JP4839882B2 - Sheet made of polyolefin resin composition, porous film, and battery separator - Google Patents

Description

  The present invention relates to a sheet comprising a polyolefin resin composition, a porous film, and a battery separator.

Porous films are used in various applications such as diapers, medical sheets, and battery separators. In particular, a porous film used as a battery separator is required to be thin and uniform. As such a porous film, a high molecular weight polyolefin having a weight average molecular weight of 5 × 10 ⁵ or more, a thermoplastic resin having a weight average molecular weight of 2 × 10 ⁴ or less, and fine particles are kneaded and formed into a sheet shape. A porous film formed by stretching the sheet is disclosed (see Patent Document 1).

JP 2002-69221 A

  However, when a porous film is produced by the method described in Patent Document 1, when a conventional inorganic filler is used, a thin portion may occur in the obtained porous film. An object of this invention is to provide the sheet | seat which consists of polyolefin resin composition containing a polyolefin resin and an inorganic filler suitable for manufacture of a homogeneous porous film without a thin part. Another object of the present invention is to provide a homogeneous porous film and laminated porous film having no thin portion. Furthermore, an object of the present invention is to provide a homogeneous battery separator having no thin portion.

  That is, the present invention is a sheet comprising a polyolefin resin composition containing 100 parts by weight of a polyolefin resin and 100 to 400 parts by weight of an inorganic filler with respect to 100 parts by weight of the polyolefin resin, The sheet is made of a polyolefin resin composition having an average particle diameter of 0.02 to 1 μm and an acid insoluble part contained in the inorganic filler of 200 ppm or less. Moreover, this invention is a laminated porous film by which the heat resistant resin layer containing ceramic powder and the heat resistant resin containing a nitrogen element is laminated | stacked on the said porous film. Furthermore, this invention is the said porous film or laminated porous film which is a battery separator.

By producing a porous film using a sheet comprising the polyolefin resin composition of the present invention, a homogeneous porous film having no thin portion can be produced. In addition, the porous film, laminated porous film and battery separator of the present invention are all homogeneous with no thin portions.

  The sheet of the present invention comprises a polyolefin resin composition containing 100 parts by weight of a polyolefin resin and 100 to 400 parts by weight of an inorganic filler with respect to 100 parts by weight of the polyolefin resin. The inorganic filler content in the polyolefin resin composition is preferably 150 to 350 parts by weight with respect to 100 parts by weight of the polyolefin resin. When the amount of the inorganic filler in the polyolefin resin composition is less than 100 parts by weight, the inorganic filler is removed when the inorganic filler is removed from the sheet made of the polyolefin resin composition to produce a porous film. It tends to be difficult to remove the agent in a short time. On the other hand, when it exceeds 300 parts by weight, the sheet strength tends to be weak and the handling tends to be difficult.

  The thickness of the sheet comprising the polyolefin resin composition in the present invention is usually 5 to 200 μm, preferably 5 to 100 μm. A sheet having such a thickness can be removed efficiently in a short time when the inorganic filler is removed from the sheet to produce a porous film, and the sheet is stretched when the sheet is stretched. It's easy to do.

  The average particle size of the inorganic filler used in the present invention is 0.02 to 1 μm, preferably 0.05 to 0.8 μm, and more preferably 0.05 to 0.5 μm. When the average particle diameter of the inorganic filler exceeds 1 μm, when the porous film is produced using a sheet made of a polyolefin resin composition containing the inorganic filler, the resulting porous film is ion-permeable. It becomes inferior to. When the average particle size of the inorganic filler is less than 0.02 μm, it is difficult to obtain a sheet in which the inorganic filler is uniformly dispersed. The porous film obtained using such a sheet has a pin Holes (small holes) and thin-walled parts are formed, or film breaks during stretching. In addition, the average particle diameter of an inorganic filler is an average value of the diameter of the particle | grains recognized in the 30000 time large visual field using a scanning electron microscope SEM (Hitachi S-4200).

  The inorganic filler used in the present invention is an inorganic filler having an acid insoluble part content of 200 ppm or less, preferably 150 ppm or less, more preferably 100 ppm or less. In the present invention, when an inorganic filler having an acid-insoluble part content of more than 200 ppm is used, a porous film can be obtained by producing a porous film using a sheet made of a polyolefin resin composition containing the inorganic filler. A thin-walled portion will occur in the adhesive film.

  Examples of the inorganic filler used in the present invention include calcium carbonate, magnesium carbonate, barium carbonate, zinc oxide, calcium oxide, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and calcium sulfate, which are essentially soluble in acids. Is mentioned. In the present invention, calcium carbonate is used because it is inexpensive, easily available in various particle sizes depending on the application, and easily removed with an acidic aqueous solution when the inorganic filler is removed in a later step. It is preferable to use precipitated calcium carbonate, and more preferable to use precipitated calcium carbonate.

