process and the battery winding process. From the viewpoint of improving the film transportability in the coating layer application process and the battery winding process, the higher E _MD / E _TD is the more preferable, but if E _MD / E _TD is greater than 6.0, the anisotropy in the MD direction and the TD direction becomes significant, and wrinkles may occur in the MD direction or tears may occur in the MD direction during film transport. In order to set E _MD / _ETD within the above range, it is preferable that the raw material composition of the film is within the range described below, and the stretching conditions during film production are within the range described below.

In the porous polyolefin film of the present invention, when the tensile strength in the longitudinal direction of the film is M _MD (unit, MPa), M _MD is preferably 180 MPa or more. The tensile strength in the MD direction is more preferably 200 MPa or more, even more preferably 240 MPa or more, and even more preferably 290 MPa or more. By setting the tensile strength to 180 MPa or more, it is possible to suppress the occurrence of film breakage in the coating layer application process and the battery winding process, and improve the transportability of the film. In addition, when the film is made into a thin film, it is possible to suppress the occurrence of short circuits due to foreign matter in the winding or in the battery, and improve the safety of the battery. From the viewpoint of improving the film transportability and improving safety, the higher the tensile strength, the more preferable it is, but there are many cases where the improvement of the porosity and the improvement of the tensile strength are a trade-off, and the upper limit is about 500 MPa. In order to set the tensile strength in the above range, it is preferable that the raw material composition of the film is set to the range described later, and the stretching conditions during film formation are set to the range described later.

The porous polyolefin film of the present invention preferably has an average pore diameter of 10 nm or more and 140 nm or less as determined by a perm porometer. By setting the average pore diameter to 10 nm or more, when used as a battery separator, the ion permeability is sufficient, and the output characteristics of the battery can be improved. By setting the average pore diameter to 140 nm or less, more preferably 45 nm or less, even more preferably 40 nm or less, and even more preferably 35 nm or less, it is possible to prevent short circuits caused by foreign matter in the battery or during winding when made into a thin film, and to improve the safety of the battery. In order to set the average pore diameter within the above range, it is preferable that the raw material composition of the film is within the range described below, and that the stretching conditions during film formation are within the range described below.

The porous polyolefin film of the present invention preferably has a ratio of the average pore size to the maximum pore size, (average pore size)/(maximum pore size), of 0.70 or more, as determined by a perm porometer. When the (average pore size)/(maximum pore size) is 0.70 or more, the pore size distribution is sufficiently uniform, so that when the film is used as a battery separator, good battery output can be obtained and short circuits due to dendrite formation can be suppressed. The (average pore size)/(maximum pore size) is more preferably 0.75 or more, even more preferably 0.78 or more, and even more preferably 0.80 or more. The upper limit of the (average pore size)/(maximum pore size) is 1.0.

The thickness of the porous polyolefin film of the present invention is preferably 15 μm or less. The thickness is more preferably 10 μm or less, even more preferably 7 μm or less, and even more preferably 6 μm or less. By making the thickness 15 μm or less, the ion permeability is sufficient, especially when used as a battery separator for rapid charging, and the output characteristics of the battery are improved. From the viewpoint of improving the battery characteristics, the thinner the thickness, the more preferable it is. However, if the thickness is thin, short circuits are likely to occur during winding or due to foreign matter in the battery, and the safety of the battery may be reduced, so the lower limit is about 3 μm. The thickness can be adjusted by the screw rotation speed of the extruder, the width of the unstretched sheet, the film-forming speed, the stretching ratio, etc., within a range that does not deteriorate other physical properties.

Next, we will explain the raw materials for the porous polyolefin film of the present invention, but the raw materials are not necessarily limited to these.

The porous polyolefin film of the present invention is a film containing polyolefin as the main component. Here, in the present invention, "main component" means that the proportion of a specific component in the total components is 50% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and even more preferably 99% by mass or more.

The polyolefin resin used in the present invention is preferably a polyolefin, and may be a polyolefin composition. Examples of polyolefins include polyethylene and polypropylene, and two or more of these may be blended together.

As for the polyolefin constituting the porous polyolefin film, it is preferable to use polyethylene. The content of polyethylene is preferably 90% by mass or more, more preferably 95% by mass or more, and particularly preferably 99% by mass or more, when the total polyolefin is taken as 100% by mass. By setting the polyethylene content within the above range, it is possible to obtain a highly uniform microporous film with little or no phase separation of polymer species.

