Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
  • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
  • H01M50/489—Separators, membranes, diaphragms or spacing elements inside the cells, characterised by their physical properties, e.g. swelling degree, hydrophilicity or shut down properties
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
  • B01D67/0002—Organic membrane manufacture
  • B01D67/0023—Organic membrane manufacture by inducing porosity into non porous precursor membranes
  • B01D67/0025—Organic membrane manufacture by inducing porosity into non porous precursor membranes by mechanical treatment, e.g. pore-stretching
  • B01D67/0027—Organic membrane manufacture by inducing porosity into non porous precursor membranes by mechanical treatment, e.g. pore-stretching by stretching
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
  • B01D69/02—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor characterised by their properties
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
  • B01D69/12—Composite membranes; Ultra-thin membranes
  • B01D69/1213—Laminated layers
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
  • B01D69/12—Composite membranes; Ultra-thin membranes
  • B01D69/1216—Three or more layers
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
  • B01D71/06—Organic material
  • B01D71/26—Polyalkenes
  • B01D71/261—Polyethylene
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
  • B01D71/06—Organic material
  • B01D71/26—Polyalkenes
  • B01D71/262—Polypropylene
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C55/00—Shaping by stretching, e.g. drawing through a die; Apparatus therefor
  • B29C55/005—Shaping by stretching, e.g. drawing through a die; Apparatus therefor characterised by the choice of materials
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M10/00—Secondary cells; Manufacture thereof
  • H01M10/05—Accumulators with non-aqueous electrolyte
  • H01M10/052—Li-accumulators
  • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
  • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
  • H01M50/403—Manufacturing processes of separators, membranes or diaphragms
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
  • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
  • H01M50/403—Manufacturing processes of separators, membranes or diaphragms
  • H01M50/406—Moulding; Embossing; Cutting
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
  • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
  • H01M50/409—Separators, membranes or diaphragms characterised by the material
  • H01M50/411—Organic material
  • H01M50/414—Synthetic resins, e.g. thermoplastics or thermosetting resins
  • H01M50/417—Polyolefins
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
  • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
  • H01M50/409—Separators, membranes or diaphragms characterised by the material
  • H01M50/449—Separators, membranes or diaphragms characterised by the material having a layered structure
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
  • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
  • H01M50/409—Separators, membranes or diaphragms characterised by the material
  • H01M50/449—Separators, membranes or diaphragms characterised by the material having a layered structure
  • H01M50/457—Separators, membranes or diaphragms characterised by the material having a layered structure comprising three or more layers
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
  • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
  • H01M50/489—Separators, membranes, diaphragms or spacing elements inside the cells, characterised by their physical properties, e.g. swelling degree, hydrophilicity or shut down properties
  • H01M50/491—Porosity
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
  • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
  • H01M50/489—Separators, membranes, diaphragms or spacing elements inside the cells, characterised by their physical properties, e.g. swelling degree, hydrophilicity or shut down properties
  • H01M50/494—Tensile strength
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2323/00—Details relating to membrane preparation
  • B01D2323/12—Specific ratios of components used
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2325/00—Details relating to properties of membranes
  • B01D2325/02—Details relating to pores or porosity of the membranes
  • B01D2325/0281—Fibril, or microfibril structures
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2325/00—Details relating to properties of membranes
  • B01D2325/02—Details relating to pores or porosity of the membranes
  • B01D2325/0283—Pore size
  • B01D2325/02832—1-10 nm
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2325/00—Details relating to properties of membranes
  • B01D2325/02—Details relating to pores or porosity of the membranes
  • B01D2325/0283—Pore size
  • B01D2325/02833—Pore size more than 10 and up to 100 nm
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2325/00—Details relating to properties of membranes
  • B01D2325/02—Details relating to pores or porosity of the membranes
  • B01D2325/0283—Pore size
  • B01D2325/02834—Pore size more than 0.1 and up to 1 µm
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2325/00—Details relating to properties of membranes
  • B01D2325/06—Surface irregularities
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2325/00—Details relating to properties of membranes
  • B01D2325/22—Thermal or heat-resistance properties
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2325/00—Details relating to properties of membranes
  • B01D2325/24—Mechanical properties, e.g. strength
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2325/00—Details relating to properties of membranes
  • B01D2325/34—Molecular weight or degree of polymerisation
  • B01D2325/341—At least two polymers of same structure but different molecular weight
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
  • B29K2023/00—Use of polyalkenes or derivatives thereof as moulding material
  • B29K2023/04—Polymers of ethylene
  • B29K2023/06—PE, i.e. polyethylene
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
  • B29K2105/00—Condition, form or state of moulded material or of the material to be shaped
  • B29K2105/04—Condition, form or state of moulded material or of the material to be shaped cellular or porous
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29L—INDEXING SCHEME ASSOCIATED WITH SUBCLASS B29C, RELATING TO PARTICULAR ARTICLES
  • B29L2031/00—Other particular articles
  • B29L2031/755—Membranes, diaphragms
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/10—Energy storage using batteries

Definitions

• the present invention relates to a microporous membrane comprising one or more polymer, the membrane having suitable permeability, mechanical strength, heat shrinkage resistance, compression resistance and electrolytic solution absorption.
• the invention also relates to a method for producing such a microporous membrane, a method for producing a battery using such a membrane as a battery separator, and a method for using such a battery as a source of sink of electric charge.
• Microporous membranes are useful as separators to prevent electrode contact in, e.g., electrochemical cells such as fuel cells and batteries.
• microporous membranes can be used as separators in primary and secondary batteries such as lithium ion secondary batteries, lithium-polymer secondary batteries, nickel-hydrogen secondary batteries, nickel-cadmium secondary batteries, nickel-zinc secondary batteries, silver-zinc secondary batteries, etc.
• primary and secondary batteries such as lithium ion secondary batteries, lithium-polymer secondary batteries, nickel-hydrogen secondary batteries, nickel-cadmium secondary batteries, nickel-zinc secondary batteries, silver-zinc secondary batteries, etc.
• the membrane's performance significantly affects the battery's properties, productivity, and safety.
• the microporous polyolef ⁇ n membrane should have appropriate permeability, mechanical properties, heat resistance, dimensional stability, shutdown properties, meltdown properties, etc.
• High separator permeability is desirable for high capacity of batteries.
• a separator with high mechanical strength is more durable, and is also desirable for improved battery assembly and fabrication properties.
• JP6-240036A discloses a microporous polyolef ⁇ n membrane having improved pore diameter and a singular pore diameter distribution.
• the membrane is made from a polyethylene resin containing 1% or more by mass of ultra-high-molecular- weight polyethylene having a weight-average molecular weight ("Mw") of 7 x 10 5 or more, the polyethylene resin having a molecular weight distribution (weight-average molecular weight/number-average molecular weight) of 10-300, and the microporous polyolef ⁇ n membrane having a porosity of 35-95%, an average penetrating pore diameter of 0.05-0.2 ⁇ m, a rupture strength (15-mm width) of 0.2 kg or more, and a pore diameter distribution (maximum pore diameter/average penetrating pore diameter) of 1.5 or less.
• Mw weight-average molecular weight
• This microporous membrane is produced by extruding a melt-blend of the above polyethylene resin and a membrane- forming solvent through a die, stretching a gel-like sheet obtained by cooling at a temperature from the crystal dispersion temperature ("Ted") of the above polyethylene resin to the melting point +10 0 C, removing the membrane-forming solvent from the gel-like sheet, re-stretching the resultant membrane to 1.5-3 fold as an area magnification at a temperature of the melting point of the above polyethylene resin - 10 0 C or less, and heat-setting it at a temperature from the crystal dispersion temperature of the above polyethylene resin to the melting point.
• Ted crystal dispersion temperature
• WO 1999/48959 discloses a microporous polyolefin membrane having suitable strength and permeability, as well as a uniformly porous surface without local permeability variations.
• the membrane is made of a polyolefm resin, for instance, high-density polyethylene, having Mw of 50,000 or more and less than 5,000,000, and a molecular weight distribution of 1 or more to less than 30, which has a network structure with fine gaps formed by uniformly dispersed micro-fibrils, having an average micro-fibril size of 20-100 nm and an average micro-fibril distance of 40-400 nm.
• This microporous membrane is produced by extruding a melt-blend of the above polyolefin resin and a membrane-forming solvent through a die, stretching a gel- like sheet obtained by cooling at a temperature of the melting point of the above polyolefin resin -5O 0 C or higher and lower than the melting point, removing the membrane-forming solvent from the gel-like sheet, re-stretching it to 1.1-5 fold at a temperature in the range of the melting point of the above polyolefin resin -50 0 C up to the melting point, and heat-setting it at a temperature from the crystal dispersion temperature of the above polyolefin resin to the melting point.
• WO 2000/20492 discloses a microporous polyolefin membrane of improved permeability which is characterized by fine polyethylene fibrils having Mw of 5 x 10 5 or more or a composition comprising such polyethylene.
• the microporous polyolefin membrane has an average pore diameter of 0.05-5 ⁇ m, and the percentage of lamellas at angles ⁇ of 80-100° relative to a membrane surface is 40% or more in longitudinal and transverse cross sections.
• This polyethylene composition comprises 1-69% by weight of ultra-high-molecular- weight polyethylene having a weight-average molecular weight of 7 x 10 5 or more, 98-1% by weight of high-density polyethylene, and 1 -30% by weight of low-density polyethylene.
• This microporous membrane is produced by extruding a melt-blend of the above polyethylene or its composition and a membrane-forming solvent through a die, stretching a gel-like sheet obtained by cooling, heat-setting it at a temperature from the crystal dispersion temperature of the above polyethylene or its composition to (melting point +3O 0 C), and removing the membrane-forming solvent.
• WO 2002/072248 discloses a microporous membrane having improved permeability, particle-blocking properties and strength.
• the membrane is made using a polyethylene resin having Mw of less than 380,000.
• the membrane has a porosity of 50-95%, an average pore diameter in the range of 0.01-1 ⁇ m.
• the microporous membrane has a three-dimensional network skeleton formed by micro-fibrils having a average diameter of 0.2-1 ⁇ m connected to each other throughout the overall microporous membrane, and openings defined by the skeleton to have an average diameter of 0.1 ⁇ m or more and less than 3 ⁇ m.
• This microporous membrane is produced by extruding a melt-blend of the above polyethylene resin and a membrane-forming solvent through a die, removing the membrane-forming solvent from a gel-like sheet obtained by cooling, stretching it to 2-4 fold at a temperature of 20-140 0 C, and heat-treating the stretched membrane at a temperature of 80-140 0 C.
