Nonwoven fabric and surface sheet for absorbent article using same

Abstract

Provided is nonwoven fabric excellent in liquid permeability for a high-viscosity liquid and in mechanical properties. The nonwoven fabric comprises heat bonding composite fibers with heat bonded intersections, wherein the specific volume of the nonwoven fabric is 60-95 cm3 / g, and the fiber density ratio between a high density part and a low density part in the nonwoven fabric is 1.4 or less. The heat bonding composite fibers constituting the nonwoven fabric are distributed in a state in which variation in density of the fibers is small in the thickness direction of the nonwoven fabric, and the fibers form bonding points at an appropriate density.

Description

Nonwoven fabric and surface sheet for absorbent article using the same

[0001] The present invention relates to a nonwoven fabric that has excellent mechanical properties and permeability to highly viscous liquids.

[0002] Hygiene materials such as disposable diapers and napkins are required to have excellent bulk and flexibility, minimal roughness when rubbed against the skin, and permeability to various liquids such as urine, loose stools, and menstrual blood.

[0003] Through-air nonwoven fabrics are widely used as such nonwoven fabrics. Through-air nonwoven fabrics are obtained by heat-treating a web made of composite fibers composed of at least two thermoplastic resins with different melting points. Known methods for heat-treating a web include thermally bonding composite fibers together using a heat treatment device (e.g., a hot air penetration heat treatment machine or a hot air blowing heat treatment machine) equipped with a transport support that supports and transports the web. However, because through-air nonwoven fabrics are produced by blowing hot air, the web is subjected to pressure from the hot air, which can result in a loss of bulk and flexibility (see Patent Document 1). Furthermore, because the nonwoven fabric is produced using the above-described manufacturing method, the fiber density on the transport support side of the nonwoven fabric is relatively high, which can easily impair the permeability of high-viscosity liquids (particularly when the liquid is repeatedly passed through or when a large amount of liquid is passed through). Furthermore, using composite fibers with a small fiber diameter improves surface smoothness while also making the density of the nonwoven fabric more pronounced.

[0004] In view of these problems, the present applicant previously proposed a nonwoven fabric that is bulky, has excellent flexibility, and has relatively high strength, and that has excellent permeability to high-viscosity liquids, by heat-fusing the intersections of thermally adhesive conjugate fibers under no pressure using superheated steam gas (Patent Document 2). However, although the nonwoven fabric obtained in this manner has excellent permeability to high-viscosity liquids, there is room for improvement in mechanical properties such as strength and resistance to crushing under load.

[0005] JP 2019-80907 A International Publication No. 2022 / 202142

[0006] The present invention has been made against the background of the above-mentioned conventional technology, and an object of the present invention is to provide a nonwoven fabric that is excellent in liquid permeability for high-viscosity liquids and mechanical properties.

[0007] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result have found that a nonwoven fabric having excellent mechanical properties and excellent permeability to high-viscosity liquids can be obtained by distributing the thermal adhesive conjugate fibers constituting the nonwoven fabric in a state in which there is little variation in density in the thickness direction of the nonwoven fabric and by forming bonded points between the fibers at an appropriate density, which has led to the completion of the present invention.

[0008] That is, the present invention has the following features: [1] A nonwoven fabric containing thermally adhesive composite fibers whose intersections are thermally bonded, the nonwoven fabric having a specific volume of 60 to 95 cm ３ / g, and the fiber density ratio of the high density portion to the low density portion in the nonwoven fabric is 1.4 or less. [2] The fiber density of the nonwoven fabric is 10 to 50 fibers / mm ２ [3] The nonwoven fabric according to [1], wherein the fiber diameter of the thermal adhesive conjugate fibers is 5 to 20 μm. [4] A method for producing a nonwoven fabric, comprising the steps of forming a web containing thermal adhesive conjugate fibers, consolidating the web, and thermally bonding intersections of the thermal adhesive conjugate fibers without pressure. [5] A topsheet for absorbent articles using the nonwoven fabric according to any one of [1] to [3]. [6] An absorbent article using the nonwoven fabric according to any one of [1] to [3].

[0009] According to the present invention, a nonwoven fabric having excellent mechanical properties and high-viscosity liquid permeability can be provided.

[0010] The nonwoven fabric of the present invention contains thermally bonded composite fibers whose intersections are thermally bonded, and has a specific volume of 60 to 95 cm ３ / g, and the fiber density ratio between the high density portion and the low density portion is 1.4 or less. ３By setting the fiber density ratio to 1.4 or less, the thermal adhesive conjugate fibers are distributed in a state with little variation in density in the thickness direction of the nonwoven fabric, thereby improving the permeability of highly viscous liquids.

