COPOLYMERIZED POLYPHENYLENE SULFIDE FIBERS

Abstract

In order to provide a copolymerized polyphenylene sulfide fiber that is thin, has a low heat shrinkage rate, and is suitable for a use as a paper-making binder having excellent weldability, a copolymerized polyphenylene sulfide fiber is characterized by containing a copolymerized polyphenylene sulfide that has a p-phenylene sulfide unit as a main component and contains 3 mol % or more and 40 mol % or less of a m-phenylene sulfide unit in a repeating unit, and having a degree of crystallization of 10.0% or more and 30.0% or less, an average fiber diameter of 5 μm or more and 25 μm or less, and further a shrinkage rate in 98° C. hot water of 25.0% or less.

Claims

1. A copolymerized polyphenylene sulfide fiber comprising a copolymerized polyphenylene sulfide that has a p-phenylene sulfide unit as a main component and contains 3 mol % or more and 40 mol % or less of a m-phenylene sulfide unit in a repeating unit, and having a degree of crystallization of 10.0% or more and 30.0% or less, an average fiber diameter of 5 μm or more and 25 μm or less, and further a shrinkage rate in 98° C. hot water of 25.0% or less.

2. The copolymerized polyphenylene sulfide fiber according to claim 1, having a birefringence of 0.18 or more and 0.40 or less.

3. The copolymerized polyphenylene sulfide fiber according to claim 1, having a melting point of 200° C. or higher and 260° C. or lower.

4. The copolymerized polyphenylene sulfide fiber according to claim 1, having a CV value of a fiber diameter of 10.0% or less.

5. The copolymerized polyphenylene sulfide fiber according to claim 1, having a strength of 2.0 cN/dtex or more and an elongation of 50% or less.

Description

EXAMPLES

[0076] Hereinafter, the copolymerized polyphenylene sulfide fiber according to the present invention is more specifically described by way of examples. The values of the characteristics in the examples were obtained by the following methods.

A. Melt Mass-Flow Rate:

[0077] The melt mass-flow rate of the copolymerized polyphenylene sulfide was measured using MELT INDEXER (F-F01 manufactured by Toyo Seiki Seisaku-sho, Ltd.) according to the above-described method (JIS K7210-1: 2014, chapter 8, method A: mass measuring method, load of 5.0 kg and temperature of 315° C.)

B. Average Fiber Diameter:

[0078] 100 fibers extracted arbitrarily from cut fibers of the obtained copolymerized polyphenylene sulfide fiber were each measured for the fiber diameter (μm) on a cross-section of the fiber using an optical microscope (BH2 manufactured by Olympus Corporation) under the conditions of an objective lens magnification of 40 times and an eyepiece lens magnification of 10 times, and an arithmetic average value of the measured values was obtained and defined as an average fiber diameter (μm). When the fiber had a modified cross-section, the diameter of a true circle obtained by converting the area of the cross-section was defined as the average fiber diameter (μm).

C. Strength and Elongation:

[0079] The strength and the elongation of the copolymerized polyphenylene sulfide fiber were measured according to the above-described method (JIS L1013: 2010, 8.5 tensile strength and degree of elongation, sample length of 200 mm, tensile speed of 200 mm/min, arithmetic average value in five measurements per one level) using TENSILON (UTM-III-100 manufactured by ORIENTEC CO., LTD.).

D. Melting Point:

[0080] The melting point of the copolymerized polyphenylene sulfide fiber was obtained by raising the temperature of the obtained fiber with a DSC (Q1000 manufactured by TA instruments) from 30° C. to 320° C. at 16° C./min, measuring the temperature at the top of a melting peak (endothermic peak) observed at a temperature of 200° C. or higher in the obtained DSC curve, performing the measurement three times per one level, and obtaining an arithmetic average value of the measured values.

E. Degree of Crystallization:

[0081]
degree of crystallization(%)={(ΔHm−ΔHc)/146.2}×100

[0082] The degree of crystallization (%) of the copolymerized polyphenylene sulfide fiber was obtained by calculating a heat of crystal melting ΔHm from the area of a melting peak in a DSC curve obtained under the same conditions as in the above-described melting-point measurement, next calculating, when an exothermic peak is observed, a heat of crystallization ΔHc from the area of the exothermic peak, obtaining a quotient by dividing the difference between the ΔHm and the ΔHc by a heat of melting (146.2 J/g) of a perfect crystal poly(p-phenylene sulfide), performing the measurement three times per one level, and obtaining an arithmetic average value of the quotients.

