Splittable composite fiber, its manufacturing method, and fiber assembly and separator material using the same

Abstract

To provide a split type composite fiber having excellent productivity and water dispersibility and suppressed thermal shrinkage on heating, a manufacturing method thereof, and a fiber assembly using the same.SOLUTION: Disclosed is a split type composite fiber involving a first component involving polypropylene and a second component involving high-density polyethylene, wherein the split type composite fiber is characterized by that the polypropylene has a z-average molecular weight (Mz) after spinning is 450,000 or over and 1,000,000 or under and a Q value after spinning of 2.4 or over and 6 or under, and a temperature difference between an extrapolation melting initiation temperature (Tim) of the polypropylene and a melting peak temperature (Tpm) of the polypropylene is 7.3°C or over.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a splittable conjugate fiber, a method for producing the same, and a fiber assembly and a separator material using the same. [Background technology]

[0002] Because fiber assemblies containing ultrafine fibers have a dense structure, they are widely used in battery separators, RO membrane supports, and various filter media. Battery separators and RO membrane supports are sometimes used while immersed in a highly alkaline solution, and for such applications, fiber assemblies using ultrafine fibers or heat-bondable fibers made of polyolefin resins such as polypropylene and polyethylene are often used.

[0003] Methods for obtaining ultrafine fibers include a method for directly producing ultrafine single fibers by spinning, a method for obtaining ultrafine fibers by leaching the sea component from a sea-island composite fiber, and a method for obtaining ultrafine fibers by splitting a splittable composite fiber. The method for obtaining ultrafine fibers by splitting a splittable composite fiber is easier to obtain ultrafine fibers than the method for directly producing ultrafine fibers by spinning or the method for leaching the sea component from a sea-island composite fiber, but improving the split rate remains an issue. In particular, when combining homologous polymers, such as polyolefin resins, the split rate is likely to decrease due to the good compatibility between the resins.

[0004] Various splittable conjugate fibers combining polyolefin resins have been proposed, particularly splittable conjugate fibers combining relatively inexpensive polypropylene and polyethylene. For example, Patent Document 1 discloses a splittable conjugate fiber in which the polypropylene has a Q value (the ratio Mw / Mn of the weight-average molecular weight Mw to the number-average molecular weight Mn) of at least 5 and the fiber cross section has a hollow central portion. Patent Document 1 uses polypropylene with a Q value of at least 5, selects polyethylene so that the ratio (Q1 / Q2) of the Q value of the polypropylene (Q1) to the Q value of the polyethylene (Q2) is at least 1.0, performs melt spinning, and draws the resulting undrawn fiber at a draw ratio of 5 or more to obtain a splittable conjugate fiber with a high split ratio. Patent Document 1 describes drawing the undrawn fiber at a high draw ratio to make the polypropylene constituting the splittable conjugate fiber highly crystalline, resulting in a splittable conjugate fiber that is easily splittable.

[0005] Patent Document 2 discloses a splittable conjugate fiber including a first segment and a second segment, wherein the first segment is a resin segment made of a first component, the second segment is a sheath-core resin segment having a cross-sectional structure in which the first component is a core component and the second component is a sheath-core resin segment, the first component is a resin component containing 50% by mass or more of polypropylene resin, and the second component is a resin component containing 50% by mass or more of polyethylene resin, the polypropylene resin having a weight-average molecular weight (Mw) to number-average molecular weight (Mn) ratio Mw / Mn of 6 or less, and when the resulting splittable conjugate fiber is subjected to differential scanning calorimetry (DSC), the DSC curve shows a double-peak melting peak for the polypropylene resin. The cross-sectional structure of the second segment of this splittable conjugate fiber is sheath-core, and since ultrafine sheath-core conjugate fibers are obtained after splitting, a fiber assembly with a dense structure is easily obtained. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-220740 [Patent Document 2] International Publication No. 2018 / 181909 Summary of the Invention [Problem to be solved by the invention]

[0007] However, because the splittable conjugate fiber described in Patent Document 1 is drawn at a high draw ratio, the resulting splittable conjugate fiber may contain polypropylene in an excessively drawn state (overdrawn state). Overdrawn polypropylene has distortions and fractures in the crystalline phase due to excessive drawing, and heating causes the polypropylene to recover from the distortions or return to the crystalline phase. When a fiber assembly such as a battery separator is produced using a splittable conjugate fiber containing overdrawn polypropylene, heating the fibers constituting the fiber assembly for thermal bonding causes the distortions and fractures in the crystalline phase to shrink and return to the original crystalline phase, resulting in significant thermal shrinkage of the fiber assembly or the occurrence of wrinkles or cracks. The splittable conjugate fiber described in Patent Document 2 has a core-sheath cross-sectional structure of the second segment, which results in a more complex cross-sectional structure than typical splittable conjugate fibers in which both the first and second segments have a single cross-sectional structure. This makes it difficult to obtain uniform fiber cross-sectional shapes during melt spinning, potentially resulting in poor productivity.

[0008] Furthermore, as seen in Patent Document 1, in conjugate fibers containing polypropylene and high-density polyethylene, it is known that the drawn filaments are heat-treated at temperatures of 110°C or higher using steam or a heated metal roll to promote crystallization of amorphous polypropylene or to eliminate the overstretched state of overstretched polypropylene remaining in the drawn filaments. While such heat treatment of drawn filaments achieves the above-mentioned objectives, exposure of the drawn filaments to high temperatures softens the thermoplastic resin constituting the filaments, and the softened thermoplastic resin can cause pseudo-fusion between the filaments. When pseudo-fusion occurs, the pseudo-fused portions become thick fibers, and when the resulting fibers are used to form a fiber assembly, these portions may cause poor formation or other problems. In splittable conjugate fibers containing polypropylene and high-density polyethylene, high-density polyethylene has a lower melting point than polypropylene, so heat treatment at temperatures of 110°C or higher is likely to cause pseudo-fusion. Therefore, there is a demand for splittable conjugate fibers containing polypropylene and high-density polyethylene that are resistant to heat shrinkage even without heat treatment at 110°C or higher (including treatments such as drawing and heat setting), have excellent dispersibility when placed in water, and can be split to produce ultrafine fibers by splitting.

[0009] In order to solve the above-mentioned conventional problems, the present invention provides a splittable conjugate fiber that is excellent in productivity and dispersibility in water and that is suppressed in thermal shrinkage when heated, a method for producing the same, and a fiber assembly and a separator material that use the same. [Means for solving the problem]

[0010] The present invention includes a plurality of first segments and a plurality of second segments, and the first segments and the second segments are alternately arranged in the fiber cross section, the first segment is a single-type resin segment consisting of a first component, the second segment is a single-type resin segment consisting of a second component, The first component contains 50% by mass or more of polypropylene, the second component contains 50% by mass or more of high-density polyethylene; The z-average molecular weight (Mz) of the polypropylene after spinning is 450,000 or more and 1,000,000 or less, The polypropylene has a Q value (ratio of weight average molecular weight (Mw) to number average molecular weight (Mn) Mw / Mn) after spinning of 2.4 or more and 6 or less, The extrapolated melting onset temperature (T) of polypropylene measured at a heating rate of 20°C per minute in the differential scanning calorimetry (DSC) measurement method specified in JIS K 7121 (1987) im ) and the melting peak temperature (T pm ) temperature difference (T pm -T im ) is 7.3°C or higher.

[0011] The present invention also provides a method for producing a splittable conjugate fiber comprising a plurality of first segments and a plurality of second segments, the first segments and the second segments being alternately arranged in the fiber cross section, the first segment is a single-type resin segment consisting of a first component, the second segment is a single-type resin segment consisting of a second component, The first component contains 50% by mass or more of polypropylene having a z-average molecular weight (Mz) before spinning of 500,000 or more and 1,500,000 or less and a Q value (the ratio of weight average molecular weight (Mw) to number average molecular weight (Mn) Mw / Mn) before spinning of 2.6 or more and 8 or less, the second component contains 50% by mass or more of high-density polyethylene; Attaching a split-type multi-component spinning nozzle to a melt spinning machine, the split-type multi-component spinning nozzle including a plurality of first segments and a plurality of second segments, the first segments and the second segments being alternately arranged in a fiber cross section; Melting each of the first component and the second component and supplying them to a first segment and a second segment of a split-type multi-component spinning nozzle attached to a melt spinning machine, respectively, to perform melt spinning; The molten resin discharged from the split-type composite spinning nozzle is taken up at a spinning speed of 300 m / min or more and 2500 m / min or less to obtain an undrawn fiber tow in which the first component and the second component are solidified and which has a single fiber fineness of 1 dtex or more and 15 dtex or less. The method includes drawing the undrawn fiber tow at a drawing temperature of 60°C or higher and 120°C or lower and a total draw ratio of 2 times or higher and 10 times or lower to obtain a drawn filament, The undrawn fiber tow is characterized in that, in an X-ray diffraction chart obtained by measurement using an X-ray diffraction method (XRD), the diffraction peaks of polypropylene, namely, diffraction peak PP1 at 2θ=14.2±0.5°, diffraction peak PP2 at 2θ=17±0.5°, and diffraction peak PP3 at 2θ=18.6±0.5°, and the diffraction peak of high-density polyethylene, diffraction peak PE1 at 2θ=21.6±0.5°, satisfy the following (1) to (4): (1) The half-width of the diffraction peak PP1 is 0.835° or more (2) The half-width of the diffraction peak PP2 is 0.835° or more. (3) The half-width of the diffraction peak PP3 is 0.955° or more. (4) Diffraction intensity of diffraction peak PE1 (I PE ) and the ratio I of the sum of the diffraction intensities of the diffraction peaks PP1, PP2, PP3, and PE1 (I) PE / I is 0.105 or more and less than 0.750

[0012] The present invention also relates to a fiber assembly containing 10% by mass or more of the splittable conjugate fibers.

[0013] The present invention also relates to a separator material comprising the fiber assembly. [Effects of the Invention]

[0014] The present invention can provide a splittable conjugate fiber that is excellent in productivity and dispersibility in water and that is suppressed in thermal shrinkage when heated, a method for producing the same, and a fiber assembly and a separator material that use the same. [Brief explanation of the drawings]

[0015] [Figure 1] 1A to 1D are schematic cross-sectional views illustrating examples of the cross section of a splittable conjugate fiber. [Figure 2] FIG. 2 is a cross-sectional view showing an example of a melt spinning apparatus used in the method for producing splittable conjugate fibers. [Figure 3] FIG. 3 is a schematic diagram of a sample used when measuring the area shrinkage rate of a fiber structure. [Figure 4] FIG. 4 is a schematic diagram showing the drying and heat-bonding steps in producing a wetlaid nonwoven fabric. [Figure 5] FIG. 5 is a schematic diagram of a net box used in the thermal bonding process when producing a wetlaid nonwoven fabric. [Figure 6] FIG. 6 is a differential scanning calorimetry (DSC) curve showing how to determine the temperature difference (Tpm-Tim) between the extrapolated melting onset temperature (Tim) and the melting peak temperature (Tpm) of polypropylene in DSC performed using the splittable conjugate fiber of Example 1. [Figure 7] FIG. 7 is a differential scanning calorimetry (DSC) curve showing the endothermic peak measured in the temperature range of about 115°C to about 145°C in DSC performed using the splittable conjugate fiber of Example 1, and how to determine the heat of fusion. [Figure 8] FIG. 8 is a differential scanning calorimetry (DSC) curve showing the endothermic peak measured in the temperature range of about 145°C to about 185°C in DSC performed using the splittable conjugate fiber of Example 1, and how to determine the heat of fusion of the endothermic peak. DETAILED DESCRIPTION OF THE INVENTION

[0016] The inventors of the present invention conducted extensive research to solve the above-mentioned problems. As a result, they found that by using polypropylene with a high z-average molecular weight before spinning and a Q value (Mw / Mn) within a certain range for the first segment and high-density polyethylene for the second segment, and by reducing the crystalline phase in the undrawn fiber tow, the resulting splittable conjugate fiber has excellent water dispersibility and reduced thermal shrinkage when heated. The number-average molecular weight Mn, weight-average molecular weight Mw, z-average molecular weight Mz, and Q value (Mw / Mn) of the polypropylene resin may differ before and after spinning.

[0017] Specifically, the fiber comprises a plurality of first segments and a plurality of second segments, the first segments and the second segments being alternately arranged in the fiber cross section, the first segments being single-type resin segments made of a first component, the second segments being single-type resin segments made of a second component, the first component containing 50% by mass or more of polypropylene, the second component containing 50% by mass or more of high-density polyethylene, the z-average molecular weight (Mz) of the polypropylene after spinning is 450,000 or more and 1,000,000 or less, the Q value (the ratio of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn) Mw / Mn) of the polypropylene after spinning is 2.4 or more and 6 or less, and the extrapolated melting onset temperature (T im ) and the melting peak temperature (T pm ) temperature difference (T pm -T im ) of 7.3°C or higher, the splittable conjugate fiber not only has excellent productivity but also excellent dispersibility when put into water, thereby improving the productivity and yield of wetlaid nonwoven fabrics using the splittable conjugate fiber. In addition, the thermal shrinkage of the polypropylene is suppressed when heated to thermally bond the fibers together, thereby suppressing the thermal shrinkage of the fiber assembly and making it less likely to wrinkle or crack.

[0018] (Splittable composite fiber) 1A to 1D, the splittable conjugate fiber 10 includes a first segment 11 and a second segment 12. The first segment is a single-type resin segment made of a first component, and the second segment is a single-type resin segment made of a second component. The first component contains 50% by mass or more of polypropylene, and the second component contains 50% by mass or more of high-density polyethylene. The splittable conjugate fiber of the present invention preferably has a cross-sectional structure in which the first segments 11 and second segments 12 constituting the splittable conjugate fiber are radially and alternately arranged.

[0019] <First component> The first component contains 50% by mass or more of polypropylene, preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more. It is particularly preferred that the first component consists essentially of polypropylene. Here, the term "substantially" takes into consideration the fact that resins provided as products typically contain additives such as stabilizers, and / or that various additives are added during fiber production, making it difficult to obtain fibers consisting solely of polypropylene and containing no other components. Typically, the first component may contain up to 15% by mass of additives.

