JP7676819B2 - Spunbond nonwoven fabric - Google Patents

Description

The present invention relates to a spunbond nonwoven fabric.

In general, nonwoven fabrics for sanitary materials such as disposable diapers and sanitary napkins are required to have texture, feel, flexibility, and high productivity. Furthermore, in recent years, there has been a demand for nonwoven fabrics with high uniformity to ensure processing stability during ultrasonic bonding, which is often used in the manufacturing process of disposable diapers and sanitary napkins. In addition, in recent years, there has been a demand for nonwoven fabrics with high whiteness to prevent nonwoven fabrics from being shiny or see-through. In addition, there is a strong demand for environmental consideration in nonwoven fabrics for sanitary materials, which are mainly used for disposable purposes, and from the perspective of zero emissions, there is an increasing demand for low basis weight products in order to reduce material usage. Furthermore, when used in applications where items are expected to be used in a wet state, such as in swimming pool diapers, medical gowns, and chemical protective clothing, or when washing masks and other items, some of the fibers can fall off due to friction and flow into rivers, lakes, and oceans via wastewater. These fibers do not decompose easily in the natural environment, and this can lead to the so-called microplastic problem, which could have a significant impact on aquatic life. As a result, there is a growing demand for nonwoven fabrics that shed less fibers.

For example, Patent Documents 1 and 2 propose highly uniform nonwoven fabrics in which a resin with a relatively high melt flow rate (hereinafter sometimes abbreviated as MFR) is used and the fiber diameter of the fibers that make up the nonwoven fabric is narrowed.

On the other hand, as a nonwoven fabric with high whiteness, for example, Patent Document 3 proposes a nonwoven fabric to which titanium oxide particles have been added.

International Publication No. 2007/091444 JP 2011-099196 A WO 97/049853

However, the technology disclosed in Patent Document 1 is a technology that sets the single fiber fineness to 1.5 denier or less by setting the draft ratio, calculated from the hole diameter and fiber diameter, to 1500 or more. In this technology, a raw material with a large MFR, i.e., low viscosity, is spun using a nozzle with a large hole diameter. As a result, it is difficult to apply nozzle pressure, and uniform spinning is not possible, which makes it easy for thread breakage and uneven fiber diameter to occur, making it difficult to obtain a stable, uniform nonwoven fabric, and there is no mention of environmental considerations.

Patent Document 2 also describes a technology that uses a resin containing polypropylene with an MFR of 100 to 2000 g/10 min, but the fiber diameter shown in the examples is very large at 19 μm or more, and it is not possible to obtain a uniform nonwoven fabric. There is also no mention of environmental considerations.

As a result of further investigations, the inventors have found that a nonwoven fabric with a very high degree of whiteness can be obtained by increasing the number of fibers per unit area to increase the fiber surface area, or by adding pigment particles such as titanium oxide, as disclosed in Patent Document 3, to the raw material with a relatively high melt flow rate, as described in Patent Documents 1 and 2.

However, it was found that as the basis weight of a nonwoven fabric is reduced, unevenness in whiteness becomes evident between areas of the nonwoven fabric where the fibers are dense and areas where they are sparse. It was found that such nonwoven fabrics have a high whiteness, but because of the high whiteness, the unevenness in appearance is noticeable, making it difficult to obtain a nonwoven fabric with a sufficiently luxurious feel. In addition, nonwoven fabrics with sparse and low uniformity fibers suffer from a large amount of weight loss due to abrasion when wet, and a large amount of fiber fall off in a humid environment.

In view of the above problems, the object of the present invention is to provide an environmentally friendly spunbond nonwoven fabric that has a low basis weight and high whiteness, excellent whiteness uniformity, and little fiber shedding when wet.

As a result of intensive research conducted by the inventors to solve the above problems, it was discovered that by reducing the average fiber diameter of the fibers that make up the spunbond nonwoven fabric and dispersing the positions through which the drawing yarn passes in the spunbond spinning drawing section to prevent bias in the yarn path, the uniformity of the nonwoven fabric is improved, and even when a nonwoven fabric with high whiteness is reduced in basis weight, excellent uniformity, i.e., whiteness unevenness is small, and as a result, a good appearance can be maintained.

The present invention has been completed based on these findings and has the following configuration: That is, the spunbond nonwoven fabric of the present invention is a spunbond nonwoven fabric made of fibers made of a polyolefin resin, the fibers having an average single fiber diameter of 6.5 μm to 14.5 μm, the basis weight of the spunbond nonwoven fabric being 8 g/ ^m2 to ³⁰ g/m2, the whiteness of the spunbond nonwoven fabric being 40% to 95%, and the whiteness unevenness being 2% to 6%.

According to a preferred embodiment of the spunbond nonwoven fabric of the present invention, the melt flow rate of the polyolefin resin is 155 g/10 min or more and 500 g/10 min or less.

According to a preferred embodiment of the spunbond nonwoven fabric of the present invention, the weight loss due to wet friction is 1 mg or more and 30 mg or less.

The method for producing the spunbond nonwoven fabric of the present invention uses a polyolefin resin with a melt flow rate of 155 to 500 g/10 min, extrudes the resin from a spinneret, cools it to solidify it, and pulls and stretches it in an ejector at a spinning speed of 3,500 m/min to 6,500 m/min, with the spinning speed varying by 1.5 to 3% from the average spinning speed, to obtain long fibers. The long fibers are collected on a moving net to form a nonwoven web, which is then thermally bonded.

