JP7746846B2 - spunbond nonwoven fabric - Google Patents

Description

The present invention relates to a spunbond nonwoven fabric that can be suitably used for hygiene materials such as disposable diapers and sanitary napkins, as well as masks.

Spunbond nonwoven fabrics are often used in hygiene materials such as disposable diapers and sanitary napkins, as well as masks, due to their low cost and the ease with which they can be made to achieve the appropriate flexibility and physical properties for practical use. Since these products are often disposed of after a single use and incinerated or landfilled, there is a need to reduce the amount of plastic used from an environmental perspective.

Reducing the amount of plastic in spunbond nonwoven fabrics specifically means reducing the basis weight (g/ ^m2 ), which is the mass of nonwoven fabric per 1 ^m2 . However, the lower the basis weight, the lower the tensile strength, which is a physical property of nonwoven fabrics, generally becomes, and problems such as tearing during actual use become apparent. Therefore, various studies are being conducted on ways to improve the strength of spunbond nonwoven fabrics.

Two methods are generally considered for improving the strength of spunbonded nonwoven fabrics: improving the strength of the yarn and improving thermal adhesiveness. As an example of the former, Patent Document 1 describes adding 0.1 to 1 part by weight of high-melt-strength polypropylene to a polypropylene resin with a molecular weight distribution of less than 3, thereby achieving both yarn tenacity and ease of extensibility. As an example of the latter, Patent Document 2 describes controlling the melt mass-flow rate and spinning conditions of the polypropylene resin to reduce the crystallite size and set the melting point and heat of fusion within specific ranges, thereby improving thermal adhesiveness during the spunbonded nonwoven fabric manufacturing process.

Special Publication No. 2008-523231 Japanese Patent Application Laid-Open No. 2019-196576

However, while the method disclosed in Patent Document 1 can improve the strength of the yarn to a certain extent, the small amount of high melt strength polypropylene added does not affect thermal adhesion, making it difficult to significantly improve the strength of the spunbond nonwoven fabric. On the other hand, the method disclosed in Patent Document 2 can improve thermal adhesion to a certain extent, but the yarn strength is low due to the small crystallite size, making it difficult to significantly improve the strength of the spunbond nonwoven fabric.

In view of the above-mentioned problems, the object of the present invention is to provide a high-strength spunbond nonwoven fabric that can withstand practical use even when the fabric has a low basis weight.

As a result of extensive research to achieve the above-mentioned objective, the inventors discovered that by setting the branching index g of the polypropylene resin in the fibers that make up the spunbond nonwoven fabric within a specific range, it is possible to improve both the strength and thermal adhesion of the fibers that make up the spunbond nonwoven fabric, thereby producing a spunbond nonwoven fabric with higher strength than ever before.

The present invention was completed based on these findings, and provides the following inventions:

The spunbond nonwoven fabric of the present invention is a spunbond nonwoven fabric composed of fibers made of polypropylene-based resin, and the branching index g of the polypropylene-based resin that makes up the fibers is 0.950 or more and 0.990 or less.

In a preferred embodiment of the spunbond nonwoven fabric of the present invention, the average single fiber diameter of the fibers constituting the spunbond nonwoven fabric is 6.5 μm or more and 14.5 μm or less.

In a preferred embodiment of the spunbond nonwoven fabric of the present invention, the polypropylene resin constituting the fibers has a melt flow rate of 155 g/10 min or more and 850 g/10 min or less.

In a preferred embodiment of the spunbond nonwoven fabric of the present invention, the mesopentad fraction (mmmm) of the polypropylene resin constituting the fibers is 0.950 or more and 0.995 or less.

According to a preferred embodiment of the spunbonded nonwoven fabric of the present invention, the basis weight of the spunbonded nonwoven fabric is 5 g/m ² or more and 100 g/m ² or less.

The present invention makes it possible to obtain spunbond nonwoven fabrics with higher strength than conventional spunbond nonwoven fabrics. Therefore, even when the spunbond nonwoven fabric has a relatively low basis weight, it still has physical properties that can withstand practical use. As an environmentally friendly nonwoven fabric that uses less plastic raw materials, it can be suitably used in hygiene materials such as disposable diapers and sanitary napkins, as well as masks.

The spunbond nonwoven fabric of the present invention is a spunbond nonwoven fabric composed of fibers made of polypropylene-based resin, and the branching index g of the polypropylene resin that makes up the fibers is 0.950 or more and 0.990 or less.

These components of the present invention are described in detail below, but the present invention is not limited in any way to the scope described below as long as it does not deviate from the gist of the invention.

[Polypropylene resin]
The spunbond nonwoven fabric of the present invention is composed of fibers whose main component is polypropylene-based resin. Polypropylene-based resin is preferred because it has superior spinnability and strength properties compared to other polyolefin-based resins such as polyethylene-based resins. Examples of polypropylene-based resins used in the present invention include propylene homopolymers and copolymers of propylene and various α-olefins.

The polypropylene-based resin constituting the fiber of the present invention preferably contains propylene homopolymers in an amount of 60% by mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more. This maintains good spinnability and improves strength.

This polypropylene-based resin may be a mixture of two or more types, or a resin composition containing other olefin-based resins such as polyethylene and poly-4-methyl-1-pentene, or a thermoplastic elastomer, etc., may also be used.

The melting point of this polypropylene resin is preferably 80°C or higher and 200°C or lower. By setting the melting point to 80°C or higher, more preferably 100°C or higher, and even more preferably 120°C or higher, it becomes easier to achieve heat resistance that is practically usable. By setting the melting point to 200°C or lower, more preferably 180°C or lower, it becomes easier to cool the yarn discharged from the spinneret, suppressing fusion between fibers and facilitating stable spinning.

Furthermore, this polypropylene-based resin may contain a fatty acid amide compound having 23 to 50 carbon atoms to improve slipperiness and flexibility. By mixing the fatty acid amide compound with the polypropylene-based resin with preferably 23 or more carbon atoms, and more preferably 30 or more carbon atoms, excessive exposure of the fatty acid amide compound on the fiber surface is suppressed, resulting in excellent spinnability and processing stability and maintaining high productivity. On the other hand, by mixing the fatty acid amide compound with preferably 50 or less carbon atoms, and more preferably 42 or less carbon atoms, the fatty acid amide compound can more easily migrate to the fiber surface, imparting slipperiness and flexibility to the spunbond nonwoven fabric.

Examples of fatty acid amide compounds having 23 to 50 carbon atoms used in the present invention include saturated fatty acid monoamide compounds, saturated fatty acid diamide compounds, unsaturated fatty acid monoamide compounds, and unsaturated fatty acid diamide compounds.

