JP2021181524A - Method of producing microporous film - Google Patents

Abstract

To provide a method of producing a microporous film which improves quality, productivity, and yield of the microporous film.SOLUTION: When a time from when a polyolefin resin and a plasticizer are charged into an extruder till extruded through a nozzle is a residence time, the residence time of a polyolefin solution is 10 min. or more and 120 min. or less, when a resin temperature of the polyolefin solution is lower than an oxidation-initiating temperature, the oxidation-inducing time of the polyolefin solution is 20% or more and 300% or less of the residence time, and when a resin temperature of the polyolefin solution is higher than an oxidation-initiating temperature of the polyolefin solution, the oxidation-inducing time of the polyolefin solution is 60% or more and 300% or less of the residence time.SELECTED DRAWING: Figure 1

Description

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

The microporous membrane is a membrane having various pore diameters and pore shapes, and is used in various fields such as filters for filtration membranes and dialysis membranes, separators for batteries, and separators for electrolytic capacitors. Among these, microporous membranes made of polyolefin as a resin material are widely used as separators for secondary batteries because they are excellent in chemical resistance, insulating property, mechanical strength, etc., and have shutdown characteristics. In these applications, abnormalities in the properties of the resin during the manufacturing process, such as low molecular weight, discoloration, and generation of carbides, greatly affect the quality of the product.

Polyolefins are film-formed by melt-kneading, but if oxygen is mixed in during melting, the resin undergoes thermal oxidative deterioration due to heat during heating and melting and shear heat generation during kneading, resulting in lower molecular weight due to molecular chain breakage and resin. It is known to cause discoloration of the product, occurrence of product defects due to the formation of carbides, and decrease in the strength of the product.

As measures to prevent deterioration of the resin, conditions such as flowing nitrogen through the extruder as described in Patent Document 1, adding an antioxidant as described in Patent Documents 1 and 2, and kneading at a temperature below the deterioration temperature are set. Although some measures can be taken, in general, the addition of an antioxidant, which is particularly convenient and can maintain a good dispersion state, is often used.

However, if the amount of the antioxidant added is less than the appropriate amount, the resin deteriorates, leading to a decrease in strength and the occurrence of defects. Further, if the amount of the antioxidant is larger than the appropriate amount, there is a risk that the antioxidant will bleed out in the film forming process as described in Patent Document 1, and there are problems such as contamination of the process and deterioration of productivity, yield and quality. .. Therefore, it is necessary to add an appropriate amount of the antioxidant during the production of the microporous polyolefin membrane.

Patent Document 1 states that by adding 0.05 to 5% by mass of an antioxidant to a polyolefin resin, it is possible to provide a microporous polyolefin membrane that can be operated for a long period of time without deterioration. Further, in Patent Document 2, by setting the oxidation induction time of the film sample in an oxygen atmosphere of 200 ° C. to 7 minutes or more, it is possible to suppress oxidative deterioration and suppress deterioration of the mechanical strength of the film over time. It is said that it can be done.

Japanese Patent No. 5520313 Japanese Unexamined Patent Publication No. 2017-57238

Patent Document 1 defines an appropriate amount of antioxidant as a mass ratio to a polyolefin resin. However, since the appropriate amount of the antioxidant can change depending on the type, molecular weight, composition, and manufacturing conditions of the resin, it can be said that the range is not sufficiently defined under the conditions of Patent Document 1. For example, when a polyolefin resin is used, the higher the molecular weight of the resin, the greater the influence on the physical properties due to deterioration. Further, when kneading using the same raw material, if the discharge amount is large, the resin is sheared and the resin temperature rises, so that the resin deterioration tends to proceed. If the appropriate amount is specified only by the amount of the antioxidant, these effects will be eliminated, so it is necessary to specify the amount including other parameters. In Patent Document 2, the oxidation induction time is adopted as a method for defining an appropriate antioxidant amount, but the oxidation induction time adopts the value of the film which is the final product, and is a process using an intermediate product in film formation. The impact on pollution is not taken into account.

Based on the above, it is an object of the present invention to improve the quality, productivity, and yield of the microporous membrane by adjusting the amount of the antioxidant according to each condition in the process of manufacturing the microporous membrane.

Therefore, the present inventors have diligently studied and measured the oxidation induction time of the polyolefin solution collected at the outlet of the twin-screw extruder in the step of kneading the resin and the plasticizer to form the kneaded product, and the oxidation induction time and the residence time. The present invention has been found to be superior to the conventional method for producing a microporous film in that it can be produced with a minimum amount of antioxidant while suppressing deterioration of the resin by adjusting the ratio of bottom.