  The inorganic filler used in the present invention is preferably subjected to a surface treatment from the viewpoint of dispersibility in a polyolefin-based resin and stretchability when a sheet containing the inorganic filler is stretched. Examples of the surface treatment agent include higher fatty acids such as palmitic acid, oleic acid, stearic acid, octadecanoic acid, lauric acid, myristic acid, mixtures thereof, and metal salts thereof. In addition to these, a small amount of higher fatty acid esters such as ethyl oleate and ethyl stearate may be used in combination as a surface treatment agent. It is preferable that the surface treatment agent used for the surface treatment of the inorganic filler is about 1 to 10 parts by weight with respect to 100 parts by weight of the inorganic filler.

It is estimated that the acid insoluble part contained in the inorganic filler is an impurity contained in the raw material itself or an impurity mixed in the inorganic filler manufacturing process. Although the amount of the acid-insoluble part contained in the inorganic filler is extremely small, many of them have a relatively large diameter. In order to obtain an inorganic filler having an acid-insoluble portion of 200 ppm or less, for example, an inorganic filler having a small particle size obtained by classifying commercially available inorganic fine particles into one having a large particle size and one having a small particle size can be mentioned. . The classification method is not particularly limited, and a dry classifier such as a well-known air classifier, mesh classifier, or cyclone classifier can be used, but an air classifier is preferably used. In wind classification, an inorganic filler is put into a classification rotor, the classification rotor is rotated, and centrifugal force and centripetal force due to airflow are applied to the inorganic fine particles. It is a method of dividing into fine powder. The classification point can be easily changed by changing the air volume and the rotation speed. The classification point is preferably 0.5 to 50 μm, and more preferably 1 to 20 μm. The classification point can be determined by the following equation.
D = c / ν√ (Q / ρ)
Where D: classification point (μm) c: constant, ν: rotational speed (rmp), Q: air volume (m ² / h)
In the above equation, the constant c is a value determined by the apparatus.

  Examples of the polyolefin resin used in the present invention include homopolymers of olefins such as ethylene, propylene, butene, and hexene, copolymers of these monomers, and copolymers of these monomers and non-olefin monomers. Specific examples of the polyolefin resin include low density polyethylene, linear polyethylene (ethylene-α-olefin copolymer), ethylene resin such as high density polyethylene, and propylene resin such as polypropylene and ethylene-propylene copolymer. Examples include poly (4-methylpentene-1), poly (butene-1), and ethylene-vinyl acetate copolymer. When the porous film of the present invention is used as a battery separator, the polyolefin resin is preferably an ethylene resin because the shutdown temperature can be about 120 to 150 ° C.

  The polyolefin resin used in the present invention preferably contains a polyolefin having a molecular chain length of 2850 nm or more (hereinafter referred to as an ultra-high molecular chain length polyolefin). A porous film obtained by removing inorganic filler from a sheet made of a polyolefin resin composition containing the polyolefin resin by using a polyolefin resin containing such an ultra-high molecular chain length polyolefin has high strength. In particular, when used as a battery separator, a battery having a low internal resistance can be obtained. The ultra high molecular chain length polyolefin contained in the polyolefin resin is preferably 10% by weight or more in the polyolefin resin, more preferably 20% by weight or more, and further preferably 30% by weight or more. preferable.

  The polyolefin resin composition used in the present invention preferably contains an olefin wax having a weight average molecular weight of 700 to 6000. Olefinic wax is usually solid at 25 ° C. The polyolefin resin composition containing a wax has improved stretchability, and the resulting porous film has excellent strength. The content of the olefin wax in the polyolefin resin composition is preferably 5 to 100 parts by weight, and preferably 10 to 70 parts by weight with respect to 100 parts by weight of the polyolefin resin contained in the resin composition. Is more preferable.

  Examples of olefin waxes include ethylene resin waxes such as low density polyethylene, linear polyethylene (ethylene-α-olefin copolymer), high density polyethylene, and propylene resin waxes such as polypropylene and ethylene-propylene copolymer. , Poly (4-methylpentene-1), poly (butene-1), and ethylene-vinyl acetate copolymer wax.

  In the present invention, the molecular chain length, weight average molecular chain length, molecular weight, and weight average molecular weight of a polyolefin resin or olefin wax can be measured by GPC (gel permeation chromatography). In addition, the content (% by weight) of the ultra-high molecular chain length polyolefin in the specific polyolefin resin can be obtained by integration of a molecular weight distribution curve obtained by GPC measurement.

