KR101143106B1 - Microporous polymer membrane - Google Patents

Abstract

The present invention comprises a differential pore volume comprising polyethylene and having an area under the curve for the pore diameter range of about 100 nm to about 1,000 nm that is at least 25% of the total area under the curve for the pore diameter range of about 10 nm to about 1,000 nm. It relates to a microporous membrane having a curve.

Description

Microporous Polymer Membrane {MICROPOROUS POLYMER MEMBRANE}

The present invention relates to microporous membranes comprising one or more polymers having suitable permeability, mechanical strength, heat shrinkage resistance, compression resistance and electrolyte absorption. The invention also relates to a method of making the microporous membrane, a method of making a battery using the membrane as a battery separator and a method of using the battery as a source or sink of charge.

Microporous membranes are useful as separators for preventing electrode contact in electrochemical cells such as, for example, fuel cells and batteries. For example, the microporous membrane can be made from primary and secondary batteries such as lithium ion secondary batteries, lithium-polymer secondary batteries, nickel-hydrogen secondary batteries, nickel-cadmium secondary batteries, nickel-zinc secondary batteries, silver- It can be used as a separator in zinc secondary batteries. When the microporous polyolefin membrane is used as a battery separator, especially for lithium ion batteries, the performance of the membrane has a significant impact on the battery's properties, productivity and safety. Thus, the microporous polyolefin membrane must have appropriate permeability, mechanical properties, heat resistance, dimensional stability, shutdown properties, meltdown properties, and the like. As is known, such batteries need to have a relatively low shutdown temperature and a relatively high meltdown temperature to improve their battery safety, especially for batteries exposed to high temperatures under operating conditions. High separator permeability is desirable for high capacity of batteries. Membranes with high mechanical strength are more durable and are also desirable for improving battery assembly and manufacturing properties.

In order to improve the properties of microporous polyolefin membranes used as battery separators, it has been proposed to optimize the material composition, stretching conditions, heat treatment conditions and the like. For example, JP6-240036A discloses microporous polyolefin membranes with improved pore diameter and single pore diameter distribution. The membrane is made from a polyethylene resin containing 1% by weight or more of ultra high molecular weight polyethylene having a weight average molecular weight (“Mw”) of 7 × 10 ⁵ or more, wherein the polyethylene resin has a molecular weight distribution (weight average molecular weight / number average molecular weight) of 10 To 300, the microporous polyolefin membrane has a porosity of 35 to 95%, an average penetrating pore diameter of 0.05 to 0.2 μm, a breaking strength (15 mm width) of 0.2 kg or more, and a pore diameter distribution (maximum pore diameter / average). Through pore diameter) is 1.5 or less. This microporous membrane is obtained by extruding a melt-blend of the polyethylene resin and a film-forming solvent through a die and cooling the gel resin at a crystal dispersion temperature (“Tcd”) to a melting point + 10 ° C. of the polyethylene resin. Stretching the sheet, removing the film-forming solvent from the gel-type sheet, and re-drawing the resulting film at an area magnification of 1.5 to 3 times at a melting point of the polyethylene resin-10 ° C or lower. It is prepared by heat-fixing it at the crystal dispersion temperature of the said polyethylene resin to the temperature of melting | fusing point.

WO 1999/48959 discloses microporous polyolefin membranes having not only adequate strength and permeability but also a uniform porous surface without local permeability variations. This film is a fine film having a Mw of 50,000 or more and less than 5,000,000, a molecular weight distribution of 1 or more and less than 30, an average micro-fibril size of 20 to 100 nm and an average micro-fibril distance of 40 to 400 nm. It is made of polyolefin resin such as high density polyethylene having a network structure with fine gaps formed by uniformly dispersing the brill. This microporous membrane stretches a gel-like sheet obtained by extruding a melt-blend of the polyolefin resin and a film-forming solvent through a die and cooling the polyolefin resin at a melting point of 50 ° C. or higher to less than the melting point. Remove the film-forming solvent from the gel-type sheet, and re-stretch it 1.1 to 5 times at a temperature of the polyolefin resin at a melting point minus 50 ° C. or more and below the melting point, It is prepared by heat-fixing at the temperature of the melting point.

WO 2000/20492 discloses microporous polyolefin membranes with improved permeability characterized by fine polyethylene fibrils having Mw of at least 5 × 10 ⁵ or compositions comprising such polyethylenes. The microporous polyolefin membranes have an average pore diameter of 0.05 to 5 μm and a ratio of lamellae with an angle θ of 80 to 100 ° relative to the membrane surface of at least 40% in the longitudinal and transverse cross sections. Such polyethylene compositions comprise from 1 to 69 weight percent of ultra-high molecular weight polyethylene, from 1 to 98 weight percent of high density polyethylene and from 1 to 30 weight percent of low density polyethylene with a weight average molecular weight of at least 7 × 10 ⁵ . Such microporous membranes extrude a melt-blend of the polyethylene or composition thereof and the film-forming solvent through a die and draw a gel-like sheet obtained by cooling, which is determined by the crystal dispersion temperature of the polyethylene or composition thereof to It is prepared by heat-setting at a temperature of melting point + 30 ° C. and removing the film-forming solvent.

WO 2002/072248 discloses microporous membranes with improved permeability, particle-blocking properties and strength. This membrane is made using polyethylene resin with Mw less than 380,000. The membrane has a porosity of 50 to 95% and an average pore diameter in the range of 0.01 to 1 μm. Such microporous membranes have a three-dimensional network backbone formed by micro-fibrils having an average diameter of 0.2 to 1 μm connected to each other over the entire microporous membrane, said skeleton having an average diameter of at least 0.1 μm and less than 3 μm. It has a void defined by. This microporous membrane extrudes a melt-blend of the polyethylene resin and the film-forming solvent through a die, and removes the film-forming solvent from the gel-type sheet obtained by cooling, and the temperature is 20 to 140 ° C. Prepared by stretching 2 to 4 times at and heat-fixing the stretched film at a temperature of 80 to 140 ° C.

Altering the internal structure of the membrane is said to have a desirable effect on at least some of the potentially desired membrane properties. In this regard, it has been proposed to prepare a membrane from a composition comprising an inorganic oxide such as silicon oxide and then leaching it from the finished membrane using an inorganic base. This method is undesirable because it inevitably leaves unwanted residues of silicon oxide in the film.

WO 2005/113657 discloses microporous polyolefin membranes having suitable shutdown properties, meltdown properties, dimensional stability and high temperature strength. This membrane comprises (a) a component having a molecular weight of 10,000 or less, a Mw / Mn ratio (where Mn is the number-average molecular weight of the polyethylene resin) of 11 to 100 and a viscosity-average molecular weight (“Mv”) of 100,000 to 1,000,000. To 60% by weight of polyethylene resin, and (b) polyolefin comprising polypropylene. This film has a porosity of 20 to 95% and a heat shrinkage of 100% or less. Such microporous polyolefin membranes are formed by extruding a melt-blend of the polyolefin and membrane-forming solvent through a die, stretching the gel-type sheet obtained by cooling, removing the membrane-forming solvent, and annealing the sheet. Are manufactured.

Battery separator properties such as permeability, mechanical strength, dimensional stability, thermal expansion, shutdown properties and meltdown properties are generally considered important. In addition to these characteristics, battery cycle characteristics such as separator characteristics and electrolyte retention characteristics related to battery productivity, such as electrolyte absorption, are also important. It may be important to simultaneously balance or optimize two or more of these properties, such as thermal expansion and electrolyte retention properties. For example, an electrode for a lithium ion battery expands and contracts as lithium intrudes and escapes, and the expansion rate becomes larger due to an increase in battery capacity. Since the separator is compressed at the time of electrode expansion, the separator should reduce the amount of electrolyte retention as low as possible.

Further improved microporous membranes are disclosed in JP6-240036A, WO 1999/48959, WO 2000/20492, WO 2002/072248 and WO 2005/113657, but in particular membrane permeability, mechanical strength, heat shrinkage resistance, compression resistance and electrolyte absorbency. Further improvement is still needed. Therefore, it is desirable to form a battery separator from a microporous membrane having improved permeability, mechanical strength, heat shrinkage resistance, compression resistance, and electrolyte absorption.

In one embodiment, the present invention relates to the discovery of microporous membranes having improved permeability, mechanical strength, heat shrinkage resistance, compression resistance and electrolyte absorption. The strength of the microporous membrane is achieved by having a sufficient number of pores having a pore size in the range of about 100 nm to about 1,000 nm to achieve a desired electrolyte uptake and having a sufficient number of pores having a pore size in the range of about 1 nm to about 100 nm. Microporous membranes have been found that do not significantly degrade.

In one embodiment, the invention relates to micropores comprising pores characterized in that at least about 25% of the area under the differential pore volume curve has a pore size distribution associated with pores having a diameter in the range from about 100 nm to about 1,000 nm. It relates to a porous polyolefin membrane.

로 표시되며, 여기서 Vp는 기공 부피이고 r은 기공 반경이다. Log r은, 밑이 10인 반경 r의 로그를 나타낸다. 상기 곡선을 기공 직경의 로그 함수로서 도식화하는 것이 편리하다. Here, the differential pore volume is

Where Vp is the pore volume and r is the pore radius. Log r represents the logarithm of the radius r of base 10. It is convenient to plot the curve as a logarithmic function of pore diameter.

In related embodiments, the differential pore volume curve as a function of pore size (eg, pore diameter when the pores are approximately cylindrical) is multimodal, ie characterized by two or more peaks. Such membranes have been found to improve permeability, mechanical strength, heat shrinkage resistance, compression resistance and electrolyte absorption. The microporous membrane can be obtained by extruding a polyolefin solution through one or more dies.

The polyolefin solution can be prepared from one or more diluents and polyolefin resin. The polyolefin solution may be prepared from polyethylene resin.

In one embodiment, the polyethylene resin comprises (a) a first polyethylene resin having a weight average molecular weight ranging from about 1 × 10 ⁴ to about 5 × 10 ⁵ , and (b) a second polyethylene having a weight average molecular weight of at least 1 × 10 ^6. 7 wt% or less of resin, wherein the wt% is based on the combined weight of the resins (a) and (b).

In another embodiment, the polyolefin solution is prepared from polyethylene resin and polypropylene resin. For example, the polyolefin solution can be prepared from up to 25 weight percent of (c) polypropylene resin, wherein the weight percent is selected from (a) the first and (b) second polyethylene resins and (c) the polyolefin in the polyolefin solution. Based on the combined weight of propylene. In one embodiment, the polyolefin resin consists essentially of polyethylene (eg, first polyethylene); In another embodiment, the polyethylene resin consists essentially of polyethylene. The polyolefin solution portion that is not polyolefin may be a diluent and / or additive, some of which are described below.

After extruding the polyolefin solution, it can be cooled to produce a cooled extrudate, where the extrudate can be, for example, in the form of a gel-type casting or sheet. The extrudate or cooled extrudate may be stretched in one or more directions, and at least a portion of the diluent may be removed to form a porous sheet. The porous sheet may be stretched at a magnification in the range of 1.1 to 1.8 times in one or more directions at an elevated temperature to form a stretched porous membrane. This step of drawing the porous sheet may be referred to as a "second" drawing step to distinguish it from drawing the extrudate or cooled extrudate. Thus, the elevated temperature during the second stretching step can be referred to as the second stretching temperature. After the second stretching, the stretched porous sheet may be heat-fixed at a heat-fixing temperature in a range of the second stretching temperature ± 6 ° C to form a porous membrane. In one embodiment, the heat-setting temperature is in the range of the second stretching temperature ± 5 ° C, ± 3 ° C or ± 2 ° C.

