JPS5832171B2 - Method for manufacturing porous membrane - Google Patents

Description

[Detailed description of the invention] The present invention consists of a polyolefin resin and an inorganic fine powder,
The present invention relates to a method for manufacturing a porous membrane having many fine pores and excellent mechanical strength.

The present invention also relates to the production of a porous membrane that is water-wettable, exhibits an unprecedentedly low electrical resistance in an electrolytic solution, and is useful as a storage battery separator or an electrolytic diaphragm.

Conventionally, (1) a method for producing a porous membrane comprising kneading a resin solvent, a plasticizer, and silica with vinyl chloride resin, molding, and drying (Japanese Patent Publication No. 37-2922), (2) vinyl chloride resin powder. A method for producing a porous membrane by sintering a mixture of and fine silica powder (Japanese Patent Publication No. 35-3092), (
3) SLMI = o, a method for producing a porous membrane comprising melt-molding a mixture of a high molecular weight polyolefin resin having a weight average molecular weight of 300,000 or more and silica process oil, and then extracting the process oil (Japanese Patent Publication No. 45-320
97) etc. are known.

When porous membranes are used as battery separators, low electrical resistance is required in the electrolyte.

Especially in recent years, as a high-performance separator, at least 0.001Ω
It is desired that the electrical resistance be 7 dm" or less (measured according to JISC-2313), preferably 0.0006 Ωa, j/sheet or less.

In order to satisfy this requirement, for films with a film thickness of 0.2% or more, the electrical resistance per 0.1% thickness must be reduced to 0.
0003 Ωdm' or lower than ever before, or 0.
A film having an electrical resistance of 0003Ωdm2/Q and a thickness of 1 mm or more needs to be a thin film of 0.2% or less.

However, the porous vinyl chloride resin membranes produced in (1) and (2) lack mechanical strength and flexibility, so if the thickness is 0.4 mm or less, a practically effective membrane cannot be obtained. .4 or higher, the electrical resistance is 0.001Ωdi
``With 7 or more membranes, it is impossible to obtain a membrane that meets the requirements for a high-performance diaphragm.

In addition, (3) has a molecular weight of 30 to give flexibility to the membrane.
0,000 or more, a high molecular weight polyolefin with SLMI=O is used.

This solves the problem of flexibility, but the moldability is poor, and the electrical resistance per unit thickness is 0.0003 Ωd m / 0.
．． 0.0006 Ωdm"7 because it is higher than 1 mrn
It is difficult to produce high-performance battery separators with electrical resistances of less than 100 nm.

The inventors of the present invention have conducted extensive research to realize a porous membrane with practically sufficient mechanical strength and flexibility, and low electrical resistance.

We have now invented a method for producing a porous membrane that has unprecedentedly low electrical resistance, good thin film formability, sufficient mechanical strength, and flexibility.

The present invention makes it possible to achieve an electrical resistance as low as 0.0003 Ωd m"/0.1 or less, and to make the film as thin as 0.05%, dramatically reducing the electrical resistance to 1/10 of conventional products. This made it possible to realize a small, high-performance storage battery separator.

That is, the present invention provides polyolefin resins having a weight average molecular weight of less than 300,000 and a number average molecular weight of 15,000 or more.
60% by volume, 7 to 42% by volume of inorganic fine powder, polyolefin:inorganic fine powder = 2:3 to 9:1, and 30 to 75% by volume of organic liquid having a solubility parameter (SP value) of 8.4 to 9.9. This is a method for manufacturing a porous membrane by mixing the mixture, melt-molding the mixture, and then extracting the organic liquid from the molded product using a solvent for the organic liquid.

The first feature of the present invention lies in the selection of raw materials.

The polyolefin resin needs to have a weight average molecular weight (Mw) of at least less than 300,000, preferably 250,000 or less, and a number average molecular weight (Mw) of at least 250,000 or less.
n) is required to be 15,000 or more, preferably 17,000 or more.

When a polyolefin resin having Mn of less than 15,000 is used, the resulting porous membrane has an extremely small elongation of a few percent and is brittle (-, it cannot be put to practical use.

