JP7427975B2 - Manufacturing method of polyolefin microporous membrane - Google Patents

Description

The present invention relates to a method for producing a microporous polyolefin membrane.

Microporous membranes have various pore sizes, pore shapes, and pore numbers, and because of the characteristics that can be expressed due to their unique structure, they are used in a variety of applications such as filters such as filtration membranes and dialysis membranes, battery separators, and electrolytic capacitor separators. Used in the fields of Among these, microporous membranes made of polyolefin as a resin material are widely used as separators for secondary batteries because they have excellent chemical resistance, insulation properties, mechanical strength, etc., and have shutdown characteristics. In these applications, in order to ensure battery safety, the uniformity of the physical properties of the microporous membrane and the presence or absence of defects greatly affect the quality of the product.
One example of a microporous membrane manufacturing process for controlling these properties is a melt-kneading process in which a resin as a raw material and a plasticizer are melt-kneaded to create a resin composition. In this step, if the resin/plasticizer is not well dispersed, uneven kneading of the resin composition will occur, leading to uneven physical properties and defects in the microporous membrane.
For example, Patent Document 1 proposes introducing a diluent into an extruder at least two places in order to improve the miscibility of the polymer and the diluent. However, in the method disclosed in Patent Document 1, there is room for improvement in terms of improving mixability and suppressing the occurrence of defects.

Special Publication No. 2011-500368

In wet film formation, when two or more types of polyolefin resins are melt-kneaded, there is a problem in that the film after film formation has granular defects due to poor kneading of the polyolefin resin composition. If a film is formed with these granular defects occurring during the melt-kneading stage, it may lead to poor appearance of the film, local stretching unevenness, or film breakage, which is one of the causes of reduced productivity. This drawback tends to become more noticeable as the extruder discharge rate increases and the composition of the resin composition increases, and is one of the constraints on improving production capacity. In view of the above problems, an object of the present invention is to provide a method for producing a microporous polyolefin membrane that improves the grain defects of the film.

Under the circumstances of the previous period, the inventors of the present invention conducted extensive studies and discovered that by controlling the temperature of the plasticizer added to the extruder, the granular defects caused by poor kneading can be improved.
That is, the present invention is as follows.
A) A step of charging a polyolefin resin and a plasticizer into an extruder, melting and kneading them to form a polyolefin resin composition, and discharging the polyolefin resin composition from a die in the form of a sheet;
B) a step of stretching the polyolefin resin composition discharged into a sheet, and C) a step of removing the plasticizer from the stretched polyolefin resin composition.
A method for producing a microporous polyolefin membrane, which satisfies a) and b) below.
a) In the step A), the plasticizer is added in two or more times, and each injection is performed at a different stage,
i) At least the first injection of plasticizer is carried out by adjusting the temperature of the plasticizer to a temperature of -40°C or more, the swelling temperature of the polyolefin resin composition, and -5°C or less, the swelling temperature of the polyolefin resin composition, and
ii) Adjust the mass of the plasticizer to be added at the first time of adding the plasticizer to an amount that is 2.0 times or more and 3.0 times or less than the mass of the polyolefin resin.b) The polyolefin resin is Contains at least two types selected from the group consisting of high-density polyethylene (polyethylene with a density of 0.94 or more), ultra-high molecular weight polyethylene (polyethylene with a weight average molecular weight (Mw) of 1 x 10 ⁶ or more), and polypropylene, and When the total mass of all polyolefin resins is 100 parts by mass, the content of polyolefin resin having a polystyrene equivalent molecular weight of 1×10 ⁶ or more is 10 parts by mass or more and less than 50 parts by mass.

According to the method for producing a microporous polyolefin membrane of the present invention, a microporous polyolefin membrane with suppressed defects can be obtained. Furthermore, the discharge stability of the extruder can be improved and the discharge capacity of the extruder can be improved.

FIG. 3 is a diagram showing the swelling behavior of polyolefin resin with respect to temperature. (a) Clip-stretched photograph of the polyolefin resin composition obtained in Example 1. (b) Clip-stretched photograph of the polyolefin resin composition obtained in Comparative Example 1. The schematic diagram which shows the plasticizer supply position when adding a plasticizer to an extruder in two parts.

The present invention will be described below based on preferred embodiments. It should be noted that the present invention is not limited to these embodiments, and various modifications may be made within the scope of achieving the purpose of the invention and not departing from the gist of the invention.
As used herein, a microporous polyolefin membrane refers to a microporous membrane containing a polyolefin resin as a main component, preferably a microporous membrane containing 90% by mass or more of a polyolefin resin based on the total mass of the microporous membrane. . Furthermore, a microporous membrane has a large number of micropores inside the membrane, and has a structure in which these micropores are connected, allowing at least one of gas and liquid to pass from one surface to the other. It refers to the membrane that is