• WO 2005/113657 discloses a microporous polyolefm membrane having suitable shutdown properties, meltdown properties, dimensional stability, and high-temperature strength.
• the membrane is made using a polyolef ⁇ n composition comprising (a) a polyethylene resin containing 8-60% by mass of a component having a molecular weight of 10,000 or less, and an Mw/Mn ratio of 11-100, wherein Mn is the number-average molecular weight of the polyethylene resin, and a viscosity-average molecular weight ("Mv”) of 100,000-1,000,000, and (b) polypropylene.
• the membrane has a porosity of 20-95%, and a heat shrinkage ratio of 10% or less at 100 0 C.
• This microporous polyolefm membrane is produced by extruding a melt-blend of the above polyolef ⁇ n and a membrane-forming solvent through a die, stretching a gel-like sheet obtained by cooling, removing the membrane-forming solvent, and annealing the sheet.
• Battery separator properties such as permeability, mechanical strength, dimensional stability, thermal expansion, shutdown properties and meltdown properties are generally considered important.
• separator properties related to battery productivity such as electrolytic solution absorption, and battery cyclability, such as electrolytic solution retention properties, are also considered to be important. It can be important to simultaneously balance or optimize two or more of these properties, such as thermal expansion and electrolyte retention.
• electrodes for lithium ion batteries expand and shrink according to the intrusion and departure of lithium, and an increase in battery capacity leads to larger expansion ratios. Because separators are compressed when the electrodes expand, it is desired that the separators when compressed suffer as little a decrease as possible in electrolytic solution retention.
• microporous membranes are disclosed in JP6-240036A, WO 1999/48959, WO 2000/20492, WO 2002/072248, and WO 2005/113657, further improvements are still needed particularly in membrane permeability, mechanical strength, heat shrinkage resistance, compression resistance, and electrolytic solution absorption properties. It is thus desired to form battery separators from microporous membranes having improved permeability, mechanical strength, heat shrinkage resistance, compression resistance and electrolytic solution absorption.
• the invention relates to the discovery of a microporous membrane having improved permeability, mechanical strength, heat shrinkage resistance, compression resistance and electrolytic solution absorption properties.
• a microporous membrane has been discovered which has a sufficient number of pores having a pore size in the range of about 100 nm to about 1000 nm to achieve the desired amount of electrolytic solution absorption and a sufficient number of pores having a pore size in the range of about 1 nm to about 100 nm so that the strength of the microporous membrane is not significantly degraded.
• the invention relates to a microporous polyolefm membrane comprising pores characterized by a pore size distribution such at least about 25% of the area under the differential pore volume curve is associated with pores having a diameter in the range of about 100 nm to about 1,000 nm.
• differential pore volume is expressed as ⁇ - ⁇ - ⁇ where Vp is the dLog ⁇ r) pore volume, and r is the pore radius. Log r is the base 10 logarithm of the radius r. It is convenient to plot the curve as a function of the logarithm of the pore diameter.
• the curve of differential pore volume as a function of pore size e.g., pore diameter when the pores are approximately cylindrical
• pore size e.g., pore diameter when the pores are approximately cylindrical
• the microporous membrane can be obtained by extruding a polyolefm solution through one or more dies.
• the polyolefm solution can be produced from one or more diluents and polyolefm resin.
• the polyolefin solution can be produced from polyethylene resin.
• the polyethylene resins comprise
• the polyolefin solution is produced from polyethylene resin and polypropylene resin.
• the polyolefin solution can be produced from (c) 25% or less by mass of a polypropylene resin, the mass percent being based on the combined mass of the (a) first and (b) second polyethylene and the mass of (c) polypropylene in the polyolefin solution.
• the polyolefin resin consists essentially of the polyethylene (e.g., the first polyethylene); in another embodiment, the polyethylene resin consists essentially of the polyethylene.
• the portion of the polyolefin solution that is not polyolefin can be diluent and/or additives, some of which are described below.
• the polyolefin solution After the polyolefin solution is extruded, it can be cooled to make a cooled extrudate, which can be in the form of a gel-like molding or sheet for example.
• the extrudate or cooled extrudate can be stretched in at least one direction, and at least a portion of the diluent removed in order to form a porous sheet.
• the porous sheet can be stretched at an elevated temperature to a magnification ranging from 1.1 to 1.8 fold in at least one direction to form a stretched porous sheet. This step of stretching the porous sheet can be referred to as a "second" stretching step to distinguish is from the step of stretching the extrudate or cooled extrudate.
• the elevated temperature during the second stretching step can be referred to as the second stretching temperature.
• the stretched porous sheet can be heat-set at a heat-setting temperature in a range of the second stretching temperature ⁇ 6°C in order to form the microporous membrane.
• the heat-setting temperature in a range of the second stretching temperature ⁇ 5°C, or ⁇ 3°C, or ⁇ 2°C.
• the microporous polyolefin membrane is characterized by a differential pore volume curve (sometimes referred to conventionally as a pore size distribution curve) having at least two modes.
• One mode or peak generally is associated with dense domains in the membrane having pore sizes of about 0.01 to about 0.08 ⁇ m.
• At least a second mode corresponds to coarser domains having pore sizes a range of about 0.08 ⁇ m to about 1.5 ⁇ m. See Figures 1 and 2, for example.
• the modes may be present as distinct peaks in the differential pore volume curve, but this is not required since the modes can manifest themselves as shoulders or inflections on the curve depending, e.g., on the level of resolution selected for the pore size measurement, the rate of mercury intrusion, the amount of pressure applied, etc..
• the pore volume ratio of the dense domains (relatively small pores) to the coarse domains relatively (large pores) is 0.5 to 49.
• the microporous polyolef ⁇ n membrane has surface roughness of in the range of about 3 x 10 2 nm to about 3 x 10 3 nm. It is believed that membranes having a surface roughness within this range have a sufficiently large contact area with an electrolytic solution when used as a battery separator, exhibiting suitable electrolytic solution absorption characteristics.
• the microporous polyolefin membrane is made by a method comprising the steps of (1) melt-blending (a) a polyethylene resin containing 7% or less by mass of ultra-high-molecular- weight polyethylene having a weight-average molecular weight of 1 x 10 6 or more based on the mass of the polyethylene resin and (b) a membrane-forming solvent, in order to form a polyolef ⁇ n solution, with the solution preferably having a solvent concentration of 25-50% by mass based on the mass of the polyolefin solution, (2) extruding the polyolef ⁇ n solution through a die to form an extrudate, (3) cooling the extrudate to form a gel-like sheet, (4) stretching the gel-like sheet at a first stretching temperature in a range of the crystal dispersion temperature ("Ted") of the polyethylene resin to Ted +3O 0 C, (5) removing the membrane-forming solvent from the stretched gel-like sheet to form a membrane, (6) stretching the membrane
• Fig. 1 shows the differential pore volume curve ⁇ - ⁇ - ⁇ for the sample of Example dLog[r)
• Fig. 2 shows the differential pore volume curve ⁇ - for the sample of Example dLog[r)
• the total area under the curve over the range of pores sizes from about 1 nm to about 1000 nm is 1.07.
• the area under the curve over the range of pores sizes from about 100 nm to about 1000 nm is 0.502. Accordingly, about 47% of the area under the curve is associated with pores having pore sizes in the range of about 100 to about 1000 nm.
• Fig. 3 shows the differential pore volume curve ⁇ - ⁇ for the sample of dLog ⁇ r
• Comparative Example 2 The total area under the curve over the range of pores sizes from about 1 nm to about 1000 nm is 0.48. The area under the curve over the range of pores sizes from about 100 nm to about 1000 nm is 0.075. Accordingly, about 16% of the area under the curve is associated with pores having pore sizes in the range of about 100 to about 1000 nm.
• Fig. 4 shows illustrates mercury intrusion porosimetry.
• Fig. 5 shows a Vp (pore volume) curve for a microporous membrane having a hybrid structure (from Example7).
• Fig. 6 shows the differential pore volume curve - ⁇ - ⁇ for a microporous dLog[r) membrane that does not have a hybrid structure.
• the membrane contains polyethylene but no polypropylene.
• Fig. 7 shows the differential pore volume curve ⁇ - ⁇ - ⁇ for a microporous dLog[r) membrane that has a hybrid structure.
• the present inventions relates to a method for making a microporous film having enhanced properties, especially electrolyte injection and compression properties.
• certain specific polyethylene resins and, optionally, a certain specific polypropylene resin can be combined, e.g. by melt-blending, to form a polyolefm composition.
• the polyolefm composition comprises polyethylene, e.g., (a) a first polyethylene resin having a weight average molecular weight of from about 2.5 x 10 5 to about 5 x 10 5 .
• the polyolefm composition comprises polyethylene and polypropylene.
• the polyolefm composition is produced from (a) the first polyethylene resin, (b) from about 0% to about 7 %, of a second polyethylene resin having a weight average molecular weight of from about 5 x 10 5 to about 1 x 10 5 , and (c) from 0% to about 25 % polypropylene resin having a weight average molecular weight of from about 3 x 10 5 to about 1.5 x 10 6 .
• the polyolefm composition is produced from polyethylene resin and 25% by mass to 65% by mass, or 30% to 55% by mass, of polypropylene resin, based on the mass of the polyolef ⁇ n composition.
• the microporous membrane is produced from a polyolef ⁇ n solution comprising diluent and polyolef ⁇ n.
• the polyolefm can comprise polyethylene or optionally polyethylene and polypropylene.
• suitable polyethylenes and polypropylenes will now be described.
• the first polyethylene has a weight average molecular weight Mw in the range of about 1 x 10 4 to about 5 x 10 5 .
• the first polyethylene can comprise at least one of high-density polyethylene ("HDPE"), medium-density polyethylene, branched low-density polyethylene and linear low-density polyethylene.
• HDPE high-density polyethylene
• the Mw of the HDPE can range, e.g., from about 1 x 10 5 to about 5 x 10 5 , or from about 2 x 10 5 to about 4 x 10 5 .
• the first polyethylene resin can be, for example, a high density polyethylene (HDPE) resin having a weight average molecular weight of from 2.5 x 10 5 to 5 x 10 5 and a molecular weight distribution of from about 5 to about 100.
• a non- limiting example of the first polyethylene resin for use herein is one that has a weight average molecular weight of from about 2.5 x 10 5 to about 4 x 10 5 and a molecular weight distribution of form about 7 to about 50.
