JPH05247253A - Single layer porous membrane - Google Patents

Abstract

(57) [Summary] [Object] To provide a monolayer porous polymer membrane having the property of losing its permeability when heated to a predetermined temperature. A single-layer porous polymer film according to the present invention is composed of a first polymer having a predetermined melting temperature,
A porous polymer layer having micropores therein, and a second polymer existing inside the micropores of the porous polymer layer and having a predetermined melting temperature lower than the melting temperature of the first polymer. And the relative amount of the second polymer with respect to the first polymer is such that before heating above a certain temperature, the second polymer exists in a state in which the second polymer does not block the micropores. It is characterized in that the amount is such that, when heated above the temperature, the second polymer melts to close the inside of the fine pores and lose its permeability.

Description

Detailed Description of the Invention

[0001]

[Object of the Invention]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a porous film having a single-layer structure, which is extremely useful when used as, for example, a separator for lithium batteries. More specifically, the present invention provides
The present invention relates to a porous membrane having a single-layer structure, which has a property of losing its permeability when heated above a predetermined temperature.

[0002]

2. Description of the Related Art Porous polymer membranes having a fine pore structure inside are widely used in the field of filtration and as various separation membranes. Various ideas have been made to give various characteristics to. For example,
Stretching to improve the strength of the microporous membrane,
When used in filtration, in order to improve the affinity with the filtrate, treatment with a surfactant has been made.
Further, it has been conventionally performed that various functional groups are grafted or copolymerized on the surface and inside of the porous membrane so as to have an ion exchange function.

Among the applications of such porous polymer membranes, there are applications in which it is desirable to give the property that the permeability of the porous polymer membrane is lost above a certain temperature. For example, when the porous polymer film is used as a separator for a rechargeable lithium secondary battery, etc., when the temperature inside the battery rises sharply due to heat generation during charging or a short circuit between electrodes, The pressure inside the battery may rise excessively due to the solvent vapor used, which may cause explosion or fire.Therefore, when the temperature inside the battery rises above a certain temperature, the ion permeability of the separator membrane disappears. It is desirable to have a property of interrupting the electric current.

As a method for imparting such a property to a porous polymer film, conventionally, a porous polymer is formed by using a single polymer having a constant melting point, and the film is formed at a temperature above the temperature. What constitutes a polymer itself is melted to close the pores existing therein, or a porous membrane of a polymer having a high melting point is used as a support layer,
It is considered that a porous film of a polymer having a low melting point is laminated on the support layer, and the low melting point polymer is melted at a temperature higher than the melting point of the low melting point polymer to form an impermeable layer. Has been. As an example of the former, 130
Lithium secondary batteries equipped with a separator made of a specific polyethylene that melts at a temperature slightly above 0 ° C have recently begun to appear on the market. In such a battery, when the temperature inside the battery abnormally rises and becomes equal to or higher than the melting point of the polyethylene that constitutes the separator, the polyethylene melts and closes the micropores inside so that ions do not flow between the cathode and the anode. To prevent the danger of explosion. Also, as an example of the latter, there is JP-A-62-1085.
No. 7 discloses a battery separator composed of a polyolefin-based multi-layered porous film, and JP-A-3-59947 discloses a battery separator in which a crosslinked polyethylene film and an uncrosslinked polyethylene film are laminated. It is disclosed. Further, in JP-A-2-75152, for a lithium battery composed of a porous membrane in which a layer of a fusible non-woven fabric is heat-sealed on a support of a porous or microporous polymer to form a multilayer. A separator is disclosed. For reference, the cross-sectional structure of the latter porous membrane, that is, a porous membrane having a multilayer structure in which a fusible porous polymer layer is laminated on a porous polymer layer serving as a support layer is schematically shown in FIG. ..

However, these conventional porous polymer membranes have the following problems.

First, in the former porous membrane, that is, in the case where a polymer having a constant melting point is used to form the porous membrane, the polymer is melted at a temperature above the temperature to close the pores, Considering the film strength, it is necessary to set the melting point of the polymer above a certain level,
For example, when it is used as a separator for a lithium secondary battery, safety cannot be secured sufficiently. That is, in order to prevent the danger associated with heating in the lithium secondary battery, it is required that the current flow path be completely closed when the temperature inside the battery abnormally rises to a temperature of at least about 130 ° C. In order to do so, it is necessary to use a polymer at which the micropores are completely closed, that is to say they start to melt at temperatures below 130 ° C. However, since such a polymer has a low membrane strength and cannot be used as a battery separator, a polymer having a melting point of 130 ° C. or higher is actually used.
For this reason, the current commercially available batteries do not completely close the flow path of the current even when the temperature reaches 130 ° C. or higher, and the risk of explosion is still included.

