JP2002367589A - Polyolefin separator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polyolefin separator with high strength, low pore-blocking temperature, and low shrinkage percentage. SOLUTION: This polyolefin separator is composed of polyolefin resin containing high molecular weight polyethylene having a viscosity average molecular weight of 500,000 or more and polyethylene having a viscosity average molecular weight of less than 500,000, and has a piercing strength of 20 g/μm or more, a pore blocking temperature of 135 deg.C or less, and a shrinkage percentage of 30% or less at 135 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polyolefin separator, and more particularly to a polyolefin separator containing a high molecular weight polyethylene suitable for a lithium ion secondary battery or the like.

[0002]

2. Description of the Related Art Conventionally, polyolefin separators have been used as separators for batteries such as lithium ion secondary batteries. In recent years, polyolefin separators have been used to reduce the size of devices such as mobile phones, notebook personal computers, and information terminals that use batteries. With the advancement of functions, a smaller and larger-capacity battery is required. In order to increase the capacity while reducing the size of the battery, it is necessary to fill the inside of the battery with more electrode active material and electrolyte.Thus, the thickness of the polyolefin separator used in the battery must be reduced. Have been. On the other hand, as the capacity of the battery increases, the energy stored inside the battery increases, so that it is required to further enhance the function of the polyolefin separator related to the safety of the battery. The following functions are required as functions of the polyolefin separator related to battery safety.

[0003] (a) To reduce short-circuit failure in the manufacture of a battery, high strength at normal temperature, and (b) to prevent a short circuit inside the battery to a higher temperature when the battery is heated,
High strength at high temperature, (c) when battery is overcharged, shutdown function to cut off current by closing the hole of separator quickly at lower temperature,
(D) Dimensional stability up to high temperatures to prevent the separator from shrinking and contact between the positive and negative electrodes inside the battery when the battery is heated or overcharged and the temperature rises.

The strength at room temperature of (a) and the strength at high temperature of (b) tend to decrease as the thickness of the polyolefin separator decreases, and the strength of the polyolefin separator at room temperature and the strength at high temperatures are increased. Therefore, it has been conventionally proposed to use a high molecular weight polyethylene having an average molecular weight exceeding 300,000. Since high molecular weight polyethylene has a very high melt viscosity and it is difficult to extrude a single high molecular weight polyethylene,
Japanese Patent No. 42035 discloses a separator in which high molecular weight polyethylene having an average molecular weight of 500,000 or more is dissolved in a solvent such as liquid paraffin, molded, and then the solvent is extracted to make the separator porous. A separator has been proposed in which a plasticizer such as stearyl alcohol is mixed with high-molecular-weight polyethylene having an average molecular weight of 400,000 or more and molded, and then the plasticizer is extracted to make the separator porous.

[0005] However, the separator made of only high molecular weight polyethylene proposed in JP-A-60-242035 or JP-A-60-255107 has a higher pore closing temperature than a separator having a small molecular weight. Tend to be. As a separator having an improved shutdown function of a high molecular weight polyethylene separator, a polyethylene separator containing a high molecular weight polyethylene and a low molecular weight polyethylene has been proposed.

For example, JP-A-2-21559 discloses a separator formed by mixing a polyethylene having an average molecular weight of 1,000,000 or more, a polyethylene having an average molecular weight of 300,000 or less and a plasticizer, and extracting the plasticizer to make it porous. JP-A-3-105851 discloses a high molecular weight polyethylene having a wide molecular weight distribution in which the value of weight average molecular weight / number average molecular weight is 10 or more, which means that the larger the value is, the wider the molecular weight distribution is. Is formed by forming a solution in which is dissolved in liquid paraffin, and then extracting the liquid paraffin to make the separator porous.
No. 9563 discloses that a plasticizer such as stearyl alcohol is added to a resin composed of a high molecular weight polyethylene having an average molecular weight of 500,000 or more and a polyethylene having an average molecular weight of less than 500,000, and then the plasticizer is extracted to make the resin porous. Separated separators have been proposed.

Each of the separators proposed in these patents is a separator formed by mixing a solvent or a plasticizer, which is extracted later to make it porous, with polyethylene. Stretching to be performed is performed after extracting and removing a solvent or a plasticizer to form a porous layer, and may be performed while including a solvent or a plasticizer, but the solvent or a plasticizer is extracted and removed. It is difficult to perform the stretching in a molten state even when the stretching is performed later or when the stretching is performed while containing the solvent or the plasticizer.

In the case of stretching after extracting and removing a solvent or a plasticizer to form a porous body, if the stretching is performed at a temperature higher than the melting point of polyethylene, the pores are closed and the porous structure is lost. The publication describes that stretching is performed at 10 to 130 ° C., which is a temperature lower than the melting point. When the stretched separator is heated at a temperature lower than the melting point of polyethylene, the separator starts to shrink before the polyethylene is melted and the pores of the separator are closed, so that the battery using such a separator is heated. If the battery is overcharged or overcharged, the separator shrinks before the current is shut off due to clogging of the holes in the separator, and the positive electrode and the negative electrode inside the battery tend to short-circuit.