According to the present invention, a spunbond nonwoven fabric can be obtained that has a low basis weight and high whiteness, excellent whiteness uniformity, and is also environmentally friendly with minimal fiber shedding when wet.

The spunbond nonwoven fabric of the present invention is a spunbond nonwoven fabric made of fibers made of a polyolefin resin, wherein the average single fiber diameter of the fibers is 6.5 μm or more and 14.5 μm or less, the basis weight of the spunbond nonwoven fabric is 8 g/ ^m2 or more and 30 g/ ^m2 or less, the whiteness of the spunbond nonwoven fabric is 40% or more and 95% or less, and the whiteness unevenness is 2% or more and 6% or less.

The details of this are explained below.

[Polyolefin resin]
Examples of the polyolefin resin used in the present invention include polypropylene resin and polyethylene resin.

Examples of polypropylene-based resins include homopolymers of propylene and copolymers of propylene and various α-olefins. Examples of polyethylene-based resins include homopolymers of ethylene and copolymers of ethylene and various α-olefins. Among these, polypropylene-based resins are preferably used because of their spinnability and strength properties.

In addition, the polyolefin resin used in the present invention may be a mixture of two or more polyolefin resins. Furthermore, as long as the effects of the present invention are not impaired, a resin mixed with a resin composition containing other thermoplastic resins or thermoplastic elastomers can also be used as the polyolefin resin.

The melting point of the polyolefin resin used in the present invention is preferably 80°C or higher and 200°C or lower. By making the melting point of the polyolefin resin 80°C or higher, more preferably 100°C or higher, the heat resistance of the spunbond nonwoven fabric can be improved and the heat resistance can be made more suitable for practical use. On the other hand, by making the melting point of the polyolefin resin 200°C or lower, more preferably 180°C or lower, it is possible to easily cool the yarn discharged from the spinneret, and it is possible to suppress fusion between the fibers and facilitate stable spinning.

In the present invention, the MFR of the polyolefin resin is preferably 155 g/10 min or more and 500 g/10 min or less. By setting the MFR of the polyolefin resin to 155 g/10 min or more, even if the fiber is stretched at a high spinning speed to increase productivity, the viscosity is low, so that the fiber can easily follow the deformation, enabling stable spinning. Furthermore, since it is possible to stretch at a high spinning speed, the crystal orientation of the fibers constituting the nonwoven fabric can be adjusted, and a fiber with high mechanical strength and a spunbonded nonwoven fabric can be obtained. On the other hand, by setting the MFR of the polyolefin resin to 500 g/10 min or less, more preferably 400 g/10 min or less, and even more preferably 300 g/10 min or less, the decrease in strength due to low viscosity can be suppressed.

In the present invention, MFR refers to the value measured using an extrusion plastometer in accordance with ASTM D1238 under conditions of a load of 2.16 kg and a measurement temperature of 230°C.

The polyolefin resin used in the present invention may contain additives such as antioxidants, weather stabilizers, light stabilizers, antistatic agents, spinning agents, antiblocking agents, lubricants, nucleating agents, pigment particles, and other polymers as necessary, within the scope of the invention.

The spunbond nonwoven fabric of the present invention may contain a pigment such as titanium oxide particles in all or part of the polyolefin resin, and more preferably contains it only in the core component. However, unless otherwise specified, the "particles" of the pigment particles in the present invention refer to those having a number average particle size of 0.04 μm or more.

Here, the particles used as pigments are not particularly limited, but examples include inorganic particles and organic particles. Inorganic particles include wet and dry silica, colloidal silica, aluminum silicate, calcium carbonate, calcium phosphate, aluminum oxide, titanium oxide, barium sulfate, zinc oxide, zinc sulfate, and the like. Organic particles include particles containing styrene, silicone, acrylic acids, methacrylic acids, polyesters, and the like as constituents. Among these, it is preferable to use titanium oxide particles from the standpoint of appearance and economic efficiency. Two or more of these particles may be used in combination.

In order to improve the flexibility of the spunbonded nonwoven fabric, the polyolefin resin used in the present invention preferably contains a fatty acid amide compound having 23 to 50 carbon atoms. It is known that the speed at which the fatty acid amide compound moves to the fiber surface changes depending on the carbon number of the fatty acid amide compound mixed into the polyolefin resin. By making the carbon number of the fatty acid amide compound preferably 23 or more, more preferably 30 or more, it is possible to suppress the fatty acid amide compound from appearing excessively on the fiber surface, and to maintain excellent spinnability and processing stability and high productivity. In addition, by making the carbon number of the fatty acid amide compound preferably 50 or less, more preferably 42 or less, it is possible to easily make the fatty acid amide compound appear on the fiber surface, and to impart slipperiness and flexibility suitable for high-speed production of spunbonded nonwoven fabric.

Examples of fatty acid amide compounds having 23 to 50 carbon atoms include saturated fatty acid monoamide compounds, saturated fatty acid diamide compounds, unsaturated fatty acid monoamide compounds, and unsaturated fatty acid diamide compounds. Specifically, examples of fatty acid amide compounds having 23 to 50 carbon atoms include tetradocosanoic acid amide, hexadocosanoic acid amide, octadocosanoic acid amide, nervonic acid amide, tetracosane pentane amide, nisinic acid amide, ethylene bislauric acid amide, methylene bislauric acid amide, ethylene bisstearic acid amide, ethylene bishydroxystearic acid amide, ethylene bisbehenic acid amide, hexamethylene bisstearic acid amide, hexamethylene bisbehenic acid amide, hexamethylene hydroxystearic acid amide, distearyl adipic acid amide, distearyl sebacic acid amide, ethylene bisoleic acid amide, ethylene biserucic acid amide, and hexamethylene bisoleic acid amide, and these can also be used in combination.