Specific 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, tetracosapentaenoic acid 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. Among these, ethylene bisstearic acid amide is preferred because it has excellent thermal stability, allowing it to be melt-spun, and can produce spunbond nonwoven fabrics with excellent lubricity and flexibility while maintaining high spinning stability.

In the present invention, the amount of fatty acid amide compound contained in the polypropylene-based resin that constitutes the fiber (hereinafter sometimes simply referred to as the fatty acid amide compound content) is preferably 0.01% to 5% by mass. By setting the fatty acid amide compound content to 0.01% by mass or more, and more preferably 0.1% by mass or more, it is possible to impart appropriate slip properties and flexibility. On the other hand, by setting the fatty acid amide compound content to 5% by mass or less, more preferably 3% by mass or less, and even more preferably 1% by mass or less, it is possible to suppress a decrease in spinnability.

The content of fatty acid amide compound here refers to the ratio (%) of the mass of fatty acid amide compound to the mass of the entire fiber made of polypropylene resin. In other words, for example, if the fiber is a sheath-core composite fiber and only the sheath component of the core-sheath composite fiber contains a fatty acid amide compound, the ratio of the mass of fatty acid amide compound to the entire mass of the sheath-core composite fiber is still calculated.

One method for measuring the content of this fatty acid amide compound is to extract the additive from the fiber with a solvent and then quantitatively analyze it using liquid chromatography/mass spectrometry (LS/MS). The extraction solvent is selected appropriately depending on the type of fatty acid amide compound. For example, in the case of ethylene bisstearamide, a method using a chloroform-methanol mixture is one example.

To impart other properties to the polypropylene-based resin that constitutes the fiber of the present invention, commonly used additives such as antioxidants, weathering stabilizers, light stabilizers, heat stabilizers, antistatic agents, antistatic aids, spinning agents, antiblocking agents, crystal nucleating agents, and pigments, or other polymers, can be added as needed, provided the effects of the present invention are not impaired.

The polypropylene-based resin constituting the fibers of the present invention is a polypropylene-based resin having long-chain branches in its main chain. A polypropylene-based resin having long-chain branches in its main chain is a polypropylene having polypropylene chains branched from the polypropylene main chain. By using polypropylene having long-chain branches in its main chain, the long-chain branches act as tie molecules that pseudo-crosslink between microcrystals from the melt extrusion stage, preferentially forming fine crystals, particularly on the fiber surface, in the subsequent spinning process. The fine crystal size and pseudo-crosslinking between microcrystals make it difficult for crystal orientation to proceed, lowering the softening temperature of the fiber surface. As a result, the fibers are firmly fused together during the spunbonding thermal bonding process, improving the strength of the spunbond nonwoven fabric.

In the present invention, the branching index g of the polypropylene resin constituting the fiber is 0.950 or more and 0.990 or less. The branching index g of this polypropylene resin is an index indicating the degree of branching of the long chain of the polypropylene resin, and is described, for example, in "Developments in Polymer Characterization-4" (J.V. Dawkins, Applied Science Publishers, 1983). A branching index g of 0.950 or more, preferably 0.965 or more, and more preferably 0.980 or more, prevents the melt tension from becoming too high, ensuring stable spinning without insufficient stretchability during melt spinning and drawing. This is thought to be because the polypropylene contains long-chain branches in the main chain structure, which, upon cooling and solidification, promote the entanglement of tie molecules in the amorphous phase that penetrate the microcrystals in the system (a pseudo-crosslinking effect between microcrystals). During the subsequent drawing process, the drawing stress is transmitted uniformly throughout the system (leading to more uniform drawing), reducing the number of voids that form inside the fibers and suppressing yarn breakage. Furthermore, a branching index g of 0.990 or less lowers the softening temperature through the effect of forming microcrystals and inhibiting crystal orientation, thereby firmly fusing the fibers together during the spunbonding thermal bonding process and improving the strength of the spunbond nonwoven fabric.

In the present invention, the branching index g of the polypropylene resin is measured by the following method.
(1) Take three 10 mg samples from the spunbond nonwoven fabric.
(2) The sample is dissolved in 1,2,4-trichlorobenzene to a concentration of 1 mg/mL.
(3) Using a gel permeation chromatography (GPC) device (e.g., Alliance GPC/V2000 manufactured by Waters) equipped with a differential refractometer (RI) and a viscosity detector (Viscometer) and GPCV analysis software compatible with the device (e.g., Millennium ³² manufactured by Waters), the intrinsic viscosity of the sample is measured under the following measurement conditions. The arithmetic mean value of the intrinsic viscosity thus obtained is rounded to three decimal places to obtain [η] _br .
Mobile phase solvent: 1,2,4-trichlorobenzene Flow rate: 1 mL/min Column: Two connected "GMHHR-H(S)HT" columns manufactured by Tosoh Corporation Column temperature, sample injection temperature, and detector temperatures: 140°C
Sample injection volume (sample loop volume): 0.2175 mL for the Waters Alliance GPC/V2000.
(4) The weight average molecular weight of the sample is measured by low angle laser light scattering photometry.
(5) Under the same conditions as in (3), the intrinsic viscosity of a linear polypropylene resin having a weight-average molecular weight equivalent to that obtained in (4) is measured. Alternatively, if it is not possible to measure the intrinsic viscosity of a linear polypropylene resin having a weight-average molecular weight equivalent to that obtained in (4), the intrinsic viscosity of a linear polypropylene resin is linearly related to the logarithm of its molecular weight, as is known from the Mark-Houwink-Sakurada equation. Therefore, the intrinsic viscosity of a commercially available linear polypropylene resin (e.g., "Novatec PP (registered trademark) FY6" manufactured by Japan Polypropylene Corporation) can be measured, and the intrinsic viscosity can be appropriately extrapolated to the low molecular weight side or the high molecular weight side to obtain a numerical value. The arithmetic mean value of the intrinsic viscosity thus obtained is rounded to four decimal places to obtain [η] _lin .
(6) Calculate the branching index g using the following formula (1), and round off the obtained value to the fourth decimal place.

g=[η] _br /[η] _lin ...(1)
Generally, when a long chain branch structure is introduced into a polymer molecule, the radius of gyration becomes smaller compared to a linear polymer molecule of the same molecular weight. Since a smaller radius of gyration results in a smaller intrinsic viscosity, the branching index g, which is the ratio ([η] br /[η] _lin ) of the intrinsic viscosity of the branched polymer ([η] _br ) to the intrinsic viscosity ([η] _lin ) of a _linear polypropylene of the same molecular weight, tends to decrease as the long chain branch structure is introduced.