When stretching while containing a solvent and a plasticizer,
Mixing a solvent or a plasticizer lowers the melting point of the high molecular weight polyethylene, so that it is difficult to stretch the film at a temperature higher than the original melting point of the high molecular weight polyethylene. For example, in the example of JP-A-3-1055851, stretching is performed at 115 ° C. or 125 ° C. lower than the melting point of high molecular weight polyethylene. As described above, the separator stretched at a temperature lower than the melting point of the high molecular weight polyethylene is disclosed in
As with the separator of JP-A-21559, there is a tendency for the separator to shrink at a temperature lower than the temperature at which the pores of the separator are closed.

On the other hand, Japanese Patent Application Laid-Open No. 7-29563 proposes a separator in which a high molecular weight polyethylene mixed with a plasticizer is stretched in a molten state. However, the molecules of the molten high molecular weight polyethylene are diluted with the plasticizer. Therefore, since the entanglement between the molecules is small and the stretching force is not easily transmitted to the whole of the high molecular weight polyethylene, the orientation degree of the high molecular weight polyethylene molecules is small and the separator tends to have a low strength. The puncture strength per 25 μm thickness of the separator described in Examples of JP-A-7-29563 is at most 254 g, and a separator obtained by stretching a high-molecular-weight polyethylene containing a plasticizer in a molten state,
It is difficult to use for a large capacity battery that requires a thinner separator. As described above, it is difficult for a conventional separator formed by mixing a solvent or a plasticizer to a high molecular weight polyethylene to have both high strength and dimensional stability at high temperatures.

Recently, there has been proposed a separator obtained by extruding and stretching high molecular weight polyethylene without mixing a solvent or a plasticizer. JP-A-11-291317
JP, JP-A-2000-119432, JP-A-20
Japanese Patent Application Laid-Open No. 00-143867 discloses a separator extruded by mixing a non-reactive gas with a high molecular weight polyethylene such as carbon dioxide or nitrogen gas.
No. 36 discloses extrusion molding of high-molecular-weight polyethylene having an intrinsic viscosity of 2.5 dl / g or more and a weight-average molecular weight / number-average molecular weight of 10 or less measured at 135 ° C. in a decalin solution without mixing a solvent or a plasticizer. Separators have been proposed.

However, Japanese Patent Application Laid-Open Nos. 11-291317, 2000-119432 and 2000
In the separator proposed in Japanese Patent No. -143867, the high molecular weight polyethylene extruded together with the non-reactive gas has already formed a porous structure before being stretched. Doing so will result in loss of the porous structure. Therefore, JP-A-11-291317 and JP-A-2000-1194
No. 32, JP-A-2000-143867,
The stretching is performed at a temperature lower than the melting point of the high molecular weight polyethylene, and it is difficult to reduce the shrinkage at the temperature at which the pores of the separator are closed.

On the other hand, the separator proposed in Japanese Patent Application Laid-Open No. H11-302436 extrudes a high-molecular-weight polyethylene without mixing a solvent or a plasticizer and forms the extruded molten high-molecular-weight polyethylene into an inflation film. This is a separator obtained by forming a non-porous film by stretching by a method, and subjecting the obtained non-porous high molecular weight polyethylene film to a porous treatment. High molecular weight polyethylene without solvent or plasticizer,
It is well known that stretching is possible even in the molten state because the molecules are entangled in the molten state and the stretching force is transmitted to the entire high molecular weight polyethylene. When the pores are closed, the separator obtained by making this porous has a small shrinkage.

Further, when a high molecular weight polyethylene is stretched at a temperature higher than the melting point, a crystal structure generally called a shish-kebab structure, which is composed of an extended chain crystal having a high melting point and a folded chain crystal having a low melting point, is conventionally formed. Higher molecular weight polyethylene separators stretched at temperatures above the melting point are more known and have pores at lower temperatures because they have crystals that melt at lower temperatures compared to high molecular weight polyethylene separators stretched at lower temperatures. Has the feature that it is blocked.

The shish-kebab structure of high-molecular-weight polyethylene is described in, for example, Kell, as reviewed in The Society of Polymer Science, published in 1993.
et al. discovered the structure of fibrous crystals due to the flow orientation of a solution of high molecular weight polyethylene,
It is well known that they have been discovered as a flow-oriented crystal structure of a melt of a high molecular weight polyethylene by I. et al. All of the crystals having a shish kebab structure discovered by Keller and Oddel are crystals obtained when a high-molecular-weight polyethylene is stretched in one direction. However, such a crystal as a separator proposed in JP-A-11-302436 is used. The fact that a structure composed of extended chain crystals and folded chain crystals can be formed even with high molecular weight polyethylene stretched in the axial direction has been reported by Iida et al. 34, no. From page 653 of 9 to 6
It is reported over page 59, and page 657 also reports the vein-like fibril structure observed in high molecular weight polyethylene separators.