In the present invention, among these fatty acid amide compounds, ethylene bisstearamide, which is a saturated fatty acid diamide compound, is preferably used. Ethylene bisstearamide has excellent thermal stability and is therefore not easily decomposed even when melt-spinning is performed. Therefore, by using fibers made of a polyolefin resin containing this ethylene bisstearamide, it becomes easier to obtain a spunbond nonwoven fabric that is excellent in flexibility while maintaining high productivity.

In the present invention, the content of the fatty acid amide compound in the polyolefin resin is preferably 0.01% by mass or more and 5.00% by mass or less. By making the content of the fatty acid amide compound 0.01% by mass or more, more preferably 0.10% by mass or more, a lubricant effect is exerted, and it is easy to impart a smooth touch and flexibility. On the other hand, by making it 5.00% by mass or less, more preferably 3.00% by mass or less, and even more preferably 1% by mass or less, it is easy to suppress deterioration of uniformity due to the blowing away of fibers during spraying.

The content of fatty acid amide compound here refers to the mass percentage of fatty acid amide compound contained in the polyolefin resin. For example, even if fatty acid amide compound is contained only in the sheath component constituting the core-sheath composite fiber, the content ratio to the total amount of the core-sheath component is calculated.

[fiber]
The spunbond nonwoven fabric of the present invention is composed of fibers made of the above-mentioned polyolefin resin.

It is important that the average single fiber diameter of this fiber is 6.5 μm or more and 14.5 μm or less. By making the average single fiber diameter of the polyolefin fiber 6.5 μm or more, preferably 7.5 μm or more, and more preferably 8.4 μm or more, a nonwoven fabric with high uniformity that can be stably produced can be obtained. On the other hand, by making the average single fiber diameter of the polyolefin fiber 14.5 μm or less, preferably 13.5 μm or less, and more preferably 11.8 μm or less, a high level of uniformity can be achieved.

The method for measuring the average single fiber diameter in the present invention is as described below.

Moreover, it is more preferable that the number of exposed pigment particles on the fiber surface of the above-mentioned fiber is 0 (pieces/100 fibers) or more and 10 (pieces/100 fibers) or less. By making the number of exposed pigment particles on the fiber surface 0 (pieces/100 fibers) or more and 10 (pieces/100 fibers) or less, a nonwoven fabric having good thermal adhesion, excellent strength, and a smooth fiber surface with an excellent texture and a good feel can be obtained. Furthermore, by making the number of exposed pigment particles on the fiber surface 10 (pieces/100 fibers) or less, preferably 5 (pieces/100 fibers) or less, and more preferably 3 (pieces/100 fibers) or less, it is possible to prevent the exposed pigment particles on the fiber surface from scratching metal rolls and the like used in the manufacturing process, and also to obtain a nonwoven fabric having excellent strength without inhibiting thermal adhesion.

The number of exposed pigment particles on the fiber surface is determined by randomly taking 10 or more small sample pieces from the nonwoven fabric, taking surface photographs at 2000x magnification using a scanning electron microscope (for example, Keyence Corporation's "VHX-D500"), observing fiber surface images with fiber lengths of 50 μm or more from each sample until a total of 100 fibers are observed, and counting the number of pigment particles with a particle diameter of 0.1 μm or more exposed on the surface of each fiber.

[Spunbond nonwoven fabric]
The spunbond nonwoven fabric of the present invention is made of the above-mentioned fibers.

The basis weight of the spunbonded nonwoven fabric of the present invention is 8 g/ ^m2 or more and 30 g/ ^m2 or less. By setting the basis weight to 10 g/ ^m2 or more, more preferably 15 g/ ^m2 or more, a spunbonded nonwoven fabric having high mechanical strength can be obtained. On the other hand, when the nonwoven fabric is used for sanitary materials, by setting the basis weight to 25 g/m2 or ^less , more preferably 20 g/m2 or ^less , and even more preferably ¹⁵ g/m2 or less, a spunbonded nonwoven fabric having appropriate flexibility suitable for sanitary materials can be obtained.

In the present invention, the basis weight of a spunbond nonwoven fabric refers to a value calculated based on "6.2 Mass per unit area" of JIS L1913:2010 "Testing methods for general nonwoven fabrics," in which three test pieces of 20 cm x 25 cm are taken per meter of sample width, and the mass (g) of each piece is measured under standard conditions, and the arithmetic average value is expressed as mass per ^m2 (g/ ^m2 ).

It is important that the whiteness of the spunbond nonwoven fabric of the present invention is 40% or more and 95% or less. By making the whiteness 40% or more, preferably 50% or more, and more preferably 70% or more, the shine and transparency of the spunbond nonwoven fabric can be eliminated, making it particularly suitable for use as a sanitary material. On the other hand, by making the whiteness 95% or less, the exposure of pigment particles on the fiber surface can be suppressed.

For example, the whiteness can be adjusted to the above range by decreasing the average fiber diameter of the fibers constituting the spunbonded nonwoven fabric layer, thereby increasing the surface area of the spunbonded nonwoven fabric and increasing the amount of reflected light. Whiteness can also be increased by increasing the content of pigment particles in the polyolefin resin. Furthermore, whiteness can be increased by reducing the heat-pressing rate, the heat-pressing temperature, and the line pressure during heat-pressing as heat-pressing conditions.