[0011] (Thermal adhesive conjugate fiber) The thermal adhesive conjugate fiber used in the present invention is not particularly limited as long as it can be melted by heat to form adhesive points, and its conjugate form can be exemplified by concentric sheath-core conjugate fiber, eccentric sheath-core conjugate fiber, or side-by-side conjugate fiber. In addition, the cross-sectional shape of the conjugate fiber is not particularly limited, and any of round shapes such as circle or ellipse, angular shapes such as triangle or square, irregular shapes such as star or octave, segmented shapes, and hollow shapes can be used.

[0012] The thermoplastic resin constituting the thermally bondable composite fiber is not particularly limited, and examples thereof include polyethylene-based resins such as low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and high-density polyethylene (HDPE); polypropylene-based resins such as crystalline polypropylene (PP) and copolymers of propylene and α-olefins (excluding propylene) (Co-PP) containing propylene as the main component; polyester-based resins such as polyethylene terephthalate (PET), polybutylene terephthalate, copolymerized polyethylene terephthalate (Co-PET), polylactic acid, polyglycolic acid, and polybutylene succinate; polyvinyl alcohol-based resins, polyvinyl acetate-based resins, acrylic resins, polystyrene-based resins, polyurethane-based resins, polyamide-based resins, and fluorine-based resins. Among these, polyethylene-based resins, polypropylene-based resins, and polyester-based resins are preferably used because of their excellent processability. The combination of thermoplastic resins constituting the thermally bondable conjugate fiber is not particularly limited, but from the viewpoint of widening the processing temperature range, the difference in melting point is preferably 10°C or more, more preferably 30°C or more, and even more preferably 50°C or more. Specific examples of combinations of high-melting point component / low-melting point component of thermoplastic resins include PP / HDPE, PP / LLDPE, PP / Co-PP, PET / HDPE, PET / LLDPE, PET / Co-PET, and PET / PP. From the viewpoints of texture, raw material costs, production stability, etc., the combination of PP / HDPE or PET / HDPE is preferred, and the combination of PET / HDPE is more preferred. Furthermore, from the viewpoint of thermal adhesion, the low-melting point component preferably occupies 50% or more, and more preferably 70% or more, of the surface of the thermally bondable conjugate fiber.

[0013] The volume ratio of the low-melting point component to the high-melting point component is not particularly limited, but a high proportion of the low-melting point component improves the strength of the bonded points between the thermal adhesive conjugate fibers, making it easier to improve the strength of the nonwoven fabric, while a high proportion of the high-melting point component improves the bulkiness and flexibility of the nonwoven fabric and makes it less susceptible to changes in thickness due to load. From this perspective, the volume ratio of the low-melting point component to the high-melting point component is preferably 20 / 80 to 80 / 20, and more preferably 30 / 70 to 70 / 30.

[0014] The thermoplastic resin constituting the thermal adhesive conjugate fiber may contain additives such as antioxidants, light stabilizers, ultraviolet absorbers, neutralizing agents, nucleating agents, epoxy stabilizers, lubricants, antibacterial agents, deodorizers, flame retardants, antistatic agents, pigments, or plasticizers, as needed, within the range that does not impair the effects of the present invention.

[0015] The fiber diameter of the thermal adhesive conjugate fiber used in the present invention is not particularly limited, but in order to make the nonwoven fabric less likely to be crushed by a load and to reduce the fiber density and improve the permeability of high-viscosity liquids, it is preferably 5 μm or more, more preferably 6 μm or more, and even more preferably 8 μm or more. In addition, in order to reduce roughness and increase the sense of smoothness, the fiber diameter is preferably 20 μm or less, more preferably 16 μm or less, and even more preferably 14 μm or less.

[0016] The fiber length of the thermal adhesive conjugate fiber is not particularly limited, but in order to increase the entanglement of the fibers and improve the shape retention of the web and the strength of the nonwoven fabric, it is preferably 3 mm or more, more preferably 20 mm or more, and even more preferably 30 mm or more. Furthermore, in order to make it easier for the fibers to orient in the thickness direction of the nonwoven fabric and improve bulkiness and flexibility, the fiber length is preferably 200 mm or less, more preferably 120 mm or less, and even more preferably 60 mm or less.

[0017] The crimp of the thermal adhesive conjugate fiber is not particularly limited, and crimp properties such as the number of crimps, crimp rate, residual crimp rate, and crimp modulus may be appropriately selected in consideration of the bulkiness, flexibility, mechanical properties, etc. of the nonwoven fabric. The shape of the crimp is also not particularly limited, and may be appropriately selected from zigzag mechanical crimps, spiral crimps, ohmic crimps, and the like.

