JP2003157826A - Separator for battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separator for a battery having high heat resistance, an excellent mechanical characteristic, excellent air permeability and sufficient thickness. SOLUTION: This separator for a battery is formed of a nonwoven fabric containing a crystallized thermoplastic polyimide short fiber as a main constituent and characterized by that the thermoplastic polyimide has a repeating unit expressed by following structural formula (1): The manufacturing method of the separator for a battery is characterized by including a process for carrying out wet-sheet-forming by fabricating slurry containing the crystallized thermoplastic polyimide short fiber, and a process for heating and pressing it at a temperature higher than a glass transition point of the thermoplastic polyimide and lower than the melting point thereof.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a battery separator. More specifically, it relates to a separator for an alkaline secondary battery, which has excellent heat resistance, mechanical properties, and air permeability.

[0002]

2. Description of the Related Art Nickel-cadmium batteries and nickel-hydrogen batteries have been well known and used as alkaline secondary batteries. These batteries are composed of positive and negative electrodes, an electrolyte, a separator, a container, and the like. The performance required for alkaline secondary battery separators is alkali resistance, electrical isolation between positive and negative electrodes, mechanical characteristics, gas / ion permeability, electrolyte permeability, liquid retention, etc. can give. As such a separator,
Nonwoven fabrics made of polyolefin resins such as polyethylene and polypropylene having excellent alkali resistance and aliphatic polyamide resins such as nylon 6 having both alkali resistance and hydrophilicity are often used.

However, the above resins have poor heat resistance, and due to rapid charging or the like, when locally heated to a high temperature, oxidative decomposition occurs and the separator is partially damaged, which shortens the cycle life of the battery. There was a problem.

Therefore, a separator excellent in heat resistance has been actively developed. For example, Japanese Patent Laid-Open No. 7-3757.
Japanese Patent Laid-Open No. 1-2004 proposes a battery separator composed of a paper-like sheet prepared by mixing and paper-making meta-aramid fibrids and meta-aramid short fibers. Although this separator has excellent heat resistance, it uses a fibrid, so if you increase the thickness to ensure electrical isolation between the positive and negative electrodes, the air permeability will decrease and It has been difficult to satisfy the thickness, air permeability and strength at the same time, as the strength decreases when the proportion of short meta-aramid fibers is increased to secure the property.

[0005]

In view of the above situation, the present invention provides a battery separator having high heat resistance, excellent mechanical properties and air permeability, and a sufficient thickness. This is an issue.

[0006]

Means for Solving the Problems The inventors of the present invention have conducted extensive studies to solve the above problems, and as a result, crystallized heat obtained by spinning and drawing using a thermoplastic polyimide having crystallinity. The present inventors have found that a non-woven fabric containing plastic polyimide short fibers as a main constituent is suitable as a battery separator that solves the above problems, and have completed the present invention.

That is, the present invention is directed to a battery separator made of a non-woven fabric containing crystallized thermoplastic polyimide short fibers as a main constituent, and further, in such a battery separator, A gist of a battery separator is characterized in that the plastic polyimide has a repeating unit represented by the following structural formula (1).

[0008]

[Chemical 2]

Further, the method includes the steps of preparing a slurry containing crystallized thermoplastic polyimide short fibers and performing wet papermaking, and heating and pressurizing at a temperature not lower than the glass transition point and not higher than the melting point of the thermoplastic polyimide. The gist of the present invention is the method for manufacturing a battery separator described above.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below.
The battery separator of the present invention is made of a nonwoven fabric containing polyimide short fibers as a main constituent. The polyimide forming the polyimide short fibers mainly used in the present invention is a thermoplastic polyimide having crystallinity.
After melt-spinning the thermoplastic polyimide having crystallinity,
By heating and stretching to increase the crystallization and the orientation of the crystal part, a crystallized polyimide fiber is obtained. In the present invention, the crystallized polyimide fiber thus obtained is cut into short fibers.

