JP5411550B2 - Polyolefin microporous membrane - Google Patents

Description

  The present invention relates to a polyolefin microporous membrane.

Polyolefin microporous membranes are used in microfiltration membranes, separators for electronic devices such as batteries, capacitors and capacitors, and materials for fuel cells.
Among the electronic devices, when used as a separator for a lithium ion battery, a characteristic required for a polyolefin microporous film is excellent heat resistance. The heat resistance refers to low heat shrinkage, in which heat shrinkage in the vicinity of the melting point is low from the crystal dispersion temperature of the separator, and film breakage resistance, in which the film does not break even when the separator is heated to the melting point or higher.
Among them, a method of blending polypropylene or the like with a microporous film containing polyethylene as a main component is conventionally known as a technique for improving the tear resistance of a polyolefin microporous film. Since polypropylene generally has a crystalline melting point of 160 ° C. or higher, it is considered that the membrane shape can be maintained even when the polyolefin microporous membrane is heated to a temperature higher than that of polyethylene.

Patent Document 1 discloses a polyolefin microporous membrane containing 5 to 35% by weight of polypropylene having an ethylene content of 1.0% by weight or less.
Patent Document 2 discloses a polyolefin microporous film characterized by containing polypropylene having a crystalline melting point of 163 ° C. or higher.
Patent Document 3 discloses a microporous membrane made of polyolefin containing polypropylene having a crystal melting heat of 90 J / g or more.
Patent Document 4 discloses a polyolefin microporous membrane containing an ethylene-propylene copolymer having a propylene fraction of less than 50 wt%.

Japanese Patent No. 3699561 Japanese Patent No. 4195810 Japanese Patent No. 4121284 Japanese Patent No. 3281866

However, the polyolefin microporous films described in Patent Documents 1 to 4 still have room for improvement from the viewpoint of heat resistance.
Therefore, the present invention has been made in view of the above circumstances, and an object thereof is to provide a polyolefin microporous film having excellent heat resistance.

  The present inventors have intensively studied to solve the above problems. As a result, it has been surprisingly found that the above-mentioned problems can be achieved by blending specific polypropylenes, and the present invention has been achieved.

That is, the present invention is as follows.
[1]
In a polyolefin microporous membrane comprising a polyolefin composition containing 50 to 97% by mass of polyethylene and 3 to 50% by mass of polypropylene,
The polypropylene has the following components (A) and (B):
(A) a homopolypropylene having a propylene fraction of more than 99% by mass and / or a propylene copolymer having a propylene fraction of 99% by mass or less and a crystal melting point of 155 ° C. or more (a),
(B) a propylene copolymer (b) having a propylene fraction of 99% by mass or less and a crystal melting point of less than 155 ° C.,
A microporous membrane made of polyolefin.
[2]
The polyolefin microporous membrane according to [1], wherein the polypropylene copolymer (a) has a propylene fraction of 75 to 99 mass%.
[3]
The polyolefin microporous membrane according to [1] or [2], wherein the propylene copolymer (b) has a propylene fraction of 60 to 99% by mass.
[4]
The proportion of the propylene copolymer (b) in the total amount of the homopolypropylene, the polypropylene copolymer (a) and the polypropylene copolymer (b) is 20 to 80% by mass, [1] to [ 3] The polyolefin microporous membrane according to any one of [3].
[5]
The polyolefin microporous membrane according to any one of [1] to [4], wherein the propylene copolymer (b) has a heat of crystal fusion of 40 to 85 J / g.
[6]
The polyolefin microporous membrane according to any one of [1] to [5], wherein the propylene copolymer (b) has a crystalline melting point of 110 to 153 ° C.

  The polyolefin microporous membrane of the present invention is excellent in heat resistance.

  Hereinafter, the best mode for carrying out the present invention (hereinafter abbreviated as “this embodiment”) will be described in detail. In addition, this invention is not limited to the following embodiment, It can implement by changing variously within the range of the summary.

The polyolefin microporous membrane of the present embodiment (hereinafter sometimes simply referred to as “microporous membrane”) is 50 to 97% by mass of polyethylene (hereinafter sometimes abbreviated as “PE”) and polypropylene. (Hereinafter, may be abbreviated as “PP”) and a polyolefin composition containing 3 to 50% by mass.
The proportion of PE in the polyolefin composition is preferably 70 to 95% by mass, more preferably 70 to 93% by mass.
Moreover, as a ratio for which PP accounts to a polyolefin composition, Preferably it is 5-30 mass%, More preferably, it is 7-30 mass%.
Furthermore, the mass ratio of PE and PP in the polyolefin composition is preferably PE: PP = 50: 50 to 97: 3, more preferably 70:30 to 95: 5, and still more preferably 70: 30-93: 7.

  The polyolefin composition contains α-olefin homopolymers such as polybutene, polymethylpentene, and polyoctene, and polyesters such as polyethylene terephthalate and polyethylene naphthalate within a range of 10% by mass or less based on the total mass of PE and PP. It is possible to blend resins and various engineering resins.

