JP7767967B2 - Polyphenylene sulfide fiber - Google Patents

Description

The present invention relates to polyphenylene sulfide fibers suitable for wet-laid nonwoven fabric applications.

Polyphenylene sulfide (hereinafter sometimes abbreviated as PPS) resin 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 insulation, and battery separators.

Nonwoven fabrics that require high density and thin film are used for electrical insulating paper and battery separator applications. In recent years, demand has been growing for electrical insulating materials and battery separators that can be used in high-temperature environments, and PPS nonwoven fabrics with excellent heat and chemical resistance have attracted attention. Furthermore, as battery materials and other products become smaller and lighter, there is a growing demand for thinner films and lower basis weight.

However, while various efforts have been made to reduce the thickness and basis weight of PPS nonwoven fabrics, problems remain, such as reduced mechanical strength and reduced uniformity. Therefore, various proposals have been made regarding PPS fibers to solve these problems.

For example, a PPS wet-laid nonwoven fabric composed of stretched PPS fibers and unstretched PPS fibers as binder fibers for stretching and thermocompression bonding has been proposed (see Patent Document 1).

In addition, a melt-blown nonwoven fabric with an average fiber diameter of 8 to 35 μm has been proposed using the melt-blowing method. This proposal produces a thin melt-blown nonwoven fabric with excellent productivity. (See Patent Document 2.)

Furthermore, a method has been proposed for obtaining PPS fibers with fineness and excellent mechanical strength by electrospinning PPS with a specific Na content. This proposal has indeed produced ultrafine fibers of 1 μm (approximately 0.01 dtex) or less and high strength fibers of 5.5 cN/dtex or more, resulting in thin PPS nonwoven fabrics. (See Patent Document 3.)

Japanese Unexamined Patent Publication No. 7-189169 Japanese Patent Application Publication No. 11-315462 JP 2015-67919 A

However, in Patent Document 1, the PPS fibers have a large fiber diameter, making it difficult to produce a thin wetlaid nonwoven sheet. In addition, there are issues with the poor dispersibility of the cut fibers during papermaking, making it impossible to produce a sufficiently thin sheet or achieve a low basis weight.

Furthermore, the nonwoven fabric in Patent Document 2 is made using a melt-blown method, and the crystallinity of the fibers that make it up is low, so it has poor heat resistance and dimensional stability, and does not achieve the mechanical strength of a nonwoven fabric.

Furthermore, Patent Document 3 uses a special spinning method called electrospinning, which results in inferior fiber productivity compared to spinning methods such as melt spinning.

The problem that this invention aims to solve is to provide a PPS fiber that enables the thinning of nonwoven fabrics and the improvement of their mechanical strength without compromising fiber productivity or nonwoven fabric productivity.

We have discovered that the following are important factors in providing PPS fibers that can be made thinner and have improved mechanical strength without sacrificing fiber productivity or nonwoven fabric productivity.

That is, the present invention is as follows.
1. A polyphenylene sulfide fiber in the form of a drawn yarn having a polymer chain end terminated with a COOH group, a melt flow rate (MFR) at 315°C of 300 to 800 g/10 min, and a single fiber fineness of 0.30 to 1.20 dtex.
Furthermore, preferred embodiments of the present invention are as follows.
2. The polyphenylene sulfide fiber according to 1 above, wherein the drawn yarn has a strength of 5.0 to 7.0 cN/dtex.
3. The polyphenylene sulfide fiber according to 1 or 2 above, wherein the sum of the crystallinity and rigid amorphous content of the drawn yarn is 50 to 100%.
4. A wetlaid nonwoven fabric containing at least 10% or more of the polyphenylene sulfide fiber described in any one of 1 to 3 above.
5. The wetlaid nonwoven fabric according to 4 above, further comprising undrawn yarns of polyphenylene sulfide fibers as binder fibers, the undrawn yarns having a single fiber fineness of 0.9 to 3.0 dtex.
6. The wetlaid nonwoven fabric according to 4 or 5 above, wherein the dry heat shrinkage of the undrawn yarn is 60% or less.
7. The wetlaid nonwoven fabric according to any one of 4 to 6 above, wherein the crystallinity of the undrawn yarn is 20% or less.

The PPS fiber of the present invention enables thinner films and improved mechanical strength without compromising fiber productivity or nonwoven fabric productivity.