[0020] The polypropylene has a z-average molecular weight after spinning of 450,000 or more and 1,000,000 or less. When the z-average molecular weight Mz after spinning is within the above range, it can be said that the polypropylene contained in the first component contains a large amount of high molecular weight components. In the splittable conjugate fiber of the present invention, to obtain a first component containing polypropylene having a z-average molecular weight after spinning of 450,000 or more and 1,000,000 or less, as described below, the z-average molecular weight Mz of the polypropylene before spinning is set to 500,000 or more and 1,500,000 or less. By melt-spinning such a first component containing polypropylene, the first component contains a large amount of high molecular weight components even at the melt-spinning stage. During melt spinning, when the first component containing molten polypropylene is discharged from the spinning nozzle and cooled while being withdrawn, the polypropylene tends to become a resin component containing more amorphous phases and smectic crystals (also called smectic crystals, mesophases, or stable intermediate phases) than crystalline phases. When the polypropylene contained in the undrawn fiber tow obtained by melt spinning is in this state, the first component containing polypropylene is in a state where it can be easily drawn, so that it is less likely to be overdrawn during the drawing process and is less likely to inhibit the drawing of the second component containing high-density polyethylene. As a result, the undrawn fiber tow is sufficiently drawn without being overdrawn, even at low temperatures below 110°C, particularly at drawing temperatures of around 100°C, which is the maximum temperature for wet drawing using hot water as a heat medium. As a result, the first component containing 50% by mass or more of polypropylene contained in the resulting splittable conjugate fiber is not overdrawn and is sufficiently crystallized, and fusion and pseudo-fusion by the second component containing high-density polyethylene are less likely to occur, resulting in a fiber that is highly dispersible in water and less susceptible to heat shrinkage even when heated. The z-average molecular weight of the polypropylene contained in the first component after spinning is preferably 500,000 or more and 980,000 or less, preferably 550,000 or more and 960,000 or less, preferably 700,000 or more and 950,000 or less, more preferably 750,000 or more and 920,000 or less, and particularly preferably 770,000 or more and 900,000 or less.

[0021] The polypropylene has a Q value (Mw / Mn, the ratio of weight-average molecular weight (Mw) to number-average molecular weight (Mn)) after spinning of 2.4 or more and 6 or less. When the Q value of the polypropylene after spinning is 2.4 or more and 6 or less, the polypropylene contained in the first component of the splittable conjugate fiber has a high polydispersity and contains a large amount of low-molecular-weight and high-molecular-weight components. In order to obtain the first component of the splittable conjugate fiber of the present invention, as described below, if the first component is melt-spun so that it contains polypropylene having a Q value of 2.6 or more and 8 or less before spinning, the first component will have a high polydispersity and contain a large amount of low-molecular-weight and high-molecular-weight components even at the melt-spinning stage, and by cooling the molten resin during melt spinning, the first component containing polypropylene will become a resin component with a high proportion of amorphous phases and smectic crystals. When the polypropylene contained in the first component is in such a state, the undrawn fiber tow obtained by melt spinning will have excellent stretchability and will not only be easily stretched in the drawing process, but will also be less likely to produce overdrawn polypropylene. The Q value of the polypropylene after spinning is preferably 2.8 or more and 6 or less, more preferably 3.0 or more and 5.8 or less, more preferably 3.2 or more and 5.6 or less, and particularly preferably 3.5 or more and 5.4 or less.

[0022] The polypropylene has a melt mass flow rate (hereinafter simply referred to as "MFR230"; measurement temperature 230°C, load 2.16 kgf (21.18 N)) according to JIS K 7210:2014, but is preferably 3 g / 10 min to 50 g / 10 min, more preferably 5 g / 10 min to 40 g / 10 min, particularly preferably 8 g / 10 min to 35 g / 10 min, and most preferably 10 g / 10 min to 35 g / 10 min. When MFR230 is within the above range, yarn breakage during spinning is unlikely to occur.

[0023] The polypropylene is not particularly limited, and examples thereof include propylene homopolymers (homopolymers containing propylene as a monomer), copolymers of propylene with other copolymerizable monomers, and mixtures thereof. The propylene copolymer may be a random copolymer or a block copolymer. Examples of the propylene copolymer include copolymers of propylene with at least one α-olefin selected from the group consisting of ethylene and α-olefins having 4 or more carbon atoms. Examples of the α-olefins having 4 or more carbon atoms include, but are not limited to, 1-butene, 1-pentene, 3,3-dimethyl-1-butene, 4-methyl-1-pentene, 4,4-dimethyl-1-pentene, 1-decene, 1-dodecene, 1-tetradecene, and 1-octadecene. The propylene content in the copolymer is preferably 50% by mass or more. Among the polypropylene resins, propylene homopolymers are particularly preferred in terms of processability and economic efficiency (production costs).

[0024] <Second component> The second component contains 50% by mass or more of high-density polyethylene, preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more. It is particularly preferred that the second component consists essentially of high-density polyethylene. Here, the term "substantially" takes into consideration that resins provided as products typically contain additives such as stabilizers, and / or various additives are added during fiber production, making it difficult to obtain fibers consisting solely of high-density polyethylene and containing no other components. Typically, the second component may contain up to 15% by mass of additives.

[0025] The high-density polyethylene may be an ethylene homopolymer (a single polymer containing ethylene as a monomer) or a copolymer containing ethylene as a main monomer. 3or more is preferable. The high-density polyethylene is not particularly limited as long as it can provide the splittable conjugate fiber intended by the present invention. The high-density polyethylene may be an ethylene homopolymer, a high-density polyethylene in which the crystallinity has been intentionally reduced by adding and copolymerizing a small amount of an α-olefin (e.g., 1-butene) to ethylene, a random copolymer containing ethylene as a monomer, a block copolymer, a graft copolymer, or a mixture thereof. Examples of the random copolymer, block copolymer, and graft copolymer include copolymers of ethylene and at least one α-olefin selected from the group consisting of α-olefins having 3 or more carbon atoms. The density of the high-density polyethylene is 0.945 g / cm. 3 More preferably, it is 0.95 g / cm or more. 3 The upper limit of the density of the high-density polyethylene is not particularly limited, but is preferably 0.97 g / cm. 3 It may be less than or equal to 0.965 g / cm 3 It may be the following:

[0026] The α-olefin having 3 or more carbon atoms is not particularly limited as long as the desired splittable conjugate fiber of the present invention can be obtained. Examples include propylene, 1-butene, 1-pentene, 3,3-dimethyl-1-butene, 4-methyl-1-pentene, 4,4-dimethyl-1-pentene, 1-decene, 1-dodecene, 1-tetradecene, and 1-octadecene. The ethylene content in the copolymer is preferably 50% by mass or more. Considering ease of production and economic efficiency (production cost), it is preferable to use, as the high-density polyethylene, an ethylene homopolymer or an ethylene copolymer containing 5 mol% or less of an α-olefin monomer other than ethylene. These high-density polyethylenes may be used alone or in combination of two or more.

[0027] The melt mass-flow rate (hereinafter also referred to as "MFR190"; measurement temperature 190°C, load 2.16 kgf (21.18 N)) of the high-density polyethylene in accordance with JIS K 7210:2014 is not particularly limited, but the MFR190 is preferably 5 g / 10 min or more and 40 g / 10 min or less, more preferably 8 g / 10 min or more and 30 g / 10 min or less, and particularly preferably 10 g / 10 min or more and 25 g / 10 min or less. When the MFR190 of the high-density polyethylene is in the range of 5 g / 10 min or more and 40 g / 10 min or less, the productivity of splittable conjugate fibers is further improved.

[0028] The number of divisions of the splittable conjugate fiber (i.e., the total number of first segments and second segments) can be determined depending on the fineness of the splittable conjugate fiber and the fineness of the ultrafine fiber. For example, it is preferably 4 to 36, more preferably 6 to 30, and particularly preferably 8 to 24. The splittable conjugate fiber tends to be more easily splittable as the number of divisions decreases, but if the number of interfaces between the first and second segments is too small, it tends to be difficult to obtain fine fibers. Furthermore, if the number of divisions is too small, it is necessary to reduce the fineness of the splittable conjugate fiber to obtain ultrafine fibers of a specified fineness, which may result in poor fiber productivity or make spinning difficult. A large number of divisions increases the number of interfaces between the first and second segments, making it easier to obtain fine fibers, but it may be difficult to obtain a cross section in which the first and second segments are uniformly arranged during melt spinning.

[0029] The splittable conjugate fiber may have a hollow portion 13 formed in the center of the fiber cross section, as shown in FIGS. 1A and 1C, or may have a so-called solid cross section, in which no hollow portion is present in the fiber cross section, as shown in FIGS. 1B and 1D. Here, the central portion of the fiber cross section refers to the vicinity of the approximate center of the fiber cross section. When the splittable conjugate fiber is a so-called hollow splittable conjugate fiber having a hollow portion in the fiber cross section, the hollow portion may be located not centrally (concentrically) but eccentrically, as long as the central portion is hollow. However, from the viewpoint of productivity, a concentric position is preferable. The shape of the hollow portion may also be circular, elliptical, or irregular. The presence of a hollow portion reduces the contact area between the first segment and the second segment, improving splittability compared to splittable conjugate fibers with a solid cross section that do not have a hollow portion in the fiber center. This allows for high splittability even with low impacts, such as low-pressure hydroentanglement treatment, defibration during slurry preparation in a wet papermaking method, and beating treatment. When the splittable conjugate fiber is a so-called solid splittable conjugate fiber that does not have a hollow portion in the fiber cross section, the cross-sectional shape of the ultrafine fibers obtained by splitting has a flat cross-sectional shape, and therefore, when a nonwoven fabric containing these ultrafine fibers is used as a battery separator, the battery separator tends to be one that is less likely to suffer from electrolyte depletion and has high liquid retention. Furthermore, ultrafine fibers with a flat cross-sectional shape have excellent scraping properties when wiping away dirt, so a nonwoven fabric obtained from a fiber assembly containing this solid splittable conjugate fiber is suitable for personal wiping materials and object wiping materials.

[0030] When the splittable conjugate fiber has hollow portions, the hollow ratio may be determined depending on the division ratio and the cross-sectional shape of the ultrafine fibers. The hollow ratio is the ratio of the area of ​​hollow portions to the cross section of the fiber. For example, the hollow ratio is preferably about 1% to 50%, and more preferably about 5% to 40%. More specifically, when the number of divisions is 6 to 10, the hollow ratio is preferably 5% to 20%, and when the number of divisions is 12 to 36, the hollow ratio is preferably 10% to 40%. If the hollow ratio is too low, it is difficult to obtain a significant effect when hollow portions are provided. If the hollow ratio is too high, the splittable conjugate fiber may split during the drawing process or the opening process, which may reduce handleability.

[0031] The composite ratio (volume ratio of first component / second component) of the first segment to the second segment is not particularly limited, but considering the productivity of the fiber and the splittability of the resulting splittable composite fiber, it is preferably 30 / 70 to 70 / 30, more preferably 40 / 60 to 60 / 40, and particularly preferably 45 / 55 to 55 / 45.

[0032] The single fiber fineness of the splittable conjugate fiber (before splitting) is not particularly limited, but is preferably 0.3 dtex to 5 dtex, more preferably 0.5 dtex to 3 dtex, particularly preferably 0.8 dtex to 2.5 dtex, and most preferably 1 dtex to 2 dtex. If the single fiber fineness of the splittable conjugate fiber before splitting is less than 0.3 dtex, spinning may become unstable, and the productivity of the fiber and therefore the fiber assembly may decrease. Similarly, if the single fiber fineness of the splittable conjugate fiber before splitting exceeds 5 dtex, not only may spinning become unstable, but the fineness of the fibers obtained after splitting may not be sufficiently small.

[0033] The splittable conjugate fiber preferably has a single fiber strength of 2.5 cN / dtex or more before splitting, more preferably 2.7 cN / dtex or more, particularly preferably 2.9 cN / dtex or more, and most preferably 3.0 cN / dtex or more. The upper limit of the single fiber strength before splitting is preferably 10 cN / dtex or less, more preferably 8.0 cN / dtex or less, particularly preferably 7.5 cN / dtex or less, and most preferably 6.8 cN / dtex or less. When the splittable conjugate fiber has a single fiber strength of 2.5 cN / dtex or more, not only can a fiber assembly having excellent puncture strength be obtained, but the polypropylene or polyethylene constituting the splittable conjugate fiber is highly crystallized, making it easier to obtain a splittable conjugate fiber with good splittability. In the present invention, the single fiber strength is measured in accordance with JIS L 1015.

[0034] The splittable conjugate fiber preferably has an elongation of 100% or less before splitting, more preferably 80% or less, and even more preferably 70% or less. When the splittable conjugate fiber has a fiber elongation of 100% or less, a fiber assembly that is resistant to elongation due to external forces can be obtained. In particular, when the splittable conjugate fiber is used as a fiber constituting a battery separator, elongation during the manufacturing process can be suppressed, resulting in a battery separator with little shrinkage. In the present invention, the elongation is measured in accordance with JIS L 1015. The lower limit of the elongation of the splittable conjugate fiber before splitting is not particularly limited, but the elongation is preferably 15% or more, more preferably 20% or more, and particularly preferably 25% or more.

[0035] The splittable conjugate fiber is a splittable conjugate fiber comprising a first component containing polypropylene having a z-average molecular weight (Mz) of 450,000 to 1,000,000 after spinning and a Q value of 2.4 to 6 after spinning, and a second component containing high-density polyethylene. In the first component constituting the splittable conjugate fiber, when the z-average molecular weight and Q value of the polypropylene contained in the first component are within the above-mentioned ranges after spinning, the polypropylene contained in the first component has a high polydispersity and is in a state containing low-molecular-weight and high-molecular-weight components (particularly high-molecular-weight components). The splittable conjugate fiber of the present invention can be produced by melt-spinning a first component containing polypropylene having a z-average molecular weight of 500,000 to 1,500,000 before spinning and a Q value of 2.6 to 8 before spinning, and a second component containing high-density polyethylene. When producing splittable conjugate fibers, if the z-average molecular weight and Q value of the polypropylene contained in the first component constituting the splittable conjugate fiber are within the above ranges before spinning, the polypropylene contained in the first segment will have a low crystalline phase and be easily drawn during melt spinning, as described below. By drawing the undrawn fiber tow in this state at a temperature that is unlikely to cause softening or fusion of the high-density polyethylene, the polypropylene is sufficiently crystallized without becoming overdrawn. Therefore, when the resulting splittable conjugate fiber is heated, the heat shrinkage caused by overdrawn polypropylene or amorphous polypropylene is reduced. Furthermore, because fusion between fibers does not occur due to softening or melting of the high-density polyethylene, the splittable conjugate fiber has excellent dispersibility when dispersed in water. During melt spinning, the polypropylene contained in the undrawn fiber tow obtained after melt spinning is made to have a low crystalline phase, in other words, a high amorphous phase and smectic crystals. It is presumed that these phases of the polypropylene become crystalline phases with small crystallite sizes during the drawing process.It is thought that crystalline phases with small crystallite sizes are more easily melted than crystalline phases with large crystallite sizes (crystalline phases that were generated during the melt spinning stage or crystalline phases that have grown in grain size during the spinning and drawing processes). Therefore, when performing differential scanning calorimetry (DSC) using the obtained splittable composite fiber, the heating rate is increased, and the extrapolated melting onset temperature during melting of polypropylene, which begins at around 150°C, and the melting peak temperature of polypropylene are measured, and this can be confirmed by calculating the temperature difference.