In one embodiment, the porous polyolefin membrane is characterized by differential pore volume curves (sometimes commonly referred to as pore size distribution curves) having two or more modes. One mode or peak generally relates to the dense domains in the membrane having a pore size of about 0.01 to about 0.08 μm. At least the second mode corresponds to coarser domains having a pore size in the range from about 0.08 μm to about 1.5 μm. See, eg, FIGS. 1 and 2. The mode may be present as a distinct peak in the differential pore volume curve, but this mode may appear as a curved shoulder or curve depending on the level of dissolution selected for, for example, pore size measurement, mercury penetration rate, amount of pressure applied, and the like. It is unnecessary because it can be. In one embodiment, the pore volume ratio of the dense domain (relative small pores) to the coarse domain (relative large pores) is between 0.5 and 49. In one embodiment, the microporous polyolefin membrane has a surface roughness ranging from 3 × 10 ² nm to about 3 × 10 ³ nm. A membrane having a surface roughness within the above range is considered to exhibit suitable electrolyte absorbency by having a sufficiently large contact area with the electrolyte when used as a battery separator.

In one embodiment, the microporous polyolefin membrane comprises (1) (a) a polyethylene resin containing up to 7% by weight of ultra high molecular weight polyethylene having a weight-average molecular weight of at least 1 × 10 ⁶ based on the weight of the polyethylene resin and (b) Melt-blending the membrane-forming solvent to form a polyolefin solution, preferably having a solvent concentration of 25-50% by weight based on the weight of the polyolefin solution; (2) extruding the polyolefin solution through a die to form an extrudate; (3) cooling the extrudate to form a gel-like sheet; (4) stretching the gel-type sheet at a first stretching temperature in the range of crystal dispersion temperature ("Tcd") to Tcd + 30 ° C of the polyethylene resin; (5) removing the film-forming solvent from the stretched gel-type sheet to form a film; (6) stretching the film at a second stretching temperature in one or more directions at a magnification of 1.1 to 1.8 times to form a stretched film; And (7) heat-fixing the stretched membrane at a heat-fixing temperature in the range of Tcd to Tm to form a microporous polyolefin membrane.

을 나타낸 것이다. 약 1 nm 내지 약 1,000 nm의 기공 크기 범위에 대한 곡선 아래의 총 면적은 1.0이다. 약 100 nm 내지 약 1,000 nm의 기공 크기 범위에 대한 곡선 아래의 면적은 0.43이다. 따라서, 곡선 아래 면적의 약 43%는 약 100 내지 약 1,000 nm 범위의 기공 크기를 갖는 기공에 관한 것이다.
도 2는 실시예 7의 샘플에 대한 미분 기공 부피 곡선

을 나타낸 것이다. 약 1 nm 내지 약 1,000 nm의 기공 크기 범위에 대한 곡선 아래의 총 면적은 1.07이다. 약 100 nm 내지 약 1,000 nm의 기공 크기 범위에 대한 곡선 아래의 면적은 0.502이다. 따라서, 곡선 아래 면적의 약 47%는 약 100 내지 약 1,000 nm 범위의 기공 크기를 갖는 기공에 관한 것이다.
도 3은 비교예 2의 샘플에 대한 미분 기공 부피 곡선

을 나타낸 것이다. 약 1 nm 내지 약 1,000 nm의 기공 크기 범위에 대한 곡선 아래의 총 면적은 0.48이다. 약 100 nm 내지 약 1,000 nm의 기공 크기 범위에 대한 곡선 아래의 면적은 0.075이다. 따라서, 곡선 아래 면적의 약 16%는 약 100 내지 약 1,000 nm 범위의 기공 크기를 갖는 기공에 관한 것이다.
도 4는 수은 침투를 이용한 기공률 측정원리(mercury intrusion porosimetry)를 나타낸 것이다.
도 5는 (실시예 7로부터) 하이브리드 구조를 가진 미세다공성 막에 대한 Vp(기공 부피) 곡선을 나타낸 것이다.
도 6은 하이브리드 구조를 갖지 않는 미세다공성 막에 대한 미분 기공 부피 곡선

을 나타낸 것이다. 상기 막은 폴리에틸렌을 함유하지만 폴리프로필렌을 함유하지 않는다.
도 7은 하이브리드 구조를 갖는 미세다공성 막에 대한 미분 기공 부피 곡선

을 나타낸 것이다. 1 is the differential pore volume curve for the sample of Example 3

It is shown. The total area under the curve for the pore size range of about 1 nm to about 1,000 nm is 1.0. The area under the curve for the pore size range of about 100 nm to about 1,000 nm is 0.43. Thus, about 43% of the area under the curve relates to pores having a pore size in the range of about 100 to about 1,000 nm.
2 is the differential pore volume curve for the sample of Example 7

It is shown. The total area under the curve for the pore size range of about 1 nm to about 1,000 nm is 1.07. The area under the curve for the pore size range of about 100 nm to about 1,000 nm is 0.502. Thus, about 47% of the area under the curve relates to pores having a pore size in the range of about 100 to about 1,000 nm.
3 is the differential pore volume curve for the sample of Comparative Example 2

It is shown. The total area under the curve for the pore size range of about 1 nm to about 1,000 nm is 0.48. The area under the curve for the pore size range of about 100 nm to about 1,000 nm is 0.075. Thus, about 16% of the area under the curve relates to pores having a pore size in the range of about 100 to about 1,000 nm.
Figure 4 shows the mercury intrusion porosimetry using mercury infiltration.
FIG. 5 shows the Vp (pore volume) curves for microporous membranes with hybrid structures (from Example 7).
6 shows differential pore volume curves for microporous membranes without hybrid structure

It is shown. The membrane contains polyethylene but no polypropylene.
7 shows differential pore volume curves for a microporous membrane having a hybrid structure.

It is shown.

[One] Melt  Produce

The present invention relates to a process for producing microporous films having improved properties, in particular electrolyte injection and compression properties. In one embodiment, certain polyethylene resins and optionally certain polypropylene resins may be combined to form a polyolefin composition, for example by melt-blending.

In one embodiment, the polyolefin composition comprises polyethylene, for example (a) a first polyethylene resin having a weight average molecular weight of about 2.5 × 10 ⁵ to about 5 × 10 ⁵ . In another embodiment, the polyolefin composition comprises polyethylene and polypropylene. In another embodiment, the polyolefin composition comprises (a) the first polyethylene resin, (b) about 0% to about 7% of a second polyethylene resin having a weight average molecular weight of about 5 × 10 ⁵ to about 1 × 10 ⁶ , And (c) 0% to about 25% of a polypropylene resin having a weight average molecular weight of about 3 × 10 ⁵ to about 1.5 × 10 ⁶ .

In another embodiment, the polyolefin composition is prepared from polyethylene resin and from 25 weight percent to 65 weight percent, or from 30 weight percent to 55 weight percent polypropylene resin, based on the weight of the polyolefin composition.

(a) polyethylene resin

The microporous membrane is made from a polyolefin solution comprising a diluent and a polyolefin. The polyolefin may comprise polyethylene or optionally polyethylene and polypropylene. Non-limiting examples of suitable polyethylenes and polypropylenes will be described below.

(i) composition

The first polyethylene has a weight average molecular weight (Mw) in the range of about 1 × 10 ⁴ to about 5 × 10 ⁵ . The first polyethylene may comprise one or more of high density polyethylene (“HDPE”), medium-density polyethylene, branched low-density polyethylene, and linear low-density polyethylene. When the first polyethylene is HDPE, the Mw of the HDPE may be in the range of about 1 × 10 ⁵ to about 5 × 10 ⁵ , or about 2 × 10 ⁵ to about 4 × 10 ⁵ , for example.

The first polyethylene resin may be, for example, a high density polyethylene (HDPE) resin having a weight average molecular weight of 2.5 × 10 ⁵ to 5 × 10 ⁵ and a molecular weight distribution of about 5 to about 100. Non-limiting examples of first polyethylene resins as used herein are those having a weight average molecular weight of about 2.5 × 10 ⁵ to about 4 × 10 ⁵ and a molecular weight distribution of about 7 to about 50. The first polyethylene resin may be an ethylene homopolymer or an ethylene / α-olefin copolymer containing a small amount, such as about 5 mol%, of the third α-olefin. The third α-olefin that is not ethylene is preferably propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate or styrene, or Combination of these. Such copolymers are preferably prepared using single-site catalysts.

The second polyethylene resin such as ultra-high molecular weight polyethylene (UHMWPE) resin has a weight average molecular weight of greater than 5 × 10 ⁵ . The Mw of the second polyethylene can range from about 1 × 10 ⁶ to about 15 × 10 ⁶ , about 1 × 10 ⁶ to about 5 × 10 ⁶ , or about 1 × 10 ⁶ to about 3 × 10 ⁶ , for example. The second polyethylene resin may be an ethylene homopolymer or an ethylene / α-olefin copolymer containing a small amount, such as about 5 mol%, of the third α-olefin. The third α-olefin that is not ethylene is for example propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate or styrene, or Combinations thereof. Such copolymers are preferably prepared using single-site catalysts.

(ii) molecular weight distribution Mw / Mn

Mw / Mn is a measure of molecular weight distribution. The larger the value, the wider the molecular weight distribution. As used herein, the Mw / Mn of the entire polyethylene composition is preferably about 5 to about 100, such as about 7 to about 50. If Mw / Mn is less than 5, the ratio of high molecular weight components is too high to facilitate melt extrusion. On the other hand, when Mw / Mn exceeds 100, the proportion of the low molecular weight component is too high to reduce the strength of the resulting microporous membrane. Mw / Mn of polyethylene (homopolymer or ethylene / α-olefin copolymer) can be appropriately controlled by multi-step polymerization. The multi-stage polymerization process is preferably a two-stage polymerization process comprising forming a high molecular weight polymer component in a first step and a low molecular weight polymer component in a second step. For polyethylene compositions as used herein, the larger the Mw / Mn, the larger the Mw difference between the high molecular weight polyethylene and the low molecular weight polyethylene, and vice versa. Mw / Mn of the polyethylene composition may be appropriately adjusted by the above molecular weight and component mixing ratio.

(b) polypropylene resin

The polyolefin solution may optionally contain a specific polypropylene resin. For example, the polypropylene resin optionally used herein has a weight average molecular weight of about 3 × 10 ⁵ to about 1.5 × 10 ⁶ , such as 6 × 10 ⁵ to about 1.5 × 10 ⁶ and a heat of fusion of at least 80 J / g. And, in non-limiting examples, from about 80 to about 120 J / g and a molecular weight distribution from about 1 to about 100, such as from about 1.1 to about 50, a propylene homopolymer or a copolymer of propylene with another, ie, fourth, olefin Homopolymers are preferred. Optionally, the polypropylene is isotactic polypropylene having a melting peak (second melt) of about 160 ° C. or more. Optionally, the polypropylene has a Trouton ratio measured at a temperature of about 230 ° C. and a strain rate of 25 seconds ⁻¹ or greater. Optionally, the polypropylene has a tensile viscosity of at least about 50,000 Pa at a temperature of 230 ° C. and a strain rate of 25 seconds ⁻¹ . When the polypropylene is a copolymer, the copolymer may be a random or block copolymer. The fourth olefin, which is an olefin other than propylene, is an α-olefin such as ethylene, butene-1, pentene-1, hexene-1,4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate, styrene, etc. And diolefins such as butadiene, 1,5-hexadiene, 1,7-octadiene, 1,9-decadiene and the like. The proportion of the fourth olefin in the propylene copolymer is preferably in a range that does not reduce the properties of the microporous membrane such as heat resistance, compression resistance, heat shrinkage resistance, etc., preferably less than about 10 mol%, such as about 0 to about 10 mol. Less than%.