In order to solve the above problems, the present invention
(A) Step of supplying the polyolefin resin and the plasticizer to the extruder and melt-kneading to prepare the polyolefin solution (b) Step of removing the gas dissolved in the polyolefin solution (c) Discharging the polyolefin solution from the extruder Step of extruding into sheet shape through mouthpiece (d) Step of cooling extruded sheet-like polyolefin solution to form gel sheet (e) Step of stretching gel sheet (f) Removal of plasticizer from stretched gel sheet and drying In a method for producing a microporous film, which comprises a step of forming a microporous film.
When the residence time is the time from when the polyolefin resin and the plasticizer are put into the extruder to when it is extruded through the mouthpiece after melt-kneading, the residence time is in the range of 10 to 120 minutes, and the discharged polyolefin solution. When the resin temperature of the polyolefin solution is lower than the oxidation start temperature of the polyolefin solution, the oxidation induction time of the polyolefin solution is 20 to 300% of the residence time, and the resin temperature of the discharged polyolefin solution is higher than the oxidation start temperature of the polyolefin solution. Provided is a method for producing a microporous film, characterized in that the oxidation induction time of the polyolefin solution is 50 to 300% of the residence time at high temperatures.

In order to solve the above problems, the present invention provides a method for producing a microporous membrane, wherein the resin temperature of the polyolefin solution discharged from the outlet of the extruder is in the range of 150 to 250 ° C.

In order to solve the above problems, the present invention, the polyolefin resin to provide a method for manufacturing a microporous film, characterized in that the weight average molecular weight ^of 1 × 10 ⁴ or more 8 × 10 ⁶ or less polyethylene.

In order to solve the above problems, the present invention, the polyolefin resin is a composition comprising a weight-average molecular weight 1 × 10 of less than ⁶ polyolefin and a weight average molecular weight 1 × 10 ⁶ or more polyolefins, the weight in the composition to provide a method of manufacturing a polyolefin microporous membrane, wherein the amount of the polyolefin of average molecular weight of less than 1 × 10 ⁶ is 50 wt% to 99 wt%.

According to the present invention, a microporous membrane suitable as a separator for a secondary battery can be produced in a process in which the yield, quality and yield are improved.

The figure which showed the method of defining the oxidation induction time using TG / DTA.

Hereinafter, the present invention will be described based on a preferred embodiment. In the present specification, the microporous membrane has a large number of micropores inside the membrane and has a structure in which these micropores are connected, and gas and liquid are transferred from one surface to the other. A membrane that allows at least one to pass through. Further, the polyolefin microporous membrane means a microporous membrane containing polyolefin as a main component, and preferably refers to a microporous membrane containing 90% by mass or more of polyolefin with respect to the total amount of the microporous membrane.

The method for producing a microporous polyolefin membrane of the present invention is
(A) Step of supplying the polyolefin resin and the plasticizer to the extruder and melt-kneading to prepare the polyolefin solution (b) Step of removing the gas dissolved in the polyolefin solution (c) Discharging the polyolefin solution from the extruder Step of extruding into sheet shape through mouthpiece (d) Step of cooling extruded sheet-like polyolefin solution to form gel sheet (e) Step of stretching gel sheet (f) Removal of plasticizer from stretched gel sheet and drying Has a step of forming a microporous film.
When the residence time is the time from when the raw material is put into the extruder and extruded through the mouthpiece after melt-kneading, the residence time of the polyolefin solution is in the range of 10 minutes or more and 120 minutes or less, and is discharged from the extruder outlet. When the resin temperature of the obtained polyolefin solution is lower than the oxidation start temperature of the polyolefin solution, the oxidation induction time of the polyolefin solution is 20% or more and 300% or less of the residence time, and the resin temperature of the discharged polyolefin solution is that of the polyolefin solution. When the temperature is higher than the oxidation start temperature, the oxidation induction time of the polyolefin solution satisfies 60% or more and 300% or less of the residence time.

When the resin temperature of the polyolefin solution discharged from the outlet of the extruder is lower than the oxidation start temperature of the polyolefin solution, the oxidation induction time of the polyolefin solution is 20% or more and 300% or less of the residence time, and the resin of the ejected polyolefin solution. When the temperature is higher than the oxidation start temperature of the polyolefin solution, it is important to adjust the oxidation induction time of the polyolefin solution to be 50% or more and 300% or less of the residence time. By setting the oxidation induction time of the polyolefin solution within the above range, a high-quality microporous polyolefin membrane can be stably produced. The oxidation induction time of the polyolefin solution can be adjusted, for example, by the type and amount of the antioxidant and the combination of the antioxidants.

Here, the oxidation induction time will be described in detail. The oxidation induction time is the time when the equilibrium between the generation rate and the extinction rate of the peroxide generated in the plastic is lost and the amount of heat rapidly increases. The sample is heated to a constant temperature in a nitrogen atmosphere and then maintained at a constant temperature. Next, the sample is made oxidizable by changing the atmosphere to air or oxygen. As a result, an oxidation reaction occurs in the sample in a state where the effect of the antioxidant is lost, and an exothermic peak is observed. The time from when the atmosphere is changed to air or oxygen until the oxidation reaction starts to occur is the oxidation induction time.

The oxidation start temperature is a temperature at which heat generation due to the oxidation reaction starts. The change in calorific value when the temperature is raised at a constant rate under an air or oxygen atmosphere is measured, and the temperature at which the calorific value begins to increase due to the oxidation of the resin is the oxidation start temperature.

When a polymer is subjected to the action of heat or shear stress, hydrogen is generally extracted from a carbon-hydrogen bond to generate a radical. In the coexistence of oxygen, this radical reacts with oxygen to generate a peroxy radical, and the peroxy radical produces a hydroperoxide and a radical by extracting hydrogen from a carbon-hydrogen bond. By repeating this reaction, the polymer has a low molecular weight and causes oxidative deterioration.

