Nonwoven fabric

Abstract

To provide a nonwoven fabric that is constituted of ultra fine fibers and has stretchability.SOLUTION: A nonwoven fabric is constituted of a fiber including polyolefin resin and having 5 μm or less of a median fiber diameter and satisfies the following physical properties (a) and (b): (a) viscosity in a molten state at 200°C is 15 Pa s or less at 0.1 s-1 of shearing speed; and (b) storage rigidity modulus is lower than loss rigidity modulus in a temperature range of 200°C or less and 50°C or more and the storage rigidity modulus is higher than the loss rigidity modulus in a temperature range of 38°C or less and 10°C or more on dynamic viscoelasticity curve obtained by setting a temperature falling rate at 4°C / minute from the molten state at 200°C.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a nonwoven fabric. [Background technology]

[0002] Nonwoven fabrics are used in a variety of applications, taking advantage of their excellent properties such as breathability and flexibility. In addition, technologies have been proposed to impart elasticity to nonwoven fabrics, either alone or in combination with other materials, to enhance their applicability to disposable diapers, sanitary products, sanitary masks, bandages, supports, medical tapes, and the like. As an example of a stretchable nonwoven fabric, Patent Document 1 describes a multi-layer stretchable nonwoven fabric in which a sticky meltblown nonwoven fabric is laminated and bonded onto a non-sticky meltblown nonwoven fabric. Both of these meltblown nonwoven fabrics are made of thermoplastic elastomers and have stretchability. This multi-layer stretchable nonwoven fabric is said to be able to suppress the phenomenon of sticking to manufacturing equipment without reducing the stretchability, even if it has stickiness during production. Furthermore, Patent Document 2 describes a nonwoven fabric that is mainly made of thermoplastic resin fibers and has different elongation rates in one direction and in the direction perpendicular thereto. According to Patent Document 2, by controlling the elongation rate in this way, a nonwoven fabric that is stretchable only in one direction can be obtained without subsequent processing of the nonwoven fabric.

[0003] Nonwoven fabrics made by depositing ultrafine fibers (fiber diameter of 5 μm or less, for example) have been attracting attention in recent years. As an example of a spinning technique for uniformly manufacturing nonwoven fabrics made of such fine fibers, a melt spinning method such as an electrospinning method is used. Nonwoven fabrics containing such ultrafine fibers are expected to be used for various purposes in the future, and intensive studies are being conducted for industrialization. For example, the nanofiber sheet described in Patent Document 3 is intended to be attached to the skin for cosmetic purposes. Patent Document 3 describes a technique for controlling the rigidity and slipperiness of the nanofiber sheet itself against the skin, from the viewpoint of maintaining the smooth state of the nanofiber sheet itself and maintaining the ability to conceal wrinkles on the skin even if wrinkles occur on the skin surface due to changes in facial expression, etc. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 10-237752 [Patent Document 2] Japanese Patent Application Publication No. 9-279460 [Patent Document 3] Patent Publication No. 2021-54734 Summary of the Invention [Problem to be solved by the invention]

[0005] There has been a demand for imparting elasticity to nonwoven fabrics made of ultrafine fibers as described in Patent Document 3 in order to further improve adhesion to objects such as the skin. However, nonwoven fabrics made of ultrafine fibers are not strong enough due to the fineness of the constituent fibers, and there is room for improvement in imparting the desired elasticity.

[0006] The present invention relates to a nonwoven fabric which is made of ultrafine fibers and exhibits elasticity. [Means for solving the problem]

[0007] As a result of extensive investigations, the inventors have found that by employing ultrafine fibers containing a polyolefin resin as the constituent fibers of a nonwoven fabric, and controlling the physical properties of the nonwoven fabric or its constituent fibers so that the nonwoven fabric or the fibers exhibit a specific low viscosity in a molten state at a specific high temperature, and that in the dynamic viscoelasticity curve obtained upon cooling from this molten state, the temperature range where the storage modulus (G') is greater than the loss modulus (G'') is within a specific low temperature range, a nonwoven fabric that is composed of ultrafine fibers and exhibits desired excellent stretchability can be realized by a manufacturing method via electrospinning. The present invention was completed based on these findings and through further investigations.

[0008] The present invention provides a nonwoven fabric composed of fibers containing a polyolefin resin and having a median fiber diameter of 5 μm or less. The nonwoven fabric of the present invention preferably satisfies the following physical properties (a) and (b). (a) Viscosity in the molten state at 200°C at a shear rate of 0.1 s -1 is less than or equal to 15 Pa·s; (b) In a dynamic viscoelasticity curve obtained from a molten state of 200°C at a cooling rate of 4°C / min, the storage modulus is lower than the loss modulus in the temperature range of 200°C to 50°C, and the storage modulus is higher than the loss modulus in the temperature range of 38°C to 10°C. Furthermore, the nonwoven fabric of the present invention preferably has a hysteresis loss of 85% or less when elongated by 30%. The nonwoven fabric of the present invention preferably has a breaking elongation of 90% or more. Furthermore, the nonwoven fabric of the present invention preferably has a tensile strength per meter width normalized by basis weight according to the following formula of 2 N·m / g or more and 4 N·m / g or less. [Tensile strength normalized by basis weight] = {[Tensile strength of test piece (N)] / [Test piece width (m)]} / [Basis weight (g / m 2 )]

[0009] The present invention also provides a method for producing a nonwoven fabric, comprising the steps of spinning a molten material into fibers having a median fiber diameter of 5 μm or less by an electrospinning method, and collecting the fibers with a collector having a surface temperature set to 40° C. or higher and 70° C. or lower. The molten material preferably contains a polyolefin resin. Moreover, the melt preferably satisfies the following physical properties (a) and (b). (a) Viscosity in the molten state at 200°C at a shear rate of 0.1 s -1 is less than or equal to 15 Pa·s; (b) In a dynamic viscoelasticity curve obtained from a molten state of 200°C at a cooling rate of 4°C / min, the storage modulus is lower than the loss modulus in the temperature range of 200°C to 50°C, and the storage modulus is higher than the loss modulus in the temperature range of 38°C to 10°C. Effect of the Invention

[0010] The nonwoven fabric of the present invention exhibits desired excellent stretchability even though it is composed of ultrafine fibers. The nonwoven fabric can be suitably produced by the method for producing the nonwoven fabric of the present invention. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a photograph substituted for a drawing, showing an example of an observation image used when measuring the ratio of the number of fusion-bonded points to the number of fiber intersections. [Diagram 2] FIG. 2 is a graph showing dynamic viscoelasticity curves obtained by molten each of the nonwoven fabrics of Example 1 at 200° C. and cooling the fabric at a rate of 4° C. / min. [Diagram 3] FIG. 3 is a graph showing dynamic viscoelasticity curves obtained by cooling each of the nonwoven fabrics of Example 2 in a molten state at 200° C. at a temperature decreasing rate of 4° C. / min. [Figure 4] FIG. 4 is a graph showing dynamic viscoelasticity curves obtained by placing each nonwoven fabric of Comparative Example 2 in a molten state at 200° C. and lowering the temperature at a rate of 4° C. / min. [Diagram 5] FIG. 5 is a graph showing dynamic viscoelasticity curves obtained by placing each of the nonwoven fabrics of Comparative Example 3 in a molten state at 200° C. and lowering the temperature at a rate of 4° C. / min. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] A preferred embodiment of the nonwoven fabric of the present invention will now be described. The present invention provides a nonwoven fabric composed of fibers containing a polyolefin resin and having a median fiber diameter of 5 μm or less. The nonwoven fabric of the present invention preferably satisfies the following physical properties (a) and (b). (a) Viscosity in the molten state at 200°C at a shear rate of 0.1 s -1 is less than or equal to 15 Pa·s; (b) In a dynamic viscoelasticity curve obtained from a molten state of 200°C at a cooling rate of 4°C / min, the storage modulus is lower than the loss modulus in the temperature range of 200°C to 50°C, and the storage modulus is higher than the loss modulus in the temperature range of 38°C to 10°C.

