TW202441044A - Core-sheath composite fiber and nonwoven fabric formed by the core-sheath composite fiber - Google Patents

Abstract

The purpose of the present invention is to provide a core-sheath composite fiber with excellent white concealing property without using titanium oxide. The core-sheath composite fiber of the present invention is composed of a core component including a polyester resin and a sheath component including a polyolefin resin, wherein the polyolefin resin has a melting point lower than that of the polyester resin by more than 20°C, and one or both of the core component and the sheath component contain more than 1.0% by mass and less than 30.0% by mass of calcium carbonate in 100% by mass of each component, and the maximum peak temperature of crystal melting of the core component measured by a differential scanning calorimeter is less than 255°C, and the L value indicating the hue of the core-sheath composite fiber is more than 91, and the b value is less than 5.0.

Description

Core-sheath composite fiber and nonwoven fabric formed by the core-sheath composite fiber

The present invention relates to a heat-bonded composite fiber which maintains the processability of a non-woven fabric and has excellent safety, concealment and sustainability for use in absorbent articles for sanitary materials such as diapers, sanitary napkins, pads, medical and sanitary materials, materials related to life, general medical materials, bedding materials, filters, nursing products, and pet products, and also to a non-woven fabric formed from the heat-bonded composite fiber.

Some nonwoven fabrics used for sanitary purposes such as diapers and sanitary napkins are obtained by heat-bonding a core-sheath structure of two-component resin raw cotton in an air-through manner, and are required to have a white color tone that gives a clean impression and concealment properties for concealing the color of soaked blood and urine.

Generally speaking, in order to improve the concealment of the fibers that make up non-woven fabrics, titanium oxide is contained in the resin. However, titanium oxide was classified as carcinogenicity category 2 (inhalation) by the European CLP (Classification, Labelling and Packaging) rules in October 2021. For example, products containing more than 1% by mass of titanium oxide require specific warnings and labeling, indicating that there are concerns about safety.

Titanium oxide is widely used as a white pigment for resin additives due to its high refractive index and high stability, accounting for 70% of the white raw materials currently in use. Other white pigments include zinc oxide, but compared to titanium oxide, it is less safe and stable, so it is considered difficult to replace it in the use of sanitary materials. In addition, titanium oxide is an underground resource (mineral resource), and its sustainability is poor compared to biological resources that belong to environmental recycling materials.

On the other hand, although there are many reports of adding inorganic particles and talc other than titanium oxide and zinc oxide to resins to improve the transparency and softness of fibers and nonwovens, the refractive index of inorganic particles and talc themselves is low and close to that of polyester resins. Therefore, it is considered that if only inorganic particles and talc are added to the resin, it is not possible to expect the same level of white color and hiding power as titanium oxide.

For example, in Patent Document 1, calcium carbonate is used to make nonwoven fabrics soft to the touch and highly rigid, but the concealing property and color of the fibers are not discussed.

According to Patent Document 2, a multifilament fiber is provided whose mechanical hardness and thermal conductivity are improved and whose opacity is increased by calcium carbonate. However, the color is not discussed.

In Patent Document 3, in order to improve the see-through property (concealment property) of the core-sheath composite fiber of two components, inorganic particles are contained therein, and the amount of inorganic particles added to the core is 0.10 mass % to 10 mass %, and the amount of inorganic particles added to the sheath is 0.050 mass % to 1.0 mass %. The function is expressed by containing inorganic particles in both the core and the sheath, and titanium oxide is mainly used as the inorganic particles. In one of the embodiments, there is an example of using calcium carbonate in the core and aluminum oxide in the sheath, but it is not discussed to use only calcium carbonate to improve the see-through property.

In Patent Document 4, in recycled polyester core-sheath composite fibers, the hue (L value, b value) is improved by using recycled PET (polyethylene terephthalate) in the core and adding 3% by mass or more of titanium oxide, but it is not discussed whether other inorganic particles or talc are used to improve the hue. As shown in Patent Documents 3 and 4, titanium oxide is not suitable for use in plastic reduction by reducing the amount of resin by adding it at a high concentration. [Prior Art Document] [Patent Document]

[Patent Document 1] Japanese Patent Publication No. 2020-90771. [Patent Document 2] Japanese Patent Publication No. 2019-112759. [Patent Document 3] Japanese Patent Publication No. 2021-55231. [Patent Document 4] Japanese Patent Publication No. 2006-328600.

[The problem that the invention wants to solve]

As mentioned above, in order to solve the safety and sustainability shortcomings of titanium oxide, which is a white pigment used in the past, and to use it for plastic volume reduction by adding it at a high concentration, it is better to add other inorganic particles to replace titanium oxide, thereby providing a fiber that maintains post-processing properties for non-woven fabrics, etc., does not significantly affect the quality of the fiber, reduces the amount of resin used, and can contribute to reducing the environmental load. Furthermore, it is also better to provide a safe and sustainable fiber by using inorganic substances that can be expected to be a bioresource material that is environmentally recyclable. The purpose of the present invention is to provide a core-sheath composite fiber with excellent white concealing property without using titanium oxide, and a nonwoven fabric formed by the core-sheath composite fiber. [Means for solving the problem]

The core-sheath composite fiber of the present invention capable of achieving the above-mentioned purpose refers to a fiber having the following characteristics. The core-sheath composite fiber of the present invention is composed of a core component including a polyester resin and a sheath component including a polyolefin resin, wherein the polyolefin resin has a melting point that is 20°C or more lower than that of the polyester resin, and the core-sheath composite fiber is characterized in that: one or both of the core component and the sheath component contain more than 1.0% by mass and less than 30.0% by mass of calcium carbonate in 100% by mass of each component, the maximum peak temperature of crystal melting of the core component measured by a differential scanning calorimeter is less than 255°C, and the L value indicating the hue of the core-sheath composite fiber is greater than 91 and the b value is less than 5.0.

It is also preferred that the peak temperature of the core component crystallization upon cooling measured by a differential scanning calorimeter is 190°C to 220°C.

Preferably, the polyester resin comprises polyethylene terephthalate, and the polyolefin resin comprises polyethylene.

Preferably, the content of the polyester resin is 40 mass % or more of the entire fiber.

