JPH01154457A - Nonwoven fabric for battery separator - Google Patents

Abstract

PURPOSE:To increase the absorbency and retainability of an electrolyte and to decrease deterioration in quality with time by occupying 30% or more of the whole surface area of single fibers which constitute a nonwoven fabric by fibers of specific saponified ethylene-vinyl acetate copolymer. CONSTITUTION:Fibers of saponified ethylene-vinyl acetate copolymer in which the copolymerization ratio of ethylene is 25-60 molar percent and saponification degree is 90% or more are mixed so as to occupy 30% or more of the whole surface srea of single fibers which constitute a nonwoven fabric. If the copolymerization ratio of ethylene is less than 25 molar percent, the production of the copolymer is difficult, and if it exceeds 60 molar percent, the absorbency and retainability of an electrolyte are in sufficient. If saponification degree is less than 90%, fiber forming capability and other performance are not satisfied. The retainability and absorbency of the electrolyte are increased and deterioration in quality in the electrolyte with time is decreased.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a nonwoven fabric battery separator containing fibers of saponified ethylene-vinyl acetate copolymer.

[Conventional technology, problems]

Traditionally, separators for alkaline batteries have been made of cotton,
Nonwoven fabrics made of nylon, polypropylene, polyethylene, viscose rayon, etc. have been used. Also,
In alkaline storage batteries (secondary batteries), nonwoven fabrics made of nylon fibers are preferably used as separators.

Furthermore, polyolefin-based polypropylene nonwoven fabric is used in lithium batteries.

-C-wise, nonwoven fabrics used in battery separators must have excellent liquid absorption and retention properties, chemical resistance, and good gas permeability for nickel-cadmira and alkaline storage batteries, as well as oxidation resistance. They are also required to be excellent.

However, fiber materials that have excellent liquid absorption properties, that is, electrolyte absorption and retention properties, have chemical resistance, that is, they are easily susceptible to quality deterioration over time due to being immersed in electrolyte solution, which makes it difficult to maintain battery performance. Adversely affect. On the other hand, fibers with excellent chemical resistance, such as polypropylene fibers, have poor liquid absorbing and liquid retaining properties, which is unfavorable in terms of battery performance. In order to improve these, methods of adding surfactants and plasma treatment have been proposed, but the current situation is that they are still not sufficient.

[Purpose of the invention]

In view of the above-mentioned problems, the present invention aims to provide a separator made of a nonwoven fabric that has excellent liquid absorbing and liquid retaining properties for electrolytic solutions and exhibits little quality deterioration over time.

[Means and operations for solving the problems] The present inventor has made extensive studies to achieve the above-mentioned object, and as a result, has arrived at the present invention. In other words, the copolymerization ratio of ethylene is 2.
Fibers made of saponified ethylene-vinyl acetate copolymer (hereinafter referred to as saponified F, VA) with a saponification degree of 5 to 60 mol% and a saponification degree of 90% or less account for the total surface area of single fibers constituting the nonwoven fabric. The present invention provides a nonwoven fabric for a battery separator that has excellent liquid absorption and retention properties for battery electrolyte by arranging it in a nonwoven fabric so as to account for at least 30% of the nonwoven fabric, and has little quality deterioration over time. .

Here, the copolymerization ratio of ethylene is 25 to 60
If it is less than 25 mol%, it will be difficult to produce the copolymer (and if it exceeds 60 mol%, it will not be possible to achieve the liquid absorbency and liquid retention that are the objectives of the present invention. It is difficult to obtain a separator with excellent quality. Further, the degree of saponification is required to be 90% or more, and if it is less than 90%, problems will occur in the fiber forming ability and other problems.

In order to obtain a separator with excellent liquid absorption and liquid retention properties, the saponified IEVΔ fibers should be configured to occupy at least 30% or more, preferably 50% or more, of the total surface area of the single fibers constituting the nonwoven fabric. is necessary.