When an antioxidant is added to a resin in the process of producing a microporous polyolefin membrane, the antioxidant captures radicals and stops the reaction of producing peroxy radicals, so that deterioration of the resin can be suppressed. However, the more the antioxidant is consumed during melt-kneading, the shorter the oxidation induction time is, and the more easily the resin is deteriorated. On the other hand, if a large amount of the antioxidant remains in the resin, there is a risk that the antioxidant bleeds out during the process and contaminates the process, resulting in a decrease in productivity. That is, the closer the oxidation induction time is to the residence time, the less the deterioration is, and the film can be formed without polluting the subsequent steps.

Hereinafter, each step of the method for producing a microporous polyolefin membrane of the present invention will be described in order.
(A) Step of supplying the polyolefin resin and the plasticizer to the extruder and melt-kneading to prepare the polyolefin solution In this step, the polyolefin resin is supplied 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 characteristics. 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 types of polyethylene can be used. For example, high-density polyethylene, medium-density polyethylene, branched low-density polyethylene, linear low-density polyethylene and the like are used. The polyethylene may be a homopolymer of ethylene or a copolymer of ethylene and another α-olefin. Examples of the α-olefin include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, styrene and the like.

The polyolefin resin can contain high density polyethylene (HDPE) (density: 0.920 g / m ³ or more and 0.970 g / m ³ or less). When high-density polyethylene is contained, it is excellent in melt extrusion characteristics and uniform drawing processing characteristics. The weight average molecular weight of high-density polyethylene used as the raw material (Mw) of, for example, about less than 1 × 10 ⁴ or more 1 × 10 ^6. Here, Mw is a value measured in terms of polystyrene by gel permeation chromatography (GPC).

The content of the high-density polyethylene is, for example, preferably 50% by mass or more with respect to 100% by mass of the entire polyolefin resin. The upper limit of the content of the high-density polyethylene is preferably, for example, 100% by mass or less, and more preferably 90% by mass or less when other components are contained.

Further, the polyolefin resin can contain ultra-high molecular weight polyethylene (UHMwPE). Ultra high molecular weight polyethylene used as a raw material has a weight average molecular weight (Mw) of 1 × 10 ⁶ or more (over one million), preferably 1 × 10 ⁶ or more 8 × 10 ⁶ or less. When the Mw of the ultra-high molecular weight polyethylene is in the above range, the moldability is good. The ultra-high molecular weight polyethylene may be used alone or in combination of two or more, and for example, two or more types of ultra-high molecular weight polyethylene having different Mw may be mixed and used.

The ultra-high molecular weight polyethylene can be contained, for example, in an amount of more than 0% by mass 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 the ultra-high molecular weight polyethylene is 10% by mass or more and 60% by mass or less, the Mw of the obtained polyolefin microporous membrane can be easily controlled within a specific range described later, and the productivity such as extrusion kneading can be improved. Tends to be better. Further, when the ultra-high molecular weight polyethylene is contained, high mechanical strength can be obtained even when the microporous polyolefin membrane is thinned.

When the polyolefin resin contains both high density polyethylene and ultra high molecular weight polyethylene, the ratio of the ultra high molecular weight polyethylene to the high density polyethylene is preferable in order to increase the strength of the film and to uniformly disperse the polyethylene solution. It is 5 to 60% by mass, more preferably 5 to 50% by mass, and particularly preferably 10 to 40% by mass.

The polypropylene may be, for example, a homopolymer of propylene, a copolymer of propylene and another α-olefin and / or a diolefin (propylene copolymer), or a mixture thereof. From the viewpoint of mechanical strength and miniaturization of through-hole diameter, it is preferable to use a homopolymer of propylene. The content of polypropylene is preferably 2.5% by mass or more with respect to 100% by mass of the entire polyolefin resin from the viewpoint of improving heat resistance. Further, it is preferable that the content is 15% by mass or less with respect to 100% by mass of the entire polyolefin resin from the viewpoint of suppressing an increase in the shutdown temperature.

Further, the polyolefin resin can contain polyolefins other than polyethylene and polypropylene and other resin components, if necessary. As the other resin component, for example, a heat-resistant resin or the like can be used. Further, the polyolefin microporous membrane is provided with various additions such as a heat stabilizer, an antistatic agent, an ultraviolet absorber, an antiblocking agent and a filler, a crystal nucleating agent, and a crystallization retarder, as long as the effects of the present invention are not impaired. The agent may be contained.

The plasticizer is not limited as long as it is a good solvent for the polyolefin resin, but is, for example, an aliphatic or cyclic hydrocarbon such as nonane, decane, decalin, p-xylene, undecane, dodecane, liquid paraffin, or a boiling point. However, mineral oil distillates and the like corresponding to these can be used. Of these, liquid paraffin, decalin, and p-xylene are preferable from the viewpoint of compatibility with polyolefin.

The plasticizer described above preferably has a viscosity at 25 ° C. of 0.03 to 0.5 Pa · s, particularly 0.05 to 0.2 Pa · s. If the viscosity at 25 ° C. is less than 0.03 Pa · s, non-uniform discharge occurs and kneading is difficult, while if it exceeds 0.5 Pa · s, the deplasticizer in the subsequent step becomes difficult.

