JP2002265657A - Polyolefin porous sheet and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a homogeneous polyolefin porous sheet having a high strength and a high porosity and suitable for uses such as a separator for cells or batteries. SOLUTION: This polyolefin porous sheet is obtained by heating and sintering a polyolefin powder having a median diameter within the range of 30-200 μm, a geometrical standard deviation of the particle diameter within the range of 0.68-0.95 and an average particle circularity within the range of 0.86-1 at a temperature of the melting point thereof or above. The resultant polyolefin porous sheet has a porosity within the range of 25-60 vol.%, a bubble point pore diameter within the range of 20-70 μm and >=1500 N/cm<2> tensile breaking strength.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a porous sheet and a method for producing the same, and more particularly, to a porous sheet which can be preferably used as a separator constituting an electric vehicle driving (running) battery, and a method for producing the same. Things.

[0002]

2. Description of the Related Art In recent years, alkaline secondary batteries represented by nickel-cadmium (nickel) batteries and nickel-metal hydride batteries have been used not only as small batteries for electric and electronic equipment but also for driving (running) electric vehicles. It is also expected to be used as a power supply.

[0003] A separator used in such an alkaline secondary battery is required to have good air permeability and ion permeability. For example, in a nickel-cadmium battery and a nickel-metal hydride battery, it is necessary to consume oxygen gas or hydrogen gas generated at an electrode at a counter electrode to prevent the internal pressure of the battery from increasing. Therefore, a separator that can sufficiently transmit oxygen gas and hydrogen gas, that is, a separator having good air permeability is required. In addition, the ion permeability is closely related to the output characteristics of the battery. In particular, batteries that require high output, such as electric vehicle driving (running) batteries, require a separator having good ion permeability. Become.
The porosity of the separator greatly affects the air permeability and the ion permeability. The larger the value, the better the characteristics.

As a separator for an alkaline secondary battery,
A hydrophilic non-woven fabric such as nylon, a polyolefin non-woven fabric subjected to a hydrophilizing treatment, and the like are used (for example, JP-A-4-167355). It has also been proposed that ultrahigh molecular weight polyolefin particles such as ultrahigh molecular weight polyethylene are three-dimensionally connected by fusion, and a porous sheet having a porous structure formed by voids between the particles is used as a separator. (For example, see Japanese Patent Application Laid-Open No. 8-77997).
No.). This porous sheet, compared to the nonwoven fabric,
It has excellent mechanical strength and has the advantage that it is not easily crushed even when electrode expansion of the battery occurs, and it is possible to impart good cycle characteristics to the battery.

[0005]

The above-mentioned porous sheet comprises:
Usually, ultra-high molecular weight polyolefin particles are filled in a mold, sintered to form a block-shaped sintered body, and the sintered body is cut into a sheet having a desired thickness. . However, it is difficult to obtain a sufficient packing density when the ultrahigh molecular weight polyolefin particles are filled in a mold, and thus there is a problem that large voids remain inside. The porous sheet obtained by sintering such a filler has excessively large pores and cracks and lacks homogeneity.

Further, as a method for improving the porosity and strength, it is conceivable that the porous sheet is subjected to a stretching treatment. However, when the porous sheet is stretched, if there is an excessively large hole in the sheet, the sheet may be damaged starting from the hole, and the sheet is stretched to a magnification that can achieve a desired porosity and strength. There was a problem that it was difficult to do.

[0007] In a battery using such a porous sheet as a separator, the electrode particles break through the separator when wound with the electrode, and the electrodes are likely to contact with each other to cause an internal short circuit. Further, when charge and discharge are repeated by repeated use of the battery, there is a problem that an internal short circuit easily occurs due to expansion and contraction of the electrode due to charge and discharge.

[0008] An object of the present invention is to provide a porous sheet capable of increasing porosity and strength by stretching and a method for producing the same. It is another object of the present invention to provide a battery separator and a battery in which a short circuit is less likely to occur inside the electrode, and the performance of the battery is less likely to deteriorate due to repeated use.

[0009]

In order to achieve the above object, the first polyolefin porous sheet of the present invention has a porosity in the range of 25 to 60% by volume and a bubble point pore diameter of 20 to 70 μm. And a tensile fracture strength of 1500 N / cm ² or more.