Further, in the latter porous membrane, that is, in the multilayer structure in which a polymer porous membrane having a low melting point is laminated on a polymer porous membrane having a high melting point, when the membranes are laminated, There were many problems that the multilayered film was curved to one side or peeled off due to the difference in heat shrinkage. Also, when the temperature rises, due to the difference in the coefficient of thermal expansion of the film, the film wrinkles or peels off between the films,
Problems often occurred with the condition of the membrane. Furthermore, when laminating porous films having different melting temperatures,
In some cases, the skin layer may be formed between the films due to the heating at the time of adhesion, and the permeability of the multilayer film may be lost, or the pores may be blocked by the skin layer. Furthermore, in order to increase the strength of the multilayer porous film while maintaining a predetermined film thickness, it is necessary to use a high molecular weight polyolefin as the support layer and increase the film thickness of the support layer.
As a result, it is necessary to make the film thickness of the low melting temperature layer extremely thin, but it was technically extremely difficult to uniformly bond the very thin porous film.

[0008]

[Constitution of the invention]

The present inventors have solved the above-mentioned conventional problems in developing a porous polymer membrane having a property that its permeability disappears at a predetermined temperature or higher. As a result of intensive research, it is composed of a first polymer having a predetermined melting temperature,
A porous polymer layer having micropores therein, and a second polymer existing inside the micropores of the porous polymer layer and having a predetermined melting temperature lower than the melting temperature of the first polymer. And the relative amount of the second polymer with respect to the first polymer is such that before heating above a certain temperature, the second polymer exists in a state in which the second polymer does not block the micropores. A porous polymer membrane having a single-layer structure, characterized in that the second polymer is melted when heated above a temperature to block the inside of the fine pores and lose its permeability. The inventors have found that the desired properties of can be obtained and can solve the various problems described above, and have completed the present invention.

That is, the porous polymer membrane of the present invention is composed of the first polymer having a predetermined melting temperature, and has a porous polymer layer having fine pores therein, and the fine pores of the porous polymer layer. It is characterized by having a single-layer structure composed of a second polymer having a predetermined melting temperature lower than the melting temperature of the first polymer, which is present inside, and is heated to a certain temperature or higher. Prior to this, the second polymer exists in a state where it does not block the fine pores, but when heated above a certain temperature, the second polymer melts and closes the inside of the fine pores. However, it has the property of eliminating the permeability of the porous membrane.

FIG. 1 shows a schematic cross section of a single layer porous membrane of the present invention. As is clear from the figure, in the porous membrane according to the present invention, in the wall portion of the micropores existing inside the porous polymer layer of the first polymer having a predetermined melting temperature which constitutes the membrane, A layer of a second fusible polymer having a predetermined melting temperature that is lower than the melting temperature of the first polymer is deposited in an amount that does not block the micropores. In a normal state, ions and the like can move through the micropores (Fig. 1-a), but when the temperature rises abnormally to the melting point of the second fusible polymer or higher, the second polymer is It melts and becomes a fluid state, blocking the micropores and blocking movement of ions and the like through the micropores (FIG. 1-b).

In the present invention, what is used as the first polymer constituting the porous polymer layer may be any one as long as it has a melting temperature high enough to withstand use as a porous membrane. It is considered possible, but those having high crystallinity and high tensile strength when formed into a film are preferable. Further, as long as the object of the present invention is not violated, it is possible to improve the strength and rigidity by crosslinking. Regarding the polymer that constitutes the porous polymer layer, it is generally desirable to have heat resistance and high strength, depending on its application,
It is considered preferable to increase the crystallinity, increase the molecular weight, or introduce physical crosslinking. However, it should be noted that if the molecular weight or the degree of cross-linking is too high, the flexibility of the film itself is impaired, which is not desirable in some applications. As the crosslinking method, in addition to known crosslinking with electron beam or radiation, crosslinking with a silane coupling agent or a peroxide can be used.

An example of the first polymer having such properties is a molecular weight of about 3 having a melting point of 150 to 180 ° C.
10,000 to 800,000 polypropylene, for example, Hipol under the trade name of Mitsui Nisseki Polymer Co., Ltd., Noblen under the trade name of Mitsui Toatsu Chemical Co., Ltd., Chisso Polypro under the trade name of Chisso Co., Idemitsu Petroleum Chemical name: Idemitsu Polypro, Mitsubishi Yuka Co., Ltd .: trade name Mitsubishi Polypro, Tonen Chemical Co., Ltd .: trade name Tonen Polypro, etc. It goes without saying that other polymers having a can be widely used.