[0018] (Nonwoven Fabric) The fiber density of the nonwoven fabric in the present invention is not particularly limited, but in order to increase the number of intersections between fibers and improve the strength of the nonwoven fabric, it is preferable to use a fiber density of 10 fibers / mm ２ It is preferable that the number of fibers is 12 or more per mm. ２ More preferably, it is 20 fibers / mm or more. ２ In order to improve the permeability of high-viscosity liquids, the fiber density is preferably 50 fibers / mm ２ Preferably, the number of fibers is 40 or less per mm. ２ More preferably, it is 35 lines / mm or less. ２ It is even more preferable that:

[0019] The nonwoven fabric of the present invention has a fiber density ratio between high-density and low-density regions of 1.4 or less. This means that the thermally adhesive conjugate fibers are distributed in the thickness direction of the nonwoven fabric with little variation in density, which can improve the permeability of highly viscous liquids. From this perspective, the fiber density ratio is more preferably 1.3 or less, and even more preferably 1.2 or less. The lower limit of the fiber density ratio is 1.0, and the closer to 1.0 the ratio is, the more uniform the fiber density becomes, i.e., the closer to the ideal state in the present invention.

[0020] Here, the fiber density ratio in this specification is expressed as the ratio of the fiber densities of the high-density portion and the low-density portion in each layer. Furthermore, the "high-density portion" and the "low-density portion" refer to the upper and lower portions of each layer divided into equal halves in the thickness direction, and the higher fiber density is referred to as the "high-density portion" and the lower fiber density is referred to as the "low-density portion" when the fiber densities are measured.

[0021] In addition, the term "fiber density" as used herein refers to the number of fibers per unit area in the cross section of the nonwoven fabric, and is expressed in units of, for example, fibers / mm ２The method for measuring the fiber density will be explained in detail in the Examples.

[0022] The fiber density of the nonwoven fabric and the fiber density ratio between the high density and low density regions can be controlled by, for example, the fiber diameter of the thermally adhesive composite fiber and the air speed and pressure of the heat medium in the thermal bonding step described below.

[0023] The basis weight of the nonwoven fabric is not particularly limited, but in order to improve the mechanical properties and to make it difficult for the passed liquid to return, it is preferably 5 g / m ２ It is preferable that the content is 8 g / m or more. ２ More preferably, it is 10 g / m or more. ２ Furthermore, in order to prevent the thickness of the nonwoven fabric from being too thick, to easily maintain drapeability, to make it difficult for liquid to be retained when liquid is passed through, and to improve dryness, it is preferable that the basis weight is 60 g / m or more. ２ It is preferable that the density is 50 g / m or less. ２ More preferably, it is 40 g / m or less. ２ It is even more preferable that:

[0024] The thickness of the nonwoven fabric is not particularly limited, but in order to improve bulkiness and flexibility, the thickness of the nonwoven fabric is preferably 0.5 mm or more, more preferably 1 mm or more, and even more preferably 1.5 mm or more. In order to improve drapeability, make it difficult for liquid to be retained when liquid is passed through the nonwoven fabric, and improve dryness, the thickness is preferably 10 mm or less, more preferably 6 mm or less, and even more preferably 5 mm or less.

[0025] The specific volume of the nonwoven fabric in the present invention is 60 to 95 cm ３ This results in an appropriate density of bonding points between the thermally bondable composite fibers, resulting in a good balance between mechanical properties and permeability to high-viscosity liquids. ３ / g or more, the permeability of high viscosity liquids is improved, and the bulkiness and flexibility are excellent, and the thickness of the fiber is 95 cm ３ / g or less, the mechanical properties are improved. From this viewpoint, the specific volume of the nonwoven fabric is 65 to 90 cm ３ / g, and 70 to 85 cm ３ The specific volume of the nonwoven fabric can be controlled by the temperature, pressure, and time in the consolidation step described below, and the temperature, air velocity, and treatment time of the heat medium in the thermal bonding step.

[0026] The thickness retention rate of the nonwoven fabric under load is not particularly limited, but is preferably 50% or more, and more preferably 75% or more, in order to improve cushioning properties and to maintain the permeability of high-viscosity liquids even when worn (under load) in a diaper, etc. The method for measuring the thickness retention rate under load will be described in detail in the Examples.

[0027] The tensile strength per unit area weight of the nonwoven fabric is not particularly limited, but in order to make the nonwoven fabric less likely to fluff or break when force is applied during processing into diapers or when the nonwoven fabric is worn, it is preferably 1.5 N / 50 mm or more, and more preferably 2.0 N / 50 mm or more.

[0028] The 5% strength per unit area weight of the nonwoven fabric is not particularly limited, but in order to make the nonwoven fabric less likely to stretch when force is applied during processing into diapers, etc., and to improve processing stability, it is preferably 30 mN / 50 mm or more, and more preferably 50 mN / 50 mm or more.

[0029] The mean deviation (MMD) of the coefficient of friction of the nonwoven fabric is not particularly limited, but in order to make the surface of the nonwoven fabric more easily felt, it is preferably 0.0080 or less, more preferably 0.0070 or less, and even more preferably 0.0060 or less. The method for measuring the mean deviation of the coefficient of friction will be described in detail in the Examples.

