Core-sheath conjugate fiber and nonwoven fabric formed therefrom

Abstract

The purpose of the present invention is to provide a core-sheath conjugate fiber or the like in which yarn breakage and sheath peeling are suppressed and which has a high degree of profile after thermal fusion.　The present invention is a core-sheath conjugate fiber composed of a core containing a thermoplastic resin of a first component and a sheath containing a thermoplastic resin of a second component having a melting point lower than the melting point of the resin by 40°C or more. The core-sheath conjugate fiber is characterized in that the cross-section of the fiber has a Y-shape both in the core and the entire cross-section; the content of the core is 45-80% by mass inclusive relative to the whole fiber; in the cross-section of the fiber, a sheath thickness ratio represented by (DO2-DO1) / (DI2-DI1) is 1.5-2.5 inclusive; a degree of core profile represented by DO1 / DI1 is 2.1-4.0 inclusive; an elongation at the maximum point of the fiber as measured by JIS L 1013 8.5.1 is 21% or more; the tip of a core projection is in a triangle connecting two deepest sheath recesses and the tip of the sheath projection adjacent thereto; and the core projection has such a shape that the dimension in the width direction becomes smaller from the base to the tip.

Description

Core-sheath composite fiber and nonwoven fabric formed therefrom

[0001] The present invention relates to a core-sheath composite fiber and a nonwoven fabric formed therefrom, which are used for absorbent articles for hygiene such as sanitary napkins and diapers, medical hygiene materials, daily living materials, general medical materials, bedding materials, nursing care products, and pet products.

[0002] While the basic performance of sanitary napkins and diapers has improved, there has been a demand for even higher functionality in recent years. For example, the top sheet needs to feel dry when worn, and in the case of sanitary napkins, the function of maintaining a dry feeling by quickly transferring absorbed menstrual blood from the top sheet to the lower layer is one example. For example, in the clothing field, a Y-shaped cross section of the fiber is known to reduce the contact area with the skin, imparting a dry touch, and the increased surface area allows capillary action to quickly diffuse and dry sweat, resulting in a so-called sweat-absorbing and quick-drying function.

[0003] However, there are very few examples of Y-shaped cross sections being used in bicomponent thermally bonded composite fibers used in top sheets that come into direct contact with the skin, such as in sanitary napkins and diapers. One reason for this is that, unlike monocomponent fibers for clothing, bicomponent composite fibers for sanitary materials can experience problems with the core and sheath peeling during production and handling. In the case of round or hollow cross sections, the sheath resin has no geometric freedom to peel from the core resin, so there is little concern about peeling. However, when the cross section is Y-shaped, the sheath resin has freedom to peel from the core resin, mainly at the recesses, making it more susceptible to peeling during actual fiber production and processing, making handling difficult.

[0004] Another reason is that during air-through processing, i.e., when the sheath resin is melted and the fibers are thermally fused together, the sheath resin tends to curl up due to surface tension, which reduces the degree of irregularity of the fiber cross section after air-through processing; in some cases, the sheath resin fills the recess in the Y-shaped cross section formed by the core resin, resulting in an almost round cross section, making it difficult to fully demonstrate the performance expected of the Y-shaped cross section after air-through processing into a nonwoven fabric.

[0005] The following documents are known as prior art related to modified cross-section fibers and nonwoven fabrics. Patent Document 1 describes a bicomponent trilobal staple fiber or short-cut fiber containing a core and a sheath, i.e., a core-sheath structure, for obtaining a nonwoven fabric having excellent flexibility and thermal bonding properties. Patent Document 2 describes a modified cross-section conjugate fiber having 3 to 16 convex portions for use as a thermally bondable conjugate fiber for separator materials, although this is not in the field of sanitary materials.

[0006] Special Publication No. 2019-515150 Patent No. 5961162

[0007] However, for example, Patent Document 1 does not disclose a suitable cross-sectional shape for achieving the performance attributed to the Y-shaped cross section after heat fusion, and states that it is preferable for the sheath to have a constant thickness. However, in the case of such a fiber, the sheath resin tends to gather in the center due to surface tension after heat fusion, which raises concerns that the degree of irregularity of the fiber will be significantly reduced after heat fusion. Furthermore, since the sheath thickness is constant around the entire periphery of the fiber cross section, it can be said that the degree of irregularity of the entire fiber is small, and it is thought that a nonwoven fabric formed from this fiber will not provide the desired feel to the touch.

[0008] Patent Document 2 proposes a fiber cross-sectional shape that is easy to flatten in order to firmly bond the fibers together in order to improve the density and mechanical properties of the separator material. However, for sanitary materials, cross-sectional properties that allow gaps to be formed between the fibers are required, and in particular, it is necessary to make the degree of fiber irregularity appropriate.

[0009] The present inventors have found that Y-shaped sheath-core conjugate fibers have problems such as yarn breakage, sheath peeling, and a decrease in the degree of modification after heat fusion, and have proceeded to develop a sheath-core conjugate fiber that can solve these problems. That is, an object of the present invention is to provide a sheath-core conjugate fiber that is suppressed in terms of yarn breakage and sheath peeling and has a high degree of modification after heat fusion, and a nonwoven fabric formed therefrom.