The method for producing a polyolefin microporous membrane of the present invention includes the steps of: A) charging a polyolefin resin and a plasticizer into an extruder, melt-kneading them to form a polyolefin resin composition, and discharging the polyolefin resin composition in a sheet form from a die; The method includes B) a step of stretching a polyolefin resin composition discharged into a sheet, and C) a step of removing a plasticizer from the stretched polyolefin resin composition. In the step of kneading in an extruder, a plasticizer is added to the polyolefin resin, and when heated, the polyolefin resin absorbs the plasticizer and swells. The extruder goes through a step of swelling the polyolefin resin charged with a plasticizer, melts and kneads the polyolefin resin composition, and discharges it from the extruder. The temperature and time required for swelling (swelling properties) differ depending on the polyolefin resin, so if two or more types of polyolefin resins are used, the difference in swelling properties may result in a polyolefin resin that cannot absorb the plasticizer sufficiently. Appears in the appearance of the film as grainy defects. By appropriately controlling the temperature of the added plasticizer and the ratio of the plasticizer to the polyolefin resin, it is possible to uniformly swell two or more types of polyolefin resins inside the extruder and eliminate granular defects. The method for producing a polyolefin microporous membrane of the present invention includes:
a) In the step A), the plasticizer is added in two or more times, and each injection is carried out at a different stage; It is necessary to adjust the temperature so that the swelling temperature of the composition is -40°C or higher and the swelling temperature of the polyolefin resin composition is -5°C or lower. In order to uniformly knead the resin composition and improve the appearance of the film, and to stably discharge the polyolefin composition from the extruder, it is necessary to divide the plasticizer into two or more doses and add them at different stages. It is necessary, and it is particularly preferable to add it in two doses.
In addition, from the viewpoint of uniform dispersion and uniform swelling of the plasticizer and polyolefin resin, it is necessary to add a plasticizer whose swelling temperature is adjusted to a range of -40°C to -5°C. If the temperature of the plasticizer added is too high, there will be no time for the plasticizer and polyolefin resin to uniformly disperse before swelling, resulting in granular defects. On the other hand, if the temperature of the plasticizer is too low, the swelling time becomes long and only the polyolefin resin with a low swelling temperature swells preferentially, causing granular defects. When the temperature of the plasticizer is low, there is a problem in addition to the granular form that the polyolefin resin cannot swell, resulting in fluctuations in discharge due to a decrease in gripping force inside the extruder. From the viewpoint of uniform swelling of the plasticizer and polyolefin resin, it is preferable that the swelling temperature is adjusted to a range of -40°C to -5°C when the plasticizer is initially added. The temperature of the plasticizer can be measured with a contact thermometer. In the present invention, the swelling temperature refers to a temperature determined by the method described in the Examples section below.
The swelling temperature of a resin composition is the initial temperature at which the torque increases by 1 N m during kneading due to the polyolefin resin absorbing a plasticizer and swelling, and the swelling temperature can be measured experimentally. It is. FIG. 1 shows an example of the results of a kneading test involving a swelling reaction of a polyolefin resin composition. When the resin composition is kneaded while increasing the temperature, the polyolefin resin absorbs the plasticizer and swells, thereby expanding the volume of the resin composition and increasing the torque. Since the torque reached 1 N·m for the first time at 119°C, the swelling temperature was measured to be 119°C. Details of the measurement method will be explained in Examples below.
Furthermore, in the method for producing a polyolefin microporous membrane of the present invention, ii) the mass of the plasticizer to be added in the first step of adding the plasticizer is 2.0 times or more and 3.0 times or less the mass of the polyolefin resin composition. It is necessary to adjust the amount and add it. If the amount of plasticizer added is small, the polyolefin resin will not be able to absorb the plasticizer uniformly, resulting in granular defects. On the other hand, if the amount of plasticizer added is large, the polyolefin resin will not be able to absorb the plasticizer completely in the swelling zone, and the excessive amount of plasticizer will cause a drop in grip strength, causing fluctuations in ejection.
Furthermore, in the method for producing a microporous polyolefin membrane of the present invention, b) the polyolefin resin is high-density polyethylene (polyethylene with a density of 0.94 or more), ultra-high molecular weight polyethylene (with a weight average molecular weight (Mw) of 1 x 10 ⁶ or more), When the total mass of all polyolefin resins is 100 parts by mass, the content of polyolefin resins is 1 x 10 6 or more and has a molecular weight of 1 x 10 ⁶ or more in terms of polystyrene. , 10 parts by mass or more and less than 50 parts by mass. If the content of the polyolefin resin having a polystyrene-equivalent molecular weight of 1×10 ⁶ or more is less than 10 parts by mass, the mechanical strength of the microporous membrane decreases and the structure becomes coarse. If the amount is 50 parts by mass or more, the fluidity during extrusion molding will decrease, making it difficult to make the resin composition uniform during kneading. When the polyolefin resin includes both high-density polyethylene and ultra-high molecular weight polyethylene, the ratio of ultra-high molecular weight polyethylene to high-density polyethylene is preferably adjusted to increase the strength of the film and to uniformly disperse the polyethylene solution. The amount is 5 to 60% by weight, more preferably 10 to 50% by weight, particularly preferably 15 to 40% by weight.
In the present invention, a microporous membrane film free of grain defects can be produced through the following steps a) to h).

a) Feeding the polyolefin resin into the extruder.

b) heating the supplied polyolefin resin;

c) A preheated plasticizer is added to the heated polyolefin resin and mixed and kneaded to prepare a polyolefin resin composition. As this mixing/kneading process,
c-1) a step of adding and kneading the plasticizer to the heated polyolefin (first kneading);
c-2) A step of further adding and kneading a plasticizer to the polyolefin resin composition after the first kneading (second kneading) is performed.

d) The polyolefin resin composition after kneading is extruded into a sheet form from an extruder through a die.

e) Cooling the extruded polyolefin resin composition into a sheet to form a gel sheet. f) Stretching the gel sheet.

g) Remove the plasticizer from the stretched gel sheet to form a microporous membrane.