[polyethylene]
Examples of PE include homopolymers of ethylene and copolymers of ethylene and α-olefins such as propylene, butene, pentene, hexene, and octene.
The density of PE is preferably 0.90~0.98g / cm ^3, more preferably 0.93~0.97g / cm ^3, more preferably a 0.94~0.96g / cm ^3. The density is preferably 0.90 or more from the viewpoint of satisfactorily realizing the microporous membrane.
In addition, the α-olefin content with respect to the ethylene unit in the case of using the α-olefin copolymer is preferably 0.1 mol% or more from the viewpoint of lowering the fuse temperature of the polyolefin microporous film. From the viewpoint of preventing a decrease in permeability of the microporous membrane due to a decrease in crystallinity, it is preferably 2 mol% or less. More preferably, it is 0.1-1 mol%.
The polymerization catalyst for polyethylene is not particularly limited, but a Ziegler-Natta catalyst, a Phillips catalyst, a metallocene catalyst, or the like can be used.

The viscosity average molecular weight (Mv) of PE is preferably 100,000 to 10,000,000, more preferably 250,000 to 3,000,000, still more preferably 250,000 to 2,000,000. It is also a preferable aspect to blend two or more kinds of PEs.
For example, PE having a viscosity average molecular weight (Mv) of 50,000 to 400,000 (hereinafter sometimes abbreviated as “PEa”) is preferably used from the viewpoint of improving shutdown characteristics. From the viewpoint of improving heat resistance, PE having Mv greater than 400,000 and up to 10 million (hereinafter sometimes abbreviated as “PEb”) is preferably used.
The total polyethylene in the polyolefin constituting the microporous film is 100 wt%, and the blending ratio of PEa and PEb is PEa: PEb = 10: 90 to 90:10. From the viewpoint of compatibility, the ratio is more preferably 20:80 to 80:20, still more preferably 30:70 to 70:30.

  The Mv of PEa is more preferably 100,000 to 350,000, and even more preferably 150,000 to 300,000. A Mv of PEa of 50,000 or more is preferable because a microporous film with high mechanical strength can be obtained. A Mv of 400,000 or less is preferable because the stretching stress relaxation at the time of melting the separator becomes remarkable and leads to an improvement in safety in the battery safety test.

  The Mv of PEb is preferably greater than 400,000 and less than or equal to 10 million, more preferably 500,000 to less than 3 million, and still more preferably 500,000 to 2 million. If the Mb of PEb is 400,000 or more, the mechanical strength can be improved, and if the Mv is less than 10 million, the shrinkage stress at the time of melting the separator can be reduced. PEb can be used by blending several types of polyethylene having different Mv's.

The molecular weight distribution (weight average molecular weight (Mw) / number average molecular weight (Mn)) of polyethylene is preferably from 3 to 20, more preferably from 5 to 15, and even more preferably 6 from the viewpoint of moldability of the microporous membrane. -10. If necessary, the molecular weight distribution can be adjusted in the range of approximately 10 to 60 by means of two-stage polymerization, blending, or the like.
The molecular weight distribution in the present embodiment is a value measured by gel permeation chromatography (GPC).

[polypropylene]
The PP includes the following PPs (i) to (iii):
(I) Homopolypropylene having a propylene fraction of greater than 99% by mass (hereinafter sometimes abbreviated as “homoPP”),
(Ii) a propylene copolymer (a) having a propylene fraction of 99% by mass or less and a crystal melting point of 155 ° C. or more (hereinafter sometimes abbreviated as “PP copolymer (a)”),
(Iii) a propylene copolymer (b) having a propylene fraction of 99% by mass or less and a crystal melting point of less than 155 ° C. (hereinafter sometimes abbreviated as “PP copolymer (b)”),
Is mentioned.

The present inventors blended homo-PP and / or PP copolymer (a) and PP copolymer (b) as PP in a composition in which PE and PP are mixed at a certain ratio. As a result, it has been found that the heat resistance of the polyolefin microporous membrane is improved even if there are few crystal components attributed to propylene effective for maintaining the shape when the microporous membrane is exposed to high temperatures.
The reason for this is not clear, but in the PP copolymer (b), other monomers copolymerized with propylene are relatively randomly distributed in the molecule, and therefore homo PP and / or PP copolymer. The presence of the PP copolymer (b) in the interface region between (a) and PE is presumed to be due to the improved compatibility between PE and PP.

  Homo PP and PP copolymer (a) can be used alone or in combination. The blend ratio is preferably 20:80 to 80:20, more preferably 30:70 to 70:30, as the mass ratio of the homo PP: PP copolymer (a).

  The ratio of the PP copolymer (b) to PP in the polyolefin microporous membrane is preferably 20 to 80% by mass, more preferably from the viewpoint of improving heat resistance by improving compatibility between PE and PP. It is 25-70 mass%, More preferably, it is 25-60 mass%, Most preferably, it is 25-50 mass%.

Hereinafter, the PPs (i) to (iii) will be described in more detail.
(I) Homo PP
The propylene fraction of the homo PP is greater than 99% by mass from the viewpoint of improving the membrane strength of the microporous membrane. Examples of the comonomer include ethylene, hexene, pentene, octene, and the like.
Examples of such homo PP include isotactic PP, syndiotactic PP, and atactic PP. Among them, isotactic PP is particularly preferable from the viewpoint of high crystallinity of the microporous film.
The Mv of homo PP is preferably 100,000 or more from the viewpoint of mechanical strength of the resulting microporous film, and preferably less than 1,000,000 from the viewpoint of moldability. More preferably, it is 200,000-800,000, More preferably, it is 400,000-800,000.
The density of homo PP is preferably 0.88 to 0.92 g / cm ³ , more preferably 0.9 to 0.913 g / cm 3. From the viewpoint of facilitating the formation of a microporous membrane, the density is preferably 0.88 or more, and polypropylene having a density greater than 0.92 is difficult to obtain.