[0010] The core-sheath composite fiber of the present invention, which has achieved the above object, has the following features. The sheath-core composite fiber of the present invention is a sheath-core composite fiber composed of a core containing a first component thermoplastic resin and a sheath containing a second component thermoplastic resin having a melting point 40°C or more lower than the melting point of the first resin, wherein the core and the entire cross section of the fiber both have a Y-shape, the content of the core is 45% by mass or more and 80% by mass or less with respect to the entire fiber, where in the fiber cross section, the diameter of the inscribed circle of the core is DI1, the diameter of the circumscribed circle of the core is DO1, the diameter of the inscribed circle of the entire cross section is DI2, and the diameter of the circumscribed circle of the entire cross section is DO2, the sheath thickness ratio expressed as (DO2-DO1) / (DI2-DI1) is 1.5 or more and 2.5 or less, the core irregularity expressed as DO1 / DI1 is 2.1 or more and 4.0 or less, and the maximum elongation of the fiber measured in accordance with JIS L1013 8.5.1 is 21% or more, The tip of the core protrusion is located within a triangle connecting the tip of the sheath protrusion and the two adjacent sheath most recessed parts, and the core protrusion has a shape in which the width dimension decreases from the base to the tip.

[0011] In the present invention, the single yarn fineness is preferably 1.2 dtex or more and 10.0 dtex or less. It is also preferable that the maximum point stress of the fiber, measured according to JIS L1013 8.5.1, is 2.0 cN / dtex or more. It is also a preferable requirement that the overall cross-sectional deformation degree, expressed as DO2 / DI2, after treating the fiber for 30 seconds under a load of 0.33 kgf / dtex at a temperature 5°C higher than the melting point of the thermoplastic resin of the second component is 1.7 or more. Another aspect of the present invention includes a nonwoven fabric formed from the core-sheath composite fiber.

[0012] According to the present invention, it is possible to provide a core-sheath composite fiber that is suppressed in terms of yarn breakage and sheath peeling and has a high degree of modification upon heat fusion bonding, and a nonwoven fabric formed therefrom.

[0013] Furthermore, according to the present invention, while maintaining the processability of the nonwoven fabric, the cross-sectional shape can enhance capillary action by increasing the surface area after processing the nonwoven fabric, and the contact area with the skin can be reduced, resulting in a dry touch.

[0014] Fig. 1 is a diagram showing the circumscribing circle and inscribing circle of the core, the circumscribing circle and inscribing circle of the entire cross section, and their diameters in the cross section of a sheath-core composite fiber according to one embodiment of the present invention. Fig. 2 is a diagram showing the relationship between the convex portions and concave portions of the entire cross section and the convex portions of the core in the cross section of a sheath-core composite fiber according to one embodiment of the present invention. Fig. 3 is a diagram showing the relationship between the convex portions and concave portions of the core in the cross section of a sheath-core composite fiber according to one embodiment of the present invention.

[0015] The present invention will be described in more detail below based on the following embodiments. However, the present invention is not limited to the following embodiments, and can be implemented with appropriate modifications within the scope of the above and below-described purposes, and all such modifications are included within the technical scope of the present invention. For convenience, hatching and component symbols may be omitted in the drawings. In such cases, reference should be made to the specification and other drawings. The dimensions of various components in the drawings may differ from their actual dimensions, as priority is given to helping understand the features of the present invention.

[0016] The sheath-core composite fiber of the present invention is a sheath-core composite fiber composed of a core containing a first component thermoplastic resin and a sheath containing a second component thermoplastic resin having a melting point 40°C or more lower than the melting point of the first resin, wherein the core and the entire cross section of the fiber both have a Y-shape, the content of the core is 45% by mass or more and 80% by mass or less with respect to the entire fiber, where in the fiber cross section, the diameter of the inscribed circle of the core is DI1, the diameter of the circumscribed circle of the core is DO1, the diameter of the inscribed circle of the entire cross section is DI2, and the diameter of the circumscribed circle of the entire cross section is DO2, the sheath thickness ratio expressed as (DO2-DO1) / (DI2-DI1) is 1.5 or more and 2.5 or less, the core irregularity expressed as DO1 / DI1 is 2.1 or more and 4.0 or less, and the maximum elongation of the fiber measured in accordance with JIS L1013 8.5.1 is 21% or more, The tip of the core convex portion is located within a triangle connecting the tip of the sheath convex portion and the two adjacent sheath-most concave portions, and the core convex portion has a shape in which the dimension in the width direction decreases from the base to the tip. The sheath-core composite fiber of the present invention has the above structure in the fiber cross section and is endowed with the above physical properties, thereby suppressing yarn breakage and sheath peeling and enabling a high degree of modification after heat fusion bonding. Furthermore, nonwoven fabrics formed from such sheath-core composite fibers can exhibit a good contact feel, such as a dry touch and a smooth feel.

[0017] <Structure of Sheath-Core Composite Fiber> The structure of the sheath-core composite fiber of the present invention will be described with reference to Fig. 1. Fig. 1 shows a cross section of a sheath-core composite fiber 10 composed of a core 1 and a sheath 2 covering the core 1 (hereinafter, the sheath 2 is also referred to as the entire cross section).

[0018] In the cross section of the fiber, both the core 1 and the sheath 2 have a Y-shape, i.e., the core 1 and the sheath 2 (overall cross section) have three convex portions. It is preferable that the core 1 and the sheath 2 have the same center of gravity (center).

[0019] The Y-shaped protrusions of the core 1 and the entire cross section are preferably evenly spaced when viewed from the center of the core 1, and the angles formed by the lines connecting the center to the tip of each protrusion are preferably 120°. The angles formed by the lines connecting the center to the tip of each protrusion are preferably the same, but may be different. If the angles formed by the lines connecting the center to the tip of each protrusion are different, the angles may differ, preferably within a range of 120°±10°, more preferably within a range of 120°±5°.