[0036] The extrapolated melting temperature (T im ) and melting peak temperature (T pm ) is determined by differential scanning calorimetry (DSC). DSC measurements are performed in accordance with JIS K 7121 (1987). That is, 3 mg of splittable conjugate fiber as a sample is filled into a sample holder. Then, differential scanning calorimetry is performed by heating the splittable conjugate fiber to a temperature at least 30°C higher than the melting point of the thermoplastic resin with the highest melting point among the thermoplastic resins contained in the splittable conjugate fiber. For example, the heating rate is set to 20°C per minute, and the temperature is raised from 20°C to 300°C.

[0037] The extrapolated melting onset temperature (T) of polypropylene was calculated from the differential scanning calorimetry (DSC) curve obtained by splittable composite fibers. im ) and melting peak temperature (T pm When the sample is heated, the melting of polypropylene begins at around 150°C and ends at around 180°C. At this time, the extrapolated melting onset temperature (T im For example, as shown in Figure 6, the extrapolated melting onset temperature (T im ) The melting peak temperature of polypropylene (T pm ) is the temperature at the apex of the melting peak, as shown in Figure 6. When the melting peak of polypropylene has multiple apexes, the temperature of the apex measured at the highest temperature is taken as the melting peak temperature.

[0038] In the splittable composite fiber, the extrapolated melting initiation temperature (T im ) and the melting peak temperature (T pm ) temperature difference (T pm -T im ) is 7.3°C or higher. pm -T im ) is 7.3°C or higher, it can be assumed that this means that, as mentioned above, the polypropylene in the splittable conjugate fiber contains a relatively large amount of phases with small crystallite sizes. This is presumably due to the fact that not only is the drawability of the undrawn fiber tow increased by using polypropylene with a z-average molecular weight and Q value that satisfy specific ranges before spinning and relatively increasing the proportion of amorphous and Smectic phases of the polypropylene in the undrawn fiber tow, but also because the undrawn fiber tow, which contains a relatively small amount of crystalline phase, is drawn, it is difficult for crystals to grow around the crystalline phase contained in the undrawn tow as a nucleus, and much of the amorphous and Smectic phases are converted into phases with small crystallite sizes. pm -T im ) is preferably 7.7°C or higher, more preferably 8.0°C or higher, and particularly preferably 8.4°C or higher. pm -T im ) is not particularly limited to an upper limit, but is preferably 20°C or lower.

[0039] The splittable conjugate fiber is a splittable conjugate fiber containing a first component containing polypropylene that satisfies specific conditions and a second component containing high-density polyethylene. It is obtained by drawing an undrawn fiber tow in which the polypropylene has been reduced to a state with a low crystalline phase during melt spinning. Because the polypropylene contained in the undrawn fiber tow has a low crystalline phase, the high-density polyethylene, which is softer than polypropylene, is not inhibited by the polypropylene during drawing. Therefore, the high-density polyethylene is sufficiently drawn in the drawing process, leading to increased orientation and a high degree of crystallinity. This can be confirmed by performing differential scanning calorimetry (DSC) using the resulting splittable conjugate fiber to determine the heat of fusion of the high-density polyethylene.

[0040] The heat of fusion of the high-density polyethylene and the heat of fusion of the polypropylene contained in the splittable conjugate fiber are determined by differential scanning calorimetry (DSC). DSC measurements are performed in accordance with JIS K 7121 (1987). First, the mass proportions of polypropylene and high-density polyethylene are determined for the splittable conjugate fiber to be measured by DSC. The content of polypropylene and high-density polyethylene is determined by determining the ratio (volume ratio) of the first segment to the second segment from the manufacturing conditions of the splittable conjugate fiber to be measured by DSC, specifically, if the extrusion amount per unit time of the molten polypropylene and high-density polyethylene during melt spinning is known. The mass proportions of polypropylene and high-density polyethylene in the splittable conjugate fiber can then be determined from the densities of the polypropylene and high-density polyethylene, the mass proportion of polypropylene in the first component, and the mass proportion of high-density polyethylene in the second component.

[0041] When the detailed manufacturing conditions of a splittable conjugate fiber to be subjected to differential scanning calorimetry (DSC) are unknown, the mass proportions of polypropylene and high-density polyethylene in the splittable conjugate fiber can be determined by a known method. One method for determining the mass proportions of polypropylene and high-density polyethylene in a splittable conjugate fiber involves printing a cross section of the splittable conjugate fiber to be subjected to differential scanning calorimetry (DSC) using a scanning electron microscope at 500x to 2000x magnification, cutting out the printouts for each first segment and each second segment, measuring the total mass of the paper pieces for the first segment and the total mass of the paper pieces for the second segment, and using this ratio as the ratio (volume ratio) of the first segment to the second segment. The mass proportions of polypropylene and high-density polyethylene in the splittable conjugate fiber can then be determined from the densities of the polypropylene and high-density polyethylene, the mass proportion of polypropylene in the first component, and the mass proportion of high-density polyethylene in the second component. Other methods for determining the mass proportions of polypropylene and high-density polyethylene in splittable composite fibers include a combination of known analytical methods, such as gel permeation chromatography (GPC), cross fractionation chromatography (CFC), and applications of these methods such as temperature rising elution fractionation (TREF) and crystallization elution fractionation (CEF), as well as high-temperature liquid chromatography (also known as high-temperature LC) utilizing differences in interactions with graphite carbon columns, such as solvent gradient chromatography (SGIC) and temperature gradient chromatography (TGIC).

[0042] Next, the splittable composite fiber whose mass ratio of polypropylene and high density polyethylene in the splittable composite fiber has been determined is used to perform DSC measurement in accordance with JIS K 7121 (1987). pm -T im The heating rate is set at 20°C per minute, the same as the method for determining the (T pm -T imAs in the case of measuring the thermal expansion coefficient (Tc), 3 mg of splittable conjugate fiber as a sample is loaded into a sample holder. The splittable conjugate fiber is then heated to a temperature at least 30°C higher than the melting point of the thermoplastic resin with the highest melting point among the thermoplastic resins contained in the splittable conjugate fiber, and differential scanning calorimetry is performed.

[0043] From the DSC curve obtained, the heat of fusion of polypropylene (ΔH PP ) and the heat of fusion of polyethylene (ΔH PE ) is determined. At least two endothermic peaks are present in the DSC curve. That is, an endothermic peak associated with the melting of high-density polyethylene observed at about 115°C to about 145°C (the melting peak temperature varies depending on the type of high-density polyethylene), and an endothermic peak associated with the melting of polypropylene observed at about 145°C to about 185°C (the melting peak temperature varies depending on the type of polypropylene).

[0044] In differential scanning calorimetry (DSC measurement), for example, as shown in FIGS. 7 and 8, a DSC curve (DSC chart) and a low-temperature baseline (BL LT ) from its high-temperature end to the high-temperature baseline (BL HT ) and a straight line (BL E) is used to calculate the heat of fusion at the endothermic peak. The heat of fusion measured at each endothermic peak is measured for the entire sample, i.e., the heat of fusion per 1 mg of splittable conjugate fiber. However, it is high-density polyethylene that undergoes a phase change and melts in the temperature range from 115°C to 145°C, while polypropylene, which generally has a melting point of 150°C or higher, is thought to undergo almost no phase change in this temperature range. Therefore, as shown in Figure 7, the endothermic peak and its heat of fusion measured in the temperature range from 115°C to 145°C are converted from the heat of fusion per 1 mg of splittable conjugate fiber to the heat of fusion per 1 mg of high-density polyethylene. That is, the heat of fusion per 1 mg of high-density polyethylene is calculated by dividing the heat of fusion measured in the temperature range from approximately 115°C to approximately 145°C by the pre-calculated mass fraction of high-density polyethylene in the splittable conjugate fiber, based on the following mathematical formula (1): For example, if the heat of fusion of the melting peak that appears at approximately 115°C to approximately 145°C is 75 mJ / mg and the content of high-density polyethylene in the splittable conjugate fiber is 50% by mass, the heat of fusion per 1 mg of high-density polyethylene contained in the splittable conjugate fiber is 150 mJ / mg.

number

[0045] Furthermore, in the temperature range from about 145°C to about 185°C, high-density polyethylene, which generally has a melting point of 140°C or less, is already melted and is thought to undergo little phase change. It is polypropylene that undergoes a phase change and melts in this temperature range. Therefore, as shown in Figure 8, the endothermic peak and heat of fusion that appeared between about 145°C and about 185°C are converted from the heat of fusion per 1 mg of splittable conjugate fiber to the heat of fusion per 1 mg of polypropylene. The conversion method is the same as that used to determine the heat of fusion of high-density polyethylene. The heat of fusion per 1 mg of splittable conjugate fiber measured between about 145°C and about 185°C is divided by the pre-calculated polypropylene content (mass%) in the splittable conjugate fiber to determine the heat of fusion per 1 mg of polypropylene. For example, based on the following mathematical formula (2), if the heat of fusion of the melting peak that appears at approximately 145°C to approximately 185°C is 50 mJ / mg and the polypropylene content in the conjugate fiber is 50 mass%, the heat of fusion per 1 mg of polypropylene contained in the splittable conjugate fiber is 100 mJ / mg.

number

[0046] In the splittable conjugate fiber, the heat of fusion (ΔH PE ) is preferably 125 mJ / mg or more. When the heat of fusion per mg of high-density polyethylene contained in the splittable conjugate fiber is 125 mJ / mg or more, it is estimated that the crystallization of the high-density polyethylene contained in the second segment of the resulting splittable conjugate fiber has progressed sufficiently, and by performing a splitting process such as stirring a slurry containing the splittable conjugate fiber in a papermaking process or spraying a high-pressure water stream onto a fiber assembly containing the splittable conjugate fiber, the splittable conjugate fiber is easily split into ultrafine fibers obtained by separating the first segment and ultrafine fibers obtained by separating the second segment. The heat of fusion (ΔH PE) is preferably 130 mJ / mg or more, more preferably 135 mJ / mg or more, particularly preferably 140 mJ / mg or more, and most preferably 145 mJ / mg or more. The heat of fusion (ΔH PE ) is not particularly limited, but is, for example, 280 mJ / mg or less.

[0047] In the splittable conjugate fiber, the dry heat dimensional change at 140°C measured in accordance with JIS L 1015 8.15 dimensional change is preferably 10.0% or less, more preferably 9.7% or less, and even more preferably 9.5% or less. When the dry heat dimensional change is within the above range, heat shrinkage during heating is suppressed and processability during heat treatment is also excellent.

[0048] In the splittable conjugate fiber, the 140°C areal shrinkage of a wetlaid nonwoven fabric made of 100% by mass of splittable conjugate fiber is preferably 20% or less, more preferably 15% or less, and even more preferably 10% or less. When the areal shrinkage is within the above range, heat shrinkage upon heating is suppressed and processability during heat treatment is also excellent. The smaller the 140°C areal shrinkage of a wetlaid nonwoven fabric made of 100% by mass of splittable conjugate fiber, the better; therefore, it may be 0%, but may also be 0.5% or more, 0.8% or more, or 1.0% or more. The 140°C areal shrinkage can be measured as described in the Examples.

[0049] (ultrafine fiber) The splittable conjugate fiber can be split to form ultrafine fibers derived from the first segment, i.e., ultrafine fibers made of a first component containing 50% by mass or more of polypropylene (also referred to as ultrafine fibers A), and ultrafine fibers derived from the second segment, i.e., ultrafine fibers made of a second component containing 50% by mass or more of high-density polyethylene (also referred to as ultrafine fibers B). That is, the segments constituting the splittable conjugate fiber become independent as the splittable conjugate fiber is split, and each form an ultrafine fiber.

[0050] The ultrafine fibers A and / or the ultrafine fibers B preferably have a single fiber fineness of less than 0.6 dtex, more preferably less than 0.4 dtex. When the single fiber fineness of the ultrafine fibers is less than 0.6 dtex, a thin fiber assembly is easily obtained. The single fiber finenesses of the ultrafine fibers A and B may be different from each other, and the lower limit of the single fiber fineness of each ultrafine fiber is preferably 0.02 dtex or more.

[0051] (Method of manufacturing splittable composite fiber) Next, a method for producing the splittable conjugate fiber of the present invention will be described. The splittable conjugate fiber can be obtained, for example, by supplying a first component containing 50% by mass or more of polypropylene to the first segments and a second component containing 50% by mass or more of high-density polyethylene to the second segments using a splittable conjugate spinning nozzle that includes a plurality of first segments and a plurality of second segments, the first segments and the second segments being arranged alternately in the fiber cross section, melt-spinning these to form undrawn spun filaments (undrawn fiber tows), and drawing the undrawn fiber tows obtained.

[0052] Specifically, a split-type conjugate spinning nozzle capable of obtaining a predetermined fiber cross section is first attached to a melt spinning machine, and a first component containing 50% by mass of polypropylene and a second component containing 50% by mass of high-density polyethylene are melted at a spinning temperature of 200°C to 350°C so that the first and second segments are adjacent and separated from each other in the fiber cross section. The melts are then fed into the first and second segments of the split-type conjugate spinning nozzle and extruded (i.e., melt-spun) to obtain undrawn fiber tows, i.e., undrawn spun filaments. The polypropylene contained in the first component has a z-average molecular weight (Mz) of 500,000 to 1,500,000 before spinning and a Q value of 2.6 to 8 before spinning. When the first component contains 50% by mass or more of polypropylene having the above-mentioned pre-spinning z-average molecular weight and pre-spinning Q value, the molten resin (the molten first component and second component) extruded from the spinning nozzle during melt spinning is withdrawn and cooled, preferably immediately after the molten resin is extruded from the spinning nozzle and spun as an undrawn fiber tow (spun filament) as described below (for example, at a position 25 mm or more but less than 85 mm from the spinning nozzle), the polypropylene will contain a large amount of amorphous phase and smectic crystals, in other words, will have a low content of crystalline phase (α crystals), resulting in an undrawn fiber tow that is easily drawn in the drawing process.

