High Melting Point Resin Fibers And Nonwoven Fabric

Abstract

Provided are: a high-melting-point resin fiber having heat resistance and solvent resistance, offering excellent workability/formability, and having a diameter of 4 .mu.m or less; and a nonwoven fabric including the high-melting-point resin fiber. Also provided is a method for efficiently producing a high-melting-point resin fiber having a diameter of 4 .mu.m or less, via laser melt electrospinning. The high-melting-point resin fiber according to the present invention includes a resin having a melting point of 250.degree. C. or higher and has a diameter of 4 .mu.m or less. In the high-melting-point resin fiber, the resin having a melting point of 250.degree. C. or higher is preferably a PEEK. The fiber preferably has a degree of crystallinity of 30% or less.

Description

TECHNICAL FIELD

[0001] The present invention relates to high-melting-point resin fibers and nonwoven fabrics each formed via melt electrospinning.

[0002] More specifically, the present invention relates to a high-melting-point resin fiber formed by processing a high-melting-point resin (film) into an ultrafine fiber via melt electrospinning using laser light as a heating means (laser melt electrospinning), where the ultrafine fiber has a diameter of 4 .mu.m or less; and to a nonwoven fabric including the high-melting-point resin fiber. This application claims priority to Japanese Patent Application No. 2014-213828, filed Oct. 20, 2014 to Japan, the entire contents of which are incorporated herein by reference.

BACKGROUND ART

[0003] Fibers having a fiber diameter on the order of submicrons or nanometers contribute to development of novel materials using their large specific surface areas and their fibrous form and have received attention. Melt electrospinning is proposed as an exemplary technique for producing such fibers. The melt electrospinning is an electrospinning technique in which a high voltage is applied to a polymer melt to form fibers.

[0004] For example, Patent Literature (PTL) 1 proposes a method using laser melt electrospinning as a melt electrospinning technique. With this method, fibers are produced via a heating-melting step and an electrospinning step. In the heating-melting step, laser light (laser beam) is applied to a thermoplastic resin to heat and melt the thermoplastic resin. In the electrospinning step, a voltage is applied to a molten zone of the thermoplastic resin to elongate a fiber, and the elongated fiber is collected on a collector. According to this method, the fibers are produced by preparing or using a linear resin article as a spinning material and allowing the linear resin article to eject a fiber from the tip of the linear resin article.

[0005] PTL 2 discloses a method using the laser melt electrospinning technique. In this method, linear laser light is applied to a sheet-like article including a thermoplastic resin to heat and melt an edge of the sheet-like article linearly to form a molten zone. With this, a potential difference is provided between the molten zone and a metallic collector to form needle protrusions in the thermally molten zone of the sheet-like article. Fibers ejected from the needle protrusions are allowed to fly toward the metallic collector and are collected on the metallic collector or on a collector disposed between the molten zone and the metallic collector.

[0006] High-melting-point resins such as poly(ether ether ketone)s (PEEKs), poly(phenylene sulfide)s (PPSs), and polyamideimides (PAIS) have heat resistance, flame retardancy, chemical resistance, and impact resistance, are also called "super engineering plastics", and are widely used in applications in the automotive and electric/electronic fields. Among them, PEEKs have a melting point of 334.degree. C. and have such super heat resistance as to be continuously usable at 250.degree. C. The PEEKs are aromatic plastics which have approximately highest heat resistance among thermoplastic resins and have chemical resistance typically to organic solvents.

[0007] These high-melting-point resins have been formed into fibers typically by meltblowing technique. This technique, however, gives fibers having diameters of several micrometers to several tens of micrometers and can hardly give more ultrafine nanofibers. Such more ultrafine nanofibers have been obtained typically by the electrospinning technique. This technique, however, is not applicable to the PEEKs and other super engineering plastics, which have high melting points. This is because most of these super engineering plastics are insoluble typically in organic solvents.

CITATION LIST

Patent Literature

[0008] PTL 1: Japanese Unexamined Patent Application Publication (JP-A) No. 2007-239114

[0009] PTL 2: JP-A No. 2010-275661

SUMMARY OF INVENTION

Technical Problem

[0010] In contrast, the laser melt electrospinning technique disclosed in PTL 2 does not control the degree of crystallinity of the material film and tends to fail to perform the working at a higher rate when the material film (sheet) has a high degree of crystallinity. In addition, the technique gives fibers which have diameters of 5 .mu.m or more and which have high degrees of crystallinity, and this impedes working/forming of the fibers.

[0011] Accordingly, the present invention has an object to provide a high-melting-point resin fiber that has heat resistance and solvent resistance, offers excellent workability/formability, and has a diameter of 4 .mu.m or less; and to provide a nonwoven fabric formed from the high-melting-point resin fiber. The present invention has another object to provide a method for efficiently producing a high-melting-point resin fiber having a diameter of 4 .mu.m or less, via laser melt electrospinning.

Solution to Problem

[0012] After intensive investigations to achieve the objects, the inventors of the present invention found that laser melt electrospinning, when employing a material polymer sheet including a resin having a melting point of 250.degree. C. or higher, can give a high-melting-point resin fiber which includes the resin having a melting point of 250.degree. C. or higher and which has a diameter of 4 .mu.m or less. The present invention has been made on the basis of these findings.

[0013] Specifically, the present invention provides a high-melting-point resin fiber that includes a resin having a melting point of 250.degree. C. or higher and has a diameter of 4 .mu.m or less.

[0014] In the high-melting-point resin fiber according to the present invention, the resin having a melting point of 250.degree. C. or higher is preferably a PEEK.

[0015] The high-melting-point resin fiber according to the present invention preferably has a degree of crystallinity of 30% or less.

[0016] The present invention also provides a method for producing a high-melting-point resin fiber. The method includes applying planar laser light to a polymer sheet including a noncrystalline high-melting-point resin to heat and melt an edge of the polymer sheet linearly to thereby form a band-like molten zone. With this, a potential difference is provided between the band-like molten zone of the polymer sheet and a fiber collecting plate to form a needle protrusion in the band-like molten zone of the polymer sheet and to allow a fiber ejected from the needle protrusion to fly toward the fiber collecting plate. The fiber is collected on the fiber collecting plate or on a collector disposed between the molten zone and the fiber collecting plate, to give the high-melting-point resin fiber.

