TWI894436B - Spunbond nonwovens and core-sheath composite fibers - Google Patents

Abstract

The present invention is to provide a spunbond nonwoven fabric that exhibits excellent softness and skin feel, a uniform texture, sufficient strength to withstand practical use, and excellent productivity. The present invention relates to a spunbond nonwoven fabric composed of core-sheath composite fibers primarily composed of a polyethylene resin. The spunbond nonwoven fabric has fused portions and non-fused portions. The ratio of the alignment parameter Ofs of the sheath component of the core-sheath composite fibers in the non-fused portions to the alignment parameter Ofc of the core component of the core-sheath composite fibers in the non-fused portions (Ofs/Ofc) is 0.10 to 0.90.

Description

Spunbond nonwovens and core-sheath composite fibers

The present invention relates to polyethylene spunbond nonwoven fabric and core-sheath composite fiber.

Generally speaking, nonwoven fabrics used in sanitary materials such as disposable diapers and sanitary napkins require skin-friendly feel, softness, and high productivity. In particular, the top sheet of disposable diapers, which comes into direct contact with the skin, is one of the applications that places these high demands.

As a means of improving skin-touch and softness, the use of polyethylene, which has a lower elastic modulus and coefficient of friction than polypropylene, has been considered. For example, a polyethylene spunbond nonwoven fabric has been proposed, comprising a resin composition blended with linear low-density polyethylene of varying densities (see Patent Document 1).

In addition, a polyethylene spunbond nonwoven fabric has been proposed. It is composed of polyethylene fibers with a density of 0.930-0.965 g/ ^cm3 and an average single fiber diameter of 8.0-16.5 μm. Its complex viscosity at a temperature of 230°C and a pressure of 6.23 rad/sec is less than 90 Pa·sec (see Patent Document 2).

Indeed, these non-woven fabrics are highly flexible due to the properties of polyethylene resin.

[Prior Art Literature] [Patent Literature]

Patent Document 1: Japanese Patent Application Publication No. 2008-274445

Patent Document 2: Japanese Patent Application Publication No. 2019-26954

However, imparting sufficient strength to spunbond nonwovens made of polyethylene resin has been a major challenge. Even with the methods disclosed in Patent Document 1 or Patent Document 2, achieving strength sufficient for practical use has been difficult.

Therefore, an object of the present invention is to provide a spunbond nonwoven fabric that has excellent softness and skin feel, a uniform texture, sufficient strength to withstand practical use, and excellent productivity.

Another object of the present invention is to provide a composite fiber that has excellent softness and skin feel, as well as excellent spinning stability and thermal adhesion.

The spunbond nonwoven fabric of the present invention is composed of a core-sheath composite fiber having a polyethylene resin as a main component. The spunbond nonwoven fabric has a fused portion and a non-fused portion. The ratio of the alignment parameter Ofs of the sheath component of the core-sheath composite fiber in the non-fused portion to the alignment parameter Ofc of the core component of the core-sheath composite fiber in the non-fused portion (Ofs/Ofc) is 0.10 to 0.90.

According to a preferred embodiment of the spunbond nonwoven fabric of the present invention, the sheath component of the core-sheath composite fiber in the welded portion has an orientation parameter Obs of 1.2 to 3.0, and the core component of the core-sheath composite fiber in the welded portion has an orientation parameter Obc of 2.0 to 10.0.

According to a preferred embodiment of the spunbond nonwoven fabric of the present invention, the solid density of the core-sheath composite fiber is not less than 0.935 g/cm ³ and not more than 0.970 g/cm ³ .

According to a preferred embodiment of the spunbond nonwoven fabric of the present invention, the aforementioned Ofs is not less than 2.0 and not more than 8.0.

According to a preferred embodiment of the spunbond nonwoven fabric of the present invention, the spunbond nonwoven fabric has a single melting peak temperature within the range of 100°C to 150°C in differential scanning calorimetry.

According to a preferred embodiment of the spunbond nonwoven fabric of the present invention, the tensile strength in the transverse direction per unit area weight of the spunbond nonwoven fabric is greater than 0.20 (N/25mm)/(g/m ² ).

According to a preferred embodiment of the spunbond nonwoven fabric of the present invention, the stress at 5% elongation in the longitudinal direction per unit area weight of the spunbond nonwoven fabric is greater than 0.20 (N/25mm)/(g/m ² ).

Furthermore, the core-sheath composite fiber of the present invention is a core-sheath composite fiber having a polyethylene resin as a main component, and the ratio of the alignment parameter Ofs of the sheath component of the core-sheath composite fiber to the alignment parameter Ofc of the core component of the core-sheath composite fiber, Ofs/Ofc, is 0.1 to 0.9.

According to a preferred embodiment of the core-sheath composite fiber of the present invention, the solid density of the core-sheath composite fiber is not less than 0.935 g/cm ³ and not more than 0.970 g/cm ³ .

According to a preferred embodiment of the core-sheath composite fiber of the present invention, the aforementioned Ofs is greater than or equal to 2 and less than or equal to 8.

According to a preferred embodiment of the core-sheath composite fiber of the present invention, the core-sheath composite fiber has a single melting peak temperature within the range of 100°C to 150°C in differential scanning calorimetry.

The present invention provides a polyethylene spunbond nonwoven fabric that exhibits excellent softness, skin-feeling, uniform texture, sufficient strength for practical use, and excellent productivity. Due to these properties, the spunbond nonwoven fabric of the present invention is particularly suitable for use as a sanitary material.

Furthermore, the present invention provides a core-sheath composite fiber that exhibits excellent softness and skin-feel, along with excellent yarn stability and thermal adhesion. Spunbond nonwoven fabrics made from the core-sheath composite fiber of the present invention possess these excellent properties.

[Form used to implement the invention]

The spunbond nonwoven fabric of the present invention is composed of a core-sheath composite fiber having a polyethylene resin as a main component. The spunbond nonwoven fabric has a fused portion and a non-fused portion. The ratio of the alignment parameter Ofs of the sheath component of the core-sheath composite fiber in the non-fused portion to the alignment parameter Ofc of the core component of the core-sheath composite fiber in the non-fused portion (Ofs/Ofc) is 0.10 to 0.90.

This results in a polyethylene spunbond nonwoven fabric that is soft and has an excellent skin feel, a uniform texture, sufficient strength for practical use, and excellent productivity.

Furthermore, the core-sheath composite fiber of the present invention comprises a polyethylene resin as a main component, and the ratio of the orientation parameter Ofs of the sheath component of the core-sheath composite fiber in the non-welded portion to the orientation parameter Ofc of the core component of the core-sheath composite fiber (Ofs/Ofc) is 0.10 to 0.90.

This creates a core-sheath composite fiber that offers excellent softness and skin feel, along with excellent yarn stability and thermal adhesion. Spunbond nonwovens made from the core-sheath composite fiber of the present invention can be made into polyethylene spunbond nonwovens that are soft, have an excellent skin feel, a uniform texture, possess sufficient strength for practical use, and offer excellent productivity.

The following describes in detail the constituent elements of the present invention. However, the present invention is not limited to the scope of the following description as long as it does not exceed the gist of the invention.

[Polyethylene resin]

The core-sheath composite fiber and the core-sheath composite fiber constituting the spunbonded nonwoven fabric of the present invention (hereinafter sometimes collectively referred to as the "core-sheath composite fiber of the present invention") are composed primarily of a polyethylene resin. By using a polyethylene resin as the primary component, the core-sheath composite fiber exhibits both excellent spinning stability and thermal adhesion. Furthermore, the resulting spunbonded nonwoven fabric exhibits excellent softness and skin-friendly feel.

Polyethylene resins are resins containing ethylene units as repeating units. Examples include ethylene homopolymers and copolymers of ethylene and various α-olefins. Of these, ethylene homopolymers are preferred to prevent degradation of yarn stability and strength.

When using copolymers of ethylene and various α-olefins, heptene or octene are preferred as the copolymer component due to their excellent spinning stability, with octene being more preferred. Furthermore, to prevent a decrease in spinning stability and strength, the copolymerization ratio is preferably 5 mol% or less, more preferably 3 mol% or less, and even more preferably 1 mol% or less.

The polyethylene resin used in the present invention preferably has an ethylene homopolymer content of 60% by mass or greater, more preferably 70% by mass or greater, and even more preferably 80% by mass or greater. This maintains good spinnability and enhances strength.

Examples of the polyethylene resin used in the present invention include medium-density polyethylene, high-density polyethylene (hereinafter sometimes referred to as HDPE), and linear low-density polyethylene (hereinafter sometimes referred to as LLDPE). LLDPE is preferred due to its excellent spinnability.

Furthermore, the polyethylene resin used in the present invention may be a mixture of two or more types. Resin compositions containing other polyolefin resins such as polypropylene and poly-4-methyl-1-pentene, thermoplastic elastomers, low-melting polyesters, and low-melting polyamides may also be used. However, to fully demonstrate the properties of polyethylene, the proportion of the other thermoplastic resins mixed is preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 1% by mass or less.

The polyethylene resin used in the present invention preferably contains a fatty acid amide compound with a carbon number of 23 or more and 50 or less to enhance skin feel and softness. By preferably setting the carbon number of the fatty acid amide compound to 23 or more, more preferably 30 or more, excessive exposure of the fatty acid amide compound to the fiber surface is suppressed, resulting in excellent spinnability and processing stability, while maintaining high productivity. Furthermore, by preferably setting the carbon number of the fatty acid amide compound to 50 or less, more preferably 42 or less, the fatty acid amide compound is more easily transferred to the fiber surface, imparting slipperiness and softness to the spunbonded nonwoven fabric.