[0013] The fibers (constituent fibers) constituting the nonwoven fabric preferably contain a polyolefin resin. The fibers constituting the nonwoven fabric may be composed of polyolefin resin alone, or may contain, in addition to polyolefin resin, resins other than polyolefin resin within a range that does not impair the effects of the present invention. In addition, various additives may be contained as necessary. Such additives may include one or more selected from surfactants, silicone oils, modified silicone oils, waxes, and inorganic salts. It is preferable that the nonwoven fabric contains polyolefin resin as the main material (substance) in the fibers constituting the nonwoven fabric, and the content is preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, even more preferably 92% by mass or more, and even more preferably 94% by mass or more. Furthermore, based on the resin components constituting the nonwoven fabric, it is preferable that all of the resin components are polyolefin resins (all of the polymers constituting the nonwoven fabric are polyolefins, i.e., the content is 100% by mass). The content of the thermoplastic resin in the resin component can be measured by the following method. Specifically, the resin component is subjected to various analyses such as NMR (nuclear magnetic resonance) analysis and IR (infrared spectroscopy) analysis, and the structure of the molecular skeleton and the functional group structure at the end of the molecular structure are identified based on the positions of each signal and spectrum obtained by these analyses. This identifies the type of resin contained. The amount of the thermoplastic resin contained in the resin component is calculated from the intensity of the measured value indicating the molecular structure corresponding to each thermoplastic resin. The calculated values ​​are then summed up to measure the content of the thermoplastic resin contained in the resin composition.

[0014] The polyolefin resin preferably contains one or more selected from an olefin homopolymer and / or an olefin copolymer. An olefin homopolymer means a polymer of one type of olefin. An olefin copolymer refers to a copolymer of an olefin and an olefin having a different chemical structure (e.g., an α-olefin having 3 or more carbon atoms, such as propylene, 1-octene, or 1-butene), and a copolymer of an olefin and a compound having a carbon-carbon double bond other than an olefin (e.g., a vinyl compound, such as vinyl acetate or vinyl alcohol). Among them, the above-mentioned polyolefin resin preferably contains one or more selected from polypropylene resins and ethylene-α-olefin copolymer resins (α-olefins having 3 or more carbon atoms, including propylene) from the viewpoint of easily changing the crystallinity by controlling the stereoregularity during synthesis without complexly changing the monomer structure, and polypropylene resin is more preferable. The type of resin contained in the resin component is identified according to the above-mentioned measurement method.

[0015] The melting point of the polyolefin resin is not particularly limited as long as it is in a molten state at 200° C. The melting point of the polyolefin resin is preferably 180° C. or lower, more preferably 170° C. or lower, and even more preferably 160° C. or lower. The melting point of the polyolefin resin is usually 50° C. or higher, and preferably 60° C. or higher.

[0016] The constituent fibers of the nonwoven fabric include ultrafine fibers, which mean fibers having a fiber diameter of 5 μm or less. Such ultrafine fibers can be formed by electrospinning, as described below. From the viewpoint of improving the adhesion of the nonwoven fabric to the surface on which it acts, the median fiber diameter of the nonwoven fabric is preferably 3 μm or less, more preferably 2 μm or less, and even more preferably 1 μm or less. From the viewpoint of improving the function of retaining and fixing the functional liquid on the acting surface of the nonwoven fabric and the strength of the nonwoven fabric, the median fiber diameter of the nonwoven fabric is preferably 0.05 μm or more, more preferably 0.1 μm or more, even more preferably 0.3 μm or more, even more preferably 0.4 μm or more, and particularly preferably 0.5 μm or more.

[0017] <Method of measuring median fiber diameter> (1) Cut the nonwoven fabric to be measured to a size of 10 mm x 10 mm. This is attached to a scanning electron microscope sample stage (manufactured by Oken Shoji Co., Ltd.) using conductive carbon double-sided tape (manufactured by Oken Shoji Co., Ltd.). (2) The sample stage with the nonwoven fabric attached is placed in a sputtering device (Ion Sputter E-1030, Hitachi High-Tech Corporation), and platinum-palladium (Pt-Pd) is evaporated under an argon gas atmosphere at a reduced pressure of 6 Pa. The distance between the nonwoven fabric mounting surface and the Pt-Pd electrode is 30 mm, the evaporation time is 80 seconds, and the evaporation current is 30 mA. (3) The sample stage is placed in a scanning electron microscope (SEM) (Hitachi High-Tech Corporation, S-4300SE / N) and images are obtained in high-resolution mode (accelerating voltage: 5 kV, work distance: 10 mm, observation magnification 500x or 1000x). Images are obtained from a total of 15 locations on the same sample by changing the observation location. (4) From the observation image acquired in (3), the fiber diameter of the nonwoven fabric is measured using image analysis software (WinRooF2015 manufactured by Mitani Shoji Co., Ltd.). From the measured fiber diameters of a total of 600 fibers, the number average diameter, the several 10% diameter (D10) (from the thin diameter side), the several 50% diameter (median diameter), and the several 90% diameter (D90) are tallied. Of these, the median diameter is taken as the representative value of the fiber diameter.

[0018] The nonwoven fabric of the present invention preferably satisfies the above physical properties (a) and (b). Note that, "the nonwoven fabric satisfies the above physical properties (a) and (b)" is synonymous with "the molten material in which the nonwoven fabric is dissolved or the molten material before spinning satisfies the above physical properties (a) and (b)."

[0019] (Physical properties (a)) The nonwoven fabric of the present invention has a viscosity of 0.1 s when melted at 200°C. -1 By exhibiting such a viscosity, the constituent fibers of the nonwoven fabric of the present invention can be ultrafine fibers having a median fiber diameter of 5 μm or less. The viscosity in the molten state at 200°C is set at a shear rate of 0.1 s in order to draw the molten liquid with less energy during melt spinning. -1 The viscosity is preferably 15 Pa·s or less, more preferably 13 Pa·s or less, and even more preferably 10 Pa·s or less. In addition, the viscosity is set at a shear rate of 0.1 s in order to prevent the molten liquid from being torn off when stretched during melt spinning. -1 The viscosity is preferably 3 Pa·s or more, more preferably 4 Pa·s or more, and even more preferably 5 Pa·s or more.