[0017] In the method according to the present invention for producing a high-melting-point resin fiber, the polymer sheet is preferably fed at a speed of 2 to 20 ram/min.

[0018] In the method according to the present invention for producing a high-melting-point resin fiber, the potential difference is preferably 0.1 to 30 kV/cm.

[0019] In the method according to the present invention for producing a high-melting-point resin fiber, the polymer sheet preferably has a viscosity of 800 Pas or less as measured at a temperature of 400.degree. C. and a shear rate of 121.6 s.sup.-1.

[0020] The present invention also provides a nonwoven fabric including the high-melting-point resin fiber according to the present invention.

[0021] Specifically, the present invention relates to the followings.

[0022] (1) The present invention relates to a high-melting-point resin fiber which includes a resin having a melting point of 250.degree. C. or higher and which has a diameter of 4 .mu.m or less.

[0023] (2) In the high-melting-point resin fiber according to (1), the resin having a melting point of 250.degree. C. or higher may be a PEEK.

[0024] (3) The high-melting-point resin fiber according to one of (1) and (2) may have a degree of crystallinity of 30% or less.

[0025] (4) The high-melting-point resin fiber according to any one of (1) to (3) may have an average fiber diameter as an assembly of 4 .mu.m or less.

[0026] (5) The present invention also relates to a method for producing the high-melting-point resin fiber according to any one of (1) to (4). The method includes applying planar laser light to a polymer sheet including a noncrystalline high-melting-point resin to heat and melt an edge of the polymer sheet linearly to thereby form a band-like molten zone. With this, a potential difference is provided between the band-like molten zone and a fiber collecting plate to form a needle protrusion in the band-like molten zone of the polymer sheet and to allow a fiber ejected from the needle protrusion to fly toward the fiber collecting plate. The fiber is collected on the fiber collecting plate or on a collector disposed between the molten zone and the fiber collecting plate. Thus, the high-melting-point resin fiber is obtained.

[0027] (6) In the method according to (5) for producing a high-melting-point resin fiber, the polymer sheet may be fed at a speed of 2 to 20 mm/min.

[0028] (7) In the method according to one of (5) and (6) for producing a high-melting-point resin fiber, the potential difference may be 0.1 to 30 kV/cm.

[0029] (8) In the method according to any one of (5) to (7) for producing a high-melting-point resin fiber, the polymer sheet may have a viscosity of 800 Pas or less as measured at a temperature of 400.degree. C. and a shear rate of 121.6 5.sup.-1.

[0030] (9) In the method according to any one of (5) to (8) for producing a high-melting-point resin fiber, the planar laser light may be applied at a power of 5 to 100 W per 13 cm.

[0031] (10) In the method according to any one of (5) to (9) for producing a high-melting-point resin fiber, the polymer sheet may have a degree of crystallinity of 25% or less.

[0032] (11) The present invention also relates to a nonwoven fabric derived from the high-melting-point resin fiber according to any one of (1) to (4).

Advantageous Effects of Invention

[0033] The high-melting-point resin fiber according to the present invention has excellent workability because of having a low degree of crystallinity and also has heat resistance and chemical resistance at excellent levels because of using a high-melting-point resin such as a PEEK. The nonwoven fabric derived from the fiber offers excellent separability when used typically as a cell (battery) separator or a filter for medical materials because it includes extremely ultrafine fibers. In addition, the nonwoven fabric also has durability, heat resistance, and chemical resistance at excellent levels because of being derived from (formed from) a material high-melting-point resin such as a PEEK.

BRIEF DESCRIPTION OF DRAWINGS

[0034] FIG. 1 is a schematic diagram schematically illustrating a method according to an embodiment of the present invention for producing a high-melting-point resin fiber;

[0035] FIG. 2 is a schematic view of Taylor cones formed in a band-like molten zone; and

[0036] FIG. 3 is a schematic cross-sectional view of nonwoven fabric production equipment using the method according to the present invention for producing a high-melting-point resin fiber.

DESCRIPTION OF EMBODIMENTS

[0037] High-Melting-Point Resin Fibers

[0038] The high-melting-point resin fibers according to the present invention are ultrafine fibers having small fiber diameters and have diameters of 4 .mu.m or less. The fibers may have diameters of preferably 3 .mu.m or less (0.1 to 3 .mu.m), and more preferably 2 .mu.m or less. The ultrafine fibers having diameters as above may include fibers having fiber diameters of typically about 50 to about 1000 nm. The diameters of the fibers can be adjusted by adjusting conditions of the after-mentioned method for producing a high-melting-point resin fiber as appropriate. Examples of the conditions include the polymer sheet thickness, the polymer sheet feed speed, and the laser intensity. The diameters of the high-melting-point resin fibers can be measured typically using an electron microscope.

[0039] The high-melting-point resin fibers according to the present invention include a resin having a melting point of 250.degree. C. or higher. The high-melting-point resin fibers have melting points of preferably 260.degree. C. or higher, more preferably 270.degree. C. or higher, and furthermore preferably 280.degree. C. or higher. Non-limiting examples of such resin having a melting point of 250.degree. C. or higher include poly(ether ether ketone)s (PEEKs) (melting point: 334.degree. C.), poly(phenylene sulfide)s (PPSs) (melting point: 290.degree. C.), polyamideimides (PAIS) (melting point: 300.degree. C.), polytetrafluoroethylenes (PTFEs) (melting point: 327.degree. C.), silicon resins (melting point: about 300.degree. C.), fluorocarbon resins (melting point: 327.degree. C.), and liquid-crystal polymers (melting point: 260.degree. C. to 300.degree. C.). Among them, PEEKs are preferred because of having a high melting point and offering heat resistance and solvent resistance at excellent levels.