Examples of the fatty acid amide compound having 23 or more and 50 or less 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.

More specifically, the following can be cited: tetracosamide, hexacosamide, octacosamide, tetracosenoic acid amide, tetracosopentaenoic acid amide, tetracosahexenoic acid amide, ethylene bislaurate amide, methylene bislaurate amide, ethylene bisstearamide, ethylene bishydroxystearamide, ethylene bis Behenamide, hexamethylenebisstearamide, hexamethylenebisbehenamide, hexamethylenehydroxystearamide, distearyladipamide, distearylsebacate, ethylenebisoleamide, ethylenebiserucamide, and hexamethylenebisoleamide, etc. These can also be used in combination.

In the present invention, among the fatty acid amide compounds, ethylenyl bisstearamide, a saturated fatty acid diamide compound, is particularly preferred because it can impart high slip and softness, as well as excellent spinnability.

In the present invention, the fatty acid amide compound is preferably added to the polyethylene resin in an amount of 0.01% to 5% by mass. By preferably adding the fatty acid amide compound in an amount of 0.01% to 5% by mass, more preferably 0.1% to 3% by mass, and even more preferably 0.1% to 1% by mass, the yarn can be imparted with appropriate slipperiness and softness while maintaining spinnability.

The amount added here refers to the mass fraction of the fatty acid amide compound in the total polyethylene resin constituting the spunbonded nonwoven fabric of this invention. For example, even when the fatty acid amide compound is added only to the sheath component of a core-sheath composite fiber, the added ratio relative to the total mass of the core-sheath component is calculated.

One method for determining the amount of fatty acid amide compounds added to fibers made from polyethylene resins is to extract the additive from the fibers using a solvent and then perform quantitative analysis using liquid chromatography-mass spectrometry (LC/MS). The extraction solvent is appropriately selected depending on the type of fatty acid amide compound. For example, in the case of ethylene bis(stearic acid)amide, a chloroform-methanol mixture can be used.

The polyethylene resin used in the present invention may contain commonly used additives such as antioxidants, weathering stabilizers, light stabilizers, heat stabilizers, antistatic agents, charging aids, antifogging agents, anti-blocking agents, lubricants including polyethylene wax, crystallization nucleating agents, pigments, or other polymers, as needed, without impairing the effectiveness of the present invention.

The polyethylene resin used in the present invention preferably has a melting point (Tmr) of 100°C to 150°C. By setting the Tmr preferably above 100°C, more preferably above 110°C, and even more preferably above 120°C, heat resistance sufficient for practical use is easily achieved. Furthermore, by setting the Tmr preferably below 150°C, more preferably below 140°C, and even more preferably below 135°C, the yarn ejected from the spinneret is easily cooled, preventing fiber fusion, and enabling stable spinning even with thin fiber diameters. The melting point (Tmr) referred to herein refers to the maximum melting peak temperature of the resin as measured by differential scanning calorimetry (DSC).

The polyethylene resin used in the present invention preferably has a melt flow rate (MFR) of 1 g/10 min to 300 g/10 min. By setting the MFR of the polyethylene resin preferably to 1 g/10 min or higher, more preferably 10 g/10 min or higher, and even more preferably 30 g/10 min or higher, even fine fibers can be spun stably, resulting in a spunbond nonwoven fabric with excellent skin feel, uniform texture, and sufficient strength for practical use. On the other hand, by preferably setting the MFR of the polyethylene resin to 300g/10min or less, a decrease in the strength of the single yarn can be suppressed, while also preventing excessive softening during heat bonding, which can lead to operational problems such as adhesion to hot rolls.

The core-sheath composite fiber of the present invention preferably has a polyethylene resin core component with an MFR of 1 g/10 min to 100 g/10 min. With an MFR of 1 g/10 min or greater, more preferably 10 g/10 min or greater, and even more preferably 30 g/10 min or greater, even fine fibers can be spun reliably, resulting in a spunbond nonwoven fabric with excellent skin feel, uniform texture, and sufficient strength for practical use. On the other hand, by setting the polyethylene resin's MFR to preferably 100 g/10 min or less, more preferably 80 g/10 min or less, and even more preferably 60 g/10 min or less, a decrease in the single yarn strength of the core-sheath composite fiber can be suppressed, resulting in a spunbond nonwoven fabric with sufficient strength for practical use.

In the core-sheath composite fiber of the present invention, the MFR of the polyethylene resin of the sheath component is preferably 5 g/10 min to 200 g/10 min greater than the MFR of the polyethylene resin of the core component. By having the MFR of the polyethylene resin of the sheath component be preferably 5 g/10 min or greater, more preferably 10 g/10 min or greater, and even more preferably 20 g/10 min or greater than the MFR of the polyethylene resin of the core component, spinning stress can be concentrated on the core component during spinning, promoting molecular alignment in the core component while suppressing molecular alignment in the sheath component. On the other hand, if the MFR of the polyethylene resin in the sheath component exceeds the MFR of the polyethylene resin in the core component by more than 200 g/10 min, the single yarn strength of the core-sheath composite fiber will decrease. Furthermore, the fiber will tend to soften excessively during heat bonding, causing problems with attaching to hot rolls, etc., which is undesirable.

The MFR of polyethylene resins is measured using ASTM D1238 (Method A). This standard specifies that polyethylene be measured under a load of 2.16 kg and a temperature of 190°C. The polyethylene resins of the present invention were also measured under the same load and temperature.

Of course, the MFR of the polyethylene resin used in the present invention can be adjusted by blending two or more resins with different MFRs in any ratio. In this case, the MFR of the blended resin, which accounts for the largest mass fraction of the primary polyethylene resin, is preferably 10-1000 g/10 min, more preferably 20-800 g/10 min, and even more preferably 30-600 g/10 min. This prevents partial viscosity variations in the blended polyethylene resin, allowing for uniform single fiber diameter and fiber density, and enabling stable spinning of even fine fibers.

Furthermore, the polyethylene resin used in the present invention preferably does not contain any agents that decompose the polyethylene resin and reduce its MFR, such as peroxides, particularly free radicals such as dialkyl peroxides. This prevents the occurrence of localized viscosity variations due to uneven decomposition or gelation, resulting in uniform single fiber density and stable spinning of even fine fibers. It also prevents deterioration of spinnability caused by bubbles generated by decomposed gases.

The polyethylene resin used in the present invention preferably has a solid density of 0.935 g/cm ³ to 0.970 g/cm ³ . By setting the solid density of the polyethylene resin preferably at least 0.935 g/cm ³ , more preferably at least 0.940 g/cm ³ , and even more preferably at least 0.945 g/cm ³ , excessive softening during heat bonding, which can cause operational problems such as adhesion to hot rolls, can be prevented. Furthermore, by setting the solid density of the polyethylene resin preferably at most 0.970 g/cm ³ , more preferably at most 0.965 g/cm ³ , and even more preferably at most 0.96 g/cm ³ , spinnability can be improved, enabling stable spinning of even fine fibers.

[Core-sheath composite fiber]

The core-sheath type composite fibers of the present invention also include sea-island type composite fibers. In the case of sea-island type composite fibers, when measuring and explaining the properties of the polyethylene resin of the core or sheath components of the composite fibers, the "sheath component" is referred to as the "sea component," and the "core component" is referred to as the "island component," and the measurements are performed.

The core-sheath composite fiber of the present invention preferably has a sheath component mass ratio of 20% to 80% by mass. With a sheath component mass ratio of preferably 20% by mass or greater, more preferably 30% by mass or greater, and particularly preferably 40% by mass or greater, the sheath components can be firmly fused together during heat welding, resulting in a spunbond nonwoven fabric with sufficient strength for practical use. On the other hand, with a sheath component mass ratio of preferably 80% by mass or less, more preferably 70% by mass or less, and particularly preferably 60% by mass or less, the proportion of the highly oriented core component can be increased, thereby improving the single yarn strength of the core-sheath composite fiber and producing a spunbond nonwoven fabric with sufficient strength for practical use.

In the core-sheath composite fiber of the present invention and the core-sheath composite fiber in the non-welded portion of the spunbonded nonwoven fabric of the present invention, the ratio of the alignment parameter Ofs of the sheath component to the alignment parameter Ofc of the core component, Ofs/Ofc, is 0.10 to 0.90.

Here, the alignment parameter of the core-sheath composite fiber of the present invention is the following index (unitless): a larger value indicates that the molecular chains of the polyethylene resin constituting the core-sheath composite fiber are more specifically aligned, while a smaller value indicates that the molecular chains are more randomly aligned. Furthermore, the alignment parameter is 1.2 when the molecular chains are completely randomly aligned.

By setting Ofs/Ofc to 0.10 or greater, preferably 0.15 or greater, and more preferably 0.20 or greater, it is possible to prevent excessive concentration of tensile stress on the inner fiber layer, where the core component is present, during spinning, thereby reducing spinning stability. On the other hand, by setting Ofs/Ofc to 0.90 or less, preferably 0.70 or less, and more preferably 0.50 or less, it is possible to soften only the fiber surface during thermal bonding. This allows the fibers to be firmly thermally bonded while preserving the molecular alignment of the inner fiber layer. Furthermore, the spunbond nonwoven fabric of the present invention can be a polyethylene spunbond nonwoven fabric that is soft, has an excellent skin feel, a uniform texture, possesses sufficient strength to withstand practical use, and has excellent productivity.

