JP2024051375A - Polyarylene sulfide fiber and nonwoven fabric made of the same - Google Patents

Abstract

To provide a polyarylene sulfide fiber having excellent dispersibility in an aqueous medium and adhesiveness.SOLUTION: A polyarylene sulfide fiber has: a heat of fusion of 15 J/g or more measured by a differential scanning calorimeter (DSC); an oxygen-containing group, which results in an oxygen content as a molar ratio O/S of 0.2 to 1.5 measured by SEM-EDX; and a fiber diameter of 3 μm or less.SELECTED DRAWING: None

Description

The present invention relates to polyarylene sulfide fibers that have excellent dispersibility in aqueous media.

Polyphenylene sulfide (PPS) has high heat resistance, chemical resistance, electrical insulation, and flame retardancy, as well as excellent mechanical properties and moldability, and is therefore widely used as a metal replacement material and a material that can withstand extreme environments. In addition, polyphenylene sulfone (PPSO), obtained by oxidizing PPS, has superior heat resistance, chemical resistance, and acid resistance compared to PPS, and also has the property of not melting due to heat. Polyarylene sulfide fibers, which are made by fiberizing the above resin, are used for applications such as bag filters, papermaking canvases, electrical insulating paper, battery separators, and various diaphragms, taking advantage of the above properties. In particular, by combining the characteristics of PPS and PPSO, such as heat resistance and chemical resistance to high-concentration alkaline solutions, with the ion permeability and gas separation properties of nonwoven fabric materials, wet-laid nonwoven fabrics made of polyarylene sulfide fibers have been developed for hydrogen production equipment diaphragms, fuel cell diaphragms, and diaphragm reinforcement materials for these diaphragms.

In recent years, there has been a demand for nonwoven fabrics used in the above applications to be thinner and have a lower basis weight in order to make devices smaller, lighter, and more powerful. Refining the fibers is an effective way to achieve a thinner fabric and lower basis weight. However, finer fibers increase the aspect ratio of the short fibers, making them more likely to become entangled, which leads to poor dispersion in the dispersion process in an aqueous medium when producing wet nonwoven fabrics, making it difficult to obtain a highly uniform nonwoven fabric. For this reason, various studies have been conducted to obtain a highly uniform nonwoven fabric even with fibers with a small fiber diameter.

On the other hand, nonwoven fabrics made of polyarylene sulfide fibers are mainly composed of skeletal fibers and binder fibers. The skeletal fibers are preferably polyarylene sulfide oriented fibers with excellent heat resistance, and the binder fibers are preferably polyarylene sulfide unoriented fibers with excellent thermal fusion properties.

In light of this background, a method has been proposed for producing PPS fibers for papermaking, which is characterized in that undrawn PPS fibers produced by melt spinning are heat-treated by passing them through hot water at 70°C or higher, and then passed in a relaxed state through a dry heat zone at 70 to 90°C for 5 to 60 minutes, with the aim of obtaining undrawn PPS fibers with good water dispersibility as binder fibers in PPS nonwoven fabrics (see Patent Document 1).

On the other hand, a method for producing a heat-resistant wet-laid nonwoven fabric has been proposed that uses PPSO, which has better heat resistance, chemical resistance, and acid resistance than PPS, and is intended to produce a thinner fabric, and is characterized by the fabric being made of PPSO fiber with a single yarn fineness of 1 dtex or less and a tensile strength of 2.0 cN/dtex or more, and undrawn PPS fiber (see Patent Document 2).

Also proposed as a technology using PPSO are PPSO nanofibers that have a fiber diameter of 0.0001 μm or more and 0.3 μm or less and that show substantially no melting peak in measurements using a differential scanning calorimeter (DSC), and a method for producing paper that contains this PPSO nanofiber (see Patent Document 3).

JP 2010-174400 A JP 2007-39840 A JP 2007-2373 A

The technology of Patent Document 1 makes it possible to obtain PPS fibers with a small thermal shrinkage rate by subjecting undrawn PPS fibers produced by a melt spinning method to a relaxation heat treatment during the manufacturing process to cause them to shrink. However, the undrawn PPS fibers obtained by this method have a substantially large fiber diameter, making it difficult to obtain thin or low-basis-weight nonwoven fabrics.