Polyolefin resins with an Mw of 300,000 or more have low fluidity during melting, resulting in poor moldability, and furthermore, the average pore diameter on the surface of the network structure formed by the polyolefin resin is 0.05μ or less, resulting in a decrease in pore area. The porosity also decreases, and the electrical resistance increases to more than 0.0003 Ωd m2/0.1 myin, which is not preferable.

As the polyolefin resin in the present invention, Mw<
300000. As long as Mn≧15,000, polyethylene, polypropylene, polybutene, a mixture thereof, or a copolymer of two or more of ethylene, propylene, butene, and hexene may be used.

Among these resins, polyethylene or a copolymer mainly composed of ethylene is particularly preferred.

The inorganic fine powder holds the organic liquid and functions as a carrier.

That is, it prevents the release of organic liquid during melt molding and facilitates molding.

Furthermore, the inorganic fine powder imparts water wettability to the porous membrane.

The inorganic fine powder is preferably a fine particle or porous particle having an average particle size of 0.005 to 0.5B and a specific surface area of 50 to 500 m/9. It is preferable that both absorb 2/3 times the volume, preferably 3 times the volume or more, and maintain a powder or granule state.

Further, when one type of inorganic fine powder is used, it is always hydrophilic, and when two or more types are used in combination, it is preferable that at least one type is hydrophilic.

Examples of the inorganic fine powder used in the present invention include finely divided silicic acid, calcium silicate, aluminum silicate, alumina, calcium carbonate, magnesium carbonate, kaolin clay, finely divided talc, titanium oxide, diatomaceous earth, and carbon black.

Finely divided silicic acid is particularly effective for use as separators for lead-acid batteries.

However, it is not limited to this.

The organic liquid used in the present invention is extracted from the molded product and imparts porosity to the molded product.

The organic liquid is required to be liquid and inert during melt molding.

Furthermore, the organic liquid has a solubility parameter (SP value) of 8.
Must be in the range of 4 to 9.9.

By using a material within this range, during melt molding, the organic liquid is appropriately dissolved in the polyolefin resin, and most of the organic liquid is adsorbed on the surface of the inorganic fine powder.

As a result, a porous membrane with good moldability, extractability, mechanical strength, and low electrical resistance can be obtained.

When the SP value of the organic liquid exceeds 9.9, the solubility in the resin decreases and the organic liquid becomes liberated during molding.

This prevents welding of resins together, resulting in poor moldability.
and the average pore diameter of the network structure of the polyolefin resin is 0.
It becomes coarse when it is 5μ or more, and its strength and elongation decrease.

Moreover, when the SP value is less than 8.4, the amount dissolved in the polyolefin resin increases and the amount adsorbed to the inorganic fine powder becomes small.

As a result, the melt is sufficiently welded, and the formability and mechanical strength are improved, but the pore-forming properties of the organic liquid are reduced, and the average pore diameter of the polyolefin structure is 0.05μ or less, and Porosity also decreases.

Furthermore, the extractability deteriorates and the extraction time also increases.

Examples of organic liquids having an SP value of 8.4 to 9.9 used in the present invention include diethyl phthalate (DEP), dibutyl phthalate (DBP), and dioctyl phthalate (
DOP), etc., sebacate esters such as dioctyl sebacate (DO8), adipate esters such as dioctyl adipate, trimellitic acid esters such as trioctyl trimellitate (TOTM), tributyl phosphate (TBP), IJN Examples include phosphoric acid esters such as octyldiphenyl acid, and glycols such as polyethylene glycol.

The manufacturing method of the present invention will be explained in more detail.

7 to 42% by volume, preferably 10 to 2% by volume based on the total volume of the inorganic fine powder, organic liquid and polyolefin resin
The following three components are mixed: 0% by volume of inorganic fine powder, 30-75% by volume, preferably 50-70% by volume of organic liquid, and 10-60% by volume, preferably 15-40% by volume of polyolefin resin.

At this time, if the amount of inorganic fine powder is less than 7% by volume, it will not be possible to adsorb the organic liquid necessary to make an effective porous membrane, the mixture will not be able to maintain a powder or granule state, and molding will be difficult. It becomes difficult.

