JP2513768B2 - Microporous polypropylene film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a novel polypropylene-based separator used for separators for electrode isolation of batteries, electrolytic capacitors, etc. (hereinafter referred to as electric elements), various filters, separation membranes and the like. It relates to a microporous film.

[Prior Art] Conventionally, as a microporous polypropylene film, polypropylene is melt-extruded under high shear to form a specific crystal structure by sheet forming, and further annealed and stretched (Japanese Patent Publication No. 46-40119). There is known a film obtained by the above method, or a film obtained by a method of blending and extruding a polyolefin, an organic liquid and inorganic fine particles, molding the mixture, and then extracting the organic liquid (Japanese Patent Publication No. 59-37292).

[Problems to be Solved by the Invention] However, the former microporous film had the following problems. That is, (1) Porosity is as low as less than 45%, and for example, when used as a separator of an electrolytic capacitor, the amount of an electrolytic solution that can be contained is limited, so that it is easy to dry up.

(2) Since it is highly uniaxially oriented, it is easy to tear, and cracks may occur at the time of slitting, or cracks may occur due to stress when exposed to low temperatures after element formation.

The latter microporous film has a high porosity of 50% or more, but is inferior in mechanical properties, and since it is used as a separator of an electric element, it easily expands when slitting on a strip of 4 to 10 mm, for example, and is stable. Then, there is a problem that an electric element of uniform quality cannot be manufactured.

The present invention has a good mechanical suitability particularly when used as a separator of an electrolytic capacitor or an electric element, has good slitability and element winding property, has a high porosity, and is excellent in long-term reliability. An object is to provide a porous polypropylene film.

[Means for solving the problem]

The present invention is a film having a large number of pores having an average pore diameter of 0.06 to 3 μm, wherein the pores form through holes in the thickness direction of the film, and the porosity is 50 to 85%, and The present invention relates to a microporous polypropylene film having a degree of orientation ΔNp of the film represented by the following formula of 0.05 to 0.30 and a breaking elongation of 70 to 200% in the longitudinal direction.

ΔNp = ΔNb / ΔNa (ΔNb is the birefringence of microporous polypropylene film, ΔNa
Is the value of the ultimate birefringence of polypropylene according to the Smuels calculation formula 33 × 10 ⁻³ ) In the present invention, polypropylene has an intrinsic viscosity (“η”) of 1.2 to 3.5 dl / g, preferably 2.3 to 3.2 dl / g. It is preferable that the melt index (MI) is 0.1 to 10 g / 10 minutes and the weight average molecular weight is more than 30 × 10 ⁴ because cold resistance and solvent resistance are balanced.

Further, an isotactic index (II) of 90% or more, preferably 96% or more is preferable because it has excellent crystallinity and excellent heat resistance and solvent resistance. Further, the melt crystallization temperature (Tmc) is preferably 106 ° C. or higher, and more preferably 110 ° C. or higher because the solvent resistance becomes good.

Further, the polypropylene may contain a component other than polypropylene, for example, an α-olefin copolymer such as ethylene or butene-1 within a range not deviating from the above characteristics, but from the viewpoint of solvent resistance and the like, the above It is preferable that the content of the copolymer component or the blend is 5 mol% or less based on the propylene unit. In addition, known additives such as lubricants, antiblocking agents, antioxidants, heat stabilizers,
You may add antistatic agents, crystal nucleating agents, etc. in the range which does not violate the purpose.

In the microporous film of the present invention, the average pore size is 0.06-3.
It is necessary that the thickness is 0.1 μm, preferably 0.1 to 2 μm.
Is. If the average pore size is too small when used as a separator for electric elements, the rate of change in equivalent series resistance (ESR) due to changes in the viscosity of the electrolytic solution increases, and electrolysis due to, for example, aging (such as dry-up) When the viscosity of the liquid increases, the ESR increases remarkably, which causes a problem in use. On the other hand, if the average pore size is too large, it is not possible to prevent the movement of the fine conductive material, which causes a problem such as an increase in leakage current or the occurrence of a short circuit.