The cross-sectional shape of the fiber may also be a concentric cross-sectional shape, an eccentric cross-sectional shape, or a hollow cross-sectional shape.

Preferably, the tensile strength measured according to JIS L1013 8.5.1 is 0.80 cN/dtex to 5.00 cN/dtex, and preferably, the elongation measured according to JIS L1013 8.5.1 is 30% to 200%.

The present invention includes a nonwoven fabric formed by the aforementioned core-sheath composite fiber. [Effect of the invention]

According to the present invention, a core-sheath composite fiber having excellent white concealing property without using titanium oxide and a nonwoven fabric formed by the core-sheath composite fiber can be provided.

That is, the core-sheath composite fiber of the present invention can maintain the same level of concealment, softness, bulkiness, and bulk recovery as the conventional fiber added with white pigment mainly composed of titanium oxide, and has excellent safety and environmental sustainability, and can be used to produce non-woven fabrics that can reduce environmental load by reducing plastic content.

Furthermore, titanium oxide has a high Mohs hardness and causes metal wear. Therefore, in the case of a fiber containing titanium oxide on the fiber surface, the metal roller of the processing machine is worn, especially during the process of winding, and continuous winding is difficult. In the present invention, by using calcium carbonate in the sheath component, the wear of the guide and the wear of the traveller during extension, which are the problems that the fiber containing titanium oxide on the fiber surface has previously been, can be reduced, and the white concealing property of the fiber can be improved. Furthermore, in the present invention, the white concealing property of the fiber can also be improved by using calcium carbonate in the core component. This makes it possible to obtain a two-component composite fiber for nonwoven fabrics that has the same level of white color, hiding power, and softness as conventional fibers that have titanium oxide added, while also having smooth fiber surface.

[Core-sheath composite fiber] The core-sheath composite fiber of the present invention is composed of a core component including a polyester resin and a sheath component including a polyolefin resin, wherein the polyolefin resin has a melting point that is 20°C or more lower than that of the polyester resin. The core-sheath composite fiber is characterized in that: one or both of the core component and the sheath component contain more than 1.0% by mass and less than 30.0% by mass of calcium carbonate in 100% by mass of each component, the maximum peak temperature of crystal melting of the core component measured by a differential scanning calorimeter is less than 255°C, and the L value indicating the hue of the core-sheath composite fiber is greater than 91 and the b value is less than 5.0. The fiber contains calcium carbonate in one or both of the core component and the sheath component. By stretching the fiber, the orientation of the polyester resin can be improved, and the crystal size of the polyester resin can be enlarged in the presence of calcium carbonate, resulting in an improvement in the whiteness (hiding property) of the core-sheath composite fiber.

The present invention includes: (i) a core-sheath composite fiber in which only the core component includes calcium carbonate (first aspect); (ii) a core-sheath composite fiber in which only the sheath component includes calcium carbonate (second aspect); and (iii) a core-sheath composite fiber in which both the core component and the sheath component include calcium carbonate (third aspect). In the following, unless it is explicitly stated as the first aspect, the second aspect, or the third aspect, it is deemed to be an explanation of all these aspects.

<Core component> The core component includes a polyester resin.

Polyester resins that exhibit strength and low shrinkage as core components include polyethylene terephthalate (melting point 255°C), polybutylene terephthalate (melting point 230°C), polytrimethylene terephthalate (melting point 230°C), polyethylene naphthalate (melting point 265°C), polybutylene naphthalate (melting point 243°C), polyhydroxyalkanoate (melting point about 180°C), polyhydroxybutyrate (melting point 175°C), polylactic acid (melting point 170°C to 175°C), etc.

Among them, the polyester resin preferably includes a polyester resin having a melting point of 200°C or higher (preferably 210°C or higher, more preferably 220°C or higher and 280°C or lower) from the viewpoints of strength and low shrinkage, and from the viewpoint of setting the melting point to be 20°C or higher than the melting point of the polyolefin resin used in the sheath component, and more preferably includes polyethylene terephthalate (melting point 255°C), polybutylene terephthalate (polybutylene terephthalate), and polybutylene terephthalate (polybutylene terephthalate). The polyester resin may be one or more of the following: 1) polyethylene terephthalate (melting point 230°C), 2) polyethylene naphthalate (melting point 265°C), 3) polyethylene naphthalate (melting point 243°C), 4) polyethylene terephthalate (melting point 255°C), 4) polyethylene naphthalate (melting point 265°C), 3) polyethylene naphthalate (melting point 243°C), 5) polyethylene terephthalate (melting point 255°C). The polyester resin may be one or more of the following:

The polyester resin may also have a predetermined intrinsic viscosity. The intrinsic viscosity (IV) of the polyester resin is preferably 0.3 dl/g to 2.0 dl/g, more preferably 0.5 dl/g to 1.5 dl/g, and even more preferably 0.55 dl/g to 0.80 dl/g. The intrinsic viscosity (IV) of the polyester resin can be obtained by grinding and drying the sample, dissolving it in a 6/4 (mass ratio) mixed solvent of phenol/1,1,2,2-tetrachloroethane, subjecting the solution to centrifugal separation to remove inorganic particles, etc., and then measuring it using an Oodel viscometer at a temperature of 30°C.

The content of the polyester resin having a melting point of 200°C or more is preferably 80% by mass or more, more preferably 90% by mass or more, more preferably 95% by mass or more, further preferably 97% by mass or more, particularly preferably 99% by mass or more, and most preferably 100% by mass in 100% by mass of the polyester resin. When the content of the polyester resin having a melting point of 200°C or more is within the above range, by extending in a state containing calcium carbonate, the crystallization of the polyester resin can be promoted and the whiteness of the fiber can be improved.

In the first and third aspects, the content of the polyester resin in the core component 100 mass% is preferably 70 mass% or more, more preferably 80 mass% or more, more preferably 90 mass% or more, and further preferably 95 mass% or more, preferably less than 99 mass%, more preferably less than 98 mass%, further preferably less than 97 mass%, and further preferably less than 96 mass%. If the content of the polyester resin is less than 70 mass%, there is a risk of reduced core strength and reduced adhesion between the core and the sheath. If the content of the polyester resin is more than 99 mass%, there is a risk that the crystallization of the polyester resin cannot be promoted and good whiteness cannot be achieved. In the aforementioned second aspect, the content of the polyester resin in the core component 100 mass % is preferably 100 mass %.