[0053] In the method for producing splittable conjugate fibers of the present invention, the z-average molecular weight Mz of the polypropylene before spinning is 500,000 or more and 1,500,000 or less. When the z-average molecular weight before spinning is within the above-mentioned range, the polypropylene contained in the first component is said to be in a state where it contains a large amount of high molecular weight components, and the polypropylene contained in the first component of the resulting splittable conjugate fiber is also in a state where it contains a large amount of high molecular weight components. The z-average molecular weight of the polypropylene before spinning is preferably 550,000 or more and 1,400,000 or less, more preferably 600,000 or more and 1,300,000 or less, more preferably 700,000 or more and 1,200,000 or less, more preferably 800,000 or more and 1,000,000 or less, and particularly preferably 820,000 or more and 980,000 or less.

[0054] The Q value of the polypropylene before spinning is 2.6 or more and 8 or less. When the Q value before spinning is within the above range, the polypropylene contained in the first component has a high polydispersity and is rich in low and high molecular weight components, and the polypropylene contained in the first component of the resulting splittable conjugate fiber also has a high polydispersity and is rich in low and high molecular weight components. The Q value of the polypropylene before spinning is preferably 2.8 or more and 7 or less, more preferably 3.0 or more and 6.5 or less, more preferably 3.2 or more and 6 or less, more preferably 3.4 or more and 5.8 or less, and particularly preferably 3.6 or more and 5.6 or less.

[0055] In the method for producing splittable conjugate fibers of the present invention, the spinning temperature (extrusion temperature) of the first component is preferably 250° C. or higher and 330° C. or lower, more preferably 260° C. or higher and 310° C. or lower, and even more preferably 260° C. or higher and 290° C. The spinning temperature (extrusion temperature) of the second component containing high-density polyethylene is not particularly limited, but is, for example, preferably 230° C. or higher and 330° C. or lower, more preferably 250° C. or higher and 300° C. or lower.

[0056] In the melt spinning, the take-up speed is not particularly limited, but is preferably 300 m / min or more and 2500 m / min or less from the viewpoint of stably obtaining spun filaments. The slower the take-up speed, the smaller the force (spinning tension) that draws the molten polypropylene, making it more difficult for the polypropylene to be oriented, and the proportion of crystalline phase contained in the polypropylene in the resulting undrawn fiber tow is likely to be smaller. On the other hand, when spinning is performed at a high speed, the fineness of the undrawn fiber tow is reduced, and the tow is sufficiently drawn even with a relatively low drawing ratio in the drawing step. This makes it easier to obtain splittable conjugate fibers with a small fineness, and also improves productivity. In melt spinning, the take-up speed is preferably 500 m / min or more and 2300 m / min or less, more preferably 600 m / min or more and 2000 m / min or less, and particularly preferably 700 m / min or more and 1800 m / min or less.

[0057] The molten resin discharged from the nozzle holes of the spinning nozzle is stretched and cooled at the same time, and the molecular chains are oriented to form an undrawn fiber tow. At this time, the molecular chains contained in the molten resin are in a random state and can move freely, but it is thought that their movement slows down as the resin cools.

[0058] In the method for producing splittable conjugate fibers of the present invention, when the molten resin extruded from the spinning nozzle is cooled while being taken up, it is preferable to take up the molten resin while cooling it at a position close to the spinning nozzle, while taking care not to cause thread breakage. By cooling the molten resin at a position close to the spinning nozzle to the extent that thread breakage does not occur and taking it up, the polypropylene contained in the undrawn fiber tow is likely to have a phase other than a crystalline phase, specifically an amorphous phase or a structure containing a large amount of smectic crystals. When an undrawn fiber tow containing a large amount of these phases is drawn, these phases are more easily drawn than the crystalline phase, so the drawing process can be carried out stably even at a high drawing ratio.

[0059] In melt spinning, the position at which the molten resin is cooled can be adjusted, for example, by the position of a forced cooling device (commonly called a chimney) that blows cooling air onto the molten resin. Adjusting the position of the forced cooling device does not require any special equipment and is easy to adjust. In the method for producing splittable conjugate fibers of the present invention, it is also preferable to adjust the position of the forced cooling device so that the polypropylene contained in the resulting undrawn fiber tow contains a large amount of amorphous phase and smectic crystals other than the crystalline phase.

[0060] In the method for producing splittable conjugate fibers of the present invention, the distance from the spinneret surface of the spinning nozzle provided in the spin pack to the cooling start point is preferably 25 mm or more and less than 85 mm. When the position of the cooling start point satisfies this range, the molten resin is cooled before it is stretched during melt spinning and molecular chain orientation progresses, so the resulting undrawn fiber tow is likely to be an undrawn fiber tow with little orientation. The distance from the spinning nozzle spinneret surface to the cooling start point is more preferably 30 mm or more and 82 mm or less, and particularly preferably 40 mm or more and 80 mm or less. In the present invention, the cooling start point refers to the position of the cooling air blown against the molten resin that is closest to the spinning nozzle spinneret surface. For example, as shown in Figure 2, when molten resin 23 is discharged from the outlet of a spinning nozzle provided in a spinning pack 21 and taken up while being cooled with cooling air to obtain an undrawn fiber tow 24, the distance from the spinning nozzle nozzle surface to the cooling start point refers to the distance L from the spinning nozzle nozzle surface that discharges the molten resin 23 to the upper end position (22) of the cooling air blown against the molten resin 23 that is closest to the spinning nozzle nozzle surface.

[0061] The cooling method of the forced cooling device is not particularly limited. It may be a uniflow-type cooling device that cools the discharged and stretched molten resin from one direction, or a circular-type cooling device that blows cooling air onto the molten resin or unstretched fiber tow from the inside to the outside, or from the outside to the inside. The gas used for the cooling air is not particularly limited, but rare gases such as argon and helium, which are stable (very low reactivity) at room temperature, nitrogen, or air are preferred. Among these, nitrogen and air, which are inexpensive to supply, are particularly preferred. The speed of the cooling air is preferably 0.2 m / s to 5 m / s, and more preferably 0.3 m / s to 3 m / s. The temperature of the cooling air is preferably low to ensure uniform cooling, but may be 40°C or lower, or 15°C to 35°C, considering the cost of adjusting the temperature.

[0062] The single fiber fineness of the obtained undrawn fiber tow is within the range of 1 dtex to 15 dtex. If the single fiber fineness of the undrawn fiber tow is less than 1 dtex, there is a tendency for frequent fiber breakage during spinning. On the other hand, if the single fiber fineness of the undrawn fiber tow exceeds 15 dtex, not only will it be impossible to obtain a splittable conjugate fiber with a small fineness unless the drawing step described below is performed at a very high draw ratio, but it will also be difficult to obtain a splittable conjugate fiber with a small fineness, and the fibers obtained by splitting the splittable conjugate fiber will also tend not to be ultrafine fibers. In order to obtain splittable conjugate fibers and ultrafine fibers with a small fineness by drawing the undrawn fiber tow, the single fiber fineness of the undrawn fiber tow is preferably 1.5 dtex to 12 dtex, more preferably 1.8 dtex to 10 dtex, and particularly preferably 2 dtex to 9 dtex.

[0063] In the method for producing a splittable conjugate fiber of the present invention, in the undrawn fiber tow obtained by melt spinning, the first segment containing polypropylene contains a crystalline phase, but is a resin segment containing a large amount of amorphous phase and smectic crystals. Furthermore, the second segment containing high-density polyethylene is crystallized and contains a large amount of crystalline phase. This state prevents the polypropylene and high-density polyethylene from interfering with each other's drawability, improving the drawability of the undrawn fiber tow. As a result, by sufficiently drawing the undrawn fiber tow, not only is the amorphous phase that causes thermal shrinkage upon heating reduced within the resulting splittable conjugate fiber, but overdrawn polypropylene (polypropylene that has been stretched excessively, causing distortion and breakage in the molecular arrangement) that causes thermal shrinkage upon heating is also less likely to remain.

[0064] In the method for producing a splittable conjugate fiber of the present invention, whether the undrawn fiber tow obtained by melt spinning is in the above-mentioned state can be determined by subjecting the undrawn fiber tow to X-ray diffraction (XRD) and examining the half-width and diffraction peak intensity obtained from the obtained diffraction peak.

[0065] Specifically, in an X-ray diffraction chart obtained by measurement using an X-ray diffraction method (XRD), the undrawn fiber tow has diffraction peaks of polypropylene, namely, diffraction peak PP1 at 2θ=14.2±0.5°, diffraction peak PP2 at 2θ=17±0.5°, and diffraction peak PP3 at 2θ=18.6±0.5°, as well as diffraction peak PE1 at 2θ=21.6±0.5°, which is a diffraction peak of high-density polyethylene, satisfying the following (1) to (4): (1) The half-width of the diffraction peak PP1 is 0.835° or more (2) The half-width of the diffraction peak PP2 is 0.835° or more. (3) The half-width of the diffraction peak PP3 is 0.955° or more. (4) Diffraction intensity of diffraction peak PE1 (I PE ) and the ratio I of the sum of the diffraction intensities of the diffraction peaks PP1, PP2, PP3, and PE1 (I) PE / I is 0.105 or more and less than 0.750

[0066] As described above, in the undrawn fiber tow, the half-widths of the polypropylene diffraction peaks PP1, PP2, and PP3 are equal to or greater than a certain value. When the half-widths of the diffraction peaks are large in X-ray diffraction analysis, the diffraction peaks are broad, and it is believed that the substance exhibiting such diffraction peaks has low crystallinity. Therefore, when the half-widths of the polypropylene diffraction peaks PP1, PP2, and PP3 are equal to or greater than a certain value, it is estimated that the polypropylene contained in the undrawn fiber tow has low crystallinity and is a resin segment containing a large amount of amorphous phases and smectic crystals. The half-width of the diffraction peak PP1 is preferably 0.835° to 2.3°, more preferably 0.850° to 2.0°, and particularly preferably 0.870° to 1.95°. The half width of diffraction peak PP2 is preferably 0.835° to 1.8°, more preferably 0.840° to 1.6°, and particularly preferably 0.850° to 1.5°. The half width of diffraction peak PP3 is more preferably 0.955° to 2.6°, more preferably 0.98° to 2.2°, and particularly preferably 1.0° to 2.0°.

[0067] In the undrawn fiber tow, as described above, the diffraction intensity (I PE ) to the sum of the diffraction intensities of the diffraction peaks of PP1, PP2, PP3 and PE1 (I, hereinafter also referred to as the total diffraction intensity (I)), PE / I is 0.105 or more and less than 0.750. PE When / I satisfies the above range, the ratio of the crystalline phases contained in the polypropylene and the high-density polyethylene contained in the undrawn fiber tow becomes optimal, and it is presumed that when such an undrawn fiber tow is drawn, the polypropylene and the high-density polyethylene do not inhibit each other's drawability, and the polypropylene and the high-density polyethylene have approximately the same drawability. PE If / I is outside the above range, it is considered that either the polypropylene or the high-density polyethylene has too much crystalline phase, and the polypropylene contains a large amount of crystalline phase (i.e., I PE When the ratio of I to I is less than 0.105, the polypropylene tends to be overstretched after stretching, and the high density polyethylene contains a large amount of crystalline phase (i.e., I PE When the ratio of I to I is 0.750 or more, the high-density polyethylene breaks before the polypropylene is sufficiently stretched, which results in insufficient stretching of the polypropylene, and is therefore likely to cause heat shrinkage when heated. PE / I is preferably 0.11 or more and 0.70 or less, more preferably 0.115 or more and 0.65 or less, and even more preferably 0.12 or more and 0.60 or less.

[0068] Next, the undrawn fiber tow is drawn to obtain a drawn filament. The drawing is performed by setting the drawing temperature within a range of 60°C or higher and lower than 110°C. The drawing is preferably performed at a temperature equal to or lower than the melting point of the resin with the lowest melting point among the resin components constituting the splittable conjugate fiber. The drawing temperature is preferably 80°C or higher and 105°C or lower, more preferably 85°C or higher and 100°C or lower, and particularly preferably 90°C or higher and lower than 100°C. When the drawing is performed at such a drawing temperature, the second component containing high-density polyethylene melts or softens during the drawing step, making it difficult for the fibers to fuse together, which tends to improve the water dispersibility of the resulting splittable conjugate fiber.

[0069] In the drawing process, although not particularly limited, a total draw ratio of 2x or more is used to enhance the crystallinity of the polypropylene and high-density polyethylene contained in the splittable conjugate fiber and more effectively suppress thermal shrinkage of the resulting splittable conjugate fiber. A total draw ratio of 2x or more promotes crystallization of the amorphous phase and smectic crystals of the polypropylene and high-density polyethylene contained in the splittable conjugate fiber, suppressing shrinkage through thermal phase transition. Furthermore, sufficient crystallization facilitates the retention of fiber treatment agents applied in subsequent processes on the fiber surface, improving dispersibility in water. Because the single fiber strength of the splittable conjugate fiber and the ultrafine fibers obtained by splitting the splittable conjugate fiber is high, fiber assemblies using these fibers have high mechanical strengths, such as tensile strength and puncture strength. Furthermore, a total draw ratio of 2x or more ensures that the fineness of the resulting splittable conjugate fiber and the ultrafine fibers obtained by splitting the splittable conjugate fiber are sufficiently small, making it easier to obtain fiber assemblies using these fibers with a dense structure and good texture. In the drawing step, the total draw ratio is preferably 3 times or more, more preferably 3.5 times or more, and particularly preferably 4 times or more. In the drawing step, from the viewpoint of productivity, i.e., to prevent frequent yarn breakage in the drawing step and to prevent a portion of the polypropylene contained in the drawn filaments from being excessively drawn and becoming overdrawn, which would cause distortion or destruction of the crystalline phase, the total draw ratio is 10 times or less, preferably 9 times or less, and more preferably 8 times or less.