The porosity Pr of the microporous film of the present invention is 50 to 85%.
It is necessary to be, and preferably 60 to 75%.
If the porosity is too low, when the electrolyte is used as a separator, the amount of the electrolyte retained is not sufficient, and the increase in ESR due to dry-up causes a large problem. On the other hand, if the porosity is too high, the mechanical properties (in particular, the rigidity in the thickness direction) deteriorate, the frequency of pinholes caused by foreign matter increases, and the short circuit occurrence rate increases.

Next, the orientation degree ΔNp of the microporous film of the present invention is 0.05 to
It is necessary to be 0.30, preferably 0.08 to 0.20
Is. If the degree of orientation ΔNp is less than 0.05, the film is likely to be mechanically deformed, resulting in poor slitability and element winding property. On the other hand, if it exceeds 0.30, it tends to tear in the uniaxial direction, the thermal dimensional stability is poor, and the cold resistance deteriorates.

Furthermore, in order to improve solvent resistance and long-term reliability, ΔNp / ΔNp / which is defined by porosity Pr (%)
(1-Pr / 100) is preferably in the range of 0.1 to 0.8.

The breaking elongation in the longitudinal direction is 70 to 200%, preferably 80 to 150%. If the elongation in the longitudinal direction is less than 70%, cracks are likely to occur when slitting a few mm in a small width, making it difficult to obtain a uniform slit product. If it exceeds 200%, there is a problem in handleability during slitting and element winding. Cause

Further, the film thickness is preferably 10 to 50 μm, more preferably 20 to 40 μm from the viewpoints of compactness and handling of the element when used as a separator of an electric element.
Is.

Further, when the microporous polypropylene film of the present invention is used as a separator for an electrolytic capacitor / electric double layer capacitor, 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, anionic or cationic surfactant, corona discharge or plasma treatment, graft treatment, ultraviolet treatment, or a combination thereof.

In the microporous polypropylene film of the present invention, the longitudinal direction is usually made to coincide with the maximum direction of the refractive index, and when slitting, it is preferable to perform along the direction so that the mechanical properties are good.

Next, a typical method for producing the microporous polypropylene film of the present invention will be described, but it is not limited to this.

That is, the intrinsic viscosity is 1.2 to 4.2 dl / g, preferably 2.6 to 3.2 d
Phthalic acid ester or phosphoric acid used as a plasticizer for 100 parts by weight of l / g polypropylene resin, such as vinyl chloride, such as an extractable room temperature organic solid such as dicyclohexyl phthalate (DCHP) or triphenyl phosphate (TPP). 90 to 250 parts by weight of ester and the like, preferably 100
~ 230 parts by weight are added. When the amount added is in this range, the film-forming property becomes good, uniform fine pores having excellent continuous through-hole properties are formed, and a microporous film having excellent mechanical properties is obtained, which is preferable. Further, a heat stabilizer, an antioxidant, a slip agent, an antistatic agent, etc. may be added to the polypropylene resin. However, adding insoluble matter such as inorganic fine particles during the manufacturing process is inferior in the uniformity of the pore size formed,
Void voids of 10 μm or more are likely to occur, so it is preferable not to add it, and even if it is added, it is preferably 5 parts by weight with respect to 100 parts by weight of the polypropylene resin.

Next, after melt-extruding the above composition, it is formed into a sheet or tube on a casting drum or in a water tank. At this time, it is taken up at a draft ratio of 5 or more, preferably 7 or more, cooled and solidified at a temperature not lower than the melting point of the organic solid and not higher than the melting and crystallization temperature of the polypropylene resin, and wound.
Here, as the draft ratio and the cooling rate are increased, a uniform phase-separated structure of the polypropylene resin and the organic solid is formed, and a uniform and fine through-hole polypropylene film having excellent continuous through-hole property can be obtained.

Next, using a good solvent for the organic solid such as trichloromethane, trichloroethane, acetone, methyl ethyl ketone, ethyl acetate, methanol, toluene, xylene, 95% or more, preferably 98% or more of the added amount of the organic solid is extracted. To do.