[0020] From the perspective of finding the circumscribing circle of the core 1, it is preferable that the three convex portions of the core 1 have the same height, but two of the convex portions of the core 1 may have the same height and the remaining convex portion may have a different height. From the perspective of finding the circumscribing circle of the core 1 or the entire cross section, it is preferable that the three convex portions of the sheath 2 or the entire cross section have the same height, but two of the convex portions of the sheath 2 or the entire cross section may have the same height and the remaining convex portion may have a different height.

[0021] The tip (most convex part) of the convex part of the lead 1 preferably overlaps with the circumscribing circle of the lead 1, and the tip (most convex part) of the convex part of the sheath 2 (entire cross section) preferably overlaps with the circumscribing circle of the sheath 2 (entire cross section). The peripheral part including the tip (most convex part) of the convex part of the lead 1 and the peripheral part including the tip (most convex part) of the convex part of the sheath 2 (entire cross section) preferably have a curved shape with the tip as the apex and both side surfaces of the tip curved with a predetermined radius of curvature.

[0022] As described below, the width of the convex portions of the core 1 and sheath 2 (overall cross section) is preferably greatest at the base and smallest at the tip, and preferably gradually decreases from the base to the tip. The convex portions of the core 1 and sheath 2 (overall cross section) preferably do not have recesses from the base to the tip. If recesses exist from the base to the tip, stress may be applied to the recesses during manufacturing or heat fusion, causing the convex portions to break.

[0023] The core 1 and sheath 2 (entire cross section) preferably have recesses between convex portions. That is, the core 1 and sheath 2 (entire cross section) preferably have three convex portions and three recesses. The peripheral portion including the most recessed portion of the recess in the core 1 and the peripheral portion including the most recessed portion of the recess in the sheath 2 (entire cross section) preferably form gently curved surfaces. The most recessed portion of the recess in the core 1 preferably overlaps with the inscribed circle of the core 1, and the most recessed portion of the recess in the sheath 2 (entire cross section) preferably overlaps with the inscribed circle of the sheath 2 (entire cross section).

[0024] The core 1 is preferably of solid construction, but may also be of hollow construction.

[0025] The cross-sectional structure of a core-sheath composite fiber is expressed based on the circumscribing and inscribing circles of the core 1 and the circumscribing and inscribing circles of the entire cross section formed by the sheath 2 covering the core 1, and their diameters.

[0026] Specifically, in Figure 1, the diameter of the circumscribing circle of the core 1 is shown as DO1, the diameter of the inscribing circle of the core 1 is shown as DI1, the diameter of the circumscribing circle of the entire cross section is shown as DO2, and the diameter of the inscribing circle of the entire cross section is shown as DI2. The cross-sectional structure of the sheath-core composite fiber is expressed in more detail based on the sheath thickness ratio ((DO2-DO1) / (DI2-DI1)), the core irregularity (DO1 / DI1), the overall cross-sectional irregularity (DO2 / DI2), and the overall cross-sectional irregularity after heat fusion bonding (DO2 / DI2). The diameters have the relationship DO2 > DO1 > DI2 > DI1. It is also preferable to satisfy the relationship (DO2-DO1) > (DI2-DI1), and the thickness of the sheath convex portions is preferably at least (DI2-DI1) and not more than (DO2-DO1), and the thickness of the tips of the sheath convex portions is preferably (DO2-DO1). The thickness of the recessed portion of the sheath is preferably greater than 0 and not greater than (DI2-DI1), and the thickness of the recessedmost portion of the sheath is preferably (DI2-DI1).

[0027] <Sheath Thickness Ratio> (DO2-DO1) / (DI2-DI1) The sheath thickness ratio may be expressed as the ratio of the sheath thickness at the tip of the convex portion across the entire cross section to the sheath thickness at the concave portion across the entire cross section, and can be regarded as an index showing the degree of deformation of the sheath thickness at the tip of the convex portion across the entire cross section relative to the sheath thickness at the concave portion across the entire cross section.

[0028] The sheath thickness ratio represented by (DO2-DO1) / (DI2-DI1) is 1.5 or more and 2.5 or less, preferably 1.55 or more and 2.45 or less, more preferably 1.60 or more and 2.40 or less, even more preferably 1.65 or more and 2.35 or less, still more preferably 1.70 or more and 2.30 or less, particularly preferably 1.75 or more and 2.25 or less, and most preferably 1.80 or more and 2.25 or less.

[0029] If the sheath thickness ratio is less than 1.5, the amount of thermoplastic resin of the second component remaining at the tips of the protrusions upon melting will be small, which may result in a small degree of irregularity for the fiber as a whole, making it difficult to obtain the effects of the Y-shaped configuration, and may reduce the feel of contact (e.g., smoothness, dryness, etc.) of a nonwoven fabric formed from the Y-shaped sheath-core composite fiber. Furthermore, the sheath thickness at the tips of the protrusions will be thin, which may cause peeling of the sheath from those points, or in severe cases, exposure of the core, resulting in a decrease in the performance of the nonwoven fabric. If the sheath thickness ratio is more than 2.5, the tips of the protrusions across the entire cross section will be too pointed, and although the degree of deformation will be high, there is a risk of the sheath breaking or peeling during production (e.g., during the crimping process).

[0030] <Core Irregularity> (DO1 / DI1) The core irregularity can be considered as an index showing the height of the core convex portion. The core irregularity expressed by DO1 / DI1 is 2.1 or more and 4.0 or less, preferably 2.2 or more and 3.9 or less, more preferably 2.3 or more and 3.8 or less, even more preferably 2.4 or more and 3.7 or less, and still more preferably 2.5 or more and 3.6 or less.