The present invention is described in detail below, along with preferred embodiments.

The PPS used in this invention refers to a polymer containing phenylene sulfide units such as p-phenylene sulfide units and m-phenylene sulfide units represented by the following structural formula (I) as repeating units.

PPS may be a homopolymer of only p-phenylene sulfide units or m-phenylene sulfide units, or a copolymer containing both p-phenylene sulfide units and m-phenylene sulfide units. It may also be a copolymer or mixture with other aromatic sulfides, as long as the effects of the present invention are not impaired.

From the standpoint of heat resistance and durability, the PPS resin used in the present invention is preferably a PPS containing at least 70 mol, and more preferably at least 90 mol, of p-phenylene sulfide units, which are repeating units represented by structural formula (I) above. In this case, it is preferable that other copolymerization components in the PPS resin are m-phenylene sulfide units or other aromatic sulfide units.

PPS generally contains metals such as Na, Ca, Mg, and Fe, and the polymer chain ends contain these metals as well as end groups derived from the polymerization raw materials and catalysts, such as Cl groups when p-dichlorobenzene is used as the polymerization raw material, or N-alkylpyridine groups or pyridine groups, which are decomposition products of N-methylpyrrolidone when N-methylpyrrolidone is used as the polymerization catalyst. Because the present invention achieves a sufficient degree of polymerization even at low viscosity, it is important to use PPS with Na at the polymer chain ends, followed by acid washing with an inorganic acid such as hydrochloric acid or an organic acid such as acetic acid, resulting in COOH-terminated PPS. PPS with COOH-terminated polymer chain ends has higher kinetics during melting compared to PPS with other terminals, even at the same degree of polymerization. Therefore, controlling the solidification behavior during extrusion improves spinning operability. Furthermore, PPS with COOH-terminated polymer chain ends has a faster crystallization rate than PPS with other ends, requiring less energy for molding operations below the melting point and near the glass transition point, and can be stretched at a high magnification, enabling the resulting fibers to be stronger and finer. In particular, the COOH-terminated PPS used in the present invention is preferably washed with a cleaning solution adjusted to a pH of 5.0 or less with acetic acid, and has a sodium content of 200 ppm or less and an ash content of 0.05 or less.

The sodium content and ash content in this invention were measured using the following method. Five grams of polymer was precisely weighed, then calcined at 500°C, and then calcined at 530°C for six hours, after which the ash content (g) was measured. The resulting ash was dissolved in hydrochloric acid, and the sodium content was measured using an atomic absorption spectrophotometer AA-6300 (Shimadzu Corporation).

The melt mass flow rate (hereinafter also referred to as MFR) of the PPS used in the present invention is preferably 300 g/10 min to 800 g/10 min. By setting the MFR to preferably 300 g/10 min or more, the fluidity during melting is increased, improving spinnability and resulting in fibers with improved operability even at finer finenesses. It is preferably 350 g/10 min or more. Furthermore, by setting the MFR to 800 g/10 min or less, PPS fibers with good mechanical properties are obtained. It is preferably 600 g/10 min or less, more preferably 500 g/10 min or less.

The MFR of PPS in this invention refers to the value measured under conditions of a load of 5.0 kg and a temperature of 315°C according to JIS K7210-1:2014 Chapter 8, Method A: Mass Measurement Method.

The PPS fiber of the present invention can be obtained by melt spinning. The PPS fiber discharged from the spinneret and taken up is called undrawn yarn, while the fiber that has been drawn in the subsequent drawing process is called drawn yarn. (Details will be provided in the manufacturing method section below.)

The single fiber fineness of the drawn yarn of the PPS fiber of the present invention (drawn yarn single fiber fineness) is preferably 0.30 to 1.20 dtex. It is preferably 0.30 to 0.90 dtex, and more preferably 0.30 to 0.70 dtex. By making the drawn yarn single fiber fineness 0.30 dtex or more, stable operability can be maintained without yarn breakage during spinning. Furthermore, by making the drawn yarn single fiber fineness preferably 0.90 dtex or less, and more preferably 0.70 dtex or less, uniformity during wet-laid nonwoven fabric formation is improved, enabling improved mechanical strength.

