Composite fibers, methods for manufacturing the same, heat-bonded nonwoven fabrics, surface sheets for absorbent articles, and absorbent articles

Abstract

To provide a composite fiber having an excellent card-passing property and a smooth touch, and capable of obtaining a heat-bonded nonwoven fabric with high adhesive strength, a method for manufacturing the same, the heat-bonded nonwoven fabric containing the same, an absorbent article surface sheet, and an absorbent article.SOLUTION: In the composite fiber, a core component and a sheath component are arranged in a substantially concentric manner. A volume ratio of the core component and the sheath component is 30 / 70 to 70 / 30. A single fiber fineness is 0.6 dtex or more and less than 2.0 dtex. The core component contains 60 mass% or more of polyester resin. The sheath component contains 60 mass% or more of high density polyethylene. A melt mass flow rate of the high density polyethylene is 13 g / 10 min to 45 g / 10 min. A crystallite size measured for a [110] face of the high density polyethylene contained in the sheath component is 20.0 to 50.0 nm. A heat of fusion (ΔHPE-HD) of the high density polyethylene measured by a differential scanning calorimetry analysis is 145.0 mJ / mg or more.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a composite fiber, a method for producing the same, a thermally adhesive nonwoven fabric containing the same, a surface sheet for absorbent articles, and absorbent articles.

Background Art

[0002] There are various types of composite fibers using two types of thermoplastic resins having different melting points. Among the composite fibers, of the two types of thermoplastic resins, the thermoplastic resin having a lower melting point is disposed outside the fiber (this resin component is referred to as a sheath component), and the other thermoplastic resin having a higher melting point is disposed inside the fiber (this resin component is referred to as a core component). The core-sheath type composite fiber is known as a fiber that can be easily adhered to other fibers by melting the thermoplastic resin disposed outside the fiber using a hot air treatment machine or a heated metal roll. A fiber web containing such a core-sheath type composite fiber easily adheres to other fibers by melting the sheath component. The thermally adhesive nonwoven fabric thus obtained becomes a nonwoven fabric excellent in bulkiness and flexibility, and is used not only for surface sheets in absorbent articles such as sanitary napkins and paper diapers, and back sheets constituting the outer portions of absorbent articles, but also for various human wiping sheets, various object wiping sheets, medical supplies, cosmetics, various absorbent materials (for example, there is an oil absorbent material that absorbs leaked oil), and various filter materials such as filter materials for liquid filters and air filters, and is used in a wide range of applications.

[0003] Among the uses of thermally adhesive nonwoven fabrics, in uses where the thermally adhesive nonwoven fabric directly touches the human skin during use, such as surface sheets and back sheets of absorbent articles, and skin covering sheets impregnated with cosmetics, it is required that the thermally adhesive nonwoven fabric be softer and have a smoother touch. Therefore, it is required to make the fineness of the single fiber of the composite fiber used for the thermally adhesive nonwoven fabric smaller.

[0004] When manufacturing heat-bonded nonwoven fabrics, there are various methods for manufacturing fiber webs containing conjugate fibers. However, in order to obtain a bulky and soft heat-bonded nonwoven fabric, a fiber web containing conjugate fibers is manufactured by a dry process, more specifically, a carding process, and the obtained fiber web is heat-treated to melt the sheath component of the conjugate fibers contained in the fiber web and bond the fibers together. This method is common. However, when manufacturing a fiber web by the carding process, the smaller the fiber diameter (the fineness of the single fiber of the fiber), the lower the carding permeability of the fiber, and the more likely the productivity of the nonwoven fabric will decrease. The reason for this is that as the fibers passing through the carding machine become fibers with a smaller single fiber fineness (diameter), the elasticity (stiffness) of the fibers decreases, and when making a fiber web with the carding machine, the fibers become entangled inside the carding machine, and it becomes easier to generate granular fiber lumps called neps.

[0005] In addition, fibers for making a fiber web with a carding machine are usually given a serrated crimp shape in order to improve the permeability inside the carding machine and make it easier to form a fiber web. Conjugate fibers are manufactured with a desired number of crimps, but these fibers are packed and shipped in a strongly compressed state, so they are in a compressed state for a long time. In addition, when using the shipped conjugate fibers, the fibers are scraped off little by little from the compressed conjugate fiber mass, and the fibers are put into the carding machine to make a fiber web, but a strong force also acts on the fibers in these processes. Therefore, when manufacturing fibers, even if the desired number of crimps is given, the shape of the crimp may collapse due to long-term storage in a compressed state and the force applied in the fiber opening process and the fiber mixing process before being put into the carding machine. Fibers with a collapsed crimp shape cannot be aligned by the cylinder roll inside the carding machine and are not easily entangled with other fibers, so they become a so-called "fly" state where they flutter without being entangled by the card wire inside the carding machine, and the productivity of the nonwoven fabric decreases. As the single fiber fineness of the fiber decreases, that is, as the fiber diameter decreases, the compressed state continues for a long time, or the shape of the crimp is easily collapsed by the strong force applied to the fibers in the fiber opening process or the fiber mixing process before being put into the carding machine. From this point as well, improvement of carding permeability is required.

[0006] In addition, heat-bonded nonwoven fabrics used in sanitary materials and medical supplies such as absorbent articles are usually required to be white in appearance to give the user a sense of cleanliness. Furthermore, among the nonwoven fabrics used in absorbent articles, the surface sheet used on the side that comes into contact with the wearer's skin is required not only to be white in appearance, but also to rapidly absorb blood (menstrual blood), urine, and fluid excrement that has been discharged from the body, as well as to have concealing properties that make the absorbed blood and excrement less visible from the surface. To enhance the whiteness of heat-bonded nonwoven fabrics and improve their concealing properties, composite fibers are manufactured using thermoplastic resins mixed with inorganic fillers (white pigments) such as titanium dioxide (also simply called titanium oxide) and zinc oxide. Synthetic fibers containing inorganic fillers tend to have reduced spinnability, as well as reduced single-fiber strength and fiber elasticity, so neps and flies are more likely to occur when the fibers are opened in a carding machine because the inorganic fillers act as foreign matter. Thus, due to the decrease in card passability associated with increasing the fineness of the fibers, and the decrease in card passability associated with the addition of inorganic fillers, composite fibers with fineness (less than 2.0 dtex) require improvement in card passability along with increasing the fineness.

[0007] Furthermore, when using heat-bonded nonwoven fabrics made with fine-denier composite fibers, particularly as sheets for absorbent articles where performance improvements are significant, it is necessary to further improve the bulkiness and permeability of the heat-bonded nonwoven fabric. Specifically, because fine-denier composite fibers tend to have small fiber diameters, heat-bonded nonwoven fabrics containing them tend to have less bulk (specific volume) compared to conventional composite fibers (i.e., composite fibers with a fineness of 2.0 dtex or higher). If the bulkiness of the heat-bonded nonwoven fabric is low, the desired tactile feel may not be obtained as a sheet for absorbent articles, and this tendency is particularly pronounced in surface sheets and back sheets of absorbent articles. In addition, heat-bonded nonwoven fabrics containing fine-denier composite fibers may not only lack bulk as described above, but because the fibers are fine and do not easily become bulky, the fiber layer containing these fine-denier composite fibers may become an overly dense fiber layer with few voids between the fibers constituting the fiber layer. In absorbent surface sheets, if the surface that comes into contact with the skin becomes too dense, it takes longer for liquids such as blood and urine to pass through the fiber layer, which can lead to poor permeability or cause liquid residue to remain on the sheet.

[0008] Numerous proposals have been made for composite fibers. Patent Document 1 discloses a heat-adhesive composite fiber produced by a manufacturing method in which the tow is heated to a predetermined temperature before crimping, then a finishing oil is sprayed on and cooled, and then crimping is applied, in order to adjust the fineness, crimp rate, and the difference between the maximum and minimum values ​​of the crimp number of the heat-adhesive composite fiber. However, the technology disclosed in Patent Document 1 has limitations in terms of production equipment and manufacturing conditions, and the fineness of the heat-adhesive fiber actually obtained is 2.4 to 3.4 dtex, and further fineness is required to improve the tactile feel. Patent Document 2 discloses a composite fiber with alkylene terephthalate with an intrinsic viscosity of 0.3 to 0.55 as the core component. The composite fiber described in Patent Document 2 yields a composite fiber with a fineness of less than 1.1 dtex, but because it uses alkylene terephthalate with a particularly low intrinsic viscosity, the types of alkylene terephthalate resins that can be used are limited. In addition, this composite fiber is characterized by its ease of tearing by hand when made into a nonwoven fabric. For applications such as heat-bonded nonwoven fabrics for hygiene materials, face masks, and filter materials, the mechanical strength of the fiber and the nonwoven fabric obtained using it may be insufficient.

[0009] Patent Document 3 discloses a heat-fusible composite fiber comprising a first component containing a polyester resin and a polyolefin resin, wherein the work of breaking when the fiber is broken is 1.6 cN·cm / dtex or more, and the ratio of breaking strength (cN / dtex) to breaking elongation (%) is 0.005 to 0.040 ([cN / dtex] / [%]). However, in the heat-fusible composite fiber described in Patent Document 3, the elongation of the fibers is all large (100% or more), making them easily stretchable and soft (Examples 1 to 5 of Patent Document 3). Therefore, when a tensile force is applied to the fiber in the direction of the fiber axis, the fiber itself can withstand the force by stretching, but because the fiber itself is easily stretchable and soft, it may twist when force is applied from various directions and become entangled with other fibers, which may easily cause neps in the carding machine. In addition, because the single fiber strength is small, when the resulting heat-fusible composite fiber is made into a fiber web, the elasticity and rigidity of the fiber web may be insufficient, which may cause problems with handling.

[0010] Patent documents 4 and 5 disclose composite fibers composed of a polyester resin core and a polyolefin resin sheath component. Patent document 4 discloses a manufacturing method in which the tow temperature is kept constant when crimping is applied, and Patent document 5 discloses a composite fiber obtained by using a polyester resin with an intrinsic viscosity of 0.60 to 0.75 as the core component and adding 7 to 12% by mass of inorganic particles to the core component. However, the composite fibers described in Patent documents 4 and 5 all have a fineness of 2.3 to 2.5 dtex, and further fineness is required to improve the tactile feel. In addition, Patent documents 4 and 5 did not consider processability when making nonwoven fabrics, such as card passability. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] Japanese Patent Publication No. 2013-133571 [Patent Document 2] Japanese Patent Publication No. 2014-201855 [Patent Document 3] Japanese Patent Publication No. 2018-172827 [Patent Document 4] Japanese Patent Publication No. 2018-135622 [Patent Document 5] Japanese Patent Publication No. 2018-159151 [Overview of the project] [Problems that the invention aims to solve]

[0012] This invention has been made in view of the above circumstances, and provides a composite fiber that, despite being a composite fiber with an unprecedented fineness (less than 2.0 dtex), has good card passability, a smooth feel, and high adhesive strength, and can produce a heat-bondable nonwoven fabric; a method for producing the same; a heat-bondable nonwoven fabric containing the same; a surface sheet for absorbent articles; and an absorbent article. [Means for solving the problem]

[0013] The present invention relates to a composite fiber comprising a core component and a sheath component, wherein the core component and the sheath component are arranged substantially concentrically, the composite ratio of the core component and the sheath component is 30 / 70 to 70 / 30 in terms of the volume ratio (core component / sheath component), the single fiber fineness is 0.6 dtex or more and less than 2.0 dtex, the core component contains 60% by mass or more of polyester resin, the sheath component contains 60% by mass or more of high-density polyethylene, the melt mass flow rate (MFR: measurement temperature 190°C, load 2.16 kgf (21.18 N)) of the high-density polyethylene is greater than 13 g / 10 min and 45 g / 10 min or less, the crystallite size measured on the

[0110] plane of the high-density polyethylene contained in the sheath component is 20.0 nm or more and 50.0 nm or less, and the heat of fusion (ΔH) of the high-density polyethylene measured by differential scanning calorimetry (DSC) is PE-HD The ) is 145.0 mJ / mg or more, and the toughness (toughness = single fiber strength [cN / dtex] × √ elongation at break [%]), which is expressed as the product of the single fiber strength and the positive square root of the elongation at break of the composite fiber, is 12.0 or more and 18.7 or less That is the case.

[0014] The present invention also relates to a method for manufacturing composite fibers, comprising the steps of: extruding a core component containing 60% by mass or more of polyester resin at a spinning temperature of 280°C to 380°C; extruding a sheath component containing 60% by mass or more of high-density polyethylene having a melt mass flow rate (MFR: measured at 190°C, load 2.16 kgf (21.18 N)) greater than 13 g / 10 min and 45 g / 10 min or less at a spinning temperature of 250°C to 350°C; supplying the core component and the sheath component to a composite nozzle such that the volume ratio of the core component to the sheath component is 30 / 70 to 70 / 30 in the fiber cross-section, and the sheath component is substantially concentrically arranged with the sheath component covering the surface of the composite fiber; and melting the extruded core component and the sheath component. The present invention relates to a method for producing composite fibers, comprising the steps of: cooling undrawn fibers in a state such that the draft ratio is 600 to 1500 while taking them up, thereby obtaining an undrawn fiber tow in which the core component and the sheath component have solidified and the single fiber fineness is 1.8 dtex to 4.5 dtex; stretching the undrawn fiber tow to 1.6 to 3.6 times its original length at a temperature of 70°C to 120°C to obtain a drawn fiber tow in which the single fiber fineness is 0.6 dtex to less than 2.0 dtex; applying a fiber treatment agent to the drawn fiber tow; heating the surface of the drawn fiber bundle to the drawn fiber tow to which the fiber treatment agent has been applied to 60°C or higher using water vapor as a medium; imparting crimp to the drawn fiber tow in which the surface temperature is 60°C or higher; and drying the crimped drawn fiber tow.

[0015] The present invention also relates to a heat-bondable nonwoven fabric containing 25% by mass or more of the aforementioned composite fibers, wherein at least some of the composite fibers are bonded together by a sheath component.

[0016] The present invention also relates to a sheet for absorbent articles containing the composite fibers, comprising a first fiber layer in contact with the skin and a second fiber layer adjacent to the first fiber layer, wherein the first fiber layer is a fiber layer containing 50% by mass or more of a first core-sheath type composite fiber, the first core-sheath type composite fiber is the composite fiber described above, the second fiber layer is a fiber layer containing 50% by mass or more of a second core-sheath type composite fiber, the second core-sheath type composite fiber is a core component containing a polyester resin, a sheath component containing a thermoplastic resin having a melting point 50°C or more lower than the melting point of the polyester resin, and a single fiber fineness of 2.2 dtex or more and 7 dtex or less, and at least a portion of the first core-sheath type composite fiber and the second core-sheath type composite fiber are heat-bonded by the sheath components of the first core-sheath type composite fiber and the second core-sheath type composite fiber.

[0017] The present invention also relates to an absorbent article comprising the heat-bonded nonwoven fabric or the surface sheet for the absorbent article. [Effects of the Invention]

[0018] The composite fiber of the present invention is a composite fiber comprising a core component and a sheath component, wherein the core component and the sheath component are arranged substantially concentrically, the composite ratio of the core component and the sheath component is 30 / 70 to 70 / 30 in terms of the volume ratio of the core component to the sheath component (core component / sheath component), the single fiber fineness is 0.6 dtex or more and less than 2.0 dtex, the core component contains 60% by mass or more of polyester resin, the sheath component contains 60% by mass or more of high-density polyethylene, the melt mass flow rate (MFR: measurement temperature 190°C, load 2.16 kgf (21.18 N)) of the high-density polyethylene is greater than 13 g / 10 min and 45 g / 10 min or less, the crystallite size measured on the

[0110] plane of the high-density polyethylene contained in the sheath component constituting the composite fiber is 20.0 nm or more and 50.0 nm or less, and the heat of fusion (ΔH) of the high-density polyethylene measured by differential scanning calorimetry (DSC) is PE-HDThis composite fiber has a saturation ratio of 145.0 mJ / mg or higher. The composite fiber has a single fiber fineness of 0.6 dtex or more and less than 2.0 dtex, resulting in a fiber with a smaller single fiber fineness and smaller fiber diameter than conventional composite fibers. When formed into a fiber aggregate such as a heat-bonded nonwoven fabric, the surface not only has a smooth and soft feel, but the fine fineness promotes diffuse reflection and scattering of light, making the appearance of the fiber aggregate appear whiter. Adding an appropriate amount of inorganic filler further improves the appearance of the fiber aggregate, as well as its opacity. In this composite fiber, the core component and the sheath component are arranged substantially concentrically, and the composite ratio of the core component to the sheath component is 30 / 70 to 70 / 30 in terms of the volume ratio (core component / sheath component). This ensures that the sheath component is uniformly present on the surface of the composite fiber, allowing for easy heat bonding of the fibers and providing a heat-bonded nonwoven fabric with high adhesive strength.

[0019] Furthermore, the composite fiber exhibits crystallization and crystal growth of the high-density polyethylene contained in the sheath component that constitutes the surface of the composite fiber. Specifically, the crystallite size measured for the

[0110] plane of the high-density polyethylene contained in the sheath component is between 20.0 nm and 50.0 nm. In addition, the heat of fusion (ΔH) of the high-density polyethylene, measured by differential scanning calorimetry (DSC), is also measured. PE-HD The heat of fusion (ΔH) of the high-density polyethylene contained in the sheath component on the surface of the composite fiber is considered to be growing crystals based on the crystallite size measured on the

[0110] plane. PE-HD Since the heat of fusion is 145.0 mJ / mg or higher, it is considered that crystallization is progressing. The high-density polyethylene contained in the sheath component satisfies the above range of crystallite size and heat of fusion, so the sheath component of the composite fiber becomes a resin component containing high-density polyethylene in which both crystal growth and crystallization are progressing. As this sheath component covers the surface of the composite fiber like a shell, the composite fiber of the present invention exhibits sufficient strength and elasticity even if the fiber diameter is small and the fineness is small, so it is thought that the twisting and excessive entanglement of the fibers and the occurrence of many neps in the carding process are reduced. However, this assumption does not limit the present invention. [Brief explanation of the drawing]

[0020] [Figure 1] Figure 1 is a schematic cross-sectional view showing the fiber cross-section of a composite fiber in one embodiment of the present invention. [Figure 2] Figures 2A and 2B are schematic diagrams showing the crimped form of a composite fiber in one embodiment of the present invention. [Figure 3] Figure 3 is a schematic cross-sectional view of an absorbent surface sheet for an absorbent article according to one embodiment of the present invention. [Modes for carrying out the invention]

[0021] The inventors, after diligent research to solve the above problems, have found a composite fiber in which the core component contains 60% by mass or more of polyester resin and the sheath component contains 60% by mass or more of high-density polyethylene having a melt mass flow rate (MFR: measured at 190°C, load 2.16 kgf (21.18 N)) greater than 13 g / 10 min and 45 g / 10 min or less, wherein the core component and the sheath component are arranged substantially concentrically, the composite ratio of the core component and the sheath component is set to 30 / 70 to 70 / 30 in terms of the volume ratio (core component / sheath component), the crystallite size measured on the

[0110] plane of the high-density polyethylene contained in the sheath component is 20.0 nm to 50.0 nm, and the heat of fusion (ΔH) of the high-density polyethylene measured by differential scanning calorimetry (DSC) is set. PE-HD We discovered that by increasing the saturation ratio to 145.0 mJ / mg or higher, the overall rigidity of the composite fibers is increased, resulting in good card passage even with fine composite fibers having a single fiber fineness of less than 2.0 dtex, and excellent tactile feel and adhesive strength when made into a heat-bonded nonwoven fabric, leading to the present invention.