The molecular chain length of the polyolefin resin is a molecular chain length in terms of polystyrene as measured by GPC (gel permeation chromatography), and more specifically is a parameter determined by the following procedure. As a mobile phase for GPC measurement, a solvent capable of dissolving both an unknown sample to be measured and standard polystyrene having a known molecular weight is used. First, GPC measurement of a plurality of types of standard polystyrenes having different molecular weights is performed to determine the retention time of each standard polystyrene. The molecular chain length of each standard polystyrene is determined using the polystyrene Q factor, and thereby the molecular chain length of each standard polystyrene and the corresponding retention time are known. The molecular weight, molecular chain length, and Q factor of standard polystyrene are in the following relationship.
Molecular weight = molecular chain length × Q factor Next, GPC measurement of an unknown sample is performed to obtain a retention time-eluting component amount curve. In the standard polystyrene GPC measurement, when the molecular chain length of the standard polystyrene that was the holding time T is L, the “polystyrene converted molecular chain length” of the component that was the holding time T in the GPC measurement of the unknown sample is L. To do. Using this relationship, the polystyrene-reduced molecular chain length distribution of the unknown sample (relation between the polystyrene-reduced molecular chain length and the eluted component amount) is determined from the retention time-eluted component amount curve of the unknown sample.

  In the present invention, the amount of polyolefin having a molecular chain length of 2850 nm or more in the polyolefin resin corresponds to a molecular chain length of 2850 nm or more with respect to the value obtained by integrating the molecular chain length-elution component amount curve obtained by the above method over the entire range. It can be obtained as a ratio of values integrated over the range to be performed.

  The method for producing the polyolefin resin composition in the present invention is not particularly limited, but a material constituting the polyolefin resin composition such as a polyolefin resin or an inorganic filler is mixed with a mixing device such as a roll, a Banbury mixer, a single screw extruder, two A polyolefin resin composition is obtained by mixing using a shaft extruder or the like. When mixing the materials, additives such as an antioxidant, a nonionic surfactant, an ultraviolet absorber, a flame retardant, a fatty acid ester, and a stabilizer may be added as necessary.

  The manufacturing method of the sheet | seat which consists of polyolefin resin composition of this invention is not specifically limited, It can manufacture by sheet | seat shaping | molding methods, such as an inflation process, a calendar process, T-die extrusion process, and a Skyf method. Since a sheet having excellent surface smoothness and high film thickness accuracy can be obtained, it is preferably produced by the following method. In particular, when the polyolefin resin in the polyolefin resin composition contains an ultra high molecular chain length polyolefin, it is preferable to produce a sheet by the following method.

  A preferred method for producing a sheet comprising a polyolefin resin composition is a polyolefin resin composition using a pair of rotational molding tools adjusted to a surface temperature higher than the melting point of the polyolefin resin contained in the polyolefin resin composition. This is a method of rolling a product. The surface temperature of the rotary forming tool is preferably (melting point + 5) ° C. or higher. Further, the upper limit of the surface temperature is preferably (melting point + 20) ° C. or less, and more preferably (melting point + 15) ° C. or less. Examples of the pair of rotary forming tools include a roll and a belt. The peripheral speeds of the two rotary forming tools do not necessarily have to be exactly the same peripheral speed, and the difference between them may be about ± 5% or less. The melting point of the polyolefin resin can be determined from the peak temperature in DSC (differential scanning calorimetry). When there are a plurality of peaks, the melting point is the peak temperature having the largest heat of fusion ΔH (J / g). When a porous film is produced using the sheet of the present invention obtained by such a method, a porous film excellent in strength, ion permeation, air permeability and the like can be obtained.

  When a polyolefin resin composition is rolled with a pair of rotary molding tools, the polyolefin resin composition discharged in a strand form from an extruder may be directly introduced between the pair of rotary molding tools, and once pelletized. The polyolefin resin composition may be used.

  The porous film of the present invention is obtained by removing the inorganic filler contained in the sheet from the polyolefin resin composition and then stretching, or removing the inorganic filler after stretching the sheet. Can do. Since the porous film which is more excellent in the uniformity of film thickness is obtained, it is preferable to manufacture by the former method. Hereinafter, the former method will be described in detail, but the liquid used for removing the inorganic filler, the removal method, and the stretching method are the same in the latter case. The sheet used when producing the porous film may be a multilayer sheet in which two or more sheets made of the polyolefin resin composition of the present invention are laminated.