The single fiber fineness of the undrawn PPS fiber of the present invention is preferably 0.90 to 3.0 dtex. Having an undrawn single fiber fineness of 0.90 dtex or more stabilizes spinning operability, while having an undrawn single fiber fineness of 3.0 dtex or less, preferably 2.2 dtex or less, improves uniformity during wet nonwoven fabric processing and enables improved mechanical strength.

The drawn yarn strength of the PPS fiber of the present invention is preferably 5.0 to 7.0 cN/dtex, and more preferably 5.5 to 6.5 cN/dtex. A strength of 5.0 cN/dtex or higher can improve the mechanical strength of the wetlaid nonwoven fabric, while a strength of 7.0 cN/dtex or lower not only improves fiber drawing operability but also makes it easier to control the crimp imparting ability of the fiber, improving dispersibility during wetlaid nonwoven fabric formation.

The dry heat shrinkage of the undrawn yarn of the PPS fiber of the present invention is preferably 60% or less. By setting it to preferably 60% or less, more preferably 50% or less, the occurrence of wrinkles during the drying step in wet-laid nonwoven fabric processing can be suppressed, and the uniformity of the basis weight can be improved. The dry heat shrinkage is calculated by the following formula from the fiber length (A) before treatment and the fiber length (B) after treatment when treated at 180°C for 20 minutes.
Dry heat shrinkage rate [%] = ((A) - (B)) / (A) x 100
When measuring the fiber length, a specified load (fineness [dtex] x 270 mg) is applied to a single fiber, and the length is calculated.

The drawn yarn of the PPS fiber of the present invention preferably has a sum of crystallinity and rigid amorphous content of 50 to 100%. By making the sum of crystallinity and rigid amorphous content 50% or more, a high-strength fiber can be obtained. Rigid amorphous refers to a state intermediate between polymer crystals and complete amorphous content, and is the amount remaining after subtracting the crystallinity (%) and mobile amorphous content (%) from the total crystal and amorphous content (100%) that constitutes the fiber, as shown in the following formula:
Rigid amorphous amount [%] = 100 [%] - Crystallinity [%] - Movable amorphous amount [%]
The mobile amorphous amount referred to in the present invention is determined by temperature-modulated DSC as described later in the Examples. The degree of crystallinity is also a value measured by the method described later.

The unstretched yarn of the PPS fiber of the present invention preferably has a crystallinity of 20% or less. By setting the crystallinity of the unstretched yarn to 20% or less, the unstretched yarn acts as an adhesion point between fibers during the drying process of wet-laid nonwoven fabric processing, making it possible to improve the paper strength of the nonwoven fabric.

It is important that the weight ratio of stretched yarn to unstretched yarn (raw cotton weight ratio) in a wetlaid nonwoven fabric made from the PPS fibers of the present invention is 20 to 80%, with the weight ratio of stretched yarn being preferably 30 to 70%. A weight ratio of stretched yarn of less than 20% is undesirable, as it will form a film and reduce tensile strength. A weight ratio of stretched yarn of more than 80% is undesirable, as it will reduce the number of bonding points between fibers and reduce the tensile strength of the wetlaid nonwoven fabric.

Next, we will explain the method for producing the PPS fiber of the present invention.

The PPS fiber of the present invention can be obtained by melt spinning using a PPS resin whose polymer chain ends are COOH terminals. First, PPS having Na at the polymer chain ends is polymerized using a known method, and then the polymer is acid-washed with an inorganic acid such as hydrochloric acid or an organic acid such as acetic acid to obtain a PPS resin whose polymer chain ends are COOH terminals. Powder or pellets of this PPS resin are melted, and the molten resin is spun from a spinneret.

In addition, because PPS resin, whose polymer chain ends in COOH, has high molecular mobility, controlling the solidification behavior and extrusion stability during spinning is important when melt-molding into fiber form. To control solidification behavior, during melt spinning, the section from the resin melt point to the top of the pack housing can be at the normally selected spinning temperature, but it is important to lower the temperature of only the lower pack housing and spinneret section, preferably by 5 to 15°C, and more preferably by 9 to 11°C, compared to the temperature of the upper pack housing. The preferred temperature for the spin block and upper pack housing is 315 to 330°C, and the preferred temperature for the lower spin block and spinneret section is 305 to 315°C.