In the present invention, the second constituent of the meltable phase
As the polymer (1), any polymer having a predetermined melting temperature lower than the melting temperature of the first polymer can be widely selected according to the application. Further, copolymers having various compositions can be used, provided that the melting temperature condition is satisfied. Examples of such a second polymer include polyethylene and copolymers thereof, for example, low density polyethylene (LDPE), for example, Mitsubishi Polyethylene LD manufactured by Mitsubishi Kasei Co., Ltd., and Mitsui Petrochemical Co., Ltd. manufactured. Misoran, Sumitomo Chemical Co., Ltd. product name Sumikasen, Mitsui Petrochemical Industry Co., Ltd. product name Ultzex, Sumitomo Seika Chemical Co., Ltd. product name Frosen, Nippon Petrochemical Co., Ltd. product name Linear low-density polyethylene (L stone, etc.)
LDPE), for example, Sumikasen-L under the trade name of Sumitomo Chemical Co., Ltd., Idemitsu Polyethylene-L under the trade name of Idemitsu Petrochemical Co., Ltd., Linilex under the trade name of Nippon Petrochemical Co., Ltd., ethylene-vinyl acetate. Copolymer, ethylene-butadiene copolymer, ethylene-pentadiene copolymer, ethylene-methacrylic acid copolymer, ethylene-acrylic acid ester copolymer, ethylene-methacrylic acid ester copolymer, ethylene-propylene copolymer , Ethylene-propylene-diene terpolymer (EPDM), ethylene-
Examples thereof include terpolymers with maleic anhydride and other monomers.

In the application using the single-layer porous polymer membrane of the present invention, the above-mentioned first property is required according to various characteristics of the membrane itself and the temperature at which it is required to eliminate the permeability and the normal operating temperature. And the second polymer can be appropriately combined and selected. For example, when the single-layer porous polymer membrane of the present invention is used as a separator of a lithium battery, polypropylene having a melting temperature of 130 ° C. or higher is used as the first polymer and about 95-120 ° C. as the second polymer. Density 0.91 with melting temperature
When using 0.94 g / cc of low density polyethylene,
The strength of the separator itself is also preferable, and a very useful separator that rapidly loses its permeability at a predetermined temperature can be obtained.

The amount ratio of the first polymer and the second polymer constituting the single-layer porous polymer membrane of the present invention depends on the first polymer in a normal state, that is, before being heated to a predetermined temperature or higher. The micropores of the constructed porous membrane are not blocked by the second polymer, and when heated to a temperature higher than the predetermined temperature, the second polymer is melted to close the micropores. It is necessary. The desirable ratio varies depending on the thickness of the porous membrane, the size and shape of the fine pores in the membrane, the method for producing the porous membrane, the method for introducing the second polymer into the fine pores, etc. Those skilled in the art can determine the optimum ratio through experiments and the like. For example, when a porous film made of the first polymer having a porosity of 50% and a thickness of 25 μm is used,
The ratio of the first polymer to the second polymer is from 50/50 to
95/5 (weight ratio) is desirable.

Further, in order to enhance the binding property between the first polymer and the second polymer, a polymer obtained by copolymerizing an unsaturated carboxylic acid with a polyolefin, such as Bondine or Mitsubishi Oil manufactured by Sumitomo Chemical Co., Ltd. Chemical name: Yukaron, a polyolefin graft material, Mitsui Petrochemical Co., Ltd. product name: Admer, Sumitomo Chemical Co., Ltd. product name: Sumifarm, Asahi Kasei Kogyo Co., Ltd. product name: A component such as Nex may be added as a binder. Furthermore, various additives such as an antioxidant, an ultraviolet absorber, a lubricant, an anti-blocking agent, etc. are added to the first and second polymers according to the present invention, if necessary, within a range that does not impair the object of the present invention. be able to.

As the method for producing the single-layer porous polymer membrane of the present invention using the above-mentioned first and second polymers, various methods known in the art can be used. The following is an example of the method for forming the structure of the single-layer porous polymer membrane of the present invention, but these are merely representative examples,
In addition to this, a wide variety of methods can be used to form the structure of the single layer porous polymer membrane of the present invention.