h) Drying the film.
Each step will be explained in order below.
a) Feeding the polyolefin resin into the extruder.
In this step, polyolefin resin is fed into the extruder from the raw material input section of the extruder. The polyolefin resin preferably contains at least one of polyethylene and polypropylene, which are inexpensive and have excellent molding properties. For example, polyethylene can be contained in an amount of 50% by mass or more based on the total amount of the microporous polyolefin membrane. The polyethylene is not particularly limited, and various polyethylenes can be used, such as high-density polyethylene, medium-density polyethylene, branched low-density polyethylene, linear low-density polyethylene, and the like. Note that polyethylene may be a homopolymer of ethylene or a copolymer of ethylene and another α-olefin. Examples of the α-olefin include propylene, 1-butene, 1-hexene, 1-pentene, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, and styrene. Furthermore, high-density polyethylene refers to polyethylene with a density of 0.94 or more, and medium-density polyethylene refers to polyethylene with a density of 0.92 to 0.94.

The polyolefin resin can contain high-density polyethylene (density: 0.940 g/m ³ or more and 0.970 g/m ³ or less). Containing high-density polyethylene provides excellent melt extrusion properties and uniform stretching properties. The weight average molecular weight (Mw) of the high density polyethylene used as a raw material is, for example, about 1×10 ⁴ or more and less than 1×10 ⁶ , preferably 1×10 ⁵ or more and less than 1×10 ⁶ . Note that Mw is a value measured in terms of polystyrene by gel permeation chromatography (GPC). The content of high-density polyethylene is, for example, 50% by mass or more based on 100% by mass of the entire polyolefin resin. The upper limit of the content of high-density polyethylene is, for example, 100% by mass or less, and when other components are included, it is, for example, 90% by mass or less.

The polyolefin resin can also include ultra-high molecular weight polyethylene. Here, ultra-high molecular weight polyethylene means polyethylene having a weight average molecular weight of 1 million or more. The ultra-high molecular weight polyethylene used as a raw material has a weight average molecular weight (Mw) of 1 x 10 ⁶ or more (1 million or more), for example 1 x 10 ⁶ or more and 8 x 10 ⁶ or less, preferably 1 x 10 ⁶ It is not less than 5×10 ⁶ or less. When the Mw of ultra-high molecular weight polyethylene is within the above range, moldability will be good. One type of ultra-high molecular weight polyethylene can be used alone or two or more types can be used in combination. For example, two or more types of ultra-high molecular weight polyethylenes having different Mw may be mixed and used.

The ultra-high molecular weight polyethylene can be contained, for example, in an amount of 0% by mass or more and 70% by mass or less, preferably 10% by mass or more and 60% by mass or less, based on 100% by mass of the entire polyolefin resin. When the content of ultra-high molecular weight polyethylene is 10% by mass or more and 60% by mass or less, the resulting microporous polyolefin membrane tends to have excellent productivity such as extrusion kneading performance. Furthermore, when ultra-high molecular weight polyethylene is contained, high mechanical strength can be obtained even when the polyolefin microporous membrane is made thin.

The polypropylene may be, for example, a homopolymer of propylene, a copolymer of propylene with other α-olefins and/or diolefins (propylene copolymer), or a mixture thereof, but the mechanical strength and From the viewpoint of miniaturizing the through-hole diameter, it is preferable to use a propylene homopolymer. The Mw of polypropylene is preferably from 1×10 ⁵ to 1×10 ⁷ , particularly preferably from 1×10 ⁶ to 5×10 ⁶ . The content of polypropylene is preferably 2.5% by mass or more based on 100% by mass of the entire polyolefin resin from the viewpoint of improving heat resistance. Further, it is preferable that the content be 20% by mass or less based on 100% by mass of the entire polyolefin resin, from the viewpoint of suppressing an increase in the shutdown temperature.

Moreover, the polyolefin resin can contain polyolefins other than polyethylene and polypropylene and other resin components as necessary. As other resin components, for example, heat-resistant resins can be used. In addition, the polyolefin microporous membrane may contain antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, antiblocking agents, fillers, crystal nucleating agents, crystallization retarders, etc., to the extent that the effects of the present invention are not impaired. Various additives such as these may also be included. Further, the polyolefin resin composition may contain a crystal nucleating agent in addition to the polyolefin resin and plasticizer described above. The crystal nucleating agent is not particularly limited, and known compound-based, particulate-based crystal nucleating agents, etc. can be used. The nucleating agent may be a masterbatch in which the nucleating agent is mixed and dispersed in a polyolefin resin in advance. When containing a nucleating agent, high mechanical strength can be obtained even without containing ultra-high molecular weight polyethylene.

In addition, when the polyolefin resin composition does not contain a crystal nucleating agent, it is preferable that the polyolefin resin contains the above-mentioned ultra-high molecular weight polyethylene and high-density polyethylene. The microporous polyolefin membrane may also include high-density polyethylene, ultra-high molecular weight polyethylene, and a crystal nucleating agent. By including these, the puncture strength can be further improved.
b) heating the supplied polyolefin resin;
In this step, the supplied polyolefin is heated inside the extruder.
The specific heating temperature varies depending on the type of polyolefin used, but for example, in the case of polyethylene, it is preferably 140 to 250 °C, especially 150 to 200 °C, and in the case of polypropylene, it is preferably 160 to 250 °C, especially 180 to 250 °C. Preferably, the temperature is 200°C.
c) A preheated plasticizer is added to the heated polyolefin resin and mixed and kneaded to prepare a polyolefin composition.