The spunbond nonwoven fabric of the present invention has a low basis weight so that it is suitable for use in sanitary materials. Therefore, it is important that the whiteness unevenness is 2% or more and 6% or less. The whiteness unevenness is 6% or less, preferably 5% or less, and more preferably 3% or less. By keeping the whiteness unevenness within the above range, a spunbond nonwoven fabric with an excellent appearance and a luxurious feel can be obtained despite its low basis weight. A whiteness unevenness of 2% or more is preferable because it can suppress slipping on the rolls during processing of the nonwoven fabric.

As a means for keeping the whiteness unevenness within the above range, for example, the whiteness unevenness can be reduced by reducing the average fiber diameter of the fibers that make up the spunbonded nonwoven fabric layer. Also, whiteness unevenness can be reduced by not excessively incorporating pigment particles into the polyolefin resin. Furthermore, whiteness unevenness can be reduced by reducing the heat bonding condition by reducing the bonding rate, the heat bonding temperature, and the line pressure during heat bonding. Whiteness unevenness can also be reduced by a method of appropriately varying the spinning speed, which will be described later.

The method for measuring whiteness and whiteness unevenness is described below.

The MFR of the spunbonded nonwoven fabric of the present invention is preferably in the range of 155 g/10 min or more and 500 g/10 min or less. By making the MFR of the spunbonded nonwoven fabric 155 g/10 min or more, even if the fibers constituting the nonwoven fabric are stretched at a high spinning speed to increase productivity, the viscosity is low, so that the fibers can easily follow the deformation, enabling stable spinning. Furthermore, since it is possible to stretch the fibers at a high spinning speed, the crystal orientation of the fibers constituting the nonwoven fabric can be adjusted, and the fibers and spunbonded nonwoven fabric can be made to have high mechanical strength. On the other hand, by making the MFR of the polyolefin resin 500 g/10 min or less, more preferably 400 g/10 min or less, and even more preferably 300 g/10 min or less, it is possible to suppress the decrease in strength due to low viscosity. The method for measuring the MFR of the spunbonded nonwoven fabric is also measured in the same manner as the method for measuring the MFR of the polyolefin resin described above.

The tensile strength in the MD direction per unit basis weight of the spunbonded nonwoven fabric of the present invention is preferably 1.0 (N/2.5 cm)/(g/m ² ) or more. By setting the tensile strength in the MD direction per unit basis weight to 1.0 (N/2.5 cm)/(g/m ² ) or more, preferably 1.1 (N/2.5 cm)/(g/m ² ) or more, more preferably 1.2 (N/2.5 cm)/(g/m ² ) or more, the fabric can withstand the process of manufacturing paper diapers and the like and can be used as a product. In addition, the upper limit is preferably 2.0 (N/2.5 cm)/(g/m ² ) or less, since if it is too high, flexibility may be impaired. The tensile strength in the MD direction is mainly adjusted by the fiber spinning speed, embossing roll compression ratio, temperature, line pressure, etc. In the present invention, the MD direction refers to the direction in which a machine runs when producing a nonwoven fabric.

Here, the tensile strength in the MD direction per unit basis weight of the spunbond nonwoven fabric is calculated by performing a tensile test at three points in the MD direction under the conditions of a sample size of 2.5 cm wide x 30 cm, a gripping distance of 20 cm, and a pulling speed of 10 cm/min in accordance with "6.3.1 Standard Time" in "6.3 Tensile Strength and Elongation (ISO Method)" of JIS L1913:2010 "Testing Methods for General Nonwoven Fabrics", and rounding off the first decimal place to the nearest whole number to calculate the tensile strength per unit basis weight from the calculated tensile strength (N/2.5 cm) and the basis weight (g/ ^m2 ) obtained above using the following formula, rounding off to the nearest whole number to the nearest whole number.
Tensile strength in the MD direction per unit basis weight=tensile strength in the MD direction (N/2.5 cm)/basis weight (g/m ² ).

The spunbond nonwoven fabric of the present invention preferably has a weight loss due to wet friction of 1 mg or more and 30 mg or less. By setting the weight loss due to wet friction to 30 mg or less, more preferably 15 mg or less, and even more preferably 5 mg or less, it is possible to reduce the amount of fallen fibers that flow into rivers. On the other hand, if the weight loss due to wet friction is less than 1 mg, the surface of the nonwoven fabric becomes hard and it is difficult for the fabric to feel soft against the skin of the user.

As a means for keeping the weight loss due to wet friction within the above range, for example, there is a method for reducing weight loss due to wet friction by increasing the uniformity of the sheet, as described below. As a means for increasing the uniformity of the sheet, for example, there is a method for varying the spinning speed within a specific range from the average spinning speed when producing a spunbond nonwoven fabric, as described below.

[Method of manufacturing spunbond nonwoven fabric]
Next, a method for producing the spunbonded nonwoven fabric of the present invention will be described.

The method for producing the spunbonded nonwoven fabric of the present invention uses a polyolefin resin with a melt flow rate of 155 to 500 g/10 min, extrudes the resin from a spinneret, cools it to solidify it, and pulls and stretches it in an ejector at a spinning speed of 3,500 m/min to 6,500 m/min, with the spinning speed varying by 1.5 to 3% from the average spinning speed, to obtain long fibers. The long fibers are collected on a moving net to form a nonwoven web, which is then thermally bonded. That is, the method for producing the spunbonded nonwoven fabric of the present invention includes the following steps (A) to (D). Each step is described below.

