JP7241279B2 - Core-sheath type composite fiber, method for producing the same, and fiber assembly containing the same - Google Patents

Description

TECHNICAL FIELD The present invention relates to a core-sheath type composite fiber containing a core component and a sheath component, a method for producing the same, and a fiber assembly containing the same. More specifically, the present invention relates to a core-sheath type composite fiber which is resistant to thermal shrinkage in heat bonding processing at a low temperature and which can provide a fiber assembly with improved dimensional stability, a method for producing the same, and a fiber assembly containing the same.

A core-sheath type composite fiber composed of a sheath component composed of a thermoplastic resin with a low melting point and a core component composed of a thermoplastic resin having a higher melting point than the thermoplastic resin of the sheath component is obtained by melting the sheath component. Because the fibers can be easily bonded to each other, not only thermal bonded nonwoven fabrics (also called thermally bonded nonwoven fabrics), but also hydroentangled nonwoven fabrics (also called spunlace nonwoven fabrics), needle punch nonwoven fabrics, airlaid nonwoven fabrics, and wet-laid nonwoven fabrics. It is widely used when bonding fibers together in various fiber aggregates such as.

When a core-sheath type composite fiber is designed with emphasis on thermal adhesiveness, it is required to use a thermoplastic resin with a low melting point for the sheath component. Therefore, the sheath component of such a core-sheath type composite fiber includes various polyethylene resins such as high-density polyethylene (Patent Document 1), copolymer polyester resins (Patent Document 2), polypropylene, and polyolefin resins such as ethylene-propylene copolymers. (Patent Document 3) and the like are widely used. Various polyethylene resins are used from the viewpoint of low-temperature adhesion and chemical resistance, and among them, high-density polyethylene is widely used. The core-sheath type conjugate fiber having high density polyethylene as the main component of the sheath component has good spinnability and drawability, and the melting point of high density polyethylene is 130 to 135 ° C. By heat-treating at , it becomes possible to bond the fibers together.

In core-sheath type conjugate fibers, in order to improve thermal adhesiveness at lower temperatures and shorten the heat treatment time, linear low-density polyethylene (hereinafter referred to as "low-density polyethylene"), a thermoplastic resin with a lower melting point than high-density polyethylene, is used. , also called PE-LLD) is proposed to be used as a sheath component of a core-sheath type composite fiber. For example, Patent Document 4 proposes a core-sheath type conjugate fiber using linear low-density polyethylene as a sheath component and polypropylene, which has a higher melting point than linear low-density polyethylene, as a core component.

However, linear low-density polyethylene is a thermoplastic resin with large heat shrinkage, and is known to undergo large heat shrinkage at temperatures near its melting point. In the core-sheath type composite fiber, when the main component of the sheath component is linear low-density polyethylene, the sheath component is melted by heat treatment at a lower temperature than the core-sheath type composite fiber in which the main component of the sheath component is high-density polyethylene. Therefore, in the process of manufacturing a thermally bonded nonwoven fabric, it is possible to lower the processing temperature when thermally bonding the fibers together or to perform the thermally bonding treatment in a shorter time. Since this core-sheath type conjugate fiber thermally shrinks at the heat treatment temperature, wrinkles and curls are likely to occur in the obtained fiber aggregate, and the dimensions of the fiber aggregate change greatly before and after the heat treatment, resulting in dimensional stability. It tends to be a fiber aggregate lacking in properties. For example, in a core-sheath type conjugate fiber composed of a resin combination of polypropylene as a core component and linear low-density polyethylene as a sheath component, polypropylene Due to the lack of rigidity of the resin, the heat shrinkage of the linear low-density polyethylene cannot be sufficiently suppressed, and heat shrinkage occurs due to heat treatment. was not

In the core-sheath type conjugate fiber, as a method for suppressing heat shrinkage during heat treatment, the undrawn fiber after melt spinning is hardly subjected to drawing treatment, in other words, the core-sheath is in a state in which the undrawn fiber is hardly drawn. It has been proposed to finish and use a type conjugate fiber (Patent Documents 5 and 6). However, since the core-sheath type conjugate fibers described in these documents are hardly drawn from an undrawn state, the core-sheath type conjugate fibers have low strength and high single fiber fineness.

Japanese Patent Application Laid-Open No. 2002-180330 Japanese Patent Application Laid-Open No. 2004-181341 JP 2011-195978 A JP-A-2002-069753 JP-A-2007-204899 Japanese Patent Application Laid-Open No. 2007-204901

As described above, in core-sheath composite fibers containing linear low-density polyethylene with excellent low-temperature adhesion as a sheath component, the heat treatment temperature, that is, the melting point of the linear low-density polyethylene, is higher than the melting point of the linear low-density polyethylene. It is required to further reduce the thermal shrinkage of fibers in a temperature range of 30°C higher.

In order to solve the above problems, the present invention is a thermal bonding process in a temperature range where linear low-density polyethylene softens or melts and adheres to other substances, more specifically in a temperature range of 120 to 150 ° C. To provide a core-sheath type conjugate fiber capable of obtaining a fiber assembly which is resistant to heat shrinkage and has improved dimensional stability, a method for producing the same, and a fiber assembly containing the same.

The present invention relates to a core-sheath type composite fiber comprising a core component and a sheath component, wherein the core component and the sheath component are arranged substantially concentrically in the core-sheath type composite fiber, and the sheath component contains 60% by mass or more of linear low-density polyethylene having a density of 0.90 g/cm ³ or more and 0.93 g/cm ^{3 or} less and a melting point of 100° C. or more and 130° C. or less; A core-sheath containing 60% by mass or more of a polyester resin having a melting point higher than the melting point of the linear low-density polyethylene by 50°C or more, and a dry heat shrinkage at 140°C of the core-sheath type composite fiber of 5.0% or less. related to type composite fibers.

The present invention also provides a core-sheath type conjugate fiber comprising a core component and a sheath component, wherein in the core-sheath type conjugate fiber, the core component and the sheath component are arranged substantially concentrically, and The sheath component contains 60% by mass or more of linear low-density polyethylene having a density of 0.90 g/cm ³ or more and 0.93 g/cm ³ or less and a melting point of 100° C. or more and 130° C. or less, and the core component. contains 60 mass% or more of a polyester resin having a melting point higher than the melting point of the linear low-density polyethylene by 50 ° C. or more, the fiber length of the core-sheath type composite fiber is 2 mm or more and 20 mm or less, and the core-sheath type The present invention relates to a core-sheath type conjugate fiber which, when the conjugate fiber is made into a wet-laid nonwoven fabric, has a 140° C. area shrinkage of 10% or less.

The present invention also provides a method for producing a core-sheath type composite fiber comprising a core component and a sheath component, which has a density of 0.90 g/cm ³ or more and 0.93 g/cm ³ or less and a melting point of 100° C. or more. A sheath component containing 60% by mass or more of a linear low-density polyethylene having a temperature of 130°C or less, and a core component containing 60% by mass or more of a polyester resin having a melting point higher than the melting point of the linear low-density polyethylene by 50°C or more. In the cross section of the core-sheath type conjugate fiber, the surface of the core-sheath type conjugate fiber is covered with the sheath component, and the center of gravity of the core component is substantially aligned with the center of gravity of the core-sheath type conjugate fiber. A step of supplying the spun filament to a composite type nozzle arranged so as to be melt-spun to obtain a spun filament, the spun filament being at least the glass transition point of the polyester resin and 15 from the melting point T _mL of the linear low-density polyethylene ℃ low temperature (T _mL -15 ° C.) or less, a step of performing a drawing process under the condition that the draw ratio is 1.2 times or more and 2.5 times or less to obtain a drawn filament, Relax at a temperature equal to or higher than the glass transition point of the polyester resin and 15° C. lower than the melting point T _mL of the linear low-density polyethylene (T _mL -15° C.) under conditions where the relaxation ratio is less than 1.0 times. The present invention relates to a method for producing a core-sheath type composite fiber including a step of heat-treating to obtain a relaxed heat-set filament.

The present invention also provides a fiber assembly containing 20% by mass or more of the core-sheath type conjugate fiber, and in the fiber assembly, at least a portion of the fibers are thermally bonded to each other by the sheath component of the core-sheath type conjugate fiber. It relates to a fiber assembly that has been.

According to the present invention, a core-sheath type conjugate fiber and a fiber assembly containing the core-sheath type conjugate fiber, which is resistant to heat shrinkage and has improved dimensional stability when thermally bonded at a temperature of 120 to 150° C., can be obtained. can provide. In addition, according to the production method of the present invention, a core-sheath type composite fiber that is resistant to heat shrinkage when subjected to heat bonding treatment at a temperature of 120 to 150° C. and that can provide a fiber aggregate with improved dimensional stability is produced. Obtainable.

FIG. 1 is a schematic diagram of a fiber cross section of a core-sheath type composite fiber in one embodiment of the present invention. FIG. 2 is a schematic diagram of the drying and thermal bonding steps in manufacturing the wet-laid nonwoven fabric used for measuring the area shrinkage in the present invention. FIG. 3 is a schematic diagram of a net box used in the thermal bonding process when producing a wet-laid nonwoven fabric used for measuring area shrinkage in the present invention. FIG. 4 shows four points (A ₀ , B ₀ ) marked on the wet papermaking web before heat bonding and four points on the wet papermaking web after heat bonding ( A ₁ , B ₁ ) is a schematic diagram showing a change. (a) It is a schematic diagram of four points marked on a wet papermaking web before heat bonding treatment. (b) A schematic diagram of four points marked on a wet-laid papermaking web after thermal bonding. FIG. 5 is a photograph showing the shape of a stainless steel column for solvent extraction used in Examples and Comparative Examples.

The present inventors have found that a fiber assembly containing core-sheath type composite fibers using a resin component containing 60% by mass or more of linear low-density polyethylene with excellent low-temperature adhesion as a sheath component, at a temperature of 120 to 150 ° C. We have made intensive studies on how to suppress the heat shrinkage of the fiber assembly and improve the dimensional stability when the heat bonding treatment is performed. As a result, in the core-sheath type conjugate fiber, by using a material that can improve the heat resistance of the conjugate fiber in addition to increasing the rigidity of the entire conjugate fiber by using a highly rigid resin component as the core component, Even if the sheath component is deformed or melted when heat treatment such as thermal bonding treatment is performed, the heat resistance of the entire fiber is increased because the core component is less likely to be deformed. It has been found that when a fiber assembly using fibers is subjected to a heat bonding treatment at a temperature of 120 to 150° C., the thermal shrinkage of the fiber assembly is suppressed and the dimensional stability is improved. Specifically, in a core-sheath type composite fiber using linear low-density polyethylene with excellent low-temperature adhesion as a sheath component, the core component is a resin component containing a polyester resin with excellent heat resistance and high rigidity of the resin itself. As a result, the heat resistance of the core component is improved compared to the core-sheath type composite fiber that uses polypropylene, which is widely used as a core component, and the heat resistance of the core-sheath type composite fiber as a whole is improved. The heat shrinkage of the core-sheath type composite fiber is reduced even in the temperature range where the heat shrinkage and melting of the sheath component is improved, and the fiber assembly containing 20% by mass or more of the core-sheath type composite fiber is a sheath component. It was found that the dimensional stability is excellent even when heat treated in the melting temperature range. And, since the sheath component containing 60% by mass or more of linear low-density polyethylene is excellent in low-temperature adhesiveness, when the core-sheath type composite fiber is used, when heat bonding is performed at a temperature of 120 to 150 ° C. It is possible to provide a fiber aggregate that is resistant to heat shrinkage and has improved dimensional stability.

