Porous film and method for producing the same

Abstract

To provide a porous film having high strength, melt-down properties and excellent permeability without load and costs on environment and facility by dry method.SOLUTION: A porous film is obtainable by drawing an ultrahigh-molecular weight polyethylene solid phase rolled film whose main component is solid powder of ultrahigh-molecular weight polyethylene having a viscosity average molecular weight (Mv) of 600,000-5,000,000. There is at least one endothermic peak between 140°C or above and below 155°C in DSC chart. Gurley air permeability measured according to JIS P8117 is 200 s / 100 ml or less.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a porous film obtained by stretching an ultra-high molecular weight polyethylene solid-phase roll-formed film (hereinafter sometimes simply referred to as "polyethylene film") formed from a raw material containing ultra-high molecular weight polyethylene as a main component, and a method for producing the same. [Background technology]

[0002] Porous films primarily composed of polyolefins such as polyethylene are widely used as separators for secondary batteries and capacitors due to their ion permeability and insulating properties, and as separation and filtration membranes such as microfiltration membranes and moisture-permeable waterproof membranes due to their chemical stability.

[0003] In recent years, demand for this porous film has been expanding as a separator for lithium-ion secondary batteries, which are widely used in various fields such as smartphones and electric vehicles. In addition to the above-mentioned properties, the reason for its use as a separator for many secondary batteries is that it has a relatively low melting point for a thermoplastic resin, resulting in shutdown characteristics and excellent mechanical properties. Furthermore, the use of ultra-high molecular weight polyolefins provides meltdown characteristics that allow the film to maintain its shape even at temperatures above its melting point, thereby preventing short circuits between electrodes and ensuring safety in secondary batteries.

[0004] Here, methods for producing olefin microporous films having fine pores can be broadly divided into dry and wet methods. The dry method involves melt-extruding a resin to produce a thin raw sheet, stabilizing the raw sheet by heat treatment or the like, and stretching the sheet primarily in a uniaxial direction under predetermined stretching conditions to form pores (see, for example, Patent Document 1). The wet method involves melt-kneading a resin with a solvent to form a sheet, stretching the sheet biaxially, and then removing the solvent to form pores (see, for example, Patent Document 2). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-88645 [Patent Document 2] Patent Publication No. 2021-14572 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the dry method described in Patent Document 1, raw sheets are obtained by melt extrusion, so ultra-high molecular weight polyolefins, which have extremely high melt viscosity and are difficult to melt extrude, cannot be used. Therefore, the strength is weak and meltdown properties cannot be achieved. In addition, to avoid the above-mentioned inconveniences, methods such as multilayering and imparting a crosslinked structure have been considered, but the strength is still insufficient, and sufficient through-holes are not formed, resulting in problems such as reduced air permeability of the porous film.

[0007] Furthermore, in the wet method described in Patent Document 2, the resin is melted and kneaded together with a solvent, which requires a subsequent step of removing the solvent, resulting in problems of increased costs and burden on the environment and facilities.

[0008] Therefore, the present invention has been made in consideration of the above problems, and aims to provide a porous film that is produced by a dry method without imposing burdens on the environment and equipment or costs, has high strength, meltdown properties, and excellent breathability. [Means for solving the problem]

[0009] In order to achieve the above object, the porous film of the present invention is a porous film obtained by stretching a solid-phase roll-formed ultra-high molecular weight polyethylene film, the main component of which is a solid powder of ultra-high molecular weight polyethylene having a viscosity average molecular weight (Mv) of 600,000 to 5,000,000, and is characterized in that it has at least one endothermic peak between 140°C and 155°C in a DSC chart and has a Gurley air permeability of 200 s / 100 ml or less as measured in accordance with JIS P 8117.

[0010] The method for producing a porous film of the present invention is characterized by comprising at least a step of supplying a solid powder of ultra-high molecular weight polyethylene having a viscosity average molecular weight (Mv) of 600,000 to 5,000,000 between a pair of rolls arranged opposite each other, forming a film by solid-phase roll rolling, and withdrawing the film, and a step of stretching the film. [Effects of the Invention]

[0011] According to the present invention, a porous film having high strength, meltdown properties and excellent breathability can be provided by a dry method without imposing burdens on the environment and facilities or increasing costs. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a schematic diagram showing an apparatus for producing an ultra-high molecular weight polyethylene solid-state rolled film according to an embodiment of the present invention. [Figure 2] FIG. 2 is a diagram illustrating an endothermic peak in a DSC chart of the porous film according to the present invention (in Example 1). [Figure 3] 1 is a scanning electron microscope (SEM) photograph of a porous film according to an embodiment of the present invention. [Figure 4] 1 is a DSC chart of the porous film in Comparative Example 8. [Figure 5] 1 is a DSC chart of the porous film in Comparative Example 9. DETAILED DESCRIPTION OF THE INVENTION

[0013] The porous film of the present invention will be specifically described below. Note that the present invention is not limited to the following embodiments, and can be appropriately modified and applied within the scope of the present invention.