[0040] The high-melting-point resin fibers according to the present invention have a degree of crystallinity of preferably 30% or less, more preferably 29% or less, and furthermore preferably 28% or less. The fibers, when having a degree of crystallinity of 30% or less, may offer excellent workability and can be easily formed or shaped typically into nonwoven fabrics, filters, and separators. The degree of crystallinity may be determined typically by X-ray diffractometry, differential scanning calorimetry using a differential scanning calorimeter (DSC), or densimetry. In this application, the degree of crystallinity is calculated from the amounts of heat determined by differential scanning calorimetry according to the method described in experimental examples.

[0041] The high-melting-point resin fibers according to the present invention are preferably obtained by the after-mentioned method for producing a high-melting-point resin fiber, using a noncrystalline high-melting-point resin as a material polymer sheet.

[0042] The noncrystalline high-melting-point resin (sheet), which is a polymer sheet, has a thickness of typically 0.01 to 10 mm, and preferably 0.05 to 5.0 mm. The polymer sheet, when having a thickness within the range, may contribute to easy production of the high-melting-point resin fibers as mentioned below.

[0043] The high-melting-point resin fibers according to the present invention may have an average fiber diameter as an assembly not limited, but preferably 4 .mu.m or less (0.1 to 4 .mu.m), more preferably 3 .mu.m or less, and furthermore preferably 2 .mu.m or less. The average fiber diameter may be determined typically by taking two or more (e.g., ten) images of fibers using a scanning electron microscope, measuring diameters of about ten optional fibers per image in the images typically using an image processing software, and averaging the measured diameters.

[0044] Method for Producing High-Melting-Point Resin Fibers

[0045] The high-melting-point resin fibers according to the present invention are preferably produced using the laser melt electrospinning below. Specifically, the laser melt electrospinning is performed in the following manner.

[0046] The method according to the present invention for producing a high-melting-point resin fiber (via laser melt electrospinning) will be illustrated with reference to the attached drawings. FIG. 1 is a schematic diagram schematically illustrating a method according to an embodiment for producing a high-melting-point resin fiber. According to the method for producing a high-melting-point resin fiber, the fibers are produced in the following manner. Band-like laser light (laser beam) is applied to a polymer sheet including a high-melting-point resin (sheet) to heat and melt an edge of the polymer sheet linearly to thereby form a band-like molten zone. With this, a potential difference is provided between the band-like molten zone and a fiber collecting plate to form needle protrusions in the band-like molten zone of the polymer sheet and to allow fibers ejected from the needle protrusions to fly toward the fiber collecting plate. The fibers are collected on the fiber collecting plate or on a collector disposed between the molten zone and the fiber collecting plate.

[0047] In the method for producing a high-melting-point resin fiber illustrated in FIG. 1, laser light having a spot cross section emitted from a laser source 1 is converted into planar laser light 5 having a linear cross section by the working of a light-path controller. The light-path controller includes a beam-expander-homogenizer 2, a collimation lens 3, and a cylindrical lens group 4. The planar laser light 5 is applied to a band-like melt zone 6a of a polymer sheet 6, where the polymer sheet 6 is held by a holder 7. With this, a voltage is applied from a high voltage generator 10 to form a potential difference between the band-like melt zone 6a and a fiber collecting plate 8 disposed under the polymer sheet 6. A thermography 9 observes the temperature of the band-like melt zone 6a so as to optimize conditions such as the voltage and the laser light to be applied.

[0048] In the embodiment illustrated in FIG. 1, the holder 7 that holds the polymer sheet 6 functions also as an electrode and, when receives the voltage from the high voltage generator 10, imparts an electric charge to the band-like melt zone 6a of the polymer sheet 6. The fiber collecting plate 8 has a surface electric resistance approximately equal to those of metals. The fiber collecting plate 8 may be in the form selected typically from plate, roller, belt, net, sawlike, wave, needle, and linear shapes. The light-path controller is an assembly of optical components and typically includes the beam-expander-homogenizer 2, the collimation lens 3, and the cylindrical lens group 4. The light-path controller can convert the spot laser light into the planar laser light 5.

[0049] In the embodiment illustrated in FIG. 1, the planar laser light 5 is applied to heat and melt the band-like melt zone 6a of the polymer sheet 6 to thereby form a heated, molten zone, and an electric charge is imparted to the molten zone. As illustrated in FIG. 2, electrical charge accumulates and repels on the surface of the band-like melt zone 6a to which the electric charge is imparted, and this causes needle protrusions (Taylor cones) 6b to gradually form on the surface. When the repulsive force of the electric charge exceeds the surface tension, the molten thermoplastic resin is ejected as fibers from the tips of the Taylor cones toward the fiber collecting plate 8 by the action of electrostatic attraction. Namely, fibers are formed from the needle protrusions 6b and fly toward the fiber collecting plate 8. As a result, the fibers elongate and are collected by the fiber collecting plate 8. In an embodiment, a collector is disposed on or over the fiber collecting plate 8. In this embodiment, the fibers are collected on the collector. Specifically, in the method according to the present invention for producing a high-melting-point resin fiber, a member for collecting fibers may be the fiber collecting plate itself, or not the fiber collecting plate, but a collector (collecting member) disposed on the fiber collecting plate. FIG. 2 is a schematic view of Taylor cones formed in the band-like melt zone 6a.

[0050] The number (intervals) of the Taylor cones as illustrated in FIG. 2 can be controlled by changing the thickness of the polymer sheet 6 as appropriate. The "growth" of a Taylor cone refers to increase of the height (h in FIG. 2) of the Taylor cone.

[0051] The number of the Taylor cones is not limited, but is preferably 1 or more per 2 cm of the heated, molten zone of the polymer sheet and is more preferably 1 to 100 per 2 cm. This is because as follows. Fiber production, if using Taylor cones present in a number of 1 per 2 cm or less, is not preferred from the viewpoints of uniformity ratio and production amount of the nonwoven fabric. In consideration of this, the Taylor cones are preferably present in a larger number. However, fiber production using Taylor cones present in a number of 100 per 2 cm or more, may cause lower uniformity ratio because of electric repulsion between Taylor cones. The number of Taylor cones is more preferably 1 to 50 per 2 cm, and particularly preferably 2 to 10 per 2 cm.