[0022] (Composite fiber) The composite fiber of the present invention is a core-sheath type composite fiber comprising a core component and a sheath component, wherein the core component and the sheath component are arranged substantially concentrically in a concentric structure.

[0023] (core component) The core component of the composite fiber of the present invention contains 60% by mass or more of polyester resin. Preferably, the core component contains 75% by mass or more of polyester resin, more preferably 85% by mass or more, and particularly preferably 90% by mass or more. There is no particular upper limit to the amount of polyester resin contained in the core component, and the core component may be configured such that all of the resin components are polyester resin, that is, the core component may be configured such that all of the thermoplastic resin, excluding inorganic fillers and the like described later, is polyester resin. The polyester resin contained in the core component may be one type or two or more types.

[0024] While the polyester resin is not particularly limited, it is preferable that it has a melting point 50°C or more higher than the melting point of the high-density polyethylene contained in the sheath component described later. Having a polyester resin with a melting point 50°C or more higher than the melting point of the high-density polyethylene contained in the sheath component not only improves spinnability during melt spinning, but also results in appropriate strength for the individual fibers of the resulting composite fibers and the heat-bonded nonwoven fabric containing the composite fibers. It is more preferable that the polyester resin has a melting point 80°C or more higher than the melting point of the high-density polyethylene contained in the sheath component, and even more preferable that it has a melting point 100°C or more higher.

[0025] The polyester resin is not particularly limited, and either aliphatic polyester resins or aromatic polyester resins can be used. Examples of polyester resins include polylactic acid (PLA), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), and polyethylene naphthalate (PEN). The polyester resin is preferably an aromatic polyester resin, and more preferably at least one polyester resin selected from the group consisting of polyethylene terephthalate, polybutylene terephthalate, and polytrimethylene terephthalate, having a melting point 50°C or higher, more preferably 80°C or higher, than the melting point of the high-density polyethylene contained in the sheath component. This is because polyethylene terephthalate is cheaper than polytrimethylene terephthalate or polybutylene terephthalate, and the resin itself has high rigidity, which gives stiffness to the fibers. As a result, the resulting composite fibers have a fineness of less than 2.0 dtex while still possessing appropriate rigidity, making it easier for the composite fibers to pass through the card smoothly.

[0026] When the core component contains 60% by mass or more of polyethylene terephthalate, the intrinsic viscosity of the polyethylene terephthalate is preferably greater than 0.55 dL / g and less than or equal to 0.75 dL / g. Intrinsic viscosity, also called intrinsic viscosity, depends on the molecular weight of polyethylene terephthalate. If the intrinsic viscosity of polyethylene terephthalate is 0.55 dL / g or less, the core component may lack sufficient strength and rigidity due to the small molecular weight of polyethylene terephthalate, potentially resulting in low single fiber strength of the resulting composite fiber or fibers that have difficulty maintaining their crimped shape. On the other hand, if the intrinsic viscosity exceeds 0.75 dL / g, the viscosity when polyethylene terephthalate is melted becomes too high, potentially reducing spinnability during melt spinning. The intrinsic viscosity of polyethylene terephthalate is preferably between 0.58 dL / g and 0.70 dL / g, and more preferably between 0.60 dL / g and 0.68 dL / g.

[0027] The number average molecular weight of the polyethylene terephthalate resin is not particularly limited, but it is preferable that the number average molecular weight of the polyethylene terephthalate resin contained in the core component is 2500 or more and 6500 or less. When the number average molecular weight of the polyethylene terephthalate in the core component satisfies the above range, the resulting composite fiber has a resin component with appropriate elasticity, so that even with a fineness of less than 2.0 dtex, the carding properties are good, and the heat-bonded nonwoven fabric containing the composite fiber tends to have a superior feel. It is more preferable that the number average molecular weight of the polyethylene terephthalate resin is 3000 or more and 6000 or less, and particularly preferable that it is 3500 or more and 5500 or less.

[0028] The weight-average molecular weight of the polyethylene terephthalate resin is not particularly limited, but it is preferable that the weight-average molecular weight of the polyethylene terephthalate resin contained in the core component is 6,000 to 18,000. When the weight-average molecular weight of the polyethylene terephthalate core component satisfies the above range, the resulting composite fiber has a resin component with appropriate elasticity, resulting in good cardability even at fineness of less than 2.0 dtex, and the heat-bonded nonwoven fabric containing the composite fiber tends to have a superior feel. It is more preferable that the weight-average molecular weight of the polyethylene terephthalate resin is 8,000 to 15,000, and particularly preferable that it is 9,000 to 14,000.

[0029] In the present invention, it is preferable to prepare polyethylene terephthalate as a raw material having a number average molecular weight (Mn) of 2,500 to 27,000 and a weight average molecular weight (Mw) of 6,000 to 80,000, and to melt spin it as the core component of the composite fiber at the spinning temperature described later, because the polyethylene terephthalate contained in the core component of the composite fiber is more likely to satisfy the above-mentioned average molecular weights. Alternatively, it is preferable to prepare polyethylene terephthalate as a raw material having an intrinsic viscosity (also called the IV value) greater than 0.55 dL / g and 0.8 dL / g or less, preferably 0.55 dL / g to 0.75 dL / g, and more preferably 0.6 dL / g to 0.7 dL / g, and to melt spin it as the core component of the composite fiber at the spinning temperature described later, because the polyethylene terephthalate contained in the core component of the composite fiber is more likely to satisfy the above-mentioned average molecular weights.

[0030] The core component described above may include thermoplastic resins other than the polyester resin described above, as long as it does not impair the function of the present invention. The thermoplastic resin other than the polyester resin is not particularly limited, but examples include polyolefin resin, polyamide resin, polycarbonate, and polystyrene.

[0031] Furthermore, various known additives can be added to the core component, provided that the effects of the present invention are not hindered and the productivity of fibers, productivity of fiber aggregates, heat adhesion, and tactile properties 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 ultraviolet absorbers. It is preferable that such additives are included in the core component in an amount of 10% or less by mass of the total mass of the core component.

[0032] (sheath component) In the composite fiber of the present invention, the sheath component contains 60% by mass or more of high-density polyethylene. In the present invention, high-density polyethylene (also referred to as PE-HD or HDPE) has a density of 0.94 g / cm³ as measured in accordance with JIS K 7112 (1999). 3 The above refers to polyethylene. High-density polyethylene has a higher density compared to other polyethylenes such as low-density polyethylene and linear low-density polyethylene, which tends to result in composite fibers that are highly rigid, have good carding properties and crimping properties, and the resulting heat-bondable nonwoven fabric tends to be bulky. The content of high-density polyethylene in the above sheath component is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more. Particularly preferably, in the sheath component, all thermoplastic resin components except for the inorganic filler described later are high-density polyethylene.

[0033] In the above-mentioned composite fiber, the high-density polyethylene contained in the sheath component has a melt mass flow rate (MFR: measurement temperature 190°C, load 2.16 kgf (21.18 N), hereinafter also referred to as MFR190) greater than 13 g / 10 min and 45 g / 10 min or less, as measured in accordance with JIS K 7210-1 (2014). Having the MFR190 of the high-density polyethylene within the above range not only improves spinnability and stretchability, but also ensures that the sheath component of the resulting composite fiber has sufficient rigidity to pass through the carding machine, resulting in good carding properties for the composite fiber. The melt mass flow rate of the high-density polyethylene is preferably 15 g / 10 min or more and 40 g / 10 min or less, more preferably 18 g / 10 min or more and 35 g / 10 min or less, and particularly preferably 18 g / 10 min or more and 32 g / 10 min or less.

[0034] The composite fiber of the present invention has a surface composed of a sheath component containing 60% by mass or more of the high-density polyethylene described above. Therefore, the heat-adhesion of the composite fiber mainly depends on the fluidity when the high-density polyethylene melts. Furthermore, the strength of the heat-adhesion nonwoven fabric using the composite fiber mainly depends on the strength of the heat-bonding points between the constituent fibers that are formed when the sheath component melts and heat-bonds during heat treatment. When the MFR190 of the high-density polyethylene satisfies the above range, the fluidity of the sheath component when melted is appropriately suppressed. As a result, when a fiber web containing the composite fiber is heat-treated near the melting point of the high-density polyethylene, the entire sheath component of the composite fiber melts, but because the fluidity is suppressed, it does not flow easily. As a result, the thickness of the sheath component becomes uniform, and heat-bonding points with uniform adhesive strength are formed between the constituent fibers at all bonding points, and it is presumed that the strength of the resulting heat-adhesion nonwoven fabric will be sufficiently high. When the MFR190 of high-density polyethylene exceeds 45 g / 10 min, the sheath component tends to flow more easily during heat treatment, which can lead to uneven thickness of the sheath component in composite fibers. This may result in the formation of heat-bonded points with low adhesive strength within the nonwoven fabric, where areas with thin sheath components are heat-bonded. Consequently, when the nonwoven fabric is pulled in the longitudinal and / or transverse directions, or when friction is applied by rubbing the surface of the nonwoven fabric, the weakly bonded points are more likely to detach, potentially leading to insufficient strength of the nonwoven fabric or causing fuzzing. On the other hand, when the MFR190 of high-density polyethylene is 13 g / 10 min or less, the fluidity of the sheath component is too low, which may reduce spinning take-up and stretchability.

[0035] In the sheath component of the composite fiber described above, the melting point of the high-density polyethylene is not particularly limited, but considering the carding properties of the composite fiber, as well as the productivity, strength, and heat resistance of the heat-bonded nonwoven fabric, the melting point of the high-density polyethylene is preferably 125°C to 140°C, and more preferably 128°C to 138°C. In the present invention, the melting point of high-density polyethylene refers to the melting peak temperature measured in accordance with JIS K 7121 (1987).

[0036] In the composite fiber of the present invention, the sheath component may contain resins other than the high-density polyethylene described above, as long as it does not impair the effects of the present invention. The resins other than high-density polyethylene are not particularly limited, but examples include polyolefin resins other than high-density polyethylene, polyester resins, polyamide resins, polycarbonates, polystyrene, etc. The polyolefin resins other than high-density polyethylene are not particularly limited, but examples include polypropylene, medium-density polyethylene, low-density polyethylene, linear low-density polyethylene, polymethylpentene, polybutene-1, copolymers or graft polymers of these with at least one selected from the group consisting of acrylic acid, methacrylic acid, maleic acid, and other unsaturated carboxylic acids, esters of unsaturated carboxylic acids such as acrylic acid esters, methacrylic acid esters, and maleic acid esters, anhydrides of unsaturated carboxylic acids such as acrylic acid anhydride, methacrylic acid anhydride, and maleic acid anhydride, as well as elastomers thereof. The polyester resins mentioned above are not particularly limited, but examples include polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polylactic acid, copolymers thereof with acid components such as isophthalic acid, succinic acid, and adipic acid, glycol components such as 1,4-butanediol and 1,6-hexanediol, polytetramethylene glycol, polyoxymethylene glycol, etc., and elastomers thereof. The polyamide resins mentioned above are not particularly limited, but examples include nylon 6, nylon 66, nylon 11, and nylon 12. Furthermore, various known additives can be added to the sheath components as long as the effects of the present invention are not hindered and do not affect fiber productivity, nonwoven fabric productivity, heat adhesion, or tactile feel. Additives that can be added to the above sheath components include known crystal nucleating agents, antistatic agents, pigments, matting agents, heat stabilizers, light stabilizers, anti-fusing agents (such as talc and calcium stearate), flame retardants, antibacterial agents, lubricants, plasticizers, softeners, antioxidants, and ultraviolet absorbers.

[0037] In the composite fiber of the present invention, the cross-sectional structure is a concentric circular structure in which the center of gravity of the core component substantially coincides with the center of gravity of the composite fiber. That is, in the fiber cross-section, the center of gravity of the core component does not substantially deviate from the center of gravity of the composite fiber. Figure 1 is a schematic diagram of the fiber cross-section of a composite fiber for absorbent articles with a concentric circular structure. A sheath component 1 is arranged around the core component 2, and the sheath component 1 surrounds the core component 2, so that in the composite fiber 10, the fiber surface other than the cut surface is covered with the sheath component 1. As a result, when a fiber web composed of composite fibers is heat-bonded, the surface of the sheath component 1 melts and heat-bonds the fibers together. In the composite fiber 10, since the core component 2 is not eccentric, i.e., has a concentric circular structure, the thickness of the sheath component 1 in the fiber cross-section is substantially constant at all points in the fiber cross-section. As a result, when a fiber web composed of composite fibers is heat-treated, the sheath component on the fiber surface of the composite fiber softens and melts, and regardless of which part of the composite fiber other fibers come into contact with, a heat-bonding point of uniform strength is formed. Therefore, the heat-bonded nonwoven fabric using the above composite fibers has high adhesive strength, is resistant to friction, and is less prone to fraying. The center of gravity position 3 of the core component 2 is not substantially deviated from the center of gravity position 4 of the composite fiber 10. The statement that the center of gravity position of the core component is not substantially deviated from the center of gravity position of the composite fiber means that the deviation rate (hereinafter also referred to as the eccentricity rate) determined by the following method is 10% or less, preferably 7% or less, particularly preferably 5% or less, and most preferably 3% or less.

[0038] <Eccentricity> The fiber cross-section of the composite fiber 10 was magnified and photographed using a scanning electron microscope, and the centroid position 3 of the core component 2 was defined as C1, and the centroid position 4 of the composite fiber 10 was defined as C f Let the radius of the composite fiber 10 be r f In this case, the calculation is performed using formula 1 below.

[0039]

number

[0040] In the above-mentioned composite fiber, the composite ratio of core component to sheath component is 30 / 70 to 70 / 30 in terms of the volume ratio of core component to sheath component. The core component affects the elasticity of the composite fiber, and the sheath component affects the adhesive strength, feel, and hardness of the heat-bonded nonwoven fabric containing the composite fiber. When the composite ratio of core component to sheath component in the above-mentioned composite fiber is 30 / 70 to 70 / 30, it is possible to achieve both the carding properties of the composite fiber and the adhesive strength and feel of the heat-bonded nonwoven fabric containing the composite fiber. If the amount of sheath component becomes too large, the proportion of the sheath component covering the fiber surface, i.e., high-density polyethylene with a lower melting point, in the composite fiber increases, which means that the resin extruded from the nozzle during melt spinning may not cool sufficiently before being withdrawn, potentially leading to a large number of fused fibers or frequent thread breakage. Furthermore, even if composite fibers are obtained, the heat-bonded nonwoven fabric using these composite fibers may have a higher nonwoven strength because the proportion of sheath components, i.e., resin components that contribute to heat bonding, is large. However, this may result in a hard texture. On the other hand, if the core component is too large, the proportion of sheath components that contribute to heat bonding between the constituent fibers is small, and the sheath components exist as a thin layer covering the circumferential surface of the composite fibers. Therefore, even if heat treatment is performed to form heat bonding points between the constituent fibers, the heat bonding points are small and easily come apart by external force. This may result in a lower nonwoven strength or easy fraying when friction is applied to the nonwoven fabric. In the above composite fibers, the composite ratio, which is the ratio of core components to sheath components, is preferably 30 / 70 to 60 / 40 in terms of the volume ratio of core components to sheath components, more preferably 33 / 67 to 55 / 45, particularly preferably 35 / 65 to 50 / 50, and most preferably 35 / 65 to 48 / 52.

[0041] In the above-mentioned composite fiber, the shape of the fiber cross-section of the core component may be any shape other than circular, such as elliptical, Y-shaped, X-shaped, grid-shaped, polygonal, or star-shaped. The shape of the fiber cross-section of the composite fiber may be any shape other than circular, such as elliptical, Y-shaped, X-shaped, grid-shaped, polygonal, or star-shaped, or it may be hollow.

[0042] In the above composite fiber, the crystallite size measured for the (0110) plane of the high-density polyethylene contained in the sheath component is 20.0 nm or more and 50.0 nm or less. The crystallite size, also called the crystallite diameter, is the size of the smallest microcrystalline unit forming the crystal. Since the crystallite size is inversely proportional to the half-width of the diffraction peak in the X-ray diffraction (XRD) of the object, if the crystallite size is large, that is, if the crystallinity is high, the half-width of the diffraction peak becomes small, and if the crystallite size is small, that is, if the crystallinity is low, the half-width of the diffraction peak becomes large. In the above composite fiber, the crystallite size measured for the (0110) plane of the high-density polyethylene contained in the sheath component is preferably 22.0 nm or more and 45.0 nm or less, more preferably 24.0 nm or more and 40.0 nm or less, and particularly preferably 24.5 nm or more and 37.5 nm or less.

[0043] In the above composite fiber, the crystallite size measured for the (0200) plane of the high-density polyethylene contained in the sheath component is not particularly limited, but preferably, the crystallite size measured for the (0200) plane is 12.0 nm or more and 35.0 nm or less. More preferably, the crystallite size measured for the (0200) plane is 16.0 nm or more and 30.0 nm or less, particularly preferably 18.0 nm or more and 27.5 nm or less, and most preferably 18.5 nm or more and 25.0 nm or less.

[0044] For the above crystallite size, wide-angle X-ray diffraction measurement is performed on the object, and from the obtained 2θ-θ intensity data, the half-width of the diffraction peak of the target crystal plane is measured, and it can be calculated based on the following formula 2 from the half-width.