[0020] <Method for measuring viscosity in a molten state at 200℃> The nonwoven fabric to be measured is cut to an arbitrary size so that the mass is 20 g. The nonwoven fabric is placed in a frame with a thickness of 1 mm and a hole with a length of 130 mm and a width of 80 mm, and is heat-pressed at 200°C and cooled-pressed at 15°C to obtain a resin press plate with a size of 130 mm x 80 mm x 1 mm. A specimen is obtained by cutting this plate to a size of 50 mm x 50 mm x 1 mm with a cutter. The specimen is set on a rotating disk-type jig (parallel plates, diameter: 50 mm, distance between plates: 1 mm) installed on a rotational rheometer (model number: MCR302) manufactured by Anton Paar. Before setting the specimen, the rotating disk-type jig is heated to 200°C in advance, and after the specimen is set, it is melted by the heat of the plates so that it completely fills the set geometric dimensions of the jig (diameter: 50 mm, distance between plates: 1 mm). The molten liquid from the specimen that protrudes from the set geometric dimensions of the jig is collected using a stainless steel spatula. After placing the sample in the above jig, it is enclosed in a dedicated heat-retention chamber (CTD 450 / TD Ready manufactured by Anton Paar). The inside of the heat-retention chamber is filled with heated nitrogen, and the temperature inside the chamber is kept stable at 200 °C. Then, the shear rate is reduced to 0.1 s -1 Rotate the upper fixture of the jig at 0.1 s for at least 100 s. -1 A shear rate of 100 seconds is applied to the specimen. The viscosity at 100 seconds from the start of measurement is taken as the measured value. When the sheets are laminated, the fiber layers are peeled off, and the nonwoven fabric to be measured is taken out and subjected to the above-mentioned operation. This procedure is similar for other measurements.

[0021] (Physical properties (b)) In the nonwoven fabric of the present invention, in a dynamic viscoelasticity curve obtained by cooling from a molten state at 200°C at a temperature decreasing rate of 4°C / min, it is preferable that the storage rigidity modulus (G') is lower than the loss rigidity modulus (G'') at least in the temperature range of 200°C or lower and 50°C or higher. It is also preferable that the storage rigidity is higher than the loss rigidity at least in a temperature range of 10° C. or higher to 38° C. In the temperature range of 10° C. or higher to 38° C., the storage rigidity is 10 5 Pa or more 10 8It is more preferable that the viscosity is equal to or less than Pa. That is, the nonwoven fabric of the present invention has a lower temperature range in the dynamic viscoelasticity curve where the values ​​of the storage modulus and loss modulus are reversed, compared to resins that have been used as constituent fibers of conventional nonwoven fabrics. This means that the dynamic viscoelasticity profile exhibits a rubbery state at the low temperature range (for example, a specific example shown in FIG. 2). In the dynamic viscoelasticity characteristic of such unique physical properties, a nonwoven fabric in which the relationship between the storage modulus and the loss modulus is controlled to the physical property (b) means that the ultrafine fibers have elasticity in a rubbery state. In addition, it means that the nonwoven fabric of the present invention has fusion points (hereinafter also referred to as crosslinking points) at the intersections of the ultrafine fibers. This means that the nonwoven fabric of the present invention can be stretched due to the elasticity of the ultrafine fibers, and moreover, the nonwoven fabric of the present invention exhibits a shrinking behavior because breakage during stretching is unlikely to occur. This means that the manufacturing process of the nonwoven fabric of the present invention will be as follows: That is, even if the temperature is not raised too much, the ultrafine fibers are likely to fuse together at their intersections in a flowing state while maintaining the shape of the ultrafine fibers. At the stage of forming a nonwoven fabric, the ultrafine fibers have elasticity in the above-mentioned rubber state, and the density of the crosslinking points of the ultrafine fibers is increased to form a finer fiber network structure, thereby achieving the desired excellent elasticity. The nonwoven fabric of the present invention is more likely to exhibit good stretchability under the temperature conditions in which it is used in daily life due to the dynamic viscoelasticity characteristic of property (b), and therefore can be used in a variety of applications as a stretchable nonwoven fabric made of ultrafine fibers. In the present invention, the nonwoven fabric "has elasticity" means that the hysteresis loss when the nonwoven fabric is stretched by 30% is 85% or less. The hysteresis loss will be described in detail later.

[0022] <Method of measuring storage modulus and loss modulus (dynamic viscoelasticity curve)> A resin press plate is obtained from the nonwoven fabric in the same manner as in the above <Method for measuring viscosity in a molten state at 200°C>. A specimen is cut from this plate with a cutter to 50 mm x 50 mm x 1 mm thick (specimen type A), and a specimen is cut to 10 mm x 10 mm x 1 mm thick (specimen type B). Specimen type A For specimen type A, set it on a rotating disk-type jig attached to a rotational rheometer in the same manner as in the above <Method for measuring viscosity in a molten state at 200°C>. Surround it in an insulation chamber and wait until the temperature inside the insulation chamber stabilizes at 200°C with heated nitrogen. Next, set the strain amplitude to 2% (strain amplitude: 0.02) and the angular frequency to 6.28 rad / s (1 Hz). Perform dynamic viscoelasticity measurement at a temperature drop rate of 4°C / min in any temperature range from 200°C to 50°C to obtain the storage modulus (G') and loss modulus (G"). For specimens that may overload the detector of the rotational rheometer due to the effects of hardening associated with the change in physical properties from a fluid state to a crystallization or rubber-like state in the temperature range from 130°C to 50°C, the following should be done: Stop measurement at the temperature where the apparent measured values ​​of storage modulus (G') and loss modulus (G") rise to around 105 Pa. Set up a measurement program to raise the temperature to 200°C within 15 minutes under the application of dynamic strain with a strain amplitude of 0.1% (strain amplitude: 0.001) and angular frequency of 0.1 rad / s. Specimen type B For specimen type B, set it on a rotating disk-type jig (parallel plates, diameter: 8 mm, distance between plates: 1 mm) attached to the above-mentioned rotational rheometer. Surround it in an insulation chamber and wait until the temperature inside the insulation chamber stabilizes at 200°C with heated nitrogen. Next, set the angular frequency to 6.28 rad / s (1 Hz) so that the strain amplitude can be switched from 10% (strain amplitude 0.1) to 0.1% (strain amplitude 0.001) during the measurement as described below, and perform dynamic viscoelasticity measurement at a temperature drop rate of 4°C / min in the temperature range from 200°C to -25°C to obtain the storage modulus (G') and loss modulus (G"). Regarding the switching of strain amplitude, for specimens that may overload the detector of the rotational rheometer mentioned above due to the hardening effect associated with the change in physical properties from a fluid state to a crystallization or rubber-like state in the temperature range from 130°C to 40°C, do the following: Set the measurement conditions so that the strain amplitude can be switched to 0.1% (strain amplitude: 0.001) at the temperature where the apparent measured values ​​of the storage modulus (G') and loss modulus (G") rise to around 105 Pa. The temperature at which the strain amplitude is switched is set as follows: The crystallization temperature from the molten state in differential scanning calorimetry for the nonwoven fabric to be measured is obtained through a preliminary experiment. Also, reference is made to catalog data on the physical properties of the resin raw material used for the nonwoven fabric to be measured. The temperature is set based on these values. In addition, after reaching -25°C, a measurement program is set up to raise the temperature to 200°C within 15 minutes. The specimen is quickly heated and melted while applying dynamic strain with a strain amplitude of 0.1% (strain amplitude: 0.001) and an angular frequency of 0.1 rad / s, and measures are taken to reduce overload on the detector of the rotational rheometer. The reason why the same specimens of different sizes, specimen forms A and B, were prepared as described above and set in corresponding rotating disk-type jigs to measure dynamic viscoelasticity is that it is impossible to capture the change in physical properties of the nonwoven fabric of the present invention from a fluid state to a rubbery state, crystalline state, or glassy state using a rotating disk-type jig of one type of size. For the storage modulus (G') and loss modulus (G") obtained for each specimen form, the phase difference between the dynamic strain and dynamic stress on which the data is based and the detected torque value are examined, and after thinning out the data outside the detection limit range, the data are overlaid to obtain a temperature profile of each modulus.