From the viewpoint of core strength, the content of the polyester resin is preferably 40% by mass or more of the entire fiber, more preferably 45% by mass or more and 75% by mass or less, further preferably 50% by mass or more and 70% by mass or less, and further preferably 55% by mass or more and 65% by mass or less.

In the first aspect and the third aspect, the calcium carbonate contained in the core component may be heavy calcium carbonate, light calcium carbonate, or a mixture thereof.

Heavy calcium carbonate can also be obtained by crushing and grading natural materials such as limestone, marble, calcite, and chalk, and is also called natural calcium carbonate. Light calcium carbonate can also be obtained by crystallizing in a liquid through a chemical reaction (for example, the reaction between calcium hydroxide and carbonic acid gas), and is also called synthetic calcium carbonate or precipitated calcium carbonate. Calcium carbonate can also be surface treated with fatty acids, silicones, or resins. As fatty acids, stearic acid, palmitic acid, myristic acid, lauric acid, etc. can be listed, and it can also be used as a salt or ester of fatty acids. As resins, polyacrylates, polydiallyldimethylammonium chloride, etc. can be listed.

The calcium carbonate preferably has a predetermined average particle size d50 and a top cut particle size d90. The average particle size d50 means a particle size smaller than the value expressed by 50% by mass of all particles, and the top cut particle size d90 means a particle size smaller than the value expressed by 90% by mass of all particles.

The average particle size d50 of the calcium carbonate used in the core is preferably 0.02μm to 3.0μm, more preferably 0.35μm to 1.75μm, more preferably 0.40μm to 1.50μm, and further preferably 0.45μm to 1.25μm. The top cut particle size d90 of the calcium carbonate is preferably 5.0μm or less, more preferably 4.0μm or less, more preferably 3.0μm or less, further preferably 2.0μm or less, particularly preferably 1.0μm or less, and preferably 0.3μm or more or 0.5μm or more.

In the first and third aspects, the content of calcium carbonate in the core component is preferably more than 1.0 mass % to less than 30.0 mass %, more preferably more than 2.0 mass % to less than 25.0 mass %, more preferably more than 3.0 mass % to less than 20.0 mass %, and further preferably more than 4.0 mass % to less than 15.0 mass %. If the content of calcium carbonate is less than 1.0 mass %, there is a risk that the crystallization of the polyester resin cannot be promoted and the L value of the obtained fiber will decrease. If the content of calcium carbonate exceeds 30.0 mass %, there is a risk that the pressure increase rate of the filter used in spinning will increase, and the adhesion of the interface between the core and the sheath will decrease, resulting in nap and fiber breakage.

As long as the core component can exert the effect of the present invention, it may also contain components other than polyester resin and calcium carbonate, and additives such as antioxidants, antistatic agents, anti-adhesive agents, pigments, thermal stabilizers, ultraviolet absorbers, and lubricants may also be added within the range that does not hinder the effect of the present invention.

In the aforementioned first aspect and the aforementioned third aspect, the content of the polyester resin and calcium carbonate contained in the core component is preferably 90% by mass or more, more preferably 95% by mass or more, more preferably 97% by mass or more, further preferably 99% by mass or more, and particularly preferably 100% by mass, in 100% by mass of the core component. The core component preferably does not substantially contain titanium oxide, and the content of titanium oxide is preferably 0.5% by mass or less, further preferably 0.1% by mass or less, and further preferably 0% by mass, in 100% by mass of the core component.

In the aforementioned first to third aspects, the core component (preferably a polyester resin containing calcium carbonate or a polyester resin not containing calcium carbonate) may also have a predetermined maximum peak temperature of crystallization melting measured by a differential scanning calorimeter, and the maximum peak temperature of crystallization melting of the core component is preferably less than 255°C, more preferably 243°C to 254°C, more preferably 244°C to 253°C, and further preferably 245°C to 252°C. The maximum peak temperature of crystallization melting in the above range is observed when the core component is stretched regardless of the presence or absence of calcium carbonate, which can promote the crystallization of the polyester resin and improve the whiteness of the fiber. In the case of no stretching, the maximum peak temperature of crystal melting of the core component becomes 255°C or more, which fails to promote the crystallization of the polyester resin and improve the whiteness of the fiber. In addition, if the content of calcium carbonate in the core component increases within the above range, the maximum peak temperature of crystal melting tends to increase within the above temperature range. The differential scanning calorimeter can be, for example, a differential scanning calorimeter manufactured by TA Instruments. The crystal melting peak temperature is measured, for example, at a temperature increase rate of 10°C/min, and the temperature giving the extreme value of the measured melting endothermic curve is set as the maximum peak temperature of crystal melting. The cooling crystallization peak temperature described later is measured, for example, at a cooling rate of 10°C/min, and the temperature at the extreme value of the measured crystallization heat curve is set as the cooling crystallization peak temperature.

In the first to third aspects, the core component may also have a predetermined cooling crystallization peak temperature measured by a differential scanning calorimeter, and the cooling crystallization peak temperature of the core component is preferably 200° C. to 220° C., more preferably 202° C. to 218° C., still more preferably 204° C. to 216° C., and still more preferably 206° C. to 214° C. If the cooling crystallization peak temperature is within the above range, there is a tendency to form unoriented micro crystals and promote crystallization of the polyester resin.

<Sheath component> The sheath component includes a polyolefin resin having a melting point that is 20°C or more lower than that of the polyester resin.

The polyolefin resin may be any of a polyolefin homopolymer or a polyolefin copolymer. The carbon number of the olefin monomer constituting the polyolefin resin is, for example, 2 to 20, preferably 2 to 15, more preferably 2 to 10, and even more preferably 2 to 5. As the olefin monomer, there can be listed: ethylene, propylene, butene, pentene, hexene, heptene, octene, nonene, decaene, etc. Among them, polyolefin resins are those with ethylene, propylene, butene, pentene, hexene and other olefin monomers as the main constituent units (preferably 60 mol% or more, more preferably 70 mol% or more, even more preferably 80 mol% or more, further preferably 90 mol% or more, particularly preferably 95 mol% or more, preferably less than 100 mol% or less than 99 mol% in 100 mol% of the total constituent units).