On the other hand, the technology of Patent Document 2 relates to a wetlaid nonwoven fabric using PPSO fiber having a fineness of 1 dtex or less and undrawn PPS fiber as a binder fiber. This technology makes it possible to achieve higher heat resistance, chemical resistance, and the like than conventional PPS nonwoven fabrics, but the aspect ratio (fiber length/fiber diameter) of the short fibers exemplified is low, so that they are easily dispersed in water, and the fiber diameter is large, so that it is difficult to obtain a thin or low basis weight nonwoven fabric.
In addition, the technology of Patent Document 3 has a small fiber diameter of 0.0001 μm or more and 0.3 μm or less, but the nonwoven fabric obtained is thick and has low tensile strength due to poor dispersibility, and is therefore insufficient.

The object of the present invention is to provide polyarylene sulfide fibers that maintain excellent dispersibility and adhesiveness, in consideration of the above circumstances.

As a result of intensive research conducted by the inventors to achieve the above object, they discovered that by subjecting PPS fibers to an oxidation treatment to introduce sulfonic groups into the fibers, it is possible to obtain polyarylene sulfide fibers that have good dispersibility in aqueous media even when the fiber diameter is made small, and that have both excellent dispersibility and binder performance suitable for papermaking, and thus completed the present invention.

In other words, the present invention aims to solve the above problems, and the polyarylene sulfide fiber of the present invention has a fiber diameter of 3.0 μm or less, a heat of fusion calculated by differential scanning calorimetry of 15 J/g or more, has oxygen-containing groups, and has an oxygen content of 0.2 to 1.5 as determined by SEM-EDX (Scanning Electron Microscope Energy Dispersive X-ray Spectroscopy) as a molar ratio O/S.

According to the present invention, it is possible to obtain polyarylene sulfide fibers that have excellent dispersibility in aqueous media and binder performance even when the fiber diameter is small. Furthermore, the polyarylene sulfide fibers of the present invention can provide high-quality nonwoven fabrics with high binder fiber dispersibility.

The polyarylene sulfide fiber of the present invention has a fiber diameter of 3.0 μm or less, a heat of fusion calculated by differential scanning calorimetry of 15 J/g or more, has oxygen-containing groups, and has a molar ratio O/S value determined by SEM-EDX of 0.2 to 1.5.

The components are described in detail below, but the present invention is not limited to the scope described below as long as it does not exceed the gist of the invention.

The polyarylene sulfide used in the present invention is a homopolymer or copolymer having a repeating unit of -(Ar-S)-. Examples of Ar include structural units represented by the following formulas (1) to (12).

(In formulas (1) to (10), R1 and R2 are substituents selected from hydrogen, an alkyl group, an alkoxy group, and a halogen group, and R1 and R2 may be the same or different.)
The repeating unit of the polyarylene sulfide used in the present invention is preferably a p-arylene sulfide unit having Ar represented by the above formula (1) or (2), and representative examples thereof include polyphenylene sulfide (PPS), polyphenylene sulfide sulfone, polyphenylene sulfide ketone, etc., and excellent heat resistance is obtained by using a resin containing, as the main structural unit of the polymer, preferably 60 mol % or more, more preferably 70 mol % or more, and even more preferably 80 mol % or more of p-phenylene units represented by the following structural formula (13). In addition, when processed into fibers and nonwoven fabrics, the resin has excellent strength.

The polyarylene sulfide fiber of the present invention is characterized by having an oxygen-containing group. Examples of functional groups having an oxygen atom include sulfone groups, ether groups, sulfonic acid groups, and carbonyl groups, and these may be introduced by copolymerization. Representative examples include polyarylene sulfide sulfone, polyarylene sulfide ketone, random copolymers thereof, block copolymers thereof, and mixtures thereof. These may be polymerized in advance and mixed with the above-mentioned PPS for use. Alternatively, the PPS fiber may be modified by the method described below to introduce oxygen-containing groups.

The polyarylene sulfide fiber according to the present invention may contain various additives, such as inorganic substances such as titanium oxide, silica, barium oxide, and calcium carbonate, colorants such as carbon black, dyes, and pigments, flame retardants, fluorescent whitening agents, antioxidants, and ultraviolet absorbing agents, to the extent that the effects of the present invention are not impaired.

The polyarylene sulfide fiber of the present invention may be a single component fiber or a composite fiber in which two or more types of resins are combined. When the polyarylene sulfide fiber is a composite fiber, the composite form is not particularly limited as long as it does not impair the effects of the present invention, and can be appropriately selected from a core-sheath type, a sea-island type, a side-by-side type, an eccentric core-sheath type, a blend type, etc.