  When removing the inorganic filler from the sheet made of the polyolefin resin composition, a liquid is used. Although the liquid to be used is suitably selected according to the kind of inorganic filler in a sheet | seat, when an inorganic filler is a calcium carbonate, acidic aqueous solution can be used. Examples of the method for removing the inorganic filler include a method in which the liquid is showered on the sheet, a method in which the sheet is immersed in a tank containing the liquid, and the like. The method of removing the inorganic filler by the liquid may be batch type or continuous type, but from the viewpoint of productivity, the continuous type is preferable. The method of conveying a sheet | seat and making it pass in a liquid is mentioned. When the liquid is an acidic or alkaline aqueous solution, the sheet from which the inorganic filler has been removed is preferably further washed with water. The degree of washing depends on the use of the porous film, but usually the washing may be carried out to such an extent that dissolved salt or the like does not precipitate. The sheet from which the inorganic filler has been removed is usually dried within a time and temperature range in which the physical properties of the sheet do not change. It is preferable that about 100 to 20000 ppm of the inorganic filler remains in the sheet from which the inorganic filler has been removed by the method of the present invention. A film in which a small amount of the inorganic filler remains is stretched by a method described later to form a porous film, and when used as a battery separator, even if the polyolefin resin constituting the porous sheet melts, a short circuit between the electrodes Expected to prevent the effects. Moreover, the porous film obtained by extending | stretching the sheet | seat with which a small amount of inorganic fillers remain | survive is excellent in permeability rather than the case where an inorganic filler is removed completely. The reason for this is not clear, but it is thought that the film is hardly crushed in the film thickness direction due to the trace amount of filler remaining in the film.

  In the case where the inorganic filler is calcium carbonate and an acidic aqueous solution is used when removing calcium carbonate from the sheet, a surfactant, methanol, ethanol, isopropanol, It is preferable to add a small amount of a water-soluble organic solvent such as acetone or N-methylpyrrolidone, but it is preferable to add a surfactant without adding an organic solvent from the viewpoint of the environment. Examples of the surfactant include known nonionic surfactants, cationic surfactants, anionic surfactants, and the like, but nonionic surfactants are preferable. Nonionic surfactants have the advantage that they are difficult to hydrolyze even when the aqueous liquid is strongly alkaline (pH 11 or higher) or strongly acidic (pH 3 or lower). Examples of the nonionic surfactant include polyoxyethylene alkyl ether, polyoxyethylene-polyoxypropylene alkyl ether, polyoxyethylene alkylphenyl ether, polyethylene glycol fatty acid ester, polyoxyethylene alkylamine / fatty acid amide, and the like. . The amount of the nonionic surfactant added to the aqueous liquid is 0.05 to 10 from the balance between the effect of increasing the inorganic filler removal rate and the efficiency of removing the surfactant from the film after removing the inorganic filler. It is preferable to set it as weight%.

  The hydrophilic / lipophilic balance (HLB) of the nonionic surfactant used in the present invention is preferably in the range of 3 to 18, more preferably in the range of 5 to 15. HLB is a value indicating a balance between hydrophilicity and hydrophobic strength. If the HLB is too small, the solubility in water tends to be poor. Conversely, if the HLB is too large, the solubility in water is sufficient, but the hydrophobicity is low, so it tends to take time to penetrate into the sheet. is there.

HLB can be calculated by the following Griffin equation.
HLB = ((molecular weight of hydrophilic group in surfactant) / (molecular weight of entire surfactant)) × (100/5)
For surfactant HLB whose HLB cannot be calculated using the Griffin equation, oil is emulsified with the surfactant whose HLB is unknown, and a plurality of surfactants whose HLB is known (those with different HLB values are used) ) By emulsifying the same oil and comparing them. The HLB of the surfactant with the known HLB in which the oil emulsification state is the same as the surfactant with the unknown HLB is defined as the HLB of the surfactant with the unknown HLB.

  A porous film can be produced by stretching a sheet obtained by removing the inorganic filler from the sheet made of the polyolefin resin composition by the method as described above, using a tenter, a roll, an autograph or the like. From the viewpoint of air permeability of the obtained porous film, the draw ratio is preferably 2 to 12 times, and more preferably 4 to 10 times. The stretching temperature is usually a temperature not lower than the softening point and not higher than the melting point of the polyolefin resin, preferably in the range of (melting point−50) ° C. to melting point. By stretching at a temperature in such a range, a porous film excellent in air permeability and ion permeability can be obtained. For example, when the polyolefin resin composition to be used is composed of a polyolefin resin mainly composed of polyethylene, the stretching temperature is preferably 80 to 130 ° C, more preferably 90 to 115 ° C. Moreover, it is preferable to heat set after extending | stretching. The heat setting temperature is preferably performed at a temperature lower than the melting point of the polyolefin resin.

  The porous film of the present invention obtained by the method as described above has no thin part and is homogeneous, and is suitable for a battery separator.