In the present invention, it is important that the polyimide forming the polyimide short fibers is thermoplastic.
That is, since the polyimide short fibers mainly constituting the nonwoven fabric which is the battery separator of the present invention are thermoplastic,
This is because the entanglement points of the polyimide short fibers can be bonded by an appropriate heat treatment, and as a result, the mechanical strength of the nonwoven fabric can be increased.

In the present invention, it is also important that the thermoplastic polyimide short fibers are crystallized. Because the polyimide is generally weak to alkali, it is not suitable for the use of a concentrated alkaline aqueous solution as in the present invention, but the polyimide with advanced crystallization is more chemically resistant than the polyimide with less advanced crystallization. This is because the present inventors have found that they are particularly suitable for alkaline batteries because they have excellent properties, especially alkali resistance. In addition,
In the present invention, “crystallized” means that the crystallinity measured by X-ray diffractometry is 15% or more, preferably 20% or more, and more preferably 25% or more.
If the crystallinity is less than 15%, the alkali resistance decreases.

As the thermoplastic polyimide short fibers, in addition to being crystallized, it is preferable that the crystal parts are highly oriented. This makes it possible to obtain polyimide short fibers which are not only excellent in alkali resistance, but also have small dimensional changes during heating and cooling, and are excellent in thermal dimensional stability. At this time, the degree of orientation of the crystal part is preferably 80% or more.

The chemical structure of the thermoplastic polyimide used to obtain the crystallized thermoplastic polyimide short fibers as described above preferably has a chemical structure represented by the following structural formula (1) as a repeating unit. The polyimide having such a chemical structure is a crystalline polyimide resin having excellent heat resistance as well as excellent thermal processability due to its thermoplasticity.

[0015]

[Chemical 3]

As a specific example of the polyimide having such a chemical structure, "Aurum" (trade name) manufactured by Mitsui Chemicals, Inc. is commercially available, and the glass transition point by the DSC method is 250 ° C. and the melting point is 388 ° C. It has characteristics.

When the above-mentioned thermoplastic polyimide is formed into fibers, polyimides having various chemical structures may be copolymerized or blended for the purpose of improving various properties, and the effects of the present invention can be obtained. Other polymers such as polyarylate, polyolefin, polyamide, polyphenylene sulfide, polyether ether ketone, and fluororesin may be blended within a range not impairing, and further inorganic fillers such as titanium oxide, alumina, silica and carbon black. You may mix | blend. Therefore, the thermoplastic polyimide short fibers may contain these auxiliary raw materials.

In the form of the polyimide short fibers mainly used for constituting the nonwoven fabric in the present invention and other short fibers used as necessary, the average fiber length is preferably 1 to 15 mm, The average fiber diameter is preferably 3 to 30 μm. If the average fiber length is less than 1 mm, the strength of the nonwoven fabric is insufficient, and if it exceeds 15 mm, it is difficult to produce a uniform nonwoven fabric, which is not preferable. On the other hand, if the average fiber diameter is less than 3 μm, the strength of the nonwoven fabric tends to be insufficient, and if it exceeds 30 μm, the voids in the nonwoven fabric become large and the function as a separator tends to deteriorate, which is not preferable. The cross-sectional shape of the above-mentioned short fibers is usually circular, but is not particularly limited to this and may have a modified cross-section or an irregular shape.

In the present invention, the non-woven fabric serving as the battery separator is a non-woven fabric composed only of the above-mentioned crystallized thermoplastic polyimide short fibers, whereby the object of the present invention can be effectively achieved. However, the present invention is not particularly limited to this, and for the purpose of facilitating the papermaking of the non-woven fabric and improving various characteristics of the battery separator, a short fiber or pulp-like material made of other organic or inorganic material, for example, having crystallinity. Polyimide other than thermoplastic polyimide, polyethylene, polypropylene, aramid, polyester, polyetheretherketone, polyphenylene sulfide, polyparaphenylene benzobisoxazole, carbon, glass, alumina, short fibers or pulp made of silica, etc., It may be a non-woven fabric in which various particulates (fillers) are mixed.

