Nonwoven fabric

Abstract

To provide a nonwoven fabric composed of ultrafine fibers and exhibiting stretchability.SOLUTION: The nonwoven fabric satisfies the following physical properties (a) and (b): (a) a viscosity in a molten state at 200 °C. is 1s·s or less at a shear rate of 0. 15Pa - 1; and (b) in a dynamic viscoelasticity curve obtained at a temperature decrease rate of 4 °C. / min from a molten state at 200 °C., a storage rigidity modulus is lower than a loss rigidity modulus in the entire temperature range of 200 °C. or lower and 80 °C. or higher, and the storage rigidity modulus is higher than the loss rigidity modulus in the entire temperature range of 70 °C. or lower and 20 °C. or higher.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to nonwoven fabrics. [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 stretchability to nonwoven fabrics, either alone or in combination with other materials, to enhance their applicability to disposable diapers, sanitary products, sanitary masks, adhesive bandages, supports, medical tapes, and the like. For example, Patent Document 1 describes a multilayer stretchable nonwoven fabric in which a non-adhesive meltblown nonwoven fabric is laminated and bonded to a sticky meltblown nonwoven fabric. Both meltblown nonwoven fabrics are made of thermoplastic elastomers and have stretchability. This multilayer stretchable nonwoven fabric is said to be able to suppress sticking to manufacturing equipment without sacrificing stretchability, even if it has adhesiveness during production. Furthermore, Patent Document 2 describes a nonwoven fabric made mainly of thermoplastic resin fibers, which has different elongation rates in one direction and in the direction perpendicular to the direction. According to Patent Document 2, by controlling the elongation rate in this way, a nonwoven fabric having elasticity in only one direction can be obtained without subsequent processing of the nonwoven fabric.

[0003] Nonwoven fabrics made by depositing ultrafine fibers (e.g., fiber diameters of 5 μm or less) have been gaining attention in recent years. Melt spinning methods, such as electrospinning, are used as an example of a spinning technique for uniformly producing nonwoven fabrics made of such fine fibers. Nonwoven fabrics containing such ultrafine fibers are expected to be used in a variety of applications in the future, and extensive research is being conducted toward their commercialization. For example, the nanofiber sheet described in Patent Document 3 is intended to be applied 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, with the aim of maintaining the smoothness of the nanofiber sheet itself and maintaining its ability to conceal wrinkles on the skin, even when wrinkles appear 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 growing demand for nonwoven fabrics made of ultrafine fibers, such as that described in Patent Document 3, to be given stretchability in order to further enhance 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 stretchability.

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

[0007] As a result of extensive research, the present 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 exhibits a specific low viscosity in a molten state at a specific high temperature, and that the temperature range in which the storage modulus (G') is greater than the loss modulus (G'') in the dynamic viscoelasticity curve obtained upon cooling from this molten state is within a specific low-temperature range, a nonwoven fabric can be produced that is composed of ultrafine fibers and exhibits the desired excellent stretchability through a manufacturing process via electrospinning. The present invention was completed based on these findings and through further research.

[0008] The present invention provides a nonwoven fabric made 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 and is 15 Pa·s or less; (b) In the dynamic viscoelasticity curve obtained from a molten state at 200°C with a cooling rate of 4°C / min, the storage modulus is lower than the loss modulus over the entire temperature range from 200°C to 80°C, and the storage modulus is higher than the loss modulus over the entire temperature range from 70°C to 20°C.

[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 using a collector whose surface temperature is set to 40°C or higher and 80°C or lower. The melt preferably contains a polyolefin resin. 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 and is 15 Pa·s or less; (b) In the dynamic viscoelasticity curve obtained from a molten state at 200°C with a cooling rate of 4°C / min, the storage modulus is lower than the loss modulus over the entire temperature range from 200°C to 80°C, and the storage modulus is higher than the loss modulus over the entire temperature range from 70°C to 20°C. [Effects of the Invention]