The alignment parameter Ofs of the sheath component of the core-sheath type composite fiber of the present invention and the alignment parameter Ofc of the core component of the core-sheath type composite fiber are measured using the following method.

(1) Use bisphenol epoxy resin to embed the core-sheath composite fiber or spunbond nonwoven fabric sample.

(2) After the resin has hardened, slices are cut using a microtome with a thickness of 2 μm. The slices are cut obliquely from the fiber axis so that the cut surface becomes an ellipse. The thickness of the minor axis of the ellipse is then selected to represent a certain thickness and measured. Furthermore, since the cutting angle is set within 4°, the film thickness can be considered parallel to the fiber axis within a 2 μm film thickness.

When the sample is spunbond nonwoven fabric,

(2) After the resin has hardened, slice the nonwoven fabric using a microtome so that the center of the non-welded portion (at a location approximately equidistant from the surrounding welded portions) is the cross-section. The slice thickness is set to 2 μm. Select the composite fiber in the non-welded portion and the cutting angle is within 4° from the fiber axis for subsequent measurements.

(3) In a section of a core-sheath composite fiber, polarized light parallel to the fiber axis is incident from the fiber surface to the center, and Raman spectrum line measurement is performed.

(4) Calculate the Raman band intensities _I1130 and _I1060 at the core and sheath components, respectively, near 1130 cm ^-1 and 1060 cm ^-1 . Calculate the alignment parameter from the ratio of these intensities using the following formula (d). When the core component is divided into multiple independent regions, measure the alignment parameter for all regions and use the highest value.

Orientation parameter = I ₁₁₃₀ /I ₁₀₆₀ ‧‧‧(d).

(5) In the axial direction of the core-sheath composite fiber, change the position and perform the same measurement at 3 locations, calculate the average value of the orientation parameter, and round off the second decimal place.

When the sample is spunbond nonwoven fabric,

(5) For different non-welded parts of the spunbond nonwoven fabric, perform the same measurement at three locations, calculate the average value of the orientation parameter, and round off the second decimal place.

The core-sheath composite fiber of the present invention and the core-sheath composite fiber in the non-welded portion of the spunbonded nonwoven fabric of the present invention preferably have an orientation parameter, Ofs, of the sheath component of 2 to 8. An Ofs of preferably 2.0 or greater, more preferably 2.5 or greater, and particularly preferably 3.0 or greater, prevents excessive softening of the fiber surface during thermal bonding, which can cause operational problems such as adhesion to hot rolls. On the other hand, an Ofs of preferably 8.0 or less, more preferably 7.0 or less, and particularly preferably 6.0 or less, facilitates softening of the fiber surface during thermal bonding, enabling strong thermal bonding between the fibers. This results in a spunbonded nonwoven fabric with sufficient strength to withstand practical use.

Ofs can be controlled by the MFR, melting point, additives, mass ratio of the sheath component of the composite fiber, and/or the spinning temperature and spinning speed described below, etc. of the polyethylene resin.

The core-sheath composite fiber of the present invention and the core-sheath composite fiber in the non-welded portion of the spunbonded nonwoven fabric of the present invention preferably have an orientation parameter Ofc of 6 to 18 for the core component. An Ofc of preferably 6.0 or greater, more preferably 7.0 or greater, and even more preferably 8.0 or greater, enhances the strength of the fiber's inner layer, resulting in a spunbonded nonwoven fabric with sufficient strength to withstand practical use after thermal bonding. Furthermore, excessive softening of the fiber surface during thermal bonding, which can lead to problems with adhesion to hot rollers, etc., can be prevented. On the other hand, by setting Ofc to preferably 18.0 or less, more preferably 16.0 or less, and even more preferably 14.0 or less, excessive tensile stress concentration on the inner layer of the fiber during spinning can be suppressed, thereby improving spinning stability.

Ofc can be controlled by the MFR, melting point, additives, mass ratio of the core component of the composite fiber, and/or the spinning temperature and spinning speed described below, etc. of the polyethylene resin.

In the core-sheath composite fiber of the present invention and the core-sheath composite fiber of the non-welded portion of the spunbonded nonwoven fabric of the present invention, the softening temperature Tss (°C) of the surface layer and the softening temperature Tsc (°C) of the inner layer preferably satisfy the following formula (a).

(Tss+5)≦Tsc≦(Tss+30)‧‧‧(a).

By setting the Tsc (°C) preferably at or above (Tss + 5)°C, more preferably at or above (Tss + 7)°C, and even more preferably at or above (Tss + 10)°C, only the components forming the fiber surface layer can be softened during thermal bonding. Furthermore, this allows the fibers to be firmly thermally bonded together while retaining the molecular alignment of the fiber's inner layer, resulting in a spunbond nonwoven fabric with sufficient strength to withstand practical use. On the other hand, by setting the Tsc (°C) preferably at or below (Tss + 30)°C, more preferably at or below (Tss + 25)°C, and even more preferably at or below (Tss + 20)°C, excessive softening of the fiber surface during thermal bonding, which could cause problems with adhesion to hot rollers, etc.

Tss (°C) and Tsc (°C) were calculated using the following procedure using nanoscale-thermomechanical analysis (nano-TMA). Nano-TMA is capable of thermal analysis in the submicron range and uses a temperature sensor equipped with a heater mounted on the probe (cantilever) of an atomic force microscope (AFM).

In the spunbonded nonwoven fabric of the present invention, the Tss (°C) and Tsc (°C) of the non-welded portion are measured and calculated using the following procedure after collecting 20 core-sheath composite fibers from the non-welded portion of the spunbonded nonwoven fabric.

(1) Fix the core-sheath composite fiber on the sample table and fix the AFM probe with a heater and temperature sensor near the center of the fiber diameter.

(2) Raise the probe temperature from 25°C to 150°C at a rate of 10°C/s and measure the change in probe height (a.u.).

(3) Measure the temperature (softening temperature (°C)) when the probe is inserted into the sample based on the change in probe height. The values observed from the lowest temperature are designated as Ts1, Ts2, Ts3, etc.

(4) Perform the same measurement on 20 fibers, round off the average value of Ts1 to the second decimal place and use it as Tss (℃). Also, round off the average value of Ts2 to the second decimal place and use it as Tsc (℃). In addition, Ts2 may not be observed in some core-sheath composite fibers due to the contact position of the AFM probe. In this case, only the observed Ts2 is averaged to obtain the softening temperature of the inner layer Tsc (℃).

Tss and Tsc can be controlled by the MFR, melting point, additives, mass ratio of the sheath component of the core-sheath composite fiber, and/or the spinning temperature and spinning speed described below.

The core-sheath type composite fiber and the spunbond nonwoven fabric of the present invention preferably have a single melting peak temperature Tm in differential scanning calorimetry (DSC). Moreover, in the present invention, the so-called "core-sheath type composite fiber has a single melting peak temperature Tm in differential scanning calorimetry" and "spunbond nonwoven fabric has a single melting peak temperature Tm in differential scanning calorimetry" means that the melting endothermic peak described in (3) of the following measurement method is substantially only observed as a single peak. This allows the core-sheath composite fiber of the present invention, when used as a fiber constituting a spunbonded nonwoven fabric, for example, to be heat-bonded without causing operational problems such as low-melting-point components melting and adhering to a heat roller. The fibers can be firmly heat-bonded at a sufficient temperature, making it easier to obtain a spunbonded nonwoven fabric with sufficient strength for practical use.

The melting peak temperature (Tm) of core-sheath composite fibers or spunbond nonwoven fabrics obtained by differential scanning calorimetry (DSC) is calculated using the following procedure.

(1) Take a sample of 0.5~5mg of core-sheath composite fiber or spunbond nonwoven fiber sheet.

(2) Using differential scanning calorimetry (DSC), the temperature was raised from room temperature to 200°C at a heating rate of 20°C/min to obtain the DSC curve.

(3) Read the peak temperature of the melting endothermic peak from the DSC curve and use it as the melting peak temperature Tm (℃).

Furthermore, when the core-sheath composite fiber of the present invention is used as the fiber constituting the spunbonded nonwoven fabric of the present invention, the Tm of the core-sheath composite fiber and the Tm of the spunbonded nonwoven fabric are considered to be the same value.

Furthermore, the core-sheath composite fiber and the spunbonded nonwoven fabric of the present invention preferably satisfy the following formulas (b) and (c).

100≦Tm≦150‧‧‧(b)

(Tm-40)≦Tss≦(Tm-10)‧‧‧(c)

This enables the production of core-sheath composite fibers and spunbond nonwovens that possess heat resistance and strength sufficient for practical use, as well as excellent spinning stability and handling stability.

First, regarding formula (b), the melting peak temperature Tm (°C) of the core-sheath composite fiber, as measured by differential scanning calorimetry (DSC), is preferably 100°C or higher and 150°C or lower. A melting peak temperature Tm (°C) of preferably 100°C or higher, more preferably 110°C or higher, and particularly preferably 120°C or higher, imparts heat resistance sufficient for practical use. Furthermore, a melting peak temperature Tm (°C) of preferably 150°C or lower, more preferably 140°C or lower, and particularly preferably 135°C or lower facilitates cooling of the yarn ejected from the spinneret, suppresses fusion between fibers, and facilitates stable spinning even with thin fiber diameters.