The cross-sectional shape of the polyarylene sulfide fiber of the present invention is not limited in any way, and can be any shape, including a round cross section, a multi-lobal cross section such as a triangular cross section, a flat cross section, an S-shaped cross section, a cross cross section, and a hollow cross section.

The polyarylene sulfide fiber of the present invention has a heat of fusion of 15 J/g or more as calculated by differential scanning calorimetry. By setting the heat of fusion to 15 J/g or more, preferably 20 J/g or more, the fiber has excellent adhesiveness and is suitable for use as a binder fiber. Furthermore, by setting the heat of fusion to 42 J/g or less, preferably 40 J/g or less, more preferably 30 J/g or less, the polyarylene sulfide fiber has excellent dispersibility in aqueous media.

The heat of fusion (J/g) of the fiber referred to here is determined as follows.
(1) Approximately 5 mg of fiber is weighed out using an electronic balance, and the fiber is then set in a differential scanning calorimeter. Differential scanning calorimeter measurement is performed under nitrogen at a heating rate of 16°C/min in a measurement temperature range of 50 to 300°C.
(2) Calculate the heat of crystal fusion ΔHm (J/g) from the area of the endothermic peak in the obtained measurement result (DSC curve). When multiple endothermic peaks are observed, ΔHm is calculated from the total area of all the peaks, and when no endothermic peak is observed, ΔHm is taken as 0 (J/g).
(3) For each level, change the measurement position and perform three measurements, and calculate the simple number average value to obtain the heat of fusion (J/g).

The heat of fusion (J/g) can be adjusted to the above range, for example, by making the content of crystalline PPS in the polyarylene sulfide that constitutes the fiber 35% or more.

The polyarylene sulfide fiber of the present invention has an oxygen-containing group, and the molar ratio O/S value determined by SEM-EDX is 0.2 to 1.5. By having an oxygen-containing group, which is a hydrophilic group, in a portion of the fiber and setting the molar ratio O/S value to 0.2 to 1.5, the fiber can have excellent dispersibility in aqueous media. In the polyarylene sulfide fiber of the present invention, the molar ratio O/S is preferably 0.4 or more, and more preferably 0.5 or more, to obtain a polyarylene sulfide fiber with excellent dispersibility in aqueous media. In addition, the molar ratio O/S is preferably 1.3 or less, and more preferably 1.0 or less, to obtain a polyarylene sulfide fiber with excellent adhesiveness suitable for binder fibers.

Methods for adjusting the molar ratio O/S to the above range include a method of using, as a core-sheath fiber, a polyarylene sulfide in which a component containing an O atom (oxygen-containing group) is copolymerized in the sheath component, and a method of subjecting the fiber to plasma treatment, corona treatment, oxidation treatment, UV treatment, or the like to introduce an oxygen-containing group (converting PPS into PPSO).
The fiber diameter of the polyarylene sulfide fiber of the present invention is 3.0 μm or less. By making the fiber diameter 3.0 μm or less, preferably 2.0 μm or less, the number of fibers constituting the same fineness and the same basis weight increases, so that the number of bonding points increases when used as a binder, and a nonwoven fabric with excellent strength is obtained. In addition, by making the fiber diameter larger than 0.3 μm, more preferably larger than 0.5 μm, a polyarylene sulfide fiber with excellent dispersibility can be obtained.

The fiber diameter (μm) of the fiber is determined as follows.
(1) An image of the cross section of the fiber is taken using a scanning electron microscope at a magnification such that a single fiber can be observed.
(2) Using the captured image and image analysis software, the area Af (μm2) formed by the cross-sectional contour of the single fiber is measured, and the diameter of a perfect circle having the same area as this area Af is calculated.
(3) This is measured for 100 randomly selected fibers, a simple number average is calculated to obtain the average fiber diameter (μm), and the average fiber diameter is rounded off to one decimal place.

The fiber diameter can be adjusted to the above range by the method described below, but other methods may be used as long as they do not impede the objectives of the present invention.

The nonwoven fabric of the present invention is a nonwoven fabric containing the polyarylene sulfide fiber of the present invention. In the nonwoven fabric of the present invention, the type of nonwoven fabric is not particularly limited, and examples include spunbond nonwoven fabric, meltblown nonwoven fabric, spunlace nonwoven fabric, needle punched nonwoven fabric, papermaking nonwoven fabric, etc., among which papermaking nonwoven fabric is preferred because it is easy to make it have a low basis weight and a thin fabric.