[0031] If the core irregularity degree is less than 2.1, the degree of irregularity of the entire fiber cannot be increased, and the degree of irregularity may further decrease after heat fusion, possibly resulting in a decrease in the touch feel (e.g., smoothness) of the nonwoven fabric formed from the Y-shaped sheath composite fibers. If the core irregularity degree is more than 4.0, the tips of the core convex portions may become pointed, making it impossible to increase the sheath ratio at the tips of the fiber convex portions. Furthermore, when a nonwoven fabric is produced by air-through, the core may be exposed, potentially resulting in a decrease in the properties of the nonwoven fabric. Furthermore, during the crimping process using a crimper, the sheath may peel off, the convex portions may break, or the entire cross section may become flat, making it difficult to achieve the effects of the Y-shaped shape.

[0032] <Overall Cross-Section Irregularity> (DO2 / DI2) The overall cross-sectional irregularity can be regarded as an index showing the height of the protrusions of the fiber. The overall cross-sectional irregularity expressed by DO2 / DI2 is preferably 2.0 or more and 4.5 or less, more preferably 2.1 or more and 4.2 or less, even more preferably 2.2 or more and 3.9 or less, and still more preferably 2.3 or more and 3.6 or less.

[0033] If the overall cross-sectional irregularity is less than 2.0, the overall irregularity of the sheath-core conjugate fiber is low, and the irregularity becomes even lower after heat fusion, which may result in a decrease in the contact feel (e.g., smoothness, dryness, etc.) of the nonwoven fabric formed from the sheath-core conjugate fiber. On the other hand, if the overall cross-sectional irregularity is more than 4.5, the overall irregularity of the sheath-core conjugate fiber is high, but there is a risk of the sheath peeling off or breaking.

[0034] <Overall Cross-Section Irregularity After Heat Fusion Bonding> (DO2 / DI2) Since the core-sheath composite fiber of the present invention is heat-fused during the formation of a nonwoven fabric, it is preferable that the entire cross-section has a predetermined irregularity after heat fusion, from the viewpoint of providing a good contact feel to the nonwoven fabric. Although the irregularity of the entire cross-section decreases because the thermoplastic resin of the second component constituting the sheath tends to become round due to surface tension caused by thermal history, it is sufficient for the fiber to have a relatively good irregularity.

[0035] The overall cross-sectional irregularity after heat treatment under the following conditions is preferably 1.7 or more, more preferably 1.8 or more and 4.0 or less, even more preferably 2.0 or more and 3.7 or less, and even more preferably 2.2 or more and 3.4 or less. Heat treatment conditions include treating the fiber under a load of 0.33 kgf / dtex at a temperature 5°C higher than the melting point of the thermoplastic resin of the second component for 30 seconds.

[0036] If the overall cross-sectional irregularity after heat treatment is less than 1.7, the overall irregularity of the sheath-core conjugate fiber will be small, making it difficult to obtain capillary action due to an increase in surface area, and there is a risk that the contact feel (e.g., smooth feel, dry touch feel) of the nonwoven fabric formed from the sheath-core conjugate fiber will be reduced. On the other hand, if the overall cross-sectional irregularity after heat treatment is more than 4.0, the overall irregularity of the sheath-core conjugate fiber will be too high, and there is a risk that the sheath will peel off or break.

[0037] Next, the relationship between the sheath (overall cross section) convex portions, concave portions, and core convex portions will be described with reference to Figure 2. In Figure 2, the tip 3 of the sheath convex portion overlaps the circumscribing circle of the entire cross section, the sheath most concave portion 4 overlaps the inscribing circle of the entire cross section, and the tip 5 of the core convex portion overlaps the circumscribing circle of the core. As shown in Figure 2, the tip 5 of the core convex portion falls within a triangle connecting the tip 3 of the sheath convex portion and the two adjacent most concave portions 4 of the sheath. The tip 5 of the core convex portion and the tip 3 of the sheath convex portion preferably exist on a straight line connecting the tip 5 and the center of the circumscribing circle and the center of the inscribing circle. In this way, by making the distance between the tip 5 of the core convex portion and the tip 3 of the sheath convex portion as close as possible, it is possible to suppress sheath peeling during manufacturing or post-processing. If the tip 5 of the core convex portion does not fall within the triangle, the core irregularity will not be high, and the fiber irregularity will not be high either, which may result in a deterioration in the properties of the nonwoven fabric.

[0038] Furthermore, the relationship between the core convex portion and the core concave portion will be explained with reference to Figure 3. The tip 5 of the core convex portion overlaps the circumscribing circle of the core, and the core most concave portion 6 overlaps the inscribing circle of the core. The core convex portion has a shape in which the dimension in the width direction decreases from the base to the tip. If the dimension in the width direction becomes locally smaller at the base of the core or from the base to the tip of the core, there is a risk that the sheath will break or peel off due to external force during manufacturing or post-processing, starting from the small dimension portion. In addition, there is a risk that the contact feel (e.g., smooth feel, dry touch feel) of a nonwoven fabric formed from the core-sheath composite fiber will be reduced.

[0039] <Composition of core-sheath composite fiber> The core-sheath composite fiber of the present invention contains a first component thermoplastic resin in the core and a second component thermoplastic resin in the sheath, and the first component thermoplastic resin has a melting point that is 40°C or more higher than that of the second component thermoplastic resin.

[0040] <Core> The melting point of the thermoplastic resin of the first component constituting the core is preferably at least 50° C. higher than the melting point of the thermoplastic resin of the second component, and more preferably at least 60° C. If the melting point of the thermoplastic resin of the first component is less than the melting point of the thermoplastic resin of the second component + 40° C., the fibers may soften during air-through and be pulled by the surface tension of the sheath, resulting in a nonwoven fabric formed with narrow inter-fiber distances, and the properties of the nonwoven fabric may be insufficient.