(Ii) PP copolymer (a)
As the PP copolymer (a), a resin obtained from a Ziegler-Natta catalyst, a metallocene catalyst, or a living polymerization catalyst can be used. Resins obtained from metallocene catalysts and other living polymerization catalysts are preferred from the viewpoint of high crystallinity resulting from the uniformity of composition distribution.
The copolymerization mode may be either block copolymerization or random copolymerization, but block copolymerization is preferred from the viewpoint of high crystallinity and crystal melting point.
As a block copolymer polymerization method, for example, sequential copolymerization in which monomers are separately added, a method in which monomers are simultaneously added to form a block copolymer using a copolymerization reactivity ratio, and the like can be used. it can.
The block copolymer is composed of at least two types of monomers including propylene, and is not particularly limited as long as it has one or more domains in which PP blocks are linked. A diblock copolymer is preferable from the viewpoint of availability. As a polymerization method of the random copolymer, for example, a statistical random copolymerization method using monomers at the same time and utilizing a copolymerization reactivity ratio can be used.

  The propylene fraction of the PP copolymer (a) is 99% by mass or less from the viewpoint of a high crystalline melting point, preferably 75 to 99% by mass, more preferably 85 to 98% by mass, and still more preferably 90 to 90%. 97% by mass. Examples of the comonomer include ethylene, hexene, pentene, octene and the like, and ethylene is preferable from the viewpoint of improving the mechanical strength of the microporous membrane.

The crystal melting point of the PP copolymer (a) is 155 ° C. or higher, and is preferably 158 to 170 ° C., more preferably 160 to 170 ° C. from the viewpoint of improving the permeability of the microporous membrane.
In addition, the heat of crystal melting of the PP copolymer (a) is preferably 80 J / g or more, more preferably 85 to 100 J / g, still more preferably 90 to 100 J / g, from the viewpoint of battery safety. .

  The Mv of the PP copolymer (a) is preferably 100,000 or more from the viewpoint of mechanical strength of the resulting microporous film, and preferably less than 1,000,000 from the viewpoint of moldability. More preferably, it is 200,000-800,000, More preferably, it is 400,000-800,000.

The density of the PP copolymer (a) is preferably 0.88 to 0.92 g / cm ³ , more preferably 0.9 to 0.913 g / cm 3. A density of 0.88 or more is preferable because it is difficult to make the microporous film porous.

(Iii) PP copolymer (b)
As the PP copolymer (b), a resin obtained from a Ziegler-Natta catalyst, a metallocene catalyst, or a living polymerization catalyst can be used. Resins obtained from metallocene catalysts and other living polymerization catalysts are preferred from the viewpoint of high crystallinity resulting from the uniformity of composition distribution.
The copolymerization mode may be either block copolymerization or random copolymerization, but is preferably a random copolymer from the viewpoint of improving compatibility. As a polymerization method, the same method as that for the PP copolymer (a) can be used.

The propylene fraction of the PP copolymer (b) is 99% by mass or less, preferably 60 to 99% by mass, more preferably 80 to 99% by mass from the viewpoint of improving heat resistance by improving compatibility between PE and PP. %, More preferably 85 to 98% by mass, and most preferably 90 to 98% by mass.
The comonomer includes ethylene, hexene, pentene, octene, and the like, and ethylene is preferable from the viewpoint of improving the mechanical strength of the microporous membrane.

The crystal melting point of the PP copolymer (b) is less than 155 ° C from the viewpoint of improving the heat resistance of the microporous membrane, more preferably from 110 to 153 ° C, and even more preferably from 120 to 150 ° C.
Further, the heat of crystal fusion of the PP copolymer (b) is preferably 85 J / g or less, more preferably 40 to 85 J / g, and still more preferably 50 to 50 J / g, from the viewpoint of maintaining the pore structure and low shutdown characteristics. 80 J / g or less.

  The Mv of the PP copolymer (b) is preferably selected within a range of ± 25% with respect to the molecular weight of the homo PP and / or PP copolymer (a) to be blended, since the dispersibility becomes good. Specifically, it is preferably 100,000 or more from the viewpoint of the mechanical strength of the microporous film, and preferably less than 1,000,000 from the viewpoint of moldability. More preferably, it is 200,000-800,000, More preferably, it is 400,000-800,000.

The density of the PP copolymer (b) is preferably 0.88 to 0.92 g / cm ³ , more preferably 0.9 to 0.913 g / cm 3. A density of 0.88 or more is preferable because it is difficult to make the microporous film porous.

[Properties of polyolefin microporous membrane]
The Mv of the polyolefin microporous membrane is preferably 100,000 or more and 1.2 million or less. More preferably, it is 300,000 or more and 800,000 or less. If the Mv is less than 100,000, the film resistance at melting may not be sufficient, and if it exceeds 1,200,000, the extrusion process becomes difficult, the relaxation of the shrinking force during melting is slow, and the heat resistance is poor. There is a fear.

