JP7400366B2 - Porous polyolefin film, battery separator and secondary battery using the same - Google Patents

Description

The present invention relates to a porous polyolefin film with excellent safety, a battery separator using the same, and a secondary battery.

Porous polyolefin films are used in a wide range of fields, including battery separators, various separation membranes, water treatment membranes, and capacitor separators. Porous polyolefin films are particularly insoluble in organic solvents, have excellent mechanical properties, and have excellent shutdown properties that block pores in the event of abnormal heat generation, cutting off current and suppressing excessive temperature rise. Suitable for use in battery separators.

In recent years, lithium-ion batteries have become increasingly high in energy density, high in capacity, and high in output, mainly for automotive applications. The characteristics required for separators are also becoming higher. In particular, since the separator is wound under tension, high strength is required to prevent membrane rupture during winding.

In addition, shrinkage that occurs when the separator becomes molten at a temperature higher than the shutdown temperature may cause a short circuit in the lateral direction (TD direction) of the electrodes, accelerating thermal runaway of the battery. Low thermal shrinkage is also important for ensuring battery safety. Generally, in the production of separators, it is known that the strength is increased by imparting orientation to the separator through stretching, etc. However, the higher the strength is achieved by stretching at a higher magnification, the higher the shrinkage rate at high temperatures becomes. It was difficult to achieve both strength and low heat shrinkage.

For example, Patent Document 1 discloses a method in which the strength is increased by increasing the resin concentration, and the shrinkage rate in the lateral direction is lowered by increasing the stretching ratio in the longitudinal direction than in the lateral direction. has been done.

Further, Patent Document 2 discloses a method of adjusting the molecular weight of raw materials, resin concentration, and stretching conditions in order to improve the balance of physical properties as a separator such as strength, shrinkage rate, and shutdown characteristics.

Patent Document 3 discloses a method of crosslinking polyethylene for the purpose of improving heat resistance and reducing heat shrinkage.

Patent No. 5595459 Patent No. 4808935 Patent No. 4189961

However, in the method disclosed in Patent Document 1 and the method disclosed in Patent Document 2, there is a limit to increasing the resin concentration and the stretching conditions are also limited, so it is possible to achieve both high strength and low heat shrinkage rate. was insufficient. In addition, in the method disclosed in Patent Document 3, although the shrinkage force during melting is small, the strength is low when achieving a sufficiently low shrinkage rate to ensure safety. Balancing the two was not enough.

An object of the present invention is to provide a porous polyolefin film that has both high strength and low heat shrinkage rate that could not be achieved with conventional technology, and also has safety, and a battery separator and a secondary battery using the same. It is in.

In order to solve the above problems, the present invention employs the following configuration. That is,
(1) A porous polyolefin film characterized by having a puncture strength of 350 mN/μm or more based on a porosity of 50%, and a maximum melt shrinkage rate in the width direction (TD direction) of 1% or more and 25% or less .
(2) The porous polyolefin film according to (1), which has a porosity of 40% or more.
(3) The porous polyolefin film according to (1) or (2), which has an average pore diameter of 40 nm or less.
(4) Any of (1) to (3), characterized in that the tensile elongation at break in the machine direction (MD direction) and the tensile elongation at break in the width direction (TD direction) are both 80% or more and 200% or less. A porous polyolefin film according to claim 1.
(5) The porous polyolefin film according to any one of (1) to (4), which has a film thickness of 15 μm or less.
(6) The average tensile strength at break, which is the average value of the tensile strength at break in the machine direction (MD direction) and the tensile strength at break in the width direction (TD direction), is 150 MPa or more, and the tensile elongation at break in the MD direction and the TD direction The average tensile elongation at break is 90% or more, and the average tensile elongation at break (MPa) x the average tensile elongation at break (%) is 15000 (MPa x %) or more. The porous polyolefin film according to any one of (1) to (5), characterized in that:
(7) The porous polyolefin film according to any one of (1) to (6), characterized in that the film contains a polyethylene resin as a main component.
(8) A battery separator using the porous polyolefin film according to any one of (1) to (7).
(9) A secondary battery using the battery separator described in (8).

The porous polyolefin film of the present invention has both high strength and low thermal shrinkage, and more effectively prevents short circuits during meltdown when used as a battery separator than porous polyolefin films obtained by conventional techniques. be able to.

Furthermore, since the porous polyolefin film of the present invention has a small average pore diameter, it is difficult for dendrites to penetrate through it, and a battery separator using the porous polyolefin film of the present invention is a battery separator with excellent safety.

The porous polyolefin film of the present invention has a puncture strength of 350 mN/μm or more based on a porosity of 50%, and a maximum melt shrinkage rate in the width direction (TD direction) of 1% or more and 25% or less. .

The puncture strength in terms of porosity of 50% is the puncture strength of a porous polyolefin film with a thickness of T1 (μm) and a porosity of P1 (%) using a needle with a diameter of 1 mm and a spherical tip (radius of curvature R: 0.5 mm). The maximum load L1 (N) when pierced at a speed of 2 mm/sec was measured, and the value obtained by the formula: L2 = L1 / T1 × 50 / (100 - P1) (when the porosity is 50%) (value converted to maximum load L2 per 1 μm film thickness).

The puncture strength of the porous polyolefin film of the present invention in terms of 50% porosity is 350 mN/μm or more, preferably 380 mN/μm or more, more preferably 400 mN/μm or more, and still more preferably 420 mN/μm or more. Further, the upper limit of the puncture strength is 2500 mN/μm or less, more preferably 2000 mN/μm or less.

If the puncture strength is 350 mN/μm or more based on a porosity of 50%, when used as a battery separator, the separator is less likely to rupture during winding or due to foreign matter, resulting in short circuits, which improves battery safety. can be increased. If the puncture strength based on porosity of 50% is less than 350 mN/μm, when used as a battery separator, the separator is likely to rupture during winding or due to foreign matter, causing short circuits, resulting in poor battery safety. It becomes like this.

Further, the porous polyolefin film of the present invention has a maximum melt shrinkage rate in the TD direction of 1% or more and 25% or less.

In addition, in this specification, MD direction (machine direction) means the direction in which the film is extruded when forming the porous polyolefin film of the present invention (film forming direction), and TD direction (width direction) means a direction perpendicular to the MD direction on the film.

The maximum melt shrinkage rate is a value determined by the following method using a thermomechanical analyzer. That is, the temperature is scanned under the conditions of sample shape: width 3 mm x length 10 mm, initial load: 19.6 mN, temperature scanning range: 30 to 210° C., and heating rate: 5° C./min. A plurality of different regions are randomly extracted from the same porous polyolefin film. In each of the plurality of regions, the amount of shrinkage in the TD direction is measured at three points each, and the shrinkage rate at the point where the dimension has shrunk the most is defined as the maximum melt shrinkage rate. The average value of the maximum melt shrinkage rates found in each region is defined as the "maximum melt shrinkage rate in the TD direction."