Although saponified EVA fiber can be used as a separator by itself as a nonwoven fabric, it can also be used in combination with other fibers, thereby improving the strength and stiffness of a nonwoven fabric made only of saponified EVA fiber. can. When used in combination with other fibers, it is possible to form fibers of other normal shapes and mix them with this, but it is possible to use a high melting point component such as polypropylene and a low melting point component such as polyethylene. Do you have it? Form a synthetic fiber M and saponify it E
■△It can be mixed with fibers, heat treated at a temperature where the low melting point component melts, and fused with the low melting point component in step 7. In addition to side-by-side type conjugate fibers, the conjugate fibers are preferably conjugate fibers having a core made of a high melting point component and a sheath made of a low melting point component. In addition to the reinforcing effect of the high melting point component, a separator of excellent quality can be obtained due to the excellent fusion of the low melting point component.Furthermore, the saponified EVA fiber may be a composite fiber, and the saponified E, V A fiber/polyethylene composite fiber can be used as the saponified EVA fiber. Composite fibers of saponified EVA and other resins can be used, such as (indicates a composite fiber composed of saponified EVA and polyethylene; the same applies hereinafter), saponified EVA/polypropylene composite fiber, and saponified EVA/polyamide composite fiber. These composite fibers may be used in combination with saponified EVA fibers, or any combination of composite fibers may be used in combination. These saponified EVA composite fibers are easier to spin and draw than fibers made only of saponified EVA (and also impart shape stability to nonwoven fabrics, which is extremely convenient for achieving the purpose of the present invention. When used as a component of a composite fiber, it refers to one in which at least 30% or more of the surface area of the composite fiber is occupied by saponified EVA.

Regarding the surface area ratio when using composite fibers,
For example, a saponified EVA fiber with a diameter of 20 microns and a side-by-side (bimetallic) type composite fiber with a circular cross section, which also has a diameter of 20 microns and has polypropylene and polyethylene adjacent to each other in a semicircular shape, are used as units. It is assumed that a web is formed by mixing the same number of fibers per volume, and then heat-treated to obtain a nonwoven fabric. In this case, saponification I in the total surface area of single fibers constituting the nonwoven fabric
The percentage of KVA fiber surface area is 50%, and the surface areas occupied by the polypropylene and polyethylene components are each 25%. The polyethylene component is heat-treated and melted during the nonwoven fabric manufacturing process, and when the nonwoven fabric is completed, it deforms and does not maintain its original semicircular shape, resulting in a large change in surface area. The surface area ratio referred to above is based on the state when the cotton is blended.

Next, various methods are adopted for arranging the saponified EVA fibers in the nonwoven fabric.
Lol! Non-woven fabrics can be manufactured from 100% i-fibers or saponified EVA fibers and other fibers using the card method, cross-layer method, random universal method, spunbond method, and even the melt-blown method described in USP 3,849,241. A web is formed using a commonly used method, and the fibers are entangled with a high-pressure water stream as described in Japanese Patent Publication No. 47-18069, or heat or high frequency is applied to the web to fuse the low melting point components of the fibers. Alternatively, a method such as intertwining the fibers by needle punching may be used, but the method is not limited to this, and various other methods may be used.

Example 1 Using saponified EVA with an ethylene copolymerization ratio of 30 mol % and a saponification degree of approximately 100%, a web having fibers with an average diameter of 9 microns was obtained by a melt-blowing method, and a web with a thickness of 0.5 microns was obtained by thermal calendering. A 16 mm nonwoven fabric was obtained. The basis weight of the nonwoven fabric was 55 g/rrr. The surface area of the single fibers that make up this nonwoven fabric is 100% saponified EVA.
Table 1 shows the properties of the nonwoven fabric. As shown in Table 1, the nonwoven fabric prepared as described above not only has excellent liquid retention and absorption properties, but also has excellent deterioration over time, giving good results as a battery separator. was gotten.

Example 2 A web consisting of single fibers having an average diameter of about 9 microns was assembled on a wire mesh belt using a melt-blown method using saponified IEVA having an ethylene copolymerization ratio of 35 mol % and a saponification degree of 99%. At this time, the copolymerization ratio of ethylene is 40 mol% and the degree of saponification is 100%.
Saponified EVA and polyethylene are composite-spun into a zide-by-side (bimetallic) type so that 50% of the total surface area of the single fiber is occupied by the saponified EVA, and the resulting fiber has a diameter of 15 microns after drawing heat treatment and cutting. Saponified EVA/polyethylene composite fibers were sprayed onto the wire mesh belt.

i.e., saponified EVA with a fiber diameter of approximately 9 microns.
The fibers and saponified EVA/polyethylene composite fibers having a fiber diameter of 15 microns were mixed on a wire mesh belt. The obtained web was thermally calendered at 158°C to have a thickness of 0.2 mm and a basis weight of 90 g/rrf.
A nonwoven fabric having the following properties was obtained. Saponified IEVA fiber and saponified E
The VA/polyethylene composite fibers were mixed in a 7:3 ratio by weight, respectively.