Next, in order to bring the degree of orientation ΔNp of the microporous fill into the range of the present invention, the extraction film is continuously heated in the temperature range from the melting crystallization temperature of the polypropylene to the melting point of -5 ° C for 1 to 60 seconds. After heat treatment, at least uniaxially 1.5 at the temperature above the glass transition point of polypropylene resin and below the melting point -10 ° C.
It is preferable to stretch to 6 times. At that time, the thickness change rate (thickness after stretching / thickness before stretching) is preferably 0.2 to 0.5, and the stretching speed is 10,000% / min or less, preferably 500.
The step of stretching at a low speed of 0% / minute or less is preferably the final step. That is, in order to obtain the film of the present invention, only the low speed stretching step may be performed, but a method of performing the low speed stretching step after the usual high speed stretching step may be used. It is preferable to be orthogonal to. Furthermore, it is preferable to heat-set after stretching in a temperature range of not lower than the melting crystallization temperature of polypropylene and not higher than the melting point of -5 ° C.

The microporous polypropylene film thus obtained not only has excellent properties as a separator for electric elements such as electrolytic capacitors, batteries and electric double layer capacitors, but also has uniform pore size, mechanical properties and solvent resistance. Because of its excellent performance, it also exhibits excellent properties for microfilters, moisture-permeable waterproofing applications, etc.

[Characteristic measuring method and effect evaluating method] Next, the measuring method and the evaluating method relating to the present invention will be summarized.

(1) Intrinsic viscosity (“η”) In accordance with ASTM D 1601, 0.1 g of a sample is completely dissolved in 100 ml of tetralin at 135 ° C, and this solution is measured with a viscometer in a thermostat at 135 ° C to obtain a specific viscosity. The intrinsic viscosity is calculated from S according to the following formula.

[Η] = S / {0.1 × (1 + 0.22 × S)} (2) Melt index (MI) Measured according to ASTM-D1238-62T.

 The unit is g / 10 minutes.

(3) Isotactic index (II) A sample is vacuum dried at 130 ° C. for 2 hours. Weight W from now on
Take (mg) sample, place in Soxhlet extractor and extract with boiling n-heptane for 12 hours.

Next, this sample was 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 is obtained by the following equation.

II (%) = (W '/ W) x 100 (4) Melting point and melting crystallization temperature Using a scanning calorimeter DSC-2 type (manufactured by Perkin Elmer), 5 mg of a sample was heated under a nitrogen stream in a heating rate. Measure from room temperature at 20 ° C / min and take the endothermic peak temperature associated with melting as the melting point.

Continue to raise the temperature to 280 ℃ and hold for 5 minutes, then at 20 ℃
The peak temperature of latent heat associated with the crystallization of polyolefin is taken as the melt crystallization temperature in the process of lowering the temperature at the rate of decrease of / min.

(5) Average pore size Mercury porosimeter (PORE SIZER 93 manufactured by MICROMERITICS)
00), the distribution of micropore diameters was measured, and the number average was taken as the average pore diameter.

(6) Breaking elongation The breaking elongation in the longitudinal direction (MD) of the sample is measured according to JIS K6782 and expressed in%.

(7) Porosity (Pr) A sample (10 x 10 cm) was immersed in flowing paraffin for 24 hours,
The weight (W ₂ ) of the surface layer after the fluid paraffin was sufficiently wiped off was measured, and the pore volume (V0) was calculated from the weight (W ₁ ) of the sample before immersion and the density (ρ) of the fluid paraffin by the following formula. Ask.

V ₀ = (W ₂ / W ₁ ) / ρ Porosity (Pr) is calculated from apparent volume (value calculated from thickness and dimensions) V and pore volume V ₀ .

Pr = V ₀ / V × 100 (%) (8) Degree of orientation ΔNp A sample is impregnated with fluidized paraffin to replace most of its pores. The retardation (R) [nm] of the treated sample is measured with a Berek compensator, and the birefringence (ΔNb) is calculated by the following formula.

ΔNb = R / d Here, d represents the film thickness (nm), and the thickness after the retardation measurement was measured with a dial gauge type thickness meter made by PEACOCK [flat head (stylus diameter 5 mm)]. Next, the value of the ultimate birefringence of polypropylene (ΔNa) according to the Smuels formula
From 33 × 10 ⁻³ , the orientation degree (ΔNp) of the microporous film is calculated by the following formula.