The porous polyolefin film preferably contains ultra-high molecular weight polyethylene (hereinafter, also simply referred to as "UHMWPE") having a weight average molecular weight of 1.0 x 10 ⁶ or more as a raw material. The content of UHMWPE is preferably 40% by mass or more, more preferably 50% by mass or more, and even more preferably 60% by mass or more, based on 100% by mass of the total mass of polyolefin. By containing an ultra-high molecular weight polyolefin resin as a raw material, it is possible to make the pores finer and to improve heat resistance, and further to improve strength and elongation.

The porous polyolefin film of the present invention is preferably 35% by mass or more of polyolefin components having a molecular weight of 1.0 x 10 ⁶ or more when the molecular weight distribution by weight of the film is measured by gel permeation chromatography (GPC) after being formed into a film under the film-forming conditions described below. By containing 35% by mass or more, more preferably 40% by mass or more, even more preferably 45% by mass or more, and even more preferably 50% by mass or more of polyolefin components having a molecular weight of 1.0 x 10 ⁶ or more in the film, a porous polyolefin film excellent in puncture strength and tensile modulus can be obtained, and safety can be improved.

In addition, the porous polyolefin film of the present invention may contain various additives such as antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, and even antiblocking agents and fillers, as long as the effects of the present invention are not impaired. In particular, it is preferable to add an antioxidant in order to suppress oxidative deterioration of the polyethylene resin due to its thermal history. As the antioxidant, it is preferable to use one or more selected from, for example, 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4), 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene (for example, BASF's "Irganox" (registered trademark) 1330: molecular weight 775.2), and tetrakis[methylene-3(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane (for example, BASF's "Irganox" (registered trademark) 1010: molecular weight 1177.7). Appropriate selection of the type and amount of antioxidant and heat stabilizer added is important for adjusting or enhancing the properties of the porous film.

The porous polyolefin film of the present invention is obtained by biaxially stretching the above-mentioned raw material. The biaxial stretching method can be any of the inflation method, simultaneous biaxial stretching method, and sequential biaxial stretching method, but among them, it is preferable to adopt the simultaneous biaxial stretching method or the sequential biaxial stretching method in terms of film formation stability, thickness uniformity, and control of high rigidity and dimensional stability of the film.

Next, the method for producing the porous polyolefin film of the present invention will be described, but is not necessarily limited thereto. The method for producing the porous polyolefin film of the present invention comprises the following steps (a) to (e).
(a) A polyolefin solution is prepared by kneading and dissolving polymer materials including a polyolefin alone, a polyolefin mixture, a polyolefin solvent mixture (plasticizer), additives, and a polyolefin mixture (preparation of polyolefin solution).
(b) extruding the melt, forming it into a sheet, and cooling and solidifying it (forming an extrudate and forming a gel-like sheet);
(c) The obtained sheet is wet-stretched by a roll method or a tenter method (wet-stretching step).
(d) Thereafter, the plasticizer is extracted from the obtained stretched film, and the film is dried (washing and drying step).
(e) Next, dry re-stretching/heat treatment is carried out (dry re-stretching/heat treatment step).

Each process is explained below.

(a) Preparation of polyolefin solution A polyolefin solution is prepared by heating and dissolving a polyolefin resin in a plasticizer. The plasticizer is not particularly limited as long as it is a solvent that can sufficiently dissolve polyethylene, but in order to enable relatively high-magnification stretching, it is preferable that the solvent is liquid at room temperature. Examples of the solvent include aliphatic, cycloaliphatic, or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, and liquid paraffin, and mineral oil fractions having boiling points corresponding thereto, as well as phthalic acid esters that are liquid at room temperature, such as dibutyl phthalate and dioctyl phthalate. In order to obtain a gel-like sheet with a stable content of liquid solvent, it is preferable to use a non-volatile liquid solvent such as liquid paraffin. A solvent that is miscible with polyethylene in a melt-kneaded state but is solid at room temperature may be mixed with the liquid solvent. Examples of such solid solvents include stearyl alcohol, ceryl alcohol, and paraffin wax. However, if only a solid solvent is used, there is a risk of uneven stretching and the like occurring.

The blending ratio of polyolefin resin and plasticizer is 100% by mass of the total of polyolefin resin and plasticizer, and the content of polyolefin resin may be appropriately selected within a range that does not impair moldability, but 5 to 50% by mass is preferable. By making the polyolefin resin 5% by mass or more (i.e., plasticizer 95% by mass or less), swelling and necking at the outlet of the die can be kept small when molding into a sheet, and deterioration of sheet moldability and deterioration of film formability can be prevented. On the other hand, by making the polyolefin resin 50% by mass or less (i.e., plasticizer 50% by mass or more), large shrinkage in the thickness direction can be suppressed and moldability can be improved.