Next, regarding formula (c), the softening temperature Tss (°C) of the surface layer of the core-sheath composite fiber is preferably not less than (Tm-40)°C and not more than (Tm-10)°C. By setting the softening temperature Tss (°C) of the surface layer to preferably not less than (Tm-40)°C, more preferably not less than (Tm-35)°C, and even more preferably not less than (Tm-30)°C, excessive softening of the fiber surface during thermal bonding, which could lead to operational problems such as adhesion to a hot roll, can be prevented. On the other hand, by keeping Tss (°C) preferably below (Tm - 10)°C, more preferably below (Tm - 15)°C, and even more preferably below (Tm - 20)°C, the fibers can be firmly bonded together during heat bonding, resulting in a spunbond nonwoven fabric with sufficient strength to withstand practical use.

Furthermore, the core-sheath composite fiber of the present invention melts after softening, so the softening temperature Tsc (°C) of the inner layer is less than the melting peak temperature Tm (°C) measured by differential scanning calorimetry (DSC). Moreover, the softening temperature Tsc (°C) of the inner layer of the core-sheath composite fiber is preferably above (Tm-20)°C and below (Tm-1)°C. By having the softening temperature Tsc (°C) of the inner layer preferably above (Tm-20)°C, more preferably above (Tm-15)°C, and even more preferably above (Tm-10)°C, the strength of the inner layer of the fiber can be increased, resulting in a spunbond nonwoven fabric having strength sufficient to withstand practical use after heat bonding. On the other hand, by keeping Tsc (°C) preferably below (Tm-1)°C, more preferably below (Tm-3)°C, and even more preferably below (Tm-5)°C, the fibers can be firmly bonded together during thermal bonding, resulting in a spunbond nonwoven fabric with sufficient strength to withstand practical use.

The core-sheath composite fibers of the present invention may be formed in a concentric core-sheath, eccentric core-sheath, or island-in-the-sea configuration. A core-sheath composite fiber is preferred due to its excellent spinnability and ability to uniformly bond the fibers together by heat welding, and a concentric core-sheath composite fiber is even more preferred.

The cross-sectional shape of the core-sheath composite fiber of the present invention can be round, flat, or irregularly shaped, such as Y-shaped or C-shaped. A round cross-section is preferred because it avoids the bending difficulties associated with flat or irregular cross-sections and allows for a spunbond nonwoven fabric that utilizes the softness of polyethylene resin. While a hollow cross-section is also possible, a solid cross-section is preferred because it offers excellent spinnability and allows for stable spinning even with fine fiber diameters.

The core-sheath composite fiber of the present invention preferably has an average single fiber fineness of 0.5 dtex to 3.0 dtex. By setting the average single fiber fineness preferably to 0.5 dtex or higher, more preferably to 0.6 dtex or higher, and particularly preferably to 0.7 dtex or higher, a decrease in spinnability can be prevented, resulting in a spunbond nonwoven fabric with excellent production stability. On the other hand, by setting the average single fiber fineness preferably to 3.0 dtex or lower, more preferably to 2.4 dtex or lower, and particularly preferably to 2.0 dtex or lower, a spunbond nonwoven fabric with excellent skin feel, uniform texture, and sufficient strength for practical use can be achieved.

The average single fiber fineness can be controlled by the spinning temperature, single hole output, spinning speed, etc., which will be described later.

The core-sheath composite fiber of the present invention preferably has an average single fiber diameter of 8 to 20 μm. By setting the average single fiber diameter preferably to 8 μm or greater, more preferably 9 μm or greater, and even more preferably 10 μm or greater, a decrease in spinnability can be prevented, resulting in a spunbond nonwoven fabric with excellent stability. On the other hand, by setting the average single fiber diameter preferably to 20 μm or less, more preferably 18 μm or less, and even more preferably 16 μm or less, a spunbond nonwoven fabric with excellent skin feel, uniform texture, and sufficient strength for practical use can be achieved.

Furthermore, in the present invention, the average single fiber diameter (μm) of the core-sheath composite fiber is the value calculated using the following procedure.

(1) For core-sheath composite fibers, take surface photographs at 500-2000x magnification using a microscope or scanning electron microscope, and measure the width (diameter) of a total of 100 different core-sheath composite fibers. If the cross-section of the core-sheath composite fiber is irregular, measure the cross-sectional area and find the diameter of a perfect circle with the same cross-sectional area.

(2) The average of the 100 measured diameters is rounded off to the second decimal place and is taken as the average single fiber diameter (μm).

Furthermore, the average single fiber diameter (μm) of the core-sheath composite fiber constituting the aforementioned spunbonded nonwoven fabric of the present invention is a value calculated using the following procedure.

(1) Randomly collect 10 small samples (100×100 mm) from spunbond nonwoven fabric.

(2) Take surface photographs at 500-2000x magnification using a microscope or scanning electron microscope. Measure the width (diameter) of the non-welded portion of the core-sheath composite fiber for 10 fibers from each sample and a total of 100 fibers. If the cross-section of the core-sheath composite fiber is irregular, measure the cross-sectional area and find the diameter of a perfect circle with the same cross-sectional area.

(3) The average of the 100 measured diameters is rounded off to the second decimal place and is taken as the average single fiber diameter (μm).

The average single fiber diameter can be controlled by the spinning temperature, single hole output, spinning speed, etc., which will be described later.

The core-sheath composite fiber of the present invention preferably has a solid density of 0.935 g/cm ³ to 0.970 g/cm ³ . By setting the solid density of the polyethylene resin preferably at least 0.935 g/cm ³ , more preferably at least 0.940 g/cm ³ , and particularly preferably at least 0.945 g/cm ³ , excessive softening during heat bonding, which can cause operational problems such as adhesion to hot rolls, can be prevented. Furthermore, by setting the solid density of the polyethylene resin preferably at most 0.970 g/cm ³ , more preferably at most 0.965 g/cm ³ , and particularly preferably at most 0.960 g/cm ³ , spinnability can be improved, enabling stable spinning of even fine fibers.

Furthermore, in the present invention, the solid density (g/cm ³ ) of the composite fiber is a value calculated by the following procedure.

(1) Immerse the composite fiber test piece in ethanol, wash it, and dry it in the air.

(2) For composite fiber test pieces, the density was determined by the sinking and floating method using a water-ethanol mixture system.

(3) Perform the same measurement five times using different test pieces, average the measured density values (g/cm ³ ), round off to the fourth decimal place and use it as the solid density (g/cm ³ ) of the composite fiber.

The solid density (g/cm ³ ) of the composite fiber constituting the spunbonded nonwoven fabric was calculated using the following procedure.

(1) Randomly collect 5 small pieces from the spunbond nonwoven fabric.

(2) Wash the small piece in ethanol and dry it in the air.

(3) For small pieces of spunbond nonwoven fabric, the density is determined by the floatation method using a water-ethanol mixture system.

(4) Carry out the same measurement on 5 small pieces, average the measured density value (g/cm ³ ), and round off to the fourth decimal place to obtain the solid density (g/cm ³ ) of the composite fiber.

[Spunbond Nonwoven Fabric]

The spunbond nonwoven fabric of the present invention is composed of a core-sheath composite fiber with polyethylene resin as the main component.

The spunbonded nonwoven fabric of the present invention has welded and non-welded areas. This configuration allows the fabric to maintain the softness and skin-touch feel of the polyethylene resin while also possessing sufficient strength for practical use. The welded areas are where the core-sheath composite fibers are welded together, while the non-welded areas are where the core-sheath composite fibers remain unfused, maintaining their cross-sectional shape.

In the spunbonded nonwoven fabric of the present invention, the sheath component of the core-sheath composite fiber in the aforementioned welded portion preferably has an orientation parameter, Obs, of 1.2 to 3.0. When Obs is preferably 1.2, the molecular chains are in a completely random orientation state, and values below this value are not acceptable. Furthermore, by preferably maintaining the sheath component orientation parameter, Obs, at 3.0 or less, more preferably 2.5 or less, and even more preferably 2.0 or less, the sheath components forming the fiber surface layer are firmly thermally bonded to each other, resulting in a spunbonded nonwoven fabric with sufficient strength to withstand practical use.

The orientation parameter Obs of the sheath component in the core-sheath composite fiber at the welded portion can be controlled by appropriately adjusting the orientation parameter Ofs of the sheath component of the core-sheath composite fiber and/or the heat-bonding conditions (temperature, linear pressure, etc.) described below.

In the spunbonded nonwoven fabric of the present invention, the core component of the core-sheath composite fiber in the aforementioned welded portion preferably has an orientation parameter, Obc, of 2 to 10. With an Obc of preferably 2.0 or greater, more preferably 2.5 or greater, and particularly preferably 3.0 or greater, the core component's strength is enhanced, resulting in a spunbonded nonwoven fabric with sufficient strength for practical use. Furthermore, excessive softening of the fiber surface during thermal bonding, which can cause problems with adhesion to hot rollers, etc., is prevented. Furthermore, with an Obc of preferably 10.0 or less, more preferably 9.0 or less, and particularly preferably 8.0 or less, excessive tensile stress concentration on the core component during spinning is suppressed, improving spinning stability.

The orientation parameter Obc of the core component in the core-sheath composite fiber at the welded portion can be controlled by appropriately adjusting the orientation parameter Ofc of the core component of the core-sheath composite fiber and/or the heat-bonding conditions (temperature, linear pressure, etc.) described below.

