KR100517204B1 - Manufacturing method of polyolefin microporous membrane - Google Patents

Abstract

The present invention is a method for producing a polyolefin microporous membrane having a relatively large pore diameter and excellent permeability with a sharp pore size distribution for use in water treatment, microfiltration membrane, and the like, and contains an ultrahigh molecular weight polyolefin having a weight average molecular weight of 5 × 10 ⁵ or more. A solution consisting of 5 to 40% by weight of a polyolefin composition and 95 to 60% by weight of a solvent is prepared, and the solution obtained is extruded by a die lip, and the sheet obtained by quenching is stretched to a surface magnification of 1.01 to 1.4 times. It is a manufacturing method of the polyolefin microporous membrane.

Description

Method for producing polyolefin microporous membrane

The present invention relates to a method for producing a microporous membrane made of a polyolefin composition containing ultra high molecular weight polyolefin, and more particularly, to a highly permeable polyolefin microporous membrane.

Microporous membranes are used in various applications such as battery separators, electrolytic capacitor diaphragms, various filters, moisture permeable waterproof medical, reverse osmosis filtration membranes, ultrafiltration membranes, and fine filtration membranes.

Conventionally, a method of obtaining a microporous membrane by mixing an organic medium and an inorganic powder such as silica with an organic medium and fine powder silica and then extruding the organic medium and an inorganic powder is known. Is largely dependent on the particle diameter of the inorganic powder, and its control was difficult.

In addition, various methods for producing a high strength microporous membrane using ultra high molecular weight polyolefin have been proposed. For example, Japanese Patent Application Laid-Open Publication No. 60-242035, Japanese Patent Application Laid-Open No. 61-195132, Japanese Patent Application Laid-Open No. 61-195133, Japanese Patent Application Laid-Open No. 63-39602, Japanese Patent Application Laid-Open No. 63-273651, etc. A method of forming a microporous membrane by forming a gel sheet from a solution in which a composition is heated and dissolved in a solvent, heating the gel sheet and extruding the solvent, has been described. It is characterized by a narrow diameter distribution and a small hole diameter, which is suitable for battery separators and the like, but a microporous membrane having a large hole diameter could not be obtained.

In recent years, various permeable microporous membranes are required for each use, and even for the same use, control of the permeability of the membrane is desired in order to improve certain characteristics. However, in the prior invention, a large number of fine holes are formed by the stretching method, and the hole diameter is small, and the microporous membrane, which is a large hole diameter used for water treatment, microfiltration membrane and the like, cannot be obtained, and the development thereof is desired. It was.

MEANS TO SOLVE THE PROBLEM The present inventors extrude the solution with the solvent using the composition containing an ultra high molecular weight polyolefin, and perform a low magnification of a specific range after quenching, and it has a small thermal contraction rate, a large hole diameter, and is sharp. It was found that a microporous membrane having a pore size distribution was obtained, and the present invention was reached.

That is, the present invention is to prepare a solution consisting of 5 to 40% by weight of a polyolefin composition containing an ultra-high molecular weight polyolefin having a weight average molecular weight of 5 × 10 ⁵ or more and 95 to 60% by weight of a solvent, and extruding the solution by a die lip And a sheet obtained by quenching is stretched at a plane magnification of 1.01 to 1.4 times, which is a method for producing a polyolefin microporous membrane.

(The best form to carry out invention)

This invention is demonstrated in detail below.

The polyolefin composition used in the production of the polyolefin microporous membrane of the present invention comprises an ultrahigh molecular weight polyolefin having a weight average molecular weight of 5 × 10 ⁵ or more, preferably an ultrahigh molecular weight polyolefin having a weight average molecular weight of 1 × 10 ⁶ to 15 × 10 ⁶ . It is a polyolefin composition containing 10% by weight or more and having a weight average molecular weight / number average molecular weight, more preferably an ultrahigh molecular weight polyolefin (A) having a weight average molecular weight of 5 × 10 ⁵ or more and a weight average molecular weight of less than 5 × 10 ^5. It is a mixture of polyolefin (B) and consists of a polyolefin composition whose weight ratio of (B) / (A) is 0.2-20, Preferably it is 0.5-10. When the weight ratio of (B) / (A) in the polyolefin composition is less than 0.2, shrinkage in the thickness direction of the obtained gel-like sheet easily occurs, permeability decreases, solution viscosity increases, and moldability decreases. Moreover, when the weight ratio of (B) / (A) exceeds 20, a low molecular weight component will increase, a gel structure will become compact, and the permeability of the obtained microporous membrane will fall.