Next, a preferred embodiment for producing the polyarylene sulfide fiber of the present invention will be specifically described.

The polyarylene sulfide used as the raw material is preferably dried before being subjected to melt spinning in order to prevent water contamination and to remove oligomers. Drying can 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 polyarylene sulfide passes through a pipe, is metered by a metering device such as a gear pump, passes through a filter to remove foreign matter, and is then guided to a spinneret. At this time, the temperature from the polymer pipe to the spinneret (spinning temperature) is preferably 290°C or higher to increase fluidity, and 380°C or lower to suppress thermal decomposition of the polymer.

The polyarylene sulfide fiber of the present invention can be obtained from an islands-in-the-sea type composite fiber having an islands-in-the-sea type composite cross section in which polyarylene sulfide is arranged in the island component.

A readily soluble polymer is preferably used for the sea component of the islands-in-the-sea composite fiber. The readily soluble polymer referred to here is selected from melt-moldable polymers and copolymers thereof, such as polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, polytrimethylene terephthalate, polypropylene, polyolefin, polycarbonate, polyacrylate, polyamide, polylactic acid, and thermoplastic polyurethane.

The islands-in-sea composite fiber used in the production of the present invention is prepared by melting the island component polyarylene sulfide (e.g., PPS) and the sea component polymer separately, then passing them through polymer piping, measuring them with a known measuring device such as a gear pump, passing them through a filter to remove foreign matter, and then leading them to a spinneret. The respective polymers led to the spinneret are merged in an arbitrary composite form within the spinneret and discharged from the spinneret holes as islands-in-sea composite fiber.

The spinneret used for extrusion preferably has a nozzle hole diameter D of 0.1 mm or more and 0.6 mm or less, 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 or more and 10 or less.

The shape and cross-sectional area of the island components in an islands-in-the-sea composite fiber may be the same as the area of the fiber cross section calculated from the fiber shape and average fiber diameter. For example, by setting the diameter of the island component polyarylene sulfide to 2.0 μm, a round cross-section fiber with an average fiber diameter of 2.0 μm can be produced.

The number of island components in the islands-in-the-sea composite fiber is not particularly limited, but is preferably 20 or more and 2000 or less. By making the number of island components preferably 20 or more, more preferably 100 or more, and even more preferably 200 or more, the diameter of the island components can be made small, making it possible to reduce the fiber diameter of the polyarylene sulfide fiber. In addition, by making the number of island components preferably 2000 or less, more preferably 1500 or less, the fiber cross-section formability is improved, making it possible to obtain fibers with small variation in fiber diameter.

The islands-in-the-sea composite fiber discharged from the nozzle holes is cooled and solidified by blowing cooling air (air) onto it. 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 preferred 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 fibers with a highly uniform fiber diameter.

It is also preferable to flow the cooling air in a direction approximately perpendicular to the fiber axis through 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 fineness, 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 linear uniformity and productivity, and 1500 m/min or less to prevent the orientation of molecular chains.

The unstretched fibers thus obtained may be subjected to the next process as unstretched fibers or may be stretched.

If necessary, the obtained fibers may be crimped using a crimper and the shape may be fixed using a setter at a temperature of 70°C or less. By crimping the fibers, the fibers become entangled, 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 crimps/25 mm or more and 15 crimps/25 mm or less. By setting the number of crimps to 2 crimps/25 mm or more, the fibers are more likely to be entangled with each other, resulting in a nonwoven fabric with excellent mechanical strength. Furthermore, by setting the number of crimps to 15 crimps/25 mm or less, the dispersibility of the fibers in the dispersion liquid is improved, resulting in a homogeneous nonwoven fabric.

In order to obtain the polyarylene sulfide fiber of the present invention, the above-mentioned islands-in-the-sea composite fiber may be immersed in a solvent capable of dissolving the sea component to remove the easily soluble polymer. When the easily soluble polymer is copolymerized polyethylene terephthalate, an alkaline aqueous solution such as an aqueous sodium hydroxide solution may be used as the solvent. In this case, the bath ratio of the islands-in-the-sea composite fiber to the alkaline aqueous solution (mass (g) of islands-in-the-sea composite fiber/mass (g) of the alkaline aqueous solution) is preferably 1/10,000 or more and 1/5 or less, more preferably 1/5,000 or more and 1/10 or less. By keeping it within this range, it is possible to prevent the fibers from unnecessarily entangling with each other when the sea component is dissolved.

