JP4245239B2 - Porous film - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a porous film. More specifically, the present invention relates to a porous film that is preferably used as a battery separator or the like that is disposed between positive and negative electrodes of a battery to isolate them.
[0002]
[Prior art]
The porous film is used for battery separators, electrolytic capacitor diaphragms, moisture-permeable waterproof materials, various filters, and the like. In particular, battery separators are attracting attention as important components for lithium secondary batteries that are lightweight, have high electromotive force and high energy, and have little self-discharge. Is also expected.
[0003]
Since lithium lithium and lithium ions are used in the lithium secondary battery, a protic solvent cannot be used, and an aprotic solvent is used. For example, a solution obtained by dissolving an electrolyte such as LiPF ₆ , LiBF ₄ , or LiClO _{4 in} an organic solvent such as propylene carbonate, γ-butyrolactone, or sulfolane is used. These organic solvents have high surface tension, and the electrolyte solution is not sufficiently impregnated into the pores of the porous film made of polyolefin resin, which is usually used as a separator. There is a problem that resistance increases. In addition, even if the electrolyte is impregnated in the pores immediately after the battery is manufactured, the electrolyte is dissipated from the separator pores due to expansion and contraction of the electrode during repeated charge and discharge, and the local liquid It is a problem that the battery internal resistance may increase due to withering.
[0004]
As means for solving such a problem, for example, Japanese Patent Laid-Open No. 5-94812 proposes to treat the internal surface of the pores of the polyolefin microporous membrane and the membrane surface with a surfactant. However, with such means, the treated surfactant may elute into the electrolyte, increasing the internal resistance of the battery or causing a redox reaction on the electrode, which may deteriorate the characteristics of the battery. Is done.
[0005]
[Problems to be solved by the invention]
It is an object of the present invention to provide a porous film that, when used as a battery separator, has excellent wettability with respect to an electrolyte used in a battery, and can provide a battery with high cycle characteristics and safety. There is to do.
[0006]
[Means for Solving the Problems]
The gist of the present invention relates to a porous film for a battery separator comprising 70 to 98% by weight of a polyolefin resin and 2 to 30% by weight of an ethylene-ethyl acrylate copolymer resin and / or an ethylene-methyl acrylate copolymer resin. .
[0007]
DETAILED DESCRIPTION OF THE INVENTION
The porous film of the present invention includes an ethylene-ethyl acrylate copolymer resin and / or an ethylene-methyl acrylate copolymer resin (hereinafter abbreviated as an ethylene-alkyl acrylate copolymer resin) in addition to the polyolefin resin as a matrix. It has a great feature in that it contains. The polyolefin resin alone is insufficient in wettability with respect to an electrolyte solution using an aprotic organic solvent used as an electrolyte solution for a battery, but it contains an ethylene-alkyl acrylate copolymer resin. Remarkably improved. In addition, since the ethylene-alkyl acrylate copolymer resin is integrated with the polyolefin resin, the ethylene-alkyl acrylate copolymer resin is electrolyzed even when the porous film of the present invention is used as a battery separator. It does not elute into the liquid and deteriorates battery characteristics. This is considered to be due to the alkyl acrylate group of the ethylene-alkyl acrylate copolymer resin.
[0008]
The content of the alkyl acrylate in the ethylene-alkyl acrylate copolymer resin is usually preferably 5 to 35% by weight, and more preferably 9 to 25% by weight. The content of the alkyl acrylate is preferably 5% by weight or more in order to obtain the effect of improving the wettability, and 35% by weight or less in order to increase the compatibility with the polyolefin resin and prevent the local wettability from being improved. Is preferred. On the other hand, the ethylene content is preferably 65 to 95% by weight, and more preferably 75 to 91% by weight.
[0009]
The density of the ethylene-alkyl acrylate copolymer resin is preferably 920 to 950 kg / m ³ .
[0010]
The melting point of the ethylene-alkyl acrylate copolymer resin is preferably 55 to 100 ° C, and the melt flow rate (hereinafter referred to as MFR) under the conditions of a temperature of 190 ° C and a load of 2.16 kgf is 0.5 to 100 g / 10 min is preferred.