F. Shrinkage Rate in Hot Water (98° C.)

[0083]
shrinkage rate in hot water=(L1−L2)/L1×100

[0084] One fiber from cut fibers of the obtained fiber was arbitrarily extracted, measured for an initial length L1, and measured for a posttreatment length L2 after immersed in 98° C. hot water for 20 minutes, then taken out from the hot water, and naturally dried, a quotient was obtained by dividing the difference between the L1 and the L2 by the initial length L1, the measurement was performed three times per one level, and an arithmetic average value of the quotients was obtained.

G. Birefringence:

[0085] The birefringence of the copolymerized polyphenylene sulfide fiber was obtained by observing, with respect to each of 10 fibers arbitrarily extracted from cut fibers, a side surface of the single fiber using an optical microscope (BX53M manufactured by Olympus Corporation) to measure the retardation and the optical path length and calculate the birefringence, then obtaining an arithmetic average value of the calculated values, and defining the arithmetic average value as the birefringence.

H. Fiber Weldability Test:

[0086] A hank with 20 winds was made for each of the copolymerized polyphenylene sulfide fiber obtained by the method described in each of the examples and comparative examples and a poly(p-phenylene sulfide) fiber that was obtained by the same method as in Example 1 except that the heat setting temperature was set at 230° C. and consisted of only the p-phenylene sulfide unit, a doubling yarn was produced by combining the two types of fibers, and this doubling yarn was further twisted at 30 T/30 cm using a twist counter (Maeda-type manual twist counter) to produce a twisted yarn. This twisted yarn was subjected to thermocompression bonding for 3 minutes by a flatbed heat press machine set at 230° C., and the fibers were welded to each other. Thereafter, the yarn was subjected to a peeling test using TENSILON (UTM-III-100 manufactured by ORIENTEC CO., LTD.) under the conditions of a sample length of 30 mm and a tensile speed of 50 mm/min, and an average value of maximum six values of the peel stress of the welded fibers was obtained as welding strength.

I. CV Value of Fiber Diameter:

[0087]
CV value of fiber diameter(%)=(standard deviation of fiber diameter)/(arithmetic average value of fiber diameter)×100

[0088] 100 fibers arbitrarily extracted from cut fibers of the obtained copolymerized polyphenylene sulfide fiber were each measured for the fiber diameter (μm) on a cross-section of the fiber using an optical microscope (BH2 manufactured by Olympus Corporation) under the conditions of an objective lens magnification of 40 times and an eyepiece lens magnification of 10 times, and the CV value of the fiber diameter was obtained by the equation. When the fiber had a modified cross-section, the diameter of a true circle obtained by converting the area of the cross-section was defined as the fiber diameter (μm).

J. Water Dispersibility:

[0089] The copolymerized polyphenylene sulfide cut fibers (binder fibers) obtained by the method described in each of the examples and comparative examples and poly(p-phenylene sulfide) cut fibers (main constituent fibers) consisting of only the p-phenylene sulfide unit having a fiber diameter of 11 μm and a cut length of 6 mm were mixed in a dispersion at a ratio of 20:80 mass % to prepare a solution having a fiber concentration of 0.4 mass %. The solution was stirred with a mixer at 13600 rpm for 10 seconds, then left to stand for 1 minute, and confirmed by the visual inspection. The solution hardly containing a fiber bundle was represented by ∘ and the solution containing a fiber bundle was represented by x.

K. Evaluation of Number of Dry Creases

[0090] The wet nonwoven fabric produced in each of the examples and comparative examples was cut into a size of 100 cm×100 cm, and then the wet nonwoven fabric undried was put into a hot-air drier and subjected to dry treatment at a temperature of 230° C. and a treatment time of 2.5 minutes. Five locations of an area of 10 cm×10 cm were arbitrarily extracted per one level in the dry-treated wet nonwoven fabric, the number of dry creases was counted using an optical microscope (BH-2 manufactured by Olympus Corporation) under the conditions of an objective lens magnification of 40 times and an eyepiece lens magnification of 10 times, and an arithmetic average value of the five locations was defined as the number of dry creases (creases/100 cm.sup.2).

