JP2024017923A - Substrate for liquid filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a substrate for a liquid filter that can reduce fluctuations in filtration time even with small apertures.

SOLUTION: This substrate for a liquid filter includes a microporous polyolefin membrane, wherein: the average value of pore diameters of the microporous polyolefin membrane is 1 nm-50 nm; the average water flow rate value of the microporous polyolefin membrane is 0.003 L/min/ft²/psi to 0.180 L/min/ft²/psi; and the variation coefficient of the water flow rate of the microporous polyolefin membrane is at most 0.100.

SELECTED DRAWING: None

COPYRIGHT: (C)2024,JPO&INPIT

Description

The present disclosure relates to substrates for liquid filters.

In recent years, electronic devices have become increasingly smaller and more sophisticated. In particular, digital devices and mobile terminals, typified by personal computers and smartphones, have undergone dramatic evolution. It is a well-known fact that among the various technologies that drive and support this evolution, technological innovation in the semiconductor industry plays a major role. In recent years, in the semiconductor industry, there has been competition in the development of interconnect pattern dimensions of less than 10 nm, and each company is rushing to build cutting-edge production lines.

The lithography process is a process of forming patterns in semiconductor component manufacturing. As patterns become finer in recent years, very advanced techniques are required not only for the properties of the chemical liquid itself used in the lithography process, but also for the handling of the chemical liquid until it is applied onto the wafer.

A highly prepared chemical solution is filtered through a dense liquid filter immediately before being coated onto a wafer, to remove minute foreign matter and gel-like foreign matter derived from the chemical solution, which have a large effect on pattern formation and yield. In the formation of state-of-the-art wiring patterns with a pattern size of less than 10 nm, as the wiring pattern size becomes smaller from 10 nm to several nanometers, finer pore diameters and smaller pores are needed to collect fine foreign particles of 5 nm or less in the chemical solution. A liquid filter that has both good permeability is needed. For this reason, filter manufacturers are actively developing liquid filters for cutting-edge semiconductors.
As liquid filter substrates applicable to dense liquid filters, liquid filter substrates containing microporous polyolefin membranes with small pore diameters are known (see, for example, Patent Documents 1 to 7).

Japanese Patent Application Publication No. 2014-218563 Japanese Patent Application Publication No. 2014-217800 Patent No. 5684951 Patent No. 5684952 Patent No. 5684953 Japanese Patent Application Publication No. 2018-167198 Patent No. 6805371

In a liquid filter base material having a micropore size of 50 nm or less, the problem of variation in filtration time in the width direction of the liquid filter base material is likely to occur. If the filtration time of a liquid filter is not uniform, for example, the pressure applied in the width direction of the filter is likely to be uneven, such as low pressure at the center of the filter and high pressure at the ends of the filter, and the liquid cannot be filtered uniformly throughout the filter. As a result, there is a concern that filter performance and filtration life may be reduced, and semiconductor manufacturing yield may be reduced.
The present disclosure has been made in view of the above-mentioned conventional problems, and an object of the present disclosure is to provide a base material for a liquid filter that can suppress variations in filtration time even with small pore diameters.

Specific means for achieving the above object are as follows.
<1> A liquid filter base material comprising a polyolefin microporous membrane,
The average value of the pore diameter of the polyolefin microporous membrane is 1 nm to 50 nm,
The average value of the water flow rate of the polyolefin microporous membrane is 0.003 L/min/ft ² /psi to 0.180 L/min/ft ² /psi,
A base material for a liquid filter, wherein the polyolefin microporous membrane has a coefficient of variation of water flow rate of 0.100 or less.
<2> The base material for a liquid filter according to <1>, wherein the microporous polyolefin membrane has a coefficient of variation in pore diameter of 0.100 or less.
<3> The base material for a liquid filter according to <1> or <2>, wherein the polyolefin microporous membrane has a coefficient of variation in film thickness of 0.100 or less.
<4> The base material for a liquid filter according to any one of <1> to <3>, wherein the polyolefin microporous membrane has a coefficient of variation of porosity of 0.100 or less.
<5> The substrate for a liquid filter according to any one of <1> to <4>, wherein the polyolefin microporous membrane has an average film thickness of 3 μm to 20 μm.
<6> The substrate for a liquid filter according to any one of <1> to <5>, wherein the microporous polyolefin membrane has an average porosity of 35% to 60%.

According to the present disclosure, it is possible to provide a base material for a liquid filter that can suppress variations in filtration time even with small pore diameters.

Embodiments of the present disclosure will be described in detail below. However, the present disclosure is not limited to the following embodiments. In the following embodiments, the constituent elements (including elemental steps and the like) are not essential unless otherwise specified. The same applies to numerical values and their ranges, and they do not limit the present disclosure.

In this disclosure, the term "step" includes not only a step that is independent from other steps, but also a step that cannot be clearly distinguished from other steps, as long as the purpose of the step is achieved. .
In the present disclosure, numerical ranges indicated using "~" include the numerical values written before and after "~" as minimum and maximum values, respectively.
In the numerical ranges described step by step in this disclosure, the upper limit or lower limit described in one numerical range may be replaced with the upper limit or lower limit of another numerical range described step by step. . Furthermore, in the numerical ranges described in this disclosure, the upper limit or lower limit of the numerical range may be replaced with the values shown in the Examples.
In the present disclosure, each component may contain multiple types of applicable substances. If there are multiple types of substances corresponding to each component in the composition, the content rate or content of each component is the total content rate or content of the multiple types of substances present in the composition, unless otherwise specified. means quantity.
In this disclosure, the term "layer" or "film" refers to the case where the layer or film is formed only in a part of the region, in addition to the case where the layer or film is formed in the entire region when observing the region where the layer or film is present. This also includes cases where it is formed.

In the present disclosure, regarding the microporous polyolefin membrane, the term "longitudinal direction" refers to the longitudinal direction of the microporous polyolefin membrane produced in a long shape, and the term "width direction" refers to the longitudinal direction of the microporous polyolefin membrane. means the direction perpendicular to Hereinafter, the "width direction" will also be referred to as "TD", and the "longitudinal direction" will also be referred to as "MD".
Moreover, the "width direction" in the base material for a liquid filter means the same direction as the direction orthogonal to the longitudinal direction in the polyolefin microporous membrane.

[Base material for liquid filter]
The substrate for a liquid filter of the present disclosure is a substrate for a liquid filter containing a microporous polyolefin membrane, wherein the average pore diameter of the microporous polyolefin membrane is 1 nm to 50 nm, and the microporous polyolefin membrane has a The average value of the flow rate is 0.003 L/min/ft ² /psi to 0.180 L/min/ft ² /psi, and the coefficient of variation of the water flow rate of the polyolefin microporous membrane is 0.100 or less. It is.
According to the liquid filter base material of the present disclosure, variations in filtration time can be suppressed even with small pore diameters. Although the reason is not clear, it is inferred as follows.
In the liquid filter base material of the present disclosure, since the average value of the water flow rate of the polyolefin microporous membrane is 0.003 L/min/ft ² /psi to 0.180 L/min/ft ² /psi, it can be used as a liquid filter. This not only makes it easier to obtain sufficient water permeability, but also makes it easier to maintain stability in liquid feeding through the filter over a long period of time. Further, in the liquid filter base material of the present disclosure, since the average value of the pore diameter of the polyolefin microporous membrane is 1 nm to 50 nm, it is possible to obtain sufficient liquid permeability and, for example, to prevent microparticles of about 10 nm or less. It can be collected to a very high degree. From the above, it is inferred that the liquid filter base material of the present disclosure can effectively function as a liquid filter for advanced semiconductors.
On the other hand, when the average value of the pore diameter of the polyolefin microporous membrane is as small as 50 nm or less, the filtration time tends to vary in the width direction of the liquid filter substrate. However, in the present disclosure, since the coefficient of variation of the water flow rate of the polyolefin microporous membrane is set to 0.100 or less, it is presumed that the pressure applied in the width direction of the filter is less likely to become uneven, and variations in filtration time are suppressed.
From the above, according to the liquid filter base material of the present disclosure, it becomes easier to filter liquid uniformly over the entire filter, and as a result, deterioration of filter performance and decrease in filtration life tend to be more likely to be suppressed.

(Polyolefin microporous membrane)
The liquid filter substrate of the present disclosure includes a microporous polyolefin membrane. The liquid filter base material of the present disclosure may be a base material consisting only of a microporous polyolefin membrane, or a base material consisting of a microporous polyolefin membrane and another layer (for example, a coating layer or a porous base material). It may be a base material. Moreover, the polyolefin microporous membrane may be a laminated membrane having a structure in which a plurality of polyolefin porous layers are laminated.
A microporous polyolefin membrane is a microporous membrane containing polyolefin. Here, a microporous membrane has a structure in which it has many micropores inside and these micropores are connected, allowing gas or liquid to pass from one surface to the other. It means a membrane.
In addition, in the polyolefin microporous membrane, it is preferable that the polyolefin is contained in an amount of 90 parts by mass or more based on 100 parts by mass of the polyolefin microporous membrane, and the remainder is an organic filler and an inorganic filler within a range that does not affect the effects of the present disclosure. , surfactants, and other additives may also be included.

(pore diameter)
In the present disclosure, the polyolefin microporous membrane has an average pore diameter of 1 nm to 50 nm. An average value of pore diameter of 1 nm to 50 nm indicates a small pore diameter. If the average value of the pore diameter of the polyolefin microporous membrane is 1 nm or more, sufficient liquid permeability as a liquid filter can be obtained. From such a viewpoint, the average value of the pore diameter is preferably 10 nm or more, more preferably 11 nm or more, and even more preferably 13 nm or more. On the other hand, if the average value of the pore diameter of the polyolefin microporous membrane is 50 nm or less, it is possible to trap particles as small as, for example, about 10 nm to a very high degree. From such a viewpoint, the average value of the pore diameter is preferably 40 nm or less, more preferably 30 nm or less, further preferably 25 nm or less, and particularly preferably 20 nm or less.

In the present disclosure, the polyolefin microporous membrane preferably has a pore diameter variation coefficient of 0.100 or less, more preferably 0.095 or less, and 0.093 or less, from the viewpoint of further suppressing variations in filtration time. It is more preferably at most 0.085, particularly preferably at most 0.085. The coefficient of variation of the pore diameter may be 0.0015 or more, preferably 0.020 or more. The coefficient of variation of pore diameter is preferably 0.0015 to 0.100.

In the present disclosure, the value of the pore diameter of the polyolefin microporous membrane, its average value, and coefficient of variation refer to the values determined by the method described in the Examples section.

(water flow rate)
In the present disclosure, the polyolefin microporous membrane has an average water flow rate of 0.003 L/min/ft ² /psi to 0.180 L/min/ft ² /psi. If the average value of the water flow rate of the polyolefin microporous membrane is 0.003 L/min/ft ² /psi or more, not only will it be easy to obtain sufficient water permeability as a liquid filter, but also the flow rate of liquid through the filter will be improved. Stability (e.g., stability of power load to maintain a constant flow rate or stability of flow rate under a constant flow pressure (constant power load)) is easily achieved over a long period of time. Therefore, it is preferable. From such a viewpoint, the average value of the water flow rate of the polyolefin microporous membrane is preferably 0.004 L/min/ft ² /psi or more, and more preferably 0.005 L/min/ft ² /psi or more. On the other hand, when the average value of the water flow rate of the polyolefin microporous membrane is 0.180 L/min/ft ² /psi or less, it is preferable because, for example, it becomes easy to highly collect microparticles of about 10 nm or less. From this viewpoint, the average value of the water flow rate of the polyolefin microporous membrane is preferably 0.150 L/min/ft ² /psi or less, more preferably 0.100 L/min/ft ² /psi or less.

In the present disclosure, the microporous polyolefin membrane preferably has a coefficient of variation of water flow rate of 0.100 or less, more preferably 0.095 or less, from the viewpoint of further suppressing variations in filtration time. It is more preferably 0.090 or less, particularly preferably 0.080 or less. The coefficient of variation of the water flow rate may be 0.010 or more, preferably 0.015 or more. The coefficient of variation of the water flow rate is preferably 0.010 to 0.100.

In the present disclosure, the value of the water flow rate of the polyolefin microporous membrane, its average value, and coefficient of variation refer to the values determined by the method described in the Examples section.