Obs and Obc are measured using the following procedure.

(1) The spunbond nonwoven fabric sample was embedded in a bisphenol epoxy resin.

(2) After the resin has hardened, slice the nonwoven fabric using a microtome so that the center of the welded portion forms the cutting surface. The slice thickness is set to 2 μm. Select a location with a cutting angle within 4° from the fiber axis for subsequent measurements. If the direction of the fiber axis is difficult to identify, rotate the polarization direction by 15 degrees at the same point, obtain polarized Raman spectra in each direction, and take the direction with the largest orientation parameter as the fiber axis direction.

(3) Polarized light parallel to the fiber axis is incident on the center of the core-sheath composite fiber slice at the welded portion, and Raman spectrum line measurement is performed.

(4) Calculate the Raman band intensities _I1130 and _I1060 at the sheath and core components of the core-sheath composite fiber at the welded portion, respectively, near 1130 cm ^-1 and 1060 cm ^-1 . Calculate the orientation parameter from the intensity ratio using the following formula (d). When the core component is divided into multiple independent regions, measure the orientation parameter for all regions and use the highest value.

Orientation parameter = I ₁₁₃₀ /I ₁₀₆₀ ‧‧‧(d).

(5) For different welded parts of the spunbond nonwoven fabric, perform the same measurement at three locations, calculate the average value of the orientation parameter, and round off the second decimal place.

The spunbond nonwoven fabric of the present invention preferably has a surface roughness (SMD) of 1.0 to 3.0 μm on at least one side as measured by the KES method. Since the surface roughness (SMD) as measured by the KES method is preferably 1.0 μm or greater, more preferably 1.3 μm or greater, and even more preferably 1.6 μm or greater, the spunbond nonwoven fabric can be prevented from becoming overly dense, which could degrade its feel or impair its softness. Furthermore, since the surface roughness (SMD) as measured by the KES method is preferably 3.0 μm or less, more preferably 2.8 μm or less, and even more preferably 2.5 μm or less, the fabric can be smooth, have minimal roughness, and provide excellent skin-feeling.

The surface roughness SMD measured by the KES method can be controlled by appropriately adjusting the average single fiber diameter of the core-sheath composite fiber, the texture of the spunbonded nonwoven fabric, and/or the heat-bonding conditions (bonding area shape, compression ratio, temperature, linear pressure, etc.) described below.

Furthermore, the surface roughness SMD measured by the KES method in the present invention is measured as follows.

(1) From the spunbond nonwoven fabric, collect three test pieces with a width of 200mm x 200mm at equal intervals in the width direction of the spunbond nonwoven fabric.

(2) Place the test piece on the test bench.

(3) Use a surface roughness measurement contact tip (material: Φ0.5mm piano wire, contact length: 5mm) with a load of 10gf to scan the surface of the test piece and measure the average deviation of the surface roughness.

(4) Perform the above measurements on all test pieces in the longitudinal direction (length direction of the nonwoven fabric) and transverse direction (width direction of the nonwoven fabric). Average the average deviation of the total of 6 points and round off to the second decimal place to obtain the surface roughness SMD (μm).

The spunbond nonwoven fabric of the present invention preferably has a coefficient of friction (MIU) of 0.01 to 0.30 as measured by the KES method. A coefficient of friction (MIU) of preferably 0.30 or less, more preferably 0.20 or less, and particularly preferably 0.15 or less improves the slipperiness of the nonwoven surface, resulting in a spunbond nonwoven fabric with excellent skin-feeling. Furthermore, a coefficient of friction (MIU) of preferably 0.01 or greater, more preferably 0.03 or greater, and particularly preferably 0.05 or greater, prevents the spun yarns from slipping against each other when collected on a collecting conveyor, thereby reducing texture uniformity.

The coefficient of friction (MIU) measured by the KES method can be controlled by appropriately adjusting the additives in the polyethylene resin, the average single fiber diameter of the core-sheath composite fiber, the texture of the spunbond nonwoven fabric, and/or the thermal bonding conditions (bonding area shape, compression ratio, temperature, and linear pressure, etc.) described below.

Furthermore, the friction coefficient MIU measured by the KES method in the present invention is measured as follows.

(1) From the spunbond nonwoven fabric, collect three test pieces with a width of 200mm x 200mm at equal intervals in the width direction of the spunbond nonwoven fabric.

(2) Place the test piece on the test bench.

(3) Scan the surface of the test piece with a contact friction head (material: Φ0.5mm piano wire (20 parallel pieces), contact area: ^1cm2 ) with a load of 50gf to measure the friction coefficient.

(4) Carry out the above measurements on all test pieces in the longitudinal direction (length direction of the nonwoven fabric) and transverse direction (width direction of the nonwoven fabric), average the average deviation of these 6 points, round off to the fourth decimal place, and use it as the friction coefficient MIU.

The MFR of the spunbond nonwoven fabric of the present invention is preferably between 1g/10min and 300g/10min. With an MFR of preferably 1g/10min or higher, more preferably 10g/10min or higher, and even more preferably 30g/10min or higher, the nonwoven fabric can be spun stably even with fine fibers, resulting in a spunbond nonwoven fabric with excellent skin feel, uniform texture, and sufficient strength for practical use. On the other hand, by preferably having an MFR of 300g/10min or lower for the polyethylene resin, a decrease in strength can be suppressed, preventing excessive softening during heat bonding, which can lead to problems with adhesion to hot rollers, etc.

The MFR of the spunbond nonwoven fabric of this invention is measured using ASTM D1238 (Method A). According to this standard, polyethylene is measured under a load of 2.16 kg and a temperature of 190°C.

The spunbond nonwoven fabric of the present invention preferably has a weight per unit area of 10 g/ ^m² to 100 g/ ^m² . With a weight per unit area of preferably 10 g/ ^m² or greater, more preferably 13 g/ ^m² or greater, and particularly preferably 15 g/m² or ^greater , the fabric can have sufficient strength for practical use. On the other hand, with a weight per unit area of preferably 100 g/ ^m² or less, more preferably 50 g/ ^m² or less, and particularly preferably 30 g/ ^m² or less, the fabric can have a softness suitable for use as a nonwoven fabric for sanitary purposes.

Furthermore, in this invention, the unit area weight of spunbond nonwoven fabrics is determined using the following procedure in accordance with "6.2 Mass per unit area" of JIS L1913:2010, "Test methods for general nonwoven fabrics."

(1) Collect three 20cm x 25cm test pieces for every 1m of sample width.

(2) Weigh the respective masses (g) under standard conditions.

(3) The average value is expressed as mass per 1m ² (g/m ² ).

The spunbond nonwoven fabric of the present invention preferably has a thickness of 0.05mm to 1.5mm. This preferred thickness of 0.05mm to 1.5mm, more preferably 0.08mm to 1.0mm, and even more preferably 0.10mm to 0.8mm, provides softness and moderate cushioning properties, making it particularly suitable for use as a sanitary material, particularly in disposable diapers.

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

(1) Using a 10mm diameter pressure head, measure the thickness of the nonwoven fabric at 10 points per meter at equal intervals of 0.01mm in the width direction at a load of 10kPa.

(2) Round off the average of the above ten points to the third decimal place.

Furthermore, the apparent density of the spunbond nonwoven fabric of the present invention is preferably between 0.05 g/ ^cm³ and 0.30 g/ ^cm³ . By preferably keeping the apparent density below 0.30 g/ ^cm³ , more preferably below 0.25 g/ ^cm³ , and even more preferably below 0.20 g/ ^cm³ , dense packing of fibers, which could impair the softness of the spunbond nonwoven fabric, is prevented. On the other hand, by preferably keeping the apparent density above 0.05 g/ ^cm³ , more preferably above 0.08 g/ ^cm³ , and even more preferably above 0.10 g/ ^cm³ , fuzzing and interlayer delamination are suppressed, resulting in a spunbond nonwoven fabric with the strength and handleability required for practical use.

The apparent density can be controlled by appropriately adjusting the average single fiber diameter of the core-sheath composite fiber and/or the heat-bonding conditions (bonding shape, compression ratio, temperature, linear pressure, etc.) described below.

Furthermore, in the present invention, the apparent density (g/cm ³ ) is calculated from the unit area weight and thickness before rounding according to the following formula, with the third decimal place rounded off.

Apparent density (g/cm ³ ) = [unit area weight (g/m ² )] / [thickness (mm)] × 10 ^-3 ... (formula).

The stiffness of the spunbond nonwoven fabric of the present invention is preferably 60 mm or less. With a stiffness of 60 mm or less, more preferably 50 mm or less, and even more preferably 40 mm or less, the spunbond nonwoven fabric for sanitary use can achieve excellent softness, making it particularly suitable for use in disposable diapers. However, since extremely low stiffness results in poor handling, a stiffness of 10 mm or greater is preferred.

Stiffness can be controlled by appropriately adjusting the MFR of the polyethylene resin, additives, the average single fiber diameter of the core-sheath composite fiber, the unit area weight of the spunbonded nonwoven fabric, the ratio of the sheath component orientation parameter Ofs in the non-welded portion of the core-sheath composite fiber to the core component orientation parameter Ofc in the non-welded portion (Ofs/Ofc), and/or the heat-bonding conditions described below (bonding portion shape, press-bonding ratio, temperature, and linear pressure, etc.).