[0010] The nonwoven fabric of the present invention exhibits the desired excellent stretchability despite being composed of ultrafine fibers. The nonwoven fabric can be suitably produced by the method for producing the nonwoven fabric of the present invention. [Brief explanation 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. [Figure 2] FIG. 2 is a graph showing the dynamic viscoelasticity curves obtained by melting each nonwoven fabric of Example 1 at 200° C. and cooling it at a rate of 4° C. / min. [Figure 3] FIG. 3 is a graph showing the dynamic viscoelasticity curves obtained by melting each nonwoven fabric of Comparative Example 1 at 200° C. and cooling it at a rate of 4° C. / min. [Figure 4] FIG. 4 is a graph showing the dynamic viscoelasticity curves obtained by melting each nonwoven fabric of Comparative Example 2 at 200° C. and cooling it at a rate of 4° C. / min. DETAILED DESCRIPTION OF THE INVENTION

[0012] A preferred embodiment of the nonwoven fabric of the present invention will now be described. The present invention provides a nonwoven fabric made of polyolefin resin-containing fibers 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 and is 15 Pa·s or less; (b) In the dynamic viscoelasticity curve obtained from a molten state at 200°C with a cooling rate of 4°C / min, the storage modulus is lower than the loss modulus over the entire temperature range from 200°C to 80°C, and the storage modulus is higher than the loss modulus over the entire temperature range from 70°C to 20°C.

[0013] The fibers (constituent fibers) that make up the nonwoven fabric preferably contain a polyolefin resin. The constituent fibers of this nonwoven fabric may be composed solely of polyolefin resin, or may contain, in addition to polyolefin resin, resins other than polyolefin resin as long as the effects of the present invention are not impaired. Various additives may also be added as needed. Such additives may include one or more selected from surfactants, silicone oils, modified silicone oils, waxes, and inorganic salts. It is preferred that the nonwoven fabric contains polyolefin resin as the main component (substance) of its constituent fibers, with the content being 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 preferred that all of the resin components are polyolefin resins (all of the polymers constituting the nonwoven fabric are polyolefins, i.e., a content of 100% by mass). The content of 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 molecular skeleton structure and the terminal functional group structure 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 thermoplastic resin contained in the resin component is calculated from the intensity of the measured values ​​indicating the molecular structures corresponding to various thermoplastic resins. The content of thermoplastic resin contained in the resin composition can then be measured by adding up the calculated values.

[0014] The polyolefin resin preferably contains one or more selected from olefin homopolymers and / or olefin copolymers. 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 (for example, 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 (for example, a vinyl compound, such as vinyl acetate or vinyl alcohol). Among these, the polyolefin resin preferably contains one or more selected from polypropylene resins or ethylene-α-olefin copolymer resins (α-olefins having 3 or more carbon atoms, including propylene), from the viewpoint that the crystallinity can be easily changed by controlling the stereoregularity during synthesis without changing the monomer structure in a complex manner, and polypropylene resins are more preferred. 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 with a fiber diameter of 5 μm or less. Such ultrafine fibers can be formed by electrospinning, as described below. 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 adhesion of the nonwoven fabric to the surface on which it acts. 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, and even more preferably 0.4 μm or more, from the viewpoint of improving the function of retaining and fixing the functional liquid on the surface of the nonwoven fabric to be acted upon and the strength of the nonwoven fabric.

[0017] <Method for measuring median fiber diameter> (1) Cut the nonwoven fabric to be measured into a 10 mm x 10 mm piece. This piece is attached to a scanning electron microscope (SEM) specimen stage (Oken Shoji Co., Ltd.) using conductive carbon double-sided tape (Oken Shoji Co., Ltd.). (2) The sample stage with the nonwoven fabric attached is placed in a sputtering device (Hitachi High-Tech Corporation, Ion Sputter E-1030) and platinum-palladium (Pt-Pd) deposition is performed 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 deposition time is 80 seconds, and the current value during deposition is 30 mA. (3) The sample stage is placed in an SEM (Hitachi High-Tech Corporation, S-4300SE / N) and observation images are acquired in high-resolution mode (accelerating voltage: 5 kV, workpiece distance: 10 mm, observation magnification: 500x or 1000x). Observation images are acquired 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 few 10% diameter (D10) (from the smallest diameter side), the few 50% diameter (median diameter), and the few 90% diameter (D90) are tallied. Of these, the median diameter is used 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 product in which the nonwoven fabric is dissolved or the molten product 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 with 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. The viscosity is set at a shear rate of 0.1 s to prevent the molten liquid from being torn off when stretched during melt spinning. -1 The viscosity is preferably 3 Pa·s or more, and more preferably 4 Pa·s or more.