(film thickness)
In the present disclosure, the average thickness of the polyolefin microporous membrane is preferably 3 μm to 20 μm. When the average film thickness of the polyolefin microporous membrane is 3 μm or more, sufficient mechanical strength is easily obtained, and handling properties during processing of the polyolefin microporous membrane and durability during long-term use of the filter cartridge are obtained. This is preferable because it becomes easier. From this viewpoint, the average thickness of the polyolefin microporous membrane is more preferably 4 μm or more, further preferably 5 μm or more, and particularly preferably 6 μm or more. On the other hand, when the average value of the membrane thickness is 20 μm or less, not only the variation in thickness in the width direction is reduced with a single microporous polyolefin membrane, but also sufficient liquid permeability can be obtained, and the This is preferable because it becomes easier to obtain a larger filtration area in the filter cartridge, and it becomes easier to design the flow rate and structure of the filter during processing of the microporous polyolefin membrane. From this point of view, the average thickness of the polyolefin microporous membrane is more preferably 18 μm or less, further preferably 16 μm or less, and particularly preferably 14 μm or less.

For example, assuming that a filter cartridge is housed in a housing of the same size, the thinner the filter medium (the entire constituent material including the filter base material) is, the larger the filter area can be, so it can be used as a liquid filter. Favorable high flow rate and low filtration pressure designs are possible. That is, the liquid filter can be designed so that the filtration pressure is low when the same flow rate is desired to be maintained, and the flow rate is high when the same filtration pressure is desired to be maintained. In particular, as the filtration pressure decreases, foreign substances that have been captured will be continuously exposed to the filtration pressure inside the filter medium, increasing the probability that they will be pushed out of the filter medium together with the filtrate over time and leak out. It is expected that the effect will be a significant reduction. In addition, by lowering the filtration pressure, it is expected that the probability that gas dissolved in the liquid to be filtered will appear as minute bubbles due to the pressure difference before and after filtration (pressure drop after filtration) will be significantly reduced. can. Furthermore, the effect of improving the filtration yield of the object to be filtered, such as a chemical liquid, and the effect of maintaining the quality of the object to be filtered, such as a chemical liquid, at a high level over a long period of time can be expected.

On the other hand, the thinner the filter medium, the lower the strength and durability of the filter medium. ), it is also possible to adjust the durability and flow rate design while reinforcing it.

In the present disclosure, the microporous polyolefin membrane preferably has a coefficient of variation in film thickness of 0.100 or less, more preferably 0.095 or less, from the viewpoint of further suppressing variations in filtration time. It is more preferably 090 or less, and particularly preferably 0.065 or less. The coefficient of variation of film thickness may be 0.0015 or more, preferably 0.020 or more. The coefficient of variation in film thickness is preferably 0.0015 to 0.100.

In the present disclosure, the value of the film thickness of the polyolefin microporous membrane, its average value, and coefficient of variation refer to the values determined by the method described in the Examples section.

(porosity)
In the present disclosure, the average value of the porosity of the polyolefin microporous membrane is preferably 35% to 60%. When the average value of the porosity of the polyolefin microporous membrane is 35% or more, the liquid permeability will be good, and from this point of view, it is more preferably 36% or more, even more preferably 38% or more, and 40% or more. is particularly preferred. On the other hand, it is preferable that the average value of the porosity is 60% or less because the mechanical strength of the polyolefin microporous membrane is good and the handling property is also improved. From this point of view, the average value of the porosity of the polyolefin microporous membrane is more preferably 58% or less, and even more preferably 55% or less.

In the present disclosure, the polyolefin microporous membrane preferably has a porosity variation coefficient of 0.100 or less, more preferably 0.095 or less, from the viewpoint of further suppressing variations in filtration time. It is more preferably .090 or less, and particularly preferably 0.075 or less. The coefficient of variation of porosity may be 0.020 or more, preferably 0.025 or more. The coefficient of variation of porosity is preferably 0.020 to 0.100.

In the present disclosure, the porosity value of a microporous polyolefin membrane, its average value, and coefficient of variation refer to values determined by the method described in the Examples section.

(calcium content)
In the present disclosure, it is preferable that the calcium content in the polyolefin microporous membrane is 2000 ppb or less. When the calcium content is 2000 ppb or less, not only the cleaning time when manufacturing a filter cartridge can be significantly shortened, but also the amount of solvent such as water used during cleaning can be greatly reduced. From this viewpoint, the calcium content of the polyolefin microporous membrane is more preferably 1500 ppb or less, even more preferably 1300 ppb or less, particularly preferably 1000 ppb or less, and extremely preferably 800 ppb or less. On the other hand, it is preferable for the calcium content to be as low as possible from the viewpoint of manufacturing efficiency of filter cartridges. However, in reality, it is difficult to reduce the calcium content because manufacturing raw materials may contain trace amounts of calcium, or trace amounts of calcium may be mixed in during the manufacturing process of polyolefin microporous membranes. The calcium content of the polyolefin microporous membrane may be 0 ppb or more, 1 ppb or more, 10 ppb or more, or 50 ppb. If the calcium content is 2000 ppb or less, the manufacturing efficiency of filter cartridges can be significantly improved.

Further, in the present disclosure, a polyolefin microporous membrane having minute pores with an average pore diameter of 1 nm to 50 nm is applied. In a polyolefin microporous membrane having such minute pores, the specific surface area becomes significantly large, and the amount of metal ions and the like eluted tends to increase. However, if the calcium content is 2000 ppb or less, even a microporous polyolefin membrane having such minute pores can significantly improve the manufacturing efficiency of the filter cartridge. In addition, if the calcium content of the polyolefin microporous membrane is 1 ppb or more, chloride ions derived from the polymerization catalyst that may remain in trace amounts in the polyolefin can be sufficiently neutralized, and stainless steel etc. This eliminates the risk of corroding the pipes. From such a viewpoint, the calcium content of the polyolefin microporous membrane is preferably 10 ppb or more, more preferably 50 ppb or more.

In the present disclosure, the method for adjusting the calcium content of the polyolefin microporous membrane to 2000 ppb or less is not particularly limited. Examples include using a polyolefin microporous membrane, and washing it with an acid etc. for a long time after producing the microporous polyolefin membrane. From the viewpoint of sufficiently reducing the calcium content to the inside of the polyolefin microporous membrane, it is preferable to use a polyolefin raw material with a low calcium content.

In the present disclosure, the calcium content in the polyolefin microporous membrane refers to a value determined by the method described in the Examples section.

Furthermore, the method for controlling the porous structure of the polyolefin microporous membrane is not particularly limited. For example, the composition of the polyolefin resin, the concentration of the polyolefin resin in the raw material, the mixing ratio if multiple solvents are mixed in the raw material, the heating temperature for squeezing out the solvent inside the extruded sheet, the pressing pressure, and the heating temperature. It can be controlled by time, stretching ratio, heat treatment (heat setting) temperature after stretching, immersion time in extraction solvent, temperature and time of annealing treatment, etc.

(Polyolefin)
In the present disclosure, as the polyolefin constituting the microporous polyolefin membrane, for example, a homopolymer or copolymer of polyethylene, polypropylene, polybutylene, polymethylpentene, etc., or a mixture of two or more thereof can be used. Among these, polyethylene is preferred. As the polyethylene, high-density polyethylene, a mixture of high-density polyethylene and ultra-high molecular weight polyethylene, etc. are suitable. Further, polyethylene and other components may be used in combination. Examples of components other than polyethylene include polypropylene, polybutylene, polymethylpentene, and copolymers of polypropylene and polyethylene. Further, as the polyolefin, polyolefins having mutually different properties may be used in combination. Specifically, incompatible polyolefins with different degrees of polymerization or branching may be used in combination, or in other words, polyolefins with different crystallinity, stretchability, or molecular orientation may be used in combination. good.

In particular, the polyolefin used in the present disclosure is a polyethylene composition obtained by mixing a high molecular weight polyethylene with a weight average molecular weight of 3 million to 6 million, and a low molecular weight polyethylene with a weight average molecular weight of 200,000 to 800,000. It is preferable to use This has the effect of forming a network structure due to fibrillation during stretching and increasing the pore generation rate by blending appropriate amounts of two or more types of polyethylene.
In particular, the blending ratio of high molecular weight polyethylene and low molecular weight polyethylene is preferably 15:85 to 85:15, more preferably 19:81 to 81:19 in terms of mass ratio.
As the low molecular weight polyethylene, high density polyethylene having a density of 0.92 g/cm ³ to 0.98 g/cm ³ is preferable.

The weight average molecular weight was determined by heating and dissolving a sample of the polyolefin microporous membrane in o-dichlorobenzene and performing GPC (Waters Alliance GPC 2000 model, column: GMH6-HT and GMH6-HTL) at a column temperature of 135°C. , by measuring at a flow rate of 1.0 mL/min. Monodisperse polystyrene (manufactured by Tosoh Corporation) can be used for molecular weight calibration.

In the present disclosure, the polyolefin constituting the polyolefin microporous membrane preferably has a calcium content of 0 ppb or more and 1000 ppb or less, more preferably 50 ppb or more and 500 ppb or less, and even more preferably 100 ppb or more and 300 ppb or less.
A method for adjusting the calcium content to 1000 ppb or less includes a method of adjusting the amount of metal soap (such as calcium stearate) added to the polyolefin after polymerization. Another example is a method in which commercially available polyolefin raw materials are washed with acid.
In the present disclosure, the calcium content of polyolefin refers to a value determined by the method described in the Examples section.

<Liquid filter>
The above-described base material for a liquid filter of the present disclosure is suitably processed to impart affinity with a chemical solution, and then processed into a cartridge shape, which can be used as a liquid filter. A liquid filter is a device for removing particles made of at least one of an organic substance and an inorganic substance from a liquid to be treated that contains or may contain particles. The particles exist in solid or gel form in the liquid to be treated. The liquid filter of the present disclosure is suitable for removing very fine particles with a particle size of about several nanometers. Further, the liquid filter can be used not only in semiconductor manufacturing processes but also in other manufacturing processes such as display manufacturing and polishing.

Porous base materials such as polytetrafluoroethylene are well known as base materials for liquid filters. When a base material containing the polyolefin microporous membrane of the present disclosure described above is used as a base material for a liquid filter, it has better affinity with a chemical solution than a polytetrafluoroethylene porous base material. Therefore, for example, it is easier to process the filter to make it compatible with the chemical liquid, and when filling the filter with the chemical liquid when loading the filter cartridge into the filter housing and starting the filtration of the chemical liquid, it is possible to This results in effects such as less air pockets forming and improved filtration yield of chemical solutions. Furthermore, since the polyethylene structure itself does not contain halogen elements, used filter cartridges are easy to handle and have the effect of reducing environmental impact.

<Method for producing microporous polyolefin membrane and method for controlling pore structure>
The polyolefin microporous membrane included in the liquid filter base material of the present disclosure can be preferably manufactured by the method shown below. That is,
(I) preparing a solution containing a polyolefin composition and a solvent;
(II) melt-kneading the prepared solution, extruding the obtained melt-kneaded product from a die, cooling and solidifying it to obtain a gel-like molded product,
(III) Squeezing out some of the solvent from the gel-like molded product in advance;
(IV) stretching the gel-like molded product from which some of the solvent has been squeezed out in at least one direction;
(V) a step of extracting and cleaning the solvent from the inside of the stretched intermediate molded product;
It can be preferably manufactured by sequentially carrying out the following steps.

In step (I), a solution containing a polyolefin composition and a solvent is prepared. The solvent is preferably a volatile solvent having a boiling point of at least less than 210° C. at atmospheric pressure. The solution is preferably a thermoreversible sol-gel solution. Specifically, in step (I), the polyolefin composition is heated and dissolved in a solvent to form a sol, thereby preparing a thermoreversible sol-gel solution. The volatile solvent with a boiling point of less than 210°C at atmospheric pressure is not particularly limited as long as it can swell or dissolve polyolefin, such as tetralin, ethylene glycol, decalin, toluene, xylene, diethyltriamine, and ethylenediamine. Preferred examples include liquid solvents such as , dimethyl sulfoxide, and hexane. These may be used alone or in combination of two or more. Among these, decalin or xylene is preferred. In addition to the above-mentioned volatile solvents with boiling points below 210°C at atmospheric pressure, non-volatile solvents with boiling points above 210°C, such as liquid paraffin, paraffin oil, mineral oil, and castor oil, are used to prepare this solution. It can also be included.