In addition, the present inventors focused on the cause of thermal shrinkage of linear low-density polyethylene, and on the method of producing a core-sheath type composite fiber composed of a sheath component containing 60% by mass or more of linear low-density polyethylene. Then, after the drawing step of adjusting the undrawn fiber obtained by melt spinning to a desired single fiber fineness, a relaxation heat treatment is performed under specific conditions to obtain a sheath containing 60% by mass or more of linear low-density polyethylene. Even if the core-sheath type conjugate fiber is composed of components, when manufacturing a fiber assembly containing the core-sheath type conjugate fiber, the temperature range above the melting point of the linear low density polyethylene, specifically 120 It was found that the heat shrinkage of the core-sheath type composite fiber is suppressed even when the heat bonding treatment is performed in a temperature range of up to 150°C, and the dimensional stability of the fiber assembly is improved.

Crystalline thermoplastic resins such as linear low-density polyethylene and high-density polyethylene have a mixture of crystalline and amorphous parts when made into fibers. It has a structure in which the parts are connected. When a crystalline thermoplastic resin fiber is drawn at a temperature between the glass transition temperature and the melting point, the tie molecules in the amorphous portion are oriented in the drawing direction and are drawn in the drawing direction. When exposed to a temperature range above the glass transition temperature of the resin, the tie molecule will undergo thermal vibration. As the temperature rises, the thermal vibration of the tie molecules becomes more active, and as the temperature approaches the melting point, the relaxation of the tie molecules becomes maximum and thermal contraction increases. Comparing high-density polyethylene and linear low-density polyethylene, it is believed that linear low-density polyethylene contains more tie molecules than high-density polyethylene and thus exhibits greater thermal shrinkage.

Therefore, the present inventors diligently studied the conditions for the relaxation heat treatment after the drawing process, and found that the temperature is 15°C or more lower than the melting point (T _mL ) of the linear low-density polyethylene contained in the sheath component of the core-sheath type composite fiber. By performing relaxation heat treatment under conditions where the tension applied to the fiber is as small as possible at a temperature, the core-sheath type composite fiber with suppressed heat shrinkage and the dimensional stability obtained by using it are improved. A fiber assembly was completed. In the core-sheath type conjugate fiber and the fiber assembly of the present invention, as a factor for suppressing heat shrinkage and improving dimensional stability, the linear low density polyethylene contained in the sheath component of the core-sheath type conjugate fiber is stretched. It is presumed that the orientation of the tie molecules generated in the process is sufficiently relaxed by the relaxation heat treatment, so that even if the heat treatment is performed at a temperature higher than the melting point of the linear low-density polyethylene, thermal shrinkage is less likely to occur. ing. However, this presumption does not limit the present invention.

The core-sheath type conjugate fiber of the present invention thus obtained is obtained, for example, by including 20% by mass or more of the core-sheath type conjugate fiber in the fiber assembly, and the fiber is formed by the sheath component of the core-sheath type conjugate fiber. By thermally bonding at least a part of them together, sufficient bonding strength can be obtained even if the thermal bonding process is performed at a low temperature. In addition, since heat shrinkage of the core-sheath type conjugate fibers is less likely to occur in the thermal bonding step, the obtained fiber assembly has a good texture and wrinkles are suppressed. The resulting fiber assembly has excellent rigidity and stiffness because the core component of the core-sheath type conjugate fiber contains a polyester resin, and when subjected to post-processing such as folding into a pleat shape. Excellent handleability.

The core-sheath type composite fiber has a 140° C. dry heat shrinkage of 5.0% or less. When the 140° C. dry heat shrinkage of the core-sheath type conjugate fiber is 5.0% or less, the fiber web containing the core-sheath type conjugate fiber is heat-treated to thermally bond the fibers contained in the fiber web. , even when exposed to a temperature range where linear low-density polyethylene is sufficiently melted, shrinkage of fibers due to heat is suppressed, so fiber aggregates containing core-sheath type conjugate fibers are prevented from wrinkling and curling, and texture is improved. can be a good fiber aggregate. Such a core-sheath type conjugate fiber is not particularly limited. By performing relaxation heat treatment at a certain temperature and relaxation ratio, distortion and strain generated in the molecular chains in the thermoplastic resin that constitutes the core-sheath type composite fiber during the drawing process, especially for tie molecules of linear low-density polyethylene, It can be obtained by relaxing the orientation generated in the stretching process. The core-sheath type conjugate fiber preferably has a dry heat shrinkage at 140°C of 4.8% or less, more preferably 4.5% or less, further preferably 4.2% or less, 4.0% or less is particularly preferable. The 140° C. dry heat shrinkage ratio of the core-sheath type conjugate fiber is preferably as small as possible. It may be 0% or more.

In the present invention, the 140° C. dry heat shrinkage of the core-sheath type composite fiber is measured as follows.
<140°C dry heat shrinkage rate>
A sample obtained by forming a core-sheath type composite fiber into a loop of 10 cm in length and tying the ends of the yarn (adjust the total fineness of the sample in which a plurality of fibers are bundled to be 55 to 56 dtex) is added with 0. An initial load of 26 cN / dtex is applied, the thermal stress tester KE-2LS (manufactured by Kanebo Engineering Co., Ltd.) is set to the displacement measurement mode, and the temperature is raised from room temperature to 170 ° C. at a heating rate of 60 ° C./min. The thermal shrinkage rate of the sample is measured at 140°C, and the heat treatment shrinkage rate of the sample at 140°C is defined as the 140°C dry heat shrinkage rate of the core-sheath type composite fiber.

When the core-sheath type conjugate fiber has a fiber length of 2 mm or more and 20 mm or less, the 140° C. area shrinkage of a wet-laid nonwoven fabric composed of 100% by mass of the core-sheath type conjugate fiber is 10% or less. When the 140° C. area shrinkage of the wet-laid nonwoven fabric is 10% or less, the linear low-density polyethylene contained in the sheath component is sufficiently contained in the fiber assembly containing 20% by mass or more of the core-sheath type conjugate fiber. Even if the heat bonding treatment is performed at a temperature of 120 to 150° C., which is the softening and melting temperature, the fibers do not easily shrink due to heat. Therefore, in wet-laid nonwoven fabrics and air-laid nonwoven fabrics in which fibers forming a fiber aggregate are often thermally bonded, the resulting nonwoven fabric is excellent in processability and uniformity. The 140° C. area shrinkage of the wet-laid nonwoven fabric composed of 100% by mass of the core-sheath type composite fiber is preferably 8.0% or less, more preferably 7.0% or less, and 6.0% or less. It is more preferable that the content is 5.0% or less, and particularly preferably 5.0% or less. The 140° C. area shrinkage ratio of the wet-laid nonwoven fabric composed of 100% by mass of the core-sheath type composite fiber is preferably as small as possible, so it may be 0%, but may be 0.5% or more, or 0.8% or more. It may be 1.0% or more.

In the present invention, the 140° C. area shrinkage of the wet-laid nonwoven fabric is measured as follows.
<140°C Area Shrinkage Rate>
(1) Weigh 3.15 g of the fiber to be measured in an absolute dry state.
(2) The weighed fibers are added to 1 liter of tap water to form a slurry, and the slurry is stirred with a pulper at a rotation speed of 1000 rpm for 1 minute to uniformly disperse the fibers in the water.
(3) Tap water is added to the pulper-stirred slurry to make 16 liters of slurry.
(4) The slurry diluted to 16 liters is poured into a 250 mm square frame lined with a metal mesh (200 mesh), wet paper is made, and the weight is 50 g/cm ² when dried. to a wet-laid papermaking web.
(5) The obtained wet papermaking web is sandwiched between filter papers and dehydrated by applying a pressure of 3.5 kg/cm ² for 2 minutes.
(6) A total of four points were marked with oil-based ink, one each in length and width, two points having an interval of 200 mm, which were symmetrical at the center of the wet papermaking web after dehydration. As shown in FIG. 4A, the initial vertical and horizontal intervals between two points are A ₀ and B ₀ respectively. Both A ₀ and B ₀ are 200 mm.
(7) As shown in FIG. 2, the marked wet papermaking web is subjected to hot air 25 at a temperature of 140° C. from the upper side of the wet papermaking web 23 in a hot air dryer 20 equipped with a conveyor net 21. A hot air drying treatment was carried out at a wind speed of 0.9 m/s for 30 seconds to completely dry the wet papermaking web, and at the same time, the sheath component was melted and the fibers were bonded together to obtain a heat-bonded nonwoven fabric.
At this time,
(a) As shown in FIG. 2, first, a polyethylene terephthalate net 22 (filament diameter: 0.3 mm, 35 mesh×25 mesh) is laid on a conveyor net 21, and a wet papermaking web 23 is laid thereon. was placed.
(b) In order to prevent the wet papermaking web 23 from being blown away by the hot air, square timbers (with a cross section of 15 mm on each side) are used so that the outer dimensions shown in FIG. A polyester resin net box 24 having the above-described polyethylene terephthalate net stretched over five surfaces excluding the bottom surface was covered with a wet papermaking web 23 on a wooden frame prepared using a square).
(8) The distance between the two points after the formation of the nonwoven fabric was measured, and as shown in _{FIG .} .
(9) Determine the 140° C. area shrinkage ratio based on the following formula.
140° C. area shrinkage (%)={(A ₀ ×B ₀ )−(A ₁ ×B ₁ )}÷(A ₀ ×B ₀ )×100

The core-sheath type conjugate fiber includes a core component and a sheath component, and is a core-sheath type conjugate fiber having a concentric structure in which the core component and the sheath component are arranged substantially concentrically.

(sheath component)
The sheath component is a straight chain having a density of 0.90 g/cm ³ or more and 0.93 g/cm ³ or less and a melting point of 100° C. or more and 130° C. or less, where the total mass of the sheath component is 100% by mass. 60% by mass or more of low-density polyethylene. Linear low-density polyethylene is a thermoplastic resin with a low melting point among polyolefin resins, and a core-sheath obtained by including 60% by mass or more of linear low-density polyethylene in the sheath component of the core-sheath type composite fiber. The type conjugated fiber becomes a core-sheath type conjugated fiber with excellent low-temperature adhesion, and by heat-treating a fiber assembly containing 20% by mass or more of this fiber at a temperature equal to or higher than the melting point of the linear low-density polyethylene, the fibers are sufficiently bonded to each other. It is possible to obtain a fiber assembly thermally bonded to a fiber by heat treatment at a temperature lower than that when using core-sheath type conjugate fibers whose sheath component is high-density polyethylene or polypropylene. In the sheath component, the linear low-density polyethylene content is preferably 75% by mass or more, more preferably 85% by mass or more, and still more preferably 90% by mass or more. The upper limit of the linear low-density polyethylene contained in the sheath component is not particularly limited, and the sheath component may have a structure in which all the resin components are linear low-density polyethylene. The linear low-density polyethylene contained in the sheath component may be one type, or two or more types.