[0010] Such a polyolefin porous sheet has an appropriate porosity and strength, and does not have excessively large pores. Therefore, it is possible to increase porosity and strength by stretching while suppressing breakage of the sheet.

The porosity p [volume%] is determined based on the single-sided area S [cm ² ], thickness d [cm] and mass m of the porous sheet.
[G] and the density r [g / cm ³ ] of the sheet-forming material can be determined by the following equation (1).

P = [1-{(m / r) / (S × d)}] × 100 (1)

The bubble point pore diameter is measured, for example, as a measuring instrument by PEROUS MATERIALS, manufactured by PE.
RM-POROMETER was used, and a fluorinated solvent (trade name “FC-4, manufactured by 3M”) was used as the impregnating solvent.
0 ", surface tension of 1.6 × 10 ^-2 N / m) and AS
It can be measured based on TM E128-94.

The tensile breaking strength is, for example, 10 mm in width.
Can be measured based on JIS-K7127 using the No. 1 test side under the conditions of a distance between marked lines of 100 mm and a tensile speed of 50 mm / min.

As described above, the porosity of the first polyolefin porous sheet is in the range of 25 to 70% by volume. If it is less than 25% by volume, it is difficult to obtain a sufficient porosity after stretching. If it exceeds 70% by volume,
The mechanical strength may be reduced by stretching. The range of the porosity is preferably 30 to 50% by volume.

In the first polyolefin porous sheet, the bubble point pore diameter is in the range of 20 to 70 μm. If it is less than 20 μm, the porosity is unlikely to increase even when stretched. On the other hand, if it exceeds 70 μm, the sheet may be damaged at the time of stretching, or a hole having an excessively large hole diameter may be generated. The range of the bubble point pore diameter is preferably 30 to 50 μm.

In the first polyolefin porous sheet, the tensile fracture strength is 1500 N / cm ² or more. If it is less than 1500 N / cm ² , the sheet may be damaged at the time of stretching, or a hole having an excessively large hole diameter may be generated. The upper limit of the tensile breaking strength is not particularly limited, but is, for example, 5000 N / cm ² or less.

The first polyolefin porous sheet is preferably a sheet in which a plurality of polyolefin particles are interconnected and a porous structure is formed by voids between the particles. In particular, the median diameter is in the range of 30 to 200 μm, and the geometric standard deviation of the particle diameter is 0.68 to 0.9.
5 and the average particle circularity is 0.86-1.
It is preferable that the polyolefin particles in the range of (1) are connected to each other by fusion, and a porous structure is formed by voids between the particles. The definition and measurement method of the median diameter, the geometric standard deviation of the particle diameter and the average particle circularity will be described later.

In the first porous sheet, the polyolefin is preferably an ultra-high molecular weight polyolefin. The viscosity average molecular weight of the ultrahigh molecular weight polyolefin is, for example, 500,000 to 16,000,000, preferably 100
It is 10,000 to 10,000,000, more preferably 2,000,000 to 5,000,000. Further, the polyolefin is preferably polyethylene or polypropylene, particularly preferably high-density polyethylene or high-density polypropylene.

Although the first polyolefin porous sheet can be used as it is for a battery separator or the like, it is possible to obtain a porous sheet having higher strength and higher porosity by performing a stretching treatment. Can be.

The second polyolefin porous sheet of the present invention is obtained by stretching the first polyolefin porous sheet. Such a polyolefin porous sheet has characteristics of high strength and high porosity. This second polyolefin porous sheet is
It is suitable for use as a battery separator, and is preferably used as a separator for an alkaline secondary battery.
Among them, it is particularly preferable to use it as a separator for an alkaline secondary battery for an electric vehicle drive (running) power supply.

The second porous sheet preferably has a bubble point pore diameter in a range of 40 to 100 μm. When the thickness is 40 μm or more, sufficient air permeability and ion permeability are easily obtained, and when the thickness is 100 μm or less, sufficient mechanical strength is easily obtained. The range of the bubble point pore diameter in the second porous sheet is more preferably 50 to 80 μm. The method for measuring the bubble point pore diameter is as described above.

