TW202412928A - Substrate for liquid filter - Google Patents

Abstract

A substrate for liquid filter comprises a polyolefin microporous film in which an average pore size of the polyolefin microporous film is from 1 nm to 50 nm, an average water flow rate of the polyolefin microporous film is from 0.003L/min/ft ²/psi to 0.180L/min/ft ²/psi, and a coefficient of variation of the flow rate of the polyolefin microporous film is 0.100 or less.

Description

Liquid filter substrate

The present invention relates to a substrate for a liquid filter.

In recent years, electronic devices have become increasingly miniaturized and high-performance. In particular, digital devices and mobile terminals represented by personal computers and mobile phones have made great progress. Among the various technologies that lead and support this progress, the technological innovation of the semiconductor industry has played an important role. In recent years, in the semiconductor industry, various companies have been competing to develop areas with wiring pattern sizes less than 10nm, and are eager to build the most advanced production lines.

The lithography step is a step for forming patterns in the manufacture of semiconductor components. With the miniaturization of patterns in recent years, not only the properties of the chemical solution used in the lithography step itself, but also the handling of the chemical solution before it is applied to the wafer requires extremely advanced technology.

The highly formulated solution is filtered through a fine liquid filter before being applied to the wafer to remove tiny impurities and gel-like impurities from the solution that will greatly affect pattern formation or yield. When forming the most advanced wiring patterns with a pattern size of less than 10nm, as the wiring pattern size shrinks to 10nm to several nm, in order to capture tiny impurities below 5nm in the solution, a liquid filter with a smaller pore size and good permeability is required. Therefore, various filter manufacturers are committed to developing liquid filters for cutting-edge semiconductors. As a liquid filter substrate applicable to a dense liquid filter, a liquid filter substrate containing a small pore size polyolefin microporous membrane is known (for example, refer to Patent Documents 1 to 7).

Patent document 1: Japanese Patent Publication No. 2014-218563 Patent document 2: Japanese Patent Publication No. 2014-217800 Patent document 3: Japanese Patent Publication No. 5684951 Patent document 4: Japanese Patent Publication No. 5684952 Patent document 5: Japanese Patent Publication No. 5684953 Patent document 6: Japanese Patent Publication No. 2018-167198 Patent document 7: Japanese Patent Publication No. 6805371

[The problem that the invention wants to solve]

Liquid filter substrates with micropores of 50nm or less are prone to uneven filtration time in the width direction of the liquid filter substrate. When the filtration time of the liquid filter is uneven, for example, low pressure is formed in the center of the filter and high pressure is formed at the ends of the filter. In this way, the pressure applied to the width direction of the filter is uneven, and the liquid cannot be filtered uniformly throughout the filter. As a result, there is a risk of reduced filter performance or filtration life, or reduced semiconductor manufacturing yield. This case was completed in view of the above-mentioned known issues, and the purpose is to provide a liquid filter substrate that can suppress uneven filtration time even with a small pore size. [Methods to solve the problem]

Specific means for achieving the above-mentioned problems are as follows. <1> A liquid filter substrate, which is a liquid filter substrate comprising a polyolefin microporous membrane, wherein the average value of the pore size 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, and the coefficient of variation of the water flow rate of the polyolefin microporous membrane is 0.100 or less. <2> The liquid filter substrate as described in <1>, wherein the coefficient of variation of the pore size of the polyolefin microporous membrane is 0.100 or less. <3> A substrate for a liquid filter as described in <1> or <2>, wherein the coefficient of variation of the film thickness of the aforementioned polyolefin microporous membrane is less than 0.100. <4> A substrate for a liquid filter as described in any one of <1> to <3>, wherein the coefficient of variation of the porosity of the aforementioned polyolefin microporous membrane is less than 0.100. <5> A substrate for a liquid filter as described in any one of <1> to <4>, wherein the average value of the film thickness of the aforementioned polyolefin microporous membrane is 3μm to 20μm. <6> A substrate for a liquid filter as described in any one of <1> to <5>, wherein the average value of the porosity of the aforementioned polyolefin microporous membrane is 35% to 60%. [Effect of the Invention]

According to the present invention, a liquid filter substrate can be provided which can suppress the unevenness of the filtration time even when the pore size is small.

[Form of implementing the invention]

The following is a detailed description of the implementation of this case. However, this case is not limited to the following implementation. In the following implementation, the constituent elements (including element steps, etc.) are not necessary unless otherwise specified. The same applies to numerical values and their ranges, which are not intended to limit this case.

The term "step" in this case includes steps that are independent of other steps and cannot be clearly distinguished from other steps as long as the purpose of the step can be achieved. The numerical range represented by "～" in this case includes the numerical values recorded before and after "～" as the minimum and maximum values, respectively. In the numerical range recorded in a stepwise manner in this case, the upper limit or lower limit recorded in one numerical range can be replaced by the upper limit or lower limit of other numerical ranges recorded in a stepwise manner. In addition, the upper limit or lower limit of the numerical range recorded in this case can also be replaced by the value shown in the embodiment. In this case, each component can also include multiple corresponding substances. When there are multiple substances corresponding to each component in the composition, the content rate or content of each component, unless otherwise specified, means the total content rate or content of the multiple substances in the composition. The term "layer" or "film" in this case includes the case where the layer or film is formed in the entire region when the region where the layer or film exists is observed, and also includes the case where the layer or film is formed in only part of the region.

In this case, regarding the polyolefin microporous membrane, the "length direction" refers to the long side direction of the polyolefin microporous membrane made into an elongated shape, and the "width direction" refers to the direction perpendicular to the length direction of the polyolefin microporous membrane. Hereinafter, the "width direction" is also referred to as "TD" and the "length direction" is also referred to as "MD". In addition, the "width direction" of the substrate for the liquid filter refers to the same direction as the direction perpendicular to the length direction of the polyolefin microporous membrane.

[Liquid filter substrate] The liquid filter substrate of the present invention is a liquid filter substrate comprising a polyolefin microporous membrane, wherein the average value of the pore size 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, and the coefficient of variation of the water flow rate of the polyolefin microporous membrane is 0.100 or less. According to the liquid filter substrate of the present invention, the non-uniformity of the filtration time can be suppressed even with a small pore size. The reason is not clear, but it can be inferred as follows. The substrate for liquid filter of the present invention is designed to have an average water flow rate of 0.003L/min/ ^ft2/ psi to 0.180L/min/ ^ft2 /psi through a polyolefin microporous membrane. It is not only easy to fully obtain the water permeability of the liquid filter itself, but also easy to obtain the stability of liquid transmission through the filter for a long time. In addition, the substrate for liquid filter of the present invention is designed to have an average pore size of 1nm to 50nm through a polyolefin microporous membrane. It can obtain sufficient liquid permeability and can capture extremely small particles, such as about 10nm or less. From the above, it can be inferred that the substrate for liquid filter of the present invention can effectively function as a liquid filter for cutting-edge semiconductors. On the other hand, when the average pore size of the polyolefin microporous membrane is less than 50 nm and the pore size is small, there is a tendency that the filtration time in the width direction of the liquid filter substrate is easily uneven. However, in this case, since the coefficient of variation of the water flow of the polyolefin microporous membrane is designed to be less than 0.100, the pressure applied to the width direction of the filter is not easy to be uneven, and it is inferred that the uneven filtration time can be suppressed. As described above, according to the liquid filter substrate of this case, it is easy to filter the liquid uniformly throughout the filter, and there is a tendency to more easily suppress the degradation of the filter performance or the reduction of the filtration life.

(Polyolefin microporous membrane) The substrate for liquid filter in this case includes a polyolefin microporous membrane. The substrate for liquid filter in this case may be a substrate composed only of a polyolefin microporous membrane, or a substrate formed by laminating a polyolefin microporous membrane and other layers (such as a coating layer or a porous substrate). In addition, the polyolefin microporous membrane may also be a laminated membrane having a structure in which multiple polyolefin porous layers are laminated. The polyolefin microporous membrane is a microporous membrane composed of polyolefin. The microporous membrane referred to here refers to a membrane having a structure in which a plurality of micropores are formed inside and these micropores are connected, and a gas or liquid can pass from one side to the other side. In addition, in the polyolefin microporous membrane, the polyolefin preferably contains 90 parts by mass or more relative to 100 parts by mass of the polyolefin microporous membrane; the remainder may also contain additives such as organic fillers, inorganic fillers, and surfactants within a range that does not affect the effect of the present invention.

(Pore diameter) In the present case, the average pore diameter of the polyolefin microporous membrane is 1nm to 50nm. The average pore diameter of 1nm to 50nm indicates a small pore diameter. If the average pore diameter of the polyolefin microporous membrane is 1nm or more, sufficient liquid permeability of the liquid filter itself can be obtained. Based on this viewpoint, the average pore diameter is preferably 10nm or more, more preferably 11nm or more, and more preferably 13nm or more. On the other hand, if the average pore diameter of the polyolefin microporous membrane is 50nm or less, microparticles of about 10nm can be captured to a very high degree. Based on this viewpoint, the average pore diameter is preferably 40nm or less, more preferably 30nm or less, and more preferably 25nm or less, and particularly preferably 20nm or less.

In the present case, the polyolefin microporous membrane preferably has a pore size variation coefficient of 0.100 or less, more preferably 0.095 or less, still more preferably 0.093 or less, and particularly preferably 0.085 or less, from the viewpoint of further suppressing the unevenness of the filtration time. The pore size variation coefficient may be 0.0015 or more, preferably 0.020 or more, and more preferably 0.024 or more. The pore size variation coefficient is preferably 0.0015 to 0.100.

In the present case, the pore size value of the polyolefin microporous membrane and its average value and coefficient of variation refer to the values obtained by the method described in the Example column.

(Water flow rate) In the present case, the average water flow rate of the polyolefin microporous membrane is 0.003L/min/ft ^{2 /} psi to 0.180L/min/ft ² /psi. If the average water flow rate of the polyolefin microporous membrane is 0.003L/min/ft ² /psi or more, it is not only easy to fully obtain the water permeability of the liquid filter itself, but also easy to obtain the stability of liquid transmission through the filter for a long time (for example, the stability of the dynamic load for maintaining a certain liquid transmission volume or the stability of the liquid transmission volume under a certain liquid transmission pressure (a certain dynamic load)). Based on this viewpoint, the average 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 water flow rate of the polyolefin microporous membrane is 0.180 L/min/ft ² /psi or less, it is more preferable because it is easier to highly capture microparticles of about 10 nm or less. Based on this viewpoint, the average water flow rate of the polyolefin microporous membrane is preferably 0.150 L/min/ft ² /psi or less, and more preferably 0.100 L/min/ft ² /psi or less.

In the present case, the polyolefin microporous membrane has a coefficient of variation of water flow of 0.100 or less, preferably 0.095 or less, more preferably 0.090 or less, and even more preferably 0.080 or less, from the viewpoint of further suppressing the unevenness of the filtration time. The coefficient of variation of water flow may be 0.010 or more, preferably 0.015 or more, and more preferably 0.018 or more. The coefficient of variation of water flow is preferably 0.010 to 0.100.