• the first polyethylene resin can be an ethylene homopolymer, or an ethylene/ ⁇ -olefm copolymer, such as, for example, one containing a small amount, e.g. about 5 mole %, of a third ⁇ -olefm.
• the third ⁇ -olefin which is not ethylene, is preferably propylene, butene-1, pentene-1, hexene-1, 4-methylpentene- 1 , octene-1, vinyl acetate, methyl methacrylate, or styrene or combinations thereof.
• Such copolymer is preferably produced using a single-site catalyst.
• the second polyethylene resin for example an ultra-high molecular weight polyethylene (UHMWPE) resin, has a weight average molecular weight that is greater than 5 x 10 5 .
• UHMWPE ultra-high molecular weight polyethylene
• the Mw of the second polyethylene can be, for example, in the range of about 1 x 10 6 to 15 x 10 6 , or about 1 x 10 6 to 5 x 10 6 , or about 1 x 10 6 to 3 x 10 6 .
• the second polyethylene resin can be an ethylene homopolymer, or an ethylene/ ⁇ -olefin copolymer, such as, for example, one containing a small amount, e.g. about 5 mole %, of a third ⁇ -olefin.
• the third ⁇ -olefin which is not ethylene, can be, for example, propylene, butene-1, pentene-1, hexene-1, 4-methylpentene- 1 , octene-1, vinyl acetate, methyl methacrylate, or styrene or combinations thereof.
• Such copolymer is preferably produced using a single-site catalyst.
• Mw/Mn is a measure of molecular weight distribution. The larger this value, the wider the molecular weight distribution.
• the Mw/Mn of the overall polyethylene composition for use herein is preferably from about 5 to about 100, for example from about 7 to about 50. When the Mw/Mn is less than 5, the percentage of a higher molecular weight component is too high to conduct melt extrusion easily. On the other hand, when the Mw/Mn is more than 100, the percentage of a lower molecular weight component is too high, resulting in decrease in the strength of the resulting microporous membrane.
• the Mw/Mn of polyethylene can be properly controlled by a multi-stage polymerization.
• the multi-stage polymerization method is preferably a two-stage polymerization method comprising forming a high molecular weight polymer component in the first stage, and forming a low molecular weight polymer component in the second stage.
• the larger the Mw/Mn the larger difference in Mw exists between higher molecular weight polyethylene and lower molecular weight polyethylene, and vice versa.
• the Mw/Mn of the polyethylene composition can be properly controlled by the molecular weights and mixing ratios of components.
• the polyolefm solution can optionally contain polypropylene resin.
• the polypropylene resin for optional use herein can have a weight average molecular weight of from about 3 x 10 5 to about 1.5 x 10 6 , for example from about 6 x 10 5 to about 1.5 x 10 6 , a heat of fusion of 80 J/g or higher, for non- limiting example from about 80 to about 120 J/g, and a molecular weight distribution of from about 1 to about 100, for example from about 1.1 to about 50, and can be a propylene homopolymer or a copolymer of propylene and another, i.e. a fourth, olefin, though the homopolymer is preferable.
• the polypropylene is isotactic polypropylene having a melting peak (second melt) of at least about 160 0 C.
• the polypropylene has a Trouton's ratio of 15 or more when measured at a temperature of 230 0 C and a strain rate of 25 sec "1 .
• the polypropylene has an elongational viscosity of 50,000 Pa sec or more at a temperature of 230 0 C and a strain rate of 25 sec "1 .
• the copolymer may be a random or block copolymer.
• the fourth olefin which is an olefin other than propylene, includes ⁇ -olefms such as ethylene, butene-1, pentene-1, hexene-1, 4-methylpentene-l, octene-1, vinyl acetate, methyl methacrylate, styrene, etc., and diolef ⁇ ns such as butadiene, 1,5-hexadiene, 1,7-octadiene, 1 ,9-decadiene, etc.
• ⁇ -olefms such as ethylene, butene-1, pentene-1, hexene-1, 4-methylpentene-l, octene-1, vinyl acetate, methyl methacrylate, styrene, etc.
• diolef ⁇ ns such as butadiene, 1,5-hexadiene, 1,7-octadiene, 1 ,9-decadiene, etc.
• the percentage of the fourth olefin in the propylene copolymer is preferably in a range that does not deteriorate the properties of the microporous membrane such as heat resistance, compression resistance, heat shrinkage resistance, etc., and is preferably less than about 10 mole %, e.g. from about 0 to less than about 10 mole %.
• the amount of polypropylene resin in the polyolef ⁇ n composition is 55% or less by mass, or 40% or less by mass, or 25 % or less by mass based on 100 % of the mass of the polyolefm composition.
• the percentage of polypropylene resin may be, for example, from about 5 to about 20 % by mass, and for further example from about 7 to about 15 % by mass of the polyolef ⁇ n composition.
• the polyolefm solution can contain (a) additional polyolefm and/or (b) heat-resistant polymer resins having melting points or glass transition temperatures (Tg) of about 170 0 C or higher, in amounts not deteriorating the properties of the microporous membrane, for example 10 % or less by mass based on the polyolef ⁇ n composition. It is not necessary to add inorganic oxides such as silicon oxides to the polyolefm solution, and in an embodiment, the polyolefm solution does not contain added inorganic oxides. In a related embodiment, the polyolefm solution does not contain a significant amount of inorganic oxides such as silicon oxides.
• the phrase " does not contain a significant amount of inorganic oxides” means that the content of such oxides is less than 0.1 wt%, or less than 0.01 wt%, less than 0.001 wt%, or less than 100 ppmw, based on the weight of the polyolefrn solution.
• the additional polyolef ⁇ n can be at least one of (a) polybutene- 1 , polypentene- 1 , poly-4-methylpentene-l, polyhexene-1, polyoctene- 1 , polyvinyl acetate, polymethyl methacrylate, polystyrene and an ethylene/ ⁇ -olefm copolymer, each of which may have an Mw of forml x 10 4 to 4 x 10 6 , and (b) a polyethylene wax having an Mw of form 1 x 10 3 to 1 x 10 4 .
• Polybutene- 1, polypentene- 1, poly-4-methylpentene-l, polyhexene-1, polyoctene- 1 , polyvinyl acetate, polymethyl methacrylate and polystyrene are not restricted to homopolymers, but may be copolymers containing still other ⁇ -olefms.
• the heat-resistant resins can be, for example, (a) amorphous resins having melting points of about 170 0 C or higher, which may be partially crystalline, and (b) completely amorphous resins having Tg of about 170 0 C or higher and mixtures thereof.
• the melting point and Tg are determined by differential scanning calorimetry (DSC) according to method JIS K7121.
• the heat-resistant resins include polyesters such as polybutylene terephthalate (melting point: about 160-230 0 C), polyethylene terephthalate (melting point: about 250-270 0 C), etc., fluororesins, polyamides (melting point: 215-265°C), polyarylene sulfide, polyimides (Tg: 280 0 C or higher), polyamide imides (Tg: 280 0 C), polyether sulfone (Tg: 223 0 C), polyetheretherketone (melting point: 334 0 C), polycarbonates (melting point: 220-240 0 C), cellulose acetate (melting point: 220 0 C), cellulose triacetate (melting point: 300 0 C), polysulfone (Tg: 190 0 C), polyetherimide (melting point: 216 0 C), etc.
• polyesters such as polybutylene terephthalate (melting point: about 160-230 0 C), poly
• the total amount of the additional polyolef ⁇ n and the heat-resistant resin is preferably 20 % or less, for example from about 0 to about 20 %, by mass per 100 % by mass of the polyolef ⁇ n solution.
• the present invention relates to a method for producing the microporous membrane comprising the steps of (1) combining certain specific polyolefms (generally in the form of polyolefin resins) and at least one solvent or diluent to form a polyolefin solution, (2) extruding the polyolefin solution through a die to form an extrudate, (3) cooling the extrudate to form a cooled extrudate, (4) stretching the cooled extrudate at a certain specific temperature to form a stretched sheet, (5) removing the solvent or diluent from the stretched sheet to form a solvent/diluent-removed membrane, (6) stretching the solvent/diluent-removed membrane at a certain specific temperature and to a certain specific magnification to form a stretched membrane, and (7) heat-setting the stretched membrane to form the microporous membrane.
• certain specific polyolefms generally in the form of polyolefin resins
• solvent or diluent to form
• a heat-setting treatment step (4i), a heat roll treatment step (4ii), and/or a hot solvent treatment step (4iii) may be conducted between the steps (4) and (5), if desired.
• a heat-setting treatment step (5i) may be conducted between the steps (5) and (6).
• a step (5ii) of cross-linking with ionizing radiations following step (5i) prior to step (6), and a hydrophilizing treatment step (7i) and a surface-coating treatment step (7ii) may be conducted after the step (7), if desired.
• the polyolef ⁇ n resins may be combined with at least one solvent or diluent to prepare a polyolef ⁇ n solution.
• the polyolef ⁇ n resins may be combined, for example, by melt-blending, dry mixing, etc., to make a polyolef ⁇ n composition, which is then combined with at least one solvent or diluent to prepare a polyolef ⁇ n solution.
• the polyolef ⁇ n solution may contain, if desired, various additives such as anti-oxidants, fine silicate powder (pore-forming material), etc., in amounts which do not deteriorate the properties of the present invention.
• the diluent or solvent e.g.
• a membrane-forming solvent is preferably liquid at room temperature.
• the liquid solvents can be, for example, aliphatic, alicyclic or aromatic hydrocarbons such as nonane, decane, decalin, p-xylene, undecane, dodecane, liquid paraffin, mineral oil distillates having boiling points comparable to those of the above hydrocarbons, and phthalates liquid at room temperature such as dibutyl phthalate, dioctyl phthalate, etc.
• a non-volatile liquid solvent such as liquid paraffin.
• one or more solid solvents which are miscible with the polyolef ⁇ n composition during, for example, melt-blending, but solid at room temperature may be added to the liquid solvent.
• Such solid solvents are preferably stearyl alcohol, ceryl alcohol, paraffin waxes, etc.
• solid solvent can be used without liquid solvent. However, when only the solid solvent is used, uneven stretching, etc., can occur.
• the viscosity of the liquid solvent is preferably from about 30 to about 500 cSt, more preferably from about 30 to about 200 cSt, when measured at a temperature of 25 0 C.
• the viscosity at 25 0 C is less than 30 cSt
• the polyolefm solution may foam, resulting in difficulty in blending.