[0070] In the drawing step, the total draw ratio is preferably 60% to 98% of the maximum draw ratio, more preferably 70% to 97%, and particularly preferably 80% to 95%. When the total draw ratio is 60% to 98% of the maximum draw ratio, the polypropylene contained in the undrawn fiber tow is less likely to be overdrawn during drawing, and the resulting splittable conjugate fiber has excellent water dispersibility and exhibits little thermal shrinkage even when heated.

[0071] Furthermore, in the drawing process, in order to facilitate crystallization of the polypropylene and high-density polyethylene contained in the undrawn fiber tow and to obtain splittable conjugate fibers with sufficiently fine fineness, the total draw ratio is preferably 60% to 98% of the maximum draw ratio and 3.5 to 8 times. The drawing method may be either a wet drawing method or a dry drawing method. Air, steam, water, oils such as glycerin, and the like can be used as the heat medium. In the wet drawing method, drawing can be performed while heating in a liquid, for example, drawing can be performed in hot water or warm water. In the dry drawing method, drawing can be performed while heating in a high-temperature gas or with a high-temperature metal roll. Steam drawing, in which the fiber is drawn while heated using steam as the heat medium under normal or pressurized conditions, may also be used.

[0072] The stretching process may be a single-stage stretching process or a multi-stage stretching process with two or more stages. In the case of a single-stage stretching process, the total stretching ratio is the stretching ratio of the single-stage stretching. In the case of a multi-stage stretching process, the total stretching ratio can be calculated by multiplying the stretching ratios of each stage. For example, if a stretching process is performed at 85°C with a stretching ratio of 1.5 times, and then a stretching process is performed at 90°C with a stretching ratio of 4 times, the total stretching ratio in this manufacturing method is 6 times.

[0073] In the present invention, the "maximum stretching ratio (V max )" is measured as follows. Melt spinning is performed using a split-type multi-component spinning nozzle, and the resulting undrawn fiber tow is subjected to wet drawing in a drawing tank filled with warm water adjusted to the drawing temperature. The drawing tank is then filled with warm water adjusted to the drawing temperature. At this time, the delivery speed (V1) of the roll that delivers the undrawn fiber tow is set to 10 m / min, and the take-up speed (V2) of the metal roll on the take-up side is gradually increased from 10 m / min. The take-up speed of the metal roll on the take-up side when the undrawn fiber tow breaks is defined as the maximum drawing speed, and the ratio (V2 / V1) of the maximum drawing speed to the delivery speed of the roll that delivers the undrawn fiber tow is determined, and the obtained speed ratio is used as the maximum draw ratio (V maxWhen the stretching treatment is carried out by dry stretching, the maximum stretch ratio under those conditions can be similarly determined by adjusting the temperature of the heat medium (for example, the surface of a metal roll) during the stretching treatment to the same temperature as during the stretching treatment, and gradually increasing the winding speed in the same manner as above.

[0074] In the case of a single drawing process, i.e., one-stage drawing, or in the case of multiple drawing processes at the same drawing temperature using the same drawing method (for example, three repeated dry drawing processes at 90°C and 2x, or 1.5x dry drawing at 90°C followed by 4x dry drawing at 90°C), the maximum draw ratio can be measured using the same method and temperature as in the drawing process. In the case of multiple drawing processes, i.e., multi-stage drawing, in which the spun filaments are drawn and the drawing temperatures differ depending on the drawing process, the maximum draw ratio is measured using the same drawing method and at the same temperature as in the drawing process performed at a higher drawing temperature.

[0075] When undrawn fiber tow is drawn by a so-called multi-stage drawing process in which drawing treatment is performed multiple times, and the drawing temperature is the same in all drawing processes but the drawing methods are different (specifically, an example is a case in which steam drawing is performed in steam at 100°C at a draw ratio of 3, and then dry drawing is performed using metal rolls heated to 100°C at a draw ratio of 2), the maximum draw ratio is measured by both methods (in this example, steam drawing at 100°C and dry drawing using metal rolls at 100°C), and the larger maximum draw ratio is taken as the maximum draw ratio under those manufacturing conditions.

[0076] A predetermined amount of fiber treatment agent is applied to the resulting drawn filaments as needed, and mechanical crimping is further performed using a crimper (crimping device) as needed. When producing nonwoven fabrics using a wet papermaking process, the fiber treatment agent facilitates dispersion of the fibers in water, etc. Furthermore, applying an external force to the fiber surface to which the fiber treatment agent has been applied (such as the force applied when crimping using a crimper) to impregnate the fibers with the fiber treatment agent further improves dispersibility in water, etc. The number of crimps is preferably in the range of 5 crimps / 25 mm to 30 crimps / 25 mm, and more preferably in the range of 10 crimps / 25 mm to 20 crimps / 25 mm. A crimp number of 5 crimps / 25 mm or more improves splittability due to the application of external force by the crimper, while a crimp number of 30 crimps / 25 mm or less reduces or eliminates fiber aggregation and clumping.

[0077] The drawn filaments after application of the fiber treatment agent (or in a wet state without application of the fiber treatment agent) are dried at a temperature in the range of 80°C to 110°C for several seconds to about 30 minutes to dry the fibers. The drying process may be omitted in some cases. Thereafter, the drawn filaments are cut, as necessary, to a fiber length of 1 mm to 100 mm, preferably 2 mm to 70 mm.

[0078] When producing a drylaid nonwoven fabric containing splittable conjugate fibers, it is necessary to dry the drawn filaments after applying a fiber treatment agent (or in a wet state without applying a fiber treatment agent). When drying the drawn filaments, the drawn filaments can be dried in a relaxed state at a temperature of 60°C or higher and lower than 110°C. After the drawn filaments are sufficiently dried, they are cut to a predetermined length to become splittable conjugate fibers suitable for producing drylaid nonwoven fabrics. When producing a drylaid nonwoven fabric using the splittable conjugate fibers of the present invention, the fiber length is not particularly limited, but is preferably, for example, 20 mm or higher and 100 mm or lower. A fiber length of 20 mm or higher and 100 mm or lower stabilizes productivity when producing drylaid nonwoven fabrics. When producing a drylaid nonwoven fabric, the fiber length is more preferably 25 mm or higher and 80 mm or lower, and particularly preferably 25 mm or higher and 65 mm or lower.

[0079] When the resulting splittable conjugate fiber is made into a fiber assembly for a wetlaid nonwoven fabric, the splittable conjugate fiber may be in a wet or dry state, but is preferably in a wet state in consideration of dispersibility in water. Therefore, the drawn filaments after application of a fiber treatment agent (or in a wet state without application of a fiber treatment agent) do not need to be dried, and are cut to the desired fiber length while still wet. When a wetlaid nonwoven fabric is produced using the splittable conjugate fiber of the present invention, the fiber length is not particularly limited, but is preferably, for example, 1 mm or more and less than 20 mm. A fiber length of 1 mm or more and less than 20 mm ensures good dispersibility in water when producing a wetlaid nonwoven fabric. The fiber length when producing a wetlaid nonwoven fabric is more preferably 2 mm or more and 15 mm or less, particularly preferably 3 mm or more and 12 mm or less, and most preferably 3 mm or more and 10 mm or less.

[0080] (fiber assembly) Next, a fiber assembly containing the splittable conjugate fiber of the present invention will be described. The form of the fiber assembly is not particularly limited, but examples thereof include woven fabrics, knitted fabrics, and nonwoven fabrics. The form of the fiber web of the nonwoven fabric is also not particularly limited, and examples thereof include a carded web formed by a carding method, an air-laid web formed by an air-laid method, and a wet-laid web formed by a wet-laid papermaking method.

[0081] In the fiber assembly, the content of the splittable conjugate fiber is preferably 10% by mass or more, more preferably 15% by mass or more, and particularly preferably 20% by mass or more. When the content of the splittable conjugate fiber is 10% by mass or more, the proportion of the splittable conjugate fiber in the fiber assembly, for example, a nonwoven fabric, is high, and a dense nonwoven fabric tends to be obtained. There is no particular upper limit to the content of the splittable conjugate fiber in the fiber assembly. However, in the case of fiber assemblies for various wiping applications such as personal and / or object wipers, fiber assemblies for battery separators used in various secondary batteries such as nickel-metal hydride batteries, and fiber assemblies for filtration layers of various filters such as cartridge filters and laminated filters, where a certain degree of void space between the constituent fibers and the associated breathability and liquid permeability are required, the content of the splittable conjugate fiber relative to the entire fiber assembly is preferably 90% by mass or less, more preferably 75% by mass or less, and particularly preferably 50% by mass or less. When the proportion of splittable conjugate fibers in a fiber assembly such as a drylaid nonwoven fabric or a wetlaid nonwoven fabric is 90% by mass or less, the proportion of ultrafine fibers derived from the splittable conjugate fibers in the resulting fiber assembly is not too high, and there is no risk of the fiber assembly becoming an unnecessarily dense nonwoven fabric. Furthermore, when attempting to produce a wetlaid nonwoven fabric using the splittable conjugate fibers, if the content of the splittable conjugate fibers is 90% by mass or less, the floating seed phenomenon in which unsplittable conjugate fibers float on the surface of the slurry during the defibration treatment in the wetlaid papermaking process is unlikely to occur, resulting in good processability. Furthermore, there is little entanglement between the splittable conjugate fibers themselves and between the ultrafine fibers and other fibers, and there is little fiber balling, making it easier to obtain a nonwoven fabric with a uniform texture. If a particularly dense fiber assembly with few inter-fiber voids is required, it is preferable that the fiber assembly used for the above-mentioned applications has a high content of splittable conjugate fibers, and the fiber assembly may have a content of the splittable conjugate fibers exceeding 90% by mass or may be composed only of the splittable conjugate fibers.

[0082] The fiber materials to be mixed in the fiber assembly other than the splittable conjugate fibers are not particularly limited. Examples include cellulosic fibers such as cotton, pulp, hemp, viscose rayon, and Tencel®; polyester fibers such as polyethylene terephthalate, polybutylene terephthalate, polytrimethylene terephthalate, polyethylene naphthalate, polylactic acid, and polybutylene succinate; polyamide fibers such as nylon 6, nylon 66, nylon 11, and nylon 12; and acrylic fibers. Other examples include polyethylene monofilaments such as low-density polyethylene, high-density polyethylene, and linear low-density polyethylene; polypropylene monofilaments such as isotactic, atactic, and syndiotactic polypropylenes polymerized using a conventional Ziegler-Natta catalyst or metallocene catalyst; copolymers of these polyolefin monomers; and polyolefins polymerized using a metallocene catalyst (also known as a Kaminsky catalyst) to polymerize these polyolefins. Other examples include fibers composed of engineering plastics such as polycarbonate, polyacetal, polystyrene, and cyclic polyolefins. The above-mentioned fibers can be used alone or in combination of two or more. The fiber shape is not particularly limited, and examples thereof include single fibers, sheath-core composite fibers, eccentric sheath-core composite fibers, multi-core sheath-core composite fibers, side-by-side composite fibers, islands-in-the-sea composite fibers, and splittable composite fibers. The fiber cross section may be circular or irregular.

[0083] The fiber assembly may contain binder fibers in an amount of 90% by mass or less. The binder fiber content in the fiber assembly is more preferably 85% by mass or less, and even more preferably 80% by mass or less. The binder fibers (thermal bonding fibers) may be, for example, sheath-core composite fibers. A fiber assembly containing thermal bonding fibers bonds the constituent fibers together, resulting in a fiber assembly with excellent tensile strength and puncture strength.

[0084] The fiber assembly preferably contains 10% by mass or more of ultrafine fibers formed by splitting splittable conjugate fibers. That is, the fiber assembly preferably contains 10% by mass or more of ultrafine fibers A and ultrafine fibers B combined. The fiber assembly more preferably contains 20% by mass or more of ultrafine fibers, and even more preferably 25% by mass or more of ultrafine fibers. The preferred upper limit is 100% by mass. A high proportion of splittable conjugate fibers in the fiber assembly tends to make it easier to obtain a dense nonwoven fabric.

[0085] In the fiber assembly, the splittable conjugate fibers can be split by applying a physical impact. For example, they can be split by a hydroentanglement treatment (spraying a high-pressure water stream). Alternatively, when a nonwoven fabric is produced by a wet papermaking method, they can be split by utilizing the impact they receive during the defibration treatment during papermaking.

[0086] The hydroentanglement treatment can be carried out, for example, by spraying a columnar water stream at a water pressure of 3 MPa to 20 MPa onto the front and back of the fiber web one or more times from a nozzle having orifices with a hole diameter of 0.05 mm to 0.5 mm arranged at intervals of 0.5 mm to 1.5 mm. The splittable conjugate fiber can be split even at a water pressure of 10 MPa or less, 8 MPa or less, or 6 MPa or less.

[0087] The method for producing the fiber assembly will be described using a nonwoven fabric as an example. Nonwoven fabrics can be produced by preparing a fiber web according to a known method, and then, if necessary, subjecting the web to heat treatment to thermally bond the fibers. If necessary, the fiber web may also be subjected to a fiber entanglement treatment. First, the fiber web can be produced, for example, by a dry method such as a carding method or an air-laying method using splittable conjugate fibers with a fiber length of 10 mm to 100 mm, or by a wet papermaking method using splittable conjugate fibers with a fiber length of 2 mm to 20 mm. For applications such as personal and / or object wipers and filters, nonwoven fabrics produced by a dry method such as a carding method or an air-laying method are preferred. This is because nonwoven fabrics produced by a dry method have a soft texture and an appropriate density. Furthermore, for applications such as battery separators, nonwoven fabrics produced from a wet-laid papermaking web are preferred. This is because nonwoven fabrics produced using a wet-laid papermaking web are generally dense and have good texture. Furthermore, in the wet papermaking method, by adjusting the conditions of the cleavage treatment during papermaking, it is possible to split the splittable conjugate fiber at a desired split ratio by the cleavage treatment alone.

[0088] The fiber web may then be subjected to a thermal bonding treatment. For example, core-sheath composite fibers (binder fibers) may be added to the splittable composite fibers, and the fibers may be bonded together by the sheath component of the core-sheath composite fibers. The conditions for the thermal bonding treatment may be appropriately selected depending on the basis weight of the fiber web, the cross-sectional shape of the ultrafine fibers, and the type of resin constituting the fibers contained in the nonwoven fabric. For example, a cylinder dryer, a hot air blowing machine, a heat roll processing machine, or a hot embossing machine may be used as a heat treatment machine. A cylinder dryer is particularly preferred because it can thermally bond the fibers together while adjusting the thickness of the nonwoven fabric. The heat treatment temperature of the cylinder dryer is preferably 80°C to 160°C when the sheath component of the binder fiber is an ethylene-vinyl alcohol copolymer, 100°C to 160°C when the sheath component of the binder fiber is various polyethylenes, and 130°C to 145°C when the sheath component of the binder fiber is high-density polyethylene.