  The film thickness of the polyolefin microporous film is preferably 1 μm or more from the viewpoint of imparting appropriate mechanical strength, and preferably 50 μm or less from the viewpoint of permeability. More preferably, it is 5-30 micrometers, More preferably, it is 7-25 micrometers.

  The porosity of the polyolefin microporous membrane is preferably 20% or more from the viewpoint of improving battery performance when used as a battery separator, and preferably 60% from the viewpoint of preventing reduction in mechanical strength. More preferably, it is 30%-55%, More preferably, it is 35%-less than 50%.

  The air permeability of the polyolefin microporous membrane is preferably 50 to 2000 seconds, more preferably 80 to 1000 seconds, and still more preferably 100 to 600 seconds. 80 seconds or more are preferable from the viewpoint of reducing the voltage defect rate when used as a battery separator, and 2000 seconds or less are preferable from the viewpoint of permeability as a polyolefin microporous membrane.

  The puncture strength is preferably 1 to 10N, more preferably 2 to 8N, and further preferably 3 to 7N. 1N or more is preferable from the viewpoint of improving the reliability of the separator, and 10N or less is preferable from the viewpoint of preventing an increase in thermal shrinkage due to excessive stretching orientation.

  The fuse temperature of the polyolefin microporous film is preferably 150 ° C. or lower under a temperature rising condition of 2 ° C./min from the viewpoint of improving safety at the time of battery temperature rising. More preferably, it is 145 degrees C or less, More preferably, it is 142 degrees C or less. The temperature is preferably 100 ° C. or higher, more preferably 120 ° C. or higher, and still more preferably 130 ° C. or higher from the viewpoint of preventing the battery from being fused in the usage environment.

  The short-circuit temperature of the polyolefin microporous membrane is preferably 190 ° C. or higher, more preferably 195 ° C. or higher, more preferably 195 ° C. or higher, from the viewpoint of safety and heat resistance when the battery is heated. 200 ° C. or higher.

  The microporous membrane made of polyolefin has a dynamic viscoelasticity measurement between each stress relaxation (tan δ) peak attributed to polyethylene and homo-PP and / or PP copolymer (a) (about 140 to 160 ° C.). It has a relaxation peak attributed to the PP copolymer (b). For example, using a dynamic viscoelasticity measuring machine manufactured by IT Measurement Control Co., Ltd., DVA (registered trademark) -200, based on JIS K-7244, sample length is 20 mm, sample width is 3 mm, heating rate is 10 ° C./min. It can be measured by measuring from 25 ° C. to 180 ° C. in a tensile mode under the conditions of a measurement frequency of 35 Hz, a strain of 0.05%, and a static power ratio of 1.5.

Hereinafter, a preferred method for producing a polyolefin microporous membrane will be described.
The method for producing a polyolefin microporous membrane includes the following steps (a) to (d):
(A) After melt-kneading a mixture containing at least a polyolefin composition (hereinafter referred to as a raw material) and a plasticizer that forms a uniform solution at a temperature equal to or higher than the melting point, it is extruded, cooled and solidified to form a sheet. Process (extrusion film forming process),
(B) Biaxial stretching process (stretching process),
(C) a step of extracting a plasticizer (plasticizer extraction step),
(D) It is preferable to include a heat setting step.

In addition, although there is no limitation in particular about the order and frequency | count of (a)-(d) process, Preferably the following 3 types are mentioned.
1. (A) Step → (b) Step → (c) Step → (d) Step 2. (A) Step → (b) Step → (c) Step → (b) Step → (d) Step 3. (A) Step → (c) Step → (b) Step → (d) Step Among the above, more preferably 1, 2, and most preferably 1.

(A) Extrusion film-forming process A plasticizer refers to a non-volatile solvent capable of forming a uniform solution at a temperature equal to or higher than the melting point of polyolefin and polyolefin. Examples thereof include hydrocarbons such as liquid paraffin and paraffin wax, di-2-ethylhexyl phthalate (DOP), diisodecyl phthalate, and diheptyl phthalate. Among these, liquid paraffin is preferable.
It is also a preferred method to add inorganic fine particles in the step. As the inorganic fine particles, silica, alumina, titania, zirconia, calcium carbonate, barium carbonate and the like can be used. Of these, silica and alumina are preferable from the viewpoint of uniformity in melt kneading. When using inorganic fine particles, it is preferable to mix and granulate the raw material, the plasticizer and the inorganic fine particles with a Henschel mixer or the like from the viewpoint of uniformly dispersing the inorganic fine particles. Extracting the inorganic fine particles is preferable when obtaining a polyolefin microporous membrane having a large pore size and excellent permeability. Examples of the method for extracting the inorganic fine particles include a method in which the fine particles are immersed or brought into contact with a liquid in which the inorganic fine particles are dissolved. The inorganic fine particles are preferably extracted between the steps (c) and (d). A method in which inorganic fine particles are not extracted is preferable from the viewpoint of improving the compression resistance of the microporous membrane by the inorganic fine particles.