The linear low-density polyethylene has a density of 0.90 g/cm ³ or more and 0.93 g/cm ³ or less. The density of the linear low-density polyethylene is preferably 0.905 g/cm ³ or more and 0.925 g/cm ³ or less, more preferably 0.91 g/cm ³ or more and 0.923 g/cm ³ or less. , 0.912 cm ³ or more and 0.92 g/cm ³ or less. In the present invention, the density of linear low-density polyethylene is measured according to JIS K 7112 (1999). In the present invention, when the density of the linear low-density polyethylene contained in the sheath component is within the range of 0.90 g/cm ³ or more and 0.93 g/cm ³ or less, it simply becomes easier to exhibit an appropriate melting point, which will be described later. In addition, the melt mass flow rate (MFR), which is the fluidity of linear low-density polyethylene when melted, becomes moderate, which not only improves the productivity of the melt spinning and drawing processes, but also reduces the heat of the fiber assembly. In the bonding step, sufficient thermal bonding is achieved even at a relatively low heat treatment temperature, and sticking to the production line is unlikely to occur. In the present invention, the density of the linear low-density polyethylene may be measured before spinning or after spinning.

The linear low-density polyethylene has a melting point of 100°C or higher and 130°C or lower. When the melting point of the linear low-density polyethylene contained in the sheath component satisfies the above range, the productivity of the drawing process is improved when producing the core-sheath type composite fiber, and the drawing temperature can be adjusted. , the fibers are less likely to fuse together during the drawing process. In addition, by heat-treating the fiber web containing the obtained core-sheath type conjugate fibers at a temperature of 120 to 150° C., the linear low-density polyethylene is easily melted, and the fibers constituting the fiber web are strengthened. Therefore, it is possible to produce a fiber aggregate having high adhesive strength by heat treatment at a lower temperature than when a core-sheath type composite fiber of high-density polyethylene or polypropylene is used as a sheath component. In the present invention, the melting point of linear low-density polyethylene is measured according to JIS K 7121 (1987). In the present invention, the melting point of the linear low-density polyethylene may be measured before spinning or after spinning.

The linear low-density polyethylene is a copolymer obtained by copolymerizing ethylene and α-olefin. The α-olefin is generally preferably an α-olefin having 3 to 12 carbon atoms. Specific examples of α-olefins having 3 to 12 carbon atoms include propylene, butene-1, pentene-1, 4-methylpentene-1, hexene-1, 4-methylhexene-1, heptene-1, Mention may be made of octene-1, nonene-1, decene-1, dodecene-1 and mixtures thereof. Among these, one or more selected from the group consisting of propylene, butene-1, 4-methylpentene-1, hexene-1, 4-methylhexene-1 and octene-1 is preferable, and butene-1 and hexene-1 One or more selected from the group consisting of is more preferable.

The α-olefin content in the linear low-density polyethylene is preferably 1 mol % or more and 10 mol % or less, more preferably 2 mol % or more and 5 mol % or less. A low α-olefin content may impair the flexibility of the fiber. If the α-olefin content is too high, the crystallinity will deteriorate and the fibers may fuse with each other during fiberization.

A linear low-density polyethylene having the above density and melting point can be easily obtained by copolymerizing ethylene and an α-olefin using a metallocene catalyst. The linear low-density polyethylene used for the sheath component has a density of 0.90 g/cm ³ or more and 0.93 g/cm ³ or less and a melting point of 100° C. or more and 130° C. or less, polymerized using a metallocene catalyst. For example, one polymerized using a Ziegler-Natta catalyst may be used.

The linear low-density polyethylene is not particularly limited, but the melt spinnability of the core-sheath type composite fiber and the heat adhesion property (heat Considering sealability), the melt mass flow rate (MFR) is preferably in the range of 1 g/10 min or more and 60 g/10 min or less. In the present invention, the melt mass flow rate (MFR) of linear low-density polyethylene is measured according to JIS K 7210-1 (2014) under conditions of a measurement temperature of 190 ° C. and a load of 2.16 kgf (21.18 N). be. The higher the MFR, the higher the fluidity of the melted linear low-density polyethylene. Therefore, when the fiber web containing the core-sheath type composite fiber is heat-treated, the thermal bonding treatment is performed at a lower temperature or in a shorter time. However, if the MFR is too large, the solidification speed of the sheath component during spinning becomes slow, and the fibers are likely to fuse together. In addition, the fluidity of the linear low-density polyethylene is too high in the heat bonding process, which may cause the fiber web to stick to the production line, or the molten resin to flow out and contaminate the production line. On the other hand, when the MFR is small, fusion between fibers is less likely to occur due to insufficient cooling during melt spinning. In addition, due to the low fluidity of linear low-density polyethylene, when heat-treating a fiber web containing core-sheath type conjugate fibers, the fiber web sticks to the production line and the molten resin flows out, which can hinder production. Although it is difficult to contaminate the line, the flowability of the linear low-density polyethylene is too small in the thermal bonding process, so the fibers that make up the fiber web are not sufficiently bonded to each other, and the resulting fiber There is a possibility that the mechanical properties such as the tensile strength of the aggregate may be easily lowered, and the fiber aggregate may easily become fuzzy due to friction. The MFR of the linear low-density polyethylene is preferably 2 g/10 min or more and 30 g/10 min or less, more preferably 5 g/10 min or more and 20 g/10 min or less, and 5 g/10 min or more and 15 g/10 min or less. is particularly preferred. In the present invention, the MFR of linear low-density polyethylene may be measured before spinning or may be measured after spinning.

The linear low-density polyethylene preferably has a weight average molecular weight (Mw) to number average molecular weight (Mn) ratio (Q value: Mw/Mn) of 5 or less. The Q value is more preferably 2 or more and 4 or less, and still more preferably 2.5 or more and 3.5 or less. When the Q value is 5 or less, the width of the molecular weight distribution of the linear low-density polyethylene is narrow, and by including the linear low-density polyethylene in the sheath component of the core-sheath type conjugate fiber, the core-sheath type conjugate fiber can be obtained. In addition to improving the productivity of the core-sheath type composite fiber, the fiber aggregate containing the core-sheath type composite fiber is excellent in mechanical properties such as elongation and impact resistance and heat sealability. In the present invention, the weight average molecular weight (Mw), number average molecular weight (Mn) and Q value of the linear low density polyethylene may be measured before spinning or after spinning. There may be.

The sheath component may contain a thermoplastic resin other than the linear low-density polyethylene as long as it does not impair the effects of the present invention. Examples of thermoplastic resins other than linear low-density polyethylene include, but are not limited to, polyolefin resins other than linear low-density polyethylene, polyester resins, polyamide resins, polycarbonate, and polystyrene. Polyolefin resins other than linear low-density polyethylene are not particularly limited, but examples include polypropylene, high-density polyethylene, medium-density polyethylene, low-density polyethylene, polymethylpentene, polybutene-1, and these together with acrylic acid and methacrylic acid. Acids, unsaturated carboxylic acids such as maleic acid, esters of unsaturated carboxylic acids such as acrylic acid esters, methacrylic acid esters and maleic acid esters, unsaturated carboxylic acids such as acrylic anhydride, methacrylic anhydride and maleic anhydride Examples thereof include those obtained by copolymerizing at least one selected from the group consisting of acid anhydrides, those obtained by graft polymerization, and elastomers thereof. Examples of polyester resins include, but are not limited to, polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polylactic acid, and acid components such as isophthalic acid, succinic acid and adipic acid, and 1, Glycol components such as 4-butanediol and 1,6-hexanediol, copolymers with polytetramethylene glycol, polyoxymethylene glycol and the like, and elastomers thereof can be mentioned. Examples of polyamide resins include, but are not particularly limited to, nylon 6, nylon 66, nylon 11, and nylon 12. In addition, various known additives can be added to the sheath component as long as the effects of the present invention are not hindered and the fiber productivity, nonwoven fabric productivity, thermal adhesiveness, and touch are not affected. be. Additives that can be added to the sheath component include known crystal nucleating agents, antistatic agents, pigments, matting agents, heat stabilizers, light stabilizers, flame retardants, antibacterial agents, lubricants, plasticizers, softening agents, oxidizing agents, Inhibitors, UV absorbers, and the like can be included. Such additives are preferably included in the sheath component in an amount not greater than 10% by weight of the total sheath component.

(core component)
The core component is a component containing 60% by mass or more of a polyester resin having a melting point higher by 50° C. or more than the melting point of the linear low-density polyethylene constituting the sheath component. The core component preferably contains 75% by mass or more of the polyester resin, more preferably 85% by mass or more, and particularly preferably 90% by mass or more. The upper limit of the polyester resin contained in the core component is not particularly limited, and in the core component, all the resin components may be polyester resin. The polyester resin contained in the core component and having a melting point higher by 50° C. or more than the melting point of the linear low-density polyethylene of the sheath component may be one kind or two or more kinds.

When the polyester resin is made into a fiber, heat shrinkage can be suppressed by performing an appropriate heat treatment after drawing. A polyester resin with suppressed heat shrinkage is less likely to undergo heat shrinkage at a temperature near the melting point of the linear low-density polyethylene constituting the sheath component, that is, in a temperature range of 100 ° C. to 150 ° C., so this polyester resin and the heat shrinkage of the entire core-sheath type composite fiber composed of linear low-density polyethylene can be suppressed. In addition, polyester resins are less expensive than other thermoplastic resins, and have high rigidity, so that the obtained core-sheath type conjugate fibers can have rigidity (feeling of stiffness).

Polyester resins include, for example, polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), and polyethylene naphthalate (PEN). The melting point of the polyester resin is 50° C. or more higher than the melting point of the linear low-density polyethylene contained in the sheath component. When two or more types of linear low-density polyethylene are contained in the sheath component, the melting point of the polyester resin is 50°C or more higher than the melting point of the linear low-density polyethylene having the highest melting point. . The melting point of the polyester resin is preferably 80° C. or higher than the melting point of the linear low-density polyethylene, and more preferably 100° C. or higher than the linear low-density polyethylene.

The core component may contain a thermoplastic resin other than the polyester resin as long as it does not impair the effects of the present invention. The thermoplastic resin other than the polyester resin is not particularly limited. low-melting point polyester resins that do not satisfy the condition that the melting point is 50° C. or higher).

Further, various known additives are added to the core component as long as the effects of the present invention are not hindered and the productivity of the fiber, the productivity of the fiber assembly, the thermal adhesiveness, and the touch are not affected. Is possible. Additives that can be added to the core component include known crystal nucleating agents, antistatic agents, pigments, matting agents, heat stabilizers, light stabilizers, flame retardants, antibacterial agents, lubricants, plasticizers, softeners, oxidizing agents, Inhibitors, UV absorbers, and the like can be included. Such additives are preferably included in the core component in an amount of 10% by weight or less of the total core component.

In the core-sheath type conjugate fiber of the present invention, the core-sheath ratio (core component/sheath component), which is the volume ratio of the core component to the sheath component, is preferably 8/2 to 2/8 (volume ratio). More preferably 7/3 to 3/7, still more preferably 6/4 to 4/6. When the core-sheath type conjugate fiber of the present invention is used to produce a fiber aggregate in which at least a part of the fibers are bonded to each other by the sheath component of the core-sheath type conjugate fiber, the core component is the mechanical strength of the fiber aggregate. It contributes to physical strength (breaking strength, stiffness, etc.), and the sheath component bonds the fibers contained in the fiber aggregate. Therefore, when the core-sheath ratio of the core-sheath type conjugate fiber is 8/2 to 2/8, when fabricating the fiber assembly, the fibers constituting the fiber assembly are heated at a relatively low heat treatment temperature for a short time. can be firmly adhered by heat treatment. In addition, the obtained fiber assembly has excellent mechanical strength because the constituent fibers are strongly thermally bonded to each other by the linear low-density polyethylene contained in the sheath component constituting the core-sheath type composite fiber. become a thing.

