JP2001020195A - Sheet material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sheet material having excellent performances especially as a separator preventing the positive and negative poles from short-circuiting in metal refining using an acidic electrolyte, electric double layer capacitors and secondary batteries. SOLUTION: This sheet material is produced by combining fibrillated acrylic fibers made from at least 97 wt.% acrylonitrile(AN) and having a freeness of <=450 mL with polyacrylonitrile fibers (PAN fiber) made of polyacrylonitrile. Since the sheet material has a small maximum pore diameter, it can advantageously prevent short-circuit between the positive pole and the negative pole, has high resistance to electrolyte and also high affinity for electrolyte, enables thin sheets to easily be formed and therefore can impart sheet-like materials having low internal resistance.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sheet material mainly used for metal refining, an electric double layer capacitor, and an electrode separator of a secondary battery. More specifically, the present invention relates to a sheet-shaped material that has a low internal resistance and is stable over a long period of time because of its excellent liquid retention properties and affinity with the electrolyte, resistance to the electrolyte and oxidation resistance.

[0002]

2. Description of the Related Art Conventionally, in metal refining, electric double layer capacitors, and secondary batteries, a sheet-like separator has been used to prevent an electrical short circuit between a positive electrode and a negative electrode. For this separator, a material having an electrolytic solution resistance corresponding to the liquid property of the electrolytic solution to be used is selected and used. In particular, metal refining using acidic electrolytes, electric double layer capacitors,
In secondary batteries, inorganic fibers such as glass, which has excellent acid resistance, and synthetic fibers such as polypropylene, polyethylene, and polyester, have been used. Many problems have been left in the development of separators that satisfy each other or all of their properties such as resistance to electrolytes, oxidation resistance of the positive electrode during charging, and processability during the production of the separator.

[0003] That is, since the glass fiber has a hydrophilic surface and a very fine fiber can be obtained, the hermetic glass fiber is particularly important in the amount of liquid absorption (indicator of initial compatibility with the electrolyte). Has been used for a separator for a secondary battery using an acidic electrolytic solution. However, glass fibers are fragile, and it is difficult not only to manufacture thin sheets, but also the sheet strength is extremely weak, which hinders the assembly work of capacitors and secondary batteries. On the other hand, for example, Japanese Patent Application Laid-Open No. 56-9996
No. 8 proposes mixing glass fiber with 10% by weight or less of fibrillated synthetic fiber having a freeness of 350 cc or less to form a sheet. However, even with such a method, particularly for high-performance secondary batteries and electric double-layer capacitors that require high sheet thickness, thinning that satisfies sufficient sheet strength has not been successful. .

On the other hand, a sheet made of a nonwoven fabric, a woven fabric, or a film of polypropylene or polyethylene,
A thin, high-strength sheet can be produced, and it is stable for a long time even in an electrolytic solution, and is used in various secondary batteries and electric double layer capacitors. However, polypropylene and polyethylene have extremely low affinity for an electrolytic solution, and therefore increase internal resistance, which has been a problem.
In order to remedy this drawback, a method of impregnating these sheets with a surfactant has been proposed. However, there is a problem that the performance is deteriorated due to the contamination of the electrolyte by the surfactant and that long-term performance cannot be maintained. is there. In addition, 6-1013
In Japanese Patent No. 23, there is an attempt to improve affinity to an electrolytic solution and liquid retention by treatment with concentrated sulfuric acid, but there is a problem that fibers are damaged by concentrated sulfuric acid and strength is reduced.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a metal separator using an acidic electrolyte, an electric double layer capacitor, and a separator having a sufficient electrolytic resistance as a separator for preventing a short circuit between a positive electrode and a negative electrode in a secondary battery. An object of the present invention is to provide a sheet material which has a liquid property, has a high affinity with an electrolytic solution, and is easy to prepare an extremely thin sheet, so that a low internal resistance can be achieved.