Examples of polyolefin resins include low-density polyethylene (melting point 100°C to 115°C), high-density polyethylene (melting point 125°C to 137°C), ultra-high molecular weight polyethylene (melting point 135°C to 140°C), polypropylene (melting point 165°C), polybutene-1 (melting point 126°C to 128°C), 1,2-polybutadiene (melting point 71°C to 105°C), etc.

When a polyolefin copolymer is used, a copolymer obtained by copolymerizing the above-mentioned substance with a small amount of α-olefin such as ethylene, propylene, butene, hexene, octene, etc. can be used as a copolymer component.

Among them, the polyolefin resin preferably has a melting point of 180° C. or less (preferably 70° C. or more and 170° C. or less), more preferably comprises polyethylene and polypropylene, and even more preferably comprises polyethylene. The polyolefin resin may be one kind or two or more kinds.

The polyolefin resin may also have a predetermined melt flow rate (MFR) measured according to JIS K 6922-2. The melt flow rate (MFR) of the polyolefin resin is preferably 8 g/10 min to 25 g/10 min, more preferably 9 g/10 min to 21 g/10 min under the conditions of a load of 2.16 kg and a temperature of 190° C.

The polyolefin resin may also have a predetermined density, low density polyethylene having a density of 0.91 g/cm ³ to 0.92 g/cm ³ , high density polyethylene having a density of 0.94 g/cm ³ to 0.965 g/cm ³ , and polypropylene having a density of 0.90 g/cm ³ to 0.91 g/cm ³ .

The content of the polyolefin resin having a melting point of 180°C or less is preferably 80% by mass or more, more preferably 90% by mass or more, further preferably 95% by mass or more, further preferably 99% by mass or more, and particularly preferably 100% by mass in 100% by mass of the polyolefin resin. When the content of the polyolefin resin is within the above range, a nonwoven fabric having high adhesion between fibers can be produced by thermal fusion.

In the aforementioned second and third aspects, the content of the polyolefin resin in 100% by mass of the sheath component is preferably 70% by mass or more, more preferably 75% by mass or more, even more preferably 80% by mass or more, and even more preferably 85% by mass or more, preferably 99% by mass or less, more preferably 98% by mass or less, even more preferably 97% by mass or less, and even more preferably 96% by mass or less. In the aforementioned first aspect, the content of the polyolefin resin in 100% by mass of the sheath component is preferably 100% by mass.

The content of the polyolefin resin is preferably at least 20% by mass of the entire fiber, more preferably 25% by mass to 55% by mass, even more preferably 30% by mass to 50% by mass, and even more preferably 35% by mass to 45% by mass.

In the second aspect and the third aspect, the calcium carbonate contained in the sheath component may be heavy calcium carbonate, light calcium carbonate, or a mixture thereof, similar to the calcium carbonate used in the core component.

Heavy calcium carbonate can also be obtained by crushing and grading natural materials such as limestone, marble, calcite, and chalk, and is also called natural calcium carbonate. Light calcium carbonate can also be obtained by crystallizing in a liquid through a chemical reaction (for example, by the reaction of calcium hydroxide and carbonic acid gas), and is also called synthetic calcium carbonate or precipitated calcium carbonate. Calcium carbonate can also be surface treated with fatty acids, silicones, or resins. As fatty acids, stearic acid, palmitic acid, myristic acid, lauric acid, etc. can be listed, and it can also be used as a salt or ester of fatty acids. As resins, polyacrylates, polydiallyldimethylammonium chloride, etc. can be listed.

The average particle size d50 of the calcium carbonate used in the sheath component is preferably 0.02μm to 3.0μm, more preferably 0.10μm to 2.0μm, and even more preferably 0.20μm to 1.0μm. The top cut particle size d90 of the calcium carbonate used in the sheath component is preferably 5.0μm or less, more preferably 4.0μm or less, even more preferably 3.0μm or less, further preferably 2.0μm or less, particularly preferably 1.0μm or less, and preferably 0.3μm or more or 0.5μm or more.

In the aforementioned second aspect and the aforementioned third aspect, the content of calcium carbonate in the sheath component is preferably more than 1.0 mass % and less than 30.0 mass %, more preferably more than 2.0 mass % and less than 25.0 mass %, still more preferably more than 3.0 mass % and less than 20.0 mass %, and further preferably more than 4.0 mass % and less than 15.0 mass %. If the content of calcium carbonate is less than 1.0 mass %, there is a risk that the crystallization of the polyolefin resin cannot be promoted and the L value of the obtained fiber will become smaller. If the content of calcium carbonate exceeds 30.0 mass %, there is a risk that the pressure increase rate of the filter used in spinning will increase, and the adhesion of the interface between the core and the sheath will decrease, resulting in the risk of lint and fiber breakage.

The calcium carbonate used in the core component and the calcium carbonate used in the sheath component may be the same or different.

As long as the sheath component can exert the effect of the present invention, it may also contain ingredients other than polyolefin resin and calcium carbonate, and may also add antioxidants, antistatic agents, anti-adhesive agents, pigments, thermal stabilizers, ultraviolet absorbers, lubricants and other additives within the range that does not hinder the effect of the present invention.

In the aforementioned second aspect and the aforementioned third aspect, the content of the polyolefin resin and calcium carbonate contained in the sheath component is preferably 90 mass% or more, more preferably 95 mass% or more, more preferably 97 mass% or more, further preferably 99 mass% or more, and particularly preferably 100 mass% in 100 mass% of the sheath component. The sheath component preferably does not substantially contain titanium oxide, and the content of titanium oxide in the sheath component is preferably 0.5 mass% or less, further preferably 0.1 mass% or less, and further preferably 0 mass% in 100 mass% of the sheath component.