In one embodiment, the amount of polypropylene resin in the polyolefin composition is 55 wt% or less, 40 wt% or less, or 25 wt% or less based on 100 wt% of the polyolefin composition. In this embodiment, the proportion of the polypropylene resin can be, for example, about 5 to about 20 weight percent of the polyolefin composition, in other examples about 7 to about 15 weight percent.

(2) other ingredients

In addition to the above components, the polyolefin solution may comprise (a) an additional polyolefin and / or (b) a heat-resistant polymer resin having a melting point or glass transition temperature (Tg) of about 170 ° C. or higher, such as to not impair the properties of the microporous membrane. It may be contained in an amount of 10% by weight or less based on the polyolefin composition. It is not necessary to add an inorganic oxide such as silicon oxide to the polyolefin solution, and in one embodiment the polyolefin solution does not contain added inorganic oxides. In related embodiments, the polyolefin solution does not contain significant amounts of inorganic oxides such as silicon oxides. As used herein, the expression "does not contain an equivalent amount of inorganic oxide" means that the content of the oxide is less than 0.1%, less than 0.01%, less than 0.001%, or less than 100 ppmw, based on the weight of the polyolefin solution. .

(a) additional polyolefins

The additional polyolefins are (a) polybutene-1, polypentene-1, poly-4-methylpentene-1, polyhexene-1, polyoctene-1, poly having Mw of 1 × 10 ⁴ to 4 × 10 ⁶ , respectively. Vinyl acetate, polymethyl methacrylate, polystyrene and ethylene / α-olefin copolymers and (b) one or more of polyethylene wax with Mw of 1 × 10 ³ to 1 × 10 ⁴ . Polybutene-1, polypentene-1, poly-4-methylpentene-1, polyhexene-1, polyoctene-1, polyvinyl acetate, polymethyl methacrylate and polystyrene are not only homopolymers but also other α-olefins. It may be a copolymer containing.

(b) heat resistant resin

The heat resistant resin may be, for example, (a) an amorphous resin having a melting point of at least about 170 ° C., which may be partially crystalline, and (b) a completely amorphous resin having a Tg of at least about 170 ° C., and mixtures thereof. The melting point and Tg are determined by differential scanning calorimetry (DSC) according to the method JIS K7121. Specific examples of heat resistant resins include polyesters such as polybutylene terephthalate (melting point: about 160 to 230 ° C.), polyethylene terephthalate (melting point: about 250 to 270 ° C.) and the like, fluororesins, polyamide (melting point: 215 to 265). ° C), polyarylene sulfide, polyimide (Tg: 280 ° C or higher), polyamide imide (Tg: 280 ° C), polyether sulfone (Tg: 223 ° C), polyether ether ketone (melting point: 334 ° C) ° C), polycarbonate (melting point: 220 to 240 ° C), cellulose acetate (melting point: 220 ° C), cellulose triacetate (melting point: 300 ° C), polysulfone (Tg: 190 ° C), polyetherimide (melting point: 216 ℃) and the like.

(c) content

The total amount of the additional polyolefin and the heat resistant resin is preferably 20% by weight or less, such as about 0 to about 20% by weight, based on 100% by weight of the polyolefin solution.

[2] methods for producing a microporous membrane

The present invention comprises the steps of (1) combining a specific polyolefin (typically in the form of a polyolefin resin) with one or more solvents or diluents to form a polyolefin solution; (2) extruding the polyolefin solution through a die to form an extrudate; (3) cooling the extrudate to form a cooled extrudate; (4) stretching the cooled extrudate at a predetermined temperature to form a stretched sheet; (5) removing the solvent or diluent from the stretched sheet to form a solvent / diluent-removed film; (6) stretching the solvent / diluent-removed film at a predetermined magnification at a predetermined temperature to form a stretched film; And (7) heat-fixing the stretched membrane to form a microporous membrane.

If necessary, a heat-fixing treatment step 4i, a hot rolling treatment step 4ii and / or a hot solvent treatment step 4iii can be carried out between the steps (4) and (5). The heat-fixing treatment step 5i can be carried out between the above steps (5) and (6). If necessary, after step (5i) and before step (6), cross-link step (5ii) with ionizing radiation, and after step (7), hydrophilization treatment step (7i) and surface-coating treatment step (7ii) Can be carried out.

(1) Preparation of Polyolefin Solution

The polyolefin resin may be combined with one or more solvents or diluents to prepare a polyolefin solution. Alternatively, the polyolefin resin can be combined to form a polyolefin composition, for example by melt-blending, dry mixing or the like, and then combined with one or more solvents or diluents to produce a polyolefin solution. The polyolefin solution may, if necessary, contain various additives such as antioxidants, fine silicate powders (pore-forming materials) and the like in amounts that do not impair the properties of the invention.

In order to be able to draw at a relatively high magnification, the diluent or solvent such as the film-forming solvent is preferably a liquid at room temperature. The liquid solvent is, for example, aliphatic, cycloaliphatic or aromatic hydrocarbons such as nonane, decane, decalin, p-xylene, undecane, dodecane, liquid paraffin, mineral oil distillates with boiling points corresponding to these hydrocarbons, and liquid at room temperature. Phosphorus phthalates such as dibutyl phthalate, dioctyl phthalate and the like. In order to most effectively obtain extrudate having a stable solvent content, it is preferable to use a non-volatile liquid solvent such as liquid paraffin. In one embodiment, one or more solid solvents may be added to the liquid solvent that are miscible with the polyolefin composition but are solid at room temperature, for example, during melt blending. Such solid solvents are preferably stearyl alcohol, seryl alcohol, paraffin wax and the like. In another embodiment, the solid solvent can be used without the liquid solvent. However, if only a solid solvent is used, stretching nonuniformity may occur.

The viscosity of the liquid solvent is preferably about 30 to about 500 cSt, more preferably about 30 to about 200 cSt at a temperature of 25 ° C. If the viscosity at 25 ° C. is less than 30 cSt, the polyolefin solution may be easily foamed and may be difficult to blend. On the other hand, if the viscosity is greater than 500 cSt, removal of the liquid solvent may be difficult.

Although not particularly limited, uniform melt-blending of the polyolefin solution is preferably carried out in a twin screw extruder to produce a highly concentrated polyolefin solution. The diluent or solvent such as film-forming solvent may be added before the start of melt-blending or fed to the twin screw extruder in the middle of the blending, but the latter is preferred.

The melt-blending of the polyolefin solution is preferably in the range of the melting point (“Tm”) + 10 ° C. to Tm + 120 ° C. of the polyethylene resin. The melting point can be measured by differential scanning calorimetry (DSC) according to JIS K7121. In one embodiment, especially when the polyolefin resin has a melting point of about 130 to about 140 ° C., the melt-blending temperature is about 140 to about 250 ° C., more preferably about 170 to about 240 ° C.

In order to obtain a good hybrid structure, the concentration of the polyolefin composition in the polyolefin solution is preferably about 25 to about 50% by weight, more preferably about 25 to about 45% by weight based on the weight of the polyolefin solution.

The ratio (L / D) of the screw length (L) to the screw diameter (D) of the twin screw extruder is preferably in the range of about 20 to about 100, more preferably about 35 to about 70. If the L / D is less than 20, melt-blending may be inefficient. If the L / D is greater than 100, the residence time of the polyolefin solution in the twin screw extruder may be too long. In this case, since the molecular weight of a film | membrane falls by excessive shearing and heating, it is not preferable. The cylinder bore of the twin screw extruder is preferably about 40 to about 100 mm.

In the twin screw extruder, the ratio (Q / Ns) of the amount of polyolefin solution (Q, kg / h) to the screw rotation speed (Ns, rpm) is preferably from about 0.1 to about 0.55 kg / h / rpm. . If Q / Ns is less than 0.1 kg / h / rpm, the polyolefin may be damaged by shearing, resulting in lower strength and meltdown temperatures. If Q / Ns is greater than 0.55 kg / h / rpm, uniform blending cannot be achieved. More preferably, the Q / Ns is about 0.2 to about 0.5 kg / h / rpm. Screw rotation speed Ns becomes like this. Preferably it is 180 rpm or more. Although not particularly limited, the upper limit of the screw rotation speed Ns is preferably about 500 rpm.

(2) extrusion

The components of the polyolefin solution can be melt-blended in an extruder and extruded from the die. In other embodiments, the components of the polyolefin solution may be extruded and pelletized. In such embodiments, the pellets can be melt-blended and extruded in a second extrusion to produce a gel-type casting or sheet. In any embodiment, the die may be a sheet-forming die having a rectangular orifice, a double-cylindrical type, a hollow die, an inflation die, and the like. The die gap is not critical, but for sheet-forming dies, the die gap is preferably about 0.1 to about 5 mm. The extrusion temperature is preferably about 140 to about 250 ° C., and the extrusion rate is preferably about 0.2 to about 15 m / min.

(3) forming a cooled extrudate

The extrudate from the die is cooled to form a cooled extrudate, generally in the form of a gel-type casting or sheet. Preferably, the extrudate has a high polyolefin content. Cooling is preferably carried out to at least the gelling temperature at a cooling rate of at least about 50 ° C./minute. Cooling is preferably carried out up to about 25 ° C. This cooling fixes the micro-phase of the polyolefin separated by the film-forming solvent. In general, slowing the cooling rate can increase the pseudo-cell units of the gel-like sheet and make the higher order structures coarser. Increasing the cooling rate, on the other hand, results in a denser cell unit. If the cooling rate is less than 50 DEG C / min, the degree of crystallinity rises, making it difficult to provide a gel-type sheet suitable for stretching. Cooling methods that can be used include: contacting the extrudate with a refrigerant such as cold air, cooling water, and the like; And a method of contacting the extrudate with a cooling roller.

By high polyolefin content is meant that the cooled extrudate comprises at least about 25%, such as from about 25 to about 50%, of polyolefin derived from the resin of the polyolefin composition based on the weight of the cooled extrudate. If the polyolefin content is less than about 25% of the cooled extrudate, it means that it becomes more difficult to form a hybrid microporous membrane having both small and large pores. If the polyolefin content is higher than about 50%, the viscosity is higher, making it more difficult to form the desired hybrid structure. The cooled extrudate preferably has a polyolefin content that is at least as high as that of the polyolefin solution.

(4) drawing the cooled extrudate

The cooled extrudate, usually in the form of gel-shaped castings or sheets of high polyolefin content, is then drawn in one or more directions. While not wishing to be bound by any theory or model, it is believed that the gel-like sheet can be uniformly stretched because it contains a film-forming solvent. The gel-like sheet is drawn at a predetermined magnification after heating, preferably by, for example, a tenter method, a roll method, an inflation method, or a combination thereof. The stretching may be carried out in a uniaxial or biaxial manner, but biaxial stretching is preferred. In the case of biaxial stretching, simultaneous biaxial stretching, sequential stretching or multi-stage stretching (eg, a combination of simultaneous biaxial stretching and sequential stretching) may be used, but simultaneous biaxial stretching is preferred. The amount of stretching does not need to be the same in either direction.

The draw ratio of this first stretching step may be, for example, 2 times or more, preferably 3 to 30 times, in the case of uniaxial stretching. In the case of biaxial stretching, the draw ratio can be, for example, 3 times or more in any direction, i.e., 9 times or more, preferably 16 times or more, more preferably 25 times or more, for example 49 times or more, in area magnification. Examples for this first stretching step will include stretching from about 9 times to about 400 times. Further examples will be about 16 to about 49 fold stretching. In addition, the amount of stretching does not need to be the same in either direction. If the area magnification is 9 times or more, the pin puncture strength of the microporous membrane can be improved. When the area magnification is 400 times or more, it may be difficult to deal with stretching apparatuses, stretching operations and the like including large-scale stretching apparatuses.

