阅读说明：本技术 无纺布及电池用隔膜 (Nonwoven fabric and battery separator ) 是由 田中尚树 目黑祐树 于 2018-05-18 设计创作，主要内容包括：提供一种无纺布及电池用隔膜,该无纺布即使厚度变薄,处理性也良好,并且具有与以往的产品同等或更高的拉伸强度。对于通过由芯部(2)与覆盖该芯部(2)的鞘部(3)形成的芯鞘结构的复合纤维(1)构成的无纺布而言,由第1聚烯烃系树脂形成复合纤维(1)的芯部(2),并且由熔点比第1聚烯烃系树脂低的第2聚烯烃系树脂形成复合纤维(1)的鞘部(3),将多个复合纤维(1)通过使其鞘部(3)彼此互相熔接而一体化,将单位面积重量设为3g/m<Sup>2</Sup>以上,将通过下述算式(I)算出的变形量S设为50mm<Sup>2</Sup>以下。S(mm<Sup>2</Sup>)＝(MD<Sub>1</Sub>-MD<Sub>0</Sub>)×(CD<Sub>0</Sub>-CD<Sub>1</Sub>)…(I)。(Provided are a nonwoven fabric and a battery separator, wherein the nonwoven fabric has good handleability even when the thickness is reduced, and has tensile strength equal to or higher than that of a conventional product. A nonwoven fabric comprising a core-sheath composite fiber (1) composed of a core part (2) and a sheath part (3) covering the core part (2), wherein the core part (2) of the composite fiber (1) is formed of a 1 st polyolefin resin, the sheath part (3) of the composite fiber (1) is formed of a 2 nd polyolefin resin having a lower melting point than the 1 st polyolefin resin, and a plurality of composite fibers (1) are integrated by mutually welding the sheath parts (3) thereof, and the unit area weight is setIs 3g/m 2 The deformation S calculated by the following formula (I) was set to 50mm as described above 2 The following. S (mm) 2 )＝(MD 1 ‑MD 0 )×(CD 0 ‑CD 1 )…(I)。)

1. A nonwoven fabric comprising a core-sheath structure of composite fibers each composed of a core and a sheath covering the core and integrated by fusing the sheath to each other, the nonwoven fabric having a thickness of 60 [ mu ] m or less,

the composite fiber has a fiber diameter of 3.4 to 4.5 [ mu ] m, a core part formed of a 1 st polyolefin resin, and a sheath part formed of a 2 nd polyolefin resin having a melting point lower than that of the 1 st polyolefin resin,

the weight per unit area of the non-woven fabric is 3-20 g/m²，

Effective length for MD Direction (MD)₀) Is 100mm, effective length in CD Direction (CD)₀) The length in the MD direction of a 100mm sample when a tensile stress of 20N is applied in the MD direction in an environment of room temperature 25 ℃ and humidity 50% RH is defined as the MD direction length₁(mm) and the length in the CD direction is set as CD₁(mm), the deformation S calculated by the following formula (I) is 50mm²In the following, the following description is given,

S(mm²)＝(MD₁-MD₀)×(CD₀-CD₁)…(I)。

2. the nonwoven fabric according to claim 1, wherein,

the 1 st polyolefin resin of the conjugate fiber is a polypropylene resin,

the 2 nd polyolefin resin is a polyethylene resin,

the strength of the composite fiber is 4cN/dTex or more.

3. The nonwoven fabric according to claim 1 or 2, having a maximum pore size of 50 μm or less and an average pore size of 5 to 25 μm.

4. The nonwoven fabric according to any one of claims 1 to 3, which has a tensile strength per unit area weight in the CD direction and the MD direction of 0.6(N/15 mm)/(g/m)²) The above.

5. A battery separator using the nonwoven fabric according to any one of claims 1 to 4.

6. The battery separator according to claim 5, wherein the battery separator is immersed in an electrolyte solution in which LiPF is dissolved in a solvent in which ethylene carbonate and methyl ethyl carbonate are mixed at a volume ratio of ethylene carbonate to methyl ethyl carbonate of 3: 7, and the film resistance value measured by sandwiching the battery separator with an aluminum plate is 0.002 to 0.007 Ω/μm₆And adjusted to 1.1 mol/L.