The maximum melt shrinkage rate in the TD direction is 1% or more and 25% or less, preferably 1% or more and 23% or less, more preferably 1% or more and 20% or less, and even more preferably 1% or more and 15% or less. Most preferably, it is 1% or more and 10% or less. When the maximum melting shrinkage rate in the TD direction is within the above range, when used as a battery separator, even if abnormal heat is generated locally within the battery, internal short circuits can be prevented, resulting in a separator with excellent safety. becomes.

Furthermore, the porosity of the porous polyolefin film of the present invention is preferably 40% or more, more preferably 43% or more, from the viewpoint of ion permeability and electrolyte content when used as a battery separator. , more preferably 46% or more. Further, the upper limit of the porosity is preferably 60% or less from the viewpoint of ensuring mechanical strength. The porosity of the porous polyolefin film of the present invention can be adjusted by adjusting the resin concentration during film formation and the stretching temperature during stretching.

The average pore diameter of the porous polyolefin film of the present invention is preferably 40 nm or less, more preferably 38 nm or less, and still more preferably 35 nm or less. When the average pore diameter is 40 nm or less, it becomes difficult for dendrites to penetrate, and short circuits of the battery can be prevented. If the average pore diameter is larger than 40 nm, the dendrites will easily penetrate, leading to a short circuit in the battery. Therefore, from the viewpoint of safety, the average pore diameter is preferably 40 nm or less.

Further, the lower limit of the average pore diameter is preferably 5 nm or more, more preferably 10 nm or more. If the average pore size is smaller than this range, when used as a battery separator, it will be difficult for ions to pass through, leading to deterioration of battery characteristics. Therefore, from the viewpoint of rate characteristics, the average pore size is preferably 5 nm or more. The average pore diameter of the porous polyolefin film of the present invention can be adjusted by the molecular weight and resin concentration of the raw polyolefin, the kneading conditions during kneading, and the stretching temperature during stretching.

Further, the porous polyolefin film of the present invention has excellent mechanical strength, and it is preferable that both the tensile strength at break and the tensile elongation at break obtained by a tensile test are high values.

Regarding the tensile elongation at break, it is preferably 80% or more in both the MD direction and the TD direction, and more preferably 95% or more. By setting the tensile elongation at break in the MD direction and the tensile elongation at break in the TD direction to 80% or more, the film becomes difficult to break even when high tension is applied, such as during winding, and short circuits can be prevented. Further, the upper limit of the tensile elongation at break is preferably 200% or less in both the MD direction and the TD direction. A tensile elongation of 200% or less is preferable because it tends to be less likely to deform when stored in a wound state for a long period of time, resulting in excellent long-term storage stability.

In addition, the average value of the tensile elongation at break in the MD direction and the tensile elongation at break in the TD direction ((tensile elongation at break in the MD direction + tensile elongation at break in the TD direction)/2) (hereinafter referred to as "average elongation at break") ) is preferably 90% or more, more preferably 100% or more, even more preferably 110% or more. By setting the average tensile elongation at break to 90% or more, the film becomes difficult to break even when high tension is applied, such as during winding, and short circuits can be prevented. Further, the upper limit of the average tensile elongation at break is preferably 300% or less. It is preferable that the average tensile elongation is 300% or less because it tends to be difficult to deform when stored in a wound state for a long period of time and has excellent long-term storage stability.

The tensile elongation at break of the porous polyolefin film of the present invention can be adjusted by adjusting the molecular weight, resin concentration, crystallinity, and stretching ratio of the polyolefin.

Regarding the tensile strength at break, the average value of the tensile strength at break in the MD direction and the tensile strength at break in the TD direction ((tensile strength at break in the MD direction + tensile strength at break in the TD direction)/2) (hereinafter referred to as "average tensile strength at break") ) is preferably 150 MPa or more, more preferably 170 MPa or more, still more preferably 200 MPa or more, most preferably 220 MPa or more. Further, the upper limit of the average tensile strength at break is preferably 1500 MPa or less, more preferably 1200 MPa or less. By having an average tensile strength at break of 150 MPa or more, membrane rupture can be prevented when hot pressing is performed under high pressure in the battery manufacturing process. If the average tensile strength at break is lower than this range, there is a risk that the membrane will rupture when hot pressed under high pressure in the battery manufacturing process, so it is preferable that the average tensile strength at break is higher. The average tensile strength at break in the present invention can be adjusted by the molecular weight and crystallinity of the resin used, and the stretching ratio during stretching.

As mentioned above, the porous polyolefin film of the present invention preferably has high values for both the average tensile strength at break and the tensile elongation at break, and the value of average tensile strength at break (MPa) x average tensile elongation at break (%) is It is preferably 15,000 (MPa x %) or more, more preferably 17,500 (MPa x %) or more, still more preferably 20,000 (MPa x %) or more. Further, the upper limit of this value is preferably 750,000 (MPa x %) or less, more preferably 700,000 (MPa x %) or less. The higher this value is, the higher the toughness can be exhibited, and if the value of average tensile strength at break (MPa) x average tensile elongation at break (%) is less than 15000 (MPa x %), coating or battery creation This is not preferable because it cannot withstand the tension that is sometimes applied, increasing the possibility that the film will rupture and short-circuit.

Furthermore, the average tensile strength at break and the average tensile elongation at break are in a trade-off relationship, and conventional techniques have not been able to obtain high toughness films with high values for both the average tensile strength at break and the average tensile elongation at break. The product of the average tensile strength at break and the average tensile elongation at break of the porous polyolefin film of the present invention can be adjusted by adjusting the molecular weight, resin concentration, crystallinity, and stretching ratio of the raw polyolefin.

The upper limit of the thickness of the porous polyolefin film of the present invention is preferably 15 μm or less, more preferably 13 μm or less, and even more preferably 10 μm or less. The lower limit of the film thickness is 1 μm or more, more preferably 3 μm or more. When the film thickness is 15 μm or less, ion permeability is improved, leading to higher capacity of the battery. If the film thickness is less than 1 μm, the strength is low and the film is likely to rupture, and the resistance against acicular foreign matter such as dendrites is low, which is not preferable. Moreover, if the film thickness is too thick, the ion permeability will be poor, making it impossible to increase the capacity. The thickness of the porous polyolefin film of the present invention can be adjusted by adjusting the width of the unstretched sheet, the film forming speed, and the stretching ratio during stretching.

Moreover, it is preferable that the porous polyolefin film of the present invention has a polyolefin resin as a main component. The term "main component" as used herein means that the proportion of the total component amount is 50% by mass or more.

Examples of the polyolefin resin in the present invention include polyethylene, polypropylene, poly(4-methyl-pentene-1), ethylene-propylene copolymer, polytetrafluoroethylene, polytrifluorochloroethylene, polyvinylidene fluoride, polychloride Examples include vinylidene, polyvinyl fluoride, polyvinyl chloride, polysulfone, etc., and they may be used alone or in combination of two or more. In particular, when used as a battery separator, it is preferable to use polyethylene resin from the viewpoint of shutdown characteristics, etc., and the content of polyethylene resin is preferably 90% by mass or more in the polyolefin resin, and preferably 95% by mass or more. It is more preferable that there be.