In the core-sheath type conjugate fiber, the cross-sectional structure is a concentric structure in which the center of gravity of the core component is substantially aligned with the center of gravity of the core-sheath type conjugate fiber. That is, in the cross section of the fiber, the center of gravity of the core component does not substantially deviate from the center of gravity of the sheath-core composite fiber. FIG. 1 is a schematic diagram of a fiber cross section of a core-sheath type conjugate fiber having a concentric structure. The sheath component 1 is arranged around the core component 2, and the sheath component 1 surrounds the core component 2, so that in the core-sheath type composite fiber 10, the fiber surface other than the cut surface is covered with the sheath component 1. . Thus, by heat-treating the fiber web containing the core-sheath type conjugate fibers at a temperature higher than the melting point of the linear low-density polyethylene, at least the surface of the sheath component 1 is melted, and the fibers constituting the fiber web are melted. to be hot glued.

In the core-sheath type composite fiber 10, the core component 2 is not eccentric, that is, has a concentric structure, so the thickness of the sheath component 1 in the cross section of the fiber is almost constant at any point in the cross section of the fiber. there is As a result, when a fiber web composed of core-sheath type conjugate fibers is heat-treated, any part of the core-sheath type conjugate fibers in which the sheath component on the fiber surface is softened or melted is in contact with other fibers. Also, since heat-bonding points of uniform strength are formed, a fiber assembly including a heat-bonded nonwoven fabric using the core-sheath-type composite fiber of the present invention has high bonding strength, is resistant to friction, and is less likely to become fuzzy. Become. In the sheath-core composite fiber of the present invention, the center-of-gravity position 3 of the core component 2 does not substantially deviate from the center-of-gravity position 4 of the core-sheath composite fiber 10 . The position of the center of gravity of the core component does not substantially deviate from the position of the center of gravity of the core-sheath type conjugate fiber means the ratio of deviation of the center of gravity of the core component relative to the position of the center of gravity of the core-sheath type conjugate fiber ( hereinafter also referred to as eccentricity) is 8% or less, preferably 6% or less, more preferably 5% or less, and still more preferably 3% or less. In the core-sheath type conjugate fiber of the present invention, the core component and the sheath component have a concentric structure, that is, the center of gravity of the core component does not substantially deviate from the center of gravity of the core-sheath type conjugate fiber. Even if the molecular structure of the linear low-density polyethylene resin contained in the sheath component has a strain that causes heat shrinkage due to heating, the effect of the heat shrinkage remains on the core-sheath type composite fiber. The heat shrinkage rate of the conjugate fiber tends to be small.

<Eccentricity>
A fiber cross section of the core-sheath type composite fiber 10 is photographed by an enlarged scanning electron microscope or the like, and the center of gravity position 3 of the core component 2 is set to _C1 , the center of gravity position 4 of the core-sheath type composite fiber 10 is set to _Cf , and the core-sheath type When the radius 5 of the composite fiber 10 is r _f , it is calculated by the following formula 1.

[Number 1]
Eccentricity (%) = [(C _f - C ₁ )/r _f ] x 100

In the core-sheath type composite fiber, the shape of the core component in the cross section of the fiber may be not only circular but also an irregular shape such as elliptical, Y-shaped, X-shaped, I-shaped, polygonal, or star-shaped. The shape of the cross section of the fiber may be not only circular but also irregular shapes such as elliptical, Y-shaped, X-shaped, I-shaped, polygonal, star-shaped, or hollow.

The core-sheath type conjugate fiber is not particularly limited, but preferably has a single fiber fineness of 0.5 dtex or more and 5.6 dtex or less. When the single fiber fineness is 5.6 dtex or less, the fiber assembly using the core-sheath type composite fibers has excellent productivity and uniform texture. In addition, when the single fiber fineness is 0.5 dtex or more, the productivity of the core-sheath type composite fiber and the fiber assembly is excellent, and in particular, when producing a wet-laid nonwoven fabric containing the core-sheath type composite fiber, the core-sheath type composite Excellent dispersibility of fibers in water. The single fiber fineness of the core-sheath type composite fiber is more preferably 0.6 dtex or more and 5.0 dtex or less, further preferably 0.7 dtex or more and 4.5 dtex or less, and 0.8 dtex or more and 3.0 dtex or less. is particularly preferred.

The core-sheath type conjugate fiber can be suitably used as a short fiber having a fiber length of 2 mm or more and 20 mm or less. When the fiber length satisfies this range, a fiber aggregate using the core-sheath type conjugate fiber, preferably a fiber aggregate produced by a fiber web by a wet method or an air-laid method, can be produced in the production stage of the fiber web. Before the fibers are formed into a sheet, the fibers are entangled with each other and are less likely to become spherical or fiber bundle foreign matter, so the productivity of the fiber web is improved, and the texture of the obtained fiber aggregate is likely to be uniform and good. . When used for a fiber web by a wet method or an airlaid method, the fiber length of the core-sheath type conjugate fiber is preferably 2 mm or more and 18 mm or less, more preferably 3 mm or more and 15 mm or less, and still more preferably 3 mm or more and 12 mm or less. be.

The fiber length of the core-sheath type conjugate fiber is selected depending on the method of manufacturing the fiber web, and crimp may be imparted in some cases. For example, when a fiber web is produced by the air-laid method, the crimped shape of the fiber may be planar zigzag crimp (also referred to as mechanical crimp or sawtooth crimp), omega (Ω) crimp (wavy crimp). (also referred to as crimp), spiral crimp (also referred to as helical crimp), and a crimped shape in which these crimped shapes are mixed. When crimping is applied, it is preferable to apply crimps so that the number of crimps is 15 crimps/25 mm or less and the crimp ratio is 15% or less. It may be in a substantially non-crimped state where no crimping is applied. In the present invention, "substantially no crimp" refers to a state in which the number of crimps is 3 crimps/25 mm or less. In the case of producing a fibrous web by a wet method (papermaking method), if the resulting wet-laid fibrous web and the wet-laid nonwoven fabric produced using the same are to be bulky, the number of crimps is 5 crests/25 mm or less. Although the crimp may be applied such that the crimp rate is 10% or less, when the fiber assembly containing the core-sheath type composite fiber is produced by a wet method, it is substantially uncrimped. state is preferred.

The core-sheath type conjugate fiber is not particularly limited, but for example, from the viewpoint of maintaining the mechanical properties (such as tensile strength, tear strength, and puncture strength) of a fiber aggregate containing the core-sheath type conjugate fiber. Therefore, the single fiber strength is preferably greater than 1.8 cN/dtex and 4.0 cN/dtex or less, more preferably 2 cN/dtex or more and 3.6 cN/dtex or less, and particularly preferably 2.1 cN/dtex or more. It is 3.4 cN/dtex or less. Although the core-sheath type conjugate fiber is not particularly limited, it is preferable that the elongation at break is 20% or more and 200% or less from the viewpoint of maintaining the mechanical properties of the fiber aggregate containing the core-sheath type conjugate fiber. , more preferably 30% or more and 160% or less.

As described above, the core-sheath type conjugate fiber of the present invention is characterized by small heat shrinkage at the temperature at which the sheath component melts, so that a fiber aggregate having excellent dimensional stability can be easily obtained. Therefore, the core-sheath type conjugate fiber of the present invention can be made into a dry nonwoven fabric (a fiber web by a carding method or an air laying method) by adjusting the single fiber fineness, fiber length, number of crimps, crimp ratio, single fiber strength and elongation. Examples include hydroentangled nonwoven fabrics, thermal bonded nonwoven fabrics, needle-punched nonwoven fabrics, chemical bonded nonwoven fabrics and air-laid nonwoven fabrics.), wet-laid nonwoven fabrics. It is preferably used as a core-sheath type composite fiber for wet-laid nonwoven fabric.

In general, fibers used in dry-laid nonwoven fabrics are stretched, coated with a fiber treatment agent for the purpose of improving fiber opening properties, and then subjected to a drying step to dry the fiber treatment agent. The drying process is carried out under tensionless conditions in a dryer adjusted to a temperature of 60-120°C. In the fiber produced through the drying process, the strain generated in the molecular chains in the thermoplastic resin constituting the fiber, especially the tie molecule of the thermoplastic resin, generated in the drawing process is relaxed to some extent. thermal shrinkage tends to be small.

On the other hand, fibers used for wet-laid nonwoven fabrics are usually used without being dried in order to increase the dispersibility of the fibers in water. In other words, the fibers used in the wet-laid nonwoven fabric are stretched, coated with a fiber treatment agent for the purpose of improving dispersibility in water, and then cut into desired fiber lengths while the fibers are wet. be. If the stretched fibers are not heat-treated in a relaxed state even once, the distortion and distortion generated in the molecular chains in the thermoplastic resin that constitutes the fibers generated in the stretching process remain. If the distortion or strain is relaxed in the drying process or heat bonding treatment during production, the obtained wet-laid nonwoven fabric may have wrinkles or the dimensions may not be stable. The core-sheath type conjugate fiber of the present invention and the method for producing the core-sheath type conjugate fiber described later do not require a drying step in a non-tension state, which is generally performed in the production of fibers used in dry-laid nonwoven fabrics. A core-sheath type composite fiber and a method for manufacturing the same, which are particularly suitable as fibers for wet-laid nonwoven fabrics that require the use of fibers in a wet state, because they are a core-sheath type composite fiber with suppressed heat shrinkage and a method for manufacturing the same. be.

The method for producing the core-sheath type composite fiber of the present invention will be described below. Although the core-sheath type conjugate fiber is not particularly limited, it can be produced, for example, as follows.

First, a sheath component containing 60% by mass or more of the linear low-density polyethylene, and a core component containing 60% by mass or more of a polyester resin having a melting point higher than the melting point T _mL of the linear low-density polyethylene by 50°C or more. prepare. Next, in the cross section of the fiber, the composite nozzle is arranged so that the surface of the core-sheath type composite fiber is covered with the sheath component, and the center of gravity of the core component coincides with the center of gravity of the core-sheath type composite fiber. For example, a sheath component and a core component are supplied to a concentric core-sheath type composite nozzle, and melt spinning is performed. At this time, for example, the temperature (spinning temperature) at which the core component is melted and extruded is 270° C. or higher and 350° C. or lower, and the temperature at which the sheath component is melted and extruded (spinning temperature) is 230° C. or higher and 350° C. or lower. is melt-spun at a temperature of 260° C. or higher and 350° C. or lower. A filament spun from a composite nozzle is taken up to obtain a spun filament. When the take-up speed is 500 m/min or more and 2500 m/min or less, a spun filament having a single fiber fineness of 1.0 dtex or more and 8 dtex or less can be easily obtained. This is preferable because a core-sheath type composite fiber having a single fiber fineness can be produced.