[0014] The porous film of the present invention is a porous film obtained by stretching a solid-phase roll-formed ultra-high molecular weight polyethylene film containing solid powder ultra-high molecular weight polyethylene as a main component.

[0015] <Ultra-high molecular weight polyethylene> The ultra-high molecular weight polyethylene used in the present invention is a solid powder, and one with a viscosity-average molecular weight (Mv) of 600,000 to 5,000,000 can be used. This is because, if the viscosity-average molecular weight (Mv) is less than 600,000, the melt viscosity is too low, causing the resin to adhere to the rolls, making it difficult to form by roll rolling. Even if a raw film is produced by extrusion molding, stretching the film in the machine axis (longitudinal) direction (hereinafter referred to as "MD") of the film (described below) at a temperature near the melting point (e.g., 130°C) can cause the film to melt through, making MD stretching difficult. Furthermore, if the viscosity-average molecular weight (Mv) is greater than 5,000,000, the melt viscosity is too high, making it difficult to form by solid-state roll rolling with rolls of a typical size.

[0016] In other words, by using a solid powder of ultra-high molecular weight polyethylene with a viscosity average molecular weight (Mv) of 600,000 to 5,000,000, it is possible to ensure formability by roll rolling, and also to improve the strength of the porous film and impart meltdown properties (i.e., the ability to maintain shape even at temperatures above the melting point) without imposing burdens on the environment and equipment or costs by using a dry method.

[0017] As the ultra-high molecular weight polyethylene, for example, a commercially available product such as Mipelon XM220 (manufactured by Mitsui Chemicals, Inc., viscosity average molecular weight: 2 million) can be used.

[0018] Furthermore, cross-linked polyethylene cross-linked by a cross-linking agent or electron beam irradiation, or a polymerized one (synthesized one) having a viscosity average molecular weight of 600,000 to 5,000,000 may also be used.

[0019] The viscosity average molecular weight is preferably 700,000 to 4,000,000, more preferably 800,000 to 3,000,000, and even more preferably 1,000,000 to 2,000,000.

[0020] The "viscosity average molecular weight" is calculated in accordance with JIS K 7367-3:1999.

[0021] The ultra-high molecular weight polyethylene preferably has a melting point of 140 to 150°C in the form of solid powder immediately after polymerization, more preferably 140 to 145°C.

[0022] The "melting point" mentioned above is measured in accordance with JIS K 7121:1987, and is determined by measuring the temperature at which the main endothermic peak appears using a differential scanning calorimeter.

[0023] <Other ingredients> The porous film of the present invention may contain various additives in addition to the main component, ultra-high molecular weight polyethylene. The additives may be known additives commonly used in polyethylene films, such as calcium stearate (metallic soap), higher aliphatic alcohols such as stearyl alcohol and ceryl alcohol, n-alkanes such as n-decane and n-dodecane, liquid paraffin, kerosene, and paraffin wax. These additives may be used alone or in combination of two or more.

[0024] <Porous film> The blending ratio of the ultra-high molecular weight polyethylene to the additive in the porous film is not particularly limited as long as it does not impair the characteristics of the porous film of the present invention, but from the viewpoint of suppressing a decrease in the mechanical strength (tensile strength) of the porous film caused by the additive, the blending amount of the ultra-high molecular weight polyethylene relative to the entire porous film is preferably 90% by mass or more, more preferably 95% by mass or more, and particularly preferably 98% by mass or more. Also, a porous film consisting of only ultra-high molecular weight polyethylene (i.e., 100% by mass) without containing processing aids etc. can be provided.

[0025] The thickness of the porous film of the present invention is preferably 30 μm or less, more preferably 25 μm or less, and even more preferably 20 μm or less. When the thickness is 30 μm or less, for example, when the film is used as a separator for a lithium ion secondary battery, the distance between electrodes can be shortened by making the separator thinner, which can contribute to increasing the capacity and power of the lithium ion secondary battery.

[0026] Furthermore, the porous film of the present invention has at least one endothermic peak in DSC measurement between 140° C. and 155° C. More specifically, as shown in the DSC chart of Fig. 2, the porous film of the present invention has two endothermic peaks, the endothermic peak near 140 to 145° C. being the melting peak of extended chain crystals (orthorhombic), and the endothermic peak near 150 to 155° C. being the transition peak of extended chain crystals (orthorhombic to hexagonal).