The plasticizer is not limited as long as it is a good solvent for the polyolefin resin, and examples thereof include aliphatic or cyclic hydrocarbons such as nonane, decane, decalin, p-xylene, undecane, dodecane, and liquid paraffin, or boiling point Mineral oil fractions and the like corresponding to these can be used.

The above-mentioned plasticizer preferably has a viscosity of 0.03 to 0.5 Pa·s, particularly 0.05 to 0.2 Pa·s at 25°C. If the viscosity at 25° C. is less than 0.03 Pa·s, non-uniform discharge will occur and kneading will be difficult, while if it exceeds 0.5 Pa·s, deplasticizing in the subsequent process will not be easy. Among the plasticizers, liquid paraffin, decalin, and paraxylene are preferred from the viewpoint of compatibility with polyolefins, and liquid paraffin is particularly preferred from the viewpoint of volatility resistance at high temperatures.

The plasticizer is added to the polyolefin resin in order to uniformly knead the resin composition and improve the appearance of the film, and to stably discharge the polyolefin composition from the extruder. It is preferable to add the plasticizer before the polyolefin resin is heated and melted, and the second and subsequent additions of the plasticizer should be carried out in a state where the first added plasticizer and the polyolefin resin are uniformly mixed and melted (in this state). It is preferable to carry out the process after reaching the first melt-mixing state (referred to as the first melt-mixing state), and it is particularly preferable to carry out the process in two stages.
The case of kneading in two stages will be described below.

The kneading after adding the plasticizer in the first stage (c-1 step) is also called "first kneading", and the kneading after adding the plasticizer in the second step (c-2 step) is also called "first kneading". 2 kneading.

The timing of adding the plasticizer in the first step is preferably before the polyolefin resin is heated and melted. If a plasticizer is added after the resin has been melted, it will be difficult to uniformly disperse the resin and plasticizer, leading to uneven dispersion and granular defects. Whether or not the resin has yet to melt is confirmed by measuring the temperature of the raw material to be transported using a thermometer installed in the extruder. The temperature of the raw material to be transported is compared with the melting point of the raw material, and if the temperature of the raw material is below the melting point, it can be determined that the resin has not yet melted. Usually, if the raw material is polyethylene, it has a melting point of 120 to 140°C, and if polypropylene is used, it has a melting point of 140 to 170°C, so the first step of addition of the plasticizer can be carried out at a temperature below the melting point.
In addition, in order to achieve uniform swelling, the temperature of the plasticizer added in the first step should be between -40°C and -40°C, relative to the swelling temperature of the resin composition when the polyolefin resin and plasticizer are mixed. The temperature is preferably within the range of 5°C. If the temperature of the added plasticizer is too high, there will be no time for the plasticizer and polyolefin resin to uniformly disperse before swelling, resulting in granular defects. On the other hand, if the plasticizer temperature is too low, the swelling time becomes long and only the polyolefin resin with a low swelling temperature swells preferentially, causing granular defects. When the plasticizer temperature is low, there is a problem in addition to the granularity that the polyolefin resin cannot swell, resulting in fluctuations in discharge due to a decrease in gripping force inside the extruder. Therefore, the temperature of the plasticizer added in the first stage can be set, for example, to 70°C or more and 130°C or less.

The amount of plasticizer added in the first step needs to be 2.0 to 3.0 times the total amount of polyolefin resin added. If the amount of plasticizer added is small, the polyolefin resin, which has a high swelling temperature, will not be able to absorb the plasticizer uniformly, resulting in granular defects. On the other hand, if the amount of plasticizer added is large, the polyolefin resin will not be able to absorb the plasticizer completely in the swelling zone, and the excessive amount of plasticizer will cause discharge fluctuations due to a decrease in grip strength.
The timing of adding the plasticizer in the second stage is after the first melt-mixing state, that is, after the first kneading is completed and the initially added plasticizer and polyolefin resin are uniformly mixed and melted. Conditions after this are preferable. The first melt-mixed state is confirmed by visually checking the resin composition being transported. When uniformly mixed and melted, it becomes a colorless and transparent liquid; however, when it is not uniform, undissolved resin, lumps, etc. may occur, and the resin may not become transparent.

The temperature of the plasticizer added in the second step is preferably 30 to 100°C, more preferably 40 to 90°C, and even more preferably Control in the range of 50 to 70°C.

Regarding the addition ratio when adding plasticizer in multiple stages, from the viewpoint of controlling the appearance of the film, the amount of plasticizer added to the extruder in the last step is relative to the total amount of plasticizer fed into the extruder. It is preferably 5% by mass or more, more preferably 5 to 40% by mass, even more preferably 10 to 40% by mass, and even more preferably 10 to 30% by mass.

The blending ratio of the polyolefin resin and plasticizer (total amount) is 10 to 33 mass%, preferably 15 to 30 mass% of the polyolefin resin, with the total of the polyolefin resin and plasticizer being 100 mass%. If the polyolefin resin is less than 10% by mass, that is, if the plasticizer is more than 90% by mass, when molded into a sheet, swell or neck-in will be large at the outlet of the die, making it difficult to mold the sheet. On the other hand, if the polyolefin resin exceeds 33% by mass, that is, if the plasticizer content is less than 67% by mass, it becomes difficult to prepare a uniform solution.
d) The polyolefin resin composition after kneading is extruded into a sheet form from an extruder through a die.
The mixed and kneaded polyolefin resin composition is extruded from an extruder through a die. The extrusion speed of the polyolefin resin composition discharged from the die is usually 1 to 10 m/min.

