JP7703835B2 - Method for producing polyphenylene sulfide composite fiber and wet-laid nonwoven fabric - Google Patents

Description

The present invention relates to polyphenylene sulfide composite fibers, their manufacturing method, and nonwoven fabrics.

Polyphenylene sulfide has high heat resistance, chemical resistance, electrical insulation, and flame retardancy, and these properties are utilized in a variety of applications, such as bag filters, papermaking canvas, electrical insulating paper, battery separators, and various diaphragms.

In recent years, however, there has been a growing interest in equipment that can be used in extreme environments and for aerospace applications, and the motors and batteries used in these devices are also required to have high environmental resistance. In this environment, there is an increasing demand for electrical insulating paper, battery separators, and various diaphragms that can be used in high-temperature environments, and nonwoven fabrics containing polyphenylene sulfide fibers, which have excellent heat and chemical resistance, are attracting attention.

However, in the case of nonwoven fabrics made only of stretched polyphenylene sulfide fibers, the adhesion between the fibers is low, so there is a problem in that it is not possible to obtain a nonwoven fabric with mechanical properties that can withstand practical use. Therefore, in order to solve the above problem, various proposals have been made regarding polyphenylene sulfide fibers and nonwoven fabrics containing them.

For example, a polyphenylene sulfide nonwoven fabric composed of stretched polyphenylene sulfide fibers and unstretched polyphenylene sulfide fibers has been proposed (see Patent Document 1). According to this technology, the unstretched polyphenylene sulfide acts as a binder during thermal bonding, making it possible to obtain a polyphenylene sulfide nonwoven fabric with high mechanical properties.

A low-thermal shrinkage binder fiber has also been proposed in which unstretched polyphenylene sulfide fibers are pre-heat-treated at a temperature below the crystallization temperature to suppress thermal shrinkage during the drying process (see Patent Document 2). This technology reduces the wrinkling and swelling of the nonwoven fabric that occurs during the drying process, and the resulting nonwoven fabric has excellent thermal dimensional stability.

Furthermore, a binder fiber with excellent adhesiveness and a fine fiber diameter has been proposed, which is made by stretching unstretched polyphenylene sulfide fibers in an ethylene glycol bath at 110°C to reduce the diameter (see Patent Document 3). This technology makes it possible to reduce the fiber diameter while maintaining low molecular chain orientation, and to obtain a homogeneous polyphenylene sulfide nonwoven fabric with a small fiber diameter CV value within the wet nonwoven fabric, even at a low basis weight.

Japanese Unexamined Patent Publication No. 7-189169 JP 2010-77544 A JP 2007-39840 A

However, in the technology disclosed in Patent Document 1, the unstretched polyphenylene sulfide fibers used as binder fibers contribute to adhesion, resulting in a nonwoven fabric with high mechanical properties. However, the extremely high thermal shrinkage rate of the unstretched polyphenylene sulfide fibers causes the fibers to shrink during the drying process, resulting in wrinkles and swelling in the nonwoven fabric. Furthermore, the presence of unstretched polyphenylene sulfide fibers with a larger fiber diameter than the skeletal fibers in the stretched polyphenylene sulfide fibers that serve as the skeletal fibers causes unevenness in the basis weight of the nonwoven fabric, impairing the homogeneity of the nonwoven fabric.

In addition, in the technology disclosed in Patent Document 2, the heat shrinkage during the drying process can be suppressed by heat treating the unstretched polyphenylene sulfide fibers, but there is an issue that the heat treatment causes thermal crystallization, which reduces the adhesiveness of the binder fibers and reduces the mechanical properties of the nonwoven fabric. Also, as in Patent Document 1, there is an issue that the fiber diameter is large because it is in an unstretched state, which causes unevenness in the basis weight within the nonwoven fabric and impairs the homogeneity of the nonwoven fabric.

In addition, in the technology disclosed in Patent Document 3, unstretched polyphenylene sulfide fibers are stretched in an ethylene glycol bath at 110°C to obtain fibers with a low degree of orientation and a small fiber diameter. However, since this method involves stretching without oriented crystallization, the thermal shrinkage rate of the obtained fibers is extremely high, and the fibers shrink during the drying process, causing wrinkles and swelling in the nonwoven fabric. In addition, since the fibers obtained by this method have low mechanical strength, there is also the issue that when these fibers are used as binder fibers, the mechanical properties of the nonwoven fabric are reduced.

Thus, polyphenylene sulfide fibers that are suitable as fibers for constituting nonwoven fabrics, have a small fiber diameter, high adhesiveness and mechanical strength, and have excellent dimensional stability, and nonwoven fabrics containing the same have not been reported to date.

As a result of further investigations, the inventors of the present invention found that when a polyphenylene sulfide resin consisting only of p-phenylene sulfide units is used, the dimensional stability is improved by crystallizing the resulting fiber through heat treatment, but at the same time, the problem of reduced thermal adhesion due to a reduction in the amorphous portion arises, and it is difficult to achieve both dimensional stability and thermal adhesion.

The inventors therefore conducted extensive research and discovered that by forming a core-sheath composite cross section with a copolymerized polyphenylene sulfide resin with low crystallinity in the sheath and a polyphenylene sulfide resin with excellent mechanical properties in the core, and then stretching and heat setting under specific conditions after spinning, it is possible to obtain polyphenylene sulfide composite fibers that are not only excellent in heat resistance and chemical resistance, but also have a small fiber diameter, high adhesiveness and mechanical strength, and excellent dimensional stability, and that nonwoven fabrics containing the same have good mechanical properties.

That is, the present invention is intended to solve the above-mentioned problems, and the polyphenylene sulfide conjugate fiber of the present invention is a core-sheath type conjugate fiber for papermaking, characterized in that component A is a polyphenylene sulfide having 97 mol % or more of repeating units composed of p-phenylene sulfide units, and component B is a copolymerized polyphenylene sulfide having 60 to 97 mol % of repeating units composed of p-phenylene sulfide units and 3 to 40 mol % of m-phenylene sulfide units, in which component A is disposed as a core and component B is disposed as a sheath , and the average fiber length is 1 to 100 mm.

According to a preferred embodiment of the polyphenylene sulfide composite fiber for papermaking of the present invention, the area ratio of the core in the cross section of the core-sheath type composite fiber is 75 to 95%.

According to a preferred embodiment of the polyphenylene sulfide conjugate fiber for papermaking of the present invention, the birefringence of the polyphenylene sulfide conjugate fiber is 0.18 to 0.40.