Further, in the second porous sheet,
The porosity is, for example, 40 to 70% by volume, preferably 45 to 70% by volume.
It is in the range of 65% by volume. Further, the tensile fracture strength is, for example, 2,000 to 10,000 N / cm ² or more. In addition,
The methods for measuring the porosity and the tensile strength are as described above.

Next, according to the method for producing a polyolefin porous sheet of the present invention, the median diameter is in the range of 30 to 200 μm, the geometric standard deviation of the particle diameter is in the range of 0.68 to 0.95, and And a step of heating and sintering a polyolefin powder having an average particle circularity in the range of 0.86 to 1 at a temperature not lower than its melting point. According to such a manufacturing method, the first polyolefin porous sheet having the above-described physical properties can be manufactured.

The median diameter and the geometric standard deviation of the particle diameter are values calculated from the result of measuring the particle diameter distribution of the polyolefin powder. In the particle size distribution,
When the number of particles having a particle diameter equal to or less than a specific value is X% of the total number of particles, and the specific value is expressed as _DpX , the median diameter is a value equal to _Dp50 . The geometric standard deviation σ _g of the particle diameter is a value calculated by the following equation (2).

Σ _g = D _p15.9 / D _p50 (2)

Here, the particle diameter is a sphere equivalent diameter which is the diameter of a sphere having the same volume as the particle.

The particle size distribution can be measured by a light scattering method. Specifically, for example, it can be measured by dispersing a polyolefin powder in a surfactant aqueous solution and using LA-910 manufactured by HORIBA, Ltd. as a measuring instrument.

The average particle circularity is an average value of the particle circularity. The particle circularity is determined by the following equation. Particle circularity = (circumference of a circle having the same area as the projected area of the particle) /
(Projected contour length of particles) The particle circularity is, for example, manufactured by Toa Medical Electronics,
Flow type particle image analyzer FPIA-1000 (product name)
Can be measured.

As described above, in the manufacturing method,
The median diameter is in the range of 30-200 μm. Outside of this range, excessively large pores and cracks are likely to occur in the obtained sintered body, making it difficult to obtain a homogeneous polyolefin porous sheet. The range of the median diameter is preferably 40 to 150 μm, more preferably 50 to 150 μm.
１００100 μm.

In the above production method, the geometric standard deviation of the particle diameter is in the range of 0.68 to 0.95. When using particles having a wide particle size distribution such as deviating from this range,
Excessively large pores and cracks are likely to occur in the obtained sintered body, and it becomes difficult to obtain a homogeneous polyolefin porous sheet. The geometric standard deviation of the particle diameter is preferably from 0.7 to 0.9.

In the above-mentioned production method, the average particle circularity is in a range of 0.86-1. Outside of this range, excessively large pores and cracks are likely to occur in the obtained sintered body, making it difficult to obtain a homogeneous polyolefin porous sheet. The average particle circularity is preferably from 0.9 to 1, more preferably from 0.92 to 0.98.
It is.

It is preferable that the production method further includes a step of stretching the obtained polyolefin porous sheet. According to this preferred example, the second polyolefin porous sheet can be manufactured.

It is preferable that the production method further includes a step of hydrophilizing the obtained polyolefin porous sheet.

Next, the battery separator of the present invention uses the first polyolefin porous sheet or the second polyolefin porous sheet. The battery separator of the present invention is preferably an alkaline secondary battery separator, and more preferably an electric vehicle driving (running) power source alkaline secondary battery separator.

The battery of the present invention is a battery in which a separator is interposed between the positive and negative electrodes, and uses the polyolefin porous sheet of the present invention for the separator.
The battery of the present invention is preferably an alkaline secondary battery,
More preferred is an alkaline secondary battery for an electric vehicle drive (running) power supply.

[0037]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an example of a method for producing a polyolefin porous sheet according to the present invention will be described.

The first polyolefin porous sheet can be manufactured by sintering a polyolefin powder. The polyolefin powder has a specific median diameter, geometric standard deviation, and average particle circularity as described above. Such a polyolefin powder is commercially available from, for example, Chikona, Mitsui Chemicals, and the like. Further, the median diameter can be controlled by a classification operation of the particles.