[0006]

The object of the present invention has been attained by combining two types of acrylic fibers having a high acrylonitrile ratio. That is, the present invention relates to a sheet-like material obtained by combining a polyacrylonitrile fiber (PAN fiber) composed of polyacrylonitrile and a fibrillated acrylic fiber comprising at least 97% by weight of acrylonitrile (AN) and having a freeness of 450 ml or less. is there.
In addition, the sheet-like material is that the freeness of the fibrillated acrylic fiber is 200 ml or less, the PAN fiber is made of PAN having a weight average molecular weight of 150,000 or more, the fibrillated acrylic fiber is polyacrylonitrile, and the fibrillated acrylic fiber and PAN are used. When the ionic group of the polymer constituting the fiber is 0.025 mmol / g or less with respect to the polymer, the object of the invention is more highly achieved.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.
The acrylonitrile-based polymer, which is a raw material of the fibrillated acrylic fiber used in the present invention, is not particularly limited as long as the acrylonitrile (hereinafter, also referred to as AN) ratio is 97% by weight or more in the whole fiber, and is not particularly limited. A copolymer with a comonomer or a mixture thereof can be used. Here, the AN ratio is the weight% of acrylonitrile based on the polymerizable component in the acrylonitrile-based polymer constituting the fibrillated acrylic fiber.

When the fibrillated acrylic fiber is a fiber in which two or more acrylonitrile polymers are mixed, all of the two or more acrylonitrile polymers are A
The N ratio does not need to be 97% by weight or more, and the AN ratio of the mixed polymer as a whole may be 97% by weight or more. Also, two or more types of fibrillated acrylic fibers having different AN ratios can be mixed.
All of the fibrillated acrylic fibers of more than one kind have an AN ratio of 9
It is not necessary to be 7% by weight or more, and it is sufficient that the AN ratio of the mixed fibers as a whole becomes 97% by weight or more.

[0009] When the acrylonitrile ratio is less than 97% by weight, the inherent chemical resistance and heat resistance of the acrylonitrile-based fiber are reduced. Physical properties are getting worse,
Deterioration easily occurs due to oxygen or the like generated at the positive electrode during charging. This long-term electrolyte resistance, heat resistance, and oxidation resistance
When the fibrillated acrylic fiber is made of polyacrylonitrile containing no copolymerization component, it shows particularly outstanding performance and is preferable.

The comonomer used in the copolymerization is not particularly limited as long as it can copolymerize with acrylonitrile, such as another polymerizable unsaturated vinyl compound.
C4 alkyl acrylate, alkyl methacrylate, acrylic acid, methacrylic acid, methacrylonitrile,
Acrylamide, vinyl acetate, vinyl chloride, vinyl bromide, vinyl fluoride, vinylidene chloride, vinylidene bromide,
Examples include styrene, styrene sulfonic acid, allyl sulfonic acid, methallyl sulfonic acid, styrene sulfonic acid salt, allyl sulfonic acid salt, methallyl sulfonic acid salt, ethylene, propylene and the like.

In order to produce an acrylic fiber as a raw material of a fibrillated acrylic fiber from such an acrylonitrile-based polymer, known spinning techniques such as a wet spinning method, a dry-wet spinning method and a dry spinning method can be applied. The fibrillated acrylic fiber can be produced by beating a predetermined length of the acrylic fiber with a known beating machine such as a beater or a refiner. Alternatively, flash spinning or the like in which a spinning solution is jetted at a high pressure can also be used.

When beaten with a beater or a refiner, an acrylic fiber which is easily fibrillated is industrially advantageous. In order to prepare such an acrylic fiber, for example, after mixing two or more acrylonitrile-based polymers, the mixture is dissolved in a known solvent to prepare a spinning dope that easily causes phase separation, and wet or dry-wet. Examples of the method include spinning and wet spinning or dry-wet spinning of the spinning solution, and then increasing the molecular orientation of the fiber by a higher degree of drawing or the like to facilitate fibrillation.