Here, the types of polyethylene resin include high-density polyethylene resin with a density exceeding 0.94 g/cm ³ , medium-density polyethylene resin with a density in the range of 0.93 to 0.94 g/cm ³ , and density polyethylene resin with a density of 0.94 g/cm 3 . Examples include low-density polyethylene resins with a density lower than .93 g/cm ³ and linear low-density polyethylene resins, which may be used alone or as a mixture.

In addition, as the polyethylene resin, (I) ethylene homopolymer, or (II) copolymer of ethylene and comonomer such as propylene, butene-1, hexene-1, etc., and mixtures thereof can be used, but economically and From the viewpoint of film strength, ethylene homopolymer is preferred.

As the main raw material for the porous polyolefin film of the present invention, it is preferable to use a blend of two or more types of polyolefin resins. As an auxiliary raw material for the porous polyolefin film of the present invention, it is recommended to use ultra-high molecular weight polyethylene resin (UHPE) with a weight average molecular weight (Mw) of 1.0 x 10 ⁶ or more to improve mechanical strength and refine the pore size. , is preferable from the viewpoint of high heat resistance. Details of the ultra-high molecular weight polyethylene resin will be described later.

The polyolefin resin used as the main raw material for the porous polyolefin film of the present invention preferably has a weight average molecular weight (Mw) of 1.0 x 10 ⁵ or more, and the weight average molecular weight (Mw) is 1.5 x 10 ⁵ The above is more preferable, and 1.8×10 ⁵ or more is even more preferable. The upper limit of the weight average molecular weight (Mw) is preferably less than 1.0×10 ⁶ , more preferably 5.0×10 ⁵ or less, and even more preferably 3.5×10 ⁵ or less.

When the Mw is 1.0×10 ⁵ or more, the molecular weight is large, and it is possible to prevent pore clogging during heat treatment due to a high melting point due to orientation during stretching and a low melting point of the raw material, and a decrease in porosity. , it is possible to prevent deterioration of output characteristics. If Mw is less than 1.0 x ¹⁰⁵ , the molecular weight will be small, and the orientation during stretching will increase the melting point, and the lower melting point of the raw material will cause pore clogging during heat treatment, resulting in a lower porosity and lower output. Characteristics may deteriorate. When the Mw is less than 1.0×10 ⁶ , it is possible to prevent the shutdown temperature from increasing due to a higher melting point due to an increase in molecular weight.

Further, the specific Mw of the ultra-high molecular weight polyethylene resin contained in the porous polyolefin film of the present invention is preferably 1.5 x 10 ⁶ or more, more preferably 1.8 x 10 ⁶ or more. Further, the upper limit thereof is preferably 5.0×10 ⁶ or less, more preferably 4.0×10 ⁶ or less. When Mw is 1.5×10 ⁶ or more, mechanical strength can be increased. By setting Mw to 5.0×10 ⁶ or less, it is possible to prevent the melt viscosity during kneading from increasing and moldability from worsening.

Further, the upper limit of the content of the ultra-high molecular weight polyethylene resin in the polyolefin resin is preferably 50% by mass or less, more preferably 40% by mass or less. By controlling the content of the ultra-high molecular weight polyethylene resin to 50% by mass or less, deterioration of moldability during film formation can be prevented. Further, the lower limit of the content of the ultra-high molecular weight polyethylene resin is preferably 2% by mass or more, more preferably 5% by mass or more, and even more preferably 10% by mass or more. By setting the content of the ultra-high molecular weight polyethylene resin to 2% by mass or more, it is possible to effectively improve mechanical strength, refine the pore diameter, and increase heat resistance.

The crystallinity of the ultra-high molecular weight polyethylene resin contributes to the strength and shrinkability of the resulting porous polyolefin film. The ultra-high molecular weight polyethylene resin used in the present invention is preferably highly crystalline and has a melting point of 140°C or higher. The melting point is more preferably 141°C or higher, even more preferably 143°C or higher. Further, when the melting point is within the above range, it is possible to achieve both high strength and low heat shrinkage, and the higher the melting point, the more preferable.

Further, the crystallinity of the ultra-high molecular weight polyethylene resin can be judged by the heat of fusion ΔHm shown below, and the larger ΔHm is, the higher the crystallinity becomes, leading to higher strength and lower thermal shrinkage rate. The ultra-high molecular weight polyethylene resin used in the present invention preferably has a ΔHm of 170 J/g or more, more preferably 180 J/g or more, and even more preferably 185 J/g or more. Moreover, the upper limit of ΔHm is 293 J/g, which theoretically corresponds to 100% crystallinity.

The heat of fusion ΔHm can be determined using a scanning differential calorimeter (DSC). The sample is placed in a DSC sample holder, held at 30°C for 1 minute in a nitrogen atmosphere, and heated to 230°C at a rate of 10°C/min. A straight line passing through the point at 60°C and the point at 180°C on the DSC curve (melting curve) obtained during the temperature raising process is drawn as the baseline, and the amount of heat (unit: :J) and dividing this by the weight of the sample (unit: g), the heat of fusion ΔHm (unit: J/g) can be obtained.

Moreover, the polyolefin resin can contain other resin components other than the polyethylene resin, if necessary. Other resin components are preferably heat-resistant resins, such as crystalline resins (including partially crystalline resins) with a melting point of 150°C or higher, and/or glass. Examples include amorphous resins having a point transition (Tg) of 150° C. or higher. Here, Tg is a value measured in accordance with JIS K7121.

Specific examples of other resin components include polyester, polymethylpentene [PMP or TPX (transparent polymer PAS), vinylidene fluoride homopolymers such as polyvinylidene fluoride (PVDF), fluorinated olefins such as polytetrafluoroethylene (PTFE), and fluororesins such as their copolymers, polystyrene (PS, melting point: 230°C) ), polyvinyl alcohol (PVA, melting point: 220-240°C), polyimide (PI, Tg: 280°C or higher), polyamideimide (PAI, Tg: 280°C), polyether sulfone (PES, Tg: 223°C), Polyetheretherketone (PEEK, melting point: 334°C), polycarbonate (PC, melting point: 220-240°C), cellulose acetate (melting point: 220°C), cellulose triacetate (melting point: 300°C), polysulfone (Tg: 190°C) , polyetherimide (melting point: 216°C), and the like. The other resin components are not limited to those made of a single resin component, but may be made of a plurality of resin components.

The preferred weight average molecular weight (Mw) of the other resin components varies depending on the type of resin, but is generally 1×10 ⁴ to 1×10 ⁶ , more preferably 1×10 ⁴ to 7×10 ⁵ . Further, the content of other resin components in the polyolefin resin may be adjusted as appropriate without departing from the spirit of the present invention, but the content of other resin components in the polyolefin resin may be approximately 10% by mass or less.

Further, as other resin components, other polyolefins other than the polyethylene resin may be included as needed, such as polybutene-1 polybutene-1 and polypentene having Mw of 1.0×10 ⁴ to 4.0×10 ⁶ . At least one selected from the group consisting of polyethylene waxes having an Mw of 1.0×10 ³ to 1.0×10 ⁴ may be used.