As a method for making a porous material without using a solvent or a plasticizer, Japanese Unexamined Patent Publication (Kokai) No. 55-161830 discloses a method in which a non-porous polyolefin film is brought into contact with a solvent capable of dissolving an amorphous portion. There has been proposed a method of forming pores in a conductive portion, and describes that a stretched polyolefin film is easier to open. The crystal orientation when a high molecular weight polyethylene is stretched is described in JP-A-58-893.
As described in Japanese Patent Publication No. 26, the lamellar surface of the crystal of high molecular weight polyethylene is oriented perpendicular to the stretching direction, and the a-axis of the crystal is oriented perpendicular to the stretching direction.

From these facts, it is considered that the reason why the high-molecular-weight polyethylene stretched at a temperature equal to or higher than the melting point is easy to open in the method of opening the hole by contacting the amorphous portion of the polyethylene with a solvent dissolving the same is as follows. Can be Polyethylene crystals tend to spread in the direction of the a-axis between the crystals, and if the a-axis of the crystals is aligned in a direction perpendicular to the stretching direction by stretching, the directions in which the crystals open are aligned, so the crystal It is considered that the solvent dissolving the amorphous portion easily penetrates into the amorphous portion existing between the crystal and the crystal.

In the non-stretched high molecular weight polyethylene, since the direction of the a-axis of the crystal is not uniform, the solvent for dissolving the amorphous portion penetrates into the amorphous portion between the crystals, and the As the spacing in the a-axis direction increases, the crystals often collide with each other, and it is considered that the solvent that dissolves the amorphous portion hardly permeates. The separator proposed in Japanese Patent Application Laid-Open No. H11-302436 also discloses that a porous nonporous high molecular weight polyethylene film is made porous by contacting a solvent that dissolves the amorphous portion of polyethylene. However, JP-A-11-302436 also discloses that
It is described that the orientation of the crystal a-axis of the stretched high molecular weight polyethylene film in a direction perpendicular to the film surface is likely to make the film porous, and high molecular weight polyethylene crystallizes when stretched at a temperature higher than the melting point. Since the a-axis of the crystal in which the gap easily spreads is aligned in the same direction, it can be said that it is easy to make the amorphous portion porous by a method of contacting the amorphous portion with a solvent that dissolves the amorphous portion.

Japanese Patent Application Laid-Open No. 11-302436 describes
The intrinsic viscosity measured with a decalin solution at 5 ° C. is 2.5 dl /
This is a polyolefin separator having a weight average molecular weight / number average molecular weight of 10 or less and a molecular weight distribution narrow in order to improve high-temperature strength. In Japanese Patent Application Laid-Open No. 11-302436, in order to obtain a polyolefin having a weight average molecular weight / number average molecular weight of 10 or less, molecules of a high molecular weight component in the polyolefin are cut by extrusion molding. It is well known that molecules with higher molecular weights are more likely to be cleaved by molecules.Because molecules with larger molecular weights have longer lengths, the probability of breaking bonds between atoms constituting the main chain of the molecule increases. It is considered.

Usually, polyethylene has a distribution in the molecular weight. If the extruder screw is rotated at an unnecessarily high speed or the temperature of the extruder cylinder is set to an unnecessarily high temperature during extrusion molding, excessive shearing force or excessive shearing force is generated. It is known that the molecular weight of the high molecular weight component in the polymer is cut by the heat energy, the high molecular weight component of the molecular weight distribution decreases, and the average molecular weight of the extruded polymer decreases and the molecular weight distribution narrows at the same time. I have.

The fact that the separator proposed in JP-A-11-302436 utilizes this well-known phenomenon in order to adjust the weight average molecular weight / number average molecular weight to 10 or less is disclosed in JP-A-11-302436. It becomes clear when the intrinsic viscosity and the weight average molecular weight / number average molecular weight of the examples of JP-A-11-302436 are examined in detail. That is, JP
In Example 2 of 1-302436, the limiting viscosity is 1
Example 3 shows that a high molecular weight polyethylene of 4.1 dl / g is extruded by an extruder to obtain a polyethylene having a limiting viscosity of 7.8 dl / g and a weight average molecular weight / number average molecular weight of 3.1. Has an intrinsic viscosity of 18.2 dl / g
Of high molecular weight polyethylene having an intrinsic viscosity of 3.
It is described that a polyethylene having a weight average molecular weight / number average molecular weight of 1.9 reduced to 8 dl / g can be obtained.

The intrinsic viscosity is a value proportional to the average molecular weight. Since the intrinsic viscosity is reduced by extruding high-molecular-weight polyethylene, the molecules of high-molecular-weight polyethylene are cut by extrusion to lower the average molecular weight. It is clear that you are. Example 2
Comparing the degree of decrease in intrinsic viscosity by extrusion molding of Example 3 with the value of weight average molecular weight / number average molecular weight, the degree of decrease in intrinsic viscosity was 18.2 dl in Example 3.
/ G to 3.8 dl / g, and the limiting viscosity of Example 3 is larger than that of Example 2 from 14.1 dl / g to 7.8 dl / g. I understand. On the other hand, the weight-average molecular weight / number-average molecular weight was 1.9 in Example 3, which means that the weight-average molecular weight / number-average molecular weight was smaller than 3.1 in Example 2.