• the viscosity is more than 500 cSt, the removal of the liquid solvent can be difficult.
• the uniform melt-blending of the polyolefm solution is preferably conducted in a double-screw extruder to prepare a high concentration polyolefm solution.
• the diluent or solvent e.g. a membrane-forming solvent, may be added before starting melt-blending, or supplied to the double-screw extruder in an intermediate portion during blending, though the latter is preferable.
• the melt-blending temperature of the polyolefm solution is preferably in a range of the melting point ("Tm") of the polyethylene resin +10 0 C to Tm +120 0 C.
• the melting point can be measured by differential scanning calorimetry (DSC) according to JIS K7121.
• the melt-blending temperature is from about 140 to about 250 0 C, more preferably from about 170 to about 240 0 C, particularly where the polyethylene resin has a melting point of about 130 to about 140 0 C.
• the concentration of the polyolefm composition in the polyolefm solution is preferably from about 25 to about 50 % by mass, more preferably from about 25 to about 45 % by mass, based on the mass of the polyolefm solution.
• the ratio L/D of the screw length L to the screw diameter D in the double-screw extruder is preferably in a range of from about 20 to about 100, more preferably in a range of from about 35 to about 70. When L/D is less than 20, melt-blending can be inefficient. When L/D is more than 100, the residence time of the polyolef ⁇ n solution in the double-screw extruder can be too long.
• the cylinder of the double-screw extruder preferably has an inner diameter of from about 40 to about 100 mm.
• the ratio Q/Ns of the amount Q (kg/h) of the polyolef ⁇ n solution charged to the number of revolution Ns (rpm) of a screw is preferably from about 0.1 to about 0.55 kg/h/rpm.
• Q/Ns is less than 0.1 kg/h/rpm, the polyolefm can be damaged by shearing, resulting in decrease in strength and meltdown temperature.
• Q/Ns When Q/Ns is more than 0.55 kg/h/rpm, uniform blending cannot be achieved. Q/Ns is more preferably from about 0.2 to about 0.5 kg/h/rpm.
• the number of revolutions Ns of the screw is preferably 180 rpm or more. Though not particularly critical, the upper limit of the number of revolutions Ns of the screw is preferably about 500 rpm.
• the components of the polyolef ⁇ n solution can be melt-blended in the extruder and extruded from a die.
• the components of the polyolef ⁇ n solution can be extruded and then pelletized.
• the pellets can be melt-blended and extruded in a second extrusion to make a gel-like molding or sheet.
• the die can be a sheet-forming die having a rectangular orifice, a double-cylindrical, hollow die, an inflation die, etc.
• the die gap is not critical, in the case of a sheet- forming die, the die gap is preferably from about 0.1 to about 5 mm.
• the extrusion temperature is preferably from about 140 to about 250 0 C, and the extruding speed is preferably from about 0.2 to about 15 m/minute.
• the extrudate from the die is cooled to form a cooled extrudate, generally in the gel-like molding or sheet.
• the extrudate has a high polyolef ⁇ n content. Cooling is preferably conducted at least to a gelation temperature at a cooling rate of about 50 °C/minute or more. Cooling is preferably conducted to about 25 0 C or lower. Such cooling sets the micro-phase of the polyolef ⁇ n separated by the membrane-forming solvent. Generally, the slower cooling rate provides the gel-like sheet with larger pseudo-cell units, resulting in a coarser higher-order structure. On the other hand, a higher cooling rate results in denser cell units.
• a cooling rate of less than 50 °C/minute can lead to increased crystallinity, making it more difficult to provide the gel-like sheet with suitable stretchability.
• Usable cooling methods include bringing the extrudate into contact with a cooling medium such as cooling air, cooling water, etc.; bringing the extrudate into contact with cooling rollers; etc.
• the cooled extrudate comprises at least about 25 %, for example from about 25 to about 50 %, polyolef ⁇ n derived from the resins of the polyolef ⁇ n composition, based on the mass of the cooled extrudate. It is believed that a polyolef ⁇ n content of less than about 25 % of the cooled extrudate makes it more difficult to form a hybrid microporous membrane structure having both small and large pores. A polyolef ⁇ n content of more than about 50 % leads to higher viscosity which makes it more difficult to form the desired hybrid structure.
• the cooled extrudate preferably has a polyolef ⁇ n content at least as high as that of the polyolef ⁇ n solution.
• the cooled extrudate generally in the form of a high polyolefin content gel-like molding or sheet, is then stretched in at least one direction. While not wishing to be bound by any theory or model, it is believed that the gel-like sheet can be uniformly stretched because the sheet contains the membrane-forming solvent.
• the gel-like sheet is preferably stretched to a predetermined magnification after heating by, for example, a tenter method, a roll method, an inflation method or a combination thereof.
• the stretching may be conducted monoaxially or biaxially, though the biaxial stretching is preferable.
• any of simultaneous biaxial stretching, sequential stretching or multi-stage stretching may be used, though the simultaneous biaxial stretching is preferable.
• the amount of stretch in either direction need not be the same.
• the stretching magnification of this first stretching step can be, for example, 2 fold or more, preferably 3 to 30 fold in the case of monoaxial stretching.
• the stretching magnification can be, for example, 3 fold or more in any direction, namely 9 fold or more, preferably 16 fold or more, more preferably 25 fold or more, e.g. 49 fold or more, in area magnification.
• An example for this first stretching step would include stretching from about 9 fold to about 400 fold.
• a further example would be stretching from about 16 to about 49 fold. Again, the amount of stretch in either direction need not be the same.
• the area magnification of 9 fold or more the pin puncture strength of the microporous membrane is improved.
• stretching apparatuses, stretching operations, etc. involve large-sized stretching apparatuses, which can be difficult to operate.
• a relatively high stretching temperature is used in this first stretching step, preferably from about the crystal dispersion temperature ("Ted") of the combined polyethylene content of the cooled extrudate to about Ted + 30 0 C, e.g. in a range of Ted of the combined polyethylene content to Ted + 25 0 C, more specifically in a range of Ted + 10 0 C to Ted + 25 0 C, most specifically in a range of Ted + 15 0 C to Ted + 25 0 C.
• Ted crystal dispersion temperature
• the crystal dispersion temperature is determined by measuring the temperature characteristics of dynamic viscoelasticity according to ASTM D 4065. Because the combined polyethylene content herein has a crystal dispersion temperature of about 90 to 100 0 C, the stretching temperature is from about 90 to 125 0 C; preferably form about 100 to 125 0 C, more preferably from 105 to 125 0 C.
• the above stretching causes cleavage between polyolefm, e.g. polyethylene, lamellas, making the polyolefin phases finer and forming large numbers of fibrils.
• the fibrils form a three-dimensional network structure.
• the stretching is believed to improve the mechanical strength of the microporous membrane and expands its pores, making the microporous membrane suitable for use as a battery separator.
• stretching may be conducted with a temperature distribution in a thickness direction, to provide the microporous membrane with further improved mechanical strength.
• the detailed description of this method is given by Japanese Patent 3347854.
• a second solvent also called a "washing solvent”
• a second solvent also called a "washing solvent”
• the removal of the solvent or diluent provides a microporous membrane.
• the removal of the solvent or diluent can be conducted by using one or more suitable washing solvents, i.e., one capable of displacing the liquid solvent from the membrane.
• washing solvents include volatile solvents, e.g., saturated hydrocarbons such as pentane, hexane, heptane, etc., chlorinated hydrocarbons such as methylene chloride, carbon tetrachloride, etc., ethers such as diethyl ether, dioxane, etc., ketones such as methyl ethyl ketone, etc., linear fluorocarbons such as trifluoroethane, CeF 14 , etc., cyclic hydrofluorocarbons such as C5H3F7, etc., hydrofluoroethers such as C 4 F 9 OCH 3 , C 4 F 9 OC 2 H 5 , etc., perfluoroethers such as C 4 F 9 OCF 3 , C 4 F 9 OC 2 F 5 , etc., and mixtures thereof.
• volatile solvents e.g., saturated hydrocarbons such as pentane, hexane, heptane, etc.,
• the washing of the stretched membrane can be conducted by immersion in the washing solvent and/or showering with the washing solvent.
• the washing solvent used is preferably from about 300 to about 30,000 parts by mass per 100 parts by mass of the stretched membrane.
• the washing temperature is usually from about 15 to about 30 0 C, and if desired, heating may be conducted during washing.
• the heating temperature during washing is preferably about 80 0 C or lower. Washing is preferably conducted until the amount of the remaining liquid solvent becomes less than 1 % by mass of the amount of liquid solvent that was present in polyolefm solution prior to extrusion.
• the microporous membrane deprived of the diluent or solvent can be dried by a heat-drying method, a wind-drying (e.g., air drying using moving air) method, etc., to remove remaining volatile components from the membrane, e.g. washing solvent.
• a heat-drying method e.g., a wind-drying (e.g., air drying using moving air) method, etc.
• Any drying method capable of removing a significant amount of the washing solvent can be used.
• substantially all of the washing solvent is removed during drying.
• the drying temperature is preferably equal to or lower than Ted, more preferably 5 0 C or more lower than Ted.
• Drying is conducted until the remaining washing solvent becomes preferably 5 % or less by mass, more preferably 3 % or less by mass, per 100 % by mass (on a dry basis) of the microporous membrane. Insufficient drying undesirably can lead to decrease in the porosity of the microporous membrane by the subsequent heat treatment, resulting in poor permeability.
• the dried membrane is stretched in a second stretching step (re-stretched) at least monoaxially at high magnification.
• the re-stretching of the membrane can be conducted, for example, while heating, by a tenter method, etc., as in the first stretching step.
• the re-stretching may be monoaxial or biaxial. In the case of biaxial stretching, any one of simultaneous biaxial stretching or sequential stretching may be used, though the simultaneous biaxial stretching is preferable.
• the directions of MD and TD (where MD means "machine direction", i.e., the direction of membrane travel during processing, and TD means "transverse direction", i.e., a direction orthogonal to both the MD and the horizontal surface of the membrane) in the re-stretching is usually the same as those in the stretching of the cooled extrudate. In the present invention, however, the re-stretching is actually somewhat greater than that used in the stretching of the cooled extrudate. Stretching magnification in this step is from about 1.1 to about 1.8 fold in at least one direction, for example from about 1.2 to about 1.6 fold.