(Process (A))
In the method for producing a spunbonded nonwoven fabric of the present invention, a polyolefin resin (P1) having an MFR of 155 g/10 min or more and 500 g/10 min or less is used.

In the method for producing a spunbonded nonwoven fabric of the present invention, it is preferable that the polyolefin resin (P1) contains pigment particles. In particular, it is more preferable that the pigment particles are titanium oxide. Specific examples of pigments and the reasons why they are preferable are as described above.

In the method for producing the spunbonded nonwoven fabric of the present invention, the content of the pigment particles is preferably 0.05 to 5.00% by mass in 100% by mass of the polyolefin resin (P1). The content of the pigment particles can be adjusted appropriately according to the desired whiteness, but a high whiteness value can be achieved by setting the content of the pigment particles to 0.05% by mass or more, more preferably 0.30% by mass or more, and even more preferably 0.50% by mass or more. On the other hand, high levels of spinnability, whiteness, and strength can be achieved by setting the content of the pigment particles to 5.00% by mass or less, more preferably 3.00% by mass or less, and even more preferably 2.00% by mass or less.

When polyolefin resins containing pigment particles are used in core-sheath composite fibers as described below, they are preferably used as the core component.

(Process (B))
In the method for producing a spunbonded nonwoven fabric of the present invention, the fibers are formed by melting and weighing in an extruder, supplying the fibers to a spinneret, and spinning out threads. In the present invention, a composite spinneret may be used, and the core component may be the polyolefin-based resin (P1), and the sheath component may be a polyolefin-based resin (P2) having a different composition from P1 to form a sheath-core composite fiber. To obtain a sheath-core composite fiber, the core component and the sheath component are melted and weighed in separate extruders, supplied to a composite spinneret, and spun out as a sheath-core composite fiber.

In this process, the polyolefin resin from step (A) is discharged from the spinneret, cooled and solidified, and then pulled and stretched by an ejector at a spinning speed of 3,500 m/min to 6,500 m/min to obtain long fibers.

The spinneret and ejector can be of various shapes, including round and rectangular. Among them, the combination of a rectangular spinneret and a rectangular ejector is preferred because it uses a relatively small amount of compressed air and is less likely to cause fusion or abrasion between the yarns.

In the present invention, the spinning temperature when melting and spinning the polyolefin resin is preferably 200°C or higher and 270°C or lower. By setting the spinning temperature to 200°C or higher, more preferably 210°C or higher, and even more preferably 220°C or higher, a stable molten state can be achieved, and excellent spinning stability can be obtained. On the other hand, by setting the spinning temperature to 270°C or lower, more preferably 260°C or lower, and even more preferably 250°C or lower, it is possible to easily cool the yarn discharged from the spinneret, and it is possible to suppress fusion between the fibers and facilitate stable spinning.

The hole diameter of the spinneret is not particularly specified, but since the polyolefin resin used in the present invention has a relatively high MFR, the hole diameter is preferably 0.5 mm or less, more preferably 0.4 mm or less, and even more preferably 0.3 mm or less. If a thin fiber is spun with a spinneret having a large hole diameter, it is not preferable because it is difficult to apply spinneret back pressure, which causes fiber unevenness due to poor discharge, non-uniformity of the texture (uneven thickness), and even yarn breakage. A preferable embodiment is one in which the nozzle diameter is less than 1500 in the following relationship between the nozzle diameter and the average single fiber diameter.
(Nozzle diameter (mm ² ))/(Average single fiber diameter (mm ² ))<1500
Next, the spun long fiber yarn is cooled. Methods for cooling the spun yarn include, for example, forcibly blowing cold air onto the yarn, naturally cooling at the ambient temperature around the yarn, and adjusting the distance between the spinneret and the ejector. Alternatively, a combination of these methods can be used. The cooling conditions can be appropriately adjusted in consideration of the discharge rate per hole of the spinneret, the spinning temperature, the ambient temperature, and the like.

Next, the cooled and solidified yarn is pulled and stretched by compressed air ejected from the ejector.

The spinning speed is preferably 3500 m/min or more and 6500 m/min or less. By setting the spinning speed to 3500 or more, more preferably 4000 m/min or more, high productivity is achieved, and the orientation and crystallization of the fibers progresses, allowing long fibers with high strength to be obtained. As a result, nonwoven fabrics made of high-strength fibers also have excellent strength. On the other hand, by setting the spinning speed to 6500 m/min or less, spinning can be carried out stably.

By increasing the pulling force of the ejector on the yarn and increasing the spinning speed, it is possible to obtain finer, stronger fibers. On the other hand, if the pulling force of the ejector on the yarn is too strong, the position through which the yarn passes inside the ejector tends to be uneven, and the resulting sheet may not be sufficiently uniform.

In the manufacturing method of the spunbonded nonwoven fabric of the present invention, the spinning speed is made to vary by 1.5 to 3% from the average value of the spinning speed. The variation here refers to the variation of the spinning speed within one minute. By making the variation of the spinning speed 1.5% or more, the positions through which the yarn passes in the ejector can be appropriately dispersed, thereby making it possible to further improve the uniformity of the sheet, reduce unevenness in whiteness, and easily obtain a sheet with excellent appearance and defect visibility. Furthermore, the high uniformity of the sheet reduces surface irregularities and can suppress fiber shedding during wet friction. The variation of the spinning speed is more preferably 2.0% or more, and even more preferably 2.5% or more. In addition, by making the variation of the spinning speed 3% or less, it is possible to prevent thread breakage caused by deterioration of spinnability due to variation of the spinning speed. As a means for making the spinning speed vary within the above range, for example, the ejector pressure for traction can be periodically varied.