The weight-average molecular weight (Mw) of the polypropylene resin constituting the fiber according to the present invention is preferably 1.0 x ¹⁰ or more. By setting the weight-average molecular weight (Mw) of this polypropylene resin to 1.0 x ¹⁰ or more, preferably 1.5 x ¹⁰ or more, and more preferably 2.0 x ¹⁰ or more, it is possible to sufficiently obtain the pseudo-crosslinking effect between microcrystals. There is no particular upper limit to the weight-average molecular weight (Mw) of the polypropylene resin as long as the effects of the present invention are achieved, but from the viewpoint of melt extrusion properties, for example, it is preferably 5.0 x ¹⁰ or less. The weight average molecular weight (Mw) of polypropylene can be measured by, for example, the method disclosed in M. L. McConnell, "POLYMER MOLECULAR WEIGHTS AND MOLECULAR WEIGHT DISTRIBUTIONS BY LOW-ANGLE LASER LIGHT SCATTERING," American Laboratory, May 1978, Vol. 10, pp. 63-75, i.e., low-angle laser light scattering photometry.

The melt flow rate (hereinafter sometimes referred to as MFR) of the polypropylene-based resin that constitutes the fibers in this invention is preferably 155 g/10 min or more and 850 g/10 min or less. By setting the MFR to 155 g/10 min or more, more preferably 170 g/10 min or more, and even more preferably 190 g/min or more, a spunbond nonwoven fabric with excellent texture, flexibility, and spinnability during production, and with a uniform texture, can be obtained. On the other hand, by setting the MFR to 850 g/10 min or less, more preferably 600 g/10 min or less, and even more preferably 400 g/10 min or less, a decrease in the strength of the spunbond nonwoven fabric can be suppressed.

The MFR of polypropylene resins is measured according to ASTM D1238 (Method A). This standard stipulates that polypropylene be measured under a load of 2.16 kg at a temperature of 230°C.

The MFR of the polypropylene resin can also be adjusted by blending two or more polypropylene resins with different MFRs in any desired ratio. In this case, the MFR of the resin blended with the main polypropylene resin is preferably 10 g/10 min or more, more preferably 20 g/10 min or more, and even more preferably 30 g/10 min or more. It is also preferably 1000 g/10 min or less, more preferably 800 g/10 min or less, and even more preferably 600 g/10 min or less. By keeping the MFR within the above range, partial viscosity variations in the blended polypropylene resin can be suppressed, preventing non-uniform fineness and deterioration of spinnability.

The MFR can be controlled by the weight-average molecular weight of the polypropylene resin. The higher the weight-average molecular weight of the polypropylene resin, the smaller the MFR. Alternatively, the MFR can be adjusted by adding at least one radical generator selected from the group consisting of organic peroxides, azo compounds, and nitroxides to the polypropylene resin and decomposing the polypropylene resin.

The mesopentad fraction (mmmm) of the polypropylene resin constituting the fibers in this invention is preferably 0.950 or more and 0.995 or less. It is more preferably 0.965 or more, and particularly preferably 0.980 or more.

The mesopentad fraction (mmmm) is an index of the stereoregularity of the crystalline phase of polypropylene measured by nuclear magnetic resonance (NMR). A higher value indicates higher stereoregularity, and the degree of crystallinity and melting point also increase. When the mesopentad fraction of a polypropylene resin is 0.950 or higher, the high crystallinity improves the strength of the spunbond yarn, providing the spunbond nonwoven fabric with sufficient mechanical strength for practical use. When the mesopentad fraction is 0.995 or lower, spinnability during melt spinning is maintained, enabling stable spunbond nonwoven fabrics to be obtained. This is because the preferential formation and growth of microcrystalline nuclei prevents yarn sway during fineness reduction and prevents yarns from fusing together even when they come into contact with each other, thereby preventing yarn breakage due to the yarn wrapping around surrounding yarns.

The mesopentad fraction (mmmm) is the isotactic fraction in pentad units in a polypropylene molecular chain measured using ^13C nuclear magnetic resonance ( ^13C -NMR), and is measured by a well-known method (for example, the method described in A. Zambelli, "Macromolecules", 1973, Vol. 6, p. 625, or the same, "Macromolecules", 1975, Vol. 8, p. 687), and is measured as the intensity fraction of the [mmmm] peak in all absorption peaks in the methyl carbon region of the ^13C -NMR spectrum.

[Fibers constituting spunbond nonwoven fabric]
The fibers constituting the spunbonded nonwoven fabric of the present invention preferably have an average single fiber diameter of 6.5 μm or more and 14.5 μm or less. By making the average single fiber diameter preferably 6.5 μm or more, more preferably 7.5 μm or more, and even more preferably 8.4 μm or more, a spunbonded nonwoven fabric with a smooth surface feel suitable for sanitary materials, masks, etc. can be obtained. During production, a decrease in spinnability can be prevented, and spunbonded nonwoven fabrics of good quality can be stably produced. On the other hand, by making the average single fiber diameter preferably 14.5 μm or less, more preferably 11.7 μm or less, and even more preferably 11.2 μm or less, single fibers are more likely to fuse together, increasing the number of fused fiber sites per unit area, and therefore a spunbonded nonwoven fabric with high strength can be obtained.

The average single fiber diameter (μm) of the fibers constituting the spunbonded nonwoven fabric of the present invention is calculated by the following procedure.
(1) Ten small sample pieces are randomly taken from the spunbond nonwoven fabric.
(2) Surface photographs are taken at a magnification of 500 to 1000 times using a scanning electron microscope (SEM, for example, "VHX-D500" manufactured by Keyence Corporation), and the widths of 10 fibers in each surface photograph, a total of 100 fibers, are measured.
(3) The arithmetic mean value (μm) of the measured values of 100 fibers was taken to one decimal place as the average single fiber diameter (μm).

The coefficient of variation of the single fiber diameter of the fibers constituting the spunbond nonwoven fabric (hereinafter sometimes simply referred to as the CV value of the single fiber diameter) is preferably 7% or less. By setting the CV value of the single fiber diameter to preferably 7% or less, more preferably 6% or less, and even more preferably 5% or less, it is possible to prevent the surface from feeling rough and to produce a highly uniform spunbond nonwoven fabric. The CV value of the single fiber diameter is determined by the uniformity of the spinneret back pressure, yarn cooling conditions, and drawing conditions, and can be controlled by appropriately adjusting these.

[Spunbond nonwoven fabric]
The basis weight of the spunbond nonwoven fabric of the present invention is preferably 5 g/ ^m2 or more and 100 g/ ^m2 or less. By setting the basis weight to preferably 5 g/m2 or ^more , more preferably 10 g/ ^m2 or more, and even more preferably 15 g/m2 or ^more , it is possible to obtain a spunbond nonwoven fabric with mechanical strength sufficient for practical use. On the other hand, by setting the basis weight to preferably 100 g/m2 or ^less , more preferably 50 g/m2 or ^less , and even more preferably 30 g/m2 or ^less , it is possible to obtain a spunbond nonwoven fabric with appropriate flexibility suitable for practical use of the nonwoven fabric.