As the cap, a sheet cap usually having a rectangular cap shape is used. When a sheet die is used, the die gap is usually 0.1 to 5 mm, and the extrusion is heated to 140 to 250°C.
e) A gel-like sheet is formed by cooling the polyolefin resin composition extruded into a sheet from a die (die).

As a method for forming a gel-like sheet, for example, methods disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. Cooling is preferably carried out at a rate of 50° C./min or more until at least the gelling temperature. Preferably, cooling is performed to 25°C or lower. Cooling makes it possible to immobilize the polyolefin microphase separated by the plasticizer. When the cooling rate is within the above range, the degree of crystallinity is maintained within an appropriate range, resulting in a gel-like sheet suitable for stretching. As a cooling method, a method of contacting with a refrigerant such as cold air or cooling water, a method of contacting with a cast roll, etc. can be used, but it is preferable to bring the material into contact with a roll cooled with a refrigerant.

f) Stretching the gel-like sheet. Stretching of a gel-like sheet is also called wet stretching. Wet stretching is performed in at least one direction. Since the gel-like sheet contains a plasticizer, it can be stretched uniformly. After heating, the gel-like sheet is preferably stretched at a predetermined magnification by a tenter method, a roll method, an inflation method, or a combination thereof. The stretching may be uniaxial or biaxial, but biaxial stretching is preferred. In the case of biaxial stretching, any of simultaneous biaxial stretching, sequential stretching, and multistage stretching (for example, a combination of simultaneous biaxial stretching and sequential stretching) may be used.

The final areal stretching ratio (area ratio) in wet stretching is preferably 3 times or more, more preferably 4 times or more and 30 times or less, in the case of uniaxial stretching, for example. Moreover, in the case of biaxial stretching, 9 times or more is preferable, 16 times or more is more preferable, and even more preferably 25 times or more. The upper limit of the final area stretching ratio in wet stretching is preferably 100 times or less, more preferably 64 times or less. Further, the final stretching ratio in wet stretching is preferably 3 times or more in both the MD direction (machine direction) and the TD direction (width direction), and may be the same or different in the MD direction and the TD direction. When the stretching ratio is 5 times or more, an improvement in puncture strength can be expected. Note that the stretching ratio in this step refers to the stretching ratio of the gel-like sheet immediately before being subjected to the next step, with reference to the gel-like sheet immediately before this step. Further, the TD direction is a direction perpendicular to (crosses) the MD direction when the microporous membrane is viewed in a plane.

The stretching temperature is preferably within the range of crystal dispersion temperature (Tcd) of the polyolefin resin to (Tcd) + 30 °C, and within the range of crystal dispersion temperature (Tcd) + 5 °C to crystal dispersion temperature (Tcd) + 28 °C. It is more preferable that the temperature be within the range of (Tcd)+10°C to (Tcd)+26°C. When the stretching temperature is within the above range, membrane rupture due to stretching of the polyolefin resin is suppressed, and stretching at a high magnification is possible. Here, the crystal dispersion temperature (Tcd) refers to a value determined by measuring the temperature characteristics of dynamic viscoelasticity based on ASTM D4065. The ultra-high molecular weight polyethylene, polyethylenes other than ultra-high molecular weight polyethylene, and polyethylene compositions have a crystal dispersion temperature of about 90-100°C. Therefore, when polyethylene is used as a raw material, the stretching temperature can be, for example, 90°C or more and 130°C or less.

By the stretching described above, cleavage occurs between the polyethylene lamellae, the polyethylene phase becomes finer, and a large number of fibrils are formed. Fibrils form a three-dimensionally irregularly connected network structure. Stretching improves the mechanical strength and enlarges the pores, but if stretching is performed under appropriate conditions, it is possible to control the through-hole diameter and have a high porosity even with a thin film thickness. Therefore, it is suitable for safer and higher performance battery separators.

g) Remove the plasticizer from the stretched gel-like sheet to obtain a microporous membrane (film). The plasticizer is removed by washing with a washing solvent. The polyolefin phase is phase-separated from the plasticizer, so when the plasticizer is removed, a porous structure consisting of fibrils forming a fine three-dimensional network structure and having three-dimensionally irregularly communicating pores (voids) is created. A membrane is obtained. Since the cleaning solvent and the method for removing the plasticizer (film-forming solvent) using the same are well known, the explanation thereof will be omitted. For example, methods disclosed in Japanese Patent Publication No. 6-104736 and Japanese Patent Application Laid-open No. 2002-256099 can be used.

h) The microporous membrane from which the plasticizer has been removed is dried by a heat drying method or an air drying method. The drying temperature is preferably below the crystal dispersion temperature (Tcd) of the polyolefin resin, particularly preferably 5° C. or more lower than (Tcd). Drying is preferably carried out until the residual cleaning solvent becomes 5% by mass or less, more preferably 3% by mass or less, based on 100% by mass (dry weight) of the microporous membrane film. When the residual cleaning solvent is within the above range, the porosity of the microporous polyolefin membrane is maintained during the subsequent stretching and heat treatment steps of the microporous membrane film, and deterioration of permeability is suppressed.

Further, the film after drying may be stretched by an areal stretching ratio (area stretching ratio). Stretching of the film after drying is also referred to as dry stretching. The dried microporous membrane film is dry stretched in at least one direction. Dry stretching of the microporous membrane film can be carried out by the tenter method or the like in the same manner as described above while heating. The stretching may be uniaxial or biaxial. In the case of biaxial stretching, either simultaneous biaxial stretching or sequential stretching may be used, but sequential stretching is preferable. In the case of sequential stretching, it is preferable to stretch in the MD direction and then continuously stretch in the TD direction.