In the solution of step (I), the concentration of the polyolefin composition is preferably 15% by mass to 40% by mass, and 20% by mass from the viewpoint of controlling the liquid permeation performance of the polyolefin microporous membrane and the removal performance as a filter medium. More preferably, the amount is from % by mass to 30% by mass. When the concentration of the polyolefin composition is 15% by mass or more, the mechanical strength tends not to become too low, so handling properties are maintained well, and furthermore, the frequency of cracking is suppressed in the production of microporous polyolefin membranes. There is a tendency to Moreover, when the concentration of the polyolefin composition is 40% by mass or less, pores tend to be easily formed.

In step (II), the solution prepared in step (I) is melt-kneaded, the resulting melt-kneaded product is extruded through a die, and is cooled and solidified to obtain a gel-like molded product. Preferably, the polyolefin composition is extruded through a die at a temperature ranging from the melting point to 65° C. higher than the melting point to obtain an extrudate, and then the extrudate is cooled to obtain a gel-like molded product. The molded product is preferably shaped into a sheet.
The method for cooling the extrudate is not particularly limited, and may be a method by quenching in water or an organic solvent, or a method by casting onto a cooled metal roll. Generally, a method of quenching with water or the volatile solvent used in the sol-gel solution is used. The cooling temperature is preferably 10°C to 40°C. Note that it is preferable to produce the gel-like sheet while providing a water flow on the surface layer of the water bath to prevent the mixed solvent released from the gelled sheet in the water bath and floating on the water surface from adhering to the sheet again.

Step (III) is a step of squeezing out a portion of the solvent in the gel-like molded product before stretching the gel-like molded product in at least one direction. Step (III) can be suitably carried out by, for example, applying pressure to the surface of the gel-like molded product by passing it through a gap between two upper and lower belts or rollers. The amount of solvent to be squeezed out needs to be adjusted depending on the liquid permeation performance and the removal performance of the object to be filtered required of the polyolefin microporous membrane. Specifically, the amount of solvent to be squeezed out can be adjusted to an appropriate range by the pressing force between the upper and lower belts or rollers, the temperature of the squeezing step, and the number of times of pressing. Note that the pressure applied to the gel-like molded product is preferably set in the range of 0.01 MPa to 0.5 MPa using a roller or the like. A range of 0.05 MPa to 0.2 MPa is more preferred. The squeezing temperature is preferably 40°C to 100°C. Further, the number of times of pressing depends on the allowable space of the equipment, so it is possible to set it without any particular restriction. Note that, if necessary, one or more stages of preheating may be performed before squeezing out the solvent to remove a portion of the volatile solvent from within the sheet. In that case, the preheating temperature is preferably 50°C to 100°C. Further, when performing this preheating, the time is preferably 5 minutes to 9 minutes per stage. In this case, the amount of volatile solvent to be removed is adjusted by the conveyance distance and conveyance speed in the heating device.

Step (IV) is a step of stretching the gel-like molded product in at least one direction. The stretching in step (IV) is preferably biaxial stretching, and either sequential biaxial stretching in which longitudinal stretching and transverse stretching are carried out separately, or simultaneous biaxial stretching in which longitudinal stretching and transverse stretching are carried out simultaneously, is suitable. It is possible to use In addition, a method of stretching in the longitudinal direction multiple times and then stretching in the transverse direction, a method of stretching in the longitudinal direction and then stretching in the transverse direction multiple times, a method of sequentially biaxially stretching and then further stretching once in the longitudinal direction and/or the transverse direction A method of stretching multiple times is also preferred.

The stretching ratio (the product of the longitudinal stretching ratio and the transverse stretching ratio) is preferably 40 times to 10 times, more preferably 50 times, from the viewpoint of controlling the liquid permeation performance and the removal performance of the object to be filtered of the polyolefin microporous membrane. It is from 100 times to 100 times. When the stretching ratio is 40 times or more, it tends to be easier to suppress the occurrence of thickness unevenness in the microporous polyolefin membrane. If the stretching ratio is 105 times or less, the frequency of cracking during the production of a microporous polyolefin membrane tends to be low. It is preferable that the stretching is carried out with the solvent remaining in a suitable state. The stretching temperature is preferably 80°C to 125°C. Particularly preferred is 100°C to 120°C.

Further, a heat setting treatment may be performed subsequent to the stretching step (IV).
The heat fixation temperature is preferably 100° C. to 143° C. from the viewpoint of controlling the liquid permeation performance and the removal performance of the object to be filtered of the microporous polyolefin membrane. More preferably 105°C to 138°C. If the heat setting temperature is 100° C. or higher, the liquid permeation performance of the polyolefin microporous membrane tends to be further improved. When the heat setting temperature is 143° C. or lower, the removal performance of the polyolefin microporous membrane for filtering objects tends to be further improved.

Step (V) is a step of washing the stretched intermediate molded product in order to extract the solvent from inside the stretched intermediate molded product. Here, in step (V), in order to extract the solvent from the inside of the stretched intermediate molded product (stretched film), it is preferable to wash with a halogenated hydrocarbon such as methylene chloride or a hydrocarbon solvent such as hexane. . When washing the intermediate molded product by immersing it in a tank containing a solvent, it is preferable to take a time of 20 seconds to 150 seconds in order to obtain a microporous polyolefin membrane with a small amount of eluted matter, and more preferably 25 seconds. 150 seconds, particularly preferably 30 seconds to 120 seconds. Furthermore, in order to further increase the cleaning effect, divide the tank into several stages, pour the cleaning solvent from the downstream side of the polyolefin microporous membrane transport process, and flow the cleaning solvent towards the upstream side of the transport process. Preferably, the purity of the cleaning solvent in the downstream tank is higher than that in the upstream tank. When the tank storing the solvent is divided into several stages, it may be two tanks or three or more tanks. From the viewpoint of making the purity gradient of the cleaning solvent in each tank more gradual, the number of tanks storing the solvent is preferably three or more.
Further, depending on the required performance of the polyolefin microporous membrane, heat setting may be performed by annealing treatment (heat treatment). Note that the annealing treatment is preferably performed at a temperature of 50° C. to 150° C., more preferably 60° C. to 140° C., from the viewpoint of transportability in the process. From the viewpoint of suppressing shrinkage of the microporous polyolefin membrane over time, the annealing treatment is preferably performed at 100°C to 130°C.
When performing the annealing treatment, it is preferable to suppress shrinkage of the polyolefin microporous membrane in the width direction. By suppressing shrinkage in the width direction, the coefficient of variation of water flow rate, etc. for the polyolefin microporous membrane tends to become smaller. For example, by performing the annealing treatment with both widthwise ends of the polyolefin microporous membrane fixed with clips or the like, shrinkage of the polyolefin microporous membrane in the width direction tends to be suppressed.
In addition, the pore diameter, film thickness, porosity, and water flow rate of the polyolefin microporous membrane are determined by the polyolefin composition, the concentration of the polyolefin composition in the polyolefin solution, the method for making the base tape, the method for removing the solvent in the polyolefin solution, and the base tape composition. By appropriately adjusting the stretching conditions, heat treatment conditions, etc. of the tape, it can be set within a predetermined range.

Examples, comparative examples, and various measurement methods of the present disclosure will be described below, but the present disclosure is not limited to these examples in any way.

[Measuring method]
(film thickness)
The film thickness of the polyethylene microporous membrane was measured using a contact type film thickness meter (manufactured by Mitutoyo Co., Ltd.). Here, a cylindrical contact terminal with a bottom surface having a diameter of 0.5 cm was used. The measurement pressure was 0.1N.
-Average value-
Along the width direction (TD direction) of the polyolefin microporous membrane, the film thickness was measured at 11 points in total at 5 points at 26 mm intervals from the center of the product width to each end face from the center of the product width as described above. However, the value calculated as the arithmetic mean of the film thickness at each measurement location was taken as the average value of the film thickness.
-Coefficient of variation-
Along the width direction (TD direction) of the polyethylene microporous membrane, measure the film thickness at 11 points in total at 5 points at 26 mm intervals from the center of the product width to each end face from the center as described above. Then, the standard deviation σTD1 was determined based on the film thickness value at each measurement location. Next, the coefficient of variation CVTD1 was obtained by dividing σTD1 by the average value of the film thicknesses at 11 points in total. The same operation was repeated nine more times every 50 mm along the longitudinal direction of the polyethylene microporous membrane, and the coefficients of variation CVTD1 to CVTD10 for a total of 10 times were calculated. The arithmetic mean of the obtained coefficients of variation CVTD1 to CVTD10 was taken as the coefficient of variation of the film thickness.

(porosity)
The porosity (ε) of the polyolefin microporous membrane was calculated using the following formula.
ε (%) = {1-Ws/(ds・t)}×100
Ws: Fabric weight of polyolefin microporous membrane (g/m ² )
ds: True density of polyolefin (g/cm ³ )
t: Film thickness of polyolefin microporous membrane (μm)
The basis weight of the polyolefin microporous membrane was determined by cutting out a sample into 2 cm x 2 cm, measuring its mass, and dividing the mass by the area.
-Average value-
Along the width direction (TD direction) of the polyethylene microporous membrane, cut out 2 cm x 2 cm samples at 7 points in total, 3 points each at 20 mm intervals from the center of the product width to each end face from the center of the product width. Porosity was measured on the samples as described above. The value calculated as the arithmetic mean of each measurement sample was taken as the average value of porosity.
-Coefficient of variation-
Along the width direction (TD direction) of the polyethylene microporous membrane, the porosity was measured at 7 points in total at 3 points at 20 mm intervals from the center of the product width to each end face from the center of the product width as described above. The standard deviation σTD1 was determined based on the porosity value at each measurement location. Next, the coefficient of variation CVTD1 was determined by dividing σTD1 by the average value of the porosity at seven points in total. The same operation was repeated nine more times at intervals of 50 mm along the longitudinal direction of the polyethylene microporous membrane, and the coefficients of variation CVTD1 to CVTD10 for a total of 10 times were calculated. The arithmetic mean of the obtained coefficients of variation CVTD1 to CVTD10 was taken as the coefficient of variation of porosity.

(pore diameter)
The average flow rate pore diameter measured by the following method was taken as the pore diameter of the polyethylene microporous membrane.
The average flow pore size of a polyethylene microporous membrane by gas/liquid phase substitution was determined using a pore size distribution measurement test method [half-dry method] using a PMI porous material automatic pore size distribution measurement system [Capillary Flow Porometer]. (ASTM E1294-89)]. The test solution used was a fluorine-based inert liquid (trade name: Fluorinert) (interfacial tension value: 16.0 dyne/cm), the measurement temperature was 25°C, and the measurement pressure was within the range of 0psi to 500psi. Measurements were carried out under the following conditions.
・Bubble point parameters: BUBLFLOW = 50, F/PT = 100, MINBPPRES = 0, ZEROTIME = 1 PULSEDELAY = 2
・Wet parameters: V2INCR = 15, PREGINC = 0.9, MINEQTIME = 30, PRESSLEW = 30, FLOWSLEW = 30, EQITER = 50, AVEITER = 10, MAXPDIF = 1, MAXFDIF = 30
・Dry parameters: V2INCR = 40, PREGINC = 2.4, MINEQTIME = 30, PRESSLEW = 30, FLOWSLEW = 30, EQITER = 40, AVEITER = 10, MAXPDIF = 1, MAXFDIF = 30
-Average value-
Along the width direction (TD direction) of the polyethylene microporous membrane, the pore diameter was measured as described above at two points each at 50 mm intervals from the center of the product width toward each end face. , the value calculated as the arithmetic mean of the pore diameters at each measurement location was taken as the average value of the pore diameters.
-Coefficient of variation-
Along the width direction (TD direction) of the polyethylene microporous membrane, the pore diameter was measured as described above at two points each at 50 mm intervals from the center of the product width toward each end face. , the standard deviation σTD1 was determined based on the value of the pore diameter at each measurement location. Next, the coefficient of variation CVTD1 was determined by dividing σTD1 by the average value of the pore diameters at five points in total. The same operation was repeated nine more times at intervals of 50 mm along the longitudinal direction of the polyethylene microporous membrane, and the coefficients of variation CVTD1 to CVTD10 for a total of 10 times were calculated. The arithmetic mean of the obtained coefficients of variation CVTD1 to CVTD10 was taken as the coefficient of variation of the pore diameter.

