JPH0778145B2 - Polyolefin microporous film and electrolytic solution separator comprising the same - Google Patents

Description

TECHNICAL FIELD The present invention relates to a novel polyolefin film and an electrolytic solution separator comprising the same.

[Prior Art] As electrolytic solution separators used in electrolytic capacitors, lithium batteries, batteries, etc., electrolytic papers such as kraft paper and manila paper have been used for a long time, but recently, for example, Japanese Utility Model Publication No. 59-140429. JP-A-61-13614 and JP-B-59-3729
It has been proposed to use microporous polyolefin films, as described in No. 2.

[Disadvantages of Prior Art] Polyolefin microporous film is preferable for reducing internal resistance because it has excellent pore uniformity and continuity as compared with conventional electrolytic paper. Since it is inferior, it has a drawback that the change over time in the electrical characteristics is large.

[Problems to be Solved by the Invention] An object of the present invention is, when used as an electrolytic solution separator, excellent in retaining force of an electrolytic solution, small change in electrical characteristics over time, and equivalent series resistance (ESR). It is to provide a small polyolefin microporous film.

[Means for Solving the Problems] As a result of earnest research, the inventors of the present application have made the equivalent porosity substantially lower by making the apparent porosity of the surface layer of the film smaller than the average porosity of all layers. The inventors have completed the present invention by finding that the holding power of the electrolytic solution can be significantly increased without increasing the above.

That is, the invention has an apparent porosity on the surface of 10% to 45%, an average porosity of all layers of 50% to 85%, a liquid paraffin permeation time of 5 seconds or less, and Provided is a polyolefin microporous film having an average pore diameter of 0.05 μm to 5 μm and an apparent average pore diameter on the surface of 0.01 μm to 4 μm, and an electrolytic solution separator comprising the same.

[Detailed Description of the Invention] As described above, the polyolefin microporous film of the present invention has an apparent porosity of 10% to 45% on its surface,
It is preferably 20% to 40%. If the apparent porosity on the surface is less than 10%, the equivalent series resistance will increase when used as an electrolyte solution separator, and if it is greater than 45%, the electrolyte retention will decrease and there is a high risk of dry-up. Become. The apparent porosity of the film surface may be the same or different on the front side and the back side of the film. The average porosity of the entire film is 50% to 85%, preferably 60% to 75%. If the average porosity of the whole is less than 50%, the electrolytic solution holding power decreases, and the probability of dry-up increases, and if it exceeds 85%, the mechanical strength decreases and the short-circuit occurrence rate increases.

The liquid paraffin permeation time of the film is 5 seconds or less,
It is preferably 3 seconds or less. If the liquid paraffin permeation time exceeds 5 seconds, the equivalent series resistance increases. There is no particular lower limit for the liquid paraffin permeation time, but if the permeation time is less than 0.5 seconds, the mechanical properties may be low, so the permeation time is 0.
5 seconds or more is desirable.

From the viewpoint of equivalent series resistance and occurrence rate of short circuit, the average pore diameter of the middle layer of the film is set to 0.05 μm to 5 μm, especially
It is preferably 1 μm to 3 μm. Further, the apparent average pore diameter on the surface is 0.01 μm to 4 μm, and particularly preferably 0.05 μm to 2 μm, from the viewpoint of holding power of the electrolytic solution and equivalent series resistance.

The thickness of the film is 10 μm from the viewpoint of handleability and electrical characteristics.
To 50 μm, particularly preferably 20 to 40 μm.

The polyolefin referred to in the present invention is ethylene, propylene, butene, pentene, homopolymers or copolymers of α-olefins represented by hexene and blends thereof, among which polyethylene, polypropylene is heat resistant, It is often used because it has excellent cold resistance and mechanical properties. Furthermore, among polypropylene, the intrinsic viscosity ([η]) is 1.8 to 3.3 dl / g, more preferably 2.7 to 3.1 dl / g, and the isotactic index (II).
Is preferably 93% or more because it is excellent in dimensional stability and solvent resistance. Further, from the viewpoint of heat resistance against heat applied during element production, the melting point of the polyolefin is preferably 120 ° C. or higher.

The polyolefin microporous film of the present invention can be manufactured as follows. Polyolefin resin 10
In 0 parts by volume, from 80 parts by volume to 180 parts by volume of organic solids such as phthalates or phosphates used as plasticizers such as vinyl chloride such as dicyclohexyl phthalate (DCHP) or triphenylene phosphate (TPP), Preferably from 90 parts by volume to 160 parts by volume, after melt extrusion, trichloromethane, trichloroethane,
It is preferable to extract 95% or more, and more preferably 98% or more of the added amount of the organic solid using a good organic fixed solvent such as acetone, methyl ethyl ketone, ethyl acetate, methanol, toluene, xylene. Next, roll drawing is carried out at a temperature not lower than the glass transition point of the polyolefin and having a melting point of -10 ° C. At this time, in order to reduce the porosity of the surface, the film is nipped on either the low speed roll or the high speed roll, preferably both. A suitable pressure for the nip is a linear pressure of 2 kg / cm to 50 kg / cm. After stretching,
It is preferable to heat-set in the temperature range of melting point -5 ° C or higher than the melting crystallization temperature of the polyolefin.