According to a preferred embodiment of the polyphenylene sulfide conjugate fiber for papermaking of the present invention, the polyphenylene sulfide conjugate fiber has an average fiber diameter of 25 μm or less, a strength of 2.0 cN/dtex or more, and an elongation of 50% or less.

The polyphenylene sulfide composite fiber of the present invention is produced by a method characterized in that the melt mass-flow rate MFR(A) of the component A used as a raw material is 50 to 250 g/10 min, and the melt mass-flow rate MFR(B) of the component B is 10 g/10 min or more higher than MFR(A).

The nonwoven fabric of the present invention is a wetlaid nonwoven fabric comprising any one of the polyphenylene sulfide conjugate fibers described above.

The polyphenylene sulfide composite fiber of the present invention is a core-sheath composite fiber in which polyphenylene sulfide, of which 97 mol % or more of the repeating units are p-phenylene sulfide units, is arranged as a core, and copolymerized polyphenylene sulfide, of which 60 to 97 mol % of the repeating units are p-phenylene sulfide units and 3 to 40 mol % are m-phenylene sulfide units, is arranged as a sheath. This not only provides polyphenylene sulfide composite fibers with excellent heat resistance and chemical resistance, but also a fine fiber diameter, high adhesiveness and mechanical strength, and excellent dimensional stability, and can provide nonwoven fabrics with good mechanical properties.

The polyphenylene sulfide composite fiber of the present invention is a core-sheath type composite fiber in which component A is polyphenylene sulfide (hereinafter sometimes abbreviated as polyphenylene sulfide (A)) in which 97 mol % or more of the repeating units are p-phenylene sulfide units, and component B is copolymerized polyphenylene sulfide (hereinafter sometimes abbreviated as copolymerized polyphenylene sulfide (B)) in which 60 to 97 mol % of the repeating units are p-phenylene sulfide units and 3 to 40 mol % are m-phenylene sulfide units, component A is arranged in the core and component B is arranged in the sheath, and the polyphenylene sulfide composite fiber is characterized in that it has an average fiber length of 1 to 100 mm. The polyphenylene sulfide composite fiber of the present invention will be described in detail below.

[Polyphenylene sulfide (A)]
It is important that the polyphenylene sulfide (A) used in the present invention is composed of p-phenylene sulfide units represented by structural formula (1) in an amount of 97 mol % or more of the repeating units. By making the p-phenylene sulfide units 97 mol % or more, preferably 98 mol % or more of the repeating units, a fiber is obtained that is excellent not only in chemical resistance and heat resistance but also in spinnability and mechanical properties. Examples of the remaining repeating units other than the p-phenylene sulfide units include aromatic sulfides such as triphenylene sulfide and biphenylene sulfide, or alkyl-substituted or halogen-substituted products thereof.

The polyphenylene sulfide (A) used in the present invention can be blended with other thermoplastic resins within the range that does not impair the effects of the present invention. Examples of thermoplastic resins include copolymerized polyphenylene sulfide in which m-phenylene sulfide units are copolymerized, and various thermoplastic resins such as polyetherimide, polyethersulfone, polysulfone, polyphenylene ether, polyester, polyarylate, polyamide, polyamideimide, polycarbonate, polyolefin, and polyetheretherketone. In order to fully express the properties of the polyphenylene sulfide (A) of the present invention, the mass ratio of the blendable thermoplastic resin is preferably 5 mass% or less based on the blended composition, more preferably 3 mass% or less, and even more preferably 1 mass% or less. Note that the blending referred to here means melt mixing and kneading of two or more resin components, and is different from the composite technology in which two or more resin components are arranged at any position in the fiber cross section during spinning.

The polyphenylene sulfide (A) used in the present invention may contain various additives, such as various metal oxides, inorganic substances such as kaolin and silica, pigments for coloring, delustering agents, flame retardants, antioxidants, ultraviolet absorbers, infrared absorbers, crystal nucleating agents, fluorescent brighteners, end group blocking agents, and compatibilizers, within the range that does not impair the effects of the present invention.

The polyphenylene sulfide (A) used in the present invention preferably has a melting point (hereinafter sometimes abbreviated as Tm(A)) of 260 to 300°C. By setting Tm(A) to preferably 260°C or higher, more preferably 270°C or higher, a polyphenylene sulfide composite fiber with excellent heat resistance is obtained. Furthermore, by setting Tm(A) to preferably 300°C or lower, more preferably 290°C or lower, it becomes easier to set the temperature conditions during fiber production. Note that Tm(A) refers to a value measured by the method described in section B of the Examples column.

[Copolymerized polyphenylene sulfide (B)]
It is important that the copolymerized polyphenylene sulfide (B) used in the present invention is composed of 60 to 97 mol % of the repeating units being p-phenylene sulfide units represented by structural formula (1). By making the p-phenylene sulfide units 60 mol % or more, preferably 70 mol % or more, a polyphenylene sulfide composite fiber having excellent heat resistance and chemical resistance is obtained. Furthermore, by making the p-phenylene sulfide units 97 mol % or less, preferably 95 mol % or less, the crystallinity of the copolymerized polyphenylene sulfide is reduced, and therefore a polyphenylene sulfide composite fiber having excellent adhesiveness is obtained.

It is also important that the copolymerized polyphenylene sulfide (B) used in the present invention consists of 3 to 40 mol% of the repeating units being m-phenylene sulfide units represented by structural formula (2). By making the m-phenylene sulfide units 3 mol% or more, preferably 5 mol% or more, the crystallinity of the copolymerized polyphenylene sulfide decreases, resulting in a polyphenylene sulfide composite fiber with excellent adhesiveness. Furthermore, by making the m-phenylene sulfide units 40 mol% or less, preferably 30 mol% or less, a polyphenylene sulfide composite fiber with excellent heat resistance and chemical resistance is obtained.

The molar fraction of p-phenylene sulfide units and m-phenylene sulfide units in the copolymerized polyphenylene sulfide (B) used in the present invention can be measured by infrared spectroscopy.

The copolymerization mode in the copolymerized polyphenylene sulfide (B) used in the present invention can be random copolymerization or block copolymerization, but random copolymerization is preferably used because it is easy to control the melting point.

The copolymerized polyphenylene sulfide (B) used in the present invention can contain other copolymerization components within a range that does not impair the effects of the present invention. Examples of other copolymerization components include aromatic sulfides such as triphenylene sulfide and biphenylene sulfide, or alkyl-substituted or halogen-substituted derivatives thereof. In order to fully express the properties of the copolymerized polyphenylene sulfide (B) of the present invention, the mass ratio of the other copolymerization components is preferably 5 mass% or less, more preferably 3 mass% or less, and even more preferably 1 mass% or less.