[0023] (hysteresis loss) The nonwoven fabric of the present invention preferably has a hysteresis loss of 85% or less at 30% elongation. In the present invention, the state in which the nonwoven fabric is not stretched is defined as 0% elongation. Therefore, for example, 100% elongation means that the length of the nonwoven fabric is doubled. The same applies to the breaking elongation described below. The hysteresis loss means the rate (%) of mechanical energy loss in one cycle of deformation and recovery when a tensile stress is applied to a nonwoven fabric. The nonwoven fabric of the present invention can exhibit desired stretchability by having a physical property in which the hysteresis loss at 30% elongation is 85% or less. From the viewpoint of improving the shrinkage response when the nonwoven fabric is pulled in a desired direction and then released immediately thereafter, the hysteresis loss is preferably 85% or less, more preferably 80% or less, and even more preferably 75% or less. From the viewpoint of maintaining the ability of the nonwoven fabric to conform to the surface on which it acts, the hysteresis loss is preferably 50% or more, more preferably 60% or more, and even more preferably 70% or more.

[0024] <Method of measuring hysteresis loss> The nonwoven fabric to be measured is cut in any direction to a width of 20 mm and a length of 100 mm, and then subjected to a tensile tester (width between upper and lower chucks: 50 mm). One cycle of 30% elongation to 0% return is performed at a load cell speed of 100 mm / min, and the hysteresis loss is calculated by dividing the area obtained in the stroke-load curve at the time of elongation by the area obtained in the stroke-load curve at 0% return. The measurement temperature is 23°C.

[0025] (Elongation at break) The nonwoven fabric of the present invention preferably has a breaking elongation (breaking elongation) of 90% or more. Although the nonwoven fabric of the present invention uses ultrafine fibers as its constituent fibers, as described above, the density of crosslinking points between fibers in the nonwoven fabric structure can be increased due to the material properties of the constituent fibers, making it less likely to break even when pulled, and achieving good breaking elongation. From the viewpoint of further enhancing the above-mentioned effects, the breaking elongation of the nonwoven fabric of the present invention is preferably 95% or more, more preferably 100% or more, and even more preferably 120% or more. The breaking elongation of the nonwoven fabric of the present invention is preferably 400% or less, more preferably 350% or less, and even more preferably 300% or less, from the viewpoint that, for example, when the nonwoven fabric is laminated with another elastic nonwoven fabric to obtain an elastic laminated nonwoven fabric, the elastic laminated nonwoven fabric stretches as a unit without delamination.

[0026] <Method of measuring breaking elongation> A cut sample of the nonwoven fabric of the same size as in the above <Method of measuring hysteresis loss> is placed in a tensile tester and measured at a pulling speed of 100 mm / min. The measurement temperature is 23°C. Based on this measured value, the breaking elongation is calculated as (length of the sample at break--original length of the sample) / (original length of the sample)×100.

[0027] (Tensile strength) The nonwoven fabric of the present invention preferably has a tensile strength per meter width normalized by basis weight according to the following formula of 2 N·m / g or more and 4 N·m / g or less. [Tensile strength normalized by basis weight] = {[Tensile strength of test piece (N)] / [Test piece width (m)]} / [Basis weight (g / m 2 )] Although the nonwoven fabric of the present invention uses ultrafine fibers as its constituent fibers, as described above, the density of crosslinking points between the fibers in the nonwoven fabric structure can be increased due to the material properties of the constituent fibers, thereby further increasing the tensile strength. The tensile strength normalized by the above basis weight is preferably 2 N·m / g or more, and more preferably 2.5 N·m / g or more, from the viewpoint of maintaining the shape of the nonwoven fabric. The tensile strength normalized by the above basis weight is preferably 4 N·m / g or less, and more preferably 3.5 N·m / g or less, from the viewpoint of improving cuttability in the nonwoven fabric processing step.

[0028] <Method of measuring tensile strength normalized by basis weight> The tensile strength value is measured at the same time as the measurement of the breaking elongation above. The tensile strength value measured at the same time is obtained by dividing the test piece basis weight x test piece width (20 mm in this case). At that time, the unit is switched between mm and m so that the unit becomes N·m / g. The measurement temperature is 23°C. In the measurement of tensile strength normalized by basis weight, the test piece width is not limited to 20 mm, and any test width may be used.

[0029] In the nonwoven fabric of the present invention, the requirements for hysteresis loss, elongation at break, and tensile strength all fall within the specific ranges described above, and the characteristics of each physical property act synergistically to exhibit excellent stretchability and excellent conformability to the target surface.

[0030] Since the nonwoven fabric of the present invention is composed of the ultrafine fibers, the electrospinning method is preferably adopted for preparing the fibers, and the nonwoven fabric is formed directly from the electrospinning process. The nonwoven fabric composed of such ultrafine fibers does not substantially exhibit a strong directional dependency, and satisfies the above-mentioned hysteresis loss, breaking elongation, and tensile strength even when stretched in any direction.

[0031] In the nonwoven fabric of the present invention, the polyolefin resin constituting the fibers preferably has a crystallinity of 10% or less, which makes it easier to achieve physical properties of the resulting nonwoven fabric that satisfy the above physical properties (a) and (b). The crystallinity is practically 10.5% or less. From the viewpoint of improving heat resistance, the crystallinity is more preferably 5% or more, and further preferably 7% or more.