The content of calcium carbonate is preferably 1% to 25% by mass, more preferably 1.5% to 20% by mass, and even more preferably 2.0% to 15% by mass in 100% by mass of the core-sheath composite fiber. If the content of calcium carbonate in the core-sheath composite fiber is set to the above range, the volume reduction of the polyester resin and the polyolefin resin can be promoted, and if further extended, the whiteness of the core-sheath composite fiber can be promoted.

In the core-sheath composite fiber of the present invention, the mass ratio of the core component/sheath component is preferably less than 7.0, more preferably less than 5.0, more preferably less than 4.0, further preferably less than 3.0, particularly preferably less than 2.0, and preferably greater than 0.2 or greater than 0.3. By setting the mass ratio of the core component/sheath component to the above range, the strength and elongation of the fiber suitable for nonwoven fabrics can be set.

The polyester resin, polyolefin resin, and both of them used in the present invention may contain more than 1.0 mass % of calcium carbonate (relative to the mass of each resin). Calcium carbonate is contained in order to improve the concealing property (i.e., whiteness) of the fiber by controlling the crystal size formed by the resin. If it is less than 1.0 mass %, the whiteness improvement when making a nonwoven fabric becomes insufficient.

The single filament fineness of the core-sheath composite fiber is preferably 0.1 dtex to 8.0 dtex, more preferably 0.2 dtex to 4.8 dtex, still more preferably 0.5 dtex to 4.6 dtex, still more preferably 1.0 dtex to 4.4 dtex, and particularly preferably 1.5 dtex to 4.2 dtex. The single filament fineness of the core-sheath composite fiber can be measured according to JIS L-1095 9.4.1.

The cross-sectional shape of the core-sheath composite fiber is preferably a concentric cross-sectional shape, an eccentric cross-sectional shape, or a hollow cross-sectional shape, and more preferably a concentric cross-sectional shape. The concentric cross-sectional shape means that the center of gravity of the core and the center of gravity of the sheath are the same, the eccentric cross-sectional shape means that the center of gravity of the core and the center of gravity of the sheath are different, and the hollow cross-sectional shape means that the core and the sheath have a hollow structure.

The core-sheath composite fiber of the present invention has a predetermined L value and b value of Hunter's color difference formula as appropriate whiteness for appearance quality. That is, the L value representing the hue of the core-sheath composite fiber is 91 or more, and the b value is 5.0 or less. The L value is preferably 91.5 or more, more preferably 92 or more, more preferably 92.5 or more, and further preferably 93 or more, and preferably 99 or less or 98 or less. If the L value is less than 91, it is not suitable for nonwoven fabrics of sanitary materials.

The b value is preferably 4.5 or less, more preferably 4.0 or less, still more preferably 3.5 or less, still more preferably 3.0 or less, particularly preferably 2.5 or less, and most preferably 2.0 or less. If the b value exceeds 5.0, the color tone is yellowish and is not suitable for nonwoven fabrics of sanitary materials. The L value and the b value can be measured, for example, by using a sample made of core-sheath composite fibers rolled up in a way that the color of the cardboard is not visible to the eye relative to a black cardboard, and using a colorimeter (e.g., CM-3700d, a spectrophotometer manufactured by MINOLTA).

The core-sheath composite fiber of the present invention may also have a predetermined strength and elongation. The strength of the fiber is preferably 0.80 cN/dtex to 5.00 cN/dtex, more preferably 0.85 cN/dtex to 4.80 cN/dtex, more preferably 0.90 cN/dtex to 4.60 cN/dtex, further preferably 0.95 cN/dtex to 4.40 cN/dtex, and particularly preferably 1.00 cN/dtex to 4.20 cN/dtex, in terms of tensile strength measured in accordance with JIS L1013 8.5.1.

The elongation of the fiber is preferably 30% to 200%, more preferably 35% to 190%, still more preferably 40% to 180%, still more preferably 45% to 170%, and particularly preferably 50% to 160%, as measured in accordance with JIS L1013 8.5.1. The core-sheath composite fiber of the present invention has an elongation equal to or greater than that of a general fiber.

[Preparation method of core-sheath composite fiber] The core-sheath composite fiber of the present invention preferably uses a polyester resin as a core component and a polyolefin resin as a sheath component, uses calcium carbonate in one or both of the core component and the sheath component, melt-spins into a core-sheath shape, and at least includes a step of preparing undrawn yarns and a step of drawing the undrawn yarns under heating conditions. The polyester resin, polyolefin resin, and calcium carbonate are used in the amounts as described above.

Calcium carbonate can be added in a predetermined amount and stirred during or immediately after the polymerization of the resin, and then dispersed by fragmenting while extruding with an extruder, or by spinning in a molten state of the resin. Methods for uniformly dispersing calcium carbonate in the resin include a circulating dispersion method and a masterbatch method. The circulating dispersion method can also be a method in which a predetermined amount of calcium carbonate is added immediately after the polymerization of the resin and the resin melt is circulated to disperse the calcium carbonate. The masterbatch method can also be a method in which a predetermined amount of calcium carbonate is wetted with water, a solvent, etc. in advance, mixed with a predetermined resin using a pressure kneader, and the calcium carbonate is dispersed while separating the water and the solvent.

<Melt spinning step> It is preferred that one or both of the polyester resin and the polyolefin resin contain calcium carbonate, and the polyester resin and the polyolefin resin are melted separately, and the molten resin is ejected from a core-sheath nozzle through a spinning pack. The nozzle preferably has 10 to 1000 holes, more preferably 50 to 800 holes, more preferably 100 to 600 holes, and more preferably 200 to 400 holes. The spinning is performed at a spinning temperature 10°C to 30°C higher than the melting point of the polyester resin. Then, preferably, air at 10°C to 25°C and preferably at an air volume of 50m/min to 100m/min is applied to the spinning line for cooling, and the spinning oil is applied. It is also preferably at a take-up speed (haul-off speed). The unstretched wire is retracted into a container (e.g., a can) at a speed of 1000m/min to 1500m/min.