The content of the polyolefin resin other than the polyethylene resin can be adjusted as appropriate within a range that does not impair the effects of the present invention, but it is preferably 10 parts by mass or less, and 5 parts by mass, based on 100 parts by mass of the polyolefin resin. It is more preferably at most 1 part by mass, even more preferably at most 1 part by mass.

Furthermore, if necessary, various additives such as antioxidants, ultraviolet absorbers, pigments, and dyes can be blended within the range that does not impair the purpose of the present invention.

When additives are added to the polyolefin resin, the amount thereof is preferably 0.01 parts by weight to 10 parts by weight based on 100 parts by weight of the polyolefin resin. When the blending amount of the additive is 0.01 part by mass or more, a sufficient effect can be obtained and the amount added during production can be easily controlled. Moreover, since the amount of additives is 10 parts by mass or less, it is advantageous in terms of economy.

The high mechanical strength, low thermal shrinkage rate, high porosity, and small pore diameter that the porous polyolefin film of the present invention possesses include, for example, materials having a high melting point and an ultra-high molecular weight of 1.0 x 10 ⁶ or more. It can be obtained by using a polyethylene resin, kneading it to uniformly disperse a high melting point ultra-high molecular weight polyethylene resin having poor dispersibility, and then stretching at a high magnification.

The method for producing a porous polyolefin film of the present invention preferably includes the following steps (1) to (5), may further include the following step (6), and further includes the following steps (7) and/or Or (8) may also be included.

(1) A step of melt-kneading the polyolefin resin and a film-forming solvent to prepare a polyolefin resin composition. (2) A step of extruding the polyolefin resin composition and cooling it to form a gel-like sheet. (3) A step of forming the gel-like sheet by extruding the polyolefin resin composition and cooling it. A first stretching step of stretching the sheet (4) A step of removing the film-forming solvent from the gel-like sheet after the stretching (5) A step of drying the sheet after the film-forming solvent has been removed (6) After the drying (7) A step of heat-treating the sheet after the drying; (8) A step of subjecting the sheet after the stretching step to a crosslinking treatment and/or a hydrophilic treatment.

Each step will be explained below.
(1) Preparation process of polyolefin resin composition Polyolefin resin is melt-kneaded to prepare a polyolefin resin composition. As for the preparation method, it is important to improve the dispersibility of the highly crystalline ultra-high molecular weight polyolefin resin, and for example, the raw materials can be kneaded using a batch type kneader capable of high shear kneading.

The lower limit of the temperature during kneading is preferably 170°C or higher, more preferably 175°C or higher, even more preferably 180°C or higher. By setting the temperature during kneading to 170° C. or higher, the viscosity of the molten resin can be lowered and the ultra-high molecular weight polyolefin resin, which is difficult to uniformly knead, can be uniformly dispersed. Furthermore, the upper limit of the temperature during kneading is preferably 250°C or lower, more preferably 220°C or lower, even more preferably 200°C or lower, and particularly preferably 185°C or lower. . By setting the temperature during kneading to 250° C. or lower, it is possible to prevent the strength of the porous polyolefin film obtained from decomposing the resin from decreasing.

The lower limit of the rotation speed during kneading is preferably 100 rpm or more, more preferably 120 rpm or more, even more preferably 130 rpm or more, and particularly preferably 150 rpm or more. By setting the rotation speed during kneading to 100 rpm or more, sufficient shear can be applied to the resin, and the ultra-high molecular weight polyolefin resin can be uniformly dispersed. Therefore, it is possible to suppress the generation of unmelted resin in the raw material resin in the resulting porous polyolefin film, that is, to prevent a decrease in puncture strength and tensile strength at break. Further, the upper limit of the rotation speed during kneading is preferably 250 rpm or less, more preferably 230 rpm or less, and even more preferably 200 rpm or less. By setting the rotational speed during kneading to 250 rpm or less, it is possible to prevent the molecular chains of the raw materials from breaking during kneading and reducing the strength of the resulting polyolefin film.

The lower limit of the kneading time is preferably 5 minutes or more, more preferably 10 minutes or more, and even more preferably 15 minutes or more. By setting the kneading time to 5 minutes or more, the ultra-high molecular weight polyolefin resin can be uniformly dispersed. Further, the upper limit of the kneading time is preferably 25 minutes or less, more preferably 20 minutes or less. By setting the kneading time to 25 minutes or less, it is possible to prevent the strength of the porous polyolefin film obtained from being reduced due to decomposition and deterioration of the raw materials during kneading.

Furthermore, it is preferable to add the raw materials in two or more parts rather than adding them all at once. By pre-kneading the ultra-high molecular weight polyolefin resin and the film-forming solvent and then adding other ingredients, the ultra-high molecular weight polyolefin resin is uniformly dispersed, resulting in a porous polyolefin film with low shrinkage and high This leads to increased strength, higher porosity, and smaller diameter.

The film-forming solvent is not particularly limited as long as it can sufficiently dissolve the polyolefin resin, but it is preferable that the film-forming solvent is liquid at room temperature in order to enable relatively high stretching. Film forming solvents include aliphatic, cycloaliphatic or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, liquid paraffin, mineral oil fractions with boiling points corresponding to these, and Examples include 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 liquid solvent content, it is preferable to use a nonvolatile liquid solvent such as liquid paraffin as the film-forming solvent. A solvent that is miscible with the polyolefin resin in the 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, uneven stretching may occur.

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

In the polyolefin resin composition, the blending ratio of the polyolefin resin and the film-forming solvent is such that the polyolefin resin content is preferably 15 parts by mass or more with respect to a total of 100 parts by mass of the polyolefin resin and the film-forming solvent. It is more preferably 20 parts by mass or more, even more preferably 22 parts by mass or more, and most preferably 25 parts by mass or more. Further, the content is preferably 60 parts by mass or less, more preferably 50 parts by mass or less, and even more preferably 45 parts by mass or less.

By setting the content to 15 parts by mass or more, the porosity of the resulting porous polyolefin film can be lowered, and the strength of the film can be increased. Moreover, by setting the content to 60 parts by mass or less, bleed-out of the solvent can be prevented and uniform film formation and stretching can be achieved.

(2) Step of forming a gel-like sheet The polyolefin resin composition is fed from an extruder to a die and extruded into a sheet. A plurality of polyolefin resin compositions having the same or different compositions may be fed from an extruder to one die, where they may be laminated into layers and extruded into a sheet.

The extrusion method may be either a flat die method or an inflation method. The extrusion temperature is preferably 140 to 250°C, and the extrusion speed is preferably 0.2 to 15 m/min. The film thickness can be adjusted by adjusting the amount of each extrusion of the polyolefin resin composition.

As the extrusion method, for example, methods disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used.

A gel-like sheet is formed by cooling the obtained extruded body. As a method for forming a gel-like sheet, for example, methods disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. Cooling is preferably carried out at a rate of at least 50°C/min, more preferably at least 100°C/min, even more preferably at least 150°C/min, until at least the gelation temperature is reached. It is said that the gel-like sheet is preferably cooled to 50°C or lower, more preferably 40°C or lower, even more preferably 30°C or lower, particularly preferably 20°C or lower.