In this case, the alkali concentration of the alkaline aqueous solution is preferably 0.1% by mass or more and 5.0% by mass or less, and more preferably 0.5% by mass or more and 3% by mass or less. By keeping it within this range, dissolution of the sea component can be completed in a short time.

In addition, it is preferable to add a surfactant to the alkaline aqueous solution as a dissolution promoter for the sea component. Examples of surfactants include cationic surfactants, anionic surfactants, and nonionic surfactants. When polyester is used as the easily soluble polymer, cationic surfactants are preferably used. In addition, the surfactant concentration in the alkaline solution is preferably 1% by mass or more and 10% by mass or less, and more preferably 3% by mass or more and 8% by mass or less, based on the weight of the fiber before dissolution.

The polyarylene sulfide fiber of the present invention can be obtained by subjecting the fiber obtained by melt spinning as described above to a treatment for introducing oxygen-containing groups, preferably an oxidation treatment with an organic peroxide. Examples of the organic peroxide used here include performic acid, peracetic acid, perbenzoic acid, perpropionic acid, perbutyric acid, methachloroperbenzoic acid, pertrichloroacetic acid, pertrifluoroacetic acid, and perphthalic acid. Among these, peracetic acid is preferred because of its fast reaction rate and ease of handling. The introduction of oxygen-containing groups into the polyarylene sulfide fiber is achieved by immersing the polyarylene sulfide fiber in an organic peroxide solution. The treatment conditions vary depending on the fineness or specific surface area of the fiber, or the reaction rate of the organic peroxide used, but for example, when peracetic acid is used for fine fibers with a fiber diameter of 3.0 μm or less, the molar ratio O/S can be set to 0.2 to 1.5 even at room temperature.

The polyarylene sulfide fibers of the present invention can be used as long fibers, but they can also be cut to a certain length to produce short fibers before being processed into nonwoven fabrics, etc.

When the polyarylene sulfide fiber of the present invention is used as a short fiber, the average fiber length is preferably 0.1 mm or more and 10.0 mm or less. By setting the average fiber length to preferably 0.1 mm or more, more preferably 0.2 mm or more, and even more preferably 0.3 mm or more, the fibers are less likely to fall off during the processing step into a nonwoven fabric. In addition, by setting the average fiber length to preferably 10.0 mm or less, more preferably 7.0 mm or less, and even more preferably 3.0 mm or less, the fibers are less likely to entangle with each other and the dispersion of the fibers is improved, so that the uniformity of the basis weight is improved when the nonwoven fabric is made.

The average fiber length of the fibers referred to here is determined based on "8.4.1. Average fiber length (c direct method)" of JIS L1015:2010 "Test methods for synthetic fiber staples."

When the polyarylene sulfide fiber of the present invention is used as a short fiber, the aspect ratio (L/D) of the fiber length (L) to the fiber diameter is preferably 5000 or less. By setting the aspect ratio to preferably 3000 or less, and more preferably 1000 or less, the fibers are less likely to become entangled and the dispersion of the fibers is improved, thereby improving the uniformity of the basis weight when made into a nonwoven fabric.

The polyarylene sulfide fiber of the present invention not only has excellent long-term heat resistance, but also chemical resistance, mechanical properties, electrical insulation, and flame retardancy. Therefore, the nonwoven fabric containing the polyarylene sulfide fiber of the present invention can be used for a variety of applications, including diaphragm applications such as hydrogen production device diaphragms, battery diaphragms, electrode diaphragms, and diaphragm reinforcement materials for these diaphragms, filter applications such as bag filters, chemical liquid 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, papermaking felt, heat-resistant felt, release material, papermaking dryer canvas, heart patch, artificial skin, printed circuit board substrate, copy rolling cleaner, ion exchange substrate, oil retention material, insulation material, cushioning material, and net conveyor, by utilizing these characteristics, but is not limited to these applications.

Next, a preferred embodiment for producing a nonwoven fabric containing the polyarylene sulfide fiber of the present invention will be specifically described.

