HEAT-RESISTANT SYNTHETIC RESIN MICROPOROUS FILM, SEPARATOR FOR NON-AQUEOUS LIQUID ELECTROLYTE SECONDARY BATTERY, NON-AQUEOUS LIQUID ELECTROLYTE SECONDARY BATTERY, AND METHOD FOR PRODUCING HEAT-RESISTANT SYNTHETIC RESIN MICROPOROUS FILM

Abstract

Provided are a heat-resistant synthetic resin microporous film having enhanced heat resistance while having reduced deterioration of mechanical strength, and a method for producing the same. Disclosed is a heat-resistant synthetic resin microporous film which includes a synthetic resin microporous film containing a synthetic resin; and a coating layer formed on at least a portion of the surface of the synthetic resin microporous film and containing a polymer of a polymerizable compound having a bifunctional or higher-functional radical polymerizable functional group, the heat-resistant synthetic resin microporous film having a surface aperture ratio of 30% to 55%, gas permeability of 50 sec/100 mL to 600 sec/100 mL, a maximum thermal shrinkage obtainable when the film is heated from 25 C. to 180 C. at a rate of temperature increase of 5 C./min, of 20% or less, and a piercing strength of 0.7 N or more.

Claims

1. A heat-resistant synthetic resin microporous film comprising: a synthetic resin microporous film containing a synthetic resin; and a coating layer formed on at least a portion of the surface of the synthetic resin microporous film and containing a polymer of a polymerizable compound having a bifunctional or higher-functional radical polymerizable functional group, the heat-resistant synthetic resin microporous film having a surface aperture ratio of 30% to 55%; gas permeability of 50 sec/100 mL to 600 sec/100 mL; a maximum thermal shrinkage obtainable when the film is heated from 25 C. to 180 C. at a rate of temperature increase of 5 C./min, of 20% or less; and a piercing strength of 0.7 N or more.

2. The heat-resistant synthetic resin microporous film according to claim 1, wherein the piercing strength is 1.0 N or more.

3. The heat-resistant synthetic resin microporous film according to claim 1, wherein the polymerizable compound is at least one selected from the group consisting of a polyfunctional (meth)acrylate modification product, a dendritic polymer having bifunctional or higher-functional (meth)acryloyl groups, and a urethane (meth)acrylate oligomer having a bifunctional or higher-functional (meth)acryloyl group.

4. A heat-resistant synthetic resin microporous film comprising: a synthetic resin microporous film containing a synthetic resin; and a coating layer formed on at least a portion of the surface of the synthetic resin microporous film and containing a polymer of a polymerizable compound having a bifunctional or higher-functional radical polymerizable functional group, the polymerizable compound being at least one selected from the group consisting of a polyfunctional (meth)acrylate modification product, a dendritic polymer having bifunctional or higher-functional (meth)acryloyl groups, and a urethane (meth)acrylate oligomer having a bifunctional or higher-functional (meth)acryloyl group, the heat-resistant synthetic resin microporous film having a surface aperture ratio of 30% to 55%; gas permeability of 50 sec/100 mL to 600 sec/100 mL; and a maximum thermal shrinkage obtainable when the film is heated from 25 C. to 180 C. at a rate of temperature increase of 5 C./min, of 20% or less.

5. The heat-resistant synthetic resin microporous film according to claim 1, wherein the coating layer contains a polymer obtained by polymerizing the polymerizable compound having a bifunctional or higher-functional radical polymerizable functional group by irradiation of active energy radiation.

6. The heat-resistant synthetic resin microporous film according to claim 1, wherein the gel fraction is 5% by weight or more.

7. The heat-resistant synthetic resin microporous film according to claim 1, wherein the synthetic resin includes a propylene-based resin.

8. A separator for a non-aqueous liquid electrolyte secondary battery, comprising the heat-resistant synthetic resin microporous film according to claim 1.

9. A non-aqueous liquid electrolyte secondary battery comprising the separator for a non-aqueous liquid electrolyte secondary battery according to claim 8.