(water flow rate)
The polyolefin microporous membrane was immersed in ethanol and dried at room temperature (24°C). This polyolefin microporous membrane was cut into 40 mm square pieces and set in a stainless steel liquid permeable cell (liquid permeable area S: 10.75 cm ² ) with a diameter of 37 mm. After moistening the polyolefin microporous membrane on the liquid permeable cell with a small amount (0.5 ml) of ethanol, a pre-measured amount of pure water V (100 ml) is allowed to permeate at a differential pressure of 90 kPa, so that the entire amount of pure water permeates. The time Tl (min) required for this was measured. From the amount of pure water and the time required for permeation of pure water, the permeation amount Vs (L/min/ft ² /psi) per unit time (min)/unit area (ft ² ) under 1 psi differential pressure is as follows: Calculated from the formula. The measurement was performed under a temperature atmosphere of room temperature 24°C.
Vs=(V/1000)/Tl/(S/929.03)/(90/6.895)
-Average value-
Along the width direction (TD direction) of the polyolefin microporous membrane, a 40 mm square sample was cut out at 2 points each at 15 mm intervals from the center of the product width to each end face from the center of the product width, and this sample was cut out. The water flow rate was measured as described above. The value calculated as the arithmetic mean of each measurement sample was taken as the average value of the water flow rate.
-Coefficient of variation-
Along the width direction (TD direction) of the polyolefin microporous membrane, measure the water flow rate at 5 points in total, including the center of the product width and 2 points at 15 mm intervals from the center to each end face as described above. Then, the standard deviation σVs1 was determined based on the value of the water flow rate at each measurement location. Next, the coefficient of variation CVVs1 was obtained by dividing σVs1 by the average value of the water flow rates at five points in total. The same operation was repeated 9 more times every 50 mm along the longitudinal direction of the polyethylene microporous membrane, and the coefficients of variation CVVs1 to CVVs10 for a total of 10 times were calculated. The arithmetic mean of the obtained coefficients of variation CVVs1 to CVVs10 was taken as the coefficient of variation of the water flow rate.

(Variation in filtration time)
An aqueous solution containing metal colloid (gold colloid) with an average particle diameter of 5 nm manufactured by Funakoshi Co., Ltd. at a particle concentration of 40 ppb was used as a test liquid, which is a filtration fluid.
In order to evaluate the filtration life, the polyethylene microporous membrane was cut into 5 cm square pieces from three locations along the TD direction of the polyethylene microporous membrane, including the center of the product width and both end faces. The cut polyethylene microporous membrane was set in a stainless steel liquid permeable cell (liquid permeable area S: 10.75 cm ² ) with a diameter of 37 mm. After wetting the polyethylene microporous membrane on the liquid permeable cell with a small amount (0.5 ml) of ethanol, 200 ml of the test liquid was filtered through it at a differential pressure of 0.1 MPa, and the amount required for the entire amount of the test liquid to pass was The time TF was measured. When the value of the shortest filtration time was set as TF to 1, the case where the ratio of the longest value of the filtration time was 1.2 or less was evaluated as A, and the case where it exceeded 1.3 was evaluated as B.

(Catching performance)
From the metal colloid concentration (M1) of the above-mentioned test liquid before measuring the filtration time and the metal colloid concentration (M2) of the filtrate that passed through the polyethylene microporous membrane used in the measurement of the filtration time, the particle size can be calculated using the following formula. The collection rate was determined. The arithmetic mean value of the collection rates measured at three locations along the TD direction of the microporous polyethylene membrane, at the center of the product width and on both end faces, was calculated and used as the collection rate of the microporous polyethylene membrane. The metal concentration of the solution was determined using the ICP-OES method (high-frequency inductively coupled plasma optical emission spectrometry, equipment name: Agilent-ICP-OES-5100, Agilent Technologies, Inc.), by diluting the standard reagent and adjusting the concentration from 0 ppb to 100 ppb. The metal concentration was determined using a calibration curve with 5 or more points in the concentration range. The collection performance was evaluated based on the following criteria.
Collection rate (%) = ((M1-M2)/(M1)) x 100
-Evaluation criteria-
・A: When the collection rate is 80% or more ・B: When the collection rate is 60% or more and less than 80% ・C: When the collection rate is 30% or more and less than 60%

(Calcium (Ca) content)
Accurately weigh 0.1 g of the measurement sample into a fluororesin container, add ultra-high purity nitric acid, perform microwave decomposition, and perform ICP (high frequency inductively coupled plasma) mass spectrometry (ICP-MS method) Instrument name: Agilent 7500cs, Agilent・Calcium content was quantified in ppb order by Technology Co., Ltd.).

(Amount of calcium (Ca) eluted from porous membrane)
A 4000 cm ² sample of a polyolefin microporous membrane from which surface dust had been removed was placed in a fluororesin container, and 200 g of a 10% by mass hydrochloric acid extract containing 60% by mass of isopropyl alcohol was poured into the container. After immersing the sample in this container for 24 hours, the Ca concentration in the liquid was measured using the ICP-OES method (high-frequency inductively coupled plasma optical emission spectrometry, equipment name: Agilent-ICP-OES-5100, Agilent Technologies, Inc.) The amount was quantified to the order of 0.1 ppb. The amount of Ca eluted from the microporous polyethylene membrane was calculated from the mass of the extract, the surface area of the microporous polyethylene membrane, and the Ca concentration in the extract, and was determined in μg/m ² .

(Production method of polyethylene microporous membrane)
Below, details of the manufacturing method of the polyethylene microporous membrane in each Example and Comparative Example will be mentioned.
The main differences in the manufacturing method between Examples and Comparative Examples are that in Examples there are three methylene chloride baths, while in Comparative Examples there are two baths, and in Examples when heat treatment is carried out, TD The width is fixed constant in the direction, whereas the width is not fixed in the comparative example.

<Example 1>
A polyethylene composition prepared by mixing 22 parts by mass of ultra-high molecular weight polyethylene with a weight average molecular weight of 4.6 million and a Ca content of 140 ppb and 5 parts by mass of high density polyethylene with a weight average molecular weight of 500,000 and a Ca content of 230 ppb was used. there was. A polyethylene solution was prepared by mixing the polyethylene composition with a mixed solvent of 72 parts by mass of liquid paraffin and 1 part by mass of decalin (decahydronaphthalene), which had been prepared in advance so that the concentration of the total amount of polyethylene resin was 27% by mass.
This polyethylene solution is extruded into a sheet form through a die at a temperature of 190°C, and then the extrudate is cooled in a water bath at 20°C, a water stream is provided on the surface layer of the water bath, and the gelled sheet is released from the water bath. A gel-like sheet (base tape) was prepared while making sure that the mixed solvent floating on the water surface did not adhere to the sheet again. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decalin from within the base tape, it was subsequently conveyed on a roller heated to 90°C while applying a pressure of 0.05 MPa. A portion of the liquid paraffin was removed from within the base tape. Thereafter, the base tape was stretched in the longitudinal direction at a temperature of 100°C at a ratio of 5 times (longitudinal stretching), then in the width direction at a temperature of 112°C at a ratio of 15 times (transverse stretching), and then at a temperature of 115°C. Heat treatment (heat fixation) was performed at.
Next, liquid paraffin was extracted while continuously immersing the heat-treated base tape in methylene chloride baths divided into three baths for 30 seconds each. In addition, when the side where immersion starts is the first tank and the side where immersion ends is the third tank, the purity of the cleaning solvent is (low) 1st layer < 2nd tank < 3rd layer (high) . Thereafter, methylene chloride was removed by drying at 40°C, heat treatment was performed in the TD direction at 110°C for 1 minute while fixing the width constant in the TD direction, and cooling was performed at 60°C for 20 seconds to form polyolefins. A substrate for a liquid filter consisting of a microporous membrane was obtained.
Table 1 shows the physical properties of the obtained liquid filter base material. Note that the physical properties of the following Examples and Comparative Examples are also shown in Table 1 or Table 2.

<Example 2>
A substrate for a liquid filter made of a microporous polyolefin membrane was obtained in the same manner as in Example 1 except that the longitudinal stretching ratio was increased to 6 times.

<Example 3>
A substrate for a liquid filter made of a microporous polyolefin membrane was obtained in the same manner as in Example 1, except that the longitudinal stretch ratio was 6.5 times and the transverse stretch ratio was 13.8 times.

<Example 4>
A polyethylene composition prepared by mixing 21 parts by mass of ultra-high molecular weight polyethylene with a weight average molecular weight of 4.6 million and a Ca content of 110 ppb and 5 parts by mass of high density polyethylene with a weight average molecular weight of 500,000 and a Ca content of 230 ppb was used. there was. A polyethylene solution was prepared by mixing the polyethylene composition with a mixed solvent of 73 parts by mass of liquid paraffin and 1 part by mass of decalin prepared in advance so that the total concentration of polyethylene resin was 26% by mass.
This polyethylene solution is extruded into a sheet form through a die at a temperature of 190°C, and then the extrudate is cooled in a water bath at 20°C, a water stream is provided on the surface layer of the water bath, and the gelled sheet is released from the water bath. The base tape was manufactured while making sure that the mixed solvent floating on the water surface did not adhere to the sheet again. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decalin from within the base tape, it was subsequently conveyed on a roller heated to 90°C while applying a pressure of 0.05 MPa. A portion of the liquid paraffin was removed from within the base tape. Thereafter, the base tape was stretched in the longitudinal direction at a temperature of 100°C at a ratio of 5 times (longitudinal stretching), then in the width direction at a temperature of 115°C at a ratio of 15 times (transverse stretching), and then at a temperature of 125°C. Heat treatment (heat fixation) was performed at.
Next, liquid paraffin was extracted while continuously immersing the heat-treated base tape in methylene chloride baths divided into three baths for 30 seconds each. In addition, when the side where immersion starts is the first tank and the side where immersion ends is the third tank, the purity of the cleaning solvent is (low) 1st layer < 2nd tank < 3rd layer (high) . Thereafter, methylene chloride was removed by drying at 40°C, heat treatment was performed in the TD direction at 120°C for 1 minute while fixing the width constant in the TD direction, and cooling was performed at 60°C for 20 seconds to form polyolefins. A liquid filter base material made of a microporous membrane was obtained.

<Example 5>
A polyethylene composition prepared by mixing 18 parts by mass of ultra-high molecular weight polyethylene with a weight average molecular weight of 4.6 million and a Ca content of 140 ppb and 5 parts by mass of high density polyethylene with a weight average molecular weight of 500,000 and a Ca content of 230 ppb was used. there was. A polyethylene solution was prepared by mixing the polyethylene composition with a mixed solvent of 76 parts by mass of liquid paraffin and 1 part by mass of decalin prepared in advance so that the concentration of the total amount of polyethylene resin was 23% by mass.
This polyethylene solution is extruded into a sheet form through a die at a temperature of 190°C, and then the extrudate is cooled in a water bath at 20°C, a water stream is provided on the surface layer of the water bath, and the sheet gelled in the water bath is released. The base tape was manufactured while making sure that the mixed solvent floating on the water surface did not adhere to the sheet again. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decalin from the base tape, it was subsequently conveyed on a roller heated to 90°C while applying a pressure of 0.05 MPa. A portion of the liquid paraffin was removed from within the base tape. Thereafter, the base tape was stretched in the longitudinal direction at a temperature of 100°C and a magnification of 4 times (longitudinal stretching), then in the width direction at a temperature of 115°C and a magnification of 15 times (transverse stretching), and then at a temperature of 125°C. Heat treatment (heat fixation) was performed at.
Next, liquid paraffin was extracted while continuously immersing the heat-treated base tape in methylene chloride baths divided into three baths for 30 seconds each. In addition, when the side where immersion starts is set as the first tank and the side where immersion ends is set as the third tank, the purity of the cleaning solvent is (low) 1st layer < 2nd tank < 3rd layer (high) . Thereafter, methylene chloride was removed by drying at 40°C, heat treatment was performed at 120°C for 1 minute in the TD direction while fixing the width constant in the TD direction, and cooling was performed at 60°C for 20 seconds to form polyolefins. A liquid filter base material made of a microporous membrane was obtained.