[0011]
In the porous film of the present invention, as a polyolefin resin as a matrix, when the porous film is used as a battery separator, if the strength is low, the positive electrode and the negative electrode come into contact with each other due to breakage, causing an internal short circuit and functioning as a battery. Since it is eliminated, the one having as high a film strength as possible is preferable. In this respect, the polyolefin resin preferably contains an ultrahigh molecular weight polyolefin resin. Examples of the ultrahigh molecular weight polyolefin resin include homopolymers and copolymers of olefins such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, and 1-hexene, and mixtures thereof. Then, from the viewpoint of increasing the strength of the resulting porous film, an ultrahigh molecular weight polyethylene resin is preferred.
[0012]
The weight average molecular weight of the ultrahigh molecular weight polyolefin resin is 5 × 10 ⁵ or more, preferably 1 × 10 ^{6 to} 20 × 10 ⁶ , more preferably 1 × 10 ⁶ to 15 × 10 ⁶ .
[0013]
The ultrahigh molecular weight polyolefin resin having a weight average molecular weight of 5 × 10 ⁵ or more, preferably 1 × 10 ⁶ or more is contained in the polyolefin resin in an amount of 1% by weight or more, preferably 30% by weight or more, more preferably 50 to 100% by weight. It is desirable that
[0014]
Examples of the resin that may be contained in the polyolefin resin in addition to the ultrahigh molecular weight polyolefin resin include the same olefin homopolymers, copolymers, and mixtures thereof as described above. Among these, high density polyethylene Resins are preferred. The weight average molecular weight of these resins is preferably less than 5 × 10 ⁵ , more preferably 1 × 10 ⁴ or more and less than 5 × 10 ⁵ , and particularly preferably 1 × 10 ^{4 to} 3 × 10 ⁵ .
[0015]
The composition of the polyolefin resin and the ethylene-alkyl acrylate copolymer resin is such that the polyolefin resin is 70 to 98% by weight, preferably 85 to 95% by weight, the ethylene-alkyl acrylate copolymer resin is 2 to 30% by weight, Preferably it is 5 to 15% by weight. When the blending amount of the ethylene-alkyl acrylate copolymer resin is less than 2% by weight, the wettability is not sufficiently improved, and when it exceeds 30% by weight, the strength and heat resistance of the porous film are insufficient. is there.
[0016]
The thickness of the porous film of the present invention is preferably 1 to 60 μm, more preferably 5 to 45 μm, and the porosity is preferably 25 to 70%.
[0017]
The porous film of the present invention was obtained by, for example, melt-kneading a resin composition containing a polyolefin resin, an ethylene-alkyl acrylate copolymer resin and a solvent, and molding the obtained melt-kneaded product into a sheet shape. It is obtained by rolling and removing the solvent after rolling the sheet-like molded product.
[0018]
The solvent used for the preparation of the resin composition is not particularly limited as long as it is excellent in solubility of polyolefin resin and ethylene-alkyl acrylate copolymer resin, but those having a freezing point of −10 ° C. or less are preferably used. It is done. Preferable specific examples of such a solvent include, for example, aliphatic or alicyclic hydrocarbons such as decane, decalin, and liquid paraffin, and mineral oil fractions having boiling points corresponding to these, among which liquid paraffin and the like Non-volatile solvents having a freezing point of −45 to −10 ° C. and a kinematic viscosity at 40 ° C. of 65 cst or less are more preferable.
[0019]
The mixing ratio of the polyolefin resin, ethylene-alkyl acrylate copolymer resin and solvent varies depending on the type of resin, solubility, kneading temperature, etc., so it cannot be determined unconditionally, but the resulting slurry resin composition is melt-kneaded. The amount of the polyolefin resin and the ethylene-alkyl acrylate copolymer resin is generally preferably 10 to 30% by weight of the resin composition.