In the above production method, the apparent porosity of the surface was reduced by nipping the film on a roll during stretching, but the melting point of the polyolefin before or after stretching the film was −50 ° C.
Therefore, the apparent porosity of the surface can also be reduced by nipping with a heating roll having a melting point of −5 ° C. at the above pressure. Furthermore, the apparent porosity of the surface is also small by preparing one with a relatively small amount of the organic solid and a large amount of the organic solid, and coextruding three layers so that the one with a large amount of the compound is in the center. It's time to get a film.

The polyolefin microporous membrane of the present invention is preferably composed only of polyolefin from the viewpoint of performance,
If the above-mentioned apparent porosity, average porosity and liquid paraffin permeation time are within the scope of the present invention, it may be possible to contain a trace amount of impurities, and, for example, a heat stabilizer,
Additives such as antioxidants, slip agents, antistatic agents, and monomers other than olefins may be added in small amounts. The term "polyolefin microporous film" used in the claims is meant to include a polyolefin microporous film containing such impurities and additives.

When the polyolefin microporous film is used as the electrolytic separator, it is preferable that a hydrophilic treatment is performed in order to improve the affinity with the electrolytic solution. The hydrophilic treatment can be performed by coating with a nonionic surfactant, an anionic or cationic surfactant, corona or plasma treatment, graft treatment, ultraviolet treatment, or a combination thereof.

[Effect of the Invention] In the polyolefin microporous film of the present invention, although the average porosity of the entire film is as high as 50% to 85%, the porosity of the surface is low, and the film middle layer portion and the surface have Since the average pore diameter of 1 is set in the optimum range from the viewpoint of equivalent series resistance and the like, when used as an electrolyte separator, the electrolyte retainability is excellent and the change in electrical characteristics over time is small. Also,
Since the liquid paraffin permeation time is 5 seconds or less, the equivalent series resistance is low. Furthermore, since the surface layer structure of the film is firm, the pinhole resistance is also good.

[Characteristic Measuring Method and Effect Evaluation Method] Next, the characteristic measuring method and evaluation method relating to the present invention will be summarized.

(1) Intrinsic viscosity ([η]) 0.1 g of a sample was completely dissolved in 100 ml of tetralin at 135 ° C, and this solution was measured with a viscometer in a constant temperature bath at 135 ° C to obtain a specific viscosity S.
The intrinsic viscosity is calculated according to the following equation.

[Η] = S / {0.1 × (1 + 0.22 × S)} (2) Isotactic index (II) The sample is vacuum dried at 130 ° C. for 2 hours. From now on, weight W (m
Take the sample of g), put it in a Soxhlet extractor, and boil it n
-Extract for 12 hours with peptane. Next, this sample is taken out, thoroughly washed with acetone, and then vacuum dried at 130 ° C. for 6 hours, and then the weight W- (mg) is measured and calculated by the following formula.

II (%) = (W ⁻ / W) × 100 (3) Melting point of polyolefin Scanning calorific value DSC-2 type (manufactured by Perkin Elmer Co., Ltd.) was used, and 5 mg of the sample was heated under nitrogen stream at a heating rate of 20 ° C. / It is measured from room temperature in minutes, and the endothermic peak temperature associated with melting is taken as the melting point.

(4) Average Pore Diameter and Apparent Porosity The area of each hole was calculated by observing the sample surface with a scanning electron microscope (SEM), and the average value (Sab) and sum (Sδ) were calculated.
It is calculated by the following formula.

Porosity (%) = Sδ / S × 100 (where S is the observed area) The average porosity of the middle layer of the film is determined by slicing the center in the thickness direction after freezing the film to liquid nitrogen temperature. Then
The sliced surface was observed and calculated by the above method.

(5) Liquid paraffin permeation time Samples and liquid paraffin were placed in an atmosphere of 25 ° C and 50% humidity.
After holding for 24 hours, place the sample on a horizontal surface and
Liquid paraffin 0.03 to 0.06 g is dropped naturally from a height of ~ 20 mm.

At this time, when the liquid paraffin touches the sample surface,
The time taken to permeate the sample and wet the opposite surface is measured and used as the liquid paraffin permeation time (sec).

The liquid paraffin used has a viscosity of 77 ± 1 centistokes at 37.8 ° C specified in JIS K 9003.

Liquid paraffin is dropped on the sample, and the time until the opposite surface is wetted is defined as the liquid paraffin permeation time (second).

(6) Porosity (Pr) A sample (10 cm x 10 cm) was immersed in liquid paraffin for 24 hours,
The weight (W ₂ ) of the surface layer after the liquid paraffin was sufficiently wiped off was measured, and the pore volume (V ₀ ) was calculated from the weight (W ₁ ) of the sample before immersion and the density (ρ) of the liquid paraffin as follows. Ask by.