[0052] Non-limiting examples of the laser source include YAG laser, carbon dioxide gas (CO.sub.2) laser, argon laser, excimer laser, and helium-cadmium laser. Among them, carbon dioxide gas laser is preferred because of having high power source efficiency and being capable of highly melting PEEK resins. The laser light may have a wavelength of typically about 200 nm to about 20 .mu.m, preferably about 500 nm to about 18 .mu.m, and furthermore preferably about 5 to about 15 .mu.m.

[0053] The laser light, when to be applied as planar laser light (laser sheet) in the method for producing a high-melting-point resin fiber, preferably has a thickness (sheet thickness) of about 0.5 to about 10 mm. The laser light, if having a thickness of less than 0.5 mm, may fail to contribute to the formation of Taylor cones. The laser light, if having a thickness of greater than 10 mm, may cause deterioration of the material because of longer residence time in melting.

[0054] The power (output) of the laser light may be controlled within such a range that the band-like molten zone has a temperature equal to or higher than the melting point of the thermoplastic resin and equal to or lower than the ignition point of the polymer sheet. The power is preferably high from the viewpoint of allowing the ejected fibers to have small fiber diameters. The specific power of the laser light can be selected as appropriate according typically to properties (such as melting point and limiting oxygen index (LOI)) and shape of the thermoplastic resin to be used, and to the feed speed of the polymer sheet. The power is generally about 5 to about 100 W per 13 cm, preferably 20 to 60 W per 13 cm, and furthermore preferably 30 to 50 W per 13 cm. The intensity of the laser light is the output (power) of the spot beam emitted from the laser source.

[0055] The temperature of the band-like molten zone is not limited, as long as being equal to or higher than the melting point of the high-melting-point resin and equal to or lower than the ignition point of the resin, but is generally about 300.degree. C. to about 600.degree. C., and preferably 350.degree. C. to 500.degree. C.

[0056] In the method for producing a high-melting-point resin fiber according to the embodiment of the present invention as illustrated in FIG. 1, the laser light is applied from only one direction to the band-like molten zone (edge) of the polymer sheet. In another embodiment, the laser light may be applied typically from two directions through a reflector to the band-like molten zone (edge) of the polymer sheet. This configuration contributes to more uniform melting of the edge of the sheet-like article (sheet) even when the sheet has a large thickness.

[0057] The potential difference to be generated between the edge of the polymer sheet and the collector in the method for producing a high-melting-point resin fiber is preferably of a high voltage within a range not causing discharge. The potential difference can be selected as appropriate according typically to the required fiber diameter, the distance between the electrode and the collector, and the irradiance of the laser light, and is generally about 0.1 to about 30 kV/cm, preferably 0.5 to 20 kV/cm, and more preferably 1 to 10 kV/cm.

[0058] The voltage may be applied to the molten zone of the polymer sheet by a direct application method, in which the portion to be irradiated with the laser light (band-like molten zone of the polymer sheet) is coincident with an electrode unit for imparting the electric charge. However, the voltage is preferably applied by an indirect application method, in which the portion to be irradiated with the laser light is disposed at a position different from the position of the electrode unit for imparting the electric charge. The indirect application method is preferred because the equipment can be prepared easily and simply, the laser light can be effectively converted into thermal energy, and the reflection direction of the laser light can be easily controlled to offer high safety. Among such indirect application methods, preferred is a method in which the portion to be irradiated with the laser light is disposed downstream in the feeding direction of the polymer sheet. In particular, in a preferred embodiment of the production method, the planar laser light is applied to the polymer sheet downstream from the electrode unit, and the distance between the electrode unit and the portion to be irradiated with the laser light (e.g., the distance between the lower end of the electrode unit and the upper outer periphery of the planar laser light) is controlled within a specific range (e.g., about 10 mm or less). This distance can be selected according typically to the electric conductivity, thermal conductivity, and glass transition point of a PEEK resin, and the irradiance of the laser light. The distance is typically about 0.5 to about 10 mm, preferably about 1 to about 8 mm, more preferably about 1.5 to about 7 mm, and particularly preferably about 2 to about 5 mm. When the two portions are disposed at a distance within this range, the resin adjacent to the portion to be irradiated with the laser light offers higher molecular mobility and can receive sufficient electric charge in a molten state. This contributes to better productivity.

[0059] The distance between the edge of the polymer sheet (tip of a Taylor cone) and the collector is not limited and may be generally 5 mm or more. For efficient production of ultrafine fibers, the distance is preferably about 10 to about 300 mm, more preferably about 15 to about 200 mm, furthermore preferably about 50 to about 150 mm, and particularly preferably about 80 to about 120 mm.

[0060] The polymer sheet, when fed continuously, may be fed at a feed speed not limited, but generally about 2 to about 20 mm/min, preferably 3 to 15 ram/min, and more preferably 4 to 10 mm/min. The feeding of the polymer sheet at a higher speed contributes to higher productivity. However, the feeding, if performed at an excessively high speed, may impede sufficient melting of the resin adjacent to the portion to be irradiated with the laser light and may impede fiber production. In contrast, the feeding, if performed at an excessively low speed, may cause decomposition of the high-melting-point resin and lower productivity.

[0061] The space between the edge of the polymer sheet and the collector may be in an inert gas atmosphere in the production method. The presence of the inert gas atmosphere in the space restrains the ignition of the fibers and allows the laser light to be applied at a higher power. Non-limiting examples of the inert gas include nitrogen gas, helium gas, argon gas, and carbon dioxide gas. Among them, nitrogen gas is generally used. In addition, the use of the inert gas can restrain oxidation reactions in the band-like molten zone.