On the other hand, if it exceeds 42% by volume, the fluidity during melting will be poor and the resulting molded product will be brittle (unable to be put to practical use).

Furthermore, if the amount of the organic liquid is less than 30% by volume, the contribution rate of the organic liquid to pore formation decreases, and the porosity of the resulting porous membrane is less than 30%, making it substantially effective as a porous membrane. is not obtained.

On the other hand, if it exceeds 75% by volume, molding becomes difficult and a porous membrane with high mechanical strength cannot be obtained.

If the polyolefin resin is less than 10% by volume, the resin content is too small, resulting in low strength and poor moldability.

If it exceeds 60% by volume, it is not preferable because a porous membrane with a low porosity cannot be obtained.

The composition used in the present invention is mainly composed of three components: a polyolefin resin, an inorganic fine powder, and an organic liquid.

However, there is no problem in adding other lubricants, antioxidants, ultraviolet absorbers, plasticizers, molding aids, etc. as necessary, as long as they do not significantly impair the effects of the present invention.

A conventional mixing method using a blender such as a Henschelmigisser V-blender or a ribbon blender is sufficient for mixing these three components.

Rather than mixing the three components at the same time, the order of mixing the three components is to first mix the inorganic fine powder and the organic liquid so that the inorganic fine powder sufficiently absorbs the organic liquid, and then mix the polyolefin resin in (. The purpose of mixing is to improve melt formability,
This is effective in improving the porosity and mechanical strength of the resulting porous membrane.

This mixture is kneaded using a melt kneading device such as an extruder, a Banbury mixer with 1 and 2 rolls, or a kneader.

The obtained kneaded material is molded into a film shape by a melt molding method, and the melt molding method used in the method of the present invention is as follows:
Examples include extrusion molding such as the T-die method and inflation method, calendar molding, compression molding, and injection molding.

It is also possible to directly mold the mixture using an extruder, kneader-ruder, or other device having both kneading and extrusion functions.

These molding methods mold the ternary mixture into a 0.05-1% film.

In particular, T-die extrusion molding is particularly effective for forming thin films of 0.10 to 0.30%.

The organic liquid is extracted from the obtained film-like molded product at a temperature below the melting point of the polyolefin resin using a solvent for the organic liquid used.

However, the solvent must not substantially dissolve the polyolefin resin.

Extraction is easily carried out using common extraction methods for membrane-like materials, such as a batch method or a countercurrent multi-stage method.

Solvents used for extraction include general solvents such as alcohols such as methanol, ethanol, and Improvil alcohol, ketones such as acetone, and chlorinated hydrocarbons such as 1,1,1-)'J chloroethane and trichlorethylene. That's enough.

Furthermore, the organic liquid is allowed to remain in the porous membrane after extraction to the extent that it does not impair the performance of the membrane.

However, if the residual amount is large, the porosity of the membrane decreases, which is not preferable.

The amount of organic liquid remaining in the porous membrane is 5% by volume or less, preferably 2% or less.

After the extraction is completed, the solvent is removed by drying the film at a temperature below the melting point of the polyolefin resin.

Drying is carried out under normal pressure or reduced pressure by a common method such as hot air or heated rolls.

The porous membrane produced according to the present invention is substantially formed of a mixture of polyolefin resin and inorganic fine powder and 30 to 75% pores.

The composition of the polyolefin resin and inorganic fine powder is 40 to 90% by volume of polyolefin resin and io to 60% by volume of inorganic fine powder.
% by volume, organic liquid O~5% by volume.

For applications that require flexibility, moisture resistance, and high porosity, such as storage battery separators, a composition of 50 to 80 volume % polyolefin resin, 20 to 50 volume % inorganic fine powder, and 0 to 2 volume % organic liquid is used. Preferably within that range.

The structure of the porous membrane produced according to the present invention is formed by continuous pores formed by a polyolefin network structure and inorganic fine powder filled in the pores.

The pore diameter of this polyolefin resin network structure is determined by the weight average molecular weight of the polyolefin resin and the SP value of the organic liquid.