The blending ratio of the polyolefin resin and the plasticizer is preferably 10 to 50% by mass, more preferably 15 to 40% by mass, with the total of the polyolefin resin and the plasticizer being 100% by mass. When the amount of polyolefin is less than 10% by mass (when the amount of plasticizer is 90% by mass or more), when forming into a sheet, swells and neck-ins are large at the outlet of the base, which makes it difficult to form the sheet. On the other hand, when the amount of the polyolefin resin exceeds 50% by mass (when the amount of the plasticizer is 50% by mass or less), it becomes difficult to prepare a uniform solution.

Further, the polyolefin solution may contain an antioxidant in addition to the above-mentioned polyolefin resin and plasticizer. The content of the antioxidant is preferably 0.05% by mass or more and less than 5% by mass with respect to 100% by mass of the entire polyolefin resin. If it is less than 0.05% by mass, the polyolefin solution may deteriorate during kneading. If it is 5% by mass or more, the antioxidant may bleed out during the film forming process, which may contaminate the process or clog the intake pipe.

Examples of the antioxidant include dibutylhydroxytoluene, pentaerythyl-tetrakis [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate], and octadecyl-3- (3,5-di-). Examples thereof include, but are not limited to, t-butyl-4-hydroxyphenyl) propionate. These hindered phenolic antioxidants may be used alone or in admixture of two or more.

Further, the polyolefin solution may contain a crystal nucleating agent in addition to the above-mentioned polyolefin resin and plasticizer. The crystal nucleating agent is not particularly limited, and known compound-based or fine particle-based crystal nucleating agents 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 a nucleating agent is contained, high mechanical strength and a low stress intensity factor (K value) can be obtained without containing ultra-high molecular weight polyethylene.

When the polyolefin solution does not contain a crystal nucleating agent, the polyolefin resin preferably contains the above-mentioned ultra-high molecular weight polyethylene and high-density polyethylene. Further, the polyolefin microporous membrane may contain high-density polyethylene, ultra-high molecular weight polyethylene and a crystal nucleating agent. By including these, the puncture strength can be further improved.

It is preferable to add the antioxidant before melting the polyolefin resin. The amount added is preferably 0.05% or more and less than 5% by mass with respect to 100% by mass of the entire polyolefin resin. However, since the susceptibility of the resin to deterioration varies depending on the process conditions such as the kneading temperature, the discharge amount, the residence time, and the residence temperature, it is necessary to reflect the process conditions in determining the appropriate amount of the antioxidant. Therefore, in the present application, the appropriate amount of the antioxidant is determined by the ratio of the oxidation induction time and the residence time. The larger the amount of the antioxidant added, the longer the oxidation induction time becomes because the deterioration is less likely to proceed. When the residence temperature is lower than the oxidation start temperature, it is important to determine the amount of the antioxidant so that the oxidation induction time is 20% or more and 300% or less with respect to the residence time. Further, when the residence temperature is higher than the oxidation start temperature, the deterioration of the resin tends to progress remarkably, but the appropriate amount of the antioxidant at this time is specified as the oxidation induction time of 50% or more and 300% or less with respect to the residence time. It can be decided by doing. By determining an appropriate amount of the antioxidant by the above method, deterioration of the polyolefin solution during melt-kneading can be suppressed without contaminating the process.

When the polyolefin resin and plasticizer supplied to the extruder are heated, the specific heating temperature depends on the type of polyolefin resin used. The polyethylene composition has a melting point of about 130-140 ° C. The lower limit of the heating temperature of the resin is preferably the melting point of the polyethylene composition of + 10 ° C., more preferably + 20 ° C., and the upper limit of the heating temperature is preferably the melting point of the polyethylene composition of + 120 ° C., more preferably + 100 ° C. That is, when the polyethylene composition is used, the set temperature of the extruder at the time of melt-kneading is preferably in the range of 140 to 250 ° C, more preferably 150 to 230 ° C. Similarly, in the case of a polypropylene solution, the temperature is preferably 160 to 250 ° C, more preferably 180 to 200 ° C.

Further, due to the influence of shear heat generation due to screw rotation, the temperature of the resin collected at the outlet of the twin-screw extruder becomes higher than the kneading temperature. The resin temperature of the polyethylene composition is preferably 150 to 250 ° C, more preferably 160 to 250 ° C, in order to suppress resin deterioration and ensure a good dispersed state. In the case of the polypropylene composition, the resin temperature is preferably 170 to 250 ° C, more preferably 190 to 220 ° C.

The resin temperature of the polyolefin solution discharged from the extruder outlet is preferably 150 to 250 ° C. The oxidation start temperature of the polyolefin solution discharged from the extruder outlet is preferably 180 to 270 ° C. In particular, in the case of a polyethylene solution using a polyethylene composition, the oxidation start temperature varies depending on the molecular weight and the amount of the antioxidant added, but is preferably about 200 to 260 ° C.