[0062] The space may be heated. This allows the resulting fibers to have smaller fiber diameters. Specifically, heating of the air or inert gas in the space can restrain abrupt temperature fall of fibers under growing, and this promotes growth or extension of the fibers to give more ultrafine fibers. The heating may be performed typically by using a heater (such as a halogen heater) or by applying laser light. The heating temperature may be selected typically within the range of from 50.degree. C. to lower than the ignition point of the resin. In consideration of spinnability, the heating temperature is preferably lower than the melting point of the resin.

[0063] Polymer Sheet

[0064] The noncrystalline polymer sheet has a degree of crystallinity of typically preferably 25% or less, more preferably 20% or less, and furthermore preferably 15% or less. The polymer sheet, when having a degree of crystallinity of 25% or less, may give high-melting-point resin fibers having a low degree of crystallinity. The degree of crystallinity of the polymer sheet may be determined by the same method as the degree of crystallinity of the high-melting-point resin fibers.

[0065] As used herein, the term "noncrystallinity" refers to such a property of a resin that the resin has a bulky molecular chain (molecular chain with large steric hindrance) in its molecular frame, thereby cannot take a regular molecular arrangement during process of cooling and solidifying from a molten state, but takes a random molecular arrangement even in a solidified state.

[0066] The noncrystalline (amorphous) polymer sheet preferably has a low viscosity for the formation of ultrafine fibers such as nanofibers and may have a viscosity of typically preferably 800 Pas or less (50 to 800 PaS), more preferably 600 Pas or less, and furthermore preferably 400 Pas or less, as measured at a temperature of 400.degree. C. and a shear rate of 121.6 5s.sup.1. The viscosity at a temperature of 400.degree. C. may be determined by the method described in the experimental examples using a capillary rheometer (trade name Capillograph 1D, supplied by Toyo Seiki Seisaku-Sho Ltd.). The shear rate may also be determined using such a capillary rheometer.

[0067] The noncrystalline polymer sheet may be produced typically by heating, melting, and molding a noncrystalline resin in the form of chips into a sheet typically using a T-die extruder. The noncrystalline resin in the form of chips may be selected from commercial products, a preferred, but non-limiting example of which is one available under the trade name of VESTAKEEP 1000G (supplied by Daicel-Evonik Ltd.). The heating temperature in the T-die extruder has only to be equal to or higher than the melting point of the resin and is typically 350.degree. C. to 400.degree. C.

[0068] The noncrystalline polymer sheet may contain any of various additives for use in fibers. Non-limiting examples of the additives include stabilizers (such as antioxidants, ultraviolet absorbers, and thermal stabilizers), flame retardants, antistatic agents, colorants, fillers, lubricants, antimicrobial agents, insect/tick repellents, antifungal agents, flatting agents, heat storage media, flavors, fluorescent brighteners, wetting agents, plasticizers, thickeners, dispersants, blowing agents, and surfactants. The polymer sheet may contain each of different additives alone or in combination.

[0069] Among these additives, for example, one or more surfactants are preferably used. Assume that a high voltage is applied to the polymer sheet to inject electric charge into the polymer sheet. In this case, since including the high-melting-point resin, the polymer sheet offers high electric insulation and resists injection of the electric charge into a thermally molten zone having a lower electric resistance. However, the use of a surfactant allows the fiber having high electric insulation to have a lower electric resistance in its surface, and this allows the electric charge to be injected sufficiently into the thermally molten zone. Assume that the polymer sheet includes multiple components. In this case, the impartment of an additive such as a surfactant is effective for phase separation upon application of a high voltage to the polymer sheet to inject electric charge into the sheet.

[0070] Each of these additives may be used in a proportion of 50 parts by mass or less, typically about 0.01 to about 30 parts by mass, and preferably about 0.1 to about 5 parts by mass, per 100 parts by mass of the resin in the polymer sheet.

[0071] Nonwoven Fabric

[0072] The nonwoven fabric according to the present invention may be one produced by the after-mentioned production method, or one which is derived from the high-melting-point resin fibers according to the present invention, but which is produced by another method.

[0073] The thickness of the nonwoven fabric according to the present invention may be selected as appropriate according to the intended use within the range of about 0.0001 to about 100 mm, and is generally about 0.001 to about 50 mm, preferably about 0.01 to about 15 mm, and more preferably about 0.05 to about 1 mm. The mass per unit area (METSUKE) of the nonwoven fabric may also be selected according to the intended use and is generally about 0.001 to about 100 g/m.sup.2, preferably about 0.05 to about 50 g/m.sup.2, and more preferably about 0.1 to about 10 g/m.sup.2. The fiber diameter, thickness, mass per unit area, and other geometries of the nonwoven fabric according to the present invention to be produced can be controlled by regulating conditions such as the sheet feed speed, laser intensity, and collector traveling speed in the after-mentioned nonwoven fabric production method.

[0074] The nonwoven fabric according to the present invention may be subjected to an after-processing according to the purpose. Non-limiting examples of the after-processing include electrification treatment via electretizing, plasma discharge treatment, corona discharge treatment, sulfonation treatment, and hydrophilization treatment typically via graft polymerization. The nonwoven fabric may be further subjected to secondary processing (fabricating) and/or may be laminated and integrated with another material. Non-limiting examples of the other material include other nonwoven fabrics (such as spunbond nonwoven fabrics), woven knitted fabrics, films, plates, and substrates.

[0075] Nonwoven Fabric Production Method

[0076] Next, an exemplary method for producing a nonwoven fabric will be illustrated. The nonwoven fabric production method as mentioned below can continuously produce high-melting-point resin fibers while moving the collection position of fibers with time, where the fibers are allowed to fly toward the fiber collecting plate.

[0077] Exemplary techniques for moving the collecting position of the fibers with time, where the fibers are allowed to fly toward the fiber collecting plate, include (1) the technique of moving the collector (or the fiber collecting plate when the fiber collecting plate itself functions as a collector), (2) the technique of moving the position at which the polymer sheet is held, (3) the technique of allowing mechanical, magnetic, or electric force to act upon fibers flying from the Taylor cones toward the collector, such as the technique of blowing air to the fibers during flying, and (4) a technique as any selective combination of the techniques (1) to (3).