The copolymerized polyphenylene sulfide (B) used in the present invention can be blended with a thermoplastic resin within a range that does not impair the effects of the present invention. Examples of the thermoplastic resin include various thermoplastic resins such as polyphenylene sulfide, polyetherimide, polyethersulfone, polysulfone, polyphenylene ether, polyester, polyarylate, polyamide, polyamideimide, polycarbonate, polyolefin, and polyetheretherketone, whose repeating units are composed only of p-phenylene sulfide units. In order to fully express the properties of the copolymerized polyphenylene sulfide (B) of the present invention, the mass ratio of the blendable thermoplastic resin is preferably 5 mass% or less based on the blended composition, more preferably 3 mass% or less, and even more preferably 1 mass% or less. Note that the blending referred to here means melt mixing and kneading of two or more resin components, and is different from the composite technology in which two or more resin components are arranged at any position in the fiber cross section during spinning.

The copolymerized polyphenylene sulfide (B) used in the present invention can contain various additives, such as various metal oxides, inorganic substances such as kaolin and silica, pigments for coloring, delustering agents, flame retardants, antioxidants, ultraviolet absorbers, infrared absorbers, crystal nucleating agents, fluorescent brighteners, end group blocking agents, and compatibilizers, within the range that does not impair the effects of the present invention.

The copolymerized polyphenylene sulfide (B) used in the present invention preferably has a melting point (hereinafter sometimes abbreviated as Tm(B)) of 200 to 270°C. By setting Tm(B) to preferably 200°C or higher, more preferably 210°C or higher, a polyphenylene sulfide composite fiber with excellent heat resistance is obtained. Furthermore, by setting Tm(B) to preferably 270°C or lower, more preferably 260°C or lower, and even more preferably 255°C or lower, a polyphenylene sulfide composite fiber with high adhesiveness is obtained. Note that Tm(B) refers to a value measured by the method described in section B of the Examples column.

[Polyphenylene sulfide composite fiber]
It is important that the polyphenylene sulfide composite fiber of the present invention is a core-sheath type composite fiber having polyphenylene sulfide (A) as the core and copolymerized polyphenylene sulfide (B) as the sheath. By using the core-sheath type composite fiber, the polyphenylene sulfide (A) in the core becomes the skeleton, thereby exhibiting excellent mechanical strength, and the copolymerized polyphenylene sulfide (B) in the sheath becomes the thermal bonding part, thereby resulting in a polyphenylene sulfide composite fiber with high adhesiveness.

The cross-sectional shape of the polyphenylene sulfide composite fiber of the present invention is not limited in any way, and can be any shape such as a round cross section, a multi-lobed cross section such as a triangular cross section, a flat cross section, an S-shaped cross section, a cross-shaped cross section, a hollow cross section, etc. However, when it is necessary to disperse short fibers in a medium for papermaking applications, resin reinforcement applications, etc., a round cross section is preferred from the viewpoint of fiber dispersion.

The polyphenyl sulfide composite fiber of the present invention is a short fiber, and it is important that the average fiber length is 1 to 100 mm. By making the average fiber length 1 mm or more, preferably 3 mm or more, and more preferably 4 mm or more, the fibers are appropriately entangled during nonwoven fabric processing, and the obtained nonwoven fabric has excellent mechanical strength. Furthermore, by making the average fiber length 100 mm or less, preferably 60 mm or less, and more preferably 30 mm or less, it is possible to prevent unevenness in basis weight due to excessive entanglement during nonwoven fabric processing, and to obtain a homogeneous nonwoven fabric with few wrinkles. Note that the average fiber length in the present invention refers to a value measured by the method described in Section C of the Examples column.

In the polyphenylene sulfide composite fiber of the present invention, the area ratio of the core to the total cross-sectional area in the cross section of the core-sheath composite fiber is preferably 75 to 95%. By setting the area ratio of the core to be preferably 75% or more, more preferably 85% or more, the proportion of the core that becomes the skeleton increases, resulting in a polyphenylene sulfide composite fiber with excellent dimensional stability. Furthermore, by setting the area ratio of the core to be preferably 95% or less, more preferably 92% or less, it is possible to prevent exposure of the core components to the fiber surface and to produce a polyphenylene sulfide composite fiber with high adhesiveness. The area ratio of the core in the present invention refers to the value measured by the method described in Section E of the Examples column.

The polyphenylene sulfide composite fiber of the present invention preferably has a birefringence of 0.18 to 0.40. By making the birefringence preferably 0.18 or more, more preferably 0.22 or more, the fiber has a highly oriented molecular chain, resulting in a polyphenylene sulfide composite fiber with excellent mechanical properties. Furthermore, by making the birefringence preferably 0.40 or less, more preferably 0.30 or less, excessive oriented crystallization of the copolymerized polyphenylene sulfide (B) in the sheath can be prevented, resulting in a polyphenylene sulfide composite fiber with high adhesiveness. The birefringence of the fiber in the present invention refers to a value measured by the method described in section F of the Examples column.

The polyphenylene sulfide composite fiber of the present invention preferably has an average fiber diameter of 25 μm or less. By setting the average fiber diameter to preferably 25 μm or less, more preferably 15 μm or less, and even more preferably 10 μm or less, uneven cooling during melt spinning is less likely to occur, resulting in a polyphenylene sulfide composite fiber with excellent spinnability and good fiber diameter uniformity. The average fiber diameter in the present invention refers to the value measured by the method described in section D of the Examples.

The polyphenylene sulfide fiber of the present invention preferably has a strength of 2.0 cN/dtex or more. By making the strength preferably 2.0 cN/dtex or more, more preferably 3.0 cN/dtex or more, the handleability during nonwoven fabric processing is improved and the mechanical strength of the resulting nonwoven fabric is improved. In addition, there is no particular upper limit to the strength, but the upper limit that can be achieved industrially is about 7.0 cN/dtex.

The polyphenylene sulfide composite fiber of the present invention preferably has an elongation of 50% or less. By setting the elongation to preferably 50% or less, more preferably 40% or less, the fiber has a sufficiently high degree of molecular orientation, making it less susceptible to plastic deformation (stretching), and improving the handleability during nonwoven fabric processing. There is no particular lower limit to the elongation, but the industrially achievable lower limit is about 8%. Note that the strength and elongation in the present invention refer to values measured by the method described in section G of the Examples.