Technical Field

The present invention relates to a nonwoven fabric and a battery separator using the same. More specifically, the present invention relates to a nonwoven fabric and a battery separator using a composite fiber having a core-sheath structure.

Background

Composite fibers having a core-sheath structure formed using a thermoplastic resin are integrated by heating and fusing the fibers to each other, and therefore, nonwoven fabrics can be produced without using an adhesive or the like, and are mainly used as materials for nonwoven fabrics for sanitary goods and food fields. On the other hand, a composite fiber using a polyolefin resin for the core portion and the sheath portion is also used as a battery separator because of its excellent chemical resistance.

On the other hand, a separator for a battery needs to be further thinned in order to improve battery performance, but in the case of a nonwoven fabric, there is a problem that sufficient strength, handleability, and the like cannot be obtained when the thickness is made thin. Therefore, a technique of reducing the thickness of a nonwoven fabric without lowering the mechanical strength thereof has been studied (for example, see patent documents 1 and 2).

Patent document 1 proposes a nonwoven fabric that can maintain sufficient strength even when made thin, by using polyphenylene sulfide (PPS) fibers having a single fiber fineness of 0.5dtex or less and a tensile strength of 3.0cN/dtex or more. Patent document 2 proposes a nonwoven fabric that can achieve both air permeability and mechanical strength even when the thickness is 20 μm or less by using a combination of cellulose fibers having an average fiber diameter of 0.1 to 50 μm and polyolefin fibers having an average fiber diameter of 1.5 μm or less.

Disclosure of Invention

Problems to be solved by the invention

However, the aforementioned prior art has the following problems. First, since the nonwoven fabric described in patent document 1 uses PPS fibers having a high melting point and 285 ℃, it is necessary to secure a heat application time by lowering a linear velocity while adjusting a dryer to a high temperature in order to fuse the fibers in a papermaking step by a wet method. Therefore, the nonwoven fabric described in patent document 1 has lower productivity than a nonwoven fabric using raw material fibers made of a polyolefin resin. Further, the nonwoven fabric described in patent document 1 cannot be expected to have a function (shutdown function) of stopping ion conductivity before thermal runaway of the battery, and therefore, the nonwoven fabric has insufficient properties as a battery separator.

On the other hand, since the nonwoven fabric described in patent document 2 uses cellulose fibers having a hydrophilic surface, moisture is easily absorbed, and when the nonwoven fabric is used for a separator of a nonaqueous battery, a careful drying treatment is required before mounting. In addition, the ultrafine polyolefin fibers produced by the pulverization treatment described in patent document 2 fibrillate, and the coarse fineness fibers and the ultrafine fineness fibers are mixed. Therefore, the nonwoven fabric described in patent document 2 is likely to vary in pore size, thickness, smoothness, and the like, and this effect becomes remarkable when the thickness is reduced. Thus, a nonwoven fabric having sufficient strength even with a thickness of 60 μm or less, suitable for a battery separator, and excellent in handling properties has not yet been realized.

Accordingly, the present invention aims to: provided are a nonwoven fabric and a battery separator, which have good handleability even when the thickness is reduced and have tensile strength equal to or higher than that of conventional products.

Means for solving the problems

The nonwoven fabric of the present invention is a nonwoven fabric in which a composite fiber having a core-sheath structure comprising a core part and a sheath part covering the core part is passed through the sheath partA core part formed of a 1 st polyolefin resin and a sheath part formed of a 2 nd polyolefin resin having a melting point lower than that of the 1 st polyolefin resin, the core part having a fiber diameter of 3.4 to 4.5 μm, the sheath part being fused and integrated with each other and having a thickness of 60 μm or less; the weight per unit area of the non-woven fabric is 3-20 g/m²(ii) a Effective length for MD Direction (MD)₀) Is 100mm, effective length in CD Direction (CD)₀) The length in the MD direction of a 100mm sample when a tensile stress of 20N is applied in the MD direction in an environment of room temperature 25 ℃ and humidity 50% RH is defined as the MD direction length₁(mm) and the length in the CD direction is set as CD₁(mm), the deformation S calculated by the following equation 1 is 50mm²The following.