These facts show that the average molecular weight of Example 3 is greatly reduced by extrusion molding and the molecular weight distribution is narrower in Example 3. From the above, the high-molecular-weight polyethylene separator proposed in Japanese Patent Application Laid-Open No. H11-302436 is capable of actively cutting molecules of high-molecular-weight components when extruding high-molecular-weight polyethylene having a distribution in molecular weight. It can be seen from the result that the separator had a value of weight average molecular weight / number average molecular weight of 10 or less.

When extruding high molecular weight polyethylene in this way, the separator disclosed in JP-A-11-302436, in which the weight average molecular weight / number average molecular weight is adjusted to a value of 10 or less by actively cutting the molecules, ,
Even when high-molecular-weight polyethylene is used as the raw material resin, the average molecular weight is reduced by extrusion, so that the degree of improvement in strength is smaller than that of a conventional separator having a small average molecular weight.

[0025]

There has been a demand for a separator having high strength, a low pore closing temperature, a small shrinkage at high temperatures and a small thickness, which can be suitably used for a battery having a large capacity. None of the more proposed high molecular weight polyolefin separators satisfy all of these properties. An object of the present invention is to provide a separator having a high strength from room temperature to a high temperature, a low pore closing temperature, and a small shrinkage at a high temperature.

[0026]

Means for Solving the Problems The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, the average molecular weight was 50
It has been found that a separator comprising a polyolefin resin containing 10,000 or more high-molecular-weight polyethylenes and a polyethylene having an average molecular weight of less than 500,000 has a high strength, a low pore closing temperature and a small shrinkage at a high temperature, and forms the present invention. Reached. That is, the present invention is as follows.

(1) A separator comprising a high molecular weight polyethylene having a viscosity average molecular weight of 500,000 or more and a polyolefin resin containing a polyethylene having a viscosity average molecular weight of less than 500,000, having a puncture strength of 20 g / μm or more and a pore closing temperature of A polyolefin separator having a shrinkage at 135 ° C or lower and 135 ° C of 30% or lower. (2) A polyolefin resin containing a high-molecular-weight polyethylene having a viscosity average molecular weight of 500,000 or more and a polyethylene having a viscosity-average molecular weight of less than 500,000 extruded using a co-rotating twin-screw extruder at a temperature not lower than the melting point of polyethylene. The polyolefin separator according to (1), wherein the non-porous film obtained by axial stretching is made porous.

[0028]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be specifically described below along a manufacturing process. (1) Polyolefin resin The polyolefin resin used in the present invention is polyethylene having a viscosity average molecular weight of 500,000 or more and polyethylene having a viscosity average molecular weight of 5
A polyolefin resin containing a polyethylene resin made of polyethylene having a molecular weight of less than 100,000, and the polyethylene resin may be used by mixing a polyethylene having a different viscosity average molecular weight by using a stirrer such as a high-speed mixer after individually polymerizing the polyethylenes. And a polyethylene having a viscosity average molecular weight of 500,000 or more and a viscosity average molecular weight of 5
A polyethylene resin composed of less than 10,000 polyethylene may be used after polymerization.

When polyethylene having a viscosity-average molecular weight of 500,000 or more is used alone, it is difficult to stretch to a high magnification at a temperature higher than the melting point because the entanglement between molecules in the molten state is too large. The strength of the resulting separator is small, and the strength of the resulting separator is small. On the other hand, when polyethylene having a viscosity average molecular weight of less than 500,000 is used alone, it is difficult to perform stretching because the viscosity in a molten state is small. In order to be able to be stretched to a high magnification at a temperature higher than the melting point and to increase the strength of the stretched separator, the viscosity average molecular weight is 5
100,000 or more high molecular weight polyethylene from 10 wt% to 90
It is preferable to use a polyethylene resin containing wt%.

The viscosity average molecular weight of the polyethylene in the present invention is Mv obtained by the following method. The viscosity average molecular weight (Mv) was calculated from the following equation by measuring [η] at a measurement temperature of 135 ° C. using decalin as a solvent. [Η] = 0.0068 × Mv ^0.67 Various additives such as an antioxidant, a nucleating agent, and an inorganic filler may be added to the polyolefin resin used in the present invention, if necessary. In addition, in order to improve performance such as heat resistance and strength, in addition to polyethylene resin, blended modified polyethylene, modified polyphenylene ether resin, polyamide resin, etc. grafted with maleic anhydride etc. are used for polyolefin resin. In order to obtain a uniform mixture of these resins and polyethylene resin, a compatibilizer can be added as necessary.

(2) Extrusion The extruder used in the present invention is a co-rotating twin-screw for uniformly melting and mixing polyethylene having different viscosity average molecular weights or for uniformly melting and mixing polyethylene and other resins. Extruders are preferred. The single-screw extruder has a smaller mixing and kneading capacity than a co-rotating twin-screw extruder, so if a single-screw extruder is used, the viscosity-average molecular weights of different polyethylenes can be mixed or mixed with polymers other than polyethylene. Is extruded with insufficient mixing, and some of the polyethylene with a large viscosity average molecular weight is extruded in a granular form without melting, or the interface of the raw polymer particles remains and is extruded and breaks in the stretching process It is easy to cause a problem that it is easy to perform.