[0020]
It should be noted that additives such as an antioxidant, an ultraviolet absorber, a dye, a nucleating agent, a pigment, and an antistatic agent are added to the resin composition as necessary within a range that does not impair the object of the present invention. be able to.
[0021]
The melt kneading of the resin composition is preferably performed by applying a sufficient shearing force to the resin composition in order to obtain sufficient entanglement of the polymer chains of the polyolefin resin and the ethylene-alkyl acrylate copolymer resin. Therefore, for the melt-kneading of the resin composition, a kneader or a twin screw extruder that can give a strong shearing force to the mixture is usually preferably used.
[0022]
The temperature at which the resin composition is melt-kneaded is not particularly limited as long as it is under an appropriate temperature condition, but is preferably 115 to 185 ° C. The temperature at the time of melt-kneading is preferably 115 ° C. or higher in order to sufficiently knead the resin composition and obtain sufficient entanglement of the polymer chains of the polyolefin resin and the ethylene-alkyl acrylate copolymer resin, with an appropriate viscosity. And in order to make sufficient shearing force act on a resin composition, 185 degrees C or less is preferable.
[0023]
The method of forming the melt-kneaded material into a sheet is not particularly limited. For example, the melt-kneaded material may be rapidly cooled by being sandwiched between metal plates cooled with dry ice and rapidly cooled to crystallize, and a T-die or the like is attached. After forming into a sheet shape using an extruder or the like, it may be cooled and crystallized.
[0024]
In addition, not only the surface layer of the obtained sheet-like molded product, but also finely crystallize the polyolefin resin and the ethylene-alkyl acrylate copolymer resin to the center, and have high strength, high porosity, uniform and dense In order to obtain a porous film having a pore structure, the melt-kneaded product is heated to a temperature below the freezing point of the solvent used in the resin composition, preferably −45 to −10 ° C., more preferably −40 to −15 ° C. It is desirable to cool, and the average cooling rate is preferably 50 ° C./min or more.
[0025]
The thickness of the sheet-like molded product is usually preferably 1 to 20 mm, and more preferably 2 to 10 mm.
[0026]
The sheet-like molded product thus obtained is stretched by melt-kneading, and the intertwined fibril fibers return to a hair-globule to form thick fibers and large through-holes are formed in the sheet-like molded product. In order to maintain the crystal structure of the polyolefin resin and the ethylene-alkyl acrylate copolymer resin by immediately subjecting to a rolling treatment described later or storing at a temperature below the freezing point of the solvent used. preferable.
[0027]
The rolling process can be performed using, for example, a heat press. By rolling, the thickness of the sheet-like molded product can be made uniform, and a porous film having higher strength can be obtained. 0.1-3 mm is preferable and, as for the thickness of the sheet-like molding after a rolling process, 0.2-0.5 mm is more preferable.
[0028]
The stretching method is not particularly limited, and may be a normal tenter method, roll method, inflation method, or a combination of these methods. In addition, any method such as uniaxial stretching or biaxial stretching can be applied. In the case of biaxial stretching, either longitudinal or transverse simultaneous stretching or sequential stretching may be used, but longitudinal and transverse simultaneous stretching is preferable.
[0029]
The stretch ratio of the rolled sheet-like molded product is preferably 2 to 20 times in one direction, and the surface stretch ratio is preferably 4 to 400 times.
[0030]
The temperature during the rolling treatment and the stretching treatment is preferably a temperature of the melting point of the polyolefin resin of −30 ° C. or more and a melting point of the polyolefin resin of + 5 ° C. or less, and a temperature of the melting point of the polyolefin resin of −25 ° C. or more and the melting point of the polyolefin resin or less. More preferred. In the present specification, the melting point of the polyolefin resin refers to an onset temperature in DSC measurement. As other rolling treatment and stretching treatment conditions, commonly used known conditions can be adopted.