L. Tensile Strength of Wet Nonwoven Fabric:

[0091] The wet nonwoven fabric produced in each of the examples and comparative examples was measured for a value of the maximum point load using TENSILON (UTM-III-100 manufactured by ORIENTEC CO., LTD.) at a sample width of 15 mm, an initial length of 20 mm, and a tensile speed of 20 mm/min, and an arithmetic average value of five measurements was defined as tensile strength (N/15 mm).

Example 1

[0092] A random copolymerized polyphenylene sulfide having a melt mass-flow rate of 175 g/10 min and containing 90 mol % of the p-phenylene sulfide unit and 10 mol % of the m-phenylene sulfide unit was vacuum-dried at 150° C. for 12 hours and subjected to melt spinning at a spinning temperature of 330° C. In the melt spinning, the copolymerized polyphenylene sulfide was supplied to a spinning pack while melt-extruded by a twin screw extruder and weighed by a gear pump. Thereafter, the copolymerized polyphenylene sulfide was discharged from a spinneret having 36 holes each with a hole diameter D of 0.23 mm and a land length L of 0.3 mm, under the condition of a single-hole discharge rate of 0.5 g/min. The spinneret that was used had a straight hole as an introduction hole positioned directly above the spinneret holes, and had a tapered connection portion between the introduction hole and the spinneret holes.

[0093] The copolymerized polyphenylene sulfide discharged from the spinneret was made to pass a 50-mm warm region and then air-cooled over 1.0 m, using a uniflow cooling device, under the conditions of a temperature of 25° C. and a wind speed of 18 m/min. Thereafter, an oil agent was imparted, and all the 36 filaments were wound up by a winder via a first godet roller and a second godet roller at 1000 m/min to give undrawn fibers.

[0094] The undrawn fiber was taken up by a feed roller equipped with a nip roller, strained between the feed roller and a first roller, and then made to go around the first roller and a second roller heated respectively at 90° C. and 100° C. six times for performing heat drawing. Further, the drawn fiber was made to go around a third roller heated at 170° C. six times and thus heat-set. The drawing ratio was 3.5 times, and the drawn heat-set fiber was, after the third roller, taken up by a non-heated roller at a peripheral speed of 400 m/min and then wound up by a winder to give a copolymerized polyphenylene sulfide fiber.

[0095] The obtained copolymerized polyphenylene sulfide fiber was cut by a cutter to give copolymerized polyphenylene sulfide cut fibers having a cut length of 6 mm. The cut fibers and poly(p-phenylene sulfide) cut fibers consisting of only the p-phenylene sulfide unit and having a fiber diameter of 11 μm and a cut length of 6 mm were mixed in a paper-making dispersion at a ratio of 20:80 mass % to prepare a paper-making solution having a fiber concentration of 0.4 mass %. This paper-making solution was supplied to a simple paper-making machine to give a wet nonwoven fabric having a basis weight of 50 g/m.sup.2. Further, the wet nonwoven fabric was put into a hot-air drier at 120° C., treated for 3 minutes, then air-cooled, and then subjected to thermocompression bonding using a flatbed heat press machine at 230° C., at a pressing pressure of 1.5 MPa for 3 minutes.

[0096] Table 1 shows evaluation results of the obtained copolymerized polyphenylene sulfide fiber. Table 1 shows that the obtained copolymerized polyphenylene sulfide fiber had an average fiber diameter of 11 μm, a degree of crystallization of 26.4%, a shrinkage rate in hot water of 3.4%, a strength of 4.0 cN/dtex, an elongation of 28%, a welding strength of 0.021 N, a birefringence of 0.25, and a CV value of the fiber diameter of 5.2%, and had good water dispersibility during the preparation of the paper-making solution. Further, the wet nonwoven fabric obtained as described above had 0.8 dry creases/100 cm.sup.2 when dried and a tensile strength of 24 N/15 mm, and thus the wet nonwoven fabric had excellent thermal dimensional stability and mechanical characteristics.