In the present invention, in order to stretch a polyolefin resin containing no solvent or plasticizer in a molten state, the polyolefin resin is extruded without mixing a solvent or a plasticizer. In order to reduce the degree to which the polyolefin resin decreases, the viscosity of the molten polyolefin resin mixed with carbon dioxide having an effect of plasticizing the polyolefin resin can be reduced, and the viscous heat generation during extrusion molding can be reduced. Carbon dioxide can be mixed by injecting carbon dioxide through an injection hole provided in the extruder cylinder and mixing it with the polyolefin resin in the extruder. Together with the pressurized container.

When extrusion molding is performed by mixing carbon dioxide gas, the polyolefin resin is extruded from the extruder without foaming the polyolefin resin or by removing carbon dioxide gas from the polyolefin resin before stretching. Can be prevented from foaming when extruded from the extruder into the atmosphere or foaming when the polyolefin resin is heated in the stretching step, making it difficult to stretch the polyolefin resin in a molten state.

To extrude the polyolefin resin from the extruder without foaming, a method in which a vent port is provided on the outlet side of the extruder and carbon dioxide gas is discharged from the extruder and extruded,
Alternatively, it can be carried out by cooling the die attached to the tip of the extruder, and solidifying and extruding the polyolefin containing carbon dioxide gas so as not to foam. When solidified and extruded high molecular weight polyolefin containing carbon dioxide,
By storing the carbon dioxide gas until it is diffused from the high molecular weight polyolefin resin and then stretching it, a nonporous film can be obtained.

(3) Stretching In the present invention, the extruded high molecular weight polyolefin is stretched to obtain a nonporous film in order to obtain a thin film or to orient the polyolefin molecules to increase the strength of the film. Stretching may be performed by a method of injecting a gas such as air into a resin extruded into a tubular shape generally called tubular stretching and expanding the resin, or into a sheet shape generally called flat stretching. The extruded resin may be stretched in one or both of the length and width directions. In the case of flat stretching, both the length direction and the width direction may be simultaneously stretched, or the length direction and the width direction may be sequentially stretched.

In the present invention, the stretching is carried out by reducing the shrinkage of the separator when the polyethylene contained in the polyolefin resin is melted and the pores of the separator are closed, and by the low melting point crystal and the high melting point crystal of polyethylene. In order to form a structure, it is preferable to carry out at a temperature higher than the melting point of polyethylene. As the stretching temperature is increased, the lower limit temperature at which the stretched separator shrinks is increased, but the degree of strength improvement by stretching is reduced, so that both the dimensional stability and strength at high temperature of the separator are satisfied. Thus, the temperature at which the stretching is performed is set. For example, in the case of a polyethylene resin, it is preferable to perform stretching in a range of about 135 ° C. to about 150 ° C.

When extruding a polyolefin resin containing no carbon dioxide gas using an extruder provided with the above vent port, stretching can be performed in the same manner as in forming a normal film or sheet. In the case of tubular stretching, the molten polyolefin resin is extruded into a cylindrical shape from a circular die attached to the tip of the extruder, and cooled to a temperature at which the cylindrical molten polyolefin resin is stretched using a cooling device such as an air cooling ring. The film is cooled and stretched.

In the case of flat stretching, a polyolefin resin extruded in a molten state from a sheet forming die such as a T-die is once cooled and solidified using a cooling roller or the like, and then the polyolefin resin is heated using a heating roller or an infrared heater. The film is stretched by heating to a predetermined temperature.
When extruding with carbon dioxide gas from the extruder, cool the die attached to the extruder tip to solidify the polyolefin resin and extrude it without foaming, until the carbon dioxide gas is diffused from the solidified polyolefin resin By stretching after storing, stretching can be performed at a temperature equal to or higher than the melting point without foaming.

(4) Porous treatment In the porous treatment for forming micropores in the nonporous film obtained by stretching the polyolefin resin, the crystal of the nonporous film is cleaved by stretching. It can be carried out by a method or a method in which the amorphous portion of the polyolefin resin is brought into contact with a solvent capable of selectively dissolving or melting to form pores. When performing the porosity treatment by stretching, the porosity can be easily increased by heat-treating the stretched polyolefin to increase the crystallinity. The stretching may be performed once or divided into a plurality of times, and the stretching between the lamellar crystals of the polyolefin resin is performed by passing through a pair of nip rolls having a speed difference generally called a roll stretching method and stretching in the machine axis direction. Cracks can be generated in the amorphous portion of

After stretching in the machine axis direction, the film can be stretched in a direction perpendicular to the machine axis direction using a clip tenter or the like to increase the hole diameter or adjust the thickness. In addition, by holding the temperature exceeding the stretching temperature while fixing the end of the film porous by stretching,
The dimensional stability of the separator can be increased by removing the strain caused by the process for making the porous body. The process of making the nonporous polyolefin resin film porous by bringing the amorphous portion of the polyolefin resin into contact with a solvent capable of selectively dissolving or melting can be performed, for example, as follows. The solvent (a) that selectively dissolves or melts the amorphous portion of the polyolefin resin is heated and put into a liquid tank, and the nonporous polyolefin resin film is swelled by dipping in the solvent (a) in the liquid tank. After that, take out from the liquid tank, wash with a liquid (b) that is compatible with the solvent (a) and does not dissolve the polyolefin resin, remove the solvent (a), and then dry to obtain a porous polyolefin resin. Can be.