[0031]
The solvent removal treatment is a step of removing the solvent from the sheet-like molded product to form a porous structure, and can be performed, for example, by washing the sheet-like molded product with a solvent to remove the remaining solvent. The solvent can be appropriately selected according to the solvent used for the preparation of the resin composition. Specifically, hydrocarbons such as pentane, hexane, heptane and decane, chlorination such as methylene chloride and carbon tetrachloride, etc. Easily volatile solvents such as hydrocarbons, ethers such as diethyl ether, dioxane and the like can be mentioned, and these can be used alone or in admixture of two or more. The method of desolvation treatment using such a solvent is not particularly limited, and examples thereof include a method of extracting a solvent by immersing a sheet-like molded product in a solvent, a method of showering the solvent on the sheet-like molded product, and the like. .
[0032]
In the present invention, for example, the sheet-shaped molded product may be subjected to a solvent removal treatment and then subjected to a stretching treatment, or the sheet-shaped molded material may be stretched as it is, and then the solvent removal treatment may be performed. Alternatively, a mode in which the solvent removal treatment is performed before the stretching treatment, and the solvent removal treatment is performed again after the stretching treatment to remove the residual solvent may be employed.
[0033]
The porous film of the present invention is obtained by subjecting the porous film thus obtained to a heat-setting treatment for preventing thermal contraction of the film, if necessary, and fixing the shape. There may be.
[0034]
Since the porous film of the present invention is very excellent in wettability with respect to the electrolyte of a battery that is usually used, it can be suitably used as a battery separator.
[0035]
【Example】
EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited to these examples. Various characteristics are measured as follows.
[0036]
[Melting point]
A differential scanning calorimeter “DSC-200” manufactured by Seiko Denshi Kogyo Co., Ltd. is used and measured at a rate of temperature increase of 10 ° C./min, and the onset temperature is defined as the melting point.
[0037]
[MFR]
Based on a method according to JIS K7210, the measurement is performed under conditions of a temperature of 190 ° C. and a load of 2.16 kgf.
[0038]
[Film thickness]
Using a 1/10000 mm thickness gauge, the thickness of the porous film is arbitrarily measured at five points in the range from the end to the center, and the average value is obtained.
[0039]
[Porosity]
The weight is calculated from the weight W (g) per area S (cm ² ) of the porous film, the film thickness t (μm), and the resin density d (g / cm ³ ) by the following equation.
[0040]
Porosity (%) = (1-10 ⁴ × W / S / t / d) × 100
[0041]
[Puncture strength]
The piercing strength is determined by performing a needle piercing test using a compression tester “KES-G5” manufactured by Kato Tech Co., Ltd., reading the maximum load from the load displacement curve obtained by the measurement, did. A needle having a diameter of 1.0 mm and a tip curvature radius of 0.5 mm was used, and the piercing speed was 2 mm / second.
[0042]
Example 1
15 parts by weight of ultrahigh molecular weight polyethylene resin (weight average molecular weight: 2 × 10 ⁶ , melting point: 133 ° C.), ethylene-ethyl acrylate copolymer (ethyl acrylate content: 25% by weight, MFR: 5 g / 10 min, density) : 1 part by weight of 940 kg / m ³ , melting point: 75 ° C. was added to 85 parts by weight of liquid paraffin (solidifying point: −15 ° C., kinematic viscosity at 40 ° C .: 59 cst), and mixed uniformly in a slurry. The resin composition was charged into a small kneader, heated at a temperature of 160 ° C. for about 60 minutes, and melt-kneaded to obtain a melt-kneaded product composed of an ultrahigh molecular weight polyethylene resin, an ethylene-ethyl acrylate copolymer resin, and a solvent. This melt-kneaded product was sandwiched between pre-cooled metal plates, rapidly cooled to −15 ° C., and ultrahigh molecular weight polyethylene was crystallized to obtain a gel-like sheet-like molded product having a thickness of 6 mm. Next, the obtained sheet-like molded product was rolled by a heat press at a temperature of 115 ° C. to a thickness of about 0.2 mm, and then simultaneously biaxially stretched 4 × 4 times in length and width at a temperature of about 120 ° C. did. The stretched sheet-like molded product was immersed in hexane, and liquid paraffin as a solvent was extracted (that is, desolventized) to obtain a porous film made of ultrahigh molecular weight polyethylene and an ethylene-ethyl acrylate copolymer. Furthermore, this porous film was fixed to a frame made of SUS and subjected to heat setting at 134 ° C. for 1 hour to obtain a porous film having a thickness of 20 μm and a porosity of 30%. The resulting porous film had a puncture strength of 1200 gf per 25 μm thickness.