[0041] The thermoplastic resin of the first component may be any resin that exhibits the strength and low shrinkage required for the core component, and examples thereof include polyester-based resins such as polyethylene terephthalate (melting point 255°C), polybutylene terephthalate (melting point 230°C), polytrimethylene terephthalate (melting point 230°C), polyethylene naphthalate (melting point 265°C), and polybutylene naphthalate (melting point 243°C); and polyamide-based resins such as nylon 6 (melting point 220°C), nylon 66 (melting point 265°C), and nylon 56 (melting point 255°C).

[0042] Among these, from the viewpoints of strength and low shrinkage and of using a resin with a melting point 40°C or higher than that of the thermoplastic resin of the second component used in the sheath component, the thermoplastic resin of the first component preferably contains a polyester resin having a melting point of 210°C or higher (preferably 220°C or higher, more preferably 230°C to 280°C), more preferably polyethylene terephthalate (melting point 255°C), polybutylene terephthalate (melting point 230°C), polytrimethylene terephthalate (melting point 230°C), polyethylene naphthalate (melting point 265°C), or polybutylene naphthalate (melting point 243°C), even more preferably polyethylene terephthalate (melting point 255°C), polyethylene naphthalate (melting point 265°C), or polybutylene naphthalate (melting point 243°C), and particularly preferably polyethylene terephthalate. The thermoplastic resin of the first component may be one type or two or more types.

[0043] The thermoplastic resin of the first component may have a predetermined intrinsic viscosity, and the intrinsic viscosity (IV) of the thermoplastic resin of the first component is preferably 0.3 to 2.0 dl / g, more preferably 0.5 to 1.5 dl / g, and even more preferably 0.55 to 0.80 dl / g. The intrinsic viscosity (IV) of the thermoplastic resin of the first component can be determined by pulverizing and drying a sample, dissolving it in a 6 / 4 (mass ratio) mixed solvent of phenol and 1,1,2,2-tetrachloroethane, centrifuging the solution to remove inorganic particles and the like, and then measuring the IV at a temperature of 30°C using an Ubbelohde viscometer.

[0044] The content of the thermoplastic resin of the first component having a melting point of 210°C or higher is preferably 80% by mass or higher, more preferably 90% by mass or higher, even more preferably 95% by mass or higher, even more preferably 97% by mass or higher, particularly preferably 99% by mass or higher, and most preferably 100% by mass, based on 100% by mass of the thermoplastic resin of the first component. When the content of the thermoplastic resin of the first component having a melting point of 200°C or higher is within the above range, the strength of the core can be improved.

[0045] The content of the thermoplastic resin of the first component, based on 100% by mass of the core, is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, still more preferably 98% by mass or more, and preferably 100% by mass or less, more preferably 98% by mass or less, even more preferably 97% by mass or less, and still more preferably 96% by mass or less. If the content of the thermoplastic resin of the first component is less than 80% by mass, the strength of the core may decrease and the adhesion between the core and sheath may decrease.

[0046] From the viewpoint of core strength, the content of the thermoplastic resin in the first component is 45% by mass or more and 80% by mass or less, preferably 47% by mass or more and 75% by mass or less, and more preferably 50% by mass or more and 70% by mass or less, based on the total fiber mass. If the content of the thermoplastic resin in the first component is less than 45% by mass, the surface tension of the thermoplastic resin in the second component melts during the air-through thermal bonding process in nonwoven fabric processing, causing the overall fiber irregularity to decrease. This may result in the resulting nonwoven fabric failing to achieve the enhanced capillary action expected from the Y-shaped cross-section and the dry touch feel due to the reduced contact area with the skin. If the content of the thermoplastic resin in the first component is more than 80% by mass, the thermoplastic resin in the second component constituting the sheath may peel off from the thermoplastic resin in the first component constituting the core during spinning. Furthermore, if the core composed of the thermoplastic resin in the first component is exposed, the oil applied to the sheath surface may adhere to the core, reducing the hydrophilicity of the fiber and degrading the properties of the nonwoven fabric.

[0047] The core may contain inorganic particles that modify the properties of the thermoplastic resin of the first component, such as titanium oxide, calcium carbonate, talc, silica, and aluminum oxide.

[0048] The content of inorganic particles is preferably 0 to 15% by mass, more preferably 1 to 15% by mass, even more preferably 2 to 13% by mass, and even more preferably 3 to 11% by mass, based on 100% by mass of the core.

[0049] The core may contain components other than those described above as long as the effects of the present invention are achieved, and additives such as antioxidants, antistatic agents, antiblocking agents, pigments, heat stabilizers, UV absorbers, and lubricants may be added within limits that do not impair the effects of the present invention.

[0050] <Sheath> The thermoplastic resin of the second component constituting the sheath is not particularly limited as long as it has a melting point that is 40°C or more lower than the thermoplastic resin of the first component. Examples include polyolefin resins such as polyethylenes such as low-density polyethylene (melting point 100°C to 115°C), medium-density polyethylene (melting point 115°C to 125°C), high-density polyethylene (melting point 125°C to 137°C), and ultra-high molecular weight polyethylene (melting point 135°C to 140°C), and polypropylene (melting point 165°C); and polyester resins such as polylactic acid (melting point 170°C to 175°C), polybutylene succinate (melting point 115°C), polycaprolactone (melting point 55°C to 60°C), polyhydroxyalkanoate (melting point up to 180°C), and polybutylene adipate terephthalate (melting point 225°C to 228°C).

[0051] Among these, the thermoplastic resin of the second component preferably has a melting point of 170°C or lower (preferably 160°C or lower and 70°C or higher), more preferably contains polyethylene or polypropylene, and even more preferably contains polyethylene. The thermoplastic resin of the second component may be one type or two or more types.