[0045]

Equation

[0046] In the above composite fiber, the heat of fusion (ΔH) of high-density polyethylene measured by differential scanning calorimetry (DSC) PE-HD The heat of fusion of the high-density polyethylene contained in the sheath component is 145.0 mJ / mg or more. Since the heat of fusion of the high-density polyethylene contained in the sheath component is 145.0 mJ / mg or more, it can be said that the crystallization of this high-density polyethylene is sufficiently advanced. As described above, the crystallite size of the high-density polyethylene measured on the

[0110] plane is between 20.0 nm and 50.0 nm. Therefore, since the high-density polyethylene of the sheath component satisfies this range of crystallite size and heat of fusion, it is considered that both crystal growth and crystallization are sufficiently advanced. As crystal growth and crystallization progress, the sheath component of the composite fiber becomes a highly rigid resin component. This imparts strong rigidity to the composite fiber, making it less prone to excessive twisting inside the card and less likely to generate neps, even at fine deniers. In addition, by firmly fixing the crimp shape to such a resin component with sufficiently advanced crystal growth and crystallization, the crimp shape becomes less likely to collapse, further improving card passability. (ΔH) PE-HD The heat of fusion (ΔH) of high-density polyethylene is preferably 148.0 mJ / mg or more, more preferably 150.0 mJ / mg or more, particularly preferably 152.0 mJ / mg or more, and most preferably 155.0 mJ / mg or more. PE-HD The upper limit of ) is not particularly limited, but is preferably 210.0 mJ / mg or less, more preferably 200.0 mJ / mg or less, particularly preferably 195.0 mJ / mg or less, and most preferably 190.0 mJ / mg or less.

[0047] The heat of fusion of the above high-density polyethylene (ΔH PE-HD ) is measured using the following procedure. First, to determine the heat of fusion of high-density polyethylene, the core-sheath ratio (volume ratio) of the composite fiber is converted to a core-sheath ratio (mass ratio) based on the density and amount of thermoplastic resin and inorganic filler constituting the core and sheath components. The proportion of high-density polyethylene in the composite fiber (mass ratio of high-density polyethylene) is then determined from the proportion of inorganic filler contained in the sheath component. Next, differential scanning calorimetry is performed on the composite fiber sample based on JIS K 7121 (1987) Method for Measuring Transition Temperatures of Plastics. Differential scanning calorimetry reveals an endothermic peak with a melting peak temperature in the 125°C to 140°C temperature range (endothermic heat associated with melting is observed from approximately 120°C, reaching the melting peak temperature between 125°C and 140°C, and ending at approximately 150°C). From the heat of fusion (ΔH) measured between approximately 120°C and 150°C, the heat of fusion (ΔH) of the high-density polyethylene contained in the composite fiber is determined. PE-HD ) is calculated using the following formula 3.

[0048]

number

[0049] The composite fiber of the present invention has a single fiber fineness of 0.6 dtex or more and less than 2.0 dtex. Having a single fiber fineness of less than 2.0 dtex results in a heat-bonded nonwoven fabric containing the composite fiber that is smooth to the touch and soft. Furthermore, because of the low single fiber fineness, for the same basis weight, the number of fibers constituting the nonwoven fabric is greater than that of a nonwoven fabric composed of fibers with a higher single fiber fineness. This results in a denser, more opaque nonwoven fabric. If the single fiber fineness of the composite fiber is 2.0 dtex or more, it becomes difficult to obtain a soft, smooth, and opaque nonwoven fabric. The single fiber fineness of the composite fiber is preferably 1.8 dtex or less, more preferably 1.7 dtex or less, particularly preferably 1.6 dtex or less, and most preferably 1.5 dtex or less. In the composite fiber, a single fiber fineness of 0.6 dtex or more improves the carding properties of the composite fiber and increases productivity. The single fiber fineness of the above-mentioned composite fiber is preferably 0.8 dtex or higher, more preferably 1.0 dtex or higher, and particularly preferably 1.1 dtex or higher. The single fiber fineness of the above-mentioned composite fiber can be adjusted by adjusting the single fiber fineness and draw ratio of the undrawn fiber tow described later, so that composite fibers with a single fiber fineness of 0.6 dtex or higher and less than 2.0 dtex can be manufactured at any fineness within the above range.

[0050] The single-fiber strength of the composite fiber is not particularly limited, but is preferably between 1.5 cN / dtex and 5.0 cN / dtex. When the single-fiber strength of the composite fiber satisfies the above range, the resulting composite fiber will have appropriate strength and appropriate rigidity, resulting in good carding properties for the composite fiber and good handling of the fiber web during nonwoven fabric production. The single-fiber strength of the composite fiber is more preferably between 1.6 cN / dtex and 4.0 cN / dtex, particularly preferably between 1.8 cN / dtex and 3.8 cN / dtex, and most preferably between 2.0 cN / dtex and 3.5 cN / dtex.

[0051] The elongation at break of the composite fiber is not particularly limited, but it is preferable that it is between 20% and 150%. When the elongation at break of the composite fiber satisfies the above range, the resulting composite fiber will have appropriate strength and appropriate rigidity, resulting in good carding properties of the composite fiber and good handling of the fiber web during nonwoven fabric production. The elongation at break of the composite fiber is more preferably between 25% and 120%, even more preferably between 25% and 100%, particularly preferably between 30% and 80%, and most preferably between 30% and 60%. In this invention, the single fiber strength and elongation at break of the composite fiber are measured in accordance with JIS L 1015 (2010).

[0052] In the above-mentioned composite fiber, it is preferable that the ratio of single fiber strength to elongation at break (single fiber strength [cN / dtex] / elongation at break [%]), measured in accordance with JIS L 1015 (2010), is greater than 0.04 and less than or equal to 0.12. The ratio of single fiber strength to elongation at break increases as the composite fiber has high strength and low elongation, and decreases as the composite fiber has low strength and high elongation. When the ratio of single fiber strength to elongation at break (single fiber strength / elongation at break) of the composite fiber satisfies the above range, the composite fiber becomes a fiber with a balanced single fiber strength and elongation at break, possessing appropriate elasticity and rigidity, resulting in excellent card passage and a fiber web that is easy to handle. The ratio of single fiber strength to elongation at break of the composite fiber (single fiber strength / elongation at break) is more preferably 0.05 or more and 0.12 or less, more preferably 0.06 or more and 0.11 or less, particularly preferably 0.07 or more and 0.10 or less, and most preferably 0.075 or more and 0.098 or less.

[0053] As an index for evaluating the elasticity and stiffness of a composite fiber from its single-fiber strength and elongation at break, there is the product of the positive square root (√elongation) of the single-fiber strength and elongation at break (hereinafter also referred to as toughness). In the composite fiber of the present invention, the toughness is preferably 12.0 or more and 20.0 or less. When the toughness of the composite fiber satisfies the above range, similar to the ratio of single-fiber strength to elongation at break described above, the composite fiber becomes a fiber with a balanced strength and elongation, possessing appropriate elasticity and stiffness, resulting in excellent card passage and a fiber web that is easy to handle. In the composite fiber of the present invention, the toughness is more preferably 15.0 or more and 19.0 or less, particularly preferably 16.0 or more and 18.5 or less, and most preferably 16.5 or more and 18.5 or less.

[0054] The fiber length of the above-mentioned composite fiber is not particularly limited, but is preferably 25 mm to 50 mm. This is because when the fiber length is within this range, the composite fiber has excellent carding properties even at fine denier, and a well-formed fiber web (card web) can be manufactured. If the fiber length is less than 25 mm, the fiber length is too short and it is likely to fail to catch on the card, resulting in a so-called "fly" state, which may prevent the manufacture of a card web. If the fiber length exceeds 50 mm, the composite fiber may get caught too much on the wire of the carding machine, or the composite fibers may easily entangle with each other, resulting in a large number of neps, which may prevent the manufacture of a card web. The fiber length of the above-mentioned composite fiber is more preferably 27 mm to 48 mm, even more preferably 28 mm to 46 mm, and particularly preferably 28 mm to 40 mm.

[0055] The above-mentioned composite fiber has at least one type of crimp selected from the group consisting mainly of sawtooth crimp (also called mechanical crimp) shown in Figure 2A and wave-shaped crimp shown in Figure 2B, and it is preferable that the number of crimps is 5 or more and 28 or less per 25 mm. A more preferable number of crimps is 8 or more and 25 or less per 25 mm, and an even more preferable number of crimps is 10 or more and 20 or less per 25 mm. Furthermore, from the viewpoint of card passability of the composite fiber and the feel and bulk recovery of the heat-bonded nonwoven fabric containing the composite fiber, the crimp rate of the above-mentioned composite fiber is preferably 5% or more and 20% or less, more preferably 6% or more and 18%, and even more preferably 6.5% or more and 16% or less.

[0056] As described above, the composite fibers of the present invention can be modified by adding various known additives to the core and sheath components, provided that the effects of the present invention are not hindered and the composite fibers do not affect fiber productivity, nonwoven fabric productivity, heat adhesion, or tactile feel. These additives may 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. Among these, when obtaining a nonwoven fabric for absorbent articles using the composite fibers of the present invention, it is preferable that the composite fibers contain an inorganic filler. This is because nonwoven fabrics for absorbent articles require not only a white appearance but also opacity to conceal the color of absorbed menstrual blood, urine, or loose stools. The amount of inorganic filler contained in the composite fibers is not particularly limited, but it is preferable that the inorganic filler be contained in an amount of 0.5% to 10% by mass per 100% by mass of the composite fibers. Including an inorganic filler within the above-mentioned range results in superior whiteness of the heat-adhesive nonwoven fabric containing the composite fibers. In addition, since the single fiber fineness of the composite fiber is less than 2.0 dtex, compared to composite fibers with a single fiber fineness of 2.0 dtex or more, the number of fibers constituting the nonwoven fabric increases for the same basis weight, so the surface of the heat-bonded nonwoven fabric tends to appear whiter. The amount of inorganic filler contained in the composite fiber is preferably 0.8% to 8% by mass of the composite fiber, more preferably 1% to 6% by mass, particularly preferably 1.3% to 5% by mass, and most preferably 1.5% to 4.5% by mass.

[0057] The inorganic filler described above is preferably an inorganic powder with high whiteness, as it makes the composite fiber appear white and enhances the opacity when the heat-bonded nonwoven fabric containing this composite fiber is used as a surface sheet for an absorbent article. Specifically, white inorganic powders such as titanium dioxide, zinc oxide, barium sulfate, calcium carbonate, magnesium oxide, silica (silicon dioxide), mica, zeolite, and talc can be incorporated into the composite fiber as an inorganic filler. The inorganic filler preferably contains at least one selected from the group consisting of titanium dioxide, zinc oxide, calcium carbonate, barium sulfate, silica, and talc, more preferably contains at least titanium dioxide, and is particularly preferably contains substantially only titanium dioxide as the inorganic filler.

[0058] The inorganic filler described above may be contained in either the sheath component or the core component that constitutes the composite fiber, or in both. However, from the viewpoint of the productivity of the composite fiber and the properties of the composite fiber and the nonwoven fabric manufactured using the composite fiber, it is preferable to contain the inorganic filler in at least the core component, and more preferable to contain the inorganic filler only in the core component. It is presumed that by containing the inorganic filler in at least the core component, the composite fiber and the nonwoven fabric containing the composite fiber tend to appear whiter, improving opacity, and also suppressing the excessive hardening of the core component containing a highly rigid polyester resin. The amount of inorganic filler contained in the core component is preferably 2% by mass or more and 10% by mass or less, more preferably 3% by mass or more and 10% by mass or less, particularly preferably 4.5% by mass or more and 8% by mass or less, and most preferably 5% by mass or more and 7.5% by mass or less, when the core component is considered as 100% by mass.

[0059] As described above, the composite fiber of the present invention promotes the crystallization of high-density polyethylene contained in the sheath component, resulting in a predetermined crystallite size. As a result, the crystallized and crystalline high-density polyethylene exists as a shell covering the surface of the composite fiber. Consequently, the overall rigidity of the composite fiber is increased, making it less likely for the fibers to entangle even in fine-denier composite fibers, and reducing the occurrence of neps. It is presumed that if the sheath component contains inorganic fillers, the presence of inorganic fillers will inhibit the crystallization of high-density polyethylene and the growth of crystalline parts. Therefore, the sheath component either does not contain inorganic fillers, or if it does, it contains only a small amount, for example, 5% by mass or less, preferably 3% by mass or less, more preferably 1% by mass or less, and particularly preferably 0.5% by mass or less, when the sheath component is considered to be 100% by mass.

[0060] (Manufacturing method) The method for producing the composite fibers of the present invention will be described below.

[0061] First, a core component is prepared containing 60% by mass or more of polyethylene terephthalate, which is a polyester resin, preferably having a number average molecular weight (Mn) of 2500 to 27000 and a weight average molecular weight (Mw) of 6000 to 80000, or an intrinsic viscosity greater than 0.55 dL / g and 0.8 dL / g or less. A sheath component is prepared containing 60% by mass or more of high-density polyethylene, which has a melt mass flow rate (MFR: measured at a temperature of 190°C, load of 2.16 kgf (21.18 N)) greater than 13 g / 10 min and 45 g / 10 min or less. The polyethylene terephthalate can preferably be used if it satisfies the preferred range of average molecular weight or the preferred range of IV value, but it is more preferable if it satisfies both the preferred range of average molecular weight and the preferred range of IV value. Next, the sheath component and core component are supplied to a composite nozzle, such as a concentric core-sheath composite nozzle, which is arranged so that in the fiber cross-section, the sheath component covers the surface of the composite fiber and the center of gravity of the core component coincides with the center of gravity of the composite fiber, creating a concentric circular structure, and melt spinning is performed. At this time, for example, the temperature at which the core component is melted and extruded (spinning temperature) is set to 280°C or higher and 380°C or lower, the temperature at which the sheath component is melted and extruded (spinning temperature) is set to 250°C or higher and 350°C or lower, and the temperature of the composite nozzle is set to 250°C or higher and 350°C or lower for melt spinning.

[0062] In the above-described concentric core-sheath type composite nozzle (hereinafter simply referred to as the nozzle), the number of holes (hereinafter referred to as the number of holes) provided in the nozzle for melt-spinning the molten core component and sheath component is not particularly limited. However, considering the effect on the draft ratio described later and the fact that melt spinning is performed under conditions where the draft ratio is high, the number of holes is preferably 300 to 5000, and preferably 450 to 3500. By satisfying the above range for the number of holes, it becomes possible to perform melt spinning under conditions where the draft ratio is high in a stable state.

[0063] In the above-described nozzle, the diameter of the hole (hereinafter referred to as the hole diameter) provided in the nozzle for melt-spinning the molten core and sheath components is not particularly limited. However, considering the effect on the draft ratio described later and the fact that melt-spinning is performed under conditions where the draft ratio is high, the hole diameter is preferably 0.2 mm or more and 0.8 mm or less, and more preferably 0.25 mm or more and 0.75 mm or less. By satisfying the above range for the hole diameter, it becomes possible to perform melt-spinning under conditions where the draft ratio is high in a stable state.

[0064] Melt spinning is performed by extruding the molten core and sheath components through the holes provided in the nozzle. At this time, the value obtained by dividing the total amount of resin extruded from the nozzle per minute by the number of holes, that is, the amount of molten core and sheath components extruded per minute per hole (hereinafter referred to as the resin discharge amount per hole), is not particularly limited, but is preferably 0.2 g / min or more and 1 g / min or less, and more preferably 0.25 g / min or more and 0.8 g / min or less. By satisfying the above range for the resin discharge amount per hole, it becomes possible to perform melt spinning under conditions where the draft ratio is high in a stable state.

[0065] Undrawn fiber tow is obtained by rapidly drawing in and cooling the molten core and sheath components extruded through the holes provided in the nozzle. At this time, the speed at which the molten core and sheath components are drawn in (hereinafter referred to as the draw speed) is not particularly limited, but is preferably 500 m / min or more and 2500 m / min or less, more preferably 600 m / min or more and 2300 m / min or less, and particularly preferably 650 m / min or more and 2000 m / min or less. By keeping the draw speed within the above range, it becomes possible to melt spin under conditions where the draft ratio is high in a stable state.

[0066] In producing the composite fibers of the present invention, melt spinning is performed using the method described above to obtain a bundle of undrawn composite fibers (undrawn fiber tow) consisting of a core component and a sheath component. In the production method of the present invention, melt spinning is performed under conditions with a high draft ratio. In the production method for obtaining the composite fibers of the present invention, the draft ratio is 600 to 1500. When the draft ratio is within the above range, strong tension is applied to the molten core component and sheath component along the length direction of the fiber during melt spinning. In particular, the strong tension applied to the sheath component (i.e., high-density polyethylene) that constitutes the outside of the undrawn fiber tow promotes the crystallization of high-density polyethylene, and in the undrawn fiber tow that has been taken up after melt spinning is completed, the high-density polyethylene undergoes crystallization and growth, and tends to become larger in crystallite size. The draft ratio is preferably 620 to 1400, more preferably 650 to 1300, and particularly preferably 660 to 1250.

[0067] In this invention, the draft ratio is calculated using the following formula 4.

[0068]

number

[0069] The undrawn fiber tow produced by the method described above preferably has a single fiber fineness of 1.8 dtex or more and 4.5 dtex or less. By satisfying the above range with the undrawn fiber tow obtained by melt spinning, and then drawing it at an appropriate drawing ratio in the drawing process described later to produce a drawn fiber tow, it is possible to stably produce a composite fiber with appropriate rigidity and elasticity, having a single fiber fineness of 0.6 dtex to less than 2.0 dtex, in which the crystallization of the core and sheath components of the undrawn fiber tow is further advanced. The single fiber fineness of the undrawn fiber tow is more preferably 2.0 dtex or more and 4.2 dtex or less, particularly preferably 2.2 dtex or more and 4.0 dtex or less, and most preferably 2.2 dtex or more and 3.8 dtex or less.

[0070] The undrawn fiber tow produced by the method described above preferably has an elongation of 100% to 400%. When the elongation of the undrawn fiber tow satisfies the above range, the crystallization of the core and sheath components of the undrawn fiber tow becomes appropriate. By drawing it at an appropriate drawing ratio in the drawing process described later, the crystallization of the core and sheath components of the undrawn fiber tow becomes further advanced, resulting in a drawn fiber tow, which allows for the stable production of composite fibers with appropriate rigidity and elasticity and a single fiber fineness of 0.6 dtex to less than 2.0 dtex. The elongation of the undrawn fiber tow is more preferably 120% to 300%, and particularly preferably 140% to 250%.