(3) First Stretching Step Next, the obtained gel sheet is stretched in at least one direction. After heating, the gel-like sheet is preferably stretched at a predetermined magnification by a tenter method, a roll method, an inflation method, or a combination thereof. Although the stretching may be uniaxial or biaxial stretching, biaxial stretching is preferred from the viewpoint of high strength and high productivity. In the case of biaxial stretching, any of simultaneous biaxial stretching, sequential stretching, and multistage stretching (for example, a combination of simultaneous biaxial stretching and sequential stretching) may be used.

In the case of uniaxial stretching, the stretching ratio (area stretching ratio) in this step is preferably 5 times or more, more preferably 10 to 100 times. In the case of biaxial stretching, it is preferably 25 times or more, more preferably 30 times or more, even more preferably 45 times or more, and particularly preferably 75 times or more.

Further, the stretching ratio in both the machine direction and the width direction (MD direction and TD direction) is preferably 5 times or more, more preferably 6 times or more. The stretching ratios in the MD direction and the TD direction may be the same or different.

It is preferable that the stretching ratio is 25 times or more, since the mechanical strength can be increased. Moreover, if the stretching ratio is 150 times or more, the possibility of membrane rupture increases, which is not preferable. Note that the stretching ratio in this step refers to the area stretching ratio of the porous polyolefin film immediately before being subjected to the next step, based on the porous polyolefin film immediately before this step.

The stretching temperature in this step is preferably within the range of crystal dispersion temperature (T _CD ) of the polyolefin resin to T _CD +30°C, and from crystal dispersion temperature (T _CD ) +5°C to crystal dispersion temperature (T _CD ) +28°C. More preferably, the temperature is within the range of , and particularly preferably within the range of T _CD +10°C to T _CD +26°C. When the stretching temperature is within the above range, membrane rupture due to stretching of the polyolefin resin is suppressed, and stretching at a high magnification is possible.

The crystal dispersion temperature (T _CD ) is determined by measuring the temperature characteristics of dynamic viscoelasticity according to ASTM D4065. Ultra-high molecular weight polyethylene resins, polyethylene resins other than ultra-high molecular weight polyethylene resins, and polyethylene resin compositions have a crystal dispersion temperature of about 100 to 110°C, so the stretching temperature is preferably 90 to 130°C, more preferably The temperature is 110 to 120°C, more preferably 114 to 117°C.

Due to the stretching described above, cleavage occurs between the crystal lamellae of the polyethylene resin, the polyethylene resin phase becomes fine, and a large number of fibrils are formed. Fibrils form a three-dimensionally irregularly connected network structure.

(4) Removal process of film-forming solvent The film-forming solvent is removed (cleaned) using a cleaning solvent. Since the polyolefin phase is phase-separated from the film-forming solvent phase, when the film-forming solvent is removed, pores (voids) consisting of fibrils forming a fine three-dimensional network structure and irregularly communicating three-dimensionally are formed. A porous membrane is obtained. Since the cleaning solvent and the method for removing the film-forming solvent using the same are well known, the explanation thereof will be omitted. For example, methods disclosed in Japanese Patent No. 2132327 and Japanese Patent Application Laid-open No. 2002-256099 can be used.

(5) Drying process The porous polyolefin film from which the film-forming solvent has been removed is dried by a heat drying method or an air drying method. The drying temperature is preferably below the crystal dispersion temperature (T _CD ) of the polyolefin resin, particularly preferably 5° C. or more lower than T _CD . Drying is preferably carried out until the remaining cleaning solvent becomes 5 parts by mass or less, more preferably 3 parts by mass or less, based on 100 parts by mass (dry weight) of the porous polyolefin film.

(6) Second Stretching Step The dried porous polyolefin film is preferably stretched in at least one direction. The porous polyolefin film can be stretched by the tenter method or the like in the same manner as described above while being heated. The stretching may be uniaxial or biaxial. In the case of biaxial stretching, either simultaneous biaxial stretching or sequential stretching may be used.

The stretching temperature in this step is not particularly limited, but is usually 90 to 135°C, preferably 95 to 130°C.

The lower limit of the stretching ratio in the uniaxial direction (area stretching ratio) of the porous polyolefin film in this step is preferably 1.0 times or more, more preferably 1.1 times or more, and even more preferably 1. That's more than double. Further, it is preferable that the upper limit of the stretching ratio in the uniaxial direction is 5.0 times or less. In the case of uniaxial stretching, the stretching ratio is 1.0 to 5.0 times in the MD direction or TD direction.

In the case of biaxial stretching, the lower limit of the area stretching ratio is preferably 1.0 times or more, more preferably 1.1 times or more, still more preferably 1.2 times or more. In the case of biaxial stretching, the upper limit of the area stretching ratio is preferably 16.0 times or less, and it is 1.0 to 4.0 times in the MD direction and TD direction, and the stretching ratios in the MD direction and TD direction are mutually It can be the same or different. Note that the stretching ratio in this step refers to the stretching ratio of the porous polyolefin film immediately before being subjected to the next step, based on the porous polyolefin film immediately before this step.

(7) Heat treatment step The porous polyolefin film after drying can also be heat treated. Heat treatment stabilizes the crystals and homogenizes the lamellae. As the heat treatment method, heat setting treatment and/or heat relaxation treatment can be used. The heat setting treatment is a heat treatment in which the film is heated while maintaining its dimensions so as not to change. Thermal relaxation treatment is a heat treatment in which the film is thermally contracted in the MD direction or the TD direction while being heated. The heat setting treatment is preferably carried out by a tenter method or a roll method. For example, as a thermal relaxation treatment method, there is a method disclosed in JP-A No. 2002-256099. The heat treatment temperature is preferably within the range of T _CD to melting point of the polyolefin resin.

(8) Crosslinking treatment step, hydrophilization treatment step Further, the porous polyolefin film after bonding or stretching may be further subjected to crosslinking treatment and/or hydrophilization treatment.

For example, crosslinking treatment is performed by irradiating a porous polyolefin film with ionizing radiation such as A rays, B rays, Γ rays, and electron beams. In the case of electron beam irradiation, an electron beam dose of 0.1 to 100 MRAD is preferred, and an accelerating voltage of 100 to 300 KV is preferred. Crosslinking increases the meltdown temperature of porous polyolefin films.

The hydrophilization treatment can be performed by monomer grafting, surfactant treatment, corona discharge, etc. Monomer grafting is preferably carried out after crosslinking treatment.

Furthermore, a porous layer may be provided on at least one surface of the porous polyolefin film of the present invention to form a laminated porous polyolefin film. Examples of the porous layer include a porous layer formed using a filler-containing resin solution or a heat-resistant resin solution containing a filler and a resin binder.

Furthermore, the porous polyolefin film of the present invention can be suitably used for both batteries using an aqueous electrolyte and batteries using a non-aqueous electrolyte. Specifically, it can be preferably used as a separator for secondary batteries such as nickel-hydrogen batteries, nickel-cadmium batteries, nickel-zinc batteries, silver-zinc batteries, lithium secondary batteries, and lithium polymer secondary batteries. Among these, it is preferable to use it as a separator for lithium ion secondary batteries.