The fibrillated acrylic fiber used in the present invention has a freeness of 450 ml or less, more preferably 200 ml or less.
ml or less. Here, the freeness is JISP
8121 refers to Canadian standard freeness measured according to the method described in 8121. If the freeness exceeds 450 ml, the entanglement of the fibers decreases, and not only does the sheet strength decrease,
Since the mesh structure of the sheet formed by the fibrillated acrylic fibers becomes coarse and the maximum pore diameter increases, the electrodes are likely to be short-circuited. As the sheet becomes thinner with the demand for lower internal resistance and compactness of the sheet, it is necessary to further reduce the maximum pore size and increase the sheet strength, and the freeness of the fibrillated acrylic fiber that meets such demands is ,
It is 200 ml or less.

There is no limitation on the lower limit of the freeness, and the smaller the freeness, the easier it is to make the sheet thinner while maintaining the sheet strength. However, in practice, the fibrillated acrylic fiber itself and the formation (operability) of the sheet-like material itself are not industrial, so that at least 50 ml or more is recommended.

As described above, the fibrillated acrylic fiber can be obtained by cutting the acrylic fiber into a predetermined length and beating it with a known beating machine such as a beater or a refiner. Acrylic fibers can be adjusted by increasing or decreasing the number of passes of a beater or refiner.

The sheet material of the present invention is a composite of the above-mentioned fibrillated acrylic fiber and PAN fiber made of polyacrylonitrile. There is no limitation on how to combine,
Mixing fibrillated acrylic fiber and PAN fiber to form a web, forming a separate sheet with each fiber and laminating them, or sandwiching another fiber layer with a certain fiber layer, etc. Is adopted according to the purpose. The easiest way is to use a wet papermaking method as described later. Needless to say, fibers of another material may be used in combination at the time of these composites.

By the way, the fibrillated acrylic fiber is
While giving strength to the sheet and playing a role in densifying the sheet, the PAN fiber plays a role as a skeleton of the sheet. The skeleton of the sheet is required to have excellent chemical resistance, heat resistance, oxidation resistance, and dimensional stability that are stable over a long period of time even under a high temperature range of the electrolytic solution. Separators used in high-performance electric double-layer capacitors and secondary batteries are required to have even stricter performance.To achieve this purpose, PAN fibers made of polyacrylonitrile containing no copolymer are required. Required. In the present invention,
When the PAN fiber is made of PAN having a weight average molecular weight of 150,000 or more, heat resistance and chemical resistance are further improved, and even a thinned sheet is preferable because it is stable for a long period of time. The weight average molecular weight (M
w) is Mw ^determined by [η] = 3.35 × 10 ⁻⁴ Mw ^−0.72 using intrinsic viscosity [η] at 30 ° C. using DMF as a solvent.

The polymer constituting the fibrillated acrylic fiber and the PAN fiber used in the present invention can be produced by a known technique such as solution polymerization or aqueous suspension polymerization. There is no particular limitation. However, the ionic group contained in the polymer was 0.025 mmol /
By setting it to g or less, more favorable electrolytic solution resistance is exhibited, and even a very thin sheet can provide stable electrolytic solution resistance for a long period of time. The ionic group referred to here, as a polymerization initiator, a polymerization initiator having an ionic group,
For example, when a redox catalyst such as 4-4 ′ azobis (4-cyanovaleric acid) or potassium persulfate or ammonium persulfate and sodium sulfite is used, an ionic group introduced to the terminal of the polymer and a fibrillated acrylic It is introduced by a comonomer having an ionic group, such as acrylic acid or styrene sulfonic acid, which is used when the fiber is not polyacrylonitrile. Of course, a polymerization initiator which does not introduce an ionic group into the polymer, for example, 2,
Adopting 2′-azobisisobutyronitrile or 2,2′-azobis-2,4-dimethylvaleronitrile,
Needless to say, it is preferable to employ a comonomer which does not introduce an ionic group into the polymer, for example, vinyl acetate, styrene or polyacrylonitrile as the fibrillated acrylic fiber.