Further, the film after drying or the polyolefin microporous membrane after dry stretching may be subjected to heat treatment. Heat treatment stabilizes the crystals and homogenizes the lamellae. As the heat treatment method, at least one of heat setting treatment and heat relaxation treatment can be used. The heat fixing treatment is a heat treatment in which both ends of the film in the TD direction are heated while being held so that the dimensions of the film in the TD direction do not change. The heat setting treatment is preferably carried out by a tenter method or a roll method. Thermal relaxation treatment is a heat treatment in which the film is thermally contracted in the MD direction or the TD direction while being heated. For example, as a thermal relaxation treatment method, there is a method disclosed in JP-A No. 2002-256099. The heat treatment temperature is preferably within the range of (Tcd) to (Tm: melting point) of the polyolefin resin.

Further, the polyolefin microporous membrane after heat treatment can be further subjected to crosslinking treatment and hydrophilic treatment. For example, crosslinking treatment is performed by irradiating the microporous membrane with ionizing radiation such as α rays, β rays, γ rays, and electron beams. In the case of electron beam irradiation, an electron beam dose of 0.1 to 100 Mrad is preferred, and an accelerating voltage of 100 to 300 kV is preferred. The crosslinking treatment increases the meltdown temperature of the microporous membrane. Further, the hydrophilic treatment can be performed by monomer grafting, surfactant treatment, corona discharge, etc. Monomer grafting is preferably carried out after crosslinking treatment.
The microporous polyolefin membrane may be a single layer, but it may also be a laminate of one or more layers of microporous polyolefin membranes. When the polyolefin microporous membrane has two or more layers of polyolefin microporous membranes (hereinafter also referred to as multilayer polyolefin microporous membrane), the compositions of the polyolefin resins constituting each layer may be the same composition or different compositions. .

The polyolefin microporous membrane of the present invention may be provided with a porous layer or coating layer other than the polyolefin resin.
Hereinafter, the present invention will be explained in more detail with reference to Examples. Note that the present invention is not limited to these examples.
The measurement method and evaluation method are as follows.

- Appearance of molten resin The raw polyolefin resin and plasticizer were put into an extruder, mixed and kneaded, and the resulting polyolefin resin composition was extruded from the extruder and collected as a sample. The sample in a molten state was held between 10 cm wide clips with a thickness of about 1 mm to create a 1 m stretched film-like sample for appearance observation, and granular defects were evaluated by visual observation. Count the number of granular defects consisting of 0.5 to 1 mm squares found in a 10 cm x 1 m sample, ◎ (excellent) if the number of defects is 3 or less, 〇 if there are more than 3 and 5 or less defects. (Good), those with more than 5 but less than 10 were evaluated as Δ (Acceptable), and those with more than 10 were evaluated as × (Poor). FIG. 2 shows a photograph of an example of the appearance of the molten resin. The appearance of the molten resin in FIG. 2(a) has almost no granular defects, so the evaluation is ◎, and the appearance of the molten resin in FIG. 2(b) has many granular defects, so the evaluation is ×.

- Film Appearance The prepared microporous film was cut into a size of 95 mm x 95 mm, spread without wrinkles on an illuminated magnifying glass loupe (manufactured by PEAK, ILLUMINATION LUPE), and the microporous film was visually observed using transmitted light. Count the transparent spots with a major diameter of 0.3 cm or more, ◎ (excellent) if the number of spots is 3 or less, ○ (good) if there are more than 3 and less than 5, and ○ (good) if there are more than 5 and less than 10. Those with more than 10 items were evaluated as × (unacceptable).

・Discharge fluctuation amount (%)
The raw material polyolefin resin and plasticizer were put into an extruder, the mixed and kneaded polyolefin resin composition was extruded from the extruder, and the weight of the extruded polyolefin resin composition was weighed every minute, and the weight was measured 10 times. , the average value and standard deviation of the amount of extrusion per minute were calculated. The ratio of the standard deviation of the extrusion amount divided by the average value was calculated and evaluated as the amount of extrusion variation. ◎ (excellent) for extrusion variation of 1% or less, ○ (good) for 1 to 2%, △ (acceptable) for 2 to 5%, and × (unacceptable) for 5% or more. did.

