JP2882675B2 - Separator sheet - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to separators such as battery separators, electrolytic capacitors, and electric double layer capacitors.

(Prior Art) The role of the separator is to prevent positive and negative short-circuiting inside the battery and to transmit ions. The properties required as a separator include hydrophilicity and chemical resistance, and high mechanical strength.

Conventionally, electrolytic capacitors, electric double layer capacitors,
Electrolytic paper (such as kraft paper) is used as a material for battery separators, but these have the disadvantage of low strength.

It is also known to use a highly hydrophilic material such as natural fiber, rayon, nylon or the like as a nonwoven fabric and use it as a separator, but these have a drawback of poor chemical resistance (alkali or acid).

In addition, as a separator having a low rate of short shots and having excellent chemical resistance, there is a separator formed by stretching a polyolefin (for example, polypropylene) film to form fine pores. Is lacking.

A method of hydrophilizing polypropylene by mixing it with a hydrophilic additive is also being studied, but this method elutes the additive from the polypropylene, and the hydrophilicity imparted to the polypropylene disappears in a short period of time. As a result, there is a drawback that the electrolyte cannot be retained, the ions cannot move, and the life of the battery is shortened.

(Problems to be Solved by the Invention) It is an object of the present invention to provide a separator sheet having excellent hydrophilicity and chemical resistance and further having excellent mechanical performance.

(Means for Solving the Problems) That is, the present invention comprises polypropylene fibers having a fineness of 0.001 to 0.3 denier, a thickness of 50 to 130 μm, and a basis weight of 3 μm.
It shall apply in sheet 0~70g / m ^2, the average porosity of the intermediate layer of the sheet, the average porosity and the entire sheet average porosity of the portion between the seat surface and the surface with the depth position of 10μm respectively 45 to 75%, 1 to 20% and 40 to 70%, and the surface of the polypropylene fiber present on the sheet surface is
The separator sheet has an O / C ratio of 0.1 to 0.4 as measured by ESCA.

The polypropylene used in the present invention may contain an inorganic substance or the like, and a nonwoven fabric made of a fiber obtained by a melt blown method or the like is effective. The average fineness of the fibers forming the sheet is in the range of 0.001 to 0.3 dr. If it is less than 0.001dr, the ion permeability may be poor and the discharge property may be poor. If it is more than 0.3dr, the network structure becomes coarse when formed into a sheet, so that the sheet is easily shot. When the thickness of the sheet is increased to make the network structure dense, it becomes difficult to incorporate the sheet inside a compact capacitor.

Examples of the method for producing a fiber having a diameter of 0.001 to 0.3 dr include an extraction method, a straight spinning method, and a melt blown method.

The sheet used for the separator preferably has a microporous network structure having a winding path in the cross-sectional direction. This is because ions can pass between the two poles and short shots can be prevented. The structure of the sheet is not limited to woven fabric, knitted fabric, non-woven fabric and the like as long as it satisfies the above. The thickness of the sheet is in the range of 50 to 130 μm, and if it is less than 50 μm, the processability during the production of the separator is poor, and the sheet may be broken or perforated during processing. 130
If it is more than μm, there is a problem that it becomes difficult to use a compact condenser.

The void ratio needs to have a gradient in the cross-sectional direction of the sheet from the viewpoint of holding the electrolyte. The reason for this is that if the void ratio of the intermediate layer of the sheet is large and the void ratio of the layer near the surface is small, the electrolyte is less likely to be adsorbed on the electrode.

The average vacancy rate of the middle layer of the sheet must be 45 to 75%,
When the content is less than 45%, the retention of the electrolyte is deteriorated, and dry-up is easy. On the other hand, when the content is 75% or more, there is a problem that a short is likely to be caused, the strength is reduced, and the processability in producing the separator is deteriorated. The intermediate layer of the sheet means a layer 5 μm above and 5 μm below the center in the thickness direction. The average porosity in the portion between the surface of the sheet and a position at a depth of 10 μm from the surface needs to be 1 to 20%, and if it is 1% or less, the ion permeability is poor, so that there is no battery performance.
If it exceeds 20%, the electrolyte is likely to be adsorbed to the electrodes, and the life of the capacitor is shortened.