The sheet material of the present invention comprises the above-mentioned fibrillated acrylic fiber and PAN fiber, and the mixing ratio thereof is not particularly limited. It is preferred to use. If it is less than 5% by weight, the strength of the sheet will be weak and the sheet will not be dense. On the other hand, if it exceeds 30% by weight, the resistance to the electrolytic solution is deteriorated and the dimensional stability is deteriorated. Regarding the method for producing the sheet material of the present invention, the fibrillated acrylic fiber and the PAN fiber cut to a length of about 2 to 10 mm are dispersed in water and subjected to ordinary wet papermaking or crimping. A known sheet forming technique such as nonwoven fabric processing in which a web is formed from PAN fibers subjected to the above-mentioned step, and a sheet is formed by a method such as water punching after the fibrillated acrylic fibers are sprinkled. Further, the sheet-like material of the present invention may contain other inorganic fibers, organic synthetic fibers, inorganic powders or the like according to the purpose, or a resin may be added to the surface of the sheet in order to further increase the electrolytic solution resistance if necessary. An emulsion or the like can be applied or sprayed. However, when an organic synthetic fiber or a resin emulsion is used in combination, the affinity of the fibrillated acrylic fiber or the PAN fiber with the electrolyte originally contained is not impaired. 20% by weight of
Is preferably not exceeded.

[0020]

EXAMPLES The following examples are provided to facilitate understanding of the present invention, but these are merely examples, and the gist of the present invention is not limited thereto. In the examples, parts and percentages are indicated by weight unless otherwise specified.

Example of Production of Fibrillated Acrylic Fiber An acrylonitrile polymer having an AN ratio and an ionic group content different from each other was dissolved in an aqueous solution of rhoda soda according to a conventional method, and an acrylic fiber was produced by a wet spinning method. Here, in the synthesis of the acrylonitrile-based polymer, a redox catalyst of methyl acrylate as a comonomer and ammonium persulfate and sodium sulfite as a polymerization initiator was used. The acrylic fiber was beaten with a beater under different conditions. 1
~ 6 fibrillated acrylic fibers were obtained. No. 1 of these fibrillated acrylic fibers. , AN ratio, ionic group content and freeness are shown in Table 1. In Table 1, the freeness was measured according to the method described in JISP8121.
The ionic group was calculated by the following equation by measuring the weight percent of the sulfur derived from the catalyst system in the polymer per polymer by atomic absorption analysis. Ionic group (mmol / g) = (wt% sulfur / 100)
/ 32

[0022]

[Table 1]

Production Example of PAN Fiber According to a conventional method, polyacrylonitrile having a different weight average molecular weight and an ionic group content was dissolved in an aqueous solution of rodane soda, and the solution was subjected to wet spinning. 1 to 3 PAN fibers were produced.
Here, in the synthesis of polyacrylonitrile, a redox catalyst of ammonium persulfate and sodium sulfite was used as a polymerization initiator. No. of these PAN fibers. , Weight average molecular weight and ionic group content are shown in Table 2.

[0024]

[Table 2]

Examples 1 to 6 20 parts of the fibrillated acrylic fiber of the present invention shown in Table 1 and 80 parts of the PAN fiber of the present invention shown in Table 2 cut to 6 mm were uniformly dispersed in water so as to be 0.2%. Then, wet papermaking was performed using a circular paper machine, and then subjected to a heat calendering treatment at 130 ° C. to obtain sheet-like materials Examples 1 to 6 of the present invention.

Table 3 shows the bulk density, liquid retention, maximum pore size, tensile strength, and tensile strength after the acid resistance test of these sheet materials.
Shown in Here, each physical property value of Table 3 was measured as follows. 1. Maximum pore diameter: measured by the bubble point method described in ASTM-F-316. 2. Liquid retention rate: A sample of 10 cm × 10 cm was collected and precisely weighed, and the value was designated as W1. Next, this sample was immersed in a 35% dilute sulfuric acid solution at 25 ° C. for 5 minutes, and then excess dilute sulfuric acid was cut off for 2 minutes, and the weight was measured. The liquid retention was calculated by the following equation. 2. Liquid retention rate (%) = (W2−W1) / W1 × 100 Tensile strength; prepared sheet 2cm wide, 20c long
The test piece was cut into m, and measured with a Tensilon measuring machine in accordance with JIS L1096A method. Here, the grip holding length was 15 cm (vertical direction), and the pulling speed was 10 cm / min. 4. Tensile strength after acid resistance test: A sample prepared in the same manner as in the above tensile strength measurement was placed in a closed container, filled with 35% dilute sulfuric acid, kept at 50 ° C for 500 hours, washed with water, and air-dried to obtain a test specimen. And 3. The tensile strength was measured in the same manner.