[0078] Among them, the technique (1), namely, the technique of moving the collector is desirable. This is because the technique (1) contributes to easy simplification of the configuration of the equipment and to easy control of the geometries (such as thickness and mass per unit area) of the nonwoven fabric to be produced. The nonwoven fabric production method according to an embodiment using the technique (1) will be illustrated in detail below.

[0079] The nonwoven fabric production method using the technique (1) is based on the production method illustrated in FIG. 1, in which a collector is placed on the fiber collecting plate 8. While moving the collector in a direction (right hand or left hand in the figure) perpendicular to the width direction of the polymer sheet 6, the high-melting-point resin fibers according to the present invention are produced continuously. The collector may travel at a constant speed, or at speed varying with time, or may be moved and stopped repeatedly. To continuously produce the high-melting-point resin fibers according to the present invention, the polymer sheet 6 may be continuously fed toward the fiber collecting plate 8 (toward the collector) with the progress of the fiber production process, as has been described above. The speed (feed speed) of the continuous feeding of the polymer sheet is as described in the method for producing a high-melting-point resin fiber.

[0080] The traveling speed of the collector on or over the fiber collecting plate 8 is not limited, may be selected as appropriate in consideration typically of the mass per unit area of the fiber sheet to be produced, and is generally about 10 to about 2000 ram/min. For example, assume that a polymer sheet having a mass per unit area of 1000 g/m.sup.2 is fed at a feed speed of 0.5 ram/min. In this case, a nonwoven fabric having a mass per unit area of about 0.5 g/m.sup.2 can be continuously produced by setting the traveling speed of the collector at about 1000 mm/min.

[0081] FIG. 3 is a schematic cross-sectional view of nonwoven fabric production equipment using the method for producing a high-melting-point resin fiber as illustrated in FIG. 1. The equipment illustrated in FIG. 3 includes a laser source 11; a light-path controller 12; a polymer sheet feeder 13 capable of continuously feeding the polymer sheet 6; and a cabinet 23. The cabinet 23 houses a holder 16 that holds the polymer sheet 6; an electrode 17 that imparts electric charge to the polymer sheet 6; a collector 22 that collects fibers; a fiber collecting plate 14 that is arranged so as to face the electrode 17 through a band-like melt zone (edge) 6a of the polymer sheet 6 and the collector 22; and a heater 15. The equipment also includes high voltage generators 20a and 20b that apply voltages independently to the electrode 17 and the fiber collecting plate 14; and a pulley 21 for moving the collector 22. The light-path controller 12 is an assembly of optical components as described above and includes, for example, the beam-expander-homogenizer 2, the collimation lens 3, and the cylindrical lens group 4 as illustrated in FIG. 1.

[0082] With reference to FIG. 3, the planar laser light 5 exits from the laser source 11, travels via the light-path controller 12, is introduced into the cabinet 23, and is applied to the band-like melt zone (edge) 6a of the polymer sheet 6. The polymer sheet feeder 13 is mounted on the cabinet 23 and includes a motor, and a mechanism that converts the rotation of the motor into a rectilinear motion. The polymer sheet feeder 13 accepts the polymer sheet 6 and continuously feeds the same into the cabinet 23. In contrast, the lower part of the polymer sheet 6 is held by the holder 16 to which the electrode 17 is mounted. The polymer sheet 6 and the electrode 17 are always in contact with each other, and this allows electric charge to be imparted to the polymer sheet 6 when a voltage is applied to the electrode 17.

[0083] The fiber collecting plate 14 pairs up with the electrode 17 and functions as a counter electrode to the electrode 17. The fiber collecting plate 14 is disposed at such a position as to face the electrode 17 through the band-like melt zone (edge) 6a of the polymer sheet 6 and the collector 23. This configuration gives a potential difference between the band-like melt zone (edge) 6a of the polymer sheet 6 and the collector 22 when voltages are applied to the electrode 17 and the fiber collecting plate 14. The high voltage generators 20a and 20b are coupled respectively to the electrode 17 and to the fiber collecting plate 14 and apply voltages to the electrode 17 and the fiber collecting plate 14. In the nonwoven fabric production equipment, the electrode 17 is a positive electrode, and the fiber collecting plate 14 is a negative electrode. The reverse configuration will also do. The collector 22 is a conveyor belt including the pulley 21 and a conveyor belt, and the conveyor belt itself corresponds to the collector 22. Accordingly, the collector 22 (conveyor belt) travels to a predetermined direction (e.g., right hand in the figure) with the driving of the pulley 21.

[0084] The nonwoven fabric production equipment illustrated in FIG. 3 includes the heater 15 and can heat fibers ejected and elongated from the band-like melt zone (edge) 6a of the polymer sheet 6 toward the collector 23. The equipment also includes a laser light absorber 19 and a heat absorber 18 in the cabinet 23.

[0085] In the nonwoven fabric production equipment illustrated in FIG. 3, while voltages are applied to both the electrode 17 and the fiber collecting plate 14 and while the polymer sheet 6 is fed by the working of the polymer sheet feeder 13 and the holder 16, the planar laser light 5 is applied to the band-like melt zone (edge) 6a of the polymer sheet 6. This allows Taylor cones to form in the band-like melt zone (edge) 6a of the polymer sheet 6, allows the Taylor cones to eject fibers, and allows the fibers to fly (to be jetted) toward the fiber collecting plate 14. As a result, elongated fibers are collected by the collector 22, as has been described above. Then, while the polymer sheet 6 is continuously fed (while fibers are continuously ejected), the collector 22 is moved, to yield a nonwoven fabric on the collector 22.

[0086] In the nonwoven fabric production equipment illustrated in FIG. 3, the collector 22 is a sheet-like member. In this equipment, the collector 22 is not limited, as long as being in the form of a sheet, but may be made of a material selected typically from paper, films, various woven fabrics, nonwoven fabrics, and meshes. The collector may also be made of a metal, or may be a sheet or belt having a surface electric resistance equivalent to such metals.