In the present invention, the basis weight of the spunbond nonwoven fabric is determined in accordance with "6.2 Mass per unit area" of JIS L1913:2010 "Testing methods for general nonwoven fabrics" and is measured by the following procedure.
(1) Take three 20cm x 25cm test pieces per meter of sample width.
(2) Measure the mass (g) of each at standard conditions.
(3) The arithmetic mean value is expressed as mass per 1 m ² (g/m ² ) and rounded to the nearest whole number.

The thickness of the spunbond nonwoven fabric of the present invention is preferably 0.05 mm or more and 1.5 mm or less. By having a thickness of preferably 0.05 mm or more, more preferably 0.08 mm or more, and even more preferably 0.1 mm or more, or by having a thickness of preferably 1.5 mm or less, more preferably 1 mm or less, and even more preferably 0.8 mm or less, the spunbond nonwoven fabric can be provided with flexibility and appropriate cushioning properties, making it suitable for practical use.

In this invention, the thickness (mm) of the spunbond nonwoven fabric is the value measured in accordance with "5.1 Thickness" of JIS L1906:2000 "Testing methods for general long-fiber nonwoven fabrics."

The apparent density of the spunbonded nonwoven fabric of the present invention is preferably 0.05 g/cm ^{or more} and 0.30 g/cm ^or less. By setting the apparent density of the spunbonded nonwoven fabric to preferably 0.30 g/cm or ^less , more preferably 0.25 g/cm ^{or less} , and even more preferably 0.20 g/cm ^or less, the spunbonded nonwoven fabric can exhibit a soft texture without impairing its strength.

On the other hand, by setting the apparent density of the spunbond nonwoven fabric to preferably 0.05 g/ ^cm3 or more, more preferably 0.08 g/ ^cm3 or more, and even more preferably 0.10 g/ ^cm3 or more, it is possible to suppress the occurrence of fluffing and delamination, and to obtain a spunbond nonwoven fabric having strength sufficient for practical use.

In the present invention, the apparent density (g/cm ³ ) is calculated from the basis weight and thickness before rounding according to the following formula, and is rounded to two decimal places: apparent density (g/cm ³ ) = [basis weight (g/m ² )] / [thickness (mm)] × 10 ^-3 .

In the present invention, the tensile strength per unit area of the spunbond nonwoven fabric is determined in accordance with "6.3 Tensile strength and elongation (ISO method)" of JIS L1913:2010 "Testing methods for general nonwoven fabrics" and is measured by the following procedure.
(1) Three 50 mm x 300 mm test pieces are taken per meter of nonwoven fabric width, with the long side facing the machine direction (lengthwise direction of the nonwoven fabric) and the cross direction (widthwise direction of the nonwoven fabric).
(2) The test piece is set in the tensile tester with a gripping distance of 200 mm.
(3) A tensile test is carried out at a tensile speed of 100 mm/min, and the maximum strength is measured.
(4) The average value of the maximum strength measured for each test piece is found, and the tensile strength per unit area is calculated using the following formula, and rounded to one decimal place: Tensile strength per unit area ((N/25 mm)/(g/m ² )) = [average maximum strength (N/25 mm)]/unit area (g/m ² ).

The spunbond nonwoven fabric of the present invention more preferably has a longitudinal tensile strength per basis weight of 1.4 (N/25 mm)/(g/ ^m2 ) or more and 3.0 (N/25 mm)/(g/ ^m2 ) or less. A tensile strength per basis weight of preferably 1.4 (N/25 mm)/(g/ ^m2 ) or more, more preferably 1.5 (N/25 mm)/(g/ ^m2 ) or more, and even more preferably 1.6 (N/25 mm)/(g/ ^m2 ) or more can provide a spunbond nonwoven fabric with strength sufficient for practical use. On the other hand, a transverse tensile strength per basis weight of preferably 3.0 (N/25 mm)/(g/ ^m2 ) or less can prevent the flexibility of the spunbond nonwoven fabric from decreasing and the texture from being impaired. The tensile strength in the longitudinal direction per unit weight can be controlled by appropriately adjusting the MFR of the polypropylene resin, additives, average single fiber diameter, and/or the spinning speed and thermal bonding conditions (shape of the bonded part, compression rate, temperature, linear pressure, etc.) described below.

The tensile strength in the transverse direction per basis weight of the spunbonded nonwoven fabric of the present invention is more preferably 0.4 (N/25 mm)/(g/ ^m2 ) or more and 2.0 (N/25 mm)/(g/ ^m2 ) or less. A tensile strength per basis weight of preferably 0.4 (N/25 mm)/(g/ ^m2 ) or more, more preferably 0.6 (N/25 mm)/(g/ ^m2 ) or more, and even more preferably 0.8 (N/25 mm)/(g/ ^m2 ) or more can provide a spunbonded nonwoven fabric with strength sufficient for practical use. On the other hand, a tensile strength per basis weight of preferably 2.0 (N/25 mm)/(g/ ^m2 ) or less in the transverse direction can prevent a decrease in the flexibility of the spunbonded nonwoven fabric and a loss of texture. Spunbonded nonwoven fabrics have tensile strengths in both the longitudinal and transverse directions, but since the transverse tensile strength is generally smaller than the longitudinal tensile strength, a spunbonded nonwoven fabric having practical strength in the longitudinal direction can be obtained by setting the transverse tensile strength per basis weight to between 0.4 (N/25 mm)/(g/ ^m2 ) and 2.0 (N/25 mm)/(g/ ^m2 ). The transverse tensile strength per basis weight can be controlled by appropriately adjusting the MFR of the polypropylene resin, additives, average single fiber diameter, and/or the spinning speed and thermal bonding conditions (shape of the bonded joint, compression ratio, temperature, linear pressure, etc.) described below.

The spunbond nonwoven fabric of the present invention has physical properties that allow it to withstand practical use even when its basis weight is reduced. As an environmentally friendly nonwoven fabric that reduces the amount of plastic raw materials used, it can be used in a wide range of applications, including hygiene materials, medical materials, household goods, and industrial materials. It is particularly suitable for hygiene materials such as disposable diapers, sanitary products, and base fabrics for compresses, and for medical materials such as protective clothing and surgical gowns.

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

The spunbond nonwoven fabric of the present invention is a long-fiber nonwoven fabric produced by the spunbond method. The spunbond method not only offers excellent productivity and mechanical strength, but also suppresses the fuzzing and fiber shedding that often occurs with short-fiber nonwoven fabrics. Furthermore, laminating multiple layers of collected spunbond nonwoven fiber webs or thermocompression-bonded spunbond nonwoven fabrics (both denoted as S) in SS, SSS, and SSSS layers is a preferred embodiment, as it improves productivity and uniformity of fabrication.