[0030] The nonwoven fabric may contain fibers other than the thermally adhesive composite fibers described above, provided that the effects of the present invention are not impaired. Examples of fibers other than thermally adhesive composite fibers include, but are not limited to, natural fibers (such as wood fibers), recycled fibers (such as rayon), semi-synthetic fibers (such as acetate), chemical fibers, and synthetic fibers (such as polyester fibers, acrylic fibers, nylon fibers, and vinyl chloride fibers) that do not have thermal adhesive properties (hereinafter referred to as "non-thermally adhesive fibers"). When fibers other than thermally adhesive composite fibers are contained, the proportion of such fibers is not limited as long as the effects of the present invention are not impaired, and can be, for example, 1 to 30% by weight, and preferably 3 to 15% by weight.

[0031] The nonwoven fabric of the present invention may consist of one type of (single-layer) nonwoven fabric, or may consist of two or more types of (multi-layer) nonwoven fabrics differing in fineness, composition, density, etc. In the case of a multi-layer nonwoven fabric, for example, by laminating layers of different fineness, the size of the gaps formed between the fibers can be varied in the thickness direction of the nonwoven fabric, thereby controlling the liquid permeability. The method for producing a multi-layer nonwoven fabric is not particularly limited, but an example is a method in which webs containing thermally adhesive composite fibers differing in fineness, composition, density, etc. are overlapped and subjected to a thermal bonding treatment.

[0032] Furthermore, the nonwoven fabric of the present invention may be laminated with a nonwoven fabric, film, or sheet other than the nonwoven fabric of the present invention, such as, but not limited to, a through-air nonwoven fabric, a spunbond nonwoven fabric, a meltblown nonwoven fabric, a spunlace nonwoven fabric, a needle-punched nonwoven fabric, a film, a mesh, or a net. By laminating, it is possible to control liquid permeability, liquid permeation rate, liquid return, processability, etc. The method for laminating may be, but is not limited to, a method of laminating with an adhesive such as a hot melt, or a method of laminating with thermal bonding such as a through-air method or a hot embossing method.

[0033] The nonwoven fabric of the present invention may be treated with a fiber treatment agent, such as a general hydrophilic, durable hydrophilic, or water-repellent agent, to enhance the permeability of highly viscous liquids. The use of a general hydrophilic fiber treatment agent facilitates the production of a nonwoven fabric with excellent initial liquid permeability, while the use of a durable hydrophilic fiber treatment agent facilitates the production of a nonwoven fabric in which the liquid permeability is less likely to decrease even after repeated liquid passage. Examples of such general hydrophilic and durable hydrophilic fiber treatment agents include those disclosed in Japanese Patent Laid-Open Publication No. 2006-239127. The method for applying the fiber treatment agent to the nonwoven fabric is not particularly limited, and examples include a method using a thermally adhesive composite fiber to which the fiber treatment agent has been applied, and a method for applying the fiber treatment agent to the nonwoven fabric by spraying, immersing, or coating the fiber treatment agent. The amount of the fiber treatment agent applied is not particularly limited, and examples include 0.1 to 3 wt % based on the weight of the nonwoven fabric. If the content is 0.1% by weight or more, the permeability of high-viscosity liquids is easily improved, and if the content is 3% by weight or less, an effect commensurate with the use is easily obtained.

[0034] The nonwoven fabric may be subjected to antistatic treatment, water-repellent treatment, hydrophilic treatment, antibacterial treatment, ultraviolet absorbing treatment, near-infrared absorbing treatment, electret treatment, or the like depending on the purpose, as long as the effects of the present invention are not impaired.

[0035] (Method for manufacturing nonwoven fabric) The method for manufacturing a nonwoven fabric of the present invention includes a step of forming a web containing thermally adhesive conjugate fibers (hereinafter may be referred to as the "web formation step"), a step of consolidating the web (hereinafter may be referred to as the "consolidation step"), and a step of thermally bonding the intersections of the thermally adhesive conjugate fibers without pressure (hereinafter may be referred to as the "thermal bonding step"). According to this method, the intersections of the fibers can be thermally bonded with little variation in density in the thickness direction and so as to achieve a specific specific volume, and a nonwoven fabric can be obtained that has excellent mechanical properties and excellent permeability to high-viscosity liquids.

[0036] (Web Forming Step) The method for forming a web containing thermally bondable conjugate fibers is not particularly limited, and may be a short fiber web forming method using short fibers (staples or chopped), such as a carding method, air-laid method, or wet method, or a long fiber web forming method, such as a spunbonding method, melt-blown method, or tow-spreading method, but from the viewpoint of making it easier to obtain the target specific volume, the carding method or air-laid method is preferred, and the carding method is more preferred. In the present invention, the term "web" refers to a fiber assembly in which the fibers are somewhat entangled, and means a state in which the intersections of the thermally bondable conjugate fibers are not bonded.