[0020] <Method for measuring viscosity in a molten state at 200°C> The nonwoven fabric to be measured was cut to an arbitrary size, weighing 20 g. The nonwoven fabric was placed in a 1 mm thick frame with a 130 mm long and 80 mm wide hole. It was then heat-pressed at 200 °C and cooled at 15 °C to obtain a resin press plate measuring 130 mm x 80 mm x 1 mm. A 50 mm x 50 mm x 1 mm specimen was then cut from this plate using a cutter. The specimen was then placed on a rotating disk-type jig (parallel plates, diameter: 50 mm, plate spacing: 1 mm) mounted on an Anton Paar rotational rheometer (model number: MCR302). The rotating disk-type jig was preheated to 200 °C before the specimen was placed on the jig. After the specimen was placed on the jig, the heat from the plates melted the specimen, filling the jig's set geometric dimensions (diameter: 50 mm, plate spacing: 1 mm) completely. Any molten liquid from the specimen that spilled over the set geometric dimensions of the jig was collected using a stainless steel spatula. After placing the sample in the jig, it is enclosed in a dedicated heat chamber (CTD 450 / TD Ready manufactured by Anton Paar). The inside of the heat chamber is filled with heated nitrogen, and the temperature inside the chamber is allowed to stabilize at 200 °C. After that, 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. -1A shear rate of 100 seconds is applied to the specimen. The viscosity measured 100 seconds after the start of measurement is the measured value. When the sheets are stacked, the fiber layers are peeled off, and the nonwoven fabric to be measured is taken out and subjected to the above-mentioned procedure. 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 rate of 4°C / min, it is preferable that the storage modulus (G') is lower than the loss modulus (G'') over the entire temperature range from 200°C to 80°C. It is also preferable that the storage rigidity modulus is higher than the loss rigidity modulus at least over the entire temperature range from 70° C. to 20° C. It is more preferable that the storage rigidity modulus is higher than the loss rigidity modulus over the entire temperature range from 70° C. to 40° C. 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 nonwoven fabric has a dynamic viscoelasticity profile that exhibits a rubbery state at the lower temperature range (for example, the specific example shown in Figure 2). In the dynamic viscoelasticity characteristics 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, this 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, it is less likely to break during stretching, and it also exhibits shrinkage behavior. This means that the manufacturing process of the nonwoven fabric of the present invention is as follows: Namely, the melt viscosity is low over the entire temperature range of 200°C or lower and 80°C or lower, allowing for more efficient fiber diameter reduction. Furthermore, even without raising the temperature too much, the ultrafine fibers are easily fused together at their intersections in a fluid state while maintaining their shape. At the stage of forming the nonwoven fabric, the ultrafine fibers are in the aforementioned rubbery state and have elasticity, and the density of crosslinking points of the ultrafine fibers is increased, forming a finer fiber network structure, thereby achieving the desired excellent elasticity. The nonwoven fabric of the present invention exhibits excellent stretchability under the temperature conditions of everyday use due to the dynamic viscoelasticity of property (b), making it suitable for a variety of uses as a stretchable nonwoven fabric made of ultrafine fibers. In the present invention, the term "stretchable" means that the hysteresis loss when the nonwoven fabric is stretched 30% and then immediately returned to 0% elongation is 85% or less. Hysteresis loss will be described in detail later.

