Core-sheath type composite fiber, method for manufacturing same, and fiber aggregate including same

Abstract

The present invention pertains to core-sheath type composite fibers in each of which a core component and a sheath component are concentrically arranged substantially. The sheath component contains 70 mass% or more of a high density polyethylene. The core component contains 70 mass% or more of a polypropylene. In a differential scanning calorimetry (DSC) method as specified by JIS K 7121:1987, the difference in temperature (melting peak temperature-extrapolated melting starting temperature: Tpm-Tim) between a melting peak temperature (Tpm) of the polypropylene and an extrapolated melting starting temperature (Tim) of the polypropylene as measured at a heating rate of 20°C per minute is 12.0°C or more.

Description

Core-sheath composite fiber, its manufacturing method, and fiber assembly containing the same

[0001] The present invention relates to a core-sheath type composite fiber that can be used as a constituent fiber for various sheets such as battery separators, a method for producing the same, and a fiber assembly containing the same.

[0002] Fiber assemblies containing ultrafine fibers have a dense structure and are therefore widely used in battery separators, RO membrane supports, filter media for various filters, etc. Battery separators and RO membrane supports are sometimes used in a state where they are immersed in a strongly alkaline solution, and for such applications, fiber assemblies using ultrafine fibers or thermally adhesive fibers made of polyolefin resins such as polypropylene and polyethylene are often used.

[0003] In particular, for battery separator applications, high battery or capacitor capacity is required, necessitating an increase in the amount of active material in the electrode. This inevitably reduces the volume occupied by the separator. In addition to the trend toward finer single-filament fibers, high strength and dimensional stability (low thermal shrinkage) are also required. Patent Document 1, for example, describes a high-strength, low-shrinkage polyolefin fiber. The fiber is obtained by stretching a melt-spun composite undrawn yarn having a crystalline propylene-based polymer core and an olefin-based polymer other than the crystalline propylene-based polymer as a sheath material in pressurized saturated steam at a temperature of 100°C or higher but lower than the melting point of the sheath material. The stretched composite fiber has a breaking strength of more than 5.74 cN / dtex, an elongation of 30% or less, and a Young's modulus of 43.1 cN / dtex or higher. However, Patent Document 1 requires the fiber to be stretched under special conditions using pressurized saturated steam, which is cumbersome.

[0004] Patent Document 2 describes a high-strength, low-shrinkage polyolefin fiber that can be obtained without drawing under special conditions using pressurized saturated steam, which is a heat-fusible fiber obtained by drawing an undrawn yarn having a crystalline propylene polymer as a core component and an olefin polymer having a lower melting point than the core component as a sheath component, the heat-fusible fiber having a fiber breaking strength of 6.0 cN / dtex or more and a dry heat shrinkage at 145°C of 20% or less as measured in accordance with JIS L 1015. Patent Document 3 also describes a high-strength polypropylene staple fiber that is made of 40% by mass or more of isotactic polypropylene having a melt flow rate (MFR) of 30 to 100 g / 10 min, the entire fiber being made of polyolefin resin, and has a strength of 5.5 to 8.0 cN / dtex and a dry heat shrinkage at 120°C of 0.0 to 10.0%.

[0005] JP 2002-180330 A JP 2007-107143 A JP 2014-196577 A

[0006] However, the fibers described in Patent Documents 1 to 3 have the problem that they are not sufficiently inhibited from thermal shrinkage, such as shrinkage at 145° C. at which polyethylene is completely melted.

[0007] In order to solve the above-mentioned problems of the conventional art, the present invention provides a core-sheath type conjugate fiber having high strength and low thermal shrinkage, a method for producing the same, and a fiber assembly containing the same.

[0008] The present invention provides a core-sheath type composite fiber in which a core component and a sheath component are arranged substantially concentrically, the sheath component containing 70% by mass or more of high-density polyethylene, and the core component containing 70% by mass or more of polypropylene, and the extrapolated melting onset temperature (T ) of polypropylene measured at a heating rate of 20°C per minute in a differential scanning calorimetry (DSC) method specified in JIS K 7121:1987 is 0.015 to 0.15. im ) and the melting peak temperature of polypropylene (T pm ) temperature difference (melting peak temperature - extrapolated melting onset temperature: T pm -T im ) is 12.0°C or higher.

[0009] The present invention also provides a method for producing a sheath-core composite fiber in which a core component and a sheath component are arranged substantially concentrically, the method comprising the steps of: supplying a sheath component containing 70% by mass or more of high-density polyethylene and a core component containing 70% by mass or more of polypropylene to a composite spinning nozzle arranged so that, in the cross section of the sheath-core composite fiber, the sheath component covers the surface of the core component and the center of gravity of the core component substantially coincides with the center of gravity of the sheath-core composite fiber; and a step of drawing the undrawn fiber tow at a drawing temperature of 60°C to 110°C under conditions where the total draw ratio is 2.7 to 10.0, thereby obtaining a sheath-core composite fiber.

[0010] The present invention also relates to a fiber structure containing 10% by mass or more of the core-sheath type composite fiber.

[0011] The present invention can provide a sheath-core conjugate fiber having high strength and low thermal shrinkage, and a fiber assembly using the same. Furthermore, the production method of the present invention can provide a sheath-core conjugate fiber having high strength and low thermal shrinkage.

[0012] 1 is a schematic cross-sectional view illustrating an example of a cross section of a sheath-core composite fiber. In differential scanning calorimetry (DSC) performed using an example of a sheath-core composite fiber, the extrapolated melting onset temperature (T im ) and melting peak temperature (T pm ) temperature difference (T pm -T im1 is a DSC curve showing how to determine the endothermic peak and heat of fusion measured in a temperature range from about 115°C to about 145°C in differential scanning calorimetry (DSC) performed using an example of a sheath-core composite fiber. FIG. 2 is a DSC curve showing how to determine the endothermic peak and heat of fusion measured in a temperature range from about 145°C to about 185°C in differential scanning calorimetry (DSC) performed using an example of a sheath-core composite fiber. FIG. 3 is a cross-sectional view showing an example of a melt spinning apparatus used in a method for producing a sheath-core composite fiber.

[0013] The inventors of the present invention have conducted extensive research to solve the above-mentioned problems. As a result, they have found that in a core-sheath composite fiber, high-density polyethylene is used as the sheath component and polypropylene is used as the core component, and the extrapolated melting temperature (T im ) and melting peak temperature (T pm ) temperature difference (melting peak temperature - extrapolated melting onset temperature: T pm -T im ) at a temperature equal to or higher than a predetermined temperature, a core-sheath type composite fiber having high strength and low thermal shrinkage can be obtained.

[0014] Specifically, high-density polyethylene is used for the sheath component and polypropylene is used for the core component. When melt spinning, the molten core component and sheath component extruded from a composite spinning nozzle are rapidly cooled and taken up at a predetermined spinning speed to obtain an undrawn fiber tow with low crystallinity. The undrawn fiber tow is then drawn under predetermined conditions to obtain the extrapolated melting onset temperature (T im ) and melting peak temperature (T pm ) temperature difference (melting peak temperature - extrapolated melting onset temperature: T pm -T im ) at a temperature above a predetermined temperature, a core-sheath type composite fiber having high strength and low thermal shrinkage can be obtained.

[0015] (Core Component) The core component contains 70% by mass or more of polypropylene, preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more. It is particularly preferable that the core component consists essentially of polypropylene. Here, the term "substantially" takes into consideration that resins provided as products usually contain additives such as stabilizers, and / or various additives are added during fiber production, making it difficult to obtain fibers consisting only of polypropylene and containing no other components. Typically, the core component may contain up to 15% by mass of additives.

[0016] The ratio Mw / Mn (hereinafter also referred to as Q value) of the weight average molecular weight (Mw) to the number average molecular weight (Mn) of the polypropylene is preferably 2.4 to 6, more preferably 2.6 to 5.8, even more preferably 2.8 to 5.6, and particularly preferably 3.0 to 5.0. When the Q value of the polypropylene is within the above range, the core component has a moderately large polydispersity and contains a large amount of low molecular weight and high molecular weight components during the melt spinning stage. During melt spinning, by cooling the molten resin, the core component containing polypropylene becomes a resin component with a large proportion of amorphous phase and smectic crystals. When the polypropylene contained in the core component is in such a state, the undrawn fiber tow obtained by melt spinning has excellent drawability and is not only easily drawn in the drawing process, but also is less likely to produce overdrawn polypropylene.

[0017] The melt flow rate of polypropylene in accordance with JIS K 7210-1:2014 (hereinafter also referred to simply as "MFR230"; measurement temperature: 230°C, load: 2.16 kgf (21.18 N)) is not particularly limited, but from the viewpoint of achieving both spinnability and fiber strength, it is preferably 3 g / 10 min or more and 35 g / 10 min or less, more preferably 4 g / 10 min or more and 25 g / 10 min or less, and particularly preferably 5 g / 10 min or more and 9.8 g / 10 min or less.

[0018] The polypropylene is not particularly limited, and examples thereof include propylene homopolymers (homopolymers containing propylene as a monomer), copolymers of propylene with other copolymerizable monomers, and mixtures thereof. Propylene-based copolymers may be random copolymers or block copolymers. Examples of propylene-based copolymers include copolymers of propylene with at least one α-olefin selected from the group consisting of ethylene and α-olefins having 4 or more carbon atoms. Examples of α-olefins having 4 or more carbon atoms include, but are not limited to, 1-butene, 1-pentene, 3,3-dimethyl-1-butene, 4-methyl-1-pentene, 4,4-dimethyl-1-pentene, 1-decene, 1-dodecene, 1-tetradecene, and 1-octadecene. The propylene content in the propylene-based copolymer is preferably 50% by mass or more. Among polypropylenes, propylene homopolymers are particularly preferred in terms of fiber strength, processability, and economics (production costs).

[0019] The core component may contain a thermoplastic resin other than polypropylene, provided that the effects of the present invention are not impaired. Examples of thermoplastic resins other than polypropylene include, but are not limited to, polyolefin resins other than polypropylene, polyamide resins, polycarbonates, polystyrenes, and polyester resins. Furthermore, various known additives can be added to the core component, provided that the effects of the present invention are not impaired and the productivity of the fiber, the productivity of the fiber assembly, thermal adhesiveness, and tactile feel are not affected. Examples of additives that can be added to the core component include known crystal nucleating agents, antistatic agents, pigments, matting agents, heat stabilizers, light stabilizers, flame retardants, antibacterial agents, lubricants, plasticizers, softeners, antioxidants, and UV absorbers. It is preferable that such additives be included in the core component so that they account for 10% by mass or less of the total core component.

[0020] (Sheath Component) The sheath component contains 70% by mass or more, preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more of high-density polyethylene. It is particularly preferable that the sheath component consists essentially of high-density polyethylene. Here, the term "substantially" takes into consideration the fact that resins provided as products usually contain additives such as stabilizers and / or various additives are added during fiber production, making it difficult to obtain fibers consisting only of high-density polyethylene and containing no other components. Typically, the sheath component may contain up to 15% by mass of additives.

[0021] The high-density polyethylene may be an ethylene homopolymer (a single polymer in which ethylene is a monomer) or a copolymer containing ethylene as the main monomer, but it is preferable that the density of the high-density polyethylene is 0.94 g / cm 3 or more is preferable. The high-density polyethylene is not particularly limited as long as it can produce the core-sheath type composite fiber targeted by the present invention. The high-density polyethylene may be an ethylene homopolymer, a high-density polyethylene in which the crystallinity has been intentionally reduced by adding and copolymerizing a small amount of an α-olefin (e.g., 1-butene) to ethylene, a random copolymer containing ethylene as a monomer, a block copolymer, a graft copolymer, or a mixture thereof. Examples of random copolymers, block copolymers, and graft copolymers include copolymers of ethylene and at least one α-olefin selected from the group consisting of α-olefins having 3 or more carbon atoms. The density of the high-density polyethylene is 0.945 g / cm 3 More preferably, it is 0.95 g / cm or more. 3 The upper limit of the density of the high-density polyethylene is not particularly limited, but is preferably 0.97 g / cm. 3 or less, and may be 0.965 g / cm 3 It may be the following:

[0022] The α-olefin having 3 or more carbon atoms is not particularly limited as long as it can produce the core-sheath composite fiber desired by the present invention, but examples include propylene, 1-butene, 1-pentene, 3,3-dimethyl-1-butene, 4-methyl-1-pentene, 4,4-dimethyl-1-pentene, 1-decene, 1-dodecene, 1-tetradecene, and 1-octadecene. The ethylene content in the ethylene-based copolymer is preferably 50% by mass or more. In consideration of ease of production and economic efficiency (production cost), it is preferable to use, as the high-density polyethylene, an ethylene homopolymer or an ethylene-based copolymer having a proportion of α-olefin monomers other than ethylene of 5 mol% or less. These high-density polyethylenes may be used alone or in combination of two or more.