Examples of the polyolefins include crystalline homopolymers, dimers, or copolymers obtained by polymerizing ethylene, propylene, 1-butene, 4-methyl-pentene-1, 1-hexene, and the like, and mixtures thereof. Among these, polyolefins, polyethylenes (particularly high density polyethylenes), and compositions thereof are preferable.

The molecular weight distribution (weight average molecular weight / number average molecular weight) of the polyolefin composition is 300 or less, particularly preferably 5 to 50. When the molecular weight distribution exceeds 300, breakage due to low molecular weight components occurs and the strength of the whole film is lowered, which is not preferable. Ultra-high molecular weight polyolefin having a weight average molecular weight of 5 × 10 ⁵ or more and polyolefin having a weight average molecular weight of less than 5 × 10 ⁵ can be obtained by appropriately mixing the molecular weight distribution so as to fall within the above range.

Moreover, it is preferable that it is 1 * 10 <^4> or more as a minimum of molecular weight of polyolefin (weight average molecular weight less than 5 * 10 <^5> ) other than ultra high molecular weight polyolefin in a polyolefin composition. When the polyolefin having a weight average molecular weight of less than 1 × 10 ⁴ is used, breakage is likely to occur and the desired microporous membrane cannot be obtained, which is not preferable. Therefore, it is preferable to mix | blend the polyolefin whose weight average molecular weight is 1 * 10 <^4> or less than 5 * 10 <^5> in an ultrahigh molecular weight polyolefin.

In addition, various additives such as antioxidants, ultraviolet absorbers, blocking blockers, pigments, dyes, inorganic fillers, and the like may be added to the polyolefin containing the ultrahigh molecular weight components as described above in a range that does not impair the object of the present invention. have.

In the method for producing a microporous membrane of the present invention, a solution is prepared by dissolving the polyolefin composition described above in a solvent. As this solvent, aliphatic or cyclic hydrocarbons such as nonane, decane, decalin, p-xylene, undecane, dodecane, liquid paraffin, or mineral oil having a boiling point corresponding thereto can be used. Moreover, as viscosity of this solvent, it is preferable that the viscosity in 25 degreeC is 30-500 cSt, especially 50-200 cSt. If the viscosity at 25 ° C. is less than 30 cSt, non-uniform discharge occurs and kneading is difficult. On the other hand, if the viscosity is higher than 500 cSt, the desolvent in the subsequent step is not easy.

The heat dissolution is performed by stirring the polyolefin composition at a temperature at which the polyolefin composition is completely dissolved in a solvent, or by homogeneously mixing and dissolving the polyolefin composition in an extruder. In the case of dissolving while stirring in a solvent, the temperature varies depending on the polymer and the solvent to be used. When preparing a microporous membrane in the high concentration solution of a polyolefin composition, it is preferable to melt | dissolve in an extruder.

When dissolving in an extruder, the above-mentioned polyolefin composition is first supplied to an extruder, and it melt | dissolves. Although melting temperature changes with kinds of polyolefin to be used, melting | fusing point + 30-100 degreeC of a polyolefin is preferable. For example, in the case of polyethylene, it is preferable that it is 160-230 degreeC, especially 170-200 degreeC, and in the case of polypropylene, it is preferable that it is 190-270 degreeC, especially 190-250 degreeC. Next, the liquid solvent is supplied to the molten polyolefin composition in the middle of the extruder.

The blending ratio of the polyolefin composition and the solvent is 100% by weight of the total polyolefin composition and the solvent, 5 to 40% by weight of the polyolefin composition, preferably 10 to 30% by weight, 95 to 60% by weight of the solvent, Preferably it is 90-70 weight%. If the polyolefin composition is less than 5% by weight (when the solvent exceeds 95% by weight), when forming into a sheet shape, swelling and neck in at the die exit are large, and thus the moldability and self-supportability of the sheet become difficult. On the other hand, when the polyolefin composition exceeds 40% by weight (when the solvent is less than 60% by weight), the shrinkage in the thickness direction becomes large, the porosity decreases, and a microporous membrane having a large pore diameter cannot be obtained. In addition, molding processability is also lowered. By changing the concentration in this range, the permeability of the membrane can be adjusted.

Next, the heating solution of the polyolefin composition melt-kneaded in this way is extruded through an extruder so as to be extruded and molded into a film thickness of 5 to 250 µm in a final product.