In the method for producing a core-sheath type composite fiber of the present invention, the single fiber fineness of the spun filament is not particularly limited as long as it is 1.0 dtex or more and 15 dtex or less. However, although it depends on the application of the core-sheath type conjugate fiber, it is preferable that the single fiber fineness of the spun filament is small. Since the single fiber fineness of the spun filament is close to the single fiber fineness of the final core-sheath type composite fiber, the draw ratio in the drawing step described later can be set from low to medium. When the draw ratio is low to medium, the orientation of tie molecules generated in the linear low-density polyethylene in the drawing process can be suppressed, and the heat shrinkage of the obtained core-sheath type composite fiber can be easily suppressed. The single fiber fineness of the spun filament is preferably 1.0 dtex or more and 8.0 dtex or less, more preferably 1.2 dtex or more and less than 5.6 dtex, further preferably 1.4 dtex or more and 5.2 dtex or less, 1.6 dtex or more and 5.0 dtex or less is particularly preferable, and 1.8 dtex or more and 4.8 dtex or less is most preferable.

Next, the obtained spun filaments are stretched at a temperature (T _mL −15° C.) that is 15° C. lower than the melting point (T _mL ) of the linear low-density polyethylene and higher than the glass transition temperature of the polyester resin. Stretching is performed under the condition that the magnification is 1.4 times or more and 2.5 times or less. The lower limit of the stretching temperature is preferably 60°C or higher, more preferably 70°C or higher, and particularly preferably 75°C or higher. A preferable upper limit of the stretching temperature is a temperature 20° C. lower than the melting point T _mL of the linear low-density polyethylene (T _mL −20° C.), and a more preferable upper limit of the stretching temperature is the melting point of the linear low-density polyethylene. 25° C. lower (T _mL −25° C.) or less. When the drawing temperature is lower than the glass transition temperature of the polyester resin, the polyester resin is not sufficiently drawn, which may deteriorate the processability of the fiber. If the drawing temperature is higher than the melting point T _mL −15° C. of the linear low-density polyethylene, the fibers tend to fuse together. The lower limit of the draw ratio is preferably 1.6 times or more, more preferably 1.7 times or more, and particularly preferably 1.8 times or more. The upper limit of the draw ratio is preferably 2.3 times or less, more preferably 2.2 times or less. If the draw ratio is less than 1.2 times, sufficient tension is not applied to the spun filaments, making it difficult to adjust the spun filaments to the desired single fiber fineness. When the draw ratio exceeds 2.5 times, the tension applied to the spun filament increases due to the high draw ratio, and the molecular chains of the thermoplastic resin constituting the spun filament, especially the orientation present in the linear low-density polyethylene. The proportion of the tie molecules becomes large, and even if the relaxation heat treatment described later is performed, the orientation of the tie molecules is not sufficiently relaxed, and in a temperature range above the melting point of the linear low-density polyethylene, such as 120 to 150 ° C., the core-sheath type Composite fibers and fiber aggregates containing them may undergo significant heat shrinkage. The draw ratio is expressed as _V2 /V1, where _{V1 is} the feed rate of the spun filament and _V2 is the take-up rate of the drawn filament in the drawing step.

The drawing method is not particularly limited, and known drawing processes such as wet drawing in which the film is drawn while being heated in a hot liquid such as hot water, and dry drawing in which the film is drawn while being heated in a hot gas or with hot metal rolls. However, it is preferable to draw the spun filaments by a wet drawing treatment using a liquid heated to a predetermined temperature such as hot water as a medium. The drawing process may be a so-called one-stage drawing in which the drawing process is performed in only one stage, a two-stage drawing in which the drawing process is performed in two stages, or a multi-stage drawing in which the drawing process includes more than two stages.

Next, the stretched filament has a relaxation ratio of 1 at a temperature above the glass transition point of the polyester resin and 15 ° C. lower than the melting point T _mL of the linear low-density polyethylene (T _mL -15 ° C.). The relaxation heat treatment is performed under the condition of less than 0.0 times to obtain a relaxation heat-set filament. In this relaxation heat treatment step, the lower limit of the temperature of the relaxation heat treatment is preferably 60° C. or higher, more preferably 70° C. or higher, and particularly preferably 75° C. or higher. The upper limit of the preferred relaxation heat treatment temperature is 20° C. lower than the melting point T _mL of the linear low-density polyethylene (T _mL −20° C.), and the more preferred upper limit of the relaxation heat treatment temperature is the linear low-density polyethylene. It is 25° C. lower than the melting point T _mL of polyethylene (T _mL −25° C.) or lower. If the temperature of the relaxation heat treatment is lower than the glass transition temperature of the polyester resin, the processability of the relaxation heat treatment may be deteriorated. The thermal energy given to the linear low-density polyethylene (molecular chain) is reduced, and a fiber that suppresses heat shrinkage when exposed to a temperature range near the melting point of the linear low-density polyethylene, such as 120 to 150 ° C., can be obtained. It may become difficult. If the temperature of the relaxation heat treatment exceeds (T _mL -15°C), the fibers tend to fuse together.

In the relaxation heat treatment step, the tension applied to the drawn filament is preferably smaller, but in order to improve productivity and prevent the drawn filament and the relaxation heat-set filament from winding around the production equipment, the relaxation ratio should be 0.50 times or more. is preferably 0.70 times or more, more preferably 0.75 times or more, and particularly preferably 0.78 times or more. The upper limit of the relaxation ratio is preferably 0.98 times or less, more preferably 0.96 times or less, and particularly preferably 0.95 times or less. The relaxation ratio is expressed by _V4 / _V3 , where V3 is the draw-out speed of the drawn filament and _V4 is the take-up speed _of the relaxation heat-set filament in the relaxation heat treatment step. When the drawing step and the relaxation heat treatment step are performed continuously, the draw-out speed _V3 of the drawn filaments in the relaxation heat treatment step is the same speed as the take-up speed _V2 of the drawn filaments in the drawing step. When the core-sheath type conjugate fiber is continuously produced, the relaxation ratio in the relaxation heat treatment step is within the above range due to production facilities. After the filament is wound on the bobbin, when the relaxation heat treatment is performed by immersing the drawn filament together with the bobbin in a hot water bath adjusted to a predetermined temperature, the relaxation ratio is substantially 0 times, that is, without tension. It may be a relaxation heat treatment. The relaxation heat treatment method is not particularly limited, and may be wet treatment while heating with a high-temperature liquid such as hot water, or dry treatment while heating in a high-temperature gas or with a high-temperature metal roll. However, it is preferable to apply a relaxation heat treatment to the drawn filaments by a wet treatment using a liquid heated to a predetermined temperature such as hot water as a medium. The relaxation heat treatment step may be a so-called one-step relaxation heat treatment in which the relaxation heat treatment is performed in only one step, a two-step relaxation heat treatment in two steps, or a multi-step relaxation heat treatment in which the stretching step exceeds two steps. good too.

In the above-described method for producing a core-sheath type composite fiber, the spun filaments obtained by melt spinning are stretched at a predetermined temperature at a predetermined draw ratio, and then subjected to a relaxation heat treatment at a predetermined temperature at a predetermined relaxation ratio. By performing the stretching process, the distortion and strain generated in the molecular chains of the thermoplastic resin that constitutes the stretched filament are relaxed, and the 140 ° C. dry heat shrinkage rate is 5.0% or less, or the 140 ° C. area shrinkage rate is A core-sheath type composite fiber with a content of 10.0% or less is easily obtained. Both the drawing treatment and the relaxation heat treatment may be performed multiple times. Therefore, it is preferable to perform the relaxation heat treatment at least once after performing the stretching treatment once.

Next, if necessary, crimps may be applied using a known crimper such as a stuffing box type crimper under the condition that the number of crimps is 15/25 mm or less. However, when the core-sheath type composite fiber of the present invention is substantially uncrimped, it may not be crimped. Before or after crimping, and before and after the step of cutting the relaxation heat-set filaments to a predetermined length, which will be described later, the filaments may be treated with a fiber treatment agent, if necessary. Alternatively, if the core-sheath type conjugate fiber is to be formed in an uncrimped state, after the relaxation heat treatment and before and after the step of cutting the relaxation heat-set filament to a predetermined length, which will be described later, if necessary, If desired, it may be treated with a fiber treatment agent. The type of fiber treatment agent is not particularly limited. Since fibers are required to disperse uniformly without clumping in water, the fiber treatment agent preferably contains various antistatic surfactants or highly hydrophilic surfactants.

When various fiber treatment agents are applied to the core-sheath type conjugate fiber of the present invention, the method of application is not particularly limited. (spraying method), coating method, etc., but since the fiber treatment agent can be uniformly adhered to the bundle of the relaxation heat-set filaments, the relaxation heat-set filaments are diluted with the desired fiber treatment agent. A method of impregnating in an aqueous solution (hereinafter referred to as a fiber treatment agent solution), squeezing off excess fiber treatment agent solution with a mangle roll such as a metal roll or a rubber roll, and adjusting the moisture content to a desired level (impregnation method). is preferred.

The fiber treatment agent solution for impregnating the relaxation heat-set filaments should be diluted to an aqueous solution with a fiber treatment agent, in other words, an active ingredient other than water, such as a surfactant, at a concentration of 0.5% by mass or more and 10% by mass or less. is preferred. When the concentration is less than 0.5% by mass, the amount of the fiber treatment agent remaining on the surface of the core-sheath type composite fiber is small, and in the step of forming a fiber assembly, water dispersibility, fiber opening property, and card passability are improved. may deteriorate and productivity may decline. If the concentration exceeds 10% by mass, the concentration and viscosity of the fiber treatment agent liquid will increase, and the productivity of the core-sheath type composite fiber will deteriorate, and the production cost will increase due to the loss of the fiber treatment agent. .

When the core-sheath type conjugate fiber of the present invention is used in a wet nonwoven fabric, the core-sheath type conjugate fiber may be dried and used in an absolutely dry state. After drying, it is preferable to squeeze off excess fiber treatment liquid, cut the fibers into desired fiber lengths in a wet state, and use them for the production of wet-laid nonwoven fabrics. At this time, the core-sheath type composite fiber from which the excess fiber treatment agent was squeezed out has a moisture content of 15 mass, which is the ratio of water adhering to the fiber, when the mass of the core-sheath type composite fiber is 100% by mass. % or more and 50 mass % or less. If the moisture content is less than 15% by mass, the core-sheath type composite fiber is likely to dry, that is, the fiber dries while the core-sheath type composite fiber is stored, and the dispersibility in water deteriorates when it is made into a wet-laid nonwoven fabric. As a result, the resulting wet-laid nonwoven fabric (fiber aggregate) tends to generate fiber clumps. If the moisture content exceeds 50% by mass, the core-sheath type composite fiber is impregnated with an excessive amount of fiber treatment liquid, which may contaminate the production line, and the core-sheath type composite fiber contains a large amount of water. Since it is in such a state, it is difficult to handle. The moisture content is preferably 18% by mass or more and 45% by mass or less, and particularly preferably 20% by mass or more and 40% by mass or less.