[0023] The melt flow rate of the high-density polyethylene in accordance with JIS K 7210-1:2014 (hereinafter also referred to as "MFR190"; measurement temperature: 190°C, load: 2.16 kgf (21.18 N)) is not particularly limited, but is preferably 5 g / 10 min or more and 40 g / 10 min or less, more preferably 8 g / 10 min or more and 30 g / 10 min or less, and particularly preferably 10 g / 10 min or more and 25 g / 10 min or less. When the MFR190 of the high-density polyethylene is in the above-mentioned range, the productivity of the core-sheath type composite fiber is further improved.

[0024] The sheath component may contain a thermoplastic resin other than high-density polyethylene, provided that the effects of the present invention are not impaired. Examples of thermoplastic resins other than high-density polyethylene include, but are not limited to, polyolefin resins other than high-density polyethylene, polyester resins, polyamide resins, polycarbonates, and polystyrenes. Furthermore, various known additives can be added to the sheath component, provided that the effects of the present invention are not impaired and the fiber productivity, nonwoven fabric productivity, thermal adhesiveness, and tactile feel are not affected. Examples of additives that can be added to the sheath component include known crystal nucleating agents, antistatic agents, pigments, matting agents, heat stabilizers, light stabilizers, flame retardants, antibacterial agents, lubricants, plasticizers, softeners, antioxidants, and UV absorbers. It is preferable that such additives be included in the sheath component so that they account for 10% by mass or less of the total sheath component.

[0025] (Sheath-core composite fiber) A sheath-core composite fiber is a concentrically structured composite fiber that includes a core component and a sheath component, and the core component and the sheath component are arranged substantially concentrically. That is, in the fiber cross section, the center of gravity of the core component is not substantially shifted from the center of gravity of the sheath-core composite fiber. Figure 1 is a schematic diagram of the fiber cross section of an example of a sheath-core composite fiber with a concentric structure. The sheath component 1 is arranged around the core component 2, and the sheath component 1 surrounds the core component 2, so that in the sheath-core composite fiber 10, the fiber surface other than the cut surface is covered by the sheath component 1. The center of gravity 3 of the core component 2 is not substantially shifted from the center of gravity 4 of the sheath-core composite fiber 10. "The center of gravity of the core component is not substantially displaced from the center of gravity of the sheath-core composite fiber" means that the ratio of the displacement of the center of gravity of the core component relative to the center of gravity of the sheath-core composite fiber (hereinafter also referred to as eccentricity), as determined by the method described below, is 8% or less, preferably 6% or less, more preferably 5% or less, and even more preferably 3% or less. In the sheath-core composite fiber of one or more embodiments of the present invention, the core component and the sheath component have a concentric structure, i.e., the center of gravity of the core component is not substantially displaced from the center of gravity of the sheath-core composite fiber. Therefore, even if the molecular structure of the high-density polyethylene resin contained in the sheath component of the sheath-core composite fiber contains distortion that causes thermal shrinkage upon heating, the sheath-core composite fiber is less susceptible to this influence, and the thermal shrinkage rate of the sheath-core composite fiber when heated is likely to be small. This is also preferable because it improves the adhesive strength of a fiber assembly produced using this fiber.

[0026] <Eccentricity Ratio> The cross section of the sheath-core composite fiber 10 is photographed enlarged using a scanning electron microscope or the like, and the eccentricity ratio is calculated using the following formula, where C1 is the center of gravity position 3 of the core component 2, Cf is the center of gravity position 4 of the sheath-core composite fiber 10, and rf is the radius 5 of the sheath-core composite fiber 10. Eccentricity ratio (%) = [(Cf - C1) / rf] x 100

[0027] In sheath-core composite fibers, the cross-sectional shape of the fiber and the cross-sectional shape of the core component may be either circular or irregular, such as elliptical, Y-shaped, X-shaped, square, polygonal, or star-shaped.

[0028] The composite ratio of the core component to the sheath component (volume ratio of core component / sheath component) is not particularly limited, but in consideration of fiber productivity, it is preferably 35 / 65 or more and 75 / 25 or less, more preferably 40 / 60 or more and 70 / 30 or less, and particularly preferably 45 / 55 or more and 65 / 35 or less.

[0029] The single fiber fineness of the sheath-core conjugate fiber is not particularly limited, but is preferably 0.1 dtex or more and 1.1 dtex or less, more preferably 0.2 dtex or more and 1.0 dtex or less, and particularly preferably 0.3 dtex or more and 0.9 dtex or less. When the single fiber fineness of the sheath-core conjugate fiber is within the above-mentioned range, a fiber assembly such as a nonwoven fabric having a dense structure is easily obtained.

[0030] The core-sheath type composite fiber preferably has a single fiber strength of 4 cN / dtex or more and 12.0 cN / dtex or less, more preferably 4.1 cN / dtex or more and 11.8 cN / dtex or less, even more preferably 4.2 cN / dtex or more and 11.6 cN / dtex or less, and particularly preferably 4.3 cN / dtex or more and 11.4 cN / dtex or less. When the single fiber strength of the core-sheath type composite fiber is within the above-mentioned range, a fiber assembly having excellent puncture strength can be obtained. In the present invention, the single fiber strength is measured in accordance with JIS L 1015:2021.

[0031] The sheath-core composite fiber preferably has an elongation of 100% or less, more preferably 70% or less, and even more preferably 50% or less. When the sheath-core composite fiber has an elongation of 100% or less, a fiber assembly that is resistant to elongation due to external forces can be obtained. In particular, when the fiber is used as a fiber constituting a battery separator, elongation during the manufacturing process can be suppressed, resulting in a battery separator with little shrinkage. In the present invention, the elongation is measured in accordance with JIS L 1015:2021. The lower limit of the elongation of the sheath-core composite fiber is not particularly limited, but is preferably 15% or more, more preferably 20% or more, and particularly preferably 25% or more.

[0032] The core-sheath composite fiber has a Young's modulus of 3000 N / mm 2More than 9000N / mm 2 Preferably, it is 3200 N / mm or less. 2 More than 8250N / mm 2 More preferably, it is 3500 N / mm or less. 2 More than 7900N / mm 2 More preferably, it is 4000 N / mm or less. 2 More than 7500N / mm 2 It is even more preferable that the Young's modulus of the core-sheath type composite fiber is within the above-mentioned range, a fiber assembly having excellent puncture strength can be obtained.

[0033] In the core-sheath type composite fiber, the extrapolated melting initiation temperature (T im ) and melting peak temperature (T pm ) is determined by differential scanning calorimetry (DSC). DSC measurements are performed in accordance with JIS K 7121:1987. That is, 3 mg of a core-sheath type composite fiber as a sample is filled into a sample holder. Then, differential scanning calorimetry is performed by heating the core-sheath type composite fiber to a temperature that is at least 30°C higher than the melting point of the thermoplastic resin with the highest melting point among the thermoplastic resins contained in the fiber. For example, the heating rate is set to 20°C per minute, and the temperature is raised from 20°C to 300°C.

[0034] The extrapolated melting onset temperature (T im ) and melting peak temperature (T pm When the sample is heated, the melting of polypropylene begins at around 150°C and ends at around 180°C. At this time, the extrapolated melting onset temperature (T im For example, as shown in FIG. 2, the temperature at the intersection of a straight line extending the low-temperature side baseline of the melting peak of polypropylene toward the high-temperature side and a tangent line drawn at the point where the slope of the curve on the low-temperature side of the melting peak is maximum is the extrapolated melting onset temperature (T im The melting peak temperature of polypropylene (T pm ) is the temperature at the apex of the melting peak, as shown in Figure 2. When the melting peak of polypropylene has multiple apexes, the temperature of the apex measured at the highest temperature is taken as the melting peak temperature.

[0035] In the core-sheath type composite fiber, the extrapolated melting initiation temperature (T im ) and the melting peak temperature of polypropylene (T pm ) temperature difference (T pm -T im ) is 12°C or higher. pm -T im ) of 12°C or higher can be presumed to mean that the polypropylene in the core-sheath composite fiber contains a relatively large amount of phases with small crystallite sizes. This is because, when melt-spinning is performed using high-density polyethylene for the sheath component and polypropylene for the core component, the molten core component and sheath component extruded from the composite spinning nozzle are rapidly cooled while being taken up at a predetermined spinning speed, thereby reducing the crystallinity of the undrawn fiber tow and relatively increasing the proportion of amorphous and Smectic phases of polypropylene in the undrawn fiber tow. Therefore, in addition to improving the drawability of the undrawn fiber tow, since an undrawn fiber tow with a relatively small crystalline phase is drawn, crystals do not easily grow around the crystalline phase contained in the undrawn tow as nuclei, and it is presumed that much of the amorphous and Smectic phases change to phases with small crystallite sizes. Then, by drawing the undrawn fiber tow under predetermined conditions, the extrapolated melting onset temperature (T im ) and the melting peak temperature of polypropylene (T pm ) temperature difference (melting peak temperature - extrapolated melting onset temperature: T pm -T im A core-sheath type composite fiber having high strength and low thermal shrinkage and a T pm -T im ) is preferably 12.5°C or higher, more preferably 13.0°C or higher, even more preferably 13.5°C or higher, and particularly preferably 14.0°C or higher. pm -T im The upper limit of the temperature is not particularly limited, but may be 20° C. or lower, for example, from the viewpoint that when the crystal size distribution of the crystalline phase is wide, stretchability decreases in areas with large crystal sizes.

[0036] The sheath-core composite fiber is a core-sheath composite fiber containing a core component containing polypropylene and a sheath component containing high-density polyethylene, and is obtained by drawing an undrawn fiber tow in which the polypropylene has been made into a state with a low crystalline phase during melt spinning. Because the polypropylene contained in the undrawn fiber tow has a low crystalline phase, the high-density polyethylene, which is softer than polypropylene, is not inhibited by the polypropylene during drawing. Therefore, the high-density polyethylene is sufficiently drawn in the drawing process, leading to increased orientation and a high degree of crystallinity. This can be confirmed by performing differential scanning calorimetry (DSC) using the obtained sheath-core composite fiber to determine the heat of fusion (also called melting energy) of the high-density polyethylene.

[0037] The heat of fusion of the high-density polyethylene and the heat of fusion of the polypropylene contained in the sheath-core composite fiber are determined by differential scanning calorimetry (DSC). The DSC measurement is performed in accordance with JIS K 7121:1987. First, the mass proportions of polypropylene and high-density polyethylene are determined for the sheath-core composite fiber to be measured by DSC. The content of polypropylene and high-density polyethylene is determined by determining the ratio (volume ratio) of the core component to the sheath component from the production conditions of the sheath-core composite fiber to be measured by DSC, specifically, when the extrusion amounts per unit time of the molten polypropylene and high-density polyethylene during melt spinning are known, and then the mass proportions of polypropylene and high-density polyethylene in the sheath-core composite fiber are determined from the densities of the polypropylene and high-density polyethylene, the mass proportion of polypropylene in the core component, and the mass proportion of high-density polyethylene in the sheath component.

[0038] When the detailed manufacturing conditions of a sheath-core conjugate fiber to be subjected to differential scanning calorimetry (DSC) are unknown, the mass proportions of polypropylene and high-density polyethylene in the sheath-core conjugate fiber can be determined by a known method. One method for determining the mass proportions of polypropylene and high-density polyethylene in the sheath-core conjugate fiber involves printing a cross section of the sheath-core conjugate fiber to be subjected to differential scanning calorimetry (DSC) using a scanning electron microscope at a magnification of 500 to 2000 times, cutting the printout into a core component and a sheath component, measuring the total mass of the paper pieces of the core component and the total mass of the paper pieces of the sheath component, and using this ratio as the ratio (volume ratio) of the core component to the sheath component, and then determining the mass proportions of polypropylene and high-density polyethylene in the sheath-core conjugate fiber from the densities of the polypropylene and high-density polyethylene, the mass proportion of polypropylene in the core component, and the mass proportion of high-density polyethylene in the sheath component. Other examples of methods for determining the mass proportions of polypropylene and high-density polyethylene in a core-sheath type composite fiber include a combination of known analytical methods, such as gel permeation chromatography (GPC), cross fractionation chromatography (CFC), and applications of these methods such as temperature rising elution fractionation (TREF), crystallization elution fractionation (CEF), solvent gradient chromatography (SGIC), and high-temperature liquid chromatography (also referred to as high-temperature LC) utilizing differences in interactions with graphite carbon columns, such as temperature gradient chromatography (TGIC).

[0039] Next, the core-sheath type composite fiber whose mass ratio of polypropylene and high-density polyethylene in the core-sheath type composite fiber has been determined is used to perform DSC measurement in accordance with JIS K 7121:1987. pm -T im The heating rate is set at 20°C per minute, the same as the method for determining the (T pm -T im As in the case of measuring the sheath-core composite fiber, 3 mg of the sample is filled into a sample holder. The sheath-core composite fiber is then heated to a temperature at least 30°C higher than the melting point of the thermoplastic resin with the highest melting point among the thermoplastic resins contained in the fiber, and differential scanning calorimetry is performed.