As the die, a rectangular die having a sheet die is usually used, but a hollow cylindrical die, an inflation die, or the like can also be used. The die gap in the case of using a sheet die is usually 0.1 to 5 mm, and is heated to 140 to 250 ° C at the time of extrusion molding. At this time, the extrusion speed is usually 20 to 30 cm / min to 15 m / min.

In this way, the solution extruded from the die is formed into a gel-like molded product by quenching. Cooling is carried out at a rate of at least 50 ° C / min up to 90 ° C or less, preferably up to 80-30 ° C. As a cooling method of a gel-like sheet | seat, the method of making direct contact with cold air, cooling water, another cooling medium, the method of contacting the roll cooled with a refrigerant | coolant, etc. can be used, but the method of using a cooling roll is preferable.

If the cooling rate is slow, the higher-order structure of the gel-formed product is denser, and the dummy cell units forming it are also larger, but if the cooling rate is faster, the cell units are denser. If the cooling rate is less than 50 ° C./min, the gel structure becomes closer to the independent cloth, and the degree of crystallinity also increases, making it difficult to remove the solvent.

Next, this gel-shaped molded product is stretched. Stretching is performed at a predetermined magnification by heating the gel-shaped molded article and combining a usual tenter method, a roll method, rolling, or these methods. The stretching may be uniaxial stretching or biaxial stretching. In the case of biaxial stretching, either longitudinal stretching or sequential stretching may be used, but simultaneous biaxial stretching is particularly preferable.

The stretching temperature is below the crystal melting point of + 10 ° C at the crystal dispersion temperature of the polyolefin, preferably below the crystal melting point at the crystal dispersion temperature. For example, polyethylene composition is 90-130 degreeC, More preferably, it is the range of 90-120 degreeC. When extending | stretching temperature exceeds melting | fusing point +10 degreeC, the orientation of the molecular chain by extending | stretching is impossible by melting of resin. In addition, when the stretching temperature is less than the crystal dispersion temperature, the softening of the resin is insufficient, the film tends to rupture during stretching, and the stretching magnification cannot be controlled.

The draw ratio needs to be 1.01 to 1.4 times, preferably 1.05 to 1.25 times, in terms of face magnification. By controlling the surface magnification, it is possible to obtain a microporous membrane having a pore diameter of 0.05 to 0.5 m and a sharp pore size distribution. Stretching is not performed, and the hole diameter is large, but the hole diameter distribution is widened, which is not preferable. When the draw ratio is more than 1.4 times, the lamellar of the polyolefin forming the gel structure is cleaved and fibrilsed so that the pore diameter becomes small, and a large pore diameter microporous membrane cannot be obtained.

Moreover, the obtained molded object is wash | cleaned with a solvent and the residual solvent is removed. Examples of the cleaning solvent include volatile hydrocarbons such as hydrocarbons such as pentane, hexane and heptane, chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride and fluorocarbons such as ethane trifluoride, ethers such as diethyl ether and dioxane, and the like. Can be used. These solvents are appropriately selected depending on the solvent used for dissolving the polyolefin composition, and used alone or in combination. The washing method can be performed by a method of immersing and extracting a solvent, a method of showering a solvent, a method by a combination thereof, or the like.

The cleaning as described above is performed until the residual solvent in the molded product is less than 1% by weight. Thereafter, the cleaning solvent is dried, but the drying method of the cleaning solvent can be performed by a method such as heat drying or air drying. The dry stretched molded product is preferably heat-set in the temperature range of crystal dispersion temperature to melting point.

The polyolefin microporous membrane produced as mentioned above is a highly permeable membrane whose air permeability is 5-250 sec / 100 cc, porosity 35-95%, and average through-hole diameter are 0.05-0.5 micrometer.

And the obtained polyethylene microporous membrane can further perform surface modification, such as hydrophilization treatment, such as plasma irradiation, surfactant impregnation, and surface graft, as needed.

Although an Example is given and this invention is demonstrated to it in more detail below, this invention is not specifically limited to an Example. And the test method in an Example is as follows.

(1) Film thickness: The cross section is measured by a scanning electron microscope.

(2) Air permeability: It measured based on JISP8117.

(3) Average pore size: measured by Coulter Prometer (manufactured by Coulter).

(4) Hole diameter distribution: It was calculated from the standard deviation of the hole diameter distribution in the hole diameter and the number of holes.

(5) Thermal contraction rate: It was measured by blast furnace for 8 hours in 105 degreeC atmosphere.