[0087] In the nonwoven fabric production equipment illustrated in FIG. 3, materials to constitute the electrode 17 and the fiber collecting plate 14 may be selected from conductive materials (generally, metal components). Non-limiting examples of such materials include elementary metals including Group 6A elements such as chromium; Group 8 metal elements such as platinum; Group 1B elements such as copper and silver; Group 2B elements such as zinc; and Group 3B elements such as aluminum. The examples also include alloys of these metals (such as aluminum alloys and stainless alloys (stainless steels)), and compounds including these metals (exemplified by metal oxides such as silver oxide and aluminum oxide). Each of different metal components as above may be used alone or in combination. Among these metal components, particularly preferred examples are copper, silver, aluminum, and stainless steels. The shape of the fiber collecting plate 14 is exemplified by, but not limited to, plate, roller, belt, net, sawlike, wave, needle, and linear shapes. Among these shapes, plate and roller shapes are particularly preferred. Non-limiting examples of the laser light absorber 19 include metals and porous ceramics each coated with a black body. Non-limiting examples of the heat absorber 18 include black ceramics. With the equipment as mentioned above, the high-melting-point resin fibers and the nonwoven fabric according to the present invention can be efficiently produced.

EXAMPLES

[0088] The present invention will be illustrated in further detail with reference to several examples below. It should be noted, however, that the examples are by no means intended to limit the scope of the present invention.

[0089] Polymer Sheet Preparation

[0090] Polymer sheets A to D were prepared in the following manner.

[0091] Sample PEEK resins in the form of chips as mentioned below, which are high-melting-point resins, were each extruded into sheets using the LABO PLASTOMILL T-Die Extruder (supplied by Toyo Seiki Seisaku-Sho Ltd.) with a T-die having a die width of 150 mm and a lip width of 0.4 mm, at an extrusion temperature of 345.degree. C. to 360.degree. C. The extruded sheets were coiled at a haul-off roller temperature of 140.degree. C. and a coiling speed of 1.0 to 2.0 m/min, and yielded the polymer sheets A to D each having a thickness of 0.1 mm. The polymer sheets B, C, and D were subjected to a heat treatment at 230.degree. C. for 20 min after the molding using the extruder.

[0092] The prepared polymer sheet had degrees of crystallinity and viscosities as measured at a temperature of 400.degree. C. and a shear rate of 121.6 s.sup.-1 as follows. The viscosities were measured by the after-mentioned polymer sheet viscosity measuring method, and the degrees of crystallinity were determined by the same method as in the degree of crystallinity of high-melting-point resin fibers as mentioned below.

[0093] Polymer sheet A: VESTAKEEP 1000G (noncrystalline sample original sheet: having a degree of crystallinity of 12.7% and a viscosity of 151 Pas)

[0094] Polymer sheet B: VESTAKEEP 1000G (crystalline sample original sheet: having a degree of crystallinity of 35.5% and a viscosity of 151 Pas)

[0095] Polymer sheet C: VESTAKEEP 3300G (crystalline sample original sheet: having a degree of crystallinity of 36.7% and a viscosity of 761 Pas)

[0096] Polymer sheet D: VESTAKEEP 4000G (crystalline sample original sheet: having a degree of crystallinity of 37.7% and a viscosity of 1012 Pas)

[0097] Polymer Sheet Viscosity Measuring Method

[0098] The viscosity as measured at a temperature of 400.degree. C. and a shear rate of 121.6 s.sup.-1 was measured using the capillary rheometer (trade name Capillograph 1D, supplied by Toyo Seiki Seisaku-Sho Ltd.) with a jig having a capillary diameter of 1 mm and a length of 10 mm.

[0099] Next, high-melting-point resin fibers were produced according to Examples 1 to 3 and Comparative Examples 1 to 5 by the following method using the polymer sheets A to D prepared by the above-mentioned method. In Examples 1 to 3 and Comparative Examples 1 to 5, the polymer sheets used, the polymer sheet feed speeds, the laser light powers, and degrees of crystallinity and fiber sizes (diameters) of the resulting high-melting-point resin fibers are as given in Table 1. No fiber was obtained according to Comparative Examples 2 and 5.

[0100] High-Melting-Point Resin Fiber Production

[0101] The high-melting-point resin fibers according to Examples 1 to 3 and Comparative Examples 1 to 5 were produced using the equipment schematically illustrated in FIG. 1.

[0102] The laser source 1 of the equipment illustrated in FIG. 1 used herein was a CO.sub.2 laser system (supplied by Universal Laser Systems, Inc., having a wavelength of 10.6 .mu.m and a power of 45 W, with air cooling, and having a beam diameter of 4 mm). In the equipment, there were arranged a beam expander with 2.5-fold magnification and a homogenizer (having an incident beam diameter of 12 mm (designed value) and an outgoing beam diameter of 12 mm (designed value)) both as the beam-expander-homogenizer 2; a collimation lens (having an incident beam diameter of 12 mm (designed value) and an outgoing beam diameter of 12 mm (designed value)) as the collimation lens 3; and a cylindrical lens group (plano-concave lenses, f-30 mm) and a cylindrical lens group (plano-convex lenses, f-300 mm) both as the cylindrical lens group 4 in the specified sequence at predetermined positions. By passing through these light-path controllers, the spot-like laser light was converted into planar laser light having a width of about 150 mm and a thickness of about 1.4 mm and was irradiated to the band-like melt zone (edge) 6a of the polymer sheet 6. This gave the high-melting-point resin fibers.

[0103] Method for Measuring Degree of Crystallinity of High-Melting-Point Resin Fiber

[0104] The degree of crystallinity of each high-melting-point resin fiber was calculated from the amounts of heat determined by differential scanning calorimetry.