The viscosity of the liquid solvent is preferably 2×10 ⁻⁵ to 2×10 ⁻⁴ m ² /s at 40° C. If the viscosity at 40° C. is 2×10 ⁻⁵ m ² /s or more, the sheet extruded from the die of the polyolefin solution is less likely to become non-uniform. On the other hand, if the viscosity is 2×10 ⁻⁴ m ² /s or less, the liquid solvent is easily removed. The viscosity of the liquid solvent is measured at 40° C. using an Ubbelohde viscometer.

(b) Formation of extrudate and formation of gel-like sheet The homogeneous melt kneading of the polyolefin solution is not particularly limited, but is preferably carried out in a twin-screw extruder when a high-concentration polyolefin solution is to be prepared. If necessary, various additives such as an antioxidant may be added within a range that does not impair the effects of the present invention. In particular, it is preferable to add an antioxidant to prevent oxidation of polyethylene.

In the extruder, the polyolefin solution is mixed uniformly at a temperature at which the polyolefin resin is completely melted. The melt-kneading temperature varies depending on the polyolefin resin used, but is preferably (melting point of polyolefin resin + 10°C) to (melting point of polyolefin resin + 120°C). More preferably, it is (melting point of polyolefin resin + 20°C) to (melting point of polyolefin resin + 100°C). Here, the melting point refers to the value measured by DSC based on JIS K7121 (1987) (hereinafter the same). For example, in the case of polyethylene, the melt-kneading temperature is preferably in the range of 140 to 250°C. More preferably, it is 160 to 230°C, and even more preferably 170 to 200°C. Specifically, since the polyethylene composition has a melting point of about 130 to 140°C, the melt-kneading temperature is preferably 140 to 250°C, and even more preferably 180 to 230°C.

From the viewpoint of suppressing deterioration of the resin, a lower melt-kneading temperature is preferable, but if the temperature is lower than the above-mentioned temperature, unmelted material may be generated in the extrudate extruded from the die, which may cause film breakage in the subsequent stretching process, and if the temperature is higher than the above-mentioned temperature, the thermal decomposition of the polyolefin becomes intense, and the physical properties of the resulting porous film, such as strength and porosity, may be deteriorated. In addition, the decomposition products may precipitate on the chill roll or the rolls in the stretching process, and adhere to the sheet, leading to a deterioration in the appearance. Therefore, it is preferable to knead within the above range.

Next, the resulting extrudate is cooled to obtain a gel-like sheet, and the polyethylene microphase separated by the solvent can be fixed by cooling. In the cooling step, it is preferable to cool to 10 to 50°C. This is because it is preferable to set the final cooling temperature below the crystallization end temperature, and by making the high-order structure finer, it becomes easier to perform uniform stretching in the subsequent stretching. Therefore, it is preferable to cool at a rate of 30°C/min or more at least until the temperature is below the gelation temperature. If the cooling rate is less than 30°C/min, the crystallization degree increases and it is difficult to obtain a gel-like sheet suitable for stretching. In general, if the cooling rate is slow, relatively large crystals are formed, so the high-order structure of the gel-like sheet becomes coarse, and the gel structure that forms it also becomes large. On the other hand, if the cooling rate is fast, relatively small crystals are formed, so the high-order structure of the gel-like sheet becomes dense, which leads to uniform stretching and high toughness of the film.

Cooling methods include direct contact with cold air, cooling water, or other cooling media, contact with a roll cooled with a refrigerant, or using a casting drum, etc.

The polyolefin porous film of the present invention is not limited to a single layer, but may be a laminate. There is no particular limit to the number of layers, and it may be a two-layer laminate or a three-layer laminate or more. As described above, the laminated portion may contain desired resins in addition to polyethylene to the extent that the effect of the present invention is not impaired. As a method for making a polyolefin porous film into a laminate, a conventional method can be used, but for example, there is a method of forming a laminate by preparing desired resins as needed, separately feeding these resins to an extruder and melting them at a desired temperature, joining them in a polymer tube or die, and extruding them from a slit-shaped die at the desired lamination thickness.

(c) Wet stretching process The obtained gel-like sheet (including laminated sheet) is stretched. Examples of the stretching method used include MD uniaxial stretching by a roll stretching machine, TD uniaxial stretching by a tenter, sequential biaxial stretching by a combination of a roll stretching machine and a tenter, or a combination of a tenter and a tenter, and simultaneous biaxial stretching by a simultaneous biaxial tenter. The stretching ratio varies depending on the thickness of the gel-like sheet from the viewpoint of the uniformity of the film thickness, but it is preferable to stretch the sheet 5 times or more in any direction. The area ratio is preferably 25 times or more, more preferably 36 times or more, and even more preferably 49 times or more. By setting the area ratio to 25 times or more, the stretching can be sufficient and the uniformity of the film can be sufficient, and a porous film excellent in terms of strength can be obtained.