The polyphenylene sulfide composite fiber of the present invention is not only excellent in heat resistance and chemical resistance, but also has a small fiber diameter, high adhesiveness and mechanical strength, and excellent dimensional stability. Taking advantage of these characteristics, it can be used in a wide range of applications, including filter applications such as bag filters, chemical filters, food filters, chemical filters, oil filters, engine oil filters, and air purifier filters, paper applications such as electrical insulating paper, heat-resistant workwear applications such as firefighting suits, safety clothing, laboratory workwear, thermal clothing, flame-retardant clothing, high-visibility textiles, and felt for papermaking. It can be used for a variety of purposes, including paper, sewing thread, heat-resistant felt, release material, paper dryer canvas, battery separators, electrode separators, various diaphragms, heart patches, artificial blood vessels, artificial skin, printed circuit board substrates, copy rolling cleaners, ion exchange substrates, oil retention materials, heat insulation materials, cushioning materials, brushes, net conveyors, motor binding threads, and motor binder tapes, but is particularly preferably used in the form of a nonwoven fabric, and is suitable for use in paper applications such as electrical insulating paper, battery separators, and various diaphragm applications.

[Nonwoven fabric]
The nonwoven fabric of the present invention is a nonwoven fabric containing the polyphenylene sulfide composite fiber of the present invention. By containing the polyphenylene sulfide composite fiber of the present invention, the nonwoven fabric has good heat resistance and chemical resistance. In addition, since the nonwoven fabric has a structure in which a binder part with excellent adhesiveness is uniformly arranged around a skeleton part with excellent mechanical strength, the nonwoven fabric has little unevenness in basis weight and is homogeneous. Furthermore, since the unevenness in adhesion between fibers is reduced, the nonwoven fabric has good mechanical properties.

The nonwoven fabric of the present invention preferably has a basis weight of 3 to 100 g/ ^m2 . By setting the basis weight to preferably 3 g/m2 or ^more , more preferably 5 g/m2 or ^more , and even more preferably 10 g/m2 or ^more , breaks during the papermaking process are less likely to occur, improving processability. In addition, by setting the basis weight to preferably 100 g/m2 or ^less , more preferably 80 g/m2 or ^less , and even more preferably 60 g/m2 or ^less , a nonwoven fabric that is lightweight and has excellent flexibility is obtained. The basis weight of the nonwoven fabric in the present invention refers to a value measured by the method described in the example section H.

The nonwoven fabric of the present invention preferably has a tensile strength of 5 to 300 N/15 mm. By setting the tensile strength to preferably 5 N/15 mm or more, more preferably 10 N/15 mm or more, and even more preferably 15 N/15 mm or more, breakage during the papermaking process is less likely to occur. In addition, the nonwoven fabric is less likely to break even when tensile stress is applied during subsequent processes or during use. In addition, by setting the tensile strength to preferably 300 N/15 mm or less, more preferably 250 N/15 mm or less, and even more preferably 200 N/15 mm or less, the flexibility of the nonwoven fabric can be maintained, resulting in a nonwoven fabric with excellent assembly properties when used for diaphragms, etc. The tensile strength of the nonwoven fabric in the present invention refers to a value measured by the method described in section J of the Examples column.

The nonwoven fabric of the present invention preferably has a tear strength of 20 to 1000 gf. By setting the tear strength to preferably 20 gf or more, more preferably 50 gf or more, and even more preferably 100 gf or more, even if a crack occurs in the nonwoven fabric during processing, the crack is less likely to propagate throughout the entire nonwoven fabric, resulting in a nonwoven fabric that is less likely to tear. Furthermore, by setting the tear strength to preferably 1000 gf or less, more preferably 900 gf or less, and even more preferably 800 gf or less, problems are less likely to occur when cutting the nonwoven fabric. The tear strength of the nonwoven fabric in the present invention refers to a value measured by the method described in section K of the Examples column.

The nonwoven fabric of the present invention preferably has an air permeability of 0.01 cc/cm ² /s to 300 cc/cm ² /s. By setting the air permeability to preferably 0.01 cc/cm ² /s or more, more preferably 0.10 cc/cm ² /s or more, and even more preferably 1.0 cc/cm ² /s or more, the permeability of ions and the like is improved when used as a diaphragm or the like. Furthermore, by setting the air permeability to preferably 300 cc/cm ² /s or less, more preferably 250 cc/cm ² /s or less, and even more preferably 200 cc/cm ² /s or less, the nonwoven fabric has sufficient mechanical strength, and is less likely to break during the papermaking process, improving process passability. Furthermore, when used as a diaphragm or the like, tears and cracks are less likely to occur during assembly. The air permeability of the nonwoven fabric in the present invention refers to a value measured by the method described in item L of the Examples column.

The nonwoven fabric of the present invention preferably has a thickness of 5 μm to 300 μm. By making the thickness preferably 5 μm or more, more preferably 10 μm or more, and even more preferably 15 μm or more, breaks during the papermaking process are less likely to occur, improving process passability. Furthermore, by making the thickness preferably 300 μm or less, more preferably 250 μm or less, and even more preferably 200 μm or less, the permeability of ions and the like when used as a diaphragm or the like is improved. Furthermore, when mounted in a device as a diaphragm or the like, the volume occupied by the nonwoven fabric is reduced, making it possible to realize a smaller and lighter device. The thickness of the nonwoven fabric in the present invention refers to the value measured by the method described in item M in the Examples column.

[Production method of polyphenylene sulfide composite fiber]
The method for producing polyphenylene sulfide composite fiber of the present invention is characterized in that, for the polyphenylene sulfide (A) and the copolymerized polyphenylene sulfide (B) used as raw materials, the melt mass flow rate of the polyphenylene sulfide (A) (hereinafter sometimes abbreviated as MFR(A)) is 50 to 250 g/10 min, and the melt mass flow rate of the copolymerized polyphenylene sulfide (B) (hereinafter sometimes abbreviated as MFR(B)) is greater than MFR(A). The method for producing polyphenylene sulfide composite fiber of the present invention will be described in detail below.

In the method for producing the polyphenylene sulfide composite fiber of the present invention, it is preferable to use polyphenylene sulfide (A) having an MFR (A) of 50 to 250 g/10 min. By setting the MFR (A) to 50 g/10 min or more, preferably 80 g/10 min or more, more preferably 100 g/10 min or more, the flowability during melting is increased, improving spinnability and resulting in a polyphenylene sulfide composite fiber with excellent fiber diameter uniformity. In addition, by setting the MFR (A) to 250 g/10 min or less, preferably 230 g/10 min or less, more preferably 200 g/10 min or less, the polyphenylene sulfide composite fiber has good mechanical strength. The melt mass flow rate in the present invention refers to a value measured by the method described in the section A in the Examples.