[0027] Furthermore, the solid-phase rolled film (raw film) before stretching has at least two endothermic peaks between 130 and 160°C, and the endothermic peak around 130 to 135°C is the melting peak of lamellar crystals.

[0028] Since the heat of fusion of extended chain crystals is smaller than that of lamellar crystals, only the transition peak (endothermic peak around 150 to 155°C) may be observed.

[0029] In the DSC chart shown in FIG. 2, the porous film of the present invention has two endothermic peaks: a melting peak of extended chain crystals at around 140 to 145°C and a transition peak of extended chain crystals at around 150 to 155°C (i.e., at least one endothermic peak between 140°C and 155°C), which is thought to be the reason for its strength.

[0030] The "DSC chart" mentioned above refers to one measured in accordance with JIS K7121.

[0031] In addition, the porous film of the present invention preferably has a tensile breaking stress of 100 MPa or more in both MD and the direction perpendicular thereto (hereinafter referred to as "TD"). If the tensile breaking stress is 100 MPa or more, the porous film can exhibit high durability against elongation stress and bending stress, and therefore can provide a porous film with excellent mechanical strength.

[0032] The above-mentioned "tensile breaking stress" refers to the stress measured in accordance with JIS K 7127.

[0033] Furthermore, the porous film of the present invention preferably has a Gurley air permeability of 200 s / 100 ml or less, more preferably 100 s / 100 ml or less, and even more preferably 50 s / 100 ml or less, measured in accordance with JIS P 8117. If the Gurley air permeability is 200 s / 100 ml or less, sufficient through-pores are formed, so that the film can function well when used as a separator for a secondary battery or a separation membrane / filtration membrane.

[0034] The porous film of the present invention preferably has a puncture strength of 2 N or more. If the puncture strength is 2 N or more, when the film is used as a separator for a secondary battery, for example, the separator can be prevented from being punctured by a foreign object or an impact, which can prevent a short circuit between the positive electrode and the negative electrode.

[0035] The above-mentioned "puncture strength" refers to the strength measured in accordance with JIS Z 1707.

[0036] In addition, the porous film of the present invention preferably has a median pore diameter of 1 μm or less as measured by mercury intrusion porosimetry. If the median pore diameter of the porous film is 1 μm or less, when the film is used as, for example, a secondary battery separator, short-circuiting between the positive electrode and the negative electrode due to the passage of a conductive material can be prevented.

[0037] As described above, the porous film of the present invention is produced by a dry method without imposing burdens or costs on the environment and facilities, has excellent breathability, high mechanical strength, and meltdown properties, and can therefore be suitably used as, for example, separation membranes and filtration membranes such as microfiltration membranes and moisture-permeable waterproof membranes, separators for secondary batteries, and separators for capacitors.

[0038] <Method of manufacturing porous film> Next, the method for producing the porous film of the present invention will be described in detail.

[0039] The porous film of the present invention is produced by first forming a raw material containing, as a main component, the above-mentioned ultra-high molecular weight polyethylene having a viscosity average molecular weight of 600,000 to 5,000,000 alone or a mixture of the ultra-high molecular weight polyethylene with the above-mentioned additives, as a solid powder, into a film using a rolling mill to produce an ultra-high molecular weight polyethylene solid-state rolled film, and then stretching the ultra-high molecular weight polyethylene solid-state rolled film produced by this solid-state rolled rolling.

[0040] The method for producing an ultrahigh molecular weight polyethylene solid-phase roll-milled film according to the present invention is a dry method for producing a film in which no processing aids are used or which does not include a step for removing the processing aids later, as opposed to a wet method in which processing aids are used and then removed.

[0041] FIG. 1 is a schematic diagram showing an apparatus (rolling mill) for producing an ultra-high molecular weight polyethylene film according to this embodiment.

[0042] 1, in a roll mill 10, a solid powder of ultra-high molecular weight polyethylene M is supplied from an outlet 1a of the raw material supply machine 1 to a pair of rolls 2 and 3 that constitute the roll mill, where the raw material ultra-high molecular weight polyethylene M is sandwiched and passed between the pair of rolls 2 and 3 to form a film. While forming a film, the film is transported in the direction of arrow Y via transfer rolls 4 to 7 and taken up in the direction of arrow Z by a take-up roll 8, thereby obtaining an ultra-high molecular weight polyethylene solid-phase roll-rolled film P.

[0043] 1, the pair of rolls 2 and 3 are provided in the supply direction of the ultra-high molecular weight polyethylene M (the direction of the arrow X in the figure) and are arranged opposite each other. The pair of rolls 2 and 3 are also provided at a predetermined distance from each other.