<Stretching step> The obtained unstretched yarn is preferably stretched using a liquid phase or a gas phase at 60°C to 130°C, preferably at a stretching ratio of 1.5 to 6.0 times, more preferably at 1.5 to 5.0 times, more preferably at 1.5 to 4.0 times, and more preferably at 1.5 to 3.0 times. In particular, the unstretched yarn is preferably stretched at a stretching ratio of 1.55 to 3.0 times, more preferably at 1.60 to 2.9 times, and more preferably at 1.70 to 2.8 times, from the viewpoint of further increasing the L value and further reducing the maximum peak temperature of crystal melting. The stretching can be one stage or two or more stages. In the case of two stages, the first stage is preferably performed at a temperature of 60°C to 90°C (preferably 60°C to 80°C) and a stretching ratio of 2.0 to 4.0 times (preferably 3.6 times or less, more preferably 3.2 times or less). The second stage is preferably performed at a temperature of 90°C to 130°C (preferably 100°C to 120°C) and a stretching ratio of 0.8 to 1.2 times (preferably 0.9 to 1.1 times).

In addition to the above steps, the manufacturing method of the core-sheath composite fiber of the present invention may also include a crimping step, a post-heat treatment step, and a cutting step. The crimping step may be performed using a crimping device such as a stuffer box crimper. The post-heat treatment step may be, for example, a step of treating the obtained fiber at a temperature of 90°C to 140°C (preferably 100°C to 130°C) without applying tension.

When the core-sheath composite fiber is cut into short fibers, the average length of the short fibers is preferably 30 mm to 80 mm, more preferably 31 mm to 70 mm, and even more preferably 32 mm to 60 mm.

The core-sheath composite fiber may also be surface treated with a hydrophilic or hydrophobic oil, and the oil may be treated in any step such as after cutting or drying.

By adding calcium carbonate to a polyester resin and performing a stretching and crystallization treatment, the crystal structure of the stretched yarn can be controlled, and a core-sheath composite fiber having both whiteness (white color tone, concealment, and anti-transmission) and mechanical properties required for raw cotton for non-woven fabrics can be obtained. In the first and third aspects of the present invention, the mechanism for improving whiteness is shown in (a) to (c) in Figure 4. As shown in (a) in Figure 4, the high molecular chain (polymer chain) contained in the unstretched polyester resin has a random orientation. As shown in (b) of Figure 4, when inorganic particles such as calcium carbonate do not exist, in the process of necking deformation of the unstretched filament by stretching (stretching and crystallization process), the unoriented crystals (large crystals such as spherulites) grow and become larger, acting as a factor that hinders the orientation of the polymer chains contained in the polyester resin (the effect of hindering the orientation crystallization of the polymer chain becomes larger). Furthermore, the crystal size of the polymer chain becomes smaller, and visible light is not easily diffused (the whiteness of the fiber decreases). In contrast, as shown in (c) of FIG. 4 , by adding inorganic particles such as calcium carbonate, calcium carbonate acts as a nucleating agent for the PET resin, which can miniaturize the size of the unoriented crystals formed in the system (due to the effect of the nucleating agent, the large crystals formed during the crystallization process become smaller, and the effect of hindering the oriented crystallization of the polymer chain becomes smaller, thereby promoting the crystallization of unoriented and tiny crystals). Thus, compared with the system without calcium carbonate addition, a straight and uniform stress can be applied to the unstretched filaments, so that crystallization accompanied by orientation can proceed, and stretched filaments with a crystal size that has visible light diffusion properties can be obtained (the larger the crystal size, the higher the visible light diffusion property and the higher the whiteness). The phenomenon that calcium carbonate acts as a nucleating agent for PET can be inferred from the cooling crystallization peak temperature of differential scanning calorimetry (DSC) (Figures 2 to 3). The higher the concentration of calcium carbonate, the higher the temperature side of the cooling crystallization peak temperature, suggesting that the nucleating agent function promotes the formation of unoriented and tiny crystals.

In a system to which calcium carbonate is added, the phenomenon that the crystal size of the stretched filament increases according to the stretching and crystallization conditions can be inferred from the results of differential scanning calorimetry (DSC). After adding calcium carbonate, in the DSC curve of the unstretched comparative example 3 (Figure 1), the crystal melting curve derived from PET appears as a wide absorption curve with a peak near 255°C. This is a crystal melting curve of unoriented crystals (spherulites) not accompanied by the orientation of the polymer chains contained in the polyester resin. In contrast, in the DSC curve of Example 1 (Figure 2), in addition to the wide absorption curve with a peak near 255°C, another absorption curve with a maximum peak temperature near 250°C appears. This is a crystal melting curve that is generated by stretching and originates from the crystallization accompanying the orientation of the polymer chain contained in the polyester resin. When the stretching ratio is gradually increased to the vicinity of the fracture region, the wide absorption curve with a peak near 255°C attenuates and no peak near 255°C is confirmed (only the absorption curve with a maximum peak temperature near 250°C remains). The whiteness of the stretched yarn in this state is excellent, but since the elongation is significantly reduced, it is not suitable for non-woven fabric applications. Therefore, in order to obtain a stretched yarn with both excellent whiteness and mechanical properties, it is important to measure the maximum peak temperature of the crystal melting of the stretched yarn at a temperature lower than 255°C.

By increasing the concentration of calcium carbonate, the formation of crystals accompanying alignment can be promoted, and whiteness can be improved. In the DSC curve of Example 1 (Figure 2), the maximum peak temperature of crystal melting of crystals accompanying alignment (crystals with high diffusivity of visible light) was measured at 250°C. In contrast, in Example 5 (Figure 3) where the concentration of calcium carbonate added was higher, the maximum peak temperature of crystal melting of crystals accompanying alignment was measured at 252°C. The fact that the crystal melting peak shifted to the high temperature side suggests that the crystal size of crystals accompanying alignment increases with the increase in the concentration of calcium carbonate added.

The core-sheath composite fiber of the present invention may also be long fiber (filament), short fiber (staple), or a combination thereof.

The present invention includes a nonwoven fabric formed of core-sheath composite fibers. The nonwoven fabric is preferably formed by conventionally known fleece forming and fleece bonding. Fleece forming can be performed by dry method, wet method, spunbond method, melt flow method, etc., and fleece bonding can be performed by thermal bonding method, chemical bonding method, needle punch method, spunlace method, stitch bonding method, steam jet method, etc. Fleece forming and fleece bonding can be selected from the above methods according to the desired nonwoven fabric.