10. A method for producing a heat-resistant synthetic resin microporous film, the method comprising coating at least a portion of the surface of a synthetic resin microporous film containing a synthetic resin with a polymerizable compound having a bifunctional or higher-functional radical polymerizable functional group, and then irradiating the synthetic resin microporous film with active energy radiation.

11. The method for producing a heat-resistant synthetic resin microporous film according to claim 10, wherein the polymerizable compound is at least one selected from the group consisting of a polyfunctional (meth)acrylate modification product, a dendritic polymer having a bifunctional or higher-functional (meth)acryloyl group, and a urethane (meth)acrylate oligomer having a bifunctional or higher-functional (meth)acryloyl group.

Description

DESCRIPTION OF EMBODIMENTS

[0202] Hereinafter, the invention is explained more specifically using Examples; however, the invention is not intended to be limited to these Examples.

EXAMPLES

Example 1

[0203] 1. Production of Homopolypropylene Microporous Film

[0204] (Extrusion Step)

[0205] A homopolypropylene (weight average molecular weight 413,000, molecular weight distribution 9.3, melting point 163 C., heat of fusion 96 mJ/mg) was supplied to an extruder and was melt kneaded at a resin temperature of 200 C. The homopolypropylene was extruded into a film form through a T-die installed at the tip of the extruder, and was cooled until the surface temperature reached 30 C. Thus, a homopolypropylene film (thickness 30 m) was obtained. Meanwhile, the amount of extrusion was 9 kg/hour, the film forming speed was 22 m/min, and the draw ratio was 83.

[0206] (Aging Step)

[0207] The homopolypropylene film thus obtained was aged by leaving the film to stand for 24 hours in an air heating furnace at an ambient temperature of 150 C.

[0208] (First Stretching Step)

[0209] The aged homopolypropylene film was uniaxially stretched in the extrusion direction only using a uniaxial stretching apparatus, at a stretch ratio of 1.2 times at a stretching speed of 50%/min under the condition of a surface temperature of 23 C.

[0210] (Second Stretching Step)

[0211] Subsequently, the homopolypropylene film was uniaxially stretched in the extrusion direction only using a uniaxial stretching apparatus, at a stretch ratio of 2 times at a stretching speed of 42%/min under the condition of a surface temperature of 120 C.

[0212] (Annealing Step)

[0213] Thereafter, the homopolypropylene film was heated over 10 minutes such that the surface temperature reached 130 C., and no tension was applied to the homopolypropylene film. The homopolypropylene film was subjected to annealing, and thus a homopolypropylene microporous film (thickness 25 m) was obtained. Meanwhile, the shrinkage of the homopolypropylene film at the time of annealing was adjusted to 20%.

[0214] The homopolypropylene microporous film thus obtained had gas permeability of 110 sec/100 mL, a surface aperture ratio of 40%, a maximum major axis of the opening end of a micropore of 600 nm, an average major axis of the opening ends of the micropores of 360 nm, and a pore density of 30 pores/m.sup.2.

[0215] 2. Formation of Coating Layer

[0216] (Coating Step)

[0217] A coating liquid containing 90% by weight of ethyl acetate as a solvent and 10% by weight of an ethylene oxide modification product of trimethylolpropane tri(meth)acrylate (number of radical polymerizable functional groups in one molecule: 3, average number of added moles of ethylene oxide: 3.5 moles, trade name: VISCOAT #360 manufactured by Osaka Organic Chemical Industry, Ltd.) as a polymerizable compound, was prepared. Subsequently, the homopolypropylene microporous film surface was coated with the coating liquid, and then the homopolypropylene microporous film was heated for 2 minutes at 80 C. to remove the solvent. Thereby, the polymerizable compound was attached over the entire surface of the homopolypropylene microporous film.