  The ratio of the polyolefin resin to the total amount of raw material, plasticizer and inorganic fine particles (hereinafter referred to as polymer concentration) is preferably 10% by mass or more from the viewpoint of molding processability during film formation, and from the viewpoint of permeability of the microporous membrane To 90% by mass or less. More preferably, it is 20-60 mass%, More preferably, it is 30-50 mass%. Further, the ratio of the plasticizer to the total amount of the plasticizer and the inorganic fine particles is preferably 50% by mass or more from the viewpoint of preventing the deterioration of the quality due to the aggregation of the inorganic fine particles, and preferably 80% or less from the viewpoint of imparting an appropriate pore size and permeability. . More preferably, it is 60-75 mass%.

Examples of the melt-kneading method include a method of mixing with a Henschel mixer, a ribbon blender, a tumbler blender or the like and then melt-kneading with a screw extruder such as a single screw extruder or a twin screw extruder, a kneader, or a Banbury mixer. As a method of melt kneading, it is preferable to carry out with an extruder capable of continuous operation.
From the viewpoint of improving the compatibility between PE and PP, a twin screw extruder is more preferable. The plasticizer may be mixed with the raw material by the Henschel mixer or the like. Moreover, you may feed directly to an extruder at the time of melt-kneading. Moreover, it is preferable from a viewpoint which prevents Mv fall of a raw material to carry out by the nitrogen atmosphere inside the mixer, feeder, and extruder at the time of melt-kneading. In addition, homo PP and / or PP copolymer (a) and PP copolymer (b) are kneaded together with a plasticizer in another extruder and continuously in a molten state to an extruder kneading PE. A method of feeding and kneading / dispersing is also a preferred mode.

  The temperature during melt-kneading is preferably 140 ° C. or higher, more preferably 160 ° C. or higher, and still more preferably 180 ° C. or higher from the viewpoint of dispersibility. Further, from the viewpoint of preventing Mv of the raw material, it is preferably 280 ° C. or lower, more preferably 260 ° C. or lower, further preferably 250 ° C. or lower, and most preferably 240 ° C. or lower. From the viewpoint of preventing a decrease in Mv of the raw material in the melt-kneading, it is preferable to add an antioxidant or a heat deterioration inhibitor. The addition amount is preferably 0.2% by mass or more with respect to the total amount of the raw material polyolefin from the viewpoint of preventing Mv reduction of the raw material, and preferably 3% by mass or less from the viewpoint of economy. More preferably, it is 0.3-3 mass% or less, More preferably, it is 0.5-2 mass%, Most preferably, it is 0.6-2 mass%. These conditions are preferable because Mv of the raw material can be prevented from substantially decreasing.

  Examples of a method for forming the kneaded material obtained by melt kneading into a sheet include a method in which the melt is solidified by cooling. Examples of the cooling method include a method of directly contacting a cooling medium such as cold air or cooling water, a method of contacting a roll or press machine cooled by a refrigerant, and the like. A method of contacting with a roll cooled by a refrigerant or a press is preferable in terms of excellent thickness control.

(B) Stretching step The stretching step is a step of stretching in the biaxial direction, and includes sequential biaxial stretching by a combination of a roll stretching machine and a tenter, simultaneous biaxial stretching by simultaneous biaxial tenter or inflation molding, and the like. From the viewpoint of increasing the tensile strength at break, simultaneous biaxial stretching is preferred.

  The stretching step can be performed before stretching (stretching before extraction), or after (stretching after extraction), or before and after (stretching before and after extraction). Among them, the pre-extraction stretching is preferable in order to improve the mechanical strength of the polyolefin microporous film obtained.

  The drawing surface magnification is preferably 20 times or more from the viewpoint of improving the mechanical strength of the polyolefin microporous membrane and further improving the film thickness distribution of the polyolefin microporous membrane, in order to prevent an increase in heat shrinkage stress due to excessive stretching. 100 times or less is preferable. More preferably, it is 35-60 times, More preferably, it is 40-55 times.

The stretching temperature can be selected with reference to the polymer concentration. The stretching temperature in stretching before extraction is preferably 110 ° C. or higher from the viewpoint of preventing breakage due to excessive stretching stress, and preferably 140 ° C. or lower from the viewpoint of the strength of the microporous film. More preferably, it is 115-130 degreeC, More preferably, it is 118-130 degreeC. The stretching temperature in the post-extraction stretching is preferably 100 ° C to 170 ° C, more preferably 110 to 150 ° C, and still more preferably 110 to 140 ° C.
As the stretching conditions for stretching before and after extraction, the conditions for stretching before extraction and stretching after extraction can be used.

(C) Plasticizer extraction step The extraction solvent is a poor solvent for the polyolefin constituting the membrane and a good solvent for the plasticizer, and has a boiling point lower than the melting point of the polyolefin constituting the membrane. Is desirable. Examples of such an extraction solvent include hydrocarbons such as n-hexane and cyclohexane, halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane, and non-chlorine-based solvents such as hydrofluoroether and hydrofluorocarbon. Examples thereof include halogenated solvents, alcohols such as ethanol and isopropanol, ethers such as diethyl ether and tetrahydrofuran, and ketones such as acetone and methyl ethyl ketone. It selects from these suitably, and it uses individually or in mixture. Of these, methylene chloride and methyl ethyl ketone are preferred.