In one embodiment, the draw temperature in the first draw step is relatively high, preferably from approximately crystal dispersion temperature (“Tcd”) to about Tcd + 30 ° C. of the combined polyethylene content of the cooled extrudate, such as a combination Polyethylene in the range of Tcd to Tcd + 25 ° C, more specifically in the range of Tcd + 10 ° C to Tcd + 25 ° C, most specifically Tcd + 15 ° C to Tcd + 25 ° C. If the stretching temperature is less than Tcd, the softening of the combined polyethylene contents becomes insufficient so that the gel-like sheet is easily broken by the stretching and high-magnification stretching cannot be achieved.

The crystal dispersion temperature is determined by measuring the temperature characteristic of dynamic viscoelasticity according to ASTM D 4065. The combined polyethylene content of the present invention has a crystal dispersion temperature of about 90 to 100 ° C, so that the stretching temperature is 90 to 125 ° C, preferably about 100 to 125 ° C, more preferably 105 to 125 ° C.

The stretching may cause cleavage between polyolefins such as polyethylene lamellar and further refine the polyolefin phases or form a plurality of fibrils. The fibrils form a three-dimensional network structure. It is believed that the stretching improves the mechanical strength of the microporous membrane and the pores thereof expand to produce a microporous membrane suitable for use as a battery separator.

Depending on the desired properties, stretching may be performed by placing the temperature distribution in the film thickness direction, thereby providing a microporous membrane having further improved mechanical strength. The detailed method is described in Japanese Patent No. 3347854.

(5) removal of solvents or diluents

A second solvent, also known as a "wash solvent," is used to remove (wash, wash or dissolve) at least a portion of the solvent or diluent. The polyolefin composition phase is phase-separated from the membrane-forming solvent phase, so removing the solvent or diluent yields a microporous membrane. Removal of the solvent or diluent can be effected by being able to remove one or more suitable washing solvents, ie liquid solvents from the membrane. Examples of washing solvents include volatile solvents such as saturated hydrocarbons such as pentane, hexane, heptane, etc., chlorinated hydrocarbons such as methylene chloride, carbon tetrachloride, etc. ethers such as diethyl ether, dioxane, etc. ketones such as methyl ethyl ketone, linear fluorocarbons such as Trifluoroethane, C ₆ F _14, and the like, cyclic hydrofluorocarbons such as C ₅ H ₃ F ₇ , hydrofluoroethers such as C ₄ F ₉ OCH ₃ , C ₄ F ₉ OC ₂ H _5, and the like, perfluoro Rotors such as C ₄ F ₉ OCF ₃ , C ₄ F ₉ OC ₂ F ₅ , and the like and mixtures thereof.

The washing of the stretched membrane can be carried out by a method of dipping in the washing solvent and / or showering with the washing solvent. The washing solvent used is preferably about 300 to about 30,000 parts by weight based on 100 parts by weight of the stretched membrane. The washing temperature is usually about 15 to about 30 [deg.] C. and may be heated during washing, if necessary. The heating temperature during the washing is preferably about 80 ° C. or less. Washing is preferably carried out until the amount of residual liquid solvent is less than 1% by weight of the amount of liquid solvent present in the polyolefin solution prior to extrusion.

Microporous membranes from which diluents or solvents, such as membrane-forming solvents have been removed, can be dried by heat drying or air drying (eg, air drying with mobile air) to remove residual volatile components such as washing solvents from the membrane. Any drying method may be used which can remove a significant amount of the washing solvent. Preferably substantially all of the washing solvent is removed during drying. The drying temperature is preferably at most Tcd, more preferably at least 5 ° C lower than Tcd. Drying is carried out with respect to 100% by weight of the microporous membrane (based on dry weight) until the residual washing solvent is preferably 5% by weight or less, more preferably 3% by weight or less. If the drying is insufficient, subsequent heat treatment may reduce the porosity of the microporous membrane and result in poor permeability.

(6) stretching of dried membrane

The dried membrane is stretched (re-stretched) at high magnification in at least one axial direction in the second stretching step. Re-stretching of the membrane may be carried out while heating, for example by a tenter method or the like as in the first stretching step. Re-drawing may be uniaxial or biaxial stretching. In the case of biaxial stretching, either simultaneous biaxial stretching or sequential stretching may be used, but simultaneous biaxial stretching is preferred. Since re-stretching is usually carried out on long sheet-like membranes obtained from stretched gel-like sheets, the MD and TD directions in the re-stretching, where MD is the "machine direction", ie processing Means the direction of movement of the membrane in the middle, and TD means the "transverse direction", ie, the direction perpendicular to both the MD and the horizontal plane of the membrane). In the present invention, however, the re-stretching is actually slightly larger than that used in the stretching of the cooled extrudate. The draw ratio in this step is from 1.1 to about 1.8 times in at least one direction, such as from about 1.2 to about 1.6 times. Stretching does not need to have the same magnification in each direction. If the stretching in step (4) of the method of the invention is smaller in the range of about 9 to about 400 times, the stretching in step (6) of the method of the invention should be larger in the range of about 1.1 to about 1.8 times . Similarly, if the stretching in step (4) of the process of the invention is greater in the range of about 9 to about 400 times, the stretching in step (6) of the process of the invention should be smaller in the range of about 1.1 to about 1.8 times do.

Said second stretching or re-drawing is preferably carried out at a second temperature below Tm, more preferably in the range of Tcd to Tm. When the second stretching temperature exceeds Tm, the melt viscosity is generally too low, so that good stretching cannot be performed, and as a result, it is considered that the transmittance is lowered. If the second stretching temperature is less than Tcd, the softening of the polyolefin is insufficient, the film is easily broken by stretching, and cannot be stretched uniformly. In one embodiment, the second stretching temperature is typically about 90 to about 135 ° C, preferably about 95 to about 130 ° C.

As mentioned above, the uniaxial draw ratio of the membrane in this step is preferably about 1.1 to about 1.8 times. Magnifications of 1.1 to 1.8 times generally provide a membrane of the invention having a hybrid structure with a large average pore size. In the case of uniaxial stretching, it can be 1.1-1.8 times in a longitudinal direction or a lateral direction. In the case of biaxial stretching, when the draw ratio in the two directions is 1.1 to 1.8 times, the film can be stretched at the same or different magnification in each stretching direction, but the same is preferable.

If the second draw ratio of the membrane is less than 1.1 times, it is considered that the hybrid structure is not formed and the membrane permeability, electrolyte absorption and compression resistance are poor. When the second draw ratio exceeds 1.8 times, it is considered that the formed fibrils become too fine and the heat shrinkage resistance and electrolyte absorbency of the film decrease. More preferably, the second draw ratio is 1.2 to 1.6 times.

It is preferable that a draw speed is 3% / sec or more in a draw direction. In the case of uniaxial stretching, the stretching speed is preferably 3% / second or more in the longitudinal or transverse direction. In the case of biaxial stretching, the stretching speed is preferably 3% / sec or more in both the longitudinal direction and the transverse direction. If the stretching speed is less than 3% / second, the membrane permeability is reduced, and the film in the transverse direction (in particular, the air permeability) provides a more nonuniform film. The stretching speed is preferably 5% / second or more, more preferably 10% / second or more. Although not particularly limited, the upper limit of the stretching speed is preferably 50% / second to prevent breakage of the film.

(7) heat treatment

The re-stretched film is heat treated (heat-fixed) to stabilize the crystals in the film and to make the lamellae uniform. Heat-fixing is preferably carried out by a tenter method or a roll method. The heat-setting temperature may be, for example, in the range of the second drawing temperature ± 5 ° C. of the membrane, or in the range of the second drawing temperature ± 3 ° C. of the membrane. If the heat-setting temperature is too low, the pin puncture strength, tensile breaking strength, tensile breaking elongation and heat shrinkage resistance of the membrane are lowered, and conversely, if the heat-fixing temperature is too high, the membrane permeability is considered to be lowered.

After the heat-fixing step, an annealing treatment may be performed. Annealing is heat treatment in the absence of a load on the microporous membrane, and can be carried out using a heating chamber having, for example, a belt conveyor or an air-lift heating chamber. The annealing may also be carried out continuously after the heat-fixing treatment and relaxation of the tenter. The annealing temperature is preferably at most Tm, more preferably in the range of about 60 ° C to about Tm-5 ° C. Annealing is believed to provide a microporous membrane with high permeability and strength. Optionally, the membrane is annealed without prior heat-fixing. In one embodiment, the heat-fixing of step (7) is optional.

(8) heat-fix treatment of elongated sheets

The sheet drawn between steps (4) and (5) can be heat-fixed in a range that does not impair the properties of the microporous membrane. The heat-fixing method can be carried out in the same manner as in step (7) above.

(9) hot rolling treatment

After any of the above steps (4) to (7), at least one side of the sheet drawn from step (4) may be contacted with one or more heat rollers. The roller temperature is preferably in the range of Tcd + 10 ° C. to Tm. The contact time of the stretched sheet and the heat roller is preferably from about 0.5 seconds to about 1 minute. The heat roller may have a flat or uneven surface. The heat roller may have a suction function for removing the solvent. Although not particularly limited, one example of a roller-heating system may include holding heated oil in contact with the roller surface.

(10) high temperature solvent treatment

Between steps (4) and (5), the stretched sheet can be in contact with the hot solvent. Hot solvent treatment forms fibrils formed by stretching in the form of leaf veins with relatively thick fiber stems to provide microporous membranes with large pore sizes and moderate strength and permeability. By "lobe vein" it is meant that the fibrils have coarse fiber stems and thin fibers extending from the stems into complex network structures. Details of the high temperature solvent treatment method are described in WO 2000/20493.

(11) Heat-fixing of Membranes Containing Washing Solvents

Between steps (5) and (6), the microporous membrane containing the washing solvent can be heat-fixed to such an extent that it does not impair the properties of the microporous membrane. The heat-fixing method may be the same as described in step (7) above.

(12) cross-linking

Heat-fixed microporous membranes can be cross-linked by ionizing radiation, such as α-rays, β-rays, γ-rays, electron beams, and the like. In the case of electron beam irradiation, the electron dose is preferably about 0.1 to about 100 Mrad and the acceleration voltage is preferably about 100 to about 300 kV. The cross-linking treatment raises the meltdown temperature of the microporous membrane.

(13) hydrophilic treatment

Heat-fixed microporous membranes can be hydrophilized (treatment to make the membrane more hydrophilic). The hydrophilization treatment can be monomer-grafting treatment, surfactant treatment, corona-discharge treatment, or the like. The monomer-grafting treatment is preferably carried out after the cross-linking treatment.

For surfactant treatments that hydrophilize heat-immobilized microporous membranes, any of non-ionic surfactants, cationic surfactants, anionic surfactants and amphoteric surfactants can be used, but non-ionic surfactants. Is preferred. The microporous membrane may be immersed in a solution of a surfactant such as water or a lower alcohol such as methanol, ethanol, isopropyl alcohol, or the like, or may be coated with the solution by a doctor blade method.

(14) surface-coating treatment

Although not required, the heat-fixed microporous membrane formed from step (7) may be prepared with porous polypropylene, porous fluororesins such as polyvinylidene fluoride and polytetrafluoroethylene, porous polyimide, porous polyphenylene sulfide, and the like. Coating can improve the meltdown properties of the membrane used as a battery separator. The polypropylene used in the coating preferably has a Mw of about 5,000 to about 500,000 and a solubility of at least about 0.5 g in 100 g of toluene at 25 ° C. Such polypropylenes more preferably have a racemic diade (structural unit in which two adjacent monomer units are enantiomeric to one another) of about 0.12 to about 0.88. The surface-coating layer may, for example, apply a solution of the coating resin in a suitable solvent, remove a portion of the solvent to increase the resin concentration to form a structure in which the resin phase and the solvent phase are separated, and remove the remainder of the solvent. It can be formed by. Examples of suitable solvents for this purpose include aromatic compounds such as toluene or xylene.