The short fibers made of polyarylene sulfide fibers obtained as described above are used as binder fibers and dispersed in water together with skeletal fibers to prepare a papermaking solution. In addition, in polyarylene sulfide wet nonwoven fabrics, polyarylene sulfide oriented fibers are usually used as the skeletal fibers. In particular, PPS oriented fibers are preferred.

The fiber diameter of the skeletal fiber of the present invention is preferably 20.0 μm or less. In addition, by making the fiber diameter preferably 15.0 μm or less, more preferably 10.0 μm or less, the number of fibers constituting the same fineness and the same basis weight increases, resulting in a nonwoven fabric with excellent strength, and by making the fiber diameter of the skeletal fiber narrower, a thinner nonwoven fabric can be obtained.

The fiber length of the skeletal fiber of the present invention is preferably 3 mm or more and 7 mm or less. By making the fiber length of the skeletal fiber 3 mm or more, more preferably 4 mm or more, and even more preferably 5 mm or more, the entanglement of the fibers increases, resulting in a nonwoven fabric with excellent strength.

The ratio of binder fiber to skeletal fiber in the papermaking solution is preferably 10% by mass or more and 80% by mass or less. In addition, by setting the ratio of binder fiber to skeletal fiber to preferably 80% by mass or less, more preferably 75% by mass or less, and even more preferably 70% by mass or less, a nonwoven fabric with appropriate breathability and flexibility can be obtained. The polyarylene sulfide fiber of the present invention has oxygen-containing groups and has a molar ratio O/S value of 0.2 to 1.5 determined by measurement with SEM-EDX, so that it has excellent dispersibility in aqueous media, is uniformly dispersed in the papermaking solution, and can efficiently penetrate between the skeletal fibers.

A nonwoven fabric can be obtained by supplying the above papermaking solution to a papermaking machine. The basis weight of the resulting nonwoven fabric can be changed by adjusting the fiber concentration of the papermaking solution supplied.

The nonwoven fabric obtained as described above is preferably dried to remove moisture. The drying temperature is preferably 70°C or less to prevent a decrease in fusibility due to crystallization of the amorphous portion.

The nonwoven fabric thus obtained is heat-pressed with a flat plate heating press or a calendar roll, whereby the polyarylene sulfide fiber of the present invention causes fusion between the skeletal fibers, resulting in a nonwoven fabric with excellent mechanical properties. The heat-pressing temperature is preferably 170°C or higher and 250°C or lower, and the pressing time is preferably within 10 minutes. By setting the heat-pressing temperature to preferably 170°C or higher, the polyarylene sulfide fiber of the present invention is fused to form a nonwoven fabric with excellent mechanical properties. Furthermore, by setting the heat-pressing temperature to 250°C or lower, it is possible to suppress the occurrence of wrinkles due to the thermal shrinkage of the nonwoven fabric during heat-pressing. Furthermore, by setting the time to 10 minutes or less, it is possible to suppress the decrease in flexibility of the nonwoven fabric due to excessive crystallization.

As described above, the nonwoven fabric containing the polyarylene sulfide fiber of the present invention is obtained from a papermaking solution in which the polyarylene sulfide fiber of the present invention is uniformly dispersed without uneven distribution, and therefore the fusion between the skeletal fibers is uniform, resulting in good and uniform mechanical properties.

Next, the present invention will be described in detail based on examples. However, the present invention is not limited to these examples. In addition, in the measurement of each physical property, unless otherwise specified, the measurement was performed based on the above-mentioned method.

(1) Fiber Diameter Measurement was performed as described above using a scanning electron microscope “S-5500” manufactured by Hitachi High-Technologies Corporation as a scanning electron microscope and “WinROOF2015” manufactured by Mitani Shoji Co., Ltd. as image analysis software.

(2) Average fiber length Measurement was performed based on “8.4.1. Average fiber length (c direct method)” of JIS L1015:2010 “Test methods for chemical fiber staples.”

(3) Heat of Fusion The measurement was carried out as described above using a differential scanning calorimeter "DSC Q2000" manufactured by TA Instruments.

(4) Molar ratio of oxygen atoms to 1 mole of sulfur atoms (molar ratio O/S)
The polyarylene sulfide fibers produced in each of the Examples and Comparative Examples were subjected to measurement of the molar ratio of oxygen atoms to 1 mole of sulfur atoms at an acceleration voltage of 15 kV using a scanning electron microscope energy dispersive X-ray spectroscopy (SEM-EDX) Hitachi High-Technologies scanning electron microscope (SU1510), and the simple number average of three measured values was calculated, and the value was rounded off to two decimal places.