Here, the spinning speed is determined by calculating the spinning speed based on the following formula using the single fiber fineness (dtex) calculated by rounding off to one decimal place from the average single fiber diameter and the solid density of the resin used to determine the mass per 10,000 m of length, and the output rate of the resin extruded from a single hole of the spinneret set under each condition (hereinafter abbreviated as single hole output rate, in g/min).
Spinning speed (m/min) = (10,000 x single hole output (g/min)) / single fiber fineness (dtex)
Furthermore, as described above, normally, as the spinning speed is increased, the spinnability deteriorates and it becomes difficult to stably produce yarn. However, in the present invention, by using a polyolefin resin having an MFR in a specific range, it is possible to stably spin the intended yarn.

(Step (C))
In this process, the long fibers are collected on a moving net and formed into a nonwoven web.

In the present invention, the fibers are drawn at a high spinning speed, so the long fibers coming out of the ejector are collected in a net under controlled conditions using a high-speed airflow, resulting in a nonwoven fabric with minimal fiber entanglement and high uniformity.

(Step (D))
In this step, the nonwoven web is thermally bonded to obtain a spunbonded nonwoven fabric.

Methods for integrating nonwoven webs by thermal bonding include thermal bonding using various rolls, such as a thermal embossing roll, a pair of upper and lower rolls each with engraved (uneven) surfaces, a thermal embossing roll consisting of a combination of a roll with one flat (smooth) surface and a roll with engraved (uneven) surfaces, and a thermal calendar roll consisting of a pair of upper and lower flat (smooth) rolls.

The adhesive area ratio during thermal bonding is preferably 5% or more and 30% or less. By setting the adhesive area ratio to preferably 5% or more, more preferably 10% or more, a spunbonded nonwoven fabric with higher strength can be obtained. On the other hand, by setting the adhesive area ratio to preferably 30% or less, more preferably 20% or less, sufficient flexibility can be obtained, particularly when used as a spunbonded nonwoven fabric for sanitary materials.

The bonding area ratio here refers to the percentage of the entire nonwoven fabric where the convex parts of the upper roll and the convex parts of the lower roll overlap and come into contact with the nonwoven web, when thermal bonding is performed using a pair of rolls with projections and recesses. Also, when thermal bonding is performed using a roll with projections and a flat roll, it refers to the percentage of the entire nonwoven fabric where the convex parts of the roll with projections and recesses come into contact with the nonwoven web.

The shapes of the engravings applied to the hot embossing roll can be circles, ellipses, squares, rectangles, parallelograms, rhombuses, regular hexagons, and regular octagons.

It is preferable that the surface temperature of the heated roll is between -50°C and the melting point of the polyolefin resin used. By setting the surface temperature of the heated roll to preferably (melting point -50°C or higher), and more preferably (melting point -45°C or higher), it is possible to achieve moderate thermal bonding and maintain the shape of the nonwoven fabric. In addition, by setting the surface temperature of the heated roll to below the melting point of the polyolefin resin, excessive thermal bonding is suppressed, and sufficient flexibility can be obtained, particularly when used as a spunbond nonwoven fabric for sanitary materials.

The linear pressure of the hot embossing roll during thermal bonding is preferably 50 N/cm or more and 500 N/cm or less. By setting the linear pressure of the roll to preferably 50 N/cm or more, more preferably 100 N/cm or more, and even more preferably 150 N/cm or more, sufficient thermal bonding can be achieved, resulting in a spunbonded nonwoven fabric with high strength. On the other hand, by setting the linear pressure of the roll to preferably 500 N/cm or less, more preferably 450 N/cm or less, and even more preferably 400 N/cm or less, sufficient flexibility can be obtained, particularly when used as a nonwoven fabric for sanitary materials.

The spunbond nonwoven fabric of the present invention can improve the inspection speed in the inspection process during production, i.e., the inspection speed. According to the inventor's study, if the whiteness unevenness is large during the production of spunbond nonwoven fabric, the appearance contrast of defective parts such as thread breakage decreases, making it difficult to visually recognize defects in in-line defect inspection using a line sensor or visual inspection during the inspection process, resulting in a problem of reduced productivity. In contrast, the spunbond nonwoven fabric of the present invention has high whiteness and can reduce whiteness unevenness despite its low basis weight, so that the visibility of defective parts such as thread breakage is improved compared to conventional spunbond nonwoven fabrics, and the inspection speed in the inspection process during production can be increased. As a result, it is expected that the productivity of spunbond nonwoven fabrics will improve.

The spunbond nonwoven fabric of the present invention has excellent uniformity and appearance despite its low basis weight and high whiteness, and therefore can be suitably used for sanitary material applications such as disposable paper diapers and napkins. Furthermore, because of its high wet friction resistance, it can also be suitably used for medical gowns and chemical protective clothing that are expected to be used in wet environments, and for masks that may be washed.

Next, the present invention will be specifically described based on examples. However, the present invention is not limited to these examples. In addition, in the measurement of each physical property, unless otherwise specified, the measurement was performed based on the above-mentioned method.

(Measurement method)
(1) Average single fiber diameter (μm):
The scanning electron microscope used was a "VHX-D500" manufactured by Keyence Corporation.