In conventional PPS fiber production, L/D, defined as the quotient of the land length L of the spinneret discharge hole (the length of the straight pipe section that is the same as the spinneret discharge hole diameter) divided by the spinneret discharge hole diameter D, was typically set between 1.0 and 4.0. However, in the present invention, it is important to set L/D between 5.0 and 10.0 in order to improve discharge stability. By setting L/D between 5.0 and 10.0, the back pressure is optimized when spinning PPS, which has high molecular mobility, at 290°C to 360°C, improving discharge stability and making it possible to achieve the fineness within the claimed range of this invention.

The melt spinning machine typically used is a pressure melter spinning machine or a single-screw or twin-screw extruder-type spinning machine. To prevent gelation due to thickening during the spinning process, the heating temperature is preferably as low as possible while still being sufficient to melt the polymer; specifically, a range of 290 to 360°C is preferred. Also to prevent gelation, a nitrogen atmosphere is preferred during melting. The molten polymer is then extruded from the spinneret and cooled and solidified by blowing cooling air onto it. The cooling air speed is typically 5 to 100 m/min, and the temperature can be room temperature or lower. After cooling and solidifying, the PPS fiber is applied with an appropriate amount of oil as a sizing agent and then taken up by a specified take-up device. The take-up speed is typically in the range of 500 to 7,000 m/min.

The resulting undrawn yarn is then subjected to a drawing process to obtain a drawn yarn. In the drawing process, the yarn is preferably drawn at a drawing temperature of approximately 90 to 170°C and a draw ratio of 2 to 5 by running it through a heated bath, heated steam, or over a hot plate or heated roller. Drawing may be either one-stage or two-stage.

Fixed-length heat treatment may be performed after heat drawing. When performing fixed-length heat treatment, it is important to either keep the length of the yarn constant or allow it to relax by a few percent.

The fixed length heat treatment temperature is preferably 130°C or higher, more preferably 150°C or higher, to reduce the shrinkage rate of the raw cotton and improve processability during drying. Furthermore, by setting the temperature below 200°C, more preferably below 180°C, pseudo-adhesion between the fibers can be suppressed.

The process of drawing the undrawn yarn may be a continuous process in which drawing is carried out following spinning, or a discontinuous process in which the undrawn yarn is taken up at a predetermined speed, temporarily stored in a can or wound up, and then subjected to the drawing process. The resulting PPS fiber may be in the form of multifilament, monofilament, or staple fiber, but staple fiber is particularly preferred in the present invention. This is because staple fiber is generally produced on a larger scale than filament on an industrial scale, resulting in cost benefits.

It is preferable to apply a papermaking dispersant to PPS fibers for use as a raw material for fiber structures such as papermaking. The papermaking dispersant is typically applied to the resulting PPS fibers in a tow state using a kiss roller. The papermaking dispersant deposition rate is preferably 0.2% by mass or more and 0.8% by mass or less, based on the fiber weight. By setting the dispersant deposition rate to preferably 0.1% by mass or more, and more preferably 0.3% by mass or more, fiber dispersibility is improved, resulting in a wetlaid nonwoven fabric with excellent film thickness uniformity and basis weight CV value. Furthermore, by setting the dispersant deposition rate to preferably 0.8% by mass or less, and more preferably 0.5% by mass or less, processability is improved.

After applying the dispersant in this way, the fabric may be crimped using a crimper. By crimping the fabric, the fibers become entangled, resulting in a wet-laid nonwoven fabric with excellent mechanical properties.

The crimp count is preferably 2 crimps/25mm or more and 15 crimps/25mm or less. By setting the crimp count to 2 crimps/25mm or more, and more preferably 5 crimps/25mm or more, the fibers will become entangled, resulting in a wetlaid nonwoven fabric with excellent mechanical properties. Furthermore, by setting the crimp count to 15 crimps/25mm or less, and more preferably 12 crimps/25mm or less, the dispersion of fibers during papermaking will be improved, resulting in a wetlaid nonwoven fabric with good film thickness uniformity and basis weight CV value.

The PPS fibers obtained as described above can be dried using a setter and then cut with a cutter to obtain cut fibers. The cut length of the cut fibers is preferably 1 mm or more and 20 mm or less. By setting the cut length to preferably 1 mm or more, and more preferably 3 mm or more, the fibers will become entangled, resulting in a wetlaid nonwoven fabric with excellent mechanical properties. Furthermore, by setting the cut length to preferably 20 mm or less, and more preferably 10 mm or less, the dispersibility of the fibers during dispersion is improved, resulting in a wetlaid nonwoven fabric with good film thickness uniformity and basis weight CV value.