In the present case, the water flow rate value of the polyolefin microporous membrane and its average value and coefficient of variation refer to the values obtained by the method described in the Example column.

(Thickness) In the present case, the average thickness of the polyolefin microporous membrane is preferably 3 μm to 20 μm. When the average thickness of the polyolefin microporous membrane is 3 μm or more, it is easy to obtain sufficient mechanical strength, easy to obtain the handleability of the polyolefin microporous membrane during processing, and the durability of the filter cartridge during long-term use. Based on this viewpoint, the average thickness of the polyolefin microporous membrane is more preferably 4 μm or more, more preferably 5 μm or more, and particularly preferably 6 μm or more. On the other hand, when the average film thickness is less than 20μm, not only the thickness variation of the polyolefin microporous membrane in the width direction is less, but also sufficient liquid permeability can be obtained. In a filter box of a given size, it is easy to obtain more filtering area, and it is easier to design the flow rate and structure of the filter when processing the polyolefin microporous membrane. Based on this viewpoint, the average film thickness of the polyolefin microporous membrane is preferably less than 18μm, more preferably less than 16μm, and particularly preferably less than 14μm.

For example, assuming that the filter cartridge is housed in a housing of the same size, the thinner the filter material (including the entire constituent material of the filter substrate), the larger the filter area can be, and the higher flow rate and lower filtration pressure suitable for a liquid filter can be designed. In other words, for a liquid filter, the filtration pressure can be reduced when the same flow rate is to be maintained, and the flow rate can be increased when the same filtration pressure is to be maintained. In particular, by reducing the filtration pressure, the temporarily captured impurities will continue to be exposed to the filtration pressure inside the filter material, and thus, it is expected that the probability of leakage from the filter material as the filter liquid is squeezed out over time will be significantly reduced. In addition, by reducing the filtration pressure, it is expected that the gas dissolved in the filtered liquid will form tiny bubbles due to the pressure difference before and after filtration (pressure reduction after filtration), and the probability of their appearance will be significantly reduced. Furthermore, it is expected to have the effect of improving the filtration yield of the filtered object such as the liquid medicine, and the effect of maintaining the high quality of the filtered object such as the liquid medicine for a long time.

On the other hand, the thinner the filter material is, the lower its strength and durability are. For example, in filter design, it can be reinforced by combining it with a high-strength support with a coarse mesh (such as by overlapping and folding), while adjusting the durability and flow design.

In the present case, the polyolefin microporous membrane preferably has a thickness variation coefficient of 0.100 or less, more preferably 0.095 or less, even more preferably 0.090 or less, and particularly preferably 0.065 or less, from the viewpoint of further suppressing the unevenness of the filtration time. The thickness variation coefficient may be 0.0015 or more, preferably 0.018 or more, and more preferably 0.020 or more. The thickness variation coefficient is preferably 0.0015 to 0.100.

In the present case, the film thickness value of the polyolefin microporous film and its average value and coefficient of variation refer to the values obtained by the method described in the Example column.

(Porosity) In the present case, the average porosity of the polyolefin microporous membrane is preferably 35% to 60%. When the average porosity of the polyolefin microporous membrane is 35% or more, the liquid permeability is good; based on this viewpoint, it is more preferably 36% or more, more preferably 38% or more, and particularly preferably 40% or more. On the other hand, when the average porosity is less than 60%, the mechanical strength of the polyolefin microporous membrane is good and the handling property can be improved. Based on this viewpoint, the average porosity of the polyolefin microporous membrane is more preferably 59% or less, more preferably 58% or less, and particularly preferably 55% or less.

In the present case, the polyolefin microporous membrane preferably has a porosity variation coefficient of 0.100 or less, more preferably 0.095 or less, even more preferably 0.090 or less, and particularly preferably 0.075 or less, from the viewpoint of further suppressing the unevenness of the filtration time. The porosity variation coefficient may be 0.020 or more, preferably 0.025 or more, and more preferably 0.026 or more. The porosity variation coefficient is preferably 0.020 to 0.100.

In the present case, the porosity value of the polyolefin microporous membrane and its average value and coefficient of variation refer to the values obtained by the method described in the Example column.

(Calcium content) In this case, the calcium content in the polyolefin microporous membrane is preferably less than 2000 ppb. If the calcium content is less than 2000 ppb, not only can the washing time when manufacturing the filter cartridge be significantly shortened, but also the amount of water and other solvents used for washing can be greatly reduced. Based on this viewpoint, the calcium content of the polyolefin microporous membrane is more preferably less than 1500 ppb, more preferably less than 1300 ppb, particularly preferably less than 1000 ppb, and extremely preferably less than 800 ppb. On the other hand, the lower the calcium content, the better from the viewpoint of the manufacturing efficiency of the filter cartridge. However, in reality, it is difficult to reduce the calcium content because the manufacturing raw materials contain trace amounts of calcium or calcium is often mixed into the manufacturing process of the polyolefin microporous membrane. The calcium content of the polyolefin microporous membrane can be above 0ppb, above 1ppb, above 10ppb or 50ppb. If the calcium content is below 2000ppb, the manufacturing efficiency of the filter cartridge can be greatly improved.

Furthermore, in the present case, a polyolefin microporous membrane having micropores with an average pore size of 1nm to 50nm is used. The polyolefin microporous membrane having such micropores has a particularly large specific surface area, and the amount of eluted metal ions tends to increase. However, if the calcium content is below 2000ppb, the manufacturing efficiency of the filter cartridge can be greatly improved even for a polyolefin microporous membrane having such micropores. Furthermore, if the calcium content of the polyolefin microporous membrane is above 1ppb, the chloride ions originating from the polymerization catalyst that may remain in the polyolefin in trace amounts can be fully neutralized, and there is no concern about corrosion of stainless steel piping in the polyolefin microporous membrane manufacturing equipment. Based on this viewpoint, the calcium content of the polyolefin microporous membrane is preferably 10 ppb or more, more preferably 50 ppb or more.

In addition, in the present case, the method for adjusting the calcium content of the polyolefin microporous membrane to 2000 ppb or less is not particularly limited, and examples thereof include using a polyolefin raw material constituting the polyolefin microporous membrane having a calcium content of 0 ppb or more and 1000 ppb or less, washing the polyolefin microporous membrane with an acid or the like for a long time after production, etc. From the viewpoint of being able to sufficiently reduce 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 case, the calcium content in the polyolefin microporous membrane refers to the value obtained by the method described in the Example column.

The method for controlling the porous structure of the polyolefin microporous membrane is not particularly limited. For example, it can be controlled based on the composition of the polyolefin resin, the concentration of the polyolefin resin in the raw material, the mixing ratio when a plurality of solvents are mixed in the raw material, the heating temperature of the solvent used to extrude the inside of the extruded sheet, the extrusion pressure, the heating time, the stretching ratio, the heat treatment (heat setting) temperature after stretching, the time of immersion in the extraction solvent, the temperature and time of the annealing treatment, etc.

(Polyolefin) In the present case, the polyolefin constituting the polyolefin microporous membrane may be a homopolymer or copolymer such as polyethylene, polypropylene, polybutene, polymethylpentene, or a mixture of two or more thereof. Among them, polyethylene is preferred. Polyethylene is preferably high-density polyethylene, a mixture of high-density polyethylene and ultra-high molecular weight polyethylene, etc. In addition, polyethylene and components other than polyethylene may also be used in combination. Components other than polyethylene include, for example, polypropylene, polybutene, polymethylpentene, copolymers of polypropylene and polyethylene, etc. In addition, polyolefins having different properties may be used in combination. Specifically, polyolefins having different polymerization degrees or branching degrees and lacking mutual solubility may be used in combination, in other words, polyolefins having different crystallinity, stretchability or molecular orientation may be used in combination.

In particular, the polyolefin used in this case is preferably a polyethylene composition formed 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. This has the effect of increasing the void generation rate by appropriately mixing two or more polyethylenes, and forming a network structure accompanied by fibrillation during stretching. In particular, the mixing ratio of high molecular weight polyethylene to low molecular weight polyethylene is preferably 15:85 to 85:15, and more preferably 19:81 to 81:19 in terms of mass ratio. The low molecular weight polyethylene is preferably a high density polyethylene with a density of 0.92 g/cm ³ to 0.98 g/cm ³ .

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

In the present case, the polyolefin constituting the polyolefin microporous membrane preferably has a calcium content of 0 ppb or more, more preferably 50 ppb or more, and even more preferably 100 ppb or more. In addition, the polyolefin constituting the polyolefin microporous membrane preferably has a calcium content of 1000 ppb or less, more preferably 500 ppb or less, and even more preferably 300 ppb or less. The method for adjusting the calcium content to 1000 ppb or less can be cited as a method of adjusting the amount of metal soap (calcium stearate, etc.) added to the polyolefin after polymerization. In addition, a method of adjusting the commercially available polyolefin raw material by acid washing can also be cited. In the present case, the calcium content of the polyolefin refers to the value obtained by the method described in the embodiment column.

<Liquid filter> The liquid filter substrate of the present case can be processed into a cartridge shape after being appropriately processed to impart affinity with the liquid medicine, and then used as a liquid filter. The liquid filter is a device for removing particles composed of at least one of organic and inorganic substances, or a particle from a treated liquid that may contain the particle. The particle exists in a solid or gel state in the treated liquid. The liquid filter of the present case is suitable for removing extremely small particles with a particle size of about several nanometers. In addition, the liquid filter can be used not only in the semiconductor manufacturing step, but also in other manufacturing steps such as display manufacturing and grinding.

As substrates for liquid filters, porous substrates such as polytetrafluoroethylene are widely known. When a substrate including the polyolefin microporous membrane of the present case is used as a substrate for a liquid filter, it has better affinity with the liquid medicine than a porous polytetrafluoroethylene substrate. Therefore, it is possible to achieve effects such as easy processing to impart affinity between the filter and the liquid medicine, and when the filter cartridge is filled in the filter housing and the filtration of the liquid medicine begins, air is not easily retained in the filter cartridge when the filter is filled with the liquid medicine, and the filtration yield of the liquid medicine is good. Furthermore, since the polyethylene structure itself does not contain halogen elements, it is easy to dispose of the used filter cartridge, thereby reducing the environmental burden.

<Method for manufacturing polyolefin microporous membrane and method for controlling pore structure> The polyolefin microporous membrane contained in the substrate for liquid filter of the present invention can be conveniently manufactured by the method shown below. That is, it can be conveniently manufactured by sequentially implementing: (I) a step of preparing a solution containing a polyolefin component and a solvent; (II) a step of melt-kneading the prepared solution, extruding the obtained melt-kneaded product from a mold, and cooling and solidifying it to obtain a gel-like molded product; (III) a step of pre-extruding a part of the solvent from the gel-like molded product; (IV) a step of stretching the gel-like molded product from which a part of the solvent has been extruded in at least one direction; (V) a step of extracting and washing away the solvent from the inside of the molded product during stretching.

In step (I), a solution comprising a polyolefin component and a solvent is prepared. The solvent is preferably a volatile solvent having a boiling point of at least less than 210° C. under atmospheric pressure. The solution is preferably a thermoreversible sol-gel solution. In step (I), specifically, the thermoreversible sol-gel solution is prepared by heating and dissolving the polyolefin component in a solvent to form a sol. The volatile solvent having a boiling point of less than 210°C under atmospheric pressure is not particularly limited as long as it can swell or dissolve the polyolefin, and preferably includes liquid solvents such as tetrahydronaphthalene, ethylene glycol, decahydronaphthalene, toluene, xylene, diethyltriamine, ethylenediamine, dimethyl sulfoxide, and hexane. These can be used alone or in combination of two or more. Among them, decahydronaphthalene or xylene is preferred. In addition, when preparing this solution, in addition to the above-mentioned volatile solvent having a boiling point of less than 210°C under atmospheric pressure, a non-volatile solvent having a boiling point of more than 210°C such as liquid paraffin, paraffin oil, mineral oil, and castor oil may also be contained.

In the solution of step (I), the concentration of the polyolefin component is preferably 15% to 40% by mass, and more preferably 20% to 30% by mass, from the viewpoint of controlling the liquid permeability of the polyolefin microporous membrane and the removal performance of the filter material itself. If the concentration of the polyolefin component is 15% by mass or more, the mechanical strength tends not to be too low and the handling properties can be well maintained, and the frequency of cracking during the film formation of the polyolefin microporous membrane tends to be suppressed. In addition, if the concentration of the polyolefin component is 40% by mass or less, pores tend to be more easily formed.

Step (II) is to melt-knead the solution prepared in step (I), extrude the obtained molten kneaded product from a mold, and cool and solidify it to obtain a gel-like molded product. Preferably, the extrudate is extruded from a mold at a temperature range of the melting point of the polyolefin composition or 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 formed into a sheet. The cooling method of the extrudate is not particularly limited, and it can be a method of quenching with water or an organic solvent, or a method of casting with a cooled metal roll. Generally speaking, a method of quenching with a volatile solvent used when using water or a sol-gel solution is used. The cooling temperature is preferably 10℃～40℃. In addition, it is better to set a water flow on the surface of the water bath to prevent the mixed solvent released from the gelled thin sheet in the water bath and floating on the water surface from adhering to the thin sheet again while making a gel-like thin sheet.

Step (III) is a step of extruding part of the solvent in the gel-like formed article before stretching the gel-like formed article in at least one direction. Step (III) can be appropriately performed, for example, by applying pressure to the surface of the gel-like formed article by passing the gel-like formed article through the gap between two upper and lower belts or rollers. The amount of the extruded solvent needs to be adjusted according to the liquid permeability and the removal performance of the filtered object required by the polyolefin microporous membrane. Specifically, the amount of the extruded solvent can be adjusted within an appropriate range according to the extrusion pressure between the upper and lower belts or rollers, the temperature of the extrusion step, and the number of extrusions. In addition, it is preferred to set the pressure to which the gel-like molded product is subjected to within the range of 0.01MPa to 0.5MPa using a roller or the like. It is more preferred to set it within the range of 0.05MPa to 0.2MPa. The extrusion temperature is preferably 40°C to 100°C. Furthermore, the number of extrusions varies with the allowable space of the equipment and can be set without special restrictions. Furthermore, as required, one or more stages of preheating can be performed before the solvent is extruded to remove part of the volatile solvent from the sheet. At this time, the preheating temperature is preferably 50°C to 100°C. Furthermore, during this preheating, the time is preferably 5 minutes to 9 minutes per stage. At this time, the amount of volatile solvent removed is adjusted by the transport distance and transport speed of the heating device.

Step (IV) is a step of stretching the gel-like formed article in at least one direction. The stretching in step (IV) is preferably biaxial stretching. Any method of sequential biaxial stretching in which longitudinal stretching and transverse stretching are performed separately or simultaneous biaxial stretching in which longitudinal stretching and transverse stretching are performed simultaneously can be used appropriately. Also preferred are methods of stretching in the longitudinal direction multiple times and then stretching in the transverse direction, methods of stretching in the longitudinal direction and in the transverse direction multiple times, and methods of stretching in the longitudinal direction and/or in the transverse direction once or more after sequential biaxial stretching.

The stretching ratio (the product of the longitudinal stretching ratio and the transverse stretching ratio) is preferably 40 to 105 times, and more preferably 50 to 100 times, from the viewpoint of controlling the liquid permeability of the polyolefin microporous membrane and the removal performance of the filtered object. If the stretching ratio is 40 times or more, it tends to be easy to suppress the occurrence of uneven thickness of the polyolefin microporous membrane. If the stretching ratio is 105 times or less, it tends to reduce the frequency of rupture when manufacturing the polyolefin microporous membrane. Stretching is preferably carried out under the condition that the solvent remains in an appropriate state. The stretching temperature is preferably 80°C to 125°C. 100°C to 120°C is particularly preferred.

And after the stretching step (IV), a heat fixation treatment can be performed. The heat fixation temperature is preferably 100°C to 143°C from the viewpoint of controlling the liquid permeability of the polyolefin microporous membrane and the removal performance of the filtered object. It is more preferably 105°C to 138°C. If the heat fixation temperature is above 100°C, the liquid permeability of the polyolefin microporous membrane tends to be further improved. If the heat fixation temperature is below 143°C, the removal performance of the filtered object of the polyolefin microporous membrane tends to be further improved.

Step (V) is a step of washing the stretched intermediate formed product in order to extract the solvent from the inside of the stretched intermediate formed product. Here, in step (V), in order to extract the solvent from the inside of the stretched intermediate formed product (stretched film), it is preferably washed with a halogenated hydrocarbon such as dichloromethane or an alkali solvent such as hexane. When the intermediate formed product is immersed in a tank storing a solvent for washing, it is preferably 20 seconds to 150 seconds, more preferably 25 seconds to 150 seconds, and particularly preferably 30 seconds to 120 seconds because a polyolefin microporous membrane with less elution can be obtained. Furthermore, in order to further improve the cleaning effect, it is preferred to divide the storage tank into several layers, inject the cleaning solvent from the downstream side of the transport step of the polyolefin microporous membrane, and pour the cleaning solvent toward the upstream side of the transport step, so that the purity of the cleaning solvent in the downstream tank is higher than that in the upstream tank. When the storage tank for storing the solvent is divided into several layers, it can be 2 tanks or 3 tanks or more. Based on the viewpoint of further reducing the purity gradient of the cleaning solvent in each tank, the storage tank for storing the solvent is preferably 3 tanks or more. In addition, under certain performance requirements for the polyolefin microporous membrane, heat setting can also be performed by annealing treatment (heat treatment). In addition, the annealing treatment is preferably carried out at 50°C to 150°C, and more preferably at 60°C to 140°C, based on the viewpoint of transportability in the step. From the viewpoint of suppressing the shrinkage of the polyolefin microporous membrane over time, the annealing treatment is preferably carried out at 100°C to 130°C. During the annealing treatment, it is preferred to suppress the shrinkage of the polyolefin microporous membrane in the width direction. By suppressing the shrinkage in the width direction, there is a tendency for the coefficient of variation of the water flow rate of the polyolefin microporous membrane to become smaller. For example, by performing the annealing treatment while fixing the two ends of the polyolefin microporous membrane in the width direction with a clamp, there is a tendency to suppress the shrinkage of the polyolefin microporous membrane in the width direction. In addition, the pore size, membrane thickness, porosity and water flow rate of the polyolefin microporous membrane can be adjusted to a predetermined range by appropriately adjusting the amount of polyolefin mixed, the concentration of the polyolefin component in the polyolefin solution, the method for preparing the substrate tape, the method for removing the solvent in the polyolefin solution, the stretching conditions of the substrate tape and the heat treatment conditions, etc. [Example]

The following describes the embodiments, comparative examples and various measurement methods of the present invention, but the present invention is not limited to these embodiments.

[Measurement method] (Film thickness) The film thickness of the polyolefin microporous membrane was measured using a contact-type film thickness meter (manufactured by Mitutoyo Co., Ltd.). Here, the contact terminal was a cylindrical object with a bottom diameter of 0.5 cm. The measuring pressure was set to 0.1 N. -Average value- Along the width direction (TD direction) of the polyolefin microporous membrane, the film thickness was measured at 11 points in total at intervals of 26 mm from the center of the product width and its center toward each end face at 5 points as described above, and the value calculated by the arithmetic average of the film thickness at each measured point was used as the average film thickness. - Coefficient of variation- Along the width direction (TD direction) of the polyolefin microporous membrane, the membrane thickness is measured at 11 points in total at intervals of 26 mm from the center of the width of the product and its center toward each end face, and the standard deviation σTD1 is calculated based on the membrane thickness value at each measurement point. Next, the coefficient of variation CVTD1 is calculated by dividing σTD1 by the average value of the membrane thickness at a total of 11 points. The same operation is further repeated 9 times every 50 mm along the length direction of the polyolefin microporous membrane, and the coefficients of variation CVTD1 to CVTD10 are calculated for a total of 10 times. The arithmetic average of the obtained coefficients of variation CVTD1 to CVTD10 is taken as the coefficient of variation of the membrane thickness.

(Porosity) The porosity (ε) of the polyolefin microporous membrane is calculated according to the following formula. ε(%)={1-Ws/(ds・t)}×100 Ws: basis weight of the polyolefin microporous membrane (g/m ² ) ds: true density of polyolefin (g/cm ³ ) t: thickness of the polyolefin microporous membrane (μm) The basis weight of the polyolefin microporous membrane is obtained by cutting a sample into 2 cm×2 cm, measuring its mass, and dividing the mass by the area. -Average value- A sample of 2 cm×2 cm is cut out at 7 points in the width direction (TD direction) of the polyolefin microporous membrane at intervals of 20 mm from the center of the width of the product and its center toward each end surface, and the porosity of this sample is measured as described above. The value calculated by the arithmetic average for each measured sample is taken as the average value of the porosity. - Coefficient of variation - Along the width direction (TD direction) of the polyolefin microporous membrane, the porosity is measured at 7 points in total at intervals of 20 mm from the center of the width of the product and its center toward each end face at every 3 points as described above, and the standard deviation σTD1 is calculated based on the porosity value at each measured point. Next, the coefficient of variation CVTD1 is calculated by dividing σTD1 by the average value of the porosity at a total of 7 points. The same operation is further repeated 9 times every 50 mm along the length direction of the polyolefin microporous membrane, and the coefficients of variation CVTD1 to CVTD10 are calculated for a total of 10 times. The arithmetic average of the obtained coefficients of variation CVTD1 to CVTD10 is taken as the coefficient of variation of the porosity.

(Pore diameter) The average flow pore diameter measured by the following method is used as the pore diameter of the polyolefin microporous membrane. The average flow pore diameter of the polyolefin microporous membrane based on gas-liquid phase substitution is measured using the pore analyzer porous material automatic pore diameter distribution measurement system [Capillary Flow Porometer] manufactured by PMI, using the pore diameter distribution measurement test method [semi-dry method (ASTM E1294-89)]. In addition, the test liquid used is a fluorine-based inert liquid (trade name: Fluorinert) (interfacial tension value: 16.0 dyne/cm), the measurement temperature is 25°C, and the measurement pressure is in the range of 0psi to 500psi under the following conditions. ・Bubble point parameters: BUBLFLOW=50, F/PT=100, MINBPPRES=0, ZEROTIME=1PULSEDELAY=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 polyolefin microporous membrane, the pore diameters of 5 points in total were measured at intervals of 50 mm from the center of the width of the product and its center toward each end face at every 2 points as described above, and the value calculated by the arithmetic average of the pore diameters at each measured location was taken as the average value of the pore diameter. -Coefficient of variation- Along the width direction (TD direction) of the polyolefin microporous membrane, the pore diameters of 5 points in total were measured at intervals of 50 mm from the center of the width of the product and its center toward each end face at every 2 points as described above, and the standard deviation σTD1 was calculated based on the pore diameter values at each measured location. Next, σTD1 was divided by the average value of the pore diameters at a total of 5 points to calculate the coefficient of variation CVTD1. The same operation was repeated 9 times every 50 mm along the length direction of the polyolefin microporous membrane, and the coefficients of variation CVTD1 to CVTD10 were calculated for a total of 10 times. The arithmetic average of the obtained coefficients of variation CVTD1 to CVTD10 was taken as the coefficient of variation of the pore size.

(Water flow rate) The polyolefin microporous membrane was immersed in ethanol and dried at room temperature (24°C). The polyolefin microporous membrane was cut into 40 mm squares and fixed on a stainless steel permeable tank with a diameter of 37 mm (permeable area S: 10.75 cm ² ). The polyolefin microporous membrane on the permeable tank was moistened with a small amount (0.5 ml) of ethanol, and then a pre-measured amount of pure water V (100 ml) was allowed to pass through at a pressure difference of 90 kPa, and the time Tl (min) required for the entire amount of pure water to pass through was measured. From the amount of pure water and the time required for pure water to pass through, the water permeation rate Vs (L/min/ft ² /psi) per unit time (min) and per unit area (ft ² ) at a pressure difference of 1 psi was calculated by the following formula. The measurement is carried out at room temperature of 24°C. Vs=(V/1000)/Tl/(S/929.03)/(90/6.895) -Average value- A 40 mm square sample is cut out at 5 points along the width direction (TD direction) of the polyolefin microporous membrane, with a spacing of 15 mm between the center of the width of the product and its center toward each end surface, and the water flow rate is measured for this sample as described above. The value calculated as the arithmetic mean of each measured sample is taken as the average value of the water flow rate. -Coefficient of variation- The water flow rate is measured at 5 points along the width direction (TD direction) of the polyolefin microporous membrane, with a spacing of 15 mm between the center of the width of the product and its center toward each end surface, and the standard deviation σVs1 is calculated based on the water flow rate value at each measurement point. Next, σVs1 was divided by the average value of the water flow rate at the five points to obtain the coefficient of variation CVVs1. The same operation was repeated nine times every 50 mm along the length of the polyolefin microporous membrane, and the coefficients of variation CVVs1 to CVVs10 were calculated for a total of ten times. The arithmetic average of the obtained coefficients of variation CVVs1 to CVVs10 was taken as the coefficient of variation of the water flow rate.

(Unevenness of Filtration Time) As the test solution of the filtration fluid, an aqueous solution containing a metal colloid (gold colloid) with an average particle size of 5 nm manufactured by Funakoshi Co., Ltd. and a particle concentration of 40 ppb was prepared for use. To evaluate the filtration life, a 5 cm square polyolefin microporous membrane was cut from the center of the product width and three locations on both end surfaces along the TD direction of the polyolefin microporous membrane. The cut polyolefin microporous membrane was fixed to a stainless steel permeable tank with a diameter of 37 mm (permeable area S: 10.75 cm ² ). After the polyolefin microporous membrane on the permeable tank is moistened with a small amount (0.5 ml) of ethanol, 200 ml of the above test solution is filtered at a pressure difference of 0.1 MPa, and the time TF required for the entire amount of the test solution to pass through is measured. When the shortest filtration time value TF is set to 1, the case where the ratio of the filtration time to the longest value is 1.2 or less is rated A, and the case where it exceeds 1.3 is rated B.

(Collection performance) The particle capture rate is calculated from the metal colloid concentration (M1) of the test solution before the filtration time measurement and the metal colloid concentration (M2) of the filtration solution through the polyolefin microporous membrane used in the filtration time measurement according to the following formula. The arithmetic mean of the capture rates measured at three locations, the center of the product width and both end surfaces, along the TD direction of the polyolefin microporous membrane is calculated as the capture rate of the polyolefin microporous membrane. The metal concentration of the solution is determined by using the ICP-OES method (high frequency inductively coupled plasma atomic emission spectrometry, instrument name: Agilent-ICP-OES-5100, Agilent Technologies Co., Ltd.) to dilute the standard reagent and obtain a calibration curve of more than 5 points in the concentration range of 0ppb to 100ppb. The capture performance is evaluated according to the following criteria. Capture rate (%) = ((M1-M2)/(M1)) × 100 -Evaluation criteria- ・A: When the capture rate is 80% or more ・B: When the capture rate is 60% or more and less than 80% ・C: When the capture rate is 30% or more and less than 60%

(Calcium (Ca) content) 0.1 g of the sample was accurately weighed in a fluororesin container, ultra-high purity nitric acid was added for microwave decomposition, and the calcium content was quantified in ppb using ICP (high frequency inductively coupled plasma) mass spectrometry (ICP-MS method, instrument name: Agilent7500cs, Agilent Technologies Co., Ltd.).

(Calcium (Ca) elution from porous membrane) ⁴⁰⁰⁰ cm2 of a polyolefin microporous membrane sample from which surface dust had been removed was placed in a fluororesin container, and 200 g of a 10 mass% hydrochloric acid extract containing 60 mass% isopropyl alcohol was injected. After the sample was immersed in the container for 24 hours, the Ca concentration in the liquid was quantified to 0.1 ppb using the ICP-OES method (high-frequency inductively coupled plasma atomic emission spectrometry, instrument name: Agilent-ICP-OES-5100, Agilent Technologies, Inc.). The Ca elution from the polyolefin microporous membrane was calculated from the mass of the extract, the surface area of the polyolefin microporous membrane, and the Ca concentration in the extract, and was obtained in μg/ ^m2 .

(Production method of polyolefin microporous membrane) The following describes the details of the production method of polyolefin microporous membrane in each embodiment and comparative example. The main differences in the production methods of the embodiment and the comparative example are: in the embodiment, the dichloromethane bath is set to 3 tanks, while in the comparative example, it is 2 tanks; and when the heat treatment is performed, the width is fixed to a certain value relative to the TD direction in the embodiment, while the width is not fixed in the comparative example.

<Example 1> A polyethylene composition is 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 with 5 parts by mass of high-density polyethylene with a weight average molecular weight of 500,000 and a Ca content of 230 ppb. A mixed solvent of 72 parts by mass of liquid wax and 1 part by mass of decahydronaphthalene (decahydronaphthalene) prepared in advance with the polyethylene composition is mixed to prepare a polyethylene solution. This polyethylene solution was extruded from a mold at a temperature of 190°C into a sheet, and then the extrudate was cooled in a water bath at 20°C, and a water flow was set on the surface 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, thereby producing a gelled sheet (base strip). The base strip was dried at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decahydronaphthalene from the base strip, and then the roll heated to 90°C was conveyed while applying a pressure of 0.05 MPa to remove part of the fluidized wax from the base strip. Afterwards, the base tape was stretched at a temperature of 100°C at a ratio of 5 times in the length direction (longitudinal stretching), then stretched at a temperature of 112°C at a ratio of 15 times in the width direction (transverse stretching), and then heat-treated at a temperature of 115°C (heat fixing). Next, the base tape that had been heat-treated was immersed in a dichloromethane bath divided into three tanks for 30 seconds each, while the liquid wax was extracted. In addition, the purity of the cleaning solvent when the side where the immersion started was the first tank and the side where the immersion was completed was the third tank (low) 1st tank < 2nd tank < 3rd tank (high). After that, the dichloromethane was removed by drying at 40°C, and the width was fixed to a certain value relative to the TD direction, while the heat treatment was performed at 110°C for 1 minute in the TD direction, and then cooled at 60°C for 20 seconds to obtain a liquid filter substrate composed of a polyolefin microporous membrane. The physical properties of the obtained liquid filter substrate are shown in Table 1. In addition, 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 composed of a polyolefin microporous membrane was obtained in the same manner as Example 1 except that the longitudinal stretching ratio was set to 6 times.

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

<Example 4> A polyethylene composition is prepared by mixing 21 parts by weight of ultra-high molecular weight polyethylene with a weight average molecular weight of 4.6 million and a Ca content of 110 ppb with 5 parts by weight of high-density polyethylene with a weight average molecular weight of 500,000 and a Ca content of 230 ppb. A mixed solvent of 73 parts by weight of liquid wax and 1 part by weight of decahydronaphthalene, which has been prepared in advance and has a concentration of 26% by weight of the total amount of polyethylene resin, is mixed with the polyethylene composition to prepare a polyethylene solution. This polyethylene solution is extruded from a mold at a temperature of 190°C into a sheet, and then the extrudate is cooled at 20°C in a water bath, and a water flow is set on the surface 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, while preparing a base tape body. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decahydronaphthalene from the base tape, the base tape was then transported while applying a pressure of 0.05 MPa to a roller heated to 90°C to remove some of the fluidized wax from the base tape. The base tape was then stretched at a temperature of 100°C at a magnification of 5 times in the length direction (longitudinal stretching), then stretched at a temperature of 115°C at a magnification of 15 times in the width direction (transverse stretching), and then heat-treated at a temperature of 125°C (heat fixing). Next, the heat-treated base tape was immersed in a methylene chloride bath divided into three tanks for 30 seconds each to extract the fluidized wax. In addition, the purity of the cleaning solvent when the side where the immersion started was the first tank and the side where the immersion was completed was the third tank (low) 1st tank < 2nd tank < 3rd tank (high). After that, the dichloromethane was removed by drying at 40°C, and the width was fixed to a certain value relative to the TD direction while heating at 120°C for 1 minute in the TD direction and cooling at 60°C for 20 seconds to obtain a liquid filter substrate composed of a polyolefin microporous membrane.

<Example 5> A polyethylene composition is prepared by mixing 18 parts by weight of ultra-high molecular weight polyethylene with a weight average molecular weight of 4.6 million and a Ca content of 140 ppb with 5 parts by weight of high-density polyethylene with a weight average molecular weight of 500,000 and a Ca content of 230 ppb. A mixed solvent of 76 parts by weight of liquid wax and 1 part by weight of decahydronaphthalene, which has been prepared in advance and has a concentration of 23% by weight of the total amount of polyethylene resin, is mixed with the polyethylene composition to prepare a polyethylene solution. This polyethylene solution is extruded from a mold at a temperature of 190°C into a sheet, and then the extrudate is cooled at 20°C in a water bath, and a water flow is set on the surface 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, while preparing a base tape body. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decahydronaphthalene from the base tape, the base tape was then transported while applying a pressure of 0.05 MPa to a roller heated to 90°C to remove some of the fluidized wax from the base tape. The base tape was then stretched at 100°C at a ratio of 4 times in the length direction (longitudinal stretching), then stretched at 115°C at a ratio of 15 times in the width direction (transverse stretching), and then heat treated at 125°C (heat fixing). Next, the heat-treated base tape was immersed in a methylene chloride bath divided into three tanks for 30 seconds each to extract the fluidized wax. In addition, the purity of the cleaning solvent when the side where the immersion started was the first tank and the side where the immersion was completed was the third tank (low) 1st tank < 2nd tank < 3rd tank (high). After that, the dichloromethane was removed by drying at 40°C, and the width was fixed to a certain value relative to the TD direction while heating at 120°C for 1 minute in the TD direction and cooling at 60°C for 20 seconds to obtain a liquid filter substrate composed of a polyolefin microporous membrane.

<Example 6> A polyethylene composition is 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 with 14 parts by mass of high-density polyethylene with a weight average molecular weight of 400,000 and a Ca content of 34 ppm. A mixed solvent of 51 parts by mass of liquid wax and 32 parts by mass of decahydronaphthalene prepared in advance with a concentration of 17% by mass of the total amount of polyethylene resin is mixed with the polyethylene composition to prepare a polyethylene solution. This polyethylene solution is extruded from a mold at a temperature of 190°C into a sheet, and then the extrudate is cooled at 20°C in a water bath, and a water flow is set on the surface 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, while preparing a base tape body. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decahydronaphthalene from the base tape, the base tape was then transported while applying a pressure of 0.05 MPa to a roller heated to 90°C to remove some of the fluidized wax from the base tape. The base tape was then stretched at a temperature of 100°C at a ratio of 4.5 times in the length direction (longitudinal stretching), then stretched at a temperature of 115°C at a ratio of 8 times in the width direction (transverse stretching), and then heat-treated at a temperature of 125°C (heat fixing). Next, the heat-treated base tape was immersed in a methylene chloride bath divided into three tanks for 30 seconds each to extract the fluidized wax. In addition, the purity of the cleaning solvent when the side where the immersion started was the first tank and the side where the immersion was completed was the third tank (low) 1st tank < 2nd tank < 3rd tank (high). After that, the dichloromethane was removed by drying at 40°C, and the width was fixed to a certain value relative to the TD direction while heating at 120°C for 1 minute in the TD direction and cooling at 60°C for 20 seconds to obtain a liquid filter substrate composed of a polyolefin microporous membrane.

<Example 7> A polyethylene composition is 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 with 5 parts by mass of high-density polyethylene with a weight average molecular weight of 400,000 and a Ca content of 34 ppm. A mixed solvent of 51 parts by mass of liquid wax and 32 parts by mass of decahydronaphthalene prepared in advance with a concentration of 17% by mass of the total amount of polyethylene resin is mixed with the polyethylene composition to prepare a polyethylene solution. This polyethylene solution is extruded from a mold at a temperature of 190°C into a sheet, and then the extrudate is cooled at 20°C in a water bath, and a water flow is set on the surface 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, while preparing a base tape body. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decahydronaphthalene from the base tape, the base tape was then transported while applying a pressure of 0.05 MPa to a roller heated to 90°C to remove some of the fluidized wax from the base tape. The base tape was then stretched at a temperature of 100°C at a ratio of 4 times in the length direction (longitudinal stretching), then stretched at a temperature of 115°C at a ratio of 9 times in the width direction (transverse stretching), and then heat treated at a temperature of 125°C (heat fixing). Next, the heat-treated base tape was immersed in a methylene chloride bath divided into three tanks for 30 seconds each to extract the fluidized wax. In addition, the purity of the cleaning solvent when the side where the immersion started was the first tank and the side where the immersion was completed was the third tank (low) 1st tank < 2nd tank < 3rd tank (high). After that, the dichloromethane was removed by drying at 40°C, and the width was fixed to a constant relative to the TD direction while heating at 120°C for 1 minute in the TD direction and cooling at 60°C for 20 seconds to obtain a liquid filter substrate composed of a polyolefin microporous membrane.

<Example 8> A polyethylene composition is 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 with 24 parts by mass of high-density polyethylene with a weight average molecular weight of 400,000 and a Ca content of 34 ppm. A mixed solvent of 68 parts by mass of liquid wax and 2 parts by mass of decahydronaphthalene prepared in advance at a concentration of 30% by mass of the total amount of polyethylene resin is mixed with the polyethylene composition to prepare a polyethylene solution. This polyethylene solution is extruded from a mold at a temperature of 190°C into a sheet, and then the extrudate is cooled at 20°C in a water bath, and a water flow is set on the surface 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, while preparing a base tape body. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decahydronaphthalene from the base tape, the base tape was then transported while applying a pressure of 0.05 MPa to a roller heated to 90°C to remove some of the fluidized wax from the base tape. The base tape was then stretched at 95°C at a ratio of 4 times in the length direction (longitudinal stretching), then stretched at 110°C at a ratio of 11 times in the width direction (transverse stretching), and then heat treated at 120°C (heat fixing). Next, the heat-treated base tape was continuously immersed in a methylene chloride bath divided into three tanks for 30 seconds each to extract the fluidized wax. In addition, the purity of the cleaning solvent when the side where the immersion started was the first tank and the side where the immersion was completed was the third tank (low) 1st tank < 2nd tank < 3rd tank (high). After that, the dichloromethane was removed by drying at 40°C, and the width was fixed to a certain value relative to the TD direction while heating at 120°C for 1 minute in the TD direction and cooling at 60°C for 20 seconds to obtain a liquid filter substrate composed of a polyolefin microporous membrane.

<Example 9> A polyethylene composition is 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 with 15 parts by mass of high-density polyethylene with a weight average molecular weight of 400,000 and a Ca content of 34 ppm. A mixed solvent of 68 parts by mass of liquid wax and 2 parts by mass of decahydronaphthalene prepared in advance at a concentration of 30% by mass of the total amount of polyethylene resin is mixed with the polyethylene composition to prepare a polyethylene solution. This polyethylene solution is extruded from a mold at a temperature of 190°C into a sheet, and then the extrudate is cooled at 20°C in a water bath, and a water flow is set on the surface 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, while preparing a base tape body. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decahydronaphthalene from the base tape, the base tape was then transported while applying a pressure of 0.05 MPa to a roller heated to 90°C to remove some of the fluidized wax from the base tape. The base tape was then stretched at a temperature of 100°C at a ratio of 4 times in the length direction (longitudinal stretching), then stretched at a temperature of 115°C at a ratio of 10 times in the width direction (transverse stretching), and then heat treated at a temperature of 125°C (heat fixing). Next, the heat-treated base tape was immersed in a methylene chloride bath divided into three tanks for 30 seconds each to extract the fluidized wax. In addition, the purity of the cleaning solvent when the side where the immersion started was the first tank and the side where the immersion was completed was the third tank (low) 1st tank < 2nd tank < 3rd tank (high). After that, the dichloromethane was removed by drying at 40°C, and the width was fixed to a certain value relative to the TD direction while heating at 120°C for 1 minute in the TD direction and cooling at 60°C for 20 seconds to obtain a liquid filter substrate composed of a polyolefin microporous membrane.

<Example 10> A polyethylene composition is prepared by mixing 17 parts by weight of ultra-high molecular weight polyethylene with a weight average molecular weight of 4.2 million and a Ca content of 31 ppm with 4 parts by weight of high-density polyethylene with a weight average molecular weight of 400,000 and a Ca content of 34 ppm. A mixed solvent of 78 parts by weight of liquid wax and 1 part by weight of decahydronaphthalene, which has been prepared in advance and has a concentration of 21% by weight of the total amount of polyethylene resin, is mixed with the polyethylene composition to prepare a polyethylene solution. This polyethylene solution is extruded from a mold at a temperature of 190°C into a sheet, and then the extrudate is cooled at 20°C in a water bath, and a water flow is set on the surface 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, while preparing a base tape body. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decahydronaphthalene from the base tape, the base tape was then transported while applying a pressure of 0.05 MPa to a roller heated to 90°C to remove some of the fluidized wax from the base tape. The base tape was then stretched at a temperature of 100°C at a ratio of 4 times in the length direction (longitudinal stretching), then stretched at a temperature of 115°C at a ratio of 8 times in the width direction (transverse stretching), and then heat-treated at a temperature of 125°C (heat fixing). Next, the heat-treated base tape was immersed in a methylene chloride bath divided into three tanks for 30 seconds each to extract the fluidized wax. In addition, the purity of the cleaning solvent when the side where the immersion started was the first tank and the side where the immersion was completed was the third tank (low) 1st tank < 2nd tank < 3rd tank (high). After that, the dichloromethane was removed by drying at 40°C, and the width was fixed to a constant relative to the TD direction while heating at 120°C for 1 minute in the TD direction and cooling at 60°C for 20 seconds to obtain a liquid filter substrate composed of a polyolefin microporous membrane.

<Comparative Example 1> A polyethylene composition was 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 with 5 parts by mass of high-density polyethylene with a weight average molecular weight of 500,000 and a Ca content of 230 ppb. A mixed solvent of 72 parts by mass of liquid wax and 1 part by mass of decahydronaphthalene, which had been prepared in advance at a concentration of 27% by mass of the total amount of the polyethylene resin, was mixed with the polyethylene composition to prepare a polyethylene solution. This polyethylene solution was extruded from a mold at a temperature of 190°C into a sheet, and then the extrudate was cooled at 20°C in a water bath, and a water flow was set on the surface 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, while preparing a base tape body. The base tape was dried at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decahydronaphthalene from the base tape, and then the tape was transported while applying a pressure of 0.05 MPa to a roller heated to 90°C to remove some of the fluidized wax from the base tape. Thereafter, the base tape was stretched at a temperature of 100°C at a ratio of 8 times in the length direction (longitudinal stretching), and then stretched at a temperature of 112°C at a ratio of 10 times in the width direction (transverse stretching), and then heat-treated at a temperature of 115°C (heat fixing). Next, the heat-treated base tape was immersed in a dichloromethane bath divided into two tanks for 30 seconds each to extract the fluidized wax. In addition, the purity of the cleaning solvent when the side where the immersion starts is the first tank and the side where the immersion is completed is the second tank (low) the first tank < the second tank (high). After that, the dichloromethane is removed by drying at 40°C, and the substrate for liquid filter composed of polyolefin microporous membrane is obtained while being transported on a roll heated to 110°C.

<Comparative Example 2> A substrate for a liquid filter composed of a polyolefin microporous membrane was prepared in the same manner as in Comparative Example 1 except that the longitudinal stretching ratio was set to 9 times.

<Comparative Example 3> A substrate for a liquid filter composed of a polyolefin microporous membrane was prepared in the same manner as in Comparative Example 1 except that the longitudinal stretching ratio was set to 10 times and the transverse stretching ratio was set to 8 times.

<Comparative Example 4> A polyethylene composition was prepared by mixing 3 parts by weight of ultra-high molecular weight polyethylene with a weight average molecular weight of 4.2 million and a Ca content of 31 ppm with 14 parts by weight of high density polyethylene with a weight average molecular weight of 400,000 and a Ca content of 34 ppm. A mixed solvent of 51 parts by weight of liquid wax and 32 parts by weight of decahydronaphthalene, which had been prepared in advance at a concentration of 17% by weight of the total amount of the polyethylene resin, was mixed with the polyethylene composition to prepare a polyethylene solution. This polyethylene solution was extruded from a mold at a temperature of 190°C into a sheet, and then the extrudate was cooled at 20°C in a water bath, and a water flow was set on the surface 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, while preparing a base tape body. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decahydronaphthalene from the base tape, the base tape was then transported while applying a pressure of 0.05 MPa to a roller heated to 90°C to remove some of the fluidized wax from the base tape. The base tape was then stretched at a temperature of 100°C at a ratio of 6 times in the length direction (longitudinal stretching), then stretched at a temperature of 115°C at a ratio of 6 times in the width direction (transverse stretching), and then heat-treated at a temperature of 125°C (heat fixing). Next, the heat-treated base tape was continuously immersed in a dichloromethane bath divided into two tanks for 30 seconds each to extract the fluidized wax. In addition, the purity of the cleaning solvent when the side where the immersion starts is the first tank and the side where the immersion is completed is the second tank (low) first tank < second tank (high). After that, the dichloromethane is removed by drying at 40°C, and the substrate for liquid filter composed of polyolefin microporous membrane is obtained while being transported on a roll heated to 120°C.

<Comparative Example 5> A polyethylene composition was 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 with 23 parts by mass of high-density polyethylene with a weight average molecular weight of 560,000 and a Ca content of 36 ppm. A mixed solvent of 69 parts by mass of liquid wax and 3 parts by mass of decahydronaphthalene, which had been prepared in advance at a concentration of 28% by mass of the total amount of the polyethylene resin, was mixed with the polyethylene composition to prepare a polyethylene solution. This polyethylene solution was extruded from a mold at a temperature of 160°C into a sheet, and then the extrudate was cooled at 25°C in a water bath, and a water flow was set on the surface 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, while preparing a base tape body. The base tape was dried at 55°C for 10 minutes and then at 95°C for 10 minutes to remove the decahydronaphthalene from the base tape. Then, the tape was transported while applying a pressure of 20kgf/m (1.96MPa) to a roller heated to 85°C to remove some of the fluidized wax from the base tape. Then, the base tape was stretched at a temperature of 100°C at a ratio of 5.8 times in the length direction (longitudinal stretching), and then stretched at a temperature of 100°C at a ratio of 14 times in the width direction (transverse stretching), and then heat-treated at a temperature of 118°C (heat fixing). Next, the heat-treated base tape was immersed in a dichloromethane bath divided into two tanks for 30 seconds each to extract the fluidized wax. In addition, the purity of the cleaning solvent when the side where the immersion starts is the first tank and the side where the immersion is completed is the second tank (low) the first tank < the second tank (high). After that, the dichloromethane is removed by drying at 45°C, and the substrate for liquid filter composed of polyolefin microporous membrane is obtained while being transported on a roll heated to 110°C.

<Comparative Example 6> In Comparative Example 5, a substrate for a liquid filter composed of a polyolefin microporous membrane was obtained in the same manner except that the substrate strip was conveyed while applying a pressure of 40 kgf/m (3.92 MPa) on a roller heated to 40°C, and after removing part of the fluidized wax from the substrate strip, the substrate was stretched at a temperature of 90°C at a ratio of 5.8 times in the length direction (longitudinal stretching), then stretched at a temperature of 90°C at a ratio of 14 times in the width direction (transverse stretching), and then heat-treated at a temperature of 124°C (heat fixing).

<Comparative Example 7> Except that in Comparative Example 5, the base belt body was conveyed while applying a pressure of 5 kgf/m (0.49 MPa) on a roller heated to 95°C, and after removing part of the fluidized wax from the base belt body, it was stretched at a temperature of 95°C in the length direction at a ratio of 6.5 times (longitudinal stretching), and then stretched at a temperature of 95°C in the width direction at a ratio of 13 times (transverse stretching), and then heat-treated at a temperature of 114°C (heat fixing), a substrate for a liquid filter composed of a polyolefin microporous membrane was obtained in the same manner.

<Comparative Example 8> In addition to the polyethylene composition in Comparative Example 5, 4 parts by weight of ultra-high molecular weight polyethylene with a weight average molecular weight of 4.6 million and a Ca content of 30 ppm and 31 parts by weight of high-density polyethylene with a weight average molecular weight of 560,000 and a Ca content of 36 ppm were used, and a mixed solvent of 55 parts by weight of liquid wax and 10 parts by weight of decahydronaphthalene with a total concentration of 35% by weight of the polyethylene resin was prepared in advance and mixed with the polyethylene. The polyethylene solution was prepared by mixing the polyolefin component, and the base tape with part of the fluidized wax removed was stretched at 100°C in the length direction at a ratio of 5 times (longitudinal stretching), and then stretched at 100°C in the width direction at a ratio of 9 times (transverse stretching), and then heat-treated at 114°C (heat fixing) to obtain a liquid filter substrate composed of a polyolefin microporous membrane in the same manner.

<Comparative Example 9> Except that in Comparative Example 5, a polyethylene composition was used in which 8 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 was mixed with 24 parts by mass of high-density polyethylene with a weight average molecular weight of 560,000 and a Ca content of 36 ppm, and a mixed solvent of 53 parts by mass of fluidized wax and 15 parts by mass of decahydronaphthalene with a concentration of 32% by mass of the total amount of polyethylene resin was prepared in advance and mixed with the polyethylene composition to prepare a polyethylene solution, and the base tape body from which part of the fluidized wax was removed was stretched at a temperature of 100°C at a ratio of 4 times in the length direction (longitudinal stretching), and then stretched at a temperature of 100°C at a ratio of 15 times in the width direction (transverse stretching) to obtain a substrate for a liquid filter composed of a polyolefin microporous membrane in the same manner.

<Comparative Example 10> A polyethylene composition was prepared by mixing 14 parts by weight of ultra-high molecular weight polyethylene with a weight average molecular weight of 4.6 million and a Ca content of 30 ppm with 6 parts by weight of high-density polyethylene with a weight average molecular weight of 560,000 and a Ca content of 36 ppm. A mixed solvent of 55 parts by weight of liquid wax and 25 parts by weight of decahydronaphthalene, which had been prepared in advance at a concentration of 20% by weight of the total amount of the polyethylene resin, was mixed with the polyethylene composition to prepare a polyethylene solution. This polyethylene solution was extruded from a mold at a temperature of 160°C into a sheet, and then the extrudate was cooled at 25°C in a water bath, and a water flow was set on the surface 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, while preparing a base tape body. The base tape was dried at 55°C for 10 minutes and then at 95°C for 10 minutes to remove the decahydronaphthalene from the base tape. Then, the tape was transported while applying a pressure of 20kgf/m (1.96MPa) to a roller heated to 85°C to remove some of the fluidized wax from the base tape. Then, the base tape was stretched at a temperature of 100°C at a magnification of 5 times in the length direction (longitudinal stretching), and then stretched at a temperature of 100°C at a magnification of 14 times in the width direction (transverse stretching), and then heat-treated at a temperature of 128°C (heat fixing). Next, the heat-treated base tape was immersed in a dichloromethane bath divided into two tanks for 30 seconds each to extract the fluidized wax. In addition, the purity of the cleaning solvent when the side where the immersion starts is the first tank and the side where the immersion is completed is the second tank (low) the first tank < the second tank (high). After that, the dichloromethane is removed by drying at 45°C, and the substrate for liquid filter composed of polyolefin microporous membrane is obtained while being transported on a roll heated to 110°C.

<Comparative Example 11> In Comparative Example 10, except that a polyethylene composition was used in which 12 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 was mixed with 8 parts by mass of high-density polyethylene with a weight average molecular weight of 560,000 and a Ca content of 36 ppm, a liquid filter substrate composed of a polyolefin microporous membrane was obtained in the same manner.

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

<Comparative Example 13> In addition to using a polyethylene composition obtained by mixing 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 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 as in Comparative Example 10, a mixed solvent of 50 parts by mass of liquid wax and 25 parts by mass of decahydronaphthalene having a concentration of 25% by mass of the total amount of the polyethylene resin prepared in advance was mixed with the polyethylene composition to prepare a polyethylene solution. This polyethylene solution was extruded in the same manner as in Comparative Example 10, and the resulting base strip was heated and dried, and then transported while applying a pressure of 10kgf/m (0.98MPa) to a roller heated to 95°C, and a liquid filter substrate composed of a polyolefin microporous membrane was obtained in the same manner except that part of the fluidized wax was removed from the base strip.

<Comparative Example 14> A polyethylene composition was prepared by mixing 3 parts by weight of ultra-high molecular weight polyethylene with a weight average molecular weight of 4.6 million and a Ca content of 30 ppm with 12 parts by weight of high density polyethylene with a weight average molecular weight of 560,000 and a Ca content of 36 ppm. A mixed solvent of 60 parts by weight of liquid wax and 25 parts by weight of decahydronaphthalene, which had been prepared in advance at a concentration of 15% by weight of the total amount of the polyethylene resin, was mixed with the polyethylene composition to prepare a polyethylene solution. This polyethylene solution was extruded from a mold at a temperature of 160°C into a sheet, and then the extrudate was cooled at 25°C in a water bath, and a water flow was set on the surface 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, while preparing a base tape body. The base tape was dried at 55°C for 10 minutes and then at 95°C for 10 minutes to remove decahydronaphthalene from the base tape. Then, the tape was transported while applying a pressure of 20 kgf/m (1.96 MPa) to a roller heated to 85°C to remove some of the fluidized wax from the base tape. Then, the base tape was stretched at a temperature of 100°C at a magnification of 6 times in the length direction (longitudinal stretching), and then stretched at a temperature of 100°C at a magnification of 12 times in the width direction (transverse stretching), and then heat-treated at a temperature of 136°C (heat fixing). Next, the heat-treated base tape was immersed in a dichloromethane bath divided into two tanks for 30 seconds each to extract the fluidized wax. In addition, the purity of the cleaning solvent when the side where the immersion starts is the first tank and the side where the immersion is completed is the second tank (low) the first tank < the second tank (high). After that, the dichloromethane is removed by drying at 45°C, and the substrate for liquid filter composed of polyolefin microporous membrane is obtained while being transported on a roll heated to 110°C.

<Comparative Example 15> In Comparative Example 14, a polyethylene composition was used in which 2 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 was mixed with 18 parts by mass of high-density polyethylene with a weight average molecular weight of 560,000 and a Ca content of 36 ppm, and a mixed solvent of 55 parts by mass of liquid wax and 25 parts by mass of decahydronaphthalene prepared in advance with a concentration of 20% by mass of the total amount of the polyethylene resin was mixed with the polyethylene composition to prepare a polyethylene solution. A liquid filter substrate composed of a polyolefin microporous membrane was obtained in the same manner.

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

<Comparative Example 17> In addition to using a polyethylene composition obtained by mixing 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 with 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 as in Comparative Example 14, a mixed solvent of 50 parts by mass of liquid wax and 25 parts by mass of decahydronaphthalene having a concentration of 25% by mass of the total amount of the polyethylene resin prepared in advance was mixed with the polyethylene composition to prepare a polyethylene solution. This polyethylene solution was extruded in the same manner as in Comparative Example 14, and the resulting base tape was heated and dried, and then transported while applying a pressure of 10 kgf/m (0.98 MPa) to a roller heated to 95°C, and a liquid filter substrate composed of a polyolefin microporous membrane was obtained in the same manner except that part of the fluidized wax was removed from the base tape.

<Comparative Example 18> A polyethylene composition was prepared by mixing 18.4 parts by weight of ultra-high molecular weight polyethylene with a weight average molecular weight of 5.1 million and a Ca content of 140 ppb with 4.6 parts by weight of high-density polyethylene with a weight average molecular weight of 650,000 and a Ca content of 270 ppb. A mixed solvent of 74.5 parts by weight of liquid wax and 2.5 parts by weight of decahydronaphthalene, which had been prepared in advance and had a concentration of 23% by weight of the total amount of the polyethylene resin, was mixed with the polyethylene composition to prepare a polyethylene solution. The polyethylene solution was extruded from a mold at 155°C into a sheet, and then the extrudate was cooled in a water bath at 20°C, while a water flow was set on the surface 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, thereby producing a base tape. The base tape was dried at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decahydronaphthalene from the base tape, and then the roll heated to 90°C was conveyed while applying a pressure of 0.05 MPa to remove part of the fluidized wax from the base tape. Afterwards, the base tape was stretched at a temperature of 115°C at a ratio of 5.8 times in the length direction (longitudinal stretching), then stretched at a temperature of 105°C at a ratio of 13 times in the width direction (transverse stretching), and then heat-treated at 130°C (heat fixing). Next, the base tape that had been heat-treated was immersed in a dichloromethane bath divided into two tanks for 30 seconds each, while the liquid wax was extracted. In addition, the purity of the cleaning solvent when the side where the immersion started was the first tank and the side where the immersion was completed was the second tank (low) first tank < second tank (high). Afterwards, the dichloromethane was removed by drying at 40°C, and the substrate was heat-treated while being transported on a roll heated to 120°C, thereby obtaining a liquid filter substrate composed of a polyolefin microporous membrane.

<Comparative Example 19> A polyethylene composition was prepared by mixing 18.4 parts by weight of ultra-high molecular weight polyethylene with a weight average molecular weight of 5.1 million and a Ca content of 140 ppb with 4.6 parts by weight of high-density polyethylene with a weight average molecular weight of 650,000 and a Ca content of 270 ppb. A mixed solvent of 75.9 parts by weight of liquid wax and 1.1 parts by weight of decahydronaphthalene, which had been prepared in advance and had a concentration of 23% by weight of the total amount of the polyethylene resin, was mixed with the polyethylene composition to prepare a polyethylene solution. The polyethylene solution was extruded from a mold at 158°C into a sheet, and then the extrudate was cooled in a water bath at 18°C, while a water flow was set on the surface 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, thereby producing a base tape. The base tape was dried at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decahydronaphthalene from the base tape, and then the roll heated to 90°C was conveyed while applying a pressure of 0.05 MPa to remove part of the flowing wax from the base tape. Afterwards, the base tape was stretched at a temperature of 95°C at a ratio of 5.8 times in the length direction (longitudinal stretching), then stretched at a temperature of 105°C at a ratio of 13 times in the width direction (transverse stretching), and then heat-treated at 140°C (heat fixing). Next, the base tape that had been heat-treated was immersed in a dichloromethane bath divided into two tanks for 30 seconds each, while the liquid wax was extracted. In addition, the purity of the cleaning solvent when the side where the immersion started was the first tank and the side where the immersion was completed was the second tank (low) first tank < second tank (high). Afterwards, the dichloromethane was removed by drying at 40°C, and the substrate was heat-treated while being transported on a roll heated to 120°C, thereby obtaining a liquid filter substrate composed of a polyolefin microporous membrane.

<Comparative Example 20> A polyethylene composition was prepared 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 with 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. A mixed solvent of 72.5 parts by mass of liquid wax and 2.5 parts by mass of decahydronaphthalene, which had been prepared in advance and had a concentration of 25% by mass of the total amount of the polyethylene resin, was mixed with the polyethylene composition to prepare a polyethylene solution. The polyethylene solution was extruded from a mold at 156°C into a sheet, and then the extrudate was cooled in a water bath at 18°C, while a water flow was set on the surface 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, thereby producing a base tape. The base tape was dried at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decahydronaphthalene from the base tape, and then the roll heated to 110°C was conveyed while applying a pressure of 0.05 MPa to remove part of the flowing wax from the base tape. Afterwards, the base tape was stretched at a temperature of 115°C at a ratio of 5.8 times in the length direction (longitudinal stretching), then stretched at a temperature of 105°C at a ratio of 13 times in the width direction (transverse stretching), and then heat-treated at 115°C (heat fixing). Next, the base tape that had been heat-treated was immersed in a dichloromethane bath divided into two tanks for 30 seconds each, while the liquid wax was extracted. In addition, the purity of the cleaning solvent when the side where the immersion started was the first tank and the side where the immersion was completed was the second tank (low) first tank < second tank (high). Afterwards, the dichloromethane was removed by drying at 40°C, and the substrate was heat-treated while being transported on a roll heated to 120°C, thereby obtaining a liquid filter substrate composed of a polyolefin microporous membrane.

<Comparative Example 21> A polyethylene composition was 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 with 24 parts by mass of high-density polyethylene with a weight average molecular weight of 650,000 and a Ca content of 270 ppb. A mixed solvent of 67.5 parts by mass of liquid wax and 2.5 parts by mass of decahydronaphthalene, which had been prepared in advance at a concentration of 30% by mass of the total amount of the polyethylene resin, was mixed with the polyethylene composition to prepare a polyethylene solution. This polyethylene solution was extruded from a mold at a temperature of 164°C into a sheet, and then the extrudate was cooled at 16°C in a water bath, and a water flow was set on the surface 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, while preparing a base tape body. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decahydronaphthalene from the base tape, the base tape was then transported while applying a pressure of 0.05 MPa to a roller heated to 30°C to remove some of the fluidized wax from the base tape. The base tape was then stretched at a temperature of 115°C at a ratio of 5.8 times in the length direction (longitudinal stretching), then stretched at a temperature of 105°C at a ratio of 13 times in the width direction (transverse stretching), and then heat-treated at 140°C (heat fixing). Next, the heat-treated base tape was immersed in a dichloromethane bath divided into two tanks for 30 seconds each to extract the fluidized wax. In addition, the purity of the cleaning solvent when the side where the immersion starts is the first tank and the side where the immersion is completed is the second tank (low) first tank < second tank (high). After that, the dichloromethane is removed by drying at 40°C, and the substrate for liquid filter composed of polyolefin microporous membrane is obtained while being transported on a roll heated to 125°C.

<Comparative Example 22> A polyethylene composition was prepared by mixing 4.6 parts by weight of ultra-high molecular weight polyethylene with a weight average molecular weight of 5.1 million and a Ca content of 140 ppb with 18.4 parts by weight of high density polyethylene with a weight average molecular weight of 650,000 and a Ca content of 270 ppb. A mixed solvent of 74.5 parts by weight of liquid wax and 2.5 parts by weight of decahydronaphthalene, which had been prepared in advance and had a concentration of 23% by weight of the total amount of the polyethylene resin, was mixed with the polyethylene composition to prepare a polyethylene solution. The polyethylene solution was extruded from a mold at 164°C into a sheet, and then the extrudate was cooled in a water bath at 16°C, and a water flow was set on the surface 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, thereby producing a base tape. The base tape was dried at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decahydronaphthalene from the base tape, and then the roll heated to 30°C was conveyed while applying a pressure of 0.05 MPa to remove part of the flowing wax from the base tape. Afterwards, the base tape was stretched at a temperature of 115°C at a ratio of 5.8 times in the length direction (longitudinal stretching), then stretched at a temperature of 105°C at a ratio of 13 times in the width direction (transverse stretching), and then heat-treated at 128°C (heat fixing). Next, the base tape that had been heat-treated was immersed in a dichloromethane bath divided into two tanks for 30 seconds each, while the liquid wax was extracted. In addition, the purity of the cleaning solvent when the side where the immersion started was the first tank and the side where the immersion was completed was the second tank (low) first tank < second tank (high). Afterwards, the dichloromethane was removed by drying at 40°C, and the substrate was heat-treated while being transported on a roll heated to 120°C, thereby obtaining a liquid filter substrate composed of a polyolefin microporous membrane.

<Comparative Example 23> A polyethylene composition was prepared 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 with 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. A mixed solvent of 74.5 parts by mass of liquid wax and 2.5 parts by mass of decahydronaphthalene, which had been prepared in advance and had a concentration of 23% by mass of the total amount of the polyethylene resin, was mixed with the polyethylene composition to prepare a polyethylene solution. The polyethylene solution was extruded from a mold at 164°C into a sheet, and then the extrudate was cooled in a water bath at 16°C, and a water flow was set on the surface 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, thereby producing a base tape. The base tape was dried at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decahydronaphthalene from the base tape, and then the roll heated to 30°C was conveyed while applying a pressure of 0.05 MPa to remove part of the flowing wax from the base tape. Afterwards, the base tape was stretched at a temperature of 115°C at a ratio of 7 times in the length direction (longitudinal stretching), then stretched at a temperature of 105°C at a ratio of 11 times in the width direction (transverse stretching), and then heat treated at 125°C (heat fixing). Next, the base tape that had been heat treated was immersed in a dichloromethane bath divided into two tanks for 30 seconds each, while the liquid wax was extracted. In addition, the purity of the cleaning solvent when the side where the immersion started was the first tank and the side where the immersion was completed was the second tank (low) first tank < second tank (high). Afterwards, the dichloromethane was removed by drying at 40°C, and the substrate was heat-treated while being transported on a roll heated to 120°C, thereby obtaining a liquid filter substrate composed of a polyolefin microporous membrane.

<Comparative Example 24> A polyethylene composition was prepared by mixing 10.0 parts by mass of ultra-high molecular weight polyethylene having a weight average molecular weight of 5.1 million and a Ca content of 140 ppb with 10.0 parts by mass of high density polyethylene having a weight average molecular weight of 650,000 and a Ca content of 270 ppb. A mixed solvent of 77.3 parts by mass of liquid wax and 2.7 parts by mass of decahydronaphthalene, which had been prepared in advance to have a concentration of 20% by mass of the total amount of the polyethylene resin, was mixed with the polyethylene composition to prepare a polyethylene solution. The polyethylene solution was extruded from a mold at 156°C into a sheet, and then the extrudate was cooled in a water bath at 16°C, while a water flow was set on the surface 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, thereby producing a base tape. The base tape was dried at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decahydronaphthalene from the base tape, and then the roll heated to 30°C was conveyed while applying a pressure of 0.05 MPa to remove part of the fluidized wax from the base tape. Afterwards, the base tape was stretched at a temperature of 115°C at a ratio of 7 times in the length direction (longitudinal stretching), then stretched at a temperature of 105°C at a ratio of 13 times in the width direction (transverse stretching), and then heat-treated at 135°C (heat fixing). Next, the base tape that had been heat-treated was immersed in a dichloromethane bath divided into two tanks for 30 seconds each, while the liquid wax was extracted. In addition, the purity of the cleaning solvent when the side where the immersion started was the first tank and the side where the immersion was completed was the second tank (low) first tank < second tank (high). Afterwards, the dichloromethane was removed by drying at 40°C, and the substrate was heat-treated while being transported on a roll heated to 120°C, thereby obtaining a liquid filter substrate composed of a polyolefin microporous membrane.