Example 1

In a polyethylene composition consisting of 80% by weight of high density polyethylene (HDPE) having a weight average molecular weight of 3.0 × 10 ⁵ and 20% by weight of ultra high molecular weight polyethylene (UHMWPE) having a weight average molecular weight of 2.5 × 10 ⁶ , an antioxidant is added per 100 parts by weight of the polyethylene composition. The polyethylene composition which added 0.375 weight part was obtained. 30 parts by weight of this polyethylene composition was introduced into a twin screw extruder (58 mm, L / D = 42, strong kneading type). Further, 70 parts by weight of the liquid paraffin was supplied from the side feeder of the twin screw extruder, and 200 ° C. Melt kneading was carried out at 200 rpm, the polyethylene solution was prepared in an extruder, extruded so that the final product might be 50-60 micrometers from the T-die installed in the tip of an extruder, and it pulled out with the cooling roll temperature-controlled at 50 degreeC, and formed the gel sheet. . Subsequently, this gel-like sheet was uniaxially stretched at 115 ° C. so that the surface magnification was 1.2 times to obtain a low magnification stretched film. The obtained membrane was washed with methylene chloride, followed by extraction and removal of the remaining liquid paraffin, followed by drying and heat treatment to obtain a polyethylene microporous membrane having a thickness of 62 µm. Table 1 shows the results of the physical property evaluation of this polyethylene microporous membrane.

Example 2

In Example 1, a microporous membrane was obtained in the same manner as in Example 1 except that the draw magnification was 1.1 times. Table 1 shows the physical properties of the obtained microporous membrane.

Example 3

In Example 1, a microporous membrane was obtained in the same manner as in Example 1 except that the draw ratio was 1.4 times the surface magnification. Table 1 shows the physical properties of the obtained microporous membrane.

Example 4

In Example 1, a microporous membrane was obtained in the same manner as in Example 1 except that the stretching ratio was coaxially stretched so that the surface magnification was 1.4 times. Table 1 shows the physical properties of the obtained microporous membrane.

(Comparative Example 1)

A microporous membrane was obtained in the same manner as in Example 1 except that the stretching magnification was biaxially stretched so that the surface magnification was 25 times. Table 1 shows the physical properties of the obtained microporous membrane.

(Comparative Example 2)

In Example 1, a microporous membrane was obtained in the same manner as in Example 1 except that the stretching ratio was biaxially stretched so that the surface magnification was four times. Table 1 shows the physical properties of the obtained microporous membrane.

(Comparative Example 3)

In Example 1, the microporous membrane was obtained like Example 1 except having uniaxially stretched the draw ratio so that surface magnification might be 4 times. Table 1 shows the physical properties of the obtained microporous membrane.

(Comparative Example 4)

In Example 1, the microporous membrane was obtained like Example 1 except not extending | stretching. Table 1 shows the physical properties of the obtained microporous membrane.

As is apparent from the Examples and Comparative Examples of Table 1, it can be seen that by performing low magnification stretching within the scope of the present invention, a microporous membrane having a low thermal contraction rate, a large hole diameter, and a sharp hole diameter distribution can be obtained. .

TABLE 1

As described above, the gel sheet obtained from the polyolefin composition solution containing the ultra high molecular weight polyolefin, which is the method of the present invention, is stretched in a specific range at a low magnification, so that it is excellent in permeability and has a sharp pore size distribution. Effective polyolefin microporous membrane can be obtained.

Claims (3)

It is a mixture of the ultrahigh molecular weight polyolefin (A) whose weight average molecular weight is 1 * 10 <^6> -15 * 10 <^6> , and the polyolefin (B) whose weight average molecular weight is 1 * 10 <^4> or more and less than 5 * 10 <^5> , (B) / (A) Was prepared by preparing a solution composed of 5 to 40% by weight of a polyolefin composition having a weight ratio of 0.2 to 20 and 95 to 60% by weight of a solvent, extruding the solution by a die lip, and quenching the sheet obtained by quenching. A method for producing a polyolefin microporous membrane, which is stretched by pear, and after removal, residual solvent is removed, dried and heat-set. The method for producing a polyolefin microporous membrane according to claim 1, wherein the draw ratio is 1.05 to 1.25 times. The method for producing a polyolefin microporous membrane according to claim 1, wherein the stretching ratio is 1.05 to 1.25 times, and after stretching, the residual solvent is removed, dried, and heat set.