[0044] The pair of rolls 2, 3, the transfer rolls 4 to 7, and the take-up roll 8 can be made of metal or rubber.

[0045] Regarding the temperature for film formation using the roll mill (the temperature of the rolls during film formation), heating near the melting point of the raw material, ultra-high molecular weight polyethylene, is a characteristic of film formation by solid-state roll rolling, and is important for forming a film without leaving any raw material on the rolls. If the melting point of the solid powder of ultra-high molecular weight polyethylene immediately after polymerization is Mp [°C], the lower limit of the film formation temperature must be Mp - 5°C or higher. The upper limit must be less than Mp + 10°C. This is because forming at a temperature higher than Mp + 10°C can result in holes being formed in the film due to aggregation of the raw material, making film formation difficult. Furthermore, forming at a temperature lower than Mp - 5°C can result in powder portions of the raw material remaining, making film formation difficult.

[0046] The "melting point" mentioned above refers to the main absorption peak temperature observed when measured using a differential scanning calorimeter (DSC) in accordance with the JIS K 7121 test method.

[0047] In the present invention, the raw material ultra-high molecular weight polyethylene is passed between a pair of rolls 2 and 3 constituting a rolling mill to perform roll molding. From the viewpoint of compressing the solid powder into a film by solid-state roll rolling, the linear pressure between the two rolls 2 and 3 constituting the pair of rolls is preferably 100 to 300 kg / cm, more preferably 30 to 250 kg / cm, and even more preferably 50 to 200 kg / cm.

[0048] Furthermore, when the roll diameter is 75 mmφ, the rolling speed is preferably set in the range of 24 to 2400 mm / min. This is because if the rolling speed exceeds 2400 mm / min, the film may break, and if the rolling speed is less than 24 mm / min, production efficiency may decrease.

[0049] The rotation speed of the roll can be set appropriately in accordance with the above-mentioned rolling speed, and when the roll diameter is 75 mmφ, it can be set to, for example, 0.1 to 1.0 rpm.

[0050] Furthermore, the rolling speed (mm / min) and the number of rotations of the rolls vary depending on the roll diameter, and are therefore not limited to the above ranges.

[0051] The roll rolling ratio is preferably set in the range of 2 to 10 times. The roll rolling ratio is calculated as follows: (1) First, the raw material solid powder is press-molded under conditions of a temperature of 180°C, a press pressure of 125 kgf, and a press time of 120 seconds to produce an unstretched film. (2) Next, the obtained unstretched film is MD-stretched to 2, 4, 5, and 6 times at 145°C to produce films, respectively. (3) This MD-uniaxially stretched film is placed in an oven at 150°C and held there for 30 minutes, after which the heat shrinkage is measured. (4) Next, a graph of the heat shrinkage and the stretch ratio is prepared, and an approximation curve is created. (5) The heat shrinkage of the film molded by the roll mill before withdrawal is measured in the same manner as in (3) above, and the stretch ratio is calculated using the approximation curve in (4) above, and this is the roll rolling ratio. A roll rolling ratio of 2 to 10 times can promote the formation of extended chain crystals necessary for porosity.

[0052] The film formed by the rolling machine is taken up by a take-up machine (take-up roll 8) (i.e., the film is taken up while being formed by rolling), and in the present invention, the take-up speed is set to be faster than the above-mentioned rolling speed. More specifically, when the rolling speed is V1 and the take-up speed is V2, it is preferable to set the take-up ratio (V2 / V1) in the range of more than 1.1 times and not more than 10 times.

[0053] When the take-up ratio (V2 / V1) is greater than 1.1 and not more than 10, it becomes possible to take up the film while applying tension to it in the film transport direction (i.e., MD, the direction of arrow Y in the figure), which promotes film orientation and improves the mechanical strength (tensile strength), and it becomes possible to obtain a thin (i.e., 100 μm or less) ultra-high molecular weight polyethylene solid-phase roll-formed film.

[0054] Next, the taken-up ultra-high molecular weight polyethylene solid-phase roll-formed film is stretched in both MD and TD directions to increase strength and make it porous, thereby producing the porous film of the present invention.

[0055] More specifically, first, the raw film, a high-molecular-weight polyethylene solid-phase rolled film, is stretched in the MD, which promotes molecular orientation in the MD, converting lamellar crystals into extended-chain crystals, and improving the strength in the MD.

[0056] The stretching temperature in the MD stretching process is 136°C or higher and lower than 160°C. This is because melt stretching at 136°C or higher forms the above-mentioned extended chain crystals, and if the temperature is lower than 136°C, the film will be below the melting point of the lamellar crystals in the raw film, and the lamellar crystals will not melt, so the film will not become soft enough to be stretched and may break. Also, if the stretching temperature is higher than 160°C, the film may melt and break.