By melting and kneading the first and second polymers constituting the single-layer porous membrane of the present invention and the third polymer which is incompatible with them to form a film, polymer molecules are separated from each other. IPN with a continuous layer intertwined with each other (int
A polymer film having an erpenetrating polymer network structure is obtained. Next, a single-layer porous polymer membrane according to the structure of the present invention can be produced by extracting only the third polymer using a solvent that dissolves only the third polymer. According to this method, the single layer porous polymer membrane of the present invention can be obtained very easily. For reference, FIG. 1.3 on page 3 of Polymer Alloy (published by Kyoritsu Shuppan, edited by The Society of Polymer Science, Takashi Inoue, Shoji Ichihara) shows a conceptual diagram of the IPN structure.

The third polymer material used in such a method is incompatible with any of the two types of polymers constituting the porous membrane of the present invention, and these polymers and IP
Any polymer can be used as long as it forms an N structure and can be extracted and removed only by a specific solvent. Here, the specific solvent means a solvent capable of dissolving only the third polymer without dissolving the first and second polymers at a predetermined temperature, for example, 50 ° C. Those skilled in the art will appreciate that the first and second
A third polymer that can be used for a particular combination of the polymers and the solvents that can be used for the combination can be determined empirically and empirically. here,
Specific examples of the third polymer that can be used as the third polymer include styrene / hydrogenated butadiene / styrene block copolymer (SEBS), such as Kraton G (trade name) manufactured by Shell Chemical Co., Ltd., and styrene / hydrogenated. Examples thereof include isoprene / styrene and styrene / hydrogenated isoprene block copolymer (SEPS), such as Septon (trade name) manufactured by Kuraray Co., Ltd. The structural formulas of SEBS and SEPS are shown in FIG. These SEBS and SEPS form an IPN structure when melt-kneaded with a polyolefin such as polypropylene or polyethylene, and after forming a film using such a kneaded product, SEBS or SEPS is extracted with a specific solvent to obtain a porous material having a through hole. A membrane can be prepared. As a specific solvent in the method, SE
Those which can be used for PS or SEPS include toluene, xylene, cyclohexane, methylcyclohexane, methyl ethyl ketone, carbon tetrachloride, chloroform and the like. Further, in addition to these solvents, solvents having solubility parameter values of 7.5 to 9.3 are considered to be solvents that roughly meet this purpose.

Although typical examples of the method for producing the single-layer porous membrane of the present invention have been given above, other known plastics such as the extrusion method by the inflation method, the T-die method, the casting method, etc. A film forming technique can be adopted. Further, stretching can be carried out in order to increase the strength of the finally obtained porous film. For example, a film produced by a predetermined film-forming method is stretched at a temperature not lower than the glass transition point of the first polymer and lower than the melting point,
After that, if necessary, the residual stress inside the film is relaxed by annealing at a temperature higher than the temperature at the time of stretching, and then the solvent is used to extract only the third polymer to obtain a porous structure with high strength. A film can be formed. At this time, the draw ratio is preferably 1.1 to 20 times, particularly preferably 1.1 to 5 times.

The porosity of the porous polymer membrane of the present invention is 80.
% Or less is desirable, and when the porosity exceeds 80%, the strength of the membrane decreases sharply and the porous membrane cannot be used practically. Further, when the porosity exceeds 80%, the amount of the second polymer must be increased in order to block the fine pores of the porous layer by melting the second polymer, and the second polymer is The compositional balance with the second polymer is completely lost, and most of the second polymer is composed of the second polymer. With this, the strength of the porous film cannot be obtained, and the porous film cannot be actually used for various purposes.

Here, the porosity is expressed by the following equation.

Porosity = (pore volume / porous membrane volume) × 100 [where, pore volume = (n-butyl alcohol porous membrane weight-absolute dry porous membrane weight) /0.81] The porosity of the porous membrane is controlled by the amount of the third polymer added in the formulation. This is because the third polymer is extracted and removed after forming the film to form pores thereafter. Here, the third polymer is preferably added in an amount of 0.5 to 9 times the total amount of the first polymer and the second polymer to obtain a porous film. When the amount of the third polymer is 0.5 times or less of the total amount of the first polymer and the second polymer, the porosity becomes low and the desired porosity cannot be obtained. If the amount of the polymer exceeds 9 times the total amount of the first polymer and the second polymer, the strength of the porous film cannot be maintained, which is not practical.

Further, by annealing the film composed of the first polymer, the second polymer and the third polymer at a temperature equal to or lower than the melting temperature of the second polymer, the porosity is higher than that of the non-annealed film. Can be raised.
This is because by annealing the film, the stress of each polymer during film formation can be relieved, and the shrinkage in the thickness direction after extracting and removing the third polymer from the film can be reduced.