[3] microporous Polyolefin  Structure, properties and composition of the membrane

(1) structure

Microporous membranes prepared by the methods described above have a relatively wide pore size distribution when shown in differential pore volume curves. Pore size distribution can be measured by conventional methods such as, for example, mercury porosimetry.

로 표시되는 미분 기공 부피를 결정할 수 있으며, 여기서 원통형 기공으로 가정시 Vp는 기공 부피이고 r은 기공 반경이다. x 축 상에 기공 직경에 의한 y 축 상에 도시하는 경우의 미분 기공 부피를 통상적으로 "기공 크기 분포"로 칭한다. 하이브리드 구조를 나타내는 막의 경우, 상기 미분 기공 부피의 약 25% 이상, 약 30% 이상, 약 40% 이상, 약 50% 이상, 또는 약 60% 이상이 크기(직경)가 약 100 나노미터 이상인 기공에 관한 것이다. 즉, 기공 직경에 대한

곡선의 경우, 약 100 나노미터 내지 약 1,000 나노미터의 기공 직경의 곡선 아래 면적의 분율은 약 10 나노미터 내지 약 1,000 나노미터의 기공 크기(또는 직경, 실린더형 기공인 경우)의 곡선 아래의 총 면적의 약 25% 이상, 약 30% 이상, 약 40% 이상, 약 50% 이상 또는 약 60% 이상이다. 한 실시양태에서, 약 100 나노미터 내지 약 1,000 나노미터의 기공 직경에 대한 곡선 아래의 면적은 약 10 나노미터 내지 약 1,000 나노미터의 기공 직경에 대한 곡선 아래의 총 면적의 약 25% 내지 약 60%, 약 30% 내지 약 55%, 또는 약 35% 내지 약 50% 범위이다.When mercury porosimetry is used to determine the pore size and pore volume distribution in a membrane, the pore diameter, pore volume and specific surface area of the membrane are typically measured. Using the above measurement method

It is possible to determine the differential pore volume, denoted by, where Vp is the pore volume and r is the pore radius. The differential pore volume when shown on the y axis by the pore diameter on the x axis is commonly referred to as "pore size distribution". In the case of a membrane exhibiting a hybrid structure, at least about 25%, at least about 30%, at least about 40%, at least about 50%, or at least about 60% of the fine pore volume has pores with a size (diameter) of at least about 100 nanometers. It is about. Ie for pore diameter

For the curve, the fraction of the area under the curve of the pore diameter of about 100 nanometers to about 1,000 nanometers is the total under the curve of the pore size (or diameter, for cylindrical pores) of about 10 nanometers to about 1,000 nanometers. At least about 25%, at least about 30%, at least about 40%, at least about 50% or at least about 60%. In one embodiment, the area under the curve for the pore diameter of about 100 nanometers to about 1,000 nanometers is from about 25% to about 60 of the total area under the curve for the pore diameter of about 10 nanometers to about 1,000 nanometers. %, About 30% to about 55%, or about 35% to about 50%.

Although not critical, the dense domains (relative small pores) and the coarse domains (relative large pores) are intertwined irregularly to form a hybrid structure in any cross section of the microporous polyolefin membrane when viewed longitudinally and transversely. The hybrid structure can be observed by transmission electron microscopy (TEM) or the like.

Since the microporous membrane of the present invention has a relatively large internal space and opening due to the coarse domain, it has appropriate permeability and electrolyte absorbency, and there is little change in air permeability during compression. These microporous membranes also have relatively small internal spaces and openings, which affect the safety of the membrane, such as shutdown temperature and shutdown rate, when used as a battery separator. Therefore, a lithium ion battery such as a lithium ion secondary battery including a separator formed by such a microporous membrane maintains high safety while having appropriate productivity and cycle characteristics.

The mercury penetration porosity measurement method used to determine the microporous membrane structure is Pore Sizer 9320 (Micromeritics Company, Ltd.), pressure range from 3.6 kPa to 207 MPa and 15 The use of a cell volume of cm 3. For this measurement, a mercury contact angle of 141.3 and a mercury surface tension of 484 dynes / cm were used. The parameters thus obtained included pore volume, surface area ratio, peak top of pore size, average pore size and porosity. The teachings of this method are described in Raymond P. Mayer and Robert A. Stowe, J. Phys. Chem. 70, 12 (1996); L.C. Drake, Ind. Eng. Chem., 41, 780 (1949); H.L. Ritter and L.C. Drake, Ind. Eng. Chem. Anal., 17, 782 (1945) and E.W. Washburn, Proc. Nat. Acad. Sci., 7, 115 (1921).

에 따른 압력(P)의 함수로서 표시될 수 있다. 이어서, 미분 기공 부피의 측정은 다음과 같은 과정을 밟을 수 있다. 먼저, 기공 내로 침투된 수은의 부피(V)를 압력(P)의 함수로서 측정한다. P의 측정값을 사용하여 상기 기술된 바와 같은 기공 반경(r)을 계산한다. P를 증분적으로 증가시키고, 각각의 P 값에서 수은의 부피를 결정한다. 이런 식으로, 상기 측정을 위해 선택된 P 범위에 의해 결정되는 바와 같은 r의 범위에 대해 도표화한 특정 r의 기공과 기공 부피의 관계를 나타내는 표를 만들 수 있다. 표에서의 r 값을 통상 Log(r)로 전환할 수 있다. 기공 부피(Vp)는 일반적으로 미세다공성 막의 그램당 ㎤로 표시된다.

로 표시되는 미분 기공 부피는 Vp 및 r의 도표 값으로부터 계산될 수 있으며, 여기서 dVp는 표에서 인접하는 Vp 값들 간의 차이에 의해 근사된 것이고, dLog(r)은 표에서 인접하는 Log(r) 값들 간의 차이에 의해 근사된 것이다. 도 5는 하이브리드 구조를 갖는 MPF(실시예 7)에 대한 Vp(기공 부피) 곡선을 나타낸 것이다.Mercury penetration porosity measurements can be briefly summarized as follows. Pressurized mercury is applied to the plane of the microporous membrane at pressure P. This pressure causes the mercury to penetrate the mercury into the pores of the microporous membrane until it reaches equilibrium. At equilibrium, the force F1 on the surface of the mercury to which the pressure P is applied is equal in magnitude to F1 but balanced against the force F2 acting in the opposite direction. See FIG. 4. According to the conventional Washbum model, the amount of work required to move mercury from the surface into the pores (W1 = 2πLγcosθ) is balanced with the work done by the counter force (W2 = Pπr ² L). In these equations, if the pores are cylindrical, L is the depth and r is the radius of the pores. The contact angle of mercury is represented by θ. Mercury's surface tension is denoted by γ and is generally known as 0.48 Nm ⁻¹ . Therefore, according to the washer boom model, the pore radius r is

Can be expressed as a function of pressure P Subsequently, the measurement of the differential pore volume may be performed as follows. First, the volume V of mercury penetrated into the pores is measured as a function of the pressure P. The measured value of P is used to calculate the pore radius r as described above. Increase P incrementally and determine the volume of mercury at each P value. In this way, a table can be made showing the relationship between the pore and pore volume of a particular r tabulated over the range of r as determined by the P range selected for the measurement. The r value in the table can be switched to Log (r) normally. The pore volume Vp is generally expressed in cm 3 per gram of microporous membrane.

The differential pore volume, denoted by, can be calculated from the plot values of Vp and r, where dVp is approximated by the difference between adjacent Vp values in the table, and dLog (r) is the adjacent Log (r) values in the table. It is approximated by the difference between them. 5 shows the Vp (pore volume) curves for MPF (Example 7) with hybrid structure.

Since the microporous membrane has a relatively large internal space and opening due to the coarse domain, it has an appropriate permeability and electrolyte absorbency and little change in air permeability during compression. Thus, lithium ion batteries, such as lithium ion secondary batteries that include separators formed by such microporous membranes, have adequate productivity and cycle characteristics.

In one embodiment, the microporous polyolefin membrane is a single-layer membrane. In other embodiments, the microporous polyolefin membrane is a multi-layer membrane. For example, in one embodiment, the multilayer microporous polyolefin membrane comprises two layers, wherein the first layer (eg, top layer) comprises a first microporous layer material, and the second layer (eg, base) Layer) comprises an independently-selected second microporous layer material. At least one of the first or second microporous layer material is characterized by a hybrid structure, ie the microporous layer material is at least about 25% of the total area under the curve over a pore diameter range of about 10 nm to about 1,000 nm. It is characterized by a differential pore volume curve having an area under the curve over a pore diameter range of about 100 nm to about 1,000 nm. In other embodiments, the microporous membrane is a multilayer microporous membrane comprising three or more layers, wherein the outer layer (also referred to as a "surface" or "shell" layer) comprises a first microporous layer material and The intermediate layer (or inner layer) comprises an independently selected second microporous layer material. The inner layer (s) of the multilayer microporous polyolefin membrane are located between the surface layers, optionally one or more inner layers are in surface contact with one or more surface layers. As in the case of two-layer membranes, one or more layers of the multilayer microporous membrane have the characteristics of a hybrid structure. When the multilayer microporous membrane has three or more layers, the multilayer microporous membrane has one or more layers comprising a first microporous layer material and one or more layers comprising an independently selected second microporous layer material. As will be appreciated by those skilled in the art, such multilayer microporous membranes can be prepared by methods such as lamination, co-extrusion, and the like. The method described in section [2] above is suitable for producing an extrudate or microporous membrane which can be laminated to form a multilayer microporous membrane. Alternatively, the method of section [2] is used together with the co-extrusion die in step (2) of the method to produce a multilayer extrudate, which is then processed according to steps (3) to (7) of the method. By this, a multilayer film can be produced. If desired, the optional steps described in the section above may be used. As discussed, one or more layers of the multilayer microporous membrane have the characteristics of a hybrid structure. The polyolefin solution used to prepare the hybrid-structured layer or layers is as described in section [2] above. The thickness of each layer of the multilayer film is independently selected. The thickness of one layer may be the same as one or more other layers, but is not required.

(2) characteristics

Microporous membranes of the present invention generally have a shutdown temperature in the range of about 130 ° C to about 140 ° C and a meltdown temperature in the range of about 145 ° C to about 200 ° C. In embodiments wherein the microporous membrane contains polyethylene and polypropylene, the meltdown temperature is generally in the range of about 160 ° C to about 200 ° C.

In a preferred embodiment, the microporous membrane has one or more of the following physical properties. A reasonable shutdown temperature and meltdown temperature range is added.

(a) air permeability of 20 to 400 seconds / 100 cm 3 (converted to value at 20 μm thickness)

When the air permeability of the membrane measured according to JIS P8117 is 20 to 400 sec / 100 cm 3, the battery having the separator formed by the microporous membrane has a moderately large capacity and good cycle characteristics. If the air permeability is less than 20 seconds / 100 cm 3, the shutdown does not occur sufficiently because the pores are so large that they are not close enough when the temperature inside the battery rises above 140 ° C. The air permeability P ₁ measured for the microporous membrane having a thickness T ₁ based on JIS P8117 is converted into the air permeability P ₂ at a thickness of 20 μm by the formula P ₂ = (P ₁ × 20) / T ₁ .

(b) 25 to 80% porosity

If the porosity is less than 25%, the microporous membrane is not considered to have good air permeability. If the porosity exceeds 80%, the battery separator formed by the microporous membrane may have insufficient strength, resulting in a short circuit of the battery electrodes.