  As a plasticizer extraction method, the plasticizer is extracted by immersing or showering the sheet obtained in the casting step or the stretching step in these extraction solvents, and then sufficiently drying.

(D) Heat setting step Heat setting is an operation of reducing heat shrinkage of a microporous polyolefin membrane by carrying out a combination of low magnification stretching and relaxation operation in a predetermined temperature atmosphere with a tenter, a roll stretching machine or the like. That is. Low magnification stretching is an area magnification of 3.0 times or less.

  The draw ratio in the low-stretch drawing is the film length direction (machine direction; hereinafter sometimes abbreviated as “MD”) and / or the film width direction (direction perpendicular to the machine direction; hereinafter abbreviated as “TD”). It is preferably 1.1 to 3.0 times, more preferably 1.3 to 2.2 times, and still more preferably 1.4 to 2.0 times. Excessive stretching is not preferable because the possibility of film breakage increases.

  The temperature during stretching is preferably 110 ° C. or higher in order to prevent membrane breakage due to stretching, and is preferably 140 ° C. or lower from the viewpoint of preventing a decrease in permeability of the polyolefin microporous membrane. More preferably, it is 115-135 degreeC, More preferably, it is 120-130 degreeC.

  The relaxation operation is an operation for slightly returning the MD and / or TD dimensions of the film. The relaxation ratio with respect to the film size during stretching is preferably 0.9 times or less, more preferably 0.87 times or less, and still more preferably 0.85 times or less, from the viewpoint of reducing thermal shrinkage. From the viewpoint of preventing a decrease in permeability due to excessive relaxation, it is 0.65 times or more, preferably 0.7 times or more, more preferably 0.75 times or more.

  The temperature during relaxation is preferably 120 ° C. or higher from the viewpoint of reducing the thermal shrinkage rate, and preferably 140 ° C. or lower from the viewpoint of preventing the permeability of the polyolefin microporous membrane from being lowered. More preferably, it is 122 degreeC-135 degreeC, More preferably, it is 125-135 degreeC.

  The TD heat-setting stretch ratio (film width after relaxation with respect to the original film width) is preferably 0.95 times or more, more preferably 1.1 times or more, from the viewpoint of preventing deterioration of the film thickness distribution. Preferably it is 1.2 times or more. From the viewpoint of reducing heat shrinkability, it is preferably 2.0 times or less, more preferably 1.9 times or less, and still more preferably 1.8 times or less. The TD heat setting draw ratio can be calculated from the TD drawing ratio and the TD relaxation ratio in the heat setting process. When the original film width is 1, the film width after relaxation (times) can be calculated by the original film width (1) × stretch ratio × relaxation ratio.

  If necessary, surface treatment such as electron beam irradiation, plasma irradiation, ion beam irradiation, surfactant coating, chemical modification or the like can be performed.

When the polyolefin microporous membrane described above is used as a battery separator, for example, a battery may be prepared by the following method.
First, the microporous membrane is formed into a vertically long shape having a width of 10 mm to 100 mm and a length of 200 mm to 2000 mm. The separator is stacked in the order of positive electrode-separator-negative electrode-separator, or negative electrode-separator-positive electrode-separator, and wound into a circular or flat spiral shape. Further, the wound body is accommodated in a battery can, and a liquid electrolyte solution or a gel electrolyte solution is injected. When using a gel electrolyte, it is also possible to use a gel impregnated in advance with a separator.

As described above, the polyolefin microporous membrane of the present embodiment is suitable for separators for electronic devices such as batteries, capacitors and capacitors, microfiltration membranes, etc., and particularly suitable as a separator for lithium ion batteries. In particular, when used as a separator for a lithium ion battery, it is possible to provide excellent battery safety. In addition, the measured values of various parameters described above are the measurement methods in the examples described later unless otherwise specified. It is a value measured according to.

  Next, the present embodiment will be described more specifically with reference to examples and comparative examples. However, the present embodiment is not limited to the following examples unless it exceeds the gist. In addition, the physical property in an Example was measured with the following method.

(1) Viscosity average molecular weight The viscosity average molecular weight of polyethylene and polyolefin microporous membrane was measured at a measurement temperature of 135 ° C. using decalin as a solvent, and calculated from the viscosity [η] by the following formula.
[Η] = 6.77 × 10 ⁻⁴ Mv ^0.67 (Chiang's formula)
For polypropylene, Mv was calculated by the following formula.
[Η] = 1.10 × 10 ⁻⁴ Mv ^0.80

(2) Film thickness (μm)
The measurement was performed at a room temperature of 23 ° C. using a micro thickness measuring instrument manufactured by Toyo Seiki, KBM (trademark).

(3) Porosity (%)
A sample of 10 cm × 10 cm square was cut from the microporous membrane, its volume (cm ³ ) and mass (g) were determined, and the film density was calculated to be 0.95 (g / cm ³ ) using the following formula.
Porosity = (1−mass / volume / 0.95) × 100
The film density was calculated as 0.95 (g / cm ³ ).

(4) Air permeability (sec)
Based on JIS P-8117, it measured with the Gurley type air permeability meter by the Toyo Seiki Co., Ltd. and G-B2 (trademark).