The spunbond nonwoven fabric of the present invention preferably has a transverse tensile strength per unit area of 0.20 (N/25mm)/(g/m ² ) or greater, more preferably 0.20 (N/25mm)/(g/m ² ) to 2.00 (N/25mm)/(g/m ² ). By having a tensile strength per unit area of preferably 0.20 (N/25mm)/(g/m ² ) or greater, more preferably 0.25 (N/25mm)/(g/m ² ) or greater, and even more preferably 0.30 (N/25mm)/(g/m ² ) or greater, the fabric can be made strong enough to withstand practical use. On the other hand, by keeping the transverse tensile strength per unit area at or below 2.00 (N/25mm)/(g/m ² ), the softness of the spunbond nonwoven fabric can be prevented from being reduced or its feel impaired. Furthermore, spunbond nonwovens have tensile strength in both the longitudinal and transverse directions, but generally speaking, the transverse tensile strength is lower than the longitudinal tensile strength. Therefore, by keeping the transverse tensile strength per unit area at 0.2-2.00 (N/25mm)/(g/m ² ), the spunbond nonwoven fabric can have practical strength in the longitudinal direction.

The transverse tensile strength per unit area weight can be controlled by appropriately adjusting the MFR of the polyethylene resin, additives, the average single fiber diameter of the core-sheath composite fiber, the ratio of the orientation parameter Ofs of the sheath component of the core-sheath composite fiber in the non-welded portion to the orientation parameter Ofc of the core component of the core-sheath composite fiber in the non-welded portion (Ofs/Ofc), and/or the spinning speed and thermal bonding conditions (bonding portion shape, press bonding ratio, temperature, and linear pressure, etc.) described below.

Furthermore, in the present invention, the transverse tensile strength per unit area weight of the spunbond nonwoven fabric is measured using the following procedure in accordance with "6.3 Tensile Strength and Elongation (ISO Method)" of JIS L1913:2010 "General Nonwoven Fabrics Test Methods."

(1) With the long side of the nonwoven fabric facing the width (the width direction of the nonwoven fabric), collect three test pieces of 25mm x 200mm for every 1m of the nonwoven fabric width.

(2) Place the specimen in the tensile testing machine with the clamps spaced 100 mm apart.

(3) Carry out a tensile test at a tensile speed of 100 mm/min to measure the maximum strength.

(4) Calculate the average value of the maximum strength measured on each test piece and calculate the tensile strength per unit area weight according to the following formula, rounding off to the third decimal place.

Transverse tensile strength per unit area ((N/25mm)/(g/m ² )) = [average value of maximum strength (N/25mm)]/unit area weight (g/m ² )… (formula).

The spunbonded nonwoven fabric of the present invention preferably has a stress per unit area weight at 5% elongation in the longitudinal direction of 0.20 (N/25mm)/(g/m ² ) or more, more preferably 0.20 (N/25mm)/(g/m ² ) to 2.00 (N/25mm)/(g/m ² ). By setting the stress per unit area at 5% elongation in the longitudinal direction to preferably 0.2 (N/25mm)/(g/m ² ) or greater, more preferably 0.25 (N/25mm)/(g/m ² ) or greater, and even more preferably 0.30 (N/25mm)/(g/m ² ) or greater, the spunbond nonwoven fabric can be prevented from stretching due to tension during production or when processed as a sanitary material, enabling stable production with high yield. Furthermore, by setting the stress per unit area at 5% elongation in the longitudinal direction to preferably 2.00 (N/25mm)/(g/m ² ) or less, the softness of the spunbond nonwoven fabric can be prevented from being reduced or its feel impaired.

The stress at 5% elongation in the longitudinal direction per unit area weight can be controlled by appropriately adjusting the MFR of the polyethylene resin, additives, the average single fiber diameter of the core-sheath composite fiber, the ratio of the orientation parameter Ofs of the sheath component of the core-sheath composite fiber in the non-welded portion to the orientation parameter Ofc of the core component of the core-sheath composite fiber in the non-welded portion (Ofs/Ofc), and/or the spinning speed and thermal bonding conditions (bonding portion shape, press bonding ratio, temperature, and linear pressure, etc.) described below.

Furthermore, in the present invention, the stress per unit area weight of the spunbond nonwoven fabric at 5% elongation in the longitudinal direction is the value measured using the following procedure in accordance with "6.3 Tensile Strength and Elongation (ISO Method)" of JIS L1913:2010 "Test Methods for General Nonwoven Fabrics."

(1) With the long side of the nonwoven fabric in the longitudinal direction (length direction of the nonwoven fabric), collect three test pieces of 25mm x 200mm for every 1m of the nonwoven fabric width.

(2) Place the specimen in the tensile testing machine with the clamps spaced 100 mm apart.

(3) Carry out a tensile test at a tensile speed of 100 mm/min and measure the stress at 5% elongation (5% elongation stress).

(4) Determine the average value of the stress at 5% elongation measured on each test piece and calculate the stress at 5% elongation in the longitudinal direction per unit area weight using the following formula, rounding off to the third decimal place.

Stress at 5% elongation in the longitudinal direction per unit area weight ((N/25mm)/(g/m ² )) = [Average stress at 5% elongation (N/25mm)]/unit area weight (g/m ² )… (Formula).

[Manufacturing method of spunbond nonwoven fabric]

Next, the preferred method for manufacturing the spunbond nonwoven fabric of the present invention is described in detail.

The spunbond nonwoven fabric of the present invention is a long-fiber nonwoven fabric produced by the spunbond method. In addition to its excellent productivity and mechanical strength, the spunbond method also suppresses the fuzzing and fiber shedding that are common with short-fiber nonwovens. Furthermore, laminating multiple layers of captured spunbond nonwoven fiber webs or heat-pressed spunbond nonwoven fabrics is preferred due to improved productivity and texture uniformity.

In the spunbond process, melted thermoplastic resin is first spun from a spinning nozzle as long fibers. An ejector draws compressed air through the fibers, stretching them. The fibers are then captured on a moving net to form a nonwoven web. The resulting nonwoven web is then thermally bonded to create a spunbond nonwoven fabric.

There are no specific restrictions on the shape of the spinning nozzle or jet; various shapes, such as round or rectangular, are possible. However, a rectangular nozzle and jet combination is preferred because it uses less compressed air, resulting in lower energy costs, is less likely to cause welding or friction between yarns, and facilitates yarn unwinding.

In the present invention, a polyethylene resin is melted and metered in an extruder, then supplied to a spinning nozzle to be spun as filaments. The spinning temperature during melt spinning of the polyethylene resin is preferably 180°C to 250°C, more preferably 190°C to 240°C, and even more preferably 200°C to 230°C. By setting the spinning temperature within this range, a stable melt state is achieved, resulting in excellent spinning stability.

The spun filament yarn is then cooled. Examples of methods for cooling the spun yarn include forcibly blowing cold air onto the yarn, naturally cooling the yarn at the ambient temperature, adjusting the distance between the spinning nozzle and the ejector, or a combination of these methods. Cooling conditions can be appropriately adjusted based on factors such as the output per hole of the spinning nozzle, the spinning temperature, and the ambient temperature.

The cooled and solidified yarn is then pulled and stretched by compressed air ejected from a jet.

The spinning speed is preferably 3000 m/min to 6000 m/min, more preferably 3500 m/min to 5500 m/min, and even more preferably 4000 m/min to 5000 m/min. Setting the spinning speed to 3000 m/min to 6000 m/min allows for high productivity and facilitates fiber orientation crystallization, resulting in high-strength long fibers. As described above, the core-sheath composite fiber of the present invention, primarily composed of a polyethylene resin, exhibits excellent spinning stability and can be produced stably even at high spinning speeds.

The resulting long fibers are then captured on a moving mesh to form a nonwoven web.

In the present invention, it is preferred that the nonwoven web be temporarily bonded to a heat-flattened roller on one side of the web. This prevents the surface of the nonwoven web from curling or fluttering during transport, thereby deteriorating the web's quality, and improves handling performance from the point of collecting the yarns to heat-pressing.

Next, by fusing the resulting nonwoven fiber web to form a fused portion, the desired spunbonded nonwoven fabric can be obtained.

There are no particular limitations on the methods for fusing nonwoven webs. Examples include heat-fusion methods using various rollers, such as a heat-embossing roller with engravings (concave and convex portions) on each of a pair of upper and lower roller surfaces, a heat-embossing roller composed of a flat (smooth) roller and a roller with engravings (concave and convex portions), and a heat-compression rolling roller composed of a pair of flat (smooth) rollers. Heat-fusion methods using ultrasonic vibrations from a horn are also possible. Furthermore, methods in which hot air is passed through the nonwoven web to soften or melt the surface of core-sheath composite fibers, thereby fusing the intersections of the fibers.

Preferred are hot embossing rolls with engravings (concavoconvex sections) on each of the upper and lower surfaces, or a combination of a roll with a flat (smooth) surface and another roll with engravings (concavoconvex sections). This approach improves productivity and allows for the provision of welded sections that enhance the strength of the spunbond nonwoven fabric, as well as non-welded sections that improve the hand and skin feel.

As the surface material for hot embossing rolls, it is best to use metal rolls in pairs to achieve a sufficient hot pressing effect and prevent the engraving (convex and concave parts) of one embossing roll from being transferred to the surface of the other roll.