V ₀ = (W ₂ −W ₁ ) / ρ Porosity (P _r ) is calculated from the apparent volume (value calculated from thickness and dimensions) V and the pore volume V ₀ by the following formula.

P _r = V ₀ / V × 100 (%) (7) Equivalent series resistance (ESR) Triethylamine and phthalic acid were dissolved in γ-butyrolactone according to the method described in JP-A-61-187221, and 3.
An electrolytic solution of 1 mS / cm was prepared. The direct current resistance component at 1 kH of the microporous film in this electrolytic solution was defined as ESR (Ω).

Here, as a comparative sample, the value (2.0Ω) of electrolytic capacitor paper (Manila paper MER2.5 50) was used as a reference, and 1.7Ω or less was ◯, 1.8 to 2.2Ω was Δ, and 2.3Ω or more was X.

The measurement conditions were as follows.

(A) Electrode: Platinum electrode (25mm square) Measuring load 240g (b) Impedance measuring machine: AG-4311 LCR METER (manufactured by Ando Electric Co., Ltd.) Measuring condition: 1kH, 5V range (8) Electrolytic capacitor test 220μF, 6.3 We fabricated 30 WV electrolytic capacitor elements, measured the weight and ESR values immediately after manufacturing (W ₁ , R ₁ ) and the values after 500 hours at 105 ° C (W ₂ , R ₂ ) and measured the change rate ( ΔW, ΔR) was measured.

ΔW and ΔR were calculated by the following equations.

ΔW (%) = (W ₂ −W ₁ ) / W ₁ × 100 ΔR (%) = (R ₂ −R ₁ ) / R ₁ × 100 [Examples of the invention] Examples and comparative examples of the present invention The effect of the present invention will be specifically described.

Example 1 As a polyolefin resin, a 40 mm extruder was used to prepare a blend of 40 wt% popipropylene powder (EB type, manufactured by Mitsui Toatsu Chemicals, Inc.) and 60 wt% dicyclohexyl phthalate (DCHP, manufactured by Osaka Organic Chemical Industry, Ltd.). Using T
It was melt-extruded from a die, introduced into a water tank, and solidified by cooling.

Next, this cast film was introduced into a 1-1-1-trichloroethane extraction tank for extraction, and subsequently stretched 3.5 times at 130 ° C. using a roll stretching device. During stretching, the film was nipped on a low speed roll and a high speed roll at a linear pressure of 5 kg / cm. Next, while allowing 5% relaxation in the longitudinal direction, heat setting was performed at 150 ° C. to obtain a film having a thickness of 30 μm.

The properties of the film thus obtained are shown in the table below. The overall average porosity is 65%, but the surface porosity is as low as 38%, so the electrolyte retention is high up to a high temperature. It can be seen that it is good, has little weight loss, and is excellent in dry-up resistance.

Comparative Example 1 In Example 1, before stretching the extracted film,
By strongly nipping with a 140 ° C mirror surface pressure roll,
A film having the following characteristics was obtained.

The film thus obtained has an overall porosity of
Although it was as large as 60%, the porosity of the surface was as low as 8%, the liquid paraffin permeation time was as slow as 6 seconds, and the ESR was inferior. Also, the reason why the ESR change in the electrolytic capacitor test is small is that the original initial ESR is extremely large.

Comparative Example 2 Polypropylene pellets (JS type, manufactured by Mitsui Toatsu Chemicals, Inc.) as a polypropylene resin were melt extruded from a T die at 220 ° C., cooled and solidified on a casting drum at a draft ratio of 50, and wound. Next, the cast film was heat-treated at 150 ° C. for 2 minutes. The film thus obtained has an elastic recovery rate of 92 as described in Japanese Examined Patent Publication No. 50-2176.
%Met. Next, the heat-treated film was stretched twice at 80 ° C. in the longitudinal direction at a stretching rate of 100% / min, further heat-treated at 140 ° C. for 5 minutes and wound.

The film thus obtained had a thickness of 25 μm, and as shown in the table, the porosity was almost the same on both the surface and all layers.
It is as low as ~ 35% and not only has a large ESR but also has a large change over time in electrolytic capacitors.

Claims (2)

[Claims] 1. An apparent porosity on the surface of 10% to 45%, an average porosity of the entire film of 50% to 85%, a liquid paraffin permeation time of 5 seconds or less, and an intermediate layer of the film. A polyolefin microporous film having an average pore diameter of 0.05 μm to 5 μm and an apparent average pore diameter on the surface of 0.01 μm to 4 μm. 2. The apparent porosity on the surface is 10% to 45%, the average porosity of the entire film is 50% to 85%, the liquid paraffin permeation time is 5 seconds or less, and An electrolytic solution separator comprising a polyolefin microporous film having an average pore diameter of 0.05 μm to 5 μm and an apparent average pore diameter on the surface of 0.01 μm to 4 μm.