[0040] From the obtained DSC curve, the heat of fusion (ΔH PP ) and the heat of fusion of polyethylene (ΔH PE The DSC curve contains at least two endothermic peaks: an endothermic peak associated with the melting of high-density polyethylene observed at about 115°C to about 145°C (the melting peak temperature varies depending on the type of high-density polyethylene), and an endothermic peak associated with the melting of polypropylene observed at about 145°C to about 185°C (the melting peak temperature varies depending on the type of polypropylene).

[0041] In differential scanning calorimetry (DSC measurement), for example, as shown in FIGS. 3 and 4, a DSC curve (DSC chart) and a low-temperature baseline (BL) are obtained at each endothermic peak. LT ) from its low-temperature end to the high-temperature baseline (BL HT ) and a straight line (BL E The heat of fusion at each endothermic peak is calculated from the area of ​​the region surrounded by the squares. In this case, the heat of fusion measured at each endothermic peak is measured as the heat of fusion of the entire sample, i.e., per 1 mg of core-sheath type composite fiber.

[0042] In DSC measurements, it is high-density polyethylene that undergoes a phase change and melts in the temperature range of 115°C to 145°C, while polypropylene, which generally has a melting point of 150°C or higher, is thought to undergo almost no phase change in this temperature range. Therefore, as shown in Figure 3, the endothermic peak and its heat of fusion measured in the temperature range of 115°C to 145°C on the DSC chart are converted from the heat of fusion per mg of sheath-core composite fiber to the heat of fusion per mg of high-density polyethylene in the sheath-core composite fiber. That is, the heat of fusion per mg of high-density polyethylene is determined by dividing the heat of fusion measured in the temperature range of about 115°C to about 145°C by the pre-calculated mass fraction of high-density polyethylene in the sheath-core composite fiber based on the following mathematical formula (1): For example, if the heat of fusion of the endothermic peak (melting peak) measured in the temperature range from 115°C to 145°C is 75 mJ / mg and the content of high-density polyethylene in the core-sheath composite fiber is 50% by mass, the heat of fusion per 1 mg of high-density polyethylene in the core-sheath composite fiber is 150 mJ / mg.

[0043]

[0044] Furthermore, in DSC measurements, in the temperature range from about 145°C to about 185°C, high-density polyethylene, which generally has a melting point of 140°C or less, is already melted and is thought to undergo little phase change. It is polypropylene that undergoes a phase change and melts in this temperature range. Therefore, as shown in Figure 4, the endothermic peak and heat of fusion measured at about 145°C to about 185°C are converted from the heat of fusion per 1 mg of sheath-core composite fiber to the heat of fusion per 1 mg of polypropylene in the sheath-core composite fiber. The conversion method is the same as when determining the heat of fusion of high-density polyethylene; the heat of fusion per 1 mg of sheath-core composite fiber measured at about 145°C to about 185°C is divided by the pre-calculated polypropylene content (mass%) in the sheath-core composite fiber to determine the heat of fusion per 1 mg of polypropylene. For example, based on the following mathematical formula (2), if the heat of fusion of the endothermic peak (melting peak) measured at approximately 145°C to approximately 185°C is 50 mJ / mg and the polypropylene content in the core-sheath composite fiber is 50 mass%, the heat of fusion per 1 mg of polypropylene in the core-sheath composite fiber is 100 mJ / mg.

[0045]

[0046] The heat of fusion (ΔH) per 1 mg of high-density polyethylene contained in the core-sheath type composite fiber is determined from a DSC curve obtained by differential scanning calorimetry at a heating rate of 20°C per minute. PE ) is preferably 125 mJ / mg or more. When the heat of fusion per mg of high-density polyethylene contained in the sheath-core composite fiber is 125 mJ / mg or more, it is estimated that the high-density polyethylene contained in the sheath component of the obtained sheath-core composite fiber has been sufficiently crystallized. The heat of fusion (ΔH PE The heat of fusion (ΔH ) per 1 mg of high-density polyethylene contained in the core-sheath type composite fiber is more preferably 135 mJ / mg or more, even more preferably 140 mJ / mg or more, and particularly preferably 145 mJ / mg or more. PE The upper limit of the total energy density is not particularly limited, but is 280 mJ / mg or less.

[0047] The heat of fusion (ΔH) per 1 mg of polypropylene contained in the core-sheath type composite fiber is determined from the DSC curve obtained by differential scanning calorimetry at a heating rate of 20°C per minute. PP The heat of fusion (ΔH ) per 1 mg of polypropylene contained in the core-sheath type composite fiber is preferably 55 mJ / mg or more, more preferably 60 mJ / mg or more, and even more preferably 65 mJ / mg or more. PP When the heat of fusion (ΔH ) per 1 mg of polypropylene contained in the core sheath type composite fiber is 55 mJ / mg or more, it is estimated that the crystallization of the polypropylene contained in the core component of the obtained core sheath type composite fiber has progressed sufficiently. PP ) is preferably 150 mJ / mg or less, more preferably 145 mJ / mg or less, and even more preferably 140 mJ / mg or less, from the viewpoint of the dimensional stability of the obtained core-sheath type composite fiber.

[0048] The core-sheath type composite fiber preferably has a dry heat dimensional change at 145°C measured in accordance with JIS L 1015:2021 of 11% or less, more preferably 10.7% or less, even more preferably 10.5% or less, even more preferably 10.3% or less, and particularly preferably 10% or less, thereby improving dimensional stability under heating conditions.

[0049] The sheath-core composite fiber preferably has a dry heat shrinkage rate at 140°C of 5.4% or less, more preferably 5.35% or less, even more preferably 5.3% or less, even more preferably 5.2% or less, and particularly preferably 5.1% or less, which improves dimensional stability under heating conditions.

[0050] (Method for producing sheath-core composite fibers) Next, a method for producing sheath-core composite fibers will be described. The sheath-core composite fibers can be obtained, for example, by supplying a sheath component containing 70% by mass or more of high-density polyethylene and a core component containing 70% by mass or more of polypropylene to a composite spinning nozzle, for example a concentric sheath-core composite spinning nozzle, which is arranged so that, in the fiber cross section, the surface of the sheath-core composite fiber is covered by the sheath component and the center of gravity of the core component substantially coincides with the center of gravity of the sheath-core composite fiber, and melt-spinning the sheath component into undrawn spun filaments (undrawn fiber tows), and then drawing the undrawn fiber tows obtained.

[0051] Specifically, a core-sheath composite spinning nozzle capable of obtaining a predetermined fiber cross section is first attached to a melt spinning machine, and the core and sheath components are supplied to the composite spinning nozzle. The core and sheath components are melt-spun at a spinning temperature (composite spinning nozzle temperature) of, for example, 200°C to 350°C. The molten core and sheath components (molten resin) extruded from the composite spinning nozzle are rapidly cooled directly below the composite spinning nozzle and taken up at a spinning speed of 300 m / min to 2300 m / min. This produces an undrawn fiber tow in which the polypropylene contains a large amount of amorphous phase and smectic crystals, in other words, a low content of crystalline phase (α crystals), which is easily drawn in the drawing process.

[0052] As described above, the Q value of the polypropylene before spinning is preferably 2.6 or more and 6 or less. When the Q value before spinning is within the above range, the polypropylene contained in the core component has a large polydispersity and is in a state where it contains a large amount of low molecular weight components and high molecular weight components, and the polypropylene contained in the core component of the obtained sheath-core composite fiber also has a large polydispersity and is in a state where it contains a large amount of low molecular weight components and high molecular weight components.

[0053] In melt spinning, the spinning temperature is preferably 250° C. or higher and 340° C. or lower, and more preferably 260° C. or higher and 330° C. or lower. The melt extrusion temperature (barrel temperature) of the core component containing polypropylene is preferably 250° C. or higher and 350° C. or lower, more preferably 255° C. or higher and 340° C. or lower, and even more preferably 260° C. or higher and 330° C. or lower. The melt extrusion temperature (barrel temperature) of the sheath component containing high-density polyethylene is not particularly limited, but is, for example, preferably 230° C. or higher and 330° C. or lower, and more preferably 250° C. or higher and 300° C. or lower.

[0054] In melt spinning, although not particularly limited, from the viewpoint of stably obtaining undrawn fibers, the molten core component and sheath component are taken up at a spinning speed (also referred to as take-up speed) of 300 m / min to 2300 m / min. The slower the take-up speed, the smaller the force (spinning tension) that tries to draw the molten polypropylene, making it more difficult for the polypropylene to be oriented, and the proportion of crystalline phase contained in the polypropylene in the resulting undrawn fiber tow is more likely to be smaller. On the other hand, when spinning at a high speed, the fineness of the undrawn fiber tow is reduced, and therefore it can be sufficiently drawn even with a relatively low drawing ratio in the drawing step, making it easier to obtain a core-sheath composite fiber with a small fineness and improving productivity. In melt spinning, the take-up speed is preferably 500 m / min to 2300 m / min, more preferably 600 m / min to 2000 m / min, and particularly preferably 700 m / min to 1800 m / min.

[0055] The molten resin discharged from the nozzle holes of the composite spinning nozzle is rapidly cooled and stretched directly below the nozzle, causing the molecular chains to become oriented, and the core and sheath components to solidify, forming an undrawn fiber tow. At this time, the molecular chains contained in the molten resin are random and able to move freely, but it is thought that their movement slows down as they are cooled.

[0056] In the method for producing a core-sheath composite fiber according to one or more embodiments of the present invention, "rapid cooling directly below the composite spinning nozzle" means cooling at a position close to the composite spinning nozzle. Specifically, it is preferable to take up the molten resin extruded from the spinning nozzle while cooling it at a position closer to the composite spinning nozzle, while taking care not to cause thread breakage. By cooling the molten resin close to the composite spinning nozzle to the extent that thread breakage does not occur and taking it up, the polypropylene contained in the obtained undrawn fiber tow tends to have a non-crystalline phase, specifically an amorphous phase or a structure containing a large amount of smectic crystals. When an undrawn fiber tow containing a large amount of these phases is drawn, these phases are more easily drawn than the crystalline phase, so the drawing process can be carried out stably even at a high drawing ratio.

[0057] In melt spinning, the position at which the molten resin is cooled can be adjusted, for example, by the position of a forced cooling device (commonly referred to as a chimney) that blows cooling air onto the molten resin. Adjusting the position of the forced cooling device does not require any special equipment and is easy to adjust. In the method for producing a core-sheath type composite fiber according to one or more embodiments of the present invention, it is also preferable to adjust the position of the forced cooling device so that the polypropylene contained in the obtained undrawn fiber tow contains a large amount of amorphous phase and smectic crystals other than the crystalline phase.

[0058] In one or more embodiments of the method for producing a core-sheath composite fiber of the present invention, the distance from the nozzle surface of the composite spinning nozzle provided in the spin pack to the cooling start point is preferably 25 mm or more and less than 85 mm. By setting the position of the cooling start point within this range, the molten resin is cooled before it is stretched during melt spinning and molecular chain orientation progresses, so the resulting undrawn fiber tow is likely to be an unoriented undrawn fiber tow. The distance from the nozzle surface of the composite spinning nozzle to the cooling start point is more preferably 30 mm or more and less than 82 mm, even more preferably 40 mm or more and less than 80 mm, even more preferably 40 mm or more and less than 70 mm, even more preferably 40 mm or more and less than 65 mm, and particularly preferably 40 mm or more and less than 60 mm. More specifically, when the nozzle hole diameter is 0.6 mm or more, the distance from the nozzle surface of the composite spinning nozzle to the cooling start point is preferably 30 mm or more and less than 70 mm, more preferably 35 mm or more and less than 65 mm, and particularly preferably 40 mm or more and less than 60 mm. Alternatively, when the nozzle hole diameter is less than 0.6 mm, the distance from the nozzle face of the composite spinning nozzle to the cooling start point is preferably 30 mm or more and 80 mm or less, more preferably 35 mm or more and 75 mm or less, even more preferably 40 mm or more and 70 mm or less, even more preferably 40 mm or more and 65 mm or less, and particularly preferably 40 mm or more and 60 mm or less. In the present invention, the cooling start point refers to the position of the cooling air blown onto the molten resin that is closest to the nozzle face of the composite spinning nozzle, and in the case of a cooling air generator in which the cooling air hits the molten resin perpendicularly, it coincides with the uppermost end of the cooling air generator. 5, when undrawn fiber tow 24 is obtained by discharging molten resin 23 from the outlet of a composite spinning nozzle provided in a spinning pack 21 and withdrawing it while cooling with cooling air, the distance from the nozzle face of the composite spinning nozzle to the cooling start point refers to the distance L from the nozzle face of the composite spinning nozzle that discharges molten resin 23 to the upper end position 22 of the cooling air that is blown onto the molten resin 23 and is closest to the nozzle face of the composite spinning nozzle. More specifically, the upper end position 22 of the cooling air coincides with the uppermost position of the cooling air generating device.