  In the present invention, at least one surface of the porous film obtained by the method as described above is coated with a coating solution containing ceramic powder and a heat-resistant resin containing nitrogen element to laminate a heat-resistant resin layer, and the laminated porosity It can be a film. The heat resistant resin layer may be provided on one side of the porous film, or may be provided on both sides. Such a laminated porous film is suitable as a separator for a non-aqueous electrolyte battery, particularly a separator for a lithium ion secondary battery, because it has excellent film thickness uniformity, heat resistance, strength, and air permeability (ion permeability). Can be used for

  The heat-resistant resin is a polymer containing a nitrogen atom in the main chain, and particularly those containing an aromatic ring are preferred from the viewpoint of heat resistance. For example, aromatic polyamide (hereinafter sometimes referred to as “aramid”), aromatic polyimide (hereinafter sometimes referred to as “polyimide”), aromatic polyamideimide and the like can be mentioned. Examples of the aramid include meta-oriented aromatic polyamide (hereinafter sometimes referred to as “meta-aramid”) and para-oriented aromatic polyamide (hereinafter sometimes referred to as “para-aramid”), and has a uniform film thickness and air permeability. Para-aramid is preferable because it is easy to form a porous heat-resistant resin layer having excellent resistance.

  Para-aramid is obtained by condensation polymerization of a para-oriented aromatic diamine and a para-oriented aromatic dicarboxylic acid halide, and the amide bond is in the para position of the aromatic ring or an oriented position equivalent thereto (for example, 4, 4′- Biphenylene, 1,5-naphthalene, 2,6-naphthalene and the like, which are substantially composed of repeating units bonded in the opposite direction (orientation positions extending coaxially or in parallel). Specifically, poly (paraphenylene terephthalamide), poly (parabenzamide, poly (4,4′-benzanilide terephthalamide), poly (paraphenylene-4,4′-biphenylenedicarboxylic amide), poly (para Para-oriented or para-oriented, such as phenylene-2,6-naphthalenedicarboxylic acid amide), poly (2-chloro-paraphenylene terephthalamide), paraphenylene terephthalamide / 2,6-dichloroparaphenylene terephthalamide copolymer The para-aramid which has a structure according to is illustrated.

In the present invention, para-aramid is dissolved in a polar organic solvent and used as a coating solution. The polar organic solvent is, for example, a polar amide solvent or a polar urea solvent, and specifically, N, N-dimethylformamide, N, N-dimethylacetamide, N-methyl-2-pyrrolidone, tetramethylurea, etc. However, it is not limited to these.
From the viewpoint of coating properties, the para-aramid is preferably a para-aramid having an intrinsic viscosity of 1.0 dl / g to 2.8 dl / g, and more preferably an intrinsic viscosity of 1.7 dl / g to 2.5 dl / g. . If the intrinsic viscosity is less than 1.0 dl / g, the strength of the formed heat-resistant resin layer may be insufficient. If the intrinsic viscosity exceeds 2.8 dl / g, it may be difficult to obtain a stable para-aramid-containing coating solution. The intrinsic viscosity here is a value measured by dissolving para-aramid once precipitated and making it into a para-aramid sulfuric acid solution, and is a value serving as a so-called index of molecular weight. From the viewpoint of coating properties, the para-aramid concentration in the coating solution is preferably 0.5 to 10% by weight.

  In order to improve the solubility of the resulting para-aramid in a solvent, it is preferable to add an alkali metal or alkaline earth metal chloride during the para-aramid polymerization. Specific examples include lithium chloride or calcium chloride, but are not limited thereto. The amount of the chloride added to the polymerization system is preferably in the range of 0.5 to 6.0 mol, more preferably in the range of 1.0 to 4.0 mol, per 1.0 mol of the amide group produced by condensation polymerization. . If the chloride is less than 0.5 mol, the solubility of the resulting para-aramid may be insufficient, and if it exceeds 6.0 mol, the amount of chloride dissolved in the solvent may be substantially exceeded, which may not be preferable. is there. In general, if the alkali metal or alkaline earth metal chloride is less than 2% by weight, the solubility of para-aramid may be insufficient. If it exceeds 10% by weight, the alkali metal or alkaline earth metal chloride may be insufficient. May not dissolve in polar organic solvents such as polar amide solvents or polar urea solvents.