This application claims the benefit of the priority of Japanese Patent Application No. 2023-058130 filed on March 31, 2023 and Japanese Patent Application No. 2023-162951 filed on September 26, 2023. All the contents of the specifications of Japanese Patent Application No. 2023-058130 filed on March 31, 2023 and Japanese Patent Application No. 2023-162951 filed on September 26, 2023 are incorporated in this application for reference. [Example]

The following examples are given to illustrate the present invention in more detail, but the present invention is not limited to the following examples, and can be implemented with appropriate changes within the scope of the subject matter of the preceding and following texts, and such changes are all included in the technical scope of the present invention. In addition, unless otherwise specified, "portion" means "portion by mass", and "%" means "% by mass". In addition, the size of the sample is based on the size described below, but in the case of a lack of sample, the sample size of a feasible size is used for measurement.

(1) Materials Polyethylene terephthalate (PET) with an intrinsic viscosity IV of 0.63 dl/g was used as the core component of the two-component core-sheath composite fiber, and high-density polyethylene (HDPE) with an MFR of 20 g/10 minutes (load 2.16 kg, temperature 190°C) was used as the sheath component. Calcium carbonate was KRS1 manufactured by Maruo Calcium Co., Ltd. for PET in the core component and YM10 manufactured by Maruo Calcium Co., Ltd. for PE (polyethylene) in the sheath component.

(2) Color tone measurement The L value and b value were measured by using a colorimeter CM-3700d manufactured by MINOLTA under the setting of excluding reflected light. The L value and b value in the present invention are measured by using a sample in which the obtained fiber is rolled onto a black cardboard and the color of the cardboard is not visible under visual observation. The L value is an indicator of the whiteness of the color, and the b value is an indicator of the yellowness of the color. If the L value is less than 85 or the b value exceeds 5.0, the color tone of the appearance is poor when used as a nonwoven fabric, and the product becomes a poor quality product with insufficient concealment. Even if the L value is 90 or less, it can have anti-see-through properties, but for nonwoven fabrics used as sanitary materials, the ideal value is 91 or more.

(3) Differential scanning calorimetry The crystal melting peak temperature was measured at a heating rate of 10°C/min using a differential scanning calorimeter manufactured by TA Instruments. The temperature giving the extreme value of the measured melting absorption curve was set as the crystal melting peak. The cooling crystallization peak temperature was measured at a cooling rate of 10°C/min.

(4) Fiber density, single filament fiber density Measured according to the method described in JIS L-1095 9.4.1. Single filament fiber density is calculated by dividing the fiber density by the filament number.

(5) Maximum point stress (also called tensile strength, expressed as strength in Table 1), maximum point elongation (also called elongation, expressed as elongation in Table 1) Measured in accordance with JIS L1013 8.5.1. The stress at the highest load is set as the maximum point stress, and the elongation at the highest load is set as the maximum point elongation.

[Example 1] Polyethylene terephthalate (PET) as a polyester resin is dried according to a common procedure, and then adjusted together with PET so that the calcium carbonate content becomes 4.0 mass %, and supplied to a composite fiber spinning machine in a manner separate from high-density polyethylene (HDPE) as a polyolefin resin (prepared separately from the aforementioned polyethylene terephthalate (PET), etc.). The composite fiber is melt-extruded at 285°C by an extruder attached to the composite fiber spinning machine, and a spinning nozzle with multiple holes is used to form a concentric core-sheath structure with a mass ratio of 40/60 (sheath/core) of the two components and PET on the core side and HDPE on the sheath side. The spun yarns are cooled by air flow, oiled by an oiling guide, bundled by a bundling guide, and then pulled by a roller at a spinning speed of 1240 m/min and wound by a take-up machine. The obtained fiber (undrawn yarn) has a single filament fineness of 5.0 dtex. Then, it is stretched to 2.4 times at a speed of 40 m/min and a stretching temperature of 70°C in a stretching device, then stretched to 0.95 times at a stretching temperature of 110°C, and heat-fixed at 120°C. Through these spinning and stretching steps, a core-sheath composite fiber with a single filament fineness of 2.2 dtex and a mass ratio of 40/60 (sheath/core) is obtained in the form of concentric circles with PET on the core side and HDPE on the sheath side.

[Example 2] After obtaining undrawn yarns by the method of Example 1, the same method as Example 1 is used except that the single yarn fineness is adjusted to 2.8 dtex by the drawing ratio.

[Example 3] After obtaining undrawn yarns by the method of Example 1, the same method as Example 1 is used except that the single yarn fineness is adjusted to 3.3 dtex by the drawing ratio.

[Example 4] Except that the calcium carbonate content of PET in Example 1 is set to 8.0 mass %, the same method as Example 1 is used.

[Example 5] Except that the calcium carbonate content of PET in Example 1 is set to 15.0 mass %, the same method as Example 1 is used.

[Example 6] The calcium carbonate content of PET is set to 10% by mass, and the calcium carbonate content of HDPE is set to 10% by mass. The composite fiber is melt-extruded at 285°C by an extruder attached to a composite fiber spinning machine. A nozzle with multiple holes is used to perform composite spinning with a mass ratio of two components of 40/60 (sheath/core) and a concentric core-sheath structure in which PET is located on the core side and HDPE is located on the sheath side. The spun yarn is cooled by air flow, oil is applied by an oiling guide, and after being bundled by a bundling guide, it is pulled into a can by a roller at a spinning speed of 1240m/min and stored. The obtained undrawn yarn was stretched to 2.7 times the ratio at a stretching temperature of 70°C in a stretching device, and then mechanically rolled, and the heat-fixed material was cut at 120°C without tension, thereby obtaining a cotton-like fiber (short fiber) with a single-filament fiber density of 2.4dtex.