[0071] Next, the obtained undrawn fiber tow is drawn at a temperature of 70°C to 120°C with a draw ratio of 1.6 to 3.6 times. A more preferable lower limit for the draw temperature is 75°C or higher, and a particularly preferable lower limit for the draw temperature is 80°C or higher. A more preferable upper limit for the draw temperature is 110°C or lower, and a particularly preferable upper limit for the draw temperature is 100°C or lower. If the draw temperature is below 70°C, the crystallization of the core and sheath components does not proceed easily, resulting in a composite fiber with sufficient rigidity and elasticity but a fine fiber that is not easily passed through the card. If the draw temperature exceeds 120°C, the fibers tend to fuse together. A more preferable lower limit for the draw ratio is 1.8 times or higher, and a particularly preferable lower limit for the draw ratio is 2.0 times or higher. A more preferable upper limit for the draw ratio is 3.4 times or lower, and a particularly preferable upper limit for the draw ratio is 3.2 times or lower. When the draw ratio is between 1.6 and 3.6 times, the crystallization of the sheath and core components progresses, resulting in fibers with good carding properties. Furthermore, fiber breakage during draw is less likely to occur, allowing for stable production.

[0072] The stretching method is not particularly limited, and known stretching processes such as wet stretching, in which the unstretched fiber tow is stretched while being heated using a high-temperature liquid such as hot water as a medium; dry stretching, in which the fiber is stretched while being heated in a high-temperature gas or with a high-temperature metal roll; and steam stretching, in which the fiber is stretched while being heated with steam of 100°C or higher at atmospheric pressure or under pressure. Among these, wet stretching using hot water or dry stretching using a high-temperature gas or a high-temperature metal roll is preferred, and wet stretching is more preferred because the tension and heat during stretching can be easily and evenly applied to the single fibers constituting the unstretched fiber tow. The stretching process may be a so-called one-stage stretch, in which there is only one stretching step, a two-stage stretch, or a multi-stage stretch with more than two stretching steps. The composite fibers of the present invention have a small single fiber fineness of less than 2.0 dtex, melt spinning is performed with a high draft ratio, and the fineness of the resulting undrawn fiber tow is small, so it is preferable to perform the process by one-stage or two-stage drawing. Furthermore, annealing treatment may be performed before or after the above drawing treatment as needed.

[0073] Next, crimping is applied to the stretched fiber tow using a known crimping machine such as a stuffing box type crimper. However, in order to apply crimping to the stretched fiber tow that is less likely to lose its crimped shape, or in other words, that is more likely to maintain its shape, crimping is applied to the stretched fiber tow while it is sufficiently heated. The following describes the process of applying crimping to the stretched fiber tow after the stretching process is complete (the crimping process).

[0074] To apply crimp to a stretched fiber tow, the stretched fiber tow is heated in order to create a crimp that is less likely to lose its shape, or in other words, to ensure that the shape and number of crimps applied are maintained for a long period of time, resulting in high durability of the crimp shape. To perform this process, a step of heating the stretched fiber tow (hereinafter also referred to as the tow heating step) is provided immediately before the crimping step. By heating the stretched fiber tow immediately before the crimping step, and then performing the crimping step on the stretched fiber tow that has reached a predetermined temperature or higher, it becomes possible to apply a crimp that is less likely to lose its shape.

[0075] When the tow of drawn fibers is heated by the tow heating process described above, the high-density polyethylene constituting the tow of drawn fibers becomes sufficiently heated, and the thermal vibrations of the crystalline and amorphous parts of the high-density polyethylene become active. Performing the crimping process in this state causes the highly crystallized high-density polyethylene to deform under active thermal vibrations, imparting a crimped shape, and ensuring that the crystalline parts of the high-density polyethylene are sufficiently crimped. After the crimping process, the tow of drawn fibers is cooled, and the crimped shape is imparted to the high-density polyethylene and fixed by the cooling, making the crimped shape of the resulting composite fiber less prone to collapse.

[0076] In the tow heating process described above, the stretched fiber tow is subjected to appropriate tension. Specifically, it is preferable to perform the tow heating process under a tensioned state where the magnification is between 0.95 and 1.3 times. This is because performing the tow heating process under tension further promotes the crystallization of high-density polyethylene. In the tow heating process described above, the means of heating are not particularly limited and may include contact with hot water, steam, dry air, or a heating roll. Any of these methods may be used, but heating with steam is preferred because it allows for uniform and rapid heating of the stretched fiber tow. The heating temperature in the tow heating process described above is preferably between 80°C and 120°C, and more preferably between 90°C and 110°C. The heating time in the tow heating process described above is not particularly limited, but is preferably between 0.5 seconds and 10 seconds, more preferably between 1 second and 5 seconds, and even more preferably between 1 second and 3 seconds.

[0077] As described above, the drawn fiber tow is heated immediately before the crimping process. As a result, the surface temperature of the drawn fiber tow immediately before the crimping process, specifically immediately before it enters a known crimping machine such as a stuffing box type crimper, is 60°C or higher. By performing the crimping process in this state, as described above, a crimp that is less likely to lose its shape, in other words, a composite fiber with a crimp that maintains its shape for a long period of time and has high shape durability can be obtained. The surface temperature of the drawn fiber tow immediately before the crimping process is preferably 70°C or higher, more preferably 75°C or higher, and particularly preferably 80°C or higher.

[0078] In the tow heating process, crimp is imparted to the sufficiently heated drawn fiber tow. In the composite fiber and its manufacturing method of the present invention, the number of crimps is not particularly limited, but it is preferable to impart crimp so that the number of crimps is 5 or more and 28 or less. If the number of crimps is less than 5 or less, the carding performance decreases, and the initial bulk and bulk recovery performance of the nonwoven fabric tend to deteriorate. On the other hand, if the number of crimps exceeds 28 or less, the carding performance decreases due to the excessive number of crimps, and the form of the nonwoven fabric deteriorates. It is more preferable that the number of crimps imparted to the drawn fiber tow be 8 or more and 25 or less, and particularly preferable that it be 10 or more and 20 or less. The crimped shape after passing through the crimping machine is not particularly limited, but it is preferable that at least one of the crimped shapes selected from sawtooth crimping and wave-shaped crimping is produced.

[0079] Since a crimping process is performed on a heated stretched fiber tow, it is preferable that the surface temperature of the stretched fiber tow immediately after the crimping process is completed, specifically when it comes out of a known crimping machine such as a stuffing box type crimper, be 50°C or higher. If the surface temperature of the stretched fiber tow immediately after the crimping process is 50°C or higher, it can be inferred that crimping has been applied to a stretched fiber tow that was sufficiently heated. It is more preferable, and particularly preferable, that the surface temperature of the stretched fiber tow immediately after the crimping process is 60°C or higher, and especially preferable, that it is 70°C or higher. In this invention, the surface temperature of the stretched fiber tow immediately before the crimping process is the average value of the temperature measured five times using a non-contact thermometer on the surface of the stretched fiber tow immediately before it enters the crimping machine. Furthermore, the temperature of the stretched fiber tow surface immediately after the crimping process is defined as the average of five temperatures measured using a non-contact thermometer on the surface of the stretched fiber tow immediately after it exits the crimping machine.

[0080] In the method for producing the composite fibers of the present invention, the drawn fiber tow (filament) may be treated with a fiber treatment agent as needed before or after crimping. By treating with a fiber treatment agent, the composite fibers can be given antistatic properties that make them less susceptible to generating static electricity. As a result, the resulting composite fibers have excellent carding properties. Furthermore, by selecting an appropriate fiber treatment agent, it is possible to impart water affinity, i.e., hydrophilicity, or water repellency to the nonwoven fabric produced using the resulting composite fibers.

[0081] The above-mentioned fiber treatment agent is not particularly limited, and known surfactants can be used as appropriate. For example, a fiber treatment agent can be used that contains one or more surfactants selected from nonionic surfactants such as sugar ester type (also called "polyhydric alcohol ester type"), fatty acid ester type, alcohol type, alkylphenol type, polyoxyethylene / polyoxypropylene block polymer type, alkylamine type, bisphenol type, polyaromatic ring type, silicone type, fluorine type, and vegetable oil type; anionic surfactants such as sulfate type, sulfonate type, carboxylic acid type, and phosphate type; cationic surfactants such as ammonium type and benzalkonium type; and amphoteric surfactants such as betaine type and glycine type.

[0082] The fiber treatment agents described above are appropriately selected depending on the application of the composite fiber. For example, if the composite fiber of the present invention is to be used as a surface sheet for an absorbent article, a fiber treatment agent containing a hydrophilic component can be selected. If it is to be used as a nonwoven fabric constituting the gathered portion or back sheet (also called the back sheet) of an absorbent article, a water-repellent fiber treatment agent that does not mix with water can be selected. It is preferable to apply the fiber treatment agent to the drawn fiber tow before tow heating. This can improve the convergence of the filaments and prevent a rapid drop in the filament temperature if applied after tow heating. The method of applying the solution containing the fiber treatment agent (treatment liquid) to the fiber surface is not particularly limited and includes known methods such as spraying, impregnation, and roll-touching. Specifically, the drawn fiber tow may be impregnated in a treatment tank filled with an aqueous solution of the fiber treatment agent, and excess aqueous solution of the fiber treatment agent may be squeezed off with a nip roll or the like. The amount of fiber treatment agent applied is not particularly limited; for example, when applying to composite fibers, it is sufficient to apply the fiber treatment agent so that the active ingredient (i.e., the ingredient remaining on the fiber surface after the water has evaporated) is between 0.03% by mass and 3% by mass relative to the fiber mass.

[0083] In the composite fiber of the present invention, the amount of fiber treatment agent attached can be measured by a rapid extraction method using the R-II type rapid residual oil extraction device manufactured by Tokai Keiki Co., Ltd. (1) 4 g of fibers cut to a predetermined length are put through a carding machine to form a fiber web, and the mass of the obtained fiber web (W f ) Measure. (2) After filling a metal cylinder (with an inner diameter of 16 mm, a length of 130 mm, and a mortar-shaped bottom with a 1 mm hole at the very bottom) with the mass of the fiber web, 10 mL of methanol is poured in from the top. (3) The methanol, which contains the dissolved fiber treatment agent that was attached to the fiber sample, drips from the hole at the bottom of the tray and is collected in an aluminum dish (mass: W1) while heating it, allowing the methanol to evaporate. The mass of the aluminum dish (W1) is measured after the aluminum dish has been thoroughly dried in a dryer and before collecting the methanol. After the methanol has completely evaporated, the mass of the aluminum dish (W2) with the remaining fiber treatment agent is measured. (4) The amount of fiber treatment agent attached to the fiber relative to its mass is calculated using the following formula.

number

[0084] Furthermore, it is preferable to perform an annealing treatment after crimping using the crimping machine described above. The annealing treatment is preferably carried out in an atmosphere such as dry heat, moist heat, or steam heat within a temperature range of 80°C to 120°C, and more preferably within a temperature range of 90°C to 120°C. Specifically, it is preferable to simplify the process by performing an annealing treatment and a drying treatment simultaneously in a dry heat atmosphere of 90°C to 120°C on the stretched fiber tow that has been crimped using the crimping machine. When the annealing treatment is carried out at a temperature of 90°C or higher, the dry heat shrinkage rate of the resulting composite fiber does not become large, and the composite fiber exhibits a clear crimp shape, resulting in a composite fiber with excellent carding properties.

[0085] The composite fibers obtained by the above method mainly have at least one type of crimp selected from the group consisting of sawtooth crimp (also called mechanical crimp) shown in Figure 2A and wave-shaped crimp shown in Figure 2B, and the number of crimps is 5 or more per 25 mm and 28 or less per 25 mm, so a flexible and smooth nonwoven fabric can be obtained without reducing carding performance, which is preferable. Then, the fibers are cut to the desired fiber length to obtain composite fibers.

[0086] The single fiber fineness of the above-mentioned composite fiber can be adjusted as desired by adjusting the single fiber fineness and draw ratio of the undrawn fiber tow. After the annealing process described above, the drawn fiber tow is cut to obtain the above-mentioned composite fiber of a predetermined length.

[0087] (thermal adhesive nonwoven fabric) Next, as an example of a nonwoven fabric containing the composite fibers of the present invention, a heat-bonded nonwoven fabric will be described along with its manufacturing method. The heat-bonded nonwoven fabric of the present invention contains 25% by mass or more of the above-mentioned composite fibers, and at least some of the above-mentioned composite fibers are bonded together by a sheath component. The above-mentioned heat-bonded nonwoven fabric can be obtained by preparing a fiber web containing 25% by mass or more of the above-mentioned composite fibers, heat-bonding the obtained fiber web, and integrating the fibers together. When other fibers are used, for example, natural fibers, regenerated fibers, refined cellulose fibers, semi-synthetic fibers, and synthetic fibers can be used as such other fibers. Examples of the above-mentioned natural fibers include cotton, silk, wool, hemp, and pulp. Examples of the above-mentioned regenerated fibers include rayon and cupro. Examples of the above-mentioned refined cellulose fibers include Tencel and lyocell. Examples of the above-mentioned semi-synthetic fibers include acetate and triacetate. Examples of the above-mentioned synthetic fibers include acrylic fibers, polyester fibers, polyamide fibers, polyolefin fibers, and polyurethane fibers. Other fibers can be selected from the above-mentioned fibers, one or more of them as appropriate depending on the application. The other fibers may be used in combination with the composite fibers of the present invention, or a fiber web made of the composite fibers of the present invention may be used by laminating a fiber web made of the other fibers.

[0088] Examples of fiber webs used in manufacturing the above-mentioned heat-bonded nonwoven fabric include parallel webs, semi-random webs, random webs, cross webs, crisscross webs, and other card webs, as well as airlaid webs. Nonwoven fabrics used in absorbent articles, especially surface sheets of absorbent articles, are required to be bulky, flexible, and have a certain amount of voids between the fibers, so the fiber web is preferably a card web. The above-mentioned heat-bonded nonwoven fabric may be made by laminating two or more different types of fiber webs from the above-mentioned fiber webs.

[0089] It is preferable to obtain a nonwoven fabric in the form of a heat-bonded nonwoven fabric by heat-treating the above fiber web to heat-bond the fibers together with sheath components. This is because the heat-bonded nonwoven fabric exhibits the effects brought about by the composite fibers of the present invention, such as the smooth texture of the nonwoven fabric surface, in a remarkable manner. In order to entangle the fibers, the fiber web may be subjected to entanglement treatments such as needle punching or water jet entanglement treatment before and / or after heat treatment, as necessary.

[0090] To obtain a heat-bondable nonwoven fabric, the fiber web is subjected to heat treatment by known heat treatment methods. Preferably, a heat treatment machine is used that does not apply much pressure, such as air pressure, to the fiber web, such as a hot air penetration heat treatment machine, a hot air blowing heat treatment machine, or an infrared heat treatment machine. The heat treatment conditions, such as the heat treatment temperature, are selected and implemented under conditions such that the sheath component is sufficiently melted and / or softened so that the fibers are joined at contact points or intersections, while the crimp is not crushed. For example, the heat treatment temperature is preferably in the range of Tm or higher and (Tm + 40°C) or lower, where Tm is the melting point of the high-density polyethylene contained in the sheath component (if multiple high-density polyethylenes are contained in the sheath component, the melting point of the high-density polyethylene with the highest melting point).

[0091] The above heat-bonded nonwoven fabric is a nonwoven fabric with a good surface texture. The surface texture of the heat-bonded nonwoven fabric can be evaluated by sensory evaluation. Furthermore, the surface texture of the heat-bonded nonwoven fabric can be measured and evaluated based on the KES (Kawabata Evaluation System) method, which is one of the methods for measuring and objectively evaluating the texture of fabrics. Specifically, the average coefficient of friction (hereinafter also referred to as MIU), the variation in the average coefficient of friction (sometimes called the average deviation of the coefficient of friction μ, hereinafter also referred to as MMD), and the standard mean deviation of surface roughness (hereinafter also referred to as SMD) are measured as characteristic values ​​of surface friction.

[0092] MIU represents the slipperiness (or resistance) of the surface, with a higher value indicating greater slipperiness. MMD represents the variability of friction, with a higher value indicating a rougher surface. SMD represents the degree of surface irregularity of the nonwoven fabric, with a higher measured SMD value indicating greater surface irregularity and a lower value indicating less surface irregularity. The surface of the heat-bonded nonwoven fabric of the present invention tends to have a relatively low MIU, and MMD and SMD tend to be particularly low compared to conventional nonwoven fabrics. Such nonwoven fabrics not only have a low frictional sensation when touched by hands or skin, but also exhibit small fluctuations in the coefficient of friction; that is, the coefficient of friction is low in all parts of the nonwoven fabric surface, giving no feeling of snagging on fingers or skin, thus providing a light, slippery feel even in contact with the skin. The instruments used to measure these surface friction characteristics are not particularly limited, as long as they are capable of measuring surface friction based on the KES method. The characteristic values ​​of surface friction can be measured using, for example, a friction tester ("KES-SE" and "KES-SESRU," both manufactured by Kato Tech Co., Ltd.) or an automated surface testing machine ("KES-FB4-AUTO-A," manufactured by Kato Tech Co., Ltd.).

[0093] The surface properties of the heat-bonded nonwoven fabric described above, specifically the surface friction of the heat-bonded nonwoven fabric, are measured on the surface opposite to the surface to which hot air was blown during the manufacturing process. This surface is the surface that was in contact with the conveying support (for example, the conveyor net used to introduce and transport the fiber web into the heat treatment machine) used to heat-treat the fiber web with hot air to create the heat-bonded nonwoven fabric. The surface that was in contact with the conveying support tends to be smoother than the surface to which hot air was blown, resulting in a smoother feel. Therefore, when this surface is used as the surface that directly contacts the wearer's skin (skin-contact surface) in the surface sheet of an absorbent article, the feel is smoother than when the hot air-blown surface is applied to the skin, improving the usability of the absorbent article. When measuring the surface friction of a heat-bonded nonwoven fabric, if it is unclear which side was the side to which hot air was blown during heat treatment, or the side that was placed on the transport support during heat treatment, the surface friction should be measured, and the side with the smaller MMD value should be used as the measurement surface.

[0094] The heat-bonded nonwoven fabric of the present invention has a smooth and soft feel. Among the surface friction characteristic values ​​based on the KES method described above, MMD (Magnetic Mold) affects the smoothness when the nonwoven fabric is touched. The nonwoven fabric containing composite fibers of the present invention not only has a small MMD but also a relatively small mean coefficient of friction (MIU), so as described above, its surface is slippery and gives a light feel when it comes into contact with the skin.