[0059] The cooling method of the forced cooling device is not particularly limited, and may be a uniflow-type cooling device that cools the discharged and stretched unstretched fibers from one direction, or a circular-type cooling device that blows cooling air onto the molten resin or unstretched fiber tow from the inside to the outside, or from the outside to the inside, of the molten resin or unstretched fiber tow. The gas used for the cooling air is not particularly limited, but rare gases such as argon and helium, which are stable (very low reactivity) at room temperature, nitrogen, or air are preferably used, and among these, nitrogen or air, which can be supplied inexpensively, is particularly preferred. The speed of the cooling air is preferably 0.2 m / s or more and 5 m / s or less, and more preferably 0.3 m / s or more and 3 m / s or less. The temperature of the cooling air is preferably low to ensure uniform cooling, but may be 40°C or less, or 15°C or more and 35°C or less, considering the cost of adjusting the temperature.

[0060] The single fiber fineness of the obtained undrawn fiber tow is in the range of 0.8 dtex or more and 3.2 dtex or less. If the single fiber fineness of the undrawn fiber tow is less than 0.8 dtex, yarn breakage tends to occur frequently during spinning. On the other hand, if the single fiber fineness of the undrawn fiber tow exceeds 3.2 dtex, it becomes difficult to obtain a fine sheath-core composite fiber unless the drawing step described below is performed at a very high draw ratio. From the viewpoint of easily obtaining a fine sheath-core composite fiber by drawing the undrawn fiber tow, the single fiber fineness of the undrawn fiber tow is preferably 1.3 dtex or more and 3.1 dtex or less, more preferably 1.5 dtex or more and 3.0 dtex or less, and particularly preferably 1.8 dtex or more and 2.9 dtex or less.

[0061] In the method for producing a sheath-core composite fiber according to one or more embodiments of the present invention, the undrawn fiber tow obtained by melt spinning has a polypropylene-containing core component that contains a crystalline phase, but is a resin segment that contains a large amount of amorphous phase or Smectic crystals, or a resin segment that contains only amorphous phase or Smectic crystals. Furthermore, the sheath component containing high-density polyethylene is crystallized to a state where a large amount of crystalline phase is present. This state prevents the polypropylene and high-density polyethylene from mutually inhibiting the drawability of the undrawn fiber tow, improving the drawability of the undrawn fiber tow. As a result, it is presumed that by sufficiently drawing the undrawn fiber tow, not only is the amorphous phase that causes thermal shrinkage upon heating reduced within the obtained sheath-core composite fiber, but overdrawn polypropylene (polypropylene that has been excessively stretched, causing distortion and breakage in the molecular arrangement) that causes thermal shrinkage upon heating is less likely to remain.

[0062] In the method for producing a splittable conjugate fiber according to one or more embodiments of the present invention, whether the undrawn fiber tow obtained by melt spinning is in the above-described state can be determined by subjecting the undrawn fiber tow to X-ray diffraction (XRD) and examining the intensity of the diffraction peaks obtained.

[0063] Specifically, in an X-ray diffraction chart obtained by measuring the undrawn fiber tow by X-ray diffraction (XRD), the diffraction peaks of polypropylene, namely, diffraction peak PP1 at 2θ = 14.2 ± 0.5°, diffraction peak PP2 at 2θ = 17 ± 0.5°, and diffraction peak PP3 at 2θ = 18.6 ± 0.5°, and the diffraction peak of high-density polyethylene, diffraction peak PE1 at 2θ = 21.6 ± 0.5°, are shown in the diffraction intensity (I PE ) and the ratio I of the sum (I) of the diffraction intensities of the diffraction peaks PP1, PP2, PP3 and PE1 PE / I satisfies the requirement that I is 0.2 or more and 1.0 or less. PE / I is preferably 0.3 or more, and more preferably 0.4 or more. When the diffraction peaks PP1, PP2, and PP3 are not detected (undetected), the diffraction intensities of these diffraction peaks are 0.

[0064] I PE When / I is 0.2 or more, particularly 0.3 or more, it can be determined that the polypropylene in the undrawn fiber has a small amount of crystalline phase, i.e., a large amount of amorphous phase and smectic crystals, and the crystalline states of the polypropylene and polyethylene are well balanced. PE When / I is 1.0, it can be determined that the polypropylene in the undrawn fiber is in an amorphous phase or in a state of only smectic crystals. When such an undrawn fiber tow is drawn, the polypropylene and the high-density polyethylene do not inhibit each other's drawability, improving the drawability of the undrawn fiber tow. As a result, it is presumed that by sufficiently drawing the undrawn fiber tow, not only is there less of the amorphous phase that causes thermal shrinkage when heated inside the resulting splittable conjugate fiber, but there is also less chance of overdrawn polypropylene (polypropylene that has been excessively stretched, causing distortion in the molecular arrangement and breakage) remaining, which causes thermal shrinkage when heated.

[0065] Next, the undrawn fiber tow is drawn to obtain drawn filaments (sheath-core composite fibers). The drawing is carried out at a temperature in the range of 60°C to 110°C. The drawing is preferably carried out at a temperature equal to or lower than the melting point of the resin with the lowest melting point among the resin components constituting the sheath-core composite fiber. The drawing temperature is preferably 70°C to 105°C, more preferably 70°C to 100°C, still more preferably 75°C to 95°C, and particularly preferably 80°C to 90°C. When the drawing is carried out at such a drawing temperature, melting and / or softening of the sheath component containing high-density polyethylene during the drawing step is suppressed, thereby suppressing fusion of the fibers to each other, and therefore the water dispersibility of the resulting sheath-core composite fiber is likely to be improved.

[0066] In the drawing step, although not particularly limited, from the viewpoint of increasing the crystallinity of the polypropylene and high-density polyethylene and obtaining a high-strength sheath-core conjugate fiber, the total draw ratio is 2.7 times or more. When the total draw ratio is 2.7 times or more, the polypropylene and high-density polyethylene contained in the sheath-core conjugate fiber are sufficiently crystallized, thereby increasing the single fiber strength of the obtained sheath-core conjugate fiber, and thus the mechanical strength, such as the tensile strength and puncture strength, of a fiber assembly using these fibers is high. In the drawing step, the total draw ratio is preferably 3.0 times or more, more preferably 3.2 times or more, even more preferably 3.5 times or more, and even more preferably 4.0 times or more. In the drawing step, from the viewpoint of productivity, i.e., suppressing frequent yarn breakage in the drawing step and suppressing a state in which a portion of the polypropylene contained in the drawn filament is excessively stretched, resulting in distortion or destruction of the crystalline phase, the total draw ratio is 10.0 times or less, preferably 8.0 times or less, and more preferably 7.5 times or less.

[0067] In the drawing step, the total draw ratio is preferably 60% to 100% of the maximum draw ratio. When the total draw ratio is 60% to 100% of the maximum draw ratio, the polypropylene contained in the undrawn fiber tow is less likely to be overdrawn during drawing, and the resulting core-sheath type composite fiber has excellent water dispersibility and exhibits little thermal shrinkage even when heated.

[0068] Furthermore, in the drawing process, in order to facilitate crystallization of the polypropylene and high-density polyethylene contained in the undrawn fiber tow and to obtain a core-sheath type composite fiber with a sufficiently small fineness, the total draw ratio is preferably 60% to 100% of the maximum draw ratio, more preferably 65% ​​to 99.5%, even more preferably 70% to 99%, and particularly preferably 80% to 95%. Furthermore, the total draw ratio is particularly preferably 2.7 times to 10.0 times while satisfying the above-mentioned range. The drawing method may be either a wet drawing method or a dry drawing method. Air, steam, water, oils such as glycerin, and the like can be used as the heat medium. In the wet drawing method, drawing can be performed while heating in a liquid, for example, in hot water or warm water. In the dry drawing method, drawing can be performed while heating in a high-temperature gas or with a high-temperature metal roll or the like. Alternatively, steam drawing may be used in which the fibers are drawn while being heated under normal pressure or under pressure using steam as a heating medium.

[0069] The stretching process may be a single-stage stretching process or a multi-stage stretching process having two or more stages. In the case of a single-stage stretching process, the total stretching ratio is the stretching ratio of the single-stage stretching process. In the case of a multi-stage stretching process, the total stretching ratio can be calculated by multiplying the stretching ratios of each stage. For example, if a stretching process is performed at 85°C with a stretching ratio of 1.5 times and then a stretching process is performed at 90°C with a stretching ratio of 4 times, the total stretching ratio in this manufacturing method is 6 times.

[0070] In the present invention, the "maximum stretching ratio (V max")" refers to the value measured as follows. Melt spinning is performed using a core-sheath type composite spinning nozzle, and the resulting undrawn fiber tow is subjected to wet drawing in a drawing tank filled with warm water adjusted to the drawing temperature, and the undrawn fiber tow is drawn in the drawing tank. In this case, the delivery speed (V1) of the roll that delivers the undrawn fiber tow is set to 10 m / min, and the take-up speed (V2) of the metal roll on the take-up side is gradually increased from 10 m / min. The take-up speed of the metal roll on the take-up side when the undrawn fiber tow breaks is defined as the maximum drawing speed, and the ratio (V2 / V1) of the maximum drawing speed to the delivery speed of the roll that delivers the undrawn fiber tow is determined, and the obtained speed ratio is defined as the maximum draw ratio (V max When the stretching treatment is carried out by dry stretching, the maximum stretch ratio under those conditions can be similarly determined by adjusting the temperature of the heat medium (for example, the surface of a metal roll) during the stretching treatment to the same temperature as during the stretching treatment, and gradually increasing the winding speed in the same manner as above.

[0071] In the case of a single-stage drawing in which the drawing process is performed, or in the case of a plurality of separate drawing processes at the same drawing temperature using the same drawing method (for example, a case where dry drawing at 90°C by 2 times is repeated three times, or a case where dry drawing at 90°C by 1.5 times is performed followed by dry drawing at 90°C by 4 times), the maximum draw ratio can be measured using the same method and temperature as in the drawing process. When the spun filaments are drawn by a plurality of stages of drawing, the maximum draw ratio is measured using the same drawing method and temperature as in the first-stage drawing process.

[0072] In the case where an undrawn fiber tow is drawn by a so-called multistage drawing in which drawing treatment is performed multiple times, and the drawing temperature is the same in all drawing treatments but the drawing methods are different (specifically, an example is a case where steam drawing is performed in steam at 100°C at a draw ratio of 3, followed by dry drawing with metal rolls heated to 100°C at a draw ratio of 2), the maximum draw ratio is measured by both methods (in this example, steam drawing at 100°C and dry drawing with metal rolls at 100°C), and the larger maximum draw ratio is taken as the maximum draw ratio under those production conditions.