[0089] As described below, when the fiber web is subjected to a hydroentanglement treatment, the thermal bonding treatment is preferably carried out before the hydroentanglement treatment. If the fibers of the fiber web are bonded together in advance and then the hydroentanglement treatment is carried out, the fibers are less likely to "escape" when hit by the high-pressure water stream, and the fibers can be tightly entangled, further accelerating the splitting of the splittable conjugate fiber. Note that the thermal bonding treatment may also be carried out after the fibers have been entangled. In other words, the order of the thermal bonding treatment and the hydroentanglement treatment is not particularly limited as long as the desired nonwoven fabric is obtained.

[0090] In the fiber assembly of the present invention, the fibers may be entangled with each other. A preferred method for entangling the fibers is hydroentanglement, in which the fibers are entangled with each other by the action of a high-pressure water stream. Hydroentanglement can firmly entangle the fibers without impairing the denseness of the entire nonwoven fabric. Furthermore, hydroentanglement can promote splitting of splittable conjugate fibers and entanglement of ultrafine fibers produced by splitting, at the same time as entangling the fibers with each other.

[0091] The conditions for the hydroentanglement treatment can be appropriately selected depending on the type and basis weight of the fiber web used, and the type and proportion of fibers contained in the fiber web. For example, 2 More than 100g / m 2 When subjecting the following wetlaid paper web to hydroentanglement, the fibrous web is placed on a support such as a plain weave structure having a mesh size of approximately 70 to 100, and a columnar water stream having a water pressure of 1 MPa to 15 MPa, more preferably 2 MPa to 10 MPa, is sprayed onto one or both sides of the fibrous web from a nozzle having orifices with a hole diameter of 0.05 mm to 0.3 mm arranged at intervals of 0.5 mm to 1.5 mm, once to 10 times. The fibrous web after hydroentanglement is subjected to a drying treatment as necessary.

[0092] The fiber assembly may be subjected to a hydrophilization treatment as needed. The hydrophilization treatment can be carried out by any method, such as fluorine gas treatment, vinyl monomer graft polymerization treatment, sulfonation treatment, discharge treatment, surfactant treatment, or hydrophilic resin application treatment. In particular, when the fiber assembly is used as a battery separator, it is preferable to subject it to a hydrophilization treatment. This is to increase the affinity with the electrolyte and improve the electrolyte retention.

[0093] The fiber assembly preferably has a basis weight of 5 g / m 2 More than 100g / m 2 More preferably, it is 10 g / m or less. 2 More than 100g / m 2 More preferably, it is 20 g / m or less. 2 More than 80g / m 2 or less, and particularly preferably 30 g / m 2 More than 60g / m 2 The weight of the fiber assembly is 5 g / m or less. 2 When the weight of the fiber assembly is 100 g / m or more, the fiber assembly has a good formation, and the fiber assembly tends to have high strength and puncture strength. 2If the thickness is less than this, the breathability of the fiber assembly will not decrease, and when the splittable conjugate fibers contained in the fiber web are split into their respective components by hydroentanglement treatment, the high-pressure water stream will more easily act uniformly on the entire fiber web, making it easier to sufficiently split the splittable conjugate fibers.

[0094] As an example of a method for producing the nonwoven fabric, a wet papermaking method is preferred, and wet papermaking can be carried out by a conventional method. First, the splittable conjugate fiber is prepared and dispersed in water to a concentration of 0.01% by mass or more and 0.6% by mass or less to prepare a slurry. The splittable conjugate fiber has excellent splittability even under weak impact forces, so it can be easily split by defibration and beating during slurry preparation. Next, the slurry is made into a wet-laid papermaking web using a short-wire, cylinder-wire, or Fourdrinier papermaking machine, or a combination of short-wire, cylinder-wire, and Fourdrinier machines. The wet-laid wet-laid web is then dried using a heat treatment machine such as a cylinder dryer, and if necessary, binder fibers may be added to bond the web while drying. Alternatively, the wet-laid web may be bonded as needed to stabilize its shape, and then subjected to a hydroentanglement treatment to split the unsplittable conjugate fibers and entangle the fibers. In wetlaid nonwoven fabrics, a split rate of 10% or more is preferable because it means that the fibers are sufficiently split by simple stirring treatment without using high-pressure water flow, such as mixer treatment (pulper treatment) in the wetlaid nonwoven fabric manufacturing process, and ultrafine fibers can be obtained. Also, even if the split rate is less than 10%, there is no problem in manufacturing various fiber assemblies such as wetlaid nonwoven fabrics, but to split splittable composite fibers to obtain ultrafine fibers, simple stirring treatment without using high-pressure water flow, such as mixer treatment in the wetlaid nonwoven fabric manufacturing process, does not result in sufficient splitting, and therefore a splitting treatment using high-pressure water flow tends to be required.

[0095] The splittable conjugate fiber of the present invention has excellent stretchability and splittability as described above, and can be used to prepare a fiber assembly such as a nonwoven fabric having a dense texture. For example, the fiber assembly is a nonwoven fabric having a basis weight of 5 g / m 2 More than 200g / m 2The following weights can be used:

[0096] (separator) The fiber assembly can be used as a separator material for battery separators, etc. The battery separator preferably contains the splittable composite fiber in an amount of 10% by mass or more and 100% by mass or less. The battery separator is preferably made of a nonwoven fabric, more preferably a wet-laid nonwoven fabric, from the viewpoints that it is easy to retain an electrolyte, maintain strength, and is as thin as possible.

[0097] The nonwoven fabric constituting the battery separator preferably has a basis weight of 5 g / m 2 More than 100g / m 2 More preferably, it is 8 g / m or less. 2 More than 80g / m 2 or less, and particularly preferably 10 g / m 2 More than 50g / m 2 The basis weight is 5g / m or less. 2 If the weight is less than 100 g / m, the density of the nonwoven fabric will vary, which may cause a short circuit when used as a battery separator. 2 If the thickness exceeds this value, the thickness of the battery separator will increase, and the amount of positive and negative electrodes in the battery will decrease accordingly.

[0098] The battery separator of the present invention is incorporated into various batteries to form the battery. For example, in a cylindrical nickel-metal hydride secondary battery, the positive and negative electrode plates can be spirally wound with the battery separator of the present invention interposed therebetween. The battery separator of the present invention may also be used in other batteries, such as nickel-cadmium secondary batteries, nickel-iron secondary batteries, and nickel-zinc secondary batteries. [Example]

[0099] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0100] First, the measurement and evaluation methods used in the examples and comparative examples will be explained.

[0101] (Number average molecular weight Mn, weight average molecular weight Mw, z-average molecular weight Mz and Q value) The number-average molecular weight (Mn), weight-average molecular weight (Mw), z-average molecular weight (Mz), and the ratio of weight-average molecular weight (Mw) to number-average molecular weight (Mn), i.e., the Q value, of polypropylene were measured by gel permeation chromatography (GPC) using a high-temperature GPC system (Polymer Laboratories PL-220) equipped with a differential refractive index detector (RI).

[0102] A 5 mg sample containing polypropylene was weighed out, and 5 mL of 1,2,4-trichlorobenzene (TCB) containing 0.1% butylhydroxytoluene (BHT) as a stabilizer and antioxidant was added to the sample. The mixture was heated to 160-170 °C and stirred for 30 minutes to dissolve the polypropylene in the solvent. The solution was then filtered through a metal filter to remove any undissolved sample or other impurities. The resulting sample solution was then injected into the gel permeation chromatograph at a flow rate of 1.0 mL / min with an injection volume of 0.2 mL (200 μL) to measure the number-average molecular weight (Mn), weight-average molecular weight (Mw), and z-average molecular weight (Mz). The Q value (Mw / Mn) was calculated from the number-average molecular weight (Mn) and weight-average molecular weight (Mw). The measurement was performed using TCB containing 0.1% BHT as the measurement solvent, one HT-G column manufactured by Shodex and two HT-806M columns manufactured by Showa Denko K.K., monodisperse polystyrene (manufactured by Tosoh) as the standard sample, the temperature of the column thermostat was set to 145°C, and a GPC data processing system (manufactured by Toray Research Center (TRC)) was used.

[0103] The Mn, Mw, Mz, and Q values ​​of the polypropylene resin before spinning were measured using the raw polypropylene resin as a measurement sample. The Mn, Mw, Mz, and Q values ​​of the polypropylene resin after spinning were measured by heating the polypropylene resin to the temperature for melt spinning, extruding the molten resin from an extruder before the spinning nozzle was attached (in other words, an extruder without a spinning nozzle), collecting a rod-shaped sample with a diameter of approximately 5 mm, and cutting it into small pieces. Note that the Mn, Mw, Mz, and Q values ​​of the polypropylene resin after spinning may be measured using the sample collected in this manner, or the resulting splittable composite fiber may be used as the sample.

[0104] (Melt Mass Flow Rate, MFR) Polypropylene was measured in accordance with JIS K 7210-1:2014 at a temperature of 230°C and a load of 21.18N (2.16kgf), and high-density polyethylene was measured in accordance with JIS K 7210-1:2014 at a temperature of 190°C and a load of 21.18N (2.16kgf).

[0105] (Diffraction peak half width and diffraction peak intensity ratio for undrawn fiber tow) To confirm the crystalline state of the polypropylene and high-density polyethylene in the undrawn fiber tow, the undrawn fiber tow was measured by wide-angle X-ray diffraction (XRD) using the following procedure. From the obtained diffraction peaks, the half-width and diffraction intensity of the diffraction peaks were determined, and the ratio of the diffraction intensity of the high-density polyethylene diffraction peak to the total diffraction intensity was calculated.

[0106] First, the obtained undrawn fiber tow was cut into a length of 3.5 cm. 25.0 mg of the cut undrawn fiber tow was weighed, and both ends were tied with enameled wire to prepare a sample. The undrawn fiber tow bundle serving as the sample was fixed in a holder perpendicular to the direction of incidence of the X-rays, and wide-angle X-ray diffraction was performed. The measurement conditions were as follows:

[0107] X-ray diffractometer: Rigaku MiniFlex II for polymers X-ray source: CuKα radiation (using Ni filter) Output: 30kV 15mA Slit system: DS:1.25°SS:1.25°mm RS:0.3mm Measurement direction: Fiber diameter scan Scanning Method: Continuous Scan Measurement range: 2θ=3~40° Step: 0.02° Scan speed: 4° / min

[0108] Wide-angle X-ray diffraction was performed under the above conditions, and from the obtained X-ray diffraction diagram (X-ray diffraction chart), the half-width (°) and peak intensity I of the diffraction peak PE1 measured at 2θ = 21.6 ± 0.5°, which is the diffraction peak of high-density polyethylene, were determined. PE The diffraction peaks of polypropylene, PP1, PP2, and PP3 were measured at 2θ = 17 ± 0.5°, and the half-width (°) and peak intensity (cps) of the diffraction peak PP1, PP2, and PP3 were measured at 2θ = 18.6 ± 0.5°. The diffraction peaks of polypropylene, PP1, PP2, PP3, and PP3 were measured at 2θ = 14.2 ± 0.5°, and the half-width (°) and peak intensity (cps) of the diffraction peak PP3 were measured at 2θ = 18.6 ± 0.5°. The sum of the diffraction intensities of the four diffraction peaks PE1, PP1, PP2, and PP3 was taken as the total diffraction intensity I (cps). The diffraction peak intensity I of high-density polyethylene was calculated. PE The value obtained by dividing by the total diffraction intensity I is the diffraction peak intensity ratio I PE / I.

[0109] (single fiber fineness) The single fiber fineness was measured in accordance with the measurement of correct fineness by the vibration method described in Appendix JB of JIS L 1015:2021 8.5.2 Measurement of fineness by the ISO method.

[0110] (Single fiber strength and elongation) The single fiber strength and elongation were measured using a tensile tester in accordance with JIS L 1015:2021, with the sample gripped at a distance of 20 mm, to measure the load and elongation at fiber breakage, and these were taken as the single fiber strength and elongation, respectively.

[0111] (Maximum stretching ratio) Each undrawn fiber tow obtained in the examples and comparative examples was subjected to a drawing treatment in which a drawing tank was filled with warm water adjusted to a drawing treatment temperature, and the tow was drawn in the drawing tank. At this time, the delivery speed (V1) of the roll that delivers the undrawn fiber tow was set to 10 m / min, and the take-up speed (V2) of the metal roll on the take-up side was gradually increased from 10 m / min. The take-up speed of the metal roll on the take-up side when the undrawn fiber tow broke was defined as the maximum drawing speed, and the ratio (V2 / V1) of the maximum drawing speed to the delivery speed of the roll that delivers the undrawn fiber tow was determined, and the obtained speed ratio was defined as the maximum draw ratio (V max When the stretching treatment is carried out by dry stretching, the maximum stretch ratio under those conditions can be similarly determined by adjusting the temperature of the heat medium (for example, the surface of a metal roll) during the stretching treatment to the same temperature as during the stretching treatment, and gradually increasing the winding speed in the same manner as above.

[0112] (Differential Scanning Calorimetry) The splittable composite fiber was subjected to differential scanning calorimetry (DSC) in accordance with JIS K 7121:1987, as described above. DSC was performed by filling 3 mg of the splittable composite fiber sample and heating it from 20°C to 300°C at a heating rate of 20°C per minute.