In addition, in the present invention, the moisture content is measured by the following method.
<Moisture content>
(1) Collect about 5 g of the fiber to be measured, and measure the exact mass (W ₁ ). At this time, the sample for measuring the moisture content may be a fiber after being cut to a desired fiber length, or a bundle of relaxation heat-set filaments after applying the fiber treatment liquid and squeezing off excess fiber treatment liquid. good.
(2) Using a high-temperature dryer set at 140° C., the sample whose mass (W ₁ ) has been accurately measured is dried for 15 minutes to completely evaporate water from the sample.
(3) Measure the mass (W ₂ ) of the sample from which water has evaporated.
(4) Calculate the moisture content (mass%) of the fiber based on the following formula Moisture content (mass%) = 100 × (W ₁ - W ₂ ) ÷ W ₂

When the moisture content of the fiber is 15% by mass or more and 50% by mass or less, depending on the concentration of the fiber treatment agent liquid, the active ingredient of the fiber treatment agent is when the weight of the core-sheath type composite fiber is 100% by weight. Since it is attached at a rate of 0.3% by mass or more and 0.5% by mass or less, it is preferable from the viewpoint of the production cost of the core-sheath type composite fiber and the productivity of the wet-laid nonwoven fabric using the core-sheath type composite fiber.

The amount of the fiber treatment agent (active ingredient such as a surfactant) adhering to the core-sheath type conjugate fiber can be estimated from the concentration of the fiber treatment agent liquid and the moisture content. The amount of the adhered fiber treatment agent (hereinafter referred to as the adhered amount of the fiber treatment agent component) is measured by the following method.

<Textile treatment component adhesion amount>
(1) Using a high-temperature dryer set at 60° C., the fibers to be measured for the amount of the fiber treatment component adhered are dried for 180 minutes.
(2) About 4 g of fiber is taken from the dried fiber as a sample, and the exact mass (W _f ) of the sample is measured.
(3) Place the sample after mass measurement in a stainless steel column for extracting oil, add 20 mL of methanol, and allow to stand for 2 minutes.
(4) Prepare a small metal dish sufficiently dried in a drier at 150° C., and measure its mass (W _b ).
(5) The sample inside the stainless steel column for oil solution extraction left for 2 minutes is pistoned for 10 minutes using an air cylinder, the fiber treatment agent attached to the sample squeezes out the extracted methanol, and the piston flows out from the outlet. The liquid is received in the small metal plate whose dry mass was measured in (4) above.
(6) After sufficiently squeezing out the methanol in the stainless steel column for extracting the oil, the small metal dish that received the extract is heated with a heater at 150°C to evaporate the methanol.
(7) Methanol evaporates and the small metal plate on which the fiber treatment agent remains is cooled at room temperature for 2 minutes, and after cooling, the mass (W _s ) of the small metal plate is measured.
(8) Determine the amount of fiber treatment component adhered (% by mass) based on the following formula: Amount of fiber treatment component adhered (% by mass) = 100 x (W _s - W _b ) ÷ W _f
The adhesion amount of the fiber treatment agent component is measured twice and taken as the average value.
The stainless steel column for oil agent extraction used for the extraction of the fiber treatment agent is a stainless steel column having the shape shown in FIG. , the length of the conical part (outlet part) at the tip is 13.0 mm, and the hole diameter of the outlet is 1.6 mm.

In the core-sheath type conjugate fiber of the present invention, the amount of the fiber treatment component adhered measured by the above method is preferably 0.07% by mass or more and 1.5% by mass or less, and is preferably 0.1% by mass or more and 1.5% by mass. It is more preferably 0 mass % or less, particularly preferably 0.2 mass % or more and 0.8 mass % or less, and most preferably 0.3 mass % or more and 0.5 mass % or less.

When the core-sheath type conjugate fiber is used in a dry-laid nonwoven fabric, it is preferable to subject the relaxation heat-set filaments to optional treatments (crimping, treatment with a fiber treatment agent) to annealing treatment. Annealing treatment is preferably performed in a dry heat atmosphere within a temperature range of 50° C. or more and 95° C. or less, more preferably within a temperature range of 60° C. or more and 90° C. or less. Specifically, after imparting a fiber treatment agent to the relaxation heat-set filament, crimping is imparted by a crimping machine, and drying treatment is performed at the same time as annealing treatment in a dry heat atmosphere of 50 ° C. or higher and 95 ° C. or lower. is preferred because it simplifies the process.

The relaxation heat-set filament is subjected to any of the above treatments, and if necessary, the relaxation heat-set filament is cut into a predetermined length to obtain a core-sheath type conjugate fiber. The method for cutting the relaxation heat-set filament is not particularly limited, and for example, it can be cut by a known method such as a rotary cutter or a guillotine cutter. When the relaxation heat-set filament is cut to form a core-sheath type conjugate fiber, the fiber length is appropriately adjusted according to the manufacturing method used when the core-sheath type conjugate fiber is made into a fiber aggregate. When the core-sheath type composite fiber is made into a fiber web by an air-laid method to form an air-laid nonwoven fabric, the fiber length is preferably 5 mm or more and 25 mm or less, more preferably 10 mm or more and 20 mm or less, and 12 mm or more and 18 mm or less. is particularly preferred. When the core-sheath type conjugate fiber is made into a fiber web by a wet method (papermaking method) to form a wet nonwoven fabric, the fiber length is preferably 2 mm or more and 18 mm or less, more preferably 3 mm or more and 15 mm or less, and 3 mm or more and 12 mm. or less, and most preferably 5 mm or more and 10 mm or less. The core-sheath type conjugate fiber is resistant to thermal shrinkage even when exposed to temperatures around the melting point of the linear low-density polyethylene contained in the sheath component, or the rate of shrinkage due to thermal shrinkage is suppressed. After obtaining a fibrous web with a carding machine such as the above, it can also be used for a dry nonwoven fabric to be a nonwoven fabric. In this case, the fiber length is preferably 20 mm or more and 75 mm or less, more preferably 25 mm or more and 70 mm or less, and particularly preferably 28 mm or more and 65 mm or less.

(Fiber assembly)
The fiber assembly of the present invention contains 20% by mass or more of the core-sheath type conjugate fibers, and at least a part of the core-sheath type conjugate fibers are bonded together by a sheath component. After obtaining a fiber web containing 20% by mass or more of the core-sheath type conjugate fiber, the fiber web is heat-treated to thermally bond the constituent fibers. A fiber web containing 20% by mass or more of the core-sheath type conjugate fiber is produced by a known method (for example, a wet method), and the obtained fiber web is heat-treated to thermally bond and integrate the constituent fibers. Since the resulting fiber aggregate contains 20% by mass or more of the core-sheath type conjugate fiber, it is heat-treated at a temperature equal to or higher than the melting point of the linear low-density polyethylene contained in the sheath component of the core-sheath type conjugate fiber. As a result, not only is it possible to obtain a heat-bonded nonwoven fabric in which the constituent fibers are sufficiently heat-bonded, but also the dimensional stability of the fiber assembly is improved because the core-sheath-type composite fiber undergoes little heat shrinkage in the heat treatment process. Well, the productivity is improved, and the yield of the obtained fiber assembly (non-defective product rate) is also improved.

The fiber assembly preferably contains the core-sheath type conjugate fibers in an amount of 25% by mass or more, more preferably 30% by mass or more, and even more preferably 40% by mass or more. The fiber assembly may be a fiber assembly consisting only of the core-sheath type conjugate fibers, in other words, a fiber assembly containing 100% by mass of the core-sheath type conjugate fibers. From the viewpoint of further increasing the air permeability and liquid permeability of the fiber aggregate and from the viewpoint of softening the texture, the core-sheath type composite fiber contained in the fiber aggregate may be 90% by mass or less, or 80% by mass. It may be less than or equal to 75% by mass or less.

The fiber assembly may be a fiber assembly containing the core-sheath type conjugate fibers and other fibers. Examples of other fibers that can be used include natural fibers, regenerated fibers, purified cellulose fibers, semisynthetic fibers, and synthetic fibers. Natural fibers include, for example, cotton, silk, wool, hemp, pulp, and the like. Examples of regenerated fibers include rayon and cupra. Purified cellulose fibers include Tencel, Lyocell, and the like. Semi-synthetic fibers include acetate, triacetate, and the like. Examples of synthetic fibers include acrylic fibers, polyester fibers, polyamide fibers, polyolefin fibers, and polyurethane fibers. As the other fibers, one or more fibers can be appropriately selected from the above-described fibers depending on the intended use. Other fibers may be used by blending with the core-sheath type composite fibers. Alternatively, a fiber web made of the core-sheath type conjugate fiber and a fiber web made of other fibers may be laminated for use.

When manufacturing the fiber assembly, first, a fiber web containing core-sheath type conjugate fibers is formed. The method for obtaining the fiber web is not particularly limited, but examples thereof include a wet papermaking method in which the core-sheath type conjugate fibers and, if necessary, other fibers are dispersed in water and formed into a sheet by a paper machine or the like. Examples of the wet papermaking method include conventionally known methods such as a short screen method, a circular net method, a fourdrinier method, a fourdrinier/circular net combination method, a short net/circular net combination method, etc. Any one of these methods can be used. The fibrous web can be formed in two ways. Other methods for producing fiber webs include, for example, an airlaid method (also referred to as an airlaid method) and a card method. In the air-laid method, the core-sheath type conjugate fibers and, if necessary, other fibers are defibrated and blended, then dispersed in the air, and then deposited on a screen such as a wire mesh to form a fiber web. Carding methods include parallel web, semi-random web, random web, cross-web, and criss-cross web. The fiber assembly may be used by laminating two or more different types of fiber webs from the fiber webs described above.

By subjecting the fiber web to a heat treatment to melt the sheath component of the core-sheath type conjugate fibers contained in the fiber web, a fiber assembly in which the fibers are thermally bonded to each other is obtained. Although the heat treatment means is not particularly limited, it is preferable to appropriately select and use the heat treatment means according to the manufacturing method of the fibrous web. If the fibrous web is produced by a wet method, a heat treatment machine capable of drying while thermally bonding the constituent fibers of the fibrous web is preferable. Examples of such a heat treatment machine include a cylinder dryer and a hot air blowing process. A machine (air-through processing machine), a heat roll processing machine, a heat embossing machine, or the like can be used. When a cylinder dryer is used as the heat treatment machine, the heat treatment is carried out by bringing the fiber web into contact with the heating roll for thermal bonding, so that the thickness of the fiber aggregate is adjusted to a desired thickness while simultaneously bonding the fibers together. It can be thermally bonded. In addition, since the fiber aggregate heat-treated by such a heat treatment machine has a thickness adjusted to some extent (that is, thermal bonding between fibers is promoted), the thickness using a calendar roll described later When adjusting, the step of adjusting the thickness is performed twice. As a result, thermal bonding between fibers proceeds more, and the mechanical properties (for example, tensile strength and puncture strength) of the fiber assembly tend to increase. The fiber assembly obtained in this way is a fiber assembly that requires high mechanical properties, and is used for separators used in various batteries, supports for various filtration membranes, filters for liquids, and filter media for air filters. can be used for If the fibrous web is produced by a method other than the wet method, the heat treatment machine for thermally bonding the constituent fibers of the fibrous web may be a hot air penetration type heat treatment machine, a hot air blowing type heat treatment machine, an infrared heat treatment machine, or the like. A heat treatment machine that does not apply much pressure to the fiber web is preferably used.