  The polyimide used in the present invention is preferably a wholly aromatic polyimide produced by condensation polymerization of an aromatic dianhydride and a diamine. Specific examples of the dianhydride include pyromellitic dianhydride, 3, 3 ′, 4, 4′-diphenylsulfone tetracarboxylic dianhydride, 3, 3 ′, 4, 4′-benzophenone tetracarboxylic acid. And acid dianhydrides, 2,2′-bis (3,4-dicarboxyphenyl) hexafluoropropane, 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride, and the like. Specific examples of the diamine include oxydianiline, paraphenylenediamine, benzophenone diamine, 3,3′-methylenedianiline, 3,3′-diaminobenzophenone, 3,3′-diaminodiphenylsulfone, 1,5 '-Naphthalenediamine and the like can be mentioned, but the present invention is not limited to these. In the present invention, a polyimide soluble in a solvent can be suitably used. An example of such a polyimide is a polycondensate polyimide of 3,3 ′, 4,4′-diphenylsulfonetetracarboxylic dianhydride and an aromatic diamine. As a polar organic solvent for dissolving polyimide, dimethyl sulfoxide, cresol, o-chlorophenol, and the like can be suitably used in addition to those exemplified as a solvent for dissolving aramid.

  In the present invention, the coating solution used for forming the heat-resistant resin layer needs to contain ceramic powder. By forming a heat-resistant resin layer using a coating solution in which ceramic powder is added to a solution having an arbitrary heat-resistant resin concentration, it is possible to form a heat-resistant resin layer having a uniform and fine porous film thickness. it can. The air permeability can be controlled by the amount of ceramic powder added. In the ceramic powder according to the present invention, the average particle diameter of the primary particles is preferably 1.0 μm or less and 0.5 μm or less from the viewpoint of the strength of the laminated porous film and the smoothness of the surface of the heat-resistant resin layer. More preferably, it is 0.1 μm or less. The average particle diameter of the primary particles is measured by a method of analyzing a photograph obtained by an electron microscope with a particle diameter measuring instrument. The content of the ceramic powder is preferably 1% by weight or more and 95% by weight or less in the laminated porous film, and more preferably 5% by weight or more and 50% by weight or less. If the ceramic powder content in the laminated porous film is too small, when the resulting porous film is used as a battery separator, the ion permeability may not be sufficient, and if it is too large, the film becomes brittle and difficult to handle. There is a case. The shape of the ceramic powder to be used is not particularly limited, and can be spherical or random.

  Examples of the ceramic powder in the present invention include ceramic powders made of electrically insulating metal oxides, metal nitrides, metal carbides, and the like. For example, powders of alumina, silica, titanium dioxide, zirconium oxide or the like are preferably used. The ceramic powders may be used alone, or two or more kinds may be mixed, or the same kind or different kinds of ceramic powders having different particle diameters may be arbitrarily mixed and used.

  The average pore diameter measured by the mercury intrusion method of the heat resistant resin layer is preferably 3 μm or less, and more preferably 1 μm or less. When the average pore diameter exceeds 3 μm, when the laminated porous film is used as a battery separator, there is a problem that short-circuiting easily occurs when carbon powder or a small piece thereof, which is the main component of the positive electrode or negative electrode, falls off. there is a possibility. The porosity of the heat resistant resin layer is preferably 30 to 80% by volume, more preferably 40 to 70% by volume. When the porosity is less than 30% by volume, when the laminated porous film is used as a battery separator, the amount of electrolyte retained may be reduced. When the porosity exceeds 80% by volume, the strength of the heat-resistant resin layer is insufficient. There is a case. The heat-resistant resin layer has a thickness of preferably 1 to 15 μm, more preferably 1 to 10 μm. When the thickness is less than 1 μm, the effect on heat resistance may be insufficient. When the thickness exceeds 15 μm, when the laminated porous film is used as a battery separator, the thickness is too thick and the electric capacity is increased. May be difficult to achieve.

As a method of laminating a heat-resistant resin layer on a porous film, a method of separately producing a heat-resistant resin layer and then laminating it with a porous film, a coating containing ceramic powder and a heat-resistant resin on at least one surface of the porous film Although the method of apply | coating a liquid and forming a heat resistant resin layer etc. are mentioned, the latter method is preferable from the surface of productivity. Specific examples of a method for forming a heat resistant resin layer by applying a coating liquid containing ceramic powder and a heat resistant resin on at least one surface of the porous film include the following steps.
(A) A slurry-like coating liquid is prepared by dispersing 1 to 1500 parts by weight of ceramic powder in 100 parts by weight of a heat-resistant resin in a polar organic solvent solution containing 100 parts by weight of a heat-resistant resin.
(B) The coating solution is applied to at least one surface of the porous film to form a coating film.
(C) The heat resistant resin is deposited from the coating film by means of humidification, solvent removal, or immersion in a solvent that does not dissolve the heat resistant resin, and then dried as necessary.
The coating liquid is preferably applied continuously by a coating apparatus described in JP-A-2001-316006 and a method described in JP-A-2001-23602.