The method for producing the first polyolefin porous sheet is not particularly limited, but the polyolefin powder is sintered, and then the sintered body is formed into a sheet, or the sintering is performed. At the same time, it can be manufactured by a method of forming a sheet. For example, a method in which polyolefin powder is filled into a shape retainer and sintered, and the sintered body is cut into a sheet, a method in which the polyolefin powder is uniformly dispersed in a solvent, and this is applied to a heat-resistant sheet and sintered. Alternatively, there is a method of using an extruder such as a screw extruder to simultaneously sinter the polyolefin powder in the die and simultaneously form the polyolefin powder into a sheet. Among them, the method using the shape retainer is preferable because the adjustment of the sheet thickness is easy. This method can be performed, for example, as follows.

First, a polyolefin powder is filled in a shape retainer. This filling can be performed, for example, by charging the polyolefin powder from a hopper through a damper into the shape retainer. At this time, it is preferable to introduce the particles with stirring so as not to cause segregation of the particles in the hopper and to improve the fluidity of the particles so that the particles can be smoothly introduced. Next, in order to obtain a porous sheet having high strength and excellent homogeneity, it is preferable to increase the density of the polyolefin powder filled inside the shape retainer. Densification can be performed, for example, by hitting, applying vibration, or applying pressure to the upper surface of the shape retainer.

Subsequently, the polyolefin powder filled in the shape retainer is sintered to form a porous body. As a sintering method, for example, a method using a hot air dryer, a method using an electric furnace, a method using a microwave heating furnace, a method using a heated steam, and the like can be used.
Among these methods, the method of sintering with heated steam is preferred because it has excellent heat conduction efficiency and can be homogeneously sintered in a short time. This method can be carried out, for example, by placing the shape retainer in a heat-resistant pressure-resistant container, exhausting the air inside the heat-resistant pressure-resistant container, and then introducing steam heated to the melting point of the powder or higher.

The sintering temperature is not particularly limited as long as it is higher than the melting point of the polyolefin used. When the polyolefin is ultrahigh molecular weight polyethylene, for example, 140 to 200 ° C., preferably 150 to 200 ° C.
180 ° C.

The porous body obtained by sintering is cut by a cutting lathe or the like to obtain the first polyolefin porous sheet. In this first polyolefin porous sheet, a plurality of polyolefin particles are interconnected, and a porous structure is formed by voids between the particles.

Further, the second polyolefin porous sheet is obtained by stretching the first polyolefin porous sheet.

The stretching method is not particularly limited, and may be uniaxial stretching or biaxial stretching. For example, adopting a method such as uniaxial stretching using a roll, sequential biaxial stretching using a combination of a roll and a tenter, sequential biaxial stretching or simultaneous biaxial stretching using a tenter, rolling using a roll or a belt press, or the like. Can be. Among these, sequential biaxial stretching and simultaneous biaxial stretching using a tenter are preferred because a porous sheet excellent in homogeneity can be obtained.

The stretching temperature is not particularly limited, but is preferably equal to or lower than the melting point of the polyolefin used. When the polyolefin is ultrahigh molecular weight polyethylene, for example, 70 to 180 ° C., preferably 100 to 180 ° C.
To 140 ° C, more preferably 110 to 130 ° C.
Also, the stretching ratio is not particularly limited, but at least in the uniaxial direction, for example, 1.1 to 5
Times, preferably 1.2-4 times, more preferably 1.3-times.
2.5 times. However, when the stretching ratio is L0 in the stretching direction of the porous sheet before stretching and L1 in the stretching direction of the porous sheet after stretching,
It is a value represented by L1 / L0.

Since the first and second polyolefin porous sheets are hydrophobic, they are preferably subjected to a hydrophilizing treatment when used for a separator for an alkaline battery or the like.
The method of the hydrophilization treatment is not particularly limited, and for example, a method such as an impregnation treatment with a surfactant or a water-insoluble polymer, a graft polymerization treatment of a hydrophilic monomer, a corona discharge treatment, a sulfonation treatment, and a plasma treatment can be used. .

In addition, the battery of the present invention can be used as a battery for driving (running) electric tools or electric vehicles requiring high output by using the polyolefin porous sheet of the present invention as a separator interposed between the positive and negative electrodes. Suitable. Other configurations of the battery of the present invention are not particularly limited, and the battery can be manufactured by an ordinary method.