The average surface pore diameter of the polyolefin resin network structure is 0.
05-0.5μ, preferably 0.08-0.3μ.

Further, the average pore diameter of the porous membrane filled with inorganic fine powder is in the range of 0.01 to 0.1 μ.

Further, the porosity of the porous membrane according to the present invention is at least 30 to 75.
%, preferably 50 to 70% for applications such as storage battery separators.

The porous membrane according to the present invention has a thickness of 0.05 to 1 inch.

In view of sufficient film strength and electrical resistance, a film thickness of 0.10 to 0.30% is particularly preferred for applications such as storage battery separators.

As described above, the porous membrane according to the present invention is a highly porous thin membrane with a fine network structure, has good mechanical strength and excellent flexibility, and has an unprecedentedly low electrical resistance of 0.0006 Ωdm2/sheet. By using the porous membrane of the present invention as a separator, it has become possible to significantly improve the performance of lead-acid batteries compared to those of the same capacity.

In addition, the porous membrane according to the present invention can be used not only as a storage battery separator but also as a filter, packaging material, liquid retention material, fragrance preservation material, etc.
・I can do things.

Next, Examples and Comparative Examples will be shown to clarify the effects of the present invention.

However, the present invention is not limited to these examples.

In addition, the various physical properties shown in the specification and examples of the present invention were measured by the following measurement method.

Weight average molecular weight (MW) Number average molecular weight (MP) GPC measuring device - Waters Model 1 200
Column - Toyo Soda G70008~3000S Solvent - Trichlorobenzene measurement temperature - 135°C Viscosity average molecular weight (Mv) (Mv≠buW) Solvent - Decalin measurement temperature - 135°C [η) = 6.20X10-4Mv' (Chiang
The weight average molecular weight of polyethylene with SLMI=O was calculated based on the property.

Composition ratio (volume%) Calculated from the value obtained by dividing the added weight of each composition by the true specific gravity.

Porosity (%) Porosity - pore volume / porous membrane volume x 100 pore volume - water content - bone dry weight Polyolefin network structure surface average pore diameter (μ) Inorganic fine powder was extracted and removed from the porous membrane Openings 20 observed in a scanning electron micrograph of the surface of a polyolefin network structure
Calculated by weighted average of the long axis and short axis of 0 pieces.

Specific surface area (m2/g) Porous membrane average pore diameter (μ) measured by BET adsorption method Calculated using the following formula d: Diameter (μ) S-specific surface area (m”/y) V: Pore volume (ml/g) Fracture Strength (kg/m”), elongation at break (%) Measured using an Instron tensile tester according to ASTM D-882.

(Strain rate 2.0 degrees/mmmη) Electrical resistance (Ωdm/piece, Ωdrr170.1 mm
) Measured according to JIS-C-2313.

Air permeability (sec/1100rIll) ) Measured by AST D-726 Method A.

Bending strength measured according to JIS-P-8115 MIT type testing machine Tension 0.3 kg/15 width SLMI: ASTM-D-1238-65T Measured according to condition E Dissolution parameter (SP value) Calculated according to the following formula d: Specific gravity G: Molar traction constant M: Molecular weight Example 1 Finely divided silicic acid [Aerosil #200 (trade name), specific surface area 1
75 m"/?, average particle size 16 mμ] 15% by volume and 61% by volume of dioctyl phthalate (DOP) were mixed in a Henschel mixer, and to this, Mw = 85000, Mn = 21
000, polyethylene C5untec with SLMI=1
S-360 (trade name)] 24% by volume of resin was added and mixed again using a Henschel mixer.

The mixture was kneaded into pellets using a 30%φ twin screw extruder.

The pellets were molded using a film manufacturing device equipped with a 30%φ twin-screw extruder and a 420% width T-die.

The extrusion amount at this time was 12 kg/Hr, and the take-up speed was 2 m.
The capacity was 7mlyr, and the resin pressure was 70kg/crA.

The formed membrane was immersed in 1,1,1-trichloroethane (chlorocene) for 5 minutes to extract DOP.