(5) Puncture strength (N)
Using a handy compression tester KES-G5 (trademark) manufactured by Kato Tech, the microporous membrane was fixed with a sample holder having a diameter of 11.3 mm at the opening. Next, the center of the fixed microporous membrane is subjected to a piercing test in a 25 ° C. atmosphere at a needle radius of curvature of 0.5 mm and a piercing speed of 2 mm / sec. (N) was obtained.

(6) Fuse temperature (° C) / Short temperature (° C)
FIG. 1A shows a schematic diagram of a fuse temperature measuring apparatus. 1 is a microporous film, 2A and 2B are 10-micrometer-thick nickel foils, and 3A and 3B are glass plates. 4 is an electric resistance measuring device (LCR meter “AG-4411” (trademark) manufactured by Ando Electric Co., Ltd.), which is connected to the nickel foils 2A and 2B. A thermocouple 5 is connected to the thermometer 6. A data collector 7 is connected to the electric resistance device 4 and the thermometer 6. 8 is an oven that heats the microporous membrane.
More specifically, as shown in FIG. 1 (B), the microporous film 1 is overlaid on the nickel foil 2A, and the “Teflon (registered trademark)” tape (shaded portion in the figure) is vertically attached to the nickel foil 2A. Fix it. The microporous membrane 1 is impregnated with a 1 mol / liter lithium borofluoride solution (solvent: propylene carbonate / ethylene carbonate / γ-butyllactone = 1/1/2) as an electrolytic solution. As shown in FIG. 1C, “Teflon (registered trademark)” tape (shaded portion in the figure) is pasted on the nickel foil 2B, and masking is performed by leaving a window portion of 15 mm × 10 mm in the central portion of the foil 2B. did.
The nickel foil 2A and the nickel foil 2B were overlapped with each other so as to sandwich the microporous film 1, and two nickel foils were sandwiched by the glass plates 3A and 3B from both sides thereof. At this time, the window portion of the foil 2B and the microporous membrane 1 were positioned to face each other.
Two glass plates were fixed by pinching with a commercially available double clip. The thermocouple 5 was fixed to a glass plate with “Teflon (registered trademark)” tape.
Temperature and electric resistance are continuously measured with such an apparatus. The temperature was raised from 25 ° C. to 200 ° C. at a rate of 2 ° C./min, and the electrical resistance value was measured at 1 V and 1 kHz alternating current. The fuse temperature was defined as the temperature at which the electrical resistance value of the microporous film reached 10 ³ Ω. The temperature at which the electrical resistance value again fell below 10 ³ Ω after the fuse was defined as the short-circuit temperature.

(7) Crystal melting point (° C) of PP, heat of crystal fusion (J / g)
3 mg of PP is taken, put in an aluminum sample pan having a diameter of 5 mm, and sealed with a crimping cover to prepare a measurement sample.
The sample was heated at a rate of 10 ° C./min from 30 to 200 ° C. in a nitrogen atmosphere using a differential scanning calorimeter (hereinafter abbreviated as DSC) (product name: DSC-60A) manufactured by Shimadzu Corporation (1st heat). ) And kept at 200 ° C. for 5 minutes. Furthermore, after cooling to 200-30 ° C. at 10 ° C./min, the temperature was maintained at 30 ° C. for 5 minutes, and then the temperature was raised again to 30-200 ° C. at 10 ° C./min (2nd heat) to obtain a DSC curve .
It was set as the melting peak temperature obtained by the method described in JIS-K7121 in the DSC curve with the exotherm obtained by the 2nd heat as the positive direction.
Regarding the heat of crystal fusion, in the case of PP copolymer (b), the area of the region surrounded by the DSC curve and the line connecting 175 ° C. and 90 ° C. on the obtained 2nd heat DSC curve is expressed as the heat of crystal fusion ( J / g). In the case of the homo PP and PP copolymer (a), the area of the region surrounded by the DSC curve and the line connecting 175 ° C. and 110 ° C. on the DSC curve was defined as the heat of crystal fusion (J / g).

(8) PP melting heat (J / g) in polyolefin microporous membrane
A polyolefin microporous membrane was punched out into a circle with a diameter of 5 mm and overlapped to give 3 mg, which was measured in the same manner as in (7) above. Using the obtained 1st heat DSC curve, the area of the region surrounded by the curve connecting 110 ° C. and 175 ° C. on the DSC curve, the DSC curve, and the straight line of temperature = 150 ° C. was calculated.

(9) Ethylene content (% by mass)
^{The 13} C-NMR spectrum was determined based on the method described in Macromolecules 1982, 15, 1150-1152. A sample was prepared by uniformly dissolving about 200 mg of PP resin in 3 ml of orthodichlorobenzene in a 10 mmφ test tube, and the ¹³ C-NMR spectrum of the sample was measured under the following conditions.
Measurement temperature: 135 ° C
Pulse repetition time: 10 seconds Pulse width: 45 °
Integration count: 2500 times

(10) Fracture temperature A test in which a polyolefin microporous membrane cut to 60 mm in MD and 60 mm in TD is fixed to a 50 mm square metal frame, placed in an oven adjusted to a predetermined temperature, and heated for 10 minutes. went. The temperature was changed every 1 ° C. until breakage, and the test was repeated using a new microporous membrane each time. The temperature at which a hole having a diameter of 1 mm or more opened in the heated membrane was defined as the membrane breaking temperature. The test was performed using an oven having a structure in which the temperature inside the oven did not change due to the influence of outside air when the sample was charged.