[0105] The differential scanning calorimetry was performed using a differential scanning calorimeter (DSC Q2000, supplied by TA) with alumina as a reference material in a nitrogen atmosphere at temperatures in the range of 0.degree. C. to 420.degree. C., and at a rate of temperature rise of 20.degree. C/min.

[0106] On the basis of the amounts of heat determined by the differential scanning calorimetry, the degree of crystallinity was determined according to the expression:

Degree of crystallinity (%)=[(Heat of fusion of sample)-(Heat of recrystallization of sample)]/(Heat of fusion of perfect crystal (130 J/g)).times.100

TABLE-US-00001 TABLE 1 Voltage Feed Laser Degree of Fiber size difference speed power crystal- (diameter) Resin (kV/cm) (mm/min) (W) linity (.mu.m) Example 1 Polymer sheet A 6 6 61 24% 0.7 Example 2 Polymer sheet A 5 4 61 24% 1 Example 3 Polymer sheet A 4 4 61 24% 2 Comparative Example 1 Polymer sheet A 4 1 61 33% 5 Comparative Example 2 Polymer sheet B 4 4 61 -- not fiberized Comparative Example 3 Polymer sheet B 4 1 61 36% 7 Comparative Example 4 Polymer sheet C 4 1 61 36% 8 Comparative Example 5 Polymer sheet D 4 1 61 -- not fiberized

REFERENCE SIGNS LIST

[0107] 1 laser source [0108] 2 beam-expander-homogenizer [0109] 3 collimation lens [0110] 4 cylindrical lens group [0111] 5 planar laser light [0112] 6 polymer sheet [0113] 6a band-like melt zone [0114] 6b needle protrusion [0115] 7 holder [0116] 8 fiber collecting plate [0117] 9 thermography [0118] 10 high voltage generator [0119] 11 laser source [0120] 12 light-path controller [0121] 13 polymer sheet feeder [0122] 14 fiber collecting plate [0123] 15 heater [0124] 16 holder [0125] 17 electrode [0126] 18 heat absorber [0127] 19 laser light absorber [0128] 20a high voltage generator [0129] 20b high voltage generator [0130] 21 pulley [0131] 22 collector [0132] 23 cabinet

INDUSTRIAL APPLICABILITY

[0133] The high-melting-point resin fibers according to the present invention are extremely fine, have heat resistance and chemical resistance at excellent levels, and give nonwoven fabrics which are useful typically as fuel cell separators, filters for medical materials, and space materials.

Claims

1. A high-melting-point resin fiber comprising a resin having a melting point of 250.degree. C. or higher, the fiber having a diameter of 4 .mu.m or less.

2. The high-melting-point resin fiber according to claim 1, wherein the resin having a melting point of 250.degree. C. or higher comprises a poly(ether ether ketone) (PEEK).

3. The high-melting-point resin fiber according to claim 1, which has a degree of crystallinity of 30% or less.

4. A method for producing a high-melting-point resin fiber, the method comprising: applying planar laser light to a polymer sheet comprising a noncrystalline high-melting-point resin to heat and melt an edge of the polymer sheet linearly to thereby form a band-like molten zone; providing a potential difference between the band-like molten zone and a fiber collecting plate to form a needle protrusion in the band-like molten zone of the polymer sheet and to allow a fiber ejected from the needle protrusion to fly toward the fiber collecting plate; and collecting the fiber on the fiber collecting plate or on a collector disposed between the molten zone and the fiber collecting plate to give the high-melting-point resin fiber according to claim 1.

5. The method according to claim 4 for producing a high-melting-point resin fiber, wherein the polymer sheet is fed at a speed of 2 to 20 mm/min.

6. The method according to claim 4 for producing a high-melting-point resin fiber, wherein the potential difference is 0.1 to 30 kV/cm.

7. The method according to claim 4 for producing a high-melting-point resin fiber, wherein the polymer sheet has a viscosity of 800 Pas or less as measured at a temperature of 400.degree. C. and a shear rate of 121.6 s.sup.-1.

8. A nonwoven fabric comprising the high-melting-point resin fiber according to claim 1.

9. The high-melting-point resin fiber according to claim 2, which has a degree of crystallinity of 30% or less.

10. A method for producing a high-melting-point resin fiber, the method comprising: applying planar laser light to a polymer sheet comprising a noncrystalline high-melting-point resin to heat and melt an edge of the polymer sheet linearly to thereby form a band-like molten zone; providing a potential difference between the band-like molten zone and a fiber collecting plate to form a needle protrusion in the band-like molten zone of the polymer sheet and to allow a fiber ejected from the needle protrusion to fly toward the fiber collecting plate; and collecting the fiber on the fiber collecting plate or on a collector disposed between the molten zone and the fiber collecting plate to give the high-melting-point resin fiber according to claim 2.

11. A method for producing a high-melting-point resin fiber, the method comprising: applying planar laser light to a polymer sheet comprising a noncrystalline high-melting-point resin to heat and melt an edge of the polymer sheet linearly to thereby form a band-like molten zone; providing a potential difference between the band-like molten zone and a fiber collecting plate to form a needle protrusion in the band-like molten zone of the polymer sheet and to allow a fiber ejected from the needle protrusion to fly toward the fiber collecting plate; and collecting the fiber on the fiber collecting plate or on a collector disposed between the molten zone and the fiber collecting plate to give the high-melting-point resin fiber according to claim 3.

12. The method according to claim 5 for producing a high-melting-point resin fiber, wherein the potential difference is 0.1 to 30 kV/cm.

13. The method according to claim 5 for producing a high-melting-point resin fiber, wherein the polymer sheet has a viscosity of 800 Pas or less as measured at a temperature of 400.degree. C. and a shear rate of 121.6 s.sup.-1.

14. The method according to claim 6 for producing a high-melting-point resin fiber, wherein the polymer sheet has a viscosity of 800 Pas or less as measured at a temperature of 400.degree. C. and a shear rate of 121.6 s.sup.-1.

15. A nonwoven fabric comprising the high-melting-point resin fiber according to claim 2.

16. A nonwoven fabric comprising the high-melting-point resin fiber according to claim 3.