[0022] <Method for 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-mentioned <Method for measuring viscosity in a molten state at 200°C>. This plate is cut with a cutter to obtain a specimen of 50 mm x 50 mm x 1 mm thick (specimen type A), and a specimen of 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 jig attached to a rotational rheometer, as in the above <Method for measuring viscosity in a molten state at 200°C>. Enclose it in an insulating chamber and wait until the temperature inside the 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 measurements at a temperature drop rate of 4°C / min within a range of any temperature between 200°C and 50°C, and obtain the storage modulus (G') and loss modulus (G"). In addition, for specimens that may overload the detector of the rotational rheometer due to the effect of hardening accompanying the change in physical properties from a fluid state to a crystallized or rubbery state in the temperature range from 130°C to 50°C, the following shall be done: When the apparent measured values ​​of the storage modulus (G') and loss modulus (G") are 10 5 Measurement is stopped when the temperature rises to around Pa. A measurement program is set up 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 an angular frequency of 0.1 rad / s. Specimen type B For specimen type B, place it on a rotating disk-type jig (parallel plates, diameter: 8 mm, distance between plates: 1 mm) attached to the rotational rheometer. Place it in an insulated chamber and wait until the temperature inside the chamber stabilizes at 200°C using heated nitrogen. Next, set the strain amplitude to 6.28 rad / s (1 Hz) so that it can be switched from 10% (strain amplitude 0.1) to 0.1% (strain amplitude 0.001) during the measurement, as described below, and further set the angular frequency to 6.28 rad / s (1 Hz). Perform dynamic viscoelasticity measurements in the temperature range from 200°C to -25°C at a cooling rate of 4°C / min 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 due to the influence of hardening accompanying the change in physical properties from a fluid state to a crystallized or rubbery state in the temperature range from 130°C to 40°C, the following should be done: When the apparent measured values ​​of the storage modulus (G') and loss modulus (G") are 10 5 The measurement conditions are set so that the strain amplitude is switched to 0.1% (strain amplitude: 0.001) when the temperature rises to around 100 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, catalog data on the physical properties of the resin raw material used for the nonwoven fabric to be measured is referenced. The temperature is set based on these values. Furthermore, 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, taking measures to reduce overload on the detector of the rotational rheometer. As described above, the reason why specimen forms A and B of the same specimen but different sizes were prepared and set in the corresponding rotating disk-type jig 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, the phase difference between dynamic strain and dynamic stress, which are the basis of the data, and the detected torque value are carefully examined, and after thinning out data outside the detection limit range, the data are overlaid to obtain a temperature profile of each modulus.

[0023] (Storage stiffness) In the nonwoven fabric of the present invention, the storage rigidity is 1.5 × 10 in the temperature range of 20 ° C. or higher and 40 ° C. or lower. 7 Pa or more 1.0×10 8 It is preferable that the elastic modulus is 10 Pa or less. This allows the nonwoven fabric of the present invention to maintain a relatively hard rubbery state over a wide temperature environment, from 20° C. to 40° C., which is a temperature range that takes into account the real world. As a result, the nonwoven fabric of the present invention is less likely to break when stretched from the fusion points at the intersections of the ultrafine fibers, and repeated stretching behavior can be fully achieved. The storage rigidity in the temperature range of 20°C to 40°C is 1.5 x 10 7 Pa or more is preferable, 1.7 × 10 7 Pa or more is more preferable, 2.0 × 10 7 Pa or more is more preferable. The storage rigidity in the temperature range of 20°C or higher and 40°C or lower is 1.0 x 10 8 Pa or less is preferred.

[0024] (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, a state in which the nonwoven fabric is not stretched is referred to as 0% stretch. Therefore, for example, 100% stretch refers to a state in which the length of the nonwoven fabric has doubled. The hysteresis loss refers to 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 immediately released, this 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, this hysteresis loss is preferably 50% or more, more preferably 60% or more, and even more preferably 70% or more.

[0025] <Method for measuring hysteresis loss> The nonwoven fabric to be measured is cut in any direction to a size of 20 mm wide x 100 mm long and placed in a tensile tester (50 mm between the upper and lower chucks). A cycle of 30% elongation to 0% return is performed at a load cell speed of 100 mm / min. Hysteresis loss is calculated by subtracting the area (S2) obtained from the stroke-load curve at 0% return from the area (S1) obtained from the stroke-load curve at elongation, and dividing the result by the area (S1) to calculate the hysteresis loss using the following formula (I). The measurement temperature is 23°C. Hysteresis loss = (S1-S2) / S1 (I)

[0026] (tensile strength) The nonwoven fabric of the present invention preferably has a tensile strength per meter width normalized by basis weight, as determined by the following formula (II), 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 / m2 )] (II) 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 within 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, 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 from the viewpoint of improving the cuttability in the nonwoven fabric processing step.