[0057] Furthermore, the stretching ratio in the MD stretching treatment is preferably 1.0 to 20 times, more preferably 1.1 to 15 times, even more preferably 1.2 to 10 times, and particularly preferably 1.3 to 5 times. When the stretching ratio in the MD stretching treatment is 1.0 to 20 times, it is presumed that MD molecular orientation progresses, lamellar crystals change to extended chain crystals, and the MD strength of the film after MD stretching treatment is developed. Note that the "stretching ratio" here refers to the ratio in the stretching direction (i.e., MD) of the length of the film after stretching to the length of the film before stretching.

[0058] The total MD stretch ratio is preferably 16 to 35 times, more preferably 20 to 30 times. The total MD stretch ratio is the product of the roll stretch ratio, the take-up ratio, and the stretch ratio in the MD stretching treatment. If the total MD stretch ratio is less than 16 times, extended chain crystals may not be sufficiently formed, and if it is more than 35 times, the film may break when stretched.

[0059] Next, the film that has been stretched in the MD is stretched in the TD, whereby molecular orientation in the TD progresses, the strength in the TD improves, and the extended chain crystals are cleaved, forming pores 21 in the scanning electron microscope (trade name: Schottky Scanning Electron Microscope SU5000, manufactured by Hitachi High-Tech Science Corporation) photograph shown in FIG. 3, thereby producing a porous film 20.

[0060] The stretching temperature in the TD stretching process is 130°C or higher, and is lower than the endothermic peak temperature of the extended chain crystals in the DSC chart (e.g., 145°C or 150°C). This is because, if the temperature is lower than 130°C, the amorphous portions between the extended chain crystals or lamellar crystals will not soften sufficiently, presumably causing cracks and resulting in breakage. Furthermore, if the temperature is higher than the endothermic peak temperature of the extended chain crystals in the DSC chart, the extended chain crystals may dissolve, blocking the pores and making it difficult to form porous films.

[0061] The stretching ratio in the TD stretching treatment is 2 to 10 times. This is because if the stretching ratio is less than 2 times, the molecular orientation in the TD does not proceed sufficiently, and the TD strength of the film may not be developed, or the extended chain crystals may not be cleaved, making it difficult to form pores. If the stretching ratio is more than 10 times, it may be necessary to increase the size of the stretching device (i.e., increase the width of the stretching device), which may make the stretching treatment difficult. Note that the "stretching ratio" here refers to the multiple of the length of the film after stretching in the stretching direction (i.e., TD) relative to the length of the film before stretching.

[0062] The stretching method in MD and TD is not particularly limited, and examples thereof include roll stretching and tenter stretching.

[0063] The porous film of the present invention produced by the above-mentioned method has a tensile breaking stress of 100 MPa or more as measured in accordance with JIS K 7127 and a Gurley air permeability of 200 s / 100 ml or less as measured in accordance with JIS P 8117, and therefore can have excellent mechanical strength and breathability.

[0064] The thickness of the raw film before stretching is preferably 30 to 300 μm, more preferably 50 to 200 μm. If the thickness of the raw film is 30 μm or more, handling properties such as wrinkles during winding and trimming ease during slitting can be ensured. If the thickness of the raw film is 300 μm or less, the porous film after stretching can have sufficient breathability.

[0065] According to the above method, in the present invention, a porous film having high strength, meltdown properties and excellent breathability can be obtained by a dry method without imposing burdens on the environment and facilities and without imposing costs. [Example]

[0066] The present invention will be described below based on examples. However, the present invention is not limited to these examples, and these examples can be modified or changed based on the spirit of the present invention, and such modifications are not excluded from the scope of the present invention.

[0067] Example 1 <Preparation of porous film> Ultra-high molecular weight polyethylene (Mitsui Chemicals, Inc., trade name: Mibelon XM220, viscosity average molecular weight: 2 million, melting point: 143°C) was used as the raw material, and was formed into a film using a rolling mill (roll diameter: 75 mm) under the manufacturing conditions shown in Table 1, to produce a polyethylene film (raw film before perforation) with a thickness of 98 μm.

[0068] Next, this raw film was stretched in the MD under the conditions of the stretching temperature and stretch ratio shown in Table 1, and then stretched in the TD to make the raw film porous, thereby producing a porous film (thickness: 15 μm) with multiple pores formed.

[0069] <Gurley air permeability measurement> In accordance with JIS P 8117, the Gurley air permeability [s / 100ml] of the fabricated porous film was measured using an air permeability tester (manufactured by Asahi Seiko Co., Ltd., product name: Digital Oken type air permeability tester, EG01-6-1MR). The above results are shown in Table 1.