The stretching temperature is preferably set to the melting point of the gel-like sheet + 10°C or less, and more preferably in the range of (crystal dispersion temperature T _cd of polyolefin resin) to (melting point of gel-like sheet + 5°C). Specifically, since the polyethylene composition has a crystal dispersion temperature of about 90 to 100°C, the stretching temperature is preferably 90 to 125°C, more preferably 90 to 120°C. The crystal dispersion temperature T _cd is determined from the temperature characteristics of dynamic viscoelasticity measured according to ASTM D4065. Alternatively, it may be determined from NMR. By setting the temperature at 90°C or more, the pores are sufficiently opened, making it easier to obtain a uniform membrane thickness, and the porosity is also improved. By setting the temperature at 125°C or less, melting of the sheet and blocking of the pores can be prevented.

The above-mentioned stretching causes the higher-order structure formed in the gel sheet to split, the crystal phase becomes finer, and numerous fibrils are formed. The fibrils form a three-dimensionally irregularly connected network structure. The stretching improves the mechanical strength and enlarges the pores, making it suitable for battery separators.

(d) Washing and drying process Next, the solvent remaining in the gel-like sheet is removed using a washing solvent. Since the polyethylene phase and the solvent phase are separated, a porous film is obtained by removing the solvent. Examples of washing solvents include saturated hydrocarbons such as pentane, hexane, and heptane, chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, ethers such as diethyl ether and dioxane, ketones such as methyl ethyl ketone, and chain fluorocarbons such as trifluoroethane. These washing solvents have low surface tension (for example, 24 mN/m or less at 25°C). By using a washing solvent with low surface tension, the shrinkage of the network structure forming the micropores is suppressed by the surface tension of the air-liquid interface during drying after washing, and a porous film having porosity and permeability is obtained. These washing solvents are appropriately selected according to the plasticizer and used alone or in combination.

The cleaning method can be a method of immersing the gel-like sheet in a cleaning solvent and extracting it, a method of showering the cleaning solvent on the gel-like sheet, or a combination of these methods. The amount of cleaning solvent used varies depending on the cleaning method, but is generally preferably 300 parts by mass or more per 100 parts by mass of the gel-like sheet. The cleaning temperature may be 15 to 30°C, and is heated to 80°C or less as necessary. At this time, the gel-like sheet is immersed in the cleaning solvent for a longer period of time, from the viewpoint of enhancing the cleaning effect of the solvent, preventing the physical properties of the resulting porous film from becoming uneven in the TD and/or MD directions, and improving the mechanical and electrical properties of the porous film. The above-mentioned cleaning is preferably performed until the residual solvent in the gel-like sheet after cleaning, i.e., the porous film, is less than 1% by mass.

Then, in the drying process, the solvent in the porous film is dried and removed. There are no particular limitations on the drying method, and methods using metal heating rolls or hot air can be selected. The drying temperature is preferably 40 to 100°C, and more preferably 40 to 80°C. By drying thoroughly, it is possible to prevent the porosity of the porous film from decreasing and the permeability from worsening during the subsequent heat treatment.

(e) Dry re-stretching/heat treatment process The dried porous film is stretched (dry re-stretching) at least in one direction. Dry re-stretching promotes fibril cleavage, increases porosity and average pore size, and produces a porous film with excellent permeability. In addition, the orientation of polyolefin crystals is promoted, and the puncture strength, tensile modulus, and tensile strength are increased, resulting in a porous film with excellent safety and process transportability.

Dry re-stretching can be performed in the same manner as the above-mentioned stretching while heating the porous film, by MD uniaxial stretching using a roll stretching machine, TD uniaxial stretching using a tenter, sequential biaxial stretching using a combination of a roll stretching machine and a tenter or a combination of a tenter and a tenter, or simultaneous biaxial stretching using a simultaneous biaxial tenter, etc.

The re-stretching temperature is preferably equal to or lower than the melting point of the polyethylene composition, and more preferably within the range of (T _cd -20°C) to the melting point. Specifically, the temperature is preferably 70 to 135°C, more preferably 110 to 132°C, and even more preferably 120 to 130°C.

The dry re-stretching ratio is preferably 1.40 or more in the MD direction. By setting the MD re-stretching ratio to 1.40 or more, more preferably 2.00 or more, even more preferably 2.50 or more, and even more preferably 3.00 or more, the orientation of the polyolefin crystals in the MD direction proceeds, and a film with excellent tensile modulus and tensile strength in the MD direction and excellent process transportability is obtained.