[ mathematical formula 1]

S(mm²)＝(MD₁-MD₀)×(CD₀-CD₁)

As the conjugate fiber, a conjugate fiber in which the 1 st polyolefin resin is a polypropylene resin, the 2 nd polyolefin resin is a polyethylene resin, and the strength is 4cN/dTex or more can be used.

The nonwoven fabric of the present invention has, for example, a maximum pore diameter of 50 μm or less and an average pore diameter of 5 to 25 μm.

The nonwoven fabric of the present invention may have a tensile strength per unit area weight in the CD direction and/or MD direction of 0.6(N/15 mm)/(g/m)²) The above.

The battery separator of the present invention uses the aforementioned nonwoven fabric.

The battery diaphragm can be used for setting the membrane resistance value of 0.002-0.007 omega/mum measured by being clamped by an aluminum plate after being immersed in the following electrolyte, wherein ethylene carbonate and methyl ethyl carbonate are used as the ethylene carbonate according to the volume ratio: ethyl methyl carbonate ═ 3: 7 in a solvent containing a mixture of the above components₆And adjusted to 1.1 mol/L.

In the present invention, "weight per unit area" is a mass per unit area, "CD direction" is a width direction of the nonwoven fabric, and "MD direction" is a machine (length) direction of the nonwoven fabric. The "effective length" is the length of a portion actually subjected to evaluation, and does not include a portion held by a jig or the like.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, since the composite fiber having a core-sheath structure made of a polyolefin resin is used, the basis weight and the deformation amount S are set within specific ranges, and therefore, a nonwoven fabric and a battery separator having tensile strength equivalent to or higher than those of conventional products can be obtained even when the thickness is reduced.

Drawings

Fig. 1 is a cross-sectional view showing a composite fiber structure constituting a nonwoven fabric according to embodiment 1 of the present invention.

Detailed Description

Hereinafter, specific embodiments of the present invention will be described in detail with reference to the accompanying drawings. The present invention is not limited to the embodiments described below.

(embodiment 1)

The nonwoven fabric according to embodiment 1 of the present invention is obtained by integrating composite fibers having a core-sheath structure made of a polyolefin resin by welding sheath portions thereof to each other, and has a basis weight of 3g/m²Above, the deformation amount is 50mm²The following.

[ conjugate fiber ]

Fig. 1 is a sectional view showing a composite fiber structure constituting the nonwoven fabric of the present embodiment. As shown in fig. 1, a composite fiber 1, which is a nonwoven fabric raw fiber of the present embodiment, is a fiber having a so-called "core-sheath structure" in which a core portion 2 and a sheath portion 3 covering the core portion 2 are integrated. The core-sheath structure of the composite fiber 1 may be an eccentric core structure. Since the composite fiber of the core-sheath structure can change the fusion area between the fibers and the fiber diameter by adjusting the ratio (cross-sectional area ratio) of the sheath to the core, the characteristics such as strength, pore diameter, and thickness of the nonwoven fabric can be easily changed by using the composite fiber of the core-sheath structure as a raw material.

The core portion 2 and the sheath portion 3 of the composite fiber 1 are formed of polyolefin resins having different melting points, and the sheath portion 3 is formed of a polyolefin resin (also referred to as a "2 nd polyolefin resin") having a melting point lower than that of the polyolefin resin (also referred to as a "1 st polyolefin resin") constituting the core portion 2.

The kind of the polyolefin resin constituting the core portion 2 and the sheath portion 3 is not particularly limited, but for example, the 1 st polyolefin resin constituting the core portion 2 may be a polypropylene resin, and the 2 nd polyolefin resin constituting the sheath portion 3 may be a polyethylene. Since the composite fibers produced by the combination have appropriate differences in melting points, it is possible to produce a nonwoven fabric in which the sheath portions 3 are easily welded to each other and which has excellent strength. The 1 st polyolefin-based resin and the 2 nd polyolefin-based resin may be a polymer alloy obtained by mixing 2 or more kinds of polyolefins, and various additives such as an antioxidant, a neutralizer, a light stabilizer, a lubricant, and an antistatic agent may be blended.