[0027]

[Table 3]

Examples 1 to 6 all have excellent liquid retention, and are sheet materials balanced with respect to the maximum pore diameter and the tensile strength after the acid resistance test. Examples 2 and 4 whose degree is 200 ml or less
No. 6 to No. 6 are particularly excellent sheet-like materials having a small maximum pore diameter and having no problem in electrode short circuit. Examples 2 to 6 in which the PAN fiber had a weight average molecular weight of 150,000 or more had high tensile strength and tensile strength after the acid resistance test, and tensile strength after the acid resistance test in Examples 5 to 6 in which the fibrillated acrylic fiber was polyacrylonitrile. Is even better, and among them, it is clear that Example 6 in which the ionic group of the fibrillated acrylic fiber and the PAN fiber is 0.025 mmol / g or less has extremely excellent electrolytic solution resistance.

Comparative Examples 1 to 5 Comparative materials A shown in Table 4 were used in place of the fibrillated acrylic fibers used in the examples, and Comparative materials B shown in Table 5 were used in place of the PAN fibers. Then, papermaking was performed in the same manner as in the examples to obtain sheet materials of Comparative Examples 1 to 5. Table 6 shows the bulk density, the maximum pore diameter, the liquid retention ratio, the tensile strength, and the tensile strength after the acid resistance test.

[0030]

[Table 4]

[0031]

[Table 5]

[0032]

[Table 6]

In Comparative Example 1, the tensile strength after the acid resistance test was particularly low because the acrylic fiber used in place of the PAN fiber had a low AN ratio. In Comparative Example 2, the freeness of the fibrillated acrylic fiber was high, and an appropriate maximum pore size was not obtained.
In Comparative Example 3, the tensile strength after the acid resistance test was low because the AN ratio of the fibrillated acrylic fiber was low. In Comparative Example 4,
Since glass fibers are used, the tensile strength of the sheet material is low, and there is a problem in workability. In Comparative Example 5, since the polypropylene material was used, the tensile strength after the acid resistance test was excellent, but the liquid retention was extremely poor.

[0034]

As described above, the sheet-like material of the present invention has a small maximum pore diameter, which is advantageous for preventing short-circuit between the positive electrode and the negative electrode, has a high electrolytic solution resistance, and has a high electrolytic solution resistance. It has a high affinity with, and it is easy to prepare an extremely thin sheet, so that low internal resistance can be achieved. Such a sheet material can be suitably used as an electrode separator of a metal refining using an acidic electrolyte, an electric double layer capacitor, and a secondary battery. Providing such a sheet-like material is a remarkable point of the present invention, and has great industrial significance.

Claims (4)

[Claims] 1. A sheet material comprising a composite of a fibrillated acrylic fiber comprising at least 97% by weight of acrylonitrile (AN) and having a freeness of 450 ml or less, and a polyacrylonitrile fiber (PAN fiber) comprising polyacrylonitrile. 2. The fibrillated acrylic fiber has a freeness of 20.
The sheet-like material according to claim 1, wherein the PAN fiber is made of PAN having a weight average molecular weight of 150,000 or more, with a volume of 0 ml or less. 3. The sheet material according to claim 1, wherein the fibrillated acrylic fiber is made of polyacrylonitrile. 4. The method according to claim 1, wherein the ionic group of the polymer constituting the fibrillated acrylic fiber and the PAN fiber is 0.025 mmol / g or less based on the polymer. A sheet material according to any one of the above.