The upper limit of the re-stretching ratio in the MD direction varies depending on the resin composition of the film and the wet stretching ratio, but is preferably 5.00 times or less. By setting the re-stretching ratio in the MD direction to 5.00 times or less, it is possible to suppress crushing in the thickness direction and a decrease in porosity. In addition, it is possible to suppress excessive anisotropy in the MD and TD directions, and to suppress the occurrence of wrinkles in the MD direction or tears in the MD direction during film transport.

The final re-stretching ratio in the TD direction is preferably selected so that the product of the re-stretching ratio in the MD direction and the final dry re-stretching ratio (area ratio) satisfies the conditions described below, but as individual conditions, it is preferably 1.40 times or more, more preferably 2.00 times or more, even more preferably 2.50 times or more, and even more preferably 3.00 times or more. The upper limit of the final re-stretching ratio in the TD direction varies depending on the resin composition of the film and the wet stretching ratio, but is preferably 5.00 times or less. By setting the final re-stretching ratio in the TD direction to 5.00 times or less, it is possible to suppress the occurrence of crushing in the thickness direction and prevent a decrease in porosity. In addition, it is possible to suppress the film from breaking, enabling stable film production.

The product of the dry re-stretching ratio in the MD direction and the final dry re-stretching ratio in the TD direction is preferably 2.00 times or more, more preferably 4.00 times or more, even more preferably 6.00 times or more, and even more preferably 8.00 times or more. The upper limit of the product of the dry re-stretching ratio in the MD direction and the final dry re-stretching ratio in the TD direction varies depending on the resin composition of the film and the wet stretching ratio, but is preferably 15.00 times or less. By setting the product of the dry re-stretching ratio in the MD direction and the final dry re-stretching ratio in the TD direction to 15.00 times or less, it is possible to suppress the occurrence of crushing in the thickness direction and prevent a decrease in porosity. In addition, it is possible to suppress the film from breaking, enabling stable film production.

The ratio of the dry re-stretching ratio to the wet stretching ratio (dry re-stretching ratio/wet stretching ratio) is preferably 0.16 or more, more preferably 0.20 or more, even more preferably 0.25 or more, and even more preferably 0.30 or more. The upper limit of the dry re-stretching ratio/wet stretching ratio varies depending on the resin composition of the film and the wet stretching ratio, but is preferably 0.60 or less. By setting the dry re-stretching ratio/wet stretching ratio to 0.60 or less, it is possible to suppress the occurrence of crushing in the thickness direction and prevent a decrease in porosity. It also suppresses the film from breaking, enabling stable film production.

It is also preferable that the product of the areal magnification of the wet stretching and the areal magnification of the dry re-stretching (total areal magnification) is 100 times or more. By making the total areal magnification 100 times or more, more preferably 125 times or more, even more preferably 150 times or more, and even more preferably 200 times or more, fibril cleavage is promoted, the porosity increases, and a film with excellent permeability is obtained.

The upper limit of the total area magnification varies depending on the resin composition of the film, but it is preferable to set it to 400 times or less. By setting the total area magnification to 400 times or less, it is possible to suppress the occurrence of crushing in the thickness direction and prevent a decrease in porosity. It also suppresses the film from rupturing, making it possible to perform stable film production.

(f) Other steps Furthermore, depending on other applications, the porous film may be subjected to hydrophilization treatment. The hydrophilization treatment may be performed by monomer grafting, surfactant treatment, corona discharge, or the like. The monomer grafting is preferably performed after the crosslinking treatment. The crosslinking treatment is preferably performed by irradiating the porous polyolefin film with ionizing radiation such as α-rays, β-rays, γ-rays, and electron beams. In the case of electron beam irradiation, the electron beam dose is preferably 0.1 to 100 Mrad, and the acceleration voltage is preferably 100 to 300 kV. The crosslinking treatment increases the meltdown temperature of the porous polyolefin film.

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

The porous polyolefin film may be surface-coated with a porous fluorine resin such as polyvinylidene fluoride or polytetrafluoroethylene, or a porous material such as polyimide or polyphenylene sulfide, or may be coated with an inorganic material such as ceramic, in order to improve the meltdown characteristics and heat resistance when used as a battery separator.

The porous polyolefin film obtained in the above manner can be used for various applications such as filters, fuel cell separators, and capacitor separators. However, when used as a battery separator, it has excellent output characteristics, safety, and process transportability, and is therefore preferably used as a battery separator for secondary batteries that require high energy density, high capacity, and high output, such as those used in electric vehicles.