[0049]

Embodiments of the present invention will be described below. In addition, about each physical-property value, it measured and calculated by each said method. The sheet thickness was measured using a 1/1000 mm dial gauge.

Example 1 First, an upper opening type cylindrical shape retainer for filling polyolefin powder was prepared. This shape retainer uses a 100 μm thick stainless steel plate,
A cylinder having an inner diameter of 15 cm and a height of 30 cm was prepared, and this cylinder was prepared by placing the cylinder on a polytetrafluoroethylene plate having a thickness of 5 mm. Next, ultra-high molecular weight polyethylene powder A (trade name “GUR4186” manufactured by Ticona Co., Ltd.)
Viscosity average molecular weight 3 million, median diameter 70 μm, geometric standard deviation of particle diameter 0.71, average particle circularity 0.94, melting point 1
(40 ° C.) was filled in the shape-retaining tool, and the shape-retaining tool was placed in a metal heat-resistant and pressure-resistant container (including a steam introducing pipe and an opening / closing valve thereof). Further, a load was applied to the upper surface of the shape retainer, and while the pressure was kept constant (7.8 kPa), the inside of the heat-resistant and pressure-resistant container was evacuated with a vacuum pump to adjust the atmospheric pressure to 3.3 kPa. After stopping the vacuum pump, the valve was opened and steam (temperature 180 ° C., pressure 10 kP
Heat sintering was performed for 2 hours while introducing a). During this time, the internal temperature of the heat and pressure resistant container was around 167 ° C. Thereafter, the mixture was allowed to cool to obtain a cylindrical ultra-high molecular weight polyethylene porous body, and this porous body was cut by a cutting lathe to obtain a 200 μm-thick porous sheet.

(Example 2) A porous sheet obtained in the same manner as in Example 1 was subjected to a batch stretching machine at 130 ° C.
Stretched uniaxially at a stretch ratio of 3.2 times with a thickness of 140
A μm porous sheet was obtained.

(Example 3) Ultra high molecular weight polyethylene powder B (trade name "Miperon XM", manufactured by Mitsui Chemicals, Inc., viscosity average molecular weight 3)
100,000, median diameter 30 μm, geometric standard deviation of particle diameter 0.6
9, a 200 μm-thick porous sheet was obtained in the same manner as in Example 1 except that the average particle circularity was 0.95 and the melting point was 140 ° C.).

(Example 4) A porous sheet obtained in the same manner as in Example 3 was subjected to a batch stretching machine at 130 ° C.
The film was stretched in the biaxial direction at a stretch ratio of 3.2 times each in length and width to obtain a porous sheet having a thickness of 120 μm.

Comparative Example 1 Ultra-high molecular weight polyethylene powder C (trade name “Hostaren GUR4126”, manufactured by Hoechst, viscosity average molecular weight 3,000,000, median diameter 40 μm, particle diameter) was used in place of the ultra-high molecular weight polyethylene powder A. Standard deviation 0.65, average particle circularity 0.84, melting point 140
C)), except that a porous sheet having a thickness of 200 µm was obtained in the same manner as in Example 1.

(Comparative Example 2) A porous sheet obtained in the same manner as in Comparative Example 1 was subjected to a batch stretching machine at 130 ° C.
The film is stretched biaxially at a stretching ratio of 2 times each in the vertical and horizontal directions, and has a thickness of 13
A 0 μm porous sheet was obtained.

The porosity, tensile fracture strength and bubble point pore diameter of the porous sheets obtained in the above Examples and Comparative Examples were measured. The results are shown in Table 1 below.

[Table 1] Example Comparative Example 1 2 3 4 1 2 Porosity 28 58 26 54 34 65 [% by volume] Tensile breaking strength 2100 2900 2300 3500 1900 1900 [N / cm ² ] Perforated pore diameter 35 70 30 65 90 1700 [μm]

Next, the positive electrode, the porous sheet, the negative electrode, and the porous sheet are superposed in this order, and the electrode and the porous sheet are stacked by using a strip automatic winder BHW-1 (manufactured by Yowa). Was produced. In addition, as a positive electrode,
Using the sheets filled with nickel hydroxide to nickel foam, as the negative electrode was used a sheet of AB ₅ -type hydrogen absorbing alloy into a paste using coated on iron punched metal of polyvinylidene fluoride. Ten wound bodies were manufactured for each, and the presence or absence of a short circuit was checked using a tester. Table 2 shows the results.