<Example 6>
A polyethylene composition prepared by mixing 3 parts by mass of ultra-high molecular weight polyethylene with a weight average molecular weight of 4.2 million and a Ca content of 31 ppm and 14 parts by mass of high density polyethylene with a weight average molecular weight of 400,000 and a Ca content of 34 ppm was used. there was. A polyethylene solution was prepared by mixing the polyethylene composition with a mixed solvent of 51 parts by mass of liquid paraffin and 32 parts by mass of decalin prepared in advance so that the concentration of the total amount of polyethylene resin was 17% by mass.
This polyethylene solution is extruded into a sheet form through a die at a temperature of 190°C, and then the extrudate is cooled in a water bath at 20°C, a water stream is provided on the surface layer of the water bath, and the gelled sheet is released from the water bath. The base tape was manufactured while making sure that the mixed solvent floating on the water surface did not adhere to the sheet again. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decalin from within the base tape, it was subsequently conveyed on a roller heated to 90°C while applying a pressure of 0.05 MPa. A portion of the liquid paraffin was removed from within the base tape. Thereafter, the base tape was stretched in the longitudinal direction at a temperature of 100°C at a magnification of 4.5 times (longitudinal stretching), then in the width direction at a temperature of 115°C at a magnification of 8 times (horizontal stretching); Heat treatment (heat fixation) was performed at 125°C.
Next, liquid paraffin was extracted while continuously immersing the heat-treated base tape in methylene chloride baths divided into three baths for 30 seconds each. In addition, when the side where immersion starts is set as the first tank and the side where immersion ends is set as the third tank, the purity of the cleaning solvent is (low) 1st layer < 2nd tank < 3rd layer (high) . Thereafter, methylene chloride was removed by drying at 40°C, heat treatment was performed at 120°C for 1 minute in the TD direction while fixing the width constant in the TD direction, and cooling was performed at 60°C for 20 seconds to form polyolefins. A substrate for a liquid filter consisting of a microporous membrane was obtained.

<Example 7>
A polyethylene composition prepared by mixing 12 parts by mass of ultra-high molecular weight polyethylene with a weight average molecular weight of 4.2 million and a Ca content of 31 ppm and 5 parts by mass of high density polyethylene with a weight average molecular weight of 400,000 and a Ca content of 34 ppm was used. there was. A polyethylene solution was prepared by mixing the polyethylene composition with a mixed solvent of 51 parts by mass of liquid paraffin and 32 parts by mass of decalin prepared in advance so that the concentration of the total amount of polyethylene resin was 17% by mass.
This polyethylene solution is extruded into a sheet form through a die at a temperature of 190°C, and then the extrudate is cooled in a water bath at 20°C, a water stream is provided on the surface layer of the water bath, and the gelled sheet is released from the water bath. The base tape was manufactured while making sure that the mixed solvent floating on the water surface did not adhere to the sheet again. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decalin from within the base tape, it was subsequently conveyed on a roller heated to 90°C while applying a pressure of 0.05 MPa. A portion of the liquid paraffin was removed from within the base tape. Thereafter, the base tape was stretched in the longitudinal direction at a temperature of 100°C and a magnification of 4 times (longitudinal stretching), then in the width direction at a temperature of 115°C and a magnification of 9 times (transverse stretching), and then at a temperature of 125°C. Heat treatment (heat fixation) was performed at.
Next, liquid paraffin was extracted while continuously immersing the heat-treated base tape in methylene chloride baths divided into three baths for 30 seconds each. In addition, when the side where immersion starts is the first tank and the side where immersion ends is the third tank, the purity of the cleaning solvent is (low) 1st layer < 2nd tank < 3rd layer (high) . Thereafter, methylene chloride was removed by drying at 40°C, heat treatment was performed in the TD direction at 120°C for 1 minute while fixing the width constant in the TD direction, and cooling was performed at 60°C for 20 seconds to form polyolefins. A substrate for a liquid filter consisting of a microporous membrane was obtained.

<Example 8>
A polyethylene composition prepared by mixing 6 parts by mass of ultra-high molecular weight polyethylene with a weight average molecular weight of 4.2 million and a Ca content of 31 ppm and 24 parts by mass of high-density polyethylene with a weight average molecular weight of 400,000 and a Ca content of 34 ppm was used. there was. A polyethylene solution was prepared by mixing the polyethylene composition with a mixed solvent of 68 parts by mass of liquid paraffin and 2 parts by mass of decalin, which had been prepared in advance so that the concentration of the total amount of polyethylene resin was 30% by mass.
This polyethylene solution is extruded into a sheet form through a die at a temperature of 190°C, and then the extrudate is cooled in a water bath at 20°C, a water stream is provided on the surface layer of the water bath, and the gelled sheet is released from the water bath. The base tape was manufactured while making sure that the mixed solvent floating on the water surface did not adhere to the sheet again. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decalin from within the base tape, it was subsequently conveyed on a roller heated to 90°C while applying a pressure of 0.05 MPa. A portion of the liquid paraffin was removed from within the base tape. Thereafter, the base tape was stretched in the longitudinal direction at a temperature of 95°C at a magnification of 4 times (longitudinal stretching), then in the width direction at a temperature of 110°C at a magnification of 11 times (transverse stretching), and then at a temperature of 120°C. Heat treatment (heat fixation) was performed at.
Next, liquid paraffin was extracted while continuously immersing the heat-treated base tape in methylene chloride baths divided into three baths for 30 seconds each. In addition, when the side where immersion starts is the first tank and the side where immersion ends is the third tank, the purity of the cleaning solvent is (low) 1st layer < 2nd tank < 3rd layer (high) . Thereafter, methylene chloride was removed by drying at 40°C, heat treatment was performed in the TD direction at 120°C for 1 minute while fixing the width constant in the TD direction, and cooling was performed at 60°C for 20 seconds to form polyolefins. A liquid filter base material made of a microporous membrane was obtained.

<Example 9>
A polyethylene composition prepared by mixing 15 parts by mass of ultra-high molecular weight polyethylene with a weight average molecular weight of 4.2 million and a Ca content of 31 ppm and 15 parts by mass of high density polyethylene with a weight average molecular weight of 400,000 and a Ca content of 34 ppm was used. there was. A polyethylene solution was prepared by mixing the polyethylene composition with a mixed solvent of 68 parts by mass of liquid paraffin and 2 parts by mass of decalin, which had been prepared in advance so that the concentration of the total amount of polyethylene resin was 30% by mass.
This polyethylene solution is extruded into a sheet form through a die at a temperature of 190°C, and then the extrudate is cooled in a water bath at 20°C, a water stream is provided on the surface layer of the water bath, and the gelled sheet is released from the water bath. The base tape was manufactured while making sure that the mixed solvent floating on the water surface did not adhere to the sheet again. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decalin from within the base tape, it was subsequently conveyed on a roller heated to 90°C while applying a pressure of 0.05 MPa. A portion of the liquid paraffin was removed from within the base tape. Thereafter, the base tape was stretched in the longitudinal direction at a temperature of 100°C and a magnification of 4 times (longitudinal stretching), then in the width direction at a temperature of 115°C and a magnification of 10 times (transverse stretching), and then at a temperature of 125°C. Heat treatment (heat fixation) was performed at.
Next, liquid paraffin was extracted while continuously immersing the heat-treated base tape in methylene chloride baths divided into three baths for 30 seconds each. In addition, when the side where immersion starts is the first tank and the side where immersion ends is the third tank, the purity of the cleaning solvent is (low) 1st layer < 2nd tank < 3rd layer (high) . Thereafter, methylene chloride was removed by drying at 40°C, heat treatment was performed in the TD direction at 120°C for 1 minute while fixing the width constant in the TD direction, and cooling was performed at 60°C for 20 seconds to form polyolefins. A liquid filter base material made of a microporous membrane was obtained.

<Example 10>
A polyethylene composition prepared by mixing 17 parts by mass of ultra-high molecular weight polyethylene with a weight average molecular weight of 4.2 million and a Ca content of 31 ppm and 4 parts by mass of high density polyethylene with a weight average molecular weight of 400,000 and a Ca content of 34 ppm was used. there was. A polyethylene solution was prepared by mixing the polyethylene composition with a mixed solvent of 78 parts by mass of liquid paraffin and 1 part by mass of decalin prepared in advance so that the concentration of the total amount of polyethylene resin was 21% by mass.
This polyethylene solution is extruded into a sheet form through a die at a temperature of 190°C, and then the extrudate is cooled in a water bath at 20°C, a water stream is provided on the surface layer of the water bath, and the sheet gelled in the water bath is released. The base tape was manufactured while making sure that the mixed solvent floating on the water surface did not adhere to the sheet again. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decalin from the base tape, it was subsequently conveyed on a roller heated to 90°C while applying a pressure of 0.05 MPa. A portion of the liquid paraffin was removed from within the base tape. Thereafter, the base tape was stretched in the longitudinal direction at a temperature of 100°C and a magnification of 4 times (longitudinal stretching), then in the width direction at a temperature of 115°C and a magnification of 8 times (transverse stretching), and then at a temperature of 125°C. Heat treatment (heat fixation) was performed at.
Next, liquid paraffin was extracted while continuously immersing the heat-treated base tape in methylene chloride baths divided into three baths for 30 seconds each. In addition, when the side where immersion starts is set as the first tank and the side where immersion ends is set as the third tank, the purity of the cleaning solvent is (low) 1st layer < 2nd tank < 3rd layer (high) . Thereafter, methylene chloride was removed by drying at 40°C, heat treatment was performed at 120°C for 1 minute in the TD direction while fixing the width constant in the TD direction, and cooling was performed at 60°C for 20 seconds to form polyolefins. A substrate for a liquid filter consisting of a microporous membrane was obtained.

<Comparative example 1>
A polyethylene composition prepared by mixing 22 parts by mass of ultra-high molecular weight polyethylene with a weight average molecular weight of 4.6 million and a Ca content of 140 ppb and 5 parts by mass of high density polyethylene with a weight average molecular weight of 500,000 and a Ca content of 230 ppb was used. there was. A polyethylene solution was prepared by mixing the polyethylene composition with a mixed solvent of 72 parts by mass of liquid paraffin and 1 part by mass of decalin prepared in advance so that the concentration of the total amount of polyethylene resin was 27% by mass.
This polyethylene solution is extruded into a sheet form through a die at a temperature of 190°C, and then the extrudate is cooled in a water bath at 20°C, a water stream is provided on the surface layer of the water bath, and the gelled sheet is released from the water bath. The base tape was manufactured while making sure that the mixed solvent floating on the water surface did not adhere to the sheet again. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decalin from within the base tape, it was subsequently conveyed on a roller heated to 90°C while applying a pressure of 0.05 MPa. A portion of the liquid paraffin was removed from within the base tape. Thereafter, the base tape was stretched in the longitudinal direction at a temperature of 100°C and a magnification of 8 times (longitudinal stretching), then in the width direction at a temperature of 112°C and a magnification of 10 times (transverse stretching), and then at a temperature of 115°C. Heat treatment (heat fixation) was performed at.
Next, liquid paraffin was extracted while continuously immersing the heat-treated base tape in two separate methylene chloride baths for 30 seconds each. Note that the purity of the cleaning solvent when the side where immersion starts is set as the first tank and the side where immersion ends is set as the second tank is (low) first layer<second tank (high). Thereafter, methylene chloride was removed by drying at 40° C., and heat treatment was performed while conveying on rollers heated to 110° C., thereby obtaining a base material for a liquid filter made of a microporous polyolefin membrane.

<Comparative example 2>
A liquid filter substrate made of a microporous polyolefin membrane was produced in the same manner as in Comparative Example 1 except that the longitudinal stretching ratio was 9 times.

<Comparative example 3>
A substrate for a liquid filter made of a microporous polyolefin membrane was produced in the same manner as in Comparative Example 1, except that the longitudinal stretch ratio was 10 times and the transverse stretch ratio was 8 times.

<Comparative example 4>
A polyethylene composition prepared by mixing 3 parts by mass of ultra-high molecular weight polyethylene with a weight average molecular weight of 4.2 million and a Ca content of 31 ppm and 14 parts by mass of high density polyethylene with a weight average molecular weight of 400,000 and a Ca content of 34 ppm was used. there was. A polyethylene solution was prepared by mixing the polyethylene composition with a mixed solvent of 51 parts by mass of liquid paraffin and 32 parts by mass of decalin prepared in advance so that the concentration of the total amount of polyethylene resin was 17% by mass.
This polyethylene solution is extruded into a sheet form through a die at a temperature of 190°C, and then the extrudate is cooled in a water bath at 20°C, a water stream is provided on the surface layer of the water bath, and the sheet gelled in the water bath is released. The base tape was manufactured while making sure that the mixed solvent floating on the water surface did not adhere to the sheet again. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decalin from the base tape, it was subsequently conveyed on a roller heated to 90°C while applying a pressure of 0.05 MPa. A portion of the liquid paraffin was removed from within the base tape. Thereafter, the base tape was stretched in the longitudinal direction at a temperature of 100°C at a ratio of 6 times (longitudinal stretching), then in the width direction at a temperature of 115°C at a ratio of 6 times (transverse stretching), and then at a temperature of 125°C. Heat treatment (heat fixation) was performed at.
Next, liquid paraffin was extracted while continuously immersing the heat-treated base tape in two separate methylene chloride baths for 30 seconds each. In addition, when the side where immersion is started is set as the first tank and the side where immersion is finished is set as the second tank, the purity of the cleaning solvent is (low) first layer<second tank (high). Thereafter, methylene chloride was removed by drying at 40° C., and heat treatment was performed while conveying on a roller heated to 120° C., thereby obtaining a base material for a liquid filter made of a microporous polyolefin membrane.