The embossing area ratio achieved by such a hot embossing roll is preferably 5-30%. By setting the bonding area ratio to preferably 5% or higher, more preferably 8% or higher, and particularly preferably 10% or higher, a spunbond nonwoven fabric can be provided with sufficient strength for practical use. On the other hand, by setting the bonding area ratio to preferably 30% or lower, more preferably 25% or lower, and particularly preferably 20% or lower, a spunbond nonwoven fabric for sanitary use can be provided with a degree of softness particularly suitable for use in disposable diapers. When ultrasonic bonding is used, the bonding area ratio is preferably within the same range.

The "bonding area ratio" mentioned here refers to the percentage of the bonded area to the total nonwoven fabric. Specifically, when heat-bonding is performed using a pair of convex-concave rollers, this refers to the percentage of the area where the convex portions of the upper and lower rollers overlap and contact the nonwoven web (the bonded area). Furthermore, when heat-bonding is performed using a convex-concave roller and a flat roller, this refers to the percentage of the area where the convex portions of the convex-concave roller contact the nonwoven web (the bonded area). Furthermore, when ultrasonic bonding is performed, this refers to the percentage of the area thermally fused by ultrasonic processing (the bonded area) to the total nonwoven fabric. During thermal bonding, sufficient heat is applied to the bonded portion, and when the core-sheath composite fibers in the bonded portion are completely fused, the areas of the bonded portion and the fused portion can be considered equal.

The shape of the bonded portions created by hot embossing rolls or ultrasonic bonding is not particularly limited; for example, circular, elliptical, square, rectangular, parallelogram, rhombus, regular hexagon, and regular octagonal shapes can be used. Furthermore, the bonded portions are preferably uniformly spaced at regular intervals in both the lengthwise (conveying) and widthwise directions of the spunbond nonwoven fabric. This reduces strength variations within the spunbond nonwoven fabric.

The surface temperature of the heat embossing roller during heat bonding is preferably set between 30°C lower and 10°C higher than the melting point (Tm) of the thermoplastic resin used, that is, between Tm-30°C and Tm+10°C. By setting the heat roller surface temperature above Tm-30°C, more preferably above Tm-20°C, and even more preferably above Tm-10°C, a strong heat bond is achieved, resulting in a spunbond nonwoven fabric with sufficient strength for practical use. Furthermore, by setting the surface temperature of the heat embossing roller preferably below Tm + 10°C, more preferably below Tm + 5°C, and even more preferably below Tm, excessive heat bonding can be suppressed, resulting in a spunbond nonwoven fabric for sanitary use having the appropriate softness, particularly suitable for use in disposable diapers.

The linear pressure of the embossing roller during heat bonding is preferably set to 50 N/cm to 500 N/cm. By setting the linear pressure of the roller to preferably 50 N/cm or higher, more preferably 100 N/cm or higher, and even more preferably 150 N/cm or higher, strong heat bonding is achieved, resulting in a spunbond nonwoven fabric with sufficient strength for practical use. On the other hand, by setting the linear pressure of the embossing roller to preferably 500 N/cm or lower, more preferably 400 N/cm or lower, and even more preferably 300 N/cm or lower, the spunbond nonwoven fabric for sanitary use can be made with a degree of softness that is particularly suitable for use in disposable diapers.

Furthermore, in the present invention, to adjust the thickness of the spunbond nonwoven fabric, heat-compression bonding can be performed using a pair of upper and lower flat rollers before and/or after the heat-compression bonding using the heat-embossing rollers. The pair of upper and lower flat rollers refers to metal rollers or elastic rollers with no uneven surfaces. Metal rollers can be paired with metal rollers, or with elastic rollers.

The elastic roller referred to here is a roller made of a material that is more elastic than a metal roller. Examples of elastic rollers include paper rollers made of paper, cotton, and aromatic polyamide paper, as well as resin rollers made of urethane resins, epoxy resins, silicone resins, polyester resins, hard rubber, and mixtures thereof.

The spunbond nonwoven fabric of this invention is suitable for a wide range of applications, including sanitary materials, medical materials, household materials, and industrial materials, due to its softness, excellent skin feel, uniform texture, sufficient strength to withstand practical use, and excellent productivity. In particular, it can be used as a base fabric for disposable diapers, sanitary products, and wet cloths in sanitary applications, and as protective clothing and surgical gowns in medical applications.

[Example]

Next, the spunbonded nonwoven fabric of the present invention will be described in detail based on the following examples. However, the present invention is not limited to these examples. Furthermore, the physical properties measured were performed according to the aforementioned methods unless otherwise specified.

[Measurement Method] (1) Resin melt flow rate (MFR) (g/10 minutes)

The resin's MFR is measured under a load of 2.16 kg and a temperature of 190°C.

(2) Average single fiber diameter of core-sheath composite fibers constituting spunbond nonwovens (μm)

Measurements were performed using the aforementioned method using a KEYENCE electron microscope "VHX-D500."

(3) Solid density of composite fibers constituting spunbond nonwoven fabrics (g/cm ³ )

The solid density of the composite fiber was measured using the aforementioned method.

(4) Spinning speed (m/min)

Based on the above average single fiber diameter and the solid density of the resin used, the average single fiber fineness (dtex) was calculated by rounding off to the second decimal place, taking the mass per 10,000 m of length as the average single fiber fineness. The spinning speed was calculated using the following formula based on the average single fiber fineness and the resin output per spindle hole (hereinafter referred to as "single hole output") (g/min) under various spindle conditions.

Spinning speed (m/min) = (10000 × [single hole output (g/min)]) / [average single fiber fineness (dtex)]… (formula).

(5) Softening temperature of core-sheath composite fiber (℃) and softening temperature of core-sheath composite fiber in non-welded portion of spunbond nonwoven fabric (℃)

Measurements were performed using the Nano-TA2 instrument from Analysis Instruments, the Nano-R AFM from PACIFIC NANOTECHNOLOGY, and the PNI-AN2-300 probe from Analysis Instruments. Measurement conditions were as follows.

‧Measurement method: nano-TMA (nanothermomechanical analysis)

‧Measurement temperature: 25~150℃

‧Heating rate: 10°C/second (600°C/minute)

‧Measurement environment: in the atmosphere.

(6) Orientation parameters of core-sheath composite fibers, orientation parameters of core-sheath composite fibers in the non-welded portion of spunbond nonwoven fabrics, and orientation parameters of core-sheath composite fibers in the welded portion of spunbond nonwoven fabrics

The measurement was performed using the T-64000 triple Raman spectrometer manufactured by Atago Bussan Co., Ltd. The measurement conditions were as follows.

‧Measurement mode: Micro-Raman (polarization measurement)

‧Objective lens: ×100

‧Beam diameter: 1μm

‧Light source: Ar ⁺ laser/514.5nm

Laser power: 100mW

‧Diffraction Grating: Single 1800gr/mm

Cross stitch: 100μm

‧Detector: CCD/Jobin Yvon 1024×256.

(7) Melting peak temperature Tm (℃) of spunbond nonwoven fabric

The measurement was performed using the DSC8500 manufactured by Perkin-Elmer using the aforementioned method. The measurement conditions were as follows.

‧Environment inside the device: Nitrogen (20mL/min)

Temperature and heat correction: High-purity indium (Tm=156.61°C, ΔHm=28.70 J/g)

Temperature range: 20°C~200°C

‧Heating rate: 20℃/min

‧Amount of sample: about 0.5~4mg

‧Sample container: Standard aluminum container.

(8) Longitudinal stiffness of spunbond nonwoven fabric (mm)

The stiffness of spunbond nonwovens is measured in the longitudinal direction (lengthwise) of the fabric according to the method described in JIS L1913:2010, "Test Methods for General Nonwoven Fabrics," "6.7 Stiffness (JIS and ISO Methods)," "6.7.4 Gray Method." For all spunbond nonwovens, the stiffness in the longitudinal direction (lengthwise) is greater than the stiffness in the transverse direction (widthwise). A stiffness in the longitudinal direction of 50 mm or less is considered acceptable.

(9) Tensile strength per unit area weight and stress at 5% elongation per unit area weight of spunbond nonwoven fabric (N/25mm/(g/ ^m2 ))

The measurement was performed using the RTG-1250 manufactured by A&D Co., Ltd. using the aforementioned method. A tensile strength in the transverse direction of 0.2 (N/25mm)/(g/m ² ) or greater per unit area weight was considered acceptable, and a stress at 5% elongation in the transverse direction of 0.2 (N/25mm)/(g/m ² ) or greater per unit area weight was considered acceptable.

[Example 1]

A polyethylene resin composed of a linear low-density polyethylene (LLDPE) homopolymer with a melt flow rate (MFR) of 30 g/10 min, a melting point of 128°C, and a solid density of 0.955 g/ ^cm³ was used as the core component, and a polyethylene resin composed of a LLDPE homopolymer with an MFR of 60 g/10 min, a melting point of 127°C, and a solid density of 0.940 g/ ^cm³ was used as the sheath component. Each was melted in an extruder and spun from a spinning nozzle with a hole diameter of 0.40 mm and a hole depth of 8 mm at a spinning temperature of 220°C and a single hole discharge rate of 0.50 g/min, with a sheath component ratio of 40% by mass to produce a concentric core-sheath type composite fiber.