[0095] Furthermore, depending on the composite fiber, when the surface of the nonwoven fabric containing that composite fiber is evaluated using the KES method, some exhibit a high MIU and a low MMD value. Such nonwoven fabrics transmit relatively large friction to the fingers and skin without significant fluctuations, resulting in a "moist" or "slippery" feel that provides a sense of friction within a smooth texture. Such nonwoven fabrics are also desirable for use in absorbent materials, and therefore, it is considered desirable for nonwoven fabrics used in absorbent materials to have the smallest possible variation in the average coefficient of friction (MMD).

[0096] The above heat-bonded nonwoven fabric preferably has a mean coefficient of friction variation (MMD) of 0.1 or less, more preferably 0.05 or less, even more preferably 0.01 or less, and particularly preferably 0.008 or less. The mean coefficient of friction variation (MMD) of 0.008 or less, measured when the nonwoven fabric is completely dry, does not have a particularly limited lower limit, and is preferable as it approaches 0, but may be 0.003 or more, or 0.005 or more.

[0097] The above heat-bonded nonwoven fabric preferably has a standard mean deviation (SMD) of surface roughness on the surface of the nonwoven fabric measured when the nonwoven fabric is sufficiently dry, which is 4 or less, more preferably 3.5 or less, even more preferably 3.2 or less, and particularly preferably 3 or less. The lower limit of the standard mean deviation (SMD) of surface roughness on the surface of the nonwoven fabric measured when the nonwoven fabric is dry is not particularly limited, and is preferably close to 0, but may be 0.5 or more, 1 or more, or 1.5 or more.

[0098] The above nonwoven fabric preferably has an average coefficient of friction (MIU) of 0.25 or less, more preferably 0.24 or less, and even more preferably 0.23 or less, measured when the nonwoven fabric is thoroughly dried. The lower limit of the average coefficient of friction (MIU) of the nonwoven fabric measured when the nonwoven fabric is dried is not particularly limited, and it is preferable that it is as close to 0 as possible, but it may be 0.05 or more, or 0.1 or more.

[0099] The heat-bonded nonwoven fabric of the present invention (i.e., a heat-bonded nonwoven fabric containing 25% by mass or more of the composite fibers of the present invention) is soft throughout and provides a smooth feel when the surface of the nonwoven fabric is touched. The above heat-bonded nonwoven fabric can be preferably used as a surface sheet for various absorbent articles such as sanitary napkins, infant diapers, adult diapers, animal diapers including mammals, panty liners, and incontinence liners. The above nonwoven fabric may also be used as a back sheet for infant diapers and adult diapers, which are touched from the outside. Furthermore, the above nonwoven fabric can also be used as a sheet constituting various absorbent articles (hereinafter also referred to as an absorbent article sheet), for example, a second sheet placed directly beneath the surface sheet, a liquid diffusion sheet, and a sheet generally called a core wrap sheet that encloses the absorbent material. When using the heat-bonded nonwoven fabric of the present invention as an absorbent article sheet, it is particularly preferable to contain 20% by mass or more of the composite fibers of the present invention on the skin-contacting surface. Furthermore, in order to take advantage of the overall flexibility and opacity of the nonwoven fabric, the heat-bonded nonwoven fabric of the present invention can also be preferably used in absorbent articles, for example, in the so-called second sheet located directly beneath the surface sheet, on the absorbent side rather than the surface sheet that comes into direct contact with the skin.

[0100] The basis weight of the heat-bondable nonwoven fabric of the present invention is not particularly limited to 5 g / m². 2 More than 70g / m 2 Preferably, it is 8 g / m 2 More than 60g / m 2 More preferably, the following is true: 10 g / m 2 More than 55g / m 2 It is even more preferable that the following conditions apply: 15 g / m 2 More than 50g / m 2 The following is particularly preferable. However, the basis weight of the heat-bonded nonwoven fabric of the present invention may be outside these ranges depending on the application of the heat-bonded nonwoven fabric. Furthermore, when the above heat-bonded nonwoven fabric is used for various applications, such as the surface sheet of various disposable diapers and sanitary napkins, the back sheet of various disposable diapers, or the second sheet placed directly beneath the surface sheet of absorbent articles, the basis weight is appropriately selected according to the application.

[0101] When the above heat-bonded nonwoven fabric is used as a surface sheet for an absorbent article, the heat-bonded nonwoven fabric contains 25% by mass or more of the above-mentioned composite fibers. Preferably, the heat-bonded nonwoven fabric contains 30% by mass or more of the above-mentioned composite fibers, and more preferably 40% by mass or more. This is because, when the proportion of the above-mentioned composite fibers is within the above range in the heat-bonded nonwoven fabric, it is easy to obtain a nonwoven fabric that has excellent surface feel and feels soft and smooth to the touch. In the heat-bonded nonwoven fabric, the content of the above-mentioned composite fibers may be 100% by mass, 90% by mass or less, or 80% by mass or less.

[0102] The above heat-bonded nonwoven fabric is preferably, more preferably, 20 N / 5 cm or more, even more preferably 25 N / 5 cm or more, and particularly preferably 28 N / 5 cm or more, from the viewpoint of the strength required when used as a nonwoven fabric constituting an absorbent article (e.g., a surface sheet or back sheet), the prevention of surface fuzzing due to friction during use, and the soft feel when touched.

[0103] By using a heat-bonded nonwoven fabric containing 25% by mass or more of the composite fibers of the present invention, a heat-bonded nonwoven fabric with a smooth texture and soft feel can be obtained. Such a heat-bonded nonwoven fabric can be used for various sheets that make up absorbent articles, such as surface sheets, second sheets (also called liquid diffusion sheets), core wrap sheets that enclose absorbents, and back sheets that form the outer surface of infant diapers and adult diapers. When obtaining a surface sheet for various absorbent articles using the composite fibers of the present invention, a laminated nonwoven fabric can be made in which the fiber layer containing the composite fibers is the surface that comes into contact with the wearer's skin, and another fiber layer is provided below that layer, resulting in a surface sheet with excellent feel and liquid absorption performance. The surface sheet for absorbent articles containing the composite fibers of the present invention will be described in detail below.

[0104] (Surface sheet for absorbent articles) The present inventors have diligently studied how to improve the tactile feel, bulkiness, and liquid absorption properties of surface sheets for absorbent articles, and have found that in a surface sheet for absorbent articles comprising a first fiber layer that comes into contact with the skin and a second fiber layer adjacent to the first fiber layer, the first fiber layer is a fiber layer containing 50% by mass or more of a first core-sheath type composite fiber, and the second fiber layer is a fiber layer containing 50% by mass or more of a second core-sheath type composite fiber, and the first core-sheath type composite fiber is the composite fiber of the present invention described above, having a single fiber fineness of 0.6 dtex or more and less than 2.0 dtex, and the second core-sheath We found that by using a composite fiber with a core component containing polyester resin and a sheath component containing a thermoplastic resin having a melting point 50°C or more lower than the melting point of the polyester resin, and a single fiber fineness of 2.2 dtex to 7 dtex, and by heat-bonding at least a portion of the first core-sheath composite fiber and the second core-sheath composite fiber using the sheath components of the first and second core-sheath composite fibers, the surface sheet for absorbent articles has a smooth feel and good liquid absorption characteristics such as run-off and liquid absorption rate.

[0105] As described above, in the above-mentioned surface sheet for absorbent articles, the fineness of the first core-sheath type composite fiber constituting the first fiber layer that comes into contact with the skin and the fineness of the second core-sheath type composite fiber constituting the second fiber layer adjacent to the first fiber layer are set to a specific range, and the fineness of the first core-sheath type composite fiber is made smaller than that of the second core-sheath type composite fiber. In addition, preferably, when comparing the fiber treatment agent attached to the surface of the first core-sheath type composite fiber with the fiber treatment agent attached to the surface of the second core-sheath type composite fiber, it has been found that by using a fiber treatment agent with low hydrophilicity attached to the surface of the first core-sheath type composite fiber and a fiber treatment agent with high hydrophilicity attached to the surface of the second core-sheath type composite fiber, the surface sheet for absorbent articles has a smoother feel and improved liquid absorption characteristics such as run-off and liquid absorption rate.

[0106] The absorbent surface sheet of the present invention includes a first fiber layer that comes into contact with the skin and a second fiber layer adjacent to the first fiber layer. Figure 3 is a schematic cross-sectional view of an absorbent surface sheet of one embodiment of the present invention. As shown in Figure 3, the absorbent surface sheet 30 is composed of a first fiber layer 31 and a second fiber layer 32 adjacent to the first fiber layer 31.

[0107] (First fiber layer) The first fiber layer is a fiber layer containing 50% by mass or more of the first core-sheath composite fiber, and the composite fiber of the present invention is used as the first core-sheath composite fiber. The composite fiber of the present invention has been specifically described above, and the content regarding the composite fiber of the present invention can be directly applied by simply replacing "composite fiber" with "first core-sheath composite fiber," and a specific explanation of the first core-sheath composite fiber will be omitted.

[0108] From the viewpoint of excellent tactile properties and liquid absorption characteristics, the first fiber layer preferably contains 60% by mass or more of the first core-sheath type composite fiber, more preferably 70% by mass or more of the first core-sheath type composite fiber, even more preferably 80% by mass or more of the first core-sheath type composite fiber, particularly preferably 90% by mass or more of the first core-sheath type composite fiber, and most preferably consists of 100% by mass of the first core-sheath type composite fiber. When other fibers are included in the first fiber layer in addition to the first core-sheath type composite fiber, other fibers such as natural fibers, regenerated fibers, and synthetic fibers can be used. Examples of natural fibers include cotton, silk, wool, hemp, and pulp. Examples of regenerated fibers include rayon and cupro. Examples of synthetic fibers include acrylic fibers, polyester fibers, polyamide fibers, polyolefin fibers, and polyurethane fibers. As other fibers, one or more types of fibers can be appropriately selected from the above-mentioned fibers depending on the application.

[0109] (Second fiber layer) The second fiber layer is a fiber layer containing 50% by mass or more of a second core-sheath type composite fiber, the core component of which contains polyester resin and the sheath component of which contains a thermoplastic resin having a melting point 50°C or more lower than the melting point of the polyester resin. From the viewpoint of excellent liquid absorption properties, the second fiber layer preferably contains 60% by mass or more of the second core-sheath type composite fiber, more preferably 70% by mass or more, even more preferably 80% by mass or more, particularly preferably 90% by mass or more, and most preferably consists of 100% by mass of the second core-sheath type composite fiber. When other fibers are to be included in the second fiber layer in addition to the second core-sheath type composite fiber, the fibers exemplified when other fibers are to be included in the first fiber layer in addition to the first core-sheath type composite fiber can also be included in the second fiber layer. The other fibers can be one or more types of known fibers, including the fibers described above, which can be appropriately selected according to the application.

[0110] The fineness of the second core-sheath type composite fiber is between 2.2 dtex and 7 dtex. By making the fineness of the second core-sheath type composite fiber constituting the second fiber layer greater than that of the first core-sheath type composite fiber constituting the first fiber layer, the surface sheet for absorbent articles will have appropriate cushioning properties, a smooth feel, and good liquid absorption characteristics. If the fineness of the second core-sheath type composite fiber is less than 2.2 dtex, the number of constituent fibers in the second fiber layer will be relatively large due to the low fineness of the second core-sheath type composite fiber, resulting in a dense structure in the second fiber layer that makes it difficult to absorb excretions such as menstrual blood and urine. On the other hand, if the fineness of the second core-sheath type composite fiber exceeds 7 dtex, the number of constituent fibers in the second fiber layer will be relatively small due to the high fineness of the second core-sheath type composite fiber, resulting in a sparse structure in the second fiber layer that makes it difficult to absorb excretions such as menstrual blood and urine. The fineness of the second core-sheath type composite fiber is more preferably 2.5 dtex or more and 6 dtex or less, even more preferably 3 dtex or more and 5.6 dtex or less, and most preferably 3.6 dtex or more and 4.8 dtex or less.

[0111] In the second core-sheath type composite fiber, the core component preferably contains 50% by mass or more of polyester resin, more preferably 60% by mass or more, even more preferably 70% by mass or more, and particularly preferably 80% by mass or more. The core component containing 50% by mass or more of polyester resin improves the carding properties of the second core-sheath type composite fiber. The polyester resin is not particularly limited, but examples include polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polylactic acid, copolymers of these with acid components such as isophthalic acid, succinic acid, and adipic acid, glycol components such as 1,4-butanediol and 1,6-hexanediol, polytetramethylene glycol, polyoxymethylene glycol, and elastomers thereof. From the viewpoint of bulkiness, cushioning, and liquid absorption rate of the surface sheet for absorbent articles, the polyester resin is preferably polyethylene terephthalate (hereinafter also referred to as PET).

[0112] In the second core-sheath type composite fiber, the thermoplastic resin having a melting point 50°C or more lower than the polyester resin contained in the core component is not particularly limited, but it is preferable to use high-density polyethylene. The inclusion of high-density polyethylene in the sheath component of the second core-sheath type composite fiber makes the second core-sheath type composite fiber more rigid, and improves its carding and crimping properties. The high-density polyethylene content in the sheath component of the second core-sheath type composite fiber is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and particularly preferably 100% by mass. As the high-density polyethylene, the high-density polyethylene that can be used in the sheath component of the first core-sheath type composite fiber described above can be used. It is preferable that the high-density polyethylene contained in the sheath component of the first core-sheath type composite fiber and the high-density polyethylene contained in the sheath component of the second core-sheath type composite fiber have approximately the same melting point. This facilitates heat bonding between the first core-sheath type composite fiber and the second core-sheath type composite fiber using the sheath components of the first and second core-sheath type composite fibers.

[0113] The second core-sheath type composite fiber can be any composite fiber in which the core component contains a polyester resin and the sheath component contains a thermoplastic resin having a melting point 50°C or more lower than the melting point of the polyester resin. In the second core-sheath type composite fiber, the arrangement of the core component and sheath component in its cross-section can be arbitrary. That is, the second core-sheath type composite fiber may be a core-sheath type composite fiber with a concentric structure in which the core component and sheath component are arranged concentrically as shown in Figure 1, or an eccentric core-sheath type composite fiber in which the center of gravity of the core component is offset from the center of the fiber, or a side-by-side type (parallel type) composite fiber in which the core component and sheath component are arranged side by side. From the viewpoint of the texture, bulkiness, and cushioning properties of the surface sheet for absorbent articles obtained, the cross-section of the second core-sheath type composite fiber is preferably a core-sheath type composite fiber with a concentric structure in which the core component and sheath component are arranged concentrically, or an eccentric core-sheath type composite fiber (excluding the side-by-side type), and more preferably a core-sheath type composite fiber with a concentric structure.

[0114] In the second core-sheath type composite fiber, the shape of the fiber cross-section of the core component may be any shape other than circular, such as elliptical, Y-shaped, X-shaped, polygonal, or star-shaped. The shape of the fiber cross-section of the composite fiber may be any shape other than circular, such as elliptical, Y-shaped, X-shaped, polygonal, or star-shaped, or it may be hollow.

[0115] The fiber length of the second core-sheath type composite fiber is not particularly limited, and may be, for example, 76 mm or less. From the viewpoint of processability when manufacturing surface sheets for absorbent articles, the fiber length is preferably 35 mm or more and 65 mm or less, more preferably 40 mm or more and 60 mm or less, and even more preferably 44 mm or more and 55 mm or less.

[0116] In the absorbent surface sheet of the present invention, at least a portion of the first core-sheath type composite fiber and the second core-sheath type composite fiber are heat-bonded by the sheath component of the first core-sheath type composite fiber and the second core-sheath type composite fiber. A first fiber web containing 50% by mass or more of the first core-sheath type composite fiber and a second fiber web containing 50% by mass or more of the second core-sheath type composite fiber are laminated, and the laminated fiber web is heat-treated to heat-bond at least a portion of the first core-sheath type composite fiber and the second core-sheath type composite fiber by the sheath component.

[0117] Examples of the fiber webs mentioned above include parallel webs, semi-random webs, random webs, cross webs, and crisscross webs, as well as card webs and airlaid webs. Since surface sheets for absorbent articles require bulkiness, flexibility, and a certain amount of void space between fibers, the fiber web is preferably a card web. The first fiber layer and the second fiber layer may be made of different types of fiber webs.

[0118] By heat-treating the fiber web of the above-described laminated structure, the first core-sheath type composite fiber and the second core-sheath type composite fiber are heat-bonded together by the sheath components of the first core-sheath type composite fiber and the second core-sheath type composite fiber, thereby obtaining the surface sheet for absorbent articles of the present invention in the form of a heat-bonded nonwoven fabric including a first fiber layer (first fiber web) and a second fiber layer (second fiber web). This is because the form of a heat-bonded nonwoven fabric exhibits remarkable effects such as flexibility in the thickness direction, bulk recovery, and a smooth texture on the nonwoven fabric surface. In order to entangle the fibers, the fiber web may be subjected to entanglement treatments such as needle punching or water flow entanglement treatment before and / or after heat treatment, as necessary. The first fiber web and the second fiber web may be entangled with each other near the boundary.

[0119] The above heat treatment can be performed using a known heat treatment machine. For example, heat treatment machines that do not apply much pressure, such as wind pressure, to the fiber web are preferably used, such as hot air penetration heat treatment machines, hot air blowing heat treatment machines, and infrared heat treatment machines. The heat treatment conditions, such as the heat treatment temperature, are such that the sheath component is sufficiently melted and / or softened so that the fibers are joined together at contact points or intersections. For example, the heat treatment temperature is preferably in the range of Tm or higher and (Tm + 40°C) or lower, where Tm is the melting point of the high-density polyethylene contained in the sheath component before spinning (if multiple high-density polyethylenes are contained in the sheath component, the melting point of the high-density polyethylene with the highest melting point). A more preferable range for the heat treatment temperature is (Tm + 5°C) or higher and (Tm + 30°C) or lower.

[0120] In the above-mentioned surface sheet for absorbent articles, from the viewpoint of liquid absorption characteristics, it is preferable that the basis weight of the first fiber layer is lower than that of the second fiber layer. From the viewpoint of minimizing liquid backflow and having excellent resistance to wet back, the basis weight of the first fiber layer is 4 g / m². 2 More than 18g / m 2 Preferably, it is 5 g / m 2 More than 15g / m 2 It is more preferable that the following is the case: 6 g / m 2 More than 12g / m 2 The following is particularly preferred: 8 g / m 2 More than 12g / m2 The following is most preferable. Furthermore, from the viewpoint of minimizing liquid return and having excellent wet-back resistance, the basis weight of the second fiber layer should be 8 g / m². 2 More than 45g / m 2 Preferably, it is 8 g / m 2 More than 35g / m 2 More preferably, the following is true: 10 g / m 2 More than 30g / m 2 The following is particularly preferable: 10 g / m 2 More than 25g / m 2 The following is most preferable: The overall basis weight of the surface sheet for absorbent articles is 12 g / m². 2 More than 60g / m 2 Preferably, it is 15 g / m 2 More than 50g / m 2 More preferably, the following is true: 15 g / m 2 More than 40g / m 2 It is particularly preferable that the following conditions are met: 18 g / m 2 More than 30g / m 2 The following is most preferable:

[0121] In the above-mentioned surface sheet for absorbent articles (laminated nonwoven fabric), from the viewpoint of superior tactile feel, the variation in average friction coefficient (MMD) of the surface of the first fiber layer, measured when the nonwoven fabric is sufficiently dried, is preferably 0.1 or less, more preferably 0.05 or less, even more preferably 0.01 or less, and particularly preferably 0.008 or less. The lower limit of the variation in average friction coefficient (MMD) measured when the nonwoven fabric is dried is not particularly limited, and the closer it is to 0, the better, but it may be 0.003 or more, or 0.005 or more.