However, in order to achieve the object of the present invention,
In the nonwoven fabric that serves as the battery separator of the present invention, the content of the crystallized thermoplastic polyimide short fibers is preferably 30% by mass or more, and further 7
It is preferably 0% by mass or more. Here, in calculating the content rate, when the crystallized thermoplastic polyimide short fiber contains an auxiliary material other than the thermoplastic polyimide having crystallinity, the mass of such an auxiliary material is the thermoplastic polyimide short fiber. Calculate by subtracting from the mass of.

The battery separator of the present invention is composed of a non-woven fabric whose main component is the thermoplastic polyimide short fibers crystallized as described above, and its basis weight (mass per unit area) is It may be appropriately set in consideration of mechanical properties, air permeability, liquid retention, etc.
100 g / m ² is preferred. In addition, the thickness is usually preferably 50 to 300 μm from the viewpoints of air permeability, liquid retention, and electrical isolation between the positive and negative electrodes.

The apparent density of the battery separator of the present invention is preferably in the range of 0.40 to 1.0 g / cm ³ . If the apparent density is less than 0.40 g / cm ³ , the mechanical strength of the separator tends to be insufficient, which is not preferable. On the other hand, if the apparent density exceeds 1.0 g / cm ³ , the porosity decreases,
It is not preferable because the air permeability and the liquid retaining property tend to be poor. The apparent density is obtained by measuring the dimensions and thickness of the vertical and horizontal sides, calculating the occupied volume, and dividing the mass by the occupied volume.

The air permeability of the battery separator of the present invention is preferably 10 cm ³ / cm ² · sec or more. When the air permeability is less than 10 cm ³ / cm ² · sec, the permeability of oxygen gas generated from the positive electrode during overcharge is lowered, and the internal pressure is likely to rise as the charge / discharge cycle proceeds, which is preferable. Absent.

The tensile strength, which is an index of the mechanical strength of the battery separator of the present invention, is preferably 1.5 kN / m or more. If the tensile strength is less than 1.5 kN / m, there is a high possibility of breakage when incorporated into a battery, which is not preferable.

Next, a method for producing the battery separator of the present invention will be described. First, the method for papermaking a nonwoven fabric using polyimide short fibers is not particularly limited, and may be carried out by a conventionally known dry or wet papermaking method, but particularly wet papermaking method, uniformity of thickness and texture It is preferably used because it is suitable for obtaining a short fiber nonwoven fabric excellent in The wet papermaking apparatus used in the wet papermaking method is not particularly limited, and a known paper machine or the like may be used, but specific examples thereof are a cylinder type wet paper machine and a short net type wet paper machine. , A short-net inclined type wet paper machine, a long-net inclined type wet paper machine, and the like. It is preferable that a hot air type, a contact type, or a radiant type dryer is provided alongside these wet papermaking apparatuses.

In the step of performing wet papermaking, the above polyimide short fibers and other components to be blended as necessary are mixed with a dispersion medium such as water and uniformly dispersed using a pulper or the like to prepare a slurry. It is prepared, wet-processed using the above apparatus, and then dried. At this time, for the purpose of improving the uniformity, smoothness and the like of the obtained nonwoven fabric, a process of shrinking the polyimide short fibers with a refiner, beater or the like may be performed in advance. For the purpose of increasing the strength of the nonwoven fabric, the nonwoven fabric may be impregnated or sprayed with an emulsion or a solution containing a polyimide precursor, an epoxy resin or the like. In this case, it may be interposed between the papermaking section and the drying section of the wet papermaking machine. An impregnation device or a spray device may be provided.