Example 2

[0097] A copolymerized polyphenylene sulfide fiber and a wet nonwoven fabric were obtained by the same method as in Example 1 except that a random copolymerized polyphenylene sulfide containing 75 mol % of the p-phenylene sulfide unit and 25 mol % of the m-phenylene sulfide unit was used.

[0098] Table 1 shows evaluation results of the obtained copolymerized polyphenylene sulfide fiber. Table 1 shows that the obtained copolymerized polyphenylene sulfide fiber had an average fiber diameter of 11 μm, a degree of crystallization of 25.8%, a shrinkage rate in hot water of 4.0%, a strength of 3.5 cN/dtex, an elongation of 26%, a welding strength of 0.044 N, a birefringence of 0.24, and a CV value of the fiber diameter of 5.5%, and had good water dispersibility during the preparation of the paper-making solution. Further, the wet nonwoven fabric obtained as described above had 0.9 dry creases/100 cm.sup.2 when dried and a tensile strength of 28 N/15 mm, and thus the wet nonwoven fabric had excellent thermal dimensional stability and mechanical characteristics.

Example 3

[0099] A copolymerized polyphenylene sulfide fiber and a wet nonwoven fabric were obtained by the same method as in Example 1 except that a random copolymerized polyphenylene sulfide containing 65 mol % of the p-phenylene sulfide unit and 35 mol % of the m-phenylene sulfide unit was used.

[0100] Table 1 shows evaluation results of the obtained copolymerized polyphenylene sulfide fiber. Table 1 shows that the obtained copolymerized polyphenylene sulfide fiber had an average fiber diameter of 11 μm, a degree of crystallization of 25.2%, a shrinkage rate in hot water of 4.4%, a strength of 3.0 cN/dtex, an elongation of 22%, a welding strength of 0.055 N, a birefringence of 0.24, and a CV value of the fiber diameter of 6.0%, and had good water dispersibility during the preparation of the paper-making solution. Further, the wet nonwoven fabric obtained as described above had 1.5 dry creases/100 cm.sup.2 when dried and a tensile strength of 32 N/15 mm, and thus the wet nonwoven fabric had excellent thermal dimensional stability and mechanical characteristics.

Example 4

[0101] A copolymerized polyphenylene sulfide fiber and a wet nonwoven fabric were obtained by the same method as in Example 1 except that a random copolymerized polyphenylene sulfide containing 95 mol % of the p-phenylene sulfide unit and 5 mol % of the m-phenylene sulfide unit was used.

[0102] Table 1 shows evaluation results of the obtained copolymerized polyphenylene sulfide fiber. Table 1 shows that the obtained copolymerized polyphenylene sulfide fiber had an average fiber diameter of 11 μm, a degree of crystallization of 28.5%, a shrinkage rate in hot water of 3.0%, a strength of 4.3 cN/dtex, an elongation of 29%, a welding strength of 0.011 N, a birefringence of 0.25, and a CV value of the fiber diameter of 5.0%, and had good water dispersibility during the preparation of the paper-making solution. Further, the wet nonwoven fabric obtained as described above had 0.7 dry creases/100 cm.sup.2 when dried and a tensile strength of 20 N/15 mm, and thus the wet nonwoven fabric had excellent thermal dimensional stability and mechanical characteristics.

Comparative Example 1

[0103] A poly(p-phenylene sulfide) fiber and a wet nonwoven fabric were obtained by the same method as in Example 1 except that a poly(p-phenylene sulfide) consisting of only the p-phenylene sulfide unit was used.

[0104] Table 1 shows evaluation results of the obtained poly(p-phenylene sulfide) fiber. Table 1 shows that the obtained poly(p-phenylene sulfide) fiber had an average fiber diameter of 11 μm, a degree of crystallization of 35.6%, a shrinkage rate in hot water of 1.6%, a strength of 4.6 cN/dtex, an elongation of 28%, a welding strength of 0.001 N, a birefringence of 0.25, and a CV value of the fiber diameter of 4.8%, and had good water dispersibility during the preparation of the paper-making solution. Further, the wet nonwoven fabric obtained as described above had 0 dry creases/100 cm.sup.2 when dried and a tensile strength of 3 N/15 mm, and thus the wet nonwoven fabric had excellent thermal dimensional stability but low mechanical characteristics.