[0027] <Method for measuring tensile strength normalized by basis weight> A cut sample of nonwoven fabric of the same size as in the above <Method for measuring hysteresis loss> is placed in a tensile tester and pulled at a speed of 100 mm / min to measure the tensile strength value. The measured tensile strength value is obtained by dividing the measured value by the basis weight of the test piece x the test piece width (20 mm in this case). At this time, the unit is switched between mm and m so that the unit becomes N·m / g. The measurement temperature is 23°C. In measuring the tensile strength normalized by basis weight, the test piece width is not limited to 20 mm, and any test width may be used.

[0028] In the nonwoven fabric of the present invention, by satisfying all of the requirements for hysteresis loss and tensile strength within the above-mentioned specific ranges, the characteristics of each physical property act synergistically, resulting in excellent stretchability and excellent followability to the surface to which the fabric is applied.

[0029] Since the nonwoven fabric of the present invention is composed of the ultrafine fibers, the electrospinning method is preferably used for preparing the fibers, and the nonwoven fabric is formed directly from this electrospinning process. When the nonwoven fabric composed of such ultrafine fibers is obtained by fixing the conveyor of the collector in the manufacturing method described below, it does not exhibit a strong directional dependency and satisfies the above-mentioned hysteresis loss and tensile strength regardless of the direction of stretching.

[0030] In the nonwoven fabric of the present invention, the polyolefin resin constituting the fibers preferably has a heat of fusion of 20 J / g 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 heat of fusion is practically 18 J / g or less, and preferably 14 J / g or less. From the viewpoint of improving heat resistance, the heat of fusion is more preferably 10 J / g or more.

[0031] <Method for measuring the heat of fusion of polyolefin resin in nonwoven fabric> (1) Cut the nonwoven fabric to be measured into a 2 mg sample and place it in a differential scanning calorimeter sample container (Hitachi High-Tech Science Corporation, a set of an aluminum autosampler sample container P / N GAA-0065 (container size: diameter 6.8 mm, height 2.5 mm) and an aluminum autosampler cover P / N GAA0064) to use as the sample. (2) The sample obtained in (1) above is placed in a differential scanning calorimeter (DSC7000X, manufactured by Hitachi High-Tech Science Corporation) and measured from a starting temperature of 20°C to a temperature increase 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 the endothermic peaks associated with the melting of the nonwoven fabric, and the enthalpy of fusion (heat of fusion) is measured. Note that depending on the type of resin that makes up the nonwoven fabric, there may be more than one endothermic peak, and the heat of fusion at all endothermic peaks is measured. The sum of the heat of fusion at all endothermic peaks is taken as the heat of fusion of the nonwoven fabric. (4) The heat of fusion of the resin component in the nonwoven fabric is calculated from the heat of fusion of the nonwoven fabric obtained in (3) above using the following formula (III). Hr = Ha × Wa / Wr (III) Hr: Heat of fusion of the resin component in the nonwoven fabric [J / g] Ha: Heat of fusion 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 component in nonwoven fabric [%]

[0032] 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 polyolefin resin prepared at 180°C or higher is cooled at a temperature decrease rate of 4°C / min, or a polyolefin that melts in a temperature range of 110°C to 180°C when a polyolefin resin that is as hard as glass at 20°C to 30°C is heated at a temperature increase 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 physical properties described above.