[0043]
Example 2
In Example 1, instead of ethylene-ethyl acrylate copolymer, ethylene-methyl acrylate copolymer (methyl acrylate content: 20% by weight, MFR: 2 g / 10 min, density: 943 kg / m ³ , melting point: 77 ° C. ) Was used in the same manner as in Example 1 to obtain a porous film made of ultrahigh molecular weight polyethylene and ethylene-methyl acrylate copolymer having a thickness of 20 μm and a porosity of 31%. The obtained porous film had a puncture strength of 1150 gf per 25 μm thickness.
[0044]
Comparative Example 1
A porous film having a thickness of 20 μm and a porosity of 39% made of only ultrahigh molecular weight polyethylene was obtained in the same manner as in Example 1 except that the ethylene-ethyl acrylate copolymer was not used. It was. The puncture strength per 25 μm thickness of the obtained porous film was 1300 gf.
[0045]
Comparative Example 2
Except for changing the amount of the ultrahigh molecular weight polyethylene resin to 10 parts by weight and the amount of the ethylene-ethyl acrylate copolymer to 5 parts by weight, respectively, the thickness was 22 μm and the porosity was the same as in Example 1. A 27% porous film was obtained. The obtained porous film had a piercing strength per 25 μm thickness of 600 gf, which was very low.
[0046]
Test example 1
A pair of platinum mesh electrodes (40 mm x 50 mm) are placed at a distance of 3 cm, and an electrolytic solution tank in which the sample holder can be fixed in the center between the electrodes is mixed with ethylene carbonate and dimethyl carbonate as a volume ratio. A solution prepared to 2: 1 was added. Thereafter, the completely dried porous film is inserted into an electrolytic solution tank held at a dried sample holder and adjusted to 25 ° C., and is sampled every 0.2 to 0.3 seconds to elapse the membrane resistance. Changes were measured. In addition, the measurement of resistance measured the alternating current resistance of frequency 10kHz with LCR meter "KC546" (made by KOKYO ELECTRIC).
[0047]
The measurement results of the membrane resistance of the porous films obtained in Examples 1 and 2 and Comparative Example 1 are shown in FIG. From this result, the porous films obtained in Examples 1 and 2 reached the original membrane resistance in about 5 seconds, and compared with the porous film obtained in Comparative Example 1, the wettability was very good. I know that there is.
[0048]
【The invention's effect】
According to the present invention, there is provided a porous film that can be used as a battery separator and has excellent wettability with respect to an electrolytic solution used in the battery, and can obtain a battery having high cycle characteristics and safety. It became possible.
[Brief description of the drawings]
FIG. 1 is a graph showing measurement results of membrane resistance of porous films obtained in Examples 1 and 2 and Comparative Example 1. FIG. In addition, (circle), (square), and ● in a graph show the measurement result of the porous film of Example 1, Example 2, and the comparative example 1, respectively.

Claims (4)

A porous film for a battery separator comprising 70 to 98% by weight of a polyolefin resin and 2 to 30% by weight of an ethylene-ethyl acrylate copolymer resin and / or an ethylene-methyl acrylate copolymer resin. 2. The porous film according to claim 1, wherein the polyolefin resin contains 1% by weight or more of an ultrahigh molecular weight polyolefin resin having a weight average molecular weight of 5 × 10 ⁵ or more. The porous film according to claim 2, wherein the ultrahigh molecular weight polyolefin resin is an ultrahigh molecular weight polyethylene resin . The porous film according to claim 1, wherein the porous film has a thickness of 1 to 60 μm .

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