  The laminated porous film obtained by the method of the present invention has no thin part and is homogeneous. Moreover, since it is excellent also in heat resistance, intensity | strength, and air permeability (ion permeability), it can be used conveniently as a battery separator, especially a lithium ion secondary battery separator.

(1) The Gurley value (second / 100 cc) of the Gurley value film was measured with a B-type densometer (manufactured by Toyo Seiki) according to JIS P8117. The measurement was performed at 10 locations per 1 m ² of film.
(2) Film thickness Measured with Mitutoyo VL-50A according to JISK7130. The measurement was performed at 10 locations per 1 m ² of film.

(3) Measurement of molecular chain length and molecular weight by GPC A gel chromatograph Alliance GPC2000 model manufactured by Waters was used. Other conditions are shown below.
Column: TSKgel GMHHR-H (S) HT 30 cm × 2 manufactured by Tosoh Corporation, TSKgel GMH 6 -HTL 30 cm × 2
Mobile phase: o-dichlorobenzene detector: Differential refractometer flow rate: 1.0 mL / min Column temperature: 140 ° C.
Injection volume: 500 μL
After completely dissolving 30 mg of the sample in 20 mL of o-dichlorobenzene at 145 ° C., the solution was filtered through a sintered filter having a pore size of 0.45 μm, and the filtrate was used as a feed solution. The calibration curve was prepared using 16 kinds of standard polystyrenes with known molecular weights.

(4) Content measurement of acid-insoluble part in inorganic filler After precisely weighing 100 g of inorganic filler, 300 ml of methanol was added and stirred to disperse the inorganic filler. Further, 300 ml of hydrochloric acid (special grade reagent from Wako Pure Chemical Industries, Ltd.) was slowly added while stirring to dissolve the inorganic filler. After confirming that bubbles were not generated, the solution was vacuum filtered using a hydrophilic PTFE filter (pore size: 10 μm; JCW4700, manufactured by Nihon Millipore). 50 ml of methanol and 50 ml of hydrochloric acid were added to the insoluble part remaining on the filter while vacuum filtration to dissolve the undissolved inorganic filler. This operation was repeated three times. Subsequently, 50 ml of diethyl ether was added three times in order to dissolve the surface treatment agent of the inorganic filler remaining on the filter. Finally, 50 ml of acetone was added and filtered until no acetone was present. This series of operations was performed while washing undissolved inorganic filler adhering to the filter wall surface. Thereafter, the PTFE filter was vacuum dried in an oven at 80 ° C. for 3 to 4 hours, and then cooled to room temperature in a desiccator. Thereafter, the weight after drying including the filter was measured. The content of the acid insoluble part was determined by the following formula.
Acid-insoluble part content (ppm) = (weight after drying−weight of filter) / precisely weight of collected inorganic filler × 10 ⁶
(5) Average particle size of inorganic filler The particle size was measured with a scanning electron microscope SEM (S-4200 manufactured by Hitachi) at a magnification of 30000, the diameter of 100 particles was measured, and the average was defined as the average particle size (μm).
(6) Puncture strength The maximum stress (gf) when the porous film was fixed with a 12 mmφ washer and the pin was punctured at 200 mm / min was defined as the puncture strength of the film. A pin with a pin diameter of 1 mmΦ and a tip of 0.5R was used.
(7) Evaluation of pinholes and thin portions present in porous film About the obtained porous film, the number of pinholes and thin portions per 1 m ² of the film was visually measured. Specifically, the light of the fluorescent lamp is applied from one surface of the porous film, the film is observed from the other surface, and the through hole is a pinhole. The part with much light was judged to be a thin part.