The composite fiber 1 preferably has a fiber strength of 4cN/dTex or more and a heat shrinkage at 120 ℃ of 4 to 8%. When the fiber strength and the heat shrinkage rate of the composite fiber 1 are set within the above ranges, excellent characteristics can be obtained when the composite fiber is used for a battery separator. When the heat shrinkage ratio of the conjugate fiber 1 at 120 ℃ is less than 4%, the fiber may not be sufficiently stretched to obtain a desired fiber strength, and when the heat shrinkage ratio exceeds 8%, the fiber may be deformed or short-circuited due to an increase in temperature in the battery when used as a battery separator.

On the other hand, the length (cut length) of the conjugate fiber 1 may be appropriately set according to the properties required for the nonwoven fabric, but when used as a battery separator, it may be set in a range of, for example, 0.7mm to 3 mm. By setting the cut length of the conjugate fiber 1 as a raw material fiber within this range, entanglement of the fibers in the papermaking step can be suppressed, and a high-strength non-uniform nonwoven fabric can be produced. In addition, from the viewpoint of improving strength and reducing deformation, it is preferable to use a composite fiber having a fiber diameter of 3.4 to 4.5 μm for the composite fiber 1.

[ weight per unit area ]

The nonwoven fabric of the present embodiment has a weight per unit area(mass per unit area) was 3g/m²The above. Weight per unit area of less than 3g/m²In the case, sufficient strength cannot be obtained, and the handling property is lowered. In addition, the amount of deformation during stretching also increases, and the pore diameter also becomes excessively large, so that when used in a battery separator, the risk of short-circuiting or the like due to the contact of the electrode through holes increases. On the other hand, the upper limit of the basis weight of the nonwoven fabric is not particularly limited, but is preferably 20g/m from the viewpoint of ease of production²The following.

[ thickness ]

The thickness of the nonwoven fabric of the present embodiment is not particularly limited, but is preferably 60 μm or less from the viewpoint of ease of production and ensuring battery characteristics when applied to a battery separator.

[ amount of deformation ]

The nonwoven fabric of the present embodiment has a deformation amount S of 50mm calculated by the following equation 2²The following. In addition, "MD" in the following equation 2₀'and' CD₀"the effective lengths of the specimen before applying tensile stress in the MD direction and the CD direction, respectively, are 100mm in both the MD direction and the CD direction. In addition, "MD₁'and' CD₁"the effective lengths (mm) in the MD direction and the CD direction of the sample measured in an environment at room temperature of 25 ℃ and a humidity of 50% RH in a state where a tensile stress of 20N is applied in the MD direction are" described below ".

[ mathematical formula 2]

S(mm²)＝(MD₁-MD₀)×(CD₀-CD₁)

The deformation S calculated by the above equation 2 is more than 50mm²In the case, the amount of deformation with respect to tensile stress increases, the risk of contact between electrodes when used in a battery separator increases, and the yield at the time of mounting decreases. On the other hand, the deformation amount S of the nonwoven fabric is set to 50mm²Hereinafter, even in a thin nonwoven fabric having a low basis weight, various practical rigidities can be obtained.

[ tensile Strength ]

The nonwoven fabric of the present embodiment is preferably at least one of in the CD direction and the MD directionThe tensile strength per unit area weight was 0.6(N/15 mm)/(g/m)²) The above. Thus, even a thin nonwoven fabric having a thickness of 60 μm or less has a tensile strength equivalent to or higher than that of conventional products, and a battery separator having a smaller thickness and excellent performance than conventional separators can be realized. Such a thin and strong nonwoven fabric is not limited to a battery separator, but can be used in various fields such as filters for water treatment and air treatment, membrane filters, reinforcing materials, and base fabrics for electromagnetic wave shielding.

[ pore diameter ]

In particular, when the nonwoven fabric of the present embodiment is used for a battery separator, it is preferable that the maximum pore diameter is 50 μm or less and the average pore diameter is 5 to 25 μm from the viewpoint of improving battery characteristics. When the maximum pore diameter exceeds 50 μm, the active materials of the electrodes may come into contact with each other through the pores of the nonwoven fabric, and the risk of short-circuiting may increase.