ΔNp = ΔNb / ΔNa (9) ESR (equivalent series resistance) Based on the method described in JP-A No. 61-187221, γ-butyrolactone + triethylamine + phthalic acid is dissolved,
An electrolyte solution of 3.1 mS / cm (25 ° C) was prepared. 30 ~ sample
Cut into 35mm square, impregnated with the electrolyte and almost 100% of the pores
After substituting the electrolyte solution with the electrolytic solution, five sheets were stacked and sandwiched between platinum-black-treated platinum electrodes, and measurement was performed to obtain the ESR for each sheet.

Here, as a comparative sample, the ESR value of electrolytic capacitor paper (Manila paper MER2.5-50) was used as a comparison value, and a microporous film having an ESR of less than the Manila paper was marked with ○, and an ESR of the Manila paper or higher was evaluated.
A microporous membrane having a was evaluated as x.

 The measurement conditions were as follows.

(A) Electrode: Platinum black treated platinum electrode (25mm square) Measuring load 240g (b) Impedance measuring machine AG-4311 LCR METER (manufactured by Ando Electric Co., Ltd.) Measuring condition: 1kHz, 25 ℃ (10) Electrolytic capacitor test A .Handling (handling) The film is slit into a width of 8 mm and anodized Al foil and cathode Al are used.
A test was carried out by winding a foil and making an electrolytic capacitor of 220 μF, 6.3V.

At this time, the handling of the film was ranked as follows.

・ Stable element winding: ○ ・ Slitting, but disturbance when winding element: △ ・ Slitting not possible: × B. Reliability test Electrolytic capacitor element 30 obtained above The number of defective samples after aging at 85 ° C. for 500 hours was evaluated.

[Examples] Hereinafter, the present invention will be described in detail based on Examples.

Example 1 As a polypropylene resin, Mitsui Noblene EB type (manufactured by Mitsui Toatsu Chemicals, Inc., [η] = 2.8 dl / g, II = 97%)
Antioxidant (Irganox 1010,
Ciba-Geigy) 0.05 parts by weight, heat stabilizer (Yoshinox, Yoshitomi Pharmaceutical Co., Ltd.) 0.35 parts by weight, room temperature organic solid dicyclohexyl phthalate (DCHP) (Osaka Organic Chemical Industry) 155 parts by weight and a twin-screw extruder. It was melt blended and pelletized. Next, this was melt extruded from a T-die at an extrusion temperature of 220 ° C. using a 40 mmφ extruder, and a draft ratio of 7
Electrostatic application (6KV) on the casting drum at 90 ℃
While being adhered to the drum, it was cooled and solidified. The film thus obtained had a thickness of 85 μm. Next, the cast film is introduced into a trichlorethylene extraction tank for extraction,
Over 98% of the added DCHP was removed. Next, the roll heat treatment device performs heat treatment at 115 ° C. for 2 seconds, and subsequently, using a roll stretching device, the film is stretched 3.5 times in the longitudinal direction at 105 ° C., and then is stretched at 140 ° C. in the width direction at 1.5 ° C. with a stenter. The film was double-stretched, heat-set at 145 ° C. and wound.

The film thus obtained had a thickness of 25 μm, an orientation degree (ΔNp) of 0.09, and an MD breaking elongation of 119%. The main characteristics are shown in Table 1, which has a high porosity and is easy to handle.
It can be seen that the characteristics of the electrolytic capacitor are excellent.

Example 2 As a polypropylene resin, Mitsui Noblen EP type (manufactured by Mitsui Toatsu Chemicals, Inc., [η] = 3.2 dl / g, II = 97%)
0.05 parts by weight of Irganox 1010, 0.35 parts by weight of Yoshinox, and 170 parts by weight of DCHP were melt-blended with 100 parts by weight using a twin-screw extruder and pelletized. Then this
It was melt-extruded from a T-die at an extrusion temperature of 230 ° C. using a 40 mmφ extruder and introduced into a water tank at 82 ° C. with a draft ratio of 6 to be cooled and solidified. The film thus obtained had a thickness of 65 μm. Next, the cast film was introduced into a 1-1-1 trichloroethane extraction tank for extraction, and 99% or more of the added DCHP was removed.