In a lithium ion secondary battery, a positive electrode and a negative electrode are stacked with a separator in between, and the separator contains an electrolyte (electrolyte). The structure of the electrode is not particularly limited, and conventionally known structures can be used. For example, an electrode structure (coin type) in which a disk-shaped positive electrode and a negative electrode are arranged to face each other, a flat plate-shaped positive electrode and a negative electrode, etc. The electrode structure may include an electrode structure in which the electrodes are alternately laminated (a laminated type), an electrode structure in which laminated strip-shaped positive electrodes and negative electrodes are wound (a wound type), and the like.

The current collector, positive electrode, positive electrode active material, negative electrode, negative electrode active material, and electrolytic solution used in the lithium ion secondary battery are not particularly limited, and conventionally known materials can be used in appropriate combinations.

Note that the present invention is not limited to the embodiments described above, and can be implemented with various modifications within the scope of the gist.

The present invention will be explained in more detail with reference to Examples, but the embodiments of the present invention are not limited to these Examples.
The evaluation methods, analysis methods, and materials used in the Examples are as follows.

(1) Film thickness The film thickness at 5 points was measured using a contact thickness meter (Lightmatic, manufactured by Mitutoyo Co., Ltd.) at randomly selected points within a 95 mm x 95 mm area of the porous polyolefin film. The average value of the film thickness was determined.

(2) Porosity Cut a 5cm square sample from a porous polyolefin film, find its volume (cm ³ ) and weight (g), and calculate from these and the polymer density (g/cm ³ ) using the following formula. did. The above measurements were performed at three randomly selected different points in the same film, and the average value of the porosity at the three points was determined.
Porosity = [(volume - weight / polymer density) / volume] x 100

(3) Maximum pore diameter and average pore diameter Using a palm porometer (manufactured by PMI, CFP-1500A), the maximum pore diameter and average pore diameter were measured in the order of dry-up and wet-up. For wet-up, pressure is applied to a porous polyolefin film sufficiently immersed in Galwick (trade name) manufactured by PMI, which has a surface tension of 15.6 dynes/cm, and the pore diameter calculated from the pressure at which air begins to penetrate is the maximum pore diameter. And so.

The average pore diameter was calculated from the pressure at the point where the curve showing the slope of 1/2 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 pressure and average pore diameter.
d=C・γ/P

In the above formula, "d (μm)" is the average pore diameter 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.

(4) Puncture strength when porosity is 50% and film thickness is converted to 1 μm Using a puncture meter made by MARUBISHI, a needle with a diameter of 1 mm and a spherical tip (radius of curvature R: 0.5 mm) is used to measure the film thickness T1 (μm). The maximum load was measured when a porous polyolefin film with a porosity of P1 (%) was punctured at a speed of 2 mm/sec. The measured value L1 (N) of the maximum load is converted to the maximum load L2 when the film thickness is 1 μm using the formula: L2 = L1 / T1 × 50 / (100 - P1). The thickness was calculated as 1 μm. The above measurements were carried out at three randomly selected different points in the same porous polyolefin film, and the average value of the puncture strength at a porosity of 50% and a film thickness of 1 μm was determined.

(5) Weight average molecular weight (Mw)
The weight average molecular weights of the ultra-high molecular weight polyethylene resin and the high-density polyethylene resin were determined by gel permeation chromatography (GPC) under the following conditions.

・Measuring device: WATERS CORPORATION GPC-150C
・Column: SHODEX UT806M manufactured by Showa Denko Co., Ltd.
・Column temperature: 135℃
・Solvent (mobile phase): O-dichlorobenzene ・Solvent flow rate: 1.0 mL/min ・Sample concentration: 0.1 wt% (dissolution conditions: 135°C/1H)
・Injection volume: 500μL
・Detector: WATERS CORPORATION differential refractometer (RI detector)
- Calibration curve: Created using a predetermined conversion constant from a calibration curve obtained using a monodisperse polystyrene standard sample.

(6) Maximum melt shrinkage rate The maximum melt shrinkage rate was determined by the following method using a thermomechanical analyzer (TMA/SS 6100, manufactured by Seiko Instruments). That is, the temperature was scanned under the conditions of sample shape: width 3 mm x length 10 mm, initial load: 19.6 mN, temperature scanning range: 30 to 210°C, and heating rate: 5°C/min. A plurality of different arbitrary regions were randomly extracted in the same porous polyolefin film. In each of the plurality of regions, the amount of shrinkage in the TD direction was measured at three points each, and the shrinkage rate at the point where the dimension shrunk the most was defined as the maximum melt shrinkage rate. The average value of the maximum melt shrinkage rates determined in each region was defined as the "maximum melt shrinkage rate in the TD direction." In addition, the "maximum melt shrinkage rate in the MD direction" was determined in the same manner.

(7) Tensile strength at break A tensile test was conducted using a tensile testing machine (Shimadzu Autograph Model AGS-J), and the strength at break of the sample was divided by the cross-sectional area of the sample before the test to obtain the tensile strength at break. The measurement conditions were: temperature: 23±2° C., sample shape: width 10 mm x length 50 mm, distance between chucks: 20 mm, and tension speed: 100 mm/min. The above measurements were carried out at three randomly selected points in the same film in the MD and TD directions, and the average value of the three points was calculated as the tensile strength at break in each direction (MD Tensile strength at break, TD tensile strength at break), the average value of all six points was taken as the average tensile strength at break.

(8) Tensile elongation at break A tensile test was performed using a tensile testing machine (Shimadzu Autograph AGS-J model), and the tensile elongation at break was determined by the gauge distance L0 (mm) of the test piece before the test, and the time at break. It was calculated from the gauge length L (mm) using the following formula. The measurement conditions were: temperature: 23±2° C., sample shape: width 10 mm x length 50 mm, distance between chucks: 20 mm, and tension speed: 100 mm/min. The above measurements were carried out at three randomly selected points in the same film in the MD and TD directions, and the average value of the three points was calculated as the tensile elongation at break in each direction ( MD tensile elongation at break, TD tensile elongation at break), the average value of all six points was taken as the average tensile elongation at break.
Tensile elongation at break (%) = [(L-L0)/L] x 100

(9) Crystallinity, melting point The sample was placed in the sample holder of a scanning differential calorimeter (DSC-System 7 type, manufactured by Perkin Elmer, Inc.) and held at 30°C for 1 minute in a nitrogen atmosphere. It was heated to 230°C at a rate of 10°C/min. A straight line passing through the point at 60°C and the point at 180°C on the DSC curve (melting curve) obtained during the temperature raising process is drawn as the baseline, and the amount of heat (unit: :J) and divided by the weight of the sample (unit: g) to obtain the heat of fusion ΔHm (unit: J/g).
In addition, the heat of fusion ΔHm and the temperature of the minimum value in the endothermic melting curve were similarly measured as the melting point.