The raw resin used in the manufacturing method of the spunbond nonwoven fabric of the present invention is a propylene-based resin in which the main repeating unit is propylene. Here, "main repeating unit" means that the repeating unit accounts for 90 mol % or more of the entire resin. Specific examples of raw resins include propylene homopolymers and copolymers of propylene and various α-olefins. When using copolymers of propylene and various α-olefins as the raw resin, the copolymerization ratio of the various α-olefins is preferably 10 mol % or less, more preferably 5 mol % or less, and even more preferably 3 mol % or less, from the perspective of achieving high strength.

The raw resin used in the present invention may be blended with other component resins, provided that the effects of the present invention are not impaired. Examples of other component resins include polyolefin resins such as polyethylene and poly-4-methyl-1-pentene, which have melting points close to those of polypropylene, as well as low-melting-point polyester resins and low-melting-point polyamide resins. Low-crystalline olefin resins are preferred for imparting flexibility. For example, ethylene-propylene copolymers are suitable as low-crystalline olefin resins. The mass ratio of the other component resins is preferably 20% by mass or less, more preferably 10% by mass or less, in order to fully express the properties of the polypropylene resin.

The raw resin may also be a recycled resin, meaning that it has been material recycled. The resin material to be material recycled must have a melt flow rate that allows it to be melt-spun using the spunbond method. Therefore, recycled raw materials with the desired melt flow rate may be re-melted and used. If the raw resin does not have the desired melt flow rate, at least one radical generator selected from the group consisting of organic peroxides, azo compounds, and nitroxides may be added to the raw resin to reduce the molecular weight of the raw resin and adjust the melt flow rate. The use of recycled resin is desirable because it reduces the amount of virgin petrochemical raw materials used and reduces the environmental impact during the production of spunbond nonwoven fabrics.

The organic peroxides include ketone peroxides such as methyl ethyl ketone peroxide and methyl isobutyl ketone peroxide, diacyl peroxides such as dibenzoyl peroxide, di-(3,5,5-trimethylhexanoyl) peroxide, dilauroyl peroxide, didecanoyl peroxide and di-(2,4-dichlorobenzoyl) peroxide, hydroperoxides such as t-butyl hydroperoxide, cumene hydroperoxide, diisopropylbenzene hydroperoxide and 2,5-dimethylhexane-2,5-dihydroperoxide, di-t-butyl peroxide, dicumyl peroxide, 2,5-dimethyl-2,5-bis(t-butylperoxy)hexane, 2,5-dimethyl-2,5-bis(t-butylperoxy)hexyne-3, α,α'-bis(t-butylperoxide), Examples of suitable peroxycarbonates include dialkyl peroxides such as 1,1-bis(t-butylperoxy)diisopropylbenzene, peroxyketals such as 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane and 2,2-bis(t-butylperoxy)butane, alkyl peresters such as t-butylperoxyoctoate, t-butylperoxypivalate, t-butylperoxyneodecanoate and t-butylperoxybenzoate, and peroxycarbonates such as di-(2-ethylhexyl)peroxydicarbonate, diisopropyl peroxydicarbonate, bis(4-t-butylcyclohexyl)peroxydicarbonate, di-sec-butylperoxydicarbonate and t-butylperoxyisopropylcarbonate. Of these, dialkyl peroxides are preferred because of their easy control of MFR.

Furthermore, examples of the azo-based compounds include azobisisobutyronitrile and azobisisovaleronitrile.

The nitroxides mentioned above include sterically hindered hydroxylamine esters.

When directly melt-spinning the raw resin, a technique using an extruder such as a single-screw or twin-screw extruder can be applied. The radical generator may be kneaded and pelletized in an extruder before spinning to produce a polypropylene-based resin. Alternatively, the raw resin and radical generator may be kneaded together during spinning using the extruder of a spinning machine, producing the polypropylene-based resin simultaneously with melt spinning.

In the present invention, in order to achieve the branching index g within the above range, in addition to using a polypropylene resin having a branching index g that falls within the above range, other preferred methods include blending a branched polypropylene resin with a linear polypropylene resin, introducing a long-chain branched structure into polypropylene molecules as described in JP-A-62-121704, or a method such as that described in Japanese Patent No. 2869606. Of these, the method of blending a branched polypropylene resin with a linear polypropylene resin is preferred because it allows for easy adjustment of the branching index g.

The amount of branched polypropylene resin blended with the linear polypropylene resin is preferably 1% by mass or more and 20% by mass or less. By adding an amount of preferably 1% by mass or more, more preferably 2% by mass or more, and even more preferably 3% by mass or more, crystal growth in the fibers is inhibited, resulting in finer crystals. Furthermore, by making it more difficult for crystal orientation to proceed, the softening temperature of the fiber surface is lowered, allowing the fibers to fuse firmly together during the spunbond thermal bonding process, resulting in a high-strength spunbond nonwoven fabric. On the other hand, by adding an amount of preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 10% by mass or less, yarn strength can be increased while maintaining the thermal adhesiveness of the spunbond without unnecessarily inhibiting crystal growth in the fibers.

Specific examples of the branched polypropylene resin include polypropylene manufactured by Basell (e.g., type names: "PF-814," "PF-633," "PF-611," "SD-632," etc.), polypropylene manufactured by Borealis (e.g., type name: "WB130HMS," etc.), and polypropylene manufactured by Dow (e.g., type names: "D114," "D201," "D206," etc.).

The branched polypropylene referred to here refers to polypropylene having 5 or less internal tri-substituted olefins per 10,000 carbon atoms. The presence of the internal tri-substituted olefins can be confirmed by the proton ratio in the ^1H -NMR spectrum.

When melting and spinning polypropylene-based resin, the spinning temperature is preferably 200°C or higher and 270°C or lower. It is more preferably 210°C or higher, even more preferably 220°C or higher, and even more preferably 260°C or lower, and even more preferably 250°C or lower. By maintaining the spinning temperature within the above range, a stable molten state can be achieved, resulting in excellent spinning stability.

The back pressure of the spinneret is preferably 0.1 MPa or more and 6.0 MPa or less. By setting the back pressure to preferably 0.1 MPa or more, more preferably 0.3 MPa or more, and even more preferably 0.5 MPa or more, it is possible to prevent deterioration of discharge uniformity and the occurrence of fiber diameter variations. On the other hand, by setting the back pressure to 6.0 MPa or less, it is possible to prevent the spinneret from being enlarged in order to increase pressure resistance. The back pressure of the spinneret can be adjusted by the nozzle hole diameter, nozzle hole depth, spinning temperature, etc. of the spinneret, with the nozzle hole diameter having the greatest contribution.