In the method for producing the polyphenylene sulfide composite fiber of the present invention, it is preferable to use a copolymerized polyphenylene sulfide (B) having a MFR (B) greater than the MFR (A). The inventors have confirmed through their investigations that the use of a copolymerized polyphenylene sulfide (B) having a MFR (B) greater than the MFR (A) results in a polyphenylene sulfide composite fiber with excellent mechanical strength. This is because the MFR (B) is greater than the MFR (A), which can suppress the occurrence of inner and outer layer orientation differences due to the high molecular chain orientation on the fiber surface that occurs during the cooling process during spinning, and enables uniform drawing in the subsequent drawing process. Furthermore, as a result of extensive investigations, the inventors have found that the use of a copolymerized polyphenylene sulfide (B) having a MFR (B) greater than the MFR (A) results in a fiber with excellent adhesiveness. This is because excessive crystallization of the oriented fibers can be suppressed by reducing the spinning stress on the copolymerized polyphenylene sulfide (B) that forms the sheath during the fiber manufacturing process, such as spinning and drawing. As a result, the proportion of the amorphous portion that contributes to thermal adhesion can be increased, resulting in a fiber with excellent adhesiveness. Note that MFR (B) is preferably 5 g/10 min or more larger than MFR (A), and more preferably 10 g/10 min or more larger.

In the method for producing polyphenylene sulfide composite fiber of the present invention, it is preferable to use a copolymerized polyphenylene sulfide (B) having an MFR (B) of 60 to 350 g/10 min. By setting the MFR (B) to preferably 60 g/10 min or more, more preferably 80 g/10 min or more, and even more preferably 100 g/10 min or more, the fluidity during melting is increased, improving spinnability and resulting in a polyphenylene sulfide composite fiber with excellent fiber diameter uniformity. In addition, by setting the MFR (B) to preferably 350 g/10 min or less, more preferably 300 g/10 min or less, and even more preferably 280 g/10 min or less, a polyphenylene sulfide composite fiber with good mechanical strength is obtained.

The polyphenylene sulfide (A) used in the present invention can be produced, for example, by reacting an alkali metal sulfide such as sodium sulfide with p-dichlorobenzene in an organic amide solvent such as N-methyl-2-pyrrolidone to obtain polyphenylene sulfide.

As a method for producing the copolymerized polyphenylene sulfide (B) used in the present invention, for example, an alkali metal sulfide such as sodium sulfide is reacted with p-dichlorobenzene and m-dichlorobenzene in an organic amide solvent such as N-methyl-2-pyrrolidone to obtain polyphenylene sulfide.

The polyphenylene sulfide (A) and copolymerized polyphenylene sulfide (B) used in the present invention are preferably dried before being subjected to melt spinning in order to prevent water contamination and to remove oligomers, in order to improve spinnability. The drying conditions usually used are vacuum drying at 100 to 200°C for 1 to 24 hours.

In melt spinning, a melt spinning method using an extruder such as a pressure melter type, single-screw or twin-screw extruder type can be applied. The extruded polyphenylene sulfide is passed through a pipe, metered by a metering device such as a gear pump, and passed through a filter to remove foreign matter, and then guided to each spinneret. The respective polymers guided to the spinneret are joined in the spinneret with shape regulation so that polyphenylene sulfide (A) is arranged in the core and copolymerized polyphenylene sulfide (B) is arranged in the sheath, and are discharged from the nozzle hole as a core-sheath type composite fiber. At this time, the temperature from the polymer pipe to the spinneret (spinning temperature) is preferably 280°C or higher to increase fluidity, and 380°C or lower to suppress thermal decomposition of the polymer.

The spinneret used for extrusion preferably has a nozzle hole diameter D of 0.1 to 0.6 mm, and L/D, defined as the quotient of dividing the land length L of the nozzle hole (the length of the straight tube section that is the same as the nozzle hole diameter) by the hole diameter, is preferably 1 to 10.

The polyphenylene sulfide composite fiber discharged from the nozzle hole is cooled and solidified by blowing cooling air (air). The temperature of the cooling air can be determined in consideration of the balance with the cooling air speed from the viewpoint of cooling efficiency, but a preferable embodiment is 30°C or less. By setting the temperature of the cooling air to preferably 30°C or less, the solidification behavior due to cooling is stable, resulting in polyphenylene sulfide composite fiber with high fiber diameter uniformity.

The cooling air is preferably blown in a direction approximately perpendicular to the undrawn fibers discharged from the spinneret. In this case, the speed of the cooling air is preferably 10 m/min or more from the viewpoint of cooling efficiency and uniformity of fiber diameter, and is preferably 100 m/min or less from the viewpoint of spinning stability.

The cooled and solidified undrawn fibers are taken up by rollers (godet rollers) rotating at a constant speed. The take-up speed is preferably 300 m/min or more to improve fiber diameter uniformity and productivity, and 2000 m/min or less to prevent yarn breakage.

The undrawn fiber thus obtained is once wound up or taken up and then continuously subjected to the drawing process. Drawing is performed by running the fiber over a heated first roller or a heating device provided between the first and second rollers, for example, a heating bath or a hot plate. The drawing conditions are determined by the mechanical properties of the undrawn fiber obtained, but the drawing temperature is determined by the temperature of the heated first roller or the heating device provided between the first and second rollers, and the draw ratio is determined by the ratio of the peripheral speeds of the first and second rollers.

The stretching ratio in the stretching process is preferably 2.0 times or more. By setting the stretching ratio at 2.0 times or more, and more preferably at 2.5 times or more, the fiber will have highly oriented molecular chains, resulting in a polyphenylene sulfide composite fiber with excellent mechanical properties. There is no particular upper limit to the stretching ratio, but the upper limit that can be achieved industrially is about 8.0 times.

The temperature of the heated first roller or heating device in the drawing process is preferably 80 to 130°C. By setting the temperature of the first roller or heating device to 80°C or higher, the drawing point is fixed and stable drawing is possible. Furthermore, by setting the temperature of the first roller or heating device to 130°C or lower, yarn breakage during the process can be suppressed and process passability is improved. Furthermore, from the viewpoint of fixing the drawing point, the temperature of the second roller is preferably set to the temperature of the heated first roller or heating device + 20°C or lower.