The present invention will be described in detail below with reference to examples. The characteristics were measured and evaluated by the following methods. In the following, Examples 1 to 4 and 7 to 9 are to be read as Reference Examples 1 to 4 and 7 to 9.

1. Weight Average Molecular Weight (Mw) and Molecular Weight Distribution of Film The weight average molecular weight of the ultra-high molecular weight polyethylene and the high density polyethylene raw material, and the molecular weight distribution of the porous polyolefin film were determined by gel permeation chromatography (GPC) under the following conditions.
Measurement device: GPC-150C manufactured by WATERS CORPORATION
Column: SHODEX UT806M manufactured by Showa Denko K.K.
Column temperature: 135° C.
Solvent (mobile phase): O-dichlorobenzene Solvent flow rate: 1.0 mL/min Sample concentration: 0.1 wt% (dissolution conditions: 135°C/1H)
Injection volume: 500 μL
Detector: WATERS CORPORATION differential refractometer (RI detector)
Calibration curve: A calibration curve was prepared using a predetermined conversion constant from a calibration curve obtained using a monodisperse polystyrene standard sample.

2. Film Thickness The film thickness of the polyolefin microporous film was measured at five points within an area of 50 mm × 50 mm using a contact thickness meter, Litematic VL-50 (10.5 mmφ ultrahard spherical probe, measuring load 0.01 N) manufactured by Mitutoyo Corporation, and the average value was recorded as the film thickness (μm).

3. Puncture strength Using a force gauge (DS2-20N manufactured by Imada Co., Ltd.), a needle having a diameter of 1 mm and a spherical tip (radius of curvature R: 0.5 mm) was pierced into a porous film having an average thickness T ₁ (μm) at a speed of 2 mm/sec, and the maximum load L ₁ (load just before penetration, unit: N) was measured in accordance with JIS Z 1707 (2019) except for the above. The puncture strength L ₂ (N/10 μm) was calculated when the film thickness was 10 μm using the formula L ₂ = (L ₁ × 10) / T ₁ .

4. Porosity A sample was cut out from the polyolefin microporous film into a square of 50 mm x 50 mm, the thickness was measured by the above-mentioned method, and the volume of the sample was calculated from the thickness and area.The mass (g) of the sample was also measured using an electronic balance, and the mass and the polymer density (g/ ^cm3 ) were used to calculate the following formula.The above measurements were carried out at three different points in the same porous film, and the average porosity was calculated.
Porosity=[(volume−mass/polymer density)/volume]×100.
The calculation was made assuming that the polymer density was a constant value of 0.99 g/cm ³ .

5. Tensile strength and tensile modulus The tensile strength and tensile modulus in the MD and TD directions were measured using a strip-shaped test piece with a width of 10 mm according to a method conforming to ASTM D882. The test piece was fixed using double-sided tape (NW-15, manufactured by Nichiban Co., Ltd.), and the measurement was performed under the conditions of a gripping distance of 20 mm and a test speed of 100 mm/min. The tensile modulus (MPa) was obtained by dividing the slope of the curve between tensile tensions of 2 to 3 N by the cross-sectional area before the test.

6. Maximum pore size and average pore size The maximum pore size and average pore size were measured in the order of dry-up and wet-up using a perm porometer (CFP-1500A, manufactured by PMI). For wet-up, pressure was applied to a porous polyolefin film sufficiently soaked in Galwick (trade name) manufactured by PMI with a surface tension of 1.59 × 10 ^-2 N / m, and the pore size calculated from the pressure at which air began to penetrate was taken as the maximum pore size.

The average diameter was calculated from the pressure at the point where the curve showing half the slope of the pressure-flow curve in the dry-up measurement intersects with the curve in the wet-up measurement. The following formula was used to convert between pressure and pore diameter.
d = C γ / P
In the above formula, "d (μm)" is the pore size of the porous polyolefin film, "γ (mN/m)" is the surface tension of the liquid, "P (Pa)" is the pressure, and "C" is a constant determined by the wetting tension of the immersion liquid, the contact angle, etc.

The following provides a more detailed explanation using examples.