[0037] (Consolidation Step) Next, the obtained web is subjected to a consolidation treatment for the purpose of adjusting the specific volume. The consolidation treatment may be performed using a known heat press or calendering machine, and the temperature, pressure, time, etc. may be appropriately adjusted so as to obtain a nonwoven fabric with the desired specific volume. The temperature for the consolidation treatment is not particularly limited, but from the viewpoint of making it easier to obtain the desired specific volume, it is preferable to set the temperature to be equal to or lower than the melting point or softening point of the low-melting point component of the thermally adhesive conjugate fiber, so that the intersections of the thermally adhesive conjugate fiber are not bonded.

[0038] (Thermal Bonding Step) Next, the consolidated web (hereinafter sometimes referred to as the "consolidated web") is heat-treated without pressure to bond the intersections of the thermally bondable composite fibers, thereby obtaining a nonwoven fabric. Here, "without pressure" refers to a state in which the consolidated web is not substantially subjected to pressure greater than atmospheric pressure. A nonwoven fabric can be determined to be "without pressure" if its thickness is 80% or more of the thickness of the consolidated web. The heat medium used in the thermal bonding step is not particularly limited, and examples include hot air or superheated steam. However, from the viewpoint of the mechanical properties and productivity of the nonwoven fabric, it is preferable to use superheated steam. An example of the thermal bonding step is a method in which the consolidated web is introduced into a furnace filled with hot air or superheated steam using a transport conveyor or the like, and a nonwoven fabric is continuously obtained.

[0039] The temperature of the heat medium is not particularly limited, but is preferably equal to or higher than the melting point or softening point of the low-melting-point component constituting the thermally adhesive composite fiber and equal to or lower than the melting point or softening point of the high-melting-point component. The air speed at which the heat medium is blown is not particularly limited, but is preferably 0.5 m / s or less, more preferably 0.3 m / s or less, and even more preferably 0.1 m / s or less, in order to obtain a specific fiber density ratio. The pressure at which the heat medium is blown is not particularly limited, but is preferably 0.1 kPaG or less in order to obtain a specific fiber density ratio. The heat treatment may be carried out under atmospheric pressure, high pressure, low pressure, or vacuum, but is preferably carried out under atmospheric pressure in order to simplify the equipment.

[0040] The treatment time for the thermal bonding step is not particularly limited, but is preferably 60 seconds or less, and more preferably 30 seconds or less. If the treatment time is 60 seconds or less, it is possible to produce a nonwoven fabric with satisfactory productivity.

[0041] The nonwoven fabric of the present invention has excellent liquid permeability for high-viscosity liquids and mechanical properties, and therefore can be suitably used, in particular, as a topsheet for absorbent articles such as diapers, napkins, incontinence pads, pet diapers, and pet sheets.

[0042] The present invention will be described in detail below with reference to examples, but the present invention is not limited thereto. The measurement methods or definitions of the physical properties shown in the examples are as follows. All of the following physical property measurements were carried out after the nonwoven fabric was heat-treated in an oven at 110°C for 5 minutes to reset the stress of the nonwoven fabric.

[0043] <Fiber diameter of thermal adhesive composite fibers> Using a scanning electron microscope (SU-8000, manufactured by Hitachi High-Technologies Corporation), the nonwoven fabric was photographed at a magnification of 100 to 200 times. The fiber diameter of the thermal adhesive composite fibers in the nonwoven fabric was measured using image analysis software (ImageJ). The fiber diameter was taken as the average value when 50 fibers were measured.

[0044] <Fiber Density> (1) Method of Preparing Observation Samples A nonwoven fabric cut into a size of 1 cm (machine direction of the nonwoven fabric; MD) x 3 cm (cross direction of the nonwoven fabric; CD) was impregnated with a photocurable resin (UV-LED Resin Hoshi no Shizuku [Soft], manufactured by PADICO Corporation) and cured by UV irradiation. Next, using a microtome (Leica, RM2265), the cured sample was cut to a thickness of 10 μm in the machine direction, yielding an observation sample measuring 10 μm (machine direction) x 3 cm (CD) x thickness (thickness direction). (2) Measurement of fiber density A sample for observation placed on a glass slide was placed between two polarizing plates arranged in a crossed Nicol position, and an image of the microtome-cut surface of the sample for observation (3 cm (CD) × thickness (thickness direction)) was taken at 30x magnification using a microscope (KEYENCE Corporation, VHX-6000). The number of fibers in the nonwoven fabric was measured using image analysis software (ImageJ) and divided by the area to calculate the fiber density. Specifically, the image was converted to 16 bits using ImageJ, and then binarized to distinguish between fibers and non-fibers. Next, the nonwoven fabric was divided into two equal parts in the thickness direction, and the upper and lower portions were selected (5 mm (CD) × half the thickness of the nonwoven fabric (thickness direction)), and the number of fibers in the selected portions was measured using the "Analyze Particle" function of ImageJ. The obtained number of fibers was divided by the area of ​​the selected part to determine the fiber density (fibers / mm ２ ) was calculated. Here, in the upper and lower parts of the nonwoven fabric, the part with a higher fiber density was designated as the high density part, and the part with a lower fiber density was designated as the low density part. From the obtained fiber densities of each part, the fiber density of the nonwoven fabric was calculated by the following formula. The fiber density of each part was calculated as the average value when measured at two places. Fiber density (threads / mm ２ ) = (fiber density of high density part (fibers / mm ２ ) + fiber density of low density area (fibers / mm ２ ))÷2