[0032] <Method for measuring the crystallinity of polyolefin resin> (1) Cut the nonwoven fabric to be measured so that it weighs 2 mg. Place it in a sample container for differential scanning calorimeter measurement (a set of sample container for Al (aluminum) autosampler P / N GAA-0065 (container size: diameter 6.8 mm, height 2.5 mm) and cover for Al (aluminum) autosampler P / N GAA0064, manufactured by Hitachi High-Tech Science Corporation) to use it as a specimen. (2) The sample obtained in (1) is placed in a differential scanning calorimeter (DSC7000X, Hitachi High-Tech Science Corporation) and measured from a starting temperature of 20°C to a heating rate of 4°C / min up to 200°C. (3) The measurement profile obtained in (2) above is analyzed using analysis software (TA7000 Standard Analysis Version 10.3, manufactured by Hitachi High-Tech Science Corporation) to identify endothermic peaks associated with the melting of the nonwoven fabric, and the enthalpy of fusion is measured. Note that depending on the type of resin constituting the nonwoven fabric, there may be more than one endothermic peak, and the enthalpy of fusion at all endothermic peaks is measured. The sum of the enthalpy of fusion at all endothermic peaks is taken as the enthalpy of fusion of the nonwoven fabric. (4) The fusion enthalpy of the resin component in the nonwoven fabric is calculated from the fusion enthalpy of the nonwoven fabric obtained in (3) above according to the following formula (I). Hr = Ha × Wa / Wr Formula (I) Hr: Melting enthalpy of the resin component in the nonwoven fabric [J / g] Ha: Melting enthalpy of nonwoven fabric [J / g] Wa: The sum of the mass content of all components of the nonwoven fabric, i.e. 100% Wr: Mass content of resin in nonwoven fabric [%] (5) The degree of crystallinity of the resin component in the nonwoven fabric is calculated by dividing Hr obtained in (4) above by the heat of fusion of perfectly crystalline polypropylene, 209 J / g. Note that the value for the heat of fusion of perfectly crystalline polypropylene is cited from H. Bu, SZD Cheng, B. Wunderlich et al., Makromoleculare Chemie, Rapid Communication, Vol. 9, p. 75 (1988).

[0033] In the nonwoven fabric of the present invention, the proportion of crystalline polyolefin resin in the polyolefin resin constituting the fibers is preferably less than 5 mass %. In the present invention, "crystalline polyolefin" means a polyolefin that crystallizes in a temperature range of 80°C to 130°C when a molten liquid of a polyolefin resin prepared at 180°C or higher is cooled at a temperature drop rate of 4°C / min. The proportion of the crystalline polyolefin resin in the polyolefin resin constituting the fibers is more preferably 3% by mass or less, further preferably 2.5% by mass or less, and is preferably 0% by mass or more. By ensuring that the content falls within these upper limits, the physical properties of the resulting nonwoven fabric can be more easily brought to the desired properties described above.

[0034] <Method for measuring the proportion of crystalline polyolefin resin> The nonwoven fabric to be measured is cut into 10 mm x 10 mm pieces. An infrared spectrum is obtained using a total reflection infrared spectrometer (NICOLET iZ10, manufactured by Thermo Fisher Scientific, Inc., using diamond as an ATR crystal). The polyolefin resin contained in the obtained infrared spectrum is estimated using analysis software (OMNIC9.12.1002 / OMNICMC9.12.928 / OMNIC Atlus9.12.990). For estimation, a certainty rate of 90% or higher is used. Once the polyolefin resin is estimated, a commercially available crystalline polyolefin resin is obtained as a model for the corresponding polyolefin resin. For example, for crystalline polyethylene, obtain high-density polyethylene, Novatec™ HD (product number HY350) manufactured by Japan Polyethylene Co., Ltd., and for crystalline polypropylene, obtain Metocene MF650Y manufactured by Polymirae Inc. These model crystalline polyolefin resins are in the form of pellets or granules as raw materials, so depending on the size of the raw material, they are crushed in a crusher and weighed out at 2 mg. These are subjected to a differential scanning calorimeter in the same manner as in the above-mentioned <Method for measuring crystallinity of polyolefin resin>. Measurement is performed from a starting temperature of 20°C to 200°C at a heating rate of 4°C / min, and then from 200°C to 20°C at a heating rate of 4°C / min, and then differential scanning calorimetry is performed again at a heating rate of 4°C / min from 20°C to 200°C. From the obtained measurement profile, the enthalpy of fusion during the second heating is measured. On the other hand, differential scanning calorimetry is also performed on the nonwoven fabric to be measured under the same conditions as the crystalline polyolefin resin model, and the fusion enthalpy during the second heating is measured. In the differential scanning calorimetry measurement profile of the nonwoven fabric, the fusion enthalpy in the same temperature range as the crystalline polyolefin resin model is considered to be due to crystalline polyolefin, and the proportion of crystalline polyolefin resin is calculated by the following formula (II). [Proportion of crystalline polyolefin resin in nonwoven fabric] = [melting enthalpy of the crystalline polyolefin in the nonwoven fabric] / [melting enthalpy of the crystalline polyolefin resin model itself] Formula (II) In addition, since it is already known that in each comparative example in the present invention, the above-mentioned Metocene MF650Y is the only crystalline polypropylene blended, when formula (II) is applied to, for example, comparative example 1, the melting enthalpy of the nonwoven fabric is 4.7 J / g and the melting enthalpy of the crystalline polypropylene is 100.3 J / g, so the proportion of crystalline polypropylene resin in the nonwoven fabric is calculated to be 5.2%.

[0035] In the nonwoven fabric of the present invention, it is preferable that at 50% or more of the number of fiber intersections, contacting fibers are fused to each other (fused intersections refer to the above-mentioned fusion points). In other words, it is preferable that the ratio of the number of fusion points to the number of fiber intersections is 50% or more. In the nonwoven fabric of the present invention, the proportion of fusion points in the number of fiber intersections is 100% or less, preferably 60% or more, more preferably 70% or more, and even more preferably 80% or more. By ensuring that the fiber density falls within these lower limits, crosslinking points between the fibers in the nonwoven fabric are densely formed, making it possible to achieve higher stretchability even though the nonwoven fabric is composed of ultrafine fibers.