Furthermore, after passing through the second roller, the drawn fiber may be heated and heat-set by a heated third roller or a heating device provided between the second roller and the third roller. The heat-set temperature when heat-setting is preferably 210°C or less. By setting the heat-set temperature to preferably 210°C or less, more preferably 190°C or less, and even more preferably 150°C or less, excessive thermal crystallization is suppressed, resulting in a polyphenylene sulfide composite fiber with high adhesiveness.

It is preferable to apply a papermaking dispersant or the like to the polyphenylene sulfide composite fiber of the present invention as necessary. The dispersant is usually applied in a tow state using a kiss roller or guide oiling. The papermaking dispersant attachment rate is preferably 1 to 6 mass% based on the fiber mass. By setting the dispersant attachment rate to preferably 1 mass% or more, more preferably 2 mass% or more, the dispersibility of the fibers in the dispersion liquid that is the precursor of the nonwoven fabric is improved, resulting in a homogeneous nonwoven fabric. In addition, by setting the dispersant attachment rate to preferably 6 mass% or less, more preferably 5 mass% or less, the process passability during nonwoven fabric processing is improved.

Then, the polyphenylene sulfide composite fiber of the present invention may be subjected to crimping using a crimper and heat fixing using a setter. By crimping, the fibers become entangled with each other, increasing the adhesion area between the fibers, resulting in a nonwoven fabric with excellent mechanical strength.

The number of crimps in the above crimping is preferably 2 to 15 crimps/25 mm. By setting the number of crimps to 2 crimps or more, the fibers are more likely to intertwine with each other, resulting in a nonwoven fabric with excellent mechanical strength. Furthermore, by setting the number of crimps to 15 crimps or less, the dispersibility of the fibers in the dispersion liquid is improved, resulting in a homogeneous nonwoven fabric.

The heat setting temperature by the setter is preferably 80°C or higher and 100°C or lower. By setting the heat setting temperature at 80°C or higher, it is possible to fix the crimped shape, and by setting the heat setting temperature at 100°C or lower, it is possible to prevent the adhesiveness from being impaired by excessive thermal crystallization.

The resulting polyphenylene sulfide composite fiber is then cut to a predetermined length with a cutter to obtain cut fibers.

[Method of manufacturing nonwoven fabric]
The method for producing the nonwoven fabric of the present invention will be described in detail below.

The papermaking solution for producing the nonwoven fabric of the present invention is obtained by dispersing the cut fibers obtained as described above in water, either alone or in a mixture with cut fibers of other fibers in any ratio. Here, from the viewpoint of obtaining paper with excellent heat resistance, it is preferable to use cut fibers of heat-resistant fibers such as polyphenylene sulfone fibers, meta-aramid fibers, and fluorine fibers as the cut fibers of other fibers. A nonwoven fabric can be obtained by supplying this papermaking solution to a papermaking machine.

The fiber concentration of the papermaking solution is preferably 0.05 to 5% by mass. If the fiber concentration is less than 0.05% by mass, production efficiency decreases and the load on the dewatering process increases. Conversely, if the fiber concentration exceeds 5% by mass, the fiber dispersion state deteriorates, making it difficult to obtain a uniform nonwoven fabric.

The papermaking solution may contain dispersants and oils made of cationic, anionic, or nonionic surfactants to improve water dispersibility, thickeners to increase the viscosity of the dispersion and prevent the papermaking solution from flocculating, and defoamers to suppress the generation of bubbles.

The papermaking solution prepared as described above is used to make paper using a papermaking machine such as a cylinder, fourdrinier, or inclined wire papermaking machine, or a hand-made papermaking machine, and the paper is then dried using a Yankee dryer or rotary dryer to remove moisture and temporarily bond the fibers together, resulting in a dry web.

The drying temperature is preferably 80 to 150°C. Preferably, a temperature of 80°C or higher allows sufficient temporary adhesion between the fibers, resulting in a dry web with sufficient mechanical properties. A drying temperature of 150°C or lower causes excessive crystallization, resulting in a loss of adhesion in the subsequent thermocompression bonding process, and as a result, the mechanical properties of the resulting nonwoven fabric are reduced. Here, the drying temperature refers to the maximum processing temperature (ambient temperature) during drying in the papermaking process.

By heat-pressing the dry web obtained as described above, the polyphenylene sulfide composite fibers of the present invention are bonded to each other, resulting in a nonwoven fabric with excellent mechanical strength. Any means can be used for heat-pressing, but examples include heat pressing with a flat plate or a calendar. Of these, a calendar that allows continuous processing is preferred. The calendar rolls that can be used include metal-metal rolls, metal-paper rolls, and metal-rubber rolls.

The heat-compression temperature is preferably 170 to 250°C. By setting the heat-compression temperature at 170°C or higher, the fibers are bonded together by the adhesion of the copolymerized polyphenylene sulfide (B) in the sheath, resulting in a wet-laid nonwoven fabric with excellent mechanical strength. In addition, by setting the heat-compression temperature at 250°C or lower, it is possible to suppress the thermal shrinkage of the nonwoven fabric during heat-compression.

When calendar processing is used as the heat-pressing means, the linear pressure is preferably 98 to 7000 N/cm. By setting the linear pressure to 98 N/cm or more, the fibers are firmly pressed together via the copolymerized polyphenylene sulfide (B) of the sheath, resulting in a wet-laid nonwoven fabric with excellent mechanical strength. In addition, by setting the linear pressure to 7000 N/cm or less, it is possible to suppress the decrease in tear strength and air permeability caused by the constituent fibers being excessively crushed and becoming film-like. The process speed is preferably 1 to 30 m/min. By setting the process speed to 1 m/min or more, it is possible to suppress the decrease in mechanical strength of the nonwoven fabric caused by excessive thermal crystallization, and to obtain good work efficiency. In addition, by setting the process speed to 30 m/min or less, heat is also conducted to the fibers inside the nonwoven fabric, and the effect of thermal fusion of the fibers can be obtained.

The polyphenylene sulfide composite fiber and the nonwoven fabric containing the same of the present invention will be described in more detail below with reference to the following examples. Note that the respective property values in the examples were determined by the following methods.

A. Melt Mass Flow Rate:
The melt mass flow rate MFR(A) of the polyphenylene sulfide (A) and the melt mass flow rate MFR(B) of the copolymerized polyphenylene sulfide (B) were measured three times per level using a melt indexer (F-F01 manufactured by Toyo Seiki Seisaku-sho, Ltd.) in accordance with "Chapter 8, Method A: Mass Measurement Method" of JIS K7210-1:2014 at a temperature of 315°C and a load of 5.0 kg, and the arithmetic average values were determined to be MFR(A) (g/10 min) and MFR(B) (g/10 min).