[0045] <Fiber density ratio> From the fiber density in each region obtained by the above method, the fiber density ratio was calculated by the following formula: Fiber density ratio = fiber density of high-density region (fibers / mm ２ ) ÷ fiber density of low density area (fibers / mm２ )

[0046] <Basis Weight> The weight of a nonwoven fabric cut into a 150 mm x 150 mm piece was measured, and the value converted to a unit area was used as the basis weight (g / m ２ The basis weight was calculated as the average value of two measurements.

[0047] <Thickness> 0.7 gf / cm was measured using a laser thickness meter (IL-S065, manufactured by KEYENCE Corporation). ２ The thickness (mm) of the nonwoven fabric when a pressure of 68.6 Pa was applied for 5 seconds was taken as the thickness of the nonwoven fabric. The thickness was measured at 5 locations and the average value was taken as the thickness.

[0048] <Specific volume> The basis weight (g / m ２ The specific volume of the nonwoven fabric was calculated from the diameter (cm) and thickness (mm) using the following formula. The larger the specific volume, the higher the bulkiness. ３ / g) = Thickness (mm) ÷ Basis weight (g / m ２ ) x 1000

[0049] <Thickness under load> A laser thickness gauge (IL-S065, manufactured by KEYENCE Co., Ltd.) was used to measure a thickness of 3.5 gf / cm ２ The thickness (mm) of the nonwoven fabric under load was measured at five locations and the average value was used.

[0050] <Thickness Retention Rate Under Load> The thickness retention rate under load was calculated from the thickness (mm) of the nonwoven fabric obtained above and the thickness (mm) under load according to the following formula: Thickness retention rate under load (%) = Thickness under load (mm) / Thickness (mm) × 100

[0051] <Tensile strength and 5% strength> A sample measuring 50 mm in the CD direction and 150 mm in the MD direction was pulled using an autograph (AGX-J, manufactured by Shimadzu Corporation) at a chuck distance of 100 mm and a pulling rate of 100 mm / min. The maximum strength was recorded as the tensile strength (N / 50 mm) of the nonwoven fabric, and the strength at 5% elongation was recorded as the 5% strength (N / 50 mm). The tensile strength and 5% strength were taken as the average values ​​of three measurements.

[0052] <Overall Evaluation of Mechanical Properties> The thickness retention rate under load, tensile strength per unit basis weight, and 5% strength per unit basis weight obtained by the above measurement methods were scored according to the following criteria, and the mechanical properties were overall evaluated based on the total score. (1) Evaluation criteria for thickness retention rate under load Less than 50% ... 1 point 50% or more, less than 75% ... 2 points 75% or more ... 3 points (2) Evaluation criteria for tensile strength per unit basis weight Less than 1.5N / 50mm ... 1 point 1.5N / 50mm or more, less than 2.0 / 50mm ... 2 points 2.0N / 50mm or more ... 3 points (3) Evaluation criteria for 5% strength per unit basis weight Less than 30mN / 50mm ... 1 point 30mN / 50mm or more, less than 50mN / 50mm ... 2 points 50mN / 50mm or more ... 3 points (4) Overall evaluation of mechanical properties Total score of (1) to (3) is 5 points or less ... × Total score of (1) to (3) is 6 to 7 points ... ○ Total score of (1) to (3) is 8 points or more ... ◎