(5) Thickness The thickness of the wetlaid nonwoven sheet was measured in mm using a dial thickness gauge (TECLOCK SM-114, probe shape 10 mmφ, graduation 0.01 mm, measuring force 2.5 N or less). Measurements were taken at any five points per sample, and the average was rounded off to one decimal place to determine the thickness of the wetlaid nonwoven sheet.

(6) Strength of Nonwoven Fabric The nonwoven fabric produced in each Example and Comparative Example was measured for maximum point load using Tensilon (UTM-III-100 manufactured by Orientec Co., Ltd.) under the conditions of a sample width of 15 mm, an initial length of 20 mm, and a tensile speed of 20 mm/min. The strength of the nonwoven fabric (N/25 mm) was calculated by simply averaging the five measured values and rounding off to one decimal place.

(7) Basis Weight The nonwoven fabric produced in each Example and Comparative Example was cut into a 250 mm x 250 mm square, weighed, and converted into a weight per unit area ( ¹ m2). The converted weight was rounded off to the nearest integer to obtain an integer value, which was used as the basis weight of the wet-laid nonwoven sheet.

(8) Air permeability The nonwoven fabrics produced in each Example and Comparative Example were measured based on the Frazier method defined in JIS L1096:1990 6.27.1A method. Three test pieces of 25 cm x 25 cm were taken, and after conditioning to standard conditions (20°C, 65% relative humidity), measurements were taken at five selected locations, and the average value was rounded off to three significant digits to determine the air permeability. The test pressure was 125 Pa, and the aperture was 50 mm.

(9) Uniformity index of nonwoven fabric With respect to the nonwoven fabric obtained in each Example and Comparative Example, an image was taken at a magnification of 100 times under transmitted illumination using a microscope VHX-2000 manufactured by Keyence Corporation. This image was converted into a monochrome image using image processing software (WINROOF), and a luminance histogram (vertical axis: frequency (number of pixels), horizontal axis: luminance) with a series of 256 was obtained to obtain a standard deviation. The same operation was performed for 10 images, and the simple number average value of these was rounded off to one decimal place to obtain a dispersion index, which was evaluated using the following index.
"20 or less": Good dispersibility, almost no fiber bundles, and uniformly dispersed.
"More than 20 and 26 or less": some fiber bundles remain, but are dispersed to a certain extent. "More than 26": poor dispersion, many fiber bundles remain, and dispersion is poor.

[Reference Example 1]
PPS consisting of only p-phenylene sulfide units was vacuum dried at 150°C for 12 hours, and then melt-spun at a spinning temperature of 330°C. In the melt spinning, PPS was melt-extruded by a twin-screw extruder, and PPS was supplied to a spinning pack while being metered by a gear pump. Then, PPS was discharged from a nozzle having 36 discharge holes under the condition of a single-hole discharge rate of 0.2 g/min. A spinneret was used in which the inlet hole located directly above the nozzle hole was a straight hole, and the connection part between the inlet hole and the nozzle hole was tapered. The PPS discharged from the nozzle was passed through a 50 mm heat-retaining area, and then air-cooled over 1.0 m using a uniflow type cooling device under the conditions of a temperature of 25°C and a wind speed of 18 m/min. Then, an oil agent was applied, and the 36 filaments were wound by a winder through the first godet roller and the second godet roller at 1000 m/min to obtain an undrawn fiber.

The resulting undrawn fibers were then drawn 3.0 times between rollers heated to 90°C, and then heat-set with rollers heated to 200°C to obtain drawn PPS fibers with a fiber diameter of 9 μm.

The resulting PPS stretched fibers were then cut with a cutter to produce short fibers with an average fiber length of 6.0 mm.

[Example 1]
PPS consisting of only p-phenylene sulfide units was used as island components, and polyethylene terephthalate copolymerized with 5.0 mol % of 5-sodium sulfoisophthalic acid was used as sea components. The island components were shaped like circles using an islands-in-sea composite spinneret (number of islands: 1000) and the sea/island component ratio was set to 40/60. The melt extruded yarn was cooled and solidified, and an oil was applied thereto. The yarn was then wound up at a spinning speed of 1000 m/min to obtain an undrawn fiber having an islands-in-sea composite cross section (output rate per hole: 2.6 g/min).