[Example 1]
75% by mass of liquid paraffin was added to 25% by mass of a polyethylene composition consisting of 40% by mass of ultra-high molecular weight polyethylene (UHMWPE) having a weight average molecular weight Mw1 of 2.0 x 10 ⁶ and 60% by mass of high density polyethylene (HDPE) having a weight average molecular weight Mw2 of 0.3 x 10 ⁶ , and 0.7% by mass of 2,6-di-t-butyl-p-cresol and 1.1% by mass of tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate]methane were added as antioxidants based on the mass of the polyethylene being mixed, and melt-kneaded to prepare a polyethylene resin solution in a twin-screw extruder. Subsequently, this polyethylene resin solution was extruded from a sheet-forming die installed at the tip of the extruder, and the obtained sheet-like extrudate was taken up by a cooling roll at 15°C to form a gel-like molding. The gel-like molding thus obtained was subjected to wet simultaneous biaxial stretching at 115°C to a stretch ratio of 5x5, and the stretched sheet was then immersed in methylene chloride at 25°C to remove liquid paraffin, and air-dried at room temperature. The dried sheet was then subjected to MD uniaxial stretching at a temperature of 113°C and a stretch ratio of 2.00 in the MD direction using a roll stretching machine, and then tenter stretching was performed at a temperature of 130°C, a stretch ratio of 3.00 in the TD direction, a relaxation rate of 8.0%, and a final stretch ratio of 2.76 to produce a porous polyolefin film.

The raw material characteristics, film-forming conditions, and evaluation results of the obtained porous polyolefin film are shown in Table 1. This porous polyolefin film had excellent porosity, puncture strength, and MD tensile modulus, and possessed properties suitable for use as a separator for secondary batteries.

[Examples 2 to 4]
A porous polyolefin film was produced in the same manner as in Example 1, except that the raw materials shown in Table 1 were used as the raw materials for the porous polyolefin film and the film-forming conditions were changed as shown in Table 1. The evaluation results of the obtained porous polyolefin film are shown in Table 3.

[Example 5]
90% by mass of liquid paraffin was added to 10% by mass of ultra-high molecular weight polyethylene (UHMWPE) having a weight average molecular weight Mw1 of 2.0 × 10 ⁶ , and 0.7% by mass of 2,6-di-t-butyl-p-cresol and 1.1% by mass of tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane were added as antioxidants based on the mass of the polyethylene being mixed, and melt-kneaded to prepare a polyethylene resin solution in a twin-screw extruder. Subsequently, this polyethylene resin solution was extruded from a sheet forming die installed at the tip of the extruder, and the obtained sheet-like extrudate was taken up by a cooling roll at 15 ° C. to form a gel-like molding. The obtained gel-like molding was subjected to wet simultaneous biaxial stretching at 115 ° C. to 5 × 5 times, and then the stretched sheet was immersed in methylene chloride at 25 ° C. to remove the liquid paraffin, and air-dried at room temperature. The dried sheet was then subjected to sequential biaxial tenter stretching under operating conditions of a temperature of 125°C, a stretch ratio of 2.00 in the MD direction, a stretch ratio of 2.00 in the TD direction, and a heat setting temperature of 125°C, to produce a porous polyolefin film.

[Example 6]
A porous polyolefin film was produced in the same manner as in Example 5, except that the raw materials shown in Table 1 were used as the raw materials for the porous polyolefin film and the film-forming conditions were changed as shown in Table 1. The evaluation results of the obtained porous polyolefin film are shown in Table 3.

[Example 7]
80% by mass of liquid paraffin was added to 20% by mass of ultra-high molecular weight polyethylene (UHMWPE) having a weight average molecular weight Mw1 of 1.5 × 10 ⁶ , and 0.7% by mass of 2,6-di-t-butyl-p-cresol and 1.1% by mass of tetrakis [methylene-3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] methane were added as antioxidants based on the mass of the polyethylene being mixed, and melt-kneaded to prepare a polyethylene resin solution in a twin-screw extruder. Subsequently, this polyethylene resin solution was extruded from a sheet forming die installed at the tip of the extruder, and the obtained sheet-like extrudate was taken up by a cooling roll at 15 ° C. to form a gel-like molding. The obtained gel-like molding was roll-stretched at 113 ° C. to 8 times in the MD direction, and tenter-stretched at 124 ° C. to 7.4 times in the TD direction (wet sequential biaxial stretching). The stretched sheet was immersed in methylene chloride at 25° C. to remove the liquid paraffin, and then air-dried at room temperature. The dried sheet was then tenter-stretched in the TD direction at 131° C. under conditions of a stretch ratio of 1.39 times, a relaxation ratio of 6.5%, and a final stretch ratio of 1.30 times to produce a porous polyolefin film.

[Examples 8 and 9]
A porous polyolefin film was produced in the same manner as in Example 7, except that the raw materials shown in Table 1 were used as the raw materials for the porous polyolefin film and the film-forming conditions were changed as shown in Table 1. The evaluation results of the obtained porous polyolefin film are shown in Table 3.