A dispersion can be prepared by mixing the drawn cut fibers obtained by the above method and the undrawn cut fibers obtained by the above method, but omitting the drawing, in any desired ratio and dispersing them in water.

If no dispersant was added to the tow, dispersant may be added to the cut fibers at this stage.

A wetlaid nonwoven fabric can be obtained by supplying the above dispersion to a papermaking machine. Furthermore, by adjusting the fiber concentration of the supplied dispersion, the basis weight and thickness of the resulting wetlaid nonwoven fabric can be changed.

The wet-laid nonwoven fabric obtained as described above is preferably dried to remove moisture. The drying temperature is preferably 90°C or higher and 150°C or lower to prevent a decrease in fusibility due to crystallization of the amorphous portion.

By subjecting the above-mentioned wet-laid nonwoven fabric to heat-pressing using a flat-plate heating press or a calendar roll, fusion occurs with the unstretched cut fibers of the present invention, resulting in a wet-laid nonwoven fabric with excellent mechanical properties. The heat-pressing temperature is preferably 170°C or higher and 250°C or lower, and the heat-pressing time is preferably 1 minute or higher and 10 minutes or lower. By setting the heat-pressing temperature to 170°C or higher, fusion of the PPS fibers of the present invention results in a wet-laid nonwoven fabric with excellent mechanical properties. By setting the heat-pressing temperature to 250°C or lower, thermal shrinkage of the wet-laid nonwoven fabric during heat-pressing can be suppressed. Furthermore, by setting the heat-pressing time to 1 minute or longer, the entire wet-laid nonwoven fabric can be heated uniformly, resulting in a homogeneous wet-laid nonwoven fabric. By setting the heat-pressing time to 10 minutes or shorter, deterioration of the mechanical properties of the wet-laid nonwoven fabric due to excessive crystallization can be suppressed.

The PPS fiber of the present invention will be explained in more detail below using examples. Note that the physical properties in the examples were determined using the following methods.

[Measurement method]
A. Melt Flow Rate (MFR)
The melt mass flow rate of PPS was measured using a melt indexer (F-F01 manufactured by Toyo Seiki Seisakusho, Ltd.) according to the above-mentioned method (JIS K7210-1 (2014) Chapter 8 Method A: Mass measurement method, load 5.0 kg, temperature 315 ° C.).

B. Fineness Fineness was measured in accordance with JIS L1015 (2010).

C. Strength A stress-strain curve was obtained using a tensile tester ("Tensilon" manufactured by Orientec Co., Ltd.) according to the method described in JIS L1015 (2010) under the conditions of a sample length of 2 cm and a pulling speed of 2 cm/min, and the strength at break was calculated from the curve. The tensile strength was calculated by dividing the value by the fineness calculated in B above.

D. Crystallinity Differential scanning calorimetry was performed using a differential scanning calorimeter (TA Instruments DSCQ1000) under nitrogen at a heating rate of 10°C/min, and the heat of crystallization at the temperature of the observed exothermic peak was taken as ΔHc (J/g). The heat of fusion at the temperature of the endothermic peak (melting point) observed at temperatures above 200°C was taken as ΔHm (J/g). The difference between ΔHm and ΔHc was divided by the heat of fusion of fully crystalline PPS (146.2 J/g) to determine the crystallinity Xc (%) (see the following formula 1).
Xc={(ΔHm-ΔHc)/146.2}×100 (1)
<DSC>
Atmosphere: nitrogen flow (50 mL/min)
・Temperature and calorific value calibration: High purity indium ・Specific heat calibration: Sapphire ・Temperature range: 0 to 350°C
Heating rate: 10°C/min. Sample amount: 5 mg
- Sample container: Standard aluminum container.