[0033] <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 obtained infrared spectrum is analyzed using analysis software (OMNIC9.12.1002 / OMNICMC9.12.928 / OMNIC Atlus9.12.990) to estimate the polyolefin resin contained. Estimations with a certainty rate of 90% or higher are used. Once the polyolefin resin is estimated, a commercially available crystalline polyolefin resin corresponding to the estimated polyolefin resin is obtained as a model. For example, for crystalline polyethylene, obtain Novatec™ HD (product number HY350), a high-density polyethylene manufactured by Japan Polyethylene Co., Ltd., and for crystalline polypropylene, obtain Metocene MF650Y manufactured by Polymirae Co., Ltd. These model crystalline polyolefin resins are raw materials in pellet or granular form, so depending on the size of the raw material, they are crushed in a crusher and weighed out to 2 mg. These are subjected to a differential scanning calorimeter in the same manner as in the above-mentioned <Method for measuring the heat of fusion of polyolefin resin in nonwoven fabric>. Measurement is performed starting at 20°C, with a heating rate of 4°C / min up to 200°C, followed by a temperature drop rate of 4°C / min from 200°C to 20°C, and then a differential scanning calorimeter measurement is performed again from 20°C to 200°C at a heating rate of 4°C / min. The heat of fusion during the second heating run is measured from the obtained measurement profile. Meanwhile, 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 heat of fusion during the second temperature rise is measured. In the differential scanning calorimetry profile of the nonwoven fabric, the heat of fusion in the same temperature range as the crystalline polyolefin resin model is considered to be derived from crystalline polyolefin, and the proportion of crystalline polyolefin resin is calculated using the following formula (IV). [Ratio of crystalline polyolefin resin in nonwoven fabric] = [Heat of fusion of crystalline polyolefin in nonwoven fabric] / [Heat of fusion of crystalline polyolefin resin model itself] (IV) It is already known that in each comparative example of the present invention, the above-mentioned Metocene MF650Y is the only crystalline polypropylene blended. Therefore, when formula (IV) is applied to comparative example 2, for example, the enthalpy of fusion of the nonwoven fabric is 4.7 J / g and the heat of fusion 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%.

[0034] In the nonwoven fabric of the present invention, it is preferable that contacting fibers are fused to each other at more than 55% of the number of fiber intersections (fused intersections refer to the above-mentioned fused points). In other words, it is preferable that the ratio of the number of fused points to the number of fiber intersections is more than 55%. In the nonwoven fabric of the present invention, the proportion of fusion points to the total 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 keeping the thickness within these lower limits, the crosslinking points between the fibers in the nonwoven fabric are densely formed, and it is possible to achieve higher stretchability even though the nonwoven fabric is made up of ultrafine fibers.

[0035] <Method for measuring the ratio of the number of fusion points to the number of fiber intersections> The same procedures as in (1), (2), and (3) of the "Method for Measuring Median Fiber Diameter" above were performed to obtain SEM images of each nonwoven fabric at a magnification of 1000x. The same sample was observed at five different locations, each focused on the sample surface, to obtain five images. The image quality was 1280 pixels wide and 960 pixels high. Fiber intersections were marked on the obtained images using image analysis software (WinRooF2015, manufactured by Mitani Shoji Co., Ltd.). The number of fusion points at the fiber intersections was recorded, and the ratio of the number of fusion points to the total number of fiber intersections was calculated using the following formula (V). This evaluation was performed by two people: an observer and a recorder. The number of fiber intersections was tallied by the observer's manual marking process on the image analysis software. Regarding fusion points, the recorder tallied the fiber intersections that the observer identified as fusion points according to the definition of fusion points below. Percentage of fusion points [%] = Number of fusion points / Total number of fiber intersections × 100 (V) The fiber intersections and fusion points at the fiber intersections are defined as follows. (fiber intersection) In SEM observation images of nonwoven fabrics, when the image itself is magnified 3 times (300%) on a 410 mm x 260 mm monitor, a focused image is selected within a range where the width between the boundary lines of the fiber shape (the fiber width observed in the observation image) does not exceed 1 mm. Fiber intersections are defined as points where fibers intersect, points where they are in contact but not intersecting, or points where a single fiber branches off midway (e.g., the areas boxed with symbols D4, D14, and D1 in Figure 1). In some fibers, two or more fibers in the image may be in contact with each other over the entire or partial image capture area along their length, forming a fiber bundle; these are also defined as fiber intersections (e.g., the area boxed with symbol D5 in Figure 1). For example, if four fibers are lined up without any gaps in the observation image, the number of fiber intersections is defined as three. Due to the characteristics of nonwoven fabrics, there is a depth due to the thickness of the nonwoven fabric in SEM observation, with the side closer to the detector (the surface side in the specimen thickness direction) and the side further away (the specimen stage side in the specimen thickness direction). Therefore, even if fibers appear to intersect in the observation image, they may not actually be in contact with each other. As mentioned above, when the image itself is enlarged 3 times (300%) on a 410 mm x 260 mm monitor, multiple fibers that are in focus within a range where the width of the fiber shape boundary line does not exceed 1 mm are considered to be in the same positional relationship in the specimen thickness direction (depth). The intersections of fibers selected in this way are considered fiber intersections according to the above definition. (Fusion points at fiber intersections) Of 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 symbol D4 in Figure 1). (ii) A portion where the width between the boundary lines of one or more fibers associated at the intersection is wider than the width between the boundary lines other than the intersection (for example, the area enclosed by the symbol D16 in FIG. 1).