• Stretching need not be the same magnification in each direction. If stretching in step (4) of the present method is lower in the range of from about 9 to about 400, then stretching in step (6) of the present method should be higher in the range of from about 1.1 to about 1.8. Likewise, if stretching in step (4) of the present method is higher in the range of from about 9 to about 400, then stretching in step (6) of the present method should be lower in the range of from about 1.1 to about 1.8.
• the second stretching or re-stretching is conducted at a second temperature preferably equal to Tm or lower, more preferably in a range of Ted to Tm.
• Tm melting point
• Ted lower than Ted
• the second stretching temperature is usually from about 90 to about 135 0 C, preferably from about 95 to about 130 0 C.
• the monoaxial stretching magnification of the membrane in this step is preferably from about 1.1 to about 1.8 fold.
• a magnification of 1.1 to 1.8 fold generally provides the membrane of the present invention with a hybrid structure having a large average pore size.
• the magnification can be form 1.1 to 1.8 fold in a longitudinal or transverse direction.
• the membrane may be stretched at the same or different magnifications in each stretching direction, though preferably the same, as long as the stretching magnifications in both directions are within 1.1 to 1.8 fold.
• the second stretching magnification of the membrane is less than 1.1 fold, it is believed that the hybrid structure is not formed, resulting in poor permeability, electrolytic solution absorption and compression resistance in the membrane.
• the second stretching magnification is more than 1.8 fold, the fibrils formed are too fine, and it is believed that the heat shrinkage resistance and the electrolytic solution absorption characteristics of the membrane are reduced.
• This second stretching magnification is more preferably from 1.2 to 1.6 fold.
• the stretching rate is preferably 3 %/second or more in a stretching direction. In the case of monoaxial stretching, stretching rate is 3 %/second or more in a longitudinal or transverse direction. In the case of biaxial stretching, stretching rate is 3 %/second or more in both longitudinal and transverse directions.
• a stretching rate of less than 3 %/second decreases the membrane's permeability, and provides the membrane with large unevenness in properties (particularly, air permeability) in a width direction when stretched in a transverse direction.
• the stretching rate is preferably 5 %/second or more, more preferably 10 %/second or more.
• the upper limit of the stretching rate is preferably 50 %/second to prevent rupture of the membrane.
• the re-stretched membrane is thermally treated (heat-set) to stabilize crystals and make uniform lamellas in the membrane.
• the heat-setting is preferably conducted by a tenter method or a roll method.
• the heat-setting temperature can be, e.g., in a range of the second stretching temperature of the membrane ⁇ 5 0 C, or in a range of the second stretching temperature of the membrane ⁇ 3 0 C. It is believed that too low a heat-setting temperature deteriorates the membrane's pin puncture strength, tensile rupture strength, tensile rupture elongation and heat shrinkage resistance, while too high a heat-setting temperature deteriorates membrane permeability.
• An annealing treatment can be conducted after the heat-setting step.
• the annealing is a heat treatment with no load applied to the microporous membrane, and may be conducted by using, e.g., a heating chamber with a belt conveyer or an air-floating-type heating chamber.
• the annealing may also be conducted continuously after the heat-setting with the tenter slackened.
• the annealing temperature is preferably Tm or lower, more preferably in a range from about 60 0 C to about Tm -5 0 C. Annealing is believed to provide the microporous membrane with high permeability and strength.
• the membrane is annealed without prior heat-setting.
• the heat-setting of step (7) is optional.
• the stretched sheet between the steps (4) and (5) may be heat-set, provided this heat setting does not deteriorate the properties of the microporous membrane.
• the heat-setting method may be conducted the same way as described above for step (7).
• At least one surface of the stretched sheet from step (4) may be brought into contact with one or more heat rollers following any of steps (4) to (7).
• the roller temperature is preferably in a range of from Ted +10 0 C to Tm.
• the contact time of the heat roll with the stretched sheet is preferably from about 0.5 second to about 1 minute.
• the heat roll may have a flat or rough surface.
• the heat roll may have a suction functionality to remove the solvent.
• one example of a roller-heating system may comprise holding heated oil in contact with a roller surface.
• the stretched sheet may be contacted with a hot solvent between steps (4) and (5).
• a hot solvent treatment turns fibrils formed by stretching to a leaf vein form with relatively thick fiber trunks, providing the microporous membrane with large pore size and suitable strength and permeability.
• the term "leaf vein form” means that the fibrils have thick fiber trunks, and thin fibers extending in a complicated network structure from the trunks. The details of the hot solvent treatment method are described in WO 2000/20493.
• the microporous membrane containing a washing solvent between the steps (5) and (6) may be heat-set to a degree that does not deteriorate the properties of the microporous membrane.
• the heat-setting method may be the same as described above in step (7).
• the heat-set microporous membrane may be cross-linked by ionizing radiation rays such as ⁇ -rays, ⁇ -rays, ⁇ -rays, electron beams, etc.
• ionizing radiation rays such as ⁇ -rays, ⁇ -rays, ⁇ -rays, electron beams, etc.
• the amount of electron beams is preferably from about 0.1 to about 100 Mrad, and the accelerating voltage is preferably form about 100 to about 300 kV.
• the cross- linking treatment elevates the meltdown temperature of the microporous membrane.
• the heat-set microporous membrane may be subjected to a hy drophilizing treatment (a treatment that makes the membrane more hydrophilic).
• the hydrophilizing treatment may be a monomer-grafting treatment, a surfactant treatment, a corona-discharging treatment, etc.
• the monomer-grafting treatment is preferably conducted after the cross-linking treatment.
• any of nonionic surfactants, cationic surfactants, anionic surfactants and amphoteric surfactants may be used, and the nonionic surfactants are preferred.
• the microporous membrane can be dipped in a solution of the surfactant in water or a lower alcohol such as methanol, ethanol, isopropyl alcohol, etc., or coated with the solution by a doctor blade method.
• the heat-set microporous membrane resulting from step (7) can be coated with porous polypropylene, porous fluororesins such as polyvinylidene fluoride and polytetrafluoroethylene, porous polyimides, porous polyphenylene sulfide, etc., to improve meltdown properties when the membrane is used as a battery separator.
• the polypropylene used for the coating preferably has Mw of form about 5,000 to about 500,000, and a solubility of about 0.5 grams or more in 100 grams of toluene at 25 0 C.
• Such polypropylene more preferably has a racemic diade fraction of from about 0.12 to about 0.88, the racemic diade being a structural unit in which two adjacent monomer units are mirror-image isomers to each other.
• the surface-coating layer may be applied, for instance, by applying a solution of the above coating resin in a good solvent to the microporous membrane, removing part of the solvent to increase a resin concentration, thereby forming a structure in which a resin phase and a solvent phase are separated, and removing the remainder of the solvent.
• good solvents for this purpose include aromatic compounds, such as toluene or xylene.
• the microporous membrane produced by the above-described method has a relatively wide pore size distribution when plotted as a differential pore volume curve. Pore size distribution can be measured, e.g., by conventional methods such as mercury porosimetry.
• mercury porosimetry is used to measure the distribution of pore sizes and pore volume in the membrane, it is conventional to measure pore diameter, pore volume, and the specific surface area of the membrane. The measurements can be used to determine a differential pore volume expressed as ⁇ - where Vp is the pore volume, and r is the pore dLog[r) radius, assuming cylindrical pores.
• the differential pore volume when plotted on the y axis with pore diameter on the x axis is conventionally referred to as the "pore size distribution."
• pore size distribution For membranes exhibiting a hybrid structure, at least about 25%, or at least about 30%, or at least about 40%, or at least about 50%, or at least about 60%, of the differential pore volume is associated with pores that are about 100 nanometers in size (diameter) or larger. In other words, for the curve of ⁇ - vs.
• the fraction of the area under the curve from a pore dLog ⁇ r) diameter of about 100 nanometers to about 1,000 nanometers is at least about 25%, or at least about 30%, or at least about 40%, or at least about 50%, or at least about 60% of the total area under that curve for pore sizes (or diameters assuming cylindrical pores) of from about 10 nanometers to about 1,000 nanometers.
• the area under the curve for pore diameters of from about 100 nanometers to about 1,000 nanometers is in the range of about 25% to about 60%, or about 30% to about 55%, or about 35% to about 50% of the total area under the curve for pore diameters of from about 10 nanometers to about 1,000 nanometers.
• the microporous membrane of the present invention has relatively large internal space and openings due to coarse domains, it has suitable permeability and electrolytic solution absorption, with little air permeability variation when compressed.
• This microporous membrane also has relatively small internal space and openings which influence safety properties of the membrane when used as a battery separator, such as shutdown temperature and shutdown speed. Accordingly, lithium ion batteries such as lithium ion secondary batteries comprising separators formed by such microporous membrane have suitable productivity and cyclability while retaining their high safety performance.
• the mercury intrusion porosimetry method used to determine the microporous membrane structure involves use of a Pore Sizer 9320 (Micromeritics Company, Ltd.), a pressure range of from 3.6 kPa to 207 MPa, and a cell volume of 15 cm 3 .
• a contact angle of mercury of 141.3 and a surface tension of mercury of 484 dynes/cm was employed.
• the parameters obtained by this included pore volume, surface area ratio, peak top of pore size, average pore size and porosity. References teaching this method include Raymond P. Mayer and Robert A. Stowe, J. Phys. Chem.70, 12(1966); L. C.
• L is the depth and r is the radius of the pores, assuming the pores are cylindrical.
• the contact angle of the mercury is expressed as ⁇ .
• the measurement of differential pore volume can then proceed as follows. First, the volume V of mercury intruded into the pores is measure as a function of pressure P. The measured value of P is used to calculate pore radius r, as described above. P is increased incrementally, and the volume of mercury is determined at each value of P.
• a table can be constructed showing the pore volume associated with pores of a particular r, tabulated over the range of r as determined by the range of P selected for the measurement.
• the values of r in the table can be conveniently converted to Log (r).
• Pore volume Vp is generally expressed as cm 3 per gram of the microporous membrane.
• Differential pore volume expressed as ⁇ - can be calculated from the tabulated values of Vp and r, dLog ⁇ r) where dVp is approximated by the difference between adjacent values of Vp in the table, and where dLog(r) is approximated by the difference between adjacent values of Log( r) in the table.
• Figure 5 is a representative of a Vp (pore volume) curve for an MPF having a hybrid structure (Example7).
• the microporous polyolef ⁇ n membrane has relatively large internal space and openings due to coarse domains, it has suitable permeability and electrolytic solution absorption, with little air permeability variation when compressed. Accordingly, lithium ion batteries such as lithium ion secondary batteries comprising separators formed by such microporous polyolefm membrane have suitable productivity and cyclability.