As the solvent (a), hydrocarbons such as paraffin oil, lower aliphatic alcohols, lower aliphatic ketones, nitrogen-containing organic compounds, ethers, glycols, lower aliphatic esters, silicone oils and the like can be used alone or in combination. be able to. The preferred temperature of the solvent (a) depends on the type of the polyolefin resin and the solvent (a).
A temperature of 40 ° C. is preferred. The heat treatment time can be shortened if the treatment temperature is high, and the treatment time is preferably short in order to maintain the strength of the resin after being made porous.

As the liquid (b), it is preferable to use a low-boiling hydrocarbon such as hexane, a non-chlorine-containing fluorinated organic solvent such as hydrofluoroether or hydrofluorocarbon, or a ketone such as methyl ethyl ketone. To adjust the size and number of pores in the resulting separator,
It is also possible to stretch the polyolefin resin film immersed in the solvent (a) or to stretch the porous polyolefin resin film after washing with the liquid (b) and drying. When the polyolefin resin film swelled by immersion in the solvent (a) sags, the speed at which the polyolefin resin film is pulled out of the liquid tank filled with the solvent (a) is set at a rate to remove the sag. It may be larger.

The polyolefin separator of the present invention obtained as described above has both high strength, low pore closing temperature and dimensional stability in a high temperature range, and the strength of the polyolefin separator is high when manufacturing a battery. 20 g / μm or more in order to reduce short-circuit failure of
The pore closing temperature is not more than 135 ° C. in order to cut off the current promptly when the battery reaches an overcharged state, and the shrinkage rate at 135 ° C. is not more than 30% in order to prevent a short circuit when the battery is heated. is there.

The present invention will be described based on examples. The method for evaluating the physical properties of the porous film in the examples is as follows. (A) Thickness Ozaki Manufacturing Dial Gauge PEACOK No. 25
It measured using. (B) Porosity The volume of the sample was determined from the thickness and area, the mass was measured, and the porosity was determined using the following equation. Porosity (%) = (1− (mass / resin density) / volume) × 1
00 (c) Puncture strength A needle with a radius of curvature of 0.5 mm at the tip was attached to a compression tester KES-G5 manufactured by Kato Tech. And The piercing test was performed in a room adjusted to a temperature of 23 ° C. (D) Air permeability Measured using a Gurley air permeability meter based on JIS P-8117. (E) Heat shrinkage A 10 cm square sample was cut from the separator, left in an oven at a temperature of 135 ° C. for 30 minutes, then measured and averaged for the length of four sides of the sample, and measured for the length after heating. I did it. The shrinkage was determined by the following equation. Shrinkage (%) = [10 (cm) −length (cm) after heating] /
10 (cm) × 100 (f) Hole closing temperature

FIG. 1 is a schematic diagram of a measuring apparatus for measuring the pore closing temperature. 1 is a microporous membrane, 2A and 2B are 10 μm thick
, 3A and 3B are glass plates. 4 is an electric resistance measuring device (Ando Electric LCR Meter AG4311)
And is connected to the Ni foil (2A, 2B). A thermocouple 5 is connected to a thermometer 6. Reference numeral 7 denotes a data collector, which is connected to the electric resistance measuring device 4 and the thermometer 6. An oven 8 heats the microporous membrane.

More specifically, the microporous membrane 1 is impregnated with a prescribed electrolytic solution, and only the MD is fixed on the Ni foil 2A with a Teflon (registered trademark) tape as shown in FIG. 1 (B). It is fixed in the form. Figure 1 shows Ni foil 2B
As shown in (C), the mask is masked with Teflon tape except for a portion of 15 mm × 10 mm. The Ni foil 2A and the Ni foil 2B are overlapped so as to sandwich the microporous membrane 1,
Further, two Ni foils are sandwiched between the glass plates 3A and 3B from both sides. The two glass plates are fixed by being sandwiched between commercially available clips. Using the device shown in FIG. 1A, temperature and electric resistance are continuously measured. The temperature was raised at a rate of 2 ° C./min, and the electric resistance was 1 k.
It is measured by alternating current of Hz. The pore closing temperature is defined as a temperature at which the electric resistance value of the microporous membrane 1 reaches 10 ³ Ω.

The specified electrolyte composition is as follows. Solvent: propylene carbonate / ethylene carbonate / γ-butyl lactone = 1/1/2 volume% Solute: 1 mol / l of lithium borofluoride in the above solvent
Dissolved to a liter concentration.