<Comparative example 5>
A polyethylene composition prepared by mixing 5 parts by mass of ultra-high molecular weight polyethylene with a weight average molecular weight of 4.6 million and a Ca content of 30 ppm and 23 parts by mass of high density polyethylene with a weight average molecular weight of 560,000 and a Ca content of 36 ppm was used. there was. A polyethylene solution was prepared by mixing the polyethylene composition with a mixed solvent of 69 parts by mass of liquid paraffin and 3 parts by mass of decalin prepared in advance so that the concentration of the total amount of polyethylene resin was 28% by mass.
This polyethylene solution is extruded into a sheet form from a die at a temperature of 160°C, and then the extruded product is cooled in a water bath at 25°C, and a water stream is provided on the surface layer of the water bath to release it from the gelled sheet in the water bath. The base tape was manufactured while making sure that the mixed solvent floating on the water surface did not adhere to the sheet again. After drying the base tape at 55°C for 10 minutes and then at 95°C for 10 minutes to remove decalin from the base tape, it was then pressed at 20 kgf/m (1.96 MPa) on a roller heated to 85°C. A portion of the liquid paraffin was removed from within the base tape by transporting the base tape while covering it with water. Thereafter, the base tape was stretched in the longitudinal direction at a temperature of 100°C and a magnification of 5.8 times (longitudinal stretching), then in the width direction at a temperature of 100°C and a magnification of 14 times (horizontal stretching); Heat treatment (heat fixation) was performed at 118°C.
Next, liquid paraffin was extracted while continuously immersing the heat-treated base tape in two separate methylene chloride baths for 30 seconds each. Note that the purity of the cleaning solvent when the side where immersion starts is set as the first tank and the side where immersion ends is set as the second tank is (low) first layer<second tank (high). Thereafter, methylene chloride was removed by drying at 45° C., and heat treatment was performed while conveying on rollers heated to 110° C., thereby obtaining a substrate for a liquid filter made of a microporous polyolefin membrane.

<Comparative example 6>
In Comparative Example 5, the base tape was conveyed on a roller heated to 40°C while applying a pressure of 40 kgf/m (3.92 MPa) to remove a part of the liquid paraffin from within the base tape, and then the base tape was conveyed in the longitudinal direction. Stretched at a temperature of 90°C at a magnification of 5.8 times (longitudinal stretching), then stretched in the width direction at a temperature of 90°C at a magnification of 14 times (horizontal stretching), and then heat treated at a temperature of 124°C (heat setting). A substrate for a liquid filter made of a microporous polyolefin membrane was obtained in the same manner except for the following steps.

<Comparative example 7>
In Comparative Example 5, the base tape was conveyed on a roller heated to 95° C. while applying a pressure of 5 kgf/m (0.49 MPa) to remove a portion of the liquid paraffin from within the base tape, and then the base tape was conveyed in the longitudinal direction. Stretched at a temperature of 95°C with a magnification of 6.5 times (longitudinal stretching), then stretched in the width direction at a temperature of 95°C with a magnification of 13 times (horizontal stretching), and then heat treated at a temperature of 114°C (heat setting). A substrate for a liquid filter made of a microporous polyolefin membrane was obtained in the same manner except for the following steps.

<Comparative example 8>
In Comparative Example 5, 4 parts by mass of ultra-high molecular weight polyethylene having a weight average molecular weight of 4.6 million and a Ca content of 30 ppm and 31 parts by mass of high density polyethylene having a weight average molecular weight of 560,000 and a Ca content of 36 ppm were mixed. Using a polyethylene composition, mix the polyethylene composition with a mixed solvent of 55 parts by mass of liquid paraffin and 10 parts by mass of decalin prepared in advance so that the concentration of the total amount of polyethylene resin is 35% by mass to prepare a polyethylene solution. Then, the base tape from which part of the liquid paraffin had been removed was stretched in the longitudinal direction at a temperature of 100°C and a magnification of 5 times (longitudinal stretching), and then in the width direction at a temperature of 100°C and a magnification of 9 times (horizontal stretching). A base material for a liquid filter made of a microporous polyolefin membrane was obtained in the same manner except that the polyolefin microporous membrane was subjected to a heat treatment (heat setting) at a temperature of 114°C.

<Comparative example 9>
In Comparative Example 5, 8 parts by mass of ultra-high molecular weight polyethylene having a weight average molecular weight of 4.6 million and a Ca content of 30 ppm and 24 parts by mass of high density polyethylene having a weight average molecular weight of 560,000 and a Ca content of 36 ppm were mixed. Using a polyethylene composition, mix the polyethylene composition with a mixed solvent of 53 parts by mass of liquid paraffin and 15 parts by mass of decalin prepared in advance so that the concentration of the total amount of polyethylene resin is 32% by mass to prepare a polyethylene solution. Then, the base tape from which part of the liquid paraffin had been removed was stretched in the longitudinal direction at a temperature of 100°C and a magnification of 4 times (longitudinal stretching), and then in the width direction at a temperature of 100°C and a magnification of 15 times (horizontal stretching). A substrate for a liquid filter consisting of a microporous polyolefin membrane was obtained in the same manner except for stretching.

<Comparative example 10>
A polyethylene composition prepared by mixing 14 parts by mass of ultra-high molecular weight polyethylene with a weight average molecular weight of 4.6 million and a Ca content of 30 ppm and 6 parts by mass of high density polyethylene with a weight average molecular weight of 560,000 and a Ca content of 36 ppm was used. there was. A polyethylene solution was prepared by mixing the polyethylene composition with a mixed solvent of 55 parts by mass of liquid paraffin and 25 parts by mass of decalin prepared in advance so that the concentration of the total amount of polyethylene resin was 20% by mass.
This polyethylene solution is extruded into a sheet form through a die at a temperature of 160°C, and then the extruded product is cooled at 25°C in a water bath, and a water stream is provided on the surface layer of the water bath to release it from the gelled sheet in the water bath. The base tape was manufactured while making sure that the mixed solvent floating on the water surface did not adhere to the sheet again. After drying the base tape at 55°C for 10 minutes and then at 95°C for 10 minutes to remove decalin from the base tape, it was then pressed at 20 kgf/m (1.96 MPa) on a roller heated to 85°C. A portion of the liquid paraffin was removed from within the base tape by transporting the base tape while covering it with water. Thereafter, the base tape was stretched in the longitudinal direction at a temperature of 100°C and a magnification of 5 times (longitudinal stretching), then in the width direction at a temperature of 100°C and a magnification of 14 times (horizontal stretching), and then at a temperature of 128°C. Heat treatment (heat fixation) was performed.
Next, liquid paraffin was extracted while continuously immersing the heat-treated base tape in two separate methylene chloride baths for 30 seconds each. In addition, when the side where immersion is started is set as the first tank and the side where immersion is finished is set as the second tank, the purity of the cleaning solvent is (low) first layer<second tank (high). Thereafter, methylene chloride was removed by drying at 45° C., and heat treatment was performed while conveying on rollers heated to 110° C., thereby obtaining a base material for a liquid filter made of a microporous polyolefin membrane.

<Comparative Example 11>
In Comparative Example 10, 12 parts by mass of ultra-high molecular weight polyethylene having a weight average molecular weight of 4.6 million and a Ca content of 30 ppm and 8 parts by mass of high density polyethylene having a weight average molecular weight of 560,000 and a Ca content of 36 ppm were mixed. A liquid filter base material made of a microporous polyolefin membrane was obtained in the same manner except that a polyethylene composition was used.

<Comparative example 12>
A substrate for a liquid filter made of a microporous polyolefin membrane was obtained in the same manner as in Comparative Example 10, except that the heat treatment (heat setting) temperature after biaxial stretching was 134°C.

<Comparative example 13>
In Comparative Example 10, 20 parts by mass of ultra-high molecular weight polyethylene having a weight average molecular weight of 4.6 million and a Ca content of 30 ppm and 5 parts by mass of high density polyethylene having a weight average molecular weight of 560,000 and a Ca content of 36 ppm were mixed. Using a polyethylene composition, mix the polyethylene composition with a mixed solvent of 50 parts by mass of liquid paraffin and 25 parts by mass of decalin prepared in advance so that the concentration of the total amount of polyethylene resin is 25% by mass to prepare a polyethylene solution. did. This polyethylene solution was extruded in the same manner as in Comparative Example 10, and the resulting base tape was heated and dried, and then conveyed on rollers heated to 95°C while applying a pressure of 10 kgf/m (0.98 MPa) to form a base tape. A substrate for a liquid filter consisting of a microporous polyolefin membrane was obtained in the same manner except that a portion of the liquid paraffin was removed from the tape.

<Comparative example 14>
A polyethylene composition prepared by mixing 3 parts by mass of ultra-high molecular weight polyethylene with a weight average molecular weight of 4.6 million and a Ca content of 30 ppm and 12 parts by mass of high density polyethylene with a weight average molecular weight of 560,000 and a Ca content of 36 ppm was used. there was. A polyethylene solution was prepared by mixing the polyethylene composition with a mixed solvent of 60 parts by mass of liquid paraffin and 25 parts by mass of decalin prepared in advance so that the concentration of the total amount of polyethylene resin was 15% by mass.
This polyethylene solution is extruded into a sheet form through a die at a temperature of 160°C, and then the extrudate is cooled in a water bath at 25°C, and a water stream is provided on the surface layer of the water bath so that the gelled sheet is released from the sheet. A base tape was prepared while making sure that the mixed solvent floating on the water surface did not adhere to the sheet again. After drying the base tape at 55°C for 10 minutes and then at 95°C for 10 minutes to remove decalin from the base tape, it was then pressed at 20 kgf/m (1.96 MPa) on a roller heated to 85°C. A portion of the liquid paraffin was removed from within the base tape by transporting the base tape while covering it with water. Thereafter, the base tape was stretched in the longitudinal direction at a temperature of 100°C and a magnification of 6 times (longitudinal stretching), then in the width direction at a temperature of 100°C and a magnification of 12 times (transverse stretching), and then at a temperature of 136°C. Heat treatment (heat fixation) was performed.
Next, liquid paraffin was extracted while continuously immersing the heat-treated base tape in two separate methylene chloride baths for 30 seconds each. In addition, when the side where immersion is started is set as the first tank and the side where immersion is finished is set as the second tank, the purity of the cleaning solvent is (low) first layer<second tank (high). Thereafter, methylene chloride was removed by drying at 45° C., and heat treatment was performed while conveying on rollers heated to 110° C., thereby obtaining a base material for a liquid filter made of a microporous polyolefin membrane.

<Comparative example 15>
In Comparative Example 14, 2 parts by mass of ultra-high molecular weight polyethylene having a weight average molecular weight of 4.6 million and a Ca content of 30 ppm and 18 parts by mass of high density polyethylene having a weight average molecular weight of 560,000 and a Ca content of 36 ppm were mixed. Using a polyethylene composition, mix the polyethylene composition with a mixed solvent of 55 parts by mass of liquid paraffin and 25 parts by mass of decalin prepared in advance so that the concentration of the total amount of polyethylene resin is 20% by mass to prepare a polyethylene solution. A liquid filter base material made of a microporous polyolefin membrane was obtained in the same manner except for the following steps.

<Comparative example 16>
A substrate for a liquid filter made of a microporous polyolefin membrane was obtained in the same manner as in Comparative Example 14, except that the heat treatment (heat setting) temperature after biaxial stretching was 142°C.