The non-woven fabric obtained by the above wet papermaking process usually has too large a porosity as it is and tends to have insufficient mechanical strength for use as a battery separator intended by the present invention. Therefore, sufficient mechanical strength is provided by performing the steps described below. That is, above the glass transition point of the thermoplastic polyimide forming the polyimide short fibers as the main constituent of the nonwoven fabric,
The step of heating and pressurizing at a temperature below the melting point is performed. By this step, it is possible to improve the mechanical strength due to the thermal bonding action and the densification while maintaining the shape of the nonwoven fabric.

The reason why the above heating temperature is set to a temperature not lower than the glass transition point of the thermoplastic polyimide is to soften the polyimide short fibers to exert the effect of thermal bonding, and the temperature is lower than the glass transition point. , Thermal bonding is not done.
In order to enhance the effect of thermal adhesion, the heating temperature is preferably 10 ° C. or more higher than the glass transition point of the thermoplastic polyimide. At the same time, the reason for setting the heating temperature to the melting point of the thermoplastic polyimide or less is that when the melting point is exceeded, it depends on the heating time, but the polyimide short fibers are fused while losing their form as fibers due to melting, resulting in air permeability and the like. This is because the properties of the non-woven fabric are impaired, and it cannot be used as the intended battery separator. Even at a temperature below the melting point, the properties of the nonwoven fabric may be impaired depending on the heating time and pressure conditions, so the heating temperature is preferably 10 ° C. or more lower than the melting point of the thermoplastic polyimide. That is,
A preferable temperature range as the heating temperature is the range shown by the following mathematical formula.

[0029]

[Equation 1]

However, in the above equation, T is the heating temperature, T
g represents the glass transition temperature of the thermoplastic polyimide, Tm represents the melting point of the thermoplastic polyimide, and the unit is ° C.

The pressure at which the pressure is applied is not particularly limited, but the higher the pressure, the greater the effect of thermally adhering the polyimide short fibers to each other, and the air permeability of the obtained battery separator decreases. Therefore, the pressure may be appropriately set in consideration of those points, and in the present invention, a pressurizing condition that a pressure of about 0.05 to 10 MPa is usually applied to the nonwoven fabric is preferably adopted.

The apparatus for performing the heating and pressurizing step as described above is not particularly limited, and a conventionally known heating press apparatus or the like may be used, but a long nonwoven fabric is continuously heated. A calender roll device and a double belt press device capable of performing hot pressing between a pair of opposing belts made of metal or the like are preferably used from the viewpoint of being able to pressurize simultaneously. Through the above steps, the battery separator of the present invention, which is made of a nonwoven fabric containing a crystallized thermoplastic polyimide as a main constituent, is excellent in heat resistance, mechanical properties, and air permeability.

[0033]

EXAMPLES The present invention will be specifically described with reference to the following examples. Various properties were measured or evaluated by the following methods. (1) Tensile strength According to JIS-P8113, the tensile strength was measured using a universal testing machine (manufactured by Intesco). (2) Air permeability The amount of air (cm ³ / cm ² · sec) passing through the test piece was measured according to JIS-L1096 General Textile Test Method Item 8.27, Air permeability Method A (Flagille type method). (3) Alkali resistance After being immersed in a 30 mass% aqueous solution of sodium hydroxide (NaOH) or potassium hydroxide (KOH) at 23 ° C. for 200 hours, it was taken out and a change in appearance was confirmed. (4) Thickness Measured according to JIS-P8118 (paper and paperboard-testing method for thickness and density).