(c) Pin puncture strength of at least 2,000 mN (converted to a value of 20 μm thick)

The pin puncture strength of the membrane (converted to the value at the film thickness of 20 μm) was punctured by the microporous membrane at a rate of 2 mm / sec with a needle of 1 mm diameter with a spherical end (curvature radius R: 0.5 mm). When measured, the maximum load is displayed. If the pin puncture strength is less than 2,000 mN / 20 μm, a short circuit may occur in a battery having a separator formed by the microporous membrane.

(d) tensile strength at break of 40,000 kPa or more;

Tensile breaking strength of 49,000 kPa or more, measured in both the longitudinal and transverse directions (according to ASTM D-882) is a feature of durable microporous membranes that are particularly suitable when used as battery separators. The tensile strength at break is preferably 80,000 kPa or more.

(e) 100% or more tensile fracture elongation

Tensile elongation of 100% or more, measured both in the longitudinal and transverse directions (according to ASTM D-882), is a characteristic of durable microporous membranes that are particularly suitable when used as battery separators.

(f) heat shrinkage of 12% or less

If the heat shrinkage rate after exposure to 105 ° C. for 8 hours exceeds 12% in both the longitudinal and transverse directions, heat generated in the battery with the microporous separator will shrink the separator and cause a short circuit on the edge of the separator. high portential.

(g) Thickness change of 20% or less after thermal compression

The rate of change of thickness after thermal compression at 90 ° C. for 5 minutes under a pressure of 2.2 MPa is generally 20% or less relative to 100% of the thickness before compression. Batteries including microporous separators with a rate of change of thickness of 20% or less have moderately large capacity and good cycle characteristics.

(h) air permeability of 700 seconds or less after thermal compression

Microporous polyolefin membranes when heat-compressed under these conditions generally have an air permeability (Gurley value) of up to 7OO sec / 1OO cm 3. Batteries using such membranes have moderately large capacity and cycle characteristics. The air permeability is preferably 650 seconds / 1OO cm 3 or less.

(i) surface roughness of at least 3 × 10 ² nm

The surface roughness of the film measured by atomic force microscopy (AFM) in dynamic force mode is generally at least 3 × 10 ² nm (measured as the maximum height difference). The surface roughness of the film is preferably at least 3.5 × 10 ² nm. The measurement can be made using a conventional device, such as model SPA500 available from SII Nano Technology Inc. The maximum height difference is defined as (highest point on surface) minus (lowest point height) over the area of the inspection membrane.

(j) normalized electrolyte absorption rate

Using a dynamic-surface-tension-measuring device (DCAT21 with a high-precision electronic balance available from Eko Instruments Co., Ltd.), the microporous membrane sample was maintained at 18 ° C. (Electrolyte: 1 mol / L LiPF ₆ , Ethylene carbonate / dimethyl carbonate volume ratio: 3/7) and immersed in the formula: [weight increase in microporous membrane (g) / weight of microporous membrane before absorption (g)]. Measure the electrolyte absorption rate. As used herein, the term "normalized" electrolyte absorption rate means that the electrolyte absorption rate is indicated with respect to the electrolyte absorption rate measurement value of the microporous membrane of Comparative Example 1. When the electrolyte absorption rate measured for the microporous membrane having a hybrid structure is divided by the electrolyte absorption rate measured for the membrane of Comparative Example 1, the quotient, that is, the normalized electrolyte absorption rate is greater than one.

(3) Microporous Polyolefin Membrane Compositions

(1) polyolefin

The microporous polyolefin membrane generally comprises a polyolefin used to form a polyolefin solution. Small amounts of washing solvents and / or membrane-forming solvents may also generally be present in amounts of less than 1% by weight, based on the weight of the microporous polyolefin membrane. Small amounts of polyolefin molecular weight reduction can occur during the process, but this is acceptable. In one embodiment, lowering the molecular weight (if any) in the process results in an Mw / Mn value of the polyolefin in the membrane and an Mw / Mn value of the polyolefin solution of about 50% or less, about 10% or less, about 1% or less, or about 0.1%. Make it different as below.

Accordingly, embodiments of the microporous membranes of the present invention comprise (a) a first polyethylene having a weight average molecular weight of about 2.5 × 10 ⁵ to about 5 × 10 ⁵ , and optionally polyolefins (b) and (c), wherein (b) is about 0 to about 7% of the second polyethylene having a weight average molecular weight of about 5 × 10 ⁵ to about 1 × 10 ⁶ and (c) is about 0 of polypropylene having a weight average molecular weight of about 1.5 × 10 ⁶ or less To about 25%.

(a) polyethylene

(i) composition

The first polyethylene can be, for example, a high density polyethylene having a weight average molecular weight of about 2.5 × 10 ⁵ to about 5 × 10 ⁵ and a molecular weight distribution of about 5 to about 100. The first polyethylene of the membrane may be an ethylene homopolymer or an ethylene / α-olefin copolymer containing a small amount such as about 5 mol% of a third α-olefin. The third α-olefin that is not ethylene is preferably propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate or styrene, or Combination of these.

The second polyethylene, for example ultra high molecular weight polyethylene, in the membrane has a weight average molecular weight of greater than 5 × 10 ⁵ . In one embodiment, the second polyethylene has a molecular weight distribution of about 5 to about 100. The Mw of the second polyethylene can range from about 1 × 10 ⁶ to about 15 × 10 ⁶ , about 1 × 10 ⁶ to about 5 × 10 ⁶ , or about 1 × 10 ⁶ to about 3 × 10 ⁶ , for example. Non-limiting examples of the second polyethylene of the membrane are those having a weight average molecular weight of about 5 × 10 ⁵ to about 8 × 10 ⁵ and a molecular weight distribution of about 5 to about 50. The second polyethylene of the membrane may be an ethylene homopolymer or an ethylene / α-olefin copolymer containing a small amount, such as about 5 mol%, of the third α-olefin. The third α-olefin that is not ethylene is preferably propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate or styrene, or Combinations thereof.

(ii) Mw / Mn molecular weight distribution of polyethylene in the microporous membrane

Although not limited, the Mw / Mn of polyethylene in the membrane is preferably about 5 to about 100, such as about 7 to about 50. When Mw / Mn is less than 5, the ratio of high molecular weight components is too high and melt extrusion cannot be performed easily. On the other hand, when Mw / Mn exceeds 100, the proportion of the low molecular weight component is too high to reduce the strength of the resulting microporous membrane. Some degradation of Mw from the starting resin can occur during the preparation of the membranes according to the invention, for example Mw of the first polyethylene in the membrane product is higher than that of the first polyethylene resin in the polyolefin composition portion of the polyolefin solution of process step (1). Can be lower.

(b) polypropylene

Optional polypropylenes in the membrane have a weight average molecular weight of about 3 × 10 ⁵ to about 1.5 × 10 ⁶ , such as 6 × 10 ⁵ to about 1.5 × 10 ⁶ and a heat of fusion of at least 80 J / g, non-limiting examples. And from about 80 to about 120 J / g and a molecular weight distribution of from about 1 to about 100, such as from about 1.1 to about 50, and may be a propylene homopolymer or a copolymer of propylene with another, ie, fourth, olefin, but Polymers are preferred. The copolymer may be a random or block copolymer. The fourth olefin, which is an olefin other than propylene, is an α-olefin such as ethylene, butene-1, pentene-1, hexene-1,4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate, styrene, etc. And diolefins such as butadiene, 1,5-hexadiene, 1,7-octadiene, 1,9-decadiene and the like. The proportion of the fourth olefin in the propylene copolymer is preferably in a range that does not reduce the properties of the microporous polyolefin membrane such as heat resistance, compression resistance, heat shrinkage resistance, etc., preferably less than about 10 mol% such as about 0 to about 10 Less than mole%. In addition, some degradation of Mw from the starting resin can occur during the preparation of the membranes according to the invention, for example, Mw of polypropylene in the membrane product is higher than that of the polypropylene resin in the polyolefin composition portion of the polyolefin solution of process step (1). Can be lower.

The heat of fusion is determined by differential scanning calorimetry (DSC). DSC is performed using data analyzed using TA Instrument MDSC 2920 or Q1000 Tzero-DSC and standard analysis software. Typically, 3-10 mg of polymer is encapsulated in an aluminum pan and loaded into the instrument at room temperature. Samples are cooled to −130 ° C. or −70 ° C. and heated to 210 ° C. at a heating rate of 10 ° C./min to evaluate glass transition and melting behavior for the samples. The sample is held at 210 ° C. for 5 minutes to break its thermal history. Crystallization behavior is evaluated by cooling the sample from the melt to the sub-ambient temperature at a cooling rate of 10 ° C./min. The sample is held at this low temperature for 10 minutes to fully equilibrate in the solid state and achieve steady state. Second heating data is measured by heating this melt crystallized sample at 10 ° C./min. Thus, the second heating data provides phase behavior for the sample crystallized under controlled thermal history conditions. Endothermic melt transitions (first and second melt) and exothermic crystallization transitions are analyzed for transition and onset of peak temperature. The area under the curve is used to determine the heat of fusion ΔH _f .

In one embodiment, the amount of polypropylene in the membrane is 55 wt% or less, 40 wt% or less, or 25 wt% or less, based on the total weight of the polyolefin in the membrane. Too large a proportion of polypropylene in the membrane can result in a lower strength microporous membrane. The proportion of polypropylene is preferably 20% by weight or less, more preferably 15% by weight or less.

(2) other ingredients

In addition to the above components, the membrane may contain additional polyolefins and / or heat resistant polymers having a melting point or glass transition temperature (Tg) of at least about 170 ° C.

(a) additional polyolefins

The additional polyolefins are (a) polybutene-1, polypentene-1, poly-4-methylpentene-1, polyhexene-1, polyoctene-1, where Mw may be 1 × 10 ⁴ to 4 × 10 ⁶ , respectively , Polyvinyl acetate, polymethyl methacrylate, polystyrene and ethylene / α-olefin copolymers and (b) polyethylene wax with Mw of 1 × 10 ³ to 1 × 10 ⁴ . Polybutene-1, polypentene-1, poly-4-methylpentene-1, polyhexene-1, polyoctene-1, polyvinyl acetate, polymethyl methacrylate and polystyrene are not only homopolymers but also other α-olefins. It may be a copolymer containing.

(b) heat resistant polymer

The heat resistant polymer is preferably (i) an amorphous polymer having a melting point of at least about 170 ° C., which may be partially crystalline, and (ii) an amorphous polymer having a Tg of at least about 170 ° C. The melting point and Tg are determined by a differential scanning calorimeter (DSC) in accordance with JIS K7121. Examples of heat resistant polymers include polyesters such as polybutylene terephthalate (melting point: about 160 to 230 ° C.), polyethylene terephthalate (melting point: about 250 to 270 ° C.) and the like, fluororesins, polyamide (melting point: 215 to 265 ° C.). ), Polyarylene sulfide, polyimide (Tg: 280 ° C or more), polyamide imide (Tg: 280 ° C), polyether sulfone (Tg: 223 ° C), polyether ether ketone (melting point: 334 ° C) ), Polycarbonate (melting point: 220 to 240 ° C), cellulose acetate (melting point: 220 ° C), cellulose triacetate (melting point: 300 ° C), polysulfone (Tg: 190 ° C), polyetherimide (melting point: 216 ° C) ), And the like.

(c) content

The total amount of additional polyolefin and the heat resistant polymer in the membrane is preferably 20% by weight or less based on 100% by weight of the membrane.

[4] battery separators

In one embodiment, the battery separator formed from any of the microporous membranes has a thickness of about 3 to about 200 μm, about 5 to about 50 μm or about 10 to about 35 μm, but the most suitable thickness is the type of battery to be manufactured. It may be appropriately selected according to.