[0073] From the viewpoint of easily obtaining high-strength and fine-fineness core-sheath composite fibers, the drawing is preferably a multi-stage drawing process of two or more stages. The second and subsequent stages of the multi-stage drawing may be tension heat setting, in which heat treatment is performed under tension, or relaxation heat setting, in which heat treatment is performed under relaxation. That is, in this specification, multi-stage drawing may include a drawing step and a heat treatment (also referred to as heat setting) step, and specifically, may include a first drawing step (also referred to as first-stage drawing) step and a heat treatment (also referred to as heat setting) step, or may include a first drawing step, a second drawing step (also referred to as second-stage drawing) step, and a heat treatment step. In the multi-stage drawing, the drawing (heat treatment) temperature is preferably 60°C or higher and 110°C or lower, more preferably 70°C or higher and 105°C or lower, even more preferably 75°C or higher and 100°C or lower, and even more preferably 80°C or higher and 100°C or lower. In the multistage stretching, the stretching ratios of the first stretching and the second stretching may be appropriately adjusted so that the total stretching ratio is within the range of 2.7 times or more and 10.0 times or less. For example, in the multistage stretching, the stretching ratio of the first stretching is preferably 1.0 times or more and 8.0 times or less, more preferably 2.0 times or more and 7.5 times or less, even more preferably 2.2 times or more and 7.0 times or less, even more preferably 2.6 times or more and 6.0 times or less, and even more preferably 3.0 times or more and 5.0 times or less, and the stretching ratio of the second stretching may be 0.80 times or more and 10.0 times or less, 0.85 times or more and 6.0 times or less, 0.90 times or more and 4.0 times or less, or 0.95 times or more and 1.5 times or less. In the multistage stretching, the stretching ratio of the first stretching may be higher than the stretching ratio of the second stretching, and the stretching ratio of the second stretching may be higher than the stretching ratio of the first stretching. In the multistage stretching, when the first stretching is a high stretching, the stretching ratio of the first stretching is preferably 2.0 times or more and 8.0 times or less, more preferably 2.2 times or more and 7.0 times or less, even more preferably 2.6 times or more and 6.0 times or less, and even more preferably 3.0 times or more and 5.0 times or less, and the stretching ratio of the second stretching may be 0.80 times or more and 1.5 times or less, 0.85 times or more and 1.4 times or less, 0.90 times or more and 1.4 times or less, or 1.0 times or more and 1.3 times or less.More specifically, in the multistage drawing, the first drawing is preferably performed at a drawing temperature of 60° C. or higher and 110° C. or lower and a draw ratio of 2.0 times or higher and 8.0 times or lower, more preferably at a drawing temperature of 70° C. or higher and 105° C. or lower and a draw ratio of 2.2 times or higher and 7.0 times or lower, even more preferably at a drawing temperature of 75° C. or higher and 100° C. or lower and a draw ratio of 2.6 times or higher and 6.0 times or lower, and even more preferably at a drawing temperature of 80° C. or higher and 100° C. or lower and a draw ratio of 3.0 times or higher and 5.0 times or lower. More preferably, the second stretching is performed at a stretching temperature of 60° C. to 110° C. and a stretching ratio of 0.80 to 1.5, more preferably at a stretching temperature of 70° C. to 105° C. and a stretching ratio of 0.85 to 1.4, even more preferably at a stretching temperature of 75° C. to 100° C. and a stretching ratio of 0.90 to 1.4, and even more preferably at a stretching temperature of 80° C. to 100° C. and a stretching ratio of 1.0 to 1.3. Furthermore, when the second or subsequent stage of multistage stretching is a relaxation heat set, the stretching temperature (heat treatment temperature) is preferably 60° C. to 110° C., more preferably 70° C. to 105° C., and even more preferably 80° C. to 100° C., and the stretching ratio may be 0.8 to less than 1.0, or may be 0.90 to less than 1.0. In addition, in multistage stretching, if the first stretching is dry stretching, the second and subsequent stages may be dry stretching and / or tension heat setting (dry). In addition, in multistage stretching, if the first stretching is wet stretching, the second and subsequent stages may be wet stretching and / or relaxation heat setting (wet). In the case of wet stretching (including wet heat setting), the stretch ratio may be within the above-mentioned range and is not particularly limited, but is preferably 0.80 times or more and less than 4.0 times, and more preferably 0.85 times or more and 1.5 times or less. In the case of dry stretching (including dry heat setting), the stretch ratio may be within the above-mentioned range and is not particularly limited, but is preferably 0.95 times or more and 8.0 times or less, more preferably 1.0 times or more and 7.0 times or less, even more preferably 2.0 times or more and 6.0 times or less, and particularly preferably 3.0 times or more and 5.0 times or less.More specifically, the multistage stretching is dry stretching, and the first stretching preferably has a stretching temperature of 60°C to 110°C and a stretch ratio of 1.0 to 8.0, and the second stretching preferably has a stretching temperature of 60°C to 110°C and a stretch ratio of 1.0 to 1.5. Alternatively, the multistage stretching is wet stretching, and the first stretching preferably has a stretching temperature of 60°C to 110°C and a stretch ratio of 1.0 to 8.0, and the second stretching preferably has a stretching temperature of 60°C to 110°C and a stretch ratio of 0.8 to 1.0. In addition, in the multistage stretching, the stretching temperature of the first stretching may be 100°C to 110°C or may be 60°C to less than 100°C.

[0074] A predetermined amount of fiber treatment agent is applied to the obtained drawn filaments as needed, and further, mechanical crimping is imparted using a crimper (crimping device) as needed. When producing nonwoven fabrics using a wet papermaking method, the fiber treatment agent facilitates dispersion of the fibers in water, etc. Furthermore, applying an external force from the fiber surface to the fibers to which the fiber treatment agent has been applied (for example, the force applied when crimping using a crimper) to impregnate the fibers with the fiber treatment agent further improves dispersibility in water, etc. The number of crimps is preferably in the range of 5 crimps / 25 mm to 30 crimps / 25 mm, more preferably in the range of 10 crimps / 25 mm to 20 crimps / 25 mm. A crimp number of 5 crimps / 25 mm or more improves dispersibility in water, etc., while a crimp number of 30 crimps / 25 mm or less reduces or eliminates fiber aggregation and clumping.

[0075] The drawn filaments after application of the fiber treatment agent (or in a wet state without application of the fiber treatment agent) are dried at a temperature in the range of 80°C to 110°C for several seconds to about 30 minutes to dry the fibers. The drying process may be omitted in some cases. The drawn filaments are then cut to a fiber length of 1 mm to 100 mm, preferably 2 mm to 70 mm.

[0076] When producing a drylaid nonwoven fabric containing a core-sheath composite fiber, it is necessary to dry the drawn filaments after application of a fiber treatment agent (or in a wet state without application of a fiber treatment agent). When drying the drawn filaments, the drawn filaments can be dried in a relaxed state at a temperature of 60°C or higher but lower than 110°C. After the drawn filaments are sufficiently dried, they are cut to a predetermined length to become core-sheath composite fibers suitable for producing drylaid nonwoven fabrics. When producing a drylaid nonwoven fabric using the core-sheath composite fiber of one or more embodiments of the present invention, the fiber length is not particularly limited. However, when a fiber web is obtained by a carding method, the fiber length is preferably 20 mm or higher and 100 mm or lower. Having a fiber length of 20 mm or higher and 100 mm or lower stabilizes productivity when producing a drylaid nonwoven fabric. When producing a drylaid nonwoven fabric, the fiber length is more preferably 25 mm or higher and 80 mm or lower, and particularly preferably 25 mm or higher and 65 mm or lower.

[0077] When a wetlaid nonwoven fabric is obtained using a core-sheath type composite fiber, the core-sheath type composite fiber may be in a wet or dry state, but is preferably in a wet state in consideration of dispersibility in water. Therefore, the drawn filaments after application of a fiber treatment agent (or in a wet state without application of a fiber treatment agent) do not need to be dried, and are cut to the desired fiber length while still wet. When a wetlaid nonwoven fabric is produced using the core-sheath type composite fiber of one or more embodiments of the present invention, the fiber length is not particularly limited, but is preferably 1 mm or more and less than 20 mm. A fiber length of 1 mm or more and less than 20 mm ensures good dispersibility in water when producing a wetlaid nonwoven fabric. The fiber length when producing a wetlaid nonwoven fabric is more preferably 2 mm or more and 15 mm or less, particularly preferably 3 mm or more and 12 mm or less, and most preferably 3 mm or more and 10 mm or less.

[0078] (Fiber Assembly) Next, a fiber assembly containing one or more embodiments of the core-sheath composite fiber of the present invention will be described. The form of the fiber assembly is not particularly limited, but examples thereof include woven fabrics, knitted fabrics, and nonwoven fabrics. The form of the fiber web of the nonwoven fabric is also not particularly limited, and examples thereof include a carded web formed by a carding method, an air-laid web formed by an air-laid method, and a wet-laid paper web formed by a wet-laid papermaking method.

[0079] In the fiber assembly, the content of sheath-core composite fibers is preferably 10% by mass or more, more preferably 15% by mass or more, and particularly preferably 20% by mass or more. When the content of sheath-core composite fibers is 10% by mass or more, the proportion of sheath-core composite fibers in the fiber assembly, for example, a nonwoven fabric, is high, and a dense nonwoven fabric tends to be easily obtained. In the fiber assembly, the upper limit of the content of sheath-core composite fibers is not particularly limited, but in the case of fiber assemblies for various wiping purposes such as personal and / or object wipers, fiber assemblies for battery separators used in various secondary batteries such as nickel-metal hydride batteries, and fiber assemblies for filtration layers of various filters such as cartridge filters and laminated filters, where a certain degree of voids between the constituent fibers and the resulting breathability and liquid permeability are required, the content of sheath-core composite fibers relative to the entire fiber assembly is preferably 90% by mass or less, more preferably 75% by mass or less, and particularly preferably 50% by mass or less. When the proportion of sheath-core conjugate fibers in a fiber assembly such as a drylaid nonwoven fabric or a wetlaid nonwoven fabric is 90% by mass or less, the proportion of sheath-core conjugate fibers in the resulting fiber assembly will not be too high, and there is no risk of the fiber assembly becoming an unnecessarily dense nonwoven fabric. Furthermore, when a wetlaid nonwoven fabric is to be produced using the sheath-core conjugate fibers, a content of 90% by mass or less of the sheath-core conjugate fibers will be less likely to occur in the defibration treatment during slurry preparation in the wetlaid papermaking process, thereby improving processability. If a particularly dense fiber assembly with few interfiber voids is required, a high content of sheath-core conjugate fibers is preferred even for fiber assemblies used for the above-mentioned applications. A fiber assembly with a content of sheath-core conjugate fibers exceeding 90% by mass or a fiber assembly consisting solely of sheath-core conjugate fibers may also be used.

[0080] In the fiber assembly, fibers other than the core-sheath composite fibers are not particularly limited. Examples include cellulosic fibers such as cotton, pulp, hemp, viscose rayon, and Tencel®; polyester fibers such as polyethylene terephthalate, polybutylene terephthalate, polytrimethylene terephthalate, polyethylene naphthalate, polylactic acid, and polybutylene succinate; polyamide fibers such as nylon 6, nylon 66, nylon 11, and nylon 12; and acrylic fibers. Other examples include polyethylene monofilaments such as low-density polyethylene, high-density polyethylene, and linear low-density polyethylene; polypropylene monofilaments such as isotactic, atactic, and syndiotactic polypropylenes polymerized using a conventional Ziegler-Natta catalyst or a metallocene catalyst; copolymers of these polyolefin monomers; and polyolefins polymerized using a metallocene catalyst (also known as a Kaminsky catalyst). Other examples include engineering plastic fibers such as polycarbonate, polyacetal, polystyrene, and cyclic polyolefins. The above-mentioned other fibers can be used alone or in combination of two or more. The fiber form is not particularly limited, and examples thereof include single fibers, concentric sheath-core composite fibers, eccentric sheath-core composite fibers, multi-core sheath-core composite fibers, side-by-side composite fibers, and islands-in-the-sea composite fibers. The cross-sectional shape of the fiber may be circular or irregular.

[0081] The fiber assembly may contain binder fibers in an amount of 90% by mass or less. The binder fiber content in the fiber assembly is more preferably 85% by mass or less, and even more preferably 80% by mass or less. The binder fibers (thermal bonding fibers) may be, for example, sheath-core composite fibers. A fiber assembly containing thermal bonding fibers bonds the constituent fibers together, resulting in a fiber assembly with excellent tensile strength and puncture strength.

[0082] The manufacturing method of a fiber assembly will be described using a nonwoven fabric as an example. A nonwoven fabric can be produced by preparing a fiber web according to a known method, and then, if necessary, subjecting the fiber web to heat treatment to thermally bond the fibers. If necessary, the fiber web may also be subjected to a fiber entanglement treatment. First, the fiber web can be produced, for example, by a dry method such as a carding method or an air-laying method using core-sheath composite fibers with a fiber length of 10 mm to 100 mm, or by a wet papermaking method using core-sheath composite fibers with a fiber length of 2 mm to 20 mm. For applications such as personal and / or object wipers and filters, nonwoven fabrics produced by a dry method such as a carding method or an air-laying method are preferred. This is because nonwoven fabrics produced by a dry method have a soft texture and an appropriate density. Furthermore, for applications such as battery separators, nonwoven fabrics produced from a wet-laid papermaking web are preferred. This is because nonwoven fabrics produced using a wet-laid papermaking web are generally dense and have good texture.

[0083] The fiber web may then be subjected to a thermal bonding treatment. For example, the fibers may be bonded together via the sheath component of the core-sheath composite fibers. The conditions for the thermal bonding treatment can be appropriately selected depending on the basis weight of the fiber web, the cross-sectional shape of the core-sheath composite fibers, and the type of resin constituting the fibers contained in the nonwoven fabric. For example, a cylinder dryer, a hot air blowing machine, a heat roll processing machine, or a hot embossing machine can be used as the heat treatment machine. A cylinder dryer is particularly preferred because it can thermally bond the fibers together while adjusting the thickness of the nonwoven fabric. The heat treatment temperature of the cylinder dryer is, for example, preferably 80°C or higher and 160°C or lower when the sheath component of the binder fiber is an ethylene-vinyl alcohol copolymer; preferably 100°C or higher and 160°C or lower when the sheath component of the binder fiber is various polyethylenes; and preferably 130°C or higher and 145°C or lower when the sheath component of the binder fiber is high-density polyethylene.

[0084] As described below, when the fiber web is subjected to a hydroentanglement treatment, the thermal bonding treatment is preferably carried out before the hydroentanglement treatment. If the fibers of the fiber web are bonded together in advance and then the hydroentanglement treatment is carried out, the fibers are less likely to "escape" when hit by the high-pressure water stream, and the fibers can be tightly entangled. The thermal bonding treatment may also be carried out after the fibers have been entangled. In other words, the order of the thermal bonding treatment and the hydroentanglement treatment is not particularly limited as long as the desired nonwoven fabric is obtained.