[Table 2] Example Comparative Example 1 2 3 4 1 2 Number of Samples Short-Circuited 0 0 0 0 3 7 [ Number / 10]

As shown in the above Tables 1 and 2, the porous sheets of Examples 1 and 3 have moderate porosity and strength, and have uniform porosity without excessively large pores. It was confirmed that it was a quality sheet. Further, the porous sheets of Examples 2 and 4 obtained by stretching them had high porosity and strength. Furthermore, in the electrode wound body manufactured using the porous sheet of the present example,
No short circuit occurred.

[0061]

As described above, the first polyolefin porous sheet of the present invention has an appropriate porosity and strength, and has no excessively large pores. And high strength can be achieved. The second polyolefin porous sheet obtained by stretching the sheet has high porosity and strength, and has few defects such as excessively large holes. In addition, a battery using the porous sheet of the present invention as a battery separator is unlikely to cause short circuit failure.

Continued on the front page (51) Int.Cl. ⁷ Identification FI FI Theme Court II (Reference) B29K 23:00 B29K 23:00 105: 04 105: 04 B29L 7:00 B29L 7:00 31:34 31:34 C08L 23 : 02 C08L 23:02 (72) Inventor Masakatsu Urari 1-1-2 Shimohozumi, Ibaraki-shi, Osaka Nitto Denko Corporation (72) Inventor Takashi Yamamura 1-1-2 Shimohozumi, Ibaraki-shi, Osaka F-term in Nitto Denko Corporation (reference) 4F074 AA17 AB01 CA52 CC04Y CC32Y DA03 DA08 DA49 4F210 AA03 AA06 AA11A AC01 AG01 AG20 AH33 QC05 QG01 5H021 BB01 BB05 BB11 CC17 EE04 HH06 HH07 5H028 AA05

Claims (12)

[Claims] 1. A polyolefin porous sheet having a porosity in the range of 25 to 70% by volume, a bubble point pore size in the range of 20 to 70 μm, and a tensile breaking strength of 1500 N / cm ² or more. 2. The polyolefin porous sheet according to claim 1, wherein the sheet is a sheet in which a plurality of polyolefin particles are interconnected, and a porous structure is formed by voids between the particles. 3. The median diameter is in the range of 30 to 200 μm, the geometric standard deviation of the particle diameter is in the range of 0.68 to 0.95, and the average particle circularity is in the range of 0.86 to 1. The polyolefin porous sheet according to claim 2, wherein the polyolefin particles are connected to each other by fusion. 4. The polyolefin has a viscosity average molecular weight of 50.
The polyolefin porous sheet according to any one of claims 1 to 3, which is 10,000 to 16,000,000 ultrahigh molecular weight polyolefins. 5. The polyolefin is an ultra-high molecular weight polyethylene or an ultra-high molecular weight polypropylene.
5. The polyolefin porous sheet according to any one of 4. 6. A polyolefin porous sheet obtained by stretching the polyolefin porous sheet according to claim 1. 7. The bubble point pore diameter is 40 to 100 μm.
The polyolefin porous sheet according to claim 6, wherein 8. The median diameter is in the range of 30 to 200 μm, the geometric standard deviation of the particle diameter is in the range of 0.68 to 0.95, and the average particle circularity is in the range of 0.86 to 1. A method for producing a polyolefin porous sheet, comprising the step of sintering polyolefin particles at a temperature equal to or higher than the melting point of the polyolefin particles. 9. The method according to claim 8, further comprising a step of stretching the obtained polyolefin porous sheet. 10. The production method according to claim 8, further comprising a step of hydrophilizing the obtained polyolefin porous sheet. 11. A battery separator using the polyolefin porous sheet according to claim 1. 12. A battery in which a separator is interposed between positive and negative electrodes, wherein the battery uses the battery separator according to claim 11 as the separator.