B. Melting Point The melting point Tm(A) of the polyphenylene sulfide (A) and the melting point Tm(B) of the copolymerized polyphenylene sulfide (B) were measured three times per level using a differential scanning calorimeter (DSC Q2000 manufactured by TA Instruments) by heating in the range from 25°C to 320°C at a rate of 10°C/min, and the arithmetic average value of the peak top temperature of the largest endothermic peak in the obtained DSC curve was calculated, which was designated as Tm(A) (°C) and Tm(B) (°C).

C. Average Fiber Length:
One hundred polyphenylene sulfide composite fibers were randomly selected, and the distance between both ends of the fibers was measured under conditions of an objective lens of 10x and an eyepiece lens of 10x while the fibers were stretched to a degree that did not cause plastic deformation, and the arithmetic average value was calculated to obtain the average fiber length (mm).

D. Average Fiber Diameter:
One hundred polyphenylene sulfide composite fibers were randomly selected, and the fiber diameters (μm) were measured from the side of the fibers using an optical microscope (BX53M manufactured by Olympus Corporation) with an objective lens of 40x and an eyepiece lens of 10x. The arithmetic average value was calculated, and this was regarded as the average fiber diameter (μm).

E. Core area ratio:
One hundred polyphenylene sulfide composite fibers were randomly selected, and photographs of the cross sections of the fibers were taken at 1000 times magnification using a scanning electron microscope (VE-7800 manufactured by Keyence Corporation). The total cross-sectional area and the core area of each fiber were measured, and the arithmetic average values were calculated. The core area ratio was calculated by dividing the core area by the total cross-sectional area.

F. Fiber Birefringence:
Ten polyphenylene sulfide composite fibers were randomly selected, and the retardation and optical path length of each single fiber were measured using an optical microscope (BX53M manufactured by Olympus Corporation) to determine the birefringence. The arithmetic average value was calculated to determine the birefringence.

G. Strength, elongation:
The strength and elongation of the polyphenylene sulfide composite fiber were measured five times per level using a Tensilon (UTM-III-100 manufactured by Orientec Co., Ltd.) in accordance with "Chapter 8.5 Tensile strength and elongation" of JIS L1013:2010 under conditions of a sample length of 200 mm and a tensile speed of 200 mm/min, and the arithmetic average values were calculated to determine the strength (cN/dtex) and elongation (%).

H. Nonwoven fabric basis weight:
Three sample pieces of 5 cm x 5 cm were taken from the nonwoven fabric obtained in the examples, and the mass (g) of each was measured under standard conditions, and the average value was expressed as the mass per ^m2 (g/ ^m2 ).

I. Number of wrinkles in nonwoven fabric:
Using an optical microscope (Olympus BX53M) with an objective lens of 40x and an eyepiece of 10x, the number of wrinkles (/100 ^cm2 ) in the dry webs obtained in the examples was confirmed. The wrinkles in the dry web here refer to linear wrinkles that occur at locations in the dry web where the thermal dimensional change rate differs locally due to poor fiber dispersion, etc. The number of wrinkles with a total length of 0.1 cm or more was confirmed by measuring three sheets per level, and the arithmetic average value was calculated to be the number of wrinkles in the dry web.

J. Tensile strength of nonwoven fabric:
Measurements were carried out for the nonwoven fabric obtained in the examples in both the warp (the direction in which the nonwoven fabric is manufactured) and weft (the width direction in which the nonwoven fabric is manufactured). Using a Tensilon (UTM-III-100 manufactured by Orientec Co., Ltd.), the maximum point load was measured under the conditions of a sample width of 15 mm, an initial length of 20 mm, and a tensile speed of 20 mm/min. Five measurements were made for each warp and weft direction per level, and the arithmetic average value was calculated to obtain the tensile strength (N/15 mm) of the nonwoven fabric.

K. Tear strength of nonwoven fabric:
From the nonwoven fabric obtained in the examples, a test piece of 63 mm x 100 mm was measured in each of the warp (the direction of progress in the nonwoven fabric manufacturing process) and weft (the width direction in the nonwoven fabric manufacturing process). Using an Elmendorf type tear tester (manufactured by Yasuda Seiki Seisakusho), a 20 mm cut was made at a right angle at the center of both grips of the test piece, and the remaining 43 mm was torn in the warp and weft directions to measure the strength. Five measurements were made in each of the warp and weft directions for each level, and the arithmetic average value was calculated to determine the tear strength (gf) of the nonwoven fabric.

L. Breathability of nonwoven fabric:
According to the Frazier method of JIS L1913 (2010), ten sample pieces of 5 cm x 5 cm were taken from the nonwoven fabric obtained in the examples, and the air permeability of the ten test pieces was measured at a test pressure of 125 Pa using an air permeability tester (FX3300 manufactured by Textest Co., Ltd.), and the arithmetic average value was taken as the air permeability (cc/ ^cm2 /s).

M. Thickness of nonwoven fabric:
In accordance with JIS P8118 (2014), ten sample pieces measuring 5 cm x 5 cm were taken from the nonwoven fabric obtained in the examples, and the thickness of each of the ten test pieces was measured using a micrometer (manufactured by Mitutoyo Corporation), and the arithmetic average value was taken as the thickness (μm) of the nonwoven fabric.

Example 1
Polyphenylene sulfide (A) having a repeating unit of 100 mol% p-phenylene sulfide unit, MFR (A) of 160 g/10 min, and Tm (A) of 284° C., and copolymerized polyphenylene sulfide (B) having a repeating unit of 75 mol% p-phenylene sulfide unit and 25 mol% m-phenylene sulfide unit, MFR (B) of 200 g/10 min, and Tm (B) of 243° C. were vacuum dried at 150° C. for 12 hours, and then melt spun at a spinning temperature of 320° C. In the melt spinning, polyphenylene sulfide (A) and copolymerized polyphenylene sulfide (B) were melt extruded by a twin-screw extruder, and supplied to a spinning pack while being metered by a gear pump. Thereafter, polyphenylene sulfide (A) was arranged in the core portion and copolymerized polyphenylene sulfide (B) was arranged in the sheath portion in the spinneret, and the shapes of the two were regulated so that the area ratio of the core portion in the cross section of the composite fiber was 75%, and the two were joined together, and spun through a spinneret having 36 round discharge holes.

The polymer discharged from the spinneret was passed through a 50 mm heat retention area, and then air-cooled over 1.0 m using a uniflow type cooling device at a temperature of 25°C and a wind speed of 18 m/min. An oil agent was then applied, and the 36 filaments were wound on a winder via the first and second godet rollers at 1000 m/min to obtain undrawn fibers.