The desirable pore diameter of the porous polymer membrane of the present invention varies depending on the application, but is generally 0.05 μm to
10 μm is preferable. The pore size of the porous membrane is determined by how finely the third polymer is dispersed in the mixture of the first polymer and the second polymer during melt-kneading. In other words, the pore size is determined by how to reduce the structural period of the separated phase of each polymer. This is because the third polymer is extracted and removed after forming the film to form pores thereafter. Therefore, the pore diameter of the porous membrane can be adjusted by changing the temperature and shear stress during melt-kneading, and the melt viscosity of the first polymer, the second polymer and the third polymer. For example, when the first polymer, the second polymer, and the third polymer are melt-kneaded with a kneader, the kneading temperature is lowered to perform melt-kneading in a state where the melt viscosity of each polymer is high, whereby the porosity of the present invention is improved. The pore size of the membrane can be reduced. On the contrary, when the temperature at the time of melt-kneading is increased, the pore diameter of the obtained porous film becomes large. Further, when the shear stress during melt-kneading is increased, the pore diameter of the obtained porous membrane becomes smaller, and conversely when it is weakened, the pore diameter of the obtained porous membrane becomes larger. Further, when a polymer having a high molecular weight is used as the third polymer to increase the melt viscosity, a porous membrane having a small pore size can be obtained. On the contrary, when a polymer having a small molecular weight is used as the third polymer to reduce the melt viscosity, a porous film having a large pore size can be obtained. By combining these events, the pore size of the obtained porous membrane can be easily changed.

The thickness of the porous polymer film of the present invention is generally 200 μm or less, especially 2 although it depends on the use.
0 μm to 120 μm is preferable.

[0027]

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.

In the following, the film thickness is measured with a dial gauge, the average pore diameter is determined from photographs of the surface and cross section obtained by a scanning electron microscope, and the porosity is 20 mm ×. It is a value obtained by the following formula, wherein the weight of the porous polymer membrane sample cut into 20 mm was impregnated with n-butyl alcohol and the weight of the sample when it was in an absolutely dry state was obtained.

Porosity = (void volume / porous membrane volume) × 100 [where, void volume = (n-butyl alcohol porous membrane weight-absolute dry porous membrane weight) /0.81] The state of blockage inside the fine pores of the porous membrane due to the melting of the second polymer was examined by ion permeability. That is, the obtained porous membrane was heated at a predetermined temperature for a predetermined time and then placed in the apparatus shown in FIG. 4 to examine the permeation of ions. The apparatus shown in FIG. 4 is arranged so that a porous membrane can be placed between two containers. One container contains pure water, and the other container contains a 1% solution of sodium chloride and a surfactant. 50 cc of each solution was added to reduce the surface tension below the critical surface tension of the porous membrane. Due to the difference in concentration between the two solutions, the ions migrate toward the pure water through the porous membrane. The flow of ions moving into this pure water was measured with a conductivity meter over time, and compared with the amount of ion permeation before heating, depending on whether or not the permeation of ions was blocked after heating, the second polymer It was judged whether or not the inside of the fine pores of the porous film was melted and blocked. Also,
The cross section of the porous membrane was observed with a scanning electron microscope before and after the melting test to determine whether the polymer was molten.

[0030]

Example 1 Polypropylene having a melting temperature of 169 ° C. (manufactured by Mitsui Nisseki Polymer Co., Ltd., trade name Hypol B200,
Melt flow rate 0.5g / 10 minutes, density 0.91g
/ Cm ³ ) 50 parts by weight, low-density polyethylene having a melting temperature of 109 ° C. (manufactured by Nippon Petrochemical Co., Ltd., trade name Nisseki Lexron L501, melt flow rate 7.0 g / 10 minutes,
Density 0.917 g / cm ³ ) 50 parts by weight, styrene-hydrogenated isoprene-styrene block polymer (Kuraray Co., Ltd., trade name Septon 2007, melt flow rate 4 g / 10 minutes, density 0.93 g / cm ³ ) 100 parts by weight. A part was sufficiently melt-kneaded at 200 to 220 ° C. using a kneader, and a film was produced by extrusion molding with a T die. Furthermore, this film is 180 ° C, 200 kg / c
It was pressed for 1 minute under the condition of m ² to produce a film having a film thickness of 150 μm. The film was immersed in cyclohexane to completely extract and remove the styrene-hydrogenated isoprene-styrene block polymer, and the surface of the film was washed with new cyclohexane to form a single-layer porous film composed of polypropylene and polyethylene. A film was obtained. The film showed a weight loss due to the content of styrene-hydrogenated isoprene-styrene block polymer due to the weight change before and after extraction. Further, IR analysis also revealed that the porous film did not contain a styrene component and a single-layer porous film composed of polypropylene and polyethylene was obtained. The properties of the obtained film are shown in Table 1. The results of the ion permeability test are shown in Table 2.
It was shown to. From the results in Table 2, it was found that the membrane heated to 115 ° C lost the ion permeability. Electron micrographs of the cross section of the porous film before and after the heating test at 115 ° C. are shown in FIGS. 5-a and 5-b, respectively. From these photographs, it is observed that the micropores of the porous film are closed after heating.