[0036] <Method for measuring the ratio of the number of fusion points to the number of fiber intersections> The same operations as those in (1), (2), and (3) of the above-mentioned <Method of measuring median fiber diameter> are performed, and a scanning electron microscope observation image at a magnification of 1000 times is obtained for each nonwoven fabric. For the same sample, five observation images are obtained at different observation locations, with each location focused on the sample surface side. The image quality is 1280 pixels wide and 960 pixels high. From the obtained observation images, the fiber intersections are marked using image analysis software (WinRooF2015, manufactured by Mitani Shoji Co., Ltd.). The number of fusion points at the fiber intersections is recorded, and the ratio of the number of fusion points to the total number of fiber intersections is calculated using the following formula (III). This evaluation is performed by two people, an observer and a recorder, and the number of fiber intersections is counted by the observer's manual marking process on the image analysis software. Regarding the fusion points, the recorder counts the fiber intersections that the observer has determined to be fusion points according to the definition of fusion points below. Percentage of fusion points [%] = Number of fusion points / Total number of fiber intersections × 100 (Formula (III)) The fiber intersections and the fusion points of the fiber intersections are defined as follows. (Fiber intersection) In scanning electron microscope images of nonwoven fabrics, images that are in focus and in which the width between the boundaries of the fiber shape (fiber width confirmed in the observed image) does not exceed 1 mm when the image itself is enlarged three times (300%) on a 410 mm x 260 mm monitor are selected. Fiber intersections are defined as points where they cross, points where they do not cross but are in contact (contact), or points where one fiber branches off in the middle (e.g., the boxed areas D4, D14, and D1 in Figure 1). For some fibers, two or more fibers shown in the image may be in contact with each other in the whole or part of the image capture area in the longitudinal direction to form a fiber bundle, and these are also defined as fiber intersections (e.g., the boxed area D5 in Figure 1). For example, if there is a point in the observed image where four fibers are lined up without any gaps, the number of fiber intersections is defined as three. In addition, due to the characteristics of nonwoven fabric, there is a depth due to the thickness of the nonwoven fabric, which is the side closer to the detector (surface side in the specimen thickness direction) and the side farther away (sample stage side in the specimen thickness direction) in scanning electron microscope observation. Therefore, even if the fibers appear to cross in the observation image, the fibers may not be in contact with each other. As described above, when the image itself is enlarged three times (300%) on a 410 mm x 260 mm monitor, multiple fibers that are in focus within a range where the width of the boundary line of the fiber shape does not exceed 1 mm are considered to be in an equal positional relationship in the thickness direction (depth) of the specimen. The intersections of the fibers selected in this way are considered to be fiber intersections according to the above definition. (Fusion points at fiber intersections) Among the intersections defined above, the following two are defined as fusion points of fiber intersections. (i) A location where the boundary line between two or more related fibers at the intersection is not clearly visible (for example, the area enclosed by the symbol D4 in FIG. 1). (ii) A portion where the width between the boundaries of one or more fibers associated with the intersection is wider than the width between the boundaries other than the intersection (for example, the enclosed portion D16 in FIG. 1).

[0037] The basis weight of the nonwoven fabric of the present invention is 5 g / m2 from the viewpoint of increasing the capillary force and increasing the strength. 2 More than 10 g / m is preferable. 2 The above is more preferable. The basis weight of the nonwoven fabric of the present invention is 40 g / m from the viewpoint of further improving the softness and adhesion to the target object such as the skin surface. 2 Less than 30 g / m is preferable. 2 The following is more preferred:

[0038] The nonwoven fabric of the present invention is also preferably used in a laminated state with another nonwoven fabric or paper. The "another nonwoven fabric" can be appropriately selected depending on the purpose. For example, it can include one or more selected from a stretch spunbond nonwoven fabric, a non-stretch spunbond nonwoven fabric, a stretch spunlace nonwoven fabric, and a non-stretch spunlace nonwoven fabric. By overlaying the nonwoven fabric of the present invention with "another nonwoven fabric or paper" to form a laminated nonwoven fabric, it is possible to impart properties such as high strength and transferability to a target surface of the nonwoven fabric. When used as a laminated nonwoven fabric, the basis weight of the nonwoven fabric of the present invention is set to 1 g / m2 from the viewpoint of increasing the capillary force and maintaining the strength of the nonwoven fabric of the present invention while ensuring the strength with the other nonwoven fabric or paper. 2 More than 1.5 g / m is preferable. 2 More preferably, 2 g / m 2 The above is more preferable. In order to further improve the softness and adhesion to the skin surface in the laminated state, the basis weight of the nonwoven fabric of the present invention when it is made into the laminated nonwoven fabric is 8 g / m 2 Less than 6 g / m is preferable. 2 Less than 5 g / m is more preferable. 2 The following is even more preferred:

[0039] Next, the method for producing the nonwoven fabric of the present invention will be described. The nonwoven fabric of the present invention can be obtained, for example, by the following production method. The method preferably includes a step of spinning the molten material into fibers having a median fiber diameter of 5 μm or less by a melt electrospinning method, and collecting the fibers with a collector whose surface temperature is set to 40° C. or higher and 70° C. or lower. The melt preferably contains a polyolefin resin, the preferred types of which are the same as those described above. The melt preferably satisfies the following physical properties (a) and (b). (a) Viscosity in the molten state at 200°C at a shear rate of 0.1 s -1 is less than or equal to 15 Pa·s; (b) In a dynamic viscoelasticity curve obtained from a molten state of 200°C at a cooling rate of 4°C / min, the storage modulus is lower than the loss modulus in the temperature range of 200°C to 50°C, and the storage modulus is higher than the loss modulus in the temperature range of 38°C to 10°C.

[0040] In the above-mentioned production method, the explanation of the physical properties (a) and (b) of the nonwoven fabric of the present invention is applied to the physical properties (a) and (b) of the melt containing the polyolefin resin as it is.

[0041] In the above manufacturing method, it is preferable to extrude the melt satisfying the above physical properties (a) and (b) in a fibrous form from a nozzle by electrospinning toward a collector whose surface temperature is set to 40°C or more and 70°C or less. This allows the collected ultrafine fibrous melt to be in a moderately fluid state without being overly fluidized, even if it is ultrafine, and to form good fusion points at the intersections of the fibers while maintaining its fibrous shape. The melt then becomes rubbery with a decrease in temperature and becomes ultrafine fibers with elasticity, and a nonwoven fabric having elasticity can be obtained in which a network with a high density of crosslinking points of the ultrafine fibers is formed. The extruded fibrous melt can be stretched by an electric field to make it ultrafine to 5 μm or less. In this respect, the nonwoven fabric of ultrafine fibers obtained by the conventional electrospinning method does not satisfy both the requirements of the physical properties (a) and (b), and the nonwoven fabric of the present invention is difficult to obtain. Conventionally, because the requirement of the physical property (b) is not satisfied, the fiber-constituting resin is forced to crystallize and harden at a higher temperature range during the cooling step during spinning in the manufacturing process, and the fiber is inevitably made to have no elasticity. Furthermore, conventionally, in the process in which the fibers are accumulated to form a nonwoven fabric, the resin is hardened due to crystallization before accumulation, so that fusion at the fiber intersections during accumulation is difficult to occur. Therefore, when the conventional nonwoven fabric of ultrafine fibers is pulled, the fibers slide along the intersections and break, and there is no recovery force against pulling. In contrast, the nonwoven fabric of the present invention exhibits the above-mentioned physical properties (a) and (b), and thus contains stretchable ultrafine fibers, has a fiber network structure formed by fusion points, and has the desired stretchability.

[0042] The set temperature of the collector may be appropriately set within the above range depending on the physical property (b) and the like, and is preferably 40° C. or higher from the viewpoint of ensuring the fluidity of the resin. From the viewpoint of preventing the material from sticking to the collector, the temperature is preferably 70° C. or lower, more preferably 50° C. or lower, and even more preferably 45° C. or lower. Except for the above points, existing methods can be appropriately applied. It is also preferable to heat the space between the spinning nozzle and the collector to a temperature within the same range as above.