E. Rigid Amorphous Amount Temperature-modulated DSC measurement was performed using the same differential scanning calorimeter as in D above under nitrogen conditions of a heating rate of 2°C/min, a temperature amplitude of 1°C, and a temperature modulation period of 60 seconds. An auxiliary line was drawn on the baseline of the obtained chart around the glass transition temperature (Tg). The difference was taken as the specific heat difference (ΔCp). This was divided by the specific heat difference (ΔCp0: 0.2699 J/g°C) around the Tg of completely amorphous PPS to determine the mobile amorphous amount (Xma) using the following formula (2). Furthermore, the rigid amorphous amount (Xra) was calculated from the difference between the degree of crystallinity (Xc) from the whole and the mobile amorphous amount (Xma) using the following formula (3):
Xma (%) = ΔCp/ΔCp0×100 (2)
Xra (%) = 100-(Xc+Xma) (3)
<Temperature Modulated DSC>
Atmosphere: nitrogen flow (50 mL/min)
・Temperature and calorific value calibration: High purity indium ・Specific heat calibration: Sapphire ・Temperature range: 0 to 250°C
Heating rate: 2°C/min. Sample amount: 5 mg
- Sample container: Standard aluminum container.

F. Dry Heat Shrinkage The dry heat shrinkage is calculated from the fiber length (A) before treatment and the fiber length (B) after treatment at 180°C for 20 minutes using the following formula.
Dry heat shrinkage rate [%] = ((A) - (B)) / (A) x 100
When measuring the fiber length, a specified load (fineness [dtex] x 270 mg) is applied to a single fiber, and the length is calculated.

G. Spinnability In each Example and Comparative Example, spinning was carried out using 24 spindles, and the number of thread breakages per spindle was counted for spinning times of 0 to 12 hours and 12 to 24 hours. There are various possible causes of thread breakage, but the reason why the number of thread breakages is higher for 12 to 24 hours compared to 0 to 12 hours is that thread breakages are caused by dirty spinnerets. Spinnability was evaluated as ○ when the number of thread breakages per spindle during a 12-hour spinning time was less than 2, △ when it was 2 to 3, and × when it was 4 or more.

H. Drying Process Passability A water dispersion of 1% by weight of fiber was prepared from the PPS fibers with a cut length of 6 mm obtained in the Examples or Comparative Examples. A wet-laid nonwoven fabric with a basis weight of 20 g/m was obtained using a hand-made papermaking machine (a square sheet machine with automatic couching, manufactured by Kumagaya Riki Kogyo Co., Ltd.) and then couched. The wet-laid nonwoven fabric was placed in a KRK rotary dryer (standard type) manufactured by Kumagaya Riki Kogyo Co., Ltd., and treated at a temperature of 120°C for a treatment time of approximately 2.5 min per cycle, and the wrinkling of the wet-laid nonwoven fabric (drying process passability) was checked. Regarding the drying process passability, those with almost no shrinkage wrinkles and capable of continuous papermaking were rated as ○ (excellent), those with shrinkage wrinkles but sufficient to enable continuous papermaking were rated as △ (fair), and those with shrinkage wrinkles or peeling that presumably made continuous papermaking impossible were rated as × (poor).
I. Paper Strength The wet-laid nonwoven fabric obtained by method G above was subjected to thermocompression bonding for 3 minutes at a pressure of 1.5 MPa using a flat plate heating press at 230°C. The maximum point load of the obtained wet-laid nonwoven fabric was measured using a Tensilon (UTM-III-100, manufactured by Orientec Co., Ltd.) with a sample width of 25 mm, an initial length of 30 mm, and a tensile speed of 20 mm/min. The arithmetic mean value of five measurements was taken as the tensile strength (N/25 mm). Average tensile strength values of 15 N/25 mm or more were rated as ○ (excellent), 10 to 15 N/25 mm were rated as △ (fair), and values below this were rated as × (poor).

J. Overall Judgment In the above spinnability (0-12 hours, 12-24 hours) and papermaking evaluation (drying process passability, paper strength), if there was even one judgment of × (bad) among the six items, the overall judgment was recorded as × (bad), and if there were only judgments of 〇 (excellent) or △ (fair) among the six items, the overall judgment was recorded as 〇 (excellent), and 〇 (excellent) was considered a pass.