TABLE-US-00001 TABLE 1 Comparative Evaluation Items Example 1 Example 2 Example 3 Example 4 Example 1 Resin Copolymerization mole mol % 10 25 35 5 0 ratio (m-component) Melting point ° C. 253 249 247 257 282 Drawing Drawing ratio times 3.5 3.5 3.5 3.5 3.5 conditions Heat setting temperature ° C. 170 170 170 170 170 Evaluation Average fiber diameter mm 11 11 11 11 11 of Average single-yarn dtex 1.3 1.3 1.3 1.3 1.3 fiber fineness physical Degree of % 26.4 25.8 25.2 28.5 35.6 properties crystallization Shrinkage rate in 98° C. % 3.4 4.0 4.4 3.0 1.6 hot water Strength cN/dtex 4.0 3.5 3.0 4.3 4.6 Elongation % 28 26 22 29 28 Welding strength N 0.021 0.044 0.055 0.011 0.001 Birefringence — 0.25 0.24 0.24 0.25 0.25 CV of fiber diameter % 5.2 5.5 6.0 5.0 4.8 Water dispersibility — ○ ○ ○ ○ ○ Evaluation of Number of dry creases dry 0.8 0.9 1.5 0.7 0 paper making creases/ 100 cm.sup.2 Tensile strength N/15 mm 24 28 32 20 3

Example 5

[0105] A copolymerized polyphenylene sulfide fiber and a wet nonwoven fabric were obtained by the same method as in Example 1 except that the copolymerized polyphenylene sulfide described in Example 2 was used and the heat setting temperature was set at 25° C.

[0106] Table 2 shows evaluation results of the obtained copolymerized polyphenylene sulfide fiber. Table 2 shows that the obtained copolymerized polyphenylene sulfide fiber had an average fiber diameter of 11 μm, a degree of crystallization of 21.7%, a shrinkage rate in hot water of 24.0%, a strength of 3.3 cN/dtex, an elongation of 30%, a welding strength of 0.094 N, a birefringence of 0.21, and a CV value of the fiber diameter of 5.7%, and had good water dispersibility during the preparation of the paper-making solution. Further, the wet nonwoven fabric obtained as described above had 8.1 dry creases/100 cm.sup.2 when dried and a tensile strength of 34 N/15 mm, and thus the wet nonwoven fabric had excellent thermal dimensional stability and mechanical characteristics.

Example 6

[0107] A copolymerized polyphenylene sulfide fiber and a wet nonwoven fabric were obtained by the same method as in Example 1 except that the copolymerized polyphenylene sulfide described in Example 2 was used and the heat setting temperature was set at 110° C.

[0108] Table 2 shows evaluation results of the obtained copolymerized polyphenylene sulfide fiber. Table 2 shows that the obtained copolymerized polyphenylene sulfide fiber had an average fiber diameter of 11 μm, a degree of crystallization of 20.1%, a shrinkage rate in hot water of 17.2%, a strength of 3.3 cN/dtex, an elongation of 29%, a welding strength of 0.098 N, a birefringence of 0.23, and a CV value of the fiber diameter of 5.4%, and had good water dispersibility during the preparation of the paper-making solution. Further, the wet nonwoven fabric obtained as described above had 6.7 dry creases/100 cm.sup.2 when dried and a tensile strength of 35 N/15 mm, and thus the wet nonwoven fabric had excellent thermal dimensional stability and mechanical characteristics.

Example 7

[0109] A copolymerized polyphenylene sulfide fiber and a wet nonwoven fabric were obtained by the same method as in Example 1 except that the copolymerized polyphenylene sulfide described in Example 2 was used and the heat setting temperature was set at 130° C.