<Comparative example 17>
In Comparative Example 14, 6.5 parts by mass of ultra-high molecular weight polyethylene having a weight average molecular weight of 4.6 million and a Ca content of 30 ppm and 18.5 parts by mass of high density polyethylene having a weight average molecular weight of 560,000 and a Ca content of 36 ppm. Using a polyethylene composition mixed with, mix the polyethylene composition with a mixed solvent of 50 parts by mass of liquid paraffin and 25 parts by mass of decalin prepared in advance so that the concentration of the total amount of polyethylene resin is 25% by mass, A polyethylene solution was prepared. This polyethylene solution was extruded in the same manner as in Comparative Example 14, and the obtained base tape was dried by heating, and then conveyed on a roller heated to 95° C. while applying a pressure of 10 kgf/m (0.98 MPa). A substrate for a liquid filter consisting of a microporous polyolefin membrane was obtained in the same manner except that a portion of the liquid paraffin was removed from the base tape.

<Comparative example 18>
Polyethylene obtained by mixing 18.4 parts by mass of ultra-high molecular weight polyethylene with a weight average molecular weight of 5.1 million and a Ca content of 140 ppb and 4.6 parts by mass of high-density polyethylene with a weight average molecular weight of 650,000 and a Ca content of 270 ppb. The composition was used. A polyethylene solution was prepared by mixing the polyethylene composition with a mixed solvent of 74.5 parts by mass of liquid paraffin and 2.5 parts by mass of decalin prepared in advance so that the concentration of the total amount of polyethylene resin was 23% by mass.
This polyethylene solution is extruded into a sheet form through a die at a temperature of 155°C, and then the extrudate is cooled in a water bath at 20°C, and a water stream is provided on the surface layer of the water bath so that the gelled sheet is released from the sheet. A base tape was prepared while making sure that the mixed solvent floating on the water surface did not adhere to the sheet again. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decalin from within the base tape, it was subsequently conveyed on a roller heated to 90°C while applying a pressure of 0.05 MPa. A portion of the liquid paraffin was removed from within the base tape. Thereafter, the base tape was stretched in the longitudinal direction at a temperature of 115° C. and a magnification of 5.8 times (longitudinal stretching), then in the width direction at a temperature of 105° C. and a magnification of 13 times (horizontal stretching); Heat treatment (heat fixation) was performed at ℃.
Next, liquid paraffin was extracted while continuously immersing the heat-treated base tape in two separate methylene chloride baths for 30 seconds each. Note that the purity of the cleaning solvent when the side where immersion starts is set as the first tank and the side where immersion ends is set as the second tank is (low) first layer<second tank (high). Thereafter, methylene chloride was removed by drying at 40° C., and heat treatment was performed while conveying it on a roller heated to 120° C., thereby obtaining a base material for a liquid filter made of a microporous polyolefin membrane.

<Comparative Example 19>
Polyethylene obtained by mixing 18.4 parts by mass of ultra-high molecular weight polyethylene with a weight average molecular weight of 5.1 million and a Ca content of 140 ppb and 4.6 parts by mass of high-density polyethylene with a weight average molecular weight of 650,000 and a Ca content of 270 ppb. The composition was used. A polyethylene solution was prepared by mixing the polyethylene composition with a mixed solvent of 75.9 parts by mass of liquid paraffin and 1.1 parts by mass of decalin prepared in advance so that the concentration of the total amount of polyethylene resin was 23% by mass.
This polyethylene solution was extruded into a sheet form through a die at a temperature of 158°C, and then the extrudate was cooled in a water bath at 18°C, and a water stream was provided on the surface layer of the water bath so that the gelled sheet was released from the sheet. A base tape was prepared while making sure that the mixed solvent floating on the water surface did not adhere to the sheet again. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decalin from within the base tape, it was subsequently conveyed on a roller heated to 90°C while applying a pressure of 0.05 MPa. A portion of the liquid paraffin was removed from within the base tape. Thereafter, the base tape was stretched in the longitudinal direction at a temperature of 95° C. and a magnification of 5.8 times (longitudinal stretching), then in the width direction at a temperature of 105° C. and a magnification of 13 times (horizontal stretching), and then at a temperature of 140° C. Heat treatment (heat fixation) was performed at ℃.
Next, liquid paraffin was extracted while continuously immersing the heat-treated base tape in two separate methylene chloride baths for 30 seconds each. In addition, when the side where immersion is started is set as the first tank and the side where immersion is finished is set as the second tank, the purity of the cleaning solvent is (low) first layer<second tank (high). Thereafter, methylene chloride was removed by drying at 40° C., and heat treatment was performed while conveying on a roller heated to 120° C., thereby obtaining a base material for a liquid filter made of a microporous polyolefin membrane.

<Comparative example 20>
Polyethylene obtained by mixing 12.5 parts by mass of ultra-high molecular weight polyethylene with a weight average molecular weight of 5.1 million and a Ca content of 140 ppb and 12.5 parts by mass of high-density polyethylene with a weight average molecular weight of 650,000 and a Ca content of 270 ppb. The composition was used. A polyethylene solution was prepared by mixing the polyethylene composition with a mixed solvent of 72.5 parts by mass of liquid paraffin and 2.5 parts by mass of decalin prepared in advance so that the concentration of the total amount of polyethylene resin was 25% by mass.
This polyethylene solution was extruded into a sheet form through a die at a temperature of 156°C, and then the extrudate was cooled in a water bath at 18°C, and a water stream was provided on the surface layer of the water bath to release the gelled sheet from the water bath. A base tape was prepared while making sure that the mixed solvent floating on the water surface did not adhere to the sheet again. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decalin from within the base tape, it was subsequently conveyed on a roller heated to 110°C while applying a pressure of 0.05 MPa. A portion of the liquid paraffin was removed from within the base tape. Thereafter, the base tape was stretched in the longitudinal direction at a temperature of 115° C. and a magnification of 5.8 times (longitudinal stretching), and then in the width direction at a temperature of 105° C. and a magnification of 13 times (horizontal stretching). Heat treatment (heat fixation) was performed at ℃.
Next, liquid paraffin was extracted while continuously immersing the heat-treated base tape in two separate methylene chloride baths for 30 seconds each. Note that the purity of the cleaning solvent when the side where immersion starts is set as the first tank and the side where immersion ends is set as the second tank is (low) first layer<second tank (high). Thereafter, methylene chloride was removed by drying at 40° C., and heat treatment was performed while conveying it on a roller heated to 120° C., thereby obtaining a base material for a liquid filter made of a microporous polyolefin membrane.

<Comparative example 21>
A polyethylene composition prepared by mixing 6 parts by mass of ultra-high molecular weight polyethylene with a weight average molecular weight of 5.1 million and a Ca content of 140 ppb and 24 parts by mass of high density polyethylene with a weight average molecular weight of 650,000 and a Ca content of 270 ppb was used. there was. A polyethylene solution was prepared by mixing the polyethylene composition with a mixed solvent of 67.5 parts by mass of liquid paraffin and 2.5 parts by mass of decalin prepared in advance so that the concentration of the total amount of polyethylene resin was 30% by mass.
This polyethylene solution was extruded into a sheet through a die at a temperature of 164°C, and then the extrudate was cooled in a water bath at 16°C, and a water stream was provided on the surface layer of the water bath to release the gelled sheet in the water bath. A base tape was prepared while making sure that the mixed solvent floating on the water surface did not adhere to the sheet again. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decalin from the base tape, it was subsequently conveyed on a roller heated to 30°C while applying a pressure of 0.05 MPa. A portion of the liquid paraffin was removed from within the base tape. Thereafter, the base tape was stretched in the longitudinal direction at a temperature of 115° C. and a magnification of 5.8 times (longitudinal stretching), then in the width direction at a temperature of 105° C. and a magnification of 13 times (horizontal stretching), and then at a temperature of 140° C. Heat treatment (heat fixation) was performed at ℃.
Next, liquid paraffin was extracted while continuously immersing the heat-treated base tape in two separate methylene chloride baths for 30 seconds each. Note that the purity of the cleaning solvent when the side where immersion starts is set as the first tank and the side where immersion ends is set as the second tank is (low) first layer<second tank (high). Thereafter, methylene chloride was removed by drying at 40°C, and heat treatment was carried out while conveying on a roller heated to 125°C, thereby obtaining a base material for a liquid filter made of a microporous polyolefin membrane.

<Comparative example 22>
Polyethylene obtained by mixing 4.6 parts by mass of ultra-high molecular weight polyethylene with a weight average molecular weight of 5.1 million and a Ca content of 140 ppb and 18.4 parts by mass of high-density polyethylene with a weight average molecular weight of 650,000 and a Ca content of 270 ppb. The composition was used. A polyethylene solution was prepared by mixing the polyethylene composition with a mixed solvent of 74.5 parts by mass of liquid paraffin and 2.5 parts by mass of decalin prepared in advance so that the concentration of the total amount of polyethylene resin was 23% by mass.
This polyethylene solution was extruded into a sheet through a die at a temperature of 164°C, and then the extrudate was cooled in a water bath at 16°C, and a water stream was provided on the surface layer of the water bath to release the gelled sheet in the water bath. A base tape was prepared while making sure that the mixed solvent floating on the water surface did not adhere to the sheet again. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decalin from the base tape, it was subsequently conveyed on a roller heated to 30°C while applying a pressure of 0.05 MPa. A portion of the liquid paraffin was removed from within the base tape. Thereafter, the base tape was stretched in the longitudinal direction at a temperature of 115° C. and a magnification of 5.8 times (longitudinal stretching), then in the width direction at a temperature of 105° C. and a magnification of 13 times (horizontal stretching), and then at a temperature of 128° C. Heat treatment (heat fixation) was performed at ℃.
Next, liquid paraffin was extracted while continuously immersing the heat-treated base tape in two separate methylene chloride baths for 30 seconds each. Note that the purity of the cleaning solvent when the side where immersion starts is set as the first tank and the side where immersion ends is set as the second tank is (low) first layer<second tank (high). Thereafter, methylene chloride was removed by drying at 40° C., and heat treatment was performed while conveying it on a roller heated to 120° C., thereby obtaining a base material for a liquid filter made of a microporous polyolefin membrane.

<Comparative example 23>
Polyethylene obtained by mixing 4.6 parts by mass of ultra-high molecular weight polyethylene with a weight average molecular weight of 5.1 million and a Ca content of 140 ppb and 18.4 parts by mass of high-density polyethylene with a weight average molecular weight of 650,000 and a Ca content of 270 ppb. The composition was used. A polyethylene solution was prepared by mixing the polyethylene composition with a mixed solvent of 74.5 parts by mass of liquid paraffin and 2.5 parts by mass of decalin prepared in advance so that the concentration of the total amount of polyethylene resin was 23% by mass.
This polyethylene solution was extruded into a sheet through a die at a temperature of 164°C, and then the extrudate was cooled in a water bath at 16°C, and a water stream was provided on the surface layer of the water bath to release the gelled sheet in the water bath. A base tape was prepared while making sure that the mixed solvent floating on the water surface did not adhere to the sheet again. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decalin from the base tape, it was subsequently conveyed on a roller heated to 30°C while applying a pressure of 0.05 MPa. A portion of the liquid paraffin was removed from within the base tape. Thereafter, the base tape was stretched in the longitudinal direction at a temperature of 115°C and a magnification of 7 times (longitudinal stretching), then in the width direction at a temperature of 105°C and a magnification of 11 times (horizontal stretching), and then at a temperature of 125°C. Heat treatment (heat fixation) was performed.
Next, liquid paraffin was extracted while continuously immersing the heat-treated base tape in two separate methylene chloride baths for 30 seconds each. Note that the purity of the cleaning solvent when the side where immersion starts is set as the first tank and the side where immersion ends is set as the second tank is (low) first layer<second tank (high). Thereafter, methylene chloride was removed by drying at 40° C., and heat treatment was performed while conveying it on a roller heated to 120° C., thereby obtaining a base material for a liquid filter made of a microporous polyolefin membrane.