[0070] <Measurement of tensile breaking stress> In accordance with JIS K 7127, the tensile breaking stress [MPa] of the fabricated porous film was measured. More specifically, a test film of test piece type 3 dumbbell was prepared, and a tensile test was performed using a tensile tester (manufactured by Shimadzu Corporation, product name: Autograph AG-5000A) under the conditions of a temperature of 25°C and a tensile speed of 100 mm / min, and the tensile breaking stress [MPa] in the MD and TD directions was measured. The above results are shown in Table 1.

[0071] <Measurement of puncture strength> In accordance with JIS Z 1707, the puncture strength [s / N] of the fabricated porous film was measured using a film puncture jig (manufactured by Imada Co., Ltd., product name: Measuring stand EMX series, film puncture jig TKS series). The above results are shown in Table 1.

[0072] <Measurement of median diameter> Using a mercury intrusion porosimeter (manufactured by Shimadzu-Micromeritics, product name: Pore size distribution measuring device Autopore V9620), the median diameter of the pores in the porous film was measured by the mercury intrusion method. The above results are shown in Table 1.

[0073] <DSC measurement> In addition, DSC measurement of the fabricated porous film was performed. More specifically, in accordance with the JIS K 7121 test method, after collecting about 1 mg of the sample and enclosing it in a differential scanning calorimeter (manufactured by Hitachi High-Technologies Corporation, product name: DSC7000X), nitrogen was flowed at 30 cc / min as the carrier gas, and a DSC chart was obtained under the condition of a heating rate of 10°C / min. Also, under the same conditions, a DSC chart of the solid-phase roll-rolled film (original film) before stretching was obtained. The above results are shown in Figure 2.

[0074] As shown in Figure 2, the DSC curve of the prepared porous film has two endothermic peaks between 140°C and 155°C (141°C and 152°C). This indicates that the extended chain crystal melting peak is located around 140-145°C, and the extended chain crystal transition peak is located around 150-155°C. This indicates that the strength of the porous film is enhanced.

[0075] Example 2 Ultra-high molecular weight polyethylene (manufactured by Mitsui Chemicals, Inc., product name: Hi-Zex Million 145M, viscosity average molecular weight: 1.15 million, melting point: 144°C) was used as the raw material. Under the manufacturing conditions shown in Table 1, it was formed into a film using a rolling mill (roll diameter: 75 mm) to produce a 79 μm thick raw polyethylene film. This raw film was then stretched in the MD under the stretching temperature and stretch ratio conditions shown in Table 1, and then stretched in the TD to make the raw film porous, producing a porous film (thickness: 6 μm) with multiple pores.

[0076] Thereafter, the Gurley air permeability, tensile breaking stress, puncture strength, median diameter, and DSC were measured in the same manner as in Example 1. The results are shown in Table 1.

[0077] Example 3 Ultra-high molecular weight polyethylene (manufactured by Mitsui Chemicals, Inc., product name: Hi-Zex Million 320MU, viscosity average molecular weight: 3.2 million, melting point: 144°C) was used as the raw material. Under the manufacturing conditions shown in Table 1, it was formed into a film using a rolling mill (roll diameter: 75 mm) to produce a polyethylene film as a raw film with a thickness of 96 μm. This raw film was then stretched in the MD under the stretching temperature and stretch ratio conditions shown in Table 1, and then stretched in the TD to make the raw film porous, producing a porous film (thickness: 15 μm) with multiple pores formed.

[0078] Thereafter, the Gurley air permeability, tensile breaking stress, puncture strength, median diameter, and DSC were measured in the same manner as in Example 1. The results are shown in Table 1.

[0079] Example 4 A porous film was prepared in the same manner as in Example 1, except that the stretching ratio in the MD stretching treatment was changed to the conditions shown in Table 1, and the Gurley air permeability, tensile breaking stress, puncture strength, median diameter, and DSC were measured. The results are shown in Table 1.

[0080] Example 5 A porous film was prepared in the same manner as in Example 1, except that the stretching ratios in the MD and TD stretching treatments were changed to the conditions shown in Table 1, and the Gurley air permeability, tensile breaking stress, puncture strength, median diameter, and DSC measurements were carried out. The results are shown in Table 1.

[0081] Example 6 A porous film was prepared in the same manner as in Example 1, except that the stretching ratio in the TD stretching treatment was changed to the conditions shown in Table 1, and the Gurley air permeability, tensile breaking stress, puncture strength, median diameter, and DSC were measured. The results are shown in Table 1.