[0036] The basis weight of the nonwoven fabric of the present invention is 5 g / m from the viewpoint of further increasing the capillary force and further increasing the strength. 2 More than 10g / 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 enhancing softness and adhesion to the target object such as the skin surface. 2 Less than 30 g / m 2 The following is more preferred:

[0037] The nonwoven fabric of the present invention is also preferably used in a laminated state by overlapping it with another nonwoven fabric or paper. The "other 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 the 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 1.5g / m is preferable. 2 More preferably, 2 g / m 2 The above is more preferable. In addition, from the viewpoint of further enhancing the softness and adhesion to the skin surface in the laminated state, the basis weight of the nonwoven fabric of the present invention when used as the laminated nonwoven fabric is 8 g / m 2 Less than 6g / m is preferred 2 Less than 5g / m is more preferable. 2 The following is even more preferred:

[0038] 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 melt electrospinning, and collecting the fibers with a collector set at a surface temperature of 40° C. or higher and 80° 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 and is 15 Pa·s or less; (b) In the dynamic viscoelasticity curve obtained from a molten state at 200°C with a cooling rate of 4°C / min, the storage modulus is lower than the loss modulus over the entire temperature range from 200°C to 80°C, and the storage modulus is higher than the loss modulus over the entire temperature range from 70°C to 20°C. 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 polyolefin resin-containing melt.

[0039] In the above-mentioned production method, it is preferable to extrude a molten material 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 higher and 80°C or lower. This allows the collected ultrafine fibrous molten material to be in a moderately fluid state without excessive fluidity, even if it is ultrafine, and to form favorable fusion points at the intersections of the fibers while maintaining its fibrous shape. The molten material then turns into a rubbery state as the temperature is lowered, becoming elastic ultrafine fibers, and a stretchable nonwoven fabric can be obtained in which the ultrafine fibers form a network with a high density of crosslinking points. The extruded fibrous molten material can be stretched in an electric field to form ultrafine fibers of 5 μm or less. In this regard, ultrafine fiber nonwoven fabrics obtained by conventional electrospinning methods do not satisfy both the requirements of physical properties (a) and (b), making it difficult to obtain the nonwoven fabric of the present invention. Conventionally, because the requirement of 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, resulting in a fiber that is not stretchable. Furthermore, conventionally, during the process of fibers being accumulated to form a nonwoven fabric, the resin hardens as it crystallizes before accumulation, making it difficult for fusion to occur at the fiber intersections during accumulation. Therefore, when conventional ultrafine fiber nonwoven fabrics are pulled, the fibers slide along the intersections and break, and they do not exhibit recovery force against the pull. In contrast, the nonwoven fabric of the present invention exhibits the above-mentioned properties (a) and (b), and therefore contains stretchable ultrafine fibers, has a fiber network structure formed by fusion points, and has the desired stretchability. In the production method of the present invention, as described above, it is preferable to collect the spun fibers by fixing the conveyor of the collector without moving it, since this prevents the nonwoven fabric from exhibiting strong directional dependency.

[0040] The set temperature of the collector may be set appropriately 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. In addition, from the viewpoint of preventing the material from sticking to the collector, the temperature is preferably 80°C or lower, and more preferably 50°C or lower. Except for the above points, existing methods can be applied as appropriate. It is also preferable to heat the space between the spinning nozzle and the collector to a temperature within the same range as above.