Example 1 As the thermoplastic polyimide, Aurum PL450 manufactured by Mitsui Chemicals, which is a thermoplastic polyimide having crystallinity, was used. The glass transition point of this polyimide measured by DSC was 250 ° C, and the melting point was 387 ° C. The polyimide is heated to 400 ° C. to be melted, melt-spun at a spinning speed of 500 m / min, and heated and drawn at a temperature of 300 ° C. and a draw ratio of 2.4 to obtain a fiber diameter of about 15
A polyimide fiber of μm was obtained. This polyimide fiber had a crystallinity of 30% and a crystal orientation of 90% (both measured values by X-ray diffractometry), confirming that it was crystallized. This crystallized polyimide fiber has a fiber length of 5
It was cut into mm to obtain polyimide short fibers. The above polyimide short fibers were thrown into water and uniformly dispersed in water using a pulper to obtain a slurry having a concentration of 0.05% by mass. Then, using this slurry, a slanted short-net continuous paper machine (manufactured by Saito Iron Works Co., Ltd.) was used to make paper with a width of 60 cm, and then a hot air dryer attached to the paper machine
It was dried at 0 ° C. to obtain a polyimide nonwoven fabric. The polyimide non-woven fabric thus obtained was heated with a double belt press (manufactured by Sandvik Co., Ltd.) under a pressure equivalent to 0.2 MPa at 240 ° C. for 2 minutes and 300 ° C. for 5 minutes while continuously pressurizing the basis weight. A battery separator of the present invention made of a non-woven fabric of 67 g / m ² , thickness of 0.125 mm and apparent density of 0.54 g / cm ³ was obtained.

Example 2 To 60 parts by mass of the polyimide short fibers used in Example 1, 40 parts by mass of the same polyimide short fibers crimped with a beater for 30 minutes were prepared. A battery separator of the present invention was obtained in the same manner as in Example 1 except that it was prepared. This battery separator had a basis weight of 68 g / m ² , a thickness of 0.115 mm, and an apparent density of 0.59 g / cm ³ .

Comparative Example 1 The same polyimide used in Example 1 was heated to 400 ° C. to be melted, melt-spun at a spinning speed of 500 m / min, and heated and stretched at a temperature of 300 ° C. and a draw ratio of 1.2 times. As a result, a polyimide fiber having a fiber diameter of about 22 μm was obtained.
This polyimide fiber has a crystallinity of 8% and a crystal orientation of 5
It was 8% (in all cases, measured by X-ray diffraction method). This polyimide fiber was cut into a fiber length of 5 mm to obtain a polyimide short fiber. A battery separator was obtained in the same manner as in Example 1 except that the above polyimide short fibers were used. This battery separator had a basis weight of 68 g / m ² , a thickness of 0.130 mm, and an apparent density of 0.52 g / cm ³ .

The characteristics of the battery separators obtained in Examples and Comparative Examples are shown in Table 1 below.

[0038]

[Table 1]

The battery separators of the examples had high strength and high air permeability and were useful as separators for alkaline secondary batteries. On the other hand, in the case of the comparative example, since the uncrystallized polyimide fiber was used, it was inferior in alkali resistance and was unsuitable as a separator for an alkaline secondary battery.

[0040]

As described above, the battery separator of the present invention is excellent in heat resistance, mechanical properties and air permeability, and can be used as a separator for alkaline secondary batteries such as nickel-cadmium batteries and nickel-hydrogen batteries. It is suitable.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Yoshihisa Yamada             23 Uji Kozakura, Uji City, Kyoto Prefecture Unitika Ltd.             Shikisha Central Research Institute F-term (reference) 4L055 AF34 BE20 FA11 FA14 FA19                       GA01                 5H021 BB01 BB02 BB08 CC01 CC02                       EE02 HH06

Claims (3)

[Claims] 1. A battery separator comprising a non-woven fabric mainly composed of crystallized thermoplastic polyimide short fibers. 2. A thermoplastic polyimide having the following structural formula (1):
The battery separator according to claim 1, which has a repeating unit represented by: [Chemical 1] 3. A step of preparing a slurry containing crystallized thermoplastic polyimide short fibers and performing wet papermaking, and a step of heating and pressing at a temperature not lower than a glass transition point of the thermoplastic polyimide and not higher than a melting point thereof. The method for producing a battery separator according to claim 1 or 2, characterized in that.