[5] batteries

Although not particularly limited, the microporous polyolefin membrane of the present invention can be used in primary and secondary batteries, in particular lithium ion secondary batteries, lithium-polymer secondary batteries, nickel-hydrogen secondary batteries, nickel-cadmium secondary batteries, nickel Can be used as separator for zinc secondary batteries, silver-zinc secondary batteries, in particular lithium ion secondary batteries.

Lithium ion secondary batteries include a cathode and an anode stacked through a separator, which typically contains an electrolyte in the form of an electrolyte ("electrolyte"). The electrode structure is not limited. Ordinary structures are also suitable. The electrode structure may be, for example, a coin type in which a disk-type anode and a cathode face each other, a laminate type in which a planar anode and a cathode are alternately stacked, a winding type in which a ribbon-type anode and a cathode are wound, and the like.

The cathode typically includes a current collector and a positive electrode active material layer formed on the current collector and capable of absorbing and releasing lithium ions. The positive electrode active material may be an inorganic compound such as a transition metal oxide, a composite oxide of lithium and a transition metal (lithium composite oxide), a transition metal sulfide, and the like. The transition metal may be V, Mn, Fe, Co, Ni and the like. Preferred examples of the lithium composite oxide may be lithium nickelate, lithium cobalt acid, lithium manganate, α-NaFeO ₂ -based laminar flow lithium composite oxide, and the like. The anode includes a current collector and a negative electrode active material layer formed on the current collector. The negative electrode active material may be a carbonaceous material such as natural graphite, artificial graphite, coke, carbon black, or the like.

The electrolyte solution may be a solution obtained by dissolving a lithium salt in an organic solvent. Lithium salts include LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiSbF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , Li ₂ B ₁₀ Cl ₁₀ , LiN ( C ₂ F ₅ SO ₂ ) ₂ , LiPF ₄ (CF ₃ ) ₂ , LiPF ₃ (C ₂ F ₅ ) ₃ , lower aliphatic carboxylic acid lithium, LiAlCl ₄ , and the like. These lithium salts can be used alone or in combination. Organic solvents include organic solvents having high boiling point and high dielectric constant such as ethylene carbonate, propylene carbonate, ethylmethyl carbonate, γ-butyrolactone, and the like; And / or organic solvents having low boiling point and low viscosity such as tetrahydrofuran, 2-methyltetrahydrofuran, dimethoxyethane, dioxolane, diethyl carbonate, dimethyl carbonate and the like. These organic solvents may be used alone or in combination. Organic solvents having a high dielectric constant are generally high in viscosity, while organic solvents having a low viscosity have a low dielectric constant, and therefore it is preferable to use a mixture thereof.

When assembling the battery, the separator is impregnated with the electrolyte to provide ion permeability to the separator (microporous polyolefin membrane). Impregnation treatment is usually carried out by immersing the microporous membrane in the electrolyte at room temperature. When assembling a cylindrical battery, for example, a cathode sheet, a microporous separator and an anode sheet are laminated in this order, and the resulting stack is wound to form a wound electrode assembly. The production electrode assembly is inserted / formed into the battery can, and then impregnated in the electrolyte, and a battery lid is manufactured by fastening the battery lid serving as a cathode terminal having a safety valve to the battery can through a gasket.

The present invention will be described in more detail with reference to the following examples, but the scope of the present invention is not limited thereto.

Example  One

(i) 5% by weight ultrahigh molecular weight polyethylene (UHMWPE) having a weight average molecular weight (Mw) of 1.5 × 10 ⁶ and a molecular weight distribution (Mw / Mn) of 8 and (ii) Mw of 3.0 × 10 ⁵ and Mw / Mn of 8.6 Polyethylene (PE) composition comprising 95% by weight of phosphorus high density polyethylene (HDPE), and tetrakis [methylene-3- (3,5-ditert-butyl-4-hydroxyphenyl) -propionate as antioxidant ] 0.2 parts by weight of methane were dry-blended. The polyethylene in the mixture had a melting point of 135 ° C., a crystal dispersion temperature of 100 ° C., and a Mw / Mn of 10.0.

Mw and Mw / Mn of each UHMWPE and HDPE were measured by gel permeation chromatography (GPC) method under the following conditions.

Measuring device: GPC-150C, available from Waters Corporation,

Column: Shodex UT806M, available from Showa Denko K.K.,

Column temperature: 135 ° C.,

Solvent (mobile phase): o-dichlorobenzene,

Solvent flow rate: 1.0 ml / min,

Sample concentration: 0.1% by weight (dissolve at 135 ° C for 1 hour)

Injection volume: 500 μl,

Detector: a parallax refractometer available from Waters Corporation, and

Calibration curve: Generated from calibration curves of monodisperse standard polystyrene samples using predetermined conversion constants.

40 parts by weight of the resulting mixture were added into a steel-blended twin screw extruder with an internal diameter of 58 mm and an L / D of 42, and 60 parts by weight of liquid paraffin (50 cst at 40 ° C.) were fed to the twin screw extruder via a side feeder. The polyethylene solution was prepared by melt blending at 210 ° C. and 200 rpm. This polyethylene solution was extruded from a T-die mounted on the twin screw extruder. The extrudate was cooled while passing through a chill roll controlled at 40 ° C. to form a gel-like sheet.

Using a tenter-stretcher, the gel-like sheet was biaxially stretched simultaneously at 118.5 ° C., 5 times in both the longitudinal and transverse directions, at a stretching rate of 1.0 meter / minute. The stretched gel-like sheet was fixed to a 20 cm × 20 cm aluminum frame, immersed in a methylene chloride bath adjusted to 25 ° C., liquid paraffin was removed while shaking at 100 rpm for 3 minutes, and dried by air flow at room temperature. The dried membrane was re-drawn at a magnification of 1.5 times in the transverse direction at 129 ° C. by a batch stretching machine. The micro-porous polyethylene membrane was prepared by heat-fixing the re-stretched membrane at a batch stretching machine for 30 minutes at 129 ° C.

Example  2

Melting point is 135 ° C. and crystal dispersion temperature is 100 ° C., comprising 2% by weight of UHMWPE with Mw of 2.0 × 10 ⁶ and Mw / Mn 8 and 98% by weight of HDPE with Mw of 2.5 × 10 ⁵ and Mw / Mn of 8.9 And a microporous polyethylene membrane was prepared in the same manner as in Example 1, except that polyethylene having a Mw / Mn of 10.0 was used. In addition, the stretching temperature of the gel-like sheet was 119 ° C, the stretching ratio of the microporous membrane was 1.4 times, and the stretching and heat-fixing temperature of the microporous membrane were both 129.5 ° C.

Example  3

Melting point is 135 ° C. and crystal dispersion temperature is 100 ° C., comprising 2% by weight of UHMWPE with Mw of 2.0 × 10 ⁶ and Mw / Mn 8 and 98% by weight of HDPE with Mw of 3 × 10 ⁵ and Mw / Mn of 8.6 And a microporous polyethylene membrane was prepared in the same manner as in Example 1, except that polyethylene having a Mw / Mn of 10.0 was used. In addition, the draw ratio of the microporous membrane was 1.4 times, and the stretching and heat-fixing temperature of the microporous membrane were both 126 ° C.

Example  4

Melting point is 135 ° C. and crystal dispersion temperature is 100 ° C., which comprises 3% by weight of UHMWPE with Mw of 2.0 × 10 ⁶ and Mw / Mn 8 and 97% by weight of HDPE with Mw of 3.0 × 10 ⁵ and Mw / Mn of 8.6 And a microporous polyethylene membrane was prepared in the same manner as in Example 1, except that polyethylene having a Mw / Mn of 10.5 was used. In addition, the stretching temperature of the gel-type sheet was 117 ° C, the stretched gel-type sheet was heat-fixed at 122 ° C for 10 seconds, and the stretching and heat-fixing temperature of the microporous membrane were both 130 ° C.

Example  5

Using only HDPE having a Mw of 3.0 × 10 ⁵ , a Mw / Mn of 8.6, a melting point of 135 ° C., and a crystal dispersion temperature of 100 ° C. as the polyolefin, the stretching temperature of the gel-type sheet is 118 ° C., and the drawing of the microporous membrane A microporous polyethylene membrane was prepared in the same manner as in Example 1, except that the magnification was 1.4 times.

Example  6

A microporous polyethylene membrane was prepared in the same manner as in Example 1, except that the same polyethylene as in Example 4 was used. In addition, the concentration of polyethylene in the polyethylene solution was 30% by weight based on the weight of the polyethylene solution. In addition, the stretching temperature of the gel-like sheet was 118 ° C, and the stretching ratio of the microporous membrane was 1.4 times.

Example  7

3 wt% UHMWPE with Mw 2.0 × 10 ⁶ and Mw / Mn 8, 92 wt% HDPE with Mw 3.0 × 10 ⁵ and Mw / Mn 8.6, and propylene homopolymer (PP) with Mw 3.0 × 10 ⁵ A microporous polyethylene membrane was prepared in the same manner as in Example 1, except that a polyolefin composition containing 5% by weight was used. The polyethylene had a melting point of 135 ° C., a crystal dispersion temperature of 100 ° C., and a Mw / Mn of 10.5. The concentration of polyolefin in the polyolefin solution was 35% by weight based on the weight of the polyolefin solution, the drawing temperature of the gel-type sheet was 116 ° C, the drawing ratio of the microporous membrane was 1.4 times, the drawing of the microporous membrane and The heat-fixed temperature was 127 ° C. Mw of PP was measured by the same GPC method.

Comparative example  One

Melting point is 135 ° C. and crystal dispersion temperature is 100 ° C., comprising 20% by weight UHMWPE with Mw of 2.0 × 10 ⁶ and Mw / Mn 8 and 80% by weight of HDPE with Mw of 3.5 × 10 ⁵ and Mw / Mn of 8.6 And a microporous polyethylene membrane was prepared in the same manner as in Example 1, except that polyethylene having a Mw / Mn of 14.4 was used. The concentration of polyethylene in the polyethylene solution was 30% by weight, the stretching temperature of the gel-type sheet was 115 ° C, the microporous membrane containing the washing solvent was heat-fixed at 126.8 ° C for 10 seconds, and the microporous membrane was drawn And not heat-fixed.

Comparative example  2

Melting point 135 ° C. and crystal dispersion temperature 100 ° C. comprising 2 wt% UHMWPE with Mw of 2.0 × 10 ⁶ and Mw / Mn 8 and 98 wt% of HDPE with Mw of 3.5 × 10 ⁵ and Mw / Mn of 8.6 And a microporous polyethylene membrane was prepared in the same manner as in Example 1, except that polyethylene having a Mw / Mn of 10.0 was used. In addition, the concentration of the polyethylene solution was 30% by weight, the stretching temperature of the gel-type sheet was 118 ° C, and the microporous membrane was heat-fixed without stretching at 128 ° C for 10 seconds.

Comparative example  3

Melting point is 135 ° C. and crystal dispersion temperature is 100 ° C., comprising 3% by weight of UHMWPE with Mw of 2.0 × 10 ⁶ and Mw / Mn 8 and 97% by weight of HDPE with Mw of 3.5 × 10 ⁵ and Mw / Mn of 8.6 And a microporous polyethylene membrane was prepared in the same manner as in Example 1, except that polyethylene having a Mw / Mn of 10.5 was used. In addition, the concentration of the polyethylene solution was 30% by weight, the stretching temperature of the gel-type sheet was 115 ° C, the microporous membrane was stretched 2.0 times at 130 ° C, and the heat-fixing temperature was 126 ° C.