The resulting islands-in-the-sea composite fibers were then cut with a cutter to obtain short fibers having an average fiber length of 2.0 mm. The short fibers were then dissolved in a 3% by mass aqueous solution of sodium hydroxide (bath ratio 1/100) heated to 65°C to elute the sea component, thereby obtaining PPS fibers. Note that the treatment was performed by adding 5% by mass of DY-1125K (quaternary ammonium salt) manufactured by Lion Specialty Chemicals Co., Ltd., as a dissolution promoter to the aqueous sodium hydroxide solution, relative to the mass of the fibers.
The obtained PPS fiber was immersed in an aqueous solution of peracetic acid (containing 9% peracetic acid in acetic acid) at 25° C. using commercially available peracetic acid at a bath ratio of fiber to aqueous solution of peracetic acid (fiber mass (g)/aqueous solution of peracetic acid mass (g)) of 1/300 for 3 minutes, and then subjected to treatments of washing with water, neutralization and washing with water to obtain a polyarylene sulfide fiber of the present invention.

The above polyarylene sulfide fiber was mixed as the binder fiber at 40% by mass, and the PPS stretched fiber obtained in Reference Example 1 was mixed as the skeletal fiber at 60% by mass in an aqueous medium to obtain a fiber concentration of 0.4% by mass. This papermaking liquid was fed to a simple papermaking machine to obtain a wet nonwoven fabric having a basis weight of 20 g/m ^2. The wet nonwoven fabric was then placed in a hot air dryer at 120° C. for 3 minutes for drying, and then air-cooled, followed by thermocompression bonding at a pressure of 10 MPa for 3 minutes using a flat plate heating press at 220° C.

The evaluation results of the fibers and nonwoven fabric obtained in this example are shown in Table 1.

[Example 2]
A nonwoven fabric was obtained in the same manner as in Example 1, except that the amount of fiber in the dispersion (fiber concentration) was changed to 5 g/ ^m2 . The evaluation results of the obtained fiber and nonwoven fabric are shown in Table 1.

[Comparative Example 1]
A nonwoven fabric was obtained in the same manner as in Example 1, except that the time of immersion in the peracetic acid aqueous solution was changed to 300 minutes. The evaluation results of the obtained fiber and nonwoven fabric are shown in Table 1. Because the immersion time in the peracetic acid aqueous solution was long, the PPS fiber was completely converted into PPSO fiber.

[Comparative Example 2]
Except for not carrying out the step of immersion in the aqueous peracetic acid solution, a nonwoven fabric was obtained in the same manner as in Example 1. The evaluation results of the obtained fibers and nonwoven fabric are shown in Table 1.

The polyarylene sulfide nonwoven fabric obtained in Example 1 was composed of oriented PPS fibers as aggregate fibers and the polyarylene sulfide fibers of the present invention as binder fibers. Even though the binder fibers had a high aspect ratio, a portion of the fibers was modified by oxidation treatment to have oxygen-containing groups added thereto, and therefore the fibers had good dispersibility when dispersed in a dispersion. As a result, a high-quality nonwoven fabric was provided with high binder fiber dispersibility.
The polyarylene sulfide nonwoven fabric obtained in Example 2 has a lower basis weight than that of Example 1, but provides a high-quality nonwoven fabric with high dispersibility even though the binder fibers have a high aspect ratio, as in Example 1.

The polyarylene sulfide nonwoven fabric obtained in Comparative Example 1 was composed of PPS oriented fibers as aggregate fibers and PPSO fibers as binder fibers, but since the entire binder fiber was converted to PPSO, it became infusible and did not function as a binder fiber, and a nonwoven fabric with sufficient tensile strength could not be obtained.

The polyarylene sulfide nonwoven fabric obtained in Comparative Example 2 was composed of PPS fibers for both the aggregate fibers and the binder fibers. Since the binder fibers had a high aspect ratio, many fiber bundles were observed and the dispersibility in the dispersion liquid was poor, so the dispersion index was large and the unevenness was large compared to Example 1. Therefore, it was not possible to obtain a high-quality nonwoven fabric.

Claims (2)

A polyarylene sulfide fiber having a fiber diameter of 3.0 μm or less, a heat of fusion of 15 J/g or more as measured by a differential scanning calorimeter (DSC), containing oxygen-containing groups, and a molar ratio O/S of 0.2 to 1.5 as determined by SEM-EDX. A nonwoven fabric comprising the polyarylene sulfide fiber of claim 1.