It is desirable that the porosity is in a relation of contrast in the thickness direction, but in the case of non-contrast, a surface having a small porosity is taken as the sheet surface.

Also, the average vacancy rate of the entire sheet is required to be 40 to 70%, and if it is less than 40%, it is easy to dry up due to poor liquid retention of the electrolyte, and if it is more than 70%, the strength is insufficient and the process passage property is poor. There are drawbacks that make it easier to shoot.

In the present invention, the average porosity of the portion between the sheet surface and a position at a depth of 10 μm from the surface was measured as follows.

(1) A cross-sectional photograph of a cross section of the sheet, which is orthogonal to the machine direction, is taken at 10,000 times using an SEM.

(2) A 10 μ × 10 μ square is selected from the sheet surface to a depth of 10 μ, and the total area (S ₁ ) of the holes at that portion and the total area (S = 100 μ ² ) are obtained.

(3) Calculate the void ratio between 10 μm from the sheet surface from S ₁ / S.

Similarly, the average porosity of the sheet intermediate layer (the layer 5 μm above and 5 μm below the center in the thickness direction) and the average porosity of the entire sheet are calculated.

In the present invention, the sheet surface and 10 μm from the surface
It is preferable that the portion between the sheet and the depth position (sheet surface area) and the portion other than the sheet intermediate layer have an average vacancy rate higher than the sheet surface area, and more preferably, from the sheet surface area to the inside of the sheet. The sheet has a porosity gradually increasing in accordance with the following formula.

Sheet, must be the basis weight is 30~70g / m ^2, 30
When it is less than g / m ² , the strength is insufficient and the processability in producing a separator is poor, and when it is more than 70 g / m ² , the volume occupied by the battery when incorporated in a capacitor is undesirably too large.

In the present invention, the polypropylene on the surface of the polypropylene fiber on the sheet is denatured to have a CＯO group or an OH group, thereby improving the hydrophilicity. Specific methods therefor include a corona discharge treatment and a low-temperature plasma treatment, but the low-temperature plasma treatment is particularly effective. In the corona discharge treatment, the functional groups are little added, and the change in hydrophilicity with time is large.

Whether a functional group such as a C ＝ O group or an OH group is provided can be determined by performing a surface analysis using ESCA. ESCA
O / C (atomic number ratio) must be 0.1 to 0.4, and if it is less than 0.1, the hydrophilicity is poor and the sheet does not hold the electrolytic solution, and the life of the battery is shortened. Also
When the ratio is 0.4 or more, the sheet is greatly deteriorated, the strong elongation is greatly reduced, and the process passability such as breakage or breakage of the separator is deteriorated. The surface having the functional group is preferably both surfaces of the sheet, but may be one surface. In the case of one surface, it is desirable that the surface is provided on the surface of the sheet surface with a low porosity.

In the low-temperature plasma processing, a gas is introduced under vacuum, and a voltage is applied between electrodes using a high-frequency power supply under a certain degree of vacuum to generate glow discharge.

When the purpose is to provide C = O groups and OH groups on the sheet surface, the gases used are non-polymerizable gases (O ₂ , H ₂ , Ar,
He, N _{2 and the} like are preferable, and among them, oxygen or a gas containing oxygen is more preferable.

The processing vacuum degree is 0.05 considering the processing time, processing voltage, etc.
~ 1 Torr is preferred. Discharge power is preferably 0.1~5W / cm ^2, at 0.1 W / cm ² or less too much longer to achieve the object. When it is 5 W / cm ² or more, the discharge becomes unstable and it becomes difficult to perform a uniform treatment. The processing time is 5
120 seconds is preferable, and if it is 5 seconds or less, the degree of functional group provision is small, and uniform provision cannot be achieved. If the time exceeds 120 seconds, the sheet surface deteriorates, which is not preferable. The high-frequency power supply frequency is preferably 1 KHz to 13.56 MHz in view of the discharge uniformity in the electrode width direction.