[0082] (Comparative Example 1) Ultra-high molecular weight polyethylene (manufactured by Mitsui Chemicals, Inc., product name: Hi-Zex Million 030S, viscosity average molecular weight: 500,000, melting point: 143°C) was used as the raw material, and attempts were made to mold it into a film using a rolling mill (roll diameter: 75 mm) under the manufacturing conditions shown in Table 2. However, because the viscosity average molecular weight was small, the melt viscosity was low and the resin melted into the roll, making it impossible to mold.

[0083] (Comparative Example 2) A porous film was prepared in the same manner as in Example 1, except that the stretching ratio in the MD stretching treatment was changed to the conditions shown in Table 2, and the Gurley air permeability, tensile breaking stress, puncture strength, median diameter, and DSC were measured. The results are shown in Table 2.

[0084] (Comparative Example 3) A porous film was prepared in the same manner as in Example 1, except that the TD stretching treatment was not performed, and the Gurley air permeability, tensile breaking stress, puncture strength, median diameter, and DSC measurements were carried out. The results are shown in Table 2.

[0085] Comparative Example 4 An attempt was made to produce a porous film in the same manner as in Example 1 above, except that the stretching temperature in the MD stretching treatment was changed to the conditions shown in Table 2. However, because the stretching temperature in the MD stretching treatment (125°C) was low, the polyethylene film broke during the MD stretching treatment, and the TD stretching treatment could not be performed.

[0086] (Comparative Example 5) An attempt was made to produce a porous film in the same manner as in Example 1 above, except that the stretching temperature in the MD stretching treatment was changed to the conditions shown in Table 2. However, because the stretching temperature in the MD stretching treatment (160°C) was high, the polyethylene film melted and broke during the MD stretching treatment, and the TD stretching treatment could not be performed.

[0087] (Comparative Example 6) An attempt was made to produce a porous film in the same manner as in Example 1 above, except that the stretching ratio in the MD stretching treatment and the stretching temperature in the TD stretching treatment were changed to the conditions shown in Table 2. However, because the stretching temperature in the TD stretching treatment (125°C) was low, the polyethylene film broke during the TD stretching treatment.

[0088] (Comparative Example 7) A porous film was produced in the same manner as in Example 1, except that the stretching temperature in the TD stretching treatment was changed to the conditions shown in Table 2, and the Gurley air permeability, tensile breaking stress, puncture strength, median diameter, and DSC were measured. The results are shown in Table 2.

[0089] (Comparative Example 8) First, a polyethylene composition obtained by mixing 70 parts by mass of ultra-high molecular weight polyethylene with a mass-average molecular weight of 4.6 million and 30 parts by mass of high-density polyethylene with a mass-average molecular weight of 560,000 was mixed with a previously prepared mixed solvent of 55 parts by mass of liquid paraffin and 25 parts by mass of decalin (decahydronaphthalene) to prepare a polyethylene solution.

[0090] This polyethylene solution was extruded into a sheet using a die at a temperature of 160°C, and then the extrudate was cooled to 25°C in a water bath. At the same time, a water flow was created on the surface of the water bath to prevent the mixed solvent released from the gelled sheet and floating on the water surface from adhering to the sheet again, thereby producing a gel-like sheet (base tape).

[0091] Next, this base tape was dried at 55 ° C. for 10 minutes and then at 95 ° C. for 10 minutes to remove decalin from the base tape, and then the base tape was conveyed on a roller heated to 85 ° C. while applying a pressure of 20 kgf / m to remove part of the liquid paraffin from the base tape. Then, this base tape was stretched in the MD at a magnification of 5 times at a temperature of 100 ° C. in the longitudinal direction, and then stretched in the TD at a magnification of 14 times at a temperature of 100 ° C., and immediately heat-treated (heat-set) at 128 ° C. A porous film (thickness: 15 μm) was produced.

[0092] Then, the Gurley air permeability, tensile breaking stress, puncture strength, median diameter, and DSC were measured in the same manner as in Example 1. The results are shown in Table 3.

[0093] (Comparative Example 9) First, polypropylene resin with a mass average molecular weight of 590,000 was melt-extruded at a T-die temperature of 200°C. The extruded film was guided to a cooling roll at 90°C, cooled by blowing cold air at 37.2°C, and then taken up at 40 m / min to produce an unstretched polypropylene film (thickness: 14.1 μm).

[0094] Next, high-density polyethylene with a mass-average molecular weight of 320,000 was melt-extruded at a T-die temperature of 173°C. The extruded film was guided to a cooling roll at 115°C, cooled by blowing cold air at 39°C, and then taken up at 20 m / min to produce an unstretched polyethylene film (thickness: 7.6 μm).