[0043] The melt containing polyolefin resin is preferably a melt of polyolefin resin only, or a mixed melt of polyolefin resin and other components. As the polyolefin resin, it is preferable to use various ones as shown above for the nonwoven fabric. In addition, it is preferable that the melting point of the polyolefin resin is within the range as shown above for the nonwoven fabric. From the viewpoint of making it easier to charge the resin melt more uniformly in the spinning step and from the viewpoint of obtaining finer spun fibers more stably, the melt preferably contains an anionic surfactant. This anionic surfactant is preferably an anionic surfactant having a melting point exceeding 20° C. By using an anionic surfactant having a melting point exceeding 20° C. (being in a solid state at 20° C.), the anionic surfactant can be fed into a mixer without adhering to the inner wall of the feed port of the mixer when mixed with a polyolefin resin pellet raw material at a temperature of 20° C. or higher and 30° C. or lower. The content of the anionic surfactant in the above melt is preferably 10% by mass or less, more preferably 8% by mass or less, and also preferably 7% by mass or less, from the viewpoint of uniformly dispersing the resin melt containing the polyolefin resin in the matrix. Moreover, from the viewpoint of imparting a certain level of electrostatic chargeability to a melt containing a polyolefin resin, the content is preferably 1% by mass or more, more preferably 2% by mass or more, and even more preferably 3% by mass or more.

[0044] In the method for producing the nonwoven fabric of the present invention, when the nonwoven fabric produced by the above-mentioned electrospinning method is laminated on another nonwoven fabric, a means for integrating the two fabrics by embossing can be used.

[0045] In relation to the above-mentioned embodiments, the present invention further discloses the following nonwoven fabric and a method for producing the nonwoven fabric.

[0046] <1> A nonwoven fabric composed of fibers containing polyolefin resin with a median fiber diameter of 5 μm or less and satisfying the following physical properties (a) and (b): (a) Viscosity in the molten state at 200°C at a shear rate of 0.1 s -1 is less than or equal to 15 Pa·s; (b) In a dynamic viscoelasticity curve obtained from a molten state of 200°C at a cooling rate of 4°C / min, the storage modulus is lower than the loss modulus in the temperature range of 200°C to 50°C, and the storage modulus is higher than the loss modulus in the temperature range of 38°C to 10°C.

[0047] <2> A nonwoven fabric composed of fibers containing polyolefin resin with a median fiber diameter of 5 μm or less, with a hysteresis loss of 85% or less at 30% elongation, a breaking elongation of 90% or more, and a tensile strength normalized by basis weight per meter width, calculated according to the formula below, of 2 N m / g to 4 N m / g. [Tensile strength normalized by basis weight] = {[Tensile strength of test piece (N)] / [Test piece width (m)]} / [Basis weight (g / m 2 )] <3> The degree of crystallinity of the polyolefin resin is 5% or more and 10.5% or less, preferably 7% or more and 10% or less. <1> or <2> The nonwoven fabric described in <4> The proportion of crystalline polyolefin resin in the polyolefin resin is 0% by mass or more and less than 5% by mass, preferably 3% by mass or less, and more preferably 2.5% by mass or less. <1> ~ <3> 13. The nonwoven fabric according to any one of claims 1 to 12. <5> The ratio of the number of fusion points to the number of intersections of the fibers is 50% or more and 100% or less, preferably 60% or more, more preferably 70% or more, and even more preferably 80% or more. <1> ~ <4> 13. The nonwoven fabric according to any one of claims 1 to 12.

[0048] <6> The content of the polyolefin resin in the constituent fibers of the nonwoven fabric is 70% by mass or more and 100% by mass or less, preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 92% by mass or more, and particularly preferably 94% by mass or more. <1> ~ <5> 13. The nonwoven fabric according to any one of claims 1 to 12. <7> The polyolefin resin includes one or more selected from an olefin homopolymer and / or an olefin copolymer, preferably one or more selected from a polypropylene resin or a polyethylene-α-olefin copolymer (α-olefin having 3 or more carbon atoms including propylene), more preferably one or more selected from a polypropylene resin. <1> ~ <6> 13. The nonwoven fabric according to any one of claims 1 to 12. <8> The median fiber diameter of the nonwoven fabric is 0.05 μm or more and 3 μm or less, preferably 0.1 μm or more and 2 μm or less, more preferably 0.3 μm or more and 1 μm or less, even more preferably 0.4 μm or more, and even more preferably 0.5 μm or more. <1> ~ <7> 13. The nonwoven fabric according to any one of claims 1 to 12. <9> The viscosity of the nonwoven fabric in a molten state at 200°C is -1 and the viscosity is 3 Pa·s or more and 15 Pa·s or less, preferably 4 Pa·s or more and 13 Pa·s or less, and more preferably 5 Pa·s or more and 10 Pa·s or less. <1> ~ <8> 13. The nonwoven fabric according to any one of claims 1 to 12. <10> The nonwoven fabric has elasticity, and the term "elasticity" means that the hysteresis loss when the nonwoven fabric is stretched by 30% is 85% or less. <1> ~ <9> 13. The nonwoven fabric according to any one of claims 1 to 12.

[0049] <11> The nonwoven fabric has a hysteresis loss of 50% or more and 85% or less, preferably 60% or more and 80% or less, and more preferably 70% or more and 75% or less, at 30% elongation. <1> ~ <10> 13. The nonwoven fabric according to any one of claims 1 to 12. <12> The nonwoven fabric has a breaking elongation of 90% or more and 400% or less, preferably 95% or more and 350% or less, more preferably 100% or more and 300% or less, and even more preferably 120% or more and 300% or less. <1> ~ <11> 13. The nonwoven fabric according to any one of claims 1 to 12. <13> The tensile strength of the nonwoven fabric is 2 N·m / g or more and 4 N·m / g or less, and preferably 2.5 N·m / g or more and 3.5 N·m / g or less. <1> ~ <12> 13. The nonwoven fabric according to any one of claims 1 to 12.

[0050] <14> The nonwoven fabric has a basis weight of 5 g / m 2 More than 40g / m 2 less than 10 g / m 2 More than 30g / m 2 The above-mentioned <1> ~ <13> 13. The nonwoven fabric according to any one of claims 1 to 12.

[0051] <15> The above <1> ~ <14> 10. A laminated nonwoven fabric comprising the nonwoven fabric according to any one of 1 to 9 and another nonwoven fabric or paper. <16> The other nonwoven fabric includes one or more selected from a stretch spunbond nonwoven fabric, a non-stretch spunbond nonwoven fabric, a stretch spunlace nonwoven fabric, and a non-stretch spunlace nonwoven fabric. <15> The laminated nonwoven fabric according to claim 1.