<Examples 1 to 6, Comparative Examples 1 to 3>
PPS having Na at the polymer chain end was polymerized by a known method to obtain the MFR listed in Table 1, and then washed with a cleaning solution adjusted to pH 5.0 or less with acetic acid. The Na content was confirmed to be 200 ppm or less and the ash content was 0.05 or less, and the resin was determined to be COOH-terminated PPS resin. This PPS resin was vacuum dried at 150°C for 6 hours and then melt-spun at a spinning temperature of 330°C. In the melt spinning, the resin was melt-extruded using a pressure melter, and the PPS was fed into a spinning pack while being metered using a gear pump, and spun through a spinneret with an L/D of 6.25.

The fibers were then stretched at a specific draw ratio and a stretching temperature of 98°C to achieve a total fineness of approximately 100,000 dtex and the fineness and fiber properties listed in Table 1. A dispersant was applied using a kiss roller, and the fibers were dried and cut to 6 mm lengths to obtain stretched cut fibers. Furthermore, unstretched cut fibers were obtained using a method similar to the above, but omitting the stretching step. The resulting PPS cut fibers were mixed with an aqueous dispersion at a fiber concentration of 1% by weight, and a wet-laid nonwoven fabric with a basis weight of 20 g/ ^m2 was couched using a hand-made papermaking machine (a square sheet machine with automatic couching, manufactured by Kumagaya Riki Kogyo Co., Ltd.). The wet nonwoven fabric was then placed in a KRK rotary dryer (standard model) manufactured by Kumagaya Riki Kogyo Co., Ltd., and treated at 120°C for approximately 2.5 min per cycle to obtain a wet-laid nonwoven fabric. Furthermore, the wet-laid nonwoven fabric was subjected to thermocompression bonding using a flat plate heating press at 230° C. under a pressure of 1.5 MPa for 3 minutes.

The above evaluations were carried out on the resulting PPS cut fiber and wet-laid nonwoven fabric. The results are shown in the table below.

In Examples 1 to 4, PPS for papermaking was obtained that had good spinnability and water dispersibility. The resulting wetlaid nonwoven fabrics also had improved wrinkles during the drying process and sufficient paper strength. In Example 5, the fineness of the drawn yarn was thin, while the fineness of the undrawn yarn was thick, resulting in large variations in paper weight and inferior paper strength compared to wetlaid nonwoven fabrics made with finer undrawn yarn. In Example 6, undrawn yarn was used that was collected under conditions that promoted polymer orientation during spinning. In this case, the undrawn yarn had a high dry heat shrinkage rate, making it prone to wrinkles during the drying process, and its high crystallinity meant that the adhesive strength of the binder fiber was low, resulting in inferior paper strength compared to Example 1.

On the other hand, in Comparative Example 1, wrinkles occurred during drying due to the large fineness and large variations in basis weight. Furthermore, sufficient paper strength was not obtained due to the small number of constituent fibers. In Comparative Example 2, because of the Ca termination, increasing the MFR reduced the degree of polymerization of the resin, and sufficient fiber strength was not obtained. As a result, the resulting wetlaid nonwoven fabric did not have sufficient paper strength. In Comparative Example 3, because of the Ca termination and low MFR, spinning of a fine-fineness product was impaired, and fiber strength was low, so sufficient paper strength was not obtained. In Comparative Example 4, because of the Na termination, fiber strength was not obtained, and sufficient paper strength was not obtained.

Claims (7)

A polyphenylene sulfide fiber in the form of a drawn yarn having a polymer chain end terminated with a COOH group, a melt flow rate (MFR) at 315°C of 300 to 800 g/10 min, and a single fiber fineness of 0.30 to 1.20 dtex. The polyphenylene sulfide fiber of claim 1, wherein the drawn yarn has a strength of 5.0 to 7.0 cN/dtex. The polyphenylene sulfide fiber according to claim 1 or 2, wherein the sum of the crystallinity and rigid amorphous content of the drawn yarn is 50 to 100%. A wetlaid nonwoven fabric containing at least 10% or more of the polyphenylene sulfide fibers described in any one of claims 1 to 3. The wetlaid nonwoven fabric according to claim 4, further comprising undrawn polyphenylene sulfide fiber as a binder fiber, the undrawn fiber having a single fiber fineness of 0.9 to 3.0 dtex. A wetlaid nonwoven fabric according to claim 4 or 5, characterized in that the dry heat shrinkage of the unstretched yarn is 60% or less. 7. The wetlaid nonwoven fabric according to claim 4, wherein the undrawn yarn has a crystallinity of 20% or less.