[0052] The thermoplastic resin of the second component may have a predetermined melt flow rate (MFR) measured in accordance with JIS K 6922-2, and the melt flow rate (MFR) of the thermoplastic resin of the second component is preferably 8 to 25 g / 10 min, and more preferably 9 to 21 g / 10 min, under conditions of a load of 2.16 kg and a temperature of 190°C.

[0053] The content of the thermoplastic resin of the second component having a melting point of 170° C. or less is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, still more preferably 99% by mass or more, and particularly preferably 100% by mass, based on 100% by mass of the thermoplastic resin of the second component. When the content of the thermoplastic resin of the second component is within the above range, a nonwoven fabric having high adhesion between fibers by heat fusion can be produced.

[0054] The content of the thermoplastic resin of the second component is preferably 85% by mass or more, more preferably 87% by mass or more, even more preferably 90% by mass or more, still more preferably 95% by mass or more, and preferably 100% by mass or less, more preferably 98% by mass or less, even more preferably 97% by mass or less, and still more preferably 96% by mass or less, based on 100% by mass of the sheath.

[0055] The content of the thermoplastic resin of the second component is preferably 20% by mass or more and 55% by mass or less, more preferably 25% by mass or more and 50% by mass or less, even more preferably 30% by mass or more and 45% by mass or less, and even more preferably 35% by mass or more and 45% by mass or less, based on the total mass of the fibers.

[0056] The sheath may contain inorganic particles that modify the properties of the thermoplastic resin of the second component, such as titanium oxide, calcium carbonate, talc, silica, and aluminum oxide.

[0057] The content of the inorganic particles is preferably 0 to 15% by mass, more preferably 1 to 15% by mass, even more preferably 2 to 13% by mass, and even more preferably 3 to 11% by mass, based on 100% by mass of the sheath.

[0058] The sheath may contain components other than those described above as long as the effects of the present invention are achieved, and additives such as antioxidants, antistatic agents, antiblocking agents, pigments, heat stabilizers, ultraviolet absorbers, and lubricants may be added within a range that does not impair the effects of the present invention.

[0059] In the core-sheath composite fiber of the present invention, the mass ratio of the core component to the sheath component is preferably less than 7.0, more preferably 5.0 or less, even more preferably 4.0 or less, still more preferably 3.0 or less, particularly preferably 2.0 or less, and is preferably 0.2 or more or 0.3 or more. By setting the mass ratio of the core component to the sheath component within the above range, the strength and elongation of the fiber can be made appropriate for nonwoven fabrics.

[0060] <Physical Properties of Sheath-Core Composite Fiber> The sheath-core composite fiber of the present invention preferably has predetermined physical properties such as maximum point elongation, maximum point stress, and fineness. The maximum point elongation of the sheath-core composite fiber is 21% or more, preferably 25% or more, more preferably 30% or more, even more preferably 35% or more, and preferably 150% or less or 120% or less. If the maximum point elongation is lower than 21%, there is a risk of yarn breakage during spinning. Furthermore, there is a risk of yarn breakage during post-processing, and fly during carding, which may contaminate the equipment.

[0061] The maximum point stress of the core-sheath composite fiber is preferably 2.0 cN / dtex or more, more preferably 2.1 cN / dtex to 15 cN / dtex, even more preferably 2.2 cN / dtex to 12 cN / dtex, and even more preferably 2.3 cN / dtex to 10 cN / dtex. If the maximum point stress is less than 2.0 cN / dtex, thread breakage and the like may occur during post-processing, and a phenomenon known as "fly," in which fibers fly about during carding, may occur, potentially contaminating the equipment. The maximum point elongation and maximum point stress can be measured according to JIS L1013 8.5.1.

[0062] The single filament fineness of the sheath-core composite fiber is preferably 1.2 dtex or more and 10 dtex or less, more preferably 1.4 dtex or more and 8 dtex or less, even more preferably 1.6 dtex or more and 6 dtex or less, and even more preferably 1.8 dtex or more and 4 dtex or less. If the fineness is less than 1.2 dtex, the fiber may be soft and the bulk may not be large when made into a nonwoven fabric. On the other hand, if the fineness exceeds 10 dtex, the feel of the nonwoven fabric may be hard to the touch. The single filament fineness of the sheath-core composite fiber can be measured in accordance with JIS L 1095 9.4.1.

[0063] <Method for preparing sheath-core composite fiber> The method for preparing the sheath-core composite fiber of the present invention preferably includes at least a step of melt-spinning a core-sheath fiber using a first component thermoplastic resin as the core component and a second component thermoplastic resin as the sheath component to prepare an undrawn yarn, and a step of drawing the undrawn yarn under heating conditions.

[0064] For example, it is preferable to obtain undrawn yarns by a known melt spinning method using polyethylene terephthalate as the thermoplastic resin of the first component and polyethylene as the thermoplastic resin of the second component, using a nozzle with a modified cross section. The spinning temperature is preferably 10°C or more higher than the melting point of the thermoplastic resin of the first component, and more preferably 20°C or more higher than the melting point of the thermoplastic resin of the first component.

[0065] The discharge rate per hole of the nozzle may be adjusted appropriately depending on the melting points, contents, etc. of the first component thermoplastic resin and the second component thermoplastic resin, and is, for example, 0.4 to 1.0 g / min H, preferably 0.45 to 0.9 g / min H, and more preferably 0.50 to 0.8 g / min H.