The thickness of the obtained membrane was 0.135%, the porosity was 58%, and the composition of the porous membrane base material was 61.5% by volume of polyethylene resin, 38.3% by volume of finely divided silicic acid, and 1212% by volume of DOP O.

The porous membrane had a tensile strength of 29 kg/crtl, an elongation at break of 106%, and a bending strength of 2,400 times, exhibiting excellent elongation and flexibility.

Average pore size formed by resin (according to electron microscopic observation) 0.10μ
, the average pore diameter when filled with fine silicic acid is 0.0\1
2μ (measured by BET adsorption method) with a maximum pore diameter of 0.09μ? Ta.

The electrical resistance of this film was extremely low at 0.00022 Ωd, m'/sheet.

Also, when a drop of water was dropped on the membrane surface, it was instantly absorbed.

Example 2 13% by volume of finely divided silicic acid used in Example 1, 4% by volume of polyethylene para 4'-34% and trimellitic acid (TOTM,
A porous membrane was made according to Example 1 using 53% by volume (SP value 8.9).

The composition of the obtained porous membrane was 72.2% by volume of polyethylene;
The contents were 27.5% by volume of fine silicic acid and 3% by volume of TOTM O.

The porosity of the membrane was 49%, and the membrane thickness was 0.085%.

The performance of the porous membrane is as follows.

Breaking strength: 62 to 9. /crA, elongation at break 201%, folding strength 10000111 or more, electrical resistance 0.00033
Ωdrrl/sheet, and water wettability was good.

Example 3 Finely divided silicic acid [Nibsil VN-3 (trade name), Hiroku area 28
0 m”/f, average particle size i 6mtt]i 3.6% by volume, dioctyl phthalate (DOP) 60.8% by volume,
Mw=85000, Mn=21000, SLMI=
1 polyethylene resin C5untec S-360
(trade name), :125-6% by volume was used to produce a porous membrane according to Example 1.

As in Example 1, the obtained porous membrane had extremely good strength and elongation and membrane performance.

The performance is shown in Table 1. In addition, on the surface of the polyethylene network structure from which silicic acid has been extracted, there are 4 to 6 × 1 pores with an average size of 0.12μ.
There were 08 pieces/crtt.

Example 4 Mw = 180000, Mn = 17000°SLMI
=0.04 polyethylene, Mw=85000, Mn
= 21000. Polyolefin resin prepared by mixing polyethylene with SLMI=1 in a ratio of 7:3 (mixture SL
Example 3 was followed except that MI=0.1) was used.

The obtained film had improved strength compared to Example 3 and was extremely good.

The performance is shown in Table 1.

Example 5 Propylene-ethylene copolymer resin with Mw = 110,000 and Mn = 18,000 (ethylene:propylene = 99
．． Example 1 was followed except that 1:0.9) was used.

Table 1 shows the performance of the obtained membrane. Examples 6-8 Examples 6, 7, and 8 are shown in Table 1, and both exhibited good mechanical strength and membrane performance.

Comparative example I Mw=120000. Mn=11000, SLMI=
The film was formed according to Example 1 except that 0.3% polyethylene was used.

The composition of the obtained porous membrane was 61.4% by volume of polyethylene.
The content of fine powder silicic acid was 38.5% by volume.

The porosity was 56%, and the film thickness was o, 2s5z.

The performance of the porous membrane was as follows.

The tensile strength was 21 kg/crA, the elongation at break was 11%, and the folding strength was 2 times, and it lacked flexibility and was brittle.

The electrical resistance was 0.00042 Ωdi/piece.

Comparative Examples 2-3 Mw=83000. Mn=7500, SLMI=0.
A film was formed according to Example 1, except that polyethylene No. 9 was used.

The resulting membrane was extremely brittle, with an elongation at break of 2% and a bending strength of 0 times.

The membrane performance is shown in Table 1.

Similarly, Comparative Example 3 is shown in Table 1. Comparative example 4 Mw=330000. Mn=20000SLMI=0
Example 3 was followed except that polyethylene was used.