• microporous polyolefm membrane is a single-layer membrane.
• microporous polyolef ⁇ n membrane is a multi-layer membrane.
• the multi-layer, microporous polyolef ⁇ n membrane comprises two layers where the first layer (e.g., the upper layer) comprises a first microporous layer material, and the second layer (e.g., the bottom layer) comprises an independently-selected second microporous layer material.
• At least one of the first or second microporous layer materials is characterized by a hybrid structure, i.e., the microporous layer material is characterized by a differential pore volume curve having an area under the curve over the range of pore diameters of from about 100 nm to about 1,000 nm that is about 25% or more of a total area under the curve over the range of pore diameters of from about 10 nm to about 1,000 nm.
• the microporous membrane is a multi-layer, microporous membrane which comprises three or more layers, wherein the outer layers (also called the "surface” or “skin” layers) comprise the first microporous layer material and at least one intermediate (or interior) layer situated comprises an independently selected second microporous layer material.
• the interior layer(s) of the multi-layer, microporous polyolefm membrane are located between the surface layers, and optionally at least one interior layer is in planar contact with at least one surface layer.
• at least one layer of the multi-layer microporous membrane is characterized by a hybrid structure.
• the multi-layer, microporous membrane has three or more layers, the multi-layer, microporous membrane has at least one layer comprising the first microporous layer material and at least one layer comprising an independently selected second microporous layer material.
• such multilayer microporous membranes can be made by processes such as lamination, co-extrusion, etc.
• the method described above in section [2] is suitable for making extrudates or microporous membranes which can be laminated to form the multi-layer microporous membranes.
• the method of section [2] can be used with a co extrusion die in step (2) of the method to make a multi-layer extrudate which is then processed in accordance with steps 3 through 7 of the method to make the multi-layer membrane.
• the optional steps described in that section can also be used, if desired.
• at least one layer of the multi-layer microporous membrane is characterized by a hybrid structure.
• the polyolefin solution used to produce the hybrid-structure layer or layers is as described in section [2] above.
• the thickness of each layer of the multilayer membrane is independently selected. The thickness of a layer can be the same as at least one other layer, but this is not required.
• the microporous membranes of the invention generally have a shutdown temperature in the range of from about 13O 0 C to about 14O 0 C, and a meltdown temperature in the range of about 145°C to 200 0 C.
• the meltdown temperature is generally in the range of about 16O 0 C to about 200 0 C.
• the microporous polyolefin membranes have at least one of the following properties. Please add reasonable shut down and melt down temperature ranges.
• the membrane's air permeability measured according to JIS P8117 is from 20 to 400 seconds/ 100 cm 3
• batteries with separators formed by the microporous membrane have suitably large capacity and good cyclability.
• the air permeability is less than 20 seconds/ 100 cm 3 , shutdown does not sufficiently occur because pores are so large that they cannot fully close when the temperatures inside the batteries are elevated at 140 0 C or more.
• the microporous membrane When the porosity is less than 25 %, the microporous membrane is not believed to have good air permeability. When the porosity exceeds 80 %, battery separators formed by the microporous membrane are believed to have insufficient strength, which can result in the short-circuiting of battery's electrodes.
• Pin puncture strength of 2.000 mN or more (converted to the value at 20- ⁇ m thickness)
• the membrane's pin puncture strength (converted to the value at membrane thickness of 20- ⁇ m) is represented by the maximum load measured when the microporous membrane is pricked with a needle of 1 mm in diameter with a spherical end surface (radius R of curvature: 0.5 mm) at a speed of 2 mm/second.
• the pin puncture strength is less than 2,000 mN/20 ⁇ m, short-circuiting might occur in batteries with separators formed by the microporous membrane.
• a tensile rupture strength of 49,000 kPa or more in both longitudinal and transverse directions is characteristic of suitable durable microporous membranes, particularly when used as a battery separator.
• the tensile rupture strength is preferably 80,000 kPa or more.
• a tensile rupture elongation of 100% or more in both longitudinal and transverse directions is characteristic of suitably durable microporous membranes, particularly when used as a battery separator.
• the thickness variation ratio after heat compression at 9O 0 C under pressure of 2.2 MPa for 5 minutes is generally 20% or less, per 100% of the thickness before compression.
• Batteries comprising microporous membrane separators with a thickness variation ratio of 20% or less have suitably large capacity and good cyclability.
• the microporous polyolefm membrane when heat-compressed under the above conditions generally has air permeability (Gurley value) of 700 sec/100 cm 3 or less. Batteries using such membranes have suitably large capacity and cyclability.
• the air permeability is preferably 650 sec/100 cm 3 or less.
• the surface roughness of the membrane measured by an atomic force microscope (AFM) in a dynamic force mode is generally 3 x 10 ran or more (measured as the maximum height difference).
• the membrane's surface roughness is preferably 3.5 x 10 2 nm or more.
• the measurement can be made using conventional equipment, e.g., model SPA500 available from SII Nano Technology Inc.
• the maximum height difference is defined as (the height of the highest point on the surface) - (that of the lowest point) over the region of membrane examined.
• a microporous membrane sample was immersed in an electrolytic solution (electrolyte: 1 mol/L of LiPF ⁇ , solvent: ethylene carbonate/dimethyl carbonate at a volume ratio of 3/7) kept at 18°C, to determine an electrolytic solution absorption speed by the formula of [weight increment (g) of microporous membrane / weight (g) of microporous membrane before absorption].
• electrolytic solution electrolytic solution
• the "normalized” electrolytic solution absorption speed means the electrolytic solution absorption speed is expressed relative to the measured value of the electrolytic solution absorption rate in the microporous membrane of Comparative Example 1.
• the measured electrolytic solution absorption speed for a microporous membrane having a hybrid structure is divided by the electrolytic solution absorption speed measured for the membrane of Comparative Example 1 , then the quotient, i.e., the normalized electrolytic solution absorption speed, is greater than 1.
• Microporous polyolefin membrane composition (1) Polvolefin
• the microporous polyolefin membrane generally comprises the polyolefm used to form the polyolefm solution.
• a small amount of washing solvent and/or membrane-forming solvent can also be present, generally in amounts less than 1 wt% based on the weight of the microporous polyolefm membrane.
• a small amount of polyolefm molecular weight degradation might occur during processing, but this is acceptable.
• molecular weight degradation during processing if any, causes the value of Mw/Mn of the polyolefm in the membrane to differ from the Mw/Mn of the polyolefm solution by no more than about 50%, or no more than about 10%, or no more than about 1%, or no more than about 0.1%.
• the microporous membrane of the present invention comprises (a) a first polyethylene having a weight average molecular weight of from about 2.5 x 10 5 to about 5 x 10 5 , and optionally polyolef ⁇ ns (b) and (c), where (b) is from about 0 to about 7 % of a second polyethylene having a weight average molecular weight of from about 5 x 10 5 to about 1 x 10 6 , and (c) is from about 0 to about 25 % of a polypropylene having a weight average molecular weight that does not exceed about 1.5 x 10 6 .
• the first polyethylene can be, for example, a high density polyethylene having a weight average molecular weight of from about 2.5 x 10 5 to about 5 x 10 5 and a molecular weight distribution of from about 5 to about 100.
• Anon-limiting example of the first polyethylene of the membrane is one that has a weight average molecular weight of from about 2.5 x 10 5 to about 4 x 10 5 and a molecular weight distribution of form about 7 to about 50.
• the first polyethylene of the membrane can be an ethylene homopolymer, or an ethylene/ ⁇ -olefm copolymer, such as, for example, one containing a small amount, e.g. about 5 mole %, of a third ⁇ -olef ⁇ n.
• the third ⁇ -olefm which is not ethylene, is preferably propylene, butene-1, pentene-1, hexene-1, 4-methylpentene- 1 , octene-1, vinyl acetate, methyl methacrylate, or styrene or combinations thereof.
• the second polyethylene for example an ultra-high molecular weight polyethylene, in the membrane has a weight average molecular weight that is greater than about 5 x 10 5 .
• the second polyethylene has a molecular weight distribution of from about 5 to about 100.
• the Mw of the second polyethylene can be, for example, in the range of about 1 x 10 6 to 15 x 10 6 , or about 1 x 10 6 to 5 x 10 6 , or about 1 x 10 6 to 3 x 10 6 .
• Anon-limiting example of the second polyethylene of the membrane is one that has a weight average molecular weight of from about 5 x 10 5 to about 8 x 10 5 and a molecular weight distribution of form about 5 to about 50.
• the second polyethylene of the membrane can be an ethylene homopolymer, or an ethylene/ ⁇ -olefin copolymer, such as, for example, one containing a small amount, e.g. about 5 mole %, of a third ⁇ -olefin.
• the third ⁇ -olefin, which is not ethylene, is preferably propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-l, octene-1, vinyl acetate, methyl methacrylate, or styrene or combinations thereof.
• Mw /Mn of the polyethylene in the microporous membrane is preferably from about 5 to about 100, for example from about 7 to about 50.
• Mw/Mn is less than 5, the percentage of a higher molecular weight component is too high to conduct melt extrusion easily.
• Mw/Mn is more than 100, the percentage of a lower molecular weight component is too high, resulting in decrease in the strength of the resulting microporous membrane.
• Mw of the first polyethylene in the membrane product may be lower than that of the first polyethylene resin in the polyolefin composition portion of the polyolef ⁇ n solution of method step (1).
• the optional polypropylene in the membrane has a weight average molecular weight of from about 3 x 10 5 to about 1.5 x 10 6 , for example from about 6 x 10 5 to about 1.5 x 10 6 , a heat of fusion of 80 J/g or higher, for non-limiting example from about 80 to about 120 J/g, and a molecular weight distribution of from about 1 to about 100, for example from about 1.1 to about 50, and can be a propylene homopolymer or a copolymer of propylene and another, i.e. a fourth, olefin, though the homopolymer is preferable.
• the copolymer may be a random or block copolymer.
• the fourth olefin which is an olefin other than propylene, includes ⁇ -olefins such as ethylene, butene-1, pentene-1, hexene-1, 4-methylpentene-l, octene-1, vinyl acetate, methyl methacrylate, styrene, etc., and diolefms such as butadiene, 1,5-hexadiene, 1,7-octadiene, 1 ,9-decadiene, etc.
• ⁇ -olefins such as ethylene, butene-1, pentene-1, hexene-1, 4-methylpentene-l, octene-1, vinyl acetate, methyl methacrylate, styrene, etc.