When the specific gravity of saponified EVAm fiber and saponified EVA/polyethylene composite fiber is 1.18 and 1.06, respectively, the surface area ratio of the saponified EVA component to the total fiber surface area in this example is about 89%, and that of polyethylene is about 11%. %.

The thus obtained nonwoven fabric had excellent mechanical properties and processing suitability, and had the properties shown in Table 1, making it excellent for use in battery separators.

Example 3 Polypropylene is used as the core, and the copolymerization ratio of ethylene is 31
Polypropylene/saponified E with a diameter of 15 microns with a sheath of saponified EVA with a saponification degree of almost 100% in mol%
A VA composite fiber was obtained. This composite fiber is drawn and heat treated, and is a short fiber having a cutting length of 40 mm, and approximately 100% of the surface area is saponified EV△.

On the other hand, polypropylene and polyethylene are joined side-to-side so that they have approximately the same surface area, and the diameter is 19 mm.
A short polypropylene/polyethylene composite fiber having a cut length of 40 mm and a cutting length of 40 mm was also obtained. The two types of composite fibers obtained in this way were mixed by weight with 85 parts of polypropylene/saponified EVA composite fiber (specific gravity 1.05) and 1 part of polypropylene/polyethylene composite fiber (specific gravity 0.93).
Mix 5 parts of cotton and use a random weaver to give a basis weight of 70g.
/M web was obtained. At this time, the ratio of the surface area of saponified EVA to the total surface area of single fibers constituting the web is 84
%.

Next, the web is subjected to fiber entanglement using a high-pressure water stream, and further dried and calendered to a size of 0.2 mm.
It was made into a nonwoven fabric with a thickness of . This nonwoven fabric had excellent mechanical strength and was excellent in workability during battery assembly. Furthermore, the performance of the nonwoven fabric is shown in Table 1. It exhibited excellent performance when used as a separator for alkaline secondary batteries.

Comparative Example 1 In Example 3, a nonwoven fabric was obtained by repeating Example 3 using polypropylene fibers with a diameter of about 15 microns instead of the polypropylene/saponified EVΔ composite fiber. It had poor liquid properties and absorbency.

Comparative Example 2 Example 3 was repeated using nylon fibers with a diameter of about 14 microns instead of the polypropylene/saponified EVA composite fibers to obtain a nonwoven fabric. The change over time was poor.

Table 1 Note that the test method in Specification 117 is as follows.

Liquid retention: 30% KO after holding the sample (10cn+X 10cm square) in a fixed position (from one point to another, weighing ji-).
After 30 minutes of immersion in the H-containing solution, the sample was pulled out, spread on a wire mesh, left to stand for 5 minutes, weighed again, and the rate of increase in gravity before and after immersion is shown.

Liquid absorption, l'l12. A 5c+o, 10 cm long sample is taken, the lower end 10 am of the sample is immersed in 30% K ON solution, and after 10 minutes the height of the K O+1 solution wicked up into the sample is shown.

Change over time: Take a square sample of 10 cm x 10 ci,
Attach the four corners to a wire mesh and place the sample on the r side at 30% K.
The change in color of the sample relative to the blank after immersion in the ON solution and boiling the solution for 1 hour is determined visually. Note that when the solution was boiled, air bubbles were continuously fed into the solution from the lower part so that the air bubbles hit the center of the sample.

〔Effect of the invention〕

The battery separator nonwoven fabric of the present invention not only has excellent liquid retention and liquid absorption properties required for a separator, but also has excellent quality change over time in an electrolytic solution.

Claims (1)

[Claims] Fibers made of a saponified ethylene-vinyl acetate copolymer having an ethylene copolymerization ratio of 25 to 60 mol% and a saponification degree of 90% or more occupy at least 30% or more of the total surface area of single fibers constituting the nonwoven fabric. A nonwoven fabric for battery separators characterized by: