JP2020164861A - Polyolefin microporous membrane, battery separator, secondary battery and method for producing polyolefin microporous membrane - Google Patents

Abstract

To provide a polyolefin microporous membrane having excellent output characteristics and strength than conventional one.SOLUTION: There is provided a polyolefin microporous membrane, in which the a puncture strength Y (N) in terms of a film thickness of 10 μm and a surface aperture ratio W (%) satisfy the relationship Y(N)≥-0.11×W(%)+6.2, the surface aperture ratio W is 5% or more and 45% or less and a pore ratio is 40% or more and less than 60%.SELECTED DRAWING: None

Description

The present invention relates to a separation membrane used for separating substances, selective permeation, etc., and a polyolefin microporous membrane widely used as a separating material for an electrochemical reaction device such as an alkaline battery, a lithium secondary battery, a fuel cell, and a capacitor. .. In particular, the present invention is a polyolefin microporous membrane preferably used as a separator for a lithium ion battery, which exhibits superior battery characteristics as compared with a conventional polyolefin microporous membrane, and is used as a separator having high safety.

The microporous polyolefin membrane is used as a filter, a separator for a fuel cell, a separator for a capacitor, and the like. In particular, it is suitably used as a separator for lithium-ion batteries widely used in notebook personal computers, mobile phones, digital cameras, and the like. The reason for this is that the polyolefin microporous membrane has excellent mechanical strength, shutdown characteristics, and lithium ion permeation performance.

In particular, since lithium-ion secondary batteries have been used in vehicles in recent years, it is necessary to shorten the charging time and improve acceleration, and the characteristics of batteries include rapid charging (large current charging) and increased power consumption (large current discharge). ) Is required. Along with this, further improvements in output characteristics are required for separators.

Further, as the cruising range of automobiles increases, the capacity of batteries is increasing, and there is a demand for thinner separators. However, when the separator is made thinner, the strength decreases, so short circuits due to electrodes and foreign matter (foreign matter resistance) and film rupture (impact resistance) when the battery is impacted are likely to occur, and the safety of the battery is reduced. .. Therefore, higher strength than before is required for the separator.

As a method for improving output characteristics, in Patent Documents 1 and 2, polyethylene (PE) and polypropylene (PP) are blended to increase the pore ratio, control the number of pores, the pore diameter, and the curvature ratio, and improve ionic conductivity. ing.

As a method for improving the output characteristics, Patent Documents 3 and 4 describe a method for improving ionic conductivity by controlling the fibril diameter, pore diameter, and surface aperture ratio of the microporous membrane and reducing the membrane resistance. ing.

In Patent Document 5, the pore structure on the surface and the inside is controlled, the impregnation property of the electrolytic solution is improved, and the output characteristics are improved.

As a method for increasing the strength, a method of increasing the draw ratio, increasing the resin concentration, or using a high molecular weight raw material is usually taken. In Patent Document 6, in order to improve film rupture resistance, polyethylene having a molecular weight of 280,000 and polyethylene having a molecular weight of 2 million are blended, and under the condition of a resin concentration of 32 wt%, 7.0 times in the MD direction and 6. in the TD direction. A method of stretching 4 times, washing and drying, and then stretching 1.2 times (area magnification 54 times) is described.

Patent Documents 7 and 8 describe a method of stretching Hi-Zex Million 145M to 6.4 × 6.0 times (area magnification 38.4 times) under the condition of a resin concentration of 18%. Since polyethylene having a relatively high molecular weight is stretched 38.4 times, a microporous film having a pore ratio of 30 to 60% and a puncture strength of 6.0 N / 20 μm can be obtained with excellent output characteristics and safety. ing.

Japanese Patent Application Laid-Open No. 2017-140840 Japanese Patent Application Laid-Open No. 2003-231772 Japanese Patent Application Laid-Open No. 2011-192529 Japanese Patent Application Laid-Open No. 2003-86162 Japanese Patent Application Laid-Open No. 2012-48987 Japanese Patent Application Laid-Open No. 2006-124652 Japanese Patent Application Laid-Open No. 2018-147858 Japanese Patent Application Laid-Open No. 2018-147690

However, in the conventional microporous membranes as described in Patent Documents 1 and 2, the porosity is increased, and the number of pores and the through holes are increased in order to improve the output characteristics. As a result, good strength is not obtained.

The microporous membranes described in Patent Documents 3 and 4 are microporous membranes having a large pore diameter, a high porosity, and relatively thick fibrils, and good strength has not been obtained.

In Patent Document 5, it is considered that the permeation path of lithium ions tends to be non-uniform because the pore structures on the surface and the inside are different. Therefore, the conventional microporous membrane is insufficient to achieve both high output characteristics and strength required for in-vehicle applications.

The microporous membrane obtained in Patent Document 6 has a relatively high puncture strength of 5.5 to 5.9 N / 20 μm and good film rupture resistance, but has a porosity of 35 to 40%. It is a relatively low value, and the porosity ratio close to that of the high output separator as shown in Patent Documents 1 and 2 has not been obtained, and there is room for improvement in the output characteristics.

In Patent Documents 7 and 8, since the separator is required to be thinned as the capacity of the battery is increased, there is room for improvement in safety improvement due to the thinning.

In view of the above circumstances, it is an object of the present invention to provide a polyolefin microporous membrane having better output characteristics and strength than before.

As a result of diligent studies to solve the above-mentioned problems, the present inventors have made the present invention fine and uniform pores by improving the pore-opening property in the drawing step, and the conventional polyolefin microporous film. By increasing the number of holes per unit volume, the ion permeation path is increased and the output characteristics are improved, and the resin layer is uniformly stressed to form fibrils, so that the strength and heat shrinkage are improved. We have found that it is possible to achieve high safety and output characteristics that could not be achieved by technology.

In addition, as a result of diligent studies to solve the above problems, the present inventors, in the present invention, improve the pore-opening property in the drawing step to make the pores fine and uniform, and the conventional polyolefin. By increasing the number of surface pores per unit area compared to the microporous membrane, the ion permeation path is increased and the output characteristics are improved, and the resin layer is uniformly stressed and fibrillated, resulting in increased strength and heat shrinkage. It has been improved and it has been found that high safety and output characteristics that could not be achieved by the conventional technology can be realized.

Furthermore, as a result of diligent studies to solve the above problems, the present inventors, in the third embodiment of the present invention, improve the pore opening property in the drawing step to make the pores fine and uniform. Therefore, by increasing the surface aperture ratio as compared with the conventional polyolefin microporous film, the ion permeation path is increased and the output characteristics are improved, and the resin layer is uniformly stressed to form a fine and uniform fibril structure. It was found that the strength and heat shrinkage were improved, and high safety and output characteristics that could not be achieved by the conventional technology can be realized.

That is, the present invention adopts the following configuration.
<1> The piercing strength Y (N) converted to a film thickness of 10 μm and the surface aperture ratio W (%) satisfy the relationship of the following formula (1), and both the tensile breaking strength in the MD direction and the tensile breaking strength in the TD direction are both. A microporous polyolefin membrane of 180 MPa or more.
Y (N) ≧ −0.11 × W (%) +6.2 ... Equation (1)
<2> The polyolefin microporous film according to <1>, wherein the surface aperture ratio W is 5% or more and 45% or less.
<3> The polyolefin microporous membrane according to <1> or <2>, which has an average pore diameter of less than 40 nm.
<4> The polyolefin microporous membrane according to any one of <1> to <3>, which has a toughness of 35,000 (MPa ×%) or more.
<5> The microporous polyolefin membrane according to any one of <1> to <4>, wherein the piercing strength Y is 3.0 N or more.
<6> The polyolefin microporous membrane according to any one of <1> to <5>, wherein the heat shrinkage rate at 120 ° C. in the MD direction measured by thermomechanical analysis is 15% or less.
<7> The polyolefin microporous membrane according to any one of <1> to <6>, wherein the porosity is 40% or more and less than 60%.

<8> The polyolefin microporous film according to any one of <1> to <7>, wherein the polyolefin that is the microporous film is polyethylene.
<9> A battery separator comprising the polyolefin microporous membrane according to any one of <1> to <8>.
<10> A secondary battery including the battery separator according to <9>.
<11> The method for producing a microporous polyolefin membrane according to any one of <1> to <8>.
When the total of the solvent and the polyolefin resin is 100% by mass, the content ratio of the ultra-high molecular weight polyethylene having a weight average molecular weight of 1 million or more is 5 to 30% by mass, and the content ratio of the polyolefin resin is less than 30% by mass. The process of preparing a solution,
A step of forming an unstretched gel-like sheet by extruding the solution from a die and cooling and solidifying the solution.
A step of stretching the gel-like sheet at a temperature from the crystal dispersion temperature of the gel-like sheet to the melting point of the gel-like sheet + 10 ° C. so that the area magnification is 40 times or more to obtain a stretched film.
It has a step of extracting a plasticizer from the stretched film and drying the stretched film, and a step of performing at least one of heat treatment of the stretched film after drying and re-stretching of the stretched film after drying.
A method for producing a microporous polyolefin membrane.

The present invention can provide a polyolefin microporous membrane having excellent safety, output characteristics, and strength as compared with a conventional polyolefin microporous membrane.

Hereinafter, embodiments of the present invention will be described in detail, but these show examples of desirable embodiments, and the present invention is not specified in these contents. The polyolefin microporous membrane of the present invention (hereinafter, may be simply referred to as "microporous membrane".

In the polyolefin microporous film of the present invention, the puncture strength Y (N) converted to a film thickness of 10 μm and the surface aperture ratio W (%) satisfy the relationship of the following formula (1), and the surface aperture ratio W is 5% or more 45. % Or less.
Y (N) ≧ −0.11 × W (%) +6.2 ... Equation (1)

Hereinafter, the present invention will be described in more detail.
[1] Polyolefin Resin The raw material such as the resin in the microporous polyolefin membrane of the present invention does not have to have a single composition, and may be a composition in which a main raw material and an auxiliary raw material are combined.
The resin is preferably a polyolefin resin, and may be a polyolefin resin mixture (polyolefin resin composition) composed of two or more kinds of polyolefin resins.

The raw material form of the polyolefin microporous membrane is preferably a polyolefin resin, and examples of the polyolefin resin include polyethylene and polypropylene, and a single composition is more preferable.

The polyolefin resin is preferably a homopolymer such as ethylene, propylene, 1-butene, 4-methyl-1-pentene or 1-hexene, and a homopolymer of ethylene (polyethylene) is particularly preferable. Polyethylene may be a copolymer containing a homopolymer of ethylene and another α-olefin.

Other α-olefins include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, or alkene having a carbon number of more than that, vinyl acetate, methyl methacrylate, styrene, and the like. Can be mentioned.

As the polyolefin resin mixture, for example, a mixture of two or more types of ultra-high molecular weight polyethylene having different weight average molecular weights (Mw), a mixture of high density polyethylene, a mixture of medium density polyethylene, or a mixture of low density polyethylene may be used. Alternatively, a mixture of two or more polyethylenes selected from the group consisting of ultra-high molecular weight polyethylene, high density polyethylene, medium density polyethylene and low density polyethylene may be used. As the polyolefin resin mixture, Mw may be a mixture of 1 × 10 ⁶ or more ultra-high molecular weight polyethylene and Mw consists 1 × 10 ⁵ or more 9 × 10 ⁵ less than polyethylene.

The weight average molecular weight of the ultrahigh-molecular-weight polyethylene used, 1.0 × 10 ⁶ or more, preferably 1.0 × 10 ⁷ or less. The weight average molecular weight is more preferably 1.1 × 10 ⁶ or more, more preferably 1.2 × 10 ⁶ or more, particularly preferably 1.4 × 10 ⁶ or more, more preferably 1.7 × 10 ⁶ or more, most preferably 2.0 × 10 ⁶ or more. The weight average molecular weight is more preferably 8.0 × 10 ⁶ or less, more preferably 6.0 × 10 ⁶ or less, more preferably 5.0 × 10 ⁶ or less, and most preferably 4.0 × 10 ⁶ or less Is. By weight average molecular weight of 1.0 × 10 ⁶ or more, entanglement density is increased in the amorphous region, good output characteristics and strength improved uniformity of openings is obtained.

The molecular weight distribution (weight average molecular weight (Mw) / number average molecular weight (Mn)) of the ultra-high molecular weight polyethylene is preferably in the range of 3.0 to 100 from the viewpoint of mechanical strength. The lower limit of the molecular weight distribution is more preferably 4.0 or more, still more preferably 5.0 or more, more preferably 6.0 or more, and most preferably 8.0 or more. The upper limit of the molecular weight distribution is more preferably 80 or less, further preferably 50 or less, still more preferably 20 or less, and most preferably 17 or less.

Ultra-high molecular weight polyethylene may be used as the above-mentioned polyolefin resin mixture, but when used alone, the workability is inferior when the molecular weight distribution is less than 3.0, and the low molecular weight component increases when the molecular weight distribution exceeds 100. Therefore, there is a high possibility that defects and the like are likely to occur during processing.

Besides the ultra-high-molecular-weight polyethylene, for example, 1.0 × 10 ⁵ or more 1.0 × 10 than ^6, preferably such a range of ^{2 × 10 5 ~9.5 × 10 5} , the weight-average of less than 1.0 × 10 ⁶ Polyethylene having a molecular weight may be used as an auxiliary material. By using the auxiliary raw material, the molding processability is improved, the mixing ratio of the polyolefin resin and the plasticizer can be easily selected, and the overlapping density of the molecular chains can be adjusted.

The most preferred form of the polyolefin resin has a weight average molecular weight (Mw) of preferably 1.0 × 10 ⁶ or more polyolefin resins, Mw is more preferably 1.4 × 10 ⁶ or more polyolefin resins, Mw of 2.0 × 10 ⁶ or more polyolefin resins is more preferred. If the Mw of the polyolefin resin is 1.0 × 10 ⁶ or more to increase the number of entanglement of molecular chains, openability and uniform stress propagation in the stretching step can be improved. As a result, the pores are finely and uniformly opened, and at least one of the number of pores per unit volume, the surface aperture ratio, and the number of surface pores per unit area of the polyolefin microporous membrane is increased, and good output characteristics are obtained. Further, Mw is increased tie molecule number by using 1.0 × 10 ⁶ or more polyolefin resins, to increase the strength of the fibrils, improve the balance of the number of pores and a surface pore number and intensity per unit volume to be described later it can. Mw is hard to stress propagation in the stretching step is less than 1.0 × 10 ^6, there is a case where the opening is lowered.

Here, the type of polyolefin resin, high density polyethylene such as density exceeding 0.94 g / cm ^3, density polyethylene in the range density of 0.93~0.94g / cm ^3, density of 0.93g / lower cm ³ low-density polyethylene, include linear low density polyethylene, high density polyethylene having a weight-average molecular weight of 1,000,000 or more ultra-high molecular weight in terms of uniformity and film strength of the opening alone It is preferable to use it.

In the process for producing the microporous polyolefin membrane of the present invention, which will be described later, it is preferable to add a plasticizer for the purpose of improving molding processability.

The blending ratio of the polyolefin resin and the plasticizer may be appropriately selected as long as the molding processability is not impaired, but the total of the polyolefin resin and the plasticizer is 100% by mass, and the proportion of the polyolefin resin is 10 to 50% by mass. It is preferable to have. When the proportion of polyolefin resin is 10% by mass or more (the proportion of plasticizer is 90% by mass or less), swell and neck-in can be suppressed at the outlet of the base when molding into a sheet, and the formability and film forming property of the sheet can be suppressed. Is improved. On the other hand, when the proportion of the polyolefin resin is less than 50% by mass (the proportion of the plasticizer exceeds 50% by mass), the pressure increase in the film forming process can be suppressed and good molding processability can be obtained.

The proportion of the polyolefin resin is preferably 12% by mass or more, more preferably 14% by mass or more, further preferably 16% by mass or more, still more preferably 20% by mass or more. By increasing the proportion of the polyolefin resin, the overlapping density of the molecular chains in the presence of the plasticizer increases, and the uniformity of stretching is improved. As a result, a fine and uniform fibril structure is formed, and it is possible to increase the strength as the number of fibrils increases and increase the number of holes as the uniformity of the opening increases, and the safety and output characteristics, which have been in a trade-off relationship in the past, can be improved. Both are possible. In addition, good molding processability can be obtained. Further, by increasing the proportion of the polyolefin resin, an increase in the draw ratio and "crushing of pores" in the washing and drying step are suppressed, and a good porosity can be obtained.

Since the viscosity differs depending on the raw material used, the ratio used differs depending on the raw material used, but when a polyolefin resin with an Mw of 1 million or more is used, the ratio of the polyolefin resin is different from that of the polyolefin resin from the viewpoint of pressure and stretching stress in the film forming process. When the total with the agent is 100% by mass, 40% by mass or less is preferable, 30% by mass or less is more preferable, and 25% by mass or less is further preferable.

In addition, various additions of antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, blocking inhibitors, fillers, etc. are added to the microporous polyolefin membrane of the present invention as long as the effects of the present invention are not impaired. The agent may be contained. In particular, it is preferable to add an antioxidant for the purpose of suppressing oxidative deterioration due to the thermal history of the polyolefin resin.

Examples of the antioxidant include 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4), 1,3,5-trimethyl-2,4,6-tris (3,5-tris). Di-t-butyl-4-hydroxybenzyl) benzene (eg, BASF "Irganox"® 1330: molecular weight 775.2), tetrakis [methylene-3 (3,5-di-t-butyl-4) -Hydroxyphenyl) propionate] It is preferable to use one or more selected from methane (for example, "Irganox" (registered trademark) 1010: molecular weight 1177.7 manufactured by BASF).

Appropriate selection of the type and amount of antioxidant and heat stabilizer added is important for adjusting or enhancing the characteristics of the polyolefin microporous membrane.

The layer structure of the microporous polyolefin membrane of the present invention may be a single layer or a laminated layer, and the laminated layer is preferable from the viewpoint of physical property balance. The raw material, the raw material ratio, and the raw material composition may be within the above ranges. When the above raw material formulations are laminated and used as a high output characteristic layer, it is preferable that the high output characteristic layer contains 50% by mass or more in the total film thickness.

[2] Polyolefin microporous membrane

In the polyolefin microporous film of the present invention, W and Y when the surface aperture ratio is W (%) and the puncture strength in terms of film thickness of 10 μm is Y (N) satisfy the relationship of the following formula (1). The surface aperture ratio W is 5% or more and 45% or less.
Y (N) ≧ −0.11 × W (%) +6.2 ... Equation (1)

When a microporous membrane is used as a separator for a lithium ion secondary battery, if the membrane resistance of the separator is high during the charging / discharging process, the movement of Li ⁺ ions is restricted and a non-uniform reaction occurs. As a result, the holes are clogged and the output characteristics are deteriorated. Therefore, in order to reduce the film resistance of the separator, it is preferable to increase the aperture ratio (surface aperture ratio) of the separator surface.

The present inventors have derived the relationship of the above equation (1) as follows.
First, the puncture strength (N) in terms of film thickness of 10 μm was defined as the y-axis, and the surface aperture ratio (%) was defined as the x-axis, and the examples and comparative examples described later were plotted. Then, they found that the existence range is different between the plot of the polyolefin microporous membrane in which the output characteristics and the safety are compatible and the plot of the polyolefin microporous membrane in which the output characteristics and the safety are not compatible.

Then, he found that the boundary line dividing the existence range was a straight line, and when he examined the straight line, he found that it was important that the relationship of the above equation (1) was established.

Usually, the surface aperture ratio per unit volume has a correlation with the pore ratio, and in the prior art, a method of lowering the resin concentration and increasing the pore ratio is adopted. However, an increase in the porosity rate leads to a decrease in the amount of resin per unit volume, so that the strength decreases. Further, as a method for increasing the strength, a method of increasing the resin concentration, increasing the draw ratio, or using a high molecular weight raw material is usually taken. However, as described above, an increase in the resin concentration leads to a decrease in the pore ratio, and a good number of pores cannot be obtained. In addition, the increase in the draw ratio is due to the fact that as a result of the film being highly oriented, tearing occurs frequently in the film forming process and productivity is reduced, and stretching non-uniformity occurs due to the influence of Boeing and the like, resulting in impaired film uniformity. It is difficult to obtain a high surface aperture ratio because good openness cannot be obtained. In the method using a high molecular weight raw material, it is difficult to obtain discharge stability because the viscosity generally increases as the molecular weight increases. Therefore, a method of dissolving the resin in a solvent to reduce the resin concentration to form a film or a method of mixing with a raw material having a low molecular weight to form a film is adopted. However, it is difficult to obtain a gel sheet having a uniform crystal structure by these methods, and it becomes remarkable that stretching is performed in a non-uniform state. Good openness cannot be obtained and it is difficult to obtain a high surface aperture ratio. On the other hand, in the polyolefin microporous membrane of the present invention, the molecular weight and resin concentration of the raw material are within the above ranges, the number of entangled molecular chains is increased, and stress is uniformly applied to the resin layer to stretch the resin layer, thereby performing a stretching step. The stress propagation in the above is uniform, the openness is improved, and a uniform and fine pore structure is formed. As a result, the aperture ratio measured by SEM observation increases and the number of fibrils per unit area increases, so that both good ion permeability and strength can be achieved, and excellent battery safety and output characteristics can be achieved. Found to be obtained.

The aperture ratio W of the microporous membrane is measured from SEM (scanning electron microscope) observation by a method described later, and the surface aperture ratio is preferably 5% or more, more preferably 8% or more, and further 10% or more. Preferably, 12% or more is even more preferable. Further, if the surface aperture ratio is too high, dendrites are likely to be generated and the safety is lowered. Therefore, the surface aperture ratio is preferably 45% or less, more preferably 40% or less, further preferably 38% or less, and more preferably 35% or less. More preferred.

Further, in order to improve the output characteristics of the battery, it is necessary to increase the porosity of the polyolefin microporous film. However, when the porosity is increased, the amount of resin in the microporous polyolefin membrane is reduced, so that the strength of the membrane is lowered and the safety of the battery is lowered. Therefore, there is a trade-off relationship between the output characteristics of the battery and the safety, and there is a problem in achieving both the output characteristics and the safety.

Examples of the method for increasing the porosity include a method for increasing the proportion of the plasticizer in the kneading process and the sheeting process, and a method for increasing the stretching ratio in the dry re-stretching process. When the proportion of the plasticizer is increased (the resin concentration is lowered), the plasticizer layer in the sheet is large and the polyolefin resin layer is not subjected to stretching stress, and the uniformity of the opening is lowered.

Further, as described above, the amount of resin decreases due to the increase in the pore ratio, and the stiffness of the film decreases, so that the draw ratio increases and "crushing of pores" occurs in the washing and drying step. When the draw ratio increases in the dry re-stretching step, voids are likely to be formed in the polyolefin microporous film, and it becomes difficult to obtain the uniformity of the film.

The viscosity is increased by using a polyolefin resin having an Mw of 1 million or more. In the conventional formulation, when a polyolefin resin having a Mw of 1 million or more is used, the viscosity is high and it is difficult to uniformly knead the resin. Therefore, the proportion of the polyolefin resin having a Mw of 1 million or more is low and the molecular weight is low. In many cases, polyolefin resin was the main component. Furthermore, since we did not pay attention to uniform openness, the openness was not good, and both output characteristics and safety could not be achieved. In the present invention, focusing on uniform openness, it has been found that when a polyolefin resin having an Mw of 1 million or more is used, good film-forming property can be obtained even at a relatively low resin concentration, and a good porosity can be obtained. It was. This is because a polyolefin resin having an Mw of 1 million or more has a long molecular chain, so that the number of entanglements increases, stress applied to the resin layer is easily propagated in the drawing step, and the openness is improved, and the conventional polyolefin microporous At least one of the number of pores in the microporous polyolefin membrane, the aperture ratio of the surface, and the number of surface pores per unit area is increased as compared with the membrane, and the ion permeation path is increased. Since the viscosity is increased by using a polyolefin resin having an Mw of 1 million or more, good film-forming property can be obtained even at a relatively low resin concentration, and a good porosity can be obtained. Further, since the polyolefin resin having Mw of 1 million or more has a long molecular chain, the number of entanglements increases, the stress applied to the resin layer in the drawing step is easily propagated, and the pore opening property is improved. Therefore, at least one of the number of pores per unit volume, the aperture ratio of the surface, and the number of surface pores per unit area in the polyolefin microporous membrane is increased as compared with the conventional polyolefin microporous membrane, and the ion permeation path is increased. To increase.

Furthermore, we found that the strength in the polyolefin microporous membrane increases because the number of Thai molecules increases as the molecular chain length increases, and it is possible to achieve both output characteristics and safety, which have been in a trade-off relationship so far. I found that there is.

The porosity of the microporous polyolefin membrane of the present invention is preferably 40% or more, more preferably 41% or more, still more preferably 42% or more, from the viewpoint of permeation performance and electrolyte content. When the porosity is 40% or more, the balance between permeability, strength and electric field liquid content is improved, and the non-uniformity of the battery reaction is eliminated. As a result, the generation of dendrites is suppressed and the battery can be used without impairing the performance of the conventional battery, and can be suitably used as a separator for a secondary battery.

Further, by increasing the pore ratio, good output characteristics can be obtained, but the puncture strength is lowered. Therefore, the porosity is preferably less than 60%, more preferably 58% or less, still more preferably 55% or less.

From the viewpoint of preventing film rupture due to the electrode active material or the like, the puncture strength of the polyolefin microporous film in which the film thickness is converted to 10 μm is preferably 3.0 N or more, more preferably 3.2 N or more, still more preferably 3.5 N or more. 3.8N or more is even more preferable, and 4.0N or more is most preferable.

When the puncture strength is 3.0 N or more, a short circuit due to a foreign substance or the like in the battery is suppressed, and good battery safety can be obtained. From the viewpoint of improving safety, the higher the piercing strength is, the more preferable it is, and in order to increase the piercing strength, a method of increasing the resin concentration or increasing the draw ratio is taken.

However, an increase in the resin concentration means a decrease in the porosity, and the output characteristics decrease. Further, when the draw ratio is increased, the polyolefin microporous film is crushed in the film thickness direction, so that "pore crushing" and voids (defects) are formed, so that the uniformity of the battery reaction is lowered and the output characteristics are deteriorated.

In order to set the puncture strength in the above range, it is preferable that the raw material composition of the polyolefin microporous film is in the above range, and the stretching conditions at the time of forming the polyolefin microporous film are in the range described later.

The puncture strength when the film thickness is converted to 10 μm is calculated by the formula: L2 = (L1 × 10) / T1 when the puncture strength is L1 in the polyolefin microporous film having a film thickness of T1 (μm). It refers to the piercing strength L2. In the following, unless otherwise specified, the term "piercing strength" is used to mean "piercing strength when the film thickness is converted to 10 μm". By using the microporous polyolefin membrane of the present invention, it is possible to prevent the occurrence of pinholes and cracks and improve the yield at the time of battery assembly.

In the charging / discharging process, the electrolytic solution is electrochemically decomposed inside the battery to generate solid products such as organic substances and gases such as ethylene and hydrogen. These products clog the pores in the microporous polyolefin membrane, degrading battery characteristics. Further, since the decomposition reaction is likely to proceed due to the bias of the ion permeation path, it is preferable that the number of ion permeation paths is large.

Therefore, pore count number of holes per unit volume obtained in Examples described later is preferably 40 pieces / [mu] m ³ or more, more preferably 50 pieces / [mu] m ³ or more, more preferably 60 pieces / [mu] m ³ or more, 70 / μm ³ or more is more preferable, 80 pieces / μm ³ or more is more preferable, 100 pieces / μm ³ or more is particularly preferable, and 120 pieces / μm ³ or more is most preferable.

The number of pores per unit volume and the number of surface pores correlate with the porosity, and the strength decreases as the number of pores per unit volume and the number of surface pores increase. However, in the polyolefin microporous membrane of the present invention, for example. Mw is by using a 1.0 × 10 ⁶ or more of the polyolefin resin of a large molecular weight, increased number entanglement of molecular chains, has improved stress propagation is uniform openness in the stretching step, conventional polyolefin microporous The number of pores per unit volume and the number of surface pores are increasing while maintaining the same porosity as the film.

Further, the polyolefin microporous membrane of the present invention has excellent piercing strength by using a polyolefin resin having a large molecular weight and increasing the number of tie molecules, and has output characteristics and safety that have been in a trade-off relationship in the past. It is excellent in that it is compatible with.

As described above, the higher the puncture strength is, the more preferable it is from the viewpoint of battery safety. However, since the piercing strength depends on the amount of resin per unit volume, an increase in piercing strength leads to a decrease in the porosity. Therefore, there is a trade-off between piercing strength and output characteristics. By satisfying the relationship of the above equation (1), it is possible to improve the safety and output characteristics, which have been in a trade-off relationship in the past.

In the present invention, the direction parallel to the film-forming direction of the polyolefin microporous film is referred to as the film-forming direction, the longitudinal direction or the MD direction, and the direction orthogonal to the film-forming direction within the polyolefin microporous film surface is the width direction or TD. Called direction.
Since tension is applied to the battery in the MD direction, it is preferable that the shrinkage rate obtained by TMA (thermomechanical analysis) in the MD direction is low. The heat shrinkage rate in the MD direction at 120 ° C. measured by TMA is preferably 15% or less, more preferably 12% or less, further preferably 10% or less, and 8% or less. Is more preferable.

Further, the heat shrinkage rate in the TD direction at 120 ° C. measured by TMA is preferably 15% or less, more preferably 12% or less, further preferably 10% or less, and 7% or less. Is more preferable.

When the heat shrinkage rate is within the above range, it is possible to prevent the expansion of the internal short circuit and minimize the influence even when abnormal heat is generated locally. The heat shrinkage has a trade-off relationship with the aperture ratio, the number of pores per unit volume, the number of surface pores, and the puncture strength. As described above, as a formulation for improving the puncture strength, the stress applied to the polyolefin resin is uniform because it is achieved by focusing on the entanglement of molecular chains and making the stress propagation in the drawing step uniform.
Therefore, strain does not easily remain, a low heat shrinkage rate is achieved, and an excellent heat shrinkage rate and strength balance are obtained.

The tensile breaking strength in the MD direction and the TD direction (hereinafter, also simply referred to as "MD strength" and "TD strength") is preferably 180 MPa or more, more preferably 190 MPa or more, further preferably 200 MPa or more, and more preferably 210 MPa or more. More preferably, 250 MPa or more is more preferable, 260 MPa or more is even more preferable, and 270 MPa or more is most preferable.

If the MD strength and the TD strength are less than 180 MPa, a short circuit due to a foreign substance or the like in the battery is likely to occur, and the safety of the battery may be lowered.

From the viewpoint of improving safety, the higher the tensile breaking strength is, the more preferable it is, but there are many trade-offs between improving the pore ratio, aperture ratio, number of holes per unit volume, surface pores and tensile breaking strength, and MD strength. The TD strength is preferably 350 MPa or less, more preferably 300 MPa or less. In order to set the MD strength and the TD strength in the above range, it is preferable that the raw material composition of the polyolefin microporous film is in the above range, and the stretching conditions at the time of forming the polyolefin microporous film are in the range described later.

Toughness, which is a measure of impact resistance obtained from tensile breaking strength and tensile breaking elongation, is calculated by the following formula using each of tensile breaking strength and tensile breaking elongation in the MD direction and the TD direction.
Toughness (MPa x%) = MD tensile breaking strength x MD tensile breaking elongation + TD tensile breaking strength x TD tensile breaking elongation

The toughness, which is a measure of impact resistance, is preferably 20,000 (MPa ×%) or more, more preferably 25,000 (MPa ×%) or more, still more preferably 30,000 (MPa ×%) or more, most preferably from the viewpoint of impact resistance. Is 35,000 (MPa ×%) or more, particularly preferably 40,000 (MPa ×%) or more. The upper limit of toughness is preferably 500,000 (MPa ×%) or less, more preferably 400,000 (MPa ×%) or less, still more preferably 300,000 (MPa ×%) or less, because other physical properties, for example, ion permeability deteriorate. Most preferably, it is 200,000 (MPa ×%) or less.

The average pore size measured by the poromometer is preferably 10 nm or more from the viewpoint of ion permeability. When the average pore diameter is 10 nm or more, a good aperture ratio, the number of holes per unit volume and the number of surface holes can be obtained, and excellent battery characteristics can be obtained. Further, since the preferable pore ratio is determined from the viewpoint of battery characteristics, fine and uniform pores are effective for increasing the ion transmission path.

As the average pore size increases, the ion permeation path decreases, so the average pore size is preferably less than 40 nm, more preferably less than 35 nm, even more preferably less than 30 nm, even more preferably less than 28 nm, and most preferably less than 25 nm.

The surface pore size obtained from the SEM image taken by the method described in the examples is preferably 20 nm or more and 70 nm or less, more preferably 20 nm or more and 60 nm or less, and further preferably 20 nm or more and 50 nm or less from the viewpoint of puncture strength and ion permeability. is there. When the surface hole diameter is within the above range, the number of holes per unit volume and the number of surface holes can easily be within the above range, and good safety and output characteristics can be obtained.

In the polyolefin microporous membrane of the present invention, the air permeability refers to a value measured in accordance with JIS P 8117 (2009). Unless otherwise specified, the term "air permeability" is used in the present specification to mean "air permeability when the film thickness is 10 μm". When the air permeability measured in the polyolefin microporous membrane having a film thickness of T1 (μm) is P1, the air permeability P2 calculated by the formula: P2 = (P1 × 10) / T1 is set to 10 μm. Let it be the air permeability when it is done.

Preferably the air permeability (Gurley value) is of 1,000 sec / 100 cm ³ or less, more preferably 700sec / 100cm ³ or less, still more preferably 500 sec / 100 cm ³ or less, it is 350sec / 100cm ³ or less Is more preferable. When the air permeability is 1000 sec / 100 cm ³ or less, good ion permeability can be obtained and the electrical resistance can be reduced.

[3] Method for Producing Polyolefin Microporous Membrane Next, the method for producing the polyolefin microporous membrane of the present invention will be specifically described. The production method of the present invention preferably has the following steps (a) to (e).

(A) Step of melt-kneading a polymer material containing one or more kinds of polyolefin resins and, if necessary, a solvent to prepare a polyolefin resin solution (b) Extruding the solution and molding it into a sheet. Step of cooling and solidifying (c) Step of stretching the obtained sheet by a roll method or a tenter method (d) Then, a step of extracting a plasticizer from the obtained stretched film and drying the film (e) Heat treatment / re-stretching Process to do

Hereinafter, each step will be described.
(A) Step of preparing a polyolefin resin solution The above polymer material is heated and dissolved in a plasticizer to prepare a polyolefin resin solution.
The plasticizer is not particularly limited as long as it is a solvent capable of sufficiently dissolving the polyolefin resin, but the solvent is preferably a liquid at room temperature in order to enable stretching at a relatively high magnification.

The content ratio of the ultra-high molecular weight polyethylene having a weight average molecular weight of 1 million or more in the solution is 5 to 30% by mass, preferably 9 to 30% by mass, when the total of the solvent and the polyolefin resin is 100% by mass. 16 to 30% by mass is more preferable, 17 to 30% by mass is more preferable, and 19 to 30% by mass is particularly preferable. The content ratio of the polyolefin resin in the solution is less than 30% by mass, preferably 10 to 29% by mass, more preferably 15 to 29% by mass, when the total of the solvent and the polyolefin resin is 100% by mass. , 17-29% by mass is more preferable.

Solvents include aliphatic, cyclic aliphatic or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, and liquid paraffin, mineral oil distillates having corresponding boiling points, and dibutylphthalate. Examples thereof include phthalates that are liquid at room temperature, such as dioctyl phthalates.

In order to obtain a gel-like sheet having a stable liquid solvent content, it is preferable to use a non-volatile liquid solvent such as liquid paraffin.

In the melt-kneaded state, it is miscible with the polyolefin resin, but at room temperature, a solid solvent may be mixed with the liquid solvent. Examples of such a solid solvent include stearyl alcohol, ceryl alcohol, paraffin wax and the like. However, if only a solid solvent is used, uneven stretching may occur.

The viscosity of the liquid solvent is preferably 20-200 cSt at 40 ° C. When the viscosity at 40 ° C. is 20 cSt or more, the sheet obtained by extruding the polyolefin resin solution from the die is unlikely to become non-uniform. On the other hand, if the viscosity at 40 ° C. is 200 cSt or less, the liquid solvent can be easily removed. The viscosity of the liquid solvent is the viscosity measured at 40 ° C. using an Ubbelohde viscometer.

(B) Formation of Extruded Product and Formation of Gel-like Sheet The uniform melt-kneading method of the polyolefin resin solution is not particularly limited, but when a high-concentration polyolefin resin solution is desired to be prepared, the solution is extruded from a die (for example, The method (using a twin-screw extruder) is preferable. If necessary, various additives such as antioxidants may be added as long as the effects of the present invention are not impaired. In particular, it is preferable to add an antioxidant in order to prevent oxidation of the polyolefin resin.

In the extruder, the polyolefin resin solution is uniformly mixed at a temperature at which the polyolefin resin is completely melted. The melt-kneading temperature varies depending on the polyolefin resin used, but is preferably (melting point of polyolefin resin + 10 ° C.) to (melting point of polyolefin resin + 120 ° C.). More preferably, it is (melting point of polyolefin resin + 20 ° C.) to (melting point of polyolefin resin + 100 ° C.).

Here, the melting point means a value measured by DSC (Differential scanning calorimetry) based on JIS K7121 (1987) (hereinafter, the same applies). For example, in the case of polyethylene, the melt-kneading temperature is preferably in the range of 140 to 250 ° C, more preferably 160 to 230 ° C, and most preferably 170 to 200 ° C. Specifically, since the polyethylene composition has a melting point of about 130 to 140 ° C., the melt-kneading temperature is preferably 140 to 250 ° C., most preferably 180 to 230 ° C.

It is preferable that the melt-kneading temperature is low from the viewpoint of suppressing deterioration of the polyolefin resin, but if the melt-kneading temperature is lower than the above-mentioned temperature, unmelted matter is generated in the extruded product extruded from the die, and in the subsequent stretching step. It may cause film rupture. Further, when the melt-kneading temperature is higher than the above-mentioned temperature, the thermal decomposition of the polyolefin resin becomes intense, and the physical properties of the obtained polyolefin microporous film, for example, the strength and the porosity may deteriorate. Further, the decomposed product precipitates on a chill roll, a roll in the stretching process, or the like and adheres to the sheet, which leads to deterioration of the appearance. Therefore, it is preferable that the melt-kneading temperature is within the above range.

Next, a gel-like sheet is obtained by cooling the obtained extruded product, and the microphase of the polyolefin resin separated by the solvent can be immobilized by cooling. It is preferable to cool the gel sheet to 10 to 50 ° C. in the cooling step. This is because the final cooling temperature is preferably set to be equal to or lower than the crystallization end temperature, and by making the higher-order structure finer, uniform stretching can be easily performed in the subsequent stretching. Therefore, cooling is preferably performed at a rate of 30 ° C./min or higher at least up to the gelation temperature or lower.

Generally, when the cooling rate is slow, relatively large crystals are formed, so that the higher-order structure of the gel-like sheet becomes coarse, and the gel structure forming the gel structure also becomes large. On the other hand, when the cooling rate is high, relatively small crystals are formed, so that the higher-order structure of the gel-like sheet becomes dense, which leads to high toughness of the film in addition to uniform stretching.

Examples of the cooling method include a method of directly contacting the extruded product with cold air, cooling water, and other cooling media, a method of contacting the extruded product with a roll cooled with a refrigerant, a method using a casting drum, and the like. is there.

Although the case where the polyolefin microporous membrane is a single layer has been described so far, the polyolefin microporous membrane of the present invention is not limited to a single layer and may be a laminated body. The number of layers is not particularly limited, and may be two layers or three or more layers. As described above, the laminated portion may contain a desired resin in addition to the polyolefin resin to the extent that the effects of the present invention are not impaired.

As a method of forming the polyolefin microporous film as a laminate, a conventional method can be used. For example, desired resins are prepared as needed, these resins are separately fed to an extruder to melt at the desired temperature and merge in a polymer tube or die to achieve the desired stacking thickness. There is a method of forming a laminated body by extruding from a slit-shaped die or the like.

(C) Stretching Step The obtained gel-like (including laminated sheet) sheet is stretched to obtain a stretched film.
As the stretching method used, MD uniaxial stretching by a roll stretching machine, TD uniaxial stretching by a tenter, sequential biaxial stretching by a roll stretching machine and a tenter, or a combination of a tenter and a tenter, simultaneous biaxial stretching by a simultaneous biaxial tenter, etc. Can be mentioned.

The draw ratio varies depending on the thickness of the gel-like sheet from the viewpoint of uniformity of film thickness, but it is preferable to stretch 7 times or more in any direction.

The area magnification is 40 times or more, preferably 49 times or more, more preferably 60 times or more, still more preferably 80 times or more, and particularly preferably 100 times or more. If the area magnification is less than 40 times, the stretching is insufficient and the uniformity of the film is likely to be impaired, and a good number of holes per unit volume and a good number of surface holes cannot be obtained. The area magnification is preferably 150 times or less. When the area magnification is large, tearing is likely to occur frequently during the production of the polyolefin microporous film, and the productivity is lowered.

From the viewpoint of improving the stretching uniformity in the stretching step, a preferable form of the stretching ratio and the raw material composition is to wet-stretch a raw material having an Mw of 1 million or more to 8 × 8 times or more, more preferably 10 × 10 times or more. It is to stretch in a wet manner. A more preferable form is to wet-stretch a raw material having an Mw of 2 million or more by 8 × 8 times or more, and most preferably to stretch it by wet-water at 10 × 10 times or more. By satisfying the above conditions, unlike the case where a conventional low molecular weight polyolefin resin is stretched at a high magnification, a polyolefin resin having a long molecular chain of Mw 1 million or more can be produced at a high magnification in a state where the molecular chains are strongly entangled with each other. Since it is stretched, it is possible to obtain a polyolefin microporous film having a dense structure in which stretching stress is uniformly propagated. As a result, it is possible to achieve both high safety and excellent output characteristics.

The stretching temperature is in the range of (crystal dispersion temperature Tcd of the gel sheet) to (melting point of the gel sheet + 10 ° C.) and (crystal dispersion temperature Tcd of the gel sheet) to (melting point of the gel sheet + 5 ° C.). Is preferable. Specifically, since the polyethylene composition has a crystal dispersion temperature of about 90 to 110 ° C., the stretching temperature is preferably 90 to 130 ° C., more preferably 90 to 120 ° C.
The crystal dispersion temperature Tcd is obtained from the temperature characteristics of dynamic viscoelasticity measured according to ASTM D 4065 (2012).

If the stretching temperature is less than 90 ° C., the pores are insufficiently opened due to the low temperature stretching, it is difficult to obtain the uniformity of the film thickness, and the pore ratio is also low. If the stretching temperature is higher than 130 ° C., the sheet melts and the pores are likely to be closed.

Due to the above stretching, the higher-order structure formed on the gel-like sheet is cleaved, the crystal phase becomes finer, and a large number of fibrils are formed. Fibrils form a three-dimensionally irregularly connected network structure. Since the mechanical strength is improved by stretching and pores are formed, the polyolefin microporous membrane of the present invention is suitable for a battery separator.

Further, by stretching before removing the plasticizer, the polyolefin resin is sufficiently plasticized and softened, so that the higher-order structure can be cleaved smoothly and the crystal phase can be uniformly refined. it can. Further, by stretching before removing the plasticizer, cleavage is facilitated, so that strain during stretching is less likely to remain, and the heat shrinkage rate can be lowered as compared with the case of stretching after removing the plasticizer. it can.

(D) Plasticizer Extraction (Washing) / Drying Step Next, the plasticizer (solvent) remaining in the stretched film is removed using a washing solvent. Since the polyolefin resin phase and the solvent phase are separated, a polyolefin microporous film can be obtained by removing the plasticizer (solvent).

Examples of the cleaning solvent include saturated hydrocarbons such as pentane, hexane and heptane, chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, ethers such as diethyl ether and dioxane, ketones such as methyl ethyl ketone, and ethane trifluoride. Chain fluorocarbon and the like can be mentioned.

These cleaning solvents have low surface tension (eg, 24 mN / m or less at 25 ° C.). By using a cleaning solvent with a low surface tension, the network structure that forms microporous is suppressed from shrinking due to the surface tension of the gas-liquid interface during drying after cleaning, and a polyolefin microporous membrane with excellent porosity and permeability. Is obtained. These cleaning solvents are appropriately selected depending on the plasticizer and used alone or in combination.

Examples of the cleaning method include a method of immersing the stretched film in a cleaning solvent for extraction, a method of showering the stretching film with the cleaning solvent, a method using a combination thereof, and the like. The amount of the cleaning solvent used varies depending on the cleaning method, but is generally preferably 300 parts by mass or more with respect to 100 parts by mass of the stretched film.

The washing temperature may be 15 to 30 ° C., and if necessary, heat to 80 ° C. or lower. At this time, from the viewpoint of enhancing the cleaning effect of the cleaning solvent, from the viewpoint of preventing the physical properties of the obtained polyolefin microporous film (for example, the physical properties in the TD direction and / or the MD direction) from becoming non-uniform, the mechanical property of the polyolefin microporous film From the viewpoint of improving physical properties and electrical properties, the longer the stretched film is immersed in the washing solvent, the better.

The above-mentioned washing is preferably carried out until the residual solvent in the stretched film after washing, that is, the microporous polyolefin film is less than 1% by mass.

Then, in the drying step, the solvent in the stretched film is dried and removed. The drying method is not particularly limited, and a method using a metal heating roll, a method using hot air, or the like can be selected. The drying temperature is preferably 40 to 100 ° C, more preferably 40 to 80 ° C. If the drying is insufficient, the porosity of the polyolefin microporous membrane will decrease in the subsequent heat treatment, and the permeability will deteriorate.

(E) Heat treatment / re-stretching step At least one of heat treatment and re-stretching (stretching in at least one axial direction) is performed on the dried stretched film.

The heat treatment can be performed using, for example, a heat treatment in the MD direction using a roll, a heat treatment in the TD direction using a tenter, a roll and a tenter, or a combination of a tenter and a tenter.
The temperature of the heat treatment is preferably 120 to 140 ° C, more preferably 125 to 135 ° C.

The re-stretching can be performed by the tenter method or the like in the same manner as the above-mentioned stretching while heating the stretched film. The re-stretching may be uniaxial stretching or biaxial stretching. In the case of multi-stage stretching, simultaneous biaxial or sequential stretching is performed in combination.

The re-stretching temperature is preferably equal to or lower than the melting point of the stretched film, and more preferably within the range of (Tcd-20 ° C. of the stretched film) to the melting point of the stretched film. Specifically, in the case of the polyethylene composition, the re-stretching temperature is preferably 70 to 135 ° C., more preferably 110 to 132 ° C. The re-stretching temperature is most preferably 120-130 ° C.

In the case of uniaxial stretching, the re-stretching ratio is preferably 1.01 to 2.0 times, particularly preferably 1.1 to 1.6 times, and more preferably 1.2 to 1.4 times in the TD direction. .. When biaxial stretching is performed, it is preferable to stretch 1.01 to 2.0 times in the MD direction and the TD direction, respectively. The re-stretching magnification may be different in the MD direction and the TD direction. By re-stretching within the above range, the porosity and permeability are increased, the number of unopened portions of the polyolefin microporous film is reduced, and the number of pores per unit volume and the number of surface pores are increased.

From the viewpoint of heat shrinkage and wrinkles and sagging, the relaxation rate from the maximum re-stretching ratio is preferably 20% or less, more preferably 10% or less, still more preferably 5% or less. When the relaxation rate is 20% or less, a uniform fibril structure can be obtained.

(F) Other Steps Further, depending on other uses, the polyolefin microporous membrane can be hydrophilized. The hydrophilization treatment can be performed by a monomer graft, a surfactant treatment, a corona discharge, or the like. The monomer graft is preferably carried out after the cross-linking treatment.

It is preferable that the microporous polyolefin membrane is crosslinked by irradiation with ionizing radiation such as α-ray, β-ray, γ-ray, and electron beam. In the case of electron beam irradiation, an electron dose of 0.1 to 100 Mrad is preferable, and an accelerating voltage of 100 to 300 kV is preferable. The cross-linking treatment raises the meltdown temperature of the microporous polyolefin membrane.

In the case of surfactant treatment, any of nonionic surfactant, cationic surfactant, anionic surfactant or amphoteric surfactant can be used, but nonionic surfactant is preferable. The polyolefin microporous membrane is immersed in water or a solution prepared by dissolving a surfactant in a lower alcohol such as methanol, ethanol, or isopropyl alcohol, or the solution is applied to the polyolefin microporous membrane by the doctor blade method.

The porous polyethylene film of the present invention is a fluororesin porous body such as polyvinylidene fluoride or polytetrafluoroethylene, or polyimide or polyphenylene for the purpose of improving meltdown characteristics and heat resistance when used as a battery separator. Surface coating such as a porous body such as sulfide or inorganic coating such as ceramic may be applied.

The polyolefin microporous membrane obtained as described above can be used for various purposes such as a filter, a separator for a fuel cell, and a separator for a capacitor, and is particularly excellent in safety and output characteristics when used as a separator for a battery. .. Therefore, the separator can be preferably used as a battery separator for a secondary battery that requires high energy density, high capacity, and high output for electric vehicles and the like.

The present invention will be described in more detail by way of examples, but the embodiments of the present invention are not limited to these examples.
The evaluation method and analysis method used in the examples are as follows.

(1) Weight average molecular weight (Mw)
The weight average molecular weight of ultra-high molecular weight polyethylene and high-density polyethylene was determined by the gel permeation chromatography (GPC) method under the following conditions.
-Measuring device: GPC-150C manufactured by WATERS CORPORATION
-Column: SHODEX UT806M manufactured by Showa Denko KK
・ Column temperature: 135 ℃
-Solvent (mobile phase): O-dichlorobenzene-Solvent flow rate: 1.0 mL / min-Sample concentration: 0.1 wt% (dissolution condition: 135 ° C / 1H)
-Injection amount: 500 μL
-Detector: WATERS CORPORATION differential refractometer (RI detector)
-Calibration curve: Prepared from a calibration curve obtained using a monodisperse polystyrene standard sample using a predetermined conversion constant.

(2) Film thickness Five points of film thickness were measured with a contact thickness meter (Mitutoyo Co., Ltd. Lightmatic) at arbitrary randomly selected points within the range of 95 mm × 95 mm of the polyolefin microporous film, and five points were measured. The average value of the film thickness was calculated.

(3) Air permeability (sec / 100 cm ³ )
The air permeability P1 is measured with an air permeability meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.) for 100 cm ³ of gas through a polyolefin microporous membrane with a film thickness of T1, and the formula: By P2 = (P1 × 10) / T1, the air permeability P2 when the film thickness was 10 μm was calculated.
Air was used as the gas.

(4) A sample having a porosity of 5 cm square is cut out from a microporous polyolefin membrane, the volume (cm ³ ) and mass (g) of the sample are obtained, and the volume (cm ³ ) and mass (g) are calculated from them and the polymer density (g / cm ³ ) using the following equation. did. The above measurement was performed at 3 points at different random sampling points in the same polyolefin microporous membrane, and the average value of the porosity at the 3 points was obtained.
Pore ratio = [(volume-mass / polymer density) / volume] x 100

(5) Puncture strength in terms of film thickness of 10 μm Using a piercing meter manufactured by MARUBISHI, a needle with a spherical tip (radius of curvature R: 0.5 mm) and a diameter of 1 mm has a film thickness of T1 (μm) and a polyolefin microporous film of 2 mm. The maximum load when piercing at a speed of / sec was measured and used as the piercing strength.

The measured value L1 (N) of the maximum load was converted into the maximum load L2 when the film thickness was 10 μm by the formula: L2 = L1 / T1 × 10, and the puncture strength was converted into the film thickness 10 μm. The above measurement was performed at 3 points at different random sampling points in the same polyolefin microporous membrane, and the average value of the puncture strength at 3 points and the puncture strength converted to a film thickness of 10 μm was obtained.

(6) Tensile breaking strength (MPa), tensile breaking elongation (%)
Tensile test pieces with a width of 10 mm were scored from the central portion of the polyolefin microporous film in the width direction, and the average value of the measurement results measured by ASTM D882 (2012) was calculated for each of them to pull the tensile strength in the MD direction and the TD direction. The breaking strength and tensile elongation at breaking were determined.

(7) Toughness (MPa x%)
The toughness was determined according to the following formula using the tensile breaking strength and tensile breaking elongation obtained from (6).
Toughness (MPa x%) = MD tensile breaking strength x MD tensile breaking elongation + TD tensile breaking strength x TD tensile breaking elongation

(8) Heat shrinkage rate (%) at 120 ° C. in TMA
Using a thermomechanical analyzer (TMA / SS6600, manufactured by Seiko Electronics Co., Ltd.), a test piece with a length of 10 mm (MD direction) and a width of 3 mm (TD direction) is pulled in the MD direction with a constant load (2 gf). However, the temperature was raised from room temperature at a rate of 5 ° C./min, the sample length at 120 ° C. was measured, and the shrinkage rate was taken as the MD shrinkage rate at 120 ° C. in TMA. Further, while pulling a test piece having a length of 10 mm (TD direction) and a width of 3 mm (MD direction) in the TD direction with a constant load (2 gf), the shrinkage rate is measured in the same manner, and the TD shrinkage at 120 ° C. in TMA. It was a rate.

(9) Maximum hole diameter (BP diameter) and average hole diameter Using a palm porometer (manufactured by PMI, CFP-1500A), the maximum hole diameter and average hole diameter were measured in the order of Dry-up and Wet-up. For Wet-up measurement, Galwick (trade name) manufactured by PMI Co., Ltd. having a surface tension of 15.6 dynes / cm was used. Pressure was applied to the polyolefin microporous membrane sufficiently immersed in Galwick, and the pore diameter converted from the pressure at which air began to penetrate was defined as the maximum pore diameter.

The average pore diameter was converted from the pressure at the intersection of the pressure and the half slope of the flow rate curve in the Dry-up measurement and the pressure at the intersection of the Wet-up measurement curve. The following formula was used to convert the pressure and the average pore size.

d = C · γ / P
(In the above formula, "d (μm)" is the average pore size of the polyolefin microporous membrane, "γ (mN / m)" is the surface tension of the liquid, "P (Pa)" is the pressure, and "C" is a constant. .)

(10) Calculation of Surface Aperture Ratio, Number of Surface Pore, and Surface Pore Diameter The vapor-deposited polyolefin microporous film was observed with a scanning electron microscope (SEM) at an accelerating voltage of 2 kV. Pore was extracted by binarizing the captured SEM image, and the number of surface holes, the surface aperture ratio and the surface hole diameter per unit area were calculated.

The binarization process was carried out using an accelerating voltage of 2 kV, a magnification of 10000 times, an image of 11.7 μm × 9.4 μm (1280 pixels × 1024 pixels), and an 8-bit (256 gradations) gray scale image. As an image processing method, after removing noise from the above SEM image by averaging 3 pixels x 3 pixels, a dynamic binary value is set to -30 gradations from the image averaged by 21 pixels x 21 pixels. By performing the conversion process, the dark part was extracted and the binarization process was performed.

The ratio of the area occupied by the total dark part in the entire original SEM image was defined as the surface aperture ratio (SEM) (%). In addition, the number of surface holes existing per 1 μm ² was calculated from the extracted independent dark areas and used as the number of surface holes (SEM) (pieces / μm ² ).

Further, from each area of the independent pores, the average was calculated based on the following formula to obtain the surface pore diameter (SEM) (nm).
Surface hole diameter (SEM) = (total area of holes / number of surface holes / 3.14) ^0.5 x 2

Further, the value of "-0.11 x surface aperture ratio W (%) +6.2" on the right side of the equation (1) was obtained.

(11) Number of holes per unit volume The number of holes X (pieces / μm ³ ) per unit volume can be calculated by the following formula (1a).
Number of holes X (pieces / μm ³ ) = 4 × (ε/100) / (π × d ² × τ _a / L) ・ ・ ・ Equation (1a)
[Ε: Pore ratio (%), d: Average pore diameter (μm), τ _a : Curvature ratio calculated by equation (1b), L: Film thickness (μm)]

Curve rate τ _a = {d × (ε/100) × v / (3L × P _s × R _gas )} ^0.5 ... Equation (1b)
[D: average pore size (μm), ε: pore ratio (%), v: molecular velocity of air (= 500 m / sec), L: film thickness (μm), P _s : standard pressure (= 101325 Pa), R _gas : Gas permeation constant calculated by equation (1c) (m ³ / (m ² · sec · Pa)]
R _gas = 0.0001 / {Air permeability (sec / 100 cm ³ ) x (6.424 x 10 ^-4 ) x (0.01276 x P _s )} ... Equation (1c)
[Ps: Standard pressure (= 101325Pa)]

(12) Foreign matter resistance evaluation Using a tensile tester (AUTOGRAPH) << AGS-X manufactured by SHIMAZU >>, a 1.5V capacitor and a data logger, set in the order of negative electrode / polyolefin microporous film / 500 μm diameter chrome ball / aluminum foil. The simple battery was pressed under the condition of 0.3 mm / min, and the amount of transition until the battery was short-circuited was measured. The sample that did not short-circuit even with a high transfer amount had better foreign matter resistance, and the relationship between the transfer amount and the foreign matter resistance was evaluated on the following five stages.

A: The amount of transfer (mm) / separator thickness (μm) was 0.030 or more.
B: The amount of transfer (mm) / separator thickness (μm) was 0.025 or more and less than 0.030.
C: The transfer amount (mm) / separator thickness (μm) was 0.020 or more and less than 0.025.
D: The amount of transfer (mm) / separator thickness (μm) was 0.015 or more and less than 0.020.
E: The amount of transfer (mm) / separator thickness (μm) was less than 0.015.

(13) Measurement of Capacity Retention Rate at 17C In order to evaluate the rate characteristics of the polyolefin microporous membrane, the polyolefin microporous membrane is incorporated into a non-aqueous electrolyte secondary battery composed of a positive electrode, a negative electrode, a separator and an electrolyte as a separator and charged. A discharge test was performed.

NMC532 (Lithium Nickel Manganese Cobalt Composite Oxide (Li _1.05 Ni _0.50 Mn _0.29 Co _0.21) at 9.5 mg / cm ² on an aluminum foil with a width of 38 mm, a length of 33 mm and a thickness of 20 μm. O ₂ )) is laminated on a cathode, and natural graphite with a density of 1.45 g / cm ³ is laminated on a copper foil having a width of 40 mm, a length of 35 mm, and a thickness of 10 μm at a unit area mass of 5.5 mg / cm ^2. Was used. The positive electrode and the negative electrode were dried and used in a vacuum oven at 120 ° C.