[0053] <High-viscosity liquid permeation time> Artificial menstrual blood of the following composition was prepared, and the initial permeation time, repeated permeation time, and large-volume permeation time were measured. A shorter permeation time indicates higher permeability. Method for preparing artificial menstrual blood: a. 874.7 parts by weight of ion-exchanged water was mixed with 10.0 parts by weight of sodium chloride, 10.7 parts by weight of sodium carbonate, 100.0 parts by weight of glycerin, and 4.6 parts by weight of carboxymethyl cellulose, in that order, and stirred at room temperature to prepare artificial menstrual blood (viscosity 14 to 18 cP). When the viscosity of the artificial menstrual blood was outside the range of 14 to 18 cP, the viscosity was adjusted by adding ion-exchanged water and carboxymethyl cellulose as appropriate. (2) Measurement method of initial liquid passage time and repeated liquid passage time A nonwoven fabric cut to 100 mm x 100 mm was placed on a 90 mm x 90 mm absorbent (two layers of Kimtowel (trade name; Kimtowel is a registered trademark) manufactured by Nippon Paper Crecia). A liquid passage plate (a SUS plate weighing 450 g, measuring 70 mm x 70 mm x 12 mm, with a central opening of 27 mm diameter) was then placed on the nonwoven fabric. Next, 3 mL of artificial menstrual blood was dropped near the center of the opening of the liquid passage plate, at a height of 10 mm from the surface of the nonwoven fabric, and the time until the artificial menstrual blood disappeared from the surface of the nonwoven fabric (initial liquid passage time) was measured. After allowing to stand for 3 minutes after measuring the initial liquid passage time, 3 mL of artificial menstrual blood was dropped again, and the time until the artificial menstrual blood disappeared from the surface of the nonwoven fabric (repeated liquid passage time) was measured. (3) Method for measuring large-volume flow time The time until the artificial menstrual blood disappeared from the surface of the nonwoven fabric (large-volume flow time) was measured in the same manner as in the measurement of the initial flow time, except that the amount of artificial menstrual blood dropped was 6 mL.

[0054] <Overall evaluation of high-viscosity liquid permeability> The initial liquid passage time, repeated liquid passage time, and large-volume liquid passage time obtained by the above measurement methods were scored according to the following criteria, and the liquid permeability was evaluated overall based on the total score. (1) Evaluation criteria for initial liquid passage time 8 seconds or more: 1 point 5 seconds or more but less than 8 seconds: 2 points Less than 5 seconds: 3 points (2) Evaluation criteria for repeated liquid passage time 60 seconds or more: 1 point 30 seconds or more but less than 60 seconds: 2 points Less than 30 seconds: 3 points (3) Evaluation criteria for large-volume liquid passage time 30 seconds or more: 1 point 15 seconds or more but less than 30 seconds: 2 points Less than 15 seconds: 3 points (4) Overall evaluation of liquid permeability Total score of (1) to (3) is 5 points or less: × Total score of (1) to (3) is 6 to 7 points: ○ Total score of (1) to (3) is 8 points or more: ◎

[0055] <Mean deviation of coefficient of friction (MMD)> Using a surface tester (KES-FB4, manufactured by Kato Tech Co., Ltd.), the mean deviation of coefficient of friction was measured as follows. The surface of the nonwoven fabric on the high density side was used as the measurement surface, and a pressure of 10 gf / cm was applied using a contact. ２ The average deviation of the friction coefficient was measured by moving the contactor in the MD direction with a load of 0.98 kPaG (0.98 kPaG) applied, with SENS (sensitivity): High and a contactor speed: 1.0 mm / sec. The average deviation of the friction coefficient was taken as the average value of 10 measurements. The smaller the average deviation of the friction coefficient, the smoother the surface.

[0056] [Example 1] A thermally adhesive composite fiber was prepared by using a polyethylene terephthalate core (intrinsic viscosity (measured using an equal weight mixed solvent of phenol and tetrachloroethane at a concentration of 0.5 g / 100 ml and a temperature of 20°C): 0.65 dl / g, melting point 250°C) and a high density polyethylene sheath (density: 0.956 g / cm ３A concentric sheath-core composite fiber having a fineness of 1.7 dtex and a fiber length of 45 mm and a volume ratio of 50 / 50 was prepared, the fiber having a melt flow rate (190°C, load 21.18N): 16 g / 10 min, and a melting point of 130°C (0.4 wt% of the fiber weight was treated with a durable hydrophilic fiber treatment agent). The fiber was then sandwiched between kraft paper sheets and pressed in a heat press at a temperature of 90°C and a pressure of 350 gf / cm. ２ The consolidation treatment was carried out under the conditions of (34 kPaG) for 1 second. The obtained consolidated web was introduced into an oven filled with superheated steam at 180°C for 10 seconds to thermally bond the intersections of the thermally bondable composite fibers, thereby obtaining a nonwoven fabric. The wind speed of the superheated steam was less than 0.1 m / s.

[0057] Example 2 A nonwoven fabric was obtained in the same manner as in Example 1, except that the fineness of the thermal adhesive conjugate fiber was changed to 2.2 dtex and the fiber length was changed to 51 mm.

[0058] Example 3 A nonwoven fabric was obtained in the same manner as in Example 1, except that the fineness of the thermal adhesive conjugate fiber was changed to 4.4 dtex and the fiber length was changed to 51 mm.

[0059] [Comparative Example 1] A web obtained in the same manner as in Example 1 was introduced for 10 seconds into an oven filled with superheated steam at 180°C without being subjected to a consolidation treatment, and the intersections of the thermally bondable conjugate fibers were thermally bonded to obtain a nonwoven fabric. The wind speed of the superheated steam was less than 0.1 m / s.