[0048]

Example 1 A raw material resin having a viscosity average molecular weight of 109
A polyethylene mixture obtained by mixing 10,000 high-molecular-weight polyethylene and high-density polyethylene having a viscosity-average molecular weight of 390,000 in a ratio of 45 wt% and 55 wt% by a high-speed mixer was used.
For extrusion molding, the width of the gear pump and slit at the tip is 1
A co-rotating twin-screw extruder consisting of a cylinder connected to 15 cylinder blocks having an L / D ratio of 3 and attached with a slit die having a gap of 1 mm and a gap of 1 mm and a screw having a diameter of 35 mm was used.

From the feed port provided in the uppermost stream cylinder block of the twin-screw extruder, 10 kg of the polyethylene mixture was fed per hour using a quantitative feeder. Adjust the current of the electric heater of the cylinder and the slit die and the amount of cooling water so that the temperature of the cylinder and the slit of the twin-screw extruder becomes 200 ° C, and rotate the screw of the twin-screw extruder at 300 rotations per minute. The resin was extruded from the slit die by rotating at a speed. The extruded polyethylene in a molten state was taken out while being cooled and solidified by using a nip roller through which cooling water was passed, thereby producing a plate-shaped sample. Simultaneous biaxial stretching of this sample was performed using a biaxial stretching machine manufactured by Iwamoto Seisakusho. The stretching speed was 10 mm / sec. Stretching was performed by changing the stretching temperature from 120 ° C. to 150 ° C. at 5 ° C. intervals, and the maximum stretching ratio immediately before breaking at each stretching temperature was determined.

The stretching condition with the largest stretching ratio is 135
At 10 ° C., the height and width were 10 times. The thin film stretched 10 times vertically and horizontally at 135 ° C. was immersed in liquid paraffin heated to 130 ° C. for 2 minutes, immersed in methyl ethyl ketone for 24 hours to remove the liquid paraffin, and dried at room temperature and pressure for 24 hours.
The obtained separator has a viscosity average molecular weight of 620,000, a thickness of 17 μm, a porosity of 45%, an air permeability of 320 seconds, a piercing strength of 520 g, a heat shrinkage of 135 ° C. of 22%, and a pore closing temperature of 134%. ° C.

[0051]

Example 2 The same polyethylene mixed resin as in Example 1 was extruded under the same conditions except that the slit die of Example 1 was replaced with a cylindrical die having an outer diameter of an annular slit of 10 mm and an inner diameter of 8 mm. . Air compressed from a tube provided in an inner die of a cylindrical die was sent into the resin extruded into a tube, and tubular stretching was performed. An air ring is provided on the outside of the resin extruded in a tubular shape, and air adjusted to 20 ° C. is blown out of the air ring to cool the tubular resin so that tubular stretching starts at a fixed position. The temperature of the resin was adjusted. The tubular stretched thin film was folded by a deflator roll and pulled through a nip roll of a metal roller and a rubber roller.

The width of the two thin films stretched and folded is 142 mm, and the stretching ratio in the width direction is 9 times from the outer diameter of the annular slit of the cylindrical die of 10 mm. The stretching ratio in the machine direction was 9.5 times from the ratio of the linear velocity of the resin extruded from the cylindrical die, which was calculated from the area of the annular slit of the cylindrical die and the extrusion amount, to the take-up speed of the nip roll. The tubular stretched thin film was subjected to a porous treatment under the same conditions as in Example 1. The obtained separator had a viscosity average molecular weight of 630,000, a weight average molecular weight / number average molecular weight of 13, a thickness of 16 μm, a porosity of 47%, an air permeability of 290 seconds, a puncture strength of 470 g and a heat shrinkage of 135 ° C. The rate was 21% and the pore closing temperature was 133 ° C.

[0053]

Example 3 As a raw material resin, the viscosity average molecular weight was 320.
A polyethylene mixture obtained by mixing 10,000 high-molecular-weight polyethylene and high-density polyethylene having a viscosity-average molecular weight of 390,000 in a ratio of 45 wt% and 55 wt% by a high-speed mixer was used.
For extrusion molding, the width of the gear pump and slit at the tip is 1
A co-rotating twin-screw extruder consisting of a cylinder connected to 15 cylinder blocks having an L / D ratio of 3 and attached with a slit die having a gap of 1 mm and a gap of 1 mm and a screw having a diameter of 35 mm was used. From the feed port provided in the most upstream cylinder block of the twin screw extruder,
10 kg of the polyethylene mixture was supplied per hour using a quantitative feeder, and carbon dioxide gas was injected at a pressure of 15 MPa from an injection hole provided in a sixth cylinder from the most upstream side of the extruder. The carbon dioxide gas was taken out of the liquefied carbon dioxide gas cylinder placed on the measuring instrument and pressed into the extruder using a plunger pump. The injection amount of carbon dioxide was determined from the decrease in the mass of the measuring instrument, and extrusion molding was performed while adjusting the liquid feed rate of the plunger pump so that the injection amount was 1 kg per hour.

The plunger pump and the piping were cooled with ethylene glycol cooled by a refrigerator so that carbon dioxide gas was not vaporized inside the plunger pump and the piping and the amount of liquid supplied was not changed. The carbon dioxide gas injected into the extruder was discharged from a vent port provided in the third cylinder from the most downstream side of the extruder, and extruded so that the polyolefin resin extruded from the extruder did not foam. Adjust the current of the electric heater of the cylinder and the slit die and the amount of cooling water so that the temperature of the cylinder and the slit of the twin-screw extruder becomes 200 ° C, and rotate the screw of the twin-screw extruder at 300 rotations per minute. Rotated at speed.