[Example 1]
Commercially available precipitated calcium carbonate (Vigot-10 manufactured by Shiroishi Calcium Co., Ltd.) was classified using Turboflex 100ATP manufactured by Hosokawa Micron Co., Ltd. The classification conditions were a rotor speed of 11000 ppm, an air volume of 4 m ² / min (theoretical classification point 2 μm), and a raw material supply rate of 15 kg / h. As a result of classification, the recovery rate of the obtained fine powder was 85% by weight, the amount of acid-insoluble part contained in the recovered inorganic filler was 100 ppm (of which 8% was carbon), and the average particle size was 0.14 μm. there were.
100 parts by weight of a polyethylene resin (Hi-Zex Million 340M, manufactured by Mitsui Chemicals, Inc., weight average molecular chain length 17000 nm, weight average molecular weight 3 million, melting point 136 ° C.) and 100 parts by weight of the polyethylene resin are classified. The resulting fine powder of 256 parts by weight of calcium carbonate and 44 parts by weight of olefin wax powder (High Wax 110P, manufactured by Mitsui Chemicals, Inc., weight average molecular weight 1000, melting point 110 ° C.) were mixed with a Henschel mixer and then biaxially kneaded. A polyolefin resin composition was obtained by kneading at 230 ° C. in a machine. The polyolefin resin composition was rolled with a pair of rolls having a surface temperature of 150 ° C. and rotating at the same peripheral speed to produce a single layer sheet having a thickness of about 40 μm. Next, the obtained single-layer sheets were pressure-bonded with a pair of rolls having a surface temperature of 150 ° C. to prepare a sheet made of a multilayer polyolefin resin composition. The multilayer sheet had a thickness of 71 μm.
Next, the inorganic filler in the multilayer sheet was removed using the apparatus shown in FIG. The multilayer sheet is conveyed by a roll and immersed in a bath containing a hydrochloric acid aqueous solution (hydrochloric acid 2 to 4 mol / L, nonionic surfactant 0.1 to 0.5% by weight) to remove the inorganic filler, and then the sheet. Was immersed in a bath (b) containing an aqueous sodium hydroxide solution (0.1 to 2 mol / L) for neutralization. Further, the sheet was washed with water in the bath of c, and was finally brought into contact with a roll heated to 50 ° C., dried and wound up. Thereafter, the sheet was stretched 8 times with a tenter (stretching temperature 100 ° C.). Table 1 shows the physical properties of the obtained porous film.

[Example 2]
Classification was carried out in the same manner as in Example 1 except that commercially available precipitated calcium carbonate (StarVigot-15A manufactured by Shiroishi Calcium Co., Ltd.) was used. The recovery rate of fine powder obtained as a result of classification was 90% by weight, the amount of acid-insoluble part contained in the recovered inorganic filler was 70 ppm, and the average particle size was 0.24 μm.
A polyolefin resin composition and a multilayer sheet (65 μm) comprising the polyolefin resin composition were obtained in the same manner as in Example 1 except that the inorganic filler obtained by classification in this way was used. After removing the inorganic filler from the sheet, it was stretched to obtain a porous film. Table 1 shows the physical properties of the obtained porous film.

[Comparative Example 1]
Except for using commercially available precipitated calcium carbonate (Vigot-10, manufactured by Shiroishi Calcium Co., Ltd., content of acid-insoluble part: 700 ppm (including 2.4% of carbon), average particle size: 0.14 μm) without classification. Obtained a polyolefin resin composition and a multilayer sheet (81 μm) comprising the polyolefin resin composition in the same manner as in Example 1, and after removing the inorganic filler from the sheet, the film was stretched to obtain a porous film. It was. Table 1 shows the physical properties of the obtained porous film.

In an Example, it is a schematic diagram of the apparatus used when removing an inorganic filler from the sheet | seat which consists of polyolefin resin compositions.

Explanation of symbols

a: Acid solution tank
b: Alkaline aqueous solution tank
c: Aquarium
d: Guide roll
e: Drying drum (heating drum)
f: Winder
g: Sheet made of polyolefin resin composition

Claims (8)

A sheet made of a polyolefin resin composition containing 100 parts by weight of a polyolefin resin and 100 to 400 parts by weight of precipitated calcium carbonate based on 100 parts by weight of the polyolefin resin, wherein the average particle size of the precipitated calcium carbonate is 0 a .02～1Myuemu, and sheet acid insoluble portion contained in the precipitated calcium carbonate is formed of a polyolefin resin composition is 200ppm or less.   The sheet comprising the polyolefin resin composition according to claim 1, wherein the polyolefin resin in the polyolefin resin composition is a polyolefin resin containing 10 wt% or more of a polyolefin having a molecular chain length of 2850 nm or more.   The polyolefin resin composition according to claim 1 or 2, wherein the polyolefin resin composition is a polyolefin resin composition containing 5 to 100 parts by weight of an olefin wax having a weight average molecular weight of 700 to 6000 with respect to 100 parts by weight of the polyolefin resin. A sheet comprising a polyolefin resin composition. Precipitated calcium carbonate of the polyolefin resin composition is a polyolefin-based resin composition according to any one of claims 1 to 3 is a precipitated calcium carbonate obtained by air classification to classification point as 0.5~50μm A sheet consisting of The porous film obtained by extending | stretching, after removing precipitated calcium carbonate from the sheet | seat which consists of a polyolefin-type resin composition in any one of Claims 1-4 . A laminated porous film obtained by laminating a heat-resistant resin layer containing ceramic powder and a heat-resistant resin containing a nitrogen element on the porous film according to claim 5 . The porous film according to claim 5 , which is a battery separator. The laminated porous film according to claim 6 , which is a battery separator.

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