The electrode may be internal or external, but from the viewpoint of processing efficiency, the internal electrodeposition method is preferred.

As the fiber material for the separator, polypropylene which is hardly oxidized and decomposed by an acid, an alkali or the like and has good mechanical properties and heat resistance is used in the present invention, but may contain other fibers.

The sheet subjected to the hydrophilization treatment is cut and shaped into a required shape, and is used as a separator for a battery, an electrolytic capacitor, an electric double layer capacitor, and the like.

 Hereinafter, the present invention will be described with reference to examples.

Examples and Comparative Examples After non-woven polypropylene polymer by melt blown, the nonwoven fabric was calendered at 110 ° C to form a sheet, and the sheet was provided with an internal electrode type parallel plate electrode provided with a 13.56 MHz power supply. It was set in the low-temperature plasma treatment layer and the pressure was reduced. After the internal pressure reached 0.01 Torr, O ₂ gas was introduced at 30 cc / min to maintain the internal pressure at 0.3 Torr. Next, plasma treatment was performed between the electrodes at an output of 1 W / cm ² for 30 seconds. The results were as shown in Table 1.

Examples 1 to 10 had a fineness of 0.005 to 0.2 dr and a sheet thickness of 52.
～90Myu, but although the sheet basis weight in place with 35~65g / m ² is the result although the plasma treatment was rows summer, as shown in Table 1, all of the things that satisfy the performance as a separator when the battery Atsuta.

Comparative Examples 1 and 2 are the results of sheets made using fibers having finenesses of 0.5dr and 0.0008dr, respectively.
Then, I chose because the mesh of the sheet was coarse. Comparative Example 2
Then, the performance as a battery was almost unimportant. This is thought to be due to poor ion permeability.

Comparative Examples 3 and 4 each have a sheet thickness of 140 μm and 30 μm.
Although the thickness was changed to μm, in Comparative Example 3, the operation was too time-consuming to assemble the battery because it was too thick. In Comparative Example 4, the loss rate was extremely high because the separator was broken when the separator was made. Further, in Comparative Example 3, the void ratio was large, and many shots occurred.
In Comparative Example 4, the separator was dry-up due to a low average void ratio at the intermediate void ratio, and the battery performance was poor.

In Comparative Examples 5 and 6, the O / C ratio was changed to 0.05 and 0.5, respectively. In Comparative Example 5, the battery performance was poor due to poor O / C and poor hydrophilicity. In Comparative Example 6, O /
Although the hydrophilic level was good due to the large C ratio, the processability during the production of the separator was very poor.

In Comparative Example 7, the void ratio was constant in the thickness direction, but the electrolyte was adsorbed on the electrode, and the performance as a separator was poor. In addition, shortage occurred due to the large vacancy rate.

In Examples and Comparative Examples, the evaluation as a battery was performed by preparing a cup-shaped separator for each sheet according to the size of a single I-type dry battery, and mixing the manganese dioxide and graphite in an anode can that was nickel-plated on iron. The results are obtained when an alkaline dry battery was prepared using a mixture of zinc powder and an alkaline electrolyte as a cathode, after interposing a separator containing 30% KOH as an anode mixture.

Claims (1)

(57) [Claims] (1) It is composed of a polypropylene fiber having a fineness of 0.001 to 0.3 denier, a thickness of 50 to 130 μm and a basis weight of 30 to 70 g / m.
² , the average porosity of the intermediate layer of the sheet, the average porosity of the portion between the sheet surface and a position at a depth of 10 μm from the surface, and the average porosity of the entire sheet are each 45.
A separator sheet which has a O / C ratio of 0.1 to 0.4 as determined by ESCA, wherein the O / C ratio is 0.1 to 0.4%, which is 75% to 75%, 1% to 20% and 40% to 70%.