The undrawn fibers were taken up by a feed roller attached to a nip roller, and tension was applied to the undrawn fibers between the first roller and the first roller, and then the fibers were heated and drawn six times around the first and second rollers heated to 90°C and 100°C, respectively. The fibers were then heated and drawn six times around the third roller heated to 120°C, and heat-set. The draw ratio was 3.85 times, and the fibers were taken up by a non-heated roller with a peripheral speed of 400 m/min after the third roller, and 2% by mass of dispersant was applied to the fiber weight using guide oiling. Next, the fibers were crimped to 8 peaks/25 mm using a crimper, and then heat-set at 70°C for 1 minute using a setter. The fibers obtained were then cut with a cutter to obtain polyphenylene sulfide composite fibers with an average fiber length of 6 mm.

The obtained polyphenylene sulfide composite fiber was dispersed in water to prepare an aqueous dispersion with a fiber concentration of 1% by mass, and then a wet paper (10 cm square) with a basis weight of 50 g/ ^m2 was obtained using a hand-made papermaking machine (Kumagaya Riki Kogyo Co., Ltd. square sheet machine with automatic couching). The web obtained by dehydrating with a roller was placed in a dryer (Kumagaya Riki Kogyo Co., Ltd. KRK rotary dryer standard type) and treated at a temperature of 140°C for a treatment time of about 2.5 minutes per cycle to obtain a dry web. Subsequently, the other side was thermocompressed at an iron roll surface temperature of 220°C, a linear pressure of 490 N/cm, and a roll rotation speed of 3 m/min to obtain a nonwoven fabric.

[Examples 2 to 4, Comparative Example 1]
Except for changing the copolymerization ratio of the copolymerized polyphenylene sulfide (B) in the sheath and changing the draw ratio in the drawing step to 3.61 times, polyphenylene sulfide conjugated fibers and nonwoven fabrics were obtained in the same manner as in Example 1. The evaluation results of the obtained polyphenylene sulfide conjugated fibers and nonwoven fabrics are shown in Table 1.

[Example 5, Example 6, Comparative Example 2]
Except for changing the average fiber length of the polyphenylene sulfide composite fibers, polyphenylene sulfide composite fibers and nonwoven fabrics were obtained in the same manner as in Example 1. The evaluation results of the obtained polyphenylene sulfide composite fibers and nonwoven fabrics are shown in Table 1.

Example 7
Except for changing the area ratio of polyphenylene sulfide (A) disposed in the core, polyphenylene sulfide composite fibers and nonwoven fabrics were obtained in the same manner as in Example 1. The evaluation results of the obtained polyphenylene sulfide composite fibers and nonwoven fabrics are shown in Table 1.

Example 8
Polyphenylene sulfide composite fibers and nonwoven fabrics were obtained in the same manner as in Example 1, except that the draw ratio in the drawing step was 3.31 times and the fibers were taken up with unheated rollers without being heat-set. The evaluation results of the obtained polyphenylene sulfide composite fibers and nonwoven fabrics are shown in Table 1.

The polyphenylene sulfide composite fibers obtained in Examples 1 to 7 had a small number of wrinkles in the nonwoven fabric, and the nonwoven fabric also showed good tensile strength and tear strength, making them suitable polyphenylene sulfide fibers for use in nonwoven fabrics with excellent mechanical properties.

On the other hand, the polyphenylene sulfide composite fiber obtained in Comparative Example 1 had a low copolymerization ratio in the sheath and poor adhesion, which caused fibers to slip out of the fibers, resulting in low tensile strength and tear strength of the nonwoven fabric. In addition, the polyphenylene sulfide composite fiber obtained in Comparative Example 2 had a large average fiber length, which caused excessive entanglement of the fibers during processing into the nonwoven fabric, resulting in a large number of wrinkles in the resulting dry web. As described above, neither fiber was suitable as a fiber for forming a nonwoven fabric.

The polyphenylene sulfide composite fiber of the present invention is a polyphenylene sulfide composite fiber that is not only excellent in heat resistance and chemical resistance, but also has a small fiber diameter, high adhesiveness and mechanical strength, and excellent dimensional stability. By using the polyphenylene sulfide composite fiber of the present invention in the form of a nonwoven fabric, a nonwoven fabric with few drying wrinkles and good mechanical properties can be obtained, and the nonwoven fabric can be suitably used for paper applications such as electrical insulating paper, battery separators, and various diaphragms.

Claims (5)

A wetlaid nonwoven fabric comprising polyphenylene sulfide composite fibers for papermaking having an average fiber length of 1 to 100 mm, the nonwoven fabric being a core-sheath type composite fiber in which component A is a polyphenylene sulfide having 97 mol % or more of repeating units composed of p-phenylene sulfide units and component B is a copolymerized polyphenylene sulfide having 60 to 97 mol % of repeating units composed of p-phenylene sulfide units and 3 to 40 mol % of m-phenylene sulfide units, the component A being a core and the component B being a sheath, the polyphenylene sulfide composite fiber having an average fiber length of 1 to 100 mm , and the nonwoven fabric having a tear strength of 100 gf or more and 1,000 gf or less. 2. The wetlaid nonwoven fabric according to claim 1, wherein the area ratio of the core in the cross section of the polyphenylene sulfide composite fiber is 75 to 95%. 3. The wetlaid nonwoven fabric according to claim 1, wherein the polyphenylene sulfide composite fiber has a birefringence of 0.18 to 0.40. The wetlaid nonwoven fabric according to any one of claims 1 to 3, characterized in that the polyphenylene sulfide composite fiber has an average fiber diameter of 25 µm or less, a strength of 2.0 cN/dtex or more, and an elongation of 50% or less. A method for producing a polyphenylene sulfide composite fiber for papermaking, comprising: component A being polyphenylene sulfide in which 97 mol % or more of the repeating units are p-phenylene sulfide units; component B being a copolymerized polyphenylene sulfide in which 60 to 97 mol % of the repeating units are p-phenylene sulfide units and 3 to 40 mol % are m-phenylene sulfide units; component A being arranged as a core portion and component B being arranged as a sheath portion; and the method for producing a polyphenylene sulfide composite fiber for papermaking, the polyphenylene sulfide composite fiber being a core-sheath type composite fiber having an average fiber length of 1 to 100 mm; wherein component A used as a raw material has a melt mass-flow rate MFR(A) of 50 to 250 g/10 min, and component B has a melt mass-flow rate MFR(B) that is 10 g/10 min or more greater than MFR(A).