[0085] In the fiber assembly of one or more embodiments of the present invention, the fibers may be entangled. A preferred method for entangling the fibers is hydroentanglement, in which the fibers are entangled by the action of a high-pressure water jet. Hydroentanglement can firmly entangle the fibers without impairing the denseness of the entire nonwoven fabric.

[0086] The conditions for the hydroentanglement treatment can be appropriately selected depending on the type and basis weight of the fiber web used, and the type and ratio of fibers contained in the fiber web. For example, 2 More than 100g / m 2 When subjecting the following wetlaid paper web to hydroentanglement, the fibrous web is placed on a support such as a plain weave structure of about 70 mesh to 100 mesh, and a columnar water stream at a water pressure of 1 MPa to 15 MPa, more preferably 2 MPa to 10 MPa, is sprayed onto one or both sides of the fibrous web from a nozzle having orifices with a hole diameter of 0.05 mm to 0.3 mm, spaced at intervals of 0.5 mm to 1.5 mm, once to 10 times. The fibrous web after hydroentanglement is subjected to a drying treatment as necessary.

[0087] The fiber assembly may be subjected to a hydrophilization treatment as needed. The hydrophilization treatment can be carried out by any method, such as fluorine gas treatment, vinyl monomer graft polymerization treatment, sulfonation treatment, discharge treatment, surfactant treatment, and hydrophilic resin application treatment. In particular, when the fiber assembly is used as a battery separator, it is preferable to subject it to a hydrophilization treatment. This is to increase the affinity with the electrolyte and improve the liquid retention.

[0088] The fiber assembly preferably has a basis weight of 5 g / m 2 More than 100g / m 2 More preferably, it is 10 g / m or less. 2 More than 100g / m 2 More preferably, it is 20 g / m or less. 2 80g / m or more 2 and particularly preferably 30 g / m 2 60g / m or more 2 The weight of the fiber assembly is 5 g / m or less. 2 When the weight per unit area of ​​the fiber assembly is 100 g / m or more, the fiber assembly has a good formation, and the fiber assembly is likely to have high strength and puncture strength. 2 If it is equal to or less than this, the breathability of the fiber assembly will not decrease.

[0089] A preferred example of a method for producing a nonwoven fabric is a wet papermaking method, which can be carried out by a conventional method. First, core-sheath composite fibers are prepared and dispersed in water to a concentration of 0.01% by mass or more and 0.6% by mass or less to prepare a slurry. Next, the slurry is made into paper using a short-wire, cylinder, or Fourdrinier papermaking machine, or a combination of short-wire, cylinder, and Fourdrinier papermaking machines, to obtain a wet-laid papermaking web in a water-containing state. Next, the wet-laid papermaking web in a water-containing state is dried using a heat treatment machine such as a cylinder dryer, and if necessary, binder fibers may be added to bond the web together while drying.

[0090] The fiber assembly can be used as a separator material for battery separators, etc. The battery separator preferably contains 10% by mass or more and 100% by mass or less of core-sheath composite fibers. The battery separator is preferably made of a nonwoven fabric, more preferably a wet-laid nonwoven fabric, from the viewpoints that it is easy to retain an electrolyte, maintain strength, and is as thin as possible.

[0091] The nonwoven fabric constituting the battery separator preferably has a basis weight of 5 g / m 2 More than 100g / m 2 More preferably, 8 g / m 2 80g / m or more 2 It is particularly preferably 10 g / m or less. 2 50g / m or more2 The basis weight is 5 g / m or less. 2 If the weight is less than 100 g / m, the density of the nonwoven fabric will vary, which may cause a short circuit when used as a battery separator. 2 If the thickness exceeds this value, the thickness of the battery separator will increase, and the amount of positive and negative electrodes in the battery will decrease accordingly.

[0092] The battery separator according to one or more embodiments of the present invention is incorporated into various batteries to form the battery. For example, in a cylindrical nickel-metal hydride secondary battery, a positive electrode plate and a negative electrode plate can be spirally wound with the battery separator according to one or more embodiments of the present invention interposed therebetween. The battery separator according to one or more embodiments of the present invention may also be used in other batteries, such as nickel-cadmium secondary batteries, nickel-iron secondary batteries, and nickel-zinc secondary batteries.

[0093] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0094] First, the measurement and evaluation methods used in the examples and comparative examples will be explained.

[0095] (Number Average Molecular Weight Mn, Weight Average Molecular Weight Mw, and Q Value) The number average molecular weight (Mn), weight average molecular weight (Mw), and the ratio Mw / Mn of the weight average molecular weight (Mw) to the number average molecular weight (Mn), i.e., the Q value, of polypropylene were measured by gel permeation chromatography (GPC). For the measurements, a gel permeation chromatograph (high-temperature GPC apparatus, manufactured by Polymer Laboratories, model number "PL-220") equipped with a differential refractive index detector RI was used.

[0096] 5 mg of polypropylene was weighed out, and 5 mL of 1,2,4-trichlorobenzene (TCB) containing 0.1% butylhydroxytoluene (BHT) as a stabilizer and antioxidant was added to the sample. The mixture was heated to 160°C to 170°C and stirred for 30 minutes to dissolve the polypropylene in the solvent. Next, to remove foreign matter such as undissolved sample from the solution, the solution was filtered through a metal filter to obtain a measurement sample solution. The resulting measurement sample solution was injected into the gel permeation chromatograph described above at a flow rate of 1.0 mL / min and an injection volume of 0.2 mL (200 μL) to measure the number average molecular weight (Mn) and weight average molecular weight (Mw). The Q value (Mw / Mn) was calculated from the obtained number average molecular weight (Mn) and weight average molecular weight (Mw). The measurement was performed using TCB containing 0.1% BHT as the measurement solvent, one HT-G column manufactured by Shodex and two HT-806M columns manufactured by Showa Denko K.K., monodisperse polystyrene (manufactured by Tosoh) as the standard sample, the temperature of the column thermostat was set to 145°C, and a GPC data processing system (manufactured by Toray Research Center (TRC)) was used.

[0097] (Melt Mass-Flow Rate, MFR) For polypropylene, MFR was measured in accordance with JIS K 7210-1:2014 at a measurement temperature of 230°C and a load of 21.18N (2.16 kgf), and for high-density polyethylene, MFR was measured in accordance with JIS K 7210-1:2014 at a measurement temperature of 190°C and a load of 21.18N (2.16 kgf).

[0098] (Single fiber fineness) Measured in accordance with the measurement of corrected fineness by the vibration method described in Appendix JB noted in JIS L 1015:2021 8.5.2 Measurement of fineness by the ISO method.

[0099] (Single fiber strength and elongation) In accordance with JIS L 1015:2021, the load value and elongation at fiber break were measured using a tensile tester with a sample gripping distance of 20 mm, and these were taken as the single fiber strength and elongation, respectively.

[0100] (Maximum draw ratio) Each of the undrawn fiber tows obtained in the examples and comparative examples was subjected to a drawing treatment in which a drawing tank was filled with warm water adjusted to the drawing treatment temperature and the tows were drawn in the drawing tank. At this time, the feed speed (V1) of the roll that feeds the undrawn fiber tow was set to 10 m / min, and the take-up speed (V2) of the metal roll on the take-up side was gradually increased from 10 m / min. The take-up speed of the metal roll on the take-up side when the undrawn fiber tow broke was defined as the maximum drawing speed, and the ratio (V2 / V1) of the maximum drawing speed to the take-up speed of the roll that feeds the undrawn fiber tow was determined, and the obtained speed ratio was defined as the maximum draw ratio (V max When the stretching treatment was carried out by dry stretching, the maximum stretch ratio under those conditions was determined by adjusting the temperature of the heat medium (for example, the surface of a metal roll) during the stretching treatment to the same temperature as during the stretching treatment, and gradually increasing the winding speed in the same manner as above.

[0101] (Differential scanning calorimetry) The sheath-core composite fiber was subjected to differential scanning calorimetry (DSC). The differential scanning calorimetry was performed in accordance with JIS K 7121:1987, as described above, by filling 3 mg of the sheath-core composite fiber as a sample and heating it from 20°C to 300°C at a heating rate of 20°C per minute.

[0102] (Dry heat dimensional change rate) Measurement was performed in accordance with b) dry heat dimensional change rate of JIS L 1015:2021 8.15 dimensional change rate. The heat treatment temperature was 145°C, and the heat treatment time was 10 minutes. The initial load was tex x 5.88 mN, and the grip spacing was 25 mm.

[0103] (Dry Heat Shrinkage Rate) A core-sheath type composite fiber was looped into a length of 55 mm, and the ends of the fiber were tied to obtain a sample (the total fineness of the sample consisting of a bundle of fibers was adjusted to 55.6 dtex). An initial load of 0.26 cN / dtex was applied to the sample, and the heat stress tester KE-2LS (manufactured by Kanebo Engineering Co., Ltd.) was set to the displacement measurement mode, and the heat shrinkage rate was continuously measured while the temperature was raised from room temperature to 170°C at a heating rate of 60°C / min, and the heat treatment shrinkage rate of the sample at 140°C was taken as the 140°C dry heat shrinkage rate of the core-sheath type composite fiber.

[0104] (Diffraction Peak Intensity Ratio of Diffraction Peaks in Undrawn Fiber Tow) In order to confirm the crystalline state of the polypropylene and high-density polyethylene in the undrawn fiber tow, the undrawn fiber tow was measured by wide-angle X-ray diffraction (XRD) using the following procedure, and the diffraction intensities of the diffraction peaks obtained were determined, and the ratio of the diffraction intensity of the high-density polyethylene diffraction peak to the overall diffraction intensity was calculated.

[0105] First, the obtained undrawn fiber tow was cut into a length of 3.5 cm. 25.0 mg of the cut undrawn fiber tow was weighed, and both ends were tied with enameled wire to prepare a sample. The undrawn fiber tow sample bundle was fixed to a holder perpendicular to the incident direction of the X-rays, and wide-angle X-ray diffraction was performed. The measurement conditions were as follows:

[0106] X-ray diffraction equipment: Rigaku Corporation MiniFlex II for polymers X-ray source: CuKα ray (using Ni filter) Output: 30 kV 15 mA Slit system: DS: 1.25° SS: 1.25° mm RS: 0.3 mm Measurement direction: Fiber diameter direction scan Scanning method: Continuous scan Measurement range: 2θ = 2 to 45° Step: 0.02° Scan speed: 3° / min

[0107] The wide-angle X-ray diffraction method was performed under the above-mentioned conditions, and from the obtained X-ray diffraction diagram (X-ray diffraction chart), the peak intensity I of the diffraction peak PE measured at 2θ = 21.6 ± 0.5°, which is the diffraction peak of high-density polyethylene, was PE The diffraction peaks of polypropylene were determined as follows: the peak intensity (cps) of the diffraction peak PP1 measured at 2θ = 14.2 ± 0.5°, the peak intensity (cps) of the diffraction peak PP2 measured at 2θ = 17 ± 0.5°, and the peak intensity (cps) of the diffraction peak PP3 measured at 2θ = 18.6 ± 0.5°. The sum of the diffraction intensities of the four diffraction peaks PE1, PP1, PP2, and PP3 was defined as the overall diffraction intensity I (cps), and the diffraction peak intensity I of high-density polyethylene was determined as follows: PE is divided by the total diffraction intensity I to obtain the diffraction peak intensity ratio I PE / I.

[0108] In the examples and comparative examples, a propylene homopolymer (PP) shown in Table 1 below was used as the core component, and a high density polyethylene (PE) shown in Table 2 below was used as the sheath component.

[0109]

[0110]

[0111] Example 1 Production of Core-Sheath Composite Fiber PP1 was used as the core component and PE1 was prepared as the sheath component. Next, the core / sheath component composite ratio (volume ratio) was adjusted to 60 / 40, and PP1 and PE1 were charged into separate extruders. The core component was melted at 315°C and the sheath component at 270°C. The nozzle temperature was adjusted to 280°C, and the fibers were extruded through the outlet hole of a core-sheath composite spinning nozzle (hole diameter 0.6 mm). The molten core component and sheath component (molten resin) extruded through the outlet hole of the core-sheath composite spinning nozzle were rapidly cooled and withdrawn at a spinning speed of 865 m / min, yielding spun filaments (undrawn fiber tows) with a single fiber fineness of 2.6 dtex. The position of the forced cooling device was adjusted so that the distance L from the nozzle face of the composite spinning nozzle to the cooling start point (hereinafter simply referred to as the "distance from the cooling start point") was 55 mm. Then, cooling air having a temperature of 27° C. was blown onto the molten resin to cool the molten resin.

[0112] The obtained undrawn fiber tow was subjected to a first-stage dry drawing at a draw ratio of 4.1 times using a metal roll having a surface temperature of 100°C, and then to a second-stage dry drawing at a draw ratio of 1.0 times using a metal roll having a surface temperature of 100°C. The maximum draw ratio of this undrawn fiber bundle was determined by the above method and was found to be 4.3 times. The obtained drawn filament (core-sheath type composite fiber) had a single fiber fineness of 0.77 dtex. The drawn filament was cut to a fiber length of 5 mm to obtain a short core-sheath type composite fiber.