[0031]

Example 2 In the same manner as in Example 1, 60 parts of polypropylene (B200) and 40 parts of polyethylene (L501) were used.
Part, styrene-hydrogenated isoprene-styrene block polymer (Septon 2007) 100 parts was prepared, and the styrene-hydrogenated isoprene-styrene block polymer was extracted and removed with cyclohexane to obtain the porous membrane of the present invention. It was The film showed a weight loss due to the content of styrene-hydrogenated isoprene-styrene block polymer due to the weight change before and after extraction. Further, IR analysis also revealed that the porous film did not contain a styrene component and a single-layer porous film composed of polypropylene and polyethylene was obtained. The properties of the obtained film are shown in Table 1. The results of the ion permeability test are shown in Table 2. From the results of Table 2, it was found that the porous membrane lost the ion permeability after the heating test at each temperature. In addition, electron micrographs of the cross section of the porous film before and after the heating test at 115 ° C. are shown in FIGS. 6-a and 6-b, respectively. From these photographs, it is observed that the micropores of the porous film are closed after heating.

[0032]

Example 3 In the same manner as in Example 1, 70 parts of polypropylene (B200) and 30 parts of polyethylene (L501) were used.
Part, styrene-hydrogenated isoprene-styrene block polymer (Septon 2007) 100 parts was prepared, and the styrene-hydrogenated isoprene-styrene block polymer was extracted and removed with cyclohexane to obtain the porous membrane of the present invention. It was The film showed a weight loss due to the content of styrene-hydrogenated isoprene-styrene block polymer due to the weight change before and after extraction. Further, IR analysis also revealed that the porous film did not contain a styrene component and a single-layer porous film composed of polypropylene and polyethylene was obtained. The properties of the obtained film are shown in Table 1. The results of the ion permeability test are shown in Table 2. From the results in Table 2, 1
It was found that after heating to 15 ° C. and 130 ° C., the amount of ion permeation was significantly reduced, that is, the ion permeability was lost.

[0033]

Example 4 In the same manner as in Example 1, 80 parts of polypropylene (B200) and 20 parts of polyethylene (L501) were used.
Part, styrene-hydrogenated isoprene-styrene block polymer (Septon 2007) 100 parts was prepared, and the styrene-hydrogenated isoprene-styrene block polymer was extracted and removed with cyclohexane to obtain the porous membrane of the present invention. It was The film showed a weight loss due to the content of styrene-hydrogenated isoprene-styrene block polymer due to the weight change before and after extraction. Further, IR analysis also revealed that the porous film did not contain a styrene component and a single-layer porous film composed of polypropylene and polyethylene was obtained. The properties of the obtained film are shown in Table 1. The results of the ion permeability test are shown in Table 2. From the results of Table 2, it was found that the amount of ion permeation slightly decreased after heating. However, the remarkable decrease as in Examples 1 to 3 was not observed.

[0034]

Example 5 In the same manner as in Example 1, 70 parts of polypropylene (B200), 30 parts of polyethylene (manufactured by Tosoh Corporation, Petrosen 356, melt flow rate 100, density 0.914 g / cm ³ ), styrene-hydrogenation. Isoprene-styrene block polymer (Septon 200
7) A film composed of 100 parts was prepared, and the styrene-hydrogenated isoprene-styrene block polymer was extracted and removed with cyclohexane to obtain the porous membrane of the present invention. The film showed a weight loss due to the content of styrene-hydrogenated isoprene-styrene block polymer due to the weight change before and after extraction. Further, IR analysis also revealed that the porous film did not contain a styrene component and a single-layer porous film composed of polypropylene and polyethylene was obtained. The properties of the obtained film are shown in Table 1. The results of the ion permeability test are shown in Table 2.
It was shown to. From the results in Table 2, it was found that the amount of permeation of ions was significantly suppressed after heating at 105 ° C and 115 ° C.