[Example 7] Polyethylene terephthalate (PET) as a polyester resin is dried by a conventional method, and high-density polyethylene (HDPE) as a polyolefin resin is prepared separately, and the calcium carbonate content is adjusted to 5.0 mass % together with HDPE, and each is supplied to a composite fiber spinning machine. Melt extrusion is performed at 285°C by an extruder attached to the composite fiber spinning machine, and a nozzle with multiple holes is used to form a concentric core-sheath structure with PET on the core side and HDPE on the sheath side at a mass ratio of the two components of 40/60 (sheath/core). The warp spun yarns are cooled by air flow, lubricated by an oiling guide, bundled by a bundling guide, pulled by a roller at a spinning speed of 1240 m/min, and wound by a take-up machine. The obtained fiber (undrawn yarn) has a single filament fineness of 5.0 dtex. Then, it is stretched to a ratio of 2.4 times at a speed of 40 m/min and a stretching temperature of 70°C in a stretching device, stretched to 0.95 times at a stretching temperature of 110°C, and heat-fixed at 120°C. Through these spinning and stretching steps, a core-sheath composite fiber with a single filament fineness of 2.3 dtex and a mass ratio of 40/60 (sheath/core) is obtained, with PET on the core side and HDPE on the sheath side arranged in a concentric circle.

[Example 8] Example 8 was carried out in the same manner as Example 7 except that the calcium carbonate content of the HDPE in Example 7 was set to 20.0 mass %.

[Comparative Example 1] In Example 1, the same method as Example 1 was used except that the calcium carbonate content was set to 1.0 mass % and no stretching or heat treatment was performed.

[Comparative Example 2] In Example 1, except that the calcium carbonate content was set to 1.0 mass %, the same method as Example 1 was followed.

[Comparative Example 3] Example 1 was carried out in the same manner as Example 1 except that no stretching or heat treatment was performed.

[Comparative Example 4] Example 7 was carried out in the same manner as Example 7 except that no stretching or heat treatment was performed.

[Table 1] Core composition (PET) Sheath component (PE) Fiber morphology Stretching + heat treatment (120℃) Total extension ratio (times) Fiber density (dtex) Strength (cN/dtex) Elongation(%) Cooling crystallization peak temperature (℃) Crystallization melting maximum peak temperature (℃) Hue Calcium carbonate content (mass %) Calcium carbonate content (mass %) L value b-value Embodiment 1 4.0 0.0 Filamentous Fiber have 2.29 2.2 2.75 55.9 207 250.4 95.9 -0.1 2 4.0 0.0 Filamentous Fiber have 1.80 2.8 1.62 79.6 200 250.2 95.4 -0.1 3 4.0 0.0 Filamentous Fiber have 1.53 3.3 1.09 132.1 200 252.3 94.5 -0.2 4 8.0 0.0 Filamentous Fiber have 2.29 2.2 2.42 54.6 208 251.2 96.1 -0.1 5 15.0 0.0 Filamentous Fiber have 2.29 2.2 1.94 52.6 210 252.1 96.9 0.3 6 10.0 10.0 Cotton Fiber have 2.70 2.4 2.22 54.1 199 245.5 95.3 0.2 7 0.0 5.0 Filamentous Fiber have 2.29 2.3 3.06 83.1 204 249.7 95.3 0.2 8 0.0 20.0 Filamentous Fiber have 2.29 2.3 2.91 69.8 207 250.2 96.1 0.1 Comparison Example 1 1.0 0.0 Filamentous Fiber without - 5.0 - - - 255.2 90.2 -0.1 2 1.0 0.0 Filamentous Fiber have 2.29 2.2 - - - - 89.8 0.5 3 4.0 0.0 Filamentous Fiber without - 5.0 - - - 255.2 89.8 0.6 4 0.0 5.0 Filamentous Fiber without - 5.0 - - 202 255.0 89.0 -0.5 [Industry Availability]

The core-sheath composite fiber of the present invention can be suitably used in absorbent articles for sanitary materials such as diapers, sanitary napkins, and pads; medical and sanitary materials; materials related to life; general medical materials; bedding materials; filters; nursing products; and pet products.

[Figure 1] shows the results of measuring the polyester resin containing calcium carbonate as the core component obtained in Comparative Example 3 using a differential scanning calorimeter. [Figure 2] shows the results of measuring the polyester resin containing calcium carbonate as the core component obtained in Example 1 using a differential scanning calorimeter. [Figure 3] shows the results of measuring the polyester resin containing calcium carbonate as the core component obtained in Example 5 using a differential scanning calorimeter. (a) to (c) in [Figure 4] show the alignment of polymer chains contained in unstretched resin, the presence or absence of calcium carbonate addition, and the mechanism of expressing the whiteness (hiding property) of the fiber by utilizing the alignment of polymer chains contained in stretched resin.

Claims (8)

A core-sheath composite fiber is composed of a core component including a polyester resin and a sheath component including a polyolefin resin, wherein the polyolefin resin has a melting point that is 20°C or more lower than that of the polyester resin; One or both of the core component and the sheath component contain more than 1.0% by mass and less than 30.0% by mass of calcium carbonate in 100% by mass of each component; The maximum peak temperature of crystal melting of the core component measured by a differential scanning calorimeter is less than 255°C; And, the L value representing the hue of the core-sheath composite fiber is greater than 91, and the b value is less than 5.0. The core-sheath composite fiber as claimed in claim 1, wherein the peak temperature of the core component crystallization upon cooling measured by a differential scanning calorimeter is 190°C to 220°C. The core-sheath composite fiber as recited in claim 1 or 2, wherein the polyester resin comprises polyethylene terephthalate, and the polyolefin resin comprises polyethylene. The core-sheath composite fiber as claimed in claim 1 or 2, wherein the content of the polyester resin is 40 mass % or more relative to the entire core-sheath composite fiber. The core-sheath composite fiber as recited in claim 1 or 2, wherein the cross-sectional shape of the core-sheath composite fiber is a concentric cross-sectional shape, an eccentric cross-sectional shape, or a hollow cross-sectional shape. The core-sheath composite fiber as claimed in claim 1 or 2, wherein the tensile strength measured in accordance with JIS L1013 8.5.1 is 0.80 cN/dtex to 5.00 cN/dtex. A core-sheath composite fiber as claimed in claim 1 or 2, wherein the elongation measured in accordance with JIS L1013 8.5.1 is 30% to 200%. A nonwoven fabric is formed from the core-sheath composite fiber as described in claim 1 or 2.

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