[0218] (Irradiation Step)

[0219] Next, the homopolypropylene microporous film was irradiated with an electron beam at an accelerating voltage of 200 kV and an amount of irradiation of 35 kGy in a nitrogen atmosphere, and thus the polymerizable compound was polymerized. Thereby, a heat-resistant homopolypropylene microporous film in which a coating layer containing a polymer of a radical polymerizable monomer is formed on the surface of the homopolypropylene microporous film and on the wall surface of the opening ends of micropores extending to the film surface, was obtained.

Example 2

[0220] A heat-resistant homopolypropylene microporous film was produced in the same manner as in Example 1, except that a coating liquid containing 90% by weight of ethyl acetate as a solvent and 10% by weight of a dendritic polymer having bifunctional or higher-functional (meth)acryloyl groups (weight average molecular weight: 2,000, trade name: VISCOAT #1000 manufactured by Osaka Organic Chemical Industry, Ltd.) as a polymerizable compound, was used.

Example 3

[0221] A heat-resistant homopolypropylene microporous film was produced in the same manner as in Example 1, except that a coating liquid containing 90% by weight of ethyl acetate as a solvent and 10% by weight of a dendritic polymer having bifunctional or higher-functional (meth)acryloyl groups (weight average molecular weight: 20,000, trade name: SUBARU-501 manufactured by Osaka Organic Chemical Industry, Ltd.) as a polymerizable compound, was used.

Example 4

[0222] A heat-resistant homopolypropylene microporous film was produced in the same manner as in Example 1, except that a coating liquid containing 90% by weight of ethyl acetate as a solvent and 10% by weight of an ethylene oxide modification product of pentaerythritol tetraacrylate (number of radical polymerizable functional groups in one molecule: 4, average number of added moles of ethylene oxide: 4 moles, manufactured by Miwon Specialty Chemical Co., Ltd., trade name: MIRAMER M4004) as a polymerizable compound, was used.

Example 5

[0223] A heat-resistant homopolypropylene microporous film was produced in the same manner as in Example 1, except that a coating liquid containing 90% by weight of ethyl acetate as a solvent and 10% by weight of an ethylene oxide modification product of trimethylolpropane triacrylate (number of radical polymerizable functional groups in one molecule: 3, average number of added moles of ethylene oxide: 6 moles, manufactured by Miwon Specialty Chemical Co., Ltd., trade name: MIRAMER M3160) as a polymerizable compound, was used.

Example 6

[0224] A heat-resistant homopolypropylene microporous film was produced in the same manner as in Example 1, except that a coating liquid containing 90% by weight of ethyl acetate as a solvent and 10% by weight of an ethylene oxide modification product of trimethylolpropane triacrylate (number of radical polymerizable functional groups in one molecule: 3, average number of added moles of ethylene oxide: 9 moles, manufactured by Miwon Specialty Chemical Co., Ltd., trade name: MIRAMER M3190,) as a polymerizable compound, was used.

Example 7

[0225] A heat-resistant homopolypropylene microporous film was produced in the same manner as in Example 1, except that a coating liquid containing 90% by weight of ethyl acetate as a solvent and 10% by weight of a propylene oxide modification product of trimethylolpropane triacrylate (number of radical polymerizable functional groups in one molecule: 3, average number of added moles of propylene oxide: 3 moles, trade name: SR492 manufactured by Sartomer Company, Inc.) as a polymerizable compound, was used.

Example 8

[0226] A heat-resistant homopolypropylene microporous film was produced in the same manner as in Example 1, except that a coating liquid containing 90% by weight of ethyl acetate as a solvent and 10% by weight of a propylene oxide modification product of trimethylolpropane triacrylate (number of radical polymerizable functional groups in one molecule: 3, average number of added moles of propylene oxide: 6 moles, trade name: SR501 manufactured by Sartomer Company, Inc.) as a polymerizable compound, was used.

Example 9

[0227] A heat-resistant homopolypropylene microporous film was produced in the same manner as in Example 1, except that a coating liquid containing 90% by weight of ethyl acetate as a solvent and 10% by weight of an ethylene oxide modification product of glyceryl triacrylate (number of radical polymerizable functional groups in one molecule: 3, average number of added moles of ethylene oxide: 3 moles, trade name: A-GYL-3E manufactured by Shin Nakamura Chemical Co., Ltd.) as a polymerizable compound, was used.