[0060] [Comparative Example 2] A web obtained in the same manner as in Example 1 was treated for 10 seconds with hot air at 130°C and a circulating air speed of 1.0 m / sec in a hot air circulation dryer without being subjected to a consolidation treatment, thereby thermally bonding the intersections of the thermally adhesive composite fibers to obtain a nonwoven fabric.

[0061] Example 4 A nonwoven fabric was obtained in the same manner as in Example 1, except that the properties of the fiber treating agent applied to the thermal adhesive conjugate fiber were changed to general hydrophilicity.

[0062] Example 5 A nonwoven fabric was obtained in the same manner as in Example 4, except that the fineness of the thermal adhesive conjugate fiber was changed to 0.9 dtex and the fiber length was changed to 38 mm.

[0063] Example 6 A nonwoven fabric was obtained in the same manner as in Example 4, except that the fineness of the thermal adhesive conjugate fiber was changed to 0.7 dtex and the fiber length was changed to 38 mm.

[0064] [Example 7] Thermal adhesive composite fiber 1 was prepared by using a polyethylene terephthalate core (intrinsic viscosity (measured using an equal weight mixed solvent of phenol and tetrachloroethane at a concentration of 0.5 g / 100 ml and a temperature of 20°C): 0.65 dl / g, melting point 250°C) and a high density polyethylene sheath (density: 0.956 g / cm ３ Concentric sheath-core composite fibers having a fineness of 1.7 dtex and a fiber length of 45 mm, in which the same polyethylene terephthalate as in the thermal adhesive composite fiber 1 was used for the core and the same high-density polyethylene as in the thermal adhesive composite fiber 1 was used for the sheath in a volume ratio of 50 / 50, were prepared, and a web 1 made of the thermal adhesive composite fiber 2 was produced by a carding method. Concentric sheath-core composite fibers having a fineness of 4.4 dtex and a fiber length of 51 mm, in which the same polyethylene terephthalate as in the thermal adhesive composite fiber 1 was used for the core and the same high-density polyethylene as in the thermal adhesive composite fiber 1 was used for the sheath in a volume ratio of 50 / 50, were prepared, and a web 2 made of the thermal adhesive composite fiber 2 was produced by a carding method. Next, the multilayer web obtained by superposing the web 2 on the web 1 is sandwiched between kraft paper sheets, and then heated at a temperature of 90°C and a pressure of 350 gf / cm using a heat press. ２ The resulting consolidated web was placed in a furnace filled with superheated steam at 180°C for 10 seconds to thermally bond the intersections of the thermally adhesive composite fibers, thereby obtaining a multilayer nonwoven fabric in which the second nonwoven fabric was laminated on the first nonwoven fabric. The air velocity of the superheated steam was less than 0.1 m / s.

[0065] The physical properties of the nonwoven fabrics obtained in Examples 1 to 3 and Comparative Examples 1 and 2 are shown in Table 1.

[0066]

[0067] The physical properties of the nonwoven fabrics obtained in Examples 4 to 7 are shown in Table 2.

[0068]

[0069] As can be seen from Tables 1 and 2, Examples 1 to 7, which satisfy the constituent requirements of the present invention, were excellent in mechanical properties and permeability to high-viscosity liquids. Among them, Examples 1, 4, 5, 6 and 7, which used thermally adhesive composite fibers with small fiber diameters, were excellent in smoothness in addition to excellent mechanical properties and permeability to high-viscosity liquids. On the other hand, ３ / g, Comparative Example 1 had excellent permeability to high-viscosity liquids but insufficient mechanical properties. Also, Comparative Example 2, in which the fiber density ratio exceeded 1.4, had excellent mechanical properties but insufficient permeability to high-viscosity liquids.

[0070] The nonwoven fabric of the present invention has excellent liquid permeability for high-viscosity liquids and mechanical properties, and therefore can be suitably used, in particular, as a topsheet for absorbent articles such as diapers, napkins, incontinence pads, pet diapers, and pet sheets.

Claims

1. A nonwoven fabric containing thermally adhesive composite fibers whose intersections are thermally bonded, the specific volume of the nonwoven fabric being 60 to 95 cm ３ / g, and a fiber density ratio of a high density portion to a low density portion in the nonwoven fabric is 1.4 or less.

2. The fiber density of the nonwoven fabric is 10 to 50 fibers / mm ２ The nonwoven fabric of claim 1 .

3. The nonwoven fabric according to claim 1 or 2, wherein the fiber diameter of the thermally adhesive composite fiber is 5 to 20 μm.

4. A method for producing a nonwoven fabric, comprising the steps of forming a web containing thermally bondable composite fibers, consolidating the web, and thermally bonding intersections of the thermally bondable composite fibers without pressure.

5. A top sheet for absorbent articles using the nonwoven fabric according to claim 1 or 2.

6. An absorbent article using the nonwoven fabric according to claim 1 or 2.

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