・Swelling temperature (℃)
The swelling temperature of the polyolefin resin composition in each Example and Comparative Example was evaluated using the following experimental equipment and method.
Measuring device: Toyo Seiki Laboplasto Mill (Model: 3S150)
Screw shape: Banbury type Measurement temperature range: 100℃~170℃
Screw rotation speed: 50rpm
Evaluation method: 70 g of a mixture obtained by mixing a polyolefin resin and a plasticizer was used as a sample. A mixture of polyolefin and plasticizer was put into a measuring device and kneaded. Kneading is continued while raising the temperature from 100°C at a heating rate of 5°C/min, and the initial resin temperature at which the torque increases due to the swelling reaction of the polyolefin resin composition due to heating increases by 1Nm is defined as the swelling temperature. And so. The torque was measured using the kneading torque value detected on the Labo Plastomill. FIG. 1 shows the results of a kneading test for the polyolefin resin composition of Example 1. After adding the sample, the torque value showed 0 N·m when the resin temperature was 100°C, but it can be seen that an increase in the torque value due to the swelling reaction was confirmed from around 119°C after the temperature was raised. After that, a significant increase in the torque value due to melting is confirmed from around 130℃, but the swelling temperature is 119℃, which is the temperature that showed 1N・m at the time of the first confirmed torque increase. Become. In addition, when the plasticizer was added in two or more stages, the swelling temperature of the resin composition obtained by mixing the total amount of the plasticizer with the polyolefin resin was measured.
[Example 1]
To achieve the composition and conditions shown in Table 1, polyethylene with Mw of 2.0 x 10 ⁶ is used as ultra-high molecular weight polyethylene, and polyethylene with Mw of 4.0 x 10 ⁵ is used as high-density polyethylene (density: 0.940 g/m ³ ), liquid paraffin (viscosity at 25°C: 0.05 Pa·s) as a plasticizer, and tetrakis[methylene-3-(3,5-ditertiarybutyl-4-hydroxyphenyl)-propionate]methane ( (0.3 parts by mass per 100 parts by mass of polyolefin resin) was used. A polyolefin resin and an antioxidant were charged into the raw material input port 3 of the twin-screw extruder 4 shown in FIG. 3, and mixed and melt-kneaded. The liquid paraffin to be supplied to the twin-screw extruder 4 was divided into two parts, separately supplied in a heated state, and introduced at two positions shown in FIG. 80 parts by mass of the total liquid paraffin that is supplied at the plasticizer supply port (first stage) 1 to the extruder, and the total flow that is supplied at the plasticizer supply port (second stage) 2 to the extruder. Paraffin was supplied in an amount of 20 parts by mass. The temperature of the plasticizer added in the first stage was adjusted to 100°C, and the temperature of the plasticizer supplied to the second stage was adjusted to 70°C. The plasticizer supply port (first stage) 1 is provided at a position where it can be added at a timing before the resin input into the extruder is heated and melted, that is, immediately after the raw material input port 3, and the plasticizer supply port (second stage) ) 2 was provided at a position where it could be added at the timing when the first kneading was completed and the polyolefin resin and plasticizer were uniformly mixed and melted. The temperature of the raw material when added in the first stage was 90°C. The prepared polyolefin resin composition was supplied from a twin-screw extruder to a T-shaped die and extruded into a sheet. When the appearance of the extruded molten resin was evaluated, it was found that there were no granular defects and an excellent appearance (appearance evaluation of molten resin: ◎). FIG. 2(a) shows a clip-stretched photograph of the polyolefin resin composition obtained in Example 1. Thereafter, the extruded body was cooled while being taken up with a cooling roll, and a gel-like sheet was formed. The formed gel-like sheet was subjected to simultaneous biaxial stretching (wet stretching) using a tenter stretching machine under the stretching conditions shown in Table 1. The stretched gel-like sheet was fixed to a 20 cm x 20 cm aluminum frame plate, immersed in a methylene chloride bath controlled at 25 ° C., rocked at 100 rpm for 3 minutes to remove liquid paraffin, and air-dried at room temperature. A dry film was obtained. Next, this membrane was subjected to thermal relaxation treatment at 126°C to create a polyolefin microporous membrane. The evaluation results of the obtained microporous polyolefin membrane are also listed in Table 1. In Table 1, the resin concentration (wt%) in the melt-kneaded material (resin composition) indicates the mass% of the polyolefin resin relative to the total of the polyolefin resin and plasticizer. The ratio (mass ratio) of the plasticizer added in the first stage to the polyolefin resin indicates the ratio of the amount of plasticizer supplied in the first stage to the total amount of polyolefin resin input into the twin-screw extruder. mass of plasticizer added to / mass of total polyolefin resin). The stretching ratio indicates the areal stretching ratio of the polyolefin microporous membrane after stretching (before heat-setting treatment), based on the gel-like sheet before wet stretching.
[Example 2]
A fine polyolefin was prepared in the same manner as in Example 1, except that the mixing ratio of ultra-high molecular weight polyethylene with Mw of 2.0×10 ⁶ and high-density polyethylene with Mw of 4.0×10 ⁵ was changed as shown in Table 1. A porous membrane was created. The evaluation results are shown in Table 1.
[Example 3]
Table 1 shows the mixing ratio of ultra-high molecular weight polyethylene with Mw of 2.0 x 10 ⁶ , high density polyethylene with Mw of 4.0 x 10 ⁵ , liquid paraffin and the supply ratio of liquid paraffin fed to the extruder in two stages. A microporous polyolefin membrane was prepared in the same manner as in Example 1 except for the changes shown. The evaluation results are shown in Table 1.
[Example 4]
A microporous polyolefin membrane was produced in the same manner as in Example 1, except that the mixing ratio of polyolefin resin and liquid paraffin and the supply ratio of liquid paraffin supplied to the extruder in two stages were changed as shown in Table 1. The evaluation results are shown in Table 1.
[Example 5]
The mixing ratio of polypropylene with Mw of 2.0 x 10 ⁶ , high density polyethylene with Mw of 4.0 x 10 ^5, and liquid paraffin and the supply ratio of liquid paraffin fed to the extruder in two stages are shown in Table 1. A microporous polyolefin membrane was prepared in the same manner as in Example 1 except for the following changes. The evaluation results are shown in Table 1.
[Example 6]
A microporous polyolefin membrane was prepared in the same manner as in Example 1, except that the second stage heating temperature of the liquid paraffin fed into the extruder was changed as shown in Table 1. The evaluation results are shown in Table 1.
[Comparative example 1]
A microporous polyolefin membrane was prepared in the same manner as in Example 1, except that the heating temperature in the first stage of liquid paraffin supplied to the extruder in two stages was changed as shown in Table 1. FIG. 2(b) shows a photograph of the appearance of the molten resin under Comparative Example 1 conditions. Comparative Example 1 showed an appearance with many granular defects (exterior evaluation of molten resin: ×). The evaluation results are shown in Table 1.
[Comparative example 2]
A microporous polyolefin membrane was prepared in the same manner as in Example 1, except that the heating temperature in the first stage of liquid paraffin supplied to the extruder in two stages was changed as shown in Table 1. The evaluation results are shown in Table 1.
[Comparative example 3]
A microporous polyolefin membrane was prepared in the same manner as in Example 1, except that the feed ratio of liquid paraffin fed to the extruder in two stages was changed as shown in Table 1. The evaluation results are shown in Table 1.
[Comparative example 4]
The procedure was the same as in Example 1 except that the mixing ratio of ultra-high molecular weight polyethylene with Mw of 2.0 x 10 ⁶ , high density polyethylene with Mw of 4.0 x 10 ⁵ , and liquid paraffin was changed as shown in Table 1. A microporous polyolefin membrane was created. The evaluation results are shown in Table 1.
[Comparative example 5]
A microporous polyolefin membrane was produced in the same manner as in Example 1, except that the mixing ratio of polyolefin resin and liquid paraffin and the supply ratio of liquid paraffin supplied to the extruder in two stages were changed as shown in Table 1. The evaluation results are shown in Table 1.
[Comparative example 6]
A fine polyolefin was prepared in the same manner as in Example 1, except that the mixing ratio of ultra-high molecular weight polyethylene with Mw of 2.0×10 ⁶ and high-density polyethylene with Mw of 4.0×10 ⁵ was changed as shown in Table 1. A porous membrane was created. The evaluation results are shown in Table 1.