(10) Appearance The appearance of the porous polyolefin film was evaluated visually. Those with small visual variations in thickness or color were rated "○", and those with large visual variations in thickness or color were rated "x".

(11) Air permeability resistance (sec/100cm ³ /20μm)
The air permeability resistance P1 (sec. /100cm ³ ) was converted into air permeability resistance P2 when the film thickness was 20 μm using the formula: P2=(P1×20)/T1.

(Example 1)
From 4.5 parts by mass of ultra-high molecular weight polyethylene resin (UHPE) having a weight average molecular weight (Mw) of 1.5×10 ⁶ , a melting point of 140.5°C, and a heat of fusion ΔHm of 175 J/g and 75 parts by mass of liquid paraffin. The compositions were blended to obtain a mixture. The obtained mixture was put into a high shear type "Laboplast Mill" manufactured by Toyo Seiki Seisakusho, and while maintaining the screw rotation speed at 150 rpm, it was melted and kneaded at a temperature of 180 ° C. for 1 minute, so that the Mw was 2.8 × 10 ⁵ 20.5 parts by mass of high-density polyethylene resin (HDPE) was added to "Laboplasto Mill" and melt-kneaded for 9 minutes under the above conditions to prepare a polyethylene resin composition.

The obtained polyethylene resin composition was molded into a sheet-like molded body using a press at 180° C. for 5 minutes and 10 MPa. The obtained molded body was cooled at 1 MPa using a press machine whose temperature was controlled to 15° C. to form a gel-like sheet. The obtained gel-like sheet was simultaneously biaxially stretched at a stretching temperature of 115° C. and a stretching speed of 1000 mm/min so that the stretching was 7 times in the MD direction and 7 times in the TD direction. The membrane after stretching was washed in a methylene chloride washing tank whose temperature was controlled to 20° C. to remove liquid paraffin. The washed membrane was dried in a drying oven adjusted to 20°C, and heat-set in an electric oven at 125°C for 10 minutes to obtain a porous polyolefin film. Table 1 shows the resin composition and film forming conditions, and Table 3 shows the properties of the obtained film.

(Example 2)
Consisting of 5.1 parts by mass of ultra-high molecular weight polyethylene resin having a weight average molecular weight (Mw) of 2.0×10 ⁶ , a melting point of 141.0°C, and a heat of fusion ΔHm of 174 J/g, and 71.5 parts by mass of liquid paraffin. The compositions were blended to obtain a mixture. The resulting mixture was put into a high-shear type "Laboplasto Mill" and melted and kneaded at a temperature of 190°C for 1 minute while maintaining the screw rotation speed at 150 ^rpm . 23.4 parts by mass of the resin was added to Labo Plastomill and melt-kneaded for 9 minutes under the above conditions to prepare a polyethylene resin composition. A porous polyolefin film was obtained in the same manner as in Example 1 except for the above. Table 1 shows the resin composition and film forming conditions, and Table 3 shows the properties of the obtained film.

(Example 3)
A composition consisting of 7.5 parts by mass of an ultra-high molecular weight polyethylene resin having a weight average molecular weight (Mw) of 2.0×10 ⁶ , a melting point of 140.5°C, and a heat of fusion ΔHm of 188 J/g, and 75 parts by mass of liquid paraffin. were blended to obtain a mixture. The resulting mixture was put into a high-shear type "Laboplasto Mill" and melted and kneaded at a temperature of 180°C for 1 minute while maintaining the screw rotation speed at 150 ^rpm . 17.5 parts by mass of the resin was added to Labo Plastomill and melt-kneaded for 9 minutes under the above conditions to prepare a polyethylene resin composition. A porous polyolefin film was obtained in the same manner as in Example 1 except for the above. Table 1 shows the resin composition and film forming conditions, and Table 3 shows the properties of the obtained film.

(Example 4)
A composition consisting of 7.5 parts by mass of an ultra-high molecular weight polyethylene resin having a weight average molecular weight (Mw) of 2.5×10 ⁶ , a melting point of 141.7°C, and a heat of fusion ΔHm of 186 J/g, and 75 parts by mass of liquid paraffin. were blended to obtain a mixture. The resulting mixture was put into a high-shear type "Laboplasto Mill" and melted and kneaded at a temperature of 180°C for 1 minute while maintaining the screw rotation speed at 150 ^rpm . 17.5 parts by mass of the resin was added to Labo Plastomill and melt-kneaded for 9 minutes under the above conditions to prepare a polyethylene resin composition. A porous polyolefin film was obtained in the same manner as in Example 1 except for the above. Table 1 shows the resin composition and film forming conditions, and Table 3 shows the properties of the obtained film.

(Comparative example 1)
A composition consisting of 7.5 parts by mass of an ultra-high molecular weight polyethylene resin having a weight average molecular weight (Mw) of 2.5×10 ⁶ , a melting point of 138.0°C, and a heat of fusion ΔHm of 178 J/g, and 75 parts by mass of liquid paraffin. were blended to obtain a mixture. The resulting mixture was put into a high-shear type "Laboplasto Mill" and melted and kneaded at a temperature of 180°C for 1 minute while maintaining the screw rotation speed at 150 ^rpm . 17.5 parts by mass of the resin was added to Labo Plastomill and melt-kneaded for 9 minutes under the above conditions to prepare a polyethylene resin composition. A porous polyolefin film was obtained in the same manner as in Example 1 except for the above. The resin composition and film forming conditions are shown in Table 2, and the properties of the obtained film are shown in Table 4.

(Comparative example 2)
A composition consisting of 7.5 parts by mass of an ultra-high molecular weight polyethylene resin having a weight average molecular weight (Mw) of 2.5×10 ⁶ , a melting point of 140.5°C, and a heat of fusion ΔHm of 175 J/g, and 75 parts by mass of liquid paraffin. were blended to obtain a mixture. The resulting mixture was put into a high-shear type "Laboplasto Mill" and melted and kneaded at a temperature of 180°C for 1 minute while maintaining the screw rotation speed at 150 ^rpm . 17.5 parts by mass of the resin was added to Labo Plastomill and melt-kneaded for 9 minutes under the above conditions to prepare a polyethylene resin composition.

The obtained polyethylene resin composition was molded into a sheet-like molded body using a press at 180° C. for 5 minutes and 10 MPa. The obtained molded body was cooled at 1 MPa using a press machine whose temperature was controlled to 15° C. to form a gel-like sheet. The obtained gel-like sheet was simultaneously biaxially stretched at a stretching temperature of 115°C and a stretching speed of 1000 mm/min so that the stretching was 5 times in the MD direction and 5 times in the TD direction. The membrane after stretching was washed in a methylene chloride washing tank whose temperature was controlled to 20° C. to remove liquid paraffin. The washed membrane was dried in a drying oven adjusted to 20°C, and heat-set in an electric oven at 125°C for 10 minutes to obtain a porous polyolefin film. The resin composition and film forming conditions are shown in Table 2, and the properties of the obtained film are shown in Table 4.