After cooling and solidifying, the spun yarn is drawn and stretched by compressed air in a jet, where it is captured on a moving mesh, forming a spunbond nonwoven web made of polyethylene filaments. The core-sheath composite fibers that comprise this nonwoven web exhibit an average single fiber diameter of 11.6 μm and a solid density of 0.949 g/cm ³ , which translates to a spinning speed of 5000 m/min. Spinnability was excellent, with no yarn breakage observed during the one-hour spinning cycle.

The resulting nonwoven web was then heat-bonded using a pair of embossing rolls, consisting of an upper and lower roll, at a linear pressure of 300 N/cm and a heat-bonding temperature of 126°C to produce a spunbond nonwoven fabric with a unit area weight of 30 g/ ^m² .

Upper roller: Metal embossing roller with an engraved water drop pattern and an 11% bonding area ratio.

Lower roller: Metal flat roller

The resulting spunbond nonwoven fabric has a uniform texture and excellent skin feel. The evaluation results are shown in Table 1.

[Example 2]

A spunbond nonwoven fabric was obtained by the same method as in Example 1, except that the sheath component ratio was set to 50% by mass and the compressed air flow rate of the ejector was reduced. The fibers constituting the resulting spunbond nonwoven web exhibited an average single fiber diameter of 13.7 μm and a solid density of 0.948 g/cm ³ , which translates to a spinning speed of 3600 m/min. Spinnability was good, with no yarn breakage observed during the one-hour spinning process. The resulting spunbond nonwoven fabric had a uniform texture and excellent skin feel. The evaluation results are shown in Table 1.

[Example 3]

A spunbond nonwoven fabric was obtained by the same method as in Example 1, except that the sheath component ratio was set to 30% by mass and the compressed air flow rate of the ejector was reduced. The fibers constituting the resulting spunbond nonwoven web exhibited an average single fiber diameter of 15.5 μm and a solid density of 0.951 g/cm ³ , which translates to a spinning speed of 2800 m/min. Spinnability was good, with no yarn breakage observed during the one-hour spinning process. The resulting spunbond nonwoven fabric had a uniform texture and excellent skin feel. The evaluation results are shown in Table 1.

[Example 4]

A spunbond nonwoven fabric was obtained by the same method as ^{in Example 2} , except that a polyethylene resin composed of a homopolymer of LLDPE with an MFR of 30 g/10 min, a melting point of 128°C, and a solid density of 0.955 g/cm³ was used as the core component, and a polyethylene resin composed of a homopolymer of LLDPE with an MFR of ⁵⁰ g/10 min, a melting point of 128°C, and a solid density of 0.950 g/cm³ was used as the sheath component. The fibers constituting the resulting spunbond nonwoven web had an average single fiber diameter of 13.7 μm and a solid density of 0.953 g/ ^cm³ , which translated to a spinning speed of 3600 m/min. Regarding spinnability, yarn breakage occurred several times during the one-hour spinning process. The evaluation results of the resulting spunbond nonwoven fabric are shown in Table 1.

[Example 5]

A spunbond nonwoven fabric was obtained by the same method as in Example 2, except that the spinning nozzle was modified to form an eccentric core-sheath composite fiber. The fibers constituting the resulting spunbond nonwoven web exhibited an average single fiber diameter of 13.7 μm and a solid density of 0.948 g/cm ³ , which translates to a spinning speed of 3600 m/min. Spinnability was good, with no yarn breakage observed during the one-hour spinning cycle. The resulting spunbond nonwoven fabric exhibited a uniform texture and excellent skin feel. The evaluation results are shown in Table 1.

[Comparative example 1]

^A spunbond nonwoven fabric was obtained by the same method as in Example 2, except that the spinning was performed using a single-component polyethylene resin composed of a homopolymer of LLDPE with an MFR of 30 g/10 min, a melting point of 128°C, and a solid density of 0.955 g/cm³. The fibers constituting the resulting spunbond nonwoven web exhibited an average single fiber diameter of 13.9 μm and a solid density of 0.955 g/ ^cm³ , which translates to a spinning speed of 3500 m/min. Spinnability was poor, with frequent yarn breakage occurring within one hour of spinning. The evaluation results of the resulting spunbond nonwoven fabric are shown in Table 1.

[Comparative example 2]

A spunbond nonwoven fabric was obtained by the same method as ^{in Example 2} , except that a polyethylene resin composed of a homopolymer of LLDPE with an MFR of 60 g/10 min, a melting point of 127°C, and a solid density of 0.940 g/cm³ was used as a single component, and the heat-bonding temperature was set at 120°C. The fibers constituting the resulting spunbond nonwoven web exhibited an average single fiber diameter of 13.7 μm and a solid density of 0.940 g/ ^cm³ , resulting in a spinning speed of 3600 m/min. Spinnability was good, with no yarn breakage observed during the one-hour spinning cycle. Furthermore, if the heat bonding temperature was set to 126°C, the sheet broke during attachment to the heat embossing roll, making production impossible. Table 1 shows the evaluation results of the resulting spunbond nonwoven fabric.

[Comparative example 3]

A spunbond nonwoven fabric was obtained by the same method as in Example 2, except that the method disclosed in Patent Document 2 (Japanese Patent Application Laid-Open No. 2019-26954) was referenced, and a polyethylene resin composed of a homopolymer of LLDPE with an MFR of 100 g/10 min, a melting point of 115°C, and a solid density of 0.933 g/ ^cm³ was used as a single component for spinning. The fibers constituting the resulting spunbond nonwoven web had an average single fiber diameter of 15.2 μm and a solid density of 0.933 g/ ^cm³ , resulting in a spinning speed of 3500 m/min, equivalent to that of Example 1 in Patent Document 2. The spinnability was good, with no yarn breakage observed during the one-hour spinning process. The evaluation results of the resulting spunbond nonwoven fabric are shown in Table 1.

[Comparative example 4]

A spunbond nonwoven fabric was obtained by the same method as ^{in Example 2} , except that a polyethylene resin composed of a homopolymer of LLDPE with an MFR of 30 g/10 min, a melting point of 128°C, and a solid density of 0.955 g/cm³ was used as the core component, and a polyethylene resin composed of a homopolymer of LLDPE with an MFR of ⁴⁰ g/10 min, a melting point of 128°C, and a solid density of 0.950 g/cm³ was used as the sheath component. The fibers constituting the resulting spunbond nonwoven web had an average single fiber diameter of 13.7 μm and a solid density of 0.953 g/ ^cm³ , which translated to a spinning speed of 3600 m/min. Regarding spinnability, yarn breakage occurred frequently during the one-hour spinning cycle, resulting in a poor spin quality. Table 1 shows the evaluation results of the resulting spunbond nonwoven fabric.

The spunbond nonwoven fabrics of Examples 1-5, composed of core-sheath composite fibers primarily composed of polyethylene resin, and having a ratio of Ofs to Ofc in the non-welded portion (Ofs/Ofc) of 0.10 to 0.90, are soft and have an excellent skin feel, a uniform texture, sufficient strength to withstand practical use, and excellent productivity.

On the other hand, the spunbond nonwovens shown in Comparative Examples 1 to 4 have low tensile strength in the transverse direction per unit area weight or low stress at 5% elongation in the longitudinal direction per unit area weight, indicating poor strength.

without.

Claims (9)

A spunbond nonwoven fabric is formed from a core-sheath composite fiber having a polyethylene resin as a main component. The spunbond nonwoven fabric has a fused portion and a non-fused portion. The ratio of the orientation parameter Ofs of the sheath component of the core-sheath composite fiber in the non-fused portion to the orientation parameter Ofc of the core component of the core-sheath composite fiber in the non-fused portion (Ofs/Ofc) is 0.10 to 0.90. The spunbond nonwoven fabric has a single melting peak temperature (Tm) within the range of 100°C to 150°C in differential scanning calorimetry. The spunbond nonwoven fabric of claim 1, wherein the alignment parameter Obs of the sheath component of the core-sheath composite fiber in the welded portion is 1.2 to 3.0, and the alignment parameter Obc of the core component of the core-sheath composite fiber in the welded portion is 2.0 to 10.0. The spunbond nonwoven fabric of claim 1 or 2, wherein the core-sheath composite fiber has a solid density of not less than 0.935 g/cm ³ and not more than 0.970 g/cm ³ . For spunbond nonwoven fabrics as claimed in claim 1 or 2, where the Ofs is not less than 2.0 and not more than 8.0. The spunbond nonwoven fabric of claim 1 or 2, wherein the tensile strength per unit area of the spunbond nonwoven fabric in the transverse direction is 0.20 (N/25mm)/(g/m ² ) or more. For the spunbond nonwoven fabric of claim 1 or 2, the stress at 5% elongation in the longitudinal direction per unit area weight of the spunbond nonwoven fabric is 0.20 (N/25mm)/(g/ ^m2 ) or more. A core-sheath composite fiber having a polyethylene resin as a main component, wherein a ratio of an orientation parameter Ofs of the sheath component of the core-sheath composite fiber to an orientation parameter Ofc of the core component of the core-sheath composite fiber (Ofs/Ofc) is 0.10 to 0.90, and the core-sheath composite fiber has a single melting peak temperature (Tm) within a range of not less than 100°C and not more than 150°C in differential scanning calorimetry. The core-sheath composite fiber of claim 7, wherein the solid density of the polyethylene resin is not less than 0.935 g/cm ³ and not more than 0.970 g/cm ³ . The core-sheath composite fiber of claim 7 or 8, wherein the Ofs is not less than 2.0 and not more than 8.0.