The spun continuous fiber yarn is then cooled. Methods for cooling the spun yarn include, for example, forcibly blowing cold air onto the yarn, allowing it to cool naturally 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. Furthermore, the cooling conditions can be adjusted appropriately, taking into account the output per spinneret hole, the spinning temperature, the ambient temperature, and other factors.

The cooled and solidified yarn is pulled and drawn by compressed air sprayed from the ejector. The spinning speed is preferably 3,500 m/min to 6,500 m/min, more preferably 4,000 m/min to 6,500 m/min, and even more preferably 4,500 m/min to 6,500 m/min. A spinning speed of 3,500 m/min to 6,500 m/min results in high productivity and promotes fiber orientation and crystallization, making it possible to obtain high-strength long fibers. Normally, increasing the spinning speed deteriorates spinnability and makes it impossible to stably produce yarns, but by using a polypropylene-based resin with an MFR within a specific range, the intended polypropylene fiber can be spun stably.

The resulting long fibers are then collected on a moving net or similar to obtain a nonwoven fiber web. When drawn at a high spinning speed, the fibers coming out of the ejector are collected on the net under controlled conditions by the high-speed airflow, making it easier to obtain a nonwoven fabric with minimal fiber entanglement and high uniformity.

In this case, it is preferable that the spinning speed/line speed ratio is 18 or greater. By setting the spinning speed/line speed ratio to preferably 18 or greater, and more preferably 20 or greater, the fibers can be collected on the moving net while oriented in the longitudinal direction, resulting in a nonwoven fabric with high strength in the longitudinal direction.

Methods for uniformly aligning the fiber orientation of the yarn ejected from the ejector include: placing an angled flat plate between the ejector and the net to guide the yarn; providing grooves at multiple different angles on the flat plate to separate the yarn into those that fall along the plate and those that fall along the grooves, dispersing and opening the yarn in the direction of the nonwoven fiber web flow; and arranging multiple flat plates at different angles like comb teeth at the ejector outlet and causing the yarn to fall along each plate, thereby further dispersing and opening the yarn in the direction of the nonwoven fiber web flow.

Among these, the method of spreading the yarn by arranging multiple flat plates at different angles in a comb-like pattern at the ejector outlet and letting the yarn fall along each of the flat plates is a preferred embodiment for aligning the fiber orientation direction, as it efficiently disperses fine fiber diameter yarns in the flow direction of the nonwoven fiber web and allows for controlled spreading with minimal deceleration.

It is also a preferred embodiment to temporarily bond a nonwoven fiber web obtained by collecting fibers on a net by contacting one side of the web with a hot flat roll. This prevents the surface layer of the nonwoven fiber web from turning over or being blown away while being transported on the net, which could cause deterioration in formation, and improves transportability from the time the yarns are collected to the time they are thermally bonded.

The resulting nonwoven fiber web can then be thermally bonded to obtain the desired spunbond nonwoven fabric. There are no particular limitations on the method for thermally bonding the nonwoven fiber web, but examples include methods using various rolls, such as a thermal embossing roll, a pair of upper and lower rolls each with engraved (concave and convex) surfaces, a thermal embossing roll consisting of a combination of one roll with a flat (smooth) surface and the other with engraved (concave and convex) surfaces, and a thermal calender roll consisting of a pair of upper and lower flat (smooth) rolls, as well as methods using ultrasonic vibrations from a horn (ultrasonic bonding).

Among these, the preferred embodiment is to use a pair of heat embossing rolls, each of which has an engraved (uneven) surface, or a heat embossing roll consisting of a combination of one roll with a flat (smooth) surface and the other with an engraved (uneven) surface, due to their excellent productivity, ability to impart strength through the partially bonded areas, and ability to maintain the texture and feel unique to nonwoven fabrics through the non-bonded areas.

The preferred surface material for the hot embossing roll is a pair of metal rolls, in order to achieve a sufficient thermocompression effect and prevent the engraving (convex and concave portions) of one embossing roll from being transferred to the surface of the other roll.

The bonded area ratio embossed using a hot embossing roll is preferably 5% or more and 30% or less. By setting the bonded area to preferably 5% or more, more preferably 8% or more, and even more preferably 10% or more, it is possible to obtain strength sufficient for practical use as a spunbond nonwoven fabric. On the other hand, by setting the bonded area to preferably 30% or less, more preferably 25% or less, and even more preferably 20% or less, it is possible to obtain appropriate flexibility for practical use. Even when ultrasonic bonding is used, it is preferable that the bonded area ratio be in a similar range.

The bonded area ratio here refers to the proportion of the bonded area in the entire spunbonded nonwoven fabric. Specifically, when thermal bonding is performed using a pair of uneven rolls, it refers to the proportion of the entire spunbonded 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 fiber web (bonded area). When thermal bonding is performed using an uneven roll and a flat roll, it refers to the proportion of the entire spunbonded nonwoven fabric where the convex parts of the uneven roll come into contact with the nonwoven fiber web (bonded area). When ultrasonic bonding is performed, it refers to the proportion of the entire spunbonded nonwoven fabric where the parts that are thermally welded by ultrasonic processing (bonded area).

The shape of the bonded areas formed using a hot embossing roll or ultrasonic bonding is not particularly limited, but can be, for example, a circle, ellipse, square, rectangle, parallelogram, rhombus, regular hexagon, or regular octagon. Furthermore, it is preferable that the bonded areas be uniformly spaced at regular intervals in both the longitudinal direction (machine direction) and width direction of the spunbonded nonwoven fabric. This reduces variation in the strength of the spunbonded nonwoven fabric.

The surface temperature of the hot embossing roll during thermal bonding is preferably at least -50°C and no more than -10°C lower than the melting point of the polypropylene resin used (hereinafter sometimes referred to as Tm (°C)) (i.e., (Tm - 50°C) to (Tm - 10°C)). By setting the surface temperature of the hot roll to preferably at least -50°C lower than the melting point of the polypropylene resin (i.e., Tm - 50°C, hereinafter the same) and more preferably at least -45°C (Tm - 45°C), a spunbond nonwoven fabric with adequate thermal bonding strength can be obtained. Furthermore, by setting the surface temperature of the hot embossing roll to preferably at most -10°C (Tm - 10°C) and more preferably at most -15°C (Tm - 15°C) lower than the melting point of the polypropylene resin, excessive thermal bonding can be suppressed and adequate flexibility for practical use can be obtained.