• diolefms such as butadiene, 1,5-hexadiene, 1,7-octadiene, 1 ,9-decadiene, etc.
• the percentage of the fourth olefin in the propylene copolymer is preferably in a range not deteriorating the properties of the microporous polyolefin membrane such as heat resistance, compression resistance, heat shrinkage resistance, etc., and is preferably less than about 10 mole %, e.g. from about 0 to less than about 10 mole %.
• some degradation of Mw from that of the starting resins may occur during manufacturing of the membrane by the present method, for example the Mw of the polypropylene in the membrane product may be lower than that of the polypropylene resin in the polyolef ⁇ n composition portion of the polyolefin solution of method step (1).
• the heat of fusion is determined by differential scanning calorimetry (DSC).
• DSC differential scanning calorimetry
• the DSC is conducted using a TA Instrument MDSC 2920 or QlOOO Tzero-DSC and data analyzed using standard analysis software.
• 3 to 10 mg of polymer is encapsulated in an aluminum pan and loaded into the instrument at room temperature.
• the sample is cooled to either -130 0 C or -70 0 C and heated to 210 0 C at a heating rate of 10 °C/minute to evaluate the glass transition and melting behavior for the sample.
• the sample is held at 210 0 C for 5 minutes to destroy its thermal history. Crystallization behavior is evaluated by cooling the sample from the melt to sub-ambient temperature at a cooling rate of 10 °C/minute.
• Second heating data is measured by heating this melt crystallized sample at 10 °C/minute. Second heating data thus provides phase behavior for samples crystallized under controlled thermal history conditions.
• the endothermic melting transition (first and second melt) and exothermic crystallization transition are analyzed for onset of transition and peak temperature. The area under the curve is used to determine the heat of fusion ( ⁇ H f ).
• the amount of polypropylene in the membrane is 55% or less by mass, or 40% or less by mass, or 25 % or less by mass based on the total mass of polyolefin in the membrane. Too large a percentage of polypropylene in the membrane can result in the microporous membrane having lower strength.
• the percentage of polypropylene is preferably 20 % or less by mass, more preferably 15 % or less by mass.
• the membrane can contain an additional polyolefin and/or heat-resistant polymer having melting points or glass transition temperatures (Tg) of about 170 0 C or higher.
• Tg melting points or glass transition temperatures
• the additional polyolefin can be one or more of (a) polybutene- 1 , polypentene- 1 , poly-4-methylpentene-l, polyhexene-1, polyoctene- 1 , polyvinyl acetate, polymethyl methacrylate, polystyrene and an ethylene/ ⁇ -olefm copolymer, each of which may have an Mw of from 1 x 10 4 to 4 x 10 6 , and (b) a polyethylene wax having an Mw of form 1 x 10 3 to 1 x 10 4 .
• Polybutene- 1, polypentene- 1, poly-4-methylpentene-l, polyhexene-1, polyoctene- 1, polyvinyl acetate, polymethyl methacrylate and polystyrene are not restricted to homopolymers, but may be copolymers containing other ⁇ -olefms.
• the heat-resistant polymers are preferably (i) amorphous polymers having melting points of about 170 0 C or higher, which may be partially crystalline, and/or (ii) amorphous polymers having a Tg of about 170 0 C or higher.
• the melting point and Tg are determined by differential scanning calorimetry (DSC) according to JIS K7121.
• the heat-resistant polymers include polyesters such as polybutylene terephthalate (melting point: about 160 to 230 0 C), polyethylene terephthalate (melting point: about 250 to 270 0 C), etc., fluororesins, polyamides (melting point: 215 to 265 0 C), polyarylene sulfide, polyimides (Tg: 280 0 C or higher), polyamide imides (Tg: 280 0 C), polyether sulfone (Tg: 223 0 C), polyetheretherketone (melting point: 334 0 C), polycarbonates (melting point: 220 to 240 0 C), cellulose acetate (melting point: 220 0 C), cellulose triacetate (melting point: 300 0 C), polysulfone (Tg: 190 0 C), polyetherimide (melting point: 216 0 C), etc.
• polyesters such as polybutylene terephthalate (melting point: about 160
• the total amount of the additional polyolefm and the heat-resistant polymer in the membrane is preferably 20 % or less by mass per 100 % by mass of the membrane.
• the battery separator formed from any of the above microporous membranes has a thickness of 3 to 200 ⁇ m, or 5 to 50 ⁇ m, or 10 to 35 ⁇ m, though the most suitable thickness can be readily selected depending on the type of battery manufactured.
• microporous polyolefin membranes of the present invention may be used as separators for primary and secondary batteries, particularly such as lithium ion secondary batteries, lithium-polymer secondary batteries, nickel-hydrogen secondary batteries, nickel-cadmium secondary batteries, nickel-zinc secondary batteries, silver-zinc secondary batteries, particularly for lithium ion secondary batteries.
• primary and secondary batteries particularly such as lithium ion secondary batteries, lithium-polymer secondary batteries, nickel-hydrogen secondary batteries, nickel-cadmium secondary batteries, nickel-zinc secondary batteries, silver-zinc secondary batteries, particularly for lithium ion secondary batteries.
• the lithium ion secondary battery comprises a cathode and an anode laminated via a separator, and the separator contains an electrolyte, usually in the form of an electrolytic solution ("electrolyte").
• the electrode structure is not critical. Conventional structures are suitable. The electrode structure may be, for instance, a coin type in which a disc-shaped positive and anodes are opposing, a laminate type in which planar positive and anodes are alternately laminated, a toroidal type in which ribbon-shaped positive and anodes are wound, etc.
• the cathode usually comprises a current collector, and a cathodic active material layer capable of absorbing and discharging lithium ions which is formed on the current collector.
• the cathodic active materials may be inorganic compounds such as transition metal oxides, composite oxides of lithium and transition metals (lithium composite oxides), transition metal sulfides, etc.
• the transition metals may be V, Mn, Fe, Co, Ni, etc.
• Preferred examples of the lithium composite oxides are lithium nickelate, lithium cobaltate, lithium manganate, laminar lithium composite oxides based on Ci-NaFeO 2 , etc.
• the anode comprises a current collector, and a negative-electrode active material layer formed on the current collector.
• the negative-electrode active materials may be carbonaceous materials such as natural graphite, artificial graphite, coke, carbon black, etc.
• the electrolytic solution can be a solution obtained by dissolving a lithium salt in an organic solvent.
• the lithium salt may be LiClO 4 , LiPF 6 , LiAsF 6 , LiSbF 6 , LiBF 4 , LiCF 3 SO 3 , LiN(CF 3 SO 2 ) 2 , LiC(CF 3 SO 2 ) 3 , Li 2 Bi 0 Cli 0 , LiN(C 2 F 5 SO 2 ) 2 , LiPF 4 (CF 3 ) 2 , LiPF 3 (C 2 Fj) 3 , lower aliphatic carboxylates of lithium, LiAlCl 4 , etc.
• These lithium salts may be used alone or in combination.
• the organic solvent may be an organic solvent having a high boiling point and high dielectric constant such as ethylene carbonate, propylene carbonate, ethylmethyl carbonate, ⁇ -butyrolactone, etc.; and/or organic solvents having low boiling points and low viscosity such as tetrahydrofuran, 2-methyltetrahydrofuran, dimethoxyethane, dioxolane, dimethyl carbonate, dimethyl carbonate, etc. These organic solvents may be used alone or in combination. Because the organic solvents having high dielectric constants generally have high viscosity, while those having low viscosity generally have low dielectric constants, their mixtures are preferably used.
• the separator When the battery is assembled, the separator is impregnated with the electrolytic solution, so that the separator (microporous polyolef ⁇ n membrane) is provided with ion permeability.
• the impregnation treatment is usually conducted by immersing the microporous membrane in the electrolytic solution at room temperature.
• a cathode sheet, a microporous membrane separator and an anode sheet are laminated in this order, and the resultant laminate is wound to a toroidal-type electrode assembly.
• the resultant electrode assembly is charged/formed into a battery can and then impregnated with the above electrolytic solution, and the battery lid acting as a cathode terminal provided with a safety valve is caulked to the battery can via a gasket to produce a battery.
• Dry-blended were 100 parts by mass of (i) a polyethylene (PE) composition comprising 5% by mass of ultra-high-molecular- weight polyethylene (UHMWPE) having a weight-average molecular weight (Mw) of 1.5 x 10 6 and a molecular weight distribution (Mw/Mn) of 8, and (ii) 95% by mass of high-density polyethylene (HDPE) having Mw of 3.0 x 10 5 and Mw/Mn of 8.6, and 0.2 parts by mass of tetrakis[methylene-3-(3,5-ditertiary-butyl-4-hydroxyphenyl)-propionate] methane as an antioxidant.
• the polyethylene in the mixture had a melting point of 135 0 C, a crystal dispersion temperature of 100 0 C, and Mw/Mn of 10.0.
• Measurement apparatus GPC- 15 OC available from Waters Corporation,
• Calibration curve Produced from a calibration curve of a single-dispersion, standard polystyrene sample using a predetermined conversion constant.
• the gel-like sheet was simultaneously biaxially stretched at 118.5 0 C to 5 fold in both longitudinal and transverse directions at a stretching rate of 1.0 meter per minute.
• the stretched gel-like sheet was fixed to an aluminum frame of 20 cm x 20 cm, immersed in a bath of methylene chloride controlled at 25°C to remove the liquid paraffin with vibration of 100 rpm for 3 minutes, and dried by an air flow at room temperature.
• the dried membrane was re-stretched by a batch-stretching machine to a magnification of 1.5 fold in a transverse direction at 129°C.
• the re-stretched membrane which remained fixed to the batch-stretching machine, was heat-set at 129 0 C for 30 seconds to produce a microporous polyethylene membrane.
• a microporous polyethylene membrane was produced in the same manner as in Example 1, except that a polyethylene having a melting point of 135°C, a crystal dispersion temperature of 100 0 C and Mw/Mn of 10.0, which comprised 2% by mass of UHMWPE having Mw of 2.0 x 10 6 and Mw/Mn of 8, and 98% by mass of HDPE having Mw of 2.5 x 10 5 and Mw/Mn of 8.9, was used. Also, the stretching temperature of the gel-like sheet was 119°C, the stretching magnification of the microporous membrane was 1.4 fold, and the stretching and heat-setting temperatures of the microporous membrane were both 129.5 0 C.