[0052] <17> A method for producing a nonwoven fabric, comprising the steps of spinning a melt containing a polyolefin resin, which satisfies the following physical properties (a) and (b), into fibers having a median fiber diameter of 5 μm or less by an electrospinning method, and collecting the fibers with a collector whose surface temperature is set to 40° C. or more and 70° C. or less: (a) Viscosity in the molten state at 200°C at a shear rate of 0.1 s -1 is less than or equal to 15 Pa·s; (b) In a dynamic viscoelasticity curve obtained from a molten state of 200°C at a cooling rate of 4°C / min, the storage modulus is lower than the loss modulus in the temperature range of 200°C to 50°C, and the storage modulus is higher than the loss modulus in the temperature range of 38°C to 10°C.

[0053] <18> The set temperature of the collector is 40° C. or more and 50° C. or less, preferably 40° C. or more and 45° C. or less. <17> A method for producing the nonwoven fabric according to claim 1.

[0054] <19> The melt contains an anionic surfactant having a melting point of more than 20° C. <17> or <18> A method for producing the nonwoven fabric according to claim 1.

[0055] <20> The content of the anionic surfactant in the melt is 1% by mass or more and 10% by mass or less, preferably 2% by mass or more and 8% by mass or less, and more preferably 3% by mass or more and 7% by mass or less. <19> A method for producing the nonwoven fabric according to claim 1. <21> The melting point of the polyolefin resin is 60° C. or higher and 180° C. or lower, preferably 170° C. or lower, and more preferably 160° C. or lower. <17> ~ <20> 13. A method for producing the nonwoven fabric according to claim 12. EXAMPLES

[0056] The present invention will be described in more detail with reference to examples, but the present invention is not limited thereto. In the examples, "parts" and "%" are all based on mass unless otherwise specified.

[0057] [Examples and Comparative Examples] Preparation of nonwoven fabric The raw materials were melted and mixed to obtain a melt with the composition (mass%) shown in the table below. The melt was spun by electrospinning. At that time, the space between the spinning nozzle and the collector was heated to 40°C or more by hot air blown from the periphery of the spinning nozzle, and the spun fibers were collected on the collector whose surface temperature was heated to 40°C. At this time, the conveyor of the collector was fixed and not moved, and a product name: Heat-resistant cooking paper cooking sheet 30 m (cooking sheet with peeling treatment) manufactured by Shin-Etsu Polymer Co., Ltd. was attached to the electrospun fiber accumulation area on the conveyor of the collector, and a nonwoven fabric made of electrospun fibers was produced on this cooking sheet. Note that since the nonwoven fabric was obtained by fixing the conveyor of the collector, the nonwoven fabric itself does not have any particular directionality. The distance between the spinning nozzle and the collector was 600 mm. As a result, a nonwoven fabric having a basis weight of 20 to 25 g / m2 with a median fiber diameter shown in the table below was obtained. 2 By the above-mentioned manufacturing method, fusion points were formed at the fiber intersections in the nonwoven fabric layer. The obtained nonwoven fabric was peeled off from the cooking sheet without being stretched, and subjected to various measurements and analyses. The measurement and analysis results of the nonwoven fabric are shown in the table below and in Figures 2 to 5.

[0058] [Table 1]

[0059] In Examples 1 and 2, the storage rigidity modulus (G') is maintained lower than the loss rigidity modulus (G'') at least from 200°C to 50°C, and the values ​​of the storage rigidity modulus (G') and the loss rigidity modulus (G'') are reversed and the value of the storage rigidity modulus (G') is 10°C or lower at least in the lower temperature range of 38°C or lower and 10°C or higher. 5 ~10 7 The nonwoven fabrics obtained in Examples 1 and 2 were composed of ultrafine fibers and exhibited elasticity, which was a unique property of having rubber-like physical properties of 0.01 Pa. As a result, the nonwoven fabrics obtained in Examples 1 and 2 were composed of ultrafine fibers and exhibited elasticity. In contrast, in Comparative Examples 1 and 2, the storage rigidity modulus (G') exceeded the loss rigidity modulus (G'') at temperatures between 90°C and 70°C. In relation to this, the proportion of fusion points at fiber intersections during fiber accumulation (nonwoven fabric formation) was smaller than in Examples 1 and 2, and the breaking elongation was also below 90%. In Comparative Example 3, the storage rigidity modulus (G') exceeded the loss rigidity modulus (G'') at around 120°C, and the storage rigidity modulus (G') rose to 108 [Pa] or more at around 120°C, indicating crystallization behavior. In relation to this, the proportion of fusion points at fiber intersections during fiber accumulation was even smaller than in Comparative Examples 1 and 2, and the nonwoven fabric elongation was also further reduced. From the above, Comparative Example 3 did not become a nonwoven fabric having elasticity.

Claims

1. A nonwoven fabric comprising fibers containing a polyolefin resin and having a median fiber diameter of 5 μm or less, and satisfying the following physical properties (a) and (b): (a) Viscosity in a molten state at 200 °C at a shear rate of 0.1 s －１ and the viscosity is 15 Pa·s or less; (b) In a dynamic viscoelasticity curve obtained by cooling from a molten state at 200°C at a rate of 4°C / min, the storage rigidity modulus is lower than the loss rigidity modulus in the temperature range of 200°C or lower and 50°C or higher, and the storage rigidity modulus is higher than the loss rigidity modulus in the temperature range of 38°C or lower and 10°C or higher.

2. A nonwoven fabric comprising fibers containing a polyolefin resin and having a median fiber diameter of 5 μm or less, having a hysteresis loss of 85% or less at 30% elongation, a breaking elongation of 90% or more, and a tensile strength per meter width normalized by basis weight according to the following formula of 2 N·m / g or more and 4 N·m / g or less. [Tensile strength normalized by basis weight] = {[Tensile strength of test piece (N)] / [Test piece width (m)]} / [Basis weight (g / m ２ )]

3. 3. The nonwoven fabric according to claim 1, wherein the polyolefin resin has a crystallinity of 10.5% or less.

4. 3. The nonwoven fabric according to claim 1, wherein the proportion of the crystalline polyolefin resin in the polyolefin resin is less than 5% by mass.

5. 3. The nonwoven fabric according to claim 1, wherein the ratio of the number of fusion-bonded points to the number of fiber intersections is 50% or more.

6. A laminated nonwoven fabric comprising the nonwoven fabric according to claim 1 or 2 laminated with another nonwoven fabric or paper.

7. A method for producing a nonwoven fabric, comprising the steps of: spinning a melt containing a polyolefin resin, which satisfies the following physical properties (a) and (b), into fibers having a median fiber diameter of 5 μm or less by an electrospinning method; and collecting the fibers with a collector whose surface temperature is set to 40° C. or higher and 70° C. or lower: (a) Viscosity in a molten state at 200 °C at a shear rate of 0.1 s －１ and the viscosity is 15 Pa·s or less; (b) In a dynamic viscoelasticity curve obtained by cooling from a molten state at 200°C at a rate of 4°C / min, the storage rigidity modulus is lower than the loss rigidity modulus in the temperature range of 200°C or lower and 50°C or higher, and the storage rigidity modulus is higher than the loss rigidity modulus in the temperature range of 40°C or lower and 10°C or higher.

8. The method for producing a nonwoven fabric according to claim 7, wherein the melt contains an anionic surfactant having a melting point above 20°C.

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