[Comparative Example 1]
75% by mass of liquid paraffin was added to 25% by mass of a polyethylene composition consisting of 40% by mass of ultra-high molecular weight polyethylene (UHMWPE) having a weight average molecular weight Mw1 of 2.0 x 10 ⁶ and 60% by mass of high density polyethylene (HDPE) having a weight average molecular weight Mw2 of 0.3 x 10 ⁶ , and 0.7% by mass of 2,6-di-t-butyl-p-cresol and 1.1% by mass of tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane were added as antioxidants based on the mass of the polyethylene being mixed, and melt-kneaded to prepare a polyethylene resin solution in a twin-screw extruder. Subsequently, this polyethylene resin solution was extruded from a sheet-forming die installed at the tip of the extruder, and the obtained sheet-like extrudate was taken up by a cooling roll at 15°C to form a gel-like molding. The obtained gel-like molding was subjected to wet simultaneous biaxial stretching at 115°C to a stretched size of 5 x 5. The stretched sheet was then immersed in methylene chloride at 25°C to remove the liquid paraffin, and air-dried at room temperature to produce a porous polyolefin film.

[Comparative Examples 2 to 5]
A porous polyolefin film was produced in the same manner as in Comparative Example 1, except that the raw materials shown in Table 2 were used as the raw materials for the porous polyolefin film and the film-forming conditions were changed as shown in Table 1. The evaluation results of the obtained porous polyolefin film are shown in Table 4.

Example 1 was subjected to wet simultaneous biaxial stretching, and after removing the liquid paraffin, dry re-stretching was performed, resulting in a higher porosity and higher pin puncture strength than Comparative Example 1, which was not subjected to dry re-stretching. In addition, by performing dry re-stretching in the MD direction, orientation in the MD direction progresses, resulting in a high tensile modulus in the MD direction, and excellent transportability.

In Examples 2 to 4, high porosity and high pin puncture strength were obtained by dry re-stretching at a higher stretch ratio than in Example 1. In addition, by increasing the dry re-stretching ratio in the MD direction, a higher tensile modulus in the MD direction was obtained than in Example 1, and a film with a large elastic modulus ratio E _MD /E _TD and excellent transportability was obtained.

In Examples 5 to 9, 100% UHMWPE was used as the raw polyolefin resin, and high porosity, high puncture strength, and high tensile modulus in the MD direction were obtained.

The porous polyolefin films of Examples 1 to 9 are excellent in terms of output characteristics, safety, and transportability when used as battery separators.

Comparative Example 2 is a porous polyolefin film obtained by dry re-stretching only in the TD direction, as compared to Example 1. Although good puncture strength was obtained, the porosity was less than 50%, and there is room for improvement in terms of ion permeability when used as a battery separator for rapid charging applications, which will be required in the future.

Comparative Example 3 was subjected to dry sequential stretching at 3.50 times the MD and 5.43 times the TD compared to Example 1, resulting in membrane rupture.

Comparative Example 4 was insufficient in terms of porosity, puncture strength, and tensile modulus in the MD direction, since dry re-stretching was not performed as in Example 5.

Comparative Example 5 uses 100% HDPE by mass as the raw material compared to Example 5, and a good porosity is obtained, but since it does not contain UHMWPE, the puncture strength is insufficient and it is inferior in terms of safety.

Claims (8)

A porous polyolefin film having a porosity of 50% or more, a pin puncture strength of 4.0 N or more when calculated based on a film thickness of 10 μm, a tensile modulus E _MD in the longitudinal direction of 980 MPa or more, an average pore size of 10 nm or more and 35 nm or less, and a content of polyethylene components having a molecular weight of 1.0 x 10 ⁶ or more of 40 mass% or more in a molecular weight distribution by weight of the film. The porous polyolefin film according to claim 1, wherein the tensile modulus _EMD and the tensile modulus _ETD in the transverse direction are such that _EMD / _ETD is 1.0 or more. The porous polyolefin film according to claim 1 or 2, having a tensile strength in the longitudinal direction (M _MD ) of 180 MPa or more . The porous polyolefin film according to any one of claims 1 to 3 , wherein the ratio of the average pore size to the maximum pore size (average pore size)/(maximum pore size) is 0.70 or more. The porous polyolefin film according to any one of claims 1 to 4 , having a thickness of 15 µm or less. The porous polyolefin film according to any one of claims 1 to 5 , which is mainly composed of polyethylene . A battery separator comprising the porous polyolefin film according to any one of claims 1 to 6 . A secondary battery comprising the battery separator according to claim 7 .