[0066] In the method for producing a sheath-core composite fiber of the present invention, the distance from the nozzle to the cooling zone is preferably within an appropriate range. The distance from the nozzle to the cooling zone is, for example, more than 20 mm and not more than 90 mm, preferably 25 mm to 85 mm, and more preferably 30 mm to 80 mm. If the distance exceeds this range, it will be impossible to obtain a sheath-core composite fiber with an appropriate structure, and sheath peeling or the like may occur.

[0067] The resulting undrawn yarn may be drawn as necessary. The drawing method is not particularly limited, and may be one-stage or two-stage or more drawing. For example, the first stage of drawing may be performed at the glass transition temperature of the thermoplastic resin of the first component, and the second stage of drawing may be performed at a temperature equal to or higher than the glass transition temperature. The method of heating the fiber during drawing is not particularly limited, and examples include methods of heating the fiber in warm water or with a heated roll.

[0068] In the case of one-stage stretching, the stretching is preferably carried out at a temperature of 60 to 90°C (preferably 60 to 80°C) and a stretch ratio of 1.1 to 3.0 (preferably 1.2 to 2.9 times, more preferably 1.3 to 2.85 times, and even more preferably 1.4 to 2.80 times).

[0069] In the case of two-stage stretching, the first stage stretching is preferably carried out at a temperature of 60 to 90°C (preferably 60 to 80°C) and a stretch ratio of 2.0 to 4.0 (preferably 3.6 times or less, more preferably 3.2 times or less), and the second stage stretching is preferably carried out at a temperature of 90 to 130°C (preferably 100 to 120°C) and a stretch ratio of 0.8 to 1.2 (preferably 0.9 to 1.1).

[0070] The method for producing a core-sheath composite fiber of the present invention may include, in addition to the above steps, a crimping step, a post-heat treatment step, and a cutting step. The crimping step may involve crimping using a crimper such as a stuffer box crimper. The post-heat treatment step may involve treating the resulting fiber at a temperature of 90 to 140°C (preferably 100 to 130°C) without applying tension to the fiber.

[0071] When the core-sheath composite fibers are cut into short fibers, the average length of the short fibers is preferably 30 to 80 mm, more preferably 31 to 70 mm, and even more preferably 32 to 60 mm.

[0072] If necessary, a surfactant or the like may be added to the resulting drawn yarn to improve its performance. For example, when polyethylene is used as the thermoplastic resin of the second component, the fiber surface can be made hydrophilic by adding a surfactant to the surface.

[0073] The sheath-core composite fibers of the present invention may be long fibers (filaments), short fibers (staples), or a combination thereof.

[0074] The present invention includes a nonwoven fabric formed from core-sheath composite fibers. The nonwoven fabric is preferably formed by a conventionally known fleece forming or fleece bonding method. The fleece can be formed by a dry method, a wet method, a spunbonding method, a meltflow method, or the like, and the fleece can be bonded by a thermal bond method, a chemical bond method, a needle punch method, a spunlace method, a stitch bond method, a steam jet method, or the like. The fleece forming or fleece bonding method may be selected from the above methods depending on the desired nonwoven fabric.

[0075] This application claims the benefit of priority based on Japanese Patent Application No. 2023-058146, filed on March 31, 2023. The entire content of the specification of Japanese Patent Application No. 2023-058146, filed on March 31, 2023, is incorporated herein by reference.

[0076] 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, and can of course be practiced with appropriate modifications within the scope of the above and below-described aims, all of which are included within the technical scope of the present invention. In the following, unless otherwise specified, "parts" means "parts by mass" and "%" means "% by mass."

[0077] (1) Fineness, Single Yarn Fineness Measured by the method described in JIS L 1095 9.4.1. The single yarn fineness was calculated by dividing the fineness by the number of filaments.

[0078] (2) Maximum Stress and Maximum Elongation According to JIS L1013 8.5.1, the initial sample length was set to 20 mm, the stress at the maximum load was defined as the maximum stress, and the elongation at the maximum load was defined as the maximum elongation.

[0079] (3) Core Degree of Irregularity and Sheath Thickness Ratio An enlarged cross-sectional photograph of the core-sheath composite fiber was taken with an OLYMPUS system microscope BX53M (1500x), and the core degree of irregularity and the sheath thickness ratio were measured by the following method.

[0080] <Core Irregularity> The diameter of the inscribed circle of the core was designated as DI1, and the diameter of the circumscribed circle of the core was designated as DO1, and the core irregularity was calculated as the value obtained by dividing DO1 by DI1.

[0081] <Sheath Thickness Ratio> The sheath thickness ratio was calculated by dividing (DO2-DO1) by (DI2-DI1), where DI2 is the diameter of the inscribed circle of the entire cross section and DO2 is the diameter of the circumscribed circle of the entire cross section.

[0082] (4) Overall Cross-Section Degree of Irregularity After Heat Treatment The prepared fibers were treated for 30 seconds at a temperature 5°C higher than the melting point of the second component thermoplastic resin of the sheath under a load of 0.33 kgf / dtex. An enlarged cross-sectional photograph of the fiber cross section after the treatment was taken (1500x) with an OLYMPUS system microscope BX53M, and the value obtained by dividing the diameter of the circumscribed circle of the entire cross section by the diameter of the inscribed circle of the entire cross section was taken as the overall cross-section degree of irregularity after heat treatment.