In the present invention, the average single fiber diameter (μm) of the fibers constituting the spunbond nonwoven fabric layer was calculated by the following procedure: 10 pieces of 100 mm wide x 100 mm wide were randomly cut from the nonwoven fabric, and photographs were taken at 500 to 3000 times magnification using a scanning electron microscope (for example, Keyence Corporation's "VHX-D500") to measure the width of 10 single fibers from each sample, for a total of 100 single fibers. The arithmetic mean value (μm) of the measured values for the 100 fibers was rounded off to one decimal place to obtain the average single fiber diameter (μm).

(2) Whiteness, uniformity (whiteness unevenness):
The whiteness was measured by cutting the nonwoven fabric into four pieces measuring 50 mm wide x 50 mm wide, placing the four nonwoven fabric pieces on a black mount, and measuring the whiteness using a colorimeter ("CR-20" manufactured by Konica Minolta, Inc.) in accordance with ASTM E313-73.

To measure the whiteness unevenness, 16 pieces of nonwoven fabric were randomly cut out, each 50 mm wide x 50 mm long, and four pieces were cut into one set, for a total of four sets (when cutting from a sheet-like nonwoven fabric, four pieces were cut out at equal intervals in the width direction of the sheet, for a total of four sets). Each set of four pieces was stacked on a black mount, and the cut nonwoven fabric was measured at four points on each of the four corners and one point in the center, for a total of five points, using the whiteness measurement method described above. The difference between the maximum and minimum whiteness values in the measurements for a total of 20 points in the four sets was taken as the whiteness unevenness.

(3) Inspection speed:
In the visual inspection of spunbond nonwoven fabric using a rewinding device, a sheet on which 10 pseudo-defects measuring 1 mm x 1 mm were previously marked was inspected using reflected light from a fluorescent lamp so that the illuminance at a position 50 cm away from the sheet was 200 lux, the conveying speed was increased in increments of 1 m/min, and the maximum unwinding conveying speed at which all 10 pseudo-defects could be visually detected without missing any was measured, and the average inspection speed for the five monitors was calculated and used as the inspection speed. The illuminance was measured using a Mother Tool Co., Ltd. "Lux-Meter LM-102" illuminometer.

(4) Weight loss evaluation due to wet friction:
For the evaluation of weight loss due to wet friction, a Gakushin-type friction tester specified in JIS L0849 (2013) was used. First, five test pieces with a width of 5 cm and a length of 20 cm were cut out from the nonwoven fabric, dried in a dryer set at 80 ° C for 24 hours, and the initial weight: m ₀ of each test piece was measured. A water-resistant abrasive paper C34P (grain size P800) manufactured by Riken Corundum Co., Ltd., which had been immersed in pure water for 1 minute, was attached to the friction element of the Gakushin-type friction tester, and rubbed 10 times back and forth on the test piece without load. The test piece after friction was dried again in a dryer set at 80 ° C for 24 hours, and the weight after friction: m _f of each test piece was measured. The m _f of each test piece was subtracted from the m ₀ , and the average value was calculated as the weight loss (mg) due to wet friction.

Example 1
A polypropylene resin with an MFR of 240 g/10 min containing 0.90% by mass of titanium oxide particles was melted in an extruder, and the yarn was spun from a rectangular core-sheath die with a hole diameter of 0.30 mm at a spinning temperature of 235° C. and a single-hole output rate of 0.36 g/min. The yarn was cooled and solidified, and then pulled and stretched by a rectangular ejector with compressed air at an ejector pressure of 0.55 MPa. The yarn was then collected on a moving net to obtain a nonwoven web made of polypropylene long fibers. The properties of the obtained polypropylene long fibers were an average single fiber diameter of 10.4 μm, and the spinning speed calculated from this was 4675 m/min. The spinnability was good, with zero yarn breakages in one hour of spinning.

The nonwoven web was then thermally bonded at a line pressure of 300 N/cm and a thermal bonding temperature of 145°C using a pair of upper and lower thermal embossing rolls, the upper roll of which was a metal embossing roll engraved with a polka dot pattern and had a bonding area ratio of 11%, and the lower roll of which was a metal flat roll, to obtain a spunbonded nonwoven fabric having a basis weight of 12 g/ ^m2 . The obtained spunbonded nonwoven fabric was evaluated. The results are shown in Table 1.

Example 2
A spunbonded nonwoven fabric was obtained in the same manner as in Example 1, except that the amount of titanium oxide particles added was 0.30% by mass, and a polypropylene resin with an MFR of 232 g/10 min was used. The properties of the obtained polypropylene long fibers were an average single fiber diameter of 10.6 μm, and the spinning speed calculated from this was 4489 m/min. The spinnability was good, with zero thread breakages in one hour of spinning. The obtained spunbonded nonwoven fabric was evaluated. The results are shown in Table 1.

Example 3
A spunbonded nonwoven fabric was obtained in the same manner as in Example 1, except that no titanium oxide particles were added and a polypropylene resin with an MFR of 226 g/10 min was used. The properties of the obtained polypropylene long fibers were an average single fiber diameter of 10.3 μm, and the spinning speed calculated from this was 4725 m/min. The spinnability was good, with zero thread breakages in one hour of spinning. The obtained spunbonded nonwoven fabric was evaluated. The results are shown in Table 1.