When the polyolefin solution is kneaded, shear heat generation of the resin is generated in addition to the heating temperature. Polyolefins are prone to oxidative deterioration and may deteriorate at 230 ° C. or higher in an oxygen-containing atmosphere. In polyolefins, the kneading temperature greatly contributes to deterioration, and if the resin temperature of the polyolefin solution is 230 ° C. or higher, the antioxidant is immediately consumed, causing resin deterioration. If the resin deteriorates and has a low molecular weight, it may cause a poor appearance of the product. Further, the susceptibility of the resin to deterioration depends on the discharge amount, and the higher the discharge rate, the easier it is for shearing to occur, and the higher the resin temperature, the easier the deterioration progresses.

(B) Step of removing the gas dissolved in the polyolefin solution When the kneading of the polyolefin resin and the plasticizer is completed, the gas in the extruder is dissolved in the prepared polyolefin solution, and bubbles are generated due to the pressure and temperature change applied to the solution. May occur. In this step, it is preferable to suck and exhaust air bubbles that cause abnormal properties of the resin and poor appearance. Therefore, a vent hole is formed in the final stage of the extruder in which air bubbles collect in the extruder, and the inside of the extruder is exhausted by a vacuum pump via an exhaust stack. If the kneaded product contains an excessive amount of the antioxidant, the bleed-out antioxidant may enter the vent hole side and gradually clog the inside of the pipe. It has been confirmed that when the antioxidant is clogged inside the pipe, air biting occurs and the degree of deterioration when discharging from the outlet of the twin-screw extruder becomes unstable, which makes the state of the polyolefin solution unstable. ..

(C) Step of ejecting the polyolefin solution from the extruder and extruding it into a sheet through the mouthpiece The mixed / kneaded polyolefin solution is extruded from the extruder through the mouthpiece. The higher the residence temperature, the more likely it is that thermal oxidation deterioration will occur. In addition, the time from when the raw material is put into the extruder to when it reaches the mouthpiece is called the residence time. When flowing in the pipe, a velocity distribution occurs, and the residence time of the solution is approximately infinite on the wall surface of the extruder and is the smallest in the center, but the residence time used in this application is from the extruder to the base. It is the average residence time calculated by the ratio of the total volume to the discharge amount (total volume (L) from the extruder to the mouthpiece x solution density (kg / L) / set discharge amount (kg / h)). .. The residence time is preferably 10 to 120 minutes, more preferably 20 to 100 minutes. When the residence time is short, if the pipe volume is constant, the amount discharged from the extruder is large, and the rotation speed increases accordingly to cause shearing, and the temperature of the resin becomes high, so that deterioration tends to proceed. Further, since the time for staying inside the extruder is shortened, poor kneading is likely to occur. Poor kneading causes granular defects in the film after film formation, which may lead to poor appearance, local stretching unevenness, and breakage of the film, which contributes to a decrease in productivity. On the other hand, when the residence time is long, the deterioration is likely to proceed because the time of exposure to high temperature is long. In addition, if the kneading temperature is lowered by lowering the set temperature of the extruder or the screw rotation speed of the extruder is lowered due to concern about deterioration, melting failure or kneading failure may occur. In this way, complicated factors are intertwined in determining the appropriate kneading conditions, and the susceptibility to deterioration and the measures required to suppress deterioration differ depending on the residence time and the residence temperature. Oxidation induction time is an important evaluation item in adjusting this condition.

The melt-kneaded polyolefin solution discharged from the outlet of the extruder is taken out as a measurement sample, the oxidation induction time of this measurement sample is measured by the measurement method described in Examples described later, and the mixture is kneaded so that the oxidation induction time falls within a predetermined range. By adjusting the conditions of stagnation and retention, it is possible to stably supply a film without precipitation of antioxidants and deterioration of resin.

The solution discharged from the extruder outlet is transported to the mouthpiece through a pipe. At this time, if the temperature of the pipe is too low, the resin temperature is lowered and the fluidity is lowered, and if it is too high, the resin is deteriorated. Therefore, the temperature is preferably 150 to 300 ° C, more preferably 180 to 250 ° C.

As the base, it is preferable to use a base having a rectangular shape. When this base is used, the base gap is usually 0.1 to 5 mm. When extruded into a sheet through the mouthpiece, the polyolefin solution is heated to 140-250 ° C. Adhesion of low molecular weight components and carbides precipitated due to resin deterioration to the mouthpiece may cause rheumatism or scorching on the mouthpiece, which may cause product defects. Therefore, when molding the resin, it is necessary to pay attention to the deterioration state of the resin.

(D) Step of Cooling Extruded Sheet-shaped Polyolefin Solution to Form Gel Sheet As a method for forming a gel sheet, for example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. .. Cooling is preferably performed at a rate of 50 ° C./min or higher, at least up to the gelation temperature. Cooling is preferably performed to 25 ° C. or lower. By cooling, the microphase of the polyolefin separated by the plasticizer can be immobilized. When the cooling rate is within the above range, the crystallinity is maintained in an appropriate range, and the gel sheet is 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 cooling roll, or the like can be used, but it is preferable to contact with a roll cooled with the refrigerant for cooling.

During sheet molding, a phenomenon is observed in which the plasticizer bleeds out due to cooling. If the amount of the antioxidant added is excessive, not only the plasticizer but also the antioxidant is deposited on the cooling roll when the plasticizer is bleeding out. This precipitate contaminates the process and is one of the causes of product defects.