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 roll linear pressure to preferably 50 N/cm or more, more preferably 100 N/cm or more, and even more preferably 150 N/cm or more, it is possible to obtain a spunbond nonwoven fabric with adequate thermal bonding strength for practical use. On the other hand, by setting the linear pressure of the hot embossing roll to preferably 500 N/cm or less, more preferably 400 N/cm or less, and even more preferably 300 N/cm or less, it is possible to obtain adequate flexibility for practical use.

In addition, in order to adjust the thickness of the spunbond nonwoven fabric, thermal compression bonding can be performed using a thermal calendar roll consisting of a pair of upper and lower flat rolls before and/or after thermal bonding using the above-mentioned thermal embossing roll. A pair of upper and lower flat rolls refers to metal rolls or elastic rolls with smooth surfaces, and a pair of metal rolls or a pair of metal rolls and elastic rolls can be used.

Here, an elastic roll refers to a roll made of a material that has greater elasticity than a metal roll. Examples of elastic rolls include so-called paper rolls made of paper, cotton, aramid paper, etc., as well as rolls made of resins such as urethane resin, epoxy resin, silicone resin, polyester resin, hard rubber, and mixtures of these.

In the method for producing a spunbond nonwoven fabric of the present invention, a functional agent may be added to the resulting spunbond nonwoven fabric. Examples of functional agents include, but are not limited to, hydrophilic agents, water repellents, oil repellents, antistatic agents, antibacterial agents, antiviral agents, deodorizers, fragrances, and cooling agents.

There are no particular limitations on the method for applying the functional agent, and impregnation, spraying, kiss roll, etc. can be used.

Next, the spunbond nonwoven fabric of the present invention will be described in detail based on examples. However, the present invention is not limited to these examples. Furthermore, unless otherwise specified, measurements of each physical property were performed according to the methods described above.

[Measurement method]
(1) Melt flow rate (MFR) of polypropylene resin (g/10 min):
Measurement was carried out in accordance with JIS K7210-1 (2014) under conditions of 230°C and 2.16 kg.
(2) Branching index g(-) of polypropylene resin:
The branching index g of the polypropylene resin was measured according to the above-described method using a gel permeation chromatography (GPC) apparatus equipped with a differential refractometer (RI) and a viscosity detector (Viscometer), "Alliance GPC/V2000" manufactured by Waters Corporation, GPCV analysis software "Millennium ³² " manufactured by Waters Corporation, a mobile phase solvent containing 1,2,4-trichlorobenzene to which the antioxidant "Irganox 1076" manufactured by BASF Japan Ltd. was added at a concentration of 0.5 mg/mL, "Novatec PP (registered trademark) FY6" manufactured by Japan Polypropylene Corporation, and a sample injection amount (sample loop volume) of 0.2175 mL.

(3) Mesopentad fraction of polypropylene resin (mmmm) (-)
A polypropylene resin sample was dissolved in a solvent, and the mesopentad fraction (mmmm) was determined using ^13C -NMR under the following conditions: In addition to the above-mentioned literature, the measurement was performed with reference to "New Edition Polymer Analysis Handbook" edited by the Japan Society for Analytical Chemistry and the Polymer Analysis Research Forum, Kinokuniya Shoten, January 1995, pp. 609-611.
A. Measurement conditions and equipment: Bruker "DRX-500"
・Measurement nucleus: ^13C nucleus (resonance frequency: 125.8MHz)
・Measurement concentration: 10% by mass
Solvent: benzene/d-o-dichlorobenzene = mass ratio 1:3 mixed solution Measurement temperature: 130°C
Spin speed: 12Hz
NMR sample tube: 5 mm tube Pulse width: 45° (4.5 μs)
Pulse repetition time: 10 seconds Data points: 64K
Conversion count: 10,000 times Measurement mode: complete decoupling
B. Analysis Conditions Fourier transformation was performed with LB (line broadening factor) set to 1.0, and the mmmm peak was determined to be 21.86 ppm. Peak division was performed using WINFIT software (manufactured by Bruker). Peak division was performed as follows, starting from the peak on the high magnetic field side, and automatic peak fitting was performed using the accompanying software. After optimizing the peak division, the sum of the mmmm peak fractions was calculated. The above measurement was performed five times, and the average value was used as the mesopentad fraction (mmmm) of this sample.
Peak (a) mrrm
(b) mrrr
(c)rrrr
(d) rmrm
(e) rmrr
(f) mmrm
(g) mmrr
(h) rmmr
(i) mmr
(j) mmmm.

(4) Average single fiber diameter (μm):
The scanning electron microscope (SEM) used was a VHX-D500 manufactured by Keyence Corporation, and the measurements were carried out by the above-mentioned method.

(5) Weight (g/m ² ):
Based on 6.2 "Mass per unit area" of JIS L1913:2010 "Testing methods for general nonwoven fabrics," three 20 cm x 25 cm test pieces were taken per meter of sample width, and the mass (g) of each was measured under standard conditions, and the average value was expressed as mass per 1 ^m2 (g/ ^m2 ).

(6) Tensile strength per unit area of spunbond nonwoven fabric ((N/25 mm)/(g/m ² )):
The measurement was carried out using the RTG-1250 measuring device manufactured by A&D Co., Ltd., using the method described above.

[Example 1]
(Polypropylene resin)
The linear polypropylene resin used was a polypropylene having an MFR of 200 g/10 min, a branching index g of 1.000, and a mesopentad fraction (mmmm) of 0.990. The branched polypropylene resin used was a polypropylene having an MFR of 2.7 g/10 min, a branching index g of 0.600, and a mesopentad fraction (mmmm) of 0.940 (Basell's "Profax PF-814"). These resins were mixed so that the linear polypropylene resin was 97% by mass and the branched polypropylene resin was 3% by mass, resulting in a polypropylene resin having an MFR of 197 g/10 min, a branching index g of 0.988, a mesopentad fraction of 0.989, and a melting point of 160°C.

(spunbond nonwoven fabric)
The polypropylene resin was introduced into a single-screw extruder, melted at a spinning temperature of 240°C, and spun from a rectangular die with a hole diameter of 0.40 mm at a single-hole throughput rate of 0.40 g/min. The resulting yarn was cooled and solidified, and then pulled and stretched in a rectangular ejector using compressed air at a pressure of 0.45 MPa. The spinning speed was 4019 m/min. The nonwoven fiber web was then collected on a moving net to obtain a nonwoven fiber web. The resulting nonwoven fiber web was then thermally bonded under conditions of a linear pressure of 500 N/cm and a thermal bonding temperature of 140°C to obtain a spunbonded nonwoven fabric having an average single fiber diameter of 11.8 μm and a basis weight of 30 g/ ^m2 . The heat embossing rolls used were a pair of upper and lower heat embossing rolls, the upper roll being a metal embossing roll engraved with a polka dot pattern and having an adhesive area ratio of 11%, and the lower roll being a metal flat roll. The tensile strength per unit area in the machine direction and cross direction of the resulting spunbond nonwoven fabric was evaluated. The results are shown in Table 1.