[0113] (Example 2) A drawn filament having a single fiber fineness of 0.82 dtex was obtained in the same manner as in Example 1, except that the composite ratio (volume ratio) of the core component to the sheath component was set to 50 / 50 and the melt extrusion temperature (also referred to as barrel temperature) of the core component was set to 325° C. The drawn filament was cut to a fiber length of 5 mm to obtain a short core-sheath composite fiber.

[0114] (Example 3) A drawn filament having a single fiber fineness of 0.74 dtex was obtained in the same manner as in Example 1, except that PP2 was used as the core component and the melt extrusion temperature of the core component was set to 295° C. The drawn filament was cut to a fiber length of 5 mm to obtain a core-sheath type composite fiber of short fibers.

[0115] Example 4 Melt spinning was carried out in the same manner as in Example 1, except that PE2 was used as the sheath component and the melt extrusion temperature of the core component was set to 295° C., and drawn filaments having a single fiber fineness of 0.79 dtex were obtained in the same manner as in Example 1. The drawn filaments were cut to a fiber length of 5 mm, and short core-sheath composite fibers were obtained.

[0116] Example 5 A drawn filament having a single fiber fineness of 0.76 dtex was obtained in the same manner as in Example 1, except that PP3 was used as the core component and the melt extrusion temperature of the core component was set to 295° C. The drawn filament was cut to a fiber length of 5 mm to obtain a core-sheath type composite fiber of short fibers.

[0117] Example 6 A drawn filament having a single fiber fineness of 0.77 dtex was obtained in the same manner as in Example 1, except that PP4 was used as the core component and the melt extrusion temperature of the core component was set to 295° C. The drawn filament was cut to a fiber length of 5 mm to obtain a core-sheath type composite fiber of short fibers.

[0118] Example 7 A drawn filament having a single fiber fineness of 0.79 dtex was obtained in the same manner as in Example 1, except that the melt extrusion temperature of the core component was set to 295°C, a conjugate spinning nozzle with a nozzle hole diameter of 0.4 mm was used, and the draw ratio in the first drawing stage was set to 3.7 times. The drawn filament was cut to a fiber length of 5 mm, and a core-sheath type conjugate fiber having short fibers was obtained.

[0119] Example 8 A drawn filament having a single fiber fineness of 0.76 dtex was obtained in the same manner as in Example 1, except that the melt extrusion temperature of the core component was 310°C, the draw ratio in the first-stage drawing was 4.03, and the draw ratio in the second-stage drawing was 1.02. The drawn filament was cut to a fiber length of 5 mm, to obtain a core-sheath type composite fiber having short fibers.

[0120] Example 9 A drawn filament having a single fiber fineness of 0.74 dtex was obtained in the same manner as in Example 1, except that the melt extrusion temperature of the core component was set to 310°C, the first-stage drawing was performed at a draw ratio of 4.67 in hot water at 93°C, and the second-stage drawing was performed at a draw ratio of 0.87 in hot water at 91°C. The drawn filament was cut to a fiber length of 5 mm, and a short core-sheath type composite fiber was obtained.

[0121] Example 10 A drawn filament having a single fiber fineness of 0.74 dtex was obtained in the same manner as in Example 1, except that PE2 was used as the sheath component, the melt extrusion temperature of the core component was set to 310°C, the draw ratio in the first-stage drawing was set to 4.15 times, and the draw ratio in the second-stage drawing was set to 1.02 times. The drawn filament was cut to a fiber length of 5 mm, and a core-sheath type composite fiber having short fibers was obtained.

[0122] Example 11 A drawn filament having a single fiber fineness of 0.78 dtex was obtained in the same manner as in Example 1, except that the melt extrusion temperature of the core component was set to 295°C, the first-stage drawing was performed at a draw ratio of 4.0 in hot water at 88°C, and the second-stage drawing was performed at a draw ratio of 0.98 in hot water at 88°C. The drawn filament was cut to a fiber length of 5 mm, and a short core-sheath type composite fiber was obtained.

[0123] Example 12 A drawn filament having a single fiber fineness of 0.75 dtex was obtained in the same manner as in Example 1, except that the melt extrusion temperature of the core component was set to 295°C, the first-stage drawing was performed at a draw ratio of 4.25 in hot water at 88°C, and the second-stage drawing was performed at a draw ratio of 0.9 in hot water at 88°C. The drawn filament was cut to a fiber length of 5 mm, and a short core-sheath type composite fiber was obtained.

[0124] Example 13 A drawn filament having a single fiber fineness of 0.76 dtex was obtained in the same manner as in Example 1, except that the melt extrusion temperature of the core component was 295°C, the first-stage drawing was performed in hot water at 88°C at a draw ratio of 4.0, the second-stage drawing was performed in hot water at 88°C at a draw ratio of 0.98, and the third-stage drawing was performed at a draw ratio of 1.0 using a metal roll with a surface temperature of 100°C. The drawn filament was cut to a fiber length of 5 mm to obtain a short core-sheath type composite fiber.

[0125] Example 14 A drawn filament having a single fiber fineness of 0.78 dtex was obtained in the same manner as in Example 1, except that the melt extrusion temperature of the core component was 295°C, the first-stage drawing was performed in hot water at 88°C at a draw ratio of 4.25, the second-stage drawing was performed in hot water at 88°C at a draw ratio of 0.9, and the third-stage drawing was performed at a draw ratio of 1.0 using a metal roll with a surface temperature of 100°C. The drawn filament was cut to a fiber length of 5 mm to obtain a short core-sheath type composite fiber.

[0126] Example 15 A drawn filament having a single fiber fineness of 0.84 dtex was obtained in the same manner as in Example 1, except that PE2 was used as the sheath component, the melt extrusion temperature of the core component was set to 285°C, the first-stage drawing was performed at a draw ratio of 4.0 in hot water at 88°C, and the second-stage drawing was performed at a draw ratio of 0.98 in hot water at 88°C. The drawn filament was cut to a fiber length of 5 mm, and a short core-sheath type composite fiber was obtained.

[0127] Example 16 A drawn filament having a single fiber fineness of 0.77 dtex was obtained in the same manner as in Example 1, except that PE2 was used as the sheath component, the melt extrusion temperature of the core component was set to 285°C, the first-stage drawing was performed at a draw ratio of 4.25 in hot water at 88°C, and the second-stage drawing was performed at a draw ratio of 0.9 in hot water at 88°C. The drawn filament was cut to a fiber length of 5 mm, and a short core-sheath type composite fiber was obtained.

[0128] Comparative Example 1: PP3 was used as the core component and PE1 was prepared as the sheath component. Next, the composite ratio (volume ratio) of the core component to the sheath component was set to 40 / 60, and PP3 and PE1 were charged into separate extruders. The core component was melted at 260°C and the sheath component at 220°C. The nozzle temperature was set to 240°C, and the components were extruded from the outlet hole of a core-sheath composite spinning nozzle (hole diameter 0.4 mm). The molten core component and sheath component (molten resin) extruded from the outlet hole of the core-sheath composite spinning nozzle were slowly cooled and taken up at a spinning speed of 720 m / min to obtain a spun filament (undrawn fiber tow) with a single fiber fineness of 7.8 dtex. At this time, the position of the forced cooling device was adjusted so that the distance from the cooling start point was 85 mm. Then, cooling air was set to a temperature of 27°C and blown onto the molten resin to cool the molten resin.

[0129] The obtained undrawn fiber tow was subjected to a first-stage dry drawing at a draw ratio of 6.3 times using a metal roll having a surface temperature of 100°C, and then to a second-stage dry drawing at a draw ratio of 1.0 times using a metal roll having a surface temperature of 110°C. The maximum draw ratio of this undrawn fiber bundle was determined by the above method and was found to be 6.0 times. The obtained drawn filament (core-sheath type composite fiber) had a single fiber fineness of 1.43 dtex. The drawn filament was cut to a fiber length of 5 mm to obtain a short core-sheath type composite fiber.

[0130] Comparative Example 2 A drawn filament having a single fiber fineness of 0.92 dtex was obtained in the same manner as in Comparative Example 1, except that PE2 was used as the sheath component, the conjugation ratio (volume ratio) of the core component to the sheath component was 50 / 50, the melt extrusion temperature of the core component was 310°C, the nozzle temperature was 260°C, the spinning speed was 940 m / min, the drawing temperature in the first stage of drawing was 95°C and the draw ratio was 1.1, and the drawing temperature in the second stage of drawing was 90°C and the draw ratio was 5.28. The drawn filament was cut to a fiber length of 5 mm, and a short core-sheath type conjugated fiber was obtained.

[0131] Comparative Example 3 A drawn filament having a single fiber fineness of 0.82 dtex was obtained in the same manner as in Comparative Example 1, except that PP1 was used as the core component, the composite ratio (volume ratio) of the core component to the sheath component was 60 / 40, a conjugate spinning nozzle with a nozzle hole diameter of 0.6 mm was used, the melt extrusion temperature of the core component was 325°C, the melt extrusion temperature of the sheath component was 270°C, the nozzle temperature was 280°C, the spinning speed was 870 m / min, the drawing temperature in the first drawing stage was 100°C and the draw ratio was 4.1, and the drawing temperature in the second drawing stage was 100°C and the draw ratio was 1. The drawn filament was cut to a fiber length of 5 mm, and a short core-sheath composite fiber was obtained.

[0132] In the examples and comparative examples, the diffraction peak intensity ratios of the undrawn fiber tows were measured as described above, and the results are shown in Tables 3 to 7 below. The single fiber strengths and elongations of the sheath-core composite fibers of the examples and comparative examples were measured as described above, and the results are shown in Tables 3 to 7 below. In addition, the sheath-core composite fibers of the examples and comparative examples were used as samples, and the extrapolated melting onset temperatures (T im ) and the melting peak temperature of polypropylene (T pm ) and the heats of fusion of polypropylene and polyethylene were measured and evaluated as described above, and the results are shown in Tables 3 to 7. In addition, the dry heat dimensional change and 140°C dry heat shrinkage of the core-sheath type composite fibers of the Examples and Comparative Examples were measured as described above, and the results are shown in Tables 3 to 7.

[0133]

[0134]

[0135]

[0136]

[0137]

[0138] As can be seen from the data in Tables 3 to 6, the core-sheath type composite fibers of the examples have the same properties as the polypropylene (T pm -T im) was 12.0°C or higher, resulting in high single fiber strength and low thermal shrinkage upon heating. This is presumably because, during melt spinning, a sheath component containing 70% by mass or more of high-density polyethylene and a core component containing 70% by mass or more of polypropylene were used, and the molten resin extruded from a conjugate spinning nozzle and spun into an undrawn fiber tow was rapidly cooled directly below the nozzle. This caused cooling to begin before the molten polypropylene was stretched, and the polypropylene in the resulting undrawn fiber had a low crystalline phase, i.e., a high amorphous phase and smectic crystals, and the crystalline states of the polypropylene and polyethylene were well balanced, resulting in adequate crystallization of both components upon stretching. Furthermore, since the polypropylene in the fiber after stretching contained many small crystallite-sized crystals, the extrapolated melting onset temperature of the polypropylene was observed to be low. The uniform formation of a small crystallite-sized crystalline phase is presumably responsible for the fact that, even if shrinkage occurred in the amorphous portion, the crystals quickly collided with each other, contributing to the suppression of thermal shrinkage.

[0139] On the other hand, as can be seen from the data in Table 7 above, the (T pm -T im The sheath-core composite fibers of Comparative Examples 1 to 3, in which the temperature difference (T) between the extrapolated melting onset temperature and the peak melting temperature was less than 12.0°C, exhibited large heat shrinkage. These fibers contained a large amount of crystalline polypropylene phase in the undrawn fibers, and when they were drawn in this state, the crystalline phase acted as nuclei, causing the crystallites to grow large, and some of the crystalline phase was destroyed during drawing, resulting in an overdrawn state. Because the sheath-core composite fibers of Comparative Examples 1 to 3 contained such large-grown polypropylene and overdrawn polypropylene, the extrapolated melting onset temperature was observed to be high, and the temperature difference (T) between the extrapolated melting onset temperature and the peak melting temperature was large. pm -T im ) is estimated to be smaller.