[0041] The melt containing polyolefin resin is preferably a melt of polyolefin resin only, or a melt of a mixture of polyolefin resin and other components. As the polyolefin resin, it is preferable to use various types of the polyolefin resin as described above for the nonwoven fabric. In addition, it is preferable that the melting point of the polyolefin resin be in the range described 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 thinner 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. When the anionic surfactant has a melting point exceeding 20° C. (is solid at 20° C.), the anionic surfactant can be added to a mixer without adhering to the inner wall of the mixer's inlet when mixed with a polyolefin resin pellet raw material at a temperature of 20° C. to 30° C. The content of the anionic surfactant in the 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.

[0042] In the method for producing the nonwoven fabric of the present invention, when the nonwoven fabric produced by the electrospinning method is laminated on another nonwoven fabric, the nonwoven fabrics can be integrated by embossing. [Example]

[0043] The present invention will be explained in more detail based on examples, but the present invention should not be construed as being limited thereto. In the examples, "parts" and "%" are all by mass unless otherwise specified.

[0044] [Examples and Comparative Examples] Preparation of nonwoven fabric The raw materials were melt-mixed to obtain a melt with the composition (mass%) shown in the table below. This melt was then spun by electrospinning. The space between the spinning nozzle and the collector was heated to 40°C or higher by hot air blown from around the spinning nozzle, and the spun fibers were collected on a collector heated to a surface temperature of 55°C. The collector conveyor was fixed and not moving, and a 30m sheet of heat-resistant cooking paper (a peel-treated cooking sheet) manufactured by Shin-Etsu Polymer Co., Ltd. was attached to the electrospun fiber accumulation area on the collector conveyor. A nonwoven fabric made of electrospun fibers was then produced on this cooking sheet. Since the nonwoven fabric was obtained with the collector conveyor fixed, the nonwoven fabric itself does not have any particular directionality. The distance between the spinning nozzle and the collector was 600mm. This resulted in a nonwoven fabric with a basis weight of 20 to 25g / m2 and a median fiber diameter shown in the table below. 2 By the above-mentioned manufacturing method, fusion-bonded points were formed at the fiber intersections within the nonwoven fabric layer. The melting point of the polyolefin resin used in Example 1 was 100°C. The obtained nonwoven fabric was peeled off from the cooking sheet without stretching and subjected to various measurements and analyses. The measurement and analysis results of the nonwoven fabric are shown in the table below and Figures 2 to 5.

[0045] [Table 1]

[0046] In Example 1, the storage rigidity modulus (G') remained lower than the loss rigidity modulus (G'') over the entire temperature range from at least 200°C to 80°C, and the values ​​of the storage rigidity modulus (G') and the loss rigidity modulus (G'') were reversed in the lower temperature range of at least 20°C or higher to 70°C or lower. As a result, the nonwoven fabric sample obtained in Example 1 had a smaller median fiber diameter than the nonwoven fabric sample obtained in Comparative Example 1. At the same time, despite the median fiber diameter being smaller than that of Comparative Example 1, the nonwoven fabric sample in Example 1 had a hysteresis loss of 85% or less and had sufficient stretchability. In Comparative Example 2, when the temperature was below 70°C, the hardness increased and the measuring tool began to slip, making it impossible to measure the storage rigidity modulus (G'). At 70°C, the storage rigidity modulus (G') was 5.0 × 10 8 Since it is mainly composed of crystalline polypropylene, if measurements could be made below 70°C, it is estimated that the storage modulus (G') would increase as the temperature decreases, compared to the value at 70°C, which is generally considered to be a physical property of crystalline polypropylene.

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 over the entire temperature range from 200°C to 80°C, and the storage rigidity modulus is higher than the loss rigidity modulus over the entire temperature range from 70°C to 20°C.

2. The storage rigidity is 1.5×10 in a temperature range of 20° C. or higher and 40° C. or lower. ７ Pa or more 1.0×10 ８ The nonwoven fabric according to claim 1, wherein the elastic modulus is 0.05 Pa or less.

3. 3. The nonwoven fabric according to claim 1, wherein the polyolefin resin has a heat of fusion of 20 J / g 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. 3. 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 80° 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 over the entire temperature range from 200°C to 80°C, and the storage rigidity modulus is higher than the loss rigidity modulus over the entire temperature range from 70°C to 20°C.

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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