The heat treatment is performed at a temperature at which the sheath component of the core-sheath type conjugate fiber is sufficiently melted, in other words, at a temperature at which the linear low-density polyethylene contained in the sheath component is sufficiently melted and the fibers can be bonded together. do. Therefore, the temperature of the heat treatment is 5% higher than the melting point T _mL of the linear low-density polyethylene contained in the sheath component of the core-sheath type composite fiber (the linear low-density polyethylene having a higher melting point when two or more types are contained). It is preferably carried out at a temperature higher than (T _mL +5° C.), preferably at a temperature higher than (T _mL +10° C.), particularly preferably at a temperature higher than (T _mL +20° C.). Although the upper limit of the heat treatment temperature is not particularly limited, it is 30° C. lower than the melting point T _mt of the polyester resin contained in the core component of the core-sheath type composite fiber from the viewpoint of the possibility of deformation of the fiber web or fiber aggregate and the production cost. The temperature is preferably (T _mt −30° C.) or less, more preferably (T _mt −50° C.) or less, and particularly preferably (T _mt −80° C.) or less.

Although the basis weight of the fiber aggregate is not particularly limited, it is preferably 5 g/m ² or more and 100 g/m ² or less, more preferably 8 g/m ² or more and 80 g/m ² or less, and 10 g/m ² or more. It is more preferably 75 g/m ² or less, and particularly preferably 15 g/m ² or more and 70 g/m ² or less. When the fabric weight of the fiber aggregate satisfies the above range of fabric weight, the resulting fiber aggregate can be used for various purposes.

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

The measurement methods and evaluation methods used in the examples are as follows.

(Number average molecular weight Mn, mass average molecular weight Mw, and Q value of linear low-density polyethylene)
Using a cross fractionator (CFC) and Fourier transform infrared absorption spectroscopy (FT-IR), gel permeation chromatography (GPC) using ortho-dichlorobenzene (ODCB) as a measurement solvent, the number average molecular weight Mn, mass The average molecular weight Mw and the ratio of weight average molecular weight/number average molecular weight (Mw/Mn: Q value) were measured.

(Density of linear low-density polyethylene)
The density of linear low-density polyethylene was measured according to JIS K 7112 (1999).

(Melting point of linear low-density polyethylene)
The melting point of the linear low-density polyethylene was measured according to JIS K 7121 (1987), and the melting temperature was taken as the melting point of the linear low-density polyethylene.

(Melt mass flow rate (MFR) of linear low-density polyethylene)
The melt mass flow rate (MFR) of linear low-density polyethylene was measured according to JIS K 7210-1 (2014) under the measurement conditions of a measurement temperature of 190° C. and a load of 2.16 kg (21.82 N).

When measuring the number average molecular weight Mn, the weight average molecular weight Mw, the weight average molecular weight / number average molecular weight ratio (Mw / Mn: Q value), the density, the melting point, and the MFR of the linear low-density polyethylene before spinning , The resin pellets of the used linear low-density polyethylene were measured as they were as samples, and the number-average molecular weight Mn, the weight-average molecular weight Mw, the weight-average molecular weight/number-average molecular weight ratio ( Mw / Mn: Q value), density, melting point, and MFR, when performing melt spinning, the temperature of the extruder is 290 ° C., and a linear low-density A rod-shaped resin strand with a diameter of 5 to 8 mm is prepared by melting and extruding polyethylene and cooling in the air, and this rod-shaped resin strand is cut into a length of about 3 mm and measured as a sample. Alternatively, the resulting core-sheath type conjugate fiber may be used as a sample for measurement.

(Melting point of polyester resin)
The melting point of the polyester resin was measured according to JIS K 7121 (1987), and the melting temperature was taken as the melting point of the polyester resin.

(Single fiber fineness and fiber length of core-sheath type composite fiber)
The single fiber fineness of the core-sheath type composite fiber was measured according to JIS L 1015 (2010) 8.5 (vibration method). In addition, the fiber length of the core-sheath type composite fiber was measured according to JIS L 1015 (2010) 8.4.

(Core-sheath ratio (volume ratio) of core-sheath type composite fiber)
First, the cross section of the core-sheath type composite fiber for measuring the core-sheath ratio was photographed with a scanning electron microscope (SEM) at a magnification of 500 to 2500 times. At this time, when the photographed photograph was printed, the magnification was adjusted so that the diameter of one printed core-sheath type conjugate fiber was 5 to 8 cm, and the photograph was observed and photographed. Only the image of the core-sheath type composite fiber is cut out from the obtained photograph of the scanning electron microscope. The image of the cut-out core-sheath type composite fiber was cut along the boundary between the core component and the sheath component, and cut into the core component portion and the sheath component. This operation was performed on 20 fibers, and the total mass of the core components alone cut out from the 20 core-sheath type composite fibers was measured with an electronic balance. Similarly, for the sheath component, the total mass of only the sheath component cut out from the 20 core-sheath type composite fibers was measured using an electronic balance. The ratio (core/sheath) of the total mass of the portion containing only the core component and the total mass of the portion containing only the sheath component was defined as the core-sheath ratio (volume ratio).

(single fiber strength, breaking elongation)
The single fiber strength and breaking elongation of the core-sheath type composite fiber were measured according to JIS L 1015 (2010).

(Moisture content of core-sheath type composite fiber)
(1) Approximately 5 g of fiber to be measured was sampled and the exact mass (W ₁ ) was measured. The fibers measured are bundles of relaxed heat-set filaments after application of the fabric treatment solution and squeezing out excess fabric treatment solution.
(2) Using a high-temperature dryer set at 140° C., the sample whose mass (W ₁ ) was accurately measured was dried for 15 minutes to completely evaporate water from the sample.
(3) The mass (W ₂ ) of the sample with evaporated water was measured.
(4) The moisture content (% by mass) of the fiber was determined based on the following formula.
Moisture content (mass%) = 100 × (W ₁ - W ₂ ) ÷ W ₂

(Amount of fiber treatment component adhered to core-sheath type composite fiber)
(1) Using a high-temperature drier set at 60° C., the fibers for which the amount of the fiber treatment agent component adhered was measured were dried for 180 minutes.
(2) About 4 g of fiber was taken from the dried fiber as a sample, and the exact mass (W _f ) of the sample was measured.
(3) The sample after mass measurement was placed in a stainless steel column for extracting oil, 20 mL of methanol was added, and left for 2 minutes.
(4) A small metal plate sufficiently dried in a dryer at 150° C. was prepared, and its mass (W _b ) was measured.
(5) The sample inside the stainless steel column for oil solution extraction left for 2 minutes is pistoned for 10 minutes using an air cylinder, the fiber treatment agent attached to the sample squeezes out the extracted methanol, and the piston flows out from the outlet. The liquid was received in the small metal plate whose dry mass was measured in (4) above.
(6) After sufficiently squeezing out the methanol in the stainless steel column for extracting oil, the small metal plate that received the extract was heated with a heater at 150°C to evaporate the methanol.
(7) Methanol evaporated and the small metal plate with the remaining fiber treatment agent was cooled at room temperature for 2 minutes, and after cooling, the mass (W _s ) of the small metal plate was measured.
(8) A fiber treatment component adhesion amount (% by mass) was obtained based on the following formula. The adhered amount of the fiber treatment agent component was measured twice and taken as the average value.
Fiber treatment component adhesion amount (% by mass) = 100 × (W _s - W _b ) ÷ W _f
A stainless steel column having the shape shown in FIG. 5 is used as the oil extraction stainless steel column used for the extraction of the fiber treatment agent. 9 mm, the length of the tip cone (outlet) is 13.0 mm, and the hole diameter of the exit is 1.6 mm.

(140°C dry heat shrinkage)
A sample obtained by forming a core-sheath type conjugate fiber into a loop of 10 cm in length and tying the ends of the yarn (adjusted so that the total fineness of the sample in which a plurality of fibers were bundled was 55.6 dtex) was added with 0.5 dtex. An initial load of 26 cN / dtex is applied, the thermal stress tester KE-2LS (manufactured by Kanebo Engineering Co., Ltd.) is set to the displacement measurement mode, and the temperature is raised from room temperature to 170 ° C. at a heating rate of 60 ° C./min. The heat shrinkage rate of the sample was measured at 140°C, and the heat treatment shrinkage rate of the sample at 140°C was defined as the 140°C dry heat shrinkage rate of the core-sheath type composite fiber.

(140°C area shrinkage rate)
The 140° C. area shrinkage of the wet-laid nonwoven fabric was measured as follows.
<140°C Area Shrinkage Rate>
(1) The fiber to be measured was weighed so as to be 3.15 g in absolute dry state.
(2) The weighed fibers were added to 1 liter of tap water to prepare a slurry, and the slurry was stirred with a pulper at 1000 rpm for 1 minute to uniformly disperse the fibers in water.
(3) Tap water was added to the pulper-stirred slurry to make 16 liters of slurry.
(4) The slurry diluted to 16 liters is poured into a 250 mm square frame lined with a metal mesh (200 mesh), wet paper is made, and the weight is 50 g/cm ² when dried. It was made into a wet papermaking web.
(5) The obtained wet papermaking web was sandwiched between filter papers and dehydrated by applying a pressure of 3.5 kg/cm ² for 2 minutes.
(6) A total of four points were marked with oil-based ink, one each in length and width, two points having an interval of 200 mm, which were symmetrical at the center of the wet papermaking web after dehydration. As shown in FIG. 4A, the initial vertical and horizontal intervals between two points are A ₀ and B ₀ respectively. Both A ₀ and B ₀ are 200 mm. (7) As shown in FIG. 2, the marked wet papermaking web is subjected to hot air 25 at a temperature of 140° C. from the upper side of the wet papermaking web 23 in a hot air dryer 20 equipped with a conveyor net 21. A hot air drying treatment was carried out at a wind speed of 0.9 m/s for 30 seconds to completely dry the wet papermaking web, and at the same time, the sheath component was melted and the fibers were bonded together to obtain a heat-bonded nonwoven fabric.
At this time,
(a) As shown in FIG. 2, first, a polyethylene terephthalate net 22 (filament diameter: 0.3 mm, 35 mesh×25 mesh) is laid on a conveyor net 21, and a wet papermaking web 23 is laid thereon. was placed.
(b) In order to prevent the wet papermaking web 23 from being blown away by the hot air, square timbers (with a cross section of 15 mm on each side) are used so that the outer dimensions shown in FIG. A polyester resin net box 24 having the above-described polyethylene terephthalate net stretched over five surfaces excluding the bottom surface was covered with a wet papermaking web 23 on a wooden frame prepared using a square).
(8) The distance between the two points after the formation of the nonwoven fabric was measured, and as shown in _{FIG .} .
(9) A 140° C. area shrinkage rate was obtained based on the following formula.
140° C. area shrinkage (%)={(A ₀ ×B ₀ )−(A ₁ ×B ₁ )}÷(A ₀ ×B ₀ )×100

Resins used in Examples and Comparative Examples were as follows.
(1) PE-LLD: Linear low-density polyethylene polymerized with a metallocene catalyst (manufactured by Ube Maruzen Polyethylene Co., Ltd., product number “420SD”, density 0.918 g/cm ³ , Q value (Mw/Mn) 3.0 , MFR 7 g/10 min (measured at 190 ° C., load 21.18 N (2.16 kgf)), melting point 118 ° C., hexene copolymerization (hexene-1: 3.1 mol%)
(2) PET: commercially available polyethylene terephthalate resin having a melting point of 255° C. (3) PP: manufactured by Japan Polypropylene Corporation Product name: SA03 (Mn after spinning: 9.6×10 ⁴ , Mw after spinning: 2.5× 10 ⁵ , Q value (Mw/Mn) after spinning: 2.63, Melt mass flow rate (MFR): 30 g/10 min (According to JIS K 7210-1 (2014), measurement temperature: 230°C, load: 2.16 kgf (measured at 21.18N))

(Examples 1-4, Comparative Examples 1-3)
Using the resins listed in Table 1 below as the sheath component and the core component, using a concentric core-sheath type composite nozzle (600 holes), the core-sheath ratio (core/sheath) of the sheath component and the core component is 45/55. Then, melt spinning was performed by adjusting the discharge amounts of the core component and the sheath component. The spinning temperature of the sheath component, the spinning temperature of the core component and the temperature of the spinning nozzle were set to the temperatures shown in Table 1 below, and the thermoplastic resins of the core component and the sheath component were extruded through the spinning nozzle. The extruded thermoplastic resin was wound on a bobbin at a take-up speed (spinning speed) shown in Table 1 below to obtain a spun filament.