As is clear from the results in Table 1, the manufacturing method of the present embodiment suppresses abnormalities in the properties of the polyolefin composition during the manufacturing process and has excellent discharge accuracy in the extruder, resulting in improved yield and quality of the polyolefin microporous membrane. It can be produced in a process with improved yield.

The method for producing a microporous polyolefin membrane of the present invention suppresses grain defects due to poor kneading of the polyolefin resin composition in the production process, so the microporous polyolefin membrane can be produced in a process with improved yield, quality, and yield. It is suitable as a method for producing a polyolefin resin composition for a separator for a secondary battery.

1: Plasticizer supply port (1st stage)
2: Plasticizer supply port (2nd stage)
3: Raw material input port 4: Twin screw extruder

Claims (5)

A) A step of charging a polyolefin resin and a plasticizer into an extruder, melting and kneading them to form a polyolefin resin composition, and discharging the polyolefin resin composition from a die in the form of a sheet;
B) a step of stretching the polyolefin resin composition discharged into a sheet, and C) a step of removing the plasticizer from the stretched polyolefin resin composition.
A method for producing a microporous polyolefin membrane, which satisfies a) and b) below.
a) In the step A), the plasticizer is added in two or more times, and each injection is performed at a different stage,
i) At least the first injection of plasticizer is carried out by adjusting the temperature of the plasticizer to a temperature of -40°C or more, the swelling temperature of the polyolefin resin composition, and -5°C or less, the swelling temperature of the polyolefin resin composition, and
ii) Adjust the mass of the plasticizer to be added at the first time of adding the plasticizer to an amount that is 2.0 times or more and 3.0 times or less than the mass of the polyolefin resin.b) The polyolefin resin is Contains at least two types selected from the group consisting of high-density polyethylene (polyethylene with a density of 0.94 or more), ultra-high molecular weight polyethylene (polyethylene with a weight average molecular weight (Mw) of 1 x 10 ⁶ or more), and polypropylene, and When the total mass of all polyolefin resins is 100 parts by mass, the content of polyolefin resin having a polystyrene equivalent molecular weight of 1×10 ⁶ or more is 10 parts by mass or more and less than 50 parts by mass. The method for producing a microporous polyolefin membrane according to claim 1 , wherein the plasticizer is liquid paraffin having a viscosity of 0.03 to 0.5 Pa·s at 25°C. The weight average molecular weight (Mw) of the high density polyethylene is 1 x 10 ⁴ or more and less than 1 x 10 ⁶ , and the weight average molecular weight (Mw) of the ultra-high molecular weight polyethylene is 1 x 10 ⁶ or more and 8 x 10 ⁶ or less. 3. The method for producing a microporous polyolefin membrane according to claim 1 or 2, wherein the polypropylene has a weight average molecular weight (Mw) of 1 x ¹⁰⁵ or more and 1 x ¹⁰⁷ or less. The first addition of the plasticizer is performed before the polyolefin resin is heated and melted, and the second and subsequent additions of the plasticizer are performed until the plasticizer added first and the polyolefin resin are uniformly mixed and melted. The method for producing a microporous polyolefin membrane according to any one of claims 1 to 3 , characterized in that the process is carried out after reaching a state in which the polyolefin is mixed (this state is referred to as a first melt-mixing state). The production of the microporous polyolefin membrane according to claim 4, characterized in that the temperature of the plasticizer to be added after the first melt-mixing state is reached is adjusted to 30° C. or more and 100° C. or less before adding the plasticizer. Method.

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