[Example 1-1]
PEa is 46% by mass of a high-density polyethylene homopolymer with an Mv of 300,000, PEb is 47% by mass of a high-density polyethylene homopolymer with an Mv of 700,000, an Mv is 400,000, a density is 0.91 g / cm ³ , a crystal melting point is 164 ° C., and a crystal melting heat is 100 J / g. A tumbler of 2% by mass of a homo-PP and 5% by mass of a random PP having an Mv of 350,000, a density of 0.91 g / cm ³ , a crystal melting point of 148 ° C. and a crystal melting heat of 71.5 J / g as a PP copolymer (b). Dry blended using a blender. 1 part by mass of pentaerythrityltetrakis- [3-3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant is added to 99 parts by mass of the obtained polymer mixture, and a tumbler blender is again used. Used to dry blend to obtain a mixture. The atmosphere in the feeder and the biaxial stretching machine was replaced with nitrogen, and the resulting mixture was fed to the biaxial extruder through the feeder and liquid paraffin (kinematic viscosity at 37.78 ° C. 7.59 × 10 ⁻⁵ m ² / s). ) Was injected into the extruder cylinder by a plunger pump. The melt kneading was performed at a preset temperature of 220 ° C., a screw rotation speed of 180 rpm, and a discharge rate of 15 kg / h. The feeder and the pump were adjusted so that the polymer concentration in the entire mixture extruded by melt kneading was 35% by mass.
Subsequently, the melt-kneaded product was extruded from a T-die and cooled and solidified to obtain a 1850 μm sheet.
Next, this sheet was guided to a simultaneous biaxial tenter stretching machine, and biaxial stretching was performed. The stretching conditions were an MD magnification of 7.0 times, a TD magnification of 6.4 times, and a set temperature of 123 ° C.
Thereafter, the sheet was introduced into a methyl ethyl ketone bath, sufficiently immersed in methyl ethyl ketone to extract and remove liquid paraffin, and then methyl ethyl ketone was removed by drying. Further, this sheet is guided to a TD tenter, and stretched at a low magnification at a temperature of 125 ° C. and a magnification of 1.5 times, and heat-fixed at a temperature of 130 ° C. and a relaxation magnification of 0.80 times. Got. Table 1 shows the properties of the polyolefin microporous membrane.

[Examples 1-2 to 1-11, Comparative examples 1-1 to 1-8, Examples 2-1 to 2-9, Comparative examples 2-1 to 2-5]
A polyolefin microporous membrane was prepared under the conditions described in Tables 1 to 4. The conditions not described were the same as in Example 1-1. The physical properties of the obtained microporous membrane are shown in Tables 1-4.
In Examples and Comparative Examples, block PP was used as the PP copolymer (a), and random PP was used as the PP copolymer (b).
The physical properties of such block PP and random PP are as follows.
Block PP-A Mv 380,000, density 0.91 g / cm ³ , crystal melting point 166 ° C., crystal fusion heat 96 J / g, ethylene content 6% by mass as comonomer.
Block PP-B Mv 350,000, density 0.91 g / cm ³ , crystal melting point 164 ° C., heat of crystal fusion 95 J / g, ethylene content 10% by mass as comonomer.
Random PP-A Mv 340,000, density 0.91 g / cm ³ , crystal melting point 148 ° C., crystal melting heat 72 J / g, ethylene content 3% by mass as comonomer.
Random PP-B Mv 350,000, density 0.91 g / cm ³ , crystal melting point 143 ° C., crystal melting heat 61 J / g, ethylene content 4% by mass as comonomer.

  The polyolefin microporous membrane of the present invention can give excellent battery safety, particularly when used as a separator for lithium ion batteries.

Claims (5)

In a polyolefin microporous membrane comprising a polyolefin composition containing 50 to 97% by mass of polyethylene and 3 to 50% by mass of polypropylene,
The polypropylene has the following components (A) and (B):
(A) a homopolypropylene having a propylene fraction of more than 99% by mass and / or a propylene copolymer having a propylene fraction of 99% by mass or less and a crystal melting point of 155 ° C. or more (a),
(B) a propylene copolymer (b) having a propylene fraction of 60 to 99 % by mass and a crystal melting point of less than 155 ° C,
A microporous membrane made of polyolefin.   The polyolefin microporous film according to claim 1, wherein the polypropylene copolymer (a) has a propylene fraction of 75 to 99 mass%. The propylene copolymer (b) is homopolypropylene, polypropylene copolymer (a), and the proportion in the total amount of the polypropylene copolymer (b), 20 to 80 wt%, according to claim 1 or 2 The polyolefin microporous film described in 1. The polyolefin microporous membrane according to any one of claims 1 to 3 , wherein the propylene copolymer (b) has a heat of crystal fusion of 40 to 85 J / g. The polyolefin microporous membrane according to any one of claims 1 to 4 , wherein the propylene copolymer (b) has a crystal melting point of 110 to 153 ° C.