The obtained spun filaments were subjected to a drawing treatment in a drawing bath filled with hot water adjusted to the temperature shown in Table 1 below. In the drawing step, the ratio V ₂ /V ₁ of the feeding speed V1 of the spun filament to the drawing tank and the drawing speed V ₂ of the drawn filament from the drawing tank, that is, the draw ratio was as shown in Table 1 below.

Next, the stretched filaments after the stretching treatment were subjected to relaxation heat treatment in a bath for relaxation heat treatment adjusted to the temperature shown in Table 1 and filled. In the relaxation heat treatment step, the ratio V ₄ /V 3 of the delivery speed V ₃ of the drawn filament to the _relaxation heat treatment water bath and the take-up speed V ₄ of the relaxation heat set filament from the relaxation heat treatment water bath, that is, the relaxation ratio is shown in Table 1 below. was as described. In addition, since the drawn filaments taken from the drawing tank are sent to the relaxation heat treatment tank at the same speed, the drawing speed V ₂ of the drawn filaments from the drawing tank and the sending speed of the drawn filaments to the relaxation heat treatment tank _V3 is the same speed. In the relaxation heat treatment water tank, a slight tension acts on the drawn filament due to the difference between the drawing speed V ₃ of the drawn filament to the relaxation heat treatment water tank and the take-up speed V ₄ of the relaxation heat set filament from the relaxation heat treatment water tank. , the heat treatment is performed in a state in which the drawn filaments are loosened to such an extent that the drawn filaments do not wind around the delivery roller or the take-up roller. By performing heat treatment at a temperature higher than the glass transition temperature of the polyester resin contained in the core component of the core-sheath type composite fiber in a state where tension is hardly applied or the drawn filament is loose, the drawing generated in the drawing process is removed. It is believed that the strain generated in the molecular chains of the thermoplastic resin constituting the filaments is eliminated, and the arrangement of the molecular chains becomes stable.

A fiber treatment agent was applied to the relaxation heat-set filaments after the relaxation heat treatment. The fiber treatment agent is a fiber treatment agent containing a hydrophilic surfactant as an active ingredient, and the relaxation heat-set filament was impregnated in an oil bath filled with an aqueous solution of the active ingredient diluted with water to 3% by mass. Thereafter, excess water was removed by nip rolls, and the relaxation heat-set filaments were cut into predetermined lengths shown in Table 1 below to obtain core-sheath type conjugate fibers. Using the obtained relaxation heat-set filaments, the moisture content and the amount of the fiber treatment component adhered were determined by the above method. % by mass, and the adhered amount of the fiber treatment agent component was 0.35% by mass.

Table 1 below shows the properties of the core-sheath type conjugate fibers obtained in each example and each comparative example.

As can be seen from the results in Table 1, the core-sheath type conjugate fibers of Examples had a 140° C. dry heat shrinkage rate of 5.0% or less and were difficult to heat shrink. In addition, the wet-laid nonwoven fabric composed of 100% by mass of the core-sheath type composite fiber has a 140° C. area shrinkage rate of 10.0% or less, and is a fiber that is resistant to thermal shrinkage and has improved dimensional stability when subjected to heat bonding at a low temperature. A conglomerate could be obtained. This is because, in the fibers of the examples, the strain associated with the orientation of the tie molecules of the linear low-density polyethylene constituting the sheath component during the drawing treatment was relaxed by the relaxation heat treatment after the drawing treatment, and the core component is polyethylene terephthalate, which is more rigid than polyolefin resins such as polypropylene and has less heat shrinkage in the processing temperature range. This is because polyethylene terephthalate, which is rigid and has small thermal shrinkage in this temperature range, suppresses large thermal shrinkage even if the strain associated with the orientation remaining in the molecules is relaxed and thermal shrinkage occurs. Conceivable.

On the other hand, since the core-sheath type conjugate fiber of Comparative Example 1 had a relaxation ratio of 1.0 times or more during the relaxation heat treatment, distortion and distortion occurred in the molecular chains of the thermoplastic resin constituting the drawn filament due to the drawing treatment. In particular, the strain caused by the alignment of the tie molecules of the linear low-density polyethylene contained in the sheath component in the stretching process remained without being sufficiently relaxed, so the 140 ° C. dry heat shrinkage rate and 140 ° C. area shrinkage When the rate was measured, the 140°C dry heat shrinkage rate exceeded 5.0% because the strain was released, and the 140°C area shrinkage rate of the wet-laid nonwoven fabric made of 100% by mass of the core-sheath type composite fiber It exceeded 10.0%. Therefore, it is difficult to obtain a dimensionally stable fiber aggregate due to large heat shrinkage in heat bonding processing at a low temperature. In addition, since the draw ratio of the core-sheath type conjugate fiber of Comparative Example 2 was more than 2.5 times during the drawing treatment, the molecular chains of the thermoplastic resin constituting the drawn filaments were distorted by the drawing treatment. The strain, especially the strain caused by the orientation of the tie molecules of the linear low-density polyethylene contained in the sheath component in the stretching process, remained without being sufficiently relaxed, so the 140 ° C. dry heat shrinkage rate and 140 ° C. area When the shrinkage rate was measured, the 140° C. dry heat shrinkage rate exceeded 5.0% because the strain was released. In addition, since the core-sheath type conjugate fiber of Comparative Example 3 uses polypropylene as the core component, the heat shrinkage of the linear low-density polyethylene as the sheath component cannot be suppressed, and the dry heat shrinkage rate at 140° C. was over 5.0%.

1 Sheath Component 2 Core Component 3 Gravity Center Position of Core Component in Fiber Cross Section 4 Gravity Center Position in Fiber Cross Section of Core-Sheath Type Composite Fiber 5 Radius of Core-Sheath Type Composite Fiber in Fiber Cross Section 10 Core-Sheath Type Composite Fiber 20 Hot Air Dryer 21 Conveyor Net 22 Polyester resin net (35 mesh x 25 mesh)
23 wet papermaking web 24 polyester resin net box 25 hot air adjusted to a predetermined temperature

Claims (12)

A core-sheath type composite fiber comprising a core component and a sheath component,
In the core-sheath type composite fiber, the core component and the sheath component are arranged substantially concentrically,
The sheath component contains 60% by mass or more of linear low-density polyethylene having a density of 0.90 g/cm ³ or more and 0.93 g/cm ³ or less and a melting point of 100° C. or more and 130° C. or less,
The core component contains 60% by mass or more of a polyester resin having a melting point higher than the melting point of the linear low-density polyethylene by 50°C or more,
A core-sheath type conjugate fiber, wherein the 140° C. dry heat shrinkage of the core-sheath type conjugate fiber is 5.0% or less. A core-sheath type composite fiber comprising a core component and a sheath component,
In the core-sheath type composite fiber, the core component and the sheath component are arranged substantially concentrically,
The sheath component contains 60% by mass or more of linear low-density polyethylene having a density of 0.90 g/cm ³ or more and 0.93 g/cm ³ or less and a melting point of 100° C. or more and 130° C. or less,
The core component contains 60% by mass or more of a polyester resin having a melting point higher than the melting point of the linear low-density polyethylene by 50°C or more,
The fiber length of the core-sheath type composite fiber is 2 mm or more and 20 mm or less,
A core-sheath type conjugate fiber, wherein when the core-sheath type conjugate fiber is made into a wet-laid nonwoven fabric, the 140° C. area shrinkage of the wet-laid nonwoven fabric is 10% or less. The core-sheath type conjugate fiber according to claim 1 or 2, wherein the single fiber fineness of the core-sheath type conjugate fiber is 0.5 dtex or more and 5.6 dtex or less. The core-sheath type conjugate fiber according to any one of claims 1 to 3, wherein the single fiber strength of the core-sheath type conjugate fiber is more than 1.8 cN/dtex and not more than 4.0 cN/dtex. According to JIS K 7210-1 (2014) for the linear low-density polyethylene, the melt mass flow rate measured under the conditions of a measurement temperature of 190 ° C. and a load of 21.18 N is 3 g / 10 minutes or more and 25 g / 10 minutes or less. The core-sheath type composite fiber according to any one of claims 1 to 4. The core-sheath type conjugate fiber according to any one of claims 1 to 5, wherein the core-sheath type conjugate fiber is substantially crimp-free. A method for producing a core-sheath type composite fiber containing a core component and a sheath component,
a sheath component containing 60% by mass or more of linear low-density polyethylene having a density of 0.90 g/cm ³ or more and 0.93 g/cm ^{3 or} less and a melting point of 100° C. or more and 130° C. or less;
A core component containing 60% by mass or more of a polyester resin having a melting point higher than the melting point of the linear low-density polyethylene by 50°C or more,
In the cross section of the core-sheath type conjugate fiber, the surface of the core-sheath type conjugate fiber is covered with the sheath component, and the center of gravity of the core component is substantially coincident with the center of gravity of the core-sheath type conjugate fiber to form a concentric circular structure. A step of supplying to a composite nozzle arranged as follows and melt spinning to obtain a spun filament,
The spun filament is stretched at a temperature equal to or higher than the glass transition point of the polyester resin and 15° C. lower than the melting point T _mL of the linear low-density polyethylene (T _mL −15° C.), and the draw ratio is 1.2 times or higher. A step of drawing a drawn filament under conditions of 2.5 times or less,
The stretched filament is stretched at a temperature equal to or higher than the glass transition point of the polyester resin and 15° C. lower than the melting point T _mL of the linear low-density polyethylene (T _mL −15° C.), and the relaxation ratio is 1.0 times. A method for producing a core-sheath type conjugate fiber, which includes a step of performing a relaxation heat treatment under conditions of less than 1 to obtain a relaxation heat-set filament. 8. The method for producing a core-sheath type composite fiber according to claim 7, further comprising a step of cutting the relaxation heat-set filament to a fiber length of 2 mm or more and 20 mm or less. In the step of obtaining the spun filament,
9. The method for producing a core-sheath type composite fiber according to claim 7 or 8, wherein the obtained spun filament has a single fiber fineness of 1.0 dtex or more and 8 dtex or less. The method for producing a core-sheath type composite fiber according to any one of claims 7 to 9, wherein both the drawing treatment and the relaxation heat treatment are performed in a liquid medium. A fiber assembly containing 20% by mass or more of the core-sheath type composite fiber according to any one of claims 1 to 6,
A fiber assembly in which at least a part of the fibers are thermally bonded to each other by the sheath component of the core-sheath type composite fiber. 12. The fiber assembly according to claim 11, wherein said fiber assembly is a wet-laid nonwoven fabric or an air-laid nonwoven fabric.

Images