[0113] (Dispersibility in water) Assuming that the obtained splittable conjugate fiber would be used to produce a wet-laid nonwoven fabric to be used for various battery separators or RO membrane supports, the dispersibility of the splittable conjugate fiber in water was evaluated by the following procedure. (1) A stirrer was attached to a water tank (width: 100 cm, depth: 20 cm, height: 55 cm). (2) After adding 80 L of tap water to the tank, a fiber treatment agent containing dodecylphenol EO adduct as its main component was added while rotating the agitator so that the concentration of the active ingredients of the fiber treatment agent (components other than water, such as surfactants) was 0.03 mass%. (3) The splittable composite fibers (drawn filaments) after the drawing process were collected and impregnated with an aqueous solution of a fiber treatment agent containing a dodecylphenol EO adduct as the main component, the active ingredients of which (components other than water, such as surfactants including the dodecylphenol EO adduct) were adjusted to a concentration of 3.0% by mass, and the fibers were dehydrated so that the mass of the aqueous solution of the fiber treatment agent was 30% by mass relative to the mass of the fibers. The dehydrated splittable composite fiber filaments were cut to a fiber length of 5 mm. (4) 5 g of splittable composite fiber cut to a fiber length of 5 mm was weighed out and placed in a water tank, and stirred with a stirrer for 120 seconds. (5) After stirring, a flashlight was shone into the water tank, and the number of thick fibers (fused fibers) was visually confirmed and counted, and the dispersibility in water was evaluated according to the following criteria. ++: There are no or up to three fused fibers, and the dispersibility in water is extremely good. +: The number of fused fibers is 4 to 9, and the dispersibility in water is good. -: There are 10 or more fused fibers, and the dispersibility in water is poor.

[0114] (area shrinkage rate) When using splittable conjugate fibers to produce wetlaid nonwoven fabrics for use in various battery separators and RO membrane supports, the raw fibers are placed in water, paper is made, and then, while drying, hot air at about 140°C is used to melt and thermally bond some of the fibers, thereby increasing the strength of the resulting wetlaid nonwoven fabric. If the fibers undergo thermal shrinkage during this process, wrinkles and tears will occur in the resulting wetlaid nonwoven fabric. Therefore, assuming the case in which a wetlaid nonwoven fabric is produced using splittable conjugate fibers, the areal shrinkage of a wetlaid nonwoven fabric containing splittable conjugate fibers was measured as follows. (1) Splittable composite fiber was weighed out to 3.15 g in an absolute dry state. (2) The weighed fibers were added to 1 L of tap water to form a slurry, and the slurry was 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 was added to the slurry stirred with a pulper to make 16 L of slurry. (4) The slurry diluted to 16 L was poured into a 250 mm square frame covered with a metal mesh (200 mesh), and wet-laid paper was made into a sheet of paper measuring 250 mm long and 250 mm wide, with a dry basis weight of 50 g / cm. 2 The result was a wet papermaking web. (5) The obtained wet-laid paper web was sandwiched between filter paper at 3.5 kg / cm 2 The pressure was applied for 2 minutes to perform dehydration. (6) After dewatering, four points were marked with oil-based ink: two pairs of points each vertically and horizontally, spaced 200 mm apart, at the center of the wet-laid paper web. Specifically, a1, a2, b1, and b2 were marked. As shown in Figure 3A, the initial linear distance (horizontal distance) between points a1 and a2 was defined as A0, and the initial linear distance (vertical distance) between points b1 and b2 was defined as B0. Both A0 and B0 were 200 mm. (7) As shown in FIG. 4, the wetlaid paper web marked with the markings was subjected to a hot air drying process in which hot air 45 at a temperature of 140°C was blown from above the wetlaid paper web 43 at a wind speed of 0.9 m / s in a hot air dryer 40 equipped with a conveyor net 41 for 30 seconds, and the wetlaid paper web was completely dried. At the same time, the sheath component was melted to bond the fibers together, thereby obtaining a thermally bonded nonwoven fabric. At this time, (a) As shown in FIG. 4, first, a polyethylene terephthalate net 42 (filament diameter: 0.3 mm, 35 mesh x 25 mesh) was laid on a conveyor net 41, and then a wetlaid paper web 43 was placed on top of it. (b) In order to prevent the wet papermaking web 43 from being blown away by the influence of hot air, a wooden frame was made using square timber (with a cross section of 15 mm on each side) so that the outer dimensions shown in FIG. 5 were 380 mm in length (L), 380 mm in width (W), and 75 mm in height (H). A polyester resin net box 44 with the polyethylene terephthalate net stretched on five sides except the bottom was placed over the wet papermaking web 43. (8) After the nonwoven fabric was formed, the distance between the two points was measured. As shown in Figure 3B, the linear distance (horizontal distance) between the two points a1 and a2 after the heat treatment was designated as A1, and the linear distance (vertical distance) between the two points b1 and b2 was designated as B1. (9) The area shrinkage rate was calculated based on the following formula (3).

number

[0115] (Dry heat dimensional change rate) Measured in accordance with JIS-L-1015:2021 8.15 Dimensional change rate b) Dry heat dimensional change rate. The heat treatment temperature was 140°C, the heat treatment time was 10 minutes, the initial load was tex x 5.88mN, and the grip spacing was 25mm.

[0116] The following resins were used in the examples and comparative examples. <1st component: Polypropylene resin (PP)> PP1: Density 0.90g / cm 3 , Mz before spinning = 9.0 × 10 5 Propylene homopolymer with a Q value of 5.2 and MFR230 (g / 10 min) of 30 before spinning PP2: Density 0.90g / cm 3 , Mz before spinning = 8.1 × 10 5 , Propylene homopolymer with Q value before spinning = 3.6 and MFR230 (g / 10 min) = 9 PP3: Density 0.90g / cm 3 , Mz before spinning = 5.8 × 10 5 , Propylene homopolymer with Q value before spinning = 2.8 and MFR230 (g / 10 min) = 30 <Second component: High density polyethylene (PE)> PE1: Density 0.96g / cm 3 High-density polyethylene with MFR190 (g / 10 min) = 12 PE2: Density 0.96g / cm 3 High-density polyethylene with MFR190 (g / 10 min) = 20

[0117] Example 1 <Production of splittable composite fibers> PP1 was used as the first component and PE1 was prepared as the second component. Next, the composite ratio (volume ratio) of the first and second components was adjusted to 50 / 50, and PP1 and PE1 were fed into separate extruders using a hollow 16-division composite spinning nozzle as the spinning nozzle. The first component was melted at 270 ° C and the second component was melted at 270 ° C and extruded from the nozzle outlet. The nozzle temperature was set to 270 ° C, and the molten resin was withdrawn at a spinning speed of 1200 m / min. The position of the forced cooling device was adjusted so that the distance L from the spinning nozzle die face to the cooling start point (hereinafter simply referred to as the "cooling start point distance") was 55 mm. Then, cooling air was heated to 27 ° C and blown onto the molten resin to cool it.

[0118] The fibers constituting the obtained undrawn fiber tow had a single fiber fineness of 6 dtex, and the fiber cross section, as shown in Figure 1C, was gear-shaped with a hollow portion (hollow ratio 10%), and the first and second segments were arranged radially and alternately in the fiber cross section, resulting in a fiber with a cross section of 16 segments (hereinafter referred to as a hollow 16-segment type). When this undrawn fiber tow was used as a sample and measured by wide-angle X-ray diffraction (XRD) as described above, the half-width of the polypropylene was 0.935° for the diffraction peak PP1 at 2θ = 14.26, 0.901° for the diffraction peak PP2 at 2θ = 17.06, and 1.171° for the diffraction peak PP3 at 2θ = 18.56. PE The z-average molecular weight was measured using polypropylene sampled under the above conditions (extruder temperature for first component: 270°C) without attaching a spin pack (spin pack) during melt spinning. The z-average molecular weight was 8.3 × 10 5 , the Q value was 4.7.

[0119] The resulting undrawn fiber tow was wet-drawn at a draw ratio of 3.8 at 90°C (first-stage drawing, the same applies hereinafter) in a hot water bath filled with 90°C hot water, and then heat-set at a draw ratio of 1.2 at 90°C (second-stage drawing, the same applies hereinafter) in a hot water bath to obtain a drawn filament with a single fiber fineness of 1.79 dtex. The undrawn fiber tow of Example 1 had a maximum draw ratio of 4.8 and a total draw ratio of 4.6. The resulting drawn filament was immersed in an aqueous fiber treatment agent solution containing a fiber treatment agent mainly composed of a dodecylphenol EO adduct, the active ingredients of which (components other than water, such as surfactants including the dodecylphenol EO adduct) being adjusted to a concentration of 3.0% by mass, and the fiber treatment solution was dehydrated so that the mass of the aqueous fiber treatment agent was 25% by mass and the component adhesion amount of the fiber treatment agent was 0.35% by mass, relative to the mass of the fiber. The dehydrated drawn filaments were cut to a fiber length of 5 mm to obtain splittable conjugate fibers of Example 1.

[0120] Example 2 Melt spinning was performed in the same manner as in Example 1, except that the conjugation ratio (volume ratio) of the first component to the second component was 60 / 40, to obtain an undrawn fiber tow. The obtained undrawn fiber tow was wet-drawn at a draw ratio of 3.3 at 90°C using a hot water bath filled with hot water at 90°C, and then heat-set at a draw ratio of 1.2 at 90°C in a hot water bath to obtain a drawn filament having a single fiber fineness of 1.84 dtex. The undrawn fiber tow of Example 2 had a maximum draw ratio of 4.6 and a total draw ratio of 4.0. The obtained drawn filament was impregnated with an aqueous fiber treatment agent solution in the same manner as in Example 1, and the dehydrated drawn filament was cut to a fiber length of 5 mm, to obtain a splittable conjugate fiber of Example 2.

[0121] Example 3 Melt spinning was performed in the same manner as in Example 1, except that the spinning speed for withdrawing the molten resin extruded from the spinning nozzle was set to 1,440 m / min and the single fiber fineness of the fibers constituting the undrawn fiber tow was set to 5 dtex, thereby obtaining an undrawn fiber tow. The obtained undrawn fiber tow was wet-drawn at a draw ratio of 3.0 at 90°C using a hot water bath filled with hot water at 90°C, and then heat-set at a draw ratio of 1.2 at 90°C in the hot water bath to obtain a drawn filament with a single fiber fineness of 1.72 dtex. The undrawn fiber tow of Example 3 had a maximum draw ratio of 4.4 and a total draw ratio of 3.6. The obtained drawn filament was impregnated with an aqueous fiber treatment agent solution in the same manner as in Example 1, and the dehydrated drawn filament was cut to a fiber length of 5 mm, thereby obtaining the splittable conjugate fiber of Example 3.

[0122] Example 4 Melt spinning was performed in the same manner as in Example 1 to obtain an undrawn fiber tow, except that PP2 was used as the first component and the first component was melted at 270°C and extruded to obtain an undrawn fiber tow. The obtained undrawn fiber tow was wet-drawn at a draw ratio of 3.3 at 90°C using a hot water bath filled with hot water at 90°C, and then heat-set at a draw ratio of 1.2 at 90°C in a hot water bath to obtain a drawn filament having a single fiber fineness of 1.89 dtex. The undrawn fiber tow of Example 4 had a maximum draw ratio of 4.2 and a total draw ratio of 4.0. The obtained drawn filament was impregnated with an aqueous fiber treatment agent solution in the same manner as in Example 1, and the dehydrated drawn filament was cut to a fiber length of 5 mm to obtain a splittable conjugate fiber of Example 4. When melt spinning was performed, the polypropylene sample was collected under the above conditions (extruder temperature for the first component: 270°C) without a spin pack (spin pack), and the z-average molecular weight was measured. The result was 7.8 × 10 5 , the Q value was 3.3.

[0123] Example 5 Melt spinning was performed in the same manner as in Example 1, except that PE2 was used as the second component, to obtain an undrawn fiber tow. The obtained undrawn fiber tow was wet-drawn at a draw ratio of 3.3 at 90°C using a hot water bath filled with hot water at 90°C, and then heat-set at a draw ratio of 1.2 at 90°C in a hot water bath to obtain a drawn filament having a single fiber fineness of 1.56 dtex. The undrawn fiber tow of Example 5 had a maximum draw ratio of 4.6 and a total draw ratio of 4.0. The obtained drawn filament was impregnated with an aqueous fiber treatment agent solution in the same manner as in Example 1, and the dehydrated drawn filament was cut to a fiber length of 5 mm, to obtain a splittable conjugate fiber of Example 5.

[0124] Example 6 An undrawn fiber tow was produced in the same manner as in Example 1, except that PE2 was used as the second component and the spinning speed for withdrawing the molten resin extruded from the spinning nozzle was 720 m / min. The single fiber fineness of the fibers constituting the obtained undrawn fiber tow was 10 dtex. The obtained undrawn fiber tow was wet-drawn at a draw ratio of 2 at 90°C using a hot water bath filled with hot water at 90°C, and then wet-drawn at a draw ratio of 3.8 at 95°C in a hot water bath to obtain a drawn filament with a single fiber fineness of 1.57 dtex. The undrawn fiber tow of Example 6 had a maximum draw ratio of 8.0 and a total draw ratio of 7.6. The obtained drawn filaments were immersed in an aqueous solution of a fiber treatment agent containing a dodecylphenol-EO adduct as the main component, the concentration of the active ingredients (components other than water, such as surfactants) of the fiber treatment agent being adjusted to 3.0% by mass, and dehydrated so that the mass of the aqueous solution of the fiber treatment agent was 30% by mass and the amount of the fiber treatment agent component adhesion was 0.47% by mass, relative to the mass of the fiber. The dehydrated drawn filaments were cut to a fiber length of 5 mm, to obtain the splittable composite fiber of Example 6.

[0125] Example 7 Melt spinning was performed in the same manner as in Example 1 to obtain an undrawn fiber tow, except that PP3 was used as the first component, and the first component was melted at 270°C and extruded to obtain an undrawn fiber tow. The obtained undrawn fiber tow was wet-drawn at a draw ratio of 3.3 at 90°C using a hot water bath filled with hot water at 90°C, and then heat-set at a draw ratio of 1.2 at 90°C in a hot water bath to obtain a drawn filament having a single fiber fineness of 1.81 dtex. The undrawn fiber tow of Example 7 had a maximum draw ratio of 4.6 and a total draw ratio of 4.0. The obtained drawn filament was impregnated with an aqueous fiber treatment agent solution in the same manner as in Example 1, and the dehydrated drawn filament was cut to a fiber length of 5 mm to obtain the splittable conjugate fiber of Example 7. When melt spinning was performed, the polypropylene sample was collected under the above conditions (extruder temperature for the first component: 270°C) without a spin pack (spin pack), and the z-average molecular weight was measured. The result was 5.3 × 10 5 , the Q value was 2.6.

[0126] (Comparative Example 1) PP1 was used as the first component and PE1 was prepared as the second component. Next, the composite ratio (volume ratio) of the first and second components was adjusted to 50 / 50, and PP1 and PE1 were fed into separate extruders using a hollow 16-division composite spinning nozzle. The first component was melted at 270°C and the second component was melted at 270°C and extruded from the nozzle outlet. The nozzle temperature was set to 270°C, and the molten resin was withdrawn at a spinning speed of 1200 m / min. The position of the forced cooling device was adjusted so that the distance from the cooling start point was 85 mm. Then, cooling air was set to a temperature of 27°C and blown onto the molten resin to cool it.