Comparative example  4

A microporous polyethylene membrane was prepared in the same manner as in Example 1, except that the same polyethylene as in Comparative Example 1 was used. In addition, the concentration of the polyethylene solution was 30% by weight, the drawing temperature of the gel-type sheet was 118 ° C, the microporous membrane was drawn 1.4 times at 130 ° C, and the heat-setting temperature was 130 ° C.

Comparative example  5

Melting point 135 ° C. and crystal dispersion temperature 100 ° C. comprising 2 wt% UHMWPE with Mw of 2.0 × 10 ⁶ and Mw / Mn 8 and 98 wt% of MPE of 2.0 × 10 ⁵ and Mw / Mn of 8.9 And a gel-like sheet was formed in the same manner as in Example 1, except that polyethylene having a Mw / Mn of 10.1 was used. This gel-like sheet fractured when drawn 5 times at 125 ° C. simultaneously in both the longitudinal and transverse directions.

Comparative example  6

A microporous polyethylene membrane was prepared in the same manner as in Example 1, except that the same polyethylene as in Comparative Example 5 was used. In addition, the stretching temperature of the gel-type sheet was 119 ° C, the microporous membrane was drawn 1.4 times at 129.5 ° C, and the heat-fixing temperature was 120 ° C.

Comparative example  7

A microporous polyethylene membrane was prepared in the same manner as in Example 1, except that the same polyethylene as in Example 2 was used. In addition, the stretching temperature of the gel-type sheet was 119 ° C, the microporous membrane was drawn 1.4 times at 115 ° C, and the heat-fixing temperature was 134 ° C.

The properties of the microporous membranes obtained in Examples 1 to 7 and Comparative Examples 1 to 7 were measured by the above method. The results are shown in Table 1 below. Regarding the microporous membranes of Examples 3 and 7, and Comparative Example 1, their pore size distribution curves obtained by mercury permeation porosity measurement are shown in FIGS. 2 to 4.

(1) Average thickness (㎛)

The thickness of each microporous membrane was averaged by measuring with a contact thickness meter at 5 cm longitudinal intervals over 30 cm width.

(2) Air permeability (seconds / 100 cm 3/20 μm)

The air permeability P ₁ measured for each microporous membrane having a thickness T _{1 according} to JIS P8117 was converted into air permeability P ₂ at a thickness of 20 μm by the formula P ₂ = (P ₁ × 20) / T ₁ . .

(3) Porosity (%)

It was measured by gravimetric method.

(4) pin puncture strength (mN / 20 μm)

The maximum load when each microporous membrane of thickness T ₁ was punctured at a speed of 2 mm / sec with a needle having a diameter of 1 mm with a spherical surface (curvature radius R: 0.5 mm) was measured. The measured maximum load L ₁ was converted to the maximum load L ₂ at a thickness of 20 μm by the formula L ₂ = (L ¹ × 20) / T1.

(5) tensile breaking strength and tensile breaking elongation

These were measured on 10 mm wide longitudinal test specimens in accordance with ASTM D882.

(6) heat shrinkage (%)

The thermal shrinkage was determined by averaging and averaging the shrinkage of each microporous membrane in both the longitudinal and transverse directions when exposed to 105 ° C. for 8 hours.

(7) Thickness change rate after thermal compression (%)

Microporous membrane samples were placed between a pair of highly flat plates and heat-compressed with a press machine at 90 ° C. for 5 minutes under a pressure of 2.2 MPa (22 kgf / cm 2) to determine the average thickness in the same manner as above. Thickness change rate was computed by Formula: (Average thickness after compression-average thickness before compression) / (average thickness before compression) x100.

(8) air permeability after thermal compression (seconds / 100 cm 3/20 μm)

Each microporous membrane of thickness T ₁ was heat-compressed under the above conditions and measured for air permeability P ₁ in accordance with JIS P8117. The measured air permeability P ₁ was converted into air permeability P ₂ at a thickness of 20 μm by the formula P ₂ = (P ₁ × 20) / T ₁ .

(9) electrolyte absorption rate

Using a dynamic-surface-tension-measuring device (DCAT21 with a high-precision electronic balance available from Eco Instruments Co., Ltd.), the microporous membrane sample was held at 18 ° C. in an electrolyte solution (electrolyte: 1 mol / L of LiPF). ₆ , solvent: 3/7 volume ratio of ethylene carbonate / dimethyl carbonate), the electrolyte solution absorption rate was determined by the formula: [weight increase in microporous membrane (g) / weight of microporous membrane before absorption (g)]. The electrolyte absorption rate was expressed as a relative value assuming the electrolyte absorption rate in the microporous membrane of Comparative Example 1 as 1.

(10) pore size distribution

는 실시예 3의 막에 대해서는 도 1에, 실시예 7의 막에 대해서는 도 2에, 비교예 2의 막에 대해서는 도 3에 나타내었다.The pore size distribution of the microporous membrane was determined by mercury penetration porosity measurement using the method described in section [3] above. Differential pore volume as a function of pore size

1 is shown in FIG. 1 for the film of Example 3, FIG. 2 for the film of Example 7, and FIG. 3 for the film of Comparative Example 2. FIG.

(11) surface roughness

The maximum height difference of the surface measured by AFM in the dynamic force mode (DFM) was used as the surface roughness.

TABLE 1

은 100 nm 내지 1,000 nm의 기공 크기를 갖는 상당한 개수의 기공의 존재를 나타낸다. 더욱이, 도 1 및 2로부터 알 수 있는 바와 같이, 약 100 나노미터 내지 약 1,000 나노미터의 기공 직경에 대한 곡선 아래의 면적 분율은 약 10 나노미터 내지 약 1,000 나노미터의 기공 직경에 대한 곡선 아래의 총 면적의 25% 이상이다. 비교예 2의 막은 하이브리드 구조를 갖지 않으며, 도 3에 나타낸 바와 같이, 약 100 나노미터 내지 약 1,000 나노미터의 기공 직경에 대한 곡선 아래의 면적 분율은 약 10 나노미터 내지 약 1,000 나노미터의 기공 직경에 대한 곡선 아래의 총 면적의 20%보다 현저히 낮다.As shown in Table 1, each of the microporous membranes of Examples 1 to 7 has the characteristics of a hybrid structure. Ie differential pore volume curve

Indicates the presence of a significant number of pores having a pore size of 100 nm to 1,000 nm. Moreover, as can be seen from FIGS. 1 and 2, the area fraction below the curve for the pore diameter of about 100 nanometers to about 1,000 nanometers is below the curve for the pore diameter of about 10 nanometers to about 1,000 nanometers. More than 25% of the total area. The membrane of Comparative Example 2 does not have a hybrid structure, and as shown in FIG. 3, the area fraction under the curve for the pore diameter of about 100 nanometers to about 1,000 nanometers has a pore diameter of about 10 nanometers to about 1,000 nanometers. It is significantly lower than 20% of the total area under the curve for.

The microporous membranes of Examples 1 to 7 have not only proper electrolyte absorbency but also adequate air permeability, pin puncture strength, tensile breaking strength, tensile breaking elongation and heat shrinkage rate, and the thickness and air permeability change after thermal compression are small.

Comparative Example 1 was not as good as Examples 1 to 7 in air permeability, air permeability after thermal compression, and electrolyte absorbency. This is considered to be because the microporous membrane of Comparative Example 1 was obtained from a polyethylene composition containing at least 7% by weight of UHMWPE.

Comparative Example 2 was not as good as Examples 1-7 in fin puncture strength, air compression after thermal compression and electrolyte absorbency. This is considered to be because the microporous membrane was not drawn.

The membranes of Comparative Examples 3 and 4 were not as good as Examples 1 to 7 in electrolyte absorbency. This is considered to be because the draw ratio of the microporous membrane was 1.8 times or more in Comparative Example 3, and a polyethylene composition containing 7% by weight or more of UHMWPE was used in Comparative Example 4.

The microporous membrane of Comparative Example 6 was not as good as Examples 1 to 7 in pin puncture strength, tensile fracture strength, tensile fracture elongation and heat shrinkage rate. This is believed to be because the gel-like sheet had a drawing temperature that was too high and the heat-setting temperature was lower than the drawing temperature of the microporous membrane minus 5 ° C.

아래의 총 면적은 0.56이다. 100 nm 내지 1,000 nm에 대한 곡선 아래의 면적은 0.13이다. 이들 면적의 비는 약 0.23이므로, 곡선 아래 면적의 약 23%는 100 nm 내지 1,000 nm 크기 범위의 기공들과 관계된다.FIG. 6 shows differential pore volume curves for microporous membranes (comparative) that do not have a hybrid structure. The membrane contains 30% by weight ultra high molecular weight polyethylene and 70% by weight high density polyethylene. In the preparation of this membrane, the cooled extrudate was biaxially stretched at a temperature of 95 ° C., but the membrane was not subjected to the second stretching step. Differential pore volume curve for 1 nm to 1,000 nm

The total area below is 0.56. The area under the curve for 100 nm to 1,000 nm is 0.13. Since the ratio of these areas is about 0.23, about 23% of the area under the curve is related to pores ranging in size from 100 nm to 1,000 nm.

FIG. 7 shows the differential pore volume curves for microporous membranes with a hybrid structure. The membrane contains 2% by weight ultra high molecular weight polyethylene, 93% by weight high density polyethylene, and 5% by weight polypropylene. In the preparation of this membrane, the cooled extrudate was biaxially stretched at a temperature of 95 ° C. and the membrane was subjected to a second stretching step at a draw ratio of 1.4 times. The heat set temperature was 127.5 ° C.

Claims (25)

Differential pore volume comprising polyethylene and having an area under the curve for a pore size range of 100 nm to 1,000 nm that is at least 25% of the total area under the curve for a pore size range of 10 nm to 1,000 nm A microporous membrane having a curve, wherein the dense and coarse domains are irregularly entangled to form a hybrid structure in the cross section of the microporous polyolefin membrane when viewed in the longitudinal and transverse directions. The method of claim 1,
7% by weight of the polyethylene (a) a first polyethylene having a weight average molecular weight of 1 × 10 ⁴ to 5 × 10 ⁵ , and (b) a second polyethylene having a weight average molecular weight of more than 5 × 10 ⁵ (based on the weight of the microporous membrane) ) A microporous membrane comprising: The method according to claim 1 or 2,
A microporous membrane, further comprising polypropylene. The method of claim 3, wherein
Wherein the amount of polypropylene in the microporous membrane is 25% by weight or less based on the weight of the microporous membrane. The method according to claim 1 or 2,
A microporous membrane, wherein the microporous membrane has a surface roughness of 3 × 10 ² nm or more. The method of claim 2,
The first polyethylene is a high density polyethylene having a molecular weight distribution of 5 to 100, and the second polyethylene is an ultrahigh molecular weight polyethylene having a weight average molecular weight of greater than 1 × 10 ⁶ . The method of claim 3, wherein
The microporous membrane of which said polypropylene has a weight average molecular weight of 3 * 10 <^5> -1.5 * 10 <^6> . The method of claim 3, wherein
A microporous membrane wherein the polypropylene is characterized by one or more of the following:
(i) heat of fusion of 80 J / g or more,
(ii) molecular weight distribution 1-100,
(iii) isotactic tacticity,
(iv) a melting peak (second melt) of at least 160 ° C.,
(v) a Trouton ratio of at least 15 measured at a temperature of 230 ° C., or
(vi) a tensile viscosity of at least 50,000 Pa · sec at a temperature of 230 ° C. and a strain rate of 25 sec ⁻¹ . A battery comprising an anode, a cathode, an electrolyte in contact with the anode and the cathode, and at least one separator disposed between the anode and the cathode and comprising the microporous membrane of claim 1. A multilayer microporous membrane having a first layer and at least a second layer comprising the microporous membrane of claim 1. delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete

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