As the separator, a microporous polyolefin membrane having a length of 50 mm and a width of 50 mm was dried in a vacuum oven at room temperature and used. The electrolytic solution is a mixture of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate (30/35/35, volume ratio) in which vinylene carbonate (VC) and LiPF ₆ are dissolved, and the VC concentration is 0.5% by mass, LiPF _6. A solution of concentration: 1 mol / L was prepared.

A non-aqueous electrolyte secondary battery is formed by stacking a positive electrode, a separator, and a negative electrode, arranging the obtained laminate in a laminate pouch, injecting an electrolytic solution into the laminate pouch, and vacuum-sealing the laminate pouch. Made.

The prepared non-aqueous electrolyte secondary battery was charged for 10 to 15% at a temperature of 35 ° C. and 0.1 C for the first time, and left at 35 ° C. overnight (12 hours or more) for degassing. Next, CC-CV charging with a temperature of 35 ° C., a voltage range of 2.75 to 4.2 V, a charging current value of 0.1 C (termination current condition 0.02 C), and CC discharge with a discharge current value of 0.1 C were carried out. .. Next, 3 cycles of CC-CV charging (termination current condition 0.05C) with temperature 35 ° C., voltage range: 2.75 to 4.2V, charging current value 0.2C, and CC discharge with discharge current value 0.2C. The time point at which this was performed was defined as the initial stage of the non-aqueous electrolyte secondary battery.

Next, CC-CV charging with a temperature of 35 ° C., a voltage range of 2.75 to 4.2 V, a charging current value of 0.2 C (termination current condition of 0.05 C), and CC discharge with a discharge current value of 0.2 C are performed. The discharge capacity at that time was set to 0.2 C capacity. Next, after CC-CV charging (termination current condition 0.05C) at a temperature of 35 ° C., a voltage range: 2.75 to 4.2V, and a charging current value of 0.5C, a non-aqueous electrolyte secondary battery at 35 ° C. A rate test was performed at 17C (306mA, 24.48mA / cm ² ). From this result, the ratio of 17C capacity to 0.2C capacity {(17C capacity / 0.2C capacity) × 100} (%) was defined as the capacity retention rate (%).

[Example 1]
Mw as the starting material was used an ultra-high molecular weight polyethylene of 20 × 10 ^5. 90 parts by mass of liquid paraffin is added to 10 parts by mass of ultra-high molecular weight polyethylene, and 0.5 parts by mass of 2,6-di-t-butyl-p-cresol and 0.7 mass by mass based on the mass of ultra-high molecular weight polyethylene. Part of tetrakis [methylene-3- (3,5-di-t-butyl-4-hydroxyphenyl) -propionate] methane was added as an antioxidant and mixed to prepare a polyethylene resin solution.

The obtained polyethylene resin solution was put into a twin-screw extruder, kneaded at 180 ° C., supplied to a T-die, and the extruded product was cooled with a cooling roll controlled at 15 ° C. to form a gel-like sheet.

The obtained gel-like sheet was simultaneously biaxially stretched 8 times in both the longitudinal direction and the width direction at 115 ° C. by a tenter stretching machine, the sheet width was fixed as it was in the tenter stretching machine, and the sheet was held at a temperature of 115 ° C. for 10 seconds. ..

Next, the stretched gel-like sheet was immersed in a methylene chloride bath in a washing tank, and after removing liquid paraffin, it was dried to obtain a polyolefin microporous membrane. Finally, an oven composed of a plurality of zones divided in the longitudinal direction was used as the oven of the tenter stretching machine, and heat fixing was carried out at a temperature of 120 ° C. for 10 minutes without stretching.

Table 1 shows the raw material formulation and film forming conditions of the polyolefin microporous film, and Table 3 shows the evaluation results of the polyolefin microporous film.

[Examples 2 to 11 and Comparative Examples 1 to 11]
A polyolefin microporous film was prepared in the same manner as in Example 1 except that the raw material formulation and the film forming conditions were changed as shown in Tables 1 and 2.

In Tables 1 and 2, "UHPE" means ultra-high molecular weight polyethylene, and "HDPE" means high-density polyethylene.
The evaluation results of the obtained polyolefin microporous membrane are as shown in Tables 3 and 4.

As a result of comparing Comparative Examples 1 to 6, 10 and 11 with Comparative Examples 7 to 8, the molecular weight of the raw material was increased and the resin concentration was decreased to increase the strength by increasing the molecular weight and the number of surface pores by decreasing the resin concentration. Although there is a tendency for the number of plastics to increase, the air permeability deteriorates and the output characteristics are poor. Moreover, the strength was not sufficiently improved. Further, as a result of comparing Comparative Examples 7 and 8 and Comparative Example 9, although the balance between strength and air permeability was improved by increasing the draw ratio, the improvement in both foreign matter resistance and output characteristics was insufficient. ..
As a method for further improving the output characteristics, even if the content ratio of the ultra-high molecular weight polyethylene is adjusted in order to increase the number of surface pores and the ultra-high molecular weight polyethylene is used alone, the number of surface pores is increased, but no significant effect is observed. Both output characteristics and strength could not be achieved (Comparative Examples 5 and 10).
In Example 1-6, as a result of stretching at a high magnification using UHPE having a molecular weight of 2 million, the uniformity of stretching was improved, a high number of pores and a high strength balance were achieved, and good output characteristics and foreign matter resistance were obtained. .. Further, in Examples 2, 4 and 5, as a result of increasing the high draw ratio or the increase in the resin concentration as compared with Example 1, further high strength is achieved and excellent safety is obtained. In Examples 7-8, UHPE having a molecular weight of 1 million was stretched at a relatively high resin concentration (17%) at a high magnification of 10 × 10 times, resulting in a higher number of holes than in Examples 4 and Comparative Examples 5 and 6. It has a strength balance and has excellent battery characteristics and safety. This balance between the number of surface holes and the strength is a region that could not be obtained by the conventional formulation, and as a result, the foreign matter resistance and the output characteristics were good. Furthermore, a polyolefin microporous membrane that has a physical property balance that could not be obtained by conventional formulations, which usually does not deteriorate in heat shrinkage, toughness, and air permeability, which are trade-offs with high strength, is obtained. Obtained.

Claims (11)

The piercing strength Y (N) converted to a film thickness of 10 μm and the surface aperture ratio W (%) satisfy the relationship of the following formula (1), and both the tensile breaking strength in the MD direction and the tensile breaking strength in the TD direction are 180 MPa or more. A polyolefin microporous membrane.
Y (N) ≧ −0.11 × W (%) +6.2 ... Equation (1) The microporous polyolefin membrane according to claim 1, wherein the surface aperture ratio W is 5% or more and 45% or less. The polyolefin microporous membrane according to claim 1 or 2, wherein the average pore size is less than 40 nm. The polyolefin microporous membrane according to any one of claims 1 to 3, wherein the toughness is 35,000 (MPa ×%) or more. The polyolefin microporous membrane according to any one of claims 1 to 4, wherein the piercing strength Y is 3.0 N or more. The microporous polyolefin membrane according to any one of claims 1 to 5, wherein the heat shrinkage rate at 120 ° C. in the MD direction measured by thermomechanical analysis is 15% or less. The polyolefin microporous membrane according to any one of claims 1 to 6, wherein the porosity is 40% or more and less than 60%. The polyolefin microporous film according to any one of claims 1 to 7, wherein the polyolefin that is the microporous film is polyethylene. A battery separator comprising the polyolefin microporous membrane according to any one of claims 1 to 8. A secondary battery comprising the battery separator according to claim 9. The method for producing a microporous polyolefin membrane according to any one of claims 1 to 7.
When the total of the solvent and the polyolefin resin is 100% by mass, the content ratio of the ultra-high molecular weight polyethylene having a weight average molecular weight of 1 million or more is 5 to 30% by mass, and the content ratio of the polyolefin resin is less than 30% by mass. The process of preparing a solution,
A step of forming an unstretched gel-like sheet by extruding the solution from a die and cooling and solidifying the solution.
A step of stretching the gel-like sheet at a temperature from the crystal dispersion temperature of the gel-like sheet to the melting point of the gel-like sheet + 10 ° C. so that the area magnification is 40 times or more to obtain a stretched film.
It has a step of extracting a plasticizer from the stretched film and drying the stretched film, and a step of performing at least one of heat treatment of the stretched film after drying and re-stretching of the stretched film after drying.
A method for producing a microporous polyolefin membrane.