<Comparative example 24>
Polyethylene obtained by mixing 10.0 parts by mass of ultra-high molecular weight polyethylene with a weight average molecular weight of 5.1 million and a Ca content of 140 ppb and 10.0 parts by mass of high-density polyethylene with a weight average molecular weight of 650,000 and a Ca content of 270 ppb. The composition was used. A polyethylene solution was prepared by mixing the polyethylene composition with a mixed solvent of 77.3 parts by mass of liquid paraffin and 2.7 parts by mass of decalin prepared in advance so that the concentration of the total amount of polyethylene resin was 20% by mass.
This polyethylene solution was extruded into a sheet form through a die at a temperature of 156°C, and then the extrudate was cooled in a water bath at 16°C, and a water stream was provided on the surface layer of the water bath to release the gelled sheet from the water bath. A base tape was prepared while making sure that the mixed solvent floating on the water surface did not adhere to the sheet again. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decalin from the base tape, it was subsequently conveyed on a roller heated to 30°C while applying a pressure of 0.05 MPa. A portion of the liquid paraffin was removed from within the base tape. Thereafter, the base tape was stretched in the longitudinal direction at a temperature of 115°C and a magnification of 7 times (longitudinal stretching), then in the width direction at a temperature of 105°C and a magnification of 13 times (horizontal stretching), and then at a temperature of 135°C. Heat treatment (heat fixation) was performed.
Next, liquid paraffin was extracted while continuously immersing the heat-treated base tape in two separate methylene chloride baths for 30 seconds each. Note that the purity of the cleaning solvent when the side where immersion starts is set as the first tank and the side where immersion ends is set as the second tank is (low) first layer<second tank (high). Thereafter, methylene chloride was removed by drying at 40° C., and heat treatment was performed while conveying it on a roller heated to 120° C., thereby obtaining a base material for a liquid filter made of a microporous polyolefin membrane.

<Comparative example 25>
A polyethylene composition prepared by mixing 24 parts by mass of ultra-high molecular weight polyethylene with a weight average molecular weight of 5.1 million and a Ca content of 140 ppb and 6 parts by mass of high density polyethylene with a weight average molecular weight of 650,000 and a Ca content of 270 ppb was used. there was. A polyethylene solution was prepared by mixing the polyethylene composition with a mixed solvent of 67.7 parts by mass of liquid paraffin and 2.3 parts by mass of decalin prepared in advance so that the concentration of the total amount of polyethylene resin was 30% by mass.
This polyethylene solution was extruded into a sheet form through a die at a temperature of 158°C, and then the extrudate was cooled in a water bath at 16°C, and a water stream was provided on the surface layer of the water bath so that the gelled sheet was released from the sheet. A base tape was prepared while making sure that the mixed solvent floating on the water surface did not adhere to the sheet again. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decalin from the base tape, it was subsequently conveyed on a roller heated to 30°C while applying a pressure of 0.05 MPa. A portion of the liquid paraffin was removed from within the base tape. Thereafter, the base tape was stretched in the longitudinal direction at a temperature of 115°C and a magnification of 7 times (longitudinal stretching), then in the width direction at a temperature of 105°C and a magnification of 9 times (transverse stretching), and then at a temperature of 130°C. Heat treatment (heat fixation) was performed.
Next, liquid paraffin was extracted while continuously immersing the heat-treated base tape in two separate methylene chloride baths for 30 seconds each. Note that the purity of the cleaning solvent when the side where immersion starts is set as the first tank and the side where immersion ends is set as the second tank is (low) first layer<second tank (high). Thereafter, methylene chloride was removed by drying at 40°C, and heat treatment was carried out while conveying on rollers heated to 105°C, thereby obtaining a base material for a liquid filter made of a microporous polyolefin membrane.

<Comparative example 26>
A polyethylene composition prepared by mixing 15 parts by mass of ultra-high molecular weight polyethylene with a weight average molecular weight of 4.6 million and a Ca content of 30 ppm and 15 parts by mass of high density polyethylene with a weight average molecular weight of 560,000 and a Ca content of 36 ppm was used. there was. A polyethylene solution was prepared by mixing the polyethylene composition with a mixed solvent of 67.5 parts by mass of liquid paraffin and 2.5 parts by mass of decalin prepared in advance so that the concentration of the total amount of polyethylene resin was 30% by mass.
This polyethylene solution is extruded into a sheet form through a die at a temperature of 155°C, and then the extrudate is cooled in a water bath at 20°C, and a water stream is provided on the surface layer of the water bath so that the gelled sheet is released from the sheet. A base tape was prepared while making sure that the mixed solvent floating on the water surface did not adhere to the sheet again. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decalin from within the base tape, it was subsequently conveyed on a roller heated to 90°C while applying a pressure of 0.05 MPa. A portion of the liquid paraffin was removed from within the base tape. Thereafter, the base tape was stretched in the longitudinal direction at a temperature of 90° C. and a magnification of 5.9 times (longitudinal stretching), then in the width direction at a temperature of 105° C. and a magnification of 13 times (horizontal stretching); Heat treatment (heat fixation) was performed at ℃.
Next, liquid paraffin was extracted while continuously immersing the heat-treated base tape in two separate methylene chloride baths for 30 seconds each. Note that the purity of the cleaning solvent when the side where immersion starts is set as the first tank and the side where immersion ends is set as the second tank is (low) first layer<second tank (high). Thereafter, methylene chloride was removed by drying at 40° C., and heat treatment was performed while conveying it on a roller heated to 120° C., thereby obtaining a base material for a liquid filter made of a microporous polyolefin membrane.

<Comparative example 27>
A polyethylene composition prepared by mixing 18 parts by mass of ultra-high molecular weight polyethylene with a weight average molecular weight of 4.6 million and a Ca content of 30 ppm and 5 parts by mass of high density polyethylene with a weight average molecular weight of 560,000 and a Ca content of 36 ppm was used. there was. A polyethylene solution was prepared by mixing the polyethylene composition with a mixed solvent of 75.9 parts by mass of liquid paraffin and 1.1 parts by mass of decalin prepared in advance so that the concentration of the total amount of polyethylene resin was 23% by mass.
This polyethylene solution was extruded into a sheet form through a die at a temperature of 158°C, and then the extrudate was cooled in a water bath at 18°C, and a water stream was provided on the surface layer of the water bath so that the gelled sheet was released from the sheet. A base tape was prepared while making sure that the mixed solvent floating on the water surface did not adhere to the sheet again. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decalin from within the base tape, it was subsequently conveyed on a roller heated to 90°C while applying a pressure of 0.05 MPa. A portion of the liquid paraffin was removed from within the base tape. Thereafter, the base tape was stretched in the longitudinal direction at a temperature of 90° C. and a magnification of 7.0 times (longitudinal stretching), then in the width direction at a temperature of 105° C. and a magnification of 13 times (horizontal stretching); Heat treatment (heat fixation) was performed at ℃.
Next, liquid paraffin was extracted while continuously immersing the heat-treated base tape in two separate methylene chloride baths for 30 seconds each. Note that the purity of the cleaning solvent when the side where immersion starts is set as the first tank and the side where immersion ends is set as the second tank is (low) first layer<second tank (high). Thereafter, methylene chloride was removed by drying at 40° C., and heat treatment was performed while conveying it on a roller heated to 100° C., thereby obtaining a base material for a liquid filter made of a microporous polyolefin membrane.

<Comparative example 28>
A polyethylene composition is used in which 18 parts by mass of ultra-high molecular weight polyethylene having a weight average molecular weight of 4.6 million and a Ca content of 30 ppm is mixed with 5 parts by mass of high density polyethylene having a weight average molecular weight of 560,000 and a Ca content of 36 ppm. there was. A polyethylene solution was prepared by mixing the polyethylene composition with a mixed solvent of 75.9 parts by mass of liquid paraffin and 1.1 parts by mass of decalin prepared in advance so that the total concentration of polyethylene resin was 23% by mass.
This polyethylene solution was extruded into a sheet form through a die at a temperature of 156°C, and then the extrudate was cooled in a water bath at 18°C, and a water stream was provided on the surface layer of the water bath so that the gelled sheet was released from the sheet. A base tape was prepared while making sure that the mixed solvent floating on the water surface did not adhere to the sheet again. After drying the base tape at 60° C. for 7 minutes and then at 95° C. for 7 minutes to remove decalin from the base tape, it was subsequently conveyed on a roller heated to 110° C. while applying a pressure of 0.05 MPa. A portion of the liquid paraffin was removed from within the base tape. Thereafter, the base tape was stretched in the longitudinal direction at a temperature of 105° C. and a magnification of 6.5 times (longitudinal stretching), then in the width direction at a temperature of 115° C. and a magnification of 13 times (horizontal stretching); Heat treatment (heat fixation) was performed at ℃.
Next, liquid paraffin was extracted while continuously immersing the heat-treated base tape in two separate methylene chloride baths for 30 seconds each. In addition, when the side where immersion is started is set as the first tank and the side where immersion is finished is set as the second tank, the purity of the cleaning solvent is (low) first layer<second tank (high). Thereafter, methylene chloride was removed by drying at 40° C., and heat treatment was performed while conveying on a roller heated to 100° C., thereby obtaining a base material for a liquid filter made of a microporous polyolefin membrane.

<Comparative example 29>
A polyethylene composition prepared by mixing 15 parts by mass of ultra-high molecular weight polyethylene with a weight average molecular weight of 4.6 million and a Ca content of 30 ppm and 15 parts by mass of high density polyethylene with a weight average molecular weight of 560,000 and a Ca content of 36 ppm was used. there was. A polyethylene solution was prepared by mixing the polyethylene composition with a mixed solvent of 67.5 parts by mass of liquid paraffin and 2.5 parts by mass of decalin prepared in advance so that the concentration of the total amount of polyethylene resin was 30% by mass.
This polyethylene solution was extruded into a sheet through a die at a temperature of 164°C, and then the extrudate was cooled in a water bath at 16°C, and a water stream was provided on the surface layer of the water bath to release the gelled sheet in the water bath. A base tape was prepared while making sure that the mixed solvent floating on the water surface did not adhere to the sheet again. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decalin from the base tape, it was subsequently conveyed on a roller heated to 30°C while applying a pressure of 0.05 MPa. A portion of the liquid paraffin was removed from within the base tape. Thereafter, the base tape was stretched in the longitudinal direction at a temperature of 90° C. and a magnification of 5.9 times (longitudinal stretching), then in the width direction at a temperature of 105° C. and a magnification of 13 times (horizontal stretching), and then at a temperature of 120° C. Heat treatment (heat fixation) was performed at ℃.
Next, liquid paraffin was extracted while continuously immersing the heat-treated base tape in two separate methylene chloride baths for 30 seconds each. Note that the purity of the cleaning solvent when the side where immersion starts is set as the first tank and the side where immersion ends is set as the second tank is (low) first layer<second tank (high). Thereafter, methylene chloride was removed by drying at 40° C., and heat treatment was performed while conveying it on a roller heated to 120° C., thereby obtaining a base material for a liquid filter made of a microporous polyolefin membrane.

From the evaluation results shown in Tables 1 and 2, the following can be seen.
The liquid filter substrates made of the polyolefin microporous membranes of Examples 1 to 10 with a water flow rate variation coefficient of 0.100 or less are the It can be seen that the variation in filtration time is superior to that of a liquid filter base material made of a porous membrane.
The base material for liquid filters is made of the polyolefin microporous membranes of Examples 1 to 5 in which the Ca content contained in the polyethylene used as the raw material is less than 1 ppm. It can be seen that the amount of Ca eluted is suppressed compared to the liquid filter base material made of a microporous membrane.
Liquid filter substrates made of the polyolefin microporous membranes of Examples 1 to 5 and 8 to 10 with an average pore diameter of 20 nm or less are made of the polyolefin microporous membranes of Examples 6 to 7 with an average pore diameter of more than 20 nm. It can be seen that the collection performance is superior to that of the liquid filter base material.

Claims (6)

A base material for a liquid filter comprising a polyolefin microporous membrane,
The average value of the pore diameter of the polyolefin microporous membrane is 1 nm to 50 nm,
The average value of the water flow rate of the polyolefin microporous membrane is 0.003 L/min/ft ² /psi to 0.180 L/min/ft ² /psi,
A base material for a liquid filter, wherein the polyolefin microporous membrane has a coefficient of variation of water flow rate of 0.100 or less. The base material for a liquid filter according to claim 1, wherein the microporous polyolefin membrane has a coefficient of variation in pore diameter of 0.100 or less. The substrate for a liquid filter according to claim 1, wherein the microporous polyolefin membrane has a coefficient of variation in film thickness of 0.100 or less. The base material for a liquid filter according to claim 1, wherein the microporous polyolefin membrane has a coefficient of variation of porosity of 0.100 or less. The substrate for a liquid filter according to claim 1, wherein the polyolefin microporous membrane has an average thickness of 3 μm to 20 μm. The substrate for a liquid filter according to claim 1, wherein the microporous polyolefin membrane has an average porosity of 35% to 60%.