[Example 2]
In Example 1 (polypropylene resin), the linear polypropylene resin and the branched polypropylene resin were mixed in a ratio of 96% by mass and 4% by mass, respectively, resulting in a polypropylene resin with an MFR of 193 g/10 min, a branching index g of 0.984, a mesopentad fraction of 0.988, and a melting point of 160°C. In Example 1 (spunbonded nonwoven fabric), the single-hole output rate was changed to 0.50 g/min and the spinning speed to 4,858 m/min. A spunbonded nonwoven fabric with an average single fiber diameter of 12.0 μm and a basis weight of 30 g/ ^m2 was obtained in the same manner as in Example 1. The tensile strength per basis weight in the longitudinal and transverse directions of the obtained spunbonded nonwoven fabric was evaluated. The results are shown in Table 1.

[Example 3]
The same procedures as in Example 1 were repeated except that the polypropylene resin in Example 1 was changed to a mixture of 92% by mass of linear polypropylene resin and 8% by mass of branched polypropylene resin, resulting in a polypropylene resin with an MFR of 184 g/10 min, a branching index g of 0.968, a mesopentad fraction of 0.986, and a melting point of 160°C. The spunbonded nonwoven fabric in Example 1 was also changed to a single-hole output rate of 0.50 g/min and a spinning speed of 5024 m/min. A spunbonded nonwoven fabric with an average single fiber diameter of 11.8 μm and a basis weight of 30 g/ ^m2 was obtained. The tensile strength per basis weight in the longitudinal and transverse directions of the resulting spunbonded nonwoven fabric was evaluated. The results are shown in Table 1.

[Example 4]
The same procedures as in Example 1 were repeated except that in Example 1 (polypropylene resin), the linear polypropylene resin and the branched polypropylene resin were mixed in a ratio of 89% by mass and 11% by mass, respectively, resulting in a polypropylene resin with an MFR of 173 g/10 min, a branching index g of 0.956, a mesopentad fraction of 0.985, and a melting point of 160°C. In Example 1 (spunbonded nonwoven fabric), the single-hole output rate was changed to 0.60 g/min and the spinning speed to 3586 m/min. A spunbonded nonwoven fabric with an average single fiber diameter of 15.3 μm and a basis weight of 30 g/ ^m2 was obtained. The tensile strength per basis weight in the longitudinal and transverse directions of the resulting spunbonded nonwoven fabric was evaluated. The results are shown in Table 1.

[Example 5]
The same procedures as in Example 1 were repeated except that the polypropylene resin used in Example 1 was changed to a blend of 90% by mass of linear polypropylene resin and 10% by mass of branched polypropylene resin, resulting in a polypropylene resin with an MFR of 176 g/10 min, a branching index g of 0.960, a mesopentad fraction of 0.958, and a melting point of 160°C. The spunbonded nonwoven fabric used in Example 1 was also changed to a single-hole output rate of 0.60 g/min and a spinning speed of 3586 m/min. A spunbonded nonwoven fabric with an average single fiber diameter of 15.3 μm and a basis weight of 30 g/ ^m2 was obtained. The tensile strength per basis weight in the longitudinal and transverse directions of the resulting spunbonded nonwoven fabric was evaluated. The results are shown in Table 1.

[Comparative Example 1]
A spunbonded nonwoven fabric having an average monofilament diameter of 11.8 μm and a basis weight of 30 g/m2 was obtained in the same manner as in Example 1, except that in Example 1 (polypropylene-based resin), only polypropylene having an MFR of 200 g/ ^{10 min} , a branching index g of 1.000, and a mesopentad fraction (mmmm) of 0.940 was used as the linear polypropylene-based resin, and no branched polypropylene-based resin was used. The tensile strength per basis weight in the longitudinal and transverse directions of the obtained spunbonded nonwoven fabric was evaluated. The results are shown in Table 2.

[Comparative Example 2]
In Example 1 (polypropylene resin), a polypropylene with an MFR of 200 g/10 min, a branching index g of 1.000, and a mesopentad fraction (mmmm) of 0.940 was used as the linear polypropylene resin, and the mixture was changed to 85% by mass of linear polypropylene resin and 15% by mass of branched polypropylene resin, resulting in a polypropylene resin with an MFR of 166 g/10 min, a branching index g of 0.940, a mesopentad fraction of 0.940, and a melting point of 160°C. In Example 1 (spunbond nonwoven fabric), the spinning speed was changed to 3823 m/min. A spunbond nonwoven fabric with an average monofilament diameter of 12.1 μm and a basis weight of 30 g/ ^m2 was obtained in the same manner as in Example 1. The tensile strength per basis weight in the longitudinal and transverse directions of the resulting spunbond nonwoven fabric was evaluated. The results are shown in Table 2.

In the spunbonded nonwoven fabrics of Examples 1 to 5, the branching index g of the polypropylene resin was 0.950 or more and 0.990 or less, and therefore the tensile strength per basis weight was 1.4 ((N/25 mm)/(g/m ² )) or more in the longitudinal direction and 0.4 ((N/25 mm)/(g/m ² )) or more in the transverse direction, resulting in high-strength spunbonded nonwoven fabrics.

On the other hand, the spunbond nonwoven fabric of Comparative Example 1 had a branching index g of greater than 0.995 for the polypropylene-based resin, which resulted in strong adhesion during the thermal bonding process and a weak tensile strength per unit area weight. Furthermore, the spunbond nonwoven fabric of Comparative Example 2 had a branching index g of less than 0.950 for the polypropylene-based resin, which resulted in weak yarn strength and a weak tensile strength per unit area weight.

Although the present invention has been described in detail using specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.

Claims (5)

A spunbond nonwoven fabric composed of fibers made of polypropylene-based resin, wherein the branching index g of the polypropylene-based resin making up the fibers is 0.950 or more and 0.990 or less. The spunbond nonwoven fabric according to claim 1, wherein the average single fiber diameter of the fibers constituting the spunbond nonwoven fabric is 6.5 μm or more and 14.5 μm or less. The spunbond nonwoven fabric according to claim 1 or 2, wherein the melt flow rate of the polypropylene resin constituting the fibers is 155 g/10 min or more and 850 g/10 min or less. The spunbond nonwoven fabric according to any one of claims 1 to 3, wherein the mesopentad fraction (mmmm) of the polypropylene resin constituting the fibers is 0.950 or more and 0.995 or less. The spunbond nonwoven fabric according to any one of claims 1 to 4, wherein the basis weight of the spunbond nonwoven fabric is 5 g/ ^m² or more and 100 g/ ^m² or less.