[0035]

Example 6 Polypropylene having a melting temperature of 130 ° C. or higher (manufactured by Mitsui Nisseki Polymer Co., Ltd., trade name Hypol F60)
0) 80 parts by weight, low-density polyethylene having a melting temperature of 104 ° C. (trade name: FLOWCEN G801 manufactured by Sumitomo Seika Chemicals Ltd.) 2
0 parts by weight, styrene-hydrogenated isoprene-styrene block polymer (Kuraray Co., Ltd., trade name Septon 200)
6) 100 parts by weight was sufficiently melt-kneaded at 220 ° C. using a kneader, and a film was prepared by extrusion molding with a T die. Next, the film was immersed in cyclohexane for 1 hour to completely extract the styrene-hydrogenated isoprene-styrene block polymer, and the surface of the film was washed with new cyclohexane to form a single layer porous film composed of polypropylene and polyethylene. A quality film was obtained. The film showed a weight loss due to the content of styrene-hydrogenated isoprene-styrene block polymer due to the weight change before and after extraction. Also, by IR analysis,
It was found that this porous film did not contain a styrene component, and a single-layer porous film composed of polypropylene and polyethylene was obtained. The properties of the obtained film are shown in Table 1.

The porous membrane was immersed in propylene carbonate, which is a poor solvent for polyolefin, and heated at 100 ° C. The cross section of the film was observed with a scanning electron microscope. However, it was found that the micropores were blocked by heat. Electron micrographs of the cross sections of the porous film obtained in this example before and after the heating test are shown in FIGS. 7 and 8, respectively.

Table 2 shows the results of the ion permeability test of the obtained membrane. From the results in Table 2, 115 ° C
It was found that the amount of ions permeated was significantly suppressed in the heating test at.

[0038]

Example 7 Polypropylene having a melt flow rate of 1.7 (manufactured by Mitsui Toatsu Chemicals, Inc., trade name Nobren JS, melting point 170 ° C.) 80 parts by weight, low-density polyethylene having a melting temperature of 100 ° C. (Mitsui Petrochemical Co., Ltd. ) Product name Misoran A
CE30N) 20 parts by weight, styrene as in Example 1
Hydrogenated isoprene-styrene block polymer (SEP
S) 100 parts by weight was sufficiently melt-kneaded at 220 ° C. using a kneader, and a film was prepared by extrusion molding with a T die. Next, the film was stretched twice at 80 ° C. in the longitudinal direction, annealed at 100 ° C. for 5 minutes, and then rapidly cooled. Further, the film was immersed in cyclohexane for 1 hour to completely extract SEPS, and the surface of the film was washed with fresh cyclohexane to obtain a single-layer porous film composed of polypropylene and polyethylene. This film has a SEP due to the change in weight before and after extraction.
The weight loss corresponding to the S content was shown. The properties of the obtained film are shown in Table 1.

The porous film was immersed in propylene carbonate, which is a poor solvent for polyolefin, and heated at 100 ° C. The cross section of the film was observed with a scanning electron microscope. However, it was found that the micropores were blocked by heat.

Table 2 shows the results of the ion permeability test of the obtained membrane. From the results in Table 2, 115 ° C
It was found that the amount of ions permeated was significantly suppressed in the heating test at.

Table 1: Properties of each porous membrane ───────────────────────────────────── Example 1 2 3 4 5 6 7 ──────────────────────────────────── PE amount (%) 50 40 30 20 30 20 20 Porosity (%) 33.2 38.9 40.8 45.2 35.6 50 50 Pore size (μm) 1 0.8 0.8 0.8 0.9 1.2 1.2 Thickness (μm) 120 120 120 120 120 100 100 ───────── ─────────────────────────── Table 2: Temporal change in conductivity before and after heating ──────────── ──────────────────────── Conductivity (μS / cm) ─────────────────── ──────────── Time (hr) 0 1 2 3 ────────────────────────────── ────── Example 1 Before heating 1 .24 303 619 894 After heating at 115 ° C 1.24 6.55 5.14 5.47 ──────────────────────────────────── Example 2 Before heating 1.42 177 368 543 105 ° C After heating 0.77 3.82 7.8 11.05 ─────────────────────────────── Before heating 1.42 145 338 514 115 After heating 0.77 2.53 2.93 3.76 ─────────────────────────────── Before heating 1.42 276 593 903 130 ° C After heating 0.77 5.72 4.85 4.87 ──────────────────────────────────── Example 3 Before heating 1.42 211 407 590 After heating at 105 ° C 0.77 59.8 124 195 ─────────────────────────────── Before heating 1.42 159 351 504 After heating at 115 ° C 0.77 11.3 29.6 44.9 ─────────────────────────────── Before heating 0.77 127 276 415 After heating at 130 ° C 0.77 9.74 7.58 11.35 ──────────────────────────────────── Example 4 Before heating 1.51 290 630 888 After heating at 120 ° C 1.3 222 496 689 ─────────────────────────────── Before heating 1.51 306 760 1112 After heating at 140 ° C 1.9 165 382 610 ──────────────────────────────────── Example 5 Before heating 1.25 229 430 705 105 After heating at ℃ 1.62 12 21.7 31.7 ─────────────────────────────── Before heating 1.25 189 347 547 115 After heating at 1.25 14.2 20.2 28.5 ─────────────────────────────── Before heating 1.62 205 394 580 130 After heating at 1.39 54.8 125 202 ──────────────────────────────────── Example 6 Before heating 1.30 386 801 1179 115 ℃ After heating 1.30 5.44 8.09 12.4 ───────────────────────────────────── Example 7 Before heating 1.15 377 755 1168 After heating at 115 ° C. 1.17 11.2 14.5 18.5 ────────────────────────────────── --- Heating time = 1 minute for Examples 1, 2, 3, 5, 6, 7 and 10 minutes for Example 4.