[0127] The fibers constituting the obtained undrawn fiber tow had a single fiber fineness of 6 dtex, and the fiber cross section, as shown in Figure 1C, was gear-shaped with a hollow portion (hollowness 15%), resulting in a fiber with a cross section divided into 16 parts, with the first and second segments arranged radially and alternately on the fiber cross section. When this undrawn fiber tow was used as a sample and measured by wide-angle X-ray diffraction (XRD) as described above, the half-widths of the polypropylene were 0.882° for the diffraction peak at 2θ = 14.16, 0.830° for the diffraction peak at 2θ = 17.00, and 1.002° for the diffraction peak at 2θ = 18.56, and I PE / I was 0.092.

[0128] The obtained undrawn fiber tow was wet drawn at a draw ratio of 3.3 at 90°C using a hot water bath filled with hot water at 90°C, and then heat-set at a draw ratio of 1.2 in a hot water bath at 90°C to obtain a drawn filament having a single fiber fineness of 1.81 dtex. The undrawn fiber tow of Comparative Example 1 had a maximum draw ratio of 4.4 and a total draw ratio of 4.0. The obtained drawn filament was impregnated with an aqueous fiber treatment agent solution in the same manner as in Example 1, and the dehydrated drawn filament was cut to a fiber length of 5 mm to obtain a splittable conjugate fiber of Comparative Example 1.

[0129] (Comparative Example 2) Melt spinning was performed in the same manner as in Comparative Example 1, except that the conjugation ratio (volume ratio) of the first component to the second component was 60 / 40, to obtain an undrawn fiber tow. The obtained undrawn fiber tow was wet-drawn at a draw ratio of 3.3 at 90°C using a hot water bath filled with hot water at 90°C, and then heat-set at a draw ratio of 1.2 at 90°C in the hot water bath to obtain a drawn filament having a single fiber fineness of 1.83 dtex. The undrawn fiber tow of Comparative Example 2 had a maximum draw ratio of 4.5 and a total draw ratio of 4.0. The obtained drawn filament was impregnated with an aqueous fiber treatment agent solution in the same manner as in Comparative Example 1, and the dehydrated drawn filament was cut to a fiber length of 5 mm, to obtain a splittable conjugate fiber of Comparative Example 2.

[0130] (Comparative Example 3) Melt spinning was performed in the same manner as in Comparative Example 1, except that the spinning speed for withdrawing the molten resin extruded from the spinning nozzle was set to 1,440 m / min and the single fiber fineness of the fibers constituting the undrawn fiber tow was set to 5 dtex, thereby obtaining an undrawn fiber tow. The obtained undrawn fiber tow was wet-drawn at a draw ratio of 3.0 at 90°C using a hot water bath filled with hot water at 90°C, and then heat-set at a draw ratio of 1.2 at 90°C in the hot water bath to obtain a drawn filament with a single fiber fineness of 1.71 dtex. The undrawn fiber tow of Comparative Example 3 had a maximum draw ratio of 4.0 and a total draw ratio of 3.6. The obtained drawn filament was impregnated with an aqueous fiber treatment agent solution as in Comparative Example 1, and the dehydrated drawn filament was cut to a fiber length of 5 mm, thereby obtaining a splittable conjugate fiber of Comparative Example 3.

[0131] Comparative Example 4 An undrawn fiber tow was obtained by melt spinning in the same manner as in Comparative Example 1, except that PP2 was used as the first component, and the first component was melted at 270°C and extruded to obtain an undrawn fiber tow. The undrawn fiber tow was wet-drawn at a draw ratio of 3.7 at 90°C using a hot water bath filled with hot water at 90°C, and then heat-set at a draw ratio of 1.0 in the hot water bath at 90°C to obtain a drawn filament having a single fiber fineness of 1.89 dtex. The undrawn fiber tow of Comparative Example 4 had a maximum draw ratio of 3.8 and a total draw ratio of 3.7. The drawn filament was impregnated with an aqueous fiber treatment agent solution in the same manner as in Comparative Example 1, and the dehydrated drawn filament was cut to a fiber length of 5 mm to obtain a splittable conjugate fiber of Comparative Example 4.

[0132] (Comparative Example 5) Melt spinning was performed in the same manner as in Comparative Example 1, except that PE2 was used as the second component, to obtain an undrawn fiber tow. The obtained undrawn fiber tow was wet-drawn at a draw ratio of 3.3 at 90°C using a hot water bath filled with hot water at 90°C, and then heat-set at a draw ratio of 1.2 at 90°C in a hot water bath to obtain a drawn filament having a single fiber fineness of 1.6 dtex. The undrawn fiber tow of Comparative Example 5 had a maximum draw ratio of 4.5 and a total draw ratio of 4.0. The obtained drawn filament was impregnated with an aqueous fiber treatment agent solution in the same manner as in Comparative Example 1, and the dehydrated drawn filament was cut to a fiber length of 5 mm, to obtain a splittable conjugate fiber of Comparative Example 5.

[0133] The single fiber strength and elongation of the splittable conjugate fibers of the Examples and Comparative Examples were measured as described above, and the results are shown in Tables 2 and 4 below. The splittable conjugate fibers of the Examples and Comparative Examples were used as samples, and DSC measurements were carried out at a heating rate of 20°C per minute in accordance with JIS K 7121 (1987). The extrapolated melting initiation temperature (T im ) and the melting peak temperature (T pm ) and the heats of fusion of polypropylene and polyethylene were measured and evaluated as described above, and the results are shown in Tables 2 and 4 below. The dry heat dimensional change, dispersibility in water, and area shrinkage at 140°C of the splittable conjugate fibers of the Examples and Comparative Examples were measured as described above, and the results are shown in Tables 2 and 4 below. Table 1 below shows the spinning conditions of the Examples and the results of wide-angle X-ray diffraction (XRD) of the obtained undrawn fiber tows, and Table 3 shows the spinning conditions of the Comparative Examples and the results of wide-angle X-ray diffraction (XRD) of the obtained undrawn fiber tows.

[0134] [Table 1]

[0135] [Table 2]

[0136] [Table 3]

[0137] [Table 4]

[0138] As can be seen from the data in Tables 1 and 2, the splittable composite fibers of the examples have a Q value of 2.4 or more and 6 or less after spinning of the polypropylene contained in the first component, and the (T pm -T im ) was 7.3°C or higher, exhibiting excellent dispersibility in water and little thermal shrinkage when heated. This is presumably because the polypropylene contained in the first component constituting the splittable composite fiber had a z-average molecular weight of 500,000 or more and 1,500,000 or less before spinning and a Q value of 2.6 or more and 8 or less before spinning, and in addition, during melt spinning, the molten resin extruded from the spinning nozzle and spun out as an undrawn fiber tow (spun filament) was cooled immediately after spinning, specifically at a position 55 mm from the spinning nozzle, so that cooling began before the molten polypropylene was stretched, and the polypropylene in the obtained undrawn fiber had a low crystalline phase, i.e., a high amorphous phase and smectic crystals, and the crystalline states of the polypropylene and polyethylene were well balanced, so that both components were crystallized without excess or deficiency by drawing. Furthermore, since the polypropylene in the fiber contains many small crystallites after stretching, the extrapolated melting onset temperature of the polypropylene is observed to be low, and it is presumed that the uniform formation of a crystalline phase with small crystallites allows the crystals to quickly collide with each other even if shrinkage occurs in the amorphous portion, thereby contributing to suppressing thermal shrinkage.

[0139] On the other hand, as can be seen from the data in Tables 3 and 4, the (T pm -T imThe splittable conjugate fibers of Comparative Examples 1 to 5, which had a T of less than 7.3, had poor dispersibility in water and large shrinkage due to heat. These fibers contained a large amount of crystalline phase of polypropylene in the undrawn fibers, and when they were drawn in this state, the crystalline phase acted as a nucleus, causing the crystallites to grow large, and some of the crystalline phase was destroyed during drawing, resulting in an overdrawn state. Because the splittable conjugate fibers of Comparative Examples 1 to 5 contained such large-grown polypropylene and polypropylene in an overdrawn state, the extrapolated melting onset temperatures were observed to be high, and the temperature difference (T between the extrapolated melting onset temperature and the melting peak temperature) was large. pm -T im ) is estimated to be smaller.

[0140] In the splittable conjugate fibers of Comparative Examples 1 to 5, the cooling point is located farther from the spinning nozzle outlet during melt spinning. Specifically, the cooling point is located 30 mm farther than in Examples 1 to 7, which results in a longer stretching distance of the molten polypropylene compared to the manufacturing methods of Examples 1 to 7. This is presumably due to the increased crystallization of the polypropylene contained in the undrawn fiber tow and the increased crystallite size. It is believed that when undrawn fibers in this state are drawn, an amorphous phase is more likely to remain between the portions that have already become crystalline. Therefore, it is believed that the fibers of Comparative Examples 1 to 5 undergo greater thermal shrinkage when heated than the splittable conjugate fibers of the Examples, due not only to the presence of overdrawn polypropylene but also to the presence of amorphous portions remaining between the crystalline phases. [Industrial Applicability]

[0141] The splittable conjugate fiber of the present invention can be used for applications such as separators for various batteries, RO membrane supports, filter media for various filters, artificial leather, sanitary materials, and various wiping products such as wipers for people and / or objects. [Explanation of symbols]

[0142] 10. Split composite fiber 11 First Segment 12 Second Segment 13 Hollow part 20 Melt spinning equipment 21 Spin pack 22 Cooling point (upper end position of cooling air) 23 Molten Resin 24 Undrawn fiber tow 40 Hot air dryer 41 Conveyor Net 42 Polyester resin net (35 mesh x 25 mesh) 43 Wet Papermaking Web 44 Polyester resin net box 45 Hot air adjusted to a specified temperature

Claims

1. The fiber includes a plurality of first segments and a plurality of second segments, the first segments and the second segments being alternately arranged in the fiber cross section; the first segment is a single-type resin segment consisting of a first component, the second segment is a single-type resin segment consisting of a second component, The first component contains 50% by mass or more of polypropylene, the second component contains 50% by mass or more of high-density polyethylene; The z-average molecular weight (Mz) of the polypropylene after spinning is 450,000 or more and 1,000,000 or less, the Q value (ratio of weight average molecular weight (Mw) to number average molecular weight (Mn), Mw / Mn) of the polypropylene after spinning is 2.4 or more and 6 or less; The extrapolated melting onset temperature (T) of polypropylene measured at a heating rate of 20°C per minute in a differential scanning calorimetry (DSC) measurement method specified in JIS K 7121:1987. im ) and the melting peak temperature of polypropylene (T pm ) temperature difference (T pm -T im ) is 7.3°C or more.

2. The heat of fusion (ΔH ) of high-density polyethylene measured at a heating rate of 20°C per minute in a differential scanning calorimetry (DSC) measurement method specified in JIS K 7121:1987. PE 2. The splittable conjugate fiber according to claim 1, wherein the tensile strength is 125 mJ / mg or more.

3. 3. The splittable conjugate fiber according to claim 1 or 2, wherein the single fiber fineness is 2 dtex or less.

4. The splittable conjugate fiber according to any one of claims 1 to 3, wherein the dry heat dimensional change at 140 ° C. measured in accordance with JIS L 1015: 2021 8.15 dimensional change rate is 10% or less.

5. The splittable conjugate fiber according to any one of claims 1 to 4, having a fiber length of 2 mm or more and 20 mm or less, and an area shrinkage rate at 140°C of 20% or less.

6. A fiber structure comprising 10% by mass or more of the splittable conjugate fiber according to any one of claims 1 to 5.

7. A separator material comprising the fibrous structure of claim 6.

8. A method for producing a splittable conjugate fiber comprising a plurality of first segments and a plurality of second segments, the first segments and the second segments being alternately arranged in a fiber cross section, the first segment is a single-type resin segment consisting of a first component, the second segment is a single-type resin segment consisting of a second component, The first component contains 50% by mass or more of polypropylene having a z-average molecular weight (Mz) of 500,000 or more and 1,500,000 or less before spinning and a Q value (ratio of weight average molecular weight (Mw) to number average molecular weight (Mn) (Mw / Mn)) of 2.6 or more and 8 or less before spinning, the second component contains 50% by mass or more of high-density polyethylene; Attaching a split-type multi-component spinning nozzle to a melt spinning machine, the split-type multi-component spinning nozzle including a plurality of first segments and a plurality of second segments, the first segments and the second segments being alternately arranged in a fiber cross section; melting the first component and the second component, respectively, and supplying them to a first segment and a second segment of a split multi-component spinning nozzle attached to a melt spinning machine, respectively, to perform melt spinning; the molten resin discharged from the split-type composite spinning nozzle is taken up at a spinning speed of 550 m / min or more and 2500 m / min or less to obtain an undrawn fiber tow in which the first component and the second component are solidified and which has a single fiber fineness of 1 dtex or more and 15 dtex or less; The method includes drawing the undrawn fiber tow at a drawing temperature of 60°C or higher and 120°C or lower and at a total draw ratio of 2 times or higher and 10 times or lower to obtain a drawn filament, The undrawn fiber tow has a diffraction peak PP at 2θ=14.2±0.5°, which is a diffraction peak of polypropylene, in an X-ray diffraction chart obtained by measurement using an X-ray diffraction method (XRD). 1 , diffraction peak PP at 2θ=17±0.5° 2 , and a diffraction peak at 2θ=18.6±0.5° (PP 3 ), and a diffraction peak at 2θ=21.6±0.5°, which is a diffraction peak of high-density polyethylene (PE 1 ) satisfies the following (1) to (4): (1) Diffraction peak PP 1 The half width of the (2) Diffraction peak PP 2 The half width of the (3) Diffraction peak PP 3 The half width of the (4) Diffraction peak PE 1 Diffraction intensity (I PE ) and the diffraction peak PP 1 , diffraction peak PP 2 , diffraction peak PP 3 and diffraction peak PE 1 The ratio of the sum of the diffraction intensities (I) PE / I is 0.105 or more and less than 0.750

9. The method for producing a splittable conjugate fiber according to claim 8, wherein the drawing is wet drawing, and the drawing temperature is 80°C or higher and 100°C or lower.

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