On the other hand, from the viewpoint of reducing the risk of short circuit, it is preferable that the maximum pore diameter of the nonwoven fabric is smaller, but in order to make the average pore diameter less than 5 μm, it is necessary to increase the weight and density of the nonwoven fabric per unit area, or to break the pores by heat pressing or the like to bring the nonwoven fabric into a state close to a film. These methods are disadvantageous in terms of manufacturing cost, since they increase the film resistance to degrade the battery characteristics. By setting the average pore diameter within the range of 5 to 25 μm, a nonwoven fabric suitable as a battery separator can be obtained. The "maximum pore diameter" and the "average pore diameter" of the nonwoven fabric can be measured by, for example, a gas permeation method or a pore measurement method.

As described above in detail, since the nonwoven fabric of the present embodiment uses the composite fiber having the core-sheath structure made of the polyolefin resin and the tensile strength in the unit area weight and the CD direction is set to a specific value or more, even if the nonwoven fabric is thin with a thickness of 60 μm or less, the nonwoven fabric can obtain a tensile strength equivalent to or higher than that of a conventional product with a thickness of about 100 μm. The nonwoven fabric of the present embodiment is suitable for a battery separator because of its excellent strength even when the thickness is small, and the use of the nonwoven fabric can improve the handling property in the production of a battery.

(embodiment 2)

A battery separator according to embodiment 2 of the present invention uses the nonwoven fabric of embodiment 1, for example, a nonwoven fabric as a base material, and an insulating layer containing 1 or more kinds of inorganic particles is provided on at least one surface of the nonwoven fabric. The insulating layer may be formed in a state in which inorganic particles are laminated on the surface of the nonwoven fabric substrate via a binder resin or the like, or may be formed in a state in which the inorganic particles are incorporated into a part of the nonwoven fabric substrate. The insulating layer is not necessarily required for the separator, and only a nonwoven fabric may be used as the separator without providing an insulating layer.

The inorganic particles contained in the insulating layer may have insulating properties, and common inorganic particles such as boehmite particles, alumina particles, silica particles, and the like may be used. However, when the electrolyte contains a fluorine-based substance, the use of silica particles may generate hydrogen fluoride and cause erosion of the insulating layer, and therefore, boehmite particles or alumina particles are preferably used.

The thickness of the insulating layer is not particularly limited, and may be appropriately set according to the kind, performance, size, and the like of the battery, but when the insulating layer is thick, the air permeation resistance is lowered, and when the insulating layer is thin, short-circuiting due to dendrite is likely to occur. Accordingly, the thickness of the insulating layer is preferably 2 to 20 μm, and more preferably 4 to 8 μm, from the viewpoint of improving the air permeation resistance and preventing the generation of internal short circuits.

The battery separator of the present embodiment has a film resistance value of 0.002 to 0.007 Ω/μm, measured by being sandwiched between aluminum plates after being immersed in an electrolyte solution in which ethylene carbonate and ethyl methyl carbonate are ethylene carbonate in a volume ratio of: ethyl methyl carbonate ═ 3: 7 in a solvent containing a mixture of the above components₆And adjusted to 1.1 mol/L. By setting the membrane resistance value of the separator within this range, good battery characteristics can be obtained.

Since the nonwoven fabric of embodiment 1 is used for the battery separator of the present embodiment, the tensile strength is high and the handling property is excellent even when the nonwoven fabric is thin. Further, since the battery separator of the present embodiment has a low film resistance value, it is expected to improve battery characteristics when applied to a lithium ion secondary battery or the like. In particular, in a battery using Lithium Titanate (LTO) as a negative electrode, since the operating potential is much higher than the deposition potential of lithium metal, there is no possibility that the separator is punctured due to the formation of lithium dendrite and short-circuited due to the puncture, and therefore, it is not necessary to laminate an insulating layer or the like on a nonwoven fabric. As a result, the battery separator using the nonwoven fabric of embodiment 1 can contribute to a battery having a high input/output characteristic with a reduced internal resistance, while reducing the cost.