[0110] Table 2 shows evaluation results of the obtained copolymerized polyphenylene sulfide fiber. Table 2 shows that the obtained copolymerized polyphenylene sulfide fiber had an average fiber diameter of 11 μm, a degree of crystallization of 24.0%, a shrinkage rate in hot water of 7.7%, a strength of 3.5 cN/dtex, an elongation of 28%, a welding strength of 0.077 N, a birefringence of 0.24, and a CV value of the fiber diameter of 5.6%, and had good water dispersibility during the preparation of the paper-making solution. Further, the wet nonwoven fabric obtained as described above had 3.5 dry creases/100 cm.sup.2 when dried and a tensile strength of 33 N/15 mm, and thus the wet nonwoven fabric had excellent thermal dimensional stability and mechanical characteristics.

Comparative Example 2

[0111] A copolymerized polyphenylene sulfide fiber was attempted to be obtained by the same method as in Example 1 using the copolymerized polyphenylene sulfide described in Example 2 and setting the heat setting temperature at 230° C., but the yarn was melted and attached to the heating roller during the heat setting, and the copolymerized polyphenylene sulfide fiber could not be obtained.

TABLE-US-00002 TABLE 2 Comparative Evaluation items Example 5 Example 6 Example 7 Example 2 Resin Copolymerization mole mol % 25 25 25 25 ratio (m-component) Melting point ° C. 249 249 249 249 Drawing Drawing ratio times 3.5 3.5 3.5 3.5 conditions Heat setting temperature ° C. 25 110 130 230 Evaluation of Average fiber diameter mm 11 11 11 Sampling fiber Average single-yarn dtex 1.3 1.3 1.3 unavailable physical fineness properties Degree of crystallization % 21.7 20.1 24.0 Shrinkage rate in 98° C. % 24.0 17.2 7.7 hot water Strength cN/dtex 3.3 3.3 3.5 Elongation % 30 29 28 Welding strength N 0.094 0.098 0.077 Birefringence — 0.21 0.23 0.24 CV of fiber diameter % 5.7 5.4 5.6 Water dispersibility — ○ ○ ○ Evaluation of Number of dry creases dry 8.1 6.7 3.5 paper making creases/ 100 cm.sup.2 Tensile strength N/15 mm 34 35 33

Comparative Example 3

[0112] A poly(p-phenylene sulfide) fiber was obtained without heat-drawing and heat-setting an undrawn fiber obtained by the same method as in Comparative Example 1. A wet nonwoven fabric was obtained by the same method as in Comparative Example 1 using this poly(p-phenylene sulfide) fiber.

[0113] Table 3 shows evaluation results of the obtained poly(p-phenylene sulfide) fiber. Table 3 shows that the obtained poly(p-phenylene sulfide) fiber had an average fiber diameter of 20 μm, a degree of crystallization of 6.9%, a shrinkage rate in hot water of 35.6%, a strength of 1.2 cN/dtex, an elongation of 344%, a welding strength of 0.181 N, a birefringence of 0.09, and a CV value of the fiber diameter of 10.1%, and had a residual fiber bundle observed and poor water dispersibility. Further, the wet nonwoven fabric obtained as described above had 9.8 dry creases/100 cm.sup.2 when dried and a tensile strength of 49 N/15 mm, and thus the wet nonwoven fabric had good mechanical characteristics but low thermal dimensional stability with many dry creases when dried.

Comparative Example 4

[0114] A poly(p-phenylene sulfide) fiber was obtained by making the undrawnfiber obtained in Comparative Example 3 run on the heating roller at 90° C. to perform fixed-length heat treatment. A wet nonwoven fabric was obtained by the same method as in Comparative Example 3 using this poly(p-phenylene sulfide) fiber.

[0115] Table 3 shows evaluation results of the obtained poly(p-phenylene sulfide) fiber. Table 3 shows that the obtained poly(p-phenylene sulfide) fiber had an average fiber diameter of 20 μm, a degree of crystallization of 18.3%, a shrinkage rate in hot water of 4.8%, a strength of 2.2 cN/dtex, an elongation of 300%, a welding strength of 0.161 N, a birefringence of 0.11, and a CV value of the fiber diameter of 12.0%, and had a residual fiber bundle observed and poor water dispersibility. Further, the wet nonwoven fabric obtained as described above had 2.4 dry creases/100 cm.sup.2 when dried and a tensile strength of 40 N/15 mm, and thus the wet nonwoven fabric had excellent thermal dimensional stability and mechanical characteristics.