[0140] The present invention may include, for example, the following embodiments. However, the present invention is not limited to the following embodiments. [1] A core-sheath type composite fiber in which a core component and a sheath component are arranged substantially concentrically, wherein the sheath component contains 70% by mass or more of high-density polyethylene, and the core component contains 70% by mass or more of polypropylene, and the polypropylene has an extrapolated melting onset temperature (T ) of 20°C / min measured in a differential scanning calorimetry (DSC) method specified in JIS K 7121:1987 at a heating rate of 20°C / min. im ) and the melting peak temperature of polypropylene (T pm ) temperature difference (melting peak temperature - extrapolated melting onset temperature: T pm -T im) is 12.0°C or higher. [2] The sheath-core conjugate fiber according to [1], wherein the volume ratio of the core component to the sheath component (core component / sheath component) is 35 / 65 or higher and 75 / 25 or lower. [3] The sheath-core conjugate fiber according to [1] or [2], wherein the single fiber fineness is 0.1 dtex or higher and 1.1 dtex or lower, and the single fiber strength measured in accordance with JIS L 1015:2021 is 4 cN / dtex or higher and 12.0 cN / dtex or lower. [4] The sheath-core conjugate fiber according to any one of [1] to [3], wherein the polypropylene has a ratio Mw / Mn of the weight average molecular weight (Mw) to the number average molecular weight (Mn) of 2.4 or higher and 6 or lower. [5] The sheath-core conjugate fiber according to any one of [1] to [4], wherein the polypropylene has a melt flow rate (MFR: measurement temperature 230°C, load 21.18 N (2.16 kgf)) measured in accordance with JIS K 7210-1:2014 in the range of 3 g / 10 min to 35 g / 10 min. [6] The sheath-core conjugate fiber according to any one of [1] to [5], wherein the sheath-core conjugate fiber has a dry heat dimensional change at 145°C of 11% or less measured in accordance with JIS L 1015:2021. [7] The sheath-core conjugate fiber according to any one of [1] to [6], wherein the sheath-core conjugate fiber has a dry heat shrinkage at 140°C of 5.4% or less. [8] A method for producing a sheath-core composite fiber in which a core component and a sheath component are arranged substantially concentrically, comprising the steps of: supplying a sheath component containing 70% by mass or more of high-density polyethylene and a core component containing 70% by mass or more of polypropylene to a composite spinning nozzle arranged so that, in the cross section of the sheath-core composite fiber, the sheath component covers the surface of the core component and the center of gravity of the core component substantially coincides with the center of gravity of the sheath-core composite fiber; quenching the molten core component and sheath component extruded from the composite spinning nozzle immediately below the composite spinning nozzle, and taking up the molten core component and sheath component at a spinning speed of 300 m / min or more and 2300 m / min or less to obtain an undrawn fiber tow in which the core component and sheath component are solidified and which has a single fiber fineness of 0.8 dtex or more and 3.2 dtex or less; a step of drawing the undrawn fiber tow at a temperature of 60°C or higher and 110°C or lower, under conditions such that the total draw ratio is 2.7 times or higher and 10.0 times or lower, to obtain a sheath-core conjugate fiber.[9] The method for producing a sheath-core conjugate fiber according to [8], wherein the melt spinning apparatus is equipped with a forced cooling device, the molten resin discharged from the conjugate spinning nozzle is cooled by an air flow supplied from the forced cooling device, and the distance from the spinneret face of the conjugate spinning nozzle to the cooling start point is 25 mm or more and less than 85 mm.

[10] The method for producing a sheath-core conjugate fiber according to [8] or [9], wherein the drawing is a multistage drawing in two or more stages, under conditions such that the total draw ratio is 3.5 times or more and 10.0 times or less.

[11] The method for producing a sheath-core conjugate fiber according to

[10] , wherein the multistage drawing is performed at a drawing temperature of 60°C or more and 110°C or less and a draw ratio of 1.0 times or more and 8.0 times or less, and at a drawing temperature of 60°C or more and 110°C or less and a draw ratio of 0.80 times or more and 10.0 times or less.

[12] The method for producing a sheath-core composite fiber according to

[11] , wherein the multistage drawing is dry drawing, the first drawing has a drawing temperature of 60°C or higher and 110°C or lower and a draw ratio of 1.0 or higher and 8.0 or lower, and the second drawing has a drawing temperature of 60°C or higher and 110°C or lower and a draw ratio of 1.0 or higher and 1.5 or lower.

[13] The method for producing a sheath-core composite fiber according to

[11] , wherein the multistage drawing is wet drawing, the first drawing has a drawing temperature of 60°C or higher and 110°C or lower and a draw ratio of 1.0 or higher and 8.0 or lower, and the second drawing has a drawing temperature of 60°C or higher and 110°C or lower and a draw ratio of 0.8 or higher and 1.0 or lower.

[14] The method for producing a sheath-core composite fiber according to

[11] , wherein, in the multistage drawing, the first drawing has a drawing temperature of 100°C or higher and lower than 110°C and a draw ratio of 1.0 or higher and 8.0 or lower, and the second drawing has a drawing temperature of 100°C or higher and lower than 110°C and a draw ratio of 1.0 or higher and 1.5 or lower.

[15] The method for producing a sheath-core composite fiber according to

[11] , wherein, in the multistage drawing, the first drawing has a drawing temperature of 60°C or higher and lower than 100°C and a draw ratio of 1.0 or higher and 8.0 or lower, and the second drawing has a drawing temperature of 60°C or higher and lower than 110°C and a draw ratio of 0.8 or higher and 1.0 or lower.

[16] In an X-ray diffraction chart obtained by measuring the undrawn fiber tow by X-ray diffraction (XRD), the diffraction intensity (I) of the diffraction peak (PE1) at 2θ = 21.6 ± 0.5° of high-density polyethylene is relative to the sum (I) of diffraction intensities of the diffraction peak (PP1) at 2θ = 14.2 ± 0.5°, the diffraction peak (PP2) at 2θ = 17 ± 0.5°, and the diffraction peak (PP3) at 2θ = 18.6 ± 0.5° of polypropylene. PE1 ) ratio I PE1

[17] A fiber assembly comprising 10% by mass or more of the sheath-core composite fiber according to any one of [1] to [7], wherein the ratio of the sheath-core composite fiber to the total weight of the fiber is 0.20 or more and 1.0 or less.

[0141] The above-described embodiments are to be considered in all respects as merely illustrative of the present invention and not restrictive, and those skilled in the art to which the present invention pertains will be able to construct various embodiments incorporating the principles of the present invention without departing from the spirit and essential characteristics thereof, even if not expressly described herein, when taking into account the teachings set forth above.

[0142] The sheath-core composite fiber according to one or more embodiments of the present invention can be used for applications such as separators for various batteries, RO membrane supports, filter media for various filters, artificial leather, sanitary materials, and various wiping products such as wipers for personal and / or object use.

[0143] REFERENCE SIGNS LIST 1 sheath component 2 core component 3 position of center of gravity of core component 4 position of center of gravity of sheath-core composite fiber 5 radius of sheath-core composite fiber 10 sheath-core composite fiber 20 melt spinning device 21 spinning pack (spin pack) 22 cooling point (upper end position of cooling air) 23 molten resin 24 undrawn fiber tow

Claims

1. A core-sheath type composite fiber in which the core component and sheath component are arranged substantially concentrically, The aforementioned sheath component contains 70% by mass or more of high-density polyethylene. The core component contains 70% by mass or more of polypropylene, A core-sheath composite fiber in which, as specified in JIS K 7121:1987, the temperature difference (melting peak temperature - extrapolation melting onset temperature: Tpm - Tim) between the extrapolation melting onset temperature (Tim) and the melting peak temperature (Tpm) of polypropylene, measured at a heating rate of 20°C per minute using differential scanning calorimetry (DSC), is 12.0°C or greater.

2. The core-sheath type composite fiber according to claim 1, wherein the volume ratio of the core component to the sheath component (core component / sheath component) is 35 / 65 or more and 75 / 25 or less.

3. The core-sheath type composite fiber according to claim 1, wherein the single fiber fineness is 0.1 dtex or more and 1.1 dtex or less, and the single fiber strength measured in accordance with JIS L 1015:2021 is 4 cN / dtex or more and 12.0 cN / dtex or less.

4. The polypropylene is a core-sheath type composite fiber according to claim 1, wherein the ratio of weight-average molecular weight (Mw) to number-average molecular weight (Mn), Mw / Mn, is 2.4 or more and 6 or less.

5. The polypropylene is a core-sheath type composite fiber according to claim 1, wherein the melt flow rate (MFR: measured at a temperature of 230°C, with a load of 21.18 N (2.16 kgf)) measured in accordance with JIS K 7210-1:2014 is in the range of 3 g / 10 min to 35 g / 10 min.

6. The core-sheath type composite fiber according to claim 1, wherein the core-sheath type composite fiber has a dry heat dimensional change rate of 11% or less at 145°C, as measured in accordance with JIS L 1015:2021.

7. The core-sheath type composite fiber according to claim 1, wherein the dry heat shrinkage rate of the core-sheath type composite fiber at 140°C is 5.4% or less.

8. A method for manufacturing a core-sheath type composite fiber in which the core component and the sheath component are arranged substantially concentrically, A process of supplying a sheath component containing 70% by mass or more of high-density polyethylene and a core component containing 70% by mass or more of polypropylene to a composite spinning nozzle arranged such that, in the cross-section of a core-sheath type composite fiber, the sheath component covers the surface of the core-sheath type composite fiber, and the center of gravity of the core component substantially coincides with the center of gravity of the core-sheath type composite fiber, forming a concentric circular structure. A step of rapidly cooling the molten core component and sheath component extruded from the composite spinning nozzle directly below the composite spinning nozzle, and drawing the molten core component and sheath component at a spinning speed of 300 m / min to 2300 m / min to obtain an undrawn fiber tow in which the core component and sheath component have solidified and the single fiber fineness is 0.8 dtex to 3.2 dtex, and A method for producing core-sheath type composite fibers, comprising the step of stretching the aforementioned unstretched fiber tow at a temperature of 60°C to 110°C and under conditions of a total stretch ratio of 2.7 to 10.0 times to obtain core-sheath type composite fibers.

9. A method for producing a core-sheath type composite fiber according to claim 8, wherein the melt spinning apparatus is equipped with a forced cooling device, the molten resin discharged from the composite spinning nozzle is cooled by an airflow supplied from the forced cooling device, and the distance from the die surface of the composite spinning nozzle to the cooling start point is 25 mm or more and less than 85 mm.

10. The method for producing a core-sheath type composite fiber according to claim 8 or 9, wherein the stretching is a multi-stage stretching of two or more stages, and the total stretching ratio is 3.5 times or more and 10.0 times or less.

11. The aforementioned multi-stage extension is The first stretching is performed with a stretching temperature of 60°C or higher and 110°C or lower, and a stretching ratio of 1.0 times or higher and 8.0 times or lower. The method for producing a core-sheath type composite fiber according to claim 10, wherein the second stretching is performed at a stretching temperature of 60°C or higher and 110°C or lower, and the stretching ratio is 0.80 times or higher and 10.0 times or lower.

12. The aforementioned multi-stage stretching is dry stretching, The first stretching is performed with a stretching temperature of 60°C to 110°C and a stretching ratio of 1.0 to 8.0 times. The method for producing a core-sheath type composite fiber according to claim 11, wherein the second stretching is performed at a stretching temperature of 60°C or higher and 110°C or lower, and the stretching ratio is 1.0 times or higher and 1.5 times or lower.

13. The aforementioned multi-stage stretching is wet stretching. The first stretching is performed with a stretching temperature of 60°C to 110°C and a stretching ratio of 1.0 to 8.0 times. The method for producing a core-sheath type composite fiber according to claim 11, wherein the second stretching is performed at a stretching temperature of 60°C or higher and 110°C or lower, and the stretching ratio is 0.8 times or higher and 1.0 times or lower.

14. The aforementioned multi-stage extension is The first stretching is performed with a stretching temperature of 100°C or higher and 110°C or lower, and a stretching ratio of 1.0 times or higher and 8.0 times or lower. A method for producing a core-sheath type composite fiber according to claim 11, wherein the second stretching is performed at a stretching temperature of 100°C or more and 110°C or less, and the stretching ratio is 1.0 times or more and 1.5 times or less.

15. The aforementioned multi-stage extension is The first stretching is performed with a stretching temperature of 60°C or higher and less than 100°C, and a stretching ratio of 1.0 times or higher and 8.0 times or lower. A method for producing a core-sheath type composite fiber according to claim 11, wherein the second stretching is performed at a stretching temperature of 60°C or more and 110°C or less, and the stretching ratio is 0.8 times or more and 1.0 times or less.

16. In the X-ray diffraction chart obtained by X-ray diffraction (XRD) measurement of the aforementioned undrawn fiber tow, the diffraction peak of polypropylene at 2θ = 14.2 ± 0.5° is PP. 1 , diffraction peak PP at 2θ = 17 ± 0.5° 2 , and diffraction peak at 2θ = 18.6 ± 0.5° (PP 3 The diffraction peak of high-density polyethylene at 2θ = 21.6 ± 0.5° (PE) relative to the sum of diffraction intensities (I) 1 ) diffraction intensity (I PE1 ) ratio I PE1 A method for producing a core-sheath type composite fiber according to claim 8 or 9, wherein / I is 0.20 or more and 1.0 or less.

17. A fiber aggregate containing 10% by mass or more of the core-sheath type composite fiber described in any one of claims 1 to 7.