[0095] Using this unstretched polypropylene film and unstretched polyethylene film, a three-layer laminate film with a sandwich structure of two outer layers of polypropylene film and an inner layer of polyethylene film was produced as follows: The polypropylene film and polyethylene film were each unwound from three sets of raw roll sandwiches at a speed of 6.5 m / min, introduced onto a heated roll, and thermocompressed with a roll temperature of 147°C. They were then introduced onto a cooling roll at the same speed at 30°C and taken up to produce a three-layer laminate film (thickness: 35.8 μm).

[0096] Next, this three-layer laminate film was placed in a hot air circulating oven heated to 125°C and heat-treated. Next, the heat-treated laminate film was stretched in MD at a ratio of 1.18 between nipple rolls maintained at 35°C. The roll speed on the supply side was 2.8 m / min. Next, in a heat-stretching zone heated to 130°C, the film was stretched in MD at a ratio of 1.9 between rollers using the difference in roll peripheral speed, and then subsequently stretched in MD at a ratio of 1.25. Then, a porous film (thickness: 15 μm) was produced by heat-treating (heat-setting) at 133°C.

[0097] Then, the Gurley air permeability, tensile breaking stress, puncture strength, median diameter, and DSC were measured in the same manner as in Example 1. The results are shown in Table 3.

[0098] [Table 1]

[0099] [Table 2]

[0100] [Table 3]

[0101] As shown in Table 1, the porous films of Examples 1 to 6 have at least one endothermic peak between 140°C and 155°C in the DSC chart and have a Gurley air permeability of 200s / 100ml or less, and therefore have excellent breathability, high mechanical strength, and meltdown properties.

[0102] On the other hand, in Comparative Example 2, the stretch ratio in the MD stretching treatment was low, so that extended chain crystals were not sufficiently formed, and pore formation due to the cleavage of the extended chain crystals could not be achieved. Therefore, the measured Gurley air permeability was greater than 99,999 s / 100 ml, and the median diameter was not measured.

[0103] In Comparative Example 3, since the TD stretching treatment was not performed, porosity based on the cleavage of extended chain crystals could not be achieved, the measured Gurley air permeability was greater than 99,999 s / 100 ml, and the median diameter was not measured. Furthermore, the molecular orientation in the TD did not progress sufficiently, and the TD breaking strength was less than 100 MPa, indicating poor mechanical strength.

[0104] In Comparative Example 7, the stretching temperature (155°C) in the TD stretching treatment was higher than the endothermic peak temperatures (141°C, 152°C) of the extended chain crystals in the DSC chart. Therefore, the extended chain crystals dissolved during the TD stretching treatment, blocking the pores and preventing pore formation based on cleavage of the extended chain crystals. As a result, the measured Gurley air permeability was greater than 99,999 s / 100 ml, and therefore the median diameter was not measured.

[0105] In Comparative Example 8, as shown in Fig. 4, the endothermic peak temperature in the DSC chart is 138°C, and there is no endothermic peak between 140°C and 155°C. That is, since there are no extended chain crystals, the tensile breaking stress in TD is less than 100 MPa, indicating poor mechanical strength.

[0106] 5, the DSC chart of Comparative Example 9 shows an endothermic peak temperature of 161°C, and does not have at least one endothermic peak between 140°C and 155°C. That is, since there are no extended chain crystals, the tensile stress at break in TD is less than 100 MPa, indicating poor mechanical strength. [Industrial Applicability]

[0107] As explained above, the present invention is suitable for a porous film obtained by stretching a solid-phase roll-formed ultra-high molecular weight polyethylene film containing ultra-high molecular weight polyethylene as a main component. [Explanation of symbols]

[0108] 1 Raw material feeder 2-3 pairs of rolls 4~7 Transfer Rolls 8. Take-off roll 10-roll rolling mill 20 Perforated film 21 pores M Ultra-high molecular weight polyethylene P Ultra-high molecular weight polyethylene solid rolled film

Claims

1. A porous film stretched in both MD and TD directions, containing 90% by mass or more of a solid powder of ultra-high molecular weight polyethylene having a viscosity average molecular weight (Mv) of 600,000 to 5,000,000, In a DSC chart, it has at least one endothermic peak between 140°C or higher and lower than 155°C, and its Gurley air permeability measured in accordance with JIS P 8117 is 200s / 100ml or lower, A porous film characterized in that the tensile breaking stress in at least one of the MD and TD directions is 261 MPa or more.

2. 2. The porous film according to claim 1, wherein the puncture strength measured in accordance with JIS Z 1707 is 2N or more.

3. 3. The porous film according to claim 1, wherein the median diameter of the pores measured by mercury porosimetry is 1 μm or less.

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