[0122] In the above-mentioned surface sheet for absorbent articles (laminated nonwoven fabric), the first fiber layer that comes into contact with the skin preferably has a standard mean deviation (SMD) of surface roughness on the surface of the first fiber layer, measured when the nonwoven fabric is sufficiently dried, of 4 or less, more preferably 3.5 or less, even more preferably 3.2 or less, and particularly preferably 3 or less, from the viewpoint of having excellent tactile feel. The lower limit of the standard mean deviation (SMD) of surface roughness on the surface of the nonwoven fabric, measured when the nonwoven fabric is dried, is not particularly limited, and is preferably close to 0, but may be 0.5 or more, 1 or more, or 1.5 or more.

[0123] In the above-mentioned surface sheet for absorbent articles (laminated nonwoven fabric), the average coefficient of friction (MIU) of the surface of the first fiber layer that comes into contact with the skin is preferably 0.25 or less, more preferably 0.24 or less, and even more preferably 0.23 or less, as measured when the nonwoven fabric is thoroughly dried. The lower limit of the average coefficient of friction (MIU) of the surface of the nonwoven fabric measured when the nonwoven fabric is dried is not particularly limited, and it is preferable that it is as close to 0 as possible, but it may be 0.05 or more, or 0.1 or more.

[0124] The above-mentioned surface sheet for absorbent articles includes a first fiber layer that comes into contact with the skin and a second fiber layer adjacent to the first fiber layer. The first fiber layer that comes into contact with the skin contains first core-sheath type composite fibers with a fineness of less than 2.0 dtex. Fiber aggregates containing fine fibers have narrow spacing between fibers, and when liquid is absorbed into the voids between the fibers, they tend to retain the absorbed liquid. If the fiber layer that comes into contact with the skin in an absorbent article surface sheet is composed of such fibers, after absorbing urine, menstrual blood, loose stool, etc., the liquid may remain in the fiber layer, causing residue and potentially reducing the user experience.

[0125] In the absorbent sheet described above, it is preferable that the fiber treatment agent attached to the surface of the second core-sheath type composite fiber contained in the second fiber layer adjacent to the first fiber layer is a fiber treatment agent with a strong hydrophilic tendency, thereby improving the transfer of liquid to the second fiber layer. In addition, it is even more preferable that the fiber treatment agent attached to the surface of the first core-sheath type composite fiber contained in the first fiber layer is a fiber treatment agent with a weak hydrophilic tendency. By using a fiber treatment agent with moderately weak hydrophilicity attached to the surface of the first core-sheath type composite fiber, the first fiber layer quickly absorbs urine and menstrual blood discharged on the surface, but does not retain liquid between the fibers, and instead transfers the absorbed liquid to the second fiber layer, which has a stronger hydrophilicity. As a result, not only is the liquid absorption excellent, but the amount of liquid that returns can also be reduced.

[0126] Therefore, in the absorbent surface sheet of the present invention, the hydrophilicity differs between the first fiber layer and the second fiber layer, and it is preferable that the second fiber layer has stronger hydrophilicity than the first fiber layer. There are many methods for measuring the strength of hydrophilicity on the surface of an absorbent surface sheet. One method involves dropping tiny water droplets onto the fibers constituting the first and second fiber layers of the absorbent surface sheet, measuring the contact angle, and determining the strength of hydrophilicity based on the magnitude of the contact angle. However, the strength of hydrophilicity can also be measured by performing a run-off test on the surfaces of the first and second fiber layers. Details of the run-off test will be described later, but it is a method in which, after smoothing both surfaces by a predetermined method, physiological saline solution is dropped onto a nonwoven fabric tilted at 45 degrees, and the distance over which the dropped water droplets flow down until they are all absorbed into the sheet is measured, and the strength of hydrophilicity is evaluated based on that length. In the absorbent surface sheet of the present invention, the run-off (R2) measured on the surface of the second fiber layer is preferably 120 mm or less, more preferably 100 mm or less, particularly preferably 80 mm or less, and most preferably 75 mm or less. When the run-off (R2) of the second fiber layer is 120 mm or less, the hydrophilicity of the second fiber layer becomes relatively strong, and the effect of drawing liquid from the first fiber layer becomes stronger. Furthermore, it is preferable that the difference (R1-R2) between the run-off value (R1) measured on the surface of the first fiber layer and the run-off value (R2) measured on the surface of the second fiber layer is greater than 3 mm. When the difference (R1-R2) between the run-off value (R1) measured on the surface of the first fiber layer and the run-off value (R2) measured on the surface of the second fiber layer is greater than 3 mm, a difference in hydrophilicity occurs between the first and second fiber layers. As a result, urine or menstrual blood discharged onto the surface of the absorbent article's surface sheet on the first fiber layer side is absorbed into the interior of the first fiber layer and simultaneously begins to migrate to the second fiber layer. This reduces the amount of liquid contained in the first fiber layer that is in contact with the wearer's skin, improving the wearer's comfort of the absorbent article. The difference (R1-R2) between the run-off value (R1) measured on the surface of the first fiber layer and the run-off value (R2) measured on the surface of the second fiber layer is more preferably 4 mm or more, particularly preferably 5 mm or more, and most preferably 6 mm or more.

[0127] Furthermore, while the run-off value (R1) measured on the surface of the first fiber layer is not particularly limited, considering the liquid absorption performance of the first fiber itself (i.e., the speed at which it absorbs liquids such as urine and menstrual blood discharged onto the surface of the first fiber layer into the fiber layer) and the difference in hydrophilicity between the first fiber layer and the second fiber layer necessary for quickly transferring the liquids such as urine and menstrual blood absorbed into the first fiber layer to the second fiber layer, the run-off value (R1) measured on the surface of the first fiber layer is preferably 20 mm or more and 150 mm or less, more preferably 25 mm or more and 140 mm or less, and particularly preferably 30 mm or more and 130 mm or less.

[0128] In the above-described surface sheet for absorbent articles, the first fiber layer comes into contact with the skin of the wearer of the absorbent article. The first fiber layer, which includes the first core-sheath type composite fiber, comes into contact with the skin, providing a comfortable feeling to the user of the absorbent article. The above-described surface sheet for absorbent articles can be preferably used as a surface sheet for various absorbent articles such as sanitary napkins, infant diapers, adult diapers, animal diapers including mammals, panty liners, and incontinence liners.

[0129] The absorbent article of the present invention is not particularly limited, as long as it includes the aforementioned surface sheet for absorbent articles. Examples include sanitary napkins, infant diapers, adult diapers, diapers for animals including mammals, panty liners, incontinence liners, and the like. [Examples]

[0130] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0131] The measurement and evaluation methods used in this embodiment are as follows.

[0132] (Melting point of high-density polyethylene) The melting point of high-density polyethylene was determined by the melting peak temperature measured in accordance with JIS K 7121 (1987).

[0133] (Melting mass flow rate of high-density polyethylene (MFR190)) The melt mass flow rate (MFR190) of high-density polyethylene was measured according to JIS K 7210-1 (2014) under measurement conditions of a measurement temperature of 190°C and a load of 2.16 kg (21.82 N).

[0134] (Intrinsic viscosity of polyester resin) The intrinsic viscosity (intrinsic viscosity) of the polyester resin was measured in accordance with JIS K 7367-5 (2000). Specifically, 1 g of polyethylene terephthalate was dissolved in 100 mL of a mixed solvent in which phenol and 1,1,2,2-tetrachloroethane were in a mass ratio (phenol / 1,1,2,2-tetrachloroethane) of 6 / 4, and the viscosity was measured at 30°C using an Ubbelohde viscometer.

[0135] (Measurement of molecular weight distribution of polyester resin) The number-average molecular weight (Mn), weight-average molecular weight (Mw), z-average molecular weight (Mz), and Q-value (Mw / Mn), which is the ratio of Mw to Mn, of polyester resins were measured by gel permeation chromatography (GPC). A gel permeation chromatograph equipped with a differential refractive index detector (RI) was used for the measurements.

[0136] For the measurement, spun composite fibers were prepared as samples. 50 mg of the composite fibers were freeze-dried using liquid nitrogen, and the sample was collected using a 0.45 μm membrane filter and thoroughly dried. Next, 3 mg of the dried sample was weighed, and 2.5 mL of the measurement solvent (hexafluoroisopropanol with sodium trifluoroacetate added to a concentration of 5 mM: HFIP) was added to this sample and stirred at room temperature. At this time, the sheath component of the composite fibers (high-density polyethylene) and the added inorganic filler, which do not dissolve in hexafluoroisopropanol, were generated as insoluble matter. After thoroughly stirring to dissolve the polyester resin, the solution was filtered using a 0.45 μm membrane filter to obtain the measurement sample solution. The obtained measurement sample solution was injected into a gel permeation chromatograph at a flow rate of 0.2 mL / min and an injection volume of 0.02 mL / min, and the number-average molecular weight (Mn), weight-average molecular weight (Mw), and z-average molecular weight (Mz) were measured. For the measurements, one Shodex (Shodex is a registered trademark) HFIP-G column and two Shodex (Shodex is a registered trademark) HFIP-606M columns manufactured by Showa Denko K.K. were used, and the molecular weight was calibrated using monodisperse polymethyl methacrylate as the standard sample.

[0137] (Crystallite size of high-density polyethylene) The crystallite size of high-density polyethylene contained in the sheath component of the composite fiber was calculated using Scherrer's equation (Equation 2) from the diffraction peaks obtained by wide-angle X-ray diffraction using the following method.

[0138] The composite fibers were cut into 2.5 cm lengths. 12.5 mg of the cut sample was weighed, and the ends were tied together with enameled wire to form the sample. The fiber bundle, which was the sample, was fixed in a holder perpendicular to the direction of X-ray incidence, and wide-angle X-ray diffraction was performed. The measurement conditions were as follows:

[0139] X-ray diffractometer: RIGAK Corporation, Smart Lab (registered trademark) for polymers. X-ray source: CuKα rays (using a Ni filter) Output: 40kV 50mA Slit type: RS1: 15mm RS2: 20mm Measurement direction: Fiber radial scanning Scanning method: Continuous scan Measurement range: 2θ = 10~40° Step: 0.05° Scan speed: 2° / min

[0140] Furthermore, the crystallite size was calculated from the full width at half maximum of the obtained X-ray diffraction peaks using Equation 2 (Scherrer's equation) below. In Equation 2, λ, β0, and K were defined as follows.

number

[0141] (Heat of fusion of high-density polyethylene (ΔH) PE-HD (Measurement) The heat of fusion (ΔH) of high-density polyethylene contained in the sheath component of composite fibers. PE-HD The transition temperature of the plastic is measured using the following procedure, and the heat of fusion (ΔH) of the endothermic peak where the melting peak temperature is in the temperature range of 125°C to 140°C (endothermic heat associated with melting is observed from approximately 120°C, the melting peak temperature is reached between 125°C and 140°C, and the endothermic heat associated with melting ends at approximately 150°C) is used as the heat of fusion (ΔH) of the high-density polyethylene contained in the composite fiber. PE-HD It was calculated by converting it to ).

[0142] First, to determine the heat of fusion of high-density polyethylene, the core-sheath ratio (volume ratio) of the composite fiber was converted to a core-sheath ratio (mass ratio) based on the density and amount of thermoplastic resin and inorganic filler constituting the core and sheath components. The ratio of high-density polyethylene to the composite fiber (mass ratio of high-density polyethylene) was then determined from the proportion of inorganic filler contained in the sheath component. Next, differential scanning calorimetry was performed on the composite fiber sample based on JIS K 7121 (1987) Method for Measuring Transition Temperature of Plastics. A differential scanning calorimeter (Seiko Instruments Inc., product name "EXSTAR6000 / DSC6200") was used for the differential scanning calorimetry. The differential scanning calorimetry showed that endothermic reactions associated with the melting of the composite fiber were observed from approximately 120°C, with a peak melting temperature between 125°C and 140°C, and the endothermic reactions associated with the melting of high-density polyethylene ended at approximately 150°C. The heat of fusion (ΔH) was measured for the endothermic peak observed in the range of approximately 120°C to 150°C. From this heat of fusion (ΔH) measured between approximately 120°C and 150°C, the heat of fusion (ΔH) of high-density polyethylene contained in the composite fiber was calculated. PE-HD The value of ) was calculated using the following formula 3.

number

[0143] (Number of crimps and crimp ratio) Measurements were taken in accordance with JIS L 1015 (2010).

[0144] (Single fiber strength and elongation at break) The single-fiber strength and elongation at break of composite fibers were measured according to JIS L 1015 (2010) 8.7 Tensile strength and elongation. The ratio of single-fiber strength to elongation at break (single-fiber strength / elongation at break) and the product of the single-fiber strength and the positive square root of elongation at break (single-fiber strength × √elongation at break) were calculated from the single-fiber strength and elongation at break measured according to JIS L 1015 (2010).

[0145] (Single fiber fineness and fiber length of composite fibers) The single fiber fineness of the composite fibers was measured in accordance with JIS L 1015 (2010) 8.5 (vibration method). The fiber length of the composite fibers was measured in accordance with JIS L 1015 (2010) 8.4.

[0146] (Core-sheath ratio (volume ratio) of composite fibers) First, the cross-section of the composite fiber to be measured for the core-sheath ratio was photographed using a scanning electron microscope (SEM) at a magnification of 500 to 2500 times. At this time, the magnification was adjusted so that when the photograph was printed, the diameter of one composite fiber in the print would be 5 to 8 cm. Only the image of the core-sheath composite fiber was extracted from the obtained scanning electron microscope images. The extracted image of the core-sheath composite fiber was cut along the boundary between the core component and the sheath component, separating the core component portion from the sheath component portion. This process was performed for 20 fibers, and the total mass of the core component portion extracted from the above 20 core-sheath composite fibers was measured using an electronic balance. Similarly, the total mass of the sheath component portion extracted from the above 20 core-sheath composite fibers was measured using an electronic balance. The ratio of the total mass of the core component portion to the total mass of the sheath component portion (core / sheath) was defined as the core-sheath ratio (volume ratio).

[0147] (Draft comparison) The draft ratio was calculated using formula 4 below.

number

[0148] (Card passability) The carding performance of composite fibers was evaluated based on the occurrence of neps and flies when fabricating fiber webs using a carding machine, as well as the texture of the resulting fiber webs, according to the following criteria. ++: The fibers pass easily through the carding machine, and neps and flies are hardly generated, resulting in a fiber web with good texture. +: A slight amount of slubs may occur, but it does not significantly affect the texture of the fiber web. -: Poor card penetration or excessive nep formation prevents the creation of a fibrous web.

[0149] (Tensile strength of nonwoven fabric) In accordance with JIS L 1096 (2010) 8.14.1 Method A (strip method), a constant-speed tension tensile testing machine was used to perform tensile tests on the sample pieces under the conditions of a width of 5 cm, a gripping distance of 10 cm, and a tensile speed of 30 ± 2 cm / min. The load value at break (tensile strength) was measured and defined as the tensile strength. The tensile test was performed with the longitudinal direction (MD direction) of the nonwoven fabric as the tensile direction. The evaluation results are shown as the average of the values ​​measured for three samples.

[0150] (Surface texture) The surface of the nonwoven fabric was touched and evaluated according to the following evaluation criteria. ++: Very smooth. +: Smooth. -: Has a hard or rough texture.

[0151] (KES measurement method) The texture of the heat-bonded nonwoven fabric was mechanically evaluated based on the KES (Kawabata Evaluation System) method. Specifically, when measuring the average coefficient of friction (MIU) and the variation in the average coefficient of friction (MMD), a friction tester (model number KES-SE) manufactured by Kato Tech Co., Ltd. was used, with a 10 mm square piano wire sensor as the measurement sensor, and measurements were performed under a static load of 25 gf. When measuring the average deviation of surface roughness (SMD), a roughness / friction tester (model number KES-SESRU) manufactured by Kato Tech Co., Ltd. was used, with a 0.5 mm roughness sensor as the measurement sensor, and measurements were performed under a static load of 10 gf. During measurement, the measurement unit (friction element, sensor) that measures the friction of the nonwoven fabric surface was moved at a speed of 1 mm per second so as to trace the nonwoven fabric surface in a direction parallel to the longitudinal direction (MD direction) of the nonwoven fabric. For heat-bonded nonwoven fabrics using card webs, the longitudinal direction is easily identifiable because the fibers are aligned longitudinally. However, if the longitudinal direction of the nonwoven fabric to be measured is unknown, measurements are taken in an arbitrary direction and in a direction perpendicular to that direction. The smaller of these values ​​is used as the average coefficient of friction, the variation in the average coefficient of friction, and the average deviation of the surface roughness of the nonwoven fabric. Three measurements are taken, and the average value is used as the measured value (MIU, MMD, SMD) for that sample.

[0152] (Method for measuring run-off of surface sheets for absorbent articles) For absorbent surface sheets (laminated nonwoven fabrics) composed of a first fiber layer and a second fiber layer, run-off measurements were performed using the following method, and the strength of hydrophilicity on the surfaces of the first and second fiber layers was compared. (1) Prepare the required number of heat-bonded nonwoven fabric samples (18 cm in the longitudinal direction (MD direction) and 7 cm in the transverse direction (CD direction)) to measure the run-off of the first fiber layer and the run-off of the second fiber layer. To smooth the measurement surface, heat treatment is performed in a hot air penetration type heat treatment machine set to 135°C. The sample is placed with the measurement surface in contact with the conveyor net surface of the hot air penetration type heat treatment machine and processed for 9 seconds to obtain a sample with a smooth measurement surface (surface of the first fiber layer and surface of the second fiber layer). (2) Four sheets of "Kimtowels (registered trademark)" manufactured by Nippon Paper Crecia Co., Ltd. were placed on a support stand having a cross-section of a roughly perpendicular isosceles triangle with an inclined surface that makes a 45-degree angle with the horizontal plane. The nonwoven fabric to be used as a sample was then placed on top of the support stand and fixed in place so that the longitudinal direction of the nonwoven fabric made a 45-degree angle with the horizontal plane. (3) Starting 1 cm from the top edge of the nonwoven fabric surface, a total of 6 g of physiological saline was dropped using a microtube pump at a rate of 1 g / 10 sec. The position where all of the poured physiological saline was absorbed by the nonwoven fabric and the physiological saline droplet disappeared from the surface of the nonwoven fabric was measured, and the distance traveled by the physiological saline droplet across the nonwoven fabric surface between this position and the position where the physiological saline was dropped was determined. Note that in the above, a burette may be used to drop the physiological saline instead of a microtube pump.