Comparative Example 5

[0116] A copolymerized polyphenylene sulfide fiber was obtained without heat-drawing and heat-setting an undrawn fiber obtained by the same method as in Example 1. A wet nonwoven fabric was obtained by the same method as in Example 1 using this copolymerized polyphenylene sulfide fiber.

[0117] Table 3 shows evaluation results of the obtained copolymerized polyphenylene sulfide fiber. Table 3 shows that the obtained copolymerized polyphenylene sulfide fiber had an average fiber diameter of 20 μm, a degree of crystallization of 2.3%, a shrinkage rate in hot water of 45.9%, a strength of 1.1 cN/dtex, an elongation of 310%, a welding strength of 0.211 N, a birefringence of 0.09, and a CV value of the fiber diameter of 9.4%, and had a residual fiber bundle observed and poor water dispersibility. Further, the wet nonwoven fabric obtained as described above had 12.5 dry creases/100 cm.sup.2 when dried and a tensile strength of 57 N/15 mm, and thus the wet nonwoven fabric had good mechanical characteristics but low thermal dimensional stability with many dry creases when dried.

TABLE-US-00003 TABLE 3 Comparative Comparative Comparative Evaluation items Example 3 Example 4 Example 5 Resin Copolymerization mole mol % 0 0 25 ratio (m-component) Melting point ° C. 282 282 249 Drawing Drawing ratio times Undrawn Undrawn Undrawn conditions Heat setting temperature ° C. — (Fixed- — length heat treatment) Evaluation Average fiber diameter mm 20 20 20 of fiber Average single-yarn dtex 4.2 4.2 4.2 physical fineness properties Degree of crystallization % 6.9 18.3 2.3 Shrinkage rate in 98° C. % 35.6 4.8 45.9 hot water Strength cN/dtex 1.2 2.2 1.1 Elongation % 344 300 310 Welding strength N 0.181 0.161 0.211 Birefringence — 0.09 0.11 0.09 CV of fiber diameter % 10.1 12.0 9.4 Water dispersibility — X X X Evaluation Number of dry creases creases/ 9.8 2.4 12.5 of paper 100 cm.sup.2 making Tensile strength N/15 mm 49 40 57

[0118] The copolymerized polyphenylene sulfide fibers obtained in Examples 1 to 7 were each a fiber that contained a copolymerized polyphenylene sulfide containing a copolymerized m-phenylene sulfide, were thin, and had a low heat shrinkage rate and excellent weldability, and therefore the wet nonwoven fabrics each having less dry creases when dried and excellent thermal dimensional stability and mechanical characteristics could be obtained.

[0119] On the other hand, the poly(p-phenylene sulfide) fiber obtained in Comparative Example 1, which was a poly(p-phenylene sulfide) fiber consisting of only the p-phenylene sulfide unit, was thin and had a low heat shrinkage rate but had a high degree of crystallization to have poor weldability, and therefore a wet nonwoven fabric having excellent mechanical characteristics could not be obtained.

[0120] The copolymerized polyphenylene sulfide fiber (or the poly(p-phenylene sulfide) fiber) (hereinafter, collectively referred to as the polyphenylene sulfide fiber or the like) obtained in Comparative Example 3 or 5, which was an undrawn polyphenylene sulfide fiber or the like, had excellent mechanical characteristics but had a high shrinkage rate in hot water and thus poor thermal dimensional stability to generate many dry creases in the paper-making step, and therefore a wet nonwoven fabric for practical use could not be obtained.

[0121] Further, the poly(p-phenylene sulfide) fiber obtained in Comparative Example 4, which was a fiber obtained by subjecting an undrawn poly(p-phenylene sulfide) fiber consisting of only the p-phenylene sulfide unit to fixed-length heat treatment, had a low heat shrinkage rate but had a high CV value of the fiber diameter to have poor water dispersibility, and a fiber bundle was confirmed to be left in the obtained wet nonwoven fabric by visual inspection.

[0122] In Comparative Example 2, the heat setting temperature during the drawing was changed to 230° C., and a fiber that contained a polyphenylene sulfide containing a copolymerized m-phenylene sulfide was attempted to be obtained, but the yarn was melted and attached to the heating roller and the polyphenylene sulfide fiber could not be obtained.