• a microporous polyethylene membrane was produced in the same manner as in Example 1, except that polyethylene having a melting point of 135°C, a crystal dispersion temperature of 100 0 C, and Mw/Mn of 10.0, which comprised 2% by mass of UHMWPE having Mw of 2.0 x 10 6 and Mw/Mn of 8, and 98% by mass of HDPE having Mw of 3.0 x 10 5 and Mw/Mn of 8.6, was used. Also, the stretching magnification of the microporous membrane was 1.4 fold, and the stretching and heat-setting temperatures of the microporous membrane were both 126 0 C.
• a microporous polyethylene membrane was produced in the same manner as in Example 1, except that a polyethylene having a melting point of 135°C, a crystal dispersion temperature of 100 0 C, and Mw/Mn of 10.5, which comprised 3% by mass of UHMWPE having Mw of 2.0 x 10 6 and Mw/Mn of 8, and 97% by mass of HDPE having Mw of 3.0 x 10 5 and Mw/Mn of 8.6, was used. Also, the stretching temperature of the gel-like sheet was 117°C, the stretched gel-like sheet was heat-set at 122°C for 10 seconds, and the stretching and heat-setting temperatures of the microporous membrane were both 130 0 C.
• a microporous polyethylene membrane was produced in the same manner as in Example 1, except that only HDPE having Mw of 3.0 x 10 5 , Mw/Mn of 8.6, a melting point of 135°C and a crystal dispersion temperature of 100 0 C was used as a polyolefin, that the stretching temperature of the gel-like sheet was 118°C, and that the stretching magnification of the microporous membrane was 1.4 fold.
• a microporous polyethylene membrane was produced in the same manner as in Example 1 , except that the same polyethylene as in Example 4 was used. Also, the concentration of the polyethylene in the polyethylene solution was 30% by mass based on the mass of the polyethylene solution. Also, the stretching temperature of the gel-like sheet was 118°C, and the stretching magnification of the microporous membrane was 1.4 fold.
• a microporous polyolefin membrane was produced in the same manner as in Example 1, except that a polyolefin composition was used comprising 3% by mass of UHMWPE having Mw of 2.0 x 10 6 and Mw/Mn of 8, 92% by mass of HDPE having Mw of 3.0 x 10 5 and Mw/Mn of 8.6, and 5% by mass of a propylene homopolymer (PP) having Mw of 5.3 x 10 5 .
• the polyethylene had a melting point of 135°C, a crystal dispersion temperature of 100 0 C, and Mw/Mn of 10.5.
• the concentration of the polyolefin composition in the polyolefin solution was 35% by mass based on the mass of the polyolefin solution, the stretching temperature of the gel-like sheet was 116°C, the stretching magnification of the microporous membrane was 1.4 fold, and that the stretching and heat-setting temperatures of the microporous membrane were both 127°C .
• the Mw of PP was measured by a GPC method as above.
• a microporous polyethylene membrane was produced in the same manner as in Example 1, except that polyethylene having a melting point of 135°C, a crystal dispersion temperature of 100 0 C, and Mw/Mn of 14.4, which comprised 20% by mass of UHMWPE having Mw of 2.0 x 10 6 and Mw/Mn of 8, and 80% by mass of HDPE having Mw of 3.5 x 10 5 and Mw/Mn of 8.6, was used.
• the concentration of the polyethylene in the polyethylene solution was 30% by mass
• the stretching temperature of the gel-like sheet was 115 0 C
• the microporous membrane containing a washing solvent was heat-set at 126.8°C for 10 seconds
• the microporous membrane was not stretched and heat-set.
• a microporous polyethylene membrane was produced in the same manner as in Example 1, except that polyethylene having a melting point of 135°C, a crystal dispersion temperature of 100 0 C, and Mw/Mn of 10.0, which comprised 2% by mass of UHMWPE having Mw of 2.0 x 10 6 and Mw/Mn of 8, and 98% by mass of HDPE having Mw of 3.5 x 10 5 and Mw/Mn of 8.6, was used. Also, the concentration of the polyethylene solution was 30% by mass, the stretching temperature of the gel-like sheet was 118 0 C, and the microporous membrane was heat-set at 128°C for 10 seconds without being stretched.
• a microporous polyethylene membrane was produced in the same manner as in Example 1, except that polyethylene having a melting point of 135°C, a crystal dispersion temperature of 100 0 C, and Mw/Mn of 10.5, which comprised 3% by mass of UHMWPE having Mw of 2.0 x 10 6 and Mw/Mn of 8, and 97% by mass of HDPE having Mw of 3.5 x 10 5 and MwMn of 8.6, was used. Also, the concentration of the polyethylene solution was 30% by mass, the stretching temperature of the gel-like sheet was 115°C, microporous membrane was stretched to 2.0 folds at 130 0 C, and the heat-setting temperature was 126°C.
• a microporous polyethylene membrane was produced in the same manner as in Example 1 , except that the same polyethylene as in Comparative Example 1 was used. Also, the concentration of the polyethylene solution was 30% by mass, the stretching temperature of the gel-like sheet was 118°C, microporous membrane was stretched to 1.4 folds at 130 0 C, and the heat-setting temperature was 130 0 C.
• a gel-like sheet was formed in the same manner as in Example 1, except for using polyethylene having a melting point of 135 0 C, a crystal dispersion temperature of 100 0 C, and Mw/Mn of 10.1, which comprised 2% by mass of UHMWPE having Mw of 2.0 x 10 6 and Mw/Mn of 8, and 98% by mass of HDPE having Mw of 2.0 x 10 5 and Mw/Mn of 8.9.
• polyethylene having a melting point of 135 0 C, a crystal dispersion temperature of 100 0 C, and Mw/Mn of 10.1, which comprised 2% by mass of UHMWPE having Mw of 2.0 x 10 6 and Mw/Mn of 8, and 98% by mass of HDPE having Mw of 2.0 x 10 5 and Mw/Mn of 8.9.
• a microporous polyethylene membrane was produced in the same manner as in Example 1 , except that the same polyethylene as in Comparative Example 5 was used. Also, the stretching temperature of the gel-like sheet was 119°C, the microporous membrane was stretched to 1.4 fold at 129.5 0 C, and the heat-setting temperature was 12O 0 C.
• a microporous polyethylene membrane was produced in the same manner as in
• Example 1 except that the same polyethylene as in Example 2 was used. Also, the stretching temperature of the gel-like sheet was 119°C, the microporous membrane was stretched to 1.4 fold at 115°C, and the heat-setting temperature was 134°C.
• Comparative Examples 1-7 were measured by the following methods. The results are shown in
• Example 1 their pore size distribution curves obtained by mercury intrusion porosimetry are shown in Figs. 2-4.
• each microporous membrane was measured by a contact thickness meter at 5-cm longitudinal intervals over the width of 30 cm, and averaged.
• the maximum load was measured, when each microporous membrane having a thickness of Ti was pricked with a needle of 1 mm in diameter with a spherical end surface (radius R of curvature: 0.5 mm) at a speed of 2 mm/second.
• a microporous membrane sample was situated between a pair of highly flat plates, and heat-compressed by a press machine under a pressure of 2.2 MPa (22 kgf/cm 2 ) at 9O 0 C for 5 minutes, to determine an average thickness in the same manner as above.
• a thickness variation ratio was calculated by the formula of (average thickness after compression - average thickness before compression) / (average thickness before compression) x 100.
• Each microporous membrane having a thickness of Ti was heat-compressed under the above conditions, and measured with respect to air permeability Pi according to JIS P8117.
• a microporous membrane sample was immersed in an electrolytic solution (electrolyte: 1 mol/L of LiPF ⁇ , solvent: ethylene carbonate/dimethyl carbonate at a volume ratio of 3/7) kept at 18°C, to determine an electrolytic solution absorption speed by the formula of [weight increment (g) of microporous membrane / weight (g) of microporous membrane before absorption].
• the electrolytic solution absorption speed is expressed by a relative value, assuming that the electrolytic solution absorption rate in the microporous membrane of Comparative Example 1 is 1.
• Mw represents a weight-average molecular weight
• MWD represents a molecular weight distribution
• Tm represents the melting point of the polyethylene (composition)
• Ted represents the crystal dispersion temperature of the polyethylene (composition)
• SFM surface roughness measured by AFM m a dynamic force mode (DFM)
• DFM dynamic force mode
• each microporous membrane of Examples 1-7 are characterized by a hybrid structure, i.e., in the curve of differential pore volume - ⁇ - ⁇ shows dLog(r) the presence of a significant number of pores with pore sizes between lOOnm and lOOOnm. Moreover, as can be seen from Figures 1 and 2, the fraction of the area under the curve from a pore diameter of about 100 nanometers to about 1,000 nanometers is 25% or more of the total area under that curve for pore diameters of from about 10 nanometers to about 1,000 nanometers.
• the membrane of Comparative Example 2 does not have a hybrid structure, and the fraction of the area under the curve from a pore diameter of about 100 nanometers to about 1,000 nanometers is significantly less than 20% of the total area under that curve for pore diameters of from about 10 nanometers to about 1,000 nanometers, as shown by Fig. 3.
• the microporous membranes of Examples 1-7 have suitable air permeability, pin puncture strength, tensile rupture strength, tensile rupture elongation and heat shrinkage resistance, as well as suitable electrolytic solution absorption, with little variation of thickness and air permeability after heat compression.
• Comparative Example 1 did not perform as well as those of Examples 1-7 in air permeability, air permeability after heat compression, and electrolytic solution absorption. It is believed that this is the case because the microporous membrane of Comparative Example 1 was obtained from a polyethylene composition containing more than 7% by mass of UHMWPE.
• Comparative Example 2 did not perform as well as those of Examples 1-7 in pin puncture strength, air permeability after heat compression and electrolytic solution absorption. It is believed that this is the case because the microporous membrane was not stretched.
• the membranes of Comparative Examples 3 and 4 did not perform as well as those of Examples 1 -7 in electrolytic solution absorption. It is believed that this is the case because the stretching magnification of the microporous membrane was more than 1.8 fold in Comparative Example 3, and because a polyethylene composition containing more than 7% by mass of UHMWPE was used in Comparative Example 4.
• the membrane contains 2% by mass of ultra-high molecular weight polyethylene and 93% by mass of high density polyethylene, and 5% polypropylene.
• the cooled extrudate was biaxially stretched at a temperature of 95°C, and the membrane was subjected to a second stretching step at a stretching magnification of 1.4.
• the heat setting temperature was 127.5 0 C.

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