The breaking strength and elongation of the obtained membrane was 48 kg/crA, 242%.
Although this is improved compared to Example 3, the porosity is small at 51.5% and the electrical resistance is 0.00033Ωd m”10.1
mm, 0.00083Ωa, and 17 pieces.

Comparative example 5 West'w =600000. Example 3 was followed except that polyethylene with SLMI=0 was used.

Due to the large molecular weight, moldability deteriorates, and the resulting film thickness is 0.
．． It was 3Qmm.

The membrane had a strong elongation of 63 kg/crrf and 195%, and was extremely thick, but a porosity of 51% and an electrical resistance of 0.0004.
0Ωd m”/0.1 mm, 0.0012Ωd m
” / large.

Furthermore, the surface average opening diameter of the polyethylene network structure from which silicic acid was extracted was 0.03 μm, which was significantly reduced compared to 0.12 μm in Example 3.

Comparative example 6 Mw=330000. 15% by volume of polyethylene with Mn=20000, finely divided silicic acid [Nibsil VN-3 (trade name)]
A porous membrane was produced according to Example 1 using a 15 volume process oil (SP value 7.9).

Petroleum ether was used to extract the process oil.

The obtained film had both strength and elongation that were Δ smaller than those of Comparative Example 4.

The porosity is 56% compared to the amount of process oil added.
k was small and the electrical resistance was high.

Comparative Example 7 Comparative Example 6 was followed except that polyethylene resin with Mw = 600,000 was used.

Although the strength of the obtained membrane was improved, the porosity was reduced to 52% and the electrical resistance was high.

The pore size on the membrane surface becomes so small that it cannot be observed with an electron microscope.

Comparative Example 8 Example 3 was followed except that dimethyl phthalate with an SP value of 10.5 was used as the organic liquid.

The average pore diameter of the obtained membrane was determined by the network structure (shift of 0.62μ
and the breaking strength and elongation are 17.3 and ρ/crr.
It was weak and brittle at t, 40% (see Table 2).

Comparative Example 9 Example 3 was followed except that a process oil with an SP value of 7.8 was used as the organic liquid.

Petroleum ether was also used as an extractant.

In the obtained film, 5.6% of process oil remained.

Although the membrane has high strength and elongation, the porosity is small at 50% and the electrical resistance is 0.00074Ωd m”/0.11n11L.
It was big.

Furthermore, the surface pore diameter was too small to be measured using an electron microscope (see Table 2).

Example 9 A porous membrane was manufactured in accordance with Example 3 except that dibutyl phthalate (DBP) with an SP value of 9.4 was used.

The average pore diameter of the obtained membrane was 0.30μ, which was larger than that of Example 3.

Other physical properties were good.

Table 2 shows the performance of the obtained membrane. Example 10.11
, 12.13.14.15.16 Example 3 was followed except that various esters having different SP values were used as the organic liquid.

Table 2 shows the performance of the obtained membrane.

As the SP value of polyethylene approaches 7.9, there is a tendency for mechanical strength to improve and membrane performance to deteriorate.

Comparative Example 10 Mw=250000, Mn=1800 used in Example 8
Comparative Example 6 was followed except that No. 0 polyethylene was used.

After extracting the process oil with petroleum ether, the remaining amount of process oil was 5%.

The thickness of the obtained porous membrane was 0.297 mm, and the porosity was 58.
%, and the electrical resistance is 0.00084 Ωdi/sheet (0,
00028Ωd m”/0.1 mm), but the tensile strength was 19.8 kg/crr!, the elongation at break was small and brittle, 6%.

Compared to the porous membrane obtained in Example 8 using the same polyethylene, both electrical resistance and tensile strength and elongation were significantly inferior.

Claims (1)

[Claims] 1 Inorganic fine powder 7-42% by volume, SP value 8.4-9.9
organic liquid 30-75% by volume, weight average molecular weight 300
A microporous porous membrane characterized by mixing 10 to 60 volume % of a polyolefin resin having a number average molecular weight of less than 1,000 and a number average molecular weight of 15,000 or more, followed by melt molding, and then extracting an organic liquid from the molded product. Production method.