(E) Step of stretching the gel sheet Stretching of the gel sheet is also referred to as wet stretching. Wet stretching is performed in at least one axial direction. Since the gel sheet contains a film-forming solvent, it can be uniformly stretched. After heating, the gel 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 stretching or biaxial stretching, but biaxial stretching is preferable. In the case of biaxial stretching, any of simultaneous biaxial stretching, sequential stretching and multi-stage stretching (for example, a combination of simultaneous biaxial stretching and sequential stretching) may be used.

In the case of uniaxial stretching, for example, the final area stretching ratio in the wet stretching is preferably 3 times or more, more preferably 4 times or more and 30 times or less. Further, in the case of biaxial stretching, 9 times or more is preferable, 16 times or more is more preferable, and 25 times or more is further preferable. 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 draw ratio in the wet stretching is preferably 3 times or more in both the MD direction (mechanical direction) and the TD direction (width direction), and may be the same or different in the MD direction and the TD direction. When the draw ratio is 5 times or more, improvement in puncture strength can be expected. The stretching ratio in this step refers to the stretching ratio of the gel sheet immediately before being subjected to the next step with reference to the gel sheet immediately before this step. Further, the TD direction is a direction orthogonal to (intersects) the MD direction when the microporous membrane is viewed in a plane.

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

Due to the above stretching, cleavage occurs between the polyethylene lamellas inside the sheet, the polyethylene phase becomes finer, and a large number of fibrils are formed. Fibrils form a three-dimensionally irregularly connected network structure. When the sheet is stretched, the mechanical strength is improved while the pores are expanded. However, if stretching is performed under appropriate conditions, it is possible to have a high porosity even with a thin film while controlling the pore diameter.

(F) Step of removing the plasticizer from the stretched gel sheet and drying it to form a microporous film The plasticizer is removed by washing with a washing solvent. Since the polyolefin phase is phase-separated from the plasticizer phase, it is composed of fibrils that form a fine three-dimensional network structure when the plasticizer is removed, and is porous with pores (voids) that communicate irregularly in three dimensions. A membrane is obtained. Since the cleaning solvent and the method for removing the plasticizer using the cleaning solvent are known, the description thereof will be omitted.

The microporous membrane is dried by a heat drying method or an air drying method. The drying temperature is preferably not less than the crystal dispersion temperature (Tcd) of the polyolefin resin, and particularly preferably 5 ° C. or more lower than (Tcd). The drying is preferably carried out with the microporous membrane film as 100% by mass (dry mass) until the residual cleaning solvent is 5% by mass or less, and more preferably 3% by mass or less. When the residual cleaning solvent is within the above range, the porosity of the polyolefin microporous film is maintained when the subsequent microporous film film stretching step and heat treatment step are performed, and the deterioration of permeability is suppressed.

Stretching of the film after drying is also referred to as dry stretching. The dried microporous membrane film is dry-stretched at least in the uniaxial direction. The dry stretching of the microporous 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 stretching or biaxial stretching. In the case of biaxial stretching, either simultaneous biaxial stretching or sequential stretching may be used, but sequential stretching is preferred. In the case of sequential stretching, it is preferable to stretch in the MD direction and then continuously in the TD direction.
(G) Others The dried film or the polyolefin microporous film after dry stretching may be heat-treated. The heat treatment stabilizes the crystals and makes the lamella uniform. As the heat treatment method, at least one of a heat fixing treatment and a heat relaxation treatment can be used. The heat fixing process is a heat treatment in which the film is heated while holding both ends in the TD direction so that the dimensions of the film in the TD direction do not change. The heat fixing treatment is preferably performed by a tenter method or a roll method. The heat relaxation treatment is a heat treatment in which the film is heat-shrinked in the MD direction or the TD direction during heating. For example, as a heat relaxation treatment method, the method disclosed in JP-A-2002-256099 can be mentioned. The heat treatment temperature is preferably in the range of (Tcd) to (Tm: melting point) of the polyolefin resin.

Further, the microporous polyolefin membrane after the heat treatment can be further subjected to a crosslinking treatment and a hydrophilization treatment. For example, the microporous membrane is subjected to a cross-linking treatment by irradiating it with ionizing radiation such as α-rays, β-rays, γ-rays, and electron beams. In the case of electron beam irradiation, an electron dose of 0.1 to 100 Mrad is preferable, and an acceleration voltage of 100 to 300 kV is preferable. The cross-linking treatment raises the meltdown temperature of the microporous membrane. Further, the hydrophilization treatment can be performed by a monomer graft, a surfactant treatment, a corona discharge or the like. The monomer graft is preferably performed after the cross-linking treatment.

The polyolefin microporous membrane obtained by the method for producing a polyolefin microporous membrane of the present invention described above has a controlled pore diameter and has a good porosity as a microporous membrane, and thus can be used as a separator for a secondary battery. Suitable.