Subsequently, the extracted film was heat-treated for 3 seconds in an oven heated to 125 ° C., and subsequently stretched 3.5 times at a stretching speed of 5000% / min in the longitudinal direction due to the peripheral speed difference between the inlet nip roll and the outlet nip roll. It was heat set at 140 ° C and wound up.

The film thickness thus obtained was 23 μm, the degree of orientation (ΔNp) was 0.18, and the MD breaking elongation was 143%. The main characteristics are shown in Table 1, and it can be seen that the separator has excellent electric characteristics and reliability as in Example 1.

Comparative Example 1 Intrinsic viscosity [η] 1.85, II = 97.5% polypropylene
A T-die was used from a 40 mmφ extruder to melt-extrude a sheet at an extrusion temperature of 220 ° C., and a 27 μm film with a draft ratio of 40 was wound in a water tank kept at 65 ° C. Subsequently, the film was subjected to a primary heat treatment in a hot air oven under tension at 110 ° C. for 60 seconds, and subsequently stretched 2.0 times in the longitudinal direction at 65 ° C.
The film thus obtained was heat-set under tension for 10 seconds in a hot air oven kept at 145 ° C. to obtain a 25 μm microporous film. The characteristics of this film are shown in Table 1. The average pore diameter is as small as 0.04 μm and the porosity is as low as 42%. Further, it can be seen that since the orientation degree (ΔNp) is as high as 0.48, it is easy to tear during slitting and the characteristics when used as an electrolytic capacitor are inferior.

Comparative Example 2 100 parts by weight of Mitsui Noblene JS type ([η] = 2.3 dl / g, II = 97%) as a polypropylene resin, 120 parts by weight of dioctyl phthalate (DOP) as an organic liquid, and inorganic fine particles having an average particle size of 12 mμ (Aerosil 200, manufactured by Nippon Aerosil Co., Ltd.)
Melt blended and pelletized at 0 ° C. Next this is 40mm
It was melt extruded from a T-die at 220 ° C. using a φ extruder, and cooled and solidified while nipping on a casting drum at 65 ° C. with a draft ratio of 3. The film thus obtained is 300μ
It was m. Next, the film was introduced into a 1-1-1 trichloroethane extraction tank to remove 99% or more of the added DOP.

Then, using a roll stretching device, stretch the film 4.0 times in the longitudinal direction at 140 ° C, and then stretch it in the width direction at 145 ° C with a stenter.
The film was double-stretched, heat-set at 150 ° C. and wound.

The film thus obtained had a thickness of 57 μm. The characteristics are shown in Table 1, with an average pore size of 0.3 μm and a porosity of 8
5%, orientation (ΔNp) 0.01 or less, MD breaking elongation 52%, very brittle and could not be slit.

As is clear from the comparison with Examples and Comparative Examples, the microporous polypropylene film of the present invention exhibits excellent properties, especially for use as a separator for electric elements.

[Advantages of the Invention] The microporous polypropylene film of the present invention can be used not only as a separator for electric elements but also as a microfilter,
When used as a separation membrane, it has the following effects.

(1) The average pore diameter is 0.06 to 3 ηm, and the porosity is
Since it is as high as 50 to 85%, it retains electrolyte well and has excellent filtration characteristics.

(2) By setting the orientation degree ΔNp in the range of 0.05 to 0.30, not only the solvent resistance becomes good, but also the handleability becomes good, and stable characteristics can be obtained as a separator.

(3) Since it has an optimized breaking elongation, the element winding property is improved especially when it is used as a separator for electrolytic capacitors.

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Claims (1)

(57) [Claims] 1. A film having a large number of pores having an average pore diameter of 0.06 to 3 μm, wherein the pores form through holes in the thickness direction of the film, and the porosity is 50 to 85%. A microporous polypropylene film characterized in that the orientation degree ΔNp of the film represented by the following formula is 0.05 to 0.30 and the breaking elongation in the longitudinal direction is 70 to 200%. ΔNp = ΔNb / ΔNa (ΔNb is the birefringence of microporous polypropylene film, ΔNa
Is the value of the extreme birefringence of polypropylene calculated by Smuels' formula, 33 × 10 ^-3 ).