[0153] (Measurement of liquid absorption rate and liquid return amount of surface sheets for absorbent articles) (1) The following items were prepared to measure the liquid absorption rate and liquid return amount of surface sheets for absorbent articles. Absorbent material: The absorbent material was made by removing the surface sheet from a commercially available absorbent product (Kao Corporation's "Relief (registered trademark)" disposable underwear pad, Secure Fit, 2-time absorption). Physiological saline solution: A sodium chloride aqueous solution prepared to have a sodium chloride concentration of 0.9% by mass (colored blue for visibility) was used as physiological saline solution. The temperature was 37°C and the viscosity was 0.7 mPa·s. Filter paper: Manufactured by Toyo Filter Paper Co., Ltd., ADVANTEC® No.2, 10cm x 10cm Weight: 5kg Absorbent cylinder: An acrylic resin cylinder with an outer diameter of 45 mm and an inner diameter of 40 mm (a 9 cm x 9 cm acrylic resin plate with a through hole in the center is attached as a base to ensure stability when placed on the surface sheet of the absorbent material. Total mass of absorbent cylinder: 1125 g) was used to pour a predetermined amount of physiological saline solution onto the surface sheet. (2) Method The absorption rate and return volume were measured according to the following procedure. (i) The surface sheet was peeled off the above-mentioned commercially available absorbent material, and a surface sheet for absorbent materials (10 cm in the vertical direction (MD direction), 10 cm in the horizontal direction (CD direction)) for evaluating the liquid absorption rate and liquid return properties was placed on top of the remaining absorbent material. At this time, it was set so that the surface of the first fiber layer was the measurement surface. The above-mentioned liquid absorption cylinder was placed on top of the set surface sheet for absorbent materials (i.e., on top of the first fiber layer), so that physiological saline solution could be poured into the first fiber layer of the surface sheet for absorbent materials through the liquid absorption cylinder. (ii) 150 g of physiological saline was poured into the absorbent surface sheet with the above-mentioned absorbent cylinder attached, from the upper end of the absorbent cylinder. At this time, the time from when the physiological saline was poured onto the absorbent surface sheet until it was no longer visible from the surface of the absorbent surface sheet (when the physiological saline moved from the surface of the absorbent surface sheet (the surface of the first fiber layer) to the absorbent material located beneath the absorbent surface sheet, and physiological saline was no longer visible as a liquid on the surface of the absorbent surface sheet) (absorption time) was measured using a stopwatch and was defined as the absorption rate for the first time. (iii) Ten minutes after pouring in the saline solution, the absorbent cylinder was removed from the absorbent surface sheet. The filter paper (manufactured by Toyo Filter Paper Co., Ltd., ADVANTEC® No. 2), which had its mass (W0) measured in a stack of 30 sheets, was stacked again in the same way as when the mass (W0) was measured. The filter paper was placed on the absorbent surface sheet, with the location where the saline solution was poured in the center, and a weight (5 kg) was placed on top of the filter paper. It was left for 20 seconds to allow the saline solution to be absorbed by the filter paper. After 20 seconds, the weight was removed and the mass (W1) of the filter paper (30 sheets) that had absorbed the saline solution was measured. The difference in mass of the filter paper before and after absorption of the saline solution (W1-W0) was taken as the amount of liquid returned in the first batch (g). (iv) The above steps (i) to (iii) were repeated, and the absorption rate and return volume were measured for the second time. When pouring saline solution onto the absorbent surface sheet, the absorbent tube was placed on the absorbent surface sheet so as to pour the saline solution into the same position as in the first measurement, and 150g of saline solution was poured into the same position as in the first measurement.

[0154] The polyethylene terephthalate (PET) and high-density polyethylene (PE-HD) used in the examples and comparative examples are as follows. (1) PET (Commercially available polyethylene terephthalate with a melting point of 255°C and intrinsic viscosity (IV value) of 0.64, using INDORAMA's TEXTILE GRADE (SEMIDULL)) (2) PE-HD1 (melting point: 133℃, density: 0.956 g / cm³) 3 (MFR190: 22g / 10min high-density polyethylene, manufactured by Nippon Polyethylene Co., Ltd., product name "Novatec (registered trademark) HE490") (3) PE-HD2 (melting point: 136℃, density: 0.956 g / cm³) 3 (MFR190: 26g / 10min high-density polyethylene, manufactured by Nippon Polyethylene Co., Ltd., product name "Novatec (registered trademark) HE491J") (4) PE-HD3 (melting point: 135℃, density: 0.954 g / cm³) 3 (MFR190: 30g / 10min high-density polyethylene, manufactured by SK Global Chemical, part number "MM810") (5) PE-HD4 (melting point: 133℃, density: 0.956 g / cm³) 3 (MFR190: 13g / 10min high-density polyethylene, manufactured by Nippon Polyethylene Co., Ltd., product number "Novatec (registered trademark) HE481")

[0155] (Examples 1-9, Comparative Examples 1-3) The high-density polyethylene described above was used as the sheath component, and the polyethylene terephthalate described above was used as the core component. Furthermore, for the polyethylene terephthalate core component, a masterbatch was prepared by adding titanium dioxide to the same polyethylene terephthalate beforehand. The masterbatch was added so that the titanium dioxide content in the total composite fiber was as shown in Tables 1 and 2. The prepared sheath and core components were melt-spun using a concentric core-sheath composite nozzle, adjusting the discharge rate of each component so that the composite ratio (volume ratio) of the sheath and core components was as shown in Tables 1 and 2. The spinning temperature for the sheath component was 270°C or 290°C, the spinning temperature for the core component was 340°C, and the nozzle temperature was 290°C. The extruded molten filaments were taken up to achieve the draft ratios shown in Tables 1 and 2, obtaining undrawn fiber tows with the single fiber finenesses shown in Tables 1 and 2.

[0156] The obtained undrawn fiber tow was wet-drawn in 80°C hot water at the drawing ratios listed in Tables 1 and 2 to obtain drawn fiber tow. Next, the drawn fiber tow was impregnated in a treatment tank filled with an aqueous solution of a fiber treatment agent that imparts hydrophilicity (concentration of the active ingredient of the fiber treatment agent: 5% by mass). Then, the excess aqueous solution of the fiber treatment agent was squeezed off with a resin roll (nip roll) to adjust the moisture content so that the component of the fiber treatment agent was 0.3% by mass when the mass of the composite fiber was 100% by mass.

[0157] In this example, a tow heat treatment was performed on a stretched fiber tow that had been treated with a fiber treatment agent. The tow heat treatment was carried out by setting the stretched fiber tow to a tension state of 1.0 times and blowing steam set to 100°C onto the stretched fiber tow for 3 seconds.

[0158] If necessary, the tow-heat treated stretched fiber tow (comparative example 1 was not tow-heated) was mechanically crimped using a stuffing box type crimper. At this time, the surface temperature of the stretched fiber tow was measured to be 85°C just before it entered the stuffing box type crimper. The surface temperature of the stretched fiber tow was measured to be 70°C just after it came out of the stuffing box type crimper. Then, annealing and drying treatments were performed simultaneously for 15 minutes in a relaxed state using a hot air blowing device set to 110°C. After that, the stretched fiber tow was cut to the predetermined lengths shown in Tables 1 and 2 to obtain composite fibers.

[0159] (Method for manufacturing heat-bonded nonwoven fabric) Using the composite fibers obtained in the examples and comparative examples, a basis weight of 30 g / m² was applied using a roller-type carding machine. 2 A fiber web was prepared. During this process, the carding properties of the composite fibers were evaluated according to the evaluation criteria described above. The obtained fiber web was subjected to heat treatment for 10 seconds using a hot air blowing device set to 135°C to melt the sheath component and obtain a heat-bonded nonwoven fabric.

[0160] The performance of the fibers and nonwoven fabrics obtained in each example and comparative example is shown in Tables 1 and 2 below.

[0161] [Table 1]

[0162] [Table 2]

[0163] (Example 10) Undrawn fibers were obtained under the same melt-spinning conditions as in Example 9. The obtained undrawn fibers were then drawn under the same drawing conditions as in Example 9 to obtain drawn fiber tow. Next, the drawn fiber tow was impregnated in a treatment tank filled with an aqueous solution of a hydrophilic fiber treatment agent that does not have water resistance, mainly composed of C12 alkyl phosphate potassium salt (concentration of the active ingredient of the fiber treatment agent: 5% by mass). After that, the moisture content was adjusted so that the component of the fiber treatment agent was 0.3% by mass when the mass of the composite fiber was 100% by mass, by squeezing off the excess aqueous solution of the fiber treatment agent with a resin roll (nip roll). The drawn fiber tow to which the fiber treatment agent had been applied was subjected to tow heat treatment under the same conditions as in Example 9. Then, mechanical crimping was applied using a stuffing box type crimper, and annealing and drying treatments were simultaneously performed in a relaxed state for 15 minutes using a hot air blowing device set to 110°C. After that, the drawn fiber tow was cut to 45 mm to obtain composite fibers. Using this composite fiber, a weight of 20g / m² was achieved using a roller-type carding machine. 2 A fiber web was prepared. The obtained fiber web was subjected to heat treatment for 10 seconds using a hot air blowing device set to 135°C to melt the sheath component and obtain the heat-bonded nonwoven fabric of Example 10.

[0164] (Example 11) The composite fibers were manufactured under the same conditions as in Example 10, except that a water-resistant fiber treatment agent containing a C12 alkyl phosphate potassium salt was used as the fiber treatment agent. The resulting composite fibers were then used to produce a heat-bonded nonwoven fabric, which was the heat-bonded nonwoven fabric of Example 11.

[0165] (Example 12) Similar to Example 11, composite fibers and heat-bonded nonwoven fabrics were manufactured under the same conditions as in Example 10, except that a water-repellent fiber treatment agent mainly composed of C18 alkyl phosphate potassium salt was used as the fiber treatment agent when manufacturing the composite fibers, resulting in the heat-bonded nonwoven fabric of Example 12.

[0166] (Example 13) Undrawn fibers were obtained under the same melt-spinning conditions as the composite fibers of Example 1. The obtained undrawn fibers were then drawn under the same drawing conditions as in Example 1 to obtain drawn fiber tow. Next, the drawn fiber tow was impregnated in a treatment tank filled with an aqueous solution of a hydrophilic fiber treatment agent that does not have water resistance, mainly composed of C12 alkyl phosphate potassium salt (concentration of the active ingredient of the fiber treatment agent: 5% by mass). After that, the moisture content was adjusted so that the component of the fiber treatment agent was 0.3% by mass when the mass of the composite fibers was 100% by mass, by squeezing off the excess aqueous solution of the fiber treatment agent with a resin roll (nip roll). The drawn fiber tow to which the fiber treatment agent had been applied was subjected to tow heat treatment under the same conditions as in Example 1, then mechanical crimping was applied using a stuffing box type crimper, and annealing and drying treatments were simultaneously performed in a relaxed state for 15 minutes using a hot air blowing device set to 110°C. After that, the drawn fiber tow was cut to 30 mm to obtain composite fibers. Using this composite fiber, a weight of 20g / m² was achieved using a roller-type carding machine. 2 A fiber web was prepared. The obtained fiber web was subjected to heat treatment for 10 seconds using a hot air blowing device set to 135°C to melt the sheath component and obtain the heat-bonded nonwoven fabric of Example 13.

[0167] (Comparative Example 4) Using commercially available concentric core-sheath composite fibers ("NBF" manufactured by Daiwabo Polytech Co., Ltd. (NBF is a registered trademark), single fiber fineness 4.4 dtex, fiber length 51 mm, and the same hydrophilic fiber treatment agent used for the composite fibers in the production of the heat-bonded nonwoven fabrics in Examples 10 and 13 as the fiber treatment agent attached to the fiber surface), a heat-bonded nonwoven fabric was produced under the same conditions as when the heat-bonded nonwoven fabric of Example 10 was produced, to obtain the heat-bonded nonwoven fabric of Comparative Example 4.

[0168] Using the heat-bonded nonwoven fabrics of Examples 10-13 and Comparative Example 4, the surface properties of the heat-bonded nonwoven fabrics were measured and evaluated based on the KES (Kawabata Evaluation System) measurement method described above. The results are shown in Table 3.

[0169] [Table 3]

[0170] (Example 14) Using the composite fibers prepared in the same manner as in Example 13, a basis weight of 10 g / m² was obtained by using a roller-type carding machine. 2 A first fiber web was prepared. Next, a commercially available concentric core-sheath composite fiber ("NBF" manufactured by Daiwabo Polytech Co., Ltd. (NBF is a registered trademark), single fiber fineness 4.4 dtex, fiber length 51 mm, containing C12 alkyl phosphate potassium salt, with a fiber treatment agent that is more hydrophilic than the fiber treatment agent used in the composite fiber used to prepare the heat-bonded nonwoven fabric in Example 13) was used with a roller carding machine to achieve a basis weight of 15 g / m². 2 A second fiber web was prepared. Next, the second fiber web was laminated onto the first fiber web, and the resulting laminated fiber web was heat-treated for 9 seconds using a hot air penetration heat treatment machine set to 135°C to melt the sheath component of the composite fibers contained in the first and second fiber webs, thereby heat-bonding the first and second fiber webs to form a heat-bonded nonwoven fabric (basis weight 25 g / m²) containing the first and second fiber layers. 2 A laminated fiber web was obtained. At this time, the first fiber web, which becomes the first fiber layer, was heat-treated with the first fiber web in contact with the conveyor net surface of a hot air penetration type heat treatment machine, and the hot air was blown onto the laminated fiber web from the second fiber layer side. For the obtained heat-bonded nonwoven fabric, the run-off value (R1) of the surface of the first fiber layer was 51 mm, the run-off value (R2) of the second fiber layer was 40 mm, and the difference between the run-off values ​​of the first fiber layer and the second fiber layer (R1-R2) was 11 mm, confirming that the hydrophilicity of the second fiber layer was stronger than that of the first fiber layer.

[0171] (Example 15) Using the composite fibers prepared in the same manner as in Example 13, a basis weight of 10 g / m² was obtained by using a roller-type carding machine. 2A first fiber web was prepared. Next, using a commercially available concentric core-sheath composite fiber ("NBF" manufactured by Daiwabo Polytech Co., Ltd. (NBF is a registered trademark), fineness 4.4 dtex, fiber length 51 mm, using the same fiber treatment agent used for the composite fiber when preparing the heat-bonded nonwoven fabric in Example 11), a basis weight of 10 g / m² was achieved using a roller-type carding machine. 2 A second fiber web was prepared. Next, the second fiber web was laminated onto the first fiber web, and the resulting laminated fiber web was heat-treated for 15 seconds using a hot air penetration heat treatment machine set to 135°C to melt the sheath component of the composite fibers contained in the first and second fiber webs, and the first and second fiber webs were heat-bonded together to form a heat-bonded nonwoven fabric (basis weight 20g / m²) containing the first and second fiber layers. 2 A laminated fiber web was obtained. At this time, the first fiber web, which becomes the first fiber layer, was heat-treated with the first fiber web in contact with the conveyor net surface of a hot air penetration type heat treatment machine, and the hot air was blown onto the laminated fiber web from the second fiber layer side. For the obtained heat-bonded nonwoven fabric, the run-off value (R1) of the surface of the first fiber layer was 51 mm, the run-off value (R2) of the second fiber layer was 43 mm, and the difference between the run-off values ​​of the first fiber layer and the second fiber layer (R1-R2) was 8 mm, confirming that the hydrophilicity of the second fiber layer was stronger than that of the first fiber layer.

[0172] (Example 16) Using a composite fiber prepared in the same manner as in Example 10 (with the fiber length changed to 38 mm), a basis weight of 10 g / m² was obtained by using a roller-type carding machine. 2 A first fiber web was prepared. Next, using a commercially available concentric core-sheath composite fiber ("NBF" manufactured by Daiwabo Polytech Co., Ltd. (NBF is a registered trademark), fineness 4.4 dtex, fiber length 51 mm, using the same fiber treatment agent used for the composite fiber when preparing the heat-bonded nonwoven fabric in Example 11), a basis weight of 10 g / m² was achieved using a roller-type carding machine. 2A second fiber web was prepared. Next, the second fiber web was laminated onto the first fiber web, and the resulting laminated fiber web was heat-treated for 15 seconds using a hot air penetration heat treatment machine set to 135°C to melt the sheath component of the composite fibers contained in the first and second fiber webs, and the first and second fiber webs were heat-bonded together to form a heat-bonded nonwoven fabric (basis weight 20g / m²) containing the first and second fiber layers. 2 A laminated fiber web was obtained. At this time, the first fiber web, which becomes the first fiber layer, was heat-treated with the first fiber web in contact with the conveyor net surface of a hot air penetration type heat treatment machine, and the hot air was blown onto the laminated fiber web from the second fiber layer side. For the obtained heat-bonded nonwoven fabric, the run-off value (R1) of the surface of the first fiber layer was 55 mm, the run-off value (R2) of the second fiber layer was 48 mm, and the difference between the run-off values ​​of the first fiber layer and the second fiber layer (R1-R2) was 7 mm, confirming that the hydrophilicity of the second fiber layer was stronger than that of the first fiber layer.