Hereinafter, the present invention will be described in more detail with reference to Examples. The present invention is not limited to these examples.
[Example 1]
A polyethylene solution was prepared by adding a polyethylene resin, liquid paraffin as a plasticizer, and an antioxidant under the composition and conditions shown in Table 1 and melt-kneading at 180 ° C. using a twin-screw kneader. Then, it was supplied from a twin-screw extruder to a T-shaped base via a polymer pipe and extruded. Here, the resin temperature of the polyethylene solution extruded from the outlet of the twin-screw extruder was 200 ° C., the oxidation induction time was 7 minutes, and the oxidation start temperature was 225 ° C. Further, the passage time of the resin from the outlet of the twin-screw extruder to the mouthpiece was defined as the residence time, which was 26 minutes in Example 1. Then, the extruded body was cooled while being taken up by a cooling roll to form a gel sheet. This gel sheet was simultaneously biaxially stretched (wet stretched) at 115 ° C. in both the MD direction and the TD direction by a tenter stretching machine at a magnification of 5 times. The stretched gel sheet was fixed to a 20 cm × 20 cm aluminum frame plate, immersed in a methylene chloride bath heated to 25 ° C., shaken at 100 rpm for 3 minutes to remove liquid paraffin, air-dried at room temperature, and a dry film was formed. Obtained. The dry film was dry-stretched 5 times in the MD direction and 5 times in the TD direction at 126 ° C. using a batch type stretching machine. Next, this film was heat-relaxed by the tenter method while shrinking at 126 ° C. by 8%. The evaluation results of the obtained polyolefin microporous membrane are also shown in Table 1. In Table 1, UHMwPE is, Mw represents an ultra high molecular weight polyethylene of 2.0 × 10 ^6, HDPE is, Mw is 6.0 × 10 ^5, shows a high density polyethylene having a density of 0.95 g / ^{m 3} .. The resin concentration (wt%) in the solution indicates the content of the polyolefin resin with respect to the total of the polyolefin resin and the plasticizer. The amount of the antioxidant (wt%) indicates the content of the antioxidant in the polyolefin resin.
[Example 2]
As shown in Table 1, a polyolefin microporous film was prepared in the same manner as in Example 1 except that the amount of the antioxidant was changed to 1.0% by mass and the residence time was changed to 117 minutes.
[Example 3]
As shown in Table 1, a microporous polyolefin membrane was prepared in the same manner as in Example 1 except that the amount of the antioxidant was changed to 1.0% by mass and the residence time was changed to 18 minutes.
[Example 4]
As shown in Table 1, a microporous polyolefin membrane was prepared in the same manner as in Example 1 except that the amount of the antioxidant was changed to 1.0% by mass, the kneading temperature was changed to 200 ° C., and the residence time was changed to 58 minutes.
[Example 5]
As shown in Table 1, a microporous polyolefin membrane was prepared in the same manner as in Example 1 except that the amount of the antioxidant was 1.0% by mass, the kneading temperature was 200 ° C., and the residence time was changed to 23 minutes.
[Example 6]
As shown in Table 1, a microporous polyolefin membrane was prepared in the same manner as in Example 1 except that the amount of the antioxidant was changed to 1.0% by mass, the kneading temperature was changed to 200 ° C., and the residence time was changed to 92 minutes.
[Comparative Example 1]
As shown in Table 1, a polyolefin microporous film was prepared in the same manner as in Example 1 except that the residence time was changed to 80 minutes.
[Comparative Example 2]
As shown in Table 1, a microporous polyolefin membrane was prepared in the same manner as in Example 1 except that the amount of the antioxidant was changed to 5.0% by mass and the residence time was changed to 50 minutes.
[Comparative Example 3]
As shown in Table 1, a microporous polyolefin membrane was prepared in the same manner as in Example 1 except that the amount of the antioxidant was changed to 1.0% by mass and the residence time was changed to 11 minutes.
[Comparative Example 4]
As shown in Table 1, a microporous polyolefin membrane was prepared in the same manner as in Example 1 except that the amount of the antioxidant was changed to 4.0% by mass and the residence time was changed to 43 minutes.
[Comparative Example 5]
As shown in Table 1, a microporous polyolefin membrane was prepared in the same manner as in Example 1 except that the amount of the antioxidant was changed to 1.0% by mass, the kneading temperature was changed to 200 ° C., and the residence time was changed to 88 minutes.

[Comparative Example 6]
As shown in Table 1, a polyolefin microporous film was prepared in the same manner as in Example 1 except that the amount of the antioxidant was changed to 1.0% by mass, the kneading temperature was changed to 200 ° C., and the residence time was changed to 11 minutes.
The evaluation method is as follows.

[Resin temperature]
Using a contact thermometer, the temperature at the center of the polyolefin solution discharged from the outlet of the twin-screw extruder was measured and used as the resin temperature.

[Oxidation start temperature]
10 mg of the polyolefin solution taken out from the outlet of the twin-screw kneader is placed in an aluminum pan, and an oxygen-containing gas is flowed at a flow rate of 200 ml / min using TG / DTA6200 (manufactured by Seiko Instruments Co., Ltd.) at 10 ° C./min. The temperature at which the temperature was raised from 20 ° C. to 400 ° C. and the amount of heat began to increase was defined as the oxidation start temperature.