Using a nip roll for cooling by passing cooling water through the hollow, the molten polyethylene extruded from the slit die was taken out while cooling and solidifying to form a plate-shaped sample. Simultaneous biaxial stretching of this sample was performed using a biaxial stretching machine manufactured by Iwamoto Seisakusho. Stretching speed is 10
mm / sec. Stretching was performed by changing the stretching temperature from 120 ° C. to 150 ° C. at 5 ° C. intervals, and the maximum stretching ratio immediately before breaking at each stretching temperature was determined. The stretching condition with the largest stretching ratio was 135 ° C. and 10 times in length and width. 135
After immersion in liquid paraffin heated to 130 ° C for 2 minutes, the film was immersed in methyl ethyl ketone for 24 hours to remove the liquid paraffin, and dried at room temperature and pressure for 24 hours. The obtained separator had a viscosity average molecular weight of 620,000, a thickness of 17 μm, a porosity of 45%,
The air permeability was 320 seconds, the piercing strength was 520 g, the heat shrinkage at 135 ° C was 22%, and the pore closing temperature was 134 ° C.

[0056]

Comparative Example 1 A polyethylene sheet was extruded under the same conditions using the same extruder as in Example 1 except that the resin to be extruded was changed to high density polyethylene having a viscosity average molecular weight of 500,000. The weight average molecular weight of the obtained polyethylene sheet was 4
The weight average molecular weight / number average molecular weight was 70,000. An attempt was made to stretch the polyethylene sheet under the same conditions as in Example 1, but no stretching was possible.

[0057]

Comparative Example 2 The resin extruded was changed to a high molecular weight polyethylene having a viscosity average molecular weight of 2.3 million, and extruded under the same conditions using the same extruder as in Example 1 to obtain a polyethylene sheet. The viscosity average molecular weight of the obtained polyethylene sheet was 2.18 million, and the weight average molecular weight / number average molecular weight was 5. When the polyethylene sheet was stretched using the same stretching machine as in Example 1, the stretching could be performed at the highest magnification when the stretching temperature was 145 ° C., but the stretching ratio was as small as 5 × 5. Was something. The separator made porous under the same conditions as in Example 1 had a thickness of 16 μm, a porosity of 49%, an air permeability of 1300 seconds, and a piercing strength of 220.
g, shrinkage at 135 ° C. of 23%, and pore closing temperature of 135
° C.

[0058]

Comparative Example 3 The polyethylene used in Example 3 had a caliber of 3
Extruded using a single screw extruder with 0 mm and L / D ratio of 24. The cylindrical die used in Example 2 was attached to the tip of the single-screw extruder, and tubular stretching was attempted in the same manner as in Example 2. However, since the extruded cylindrical resin was broken, the tubular stretching was performed in a molten state. Could not be performed. Observation of the broken resin pieces revealed that the resin was not melted in the shape of a tub.

[0059]

The polyolefin separator of the present invention is a separator having a high strength, a low pore closing temperature, and a low shrinkage, and is particularly useful as a separator for a lithium ion secondary battery.

[Brief description of the drawings]

FIG. 1A is an overall schematic view of an apparatus for measuring a pore closing temperature. (B) It is sectional drawing in the Ni foil (2A) surface of FIG. (C) It is sectional drawing in the Ni foil (2B) surface of FIG.

[Explanation of symbols]

 1: microporous membrane 2A, 2B: Ni foil 3A, 3B: glass plate 4: electric resistance measuring device 5: thermocouple 6: thermometer 7: data collector 8: oven

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme court ゛ (Reference) // B29K 23:00 B29K 23:00 B29L 7:00 B29L 7:00 31:14 31:14 C08L 23: 00 C08L 23:00 F-term (reference) 4F074 AA17 AB01 AB05 CA01 CA02 CA03 CB03 CB17 CC02 DA08 DA22 DA24 DA49 4F210 AA03 AA05 AA06 AE10 AG01 AG20 AH81 QC05 QG01 4F212 AA04C AG01 AG20 UA04 H02 UA13 U02H04 UA13 UA15 U04 HH07

Claims (2)

[Claims] 1. A separator comprising a high molecular weight polyethylene having a viscosity average molecular weight of 500,000 or more and a polyolefin resin containing a polyethylene having a viscosity average molecular weight of less than 500,000, having a puncture strength of 20 g / μm or more and a pore closing temperature of 1 or more.
A polyolefin separator having a shrinkage at 35 ° C or lower and 135 ° C of 30% or lower. 2. A polyolefin resin containing a high-molecular-weight polyethylene having a viscosity-average molecular weight of 500,000 or more and a polyethylene having a viscosity-average molecular weight of less than 500,000 extruded using a co-rotating twin-screw extruder at a temperature not lower than the melting point of polyethylene. 2. The polyolefin separator according to claim 1, wherein the nonporous film obtained by biaxial stretching is made porous.