[0083] [Examples 1 to 5, Comparative Examples 1 to 4] Using polyethylene terephthalate with an intrinsic viscosity IV of 0.63 as the first component and high-density polyethylene (Nipolon Hard (registered trademark), manufactured by Tosoh Corporation) with an MFR of 20 g / 10 min (load 2.16 kg, temperature 190°C) as the second component, sheath-core composite fibers having a Y-shaped configuration were produced by a predetermined method. The spinning speed was 1200 m / min, and in Examples 1 to 5 and Comparative Examples 1 to 4, spinning was performed with the single-hole output rate, core mass ratio, spinning temperature, and nozzle-to-cooling zone distance shown in Table 1. The resulting undrawn yarns were drawn at various draw ratios at 70°C and heat-treated at 110°C to obtain drawn yarns. The single yarn fineness, maximum point stress, maximum point elongation, overall cross-sectional irregularity after heat treatment, and special notes of the drawn yarns are shown in Table 1.

[0084] Example 1 had a good core irregularity and sheath thickness ratio, and the overall cross-sectional irregularity after heat treatment was also good at 1.83. In Example 2, the distance from the nozzle to the cooling zone was shorter and the draw ratio was lower than in Example 1. The overall cross-sectional irregularity after heat treatment was good at 1.78.

[0085] In Example 3, the single-hole output rate was increased from Example 2 to 0.63 g / min, the core mass ratio was set to 60%, and spinning was performed at 280°C. Because the core mass ratio was high, a drawn yarn having a high maximum point stress was obtained even though the maximum point elongation was close to that of Example 2. In addition, the overall cross-sectional irregularity after heat treatment was also high. This is thought to be because the core mass ratio was high in Example 3, and the core hardly changed shape during heat treatment.

[0086] In Example 4, the single-hole output rate was increased to 0.75 g / min compared to Example 2, and spinning was performed. By increasing the draw ratio, a drawn yarn having a single yarn fineness equivalent to that of Example 2 and a high maximum point stress was obtained. Example 4 showed a higher overall cross-sectional irregularity after heat treatment than Example 2. This is thought to be because, although Examples 2 and 4 have similar sheath thickness ratios, Example 4 had a high core irregularity of 3.12.

[0087] In Example 5, the spinning temperature was higher than in Example 3, and the distance from the nozzle to the cooling zone was longer, resulting in a lower core irregularity. However, the draw ratio was set high, resulting in a thin drawn yarn with a single yarn fineness of 1.8 dtex. Compared to Example 3, the core irregularity was lower and the sheath thickness ratio was also lower, so the overall cross-sectional irregularity after heat treatment was lower than in Example 3, but still achieved 1.80.

[0088] In Comparative Example 1, the spinning temperature was higher than in Example 1, and the distance from the nozzle to the cooling zone was increased to increase the draw ratio. Because the core irregularity was low, the overall cross-sectional irregularity after heat treatment was low at 1.63.

[0089] In Comparative Example 2, the spinning temperature was lower and the draw ratio was lower than in Comparative Example 1. Although the sheath thickness ratio was 1.78, the core irregularity was low at 1.93, and therefore the overall cross-sectional irregularity after heat treatment was low at 1.50.

[0090] In Comparative Example 3, a drawn yarn having a high degree of modification and a high sheath thickness ratio was obtained by setting the core mass ratio to 30%, the spinning temperature to 270°C, and the distance from the nozzle to the cooling zone to 20 mm. This drawn yarn had a thin sheath thickness at the cross-sectional recess, and sheath peeling occurred.

[0091] In Comparative Example 4, the draw ratio was increased compared to Example 4. Although the cross-sectional overall irregularity after heat treatment was high, single yarn breakage occurred during drawing due to the low maximum point elongation.

[0092]

[0093] DI1: diameter of the inscribed circle of the core DO1: diameter of the circumscribed circle of the core DI2: diameter of the inscribed circle of the entire cross section DO2: diameter of the circumscribed circle of the entire cross section 1 Core (first component) 2 Sheath (second component) 3 Tip of the sheath convex part 4 Deepest part of the sheath concave part 5 Tip of the core convex part 6 Deepest part of the core concave part 10 Core-sheath composite fiber

Claims

1. A core-sheath composite fiber comprising a core containing a first component thermoplastic resin and a sheath containing a second component thermoplastic resin having a melting point that is 40°C or more lower than the melting point of the first component thermoplastic resin, The fiber cross section has a Y-shape both at the core and throughout the cross section, The content of the core is 45% by mass or more and 80% by mass or less of the total fiber, In the fiber cross section, if the diameter of the inscribed circle of the core is DI1, the diameter of the circumscribed circle of the core is DO1, the diameter of the inscribed circle of the entire cross section is DI2, and the diameter of the circumscribed circle of the entire cross section is DO2, a sheath thickness ratio represented by (DO2-DO1) / (DI2-DI1) is 1.5 or more and 2.5 or less; The core irregularity expressed by DO1 / DI1 is 2.1 or more and 4.0 or less, The maximum elongation of the fiber measured in accordance with JIS L1013 8.5.1 is 21% or more, The tip of the core protrusion is located within the triangle connecting the tip of the sheath protrusion and the two adjacent sheath most recessed parts. A core-sheath composite fiber characterized in that the core convex portion has a shape in which the dimension in the width direction decreases from the base to the tip.

2. 2. The core-sheath composite fiber according to claim 1, wherein the single fiber fineness is 1.2 dtex or more and 10.0 dtex or less.

3. 3. The core-sheath composite fiber according to claim 1, wherein the maximum point stress of the fiber measured in accordance with JIS L1013 8.5.1 is 2.0 cN / dtex or more.

4. 3. The core-sheath composite fiber according to claim 1, wherein the cross-sectional overall irregularity expressed by DO2 / DI2 is 1.7 or more after the fiber is treated for 30 seconds at a temperature 5°C higher than the melting point of the thermoplastic resin of the second component under a load of 0.33 gf / dtex.

5. A nonwoven fabric formed from the core-sheath composite fiber according to claim 1 or 2.