[0173] (Example 17) Using the composite fibers prepared in the same manner as in Example 13, a basis weight of 10 g / m² was obtained by using a roller-type carding machine. 2 A first fiber web was prepared. Next, using the composite fiber used in the production of the heat-bonded nonwoven fabric of Comparative Example 4 ("NBF" manufactured by Daiwabo Polytech Co., Ltd. (NBF is a registered trademark), fineness 4.4 dtex, fiber length 51 mm, using the same fiber treatment agent used for the composite fiber used in the production of the heat-bonded nonwoven fabric of Example 13), a basis weight of 10 g / m² was obtained by rolling a carding machine. 2 A second fiber web was prepared. Next, the second fiber web was laminated onto the first fiber web, and the resulting laminated fiber web was heat-treated for 15 seconds using a hot air penetration heat treatment machine set to 135°C to melt the sheath component of the composite fibers contained in the first and second fiber webs, and the first and second fiber webs were heat-bonded together to form a heat-bonded nonwoven fabric (basis weight 20g / m²) containing the first and second fiber layers. 2A laminated fiber web was obtained. At this time, the first fiber web, which will become the first fiber layer, was heat-treated with the first fiber web in contact with the conveyor net surface of a hot air penetration type heat treatment machine, and the hot air was blown onto the laminated fiber web from the second fiber layer side. For the obtained heat-bonded nonwoven fabric, the run-off value (R1) of the surface of the first fiber layer was 51 mm, the run-off value (R2) of the second fiber layer was 48 mm, and the difference between the run-off values ​​of the first fiber layer and the second fiber layer (R1-R2) was 3 mm, confirming that the hydrophilicity of the first fiber layer and the second fiber layer was about the same.

[0174] To evaluate the performance of the laminated nonwoven fabrics of Examples 14-17 as surface sheets for absorbent articles, liquid absorption tests and liquid return measurements / evaluations were performed as described above. The results are shown in Table 4.

[0175] [Table 4]

[0176] As described above, the composite fibers of Examples 1 to 9 exhibit good card passability. This is because the composite fibers of Examples 1 to 9 have a crystallite size of 20.0 nm to 50.0 nm measured on the

[0110] plane of high-density polyethylene, in addition to the heat of fusion (ΔH) of the high-density polyethylene measured by differential scanning calorimetry (DSC). PE-HD It is presumed that this is related to the fact that the heat of fusion (ΔH) of high-density polyethylene became 145.0 mJ / mg or higher. The composite fibers of Examples 1 to 9 showed a higher heat of fusion (ΔH) of high-density polyethylene compared to the composite fiber of Comparative Example 1. PE-HD Because of the large size of the crystallite size (ΔH) measured on the

[0110] plane of the high-density polyethylene in the composite fibers of Examples 1 to 9, not only did crystallization progress, with crystallite sizes of 20.0 nm or more being measured, but the crystals grew significantly. In addition, tow heating further promoted the crystallization of the high-density polyethylene, which strengthened the rigidity of the sheath component, preventing twisting even when passed through a carding machine at high speed, and thus improving card passability. Furthermore, the crystallite size and heat of fusion (ΔH) of the high-density polyethylene were also considered. PE-HDBecause the above-mentioned range is present, the fiber properties such as single fiber strength, elongation, the ratio of single fiber strength to elongation, and the product of the positive square root of single fiber strength and elongation (√elongation) tend to satisfy the above-mentioned preferred range, resulting in improved card passability.

[0177] On the other hand, the composite fiber of Comparative Example 1 is less than 2.0 dtex, but its carding performance is lower compared to the composite fiber of the Examples. This is because, unlike the composite fibers of Examples 1 to 9, the fiber of Comparative Example 1 did not undergo tow heat treatment. Therefore, when crimping was applied to the drawn fiber filament during the crimping process, the drawn fiber tow was not heated, or in other words, the thermal vibration of the crystalline and amorphous parts inside the high-density polyethylene was insufficient. As a result, the desired crimp shape was applied to the composite fiber, making it prone to losing its crimp shape over time and under pressure. Furthermore, the results of Comparative Examples 2 and 3 confirmed that too much sheath component and too little fluidity of the sheath component disrupt the balance between the fluidity and cooling rate of the core component and sheath component during melt spinning, making melt spinning impossible.

[0178] By comparing the heat-bonded nonwoven fabrics of Examples 10-13 with the heat-bonded nonwoven fabric of Comparative Example 4, it can be seen that the heat-bonded nonwoven fabrics containing the composite fibers of the present invention have superior texture, less friction when the surface of the nonwoven fabric is touched, and have a smooth feel. Specifically, the heat-bonded nonwoven fabrics of Examples 10-13 have a smaller average coefficient of friction (MIU) compared to the nonwoven fabric of Comparative Example 4, indicating that the frictional force generated when touched is smaller. In addition, the variation in the average coefficient of friction (MMD) is extremely small compared to the heat-bonded nonwoven fabric of Comparative Example 4, so the heat-bonded nonwoven fabrics of Examples 10-13 have less friction and less variation in friction, resulting in a smooth texture when the surface is touched with bare hands, without any feeling of snagging on the skin due to variations in the coefficient of friction. Furthermore, it can be seen that by using the composite fibers of the present invention, a heat-bonded nonwoven fabric with superior texture can be obtained regardless of the application. The nonwoven fabrics in Examples 10-12 were produced by selecting a fiber treatment agent used in the manufacture of the composite fibers, ranging from composite fibers using a strongly hydrophilic fiber treatment agent (Example 11) to composite fibers using a water-repellent fiber treatment agent (Example 12). Since the average coefficient of friction (MIU) and variation in average coefficient of friction (MMD) of the heat-bonded nonwoven fabrics in Examples 10-12 are smaller than those of the heat-bonded nonwoven fabric in Comparative Example 4, regardless of the type of fiber treatment agent, the resulting heat-bonded nonwoven fabrics have the smooth feel described above. Therefore, when the composite fibers of the present invention using a hydrophilic fiber treatment agent are used in heat-bonded nonwoven fabrics containing them, the feel to the touch is smooth and desirable. Furthermore, when the composite fibers of the present invention using a water-repellent fiber treatment agent are used in heat-bonded nonwoven fabrics containing them, the feel to the touch is smooth and desirable.

[0179] Table 4 shows that by using a laminated nonwoven fabric as a surface sheet for absorbent articles, in which a fiber layer containing the composite fibers of the present invention is used as the first fiber layer that comes into contact with the skin, and a fiber layer containing composite fibers with a finer density than the composite fibers of the present invention is used as the second fiber layer, an absorbent article surface sheet exhibiting excellent texture can be obtained. Specifically, in Table 4, the laminated nonwoven fabrics of Examples 14 to 17 have a small average coefficient of friction (MIU) and a small variation in the average coefficient of friction (MMD) of 0.25 or less and 0.1 or less, respectively, for the first fiber layer, which is the surface that comes into contact with the wearer's skin, indicating that the feel when it touches the skin is very smooth. In addition, in the absorbent article surface sheets of Examples 10 to 13, the hydrophilicity of the second fiber layer is stronger than that of the first fiber layer, creating a hydrophilic gradient between the second and first fiber layers, which strengthens the effect of the second fiber layer in drawing in and absorbing liquids such as urine and menstrual blood absorbed by the first fiber layer. Therefore, urine and menstrual blood absorbed by the first fiber layer migrate to the second fiber layer and then to the absorbent material adjacent to the second fiber layer. As a result, even if the first fiber layer is a dense fiber layer containing fine-fiber composite fibers, urine and menstrual blood easily migrate to the absorbent material through the second fiber layer. This suppresses a decrease in the absorption rate and an increase in the amount of fluid returned even after repeated absorption of urine and menstrual blood. On the other hand, in the absorbent surface sheet of Example 17, the hydrophilicity of the fiber treatment agent attached to the fibers constituting the first and second fiber layers is similar. As a result, the hydrophilicity gradient is small or almost nonexistent, and the first fiber layer containing fine-fiber composite fibers tends to become a dense fiber layer. This makes it easier for urine and menstrual blood absorbed by the first fiber layer to be retained, and the migration from the first fiber layer to the absorbent material is slower. Therefore, compared to the absorbent surface sheets of Examples 14-16, not only is the initial absorption rate slower, but the decrease in the absorption rate and increase in the amount of fluid returned after repeated absorption of urine and menstrual blood are greater.

[0180] The present invention includes, for example, one or more embodiments described below.

[0181] [1] A composite fiber containing a core component and a sheath component, The core component and the sheath component are arranged substantially concentrically, and the combined ratio of the core component and the sheath component is such that the volume ratio of the core component to the sheath component (core component / sheath component) is 30 / 70 to 70 / 30. The single fiber fineness is 0.6 dtex or more and less than 2.0 dtex. The core component contains 60% by mass or more of polyester resin. The aforementioned sheath component contains 60% by mass or more of high-density polyethylene. The melt mass flow rate (MFR: measured at a temperature of 190°C, with a load of 2.16 kgf (21.18 N)) of the aforementioned high-density polyethylene is greater than 13 g / 10 min and less than or equal to 45 g / 10 min. The crystallite size measured for the

[0110] plane of the high-density polyethylene contained in the sheath component is between 20.0 nm and 50.0 nm. The heat of fusion (ΔH) of the high-density polyethylene measured by differential scanning calorimetry (DSC) PE-HD A composite fiber characterized by having a saturation level of 145.0 mJ / mg or higher. [2] The single fiber strength of the composite fiber is 1.5 cN / dtex or more and 5.0 cN / dtex or less. The elongation at break of the composite fiber is 20% or more and 150% or less. The composite fiber according to [1], wherein the ratio of single fiber strength to elongation at break (single fiber strength [cN / dtex] / elongation at break [%]) is greater than 0.04 and less than or equal to 0.12. [3] The composite fiber according to [1] or [2], wherein the toughness (toughness = single fiber strength [cN / dtex] × √ elongation at break [%]), expressed as the product of the single fiber strength and the positive square root of the elongation at break, is 12.0 or more and 20.0 or less. [4] A composite fiber according to any one of [1] to [3], wherein the crystallite size measured on the

[0200] plane of the high-density polyethylene contained in the sheath component is greater than 16.7 nm and less than or equal to 30.0 nm. [5] A composite fiber as described in any of [1] to [4], having a fiber length of 25 mm or more and 50 mm or less. [6] The composite fiber according to any one of [1] to [5], wherein the composite fiber contains 0.5% by mass or more and 10% by mass or less of an inorganic filler when the mass of the composite fiber is 100% by mass. [7] A process of extruding a core component containing 60% by mass or more of polyester resin at a spinning temperature of 280°C to 380°C. A process of extruding a sheath component containing 60% by mass of high-density polyethylene, in which the melt mass flow rate (MFR: measured at 190°C, load 2.16 kgf (21.18 N)) is greater than 13 g / 10 min and 45 g / 10 min or less, at a spinning temperature of 250°C to 350°C. A process of supplying the core component and the sheath component to a composite nozzle such that the volume ratio of the core component to the sheath component (core component / sheath component) is 30 / 70 to 70 / 30, and in which the sheath component covers the surface of the composite fiber in the fiber cross-section, and the core component and the sheath component are substantially arranged concentrically. A process to obtain an undrawn fiber tow with a single fiber fineness of 1.8 dtex or more and 4.5 dtex or less, in which the core component and the sheath component have solidified, while the molten undrawn fiber consisting of the extruded core component and the sheath component is taken up and cooled at a draft ratio of 600 or more and 1500 or less. A process of stretching the aforementioned unstretched fiber tow to 1.6 to 3.6 times its original length at a temperature of 70°C to 120°C to obtain a stretched fiber tow with a single fiber fineness of 0.6 dtex to less than 2.0 dtex. A step of applying a fiber treatment agent to the stretched fiber tow, A step of heating the surface of a stretched fiber tow, to which a fiber treatment agent has been applied, to 60°C or higher using steam as a medium. A step of imparting crimp to the stretched fiber tow whose surface temperature is 60°C or higher, A process of drying the stretched fiber tow that has been given crimp, A method for producing composite fibers, including [8] The method for producing a composite fiber according to [7], wherein the step of obtaining the drawn fiber tow is wet drawing using hot water at 70°C to 100°C. [9] A heat-bondable nonwoven fabric containing 25% by mass or more of the composite fibers described in any of [1] to [6], wherein at least some of the composite fibers are bonded together by a sheath component.

[10] The heat-bonded nonwoven fabric described in [9], wherein the variation in mean coefficient of friction (MMD) measured according to the KES method is 0.01 or less.

[11] An absorbent surface sheet comprising a first fiber layer in contact with the skin and a second fiber layer adjacent to the first fiber layer, The first fiber layer is a fiber layer containing 50% by mass or more of a first core-sheath type composite fiber, and the first core-sheath composite fiber is a composite fiber according to any one of claims 1 to 6. The second fiber layer is a fiber layer containing 50% by mass or more of a second core-sheath type composite fiber, the second core-sheath type composite fiber having a core component containing a polyester resin and a sheath component containing a thermoplastic resin having a melting point 50°C or more lower than the melting point of the polyester resin, and having a single fiber fineness of 2.2 dtex or more and 7 dtex or less. An absorbent surface sheet for articles in which at least a portion of the first core-sheath type composite fiber and the second core-sheath type composite fiber are heat-bonded by the sheath component of the first core-sheath type composite fiber and the second core-sheath type composite fiber.

[12] The basis weight of the first fiber layer is 4 g / m 2 More than 18g / m 2 The following conditions apply, and the basis weight of the second fiber layer is 8 g / m². 2 More than 45g / m 2 The surface sheet for absorbent articles according to

[11] , wherein the basis weight of the second fiber layer is greater than that of the first fiber layer.

[13] Comparing the fiber treatment agent adhering to the surface of the first core-sheath type composite fiber with the fiber treatment agent adhering to the surface of the second core-sheath type composite fiber, the fiber treatment agent adhering to the surface of the first core-sheath type composite fiber is a fiber treatment agent with lower hydrophilicity. The surface sheet for absorbent articles according to

[11] or

[12] , wherein the difference (R1-R2) between the run-off of the first fiber layer (R1) and the run-off of the second fiber layer (R2) is greater than 3 mm.

[14] An absorbent article comprising a heat-bonded nonwoven fabric as described in [9] or

[10] , or a surface sheet for absorbent articles as described in any of

[11] to

[13] . [Industrial applicability]

[0182] The composite fibers of the present invention can be incorporated into a heat-bonded nonwoven fabric, which can be preferably used as a surface sheet for various absorbent articles such as sanitary napkins, infant diapers, adult diapers, animal diapers including mammals, panty liners, and incontinence liners. It can also be preferably used as a back sheet for infant diapers and adult diapers, and as a second sheet located on the absorbent side of the surface sheet in absorbent articles, for example, directly below the surface sheet. [Explanation of Symbols]

[0183] 1 Sheath component 2 core components 3. Centricular position in the fiber cross-section of the core component 4. Centricular position in the fiber cross-section of composite fibers 5. Radius of the fiber cross-section of composite fibers 10 Composite Fibers 30 Surface sheets for absorbent articles 31. First fiber layer 32 Second fiber layer

Claims

1. It is a composite fiber containing a core component and a sheath component. The core component and the sheath component are arranged substantially concentrically, the composite ratio of the core component and the sheath component is 30 / 70 to 70 / 30 in terms of the volume ratio of the core component to the sheath component (core component / sheath component), and the single fiber fineness is 0.6 dtex or more and less than 2.0 dtex. The core component contains 60% by mass or more of polyester resin. The aforementioned sheath component contains 60% by mass or more of high-density polyethylene. The melt mass flow rate (MFR: measured at a temperature of 190°C, with a load of 2.16 kgf (21.18 N)) of the aforementioned high-density polyethylene is greater than 13 g / 10 min and less than or equal to 45 g / 10 min. The crystallite size measured for the [110] plane of the high-density polyethylene contained in the sheath component is 20.0 nm or more and 50.0 nm or less. The heat of fusion (ΔH) of the high-density polyethylene measured by differential scanning calorimetry (DSC) PE-HD ) is 145.0 mJ / mg or higher, A composite fiber characterized in that its toughness (toughness = single fiber strength [cN / dtex] × √ elongation at break [%]), which is expressed as the product of the single fiber strength and the positive square root of the elongation at break, is 12.0 or more and 18.7 or less.

2. The single fiber strength of the composite fiber is 1.5 cN / dtex or more and 5.0 cN / dtex or less. The composite fiber according to claim 1, wherein the elongation at break of the composite fiber is 20% or more and 150% or less.

3. The composite fiber according to claim 1 or 2, wherein the crystallite size measured on the [200] plane of the high-density polyethylene contained in the sheath component is greater than 16.7 nm and less than or equal to 30.0 nm.

4. A composite fiber according to any one of claims 1 to 3, wherein the fiber length is 25 mm or more and 50 mm or less.

5. The composite fiber according to any one of claims 1 to 4, wherein the composite fiber contains 0.5% by mass or more and 10% by mass or less of inorganic filler when the mass of the composite fiber is 100% by mass.

6. A process of extruding a core component containing 60% by mass or more of polyester resin at a spinning temperature of 280°C to 380°C. A step of extruding a sheath component containing 60% by mass of high-density polyethylene having a melt mass flow rate (MFR: measured at 190°C, load 2.16 kgf (21.18 N)) greater than 13 g / 10 min and 45 g / 10 min or less, at a spinning temperature of 250°C to 350°C; a step of supplying the core component and the sheath component to a composite nozzle in which the sheath component covers the surface of the composite fiber in the fiber cross-section and the core component and the sheath component are substantially arranged concentrically, such that the volume ratio of the core component to the sheath component (core component / sheath component) is 30 / 70 to 70 / 30; A process to obtain an undrawn fiber tow in which the core component and the sheath component have solidified, the molten undrawn fiber consisting of the extruded core component and the sheath component is cooled while being taken up so that the draft ratio is 600 or more and 1500 or less, and the single fiber fineness is 1.8 dtex or more and 4.5 dtex or less, and the elongation at break is 100% or more and 400% or less. A step of stretching the aforementioned unstretched fiber tow at a temperature of 70°C to 120°C to 1.6 times or more to obtain a stretched fiber tow with a single fiber fineness of 0.6 dtex or more and less than 2.0 dtex. A step of applying a fiber treatment agent to the stretched fiber tow, A step of heating the surface of a stretched fiber tow, to which a fiber treatment agent has been applied, to 60°C or higher using steam as a medium. A step of imparting crimp to the stretched fiber tow whose surface temperature is 60°C or higher, and a step of drying the stretched fiber tow to which crimp has been imparted. A method for producing composite fibers, including

7. The method for producing a composite fiber according to claim 6, wherein the step of obtaining the stretched fiber tow is wet stretching using hot water at 70°C to 100°C.

8. A heat-bondable nonwoven fabric containing 25% by mass or more of the composite fibers described in any one of claims 1 to 5, wherein at least some of the composite fibers are bonded together by a sheath component.

9. The heat-bondable nonwoven fabric according to claim 8, wherein the variation in average friction coefficient (MMD) measured based on the KES method is 0.01 or less.

10. An absorbent article comprising the heat-bondable nonwoven fabric according to claim 8 or 9.

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