<Comparative Example 25> A polyethylene composition was prepared by mixing 24 parts by weight of ultra-high molecular weight polyethylene with a weight average molecular weight of 5.1 million and a Ca content of 140 ppb with 6 parts by weight of high-density polyethylene with a weight average molecular weight of 650,000 and a Ca content of 270 ppb. A mixed solvent of 67.7 parts by weight of liquid paraffin and 2.3 parts by weight of decahydronaphthalene, which had been prepared in advance at a concentration of 30% by weight of the total amount of the polyethylene resin, was mixed with the polyethylene composition to prepare a polyethylene solution. This polyethylene solution was extruded from a mold at a temperature of 158°C into a sheet, and then the extrudate was cooled at 16°C in a water bath, and a water flow was set on the surface 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, while preparing a base tape body. The base tape was dried at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decahydronaphthalene from the base tape, and then the tape was transported while applying a pressure of 0.05 MPa to a roller heated to 30°C to remove some of the fluidized wax from the base tape. The base tape was then stretched at a temperature of 115°C at a magnification of 7 times in the length direction (longitudinal stretching), and then stretched at a temperature of 105°C at a magnification of 9 times in the width direction (transverse stretching), and then heat-treated at 130°C (heat fixing). Next, the heat-treated base tape was immersed in a dichloromethane bath divided into two tanks for 30 seconds each to extract the fluidized wax. In addition, the purity of the cleaning solvent when the side where the immersion starts is the first tank and the side where the immersion is completed is the second tank (low) the first tank < the second tank (high). After that, the dichloromethane is removed by drying at 40°C, and the substrate for liquid filter composed of polyolefin microporous membrane is obtained while being transported on a roll heated to 105°C.

<Comparative Example 26> A polyethylene composition was 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 with 15 parts by mass of high density polyethylene with a weight average molecular weight of 560,000 and a Ca content of 36 ppm. A mixed solvent of 67.5 parts by mass of liquid wax and 2.5 parts by mass of decahydronaphthalene, which had been prepared in advance at a concentration of 30% by mass of the total amount of the polyethylene resin, was mixed with the polyethylene composition to prepare a polyethylene solution. This polyethylene solution was extruded from a mold at a temperature of 155°C into a sheet, and then the extrudate was cooled at 20°C in a water bath, and a water flow was set on the surface 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, while preparing a base tape body. The base tape was dried at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decahydronaphthalene from the base tape, and then the tape was transported while applying a pressure of 0.05 MPa to a roller heated to 90°C to remove some of the fluidized wax from the base tape. The base tape was then stretched at a temperature of 90°C at a ratio of 5.9 times in the length direction (longitudinal stretching), and then stretched at a temperature of 105°C at a ratio of 13 times in the width direction (transverse stretching), and then heat-treated at 130°C (heat fixing). Next, the heat-treated base tape was immersed in a dichloromethane bath divided into two tanks for 30 seconds each to extract the fluidized wax. In addition, the purity of the cleaning solvent when the side where the immersion starts is the first tank and the side where the immersion is completed is the second tank (low) the first tank < the second tank (high). After that, the dichloromethane is removed by drying at 40°C, and the substrate for liquid filter composed of polyolefin microporous membrane is obtained while being transported on a roll heated to 120°C.

<Comparative Example 27> A polyethylene composition was prepared by mixing 18 parts by weight of ultra-high molecular weight polyethylene with a weight average molecular weight of 4.6 million and a Ca content of 30 ppm with 5 parts by weight of high-density polyethylene with a weight average molecular weight of 560,000 and a Ca content of 36 ppm. A mixed solvent of 75.9 parts by weight of liquid wax and 1.1 parts by weight of decahydronaphthalene, which had been prepared in advance at a concentration of 23% by weight of the total amount of the polyethylene resin, was mixed with the polyethylene composition to prepare a polyethylene solution. This polyethylene solution was extruded from a mold at a temperature of 158°C into a sheet, and then the extrudate was cooled in a water bath at 18°C. At the same time, a water flow was set on the surface 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, while preparing a base tape body. The base tape was dried at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decahydronaphthalene from the base tape, and then the tape was transported while applying a pressure of 0.05 MPa to a roller heated to 90°C to remove some of the fluidized wax from the base tape. Thereafter, the base tape was stretched at a temperature of 90°C at a ratio of 7.0 times in the length direction (longitudinal stretching), and then stretched at a temperature of 105°C at a ratio of 13 times in the width direction (transverse stretching), and then heat-treated at 130°C (heat fixing). Next, the heat-treated base tape was immersed in a dichloromethane bath divided into two tanks for 30 seconds each to extract the fluidized wax. In addition, the purity of the cleaning solvent when the side where the immersion starts is the first tank and the side where the immersion is completed is the second tank (low) the first tank < the second tank (high). After that, the dichloromethane is removed by drying at 40°C, and the substrate for liquid filter composed of polyolefin microporous membrane is obtained while being transported on a roll heated to 100°C.

<Comparative Example 28> A polyethylene composition was prepared by mixing 18 parts by weight of ultra-high molecular weight polyethylene with a weight average molecular weight of 4.6 million and a Ca content of 30 ppm with 5 parts by weight of high-density polyethylene with a weight average molecular weight of 560,000 and a Ca content of 36 ppm. A mixed solvent of 75.9 parts by weight of liquid wax and 1.1 parts by weight of decahydronaphthalene, which had been prepared in advance at a concentration of 23% by weight of the total amount of the polyethylene resin, was mixed with the polyethylene composition to prepare a polyethylene solution. This polyethylene solution was extruded from a mold at a temperature of 156°C into a sheet, and then the extrudate was cooled at 18°C in a water bath, and a water flow was set on the surface 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, while preparing a base tape body. After drying the base tape at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decahydronaphthalene from the base tape, the base tape was then transported while applying a pressure of 0.05 MPa to a roller heated to 110°C to remove some of the fluidized wax from the base tape. The base tape was then stretched at a temperature of 105°C at a ratio of 6.5 times in the length direction (longitudinal stretching), then stretched at a temperature of 115°C at a ratio of 13 times in the width direction (transverse stretching), and then heat-treated at 130°C (heat fixing). Next, the heat-treated base tape was immersed in a dichloromethane bath divided into two tanks for 30 seconds each to extract the fluidized wax. In addition, the purity of the cleaning solvent when the side where the immersion starts is the first tank and the side where the immersion is completed is the second tank (low) the first tank < the second tank (high). After that, the dichloromethane is removed by drying at 40°C, and the substrate for liquid filter composed of polyolefin microporous membrane is obtained while being transported on a roll heated to 100°C.

<Comparative Example 29> A polyethylene composition was 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 with 15 parts by mass of high density polyethylene with a weight average molecular weight of 560,000 and a Ca content of 36 ppm. A mixed solvent of 67.5 parts by mass of liquid wax and 2.5 parts by mass of decahydronaphthalene, which had been prepared in advance at a concentration of 30% by mass of the total amount of the polyethylene resin, was mixed with the polyethylene composition to prepare a polyethylene solution. This polyethylene solution was extruded from a mold at a temperature of 164°C into a sheet, and then the extrudate was cooled at 16°C in a water bath, and a water flow was set on the surface 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, while preparing a base tape body. The base tape was dried at 60°C for 7 minutes and then at 95°C for 7 minutes to remove decahydronaphthalene from the base tape, and then the tape was transported while applying a pressure of 0.05 MPa to a roller heated to 30°C to remove some of the fluidized wax from the base tape. The base tape was then stretched at a temperature of 90°C at a ratio of 5.9 times in the length direction (longitudinal stretching), and then stretched at a temperature of 105°C at a ratio of 13 times in the width direction (transverse stretching), and then heat-treated at 120°C (heat fixing). Next, the heat-treated base tape was immersed in a dichloromethane bath divided into two tanks for 30 seconds each to extract the fluidized wax. In addition, the purity of the cleaning solvent when the side where the immersion starts is the first tank and the side where the immersion is completed is the second tank (low) the first tank < the second tank (high). After that, the dichloromethane is removed by drying at 40°C, and the substrate for liquid filter composed of polyolefin microporous membrane is obtained while being transported on a roll heated to 120°C.

From the evaluation results recorded in Tables 1 and 2, the following facts can be known. It can be seen that the liquid filter substrate composed of the polyolefin microporous membrane of Examples 1 to 10, whose coefficient of variation of water flow is less than 0.100, has a better unevenness of filtration time than the liquid filter substrate composed of the polyolefin microporous membrane of Comparative Examples 1 to 29, whose coefficient of variation of water flow exceeds 0.100. It can be seen that the liquid filter substrate composed of the polyolefin microporous membrane of Examples 1 to 5, in which the Ca content contained in the polyethylene used as the raw material is less than 1 ppm, can suppress the Ca elution amount more than the liquid filter substrate composed of the polyolefin microporous membrane of Examples 6 to 10, in which the Ca content is more than 1 ppm. It can be seen that the liquid filter substrate composed of the polyolefin microporous membrane of Examples 1 to 5 and 8 to 10, in which the average pore size is less than 20 nm, has a better collection performance than the liquid filter substrate composed of the polyolefin microporous membrane of Examples 6 to 7, in which the average pore size exceeds 20 nm.

The disclosure of Japanese Patent Application No. 2022-120889 filed on July 28, 2022 is incorporated herein by reference in its entirety. All documents, patent applications, and technical specifications described in this specification are incorporated by reference to the same extent as when they were specifically and individually described.

Claims (6)

A liquid filter substrate comprising a polyolefin microporous membrane, wherein the average pore size of the polyolefin microporous membrane is 1 nm to 50 nm, the average water flow rate of the polyolefin microporous membrane 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. The liquid filter substrate of claim 1, wherein the coefficient of variation of the pore size of the polyolefin microporous membrane is less than 0.100. A liquid filter substrate as claimed in claim 1, wherein the coefficient of variation of the film thickness of the polyolefin microporous film is less than 0.100. A liquid filter substrate as claimed in claim 1, wherein the coefficient of variation of the porosity of the polyolefin microporous membrane is less than 0.100. In the liquid filter substrate of claim 1, the average thickness of the polyolefin microporous membrane is 3 μm to 20 μm. As for the liquid filter substrate of claim 1, the average porosity of the polyolefin microporous membrane is 35% to 60%.