(Comparative example 3)
30 parts by mass of ultra-high molecular weight polyethylene resin with a weight average molecular weight (Mw) of 2.5×10 ⁶ , a melting point of 133.9°C, and a heat of fusion ΔHm of 147 J/g, and a high density Mw of 2.8×10 ⁵ A mixture was obtained by dry blending 100 parts by mass of a polyethylene resin (PE) composition consisting of 70 parts by mass of polyethylene resin. 25 parts by mass of the obtained mixture was put into a high shear type "Laboplasto Mill" manufactured by Toyo Seiki Seisakusho, and while maintaining the screw rotation speed at 150 rpm, 75 parts by mass of liquid paraffin was added to the "Laboplasto Mill", A polyethylene resin composition was prepared by melt-kneading for 10 minutes. A porous polyolefin film was obtained in the same manner as in Example 1 except for the above. The resin composition and film forming conditions are shown in Table 2, and the properties of the obtained film are shown in Table 4.

(Comparative example 4)
30 parts by mass of ultra-high molecular weight polyethylene resin with a weight average molecular weight (Mw) of 2.5×10 ⁶ , a melting point of 140.5° C., and a heat of fusion ΔHm of 179 J/g, and a high density Mw of 2.8×10 ⁵ A mixture was obtained by dry blending 100 parts by mass of a polyethylene resin (PE) composition consisting of 70 parts by mass of polyethylene resin. 25 parts by mass of the obtained mixture and 75 parts by mass of liquid paraffin were blended and put into a laboplast mill, and while maintaining the screw rotation speed at 50 rpm, they were melt-kneaded at a temperature of 190°C for 10 minutes to form a polyethylene resin composition. Prepared.

The obtained polyethylene resin composition was molded into a sheet-like molded body using a press at 180° C. for 5 minutes and 10 MPa. The obtained molded body was cooled at 1 MPa using a press machine whose temperature was controlled to 15° C. to form a gel-like sheet. The obtained gel-like sheet was simultaneously biaxially stretched at a stretching temperature of 115° C. and a stretching speed of 1000 mm/min so that the stretching was 7 times in the MD direction and 7 times in the TD direction. The membrane after stretching was washed in a methylene chloride washing tank whose temperature was controlled to 20° C. to remove liquid paraffin. The washed membrane was dried in a drying oven adjusted to 20°C, and heat-set in an electric oven at 125°C for 10 minutes to obtain a porous polyolefin film. The resin composition and film forming conditions are shown in Table 2, and the properties of the obtained film are shown in Table 4.

(Comparative example 5)
30 parts by mass of ultra-high molecular weight polyethylene resin with a weight average molecular weight (Mw) of 3.3×10 ⁶ , a melting point of 143.7° C., and a heat of fusion ΔHm of 205 J/g, and a high density Mw of 2.8×10 ⁵ A mixture was obtained by dry blending 100 parts by mass of a polyethylene resin (PE) composition consisting of 70 parts by mass of polyethylene resin. 20 parts by mass of the obtained mixture and 80 parts by mass of liquid paraffin were blended and put into a laboplast mill, and while maintaining the screw rotation speed at 50 rpm, they were melt-kneaded at a temperature of 190°C for 10 minutes to form a polyethylene resin composition. Prepared.

The obtained polyethylene resin composition was molded into a sheet-like molded body using a press at 180° C. for 5 minutes and 10 MPa. The obtained molded body was cooled at 1 MPa using a press machine whose temperature was controlled to 15° C. to form a gel-like sheet. The obtained gel-like sheet was simultaneously biaxially stretched at a stretching temperature of 115°C and a stretching speed of 1000 mm/min so that the stretching was 6 times in the MD direction and 6 times in the TD direction. A porous polyolefin film was obtained in the same manner as in Example 1 except for the above. The resin composition and film forming conditions are shown in Table 2, and the properties of the obtained film are shown in Table 4.

(Comparative example 6)
30 parts by mass of ultra-high molecular weight polyethylene resin with a weight average molecular weight (Mw) of 2.5×10 ⁶ , a melting point of 138.0° C., and a heat of fusion ΔHm of 178 J/g, and a high density Mw of 2.8×10 ⁵ A mixture was obtained by dry blending 100 parts by mass of a polyethylene resin (PE) composition consisting of 70 parts by mass of polyethylene resin. 25 parts by mass of the obtained mixture and 75 parts by mass of liquid paraffin were blended and put into a laboplast mill, and while maintaining the screw rotation speed at 150 rpm, they were melt-kneaded at a temperature of 180°C for 10 minutes to form a polyethylene resin composition. Prepared. A porous polyolefin film was obtained in the same manner as in Comparative Example 2 except for the above. The resin composition and film forming conditions are shown in Table 2, and the properties of the obtained film are shown in Table 4.

As shown in Tables 3 and 4, in Examples 1 to 4, by adding resin in portions, the resin contained ultra-high molecular weight polyethylene resin with high crystallinity and high melting point, good appearance, strength and shrinkage rate. A porous polyolefin film with excellent balance was obtained.

On the other hand, in Comparative Examples 1, 3, and 6, the ultra-high molecular weight polyethylene resin used had low crystallinity and a low melting point, so porous polyolefin films with good appearance were obtained, but they were not able to achieve both strength and shrinkage rate. was not completed. In Comparative Example 2 as well, it was not possible to achieve both strength and shrinkage rate due to low stretching ratio. In Comparative Examples 4 and 5, ultra-high molecular weight polyethylene resin with high crystallinity and high melting point was used, but since the raw materials were added all at once and kneading was insufficient, unmelted resin was scattered and the appearance was poor. was defective. In addition, the average flow pore diameter was clearly small, and it was recognized that the pore diameter of the porous polyolefin film had become smaller.

Claims (9)

A porous polyolefin film having a puncture strength of 350 mN/μm or more based on a porosity of 50%, and a maximum melt shrinkage rate in the width direction (TD direction) of 1% or more and 25% or less. The porous polyolefin film according to claim 1, having a porosity of 40% or more. The porous polyolefin film according to claim 1 or 2, wherein the porous polyolefin film has an average pore diameter of 40 nm or less. The porous material according to any one of claims 1 to 3, wherein both the tensile elongation at break in the machine direction (MD direction) and the tensile elongation at break in the width direction (TD direction) are 80% or more and 200% or less. polyolefin film. The porous polyolefin film according to any one of claims 1 to 4, characterized in that the film thickness is 15 μm or less. The average tensile strength at break, which is the average value of the tensile strength at break in the machine direction (MD direction) and the tensile strength at break in the width direction (TD direction), is 150 MPa or more, and the tensile elongation at break in the MD direction and the tensile strength at break in the TD direction are 150 MPa or more. The average tensile elongation at break, which is the average value of elongation, is 90% or more, and further, the average tensile strength at break (MPa) x the average tensile elongation at break (%) is 15000 (MPa x %) or more. The porous polyolefin film according to any one of claims 1 to 5. The porous polyolefin film according to any one of claims 1 to 6, characterized in that the porous polyolefin film contains a polyethylene resin as a main component. A battery separator using the porous polyolefin film according to any one of claims 1 to 7. A secondary battery using the battery separator according to claim 8.