As described above, the single-layer porous polymer membrane according to the present invention has the property that when it is heated to a predetermined temperature, the internal micropores are closed and the permeability is lost. Therefore, it is useful as a separator having safety against abnormal heating for lithium secondary batteries, for example. Further, even when compared with a film having a multilayer structure conventionally used for this purpose, it solves various problems included in these conventional ones. The single-layer porous polymer membrane according to the present invention can be used as a permeable membrane used in various applications where it is desired to lose its permeability at a certain temperature or higher, in addition to the use as a battery separator. Conceivable and its industrial value is extremely high.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing a cross-sectional structure of a single-layer porous polymer film according to the present invention. FIG. 1-a shows a normal state before heating to a predetermined temperature, and FIG. 1-b shows a state where the fine holes are closed after heating to a predetermined temperature.

FIG. 2 is a diagram schematically showing a cross-sectional structure of a conventional multilayered porous film having a fusible layer.

FIG. 3 Styrene / hydrogenated butadiene / styrene block copolymer (SEBS) and styrene / hydrogenated isoprene /
It is a figure which shows the structure of a styrene block copolymer (SEPS).

FIG. 4 is a schematic diagram of an apparatus for measuring ion permeation and determining whether the porous membrane is occluded.

5 is an electron micrograph at a magnification of 5000 times showing the fiber shape of the single-layer porous membrane of the present invention obtained in Example 1. FIG. FIG. 5-a is an electron micrograph showing the shape of the fiber before heating the porous membrane to 115 ° C., and FIG. 5-b is showing the shape of the fiber after heating the porous membrane to 115 ° C.

FIG. 6 is an electron micrograph at a magnification of 5000 times showing the fiber shape of the single-layer porous membrane of the present invention obtained in Example 2. FIG. 6-a is an electron micrograph showing the shape of the fiber before heating the porous membrane to 115 ° C., and FIG. 6-b is the shape of the fiber after heating the porous membrane to 115 ° C.

FIG. 7: Magnification 2 showing the shape of the fibers of the single-layer porous membrane of the present invention obtained in Example 6 before heating to 100 ° C.
It is a 000 times electron micrograph.

FIG. 8: Magnification 2 showing the shape of the fibers of the single-layer porous membrane of the present invention obtained in Example 6 after heating to 100 ° C.
It is a 000 times electron micrograph.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Yuichi Mori 1642-212 B-4, Kamariya-cho, Kanazawa-ku, Yokohama, Kanagawa

Claims (2)

[Claims] 1. A porous polymer layer composed of a first polymer having a predetermined melting temperature and having fine pores therein, and a first polymer existing inside the fine pores of the porous polymer layer. Is composed of a second polymer having a predetermined melting temperature that is lower than the melting temperature of the polymer, and the relative amount of the second polymer to the first polymer is
Before being heated above a certain temperature, the second polymer exists in a state where it does not block the fine pores, but when heated above a certain temperature, the second polymer melts and the inside of the fine pores is melted. A porous membrane having a single-layer structure, which is characterized in that the amount is such that it occludes the membrane and eliminates its permeability. 2. A first polymer having a predetermined melting temperature, a second polymer having a predetermined melting temperature lower than the melting temperature of the first polymer, and incompatible with the first and second polymers. An IPN (interpenetrating polymer networ) having a continuous layer in which polymer molecules are intertwined with each other by melt-kneading a third polymer to form a film.
k) A porous polymer having a single layer structure according to claim 1, wherein a polymer film having a structure is produced, and then only the third polymer is extracted with a solvent that dissolves only the third polymer. Membrane manufacturing method.