Example 4
A spunbonded nonwoven fabric was obtained in the same manner as in Example 1, except that the amount of titanium oxide particles added was 1.50 mass % and a polypropylene resin with an MFR of 245 g/10 min was used. The properties of the obtained polypropylene long fibers were an average single fiber diameter of 10.2 μm, and the spinning speed calculated from this was 4844 m/min. The spinnability was good, with zero thread breakage during 1 hour of spinning. The obtained spunbonded nonwoven fabric was evaluated. The results are shown in Table 1.

Example 5
A spunbonded nonwoven fabric was obtained in the same manner as in Example 4, except that 1.0% by mass of ethylene bisstearic acid amide was added as the fatty acid amide compound, and a polypropylene resin having an MFR of 249 g/10 min was used. The properties of the obtained polypropylene long fiber were an average single fiber diameter of 10.3 μm, and the spinning speed calculated from this was 4796 m/min. The spinnability was good, with zero thread breakages in one hour of spinning. The obtained spunbonded nonwoven fabric was evaluated. The results are shown in Table 1. In Table 1, EBA stands for ethylene bisstearic acid amide.

Example 6
A spunbonded nonwoven fabric was obtained in the same manner as in Example 5, except that the rectangular ejector was used to pull and stretch the fibers while varying the ejector pressure in the range of 0.54 to 0.56 MPa. The properties of the obtained polypropylene long fibers were an average single fiber diameter of 10.3 to 10.7 μm, and the spinning speed calculated from this was 4377 to 4710 m/min. The spinnability was good, with zero yarn breakages during 1 hour of spinning. The obtained spunbonded nonwoven fabric was evaluated. The results are shown in Table 1.

(Comparative Example 1)
A spunbond nonwoven fabric was obtained in the same manner as in Example 4, except that a rectangular ejector was used and the ejector pressure was set to 0.28 MPa.
The properties of the obtained polypropylene long fibers were an average single fiber diameter of 16.2 μm, and the spinning speed calculated from this was 1917 m/min. The spinnability was good, with zero yarn breakages in one hour of spinning. The obtained spunbond nonwoven fabric was evaluated. The results are shown in Table 1.

(Comparative Example 2)
Spinning was carried out in the same manner as in Example 5, except that the rectangular ejector was used to pull and draw the yarn while varying the ejector pressure in the range of 0.52 to 0.58 MPa. However, frequent yarn breakage occurred, and the yarn could not be made into a sheet.

(Comparative Example 3)
Except for using a polypropylene resin having an MFR of 35 g/10 min, spinning was carried out in the same manner as in Example 3. However, frequent yarn breakage occurred, and it was not possible to form the resulting sheet.

(Comparative Example 4)
A spunbonded nonwoven fabric was obtained in the same manner as in Comparative Example 3, except that the ejector pressure was 0.30 MPa using a rectangular ejector. The properties of the obtained polypropylene long fibers were an average single fiber diameter of 15.7 μm, and the spinning speed calculated from this was 2061 m/min. The spinnability was good, with zero thread breakage during 1 hour of spinning. The obtained spunbonded nonwoven fabric was evaluated. The results are shown in Table 1.

Examples 1 to 6 had good spinnability even at high spinning speeds, resulting in high productivity and stability. In addition, as a result of spinning at high spinning speeds in Examples 1 to 5, the whiteness was high due to uniformity achieved by finer fineness, and despite the low basis weight, the unevenness in texture was small and the whiteness unevenness was small, resulting in an excellent appearance and easily visible defects. On the other hand, as the amount of titanium oxide added increased, the unevenness in whiteness tended to increase. In Example 6, the pulling of the yarn by the ejector was appropriately changed and the passing position of the yarn path was dispersed, resulting in even smaller unevenness in whiteness despite the high whiteness. In addition, Examples 1 to 6 had little weight loss due to wet friction.

On the other hand, as shown in Comparative Example 1, when the ejector was weakly pulled and spun at a low spinning speed, the fineness was thick, so the whiteness was low and the unevenness in the texture was large, so the unevenness in the whiteness was noticeable, and the appearance was poor and the defects were difficult to visually recognize. In addition, as shown in Comparative Example 2, when the fluctuation in the pulling speed by the ejector was too large, thread breakage occurred, and a problem occurred in which stable production was not possible. As shown in Comparative Example 3, when a propylene resin with a relatively small MFR was used, thread breakage occurred at a high spinning speed, and a problem occurred in which stable production was not possible. Furthermore, as shown in Comparative Example 4, when a propylene resin with a relatively small MFR was used and pulled at an ejector pressure that allows stable spinning, the whiteness was low and the unevenness in the whiteness was large, similar to Comparative Example 1. In addition, Comparative Examples 1 and 4 had a large weight loss due to wet friction.

Claims (1)

A polyolefin resin containing 0.05 to 5.00% by mass of pigment particles and having a melt flow rate of 155 to 500 g/10 min is used,
After being discharged from the spinneret, it is cooled and solidified.
The spinning speed of the ejector is 3500 m/min or more and 6500 m/min or less, and
The spinning speed is varied by 1.5 to 3% from the average value of the spinning speed, and the spinning speed is drawn and stretched to obtain long fibers. The long fibers are collected on a moving net to form a nonwoven web.
The adhesive is thermally bonded at an adhesive area ratio of 5% to 30% and a linear pressure of a hot embossing roll of 50 N/cm to 500 N/cm .
The average single fiber diameter of the fibers is 6.5 μm or more and 14.5 μm or less,
The basis weight of the spunbond nonwoven fabric is 8 g/m2 or ^more and 30 g/m2 ^or less.
A method for producing a spunbond nonwoven fabric.