Example 10

[0228] A heat-resistant homopolypropylene microporous film was produced in the same manner as in Example 1, except that a coating liquid containing 90% by weight of ethyl acetate as a solvent and 10% by weight of an isopropylene oxide modification product of dipentaerythritol hexaacrylate (number of radical polymerizable functional groups in one molecule: 6, average number of added moles of isopropylene oxide: 6 moles, trade name: A-DPH-6P manufactured by Shin Nakamura Chemical Co., Ltd.) as a polymerizable compound, was used.

Comparative Example 1

[0229] A heat-resistant homopolypropylene microporous film was produced in the same manner as in Example 1, except that a coating liquid containing 90% by weight of ethyl acetate as a solvent, and 3.8% by weight of pentaerythritol tetrakis(3-mercaptobutyrate) [KARENZ MT (registered trademark) PE-1] and 6.2% by weight of triallyl isocyanurate (TAIC) as polymerizable compounds, was used.

[0230] [Evaluation]

[0231] For the heat-resistant homopolypropylene microporous films produced in Examples and Comparative Examples, the surface aperture ratio, the gas permeability, the maximum thermal shrinkage obtained when a film was heated from 25 C. to 180 C. at a rate of temperature increase of 5 C./min, the piercing strength, and the gel fraction were measured by the methods described above, and the results are presented in Table 1. The content of the coating layer in a heat-resistant homopolypropylene microporous film with respect to 100 parts by weight of the homopolypropylene microporous film is presented in Table 1.

TABLE-US-00001 TABLE 1 Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Comparative ple 1 ple 2 ple 3 ple 4 ple 5 ple 6 ple 7 ple 8 ple 9 ple10 Example 1 Blend of coating Ethyl acetate 90 90 90 90 90 90 90 90 90 90 90 liquid (wt %) VISCOAT #360 10 0 0 0 0 0 0 0 0 0 0 VISCOAT #1000 0 10 0 0 0 0 0 0 0 0 0 SUBARU-501 0 0 10 0 0 0 0 0 0 0 0 Miramer M4004 0 0 0 10 0 0 0 0 0 0 0 Miramer M3160 0 0 0 0 10 0 0 0 0 0 0 Miramer M3190 0 0 0 0 0 10 0 0 0 0 0 SR492 0 0 0 0 0 0 10 0 0 0 0 SR501 0 0 0 0 0 0 0 10 0 0 0 A-GYL-3E 0 0 0 0 0 0 0 0 10 0 0 A-DPH-6P 0 0 0 0 0 0 0 0 0 10 0 KARENZ MT 0 0 0 0 0 0 0 0 0 0 3.8 PE-1 TAIC 0 0 0 0 0 0 0 0 0 0 6.2 Heat-resistant Content of 35 36 34 35 34 33 34 35 36 35 34 homopolypropylene coating layer microporous film [parts by weight] Surface 38 39 38 37 38 39 38 39 38 38 39 aperture ratio [%] Gas 120 125 120 120 115 110 120 125 125 120 115 permeability [sec/100 mL] Maximum 15 17 17 14 19 20 18 20 15 13 36 thermal shrinkage [%] Piercing 1.0 1.1 1.0 1.1 1.0 1.2 1.0 1.1 0.9 0.7 1.4 strength [N] Gel fraction 30 32 31 30 30 32 30 31 31 33 17 [wt %]

CROSS-REFERENCE TO RELATED APPLICATIONS

[0232] This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2014-55478, filed on Mar. 18, 2014, the entire contents of which are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

[0233] The heat-resistant synthetic resin microporous film of the invention has enhanced heat resistance while having reduced deterioration of mechanical strength, and thus the heat-resistant synthetic resin microporous film can be suitably used as a separator for a non-aqueous liquid electrolyte secondary battery.