[Oxidation induction time]
10 mg of the polyolefin solution taken out from the outlet of the twin-screw kneader is placed in an aluminum pan, and nitrogen gas is flowed at a flow rate of 200 ml / min using TG / DTA6200 (manufactured by Seiko Instruments Co., Ltd.) at 20 ° C./min for 30. The temperature was raised from ° C to 200 ° C. Then, after stabilizing the temperature by flowing nitrogen gas at a flow rate of 50 ml / min for 15 minutes while maintaining 200 ° C., the nitrogen gas was switched to oxygen-containing gas and flowed at 50 ml / min, and the inflow gas was switched to oxygen-containing gas. The time at which the amount of heat began to increase was defined as the oxidation induction time.

[Deterioration of resin]
The degree of deterioration of the polyolefin resin in the polyolefin solution taken out from the outlet of the twin-screw kneader was evaluated using the melt index (MI) as an index. For the melt index, use the melt indexer F-F01 / F-W01 (manufactured by Toyo Seiki Seisakusho) for the polyolefin solution taken out from the outlet of the twin-screw kneader, according to the method A of JIS K 7210-1 (2014). The test temperature and load were set to 190 ° C. and 2.16 kgf (21.18 N), respectively, and the melt index was measured and determined (unit: g / 10 min). When the ratio of the MI value to the MI value of Example 1 when the ratio of the MI value to the MI value of Example 1 when an appropriate amount of the antioxidant is added is less than 1.0 is excellent (⊚), 1.0 or more and 1 If it is less than 3.3, it is good (◯), if it is 1.3 or more and less than 2.5, it is acceptable (Δ), and if it is 2.5 or more, it is not (×).

[Process pollution]
If the kneaded product contains an excessive amount of the antioxidant, the antioxidant may precipitate on the cast roll and cause product defects, or the intake pipe may be clogged and the process may become unstable. Therefore, by observing the clogging inside the vent hole, it is not possible if the number of days that the antioxidant is deposited on the intake pipe is less than 30 days on average per year (×), and if it is 30 days or more and less than 55 days (△). , The case of 55 days or more and less than 80 days was evaluated as good (〇), and the case of not clogging the vent hole even after 80 days or more was evaluated as excellent (⊚).

[Film appearance]
The microporous film was cut out to a size of 95 mm × 95 mm, spread on an illuminated magnifying glass loupe (ILLUMINATING LUPE manufactured by PEAK) without wrinkles, and the microporous film was observed with transmitted light. Count transparent spots with a major axis of 0.3 cm or more, and those with 3 or less spots are excellent (◎), those with more than 3 and 5 or less are good (○), those with more than 5 and 10 or less. Was evaluated as acceptable (Δ), and those exceeding 10 were evaluated as unacceptable (×).

(evaluation)
It was shown that the production methods of Examples 1 to 6 improved the resin deterioration resistance of the polyolefin solution, the process suitability, and the quality of the product because the oxidation induction time satisfied the predetermined range. From the above, it is clear that the production method of the present embodiment improves the yield, quality and yield in the production process, and improves the productivity of the polyolefin microporous film.

The method for producing a microporous polyolefin membrane of the present invention has improved yield, quality and yield in the manufacturing process, and the surface appearance of the obtained microporous polyolefin membrane is good, and is suitable for a separator for a secondary battery. Can be used for.

Claims (4)

(A) Step of supplying the polyolefin resin and the plasticizer to the extruder and melt-kneading to prepare the polyolefin solution (b) Step of removing the gas dissolved in the polyolefin solution (c) Discharging the polyolefin solution from the extruder Step of extruding into sheet shape through mouthpiece (d) Step of cooling extruded sheet-like polyolefin solution to form gel sheet (e) Step of stretching gel sheet (f) Removal of plasticizer from stretched gel sheet and drying In a method for producing a microporous film, which comprises a step of forming a microporous film.
When the residence time is the time from when the polyolefin resin and the plasticizer are put into the extruder to when it is extruded through the mouthpiece after melt-kneading, the residence time of the polyolefin solution is 10 minutes or more and 120 minutes or less, and the extruder When the resin temperature of the polyolefin solution discharged from the outlet is lower than the oxidation start temperature of the polyolefin solution, the oxidation induction time of the polyolefin solution is 20% or more and 300% or less of the residence time, and the resin temperature of the discharged polyolefin solution is A method for producing a microporous film, characterized in that the oxidation induction time of the polyolefin solution is 60% or more and 300% or less of the residence time when the temperature is higher than the oxidation start temperature of the polyolefin solution. The method for producing a microporous membrane according to claim 1, wherein the resin temperature of the polyolefin solution discharged from the outlet of the extruder is in the range of 150 to 250 ° C. The method for producing a microporous membrane according to claim 1 or 2, wherein the polyolefin resin is polyethylene having a weight average molecular weight of 1 × 10 ⁴ or more and 8 × 10 ^{6 or less.} The polyolefin resin is a composition comprising a weight-average molecular weight 1 × 10 of less than ⁶ polyolefin and a weight average molecular weight 1 × 10 ⁶ or more polyolefins, the content of the weight average molecular weight 1 × 10 under ⁶ polyolefin in the composition The method for producing a microporous polyolefin film according to any one of claims 1 to 3, wherein the amount is 50% by mass to 99% by mass.

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